Volume 20, Issue 8
December 1997, pages 1029-1108
pp 1029-1038 December 1997
The electrical and optical properties of In2O3 films prepared at room temperature by activated reactive evaporation have been studied. Hall effect measurements at room temperature show that the films have a relatively high mobility 15 cm2v−1s−1, high carrier concentration 2·97 × 1020/cm3, with a low resistivityρ = 1·35 × 10−3 ohm cm. As-prepared film is polycrystalline. It shows both direct and indirect allowed transitions with band gaps of 3·52eV and 2·94eV respectively.
pp 1039-1048 December 1997
Lead niobates belong to orthorhombic symmetry and acquire tetragonal symmetry at 570°C. Some lead-based ceramic compounds have been synthesized and their structural and ferroelectric properties studied. Strontium, barium and potassium were used to reduce the transition temperature and to control the grain growth, thereby electrical properties. The transition temperature was found to decrease from 465°C to 282°C with dopants. The transition temperature was drastically reduced with the addition of barium. This can be attributed to the suitability of doping ion in determining the transition temperature and other piezoelectric constants. Powder X-ray diffraction studies were carried out and orthorhombic symmetry was confirmed. The lattice constants were calculated through diffraction patterns. The average grain size was measured from the scanning electron microscopy (SEM) pictures taken on fractured surfaces of the samples.
pp 1049-1058 December 1997
When milling micrometer thin Nd2Fe14B platelets, of an average 1–2 mm diameter, in toluene in a closed reactor, part of the toluene decomposes at the surface of the platelets and yields nascent hydrogen and carbon/low hydrocarbons. The hydrogen diffuses into the Nd2Fe14B platelets and the carbon forms a thin surface passivation layer of the platelets, forming the stable Nd2Fe14BHx,x ≤ 5, hydride at room temperature. On heating in a calorimeter, the hydrogen desorbs off the sample with a well-defined endotherm between 370 and 425 K. An N2 gas atmosphere, if used during the heating, facilitates the H-desorption process with the modified kinetic parameters. For example, the enthalpy of the H-desorption ΔH and the related activation energyEa have the measured values ΔH = 153 J/g andEa = 58·2 kJ/mol in argon and ΔH = 256 J/g andEa = 41·6 kJ/mol in N2. It is argued that N2 gas has a fast reaction with the H atoms desorbing off the thin sample platelets and forms NH3 gas with an instantaneous decrease of the total external gas pressure at the sample. This supports the fast desorption of H atoms in the sample with the modified desorption kinetics in N2 gas.
pp 1059-1068 December 1997
Lanthanum samarium oxalate (LSO) single crystals are grown in silica gels by the diffusion of a mixture of aqueous solutions of lanthanum nitrate and samarium nitrate into the test tube having the set gel impregnated with oxalic acid. Tabular crystals of LSO having well defined hexagonal basal planes are observed at different depths inside the gel. LSO crystals grown by this method are colourless and transparent. Laue transmission X-ray diffraction pattern of LSO reveals well defined spots with two-fold symmetry along the horizontal axis. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) support that LSO loses water around 120°C, and CO and CO2 around 350–450°C. The infrared (IR) absorption spectrum of LSO establishes the presence of oxalate (C2 O4)2− ions. Energy dispersive X-ray analysis (EDAX) confirms the presence of La and Sm in the sample. X-ray photoelectron spectroscopic (XPS) studies of LSO confirm the presence of La and Sm in their respective oxide states. An empirical structure for LSO has been proposed on the basis of these findings.
pp 1069-1077 December 1997
Implantations were carried out on gel-grown potassium dihydrogen orthophosphate (KDP) and those doped with magnesium oxide (MgO) single crystals using 100 MeV Ag+ heavy-ion beam of 15 UD 16 MV pelletron accelerator. To conduct a comparative study, measurements were carried out in the temperature range 243 K-403 K at frequencies ranging from 1 kHz-1 MHz on irradiated and nonirradiated nonlinear samples. It was observed that the mechanism of dielectric behaviour varied with frequency, temperature and ion irradiation. Further, implantation produced erratic variation in the conductivity both in the intrinsic and extrinsic regions, and also in the dielectric behaviour of the substance. The property of sensitive dependence on initial conditions, namely, chaos had set in after ion irradiation. However, the doping effect had not completely terminated the above transition, leading to chaos in the nonlinear medium.
pp 1079-1083 December 1997
Metal-semiconductor contacts are drawing increasing attention due to their potential for applications in devices and integrated circuits. Experimentally, lower barrier heights have been reported more often for metallic contacts onp-type semiconductors. Here we report our results regarding barrier height of zinc onp-type germanium. The result is discussed in light of the mechanisms which could reduce the pinning effect in such contacts which are normally thought to be responsible for the observation of low barrier heights onp-type semiconductors.
pp 1085-1087 December 1997
Top layer of spray pyrolyzedn-type CdS has been converted intop-type by diffusion of copper which resulted in the formation of homojunction. This is achieved by annealing CdS/Cu bilayer film. The nature of diffusion of copper atoms into CdS has been studied using auger electron spectroscopy (AES).
pp 1089-1096 December 1997
Thermal behaviour of gel-grown pure and mixed rare earth tartrates of yttrium and samarium is investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal behaviour suggests that the materials are unstable at lower energies and pass through various stages of decomposition, decomposing to respective rare earth oxides which remain stable on further heating. It is estimated that both pure yttrium and pure samarium tartrate crystals carry eight waters of hydration, while mixed yttrium samarium tartrate crystals carry six waters of hydration. Critical examination of TG and DSC curves shows that the initial decompositions are endothermic and the latter are exothermic. Thermal kinetics of these materials has been worked out using Horowitz-Metzger, Piloyan-Novikova and Coats-Redfern equations. Application of these equations to these materials yields values of activation energy, order of reaction and frequency factor which are in reasonably good agreement.
pp 1097-1102 December 1997
Pure lead germanate (LG), 0·1 wt% Nd+3-doped lead germanate (LG.1Nd+3), 0·2wt% Nd+3-doped lead germanate (LG.2Nd+3) were grown by controlled cooling of melt. Variation of d.c. conductivity was studied in the temperature range 293°K–523°K. Conductivity increases with increasing temperature, showing semiconducting behaviour for all the compositions under study. The activation energy in ferroelectric phase is larger than in paraelectric phase for all the compositions. The result suggests polaron hopping to be a significant contributor to conduction mechanism.
pp 1103-1108 December 1997
Microhardness anisotropy in single crystals of anthracene and phenanthrene has been observed by the Knoop indentation technique. The Knoop hardness variation with respect to the  direction shows opposite trends. Both these crystals have the same crystal structure and space group. The present observations are explained in terms of orientation and disposition of molecules in the unit cells.
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