NL2035034B1

NL2035034B1 - Alumina ceramic and preparation method thereof

Info

Publication number: NL2035034B1
Application number: NL2035034A
Authority: NL
Inventors: Wang Jinfang; Sun Shibo; Li Weiwei; Ding Longqi; Xue Na; Zhu Liu; Deng Minjie
Original assignee: Taizhou Clean Carbon Tech Co Ltd; Univ Taizhou
Priority date: 2022-06-08
Filing date: 2023-06-08
Publication date: 2024-02-02
Also published as: NL2035034A; CN114835473B; CN114835473A

Abstract

The present invention provides an alumina ceramic and a preparation method thereof, falling within the technical field of alumina ceramics. In the present invention, the addition of Y203 can form eutectic with alumina and reduce a sintering temperature; and the addition of Al(OH)3 can further reduce the sintering temperature. Then the product is mixed with saturated oxalic acid solution, and Y203 is slightly soluble in oxalic acid solution. During cold sintering, oxalic acid crystals are precipitated with the evaporation of water, and react with Y203 to generate yttrium oxalate, which plays a role in nucleation and promotes the nucleation of alumina, thereby reducing the sintering temperature. The compactness can be improved by low temperature heat preservation and hot-pressed sintering, thereby improving the mechanical property.

Description

ALUMINA CERAMIC AND PREPARATION METHOD THEREOF

TECHNICAL FIELD

[01] The present invention belongs to the technical field of alumina ceramics, in particular to an alumina ceramic and a preparation method thereof.

BACKGROUND ART

[02] Alumina ceramics are widely used in electronics, electrical appliances, machinery, chemicals, textiles, automobiles, metallurgy, aerospace and other industries because of their excellent comprehensive properties such as high mechanical strength, high hardness, low high-frequency dielectric loss, high high-temperature insulation resistance and good chemical corrosion resistance, as well as the advantages of wide source of raw materials, relatively cheap price and mature processing and manufacturing technology. Alumina ceramics have become one of the most widely used advanced ceramic materials in the world.

[03] Alumina has a strong ionic bond and a melting point of 2050°C, which requires a higher sintering temperature. The production process of alumina ceramics needs to consume a lot of energy and high calorific value fuel, and also needs to consume a lot of high temperature firing of high-grade refractory materials (kiln furniture, furnace materials and the like) and high-temperature heating elements, which limits the development and application of alumina ceramics. Moreover, excessive sintering temperature makes grains of main crystal phases of ceramics grow larger and residual pores gather and grow, which leads to the decrease of mechanical property of material. Therefore, how to reduce the sintering temperature of alumina ceramics has always been an important issue that enterprises are concerned about and urgently need to solve. At present, the sintering temperature of alumina ceramics is mainly reduced by adding sintering aids (T102, Cr203, MnO: and the like), which can only reduce the sintering temperature to 1600-1650°C, and the prepared alumina ceramics have low mechanical properties, which still limits the development and application of alumina ceramics. Therefore, how to further reduce the sintering temperature of alumina ceramics and improve the mechanical properties of alumina ceramics has become urgent problems in the field.

SUMMARY

[04] The present invention aims to provide an alumina ceramic and a preparation method thereof. The preparation method provided by the present invention has a sintering temperature of 1350-1400°C when preparing an alumina ceramic, and the obtained alumina ceramic has a high mechanical property.

[05] In order to achieve the above object, the present invention provides the following technical schemes.

[06] The present invention provides a preparation method for an alumina ceramic, including the following steps:

[07] (1) mixing Al203, AI(OH)s and Y20: to obtain mixed powder;

[08] (2) mixing the mixed powder obtained in step (1) with a saturated oxalic acid solution to obtain a mixed slurry;

[09] (3) performing cold sintering on the mixed slurry obtained in step (2) to obtain a first sintered body, the cold sintering being performed at a sintering pressure of 300-420 MPa, and including a first cold sintering at 100-120°C and a second cold sintering at 200-250°C;

[19] (4) performing heat preservation on the first sintered body obtained in step (3) at 100-150°C for 3-4 h to obtain a second sintered body; and

[11] (5) performing hot-pressed sintering on the second sintered body obtained in step (4) to obtain the alumina ceramic, the hot-pressed sintering being performed at a pressure of 25-30 MPa, and including a first hot-pressed sintering at 250-400°C, a second hot-pressed sintering at 700-900°C, a third hot-pressed sintering at 1200- 1300°C, and a fourth hot-pressed sintering at 1350-1400°C.

