Silicon carbide-containing materials, precursor compositions, and methods for making same

Abstract

A method for producing a precursor composition of a silicon carbide-containing material is disclosed, which comprises mixing nanoscale silicon dioxide, particularly fumed silica, with nanoscale carbon, particularly carbon black. Also disclosed are the precursor composition thus produced, a method for producing a silicon carbide-containing material from the precursor composition, and the silicon carbide-containing material thus produced.

Description

[Background technology]

[0001] The present invention relates to silicon carbide-containing materials and their production, including precursor compositions for use in such production methods and methods for producing such precursor compositions.

[0002] In the well-known Degussa process, SiC is produced from a dry mixture of SiO2 and C by carbothermal reaction. In particular, silicon carbide is produced from compounds containing silicon dioxide by carbothermal reduction. In particular, sand, various silicas, or silane hydrolysates can be used as silicon dioxide-containing compounds. Organic binder systems such as sugar or phenolic resins are often used as carbon-containing materials for carbothermal reduction. For this, silicon dioxide and carbon-containing materials are usually heated to temperatures above 1400 ° C. By using particulate silicon dioxide, silicon carbide layers or silicon carbide granules are usually produced.

[0003] However, the Degussa process is not very effective in practice: the SiO2-C mixture used is loose and the reactants are too far apart from each other, so the carbothermal reaction to SiC proceeds only incompletely. The reaction produces large amounts of CO and CO2, and the SiC produced contains excess Si. This is unsatisfactory from the quality, economic and ecological points of view.

[0004] The manufacture of silicon carbide-containing structures using additive manufacturing is also known. For example, silicon carbide-containing structures can be produced from precursors containing silicon and carbon by using a powder bed process with laser-induced so-called selective synthetic crystallization. Selective synthetic crystallization is disclosed, for example, in DE 102017110362 A1 and DE 102015105085 A1. In this process, precursor granules based on silane hydrolysates and sugars, and possibly other additives, are used in a powder bed and are selectively converted by a laser beam into silicon carbide or silicon carbide alloys. In a sol-gel process, precursor granules are produced, for example, by mixing silicate, sugar solution, alcohol, and other additives to form a sol, which is gelled at about 70 ° C. This is followed by drying at about 200 ° C and pyrolysis at about 1000 ° C.

[0005] Electrodes, especially anodes for lithium-ion batteries, can be made from nano- or microcrystalline SiC, or from amorphous SiC. To improve battery efficiency, the goal is to increase the surface area and to incorporate lithium more efficiently into the electrode material, for example by using nano- or microstructured SiC. An example is a nanostructured silicon carbide foam made of interconnected silicon carbide fibers forming an open-cell foam. Depending on the reaction conditions, in particular the temperature regime in the furnace, either isolated silicon carbide fibers or nanostructured silicon carbide-containing foams can be obtained from suitable precursor materials. A method for producing such electrode materials from precursor granules containing silane hydrolysates and sugars is described in DE 102014116868 A1. A corresponding method for producing silicon carbide-containing fibers and foams from liquid or gaseous precursors is described in DE 102017114243 A1.

[0006] However, the use of precursors containing sugar as a carbon source has disadvantages. Although the use of sugar, especially liquid sugar solutions, in the preparation of precursor granules achieves good and homogeneous mixing with the silicon-containing starting material, sugars, due to their high hydrogen and oxygen content, tend to release a lot of gases during the decomposition reaction of the carbothermal reduction process. Most of these gases are gases that have a negative impact on the climate, such as CO2 and methane. This also leads to a certain waste of resources. The release of gases also hinders additive manufacturing by site-selective formation and deposition of silicon carbide, since the production of strong gases makes site-selective deposition of silicon carbide more difficult.

[0007] In the adjacent field of silicon nitride production, U.S. Patent Application Publication No. 2020 / 0038955 discloses a method for converting gritty silicon dioxide to silicon nitride and carbon monoxide by laser irradiation in a nitrogen atmosphere and with small amounts of graphite present in the powder bed. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] DE 102017110362 [Patent Document 2] DE 102015105085 A1 [Patent Document 3] DE 102014116868 A1 [Patent Document 4] DE 102017114243 [Patent Document 5] US Patent Application Publication No. 2020 / 0038955 Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the above-mentioned prior art, it is an object of the present invention to provide a silicon carbide-containing material and a method for its production, as well as a precursor composition for use in such a production method and a method for producing the precursor composition which is more efficient and allows for more precise control of the properties of the silicon carbide-containing material produced. [Means for solving the problem]

[0010] The solution to this problem is achieved by silicon carbide-containing materials, methods and precursor compositions as specified in the appended claims.

