Metal Nitride Fibers, Textiles and Shapes and Process for Producing them

Beginning of the patent period: August 15, 1969.

The invention relates to a process for the production of metal nitride fibers and textiles.

Heretofore, there has not been a completely satisfactory process for producing articles of inorganic material having a predetermined irregular or complicated shape. Previous methods required machining or other molding techniques. The processes would be difficult and complicated, or the final shape of the article would be difficult to control.

The object of this invention is an improved, reproducible process for producing metal nitride fibers having diameters of less than 30 μ, preferably 2 to 30 μ, and a length / diameter ratio greater than 400, the metal nitride fibers having uniform diameters, straight, smooth surfaces and no deformations should have. This method should not require high pressure techniques. The fibers should be flexible and have high strength at high temperatures. By the method according to the invention also various textile forms, such as staple fibers, continuous tow and yarn, woven products, cotton wool and felts, which consist of metal nitride fibers, are produced.

The inventive method is that

a) in a conventional manner, a preformed organic polymeric fiber or textile is allowed to swell by immersion in a solution of a metal compound and the amorphous regions between the crystallites of the fibers are thereby filled with the metal salt solution, whereupon the excess metal salt solution is removed from the surface and the material is dried,

b) the polymer with the metal compound soaked is first heated to a temperature of between about 300 and about 400 ° C at a rate sufficiently low that decomposition products of the polymer can escape without adversely affecting the physical structure of the fiber, the heating being sustained for a sufficient time to replace the organic structure of said polymer leaving a carbon-containing residue, containing said metal in finely dispersed form, and

c) the remainder of the first heating step (b) is heated in known manner in contact with vapors of nitrogen-containing compounds to a temperature of between about 650 and about 2000 ° C to convert the metal throughout the remainder of the nitrogen compound to a Metal nitride fiber or a textile to achieve.

The process of the present invention provides microcrystalline metal nitride material having substantially the same shape as the original organic polymer.

Theory and mechanism of this process may be as follows: Organic polymers, such as

Rayon fibers consist microscopically of extremely small crystallites of cellulose chains (micelles or

Microfibrils) held together in a matrix of amorphous cellulose. The crystallites, which in Rayongarnen with large tensile strength a diameter of approximately 40 Ä and a length of

Nr.280463

250 Ä are parallel to the axis of the rayon fiber and when dry about 20 Ä apart. A fiber of 1 denier (1 g weight per 9000 m length) has several million crystallites over its cross section. When the fiber is immersed in a solvent such as water or an aqueous solution, it swells laterally to open cavities, the amorphous regions increase, and the distance of the crystallites becomes about 50 Å (in the case of rayon). The dissolved metal compound, such as. A metal salt, enters the swollen amorphous regions, which make up about 85% of the bulk of the swollen rayon, and is trapped in these amorphous regions between the crystallites as the solvent is evaporated from the fibers.

The metal compounds do not crystallize upon drying of the organic polymer, as would otherwise be expected upon drying of a solution, since they are effectively suspended and separated between the crystallites as islands about 50 Å in size.

Of the organic polymers, two or more Metallver compounds can be absorbed by the same solution, so that nitrides can be prepared from more than one metal. In a first approximation, most metal compounds enter the polymer cavities directly proportional to their solution concentration, thereby providing easy control of the relative loading of the organic polymer with the metal compounds. Because of the blocking effect of the organic crystallites, the metal compounds can not separate or crystallize during drying. Because they are finely divided, the metal compounds and later the components of the polymeric residue are highly reactive and can undergo the necessary chemical reactions to form the desired metal nitride products at relatively low temperatures.