[12] Preferably, a mass of Al(OH); in the step (1) is 5-15% of a mass of the mixed powder.

[13] Preferably, a mass of Y20: in the step (1) is 1-2% of a mass of the mixed powder.

[14] Preferably, the mixing in the step (1) is ball-mill mixing.

[15] Preferably, the ball-mill mixing is performed at a rotation speed of 200-250 r/min for 6-10 h.

[16] Preferably, in the step (2), a ratio of the mass of the mixed powder to a volume of the saturated oxalic acid solution 1s 10 g: (1-1.5) mL.

[17] Preferably, the first cold sintering and the second cold sintering in the step (3) are independently performed heat preservation for 0.3-1 h.

[18] Preferably, the second hot-pressed sintering in the step (5) is performed at a temperature of 800-900°C.

[19] Preferably, in the step (5), the first hot-pressed sintering, the second hot- pressed sintering, the third hot-pressed sintering and the fourth hot-pressed sintering are independently performed heat preservation for 0.5-1 h.

[20] The present invention also provides an alumina ceramic prepared by the preparation method described in the above technical scheme.

[21] The present invention provides a preparation method for an alumina ceramic, including the following steps: mixing Al>Os, Al(OH): and Y20s to obtain mixed powder; mixing the mixed powder obtained in step (1) with a saturated oxalic acid solution to obtain a mixed slurry; performing cold sintering on the mixed slurry obtained in step (2) to obtain a first sintered body, the cold sintering being performed at a sintering pressure of 300-420 MPa, and including a first cold sintering at 100- 120°C and a second cold sintering at 200-250°C; performing heat preservation on the first sintered body obtained in step (3) at 100-150°C for 3-4 h to obtain a second sintered body; and performing hot-pressed sintering on the second sintered body obtained in step (4) to obtain the alumina ceramic, the hot-pressed sintering being performed at a pressure of 25-30 MPa, and including a first hot-pressed sintering at 250-400°C, a second hot-pressed sintering at 700-900°C, a third hot-pressed sintering at 1200-1300°C, and a fourth hot-pressed sintering at 1350-1400°C. In the present invention, the addition of Y20s can form eutectic with alumina and reduce the sintering temperature; and the addition of AI(OH)s can further reduce the sintering temperature. Then the product is mixed with saturated oxalic acid solution, and Y20:3 is slightly soluble in oxalic acid solution. During cold sintering, oxalic acid crystals are precipitated with the evaporation of water, and react with Y20: to generate yttrium oxalate, which plays a role in nucleation and promotes the nucleation of alumina, thereby reducing the sintering temperature. The compactness of alumina ceramic can be improved by low temperature heat preservation and hot-pressed sintering, thereby improving the mechanical property of alumina ceramic. The experimental results show that the sintering temperature can be reduced to 1350-1400°C when preparing the alumina ceramic according to the present invention, and the obtained alumina ceramic has a microhardness of 1870-2040 HV, a fracture toughness of 6.2-6.5 MPa:m!’? and a bending strength of 510-535 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

[22] FIG. 1 is a scanning electron microscope fracture morphology diagram of an alumina ceramic prepared in Example 1;

[23] FIG. 2 is a scanning electron microscope fracture morphology diagram of an alumina ceramic prepared in Comparative Example 1; and

[24] FIG. 3 is a scanning electron microscope fracture morphology diagram of an alumina ceramic prepared in Example 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[25] The present invention provides a preparation method for an alumina ceramic, including the following steps.

[26] (1) Al,O3, AI(OH)3 and Y20: are mixed to obtain mixed powder.

[27] (2) The mixed powder obtained 1n step (1) are mixed with a saturated oxalic acid solution to obtain a mixed slurry.

