Silicon carbide-quartz composite ceramic crucible and gel-casting method thereof

Abstract

The application discloses a silicon carbide-quartz composite ceramic crucible and a gel-casting forming method thereof. The crucible is composed of SiC aggregate and SiO2, wherein the SiC aggregate accounts for 25-45 vol% of the total volume of the crucible, the SiO2 accounts for 55-75 vol% of the total volume of the crucible, the surface of the SiC aggregate is covered with an amorphous SiO2 transition layer, and the thickness of the SiO2 transition layer is 50-200 nm. The silicon carbide-quartz composite crucible prepared by covering the surface of the SiC aggregate with the amorphous SiO2 transition layer has obviously improved thermal conductivity, and no abnormal silicon leakage occurs in ingot casting application, and crystallization of the crucible after the ingot is taken out is obviously improved.

Description

Technical Field

[0001] This invention belongs to the field of quartz crucible preparation technology, specifically relating to a silicon carbide-quartz composite ceramic crucible and its casting method. Background Technology

[0002] Quartz ceramic crucibles are key components of polycrystalline silicon ingot casting furnaces used in solar cells. As containers for polycrystalline silicon raw materials, they must operate continuously at temperatures above 1500℃ for over 50 hours to melt and produce polycrystalline silicon ingots for solar cell manufacturing. The production of polycrystalline silicon ingots employs a directional solidification method. After the silicon material is melted, a specific cooling method is used to reduce the heat at the bottom of the crucible, establishing unidirectional heat dissipation, thus causing the molten silicon to solidify sequentially in the direction opposite to the heat flow. Currently, quartz crucibles are mainly integral crucibles produced using the injection solidification process. The raw material used in quartz crucibles is fused silica glass. Quartz glass has a low thermal conductivity (0.6-1.7 W / (m·K), therefore, using quartz ceramic crucibles results in longer silicon ingot production cycles and poor thermal shock resistance.

[0003] To address the problems of low thermal conductivity, poor thermal shock resistance, and high silicon leakage rate in existing fused silica ceramic crucibles, SiC particles have been added to SiO2. However, the thermal expansion coefficients of SiC and SiO2 are mismatched (difference of 8 times), resulting in high interfacial stress; the SiC and SiO2 slurry have poor compatibility, leading to severe sedimentation and stratification. Another approach is to prepare SiC crucibles, but this requires extremely high sintering temperatures (2150°C), multi-component sintering aids such as Y2O3-Al2O3-B4C-C, residual free Si (8-15%), high cost, and complex processes. Summary of the Invention

[0004] The purpose of this invention is to provide a silicon carbide-quartz composite ceramic crucible and its casting method. An amorphous SiO2 transition layer is covered on the surface of SiC aggregate, and the resulting silicon carbide-quartz composite crucible has significantly improved thermal conductivity. In ingot casting applications, no silicon leakage abnormality is observed, and crystallization after the crucible is removed from the ingot is significantly improved.

[0005] To achieve the above objectives, the present invention proposes a silicon carbide-quartz composite ceramic crucible for polycrystalline silicon ingot casting. The crucible is composed of SiC aggregate and SiO2, wherein the SiC aggregate accounts for 25-45 vol% of the total volume of the crucible, and the SiO2 accounts for 55-75 vol% of the total volume of the crucible. The surface of the SiC aggregate is covered with an amorphous SiO2 transition layer with a thickness of 50-200 nm.

[0006] Preferably, the room temperature thermal conductivity of the crucible is 10-50 W / (m·K); the bulk density of the crucible is 1.95-2.20 g / cm³.

[0007] This invention, by hiatus, proposes a method for casting a silicon carbide-quartz composite ceramic crucible, comprising the following steps:

[0008] S1. Pre-oxidation of SiC aggregate surface: α-SiC powder is calcined in air at 750-950℃ for 1-4 hours to generate an amorphous SiO2 transition layer on the surface of SiC particles in situ, thus obtaining pre-oxidized SiC aggregate.

[0009] S2. Preparation of AM-MBAM premix: Dissolve acrylamide (AM) monomer and N,N'-methylenebisacrylamide (MBAM) crosslinking agent in deionized water to obtain a premix, and adjust the pH of the premix to 3-6.

