High-temperature electrically insulating alumina ceramic with high-density grain boundary network and method of preparation and use thereof

Abstract

This invention belongs to the technical field of advanced ceramic materials and high-temperature electrical insulation materials, and relates to a high-temperature electrical insulating alumina ceramic with a high-density grain boundary network, its preparation method, and its application. This alumina ceramic has a relative density of not less than 99.5% and an average grain size of less than or equal to 0.53 μm. High-purity nanomaterials are used. α Using alumina powder as raw material, a homogeneous, high-density green body is obtained through dispersant dispersion, pH adjustment, high-solids slurry control, and centrifugal casting. This green body is then subjected to a three-step pressureless sintering process, prioritizing re-homogenization, to obtain a highly dense, fine-grained alumina ceramic. This alumina ceramic, without the introduction of additional sintering aids, dopants, or a high-resistivity second phase, forms a high-density grain boundary network, which is beneficial for suppressing high-temperature carrier migration across grain boundaries. Compared to coarse-grained alumina ceramics, it exhibits higher volume resistivity and electrical insulation stability in the temperature range from room temperature to 400°C, making it suitable for high-temperature electrical insulation components such as nuclear detectors, high-temperature electronic devices, and sensors.

Description

Technical Field

[0001] This invention belongs to the technical field of advanced ceramic materials and high-temperature electrical insulation materials, specifically relating to a high-temperature electrical insulating alumina ceramic with a high-density grain boundary network, its preparation method, and its application. Background Technology

[0002] Alumina, as a wide bandgap oxide (approximately 8.8 eV), has an extremely low intrinsic carrier concentration and a volume resistivity of up to 10⁻⁶ at room temperature. 14 -10 16 With a volume resistivity on the order of Ω·cm and excellent chemical stability, mechanical strength, and high-temperature resistance, alumina is widely used in insulating components for nuclear detectors, supports for high-temperature electronic devices, insulating bases for sensors, and electrical insulation structures in harsh environments. However, as the service temperature increases, the generation and migration of thermally excited charge carriers in alumina intensifies, and its volume resistivity typically decreases significantly with increasing temperature, limiting its reliability in medium- and high-temperature electrical insulation environments.

[0003] Existing strategies for improving the electrical insulation properties of alumina or other oxide ceramics mainly include doping and introducing a high-resistivity second phase. These methods can alter the carrier concentration, defect state, or grain boundary composition of the material to some extent, but they also have limitations: First, doping elements or the second phase can change the intrinsic chemical composition of the material, potentially leading to segregation, migration, or interfacial instability during high-temperature service. Second, the second phase may introduce new interfacial defects between the second phase and the alumina matrix, becoming weak areas where high-temperature insulation performance deteriorates or fails. Third, the effects of doping or second-phase modulation are affected by the uniformity of component distribution, phase interface compatibility, and high-temperature thermal stability, resulting in a narrow process window. The preparation process is sensitive to raw material composition, mixing uniformity, and sintering conditions, making it difficult to guarantee the stability of high-temperature electrical insulation performance and the reproducibility of the preparation. Therefore, there is an urgent need for a modulation method that does not rely on doping or the introduction of a high-resistivity second phase to improve the high-temperature electrical insulation properties of alumina ceramics at the intrinsic microstructure level. Summary of the Invention

[0004] The purpose of this invention is to provide a high-temperature electrically insulating alumina ceramic with a high-density grain boundary network, in order to solve the technical problems of the significant decrease in volume resistivity of existing alumina ceramics under high-temperature conditions, and the difficulty in ensuring high-temperature electrical insulation stability and reproducibility of existing doping and high-resistivity second phase methods.

[0005] The second objective of this invention is to provide a method for preparing high-temperature electrically insulating alumina ceramics with a high-density grain boundary network.

[0006] A third objective of this invention is to provide an application of a high-temperature electrically insulating alumina ceramic with a high-density grain boundary network.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A high-temperature electrically insulating alumina ceramic with a high-density grain boundary network, wherein the alumina ceramic has a relative density greater than or equal to 99.5% and an average grain size less than or equal to 0.53 μm.

[0009] Preferably, the average grain size of the alumina ceramic is 0.42 to 0.53 μm.

[0010] Preferably, the volume resistivity of the alumina ceramic at 400°C is not less than 1×10⁻⁶. 12 Ω·cm.

[0011] Preferably, the volume resistivity of the alumina ceramic at 400°C is not less than 8.9 × 10⁻⁶. 12 Ω·cm.

[0012] Preferably, the Al2O3 content in the alumina ceramic is not less than 99.98 wt%.

