A porous tantalum carbide prepared by molten salt method and its preparation method and application

Abstract

The present invention relates to a porous tantalum carbide prepared by a molten salt method and a preparation method and application thereof. In combination with molten salt and solid phase sintering methods, firstly, an excess of molten tantalum precursor powder (such as tantalum oxide, etc.) is used to metallize a porous carbon-based material (such as porous graphite) to form porous tantalum; then, tantalum atoms diffuse and penetrate into the lattice of carbon materials such as graphite by thermal motion to obtain a porous tantalum carbide with high purity and high porosity, but there will be a partial carbon core inside; finally, carbon removal is performed in a heat treatment furnace to obtain a porous tantalum carbide with high purity and high porosity. Compared with the prior art, the porous tantalum carbide manufactured by the present invention has stronger corrosion resistance, can be recycled, has a higher porosity and does not cause void clogging.

Description

Technical Field

[0001] The invention belongs to the technical field of porous tantalum carbide preparation, and relates to a porous tantalum carbide prepared by a molten salt method, and a preparation method and application thereof. Background Art

[0002] At present, wide bandgap semiconductors represented by silicon carbide are strategic industries facing the main battlefield of the economy and major national needs. At the same time, silicon carbide semiconductors are an industry with complex processes and extremely high requirements for equipment. Among them, the preparation of silicon carbide single crystals is the most basic and most important link in the entire industrial chain.

[0003] At present, the most commonly used method for SiC crystal growth is the physical vapor transport method (PVT method). The PVT method heats silicon carbide powder at a high temperature of more than 2300°C and a low pressure close to vacuum in a closed growth chamber by induction or resistance heating, causing it to sublime and produce reaction gases containing different gas phase components such as Si, Si2C, SiC2, etc., and a silicon carbide single crystal reaction source is produced through solid-gas reaction. A silicon carbide seed crystal (seed) is set at the top of the growth chamber, and the gas phase components transported to the seed crystal are atomically deposited on the surface of the seed crystal driven by the supersaturation of the gas phase components, and grow into a silicon carbide single crystal.

[0004] In the process of preparing silicon carbide single crystals, the materials inside the growth crucible are mainly graphite and porous graphite. Among them, porous graphite can improve the temperature uniformity of the raw material area, increase the axial temperature difference in the crucible, and weaken the recrystallization of the surface of the raw material. At the same time, it can improve the atmosphere flow and improve the stability of the entire growth process, thereby effectively improving the quality of crystal growth. However, as power devices further increase the quality requirements of silicon carbide single crystal substrates, the growth of substrate materials faces the challenges of "growing fast, thick, and long". During the growth of silicon carbide crystals, porous graphite is corroded by high-temperature silicon atmosphere, resulting in fine carbon particles that float into the growing silicon carbide crystals and produce defects such as microtubes and inclusions, thereby affecting the crystal quality and yield.

[0005] In order to solve this problem, researchers are currently using CVD, spraying, sintering and other methods to form a tantalum carbide coating on the graphite surface that does not react with silicon vapor to protect the graphite substrate. TaC (tantalum carbide) has excellent thermochemical stability and excellent physical properties, and has good chemical and mechanical compatibility with graphite. The preparation of TaC coating on the graphite surface can effectively enhance its anti-oxidation, anti-corrosion, wear resistance and mechanical properties.

[0006] However, the existing technology has the following two main problems: (1) Tantalum carbide (6.3×10 -6 / K) and graphite (3.0×10 -6 / K) have a large difference in thermal expansion coefficients, which will produce relatively large thermal stress in the high-temperature crystal growth furnace, causing the tantalum carbide coating to peel off and the protective performance to deteriorate. (2) The particle size of the tantalum carbide coating produced by CVD, spraying, sintering and other methods is difficult to control, and it is easy to block the gaps on the surface of the crystal growth furnace, reducing the porosity and affecting the flow of the atmosphere. Summary of the invention

[0007] The purpose of the present invention is to provide a method for preparing porous tantalum carbide using a molten salt method and its application. Compared with a traditional graphite substrate with a tantalum carbide coating, the prepared porous tantalum carbide has stronger corrosion resistance, can be recycled, has a higher porosity and does not cause void clogging.

