A method for producing a porous beryllium oxide ceramic sheet

Abstract

The application belongs to the technical field of porous ceramic manufacturing, and particularly relates to a preparation method of a porous beryllium oxide ceramic sheet. The application is prepared by specific monomers and monomer proportions to obtain granular polymer modified polyvinyl acetal with a specific particle size of 5-8 microns. Then, beryllium oxide powder, a sintering aid and the polymer modified polyvinyl acetal are ball milled, and then cold isostatic pressing is performed. The polymer modified polyvinyl acetal ball milling simultaneously serves as a binder and a pore forming agent. After forming, sintering is performed. A large number of uniform pore structures are formed in the sintering process, excellent electrical properties are provided, and good mechanical properties are maintained.

Description

Technical Field

[0001] This invention belongs to the field of porous ceramic manufacturing technology, specifically relating to a method for preparing porous beryllium oxide ceramic sheets. Background Technology

[0002] Beryllium oxide ceramics possess high thermal conductivity, good insulation, excellent mechanical properties, low loss, and superior chemical stability, and are widely used in high-power heat sinks, electronic vacuum devices, and packaging devices. In applications requiring high reliability and high mechanical performance, such as aerospace and military electronic products, porous beryllium oxide ceramics are frequently used. Current preparation methods primarily involve using beryllium oxide as a raw material, adding a pore-forming agent, and then molding and sintering to obtain porous beryllium oxide ceramics.

[0003] CN119775044A discloses a porous beryllium oxide ceramic and its preparation method. The ceramic is made from the following raw materials in parts by weight: 65-85 parts nano-beryllium oxide, 0.33-0.43 parts nano-magnesium oxide, 0.33-0.43 parts nano-aluminum silicate, 4-8 parts polyvinyl butyral, 1-4 parts guar gum powder, and 12-30 parts anhydrous ethanol. The preparation method includes the following steps: (1) Weigh each raw material according to its weight; (2) Dissolve polyvinyl butyral in anhydrous ethanol to obtain solution A; (3) Dissolve guar gum powder in water to obtain solution B; mix solution A and solution B to obtain a mixed solution; (4) Add nano-beryllium oxide, nano-magnesium oxide, and nano-aluminum silicate to the mixed solution and disperse evenly, spray granulate, and dry to obtain powder; (5) Cold isostatically press the powder and sinter at high temperature to obtain the porous beryllium oxide ceramic. Vinyl butyral acts as both an adhesive and a pore-forming agent.

[0004] CN116514575A discloses a porous beryllium oxide electrode material, which is prepared by molding and sintering beryllium oxide and a pore-forming agent as raw materials. 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. Simultaneously, the carbon-based polymer decomposes, removes low-boiling-point substances, and shrinks to form pores. At the same time, carbon compounds form a conductive bonding layer on the inner wall of the pores, resulting in beryllium oxide ceramic with good conductivity as an electrode material. However, the most important function of beryllium oxide ceramic is as an insulating material. This patent does not discuss the application of beryllium oxide ceramic as an insulating material.

[0005] In some applications requiring further reduction in the density of beryllium oxide ceramics, porous beryllium oxide ceramics are needed, while still retaining the superior properties of dense beryllium oxide ceramics. Currently, there is very little literature on porous beryllium oxide ceramics, mainly due to concerns about toxicity and the difficulty in simultaneously achieving both strength and insulation properties in porous ceramic materials. Furthermore, high porosity does have a certain impact on the thermal conductivity and insulation properties of the material, especially when the pores are non-uniform, which significantly affects the dielectric loss tangent and breakdown strength. Pore uniformity directly affects the microstructure of the material, thus influencing the electric field distribution, carrier transport, and energy dissipation mechanisms. For example, non-uniform pores and irregular pore edges (such as sharp corners) can induce localized electric field enhancement, leading to a broadening of the relaxation time distribution and maintaining a higher tanδ value over a wider frequency range. Additionally, non-uniform pores can create low-resistance paths through interconnected pores, allowing breakdown to propagate rapidly along the pore walls. Summary of the Invention

