Gradient sintering process for improving ceramic closed pore elimination rate

Abstract

The invention discloses a gradient sintering process for improving the elimination rate of closed pores of ceramic, and belongs to the technical field of aluminum oxide ceramic manufacturing. During sintering and glue discharging, lanthanum aluminum oxide core-shell powder is added as a sintering aid, and switching from an air atmosphere to an oxygen atmosphere is utilized, so that gradient sintering is realized, the closed pores can be greatly eliminated, and the sealing performance of the ceramic is improved. The density and mechanical strength of the ceramic material are improved, so that the ceramic material is not easy to crack; the sintering aid is lanthanum oxide aluminum core-shell powder, the structure of the sintering aid is a core-shell structure with lanthanum oxide as a shell layer and gamma-aluminum oxide as a core layer, lanthanum oxide can reduce the concentration of lattice cation filling gaps or generate a grain boundary dragging effect, so that the growth rate of crystal grains is controlled, smooth removal of air holes is ensured, and under oxygen calcination, the sintering efficiency is improved. The concentration of positive ion gaps in aluminum oxide and lanthanum oxide can be increased, the grain boundary migration rate can be controlled to a certain extent in the sintering densification process, and formation of closed pores is further reduced.

Description

Technical Field

[0001] This invention belongs to the field of alumina ceramic manufacturing technology, specifically a gradient sintering process to improve the elimination rate of closed pores in ceramics. Background Technology

[0002] High-purity alumina ceramics are the most common advanced precision ceramic materials, characterized by high strength, high hardness, good wear resistance, excellent chemical stability and insulation, and are widely used in metallurgy, aerospace, chemical industry, electronics and other fields.

[0003] The development of alumina ceramics can be traced back to the early 20th century. With continuous technological advancements and improvements in ceramic processing, the performance of alumina ceramics has been continuously enhanced, and their application range has been expanding. However, the chemical bonds in alumina ceramic materials are ionic bonds, possessing strong directionality and high binding energy, resulting in difficulties in plastic deformation, high brittleness, and strong crack sensitivity. How to effectively improve the toughness of ceramics is a core issue in the research field of alumina ceramic materials.

[0004] Alumina ceramics can be toughened by refining the grain size. Grain refinement toughening refers to controlling the grain size and increasing the number of grain boundaries, which allows for matrix slippage during fracture, making crack deflection more likely and extending the crack path. Simultaneously, the multiple grains and grain boundaries bear the stress at the grain tips, preventing intracellular fracture caused by large grain size overload. While toughening ceramics, adding appropriate sintering aids can lower the sintering temperature. In the sintering process of alumina ceramics, sintering aids such as titanium dioxide, lanthanum oxide, and magnesium oxide are typically added to promote mass transfer during sintering, accelerate the removal of pores, and obtain highly dense, high-strength alumina ceramics.

[0005] Chinese patent announcement CN115231903B discloses a fabrication process for large-size high-purity ceramic substrates. This method uses micron-sized α-alumina as the main phase material and nano-γ-alumina as a sintering aid. The substrates are prepared by tape casting followed by hot pressing and sintering. Because the process of removing the binder from the green sheet before high-temperature hot pressing and sintering is adopted, the organic additives in the green sheet can be completely removed during the binder removal process. This avoids defects such as pores and gaps, as well as gas residues, caused by conventional direct lamination followed by binder removal. Although the use of γ-alumina as a sintering aid in this method can avoid the introduction of impurities, alumina usually has a high grain boundary migration rate at the end of sintering, which can easily lead to the formation of intragranular pores. The method of removing the binder before high-temperature hot pressing and sintering alone is not enough to improve the elimination rate of closed pores. Summary of the Invention

[0006] The purpose of this invention is to provide a gradient sintering process that improves the elimination rate of closed pores in ceramics. By adding lanthanum aluminum oxide core-shell powder as a sintering aid during the sintering and debinding process and utilizing the switching from an air atmosphere to an oxygen atmosphere, gradient sintering can be achieved. This process can significantly eliminate closed pores, improve the density and mechanical strength of the ceramic substrate material, and make it less prone to cracking.

