High-temperature component and method for manufacturing

Abstract

Method for producing a high-temperature component, wherein a resistance heating element (10) is formed as the high-temperature component, wherein a dimensionally stable green body of the high-temperature component is formed from a matrix material, wherein the green body is formed by means of pyrolysis of the matrix material to form the high-temperature component, wherein a material mixture of the matrix material with a carbon material is used to form the high-temperature component, wherein the green body is formed by means of an additive manufacturing process, characterized in that a thermoplastic is used as the matrix material, wherein the resistance heating element is formed with a heating conductor (11), wherein the resistance heating element is formed with an electrically non-conductive conductor carrier (12) receiving the heating conductor, wherein a further material mixture of the matrix material with a silicon material is used to form the conductor carrier.

Description

The invention relates to a method for producing a high-temperature component and to a resistance heating element, wherein a resistance heating element is formed as a high-temperature component, wherein a dimensionally stable green body of the high-temperature component is formed from a matrix material, wherein the green body is formed by pyrolysis of the matrix material to form the high-temperature component, wherein a material mixture of the matrix material with a carbon material is used for forming the high-temperature component, wherein the green body is formed by an additive manufacturing method.High-temperature components and in particular resistance heating elements are regularly used as heating elements for thermal analysis in so-called DSC ovens (differential scanning calorimetry ovens). The known resistance heating elements are therefore tubular and of one piece and are contacted on their underside at an anode and a cathode or connection surfaces. A wall of the resistance heating element is provided with two slots which are of helical design and thus form heating spirals of the resistance heating element or a heating conductor. In the region of the heating coils of the resistance heating element, a temperature of up to 1,650 °C is reached. In this case, an incandescent pattern should be distributed as homogeneously as possible over the region of the heating coils. Furthermore, a high purity of the material of the resistance heating element is of great importance, since, for example, undesirable additives may diffuse out of the resistance heating element and distort a measurement when samples are determined in purity in the DSC furnace.In a known method for producing a high-temperature component or a resistance heating element, for example, a material blank made of a fiber material is dimensionally stabilized by means of resin and is formed with final pyrolysis and infiltration of silicon in order to obtain a resistance heating element made of silicon carbide. It is also known to form a cylindrical shaped body for forming a resistance heating element by a slip process. In order to obtain a desired heating coil structure, a green body or a cylindrical shaped body formed by the slip process must be processed. A low strength of the green body in this case, as in the case of the resistance heating element formed from fibrous material, significantly restricts the machining possibilities and results in a high number of defective parts due to breakage as a result of machining and handling within the scope of the respective production method. Cracks can also result, in particular, from an inhomogeneous distribution of materials in the resistance heating element during operation.It is also known to produce resistance heating elements or a green body of a resistance heating element by layer-wise construction of a powder-resin mixture containing silicon carbide or silicon and carbon. A support matrix made of silicon carbide can then be formed by pyrolysis of the green body, said support matrix being partially filled with carbon or subsequently being filled with a carbon-containing material in further process steps in order to set a resistance of the resistance heating element. Here too, the green body consisting of the powder mixture with a binder or resin is very fragile and a series of further process steps are required following pyrolysis, for example infiltration of the resistance heating element with carbon is carried out.DE 10 2014 216 433 A1 discloses a resistance heating element and a method for producing the same. In this case, a powder mixture of silicon carbide or silicon and carbon is produced with a carbon-containing synthetic resin and is output layer by layer by means of a so-called 3D printing device in such a way that a dimensionally stable green body of the resistance heating element is obtained.A further resistance heating element and a method for producing the same are known from EP 2 694 451 B1. In this case, a molded body made of a fiber material, in particular fiber felt, is impregnated with a so-called matrix material, for example a thermoplastic resin, cured and subsequently pyrolyzed.The present invention is therefore based on the object of proposing a method for producing a high-temperature component or a resistance heating element, which method enables efficient production.This object is achieved by a method having the features of claim 1 and a resistance heating element having the features of claim 14.In the method according to the invention for producing a high-temperature component, a resistance heating element is formed as a high-temperature component, wherein a dimensionally stable green body of the high-temperature component is formed from a matrix material, wherein the green body is formed by pyrolysis of the matrix material to form the high-temperature component, wherein a material mixture of the matrix material with a carbon material is used for forming the high-temperature component, wherein the green body is formed by an additive manufacturing method, wherein a thermoplastic is used as the matrix material, wherein the resistance heating element is formed with a heating conductor, wherein the resistance heating