As the starting material for the process of the present invention, any organic polymeric material can be used if it has the extremely small crystallites described above, held together in a matrix of amorphous regions which expand upon immersion in the solvent and take up the metal compounds. Any material that consists of long-chain molecules held together by chemical crosslinking can also be used. Any cellulosic material such as rayon, cellophane, saponified cellulose acetate, cotton, wood and ramie fibers can be used. Other suitable organic materials are protein fibers (wool and silk), acrylics, polyesters, vinyl materials and polyurethanes. Certain organic substances, such as polyethylene and polypropylene, are not suitable because they can not be swollen for the absorption of the metal compounds and / or the material melts and loses its structure during pyrolysis. Rayon is preferred as the cellulosic material because of its structural uniformity, good wicking properties, and high purity.

The external shape of the metal nitride composition is substantially the same as that of the polymeric organic starting material. During the transfer of the organic fiber containing the metal compound absorbed into the metal nitride fiber, the length of the fiber shrinks to about 40 to 60% and the diameter to 25 to 35% of the original dimensions.

When a yarn is to be made of many continuous metal nitride fibers, a continuous organic yarn is used as the starting material. Similarly, in the production of a woven fabric or felt of metal nitride, a woven organic fiber material or felt is used as the starting material. Of course, woven fabrics of metal nitride can be made by conventional textile apparatus and methods using staple fibers or metal nitride yarns made in accordance with the present invention. A similar shrinkage of all dimensions also occurs in non-fibrous forms. The term preformed is intended herein to mean that the organic, polymeric material has been made into a fibrous or non-fibrous form prior to impregnation with the metal compound.

To obtain sufficient tensile strength in the finished metal nitride fiber, so many metal compounds are absorbed by the cellulosic material that at least 0.25 mole and preferably 1.0 to 2.0 moles of metal compound (s) are contained in each base cellulose. The term base mole as used herein refers to the molecular weight of a glycosidic unit of the cellulose chain (having a molecular weight of 162). For non-cellulosic materials, at least 0.1 and preferably 0.5 to 1.0 gram equivalents of metal ions of the metal compound per gram of organic polymer should be absorbed. At lower concentrations of metal compounds, too little metal salt is available in the test to form a solid article and the process is less efficient in terms of metal nitride yield per unit weight of preformed organic polymer. Another disadvantage of these low metal concentrations is that more drastic conditions are required to achieve pyrolysis.

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The Aufeaugevorgang or the impregnation of the organic material can be done in various ways. When the salt of the metal whose carbide is to be produced is well soluble in water, the organic material may be immersed in a concentrated aqueous solution of this salt.

For salts which strongly hydrolyze in aqueous solution, the acidity of the impregnating solution should preferably be not greater than 1.0 molar (of hydrogen ion) in order to prevent decomposition of the organic fiber during dipping. If desired, the acid can be neutralized with ammonia.

To increase the rate and extent of salt absorption, the organic cellulosic polymers can be pre-swollen in water. Water is also suitable for swelling protein materials. For the swelling of acrylic and polyester polymers aromatic alcohols are suitable and for vinyl and polyurethane polymers ketones.

Water is preferred for soaking the metal compounds in cellulose and protein materials, such as wool and silk, as solvents. Other solvents, such as alcohols, do not give so good swelling of the polymers and also no solubility of the chosen metal compound required for good absorption. Suitable solvents for vinyl and polyurethane polymers are esters and ketones, such as n-butyl acetate or methyl ethyl ketone. For acrylic and polyester polymers, aromatic alcohols and amines such as aniline, nitrophenol, m-cresol and p-phenylphenol are useful as solvents for the metal compound.

In order to achieve a sufficient absorption at normal temperatures (21 to 23 ° C), depending on the salts used and the type of organic polymer used, immersion times of 30 minutes to a few days are required. Dipping times greater than about 3 days in concentrated salt solutions are undesirable to fibers because they can degrade, thereby absorbing less metal compound and bonding the polymers together. When the absorption rate of the metal compound in the organic polymer is to be increased to shorten the immersion time, the solution of the metal compound may be heated up to 100 ° C.