[28] (3) Cold sintering is performed on the mixed slurry obtained in step (2) to obtain a first sintered body. The cold sintering is performed at a sintering pressure of

300-420 MPa, and includes a first cold sintering at 100-120°C and a second cold sintering at 200-250°C.

[29] (4) Heat preservation is performed on the first sintered body obtained in step (3) at 100-150°C for 3-4 h to obtain a second sintered body. 5 [BO] (5) Hot-pressed sintering is performed on the second sintered body obtained in step (4) to obtain the alumina ceramic. The hot-pressed sintering is performed at a pressure of 25-30 MPa, and includes a first hot-pressed sintering at 250-400°C, a second hot-pressed sintering at 700-900°C, a third hot-pressed sintering at 1200- 1300°C, and a fourth hot-pressed sintering at 1350-1400°C. [BI] In the present invention, Al:05, AI(OH)3 and Y2O: are mixed to obtain mixed powder. In the present invention, the addition of Y203 can form eutectic with alumina and reduce the sintering temperature; and the addition of AI(OH); can further reduce the sintering temperature.

[32] In the present invention, a purity of the Al>O; is preferably of more than or equal to 99.99%; the Al20s is preferably a- Al>Os; and an average particle size of the

Al20: is preferably 400-700 nm. The source of Al20:3 is not particularly limited in the present invention, and the commercially available products well known to those skilled in the art can be adopted.

[33] In the present invention, a purity of the Al(OH); is preferably of more than or equal to 99.99%; and an average particle size of the Al(OH); is preferably 400-700 nm. The source of AI(OH); is not particularly limited in the present invention, and the commercially available products well known to those skilled in the art can be adopted.

[34] In the present invention, a purity of the Y20s is preferably of more than or equal to 99.99%; and an average grain size of the Y20: is preferably 50-200 nm. The source of Y20: is not particularly limited in the present invention, and the commercially available products well known to those skilled in the art can be adopted.

[35] In the present invention, a mass of the Al(OH); is preferably 5-15%, more preferably 10-15%, of a mass of the mixed powder; and a mass of the Y20s is preferably 1-2%, more preferably 1.5-2%, of the mass of the mixed powder. In the present invention, the sintering temperature can be further reduced by controlling the masses of Al(OH): and Y20:.

[36] In the present invention, the mixing of Al:03, Al(OH); and Y20: is preferably ball-mill mixing. The ball-mill mixing is preferably performed at a rotation speed of 200-250 r/min for 6-10 h, more preferably 8-9 h. A mass ratio of ball to material during the ball-mill mixing is preferably (5-15): 1; and a ball-mill medium of the ball-mill mixing is preferably anhydrous ethanol. In the present invention, a mixing degree of raw materials can be improved by ball-mill mixing Al203, AI(OH): and

Y20:.

[37] In the present invention, the ball-mill mixing is preferably performed in a planetary ball mill. The source of planetary ball mill is not particularly limited in the present invention, and the instruments and equipment well known to those skilled in the art can be adopted.

[38] After the mixing of Al:03, Al(OH); and Y20:; is completed, in the present invention, mixed products are preferably dried and sieved in sequential to obtain the mixed powder.

[39] In the present invention, the drying is preferably vacuum drying at a temperature of 60°C. The drying time is not particularly limited in the present invention, which needs to be dried to constant weight.

[40] In the present invention, a sieve is preferably a 120-mesh sieve.

[41] After the mixed powder is obtained, in the present invention, the mixed powder is mixed with saturated oxalic acid solution to obtain mixed slurry.

[42] In the present invention, a ratio of the mass of the mixed powder to a volume of the saturated oxalic acid solution is preferably 10 g: (1-1.5) mL, more preferably 10 gg (1-1.2) mL. The source of saturated oxalic acid solution is not particularly limited in the present invention, and the formulation method well known to those skilled in the art can be adopted. In the present invention, the saturated oxalic acid solution can dissolve yttrium oxide in a trace amount. During subsequent cold sintering, with the evaporation of water, oxalic acid crystals are precipitated to generate a trace amount of yttrium oxalate, which plays a role in nucleation and promotes the nucleation of alumina, thereby reducing the sintering temperature. The sintering temperature can be further reduced by controlling the ratio of the mass of mixed powder to the volume of saturated oxalic acid solution.