[0007] The objective of this invention can be achieved through the following technical solutions: A gradient sintering process for improving the porosity elimination rate of ceramics includes the following steps: The ceramic substrate green body is placed in a mold and stacked, then placed in a hot press sintering furnace to remove the binder. Under the conditions of air atmosphere, heating rate of 3-4℃ / min and temperature of 600-620℃, it is held for 1-2 hours. Then, the hot press sintering furnace is evacuated and oxygen is introduced. It is heated to 1000-1100℃ in the oxygen atmosphere and held for 1-2 hours. Then, the oxygen is vented through air, the air atmosphere is restored, and it is heated to 1580-1600℃ and held for 1-2 hours. After natural cooling, the ceramic substrate is obtained, completing the gradient sintering process that improves the elimination rate of closed pores in ceramics.

[0008] Furthermore, the vacuum level inside the hot-pressing sintering furnace after vacuuming is 0.02-0.04 MPa; The oxygen flow rate is 5-6 L / min; The stacking height of the ceramic substrate green bodies is 10-12cm.

[0009] Furthermore, the specific preparation steps for the ceramic substrate green body are as follows: An α-alumina powder with an average particle size of 5 μm, anhydrous ethanol, methyl ethyl ketone, tributyl phosphate, polyvinyl butyral, polyethylene glycol, and lanthanum alumina core-shell powder were added to a ball mill and ball-milled for 30-40 min at 20-25℃ and 40-50 r / min. The mixture was then degassed under vacuum, cast into a film, vacuum dried at 60-80℃ for 1-2 h, and cut to obtain a ceramic substrate green body.

[0010] Furthermore, the ratio of α-alumina powder, anhydrous ethanol, methyl ethyl ketone, tributyl phosphate, polyvinyl butyral, polyethylene glycol, and lanthanum oxide core-shell powder is 80-90g: 200-220mL: 100-110mL: 1-2g: 2-5g: 1-2g: 2-4g.

[0011] Furthermore, the specific preparation steps for lanthanum aluminum oxide core-shell powder are as follows: Dispersed alumina powder, anhydrous ethanol, and lanthanum nitrate hexahydrate were added to a reaction vessel and stirred for 20-30 minutes at 50-60℃ and 400-500 r / min. The mixture was filtered, and the product was transferred to a muffle furnace and heated to 500-550℃ at a heating rate of 5-6℃ / min. The temperature was held for 1-2 hours, and then heated to 1600-1700℃ at a heating rate of 4-5℃ / min. The temperature was held for 3-4 hours, and the mixture was allowed to cool naturally to obtain lanthanum alumina core-shell powder.

[0012] Furthermore, the ratio of dispersed alumina powder, anhydrous ethanol, and lanthanum nitrate hexahydrate is 40-50g: 200-300mL: 20-30g.

[0013] Furthermore, the specific preparation steps for dispersed alumina powder are as follows: γ-alumina powder with an average particle size of 800 nm, γ-aminopropyltriethoxysilane, anhydrous ethanol and deionized water were added to a reaction vessel and stirred for 20-30 min at 50-60 °C and 400-500 r / min. Then hydrochloric acid was added to adjust the pH value to 3-4, and the mixture was filtered. The filter cake was washed 2-4 times with deionized water and dried under vacuum at 60-80 °C for 1-2 h to obtain dispersed alumina powder.

[0014] Furthermore, the ratio of γ-alumina powder, γ-aminopropyltriethoxysilane, anhydrous ethanol and deionized water is 50-60g: 8-10mL: 800-900mL: 300-400mL.

[0015] The beneficial effects of this invention are: 1. The present invention provides a gradient sintering process for improving the elimination rate of closed pores in ceramics. By adding lanthanum aluminum oxide core-shell powder as a sintering aid during the sintering and binder removal process, and sequentially switching between three sintering atmospheres of air atmosphere, oxygen atmosphere, and air atmosphere, gradient sintering is achieved. This process can significantly eliminate closed pores, improve the density and mechanical strength of ceramic substrate materials, and make them less prone to cracking.

[0016] 2. The sintering aid of this invention is lanthanum alumina core-shell powder, with a core-shell structure consisting of lanthanum oxide as the shell and γ-alumina as the core. Alumina typically has a high grain boundary migration rate at the end of sintering, which easily leads to the formation of intragranular pores. Lanthanum oxide can reduce the concentration of interstitial cations in the lattice or generate a grain boundary dragging effect, thereby controlling the grain growth rate and ensuring the smooth removal of pores. Furthermore, under oxygen calcination, it can increase the concentration of interstitial cations in alumina and lanthanum oxide, which can control the grain boundary migration rate to a certain extent during the sintering densification process, further reducing the formation of closed pores.

