A porous high-entropy boride ceramic material and a method of making the same

Abstract

The present application relates to a kind of porous high-entropy boride ceramic materials and its preparation method.Preparation method includes: metal oxide is mixed with reducing agent powder grinding and is pressed into green body;Green body is clamped in conductive anti-bonding layer and graphite felt, and is placed between graphite plate to form multilayer assembly structure;The space between graphite plate is controlled to compress graphite felt to 60%~80% of initial thickness;After vacuumizing, fill protection gas, and partial reduction reaction is carried out by electric heating to obtain clinker;Clinker is ground and is formed into clinker body;Multilayer assembly structure and compression condition are reused, and finished product is obtained by electric sintering.The obtained material is single-phase high-entropy solid solution structure, porosity 45%~55%, room temperature compressive strength 150MPa~190MPa, present diameter 100mm~200mm, thickness 3mm~6mm integral block, with three-dimensional interconnected open hole structure and rigid percolation network skeleton in internal.

Description

Technical Field

[0001] This invention relates to the field of high-entropy ceramic materials technology, and in particular to a porous high-entropy boride ceramic material and its preparation method. Background Technology

[0002] High-entropy boride (HEB) ceramics, as ultra-high temperature ceramic materials, possess physical properties such as high melting point, high hardness, and high strength. Furthermore, their thermal conductivity is generally lower than that of single-component boride ceramics, making them promising for applications in aerospace, extreme high-temperature thermal protection, and special thermal insulation materials. Building upon this foundation, porous HEB ceramics further combine the advantages of porous materials—lightweight and low thermal conductivity—and exhibit significant potential for control over macroscopic properties and microscopic composition, becoming an important research direction in the field of novel thermal protection materials. Among existing porous HEB ceramic preparation processes, electric field-assisted sintering (such as flash sintering or ultra-high temperature rapid sintering technology) is widely used due to its rapid heating rate and short reaction cycle. For example, existing technologies include a two-step method using metal oxides and reducing agents combined with an electrically heated element to prepare porous HEB ceramics. However, due to limitations in sintering mechanisms and assembly methods, current preparation technologies are typically limited to laboratory-scale micro-sized samples, resulting in ceramic blocks with generally small diameters (e.g., effective diameters limited to 16 mm and below), which is insufficient to meet the requirements for large-sized components in industrial applications.

[0003] When attempting to scale up the size of porous high-entropy boride ceramics directly to industrial scale (e.g., diameters exceeding 100 mm) using existing processes, a series of insurmountable kinetic and thermodynamic bottlenecks arise. First, under large-scale conditions of electric field-assisted rapid sintering, the contact resistance between the assembly structure and the green body is difficult to maintain uniformly, easily leading to edge discharge effects and distortion of the electrothermal field distribution, resulting in significant temperature gradients and uneven heating within the ceramic block. Second, large blocks accumulate enormous internal thermal stress during rapid heating and cooling. Without effective elastic buffering and heat dissipation mechanisms, this easily triggers network cracking on the material surface and even delamination in the thickness direction. Furthermore, the pore-forming raw materials release a large amount of gas during the high-temperature reaction stage. In large-scale solid matrices, if the gas escape path is obstructed or the matrix lacks sufficient mechanical support in the early stages of the reaction, the internal gas expansion force not only leads to extremely uneven pore distribution but also causes localized, unintended densification (dead pores), and in severe cases, even direct fragmentation and disintegration of the macroscopic block. For the reasons mentioned above, how to prepare porous high-entropy boride ceramic materials with macroscopic structural integrity, uniform microscopic pore distribution, and high mechanical strength under large-scale working conditions is a technical problem that urgently needs to be solved in this field.

[0004] Furthermore, on the one hand, there are differences in understanding among those skilled in the art; on the other hand, the applicant studied a large number of documents and patents when making this invention, but due to space limitations, not all details and contents were listed in detail. However, this does not mean that the present invention does not possess the features of these prior art. On the contrary, the present invention already possesses all the features of the prior art, and the applicant reserves the right to add relevant prior art to the background art. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a porous high-entropy boride ceramic material and its preparation method, thereby solving at least some of the aforementioned technical problems.

[0006] This invention also discloses a method for preparing porous high-entropy boride ceramic materials, comprising the following steps: S1. After mixing and grinding the metal oxide raw material powder and the reducing agent raw material powder, the green body is obtained by pressing. S2. The green blank is sandwiched between two conductive anti-adhesion layers, placed between two graphite felts, and then placed between two graphite plates to form a multi-layer assembly structure; the spacing between the graphite plates is controlled so that the graphite felt is compressed to 60% to 80% of its initial uncompressed thickness; after evacuating the reaction chamber, a protective gas is filled in, and the multi-layer assembly structure is heated by electricity through the graphite plates to cause the green blank to undergo a partial boronothermic reduction reaction. After cooling, clinker is obtained. S3. Grind the clinker, control the grinding parameters to make the particle size D90 of the clinker powder reach 1μm~5μm, and press the clinker powder into clinker blanks; S4. Place the clinker blank between two conductive anti-adhesion layers, two graphite felts, and two graphite plates in the same assembly sequence as in step S2; control the spacing of the graphite plates again so that the graphite felt is compressed to 60% to 80% of its initial uncompressed thickness; in a vacuum environment and filled with protective gas, sinter the clinker blank by passing electricity through the graphite plates, and obtain a porous high-entropy boride ceramic material after cooling.

[0007] To address the technical bottleneck of easily initiating thermal stress cracks and localized densification in large-size ceramic blocks during rapid sintering, this invention proposes a specific assembly architecture combining a two-step flash sintering method and controlled confined compression. Specifically, during the first and second stages of energization, the graphite felt is compressed to 60%–80% of its initial thickness. This controlled compression state allows the graphite felt to retain channels for the escape of reactive gases while increasing the contact density of the internal carbon fibers to homogenize the conductive network, thereby avoiding thermal field distortion caused by localized current overload at the edges of large-size blanks. Furthermore, the first stage induces a partial reduction reaction in the raw materials to generate highly reactive precursors. Combined with subsequent grinding to control the clinker particle size D90 to 1μm–5μm, this ensures close contact between the partially reacted high-entropy boride precursors and the unreacted raw materials. During the second energized sintering process, these closely contacted refined powders are in-situ solidified together to form a micron-scale rigid percolation network framework. The in-situ generated skeleton structure provides internal mechanical support in the later stages of sintering, resists the volume expansion force of the gas generated by the continued reaction of residual raw materials, prevents macroscopic fragmentation of the matrix, and ultimately achieves the integral forming of large-size, high-porosity, porous, high-entropy ceramic blocks.

[0008] To address the shortcomings of CN117105687A, which suffers from significant temperature gradients, edge discharge effects, and macroscopic thermal stress cracking within large-sized ceramic blocks due to limitations imposed by single solid-phase reactions or the lack of external elastic stress buffering media, this invention provides a systematic solution that combines microscopic topological reconstruction with macroscopic dynamic confining compression. Specifically, a copper electrode component with a stepped axial guiding structure guides the periodic alternating electric field generated by an external AC power source to a specific heating area within a sealed water-cooled cavity; a graphite plate component with an effective heating area larger than the diameter of the blank converts the received electric field energy into uniformly distributed heat and surface current; and a conductive anti-adhesion layer component made of hexagonal boron nitride provides physical isolation from interfacial carbon contamination while maintaining the electrical conduction path. During the partial reduction reaction under electric heating, the copper electrode component with a stepped axial guiding structure, in conjunction with the graphite plate component, achieves charge transfer by outputting electrical power that raises the temperature of the green body from room temperature to 1650℃ to 1850℃ within 50s to 80s. Under controlled compression, the graphite plate component, under the action of downward displacement, closely adheres to the conductive anti-adhesion layer component made of hexagonal boron nitride, thereby forming a uniformly conductive surface current. During the second stage of electric sintering, the conductive anti-adhesion layer component made of hexagonal boron nitride utilizes its geometric overhang structure covering the edge area of ​​the ceramic body's side surface to collaboratively maintain the dynamic gas-solid phase equilibrium microenvironment during the reaction process of the clinker body component.

