Combustion and Thermal Decomposition of Ammonium Perchlorate-Aluminium Composite Pellets: Mechanism for Aluminium Participation

Thermal behaviour of ammonium perchlorate-aluminium composites is studied using differential thermal analysis, thermogravimetry and differential scanning calorimetry. Electrical resistivity studies throw light on the mechanism of ammonium perchlorate decomposition at different aluminium contents. The differences observed in burning behaviour by earlier authors is explained in terms of porosity and thermal conductivity of the composite.

SUBSTITUTIONAL EFFECT OF Zn AND Pr IN YBa2Cu3O7-delta

The co-doping effect of Zn and Pr impurities in the compound of composition Y1-xPrxBa2[Cu1-yZny](3)O7-delta with x = 0.1, x = 0.2 and 0 <= y <= 0.1 has been investigated by analyzing the results of electrical resistivity measurements. It is found that for Pr substitution at x = 0.1, there is a minimal influence on in-plane processes, thereby slightly affecting T-c and residual resistivity rho(0), but with the resistivity slope d rho/dT becoming large for the range of y from 0.03 to 0.06, leading to a larger depinning effect. For x = 0.2 a drastic change is observed whereby rho(0) becomes abnormally large, and d rho/dT becomes negative, implying totally pinned charge stripes and no depinning. The second observation therefore suggests that Pr substitution converts the overdoped system to an optimally doped system, leading to the universal superconductor-insulator transition.

Synthesis, structural and ferromagnetic properties of La1-xKxMnO3 (0.0 <= x <= 0.25) phases by solution combustion method

We describe the solution combustion synthesis and characterization of La1-xKxMnO3 (0.0 <= x <= 0.25) perovskite phases, which is a low temperature initiated, rapid route to prepare metal oxides. As-synthesized compounds are amorphous in nature; crystallinity was observed on heating at 800 degrees C for 5 min. Structural parameters were determined by the Rietveld refinement method using powder XRD data. Parent LaMnO3 compound crystallizes in the orthorhombic structure (space group Pbnm, No. 62). Potassium substituted compounds were crystallized with rhombohedral symmetry (space group R-3c, No. 167). The ratio of the Mn3+/Mn4+ was determined by the iodometric titration. The Fourier transform infrared spectrum (FTIR) shows two absorption bands for Mn-O stretching vibration (v, mode), Mn-O-Mn deformation vibration (v(b) mode) around 600 cm(-1) and 400 cm(-1) for the compositions, x = 0.0, 0.05 and 0-10. Four-probe electrical resistivity measurements reveal a composition controlled metal to insulator transition (TM-1), the maximum TM-1 was observed for the composition La0.85K0.15MnO3 at 287 K. Room temperature vibrating sample magnetometer data indicate that for the composition up to x = 0-10, the compounds are paramagnetic whereas composition with x = 0.15, 0.20 and 0.25 show magnetic moments of 27, 29 and 30 emu/g, respectively.

Influence of Solvents on the MWCNT/Adhesive Grade Epoxy Nanocomposites Preparation

In this article, the effect of two solvents, namely dimethyl formamide (DMF) and N-methyl pyrrolidone (NMP), on the dispersion effectiveness and the resulting electrical and mechanical properties of multi-walled carbonanotubes (MNCNT) filled structural adhesive grade epoxy nanocomposites was studied. The solvents were used mainly to reduce the viscosity of the resin system to effectively disperse the nanofiller. The dispersion was carried out under vacuum using high energy sonic waves. SEM was undertaken to study the dispersion effectiveness. Electrical resistivity, tensile properties, and glass transition of the nanocomposites were studied. Between DMF and NMP, the former proved better in terms of dispersion effectiveness and the resulting electrical and mechanical properties of the nanocomposites. Addition of MWCNT into AV138M resulted in an increase in glass transition temperature irrespective of the solvent used and in both cases percolation threshold was found with respect to reduction in electrical resistivity of the nanocomposites. Less agglomeration and hence better interaction between CNT and epoxy was observed in the samples prepared using DMF compared with that using NMP.