[0013] Preferably, the alumina ceramic is not additionally incorporating sintering aids, dopants, or a high-resistivity second phase.

[0014] A method for preparing a high-temperature electrically insulating alumina ceramic with a high-density grain boundary network includes the following steps:

[0015] S1: Take high-purity alumina powder, disperse it in a water-based system, add a dispersant, and disperse it evenly to obtain an alumina dispersion slurry;

[0016] S2: The alumina dispersion slurry is centrifugally cast into shape and dried to obtain an alumina blank;

[0017] S3: Remove the dispersant from the alumina blank and then sinter it.

[0018] Preferably, the sintering in step S3 is pressureless sintering. The first step involves heating to 800–1000°C and holding for 2–5 hours. The second step involves heating to 1200–1300°C, reaching the target temperature without holding. The third step involves cooling to 1100–1200°C and holding for 10–30 hours. In conventional pressureless sintering, grain boundary diffusion and grain boundary texture migration are highly coupled, and densification is often accompanied by rapid grain coarsening, making it difficult to obtain highly dense, fine-grained ceramics. This invention employs a three-step sintering strategy, prioritizing low-temperature homogenization and using time-temperature coordinated control, to decouple densification from grain growth. First, in the low-temperature holding section, particle rearrangement induced by intergranular sintering necks is used to complete the re-homogenization of the green body. Then, high-temperature non-holding sintering is used to make the pores of the homogenized green body enter an unstable state that tends to shrink, opening the decoupling window before significant grain growth. Finally, long-term holding at a lower temperature drives densification dominated by grain boundary diffusion, while effectively suppressing grain growth caused by grain boundary texture migration.

[0019] Preferably, the heating rate in the first step is 5-15℃ / min, the heating rate in the second step is 5-15℃ / min, and the cooling rate in the third step is 5-15℃ / min.

[0020] Preferably, in step S2, the centrifugal acceleration during centrifugal casting is 5000–20000 g, and the centrifugation time is 5–30 min.

[0021] Preferably, after centrifugal casting in step S2, the supernatant is removed and the precipitate is allowed to stand for 3 to 7 days. After demolding, the precipitate is dried at 40 to 60°C until the mass of the precipitate is constant, thus obtaining the alumina blank.

[0022] Preferably, the high-purity alumina powder in step S1 is α-alumina powder with a purity of not less than 99.99%, and the average particle size of the α-alumina powder is preferably 150 to 300 nm.

[0023] By employing a high-purity alumina powder and a sintering-free preparation route, the dominant influence of doping or the second phase on the electrical behavior of grain boundaries is avoided. A strategy of increasing the grain boundary density per unit volume through grain refinement is adopted to construct a high-density grain boundary network, enabling the grain boundaries to act as a potential barrier during carrier migration, thereby improving the high-temperature electrical insulation performance of alumina ceramics.

[0024] Preferably, the alumina dispersion slurry has a solid content of 45-52 vol% and a pH of 8.5-9.5.

[0025] Preferably, in step S3, the dispersant is removed by heating the alumina blank obtained in step S2 to 450-600°C in air at a heating rate of 1-5°C / min and holding it at that temperature for 10-60 min.

[0026] Preferably, the dispersant is ammonium citrate; the amount of the dispersant added is 0.1% to 0.6% of the mass of the alumina powder.

[0027] Preferably, the method for achieving uniform dispersion is as follows: after adding alumina powder dispersed in water to a dispersant, the powder is ball-milled at 100–400 rpm for 1–6 hours.

[0028] By optimizing the selection and dosage of dispersant, the pH value of slurry, and the solid content ratio, the surface charge state of particles, the steric hindrance effect, and the dispersion stability of slurry are simultaneously controlled. This allows the deagglomerated alumina particles to form a stable surface charge structure and an effective steric hindrance layer, enabling them to co-settle in a centrifugal force field as an integral particle network. This avoids the problem of a packing gradient along the axial direction caused by particle agglomerates or preferential deposition of large particles, providing a foundation for obtaining high-quality green bodies with uniform microstructures through centrifugal casting. This allows subsequent sintering to obtain ceramic bodies with higher density and better electrical insulation properties.

[0029] Application of a high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in insulating devices.

[0030] The beneficial effects of this invention are:

[0031] The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network of the present invention does not rely on the introduction of doping elements and high-resistivity second phases, nor does it add sintering aids, and can improve its high-temperature electrical insulation performance without changing the intrinsic chemical composition of alumina ceramic.

[0032] This invention constructs a high-density grain boundary network through grain refinement, enabling grain boundaries to act as potential barriers during carrier migration. This helps to suppress the migration of carriers across grain boundaries at high temperatures, thereby improving the high-temperature volume resistivity and electrical insulation stability of alumina ceramics.

