Volume 6, Issue 1
February 1984, pages 1-89
pp 1- February 1984
pp 1-5 February 1984
Effects of fracture, stress and isothermal annealing of Fe-Ni based metallic glasses have been investigated using the ferromagnetic resonance technique.fmr linewidth is quite sensitive to changes in the magnetic and structural order in metallic glasses, andfmr lineshape seems to provide useful qualitative information on the mechanical state of these systems. Our observations are compared with recent work of Baianu and co-workers.
pp 7-12 February 1984
The presence of small amounts of Fe2+ ion in nickel zinc ferrite significantly influences some of its magnetic properties. The lattice parameter increases slightly and the increase Δa is independent ofx. The variation of magnetization with zinc concentration is explained on the basis of the Yafet-Kittel model. Increase in the Néel temperature on Fe2+ substitution in ZnxNi1-xFe2O4 is remarkable. This has been explained on the basis of a four sublattice model. Our analysis shows thatJAB (d5-d5) interaction is most affected. The influence of Fe2+ ions on the relaxation processes in the Mössbauer line shapes of Ni-Zn ferrite is also investigated and is compared with Cu2+ doped Ni-Zn ferrite.
pp 13-16 February 1984
Although a strong thermo-compression bond is formed between some metals and ceramics, the bonding mechanism is not clearly understood. In the present work57Fe Mössbauer spectroscopic study of (Al2O3 + Fe) cermet is carried out to investigate the reason of adherence at the metal-ceramic interface. The isomer shift, quadrupole splitting and linewidths of Mössbauer spectra indicate that a compound of the spinel type FeO·Al2O3 is formed but the site symmetry around Fe may be locally distorted by a small amount as compared to that for stoichiometric FeAl2O4.
pp 17-20 February 1984
The Mössbauer spectroscopy of polycrystalline Y3−xCaxFe5−xTixO12 (x=0·2, 0·5, 0·7) garnets has been studied at room temperature. The hyperfine fields both at the octahedral and tetrahedral sites decrease with increasing titanium content. The hyperfine fields both ata andd-sites are linearly proportional to magnetization. The quadrupole splitting fora-site shows no dependence on Ti content while the quadrupole splitting ford-site increases slightly withx. The results are explained on the basis of lattice distortion due to Ti substitution ona-site.
pp 21-25 February 1984
Magnetic metallic glass Fe39Ni39Mo4Si6B12 has been studied by the57Fe Mössbauer spectroscopy. Mössbauer spectra consist of broad and overlapping six-line pattern below the Curie temperature. The Curie and crystallization temperatures of this metallic glass have been determined to be 575 ± 3 K and 725 ± 3 K, respectively. The hyperfine magnetic field at room temperature is approximately 225 kOe. The reduced hyperfine fields of this sample decrease much faster than that observed for other iron-rich metallic glasses like Fe40Ni40B20, Fe80B20 etc. This behaviour is attributed to the presence of molybdenum atoms in the sample.
pp 27-31 February 1984
A detailed analysis of the available experimental data has been made for the ordered phase of ZnFe2O4. The abnormally lowTN and the observed spin arrangement have been explained on the basis of the hybridization of orbitals on the oxygen ions which gives rise to an anisotropic exchange interaction. The variation of hyperfine field and susceptibility with temperature along with the zero point reduction of the magnetic moment indicate that the ZnFe2O4 is a two-dimensional antiferromagnet.
pp 33-37 February 1984
Fe3BO6 shows no thermal hysteresis around the phase transition temperatureTSR=418 K; Fe2·88Ga0·12BO6 undergoes spin transition atTSR=403 K resulting in a reduction of theTSR. Both these features are different from the reported observations on α-Fe2O3. The two Fe3+ sites show difference in their magnetic interactions. Ga3+ replaces Fe3+ in a random manner.
