WO1990006905A1

WO1990006905A1 - Whisker-reinforced ceramic and superconductor fibers from preceramic sol-gel, liquid mix, and polymer precursors

Info

Publication number: WO1990006905A1
Application number: PCT/US1989/005810
Authority: WO
Inventors: Anna L. Baker; Jerry A. Hebert; Mark W. Shorey
Original assignee: The Boeing Company
Priority date: 1988-12-22
Filing date: 1989-12-22
Publication date: 1990-06-28

Abstract

A reinforced, nonglass-forming composition ceramic fiber, including a superconductive oxide fiber, is formed from ceramic material, prepared by a sol-gel, liquid mix, or preceramic polymer process without glass-forming materials therein, and a plurality of whiskers. The fibers have sufficient tensile strength to allow handling of the fibers without breakage.

Description

WHISKER-REINFORCED CERAMIC AND SUPERCONDUCTOR

FIBERS FROM PRECERAMIC SOL-GEL, LIQUID MIX, AND

POLYMER PRECURSORS

Technical field

The present invention relates to high-tensile strength, nonglass-forming ceramics, particularly superconductive oxide fibers. The fibers are reinforced with a plurality of whiskers, and the fiber material is prepared from preceramic sol-gel, liquid mix, or polymer precursors.

Background pf the Invention

Ceramics are polycrystalline, inorganic solid bodies consisting of one or more phases. Ceramics are made by mixing fine-grained solid powders, usually oxides such as SiQ₂, AI₂O₃, La₂O₃, most metals in the Periodic Table that form oxides or compounds thereof, and than allowing them to react in the solid state at temperatures between 1000°C and 1500ºC for periods of hours to days.

More recently developed, a sol-gel process of making ceramics has three key parts: (1) mixing various oxides in solution, often with the use of metal organic precursors; (2) forming a sol and causing it to gel aa the key step in the process to retain chemical homogeneity during drying; and (3) shaping during or after gelation into essentially final shape before firing. These steps are described by s. P. Mukherjee in "Sol-Gel Processes in Glass Science and Technology," J. of Non-Cryatalline solids 42:477-488, 1980, North-Holland Publishing Company; and by R. Roy in "Ceramics by the Solution-Sol-Gel Route," Science 238.1664-1669, 1987, which are incorporated herein by reference. The sol-gel process actually involves a five-step procedure. First, the components are dissolved in a liquid phase, such as water or a short-chain alcohol. The solutes may be either metal salts, such as nitrates or chlorides, or organometallics. The second step adjusts the activities of some species to form a dispersed solid phase. Then, a sol is formed by controlling or adjusting the pH, ionic strength, temperature, and time, as dictated by the composition of the sol. Gelling the sol into the desired shape and drying the gel completes the process. Spheres, fibers, thin sheets, coatings, or solid articles are formable. Drying usually involves heat treatment to create the desired glass or ceramic from the sol components. The heat treatment requires shorter times and lower temperatures to produce the desired product than are required for the conventional hot press techniques that use powders.

The present invention provides high-tensile strength, nonglass-forming, reinforced ceramic fibers. Previously, high-tensile strength fibers have been formed only from ceramics that contain glass-forming elements, such as SiO₂, AI₂O₃, B₂O₃, P₂O₅, and GeO₂. The sol-gel process has been used to make 10 to 20 micron diameter fibers with glass-forming elements containing principally polycrystalline AI2O3, mullite (3Al₂O₃x2SiO₂), and other polyphasic compounds. The gel is extruded or spun, and the resulting fibers are dried and fired.

The fibers produced from the sol-gel process and without glass-forming elements often have tensile strengths that are quite low and are often brittle and break when handled. Ceramics are also made by the liquid mix process which is described in "Polymeric Precursor Synthesis of Ceramic Material, " N. G. Eror and H. U. Anderson, Proceedings of Materials Research Society; Better Ceramics Through chemistry. C. J. Brinker, D. B. Clark and D. R. Ulrich (eds.), 1986, which is incorporated herein by reference.

As disclosed in U.S. Patent No. 4,543,345 (Wei), silicon carbide whiskers have been used to reinforce ceramic composites or monolithic ceramic matrices.

