CERAMIC CARBIDE FIBER, METHOD FOR MAKING THE CERAMIC CARBIDE FIBER AND APPLICATION OF THE CERAMIC CARBIDE FIBER

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a ceramic carbide fiber, a method for making the ceramic carbide fiber and an application of the ceramic carbide fiber.

2. Description of the Related Art

From the publication "Formation of Inorganic (TaC, TaN) Fibers by Thermal Decomposition of Cellulose Acetate-Tantalum Alkoxide Precursor Gel Fibres" , Journal of Applied Polymer Science, Vol. 100 (2006) 4320 - 4324 a method for making pure Tantalum Carbide (TaC) fibers by sol-gel-technique is known.

TaC is a candidate material for high temperature thermal emitter due to its high melting temperature of above of 3800° C. However, at temperatures of above of 3000° C an exponential increase of a decomposition rate due to carbon evaporation occurs. Therefore, the use of TaC as filament material of lamps is improper.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a ceramic carbide fiber, which can be used as filament material of lamps. Another object of the invention is the providing of a method for making the ceramic carbide fiber. The method should be easy and reproducible.

These objects are achieved by the inventions specified in the claims . The idea behind the invention is a modification of the known ceramic carbide fibers and a modification of the known method for making the carbide fibers.

The present invention provides a ceramic carbide fiber comprising: a carbide solid solution with at least two transition metals; the fiber comprises a fiber diameter selected from a range of 10 μm to 200 μm. Substantially the fiber diameter is selected from the range of 20 μm to 80 μm and more particularly in the range of 20 μm to 60 μm. By the presence of two ore more different kinds of transition metals the chemical stability of the ceramic carbide fiber is increased.

Additionally, the present invention provides a method for making a ceramic carbide fiber, the ceramic carbide fiber comprising a carbide solid solution with at least two transition metals; the fiber comprises a fiber diameter selected from a range of 10 μm to 200 μm, the method comprising: providing a mixture with liquid precursors of the solid carbide solution in a solvent; converting the mixture into a gel precursor solution by at least partially cross- linking of the liquid precursors; drawing a green fiber of partly cross-linked precursors out of the gel precursor solution; drying the green fiber by removing solvent and pyrolysing the dried green fiber, whereas the ceramic fiber is formed.

The resulting ceramic carbide fiber can be used as a component of a fiber mat. Due to the high stability the ceramic carbide fiber is used as a filament component of a lamp .

The method is based on the sol-gel-technique. The converting of the mixture into a gel leads to an increase of the viscosity. When a certain viscosity range is achieved a spinnable mixture results. Green fibers (gel fibers with an amorphous structure consisting of cross-linked precursors) can easily be drawn out of the spinnable mixture.

The precursors function as sources for carbon (carbon precursors) and sources for the transition metals (transition metal precursors) . Thereby not all of the precursors have to be liquid. Besides the liquid precursors solid precursors can be used, too.

Transition metal precursors include organic compounds like transition metal alkoxides as well as inorganic salts like transition metal chlorides. Carbon precursors are - for example - hydrochinone , saccharose, glycerol, ethylene glycol and phenolic resic.

Besides the providing an excellent mixture for drawing fibers the sol-gel-technique imply following special advantage: The fiber material of the resulting green fiber is extremely homogenous. The precursors are evenly distributed over the whole green fiber. By pyrolising (thermal decomposition) of the green fiber a ceramic carbide fiber with a very high chemical homogeneity results. The result is a carbide solid solution. The carbide solid solution consists of substantially just one chemical phase. Moreover, a microstructure of the resulting solid carbide solution is dense and uniform.

In a preferred embodiment the transition metals are selected from the groups 4 and 5 of the periodic table of the elements. Preferably the transition metals are selected from the group consisting of Hf, Nb, Ta, Ti and Zr. In particular with these transition metals the ceramic carbide fiber is stable at temperatures of above 3000° C. The decomposition rate is very low. The increase of the stability permits the use of the ceramic carbide fiber as filament material for lamps. With regard to the sol-gel-technique suitable precursors for these transition metals are - for example - tantalum pentaethoxide, niobium pentaethoxide, hafnium tetra- isopropoxid, tantalum pentachloride and niobium pentachloride .

