WO1991015437A1

WO1991015437A1 - Ultrafine ceramic fibers

Info

Publication number: WO1991015437A1
Application number: PCT/US1991/002029
Authority: WO
Inventors: Gary L. Messing; Derek W. Sproson; Shi Chang Zhang
Original assignee: Research Corporation Technologies, Inc.
Priority date: 1990-03-30
Filing date: 1991-03-26
Publication date: 1991-10-17

Abstract

Discontinuous, discrete ceramic fibers are prepared by atomizing a viscous sol or solution of a precursor of a metal oxide into a heated zone where the discontinuous discrete fibers are formed. Subsequent calcination of the fibers provides discontinuous discrete metal oxide ceramic fibers.

Description

¹ ULTRAFINE CERAMIC FIBERS

This invention relates to the production of ceramic fibers. More particularly, this invention relates to the production of discontinuous ultrafine ceramic fibers. This

5 invention especially relates to the production of discrete ultrafine ceramic fibers by atomizing a viscous solution of a ceramic precursor into a high temperature zone.

Ceramic fibers include a wide range of amorphous _Q and polycrystalline fibers which are used at high temper¬ atures. Chemically, ceramic fibers are generally classified as oxide or nonoxide fibers. The ceramic fibers described herein have also been referred to in the prior art as refractory fibers. Sometimes the terms have been used c interchangeably. However, those skilled in this art today appear to prefer the term ceramic fibers and that term will be employed herein. It is understood that by this desig¬ nation, those materials which can be formed into ultrafine fibers by the process described herein and which may have

2 been described heretofore as refractory or ceramic are included in the term ceramic employed herein. A discussion of ceramic fibers, their properties, uses and preparation, is presented in 20 Kirk-Othmer, Encyclopedia of Chemical Technology 65-77 (3d ed. 1982), the contents of which are rm- incorporated herein by reference.

The ceramic fibers which are the subject of this application are of the oxide type. Examples of oxide ceramic

30

5 fibers include alumina-silica fibers, high silica fibers, zirconia fibers, beryllia fibers, magnesia fibers, and alumina fibers as well as fibers prepared from compositions from which a sol can be prepared. These fibers generally xz have diameters of 0.5 to 10 urn and are manufactured in lengths ranging from 1 cm to continuous filaments. Any of the manufacturing techniques employed to produce ceramic fibers produce a product which may contain a significant quantity of the ceramic material in particulate rather than o in fiber form. These unfiberized particles are commonly referred to in the art as shot. The presence of shot is normally undesirable, since it reduces the thermal efficiency of the fibers. It follows that techniques which substan¬ tially reduce or eliminate the shot are extremely desirable Tc where thermal efficiency will be an important factor in the end use application of the fibers. Thus, the particular process employed in preparing a ceramic fiber can be a major factor in the weight percentage of shot which is found in the end product. 20 Blowing techniques, where liquid material is dropped into a high velocity jet of steam, have been utilized heretofore for preparing alumina-silica fibers. In a cooperative project of Johns-Manville Corporation and Carborundum Company, the addition of small quantities of 25 other materials to these fibers produced some desirable results. Thus, the incorporation of a small percentage of zirconia in the alumina-silica system produced a longer fiber, while a chromia modified alumina-silica fiber produced a upper temperature limit above 1400°C. A boria modified 30 alumina-silica fiber also produced a fiber which could be used continuously at temperatures above 1400°C.

A precursor process developed by Union Carbide Corporation involved the use of an organic polymer fiber as

35 1 a precursor which absorbed the dissolved metal oxides.

