US3406025A - Process for producing metal fibers, textiles and shapes - Google Patents

Description

United States Patent 3,406,025 PROCESS FOR PRODUCING METAL FIBERS, TEXTILES AND SHAPES Bernard H. Hamling, Warwick, N.Y., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Continuation-impart of applications Ser. No. 451,326, Apr. 27, 1965; Ser. No. 523,549, Jan. 28, 1966; and Ser. No. 576,840, Sept. 2, 1966. This application Dec. 19, 1966, Ser. No. 602,555

12 Claims. (Cl. 75.5)

ABSTRACT OF THE DISCLOSURE A process for producing elemental metal textiles, fibers, and shapes is described. The process involves the steps of (a) impregnating a preformed organic polymeric substrate, such as rayon fiber, with a solution of a metal compound, (b) removing the solvent, such as by evaporation, (c) heating the impregnated substrate in an oxidizing atmosphere to first carbonize and then oxidize (without igniting) the substrate to etfect removal of the substrate thereby leaving a metal oxide relic and (d) then reducing the metal oxide relic to form the elemental metal in the shape of the preformed substrate. The process is widely useful for producing many types of metal articles, and it is particularly interesting as a means for producing tungsten wire.

This application is a continuation-in-part of Ser. No. 451,326 filed Apr. 27, 1965, Ser. No. 523,549 filed Jan. 28, 1966, both now abandoned, and Ser. No. 576,840 filed on Sept. 2, 1966, all by B. H. Hamling. The foregoing applications were all continuations-in-part of Ser. No. 320,843 filed Nov. 1, 1963, now abandoned.

This invention relates to fibers, textiles and shaped articles composed of elemental metals, and to a process for producing such fibers, textiles, and shaped articles.

The prior art has prepared elemental metal fibers, for example, by a number of methods, but each is characterized by important limitations. For example, in the shaving or skinning method, Wire is pulled across a large number of chisel-cutting tools to shave otf small filaments. This method is severely limited by the high tensile strength required of the base metal and the inability to shave fibers below about 20 microns diameter in the production machines.

In the whisker method, fibers are formed from the vapor phase or by electrolysis of molten salt. This process is prohibitively expensive for large-scale production.

None of the prior art methods provide elemental metal fibers in continuous lengths, or in lengths twisted together to form yarn or cloth-type forms. For example, if the fibers are prepared by the widely used wire drawing method, such twisting and shaping must be performed after the fiber formation. This is a time-consuming and expensive procedure often precluded by the fiber tensile or shear strength.

It is an object of this invention to provide a novel process for producing a class of elemental metal fibers which are not subject to the disadvantages and limitations of elemental metal fibers previously known. Another object of the invention is to provide fibers composed of a major amount of one or more elemental metals, which fibers have diameters of 3 microns or less, length-todiameter ratios over 400 and tensile strengths in excess of 40,000 pounds per square inch, and a process for producing such fibers. A further object of the invention is to provide a relatively low temperature process for producing elemental metal fibers which have high strength and flexibility. A still further object of the invention is 3,406,025 Patented Oct. 15, 1968 "ice to provide a variety of textile forms, including staple fibers, continuous tow and yarns, woven fabrics, batting and felts, composed of elemental metal fibers.

It is another object of the invention to provide shaped metal articles, and a method for producing such articles from non-fibrous organic materials. Still another object is to provide a variety of films, tubes, cups and other shapes which are composed of elemental metal.

The novel fibers, textiles and shapes of the present invention are composed of one or, more of the elemental metals from Group I-B (copper, silver and gold), Group VI -B (chromium, molybdenum and tungsten), Group VIII of the Periodic Table (iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum) technetium, rhenium and zinc. Each of these metals is characterized by a melting point above 350 C. and the corresponding metal oxides have free energies of formation (AF) at 350 C. of up to 73 kilocalories per gram atom of oxygen, as listed in Table I.

TABLE I Oxide form AF of oxide at 350 C.

Other forms of metals such as metal oxides can be present in the elemental metal fibers, textiles, and shapes of this invention but in amounts not greater than an aggregate of 20 percent by weight.

The elemental metal fibers of this invention can exist in a wide variety of textile forms, including staple fibers inch to 3.0 inches or more in length, continuous length tow, yarn and roving, woven cloth, knits, braids, felts, papers and the like.

The process for producing the elemental metal fibers, textiles and shapes of this invention comprises the steps of (1) impregnating a preformed organic polymeric material with one or more compounds (preferably salts or hydrolysis products of salts) of metal elements which form the metal oxides of Table I, (2) heating the impregnated organic material under controlled conditions (which prevent ignition of the said organic material) to convert (pyrolyze) the organic material to predominantly carbon and thereafter remove the carbon as a carbon-containing gas, and thereby producing a fiber, textile or shape that is predominantly metal oxide or other reducible metal compound and which has essentially the same physical configuration as the original polymeric material, and (3) then heating the product of step (2) at a temperature of at least 300 C. in the presence of a reducing gas so as to reduce the metal to the elemental form.

The term preformed as used herein means that the organic polymer material has been fabricated into a fibrous or non-fibrous shape prior to impregnation with the metal compounds.

The physical form and shape of the elemental metal product is essentially the same as and is determined by the physical form of the preformed organic starting material, although considerable reduction in size takes place.

During conversion of impregnated organic fibers to the elemental metal fiber form, both the diameter and the length of the fiber shrink to approximately 40 to 60 percent of the original dimensions. Similar shrinkage in all dimensions also takes place with the non-fibrous shapes.

Where a yarn composed of a multiplicity of continuouslength elemental metal fibers is desired, a continuous filament organic yarn is employed as the starting material in the process of this invention. Similarly, where a woven fabric or felt composed of elemental metal fibers is desired, a woven organic fiber cloth or felt can be used as the starting material. Of course, metal woven textiles can be made using conventional textile equipment and techniques starting with metal staple fibers or yarns made by the process of this invention.

