IL31475A - Stabilized tetragonal zirconia fibers and textiles made therefrom - Google Patents

Description

ana cm Q*» »Bb o α'ηοιηι oasi*o n*:np-r»3 -»a»o STABILIZED TETRAGONAL ZIRCONIA FIBERS AND TEXTILES MADE THEREFROM The invention relates to zirconia fibers and textiles that are stabilized in the tetragonal form by small, carefully controlled amounts Of , oxides of the metals of Group III B of the Periodic Table, and to a process for producing the stabilized tetragonal zirconia fibers and textiles.

Zirconia (ZrOg) has a fusion point of 2677°C, and is therefore a very useful refractory material. However, pure zirconia bodies are rarely used as refractories because of a phase change that occurs near 1000°C.

Below 1000°C, pure zirconia exists in the monoclinic form, and when heated above 1000°C, it transforms to the tetragonal form. The phase change at about 1000°C is accompanied by a large volume change which would cause a pure zirconia article to shatter. However, it is known that zirconia can be stabilized by firing with certain other refractory oxides to produce the cubic form which is stable over a wide range of temperatures. For instance, zirconia is frequently stabilized in the cubic form by firing to 1700°C, or higher with 11.5 to 15 weight per cent yttrla, by firing to 1550°C or higher with 8 to 15 per cent magnesia, or by firing to 1500°C or higher with 6 to 15 weight per cent calcia. Zirconia can also be stablized in the cubic form by silica, scandia, and oxides of the rate earth metals (i.e. lanthanide metal oxides). Discussions of known methods for stabilizing zirconia can be found on pages 364^367 of "Oxide Ceramics" by Eugene Ryshkewitoh, Academic Press, New York (i960) and on pages 77^81 of "High-Temperature Technology" edited by I.E. Campbell, John Wiley and Sons, Inc., New York. Similarly, U.S. Patent 3*259*585 discloses a process for preparing stabilized zirconia sols for use in ceramic applications,' however, this patent also does not teach or suggest the subject of the present invention.

The present invention is based upon the discovery that zirconia fibers and textiles can be stabilized in the tetragonal form by small, carefully controlled amounts of oxides of metals of Group III B of the Periodic Table. scandia, yttria, oxides of the rare earth metals such as lanthana ^ ceria, and the like, and oxides of the metals of the actinide series such as uranium oxide. The preferred metal oxide stabilizers are yttria, ceria, and mixed rare earth metal oxides. Yttria is more preferred. Of course, the metal oxide stabilizers can be used in mixtures, if desired.

The stabilized tetragonal zirconic fibers and textiles are produced by the "relic process" which is described in Belgian Patent No. 697 , 315 , filed April 20, 1967 and made available to the public in late October, 1967 · Briefly, the relic process for producing stabilized tetragonal zirconia fibers and textiles comprises the steps of: (a) impregnating an organic polymeric fabric or textile with a mixture of a zirconium compound and a Group III B metal compound, and (b) heating the impregnated fabric or textile, at least partly in an oxidizing atmosphere, in order to carbonize and volatilize the organic polymer without igniting said polymer, and at the same time to insure conversion of the zirconium and Group III B metal compounds to their oxides.

The resulting stabilized zirconia fiber or textile will have the same form as the original fiber or textile, although the dimensions will be reduced.

The zirconium compound and Group III B metal compound are normally Introduced into the organic polymeric fabric or textile in solution. The solvent is preferably water, but suitable organic solvents can be used also. The action of the solvent apparently swells the amorphous matrix of the fiber polymer, thereby opening the interstices between the small crystallites of polymer chains (micelles or microfibrils). The dissolved metal compounds enter the swollen amorphous regions and become trapped there between the crystallites when the solvent Is removed.

The precursor fiber or textile can be in the form of yarn, monofilaments, knitted textiles, woven textiles, and felts and other non-woven textiles, especially wherein the individual fibers have been interlocked to some extent by processes such as needle punching.