[0033] This invention reduces preferential deposition of agglomerates and differential particle settling by using high-solids-content alumina slurry for synergistic dispersion control and centrifugal co-sedimentation, thereby obtaining homogeneous high-density green blanks.

[0034] This invention utilizes a three-step sintering process, employing low-temperature re-homogenization, high-temperature decoupling window establishment, and low-temperature diffusion densification in a step-by-step manner. This process effectively suppresses grain coarsening while achieving high densification, overcoming the contradiction of difficulty in achieving both densification and grain refinement in conventional sintering, and obtaining highly dense fine-grained alumina ceramics.

[0035] The fine-grained, high-density alumina ceramics prepared by this invention exhibit higher volume resistivity and better high-temperature electrical insulation stability compared to coarse-grained alumina ceramics in the range of room temperature to 400 °C. Attached Figure Description

[0036] Figure 1 The images show SEM images of the alumina blanks obtained by centrifugal casting in Example 1, where (a) is a low-magnification SEM image and (b) is a high-magnification SEM image.

[0037] Figure 2 The images show SEM images and particle size distribution diagrams of alumina ceramics from Examples 1 and 2, where (a) is an SEM image of alumina ceramic from Example 1, (b) is a particle size distribution diagram of alumina ceramic from Example 1, (c) is an SEM image of alumina ceramic from Example 2, and (d) is a particle size distribution diagram of alumina ceramic from Example 2.

[0038] Figure 3 The graph shows the volume resistivity test results of dense alumina ceramics of Example 1, Comparative Example 1, and Comparative Example 2 from room temperature to 400 °C.

[0039] Figure 4 SEM images and particle size distribution diagrams of alumina ceramics in Comparative Examples 1, 2, and 4 are shown below; (a) is the SEM image of alumina ceramic in Comparative Example 1, (b) is the particle size distribution diagram of alumina ceramic in Comparative Example 1, (c) is the SEM image of alumina ceramic in Comparative Example 2, (d) is the particle size distribution diagram of alumina ceramic in Comparative Example 2, (e) is the SEM image of alumina ceramic in Comparative Example 4, and (f) is the particle size distribution diagram of alumina ceramic in Comparative Example 4. Detailed Implementation

[0040] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0041] Example 1

[0042] The method for preparing high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment includes the following steps:

[0043] S1: α-Alumina powder with a purity of not less than 99.99% and an average initial particle size of 150 nm was selected. Using high-purity α-alumina powder reduces the influence of impurity ions on carrier transport and grain boundary electrical behavior; using fine-particle-size alumina powder improves the sintering activity of the powder, providing a microstructural basis for subsequent low-temperature densification and fine-grained structure formation. The α-alumina powder was dispersed in an aqueous system, and TAC was added as a dispersant to obtain an alumina slurry. The TAC addition amount was 0.3% of the α-alumina powder mass. The pH of the alumina slurry was then adjusted to 9, and the solid content of the alumina slurry was controlled at 52 vol%. The slurry was then ball-milled at 250 rpm for 3 h to obtain a high-solids-content alumina dispersion slurry. In the alumina dispersion slurry system, ball milling is used to dissociate pre-existing agglomerates in the alumina powder. Citrate ions from ammonium citrate can be adsorbed onto the alumina particle surface via carboxylate groups, altering the particle surface charge state and forming an adsorption layer with steric hindrance. The synergistic effect of pH adjustment and ammonium citrate adsorption moves the system away from the modified isoelectric point, enhancing the particle surface charge state and improving the slurry dispersion stability. Through the synergistic effect of ball milling deagglomeration, ammonium citrate adsorption, and pH adjustment, the deagglomerated alumina particles can maintain a stable dispersion state through electrostatic repulsion and steric hindrance, providing a foundation for the co-settling of dispersed particles in the form of an integral particle network during subsequent centrifugal casting.

[0044] S2: The obtained high-solids-content alumina dispersion slurry was injected into a centrifugal mold and centrifuged at 13000 g for 15 min, allowing the dispersed alumina particles to co-sediment in a centrifugal force field as a whole particle network. After centrifugation, the supernatant was removed, and the mold was allowed to stand at room temperature for 5 days before demolding. The demolded green blank was placed in a 50℃ oven and dried until constant mass was obtained, yielding a homogeneous alumina green blank with high bulk density. The relative density of the alumina green blank was approximately 66%. Figure 1 As shown, the alumina preform exhibits no significant packing density gradient in either the radial or axial direction, and its pore distribution is uniform. This homogeneous, high-density alumina preform provides a favorable microstructure starting point for the subsequent first-step low-temperature re-homogenization sintering.

