Near net shape densification of cerium composites and making nano-sized phase separated microstructures in ceramics

Abstract

A dense ceramic body includes a composite microstructure having a first solid phase and a second solid phase. The first solid phase includes cerium oxide with cerium in a Ce(IV) oxidation state, and the second solid phase includes a non-cerium-based oxide. An oxygen ion transport phase is also included, forming a continuous network throughout the body. The composite microstructure is embedded within this continuous network of the oxygen ion transport phase. The first and second solid phases are arranged in a nano-sized, phase-separated morphology.

Description

NEAR NET SHAPE DENSIFICATION OF CERIUM COMPOSITES AND MAKING NANO-SIZED PHASE SEPARATED MICROSTRUCTURES IN CERAMICSSTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0001] This invention was made with United States Government support from the National Institute of Standards and Technology (NIST), an agency of the United States Department of Commerce. The Government has certain rights in this invention.CROSS-REFERENCE TO RELATED APPLICATION

[0002] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 677,048 (filed July 30, 2024), which is herein incorporated by reference in its entirety.BACKGROUND

[0003] The present invention generally relates to the field of ceramic manufacturing, and more particularly to techniques for the near net-shape densification of ceramic composites and the controlled formation of nano-sized, phase-separated microstructures within a densified body.

[0004] The ubiquitous utility of ceramic materials, spanning applications from advanced electronics and biomedical implants to thermal barriers and ballistic armor, is a direct consequence of their exceptional properties, including high- temperature stability, extreme hardness, and chemical inertness. The conventional manufacturing pathway for producing functional ceramic components begins with the consolidation of fine powders into a predetermined shape, often referred to as a green part. This initial form is inherently porous and lacks the mechanical strength required for most end-use applications. To eliminate this porosity and achieve the desired finalproperties, the green part is subjected to a high-temperature thermal process known as sintering, wherein mass diffusion drives a densification process that fuses the powder particles together. A fundamental and unavoidable consequence of this conventional sintering is a significant volumetric shrinkage of the component.

[0005] This substantial reduction in volume is not merely an incidental byproduct of the process; it is a fundamental and pervasive challenge that complicates the fabrication of complex, high-precision ceramic parts. The shrinkage can be non- uniform, leading to deleterious effects such as warping, the formation of internal stresses, and catastrophic cracking. Such defects compromise not only the mechanical integrity and functional performance of the final part but also its aesthetic qualities. Consequently, achieving high dimensional accuracy, often with required tolerances of less than one percent, becomes an exceedingly difficult and costly endeavor. While subtractive manufacturing or machining is a common solution for shaping metallic parts to final tolerances, the inherent hardness and brittleness of densified ceramics render such post-sintering machining operations both economically impractical and technically intractable.

[0006] In response to these challenges, the field has developed several near-net-shape manufacturing techniques aimed at minimizing the dimensional change from the green state to the final densified state. These approaches include, for example, reaction bonding, the displacive compensation of porosity through infiltrating secondary phases, the use of pre-ceramic polymers, and directed metal oxidation. While these methods can successfully produce parts with reduced shrinkage, they are not without their own significant drawbacks. Many of these near- net-shape processes are highly complex, often requiring multiple intricate steps, the use of expensive or reactive precursors, and complex thermal schedules. Furthermore, they frequently result in the formation of specific ceramic-metal or ceramic-ceramic composite microstructures, which, while beneficial for some applications, may be undesirable or functionally limiting for others and severely constrains the palette of achievable material compositions. A persistent challenge also remains in creating and preserving nano-scale microstructural features, which can impart superior properties, as the high thermal energy required for densificationsimultaneously provides a strong thermodynamic driving force for the coarsening and growth of such fine features, effectively destroying them.

[0007] It is therefore an objective of the present invention to provide a novel processing route for fabricating dense ceramic bodies that simultaneously achieves near-net-shape dimensional control while enabling the in-situ formation of finely-tuned, nano-sized phase-separated microstructures, thereby overcoming the above- mentioned disadvantages of the prior art at least in part. Accordingly, fabrication techniques that leverage controlled, volume-expansive phase transformations to achieve near-net-shape densification and deliberately engineered nano-scale microstructures would represent a significant advancement and would be favorably received in the art.BRIEF DESCRIPTION

[0008] One aspect of the present invention relates to a dense ceramic body. A ceramic may be understood as an inorganic, non-metallic solid material comprising metal, non-metal, or metalloid atoms primarily held in ionic, covalent, or a mixture of the two bonds. A dense body may be understood as a solid object having a low level of porosity, typically with a relative density exceeding 95% of its theoretical maximum.

[0009] It may be provided that the body comprises a composite microstructure comprising at minimum a first solid phase and a second solid phase, wherein the first solid phase comprises cerium oxide with cerium in a Ce(IV) oxidation state and the remaining solid phases are comprised of non-cerium-based oxides. A microstructure may be understood as the fine-scale structure of a material, and a composite microstructure is one composed of two or more distinct constituent phases. A polycrystalline solid phase may be understood as a region of space throughout which all physical properties of a material are essentially uniform. Cerium is a chemical element that can exist in multiple oxidation states, with cerium (IV) oxide being a specific, stable oxide. An oxide is a chemical compound containing at least one oxygen atom, and a non-cerium-based oxide is one that does not have cerium as a primary constituent cation. This arrangement, which combines distinct material phases at an intimate level, allows for the engineering of novel, multifunctional properties in the finalbody, thereby enhancing performance by synergistically uniting the characteristics of the constituent phases, such as mechanical strength from the non-cerium-based oxide and the catalytic, electronic, thermoelectric, or ionic properties from the cerium oxide.

[0010] It may be provided that the body further comprises an oxygen ion transport phase forming a continuous network, wherein the precipitated phase or the composite microstructure is embedded in the continuous network of the oxygen ion transport phase. An oxygen ion transport phase may be understood as a material that facilitates the diffusion of oxygen ions, often referred to as a solid electrolyte, and a continuous network may be understood as an interconnected structure extending throughout the volume of the material. This arrangement provides a novel solution for manufacturing by creating highly efficient, three-dimensional pathways for oxygen transport deep within the body during its thermal processing. This improved functionality allows a solid-state reaction to proceed more efficiently and uniformly throughout the entire volume, avoiding the defects and non-uniform ity associated with slow, surface-inward reaction fronts, which leads to a final product with superior structural integrity and reliability.

[0011] It may be provided that the first solid phase and the second solid phase are arranged in a nano-sized, phase-separated morphology, the morphology having an interphase spacing of less than 100 nanometers. A morphology describes the form and structure of the constituent phases, with a nano-sized morphology indicating that characteristic features are on the scale of nanometers. Interphase spacing may be understood as the characteristic distance between adjacent domains of the different phases. This arrangement provides a new and unique solution to a long-standing materials science problem by creating an exceptionally high density of functional interfaces between the different solid phases within a fully dense body. Such high interfacial area results in enhanced performance, including improved functionality such as superior fracture toughness, as the numerous interfaces can act to deflect or arrest propagating cracks. This fine-scale structure, which is not achievable in conventionally sintered materials due to coarsening, provides a practical application for creating materials with superior mechanical, catalytic, thermal, and electrical properties.

[0012] It may be provided that the precursor material comprises a complex oxide. A complex oxide may be understood to be an inorganic material with a crystalline ionically bonded structure with a minimum of two major cationic species.

[0013] One aspect of the present invention relates to a method of forming a dense ceramic body. It may be provided that the method includes the step of obtaining a precursor powder comprising a cerium-based complex oxide having cerium stabilized in a Ce(lll) oxidation state. A precursor powder may be understood as a fine, particulate raw material from which a final product is formed. A cerium-based complex oxide is an oxide compound containing cerium and at least one other different cation, and the Ce(lll) oxidation state is a specific, lower-energy electronic state of the cerium ion that is chemically reactive and prone to oxidation.

[0014] It may be provided that Ce(IV) oxide, or CeO2, is a product phase of the eutectoid decomposition of Ce(lll) based complex oxides. It may be provided that a eutectoid reaction is understood to be a phase transformation in which a solid phase transforms into two distinct solid phases.

[0015] It may be provided that CeO2 is an inorganic compound with the fluorite crystal structure. The fluorite structure is known to provide oxygen ion transport.

[0016] One aspect of the present invention relates to the method of forming a composite microstructure by transport of oxygen through an oxygen ion transport phase. An oxygen ion transport phase is a material that facilitates the diffusion of oxygen ions. This arrangement establishes the foundational chemistry for the entire fabrication process. By starting with a precursor powder that reacts to form a product phase that has advantageous oxygen ion transport properties, a novel material system is created, providing a new and unique solution that enables a more efficient and reliable manufacturing pathway based on subsequent, precisely controlled solid-state transformations.

[0017] One aspect of the present invention relates to the use of an oxygen transport phase to facilitate fast delivery of oxygen ions to the precursor reaction front. The oxygen transport phase may be included as a second phase mixed together with the initial Ce(lll) based complex oxide, or the oxygen transport phase may form on thesurface of the precursor particles when exposed to oxidizing conditions. For example, when CeAIOs particles in a green body are exposed to sufficient oxidizing conditions, CeO2+Al2O3 will form on the surface of the particles, and this two phase composite will have an interconnected oxygen transport phase of Ce02 which facilitates further delivery of oxygen to the reaction front.

[0018] It may be provided that the method includes forming a green part from the precursor powder. A green part may be understood as a consolidated, yet porous and unfired, ceramic article that has been shaped into a desired geometry but has not yet undergone final densification. This arrangement provides a critical utility by allowing the material to be formed into a complex, near-net-shape geometry while it is still in a malleable, easily processable state. This performs the shaping task more efficiently and accurately than attempting to machine a final, fully densified, and hardened ceramic component, which is often intractable.

[0019] It may be provided that the method includes heating the green part in an oxidizing atmosphere to a sintering temperature, wherein the heating concurrently effectuates a densification of the green part via porosity removal and a eutectoid decomposition of the cerium-based complex oxide, the eutectoid decomposition transforming the cerium-based complex oxide into a composite microstructure of a first solid phase comprising cerium oxide with cerium in a Ce(IV) oxidation state and additional solid phases, and wherein the oxygen ion transport phase facilitates a substantially homogenous progression of the eutectoid decomposition throughout a volume of the green part to form a nano-sized, phase- separated morphology while mitigating coarsening of the first and second solid phases. This step represents a novel and non-obvious convergence of multiple physical and chemical phenomena within a single thermal treatment. By heating the green part in an oxidizing atmosphere, the process simultaneously triggers both the densification via porosity removal, which causes the part to shrink, and a transformative eutectoid decomposition, which involves a volume expansion. The arrangement provides for a substantially homogenous progression of this decomposition throughout the entire volume of the part, a functionality enabled by the combination of the sintering and reaction steps. The green part contains interconnected porosity that exposes the surfaces of the precursor powder to changesin the oxygen concentration in the processing atmosphere. The surfaces of the precursor powder in the consolidated green part can react to form an oxygen ion transport phase, which minimizes the length scale of reaction fronts and associated internal stresses that would otherwise lead to cracking. This enhanced performance allows for the in-situ creation of a nano-sized, phase-separated morphology while concurrently mitigating the thermodynamic tendency for these fine features to coarsen at high temperatures, thereby performing the task of creating a dense, nano-structured ceramic more efficiently and reliably, yielding a final component with superior structural integrity and precisely engineered microstructural properties that are unattainable through prior art methods.

[0020] One aspect of the present invention relates to a process for manufacturing a near-net-shape ceramic composite. A near-net-shape ceramic composite is a multicomponent ceramic article produced in a form very close to its final desired dimensions, comprising at least two distinct ceramic phases.

[0021] It may be provided that the process comprises formation of a composite microstructure by route of a redox-driven eutectoid reaction. A redox-driven reaction may be understood to be a reaction in which the oxidation state of one or more of the ions in the precursor becomes a different oxidation state in the reactant phase.

[0022] It may be provided that the process comprises a precursor material that is thermodynamically in a metastable state. A metastable state may be understood to be a state in which the material can be handled, measured, and processed without changing its properties, but on exposing the material to different thermal, electrical, or chemical potential stimuli, the material transitions to a lower energy state.

[0023] It may be provided that the process comprises providing a precursor powder comprising a cerium-based oxide material, the cerium-based oxide material characterized by a crystalline structure containing cerium in a metastable Ce(lll) oxidation state and being capable of undergoing a redox-driven eutectoid decomposition into a plurality of product phases upon oxidation. A precursor powder may be understood as a particulate starting material, and a crystalline structure refersto an ordered atomic arrangement. A redox-driven eutectoid decomposition is a solid- state reaction, initiated by a change in oxidation state, wherein a single solid phase transforms into a plurality of distinct product phases. This arrangement provides a novel and non-obvious material selection, embedding a latent, controllable potential for volumetric expansion directly within the starting material. This improved functionality of the precursor itself is foundational to the entire manufacturing process, offering a new and unique solution to counteract the inherent shrinkage of conventional ceramic processing.

[0024] It may be provided that the process includes shaping the precursor powder into a porous green part. Shaping may be understood as the act of forming the material into a specific geometry, and a porous green part is a consolidated, unfired article containing a network of voids. This arrangement provides the practical utility of defining the intricate final geometry of the component at a stage when the material is still in a compliant, easily-formed state. This allows the task of creating complex shapes to be performed more efficiently and accurately, circumventing the extreme difficulty and expense associated with machining a fully hardened and brittle final ceramic part.

