Low Temperature Synthesis

This invention relates to the preparation of ultrafine particles of metals, alloys and ceramic materials including ferrites. More specifically, the invention relates to the low temperature synthesis of such materials.

Ferrites are important materials to industry because of their wide use as magnetic materials and as catalysts. The synthesis of submicron sized ferrite powders has generally received particular attention due to a number of factors including higher green densities and improved electric, magnetic and surface properties. Low temperature syntheses of such materials have been found to generally result in finer particles but have also been the focus of much interest because of unique or unusual particle forms and chemical stoichiometry considerations.

Traditional techniques for the preparation of fine particles of ferrites and ceramic materials in general have involved repeated firings of the component oxides, hydroxides, or carbonates at high temperatures with frequent grindings and mixings. The result is usually crystalline but inhomogeneous materials having low surface area. To induce better diffusion and homogeneity, chemical precursor techniques such as, for example, electrolytic coprecipitation, coprecipitation of oxalates, spray drying of mixed sulfate solutions, and thermal decomposition of mixed metal acetate complexes have been known in the art. Two reviews have been published on the topic: R. Lal and P. Ramakrishman, Trans. Indian Cer. Soc. 38, 166, 1979, and B. K. Das, Preparation and Characterization of Materials, Honig & Rao, Eds., Academic Press, 75, 1981. The advantage in using precursor techniques has been the production of homogeneous, well mixed oxides at reduced formation temperatures. However, even though the reported synthesis temperatures for these techniques are lower than those required in conventional ceramic syntheses, there is positively a need in the art to lower synthesis temperatures further.

A typical chemical precursor, such as ferric oxalate or ferric nitrate, is an oxidizer or has an oxygen-containing anion and can be decomposed endothermically to produce an oxide residue, such as .gamma.Fe.sub.2 O.sub.3. It is well known that the time required for such decompositions can be considerably shortened if the precursor is heated in the presence of a reducing agent, such as hydrazine. The metal salts are normally reduced in solution and yield metal powders.

A relevant chemical precursor method which has recently been reported involves the preparation of ferromagnetic spinels, such as MnFe.sub.2 O.sub.4, CoFe.sub.2 O.sub.4 and NiFe.sub.2 O.sub.4, by the thermal decomposition of mixtures of pyridine and pyridine-1-oxide complexes of the corresponding metal nitrates. See R. T. Richardson, J. of Mat. Sc. 15, 2569, 1980. Using this technique, amorphous oxides in a glassy state are formed between 300.degree. and 400.degree. C. and crystallization takes place at temperatures between 800.degree. and 1000.degree. C. Due to the use of the organic pyridine, however, there is a real possibility of carbon contamination in the ferrite products.

The applicant herein has prepared and decomposed mixed metal oxalate hydrazinate precursors at temperatures as low as 120.degree. to 200.degree. C. to yield oxide spinels such as ferrites and cobaltites. See K. C. Patil, D. Gajapathy, V. R. Pai Verneker, Mater. Res. Bull. 17, 29, 1982, and J. of Mat. Sci. Letters 2, 272, 1983. The approach for these syntheses is the incorporation of an inorganic reducing fuel molecule, such as hydrazine, within the metal salt molecules yielding precursor molecules which are capable of rapid decomposition in air to the corresponding oxides.

A typical synthesis begins with the preparation of the precursor by dissolving the oxalate or oxalates (single or double salt synthesis, respectively) into excess liquid hydrazine hydrate according to the following formula:

where M may be a metal such as manganese, iron, nickel, cobalt, titanium, cadmium, magnesium, aluminum, boron, etc. and x is an integer dependent on the valency of the metal. Alternatively, the oxalate hydrazinate precursor may be prepared from one or more metal powders and ammonium oxalate as follows:

where M and x are the same as above. The precursor is thereafter recovered, placed in a reactor and heated to a temperature up to 200.degree. C. in air for 30 to 90 minutes to decompose the oxalate hydrazinate. Thus, for example, if ferric oxalate is converted into ferric oxalate hydrazinate, a coordination compound of oxalate and hydrazine, this precursor decomposes exothermically at as low a temperature as 120.degree. C. producing fine amorphous .gamma.Fe.sub.2 O.sub.3 particles which are highly magnetic and capable of high green density. Crystallization of these particles takes place at around 340.degree. C. Ultrafine particles of iron may be produced if the decomposition is carried out under nitrogen, hydrogen or vacuum conditions. Similarly, double precursor salts, such as MgFe.sub.2 (C.sub.2 O.sub.4).sub.3.5N.sub.2 H.sub.4, produce ceramic materials such as MgFe.sub.2 O.sub.4 on heating in air and alloys such as MgFe.sub.2 on heating under hydrogen or vacuum conditions.

This invention is based on the discovery that ultrafine particles of ceramic or metallic materials can be obtained at very low temperatures from a nitrate source capable of endothermic decomposition by molecularly combining under a nitrogen atmosphere the nitrate source with an inorganic reducing fuel to provide a chemical precursor for the particular ceramic or metallic material and then exothermically decomposing the precursor in a controlled atmosphere by heating to a temperature up to about 200.degree. C. below the endothermic decomposition temperature of the nitrate source.

The preferred reducing fuel compound is hydrazine and the molecular combination is obtained by treating the nitrate source with hydrazine hydrate. Since hydrazine is inorganic, there is no possibility of contamination of the products prepared according to this invention.

