Anomalous behaviour in the composition dependence of the photoacoustic properties of Si---As---Te glasses

The optical bandgap and thermal diffusivity of Si10AsxTe90−x (10 ≤ x ≤ 50) and Si15AsxTe85−x (5 ≤ x ≤ 40) glasses have been measured using the photoacoustic technique. The anomalous behaviour observed in these properties at the mean coordination number left angle bracketrright-pointing angle bracket = 2.60 is interpreted by reference to the formation and development of a layered structure in these glasses.

The effects of charge density and concentration on the composition of polyelectrolyte complexes

Polyelectrolyte complex formation involving carboxymethylcellulose and quaternized poly(vinylpyridine) as the polyions has been studied using viscosity and u.v. spectroscopic methods. The influence of charge density and molecular weight of two polycations on the composition of the complex has been investigated at two different concentrations. The charge density of the polycation is found to have different influences on the composition at different concentrations. The molecular weight of the polycation and the location of the ionic site on the polycation do not show any effect on the composition. A drastic increase in the viscosity of the polyion mixture containing quaternized poly(2-vinylpyridine) in the non-stoichiometric ratio shows evidence for the existence of the soluble polyelectrolyte complex. The results are analysed on the basis of the relative extension of the polyelectrolyte chains.

High-pressure studies on the critical composition in Ge—As—Te glasses

High-pressure resistivity measurements have been performed on G37.5AsxTe92.5-x (x = 20, 40, 45, 50 and 55) and Ge10AsxTe90-x (x = 15, 20, 35, 40, 45 and 50) glasses. The glasses show reversible metallization behaviour under pressure. The pressure derivative of the logarithm of the resistivity of the glasses is a minimum at glass compositions corresponding to the average coordination number [r] = 2.60. This behaviour is interpreted on the basis of the existence of a critical composition in glasses with a layered structure.

Composition Driven Monolayer to Bilayer Transformation in a Surfactant Intercalated Mg-Al Layered Double Hydroxide

The structure and organization of dodecyl sulfate (DDS) surfactant chains intercalated in an Mg-Al layered double hydroxide (LDH), Mg(1-x)Alx(OH)(2), with differing Al/Mg ratios has been investigated. The Mg-Al LDHs can be prepared over a range of compositions with x varying from 0.167 to 0.37 and therefore provides a simple system to study how the organization of the alkyl chains of the intercalated DDS anions change with packing density; the Al/Mg ratio or x providing a convenient handle to do so. Powder X-ray diffraction measurements showed that at high packing densities (x >= 0.3) the alkyl chains of the intercalated dodecyl sulfate ions are anchored on opposing LDH sheets and arranged as bilayers with an interlayer spacing of similar to 27 angstrom. At lower packing densities (x < 0.2) the surfactant chains form a monolayer with the alkyl chains oriented flat in the galleries with an interlayer spacing of similar to 8 angstrom. For the in between compositions, 0.2 <= x < 0.3, the material is biphasic. MD simulations were performed to understand how the anchoring density of the intercalated surfactant chains in the Mg-Al LDH-DDS affects the organization of the chains and the interlayer spacing. The simulations are able to reproduce the composition driven monolayer to bilayer transformation in the arrangement of the intercalated surfactant chains and in addition provide insights into the factors that decide the arrangement of the surfactant chains in the two situations. In the bilayer arrangement, it is the dispersive van der Waals interactions between chains in opposing layers of the anchored bilayer that is responsible for the cohesive energy of the solid whereas at lower packing densities, where a monolayer arrangement is favored, Coulomb interactions between the positively charged Mg-Al LDH sheets and the negatively charged headgroup of the DDS anion dominate.

Finite data performance of MUSIC and minimum norm methods

In the direction of arrival (DOA) estimation problem, we encounter both finite data and insufficient knowledge of array characterization. It is therefore important to study how subspace-based methods perform in such conditions. We analyze the finite data performance of the multiple signal classification (MUSIC) and minimum norm (min. norm) methods in the presence of sensor gain and phase errors, and derive expressions for the mean square error (MSE) in the DOA estimates. These expressions are first derived assuming an arbitrary array and then simplified for the special case of an uniform linear array with isotropic sensors. When they are further simplified for the case of finite data only and sensor errors only, they reduce to the recent results given in [9-12]. Computer simulations are used to verify the closeness between the predicted and simulated values of the MSE.

Electrical switching studies on Ge-Te-Tl chalcogenide glasses: Effect of thallium on the composition dependence of switching voltages

Bulk Ge(17)Te83_,JI glasses (05x.5_13), have been found to exhibit memory type electrical switching. The switching voltages (also known as threshold voltage V-th) of Ge17Te83-xTlx glasses are found to decrease with increasing thallium content. The rate of decrease of Vtry is greater at lower concentrations and \textbackslashid, falls at a slower rate for higher thallium concentrations (x 6). The addition of thallium to the Ge-Te network fragments the covalent network and introduces ionic nature to it; the reduction in network connectivity leads to the decrease in switching voltages with thallium content. The decrease in the glass transition temperatures of Ge17Te83-xTlx glasses with increasing thallium concentration supports the idea of decrease in network connectivity with TI addition. The more metallic nature of TI also contributes to the observed reduction in the switching voltages of Ge17Te83-xTlx glasses with TI content. Further, there is an interesting correlation seen between the threshold voltage V-th and the average bond energy, as a function of TI content. In addition, the switching voltages of Ge17Te83-xTlx glasses have been found to decrease with sample thickness almost linearly. The set-reset studies indicate that the Ge17Te83-xTl2 sample can be switched for more than 10 cycles, whereas other glasses could not be reset beyond two switching cycles. (C) 2010 Elsevier B.V. All rights reserved.