[0025] It may be provided that the process includes subjecting the porous green part to a thermal treatment in an oxidizing atmosphere, the thermal treatment inducing a densification process involving a volumetric contraction through porosity reduction, while concurrently initiating the eutectoid decomposition, the eutectoid decomposition generating a volumetric expansion, wherein kinetics of the volumetric contraction and the volumetric expansion are controlled such that the volumetric expansion substantially compensates for the volumetric contraction, thereby producing a densified ceramic body with a volume substantially equivalent to a volume of the porous green part. A thermal treatment is the application of heat, and an oxidizing atmosphere is one that promotes oxidation. Densification is the reduction of porosity leading to volumetric contraction, or shrinkage, while the eutectoid decomposition generates a counteracting volumetric expansion. Kinetics may be understood as the rates of these transformations. This arrangement provides a new and unique solution to the long-standing problem of sintering shrinkage by ingeniously orchestrating two opposing physical phenomena within a single, unified thermal step.The controlled balance between the kinetics of densification and decomposition allows the volumetric expansion to precisely offset the contraction, performing the task of densification more reliably and accurately by producing a final dense body with minimal dimensional change. This enhanced performance directly improves the quality and reproducibility of manufactured ceramic components, enhancing their utility by enabling the fabrication of high-precision parts with composite microstructures that were previously unattainable.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The following description cannot be considered limiting in any way. Various objectives, features, and advantages of the disclosed subject matter can be more fully appreciated with reference to the following detailed description of the disclosed subject matter when considered in connection with the following drawings, in which like reference numerals identify like elements.

[0027] FIG. 1 shows, according to some embodiments, a process flow for making block diagram illustrating an exemplary method of forming a ceramic part.

[0028] FIG. 2 shows, according to some embodiments, a schematic diagram comparing a conventional process that results in significant volumetric shrinkage with an innovative near-net-shape sintering process that results in a dimensionally stable final part.

[0029] FIG. 3 shows, according to some embodiments, a schematic diagram illustrating the underlying principle of a eutectoid decomposition reaction, where a precursor phase transforms into a plurality of product phases with a resulting net volumetric expansion.

[0030] FIG. 4 shows, according to some embodiments, a schematic diagram illustrating the underlying principle of the formation of an oxygen ion transport phase produced by a eutectoid decomposition reaction, where the surface of a precursor phase transforms into a plurality of product phases with a resulting net volumetric expansion.

[0031] FIG. 5 shows, according to some embodiments, schematic diagrams illustrating exemplary nano-sized, phase-separated morphologies achievable through the innovative process, specifically depicting a lamellar morphology and a rod-like morphology.

[0032] FIG. 6 shows, according to some embodiments, a schematic diagram contrasting an undesirable, surface-inward reaction front with the homogeneous, volume-wide reaction enabled by the presence of a continuous oxygen ion transport phase.

[0033] FIG. 7 shows X-ray diffraction (black) of CeAIOs samples processed under conditions (from top to bottom): 1 ) CeAIOs powder annealed at 1300 °C for 18 hrs. under forming gas, 2) the surface of a polished pellet heated at 10 °C / min and sintered at 1200 oC for 10 min., 3) the surface of a sintered pellet heated at 10 °C / min and sintered at 1350 °C for 10 min., and 4) the polished surface of a pellet heated at 10 °C / min and sintered at 1650 °C for 20 hrs.. The Rietveld refinements of each spectrum are provided with the tick marks representing the phases red) CeAIOs, purple) CeO2, purple) a-Al2O3, and blue) CeAlnO .

[0034] FIG. 8 shows isothermal TGA data for CeAIOs powder samples measured at 1075 °C with oxygen atmospheres (from left to right) of (179700, 46550, 23470, 15460, and 144) ppm respectively.

[0035] FIG. 9 shows percent linear strain of samples heated at constant rates to 1550 °C for 1 hr in air.

[0036] FIG. 10 shows percent linear strain of samples heated at a constant rate of 10 °C / min to 1150 °C, 1250 °C, 1350 °C, and 1450 °C in flowing argon. Once the temperature was reached for the isothermal dwell, the furnace environment was switch from argon to air.

[0037] FIG. 11 shows percent linear strain of samples heated to 1550 °C and held for 2 hr. under flowing argon. Samples were then cooled under flowing argon to a second isothermal dwell temperature. Upon reaching the second isothermal dwell temperature, the furnace environment was switched from an argon to air atmosphere.

[0038] FIG. 12 shows (left) percent theoretical densities of sintered samples measured using a geometrical method for samples, mixed CeO2 / Al2O3 precursors (triangles), CeAIOs precursors (squares), and CeAIOs precursors sintered using a two- step heating / oxidation process (circles). The size of the marker is proportional to the time at the sintering temperature with a key given (right).

[0039] FIG. 13 shows percent theoretical density versus percent change in volume during sintering for a CeO2 / Al2O3 precursor (squares) and a CeAIOs precursor with a two-step heating / oxidation sintering procedure (circles).

[0040] FIG. 14 shows secondary electron images of polished cross-sections of samples sintered at, a) CeO2 / Al2O3 sintered at 1650 °C for 2 hr., b) CeAIOs sintered at 1650 °C for 0.2 hr., c) CeAIOs sintered at 1650 °C for 20 hr., d) CeO2 / Al2O3 sintered at 1500 °C for 60 hr., e) CeAIOs sintered at 1500 °C for 60 hr., and f) CeAIOs sintered at 1550 °C for 60 hr. where the scale bar (red) is 20 pm.

[0041] FIG. 15 shows microscopy images of various sintered ceramics. The polished cross-sections represent images of samples processed under the following conditions: a) heated in air at 10 °C / min to 1550 °C and held for 60 hrs., b) heated in argon at 10 °C / min to 1350 °C and held for 10 min. in air followed by 2 hrs. at 1600 °C, c) heated in argon at 10 °C / min to 1500 °C and held for 2 hrs. followed by 18 hrs. in air, d) heated in air at 10 °C / min to 1550 °C and held for 2 hrs. followed by cooling in air, e) heated in argon at 20 °C / min to 1550 °C and held for 2 hrs. followed by cooling to 1450 °C in air and held for 2 hrs., f) heated in argon at 20 °C / min to 1550 °C and held for 2 hrs. followed by cooling to 1400 °C in air and held for 3 hrs., g-i) multiple images of the area around a Vickers indentation showing potential evidence of crack branching and crack bridging. The scale bar (red) is equal to 5 pm.

[0042] FIG. 16 shows microscopy images of a sample processed via a two- step heating and oxidation procedure, revealing a preserved, dense, sub-100 nm lamellar microstructure. This polished cross-section is from a sample heated at 10 °C / min to 1550 °C under flowing forming gas followed switching the gas to flowing air and then quenching the sample.DETAILED DESCRIPTION

[0043] A detailed description of one or more embodiments is presented herein by way of exemplification and not limitation.

[0044] Conventional fabrication of high-performance ceramic articles is fundamentally constrained by two intractable challenges. First, the standard process of sintering a porous green part into a dense final body is accompanied by significant and often non-uniform volumetric shrinkage, which leads to warping, internal stress, and cracking, thereby making the production of complex, high-tolerance components exceedingly difficult. Second, the creation and preservation of beneficial nano-scale microstructural features within a bulk, dense ceramic is a formidable materials science problem; the very thermal energy required to eliminate porosity and densify the body also drives the rapid coarsening and destruction of these fine features. Previous attempts to circumvent this by first densifying a precursor material and then inducing a solid-state transformation have proven unsuccessful, as the reaction progresses slowly from the surface inward, leading to inhomogeneous volume changes that cause catastrophic cracking and a non-uniform microstructure.

[0045] The dense ceramic body overcomes these deficiencies of dimensional instability and the inability to create and preserve uniform, nano-scale features in a bulk article.

[0046] It has been discovered that a dense ceramic body can be realized that intrinsically unites a nano-sized, phase-separated composite microstructure with a pervasive, continuous network of an oxygen ion transport phase. This unique structural arrangement provides a novel and non-obvious solution to the fundamental impediments of the prior art. One advantage of the dense ceramic body lies in its ability to be manufactured with unprecedented microstructural control. In this embodiment, the final product will consist of a minimum of three phases: an oxygen ion transport phase and two or more phases embedded in the oxygen transport phase following a eutectoid decomposition. The presence of the oxygen ion transport phase forming a continuous network, in which the precipitated phase or composite microstructure is embedded, provides an improved functionality by acting as an internal, three- dimensional conduit for oxygen ions. This allows a solid-state reaction to be initiatedhomogeneously and simultaneously throughout the entire volume of the body, rather than proceeding from the exterior surface inward. This circumvents the development of the destructive stress gradients and non-uniform reaction fronts that cause cracking and coarsening in conventional materials. In an embodiment, the final product will consist of two phases that are products of the eutectoid decomposition. One of these phases will be a Ce(IV)-based oxygen ion transport phase formed from the eutectoid decomposition of the reactant Ce(lll)-containing phase. In this embodiment, reaction fronts proceeding from the exterior surface may exist; however, the formation an efficient oxygen ion transport phase during this type of reaction has been discovered to accommodate stress gradients and facilitate fast reaction kinetics that result in solid, defect-free, parts with preserved nano-lamellar microstructures.

[0047] This improved manufacturing reliability and efficiency directly enables the creation and preservation of the nano-sized, phase-separated morphology. The arrangement of the first solid phase, comprising cerium oxide, and the second, non-cerium-based oxide phase with an interphase spacing of less than 100 nanometers results in a material with an exceptionally high density of interfaces. This structure yields enhanced performance and opens new avenues of utility. For instance, the multitude of interfaces provides superior mechanical properties, such as enhanced fracture toughness, by creating pathways to deflect and impede crack propagation. This results in a dense ceramic body that is not only manufacturable with high fidelity but also possesses a combination of structural integrity and nano-scale features that provide for superior performance in demanding mechanical, catalytic, or electronic applications previously considered unattainable for bulk ceramic components.

[0048] In an embodiment, a dense ceramic body (240) comprises a composite microstructure (245) having a first solid phase and, at minimum, a second solid phase, wherein the first solid phase comprises cerium oxide with cerium in a Ce(IV) oxidation state and the additional solid phase comprises a non-cerium-based oxide; an oxygen ion transport phase (200) forming a continuous network, wherein the precipitated phase or composite microstructure (245) is embedded in the continuous network of the oxygen ion transport phase (200); and wherein the first solid phase and the second solid phase are arranged in a nano-sized, phase-separated morphologyhaving an interphase spacing of less than 100 nanometers. In an embodiment, the oxygen ion transport phase (200) comprises an oxide material that contains oxygen vacancies and facilitates oxygen ion transport. In an embodiment, the oxygen ion transport material can be selected from a group of stabilized zirconia, doped cerium oxide, lanthanum gallate, strontium ferrite, barium zirconate, or any choice of appropriate oxygen ion conductors. In an embodiment, the second solid phase is the non-Ce(IV) containing product phase formed as a result of the redox-driven eutectoid decomposition. In an embodiment, the reactive precursor can consist of any oxide structure that contains as a major constituent Ce(lll) such as perovskite CeXOs, pyrochlore Ce2X2O?, spinel XCe2O4, scheelite CeXCM, or melilite CeXTO? compositions, where X and T are any appropriate / charge-balancing cation. In an embodiment, the nano-sized, phase-separated morphology is a rod-like morphology. In an embodiment, the composite microstructure (245) further comprises a dopant element substituting a cation in the reactant or product phases.

[0049] In an embodiment, the second solid phase can be one of many polymorphs. It may be provided that a polymorph is a chemical composition that can form a stable compound as more than one crystalline structures. In an embodiment, an example is AI2O3 comprising at least one polymorph selected from the group consisting of a- AI2O3, y- AI2O3, and Q- AI2O3.

[0050] In an embodiment, a volume of the body (240) exhibits a net change of less than 5% relative to a volume of a corresponding green-part (220) having a green density of from 55% to 75% of a theoretical density of the body (240). In an embodiment, the first solid phase is CeO2, the second solid phase is AI2O3, the oxygen ion transport phase (200) is doped ceria or doped zirconia, and the nano-sized, phase- separated morphology comprises lamellar features having an interphase spacing of less than 100 nanometers.

[0051] Referring to the figures, a dense ceramic body (240) is provided, the structure and composition of which overcome the fundamental limitations of conventionally fabricated ceramics.

[0052] The dense ceramic body (240) is defined in part by its composite microstructure (245), which is composed of, at minimum, a first continuous solid phaseand a second solid phase. The first solid phase comprises cerium oxide where the cerium ion is in the stable Ce(IV) oxidation state, while the second solid phase is a non-cerium-based oxide. A functionality of this composite microstructure (245) is to synergistically combine the distinct physical and chemical attributes of its constituent phases into a single, monolithic body. This structure is not merely a physical mixture of pre-existing powders. Instead, it is the direct result of an in-situ eutectoid decomposition of a single-phase precursor, a process that enables an exceptionally intimate and controlled dispersion of the phases. The primary benefit of this arrangement is the creation of a multifunctional material whose properties can be precisely tailored. The cerium oxide phase, for example, provides useful catalytic, redox, and ionic properties, while the non-cerium-based oxide phase typically imparts desirable mechanical characteristics such as hardness and strength, resulting in a material with enhanced performance and a broader utility than either component alone. While an implementation involves alumina (AI2O3) as the second solid phase for structural applications, a wide array of other non-cerium-based oxides may be employed to achieve different functionalities, including but not limited to chromia (Cr2O3), gallia (Ga2O3), or scandia (SC2O3). As an example of use, a dense ceramic body (240) implemented with a CeO2 1 AI2O3 composite microstructure (245) serves as a superior candidate for biomedical implants, offering a unique combination of high fracture toughness and biocompatibility.

