A heat shielding ceramic having a directional hole structure and a method of manufacturing the same

Abstract

The application discloses a heat shielding ceramic with a directional hole structure and a preparation method thereof, and belongs to the technical field of advanced heat protection ceramic materials. The method takes ABO3 type rare earth manganate powder as a ceramic matrix raw material; natural flaky graphite is selected as a sacrifice pore-forming template; the natural graphite flake is arranged in the matrix powder in a directional manner; and sectional sintering is carried out in an inert atmosphere or vacuum, so that a vertically oriented directional long and narrow multi-stage pore structure is formed in the ceramic matrix. The pore structure constructed by the application not only significantly prolongs the phonon transmission path, so that the thermal conductivity of the material is 0.4 W / (m*K) at 800 DEG C high temperature, but also optimizes the ion migration channel. In addition, the unique microstructure effectively reduces the infrared emissivity of the material, so that the average emissivity of the material in the 3-14 mu m wave band is lower than 0.3, so that the material has excellent heat insulation performance and thermal-optical performance.

Description

Technical Field

[0001] This invention relates to the field of advanced thermal protection ceramic materials technology, and in particular to a thermal shielding ceramic with an oriented pore structure and its preparation method. Background Technology

[0002] The infrared response mechanism of rare-earth manganate ceramics differs fundamentally from that of infrared-transmitting thermal barrier ceramics, such as yttrium-stabilized zirconia (YSZ). YSZ exhibits high infrared transmittance in the 3–5 μm and 8–14 μm atmospheric windows, allowing radiative heat flux to penetrate directly; while rare-earth manganates, due to their polaron conductivity mechanism at high temperatures, exhibit low infrared emissivity intrinsically, possessing excellent infrared stealth performance—a characteristic not found in conventional thermal barrier ceramics.

[0003] However, this specific system faces a unique "performance contradiction": on the one hand, the lattice thermal conductivity of dense rare-earth manganate is approaching the Debye limit, leaving very little room for further reduction in thermal conductivity through conventional doping; on the other hand, its low emissivity depends on the integrity of the polaron conductive network, while the intrinsic thermal conductivity bottleneck of this material necessitates the introduction of pores to suppress heat conduction. However, the random pore strategy used in conventional porous ceramics has inherent defects in this system: while introducing random, disordered pores can reduce thermal conductivity through phonon scattering, it also severely disrupts the continuity of the polaron conductive network, destroying the carrier transport channels upon which the low infrared emissivity depends, leading to a significant increase in emissivity and thus losing the intrinsic infrared stealth advantage of rare-earth manganate. Therefore, how to maintain the integrity of the polaron conductive network while reducing thermal conductivity is a key technical challenge that urgently needs to be solved in the field of rare-earth manganate thermal protection ceramics. Summary of the Invention

[0004] The purpose of this invention is to provide a thermally shielding ceramic with a directional pore structure and its preparation method. By constructing vertically oriented through-holes, the phonon transmission path can be extended to the maximum extent, significantly reducing the solid-state thermal conductivity. Furthermore, through multiple reflections and absorptions from the pore walls, infrared radiation can be effectively scattered and attenuated. More importantly, by precisely controlling the porosity of this directional pore structure, "on-demand design" of key performance parameters such as thermal conductivity and infrared emissivity can be achieved, thus reaching a balance between low thermal conductivity and low infrared emissivity.

[0005] To achieve the above objectives, this invention discloses a method for preparing a heat-shielding ceramic with an oriented pore structure. The method uses ABO3-type rare earth manganate powder as the ceramic matrix raw material; selects natural flake graphite as a sacrificial pore-forming template, with a volume fraction of 10%-30% of the ceramic matrix raw material; oriented natural flake graphite is arranged within the matrix powder; and then, segmented sintering is performed in an inert atmosphere or vacuum to form a vertically oriented, elongated, multi-level pore structure within the ceramic matrix. Among them, the directional, narrow, multi-level porous structure serves as an ion transport channel, thereby changing the ion conductivity of the ceramic material and achieving synergistic regulation and optimization of the infrared emissivity of the ceramic in the atmospheric window bands of wavelengths 3-5μm and 8-14μm and the thermal conductivity at 800℃. The fabricated thermal shielding ceramic has an average infrared emissivity of less than 0.3 in the atmospheric window bands of 3-5 μm and 8-14 μm, and a thermal conductivity of less than 0.4 W / (m·K) at 800℃.

