Alumina ceramic with low thermal expansion coefficient and method for producing the same

Abstract

The application belongs to the field of new materials, and discloses an alumina ceramic with low thermal expansion coefficient, which is formed by sintering raw materials, the raw materials comprising: alumina: 98.5-99.1 wt%; silicon powder: 0.15-0.25 wt%; magnesium oxide: 0.15-0.25 wt%; yttrium fluoride: 0.15-0.25 wt%; and fused crystal beta-cristobalite: 0.25-0.75 wt%. The product is composed of four-element ceramic formula of Al, Si, Mg and Y, which meets the demand of the market for 99 alumina ceramic products with low thermal expansion coefficient, and greatly improves the application value of 99 alumina ceramic in ceramic substrates and electrostatic chucks. Meanwhile, the application also discloses a preparation method of the alumina ceramic.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of special ceramics, in particular to an alumina ceramic with low thermal expansion coefficient and a preparation method thereof. BACKGROUND

[0002] With the development of science and technology in China and the continuous improvement of manufacturing level, 99 alumina ceramics are more widely used in modern industry, biological field, modern science, electronic integrated technology, ship corrosion prevention and other fields. 99 alumina ceramics have the advantages of high voltage resistance, high bending strength, good wear resistance, high hardness, and the characteristic of low thermal expansion coefficient close to 0 is more conducive to expanding the application range.

[0003] At present, the production of 99 alumina ceramics on the market generally has the following shortcomings:

[0004] 1. The ceramic grains obtained by sintering domestic alumina have the following differences compared with the ceramic grains obtained by using foreign Martin material (alumina, price 30000-40000 yuan / ton): the grain size is different, the domestic product is prone to processing edge collapse, cracking, and deformation after processing; the density of the domestic product is 3.8-3.90 g / cm 3 , the sintering is not dense, the void is large, and the sintering grain size is generally more than 30 μm; while the alumina ceramic grain size obtained by sintering Martin material is 5-20 μm, and the porosity is basically equal to 0;

[0005] Therefore, the 99 alumina ceramic has a voltage resistance of 30-35 KV / mm, especially in the vacuum ceramic tube shell, the voltage resistance of the 5-10 mm thick ceramic piece is at most 300 KV, and there is a problem of breakdown in use, which greatly restricts its application.

[0006] 2. The alumina ceramic produced by using domestic alumina as raw material has the problem of deformation in high temperature use, such as plasma ceramic body, volume expansion, bending and reduced voltage resistance at 1200℃;

[0007] 3. The alumina ceramic produced by using domestic alumina as raw material has many microcracks on the surface of the sintered ceramic piece, even has dark cracks, and directly cracks in the subsequent processing process; the effect of rapid cooling and heating is not good, for example, the ceramic plate used in electronic ceramic sintering is used below 1500℃, and cracks and deformation after cooling;

[0008] 4. The alumina ceramic produced by using domestic alumina as raw material has sintering deformation of the sintered ceramic piece, which greatly affects the size of the structure, increases the difficulty of later processing of 99 alumina ceramic, and doubles the processing cost.

[0009] The problem to be solved by the present application is: how to prepare a product with a thermal expansion coefficient of approximately 0 and superior parameters to the 99 alumina ceramic products on the market by using domestic alumina as raw material. SUMMARY

[0010] The purpose of the present application is to provide an alumina ceramic with low thermal expansion coefficient, which is composed of Al, Si, Mg and Y four elements to meet the market demand for low thermal expansion coefficient of 99 alumina ceramic products and greatly improve the application value of 99 alumina ceramic in ceramic substrate and electrostatic chuck.

[0011] Meanwhile, the present application also discloses a preparation method of the alumina ceramic.

[0012] To achieve the above purpose, the present application discloses:

[0013] An alumina ceramic with low thermal expansion coefficient is prepared by sintering the following raw materials, wherein the raw materials comprise:

[0014] alumina: 98.5-99.1wt%;

[0015] silicon powder: 0.15-0.25wt%;

[0016] magnesium oxide: 0.15-0.25wt%;

[0017] yttrium fluoride: 0.15-0.25wt%;

[0018] fused crystal beta-cristobalite: 0.25-0.75wt%.

[0019] The price of alumina used in the present application is about 5500-7500 yuan / ton, which is much lower than that of Martin material.

[0020] The present application uses silicon powder, magnesium oxide, yttrium oxide and fused crystal beta-cristobalite as modified raw materials, so that the thermal expansion coefficient of the product is approximately 0.