[0053] A further defining characteristic of the dense ceramic body (240) is the oxygen ion transport phase (200), which forms a continuous, interconnected network throughout the body. The precipitated phase or composite microstructure (245) is embedded within this pervasive network. Structurally, this phase acts as a robust matrix that encapsulates and supports the composite microstructure. Functionally, this continuous network is the critical enabler of the body's unique nanostructure; during the thermal processing, it provides a high-diffusivity pathway for oxygen ions, permitting the eutectoid decomposition to occur homogeneously and rapidly throughout the entire volume of the body, rather than being confined to a slow- moving reaction front initiating at the exterior surface. Implementation of this phase involves its incorporation into the initial precursor powder such that, upon sintering, its particles form an interconnected framework. Yttria-stabilized zirconia (YSZ) or gadolinium doped ceria (GDC) represents an exemplary implementation due to its highoxygen ion conductivity and excellent mechanical properties. For examples, a GDC / AI2O3 composite can be realized by making a perovskite precursor of (Cei- xGdx)AI03 and oxidizing the reactant to form nano-GDC / Al2O3 clusters in a GDC matrix. For example, a YSZ / CeO2 / Al2O3 composite can be realized by sintering a mixture of YSZ with reactant CeAI03 and oxidizing the material to produce nano- CeO2 / Al2O3 cluster in a YSZ matrix. The chief benefit of this continuous network is the mitigation of catastrophic failure during manufacturing, a common failure mode in the prior art, which in turn enables the very formation of the internal nano-structure. This improves the reliability of the manufacturing process and yielding a final body with superior structural integrity. For an application in solid oxide fuel cells, the continuous network of the oxygen ion transport phase (200), such as YSZ, provides the primary electrolyte function while the embedded precipitate phase or composite microstructure (245) contributes enhanced catalytic activity and mechanical stability.

[0054] The arrangement of the first solid phase and the second solid phase within the composite microstructure (245) is given by a nano-sized, phase-separated morphology, where the characteristic interphase spacing is less than 100 nanometers. The functionality of this nano-scale arrangement is to generate an exceptionally high density of interfacial boundaries between the two distinct solid phases. This specific morphology is a direct and controllable outcome of the inventive manufacturing process, wherein the homogenous reaction enabled by the oxygen ion transport phase (200) kinetically favors the formation of these fine features and prevents their subsequent coarsening. The benefits stemming from this high interfacial area are significant and diverse, leading to enhanced performance across multiple domains. Mechanically, the dense array of interfaces acts to deflect, bifurcate, and arrest propagating micro-cracks, dramatically improving the fracture toughness and reliability of the dense ceramic body (240). Chemically, it maximizes the surface area between catalytically active phases, improving the efficiency of catalytic reactions. This morphology, which cannot be achieved in bulk, dense bodies via conventional methods, represents a new and unique solution for engineering materials with superior properties. The process allows for variations in this morphology, which may be implemented as a lamellar (layered) structure or as a rod-like structure depending on the specific precursor chemistry and thermal processing conditions. In a practical use case for a high-performance cutting tool, the interphase spacing of less than 100nanometers creates a microstructure that is exceptionally resistant to fracture propagation under high stress, thereby extending the tool's operational lifetime and enhancing its durability.

[0055] In some embodiments of the dense ceramic body (240), the oxygen ion transport phase (200) comprises yttria-stabilized zirconia. This material is implemented due to its well-established high ionic conductivity at elevated temperatures and its excellent chemical and mechanical compatibility with the cerium- based oxide systems. Its function is to provide an efficient, pervasive pathway for oxygen transport, which is a prerequisite for enabling the homogeneous, volume-wide decomposition reaction. A benefit of utilizing yttria-stabilized zirconia is the enhanced reliability and efficiency of the manufacturing process, as its superior ion transport properties ensure that the eutectoid transformation proceeds uniformly, thereby mitigating the risk of defect formation and leading to a final body (240) with superior structural integrity and microstructural uniformity. A dense ceramic body (240) intended for use in a solid oxide fuel cell, for instance, would benefit from a yttria- stabilized zirconia network which serves both as the structural matrix and the primary ion-conducting electrolyte.

[0056] The composition of the second solid phase can be selected to impart specific properties to the final dense ceramic body (240). In certain embodiments, this second phase is an oxide that includes or can be chosen from the group consisting of AI2O3, Cr2O3, B2O3, SC2O3, Ga20s, Mn2O3, Fe2O3, or equivalent transition metal oxide, metalloid oxide, or alkaline metal earth oxides. Furthermore, the second is not limited to binary oxides and may be more compositionally complex such as a ternary oxide (e.g., calcium aluminates). The identity of this phase is determined by the selection of the initial cerium-based complex oxide precursor. The functionality of selecting from this group is to allow for the precise engineering of the final body's thermomechanical or catalytic properties. For example, implementing AI2O3 results in a body with high hardness and wear resistance, while implementing Cr20s can create a composite with enhanced catalytic activity. In other embodiments, the second solid phase may be selected from the group consisting of Ta2Os and Nb2Os, which expands the utility of the dense ceramic body (240) into electronic applications by incorporating phases known for their distinct dielectric properties.

[0057] The nano-sized, phase-separated morphology can be further defined to achieve specific anisotropic properties. In one implementation, the morphology is a lamellar morphology, characterized by alternating, plate-like layers of the first and second solid phases. This structure is particularly effective at impeding crack propagation, as a crack attempting to move across the layers is repeatedly deflected at the numerous interfaces, yielding a dense ceramic body (240) with significantly enhanced fracture toughness. Alternatively, the morphology may be implemented as a rod-like morphology, wherein nanoscopic rods of one or more phases are embedded within a continuous matrix of the other. This arrangement provides reinforcement that can improve the mechanical strength and damage tolerance of the body in multiple directions, offering a different pathway to enhanced performance.

[0058] Further refinement of the material's properties can be achieved through the introduction of a dopant element. The composite microstructure (245) may comprise a dopant element that substitutes a cation in any of the reactant solid phases. This doping is implemented by adding the dopant in small quantities to the initial precursor powder. The function of the dopant is to introduce controlled point defects into the crystal lattice of the product phases, which can fine-tune a variety of characteristics, including ionic conductivity, catalytic surface activity, or optical absorption, thereby providing an additional lever of control for improving the functionality of the final dense ceramic body (240).

[0059] In embodiments where the second solid phase is AI2O3, the specific crystal structure, or polymorph, of the alumina can be controlled. The AI2O3 may comprise at least one polymorph that includes or that can be selected from the group consisting of a- AI2O3, y- AI2O3, and 9- AI2O3. Since each polymorph possesses distinct properties that may be beneficial for particular applications such as catalysis, the ability to control the polymorphic content during processing provides another means to tailor the body's functional performance for a specific application.

[0060] The dimensional stability of the final article provides a significant manufacturing advantage. A volume of the dense ceramic body (240) exhibits a net change of less than 5% relative to a volume of its corresponding green-part (220), where the green-part (220) has a typical green density of from 55% to 75% of the final theoretical density. This near-net-shape outcome is a direct benefit of the process,wherein the volumetric expansion from the eutectoid reaction is kinetically balanced against the volumetric contraction from sintering. This allows the task of producing high-precision, complex-shaped components to be performed more efficiently and reliably, yielding a durable final part that does not require costly and difficult postsintering machining.

[0061] A specific, high-performance embodiment is realized when the first solid phase is CeO2, the second solid phase is AI2O3, and the oxygen ion transport phase (200) is a fluorite structured oxide phase . In this case, the nano-sized, phase- separated morphology comprises lamellar features having an interphase spacing of less than 100 nanometers. This particular combination results in a novel material with exceptional, multi-functional properties, uniting the proven biocompatibility and toughness of the CeC / AI2O3 system with an extremely fine microstructure that imparts superior resistance to fracture, making it a material with unparalleled utility for demanding applications such as next-generation medical or dental implants.

[0062] The dense ceramic body (240) and methods for its fabrication incorporate several non-conventional elements that synergistically overcome the longstanding challenges in the field of ceramic processing. These elements include the strategic use of a volume-expansive eutectoid decomposition to achieve near-net- shape manufacturing, the in-situ formation of precisely controlled nano-sized microstructures within a fully dense body, and the integral role of a continuous oxygen ion transport phase (200) that enables these outcomes.

[0063] An aspect of certain embodiments is the use of a redox-driven eutectoid decomposition to actively compensate for sintering shrinkage. The functionality of this element is to generate a predictable and controllable volumetric expansion concurrently with the volumetric contraction that occurs during densification. This is implemented by providing a precursor powder (201 ) comprising a cerium-based oxide material, such as CeAIOs, wherein the cerium is in a chemically reactive Ce(lll) oxidation state. During a single thermal treatment in an oxidizing atmosphere, two simultaneous phenomena are induced: the porous green part (220) begins to densify, which involves a volumetric contraction, while the Ce(lll) ions oxidize to Ce(IV), triggering a eutectoid decomposition of the parent phase into a plurality of product phases (e.g., CeO2 and AI2O3). The combined molar volume of these productphases is significantly greater than that of the parent precursor phase, resulting in a net volumetric expansion. A benefit of this is the achievement of a near-net-shape final part, which is a solution to the intractable problems of warping, cracking, and dimensional inaccuracy that plague conventional ceramic sintering. By carefully controlling the process kinetics, the expansion can be tailored to substantially cancel out the shrinkage, resulting in a dense ceramic body (240) with final dimensions remarkably close to those of its green part (220). Variations can be implemented by selecting different cerium-based oxide precursors (e.g., CeCrOs, CeGaOs) to modulate the magnitude of the volume expansion and the final properties of the composite microstructure (245). This improved functionality allows for the reliable and efficient manufacturing of complex, high-precision ceramic components, such as intricate turbine engine components or custom-fit medical implants, which were previously impossible to fabricate with the required fidelity.

[0064] Another non-conventional element is the in-situ formation of a nanosized, phase-separated composite microstructure (245) within the bulk of the final, fully dense ceramic body (240). The functionality of this element is to imbue the ceramic with superior material properties derived from features on the nanometer scale. This is implemented not by attempting to sinter pre-existing nano-powders, which invariably coarsen, but by creating the nano-structure as a direct product of the eutectoid decomposition. The process energetically favors the initial formation of extremely fine, organized domains of the product phases, which can be preserved by controlling the thermal profile. The benefits of this are manifested as enhanced performance in the final part. For instance, a nano-scale lamellar or rod-like morphology, with an interphase spacing of less than 100 nanometers, creates an exceptionally high density of interfaces that serve to deflect and arrest propagating cracks, dramatically improving the fracture toughness and durability of the body. This provides a practical utility for creating materials with superior mechanical strength, enhanced catalytic activity due to high surface area, or tailored electrical properties. As an example of use, a ceramic armor plate fabricated with such a nano-sized microstructure would exhibit superior ballistic performance due to its enhanced ability to dissipate impact energy.

[0065] The formation of such a delicate nano-structure within a dense body is made possible by a third non-conventional element, the incorporation of a continuous oxygen ion transport phase (200). The function of this phase, which is implemented by adding a solid electrolyte powder like yttria-stabilized zirconia (YSZ) to the initial precursor powder (201 ), is to act as a pervasive, internal conduit for oxygen throughout the entire volume of the green part (220). Alternatively, the eutectoid decomposition of CeAIOs results in the formation of a continuous oxygen transport phase CeO2. In this case, the oxygen transport network is produced by the decomposition of the precursor material. Conventionally, a redox-driven reaction in a dense body would be limited by the slow diffusion of oxygen from the exterior surface, creating a reaction front that moves inward. This invariably leads to massive stress gradients between the reacted (expanded) shell and the unreacted core, causing catastrophic cracking and preventing the formation of a uniform microstructure. The continuous oxygen ion transport phase (200) provides a novel and non-obvious solution to this problem. It allows oxygen ions to be delivered uniformly and rapidly to all regions of the body simultaneously, enabling a homogeneous, volume-wide eutectoid decomposition. The benefits includes prevention of cracking, thereby improving the reliability and scalability of the process, and it allows the reaction to complete quickly, which is essential for mitigating the thermal coarsening that would otherwise destroy the nano-sized morphology. This improved functionality makes it possible to fabricate large, defect-free, nano-structured ceramic components, a feat unattainable by prior art methods.

[0066] In an embodiment, a method of forming a dense ceramic body (240) comprises: obtaining a precursor powder (201 ) comprising a cerium-based complex oxide having cerium in a Ce(lll) oxidation state and an oxygen ion transport phase (200); forming a green part (220) from the precursor powder (201 ); and heating the green part (220) in an oxidizing atmosphere to a sintering temperature, wherein the heating concurrently effectuates a densification of the green part (220) via porosity removal and a eutectoid decomposition of the cerium-based complex oxide, the eutectoid decomposition transforming the cerium-based complex oxide into a composite microstructure (245) of a first solid phase comprising cerium oxide with cerium in a Ce(IV) oxidation state and a second solid phase, and wherein the oxygen ion transport phase (200) facilitates a substantially homogenous progression of theeutectoid decomposition throughout a volume of the green part (220) to form a nanosized, phase-separated morphology while mitigating coarsening of the first and second solid phases. In an embodiment, the cerium-based complex oxide is selected from the group consisting of any oxide structure that contains as a major constituent Ce(lll) such as perovskite CeXOs, pyrochlore 062X267, spinel XCe2O4, scheelite CeXO4 or melilite CeXTO? compositions. . Alternatively, the cerium-based complex can include alloys of multiple compositions. An example of such an alloyed complex oxide is a perovskite (A,Ce)(Bl,Bll)O3 where A is an ion with 3+ charge and B1and B" are any combination of cations with an average charge of 3+. In an embodiment, the step of obtaining a precursor powder (201 ) comprises mixing a Ce(lll) stabilized oxide powder with a powder of the oxygen ion transport phase (200), wherein the oxygen ion transport phase (200). In an embodiment, the step of forming the green part (220) utilizes a shape-forming technique selected from the group consisting of uniaxial pressing, isostatic pressing, tape casting, slip casting, gel casting, injection molding, extrusion, pressure filtration, three-dimensional printing, or any comparable ceramic shape forming technique. In an embodiment, the method further comprises mixing a binder with the precursor powder (201 ) before the forming step and removing the binder from the green part (220) before or during the heating step. In an embodiment, the step of heating the green part (220) can proceed through two routes. In an embodiment, route one comprises a two-step heating procedure having a first heating step in an inert or reducing atmosphere to a first temperature to densify the green part (220) while substantially maintaining the Ce(lll) oxidation state of the cerium-based complex oxide, and a second heating step wherein the oxidizing atmosphere is introduced at a second temperature sufficient to initiate the eutectoid decomposition throughout the volume of the densified green part (220) without inducing cracking from inhomogeneous volume expansion. In an embodiment, the second route comprises a single step process in which the material is heated only in oxidizing atmosphere promoting both densification and decomposition of the reactive Ce(lll) containing green part (220). In an embodiment, the heating is performed with control of an oxygen partial pressure of the oxidizing atmosphere to kinetically balance a rate of volume contraction from the densification with a rate of volume expansion from the eutectoid decomposition, thereby minimizing a net volume change between the green part (220) and the dense ceramic body (240). In an embodiment, the precursor powder (201 ) further comprises one or more dopant elements, and the dopant elements areincorporated into the first solid phase or the second solid phase during the eutectoid decomposition. In an embodiment, the cerium-based complex oxide is CeAIOs, the oxygen ion transport phase (200) is yttria-stabilized zirconia forming a continuous network, and the heating step is controlled to produce the dense ceramic body (240) having a nano-sized lamellar microstructure of CeO2 and AI2O3 with an interphase spacing of less than 100 nanometers.

