| dc.description.abstract | This thesis concentrates on the precursor chemistry of simple and mixed metal carboxylates, especially the citrates and tartrates. The carboxylate process is simple, convenient and most efficient to synthesize fine powders of metal oxides. The aim of the present investigation is to synthesize perovskite and structurally related metal oxides having desired powder characteristics at relatively low temperatures.
The introductory chapter begins with a brief description of the perovskite, K?NiF? and oxygen-deficient triple perovskite structures. The conventional and non-conventional approaches to synthesize metal oxides are reviewed. The merits and demerits of these approaches have been highlighted. The advantages of the citrate method over the other methods of preparation are emphasized. Various investigations on the citrate process have been analyzed and the need to prepare citrate precursors with definite compositions is stressed. The importance of a systematic study on the precursor chemistry and pathways of oxide formation are emphasized in the scope and objective of the present study.
A brief account of various physicochemical techniques employed in the present study constitutes the subject matter of Chapter 2.
The third chapter describes the preparation of new zirconyl citrate complexes and their characterization. These precursors have been employed to synthesize the tetragonal form of zirconia. A detailed study on the thermal decomposition of these complexes has been presented. The dehydration and decarboxylation of zirconyl citrate dihydrate, H?ZrO(C?H?O?)·2H?O occurs in stages. The composition of the citrate produces amorphous ZrO? and zirconyl nitrate octahydrate, H?ZrO(C?H?O?)·2H?O. However, these events are distinctly shown in DTA for these complexes. Considerable carbonization has also been observed along with the oxide formation. Oxidation of residual carbon and the crystallization of tetragonal ZrO? at about 500°C in air or oxidizing atmosphere results in well-crystallized powders. The powders thus obtained have been characterized. The citrate-derived samples consist of nanosized t-ZrO? particles. Hall plot is employed to analyze the combined effect of crystallite size (9 to 15 nm) and lattice strain (3 to 0.5 × 10?³) contribution for line broadening. Above ~15 nm size, the lattice strain is relieved and hence only the size effect decides the metastability of zirconia particles.
The t ? m transformation behaviour of citrate-derived zirconia has been analyzed using Avrami’s equation and Honig’s order-disorder theory.
The reactivity of the fine ZrO? powders obtained from citrate precursors has been successfully exploited to synthesize zirconate perovskites at relatively low temperatures and short durations. A kinetic study has also been carried out for the reaction between citrate-derived BaCO? and ZrO? samples. By adapting Carter model, an activation energy of 190–210 kJ mol?¹ has been calculated. The low-temperature formation of BaZrO? has been attributed to the fine particle nature of the reactants as well as the t ? m polymorphic transformation.
Chapter 4 gives a detailed account of the preparation of a new series of mixed metal citrate complexes, MZrO(C?H?O?)?·nH?O (M = Ba, Sr, Ca and Pb) and their thermal decomposition behaviour. These precursors have enabled the synthesis of respective perovskite oxides at temperatures as low as 650°C. The genesis of perovskite phase formation from citrate precursors has been investigated. Isothermal calcination experiments have also been carried out to isolate the intermediates for further characterization. Based on the experimental results, a thermal decomposition scheme has been proposed. The citrate precursors decompose to produce corresponding zirconate perovskites in three major stages, viz., (i) dehydration, (ii) decomposition of the citrate in a multistep process to form a mixed metal oxycarbonate intermediate, M?Zr?O?CO? (M = Ba, Sr, Ca and Pb) and (iii) decomposition of the mixed metal carbonate to form the resultant oxide, MZrO?. At no stage MCO? or ZrO? has been identified as intermediates during the course of precursor decomposition. The supporting evidences stem from TG, isothermal calcination experiments and X-ray powder diffraction techniques.
The barium titanyl citrate precursor, BaTiO(C?H?O?)·nH?O has been found to produce BaTiO? fine powders at 600°C and the details are given in Chapter 5. Thermal decomposition of the precursor results in the formation of an oxycarbonate Ba?Ti?O?CO? in the temperature range 500–550°C. The resultant BaTiO? has been found to be a mixture of cubic and tetragonal phases. Calcining the oxide above 600°C results in a complete tetragonal phase formation as inferred from XRD. The green densities of the BaTiO? compacts have been found to be about 60% of the theoretical value. Sintering these compacts at 1300°C for 2 h results in a densification of about 93% with ~0.5 ?m grain size. Increase of the sintering temperature to 1350°C results in drastic grain growth of about 40 ?m. The dependence of dielectric constant upon the grain size of citrate-derived BaTiO? has been investigated and the results compared with the alkoxide-derived samples. Pechini gel method has been adapted for the synthesis of PTCR compositions (donor doped and donor–acceptor co-doped BaTiO? (La?.???Mn?.????TiO?.???)). Results indicate good PTCR behaviour in sintered compacts.
Chapter 6 outlines the potential chelating ability of tartaric acid with metal ions to form titanyl tartrate species in aqueous solution, isolated as fine powders. The resultant precursors on thermal decomposition at 650°C produce MTiO? fine powders. Similarly, zirconyl tartrate complexes, M[ZrO(C?H?O?)?·nH?O] have been prepared at optimized conditions and decomposed to obtain corresponding zirconates at about 650°C. Here again, the formation of mixed metal oxycarbonate intermediates, M?Ti?O?CO? and M?Zr?O?CO? has been identified.
The seventh chapter concentrates on the low-temperature synthesis of cuprates crystallizing in K?NiF? and related structures. Citrate gel route has been employed to synthesize La???M?CuO? (M = Ba and Sr; 0 < x < 0.2). The powder characteristics and the sintering behaviour have been investigated. La???Gd?CuO? (0 < x < 2) compositions have been synthesized at low temperatures and short durations (750°C; 4 h). The striking feature of the citrate process is the stabilization of T* phase in the compositional regime of 0.45 < x < 0.8, T + T' phases for 0 < x < 0.45 and T' phase for 1 < x < 2.0. Presence of only the gadolinium moment in T* phase has been inferred from susceptibility measurements in the temperature range 30–300 K. All the T* and T' compositions exhibit paramagnetic behaviour down to 30 K. | |
