A method for preparing a silicon oxynitride ceramic riser

Abstract

This invention provides a method for preparing silicon oxynitride ceramic riser tubes. The method includes: selecting ingredients, adding the ingredients to a stirred mill, ball milling for 22 to 26 hours to prepare a uniform ceramic slurry; aging the prepared ceramic slurry, degassing for 22 to 26 hours, and spray drying it through a spray drying tower to prepare ceramic powder with a particle size distribution; aging the prepared ceramic powder for more than 48 hours, homogenizing it for 30 minutes using a homogenizer, and adding molding additives; sintering the ceramic powder into a silicon oxynitride ceramic riser tube; processing the sintered silicon oxynitride riser tube to the designed size using a grinding machine, and packaging it into a warehouse after passing inspection. The silicon oxynitride ceramic riser tube preparation method provided by this invention, through optimized formula, sintering, ball milling, and molding processes, enables the riser tube to achieve a flexural strength of approximately 400-500 MPa, which fully meets the domestic demand for ceramic riser tubes.

Description

Technical Field

[0001] This invention belongs to the technical field of methods for preparing silicon oxynitride ceramic riser tubes, and specifically relates to a method for preparing silicon oxynitride ceramic riser tubes. Background Technology

[0002] The riser pipe used in the aluminum casting industry is one of the key components of low-pressure casting machines. Its performance and stability directly affect the quality of the cast products and production efficiency.

[0003] The working principle of the riser tube is generally as follows: the lower end is inserted into the aluminum alloy molten metal in the holding furnace, and the upper end is connected to the casting mold cavity. Under pressure, the aluminum alloy molten metal enters the mold cavity from bottom to top to complete the casting. According to the working conditions, the riser tube must have good thermal shock resistance, resistance to corrosion by the aluminum alloy molten metal, non-wetting with the aluminum alloy molten metal, good airtightness, and resistance to thermal shock fatigue.

[0004] In existing technologies, the main riser pipes are cast iron riser pipes, aluminum titanate riser pipes, and silicon nitride ceramic riser pipes. However, cast iron riser pipes are not resistant to corrosion from molten aluminum alloys, easily introduce cast iron impurities into aluminum alloy castings, and have a short service life, affecting production efficiency. Aluminum titanate ceramics are difficult to control in terms of synthesis and sintering temperature, have low strength, and a service life of only two months. Silicon nitride ceramic riser pipes are corrosion resistant, but have relatively poor thermal shock resistance and react slightly with molten aluminum alloys containing high levels of magnesium and zinc, with a service life of about one year.

[0005] There are currently no solutions to the technical problems existing in the riser tubes mentioned above; therefore, there is an urgent need to find effective solutions to address these issues. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of the above-mentioned technologies by proposing a method for preparing a silicon oxynitride ceramic riser tube, which aims to solve one of the problems of insufficient thermal shock resistance, short service life, and poor resistance to high-temperature corrosion of aluminum alloy melt in existing riser tubes.

[0007] This invention provides a method for preparing a silicon oxynitride ceramic riser tube, the method comprising the following steps:

[0008] S1: Selected ingredients include:

[0009]

[0010]

[0011] S2: Ball milling: Add the selected silicon oxynitride powder, calcined alumina, lithium aluminate, sintering aid, dispersant, binder PVB and ethanol from S1 to a stirring mill and ball mill for 22 to 26 hours to prepare a uniform ceramic slurry.

[0012] S3: Spray drying: The prepared ceramic slurry is aged and defoamed for 22 to 26 hours, and then spray dried in a spray drying tower to produce ceramic powder that meets the particle size distribution; the moisture content of the ceramic powder is controlled below 1%;

[0013] S4: Ceramic powder homogenization: After the prepared ceramic powder has been aged for more than 48 hours, it is homogenized for 30 minutes using a homogenizer, molding additives are added, and it is packaged for later use.