[0008] The purpose of the present invention can be achieved by the following technical solutions:

[0009] In one aspect, the present invention provides a method for preparing porous tantalum carbide using a molten salt method, comprising the following steps:

[0010] S1. Wrapping and compacting an excess of tantalum precursor powder into a porous carbon-based material as a matrix, heating and heat preservation once, so that the tantalum precursor melts and infiltrates into the porous carbon-based material in a liquid state to react, thereby obtaining a crude porous tantalum carbide product;

[0011] S2. Covering the crude porous tantalum carbide product on a reaction vessel filled with silicon precursor material, heating and heat preservation for a second time, generating sublimated silicon vapor to react with residual carbon in the crude porous tantalum carbide product to obtain purified porous tantalum carbide, which is the final product.

[0012] Furthermore, the porous carbon-based material is porous graphite, porous carbon polymer, carbon nanotube composite material, carbon fiber polymer, preferably porous graphite.

[0013] Furthermore, in S1, the temperature of the first heating and heat preservation is above 1900°C, and the heat preservation time is above 30 minutes, preferably not more than 1.5 hours.

[0014] Furthermore, in S1, the system pressure during the primary heating and insulation process is maintained at 10 mbar or less.

[0015] Further, in S1, the tantalum precursor powder is one or more of tantalum oxide, tantalum chloride, tantalum fluoride, and fluorooxytantalate, and the addition amount of the tantalum precursor powder and the porous carbon-based material satisfies: the mass ratio of the tantalum precursor to the porous carbon-based material is between 3:1 and 10:1. Exemplarily, the tantalum precursor powder is preferably tantalum oxide (Ta2O5).

[0016] Furthermore, after the reaction in S1 is completed, the temperature and pressure of the system are restored to room temperature and normal pressure, and then the crude porous tantalum carbide product is taken out.

[0017] Furthermore, in S2, the secondary heating and insulation temperature is above 2200°C and the time is 6-12h.

[0018] Furthermore, in S2, the system pressure during the secondary heating and insulation process is maintained at 10 mbar or less.

[0019] Furthermore, in S2, during the secondary heating and heat preservation process, the porous tantalum carbide crude product is suspended above the silicon precursor material, and the suspension height is between 10-20 cm.

[0020] Further, the silicon precursor material is at least one of silicon carbide, silicon powder or organosilicon. More preferably, the organosilicon is tetraphenylsilicon, polysilane, polydimethylsiloxane, etc. Exemplarily, the present invention preferably uses silicon carbide powder.

[0021] Furthermore, in S1, the porous carbon-based material is pre-cleaned and dried.

[0022] In a second aspect, the present invention provides a porous tantalum carbide prepared by a molten salt method, wherein the internal porosity is >50%, the porous tantalum carbide is free of blockage, and the air passage is smooth.

[0023] In a third aspect, the present invention provides an application of porous tantalum carbide prepared by a molten salt method in the growth of silicon carbide crystals. In specific applications, it replaces the graphite or porous graphite inside the existing growth crucible to further improve the growth quality and yield of silicon carbide crystals.

[0024] The present invention is the first to combine molten salt and solid phase sintering methods. First, tantalum precursor materials such as excess molten tantalum oxide are used to metallize porous graphite to form porous tantalum. Then, tantalum atoms diffuse and penetrate into the lattice of carbon materials such as graphite by thermal motion. By controlling the sintering time, temperature, and precursor filler ratio, high-purity, high-porosity porous tantalum carbide with different weight gain and thickness can be obtained through a one-step reaction, but there will be a partial carbon core inside. Finally, decarbonization is carried out in a heat treatment furnace to obtain high-purity, high-porosity porous tantalum carbide.