[0010] S3. Preparation of SiC-SiO2 composite slurry: Add 20-45 vol% pre-oxidized SiC aggregate and 27-55 vol% multi-stage fused silica sand prepared in S1 to the premixed liquid prepared in S2 by volume fraction, with a total solid phase volume of 60-80 vol%; then add 0.5-2.0 wt% dispersant to the premixed liquid, mix and stir for 1-3 h to obtain SiC-SiO2 composite slurry;

[0011] S4. Casting and molding: Add 0.5-1.5wt% of azo water-soluble initiator to the composite slurry prepared in S3, degas under vacuum for 10-30 minutes, then pour the composite slurry into a stainless steel mold, cure at 60-90℃ for 30-120 minutes, and demold to obtain the crucible green blank.

[0012] S5. Drying and Sintering: The crucible green prepared in S4 is dried at room temperature for 24-48 hours in an environment with humidity >80%, and then dried in an oven at 80-120℃ for 12-24 hours. After that, the temperature is increased to 600℃ at a heating rate of 0.5-2℃ / min and held for 2 hours to remove the binder. The temperature is then increased to 1100-1300℃ at a heating rate of 1-3℃ / min and held for 4-12 hours to sinter. After that, the furnace is cooled to room temperature to obtain a silicon carbide-quartz composite ceramic crucible.

[0013] Preferably, in step S2, the mass fraction of the acrylamide monomer is 5-12 wt%; and the mass fraction of the N,N'-methylenebisacrylamide is 0.05-0.3 wt%.

[0014] Preferably, in step S2, citric acid or lactic acid is used to adjust the pH value of the premix.

[0015] Preferably, in step S3, the dispersant is ammonium polyacrylate; in step S4, the azo water-soluble initiator is V50.

[0016] Preferably, in step S3, the multi-stage fused silica sand comprises 20-45 wt% coarse particles, 15-30 wt% medium particles, and 30-55 wt% fine particles; wherein, the coarse particles have a d 50 The d of medium-sized particles is 400-900 μm. 50 The d of fine particles is 70-160μm. 50 It is 5-10μm.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] This invention prepares a silicon carbide-quartz composite crucible by covering the surface of SiC aggregate with an amorphous SiO2 transition layer, which significantly improves the thermal conductivity, enhances the thermal shock resistance of the crucible in polycrystalline ingot casting applications, and prevents silicon leakage abnormalities in ingot casting applications. The crystallization after the crucible is removed from the ingot is also significantly improved.

[0019] The present invention provides a silicon carbide-quartz composite crucible that can be obtained by sintering the crucible at a low temperature of 1100-1300°C, thereby reducing energy consumption. Attached Figure Description

[0020] Figure 1 This is a process flow diagram for preparing the silicon carbide-quartz composite ceramic crucible according to the present invention;

[0021] Figure 2 These are the SEM image (a-1), EDS full spectrum (a-2), and elemental surface distribution diagram (a-3: Si; a-4: O; a-5: C) of the crucible prepared in Example 1 of this invention. Detailed Implementation

[0022] The invention will now be further described with reference to the accompanying drawings.

[0023] Example 1

[0024] S1 and SiC aggregate surface pre-oxidation

[0025] α-SiC powder with a purity of ≥99.5%, Fe content <20ppm, and an average particle size of 200 μm was selected and placed in an air atmosphere box furnace. It was calcined at 850℃ for 2 hours with a heating rate of 5℃ / min to generate an amorphous SiO2 transition layer with a thickness of 50-200nm on the surface of SiC particles in situ, thus obtaining pre-oxidized SiC aggregate.

[0026] S2. Prepare AM-MBAM premix solution

[0027] 4 kg (14.25 wt%) of acrylamide (AM) monomer and 0.08 kg (0.28 wt%) of N,N'-methylenebisacrylamide crosslinking agent were dissolved in 25 L of deionized water to obtain a premix; the pH of the premix was adjusted to 4 using citric acid / lactic acid; and the mixture was stirred for 30 min until completely dissolved.

