Combustion synthesis and properties of fine particle oxide materials: studies on Ferrites

Abstract

Synthesis of oxide materials with desired structure and properties is of prime importance in Solid State Chemistry and Materials Science. Since the discovery of superconductivity in oxide ceramics, there has been renewed interest in developing novel chemical routes to the preparation of fine particle, large surface area, sinter-active oxide powders. The need for fine particles is to achieve highly dense oxide materials with controlled grain growth. The thesis aims to achieve this goal. The thesis comprises five chapters. First Chapter is devoted to the literature survey on chemical routes to the synthesis of fine particle oxide materials in general and ‘FERRITES’ in particular. The scope and objectives of the present study are highlighted. Second Chapter describes the analytical methods employed in the present study. The techniques employed are: simultaneous TG-DTA, density and surface area measurements, X-ray diffraction, particle size analysis, microscopy (SEM and TEM), IR, ESR, fluorescence spectroscopy, AC susceptibility and vibrating sample magnetometer. Preparation of fuels used in the combustion synthesis are given. They are: Oxalyl dihydrazide (ODH), Tetraformal triazine (TFTA), Malonic acid dihydrazide (MDH), Succinic acid dihydrazide (SDH), Maleic hydrazide (MH) and Carbohydrazide (CH). Third Chapter deals with the preparation of a variety of oxide materials by the combustion of metal nitrate and Oxalyl dihydrazide (ODH) redox mixtures. Oxalyl dihydrazide (CON?H?)?, used as a coolant in solid propellant combustion, is compatible with metal nitrates and the redox mixtures burn smoothly (without explosion) when ignited at 350°C to yield fine particle oxide materials. The oxides prepared by the combustion of metal nitrate–ODH redox mixtures are:

MO – where M = Mg, Ni, Cu and Zn M?O? – where M = Al, Cr and Fe M?O? – where M = Co and Mn AB?O? – where A = Mg, Mn, Fe, Co, Ni, Cu and Zn; B = Al, Cr, Mn and Co LnBO? – where Ln = La, Nd, Sm, Gd, Dy and Y; B = Al, Cr, Mn and Co Y?Al?O?? Ln?M?? – where Ln = La and Nd; M = Mn, Co, Ni and Cu La???Sr?CuO? where x = 0.1–0.3 Cr³? doped ?-Al?O?, MgAl?O? and LaAlO? Ce³? doped Y?Al?O??, CaAl??O?? and LaMgAl??O??

Oxalyl dihydrazide (ODH) appears to be a unique fuel for the combustion synthesis of all types of oxide materials. The particulate properties of aluminates and chromites prepared by the ODH process are compared with those obtained by using other fuels like TFTA, CH, MH and urea. The ODH process yields finer oxide materials. The combustion process using Oxalyl dihydrazide is particularly ideal and useful for the preparation of iron-based oxides as the combustion is controlled and the oxides are voluminous and fine. This fact has been exploited to prepare all types of ferrites. Chapter IV describes the preparation, characterization and magnetic properties of ferrites such as:

Soft ferrites, MFe?O? where M = Mg, Mn, Co, Ni, Cu and Zn Mixed and substituted ferrites: Ni-Zn, Mg-Zn, Li-Zn, Ni-Al ferrites Orthoferrites, LnFeO? where Ln = La, Nd, Sm, Gd, Dy and Y Garnets, Ln?Fe?O?? where Ln = Sm, Gd, Dy, Tb, Eu & Y Hexaferrites, MFe??O?? where M = Sr and Ba

Sintering and microstructure of some representative ferrites have been investigated. Last Chapter (Chapter V) summarizes the results and conclusions of the present investigations.