• Volume 6, Issue 2

      May 1984,   pages  129-413

    • Sophisticated equipment developed for growth and evaluation of perfection of nearly perfect crystals

      Krishan Lal Ajit Ram Verma

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      This paper reviews four major equipment developed at the National Physical Laboratory for growth and perfection evaluation of single crystals, namely (i) a crystal puller for growth of nearly perfect crystals by the Czochralski method; (ii) a microfocus x-ray generator; (iii) an x-ray diffraction topography camera; and (iv) a triple crystal x-ray diffractometer. The crystal puller can provide smooth, uniform and variable pulling rates. The maximum length of pull is nearly 60cm. Efforts have been made to isolate vibrations. Nearly perfect single crystals of KCl, KBr and NaCl with maximum diameter of ∼ 60 mm have been grown. The crystals give diffraction curves with half width in the range of 10–30 sec of arc. In the projection topographs, dislocations can be resolved and characterized. The microfocus x-ray generator is a demountable continuously evacuated system with specially designed electron gun and anode assembly. The vacuum is continuously monitored for ease of maintenance. In the point focus mode the spot size is 40 µm on the anode. X-ray topography system is a versatile equipment used for projection and section topography. It can provide 360° rotations to the specimen disc around an axis perpendicular to it. Rotations of a few sec of arc can be given to the specimen around a vertical axis. Typical diffraction curves of a dislocation-free crystal and a crystal with boundaries are shown. Well-resolved images of dislocations are shown in a topograph as an illustration. In the triple crystal x-ray diffractometer a highly collimated and monochromated Kα1 exploring x-ray beam is obtained by combining microfocus source, a special collimator and crystal monochromators of Bonse-Hart type. With this beam very narrow diffraction curves with half width of about 5 sec of arc can be recorded. Typical results of measurement of diffuse x-ray scattering (dxs) on nearly perfect silicon single crystals are discussed. It has been observed that the contribution of phonons to thedxs is negligible. Thedxs is mainly due to point defect aggregates.

    • Investigation of hydrogenous materials using neutrons

      B A Dasannacharya P S Goyal

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      Slow neutrons have proved to be a very powerful probe for examining materials in general and hydrogenous materials in particular. In this article we review various neutron scattering techniques which have been utilised to investigate different aspects of a variety of hydrogenous materials. Translationally as well as orientationally disordered materials, ferroelectrics, superconductors, metal-hydrogen systems, polymers and biological molecules have been chosen as illustrative examples.

    • Electrochemical preparation of potassium gold cyanide

      Indira Rajagopal S R Rajagopalan

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      The essential requirements for the industrial preparation of potassium gold cyanide (pgc) are: (a) high rate of dissolution and (b) smooth and uniform dissolution. Employing galvanostatic and potentiostatic polarisation data and observations on the surface topography of anodes dissolved by both the techniques, it is shown that potentiostatic dissolution of gold in potassium cyanide at +0·345 V satisfies the above requirements.

    • Hydrothermal growth of AIIBVI semiconductors

      V A Kuznetsov I P Kuzmina I M Sylvestrova

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      Hydrothermal method is used for growing ZnO, ZnS and HgS single crystals. This method is more advantageous compared to the other ones and enables one to obtain large bulky crystals of technically convenient habit with low concentration of defects. Some physical properties of these single crystals were studied and active elements for the new advanced piezoelectric, electro-optic and laser devices are reported.

    • Glass-metal particulate composites

      D Chakravorty

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      Oxide glasses containing ultrafine metal particles have interesting physical properties and have been widely used in practical systems. The various preparational techniques developed so far for making these materials are discussed. Electrical conduction in these composites is controlled by electron tunnelling between the metal islands. At high electric fields certain glasses containing bismuth granules show a memory switching effect. The latter has been explained by a particle stretching model. Optical absorption characteristics of these composites can be explained on the basis of various effective medium theories developed so far for inhomogeneous materials. Glasses containing ferromagnetic metal grains show a superparamagnetic behaviour with a transition temperature below 300 K. Improvement in the mechanical properties of glasses can be achieved by incorporating metal particles of suitable characteristics within them. In this paper the present state of understanding of all these properties is reviewed.

