V. Hari Babu

Compositional analysis and depth profile studies on undoped and doped tin oxide films prepared by spray technique

Tin oxide (TO) and fluorine doped tin oxide (FTO) thin films of different thicknesses and varying impurity concentrations have been prepared by spray technique. The XRD results are in good agreement with ASTM data. From the XPS (X-ray photoelectron spectroscopy) studies, the presence of tin and oxygen in tin oxide has been confirmed and also the ratio of Sn/O has been found out. Chemical analysis of FTO films has been carried out by SAM (Scanning Auger Microprobe) studies and from the peak to peak height (PPH) calculations, Sn, O and F concentrations have been determined. The compositional analysis of FTO films at different depths (depth propfiles) has been studied. The results have been explained.

Microhardness studies in alkali halide mixed crystals

Microhardness measurements done in KCl, KBr and in different compositions of KCl-KBr mixed crystals show that it varies nonlinearly with composition. In order to investigate the nature of defects, several techniques such as etching, ionic conductivity and dielectric loss have been employed which showed that the mixed cristals of KCl-KBr are more defective, containing a high concentration of dislocations, low-angle grain boundaries and vacancies as compared to the end products KCl and KBr. These imperfections appear to be responsible for the nonlinear variation of microhardness in mixed crystals. The microhardness studies also revealed that the difference in size of the ions constituting the mixed system are responsible for the internal strains which in turn give rise to imperfections affecting the microhardness of mixed crystals.

Growth and characterization of alkali halide mixed crystals

Abstract The study of the physical properties of alkali halide mixed crystals has been a subject of wide interest in the recent past. The main aim of this review is to provide a survey of the current state of knowledge about the nature of imperfections present and their role in understanding various properties associated with alkali halide mixed crystals. An attempt has been made to distinguish different types of mixed crystals, the conditions for the formation of a mixed crystal, the local strains that arise in the lattice due to the difference in the size of the ions that constitute the mixed crystal. The results obtained from various studies such as ionic conductivity, dielectric loss, microhardness, radiation hardening, colour centres, thermoluminescence, optical absorption etc. on alkali halide mixed crystals are presented. The non-linear variation of electrical conductivity, microhardness, half widths of many colour centre bands with composition have been discussed in terms of the concentration of various defects present in them. The aspects of the problem described include: 1. a) Dislocation density and distribution studies. 2. b) Influence of dislocations and grain boundaries on transport properties of these crystals. 3. c) Role of ionic size on microhardness. 4. d) Lesser rate of hardening in mixed crystals due to irradiation. 5. e) Stability of colour centres in mixed crystals in comparison with the end products, finally the present position and out look are summarized.

Microhardness studies in ammonium halide crystals

Microhardness studies of NH4Cl (pure and doped), NH4Br and alkali halide crystals are presented. The hardness of ammonium halides is found to be less as compared to alkali halide crystals. Doping NH4Cl crystals with copper (Cu2+) is found to increase the hardness enormously and the results obtained with various concentrations of copper are presented. The results have been analysed and the various factors contributing to the increase in hardness at lower loads have been discussed.

Effect of rare-earth Sm3+ substitution on the superconducting properties of the Bi1.7Pb0.3Sr2Ca2−xSmxCu3Oy system

Abstract The superconducting properties of the Bi1.7Pb0.3Sr2Ca2−xSmxCu3Oy system have been investigated by X-ray diffraction, electrical resistance, AC susceptibility and DC magnetization. Substitution of diluted quantities of Sm3+ for Ca2+ is found to change drastically the superconducting properties of the system, despite the weak paramagnetic nature of the Sm3+ ion. X-ray diffraction results show that the volume fraction of the high-Tc (2:2:2:3) phase decreases and that of the low-Tc (2:2:1:2) phase increases as the samarium concentration increases from x=0 to 0.05. At higher samarium concentrations, i.e. for x=0.075 and 0.1, only the low-Tc phase exists. AC susceptibility measurements (χ′ versus T) show that Tc onset does not change up to x=0.02, and then decreases down to 100 K for x=0.1. The loss peak (χ″ versus T) decreases from 108 to 104 K for x=0 to x=0.02, while for higher Sm concentrations, the χ″ peak decreases more significantly. The same trend was observed by resistivity measurements and can be explained on the basis of granular superconductivity. Magnetization measurements show that the superconducting volume (slope d M d H ) decreases with increasing samarium concentration. These results show that the Ca2+→Sm3+ substitution changes the hole carrier concentration, which in turn lowers Tc and decreases the volume fraction of the superconducting phase.