[0067] The method of forming a dense ceramic body (240), as illustrated in the accompanying figures, begins with the step of providing a precursor powder (201 ). The functionality of this initial step is to establish a specific, non-conventional material system that contains the latent potential for the subsequent transformative processes. This is implemented by providing a powder that is a composite mixture, comprising a cerium-based complex oxide and a distinct oxygen ion transport phase (200). The cerium-based complex oxide is a precisely engineered compound, such as CeAIOs, characterized by cerium ions existing in the reactive Ce(lll) oxidation state. The oxygen ion transport phase (200) is a solid electrolyte, such as yttria-stabilized zirconia (YSZ), which is intimately mixed with the complex oxide powder. A benefit is the creation of a novel precursor system; by distributing the oxygen-conducting phase throughout the initial powder, a mechanism for enabling a subsequent, volume-wide homogeneous reaction is pre-emptively embedded in the material, a unique solution that improves the efficiency and reliability of the entire manufacturing process. Variations can be implemented by selecting different perovskite-structure cerium- based oxides or alternative solid electrolyte materials to tailor the final properties of the dense ceramic body (240). For an application requiring high catalytic activity, one would obtain a precursor powder (201 ) comprising a CeCrOs complex oxide thoroughly mixed with fine YSZ particles.

[0068] Following the provision of the precursor, the method involves forming a green part (220) from the precursor powder (201 ). The functionality of this step is to define the macroscopic geometry and shape of the final component while the material is in a compliant, easily-handleable state. This is typically implemented using ceramic forming technologies, such as uniaxial pressing, isostatic pressing, tape casting, slip casting, extrusion, pressure filtration, gel casting, injection molding, three-dimensional printing, or any comparable ceramic shape forming technique, often with thetemporary use of an organic or inorganic binder to provide mechanical integrity to the porous green part (220). The benefit of this step is its immense utility, as it allows for the cost-effective creation of intricate and complex shapes that would be technologically difficult and economically prohibitive to produce by trying to machine the final, fully hardened ceramic article. For example, to fabricate a porous ceramic filter with a complex internal channel structure, a 3D printing technique would be used to form the green part (220) from a slurry containing the precursor powder (201 ).

[0069] The method also includes heating the green part (220) in an oxidizing atmosphere to a sintering temperature. The functionality of this thermal treatment is multifaceted and represents a novel and non-obvious convergence of several distinct physical phenomena. The heating concurrently effectuates two processes, densification of the green part (220) through the removal of porosity, and a redox- driven eutectoid decomposition of the cerium-based complex oxide. This decomposition transforms the parent Ce(lll)-containing oxide into a composite microstructure (245) composed of a first solid phase containing Ce(IV) oxide and a second solid phase. The oxygen ion transport phase (200), having been established as a continuous network during the initial stages of sintering, performs the critical function of facilitating a substantially homogenous progression of this eutectoid decomposition throughout the entire volume of the part. This is implemented by providing uniform, rapid access for oxygen ions to all regions of the body simultaneously. A benefit of this is the complete mitigation of the cracking and internal stress that would otherwise arise from a slow, surface-inward reaction front. This enhanced reliability, in turn, allows for the successful formation of a nano-sized, phase-separated morphology while simultaneously mitigating the thermal coarsening that would typically destroy such fine features at sintering temperatures. This performs the task of creating a dense, nano-structured ceramic more efficiently and accurately than any prior art method, yielding a final dense ceramic body (240) with superior, precisely engineered microstructural and mechanical properties. In a use case for producing a high-toughness component, a green part (220) of a CeAIOs-CeC composite is heated in air, allowing the CeO2 network to facilitate the rapid, uniform decomposition of CeAIOs into a nano-lamellar microstructure of CeO2 and AI2O3 throughout the component as it densifies, preserving the fine toughening features in the final part. In a use case for producing a thermoelectric material, (Cei-xGdx)AIO3 issintered to produce a dense material with a continuous matrix of oxygen conducting GDC with nano-lamellar inclusions of AI2O3 oxide that act as phonon deflectors resulting in improved thermal resistivity. In a use case for producing a high surface area supported catalyst, Ce(Ah-xMx)03, where M is a noble metal, is heated in oxidizing conditions to produce a partially dense / gas permeable material with a continuous matrix of oxygen conducting ceria with nano-lamellar inclusions of AI2O3 decorated with metal precipitates that produce high number densities of catalytically active triple-phase boundaries.

[0070] In certain implementations of the method, the cerium-based complex oxide is a perovskite that includes or that can be selected from the group consisting of, for example, CeAIOs, CeCrOs, CeScOs, CeGaOs, CeMnOs, CeFeOs, or any other appropriate CeXOs composition. The functionality of selecting a material with the perovskite crystal structure is to provide a well-defined and predictable starting platform for the eutectoid decomposition. These materials are thermodynamically unstable in oxidizing conditions, providing the driving force for the reaction. The provides improved reliability and controllability of the process, as the specific transformation behavior and resulting product phases can be accurately predicted based on the known crystallography and chemistry of the chosen perovskite, leading to more consistent and reproducible manufacturing outcomes.

[0071] The step of obtaining the precursor powder (201 ) may be implemented by mixing a pre-synthesized Ce(lll) stabilized oxide powder with a powder of the oxygen ion transport phase (200). This step provides a practical and efficient means of creating the necessary composite starting material. By physically blending the two distinct powders, a homogeneous distribution is achieved, which is essential for the subsequent formation of the continuous transport network. A benefit of this implementation is its utility and simplicity, allowing for the use of readily available materials to construct the novel precursor system required for the inventive process.

[0072] The formation of the green part (220) may utilize a shape-forming technique selected from the group consisting of uniaxial pressing, isostatic pressing, tape casting, slip casting, extrusion, pressure filtration, gel casting, injection molding, three-dimensional printing, or any comparable ceramic shape forming technique. This has wide applicability across a range of standard industrial manufacturing platforms.A benefit is the versatility of the method, which is not constrained to a single, specialized forming technique but is instead compatible with existing manufacturing infrastructure, thereby expanding its utility for producing a diverse array of component geometries, from thin sheets via tape casting to highly complex structures via three- dimensional printing.

[0073] To ensure the mechanical integrity of the green part (220) prior to and during the initial stages of heating, the method may further comprise mixing a binder with the precursor powder (201 ) before the forming step and subsequently removing the binder from the green part (220). The function of the binder is to provide temporary strength, preventing the collapse or fracture of the porous article. This is implemented by using a fugitive material, typically an organic polymer, that can be cleanly removed by a low-temperature burnout process before the onset of sintering. This improves the reliability of the overall process, as it ensures the shaped component maintains its intended geometry through handling and the initial thermal stages.

[0074] An embodiment of the heating step involves a heating procedure. This procedure comprises a first heating step in an inert or reducing atmosphere to a first temperature, the function of which is to densify the green part (220) while the Ce(lll) oxidation state of the cerium-based complex oxide remains substantially stable. This is followed by a second heating step wherein the oxidizing atmosphere is introduced at a second temperature sufficient to initiate the eutectoid decomposition. The benefit of this two-step process is that it decouples the densification from the volume-expansive transformation, which provides a novel and non-obvious solution to prevent the cracking that occurs when an already-dense body undergoes an inhomogeneous reaction. This improved functionality allows for the fabrication of larger and thicker defect-free components.

[0075] In certain embodiments, the heating is performed with deliberate control over the oxygen partial pressure of the oxidizing atmosphere. The function of this control is to kinetically balance the rate of volume contraction from the densification process against the rate of volume expansion from the eutectoid decomposition. This is implemented using controlled gas flow systems in the furnace. The benefit is the achievement of superior dimensional control in the final part, therebyminimizing the net volume change between the green part (220) and the dense ceramic body (240) and yielding a product with enhanced near-net-shape accuracy.

[0076] The versatility of the method may be expanded through the inclusion of one or more dopant elements within the precursor powder (201 ). These dopants are incorporated into either the first solid phase or the second solid phase during the eutectoid decomposition, with the function of further tailoring the final material properties. A benefit of this is improved functionality, as doping can be used to modify catalytic activity, ionic conductivity, or optical properties for a specific end-use application.

[0077] A specific implementation of the method where the cerium-based complex oxide is CeAIOs and the oxygen ion transport phase (200) is yttria-stabilized zirconia forming a continuous network, allows for the heating step to be controlled to produce a dense ceramic body (240) having a nano-sized lamellar microstructure of CeO2 and AI2O3 with an interphase spacing of less than 100 nanometers. This demonstrates the high degree of control afforded by the method, yielding a final product with an exceptionally fine microstructure that provides for enhanced performance, particularly superior fracture toughness.

[0078] In an embodiment, a process for manufacturing a near-net-shape ceramic composite comprises providing a precursor powder (201 ) comprising a cerium-based oxide material, the cerium-based oxide material characterized by a crystalline structure containing cerium in a metastable Ce(lll) oxidation state and being capable of undergoing a redox-driven eutectoid decomposition into a plurality of product phases upon oxidation; shaping the precursor powder (201 ) into a porous green part (220); and subjecting the porous green part (220) to a thermal treatment in an oxidizing atmosphere, the thermal treatment inducing a densification process involving a volumetric contraction through porosity reduction, while concurrently initiating the eutectoid decomposition, the eutectoid decomposition generating a volumetric expansion, wherein kinetics of the volumetric contraction and the volumetric expansion are controlled such that the volumetric expansion substantially compensates for the volumetric contraction, thereby producing a densified ceramic body (240) with a volume substantially equivalent to a volume of the porous green part (220). In an embodiment, the cerium-based oxide material is a complex oxide havinga perovskite crystal structure. In an embodiment, the cerium-based oxide material that includes or that can be selected from the group consisting of CeXOs, pyrochlore Ce2X2O?, spinel XCe2O4, scheelite CeXCM compositions, or melilite CeXTO? , other Ce(lll)-containing complex oxides and solid solutions thereof. In an embodiment, the kinetics are controlled by regulating at least one parameter that includes or that can be selected from the group consisting of, raw material powder size and morphology, a heating rate of the thermal treatment, a partial pressure of oxygen in the oxidizing atmosphere, and a dwell time at a sintering temperature. In an embodiment, the step of providing a precursor powder (201 ) further comprises synthesizing the cerium- based oxide material by heating a mixture of oxide precursors in a reducing environment sufficient to stabilize the Ce(lll) oxidation state prior to the shaping step. In an embodiment, the precursor powder (201 ) further comprises additional noncerium-based oxide powders, whereby the densified ceramic body (240) comprises a multi-phase composite material including the plurality of product phases and the additional non-cerium-based oxides. In an embodiment, the thermal treatment is a multi-step procedure comprising a first thermal step of heating the porous green part (220) in an atmosphere having a first oxygen partial pressure that is sufficiently low to inhibit the eutectoid decomposition while allowing the volumetric contraction through densification to proceed, and, at minimum, a second thermal step of increasing the oxygen partial pressure to a second oxygen partial pressure at a temperature sufficient to initiate and complete the eutectoid decomposition and its associated volumetric expansion throughout a body of the densified green part (220). In an embodiment, the volumetric expansion resulting from the eutectoid decomposition is greater than 15% relative to a volume of the cerium-based oxide material. In an embodiment, a total volumetric shrinkage of the densified ceramic body (240) relative to the porous green part (220) is less than 5%, and wherein the densified ceramic body (240) possesses a final density of at least 95% of its theoretical density.

[0079] The process for manufacturing a near-net-shape ceramic composite overcomes the deficiencies of dimensional instability and manufacturing imprecision that are inherent to conventional ceramic fabrication methods. It has been discovered that a process for manufacturing a near-net-shape ceramic composite can be achieved by ingeniously harnessing a controlled, volume-expansive chemical transformation to directly counteract the volumetric shrinkage intrinsic to thedensification process. This approach provides a novel solution that yields enhanced performance by producing components with superior dimensional fidelity. The utility of this process lies in its capacity to fabricate complex geometries that meet strict tolerance requirements, thereby reducing manufacturing costs and material waste by obviating the need for difficult post-sintering machining and by lowering rejection rates attributable to warping or fracture.