Subjecting the treated fiber to heat, burned out the organic fiber, leaving the metal oxide as a polycrystalline ceramic in the shape of the precursor polymer fiber. Employing 5 zirconium oxides permitted the development of zirconia fiber useful at temperatures in excess of 1600°C. Babcock & ilcox developed a process of spinning, blowing or drawing a viscous aqueous solution of metal salts to produce fibers which could be converted to the oxide form of the salts in a subsequent _Q heat treatment. Mixtures of metallic salts permitted the preparation of fibers of combinations of metal oxides. Imperial Chemical Industries developed a process which eliminated the requirement for the use of an organic precursor fiber. This process, known as the sol process, resulted in a significant reduction in the manufacturing cost of such oxide fibers as zirconia and alumina. In the sol process a metal salt, such as aluminum oxychloride or acetate for an alumina fiber, or zirconium oxychloride or acetate for a zirconia fiber, is dissolved to form an agueous solution ^and then is mixed with a quantity of a polymer, such as pol (vinyl alcohol) to increase its viscosity. The solution is further thickened by evaporation and then extruded through a spinneret to produce fibers which are collected on a drum where they are fired to a temperature of 800^βC to provide the desired oxide fiber. The firing removes the organic polymer thickener and produces a fiber with a diameter of 3 to 5 urn and a low degree of porosity. Where the fibers are to be used at a higher temperature, they may be heated to 1400 to 1500°C to eliminate the porosity. An alumina-silica fiber may be prepared in basically the same process from a solution of aluminum acetate mixed with a dispersion of colloidal silica and dimethylformamide. Zirconia fibers often require the addition of a material known as a phase stabilizer, since zirconia undergoes a volume change associated with a monoclinic- tetragonal crystal phase transformation at 1000 to 1200°C. This volume change limits the utility of zirconia in certain end use applications. As a result, additives have been developed for use with zirconia to stabilize its crystalline form. U.S. 4,520,114 of David describes a number of zirconia stabilizing additives including rare earth oxides, yttrium oxides and mixtures of zinc oxide and magnesia or yttria, as well as alkaline earth and lanthanide metal beta-diketonates.

U.S. 3,704,147 of Hardy discloses the preparation of zirconia or alumina fibers by spray drying a highly viscous solution of the metal oxide where the solution has a viscosity in excess of 500 cp at 25°C. The spray dryer is operated using an atomizer disk spun at speeds in excess of 35,000 rpm onto which the viscous solution is introduced. Air inlet temperatures of 200 to 250°C and air outlet temperatures of 100 to 120°C are employed. Subsequent to the separation of the fibers, they are calcined at temperatures of 700 to 1000°C. Fibers having a diameter of 1 to 10 urn and lengths of up to and exceeding 1 mm are prepared by this method.

A need exists for alternate techniques for preparing ceramic fibers, particularly those having an ultrafine diameter and which are not only discrete, but also extremely short in length.

In accordance with the present invention discontinuous, discrete ceramic fibers are prepared by employing an aqueous solution of a chemical precursor of the desired oxide composition, adjusting the solution viscosity to about 50 to about 140 cp and the surface tension of the

2 solution to about 25 to about 70 dynes/cm at ambient temperature and atomizing the viscous sol or solution into a high temperature zone. By atomizing the sol or the solution to form ligaments, a fiber is formed as a result of its high viscosity which continuously breaks off to form discrete, discontinuous fibers, for example, fibers of about 0.1 to 20 cm in length and diameters of about 1 to 5 urn, although fibers of longer or shorter length or greater or smaller diameter can be prepared by the subject method. By providing sufficient heat in the zone into which the solution is atomized, the fibers produced are rapidly dried before they contract to thicker diameter fibers or a droplet morphology. The rapid drying process results in the production of fibers, since it preserves the ligaments formed during the atomization of the solution. After being rapidly dried in the first high temperature zone, a higher degree of heat is

1 provided in a second high temperature zone to decompose or partially decompose the fiber precursor to the desired oxide. The conversion of the precursor produces the desired product of discrete, individual fibers. By only drying the fibers and delaying the higher temperature heating until a later time, the dried fibers would bond together before the subsequent heating step and would, therefore, lose the properties of these small discrete fibers which are produced in the initial zone. By regulating the time and temperature _0 i the second zone, a wide range of crystallinity can be provided.

A solution viscosity of about 50 to about 140 cp at ambient temperature have been found useful for many solutions, but it is the combination of solution viscosity, 5 solution surface tension and atomization parameters which must be coordinated to provide fibers of the desired properties. For a given chemical precursor, one of ordinary skill in the art can, without undue experimentation, adjust the viscosity and surface tension of the solution, the o atomizing conditions and the high temperature conditions to provide fibers having the desired properties.

More particularly, this invention relates to a process of preparing discontinuous discrete ceramic fibers which comprises: (a) atomizing an aqueous solution of a precursor of a metal oxide into a first high temperature zone under conditions of solution viscosity, solution surface tension, atomization and temperature in the first high temperature zone effective to form ligaments of said solution 0 and remove the aqueous phase from said ligaments so as to provide discrete, discontinuous fibers, and

5 1 (b) calcining said fivers in a second high temperature zone maintained at a temperature effective to convert the precursor to the corresponding metal oxide and maintain the discrete nature of the fibers.

5 In the accompanying drawings, Figures 1A and IB are phtomicrographs of. zirconia fibers prepared from different zirconia precursors.

