EP3277646A1

EP3277646A1 - Method for producing a nano- or microstructured foam

Info

Publication number: EP3277646A1
Application number: EP16714351.0A
Authority: EP
Inventors: Siegmund Greulich-Weber
Original assignee: Universitaet Paderborn
Current assignee: Universitaet Paderborn
Priority date: 2015-03-31
Filing date: 2016-03-30
Publication date: 2018-02-07
Also published as: DE102015104943A1; WO2016156384A1

Abstract

The invention relates to a method for producing a nano- or microstructured foam, wherein the foam is constructed of a plurality of in particular nano- or microstructured silicon carbide fibers connected to each other, characterized in that the method has the following steps: a) providing a mixture having a silicon source and a carbon source, wherein the silicon source and the carbon source are jointly present in particles of a solid granular material; b) treating the mixture provided in step a) at a temperature in a range of ≥ 1400°C to ≤ 1500°C. Such a method allows a foam formed of silicon carbide fibers to be produced simply and economically.

Description

Process for producing a nano- or micro-structured foam

The present invention relates to a process for producing a nano- or microstructured foam. The present invention further relates to a foam which may be made by such a method and which is composed of silicon carbide fibers.

Foams are known in various technical fields from the prior art. Foams can be characterized by a particularly reversible compressibility and thus by elastic properties. Conventionally, foams are produced using plastics such as polyurethane plastics, which can be foamed, for example, in the production to produce the foam structure.

For special applications, however, other materials may be advantageous to produce foams. From the document CN 102140030 A it is known, for example, to produce nanocrystalline foams of silicon carbide (SiC). Such a manufacturing method is based on silicon and carbon, in particular carbon black, as starting materials, which are first mixed, then foamed using additives and subjected to a temperature treatment and a pressure treatment. From the publication of H. Konishi, Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture University, available at www.lm-foundation.or.jp/English/abstract/62.html, is also the production of one-dimensional Silicon c arb idst ru ktu re n known. In such a method, silica and carbon monoxide are reacted together as reactants.

The methods described in these documents thus describe either a two-dimensional structure, and thus no foam, or a rather complicated and thus costly process. Furthermore, the foam produced can still offer room for improvement.

There is therefore still room for improvement in the production of foams, especially on an inorganic basis. In particular, there is further potential for improvement in terms of simplicity and cost-effectiveness of the manufacturing process.

It is therefore an object of the present invention to provide a solution by which a nano- or micro-structured foam on an inorganic basis, in particular based on silicon carbide, can be produced in a simple and cost-effective manner. The object is achieved according to the invention by a method for producing a nano- or microstructured foam with the features of claim 1. The object is further achieved by a particular nano- or micro-structured foam with the features of claim 9. The solution of the problem takes place Further, by a use with the features according to claim 14 or with the features according to claim 15. Preferred embodiments of the invention are disclosed in the subclaims, in the description, in the figure and the example, wherein further in the subclaims or in the description or the Figure or the example described or shown individually or in any combination an object of the invention, if the context does not clearly indicate the opposite.

A method for producing a nano- or microstructured foam is proposed, wherein the foam is constructed from a multiplicity of interconnected nano- or microstructured silicon carbide fibers. The method comprises the method steps:

a) providing a mixture with a silicon source and a carbon source, wherein the silicon source and the carbon source are present together in particles of a solid granules;

b) treating the mixture provided in process step a) at a temperature in a range from> 1400 ° C to <1500 ° C.

By a method described above, a foam can be represented in a particularly simple and cost-effective manner, wherein the foam by virtue of its formation of silicon carbide and in particular of silicon carbide fibers has particularly advantageous properties.

In this case, the process described here can proceed completely or individually from process steps a) and b) and in particular b), preferably under protective gas, in particular argon.

In detail, the method according to method step a) comprises firstly providing a mixture with a silicon source and a carbon source, the silicon source and the carbon source being present together in particles of a solid granulate.

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 the further process by a reaction of the carbon source with the silicon source. The silicon carbide formed is present in particular in the form of nano- or microcrystalline fibers, which form the foam and are thus connected to one another via preferably a multiplicity of connection points.

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.