[0025] The most widely used graphite-based CVD tantalum carbide has a coating that is easy to fall off due to the mismatch of thermal expansion coefficients, poor protection effect, and will cause pore blockage and decreased porosity. In contrast, the porous tantalum carbide product of the present invention has tantalum carbide materials on the surface and inside, and there is no problem of coating peeling. At the same time, the corrosion resistance and service life are greatly enhanced. In the application of silicon carbide crystal growth, there will be no corrosion and ashing, which will reduce the product quality. On the other hand, since the porous graphite substrate reacts and is consumed during the generation process of the porous tantalum carbide, the porous space structure it occupies is replaced by the generated tantalum, so that the porosity of the porous tantalum carbide does not change significantly, and there will be no pore blockage. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the reaction between porous graphite and tantalum oxide powder;

[0027] Figure 2 It is a schematic diagram of the reaction principle of the present invention;

[0028] Figure 3 A schematic diagram of the treatment for decarburization of porous tantalum carbide crude product;

[0029] Figure 4 This is an optical photograph of the porous tantalum carbide product obtained in Example 1;

[0030] Figure 5 This is the XRD diagram of the porous tantalum carbide product of Example 1;

[0031] Figure 6 This is an optical microscope surface morphology image of the porous tantalum carbide of Example 1;

[0032] Figure 7 This is a scanning electron microscope cross-sectional morphology image of the porous tantalum carbide of Example 1;

[0033] Figure 8 This is an optical microscope surface morphology image of the porous tantalum carbide of Example 2. DETAILED DESCRIPTION

[0034] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0035] In order to obtain a porous tantalum carbide product with stronger corrosion resistance, renewable use, higher porosity and no void clogging, in some embodiments, the present invention provides a method for preparing porous tantalum carbide using a molten salt method. By way of example, the porous carbon-based material is porous graphite, the tantalum precursor is tantalum oxide, and the silicon precursor material is silicon carbide. The specific process of the preparation method of the present invention is as follows.

[0036] First, the porous graphite substrate is cleaned and dried. The treated porous graphite is placed in a crucible, an excess of Ta2O5 powder is added, and the porous graphite is wrapped and compacted, such as Figure 1 At this point, ensure that the Ta2O5 powder is evenly distributed to enhance its contact with the porous graphite.

[0037] Then, the system pressure is reduced and the temperature is gradually increased at a constant power until the temperature reaches above 1900°C. In this high temperature environment, the insulation time needs to be maintained for more than 30 minutes to ensure the complete reaction. During this process, tantalum oxide will begin to melt and penetrate into the porous graphite in liquid form, and undergo redox reactions with carbon atoms in the graphite as follows:

[0038] 2Ta2O5(l)+5C(s)→4Ta(s)+5CO2(g).

[0039] The tantalum generated later forms a solid coating on the surface of the porous graphite and continues to diffuse with the carbon inside the porous graphite at high temperature: Ta(s)+C(s)→TaC(s), generating granular TaC particles. As the reaction proceeds, the particles become larger and the surface tantalum carbide gradually becomes thicker ( Figure 2 , the arrows in the figure indicate the mutual penetration direction of tantalum and carbon atoms), then as the tantalum precursor is consumed, the reaction rate slows down and eventually stops, generating a uniform, continuous porous tantalum carbide structure with a core of a residual porous graphite core and a tantalum carbide coating as the outer shell. By controlling the weight ratio of the added precursor and porous graphite, the weight gain of the porous graphite matrix can be controlled to meet the needs of different environments.

[0040] After the reaction is completed, the pressure is increased and the temperature is lowered to room temperature and atmospheric pressure, and the furnace is opened to take out the porous tantalum carbide.

[0041] The porous tantalum carbide is covered on a crucible containing silicon carbide particles, such as Figure 3 As shown, the pressure is reduced and the temperature is raised to above 2200°C at constant power and kept at this temperature for 10 hours. At this time, the sublimated silicon vapor and the carbon powder remaining in the porous tantalum carbide react as follows:

[0042] Si(g)+C(s)→SiC(g)

[0043] The silicon carbide generated by the reaction is transferred from the surface of the porous tantalum carbide to the low temperature zone for crystallization. The purpose of removing the carbon core in the porous tantalum carbide is achieved. Then the heating power is disconnected, the pressure is increased and the temperature is lowered to atmospheric pressure and room temperature, and the furnace is opened to take out the product.

[0044] The purified porous tantalum carbide is washed and dried to obtain the final product.

