Preparation method of porous beryllium oxide ceramic chip

Abstract

The invention belongs to the technical field of porous ceramic manufacturing, and particularly relates to a preparation method of a porous beryllium oxide ceramic chip. Granular polymer modified polyvinyl acetal with a specific particle size of 5-8 [mu] m is prepared through specific monomers and a monomer ratio, then beryllium oxide powder, a sintering aid and the polymer modified polyvinyl acetal are subjected to ball milling, then cold isostatic pressing molding is performed, the polymer modified polyvinyl acetal is subjected to ball milling and serves as an adhesive and a pore-forming agent at the same time, and the porous beryllium oxide is obtained. Sintering is carried out after molding, a large number of uniform pore structures are formed in the sintering process, excellent electrical properties are provided, and good mechanical properties are kept.

Description

Technical Field

[0001] The invention belongs to the technical field of porous ceramic manufacturing, and in particular relates to a method for preparing a porous beryllium oxide ceramic sheet. Background Art

[0002] Beryllium oxide ceramics have high thermal conductivity, good insulation, mechanical properties, low losses, and excellent chemical stability. They are widely used in high-power heat sinks, electronic vacuum devices, and packaging devices. Porous beryllium oxide ceramics are often used in applications requiring high reliability and high mechanical properties, such as aerospace and military electronics. Currently, the preparation method generally uses beryllium oxide as the raw material, adds a pore-forming agent, and then forms and sinters the material to produce porous beryllium oxide ceramics.

[0003] CN119775044A discloses a porous beryllium oxide ceramic and a preparation method thereof. The ceramic is prepared from the following raw materials, in parts by weight: 65-85 parts of nano-beryllium oxide, 0.33-0.43 parts of nano-magnesium oxide, 0.33-0.43 parts of nano-aluminum silicate, 4-8 parts of polyvinyl butyral, 1-4 parts of sesbania powder, and 12-30 parts of anhydrous ethanol. The preparation method comprises the following steps: (1) weighing each raw material in parts by weight for later use; (2) dissolving polyvinyl butyral in anhydrous ethanol to obtain solution A; (3) dissolving sesbania powder in water to obtain solution B; and mixing solution A and solution B to obtain a mixed solution; (4) adding nano-beryllium oxide, nano-magnesium oxide, and nano-aluminum silicate to the mixed solution, dispersing them uniformly, spray granulating, and drying to obtain a powder; and (5) cold isostatically pressing the powder and sintering it at a high temperature to obtain the porous beryllium oxide ceramic. Among them, vinyl butyral serves as both an adhesive and a pore-forming agent.

[0004] CN116514575A discloses a porous beryllium oxide electrode material, which is made from beryllium oxide and a pore-forming agent as raw materials, through molding and sintering. The pore-forming agent is a carbon-based pore-forming agent and / or an inorganic salt pore-forming agent. This patent requires the use of a large amount of pore-forming agent. The pore-forming agent decomposes the carbon-based polymer, removes low-boiling point substances, shrinks to form pores, and simultaneously, the carbon compound forms a conductive connecting layer on the inner wall of the pores, resulting in a beryllium oxide ceramic with good electrical conductivity as an electrode material. However, the most important function of beryllium oxide ceramics is as an insulating material. This patent does not discuss the application of beryllium oxide ceramics as an insulating material.

[0005] In some applications where the density of beryllium oxide ceramics needs to be further reduced, porous beryllium oxide ceramics are desirable, while still retaining the excellent properties of dense beryllium oxide ceramics. Currently, there is very little literature on porous beryllium oxide ceramics, primarily due to toxicity concerns and the difficulty in balancing the strength and insulation properties of porous ceramic materials. Furthermore, high porosity can negatively impact the thermal conductivity and insulation properties of the material, particularly pore non-uniformity, which can significantly affect the dielectric loss tangent and breakdown strength. Pore uniformity directly influences the material's microstructure, which in turn affects the electric field distribution, carrier transport, and energy dissipation mechanisms. For example, pore non-uniformity can cause localized electric field enhancement at irregular pore edges (such as sharp corners), broadening the relaxation time distribution and maintaining a higher tanδ value over a wider frequency range. Furthermore, pore non-uniformity can lead to low-resistance paths between interconnected pores, allowing breakdown to propagate rapidly along the pore walls. Summary of the Invention