[0006] Preferably, it includes the following steps: (1) Synthesis of ABO3 type rare earth manganate ceramic powder: Metal oxide powder, including rare earth oxide, strontium carbonate and cobalt oxide, was weighed according to stoichiometric ratio to synthesize mixed powder. The mixed powder was placed in a planetary ball mill jar with anhydrous ethanol and zirconium oxide microspheres at a mass ratio of 1:0.1:4 and ball milled at 400 r / min for 24 hours to obtain a uniformly mixed slurry. Subsequently, the slurry was dried at 90℃ for 10 hours by forced air drying. The dried powder was calcined in a box-type resistance furnace at 900℃ for 8 hours (heating rate 10℃ / min). After cooling in the furnace, it was ground and passed through a 300-mesh sieve to obtain a pure phase, uniform ceramic matrix powder. (2) Preparation of directional composite powder: The ceramic matrix powder obtained in step (1) is mixed with natural flake graphite in a predetermined ratio, and anhydrous ethanol is added as a dispersion medium. The graphite flakes are directionally arranged in the ceramic matrix by a controllable directional grinding operation in a single direction. The ground slurry is dried and sieved to obtain a composite powder with a directional structure. (3) Segmented sintering: The directional composite powder obtained in step (2) is sintered in segments to form vertically penetrating directional layered pores.

[0007] Preferably, in step (3), after the composite powder obtained in step (2) is pre-pressed and cold isostatically pressed to obtain a blank, it is sintered without pressure under a protective atmosphere: the temperature is raised from room temperature to 500-600℃ at a heating rate of 4-10℃ / min and held for 1-2 hours to completely remove the graphite template; then the temperature is raised to 1250-1350℃ at a heating rate of 4-10℃ / min and held for 8-15 hours to complete the sintering densification; finally, it is cooled with the furnace.

[0008] Preferably, in step (3), after the composite powder obtained in step (2) is pre-pressed and cold isostatically pressed to obtain a green body, it is subjected to pressureless sintering under a protective atmosphere: the temperature is raised to 1100℃ at a heating rate of 20-30℃ / min and held at that temperature, and an axial pressure of 25-30 MPa is applied in the 900℃-1100℃ range during the heating process. During this process, the axial pressure and high temperature work together: on the one hand, the pressure induces the flake graphite to further orient and arrange itself in the ceramic matrix and effectively fix its orientation; on the other hand, it promotes the densification sintering of the ceramic matrix to form a high-strength composite ceramic. After obtaining the composite ceramic, the natural flake graphite used as a pore-forming agent is removed by heat treatment.

[0009] Preferably, the heat treatment involves placing the composite ceramic in a muffle furnace and holding it at 700-800℃ for 4-6 hours in an air atmosphere to oxidize and remove the graphite template. After the flake graphite is ablated in situ, it forms vertically penetrating oriented layered pores that are completely consistent with the morphology and orientation of the initial template, ultimately obtaining a ceramic material with a stable oriented porous structure.

[0010] Preferably, the natural flake graphite is graphite flakes with a fixed carbon content of not less than 99%, a particle size of 10-200 micrometers, a thickness of 1-10 micrometers, and a diameter-to-thickness ratio greater than 5:1.

[0011] Preferably, step (2) directional grinding operation is to use a mortar and pestle to grind along a unidirectional and regular trajectory. By performing controllable grinding operations along a single and fixed direction, the shear force is used to make the natural flake graphite achieve a highly consistent directional arrangement in the ceramic matrix powder.

[0012] This invention also provides a heat-shielding ceramic with an oriented pore structure prepared by the above-described method, using an ABO3-type rare earth manganate as the matrix, wherein the general formula of the ABO3-type rare earth manganate is La. 0.5 Sr 0.5 MnO3 has highly interconnected pores that are distributed in a narrow, elongated layer perpendicular to the Z-axis; its porosity is 45-50%.

[0013] Preferably, the multi-level porous structure of the heat-shielding ceramic is a vertical pore structure, which is composed of layered pores formed by the agglomeration and sintering of graphite sheets between sheet-like ceramic units. The pore diameter is 5-10 μm and the pore direction is vertical.

[0014] Preferably, the multi-level porous structure of the shielding ceramic is a cross-pore structure, including: The pores are formed by the burning off of ultrafine graphite fragments and the residual gas from sintering inside the sheet-like ceramic unit. The pore size ranges from hundreds of nanometers to 1 μm, and the pore direction is intersecting. And / or layered pores formed by stacking natural graphite flakes along the Z-axis, with thicknesses of 1-3 μm and 5-10 μm, and pore orientations at 30-degree intersections; The cross-hole structure is distributed in a narrow, elongated layered pattern perpendicular to the Z-axis.

