Effect of hydrostatic pressure on the elastic moduli of As2S3 and As2Se3 glasses at 195 and 296 K

Velocities of 30 MHz longitudinal and shear ultrasonic waves have been measured in As₂S₃ and As₂Se₃ glasses as a function of hydrostatic pressure up to 1.5 kbar at 195 K and 3 kbar at 296 K. The elastic stiffness moduli are found to have relatively large, positive, pressure dependences which are about the same at both temperatures for both glasses. This behavior is attributed to the weakness of bonding between layers comprised of AsS₃ and AsS₃ pyramids.Inspection of data for a variety of glasses reveals a correlation between the value of C_L/3C_T and whether the elastic moduli are increased or decreased by pressure. (C_L is the longitudinal modulus and C_T the shear modulus.)Using the pressure dependences of the elastic moduli obtained in the present work, it is found that volume change is responsible for most of the temperature dependences of the moduli. In addition elastic gammas are obtained which are consistent with thermal Grüneisen gammas at 12 K. The pressure dependence of the volume of As₂S₃ glass at 296 K is calculated using the present results in the Murnagham equation. Agreement with volumetric data of Weir is obtained.     

Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

Thermal expansivity in semiconducting NbO2

Measurements have been made of sample lengths L_a and L_c parallel to the a- and c-crystallographic directions, respectively, in semiconducting, distorted-rutile (C⁶_4h) structure NbO₂ between 133 K and 360 K using a fused silica LVDT dilatometer system. L_a changes very slightly and goes through a minimum as a function of temperature, T, while L_c increases monotonically with T in a normal manner. This behavior is compared with that of rutile-structure compounds. The present data do not cause significant changes in the elastic stiffness moduli which were deduced previously from ultrasonic transit times using lengths extra-polated from lattice parameter data above room T. Thus the discrepancy between the elastic and calorimetric Debye temperatures reported in the literature remains. Thermal expansion coefficients and Grüneisen anharmonicity parameters are deduced. The expansion coefficients are analyzed into a quasicubic component and a term depending on the anisotropy of the Grüneisen function.     

Anomalies in the specific heat of PbF2

We have measured the heat capacity, C_p, of PbF₂ between 400 K and 800 K using a cubic, fluorite-structure, crystal as well as material recovered in the orthorhombic form at room temperature after a fluorite-structure crystal had been pressurized to 4.5 kbar. C_p of each behaved normally up to somewhat above 600 K. However C_p of the recovered material revealed that orthorhombic PbF₂ undergoes a sharp, endothermic, nonreversible transition at T = 633 K, which seems to be a transformation back to the cubic fluorite structure. The C_p of each sample exhibited a λ-like anomaly in the temperature range where a transition to the superionic state is known to occur in cubic PbF₂, thus confirming the second order nature suggested for the transition.     

Elastic anharmonicity of InP: Its relationship to the high pressure transition

Ultrasonic wave transit times have been measured in n-type InP at room temperature using hydrostatic pressures up to 4 kbar. Linear pressure dependences are found for the elastic stiffness moduli implying that at the high pressure structural-electrical transition the shear-to-bulk modulus ratio $\text{(C}{}_{11}\text{?C}{}_{12}\text{)}\text{2B}$ has a (fractional) value which fits the modified Born criterion for stability developed by Demarest $\text{et al}$. The anharmonic force constants and some of the third order elastic constants are found to be smaller the higher the transition pressure for indium III-V compounds.     

2-Dimensional particle tracking velocimetry (PTV): Technique and image processing algorithms

An automated particle track velocimeter (PTV) was constructed to determine the fluid velocity field in a transparent test section (an engine throttle body assembly is used as an example) by analyzing images of scattered light from hollow nylon particles as they move with the flow. The light from individual particles was imaged on an SIT vidicon at video rates, digitized on an FG100CD frame buffer board and analyzed automatically in an IBMPC/AT. The essence of this technique is a novel processing algorithm which converts particle track segments to flow velocity vectors at the rate of 2–3 images/minute with this hardware. An application of the technique to a gas phase flow ( 6.5 m/s) through this throttle body assembly is presented.