[0021] The mechanism is that:

[0022] The fused crystal beta-cristobalite is a cubic crystal series, and the phase generated when the temperature is greater than 1250 DEG C has a hexagonal spiral structure and a low thermal expansion coefficient of 5x10 -7 / ℃, the thermal expansion coefficient of the c-axis direction is negative, the formed [SiO4] presents a disordered crystal structure, and the fused crystal beta-cristobalite helps to form a silicon-oxygen octahedron with [AlO4];

[0023] The introduction of magnesium oxide greatly fills the vacancies in the aluminum-oxygen tetrahedron, forms silicon-oxygen and aluminum-oxygen tetrahedral connection, increases the interatomic binding energy, and at the same time reduces the sintering temperature;

[0024] The silicon powder is active silicon powder, which can replace part of oxygen in the alumina to stabilize the set alumina crystal structure;

[0025] The yttrium fluoride plays a substitution role in the formula of the present application;

[0026] The present application adopts the above formula, combines specific phases, and has a substitution role, a connection role between crystals, or a growth role of specific axes, so as to form an alumina material with low thermal expansion coefficient.

[0027] In the above alumina ceramic, the particle size of the alumina, the magnesium oxide, the yttrium fluoride, and the fused crystal β-cristobalite is 1-3 μm; and the particle size of the silicon powder is less than 1 μm.

[0028] Preferably, the purity of the alumina is ≥99.8%, the original crystal size is 300-500 nm, the α-alumina conversion rate is more than 95%, and the true density is 4.0 g / cm 3 ;

[0029] Preferably, the purity of the silicon powder is ≥99.9%;

[0030] Preferably, the purity of the magnesium oxide is ≥99.9%;

[0031] Preferably, the purity of the yttrium fluoride is ≥99.9%, and the original crystal size is 200 nm;

[0032] The purity of the fused crystal β-cristobalite is ≥99.9%, and the crystal series is cubic.

[0033] Meanwhile, the present application also discloses a preparation method of the alumina ceramic.

[0034] Step 1: the silicon powder, the magnesium oxide, and the yttrium fluoride are mixed to perform ball milling, sand milling, and drying granulation to obtain powder 1;

[0035] Step 2: the powder 1 obtained in step 1 is pre-fired and crushed to obtain pre-fired powder 1;

[0036] Step 3: the alumina and the fused crystal β-cristobalite are mixed to perform ball milling and sand milling to obtain slurry 1;

[0037] Step 4: the slurry 1 obtained in step 3 is added with a dispersant 1, and then the pre-fired powder 1 obtained in step 2 is added, and then ball milling and grinding are performed to obtain slurry 2;

[0038] Step 5: the slurry 2 obtained in step 4 is added with a dispersant 2 to perform sand milling to obtain slurry 3;

[0039] Step 6: the slurry 3 obtained in step 5 is sequentially added with ammonium citrate, a binder, and a defoaming agent to perform spray drying and granulation to obtain powder 2.

[0040] Step 7: The powder 2 obtained in step 6 is shaped and sintered to obtain an alumina ceramic.

[0041] In the above preparation method, the particle size of the powder 1 in step 1 is D50: 0.5-0.7 μm, and D97: 1.0-1.1 μm; the pre-sintering temperature in step 2 is 1200-1400 ℃, and the particle size of the pre-sintered powder 1 is 0-1 μm.

[0042] In the above preparation method, the particle size of the slurry 1 in step 3 is D50: 0.9-1.1 μm, and D97: 1.8-2.2 μm.

[0043] In step 4, the dispersant 1 is an ammonium carboxylate salt, and the amount used is 0.25-1% of the total mass of the powder; the particle size of the slurry 2 satisfies D50: 0.9-1.1 μm, and D97: 1.8-2.2 μm.

[0044] In step 5, the particle size of the slurry 3 satisfies D50: 0.6-0.7 μm, and D97: 1.0-1.1 μm; the dispersant 2 is an ammonium carboxylate salt, and the amount used is 0.25-1% of the total mass of the powder.

[0045] In step 6, the amount of ammonium citrate used is 0.1% of the total mass of the powder.

[0046] The binder is polyvinyl alcohol, and the amount used is 8-12% of the total mass of the powder.

[0047] The polyvinyl alcohol exists in the form of a polyvinyl alcohol solution, with a concentration of 10-15 wt%, an average polymerization degree of 1750-1850, and an alcoholysis degree of 87-89%.

[0048] The defoaming agent is an alcohol, and the amount used is 0.01-0.05% of the total mass of the powder.

[0049] After granulation, the specifications are: powder bulk density: >1.2.5 g / cm 3 ; powder particle size 60-80 μm: >80%; moisture content: 0.15-0.6 wt%; powder loss on ignition: 3-4.5 wt%.

[0050] In the above preparation method, in step 7, the shaping process is a cold isostatic pressing process or a dry pressing shaping process; the pressure of the cold isostatic pressing process is >100 MPa; the pressure of the dry pressing shaping process is 110-200 KN; and the sintering temperature is 1600-1680 ℃.

[0051] In the above preparation method, the sintering in step 7 is performed by an electric furnace or a natural gas sintering furnace.