[0015] The principle of this invention lies in achieving an integrated improvement in material performance through synergistic innovation in component design, microstructure control, and preparation process. In terms of composition, by designing and doping ABO3-type rare earth manganate at the A-site, the valence bond structure and band structure are controlled at the atomic scale, optimizing the intrinsic electrical and infrared optical properties of the material. In terms of structural design and preparation process, the core innovation lies in using flake graphite as a sacrificial template, combined with hot-pressing sintering. Through directional grinding, the matrix phase powder is uniformly adsorbed onto the surface of the flake graphite. During hot pressing, axial pressure induces the graphite template to align directionally. Then, the template is removed by oxidation after high-temperature sintering, constructing three specific directional layered pore structures in the ceramic matrix: vertical pore structure, intersecting pore structure, and 30-degree intersecting pore structure, all of which are directional layered pore structures with adjustable porosity. This unique structure plays a crucial role in multiphysics: In terms of thermal management, the oriented layered channels significantly extend the phonon transport path and enhance interface scattering. Combined with the nanoscale micropores formed by ultrafine graphite fragments and residual gas within the sheet-like ceramic unit, a multi-level porous structure is formed to synergistically reduce heat conduction. The thermal conductivity at 800℃ is 0.4 W / (m·K), exhibiting excellent thermal insulation performance. In terms of thermal radiation control, this structure efficiently scatters and reflects infrared radiation, resulting in an emissivity of less than 0.3 at room temperature in the 3-14 μm band, and maintaining low emissivity even at 800℃, greatly improving thermal shielding efficiency. Simultaneously, the vertically connected oriented channels provide an efficient and rapid transport path for ion migration. Combined with the high connectivity of the pores confirmed by X-ray three-dimensional reconstruction technology, this significantly improves ionic conductivity. Finally, through innovative processes such as component control and hot-pressing sintering-sacrificial template method for constructing the oriented porous structure, this invention successfully enables the material to possess the dual advantages of ultra-low thermal conductivity and low infrared emissivity, achieving an integrated structural and functional design.

[0016] Therefore, the present invention has the following beneficial effects: I. Breakthrough in Overcoming the Bottleneck of Balancing High-Temperature Performance and Low Emissivity in Materials. This invention successfully resolves the inherent contradiction between the poor high-temperature stability of metallic materials and the high infrared emissivity of inorganic ceramic materials. The prepared porous ceramic, while maintaining the inherent high-temperature resistance of the ceramic matrix, achieves highly efficient radiation shielding in the critical atmospheric window band of 3-14μm by introducing a directional pore structure. Its infrared emissivity at both room temperature and 800℃ is below 0.3, providing a new generation of thermal protection materials for high-temperature hot-end components that combine excellent thermal stability with persistently low emissivity.

[0017] II. Synergistic optimization and active control of performance are achieved through a controllably fabricated directional porous structure. This invention innovatively uses flake graphite as a template to construct directional interconnected channels with adjustable porosity. This structure not only significantly extends the phonon transport path and enhances scattering, reducing the material's thermal conductivity to below 0.4 W / (m·K), thus achieving excellent thermal insulation performance; simultaneously, the increased ion channels effectively improve ionic conductivity. Furthermore, the optimization of the material's electrical properties, combined with the synergistic effect of multiple reflections and scattering of infrared radiation by the directional pores, effectively suppresses and controls infrared emissivity.

[0018] Third, by combining the unidirectional shear force during directional grinding with the axial pressure during hot pressing sintering, a highly consistent directional arrangement of flake graphite pore-forming agents in the ceramic matrix was achieved, further forming three specific pore structures: vertical pore structure, cross pore structure, and 30-degree cross pore structure. This "double redirection" mechanism ensures precise control of pore orientation, avoiding the structural disorder caused by the random distribution of pore-forming agents in traditional methods. The hot pressing sintering process integrates forming and sintering; applying axial pressure at high temperature not only promotes the densification of the ceramic matrix but also effectively fixes the directional arrangement structure, avoiding structural relaxation or orientation disorder that may occur in traditional pressureless sintering. This method is simple, with easily controllable parameters and cost. The prepared material has good crystal structure symmetry and a stable microstructure. The directional layered pores and the dense ceramic skeleton form a stable composite structure, exhibiting excellent structural stability and long-term high-temperature service reliability.

