DE102018100681A1 - Method for producing silicon carbide - Google Patents

Abstract

The present invention relates to a method for producing crystalline silicon carbide in fiber form, comprising the method steps: a) providing a mixture with a silicon source, a carbon source and optionally a dopant, wherein at least the silicon source and the carbon source are present together in particles of a solid granules, as starting material b) treating the starting material provided in step a) at a temperature in a range of ≥1650 ° C. c) depositing fibers having a diameter d of silicon carbide on a substrate, the substrate (20) being in fluid contact with the educt and wherein the substrate (20) is heatable to a temperature which is lower than the temperature of the educt over a temperature range of ≥ 50 ° C to ≤ 100 ° C, and wherein the substrate (20) having a seed structure (22) Inoculation areas (24), wherein the Impfbereiche (24) are spaced from each other by a distance d, which is larger than the diameter d of the fibers to be produced.

Description

The present invention relates to a method for producing crystalline silicon carbide in the form of nanoscale fibers.

Silicon carbide is preferred for a variety of applications. For example, it is known to use silicon carbide as the electrode material for batteries such as lithium-ion batteries. The production of crystalline silicon carbide, in particular on a nanocrystalline or microcrystalline scale, is a complex process which requires precise control in order to produce defined silicon carbide, for example in the form of nanocrystalline fibers.

From O. Klein et al., Induction heating during SiC growth by PVT: Aspects of Axisymmetric Sinusoidal Modeling, it is known that inductive heating can be used to produce SiC single crystal based on SiC powder. However, this publication particularly aims at simulations of heat distribution by adjusting the voltage distribution in the coil. However, there is no focus on the production of silicon carbide fibers in this document.

Q.-S. Chen et al., In a large-size SiC growth system, Volume 266, Issues 1-3, pages 320-326 further discloses a simulation to investigate the crystal growth of SiC by induction based on the induction parameters when fabricated from SiC powder. However, the production of silicon carbide fibers is not described in this document.

Y. J. Wu et al., Synthesis of β-SiC nanowhiskers by high temperature evaporation of solid reactants, DOI: 10.1016 / j.matlet.2004.03.002, discloses the preparation of SiC nanowhiskers which are produced by inductive heating of silicon, silica and Graphite. There is no indication in this document for producing a fiber material.

The dissertation by Ms. B. Friedel, 3C silicon carbide based on sol-gel: The development, growth mechanisms and character of application-oriented morphologies of the wide-bandgap semiconductor are also addressed in the production of silicon carbide.

The above-described solutions, however, still offer room for improvement, in particular with regard to the production of silicon carbide as nanocrystalline fibers.

It is therefore the object of the present invention to provide a measure by which, in an improved and, in particular, cost-effective and particularly defined manner, production of silicon carbide in the form of nanocrystalline fibers can be made possible.

The object is achieved by a method having the features of claim 1. Preferred embodiments of the invention are disclosed in the subclaims, in the description and in the figure, wherein further described in the subclaims or in the description or the figure or example or features shown individually or in any combination may constitute an object of the invention unless the context clearly dictates otherwise.

1. a) Bereitstellen eines Gemisches mit einer Siliziumquelle, einer Kohlenstoffquelle und gegebenenfalls einem Dotierstoff, wobei wenigstens die Siliziumquelle und die Kohlenstoffquelle gemeinsam in Partikeln eines Feststoffgranulats vorliegen, als Edukt;
2. b) Behandeln des in Verfahrensschritt a) bereitgestellten Edukts mit einer Temperatur in einem Bereich von ≥ 1650°C, insbesondere bis ≤ 2000°C;
3. c) Abscheiden von Fasern mit einem Durchmesser dF aus Siliziumcarbid auf einem Substrat, wobei das Substrat in fluidem Kontakt zu dem Edukt positioniert ist und wobei das Substrat auf eine Temperatur heizbar ist, die niedriger ist, als die Temperatur des Edukts um einen Temperaturbereich von ≥ 50°C bis ≤ 100°C, und wobei das Substrat eine Impfstruktur mit Impfbereichen aufweist, wobei die Impfbereiche zueinander beabstandet sind um einen Abstand d₁, der größer ist, als der Durchmesser d_F der zu erzeugenden Fasern.

The present invention relates to a process for producing crystalline silicon carbide in fiber form, comprising the process steps:

1. a) providing a mixture with a silicon source, a carbon source and optionally a dopant, wherein at least the silicon source and the carbon source are present together in particles of a solid granules, as starting material;
2. b) treating the starting material provided in process step a) at a temperature in a range of ≥ 1650 ° C, in particular up to ≤ 2000 ° C;
3. c) depositing fibers of diameter dF of silicon carbide on a substrate, wherein the substrate is positioned in fluid contact with the educt and wherein the substrate is heatable to a temperature lower than the temperature of the educt by a temperature range of ≥ 50 ° C to ≤ 100 ° C, and wherein the substrate has a seed structure with seed portions, wherein the seed portions are spaced apart by a distance d ₁ , which is greater than the diameter d _{F of} the fibers to be produced.

By a method described above, silicon carbide can be produced in a particularly defined and reproducible manner, wherein the silicon carbide is obtained in fiber form and in particular as a single crystal in the cubic 3C crystal form.

In this case, the following process may be carried out completely or individually of process steps a) to c), preferably under protective gas, in particular argon, such that the process proceeds in an atmosphere which is free of oxygen.

The above-described method for producing fibrous silicon carbide comprises first according to method step a), the provision of a mixture with a silicon source, a carbon source and optionally a dopant as starting material, wherein at least the silicon source and the carbon source are present together in particles of a solid granules. In particular, it may thus be preferable for each of the particles of the solid granules to have a carbon source and a silicon source. The silicon source and the carbon source serve to be able to produce silicon carbide in a further method by a reaction of the carbon source with the silicon source. Therefore, the silicon source and the carbon source should be chosen such that they can form silicon carbide under the conditions described below, in particular at the following temperatures, such as at normal pressure (1 bar) or slight overpressure by the method described above.