However, whiskers have not previously been used in the formation of ceramic fibers.

Ceramic fibers are small-dimension filaments or threads composed of a ceramic material, e.g., alumina and silica. Ceramic fibers have been used in lightweight units for electrical, thermal and sound insulation, filtration at high temperatures, packing, and reinforcing other ceramic materials or composites.

Liquid mix processes for making ceramics initially use individual cations complexed in separate weak organic acid solutions, rather than a single solution of all the components. Weak acids, such as alphapyroxycarboxylic acids, form polybasic acid chelates with various cations, such as Ti, Zr, Cr, Mn, Ba, La, etc. The chelates undergo esterification when mixed and heated in a polyol or polyhydric alcohol, such as ethylene glycol, to form a polymeric glass which has the cations uniformly distributed throughout. Evaporated to a rigid polymeric state, the liquid mix forms a uniformly colored, tranaparent gel. The gel retains homogeneity on the atomic scale, and may be calcined at a relatively low temperature of only a few hundred degrees Celsius to a homogenous, single solid phase having predetermined stoichiometry in finegrained particles of a few hundred Angstroms.

Fibers formed from a liquid mix process, at least those fibers lacking glass-forming elements, lack sufficient tensile strength to prevent breakage when handled. Accordingly, the liquid mix process does not allow the formation of ceramic or superconductor fibers of increased length and tensile strength, but forms fibers that break apart when handled.

Preceramic polymer precursors are formed with heating of approximately 800ºC to approximately 1200°C with minimal weight loss, little chemical changes and primarily physical changes. The polymerization chemistry can involve ligand redistribution, such as the combination of two disilanes to form a trisilane and a monosilane. Moreover, a silane, disilane, or polysilane often can react with a methylated metal, such as lithium or magnesium, to form a methylated silane and a metal chloride salt. As taught in Pyrolysis and Char Chemistry of Preceramic Polymers, J, Lipowitz and R. Reaoch, G. E. LeGrow, preceramic polymers can also be formed from molecular rearrangements and from condensation-type reactions.

Ceramics have been prepared from preceramic polymer precursors of methylpolysilane (MPS), (polymethylsilyl) polysilazane (MPSZ), methylpolydisilylazane (MPOZ), dodecamethylcyclohexasilane, methylpolysilazane (MPZ), polycarbosilane (PCS), polymethylsilazane (PMZ), hydridopolysilazane (HPZ), polysilastyrene (PSS), (phenyl vinyl modified) methylpolydisilylazane (MPDZ-PhVi), (phenyl vinyl modified) methylpolysilazane (MPZ-PhVi), polycarbosilazane resin (PCZ), and vinyl functional polymethylsilane (VMPS). Nonoxide ceramic fibers produced from these polymer precursors are silicon carbide (SiC) and silicon nitride (SiNH₄).

A dodecamethylcyclohexasilane polymer precursor has been spun into an organosilicon fiber and pyrolized at temperatures above 1000ºC to produce SiC/Si₃N₄ fibers. The multiphasic fibers included fine, poorly formed Sic and Si₃N₄ crystals in a noncrystalline matrix of Si-C-O-H-N, with the SiC/Si₃N₄ content often 90 to 95 atomic percent. Only glass-forming composition ceramics, such as AI₂O₃ and B₂O₃xAl₂O₃xSiO₂ (boroaluminasilicate), have been made into fibers with acceptable tensile strength. It is desirable to make nonglass-forming ceramic fibers, such as superconductive oxide fibers, that are handled without breakage. The present invention provides such a method and product.

Summary of the Invention

The present invention is directed to nonglassforming, reinforced ceramic fibers, including superconductive oxide fibers, having improved tensile strength sufficient to allow handling of the fibers without breakage, and to methods of preparing such fibers.

Ceramic fibers are small-dimension filaments or threads composed of ceramic materials, e.g., alumina or silica.

Reinforced ceramic fibers according to the invention are comprised of a ceramic material prepared from sol-gel, liquid mix, or polymer precursors, wherein the ceramic material has a plurality of whiskers of, for example, silicon carbide dispersed therein.