[0017] 2. The sintering aid of the present invention is lanthanum alumina core-shell powder. The sintering aid is nano-sized, and its nanostructure gives it a high specific surface area and strong chemical activity. During the sintering process, it can reduce the sintering temperature. Furthermore, the γ-alumina powder, which serves as the carrier, will transform into α-alumina after sintering, accompanied by volume shrinkage. Volume shrinkage will make the ceramic material more dense in structure, but it will generate inward stress. Lanthanum oxide is coated on the surface of γ-alumina powder in the form of a uniform and dense film. When the core γ-alumina powder undergoes phase transformation and shrinkage, the outer lanthanum oxide coating layer, due to its certain elasticity, will undergo slight elastic stretching, dispersing the inward shrinkage stress and avoiding the generation of microcracks. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1: A gradient sintering process for improving the elimination rate of closed porosity in ceramics, comprising the following steps: S1: Add 50g of γ-alumina powder with an average particle size of 800nm, 8mL of γ-aminopropyltriethoxysilane, 800mL of anhydrous ethanol and 300mL of deionized water to a reaction vessel, stir for 20min at 50℃ and 400r / min, then add hydrochloric acid to adjust the pH to 3, filter, wash the filter cake twice with deionized water, and vacuum dry at 60℃ for 1h to obtain dispersed alumina powder.

[0020] S2: Add 40g of dispersed alumina powder, 200mL of anhydrous ethanol and 20g of lanthanum nitrate hexahydrate to a reaction vessel, stir for 20min at 50℃ and 400r / min, filter, transfer the product to a muffle furnace, heat to 500℃ at a heating rate of 5℃ / min, hold for 1h, then heat to 1600℃ at a heating rate of 4℃ / min, hold for 3h, and cool naturally to obtain lanthanum alumina core-shell powder.

[0021] S3: Add 80g of α-alumina powder with an average particle size of 5μm, 200mL of anhydrous ethanol, 100mL of butanone, 1g of tributyl phosphate as a dispersant, 2g of polyvinyl butyral as a binder, 1g of polyethylene glycol as a plasticizer, and 2g of lanthanum alumina core-shell powder as a sintering aid to a ball mill. Ball mill for 30min at 20℃ and 40r / min, degas under vacuum, cast into shape, vacuum dry at 60℃ for 1h, and cut to obtain a ceramic substrate green body.

[0022] S4: Place the ceramic substrate green body in the mold and stack it to 10cm. Then place it in the hot press sintering furnace to remove the glue. Under the conditions of air atmosphere, heating rate of 3℃ / min and temperature of 600℃, hold for 1h. Then, extract the air in the hot press sintering furnace to a vacuum degree of 0.02MPa, introduce oxygen at a flow rate of 5L / min, heat to 1000℃ and hold for 1h in the oxygen atmosphere. Then, remove the oxygen through air, restore the air atmosphere and heat to 1580℃ and hold for 1h. Allow it to cool naturally to obtain the ceramic substrate, thus completing the gradient sintering process to improve the sealing porosity elimination rate of ceramics.

[0023] Nitrogen has a high nitrogen content in air atmosphere. Nitrogen molecules have a large diameter and are inert gases. Their diffusion coefficient at ceramic grain boundaries is extremely low, making it difficult for them to diffuse out. This eventually leads to the formation of closed pores that cannot be eliminated. Alumina usually has a high grain boundary migration rate at the end of sintering, which easily leads to the formation of intragranular pores.

[0024] Example 2: A gradient sintering process for improving the elimination rate of closed porosity in ceramics, comprising the following steps: S1: Add 55g of γ-alumina powder with an average particle size of 800nm, 9mL of γ-aminopropyltriethoxysilane, 850mL of anhydrous ethanol and 350mL of deionized water to a reaction vessel, stir for 25min at 55℃ and 450r / min, then add hydrochloric acid to adjust the pH to 3, filter, wash the filter cake three times with deionized water, and vacuum dry at 70℃ for 1.5h to obtain dispersed alumina powder.

[0025] S2: Add 45g of dispersed alumina powder, 250mL of anhydrous ethanol and 25g of lanthanum nitrate hexahydrate to a reaction vessel, stir for 25min at 55℃ and 450r / min, filter, transfer the product to a muffle furnace, heat to 525℃ at a heating rate of 5.5℃ / min, hold for 1.5h, then heat to 1650℃ at a heating rate of 4.5℃ / min, hold for 3.5h, and cool naturally to obtain lanthanum alumina core-shell powder.