The choice of the silicon source or the carbon source is thus not fundamentally limited. For example, preferred silicon sources may be formed by a sol-gel process, as described below, and formed of, for example, tetraethyl orthosilicate (TEOS) as the silicon source and sucrose as the carbon source, but the invention is not limited to the aforementioned examples in an understandable manner.

With respect to the dopant, this can be selected based on the desired doping. The dopant (s) may hereby be added in a basically freely selectable form, for example in a production process of the solid granules, as a soluble compound or may be added elementally, for example metallic. Thus, the dopant may also be part of the solid granules.

Furthermore, insofar as a dopant is desired and this is not present in the solid granules comprising the carbon source and the silicon source and the solid granules according to method step a) is transferred to a reactor in which the temperature treatment according to process step b) takes place, the dopant as a gas be added to the reactor, wherein the mixture according to process step a) can form directly in the reactor before the temperature treatment. This may be particularly advantageous if the dopant may be present as a gas. For example, in this case, gaseous nitrogen can serve as a dopant.

As doping materials, phosphorus (P) or nitrogen (N) may be preferably used for n-type doping, or boron (B) or aluminum (Al) may be used for p-type doping. By doping can be set a sufficient electrical conductivity of the fibers produced, which allows a particularly wide variety of applications of the silicon carbide fibers produced.

It may be advantageous in the method described here in particular that the silicon carbide produced has a high purity, so that it has semiconductor properties. This is an advantage over commercial fibers (e.g., SCS, silicon carbide fibers) which, due to their limited purity, have no semiconductor properties, but may only conduct. In the case of the silicon carbide which can be produced here, the majority charge carriers (electrons or holes) can be influenced by doping with regard to the type and concentration and the energetic properties, such as the position of the defect state in the band gap.

According to method step b), the method further comprises treating the mixture provided in method step a) at a temperature in a range of ≥ 1650 ° C, for example in a range of ≥ 1650 ° C to ≤ 2000 ° C, approximately ≥ 1750 ° C to ≤ 2000 ° C, especially in a reactor. In this process step, it is possible for silicon carbide to form from the carbon source or from the silicon source of the solid granules.

With regard to the temperature to be selected, if appropriate, an exact adjustment may be advantageous since a suitable reaction temperature may vary with the doping. Thus, for example, an advantageous reaction at a nitrogen doping from 1750 ° C, run, a reaction with undoped material from 1800 ° C and at a doping with aluminum from 1850 ° C. The most suitable temperature is adjustable by simply checking the set temperature in the aforementioned range.

In particular, at the above-described temperature or at a choice of the parameters described above, it can be made possible that the starting material passes into the gas phase and form carbon species or silicon species in the gas phase, which react with one another accordingly in a deposition and so as crystalline, in particular Nanoscale fibers can be deposited.

With regard to the fiber formation, it may be advantageous that Si ₂ C and SiC ₂ may already be present in the gas phase due to the intimate mixture of silicon and carbon in the solid granules, which leads to an easier formation of SiC on the substrate. It can therefore be directly a Si-C gas, which may be present in a manner understandable to the skilled person, other gas components, such as SiO or CO, wherein the Gas components can react in particular on the substrate to silicon carbide.

It can also be made possible by adjusting the temperature that the silicon carbide produced is nanocrystalline in a particularly defined manner in the form of fibers and in detail a cubic 3C structure of the silicon carbide is made possible. In particular, when the silicon carbide (SiC) is a silicon carbide single crystal, preferably a monocrystalline cubic 3C-SIC, the monocrystalline silicon carbide fibers combine high thermal conductivity, which may be advantageous for certain applications.

Depending on the exact selected temperature thus the concrete form of the silicon carbide produced can be controlled. In detail, at a setting of the temperature in process step b) to a range of about ≥ 1750 ° C to about ≤ 2000 ° C, about ≥ 1750 ° C to about ≤ 1850 ° C, at atmospheric pressure (1bar) in particular advantageously nanostructured fibers of the silicon carbide are formed.

The fibers thus produced with a diameter d _F of silicon carbide deposit in accordance with method step c) on a substrate. Conveniently, the substrate or at least one deposition surface thereof is positioned in fluid contact with the educt.

In this case, the formation of a temperature gradient may be advantageous so that the material of the solid granules or the starting material can pass into the gas phase at a position which has a comparatively higher temperature and silicon carbide fibers can deposit at the comparatively lower temperature, such as at a deposition surface , Thus, in particular, in order to produce fibrous silicon carbide, a deposition surface may be provided, which has a reduced temperature compared to the abovementioned temperature of the educt in process step b). For example, the temperature of the deposition surface may be decreased by a temperature ranging from ≥ 50 ° C to ≦ 100 ° C, as compared with the temperature basically set in the reactor and the temperature of the reactant in the aforementioned range of ≥ 1650 °, respectively C, up to ≤2000 ° C, for example, in a range of ≥1750 ° C to ≤2000 ° C, such as from ≥1750 ° C to ≤1850 ° C. Accordingly, it is provided that the substrate can be heated to a temperature which is lower than the temperature of the educt by a temperature range of ≥ 50 ° C to ≦ 100 ° C. In this case, one or a plurality of tempering units can be provided for heating the educt and the substrate, as described below.

With regard to fibers, these may in particular be structures in which the ratio of length to diameter is at least greater than or equal to 3: 1, whereas, in contrast to fibers in the case of particles, the ratio of length to diameter is less than 3: 1.

For example, in the present application in the case of the fibers, the ratio of length to diameter can also be greater than or equal to 10: 1, in particular greater than or equal to 100: 1, for example greater than or equal to 1000: 1. For example, it is possible to produce fibers with a diameter in a range of ≥ 10 nm to ≦ 3 μm and a length of a few millimeters, for example in a range of ≥ 1 mm to ≦ 20 mm. Fibers, for example, differ from nanowhiskers (nano-rods) in particular due to the bending radius, since rods are only limitedly bendable, but fibers are very flexible.

With regard to the substrate or the deposition surface, it is further provided that the substrate or the deposition surface has a seed structure with seed regions, wherein the seed regions are spaced apart by a distance d ₁ , which is greater than the diameter d _{F of} the fiber material to be produced. In particular, this embodiment can lead to fibers can be generated in a defined manner.