[0080] The process begins with the step of providing a precursor powder (201 ) that comprises a cerium-based oxide material. The functionality of this precursor powder (201 ) is to serve as the initial building block of the component, but more critically, to contain the latent chemical potential required for the subsequent in-situ volumetric expansion. This is implemented by formulating a powder whose cerium- based oxide material is characterized by a specific crystalline structure containing cerium in a metastable Ce(lll) oxidation state. This specific electronic configuration renders the material capable of undergoing a redox-driven eutectoid decomposition into a plurality of product phases upon exposure to an oxidizing atmosphere at elevated temperatures. The primary benefit of selecting such a precursor is that it provides a novel and non-obvious starting point for the entire manufacturing scheme; the mechanism for shrinkage compensation is chemically embedded within the material itself, providing an improved functionality that is foundational to the process. Alternative implementations may involve the selection of different cerium-based oxide chemistries, such as CeAIOs or CeCrOs, to precisely tailor the magnitude of the volumetric expansion and the properties of the final composite. As an example of use, to fabricate a component with high hardness and thermal stability, one would provide a precursor powder (201 ) predominantly composed of CeAIOs, which is known to decompose into hard, refractory phases of AI2O3 and CeO2.

[0081] Subsequently, the process involves shaping the precursor powder (201 ) into a porous green part (220). The function of this step is to define the final, often intricate, geometry of the ceramic article at a stage when the material is in a compliant, powder-based form. This step is implemented using standard ceramic shape-forming techniques such as pressing, molding, casting, extrusion, or additive manufacturing methods. A significant benefit of this is its utility, as it allows for the efficient and accurate formation of complex components that would be otherwiseimpossible to create by machining a final, fully hardened ceramic. For instance, in the fabrication of a custom, patient-specific orthopedic implant, a 3D model of the implant would be used to guide a 3D printing process, shaping the precursor powder (201 ) into the required complex geometry of the porous green part (220).

[0082] The final step of the process involves subjecting the porous green part (220) to a thermal treatment in an oxidizing atmosphere. This single thermal step has a complex and multifaceted functionality, as it concurrently induces two opposing physical transformations. First, it initiates the densification process, wherein the removal of porosity causes a volumetric contraction. Simultaneously, it triggers the redox-driven eutectoid decomposition of the cerium-based oxide material, a transformation which generates a significant volumetric expansion. The process is implemented by placing the green part (220) in a furnace where the temperature, heating rate, and oxygen partial pressure are precisely controlled. The key to the process is that the kinetics of the volumetric contraction and the volumetric expansion are controlled such that the expansion substantially compensates for the contraction. The benefit of this is a novel and non-obvious solution to the fundamental problem of sintering shrinkage. By orchestrating this balance, the process performs the task of densification more reliably and accurately, thereby producing a densified ceramic body (240) with a volume substantially equivalent to that of the porous green part (220). In a practical application to create a large, thin ceramic substrate for an electronic device, the thermal treatment profile would be carefully programmed to ensure the expansion rate from the decomposition matches the shrinkage rate from densification at every stage, resulting in a final densified ceramic body (240) that is perfectly flat and free from the warping that would render a conventionally-processed part useless.

[0083] In further embodiments of the process, the cerium-based oxide material is specifically a complex oxide having a perovskite, or other well-defined complex oxide, crystal structure. The functionality of using a perovskite is to provide a starting material with a well-defined, predictable crystallographic framework that is known to be susceptible to the desired redox-driven eutectoid decomposition. This is implemented by synthesizing precursor powders such as CeAIOs or CeCrOs. The benefit is a more reliable and controllable manufacturing process; because the transformation pathways of these perovskite structures are well-understood, theresulting product phases and the magnitude of the volumetric expansion can be accurately predicted, leading to enhanced performance through greater consistency and reproducibility in the final manufactured articles.

[0084] The process offers a wide utility through the selection of the cerium- based oxide material from the group consisting of perovskite CeXOs, pyrochlore Ce2X2O?, spinel XCe2O4, scheelite CeXCM, or melilite CeXTO? compositions, other Ce(lll)-containing complex oxides and solid solutions thereof. The function of this selection is to provide a versatile material palette for tailoring the final properties of the densified ceramic body (240). This is implemented by choosing a specific compound or a solid solution of multiple compounds as the precursor powder (201 ). The benefit is a significant improvement in the functionality and applicability of the process. For instance, selecting CeAIOs as the precursor yields a final composite containing hard, refractory AI2O3 for structural applications, while selecting CeCrOs yields a composite with catalytically active Cr20s phases. Creating a solid solution, such as Ce(AI,Cr)O3, provides a means to continuously tune the properties between these end-members.

[0085] The kinetics of the opposing volumetric transformations are controlled by determining an optimized multi-step procedure. An example multi-step procedure may consider regulating at least one process parameter selected from the group consisting of heating rates of the thermal treatment, partial pressures of oxygen in the oxidizing atmosphere, and a dwell times at the sintering temperatures. The functionality of this step is to provide precise, active control over the rates of densification and decomposition. This is implemented using modem, programmable furnace systems capable of precise atmospheric and thermal profile control. The benefit is a dramatic improvement in the accuracy and reliability of achieving the near- net-shape objective. By manipulating these parameters, the process is transformed from a passive heating cycle into a dynamically engineered event, allowing for the successful fabrication of parts with varying geometries and thicknesses. For example, a lower oxygen partial pressure can be used to slow the decomposition kinetics, allowing a thick part to achieve thermal uniformity before the volumetric expansion begins.

[0086] The process may further comprise synthesizing the cerium-based oxide material by heating a mixture of oxide precursors in a reducing environment priorto the shaping step. The function of this preliminary step is to ensure that the cerium in the precursor powder (201 ) is entirely and stably in the required reactive Ce(lll) oxidation state. This is implemented by calcining simpler oxide powders (e.g., CeO2 and AI2O3) in a furnace with a reducing atmosphere, such as a hydrogen-nitrogen forming gas. A key benefit is the enhanced reliability of the main process; this presynthesis guarantees that the precursor powder (201 ) has the maximum potential for volumetric expansion, ensuring the shrinkage compensation is effective and reproducible.

[0087] The utility of the process can be expanded by providing a precursor powder (201 ) that further comprises additional non-cerium-based oxide powders. The function of these additional powders is to serve as inert fillers that are incorporated into the final microstructure, allowing for the creation of more complex composite materials. This is implemented by physically blending powders such as ZrO2 or MgAl2O4 with the reactive cerium-based precursor powder (201 ) before the shaping step. The benefit is a significant increase in the versatility of the process, enabling the fabrication of materials with properties unattainable through the decomposition products alone, such as adding a secondary phase known for its exceptional thermal insulation or fracture toughening capabilities.

[0088] For manufacturing larger or thicker components, the thermal treatment may be implemented as a multi-step procedure. This process comprises a first thermal step of heating the porous green part (220) in an atmosphere with a low oxygen partial pressure to inhibit the eutectoid decomposition while allowing densification to proceed, followed by a second thermal step where the oxygen partial pressure is increased to initiate the volumetric expansion throughout the now-dense body. The functionality of this procedure is to decouple the volumetric contraction and expansion events. The benefit is a novel solution for preventing the formation of deleterious stress gradients that would otherwise cause cracking in larger parts, thereby improving the reliability of the process and expanding its utility to a wider range of component sizes.

[0089] The kinetics of the eutectoid phase transformation is dependent on multiple controllable parameters such as the starting precursor chemistry, powder morphology, the heating rates, hold temperatures, and oxygen atmosphere.Depending on considerations of required properties of the material, the processing parameters may be chosen to optimize for a minimum volume change during the entire sintering procedure which would attempt to compensate the kinetics of the volume contraction during sintering with the kinetics of the volume expansion of the eutectoid reaction. Alternatively, in order to engineer a particular composite microstructure additional processing steps may need to be considered. For example, to avoid coarsening of nano-lamellar regions, the sample may be sintered to full density under reducing conditions that preserve the Ce(lll) precursor oxidation state resulting in a volume reduction of the part. Following densification, the sample may be exposed to oxidizing conditions at one or a series of temperatures that results in volume expansion of the sample and allows the nano-lamellar regions to precipitate for times scales that avoid coarsening. In both cases the final volume change of the material is equivalent; the difference is whether the eutectoid decomposition occurs concurrently or subsequently to the sintering process. Each process will result in different microstructures, and there are considerations that may dictate one preference of process of volume change vs. another. For example, if the material is one component of a more complex device that is being thermally processed with other materials, the volume expansion may need to be engineered to accommodate the strain of the other components.

[0090] The process can be rendered more efficient by incorporating inorganic binder (202) with the precursor powder (201 ) during the shaping step. This inorganic binder (202) comprises metal ions that convert into a useful secondary oxide phase within the densified ceramic body (240) during the thermal treatment. Its function is twofold: it provides necessary mechanical strength to the porous green part (220), and it simultaneously serves as a precursor for an in-situ reinforcing phase. This is implemented using metal salts, such as metal nitrates or acetates, as the binder. The benefit is improved functionality and process efficiency, as a step that traditionally creates a waste product (binder burnout) is transformed into a value-adding step that enhances the properties of the final product.

[0091] The process achieves enhanced performance by utilizing a volumetric expansion from the eutectoid decomposition that is greater than 15% relative to the volume of the cerium-based oxide material. The functionality of this largeexpansion is to provide a powerful counterforce to the sintering shrinkage. This is an intrinsic property of the chosen precursor chemistry. The benefit is a more robust and forgiving manufacturing process, as the significant expansion can effectively compensate for a wide range of green densities and processing conditions.

[0092] The successful implementation of the process results in a total volumetric shrinkage of the densified ceramic body (240) relative to the porous green part (220) of less than 5%, while the densified ceramic body (240) simultaneously possesses a final density of at least 95% of its theoretical density. This combination of final properties defines the superior outcome of the process: a ceramic article that is both structurally sound and dimensionally precise. This enhanced performance represents the culmination of the process's advantages, delivering a final product that meets the stringent requirements for high-performance applications.

[0093] FIG. 2 presents a schematic comparison illustrating the fundamental operational difference and a benefit of the inventive manufacturing process relative to a conventional ceramic sintering process. The figure shows two parallel process flows, a conventional process depicted in the upper portion and an exemplary embodiment in the lower portion, allowing for a direct visual assessment of their respective outcomes on a component's dimensional stability. The interconnectivity of the elements is linear and sequential, showing a starting component, a thermal process, and a final resulting component for each pathway. In the conventional process pathway, a porous green part is initially formed into a desired geometry. This green part is then subjected to a thermal process, which functionally serves to consolidate the porous structure into a dense solid. The physical structure of the resulting final dense part is shown to be significantly smaller in all dimensions than the initial green part, visually representing the substantial volumetric contraction, or shrinkage, that is an inherent and problematic consequence of conventional densification. This volumetric shrinkage is a well-known deficiency in the art and can be in the range of 30% to 50%, corresponding to linear shrinkages that can be from 10% to 20%, specifically from 12% to 18%, and more specifically from 14% to 16%. Such a large and often anisotropic change in dimension routinely leads to a loss of tolerance, component warping, the generation of deleterious internal stresses, and in manycases, catastrophic fracture of the component, thereby severely limiting the complexity and precision of parts that can be reliably manufactured.

[0094] Conversely, the lower pathway illustrates an embodiment of the disclosed process, which begins with a green part (220) of a size and shape identical to that in the conventional pathway, establishing a direct baseline for comparison. This green part (220) is subjected to the innovative sintering process, which, as disclosed herein, involves the concurrent phenomena of densification and a controlled, volume- expansive eutectoid decomposition. The functionality of this process is to achieve full densification while actively counteracting the volumetric contraction. The final dense part (240) is shown to have a physical structure with dimensions that are substantially equivalent to those of the initial green part (220), a state defined as near-net-shape. The implementation of this process relies on the specific precursor chemistries and controlled thermal and atmospheric profiles detailed in this disclosure. The benefit, as starkly illustrated by the direct comparison in FIG. 2, is the elimination of significant dimensional change, which overcomes the previously described deficiencies of the conventional art. This near-net-shape outcome allows for the fabrication of highly complex ceramic components with exceptional dimensional accuracy and fidelity to the intended design. The total volumetric shrinkage in the process herein can be less than 5%, specifically less than 2%, and more specifically less than 1 %, while still achieving a final density of over 95% of the theoretical maximum. As a variation, the process can be applied to green parts (220) of virtually any geometry, from simple pellets to intricate, latticed structures, with the expectation of similar dimensional retention. As an example of use, in the manufacturing of a multi-layer ceramic capacitor, an application demanding extreme geometric precision, the conventional process may result in delamination of the ceramic and metal electrode layers during sintering, resulting in a rejected part. The process, as depicted in the lower pathway of FIG. 2, would produce a fully dense multi-layer capacitor (240) that precisely matches the required complex geometry of the initial green part (220), thus demonstrating the process's superior functionality, reliability, and utility.

[0095] FIG. 3 provides a representation of a microstructural transformation of embodiments, specifically illustrating the principle of a eutectoid decomposition reaction that results in a net volumetric expansion. The figure shows a material systemin an initial Before state on the left and a transformed After state on the right, interconnected by a process step labeled Oxidation. The operability of the system is predicated on this transformation, which is the core physical mechanism responsible for compensating for sintering shrinkage. The Before state depicts the precursor phase, contained within a boundary representing its initial volume. This physical structure is shown as a homogeneous material composed of a single type of constituent particle, labeled with the example of CeAIO3or as a heterogeneous material composed of CeAIOs dispersed in an oxide matrix phase with good oxygen transport properties. This represents the cerium-based oxide material in its metastable, reduced Ce(lll) oxidation state prior to the primary thermal treatment. The functionality of this precursor phase is to serve as a chemically charged system, possessing a latent potential for a significant structural and volumetric change upon exposure to the correct stimulus. Its implementation involves synthesizing a singlephase powder with the desired perovskite or other suitable crystal structure, which is then formed into the green part (220). The central arrow, labeled Oxidation, represents the process step that triggers the transformation. This corresponds to the act of heating the material in an oxidizing atmosphere, which serves as the energetic and chemical impetus for the subsequent reaction. It is the critical operational link between the initial and final states of the material system. The After state on the right illustrates profound changes that occur as a result of the oxidation step. First, the material's physical structure has undergone a eutectoid decomposition, transforming from a single precursor phase into a composite microstructure comprising two distinct product phases, exemplified as CeO2and AI2O3. These product phases are shown to be intimately intermixed within the final structure. Second, the boundary representing the volume of the material is shown to be significantly larger than the initial volume, indicating a net volumetric expansion. This expansion is a direct and intended consequence of the fact that the combined molar volume of the product phases is greater than the molar volume of the precursor phase. The magnitude of this volumetric expansion is a performance characteristic of the process and can be from 10% to 25%, specifically from 15% to 20%, and more specifically from 17% to 19%.