The precursor fibers or textiles can be made from any organic polymer provided that the polymer is characterized by the above-described structure of extremely small crystallites held together in a matrix of amorphous regions which enlarge and admit the metal compounds upon immersion in the metal compounds-containing material, which is usually a solution of the said metal compounds. Cellulosic materials such as rayon, saponified cellulose acetate, cotton, linen, and the like can be used as the fiber or textile.

The fiber or textile can also be wool, silk, acrylics, polyesters, polyamides, and polyurethanes . Preferred fabrics and textiles are those made from rayon.

Many different zirconium compounds and Group III B metal compounds can be used to impregnate the precursor fiber or textile. Specific illustrative examples include zirconyl chloride, zirconium acetate, zirconium oxalate, zirconium citrate, yttrium acetate, yttrium trichloride, yttrium oxalate, and the acetates and chlorides of scandium, cerium and other rare earth metals, and uranium.

Following impregnation of the fiber or textile with the metal compounds from a solvent solution, it is preferred to remove excess solution from the surfaces of the individual fibers in order to prevent accumulation of caked metal compound. This can be done by blotting, pressure rolling, centrifugation, or the like.

The impregnated fiber or textile is then dried, for example, by steps which volatilize the organic polymeric presursor fiber or textile and form the tetragonal zirconia.

The heating step for volatilizing the precursor fiber or textile and forming the metal oxide is carried out under controlled conditions so as to avoid ignition of said fiber or textile. As an example of such controlled conditions, an impregnated cellulosic fiber or textile is heated in air to a temperature between about 350°C. and 900°C. at a rate of not more than about 100°C. per hour. Much faster heating times can also be used, as long as ignition of the precursor fiber or textile is avoided. After the initial formation of the metal oxide (which can occur at temperatures as low as about 350°-400°C. ), a bake at 500° -600°C. or higher is desirable in order to achieve maximum elimination of impurities and densification of the zirconia.

The zirconia fibers and textiles are produced in the tetragonal form and they are stabilized in that form by the presence of the Group III B metal oxide (up to about 20 per cent of the monoclinic form can be present in many cases without detrimental effect) . The zirconia known to the prior art could be prepared in a metastable tetragonal form by decomposing a zirconium compound in air at temperatures below about 600°C. (usually between about 500° and 600°C). However, such metastable zirconia reverted to the monoclinic form when heated above about 600eC, and thereafter its behavior with respect to crystallographic phase changes was identical to that of "conventional" zirconia.

The Group III B metal oxide stabilizers prevent this reversion to monoclinic form at 600eC.

The Group III B metal oxide stabilizer is used in a small stabilizing amount sufficient to maintain at least about 80 per cent of the zirconia fiber or textile in the tetragonal form when said zirconia fiber or textile is heated at about 800°C. for varies somewhat with the exact nature of the Group III B metal oxide stabilizer. For instance, it has been found that from about 0.5 to about 4, and preferably from 2 to 4, mole per cent of yttria {^2^3) » based upon moles of zirconia plus yttria, are effective stabilizing amounts. Amounts of from about 5 to about weight per cent of ceria, based upon weight of zirconia plus ceria, have been found to be effective stabilizing amounts.

Amounts of from about 5 to about 20 weight per cent of mixed rare earth oxides, based upon weight of zirconia plus mixed rare earth oxides, have been found to be effective stabilizing amounts.

The exact amount of stabilizer oxide that is desired to be used depends, to an extent, upon the end use of the zirconia fiber or textile. In uses wherein the excellent resistance of zirconia to alkali is important, and wherein the use temperature is not excessively high (e.g., from 300° to 500°p . ) , as little as 0.5 mole per cent yttria can be employed. An addition level of about 2 to 4 mole per cent yttria is desired to enable the zirconia fiber or textile to be fired to the 800° -1000°C. range in order to eliminate impurities and permit maximum densification, and to thereby achieve maximum chemical resistance. As much as or 20 per cent of monoclinlc zirconia is permissible in such uses. When the zirconia is to be used at temperatures above 1000°C, from about 2 to 4 mole per cent yttria is normally desired in order to achieve the best physical properties.