[0052] This application has at least the following beneficial effects:

[0053] This invention uses a quaternary ceramic formula composed of Al, Si, Mg, and Y. The alumina ceramic prepared by this formula has a hardness (HRA) >91.5 and a density >3.95 g / cm³. 3 With a porosity of 0%, flexural strength >415MPa, compressive strength >3100MPa, low coefficient of thermal expansion, and high density, it meets the market demand for low coefficient of thermal expansion of 99% alumina ceramic products and greatly expands the application range of 99% alumina ceramics in ceramic substrates and electrostatic chucks. Attached Figure Description

[0054] Figure 1 Microscopic photograph of Example 3;

[0055] Figure 2 Photograph of a high-temperature, high-pressure plasma generator made from alumina ceramic powder;

[0056] Figure 3 Photograph of a vacuum threaded tube shell made of alumina ceramic powder;

[0057] Figure 4 A photograph of an alumina ceramic substrate prepared from alumina ceramic powder. Detailed Implementation

[0058] The present invention will now be clearly and completely described in conjunction with embodiments thereof. It should be noted that, unless specific conditions are specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available products. Unless otherwise specified, all parts and percentages used in the embodiments of the present invention are by weight.

[0059] Part 1: Research on Optimal Process and Formulation

[0060] Example

[0061] A powder preparation process and sintering method for alumina ceramics, comprising the following steps performed sequentially:

[0062] Step 1: The silicon powder, magnesium oxide, and yttrium fluoride are ball-milled and ground into a slurry, then dried and granulated to obtain powder 1;

[0063] The target particle size of powder 1 is D50: 0.5~0.7μm, D97: 1.0~1.1μm;

[0064] Step 2: Pre-calcinate and pulverize the powder 1 obtained in Step 1 to obtain pre-calcined powder 1;

[0065] The pre-sintering temperature is 1300℃, and the particle size of the pre-sintered powder 1 is 0-1 μm;

[0066] Step 3: Ball-milling the alumina and the fused crystal β-cristobalite to obtain slurry 1;

[0067] The target particle size of the particles in the slurry 1 is D50: 0.9-1.1 μm, and D97: 1.8-2.2 μm; and the solid content of the slurry 1 is 65 wt%;

[0068] Step 4: Adding the slurry 1 obtained in Step 3 into dispersant 1, and then adding the pre-sintered powder 1 obtained in Step 2, and then ball-milling and grinding to obtain slurry 2;

[0069] The dispersant 1 is an ammonium carboxylate; and the particle size of the slurry 2 satisfies D50: 0.9-1.1 μm, and D97: 1.8-2.2 μm;

[0070] Note: The amount of each additive in this text is calculated based on the total amount of the silicon powder, the magnesium oxide, the yttrium fluoride, the alumina and the fused crystal β-cristobalite as 100 wt%;

[0071] Step 5: Adding the slurry 2 obtained in Step 4 into dispersant 2 for sand milling to obtain slurry 3; the particle size of the slurry 3 satisfies D50: 0.6-0.7 μm, and D97: 1.0-1.1 μm; the dispersant 2 is an ammonium carboxylate; and the dispersants in Step 4 and Step 5 are equal in amount;

[0072] Step 6: Adding the slurry 3 obtained in Step 5 into ammonium citrate, a binder and an antifoaming agent in sequence, and then performing spray drying granulation to obtain powder 2;

[0073] The binder is polyvinyl alcohol; the polyvinyl alcohol exists in the form of a polyvinyl alcohol solution, the concentration of the polyvinyl alcohol solution is 15 wt%, the average polymerization degree of the polyvinyl alcohol is 1750-1850, and the alcoholysis degree is 87-89%;

[0074] The antifoaming agent is glycerol;

[0075] The specification of the granulated product is: powder bulk density: >1.2.5 g / cm 3 ; powder particle size 60-80 μm: >80%; moisture: 0.15-0.6 wt%; powder loss on ignition: 3-4.5 wt%;

[0076] Step 7: Forming and sintering the powder 2 obtained in Step 6 to obtain an alumina ceramic.

[0077] The forming process is a cold isostatic pressing process; the pressure of the cold isostatic pressing process is >100 MPa; the sintering atmosphere is air; and the sintering is performed in an electric furnace at 1600℃, 1640℃ and 1680℃ to obtain three parallel samples.

[0078] The formulations of Examples 1 to 5 refer to Table 1;

[0079] Table 1 Formulation Table Unit: kg

[0080] Alumina Silica powder Magnesia Yttrium fluoride Fused crystal β-cristobalite Ammonium citrate Binder Dispersant Defoamer Example 1 300.0000 0.7614 0.7614 0.7614 2.2843 0.3046 30.4569 1.5228 0.0305 Example 2 300.0000 0.7576 0.7576 0.7576 0.7576 0.3030 30.3030 1.5152 0.0303 Example 3 300.0000 0.4545 0.4545 0.7576 1.3623 0.3030 30.3030 1.5152 0.0303 Example 4 300.0000 0.4541 0.4541 0.7568 1.0595 0.3027 30.2725 1.5136 0.0303 Example 5 300.0000 0.4541 0.4541 0.4541 1.3623 0.3027 30.2725 1.5136 0.0303

[0081] The specifications of each main raw material are as follows:

[0082] The particle sizes of the alumina, magnesium oxide, yttrium fluoride, and fused crystal β-cristobalite are 1 to 3 μm; the particle size of the silicon powder is less than 1 μm;

[0083] The purity of the alumina is 99.8%, the primary crystal size is 300 to 500 nm, the conversion rate of α-alumina is more than 95%, and the true density is 4.0 g / cm 3 ;

[0084] The purity of the magnesium oxide is 99.9%;

[0085] The purity of the yttrium fluoride is 99.9%, and the primary crystal size is 200 nm;

[0086] The purity of the silicon powder is 99.9%;

[0087] The purity of the fused crystal β-cristobalite is 99.9%, and the cubic crystal series.