pp 39-45 February 1984
Because of their several attractive features including relatively high Curie temperatures, substituted lithium ferrites have become important for applications at microwave frequencies. Néel collinear arrangement of spins onA andB sublattices is unable to satisfactorily explain the 0°K saturation moments and the observed Curie temperatures of the zinc-substituted lithium ferrites, especially at concentrations of zincz>0·3 in the formula Li0·5−z/2ZnzFe2·5−z/2O4. Rosencaig’s localized canting model has been extended and used to compute 0°K magnetic moments and Curie temperatures of these ferrites with substitution levels up toz=0·7. Reasonably good agreement between the calculated and experimental values, both for the 0°K magnetic moments and the Curie temperatures, has been obtained using exchange parameters ratios based on the valuesJaa=−20°K,Jbb=−8°K andJab=Jba=−29°K.
pp 47-52 February 1984
The liquid phase epitaxial growth of rare earth-substituted magnetic garnet films suitable for magnetic bubble domain application by both vertical and horizontal dipping using PbO and B2O3 as flux is reported. The dependence of various parameters on lead incorporation in the films present as inhomogeneity has been studied.
pp 53-57 February 1984
Thin films of yttrium iron garnet (yig) and Gd-substitutedyig of different thickness have been prepared by flash evaporation. The surfacedc andac electrical resistivity and thermopower in these films have been studied. The results are explained on the basis of Mott and Davis model in which narrow tails of localised states exist at the extreme valence and conduction bands and a band of localised levels near the middle of the gap. For the temperature range studied, the main conduction mechanism is on account of excitation of carriers into localised states at the band edges and hopping at energies close to the band tails.
pp 59-64 February 1984
Optimum conditions and experimental details for the formation of γ-Fe2O3 from goethite have been worked out. In another method, a cheap complexing medium of starch was employed for precipitating acicular ferrous oxalate, which on decomposition in nitrogen and subsequent oxidation yielded acicular γ-Fe2O3. On the basis of thermal decomposition in dry and moist nitrogen,dta, xrd, gc and thermodynamic arguments, the mechanism of decomposition was elucidated. New materials obtained by doping γ-Fe2O3 with 1–16 atomic percent magnesiu, cobalt, nickel and copper, were synthesised and characterized.
pp 65-70 February 1984
The dependence of microstructure on sintering conditions during hot pressing have been investigated and the densification mechanisms have been used to explain the observed results. The effect of addition of excess ZnO content over stoichiometry on permeability has been studied in the normal sintered Mn-Zn ferrites.
pp 71-82 February 1984
The development of suitable materials for use in mhd generator is one of the most challenging areas. The very high temperatures (∼ 2700 K) coupled with the highly corrosive seed-laden atmosphere limit the choice of materials in contact with the plasma to just a few ceramic oxides. These materials in contact with the high temperature plasmas in mhd generator are simultaneously subjected to stresses of mechanical, thermal, chemical and electromagnetic nature. The development and suitability of the different potential electrode and insulator materials such as lanthanum chromite, zirconia-based materials, alumina, magnesia etc. are reviewed with particular reference to the mhd channel materials programme in the mhd Project atbarc.
The operating experience with alumina-based ceramics and other materials under mhd operating conditions is also detailed. This includes the specifications, processing, bonding of the ceramic materials, the test programme and evaluation. Also presented are some of the novel processing techniques like plasma spraying and their application in the development of mhd materials. The advantages and limitations of each of these materials and their principal mode of degradation while in service are also discussed.
pp 83-89 February 1984
Lead zirconate titanate powders of different compositions varying from 50 to 55 atomic percent zirconium in the solid solution have been prepared by spray-drying technique. The compositions were varied by changing the zirconium-to-titanium ratio in the solid solution and also with the addition of strontium. The amorphous character of the as-prepared powders and the formation of single phase lead zirconate titanate during calcination were also confirmed by x-ray diffraction technique. These powders were then converted to piezoelectric ceramics by compaction and sintering followed by electroding and poling. Effect of the variation of zirconium to titanium ratio and strontium additions on the piezoelectric properties of the finished ceramics have been explained on the basis of improved sintering, uniform grain size, formation of morphotropic phase boundary and subsequent shift to rhombohedral structure.
Volume 42 | Issue 6
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