[0011] The present invention is based on the finding that nanoscale silicon dioxide and nanoscale carbon, in particular highly porous fumed silica (more precisely fumed silicon dioxide) and carbon black particles, can be mixed in a wide range in the process according to the invention, and that these mixtures are suitable as efficient precursor compositions or precursors for the production of silicon carbide. In particular, the electrostatic charging of the silica particles, which makes it difficult to use these materials in solid mixtures, is avoided by the use of conductive carbon black.

[0012] The particularly dense spatial attachment of carbon to nanoscale fumed silica allows an optimal carbothermal reduction of SiO2. In this way, high-quality SiC can be produced in a resource- and environmentally friendly way. The traditional CO and CO2 emissions due to the lack of spatial proximity of SiO2 and C are prevented, as well as the harmful emissions of other gases, e.g. from sugar decomposition. Raw materials and energy are used more efficiently, less harmful greenhouse gases are produced, and the produced SiC is of higher quality, since it contains fewer impurities and its structure is easier to control. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Preferred embodiments of the present invention are described below, including methods of producing precursor compositions as precursors for producing silicon carbide-containing materials, the precursor compositions so produced, methods of producing silicon carbide-containing materials by using the precursor compositions, and the silicon carbide-containing materials produced thereby.

[0014] Method for preparing a precursor composition:

[0015] Powdered nanoscale SiO2 and nanoscale carbon serve as starting materials for the preparation of the precursor composition. SiO2 and carbon each exist as particles (primary particles) with sizes (maximum diameter of each particle) of about 5-100 nm.

[0016] Examples of nanoscale SiO2 include fumed silica, such as those sold under the trade names AEROSIL, HDK, or CAB-O-SIL.

[0017] An example of nanoscale carbon is carbon black, i.e. industrial carbon black such as furnace soot, with particle sizes up to about 100 nm. Carbon black is advantageously electrically conductive, which prevents electrostatic charging of the silica particles in the mixture and results in a relatively dense mixture. Thus, high powder conductivity, i.e. a powder resistivity of 5×10 -2 Ω cm or less, preferably 1×10 -2 Highly conductive carbon black, with a conductivity of Ω·cm or less, is preferred. Carbon black is particularly economical to source, but graphene or carbon nanotubes, which have the properties mentioned above, can also be used.

[0018] First, two nanoscale components SiO2 and C, in this example fumed silica and carbon black, are mixed in the desired mass ratio. The preferred mass ratio is 1 part SiO2 (fumed silica) to 2.4-3.1 parts C (carbon black). The result is a nanoscale mixture of substances, as opposed to a sol or gel in the prior art.

[0019] A solvent is then added to the powdered mixture. A highly volatile solvent such as alcohol, preferably ethanol, is preferred. The amount of solvent is not critical as long as there is enough to ensure that the resulting mixture is easily stirrable and provides good mixing. The actual amount depends on the specific surface area of ​​the SiO2. For SiO2 particles with a BET specific surface area of ​​typically around 200 m2, 2 / g, about 2.2 to 3 parts by weight of ethanol can be added to 1 part by weight of SiO2. The miscibility helps to wash the C particles (carbon black) into the SiO2 particles (fumed silica). The nanoscale of the C particles (carbon black) is particularly important for this function.

[0020] In the above mentioned sequence of first mixing the two components SiO2 and C and then adding the solvent, the electrical properties of the components are exploited, allowing good mixing already at the drying stage and then optimized by the application of the solvent. It is also possible to change the sequence and mix one component first with the solvent and then add the other component, provided that the solvent is well miscible with one of the components and that any stabilizers required for this do not result in unacceptable properties in the silicon carbide-containing material subsequently produced.

[0021] In principle, water is also a usable solvent, but it is more difficult to achieve the purity required for many applications, such as in the manufacture of electronic devices and batteries.

[0022] Any dopant or alloying materials can be introduced into the precursor composition, optionally with a solvent, which will then be contained in the silicon carbide-containing material produced from the precursor composition.

[0023] The mixture of the two components (in this case, fumed silica and carbon black) in the solvent is continuously stirred. Continuous stirring first sweeps the carbon black macroscopically into the highly porous SiO2 nanostructure of the fumed silica, ensuring excellent penetration of the two components.