[0006] In order to solve the problem that it is difficult to balance the insulating electrical properties and mechanical properties of the material in the preparation of porous beryllium oxide ceramics in the prior art, the present invention proposes a method for preparing porous beryllium oxide ceramic sheets, which is achieved by ball milling beryllium oxide powder, sintering aid and polymer-modified polyvinyl acetal, with the polymer-modified polyvinyl acetal ball milling serving as both a binder and a pore-forming agent, cold isostatic pressing and sintering, thereby forming a rich and uniform pore structure inside the beryllium oxide ceramic, while taking into account good mechanical and electrical properties. Specifically, the present invention provides the following technical solutions to solve the above technical problems: A method for preparing a porous beryllium oxide ceramic sheet comprises the following steps: (S1) After polyvinyl alcohol and aldehyde react under acidic conditions, the pH is adjusted to be weakly alkaline, a copolymer solution is added, and the mixture is kept warm for reaction. A low-boiling point organic solvent miscible with water is added to the resulting mixed solution, and the mixture is spray-dried to obtain a granular polymer-modified polyvinyl acetal having a particle size of 5-8 μm; the mass ratio of polyvinyl alcohol to the copolymer solid component is 100:12-18; the copolymer is obtained by copolymerizing acrylamide, a long-chain alkyl quaternary ammonium salt unsaturated monomer, and polyethylene glycol diacrylamide; (S2) adding a sintering aid and an alcohol solvent to the beryllium oxide powder and performing a first-stage high-speed ball milling, adding granular polymer-modified polyvinyl acetal, and performing a second-stage low-speed ball milling to obtain a mixed powder; (S3) The mixed powder is pressed into a shape by cold isostatic pressing and calcined to obtain a porous beryllium oxide ceramic sheet.

[0007] The mass of the solid component of the copolymer is obtained by multiplying the mass of the copolymer solution by the solid content. The specific particle size of the granular polymer-modified polyvinyl acetal prepared from a specific monomer ratio is the key to the realization of the present invention. A particle size of 5-8 μm is required for the granular polymer-modified polyvinyl acetal to simultaneously function as a binder and a pore-forming agent. Excessively large or small particle sizes can adversely affect the performance of the porous beryllium oxide ceramic sheet. Furthermore, the type and ratio of monomers also have a significant impact. Acrylamide, as the monomer with the largest proportion, provides the copolymer with a high density of amide groups, providing sufficient hydrophilicity and bonding strength. The long-chain alkyl quaternary ammonium salt unsaturated monomer introduces cations and hydrophobic chains into the copolymer, forming micelles in the solution and facilitating the formation of particles of suitable size during spray drying. Polyethylene glycol diacrylamide is a crosslinking agent, and moderate crosslinking facilitates spray drying and molding. However, the degree of crosslinking should not be too high, otherwise residual carbon will easily remain during sintering, adversely affecting the insulation properties of the porous beryllium oxide ceramic sheet.

[0008] Furthermore, in step (S1), the molar ratio of acrylamide, long-chain alkyl quaternary ammonium salt unsaturated monomer, and polyethylene glycol diacrylamide is 10-15:3-5:0.5-0.7.

[0009] Furthermore, in step (S1), the long-chain alkyl quaternary ammonium salt unsaturated monomer is selected from at least one of octadecyldimethylallylammonium chloride, octadecyldimethylallylammonium bromide, hexadecyldimethylallylammonium chloride, hexadecyldimethylallylammonium bromide, tetradecyldimethylallylammonium chloride, and tetradecyldimethylallylammonium bromide; and the number average molecular weight of the polyethylene glycol diacrylamide is 200-400.