Fibers according to the invention can be prepared by sol-gel, liquid mix, or ceramic polymer techniques without the need for glass-forming components previously required for the preparation of ceramic fibers having acceptable tensile strengths.

The invention provides a method of preparing the fibers of the invention comprising mixing a sol-gel, liquid mix, or polymer ceramic composition with whiskers; extruding the mixture through an orifice to form a fiber; and heating the fiber to remove organic materials.

The present inventions is also directed to superconductive fibers and perovskite fibers, particularly those of lanthanum chromite (LaCrO3), prepared according to the methods of the invention. Uses of fibers according to the invention include, but are not limited to, reinforcement of other ceramic materials, us« as a superconductive material, and for electromagnetic shielding.

Unlike typical, commercially available ceramic oxide fibers, the oxide fibers of the invention are not so friable that they break upon handling and may be spun or woven. Brief Description of the Drawings

Figure 1 is a scanning electron micrograph (SEM) at 580 times magnification (580x) of a La_0.9Sr_0.1Cr_0.5Mn_0.5O₃ fiber without whisker reinforcement. The fiber was brittle and broke upon handling. The longest fiber was no longer than 1 cm.

Figure 2 is an SEM (600x) of an Si₃ whiskerreinforced La_0.9Sr_0.1Cr_o.5Mn_0.5O₃ fiber. Fiber lengths were consistently 12 m and did not break upon handling.

Figure 3 is a 2500x SEM enlargement of the fiber of Figure 2.

Detailed Description of the Invention

The present invention is directed to whiskerreinforced ceramic fibers, both oxide and nonoxide, having improved tensile strength. The improved strength of the fibers of the invention is obtained without inclusion of glass-forming constituents in the chemical composition previously required to obtain a ceramic fiber having acceptable tensile strength.

The ceramic fibers of the invention are reinforced with a plurality of whiskers. The whiskers should be of a small diameter material generally cylindrical in shape. The shape of the whiskers may vary considerably, but should be of sufficiently small dimension so they can be spun or extruded through an orifice. Typically, whiskers useful in the invention are between about 0.1 and about 4.0 microns in width or diameter, are chemically compatible with the ceramic material of the fiber, are stable at high temperatures, at least between about 1500ºF to 2500ºF, and have a coefficient of thermal expansion compatible with that of the ceramic material.

Whiskers useful in the invention may be of silicon carbide, silicon nitride, and aluminum oxide. The fibers may also be reinforced with other particulates having void spaces, indentations, and the like to provide support for the ceramic material, such as diatomaceous earth, or other fine particulates having the appropriate structure and compatibility. Those skilled in the art will be able to identify other materials suitable for use as whiskers in the invention based upon the structural, size, compatibility, and thermal stability requirements for incorporation into the fibers of the invention.

Diatomaceous earth is composed of the siliceous shells of diatoms, microscopic algae; the shells may have various shapes, both regular and irregular (e.g., branched or shaped like needles, squares, barrels, or triangles).

The whisker-reinforced ceramic fibers of the invention are comprised of ceramic materials prepared from sol-gel, liquid mix, or polymer precursors and a plurality of whiskers.

Metal oxide ceramics can be formed with the solgel or liquid mix precursors. The eol-gel or liquid mix precursor is of a chemical composition that yields metal oxides, such as perovskites (e.g., LaCrO₃ or LaMnO₃), ferrites, superconductive oxides, and combinations thereof after heat treatment.

Liquid mix precursors useful in the invention include the group consisting of metal carbonates, metal acetates, metal formates, organic acids, and hydroxy compounds The metal cation for the metal carbonate, metal acetate, or metal formate may be selected from the group consisting of lanthanum, manganese, barium, chromium, zirconium, titanium, yttrium, bismuth, strontium, calcium, copper, and other oxide-forming metals. Examples of suitable preceramic polymer precursors include disilane, trisilane, monosilane, polysilane, SiNHSi(CH₃)₃, Si-Si, SiNH, SiNR, SiNHR, SiCH₂SiH, SiNHSi, ((CH₃)_wSi_w)_x(NH)_v(NHSi(CH₃)₃)₂, methylpolydisilylazane, methylpolysilazane, methylpolysilane, polycarbosilane, (phenyl vinyl modified) methylpolysilizane, (phenyl vinyl modified) methylpolydisilylazane, hydridopolysilazane, (polymethylsilyl) polysilazane, dodecamethylcyclohexasilane, polysilastyrene, methylpolydisilylazane, methylpolysilazane, polycarbosilazane resin and vinyl functional polymethylsilazane.

