Discussion of enthalpy, entropy and free energy of formation of GaN

Presented in this letter is a critical discussion of a recent paper on experimental investigation of the enthalpy, entropy and free energy of formation of gallium nitride (GaN) published in this journal [T.J. Peshek, J.C. Angus, K. Kash, J. Cryst. Growth 311 (2008) 185-189]. It is shown that the experimental technique employed detects neither the equilibrium partial pressure of N-2 corresponding to the equilibrium between Ga and GaN at fixed temperatures nor the equilibrium temperature at constant pressure of N-2. The results of Peshek et al. are discussed in the light of other information on the Gibbs energy of formation available in the literature. Entropy of GaN is derived from heat-capacity measurements. Based on a critical analysis of all thermodynamic information now available, a set of optimized parameters is identified and a table of thermodynamic data for GaN developed from 298.15 to 1400 K.

Formation and Degradation of Polymeric Peroxides

An attempt has been made to bring the literature on polymeric peroxides together from all angles in order to present a comprehensive picture about them. Both polyperoxides, where the peroxide group has been attached to the main chain, and polymeric hydroperoxides, where the peroxide group is present as a side chain, have been considered. Various aspects such as formation, thermal decomposition characteristics, photodecomposition, and analysis of peroxides have been discussed.

The formation of a p-n junction in a polymer electrolyte top-gated bilayer graphene transistor

We show simultaneous p- and n-type carrier injection in a bilayer graphene channel by varying the longitudinal bias across the channel and the top-gate voltage. The top gate is applied electrochemically using solid polymer electrolyte and the gate capacitance is measured to be 1.5 microF cm(-2), a value about 125 times higher than the conventional SiO(2) back-gate capacitance. Unlike the single-layer graphene, the drain-source current does not saturate on varying the drain-source bias voltage. The energy gap opened between the valence and conduction bands using top- and back-gate geometry is estimated.

Low temperature ferrite formation using metal oxalate hydrazinate precursor

Magnesium ferrite, MgFe2O4 has been prepared at low temperatures by the thermal decomposition of a new precursor, MgFe2(C2O4)3. 5N2H4. The ferrite has been characterized by X-ray diffraction, infrared and Mössbauer spectra.

Interfacial structure and crystallographic studies of transformations in betat' and beta Cu---Zn alloys-II. martensitic formation of alpha1' plates in beta'

A TEM study of the interphase boundary structure of 9R orthorhombic alpha1' martensite formed in beta' Cu---Zn alloys shows that it consists of a single array of dislocations with Burgers vector parallel to left angle bracket110right-pointing angle beta and spaced about 3.5 nm apart. This Burgers vector lies out of the interface plane; hence the interface dislocations are glissile. Unexpectedly, though, the Burgers vectors of these dislocations are not parallel when referenced to the matrix and the martensite lattices. This finding is rationalized on published hard sphere models as a consequence of relaxation of a resultant of the Bain strain and lattice invariant shear displacements within the matrix phase.

Influence of Sm2O3 doping on formation and structure of SrBi2Nb2O9 nanocrystals in lithium borate glasses

New glasses of 16.66SrO–16.66[(1 − x)Bi2O3–xSm2O3]–16.66Nb2O5–50Li2B4O7 (0 ≤ x ≤ 0.5, in molar ratio), i.e., the pseudo-binary Sm2O3-doped SrBi2Nb2O9–Li2B4O7 glass system, giving the crystallization of Sm3+-doped SrBi2Nb2O9 nanocrystals are developed. It is found that the thermal stability of the glasses against the crystallization and the optical band gap energy increases with increasing Sm2O3 content. The formation of fluorite-type Sm3+-doped SrBi2Nb2O9 nanocrystals (diameters: 13–37 nm) with a cubic structure is confirmed in the crystallized (530 °C, 3 h) samples from X-ray powder diffraction analyses, Raman scattering spectrum measurements, and transmission electron microscope observations. The effect of Sm3+-doping on the microstructure, Raman scattering peak positions, and dielectric properties of composites comprising of fluorite-type SrBi2Nb2O9 nanocrystals and the Li2B4O7 glassy phase is clarified. It is found that fluorite-type SrBi2Nb2O9 nanocrystals transform to stable perovskite-type SrBi2Nb2O9 crystals with an orthorhombic structure by heat treatments at around 630 °C.

Resistivity noise in crystalline magnetic nanowires and its implications to domain formation and kinetics

We have investigated the time-dependent fluctuations in electrical resistance, or noise, in high quality crystalline magnetic nanowires within nanoporous templates. The noise increases exponentially with increasing temperature and magnetic field, and has been analyzed in terms of domain wall depinning within the Neel-Brown framework. The frequency-dependence of noise also indicates a crossover from nondiffusive kinetics to long-range diffusion at higher temperatures, as well as a strong collective depinning, which need to be considered when implementing these nanowires in magnetoelectronic devices.

Intermolecular interactions in the formation of aromatic hydrocarbon dimers

Both short-range and long-range intermolecular interaction energies between two aromatic hydrocarbon molecules, both in their ground state, separated by a range of interplanar distances of 3 ∼ 4 Aring, are estimated using the standard perturbation theory. The results show that aromatic hydrocarbons can form weak sandwich dimers with larger separation between them than is normally believed in their excimers. The non-sandwich form of dimer in which the long in-plane axes of the monomers are parallel and their short in-plane axes inclined, represents an unstable orientation because this form can pass to the perfect sandwich form without an energy barrier.