[43] The operation of mixing the mixed powder with the saturated oxalic acid solution is not particularly limited in the present invention, and the technical scheme for preparing the mixed material well known to those skilled in the art can be adopted.

[44] After the mixed slurry is obtained, in the present invention, the mixed slurry is performed cold sintering to obtain a first sintered body.

[45] In the present invention, the cold sintering 1s performed at a pressure of 300- 420 MPa, preferably 350-400 MPa, and includes a first cold sintering at 100-120°C (preferably 100-110°C), and a second cold sintering at 200-250°C (preferably 220- 250°C). The first cold sintering and the second cold sintering are independently performed heat preservation for 0.3-1 h, more preferably 0.5-1 h. The rate of heating to the first cold sintering temperature and the second cold sintering temperature is not particularly limited in the present invention, which can be adjusted according to the actual needs. In the present invention, by adopting cold sintering, the solvent in the mixed slurry can be evaporated, and oxalic acid crystals can be precipitated, which react with Y20: to generate yttrium oxalate. By introducing yttrium oxalate, the surface free energy distribution is effectively reduced, thereby significantly reducing the sintering temperature. The sintering temperature can be further reduced by controlling the technological parameters of cold sintering.

[46] In the present invention, after the heat preservation of the first cold sintering is completed, preferably, the temperature is directly raised to the second cold sintering temperature without cooling.

[47] After the first sintered body 1s obtained, in the present invention, the first sintered body is performed heat preservation at 100-150°C for 3-4 h to obtain the second sintered body. In the present invention, alumina blocks can be obtained by adopting low-temperature heat preservation, and the compactness is improved, so that the mechanical property of the material is improved.

[48] In the present invention, the first sintered body is preferably performed heat preservation at 100-150°C for 3-4 h without cooling to obtain a second sintered body.

[49] In the present invention, the heat preservation is preferably performed in an incubator. The source of incubator is not particularly limited in the present invention, and the instruments and equipment well known to those skilled in the art can be adopted.

[50] After the second sintered body is obtained, in the present invention, the second sintered body is performed hot-pressed sintering to obtain alumina ceramics. In the present invention, the compactness can be further improved by adopting hot- pressed sintering, so that the mechanical property of the material 1s improved.

[51] In the present invention, the second sintered body is preferably performed hot-pressed sintering without cooling to obtain the alumina ceramic.

[52] In the present invention, the hot-pressed sintering is performed at a pressure of 25-30 MPa, preferably 28-30 MPa, and includes a first hot-pressed sintering at 250- 400°C (preferably 300-350°C), a second hot-pressed sintering at 700-900°C (preferably 800-900°C), a third hot-pressed sintering at 1200-1300°C (preferably 1250- 1300°C), and a fourth hot-pressed sintering at 1350-1400°C (preferably 1350-1400°C).

In the present invention, by controlling the technological parameters of hot-pressed sintering, the grain growth of alumina ceramic in a late sintering stage can be effectively inhibited, thereby improving the compactness of alumina ceramics and further improving the mechanical property.

[53] In the present invention, the first hot-pressed sintering, the second hot- pressed sintering, the third hot-pressed sintering and the fourth hot-pressed sintering are independently and preferably performed heat preservation for 0.5-1 h, more preferably 0.6-0.8 h. The rate of heating to the first hot-pressed sintering temperature, the second hot-pressed sintering temperature, the third hot-pressed sintering temperature and the fourth hot-pressed sintering temperature is not particularly limited in the present invention, which can be adjusted according to the actual needs.

[54] In the present invention, after the heat preservation of the first hot-pressed sintering is completed, preferably, the temperature is directly raised to the second hot- pressed sintering temperature without cooling; after the heat preservation of the second hot-pressed sintering is completed, preferably, the temperature is directly raised to the third hot-pressed sintering temperature without cooling; after the heat preservation of the third hot-pressed sintering is completed, preferably, the temperature is directly raised to the fourth hot-pressed sintering temperature without cooling.

[55] After the hot-pressed sintering is completed, in the present invention, the products obtained by the hot-pressed sintering are preferably cooled, cut and polished in sequential to obtain the alumina ceramic.