[0028] S3. Preparation of SiC-SiO2 composite slurry

[0029] Raw materials: 80 kg of pre-oxidized SiC aggregate (35 vol%), 180 kg of multi-grade fused silica sand (54 kg of coarse particles + 36 kg of medium particles + 90 kg of fine particles);

[0030] Dispersant: Ammonium polyacrylate (PAA-NH4) 8 g;

[0031] Mixing: Add the above raw materials and dispersant to the mixer at 70 rpm for 2-3 hours. During the mixing process, cooling water at 10-24℃ should be introduced to cool the mixer tank and prevent the slurry from solidifying prematurely due to the temperature rise of the slurry during the mixing process.

[0032] Results: The viscosity of the composite slurry was 4000-7000 CP, and the solid volume fraction was 75 vol%.

[0033] S4, Injection Molding

[0034] Add 4g (0.4wt%) of initiator V50 to the composite slurry in S3;

[0035] Vacuum degassing: 0.05 MPa, 15 min;

[0036] Casting: A 1220×1220×630mm square stainless steel mold was used;

[0037] Curing: Heat in a water bath at 80°C for 60 minutes to obtain a complete crucible green blank.

[0038] S5. Drying and Sintering

[0039] High humidity drying at room temperature: Dry at room temperature for 48 hours in an environment with 85% humidity;

[0040] Oven drying: Dry in an oven at 100℃ for 12-24 hours;

[0041] Glue removal: Heat to 600℃ at a heating rate of 1℃ / min and hold for 2 hours to remove glue;

[0042] Sintering: Heat to 1180℃ at a heating rate of 2℃ / min and hold for 6 h;

[0043] Cooling: Cool to room temperature with the furnace to obtain a silicon carbide-quartz composite ceramic crucible.

[0044] Depend on Figure 2 As can be seen from a-1, the whole structure exhibits typical fracture or polished surface characteristics of sintered ceramics or refractory materials.

[0045] Figure 2 Energy dispersive spectroscopy analysis of a-2 confirmed the presence of carbon, indicating that silicon carbide had been successfully combined with quartz, with a content of 28.6% wt.

[0046] Figure 2 In the attached figure a-3, most of the color is red, indicating that silicon is uniformly distributed throughout the matrix; Figure 2 In the attached figure a-4, most of the oxygen is purple, but in some local areas (corresponding to the bright spot area in a-1), it appears to be denser or sparser, indicating that the distribution of oxygen highly overlaps with the distribution of silicon. This confirms that SiO2 is widely present in the matrix as a binding phase. Figure 2 In the attached diagram a-4, the green dots are relatively discrete, appearing as spots. These green spots correspond to SiC aggregate particles, indicating that the SiC aggregate is uniformly distributed within the matrix. Elemental Distribution Figure 2 (a-1-a-5) shows SiC particles (green) being encapsulated by a SiO2 matrix (purple / red mixed region), confirming that an amorphous SiO2 transition layer covers the surface of the SiC aggregate.

[0047] Example 2

[0048] The difference from Example 1 is as follows:

[0049] S1: The pre-oxidation temperature of SiC aggregate surface is 800℃;

[0050] S3: Add 20 vol% of pre-oxidized SiC aggregate to the premix;

[0051] S5: Sintering temperature is 1200℃, sintering time is 8h;

[0052] The remaining steps and parameters are the same as in Example 1.

[0053] Comparative Example 1: Pure Quartz Ceramic Crucible

[0054] The difference from Example 1 is as follows:

[0055] No SiC aggregate, i.e., excluding S1 of Example 1;

[0056] S3: The amount of multi-grade fused silica sand used is 100%;

[0057] S5: Sintering temperature is 1135℃, sintering time is 3h;

[0058] The remaining steps and parameters are the same as in Example 1.