[0045] S3: The dried green blank is heated to 500°C in air at a heating rate of 2°C / min and held for 30 min to remove the triammonium citrate dispersant, thus obtaining the debinding alumina green blank.

[0046] S4: The obtained alumina green body after debinding was subjected to pressureless three-step sintering without the addition of sintering aids. First, the temperature was increased to 1000℃ at a rate of 5℃ / min and held for 3 h to rehomogenize the green body by inducing particle rearrangement through intergranular sintering necks. Second, the temperature was increased to 1250℃ at a rate of 10℃ / min, and then held without holding to allow the green body to reach the critical relative density required for subsequent low-temperature densification, which was approximately 94%. Third, the temperature was decreased to 1150℃ at a rate of 10℃ / min and held for 20 h to allow the green body to continue densifying mainly through grain boundary diffusion, while suppressing grain boundary texture migration and the resulting grain coarsening. After holding, the temperature was decreased to room temperature at a rate of 5℃ / min to obtain high-purity, high-density, fine-grained alumina ceramic, denoted as TSS. The resulting alumina ceramic has an average grain size of 0.42 μm, a relative density of 99.8%, and an alumina content of 99.983%. It possesses a high-density grain boundary network composed of submicron grains.

[0047] The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network of this embodiment was prepared by the method described in this embodiment. The SEM image of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network of this embodiment is shown below. Figure 2 As shown.

[0048] Example 2

[0049] The method for preparing high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment includes the following steps:

[0050] S1: α-Alumina powder with a purity of not less than 99.99% and an average initial particle size of 150 nm was selected. The α-alumina powder was dispersed in an aqueous system, and TAC was added as a dispersant to obtain an alumina slurry. The amount of TAC added was 0.3% of the mass of the α-alumina powder. Then, the pH of the alumina slurry was adjusted to 9, the solid content of the alumina slurry was controlled to be 52 vol%, and ball milling was performed at 250 rpm for 3 h to obtain a high-solids-content alumina dispersion slurry.

[0051] S2: The obtained high-solids-content alumina dispersion slurry was injected into a centrifugal mold and centrifuged at 13000 g for 15 min, allowing the dispersed alumina particles to co-sediment in a centrifugal force field as a whole particle network. After centrifugation, the supernatant was removed, and the mold was allowed to stand at room temperature for 5 days before demolding. The demolded green blank was placed in a 50℃ oven and dried until the mass was constant, obtaining a homogeneous alumina green blank with high bulk density. This homogeneous high-density alumina green blank provides a favorable microstructure starting point for the subsequent first step of low-temperature re-homogenization sintering.

[0052] S3: The dried green blank is heated to 500°C in air at a heating rate of 2°C / min and held for 30 min to remove the triammonium citrate dispersant, thus obtaining the debinding alumina green blank.

[0053] S4: The obtained alumina green body after debinding is subjected to pressureless three-step sintering without the addition of sintering aids. First, the temperature is increased to 1000℃ at a rate of 5℃ / min and held for 3 hours to rehomogenize the green body by inducing particle rearrangement through intergranular sintering necks. Second, the temperature is increased to 1250℃ at a rate of 10℃ / min, and then held without holding to allow the green body to reach the critical relative density required for subsequent low-temperature densification, which is approximately 94%. Third, the temperature is decreased to 1175℃ at a rate of 10℃ / min and held for 20 hours to allow the green body to continue densifying mainly through grain boundary diffusion, while suppressing grain boundary texture migration and the resulting grain coarsening. After holding, the temperature is decreased to room temperature at a rate of 5℃ / min to obtain high-purity, high-density, fine-grained alumina ceramic. The resulting alumina ceramic has an average grain size of 0.53 μm, a relative density of 99.6%, and an alumina content of 99.982%. It possesses a high-density grain boundary network composed of submicron grains.

[0054] The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network of this embodiment was prepared by the method described in this embodiment. The SEM image of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network of this embodiment is shown below. Figure 2 As shown. The preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment differs from that in Example 1 in that, in step S4, the third step involves cooling the temperature to 1175°C at a rate of 10°C / min and holding it at that temperature for 20 hours. This temperature is 25°C higher than the holding temperature in the third step of Example 1. Under the conditions of this temperature and holding for 20 hours, the grains will gradually grow. Therefore, the average grain size of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment is larger than that in Example 1.