In a process for impregnating rayon fibers and other cellulosic fibers and films with certain important metals, the rayon is allowed to absorb water and then contacted with a metal compound to penetrate and hydrolyze the fiber or to form the absorbed water insoluble metal oxide products reacts. Typical hydrolysis reactions are as follows:

SiCl + 3HO

Z

TiCl₄ + 2 ^ 0 2 BF_S + 3HjO * HgSiO_s+ 4HCl »TiO₂ + 4 HCl * BjO_s + 6 HF.

The amount of water absorbed by the rayon fibers is easily controlled by bringing the fibers into contact with air of desired moisture content. If the rayon fibers are immersed directly in liquid water, they absorb the maximum possible amount of water. The amount of water absorbed by ViskoseRayon (textile grade) in equilibrium with moist air and liquid water at 24 ° C is shown in the following table:

<td>Relative humidity at 24 ° C</td><td>Moisture content in% of the dry fiber weight</td>

<td>10</td><td>4</td>

<td>30</td><td>8th</td>

<td>50</td><td>10</td>

<td>70-</td><td>14</td>

<td>80</td><td>17</td>

<td>90</td><td>23</td>

<td>95</td><td>30</td>

<td>100 (immersed in water)</td><td>80-110</td>

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Some hydrolyzable metal compounds are liquid under normal conditions and the water-loaded rayon can be dipped directly into the metal compound, thereby forming the product of hydrolysis in the fiber. Examples of these liquids are SiCl *, TiCl₄, VOC1_S, VC1 *. However, many hydrolysis reactions proceed very rapidly with the release of heat. The resulting harsh conditions can destroy or break the fibers. In this case, the metal compound is preferably diluted with a non-reactive, miscible liquid to avoid such conditions. Suitable non-reactive liquids are many, non-polar organic liquids such as benzene, toluene, hexane, carbon tetrachloride, chloroform. These organic liquids lower the hydrolysis rates and help to distribute the heat of reaction. Unreacted liquid metal compounds (and also diluents) can be removed from the space between the fibers by evaporation because they have high vapor pressures.

Other metal compounds that can be introduced into fibers, films, etc. by hydrolysis, but are not normally liquid, are best dissolved in a non-reactive liquid that is immiscible with water. Such metal compounds are for. B. NbCl_s, ZrCl₄, UC1₄, Suitable solvents are bromoform, carbon tetrachloride, diethyl ether and nitrobenzene.

After soaking up the metal compound (s) from a solution, it is necessary to remove the excess solution from the space between the organic fibers prior to drying so that the fibers are not bonded together by salt formed, and from the surface of the non-fibrous forms, to avoid salt accumulation on it. Leaving unswept metal or hydrolysis product on the fibers results in reduced strength and increased brittleness in the final metal nitride product. For the removal of excess solution from the organic material, it is convenient to thoroughly wipe it with absorbent paper or tissue under moderate pressure. In addition, washing, the use of high velocity gas streams, vacuum filtration and centrifugation have proven to be effective in removing the excess solution. For solutions such as 3.0 molar A1C1_SWith viscosities of more than 10 cP, increasing the temperature of the organic material to 50 to 60 ° C helps to remove the excess solution.

The organic compound containing the metal compound is then dried in some manner, such as by air drying or heating in a warm gas stream. It is desirable to rapidly dry the polymer (in 1 hour or less) to prevent the salt from migrating from the inside of the organic polymer to its surface.

When a product containing two or more metal nitrides is desired, the organic material is impregnated with two or more salts or hydrolysis products. When two or more water-soluble salts are used, the impregnation can be carried out by simply immersing in an aqueous solution containing both salts. When two nitrides are desired, one of which has been impregnated into the organic material from an aqueous solution and the second one by hydrolysis of the metal halide or oxyhalide from organic solution, a preferred method is to add first with the hydrolysis product and then with the water-soluble salt impregnate.