[0019] Fourth, this invention abandons the common practice of uniform random pore formation and constructs an ordered directional or gradient pore structure in rare earth manganate. This ordered structure, on the one hand, significantly extends the phonon path through phonon-boundary scattering, breaking through the thermal conductivity limit; on the other hand, by precisely controlling the pore size and distribution, it effectively reduces thermal conductivity while maximizing the continuity of polaron conductive channels, thereby maintaining the material's low emissivity characteristics. Simultaneously, the periodic size of the ordered pores can be matched with the infrared wavelength, further modulating radiative transmission and enhancing infrared stealth effects. This scheme, based on the synergistic design of a specific substrate (conductive network sensitivity) and ordered pores, is something that transparent ceramics such as YSZ cannot and do not need to consider, thus achieving a unique unity of improved thermal insulation performance and maintained infrared stealth functionality.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 XRD images of ceramic samples prepared in Example 1 and Comparative Example 1; Figure 2 SEM images of ceramic samples prepared for Example 1; Figure 3 SEM images of ceramic samples prepared for Example 2; Figure 4 The infrared emissivity of ceramic samples prepared in Example 1 and Comparative Example 1 at room temperature is shown, where a is the wavelength-emissivity spectrum and b is the histogram of average emissivity in the characteristic infrared band. Figure 5 The infrared emissivity of ceramic samples prepared in Example 1 and Comparative Example 1 under high temperature conditions is shown in Figure a, where a is the wavelength-emissivity spectrum and b is the histogram of average emissivity in the characteristic infrared band. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0024] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. These other embodiments are also covered within the scope of protection of this invention.

[0025] Unless otherwise specified, the materials, reagents, instruments, and equipment used in this invention are all materials, reagents, instruments, and equipment routinely used by those skilled in the art, and the testing standards all use national or international standards commonly used in the field, without further explanation.

[0026] Example 1 I. Raw material preparation and pretreatment: La2O3, SrCO3 and Mn2O3 powders were selected as ceramic matrix raw materials, and natural flake graphite was selected as sacrificial pore-forming agent.

[0027] Among them, La2O3 and Mn2O3 powders need to be dried at 100℃ for 5 hours before use to completely remove adsorbed moisture and possible hydroxides; SrCO3 powder can be used directly; the aspect ratio of natural flake graphite is 10:1 to ensure that it can effectively achieve directional alignment in subsequent processes.

[0028] II. Synthesis of ceramic matrix powder: (1) Press La 0.5 Sr 0.5To determine the stoichiometric ratio of MnO3, accurately weigh 11.297g of pretreated La2O3 powder, 10.971g of Mn2O3 powder, and 10.253g of SrCO3 powder, place them in the same container, and mix them initially to obtain the raw material powder.

[0029] (2) The above raw material powder, anhydrous ethanol solvent and zirconium oxide microspheres were placed in a planetary ball mill jar at a mass ratio of 1:0.2:4 and ball milled at a speed of 400 r / min for 12 h to obtain a highly uniform mixed slurry.

[0030] (3) Place the mixed slurry obtained in step (2) in a forced-air drying oven and dry it at 90°C for 10 hours to ensure that the solvent is completely evaporated, and obtain the dried mixed powder block.

[0031] (4) Place the dried powder block in a box-type resistance furnace and heat it to 1000℃ at a heating rate of 5℃ / min. Hold it at this temperature for 8 hours to complete the solid-phase reaction and ceramic phase synthesis. Then cool it to room temperature with the furnace.

[0032] (5) Place the calcined powder block in an agate mortar and grind it thoroughly for 1 hour. Then pass it through a 300-mesh sieve to obtain a pure phase ceramic matrix powder with uniform particle size. Seal it for later use.

[0033] III. Preparation and Orientation of Composite Powders: (6) Weigh the ceramic matrix powder obtained in step (5) and the natural flake graphite pore-forming agent precisely at a mass ratio of 6:1 and mix them initially.

[0034] (7) Transfer the pre-mixed powder into an agate mortar, add an appropriate amount of anhydrous ethanol as a dispersion medium, and then perform directional grinding. Apply a stable shear force along the same circumferential tangent direction of the mortar and continue grinding for 2 hours. During this process, the addition of anhydrous ethanol not only prevents the powder from agglomerating, but also acts as a lubricating medium, significantly reducing the mechanical damage and breakage of the flake graphite during the directional alignment process. It effectively utilizes the shear force field to force the flake graphite flakes to align in a highly consistent direction with their maximum plane parallel to the grinding direction.

[0035] (8) The composite powder after directional grinding is placed in a drying oven and dried at 80°C to remove ethanol. Then it is passed through a 200-mesh sieve to obtain a loose, uniform directional composite powder with a high degree of orientation consistency.

[0036] IV. Hot pressing sintering, directional forming, and densification of composite preforms: (9) The directional composite powder obtained in step (8) is uniformly loaded into a mold with an inner diameter of Φ40mm and sintered in a hot pressing sintering furnace. The sintering process is set as follows: under the protective atmosphere of argon or nitrogen, the temperature is increased from room temperature to 550℃ at a heating rate of 10℃ / min and held for 2h to allow the pore-forming agent to be initially carbonized and the adsorbed gas to be removed.