The stabilized tetragonal zirconia fibers and textiles of the invention have enhanced utility in many high temperature and corrosion resistant applications. The stabilized zirconia fibers and textiles can be used as battery separators, fuel cell separators, pipe liners and troughs for transferring molten metal, heat shields, and the like.

The Examples below illustrate the invention.

Example 1 A series of zirconla cloths containing varying proportions of yttria were produced by the relic process. The initial cloth substrate was square weave, textile rayon. The rayon was preswollen in IN hydrochloric acid, then rinsed in water. The cloth was impregnated by immersing it for 18 hours in 2.5 molar aqueous ZrOCl2 containing varying quantities of YCI3. The impregnated cloth was centrlfuged three times at 4000-4600 rpm in an 11-inch diameter basket for a. total of 20 minutes in order to remove excess salt solution.. The cloth was then heated in a forced air oven. The . initial temperature was 25°C, and the temperature was gradually increased over a period of 24 hours to a final temperature of 650°C. Thereafter, the cloth was fired as indicated in Table I below Table I, below, displays the YCI3 content of the Impregnatin solution, the mole per cent Y2O3 in the Zr02 cloth, and the crystallo graphic phases in the cloth after heat treating in air. The crystallographic phases were determined at room temperature by X-ray diffraction analysis. In Table I, M, T and C denote the monoclinic, tetragonal, and cubic phases, respectively, present in the Zr02 cloths.

TABLE I Crystallographic Phases of Zrt /YpO-t Cloth YCl Content Crystal Structure After: Cloth of Solution Mole Y2O3 1 hr. @ 1 hr. @ 1 hr. @ 24 hr. @ No. Gm/liter in Cloth 1000°C 1200°C l400°C l400°C 1 0 0 93#M iob#M 100#M 100#M 796T 2 3.6 I.29 90#M 100#M 100#M lOO^M #T 3 7.2 2.08 9# 3#M 96#M 100 M 91#T 57#T 4#T 4 13.9 2.73 100#T 100#T lOO^T 100#T 26.2 5 -52 10C C lOO^C 10C#C lOOgC 6 .4 .10 10C C 10C C 10C C 100 C Example 2 A free-standing zirconia thermal insulation heat shield was fabricated. The heat shield assembly consisted of two half-cylinders measuring 4.3 inches internal diameter by 7.0 inches tall, with top and bottom circular plates. The wall thickness of the cylinder varied from 0.150 to 0.200 inch. The heat shield was produced by the following procedure: Thirty mil satin weave zirconia cloth containing 2.2 mole per cent yttria stabilizer was produced from rayon by the relic process by a procedure analogous to that described in Example 1. The cloth was fired to a final temperature of 1000°C. The zirconia cloth was then impregnated with a zirconia cement and wound around a cylinder to give a six-layer laminate. The zirconia cement is a mixture of 165 parts by weight of an aqueous solution of basic zlrconyl chloride i.e., Zr0(0H)Cl« nH20, and yttrium trichloride (Sp. Gr. of the solution was 1.65 at about room temperature) in proportions to yield 4 weight per cent (2.2 mole per cent) yttria in zirconia when fired, and 100 parts by weight of a zirconia powder containing 4 weight per cent yttria. The powder was prepared by flash decomposition at 600°-800°C. of an aqueous solution of zlrconyl chloride and yttrium trichloride, followed by dry ball milling of the decomposition product for 20 hours and passing the powder through a 400 mesh screen. The weights of the cloth and cement was 340 and 220 grams, respectively. The laminate was then fired to a temperature of 1000°C, over a period of 4 hours.

The heat shield was placed inside a tungsten-heated vacuum furnace. The furnace was cycled through twenty rapid heating and cooling cycles ranging from room temperature to 850°C. to 2000°C. with no apparent damage to the heat shield other than a slight amount of warping. Unstabilized zirconia would have Example 3 Ten-mil zirconia satin weave cloth containing 4.56 weight per cent yttria (i.e., 2.5 mole per cent yttria) that was prepared by the relic process from rayon cloth, was fired to various temperatures and then immersed in 75 weight per cent' aqueous potassium hydroxide for 50 hours at 4lO°F. Table II, below, displays the firing times and temperatures (in air) and the tensile strengths before and after immersion in potassium hydroxide.