Substrate-dependent structure, microstructure, composition and properties of nanostructured TiN films

Titanium nitride films of a thickness of similar to 1.5 mu m were deposited on amorphous and crystalline substrates by DC reactive magnetron sputtering at ambient temperature with 100% nitrogen in the sputter gas. The growth of nanostructured, i.e. crystalline nano-grain sized, films at ambient temperature is demonstrated. The microstructure of the films grown on crystalline substrates reveals a larger grain size/crystallite size than that of the films deposited on amorphous substrates. Specular reflectance measurements on films deposited on different substrates indicate that the position of the Ti-N 2s band at 2.33 eV is substrate-dependent, indicating substrate-mediated stoichiometry. This clearly demonstrates that not only structure and microstructure, but also chemical composition of the films is substrate-influenced. The films deposited on amorphous substrates display lower hardness and modulus values than the films deposited on crystalline substrates, with the highest value of hardness being 19 GPa on a lanthanum aluminate substrate. (C) 2011 Elsevier Ltd. All rights reserved.

Structure and composition related properties of titania thin films

The dependence of optical constants, structure and composition of titania thin films on the process parameters has been investigated. Films were deposited using both reactive electron beam evaporation and ion Assisted Deposition(IAD). If has been observed that the refractive index of IAD films is higher than that for the reactively deposited films, without much difference in the extinction coefficient. Electron paramagnetic resonance has been used to estimate qualitatively the presence of non-stoichiometry in the films. It has been found that these spectra correlate very well the optical behaviour of the films. X-ray diffraction studies revealed that the neutral oxygen deposited films were stress free, while the IAD films showed tensile stress. The lattice parameters showed anisotropic change with ion beam parameters.

Theoretical Analysis of the Electromotive Force of a Cell Incorporating a Composition Gradient Solid Electrolyte

New composition gradient solid electrolytes have been designed for application in high temperature solid-state galvanic sensors and in thermodynamic measurements. The functionally gradient electrolyte consists of a solid solution between two or more ionic conductors with a common ion and gradual variation in composition of the other ionic species. Unequal rates of migration of the ions, caused by the presence of the concentration gradient, may result in the development of space charge, manifesting as diffusion potential. Presented is a theoretical analysis of the EMF of cells incorporating gradient solid electrolytes. An analytical expression is derived for diffusion potential, using the thermodynamics of irreversible processes, for different types of concentration gradients and boundary conditions at the electrode/electrolyte interfaces. The diffusion potential of an isothermal cell incorporating these gradient electrolytes becomes negligible if there is only one mobile ion and the transport numbers of the relatively immobile polyionic species and electrons approach zero. The analysis of the EMF of a nonisothermal cell incorporating a composition gradient solid electrolyte indicates that the cell EMF can be expressed in terms of the thermodynamic parameters at the electrodes and the Seebeck coefficient of the gradient electrolyte under standard conditions when the transport number of one of the ions approaches unity.

Low-temperature magnetoresistance of ?-phase FexNi80-xCr20 alloys near the critical composition for ferromagnetism

We present a comprehensive study of magnetoresistance (MR) of the crystalline pseudobinary ?-phase Fe alloy series FexNi80-xCr20 (50?x?66). This alloy series shows exotic magnetic phases as the composition (x) is varied. It has a critical composition for ferromagnetism at x=xc?59�60. MR was measured in the temperature range 1.7�110 K and up to a field of 7 T. The observed MR was small and the change was ?1%. The temperature dependence of MR was found to contain a positive and a negative contribution. The positive term was found to be ?H2 and it dominates at high field and high temperatures. We explain this as a manifestation of Kohler�s rule. The negative MR was found to have a quadratic dependence on magnetization M. The magnitude of the negative MR reaches a maximum as x?xc.

Pressure-induced charge transfer in lanthanum nickel ferrate (LaNi0.5Fe0.5O3) and analogies to composition-driven charge transfer in dilanthanum cuprate (La2CuO4): an infrared study

A reversible pressure-induced phase transition in lanthanum nickel ferrate (LaNi0.5Fe0.5O3) manifests itself in the infrared spectrum of the transition metal-oxygen stretching (nu(TM-O)) modes by the emergence of new peaks at pressures greater than similar to 1.4 x 10(9) Pa. Analogies to this transition are made by considering charge transfer in dilanthanum cuprate (La2CuO4) and its modification by partial substitution of copper ions by chromium ions.

Relation between integral and partial properties of ternary solutions along different composition trajectories

The relations between partial and integral properties of ternary solutions along composition trajectories suggested by Kohler, Colinet and Jacob, and along an arbitrary path are derived. The chemical potentials of the components are related to the slope of integral free energy by expressions involving the binary compositions generated by the intersections of the composition trajectory with the sides of the ternary triangle. Only along the Kohler composition trajectory it is possible to derive the integral free energy from the variation of the chemical potential of a single component with composition or vice versa. Along all other paths the differential of the integral free energy is related to two chemical potentials. The Gibbs-Duhem integration proposed by Darken for the ternary system uses the Kohler isogram. The relative merits of different limits for integration are discussed.