[0045] The above-mentioned embodiments are further described below in conjunction with specific examples.

[0046] In the following examples, porous graphite was purchased from Ningbo Hongxin New Material Technology Co., Ltd., and tantalum oxide was purchased from Hunan Jinxin Technology Co., Ltd.

[0047] The rest of the raw materials or processing techniques, unless otherwise specified, are conventional commercially available raw materials or conventional processing techniques in the art.

[0048] Example 1

[0049] The method for preparing porous tantalum carbide by using a molten salt method provided in this embodiment comprises the following steps:

[0050] S1. Clean and dry the porous graphite substrate (10 mm in diameter, 3 mm in thickness, 100 g), place it in a crucible, add 500 g of tantalum oxide (Ta2O5) powder, and wrap and compact the porous graphite as the substrate.

[0051] S2. Reduce the pressure of the system to 10 mbar, and gradually increase the temperature to 2000 °C at a constant power, and keep it at this temperature for 40 minutes, so that the tantalum oxide melts and infiltrates into the porous graphite in liquid state to react.

[0052] S3. After the reaction is completed, the pressure is increased and the temperature is lowered to reach room temperature and atmospheric pressure, and the furnace is opened to take out the crude porous tantalum carbide product.

[0053] S4. Cover the crude porous tantalum carbide product on the crucible containing silicon carbide particles, ensuring that the crude porous tantalum carbide product is suspended about 12 cm above the silicon carbide particles, reduce the pressure to 10 mbar, and increase the temperature to 2200 °C at a constant power and keep it at this temperature for 10 h.

[0054] S5. After the reaction is completed, stop heating and increase the pressure and cool down to atmospheric pressure and room temperature. Open the furnace to take out the purified porous tantalum carbide, then wash and dry it to obtain the final product.

[0055] The porous tantalum carbide product prepared above is as follows Figure 4 As shown, the surface is also subjected to XDR detection, such as Figure 5 As shown in the figure, it is consistent with the standard XRD spectrum of tantalum carbide, proving that porous graphite has been converted into tantalum carbide. Figure 6 As shown (where Figure 6 (a) and (b) are morphologies with different resolutions respectively). Golden tantalum carbide particles can be observed on the surface. The measurement shows that the internal porosity of the porous tantalum carbide is >50%, there is no internal blockage, and the airway is smooth.

[0056] Its corrosion resistance has been verified by silicon carbide crystal growth experiments. Compared with disposable porous graphite substrates, this product has no carbon core and is reusable. During the crystal growth process, porous graphite will not be corroded by silicon carbide vapor, and the phenomenon of ash entering the wafer to reduce quality will not occur. The dislocations in the silicon carbide grown using this embodiment are reduced by more than 90% compared to porous graphite. Compared with the product that directly coats tantalum carbide on a porous graphite substrate by CVD, the porous tantalum carbide product of this embodiment has no carbon core, so there is no problem of cracking of the coating due to thermal expansion mismatch during the heating and cooling process. Therefore, ordinary CVD-coated tantalum carbide will deform, peel off and crack after a single furnace use, while this embodiment can still keep the surface intact without cracks and obvious deformation after the furnace is used more than 5 times.

[0057] Comparative Example 1:

[0058] Compared with Example 1, most of the above are the same, except that the reaction with silicon carbide is omitted, that is, a porous silicon carbide crude product containing a carbon core is obtained. After crystal growth, due to the mismatch between the thermal expansion coefficients of the surface tantalum carbide and the carbon core, obvious cracking and peeling occur on the surface, and unreacted carbon (such as Figure 8 As shown, Figure 8 (a) and (b) are surface morphology images with different resolutions respectively), resulting in poor protection effect and short life.

[0059] Comparative Example 2:

[0060] Compared with Example 1, in step 2, the holding time is reduced to 20 minutes, and the rest is the same. There is blue-grey tantalum oxide residue on the surface of the obtained crude porous tantalum carbide.

[0061] Example 2

[0062] Compared with Example 1, the holding time in step 2 is increased to 1 hour, and the rest is the same, and a normal coarse porous tantalum carbide product can be obtained.