Furthermore, the shaped silicon carbide fibers may be present in particular as 3C-SiC or in hexagonal form.

The silicon source and the carbon source should advantageously be selected such that they can form silicon carbide in the form of foams forming fibers in the conditions described below, in particular at the following temperatures, for example at normal pressure (lbar) by the method described above.

In particular, in the solid, the silicon source may be pure silicon or silicon dioxide, and in the solid, the carbon source may be pure carbon, wherein the solid particles may be formed, for example, by a sol / gel process, as described in detail below. By way of example, the solid particles may consist of silicon, carbon and optionally one or more dopants as described below, or at least for the most part, in a range of> 90% by weight. According to process step b), the method further comprises treating the mixture provided in process step a) at a temperature in a range of> 1400 ° C to <1500 ° C. In this process step, silicon carbide is allowed to form from the carbon source and from the silicon source of the solid granules.

Depending on the exact selected temperature while the concrete shape of the silicon carbide produced can be controlled. In detail, with a setting of the temperature in process step b) to a range of> 1400 ° C to <1500 ° C in particular at normal pressure (lbar) nano-structured fibers of silicon carbide are formed in a particularly advantageous manner, wherein the silicon carbide especially in the form of monocrystalline 3C-SiC can form. The temperature selected may allow the resulting fibers not to be present as separate fibers but rather as a foam. In the context of the present invention, a foam is in particular a body which has a plurality of individual fibers or fiber strands, each of the fibers or fiber strands having at least one, in particular a plurality of at least two, in particular at least three, Connecting hatches, which is respectively connected to at least one connection point of another fiber. In this way, a network of individual fibers is formed, which are connected to form a stable and elastic structure and thus form the foam.

In the method described above, the formation of a temperature gradient may be advantageous, so that the material of the solid granules can pass into the gas phase at a position which has a comparatively higher temperature and silicon carbide fibers or a silicon carbide foam at the relatively lower temperature can be deposited, such as at a Abscheidoberoberfläche. Thus, in particular in order to produce a foam formed from silicon carbide fibers, a release be provided scheideoberfläche, which compared to the aforementioned temperature has a reduced temperature. For example, the temperature of the deposition surface may be reduced by a temperature that is in a range of> 30 ° C to <100 ° C, for example 50 °, compared to the temperature generally set in the reactor in the aforementioned range of> 1400 ° C to <1500 ° C.

With regard to the fiber formation described above, it may be advantageous that Si ₂ C and SiC ₂ may already be present in the gas phase due to the intimate mixture, for example at the atomic level of silicon and carbon in the solid granules, resulting in easier formation of SiC at a different location in the temperature gradient leads. It can therefore be directly a Si-C gas, which may be present in a manner understandable to the skilled person, other gas components.

In this case, by adjusting the temperature and providing the mixture by method step a), it can also be made possible that the silicon carbide produced is monocrystalline and thereby nano- or microcrystalline, 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, as described in detail below, as well as chemical and thermal durability, which is advantageous for long-term stability, with good properties of a foam.

However, hexagonal shapes of the silicon carbide are also conceivable within the scope of the present invention. For this, the molded product could be converted to the hexagonal form by a thermal treatment, for example to a range of about 2000 ° C. By the method described above, a foam is produced, which is open-pored due to its construction of a fiber structure. As a result, a variety of applications can be realized. In addition, a foam in principle and also the foam produced by a method described above further characterized an elastic behavior. Thus, the foam after a force from a certain restoring force and can thus return to its original form. Despite its fundamental stability, an elastic behavior can thus be achieved, which can further increase the field of application of the foam. In this case, the foam can remain stable even after a large number of compressions and reliefs and, in particular, can not suffer any fiber break, thus retaining its properties, which indicates a high degree of longevity.

In the method described here, a solid and nevertheless flexible composite of the fibers or a fiber network is formed from the individual fibers by the connection in the region of the points of contact of individual or a plurality of fibers, without steps by means of textile processing, the fibers must be connected to each other. This further speaks for the simplicity of the method described here.