[0010] Furthermore, in step (S1), the copolymer solution is prepared by a preparation method comprising the following steps: adding acrylamide, a long-chain alkyl quaternary ammonium salt unsaturated monomer, and polyethylene glycol diacrylamide to water 5-7 times the total weight of the monomers, heating to dissolve all the monomers, raising the temperature to 60-80° C., adding an initiator aqueous solution and a chain transfer agent, and keeping the temperature for reaction for 4-6 hours. After the reaction is completed, cooling to room temperature to obtain a copolymer aqueous solution, wherein the initiator is a persulfate, such as at least one of potassium persulfate, ammonium persulfate, and sodium persulfate, the concentration of the initiator aqueous solution is 1-5wt%, and the amount of the initiator used is 1-2wt% of the total weight of the monomers; the chain transfer agent is selected from at least one of mercaptoethanol, isopropyl alcohol, n-butanol, n-pentanol, and n-hexanol, and the amount of the chain transfer agent added is 5-10wt% of the total weight of the monomers. The purpose of adding the chain transfer agent is to prevent the molecular weight of the copolymer from being too high, which affects the water solubility, and to improve the molecular weight distribution of the copolymer, making the quality of the granular polymer-modified polyvinyl acetal more stable, which is conducive to the preparation of high-quality porous beryllium oxide ceramic sheets.

[0011] Furthermore, in step (S1), the polyvinyl alcohol has a number average molecular weight of 20,000-30,000, and the aldehyde is a mixture of a C1-4 monoaldehyde and an aromatic dialdehyde in a mass ratio of 4-7:1. The C1-4 monoaldehyde is selected from at least one of formaldehyde, acetaldehyde, propionaldehyde, and butyraldehyde, and the aromatic dialdehyde is selected from at least one of terephthalaldehyde and 2,6-dialdehyde-1,5-dihydroxynaphthalene. Furthermore, the mass ratio of the polyvinyl alcohol to the aldehyde is 100:5-8.

[0012] Furthermore, in step (S1), the ratio of the mass of polyvinyl alcohol to the mass of the copolymer solution multiplied by the solid content value is 100:12-18. There is a strong hydrogen bond interaction between the copolymer and polyvinyl acetal, and the molecular chains are interpenetrating and tightly entangled to complete the modification of the polyvinyl acetal by the copolymer. The amount of copolymer cannot be too much, otherwise the system will be too viscous and cannot be sprayed and granulated smoothly; but the amount of copolymer cannot be too little, otherwise the modification purpose cannot be achieved. After a large number of experiments, it was found that the ratio of the mass of polyvinyl alcohol and the mass of the copolymer (simplified by multiplying the mass of the copolymer solution by the solid content as the mass of the copolymer) is within the above range, and the best performance porous beryllium oxide ceramic sheet can be obtained.

[0013] Furthermore, in step (S1), the low-boiling point organic solvent miscible with water is selected from at least one of acetone, ethanol, and tetrahydrofuran. The purpose of adding the organic solvent is to adjust the atomization behavior, reduce the surface tension of the aqueous solution, make it easier to be broken into small droplets by the atomizer, and improve the atomization efficiency and the stability of the forming. It can also prevent the particles from sticking and caking, and obtain a powder with good fluidity. Adding the organic solvent is conducive to obtaining a granular polymer-modified polyvinyl acetal with good sphericity and a smooth surface. The amount of organic solvent added is 1-5% of the volume of the mixed solution, such as 2%, 3%, or 4%.

