WO2001014282A1

WO2001014282A1 - Method for the production of zirconium dioxide fibers

Info

Publication number: WO2001014282A1
Application number: PCT/EP2000/007827
Authority: WO
Inventors: Hans-Josef Sterzel
Original assignee: Basf Aktiengesellschaft
Priority date: 1999-08-24
Filing date: 2000-08-11
Publication date: 2001-03-01
Also published as: TW495569B; DE19940134A1

Abstract

The invention relates to zirconium dioxide fibers which are produced by spinning a zirconium compound solution in a mixture of formic acid and water in a molar ratio of 1:1 to 1:3 and then performing sintering.

Description

Process for the production of zirconia fibers

description

The present invention relates to a method for producing zirconia fibers. The invention further relates to a method for producing stabilized zirconium dioxide fibers, in which the tetragonal modification of the zirconium dioxide is stabilized against the conversion into the monoclinic modification by means of stabilizing additives.

Due to its properties, zirconium dioxide is being used to an increasing extent in those areas of application where the mechanical, thermal and chemical resistance of the material is required. Zirconium dioxide, for example, can be used alone or in a mixture with other oxides as a material for the production of shaped bodies or fibers made of sintered ceramic. Zirconium dioxide fibers are used, for example, in special applications for thermal insulation. Another area of application is the fiber reinforcement of metals in order to increase their strength and creep resistance. This applies in particular to fiber-reinforced titanium and titanium alloys, since titanium reacts with the aluminum oxide fibers used in other metals under certain conditions. Furthermore, zirconium dioxide fibers are good oxygen ion conductors at higher temperatures, for example above 800 ° C., and are therefore used in corresponding components such as sensors or fuel cells. In addition, zirconia fibers are inert to strong alkalis and chlorine, which makes them possible asbestos substitutes in diaphragms for chlor-alkali electrolysis.

The zirconium dioxide powder used for these sintered bodies or fibers is generally used in a stabilized form, as a result of which the tetragonal high-temperature modification is retained even when it is cooled to the application temperature. In order to achieve this stabilization, the zirconium dioxide powder is mixed with other oxides such as e.g. B. yttrium oxide, cerium oxide, rare earth oxides, calcium oxide, magnesium oxide or doped with mixtures of these oxides. So that from the stabilized zirconia, for. B. by pressing, injection molding, slip casting or spinning green parts produced the desired sintering behavior and the sintered molded body or fibers have the desired good mechanical, thermal and chemical properties, it is necessary that the added oxide is distributed as evenly as possible in the zirconia lattice. _V out for the production of non-stabilized or stabilized zirconia fiber are known.

For example, zirconia fibers are drawn from a melt of zirconia in accordance with the teaching of US-A 3944640. This method is economically unsatisfactory due to its high energy costs. T. Yogo, in J. Mater. Be. 25 (5) (1990) 2394-2398 and Y. Abe, H. Tomioka, T. Gunji, Y. Nagao and T. Misono in J. Mater. Be. Lett. 13 (13) (1994) 960-962 teach processes in which polymeric zirconium-organic compounds are formed into fibers and then converted into zirconium dioxide fibers by pyrolysis. This process is also economically unsatisfactory since the detour via the zirconium organic compound is cumbersome.

In other known processes, for example the one disclosed in DE-A 30 28 314, powdered zirconium dioxide is processed into fibers. For this purpose, stabilized zirconium dioxide particles with particle sizes between 0.1 and 1 micrometer are dispersed in an auxiliary polymer or its solution and the viscous mixture obtained is extruded through fine nozzles. The solvent is then removed by slow heating, the polymer is burned out and the finished fiber is produced by sintering at a temperature of 1300 ° C. to 1500 ° C. These processes usually lead to comparatively brittle fibers.

Still other processes, such as those in EP-A 375 158 or

EP-A 383 051 disclosed start from zirconium dioxide sols, which are usually prepared by hydrolysis of alkoxy zirconium compounds. The brine is spun and the finished fiber is produced by firing. The use of alkoxy zirconium compounds as starting material makes these processes economically unsatisfactory.

Another group of known processes starts from dissolved zirconium compounds, the solution being deformed into fibers, which are then converted to zirconium dioxide by calcination. Examples of such processes are disclosed in JP-A 04-100 921, JP-A 02-097 425 or JP-A 05-321 036. All known processes that start from dissolved zirconium compounds require careful control of the process conditions, since even small deviations lead to unsatisfactory results. In particular, the premature formation of solid particles should be avoided, since these act as predetermined breaking points in the later fiber. Another disadvantage of many such processes is that they start from chloride-containing zirconium compounds. Chloride residues in the finished fiber can permanently deteriorate their properties and also lead to corrosion problems in some areas of application.