[0096] A benefit of this transformation is its utility in manufacturing near-net- shape ceramics. The substantial and predictable volumetric expansion generated during this step is harnessed to directly counteract the volumetric contraction thatoccurs concurrently during the densification of the porous green part (220). This provides a novel and non-obvious solution to the long-standing problem of sintering shrinkage. Variations on this implementation can involve the use of different cerium- based precursors, which would result in different product phases and allow for the magnitude of the volumetric expansion to be precisely tailored for different material systems and green part densities. As an example of use in the process, a green part (220) formed from a precursor powder (201 ) of CeAIO3is heated. As the part densifies and begins to shrink, the oxidation simultaneously drives the decomposition of CeAIO3into CeO2and AI2O3, causing the microstructure to expand by approximately 18%. This expansion functionally negates the shrinkage, resulting in a final, fully dense ceramic body (240) that maintains the precise dimensions of the initial green part (220), thereby demonstrating the improved functionality and enhanced accuracy of the manufacturing method.

[0097] FIG. 4 provides a representation of a microstructural transformation of embodiments, specifically illustrating the principle of a eutectoid decomposition reaction that results in formation of an oxygen ion transport phase. The figure shows a material system in an initial Before state on the left and a partially transformed During state on the right, interconnected by a process step labeled Oxidation. The operability of the system is predicated on this transformation, which is the core physical mechanism responsible for delivering required oxygen ions to the eutectoid decomposition reaction front. The Before state depicts the precursor phase, contained within a boundary representing its initial volume. This physical structure is shown as a homogeneous material composed of a single type of constituent particle, labeled with the example of CeAIO3. This represents the cerium-based oxide material in its metastable, reduced Ce(lll) oxidation state prior to the primary thermal treatment. The functionality of this precursor phase is to serve as a chemically charged system, possessing a latent potential for a significant structural and volumetric change upon exposure to the correct stimulus. Its implementation involves synthesizing a singlephase powder with the desired perovskite or other suitable crystal structure, which is then formed into the green part (220). The central arrow, labeled Oxidation, represents the process step that triggers the transformation. This corresponds to the act of heating the material in an oxidizing atmosphere, which serves as the energetic and chemical impetus for the subsequent reaction. It is the critical operational link betweenthe initial and final states of the material system. The During state on the right illustrates profound changes that occur as a result of the oxidation step. First, the surface of the material's physical structure has undergone a eutectoid decomposition, transforming from a single precursor phase into a composite microstructure comprising two distinct product phases, exemplified as CeO2and AI2O3. These product phases are shown to be intimately intermixed with the Ce02, consisting of the oxygen ion transport phase, representing the continuous matrix phase. Second, the boundary representing the partially transformed surface volume of the material is shown to be significantly larger than the initial volume, indicating a net volumetric expansion. This expansion is a direct and intended consequence of the fact that the combined molar volume of the product phases is greater than the molar volume of the precursor phase. The magnitude of this volumetric expansion is a performance characteristic of the process and can be from 10% to 25%, specifically from 15% to 20%, and more specifically from 17% to 19%.

[0098] FIG. 5 presents schematic diagrams of two distinct, nano-sized, phase-separated morphologies that can be implemented within the composite microstructure (245) of the dense ceramic body (240). The figure illustrates the specific, controllable structural outcomes of the innovative process, which are themselves involved in the enhanced performance of the final material. The operability of these morphologies stems from the precise spatial arrangement of the first and second solid phases at the nanometer scale, which is a direct result of the kinetically controlled eutectoid decomposition reaction. The functionality of these structures is to impart superior mechanical and functional properties to the dense ceramic body (240) by creating an exceptionally high density of interfaces. The left panel of FIG. 5 depicts a lamellar morphology. The physical structure of this morphology is constituted by alternating, parallel, plate-like layers of the first solid phase and the second solid phase. These layers, or lamellae, are shown to have a consistent and periodic arrangement, with a well-defined interphase spacing. This structure is implemented as a direct product of a cooperative growth mechanism during the eutectoid decomposition, where the two product phases grow in a coupled manner with a preferred crystallographic registry. The functionality of this arrangement is to create an exceptionally high density of interfaces oriented perpendicular to the plane of the layers. The benefit of this lamellar structure is a dramatic enhancement in the fracturetoughness of the dense ceramic body (240). The numerous interfaces provide a tortuous path for any propagating crack; a crack attempting to traverse the structure must repeatedly change direction, a process that dissipates a significant amount of energy and effectively arrests the fracture. This provides a novel solution to the inherent brittleness of many ceramic materials, leading to enhanced performance and durability in high-stress environments. The interphase spacing between the lamellae is a critical parameter that can be controlled by the thermal processing conditions; this spacing can be from 1 nanometer to 100 nanometers, specifically from 5 nanometers to 50 nanometers, and more specifically from 10 nanometers to 20 nanometers. As an example of use, a cutting tool insert fabricated with this lamellar morphology would exhibit superior resistance to chipping and catastrophic failure, extending its operational lifetime.

[0099] The right panel of FIG. 5 illustrates a rod-like morphology, which represents an alternative implementation of the nano-sized, phase-separated structure. The physical structure of this morphology consists of discrete, nanoscopic rods of one or more solid phases embedded within a continuous matrix of the second solid phase. The interconnectivity is such that the matrix phase is fully continuous, while the rod phase provides reinforcement throughout the matrix volume. The function of these reinforcing rods is to provide three-dimensional strengthening and toughening, analogous to the function of rebar in concrete but at the nanometer scale. The benefit of the rod-like morphology is that it can provide more isotropic mechanical property enhancement compared to the more anisotropic nature of the lamellar structure, improving the strength and damage tolerance of the dense ceramic body (240) in multiple directions. The ability to selectively produce either a lamellar or a rodlike morphology by controlling the precursor chemistry and processing parameters demonstrates the high degree of control and improved functionality afforded by the inventive method. As a use case, a load-bearing structural component for an aerospace application that experiences complex, multi-axial stress states would benefit from the isotropic reinforcement provided by this rod-like morphology, improving its reliability and safety.

[0100] FIG. 6 shows a non-conventional role of the oxygen ion transport phase (200) in enabling the successful fabrication of a dense, nano-structured ceramicbody. The figure provides a side-by-side comparison, contrasting the outcome of a thermal process on a material lacking this phase (left panel) with the outcome on a material incorporating the innovative structure (right panel). This comparison serves to demonstrate the solution to a fundamental technical problem that has previously precluded the creation of such materials. The left panel, titled Without O2Transport Phase, illustrates the physical behavior of a conventional, monolithic ceramic precursor undergoing a redox-driven decomposition. The operability of this system is diffusion-limited; the reaction can only be initiated by the slow ingress of oxygen from the external atmosphere, a process depicted by the inward-pointing arrows. This creates a distinct Inward Reaction Front that separates an outer, already-reacted shell from an inner, unreacted core. The functionality, or more accurately the dysfunctionality, of this process is that it inherently generates massive stress gradients. As the outer shell reacts and undergoes a volumetric expansion, it exerts immense pressure on the stable, unreacted core. This mismatch inevitably leads to the formation of micro- and macro-cracks, as depicted, which compromise the structural integrity of the component and often lead to its catastrophic failure. Furthermore, the extended time required for the reaction front to slowly penetrate the entire volume ensures that any fine microstructural features that may form at the surface are subjected to prolonged high-temperature annealing, resulting in significant coarsening and the complete loss of any desired nano-scale structure.

[0101] The right panel, titled With O2Transport Phase, illustrates the operation and benefits of the innovative structure. The physical structure shown is that of a dense ceramic body (240) wherein the reactive precursor phase is embedded within a continuous, interconnected network of an oxygen ion transport phase (200). This network is the key operational element. Its functionality is to act as a pervasive, three-dimensional conduit, delivering oxygen ions rapidly and uniformly to all regions of the reactive phase simultaneously. This is implemented by formulating the initial precursor powder (201 ) as an intimate mixture of the cerium-based oxide and the oxygen transport phase (e.g., yttria-stabilized zirconia), such that the transport phase forms a percolating network upon initial densification. The volume fraction of the oxygen ion transport phase (200) can be from greater than 60%. This implementation fundamentally alters the reaction mechanism from a slow, surface-limited process to a rapid, homogeneous, volume-wide reaction, as represented by the distributedasterisks. The benefit solves the problem of processing-induced fracture. By eliminating the reaction front and its associated stress gradients, the dense ceramic body (240) remains structurally sound and free of cracks throughout the transformation. Another benefit is the preservation of the nano-sized microstructure. The rapid, homogeneous reaction allows the transformation to be completed in a much shorter time at high temperature, thereby kinetically mitigating the thermal coarsening of the newly formed product phases. This results in enhanced performance, yielding a final product that successfully combines full density with a preserved, uniform nanosized morphology. As a practical example of use, attempting to fabricate a large ceramic component for a high-temperature catalytic reactor via the conventional method would fail due to cracking. By employing the inventive structure with an interconnected YSZ network as shown in FIG. 6, a large, mechanically robust, and highly efficient catalytic component can be reliably produced, its enhanced performance directly attributable to the preserved high-surface-area nano-structure.

[0102] Established fabrication techniques for producing dense ceramic components are fundamentally constrained by the physics of sintering. Conventional methods begin with a porous green part that must be heated to high temperatures to achieve densification. This process is invariably accompanied by a substantial volumetric contraction, a physical necessity that compromises dimensional accuracy and frequently leads to component failure through warping or cracking. Furthermore, any attempt to create dense ceramics with beneficial nano-scale features via these methods is self-defeating; the very thermal energy required to eliminate porosity also provides a potent driving force for the rapid coarsening and destruction of such delicate structures. While some approaches involve chemically reactive precursors, they typically fail when applied to dense bodies because the reaction is limited to a slow, surface-inward diffusion front, which itself creates massive internal stresses and non-uniform microstructures.

[0103] The presently described solution to these issues involves the deliberate orchestration of two distinct, concurrent physical phenomena within a single, unified thermal process. The first aspect of this approach is the harnessing of a redox-driven eutectoid decomposition as an active tool for volumetric compensation. Where established methodologies treat sintering shrinkage as an unavoidableconsequence to be managed, the described process introduces a precursor material (201 ) containing cerium in a reactive Ce(lll) state. Upon heating in an oxidizing atmosphere, this precursor decomposes into product phases whose combined molar volume is significantly greater than the parent phase, generating a substantial volumetric expansion. This allows the process to achieve dimensional stability not by minimizing shrinkage, but by actively and concurrently counteracting it with a controlled expansion, thereby producing a final dense ceramic body (240) that maintains the precise dimensions of its green part (220).

[0104] Another aspect is the circumvention of kinetic and diffusion limitations that have previously prevented the formation of uniform nano-structures in bulk bodies. A technical contribution here is the incorporation of a continuous oxygen ion transport phase (200) within the precursor powder (201 ) before the thermal treatment begins. Materials fabricated through established routes, when subjected to a redox transformation, are limited by the slow diffusion of oxygen from the external atmosphere. The presently described solution fundamentally alters the reaction pathway by providing a pervasive, internal, high-speed network for oxygen transport. This structural feature transforms the reaction from a slow, two-dimensional surface front into a rapid, homogeneous, three-dimensional volume-wide event.

[0105] This unique convergence of phenomena results in a process and a resulting material that are demonstrably distinct from what was previously known. The methodology for near-net-shape manufacturing differs from previous approaches in its fundamental strategy: it does not passively accept shrinkage but actively generates a countervailing volumetric expansion that is kinetically balanced against the densification process. The methodology for creating nano-structures differs by structurally pre-empting the diffusion bottleneck. By embedding the transport phase (200), the transformation is allowed to proceed so rapidly and uniformly that it effectively outruns the thermodynamic drive for coarsening, enabling the preservation of nano-sized features with an interphase spacing of less than 100 nanometers. This allows for the fabrication of a dense ceramic body (240) that possesses a combination of full density, high dimensional fidelity, and a uniform, engineered nano-scale microstructure — a suite of characteristics previously unattainable in a single, bulk ceramic article.

[0106] The articles and processes herein are illustrated further by the following Example, which is non-limiting.EXAMPLE

[0107] Reactive Sintering of Oxide Composites by Route of a Redox-Driven Eutectoid Transition

[0108] The eutectoid decomposition reaction of perovskite CeAIOs facilitates a simultaneous volume expansion and formation of a dense composite CeO2 / Al2O3 microstructure during sintering. Extended processing times and higher sintering temperatures transforms lathe-like a-ALOs and CeO2 inclusions into coarsened, equiaxed microstructures. Dense samples were sintered with CeC / ALOs composite microstructures using two processing routes. In the first case, CeO2 and AI2O3 powders were mixed, pressed, and sintered; and in the second case, CeO2 / Al2O3 was densified through a reactive sintering route by simultaneously oxidizing CeAIOs powder and sintering the product phases combining the densification and eutectoid decomposition steps. Fine, sub-100 nm, microstructures with either lamellar or equiaxed features could be preserved in the final sintered specimens by employing a two-step heating / oxidation procedure whereby initiation of the eutectoid decomposition temperature could be selectively chosen by controlling the oxygen partial pressure during processing. Specimens sintered with finer CeC / ALOs microstructural features resulted in samples with improved mechanical properties provided by Vickers indentation hardness measurements.