Figures 2A, 2B, 2C and 2D are photomicrographs of zirconia fibers showing the effect of the concentration of ¹⁰ the precursor in the initial solution.

Figures 3A and 3B are photomicrographs showing the effect of employing a polymeric binder in the zirconia precursor solution.

Figures 4A and 4B are photomicrographs of 15 commercial zirconia fiber and Figures 4C and 4D are photomicrographs of zirconia fiber prepared by the process of the subject invention.

The present invention relates to the process for the preparation of ceramic fibers by a process described

PΩ as spray pyrolysis. The subject process prepares discrete, discontinuous fibers by employing an aqueous solution of a chemical precursor of the desired oxide composition. The viscosity and surface tension of the solution are adjusted and then the solution is sprayed or atomized into a heated

²5 cavity to produce short discrete fibers, typically, 0.1 to 20 cm in length and of narrow diameter of, typically, 1 to 5 urn. By employing the process of this invention, discontinuous ceramic fibers of such diverse materials as zirconia, zinc oxide, ferrite, alumina, silica, alumina-

30 silica, magnesia, beryllia, alumina-silica-magnesia and the

35 2 like may be prepared by employing an appropriate precursor. Aqueous solutions of the precursor in concentration of 20 to 30, preferably 22 to 25 wt.% (as the metal oxide) have been found to be useful in the instant process. Thus, when c preparing zirconia fibers, aqueous solutions of such precursors as zirconium acetate, zirconium hydroxyacetate and the like may be employed. Similarly, aluminum oxychloride, aluminum chlorate, aluminum lactate, and aluminum acetate and the like may be employed when alumina fibers are to be 0 prepared. Likewise, ZnO precursors such as zinc acetate, zinc chlorate, zinc nitrate, zinc chloride, zirconyl nitrate, zirconyl chloride and the like and ferrite precursors such as nitrates, acetates, chlorides and the like salts of iron and such other metals as manganese, zinc, nickel and the like may 5 be employed when oxide fibers of these materials are to be prepared by the process of this invention. Where desirable, mixtures of the precursor salts may be employed. One skilled in the art may select other appropriate precursors provided each will form an aqueous solution containing at least 1 wt.% o of the metal compound, as the oxide, will be compatible with any viscosity increasing polymer added to the aqueous solution and will be converted to the appropriate oxide by calcination of the discontinuous fiber prepared therefrom.

Before the solution may be sprayed or atomized into 5 the heated zone, the viscosity of the solution must be adjusted to about 50 to about 140 cp, preferably about 100 to about 125 cp at 25°C and the surface tension of the solution

2 must be adjusted to about 25 to about 70 dynes/cm ,

2 preferably about 30 to about 50 dynes/cm at 25°C. It has 0 been found that aqueous solutions of a medium molecular weight polymer, such as poly(vinyl alcohol), may be employed to increase the viscosity of the solution. A 2.5 to 7.5 wt.%

5 solution of poly(vinyl alcohol) has been found to be useful. Other polymeric materials, such as methyl cellulose, polyethylene oxide, hydroxyethyl cellulose and the like, may likewise be employed to adjust the viscosity of the solution, but poly(vinyl alcohol) is preferred. However, when the end use of the subject fibers requires high purity and, especially, freedom from sodium impurities, high purity water soluble polymers such as poly(2-ethyl-2 oxazoline) or poly(propylene carbonate), both of which are commercially available, can be substituted for the preferred poly(vinyl alcohol). The surface tension may be adjusted by means of a surfactant, such as an alcohol ethoxylate. Particularly useful are those surfactants sold under the Brij tradename by ICI Americas Inc. One particularly useful surfactant is Brij-58 where the alcohol ethoxylate is of the formula, C₁₆H₃₃tOCH₂CH₂}₂₀OH..

The heated zones which are employed to prepare the discontinuous fibers may be located in a single vessel, such as a two zone heated furnace, or a separate vessel or furnace ^may be utilized for each zone. It has been found that the initial zone into which the solution is sprayed should be operated at a temperature of approximately 300 to 400°C with the fibers being retained therein for a period sufficient to rapidly dry the fibers as they are formed. Heating periods of about 1 to about 6 seconds, preferably about 2 to about 3 seconds have been found useful for this purpose. Once the fibers have been dried, they may be collected and passed to the second zone which is operated at a higher temperature to permit calcination of the fibers. Alternately, the fibers may be continuously passed from the first zone into the second higher temperature zone for calcination. Either technique may be employed, and in one particular preferred embodiment, the heating zones are located in a vertical furnace such that the fibers are initially formed in the lower temperature zone, and once they are dried they are carried by air flow into the calcination zone. The calcination zone should be operated at a temperature of 1000 to 1400°C for effective calcination of the fibers.