Without being bound by same, the theory and mechanism of this process appears to be as follows: Microscopically, organic polymeric materials, such as cellulose, are composed of extremely small crystallites of polymer chains (micelles or microfi'brils) held together in a matrix of amorphous polymer. When the organic materials is immersed in a solvent, such as water, aqueous solutions, or organic solvents it swells, thus opening the interstices between the crystallites. The amorphous regions enlarge and the crystallite spacing increases. The dissolved metal compound such as a salt, enters the swollen amorphous regions, which is generally about 50 to 90 percent of the volume of the swollen organic material, and becomes trapped in the amorphous regions between the crystallite when the solvent is removed from the material.

The metal compounds do not crystallize upon drying of the organic material, as would normally occur upon drying most solutions, since they are effectively suspended and separated as islands, about 50 A. in size in the case of cellulose, between the polymer crystallites.

The organic polymeric material can be impregnated with two or more metal compounds from the same solvent solution, so that fibers, textiles or shapes containing more than one elemental metal can be prepared. In the first approximation, most metal compounds enter the interstices in direct proportion to their solution concentration, allowing ready control of the relative loadings of metal compounds in the organic material. Due to the blocking action of the organic crystallites, the metal compounds and later the oxides and elemental metals cannot segregate from each other nor crystallize during the subsequent steps.

Any organic polymeric material can be employed as a starting material in the process of this invention providing it 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 on immersion in the solvent. Any class of materials which are composed of long-chain molecules held together by chemical cross-links can also be used, provided these materials are capable of swelling and absorbing a solvent and provided the organic polymeric material does not melt on heating. Any cellulosic material can be employed including rayon, saponified cellulose acetate, cotton, wood and ramie, and the like. Other suitable organic materials include the protein materials (such as Wool and silk) and the man-made acrylics, polyesters, vinyls and polyurethanes. Certain organic materials, such as polyethylene and polypropylene, are not suitable for practicing the instant process because they cannot be swollen for imbibition of the metal compounds and/or the materials melt and lose their structure during pyrolysis. A preferred cellulosic material is rayon due to its structural uniformity, good imbibition characteristics and low impurity content.

Impregnation, or imbibition, of the organic matter can be carried out by several methods. Where the element which will appear in the final metal article has salts which are highly soluble in water, the impregnation step can be carried out by immersing the organic material in a concentrated aqueous solution of such salt. For example, Where a nickel fiber is desired, an organic fiber can be impregnated by immersion in an aqueous solution of nickel nitrate (Ni(NO or nickel chloride (NiCl having concentrations in the range 2.0 to 4.4 moles of salt per liter. For salts which hydrolyze (acid reaction) when dissolved in water, the acidity of the impregnating solution is preferably not greater than 1.0 molar (in hydrogen ion) in order to prevent degradation of the organic material during immersion. The acid may be neutralized with ammonia, if desired.

In order to obtain adequate strength in the final elemental metal product, cellulosic materials are imbibed with the metal compounds to the extent of at least onequarter mole and preferably 1.0 to 2.0 moles of the metal compound(s) in each base mole, of cellulose. The term base mole as used herein refers to the molecular weight of a glycosidic unit of the cellulose chain (molecular weight of 162). With non-cellulosic materials, the concentration should be at least 0.1 and preferably 0.5 to 1.0 gram equivalent of metal in the metal compound imbibing solution per gram organic material. For example, on this basis at least 183.9/58.7 or 0.31 gram tungsten metal should be employed in a solution for imbibing one gram of a non-cellulosic fiber. With lower concentrations of metal compound(s), insufficient elemental metal is available in the organic material relic for a strong article and the process becomes less etficient in terms of elemental metal yield per unit weight organic starting material.

Pre-swelling of cellulosic organic materials in water prior to immersion in concentrated imbibing solutions is preferably employed to increase both the rate and extent of salt inhibition. For acrylic and polyester materials, aromatic alcohols are suitable swelling agents, and the ketones are useful in swelling vinyl and polyurethane materials for the same purpose.

Water is the preferred solvent for metal compound-imbibing of cellulosic materials, Other solvents such as alcohols do not afford as eflicient swelling nor solubility of the selected metal compound for a high degree of imbibing. For vinyl and polyurethane materials, esters and ketones are appropriate solvents, as for example normal butyl acetate or methyl ethyl ketone. For acrylic and polyester materials, suitable solvents for the metal compound imbibition include aromatic alcohols and amines such as aniline, nitro-phenol, meta-cresol and paraphenylphenol.

Immersion times at normal room temperature (21-23 C.) required to give adequate impregnation vary from a few minutes to several days depending on the salt(s) employed and the type of organic material employed. Immersion times greater than about 3 days in concentrated salt solutions is undesirable for cellulosic material since the material may degrade, resulting in a decrease in the amount of salt absorbed, and in the case of fibers, causing the fibers to bond to each other.

To illustrate the impregnation step, viscose rayon rapidly swells and absorbs ZnCl to a large extent in concentrated solutions. Within 15 minutes water-swollen viscose rayon absorbs 3.4 moles of ZnCl per base mole of rayon from a 6.8 molar solution at 21 C. However, the rayon impregnated by this treatment swells to essentially a gel and becomes tacky. The rayon in this swollen state is too Weak to be handled and the fibers cannot be separated from each other. The preferred process for impregnating rayon with ZnCl so that the salt-loaded rayon is not degraded and fibers are not bonded together, is immersion of water-swollen rayon in 3.6 to 4.0 molar solution for 1 to 3 hours. By this treatment, viscose rayon absorbs 0.6 to 1.0' mole of ZnCl per base mole of rayon.