[0014] Further, in step (S1), polyvinyl alcohol is added to 5-7 times the mass of water, heated to 80-95 ° C, stirred until the polyvinyl alcohol is completely dissolved, hydrochloric acid is added to adjust the pH to 1-2, aldehyde is slowly added (0.5-1h is added), the reaction is 2-4h, cooled to room temperature, the pH is adjusted to 9-10 with alkali, the copolymer solution is added, the temperature is raised to 40-50 ° C, kept warm for 1-2h, cooled to room temperature, and the resulting mixed solution is spray-dried to obtain a polymer-modified polyvinyl acetal with a particle size of 5-8 μm. The process parameters of spray drying are an inlet air temperature of 80-90 ° C and an outlet air temperature of 50-60 ° C. The temperature cannot be too high, otherwise the polymer-modified polyvinyl acetal will soften and cannot be formed. The inventor unexpectedly found that 5-8 μm granular polymer-modified polyvinyl acetal is the key to achieving the purpose of the present invention. By adjusting the spray drying atomizer parameters, granular polymer-modified polyvinyl acetal with a suitable particle size can be obtained. If the particle size is too large or too small, the polymer-modified polyvinyl acetal cannot fully function as both a binder and a pore-forming agent. When the particle size is less than 5μm, the polymer-modified polyvinyl acetal primarily acts as a binder. Excessively fine polymer-modified polyvinyl acetal has a larger specific surface area and a stronger bonding effect, which deteriorates pore uniformity. When the particle size is greater than 8μm, the polymer-modified polyvinyl acetal is difficult to evenly disperse in the mixed powder, easily causing defects in pore formation and significantly reducing the mechanical properties of the porous beryllium oxide ceramic sheet. Only when the particle size is between 5-8μm, preferably in the range of 5.7-7.1μm, can the polymer-modified polyvinyl acetal fully function as both a binder and a pore-forming agent, fully dispersing in the mixed powder, facilitating the formation of a uniform pore structure, while also ensuring the mechanical strength of the porous ceramic sheet after sintering.

[0015] Furthermore, in step (S2), the beryllium oxide powder has a D50 of 10-20 μm and a beryllium oxide purity of ≥99.9%. The sintering aid is selected from at least one of magnesium oxide, manganese dioxide, aluminum oxide, and calcium oxide. The sintering aid particle size D50 is 50-200 nm and the sintering aid purity is ≥99.9%. The alcohol solvent is selected from at least one of methanol, ethanol, and isopropanol, and the amount of the alcohol solvent used is 20-30 wt% of the beryllium oxide powder. The alcohol solvent is added during ball milling to prevent the spread of highly toxic dust during the grinding process, so wet mixing ball milling is used to avoid the toxicity issues associated with dry milling.

[0016] Furthermore, in step (S2), the high-speed first-stage ball milling process is performed at a speed of 300-400 rpm for 2-3 hours, while the low-speed second-stage ball milling process is performed at a speed of 80-110 rpm for 0.5-1 hour. The high speed of the first-stage ball milling process is intended to thoroughly mix the beryllium oxide powder and the sintering aid, crushing them to a suitable particle size and ensuring that the sintering aid is evenly dispersed in the beryllium oxide. The low speed of the second-stage ball milling process is intended to further mix the polymer-modified polyvinyl acetal into the already thoroughly mixed beryllium oxide and sintering aid. Too high a speed will disrupt the size and morphology of the polymer, preventing the polymer from fully utilizing the binder and pore-forming agent.

[0017] Furthermore, in step (S2), the mass ratio of beryllium oxide powder, sintering aid, and polymer-modified polyvinyl acetal is 100:1-2:20-25.

[0018] Furthermore, in step (S3), the cold isostatic pressing process is a pressure of 140-180 MPa and a cold pressing time of 10-30 min; the calcination is carried out in an air atmosphere, the temperature is raised to 1500-1700°C and the temperature is kept at this temperature for 6-10 h, and the heating rate is not particularly limited, such as 1-20°C / min, preferably 5-10°C / min; after cooling, a porous beryllium oxide ceramic sheet is obtained.

[0019] Compared with the prior art, the present invention has achieved the following technical advancements: The present invention prepares granular polymer-modified polyvinyl acetal with a specific particle size of 5-8 μm through specific monomers and monomer ratios, and mixes the granular polymer-modified polyvinyl acetal into a mixed powder of beryllium oxide and a sintering aid by ball milling. The polymer-modified polyvinyl acetal simultaneously acts as a binder and a pore-forming agent, forming a large number of rich and uniform pore structures during the sintering process while maintaining good mechanical strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a SEM photograph of the porous beryllium oxide ceramic sheet obtained in Example 1; Figure 2 This is a SEM photograph of the porous beryllium oxide ceramic sheet obtained in Comparative Example 8. DETAILED DESCRIPTION

[0021] The technical solutions of the present invention are further explained and illustrated with reference to specific embodiments below.