A variety of techniques may be employed for spraying or atomizing the viscous solution into the initial drying zone. Spray nozzles of various types have been effectively employed, although atomization with a two-fluid nozzle employing air as a second fluid have been found effective for this purpose. A flow rate of about 12 ml/min. and a pressure of about 15 psi have been found to be useful.

Other gases such as nitrogen, argon and the like can also be used. Effective pressurized atomizers are those which provide liquid droplets of about 50 um or less. For a uniform product, it is important that the flow rate be stable and the gas pressure be maintained substantially constant.

The physical nature of the fibers produced are dependent on the interrelationship of the solution properties

(both the viscosity and the surface tension), the atomizing conditions and the temperature in the first high temperature zone. Depending on the chemical makeup of the precursor and the oxide fiber, one having ordinary skill in the art can, without undue experimentation, determine the solution viscosity, solution surface tension, atomizing parameters and the first high temperature zone conditions that should be employed to provide discrete, discontinuous ceramic fibers of the desired characteristics.

In general, the calcined fibers produced by this technique are both discrete and discontinuous, being about

0.1 to about 20 cm in length. These fibers are also ultrafine, having diameters of about 1 to about 5 um. The discrete fibers produced by this process are crystalline in nature and may be produced in a wide range of crystallinity by regulating the time and temperatures, particularly in the calcination zone. The fibers are X-ray amorphous before being calcined. However, the crystallinity can be varied from 0 to 100% through control of calcination time and temperature. The fiber diameter and length are primarily controlled by atomizer pressure, solution viscosity, solution surface tension, concentration and solution flow rate. Where the fibers removed from the calcination zone contain an incompletely decomposed precursor or are of too low crystallinity, the fibers may be heated subsequently to produce the desired phase compositions.

The process of this invention can provide ceramic fibers in either a solid or hollow configuration. In general, hollow fibers are prepared if higher concentration and high viscosity solutions are supplied to the atomizer at a higher flow rate thus attaining a higher heating rate. Conversely, by using low solution concentration and viscosity ^at l°^w flow rate, solid fibers are obtained.

The formation of solid or hollow fibers by the method of the subject invention depends on the concentration of precursors in the solution. Using low relative solution concentration allows for more shrinkage of the solution precursor ligament before the precursor precipitates. Although the presence of the polymer affects the precipitation of the precursor, one having ordinary skill in the art can determine without undue experimentation the exact conditions required to provide the desired fiber. For example, it is known that solid fibers form when the initial precursor concentration is about 22 to 24 wt.% Zr0₂ in the zirconium acetate solutions. Solid fiber formation is also enhanced by using heating rates lower than those associated with spray pyrolysis (~300°C/sec) . Further, the slower the heating rate, the larger will be the fiber diameter. However, when the heating rate is too low (e.g., 10°C/sec) the liquid precursor ligament has sufficient time to contract and spheriodize instead of forming a fiber. Hollow fibers can be obtained by starting with highly concentrated solutions up to the saturation limit of the precursor salt and employing faster heating rates than are employed for producing solid fibers.

According to the Harai equation, the fiber length * (x ) formed from a liquid is controlled by

where V is the fiber elongation velocity, R is the initial fiber radium, ' is the liquid viscosity and σ is the liquid surface tension. For the subject process, this means that fiber length is increased by increasing flow rate to the atomizer and atomizer pressure. As can be seen in the above equation, fiber diameter is decreased for the same conditions leading to increased fiber length. For the zirconia system, low flow rates yield fibers of < 2um diameter and > 2μm diameter fibers for rapid flow rates. The fiber diameter is also controlled by adjustments in the viscosity to surface tension ratio.

Where the metallic oxide of the fibers undergoes phase transformation at elevated temperatures which may produce a volume or other change which is undesirable for the desired end use, it has been found that phase stabilizers may be employed. Thus, yttria and other phase stabilizers may be added to a zirconia precursor solution to provide the meta stable zirconia crystal. The following examples illustrate the practice of the process of the subject invention. In all of these examples, the preparation of discrete, discontinuous zirconia fiber is illustrated. The preparation of zirconia fiber is made for illustrative purposes only and is not to be taken as limiting the subject invention, since those skilled in the art can readily appreciate that the process illustrated in these examples may be readily adapted to prepare metal oxide, discrete discontinuous fibers of other metals.