[0006] To address the challenge of simultaneously achieving both electrical and mechanical insulation properties in the preparation of porous beryllium oxide ceramics using existing techniques, this invention proposes a method for preparing porous beryllium oxide ceramic sheets. This method involves ball milling beryllium oxide powder, a sintering aid, and polymer-modified polyvinyl acetal. The polymer-modified polyvinyl acetal acts as both a binder and a pore-forming agent during ball milling. Cold isostatic pressing and sintering then complete the process, resulting in a rich and uniform porous structure within the beryllium oxide ceramic while simultaneously achieving good mechanical and electrical properties. Specifically, this invention provides the following technical solutions to address the aforementioned technical problems:

[0007] A method for preparing porous beryllium oxide ceramic sheets includes the following steps:

[0008] (S1) After polyvinyl alcohol and aldehyde react under acidic conditions, the pH is adjusted to weakly alkaline, a copolymer solution is added, and the reaction is carried out at a constant temperature. The resulting mixed solution is then mixed with a low-boiling-point organic solvent that is miscible with water, and spray-dried to obtain granular polymer-modified polyvinyl alcohol acetal with a particle size of 5-8 μm. The mass ratio of polyvinyl alcohol to the solid component of the copolymer is 100:12-18. The copolymer is obtained by copolymerizing acrylamide, long-chain alkyl quaternary ammonium salt unsaturated monomers, and polyethylene glycol diacrylamide.

[0009] (S2) Add sintering aid and alcohol solvent to beryllium oxide powder and perform high-speed first-stage ball milling, add granular polymer-modified polyvinyl acetal and perform low-speed second-stage ball milling to obtain mixed powder;

[0010] (S3) The mixed powder is pressed into shape by cold isostatic pressing and calcined to obtain porous beryllium oxide ceramic sheets.

[0011] The mass of the solid component of the copolymer is obtained by multiplying the mass of the copolymer solution by the solid content. The key to achieving this invention is the preparation of granular polymer-modified polyvinyl acetal with a specific particle size from a specific monomer ratio. A particle size of 5-8 μm is necessary for the granular polymer-modified polyvinyl acetal to simultaneously function as both a binder and a pore-forming agent. Particle sizes that are too large or too small will adversely affect the performance of the porous beryllium oxide ceramic sheet. Simultaneously, the type and ratio of monomers also have a significant impact. Acrylamide, as the most abundant monomer, provides the copolymer with a high density of amide groups, providing sufficient hydrophilicity and bonding strength. Long-chain alkyl quaternary ammonium salt unsaturated monomers introduce cationic and hydrophobic chains into the copolymer, forming micelles in solution, facilitating the formation of particles with suitable size during spray drying. Polyethylene glycol diacrylamide is a crosslinking agent; moderate crosslinking facilitates spray drying molding, but the degree of crosslinking should not be too high, otherwise residual carbon may be left during sintering, adversely affecting the insulation performance of the porous beryllium oxide ceramic sheet.

[0012] Further, 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.

[0013] Further, in step (S1), the long-chain alkyl quaternary ammonium salt unsaturated monomer is selected from at least one of octadecyl dimethyl allyl ammonium chloride, octadecyl dimethyl allyl ammonium bromide, hexadecyl dimethyl allyl ammonium chloride, hexadecyl dimethyl allyl ammonium bromide, tetradecyl dimethyl allyl ammonium chloride, and tetradecyl dimethyl allyl ammonium bromide; the number average molecular weight of polyethylene glycol diacrylamide is 200-400.

[0014] Further, in step (S1), the copolymer solution is prepared by a method including the following steps: acrylamide, long-chain alkyl quaternary ammonium salt unsaturated monomer, and polyethylene glycol diacrylamide are added to water at a volume 5-7 times the total mass of the monomers. The mixture is heated to completely dissolve the monomers, and the temperature is raised to 60-80°C. An initiator aqueous solution and a chain transfer agent are added, and the reaction is maintained at this temperature for 4-6 hours. After the reaction is completed, the mixture is cooled to room temperature to obtain a copolymer aqueous solution. The initiator is a persulfate, such as at least one of potassium persulfate, ammonium persulfate, and sodium persulfate, with an initiator aqueous solution concentration of 1-5 wt% and an initiator dosage of 1-2 wt% of the total monomer mass. The chain transfer agent is selected from at least one of mercaptoethanol, isopropanol, n-butanol, n-pentanol, and n-hexanol, with the chain transfer agent dosage of 5-10 wt% of the total monomer mass. The purpose of adding the chain transfer agent is to prevent the copolymer molecular weight from being too high, which would affect water solubility. In addition, it can improve the molecular weight distribution of the copolymer, making the quality of the granular polymer-modified polyvinyl acetal more stable, which is beneficial to the preparation of high-quality porous beryllium oxide ceramic sheets.