[0088] Comparative Example 1

[0089] Generally the same as Example 3, except that:

[0090] The primary crystal size of the alumina is 1.2 μm, and the others are unchanged.

[0091] Comparative Example 2

[0092] A method for preparing an alumina ceramic includes the following steps performed in sequence: one-time ball milling of all powders to a desired particle size, granulation, and sintering, and the specific steps are as follows:

[0093] Step 1: Ball milling of the silicon powder, magnesium oxide, yttrium fluoride, alumina, fused crystal β-cristobalite, and dispersant 1 to obtain slurry 1;

[0094] The target particle size of the particles in the slurry 1 is D50: 0.9 to 1.1 um, D97: 1.8 to 2.2 um, and the solid content is 65 wt%;

[0095] Step 2: Sand milling of the slurry 1 obtained in Step 1 with dispersant 2 to obtain slurry 2;

[0096] The particle size of the slurry 2 satisfies D50: 0.6 to 0.7 um, D97: 1.0 to 1.1 um;

[0097] Step 3: Add ammonium citrate, binder and defoaming agent into slurry 2, mix and spray dry granulation to obtain powder;

[0098] Step 4: Form and electric furnace sinter the powder obtained in step 3 to obtain alumina ceramic.

[0099] The forming process is cold isostatic pressing process; the pressure of the cold isostatic pressing process is > 100 Mpa; the sintering atmosphere is air; sintering at 1600℃, 1640℃ and 1680℃ to obtain three parallel samples.

[0100] The amount and type of the auxiliary agent of the present comparative example are the same as those of example 3, and the unlisted process parameters are the same as those of example 3.

[0101] Example 6

[0102] Generally the same as example 1, except that the sintering method is natural gas sintering furnace sintering.

[0103] Example 7

[0104] Generally the same as example 2, except that the sintering method is natural gas sintering furnace sintering.

[0105] Example 8

[0106] Generally the same as example 3, except that the sintering method is natural gas sintering furnace sintering.

[0107] Example 9

[0108] Generally the same as example 4, except that the sintering method is natural gas sintering furnace sintering.

[0109] Example 10

[0110] Generally the same as example 5, except that the sintering method is natural gas sintering furnace sintering.

[0111] Example 11

[0112] Generally the same as comparative example 1, except that the sintering method is natural gas sintering furnace sintering.

[0113] Example 12

[0114] Generally the same as comparative example 2, except that the sintering method is natural gas sintering furnace sintering.

[0115] Performance test:

[0116] Voltage resistance: tested by the detection center of Solid State Super Capacitor Technology (Guangzhou) Co., Ltd.;

[0117] Flexural strength: tested by flexural strength tester;

[0118] Bulk density, porosity: tested by the testing center of Solid State Super Capacitor Technology (Guangzhou) Co., Ltd.

[0119] Hardness: tested by the testing center of Solid State Super Capacitor Technology (Guangzhou) Co., Ltd.

[0120] Thermal expansion coefficient: CTI Huache Testing Company

[0121] The performance parameter tests of the examples and comparative examples are as follows.

[0122] Table 2 Performance parameters of the sample of Example 1 sintered at 1600℃, 1640℃, 1680℃ in an electric furnace

[0123] Item 1600℃ 1640℃ 1680℃ Coefficient of thermal expansion (800-1200°C, 10 -7 / °C) 25 14 17 Thermal expansion coefficient (<800°C, 10 -7 / °C) 24 17 16 Withstand voltage (KV / mm) 33 37 34 Density (g / cm 3 )]> 3.89 3.92 3.9 Flexural strength (MPa) 390 407 408 Hardness (HRA) 86~88 90~92 89~90 Compressive strength (MPa) 2598 2946 2878 Porosity (%) 0.07 0.05 0.065

[0124] Table 3 Performance parameters of the sample of Example 2 sintered at 1600℃, 1640℃, 1680℃ in an electric furnace

[0125] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 10 9 11 Thermal expansion coefficient (<800°C, 10 -7 / °C) 4 3 4 Withstand voltage (KV / mm) 36 41 40 Density (g / cm 3 )]]> 3.92 3.95 3.94 Flexural strength (MPa) 396 416 421 Hardness (HRA) 90~91 90~92 91~91.5 Compressive strength (MPa) 2882 3126 3178 Porosity (%) 0.041 0.023 0.035