[0063] Comparative Example 3:

[0064] Compared with Example 1, the holding time in step 2 was increased to 2 hours, and the rest was the same, and the obtained coarse porous tantalum carbide product was slightly deformed.

[0065] Combining the above-mentioned Example 2 and Comparative Examples 2-3, it can be seen that the reaction time is a key condition for the formation of crude porous silicon carbide. If the reaction time is short, the tantalum precursor may not react completely, and if the reaction time is long, deformation is likely to occur.

[0066] It should be pointed out that, in the present invention, different tantalum precursors can be replaced while ensuring a stable molar ratio of tantalum element to carbon element, and a porous tantalum carbide crude product can also be obtained, as shown in the following specific examples.

[0067] Example 3

[0068] Compared with Example 1, in the first step, 500 g of tantalum oxide was replaced by 800 g of tantalum chloride, and the rest was the same.

[0069] Example 4

[0070] Compared with Example 1, in the first step, 500 g of tantalum oxide is replaced by 600 g of tantalum fluoride, and the rest are the same.

[0071] In addition, the above-mentioned embodiment 1 uses silicon carbide to remove carbon by secondary heating. The present invention can also use different silicon sources to create a silicon atmosphere and remove the carbon core of porous tantalum carbide. Specific examples are as follows.

[0072] Example 5

[0073] Compared with Example 1, in the fourth step, the silicon carbide particles are replaced with silicon powder, and the rest are the same.

[0074] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for preparing porous tantalum carbide using a molten salt method, characterized in that: The following steps are involved: S1. Wrapping and compacting an excess of tantalum precursor powder into a porous carbon-based material as a matrix, heating and heat preservation once, so that the tantalum precursor melts and infiltrates into the porous carbon-based material in a liquid state to react, thereby obtaining a crude porous tantalum carbide product; S2, covering the crude porous tantalum carbide product on a reaction container containing a silicon precursor material, heating and heat preservation for a second time, generating sublimated silicon vapor to react with residual carbon in the crude porous tantalum carbide product to obtain purified porous tantalum carbide, which is the final product; In S2, the secondary heating and insulation temperature is above 2200°C and the time is 6-12h; The system pressure during the secondary heating and insulation process is below 10 mbar; During the secondary heating and heat preservation process, the porous tantalum carbide crude product is suspended above the silicon precursor material, and the suspension height is between 10-20 cm; The silicon precursor material is at least one of silicon carbide, silicon powder or organic silicon; The internal porosity of the porous tantalum carbide is >50%, there is no blockage, and the airway is smooth; In S1, the primary heating and insulation temperature is above 1900°C, and the insulation time is 30min-60min.

2. The method for preparing porous tantalum carbide by molten salt method according to claim 1, characterized in that: The porous carbon-based material is porous graphite, porous carbon polymer, carbon nanotube composite material, or carbon fiber polymer.

3. The method for preparing porous tantalum carbide by molten salt method according to claim 1, characterized in that: In S1, the system pressure during the primary heating and insulation process is below 10 mbar.

4. The method for preparing porous tantalum carbide by molten salt method according to claim 1, characterized in that: In S1, the tantalum precursor powder is one or more of tantalum oxide, tantalum chloride, tantalum fluoride, and tantalum fluoride salt, and the addition ratio of the tantalum precursor powder to the porous carbon-based material satisfies: the mass ratio of the tantalum precursor powder to the porous carbon-based material is between (3~10):

1.

5. The method for preparing porous tantalum carbide by molten salt method according to claim 1, characterized in that: After the reaction in S1 is completed, the temperature and pressure of the system are restored to room temperature and normal pressure, and then the crude porous tantalum carbide product is taken out.

6. The method for preparing porous tantalum carbide by molten salt method according to claim 1, characterized in that: The organosilicon is tetraphenylsilane, polysilane or polydimethylsiloxane.

7. A porous tantalum carbide prepared by a molten salt method, characterized in that: The product is prepared based on the method according to any one of claims 1 to 6, and has an internal porosity of >50%, no blockage, and smooth airway.

8. Use of the porous tantalum carbide prepared by the molten salt method as claimed in claim 7 in the growth of silicon carbide crystals.

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