    • Fast ion conducting lithium glasses—Review

      A R Kulkarni H S Maiti A Paul

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      For the last few years fast ion conducting lithium glasses are being studied due to their potential use in advanced electrochemical devices. A number of glass systems containing oxides, sulphides and other lithium compounds prepared by both conventional cooling and rapid quenching techniques have been reported. In this paper we review the transport properties of lithium ion conducting glasses. The special features of the ionic conduction process have been highlighted and some experimental techniques to study transport properties have been described. Some of the common observations of the properties have been discussed and finally some important problems for future development have been pointed out.

    • Lithium solid electrolytes and their applications

      D N Bose D Majumdar

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      The preparation, characterisation and applications of two systems of lithium ion conductors, lithium zinc germanate (Lisicon) and lithium germanate vanadate are described. Ionic conductivity studies include ac conductivity, thermopower andnmr which provide complementary information. High pressure studies and fabrication of a solid-state cell are also reported.

    • Electronic conduction in bulk Se1−xTex glasses at high pressures and at low temperatures

      G Parthasarathy E S R Gopal

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      The electrical resistivity of bulk Se1−xTex glasses is reported as a function of pressure (up to 8 GPa) and temperature (down to 77K). The activation energy for electronic conduction has been calculated at different pressures. The samples with 0⩽x⩽0·06 show a single activation energy throughout the temperature range of investigations. On the other hand samples with 0·08⩽x⩽0·3 show two activation energies in the different regions of temperature. The observed behaviour has been explained on the basis of band picture of amorphous semiconductors.

    • Electrical properties of polycrystalline silicon and zinc oxide semiconductors

      S N Singh S Kumari B K Das

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      Polycrystalline silicon and zinc oxide ceramic are important electronic materials. The electrical properties which determine the applications of polycrystalline silicon in integrated circuits and solar cells and that of ZnO ceramic in varistors are due primarily to grain boundary effects in them. A large amount of information in this area has already been gathered in literature but the quantitative understanding of grain boundary effects in these materials is not yet complete. In this review the important aspects of grain boundaries and their effects on transport and photoelectric properties of polycrystalline silicon and on the I–V characteristic of ZnO varistors are discussed.

    • MnO2 catalyzed carbon electrodes for dioxygen reduction in concentrated alkali

      R Manoharan A K Shukla

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      A process to depositγ-MnO2 catalytic oxide onto coconut-shell charcoal substrate is described. Current-potential curves for electroreduction of dioxygen with electrodes fabricated from this catalyzed substrate are obtained in 6M KOH under ambient conditions. The performance of these electrodes is competitive with platinized carbon electrodes.

    • Structural and magnetic investigations of some new boron containing rare earth intermetallic compounds

      S K Malik S K Dhar R Vijayaraghavan

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      By alloying boron with RPd3 (R=rare earth) compounds, new ternary alloys of the formula RPd3Bx (0 ≤x ≤ 1) have been prepared. The parent RPd3 compounds crystallise in the cubic AuCu3 type structure. The addition of boron does not change the structure but results in an expansion of the lattice. Therefore, it is likely that the small boron atoms occupy the vacant body centred position in AuCu3 type structure. It is also observed that compounds of the composition RRh3B can be formed in the cubic structure for all rare earths R though the parent RRh3 compounds exist only for R=Ce (AuCu3 type) and La, Sm, Nd and Gd (all hexagonal CeNi3 type). This points out the role of boron in stabilising new crystallographic phases. The results of susceptibility measurements on some of the RPd3B and RRh3B compounds are presented. In particular, it is noted that while GdPd3 orders antiferromagnetically withTN=6 K, GdPd3B does not order magnetically down to 4.2 K.

    • On the mechanism of pairing in thebcs theory

      C M Srivastava

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      An explanation has been offered for the mechanism of cooper pairing in thebcs theory of superconductivity on the basis of the relativistic Darwin interaction. The theory leads to an expression for critical temperature which depends on a few atomic parameters. Calculated values ofTc for elements and alloys are in good agreement with experiment.

    • Structure and superconductivity in ternary systems of compounds

      G V Subba Rao Geetha Balakrishnan

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      A survey of the known highTc ternary systems of compounds is presented. Special emphasis has been placed on the part played by the crystal structure and bonding in the exhibition of highTc superconductivity. The superconducting and other physical properties of the ternary borides have been discussed in relatively more detail. Finally, the available guidelines from crystal and solid state chemistry points of view, have been reiterated for a search for new highTc materials.