When it is desired to increase the rate of imbibition of the metal com-pounds in the organic materials to shorten immersion time, the metal compound solution may be heated to as high as C. For example, for salts which are taken up by cellulosic fibers at a slow rate, the immersion time can be shortened by raising the temperature of the salt solution to 50 C. to 65 C. Care should be exercised, however, in using elevated temperatures since many salts will grossly degrade the organic material at high temperatures. 5

Following imbibition with metal compound(s) from a solvent solution, it is necessary to remove excess solution from between the organic fibers before they dry in order to avoid bonding together of fibers by caked salt, or from the surface of non-fibrous shapes in order to prevent accum-ulation of caked salt on the surfaces of the shape. Allowing excess unimbibed metal or hydrolysis product to remain results in reduced strength and increased brittleness in the final elemental metal product. Blotting thoroughly with absorbent paper or cloth using moderate pressure is useful for removing excess solutionfrom the organic material. In addition, washing, high velocity gas streams, vacuum filtration and centrifugation have proven to be effective methods for removing excess impregnating solution. For solutions having viscosities greater than about 10 centipoises, raising the temperature of the organic material to 5060 C. aids in removing excess solution.

The impregnated organic material is then thoroughly dried by any convenient means, such as air drying or heating in a stream of warm gas. It is desirable to dry the impregnated fibers rapidly (in about one hour or less) to prevent expulsion of salt from the interior of the organic material to its surface.

When a product containing two or more elemental metals is desired, the organic material is impregnated with two or more salts or hydrolysis products. If two or more water-soluble salts are employed, the impregnation can be carried out by a single immersion in an aqueous solution containing both salts. When two metals are desired, one of which is impregnated into the organic material from aqueous solution and the second impregnated by hydrolysis of the metal halide or oxyhalide from organic solution, a preferred method is to impregnate first with the hydrolysis product and then with the water soluble salt.

In the next principal step in the process of this invention (the decomposition of the preformed organic polymeric material) the impregnated organic material is heated under controlled conditions for a time sutficient to decompose the organic structure and form a carbonaceous a relic shape containing the metal compound in finely dispersed form, and concurrently and/or subsequently to eliminate at least a portion of the carbon.

The controlled conditions must be such as to avoid 5 ignition of the organic material. For the impregnated organic materials used in the process of this invention, ignition generally takes the form of an uncontrolled temperature increase within the material rather than combustion accompanied by flame. An uncontrolled temperature increase is a rapid increase which deviates sharply from the heating pattern of the impregnated organic material and its environment. When pyrolysis conditions are properly controlled, the temperature increase in the impregnated organic material follows closely the temperature of its surroundings (atmosphere, furnace wall, and the like) even though the exact temperature of the organic material may fiucturate to temperatures both above and below the nominal temperature of the environment. If the organic material ignites or burns instead of carbonizes, the metal compound temperature rises excessively due to its contiguous relation to the organic structure. Under such circumstances it is impossible to control the temperature, and the melting point of intermediate metal compounds formed may be exceeded. Also, the metal compound may become suspended in the pyrolysis product vapors, and thus lost from the environment and unavailable to form the desired relic. When ignition is avoided the products have smoother surfaces and are stronger due to a more orderly consolidation of the metal compound particles.

In practice, a convenient way to determine whether or not ignition has taken place during the heating steps is to observe the degree of shrinkage or consolidation of the starting organic material. Where ignition has not taken place, the impregnated organic material undergoes substantial shrinkage along its longest dimension, generally in the order of 4060 percent. (Shrinkage from 10 cm. long to 5 cm. long is 50 percent shrinkage.) The metal oxide intermediate product is strong and microcrystalline, and in the case of fibrous products, highly flexible. On the other hand, where'undesirable ignition takes place during the heating steps, the degree of shrinkage is considerably less, and the resulting metal oxide intermediate product tends to be crystalline rather than mierocrystalline and is brittle and of low strength. If ignition takes place toward the end of the carbonization step, thedegree of shrinkage may still be substantial but the physical properties of the product will be less desirable. In general, the degree of shrinkage is inversely proportional to the loading of metal compound into the organic material. It has been found desirable to adjust process conditions to obtain maximum shrinkage for the particular metal compound loading in the impregnated organic polymeric material.

In practice, it has been found that ignition can be avoided by use of controlled reaction conditions, particularly conditions which avoid sharp changes in temperature, atmosphere composition and the like. Sharp changes in conditions tend to precipitate the uncontrolled temperature increases within the impregnated organic material which constitute ignition as hereinabove defined.

As an example of such controlled conditions, cellulosic fibers impregnated with metal salts are heated to a temperature between about 350 C. and 900 C. at a rate of not more than C. per hour in an atmosphere that can contain between 5 and about 25 volume percent oxidizing gas. (It is understood, of course, that where the metal compound intermediate product is to contain an oxide such as Ni O or C0 0 which has a decomposition or melting point below 900 C. the impregnated organic material is not heated above such decomposition or melting temperature.) By the time the impregnated fiber has been heated to 350 C. or above, under the abovedescribed conditions, a major portion of the cellulosic fiber will have been pyrolyzed to carbon (carbonized) and the carbon removed as a carbon-containing gas, and a major portion of the metal compound in the impregnated fiber will have been decomposed and/ or oxidized to the metal oxide form.

After the first slow heating to a temperature above 350 C. the volume percent of oxidizing gas need not be maintained at 25 volume percent or below, although there is generally no significant advantage in employing an atmosphere containing greater than 25 volume percent oxidizing gas. The preferred oxidizing gas is oxygen, although other oxidizing gases such as nitrogen dioxide and sulphur trioxide can be used if desired. The balance of the gaseous atmosphere comprises gases which are chemically non-reactive at temperatures up to 900 C. and above. Typical non-reactive gases include nitrogen, he-

0 lium, argon, neon, and the like. In the first heating step,

it is not essential to provide an oxidizing gas in many cases.

In the process of this invention, the important factor is that the process variables be controlled to avoid ignition and/ or burning or the organic portion of the impregnated organic material. Control of process variables to avoid ignition can be carried out in many ways; for example, by temperature regulation, by limiting the amount of oxidizing agent available to the impregnated material, or by the use of vacuum or inert atmosphere. It must be remembered that a certain amount of oxygen forms a part of the chemical structure of many organic materials, for example cellulose, so that a certain amount of oxidizing agent is available within the impregnated material itself. Here the use of smaller samples of impregnated material or relatively slow heating rates, particularly to temperatures up to about 350 C., can help to avoid undesirable ignition. It has been found, however, that with many systems, particularly impregnated cellulosic fibers from which excess solvent has been removed and which have been carefully dried, that rapid heating in vacuum or inert atmosphere is possible without causing ignition.