Thermoelectric power studies on the Sm substituted BPSCCO (2223) superconductors

Abstract Measurements of the thermoelectric power have been carried out on Bi 1.7 Pb 0.3 Sr 2 Ca 2−x Sm x Cu 3 O y (0.0⩽ x ⩽2.0) specimens between 77 K and 300 K. The hole-carrier concentrations were calculated from the room-temperature thermopower values and it was found to decrease with increasing Sm concentration. The temperature variation of the thermopower is similar for all the samples and has indicated a two-band conduction. The results also have suggested an increased interaction of electrons and holes with phonons at low temperatures. The thermopower has attained a maximum value just above the transition temperature and thereafter decreased with increasing temperature. The thermopower value has also undergone a sign change at a temperature which varied systematically with Sm concentration. The results are presented and discussed.

Systematics of the physical properties of compounds

A study of systematics was carried out on compounds by replacing Ca with Nd ions. The dependence of on structural parameters, oxygen content and hole carrier concentration was observed as a function of Nd concentration. X-ray diffraction patterns show the single phase (2212) nature of the samples. Lattice parameters have been calculated and the results show an increase in a-axis with a simultaneous decrease in c-axis. Oxygen content was estimated from the iodometric titration method and was shown to increase with x. The measured (0) and (onset) obtained from resistivity and susceptibility data were found to decrease with increasing Nd concentration. Vacuum annealing was found to improve . The materials exhibited superconductivity in the concentration range, and for higher concentrations of Nd they became semiconducting. In the semiconducting region, the conduction was found to be due to the 3D variable range hopping (VRH) mechanism. The initial susceptibility dM/dH relating to the superconducting volume, has shown a continuous decrease with increasing Nd concentration. The hole carrier concentration was computed from thermoelectric power data and found to decrease with increasing Nd ion concentration. The results are presented and discussed in comparison with those obtained from system.

Formation of Beaks at Dislocation Sites in Sodium Chloride Crystals

Beaks, solution channels, etc., can be obtained on NaCl cleavage faces by an etchant consisting of methanol and cadmium chloride. The beak formation is described in detail and the mechanism for beak formation is discussed. Etching parameters, such as the concentration of the poison and understuration of the etchant, have been varied, and their effect on the beak formation is studied. At higher concentrations of the poison, as well as at low undersaturations of the etchant, beak formation was found to be predominant. Vigorous agitation of the etchant was found to suppress the beak formation. Beaks are always associated with the aged dislocations. The etch pits due to fresh dislocations do not have beaks, and therefore it is concluded that segregated impurities at the aged dislocations help in the rapid dissolution along the dislocations. If the dissolution rate along the aged dislocation VAD exceeds the velocity of steps VS across the surface, beaks can be formed. Etch pits associated with the beaks appear...

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.

Effect of Ca2PbO4 on the formation of the (2223) phase in the BiPbSrCaCuO system

Abstract The effect of excess Ca2PbO4 on the superconducting properties of Bi1.7Pb0.3Sr2Ca2Cu3OyBx ( B ≡ Ca 2 PbO 4 ) is investigated through X-ray diffraction, resistivity and a.c. magnetic susceptibility measurements. The Xpray diffraction results show that the volume friction of the low T c (221s) phase decreases and that of (2223) phase increases with the addition of Ca2PbO4. For x = 0.6 and 1.5, only the (2223) phase exists. At higher Ca2PbO4 concentrations, i.e. for x = 3.0 , the low T c phase again appears along with the high T c phase. Diffraction peaks appear at d = 2.02, 2.814 and 2.85 , which are characteristic peaks of Ca2PbO4, and their intensities increase with increasing x , indicating that Ca2PbO4 exists in the lattice in the same form. However, the T c (0) values decreased gradually from 107 to 98 K with increasing Ca2PbO4 concentration. Compared with the undoped sample, the width of the transition δT c is broad for impurity-added samples and reaches a maximum value for x = 3.0 . Pure and Ca2PbO4-doped samples showed two peaks in ξ″ vs. T curves and two steps in ξ′ vs. T curves. The first peak is close to the transition temperature and corresponds to the midpoint of the first step in the ξ′ curve. The second peak appears below 98 K and this peak maximum corresponds to the midpoint of the second step. The position of both sets of peaks remained almost the same up to x = 1.5 and shifted towards lower temperatures thereafter. The role of Ca2PbO4 in the growth of the (2223) phase and the mechanism of energy loss are discussed.