• Volume 2, Issue 3

      August 1980,   pages  1-219

    • Preface

      C N R Rao E C Subbarao S Ramaseshan

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    • Separation of rare earths by ion exchange

      G S Rao G M Phatak

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      Separation of rare earths from monazite source by ion-exchange process is discussed. Optimum conditions for obtaining kilogram quantities of individual rare earths of high purity are reported.

    • Electrowinning of cerium group metals from fused chloride bath

      Sohan Singh A L Pappachan

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      Cerium group metals, namely, misch metal, lanthanum and cerium have been electrowon from their vacuum-dehydrated chlorides in fused sodium chloride-potassium chloride and lithium chloride-potassium chloride solvent. Temperature, cathode current density and composition of bath were optimised for high current efficiency and metal yield. In the case of lanthanum, current efficiency of 88% was achieved. The metals were free from inclusions and voids and analysed 99%+. The major non-rare earth metallic impurities were Fe, Mg and Al. The total gaseous impurities for lanthanum and misch metal were not more than 0·017%.

    • Preparation of rare earth-cobalt magnet alloy by reduction-diffusion process

      T S Krishnan

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      Preparation of rare earth-cobalt alloys by reduction-diffusion (R-D) process is described. The process essentially involves mixing of the rare earth oxide and cobalt/cobalt oxide powders in proper proportion and high temperature reduction of the charge in hydrogen atmosphere, followed by aqueous leaching of the reduced mass to yield the alloy powder. Comparison is made of the magnetic properties of the R-D powder with those of the powder prepared by the direct melting (DM) route and it is observed from the reported values for SmCo5 that the energy product of the R-D powder (∼ 22 MGOe) is only marginally lower than that of the Directly Melted alloy (∼ 25 MGOe). The paper also includes the results of studies carried out at the Bhabha Atomic Research Centre on the preparation of misch metal-cobalt alloy by the R-D process.

    • Analytical methods for RE-Co alloys

      T A Padmavathy Sankar H O Gupta E C Subbarao K P Gupta N R Bonda D K Goel S N Kaul A K Majumdar R C Mittal G Sarkar M V Satyanarayana K Shankara Prasad J Subramanyam E M T Velu

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      Total RE, Co and Fe in mischmetal and its cobalt alloys are determined by visual complexometric methods and instrumental x-ray fluorescence techniques. As Fe causes interference in the determinations of RE and Co and its own determination is affected by the presence of Co, it is removed by precipitation. The iron is determined from the precipitate and RE and Co from the filtrate. Accuracy of the method is checked by analysing synthetic mixtures of RE, Co and Fe. The individual REs in mischmetal are determined by x-ray fluorescence method. The analysis technique is based on comparison of the oxides of the test samples with standards (both in pressed pellet form) in the appropriate composition range. The total RE content of mischmetal determined through complexometric analysis is compared with the total RE determined through x-ray fluorescence.

    • Magnetic properties of a mischmetal-cobalt alloy

      R A Tewari

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      The magnetic properties of a mischmetal-cobalt alloy are studied. A mischmetal-64% cobalt alloy was melted in induction furnace under a protective atmosphere of argon. The melt was solidified in a magnetic field of 7 kOe, and cooled to a temperature of 950° C and was thereafter quenched in water. Specimens of suitable dimensions were machined, heat-treated in the temperature range 1000–1100° C for various lengths of time and the magnetic properties evaluated. The (BH)max value was 10·3 MGOe which was associated with aBr value of 7 kG and Hc value of 5 kOe. The results obtained offer feasibility to design new MM-Co5 magnets through developments in processing technique.

    • Wave-vector dependence of spin-wave line-width in yttrium iron garnets

      OM Prakash CM Srivastava

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      The wave-vector dependence of the spin-wave line-width in yttrium iron garnets has been studied for samples of grain diameter varying from 1·2µm to 12µm. An expression for the spin-wave line-width as a function of wave-vector and grain diameter has been obtained, which agrees satisfactorily with experiment.

    • Exchange constants in ferrimagnetic garnets

      CM Srivastava C Srinivasan R Aiyar

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      The exchange constants in the ferrimagnetic garnets R3 Fe5 O12 (R=Y3+, Gd3+ and Dy3+) have been obtained from the experimental data on saturation magnetisation and inverse susceptibility. The sign and magnitude of the exchange constants in Y3Fe5O12 have been explained on the basis of Anderson’s theory of superexchange. The low temperature magnetisation data in Dy3Fe5O12 have been explained by assuming canting on thec sublattice.

    • Systematics in the electron transport and magnetic properties of LnBO3 perovskites

      P Ganguly C N R Rao

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      Systematics in the electrical and magnetic properties of transition metal perovskites LnBO3 (Ln=rare-earth ion, B=3d transition metal) with the variation of Ln and B ions are reviewed. The electrical resistivity and activation energy of LnBO3 compounds increase with the decreasing size of the Ln3+ ion for a given B ion. The low-spin to high-spin transition temperature of Co3+ ion in LnCoO3 similarly increases with the decrease in size of Ln3+ while the magnetic ordering temperatures in LnVO3, LnFeO3, LnCrO3 and LnSrCo2O6 decreases with decreasing size of the rare-earth ion. These results may be understood in terms of the increasing acidity of the rare earth ion with decreasing size and the competition between the Ln3+ and the B3+ ions for covalency with the oxygen ions. The effect of this competition on the metal oxygen covalency and crystal field parameter is discussed in relation to the results obtained and Goodenough’s phase diagram. The possibility ofd-f exchange interaction in La1−xLnx NiO3 is also discussed in the light of ESR results.

    • Solid state studies on rare earth mixed oxide pyrochlores Er2(V1−x Fex)4/3 W2/3O7

      M A Subramanian G Aravamudan G V Subba Rao

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      Complete series of solid solutions exist in the mixed oxide pyrochlore system, Er23+ (V1−x3+ Fex3+)4/3 W23/6+O7, with the crystal structure changing from cubic to rhombohedral atx=0·25. The compounds are semiconductors at 300 K exhibitingp- orn-type behaviour depending on the value ofx. The observed resistivity and Seebeck coefficient data as a function of temperature have been interpreted in terms of a hopping mechanism.

    • Positron annihilation studies of rare-earth mixed valence compounds

      K P Gopinathan C S Sundar B Viswanathan A Bharathi

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      Positron lifetime has been measured in SmS under external pressures from 0 to ∼ 16 kbar, and in (Sm0·8Gd0·2)S at temperatures from 10 K to 300 K. An abrupt drop in the lifetime was observed at the valence transition at a pressure of 6·5 kbar in SmS and at a temperature of ∼ 120 K in (Sm0·8Gd0·2)S. Measurement of the Doppler broadening of the annihilation photons was made in (Sm0·8 Gd0·2)S as a function of temperature. The line shape parameter of the Doppler broadened spectrum also showed an abrupt change at the transition. Angular correlation of annihilation photons was measured in the semiconducting phase of SmS and in the metallic phase of (Sm0·8Tb0·2)S. The correlation profile in the mixed valence (metallic) phase was broader than that in the semi-conducting phase. These results have been explained on the basis of the electronic structure of the semiconducting and the metallic phases of these materials.

    • Higher order elastic constants of rare earth metals: gadolinium, dysprosium and erbium

      C S Menon

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      The expressions for the second and third order elastic constants of the rare earth metals, gadolinium dysprosium and erbium have been worked out and their values have been determined. The present theoretical values are compared with the experimentally observed results. It is also suggested that the third order elastic constants of these metals may be measured using ultrasonic technique under high pressures.

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