Control of conditions to avoid ignition is generally easier with fibers and fibrous materials such as textiles than with other non-fibrous shapes, such as organic foams or sponges, which have been impregnated with metal compounds. For such non-fibrous shapes it is generally preferred to use non-oxidizing atmospheres in the initial portion of the carbonization-oxidation process and to maintain the rate of heating at less than 50 C. per hour.

An oxidizing agent can thereafter be added to the atmos phere after a substantial portion of the carbonization step is completed. It is also possible to carry out the entire carbonization treatment of non-fibrous shapes by heating at rates between 10 C. and 50 C. per hour in atmospheres containing from 5 to volume percent oxygen.

For both fibrous and non-fibrous materials it is seldom necessary to employ temperatures above 1000 C.

The exact choice of reaction conditions of course depends on the shape and chemical composition of the starting organic material, and on the metal compound or metal compounds employed in the impregnation step.

The carbonization of the imbibed organic material and the removal of such carbon by volatilization (usually to CO or CO are not separate and distinct steps. When heating of the imbibed organic material is first begun, pyrolysis of the organic portion to carbon is the predominant chemical reaction. The carbonized organic material comprises predominantly carbon but also can include small amounts of residual hydrogen. However, some slight oxidation of the carbon formed and of the metal present in the imbibed fiber usually takes place. As heating continues and substantially all of the organic material is converted to carbon, oxidation of the carbon and frequently of the metal with which the fiber had been imbibed becomes the predominant reaction.

In this first heating step wherein the organic polymer is first carbonized and then oxidized, usually the metal compound is also oxidized to the metal oxide. It is not essential, however, to produce the metal oxide in this step as long as the metal is in the form of a compound that is (a) ultimately reducible to the elemental form and (b) relatively non-volatile so that the metal compound will not be volatilized during the heating steps.

Microscopic voids are produced in the fibrous or nonfibrous shape as carbon is volatilized. Maximum densification is achieved by limiting the concentration of oxidizing gas and the temperature of the shape below about 500 C. during the first heating-oxidation step. This step normally requires between 1 and 48 hours to complete, depending on the metal compound(s) and resulting intermediate compounds, e.g., metal oxide or metal oxide mixture. Many metal oxides, such as iron oxide, copper oxide and chromium oxide enhance the rate of carbon oxidation from the organic material and relatively lower temperatures are normally used when these metal oxides are present in the fiber. Volatilization of the carbon at a faster rate (promoted by either increased oxidizing gas concentration or relatively higher temperature in early stages of this step) produces a less dense and weaker elemental metal shape due to the voids remaining in the shape. Relatively high temperatures above 500 C. also initiate crystallite formation in the shape which weakens the end product, produces rough surfaces, and increases the difliculty in reducing the fiber relic to the elemental metal form in the second heating-reduction step. Higher temperatures and/or stronger reducing atmospheres are needed with crystalline fibers. For these reasons the first heating step is performed under conditions which minimize crystalline formation. Densification of the metal fiber, for example, is observed as shrinkage of both the diameter and length of the fiber during oxidation. The length-to-diameter ratio, as Well as the geometry of the fiber cross-section, remains essentially the same as the starting organic fibers during conversion.

In the final process step or" this invention, the relic from the first heating step is further heated at temperature of at least 300 C. and in the presence of a reducing gas. As previously indicated, each of the metals which can be prepared in the elemental fibrous or non-fibrous form by this invention has a free energy of formation in the oxide form of less than about 73 kilocalories per gram atom of oxygen in the oxide at 350 C. (see Table I). Also, usually most of the relics from the first heating step are at least partially'in the oxide form. Because of their relatively low free energies of formation, they may be readily reduced to the elemental form at temperatures between about 300 C. and 1400 C. The following Table II lists suitable temperatures for reducing the oxide of the various metals to the elemental form in a hydrogen atmosphere:

TABLE II Rh n: 300400 It should also be recognized that the oxide of chromium is quite difiicult to reduce to the elemental form, but may be reduced under less severe conditions when alloyed with other metals. For example, chromium oxide (Cr O requires a temperature of 12001400 C. for reduction with hydrogen by itself, but when heated in hydrogen with iron oxide or nickel oxide the reduction may be achieved at about 900 C.

There is no actual upper temperature limit on the second heating-reduction step, but as indicated in Table II, all of the elemental metal shapes may be prepared by reduction in hydrogen atmospheres at temperatures below about 1400 C.

Hydrogen gas is the preferred reducing atmosphere although other well-known reducing agents such as carbon monoxide and ammonia may be employed. With carbon monoxide the reducing temperature should be at least 600 C., and when using ammonia a temperature of at least 800 C. is most suitable.

The duration of the second heating-reduction step depends on the metal compound to be reduced, the temperature and the reducing agent. In general, this step is completed in 26 hours at the temperautre level indicated in Table II. The reduction step is preferably monitored by condensation of water from the exit gas. When water is no longer being formed the reaction is complete. For

. the preparation of elemental tungsten fibers, it is preferred to maintain the fibers at 600 C. for 4 hours so that substantially all of the carbon is removed at this level. In a continuous process, it is of course not appropriate to follow progress of the reaction by presence of Water in the exit gas.

It should be recognized that the elemental metal shapes of this invention are highly crystalline.

In a preferred embodiment of the present invention, where a textile yarn containing a multiplicity of elemental metal fibers is desired, the imbided organic fiber yarn is kept under tension during the first and second heating steps. Tensions in the to 40 gram range have proved satisfactory for keeping 3300 denier/ 1440 filament rayon yarns straight during these steps.

