R. Ravinder Reddy

Elastic behavior of a Y-Ba-Cu-O sample prepared by the MPMG method

Abstract Ultrasonic velocity measurements have been undertaken by the pulse transmission technique using YBaCuO samples prepared by the melt-powder-melt-growth (MPMG) process over a temperature range of 80–300 K. It has been observed that the sample exhibits a very high value of Youngs and rigidity moduli, thereby indicating that the material is an excellent one from the point of view of its mechanical strength. Further, the Youngs modulus apart from exhibiting a hump at around 120 K, is found to increase continuously with decreasing temperature. In the superconducting phase, however, the modulus is found to remain almost constant. It has been concluded that the observed elastic behavior, to a large extent, is analogous to that of normal oxide materials. In order to understand how far the experimentally observed behavior deviates at 120 K, the elastic moduli of the sample were also estimated theoretically and compared with the empirical values.

An analytical study of Young's moduli of HTSC at low temperatures

Abstract The ultrasonic-velocity measurements of a series of RE based and Bi based superconducting samples have been carried out by the pulse transmission technique over a temperature range of 80–300 K. The elastic moduli have been evaluated analytically by employing three different models, viz. the Watchman, Varshni and Lakkad models, over the temperature range of 5 to 300 K mainly to understand the elastic anomalies. It has been observed that the elastic moduli predicted by all the models are in agreement with the experimental ones in the temperature region 300–150 K, where the elastic anomalies are absent. On the other hand, although the elastic behaviour predicted by all four equations in the temperature range 150–80 K is almost similar, the experimental behaviour is found to deviate very much from the theoretical ones, due to the presence of anomalies in almost all the samples of the present investigation. An attempt is made to evaluate the nature and magnitude of the lattice contribution to the elastic moduli analytically. Further, the Youngs moduli of all the superconductors of the present investigation are found to vary as T 4 at low temperatures ( T Φ D /50) and at high temperatures T .

The relationship between the porosity and elastic moduli of the Bi-Pb-2212 high-Tc superconductor

A series of Bi-Pb-2212 high-Tc superconductors, having different porosities has been prepared by the solid-state reaction method. After the usual characterization of the samples by XRD, AC susceptibility, SEM, bulk-density and porosity studies, longitudinal (Vl) and shear (Vs) wave velocity measurements have also been undertaken by the pulse-transmission technique. Using the experimental values of Vl and Vs, the Youngs moduli (E) of all the samples have been calculated and are later corrected to zero porosity by employing a number of theoretical equations. The corrected moduli are compared with the literature value, which itself has been arrived at by averaging the available single-crystalline stiffness constants. It is concluded from the results that among all the formulae, Mackenzies equation and that proposed by the authors of the present investigation are in closest agreement with the literature value.

Elastic behaviour of BiPbAgCa2CuO3 superconducting composite materials

Abstract A series of Bi1.7Pb0.3Sr2Ca2Cu3O10-AxBy where A  Ag and B  Ca2CuO3 high-Tc superconductor composite materials have been prepared by the solid-state reaction method. After usual characterization by X-ray diffraction, the bulk density, porosity, electrical resistance, and a.c. susceptibility of the samples were determined and the longitudinal (V1) and shear wave (Vs) velocities were measured by the pulse transmission technique. Using these data, the elasticity parameters such as the Youngs modulus (E), rigidity modulus (G), Debye temperature (OD) and Poissons ratio (σ) have been evaluated. The variation of elastic moduli with dopant concentration has been explained in terms of binding forces between various atoms of the superconductor.

Elastic anomalies in sodium-doped Bi-2212 single-phase HTSC materials

A series of sodium-doped Bi-2212 single-phase high- superconductors have been prepared by the solid state reaction method. After the usual characterization by XRD, SEM, bulk density porosity etc, ultrasonic longitudinal velocity studies were undertaken over the temperature range 80 - 300 K using the pulse transmission technique. In the temperature range 300 - 230 K, in contrast to normal solids, the velocity of Bi - Na(0) and Bi - Na(3) samples is found to decrease with decreasing temperature (elastic softening) followed by a velocity maximum at around 150 K for all the four samples, signifying the presence of lattice instabilities. Plausible explanations are given for both the elastic softening and the velocity maxima on the basis of microstructure and ordering readjustments of the oxygen atoms among the Cu - O planes of the samples.