[0024] In a next step, the mixture is dried, preferably under continuous stirring. Depending on the process control, the drying step results in nanoscale to microscale particles of the precursor composition and the corresponding size scale SiC that is subsequently produced. For example, nanoscale SiC is advantageous for battery anodes, and microscale SiC is advantageous for AM (additive manufacturing). Solvent that is transferred to the gas phase during drying is preferably recovered by distillation or recondensation.

[0025] To form nanoscale particles of the precursor composition, which are preferred for many applications, the mixture is heated in the drying step to a temperature of up to about 100°C or only slightly above. The corresponding process parameters in the drying step of the nanoscale precursor particles or the nanoscale SiC obtained therefrom are relatively rapid stirring and heating with a low final temperature and a small temperature gradient over time. Rapid stirring here means stirring at about 2 to 10 revolutions per minute. A low final temperature and a small temperature gradient means that the mixture is initially stirred at about 100°C until the mixture appears dry, since the solvent does not substantially escape any further. However, the particles still contain residual moisture that can later cause agglomeration. This is removed by slowly increasing the temperature further, usually within about an hour, to a maximum of about 150°C or preferably to a maximum of 120°C. Higher temperatures must be avoided to prevent the carbon from being oxidized to CO2. A slight negative pressure in the reactor in which the mixture is placed is beneficial to facilitate the dehumidification and to ensure that as little oxygen as possible is present. Running the process under inert gas also supports this.

[0026] If the solvent is evaporated initially faster at higher temperatures, up to 250°C, preferably up to 200°C, with slow stirring at about one revolution per 3 minutes, microscale particles of the precursor composition are obtained. This is because the SiO2 particles combine to form larger particles. To prevent further extensive agglomeration and control the particle size, residual moisture must also be removed from these particles. For this purpose, drying is continued for about 2 hours after the initial evaporation of the solvent until the residual moisture has also escaped from the larger particles.

[0027] The mixture thus dried is the precursor composition from which a silicon carbide-containing material can be produced by the method steps further described below.

[0028] Here, pyrolysis at about 1000°C does not occur as in known processes. A significant drawback of pyrolysis is the loss of the nanoscale nature of the particles. Even if the SiO2 particles used have nanoscale dimensions, this is not preserved during pyrolysis, especially when carbohydrates such as sugars are used as carbon sources. This is because the SiO2 particles stick together during the drying process to form gradually larger particles, which harden during pyrolysis. In the method described here, this is avoided by using nanoscale carbon, such as carbon black, as the carbon source and / or by the temperature during the preparation of the precursor composition from nanoscale SiO2 and nanoscale carbon not exceeding 700°C, preferably not exceeding 400°C, more preferably not exceeding 250°C, or not exceeding 150°C when forming nanoscale particles.

[0029] Another advantage is that neither the production of the precursor from nanoscale SiO2 and nanoscale carbon nor the introduction of alloys or dopants into the precursor requires chemical reactions. Thus, alloys and dopants can be introduced into the mixture as pure elements or substances as required in the subsequent SiC material. Thus, the mixture produced preferably consists of the two components SiO2a and C, a solvent and, if necessary, alloys and doping materials.

[0030] Precursor Composition:

[0031] The precursor composition produced by the above-mentioned method comprises primarily nanoscale (or, in certain variants where the temperature during drying is higher, microscale) particles (primary particles) of carbon-embedded fumed silica, optionally including added dopants and alloying materials. The nanoscale particles have a particle size in the range of 5-1000 nm, typically in the range of about 20-200 or 1000 nm. The microscale particles have a particle size in the range of 1-1000 μm, typically in the range of about 1 or 20-200 μm.

[0032] Thus, the particles of the present precursor composition are much smaller than the granules of known precursor compositions.

[0033] Moreover, the precursor composition is much purer than known precursors made from materials such as sugars or organic binder systems as carbon sources rather than from nanoscale carbon (industrial carbon black): unavoidable impurities (residual impurities) beyond the above-mentioned intentionally added dopants or alloying agents, such as metal, Al, B or N impurities, are present in amounts of only a few ppm, typically less than 10 ppm, preferably less than 5 ppm, respectively.

[0034] Methods for producing silicon carbide-containing materials, and silicon carbide-containing materials produced by such methods:

[0035] The precursor compositions disclosed herein can be used in place of known precursors in known methods for producing silicon carbide-containing materials.