[0009] According to a preferred embodiment, the metal oxide raw material powder includes at least zirconium oxide, titanium oxide, tantalum pentoxide, and niobium pentoxide; The reducing agent raw material powder is boron powder, or a mixture of boron carbide powder and carbon powder.

[0010] To address the challenge of coordinating reaction kinetics in the ultra-high temperature synthesis of complex multi-component systems, this scheme specifically designs a matching system of five- or higher transition metal oxides and composite reducing agents. Zirconia, titanium oxide, tantalum pentoxide, and niobium pentoxide are used as essential basic components, providing the material basis for constructing multi-principal-component high-configurational-entropy solid solutions. Furthermore, a mixture of boron carbide and carbon powder is provided as an alternative reducing agent. Utilizing a carbothermic and boronic composite reduction reaction pathway, this improves upon the problems of excessively rapid local reactions or uneven degassing that may occur with a single reducing agent at ultra-high temperatures. This composite reduction system helps to regulate the release rate of reactant gases and the diffusion matching degree of the solid phase, broadening the applicability of this large-size porous ceramic preparation process to different material systems.

[0011] According to a preferred embodiment, when the reducing agent raw material powder is boron powder, its excess addition ratio is 30% to 40% of the theoretical stoichiometric ratio; When the reducing agent raw material powder is a mixture of boron carbide powder and carbon powder, the excess addition ratio of boron carbide powder is 40% to 55% of the theoretical stoichiometric ratio.

[0012] Considering the high susceptibility of light elements to vapor-phase volatilization and pyrolysis loss under ultra-high temperature electric flash combustion conditions, this invention precisely establishes the excess compensation benchmarks for different reducing agents. An excess ratio of 30%–40% is set for boron powder, and an excess ratio of 40%–55% is set for boron carbide powder, compensating for the inevitable element loss during the high-temperature stage in terms of reaction kinetics. This excess compensation mechanism maintains the reducing atmosphere in the system during the later stages of the reaction, suppresses residual transition metal oxides caused by insufficient reducing agent, and helps ensure the purity and stability of the final porous high-entropy ceramic solid solution phase structure.

[0013] According to a preferred embodiment, in step S1, the particle size D90 of the mixed powder of metal oxide and reducing agent after mixing and grinding is 1 μm to 3 μm; The process conditions for pressure molding are: holding pressure at 18MPa to 21MPa for 180s to 240s.

[0014] The initial topological packing state of the powder directly constrains the kinetic process of subsequent solid-phase in-situ reactions. Therefore, this technical solution strictly defines the characteristic particle size of the mixed powder and the pressure molding boundary of the green body. Controlling the particle size D90 of the mixed powder within the range of 1μm to 3μm increases the initial contact area at the solid-phase interface, thereby reducing the activation energy of the first-stage solid-phase diffusion reaction. Combined with a molding pressure of 18MPa to 21MPa and a corresponding holding time of 180s to 240s, the green body achieves sufficient strength for handling and assembly while retaining adequate micropores between its internal particles to facilitate venting during the initial stage of the first-stage reaction. This specific initial compaction density prevents gas stagnation and block delamination caused by excessive internal density of the green body.

[0015] According to a preferred embodiment, in steps S2 and S4, the conductive anti-adhesion layer is hexagonal boron nitride paper with a thickness of 0.1 mm to 0.2 mm; The initial uncompressed thickness of the graphite felt is 3 mm; the radial dimensions of both the conductive anti-adhesion layer and the graphite felt are more than 10 mm larger than the radial dimension of the green or calcined blank placed between them.

[0016] To overcome the unavoidable interfacial carbon contamination and edge thermal dissipation problems in large-scale reaction systems, this invention introduces a conductive anti-adhesion layer with specific thickness and geometric overhang characteristics into the multilayer assembly. The 0.1mm–0.2mm thick hexagonal boron nitride paper, while conducting surface current, also acts as a physical barrier, preventing the carbon within the graphite felt from adhering to or becoming carburized with the green body at high temperatures. Furthermore, the radial dimension of the conductive anti-adhesion layer and the graphite felt is limited to be at least 10mm larger than the green or clinker body, forming a geometrically overhanging boundary. This radially overhanging structure covers the side edge region of the ceramic body, slowing down the rate of heat loss from the edge region to the cold wall environment, improving the radial temperature gradient of the overall component, and thus reducing the risk of thermal stress cracking caused by edge supercooling shrinkage.

[0017] To clarify the spatial structure of the overhanging boundary and suppress thermal stress caused by edge heat radiation dissipation, a conductive anti-adhesion layer component with a thickness of 0.1 mm to 0.2 mm is geometrically overhanging, with a radial dimension at least 10 mm larger than the radial dimension of the green or flaked blank placed between it. This overhang connects to an initially uncompressed graphite felt component with a porous carbon fiber network, which is in a state of surface current homogenization and recombination. This allows the graphite felt component to form a path that blocks heat radiation loss with the external environment within the sealed reaction chamber, thereby suppressing edge overcooling. This radially overhanging structure spatially forms a thermally shielding epitaxial layer composed of the graphite felt and the conductive anti-adhesion layer, aiming to alleviate thermal field distortion caused by the excessively rapid rate of heat loss from the edges of large-sized components to the cold walls of the chamber.

[0018] According to a preferred embodiment, in steps S2 and S4, the operation of filling the protective gas after evacuation is specifically as follows: evacuate the gas pressure in the reaction chamber to below 10 Pa, then fill it with high-purity argon gas, and maintain the gas pressure in the reaction chamber at 103 kPa to 105 kPa during the process of electric heating or electric sintering.

[0019] In terms of constructing the reaction environment, this scheme establishes a dynamic gas-solid equilibrium microenvironment through a specific vacuuming and gas-filling sequence. Before energizing, the system is evacuated to below 10 Pa to eliminate residual air and adsorbed moisture within the pores of the assembly system and the preform, preventing unintended deep oxidation of the target oxide and ineffective consumption of the reducing agent. Subsequently, a high-purity argon gas micro-positive pressure environment of 103 kPa to 105 kPa is introduced and maintained during the reaction. This thermodynamically suppresses potential surface pyrolysis and volatilization of the products at ultra-high temperature extremes, helping to maintain the dynamic gas-solid equilibrium at the reaction interface. Simultaneously, it provides a stable dielectric environment for electric field conduction to prevent abnormal arc discharge.

[0020] According to a preferred embodiment, in step S2, the process conditions for electric heating are as follows: control the input electric power to raise the temperature of the green blank from room temperature to 1650℃ to 1850℃ within 50s to 80s, and hold the temperature for 70s to 100s. In step S3, the process conditions for pressing the clinker powder into a clinker blank are: holding the pressure at 15MPa to 28MPa for 150s to 280s.

[0021] To address the contradiction between preserving porosity and maintaining macroscopic framework strength in a single rapid sintering process, this invention cleverly utilizes the rapid heating and short holding time in the first stage to kinetically "truncate" the reduction process. By controlling the rapid heating of 50s to 80s and the short holding time of 70s to 100s, only partial boronothermic reduction of the raw materials occurs, generating some high-entropy boride precursors in situ. After grinding and pulverizing, the clinker is re-pressurized at 15MPa to 28MPa, disrupting the irregular porous structure formed by local agglomeration in the first stage. This allows the residual raw materials and the generated precursor particles to redistribute uniformly and come into close contact, forming a new reaction interface. This provides the geometrical basis for obtaining a uniform three-dimensional interconnected microporous system in the final product.