Because the fact that the Impfbereichen are spaced apart by a distance d ₁ , which is greater than the diameter d _{F of} the fiber material to be produced, it is possible in a particularly defined manner to deposit fibers. Because it can be effectively prevented that the individual precipitating fibers hinder each other in their growth or influence and thus influence the shape of the deposited fibers in no or only poorly controllable way. Thus, it can be particularly preferably made possible by such a seed structure that the fibers can be produced in a very defined manner.

It can thus be ensured that the set parameters have a defined effect on the fiber growth and not influenced by the growth of other fibers.

The above-described method offers the advantage that it enables the production of qualitatively extremely high-quality silicon carbide in the form of fibers, since very defined conditions can be set. The set parameters are not only adjustable adjustable, but also easily adaptable to the desired field of application, so that further not only qualitatively high quality but also very customizable or variable silicon carbide crystals can be produced. The parameters to be set for this include, for example, the selected temperature within the reactor or in the reaction space and the selected temperature of the substrate, including the selected temperature gradient between the reaction space and the substrate.

Another advantage, for example with regard to use in Li-ion batteries, may be that the fibers may possibly be allowed to grow directly on a carbon felt or a carbon foil or another substrate and then be further processed directly in the component, such as the battery. Further separation, purification and / or attachment processes can be dispensed with.

By a suitable selection of the parameters to be used, the properties of the produced silicon carbide crystals can be set in a simple and defined manner, such as the diameter of crystalline fibers produced, their length and crystalline quality.

The generation of the corresponding heat in process step b) can, for example, be carried out indirectly by resistance heaters made of graphite or molybdenum silicide or more preferably directly by induction, as described in more detail below.

Furthermore, it may be preferred if the high-temperature treatment according to method step b) takes place with exclusion of oxygen, in particular in an argon atmosphere.

With regard to the temperatures to be used, these should, as indicated above, be at least 1650 ° C., at least approximately 1750 ° C. In this regard, it should be ensured that the chosen temperature is not too high or too low so that crystalline silicon carbide precipitates as desired and does not precipitate or amorphously deposit or in a high temperature form of SiO ₂ such as Christobalite. Preferably, the temperature of the substrate is not below 1750 ° C, the substrate should be about 50 ° C to 100 ° C cooler than the educt.

Temperature gradients, heating ramps, hold times and cooling dynamics can be set as parameters. These can further positively influence fiber growth. In addition to the parameters already mentioned, suitable heating ramps may be in the approximate range of 500 ° C./min to 100 ° C./hour, and the silicon carbide formed may be approximately coolable to room temperature in a range of 15-20 minutes.

Further, with respect to the seed areas provided on the deposition surface, which may also be referred to as seed points, the following configurations may be advantageous.

The seed structures may comprise, as seed regions, in particular defined topological, morphological or chemical inhomogeneities provided on the deposition surface, wherein only individual ones of any combination or all of the aforementioned inhomogeneities of the material of the deposition surface may be present. These inhomogeneities may also form the seed sites of the seed structure. Thus, basically and independently of the specific embodiment, the seed areas may be the areas where fibers grow, such as spots, whereas the seed structures have one or more of the seed areas.

A topological inhomogeneity can be understood as meaning, in particular, a surface structuring of the substrate or of the deposition surface. For example, a structuring of the surface of the substrate or of the deposition surface can take place in various sizes in the nano- or macro-range. In the simplest case, this may be about a surface roughness or surface porosity. Being producible, a topological inhomogeneity can be made possible in particular by laser ablation, or focused ion beam etching or chemical etching. As a result, specifically defined structures or patterns on the deposition surface can be made possible, such as points, columns, pyramids, lines, screens, carpets or others. As a result, the vaccination areas can be particularly defined, which in turn can allow defined fiber growth in particular.

In addition, morphological inhomogeneity can be understood as meaning, in particular, a structural defect of the substrate or of the material present on the deposition surface. An example may be an offset of the deposition surface or material thereof with silicon carbide. Other examples include multi-phase system locations of a particular phase, phase orientations, or foreign materials. In particular, foreign materials are very well suited to favor fiber growth. An example here is the decoration of the surface with metal particles, which according to the VLS mechanism ("vapor-liquid-solid mechanism") as a melting droplet at the top of the Fiber act as a growth front. Examples include, for example, dopants, such as aluminum or other metals or materials that catalytically act on the one-dimensional growth. Condensation of silicon carbide on a particular crystal surface may be energetically favored here, such as salts, metals or ceramic particles. Another possibility would be a decoration of the substrate or of the deposition surface with silicon carbide, for example with silicon carbide particles, for example on a graphite substrate. Due to the comparatively soft surface of the substrate, the comparatively hard silicon carbide particles can be pressed into the deposition surface and thus fixed. In an exemplary application, this can be realized, for example, by polishing a deposition surface made of graphite, for example, with a paste containing silicon carbide particles. However, it could be more advantageous for highly pure, for example monocrystalline, silicon carbide particles to be applied to the surface. Thus, silicon carbide particles could also promote fiber growth as seed areas. Also in this embodiment, the seed areas can be particularly defined, which in turn can allow specifically defined fiber growth

With regard to chemical inhomogeneities, in the sense of the present invention, it may be understood in particular that there is a locally limited deposition of atomic monolayers, that is to say in particular a specific compound on the substrate surface or deposition surface. Examples include, for example, graphene deposition on a silicon carbide substrate or localized localization of chemical surface termination on seed sites. Examples include treatment of graphite with nitric acid, which renders the surface reactive and wettable. Such treatment may affect the adsorption of silicon carbide. In principle, a difference in surface energy, surface chemistry or surface reactivity can be made possible by chemical inhomogeneities. Examples of applying the inhomogeneities, for example, by applying nitric acid or self-assembled monolayers include, for example, dipping, spraying, spin coating, etc. Then, the control of exposure time and / or solution concentration is sufficient to adjust how much surface area has reacted , Another possibility is possibly even with a photoresist (photoresist) cover to allow local exposure.