In another useful embodiment of the invention, elemental tungsten fibers are prepared by first adding sufficient ammonium paratungstate to an aqueous solution containing between about and by weight'hydrogen peroxide at about -70 C. to provide about 700-1000 grams dissolved tungsten per liter solution. The solution is immediately cooled to about ambient temperature when the ammonium paratungstate is completely dissolved, and rayon fiber is immersed in the solution at ambient temperature within 72 hours of the cooling step. The rayon fibers swell and the interstices are opened by the solution such that the tungsten compound is imbided. The unimbided tungsten compound is removed from the outer surface of the rayon fiber and the tungsten compound-imbided fiber is dried. Next the imbided fiber is heated at least partly in an oxygen-containing atmosphere to temperatures between 350 C. to 500 C. rate of not more than 100 C. per hour to decompose the rayon fiber and evolve at least most of the carbonaceous matter therefrom to form a tungsten oxide fiber relic. The latter is further heated at temperature of 350l000 C. and in the presence of a reducing gas to reduce the tungsten to the elemental metal form. This further heating step includes a period of at least 3 hours at temperature of at least 500 C.

It appears that the hydrogen peroxide affects the quantity of ammonium paratungstate which can be dissolved in an aqueous solution. Tests have shown that insufii cient tungsten imbibition of organic fibers occurs when the same tungsten compound is simply dissolved in ammonium hydroxide solutions. That is, the resulting solution contains only about 100 grams dissolved tungsten per liter solution, and this concentration is too low to prepare strong elemental tungsten fibers from rayon.

It has been found that an aqueous solution containing 30-40% by weight hydrogen peroxide at about 50-70 C. permits 700-1000 grams tungsten to be dissolved per liter solution, and this concentration range is optimum from the standpoint of effective imbibition to the fiber interstices in the minimum time. Higher concentrations are too viscous for effective penetration by the tungsten compound. For example, using 745 grams tungsten/liter at a pH of 1.2, equilibrium imbibition of 1.3 pounds tungsten compound/ pound rayon has been reached in less than 30 minutes.

Dissolution of the ammonium paratungstate at 50-70 C. is necessary for a rapid reaction to form a highly soluble but unstable peroxy tungsten compound which may be (NH O-W O -2H O. Rapid dissolution of the ammonium paratungstate in hydrogen peroxide begins at about 60 C. which heats the mixture to 80-90 C. when no cooling is used. The resulting reaction product is relatively unstable at this temperature and for this reason the solution is immediately cooled to ambient temperature when dissolution is complete. During the dissolution, oxygen is released due to decomposition of the peroxy tungsten compound reaction product. If the solution is not cooled the pH rises to 304.0 within an hour and the tungsten precipitates from the solution as a yellow solid. It is not useable for rayon fiber imbibition in this form.

Using this process, strong lustrous fibers with 99.6% tungsten and less than 0.04% carbon have been made with remarkably high specific gravities, e.g., 17.5-18.1 gm./cc. or 91-94% of theoretical density of tungsten.

Elemental tungsten woven tape of extreme smoothness and high luster has been prepared by this process, starting with a rayon ribbon of the type sold at variety stores. Characteristics of the starting rayon ribbon and the tungsten ribbon product are as follows:

Alternatively, elemental tungsten and molybdenum fibers, textiles and shaped articles may be prepared by hydrolysis of appropriate compounds from organic solvents. These metals exist as compounds which hydrolyze or react with Water to form metal oxide products essentially insoluble in water. This chemical property is utilized to effect imbibition of organic materials with these metal oxides as described below. Suitable hydrolyzable and/or water reactive compounds include the following metal halides and oxyhalides: (l) MoCl Mo O Cl to yield MoO hydrate; and (2) WCI WCl to yield W0 hydrate (tungstic acid). The above metal halides or oxyhalides are dissolved in an organic liquid immiscible with water, such as carbon tetrachloride, chloroform, carbon disulfide, ethyl ether, or benzene, to the extent of 5 to 50 gm. of metal halide or oxyhalide per 100 ml. or organic liquid. The rayon or other organic material is exposed to air having a relative humidity between 50 and percent, in order for the material to swell by absorbing between 5 and 30 percent by weight of water. While still swollen and containing the absorbed water, the organic material is immersed in the metal halide or oxyhalide organic solution. As the metal halide or oxyhalide penetrates the moist organic material, it reacts with the water and an oxide precipitate forms directly in the organic material structure. This hydrolysis reaction is normally complete in 20 to 30 minutes. The remaining steps of the elemental fiber, textile, or shaped article preparation are as previously described.

The elemental fibers and fiber compositions of this invention have numerous uses. For example they may be employed for reinforcing plastics for use at relatively low temperatures, and metals and porcelains and other ceramic bodies for use at high temperatures.

Certain of the elements which can be prepared in the form of very small diameter fibers by this invention, are well-known catalysts for chemical reactions. For example, elemental platinum and palladium are widely used hydrocarbon conversion catalysts. The present fibers may be used for this purpose with the advantage of intimate contact with the hydrocarbon feedstock by virtue of the large fiber surface area.

Certain of the elemental metal fibers are characterized by relatively high thermal conductivity, and useful where this function is desired. For example, flexible cloth composed of nickel, silver and tungsten fibers in the woven form may be used for wearing apparel in specialty applications such as for astronauts.

Certain of these elemental metal fibers are characterized by high electrical conductivity, e.g, silver, copper, nickel and tungsten. These fibers may be woven into rugs for discharging static electricity or into fabrics for electrical heating.

Elemental tungsten is widely used as an incandescent material, and the present tungsten fibers are suitable for this purpose. An elemental tungsten yarn of this invention was heated to incandescence by passing an electric current through it, and the yarn behaved much as the filament in an electric light bulb.

The elemental tungsten fibers preferably in fabric form, are useful as resistance heating elements in high tempera- 1 l ture furnaces. Their fiexibility provides an important advantage over tungsten wire in the fabrication.

The metal shapes of this invention have a wide variety of uses. The thin films can be used as conductive sheets. The metal shapes of this invention can be used as light weight structural members, and the like. Metal shapes prepared from organic foams or sponges are also useful as filters.

For use as filters it is preferred that the metal shapes of this invention be prepared from cellulosic foams or sponges characterized by open porosity, uniform pore size and low density.

The metal films of this invention are sheets which are highly uniform in thickness and which can be as thin as microns.