[0015] Further, in step (S1), the polyvinyl alcohol has a number-average molecular weight of 20,000-30,000, and the aldehyde is a compound of C1-4 monoaldehyde and 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. Even further, the mass ratio of polyvinyl alcohol to aldehyde is 100:5-8.

[0016] Further, in step (S1), the ratio of the mass of polyvinyl alcohol to the mass of the copolymer solution multiplied by the solid content is 100:12-18. The copolymer and polyvinyl acetal exhibit strong hydrogen-bonded interactions and interpenetrating molecular chains, tightly entangled to complete the modification of polyvinyl acetal by the copolymer. The amount of copolymer used should not be too high, otherwise the system will be too viscous and difficult to spray granulate; however, the amount of copolymer used should not be too low, otherwise the modification purpose cannot be achieved. Extensive experiments have shown that feeding materials within the above-mentioned range of the ratio of polyvinyl alcohol to copolymer mass (simplified as the copolymer mass multiplied by the solid content) yields porous beryllium oxide ceramic sheets with optimal performance.

[0017] Further, 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, making it easier for the atomizer to break it into smaller droplets, thereby improving atomization efficiency and molding stability. It also helps prevent particle adhesion and agglomeration, resulting in a powder with good flowability. Adding the organic solvent is beneficial for obtaining spherical, smooth-surfaced granular polymer-modified polyvinyl acetal. The amount of organic solvent added is 1-5% of the volume of the mixed solution, for example, 2%, 3%, or 4%.

[0018] Further, in step (S1), polyvinyl alcohol is added to 5-7 times its weight of water, heated to 80-95℃, and stirred until the polyvinyl alcohol is completely dissolved. Hydrochloric acid is added to adjust the pH to 1-2, and the aldehyde is slowly added (completely added over 0.5-1 hour). The reaction is allowed to proceed for 2-4 hours, then cooled to room temperature. The pH is adjusted to 9-10 with alkali, and the copolymer solution is added. The temperature is raised to 40-50℃ and maintained for 1-2 hours. The mixture is then cooled to room temperature, and the resulting mixed solution is spray-dried to obtain polymer-modified polyvinyl acetal with a particle size of 5-8 μm. The spray-drying process parameters are an inlet air temperature of 80-90℃ and an outlet air temperature of 50-60℃. The temperature must not be too high, otherwise the polymer-modified polyvinyl acetal will soften and become unusable. The inventors unexpectedly discovered that 5-8 μm granular polymer-modified polyvinyl acetal is key to achieving the purpose of this invention. By adjusting the spray-drying atomizer parameters, granular polymer-modified polyvinyl acetal with a suitable particle size can be obtained. Neither too large nor too small a particle size can effectively allow polymer-modified polyvinyl acetal to function as both a binder and a pore-forming agent. When the particle size is <5μm, polymer-modified polyvinyl acetal mainly acts as a binder; excessively fine polymer-modified polyvinyl acetal has a larger specific surface area, resulting in strong adhesion but also poor pore uniformity. When the particle size is >8μm, polymer-modified polyvinyl acetal is difficult to disperse uniformly in the powder mixture, leading to 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 polymer-modified polyvinyl acetal effectively function as both a binder and a pore-forming agent, being fully dispersed in the powder mixture, facilitating the formation of a uniform pore structure, and also ensuring the mechanical strength of the porous ceramic sheet after sintering.

[0019] Further, in step (S2), the beryllium oxide powder has a D50 of 10-20 μm and a beryllium oxide purity ≥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 ≥99.9%; the alcohol solvent is selected from at least one of methanol, ethanol, and isopropanol; the amount of alcohol solvent used is 20-30 wt% of the beryllium oxide powder. Adding the alcohol solvent during ball milling is to avoid the diffusion of highly toxic dust during the grinding process; therefore, wet mixing ball milling is used to avoid the toxicity problems caused by dry milling.