[0022] The clinker green body component, located in the core region of the reaction assembly structure, comprises incompletely reacted residual raw material particles, generated high-entropy boride precursor particles with high reactivity, and uniformly distributed micropores constructed by mechanical pressure. Under a macroscopically confined environment where the clinker powder particle size D90 is 1μm–5μm and a holding pressure of 15MPa–28MPa is maintained, the residual raw material particles and the precursor particles establish a uniformly dispersed and multidirectionally contacting topological stacking system. This places the clinker green body component in a microscopic pre-arrangement state capable of resisting the pressure of secondary gas phase formation. The specific parameter grinding process here is not intended to pulverize the ceramic phase to destroy the crystal nuclei, but rather to disrupt the irregular macropores or blocks formed by local agglomeration in the first stage, thereby retaining submicron to micron-sized high-entropy boride precursor particles as seed points for in-situ growth into a rigid percolation network framework during the secondary reaction.

[0023] According to a preferred embodiment, in step S4, the process conditions for sintering by electric current are: controlling the input electric power to raise the temperature of the clinker blank from room temperature to 1850℃ to 2050℃ within 90s to 120s, and holding it at that temperature for 30s to 50s. After the heat preservation is completed, the input power is gradually reduced to zero within 30s to 60s and then cooled with the furnace.

[0024] After entering the second stage of sintering, this scheme adopts a significantly different, gradual heating and cooling thermal strategy compared to the first stage. Compared to the first stage, the heating time is slowed down to 90-120 seconds, allowing the rate of gas release from the remaining material to match the exhaust flow of the pressurized graphite felt buffer layer. This prevents a sudden, large-scale gas release from breaking through the forming ceramic network skeleton and causing macroscopic cracking of the block. After the holding period, the input power is gradually reduced to zero over 30-60 seconds instead of being abruptly cut off. This effectively slows down the rate of heat dissipation from the interior of the ceramic block to the surface, reduces the thermal shock effect caused by rapid cooling, and further suppresses the initiation of microcracks caused by residual thermal stress during the cooling and solidification stage.

[0025] During the sintering process involving electric field and mechanical coupling, the copper electrode component with an axial guiding structure on the upper side is in a continuously downward pressure limiting state while adjusting its axial displacement to control the spacing between the graphite plates. This results in the graphite felt component located directly below it being in a controlled deformation state where the thickness of the green or clinker blank is compressed to 60%–80% of its initial thickness. During the furnace cooling stage after the heat preservation is completed, the copper electrode component releases or reduces the rigid compression on the graphite plates through displacement adjustment. This allows the graphite felt component to utilize its own elastic recovery force to absorb the volume expansion generated by the ceramic phase transformation and dissipate residual thermal stress, thereby playing an elastic buffering and compensation role.

[0026] According to a preferred embodiment, in the step of heating or sintering a multi-layer assembly structure or a clinker blank by passing an electric current through a graphite plate, the electric field introduced is an alternating current electric field.

[0027] To address the defect that DC electric fields easily induce unidirectional migration of charged ions under ultra-high temperature driving, leading to macroscopic component segregation, this process specifies the use of AC electric fields as the driving source for heating and reaction. The periodic polarity reversal of the AC electric field drives charged ions to oscillate at high frequency in situ within the crystal lattice. This physical mechanism neutralizes the unidirectional polarization effect over large spans, eliminates the enrichment or depletion of various transition metal elements in specific regions, and helps ensure the uniform distribution of elements at the atomic scale within large-size high-entropy solid solutions larger than 100 mm.

[0028] The present invention also discloses a porous high-entropy boride ceramic material, which is prepared by the aforementioned preparation method; Porous high-entropy boride ceramic materials are single-phase high-entropy solid solution structures with a rigid permeation network skeleton formed by the in-situ solid-phase reaction of high-entropy boride particles, and a three-dimensional interconnected open-pore structure defined by the rigid permeation network skeleton. The porous high-entropy boride ceramic material is in the form of a monolithic block with a diameter of 100 mm to 200 mm and a thickness of 3 mm to 6 mm. The overall porosity of porous high-entropy boride ceramic materials is 45%–55%, and the compressive strength at room temperature is 150 MPa–190 MPa.

[0029] Thanks to the systematic process design described above, from powder control to two-step controlled flash calcination, the resulting porous high-entropy boride ceramic material overcomes the inherent barrier of traditional porous ceramics that struggle to balance high porosity and high strength. This material possesses a specific rigid permeable network framework generated by in-situ solid-phase reactions, and a three-dimensional interconnected open-pore structure defined by this framework. This specific microscopic topology endows the material with an excellent load transfer mechanism: when a large-sized block is subjected to external compressive stress, the macroscopic force can be effectively transferred and uniformly dissipated along the continuous rigid framework network, avoiding stress concentration failure at isolated, weak pore walls. Therefore, this product can achieve a high compressive strength of 150MPa–190MPa while maintaining a porosity of 45%–55% to preserve its lightweight and thermal insulation properties, solving the problem of drastic decrease in mechanical load-bearing capacity after scaling up in traditional processes.

[0030] To address the mechanical response characteristics of porous ceramic materials under room temperature compression, high-entropy boride ceramic components after secondary sintering possess submicron to micron-sized high-entropy boride solid solution particles, a rigid neck structure generated by in-situ secondary solid-phase reaction, and three-dimensionally interconnected uniform open channels. These solid solution particles, the rigid neck structure, and the open channels construct a three-dimensionally interconnected rigid permeation network framework within a large-size matrix with a thickness of 3mm to 6mm. This framework possesses the mechanical property of transmitting and dissipating local stress along a continuous spatial network under macroscopic external compressive stress. Through a two-step control method, D90 parameter reset, and specific neck-connected micro-topological morphology formed by confined space solidification, this material achieves a compressive strength of 150MPa to 190MPa while maintaining a porosity of 45% to 55%, and exhibits structural buffer strain adapted to the porous morphology before brittle fracture. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the cross-sectional structure of the reaction chamber used in the embodiments and comparative examples of the present invention; Figure 2 This is a schematic diagram of the preparation process and multilayer assembly structure of the porous high-entropy boride ceramic material of the present invention; Figure 3 The X-ray diffraction (XRD) pattern of the porous high-entropy boride ceramic material prepared in Example 1 of this invention; Figure 4 This is a scanning electron microscope (SEM) image of the porous high-entropy boride ceramic material prepared in Example 1 of the present invention; Figure 5 This is a room temperature compressive stress-strain curve of the porous high-entropy boride ceramic material prepared in Example 1 of the present invention. Figure 6 This is a macroscopic photograph of the sintered sample of Comparative Example 1 of this invention. Figure 7 This is a scanning electron microscope (SEM) image of the locally over-sintered failure sample of Comparative Example 4 of this invention. Detailed Implementation

[0032] The following is a detailed explanation with reference to the accompanying drawings.

[0033] The metal oxide raw material powders used in this embodiment include zirconium oxide (ZrO2), titanium oxide (TiO2), tantalum pentoxide (Ta2O5), and niobium pentoxide (Nb2O5). In addition to these metal oxide powders, the raw material powders may optionally include one or more of hafnium oxide (HfO2), chromium trioxide (Cr2O3), molybdenum trioxide (MoO3), vanadium pentoxide (V2O5), and tungsten trioxide (WO3). The purity of the above-mentioned metal oxide raw material powders is not less than 99%, and their initial average particle size distribution is in the range of 3 μm to 10 μm. The reducing agent raw material powders include boron powder, boron carbide powder, and carbon powder. The initial average particle size of the boron powder is 3 μm; the initial average particle size distribution of the boron carbide powder and carbon powder is in the range of 5 μm to 20 μm. The purity of the above-mentioned reducing agent raw material powders is not less than 99.0%. Anhydrous ethanol was used as the grinding medium in the experiment, and a sand mill was used for mixing and refining the raw materials.