[0026] S3: 85g of α-alumina powder with an average particle size of 5μm, 210mL of anhydrous ethanol, 105mL of butanone, 1.5g of tributyl phosphate as a dispersant, 3.5g of polyvinyl butyral as a binder, 1.5g of polyethylene glycol as a plasticizer, and 3g of lanthanum alumina core-shell powder as a sintering aid were added to a ball mill and ball-milled for 35min at 22.5℃ and 45r / min. The mixture was then degassed under vacuum, cast into a film, vacuum dried at 70℃ for 1.5h, and cut to obtain a ceramic substrate green body.

[0027] S4: Place the ceramic substrate green body in the mold and stack it to 11cm. Then place it in the hot press sintering furnace to remove the binder. Under the conditions of air atmosphere, heating rate of 3.5℃ / min and temperature of 610℃, hold for 1.5h. Then, extract the air from the hot press sintering furnace, the vacuum degree is 0.03MPa, and introduce oxygen at a flow rate of 5.5L / min. Heat to 1050℃ and hold for 1.5h. Then restore the air atmosphere, heat to 1590℃ and hold for 1.5h. Allow it to cool naturally to obtain the ceramic substrate, thus completing the gradient sintering process to improve the sealing porosity elimination rate of ceramics.

[0028] Example 3: A gradient sintering process for improving the elimination rate of closed porosity in ceramics, comprising the following steps: S1: 60g of γ-alumina powder with an average particle size of 800nm, 10mL of γ-aminopropyltriethoxysilane, 900mL of anhydrous ethanol and 400mL of deionized water were added to a reaction vessel and stirred for 30min at 60℃ and 500r / min. Then, hydrochloric acid was added to adjust the pH value to 4. The mixture was filtered, and the filter cake was washed 4 times with deionized water and dried under vacuum at 80℃ for 2h to obtain dispersed alumina powder.

[0029] S2: Add 50g of dispersed alumina powder, 300mL of anhydrous ethanol and 30g of lanthanum nitrate hexahydrate to a reaction vessel, stir for 30min at 60℃ and 500r / min, filter, transfer the product to a muffle furnace, heat to 550℃ at a heating rate of 6℃ / min, hold for 2h, then heat to 1700℃ at a heating rate of 5℃ / min, hold for 4h, and cool naturally to obtain lanthanum alumina core-shell powder.

[0030] S3: 90g of α-alumina powder with an average particle size of 5μm, 220mL of anhydrous ethanol, 110mL of butanone, 2g of tributyl phosphate as a dispersant, 5g of polyvinyl butyral as a binder, 2g of polyethylene glycol as a plasticizer, and 4g of lanthanum alumina core-shell powder as a sintering aid were added to a ball mill and ball-milled for 40min at 25℃ and 50r / min. The mixture was then degassed under vacuum, cast into shape, vacuum dried at 80℃ for 2h, and cut to obtain a ceramic substrate green body.

[0031] S4: Place the ceramic substrate green body in the mold and stack it to 12cm. Then place it in the hot press sintering furnace to remove the glue. Under the conditions of air atmosphere, heating rate of 4℃ / min and temperature of 620℃, hold for 2h. Then, extract the air from the hot press sintering furnace, the vacuum degree is 0.04MPa, and oxygen flow rate of 6L / min is introduced. Heat to 1100℃ and hold for 2h. Then restore the air atmosphere, heat to 1600℃ and hold for 2h. Allow it to cool naturally to obtain the ceramic substrate. This completes the gradient sintering process to improve the sealing porosity elimination rate of ceramics.

[0032] Comparative Example 1: Based on Example 3, the dispersed alumina powder in step S2 was replaced with the raw material γ-alumina powder in step S1, while the other steps remained unchanged, to prepare a closed ceramic and complete the gradient sintering process to improve the elimination rate of closed pores in the ceramic.

[0033] Comparative Example 2: Based on Example 3, the lanthanum oxide aluminum core-shell powder in step S3 was replaced with a mixture of γ-alumina powder and lanthanum oxide in a mass ratio of 8:2, while the other steps remained unchanged, to prepare a sealed ceramic and complete the gradient sintering process to improve the elimination rate of sealed pores in the ceramic.