[0109] It was recently reported that composite microstructures of CeO2 and AI2O3 with controlled interphase spacing on the nanometer scale could be synthesized by utilizing the eutectoid decomposition of the perovskite CeAIOs [1 ,2], These experiments revealed that the rate of decomposition and the coarsening of the resulting microstructure CeC / ALOs could be engineered by tuning the oxidizing kinetics. The rate of the eutectoid decomposition is controlled by multiple variables including the product species of the eutectoid decomposition, ion diffusivity, diffusion length, temperature, and oxygen partial pressure. For powder samples heated in air, the eutectoid reaction was observed to produce nanometer scale lamellarmicrostructures for samples annealed between 900 and 1100 °C with isothermal reaction times ranging from 105to 102s respectively. For eutectoid decompositions in this temperature range, AI2O3 tends to form as various polymorphs including y, 9, and 5-phases. Prolonged heating at higher temperatures for longer dwell times would result in more coarsened structures with eventual loss of lamellar colonies and the formation of equiaxed grains of fluorite CeO2 and corundum AI2O3.

[0110] Eutectoid decomposition reactions are well known in the steel industry. The austenite transformation can undergo either cooperative or uncooperative growth depending on the undercooling (temperature) and cooling rate of the transformation. Eutectoid decompositions can also occur in oxide materials; an example is the A^TiOs — AI2O3 + TiO2 reaction [3-5], Cerium oxide is generally used in electronic material applications in which it is employed as an oxygen conducting electrolyte, while aluminum oxide is traditionally used for its mechanical and insulating properties. The combination of these materials in ceram ic / ceramic composite microstructures leads to interesting multifunctional properties that can range widely in application space. CeO2 / Al2O3 are bio-compatible, and these composite microstructures are used in dental applications [6], Ceria on alumina supported substrates has long been studied for its high catalytic activity [7], CeO2 / Al2O3 composite microstructures can form triple point for catalysis [8,9], Nano-structured features of the CeO2 / Al2O3 system are difficult to control during processing, but size control of these features can have a pronounced effect on the catalytic activity

[0010] , Densification of CeO2 / Al2O3 composites has been demonstrated in a few cases, but coarsening of the microstructure results during high temperature processing. Ideally, the fine microstructural features reported in refs [11 ,12] could be preserved in a sintered body. Fine microstructural features in ceramics have been shown to improve properties such as fracture toughness, catalytic activity, and thermoelectricity to name a few.

[0111] The eutectoid decomposition of CeAIOs results in a significant volume expansion. Using room temperature density values of (6.64, 7.13, and 3.99) g / cm3for CeAIOs, CeO2, and a-Al2O3 respectively, the theoretical volume expansion of the CeAIO3+ 1 / 202 — CeO2 + I / 2AI2O3 reaction is 18%. Volume expansion during sintering can be used advantageously to compensate for the shrinkage associatedwith densification. This process, near-net shape densification, reduces the amount of shrinkage during sintering and results in improvements in the dimensional tolerance of the final manufactured part [13-16], Reducing the amount of shrinkage during densification can be achieved using different processing routes, examples of which include: using bimodal or multimodal particle size distributions to improve packing efficiency [17,18], porous infiltration

[0019] , chemical vapor infiltration, reaction bonding [20-22], liquid-solid displacement reactions

[0023] , pre-ceramic polymer fabrication [24,25], and directed metal oxidation

[0026]

[0112] Many of the examples of near-net shape densification processes that have been developed to reduce the amount of shrinkage that occurs during sintering are complex and require multiple steps that can involve liquid infiltration, reactive precursors, and complex heat treatments [13,14,27-29], Procedures that involve some form of reactive sintering (liquid infiltration or reactive precursors) can result in complex microstructures with undesirable properties for a given application. There are examples of combining reactive sintering with near-net shape formation [13,30], Examples of reactive sintering of oxides include: ZnAl2O4, AI-AI2O3. Some complications that can arise when trying to densify ceramic materials through a reactive sintering route include: large volumes of interconnected porosity, poor control over stoichiometry, slow kinetics, and volume changes of phases during reactions that can lead to the formation of defects.

[0113] Here we report the reactive sintering of CeO2 / Al2O3 by route of a CeAIOs eutectoid decomposition pathway. The volume expansion during the eutectoid reaction helps to decrease the amount of shrinkage produced during the sintering processes. Sintering by route of a redox activated eutectoid decomposition mechanism is differentiated from a conventional reactive sintering process in the extra degree of experimental control, the oxygen activity during sintering, that will affect the kinetics of the reaction mechanism.

[0114] Samples were processed through a mixed oxide solid state reaction route. CeO2 (Inframat Advanced Materials, 99.95%) and AI2O3 (Sumitomo Chemical Co., AA-04) were weighed and mixed using a planetary mill with stabilized zirconia media in ethanol. The dried powders were separated into two batches: one batch was for CeO2+Al2O3 sintering and one for further processing to produce a CeAIOsprecursor. The CeAIOs precursor was calcined at 1350 °C for 18 hr. in a horizontal tube furnace with flowing 95% / 5% N2 / H2 forming gas. X-ray diffraction confirmed formation of CeAIOs, and this powder was milled using the previously described milling procedure. Pellets of CeAIOs were formed by first mixing powder with a 2% by mass solution of Acryloid B72 polymer and sieving the binder modified powder through 100 mesh. This procedure was followed for both compositions by pressing powder in a 10 mm steel die followed by isostatic pressing at 200 MPa. Smaller cross-section samples, 5 mm, were also prepared for dilatometer experiments. The samples were pre-fired to burn out the binder to a temperature of 600 °C at a rate of 0.5 °C / min. These green pellets were weighed and measured to record green density prior to sintering. Dilatometry experiments were performed on samples that were pre-fired to remove organics. Experiments were performed on a Linseis DIL L75 dilatometer in air or under flowing Argon gas. Sintering of 10 mm diameter pellets was carried out in a one-step process in a muffle furnace or a two-step method by heating samples in a controlled atmosphere to promote sintering of the CeAIOs phase followed by reaction of the densified perovskite in oxidizing conditions to convert the material to a controlled microstructure of CeO2 / Al2O3.

[0115] Thermogravimetric analysis of CeAIOs was carried out by heating roughly 100 mg of powder in a platinum crucible to 1075 °C at a rate of 15 °C / min in an inert Ar environment. The mass of these samples was then measured under isothermal conditions in mixed air / Ar environments using a Netzsch STA 449 F1 thermogravimetric analyzer (TGA). The mixed flow of breathing air and Ar gases were controlled using the internal mass flow controllers of the instrument.

[0116] The dimensions and mass of sintered samples were measured, and the densities of samples with closed porosity were measured using the Archimedes method. Samples were prepped for SEM by polishing using a combination of diamond impregnated platens and diamond pastes to 1 urn final finish. Prior to sputter coating with carbon, the surfaces of these polished pellets were also analyzed with X-ray diffraction on a PANalytical X’Pert Pro. Polished cross-sections of samples were imaged using secondary electrons on a JEOL 7800F FESEM.

[0117] Selected X-ray diffraction patterns of CeAIOs powder and pellets sintered in oxidizing conditions are shown in FIG. 7. The Profex software was used toperform a Rietveld refinement of the powder diffraction profiles using reference patterns: PDF# 04-013-4067 (CeAIOs) and PDF# 00-048-0055 (CeAlnOis). The calcining conditions used in this study were performed at temperatures slightly lower than those of previous studies where single phase CeAIOs has been synthesized

[0031] , Some secondary phase peaks assigned to CeO2 can be observed in the CeAIOs powder diffraction profile. The choice to reduce the calcine temperature was made to avoid coarsening of the powder that was observed at elevated temperatures to improve the sinterability of the material used in this study. At temperatures between 1350 and 1450 °C, the CeAIOs fully decomposes into CeO2 and cr-Al2O3. At temperatures of 1450 °C and higher, cr-Al2O3 and CeO2 continue to react and form a CeAh 10i s-like phase. This cerium hexaaluminate (CeAlnOis) phase has been described in previous studies [32-35], The diffraction data in FIG. 7 shows clear evidence of the formation of a cerium hexaaluminte phase at temperatures as low as 1500 °C in oxidizing conditions.

[0118] The composite microstructure of CeO2-Al2O3 has been investigated in the literature, but knowledge gaps persist. Multiple studies on the Al2O3-nch side of the phase diagram have been published, but less has been documented on the stoichiometric CeO2-1 / 2Al2O3 and CeO2-rich compositions. The CeAlnOis phase has been reported to form in reducing conditions

[0036] , This phase, because of its anisotropic structure and inclination to form long lamellar microstructural features, has been investigated as a strengthening mechanism

[0037] , This phase is similar to Sr- and Ca- containing aluminum-rich oxide phases that act as crack-deflecting inclusions in strength toughened ceramics

[0038] , and CeAlnOis inclusions may improve toughness of alumina-toughened zirconia materials as well

[0039] ,

[0119] Samples were measured under isothermal conditions at 1075 °C under varying oxygen atmospheres to measure the effect on the kinetics of the eutectoid decomposition as a function of oxygen partial pressure. The gas mixtures were formed by varying the flow rate of Ar vs. air in through the sample chamber. The exhaust gas was sampled outside of the TGA chamber to record the relative oxygen content during the experiment. The flow rate of the combined gases was fixed at 100 mL / min, and TGA experiments with air / Ar mixtures of 0 / 100, 30 / 70, 20 / 80, 15 / 85, and 100 / 0 were recorded as (144, 15460, 23470, 46550, and 179700) ppm O2 respectivelywith the oxygen sensor. The isothermal TGA curves are plotted in FIG. 8. The theoretical increase in mass for the eutectoid decomposition is 3.72 %, and this value is approached in all cases shown in FIG. 8. As expected, the kinetics of the eutectoid decomposition are partially controlled by the availability of oxygen at the surface of the material, and lower oxygen partial pressures significantly inhibit the pace of the reaction.

[0120] Samples with both rectangular and cylindrical geometries were heated at constant heating rates in air. In FIG. 9, linear expansion curves are shown for samples heated at varying rates to 1550 °C and held for 2 hr. before cooling to room temperature at a rate of 20 °C / min. The onset of the nonlinear expansion of the sample during heating corresponds to the beginning of the eutectoid decomposition reaction. As discussed in a previous study focused on the kinetics of the decomposition reaction in powdered samples

[0040] , the ( -AI2O3 phase does not initially form; rather, the corundum phase is preceded by y-, 6, and 0-AI2O3 phases before crystalline ( -AI2O3 phases form at higher temperatures or longer annealing times (see FIG. 7.) coinciding with observable coarsening of the microstructure from lamellar CeO2 / Al2O3 nano-regions to micron-scale equiaxed grains. After the linear expansion of the sample reaches a maximum upon heating (FIG. 9), the sample begins to shrink followed by a small expansion before a final consolidation during the sintering stage of the process. This anomalous expansion that occurs prior to sintering coincides with the temperature range where alumina polymorphs with a face centered cubic packing of the oxygen sublattice, i.e., Q- and 5-AI2O3, phases begin to transform to thermodynamically stable a-Al2O3, which exhibits a hexagonally close packed oxygen sublattice

[0011] . This process of transforming lower density polymorphs or amorphous phases to crystalline a-Al2O3 would result in complex mass transport during this portion of the thermal heat treatment.

[0121] Two types of heating profiles were used to study the effect of atmosphere on the sintering kinetics of CeAIOs samples while undergoing a eutectoid decomposition. In one case, samples were heated at a constant heating rate of 20 °C / min under flowing argon gas to a dwell temperature. Once the dwell temperature was reached, the argon flow was immediately cut off and the sample was exposed to air. The linear shrinkage of samples measured using this two-step process is shownin FIG. 10. Samples that were held at temperatures of 1250 °C or below upon transitioning from a slightly reducing to oxidizing environment displayed a positive linear expansion while samples heated to temperatures above 1250 °C after transitioning from a reducing to oxidizing atmosphere only registered a linear shrinkage. This set of data points shows that the mechanism of the volume expansion of the eutectoid decomposition can be controlled by the oxygen partial pressure during sintering. The samples in FIG. 10. measured after the experiment gained in mass a total of 3.51 , 3.43, 3.50, and 3.50 % compared to the mass of the starting CeAIOs sample. This gain in mass is consistent with previous measurements made on fully reacted CeAIOs material in oxidizing conditions to drive a complete reaction to a CeO2 / Al2O3 microstructure

[0011] ,

[0122] A second heating / oxidation profile was used in which samples were heated at constant heating rates in flowing argon followed by an isothermal dwell at 1550 °C for 2 hr. This heating profile promoted densification of the CeAIOs phase. A second isothermal dwell was then conducted at a lower temperature under oxidizing conditions to promote the eutectoid reaction. The linear expansion of samples measured using this type of two-step heating / oxidation profile is shown in FIG. 11. This sintering profile was developed after observing that samples sintered to near-full density in the CeAIOs phase would crack and break apart after reheating the samples to high temperature in oxidizing conditions due to the large volume expansion of the eutectoid reaction. Cracking of the samples could be avoided by bypassing the temperature range where catastrophic expansion occurs. This process was accomplished by introducing dense samples to oxidizing conditions at temperatures above a critical threshold. The critical threshold observed in this study was between (1250 to 1350) °C. Dense CeAIOs samples oxidized above 1350 °C would transform to CeO2 / Al2O3 microstructures, maintain high theoretical densities, and show no signs of producing macro- or micro-sized cracks. Dense CeAIOs samples oxidized at or below 1250 °C would shatter into many pieces during the reoxidation process or develop large visible macro-sized cracks.