It may further be preferred that the seed regions have a spacing in a range from ≥ 20 nm to ≦ 100 μm, for example from ≥ 50 nm to ≦ 20 μm, for example from ≥ 50 nm to ≦ 100 μm, relative to one another. In particular, in this embodiment, fibers of an advantageous thickness can be produced, which can make a variety of applications possible.

It may further be preferred that the seed areas cover the deposition surface of the substrate in a range of greater than or equal to 5% to less than or equal to 74%, based on the total deposition surface area. In this embodiment, the growth of fibers may be particularly preferably made possible because the seed points favor fiber growth, but the number of seed regions is not too high, so that with the limited number of seed regions, the silicon carbide will selectively deposit on the fibers. In particular, the above-described values can be dependent on the parameters to be set or the parameters to be selected can be selected when operating the device as a function of the overlap of the deposition surface with seed regions.

It may further be preferred that the educt provided in process step a) is provided using a sol-gel process.

A sol-gel process is to be understood in a manner known per se as such a process in which starting materials of the compound to be produced, the so-called precursors, are present in a solvent. In the course of the process, the precursor reacts and, by polycondensation, first forms particles, for example of a size of <10 nm, in the solvent, which is referred to as a sol. As a result of their subsequent crosslinking, aging or temperature treatment, as at 60 ° C., a so-called gel is formed, which may have, for example, a coalesced network of the particles agglomerated in the sol, it being possible for solvent to be incorporated in the intermediate regions of the particles. From the gel, a solid can be formed by further treatment, in particular temperature treatment, in particular drying and sintering. This solid can thus be defined by the selection of the precursors, which are the starting materials of the sol-gel process, and contains the carbon source and the silicon source for silicon carbide formation, and may optionally further contain a dopant for doping the silicon carbide already present in the preparation of the sol can be added.

The sol-gel process can also be carried out completely or at least partially in a protective atmosphere, in particular in an argon atmosphere.

Furthermore, the product of the sol-gel process can in particular be the starting material provided in process step a).

• d) Bereitstellen eines Präkursorgemisches mit einer Siliziumquelle, einer Kohlenstoffquelle und gegebenenfalls einem Dotierstoff, wobei das Präkursorgemisch in einem Lösungsmittel vorliegt;
• e) Behandeln des Präkursorgemisches bei erhöhter Temperatur unter Trocknung des Präkursorgemisches; und
• f) gegebenenfalls Erhitzen des getrockneten Präkursorgemisches auf eine Temperatur in einem Bereich von ≥ 800°C bis ≤ 1200°C, insbesondere in einem Bereich von ≥ 900°C bis ≤ 1100°C.

With regard to the sol-gel process, it may preferably be provided that it has at least the following process steps:

• d) providing a precursor mixture comprising a silicon source, a carbon source, and optionally a dopant, wherein the precursor mixture is in a solvent;
• e) treating the precursor mixture at elevated temperature while drying the precursor mixture; and
• f) optionally heating the dried precursor mixture to a temperature in a range of ≥ 800 ° C to ≤ 1200 ° C, in particular in a range of ≥ 900 ° C to ≤ 1100 ° C.

The educt according to method step a) can be provided in a very defined and reproducible manner by the above-described process steps in such a way that it can form silicon carbide in fiber form during a temperature treatment or a deposition on the substrate.

In a first step, according to method step d), the precursor solution preparation takes place. In this case, in the precursor, which is the starting material for the sol-gel process, preferably a molar ratio of carbon to silicon of 4: 1, at least in a molar excess of carbon generated, so as not to allow only the stoichiometry of the reaction but also to favor the reduction by the carbon excess. For example, a carbohydrate such as sucrose may be used as the carbon source, and tetraoxysiloxane (TEOS) may be used as the silicon source. At this time, the substances may be mixed in a solvent such as ethanol or water. The concentrations of the substances used usually do not affect the reaction, but the concentration may have an influence on the following step, the formation of a sol.

Thus, for example, a highly concentrated solution is thicker and gelled faster and therefore is well suited to produce a granulate as starting material according to process step a), which may be advantageous for fiber production of silicon carbide. On the other hand, if the solution is less concentrated and thus less viscous, it can be used, for example, for single or multiple infiltration of porous structures or for coating substrates. The latter may be suitable for an alternately doped multilayer structure, which may be advantageous for example for charge transfer processes in electronic or catalytic applications. In both cases, silicon carbide fibers can be produced. For example, the fibers may be so multi-layered, such as with an inner, N-doped layer and an outer, Al-doped layer, or grow as segmented fibers.

Thus, in particular, using a sol-gel process, it may be possible for the one deposition surface to be part of a porous substrate, ie, for instance to exist in part as an inner surface of the substrate, or for a deposition surface to be part of a closed substrate, ie as an outer surface of the substrate is present. In the latter case, the Abscheidenoberfläche thus be particularly flat. In other words, it can be provided that the substrate is at least partially porous, wherein the fiber formation takes place at least partially within the pores of the substrate and / or that the substrate is at least partially closed, ie has an at least partially closed Abscheideoberfläche.

The catalyst used may in particular be an acid present in the solvent, for example hydrochloric acid. The amount of catalyst relative to the silicon precursor affects the gelation rate, that is, molecular formation of silicate networks rather than particle growth. These differences can also affect the silicon carbide end product. By way of example, a molar ratio of TEOS to water to sucrose to hydrochloric acid may be 1 to 6.5 to 0.3 to 0.06 in order to produce fibers of silicon carbide particularly advantageously in the further process.

Insofar as the silicon carbide fiber to be produced is to be doped, a dopant can be added even during the sol formation. In principle, any doping substances or dopants which are at least partially soluble or dispersible in the corresponding solvent, such as, for example, ethanol of the water, are suitable. Furthermore, in the subsequent process steps, ie even under comparatively high heat, the dopant should not be volatile, evaporate or sublime or otherwise decompose, since otherwise the further reaction steps would not be feasible.

Examples of dopants include, for example, those mentioned above.