[0034] Combination Figure 1 As shown, the reaction assembly structure is entirely embedded within a sealed cavity that defines a water-cooled environment. The entire assembly structure is symmetrically distributed axially around the central blank. In this assembly structure, the shared transmission path for mechanical loads and alternating current is as follows: the load is transmitted to the graphite plate via axially introduced copper electrodes, then sequentially penetrates the graphite felt and BN paper, ultimately acting uniformly on both ends of the blank. The upper and lower copper electrodes are respectively inserted into the top and bottom walls of the sealed cavity, with stepped axial guide structures at their penetration points. Driven by external force, the upper and lower copper electrodes move relative to each other along the central axis, thereby causing the graphite plate to apply a normal clamping force to the internal structure. The radial dimension of the graphite plate is larger than that of the graphite felt, BN paper, and blank placed within it; this radial overhanging structure defines a wide heating coverage area, thereby reducing the edge discharge effect during the sintering of large-size ceramics to provide a uniform radial thermal field. When working in a multi-layer assembly, the upper and lower graphite plates are brought closer together by adjusting the axial displacement of the copper electrodes. The relative displacement is absorbed by the structurally elastic graphite felt layer and converted into controlled volume shrinkage, thereby establishing a stable electrical conduction path while retaining a buffer gap.

[0035] The hexagonal boron nitride (BN) paper used in the reaction assembly structure has a thickness of 0.1 mm to 0.2 mm, and its diameter is at least 10 mm larger than the diameter of the internal preform material. This BN paper is responsible for electrical conductivity during the reaction and preventing the preform from sticking to the surrounding materials. The graphite felt has an initial uncompressed thickness of 3 mm, and its diameter is at least 10 mm larger than the diameter of the preform material. For the graphite plate used as the electrode and pressure interface, the effective heating area is much larger than the diameter of the preform material, so that the heating area can cover preforms with a diameter of 100 mm to 200 mm and provide a uniform thermal field. As an optional hardware size configuration, the graphite plate thickness can be set to 15 mm, and the overall dimensions can be 360 ​​mm in length and 300 mm in width, corresponding to an effective heating area size of 300 mm × 300 mm.

[0036] The experimental equipment system includes a sand mill, powder molding die, vacuum unit, and AC flash power supply. The inner diameter of the molding die is configured from 100mm to 200mm depending on the target product specifications. The vacuum unit consists of a mechanical pump and a molecular pump, capable of evacuating the reaction chamber to a vacuum level below 10Pa. High-purity argon gas, with a purity of no less than 99.99%, is used as a protective atmosphere within the chamber, and the gas pressure is maintained between 103kPa and 105kPa during the reaction process. The AC power supply conducts current through copper electrodes to the graphite plate, thereby providing the thermal and electric field environment for the assembled structure.

[0037] The phase composition of the ceramic material was determined by X-ray diffraction (XRD) to confirm the pure phase structure of the high-entropy solid solution. The microstructure and pore distribution were observed using scanning electron microscopy (SEM). Porosity was measured using Archimedes' displacement method. The compressive strength and stress-strain curves of the samples were determined using a universal pressure testing machine at room temperature, with the compression displacement rate set to a fixed value (e.g., 0.5 mm / min).

[0038] Combination Figure 2As shown, the preparation process of porous high-entropy boride ceramic materials is limited to a two-stage thermomechanical coupling process in series. Along the material flow path, metal oxides and reducing agents (boron powder or a combination of boron carbide and carbon powder) are mixed and ground, then subjected to mechanical pressure to undergo physical densification and transform into green bodies. In the first stage of electrochemical reaction synthesis, the green body is placed in a multi-layered assembly structure with axial symmetry; this assembly structure, from the outside to the inside along the normal direction, includes graphite plates, graphite felt, and BN paper. The outer graphite plates are used to introduce electric fields and mechanical loads, while the inner compressed graphite felt is used to homogenize the electrothermal conduction network, thereby driving the initial reaction of the green body. After the reaction is complete, the product is re-ground back into bulk powder, and then mechanically pressurized again to transform into a clinker green body. When entering the second stage of sintering, the clinker blank reuses the same multi-layer assembly structure; in which the intermediate grinding process microscopically resets the material contact interface, and the secondary sintering completes the remaining solid phase reaction under the isomorphic mechanical constraint, thereby solidifying the released gas channels into micron-level uniform pores of the finished product.

[0039] The raw materials and anhydrous ethanol are mixed and ground in a sand mill. The grinding time is controlled between 70 and 100 minutes to achieve a particle size D90 of 1 μm to 3 μm for the mixed powder. After grinding, the mixed powder is dried. Then, the dried mixed powder is directly injected into a circular mold with a diameter of 100 mm to 200 mm, and pressed under a pressure of 18 MPa to 21 MPa for 180 to 240 seconds to obtain a green body. This pressing process defines the initial contact density between the raw material particles.

[0040] In the first flash reaction stage, the green body is sandwiched between two sheets of BN paper with a thickness of 0.1 mm to 0.2 mm. Then, the BN paper and the green body it holds are placed between two sheets of graphite felt with a thickness of 3 mm. Finally, the entire structure is placed between two graphite plates. By adjusting the height of the upper graphite plate, the graphite felt shrinks, forming a tightly packed multi-layered structure. The vacuum unit is activated to evacuate the reaction chamber to 10 Pa, and then argon gas is introduced into the chamber to maintain the pressure at 103 kPa to 105 kPa. Alternating current is applied to both ends of the graphite plates through copper electrodes, controlling the heating power to raise the temperature of the green body from room temperature to 1650°C to 1850°C within 50 to 80 seconds, and then holding it at that temperature for 70 to 100 seconds. During this process, the green body undergoes a partial borate reduction reaction. Under the aforementioned temperature and time conditions, the raw materials only undergo partial reaction and generate a portion of high-entropy boride precursors, providing a highly active material basis for constructing a micron-scale rigid percolation network framework in subsequent sintering.

[0041] After the first reaction is complete and the material has cooled, the resulting clinker is removed and ground again in a sand mill. The grinding parameters are controlled to achieve a clinker powder particle size (D90) of 1 μm to 5 μm. This step disrupts the macroporous structure within the clinker through mechanical crushing, allowing unreacted raw materials and the generated high-entropy boride precursors to redistribute uniformly and come into close contact. This facilitates heat transfer and solid-phase diffusion, enabling in-situ bonding during subsequent sintering to form micron-sized pores and a rigid, supportive framework structure. The ground clinker powder is then injected into a circular mold with a diameter of 100 mm to 200 mm and pressed under a pressure of 15 MPa to 28 MPa for 150 to 280 seconds to obtain the clinker green body.

[0042] In the second flash calcination stage, the clinker blank is sandwiched between two sheets of BN paper with a thickness of 0.1 mm to 0.2 mm, placed between two sheets of graphite felt with a thickness of 3 mm, and finally placed between two graphite plates. This multi-layered structure is vertically placed in the order of "graphite plate—graphite felt—BN paper—clinker blank—BN paper—graphite felt—graphite plate". The height of the upper graphite plate is adjusted by mechanical limiting or displacement adjustment means, so that the graphite felt with an initial thickness of 3 mm is compressed to a thickness range of 1.8 mm to 2.4 mm, forming a passage. The cavity is evacuated again to 10 Pa and argon gas is introduced to 103 kPa to 105 kPa. Alternating current is applied through copper electrodes, so that the clinker blank is heated from room temperature to 1850 °C to 2050 °C within 90 s to 120 s, and held at that temperature for 30 s to 50 s. During this stage, as the graphite felt is compressed to 60% to 80% of its original thickness, the contact point density of the internal carbon fibers increases, effectively regulating the contact resistance in the vertical direction. This allows the alternating current to be converted into a uniform surface current and introduced into the clinker. The remaining unreacted raw materials continue to react and release gases, which escape through the permeable channels retained inside the graphite felt. Simultaneously, the precursor powder rapidly connects in situ during the secondary sintering, forming a rigid skeleton to support the pores and prevent collapse. After the holding period, the power input is gradually reduced to 0 within 30 to 60 seconds, and then the furnace is cooled to room temperature.

