E S R Gopal
Articles written in Pramana – Journal of Physics
Volume 1 Issue 6 December 1973 pp 260-268 Liquids
The phase boundary of the binary liquid system CS2+CH3NO2 is studied over nearly six decades in reduced temperature 3×10−6<ε=(
Volume 3 Issue 5 November 1974 pp 286-294 Liquids
The cylindrical approximation originally proposed by Pippard for the λ-transitions in one-component systems is extended to multicomponent systems. The application of the generalised Pippard relations to binary liquid mixtures is considered. The relations for He3 + He4 mixtures are shown to be equivalent to those obtained by earlier workers. The validity of the relation relating specific heat and thermal expansion for binary liquid systems is discussed on the basis of the measurements performed on the system methaonl +
Volume 4 Issue 5 May 1975 pp 213-217 Solids
The elastic constants
Volume 6 Issue 1 January 1976 pp 19-24 Solids
The elastic constants
Volume 7 Issue 4 October 1976 pp 266-276 Statistical Physics
Electrical resistance measurements are reported on the binary liquid mixtures CS2 + CH3CN and CS2 + CH3NO2 with special reference to the critical region. Impurity conduction seems to be the dominant mechanism for charge transport. For the liquid mixture filled at the critical composition, the resistance of the system above
Volume 11 Issue 5 November 1978 pp 519-527 Experimental Techniques and Instrumentation
A fully automated calorimeter has been designed and tested over the temperature range 200K–400K. The system may be used for measurements with an absolute accuracy of 0.2% of samples of approximate mass 50g and thermal capacity 15 JK−1. The temperature of the sample is determined by a quartz crystal thermometer of resolution 100 µK which is not in direct thermal contact with the sample. The performance of the system is illustrated by results obtained on high purity copper, distilled water and K2PbCu(NO2)6 which exhibits very sharp first order phase transitions at about 273.4K and 281.8K.
Volume 12 Issue 5 May 1979 pp 511-522 Liquids
The electrical resistance of the binary liquid system cyclohexane + acetic anhydride is measured, in the critical region, both in the pure mixture and when the mixture is doped with small amounts (≈ 100 ppm) of H2O/D2O impurities.
Volume 14 Issue 5 May 1980 pp 349-362 Instrumentation
The design and construction of precision temperature controllers, capable of tracking the temperature of the samples to within 1 mK for ramp heating rates from 0.05 to 10 K per hour, are discussed. A tutorial section on the evolution of the control loop configuration is first given. This is followed by an outline of the refinements of the basic control loop desirable in the actual implementation of the electronic controller. The novel features of the present system and its performance are then briefly discussed. Finally the inadequacy of the conventional PID controllers for this application, the estimation of the time constants of the physical system needed in the design of the electronic controllers and the pitfalls in using a simple model of the heater plus thermometer assembly with a single pole are also discussed.
Volume 17 Issue 1 July 1981 pp 25-32 Solid State Physics
The elastic constants of single crystal galena have been determined from the measured ultrasonic velocities down to liquid helium temperature. A cryostat incorporating an arrangement to inject the liquid bonding material at low temperature is described. At 5 K, the values of elastic constants are C11=14.90, C12=3.51 and C44=2.92×1010 N/m2.
Volume 17 Issue 1 July 1981 pp 33-38 Solid State Physics
Precise measurements of 10 MHz frequency longitudinal and shear wave velocities are reported in amorphous SeGe alloys near their glass transition temperature T
Volume 21 Issue 1 July 1983 pp 71-78 Chemical Physics
The electrical resistance
Volume 22 Issue 3-4 March 1984 pp 203-212 Statistical Physics
The effect of gravity on various thermodynamic properties near the gas-liquid critical point has been calculated. Using a simple equation satisfying scaling requirements, an analytic expression for density profile is obtained, using which the effect on different thermodynamic properties can be easily calculated.
Volume 23 Issue 1 July 1984 pp 17-29 Solid State Physics
The electrical resistivity of bulk Ge20Te80 has been measured as a function of pressure and temperature. At 5 GPa, an amorphous semiconductor-to-crystalline metal transition has been observed. The sample recovered from the high pressure cell, after the application of 7 GPa, has a face-centred cubic structure with a lattice constant of 6·42 A. In crystalline sample, the semiconductor-to-metal transition occurs at 7 GPa. The thermoelectric power has also been measured for glassy samples in the temperature range 300–240 K.
