The production and analysis of BaFe12 O19 for microwave applications.
This thesis describes a study into the production of polycrystalline BaFe120 19
(BaM) specimens possessing properties appropriate to microwave device applications.
Principal requirements were controlled stoichiometry, single-crystal density and a high
degree of crystallographic alignment; all of these contributing to the reduction of
ferromagnetic resonance (FMR) losses and linewidths to levels comparable with more
expensive single-crystal alternatives.
The stoichiometry of precursor BaM powder was analysed by X-ray diffraction
(XRD) and found to be critically dependent on reaction conditions and batch
reproducibility of constituent chemicals. The reaction process of BaM was analysed in
the temperature range 500 - 1200 °C and phase content elucidated. The XRD spectrum
of the single-phase BaM contained peaks not in the powder diffraction file (PDF) for
this compound. Careful analysis eliminated the possibility of impurities and second
phases. A comprehensive theoretical calculation from first principles allowed a
complete fit between the calculated and observed XRD spectra. A revised spectrum has
been proposed to the Joint Committee on Powder Diffraction Standards (JCPDS) in
order to improve the PDF data reported by earlier workers. Adaptation of the theory
has enabled the development of a tool for the determination of site preferences of
dopant systems, such as Zn2+ -Ti4+ pairs.
A novel magnetic press system was devised and constructed for the simultaneous
alignment and compaction of powders in an aqueous slurry. Sintering of compacts
resulted in relative densities up to 95 % coupled with improved alignment through grain
growth. In an improved technique, aligned compacts were pre-sintered and hot-pressed
in an oversize die, allowing viscous flow of grains. Near 100 % relative densities were
obtained at temperatures significantly lower than for conventional sintering. Alignment
was enhanced by subsequent annealing, but did not exceed levels achieved by normal
sintering. Suggestions were made for achieving higher alignments.
Microwave resonance linewidths were significantly lower than for isotropic
polycrystals and suggestions for further improvements were identified.