[0034] Comparative Example 3: Based on Example 3, the operation of introducing oxygen at a flow rate of 5-6 L / min in step S4 was omitted, so that the sintering and debinding were carried out completely in an air atmosphere, thus completing the gradient sintering process that improves the sealing and pore elimination rate of ceramics.

[0035] The γ-alumina powder was purchased from Foshan Zhengnian New Materials Co., Ltd., CAS No.: 1344-28-1.

[0036] Lanthanum oxide was purchased from Beijing Yijin New Materials Technology Co., Ltd., with an average particle size of 40 nm and grade: Co99.80.

[0037] Polyvinyl butyral was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., MW120,000-150,000, CAS No.: 63148-65-2.

[0038] Tributyl phosphate was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., CAS No.: 126-73-8.

[0039] Polyethylene glycol was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., with an average molecular weight of 20,000 and CAS number 25322-68-3.

[0040] Lanthanum nitrate hexahydrate was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., CAS No.: 10277-43-7.

[0041] The performance of the sealed ceramics prepared in Examples 1-3 and Comparative Examples 1-3 was tested. Density was tested according to GB / T 25995-2010 "Test Method for Density and Apparent Porosity of Fine Ceramics"; flexural strength was tested according to GB / T 6569-2006 "Test Method for Flexural Strength of Fine Ceramics". Cracking of the ceramic green bodies during sintering was observed visually. The results are shown in Table 1.

[0042] Table 1 Test Table of Sealing Ceramic Properties project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Density (%) 99.4 99.5 99.6 97.5 98.2 98.0 Flexural strength (MPa) 426 432 439 385 412 408 Cracking No cracks No cracks No cracks No cracks No cracks Partial cracking As can be seen from Table 1, the density and flexural strength of the sealed ceramics obtained by the gradient sintering process for improving the elimination rate of sealed pores in ceramics provided in Examples 1-3 are significantly better than those of the comparative example. Compared with the comparative example, no cracking occurred. This indicates that the gradient sintering process for improving the elimination rate of sealed pores in ceramics provided by the present invention can significantly eliminate sealed pores, improve the density and mechanical strength of ceramic materials, and make them less prone to cracking.

[0043] In Comparative Example 1, the dispersed alumina powder in step S2 was replaced with the raw material γ-alumina powder in step S1. The original γ-alumina had dense hydroxyl groups on its surface, which easily agglomerated into large particles through hydrogen bonds. When reacting with lanthanum nitrate, the contact was uneven, and lanthanum oxide could not be uniformly coated on the alumina surface, forming a defect structure with local uncoated and local overcoated areas. The specific surface area of ​​the agglomerated powder was greatly reduced, the chemical activity was weakened, and the sintering temperature could not be effectively reduced. Stress dispersion failed, and the lanthanum oxide coating was uneven. When γ-alumina was transformed into α-alumina, there was no elastic lanthanum oxide coating layer to buffer the inward stress, which led to an increase in the cracking rate of the green body.

[0044] In Comparative Example 2, the lanthanum oxide aluminum core-shell powder in step S3 was replaced with a mixture of γ-alumina powder and lanthanum oxide in a mass ratio of 8:2. Physical mixing could only disperse lanthanum oxide particles on the surface of alumina, and could not form a continuous and dense lanthanum oxide coating film. When γ-alumina was converted to α-alumina, there was no elastic layer to disperse the stress, and the cracking rate increased. Lanthanum oxide particles locally aggregated and could not uniformly exert the grain boundary drag effect. In some areas, the grains grew abnormally. During the grain growth process, the gas was trapped to form closed pores, and the density decreased.

[0045] Comparative Example 3 omitted the step of introducing oxygen at a flow rate of 5-6 L / min in step S4. In an air atmosphere, nitrogen molecules have a large diameter and a low diffusion coefficient. During sintering, they are encapsulated by the densified ceramic matrix, forming a large number of closed pores and reducing density. In an oxygen-free atmosphere, the interstitial concentration of cations in alumina and lanthanum oxide decreases, which cannot effectively inhibit grain boundary migration. The grain growth rate accelerates, and the number of intracrystalline pores increases. The closed pores and impurity residues lead to uneven stress distribution inside the ceramic. Combined with the problem of uneven sintering shrinkage under an air atmosphere, the cracking rate of the green body increases.

[0046] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention.

