Method for producing a precursor composition for producing a silicon carbide-containing material, precursor composition, and silicon carbide-containing material

Abstract

The invention relates to a method for producing a precursor composition for producing a silicon carbide-containing material, the method comprising: – mixing carbohydrates and nitrogen-free precipitated silicon dioxide.

Description

[0001] Process for producing a precursor composition for producing a silicon carbide-containing material, precursor composition and silicon carbide-containing material

[0002] The invention relates to a method for producing a precursor composition for producing a silicon carbide-containing material, a precursor composition, the use of a precursor composition, a method for producing a silicon carbide-containing material, and a silicon carbide-containing material. Furthermore, a method for producing precipitated silicon dioxide is specified.

[0003] Silicon carbide can be used, for example, for the manufacture of electrodes. To produce silicon carbide, a silicon-containing component and a carbon-containing component can be reacted with each other. For example, silicon carbide can be produced by reacting silicon dioxide with a carbon source comprising a carbohydrate at elevated temperature, as described in EP 2 334 597 A1.

[0004] Furthermore, WO 2008 / 061521 A2 discloses a method for producing an object at least partially with a silicon carbide structure from a blank made of a carbon-containing material. In a first step, the object is produced from the carbon-containing material essentially according to its desired final shape and / or final dimensions and is at least partially coated with carbon-rich silicon dioxide granules. Annealing then takes place in a protective gas atmosphere, with the silicon dioxide granules releasing silicon carbide-containing gas, which penetrates the object and partially or completely converts the carbon-containing material into silicon carbide.

[0005] For various applications of silicon carbide, e.g. in the semiconductor sector or as a catalyst, any doping would be desirable. However, silicon carbides produced using state-of-the-art technology contain nitrogen impurities or nitrogen contamination. The nitrogen contamination of silicon carbide (SiC) means that a pure, i.e. exclusively p-doping of the semiconductor cannot be achieved. The omnipresent nitrogen contamination always leads to n-type conduction, which can only be compensated for by p-doping, e.g. with Al and / or B, or doping with metals, e.g. vanadium. The subsequent removal of nitrogen, e.g. during crystallization to SiC, is difficult and involves great technical, energetic and financial expenditure. Removal of nitrogen after crystallization is no longer possible.Previously used processes rely on highly contaminated SiC, which was produced using the Acheson process. This material requires considerable effort to purify, but nitrogen still remains in the SiC.

[0006] Generally, nitrogen from the air is adsorbed onto silicon dioxide. If this occurs at room temperature, the nitrogen can be removed even at low temperatures. However, if nitrogen is incorporated into silicon dioxide at high temperatures, e.g., 1500–2500 °C, silicon nitrides are formed, which are stable at high temperatures. These nitrides can only be converted back into nitrogen-free compounds with considerable energy and time expenditure.

[0007] Against this background, the object of the invention is to provide possibilities with which the above-mentioned problems can be avoided as far as possible.

[0008] This problem is solved by the subject matter of the independent claims. The dependent claims concern embodiments.

[0009] A basic idea of ​​the present invention is to first produce a precursor that has the lowest possible nitrogen content and is ideally nitrogen-free. In other words, highly pure reactants are used. Nitrogen-free precipitated silica can preferably be used as the SiO2 source for the carbothermal reaction to SiC.

[0010] To prevent the incorporation of nitrogen from the air, precipitated silica or precipitated silicon dioxide can be produced in a nitrogen-free dispersion. Until now, there has been neither consideration nor necessity for producing precipitated silicon dioxide without nitrogen, as precipitated silicon dioxide is primarily used in areas where high purity is required but nitrogen-freeness is unimportant, such as in the food industry, pharmaceuticals, etc. Accordingly, nitrogen-containing dispersants, such as ammonia-containing dispersants, have often been used to produce precipitated silicon dioxide. The nitrogen-free precursor enables p-doping of SiC, which opens up a significantly wider range of electronic components made of SiC, similar to silicon. Any desired doping level can be achieved using the precursor.For example, catalytic and photocatalytic applications become possible.

[0011] It is also possible to produce electrically insulating SiC, i.e., undoped SiC. This differs from SiC with "counter" doping, e.g., with metals, as described in the introduction, in that electronically and optically disruptive side effects, such as the formation of undesirable recombination centers for charge carriers, can be avoided.

[0012] A first aspect of the invention relates to a process for producing a precursor composition for the production of a silicon carbide-containing material. The proposed process comprises the following steps: mixing carbohydrates and nitrogen-free precipitated silicon dioxide or nitrogen-free precipitated silica.

[0013] The precursor composition is characterized by an intensive mixture of carbohydrates and silicon dioxide. In other words, silicon dioxide and carbohydrate molecules are distributed as evenly as possible and are preferably close to each other on a molecular scale.

