A CCD based detector for quick detection of pressure induced phase transitions

Abstract

A high sensitivity CCD based two dimensional angle dispersive X-ray are a detector has been developed for quick detection of pressure induced phase transitions for a laboratory X-ray source such as a rotating anode generator. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on element Pd, intermetallics AuIn², AuGa² and low Z scatterer adamantane (C₁₀H_l6) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell (DAC) is demonstrated by reproducing the known pressure induced structural phase transitions in RbI and KI. The importance of this detector system in search of unknown phase transitions has been established by observing new structural phase transitions in In_0.25Sn_0.75 and AuGa₂. Various softwares have also been developed such as interactive location of centre of diffraction rings, radial integration and image enhancement to analyze data from this detector.     

Analysis of the nonlinear vibration of a two-mass–spring system with linear and nonlinear stiffness

An analytical approach is developed for areas of nonlinear science such as the nonlinear free vibration of a conservative, two-degree-of-freedom mass–spring system having linear and nonlinear stiffnesses. The main contribution of this research is twofold. First, it introduces the transformation of two nonlinear differential equations for a two-mass system using suitable intermediate variables into a single nonlinear differential equation and, more significantly, the treatment of a nonlinear differential system by linearization coupled with Newton’s method. Secondly, the major section is the solving of the governing nonlinear differential equation where the displacement of the two-mass system can be obtained directly from the linear second-order differential equation using a first-order variational approach. The aforementioned approach proposed by J.H. He, who actually developed the method, is exactly He’s variational method. This approach is an explicit method with high validity for resolving strong nonlinear oscillation system problems. Two examples of nonlinear two-degree-of-freedom mass–spring systems are analyzed, and verified with published results and exact solutions. The method can be easily extended to other nonlinear oscillations and so could be widely applicable in engineering and science.     

A fully automated algorithm under modified FCM framework for improved brain MR image segmentation

Automated brain magnetic resonance image (MRI) segmentation is a complex problem especially if accompanied by quality depreciating factors such as intensity inhomogeneity and noise. This article presents a new algorithm for automated segmentation of both normal and diseased brain MRI. An entropy driven homomorphic filtering technique has been employed in this work to remove the bias field. The initial cluster centers are estimated using a proposed algorithm called histogram-based local peak merger using adaptive window. Subsequently, a modified fuzzy c-mean (MFCM) technique using the neighborhood pixel considerations is applied. Finally, a new technique called neighborhood-based membership ambiguity correction (NMAC) has been used for smoothing the boundaries between different tissue classes as well as to remove small pixel level noise, which appear as misclassified pixels even after the MFCM approach. NMAC leads to much sharper boundaries between tissues and, hence, has been found to be highly effective in prominently estimating the tissue and tumor areas in a brain MR scan. The algorithm has been validated against MFCM and FMRIB software library using MRI scans from BrainWeb. Superior results to those achieved with MFCM technique have been observed along with the collateral advantages of fully automatic segmentation, faster computation and faster convergence of the objective function.     

On the nature of the dhcp to fcc transition under pressure in Pr and Pr-Th alloys

The results of electrical resistance (R), thermoelectric power (TEP) and X-ray diffraction measurements on praseodymium (Pr) and its alloys with thorium under pressure are reported. The maximum inR vsP curve exhibited by Pr persists only in the dhcp phase of PrTh alloy. X-ray measurements confirmed that in the alloys also the maximum inR vsP curve is due to the dhcp → fcc transition. Thus the behaviour of Pr and Pr-Th alloys is different from that of La and its alloys with Ce and Th where the maximum in theR vsP curve is electronic in origin and is exhibited by the dhcp, fcc and dist fcc phases.     

Structural phase transitions in Si and Ge under pressures up to 50 GPa

The crystal structures of Si and Ge were studied by energy dispersive X-ray diffraction at room temperature and pressures up to 50 GPa. Si transforms to a primitive hexagonal (Si-V) structure around 16 GPa, to an intermediate phase Si-VI between 35 and 40 GPa, and to hcp (Si-VII) around 40 GPa. In contrast, Ge remains in the β-tin structure up to at least 51 GPa. The pseudopotential method reproduces these differences in the high-pressure behavior of Si and Ge.     

Opacities of high temperature highZ plasmas

A procedure is described for computation of opacities for highZ plasmas. Bound-bound, bound-free, free-free and scattering processes are considered. The inputs for these have been obtained by solving IEEOS form of Saha’s equation. Detailed calculations of opacities have been done for tungsten and uranium up to 10 keV of temperature. Preliminary results have been presented at the Nucl. Phys. and Solid State Phys. Symp., Dept. of Atom. Energy, held at Ahmedabad, India (December 1976).     

Efficient Transformation of Furfuryl Alcohol Into Ethyl Levulinates via Alcoholysis Reaction Catalyzed by SnO2/H-Mordenite Catalyst

Catalysis Surveys from Asia - Catalytic production of ethyl levulinate by alcoholysis of furfuryl alcohol with ethanol was investigated over H-mordenite supported Sn catalyst under atmospheric N2...     

On some hydrogen bond correlations at high pressures

Structural data from in situ high pressure neutron diffractions studies are analysed to show that the measured lengths of O?H and H---O pairs in hydrogen bonds follow the correlation between them established from 0.1 MPa data on different chemical compounds. In particular, the accepted view in literature that the high pressure data on ice VIII differ from it is not supported. For compounds in which the O?H stretching frequencies red shift under pressure, it is shown that wherever structural data is available, they follow the stretching frequency versus H---O (or O---O) distance correlation. For compounds displaying blue shifts with pressure, an analogy appears to exist with ‘improper’ hydrogen bonds. Another pressure effect expected is the symmetrization of a hydrogen bond with evolution of the double-welled, hydrogen bond potential into a single-well potential. Using the results from some recent ab initio electronic structure studies, we show that the bulk modulus shows a discontinuity at this point and this can be taken as signature for hydrogen bond symmetrization. This is supported by analysing a few available experimental data.