| dc.description.abstract | Aluminous oxides form one of the major and most important classes among oxide ceramics. Their technological applications as refractories, abrasives, cutting tools (?-Al?O?, MgAl?O?), high alumina cement (CaAl?O?), catalysts (LaAlO? / CuAl?O?), lasers (Ruby, Cr³?/Al?O?, Nd³?/Y?Al?O?? - Nd:YAG), TV phosphors (Ce³?/CaAl??O??), pigments (CoAl?O?), solid electrolytes in Na-S batteries (?-Al?O?), toughened ceramics (t-ZrO?/Al?O?) and in fluorescent lamps (Ce³?/Y?Al?O??, CeMgAl??O??) make them really more attractive and useful.
The conventional ceramic method of preparing these oxides involves the solid-state reaction of the component oxides at elevated temperatures (1000°C–1600°C) over an extended period. The products obtained from this process are quite inhomogeneous and show poor reactivity as the product oxides are hard agglomerates (large particles). Several wet chemical methods such as co-precipitation, pyrolysis of metal salt solutions, spray-drying, sol-gel process, etc., have been employed to prepare reactive fine powders of these oxides. Nevertheless, they are involved and require high temperatures (>1000°C) and long time.
In the present investigation, it was considered interesting to use alternative sources of energy (such as chemical energy) to prepare these high-temperature oxides in a very short time. It is well known that the chemical energy could be released by the combustion of a proper combination of an oxidizer and a fuel. Such combustion processes have been of use in propellants, explosives and pyrotechnics. The oxidizers that have been commonly used in such processes are potassium or ammonium chlorate, perchlorate or nitrate. The fuels generally used are polymeric amides, esters and hydrocarbons.
Presently, a novel combustion process has been developed by using metal nitrates as oxidizers and certain simple amides (urea) and hydrazides (carbohydrazide) as fuels. A proper combination of metal nitrate (oxidizer) and urea/carbohydrazide (fuel) mixtures when decomposed at 500°C undergo combustion with an incandescent flame (temperature of 1000°C–1600°C) producing fine, voluminous, foamy and fluffy oxide ceramics in less than 5 minutes. The process is safe, simple, instantaneous, energy and time-saving. This process has been successfully employed to prepare a host of fine particle aluminous oxide ceramics. The preparation and properties of fine particle ?-alumina and related oxide materials form the topic of this thesis.
Chapter I deals with the detailed literature survey on ?-alumina and related oxide materials. The nomenclature, preparation, properties and important applications of aluminous oxides have been discussed. Definition and classification of fine particles, their method of preparation and characterization have also been reviewed. The scope of the present investigations is highlighted.
Chapter II describes the synthesis of fuels used in the combustion process and analytical methods employed. Principles and instrumentation of physical methods employed in the present study such as: simultaneous thermal analysis, surface area measurement, particle size analysis, X-ray powder diffraction, electron microscopy (SEM, TEM, HREM and EPMA), fluorescence spectroscopy, UV-Vis spectroscopy, electron spin resonance spectroscopy and pulsed laser decay time measurements have been discussed briefly.
Chapter III describes the preparation of fine particle ?-Al?O? by the combustion of aluminium nitrate–urea (1:2.5) mixture. The mixture when heated rapidly at 500°C, boils, froths, foams and ignites to burn with incandescence (flame temperature of ~1350°C) to yield fine, fluffy and voluminous ?-Al?O? in 5 minutes time under normal atmospheric pressure. Formation of single-phase ?-Al?O? (the high-temperature form) was confirmed by its X-ray powder diffraction pattern and the fine particle nature of the product has been investigated by surface area measurement and particle size analysis. The product (?-Al?O?) has a foam density of 0.06 g/cm³ and a tap density of 0.906 g/cm³. The surface area of the as-prepared ?-Al?O? was high with particle size ranging from 0.2 to 0.8 ?m. The average agglomerate size of ?-Al?O? is 4.3 ?m. The effect of various parameters like heating rate, mass and composition of the combustion mixture, volume of the container and nature of the fuel on the combustion behaviour have also been investigated in detail. A probable reaction mechanism for the combustion process has been proposed.
An appendix to Chapter III describes the preparation and properties of t-ZrO?/Al?O? composites obtained by the combustion of zirconyl nitrate–aluminium nitrate–urea mixtures. The process yields t-ZrO?/Al?O? composite powders, which has uniform distribution of tetragonal zirconia in ?-Al?O? as evidenced by its X-ray diffraction pattern and electron microscopic analysis (SEM, TEM, HREM and EPMA). The crystallite size of t-ZrO? is below 300 Å as required for its stabilization. The inhibited coarsening observed on these composites is of special relevance in these metastable t-ZrO?/Al?O? composites.
Chapter IV deals with the preparation of fine particle metal aluminates, MAl?O? where M = Mg, Ca, Ba, Sr and Zn, Ca?Al?O? and calcium hexaaluminate, CaAl??O?? by the combustion of corresponding metal nitrates–aluminium nitrate–urea mixtures at 500°C within 5 minutes. Metal aluminates, MAl?O? where M = Mn, Co, Ni and Cu which could not be obtained by urea process have been prepared by the combustion of corresponding metal nitrate–aluminium nitrate–carbohydrazide mixtures at 350°C. The surface area of metal aluminates prepared by carbohydrazide process were higher (40–80 m²/g) compared to those obtained by urea process (1–20 m²/g). It was also possible to prepare Using the combustion of aluminium nitrate–sodium nitrate–urea mixtures at 500°C, the formation of single-phase aluminates was confirmed by the characteristic X-ray powder diffraction patterns, and their fine particle nature was studied by SEM, TEM, particle size and surface area analysis.
Chapter V describes the preparation of rare earth orthoaluminates, LnAlO?, where Ln = La, Pr, Nd, Sm, Gd, Tb and Dy, and yttrium aluminium garnet, Y?Al?O?? (YAG), by the combustion of corresponding rare earth nitrates–aluminium nitrate–urea/carbohydrazide mixtures at 500°C. These as-prepared oxides were fine, with the particle size in the range of 0.1–0.6 ?m and an average agglomerate size of 4–5 ?m.
This instant combustion process developed for the preparation of ?-Al?O?/metal aluminates and rare earth orthoaluminates has been extended to the preparation of fine particle fluorescent aluminous oxide materials like Cr³?-doped ?-Al?O? (Ruby), MgAl?O?, LaAlO?, Y?Al?O?? and Ce³?-doped Y?Al?O?? / LaMgAl??O?? and CaAl??O?? and CeMgAl??O??. Formation of these Cr³? and Ce³? doped aluminous oxides has been confirmed by their characteristic colour, UV-Visible and fluorescence spectra as well as decay time measurements. Ruby (Cr³?/?-Al?O?) powder showed the characteristic excitation bands at 406 and 548 nm and emission band at 695 nm with the decay time of 3.6 msecs. Results of these investigations have been presented in Chapter VI. | |