The invention and its advantages will be more clearly understood by the ensuing examples.

Example 1.Tungsten cloth The solution used for impregnating was made by dissolving 249 grams of ammonium paratungstate in 200 ml. of 30% hydrogen peroxide. The solution was heated to 6070 C. at which temperature the ammonium paratungstate reacted and dissolved in 510 minutes. The solution was then rapidly cooled to room temperature and contained 745 grams of tungsten per liter, with a specific gravity of 1.89 and 1.0 pH. The rayon cloth was of 5 harness satin weave in both fill and warp directions using a textile'grade viscose rayon yarn (1650 denier/720 filaments in yarn/l ply), the cloth weight being oz./yd. A 6 inch x 18 inch piece of cloth weighing 35.6 grams was immersed in the tungsten salt solution for 3 hours. The cloth was next centrifuged of excess solution and dried in warm air. The dried cloth contained 1.30 grams of tungsten salt per gram of rayon.

The cloth was converted to the elemental tungsten metal form by first heating in air at a rate of 20 C./hour to 300 C. and maintaining the latter temperature for 4 hours. Next the cloth was further heated at a rate of 50 C./hour until 350 C. was reached, and this temperature was maintained for 4 hours. The air in the tube furnace was then purged with gaseous nitrogen before the reduction with hydrogen commenced. Hydrogen gas was passed through the furnace at a rate of 4 liters/min. (STP). The cloth was subjected to the following temperature regime: 550 C. /2 hour; 600 C.-4 hours; 700 C.1 hour and 1000 C.-1 hour. During the 1000 C. heat treatment the hydrogen was dried thoroughly by passage through a coil in a liquid nitrogen trap. The reducer cloth was uniformly bright and metallic, and the flexibility of the metal cloth was near that of the original rayon, and the metal cloth had a tear strength of 30 lbs/in. of width. An X-ray powder diffraction examination revealed that a highly crystalline tungsten phase was the only phase present in the cloth. The complete lack of a W C phase indicated that less than 0.4% carbon was present. The tungsten fibers in the cloth had a specific gravity of 18.1 gms./ cc. or 93.8% of theoretical density. The fibers had a diameter of 4-5 microns.

Example 2.Tungsten felt A section of interlocked rayon felt made from 5.0 denier rayon filaments weighing 3.7 grams and measuring 2.5 inches by 5.9 inches and approximately A inch thick was immersed in a solution containing 500 grams of tungsten per liter, the specific gravity being 1.69 and pH 0.9. After two hours immersion the felt was centrifuged and dried at 54 C. in air. The impregnated felt was converted to tungsten metal felt by heating in air at a rate of 50 C./hour to 300 C. and maintaining 300 C. for 4 hours. It was next heated at a rate of 50 C. per hour to 350 C. and kept at the latter temperature for 4 hours. The resulting black felt was then placed in a tube furnace and heated (in a 4 liter/minute hydrogen stream) rapidly to 640 C. and held at this temperature for minutes.

It was next heated to 900 C. and held at that temperature for 2 hours. The resulting elemental tungsten felt has a high luster, bright metallic appearance and Was flexible as well as strong (minimum tear strength of 2 lbs/inch width). A 53.2% weight yield was achieved based on the rayon starting weight (0.532 gram elemental tungsten per gram rayon). The area yield of the felt was 14.1% and the thickness was 0.1 inch. The felt had a bulk density of 0.58 gram/ cc. or 97% porosity. The elemental tungsten filaments in the felt had diameters of 7-8 microns.

Example 3.-Tungsten yarn and tungsten filaments The rayon yarn used was a 60 ft. length of a high tenacity viscose type (1650/4000/1), with a nominal filament denier of 0.4. It was nearly round in cross-section with 6.2 micron diameter. Even through the ply was single, two plys could be easily separated from the zerotwist yarn. One ply of the yarn was 800 denier and the other about 870 denier. The imbibition of the yarn in tungsten solution was the same as described in Example 1. After centrifuging excess solution from the yarn but before the yarn was allowed to dry, its length was cut into ten sections. Each section was individually suspended and held under tension with a 100 gram weight to keep the filaments in the yarn parallel and straight during drying. Conversion of the yarn to the elemental metal form was performed in a vertically oriented tube furnace, and 40 gram weights were hung on the yarns to keep them straight during conversion. They were first heated in air at a rate of 50 C./hour to 350 C. and held at that temperature for 4 hours. The air was then flushed with a nitrogen gas stream before hydrogen was introduced to the tube furnace. Prepurified hydrogen was passed through the furnace at a rate of 1 liter per minute (STP). The yarns were heated rapidly to 400 C. and from that temperature to 600 C. at a rate of 100 C. per hour. At 600 C. it was held for 2 hours, then heated at a rate of 200 C. per hour to 985 C. and held at that temperature of hour. During this heat treatment the section of yarn in the furnace shrank to /2 of its original length.

A 3-inch long section of the elemental tungsten metal yarn with a denier of 1015 was selected from the center portion of the yarn for tensile tests. The ends of the yarn were glued to metal tabs with epoxy resin and the yarn breaking strength measured with an instrument employing a transducer load cell for indicating the tension placed on the specimen at the time of breaking. The yarn had a breaking strength of 0.71 lb. Based on fiber area the tensile strength of the yarn was 79,000 p.s.i. A single tungsten filament /z inch long) was taken from the broken yarn and its breaking strength similarly measured using a modified analytical balance. The 2.0 micron diameter fiber supported a weight of 0.53 gram, which indicated its tensile strength to be 250,000 p.s.i.