    • Pyroelectric materials

      M R Srinivasan

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      This paper presents a review of pyroelectric materials. The physics of pyroelectric materials is described. Various pyroelectric materials are compared for their efficiency in the detection of radiation. Current trends in pyroelectric material technology are highlighted.

    • Amorphous state ferroelectricity, magnon scattering and phase stability in microparticle materials

      M S Multani P Ayyub V R Palkar M R Srinivasan V Saraswati R Vijayaraghavan DO Shah

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      Microparticles and micrograin ceramics show features distinct from the usual-sized polycrystalline materials. Amorphous state material combined with microparticle size for Pb(Zr0.51 Ti0.49)O3 mimics the dielectric behaviour of crystalline ferroelectricity in ABO3 compounds. Fine-grained Y3Fe5O12 (yig) synthesized by pressure sintering ofyig microparticles exhibit spinwave relaxation due to transit time across grain diameter. Applying microemulsion techniques for microparticle synthesis,γ-Fe2O3 has been synthesized. The phase stability fractionγ-Fe2O3/α-Fe2O3 is found to be a function of particle size. At very small sizesγ-Fe2O3 becomes amorphous, leading to interesting Mössbauer studies.

    • Radiation effects in nuclear reactor materials—correlation with structure

      P Rodriguez R Krishnan C V Sundaram

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      A review of radiation effects in nuclear reactor materials has been made; the irradiation effects have been correlated with the crystal structure of the materials. Five phenomena, irradiation hardening, irradiation embrittlement, irradiation creep, irradiation growth and void swelling that occur in materials by neutron irradiation in a reactor environment have been discussed with a view to explaining the physics of the phenomena and the engineering consequences. Metallurgical approaches for improving the irradiation performance of materials and for developing new alloys with better resistance to radiation damage have been pointed out.

    • Structural disorder and solid state transformations in single crystals of ZnxCd1−xS and ZnxMn1−xS

      M T Sebastian P Krishna

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      Single crystals of ZnxCd1−xS and ZnxMn1−xS were grown from the vapour phase at 1100°C in the rangex=0·9 to 1. X-ray characterization shows that polytypes and disordered structures occur in ZnxCd1−xS forx ≥ 0·94, whereas ZnxMn1−xS displays disordered and polytype structures in the entire rangex=0·9 to 1. It is observed that ZnxCd1−xS and ZnxMn1−xS undergo a 2H-6H solid state transformation on annealing in vacuum around 600°C. Experimental analysis of the intensity distribution along the 10·L reciprocal lattice row as recorded on a single crystal diffractometer from partially transformed crystals shows that the mechanism of the transformation cannot be explained in terms of the one-parameter models of non-random faulting reported earlier. A two-parameter theoretical model with α representing the probability of random insertion of a fault in the 2H structure and β representing the probability of the growth of the 6H nucleus, is developed both for a deformation mechanism and a layer displacement mechanism. It is found that the theoretical model of non-random deformation faulting with β ≫ α approximates the actual mechanism of transformation in these crystals.

    • The pressure-volume relation of ytterbium up to 9 GPa

      S Usha Devi A K Singh

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      The pressure-volume relation of ytterbium has been determined up to 9 Gpa using tungsten carbide opposed anvil high pressure x-ray camera. The fcc phase of ytterbium is observed between one atmosphere and 4 GPa and the bcc phase above 3·5 GPa. The bcc phase can be metastably retained down to 1 GPa by gradually decreasing the pressure from a region where only bcc phase alone is observed. The bulk modulus,B0, at zero pressure and the pressure derivative of the bulk modulus,B’0, are determined by fitting Murnaghan equation to the pressure-volume data. The following values were obtained:B0=16·3 GPa andB’0=3·6 for the fcc phase, andB0=14·7 GPa andB’0=1·5 for the bcc phase. Based on the present data it is suggested that the thermodynamic equilibrium pressure for fcc ⇆ bcc transformation in ytterbium is below 3·5 GPa. The valence change under pressure has been discussed.

    • Some recent advances in materials technology

      S Ramaseshan N Balasubramanian

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      Recent advances in five key areas of materials technology are discussed. Structural and non-structural composites, electrically-conducting polymers, materials obtained by rapid quenching, new developments in hydraulic cements and photothermal solar energy conversion are reviewed.

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