In a further preferred embodiment one of the transition metals is Ta and a proportion of Ta is selected from a range of 10 mol.% to 90 mol.%, particularly from a range of 60 mol.% to 80 mol.%.

Preferably the portion of Ta is substantially 70 mol.%. Very good results are achieved by the combination for the transition metals Ta and Nb. Preferably a second transition metal is Nb and a portion of Nb is substantially 20 mol.%. Alternatively the second transition metal is Hf and a portion of Hf is substantially 20 mol.%. More preferably there is a mixture of the second transition metals Nb and Hf with a total portion of 20 mol.%, e.g. Hf 10 mol.% and Nb mol.%.

As already mentioned, the sol-gel solution results in a spinnable mixture. In a preferred embodiment the drawing of a gel fiber of cross- linked precursors out of the gel includes therefore a spinning process. For example, the spinning process is carried out by a rotating container with a perforation. By this a continuous and rapid fabrication of green fibers is possible. Alternatively the drawing a gel fiber of cross-linked precursors out of the gel includes immersing an object into the gel. With the object the green fiber can be pulled out of the gel. For example, the object is a spatula or a glass bar.

More methods include the extrusion of the partly cross- linked solution and the gravity falling of the solution through a hole placed at the bottom of the container.

The pyrolysis is carried out at a temperature up to 2200 °C, e.g. at a temperature between 1400° C and 2200° C, and preferably in an inert atmosphere with Ar or N ₂ . Vacuum is possible, too.

The resulting ceramic fiber of a carbide solid solution with at least two transition metals enhances the performance of the ceramic carbide fibers with just one transition metal, e. g. TaC, HfC, NbC, ZrC or TiC by increasing the melting point, the corrosion resistance, the durability in harsh environments or the catalytic properties .

The main advantage of the method for making the ceramic fiber is based on the easiness of the fiber conformation, the plurality of different carbide solid solutions for the ceramic fiber, the chemical homogeneity and the good sintering of the green (organic) fiber into an inorganic one.

BRIEF DESCRIPTION OF THE DRAWING

The figure shows a picture of a ceramic carbide fiber with a solid carbide solution with the transition metals Ta and Nb.

DETAILED DESCRIPTION OF THE INVENTION

The ceramic carbide fiber is a solid carbide solution of Ta and Nb. The portion of Ta is about 80 mol . % . The portion of Nb is about 20 mol . % . The diameter of the ceramic fiber is about 50 μm (figure) .

The method for making the ceramic carbide fiber is starting with providing a mixture of liquid precursors for the carbide solid solution. The liquid transition metal precursors are tantalum pentaethoxide and niobium pentaethoxide with according amounts. Acetylacetone is added to the separate metal precursors under inert atmosphere . Acetylacetone functions as a complexing agent for the transition metal alkoxides . The complexed metal precursors are mixed. Hydrochinone is given to the mixture as a liquid carbon precursor.

The mixture increases its viscosity with the cross-linking of the liquid precursors. The viscosity of the mixture is increasing. By agitating the mixture an optimum viscosity in view of the subsequent drawing process can be adjusted, before the sol undergo the gelling.

After that, the drawing of a green fiber of cross-linked precursors out of the gel is carried out by a spinning process . Drying the drawn green fiber by removing the solvents and pyrolysing the. dried green fiber in Ar atmosphere lead to the ceramic carbide fiber.

Alternatively the ceramic carbide fiber comprises a solid carbide solution of Ta and Hf. The portion of Ta is about 80 mol.%. The portion of Nb is about 20 mol.%. In a further alternative example the ceramic carbide fiber comprises a solid carbide solution of Ta, Nb and Hf. The portion of Ta is about 80 mol.%. The portion von Hf is about 10 mol.%. The portion of Nb is about 10 mol.%.