Contribution of geophysical surveys to groundwater modelling of a porous aquifer in semiarid Niger: An overview

Subsurface geophysical surveys were carried out using a large range of methods in an unconfined sandstone aquifer in semiarid south-western Niger for improving both the conceptual model of water flow through the unsaturated zone and the parameterization of numerical a groundwater model of the aquifer. Methods included: electromagnetic mapping, electrical resistivity tomography (ERT), resistivity logging, time domain electromagnetic sounding (TDEM), and magnetic resonance sounding (MRS). Analyses of electrical conductivities, complemented by geochemical measurements, allowed us to identify preferential pathways for infiltration and drainage beneath gullies and alluvial fans. The mean water content estimated by MRS (13%) was used for computing the regional groundwater recharge from long-term change in the water table. The ranges in permeability and water content obtained with MRS allowed a reduction of the degree of freedom of aquifer parameters used in groundwater modelling.

A fluctuation-based characterization of athermal phase transitions:Application to shape memory alloys

We employ a fluctuation-based technique to investigate the athermal component associated with martensite phase transition, which is a prototype of temperature-driven structural transformation. Statistically, when the phase transition is purely athermal, we find that the temporal sequence of avalanches under constant drive is insensitive to the drive rate. We have used fluctuations in electrical resistivity or noise in nickel titanium shape memory alloys in three different forms: a thin film exhibiting well-defined transition temperatures,a highly disordered film, and a bulk wire of rectangular cross-section. Noise is studied in the realm of dynamic transition,viz.while the temperature is being ramped, which probes into the kinetics of the transformation at real time scales,and could probably stand out as a promising tool for material testing in various other systems, including nanoscale devices.

BCN:A Graphene Analogue with Remarkable Adsorptive Properties

A new analogue of graphene containing boron, carbon and nitrogen (BCN) has been obtained by the reaction of high-surface-area activated charcoal with a mixture of boric acid and urea at 900 degrees C. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy reveal the composition to be close to BCN. The X-ray diffraction pattern, high-resolution electron microscopy images and Raman spectrum indicate the presence of graphite-type layers with low sheet-to-sheet registry. Atomic force microscopy reveals the sample to consist of two to three layers of BCN, as in a few-layer graphene. BCN exhibits more electrical resistivity than graphene, but weaker magnetic features. BCN exhibits a surface area of 2911 m(2)g(-1), which is the highest value known for a BxCyNz composition. It exhibits high propensity for adsorbing CO2 (approximate to 100 wt %) at 195 K and a hydrogen uptake of 2.6 wt % at 77 K. A first-principles pseudopotential-based DFT study shows the stable structure to consist of BN3 and NB3 motifs. The calculations also suggest the strongest CO2 adsorption to occur with a binding energy of 3.7 kJ mol(-1) compared with 2.0 kJ mol(-1) on graphene.

Optimization of the distance between source and substrate for device-grade SnS films grown by the thermal evaporation technique

Tin monosulfide (SnS) films with varying distance between the source and substrate (DSS) were prepared by the thermal evaporation technique at a temperature of 300 degrees C to investigate the effect of the DSS on the physical properties. The physical properties of the as-deposited films are strongly influenced by the variation of DSS. The thickness, Sn to S at.% ratio, grain size, and root mean square (rms) roughness of the films decreased with the increase of DSS. The films grown at DSS = 10 and 15 cm exhibited nearly single-crystalline nature with low electrical resistivity. From Hall-effect measurements, it is observed that the films grown at DSS <= 15 cm have p-type conduction and the films grown at higher distances have n-type conduction due to the variation of the Sn/S ratio. The films grown at DSS = 15 cm showed higher optical band gap of 1.36 eV as compared with the films grown at other distances. The effect of the DSS on the physical properties of SnS films is discussed and reported.

Influence of finite size effect on magnetic and magnetotransport properties of La0.5Sr0.5CoO3 thin films

We present a comprehensive study of the thickness dependent structural, magnetic and magnetotransport properties of oriented La0.5Sr0.5CoO3 thin films grown on LaAlO3 by Pulsed Laser Deposition. We observe that these films undergo a reduction in Curie temperature (T-c) with a decrease in film thickness, and it is found to be primarily caused by the finite size effect since the finite scaling law [T-c(infinity) T-c(t)/T-c(infinity) = (c/t)lambda holds good over the studied thickness range. We rule out the contribution from the strain induced suppression of Curie temperature with decreasing film thickness since all the films exhibit a constant out of plane tensile strain (0.5%) irrespective of their varying thickness. However, we observe that the coercivity of the films is an order of magnitude higher than that of the bulk due to the tensile strain. In addition, we also observe an increase in the magneto resistance peak and a decrease in coercivity and electrical resistivity with an increase in film thickness. (C) 2010 Elsevier Ltd. All rights reserved.