[56] In the present invention, the cooling is preferably cooling with the furnace to room temperature.

[57] The cutting operation is not particularly limited in the present invention, which can be selected according to the actual size needs.

[58] In the present invention, the polishing is preferably performed by using a diamond grinding paste. Other polishing operations are not particularly limited in the present invention, which can be adjusted according to actual needs.

[59] In the present invention, the addition of Y20:3 can form eutectic with alumina and reduce the sintering temperature; and the addition of AI(OH): can further reduce the sintering temperature. Then the product is mixed with saturated oxalic acid solution, and Y20s is slightly soluble in oxalic acid solution. During cold sintering, oxalic acid crystals are precipitated with the evaporation of water, and react with Y>0; to generate yttrium oxalate, which plays a role in nucleation and promotes the nucleation of alumina, thereby reducing the sintering temperature. The compactness of alumina ceramics can be improved by low temperature heat preservation and hot- pressed sintering, thereby improving the mechanical property of alumina ceramic.

[60] In the present invention, o-Al:0: raw materials with an average particle size of 400-700 nm and a purity of more than or equal to 99.99% are selected, a hot-pressed sintering furnace is adopted to prepare alumina ceramics at a sintering temperature of 1350-1400°C, and the obtained alumina ceramics are polished. The temperature range of the preparation method can effectively inhibit the grain growth of alumina ceramics in a late sintering stage, so that the microhardness of alumina ceramics reaches 1870- 2040 HV, the fracture toughness reaches 6.2-6.5 MPa-m'? and the bending strength reaches 510-535 MPa.

[61] In the present invention, the sintering temperature of 1350-1400°C is adopted to prepare alumina ceramics, and compared with the traditional sintering temperature of 1600-1650°C, the energy consumption can be reduced, the energy can be saved, and the cost can be reduced; and low-temperature sintering of Al20:3 ceramics is of great significance.

[62] The present invention also provides an alumina ceramic prepared by the preparation method described in the above technical scheme.

[63] The alumina ceramic provided by the present invention has an excellent mechanical property.

[64] The technical schemes in the present invention will be described clearly and completely in the following with reference to the examples in the present invention.

Obviously, the described example is only a part of the example of the present invention, but not all the examples. Based on the examples in the present invention, all other examples obtained by ordinary technicians in the field without creative labor belong to the scope of protection of the present invention.

[65] Example 1

[66] A preparation method for an alumina ceramic included the following steps.

[67] (1) 88 g of a-Al:0; powder with an average particle size Dso of 400 nm and a purity of more than or equal to 99.99%, 10 g of Al(OH); powder with an average particle size Ds of 700 nm and a purity of more than or equal to 99.99% and 2 g of

Y20O:3 powder with an average particle size Dso of 50 nm and a purity of more than or equal to 99.99% were ball-milled and mixed in a planetary ball mill, performed vacuum drying at 60°C and sieved with a 120-mesh sieve to obtain mixed powder. The conditions were as follows: a rotation speed of the ball-mill mixing: 200 r/min; a time of the ball-mill mixing: 9 h; a mass ratio of ball to material: 10:1; and a ball-mill medium: anhydrous ethanol.

[68] (2) The mixed powder obtained in step (1) was added in a saturated oxalic acid solution to obtain a mixed slurry. A ratio of the mass of the mixed powder to a volume of the saturated oxalic acid solution was 10 g: 1.5 mL.

[69] (3) The mixed slurry obtained in step (2) was poured into a mold for cold sintering to obtain a circular first sintered body with a diameter of 25 mm and a height of 10 mm. The cold sintering was performed at a sintering pressure of 400 MPa and included heating to 120°C for a first cold sintering for 1 h, and then directly heating to 220°C for a second cold sintering for 1 h.

[70] (4) The first sintered body obtained in step (3) was placed in a heat preservation box and performed heat preservation at 100°C for 4 h to obtain a second sintered body.