[0043] The porous high-entropy boride ceramic material prepared in this embodiment presents as a disc-shaped ceramic block with a diameter of 100 mm to 200 mm and a thickness of 3 mm to 6 mm. This ceramic block exhibits structural integrity on a macroscopic scale, specifically demonstrated by the absence of penetrating thermal stress cracks on the surface and edges under an optical microscope, and the absence of delamination in the thickness direction. This macroscopic integrity stems from the elastic compensation environment provided by the aforementioned controlled-distance compression flash-firing process, which effectively releases the internal stress generated during the heating and cooling of large-size ceramics, thus maintaining a uniform and intact block morphology even within industrial-grade sizes exceeding 100 mm.

[0044] Regarding the microscopic phase fingerprint, X-ray diffraction analysis confirmed that the ceramic material is a single-phase high-entropy solid solution structure, containing no unreacted transition metal oxides or other non-target impurity phases. The five or more transition metal elements composing this high-entropy solid solution exhibit a randomized distribution at the atomic scale. Energy dispersive spectroscopy analysis revealed that the constituent metal elements largely overlap within the microscopic region, with no obvious areas of element enrichment or depletion. This homogeneity of chemical composition is beneficial for maintaining the stability of the ceramic material's physical properties under ultra-high temperature conditions.

[0045] The morphological fingerprint of pores and framework is one of the main structural features that distinguishes this ceramic material from traditional porous ceramics. The material has an overall porosity of 45% to 55%, and this porosity is a three-dimensionally interconnected open-pore structure. Microscopic observation reveals that the pores are defined by a rigid boride framework generated by the in-situ solid-state reaction during the second-stage electro-sintering process, and the pores are uniformly distributed in space. The average pore size of these pores ranges from 0.5 μm to 3 μm, exhibiting uniform micropores at the submicron or micron level. The ceramic framework is formed by the interconnection of submicron or micron-sized high-entropy boride particles through in-situ solid-state reactions. The particles are rigidly connected by neck structures generated by the reaction, thereby constructing a permeation network system with continuous thermal conduction paths and mechanical load-bearing capacity.

[0046] The coupling characteristics of structure and performance further define the identity of this product. This ceramic material maintains a moderate porosity of 45% to 55% while exhibiting a compressive strength of 150 MPa to 190 MPa. This nonlinear match between high porosity and high strength directly stems from the specific microstructure formed by the two-step process. The highly reactive precursors generated in the first stage reaction rapidly connect to form a rigid framework during the second stage of secondary pressure sintering. This framework provides mechanical support in the later stages of the reaction, effectively resisting the volume expansion forces during gas phase formation, and ultimately solidifying to form a uniformly distributed and continuous microporous structure. This specific physical characteristic allows stress to be effectively transferred and dissipated along the rigid framework network when the material is subjected to external compressive loads, thus maintaining lightweight thermal insulation properties while possessing superior mechanical reliability compared to traditional porous high-entropy ceramics.

[0047] In Example 1, a large-size nonaluminate porous high-entropy boride ceramic material was prepared using the process provided by this invention. 177.94 g of hafnium oxide powder, 104.17 g of zirconium oxide powder, 67.51 g of titanium oxide powder, 186.78 g of tantalum pentoxide powder, 112.36 g of niobium pentoxide powder, 64.25 g of chromium trioxide powder, 121.69 g of molybdenum trioxide powder, 76.88 g of vanadium pentoxide powder, and 195.99 g of tungsten trioxide powder were weighed according to the equimolar ratio of the metal elements. 394.78 g of boron powder was added to these powder raw materials. The above raw materials and anhydrous ethanol were mixed and ground in a sand mill for 90 min to obtain a mixed powder. After drying the mixed powder, 300 g was placed into a circular mold with a diameter of 150 mm, pressed at a pressure of 20 MPa and held for 200 s to obtain a green body. The green compact was sandwiched between two 160mm diameter pieces of BN paper, then placed between two 160mm diameter circular graphite felts with an initial thickness of 3mm, and subsequently between two graphite plates measuring 360mm × 300mm (with an actual heating area of ​​300mm × 300mm) and 15mm thick. The height of the upper graphite plate was adjusted, and the graphite felt was compressed to a thickness of 1.8mm through displacement control, forming a tightly packed multilayer structure and establishing a stable electrothermal conduction pathway. The reaction chamber was sealed, and a vacuum was evacuated to 10Pa using a mechanical vacuum pump and a molecular pump, followed by the introduction of argon gas to 103kPa–105kPa. Alternating current was applied to both ends of the graphite plates through copper electrodes, and the heating rate was controlled to raise the green compact to 1700℃ within 60s, holding it at that temperature for 75s. During this process, the raw material underwent a partial boronothermic reduction reaction, thereby generating a portion of high-entropy boride precursors at the microscopic level. After natural cooling, the reacted clinker was removed, ground in a sand mill for 120 minutes, and dried. The particle size D90 of the clinker powder was measured to be 3.314 μm. 254 g of the clinker powder was placed into a 150 mm diameter circular mold, pressed at 18 MPa, and held for 150 s to obtain a clinker green body. Following the same assembly method described above, the green body was placed between graphite plates, and the graphite felt was compressed again from the initial 3 mm to 1.8 mm. Under an argon protective atmosphere, the green body was heated from room temperature to 2000 °C within 90 s and held for 35 s. After the holding period, the power input was gradually reduced to 0 within 45 s, and then the furnace was cooled to room temperature. This specific depressurization rate effectively mitigates the temperature gradient of large-sized blocks, thereby suppressing ceramic cracking caused by thermal stress.

[0048] Combination Figure 3As shown, X-ray diffraction analysis was performed on the large-sized porous ceramic bulk material prepared in Example 1. The XRD pattern showed clear diffraction peaks in the diffraction angle range of 20° to 80°, with each characteristic peak corresponding to the crystal plane of the target high-entropy boride (HEB) phase. The test pattern had a flat baseline, and no residual diffraction peaks from initial raw materials such as transition metal oxides or boron powder were observed. Under the uniform electrothermal field provided by the controlled-distance compression flash calcination process, the nine-membered metal oxide system underwent a complete reduction reaction; in this process, various transition metal elements randomly occupied cation sites at the atomic scale, thereby generating a single-phase high-entropy solid solution structure with good crystallinity and no obvious local segregation.

[0049] Combination Figure 4 As shown, the fracture surface microstructure of the ceramic block was observed using a scanning electron microscope. This porous ceramic material exhibits a three-dimensional interconnected open-pore structure, where pores are defined and formed by a boride framework generated by the reaction, thus maintaining a uniform spatial distribution. The image scale indicates that the pore size is mainly concentrated in the micrometer range of 0.5 μm to 2 μm. The high-entropy boride particles constituting the ceramic framework are at the submicrometer to micrometer scale; adjacent particles are physically connected through in-situ solid-phase reactions to form neck structures, thereby constructing a rigid permeation network with mechanical support capabilities. This specific microscopic topology avoids the generation of localized unintended densification or large-scale independent pores.