An important point in the method described above may be the nano- relationship or microscale of the silicon carbide fibers produced. A nano-structured silicon carbide may be understood to mean, in particular, a silicon carbide which has a maximum spatial extent in the nanometer range, in particular of less than or equal to 100 nm, for example> 100 μm, the lower limit may be limited by the production method. Accordingly, a microstructured fiber may in particular be a fiber which has a maximum spatial extent in the micrometer range, in particular of less than or equal to ,μιη, for example> 100 nm, in at least one dimension. In particular, the scaling and in particular the diameter and the length of the fibers can be determined by the temperature. at the growth site, the set temperature gradient and the time to grow the fibers.

Basically, silicon carbide fibers offer the advantage of high robustness and resistance to a variety of chemicals and conditions, such as aggressive media, such as acetone, acids or alkalis. The method described here has the advantage that the foam produced here is highly resistant and also at high temperatures, optionally also with oxygen addition of well over 1000 ° C, such as to over 1100 ° C of oxygen and 1300 ° C in vacuo, or even stable if necessary. Furthermore, a foam which can be formed by this method is also very robust against mechanical influences, so that no fiber breaks are to be expected even with a multiple compression.

It is further advantageous that formation of a silicon oxide film (SiO ₂ ) on the surface of the silicon carbide can be prevented. Thus, no material forms by oxidation processes, which could reduce the properties of the foam in an undesired manner. Their targeted removal is just not necessary due to immediate applicability. According to the present invention, such an additional step can just be dispensed with, which can make the method particularly cost-effective.

Compared with the fibers forming the foam according to the invention, conventional, for example amorphous, SiC fibers usually have only a limited purity, such as residues of carbon. By contrast, the process described above makes it possible to produce highly pure silicon carbide fibers, which can further improve the properties of the foam and allow it to be defined in a very stable manner.

Characterized in that the fibers produced in a method described above, in particular crystalline, which advantageously has a 3C-SiC structure, the properties with Regarding stability and inertness to chemicals and conditions will be further improved.

Furthermore, the method described above makes it possible to produce a large amount of foam in a comparatively short time, simply because of its simplicity.

In summary, it is thus possible by the method described above to produce a very robust foam in a simple and therefore cost-effective manner. In a preferred embodiment of the method described above, it may be provided that the mixture provided in method 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, this mixture being referred to as sol. In the course of the process, a so-called gel is formed by drying or aging, from which a solid can be formed by further treatment, in particular temperature treatment. This solid can thus be defined by the selection of the precursors and contains the carbon source and the silicon source for the silicon carbide formation and can optionally also contain a dopant for doping the silicon carbide, which can already be added during the preparation of the sol.

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

With particular reference to an embodiment of the method described above by a sol-gel process, it can be provided that the sol-gel process has at least the following method steps: c) providing a precursor mixture comprising a silicon precursor, a carbon precursor and optionally a dopant, wherein the precursor mixture is present in a solvent;

d) treating the precursor mixture in particular at room temperature (22 ° C) elevated temperature with drying of the precursor mixture; and

e) 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.

According to method step c), therefore, the precursors can first be provided, which are processed into a solid and can subsequently serve as carbon source or as silicon source, respectively, which are provided or used in method step a). The choice of the silicon source or the carbon source or the silicon precursor and the carbon precursor is thus not fundamentally limited. Preferred silicon precursors may include, for example, silicates, such as tetraethylorthosilicate (TEOS), whereas preferred carbon precursors may include sugars, such as sucrose, to form the solid particles provided as the source of carbon and silicon source in step a). For example, a mixture of liquid sugar and tetraethyl orthosilicate dissolved in ethanol may be provided as a mixture of carbon precursor and silicon precursor in process step c), the invention being understood to be not limited to the aforementioned examples.

This can according to process step d), for example, in a temperature range close to the boiling point of the solvent, in the use of ethanol in a range of 60-70 ° C, brought to an end or gelled aging, and it is also at a temperature above the Boiling point can be dried. It may be advantageous if, during drying of the solid, particles are formed which have a maximum have diameters in a range of about> 1 μιη to <2mm, with particles in a size range of> 10 μιη to <2mm for nanocrystalline silicon carbide fibers or a foam formed therefrom are particularly preferred. The aforementioned size ranges have in particular process engineering advantages, such as preventing the increase of finer particles in a fiber production. Such a particle size can be made possible, for example, by stirring during drying, wherein the particle size may be adjustable by the agitator used, a rotational speed and the duration or strength of the stirring, as is generally known in the art.