[71] (5) Hot-pressed sintering was performed on the second sintered body obtained in step (4), then cooled to room temperature with a furnace, then cut, and polished with a diamond grinding paste to obtain the alumina ceramic. The hot-pressed sintering was performed at a pressure of 30 MPa, and included heating to 400°C for a first hot-pressed sintering for 1h, heating to 800°C for a second hot-pressed sintering for 0.5 h, then directly heating to 1300°C for a third hot-pressed sintering for 0.5 h, and finally directly heating to 1350°C for a fourth hot-pressed sintering for 1 h.

[72] The property of alumina ceramic prepared in Example 1 was tested, where a

Micro Vickers hardness was in accordance with the international standard ISO6507/1- 82; a bending strength was based on GB/T 6569-2006 standard, and a fracture toughness was calculated according to Nithara formula. The results showed that the microhardness was 1870 HV, the fracture toughness was 6.2MPa-m'?, and the bending strength was 520 MPa.

[73] Comparative Example 1

[74] A preparation method for an alumina ceramic included the following steps.

[75] (1) 88 g of a-Al20: powder with an average particle size Dso of 400 nm and a purity of more than or equal to 99.99%, 10 g of AI(OH)3 powder with an average particle size Dso of 700 nm and a purity of more than or equal to 99.99% and 2 g of

Y20: powder with an average particle size Dso of 50 nm and a purity of more than or equal to 99.99% were ball-milled and mixed in a planetary ball mill, performed vacuum drying at 60°C and sieved with a 120-mesh sieve to obtain mixed powder. The conditions were as follows: a rotation speed of the ball-mill mixing: 200 r/min; a time of the ball-mill mixing: 9 h; a mass ratio of ball to material: 10:1; and a ball-mill medium: anhydrous ethanol.

[76] (2) Hot-pressed sintering was performed on the mixed powder obtained in step (1), then cooled to room temperature with a furnace, then cut, and polished with a diamond grinding paste to obtain the alumina ceramic. The hot-pressed sintering was performed at a pressure of 30 MPa, and included heating to 800°C for 0.5 h, then directly heating to 1300°C for 0.5 h, then directly heating to 1350°C for 1 h.

[77] The tensile fractures of alumina ceramics prepared in Example 1 and

Comparative Example 1 were observed by using a scanning electron microscope, and the fracture morphology diagrams were shown in FIG. 1 and FIG. 2. As could be seen from FIG. 1 and FIG.2, compared to Comparative Example 1, the alumina ceramic prepared in Example 1 had good grain development, no pores, good compactness, no abnormal grain growth and a grain size of about 1 um.

[78] Example 2

[79] A preparation method for an alumina ceramic included the following steps.

[80] (1) 83 g of a- Al,0; powder with an average particle size Dso of 400 nm and a purity of more than or equal to 99.99%, 15 g of Al(OH); powder with an average particle size Dso of 700 nm and a purity of more than or equal to 99.99% and 2 g of

Y20; powder with an average particle size Dso of 50 nm and a purity of more than or equal to 99.99% were ball-milled and mixed in a planetary ball mill, performed vacuum drying at 60°C and sieved with a 120-mesh sieve to obtain mixed powder. The conditions were as follows: a rotation speed of the ball-mill mixing: 200 r/min; a time of the ball-mill mixing: 10 h; a mass ratio of ball to material: 10:1; and a ball-mill medium: anhydrous ethanol.

[81] (2) The mixed powder obtained in step (1) was added in a saturated oxalic acid solution to obtain a mixed slurry. A ratio of the mass of the mixed powder to a volume of the saturated oxalic acid solution was 10 g: 1.5 mL.

[82] (3) The mixed slurry obtained in step (2) was poured into a mold for cold sintering to obtain a circular first sintered body with a diameter of 25 mm and a height of 10 mm. The cold sintering was performed at a sintering pressure of 350 MPa and included heating to 120°C for first cold sintering for 1 h, and then directly heating to 250°C for second cold sintering for 1 h.

[83] (4) The first sintered body obtained in step (3) was placed in a heat preservation box and performed heat preservation at 100°C for 4 h to obtain a second sintered body.

