Simple method for preparing conductive aluminum oxide ceramic

Abstract

The invention belongs to the field of conductive aluminum oxide ceramic preparation, and particularly relates to a treatment method for performing carbon deposition infiltration on the surface of aluminum oxide ceramic to enable the aluminum oxide ceramic to have certain conductivity. According to the invention, wheat starch and corn starch are used as main raw materials and are converted into a volatile carbon source through a high-temperature carbonization process under the protection of inert gas; and placing the carbon source and aluminum oxide ceramic pre-subjected to conductive treatment in a high-temperature furnace protected by inert gas, and performing deposition carburizing treatment on the surface of the aluminum oxide ceramic at 800-900 DEG C to obtain the aluminum oxide conductive ceramic with certain conductivity. The method is low in raw material cost, low in sintering temperature of the related process, low in equipment investment, good in surface carburization uniformity of the aluminum oxide ceramic and stable in property.

Description

Technical Field

[0001] The invention belongs to the field of conductive ceramic preparation, and particularly relates to a treatment method for carburizing the surface of alumina ceramics to make them have better conductivity. Background Art

[0002] Conductive ceramics are a class of ceramic materials with remarkable electrical conductivity. Compared to metals, they exhibit higher resistance to high temperatures. Furthermore, they are far more stable against chemical corrosion, such as acid and alkali, than ordinary metals. These properties hold great potential for applications in new energy, high-end manufacturing, and electronic miniaturization. Common conductive ceramics include oxide-based conductive ceramics (such as ZnO, SnO2, and In2O3), non-oxide-based conductive ceramics (such as SiC, TiN, and LaB6), and composite conductive ceramics (such as Ag / SnO2 and C / SiC). The conductivity of these ceramics is typically based on bulk material conductivity. This involves doping the constituent materials' lattices to manipulate the band structure, altering the carrier concentration in the valence or conduction bands and imparting a certain degree of conductivity. However, lattice structure manipulation is complex, requiring demanding material synthesis conditions and requiring high equipment investment and requirements. Furthermore, achieving excellent conductivity through doping is difficult for wide-bandgap compounds (such as ZnO and SiC). Consequently, the products are typically expensive, hindering large-scale application.

[0003] Alumina ceramics offer advantages such as high hardness, high-temperature resistance, and corrosion resistance. Thanks to the abundant reserves of alumina in the Earth's crust, ease of mining, and processing, they are inexpensive and widely used in production and daily life. If alumina ceramics were to become conductive, they could potentially catalyze new technologies in fields ranging from energy and aerospace to bioelectronics. The core value of conductive alumina ceramics lies in their integrated structural and functional capabilities, enabling them to serve as load-bearing components while also actively participating in energy transmission and signal regulation. For example, as submarine hulls, they can withstand deep-sea pressure while also providing real-time monitoring of the submarine's surface status through conductive networks. Alternatively, conductive alumina ceramics can be used to construct Faraday cages, protecting precision electronic equipment from EMP interference from nuclear explosions while also providing mechanical support. Unfortunately, alumina itself is not conductive. Compared to the conductive ceramics mentioned above, the band gap of α-alumina crystals is as high as 7.3–8.8 eV, significantly higher than the band gap of SiC crystals (3.26 eV) and even significantly higher than that of diamond (5.47 eV). Therefore, it is difficult to impart conductivity through doping or other methods that affect the band structure.

[0004] Since it is difficult to achieve the goal of making the ceramic body conductive by regulating the energy band of alumina crystals, other solutions can be used to achieve the goal of making alumina ceramics conductive. Solution one is to construct a continuous conductive grid inside the alumina ceramic material to give it conductivity, such as the method adopted by patent CN118812248A to form a zirconium-nickel-lanthanum-carbon crystal structure conductive path and the method adopted by patent CN119330689A to form a conductive network of CuAlO2 phase. The advantage of this solution is that the networked conductive material is uniformly present inside the alumina ceramic material, and damage to the surface of the material such as wear or ablation will basically not affect the conductivity of the body. However, the disadvantages are also obvious. For example, the contradiction between the conductivity and mechanical properties of the alumina ceramic constructed in this way is difficult to resolve. That is, if the proportion of the conductive phase is low, the conductivity of the alumina ceramic is poor; and if the proportion of the conductive phase is high, the mechanical properties such as hardness and strength of the alumina ceramic deteriorate. Furthermore, the structural characteristics of this conductive network distributed within the alumina ceramic body dictate that the alumina ceramic body and the conductive network material must be fired together. This is especially true given the high firing temperatures typically associated with alumina ceramics, which are difficult for common conductive materials to withstand. Existing papers and patents indicate that the materials required to construct the conductive network within alumina ceramics are complex, resulting in high production costs, complex processes, and a high price tag. Therefore, without breakthroughs in new technologies or materials, large-scale application is unlikely. A second approach involves depositing a conductive coating on the surface of the alumina ceramic to impart conductivity. This approach offers the advantage of not affecting the inherent mechanical properties of the alumina ceramic itself or its established production process. However, its disadvantages are that the coating is susceptible to wear, oxidation, and corrosion, and the choice of coating material presents a dilemma regarding cost and process. Ordinary metal coatings are impractical due to their poor resistance to oxidation and corrosion, while precious metals are significantly cost-prohibitive and should be avoided unless necessary. While conductive oxides (such as ITO) are inherently inexpensive, their deposition processes, whether chemical vapor deposition or magnetron sputtering, are expensive and difficult to scale up. If there is a technical solution that can meet the requirements of corrosion resistance and oxidation resistance of deposited coatings while satisfying the simplicity of process, then this solution will surely be widely used.