[0014] S5: Sintering of ceramics: Sinter the ceramic powder in S4 into a ceramic silicon oxynitride riser tube; the sintering temperature is slowly increased at a rate of 50℃ / hour, and at 300℃ to 400℃, the heating rate is slowed down to 40 to 48℃ / hour, and the temperature is held at 300℃ to 400℃ for 6 hours, then the temperature is increased to 1650℃, held for 3 hours, and then naturally cooled to room temperature;

[0015] S6: Processing and Inspection: The fired silicon oxynitride riser tubes are processed to the design dimensions using a grinding machine, and packaged and stored after passing inspection.

[0016] Furthermore, S4 also includes isostatic pressing and lathe machining:

[0017] S41: Load the homogenized ceramic powder into the isostatic pressing mold, seal it tightly with rubber bands, and place it into the isostatic press.

[0018] S42: Pressurize to 300MPa, hold pressure for 120 seconds, slowly release pressure, remove the formed green blank, dry it in a 50℃ drying room, let it sit for a week, and then load it into a machining equipment to process it according to the design dimensions.

[0019] Furthermore, the lithium aluminate pretreatment in S1 includes: mixing lithium carbonate and aluminum hydroxide according to a preset molar ratio, and then ball milling them separately, using ethanol as a solvent, ball milling for 24 hours, removing from the mill, drying, and calcining at 1250 degrees Celsius for 6 hours to generate lithium aluminate blocks.

[0020] Furthermore, in the pretreatment process of lithium aluminate: the preset molar ratio of lithium carbonate to aluminum hydroxide is 1:1, 1:2, or 1:3; the calcined lithium aluminate lumps are pulverized into particles smaller than 1 mm, mixed with ethanol, ball-milled into a slurry smaller than 1 micrometer, dried, pulverized, and packaged and labeled; the preset molar ratio of lithium carbonate to aluminum hydroxide of 1:1 is designated as raw material A1, the preset molar ratio of lithium carbonate to aluminum hydroxide of 1:2 is designated as raw material A2, and the preset molar ratio of lithium carbonate to aluminum hydroxide of 1:2 is designated as raw material A3.

[0021] Furthermore, the proportion of lithium aluminate in S1 is selected as 3-6 parts or 2-5 parts.

[0022] Furthermore, the sintering aids include iron oxide, yttrium oxide, barium carbonate, and zinc oxide.

[0023] Furthermore, the silicon oxynitride powder is a self-propagating synthesized silicon oxynitride; the particle size D50 of the silicon oxynitride powder is less than 3 micrometers, the silicon oxynitride content is not less than 98%, and the silicon nitride content is not greater than 2%.

[0024] Furthermore, the calcined alumina is microcrystalline alumina calcined using a mineralizing agent at a calcination temperature below 1300℃; the alpha phase content of the microcrystalline alumina is not less than 98%; the original crystal size of the microcrystalline alumina is less than 100 nanometers; after calcination and pulverization, the particle size D50 of the microcrystalline alumina is controlled at 0.3-0.6 micrometers to meet the requirements of industrial-grade calcined alpha alumina.

[0025] Furthermore, the aluminum hydroxide is selected as an ultrafine powder with a mesh size of 10,000 or higher, an alumina content of more than 64%, and an alkali content of less than 0.2%.

[0026] Furthermore, the lithium carbonate is selected as chemically pure lithium carbonate; S1 also includes a mineralizer, which is selected as chemically pure kaolin, basic magnesium carbonate, iron oxide, zinc oxide, barium carbonate, yttrium oxide, cerium oxide and lanthanum oxide; the particle size of the mineralizer is controlled to be below 1 micrometer.

[0027] Furthermore, the dispersant is ammonium polyacrylate 9300.

[0028] Furthermore, the adhesive is PVB.