[0055] Example 3

[0056] The method for preparing high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment includes the following steps:

[0057] S1: α-Alumina powder with a purity of not less than 99.99% and an average initial particle size of 200 nm was selected. The α-alumina powder was dispersed in an aqueous system, and TAC was added as a dispersant to obtain an alumina slurry. The amount of TAC added was 0.5% of the mass of the α-alumina powder. Then, the pH of the alumina slurry was adjusted to 8.5, the solid content of the alumina slurry was controlled to be 45 vol%, and ball milling was performed at 200 rpm for 6 h to obtain a high-solids-content alumina dispersion slurry.

[0058] S2: The obtained high-solids-content alumina dispersion slurry was injected into a centrifugal mold and centrifuged at 20,000 g for 20 min, allowing the dispersed alumina particles to co-sediment in a centrifugal force field as a whole particle network. After centrifugation, the supernatant was removed, and the mold was allowed to stand at room temperature for 7 days before demolding. The demolded green blank was placed in a 40 ℃ oven and dried until the mass was constant to obtain a homogeneous alumina green blank with high bulk density.

[0059] S3: The dried green blank is heated to 450°C in air at a heating rate of 5°C / min and held for 60 min to remove the triammonium citrate dispersant, thus obtaining the debinding alumina green blank.

[0060] S4: The obtained alumina green body after debinding is subjected to pressureless three-step sintering without the addition of sintering aids. First, the temperature is increased to 800℃ at a rate of 5℃ / min and held for 5 h to rehomogenize the green body by inducing particle rearrangement through intergranular sintering necks. Second, the temperature is increased to 1200℃ at a rate of 15℃ / min, and then held without holding, allowing the green body to reach the critical relative density required for subsequent low-temperature densification, which is approximately 94%. Third, the temperature is decreased to 1100℃ at a rate of 10℃ / min and held for 30 h, allowing the green body to continue densifying mainly through grain boundary diffusion, while suppressing grain boundary texture migration and the resulting grain coarsening. After holding, the temperature is decreased to room temperature at a rate of 5℃ / min to obtain high-purity, high-density, fine-grained alumina ceramic.

[0061] The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment is prepared by the preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment.

[0062] Example 4

[0063] The method for preparing high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment includes the following steps:

[0064] S1: α-Alumina powder with a purity of not less than 99.99% and an average initial particle size of 300 nm was selected. Using high-purity α-alumina powder reduces the influence of impurity ions on carrier transport and grain boundary electrical behavior. The α-alumina powder was dispersed in an aqueous system, and TAC was added as a dispersant to obtain an alumina slurry. The TAC addition amount was 0.2% of the α-alumina powder mass. The pH of the alumina slurry was then adjusted to 9, and the solid content of the alumina slurry was controlled to be 50 vol%. The slurry was then ball-milled at 400 rpm for 1 h to obtain a high-solids-content alumina dispersion slurry.

[0065] S2: The obtained high-solids-content alumina dispersion slurry was injected into a centrifugal mold and centrifuged at 5000 g for 30 min, allowing the dispersed alumina particles to co-sediment in the centrifugal force field as a whole particle network. After centrifugation, the supernatant was removed, and the mold was allowed to stand at room temperature for 5 days before demolding. The demolded green blank was placed in a 60 ℃ oven and dried until the mass was constant to obtain a homogeneous alumina green blank with high bulk density.

[0066] S3: The dried green blank is heated to 600°C in air at a heating rate of 5°C / min and held for 10 min to remove the triammonium citrate dispersant, thus obtaining the debinding alumina green blank.

[0067] S4: The obtained alumina green body after debinding is subjected to pressureless three-step sintering without the addition of sintering aids. First, the temperature is increased to 900℃ at a rate of 15℃ / min and held for 4 hours to rehomogenize the green body by inducing particle rearrangement through intergranular sintering necks. Second, the temperature is increased to 1300℃ at a rate of 10℃ / min, and then held without holding to allow the green body to reach the critical relative density required for subsequent low-temperature densification, which is approximately 94%. Third, the temperature is decreased to 1200℃ at a rate of 5℃ / min and held for 20 hours to allow the green body to continue densifying mainly through grain boundary diffusion, while suppressing grain boundary texture migration and the resulting grain coarsening. After holding, the temperature is decreased to room temperature at a rate of 5℃ / min to obtain high-purity, high-density, fine-grained alumina ceramic.

[0068] The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment is prepared by the preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment.