[0022] Preparation Example 1 Acrylamide, octadecyldimethylallyl ammonium chloride, and polyethylene glycol diacrylamide (number average molecular weight 400) were added in a molar ratio of 12:4:0.6 to water 5 times the total mass of the monomers (the total mass of acrylamide, octadecyldimethylallyl ammonium chloride, and polyethylene glycol diacrylamide), and the mixture was heated to 40°C to dissolve all the monomers. The mixture was then heated to 75°C, and a 5 wt% aqueous solution of ammonium persulfate was added, the amount of ammonium persulfate added being 1 wt% of the total mass of the monomers. 5 wt% of n-butanol was also added, and the mixture was kept at 75°C for 4 h. After cooling, a copolymer aqueous solution was obtained, and the solid content of the copolymer aqueous solution was 14.7%.

[0023] Preparation Example 2 Other conditions were the same as those in Preparation Example 1, except that the monomers used were acrylamide, tetradecyldimethylallylammonium chloride, and polyethylene glycol diacrylamide (number average molecular weight 200) in a molar ratio of 15:3:0.7. The resulting copolymer aqueous solution had a solids content of 14.5%.

[0024] Preparation Example 3 Other conditions were the same as those in Preparation Example 1, except that the monomers used were acrylamide, hexadecyldimethylallylammonium chloride, and polyethylene glycol diacrylamide (number average molecular weight 300) in a molar ratio of 10:3:0.5. The resulting copolymer aqueous solution had a solids content of 14.6%.

[0025] Comparative Preparation Example 1 The other conditions were the same as those in Preparation Example 1, except that hexadecyldimethylallylammonium chloride was not added. The solid content of the obtained copolymer aqueous solution was 14.4%.

[0026] Comparative Preparation Example 2 The other conditions were the same as those in Preparation Example 1, except that polyethylene glycol diacrylamide was replaced with an equimolar amount of N,N-methylenebisacrylamide. The resulting copolymer aqueous solution had a solid content of 14.3%.

[0027] Example 1 (S1) 100 parts by mass of polyvinyl alcohol (number average molecular weight 20,000) was added to 500 parts by mass of water, and the mixture was heated to 90° C. with stirring until the polyvinyl alcohol was completely dissolved. 10 wt % hydrochloric acid was added to adjust the pH to 1, and 4 parts by mass of butyraldehyde and 1 part by mass of terephthalaldehyde were slowly added within 1 hour. After the reaction was continued for 2 hours, the mixture was cooled to room temperature, and the pH of the system was adjusted to 10 with a 5 wt % aqueous solution of NaOH. 100 parts by mass of the copolymer solution prepared in Preparation Example 1 (equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.7) was added. The mixture was heated to 40° C. and kept warm for 1 hour to obtain a mixed solution. 3% by volume of ethanol of the mixed solution was added, and the mixture was spray-dried at an air inlet temperature of 90° C. and an air outlet temperature of 55° C. The speed of the rotary atomizing disk and the atomizing pressure were adjusted to obtain granular polymer-modified polyvinyl acetal having a particle size of 5.7 μm. (S2) 100 parts by mass of beryllium oxide powder with a D50 of 17.4 μm, 1.6 parts by mass of magnesium oxide with a D50 of 155 nm, and 20 parts by mass of anhydrous ethanol were added to a ball mill, and the mixture was mixed and ball-milled at a ball-to-material ratio of 20:1 and a rotation speed of 350 rpm for 3 h; then 20 parts by mass of the granular polymer-modified polyvinyl acetal obtained in step (S1) was added, the rotation speed was reduced to 100 rpm, and the mixture was ball-milled for 0.5 h to obtain a mixed powder; (S3) The mixed powder was pressed into a shape (30 cm × 30 cm × 2 cm) by cold isostatic pressing. The cold isostatic pressing process parameters were a pressure of 160 Ma and a cold pressing time of 20 min. The formed sample was transferred to a muffle furnace, heated to 1600 ° C at a heating rate of 10 ° C / min, and calcined for 8 h to obtain a porous beryllium oxide ceramic sheet.

[0028] Example 2 Other conditions are the same as those in Example 1, except that in step (S1), the copolymer solution prepared in Preparation Example 1 is replaced by an equal mass of the copolymer solution prepared in Preparation Example 2, which is equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.5.