[0050] Combination Figure 5 The figure shows the stress-strain curve of the ceramic block during room temperature compression testing. Under compressive load, the material can withstand a maximum compressive stress of up to 187 MPa before macroscopic fracture, with a corresponding compressive strain of approximately 0.37%. During loading, the stress path within the ceramic material is as follows: external mechanical loads are input through surface nodes and transmitted and dissipated inward along the rigid continuous skeleton formed by the secondary electric sintering in situ connection. Under a measured porosity of approximately 50%, this continuous skeleton effectively resists external deformation constraints; the synergistic cooperation between the micron-sized uniform pores and the rigid neck connection prevents stress concentration at a single node, thereby enabling this lightweight porous material to maintain good mechanical load-bearing capacity and fracture resistance.

[0051] In Example 2, a pentagonal porous high-entropy boride ceramic material with a diameter of 200 mm was prepared. 180.56 g of zirconium oxide powder, 117.03 g of titanium oxide powder, 323.76 g of tantalum pentoxide powder, 194.75 g of niobium pentoxide powder, and 133.26 g of vanadium pentoxide powder were weighed according to the equimolar ratio of the metal elements. A boron carbide powder and carbon powder substitution system was used as the reducing agent, with 167.20 g of carbon powder and 283.43 g of boron carbide powder. The raw materials were ground in a sand mill for 90 min to obtain a mixed powder. 496 g of the mixed powder was placed into a circular mold with a diameter of 200 mm, pressed at a pressure of 21 MPa, and held for 240 s to obtain a green body. The green compact was sandwiched between two circular BN paper sheets with a diameter of 210 mm, then between two circular graphite felt sheets with a diameter of 210 mm and a thickness of 3 mm. It was then placed on a graphite plate with dimensions of 360 mm × 300 mm (actual heating area 300 mm × 300 mm) and a thickness of 15 mm. The height of the upper graphite plate was adjusted to compress the 3 mm thick graphite felt to 2.0 mm. The cavity environment was adjusted to a vacuum of 10 Pa and then filled with argon gas at 103 kPa–105 kPa. During the electric heating process, the power output was controlled to raise the temperature of the green compact to 1800 °C within 65 s and hold it at that temperature for 80 s. After the clinker cooled, it was removed and ground for 150 min. The particle size D90 of the clinker powder was measured to be 2.663 μm. 408 g of the clinker powder was injected into a circular mold with a diameter of 200 mm and held under pressure at 25 MPa for 280 s to obtain the clinker compact. The clinker blank was then sandwiched between two circular BN paper sheets with a diameter of 210 mm, and then between two circular graphite felt sheets with a diameter of 210 mm and a thickness of 3 mm. It was then placed on a graphite plate of the aforementioned dimensions, and the graphite felt was compressed again to 2.0 mm. The reaction chamber was sealed, and a mechanical vacuum pump and a molecular pump were used to evacuate the furnace to 10 Pa. Argon gas was then slowly introduced to 103 kPa–105 kPa. The power switch was then turned on, and alternating current was applied through the copper electrodes. The heating rate was controlled to raise the clinker blank to 2050 °C within 120 s, and held at that temperature for 45 s. The power input was then gradually reduced to 0 within 55 s, and the power switch was turned off, allowing the furnace to cool to room temperature. Performance testing showed that the 200 mm diameter circular ceramic block obtained in this embodiment was a pure phase according to XRD analysis. Its internal pores were uniformly distributed, with pore sizes mainly ranging from 1 μm to 3 μm. The overall porosity of the material was approximately 47%, and the compressive strength reached 153 MPa. This embodiment demonstrates that the controlled-distance compression flash calcination process provided by the present invention can still ensure the structural integrity and mechanical properties of large-size porous ceramics under a reducing agent substitution system.

[0052] In Comparative Example 1, the effect of the particle size D90 of the clinker powder after the first step of the reaction on the molding quality was verified. 141.20 g of titanium dioxide powder, 217.86 g of zirconium oxide powder, 234.98 g of niobium pentoxide powder, 390.64 g of tantalum pentoxide powder, 148.65 g of carbon powder, and 273.51 g of boron carbide powder were weighed. After grinding the raw materials in a sand mill for 90 min to obtain a mixed powder, 175 g was placed into a circular mold with a diameter of 120 mm, pressed at a pressure of 20 MPa and held for 200 s to obtain a green body. The green compact was sandwiched between two 130mm diameter circular BN paper sheets and two 130mm diameter graphite felt sheets with an initial thickness of 3mm, and placed on a 360mm × 300mm graphite plate (effective heating zone 300mm × 300mm, thickness 15mm). The height of the upper graphite plate was adjusted to compress the graphite felt to 1.8mm. The reaction chamber was sealed, and a mechanical vacuum pump and a molecular pump were used to evacuate to 10Pa. Argon gas was slowly introduced to 103kPa–105kPa. The power switch was then turned on, and alternating current was supplied to the graphite plate through copper electrodes. The heating power was controlled to raise the temperature of the green compact to 1800℃ within 75s, hold it at that temperature for 90s, and then the power switch was turned off, allowing it to cool naturally to obtain clinker. The clinker was ground in a sand mill for only 45 minutes, and the particle size D90 of the clinker powder was measured to be 33.45μm. 151g of the clinker powder was pressurized at 18MPa for 150s in a 120mm diameter circular mold to obtain a clinker preform. This preform was sandwiched between two 130mm diameter circular BN paper sheets and two 130mm diameter, 3mm thick circular graphite felt sheets, and placed on a 360mm × 300mm graphite plate (with an actual heating area of ​​300mm × 300mm and a thickness of 15mm). The height of the upper graphite plate was adjusted to compress the graphite felt to 1.8mm. The reaction chamber was sealed, and a mechanical vacuum pump and a molecular pump were used to evacuate to 10Pa. Argon gas was slowly introduced to 103kPa–105kPa. The power switch was then turned on, and alternating current was supplied to the graphite plate through the copper electrodes. The heating power was controlled to raise the sample to 2050℃ within 100s and hold for 40s. The power input was then gradually reduced to 0 over 45s, after which the power switch was turned off, and the sample was allowed to cool naturally. Figure 6As shown, the macroscopic physical morphology of the ceramic sample prepared in Comparative Example 1 was observed. The surface of the block exhibited irregularly interwoven network cracks and granular powdering and flaking characteristics. The evolution path of its structural instability mechanism is as follows: due to insufficient grinding time of the clinker in the first stage, the initial large particle size of up to 33.45 μm resulted in a low initial contact area between solid particles; in particular, the excessive physical gaps cut off the atomic diffusion channels and heat conduction in the in-situ reaction process, resulting in the inability to quickly connect and form a micron-level rigid percolation network with global mechanical support during the secondary sintering. In the loose matrix without skeletal constraints in the early stage (initial pore formation), the thermal stress induced by the secondary sintering and the gas expansion force generated by the reaction of residual materials lost their conduction and dissipation paths; in particular, the internally accumulated stress directly broke through the fragile local particle connections, thereby triggering multi-directional cracking and disintegration of the block at the macroscopic level. This macroscopic failure phenomenon establishes the critical significance of limiting the clinker powder particle size D90 to the range of 1 μm to 5 μm for maintaining the structural integrity of large-size ceramics.