In the next step of the process of the invention (the decomposition of the organic polymer structure), the organic polymer containing the metal compound is heated under controlled conditions, i.a. to a temperature of between 300 and 400 ° C at a rate which is sufficiently low to allow the volatile decomposition products of the polymer to escape without destroying the integrity of the polymer. This heating must be sufficiently long so that the organic structure of the polymer is destroyed and a carbonaceous residue is formed containing the metal compound in finely dispersed form.

The heating rate must also be sufficiently low to prevent ignition of the polymer. When the organic polymer burns in place of the carbonization, the temperature of the metal compound excessively increases because it is in contact with the organic structure. Under such circumstances, it is impossible to control the temperature and the melting point of the metal compounds formed may be exceeded or excessive crystallization or grain growth occurs. The metal compound may also be carried away by the vapors of the organic compound and thus no longer available for the desired residue. By preventing inflammation, the products also have smoother surfaces, are easier to bend independently of adjacent fibers, and are firmer. By very rapid heating and expelling the decomposition gases so the continuity of the polymer is repealed and it takes place excessive crystallization in the remainder, whereby no so smooth, flexible and strong metal 5

No. 280,483 nitride products are obtained as in the non-ignited, amorphous or weakly crystalline, denser polymer residue.

The first heating stage is usually carried out in a non-oxidizing inert atmosphere, such as. As nitrogen, helium, argon, neon u. similar, or in vacuum. However, if it is desired to reduce the amount of carbon remaining from the polymer pyrolysis, this first heating step will be carried out wholly or partly in an oxygen-containing atmosphere, preferably containing about 5 to 25% by volume of an oxidizing gas. The remaining gases of this atmosphere are chemically nonreactive, such as. As the above-mentioned inert gases. When using an oxygen-containing gas, part of the carbon is removed as a carbon-containing gas.

The rate of heating depends on whether the environment is inert or oxidative, in which case the control is more difficult. In a non-oxidizing atmosphere, the heating rate may be at least 100 ° C / hr or higher as long as inflammation of the polymer is prevented.

In an atmosphere containing 5 to 25% by volume of oxygen, the polymer is preferably heated at 10 to 100 ° C./h, although it can also be heated up more rapidly if the carbonaceous gases are removed. Higher oxygen concentrations may be especially useful towards the end of the first heating season. The preferred oxidizing gas is oxygen, although others such as nitrogen dioxide and sulfur trioxide may be used.

At the beginning of the heating of the polymer (even in an oxidizing atmosphere), pyrolysis of the polymer into carbon is the predominant chemical reaction. The carbonized organic polymer contains predominantly carbon, but it may also contain small amounts of residual oxygen and hydrogen. As the heating proceeds in an oxidizing atmosphere, the oxidation of the carbon becomes predominant.

In the final step (nitridation) of the process of the present invention, the polymeric remnant from the first heating and pyrolysis step in contact with a nitrogen-containing compound is further heated to a temperature between about 650 and 2000 ° C to form the metal in the carbonaceous residue to form a catalyst microcrystalline metal nitride fiber, a textile or molded article with the nitrogen compound in reaction. The nitrides formed at the relatively lower temperatures tend to be relatively amorphous, while the nitrides formed at the relatively higher temperatures tend to be microcrystalline. The heating rate in the step of nitriding is not critical; Speeds between about 200 and 1000 ° C / h have been found to be favorable. Similarly, the total time of the nitride formation step is not critical. In an intermittent method z. B. Times between about 1 and 4 h are used. Much shorter times are needed in a continuous process.

As mentioned, any metal that forms a stable nitride can be used according to the invention. Such are, for example, the elements of the groups III-B, IV-B, VB and VI-B of the Periodic Table, the lanthanides and actinides, boron, aluminum, beryllium and silicon. A group of elements which are particularly suitable because of the relatively high melting or sublimation points of their nitrides are boron, silicon, titanium, hafnium, zirconium and uranium.

Many nitrogen compounds can be used in the process of the invention as long as they are volatile at the temperatures prevailing in the nitride formation step. A particularly preferred group of nitrogen-containing compounds are nitrogen, ammonia and organic amines such as methylamine, piperidine, triethylamine and aniline.