element is formed with an electrically non-conductive conductor carrier receiving the heating conductor, wherein a further material mixture of the matrix material with a silicon material is used for forming the conductor carrier.With the method, it is possible in principle to form any desired shape of a high-temperature component. A high-temperature component is understood here to mean a component which can be used at a temperature from a range from 300° C. to 3,000 ° C. By using a thermoplastic, it can be ensured particularly well in the case of the layer-by-layer construction of the green body that the respective layers, unlike in the case of a powder mixture with a resin or the like, adhere firmly to one another, as a result of which a particularly stable green body is obtained. This green body can be handled easily without the risk of rapid fracture of the green body. Furthermore, during the subsequent pyrolysis of the green body in a furnace, the thermoplastic can be largely converted into carbon. Together with the carbon material which has been added to the thermoplastic, a high-temperature component can then be obtained which has a low porosity and thus a high carbon content. A comparatively high filling of the thermoplastic with the carbon material brings about a low electrical resistance and a reduced shrinkage and thus a better stability of the high-temperature component during pyrolysis. Overall, a number of possible scrap parts can thus be significantly reduced during the production of the high-temperature component.It is particularly advantageous if the high-temperature component is formed in one piece. This eliminates the need for mounting a plurality of components to form the high-temperature component, and the high-temperature component can be produced more easily overall. In the case of a one-piece formation of the high-temperature component, the green body can then likewise be formed in one piece.According to the invention, a resistance heating element is formed as a high-temperature component, wherein the resistance heating element is formed with a heating conductor. The heating conductor of the resistance heating element, from which the entire resistance heating element can also be formed, can be built up in layers from the thermoplastic to which a carbon material is added by the additive manufacturing method and thus be formed in its shape. Together with the carbon material which has been added to the thermoplastic, a heating conductor can then be obtained which has a low porosity and thus a high carbon content. With the method it is possible in principle to form any desired shape of a resistance heating element. However, the method is particularly suitable for forming resistance heating elements with a complex geometry and also flat or planar resistance heating elements with comparatively fine heating conductors, since these resistance heating elements can easily be destroyed in the production methods known from the prior art.According to the invention, the resistance heating element is formed with an electrically non-conductive conductor carrier receiving the heating conductor. The conductor carrier can then have dielectric or semiconductor properties (>10 4 S / cm). Because the conductor carrier can accommodate the heating conductor, the heating conductor can be arranged on the conductor carrier, embedded therein or enclosed on all sides by the conductor carrier. This is only possible by forming the green body by means of the additive manufacturing method. The heating conductor can then be formed to be particularly delicate, since the heating conductor is then supported by the conductor carrier. Even if the conductor carrier is formed to be substantially non-conductive or semi-conductive, it is unnecessary to form air gaps in the material of the green body or of the resistance heating element and thus to perform a mechanical processing, which can often be the cause of a fracture of the green body or of the resistance heating element. Also, due to the absence of air gaps between heating conductor tracks, a temperature distribution or an annealing pattern of the resistance heating element can be made more homogeneous and thus improved. The conductor carrier can have the lowest possible electrical conductivity, so that a current flow between conductor tracks of the heating conductor is avoided. In principle, however, it is still possible to form an air gap between conductor tracks of the heating conductor if this appears advantageous.According to the invention, a further material mixture of the matrix material with a silicon material is used for forming the conductor carrier. This makes it possible, by means of the additive manufacturing method, to join the material mixture together with the further material mixture easily and stably to one another on account of the matching matrix material. In the case of a layer-by-layer construction of the green body, the green body then completely consists of the matrix material, wherein the carbon material and the silicon material are then dispensed during the layer-by-layer construction depending on the configuration of the heating conductor and of the conductor carrier.The green body can be formed with the material mixture embedded in the further material mixture. Accordingly, the heating conductor can be arranged substantially within the conductor carrier. The conductor carrier then completely surrounds the heating conductor, whereby the heating conductor can be protected against oxidation and mechanical damage. For the connection of the resistance heating element, it can be provided that connection surfaces of the heating conductor are formed on the resistance heating element, which connection surfaces emerge from the conductor carrier or are not covered by the conductor carrier. Embedding the material mixture in the further material mixture is only possible by using the additive manufacturing method.The further material mixture can be used with a stoichiometric mixture of matrix material and silicon material, wherein silicon carbide can then be formed from the further material mixture during pyrolysis. Since the matrix material is a thermoplastic which is converted into carbon during pyrolysis, silicon carbide can be formed