Studies on metal hydrazinecarboxylates:Precursors to fine particle oxide materials

Abstract

Fine?particle oxide materials have a variety of applications. Simple metal oxides find use as catalysts and are more reactive in solid?state reactions. Mixed oxide materials such as ferrites and cobaltites are used in the preparation of ferromagnetic liquids, high?density materials, catalysts, etc. Preparation of fine?particle oxides involves either breaking?down or building?up methods. Of the various methods reported in the literature, the solid?solution precursor technique is known to yield stoichiometric oxides on thermal decomposition. A variety of solid solutions of metal oxalates, carbonates, etc. have been used as precursors to mixed metal oxides. These anions, when complexed with hydrazine, are known to decompose at lower temperatures to yield the oxides. Based on this, one might expect that if both hydrazine and carboxylate moieties are present in the same anion, it would easily decompose to yield fine?particle oxides; hydrazine moiety providing the exothermicity needed and carboxylate anion liberating large amounts of gases. One such anion is hydrazinecarboxylate, N?H?COO?. Although the literature contains data on simple hydrazinecarboxylate derivatives, no data is available on mixed?metal hydrazinecarboxylates and their possible use as precursors to useful ceramic oxides. Thus, it is aimed to prepare the simple and mixed?metal hydrazinecarboxylates and examine the possibility of using them as precursors to fine?particle oxide materials. Literature survey on the preparation, characterization, size analysis, and application of fine particles in general and oxide materials in particular are presented in Chapter I. The existing literature on hydrazinecarboxylates has also been briefly reviewed in this chapter. Scope of the present investigations has also been outlined in the introductory chapter. Chapter II describes the analytical procedures and instrumental methods employed in the present study. The instrumental methods used are simultaneous thermal analysis (thermogravimetry–derivative thermogravimetry–differential thermal analysis, TG–DTG–DTA), differential scanning calorimetry (DSC), vibrating?sample magnetometer (VSM), electrical?conductivity measurements, particle?size analysis (Micron Photosizer), surface?area measurement, electron microscopy, X?ray powder diffraction, infrared and Mössbauer spectra. Metal hydrazinecarboxylates are usually prepared by the reaction of aqueous solutions of metal salts with hydrazine hydrate saturated with carbon dioxide. However, the conditions have not been standardised to obtain a desired product. For example, it has been reported in the literature that such a reaction with cobalt salts gave as many as six complexes: Co(N?H?COO)?, Co(N?H?COO)?(H?O)?, Co(N?H?COO)?N?H?, Co(N?H?COO)?(N?H?)?, KCo(N?H?COO)?, and N?H?Co(N?H?COO)?·H?O. The preparation, characterization, and thermal analysis of the following complexes have been described in Chapter III:

M(N?H?COO)?·nH?O, where M = Mg, Mn (n = 2); Ca (n = 1) Ln(N?H?COO)?·3H?O, where Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Y M(N?H?COO)?(N?H?)?, where M = Mn, Fe, Co, Ni and Zn M·N?H?(N?H?COO)?·H?O, where M = Mn, Fe, Co, Ni and Zn

Fine?particle nature of the residues of thermal decomposition of metal hydrazinecarboxylate derivatives has been characterised. The conditions for obtaining any desired complex have been standardised. ??Fe?O? is the most widely used recording material. The method of preparation in vogue involves three steps which are quite involved and lengthy. A single?step route for the preparation of ??Fe?O? using iron hydrazinecarboxylate derivatives, Fe(N?H?COO)? and N?H?Fe(N?H?COO)?·H?O, as precursors is described in an appendix to Chapter III. Conversion of fine?particle Fe?O? obtained by the decomposition of iron hydrazinecarboxylate precursors into ??Fe?O? has also been discussed in the appendix. Solid solutions of hydrazinium metal hydrazinecarboxylates, N?H?M?/?Fe?/?(N?H?COO)?·H?O, M = Mg, Mn, Fe, Co, Ni and Zn, have been prepared and investigated as precursors for ultrafine?particle ferrites. These complexes decompose autocatalytically at as low a temperature as 75°C to yield ultrafine ferrites. The decomposition has also been done in melt to minimise the size of the particles and to narrow down the size distribution. The ferrites obtained thus have large surface area and achieve ~99% of theoretical density when sintered at low temperatures (~1000°C), lower than the normal sintering temperature (>1200°C). The particles are found to be superparamagnetic in nature at room temperature. The details of these investigations are discussed in Chapter IV. Nickel–zinc ferrites are commercially important as they find use in high?frequency radio?frequency coils, transformer cores, magnetic heads, etc. Solid?solution precursor technique using (N?H?)?Ni???Zn?Fe?(N?H?COO)?·H?O; x = 0.2 to 0.8 has been extended to prepare the application?rich Ni–Zn ferrites. These precursors decompose to yield fine?particle nickel–zinc ferrites with large surface area. The results of these studies are detailed in Chapter V. Chapter VI describes the preparation of fine?particle cobaltites, MCo?O?; M = Mg, Mn, Fe, Ni and Zn, by the thermal decomposition of solid?solution precursors of corresponding metal hydrazinecarboxylate derivatives, [N?H?]?/?M?/?Co?/?(N?H?COO)?·H?O, where M = Mg, Mn, Fe, Ni and Zn. The cobaltites have been characterised by particle?size analysis, electrical?conductivity measurements, etc. Nickel cobaltite obtained by this method has been tested for its electrochemical performance as an electrocatalyst in the oxygen?reduction reaction.