TABLE II Sample Maximum Hrs.ig Tensile Strength-lb/inch No. Temp.°C. Temp. Before Immerslon After Immersion 1 650 5 0.8 0.02 2 700 1 1.0 0.02 3 800 1 0.7 0.05 4 900 1 0.9 0.17 1000 1 1.0 Ο .47 The maximum retention of strength was found with the cloth that was fired to 1000°C. Unstabilized zirconia cannot be heated to temperatures nearly as high as 1000°C. without forming the monoclinic phase which is not nearly as chemically resistant as the tetragonal phase, probably because the individual crystals are much larger in the monoclinic phase. All five ZrO cloths stabilized with Y2O3 were still intact after contact with KOH and could be recovered from the KOH solution.

Example 4 A series of zirconia cloths were made by the relic process analogous to the procedure described in Example 1.

The precursor cloth was 5-harness satin-weave cloth using textile rayon yarn of 1100 denier and 480 filaments. The rayon cloths were soaked for 17 hours in an aqueous solution of zirconyl chloride having a specific gravity of 1.400 ± .002 and which containe speed of 3.0 rpm. After air- drying the cloths were again rolled once at the same conditions. The heating schedule was analogous to that described in Example 1, to a final temperature of 600°C.

Table III, below, displays the amounts of yttria and zirconia (as YCI3 and ZrOCl2) in the impregnating solutions and the weights of the rayon cloths that were used as the precursor fabric : TABLE III Gr YCl Cloth Mole Per Weight of No. Cent Y2P3 Rayon Cloth 1 0 0 312 272 2 0.5 Ο.69 307 270 3 1.0 . . 1.2-3 · 305 272 4 1 .5 1.83 304 266 . 2.0 2.52 305 266 6 2 .5 3.32 303 267 7 3.0 4. 17 301 264 8 3.5 5.39 301 267 9 4.0 6.84 297 265 5.0 11.28 289 265 11 6.0 13.93 289 266 12 12.0 31.10. 268 263 ( 1) Zirconyl chloride and yttrium trichloride are often supplied commercially labeled with an assay of the number of grams per equivalent of the oxide.

Table IV, below, displays the mole per cents of yttria found in the cloths (which, it will be noted, are not identical to the amounts that were in the impregnating solutions), and the crystallographlc phases after heat treating for the indicated times.

TABLE IV Crystallographlc Phases Present In ZrOpAgO^ Cloths After Heat Treatment In Air (M = monoclinlcj T = tetragonal; C = cubic) Y2°3 Y2°3 2 houra 1 hour 1 hour 1 hour 1 hour Content Content at at at at at mole t. # 600 °C 800 °C 1000°c 1200 °C 1400° C 0 0 91#T 6C#T Μτ 95*M 4#M 86#Μ 100#M lOO^M 0.55 1.0 91#T 8<¾6Τ 2½Τ lOO^M 10O#M 0.72 1.3 93 T 8¾&Γ 2¾6 #M 7#M 11#Μ 91^M lOO^M 0.93 1.7 93 T 89#τ 51#Τ 12 T 7#M 11#Μ ¾6Μ 88#M 10 M 1.0 1.8 92gT 90#Τ 4 Γ 18#T 8j*M 10#Μ 55#Μ 82#M 96#M 1.3 2.3 94#T 9^Τ 87#Τ 25#T 1÷¾*T 6 Μ 1$Μ 75#M 8 #M 1.·2 2.6 93t? 93# 93#Τ 24#T 8#T 7#Μ 75TM 72#M 8 1.8 3.3 9 T 93#Τ 92#Τ 49#T 46#T 6 M 7#Μ 8#Μ 1^M 4#M 9 2.4 4.3 95#T 93 Τ ½τ 97 T 100#T #M 6 Μ 3# 3.4 6.2 100#T ιοο#τ 97#Τ ιοοτ 100#T 3#Μ 11 3.8 6.9 95#T 96#τ 100 Τ 100#T 100#T # 12 7.6 13.9 lOOgC ioo#c 100#C IOO56C lOO C Table V, below, displays the tensile strengths of the zirconia cloths after heat treatment in air at the indicated temperatures.