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EXAMPLE 1

This example illustrates the preparation of zirconia fibers from different precursors.

An aqueous solution of zirconium acetate and zirconyl nitrate was prepared by dissolving a quantity of the salts in water. In a similar fashion an aqueous solution of zirconium hydroxyacetate and zirconyl nitrate was prepared. Yttrium nitrate was added to each solution to provide a Y-0-, concentration of about 5 wt.% of total oxide yield. Upon calcination this provided a yttria phase stabilizer in the zirconia fiber. The aqueous solutions were stirred until all the salts were dissolved and the solution became clear. Sufficient quantities of a 5 wt.% poly(vinyl alcohol) aqueous solution were added to each solution and the solution was then concentrated to provide a solution viscosity of about 120 cp at ambient temperature. At this viscosity the zirconia concentration in the zirconium acetate-based solution and the zirconium hydroxyacetate-based solution was 25.5 wt.% and 30.12 wt%, respectively.

Each viscous solution was atomized by means of a two-fluid nozzle employing air at a pressure of 15 psi and an aqueous solution flow rate of 12 ml/min. The solution was atomized into a vertical furnace operating at a temperature of about 400°C. As the discrete fibers were formed and dried, they were collected in a cotton bag. The fibers were deep gray or brown in color and were quite flexible. The collected fibers were then heated in a calcination zone at a temperature from 900 to 1600°C. The calcined fibers were white and cotton-like.

After calcination the fibers were characterized by X-ray diffraction and SEM. Figure 1A shows a micrograph (250X) of the fibers prepared from the acetate precursor, and Figure IB shows a micrograph (1250X) of those prepared from the hydroxyacetate precursor. Figure 1A shows a higher fiber yield, while Figure IB shows a large fraction of particulate matter or shot. In both cases, the zirconia was tetragonal phase with a grain size of 0.1 and 0.2 um at 1200 and 1400°C, respectively.

EXAMPLE 2

This example illustrates the effect of precursor concentration in the solution. In a fashion similar to that of Example 1, solutions of the acetate precursor were prepared in four weight percent concentrations, 23.5%, 24.8%, 26.6% and 27.4%, as zirconium oxide. The calcined fibers prepared from the solutions were evaluated by X-ray diffraction and SEM. Figures 2A, 2B, 2C and 2D are micrographs at a magnification of about 10OX and correspond to the solution concentrations of 23.5 to 27.4%, respectively. These micrographs suggest that the particulate fraction in the fibers increases with increasing oxide content. The zirconia was in tetragonal form with grain sizes of 0.2 and 0.4 um for calcination at 1400 and 1600°C, respectively.

EXAMPLE 3

This example illustrates the effect of the organic additive on the fiber yield. Two zirconium hydroxyacetate solutions were prepared at a viscosity at 120 cp, in a fashion similar to that of Example 1, except that one solution was prepared without any addition of poly(vinyl alcohol). The fibers were prepared by the method employed in Example 1. Figure 3A is a

10 micrograph (2000X) of the fibers prepared from the solution containing the polymer addition, and Figure 3B is a micrograph (1250X) of the fibers prepared from the solution with no polymer addition. From these micrographs it would appear that the organic film former is necessary to obtain

15 the fibers according to the process of this invention.

20

^•25

30

35 EXAMPLE 4

This example illustrates the effect of the surface tension of the aqueous solution on the subject fibers of the invention.

Two zirconium acetate precursor solutions (25 wt.% Zr0₂) were prepared at a viscosity of about 120 cp, in a fashion similar to that of Example 1, except that a nonionic surfactant was added to one solution to reduce its surface tension from 47 dynes/cm 2 to 39 dynes/cm2. The surfactant employed was an alcohol ethoxylate of the formula

^C16^H33^^OCH2^CH2^2₀ ^OH* ^{The s}P^{ec f c} surfactant was obtained from ICI Americas Inc. under the tradename Brij-58. Fibers were prepared from each solution by the method employed in Example 1. The fibers prepared from the original solution

2 (47 dynes/cm ) had a diameter of 2-3 u , while those prepared from the solution with the lower surface tension

2 (39 dynes/cm ) had a diameter of about 1 um and were visibly longer and contained less shot than those obtained from the solution having the higher surface tension.

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EXAMPLE 5

This example illustrates the difference between commercially available zirconia fibers and those prepared according to the subject invention.