[0126] Table 4 Performance parameters of the sample of Example 3 sintered at 1600℃, 1640℃, 1680℃ in an electric furnace

[0127] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 12 1~2 2~3 Thermal expansion coefficient (<800°C, 10 -7 / °C) 10 0 0 Withstand voltage (KV / mm) 35 41 40 Density (g / cm 3 )]> 3.901 3.956 3.942 Flexural strength (MPa) 390 416 421 Hardness (HRA) 90~89 90~92 91~91.5 Compressive strength (MPa) 2978 3226 3178 Porosity (%) 0.001 0.000 0.000

[0128] Table 5 Performance parameters of the sample of Example 4 sintered at 1600℃, 1640℃, 1680℃ in an electric furnace

[0129] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 16 1-2 2-3 Thermal expansion coefficient (<800°C, 10 -7 / °C) 12 2-1 3-4 Withstand voltage (KV / mm) 34 40 40.5 Density (g / cm 3 ) 3.901 3.956 3.952 Flexural strength (MPa) 396 417 419 Hardness (HRA) 90~89 90~92 91~91.5 Compressive strength (MPa) 2982 3219 3207 Porosity (%) 0.001 0.000 0.000

[0130] Table 6 Performance parameters of the sample of Example 5 sintered at 1600℃, 1640℃, 1680℃ in an electric furnace

[0131] Item 1600℃ 1640℃ 1680℃ Coefficient of thermal expansion (800-1200°C, 10 -7 / °C) 12 1-2 2-3 Thermal expansion coefficient (<800°C, 10 -7 / °C) 10 0 0 Withstand voltage (KV / mm) 35 37 37 Density (g / cm 3 )]> 3.901 3.916 3.922 Flexural strength (MPa) 408 417 414 Hardness (HRA) 90~89 90~92 91~91.5 Compressive strength (MPa) 3076 3157 3171 Porosity (%) 0.021 0.000 0.000

[0132] Table 7 Performance parameters of the sample of Comparative Example 1 sintered at 1600℃, 1640℃, 1680℃ in an electric furnace

[0133] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 20 12 10 Thermal expansion coefficient (<800°C, 10 -7 / °C) 13 7 6 Withstand voltage (KV / mm) 30 38 31 Density (g / cm 3 ) 3.895 3.916 3.902 Flexural strength (MPa) 371 395 379 Hardness (HRA) 90~89 90~91 90~91 Compressive strength (MPa) 2895 3035 2993 Porosity (%) 0.011 0.012 0.026

[0134] Table 8 Performance parameters of the sample of Comparative Example 2 sintered at 1600℃, 1640℃, 1680℃ in an electric furnace

[0135] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 18 3~6 4~9 Thermal expansion coefficient (<800°C, 10 -7 / °C) 11 0.5 0.6 Withstand voltage (KV / mm) 34 38 38 Density (g / cm 3 ) 3.90 3.942 3.934 Flexural strength (MPa) 380 411 401 Hardness (HRA) 90~89 90~91 90~91 Compressive strength (MPa) 2877 3006 2987 Porosity (%) 0.011 0.001 0.020

[0136] It can be known from Tables 2 to 8 that:

[0137] The adopted alumina raw crystal is 300-500 nm, and as the alumina content increases to 99.1% and 99.0%, the thermal expansion coefficient is 0 (Example 3 and Example 5), and the mechanical properties of all types are optimal, which indicates that the four types of elements reach a specific proportion optimization value, and confirms that the elimination of all types of pores and uniform lattice growth are achieved; in addition, the thermal expansion coefficient of Example 4 slightly increases, and the reason for this phenomenon may be that the amount of fused crystal β-cristobalite is too small.

[0138] In Examples 1-5 and Comparative Examples 1 and 2, there are larger pores at 1600°C, and all types of parameters cannot reach the maximum value, indicating that there are certain defects in the lattice growth, and at a temperature of 1680°C, there is a slight decrease in the superiority of all types of parameters, which confirms that the sintering range of 99 alumina ceramic is between 1640-1680°C, and also provides a wider sintering range for ceramic sintering.

[0139] In Examples 4 and 5, the alumina content is 99.0%-99.1%, and the content of the sintering aid slightly changes, and the thermal expansion coefficient of Example 5 is better than that of Example 4, and the slight change in the content of silicon powder, magnesium oxide, yttrium fluoride, and fused crystal β-cristobalite has a great influence on the performance of all types, thereby determining the range of the addition ratio, and there are more auxiliary raw material formula addition tests, which are not listed here. The performance of the application in terms of low thermal expansion coefficient is much higher than that of traditional alumina thermal expansion coefficient (6-8) x 10 -6 / ℃, which is improved by one order of magnitude.