Claims

1. A gradient sintering process for improving the elimination rate of sealed porosity in ceramics, characterized in that, Includes the following steps: The ceramic substrate green body is placed in a mold and stacked, then placed in a hot press sintering furnace to remove the binder. Under the conditions of air atmosphere, heating rate of 3-4℃ / min and temperature of 600-620℃, it is held for 1-2 hours. Then, the hot press sintering furnace is evacuated and oxygen is introduced. It is heated to 1000-1100℃ in the oxygen atmosphere and held for 1-2 hours. Then, the oxygen is vented through air, the air atmosphere is restored, and it is heated to 1580-1600℃ and held for 1-2 hours. After natural cooling, the ceramic substrate is obtained, completing the gradient sintering process that improves the elimination rate of closed pores in ceramics.

2. The gradient sintering process for improving the elimination rate of sealed porosity in ceramics according to claim 1, characterized in that, The vacuum level inside the hot-pressing sintering furnace after vacuuming is 0.02-0.04 MPa.

3. The gradient sintering process for improving the elimination rate of sealed porosity in ceramics according to claim 1, characterized in that, The oxygen flow rate is 5-6 L / min.

4. The gradient sintering process for improving the elimination rate of sealed porosity in ceramics according to claim 1, characterized in that, The height of the stacked ceramic substrate green bodies is 10-12cm.

5. The gradient sintering process for improving the elimination rate of sealed porosity in ceramics according to claim 1, characterized in that, The specific preparation steps for the ceramic substrate green body are as follows: An α-alumina powder with an average particle size of 5 μm, anhydrous ethanol, methyl ethyl ketone, tributyl phosphate, polyvinyl butyral, polyethylene glycol, and lanthanum alumina core-shell powder were added to a ball mill and ball-milled for 30-40 min at 20-25℃ and 40-50 r / min. The mixture was then degassed under vacuum, cast into a film, vacuum dried at 60-80℃ for 1-2 h, and cut to obtain a ceramic substrate green body.

6. The gradient sintering process for improving the elimination rate of sealed porosity in ceramics according to claim 5, characterized in that, The ratio of the amounts of α-alumina powder, anhydrous ethanol, methyl ethyl ketone, tributyl phosphate, polyvinyl butyral, polyethylene glycol, and lanthanum oxide core-shell powder is 80-90g: 200-220mL: 100-110mL: 1-2g: 2-5g: 1-2g: 2-4g.

7. The gradient sintering process for improving the elimination rate of sealed porosity in ceramics according to claim 5, characterized in that, The specific preparation steps for the lanthanum aluminum oxide core-shell powder are as follows: Dispersed alumina powder, anhydrous ethanol, and lanthanum nitrate hexahydrate were added to a reaction vessel and stirred for 20-30 minutes at 50-60℃ and 400-500 r / min. The mixture was filtered, and the product was transferred to a muffle furnace and heated to 500-550℃ at a heating rate of 5-6℃ / min. The temperature was held for 1-2 hours, and then heated to 1600-1700℃ at a heating rate of 4-5℃ / min. The temperature was held for 3-4 hours, and the mixture was allowed to cool naturally to obtain lanthanum alumina core-shell powder.

8. The gradient sintering process for improving the elimination rate of sealed porosity in ceramics according to claim 7, characterized in that, The ratio of the dispersed alumina powder, anhydrous ethanol, and lanthanum nitrate hexahydrate is 40-50g: 200-300mL: 20-30g.

9. The gradient sintering process for improving the elimination rate of sealed porosity in ceramics according to claim 7, characterized in that, The specific preparation steps for the dispersed alumina powder are as follows: γ-alumina powder with an average particle size of 800 nm, γ-aminopropyltriethoxysilane, anhydrous ethanol and deionized water were added to a reaction vessel and stirred for 20-30 min at 50-60 °C and 400-500 r / min. Then hydrochloric acid was added to adjust the pH value to 3-4, and the mixture was filtered. The filter cake was washed 2-4 times with deionized water and dried under vacuum at 60-80 °C for 1-2 h to obtain dispersed alumina powder.

10. The gradient sintering process for improving the elimination rate of sealed porosity in ceramics according to claim 9, characterized in that, The ratio of the amounts of γ-alumina powder, γ-aminopropyltriethoxysilane, anhydrous ethanol and deionized water is 50-60g: 8-10mL: 800-900mL: 300-400mL.