[0053] In Comparative Example 2, the criticality of the second-step sintering heating rate was verified. 155.32 g of titanium dioxide powder, 239.65 g of zirconium oxide powder, 258.48 g of niobium pentoxide powder, 429.70 g of tantalum pentoxide powder, 163.52 g of carbon powder, and 300.86 g of boron carbide powder were weighed. 280 g of the mixed powder was pressed in a 150 mm diameter circular mold at 20 MPa and held for 200 s to obtain a green compact. This green compact was sandwiched between two 160 mm diameter circular BN paper sheets and two 3 mm thick circular graphite felt sheets, and placed on a 360 mm × 300 mm graphite plate with an actual heating area of ​​300 mm × 300 mm and a thickness of 15 mm. The graphite felt was compressed to 1.8 mm. The reaction chamber was sealed, and a mechanical vacuum pump and a molecular pump were used to evacuate the sample to 10 Pa. Argon gas was then introduced to 103 kPa–105 kPa. The power switch was then turned on, and alternating current was applied to the graphite plate through copper electrodes. The heating power was controlled to raise the sample temperature to 1800 °C within 75 s and hold it at that temperature for 90 s. After cooling, the clinker was ground for 100 min, and the D90 was measured to be 2.766 μm. 234 g of clinker powder was pressed in a 150 mm diameter mold at 18 MPa and held for 150 s to obtain a clinker preform. This preform was sandwiched between two 160 mm diameter circular BN paper sheets and two 160 mm diameter, 3 mm thick circular graphite felt sheets, and placed on a 360 mm × 300 mm graphite plate with an actual heating area of ​​300 mm × 300 mm and a thickness of 15 mm. The height of the upper graphite plate was adjusted to compress the graphite felt to 1.8 mm. The reaction chamber was sealed, and a mechanical vacuum pump and a molecular pump were used to evacuate the sample to 10 Pa. Argon gas was then slowly introduced to 103 kPa–105 kPa. The power switch was then turned on, and alternating current was supplied to the graphite plate through the copper electrodes. The heating power was controlled to rapidly heat the sample to 2050 °C within 40 seconds, hold it at that temperature for 40 seconds, and then gradually reduce the power input to zero within 45 seconds. Afterward, the power switch was turned off, and the sample was allowed to cool naturally. The results showed that the ceramic block exhibited a fractured state. The failure was caused by excessively rapid heating, which led to the rapid generation of a large amount of gas from the residual material inside, and the resulting thermal stress exceeded the structural limits of the framework. This demonstrates the crucial role of the 90–120 s heating range.

[0054] In Comparative Example 3, the criticality of the upper limit of graphite felt compression was verified. 155.32g of titanium dioxide powder, 239.65g of zirconium oxide powder, 258.48g of niobium pentoxide powder, 429.70g of tantalum pentoxide powder, 163.52g of carbon powder, and 300.86g of boron carbide powder were weighed. 283g of the mixed powder was pressed in a 150mm diameter circular mold at 20MPa and held for 200s to obtain a green compact. This green compact was sandwiched between two 160mm diameter circular BN paper sheets and two 160mm diameter, 3mm thick circular graphite felt sheets, and assembled onto a 360mm×300mm graphite plate with an actual heating area of ​​300mm×300mm and a thickness of 15mm. The graphite felt was compressed to 2.0mm by adjusting the height of the upper graphite plate. The reaction chamber was sealed, and a mechanical vacuum pump and a molecular pump were used to evacuate the sample to 10 Pa. Argon gas was then slowly introduced to 103 kPa–105 kPa. The power switch was then turned on, and alternating current was applied to the graphite plate through the copper electrodes. The heating power was controlled to raise the sample temperature to 1800 °C within 65 s and hold it at that temperature for 80 s. The power switch was then turned off, and the sample was allowed to cool naturally. The resulting clinker was removed and ground for 150 min, with a D90 of 2.810 μm. 241 g of the clinker powder was then pressed in a 150 mm diameter mold at 22 MPa and held for 180 s to form a green body. The clinker blank was sandwiched between two 160mm diameter circular BN paper sheets and two 160mm diameter, 3mm thick circular graphite felt sheets, and placed on a 360mm × 300mm graphite plate with an actual heating area of ​​300mm × 300mm and a thickness of 15mm. By adjusting the height of the upper graphite plate, the initial 3mm graphite felt was compressed to only 2.6mm. The reaction chamber was sealed, and a mechanical vacuum pump and a molecular pump were used to evacuate the sample to 10Pa. Argon gas was then slowly introduced to 103kPa–105kPa. Subsequently, the power switch was turned on, and alternating current was supplied to the graphite plate through the copper electrodes. The heating power was controlled to raise the sample temperature from room temperature to 1950℃ within 90s and hold it at that temperature for 40s. Then, the power input was gradually reduced to 0 within 45s, and the power switch was turned off, allowing it to cool naturally. The results showed that obvious cracking occurred on the material surface, and significant delamination occurred in the thickness direction. The mechanism is that insufficient compression leads to a loose structure, causing heat loss and uneven distribution of the electrothermal field. The uneven sintering process, together with gas stress and thermal stress, leads to the failure of the sample.

[0055] In Comparative Example 4, the criticality of the lower limit of graphite felt compression was verified. The raw material ratio was the same as in Comparative Example 3. 278g of mixed powder was pressed in a 150mm diameter mold at a pressure of 20MPa and held for 200s to obtain a green body. The green body was sandwiched between two 160mm diameter circular BN paper sheets and two 160mm diameter, 3mm thick circular graphite felt sheets, and placed on a graphite plate with dimensions of 360mm×300mm, an actual heating area of ​​300mm×300mm, and a thickness of 15mm. The height of the upper graphite plate was adjusted to compress the graphite felt to 2.0mm. The reaction chamber was sealed, and a mechanical vacuum pump and a molecular pump were used to evacuate to 10Pa, followed by the slow introduction of argon gas to 103kPa~105kPa. The power switch was then turned on, and alternating current was supplied to the graphite plate through the copper electrodes. The heating power was controlled to raise the sample temperature to 1800℃ within 65s and hold it at that temperature for 80s. The power switch was then turned off, and the sample was allowed to cool naturally. The resulting clinker was removed, ground for 150min, and the D90 was measured to be 2.810μm. 240g of clinker powder was pressed in a 150mm diameter mold at a pressure of 22MPa and held for 180s to obtain a clinker blank. This clinker blank was sandwiched between two 160mm diameter circular BN paper sheets and two 160mm diameter, 3mm thick circular graphite felt sheets, and placed on a graphite plate with dimensions of 360mm×300mm, an actual heating area of ​​300mm×300mm, and a thickness of 15mm. The height of the upper graphite plate was adjusted to excessively compress the initial 3mm graphite felt to 1.5mm. The reaction chamber was sealed, and a vacuum of 10 Pa was achieved using a mechanical vacuum pump and a molecular pump, followed by slow introduction of argon gas to 103 kPa–105 kPa. Then, the power switch was turned on, and alternating current was supplied to the graphite plate through the copper electrodes. The heating power was controlled to raise the sample temperature from room temperature to 1950 °C within 90 seconds and hold it at that temperature for 40 seconds. The power input was then gradually reduced to zero over 45 seconds, after which the power switch was turned off, and the sample was allowed to cool naturally. Experimental observation revealed macroscopic fragmentation of the sample. Combined with… Figure 7As shown, the local fracture morphology of the failed sample in Comparative Example 4 was observed using scanning electron microscopy. Unlike the three-dimensional interconnected open structure of the target sample, the fracture surface of this overcompressed sample exhibits a densification characteristic of abnormally grown and highly fused polyhedral grains, with only scattered spherical closed micropores remaining at the grain boundaries. The physical transmission path of its failure mechanism is as follows: when the graphite felt is overcompressed to 1.5 mm, its internal porous carbon fiber network closes; the tightly fitted clamping interface directly blocks the outflow channel of the reaction gas in the second stage and simultaneously loses the elastic displacement margin for absorbing the volume expansion of the ceramic phase transformation. On the one hand, the trapped gas accumulates internally, forming overload pressure that causes the bulk to fracture and shrink into spherical dead holes under the action of surface tension; on the other hand, the excessively confined rigid constraint changes the local contact resistance distribution, where the abnormally concentrated electrothermal field forces accelerated solid-phase mass transfer in this region, thereby inducing abnormal grain growth and local over-densification. This microscopic morphological evidence indirectly supports the lower limit constraint that the thickness of the graphite felt after compression should not be less than 1.8 mm.