With respect to using aluminum, such as a powder, it may be the case that it is dissolved by the catalyst, such as hydrochloric acid, and AlCl _{3 is} formed. Indeed Depending on the size of the aluminum particles, these may not be completely dissolved but are protected by an AlCl ₃ shell around an aluminum core, so that they remain intact until the subsequent high-temperature treatment, ie process step b), with corresponding particles being referred to as "growth front" according to US Pat vapourliquid-solid theory could prefer the fiber growth in process step c).

Other ways of positively influencing fiber growth include, for example, alkoxides of the corresponding dopant, which additionally incorporate into the forming lattice of silicon carbide, since they have a similar structure compared to the silicon precursor and thus into the sol or the gel as Doping be incorporated.

In a next step, the above-indicated precursor sol processing with subsequent gelation or gelation and drying according to process step e) takes place. After the sol has been applied, the gelation can be in particular two steps, namely first a treatment at about 60 ° C., for example in a closed container, and then at about ≥ 60 ° C. to ≦ 120 ° C.

Depending on the viscosity, the sol produced as described above can be used for the purpose of infiltrating, coating or producing bulk material, for example for producing silicon carbide fibers on a planar or closed substrate. In particular, relatively thick solutions or sols can be gelled directly in the reaction vessel for the preparation of precursor granules. With comparatively low-viscosity sols, smooth, structured or nano- or microporous or macroporous substrates can be infiltrated from carbon, for example. For example, glassy carbon substrates, carbon fiber textiles, graphite bodies or glassy carbon foams can be mentioned here. Since a porous structure in particular provides a high or large surface, the precursor deposited thereon would have the possibility of reacting in a very efficient and defined manner in the further steps and of generating the reaction gases necessary for silicon carbide growth. In the case of simultaneous local inhomogeneities of the structure of the material of the porous structure in the sense of the vaccination regions described above, fiber growth in such infiltrated structures can be carried out in a preferred and efficient manner. For a temperature gradient, the substrate may be positioned at the junction to a cooler zone, or the fibers may grow due to local fluctuations or cooling of the reactor, such as the crucible.

For example, a porous structure could be pre-decorated with metal particles prior to treatment with the precursor sol to subsequently control fiber formation. This can be done, for example, according to the vapour-solid mechanism with metal drops as a growth front.

With regard to the gelation, in this process step the previously produced sol is exposed to an elevated temperature in the porous structure or even in a particularly closed vessel. There should be two processes, namely, a network formation of the silicate and the other the reduction of the carbon compound, such as the cleavage and reduction of sucrose by the catalyst, such as hydrochloric acid, and heat. The network formation may proceed at about 60 ° C for more than 12 hours whereas another step may proceed at 60-120 ° C for 4 hours or more. Since the chosen conditions can affect the final product, a multi-step reaction may be beneficial. Thus, treatment at elevated temperature may first be treated in a range of ≥ 40 ° C to <90 ° C, and the second step at temperatures of ≥ 60 ° C to ≤ 120 ° C, such as ≥ 90 ° C to ≤ 120 ° C. The gelation can take place in a closed container.

After the two aforementioned steps, a further step may follow, a drying of the gel. During drying, for example, the vessel can be opened and the solvent can evaporate by means of a temperature treatment at or above the boiling point. Depending on the condition, such as temperature and time, the formation of dense structures on the one side or pores, cracks or disintegration into granules of various sizes may occur because the gel structure condenses due to capillary forces of the outflowing solvent. Further, in the drying process, the decomposition of the carbohydrate proceeds. The chosen conditions can be decisive for the properties of the fiber material to be produced from silicon carbide. For example, drying can take place for 24 hours at temperatures of ≥ 100 ° C to ≦ 160 ° C. The duration and the temperatures may be dependent on the solvent used or its boiling point, and the temperature and the duration of the aforementioned step.

The gel produced as described above can then be sintered according to method step f) at a temperature in a range from ≥ 800 ° C. to ≦ 1200 ° C., in particular in a range from ≥ 900 ° C. to ≦ 1100 ° C. It may be important that in the method described herein, the sintering of the precursor gel in contrast to conventional sol-gel preparations of Silicates are carried out under exclusion of oxygen. The silicate is vitrified and the carbohydrate finally pyrolized. Relevant details for the precursor structure and thus for the final end product can be in particular the following.

First of all, the work may be in the absence of oxygen and, in particular, in a protective gas atmosphere, such as argon atmosphere, for instance under an argon stream or static.

Further, temperature control may be important, such as heating rates, temperature and duration of the temperature treatment. For example, carbon already pyrolyzes at more than 400 ° C, while the performed here glazing of the silicate only at above 800 ° C, preferably above 900 ° C, takes place. One could influence the order and progress of these two processes by the temperature of the ramp, say at slow rate of heating and keeping at 500 ° C, pyrolysis could be completed before further heating to 1000 ° C. Conversely, it would be possible with a rapid heating ramp directly to 1000 ° C., that the glass simultaneously forms for the pyrolysis of the carbon source or of the carbohydrate. The sintered precursor microstructure may be relevant to the properties of the final silicon carbide product because in the next step, the carbothermal reduction of process step a), as described above, may affect the composition and dynamics of release of the reaction gases.

Furthermore, the type of doping may be of importance as far as desired. In the following step, the dopant used, for example, in sol formation is of importance, since it must be stable, as indicated above, even at high temperatures. Another possibility would be core / shell particles in which the selected dopant is present as a core, wherein the core is covered by a high temperature stable substance, which acts as a protective shell for the core. As an example may be mentioned the compound described above, in which elemental aluminum is originally coated with Al ₂ O ₃ or by the reaction with the catalyst, such as hydrochloric acid, with AlCl ₃ .

After completion of the sintering process step a) is completed and it can be carried out following step b) a carbothermal reduction.

