A review on the synthesis of in situ aluminum based composites by thermal,mechanical and mechanical–thermal activation of chemical reactions

The aim of the present paper is to review the recent progress in the synthesis of in situ particle reinforced aluminum composites using thermal, mechanical and combined mechanical-thermal activation of aluminothermic reduction reactions. The combination of combustion synthesis (CS) and mechanosynthesis (MS) is the most recent development in the processing of advanced materials like micro and nano aluminum based composites. The combined mechanical thermal synthesis (MTS) has widened the possibilities for both CS and MS. MTS holds great potential for commercial viability and offers exciting processing route for the synthesis of advanced materials. Enhanced reaction kinetics and extended concentration limits in MTS are demonstrated by illustrating the synthesis of aluminum based nanocomposite involving Al–CeO₂.     

Dissolution processes,hydrolysis and condensation reactions during geopolymer synthesis: Part I—Low Si/Al ratio systems

Initial geopolymeric reaction processes governing dissolution of solid aluminosilicate particles in alkali solutions have been investigated using conventional experimental techniques, and the data analysed by speciation predictions of the partial charge model (PCM). For metakaolin powders activated with 5.0 M NaOH, solid-state nuclear magnetic resonance (NMR) spectra disclose the existence of monomeric [Al(OH)₄]⁻ species after two hours of dissolution, consistent with PCM predictions. However, no equivalent monomeric silicate species were observed for 5.0–10.0 M NaOH activator solutions characteristic of systems with nominal Si/Al ≤ 1. The apparent absence of monomeric silicate species suggest rapid condensation of silicate units with [Al(OH)₄]⁻ to generate aluminosilicate species, as indicated by the evolution of the shoulder at around −87 ppm in the ²⁹Si NMR spectra. Of the two possible stable silicate species [SiO₂(OH)₂]²⁻ and [SiO(OH)₃]⁻, the latter appears most likely to condense with [Al(OH)₄]⁻ to produce aluminosilicate oligomers, from which larger oligomers subsequently form through further condensation with [Al(OH)₄]⁻ leading to a gradual build up of aluminosilicate networks and a lowering of system alkalinity. This dissolution and hydrolysis sequence at the early stages of synthesis suggests a reaction path wholly consistent with predictions of the partial charge model.     

Effects of hot-carrier stress on high performance polycrystalline silicon thin film transistor with a single perpendicular grain boundary

This paper is a report on the effect of a single perpendicular grain boundary on the hot-carrier and high current stability in high performance polycrystalline silicon (poly-Si) thin film transistors (TFTs). Under a hot carrier stress condition (Vg = Vth + 1 V, Vd = 12 V), the poly-Si TFT with a single grain boundary is superior to the poly-Si without any grain boundary because of the smaller free carriers available for electric conduction. The shift of transconductance in poly-Si TFT with a single grain boundary is less than 5% after hot carrier stress during a period of 1000 s. The shift of transconductance is about 25% in the case of the poly-Si TFTs without a grain boundary in the channel. On high current stress, the poly-Si TFT without the grain boundary is less degraded than the poly-Si TFT with the grain boundary because the concentrated electric field near the drain junction is lower.     

Surface barrier diode based on zinc selenide with a passivating zinc oxide film

We propose a method of fabricating surface barrier diodes with overlapped metal junction, which provides for a significant decrease in the reverse currents and an increase in the breakdown voltage.     

An analysis of nonlinear ion drift spectrometry for gas detectors with separating chamber of planar geometry

A consistent analytical model of nonlinear ion drift spectrometry for modern gas analyzers is developed. A procedure for determining the field dependence of the ion mobility using the experimental data is described. An ionogram is calculated for the case of a flat drift chamber and polynomial character of the field dependence of the ion mobility.     

Simulations of Dynamical Ordering in Pinned Vortex Systems

We model a vortex system in a sample with bulk pinning and superficial pinning generated by a magnetic decoration. We perform a sequence of finite temperature numerical experiments in which external forces are applied to obtain, a dynamically ordered vortex lattice. We analyze the final structures and the behavior of the total energy of the system.     

Quantum Critical Ferromagnetism in Ni x Pd1- x Alloys

We analyze the non-Fermi liquid behavior in the proximity of the quantum phase transition induced by the strong polarization of the electrons due to local magnetic moments in Ni _x Pd_1-x alloys. We use the renormalization group approach introduced by Hertz–Mi11is–Moriya (HMM) to estimate the temperature dependence of the electrical resistivity for the case of three dimensional itinerant ferromagnets. We study two different cases, namely, the clean and disordered ferromagnets, and argue that at the present time more experimental data are needed to decide if the HMM theory can accurately describe the physical properties in Ni _x Pd_1-x alloys.     

Oscillations of the Electric-Dipole Echo in Glasses in a Magnetic Field

Using a simple diagram technique we derive the electric-dipole echo amplitude from two-level systems with a quadrupole nuclear moment in glasses in an external magnetic field. We show, that due to the quadrupole moment interaction of a tunneling particle with a gradient of an internal electric field, the echo amplitude experiences oscillations in rather weak magnetic fields. With an increase of the magnetic field, when the Zeeman energy becomes larger than the quadrupole energy splitting, the average echo amplitude increases and saturates (for high magnetic fields) at some level which is above the average level of echo oscillations for small magnetic fields. PACS numbers: 77.22.Ch, 61.43.Fs, 76.60.Gv, 76.60.Lz     

Temperature and Frequency Dependence of the Ultrasound Velocity in Neutron-Irradiated Quartz Crystals Above 5 K

Neutron-irradiated quartz crystal is one of the interesting materials for the study of the glassy dynamics. The temperature and frequency dependence of the sound velocity in amorphous materials at temperatures above the low-temperature maximum is not well understood. In this paper we present the results of our measurements of the variation of the ultrasound velocity (270 – 650 MHz) in neutron-irradiated quartz crystals with different irradiation doses in temperature range 5 - 35 K. We have found that the temperature dependence of the ultrasound velocity is linear in the sample with a dose < 10 ¹⁹ n/cm² and is sublinear in samples with a dose > 10 ¹⁹ n/cm² . We discuss our results in the framework of the different theoretical approaches. PACS numbers: 61.43. j, 62.65.+k, 63.     

Point Contact Spectroscopy and Temperature Dependence of Resistivity of Metallic Sodium Tungsten Bronzes - Role of Optical Phonons

In this paper we report the results of electrical resistivity (1.5≤ T ≤ 300K) and point contact spectroscopy (PCS) measurements on single crystals of metallic sodium tungsten bronze with varying sodium content. We have shown that the electron-phonon coupling function as measured through PCS can explain quantitatively the large temperature dependence of resistivity ρ seen in these materials over the entire temperature range. The electron-phonon coupling function shows predominately large peaks for phonon frequency range of 30 meV ≤ ω ≤ 100 meV which match well with the calculated optical phonons for WO₆ octahedron. The integrated electron-phonon coupling constant λ from this data is ≈ 0.25-0.45, depending on the Na content. PACS numbers: 72.15.-v, 72.80.Ga