[0037] (10) When the temperature continues to rise to 900℃, axial pressure is applied. During the process of raising the temperature to the final sintering temperature of 1100℃, the pressure is gradually increased to 27.8MPa. The axial pressure is used to induce the flake graphite to achieve a highly consistent directional arrangement in the ceramic matrix, while promoting the densification sintering of the ceramic matrix. The temperature is held at the final sintering temperature for 8 hours to ensure that the ceramic is fully sintered. (11) After sintering, the composite ceramic block is naturally cooled to room temperature in the furnace to obtain the composite ceramic. In this composite ceramic, the flake graphite is highly oriented along the direction perpendicular to the pressure, forming a composite structure in which the structural orientation is maintained.

[0038] V. Obtaining Directional Porous Ceramics: The composite ceramic block obtained in step (11) was cut into the required size of 10mm×10mm×1mm, sanded to a thickness of 0.5mm, and then placed in a muffle furnace and held at 700℃ for 4 hours in an air atmosphere to fully oxidize and remove the graphite template. After the flake graphite was ablated in situ, the space it occupied in the ceramic matrix was transformed into vertically penetrating oriented layered pores with corresponding size, morphology, and orientation, thus successfully obtaining a ceramic block with a controllable oriented porous structure.

[0039] Example 2 This embodiment provides a heat-shielding ceramic with an oriented hole structure, the preparation method of which includes the following steps: I. Raw material preparation and pretreatment: Same as in Example 1, except that the aspect ratio of natural flake graphite is 5:1.

[0040] II. Synthesis of ceramic matrix powder: Same as in Example 1.

[0041] III. Preparation and Orientation of Composite Powder: Same as in Example 1, except that in step (6), the ceramic matrix powder and the natural flake graphite pore-forming agent are precisely weighed and initially mixed at a mass ratio of 10:1.

[0042] IV. Hot pressing sintering, directional forming, and densification of composite preforms: (9) The directional composite powder obtained in step (8) is uniformly loaded into a mold with an inner diameter of Φ40mm and sintered in a hot pressing sintering furnace. The sintering process is set as follows: under the protective atmosphere of argon or nitrogen, the temperature is increased from room temperature to 550℃ at a heating rate of 10℃ / min and held for 2h to allow the pore-forming agent to be initially carbonized and the adsorbed gas to be removed.

[0043] (10) When the temperature continues to rise to 900℃, axial pressure is applied. During the process of raising the temperature to the final sintering temperature of 1100℃, the pressure is gradually increased to 27.8MPa. The axial pressure is used to induce the flake graphite to achieve a highly consistent directional arrangement in the ceramic matrix, while promoting the densification sintering of the ceramic matrix. The temperature is held at the final sintering temperature for 8 hours to ensure that the ceramic is fully sintered.

[0044] (11) After sintering, the composite ceramic block is naturally cooled to room temperature in the furnace to obtain the composite ceramic. In this composite ceramic, the flake graphite is highly oriented along the direction perpendicular to the pressure, forming a composite structure in which the structural orientation is maintained.

[0045] V. Obtaining Directional Porous Ceramics: The composite ceramic block obtained in step (11) is cut into the required size (e.g., 10mm × 10mm × 1mm), sanded to a thickness of 0.5mm, and then placed in a muffle furnace and held at 700℃ for 4 hours in an air atmosphere to fully oxidize and remove the graphite template. After the flake graphite is ablated in situ, the space it occupies in the ceramic matrix is ​​transformed into vertically penetrating oriented layered pores with corresponding size, morphology, and orientation, thus successfully obtaining a ceramic block with a controllable oriented porous structure.

[0046] Example 3 This embodiment provides a heat-shielding ceramic with an oriented hole structure, the preparation method of which includes the following steps: I. Raw material preparation and pretreatment: Same as in Example 1.

[0047] II. Synthesis of ceramic matrix powder: Same as in Example 1.

[0048] III. Preparation and Orientation of Composite Powders: Same as in Example 1.

[0049] IV. Hot pressing sintering, directional forming, and densification of composite preforms: (9) The directional composite powder obtained in step (8) is uniformly loaded into a mold with an inner diameter of Φ40mm and sintered without pressure in an atmosphere furnace. The sintering process is set as follows: under the protection of argon or nitrogen, the temperature is increased from room temperature to 550℃ at a heating rate of 5℃ / min and held for 2h to allow the pore-forming agent to be initially carbonized and the adsorbed gas to be removed.

[0050] (10) Continue to raise the temperature to 1300℃ at 5℃ / min and hold for 8 hours to ensure that the ceramic is fully sintered.

[0051] (11) After sintering, the composite ceramic block is naturally cooled to room temperature in the furnace.

