• K Govinda Rajan

      Articles written in Bulletin of Materials Science

    • Strength and electronic structure

      K Govinda Rajan

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      The main attribute of a solid is its resistance to deformation, or its ‘strength’. We discuss first the interpretation of the strength parameter. The current situation with regard to the central problem of providing a microscopic description of the strength parameter is briefly reviewed. Electrons in metals provide the cohesion, so that an understanding of the role played by electronic structure in the strength attribute should lead to practical hints for building stronger materials. The useful ‘aircraft alloy’ (Ti + Al + V) illustrates one such important relationship,viz., that the addition of a nond-character metal to ad-electron host strengthens the latter. Again, metals are distinguished from non-metals by the Fermi surface they possess, and it is interesting to examine any possible relationship between the anisotropy of the Fermi surface with the observed anisotropy in hardness (or yield strength). Next, we turn to cleavage, and point out that the assumption that it is the exact opposite of cohesion faces objections. Cohesion is an average property, whereas cleavage is a crack-tip phenomenon. Finally, among the processes familiar to the metallurgist wherein a metal is hardened, electron-moderated mechanisms have been identified in at least two cases, and we conclude with a brief account of these.

    • Growth of low dislocation density single crystals of nickel

      S K Khanna K Govinda Rajan

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      Low dislocation density single crystals of nickel have been grown at high ambient pressure by the Czochralski method. X-ray Laue picture shows that the crystals are strain-free. The dislocation density was determined to be <103/cm2 by the etching procedure. It was found that the necking and cone regions are very critical in the dislocation introduction in the crystals. An increase in the ambient pressure used during the growth seems to aid the crystal quality.

    • Preparation and characterization of boric oxide for single crystal growth of GaP and GaAs

      K Govinda Rajan N V Chandra Shekar M Sekar D Champion Christdoss Selvakumar

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      Boric oxide, used as an encapsulant, prevents loss of volatile components in the growth of compound semiconductors. As the material readily absorbs moisture, and as moisture content has to be kept below a certain level, preparation and handling of this material becomes an involved process. In the present paper we report the process developed for preparing boric oxide from boric acid and growth of cylindrical rods of the desired diameter. The grown boric oxide is characterized by thermal analysis. Infrared characterization is also a powerful method and the advantages of this technique as well as the problems faced in taking the IR spectrum are discussed.

    • Synthesis and single crystal growth of gallium phosphide by the liquid encapsulated vertical Bridgman technique

      K Govinda Rajan N V Chandra Shekar G V N Rao A J Singh R M Iyer

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      Gallium phosphide is a typical III–V compound semiconductor and is also an important electronic material. The synthesis and single crystal growth of this compound by melt methods is rendered very difficult because of the large phosphorus vapour pressure. A high pressure vessel with internal heating and a quartz reactor was first developed for the direct synthesis of gallium phosphide. The crystal growth was carried out in a second high pressure chamber rated for 100 bars gas pressure and equipped with the paraphernalia for crystal growth. Single crystals of gallium phosphide were grown from the polycrystalline starting material by the vertical Bridgman method and the vapour pressure problem was overcome by encapsulating the melt in a column of molten boric oxide. Both boron nitride and silica were employed as crucibles, and with the former, single crystal rods of 8–10 mm diameter and 10–15 mm length were obtained.

    • Preparation of single crystals of YBa2Cu3O7−x from nonstoichiometric melts

      N V Chandra Shekar J Suresh K Govinda Rajan G V N Rao

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      Studies on the single crystal growth of YBa2Cu3O7−x show that the growth conditions have not been optimised yet and they vary in many ways. Here we report the growth of single crystals of YBCO in the size range 0·5–1·2 mm from nonstoichiometric melts. We have made systematic variations in the flux composition (constituting CuO and BaCO3) in order to arrive at an optimum composition for consistently getting single crystals of size 0·5–1·2 mm. The tetragonal phase was confirmed by X-ray diffraction and single crystallinity by the Laue technique. Superconductivity was confirmed in oxygenannealed crystals.

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      Posted on October 12, 2020

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      Chemical Sciences 2020

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      Physical Sciences 2020

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