by the mixture with the silicon material during pyrolysis. In order to avoid an excess of free silicon or carbon in the conductor carrier, the stoichiometric mixture of matrix material and silicon material is then used. For the formation of silicon carbide, a mass ratio of carbon to silicon of 1:2.33 is required. In this case, it is to be taken into account which mass of carbon can be obtained during the pyrolysis of the thermoplastic. The stoichiometric mixture is therefore always also dependent on the selection of the matrix material. If pure silicon carbide can be formed, a particularly sharp separation of heating conductor and conductor carrier becomes possible. In addition, a low electrical conductivity of the conductor carrier is formed and a diffusing out of free silicon from the conductor carrier can be avoided.Thus, by means of pyrolysis, the material mixture of the heating conductor can be converted into carbon and the further material mixture of the conductor carrier can be converted into silicon carbide. By filling the matrix material or thermoplastic with carbon material or silicon material, a high density of the resistance heating element can also be achieved with a low porosity at the same time. Silicon fibers or silicon particles can be used as the silicon material, for example. The use of silicon fibers makes it possible to already form the green body in a very stable manner. The silicon fibers also prevent any cracks in the green body or resistance heating element during subsequent method steps.Carbon fibers, carbon black, graphite, graphene and / or carbon nanotubes can be used as carbon material. In particular, the exclusive use or partial admixture of graphene makes it possible to substantially improve a conductivity of the heating conductor with respect to graphite. Furthermore, a high proportion of carbon in the thermoplastic causes a reduced shrinkage of the heating conductor during pyrolysis. In addition to carbon fibers, however, other organic fibers can also be used, which can be converted into carbon in the course of pyrolysis. The carbon fibers also improve the strength properties of the green body or of the high-temperature component or resistance heating element.The fibers can preferably be short-cut fibers and can be discharged from a nozzle together with the matrix material and arranged spatially. If the fibers are extruded from the die together with the matrix material or the thermoplastic, the green body can then be formed without the aid of a mold. The fibers are deposited layer by layer together with the thermoplastic on the basis of a data model of a shape of the green body or of a heating conductor and of a conductor carrier from the nozzle. It can be provided here that this takes place with a nozzle for the material mixture and a further nozzle for the further material mixture. The die is then moved along the shape of the green body during extrusion so that the green body is generatively constructed by applying the fibers with the thermoplastic. It then becomes possible to form a fiber composite component or a green body with virtually any desired shape. The use of a mold for forming the shape is then not necessary, as a result of which the green body can also be produced more cost-effectively overall. The fiber can have a diameter of 5 μm to 30 μm, preferably of 10 μm. Fibers having these diameters are particularly well suited for extrusion from the die together with the thermoplastic.It is particularly advantageous if the high-temperature component or resistance heating element can be formed with a fiber content of 10 vol. % to 60 vol. %, preferably of up to 35 vol. %. A high proportion of fiber promotes the strength properties of the green body or of the resistance heating element.The green body can be formed by means of fused deposition modelling (FDM). In melt lamination, a grid of dots is applied to a surface using a so-called 3D printer. In this case, a wire-shaped thermoplastic is fed to the 3D printer, heated and extruded via a die, the thermoplastic then hardening by cooling to the desired position. The green body is then built up by running the respective layers line by line with the nozzle, so that a shape of the green body is formed in layers. A layer thickness can be between 0.025 mm and 1.25 mm.Polyetherimide (PEI), polyetheretherketone (PEEK), polysulfone (PSU), or polyphenylene sulfone (PPSU) can be used as the matrix material. These plastics are suitable for use by means of an additive manufacturing method and a high proportion of carbon can be obtained during pyrolysis of these plastics, for example 55 mass % for PEI and 50 mass % for PEEK.After pyrolysis, a CVD coating of the high-temperature component or resistance heating element with silicon carbide can take place. Thus, leakage of free silicon during operation of a resistance heating element can be prevented. In CVD (chemical vapor deposition) coating, a silicon carbide layer is applied to the high-temperature component or resistance heating element at, for example, 700° C. to 1,500° C. The silicon carbide layer substantially completely surrounds the high-temperature component or resistance heating element, so that any silicon enclosed in the material of the high-temperature component or resistance heating element cannot escape from the latter.After pyrolysis, a high-temperature treatment of the high-temperature component or resistance heating element can take place. The pyrolysis may be performed in a temperature range of 280° C. to 1,200° C. and the high-temperature treatment may be performed in a temperature range of 1,200° C. to 2,400° C. The high-temperature treatment can serve, inter alia, for the decomposition of oxygen and nitrogen in the high-temperature component or resistance heating element and can be carried out under vacuum or protective gas.The resistance heating element according to the invention is formed in one piece, wherein the resistance heating element is formed with a heating conductor made of carbon, wherein the resistance heating element is formed with an electrically non-conductive conductor carrier made of silicon carbide, which receives the heating conductor, wherein a zone containing both carbon and silicon carbide is formed between the heating