Internal displacement reactions in multicomponent oxides: Part I. Line compounds with narrow homogeneity range

Structure and phase transition of LaO1−xF1+2x, prepared by solid-state reaction of La2O3 and LaF3, was investigated by X-ray powder diffraction and differential scanning calorimetry for both positive and negative values of the nonstoichiometric parameter x. The electrical conductivity was investigated as a function of temperature and oxygen partial pressure using AC impedance spectroscopy. Fluoride ion was identified as the migrating species in LaOF by coulometric titration and transport number determined by Tubandt technique and EMF measurements. Activation energy for conduction in LaOF was 58.5 (±0.8) kJ/mol. Conductivity increased with increasing fluorine concentration in the oxyfluoride phase, suggesting that interstitial fluoride ions are more mobile than vacancies. Although the values of ionic conductivity of cubic LaOF are lower, the oxygen partial pressure range for predominantly ionic conduction is larger than that for the commonly used stabilized-zirconia electrolytes. Thermodynamic analysis shows that the oxyfluoride is stable in atmospheres containing diatomic oxygen. However, the oxyfluoride phase can degrade with time at high temperatures in atmospheres containing water vapor, because of the higher stability of HF compared with H2O.

Temperature-pressure-induced solid-solid < 100 > to < 110 > reorientation in FCC metallic nanowire: a molecular dynamic study

Atomistic simulation of initial < 100 > oriented FCC Cu nanowires shows a novel coupled temperature-pressure dependent reorientation from < 100 > to < 110 > phase. A temperature-pressure-induced solid-solid < 100 > to < 110 > reorientation diagram is generated for Cu nanowire with varying cross-sectional sizes. A critical pressure is reported for Cu nanowires with varying cross-sectional sizes, above which an initial < 100 > oriented nanowire shows temperature independent reorientation into the < 110 > phase. The effect of surface stresses on the < 100 > to < 110 > reorientation is also studied. The results indicate that above a critical cross-sectional size for a given temperature-pressure, < 100 > to < 110 > reorientation is not possible. It is also reported here that for a given applied pressure, an increase in temperature is required for the < 100 > to < 110 > reorientation with increasing cross-sectional size of the nanowire. The temperature-pressure-induced solid-solid < 100 > to < 110 > reorientation diagram reported in the present paper could further be used as guidelines for controlling the reorientations/shape memory in nano-scale applications of FCC metallic nanowires.

Surface pressure and heat transfer measurements in the unsteady separated hypersonic flow field over double cones

Reliable bench mark experimental database in the separated hypersonic flow regime is necessary to validate high resolution CFD codes. In this paper we report the surface pressure and heat transfer measurements carried out on double cones (first cone semi-apex angle = 15, 25 deg.; second cone semi-apex angle= 35, 68 deg.) at hypersonic speeds that will be useful for CFD code validation studies. The surface pressure measurements are carried out at nominal Mach number of 8.35 in the IISc hypersonic wind tunnel. On the other hand the surface heat transfer measurements are carried out at a nominal Mach number of 5.75 in the IISc hypersonic shock tunnel. The flow separation point on the first cone, flow reattachment on the second cone and the wild fluctuation of the transmitted shock on the second cone surface (25/68 deg. double cone) in the presence of severe adverse pressure gradient are some of the flow features captured in the measurements. The results from the CFD studies indicate good agreement with experiments in the attached flow regime while considerable differences are noticeable in the separated flow regime.

A comparative study of erbium oxide and gadolinium oxide high-k dielectric thin films grown by low-pressure metalorganic chemical vapour deposition (MOCVD) using beta-Diketonates as precursors

In this paper, a comparative study of thin films of Er2O3 and Gd2O3 grown on n-type Si(100) by low-pressure metalorganic chemical vapour deposition (MOCVD) under the identical conditions has been presented. beta-Diketonate complex of rate earth metals was used as precursor. Description on the evolution of the morphology, structure, optical, and electrical characteristics of films with respect to growth parameters and post-deposition annealing process has been presented. As-gown Gd2O3 films grow with <111> texture, whereas the texture of Er2O3 films strongly depends on the growth temperature (either <100> or <111>). Compositional analysis reveals that the Gd2O3 films grown at or above 500degreesC are carbon free whereas Er2O3 films at upto 525degreesC show the presence of heteroatoms and Er2O3 films grown above 525degreesC are carbon five. The effective dielectric constant is in the range of 7-24, while the fixed charge density is in the range - 10(11) to 10(10) CM-2 as extracted from the C-V characteristics. DC I-V study was carried out to examine the leakage behaviour of films. It reveals that the as-grown Gd2O3 film was very leakey in nature. Annealing of the films in oxidizing ambient for a period of 20 min results in a drastic improvement in the leakage behaviour. The presence of heteroatoms (such as carbon) and their effect on the properties of films are discussed.