The Example 1 fibers prepared from the acetate precursor were compared with commercial zirconia fibers known by the tradename of Zircar. Micrographs of the Zircar fibers are shown in Figures 4A and 4B, while those prepared by the subject invention are shown in Figures 4C and 4D. The Zircar fibers have a diameter greater than 10 um, while the fibers prepared by the invention have a diameter of 1 to 3 um. The Zircar fibers also exhibit a crack-like morphology, while the fibers prepared by the subject invention are solid and do not exhibit those defects. Microstructurally the fibers prepared according to the invention are much finer and have a more uniform grain size relative to the Zircar fibers in the prior art. Further, fibers of the subject invention can be produced in either solid form or with hollow cross sections by adjusting the solution concentration and viscosity as described hereinbefore.

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EXAMPLE 6

This example illustrates the use of the zirconia fibers of the subject invention. A quantity of zirconia fibers prepared according to

Example 1 were passed through compression rolls and needled into flexible blanket form and subjected to temperatures of 1200 to 1600°C. The blankets remained flexible and exhibited insulating properties comparable to those of commercially available needled blankets prepared from Zircar zirconia fibers.

Modifications and variations of the invention as hereinbefore set forth may be made without the parting from the spirit and the scope thereof. Therefore, only such limitations should be imposed on the invention as are indicated in the appended claims.

Claims

WHAT IS CLAIMED IS;

1. A method of preparing a discontinuous discrete ceramic fiber which comprises:

(a) atomizing an aqueous solution of a precursor of a metal oxide into a first high temperature zone under conditions of solution viscosity, solution surface tension, atomization and temperature in the first high temperature zone effective to form ligaments of said solution and remove the aqueous phase from said ligaments so as to provide discrete, discontinuous fibers, and 0 (b) calcining said fibers in a second high temperature zone maintained at a temperature effective to convert the precursor to the corresponding metal oxide and maintain the discrete nature of the fibers.

2. A method of Claim 1 wherein the solution has 5 a viscosity of at least about 50 to about 140 cp at ambient temperature and a surface tension of about 25 to

2 about 70 dynes/cm at ambient temperature.

3. A method according to Claim 1 or 2 wherein the discrete, discontinuous fibers have a diameter in the o range of 1 to 5 um and a length of about 0.1 to about 20 cm.

4. A method according to any of Claims 1 to 3 wherein the temperature of step (a) is about 300 to about 400°C.

5. A method according to any of Claims 1 to 4 wherein the temperature of step (b) is about 1000 to about 1600°C.

6. A method according to any of Claims 1 to 5 wherein the concentration of the precursor in the aqueous solution is about 1 to about 30 wt.%, as the oxide.

7. A method according to any of Claims 1 to 6 wherein the metal oxide is zirconia, ferrite, alumina or ZnO.

8. A method according to any of Claims 1 to 6 wherein the metal oxide is silica, alumina-silica, magnesia, beryllia or alumina-silica-magnesia.

9. A method according to any of Claims 1 to 6 wherein the precursor is zirconium acetate, zirconium hydroxyaσetate, zirconyl chloride, zirconyl nitrate or zirconium hydroxychloride and metal oxide is zirconia.

10 10. A method according to Claim 9 wherein the solution contains a quantity of yttrium oxide precursor effective to provide an amount of yttria effective to stabilize the zirconia.

11. A method according to any of Claims 1 to 6 -,[- wherein the precursor is aluminum oxychloride, aluminum chlorate, aluminum lactate or aluminum nitrate and the metal oxide is alumina.

12. A method according to any of Claims 1 to 6 wherein the precursor are nitrates, acetates, or chlorides

20 of iron and manganese, zinc or nickel and the resultant mixed metal oxide is a ferrite.

13. A method according to any of Claims 1 to 6 wherein the precursor is zinc acetate, zinc chlorate, zinc nitrate or zinc chloride and the metal oxide is ZnO.

25 14. A method according to any of Claims 1 to 13 wherein the concentration of the precursor in the aqueous solution, the solution viscosity and the heating rate are effective to provide fibers of solid cross-section configuration.

30 15. A method according to any of Claims 1 to 13

35 wherein the concentration of the precursor in the aqueous solution, the solution viscosity and the heating rate are effective to provide fibers of hollow configuration.

16. A method according to any of Claims 1 to 15 wherein the viscosity of said solution is obtained by adding effective amounts of an aqueous solution of poly(vinyl alcohol).

17. A discontinuous discrete ceramic fiber prepared according to the methods of any of Claims 1 to 16.