In a further preferred embodiment of the method, it may be provided that in the solid granules provided in accordance with method step a), carbon is present in a greater than equimolar amount with respect to silicon. In other words, the carbon content can be chosen such that in a reaction of all silicon to silicon carbide even more carbon is present. For example, carbon may be present in relation to silicon in an amount of> 1/1 to ≤ 5/1, based on the molar proportions. In particular, in this embodiment, the method can be particularly simple, as by the so taking place modification of the surface of the Siliziumcarbidfasern or silicon carbide particles by forming a carbon layer or carbon layer on the surface of an oxidation of silicon or a formation of silicon oxide particularly effective even when stored in air can be effectively prevented over a longer period. Thus, it can be effectively prevented, in particular in this embodiment, that additional steps for removing a silicon oxide layer are necessary in order to enable effective storage of lithium compounds when operating as an electrode in a lithium-ion battery or in a lithium-ion battery. It can thus be generated by a relatively simple and straightforward change in stoichiometry during the sol-gel process, a protective layer with the desired antioxidant properties. In this case, no additional step is necessary, which makes the method particularly cost-effective and time-saving feasible.

• - einen Aufnahmebehälter zum Aufnehmen eines zumindest teilweise elektrisch leitfähigen Edukts;
• - eine Induktionsspule zum Erzeugen eines magnetischen Wechselfeldes, wobei die Induktionsspule derart angeordnet ist, dass zumindest das Edukt durch einen durch das magnetische Wechselfeld induzierten Strom auf eine definierte Temperatur heizbar ist, die in einem Bereich von ≥ 1650°C, bevorzugt bis ≤ 2000°C liegt; und
• - ein Substrat zum Abscheiden von aus dem Edukt erzeugten Fasermaterial aus Siliziumcarbid, wobei das Substrat in fluidem Kontakt zu dem Edukt positioniert ist und wobei das Substrat auf eine Temperatur heizbar ist, die niedriger ist, als die Temperatur des Edukts um einen Temperaturbereich von ≥ 50°C bis ≤ 100°C, und wobei das Substrat eine Impfstruktur mit Impfbereichen aufweist, wobei die Impfbereiche zueinander beabstandet sind um einen Abstand d₁, der größer ist, als der Durchmesser d_F der zu erzeugenden Fasern.

It may further be preferred that the method is carried out in a device comprising:

• - A receptacle for receiving an at least partially electrically conductive reactant;
• - An induction coil for generating an alternating magnetic field, wherein the induction coil is arranged such that at least the starting material is heated by a induced magnetic field alternating current to a defined temperature in a range of ≥ 1650 ° C, preferably up to ≤ 2000 ° C is located; and
• a substrate for depositing silicon carbide fiber material produced from the educt, wherein the substrate is positioned in fluid contact with the educt and wherein the substrate is heatable to a temperature lower than the temperature of the educt by a temperature range of ≥ 50 ° C to ≤ 100 ° C, and wherein the substrate has a seed structure with seed portions, wherein the seed portions are spaced apart by a distance d ₁ , which is greater than the diameter d _{F of} the fibers to be produced.

In particular, such a device is suitable for being able to set the desired process parameters in a defined manner.

The receptacle serves to receive and in particular enclose an at least partially electrically conductive educt, as described in detail above. In particular, the product may be electrically conductive from a sol-gel process as described above. In order to position the educt in the receptacle this may have a material lock or be closed about by a lid. Furthermore, the receptacle can be designed as a particularly electrically conductive, formed from graphite, crucible.

For heating the starting material, an induction coil for generating electromagnetic radiation is furthermore provided as the heating device. The coil is arranged in such a way that at least the starting substance or the starting material can be heated to a defined temperature by one of an alternating electromagnetic field generated by the coil, which is in a range of ≥ 1650 ° C., approximately up to ≦ 2000 ° C, for example, in a range of ≥ 1750 ° C to ≤ 2000 ° C, such as from ≥ 1750 ° C to ≤ 1850 ° C. As a result, the starting material can pass into suitable species in the gas phase, which deposit silicon carbide from these species as nanoscale monocrystalline fibers.

Accordingly, the device further comprises a substrate for depositing fiber material produced from the educt. In particular, the substrate has a deposition surface or a plurality of deposition surfaces.

For example, it may be provided that the substrate or at least the deposition surface is formed from graphite. In particular, in this embodiment, a suitable positioning of the substrate should be ensured within the coil, so that the substrate is heated in not too strong. Further, the substrate may be made of vitreous carbon, monocrystalline silicon carbide, polycrystalline silicon carbide, another high temperature stable and reduction resistant ceramic, or a high temperature stable metal such as tungsten, tantalum or molybdenum.

In order to ensure particularly defined deposition of the silicon carbide from the gas phase, it is advantageous that the temperature in the reactor and the temperature of the substrate or the deposition surface are different from each other. In detail, it may be preferable for a sufficiently high temperature to be present within the receiving space in order to bring the corresponding substances or the educt (s) into the gas phase or to keep them in the gas phase. Thus, the interior of the receptacle serves as a reaction space having a volume in which the starting materials of Siliziumcarbidherstellung go into the gas phase or remain in the gas phase. In order to enable the desired deposition or crystallization, it may also be advantageous for the substrate to have a temperature which is reduced compared to the temperature in the reaction space, as described above.

Furthermore, it may be advantageous that the position of at least one of the receptacle and the substrate relative to the induction coil is variable by means of a displacement device. As a result, a particularly advantageous and adaptable temperature control can be made possible, which can make the formation of fibers particularly defined. In particular, a suitable temperature gradient between the educt and the substrate or the deposition surface and thus the growth region can be made possible, which in turn can enable the setting of particularly defined conditions.

As described above, the positioning of the receiving device with the starting substance or of the substrate can in particular be configured in such a way that the starting substance is arranged in the comparatively hotter zone and the substrate or its deposition surface can be positioned in a comparatively colder region, since the mass transport differs from the comparative one warmer zone takes place to the relatively colder zone and so the fiber growth can be made possible.

The aforementioned method may, for example, be suitable for producing silicon carbide fibers as electrode material for a battery, such as in particular a lithium-ion battery. Because, as explained above. Due to the good thermal properties of the fiber material, the thermal management of the battery can be improved. Furthermore, the chemical and thermal durability of the fibers may be advantageous for long-term stability, and the flexibility of silicon carbide, particularly as fibers, may be advantageous for high cycle stability. However, polycrystalline forms of the silicon carbide are also conceivable within the scope of the present invention. It is also advantageous that silicon carbide can have a high capacitance as an electrode material, so that an electrode material produced as described can further enable good performance of a battery.