TABLE V Tensile Strengths* of Zr02/Y203 Cloths After Heat Treatment in Air Y2O3 Y2O3 1 hour 1 hour 1 hour 1 hour Cloth Content Content at at at at No. mole # wt. <$> 800°C 1000°C 1200°C 1400°C 1 0 0 0.44 0.46 too weak for measurement 2 0.55 1.0 1.20 0.97 too weak for measurement 3 O.72 1.3 I.92 Ο.58 too weak for measurement 4 0.93 . 1 .7 1.28 0.53 0.46 1.0 1.8 I.45 0.90 0.82 6 1.3 2 . 3 1.91 1.48 0.77 7 1.4 2.6 2.39 2.54 0.57 0.40 8 1.8 3.3 2.38 2.98 1.48 1.04 9 2.4 4.3 2.96 2 .83 5 .61 2.84 3-4 6.2 4.22 4. 32 5.23 8.19 11 3.8 6.9 3.51 4.95 6.89 5 . 61 12 7.6 13.9 0.93 2 .56 1.84 2.63 * Reported in lb/in. Average of two pulls on each sample.

Example 5 Tetragonal Stabilization of Zlrconia Fibers with Cerium Oxide Stabilized zirconia fibers containing additions of three levels of cerium oxide were prepared in the form of woven cloth employing the same method described previously in Example 4. Cerium oxide was used as the chloride salt which is commercially available and had a purity of 99 - 9.

Three aqueous solutions (2000 ml each) used for impregnation of the fibers contained chloride salts having equivalent oxide compositions listed below: Solution Cerium Oxide (06903 Zr02 Specific Gravity No. gm./liter gm./liter of Solution 1 17.5 291 . 1.40 2 25.2 280 1.40 3 39.5 264 1.40 The rayon fibers were impregnated with the salt by immersing 266 gm. of rayon in each case in the above solutions for a period of 19 hours at 70°P .

The fibers were subsequently treated in a manner analogous to Example 4 to convert the salt-loaded rayon fibers to oxide fibers. The three fibers were determined by x-ray diffraction analysis to be composed of tetragonal zirconla. The fibers contained 6.99* 10.70 and 16.95 weight per cent cerium oxide, based upon total weight of fibers. The fibers containing the smallest amount of cerium oxide also contained 13# monoclinic zirconia. All three fibers were heated to 1000°C.

Example 6 Tetragonal Stabilization of Zirconia Fibers with Rare Earth Oxide Stabilized zirconia fibers containing additions of three levels of mixed rare earth oxides were prepared in the form of woven cloth employing the same method described in Example 4. The mixed rare earth oxide was used as the chloride salt which is commercially available and had the following composition: Rare Earth Oxide Percentage by Weight La203 24 Ce02 48 Pr60n 5 Gdg03 2 Y2°3 0.2 » equivalent oxide compositions listed below Solution Rare Earth Oxide Z >2 Specific Gravity No. gm./liter gm./liter of Solution 1 17. 3 289 1.40 2 25 . 1 279 > 1.40 3 40.0 269 1.40 The rayon fibers were impregnated with the salt mixture by immersing 266 gm. of rayon in each case in the above solutions for a period of 19 hours at 70°P.

The fibers were subsequently treated in a manner analogous to Example 4 to convert the salt loaded rayon fibers to oxide fibers. The three fibers, heated to 1000°C, were determined by x-ray diffraction analysis to be composed of only tetragonal zirconia. The zirconia fibers contained 8 . 30, 10. 30 and 16.55 weight per cent rare earth oxides, based upon total weight of fibers.

Example 7 The oxide fibers in cloth form containing the three levels of rare earth oxides (Example 6) and three levels of cerium oxide (Example 5) were tested for resistance to chemical attack by concentrated aqueous potassium hydroxide. (Potassium hydroxide is employed as the electrolyte in many secondary (re-chargable-type) batteries and fuel cells and chemical and physical stability of electrode separators used in contact with the electrolyte often at temperatures in the range of 200-500°P. is Important to the efficient operation of these devices.