[0036] In these methods, the precursor composition is heated to a temperature of about 1400°C to 2000°C, preferably 1700°C to 1900°C, and reacts to form SiC. Solid-state diffusion occurs primarily, with C diffusing into SiO2 and SiC forming in a carbothermal reaction. SiC can also be formed in the gas phase and, depending on the process, can be deposited in situ or after some transport from the gas phase.

[0037] For example, the precursor composition is advantageously used instead of precursor granules in additive manufacturing, for example in powder bed processes using laser-induced selective synthetic crystallization according to DE 102017110362 A1 and DE 102015105085 A1. The nanoscale structure of the precursor composition and the close proximity of silicon and carbon result in less gas evolution when exposed to a laser beam than when granular precursors are used, and the effect of the laser beam is more clearly localized to the beam irradiation position. This means that more precise structures can be produced with consistently high quality.

[0038] The precursor composition is also advantageously used instead of precursor granules when producing electrode materials for battery electrodes, for example in a controlled heating reactor according to DE 102014116868 A1. Electrodes, in particular anodes for lithium or sodium-ion batteries, are produced in this way. Depending on the reaction conditions, in particular the temperature regime, nano- or microcrystalline silicon carbide, discrete silicon carbide fibers or nanostructured silicon carbide-containing foams can be obtained. A particularly preferred method is the production of nanostructured silicon carbide foams as silicon carbide anode material by gas-phase deposition from a precursor composition of a powdery mixture of silicon dioxide and carbon black particles.

[0039] Due in particular to the nanoscale structure and the close proximity of silicon and carbon in the present precursor composition, the silicon carbide materials and products thus obtained are more homogeneous, more uniform in structure and of higher quality than those produced with known precursors. Moreover, due to the higher purity of the precursor composition, they are also particularly pure. Residual impurities in addition to dopants or alloying elements, such as metal, Al, B or N impurities, are present in the material in amounts of only a few ppm, typically less than 10 ppm, preferably less than 5 ppm each. In this way, for example, high purity β-SiC can be produced.

[0040] This broadens the range of applications, and the silicon carbide-containing materials produced can be used as high purity starting materials for ingot or wafer production, where they can also be P-doped, or used as battery anode materials or workpiece coatings with SiC. Additionally, the precursor compositions and silicon carbide-containing materials and products having the properties disclosed herein are advantageous independently of the disclosed methods of production.

[0041] The methods disclosed herein are readily applicable on an industrial scale.

Claims

1. 1. A method for producing a precursor composition for producing a silicon carbide-containing material, comprising: Mixing nanoscale silicon dioxide and nanoscale carbon A method comprising:

2. The method of claim 1, wherein the carbon in the nanoscale carbon is present in particles having a size of 5 to 100 nm.

3. The method of claim 1 , wherein the nanoscale carbon is carbon black.

4. The method of claim 3 wherein the carbon black is a conductive carbon black.

5. 5. The method of claim 1, wherein the nanoscale carbon has a resistivity of 5×10 Ω cm or less.

6. 5. The method of claim 1, wherein a dopant or alloying material is added to the mixture.

7. adding a solvent to the mixture of nanoscale silicon dioxide and nanoscale carbon; 5. The method of claim 1, wherein the mixture is dried at a temperature of up to 700°C.

8. The method of claim 7, wherein the drying is performed at a temperature of 150°C or less to provide a dried nanoscale precursor composition.

9. The method of claim 7 , wherein the drying is performed at a temperature of 250° C. or less to provide a dried microscale precursor composition.

10. A precursor composition produced by the method of claim 1.

11. 11. The precursor composition of claim 10, wherein the metals, Al, B, and N, are present in amounts up to 10 ppm, in excess of any added dopants or alloying materials.

12. A method for producing a silicon carbide-containing material, 11. A method of converting the precursor composition of claim 10 into said silicon carbide-containing material by heating.

13. The method of claim 12 , wherein the conversion of the precursor composition to the silicon carbide-containing material is carried out in an additive manufacturing process.

14. The method of claim 12, wherein the silicon carbide-containing material is an electrode material for a battery electrode.

15. 13. A silicon carbide-containing material produced by the method of claim 12.

16. The silicon carbide-containing material of claim 15, further comprising a dopant or alloying material.

17. 17. The silicon carbide-containing material of claim 15 or 16, comprising the metals Al, B, and N in an amount of 10 ppm or less each, and, if a dopant or alloying material is added, in an amount exceeding the added amount.