In accordance with process step e), the dried precursor mixture is then optionally heated to a temperature in a range from> 800 ° C. to <1200 ° C., in particular in a range from> 900 ° C. to <1100 ° C., for example from 1000 ° C. By means of this method step, the solid produced can be freed from impurities in particular, which can make the silicon carbide produced particularly pure. As a result, the quality of a generated fiber or a foam can be set particularly high and further defined. In addition, crystallization of the silicon carbide from the gas phase can be improved. By method step d) or optionally e) while the mixture according to step a) is provided or completed, which can be formed by the above sol-gel process particles, each having a silicon source, such as pure silicon or silicon dioxide, and a carbon source , such as pure carbon. Upon addition of a dopant during the sol-gel process, it may also be present in the particles, as described in detail below. Thus, by the sol-gel process, a mixture can be made possible on a quasi-atomic level, which significantly simplifies the production of silicon carbide. In summary, therefore, in one embodiment, a sol-gel process can be used in which the materials to be processed together form a mixture in the form of a gel and are then dried, and in a further step in a carbothermal reduction the crystallization of the silicon carbide, such as the growth of fibers, runs off. The sol-gel process, which is known per se as a process, offers a readily controllable and widely variable possibility of producing a wide variety of starting materials for the production of the fiber material according to the invention or of its starting materials.

In a further preferred embodiment of the method, it can be provided that process step b) proceeds in a reactor having a Abscheideooberfläche whose temperature is reduced relative to at least one other inner reactor surface. In particular, in this embodiment, it can be made possible in a particularly simple manner and without the need for a high apparatus design that silicon carbide separates from the gas phase in the desired manner, in particular as a foam-forming fibers by providing a temperature gradient. Because the contact with the Abscheideooberfläche can deposit silicon carbide directly from the gas phase, without the need for further funds. For example, the reactor may be configured by an upwardly open vessel, such as an upwardly open cylinder, in which the mixture is heated to the prescribed temperature in accordance with step a). Above the opening, which is directed upwards, for example, the, for example, circular and approximately rotatable deposition surface can be arranged approximately in the interior of the vessel or aligned therewith, so that the gas phase can come into contact with the deposition surface, whereby here the silicon carbide, for example in Form of nanoscale fibers or a foam formed therefrom, can separate.

With particular reference to the above-described step, depending on the desired shape of the silicon carbide to be produced, it may be advantageous that, for example, in the case of a Drying process during the sol-gel process, such as by stirring, a suitable particle size of the solid is set, as described above with respect to process step e). For example, it may be preferred if a particle size arises which is in a range of> ΙΟμιη to <2mm, for example in a range of> 25μιη to <70μιη to produce silicon carbide fibers or a foam formed therefrom.

In this case, provision may be made for the deposition surface to have a temperature which, at least in relation to at least one further inner reactor surface, has a temperature which is in the range of> 30 ° C. to <200 ° C., preferably in the range of> 50 ° C to <100 ° C, is reduced. In this embodiment, the deposition of a particular foam can be particularly effective, with such a temperature difference is also easily adjustable in terms of process technology. In principle, the further inner reactor surface may be any surface located within the reactor, such as an inner wall or, in particular, a receiving surface for receiving the mixture.

With regard to a further preferred embodiment, it may further be provided that the silicon carbide fibers are doped. This is possible by introducing a desired dopant into the fibers so as to enable electrical conductivity.