[0140] Examples 3, Comparative Example 1, and Comparative Example 2 have the same formula, and Comparative Example 1 has an alumina raw crystal size of 1.2 μm, and the thermal expansion coefficient cannot reach 0 or 10 -7 / ℃, which indicates that the size of the alumina raw crystal is extremely important for the performance of 99 alumina ceramic, and it is reflected that the smaller the raw crystal, the lower the sintering temperature, but the sintering temperature is basically above 1640°C, which is related to the production place of alumina, which is not described here;

[0141] In Comparative Example 2, the processing technology of all types of auxiliary raw materials is different, and in Example 3, the auxiliary raw materials are pre-synthesized and then combined with alumina, and the powder is reduced to nanoscale, which has the effect of controlling sintering shrinkage and making the formula more accurate. Because the content of the auxiliary raw materials used is extremely small, errors are avoided, the activation energy is reduced, and the overall shrinkage is not more than 19%.

[0142] Table 9 Performance parameters of the sample of Example 6 sintered at 1600°C, 1640°C, and 1680°C in natural gas

[0143] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 32 12 10 Thermal expansion coefficient (<800°C, 10 -7 / °C) 28 10 9 Withstand voltage (KV / mm) 34 35 34 Density (g / cm 3 ) 3.89 3.90 3.91 Flexural strength (MPa) 384 392 394 Hardness (HRA) 86~87 89~90 89~90 Compressive strength (MPa) 2608 2968 2978 Porosity (%) 0.054 0.046 0.055

[0144] Table 10 Performance parameters of samples of Example 7 sintered in natural gas at 1600°C, 1640°C, 1680°C

[0145] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 5 4 5 Thermal expansion coefficient (<800°C, 10 -7 / °C) 4 3 4 Withstand voltage (KV / mm) 37 42 41 Density (g / cm 3 ) 3.918 3.955 3.95 Flexural strength (MPa) 392 425 421 Hardness (HRA) 90~91 92~95 91~91.5 Compressive strength (MPa) 2973 3136 3178 Porosity (%) 0.013 0.003 0.015

[0146] Table 11 Performance parameters of samples of Example 8 sintered in natural gas at 1600°C, 1640°C, 1680°C

[0147] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 2 1~0.5 1~2 Thermal expansion coefficient (<800°C, 10 -7 / °C) 5 0 0 Withstand voltage (KV / mm) 37 42 41 Density (g / cm 3 ) 3.911 3.959 3.953 Flexural strength (MPa) 412 425 421 Hardness (HRA) 90~91 92~93 91~92 Compressive strength (MPa) 3078 3276 3188 Porosity (%) 0.001 0.000 0.000

[0148] Table 12 Performance parameters of samples of Example 9 sintered in natural gas at 1600°C, 1640°C, 1680°C

[0149] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 11 1~2 2~3 Thermal expansion coefficient (<800°C, 10 -7 / °C) 6 0 1~2 Withstand voltage (KV / mm) 36 40 40.5 Density (g / cm 3 ) 3.931 3.950 3.952 Flexural strength (MPa) 406 423 419 Hardness (HRA) 90~89 91~92 91~91.5 Compressive strength (MPa) 3024 3189 3167 Porosity (%) 0.001 0.000 0.000

[0150] Table 13 Performance parameters of samples of Example 10 sintered in natural gas at 1600°C, 1640°C, 1680°C

[0151] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 12 7 9 Thermal expansion coefficient (<800°C, 10 -7 / °C) 8 2 2 Withstand voltage (KV / mm) 34 34 35 Density (g / cm 3 ) 3.909 3.956 3.952 Flexural strength (MPa) 391 415 413 Hardness (HRA) 90~89 90~91 90~91 Compressive strength (MPa) 2855 3085 2973 Porosity (%) 0.011 0.008 0.026

[0152] Table 14 Performance parameters of samples of Example 11 sintered in natural gas at 1600°C, 1640°C, 1680°C

[0153] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 20 12 10 Thermal expansion coefficient (<800°C, 10 -7 / °C) 13 2 2 Withstand voltage (KV / mm) 30 34 31 Density (g / cm 3 ) 3.895 3.916 3.902 Flexural strength (MPa) 371 395 379 Hardness (HRA) 90~89 90~91 90~91 Compressive strength (MPa) 2895 3035 2993 Porosity (%) 0.031 0.012 0.021

[0154] Table 15 Performance parameters of samples of Example 12 sintered in natural gas at 1600°C, 1640°C, 1680°C

[0155] Item 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 10 2-3 3 Thermal expansion coefficient (<800°C, 10 -7 / °C) 5 0.1 0.2 Withstand voltage (KV / mm) 36 39 38 Density (g / cm 3 ) 3.917 3.942 3.964 Flexural strength (MPa) 392 415 418 Hardness (HRA) 90~89 90~91 90~91 Compressive strength (MPa) 2977 3026 3037 Porosity (%) 0.011 0.001 0.009

[0156] From Example 6 to Example 12, it is known that the porcelain pieces obtained in Example 6 to Example 12 are superior in performance to those obtained in Example 1 to Example 5, Comparative Example 1 and Comparative Example 2, and the main reason is that the sintering atmosphere is slightly reducing (CO), although the impurities in natural gas are organic compounds, which are easy to remove, and its role is that the calorific value of natural gas is high, the heat exchange becomes convection heating, which plays a key role in the removal of impurities in the raw materials. At the same time, the internal furnace pressure of the natural gas furnace is larger than that of the electric furnace, which helps to densify, and the experimental comparison of the natural gas furnace provides a basis for the production of large porcelain pieces in the future.