[0029] The method for preparing silicon oxynitride ceramic riser tubes provided by this invention, by using low liquid point materials and sintering at 1650℃, and by optimizing the ball milling, forming, and sintering processes, effectively solves the problem of high-temperature decomposition of silicon oxynitride, enabling the riser tube to achieve a flexural strength of approximately 400-500 MPa; the use of self-propagating powder, which has stable production and quality, makes large-scale mass production of silicon oxynitride ceramic riser tubes possible, fully meeting the domestic demand for ceramic riser tubes. Attached Figure Description

[0030] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0031] The present invention will be further described below with reference to the accompanying drawings:

[0032] Figure 1 This is a flowchart of a method for preparing a silicon oxynitride ceramic riser tube according to the present invention. Detailed Implementation

[0033] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0034] It should be noted that when a component is referred to as being "fixed to" or "set on" another component, it can be directly on or indirectly on that other component. When a component is referred to as being "connected to" another component, it can be directly connected to or indirectly connected to that other component.

[0035] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. "Several" means one or more, unless otherwise explicitly specified.

[0036] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0037] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0038] like Figure 1 As shown, this invention provides a method for preparing a silicon oxynitride ceramic riser, specifically a method for preparing a silicon oxynitride ceramic riser using self-propagating silicon oxynitride powder. In this application, silicon oxynitride ceramic is used because it possesses excellent thermal shock resistance, high-temperature non-wetting properties against molten aluminum alloys, excellent oxidation resistance, and excellent resistance to high-temperature corrosion from molten aluminum alloys, making it an excellent material for preparing risers for the aluminum die-casting industry. Specifically, the preparation method includes the following steps:

[0039] S1: Selected ingredients include:

[0040]

[0041]

[0042] S2: Ball milling: Add the selected silicon oxynitride powder, calcined alumina, lithium aluminate, sintering aid, dispersant, binder PVB and ethanol from S1 to a stirring mill and ball mill for 22 to 26 hours to prepare a uniform ceramic slurry.

[0043] S3: Spray drying: The prepared ceramic slurry is aged and defoamed for 22 to 26 hours, and then spray dried in a spray drying tower to produce ceramic powder with a particle size distribution; specifically, the moisture content of the ceramic powder is controlled below 1%;

[0044] S4: Ceramic powder homogenization: After the prepared ceramic powder has been aged for more than 48 hours, it is homogenized for 30 minutes using a V-type homogenizer. Molding additives are added to facilitate molding and green body processing, and then packaged for later use.

[0045] S5: Sintering of Ceramic: Since silicon oxynitride riser tubes are mostly large ceramic components, each weighing around 20 kg, the sintering of silicon oxynitride ceramics is crucial to the entire process. In this step, the homogenized ceramic powder from S4 is sintered into ceramic silicon oxynitride riser tubes. Specifically, the sintering temperature is slowly increased at a rate of 50°C / hour, and at 300°C to 400°C, the heating rate is slowed to 40 to 48°C / hour to ensure proper debinding of the silicon oxynitride ceramic riser tube. The temperature is then maintained between 500°C and 700°C. After 6 hours, the temperature is further increased to 1650℃, held for 3 hours, and then naturally cooled to room temperature. The material formulation of the silicon oxynitride ceramic riser tube preparation method provided by this invention also makes the sintering temperature lower than the decomposition temperature of silicon oxynitride (1700℃), with the current formulation having a sintering temperature of 1650℃. Specifically, the silicon oxynitride ceramic riser tube preparation method provided by this invention uses low liquid point materials, making low-temperature sintering of silicon oxynitride ceramics possible. The current formulation has a sintering temperature of 1650℃, which is lower than the decomposition temperature of silicon oxynitride material (1700℃).

[0046] S6: Processing and Inspection: The fired silicon oxynitride riser tubes may have certain dimensional deviations and need to be processed. Specifically, the fired silicon oxynitride riser tubes are processed to the design dimensions using a grinding machine. After passing the inspection, they are packaged and put into storage.

[0047] The above-mentioned method for preparing silicon oxynitride ceramic riser tubes results in riser tubes with higher strength (flexural strength of about 400-500 MPa), excellent thermal shock resistance, oxidation resistance, and significantly better non-wetting properties with molten aluminum than silicon oxynitride ceramic riser tubes.

[0048] Preferably, in conjunction with the above scheme, step S4 further includes isostatic pressing and lathe machining:

[0049] S41: Load the homogenized ceramic powder into the isostatic pressing mold, seal it tightly with rubber bands, and place it into the isostatic press.