By way of example, but in no way limiting, in process step a) a mixture with a silicon source, a carbon source and a dopant can be provided, the silicon source, the carbon source and the dopant being present together in particles of a solid granulate. With respect to the dopant, this can be selected based on the desired doping. The dopant (s) may in principle be chosen freely, for example in a production process of the solid granules be added as a soluble compound or about elementary, such as metallic, are added. Thus, the dopant may also be part of the solid granules according to process step a). Alternatively, it is also conceivable that the doping of the forming silicon carbide, such as forming fibers as 3C silicon carbide nanocrystals, as described in detail below, during the thermal treatment is carried out via the gas phase, especially if the Doping is carried out using a gaseous dopant in process step b). As doping materials, phosphorus (P) or nitrogen (N) may preferably be used for n-type doping, or boron (B) or aluminum (Al) may be used for p-type doping. By doping a particularly good electrical conductivity of the fibers or the foam can be adjusted. For example, as far as the dopant is not present in the solid granules having the carbon source and the silicon source and the solid granules according to step a) is transferred to the reactor in which the temperature treatment according to process step b) takes place, the dopant as a gas in the Reactor can be given, wherein the mixture according to process step b) 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.

It is advantageous in particular when the dopant is provided in the particles of the mixture according to method step a) when the doping materials are introduced into the wet-chemical part of the sol-gel synthesis, whereby the doping materials are introduced into the growing fibers during the thermal treatment Particles are incorporated. The doping materials can be added here either as a soluble compound or metallic added. Alternatively, as described above, it is also conceivable that the doping of the forming fibers is carried out during the thermal treatment via the gas phase. Phosphorus (P) or nitrogen (N) or boron (B) or aluminum (AI) can in turn preferably be used as doping materials.

In particular, by adding a dopant, it may be possible for the silicon carbide fibers or the foam produced to have an extremely good electrical conductivity. This can further expand the field of application of the foam produced or of the components produced therefrom.

It may also be advantageous if process step b) is carried out for a period in the range of> 30 s to <10 min. By maintaining a predetermined period of time in the formation of the foam, it may be particularly preferred to allow all the silicon carbide fibers that form to join into a highly branched network to form a foam having substantially no free or low branched silicon carbide fibers. In this case, the time period used can be selected in a way which is obvious to the person skilled in the art as a function of further reaction parameters, such as the exact temperature.

With regard to further advantages and technical features of the above-described method for producing a foam, reference is hereby explicitly made to the description of the foam, the use as well as to the figure and the example, and vice versa.

The present invention further provides a foam, wherein the foam comprises a plurality of in particular nano- or microstructured fibers, wherein the fibers comprise silicon carbide, and wherein the fibers at least partially a plurality of at least two, in particular of at least three, connection points, wherein the connection points of a first fiber are each connected to at least one connection point of another fiber. Thus, the foam is made up of a network of silicon carbide fibers, ie fibers having silicon carbide, which are each connected to other silicon carbide fibers via at least two, in particular at least three, connection points. For example, the foam can consist of silicon carbide fibers or silicon carbide fiber strands and / or the fibers can consist of silicon carbide. For example, the silicon carbide may be 3C-SiC. Furthermore, the silicon carbide may be in hexagonal form.

A foam described above can in particular by the configuration of particular monocrystalline fibers of the crystal structure 3C-SiC in a particularly advantageous manner, the properties of a foam, namely a suitable elasticity or flexibility, combine with a high stability, in particular resistance to harsh conditions, such as comprehensive high Temperatures, aggressive media and high mechanical influences. As a result, the foam can be particularly durable for a large number of applications.

The abovementioned advantages can be achieved, in particular, by the fact that the foam is produced by a process which has been developed and further developed as described in detail above.

It may further be preferred that the silicon carbide fibers have a length of several millimeters and a thickness or a diameter in the nanometer range to micrometer range. In particular, it may be provided that the silicon carbide fibers have a length in a range of> 5mm to <20mm. Additionally or alternatively, it can be provided that the silicon carbide fibers have a thickness in a range of> 10 nm to <2 μm. It has been found that, in particular, silicon carbide fibers with the parameters described above it can allow the foam formed to combine suitable elasticity with a high degree of robustness.

It may also be advantageous that the foam has a porosity in a range of> 30% to <80%, wherein the above-described values refer to the free, so not occupied by a Siliziumcarbidfaser, volume with respect to the total volume. Such porosities of the foam can also lead to the foam having particularly advantageous properties with regard to its elasticity or robustness. In addition, the field of application can be increased.