To make reinforced fibers, the liquid mix, solgel, or polymer precursor is mixed with the whiskers. The mixture may be sonicated for approximately two minutes to uniformly disperse the whiskers. Preferably, sonication is accomplished at approximately 50% power and pulsed. The mixture is extruded through an orifice from about 10 microns to about 250 microns in diameter to form a fiber. The fiber is then heated to remove the organic materials and to harden the fiber.

The heating rate is also important. This process is furnace-dependent, and if the rate of temperature increase within the furnace is too rapid, the fiber will not form properly because the sol-gel or liquid mix materials are approximately 60% organic. Organics will evaporate or dissociate from sol-gel and liquid mix materials from ambient temperature to the 600º to 700ºC range. If the rate of heating is too high, rapid evaporation could cause the material to crack, rendering it less handleable. Accordingly, it is important to have the temperature rise slowly during the heating step, for example, in the range of 2ºC to 5ºC per hour. An air, vacuum, or inert atmosphere may be used, depending upon the chemical nature of the composition. The inert atmosphere is composed of one or a combination of inert gases, such as He, Ar, Ne, etc. One of ordinary skill in the art will know which atmosphere is appropriate depending upon the chemical nature of the composition.

The fibers also can be spun out. The relatively non-friable fibers have a high-tensile strength and can be reasonably handled without breakage. Suitable liquid mix, sol-gel, and preceramic polymer materials without glassforming materials are known and are described herein in the background section.

The following examples are offered by way of illustration and not limitation.

EXAMPLE 1

A liquid mix was prepared with the following ingredients:

Ingredient Amount

1. La₂(CO₃)₃ x H₂O 25.77 g

2. SrcO₃ 1.85

3. Mn(COOH)₂.H₂0 11.32

4. Cr(COOH)2.H₂0 17.69

5. Si₃N₄ whiskers (Tateho Co. ) 5.00

6. citric acid 68.75

7. ethylene glycol 115.54

8. H₂O 240.00 ml

The first four ingredients were mixed dry and were added to a mixture of the acid, glycol , water, and whiskers. The mixture was sonicated for two minutes to disperse the whiskers, the mixture was transferred to a rotovap flask. The liquid mix was heated in the flask at just below the boiling point for 24 hours, during which reaction water was stripped off. Then 160 ml of water was added. The liquid mix was then .3tripped of water until it achieved the viscosity needed for fiber spinning. EXAMPLE 2

The whisker-loaded liquid mix of Example 1 was put into the extrusion head of a fiber spinner (University of Bradford, England) and extruded through a 125 micron orifice. The sol was heated to 60ºC in the extrusion head and extruded using 50 kg of pressure. The fiber hardened in air and was wound out on a Teflon® film-covered mandrel. The pickup wheel speed was adjusted to draw the fiber to align the whiskers. The fibers were removed from the mandrel, placed on a zirconia setter, and put into a high-temperature vacuum furnace (Deltech Inc.). The fiber was heat-treated under vacuum from ambient temperature to 650ºC at a ramp rate of 5ºC per hour, held at 650ºC for 20 minutes, then cooled to room temperature.

The resulting fiber had a composition of

La_o.9Sr_0.1Cr_0.5Mn_0.5O₃ and, in addition, 5% by volume of silicon nitride whiskers. These whisker-reinforced fibers can be used as an oxygen sensor, among other uses. EXAMPLE 3

A liquid mix is prepared in the manner described in Example 1 herein. The liquid mix forms a fiber of YBa₂Cu₃O₇ (a superconductive oxide fiber). A whisker which is chemically and thermally compatible is added to the liquid mix. The mixture is extruded according to the procedure described in Example 2. The fiber is heattreated from ambient temperature to about 950ºC at a ramp rate of 5ºC per hour in a flowing oxygen atmosphere and held for 20 minutes. The fibers are then annealed at 450ºC.