Volume 23 Issue 1 July 1984 pp 31-37 Solid State Physics
The pressure dependence of the electrical resistivity of bulk GeSe2 glass shows a semiconductor-to-metal transition at 7 GPa pressure. The high pressure phase is examined using the x-ray diffractometer and is found to be crystalline, with a face-centred cubic structure having
Volume 27 Issue 1-2 July 1986 pp 267-273 Solid State Physics
Electron and x-ray diffraction experiments on the metlt-spun Al100−
Volume 28 Issue 3 March 1987 pp 269-275 Condensed Matter Physics
The electrical capacitance of the binary liquid mixture
Volume 28 Issue 5 May 1987 pp 435-469
The article reviews the experimental techniques used in high pressure-low temperature investigations to study a variety of physico-chemical phenomena. The general principles of producing high pressures at low temperatures, the methods of measuring P and T, the materials used for construction and the diamond anvil cell (DAC) are briefly given first. Specific pieces of apparatus to measure the mechanical properties, phase equilibria, thermal properties, electrical properties, magnetic phenomena, optical and Raman/IR spectroscopic behaviour as well as Mössbauer spectra are then discussed. While instrumentation is the main emphasis of the article, a few illustrative examples of interesting observations are also indicated. Over 250 current papers are cited.
Volume 28 Issue 5 May 1987 pp 471-482
The ternary glasses of arsenic and germanium with antimony and selenium can be prepared in large sizes for optical purposes. The elastic behaviour of eight compositions of each glass has been studied down to 4.2 K using a 10 MHz ultrasonic pulse echo interferometer. The glasses have a normal elastic behaviour, with the velocities gradually increasing as the temperature is lowered. An anharmonic solid model of Lakkad satisfactorily explains the temperature variations. The elastic moduli of Ge
Volume 33 Issue 3 September 1989 pp 405-420 Crystallography
The two dimensional plane can be filled with rhombuses, so as to generate non-periodic tilings with 4, 6, 8, 10 and 12-fold symmetries. Some representative tilings constructed using the rule of inflation are shown. The numerically computed diffraction patterns for the corresponding tilings are also shown to facilitate a comparison with possible X-ray or electron diffraction pictures.
Volume 34 Issue 6 June 1990 pp 525-536
The diffraction patterns from Fibonacci quasilattices have been calculated. Finite-size effects are evaluated for weak and strong peaks. For a smaller number of scatterers (<100) there are fluctuations in the intensities of weak and strong peaks. The fluctuations in weak peaks are greater than that in strong peaks. The fluctuations in intensities of weak and strong peaks near the origin are larger than in the corresponding cases of weak and strong peaks far away from the origin. Small shifts in peak-positions are unexpectedly found, the shifts being proportional to
Volume 34 Issue 6 June 1990 pp 537-553
A method for generating aperiodic tilings with five fold symmetry is discussed here. Basic patterns formed within decagons can be used to fill two dimensional space, by matching such suitable patterns. It appears to be possible to generate perfect tilings without retracing already established coordinates imposing conditions at the initial stages of generating them. Various possible ways to generate tilings, when perfectness is not required, are discussed. The calculated diffraction patterns for some representative finite size tilings are shown. There are subtle differences in the intensities of peaks in the diffraction patterns corresponding to different finite size tilings constructed using intersecting decagons. These effects persist for a larger number of scatterers in weak peaks than in strong peaks. They are unaffected by an introduction of systematic disorder. These effects could be termed as the finite size boundary effects. There are also small shifts in the peak positions owing to the finite size effects. The possibility of formation of large approximate square cells in large tilings is shown.
Volume 36 Issue 4 April 1991 pp 1- Comments
Volume 38 Issue 4 April 1992 pp 335-341
We report here a systematic data analysis of the vapour pressure of argon at different amounts of the liquid phase to understand the thermodynamic behaviour of this inert gas around triple point. At the triple point plateau, the applied heat pulse melts a certain phase of solid argon into liquid and increases vapour pressure. It is observed that this vapour pressure attains the thermodynamic equilibrium pressure after a certain time interval. The expoential decay of the vapour pressure as a function of time at different fractions of the liquid phase shows two different features. In one region, the relexation time constant (
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