A theoretical study of anomalous elastic behaviour of Na-doped Y-1-2-3 high-Tc superconductor

In view of enhancing the T, value of Y-l-2-3 samples, Na is doped at the yttrium site. Ultrasonic longitudinal velocity studies have been carried out over the temperature range 80-300 K by the pulse transmission technique. The Young’s modulus, leaving the temperature region 200-l 10 K, is found to increase continuously with decreasing temperature. However, in the temperature region 200-I 10 K, the modulus of elasticity is found to increase in an unprecedented manner exhibiting a peak value at 140 K. A qualitative explanation based on the ordering of oxygen atoms is given. The elastic moduli of this sample at various temperatures have also been generated by using Watchman’s and Varshni’s theoretical equations by employing a standard least-squares method and are found to predict the values of C, ( T) - C,, (0) easily up to 50 K.

Anomalous elastic behaviour of Bi-Pb-Ag composites☆

Abstract Bi-Pb and Bi-Pb-Ag superconducting composites have been prepared by the solid state reaction method. After the usual characterization, ultrasonic longitudinal velocity and attenuation studies have been undertaken over a temperature range 80–300 K by the pulse transmission technique. In contrast to normal solids, the ultrasonic velocities of both the samples in the temperature range 200-100 K are found to decrease with decreasing temperature (softening), followed by a velocity maximum. The samples are also found to exhibit longitudinal attenuation peaks at temperatures = 260, 160 and 120 K. An attempt has also been made to verify whether the Wachtmans equations can theoretically explain the low-temperature behaviour of the Youngs modulus of these materials. A qualitative explanation for both the phenomena of softening of velocity as well as the occurrence of attenuation peaks is given.

The effect of porosity on elastic moduli of YBa2Cu3O7−δ high Tc superconductors

Abstract The ultrasonic velocity measurements of a series of YBa2Cu3O7−δ samples having different porosities, have been undertaken by the Pulse Transmission Technique. Using the experimental values of Youngs moduli, the void-free elastic moduli of all the samples have been arrived at theoretically by assuming that the shape of the pores is spherical. For this purpose, different types of equations viz. linear, non-linear, exponential and power types have been used. Later the computed values have been compared with the polycrystalline Youngs modulus value obtained by Voigt-Reuss-Hill (VRH) scheme using the single crystalline elastic stiffness constants available in the literature. It has been concluded that the Youngs modulus values obtained by the power equations are more accurate in predicting the void-free state of moduli of Y-Ba-Cu-O samples of present investigation, when compared with those obtained by other types of equations.

ULTRASONIC ANOMALIES IN Y–Ba–Cu–O HIGH-Tc SUPERCONDUCTING MATERIALS

The longitudinal and shear wave velocities of Y–1–2–3 superconducting materials prepared by the sol–gel method have been measured by the pulse transmission technique over a temperature range of 80–300 K. The longitudinal velocity of both the samples, in general, is found to vary in an anomalous way. The observed behaviour has been explained qualitatively on the basis of a model.

LOW TEMPERATURE ELASTIC BEHAVIOR OF POTASSIUM-DOPED Bi-2212 SINGLE PHASE HIGH Tc SUPERCONDUCTORS

A single phase Bi-2212 and two potassium-doped Bi-2212 samples have been prepared by the solid state reaction method. After the usual characterisation, the ultrasonic longitudinal velocity measurements of all the samples over a temperature range of 80–300 K were undertaken by the pulse transmission technique. The longitudinal velocity of the undoped sample is found to vary in an anomalous way followed by a maximum at 140 K. On the other hand, in the case of the two K-doped ones, the velocity is found to vary with temperature in a normal way up to 120 K followed by a maximum at 105 and 100 K respectively. A qualitative explanation for the observed behavior in all the three cases is given.