[0020] Further, in step (S2), the high-speed first-stage ball milling is performed at a speed of 300-400 rpm for 2-3 hours; the low-speed second-stage ball milling is performed at a speed of 80-110 rpm for 0.5-1 hour. The high speed of the first-stage ball milling is intended to thoroughly mix the beryllium oxide powder and sintering aid, and to break them into suitable particle sizes, ensuring that the sintering aid is uniformly dispersed in the beryllium oxide. The low speed of the second-stage ball milling is intended to further mix the already thoroughly mixed beryllium oxide and sintering aid into the polymer-modified polyvinyl acetal. Too high a speed would damage the size and morphology of the polymer, preventing it from fully functioning as a binder and pore-forming agent.

[0021] Further, in step (S2), the mass ratio of beryllium oxide powder, sintering aid, and polymer-modified polyvinyl acetal is 100:1-2:20-25.

[0022] Further, in step (S3), the cold isostatic pressing process is carried out at a pressure of 140-180 MPa for 10-30 min; calcination is carried out in an air atmosphere, with the temperature raised to 1500-1700℃ and held for 6-10 h, and the heating rate is not particularly limited, such as 1-20℃ / min, preferably 5-10℃ / min; after cooling, porous beryllium oxide ceramic sheets are obtained.

[0023] Compared with the prior art, the present invention has achieved the following technical advancements:

[0024] This invention prepares granular polymer-modified polyvinyl acetal with a specific particle size of 5-8 μm by using specific monomers and monomer ratios. The granular acetal is then mixed with a mixture of beryllium oxide and a sintering aid powder by ball milling. The polymer-modified polyvinyl acetal acts as both 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. Attached Figure Description

[0025] Figure 1 This is a SEM image of the porous beryllium oxide ceramic sheet obtained in Example 1;

[0026] Figure 2 This is a SEM image of the porous beryllium oxide ceramic sheet shown in Comparative Example 8. Detailed Implementation

[0027] The technical solution of the present invention will be further explained and described below with reference to specific embodiments.

[0028] Preparation Example 1

[0029] Acrylamide, octadecyl dimethyl allyl ammonium chloride, and polyethylene glycol diacrylamide (number average molecular weight 400) were added in a molar ratio of 12:4:0.6 to water at a volume 5 times the total mass of the monomers (acrylamide, octadecyl dimethyl allyl ammonium chloride, and polyethylene glycol diacrylamide). The mixture was heated to 40°C to dissolve all the monomers, then heated to 75°C. A 5 wt% ammonium persulfate aqueous solution was added, with the amount of ammonium persulfate being 1 wt% of the total mass of the monomers. Additionally, 5 wt% n-butanol (the total mass of the monomers) was added. The mixture was kept at 75°C for 4 hours. After cooling, a copolymer aqueous solution was obtained with a solid content of 14.7%.

[0030] Preparation Example 2

[0031] Other conditions were the same as in Preparation Example 1, except that the monomers were acrylamide, tetradecyl dimethyl allyl ammonium chloride, and polyethylene glycol diacrylamide (number average molecular weight 200) fed in a molar ratio of 15:3:0.7. The resulting copolymer aqueous solution had a solid content of 14.5%.

[0032] Preparation Example 3

[0033] Other conditions were the same as in Preparation Example 1, except that the monomers were acrylamide, hexadecyl dimethyl allyl ammonium chloride, and polyethylene glycol diacrylamide (number average molecular weight 300) fed in a molar ratio of 10:3:0.5. The resulting copolymer aqueous solution had a solid content of 14.6%.

[0034] Comparative Preparation Example 1

[0035] Other conditions were the same as in Preparation Example 1, except that hexadecyldimethylallylammonium chloride was not added. The resulting copolymer aqueous solution had a solid content of 14.4%.

[0036] Comparative Preparation Example 2

[0037] Other conditions were the same as 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%.

[0038] Example 1

[0039] (S1) 100 parts by weight of polyvinyl alcohol (number average molecular weight 20,000) were added to 500 parts by weight of water. The mixture was heated to 90°C under stirring until the polyvinyl alcohol was completely dissolved. 10 wt% hydrochloric acid was added to adjust the pH to 1. 4 parts by weight of butyraldehyde and 1 part by weight of terephthalaldehyde were slowly added over 1 hour. After the reaction continued for 2 hours, the mixture was cooled to room temperature. The pH of the system was adjusted to 10 with 5 wt% NaOH aqueous solution. 100 parts by weight of the copolymer solution prepared in Example 1 (equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.7) were added. The mixture was heated to 40°C and kept at that temperature for 1 hour to obtain a mixed solution. 3% of the volume of ethanol in the mixed solution was added, and spray drying was performed. The inlet temperature was 90°C and the outlet temperature was 55°C. The rotation speed of the rotating atomizing disk and the atomizing pressure were adjusted to obtain granular polymer-modified polyvinyl alcohol acetal with a particle size of 5.7 μm.