With respect to the thickness and / or length of the silicon carbide fibers, and further with respect to the porosity or density of the foam, these parameters may be adjustable, in particular by the chosen parameters of the manufacturing process. For example, by adjusting the temperature in the method step b) of the method described above or by a residence time of the fiber composite on a Abscheideooberschicht the formation of the foam, these properties of the foam forming fibers are influenced.

In a further preferred embodiment, it may further be provided that the foam is electrically conductive. In particular, by providing an electrically conductive foam which is composed of electrically conductive silicon carbide fibers, the field of application of the foam can be significantly increased compared to a non-doped foam. By way of example and in no way limiting, the electrically conductive fibers ¹ ¹ may have a conductivity of 0,0005Ω αη ^~ ^{~ 1} to <lff'cm ^"1, for example,> ^~ 0,005Ω ΰΐη ^{~ 1} to <Ο Ω ^ ^{'^} ^{ι' 1} In this case, the silicon carbide fibers of the foam can be provided with a desired electrical conductivity in a particularly simple manner by doping, as described in detail above with respect to the method, for example by selection or amount of the dopant used. With regard to further advantages and technical features of the above-described foam, reference is hereby explicitly made to the description of the method for producing a foam, the use, as well as to the figure and the example, and vice versa.

The present invention further provides a use of a foam as described above for producing a heat-conducting element, an electrode, a seal, such as electrically conductive or electrically insulating, a filter, in particular an electrostatic filter, an absorber, in particular for high-frequency technology, a Flame retardant material, a catalyst, or a component for flame regulation, such as a gas burner, such as a component in Gasausströmbereich, such as for gas burners. The further processing of the pure foam to a prescribed component can for example take place in that the material is brought about by cutting into a desired shape and / or introduced into a suitable periphery. Furthermore, the further processing may in principle comprise any suitable step which can finish the component in a manner known per se to a person skilled in the art, such as the provision of suitable structures, electrical connections or the like. This is in a manner understandable to the skilled person readily understandable depending on the specific shaped component.

Particularly in the case of the abovementioned components, the above-described foam comprising silicon carbide fibers of silicon carbide, for example of the crystal structure 3C-SiC, can bring advantages. For example, gaskets can be produced inexpensively even under extreme conditions, such as for high temperatures and aggressive media, and have a long service life. The same applies to thermally conductive elements or heat-conducting, which may be electrically insulating or electrically conductive depending on the doping. For example, such an element can be used to connect a Heat source, such as a high-performance electronic element, with a heat sink. With regard to the electrode, it can be doped in particular and can also be resistant to aggressive media. Flame retardant materials, flame control materials, catalysts or absorbers can also be produced particularly advantageously by the method described above.

With regard to further advantages and technical features of the above-described use, reference is hereby explicitly made to the description of the method for producing a foam, of the foam and to the figure and the example, and vice versa.

In the following, the invention will be explained by way of example with reference to a figure and a preferred exemplary embodiment, wherein the features illustrated below may represent an aspect of the invention both individually and in combination, and the invention is not limited to the following example.

It shows:

Fig. 1 A SEM image of a foam according to the present invention.

FIG. 1 shows an electron microscope (SEM) image of a foam according to the invention. 1 shows a plurality of silicon carbide fibers which are connected to one another at a plurality of connection points. Such a network of silicon carbide fibers forms a foam with particularly advantageous properties.

An exemplary method for producing such a foam is described in the following embodiment. embodiment

The example described below relates to producing a foam configured of silicon carbide fibers using a sol-gel process to form the starting mixture.

Preparation of the sol-gel-Si-C precursor: In the following, the chemical composition, sol-gel preparation with various drying steps at 70 ° C. to 200 ° C., as well as final recovery of the Si-C solid granules, are added 1000 ° C described.

Liquid sugar, tetraethyl orthosilicate and ethanol are mixed to form a sol and gelled at 60-70 ° C with exclusion of air. The composition for one batch was (a) a colloidal suspension of 135 g of tetraethyl orthosilicate (TEOS) dissolved in 168.7 g of ethanol as a silicon source and (b) a solution of 68 g of sucrose as the carbon source in 75 g of distilled water containing 37.15 g of hydrochloric acid (HCl) is added as a catalyst for forming invert sugar. Subsequently, solution (a) was mixed with the liquid sugar (b) with stirring. Alternatively, liquid sugar (invert sugar, 122g 70 ig) can be used instead of solution (b) directly. Then no water is added and only very little hydrochloric acid (5.2 g), since this is only needed to start the gelling process. This sol is aged at 50 ° C and then dried at 150 - 200 ° C.