The foregoing examples are provided for illustrative purposes and are not intended to be limiting. Various changes can be made by those skilled in the art without departing from the scope and spirit of the invention.

Claims

Claims We claim:

1. A whisker-reinforced ceramic fiber comprising a nonglass-forming sol-gel, liquid mix, or polymer ceramic material and a plurality of whiskers dispersed therein, said fiber has sufficient tensile strength to be handled without breakage.

2. The reinforced fiber of claim 1 wherein the ceramic material is a superconductive ceramic.

3. The reinforced fiber of claim 1 wherein the fiber is made from a sol-gel precursor.

4. The reinforced fiber of claim 1 wherein the fiber is made from a liquid mix precursor.

5|A The reinforced fiber of claim 4 wherein the liquid mix is selected from one or a combination of ingredients selected from the group consisting of metal carbonates, metal acetates, metal formates, organic acids, and hydroxy compounds.

6. The reinforced fiber of claim 5 wherein the organic acid is citric acid and the hydroxy compound is ethylene glycol.

7. The reinforced fiber of claim 5 wherein the metal cation from the metal carbonate, metal acetate, or metal formate is selected from the group consisting of lanthanum, manganese, barium, chromium, zirconium, titanium, yttrium, bismuth, strontium, calcium, copper, and other oxide-forming metals.

8. The reinforced fiber of claim 1 wherein the fiber is made from a preceramic polymer precursor.

9. The reinforced fiber of claim 8 wherein the preceramic polymer precursor is selected from one or a combination of ingredients selected from the group consisting of disilane, trisilane, monosilane, polysilane, SlNHSSi(CH₃)₃, ClSi(CHC₃)₃, Si-Si, SiNH, SiNR, SiNHR, SiCH₂SiH, SiNHSi,((CH₃)_wSi_w)_x(NH)_v(NHSi(CH₃)₃)₂, methylpolydisilylazane, methylpolysilazane, polycarbosilane, polymethylsilazane, hydridopolysilazane, polysilastyrene, methylpolydisilylazane, methylpolysilazane, polycarbosilazane resin, and vinyl functional polymethylsilane.

10. The whisker-reinforced ceramic fiber of claim 1 wherein said whiskers are made of a material having sufficient structure to provide support for the ceramic material.

11. The whisker-reinforced ceramic fiber of claim 12 wherein said whiskers are chemically and thermally compatible with the ceramic material.

12. A method for making whisker-reinforced, nonglass-forming ceramic fibers, comprising:

mixing a preceramic sol-gel, liquid mix, or polymer precursor without a glass-forming material with whiskers to form a mixture;

extruding the mixture through an orifice to form a fiber; and

heating the fiber to remove organic material.

13. The method of claim 12, further comprising sonicating the mixture after the mixing step and before the extruding step.

14. The method of claim 12 wherein the nonglass-forming composition ceramic is a superconductive oxide.

15. The method of claim 12 wherein the temperature rise of the heating step is from about 2ºC to about 5ºC per hour to its maximum temperature.

16. The method of claim 12 wherein the heating step is conducted in an air atmosphere, an inert atmosphere, or in a vacuum.

17. The method of claim 13 wherein said whiskers are made of a material that is chemically and thermally compatible with the ceramic material and are made of a material haying sufficient structure to provide support for the ceramic material.

18. The method of claim 12 wherein the whiskers are silicon carbide, silicon nitride, aluminum oxide, or diatomaceous earth.

19. The method of claim 13 wherein said whiskers are between about 0.1 to about 4.0 microns in diameter or width.

20. The method of claim 13 wherein said orifice is between about 10 to 250 microns.

21. The fiber of claim 1 wherein the whiskers are between about 0.1 to about 4.0 microns in width or diameter.

22. The fiber of claim 1 wherein said fiber is a perovskite.

23. The fiber of claim 1 wherein said perovskite is LaCrO_3.

24. The fiber of claim 1 wherein said perovskite is

LaMnO₃.

25. The product of the process of claim 12.