[0050] S42: Pressurize to 300MPa, hold pressure for 120 seconds, slowly release pressure, remove the formed green body, put it into a 50℃ drying room to dry, and at the same time allow the ceramic green body to slowly release stress. After one week, load it into a machining equipment and process it according to the design dimensions.

[0051] Preferably, in conjunction with the above scheme, the lithium aluminate pretreatment in S1 includes: mixing lithium carbonate and aluminum hydroxide according to a preset molar ratio, and adding them to a ball mill for ball milling, using ethanol as a solvent, ball milling for 24 hours, removing from the mill, drying, and calcining at 1250 degrees Celsius for 6 hours to generate lithium aluminate blocks.

[0052] Preferably, in conjunction with the above scheme, in this embodiment, during the pretreatment of lithium aluminate: the preset molar ratio of lithium carbonate to aluminum hydroxide is 1:1, 1:2, or 1:3; the calcined lithium aluminate lumps are pulverized into particles smaller than 1 mm, mixed with ethanol, ball-milled into a slurry smaller than 1 micrometer, dried, pulverized, and packaged and labeled; specifically, multiple sets of preset molar ratios of lithium carbonate and aluminum hydroxide can be designed for comparison; wherein, a preset molar ratio of lithium carbonate to aluminum hydroxide of 1:1 is designated as raw material A1, a preset molar ratio of lithium carbonate to aluminum hydroxide of 1:2 is designated as raw material A2, and a preset molar ratio of lithium carbonate to aluminum hydroxide of 1:2 is designated as raw material A3.

[0053] Preferably, in conjunction with the above scheme, two sets of embodiments are specifically designed based on the two raw materials, A1 and A2, as indicated:

[0054] Example 1: Add 80-90 parts of silicon oxynitride powder, 1-3 parts of calcined alumina, 3-6 parts of lithium aluminate (Al), 1-5 parts of sintering aid (containing iron oxide, yttrium oxide, barium carbonate, zinc oxide, etc.), 1 part of dispersant, 3 parts of binder PVB (calculated as dry powder, prepared as a 10% solution), and 100-150 parts of ethanol to a stirred mill according to the formula ratio. Ball mill for 24 hours until the particle size control index is reached, then remove from the mill and age. Then proceed with steps S3 to S6.

[0055] Example 2: Add 80-90 parts of silicon oxynitride, 1-3 parts of calcined alumina, 2-5 parts of lithium aluminate A2, 1-5 parts of sintering aid (including iron oxide, yttrium oxide, barium carbonate, zinc oxide, etc.), 1 part of dispersant, 3 parts of binder PVB (calculated as dry powder, prepared as a 10% solution), and 100-150 parts of alcohol to a stirred mill according to the formula ratio. Ball mill for 24 hours until the particle size control index is reached, then remove from the mill and age. Then proceed with steps S3 to S6.

[0056] Preferably, in combination with the above scheme, the proportion of lithium aluminate in S1 is selected as 3-6 parts or 2-5 parts, wherein the proportion of lithium aluminate selected as 3-6 parts is the A1 composition product; the proportion of lithium aluminate selected as 2-5 parts is the A2 composition product.

[0057] Preferably, in combination with the above scheme, the dispersant is selected as ammonium polyacrylate 9300, and the binder is selected as PVB (i.e., polyvinyl butyral).

[0058] Preferably, in combination with the above scheme, the sintering aid includes iron oxide, yttrium oxide, barium carbonate, and zinc oxide.

[0059] Preferably, in conjunction with the above scheme, the silicon oxynitride powder is self-propagating synthesized silicon oxynitride; the particle size D50 of the silicon oxynitride powder is less than 3 micrometers, the silicon oxynitride content is not less than 98%, and the silicon nitride content is not greater than 2%; in this application scheme, the use of silicon oxynitride as a riser is an original design, the coefficient of thermal expansion of silicon oxynitride is half that of silicon nitride, around 1.2, and the service life is more than two years, which is more than three times that of silicon nitride ceramic risers; the oxidation resistance and peeling performance of silicon oxynitride from molten aluminum are far superior to those of silicon nitride ceramic risers; at the same time, compared with silicon nitride ceramic risers, due to the use of self-propagating powder, the ceramic manufacturing cost is reduced by more than 20%.