[0157] The following conclusions are drawn from the various embodiments: comparing the various performance parameters, it is concluded that Example 8 is the optimal combination, and comparing Example 12 with Example 8, it is found that the pre-burning process of the auxiliary raw materials in the early stage plays an extremely important role in the maximum performance, and therefore the pre-burning process is extremely important, and the control of the particle size of the formula material in the ball mill is the key, and comparing Example 11, it is found that the alumina raw crystal 1.2 μm has a particle size of 3-5 μm, and there are problems of high sintering temperature, large pores, and low bending strength.

[0158] Item The microscope photograph of Example 3 is shown in the figure, and the pores are fewer, and the grain size is moderate, thereby improving the bending strength, voltage resistance, bulk density, and obtaining a low thermal expansion coefficient of the porcelain piece.

[0159] Second part: discussion on the importance of each raw material

[0160] Comparative Example 3

[0161] Generally the same as Example 3, except that yttrium fluoride is replaced with yttrium oxide at 0.5 times the molar equivalent, and the amount of yttrium oxide is 0.586 kg.

[0162] Comparative Example 4

[0163] Generally the same as Example 3, except that the silicon powder in step 1 is replaced with fused crystal β-cristobalite.

[0164] The above comparative examples 3-4 also use sintering at 1600°C, 1640°C, and 1680°C to obtain three parallel samples for parallel evaluation of the performance of each product.

[0165] The test results are shown in Tables 16-17;

[0166] Table 16: Performance parameters of the samples of Comparative Example 3 sintered at 1600°C, 1640°C, and 1680°C in an electric furnace

[0167] Withstand voltage (KV / mm) 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 15 3~5 8~9 Thermal expansion coefficient (<800°C, 10 -7 / °C) 6~9 2~3 5~7 Flexural strength (MPa) 35 36 37 Density (g / cm 3 ) 3.921 3.937 3.928 Hardness (HRA) 382 395 388 Compressive strength (MPa) 88~89 90~92 89~90 Porosity (%) 2838 2965 2782 Item 0.015 0.012 0.017

[0168] Table 17: Performance parameters of the samples of Comparative Example 4 sintered at 1600°C, 1640°C, and 1680°C in an electric furnace

[0169] Withstand voltage (KV / mm) 1600℃ 1640℃ 1680℃ Thermal expansion coefficient (800-1200°C, 10 -7 / °C) 16 4~5 7~8 Thermal expansion coefficient (<800°C, 10 -7 / °C) 6~8 3~4 5~7 Flexural strength (MPa) 36 38 37 Density (g / cm 3 ) 3.927 3.939 3.930 Hardness (HRA) 372 385 386 Compressive strength (MPa) 88~89 90~92 89~90 Porosity (%) 2858 2975 2792 Figure 1 0.012 0.010 0.013

[0170] Result analysis:

[0171] In the above, it is demonstrated through Examples 1-5 that slight changes in the main components can cause significant deterioration in performance; in this part, through the replacement of the key components yttrium fluoride and fused crystal β-cristobalite, it is found that the performance of the product has been further deteriorated.

[0172] From the combination of Example 4, Example 3, Comparative Example 4, it can be seen that when the fused crystal β-cristobalite is used completely, the thermal expansion coefficient (10 -7 / ℃) increases to 5~7, which further illustrates the importance of the fused crystal β-cristobalite in the present application; the basic reason is that the fused crystal β-cristobalite is replaced completely, and its crystal form is stable, which is not convenient for the synthesis of the pre-sintered powder in the early stage, that is, the fused crystal β-cristobalite is not good in the synthesis with magnesium oxide / yttrium fluoride, and cannot meet the requirements of the modifier (reducing the thermal expansion coefficient).

[0173] From the combination of Example 3 and Comparative Example 3, it can be seen that the comprehensive performance of yttrium fluoride is obviously better, and the reason is that the activity of yttrium fluoride is higher than that of yttrium oxide, and the yttrium fluoride forms a stable compound with the silicon powder / magnesium oxide to form a specific solid solution, and the three different density materials are synthesized preliminarily, and the density is unified, which is convenient for the various materials in the formula to form a relatively uniform mixture and sintering, and improves the comprehensive performance.

[0174] The alumina ceramic powder prepared in Example 3 is used to prepare various alumina ceramic devices, such as Item as shown in the figure; Withstand voltage (KV / mm) is a high-temperature and high-pressure plasma generator prepared from the alumina ceramic powder; Flexural strength (MPa) is a vacuum threaded tube shell prepared from the alumina ceramic powder; Hardness (HRA) Compressive strength (MPa) Porosity (%) Item Withstand voltage (KV / mm) Flexural strength (MPa) Hardness (HRA) Compressive strength (MPa) Porosity (%) Figure 2 to Figure 4 Figure 2 Figure 3 Figure 4 is an alumina ceramic substrate prepared from the alumina ceramic powder.