^" ü ^ -A-6X ^ 289130 teaches ^" a process for producing stabilized zirconia fibers by spinning a solution of zirconium formate containing 5 to 30% by weight of zirconium acetate, based on the total salt content of the solution, and subsequent sintering. If less zirconium acetate is used in this process, non-spinnable solutions are formed; if more is used, the strength of the fibers obtained is reduced.

DE-A 196 19 638 discloses a process for the production of optionally stabilized zirconium dioxide powder from a solution of a chloride-free zirconium compound in a formic acid / water mixture.

In view of the prior art, the task is to find a further, simple and economically satisfactory process for the production of zirconium dioxide fibers, which enables the production of good quality, optionally stabilized fibers. Accordingly, a process for the production of zirconium dioxide fibers by spinning a solution of a zirconium compound and subsequent calcination has been found, which is characterized in that a solution of a zirconium compound in a formic acid / water mixture is used.

The method according to the invention enables simple and economical production of high-quality, optionally stabilized zirconium dioxide fibers with excellent properties. The fibers can also be produced without the use of chloride-containing raw materials.

In the first step of the method according to the invention, a spinnable solution of a zirconium compound in a formic acid / water mixture is produced. For this purpose, a zirconium compound, preferably a chloride-free zirconium compound, is dissolved in a formic acid / water mixture. Zirconium oxide hydrate or basic zirconium carbonate, for example, is used as the zirconium compound. The molar ratio of formic acid to water in this solvent is 1: 1 to 1: 3, preferably 1: 1.5 to 1: 2.5 and very particularly preferably 1: 1.8 to 1: 2.2. The formic acid / water mixture preferably contains no further components. The solution can be prepared at room temperature or a slightly elevated temperature, but the use of a dissolving temperature in the range of is preferred 50 to 95 ° C, preferably in the range of 70 to 85 ° C. In general, the mixture of formic acid and water is introduced and the solids are metered in, but the reverse order is also possible. If carbonate is used, the metering rate is preferably set so that the reactor contents are not overexposed.

In principle, it is possible, by loosening z. B. 1 kg of basic zirconium carbonate in a mixture of 0.4 kg of water and 0.5 kg of formic acid to obtain a low-viscosity, clear solution. It is also possible to produce a highly viscous, also clear solution by dissolving 1 kg of basic zirconium carbonate in just 0.24 kg of water and 0.307 kg of formic acid. In order to achieve optimum spinnability, a low-viscosity solution is concentrated by concentrating the solvent, or a high-viscosity solution is diluted by adding further solvent. In order to save such additional process steps as much as possible, the zirconium compound is preferably dissolved in approximately the same amount by weight of the formic acid / water mixture.

With the method according to the invention, the production of stabilized zirconium dioxide fibers is optionally also possible. For this purpose, additives are added to the zirconium dioxide before, during or after its production, which stabilize the tetragonal modification in relation to the conversion into the monoclinic modification. For example, stabilizing compounds or precursors of such compounds, which are advantageously also soluble in the formic acid / water mixture, are mixed in as solids in the zirconium compounds used in the process according to the invention before they are dissolved in the formic acid / water mixture. Likewise, stabilizing additives or precursors of such additives can be added to the zirconium dioxide fibers produced according to the invention, for. B. by precipitation of precursors of stabilizing oxides in the form of oxide hydrates, carbonates, oxalates or similar compounds which can be converted into oxides by calcination in suspension, removal of the volatile constituents of the suspension and calcination. However, at least one stabilizing additive and / or at least one precursor of such a stabilizing additive is preferably dissolved together with the zirconium compound in the mixture of formic acid and water. An yttrium compound is preferably used as the stabilizing additive or precursor. Preference is given to using a chloride-free yttrium compound which is soluble in the mixture of formic acid and water used as solvent. Suitable yttrium-containing compounds are e.g. B. yttrium oxide or yttrium carbonate. In general, the amount of yttrium compound becomes so dimensioned so that the zirconia fibers have an yttria content of 2 to 10 mol%, preferably 2.5 to 5 mol%. In the production of stabilized zirconium dioxide fibers, the total amount of compounds to be dissolved is preferably dissolved in approximately the same amount by weight of the formic acid / water mixture.