[0052] V. Obtaining Directional Porous Ceramics: The composite ceramic block obtained in step four was cut into the required size (e.g., 10mm × 10mm × 1mm), sanded to a thickness of 0.5mm, and then placed in a muffle furnace and held at 700℃ for 4 hours in air atmosphere to fully oxidize and remove the graphite template. After in-situ ablation of the flake graphite, the space it occupied in the ceramic matrix was transformed into vertically penetrating oriented layered pores with corresponding size, morphology, and orientation, successfully obtaining a ceramic block with a controllable oriented porous structure.

[0053] Comparative Example 1 This comparative example provides a heat-shielding ceramic with an oriented pore structure, the preparation method of which includes the following steps: I. Raw Material Preparation and Pretreatment: Same as in Example 1, except that the sacrificial pore-forming agent is graphene, and the specific surface area of ​​graphene is greater than 500 m². 2 / g, number of layers less than 5.

[0054] II. Synthesis of ceramic matrix powder: Same as step two in Example 1.

[0055] III. Preparation and Orientation of Composite Powder: The same as step three in Example 1, except that step (6) involves accurately weighing and initially mixing the ceramic matrix powder obtained in step (5) with graphene at a mass ratio of 6:1.

[0056] IV. Pressing and forming of the blank: (9) The directional composite powder obtained in step (8) is uniformly loaded into a stainless steel mold with an inner diameter of Φ40mm. It is then pre-pressed on a tablet press with a pressure of 10MPa for 3 minutes to obtain a cylindrical initial blank with a certain initial strength, with a size of approximately Φ40mm×3mm.

[0057] (10) After sealing the initial billet in a vacuum-sealed bag, place it in a cold isostatic pressing equipment and hold it under an isostatic pressure of 180 MPa for 5 minutes to make the internal stress of the billet uniform and obtain a composite billet whose structural orientation is maintained. V. Sintering of Directional Porous Ceramics: (11) The composite green body obtained in step (10) was placed stably in the homogenization zone of the high-temperature sintering furnace and the pressureless sintering program was set as follows: the temperature was increased from room temperature to 500℃ at a rate of 5℃ / min. This stage helps the graphite template to decompose and burn off stably. Then the heating rate was reduced to 4℃ / min and the temperature was increased to the final sintering temperature of 1300℃. The temperature was held at this temperature for 10h to ensure that the ceramic matrix was fully sintered and densified. After sintering, the power was cut off and the sample was allowed to cool naturally to room temperature with the furnace. Finally, the sheet graphite template was completely ablated, and the space it originally occupied in the green body was transformed in situ into channels with corresponding size, morphology and orientation, and a ceramic block with a porous structure was successfully obtained.

[0058] The ceramic samples prepared in Example 1 and Comparative Example 1 were polished and cut to prepare test samples, and then the following performance characterization was performed: the phase composition of the ceramic materials was analyzed using an X-ray diffractometer, and the diffraction angle (2θ) ranged from 10° to 90°. The results are shown in the figure. Figure 1 The surface and cross-sectional morphology of the ceramic blocks in Examples 1 and 2 were observed using field emission scanning electron microscopy. The results are as follows: Figure 2 and Figure 3 As shown in Table 1, the porosity of porous ceramics was determined by the Archimedes displacement method, and the data are listed in Table 1. The infrared emissivity of the samples was measured at room temperature and high temperature using a Fourier transform infrared spectrometer and a self-made high-temperature emissivity meter, respectively. The high-temperature test range was 800℃. The test results are shown in Table 1. Figure 4 and Figure 5 The thermal conductivity of the samples at 25℃, 400℃ and 800℃ was measured using a laser thermal conductivity meter, and the data are listed in Table 2.

[0059] Figure 1 The X-ray diffraction patterns of the ceramic samples prepared in Example 1 and Comparative Example 1 are shown. As can be seen from the figures, the main diffraction peaks of all samples match the characteristic diffraction peaks of the perovskite structure LaMnO3 standard card PDF#33-0713, and no obvious impurity phase peaks appear, indicating that the solid-state reaction synthesis process successfully obtained a pure-phase ABO3 type ceramic matrix. In Example 1, after introducing a graphite pore-forming agent and undergoing hot-pressing sintering and subsequent oxidation treatment, the crystal structure of the ceramic matrix remained intact, indicating that the burn-off of the graphite template did not adversely affect the matrix phase structure.

[0060] Figure 2 This is a scanning electron microscope (SEM) image of the ceramic sample prepared in Example 1. The ceramic cross-section shows a highly oriented layered pore structure. The pores are distributed in a narrow and elongated shape along the direction perpendicular to the hot pressing (Z-axis). The pores have good connectivity and dense and continuous pore walls, indicating that the sheet graphite template achieved effective oriented alignment during the hot pressing process and was successfully replicated into oriented pores after oxidation removal.