[0005] This patent addresses the above-mentioned technical difficulties and pain points. It uses wheat starch and corn starch as raw materials, and forms a carbon source with volatile properties at high temperatures by carbonizing them at high temperatures. Under the protection of inert gases such as argon or nitrogen, it is placed in a high-temperature furnace with alumina ceramics that have been pre-treated for conductive properties. The carbon source evaporates at high temperature and deposits and penetrates into the surface of the alumina ceramics, giving the alumina ceramic material a certain degree of conductivity. At the same time, it solves the problems of anti-oxidation, anti-corrosion, low cost and simplified process, making large-scale production and application of conductive alumina ceramics possible. Summary of the Invention

[0006] This method uses wheat starch and corn starch as raw materials, forms a volatile carbon source through high-temperature carbonization, and then deposits and infiltrates the alumina ceramic surface at high temperatures in a protective gas atmosphere, imparting a certain degree of electrical conductivity to the alumina ceramic. This method offers low raw material costs, a simple process, minimal equipment investment, and great potential for application.

[0007] The present invention provides a simple method for preparing conductive alumina ceramics, characterized by comprising the following steps: (1) Wheat starch and corn starch are used as the main raw materials and are heat treated at 800-900°C for 1-2 hours in a quartz tube furnace under the protection of an inert gas such as nitrogen or argon to convert them into a carbon source with volatility at high temperature; (2) The carbon source and the pre-conductively treated alumina ceramic are placed in a quartz tube furnace under nitrogen or argon protection, carburized at 800-900°C, and cooled to obtain the conductive alumina ceramic.

[0008] Furthermore, the steps for preparing the high temperature volatile carbon source are: Wheat starch or corn starch, or a mixture of the two in varying proportions, is placed in a high-temperature crucible with a porous lid. Heat-treated in a tube furnace at 800-900°C for 1-2 hours under argon or nitrogen. For large quantities of starch, the heat treatment time can be extended, preferably to no more than 4 hours. During the heat treatment, protective gas needs to be continuously introduced and exhausted from the other end of the furnace tube to remove complex components such as water vapor and smoke generated during the starch thermal decomposition.

[0009] Furthermore, the conductive treatment process steps of alumina ceramics are as follows: The quartz boat containing the carbon source prepared in step (1) and the alumina ceramic pre-treated for conductivity are placed in a tube furnace. The volatile carbon source and the alumina ceramic are controlled to be close but not in contact. The temperature is raised to 800-900°C under the protection of argon or nitrogen. The carbon particles volatilized by the carbon source at high temperature are deposited on the surface of the alumina ceramic and penetrated at high temperature, making the surface of the alumina ceramic have a certain degree of conductivity. It should be noted that the protective gas introduced in this step can, on the one hand, protect the volatilized carbon from being oxidized, and on the other hand, allow the carbon particles volatilized by the carbon source to flow with the air flow and evenly cover the surface of the alumina ceramic. The deposition time is appropriately adjusted according to the amount of carbon source, the size of the alumina ceramic, and the requirements for its conductivity.

[0010] The beneficial effects of the present invention are: 1. Using wheat starch and corn starch as the main raw materials, a simple carbonization process is used to obtain a high-temperature volatile carbon source, and a simple diffusion deposition process is used to give the alumina ceramics electrical conductivity. The process is simple, with low equipment requirements, low investment, and low production cost. 2. It does not affect the mechanical properties of alumina ceramics and their original production process, does not increase the cost of additional equipment and debugging, and has good process adaptability; 3. The conductivity of the alumina ceramic surface can be achieved by controlling the density of surface carbon particles through deposition time, which is flexible and adjustable. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 The following are photos of alumina ceramic samples with and without carbon deposition; Figure 2 This is a scanning electron microscope image of the alumina ceramic surface without carbon deposition; Figure 3 This is a scanning electron microscope image of the carbon-deposited alumina ceramic surface. DETAILED DESCRIPTION