After the solution has been prepared, it is stirred or left to stand until it is completely clear after one or after cooling to a temperature below 30.degree. In general, this is the case after leaving at room temperature for a maximum of 20 hours. If the solution is prepared at an elevated temperature, it should be noted that the solution is formed more quickly, but is usually cloudy above 30 ° C.

The viscosity of the solution is then adjusted so that it is spinnable. For this purpose, either solvent is drawn off (preferably by means of negative pressure at room temperature) or solvent is added. The viscosity at room temperature is preferably at a shear rate of 10 s ^-1 in the range from 2000 to 10,000 dPas. This corresponds to a solids content of the spinning mass, calculated as oxides (Zr0 or possibly Zr0 / stabilizer, for example Y ₂ O ₃ ) in the range from 40 to 50% by weight.

The spinnable solution is then spun into fibers using conventional spinning techniques. The spinning process can take place in air and at room temperature. The fibers produced usually have diameters of 5 to 20 micrometers and are either drawn directly from the solution at speeds of up to 1 m / s or, following the spinning process, at speeds of up to 10 m / s to give fibers with diameters in the range from Stretched 2 to 10 microns. The so-called "green" fibers produced in this way dry very quickly due to the high solids content, with the formation of glass-like amorphous fibers which do not stick together.

The reworking of green zirconium dioxide fibers to the end product is known. If necessary, formic acid and / or water still present is removed from the green fiber (“drying”) by exposing the green fibers to a temperature of at least 100 ° C., generally a temperature below 300 ° C. is sufficient for this. The finished zirconium dioxide fibers are then produced in the usual way by calcining the optionally dried green fibers at a temperature of at least 600 ° C. If the heating rates used are unusually low to these temperatures, shift the temperature range to be applied slightly downwards, with unusually high heating rates upwards. For the technical production of continuous fibers, spinning is preferably carried out at the lower end of a heatable shaft with a length of up to 10 m, which is heated to temperatures of, for example, 1500 ° C. The stretching takes place immediately after the fibers exit the spinnerets. Finished zirconium dioxide continuous fibers are obtained at the top of the shaft. Short fibers are preferably produced in a centrifugal process, in which the solution is pressed through a rotating, cooled spinning head with spinnerets located on the circumference. The head is in an oven heated to 1200 to 1500 ° C. This process creates finished short zirconium dioxide fibers up to 2 mm long.

example

1000 g of basic zirconium carbonate (calculated zirconium dioxide content 44% by weight) and 25 g of yttrium oxide were placed in an open plastic container. At room temperature, 1000 g of a mixture of 561 g of formic acid and 439 g of water were added, whereupon violent evolution of carbon dioxide began. The mixture was kept at room temperature for 20 hours, resulting in a clear solution. This solution was concentrated using a rotary evaporator to such an extent that it was possible to pull meter-long fibers out of it with a glass rod. The solids content of the spinning mass thus obtained was 44.2% by weight, calculated on the sum of zirconium dioxide and yttrium oxide.

The fibers, drawn manually in air at room temperature with a glass rod, were heated in air in an oven to 1450 ° C, left at this temperature for 10 minutes and then cooled. Flexible zirconium dioxide fibers with diameters of 5 to 10 micrometers were created.

Claims

claims

1. A process for the production of zirconium dioxide fibers by spinning a solution of a zirconium compound and subsequent calcination, characterized in that a solution of a zirconium compound in a formic acid / water mixture is used.

2. The method according to claim 1, characterized in that the solvent is a mixture of formic acid with water in a molar ratio of 1: 1 to 1: 3.

3. The method according to claim 2, characterized in that the zirconium compound is zirconium oxide hydrate or basic zirconium carbonate.

4. The method according to claim 1, characterized in that at least one additive and / or at least one precursor of an additive is added to the zirconium dioxide before, during or after its preparation, which tetragonal modification of the zirconium dioxide in the fiber against conversion into the monoclinic Modification stabilized.

5. The method according to claim 4, characterized in that at least one yttrium compound is used as the stabilizing additive.

6. The method according to claim 4, characterized in that the zirconium compound together with the stabilizing

Additive and / or its precursor dissolves in the mixture of formic acid and water.

7. Process according to claims 3 and 4, characterized in that the concentration of zirconium and yttrium compound in the solution before spinning is 40 to 50% by weight, calculated as oxides.