B. Sivaiah

Studies on phase stability, mechanical, optical and electronic properties of a new Gd2CaZnO5 phosphor system for LEDs

A new ternary oxide Gd2CaZnO5 having interesting structural, mechanical, electronic and optical properties is synthesized and is studied in detail using density functional theory. The analysis revealed two polymorphs: orthorhombic and tetragonal; the orthorhombic phase was found to be the most stable structure under ambient conditions. A high-pressure (hydrostatic) phase transition to the tetragonal phase is predicted at about 4 GPa. This is one of very few reports that depict the phase transition of oxide materials under pressure. The calculated results are in agreement with the X-ray diffraction studies supported by Rietveld analysis. Analysis of the optical properties revealed both polymorphs to be direct-gap semiconductors with low dielectric constants. The calculated elastic constants of both phases satisfy the mechanical stability criteria. It is also identified that the half-filled 4f orbital of Gd induces a strong magnetic spin polarization in the host oxide lattice indicating that the material could be effectively used in versatile applications ranging from biomedical devices to light emitting diodes.

Synthesis of Aluminium–Graphene Nanocomposite Sintered Using Spark Plasma Sintering

Graphene (Gr) has attracted tremendous attention for the synthesis of lightweight structural nanocomposites due to its excellent properties such as high Young’s modulus (1 TPa), high fracture strength (~125 GPa) and extreme thermal conductivity (~5000 W/m/K). Fabrication of pure aluminium–graphene nanocomposite is conducted using a chemical synthesis route followed by consolidation using spark plasma sintering process. The pure aluminium powder was initially cryomilled to refine the grain structure. Subsequently, nanocomposites of Al-reduced graphene (Al-Gr) were synthesized using a chemical method employing different proportions (by volume fraction) of graphene oxide (GO) dispersed in pure aluminium powder. The synthesized powder was ball milled under optimized conditions followed by spark plasma sintering. The powder and sintered Al-Gr nanocomposites are characterized by X-ray diffraction, Raman spectroscopy, SEM and TEM microscopy. The mechanical behaviour is evaluated using the indentation hardness method. GO was found fully converted to reduced graphene during SPS. The increased proportion of reduced graphene in aluminium powder has suggested improved mechanical properties such as hardness after SPS. The structure–property correlation of synthesized Al-Gr nanocomposites is discussed with regard to the improvement in the mechanical behaviour.

Modulating the lattice dynamics of n-type Heusler compounds via tuning Ni concentration

Reducing the lattice thermal conductivity (κL) comprises one of the crucial aspects of thermoelectric research. Ternary intermetallic half Heusler compounds have revealed properties promising for thermoelectric applications. Studies have shown that self doping with Ni in Ni based half Heuslers leads to unprecedented lowering in the κL. Although the underlying physical mechanisms have not been explored in detail, with ZrNiSn as a case study, we experimentally investigate the change in κL with increase in the Ni concentration in Ni based n-type half Heusler alloys. We observe that at excess Ni doping of 3% in the half Heusler lattice, the thermal conductivity reduces by more than 60%. Our density functional theory based analysis on the ongoing phenomena reveals that at ultralow Ni doping, the localized modes of the antisite Ni defect hybridize with the acoustic modes and this plays the most dominant role in scattering of the thermal phonons leading to significant lowering in κL. Our theoretical analysis can be employed for predicting a suitable dopants that may reduce the κL prior to the synthesis of the compound in the laboratory.