[0060] Preferably, in combination with the above scheme, the calcined alumina is microcrystalline alumina calcined with a mineralizing agent at a calcination temperature below 1300℃; the alpha phase content of the microcrystalline alumina is not less than 98%; the original crystal size of the microcrystalline alumina is less than 100 nanometers; after calcination and pulverization, the particle size D50 of the microcrystalline alumina is controlled at 0.3-0.6 micrometers to meet the requirements of industrial-grade calcined alpha alumina, so as to reduce the synthesis temperature of silicon oxynitride ceramics as much as possible.

[0061] Preferably, in combination with the above scheme, aluminum hydroxide is selected as an ultrafine powder with a mesh size of 10,000 or larger, an alumina content of more than 64%, and an alkali content of less than 0.2%.

[0062] Preferably, in combination with the above scheme, the lithium carbonate is selected as chemically pure lithium carbonate; further, S1 also includes a mineralizing agent, which is selected as chemically pure kaolin, basic magnesium carbonate, iron oxide, zinc oxide, barium carbonate, yttrium oxide, cerium oxide and lanthanum oxide; specifically, the particle size of the mineralizing agent is controlled below 1 micrometer, which effectively improves the forming strength of the silicon oxynitride riser pipe.

[0063] The method for preparing silicon oxynitride ceramic riser tubes provided by this invention, by using low liquid point materials and sintering at 1650℃, and by optimizing the ball milling, forming, and sintering processes, effectively solves the problem of high-temperature decomposition of silicon oxynitride, enabling the riser tube to achieve a flexural strength of approximately 400-500 MPa; the use of self-propagating powder, which has stable production and quality, makes large-scale mass production of silicon oxynitride ceramic riser tubes possible, fully meeting the domestic demand for ceramic riser tubes.

[0064] In this application, silicon oxynitride ceramic has a low coefficient of thermal expansion, and its oxidation resistance and non-wetting properties with molten aluminum are far superior to those of silicon nitride ceramic riser tubes. Its service life is more than two years, which is more than twice that of silicon nitride ceramic riser tubes. At the same time, due to the use of self-propagating powder, the production and quality of self-propagating powder are stable, making it possible to mass-produce silicon oxynitride ceramic riser tubes.

[0065] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any person skilled in the art can make many possible variations and modifications to the technical solution of the present invention, or modify it into equivalent embodiments, without departing from the scope of the present invention. Therefore, any modifications, equivalent changes, and alterations made to the above embodiments based on the technology of the present invention without departing from the scope of the present invention are within the protection scope of the present invention.

Claims

1. A method for preparing a silicon oxynitride ceramic riser tube, characterized in that, The preparation method includes the following steps: S1: Selected ingredients include: 80-90 parts of silicon oxynitride powder; 1-3 parts of calcined alumina; Lithium aluminate 2-6 parts; 1-5 parts of sintering aid; 1 part dispersant; 3 parts of PVB adhesive; 100-150 parts ethanol; S2: Ball milling: Add the selected silicon oxynitride powder, calcined alumina, lithium aluminate, sintering aid, dispersant, binder PVB and ethanol from S1 to a stirring mill and ball mill for 22 to 26 hours to prepare a uniform ceramic slurry. S3: Spray drying: The prepared ceramic slurry is aged and defoamed for 22 to 26 hours, and then spray dried in a spray drying tower to produce ceramic powder with a particle size distribution; the moisture content of the ceramic powder is controlled below 1%; S4: Ceramic powder homogenization: After the prepared ceramic powder has been aged for more than 48 hours, it is homogenized for 30 minutes using a homogenizer, molding additives are added, and it is packaged for later use. S5: Sintering of ceramics: Sinter the ceramic powder in S4 into a ceramic silicon oxynitride riser tube; the sintering temperature is slowly increased at a rate of 50°C / hour, and at 300°C to 400°C, the heating rate is slowed down to 40 to 48°C / hour, and the temperature is held at 500°C to 700°C for 6 hours, then the temperature is increased to 1650°C, held for 3 hours, and then naturally cooled to room temperature; S6: Processing and Inspection: The fired silicon oxynitride riser tubes are processed to the design dimensions using a grinding machine, and packaged and stored after passing inspection.