The six cloths were immersed in 75-85$ potassium hydroxide ( 15-25$ H2O) held at 400°F. for a period of 360 hours. After this period they were removed from the solution and washed free of potassium hydroxide with water. All cloth specimens were recovered from the solution intact and had undergone no damage or changes TABLE VI Breaking Cloth Weight Loss Strength Mo . Oxide Stabilizer Per Cert lb./inch widt Example 6 656 Rare Earth Oxide 12.8 1.7 E ample 6 9% Rare Earth Oxide 10.4 2.8 Example 6 15$ Rare Earth Oxide 6.6 3.3 Example 5 6 Cerium Oxide 27.4 0.1 Example 5 9^ Cerium Oxide 5.4 1.8 Example 5 15# Cerium Oxide 2.5 2.8

Claims (14)

31475/2 Claims

1. . Tetragonal zferconia fiber or textile containing an oxide of ajmetal of Group III B of the periodic table where In said group III B metal oxide 1s (a) from about 0,5 to 4 mole percent yttrla , based on moles of zlrconla plus yttrla (b) from about 5 to about 20 weight per cent of ceMa , based on weight of zlrcon a plus cerla , or (c) from about 5 to about 20 weight per cent of mixed rate eartfjbxldes , based upon weight of zlrconla plus mixed rare earth oxides , the oxide being present 1n an amount sufficient to maintain at least about 80 per cent of said zlrconla fiber or texti le 1n the tetragonal form when heated at about 800°C for about one hour.

2. The tetragonal zlrconla fiber or texti le of claim 1 wherein said Group III B metal oxide 1s yttrla in an amount of from about 2 to 4 mole per cent, based upon moles of zlrconla plus yttrla .

3. A process for the preparation of a tetragonal zlrconla fiber or textile as defined 1n claim 1 , wherein an organic polymeric fiber or texti le 1s Impregnated with a mixture of a zirconium compound and a Group 9QII B metal compound , whereafter the Impeegnated fiber or textile 1s heated partly 1n , an oxidizing atmosphere without. Igniti ng said fiber or texti le.

4. A process according to Claim 3, wherein the zirconium compound and Group III B metal are Introduced I nto the organic polymeric fiber or textile 1η solution.

5. A process according to claim 4, wherein the solvent 1s water.

6. A process according to Claim 8 or 5, wherein the excess solvent 1s removed before heating.

7. A process according to any o cl aim 3 to 6, wherein the zirconla compound 1s selected from the grou consisting of zlrconyl chloride, zirconium acetate, zirconium oxalate and zirconium citrate.

8. A process according to any of Claims 3 to 7, wherein the Group III B compound is selected among the group consisting of yttrium acetate, yttrium trichloride, yttrium oxalate and the acetates and chlorides of scandium, cerium and other rate earth metals and uranium. 31475/2 .. .

9. A process according to any of Claims 3 to 8, wherein the organic polymer Is a cel luloslc material .

10. A process according to Claim 9 , wherein the cel luloslc material 1s rayon.

11. . A process according to any of Claims 3 to ίθ, wherein the heating step 1s performed by heating 1n air to a temperature between about 350°-900°C , at a rate of not more than about 100°C per hours .

12. A process according to any of Claims 3 to 11 , wherein the amount of said Group III B metal oxide util ised 1s (a) from about 0.5 to about 4 mole per cent yttrla , based on moles of zlrconla plus yttrdi« (b) from about 5 to about 20 weight per cent of cerla , based upon weight of zlrconia plus cerla , or (c) from about 5 to about 20 weight per cent of mixed rae earth oxides , based upon weight of zlrconia plus mixed rate earth oxldss.

13. A process for the preparation of a tetragonal zlrconia fiber or texti le as defined 1n claim 1 , substantial ly as hereinbefore described wi th referfice to the xamples .

14. Tetragonal zlrconia fiber or textile, whenever-produced by the process accordi ng to any of Claims 3 to 13