[0069] Example 5

[0070] The method for preparing high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment includes the following steps:

[0071] S1: α-Alumina powder with a purity of not less than 99.99% and an average initial particle size of 150 nm was selected. The α-alumina powder was dispersed in an aqueous system, and TAC was added as a dispersant to obtain an alumina slurry. The amount of TAC added was 0.1% of the mass of the α-alumina powder. Then, the pH of the alumina slurry was adjusted to 8.7, the solid content of the alumina slurry was controlled to be 52 vol%, and ball milling was performed at 100 rpm for 6 h to obtain a high-solids-content alumina dispersion slurry.

[0072] S2: The obtained high-solids-content alumina dispersion slurry was injected into a centrifugal mold and centrifuged at 15000 g for 15 min, allowing the dispersed alumina particles to co-sediment in the centrifugal force field as a whole particle network. After centrifugation, the supernatant was removed, and the mold was allowed to stand at room temperature for 5 days before demolding. The demolded green blank was placed in a 50 ℃ oven and dried until the mass was constant to obtain a homogeneous alumina green blank with high bulk density.

[0073] S3: The dried green blank is heated to 500°C in air at a heating rate of 2°C / min and held for 30 min to remove the triammonium citrate dispersant, thus obtaining the debinding alumina green blank.

[0074] S4: The obtained alumina green body after debinding is subjected to pressureless three-step sintering without the addition of sintering aids. First, the temperature is increased to 1000℃ at a rate of 10℃ / min and held for 2 hours to rehomogenize the green body by inducing particle rearrangement through intergranular sintering necks. Second, the temperature is increased to 1250℃ at a rate of 5℃ / min, and then held without holding to allow the green body to reach the critical relative density required for subsequent low-temperature densification, which is approximately 94%. Third, the temperature is decreased to 1150℃ at a rate of 10℃ / min and held for 20 hours to allow the green body to continue densifying mainly through grain boundary diffusion, while suppressing grain boundary texture migration and the resulting grain coarsening. After holding, the temperature is decreased to room temperature at a rate of 5℃ / min to obtain high-purity, high-density, fine-grained alumina ceramic.

[0075] The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment is prepared by the preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment.

[0076] Example 6

[0077] The method for preparing high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment includes the following steps:

[0078] S1: α-Alumina powder with a purity of not less than 99.99% and an average initial particle size of 150 nm was selected. The α-alumina powder was dispersed in an aqueous system, and TAC was added as a dispersant to obtain an alumina slurry. The amount of TAC added was 0.6% of the mass of the α-alumina powder. Then, the pH of the alumina slurry was adjusted to 9.2, the solid content of the alumina slurry was controlled to be 50 vol%, and ball milling was performed at 250 rpm for 3 h to obtain a high-solids-content alumina dispersion slurry.

[0079] S2: The obtained high-solids-content alumina dispersion slurry was injected into a centrifugal mold and centrifuged at 13000 g for 15 min, allowing the dispersed alumina particles to co-sediment in a centrifugal force field as a whole particle network. After centrifugation, the supernatant was removed, and the mold was allowed to stand at room temperature for 5 days before demolding. The demolded green blank was placed in a 50 ℃ oven and dried until the mass was constant to obtain a homogeneous alumina green blank with high bulk density.

[0080] S3: The dried green blank is heated to 500 ℃ in air at a heating rate of 1℃ / min and held for 30 min to remove the triammonium citrate dispersant, thus obtaining the debinding alumina green blank.

[0081] S4: The obtained alumina green body after debinding is subjected to pressureless three-step sintering without the addition of sintering aids. First, the temperature is increased to 900℃ at a rate of 10℃ / min and held for 3 hours to rehomogenize the green body by inducing particle rearrangement through intergranular sintering necks. Second, the temperature is increased to 1250℃ at a rate of 5℃ / min, and then held without holding to allow the green body to reach the critical relative density required for subsequent low-temperature densification, which is approximately 94%. Third, the temperature is decreased to 1100℃ at a rate of 15℃ / min and held for 20 hours to allow the green body to continue densifying mainly through grain boundary diffusion, while suppressing grain boundary texture migration and the resulting grain coarsening. After holding, the temperature is decreased to room temperature at a rate of 5℃ / min to obtain high-purity, high-density, fine-grained alumina ceramic.

[0082] The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment is prepared by the preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this embodiment.

[0083] Comparative Example 1

[0084] The preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this comparative example is exactly the same as that in Example 1, except that the three-step sintering process was not used; instead, sintering was carried out at 1400℃ for 30 min. The sample is designated as CS-1. The SEM image of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this comparative example is shown below. Figure 4 As shown.

[0085] Comparative Example 2

[0086] The preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this comparative example is exactly the same as that in Example 1, except that the three-step sintering process was not used; instead, sintering was carried out at 1500℃ for 20 h. The sample is designated as CS-2. The SEM image of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this comparative example is shown below. Figure 4 As shown.