[0040] (S2) 100 parts by weight of beryllium oxide powder with a D50 of 17.4 μm and 1.6 parts by weight of magnesium oxide with a D50 of 155 nm, and 20 parts by weight of anhydrous ethanol were added to a ball mill and mixed and ball-milled for 3 hours at a ball-to-powder ratio of 20:1 and a rotation speed of 350 rpm. Then, 20 parts by weight of the granular polymer-modified polyvinyl acetal obtained in step (S1) were added, the rotation speed was reduced to 100 rpm, and the mixture was ball-milled for 0.5 hours to obtain a mixed powder.

[0041] (S3) The mixed powder was pressed into shape by cold isostatic pressing (30cm×30cm×2cm). The cold isostatic pressing process parameters were 160Ma pressure and 20min cold pressing time. The shaped sample was transferred to a muffle furnace and heated to 1600℃ at a heating rate of 10℃ / min. It was calcined for 8h to obtain porous beryllium oxide ceramic sheets.

[0042] Example 2

[0043] The other conditions are the same as in Example 1, except that in step (S1), the copolymer solution prepared in Example 1 is replaced with an equal mass of the copolymer solution prepared in Example 2, which is equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.5.

[0044] Example 3

[0045] The other conditions are the same as in Example 1, except that in step (S1), the copolymer solution prepared in Preparation Example 1 is replaced with 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.

[0046] Example 4

[0047] The other conditions are the same as in Example 1, except that in step (S1), the rotation speed of the rotating atomizing disk and the atomizing gas pressure are adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 7.1 μm.

[0048] Example 5

[0049] The other conditions are the same as in Example 1, except that in step (S1), the rotation speed of the rotating atomizing disk and the atomizing gas pressure are adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 5.0 μm.

[0050] Example 6

[0051] The other conditions are the same as in Example 1, except that in step (S1), the rotation speed of the rotating atomizing disk and the atomizing gas pressure are adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 8.0 μm.

[0052] Example 7

[0053] Other conditions are the same as in Example 1, except that step (S1) is changed as follows: 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 to 1, 7 parts by mass of butyraldehyde and 1 part by mass of terephthalaldehyde are slowly added over 1 hour, the reaction is continued for 2 hours, then cooled to room temperature, the pH of the system is adjusted to 10 with 5 wt% NaOH aqueous solution, 100 parts by mass of the copolymer solution prepared in 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 kept at that temperature for 1 hour to obtain a mixed solution, 3% by volume of ethanol is added to the mixed solution, and spray drying is performed with an inlet temperature of 90°C and an outlet temperature of 55°C. The rotation speed of the rotating atomizing disk and the atomizing pressure are adjusted to obtain granular polymer-modified polyvinyl alcohol acetal with a particle size of 6.2 μm.

[0054] Example 8

[0055] The other conditions are the same as in Example 1, except that in step (S1), the amount of copolymer solution prepared in Example 1 is adjusted so that the ratio of the mass of polyvinyl alcohol to the mass of copolymer solution multiplied by the solid content value is 100:12.

[0056] Example 9

[0057] The other conditions are the same as in Example 1, except that in step (S1), the amount of copolymer solution prepared in Example 1 is adjusted so that the ratio of the mass of polyvinyl alcohol to the mass of copolymer solution multiplied by the solid content value is 100:18.

[0058] Example 10

[0059] The other conditions are the same as in Example 1, except that in step (S1), 4 parts by mass of butyraldehyde and 1 part by mass of terephthalaldehyde are replaced with 5 parts by mass of butyraldehyde.

[0060] Example 11

[0061] The other conditions are the same as in Example 1, except that in step (S1), 4 parts by mass of butyraldehyde and 1 part by mass of terephthalaldehyde are replaced with 5 parts by mass of terephthalaldehyde.

