Infrared reflectance spectra and formalism of precipitation of acicular magnetic particles in network glasses

Abstract

1–2 mm thin platelets of borate or silicate glasses precipitate peculiarly isolated magnetic particles (of micrometer sizes) of hexagonal ferrites, spinel ferrites, or garnets at reaction temperature. A small 0.5–3.0 mol % additive of Ag 2O, Bi 2O 3 or P 2O 5 in these glasses behaves as a strong catalyst in undercooling the melt (in the shape of the platelets) in a truly amorphous metastable glass state of locally ordered network of interconnected basis structural units. In a typical 35BaO-25Fe 2O 3-40B 2O 3 borate glass, for example, the ordered structure in the 1–2 mm thin platelets results in 15–52 cm −1 increase in the BO stretching vibration frequencies over the usual values in the bulk. The surface tension σ (which is modified by the additives) of the glass-liquid facilitates the chemical ordering of planar configurations of interconnected (B 3O 4.5) n → 8 boroxol rings one over other along the surface of the platelets. On isothermal annealing at 500–850°C, the planar network configurations (α) nucleate planar α-β interfaces with solid aggregates (β) of the immiscible (at this temperature) Ba 2+ and Fe 3+ cations caused in the network in the thermal induced recrystallization process of BaFe 12O 19 hexagonal ferrite. The initial shape and size of the α-β interface depends (in addition to the σ and the free-energy difference ΔG v between the two phases α and β) on the size and shape of the crystal (β) unit cell and the local structure of the glass. It therefore assumes an elongated shape in the elongated P6 3/mmc hexagonal crystal lattice (of crystallographic axial ration c/ a ∼ 3.94) of BaFe 12O 19 ferrite. That ultimately grows and precipitates in BaFe 12O 19 single crystal in the presumed acicular shape (along the c-axis) over the network, following the flow of the heat released in the reaction along the planar interface, maintained by the strong surface tension σ, surface anisotropy (δ), and α-β wetting. The results are modelled and discussed using the BO stretching or bending vibrations in the thin glass-platelets and the platelets milled (removing the rather long range atomic ordering) into powder of particle size of 1 μm or lower.