India's biodiversity hotspot under anthropogenic pressure: A case study of Nilgiri Biosphere Reserve

This paper presents data on the impact of biotic pressure in terms of grazing by livestock and wood cutting by humans on the plant community in the Nilgiri Biosphere Reserve of India. Grass, and herbaceous plant biomass, number of cattle dung piles, number of woody stems available and damaged by human activities and weed biomass were assessed at different proximity along transects radiating from village-forest boundary to forest interior to measure the ecological impact of livestock grazing and fire wood collection. The grass biomass was positively correlated to overgrazing indicating the adverse effect on natural vegetation by cattle. Woodcutting was intense along the forest boundary and significantly declined as distance increased. Similarly, weed biomass and number of thorny species declined positively with proximity from village-forest boundary and the weed biomass was significantly higher in the pastoral sites compared to residential sites. The results suggest that human impact adversely affects natural vegetation and promotes weed proliferation in forest areas adjoining human settlements in the ecologically important Nilgiri Biosphere Reserve. Continued anthropogenic pressure could cause reduction in fodder availability to large herbivores like elephants, which in turn leads to an increase in human-elephant conflict. (C) 2011 Elsevier GmbH. All rights reserved.

Pressure induced phase transition in metallic LaB6: A Raman study

We report high pressure Raman studies on single crystals of metallic LaB6 upto a pressure of 16.$ GPa. Raman spectra shows three lines at 680 cm(-1) (T-2g), 1120 cm(-1) (E-g) and 1258 cm(-1) (A(1g)), associated with the internal modes of B-6 molecule. The T-2g mode shows an asymmetric Fano line shape, arising from the interference between the phonon line and the electronic continuum. The line is fitted with I(omega) = I-0(q + epsilon(2))/(I + epsilon(2)) where epsilon = (omega - omega(0))/Gamma, omega(0) is the phonon frequency renormalised due to electron-phonon self energy corrections, Tis the width parameter proportional to the square of the matrix element of the electron-phonon interaction potential. The parameter a signifies the strength of interference. Most interestingly our pressure data for the T-2g mode shows a significant change in the slope of the mode frequency with pressure d omega(0)/dP and Gamma at 9.5 GPa. This clearly indicates that LaB6 undergoes a subtle phase transition at 9.5 GPa within the metallic phase.

Raman signatures of pressure induced electronic topological and structural transitions in Bi2Te3

We report Raman signatures of electronic topological transition (ETT) at 3.6 GPa and rhombohedral (alpha-Bi2Te3) to monoclinic (beta-Bi2Te3) structural transition at similar to 8 GPa. At the onset of ETT, a new Raman mode appears near 107 cm(-1) which is dispersionless with pressure. The structural transition at similar to 8 GPa is marked by a change in pressure derivative of A(1g) and E-g mode frequencies as well as by appearance of new modes near 115 cm(-1) and 135 cm(-1). The mode Grilneisen parameters are determined in both the alpha and beta-phases. (C) 2011 Elsevier Ltd. All rights reserved.

Synthesis of free standing carbon nanosheet using electron cyclotron resonance plasma enhanced chemical vapor deposition

Carbon nanosheets (CNSs) have been synthesized by electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition (PECVD) using a mixture of acetylene and argon gases on copper foil as the substrate. Micrometer-wide carbon sheets consisting of several atomic layers thick graphene sheets have been synthesized by controlled decomposition of carbon radicals in ECR-PECVD. Raman spectroscopy of these films revealed characteristics of a disordered graphitic sheet. Thick folded carbon-sheets and a semi transparent freestanding CNSs have been observed by scanning electron microscopy. This is a promising technique to synthesize free standing CNSs and can be used in the fabrication of nanoelecronic devices in future. (C) 2012 Elsevier B.V. All rights reserved.