[0014] Precipitated silica, also known as precipitated silica, is understood here as a synthetically produced colloidal material comprising amorphous silicon dioxide particles aggregated into larger units, which was produced by precipitation from a silicate-containing solution. Silicon dioxide produced by the Stöber synthesis is also encompassed by the term "precipitated silicon dioxide." Stöber synthesis can be used to produce monodisperse particles with definable diameters in the nm to pm range, which subsequently enables the production of monodisperse silicon carbide particles in the aforementioned size range.

[0015] Nitrogen-free means that the nitrogen content is less than 1 ppm, for example, less than 0.1 ppm or less than 0.01 ppm. A carbohydrate is defined here as a chemical compound with the general molecular formula C n H2nOn with n > 3. Within the scope of the present invention, both a specific carbohydrate and a mixture of different carbohydrates can be used.

[0016] For example, sugar, e.g., a mixture of glucose and fructose, such as invert sugar, a mixture of glucose and fructose produced by breaking down sucrose, can be used as a carbohydrate. Glucose and fructose can, for example, be present in a molar ratio of 1:1.

[0017] According to various embodiments, the carbohydrates can be used dissolved in a solvent, e.g., water. For example, an aqueous invert sugar solution can be mixed with nitrogen-free precipitated silicon dioxide.

[0018] According to further embodiments, the process may comprise drying the mixture of carbohydrates and precipitated silicon dioxide.

[0019] Drying allows solvents, such as the water from the aqueous sugar solution, to be removed, so that a solid, e.g. powdered, precursor composition can be obtained.

[0020] According to further embodiments, the mixture can be heated to a temperature between 200 °C and 500 °C, preferably between 300 °C and 400 °C. The heating can be carried out in temperature stages.

[0021] Heating can help the mixture dry faster. Depending on the selected temperature, partial pyrolysis can also occur, allowing the initial bonds between carbon and silicon to form.

[0022] Precipitated silicon dioxide can be dispersed in a nitrogen-free dispersant, e.g., water, as nitrogen-free precipitated silicon dioxide. To ensure nitrogen-free precipitated silicon dioxide, precipitation can be carried out using a base that does not contain nitrogen, such as sodium or potassium hydroxide. Bases are compounds that are capable of forming hydroxide ions (OH) in aqueous solution, thereby increasing the pH of a solution. Accordingly, the process can also involve producing nitrogen-free precipitated silicon dioxide in a nitrogen-free dispersant.

[0023] A further aspect of the invention relates to a precursor composition comprising a mixture containing carbohydrates and nitrogen-free precipitated silicon dioxide. The precursor composition can be produced, for example, by one of the methods described above.

[0024] The carbohydrate can be, for example, sugar, e.g. a mixture of glucose and fructose, such as invert sugar.

[0025] A further aspect of the invention relates to the use of such a precursor composition in a 3D printing process or in a coating process. For this purpose, the precursor composition can be dispersed in a dispersant or dissolved in a solvent.

[0026] A further aspect of the invention relates to the use of such a precursor composition for producing silicon carbide.

[0027] Another aspect of the invention relates to a method for producing a silicon carbide-containing material. The method comprises heating the precursor composition until a pyrolysis temperature is reached. In other words, the precursor composition is pyrolyzed.

[0028] The pyrolysis temperature can be in a range between 900 °C and 1200 °C, for example between 1000 °C and 1100 °C. The heating of the precursor composition can be carried out in stages. The pyrolysis can be carried out, for example, in a vacuum, e.g., at a pressure between 10' 5 and 10' 3 bar, or in an argon atmosphere.

[0029] The process enables the production of a silicon carbide-containing material with a high degree of purity. This means, for example, that virtually pure silicon carbide can be produced. The proportion of all residual impurities, e.g., N, P, B, Al, and other metals, can preferably be less than 1 ppm, more preferably less than 0.1 ppm. Optionally, the process can include introducing a reaction gas into the precursor composition. The term reaction gas refers to a gaseous composition that can react chemically with the precursor.

[0030] The reaction gas can be introduced before, after, and / or simultaneously with heating the precursor composition. The reaction gas can cause a further reduction in the nitrogen content.

[0031] The reaction gas can, for example, be an argon-hydrogen mixture, with the hydrogen volume fraction in the argon-hydrogen mixture being between 1% and 20%, for example between 5% and 10%. Preferably, both argon and hydrogen can be used as high-purity gases, e.g., with purity grade 6.0 or 7.0, i.e., a purity of >99.9999% or >99.99999%. This can further improve the quality of the resulting silicon carbide.

[0032] A further aspect of the invention relates to a silicon carbide-containing material produced by a method as described above.

[0033] The silicon carbide-containing material can, in particular, have a high degree of purity, i.e., consist of virtually pure silicon carbide. The proportion of all residual impurities, e.g., N, P, B, Al, and other metals, can preferably be less than 1 ppm, more preferably less than 0.1 ppm.

