S. Ram

Effect of nucleating agents on the crystallisation behaviour of barium hexaferrite in a borate glass

Abstract Glass-ceramics containing BaFe12O19 as the major crystalline phase have been prepared from glasses in the system BaO-Fe2O3-B2O3 by subjecting the latter to a suitable heat treatment schedule. Small concentrations (up to 1 mol%) of Ag2O, Bi2O3, P2O5 and TiO2, respectively, have been used to improve the crystallisation kinetics of the ferrite phase. Bi2O3 is found to be the most efficient nucleating agent in this process whereas TiO2 inhibits the precipitation of any ferrite phase in the glass matrix. Detailed magnetic and EPR measurements have been carried out on all the glass ceramics prepared. The data are explained on the basis of microstructural characteristics as delineated by optical and transmission electron microscopy.

Crystallisation of W-type hexagonal ferrites in an oxide glass with As2O3 as nucleation catalyst

Abstract Magnetic glass-ceramics with the W-type (BaFe18O27) hexagonal Ba-ferrite as a prominent crystalline phase have been prepared from glasses in the system BaO-Fe2O3-B2O3 by subjecting the latter to a two stage heat-treatment. The magnetic phases due to M-type (BaFe12O19) hexagonal ferrite and iron oxide have also precipitated in some of the samples. An incorporation of ≈ 1 mol% As2O3 in the glass particularly favours the nucleation and growth of the W-type ferrite microcrystals in the shape of hexagonal platelets. Detailed X-ray diffraction, magnetic and EPR measurements have been carried out for all the glass-ceramic samples. The results are explained on the basis of the microstructure. The glass-ceramic products obtained show a maximum magnetisation of ≈ 79 emu/g in the ferrite particles with coercivity Hc = 5350 Oe at room temperature (≈ 300 K) and Tc ≈ 820 K.

Magnetic and microstructural studies of Ca-hexaferrite based glass-ceramics

Abstract X-ray diffraction, magnetisation and EPR measurements have been carried out on the glass-ceramic specimens of CaOFe2O3B2O3 based glass systems. The results are explained on the basis of their microstructures. In a system, 13CaO48Fe2O339B2O3, CaFe2O4 is found to crystallise as a magnetic phase having a surprisingly large magnetisation (85 emu/g). The actual composition of this phase is believed to be Ca0.6Fe2.4O4 with Fe2+ and Fe3+ ions distributed among the different available sites in a particular way. CaFe12O19 and Ca3Fe15O25 ferrites can be crystallised with useful magnetic properties in the systems containing (1 mol%) Ag2O or P2O5 as nucleating agent. Incorporation of As2O3 (0.1 mol%) in the P2O5 system gives rise to the crystallisation of Ca3Fe15O25 particles in platelet shape with an aspect ratio ∼ 12. The crystallisation behaviour and microstructural features of the systems are consistent with the EPR linewidths and g-values.

Magnetic properties of BaFe12O19 particles with B2O3 addition

Abstract Single-phase BaFe 12 O 19 ferrite particles have been prepared by a classical ceramic method using a small additive of B 2 O 3 in the ferrite composition series. The systems show an enhanced magnetization of σ s ⋍ 77 emu/g as compared to that (σ s ⋍ 68.5 emu/g) in the composition series containing no B 2 O 3 . It is believed that B 3+ ions substitute for the tetrahedral Fe 3+ ions of the spinel block of the parent compound. Moreover, the coercivity ( H c ) is usually found to decrease on incorporation of B 2 O 3 in the composition and/or sintering the systems at the higher temperatures. The variation in particle sizes (also shape) and growth of multidomain particles are generally responsible for these features.

Spectroscopic studies of Athabasca oil sands

Abstract Various molecular species present in Athabasca oil sand were identified using X-ray powder diffraction, and i.r. and Raman spectroscopy (200–4000 cm −1 ). Acyclic paraffin, straight-chain alkane, hopane and sterane hydrocarbons were prominent. Quartz sand (SiO 2 ) grains were dispersed in thin films of water, to form silanol type ( SiO 2 + H 2 O γ Si  OH ) functional groups. The H 2 O molecules gave rise to a characteristically sharp i.r. band at 3720 cm −1 , and the silanol molecular species existed as strongly hydrogen bonded systems. This investigation also indicated the presence of carbonyl, hydroxyl and/or carboxylic acid species, in polymerized forms.

Crystallisation of yttrium-iron-garnet (YIG) in a borate glass

Glass ceramics containing YIG microcrystals have been precipitated in BaO-B2O3-Y2O3-Fe2O3-Al2O3-Bi2O3 glass. The BaFe12O19 ferrite phase also precipitates along with some non-magnetic Ba borates in a few of the glass-ceramic samples. The YIG particles precipitated are essentially in acicular shape (aspect ratio ~ 20). Magnetisation and EPR measurements have been carried out on all the specimens. The results are consistent with their microstructures.

Development of planar hexagonal Fe2-Y ferrite particles for millimeter wave devices

Abstract The platelet-like particles of Sr 2 Fe 2+ 2 Fe 3+ 12 O 22 (noted Fe 2 -Y) hexagonal ferrite have been obtained by crystallisation in a glass matrix. The structure seems thermally stable in the present system. An addition of 1 mol% Gd 2 O 3 to the starting composition (4SrO-25Fe 2 O 3 -71B 2 O 3 ) favours the crystallisation of Fe 2 -Y particles. Moreover, the Fe 2+ cations incorporate on Sr 2+ vacancies if (Fe 2 O 3 ) present in an amount greater than that of the stoichiometric ratio. Magnetic and microstructural features of the useful samples are characterised in detail. They appear particularly suitable for the use of millimeter wave device applications.

IR and visible absorption studies on KNiBr3 crystals

Abstract The absorption bands caused by the phonon vibrations and the Ni 2+ (3 d 8 ) electronic transitions observed in KNiBr 3 single crystals have been studied. The Ni 2+ ions in the present crystals appear to exist in a moderate octahedral coordination site [forming the Ni(Br) 6 -type polyhedron] which is excited in the electronic as well as in the vibration (IR) spectrum. The electronic spectrum for some of the d - d transitions indicates well-resolved vibronic structures, the frequencies of which are consistent with those observed in the IR spectrum.