[0087] Comparative Example 3

[0088] The preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this comparative example is exactly the same as that in Example 1, except that in step S4, the second sintering temperature is raised to 1100℃, which is below 1200℃. The final alumina ceramic, as observed, is ivory-colored, not corundum-colored, indicating that its density is not high and it is not suitable for the high-temperature electrically insulating applications described in this application.

[0089] Comparative Example 4

[0090] The preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this comparative example is exactly the same as that in Example 1, except that in step S4, the sintering temperature is raised to 1350℃ (higher than 1300℃) in the second step. The sample is designated as CS-3. The SEM image of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this comparative example is shown below. Figure 4 As shown.

[0091] Comparative Example 5

[0092] The preparation method of the high-temperature electrically insulating alumina ceramic with a high-density grain boundary network in this comparative example is exactly the same as that in Example 1, except that the sintering temperature in step S4 is 1050℃, which is lower than 1100℃. The final alumina ceramic, as observed, is ivory-colored, not corundum-colored, indicating that its density is not high and it is not suitable for the high-temperature electrically insulating applications described in this application.

[0093] Comparative Example 6

[0094] The preparation method of this comparative example of high-temperature electrically insulating alumina ceramic with a high-density grain boundary network is exactly the same as that in Example 1, except that the dispersant is ammonium polyacrylate. Poor dispersion stability resulted in gradients and poor uniformity in the centrifugally cast green body, leading to insufficient density of the final alumina ceramic. This demonstrates that the dispersant affects the dispersion stability of the slurry, the uniformity of the green body, and the final densification effect of the ceramic.

[0095] Experimental Example 1

[0096] The microstructure of the alumina ceramics in Examples 1, 2, 1, 2, and 4 was determined, and the test results are as follows: Figure 2 and Figure 4 As shown in Table 1.

[0097] Table 1. Average grain size of alumina ceramics in Examples 1, 2, 1, 2, and 4.

[0098]

[0099] Experimental Example 2

[0100] The relative densities of the alumina ceramics of Examples 1, 2, 1, 2, and 4 were measured, and the results are shown in Table 2.

[0101] Table 2. Relative densities of alumina ceramics in Examples 1, 2, 1, 2, and 4.

[0102]

[0103] As can be seen from Table 2, the alumina ceramics of Examples 1, 2, 1, 2, and 4 have high relative densities.

[0104] Experimental Example 3

[0105] The alumina ceramic samples from Examples 1, 2, and Comparative Examples 1, 2, and 4 were processed into polished discs with a diameter of 10 mm and a thickness of 1 mm. Conductive platinum paste was coated onto the surface of the alumina ceramic samples to ensure good electrical contact. A high-temperature resistance testing system based on the three-electrode method was used to measure the volume resistivity R of the alumina ceramic samples within the range of room temperature to 400°C. v And based on the thickness d of the alumina ceramic sample and the effective area S of the measuring electrode, according to the formula ρ v =R v S / d calculates volume resistivity ρ v The alumina ceramic samples of Examples 1, 2, and Comparative Examples 1, 2, and 4 all had a relative density of not less than 99.5%, and the consistency of their chemical composition was confirmed by ICP analysis, thus eliminating the potential interference of density and chemical composition differences on the volume resistivity test results. The volume resistivity results are shown in Table 3.

[0106] Table 3. Volume resistivity of alumina ceramics in Examples 1, 2, 1, 2, and 4.

[0107]

[0108] Note: The unit of volume resistivity in Table 3 is Ω·cm.

[0109] As can be seen from Table 3, the volume resistivity of the alumina ceramic in Example 1 is 6.7 × 10⁻⁶ at room temperature. 14 Ω·cm, 8.9×10 at 200℃ 13 Ω·cm, still maintained at 8.9×10 at 400℃ 12 The volume resistivity of the alumina ceramic is Ω·cm, exhibiting good high-temperature electrical insulation stability in the range from room temperature to 400°C. In particular, at 400°C, the volume resistivity of Example 1 is approximately 42 times that of Comparative Example 1 and approximately 278 times that of Comparative Example 2, indicating that the fine-grained high-density grain boundary network can significantly improve the high-temperature volume resistivity of alumina ceramics.