[0012] The present invention will be further described below with reference to the embodiments. Example 1

[0013] The present invention provides a simple method for preparing conductive alumina ceramics, comprising the following steps: (1) Preparation of high-temperature volatile carbon source: Weigh 20g of wheat starch into an alumina crucible, cover with a porous lid, and place the crucible in the center of a tube furnace. Flow nitrogen as a protective gas at a rate of 2 L / min. Set the heating program as follows: From room temperature, increase the temperature to 830°C over 80 minutes, then hold at 830°C for 60 minutes, then cool naturally. After cooling, remove the crucible to obtain a high-temperature volatile carbon source.

[0014] (2) Carbon deposition on the surface of alumina ceramics: The carbon source prepared in step (1) was divided into two quartz boats. The alumina ceramic (here we used self-fired alumina ceramic discs) that had been pre-treated with conductive carbon was placed in another quartz boat. Care was taken to ensure that the deposited surface did not fit tightly against the quartz boat. All three quartz boats were placed in a tube furnace, with the quartz boat containing the alumina ceramic placed between the two carbon sources. Nitrogen was introduced at a flow rate of 0.2 L / min. A heating program was set: from room temperature to 880°C over 90 minutes, the temperature was maintained at 880°C for 60 minutes, and then the furnace was stopped and allowed to cool naturally. After cooling, the quartz boat was removed to obtain the conductive alumina ceramic.

[0015] Performance test analysis: 1. Resistivity test: The resistance of the obtained conductive alumina ceramics was tested using a KDY-1 four-probe resistivity meter.

[0016] 2. Acid and alkali corrosion resistance test: The obtained alumina ceramics were immersed in 98% concentrated sulfuric acid and 6 mol / L sodium hydroxide for 60 min respectively, and the resistance before and after immersion was compared.

[0017] 3. High temperature oxidation test: The prepared conductive alumina ceramics were heated in an air atmosphere at 100°C, 200°C, 300°C, 400°C, 500°C, and 600°C for 1 hour, and the resistance before and after treatment was compared.

[0018] Test results: Table 1 shows the square resistance test data of the conductive alumina ceramic obtained in Example 1; Table 2 shows the change in resistance of the sample of Example 1 before and after acid and alkali immersion; Table 3 shows the change in resistance of the sample before and after various temperature treatments.

[0019] Table 1 Resistivity test data of alumina ceramics obtained in Example 1

[0020] Table 2 Acid and alkali tolerance test data of alumina ceramics obtained in Example 1

[0021] Table 3 High temperature oxidation resistance test data of alumina ceramics obtained in Example 1

[0022] As shown in Table 1, the sheet resistance of the six alumina ceramic sheet samples prepared in Example 1 is basically around 400Ω / □. Although it is far inferior to the conductivity of good electronic conductors such as metals, for ceramics, it does have a certain conductivity.

[0023] Table 2 shows the resistance changes after immersion in 98% concentrated sulfuric acid for one hour and then in 6 mol / L NaOH for one hour. This indicates that concentrated sulfuric acid has little effect on the conductive layer of the alumina ceramic. (The fluctuations in the resistance of the alumina ceramic cubes during the sulfuric acid test shown in the table are due to differences in the four-probe pressure measurement position during the resistance test, not to any effect of sulfuric acid on their conductivity.) However, after immersion in 6 mol / L NaOH, all three samples showed a significant increase in resistance, likely due to corrosion of the alumina by the high concentration of alkaline solution. However, the samples still maintained a certain degree of conductivity and did not fail.

[0024] Table 3 shows the results of thermal oxidation resistance tests on conductive alumina ceramic samples in air. As can be seen from Table 3, the conductivity of the samples remained largely unaffected up to 400°C. Note: The slight fluctuations in the data before 400°C are due to differences in the four-probe test position, not the effects of the heat treatment. At 500°C, the carbon on the surface of the alumina ceramic begins to oxidize, and the resistance of the samples begins to increase, but the samples do not lose their conductivity. After the heat treatment temperature reaches 600°C, all samples lose their conductivity. Example 2

[0025] The present invention provides a simple method for preparing conductive alumina ceramics, comprising the following steps: (1) Preparation of high-temperature volatile carbon source: Weigh 100g of corn starch into a crucible, cover with a perforated lid, and place in a quartz tube furnace. Flow argon shielding gas at a flow rate of 4 L / min. Set the temperature program: increase from 30°C to 900°C over 90 minutes, then hold at 900°C for 120 minutes before cooling naturally. After cooling, remove the crucible to obtain a high-temperature volatile carbon source.

