Modelling of Ion Transport in Solids with a General Bond Valence Based Force-Field

Empirical bond length - bond valence relations provide insight into the link between structure of and ion transport in solid electrolytes. Building on our earlier systematic adjustment of bond valence (BV) parameters to the bond softness, here we discuss how the squared BV mismatch can be linked to the absolute energy scale and used as a general Morse-type interaction potential for analyzing low-energy pathways in ion conducting solid or mixed conductors either by an energy landscape approach or by molecular dynamics (MD) simulations. For a wide range of Lithium oxides we could thus model ion transport revealing significant differences to an earlier geometric approach. Our novel BV-based force-field has also been applied to investigate a range of mixed conductors, focusing on cathode materials for lithium ion battery (LIB) applications to promote a systematic design of LIB cathodes that combine high energy density with high power density. To demonstrate the versatility of the new BV-based force-field it is applied in exploring various strategies to enhance the power performance of safe low cost LIB materials (LiFePO4, LiVPO4F, LiFeSO4F, etc.).Received: 11 October 2010; Revised: 26 October 2010; Accepted: 28 October 201

Electrical and Magnetic behaviour of PrFeAsO0.8F0.2 superconductor

The superconducting and ground state samples of PrFeAsO0.8F0.2 and PrFeAsO have been synthesised via easy and versatile single step solid state reaction route. X-ray & Reitveld refine parameters of the synthesised samples are in good agreement to the earlier reported value of the structure. The ground state of the pristine compound (PrFeAsO) exhibited a metallic like step in resistivity below 150K followed by another step at 12K. The former is associated with the spin density wave (SDW) like ordering of Fe spins and later to the anomalous magnetic ordering for Pr moments. Both the resistivity anomalies are absent in case of superconducting PrFeAsO0.8F0.2 sample. Detailed high field (up to 12Tesla) electrical and magnetization measurements are carried out for superconducting PrFeAsO0.8F0.2 sample. The PrFeAsO0.8F0.2 exhibited superconducting onset (Tconset) at around 47K with Tc({\rho} =0) at 38K. Though the Tconset remains nearly invariant, the Tc({\rho} =0) is decreased with applied field, and the same is around 23K under applied field of 12Tesla. The upper critical field (Hc2) is estimated from the Ginzburg Landau equation (GL) fitting, which is found to be ~ 182Tesla. Critical current density (Jc) being calculated from high field isothermal magnetization (MH) loops with the help of Beans critical state model, is found to be of the order of 103 A/cm2. Summarily, the superconductivity characterization of single step synthesised PrFeAsO0.8F0.2 superconductor is presented.Comment: 15 Pages Text + Fig

Detection of a Series of X-ray Dips Associated with a Radio Flare in GRS 1915+105

We report the detection of a series of X-ray dips in the Galactic black hole candidate GRS 1915+105 during 1999 June 6-17 from observations carried out with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment on board the Indian satellite IRS-P3. The observations were made after the source made a transition from a steady low-hard state to a chaotic state which occuered within a few hours. Dips of about 20-160 seconds duration are observed on most of the days. The X-ray emission outside the dips shows a QPO at ~ 4 Hz which has characteristics similar to the ubiquitous 0.5 - 10 Hz QPO seen during the low-hard state of the source. During the onset of dips this QPO is absent and also the energy spectrum is soft and the variability is low compared to the non-dip periods. These features gradually re-appear as the dip recovers. The onset of the occurrence of a large number of such dips followed the start of a huge radio flare of strength 0.48 Jy (at 2.25 GHz). We interpret these dips as the cause for mass ejection due to the evacuation of matter from an accretion disk around the black hole. We propose that a super-position of a large number of such dip events produces a huge radio jet in GRS 1915+105.Comment: 18 pages, 7 figures, Accepted for publication in Ap

X-ray properties of the microquasar GRS 1915+105 during a variability class transition

We present a detailed X-ray study of the microquasar GRS 1915+105 during a variability class transition observed in 2000 June with the PPCs of the Indian X-ray Astronomy Experiment. We supplement this observation with data from the RXTE archives. The source made a transition from a steady low-hard state to a regular oscillatory behaviour in the light curve known as bursts or class `rho' (Belloni et al. 2000) between 2000 May 11 and 17 and reverted back to the low-hard state on 2000 June 27. A gradual change in the burst recurrence time from about 75 s to about 40 s was observed which then increased to about 120 s during the ~ 40 days of class `rho'. The regular bursts disappeared from the X-ray light curves and the class transition was observed to occur within 1.5 hours on 2000 June 27 with the PPCs. A correlation is found between the observed QPO frequency at 5-8 Hz in the quiescent phase and the average X-ray intensity of the source during the class `rho'. We notice a strong similarity between the properties of the source during the class `rho' and those during the oscillatory phase of the observations of class `alpha'. From the timing and spectral analysis, it is found that the observed properties of the source over tens of days during the class `rho' are identical to those over a time scale of a few hundreds of seconds in the class `alpha'. Examining the light curves from the beginning of the RXTE/PCA and RXTE/ASM observations, it is found that the change of state from radio-quiet low-hard state to high state occurs through the X-ray classes `rho' and `alpha' which appear together during the state transition. It is further inferred that the source switches from low-hard state to the class `rho' through the intermediate class `alpha'.Comment: 10 pages with 9 figures, LaTex. To be appeared in MNRA

Constant amplitude and post-overload fatigue crack growth behavior in PM aluminum alloy AA 8009

A recently developed, rapidly solidified, powder metallurgy, dispersion strengthened aluminum alloy, AA 8009, was fatigue tested at room temperature in lab air. Constant amplitude/constant delta kappa and single spike overload conditions were examined. High fatigue crack growth rates and low crack closure levels compared to typical ingot metallurgy aluminum alloys were observed. It was proposed that minimal crack roughness, crack path deflection, and limited slip reversibility, resulting from ultra-fine microstructure, were responsible for the relatively poor da/dN-delta kappa performance of AA 8009 as compared to that of typical IM aluminum alloys

Two Modes of Solid State Nucleation - Ferrites, Martensites and Isothermal Transformation Curves

When a crystalline solid such as iron is cooled across a structural transition, its final microstructure depends sensitively on the cooling rate. For instance, an adiabatic cooling across the transition results in an equilibrium `ferrite', while a rapid cooling gives rise to a metastable twinned `martensite'. There exists no theoretical framework to understand the dynamics and conditions under which both these microstructures obtain. Existing theories of martensite dynamics describe this transformation in terms of elastic strain, without any explanation for the occurence of the ferrite. Here we provide evidence for the crucial role played by non-elastic variables, {\it viz.}, dynamically generated interfacial defects. A molecular dynamics (MD) simulation of a model 2-dimensional (2d) solid-state transformation reveals two distinct modes of nucleation depending on the temperature of quench. At high temperatures, defects generated at the nucleation front relax quickly giving rise to an isotropically growing `ferrite'. At low temperatures, the defects relax extremely slowly, forcing a coordinated motion of atoms along specific directions. This results in a twinned critical nucleus which grows rapidly at speeds comparable to that of sound. Based on our MD results, we propose a solid-state nucleation theory involving the elastic strain and non-elastic defects, which successfully describes the transformation to both a ferrite and a martensite. Our work provides useful insights on how to formulate a general dynamics of solid state transformations.Comment: 3 pages, 4 B/W + 2 color figure