Further applications of the silicon carbide thus produced include, for example, areas of photonics, such as solar cells, for example, which can act differently doped silicon carbide fibers, LEDs in which, for example, organic luminous surfaces can be provided in luminous textiles based on silicon carbide, or else the structural reinforcement, for example of other fibers.

In this case, the method is very effective because very large amounts of silicon carbide fibers can be produced in an exemplary apparatus or by a method described above.

With regard to further advantages and technical features of the method described above, reference is hereby explicitly made to the figure and the description of the figures, and vice versa.

The invention will now be described by way of example with reference to the accompanying drawings, in which the features shown below may each individually and in combination constitute an aspect of the invention, and the invention is not limited to the following drawing, the following description and the following embodiment ,

• 1 eine schematische Schnittansicht durch eine Vorrichtung gemäß der Erfindung; und
• 2 eine schematische Draufsicht auf eine Abscheideoberfläche eines Substrats.

Show it:

• 1 a schematic sectional view through a device according to the invention; and
• 2 a schematic plan view of a Abscheideooberfläche a substrate.

In the 1 is a device 10 for producing silicon carbide as fibers.

The device comprises a receptacle 12 for receiving an at least partially electrically conductive educt, which at the bottom of the receptacle 12 can be arranged. The receptacle 12 is designed as in particular made of graphite and thus electrically conductive ausgestalteter crucible, which is a crucible 16 and a crucible lid 18 includes. By placing the crucible lid 18 on the crucible pan 16 is the crucible or the receptacle 12 gastight sealable.

It is further contemplated that the crucible lid 18 as a substrate 20 serves for the separation of fibers. This can be easily made possible that the substrate 20 is positioned in fluid contact with the educt. Furthermore, the substrate 20 regarding 2 described.

In the 2 is shown that the substrate 20 a vaccine structure 22 with vaccination areas 24 having the vaccine areas 24 are spaced apart by a distance d1, which is greater than the diameter of the fiber material to be produced. In particular, the distance d _{1 can be} in a range from ≥ 20 nm to ≦ 100 μm, for example from ≥ 50 nm to ≦ 20 μm, for example from ≥ 50 nm to ≦ 100 μm.

The vaccine areas 24 may be formed at least in part by topological inhomogeneities, at least in part by morphological inhomogeneities and / or at least in part by chemical inhomogeneities. There may be one or a selectable combination of the aforementioned inhomogeneities. In principle, it may be advantageous if the substrate 20 was germinated with aluminum or silicon carbide.

Coming back to 1 it is shown that the device 10 an induction coil 26 for generating electromagnetic radiation. The induction coil 26 is arranged such that at least the educt is heated by the electromagnetic radiation to a defined temperature, which is in a range of ≥ 1650 ° C, for example ≥ 1750 ° C, to about ≤ 2000 ° C. Further, the substrate 20 is heatable to a temperature which is lower than the temperature of the starting material over a temperature range of ≥ 50 ° C to ≤ 100 ° C. This can be achieved by suitable positioning of starting material and substrate 20 , So for example, the crucible or the receptacle 12 inside the induction coil 26 ,

It is further shown that the position of at least the receptacle 12 relative to the induction coil 26 by means of a traversing device 28 is variable. For this purpose, according to the embodiment 1 a pull rod 30 provided on the receptacle 12 or a crucible holder 32 is articulated and this thus in particular in the axial direction in the induction coil 26 is movable.

Furthermore, it is shown that the induction coil 26 and the receptacle 12 in a housing 34 are arranged. The housing 34 can thus form a heating chamber. The housing 34 has a first viewing window 36 for a temperature measurement, such as for using a thermal camera and also has another viewing window 38 on. Furthermore, there are two gas connections 38 provided, through which protective gas, such as argon can be introduced and also a gas connection 40 for connecting a vacuum pump. The receptacle 12 can out of the case 34 removable or in the housing 34 be accessible in order to equip this with educt and / or to remove the fibers.

It is thus apparent that by suitable design of receptacles 12 and induction coil 26 and by adjusting the selected parameters, a method can be suitably controlled, in particular with respect to temperature control, to produce silicon carbide fibers. For example, a graphite crucible as a receptacle 12 in an induction coil 26 with a length of 100 mm and a diameter of 60 mm at a certain point in time produce a temperature of 1850 ° C. in the center but only 1750 ° C. in the end area or in the axial or radial outside area, since the magnetic field is in regions outside the induction coil 26 becomes weaker.

1. a) Einfügen des kohlenstoffhaltigen und siliziumhaltigen Edukts in den Aufnahmebehälter 12;
2. b) Beaufschlagen des in Verfahrensschritt a) bereitgestellten Edukts mit einer Vergasungstemperatur in dem Aufnahmebehälter 12; und
3. c) Abscheiden von kristallinem Siliziumcarbid in Faserform an dem Substrat 20 durch Einstellen einer Kristallisierungstemperatur an dem Substrat 20.

By such a device 10 Thus, as already indicated, for example, a method for producing fiber material from silicon carbide can be carried out. Such a method can have the following method steps in the broadest application:

1. a) insertion of the carbonaceous and silicon-containing educt into the receptacle 12 ;
2. b) applying the starting material provided in process step a) with a gasification temperature in the receptacle 12 ; and
3. c) depositing crystalline silicon carbide in fibrous form on the substrate 20 by setting a crystallization temperature at the substrate 20 ,

In more detail, for the production of silicon carbide as starting material, in particular a composite or a compound which contains suitable proportions of carbon and silicon, for example in a ratio of 4: 1, may be suitable. These may be, for example, organometallic precursors, silicates, elemental silicon, and / or carbohydrates or polymers. For example, at temperatures of 1650 ° C, about 1750 ° C, or more, can be generated by the induction coil 26 It uses the reducing action of carbon, firstly via elemental carbon, indirectly via carbon monoxide (CO) in the resulting gas as a carbothermal reduction. In this case, the carbon can reduce the C / Si precursor or the educt, especially SiO _{2 contained} , in contact or via the gas phase to SiO gas and subsequently to silicon carbide on the substrate 20 ,

For heating, the educt or the carbonaceous and silicon-containing precursor is thereby inserted, for example, in the graphite crucible and in the middle of the induction coil 26 positioned, which can be conveniently connected to a high frequency generator. The thus generated magnetic alternating field in the induction coil 26 induces a current in the at least partially electrically conductive educt and optionally in a conductive wall of the receptacle 12 , As a result, the educt is heated by the electrical resistance in a very defined manner.