[0062] Example 12

[0063] The other conditions are the same as in Example 1, except that in step (S2), the amount of polyvinyl acetal modified by the granular polymer obtained in step (S1) is changed from 20 parts by mass to 25 parts by mass.

[0064] Comparative Example 1

[0065] The other conditions are the same as in Example 1, except that in step (S1), the copolymer solution prepared in Example 1 is replaced with an equal mass of the copolymer solution prepared in Comparative Example 1, which is equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.4.

[0066] Comparative Example 2

[0067] The other conditions are the same as in Example 1, except that in step (S1), the copolymer solution prepared in Example 1 is replaced with an equal mass of the copolymer solution prepared in Comparative Example 2, which is equivalent to a mass ratio of polyvinyl alcohol to copolymer of 100:14.3.

[0068] Comparative Example 3

[0069] The other conditions are the same as in Example 1, except that in step (S1), the rotation speed of the rotating atomizing disk and the atomizing gas pressure are adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 4.0 μm.

[0070] Comparative Example 4

[0071] The other conditions are the same as in Example 1, except that in step (S1), the rotation speed of the rotating atomizing disk and the atomizing gas pressure are adjusted to obtain granular polymer-modified polyvinyl acetal with a particle size of 10.0 μm.

[0072] Comparative Example 5

[0073] The other conditions are the same as in Example 1, except that in step (S1), the amount of copolymer solution prepared in Example 1 is adjusted so that the ratio of the mass of polyvinyl alcohol to the mass of copolymer solution multiplied by the solid content value is 100:10.

[0074] Comparative Example 6

[0075] Other conditions are the same as in Example 1, except that step (S1) is changed as follows: 100 parts by mass of polyvinyl alcohol (number average molecular weight 20,000) are added to 500 parts by mass of water, and heated to 90°C under stirring until the polyvinyl alcohol is completely dissolved. Hydrochloric acid is added to adjust the pH to 1. Within 1 hour, 4 parts by mass of butyraldehyde and 1 part by mass of terephthalaldehyde are slowly added. After reacting for another 3 hours, the mixture is cooled to room temperature and spray-dried. The inlet temperature is 90°C and the outlet temperature is 55°C. The rotation speed of the atomizing disc and the atomizing pressure are adjusted to obtain granular polyvinyl acetal with a particle size of 5.7 μm. That is, compared with Example 1, no copolymer solution is added.

[0076] Comparative Example 7

[0077] Other conditions were the same as in Example 1, except that in step (S1), the amount of copolymer solution prepared in 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 100:22. The system was too viscous to be successfully spray-dried to obtain granular polymer-modified polyvinyl acetal.

[0078] Comparative Example 8

[0079] Other conditions were the same as in Example 1, except that step (S2) was changed 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 ball-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, Comparative Example 8 did not undergo segmented ball milling; instead, all three materials were ball-milled together at a medium rotation speed.

[0080] Comparative Example 9

[0081] The other conditions are the same as in Example 1, except that in step (S1), 3% of the volume of ethanol in the mixed solution is not added, otherwise it cannot be successfully spray-dried into granular polymer-modified polyvinyl acetal.

[0082] Figure 1 This is a SEM image of the porous beryllium oxide ceramic sheet obtained in Example 1. Figure 2 This is a SEM image of the porous beryllium oxide ceramic sheet obtained in Comparative Example 8. It can be seen that the material obtained in Example 1 has more uniform pore size. Comparative Example 8 illustrates that segmented ball milling is necessary to prepare porous beryllium oxide ceramic sheet products with uniform pore size.

[0083] Application examples

[0084] The porous beryllium oxide ceramic sheets prepared in the above embodiments and comparative examples were tested for performance, and the results are shown in Table 1 below:

[0085] Table 1 Performance Tests of Porous Beryllium Oxide Ceramic Sheets

[0086] 。

Claims

1. A method for preparing porous beryllium oxide ceramic sheets, characterized in that, Includes the following steps: (S1) After polyvinyl alcohol and aldehyde react under acidic conditions, the pH is adjusted to weakly alkaline, a copolymer solution is added, and the reaction is carried out at a constant temperature. The resulting mixed solution is then mixed with a low-boiling-point organic solvent that is miscible with water, and spray-dried to obtain granular polymer-modified polyvinyl alcohol acetal with 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, long-chain alkyl quaternary ammonium salt unsaturated monomer, and polyethylene glycol diacrylamide in a molar ratio of 10-15:3-5:0.5-0.

