• Volume 13, Issue 4

      September 1990,   pages  235-304

    • Thermodynamic properties of Pt5La, Pt5Ce, Pt5Pr, Pt5Tb and Pt5 Tm intermetallics

      K T Jacob Y Waseda

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      The Gibbs’ energies of formation of Pt5La, Pt5Ce, Pt5Pr, Pt5Tb and Pt5 Tm intermetallic compounds have been determined in the temperature range 870–1100 K using the solid state cell:$$Ta,M + MF_3 /CaF_2 /Pt_5 M + Pt + MF_3 ,Ta$$.

      The reversible emf of the cell is directly related to the Gibbs’ energy of formation of the Pt5M compound. The results can be summarized by the equations:$$\begin{gathered} \Delta G_f^ \circ \left\langle {Pt_5 La} \right\rangle = - 373,150 + 6 \cdot 60 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \Delta G_f^ \circ \left\langle {Pt_5 Ce} \right\rangle = - 367,070 + 5 \cdot 79 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \Delta G_f^ \circ \left\langle {Pt_5 Pr} \right\rangle = - 370,540 + 4 \cdot 69 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \Delta G_f^ \circ \left\langle {Pt_5 Tb} \right\rangle = - 372,280 + 4 \cdot 11 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \Delta G_f^ \circ \left\langle {Pt_5 Tm} \right\rangle = - 368,230 + 4 \cdot 89 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \end{gathered} $$ relative to the low temperature allotropic form of the lanthanide element and solid platinum as standard states The enthalpies of formation of all the Pt5M intermetallic compounds obtained in this study are in good agreement with Miedema’s model. The experimental values are more negative than those calculated using the model. The variation of the thermodynamic properties of Pt5M compounds with atomic number of the lanthanide element is discussed in relation to valence state and molar volume.

    • Role of alumina in flexure of glass/epoxy composites

      K Padmanabhan Kishore

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      The influence of particulate additions of alumina on the flexural properties of glass-fabric/epoxy composites was studied. The additions improved translaminar flexural strength, while decreasing interlaminar strength. The translaminar bending modulus showed an increasing trend whereas its interlaminar value showed a decrease, up to additions of 3 vol%. The mechanisms of deformation and the fracture features have been discussed with the aid of scanning electron microscopy.

    • Preparation of glass-nickel microcomposites byin situ reduction via sol-gel route

      G C Das A Basumallick S Mukherjee

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      Gel has been prepared with nickel chloride and glucose in the starting solution followed by heat treatment at higher temperature, where glucose decomposes to carbon and water vapour, which in turn react to form hydrogenin situ to reduce nickel chloride to metallic nickel. The presence and distribution of nickel granules in the microcomposite, has been established by transmission electron microscopy, selected area diffraction and electron paramagnetic resonance analysis.

    • Properties of reaction bonded silicon nitride obtained from slip cast preforms

      J Rakshit J Mukerji

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      Fabrication of silicon preforms of high green density (>1·2 g/cm3) by slip casting of silicon (in aqueous medium) has been studied. The nitridation product consists of 59–85% α-Si3N4, 7–22%β-Si3N4 and 7–23% Si2N2O phase. The amounts of un-nitrided silicon were negligible. The microstructure is either granular or consists of needle-like grains (α-Si3N4) and whiskers deposited in the large pores. MOR values of the specimens are almost constant up to 1000°C or 1400°C or show slight increase up to 1000°C or 1200°C. In some cases a little dip around 1200°C, then a sharp increase in MOR up to 1400°C was observed.Kic values are almost constant up to 1000°C, and thereafter increase sharply.

      Pore size distribution, existence of Si2N2O phase and oxidation of RBSN at high temperatures have been considered for the explanation of the observed behaviour.

    • Sintering and oxidation in gel-coated SiCw/Al2O3 composites

      V Saraswati

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      Boehmite gel-coated SiC whiskers have been studied as discs and as dispersed in gel and commercial alumina. The coated whiskers oxidize and react to some extent in alumina-rich composite compacts when heated in vacuum. In the absence of alumina as matrix, discs of coated whiskers turned brittle on heating above 1600°C. A density of 3.7g/cc was obtained for compacts of SiC whiskers in submicron alumina when sintered pressureless for 2 h at 1600°C. A smooth interface, compatible with the matrix, has resulted in good mechanical properties.

    • Crystallization of mica in the K2O-SiO2-MgO-MgF2 glass system

      V Saraswati K V S R Anjaneyulu

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      The growth of crystalline mica in K2O-SiO2-MgO-MgF2 glass has been studied and characterised through various techniques, viz. differential thermal analysis (DTA), X-ray diffraction (XRD), microscopy (SEM), and microhardness tests. It is observed that the addition of titania has a stronger influence on the growth of mica crystallites as compared to variation in heat treatment schedules.

    • Electrical transport in magnesium aluminate

      Tom Mathews K T Jacob J P Hajra

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      The conductivity of MgAl2O4 has been measured at 1273, 1473 and 1673 K as a function of the partial pressure of oxygen ranging from 105 to 10−14 Pa. The MgAl2O4 pellet, sandwiched between two platinum electrodes, was equilibrated with a flowing stream of either Ar + O2, CO + CO2 or Ar + H2 + H2O mixture of known composition. The gas mixture established a known oxygen partial pressure. All measurements were made at a frequency of 1 kHz. These measurements indicate pressure independent ionic conductivity in the range 1 to 10−14 Pa at 1273 K, 10−1 to 10−12 Pa at 1473 K and 10−1 to 10−4 Pa at 1673 K. The activation energy for ionic conduction is 1·48 eV, close to that for self-diffusion of Mg2+ ion in MgAl2O4 calculated from the theoretical relation of Glyde. Using the model, the energy for cation vacancy formation and activation energy for migration are estimated.

    • Pyroelectric properties of ferroelectric potassium-cesium vanadate and potassium-lithium vanadate

      A J Kulkarni A P Kashid S H Chavan

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      Measurements of pyroelectric currents and coefficients of poled sintered discs of ferroelectric solid solutions, potassium-cesium vanadate and potassium-lithium vanadate have been investigated in the temperature range covering their transition points. In these solid solutions, KxCs1−xVO3 and KxLi1−xVO3, pronounced peaks have been observed at the ferroelectric curie temperatures. The peak values of pyroelectric currents and coefficients change with change in potassium concentration in both the solid solutions.

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