[0029] Example 3 Other conditions are the same as those in Example 1, except that in step (S1), the copolymer solution prepared in Preparation Example 1 is replaced by an equal mass of the copolymer solution prepared in Preparation Example 3, which is equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.6.

[0030] Example 4 Other conditions were the same as those in Example 1, except that in step (S1), the rotation speed of the rotary atomizing disk and the atomizing air pressure were adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 7.1 μm.

[0031] Example 5 Other conditions were the same as those in Example 1, except that in step (S1), the rotation speed of the rotary atomizing disk and the atomizing air pressure were adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 5.0 μm.

[0032] Example 6 Other conditions were the same as those in Example 1, except that in step (S1), the rotation speed of the rotary atomizing disk and the atomizing air pressure were adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 8.0 μm.

[0033] Example 7 The other conditions are the same as those in Example 1, except that step (S1) is changed to: 100 parts by mass of polyvinyl alcohol (number average molecular weight 30,000) are added to 500 parts by mass of water, heated to 90°C under stirring until the polyvinyl alcohol is completely dissolved, hydrochloric acid is added to adjust the pH = 1, 7 parts by mass of butyraldehyde and 1 part by mass of terephthalaldehyde are slowly added within 1 hour, the reaction is continued for 2 hours, and then the temperature is cooled to room temperature, the pH of the system is adjusted to 10 with a 5wt% NaOH aqueous solution, 100 parts by mass of the copolymer solution prepared in Preparation Example 1 (equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.7) are added, the temperature is raised to 40°C, and the mixture is kept warm for 1 hour to obtain a mixed solution, 3% by volume of ethanol of the mixed solution is added, and spray drying is performed. The air inlet temperature is 90°C and the air outlet temperature is 55°C. The speed of the rotary atomizing disk and the atomizing pressure are adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 6.2 μm.

[0034] Example 8 Other conditions were the same as those in Example 1, except that in step (S1), the amount of the copolymer solution prepared in Preparation Example 1 was adjusted so that the ratio of the mass of polyvinyl alcohol to the mass of the copolymer solution multiplied by the solid content was 100:12.

[0035] Example 9 Other conditions were the same as those in Example 1, except that in step (S1), the amount of the copolymer solution prepared in Preparation Example 1 was adjusted so that the ratio of the mass of polyvinyl alcohol to the mass of the copolymer solution multiplied by the solid content was 100:18.

[0036] Example 10 Other conditions were the same as those in Example 1, except that in step (S1), 4 parts by mass of butyraldehyde and 1 part by mass of terephthalaldehyde were replaced by 5 parts by mass of butyraldehyde.

[0037] Example 11 Other conditions were the same as those in Example 1, except that in step (S1), 4 parts by mass of butyraldehyde and 1 part by mass of terephthalaldehyde were replaced by 5 parts by mass of terephthalaldehyde.

[0038] Example 12 Other conditions are the same as those in Example 1, except that in step (S2), the amount of the granular polymer-modified polyvinyl acetal obtained in step (S1) is changed from 20 parts by mass to 25 parts by mass.

[0039] Comparative Example 1 Other conditions are the same as those in Example 1, except that in step (S1), the copolymer solution prepared in Preparation Example 1 is replaced by an equal mass of the copolymer solution prepared in Comparative Preparation Example 1, which is equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.4.

[0040] Comparative Example 2 Other conditions are the same as those in Example 1, except that in step (S1), the copolymer solution prepared in Preparation Example 1 is replaced by an equal mass of the copolymer solution prepared in Comparative Preparation Example 2, which is equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.3.

[0041] Comparative Example 3 Other conditions were the same as those in Example 1, except that in step (S1), the rotation speed of the rotary atomizing disk and the atomizing air pressure were adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 4.0 μm.

[0042] Comparative Example 4 Other conditions were the same as those in Example 1, except that in step (S1), the rotation speed of the rotary atomizing disk and the atomizing air pressure were adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 10.0 μm.