7. The copolymer solution is prepared by a method including the following steps: acrylamide, long-chain alkyl quaternary ammonium salt unsaturated monomer, and polyethylene glycol diacrylamide are added to water at a mass of 5-7 times the total mass of the monomers, heated to dissolve all the monomers, heated to 60-80℃, an initiator aqueous solution and a chain transfer agent are added, and the reaction is carried out at a constant temperature for 4-6 hours. After the reaction is completed, the solution is cooled to room temperature to obtain the copolymer aqueous solution. (S2) Add sintering aid and solvent to beryllium oxide powder, and perform high-speed first-stage ball milling. Add granular polymer-modified polyvinyl acetal, and perform low-speed second-stage ball milling to obtain mixed powder. (S3) The mixed powder is pressed into shape by cold isostatic pressing and calcined to obtain porous beryllium oxide ceramic sheets.

2. The method for preparing porous beryllium oxide ceramic sheets according to claim 1, characterized in that, In step (S1), the long-chain alkyl quaternary ammonium salt unsaturated monomer is selected from at least one of octadecyl dimethyl allyl ammonium chloride, octadecyl dimethyl allyl ammonium bromide, hexadecyl dimethyl allyl ammonium chloride, hexadecyl dimethyl allyl ammonium bromide, tetradecyl dimethyl allyl ammonium chloride, and tetradecyl dimethyl allyl ammonium bromide; the number average molecular weight of polyethylene glycol diacrylamide is 200-400.

3. The method for preparing porous beryllium oxide ceramic sheets according to claim 1, characterized in that, In step (S1), the initiator is persulfate, the concentration of the initiator aqueous solution is 1-5 wt%, and the amount of initiator is 1-2 wt% of the total mass of the monomers; the chain transfer agent is selected from at least one of mercaptoethanol, isopropanol, n-butanol, n-pentanol, and n-hexanol, and the amount of chain transfer agent added is 5-10 wt% of the total mass of the monomers.

4. The method for preparing porous beryllium oxide ceramic sheets according to claim 1, characterized in that, In step (S1), the polyvinyl alcohol has a number average molecular weight of 20,000-30,000, and the aldehyde is a compound of C1-4 monoaldehyde and 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.

5. The method for preparing porous beryllium oxide ceramic sheets according to claim 1, characterized in that, The mass ratio of polyvinyl alcohol to aldehyde is 100:5-8.

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

7. The method for preparing porous beryllium oxide ceramic sheets according to claim 1, characterized in that, In step (S1), polyvinyl alcohol is added to 5-7 times its weight of water, heated to 80-95℃, and stirred until the polyvinyl alcohol is completely dissolved. Hydrochloric acid is added to adjust the pH to 1-2, and the aldehyde is slowly added over 0.5-1 hours. The reaction is allowed to proceed for 2-4 hours, then cooled to room temperature. The pH is adjusted to 9-10 with alkali, and the copolymer solution is added. The temperature is raised to 40-50℃ and maintained for 1-2 hours. The mixture is then cooled to room temperature, and the resulting mixed solution is spray-dried to obtain polymer-modified polyvinyl acetal with a particle size of 5-8 μm.

8. The method for preparing porous beryllium oxide ceramic sheets according to claim 1, characterized in that, The particle size of the granular polymer-modified polyvinyl acetal is 5.7-7.1 μm.

9. The method for preparing porous beryllium oxide ceramic sheets according to claim 1, characterized in that, 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, and aluminum oxide; the sintering aid has a particle size D50 of 50-200 nm and a sintering aid purity of ≥99.9%.

10. The method for preparing porous beryllium oxide ceramic sheets according to claim 1, characterized in that, In step (S2), the high-speed first-stage ball milling speed is 300-400 rpm and the ball milling time is 2-3 hours; the low-speed second-stage ball milling speed is 80-110 rpm and the ball milling time is 0.5-1 hours.

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

12. The method for preparing porous beryllium oxide ceramic sheets according to claim 1, characterized in that, In step (S3), the cold isostatic pressing process is carried out at a pressure of 140-180 MPa for 10-30 min; the calcination is carried out in an air atmosphere, with the temperature raised to 1500-1700℃ and held for 6-10 h.

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