Studies on micro explosive driven blast wave propagation in confined domains using NONEL tubes

Explosive driven micro blast waves are generated in the laboratory using NONEL tubes. The explosive mixture coated to the inner walls of the plastic Nonel tube comprises of HMX and Aluminum ( 18mg/m). The detonation is triggered electrically to generate micro blast waves from the open end of the tube. Flow visualization and over pressure measurements have been carried out to understand the propagation dynamics of these micro-blast waves in both confined and unconfined domains. The classical cubic root law used for large scale blast correlation appears to hold good even for these micro-blasts generated in the laboratory.

Spectroscopic studies of micro-explosions

NONEL tube finds vast applications in civil and military because of its safe and confined explosion technique. Spectroscopic and chemical analysis of a NONEL tube with an uniform mixture of HMX and Al is reported here. Peak temperature obtained at the open end of the NONEL tube due to the detonation of the explosive has been calculated using Planck’s radiation law. The products of the chemical reaction taking place due to the ignition of HMX + Al are characterized using FTIR spectroscopy.

Formation of Pb(Ti,Zr)O3, from thermal decomposition of oxalate precursors

Lead zir conyl oxalate hexahydrate (LZO) and lead titanyl zirconyl oxalate hydrate (LTZO) are prepared and characterized. Their thermal decompositions have been investigated by thermoanalytical and gas analysis techniques. The decomposition in air or oxygen has three steps — dehydration, decomposition of the oxalate to a carbonate and the decomposition of carbonate to PbZrO3. In non oxidising atmosphere, partial reduction of Pb(II) to Pb(0) takes place at the oxalate decomposition step. The formation of free metallic lead affects the stoichiometry of the intermediate carbonate and yields a mixture of Pb(Ti,Zr)O3 and ZrO2 as the final products. By maintaining oxidising atmosphere and low heating rate, direct preparation of stoichiometric, crystalline Pb(Ti,Zr)O3 at 550°C is possible from the corresponding oxalate precursor.

Modified ILMI algorithm for practical PID/PD implementation on a Micro Air Vehicle

This paper extends the iterative linear matrix inequality algorithm (ILMI) for systems having non-ideal PI, PD and PID implementations. The new algorithm uses the practical implementation of the feedback blocksto form the equivalent static output feedback plant. The LMI based synthesis techniques are used in the algorithm to design a multi-loop, multi-objective fixed structure control. The benefits of such a control design technique are brought out by applying it to the lateral stabilizing and tracking feedback control problem of a 30cm wingspan micro air vehicle.

Thiol-Disulfide Interchange Mediated Reversible Dendritic Megamer Formation and Dissociation

We report the formation of dynamic, reversible cross-linked dendritic megamers and their dissociation to monomeric dendrimers, through a thiol-disulfide interchange reaction. For this study, poly(alkyl aryl ether) dendrimers up to three-generations presenting thiol functionalities, were prepared. The series from zero to three generations of dendrimers were installed with 3, 6, 12, and 24 thiol functionalities at their peripheries. Upon synthesis, cross-linking of the dendrimer was accomplished through disulfide bond formation. The cross-linking of dendrimers was monitored through optical density changes at 420 nm. Dense cross-linking led to visible precipitation of dendritic megamers and the morphologies of the megamers were characterized by transmission electron microscopy. The disulfide cross-links between megamer monomers could be dissociated readily upon reduction of disulfide bond by dithiothreitol reagent. Preliminary studies show that dendritic megamers encapsulate C-60 and the efficiency of encapsulation increased with increasing generation of dendritic megamer.

Synthesis, Structural Characterization and Formation Mechanism of Ferroelectric Bismuth Vanadate Nanotubes

We report the synthesis and structural characterization of ferroelectric bismuth vanadate (Bi2VO5.5) (BVO) nanotubes within the nanoporous anodic aluminum oxide (AAO) templates via sol-gel method. The as-prepared BVO nanotubes were characterized by X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM), High-Resolution Transmission Electron Microscope (HRTEM) and the stoichiometry of the nanotubes was established by energy-dispersive X-ray spectroscopy (EDX). Postannealed (675 degrees C for 1 h), BVO nanotubes were a polycrystalline and the XRD studies confirmed the crystal structure to be orthorhombic. The uniformity in diameter and length of the nanotubes as reveled by the TEM and SEM suggested that these were influenced to a guest extent by the thickness and pore diameter of the nanoporous AAO template. EDX analysis demonstrated the formation of stoichiometric Bi2VO5.5 phase. HRTEM confirmed that the obtained BVO nanotubes were made up of nanoparticles of 5-9 nm range. The possible formation mechanism of nanotubes was elucidated.

Microbial Aspects of Environmentally Benign Iron Ore

Many types of micro-organisms inhabit iron ore deposits contributing to biogenic formation and conversion of iron oxides and associated minerals. Bacteria such as Paenibacillus polymyxa arc capable of significantly altering the surface chemical behaviour of iron ore minerals such as haematite, alumina, calcite and silica. Differing mineral surface affinities of bacterial cells and metabolic products such as proteins and polysaccharides can be utilised to induce their flotation or flocculation. Mineral-specific bioreagents such as proteins are generated when bacteria are grown in the presence of haematite, alumina, calcite and silica. Alumina-grown bacterial cells and proteins separated from such cells were found to be capable of separating alumina from haematite. Biodegradation of iron ore flotation collectors such as amines and oleates can be effectively utilised to achieve environmental control in iron ore processing mills.