[0034] Further aspects of the invention relate to a process for producing precipitated silicon dioxide and nitrogen-free precipitated silicon dioxide.

[0035] The invention will be described in more detail below, particularly with regard to preferred or advantageous aspects, using an exemplary embodiment, without, however, being limited thereby.

[0036] A horizontal mixer is primarily used to produce the precursor composition. This ensures uniform mixing of the reactants. A chopper is also integrated into the mixer, which introduces shear energy into the mixture at specific times. The exhaust gases from the mixer must be filtered and cooled. This requires a filter and a condenser. An extraction system ensures the constant flow of the process gases. Process parameters such as pressures and temperatures of the heaters and the product temperature must be continuously monitored. A batch test takes between 8 and 12 hours and involves several process phases. To ensure safety at all times, an inerting system is installed.

[0037] The starting materials used are nitrogen-free precipitated silica or nitrogen-free precipitated silicon dioxide, and a mixture of glucose and fructose, particularly invert sugar, is used as the carbon source. A suitable ratio of silicon source to carbon source is 1:1. Water is used as the solvent. The starting materials are used as aqueous solutions or dispersions.

[0038] The preparation of the precursor composition is essentially as described below.

[0039] - Mixing carbohydrates and nitrogen-free precipitated silicon dioxide: The two liquid reactants are blended together in a mixer. In the first step, mixing takes place using a defined shear energy.

[0040] - Drying of the mixture of carbohydrates and precipitated silicon dioxide: The solvent added through the liquid phase is evaporated at a temperature between 300 °C and 400 °C.

[0041] Advantageously, the process or in particular the drying further comprises the following steps:

[0042] The mixture is heated under constant shearing. This results in a sharp increase in viscosity due to evaporating solvent. The material is then ground. Pressure monitoring ensures process stability. The process is run in phases under overpressure. A dried precursor powder is obtained. The powder is then fed into a container. This process step takes place under inert gas conditions.

[0043] In a particular embodiment, the process for producing a precursor composition can also be directly followed by a process for producing a silicon carbide-containing material.

[0044] The process then provides that the obtained precursor composition is further treated as follows:

[0045] Heating a precursor composition until a pyrolysis temperature of between 1000 °C and 1100 °C is reached.

[0046] Introducing an argon-hydrogen mixture as reaction gas into the precursor composition.

[0047] After the pyrolysis process is completed, silicon carbide is obtained.

Claims

Patent claims:

1. A process for producing a precursor composition for the production of a silicon carbide-containing material, the process comprising: - Mixing carbohydrates and nitrogen-free precipitated silicon dioxide.

2. The method according to claim 1, wherein sugar is used as the carbohydrate.

3. The process according to claim 2, wherein a mixture of glucose and fructose is used as sugar.

4. The process according to claim 3, wherein invert sugar is used as the mixture.

5. A process according to any one of the preceding claims, wherein the carbohydrates are used dissolved in a solvent.

6. Method according to one of the preceding claims, comprising: - Drying the mixture of carbohydrates and precipitated silicon dioxide.

7. A process according to any one of the preceding claims, wherein the mixture is heated to a temperature between 200 °C and 500 °C, preferably between 300 °C and 400 °C.

8. A process according to any one of the preceding claims, wherein the nitrogen-free precipitated silicon dioxide used is precipitated silicon dioxide dispersed in a nitrogen-free dispersant.

9. The process of claim 8, wherein the nitrogen-free dispersant is water.

10. A precursor composition comprising a mixture containing carbohydrates and nitrogen-free precipitated silicon dioxide.

11. A precursor composition according to claim 10, wherein the carbohydrate is sugar, for example a mixture of glucose and fructose.

12. Precursor composition according to claim 11, wherein the sugar is invert sugar.

13. Use of a precursor composition according to any one of claims 10 to 12 in a 3D printing process or in a coating process.

14. Use of a precursor composition according to any one of claims 10 to 12 for the production of silicon carbide.

15. A method for producing a silicon carbide-containing material, the method comprising: - Heating a precursor composition according to any one of claims 10 to 12 until a pyrolysis temperature is reached.

16. The process according to claim 15, wherein the pyrolysis temperature is in a range between 900 °C and 1200 °C, for example between 1000 °C and 1100 °C.

17. The method according to claim 15 or 16, comprising: - Introducing a reaction gas into the precursor composition.

18. The method according to claim 17, wherein the reaction gas is an argon-hydrogen mixture.

19. The method according to claim 18, wherein the hydrogen volume fraction in the argon-hydrogen mixture is between 1% and 20%, for example between 5% and 10%.

20. A silicon carbide-containing material produced by a process according to any one of claims 15 to 19.

21. A process for producing precipitated silicon dioxide, wherein precipitated silicon dioxide is produced dispersed in a nitrogen-free dispersing agent.

22. The method according to claim 21, wherein the protective gas atmosphere is an argon atmosphere.

23. Nitrogen-free fumed silica.