[84] (5) Hot-pressed sintering was performed on the second sintered body obtained in step (4), then cooled to room temperature with a furnace, then cut, and polished with a diamond grinding paste to obtain the alumina ceramic. The hot-pressed sintering was performed at a pressure of 30 MPa, and included heating to 400°C for a first hot-pressed sintering for 1h, heating to 800°C for a second hot-pressed sintering for 0.5 h, then directly heating to 1300°C for a third hot-pressed sintering for 0.5 h, and finally directly heating to 1350°C for a fourth hot-pressed sintering for 1 h.

[85] The property of alumina ceramic prepared in Example 2 was tested, where a

Micro Vickers hardness was in accordance with the international standard ISO6507/1- 82; a bending strength was based on GB/T 6569-2006 standard, and a fracture toughness was calculated according to Niihara formula. The results showed that the microhardness was 2040 HV, the fracture toughness was 6.3 MPa-m!? and the bending strength was 525 MPa.

[86] Example 3

[87] A preparation method for an alumina ceramic includes the following steps.

[88] (1) 88 g of u-Al205 powder with an average particle size Dso of 400 nm and a purity of more than or equal to 99.99%, 10 g of Al(OH); powder with an average particle size Dso of 700 nm and a purity of more than or equal to 99.99% and 2 g of

[89] (2) The mixed powder obtained in step (1) was added in a saturated oxalic acid solution to obtain a mixed slurry. A ratio of the mass of the mixed powder to a volume of the saturated oxalic acid solution was 10 g: 1 mL.

[90] (3) The mixed slurry obtained in step (2) was poured into a mold for cold sintering to obtain a circular first sintered body with a diameter of 25 mm and a height of 10 mm. The cold sintering was performed at a sintering pressure of 400 MPa and included heating to 100°C for first cold sintering for 1 h, and then directly heating to 200°C for second cold sintering for 1 h.

[91] (4) The first sintered body obtained in step (3) was placed in a heat preservation box and performed heat preservation at 100°C for 4 h to obtain a second sintered body.

[92] (5) Hot-pressed sintering was performed on the second sintered body obtained in step (4), then cooled to room temperature with a furnace, then cut, and polished with a diamond grinding paste to obtain the alumina ceramic. The hot-pressed sintering was performed at a pressure of 30 MPa, and included heating to 400°C for a first hot-pressed sintering for 0.5 h, directly heating to 800°C for a second hot-pressed sintering for 0.5 h, then directly heating to 1300°C for a third hot-pressed sintering for 0.5 h, and finally directly heating to 1400°C for a fourth hot-pressed sintering for 1 h.

[93] The property of alumina ceramic prepared in Example 3 was tested, where a

Micro Vickers hardness was in accordance with the international standard ISO6507/1- 82; a bending strength was based on GB/T 6569-2006 standard, and a fracture toughness was calculated according to Nithara formula. The results showed that the microhardness was 1806 HV, the fracture toughness was 6.4 MPa-m!’? and the bending strength was 522 MPa.

[94] The tensile fractures of alumina ceramics prepared in Example 3 were observed by using a scanning electron microscope, and the fracture morphology diagrams were shown in FIG. 3.

[95] As could be seen from FIG. 3, the alumina ceramic prepared in Example 3 had no pores and compactness, but the grain size of which was larger than that in

Example 1.

[96] Example 4

[97] A preparation method for an alumina ceramic included the following steps.

[98] (1) 83 g of a- Al2O; powder with an average particle size Dso of 400 nm and a purity of more than or equal to 99.99%, 15 g of Al(OH): powder with an average particle size Dso of 700 nm and a purity of more than or equal to 99.99% and 2 g of

Y20; powder with an average particle size Dso of SO nm and a purity of more than or equal to 99.99% were ball-milled and mixed in a planetary ball mill, performed vacuum drying at 60°C and sieved with a 120-mesh sieve to obtain mixed powder. The conditions were as follows: a rotation speed of the ball-mill mixing: 210 r/min; a time of the ball-mill mixing: 10 h; a mass ratio of ball to material: 10:1; and a ball-mill medium: anhydrous ethanol.

[99] (2) The mixed powder obtained in step (1) was added in a saturated oxalic acid solution to obtain a mixed slurry. A ratio of the mass of the mixed powder to a volume of the saturated oxalic acid solution was 10 g: 1.5 mL.