[0110] The alumina ceramics of Examples 1 and 2 were both produced using a three-step sintering process, resulting in smaller average grain sizes. At 400°C, the alumina ceramics of Examples 1 and 2 exhibited high volume resistivity, reaching 1×10⁻⁶. 12 Ω·cm. The alumina ceramics of Comparative Example 1 and Comparative Example 2 did not employ a three-step sintering process, resulting in an average particle size in the micrometer range, failing to reach the submicrometer level. Consequently, at 400℃, the volume resistivity of the alumina ceramics of Comparative Example 1 and Comparative Example 2 could not reach 1×10⁻⁶ Ω·cm. 12 Ω·cm. Although the alumina ceramic in Comparative Example 4 employed a three-step sintering process, the temperature was higher in the first sintering step, at which point the grains had already grown. Therefore, the grain size of the alumina ceramic in Comparative Example 4 was also larger, and its volume resistivity could not reach 1×10⁻⁶ Ω·cm. 12 Ω·cm. Since Examples 1, 2, and Comparative Examples 1, 2, and 4 all achieved a high density of not less than 99.5% and had the same chemical composition, the dominant influence of differences in density and chemical composition can be basically eliminated. This indicates that the high-density grain boundary network brought about by grain refinement is the dominant factor in improving high-temperature volume resistivity and electrical insulation stability.

[0111] This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The description and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the claims.

[0112] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. The scope of patent protection of the present invention is determined by the claims. Similarly, any equivalent variations made based on the content of the specification of the present invention should also be included within the scope of protection of the present invention.

Claims

1. A high-temperature electrically insulating alumina ceramic with a high-density grain boundary network, characterized in that, The alumina ceramic has a relative density greater than or equal to 99.5% and an average grain size less than or equal to 0.53 μm.

2. The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network according to claim 1, characterized in that, The average grain size of the alumina ceramic is 0.42–0.53 μm.

3. The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network according to claim 1, characterized in that, The volume resistivity of the alumina ceramic at 400℃ is not less than 1×10⁻⁶. 12 Ω·cm.

4. The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network according to claim 1 or 3, characterized in that, The alumina ceramic has a volume resistivity of not less than 8.9 × 10⁻⁶ at 400 °C. 12 Ω·cm.

5. The high-temperature electrically insulating alumina ceramic with a high-density grain boundary network according to claim 1, characterized in that, The alumina ceramic contains no less than 99.98 wt% Al2O3; the alumina ceramic does not contain any additional sintering aids, dopants, or high-resistivity second phase.

6. A method for preparing a high-temperature electrically insulating alumina ceramic with a high-density grain boundary network as described in claim 1, characterized in that, Includes the following steps: S1: Take high-purity alumina powder, disperse it in a water-based system, add a dispersant, and disperse it evenly to obtain an alumina dispersion slurry; S2: The alumina dispersion slurry is centrifugally cast into a mold and dried to obtain an alumina blank; S3: Remove the dispersant from the alumina blank and then sinter it.

7. The method for preparing high-temperature electrically insulating alumina ceramic with a high-density grain boundary network according to claim 6, characterized in that, The sintering described in step S3 is pressureless sintering. The pressureless sintering steps include: first, heating to 800-1000℃ and holding for 2-5 hours; second, heating to 1200-1300℃ and then not holding after reaching the target temperature; and third, cooling to 1100-1200℃ and holding for 10-30 hours. The heating rate in the first step is 5-15℃ / min, the heating rate in the second step is 5-15℃ / min, and the cooling rate in the third step is 5-15℃ / min. The removal of the dispersant in step S3 involves heating the alumina blank obtained in step S2 to 450-600℃ in an air atmosphere at a heating rate of 1-5℃ / min and holding for 10-60 minutes.

8. The method for preparing high-temperature electrically insulating alumina ceramic with a high-density grain boundary network according to claim 7, characterized in that, In step S2, the centrifugal acceleration during centrifugal casting is 5000-20000g, and the centrifugation time is 5-30min. After centrifugal casting in step S2, the supernatant is removed and the precipitate is left to stand for 3-7 days. After demolding, the precipitate is dried at 40-60℃ until the precipitate mass is constant to obtain the alumina blank.

9. The method for preparing high-temperature electrically insulating alumina ceramic with a high-density grain boundary network according to claim 7, characterized in that, The high-purity alumina powder in step S1 has a purity of not less than 99.99%. α - Alumina powder, the α The alumina powder has an average particle size of 150–300 nm; the alumina dispersion slurry has a solid content of 45–52 vol% and a pH of 8.5–9.5; the dispersant is ammonium citrate; the amount of dispersant added is 0.1%–0.6% of the mass of the alumina powder; the method for uniform dispersion is as follows: after adding the alumina powder dispersed in water to the dispersant, ball milling is performed at 100–400 rpm for 1–6 h.

10. The application of a high-temperature electrically insulating alumina ceramic with a high-density grain boundary network as described in any one of claims 1-5 in insulating devices.

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