Of course, the preferred process conditions with respect to reaction temperatures and molar ratios will change somewhat with the metal and nitrogenous compound used. It is preferred to contact metal and nitrogen compounds at least in stoichiometric amounts in order to obtain strong, pliable products containing a nitride of one or more metals.

The metal nitride fibers of the invention are widely used. For example, they can be used as a nitride fiber plastic composition as heat-resistant materials, reinforcement of metals and ceramic bodies at high temperatures, especially where high strength and low weight are desired.

Since these metal nitride fibers can be processed in the form of a woven belt and continuous yarn, they can be used for yarn windings of composite structures.

The metal nitride forms of this invention are widely used. The thin films can be used as thin dielectric or heat insulating films or sheets. The erfindungsgemä6

No. 2,804,663 metal nitride molds may be used as lightweight structural parts, heat and / or electrical insulators, battery separators and the like. be used. Metal nitride molds made from organic foams or sponges are also useful as filters.

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

The metal nitride films according to the invention are sheets of uniform thickness, which can be 10 μ thin.

The following examples illustrate the process according to the invention and the metal nitride fibers in more detail:

Example 1: Thirty meters of Tyrex type rayon yarn made of 3000 continuous length and 1.1 denier yarns were dipped in an ammonium borate solution. The solution was prepared by dissolving boric acid at 23 ° C to saturation in 30% ammonium hydroxide. After immersion for 1 hour, the rayon yarn was removed by centrifugation of unabsorbed salt and dried in a stream of hot air. The dried rayon absorbed 0.334 mg borate per kg rayon.

The salt-loaded rayon yarn was heated in air at a rate of 50 ° C / h to 350 ° C and held at this temperature for 4 h. The carbonized yarn was then placed in a tube furnace and heated in an ammonia gas stream at a rate of about 100 ° C / hr to 1000 ° C. It has been observed that the nitriding reaction takes place between 650 and 800 ° G and the fibers turn brown between 800 and 1000 ° C and then dark yellow, indicating that only minor traces of carbon remained in the fibers. After the nitriding reaction, the yarn was flexible and could be processed, but shrunk to 43% of the original length, the weight being 16% of the original yarn weight. The dark yellow fibers were amorphous, as determined by the absence of X-ray diffraction lines.

When the boron nitride fibers were heated to 1300 ° C. for 1/2 h in vacuo, a weakly crystalline hexagonal structure formed. In this heat treatment, the fibers did not deform, but turned white. At 1500x magnification under the microscope, the fiber surface was smooth and no visible crystallite grain structure was seen. The fiber diameter was 2.4μ and the yarn product was electrically non-conductive.

Example 2: A 2.10 m long Tyrex rayon yarn containing 4000 fibers of 6.2 μm diameter in one piece and weighing 0.387 g was poured into an aqueous solution of ammonium decaboran, (NH₄)₂B₁₀H_{1 (jt} dipped (Inorganic Chemistry, Vol. 3, p. 444 [1964]). The solution concentration was 31 g of boron compound per 100 ml of water. The ammonium decaboran was prepared by adding Dekaboran with dimethyl sulfide to the adduct Β_1θΗ_π · 2S (CH_s)₂ reacted again with liquid ammonia to the desired water-soluble compound.

After drying, the yarn contained 0.403 grams of salt per gram of rayon. The yarn was carbonized in air by heating at 350 ° C at a rate of 50 ° C / hr and allowing it to stand at that temperature for 4 hours. Then the fibers were placed in a tube furnace in an ammonia flow of 85 dm³ / h heated to 1460 ° C for 3 h.

The yarn product was white to dark yellow, weighed 0.204 g and had good flexibility. The fiber diameter was 2 to 4 μ, and the surface was smooth. X-ray diffraction analysis showed that only hexagonal boron nitride was present, but only in weakly crystalline form.

The products of Examples 1 and 2 both contained more than 90% by weight boron nitride.