• N V Chandra Shekar

Articles written in Bulletin of Materials Science

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

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

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

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.

• Compressibility studies of RE$_6$UO$_{12}$ at extreme conditions of pressure

Rare-earth uranates-RE$_6$UO$_{12}$ are synthesized by heating mixture of uranium oxide and rare-earth oxides in 1:6 ratio above 1273 K. These compounds stabilize in rhombohedral structure at ambient. High-pressure (HP) X-ray diffraction studies reveal that the compounds are stable at lower pressures, beyond which disorder is seen to originate and compound has a tendency to amporphize at very HPs. The a-axis of the lattice is found to be more rigid as compared to c-axis because of corner sharing polyhedra along a-axis. Anomalous compressibility behaviour is seen in Gd$_6$UO$_{12}$, where sharp decrease in the bulk modulus is observed. The behaviour is against the normal trend of compressibility in RE$_6$UO$_{12}$ compounds along rare-earth cation series.

• High pressure studies on core/shell amorphous carbon nanostructures

High pressure behaviour of core/shell amorphous carbon (a-C) nanostructures was studied using synchrotron X-ray diffraction (XRD) and Raman spectroscopy up to ${\sim}$25 GPa. These nanostructures having spherical a-C shells with ${\gamma}$-Fe$_2$O$_3$ nanoparticles at the core were synthesized by catalyst-assisted lamp black method. The typical size of core (${\gamma}$-Fe$_2$O$_3$)/shell (a-C) is ${\sim}$5 nm/20 nm. Comparative XRD of the core (${\gamma}$-Fe$_2$O$_3$) in opened- and closed-shell (a-C) shows that the carbon shells do not fully transmit the applied high pressure inwards, under hydrostatic conditions. Raman spectroscopy shows that the graphitic G-mode blue shifts reversibly with applied pressure with a coefficient ${\sim}$3.5 cm$^{-1}$ GPa$^{-1}$, which is ${\sim}$25 to 40% less than that for graphite and carbon nanotubes. It is suggested that a rigid and crosslinked structure of the carbon shell might be lossy for the observed pressure drop.

• Editorial

• High pressure investigations on neutron irradiated ferroboron

This study reports the pressure effect on structural stability of neutron irradiated ferroboron systems. Ferroboron, a mixture of boron and iron, has been found to have three phases, i.e., FeB, Fe$_2$B and Fe$_3$B. Studies have been conducted on single-phase Fe$_2$B and ferroboron. Fe$_2$B adopts tetragonal structure at ambient and undergoes structural transition to orthorombhic phase at 6 GPa. Further, Fe2B is irradiated with neutrons with a fluence of ${\sim}$10$^{17}$ n cm$^{–2}$ and yields bulk modulus of 254 GPa, which is 16% enhancement as compared to unirradiated sample. The defects are estimated by the use of SRIM code. Total displacement per atom (dpa) in Fe$_2$B for the irradiation fluence is found to be 5.53${\times}$10$^{-5}$. The study also shows that phase transition seen in pristine Fe$_2$B is inhibited upon neutron irradiation under pressure up to 24 GPa. Similar result was obtained on ferroboron mixture, irradiated with a neutron fluence of 8.18${\times}$10$^{21}$ n cm$^{-2}$ with dpa of 2.8. The irradiated sample is found to be stable up to 16 GPa.

• Correction to ‘High pressure investigations on neutron irradiated ferroboron’

• # Bulletin of Materials Science

Volume 46, 2023
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Continuous Article Publishing mode

• # Dr Shanti Swarup Bhatnagar for Science and Technology

Posted on October 12, 2020

Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
Chemical Sciences 2020