[100] (3) The mixed slurry obtained in step (2) was poured into a mold for cold sintering to obtain a circular first sintered body with a diameter of 25 mm and a height of 10 mm. The cold sintering was performed at a sintering pressure of 350 MPa and included heating to 100°C for first cold sintering for 1 h, and then directly heating to 250°C for second cold sintering for 1 h.

[101] (4) The first sintered body obtained in step (3) was placed in a heat preservation box and performed heat preservation at 100°C for 4 h to obtain a second sintered body.

[102] (5) Hot-pressed sintering was performed on the second sintered body obtained in step (4), then cooled to room temperature with a furnace, then cut, and polished with a diamond grinding paste to obtain the alumina ceramic. The hot-pressed sintering was performed at a pressure of 30 MPa, and included heating to 400°C for a first hot-pressed sintering for 0.5 h, directly heating to 800°C for a second hot-pressed sintering for 0.5 h, then directly heating to 1300°C for a third hot-pressed sintering for 0.5 h, and finally directly heating to 1400°C for a fourth hot-pressed sintering for 1 h.

[103] The properties of alumina ceramics prepared in Example 4 were tested, where a Micro Vickers hardness was in accordance with the international standard

IS06507/1-82; a bending strength was based on GB/T 6569-2006 standard, and a fracture toughness was calculated according to Niihara formula. The results showed that the microhardness was 1844 HV, the fracture toughness was 6.5 MPa-m'?, and the bending strength was 535 MPa.

[104] As could be seen from the above examples, the preparation method provided by the present invention had a sintering temperature of 1350-1400°C when preparing the alumina ceramic, and the prepared alumina ceramic had a high mechanical property.

[105] The above is only the preferred embodiment of the present invention, and it should be pointed out that a person skilled in the art can make several improvements and embellishments without departing from the principle of the present invention, and these improvements and embellishments should also be regarded as the protection scope of the present invention.

Claims

Conclusions

1. Preparation method for an alumina ceramic, which includes the following steps: (1) mixing Al:O3, Al(OH): and Y2O3 to obtain mixed powder; (2) mixing the mixed powder obtained in step (1) with a saturated oxalic acid solution to obtain a mixed suspension; (3) performing cold sintering on the mixed suspension obtained from step (2) to obtain a first sintered body, cold sintering being performed at a sintering pressure of 300-420 MPa and a first cold sintering at 100-120° C and a second cold sintering at 200-250 °C; (4) performing heat preservation on the first sintered body obtained in step (3) at 100-150°C for 3-4 hours to obtain a second sintered body; and (5) performing hot-pressed sintering on the second sintered body obtained in step (4) to obtain the alumina ceramic, wherein the hot-pressed sintering is performed at a pressure of 25-30 MPa and a first hot-pressed sintering at 250-400° C, a second hot-pressed sintering at 700-900°C, a third hot-pressed sintering at 1200-1300°C and a fourth hot-pressed sintering at 1350-1400°C.

Preparation method according to claim 1, wherein a mass of Al(OH); in step (1) is 5-15% of a mass of the mixed powder.

Preparation method according to claim 1, wherein a mass of Y2O;3 in step (1) is 1-2% of a mass of the mixed powder.

The preparation method according to claim 1, wherein the mixing in step (1) is a ball mill mixing.

The preparation method according to claim 4, wherein the ball mill mixing is carried out at a rotation speed of 200-250 rpm for 6-10 hours.

The preparation method according to claim 1, wherein in step (2) a ratio of the mass of the mixed powder to a volume of the saturated oxalic acid solution is 10 g: (1-1.5) mL.

The preparation method according to claim 1, wherein the first cold sintering and the second cold sintering in step (3) are carried out independently of each other with heat preservation for 0.3-1 hour.

The preparation method according to claim 1, wherein the second hot-pressed sintering in step (5) is carried out at a temperature of 800-900°C.

The production method according to claim 1, wherein in step (5), the first hot-pressed sintering, the second hot-pressed sintering, the third hot-pressed sintering and the fourth hot-pressed sintering are carried out independently of each other heat preservation for 0.5-1 hour.

10. Aluminum oxide ceramic produced by a preparation method according to any of claims 1-9.