[0061] Figure 3 This is a cross-sectional scanning electron microscope (SEM) image of the ceramic sample prepared in Example 2. As can be seen in the image, the pore structure remains layered, but its orientation is not completely perpendicular to the hot-pressing direction (Z-axis), but rather exhibits an inclination angle of approximately 30°. The pores retain their elongated morphology, with dense ceramic matrix separating the layers. The pore walls are continuous, and the pores have good connectivity. Figure 2 compared to, Figure 3 The arrangement of the channels is relatively loose, the regularity of the layered structure is slightly reduced, and some channels show branching or local interruption.

[0062] Table 1 Porosity Results

[0063] The main reason for the difference in pore structure is that the aspect ratio of the natural flake graphite used in Example 1 is 10:1, while that in Example 2 is 5:1. In addition, the mass ratio of ceramic powder to graphite in Example 1 is 6:1, while that in Example 2 is 10:1 (with a lower amount of graphite). The larger the aspect ratio and the higher the graphite content, the easier it is for the graphite flakes to achieve a completely oriented arrangement perpendicular to the pressure direction under directional grinding and axial pressure, forming a regular vertical pore structure. However, when the aspect ratio is small and the graphite content is low, the oriented arrangement ability of the graphite template decreases, and it is more susceptible to the influence of local stress and shrinkage during high-temperature sintering and densification, resulting in a deflection and the formation of a layered pore structure with an inclination of about 30°. At the same time, the porosity also decreases from 47.2% to 35.6%.

[0064] Figure 4 and Figure 5 The results show the infrared emissivity of the samples at room temperature and 800℃, respectively. Figure 4 This indicates that, at room temperature, Example 1 has an emissivity of less than 0.3 in the 3-5 μm band and is significantly better than Comparative Example 1 in the 8-14 μm band. Figure 5 The results show that at a high temperature of 800℃, the infrared emissivity of Example 1 in the 3-5μm band remains below 0.2, demonstrating excellent high-temperature thermal radiation suppression capability. Example 1 uses natural flake graphite (diameter-to-thickness ratio 10:1) to construct a high-porosity (47.2%) vertically oriented layered hierarchical porous structure. This oriented, interconnected, narrow channel can generate multiple reflections, scatterings, and absorptions of infrared radiation in the 3-5μm and 8-14μm bands. At the same time, the vertical pore walls form an effective geometric interception effect with the incident radiation, thereby achieving a low emissivity of less than 0.3 at room temperature. In contrast, Comparative Example 1 uses graphene as a pore-forming agent. After burning, it only forms nanoscale irregular isolated pores, lacking a macroscopic-scale oriented layered structure. Its ability to scatter infrared radiation is limited. In addition, its porosity is only 19.7%, and the number of solid-gas interfaces is small, making it easy for radiation to penetrate directly. At a high temperature of 800℃, the vertically oriented pore structure of Example 1 remains stable, the ceramic matrix has excellent high-temperature phase stability, and the emissivity remains below 0.2; while in Comparative Example 1, the nanoscale micropores are prone to sintering closure or coarsening at high temperatures, and the graphene residual defects may introduce additional infrared absorption centers, causing its emissivity to increase significantly with increasing temperature.

[0065] Table 2 shows the thermal conductivity test results of Example 1 and Comparative Example 1 at 25℃, 400℃, and 800℃. The results show that the thermal conductivity of Example 1 at 25℃, 400℃, and 800℃ is significantly lower than that of Comparative Example 1, especially after reaching a high temperature of 800℃, where the thermal conductivity of Example 1 is 0.4 W·m. -1·K -1 It is lower than the 1.25 W·m of Comparative Example 1. -1 ·K -1 It exhibits excellent high-temperature thermal insulation performance.

[0066] Table 2 Thermal conductivity results

[0067] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a heat-shielding ceramic with an oriented pore structure, characterized in that, ABO3 type rare earth manganate powder is used as the ceramic matrix raw material; natural flake graphite is selected as the sacrificial pore-forming template, and the volume amount is 10%-30% of the ceramic matrix raw material; the natural graphite flakes are oriented in the ceramic matrix powder; and then segmented sintering is carried out in an inert atmosphere or vacuum to form a vertically oriented, oriented, narrow, multi-level pore structure inside the ceramic matrix. Among them, the directional, narrow, multi-level porous structure serves as an ion transport channel, thereby changing the ion conductivity of the ceramic material and achieving synergistic regulation and optimization of the infrared emissivity of the ceramic in the atmospheric window bands of wavelengths 3-5μm and 8-14μm and the thermal conductivity at 800℃. The fabricated heat-shielding ceramic has an average infrared emissivity of less than 0.3 in the atmospheric window bands of 3-5 μm and 8-14 μm, and a thermal conductivity of less than 0.4 W / (m·K) at 800℃.

