• Volume 18, Issue 1

      January 1982,   pages  1-112

    • Principle of massive support in the opposed anvil high pressure apparatus

      Mohammad Yousuf K Govinda Rajan

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      The Bridgman anvil technique offers a simple and versatile means of generating very high pressures required in solid state studies. The opposed anvil technique is based on the principle of massive support. The practical case of a gasketted anvil is considered, and an expression for the maximum pressure generated under massive support is derived in terms of the geometric parameters, the strength of the anvil material and the gasket properties. In particular, for a given maximum pressure, it is possible to calculate the taper angle, the taper height and the gasket thickness from this expression. The anvil is assumed to be in the elastic region under load. Good agreement is obtained between the calculated and the experimental values for the massive support factor (msf) for various taper angles. By choosing the proper geometry, it is possible to achieve a pressure as high as 130 kbar in an alloy steel anvil. It has been clearly found that the straight portion, where the taper ends, does not really take any part in changing the stress pattern. Thus the minimum straight portion can serve the purpose, and will result in material saving. Anvils exhibit yielding at very high pressure. It is also pointed out that a further strengthening of the anvil can extend the ultimate pressure. Several methods of further strengthening the anvils are discussed.

    • Quantum corrections to the thermodynamic properties of a two-dimensional fluid with hard-core potential

      L Rai S K Sinha

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      A simple theory, based on the physical interpretation of the reciprocal of activity, is developed to evaluate the thermodynamic properties of a two-dimensional fluid in the semi-classical limit. The theory is applied to calculate the quantum corrections to the equation of state and excess free energy of two-dimensional fluids, whose molecules interactvia hard-disc and square-well potential. It is found that the quantum effect increases with the increase of density and decrease of temperature.

    • Off-shell behaviour ofα-α interaction potentials

      N R Sharma

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      Off-shell behaviour of representativeα-α interaction potentials, both local and non-local separable, is compared through the partial wave Kowalski-Noyes half-off-shell functions. Parameters of the existing rank-one separable potentials are redetermined and an additional rank-two potential is constructed for this family. It is found that all these potentials show similar off-shell behaviour for higher partial waves. Their behaviour for low partial waves, however, particularly in the region far away from the energy-shell, is widely different. The off-shell correction for the (α, 2α) reaction at 140 MeV is calculated, as an application, and it is found that separable potentials predict a non-negligible effect.

    • Phase shift analysis ofππ scattering

      B B Deo J K Mohapatra

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      A novel approach to energy dependent phase shift analysis ofππ scattering is proposed. Optimised polynomial expansions of Roskies’ amplitudes are given. The parameters of these expansions are searched out by making a fit to the differential cross-sections. Then the transformation matrix between Roskies’ amplitudes and the conventional partial waves is used to compute the phase shifts and inelasticities. Significant improvement is observed in so far as the number of free parameters for a fixed energy phase shift analysis is concerned. Value for the effective Chew-Mandelstam coupling constant, λCM, is also estimated.

    • Electrical resistivity studies of certain quasi one-dimensional organic systems at high pressures and low temperatures

      A K Bandyopadhyay

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      We report the electrical resistivity studies of six quasi one-dimensional organic systems under high pressures up to 8 GPa and temperatures down to 77K. The room temperature resistivity of these complexes lies in the wide range from 108 ohm cm to 0.05 ohm cm, but they have common features under high pressures. The possible interpretations of these behaviours have also been discussed.

    • Calculation of phonon life time in amorphous solids

      Y P Joshi

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      We present the calculation of phonon life time at low frequencies in an amorphous solid, which is assumed to be characterized by an elasticity that exhibits spatial fluctuation. Thermodynamic Green’s function method is used to compute phonon self-energy, and an iterative method is devised to obtain an improvement upon the first order perturbation calculation. The elasticity correlation is taken to be an exponentially falling function of distance. We obtain an inverse life time that varies as the fourth power of phonon frequency for small values of the latter, and whose frequency-dependence becomes weaker and weaker as the frequency increases.

    • Concentration dependence of the mixed crystal modes of vibration

      P Nayak S N Behera

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      The concentration dependence of the reststrahl absorption in various mixed crystals exhibiting one, two and mixed mode behaviour is investigated using the coherent potential approximation (cpa). The phonon Green’s function, the impurity mode frequencies and the strength of absorption are calculated in the Einstein model from the generalizedcpa proposed by Tripathi and Behera which takes into account both mass and force constant changes. The introduction of a phenomenological concentration dependence of the force constant change parameter is shown to provide a satisfactory explanation of the concentration dependence of the experimental data for the twenty mixed crystal systems analysed. It is conjectured that the nearest neighbour force constant of an impurity atom substituted at a host site is very much different from that of a perfect crystal consisting of these impurity atoms and that both these play an important role in determining the one, two and mixed mode behaviour of the mixed crystals.

    • Solution of Ornstein-Zernike equation for one-dimensional fluids

      B B Deo B P Das A C Naik

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      The application of Wiener-Hopf factorisation procedure as adopted by Baxter has been used to solve the one-dimensional Ornstein and Zernike (oz) equation for a fluid of interacting hard rods. Exact solution is obtained for the Kac potential in the van der Waals limit. We also obtain perturbative results which agree exactly with the lowest order calculations of Kac, Uhlenbeck and Hemmer.

    • Neutral particle modelling assumptions: Theory and experiment

      R Jones

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      The neutral particle recycling models that have been used to date in numerical plasma confinement simulations are by no means equivalent but may lead, instead, to divergent results. As a consequence of this confinement studies based on individual neutral assumptions must be received with a healthy degree of scepticism.

      We go on to compare experimental observations of the time development of plasma density, temperature, and neutral pressure made in an electrostatic surface trap with numerical plasma simulations incorporating a number of alternative neutral models.

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