[0059] Table 1 Application data of silicon carbide-quartz composite crucibles

[0060]

[0061] As shown in Table 1, the thermal conductivity of the silicon carbide-quartz composite crucible prepared in Example 2 is significantly improved compared to that of Example 1 and the comparative example. This improved thermal conductivity leads to a reduction in the ingot casting cycle of 11-17 hours and a reduction in energy consumption of 869-1391 kWh. No silicon leakage was observed in the silicon carbide-quartz composite crucibles prepared in Examples 1 and 2 during ingot casting applications, and crystallization after ingot removal was significantly improved. Furthermore, the impurity data of Example 1 is superior to that of Example 2 and the comparative example.

[0062] The above are merely preferred embodiments of the present invention and do not constitute any limitation on the present invention. Any equivalent substitutions or modifications made by those skilled in the art to the technical solutions and content disclosed in the present invention without departing from the scope of the present invention shall be deemed to have remained within the protection scope of the present invention.

Claims

1. A silicon carbide-quartz composite ceramic crucible for polycrystalline silicon ingot casting, characterized in that, The crucible is composed of SiC aggregate and SiO2, wherein the SiC aggregate accounts for 25-45 vol% of the total volume of the crucible, and the SiO2 accounts for 55-75 vol% of the total volume of the crucible. The surface of the SiC aggregate is covered with an amorphous SiO2 transition layer with a thickness of 50-200 nm.

2. The silicon carbide-quartz composite ceramic crucible according to claim 1, characterized in that, The crucible has a room temperature thermal conductivity of 10-50 W / (m·K) and a bulk density of 1.95-2.20 g / cm³.

3. A method for casting a silicon carbide-quartz composite ceramic crucible as described in claim 1 or 2, characterized in that, Includes the following steps: S1. Pre-oxidation of SiC aggregate surface: α-SiC powder is calcined in air at 750-950℃ for 1-4 hours to generate an amorphous SiO2 transition layer on the surface of SiC particles in situ, thus obtaining pre-oxidized SiC aggregate. S2. Preparation of AM-MBAM premix: Dissolve acrylamide (AM) monomer and N,N'-methylenebisacrylamide (MBAM) crosslinking agent in deionized water to obtain a premix, and adjust the pH of the premix to 3-6. S3. Preparation of SiC-SiO2 composite slurry: Add 20-45 vol% pre-oxidized SiC aggregate and 27-55 vol% multi-stage fused silica sand prepared in S1 to the premixed liquid prepared in S2 by volume fraction, with a total solid phase volume of 60-80 vol%; then add 0.5-2.0 wt% dispersant to the premixed liquid, mix and stir for 1-3 h to obtain SiC-SiO2 composite slurry; S4. Casting and molding: Add 0.5-1.5wt% of azo water-soluble initiator to the composite slurry prepared in S3, degas under vacuum for 10-30 minutes, then pour the composite slurry into a stainless steel mold, cure at 60-90℃ for 30-120 minutes, and demold to obtain the crucible green blank. S5. Drying and Sintering: The crucible green prepared in S4 is dried at room temperature for 24-48 hours in an environment with humidity >80%, and then dried in an oven at 80-120℃ for 12-24 hours. After that, the temperature is increased to 600℃ at a heating rate of 0.5-2℃ / min and held for 2 hours to remove the binder. The temperature is then increased to 1100-1300℃ at a heating rate of 1-3℃ / min and held for 4-12 hours to sinter. After that, the furnace is cooled to room temperature to obtain a silicon carbide-quartz composite ceramic crucible.

4. The injection molding method according to claim 3, characterized in that, In step S2, the mass fraction of the acrylamide monomer is 5-12 wt%; the mass fraction of the N,N'-methylenebisacrylamide is 0.05-0.3 wt%.

5. The injection molding method according to claim 3, characterized in that, In step S2, citric acid or lactic acid is used to adjust the pH value of the premix.

6. The injection molding method according to claim 3, characterized in that, In step S3, the dispersant is ammonium polyacrylate; in step S4, the azo water-soluble initiator is V50.

7. The injection molding method according to claim 3, characterized in that, In step S3, the multi-stage fused silica sand comprises 20-45 wt% coarse particles, 15-30 wt% medium particles, and 30-55 wt% fine particles; wherein, the coarse particles have a d 50 The d of medium-sized particles is 400-900 μm. 50 The d of fine particles is 70-160μm. 50 It is 5-10μm.

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