conductor and the conductor carrier, wherein the resistance heating element is formed with a material gradient between the carbon of the heating conductor and the silicon carbide of the conductor carrier. A non-conductive conductor carrier is understood here to mean a conductor carrier having an electrical conductivity of at least >10 4 S / cm. In particular, because the conductor carrier made of silicon carbide accommodates the heating conductor made of carbon, the heating conductor made of carbon is mechanically stabilized by the conductor carrier. The heating conductor can then be formed comparatively thin or also in a virtually arbitrary shape which no longer has to be bonded to a shape taking account of strength properties of the heating conductor. The resistance heating element can be formed in particular using the method according to the invention. For further advantageous properties of the resistance heating element, reference is made to the description of the advantages of the method according to the invention.According to the invention, the resistance heating element is formed with a substance gradient between the carbon of the heating conductor and the silicon carbide of the conductor carrier. Accordingly, according to the invention, a zone is formed between the heating conductor and the conductor carrier, which zone contains both carbon and silicon carbide. This zone is formed by diffusion from the material of the heating conductor into the material of the conductor carrier and vice versa during pyrolysis. It is particularly advantageous if the zone is formed to be comparatively thin.The heating conductor can be embedded in the conductor carrier, preferably enclosed by the conductor carrier. As a result of this configuration of the resistance heating element, the heating conductor can be protected particularly well against cracks or breakage. Furthermore, during operation of the resistance heating element, the conductor carrier can be heated by the heating conductor, so that a particularly homogeneous glow pattern and thus a good heat distribution can be achieved.Electrical connection surfaces of the heating conductor can also be formed on the conductor carrier. The electrical connection surfaces can protrude from the conductor carrier or be formed in a plane with a surface of the conductor carrier, so that no jumps or shoulders are present in the surface. The resistance heating element can also be coated in the region of the connection surfaces by means of flame spraying. By thermal spraying of pulverulent aluminum, the connection surfaces can thus be provided with an aluminum layer which can be easily contacted electrically. Aluminum can be processed well by means of flame spraying and does not melt off the resistance heating element during operation of the latter.The resistance heating element can be formed with a round tube cross section and a helical heating conductor. The helical heating conductor can be designed in the manner of a helix or a double helix, it being possible for connection surfaces to be arranged at one end of the heating conductor. The resistance heating element can then form a shaped body in the manner of a tube with the same diameter and the same wall thickness. An embodiment of air gaps for electrically separating the helical heating conductor tracks is then no longer necessary, but air gaps can also be present in principle.Further advantageous embodiments of a resistance heating element are evident from the feature descriptions of the dependent claims referring back to method claim 1.The invention is explained in more detail below with reference to the attached drawings.The following are shown: FIG. 1 is a side view of a resistance heating element; FIG. 2 shows a perspective view of a heating conductor of the resistance heating element; FIG. 3 is a partial sectional view of the resistance heating element of FIG. 1.A combination of FIGS. 1 to 3 shows a high-temperature component or a resistance heating element 10, which is formed in one piece and consists of a heating conductor and a conductor carrier 12. The resistance heating element 10 has a tubular or round, circular cross section 14 with respect to a longitudinal axis 13. A tube wall 15 of the resistance heating element 10 is embodied to be comparatively thin. Furthermore, in the region of a lower end 16 of the resistance heating element 10, two connection surfaces 17 and 18 are formed for connecting the resistance heating element 10 to connection contacts of a connection device, not shown here, of a DSC furnace.As can be seen from FIG. 2, the heating conductor 11 shown here alone is formed with spiral heating conductor tracks 19 and 20 which run within the tube wall 15. The heating conductor tracks 19 and 20 extend from the connection surfaces 17 and 18 in the longitudinal direction as far as an upper end 21 of the resistance element 10 and meet there in the form of a ring 22 of the heating conductor 11.FIG. 3 shows a partial sectional view through the tube wall 15 along the longitudinal axis 13, wherein it can be seen here that the heating conductor tracks 19 and 20 run in the tube wall 15 or within the latter, and thus within the conductor carrier 12. The heating conductor 11 is formed in particular from a material mixture of a matrix material with a carbon material and the conductor carrier 12 from a further material mixture of the matrix material with a silicon material, wherein a thermoplastic is used as the matrix material and a green body is formed by means of an additive manufacturing method from the material mixture and simultaneously from the further material mixture and is subsequently formed by means of pyrolysis to form the resistance heating element 10. The heating conductor 11 of the resistance element 10 thus consists essentially of carbon and the conductor carrier 12 of silicon carbide. Within a very narrow boundary region 23 between the heating conductor 11 and the conductor carrier 12, a substance gradient can be formed due to the pyrolysis of the material mixture and the further material mixture. Furthermore, the resistance heating element 10 is provided with a silicon carbide layer, which is not visible here and which has been applied in the context of a CVD coating method.