[0026] (2) Carbon deposition on the surface of alumina ceramics: The carbon source prepared in step (1) was divided into two quartz boats. The alumina ceramic pre-deposited with conductive carbon was placed in another quartz boat. Care was taken to ensure that the deposited surface did not fit tightly against the quartz boat. All three quartz boats were placed in a quartz tube furnace, with the quartz boat containing the alumina ceramic placed between the two carbon sources. Nitrogen was introduced at a flow rate of 0.2 L / min. A heating program was set: 30°C was raised to 850°C over 90 minutes, then maintained at 850°C for 120 minutes, and then the furnace was stopped and cooled naturally. After cooling, the quartz boat was removed to obtain the conductive alumina ceramic.

[0027] Performance test analysis: 1. Resistivity test: The resistance of the obtained conductive alumina ceramics was tested using a KDY-1 four-probe resistivity meter.

[0028] 2. Acid and alkali corrosion resistance test: The obtained alumina ceramics were immersed in 98% concentrated sulfuric acid and 6 mol / L sodium hydroxide for 60 min respectively, and the resistance before and after immersion was compared.

[0029] 3. High temperature oxidation test: The prepared conductive alumina ceramics were heated in an air atmosphere at 100°C, 200°C, 300°C, 400°C, 500°C, and 600°C for 1 hour, and the resistance before and after treatment was compared.

[0030] Test results: Table 4 shows the square resistance test data of the conductive alumina ceramic obtained in Example 2; Table 5 shows the change in resistance of the sample of Example 2 before and after acid and alkali immersion; Table 6 shows the change in resistance of the sample before and after various temperature treatments.

[0031] Table 4 Resistivity test data of alumina ceramics obtained in Example 2

[0032] Table 5 Acid and alkali tolerance test data of alumina ceramics obtained in Example 2

[0033] Table 6 High temperature oxidation resistance test data of alumina ceramics obtained in Example 2

[0034] The data shown in Tables 4 to 6 are the square resistance and acid, alkali and high-temperature oxidation resistance of the conductive alumina ceramics prepared in Example 2. Comparing Table 4 with Table 1, it can be seen that the conductive alumina ceramics prepared in Example 2 have lower resistance than those in Example 1. This is because the amount of carbon source used in Example 2 is significantly greater than that in Example 1, and the carbon deposition time is longer. Therefore, the density of carbon particles deposited on the surface of the alumina ceramics will be higher, and its conductivity will inevitably be better, indicating that the conductive properties of the conductive ceramics prepared by this method can be regulated by the carbon source and the time of carbon deposition. In addition, comparing Table 5 with Table 2, it can be seen that with the increase in carbon deposition density, the resistance of the conductive alumina ceramics prepared in Example 2 to sodium hydroxide is significantly improved. However, the data in Table 6 show that the increase in carbon deposition density still cannot withstand high-temperature oxidation above 500°C, and its high-temperature oxidation resistance is only 400°C, the same as in Case 1.

[0035] From the above analysis, it can be seen that the simple method for preparing conductive alumina ceramics proposed in this patent enables alumina ceramics to have a certain conductivity through a simple process, and has good resistance to acid and alkali corrosion and thermal oxidation, and has certain application potential.

[0036] By adjusting the process parameters according to the present invention, conductive alumina ceramics can be easily prepared and exhibit similar performance to the above-mentioned embodiments. The above description of the present invention is illustrative. It should be noted that any simple modification without departing from the core of the present invention, as well as equivalent substitutions that can be made by those skilled in the art without inventive effort, fall within the scope of protection of the present invention.

Claims

1. A simple method for preparing conductive alumina ceramics, characterized in that: The following steps are involved: (1) Wheat starch and corn starch are used as the main raw materials and are heat treated at 800-900°C for 1-2 hours in a quartz tube furnace under the protection of an inert gas such as nitrogen or argon to convert them into a carbon source with volatility at high temperature; (2) The carbon source and the pre-conductively treated alumina ceramic are placed in a quartz tube furnace under nitrogen or argon protection, carburized at 800-900°C, and cooled to obtain the conductive alumina ceramic.

2. A simple method for preparing conductive alumina ceramics according to claim 1, characterized in that: The carbonization raw materials in step (1) are wheat starch and corn starch, and the carbonization temperature is 800-900°C. It needs to be carried out under the protection of an inert gas such as flowing nitrogen or argon.

3. A simple method for preparing conductive alumina ceramics according to claim 1, characterized in that The carburizing temperature of step (2) is 800-900°C. The carbon source and the alumina ceramic are close to each other but not in contact. Carbon particles volatilized from the carbon source at high temperature are deposited on the surface of the alumina ceramic and penetrate into the alumina ceramic at high temperature, so that the surface of the alumina ceramic has a certain conductivity.

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