Example 4.Silver cloth A piece of satin-weave rayon cloth (identical to that used in Example 1) weighing 21.1 grams and measuring 4.5 inches by 12 inches was immersed in a saturated silver nitrate solution for 15 minutes (specific gravity of the solution was 2.22 at 22 C.). The salt entered and swelled the rayon immediately. The fibers became very swollen and stretchy but were not degraded. After centrifuging excess solution from the cloth and drying, it contained 1.83 grams of silver nitrate per gram of cloth. Conversion of the silver loaded rayon to elemental metal fibers occurred readily due to the reactivity of the nitrate in the rayon as C. was approached in the heating schedule (50 C. per hour to 350 C. with holding for 4 hours at this temperature). Reaction took place between the nitrate and the rayon and heated the fibers to sufiiciently high temperature in air to cause complete decomposition and volatization of the rayon and leaving metallic silver fibers, even though this conversion was carried out in the presence of air. A sufficiently high reducing atmos- Example .Nickel yarn Five grams of 0.4 denier rayon yarn were immersed in 100 ml of 3.0 molar nickel chloride (NiCl After immersion for 18 hours the fibers were centrifuged of excess solution and dried in warm air. The dried fibers contained 0.5 gram of nickel chloride per gram of rayon.

. The fibers were, oxidized by heating in air at a rate of C./hour to 375 C. and held at that temperature for 24 hours. After this oxidizing treatment the fibers still retained their high degree of luster and were uni formly gray. The fibers evidenced no breakup at this point but were fairly Weak. They were next reduced to the elemental metal by heating in a stream of dry hydrogen at 400 C. for 0.5 hour and then slowly raising to 800 C. and maintaining that temperature for /2 hour. The reduced nickel fibers were bright metallic color, flexible and ductile. Fiber diameters were 1.8 microns $0.3 micron as measured at 1500 magnification using a calibrated filar eyepiece. A portion of these fibers were further heated in hydrogen to 900 C. There was substantially no change in the fibers but when heated still further to 1000 C. the fibers became slightly sintered together; i.e. metallurgical bonds were created at points of fiber contact. These fibers were attracted to a magnet. X-ray powder diffraction patterns showed a well-crystallized face-centered cubic phase to be the only structure present in the fibers. The metal yield of the above sample was low compared to optimum conditions0.17 gram nickel per gram starting rayon. Using 4.3 M nickel chloride and preswelling in water followed by impregnation for 24 hours, weight yields of 0.41 gram elemental nickel per gram of starting rayon have been obtained.

Example 6.-NickeI-iron alloy cloth The solution used for this imbibition contained 1.4 M nickel chloride (NiCl and 2.6 M iron chloride (Fe'Cl The solution had an iron-nickel atom ratio of 1.81, and the pH was 0.5. The cloth used for impregnation was a 2 warp x 2 fill basket weave cloth made from rayon yarn. A 10 inch x 10 inch piece weighing 36.0 grams was immersed in the solution for 24 hours, centrifuged of excess solution and dried at 47 C. The dried cloth contained 0.65 gram of mixed salt per gram of rayon, with a nickel content of 5.12% and an iron content of 8.81%. This cloth was heated rapidly in air to 350 C. and held at that temperature for 22 hours. It was next placed in a tube furnace and heated in a hydrogen stream to 800 C. in approximately 2 hours, followed by further heating to 1000 .C. at a rate of approximately 50 C. per hour. Upon removal from the furnace the elemental nickeliron alloy cloth had a bright metallic luster. The cloth was stiff due to slight sintering of the yarn.

Although no chemical analysis was made it was obvious that the cloth contained little remaining oxygen and carbon since it was readily ductile and could be hammered into a thin film with no indication of brittleness. X-ray powder diffraction analysis revealed the cloth contained a single metallic phase. No carbide or oxide phase was present.

Example 7.Nickel-chromiun alloy cloth The imbibing solution used was made by adding ml. of concentrated chromic acid solution to 100 ml. of 4.4 molar nickel chloride. The resulting solution was intended to have a relative content of chromium to nickel of 20 to 80 wt. percent and the pH of the solution was 14 0.3. The cloth used was identical to that used in Example 6. A cloth section (13.5 grams) was immersed in solution for 19 hours. After centrifuging excess solution and drying overnight the salt content of the cloth was 0.25 gram per gram of rayon. The cloth was next oxidized by heating to 400 C. in air in about 1 hour and holding at that temperature for 1 hour. The resulting oxide cloth had a dark green to black color. Reduction of the oxidized cloth was performed by heating in a tube furnace and passing dry hydrogen to 900 C. and held at this temperature for 15 minutes. The resulting cloth had a bright metallic appearance, was ductile and could be hammered into a thin film without evidence of brittleness. X-ray powder diffraction analysis indicated the cloth to be completely metallic and composed of a chromium-rich phase and a nickel-rich phase. No oxide nor carbide phases were present. This example demonstrates that the present invention facilitates the preparation of nickel-chromium fibers at relatively low temperature, that is, 880-900 C.

Example 8.--Tungsten film The starting organic film for the preparation of tungsten film was commercially-available, plain, 450 gage (33 microns thick) cellophane. Cellophane is a transparent cellulose sheeting made from viscose and is essentially identical in composition as rayon.

A piece of cellophane measuring four inches square was immersed in a 3.7 molar aqueous solution of ammonium metatungstate for a period of 20 hours. The film was removed from the solution, wiped of excess solution, and allowed to dry.

It was next pyrolyzed by heating in air at a rate of 50 C./hr. to 350 C. and holding for 4 hours at 350 C. The carbonized film was then placed in a tube furnace and heated in a flowing hydrogen atmosphere for one. half hour periods at 450, 550, 700, 800 and 1000 C.

The product of the above treatment was a smooth highly-reflective metallic film with dimensions of 2.4 inches square. The thickness of the film was 20-21 microns. A metallographic section of the film revealed the metal to be fully dense and with no observable crystalline grain structure. However, X-ray diffraction analysis indicated the film to be crystalline tungsten. Purity of the film was greater than 99% elemental tungsten.

In a preferred embodiment of the invention, tungsten fibers are prepared in the form of continuous filament yarn by a continuous process. Highly-oriented, all-skin, Tyrex ray-0n yarn is employed as the preformed organic polymeric material. The impregnating or imbibing solution is an aqueous solution of ammonium metatungstate, a compound of the formula To illustrate the impregnation step, the following table displays the amount of tungstate imbibed by various types of rayon for aqueous solutions of ammonium metatungstate at various concentrations:

TABLE.-TUNGSTEN IMBIBITION IN VARIOUS RAYON YARNS Immersion time-22 hours Solution Temperature-23 C.