Is the receptacle 12 also electrically conductive, this can thus also be heated. If it is electrically insulating, which is basically possible, such as made of aluminum oxide (Al ₂ O ₃ ), only the starting material can be heated, both variants can have application-related advantages.

1. a) Bereitstellen eines Gemisches mit einer Siliziumquelle, einer Kohlenstoffquelle und einem Dotierstoff, wobei wenigstens die Siliziumquelle und die Kohlenstoffquelle gemeinsam in Partikeln eines Feststoffgranulats vorliegen, als Edukt;
2. b) Behandeln des in Verfahrensschritt a) bereitgestellten Edukts mit einer Temperatur in einem Bereich von ≥1750°C bis ≤ 1850°C;
3. c) Abscheiden von Fasern mit einem Durchmesser d_F aus Siliziumcarbid auf einem Substrat, wobei das Substrat (20) in fluidem Kontakt zu dem Edukt positioniert ist und wobei das Substrat (20) auf eine Temperatur heizbar ist, die niedriger ist, als die Temperatur des Edukts um einen Temperaturbereich von ≥ 50°C bis ≤ 100°C, und wobei das Substrat (20) eine Impfstruktur (22) mit Impfbereichen (24) aufweist, wobei die Impfbereiche (24) zueinander beabstandet sind um einen Abstand d₁, der größer ist, als der Durchmesser d_F der zu erzeugenden Fasern.

By the device described above, therefore, a method for producing crystalline silicon carbide in fiber form is possible in detail, comprising the method steps:

1. a) providing a mixture with a silicon source, a carbon source and a dopant, wherein at least the silicon source and the carbon source are present together in particles of a solid granules, as starting material;
2. b) treating the starting material provided in process step a) at a temperature in a range of ≥1750 ° C to ≤ 1850 ° C;
3. c) depositing fibers of a diameter d _F of silicon carbide on a substrate, wherein the substrate ( 20 ) is positioned in fluid contact with the educt and wherein the substrate ( 20 ) is heatable to a temperature which is lower than the temperature of the educt over a temperature range of ≥ 50 ° C to ≤ 100 ° C, and wherein the substrate ( 20 ) a vaccine structure ( 22 ) with vaccination areas ( 24 ), wherein the vaccination areas ( 24 ) are spaced apart by a distance d ₁ , which is greater than the diameter d _{F of} the fibers to be produced.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Q.-S. Chen et al., In large-size SiC Growth System, Volume 266, Issues 1-3, pages 320-326 [0004]

Claims (10)

A method for producing crystalline silicon carbide in fiber form, comprising the method steps: a) providing a mixture with a silicon source, a carbon source and optionally a dopant, wherein at least the silicon source and the carbon source are present together in particles of a solid granules, as starting material; b) treating the educt provided in process step a) at a temperature in the range of ≥1650 ° C; c) depositing fibers having a diameter d _F of silicon carbide on a substrate (20), wherein the substrate (20) is positioned in fluid contact with the educt, and wherein the substrate (20) is heatable to a lower temperature, as the temperature of the educt over a temperature range of ≥ 50 ° C to ≤ 100 ° C, and wherein the substrate (20) has a seed structure (22) with seed regions (24), wherein the seed regions (24) are spaced apart by a distance d ₁ , which is greater than the diameter d _{F of} the fibers to be produced. Method according to Claim 1 , characterized in that the seed structure (22) comprises, at least in part, topological inhomogeneities. Method according to one of Claims 1 or 2 , characterized in that the seed structure (22) comprises, at least in part, morphological inhomogeneities. Method according to one of Claims 1 to 3 , characterized in that the seed structure (22) at least partially comprises chemical inhomogeneities. Method according to one of Claims 1 to 4 , characterized in that the starting material provided in process step a) is provided using a sol-gel process. Method according to Claim 5 characterized in that the sol-gel process comprises at least the following process steps: d) providing a precursor mixture comprising a silicon source, a carbon source and optionally a dopant, the precursor mixture being in a solvent; e) treating the precursor mixture at elevated temperature while drying the precursor mixture; and f) optionally heating the dried precursor mixture to a temperature in a range of ≥ 800 ° C to ≤ 1200 ° C, more preferably in a range of ≥ 900 ° C to ≤ 1100 ° C. Method according to Claim 6 , characterized in that the substrate (20) is at least partially porous, wherein the fiber formation takes place at least partially within the pores of the substrate (20). Method according to Claim 6 or 7 , characterized in that the substrate (20) is at least partially closed. Method according to one of Claims 1 to 8th characterized in that in the solid granules provided according to process step a), carbon with respect to silicon is present in a greater than equimolar amount, in particular in a ratio of 4: 1. Method according to one of Claims 1 to 9 , characterized in that the method is carried out in a device comprising: - a receptacle (12) for receiving an at least partially electrically conductive educt; - An induction coil (26) for generating an alternating magnetic field, wherein the induction coil (26) is arranged such that at least the starting material is heated by a induced magnetic field alternating current to a defined temperature in the range of ≥ 1650 ° C. , in particular up to ≤ 2000 ° C is; and - a substrate (20) for depositing silicon carbide fiber material produced from the reactant, wherein the substrate (20) is positioned in fluid contact with the educt, and wherein the substrate (20) is heatable to a temperature lower than the temperature of the educt over a temperature range of ≥ 50 ° C to ≤ 100 ° C, and wherein the substrate (20) has a seed structure (22) with seed regions (24), wherein the seed regions (24) are spaced apart by a distance d ₁ , which is larger than the diameter d _{F of} the fibers to be produced.

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