The nitrate source may be any one or more of the nitrate salts of metals such as iron, cobalt, nickel, manganese, chromium, titanium, zirconium, aluminum, magnesium, zinc, cadmium, copper, boron and silicon. Upon the absorption of sufficient heat, such salts are known to decompose to elemental or oxide forms of the metal or metals, depending on the particular conditions of the decomposition.

In the preparation of a simple metal oxide ceramic according to this invention, the nitrate source is the corresponding nitrate salt. For example, to obtain Fe.sub.2 O.sub.3, the nitrate source is Fe(NO.sub.3).sub.3. Such a salt is treated under nitrogen conditions with hydrazine hydrate to produce a solid reaction product. This treatment molecularly combines the nitrate salt with one or more hydrazine molecules resulting in a coordination compound of the nitrate salt and hydrazine. Thus, in the immediate example, Fe(NO.sub.3).sub.3 5N.sub.2 H.sub.4 is formed by the treatment of Fe(NO.sub.3).sub.3 with hydrazine hydrate. Other nitrate salts, such as Al(NO.sub.3).sub.3 and Zr(NO.sub.3).sub.4, react similarly with hydrazine hydrate to produce similar complexes.

For a double salt synthesis, such as is used for the preparation of ferrites according to this invention, two or more nitrate salts are simultaneously treated with hydrazine hydrate. For example, a MgFe.sub.2 (NO.sub.3).sub.6.5N.sub.2 H.sub.4 complex may be prepared by molecularly reacting a mixture of ferric nitrate and magnesium nitrate with hydrazine. The mixture must contain the ferric ion in a 2:1 molar ratio with at least one other metal ion to provide a ferrite precursor.

The solid coordination compounds thus produced provide chemical precursors for the low temperature syntheses of the desired and metallic materials. The complexes, in fact, have been found to proceed along a route to ultimate decomposition even at temperatures ranging from near room temperature to about 90.degree. C. However, the decomposition progresses rapidly as the solids are heated to 100.degree. C. and beyond and is accompanied by the liberation of heat. The peak temperatures reached during the decompositions are, on the average, about 200.degree. C. below the endothermic decomposition temperatures for the respective nitrate salts. For example, precursor compounds for the simple oxides Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3 and ZrO.sub.2 decompose exothermically in air at peak temperatures of 100.degree. C., 260.degree. C. and 500.degree. C., respectively. For the ferrites, the precursors decompose exothermically in air at a peak temperature of about 100.degree. C. The decompositions generally require less than 30 minutes.

The decomposition products are ultrafine or submicron-sized particles of the desired ceramic or metallic material, depending on the particular decomposition conditions. Thus, decompositions conducted in air result in oxygen-containing ceramics whereas those under nitrogen, hydrogen or vacuum conditions result in free metals or alloys. The latter conditions, for example, may be employed for the decomposition of double nitrates of iron and boron or titanium and boron to produce FeB and TiB.sub.2, respectively.

A mixture of Ni(NO.sub.3).sub.2.6H.sub.2 O, Zn(NO.sub.3) 6H.sub.2 O and Fe(NO.sub.3).sub.3.9H.sub.2 O in a 1:1:4 molar ratio is treated with an excess of hydrazine hydrate, N.sub.2 H.sub.4 H.sub.2 O, in an open vessel. In order to avoid a violent reaction, the addition of the first third of the hydrazine hydrate must be very slow. The color of the metal nitrate mixture turns dark brown immediately upon contact with N.sub.2 H.sub.4.H.sub.2 O. After the addition of excess N.sub.2 H.sub.4.H.sub.2 O is complete and the white fumes subside, the heterogeneous mixture of solid metal salts and the liquid N.sub.2 H.sub.4.H.sub.2 O is purged with nitrogen in a closed vessel and stirred for several days. Thereafter, the resulting double salt precursor is filtered, washed with distilled water and allowed to dry in air. The solids are then heated in air at temperatures within the range of 90 to 100.degree. C. for approximately 30 minutes to yield a nickel zinc ferrite.

The composition of the ferrite is determined by chemical, IR and X-ray analysis to be Ni.sub.0.5 Zn.sub.0.5 Fe.sub.2 O.sub.4. A picture provided by a scanning electron microscope (SEM) shows that the ferrite powders are spherical particles which are very similar to a commercial nickel zinc ferrite sold under the trade designation TT2-111 by Trans-Tech Corporation of Maryland. However, the surface area of the ferrite powders prepared according to this example is 6.2 m.sub.2 /g which is four times that of the commercial ferrite. The magnetic properties of the powders, as compared with the commercial product, are as follows:

The procedure of the preceding example is repeated except that the nitrate hydrazinate-excess hydrazine hydrate mixture is saturated with CO.sub.2 to yield Ni.sub.0.5 Zn.sub.0.5 Fe.sub.2 (N.sub.2 H.sub.3 COO).sub.6.6N.sub.2 H.sub.4. On decomposition in air, this precursor compound gives the same nickel zinc ferrite. On the other hand, if the decomposition is carried out in hydrogen or under a vacuum, an iron alloy with nickel and zinc is formed having submicron particle size.

By procedures similar to the above examples, a variety of submicron materials have been prepared, including .gamma.Fe.sub.2 O.sub.3, tetragonal zirconium oxide, amorphous Al.sub.2 O.sub.2, TiO.sub.2 and lead titanate, as well as the corresponding metals and alloys.

The invention has been described with special regard for certain preferred embodiments. Modifications and changes that would be obvious to a skilled artisan may be made in the disclosed method, however, without departing from the spirit and scope of the invention which is defined by the appended claims.