[0043] Comparative Example 5 Other conditions were the same as those in Example 1, except that in step (S1), the amount of the copolymer solution prepared in Preparation Example 1 was adjusted so that the ratio of the mass of polyvinyl alcohol to the mass of the copolymer solution multiplied by the solid content was 100:10.

[0044] Comparative Example 6 Other conditions were the same as those in Example 1, except that step (S1) was modified to: 100 parts by mass of polyvinyl alcohol (number average molecular weight 20,000) was added to 500 parts by mass of water, heated to 90°C with stirring until the polyvinyl alcohol was completely dissolved, hydrochloric acid was added to adjust the pH to 1, 4 parts by mass of butyraldehyde and 1 part by mass of terephthalaldehyde were slowly added over 1 hour, the reaction was continued for 3 hours, the temperature was cooled to room temperature, and spray-dried at an air inlet temperature of 90°C and an air outlet temperature of 55°C. The rotary atomizer speed and atomizing pressure were adjusted to obtain granular polyvinyl acetal with a particle size of 5.7 μm. That is, compared with Example 1, no copolymer solution was added.

[0045] Comparative Example 7 Other conditions were the same as in Example 1, except that in step (S1), the amount of the copolymer solution prepared in Preparation Example 1 was adjusted so that the ratio of the mass of polyvinyl alcohol to the mass of the copolymer solution multiplied by the solids content was 100:22. The system was too viscous to successfully spray-dry the resulting granular polymer-modified polyvinyl acetal.

[0046] Comparative Example 8 Other conditions were the same as in Example 1, except that step (S2) was modified to: 100 parts by mass of beryllium oxide powder with a D50 of 17.4 μm, 1.6 parts by mass of magnesium oxide with a D50 of 155 nm, and 20 parts by mass of the granular polymer-modified polyvinyl acetal obtained in step (S1) were added to a ball mill and milled for 4 hours at a ball-to-material ratio of 20:1 and a rotation speed of 200 rpm to obtain a mixed powder. That is, compared to Example 1, in Comparative Example 8, instead of performing staged ball milling, all three materials were ball milled together at a moderate rotation speed.

[0047] Comparative Example 9 Other conditions were the same as those in Example 1, except that in step (S1), 3% ethanol by volume of the mixed solution was not added, and the spray drying to form granular polymer-modified polyvinyl acetal could not be carried out smoothly.

[0048] Figure 1 1 is a SEM photograph of the porous beryllium oxide ceramic sheet obtained in Example 1. Figure 2 This is an SEM photograph of the porous beryllium oxide ceramic sheet obtained in Comparative Example 8. It can be seen that the pores of the material obtained in Example 1 are more uniform. Comparative Example 8 illustrates that segmented ball milling is required to produce a porous beryllium oxide ceramic sheet with uniform pores.

[0049] Application Examples The porous beryllium oxide ceramic sheets prepared in the above examples and comparative examples were tested for performance, and the results are shown in Table 1 below: Table 1 Performance test of porous beryllium oxide ceramic sheets .

Claims

1. A method for preparing a porous beryllium oxide ceramic sheet, characterized in that: The following steps are involved: (S1) After polyvinyl alcohol and aldehyde react under acidic conditions, the pH is adjusted to be weakly alkaline, a copolymer solution is added, and the mixture is kept warm for reaction. A low-boiling point organic solvent miscible with water is added to the resulting mixed solution, and the mixture is spray-dried to obtain a granular polymer-modified polyvinyl acetal having a particle size of 5-8 μm; the mass ratio of polyvinyl alcohol to the copolymer solid component is 100:12-18; the copolymer is obtained by copolymerizing acrylamide, a long-chain alkyl quaternary ammonium salt unsaturated monomer, and polyethylene glycol diacrylamide; (S2) adding a sintering aid and a solvent to the beryllium oxide powder and performing a first stage of high-speed ball milling, adding granular polymer-modified polyvinyl acetal and performing a second stage of low-speed ball milling to obtain a mixed powder; (S3) The mixed powder is pressed into a shape by cold isostatic pressing and calcined to obtain a porous beryllium oxide ceramic sheet.

2. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S1), the molar ratio of acrylamide, long-chain alkyl quaternary ammonium salt unsaturated monomer, and polyethylene glycol diacrylamide is 10-15:3-5:0.5-0.

7.

3. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S1), the long-chain alkyl quaternary ammonium salt unsaturated monomer is selected from at least one of octadecyldimethylallylammonium chloride, octadecyldimethylallylammonium bromide, hexadecyldimethylallylammonium chloride, hexadecyldimethylallylammonium bromide, tetradecyldimethylallylammonium chloride, and tetradecyldimethylallylammonium bromide; and the number average molecular weight of the polyethylene glycol diacrylamide is 200-400.

4. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S1), the copolymer solution is prepared by a preparation method comprising the following steps: adding acrylamide, a long-chain alkyl quaternary ammonium salt unsaturated monomer, and polyethylene glycol diacrylamide to water 5-7 times the total mass of the monomers, heating to dissolve all the monomers, raising the temperature to 60-80° C., adding an initiator aqueous solution and a chain transfer agent, and keeping the temperature for reaction for 4-6 hours. After the reaction is completed, cooling to room temperature to obtain a copolymer aqueous solution, wherein the initiator is persulfate, the concentration of the initiator aqueous solution is 1-5wt%, and the amount of the initiator used is 1-2wt% of the total mass of the monomers; and the chain transfer agent is selected from at least one of mercaptoethanol, isopropanol, n-butanol, n-pentanol, and n-hexanol, and the amount of the chain transfer agent added is 5-10wt% of the total mass of the monomers.

5. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S1), the number average molecular weight of the polyvinyl alcohol is 20,000-30,000, and the aldehyde is a compound of a C1-4 monoaldehyde and an aromatic dialdehyde in a mass ratio of 4-7:1; the C1-4 monoaldehyde is selected from at least one of formaldehyde, acetaldehyde, propionaldehyde, and butyraldehyde, and the aromatic dialdehyde is selected from at least one of terephthalaldehyde and 2,6-dialdo-1,5-dihydroxynaphthalene.

6. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: The mass ratio of polyvinyl alcohol to aldehyde is 100:5-8.

7. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S1), the low-boiling-point organic solvent miscible with water is selected from at least one of acetone, ethanol, and tetrahydrofuran; and the amount of the organic solvent added is 1-5% of the volume of the mixed solution.

8. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S1), polyvinyl alcohol is added to 5-7 times the mass of water, the temperature is raised to 80-95°C, and the polyvinyl alcohol is stirred until it is completely dissolved. Hydrochloric acid is added to adjust the pH to 1-2, and the aldehyde is slowly added over 0.5-1h. The reaction is carried out for 2-4h, the temperature is lowered to room temperature, the pH is adjusted to 9-10 with alkali, the copolymer solution is added, the temperature is raised to 40-50°C, the temperature is kept for 1-2h, and the mixed solution is spray-dried to obtain a polymer-modified polyvinyl acetal with a particle size of 5-8μm.

9. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: The particle size of the granular polymer-modified polyvinyl acetal is 5.7-7.1 μm.

10. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S2), the D50 of the beryllium oxide powder is 10-20 μm, and the purity of the beryllium oxide is ≥99.9%; the sintering aid is selected from at least one of magnesium oxide, manganese dioxide, and aluminum oxide; the particle size D50 of the sintering aid is 50-200 nm, and the purity of the sintering aid is ≥99.9%.

11. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S2), the rotation speed of the high-speed first-stage ball mill is 300-400 rpm, and the ball milling time is 2-3 hours; the rotation speed of the low-speed second-stage ball mill is 80-110 rpm, and the ball milling time is 0.5-1 hour.

12. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S2), the mass ratio of beryllium oxide powder, sintering aid, and polymer-modified polyvinyl acetal is 100:1-2:20-25.

13. The method for preparing a porous beryllium oxide ceramic sheet according to claim 1, wherein: In step (S3), the cold isostatic pressing process is a pressure of 140-180 MPa and a cold pressing time of 10-30 minutes; and the calcination is carried out in an air atmosphere by heating to 1500-1700° C. and keeping the temperature for 6-10 hours.

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