Claims

Method for producing a high-temperature component, wherein a resistance heating element (10) is formed as a high-temperature component, wherein a dimensionally stable green body of the high-temperature component is formed from a matrix material, wherein the green body is formed by pyrolysis of the matrix material to form the high-temperature component, wherein a material mixture of the matrix material with a carbon material is used for forming the high-temperature component, wherein the green body is formed by an additive manufacturing method, characterized in that a thermoplastic is used as the matrix material, wherein the resistance heating element is formed with a heating conductor (11), wherein the resistance heating element is formed with an electrically non-conductive conductor carrier (12) receiving the heating conductor, wherein a further material mixture of the matrix material with a silicon material is used for forming the conductor carrier.Method according to Claim 1, characterized in that the high-temperature component is formed in one piece.Method according to claim 1 or 2, characterised in that the green body is formed with the material mixture embedded in the further material mixture.Method according to one of the preceding claims, characterized in that the further material mixture is used with a stoichiometric mixture of matrix material and silicon material, silicon carbide being formed from the further material mixture during pyrolysis.Method according to one of the preceding claims, characterized in that the material mixture of the heating conductor (11) is converted into carbon and the further material mixture of the conductor carrier (12) is converted into silicon carbide by means of the pyrolysis.Method according to one of the preceding claims, characterized in that silicon fibres or silicon particles are used as silicon material.Method according to one of the preceding claims, characterized in that carbon fibres, carbon black, graphite, graphene and / or carbon nanotubes are used as carbon material.Method according to claim 6 or 7, characterised in that the fibres, preferably short-cut fibres, are discharged from a nozzle together with the matrix material and are spatially arranged.Method according to one of Claims 6 to 8, characterized in that the high-temperature component is formed with a fibre content of from 10% by volume to 60% by volume, preferably of up to 35% by volume.Method according to one of the preceding claims, characterized in that the green body is formed by means of fused deposition modelling (FDM).Method according to one of the preceding claims, characterized in that polyetherimide (PEI), polyetheretherketone (PEEK), polysulfone (PSU) or polyphenylene sulfone (PPSU) is used as matrix material.Method according to one of the preceding claims, characterized in that a CVD coating of the high-temperature component with silicon carbide takes place after the pyrolysis.Method according to one of the preceding claims, characterized in that a high-temperature treatment of the high-temperature component is carried out after the pyrolysis.Resistance heating element (10), wherein the resistance heating element is formed in one piece, wherein the resistance heating element is formed with a heating conductor (11) made of carbon, characterized in that the resistance heating element is formed with an electrically non-conductive conductor carrier (12) made of silicon carbide, which receives the heating conductor, wherein a zone containing both carbon and silicon carbide is formed between the heating conductor and the conductor carrier, wherein the resistance heating element is formed with a material gradient between the carbon of the heating conductor and the silicon carbide of the conductor carrier.Resistance heating element according to claim 14, characterised in that the heating conductor (11) is embedded in the conductor carrier (12), preferably enclosed by the conductor carrier.Resistance heating element according to claim 14 or 15, characterised in that electrical connection surfaces (17, 18) of the heating conductor (11) are formed on the conductor carrier (12).Resistance heating element according to one of claims 14 to 16, characterised in that the resistance heating element (10) is formed with a round tube cross-section (14) and a helical heating conductor (11).

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