[0056] This invention achieves a balanced control of the microstructure and macroscopic stress in large-size porous high-entropy boride ceramic materials through the synergistic effect of a two-step flash sintering process and a dynamic confinement structure. In the first stage, highly reactive precursor particles are generated, which, after grinding, pulverization, and redistribution, form a rigid, supportive percolation network in situ during the second-stage secondary electro-sintering. In the subsequent second-stage flash sintering, controlled compression of the graphite felt acts as an elastic buffer interface and a homogenization pathway. The degree of graphite felt compression limits the uniformity of the electrothermal field and the gas escape rate, thereby enabling the large-size ceramic mass to maintain structural integrity while forming uniform pores.

[0057] The metal oxide composition ratio and reducing agent type provided in the foregoing embodiments can be adjusted according to the target performance of the high-entropy solid solution product. When other transition metal oxides are used for equivalent substitution, or the molar ratio of oxide components is changed, as long as these changes do not lead to phase instability or significant segregation, they should be considered within the scope of the technical solution disclosed in this invention. Similarly, the specific ratio adjustment of boron powder, boron carbide, and carbon powder in the reducing agent system, while maintaining the purpose of the invention, also falls within the scope of equivalent substitution.

[0058] The temperature, pressure, time, and graphite felt compression recorded in the embodiments have specific critical significance. Fine-tuning of these parameters by those skilled in the art within the controlled compression logic framework disclosed in this invention, as long as the fine-tuned process can maintain a porosity of 45% to 55% and a crack-free bulk morphology, should be included within the scope of protection. When process parameters fluctuate outside the disclosed numerical range, and the porosity deviation of the resulting product is within ±5% and the strength deviation is within ±10%, it should be considered to fall within the scope of protection of this invention.

[0059] Mechanical limit blocks, displacement sensors, or manual screw adjustment devices used to adjust the spacing between graphite plates are equivalent in achieving the function of spacing adjustment. The vacuum control of the reaction chamber and the flow regulation method of the protective atmosphere can be optimized according to specific equipment. Various power supply systems with high-frequency power regulation capabilities, including AC power supplies or DC pulse power supplies, should not limit the scope of protection of this invention, as long as they can ensure that the oxygen content is below the set threshold and maintain a constant working gas pressure during the reaction process, regardless of their specific gas path layout or power supply type.

[0060] It should be noted that the specific embodiments described above are exemplary. Those skilled in the art can devise various solutions inspired by the disclosure of this invention, and these solutions all fall within the scope of this invention and its protection. Those skilled in the art should understand that this specification and its accompanying drawings are illustrative and not intended to limit the scope of the claims. The scope of protection of this invention is defined by the claims and their equivalents. This specification contains multiple inventive concepts; terms such as "preferredly," "according to a preferred embodiment," or "optionally" indicate that the corresponding paragraph discloses an independent concept. The applicant reserves the right to file divisional applications based on each inventive concept.

Claims

1. A method for preparing a porous high-entropy boride ceramic material, characterized in that, Includes the following steps: S1. After mixing and grinding the metal oxide raw material powder and the reducing agent raw material powder, the green body is obtained by pressing. S2. The green blank is sandwiched between two conductive anti-adhesion layers, placed between two graphite felts, and then placed between two graphite plates to form a multi-layer assembly structure; the spacing between the graphite plates is controlled so that the graphite felt is compressed to 60% to 80% of its initial uncompressed thickness; after evacuating the reaction chamber, a protective gas is filled in, and the multi-layer assembly structure is heated by electricity through the graphite plates to cause the green blank to undergo a partial boronothermic reduction reaction, and after cooling, clinker is obtained; S3. Grind the clinker, control the grinding parameters so that the particle size D90 of the clinker powder reaches 1μm to 5μm, and press the clinker powder into a clinker blank. S4. The clinker blank is placed between the two conductive anti-adhesion layers, the two graphite felts, and the two graphite plates in the same assembly sequence as in step S2; the spacing between the graphite plates is controlled again so that the graphite felt is compressed to 60% to 80% of its initial uncompressed thickness; under vacuum and protective gas filling environment, the clinker blank is sintered by passing electricity through the graphite plates, and after cooling, the porous high-entropy boride ceramic material is obtained.

2. The preparation method according to claim 1, characterized in that, The metal oxide raw material powder includes at least zirconium oxide, titanium oxide, tantalum pentoxide, and niobium pentoxide; The reducing agent raw material powder is boron powder, or a mixture of boron carbide powder and carbon powder.

3. The preparation method according to claim 1 or 2, characterized in that, When the reducing agent raw material powder is boron powder, its excess addition ratio is 30% to 40% of the theoretical stoichiometric ratio; When the reducing agent raw material powder is a mixture of boron carbide powder and carbon powder, the excess addition ratio of the boron carbide powder is 40% to 55% of the theoretical stoichiometric ratio.

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step S1, the particle size D90 of the mixed powder of metal oxide and reducing agent after mixing and grinding is 1 μm to 3 μm; The pressure molding process conditions are as follows: holding pressure at 18MPa to 21MPa for 180s to 240s.

5. The preparation method according to any one of claims 1 to 4, characterized in that, In steps S2 and S4, the conductive anti-adhesion layer is hexagonal boron nitride paper with a thickness of 0.1 mm to 0.2 mm; The initial uncompressed thickness of the graphite felt is 3 mm; the radial dimensions of both the conductive anti-adhesion layer and the graphite felt are more than 10 mm larger than the radial dimension of the green or calcined blank placed therebetween.

6. The preparation method according to any one of claims 1 to 5, characterized in that, In steps S2 and S4, the operation of filling the protective gas after vacuuming specifically involves: evacuating the gas pressure in the reaction chamber to below 10 Pa, then filling it with high-purity argon gas, and maintaining the gas pressure in the reaction chamber at 103 kPa to 105 kPa during the electric heating or electric sintering process.

7. The preparation method according to any one of claims 1 to 6, characterized in that, In step S2, the process conditions for the electric heating are: controlling the input electric power to raise the temperature of the green blank from room temperature to 1650℃ to 1850℃ within 50s to 80s, and holding it at that temperature for 70s to 100s; In step S3, the process conditions for pressing the clinker powder into a clinker blank are: holding the pressure at 15MPa to 28MPa for 150s to 280s.

8. The preparation method according to any one of claims 1 to 7, characterized in that, In step S4, the process conditions for the electric sintering are: controlling the input electric power to raise the temperature of the clinker blank from room temperature to 1850℃~2050℃ within 90s~120s, and holding it at that temperature for 30s~50s; After the heat preservation is completed, the input power is gradually reduced to zero within 30s to 60s and then cooled with the furnace.

9. The preparation method according to any one of claims 1 to 8, characterized in that, In the step of heating or sintering the multi-layer assembly structure or clinker blank by passing electricity through a graphite plate, the electric field introduced is an alternating electric field.

10. A porous high-entropy boride ceramic material, characterized in that, The porous high-entropy boride ceramic material is prepared by the preparation method according to any one of claims 1 to 9; The porous high-entropy boride ceramic material is a single-phase high-entropy solid solution structure, with a rigid permeation network skeleton formed by the in-situ solid-phase reaction of high-entropy boride particles, and a three-dimensional interconnected open structure defined by the rigid permeation network skeleton. The porous high-entropy boride ceramic material is in the form of a monolithic block, with a block diameter of 100mm to 200mm and a thickness of 3mm to 6mm. The porous high-entropy boride ceramic material has an overall porosity of 45% to 55% and a compressive strength of 150 MPa to 190 MPa at room temperature.

Images