2. The method for preparing a silicon oxynitride ceramic riser tube according to claim 1, characterized in that, S4 also includes isostatic pressing and lathe machining: S41: Load the homogenized ceramic powder into the isostatic pressing mold, seal it tightly with rubber bands, and place it into the isostatic press. S42: Pressurize to 300MPa, hold pressure for 120 seconds, slowly release pressure, remove the formed green blank, dry it in a 50°C drying room, let it sit for a week, and then load it into a machining equipment to process it according to the design dimensions.

3. The method for preparing a silicon oxynitride ceramic riser tube according to claim 1, characterized in that, The preparation method of lithium aluminate in S1 includes: mixing lithium carbonate and aluminum hydroxide according to a preset molar ratio, and then ball milling them separately, using ethanol as a solvent, ball milling for 24 hours, removing from the mill, drying, and calcining at 1250 degrees Celsius for 6 hours to generate lithium aluminate blocks.

4. The method for preparing a silicon oxynitride ceramic riser tube according to claim 3, characterized in that, In the preparation method of lithium aluminate: the preset molar ratio of lithium carbonate to aluminum hydroxide is 1:1, 1:2, or 1:3; the calcined lithium aluminate lumps are pulverized into particles smaller than 1 mm, mixed with ethanol, ball-milled into a slurry smaller than 1 micrometer, dried, pulverized, and packaged and labeled; the preset molar ratio of lithium carbonate to aluminum hydroxide of 1:1 is designated as raw material A1, the preset molar ratio of lithium carbonate to aluminum hydroxide of 1:2 is designated as raw material A2, and the preset molar ratio of lithium carbonate to aluminum hydroxide of 1:2 is designated as raw material A3.

5. The method for preparing a silicon oxynitride ceramic riser tube according to claim 1, characterized in that, The proportion of lithium aluminate in S1 is selected as 3-6 parts or 2-5 parts.

6. The method for preparing a silicon oxynitride ceramic riser tube according to claim 1, characterized in that, The sintering aids include iron oxide, yttrium oxide, barium carbonate, and zinc oxide.

7. The method for preparing a silicon oxynitride ceramic riser tube according to claim 1, characterized in that, The silicon oxynitride powder is a self-propagating synthesized silicon oxynitride; the particle size D50 of the silicon oxynitride powder is less than 3 micrometers, the silicon oxynitride content is not less than 98%, and the silicon nitride content is not greater than 2%.

8. The method for preparing a silicon oxynitride ceramic riser tube according to claim 1, characterized in that, The calcined alumina is microcrystalline alumina calcined using a mineralizing agent at a calcination temperature below 1300°C; the alpha phase content of the microcrystalline alumina is not less than 98%; the original crystal size of the microcrystalline alumina is less than 100 nanometers; after calcination and pulverization, the particle size D50 of the microcrystalline alumina is controlled at 0.3-0.6 micrometers to meet the requirements of industrial-grade calcined alpha alumina.

9. The method for preparing a silicon oxynitride ceramic riser tube according to claim 3, characterized in that, The aluminum hydroxide is selected from ultrafine powder with a mesh size of 10,000 or higher, an alumina content of more than 64%, and an alkali content of less than 0.2%.

10. The method for preparing a silicon oxynitride ceramic riser tube according to claim 3, characterized in that, The lithium carbonate is selected from chemically pure lithium carbonate; S1 also includes a mineralizing agent, which is selected from chemically pure kaolin, basic magnesium carbonate, iron oxide, zinc oxide, barium carbonate, yttrium oxide, cerium oxide, and lanthanum oxide; the particle size of the mineralizing agent is controlled to be below 1 micrometer; the dispersant is ammonium polyacrylate 9300; and the binder is PVB.

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