2. The method for preparing a heat-shielding ceramic with an oriented pore structure according to claim 1, characterized in that, Includes the following steps: (1) Synthesis of ABO3 type rare earth manganate ceramic powder: Weigh the metal oxide powder according to the stoichiometric ratio, mix the metal oxide powder with anhydrous ethanol and zirconium oxide microspheres at a mass ratio of 1:0.1:4 and ball mill at 400 r / min for 24 hours to obtain a slurry; dry the slurry at 90℃ for 10 hours, calcine it in a box-type resistance furnace at 900-1000℃ for 8 hours, grind and sieve to obtain pure phase ceramic matrix powder; (2) Preparation of directional composite powder: The ceramic matrix powder obtained in step (1) is mixed with natural flake graphite in a predetermined ratio, and anhydrous ethanol is added as a dispersion medium. The graphite flakes are directionally arranged in the ceramic matrix by a controllable directional grinding operation in a single direction. The ground slurry is dried and sieved to obtain a composite powder with a directional structure. (3) Segmented sintering: The directional composite powder obtained in step (2) is sintered in segments to form vertically penetrating directional layered pores.

3. The method for preparing a heat-shielding ceramic with an oriented pore structure according to claim 2, characterized in that, In step (3), the composite powder obtained in step (2) is pre-pressed and cold isostatically pressed to obtain a blank, and then pressureless sintering is carried out under a protective atmosphere: the temperature is raised from room temperature to 500-600℃ at a heating rate of 4-10℃ / min and held for 1-2 hours to completely remove the graphite template; then the temperature is raised to 1250-1350℃ at a heating rate of 4-10℃ / min and held for 8-15 hours to complete the sintering densification; finally, it is cooled with the furnace.

4. The method for preparing a heat-shielding ceramic with an oriented pore structure according to claim 2, characterized in that, In step (3), the composite powder obtained in step (2) is pre-pressed and cold isostatically pressed to obtain a blank, and then hot-pressed and sintered under a protective atmosphere: the temperature is raised to 1100℃ at a heating rate of 20-30℃ / min and held at that temperature, and an axial pressure of 25-30 MPa is applied in the 900℃-1100℃ range during the heating process. After obtaining the composite ceramic, the natural flake graphite used as a pore-forming agent is removed by heat treatment.

5. The method for preparing a heat-shielding ceramic with an oriented pore structure according to claim 4, characterized in that, The heat treatment involves placing the composite ceramic in a muffle furnace and holding it at 700-800℃ for 4-6 hours in an air atmosphere to oxidize and remove the pore-forming agent.

6. The method for preparing a heat-shielding ceramic with an oriented pore structure according to claim 1, characterized in that, Natural flake graphite is a type of flake graphite with a fixed carbon content of not less than 99%, a particle size of 10-200 micrometers, a thickness of 1-10 micrometers, and a diameter-to-thickness ratio greater than 5:

1.

7. The method for preparing a heat-shielding ceramic with an oriented pore structure according to claim 2, characterized in that, Step (2) Directional grinding is performed by grinding in a mortar along a unidirectional and regular trajectory. By applying shear force to the powder, the flake graphite is oriented in the ceramic powder matrix.

8. A heat-shielding ceramic with an oriented hole structure, characterized in that, The heat-shielding ceramic is prepared by the preparation method according to any one of claims 1-7, wherein the porosity is 45-50%, and ABO3-type rare earth manganate is used as the matrix raw material, wherein the ABO3-type rare earth manganate is La 0.5 Sr 0.5 MnO3.

9. A heat-shielding ceramic with an oriented hole structure according to claim 8, characterized in that, The multi-level porous structure of the heat-shielding ceramic is a vertical pore structure, which consists of layered pores formed by the sintering of natural flake graphite between the sheet-like ceramic units. The pore diameter is 5-10μm and the pore direction is vertical.

10. A heat-shielding ceramic with an oriented hole structure according to claim 8, characterized in that, The multi-level porous structure of the heat-shielding ceramic is a cross-pore structure, including: The pores are formed by the burning off of ultrafine graphite fragments and the residual gas from sintering inside the sheet-like ceramic unit. The pore size ranges from hundreds of nanometers to 1 μm, and the pore direction is intersecting. And / or layered pores formed by stacking natural graphite flakes along the Z-axis, with thicknesses of 1-3 μm and 5-10 μm, and pore orientations at 30-degree intersections; The cross-hole structure is distributed in a narrow, elongated layered pattern perpendicular to the Z-axis.

Images