Elusive superconductivity in polycrystalline samples of layered lanthanum nickelates

La2-xNiO4, La2-xSrxNiO4 and related layered nickelates have been investigated for possible presence of superconductivity. While there is clear onset of diamagnetism around 20 K in many of these nickelates, we do not, however, find any anomaly in the electrical resistivity, magnetoresistance or thermopower around 20 K. High energy spectroscopic studies show Ni to be in the 2+ oxidation state accompanied by a substantial proportion of oxygen holes.

Formation of crystalline diamond from amorphous diamond-like carbon films by pulsed laser irradiation

The formation of crystalline diamond films from amorphous diamond-like carbon films by pulsed laser irradiation with a 300 μs non-Q-switched Nd:YAG laser has been established by a combined study of transmission electron microscopy, x-ray photoelectron spectroscopy, and electrical resistivity. The films have been prepared by glow discharge decomposition of a mixture of propane, n-butane, and hydrogen in a rf plasma operating at a frequency of 13.56 MHz. Prior to laser irradiation, the films have been found to be amorphous by transmission electron microscope studies. After irradiation, the electron diffraction patterns clearly point out the formation of cubic diamond structure with a lattice spacing of 3.555 Å. However, the close similarity between diamond and graphite electron diffraction patterns could sometimes be misleading regarding the formation of a diamond structure, and hence, x-ray photoelectron spectroscopic studies have been carried out to confirm the results. A chemical shift in the C 1s core level binding energies towards higher values, viz., from 286.5 to 287.8 eV after laser irradiation, and a high electrical resistivity >1013 Ω cm are consistent with the growth of diamond structure. This novel "low-temperature, low-pressure" synthesis of diamond films offers enormous potential in terms of device compatibility with other solid-state devices.

Pressure induced incommensurate to commensurate transition in K3Cu8S6

To study the effect of hydrostatic pressure on the incommensurate lattice modulation at 153 K in K3Cu8S6, electrical resistivity measurements are done at 1.0 GPa, 1.5 GPa and 2.2 GPa. The sharp increase in resistance at 2.2 GPa is attributed to the incommensurate to commensurate transition. This is further confirmed by the non-linear I–V characteristics at 2.2 GPa showing the driven motion of the commensurate charge density wave in the presence of an external electric field.

Synthesis and characterisation of co-evaporated tin sulphide thin films

Tin sulphide films were grown at different substrate temperatures by a thermal co-evaporation technique. The crystallinity of the films was evaluated from X-ray diffraction studies. Single-phase SnS films showed a strong (040) orientation with an orthorhombic crystal structure and a grain size of 0.12 mu m. The films showed an electrical resistivity of 6.1 Omega cm with an activation energy of 0.26 eV. These films exhibited an optical band gap of 1.37 eV and had a high optical absorption coefficient (> 10(4) cm(-1)) above the band-gap energy. The results obtained were analysed to evaluate the potentiality of the co-evaporated SnS films as an absorber layer in solar photovoltaic devices.

High-pressure studies on Ge-Te glasses. Evidence for a critical composition in IV-VI chalcogenide glassy systems

The electrical resistivity of bulk GexTe100-x glasses has been measured as a function of temperature and pressure. Under high pressure, all the glasses were found to undergo sharp discontinuous transitions from glassy semiconductors to crystalline metal. Several of the observed properties such as the transition pressure, conductivity activation energy and pre-exponential factor, exhibit anomalous trends at a composition x = 20. These results suggest that the x = 20 composition in the Ge-Te system should possess salient structural features. A model based on the unusual stability of structural units is proposed for explaining the anomaly at 20 at.% Ge concentration.

Mechanical and Chemical Thresholds in IV-VI Chalcogenide Glasses

It has been possible to identify two critical compositions in the IV-VI chalcogenide glassy system GexSe100-x by the anomalous variations of the high-pressure electrical resistivity behavior. The first critical composition, the chemical threshold, refers to the stoichiometric composition. The second critical composition, identified recently as the mechanical percolation threshold, is connected with the structural rigidity of the material.