In order to obtain comparatively coarser granules in the range of a few tens of μm, temporary stirring takes place during aging and / or during drying. This granulate or powder is freed from remaining un desired reaction products at 1000 ° C in a nitrogen or argon gas stream and finally ground if necessary. There may be a modification of the SiC precursor for the purpose of doping SiC nanofibers. An n-doping can be carried out with nitrogen (for example, additives: nitric acid, ammonium chloride, potassium nitrate or melamine) or with phosphorus (exemplary additives: potassium dihydrogen phosphate or di-sodium hydrogen phosphate). A p-doping can be carried out by way of example with boron (exemplary additives: di-sodium tetraborate) or with aluminum (add .: aluminum powder). The dopants are added to the sol, the amounts are dependent on the specific additive and the desired doping concentration. Concerning the production of silicon carbide fibers, the resulting solid is heated in a high-temperature reactor, wherein the granules in a temperature range of> 1400 ° C to <1500 ° C in the gas phase passes and monocrystalline silicon carbide fibers in a temperature gradient on a rotating substrate or Depositing on a deposition surface cooler by about 50-100 ° C, wherein the fibers are bonded together to form a foam.

Claims

claims

1. A method for producing a nano- or micro-structured foam, wherein the foam is composed of a plurality of interconnected, in particular nano- or micro-structured Siliziumcarbidfasern, characterized in that the method comprises the method steps:

2. The method according to claim 1, characterized in that the provided in step a) mixture is provided using a sol-gel process.

3. The method according to claim 2, characterized in that the sol-gel process has at least the following method steps:

c) providing a precursor mixture comprising a silicon precursor, a carbon precursor and optionally a dopant, wherein the precursor mixture is present in a solvent;

d) treating the precursor mixture at elevated temperature while drying the precursor mixture; and

4. The method according to any one of claims 1 to 3, characterized in that process step b) takes place in a reactor having a Abscheideloberoberfläche whose Tem- temperature is reduced relative to at least one further inner reactor surface, in particular wherein the Abscheidenoberfläche has a temperature relative to at least one other inner reactor surface has a temperature by an amount in a range of> 30 ° C to <200 ° C, preferably in a range of> 50 ° C to <100 ° C, is reduced.

5. The method according to any one of claims 1 to 4, characterized in that the silicon carbide fibers are doped.

6. The method according to claim 5, characterized in that the doping is carried out using a gaseous dopant in step b).

7. The method according to claim 5, characterized in that in method step a) a mixture with a silicon source, a carbon source and a dopant is provided, wherein the silicon source, the carbon source and the dopant are present together in particles of a solid granules.

8. The method according to any one of claims 1 to 7, characterized in that method step b) is carried out for a period in a range of> 30s to <10min.

9. foam, characterized in that the foam has a plurality of particular nano- or micro-structured fibers, wherein the fibers comprise silicon carbide, and wherein the fibers at least partially have a plurality of at least two, in particular of at least three connection points, wherein the Connection points of a first fiber are each connected to at least one connection point of another fiber.

10. Foam according to claim 9, characterized in that the Siliziumcarbidfasern have a length in a range of> 5mm to <20mm.

11. The foam of claim 9 or 10, characterized in that the Siliziumcarbidfasern have a thickness in a range of> lOnm to <2μιη.

12. Foam according to one of claims 9 to 11, characterized in that the foam has a porosity in a range of> 30% to <80%.

13. Foam according to one of claims 9 to 12, characterized in that the foam is electrically conductive.

14. Use of a method according to any one of claims 1 to 8 for producing a foam.

15. Use of a foam according to any one of claims 9 to 13 for the manufacture of a thermally conductive element, an electrode, a gasket, a filter, an absorber, a flameproofing material, a Flammregulierungsmaterials, or a catalyst.