[0123] Samples were sintered under a variety of conditions. The sintered densities of a select number of samples sintered using a CeO2 / Al2O3 precursor and a CeAIOs precursor using a one-step or two-step heating / oxidation procedure areplotted in FIG. 12. As shown from the dilatometry data in FIG. 9, the onset temperature for sintering of the CeO2 / Al2O3 samples is much lower than the onset for sintering of the CeAIOs precursor samples. As a result, the density of the CeO2 / Al2O3 samples is much higher than the CeAIOs samples for similar sintering conditions. The densities of the samples sintered using a two-step process, as shown in FIG. 12, have densities that approach the densities of the CeO2 / Al2O3 precursor samples. When comparing the samples sintered to comparable densities using these two methods in FIG. 13, the two-step CeAIOs precursor samples undergo a smaller change in volume during sintering than the CeO2 / Al2O3 precursor samples. These results show that high densities with improved near-net shape volume contraction can be achieved by using a CeAIOs precursor with a two-step heating / oxidation procedure. It should be noted that the green densities of the pressed and debound pellets for the CeAIOs samples were lower than the CeO2 / Al2O3 precursor samples (57.2 ± 0.6 % vs. 62.3 ± 0.4 % respectively). This discrepancy can be partially attributed to the fact that the calcined and milled CeAIOs precursor powder was significantly coarser than the starting CeO2 and AI2O3 powders. Better control of the starting particle size of the CeAIOs powder would promote higher green density packing and increase the gap between the volume contraction curves of the CeAIOs vs. CeO2 / Al2O3 precursors shown in FIG. 13.

[0124] The microstructures of CeAIOs and CeO2 / Al2O3 precursor samples sintered using a one-step process are shown in FIG. 14. The samples sintered using the CeAIOs starting powder contain more visible porosity than the CeO2 / Al2O3 samples. Unlike CeO2 / Al2O3, the CeAIOs samples undergo a large volume expansion prior to sintering as shown in the dilatometry data in FIG. 9 resulting in the creation of larger pores prior to densification, and this porosity is more difficult to remove during the sintering stage resulting in lower relative densities or requiring longer sintering times that create a more coarsened microstructure.

[0125] Samples of CeAIOs precursor were additionally sintered at 1600 °C for two hours followed by a reoxidation step at lower temperature. It is important to note that the heating / oxidation profile is critical for these samples. Dense CeAIOs samples reheated from low temperature in oxidizing conditions will often crack and break apart because the reoxidation process will begin below the critical temperature of 1250 °C. To avoid cracking, samples must be heated above 1250 °C in reducingconditions before switching the gas environment from reducing to oxidizing. A wide range of reducing conditions was found to be sufficient for maintaining the CeAIOs phase to high temperatures. Unlike the calcination step where forming gas conditions were necessary to drive the CeO2+1 / 2Al2O3^CeAIO3 reaction; once formed, the CeAIOs phase could be stabilized in moderately reducing conditions maintained by flowing argon gas through the furnace. Using this two-step process, the eutectoid decomposition proceeds quickly during the high temperature reoxidation process, and the microstructure could be controlled by tuning the time / temperature conditions the sample was reoxidized under. If samples were quenched during this step of the process, a fine, sub-100 nm lamellar microstructure could be preserved similar to the microstructure produced in powder specimens in previously published results

[0011] , An example of a sample quenched following a two-step heating / oxidation procedure is shown in FIG. 16. Here, the cooperative growth of lamellar colonies is preserved in a dense sintered specimen. The resulting microstructural features are approximately an order of magnitude smaller than the initial particle size of precursor powders.

[0126] References

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[0127] The Example includes citations to scientific literature, listed immediately below the text of the Example and indicated by the use of numerals appearing in brackets in the text of the Example. This inclusion of the citations and the cited scientific literature cited are not to be construed as an admission that such scientific literature constitutes prior art or is otherwise available as prior art against the claimed invention under any patent statute or jurisprudence. Further, any citation to documents, scientific articles, or publications is solely for the purpose of providing a general historical context and is not an admission of anticipation or obviousness of the claimed subject matter. Each reference so cited is incorporated herein by reference in its entirety.

[0128] While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation. Embodiments herein can be used independently or can be combined.

[0129] All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. The ranges are continuous and thus contain every value and subset thereof in the range. Unless otherwise stated or contextually inapplicable, all percentages, when expressing a quantity, are weight percentages. The suffix (s) as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including at least one of that term (e.g., the colorant(s) includes at least one colorants). Option, optional, or optionally means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event occurs and instances where it doesnot. As used herein, combination is inclusive of blends, mixtures, alloys, reaction products, collection of elements, and the like.

[0130] As used herein, a combination thereof refers to a combination comprising at least one of the named constituents, components, compounds, or elements, optionally together with one or more of the same class of constituents, components, compounds, or elements.

[0131] All references are incorporated herein by reference.

[0132] The use of the terms a, an, and the and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. It can further be noted that the terms first, second, primary, secondary, and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. It will also be understood that, although the terms first, second, etc. are, in some instances, used herein to describe various elements, these elements should not be limited by these terms. For example, a first current could be termed a second current, and, similarly, a second current could be termed a first current, without departing from the scope of the various described embodiments. The first current and the second current are both currents, but they are not the same condition unless explicitly stated as such.

[0133] The modifier about used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity). The conjunction or is used to link objects of a list or alternatives and is not disjunctive; rather the elements can be used separately or can be combined together under appropriate circumstances.

Claims

What is claimed is:1 . A dense ceramic body (240), comprising: a composite microstructure (245) comprising a first solid phase and a second solid phase, wherein the first solid phase comprises cerium oxide with cerium in a Ce(IV) oxidation state and the second solid phase comprises a non-cerium-based oxide; an oxygen ion transport phase (200) forming a continuous network, wherein the composite microstructure (245) is embedded in the continuous network of the oxygen ion transport phase (200); and wherein the first solid phase and the second solid phase are arranged in a nano-sized, phase-separated morphology, the morphology having an interphase spacing of less than 100 nanometers.

2. The dense ceramic body (240) of claim 1 , wherein the oxygen ion transport phase (200) comprises yttria-stabilized zirconia, CeO2, or alternative oxide electrolyte material.

3. The dense ceramic body (240) of claim 1 , wherein the second solid phase is a binary, ternary or quaternary oxide.

4. The dense ceramic body (240) of claim 1 , wherein the second solid phase is selected from the group consisting of Ta2Os and Nb20s.

5. The dense ceramic body (240) of claim 1 , wherein the nano-sized, phase- separated morphology is a lamellar morphology.

6. The dense ceramic body (240) of claim 1 , wherein the nano-sized, phase- separated morphology is a rod-like morphology.

7. The dense ceramic body (240) of claim 1 , wherein the composite microstructure (245) further comprises a dopant element substituting a cation in the first solid phase, the second solid phase, or both.

8. The dense ceramic body (240) of claim 3, wherein the second solid phase is AI2O3 comprising at least one polymorph selected from the group consisting of a- AI2O3, y- AI2O3, and 9- AI2O3.

9. The dense ceramic body (240) of claim 1 , wherein a volume of the body (240) exhibits a net change of less than 5% relative to a volume of a corresponding greenpart (220) having a green density of between 55% and 75% of a theoretical density of the body (240).

10. The dense ceramic body (240) of claim 1 , wherein the first solid phase is CeO2, the second solid phase is AI2O3, the oxygen ion transport phase (200) is stabilized zirconia or doped ceria, and the nano-sized, phase-separated morphology comprises lamellar features having an interphase spacing of less than 100 nanometers.11 . A method of forming a dense ceramic body (240), the method comprising the steps of: obtaining a precursor powder (201 ) comprising a cerium-based complex oxide having cerium in a Ce(lll) oxidation state and an oxygen ion transport phase (200); forming a green part (220) from the precursor powder (201 ); andheating the green part (220) in an oxidizing atmosphere to a sintering temperature, wherein the heating concurrently effectuates a densification of the green part (220) via porosity removal and a eutectoid decomposition of the cerium-based complex oxide, the eutectoid decomposition transforming the cerium-based complex oxide into a composite microstructure (245) of a first solid phase comprising cerium oxide with cerium in a Ce(IV) oxidation state and a second solid phase, and wherein the oxygen ion transport phase (200) facilitates a substantially homogenous progression of the eutectoid decomposition throughout a volume of the green part (220) to form a nano-sized, phase-separated morphology while mitigating coarsening of the first and second solid phases.

12. The method of claim 11 , wherein the cerium-based complex oxide is a perovskite selected from the group consisting of CeAIOs, CeCrOs, CeBOs, CeScOs, CeGaOs, CeMnOs, CeTiOs, CeNiOs, CeFeOs, or any combination comprising at least one of the foregoing perovskite.

13. The method of claim 11 , wherein the step of obtaining a precursor powder (201 ) comprises mixing a Ce(lll) stabilized oxide powder with a powder of the oxygen ion transport phase (200), wherein the oxygen ion transport phase (200) comprises stabilized zirconia or doped ceria.

14. The method of claim 11 , wherein the step of forming the green part (220) utilizes a shape-forming technique selected from the group consisting of powder consolidation, tape casting, slip casting, gel casting, extrusion, pressure filtration, injection molding, and three-dimensional printing.

15. The method of claim 14, further comprising mixing a binder with the precursor powder (201 ) before the forming step and removing the binder from the green part (220) before or during the heating step.

16. The method of claim 11 , wherein the step of heating the green part (220) comprises a two-step heating procedure comprising: a first heating step in an inert or reducing atmosphere to a first temperature to densify the green part (220) while substantially maintaining the Ce(lll) oxidation state of the cerium-based complex oxide; and a second heating step wherein the oxidizing atmosphere is introduced at a second temperature sufficient to initiate the eutectoid decomposition throughout the volume of the densified green part (220) without inducing cracking from inhomogeneous volume expansion.

17. The method of claim 16, wherein the second temperature is above the temperature required to obtain the eutectoid decomposition.

18. The method of claim 11 , wherein the heating is performed with control of an oxygen partial pressure of the oxidizing atmosphere to kinetically balance a rate of volume contraction from the densification with a rate of volume expansion from the eutectoid decomposition, thereby minimizing a net volume change between the green part (220) and the dense ceramic body (240).

19. The method of claim 11 , wherein the precursor powder (201 ) further comprises one or more dopant elements, and wherein the dopant elements are incorporated into the first solid phase or the second solid phase during the eutectoid decomposition.

20. The method of claim 11 , wherein the cerium-based complex oxide is CeAIOs, the oxygen ion transport phase (200) is stabilized zirconia or doped ceriaforming a continuous network, and the heating step is controlled to produce the dense ceramic body (240) having a nano-sized lamellar microstructure of CeO2 and AI2O3 with an interphase spacing of less than 100 nanometers.

21. A process for manufacturing a near-net-shape ceramic composite, the process comprising: providing a precursor powder (201 ) comprising a cerium-based oxide material, the cerium-based oxide material characterized by a crystalline structure containing cerium in a metastable Ce(lll) oxidation state and being capable of undergoing a redox-driven eutectoid decomposition into a plurality of product phases upon oxidation; shaping the precursor powder (201 ) into a porous green part (220); and subjecting the porous green part (220) to a thermal treatment in an oxidizing atmosphere, the thermal treatment inducing a densification process involving a volumetric contraction through porosity reduction, while concurrently initiating the eutectoid decomposition, the eutectoid decomposition generating a volumetric expansion, wherein kinetics of the volumetric contraction and the volumetric expansion are controlled such that the volumetric expansion substantially compensates for the volumetric contraction, thereby producing a densified ceramic body (240) with a volume substantially equivalent to a volume of the porous green part (220).

22. The process of claim 21 , wherein the cerium-based oxide material is a complex oxide having a perovskite crystal structure.

23. The process of claim 22, wherein the cerium-based oxide material is selected from the group consisting of perovskite CeXOs, pyrochlore 062X267, spinel XCe2O4, scheelite CeXO4, or melilite CeXTO? compositions, where X and T are any appropriate / charge-balancing cation.

24. The process of claim 21 , wherein the kinetics are controlled by regulating at least one parameter selected from the group consisting of a heating rate of the thermal treatment, a partial pressure of oxygen in the oxidizing atmosphere, and a dwell time at a sintering temperature.

25. The process of claim 21 , wherein the step of providing a precursor powder (201 ) further comprises synthesizing the cerium-based oxide material by heating a mixture of oxide precursors in a reducing environment sufficient to stabilize the Ce(lll) oxidation state prior to the shaping step.

26. The process of claim 21 , wherein the precursor powder (201 ) further comprises additional non-cerium-based oxide powders, whereby the densified ceramic body (240) comprises a multi-phase composite material including the plurality of product phases and the additional non-cerium-based oxides.

27. The process of claim 21 , wherein the thermal treatment is a multi-step procedure comprising: a first thermal step of heating the porous green part (220) in an atmosphere having a first oxygen partial pressure that is sufficiently low to inhibit the eutectoid decomposition while allowing the volumetric contraction through densification to proceed; and a second thermal step of increasing the oxygen partial pressure to a second oxygen partial pressure at a temperature sufficient to initiate and complete the eutectoid decomposition and its associated volumetric expansion throughout a body of the densified green part (220).

28. The process of claim 21 , wherein the shaping step comprises incorporating an inorganic binder (202) with the precursor powder (201 ), the inorganic binder (202) comprising metal ions that are converted into a secondary oxide phase within the densified ceramic body (240) during the thermal treatment.

29. The process of claim 21 , wherein the volumetric expansion resulting from the eutectoid decomposition is greater than 15% relative to a volume of the cerium- based oxide material.

30. The process of claim 21 , wherein a total volumetric shrinkage of the densified ceramic body (240) relative to the porous green part (220) is less than 5%, and wherein the densified ceramic body (240) possesses a final density of at least 95% of its theoretical density.

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