[0175] It is apparent for those skilled in the art that the present application is not limited to the details of the above-described exemplary embodiments, and the present application can be implemented in other concrete forms without departing from the spirit or essential characteristics of the present application. Therefore, the embodiments should be considered in all aspects as exemplary and non-limiting, and the scope of the present application is defined by the appended claims rather than the above description, and it is intended to encompass all changes falling within the meaning and range of equivalent elements of the claims. Any reference signs in the claims should not be considered as limiting the involved claims.

Claims

1. An alumina ceramic with a low coefficient of thermal expansion, characterized in that, It is made by sintering the following raw materials, wherein the raw materials include: Alumina: 98.5~99.1 wt%; Silicon powder: 0.15~0.25wt%; Magnesium oxide: 0.15~0.25 wt%; Yttrium fluoride: 0.15~0.25 wt%; Fused β-cristobalite: 0.25~0.75wt%; The preparation method of the alumina ceramic includes the following steps: Step 1: Mix silicon powder, magnesium oxide, and yttrium fluoride, then ball-mill, sand-mill, dry, and granulate to obtain powder 1; Step 2: Pre-calcinate and pulverize the powder 1 obtained in Step 1 to obtain pre-calcined powder 1; Step 3: Mix alumina and molten β-cristobalite and then ball-mill and sand-mill to obtain slurry 1; Step 4: Add dispersant 1 to the slurry 1 obtained in step 3, then add the pre-calcined powder 1 obtained in step 2, and then ball mill and sand mill to obtain slurry 2; Step 5: Add dispersant 2 to the slurry 2 obtained in step 4 and mill it to obtain slurry 3; Step 6: Add ammonium citrate, binder and defoamer to the slurry 3 obtained in step 5 in sequence, and spray dry and granulate to obtain powder 2; Step 7: The powder 2 obtained in step 6 is shaped and sintered to obtain alumina ceramic.

2. The alumina ceramic according to claim 1, characterized in that, The alumina, magnesium oxide, yttrium fluoride, and fused β-cristobalite all have a particle size of 1~3μm; the silicon powder has a particle size of less than 1μm.

3. The alumina ceramic according to claim 1, characterized in that, The alumina has a purity of ≥99.8%, a primary crystal size of 300~500nm, an α-alumina conversion rate of over 95%, and a true density of 4.0g / cm³. 3 ; The silicon powder is active silicon powder, and the purity of the silicon powder is ≥99.9%. The purity of the magnesium oxide is ≥99.9%; The yttrium fluoride has a purity of ≥99.9% and a primary crystal size of 200 nm; The fused β-cristobalite has a purity of ≥99.9% and belongs to the cubic crystal series.

4. The alumina ceramic according to claim 1, characterized in that, In step 1, the particle size of powder 1 is D50: 0.5~0.7μm, D97: 1.0~1.1μm; in step 2, the pre-calcination temperature is 1200~1400℃, and the particle size of pre-calcined powder 1 is 0~1μm and not 0μm.

5. The alumina ceramic according to claim 1, characterized in that, The particle size of slurry 1 in step 3 is D50: 0.9~1.1μm, D97: 1.8~2.2μm; In step 4, dispersant 1 is an ammonium carboxylate salt, and the amount used is 0.25~1% of the total mass of the powder; the particle size of slurry 2 meets the requirements of D50: 0.9~1.1μm and D97: 1.8~2.2μm; In step 5, the particle size of slurry 3 meets the requirements of D50: 0.6~0.7μm and D97: 1.0~1.1μm, and the dispersant 2 is an ammonium carboxylate salt, with a dosage of 0.25~1% of the total mass of the powder. In step 6, the amount of ammonium citrate used is 0.1% of the total mass of the powder. The binder is polyvinyl alcohol, and its dosage is 8-12% of the total mass of the powder. The polyvinyl alcohol exists in the form of a polyvinyl alcohol solution with a concentration of 10-15 wt%, an average degree of polymerization of 1750-1850, and a degree of alcoholysis of 87-89%. The defoamer is an alcohol-based agent, and its dosage is 0.01~0.05% of the total mass of the powder. The granulated powder has a bulk density of >1.2.5 g / cm³. 3 Powder particle size 60~80μm: >80%; Moisture content: 0.15~0.6wt%; Loss on ignition of powder: 3~4.5wt%.

6. The alumina ceramic according to claim 1, characterized in that, In step 7, the forming process is cold isostatic pressing or dry pressing, etc.; the pressure of the cold isostatic pressing process is >100MPa; the pressure of the dry pressing process is 110~200kN; and the sintering temperature is 1600~1680℃.

7. The alumina ceramic according to claim 1, characterized in that, The sintering in step 7 is carried out by sintering in an electric furnace or a natural gas sintering furnace.

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