N. C. Mehra

Development and properties of surfactant-free water-dispersible Cu2ZnSnS4 nanocrystals: a material for low-cost photovoltaics.

A simple, yet novel hydrothermal method has been developed to synthesize surfactant-free Cu2ZnSnS4 nanocrystal ink in water. The environmentally friendly, 2-4 nm ultrafine particles are stable in water for several weeks. Detailed X-ray diffraction (XRD) and high-resolution transmission electron microscopy revealed the formation of single-crystalline-kesterite-phase Cu2ZnSnS4. Elemental mapping by scanning electron microscopy/energy dispersive spectrometry corroborated the presence of all four elements in a stoichiometric ratio with minor sulfur deficiency. Finally, Raman spectroscopy ruled out the possible presence of impurities of ZnS, Cu2SnS3, SnS, SnS2, Cu(2-x)S, or Sn2S3, which often interfere with the XRD and optical spectra of Cu2ZnSnS4. X-ray photoelectron spectroscopic studies of the as-synthesized samples confirmed that the oxidation states of the four elements match those of the bulk sample. Optical absorption analyses of thin film and solution samples showed high absorption efficiency (>10(4) cm(-1)) across the visible and near-infrared spectral regions and a band gap E(g) of 1.75 eV for the as-synthesized sample. A non-ohmic asymmetric rectifying response was observed in the I-V measurement at room temperature. The nonlinearity was more pronounced for this p-type semiconductor when the resistance was measured against temperature in the range 180-400 K, which was detected in the hot-point probe measurement.

EFFECT OF PROCESSING CONDITIONS ON DIELECTRIC PROPERTIES OF CaCu3Ti4O12 CERAMICS

Samples of CaCu3Ti4O12 (CCTO) ceramics were prepared by solid state reaction method. X-ray diffraction analysis confirmed single phase formation for the powder calcined at 1173 K. Sintering was done at different temperatures viz. 1348 K, 1373 K and 1398 K with a fixed heating rate of 3 K/min. Detailed study of dielectric properties was carried out for the CCTO samples sintered at 1373 K for different duration of holding times (2 h and 10 h). It is found that dielectric properties are sensitive to both sintering time and temperature. With increasing sintering time from 2 h to 10 h dielectric constant increases from ∼2.1 × 104 to ∼2.5 × 104 measured at 1 kHz at room temperature. Impedance spectroscopy has been used for separating out of grain and grain boundary contributions to the overall electrical properties.

EFFECT OF PROCESSING CONDITIONS ON ELECTRICAL PROPERTIES OF CaCu3Ti4O12 CERAMICS

CaCu3Ti4O12 (CCTO) ceramics were prepared by the solid-state reaction route. Effect of sintering time was studied on the polarization (P) versus electric field (E) behavior. Unlike conventional ferroelectric hysteresis loop, PE hysteresis behavior in CCTO ceramics was observed to exhibit ferroelectric-like loop where polarization does not saturate but gives a maximum value. Remnant polarization and maximum polarization was observed to increase with sintering time. Current (I)–voltage (V) characteristics shows a nonlinear behavior making them useful for varistor applications. Coefficient of non-linearity (α) is also found to depend on sintering duration.

Neodymium doped bismuth telluride alloys using chemical reflux method for thermoelectric application

ABSTRACT This work reports synthesis of pure and neodymium (Nd) doped bismuth telluride (Bi2Te3) alloys (Bi2-xNdxTe3, x = 0, 0.5), using reflux method. The X-ray powder diffraction for the as-synthesized Bi2Te3 indicates phase purity and crystallographic structure, and agrees well with the prescribed JCPDS data # 000–60601. The Bi2-xNdxTe3 alloys were characterized by pursuing various imaging and spectroscopic techniques including, scanning electron microscope, and UV-Vis absorption spectroscopy. After Nd doping, the micrographs show formation of a new contrast (white phase) with the prominent (201) plane of a hexagonal crystal structure.

Hot pressed pellets of thallium doped bismuth telluride alloys for thermoelectrics

ABSTRACT Thallium doped bismuth telluride alloys (Bi2-xTlxTe3) are synthesized using solvothermal method, and pellets are formed using hot-press machine by applying 60 MPa pressure at 553 K temperature for 5 min. The X-ray powder diffraction for the as-synthesized Bi2-xTlxTe3 alloys indicates phase purity and crystallographic structures. Scanning electron microscopy indicates highly compact plate-like structures for hot-pressed pellets. The energy dispersive X-ray spectrophotometer predicts a stoichiometric ratio of 2:3 for Bi and Te in the Bi2Te3. The newly prepared Bi2-xTlxTe3 pellets could be utilized as an efficient material in thermoelectric devices for power generation from waste heat.

Growth of flower-like patterns of TiO2 nanorods over FTO substrate

ABSTRACT Nanostructures such as nanoparticles, nanowires, nanorods, nanotubes have attracted great interest with the understanding of shape and size dependence in the electronic and sensing properties. This work demonstrates growth and characterization of one-dimensional nanorods of titanium-dioxide (TiO2) on the fluorine doped tin oxide (FTO) substrate by using hydrothermal method. Field effect scanning electron microscopy shows flower-like morphology, and energy-dispersive X-ray spectroscopy confirms the presence of Ti and O. The X-ray diffraction peaks agree well with the JCPDS data. Initially, the 1D nanorods are formed and high concentration of precursor assists evolution of the short branches to form flower-like structures.

Micro-structural, ferroelectric and magnetic properties of multiferroic composite system Ba0.5Sr0.5TiO3- Ni0.4Co0.2Zn0.4Fe2O4

ABSTRACT In this paper, microstructural, ferroelectric and magnetic properties of multiferroic composite system (1-x) Ba0.5Sr0.5TiO3 (BST) – (x) Ni0.4Co0.2Zn0.4Fe2O4 (NCZF), where x = 0.1, 0.2, 0.3, 0.4 molar ratio have been studied. Grains of NCZF are larger in size and more symmetrical in comparison to BST grains. Overall grain size of BST grains in the composite samples decreases with increase in NCZF content. Polarization vs Electric Field (P-E) loops of the composite samples are unsaturated. The values of remanant polarization of the composite samples increase with increase in NCZF content. The composite samples possess saturated M-H loops with the values of remanant and saturation magnetization increases with increase in NCZF content.

Temperature dependence of friction and wear performance and thermomechanical response of flyash-filled brake composites

Flyash-filled glass fibre reinforced composites have been fabricated and evaluated for drag braking performance on a chase friction testing machine following SAE J 661a standard. The friction build-up and decay phenomena in the drag braking mode as a function of drum temperature have consistently been found to be in the composites with ∼5−7.5 wt% of glass fibres, whereas the absolute friction effectiveness remained higher in the composites with maximum flyash/maximum glass fibre contents. Temperature-dependent friction performance remained broadly composition specific and qualitative correlations of performance to compositional attributes have been found to be complex. Topographical variations and their possible roles in controlling the tribological performance have been characterized by worn surface morphology. Scanning electron microscopy–energy dispersive X-ray combined analysis have revealed that at higher amount of glass fibre content in combination with flyash, the composite exhibited enhanced noise and judder propensity apart from higher Fe-content deposition on friction film. The energy dissipation ability modes as revealed from dynamic mechanical analysis showed not only a correspondence between the damping ability of the composites and compositional variations but also qualitatively established a correlation between wear and storage moduli.