Tungsten loading, grams w/grarn rayon Ammonium metatungstate Tyrex type 130, Painesville solution, sp. gr. 1,650/4,000/1 Beunit, 900/744/5 white, 1,650/720/1 (als I This yarn was 1,650 denier having 4,000 single ply filaments.

After the impregnation, the excess solution is removed, for example, by centrifugation followed by air-drying. The impregnated yarn is then subjected to the heating steps to convert it to continuous filament tungsten yarn. A continuous process can be used for this purpose.

In one illustration of such a continuous process, a sevent and one-half foot laboratory scale reactor having a one-half inch diameter reaction tube is employed. The reaction tube is divided into four sixteen-inch heating zones wherein the steps of the process are carried out. The first heating zone (Zone 1) is maintained at about 200 C., Zone 2 at about 500-600 C., Zone 3 at about 800 C., and Zone 4 at about 800-900 C. In Zones 1 and 2, carbonization of the rayon takes place and (principally in heating Zone 2) the ammonium metatungstate is decomposed into tungsten oxides. In heating Zones 3 and 4, the carbon is removed by volatilization, and the tungsten oxides are reduced to tungsten metal. A countercurrent stream of hydrogen is run through the reactor, thereby providing a hydrogen atmosphere for all of the heating steps. The hydrogen is not needed in heating Zones 1 and 2, but its presence is not detrimental and it is therefore used throughout the reactor for convenience. It is desirable to add an inert gas such as argon to the hydrogen stream between heating Zones 2 and 3 in order to help sweep out the gaseous decomposition products formed in Zones 1 and 2. Decomposition of the rayon and volatilization of the carbon is accomplished without using an oxidizing atmosphere anywhere in the reactor.

In this continuous process, it has been found that it is at times desirable to have some carbon left in the yarn as it enters the high temperature zone (i.e., Zone 3). In this case, volatilization of the last portions of the carbon is eifected simultaneously with reduction of the tungsten oxides to elemental tungsten. It has been found desirable to have some moisture in the hydrogen in order to facilitate volatilization of the carbon. For instance, at a yarn throughput rate of 150 feet/hour, 6 volume percent of H in the hydrogen gas appears to be optimum for the conditions set forth above. (The water can be added to the hydrogen simply by bubbling the hydrogen through water.) At said throughput rate of 150 feet/hour, the yarn is in each 16-inch heating zone for about 20 seconds.

Although preferred embodiments of this invention have been described in detail it is contemplated that modifications of the process and composition may be made and that some features may be employed without others, all within the scope of the invention.

What is claimed is:

1. A process for producing elemental metal fibers and textiles which comprises the steps of:

(a) impregnating a preformed organic polymeric fiber or textile with a compound of one or more of the metals of Groups I-B, VI-B or VIII of the Periodic Table, technetium, rhenium, or zinc, to produce an impregnated fiber or textile;

(b) heating said impregnated fiber or textile to a temperature sufiiciently high to carbonize and then volatize said organic polymeric fiber or textile, wherein this step (b) is carried out under conditions whereby ignition of said organic polymeric fiber or textile is avoided; and

(c) heating and product of step (b) in the presence of a reducing gas at an elevated temperature sufficient to reduce the product of step (b) to the elemental metal form.

2. Process of claim 1 wherein said organic polymeric fiber or textile is cellulosic.

3. Process of claim 2 wherein the cellulosic fiber or textile is a rayon fiber or textile.

4. Process of claim 1 wherein said metal compound is a salt that is converted into a metal oxide by said step (b).

5. Process of claim 1 in which tungsten is said metal.

6. Process of claim 1 in which silver is said metal.

7. Process of claim 1 in which nickel is said metal.

8. Process of claim 1 wherein a mixture of nickel and iron is said metal.

9. Process of claim 1 wherein a mixture of nickel and chromium is said metal.

10. Process of claim 1 wherein said preformed organic polymeric fiber or textile is impregnated with said compound by immersing said preformed organic polymeric fiber or textile in an aqueous solution of said compound.

11. A process for producing elemental tungsten fibers which comprises: i

(a) adding sufiicient ammonium-paratungstate to an aqueous solution containing between about 30% and 40% by weight hydrogen peroxide at about 5070 C. to provide about 700-1000 grams dissolved tungsten per liter solution;

(b) immediately cooling the solution to about ambient temperature when the ammonium paratungstate is completely dissolved;

(c) immersing rayon fiber in the tungsten compoundcontaining aqueous solution at ambient temperature within 72 hours of cooling step (b) thereby swelling and opening the rayon fiber interstices such that the tungsten compound is imbibed in said interstices;

(d) removing the unimbibed tungsten compound from the'outer surface of said rayon fiber and drying the tungsten compound imbibed rayon fiber;

(e) first heating the tungsten compound-imbibed rayon fiber to temperature between about 350 C. and

I 500 C. at rate of not more than 100 C. per hour for at least one hour and for sufficient duration to decomposed the rayon fiber and evolve at least most of the carbonaceous matter therefrom to form a tungsten oxide fiber relic, at least part of said first heating being performed in an oxygen-containing atmosphere; and

(f) further heating the tungsten oxide fiber relic from first heating step (e) at temperature of between about 350 C. and 1000 C. and in the presence of a reducing gas so as to reduce the tungsten to the elemental form, said further heating including a period of at least 3 hours at temperature of at least 500 C.

12. The process of claim 1 wherein said preformed organic polymeric material is rayon, wherein the metal compound is ammonium metatungstate, and wherein the reducing gas is hydrogen.

References Cited UNITED STATES PATENTS 399,174 3/ 1889 Von Welsbach 117-33.4 575,261 1/1897 Moscheles l1733.4 623,723 4/1899 Kohl et al 1l733.4 2,870,000 1/1959 Ryznar -20 3,087,233 4/ 1963 Turnbull 75200 L. DEWAYNE RUTLEDGE, Primary Examiner.