S. C. Mathur

Studies on triglycine sulphate crystals doped with nitroanilines

Single crystals of triglycine sulphate (TGS) doped with o-, p- and m-nitroanilines and L-alanine have been grown by the method of temperature lowering. Their dielectric permittivity and electrical conductivity have been studied as a function of temperature. The observed increase in the Curie temperature due to p-nitroaniline as dopant is only 1.5 degrees C but at the expense of a nine-fold reduction in the maximum value of the permittivity. It has been suggested that nitroanilines enter the TGS lattice interstitially, in contrast to L-alanine which substitutes in place of the glycine molecules.

Optical absorption spectra and energy band gap in praseodymium borophosphate glasses

There has been some emphasis recently on the optical properties of glassy and amorphous semiconductors doped with small amounts of rare earth ions [1–8]. These glasses are potential materials for lasing action. This arises from the fact that the electrons responsible for the spectral and magnetic properties of rare earth ions are 4f electrons, which are very effectively shielded from interaction with external forces by the overlaying 5S and 5P shells. Therefore their energies and the energies of the f–f transitions are not likely to be significantly affected by external fields. Some features of the band structure of glasses can be understood by an analysis of the optical absorption edge. The electronic states of the glasses interact with the incoming radiation and give rise to optical absorption. Doping of V2O5–P2O5 glasses by different rare earth oxides does not show any significant change in optical ‘‘band gap’’ Eopt (it varies from 2.20 eV to 2.37 eV only) [7]. This slight difference in calculated Eopt values for different dopants is a consequence of small differences in the observed position and slope of the absorption edge. A survey of the literature indicates that rare earth oxide glasses have not been studied for their optical band gap and related properties [9–11]. Ahmed et al. [4] studied the optical energy gap of praseodymium phosphate glass. They analysed their data on the basis of modern theoretical models of amorphous solids. There are no other reports on rare earth oxide glasses. Thus it was of some interest to study the nature of the optical energy gap of rare earth doped glasses and to see the effect of composition on the position of the absorption edge and the value of the so-called optical energy gap. The present communication reports such studies on Pr-doped B2O3– P2O5–BaO glasses. Borophosphate glasses of final composition B2O3–P2O5–BaO in the ratio of 40, 45 and 15 mol % were prepared from analytical reagent grades of H3BO3, NH4H2PO4 and BaCo3 in 10 g batches. The praseodymium oxide added to the glass was Pr6O11 (99.99%). The chemicals were obtained from E. Merck (India) Ltd, Glaxo India Ltd and Aldrich Chemical Company, USA, respectively. Batches containing 0.1, 0.3, and 0.5 mol % of Pr in the base glass materials were mixed in an agate pestle mortar for 30 min, and were thermally treated in an alumina crucible up to 1000 8C in steps of 100 8C starting from 300 8C. A hold time of 6 h was given at each temperature step. Homogeneity of the melt was ensured by stirring the melt with an alumina rod from time to time. The melt was quenched by pouring into a rectangular-shaped depression in a copper plate. Glasses contained no crystalline phases as revealed by an X-ray diffractogram of the specimen. The glass samples were polished using cerium oxide power. The absorption spectra were recorded using Perkin Elmer Lambda-4B UV/VIS Spectrophotometer in the wavelength range 200–900 nm. Four sharp absorption peaks at 445.5, 470.5, 482.8 and 591 nm were observed in the visible region. Fig. 1 shows the optical absorption spectra for several compositions of Pr-doped borophosphate glasses. On the basis of a comparison of the present spectra with the work already reported [1–3], the absorption peaks observed at 442.0, 469.6, 481.8 and 587.8 nm may be assigned as arising due to the transitions H4? P2, H4? P1, H4? P0 and H4? D2, respectively. This assignment of peaks similar to the ones existing in the literature for different materials containing Pr ions, is justified if we remember that f–f transitions are only weakly affected by the environment of the praseodymium ion [2, 12, 13]. This also explains the observed sharpness of the peaks. In crystalline materials, the Eopt value is directly obtained from the absorption. However, for amorphous materials in general and oxide glasses in particular, the optical absorption at the fundamental edge may be used to estimate Eopt values using a theory due to Davis and Mott [14]. The absorption in amorphous semiconductors obeys a quadratic relation for the inter-band non-direct transitions:

CdS and AgBr sensitized Eriochrome Black T (EBT) dye solar cells

Abstract The photovoltaic and rectification properties of CdS- and AgBr-sensitized Eriochrome Black T dye solar cells have been studied. The dependence of the short-circuit current and the open-circuit voltage on light intensity and electrode material are examined and the variations with electrode material are explained on the basis of the built-in potential developed at the metal-semiconductor interface. Conversion efficiency, fill factor, diode factor and reverse saturation current are also calculated for each cell.

Dielectric and pyroelectric studies of TGS, TGFB and TGS-TGFB system

Abstract Dielectric permittivity and pyroelectric coefficient of mixed crystals of TGS and TGFB are reported. It has been shown that L-alanine doped mixed crystals can be good elements for IR detectors.

Electric conductivity of doped triglycine sulphate crystals

crystals. Single crystals of pure and doped TGS crystals were grown from seeds by slow cooling of a saturated solution of glycine, constituent acid and dopants in the stoichiometric ratio, at an initial rate of 0.2°C day-~ to a maximum of 1.0°Cday ~, under similar growth conditions. Transparent, flawless crystals of high optical quality were cleaved and silver electroded onto (0 1 0) faces so as to make the full face electrodes. The conductivity was measured using a three terminal sample holder by a Keighley 610C TABLE I Values of d.c. conductivity at room doped TGS crystals electrometer. The geometrical dimensions of the samples were chosen so that in the region of the phase transition, the readings could be taken at the lower section of the electrometer. Currents were measured one minute after the voltage switch on. The voltage used in the measurements was 100 V cm i. The ohmic contact of electrodes was verified by the linearity of the currentvoltage characteristics. During the measurements, the temperature was stabilized to within _+ 0.05 ° C. The values of conductivity of pure and doped TGS crystals, at room temperature and transition temperature are collected in Table I. Triglycine sulphate is an association compound, therefore, amino-acid glycine is expected to play a definite role in the conduction mechanism. It is possible that there is mixed conduction in these crystals, i.e. electronic, protonic and ionic conduction. The possibility of electronic conduction as in the case of protienes [5] can be ruled out. Because the band gap of glycine is

AC conductivity and dielectric permittivity of TGS and urea doped TGS crystals at microwave frequency

Abstract The complex dielectric permittivity of pure and doped Triglycene Sulphate (TGS) crystals have been studied at microwave frequency(10GHz). Measurements have been carried out from room temperature to about 60°C, a temperature well above the curie temperature(49.1°C) for TGS. The results of measurements of the real and imaginary parts of the permittivity have been compared with those reported by earlier workers. Our results do not fully agree with the results of earlier workers. The behavior of our pure TGS crystal comes close to the behaviour of TGS crystals reported by Hill and Ichiki and is similar to the behavior of Cu++ doped TGS crystal studied by Bolko et.al. and the behavior of our urea doped TGS crystal is similar to the behavior of pure TGS crystal as reported by Pereverzeva and others.

Electrical conduction mechanism in crystal violet dye sensitised with AgI

The theory of dark conduction in dyes assumes thermal excitation from the ground state to an excited state, followed by trapping of the hole near the electrode. Electrical conductivity studies on crystal violet (CV), an organic dye of industrial importance, have been carried out in both dark and light conditions. The present paper deals with the effect of field, temperature and electrode materials on the electrical conduction in CV as well as in CV sensitised with AgI. These studies suggest that conduction is mainly due to the charge carriers injected from the electrode into the dye (Schottky-Richardson mechanism) and the electron transfer mechanism. Photo and dark activation energies in the sensitised dye have been calculated.

ac susceptibility of PbxBi2−xSr2Ca2Cu3O10 high Tc superconductors sintered for different durations (abstract)

ac magnetic susceptibility measurements is one of the processes by which the superconducting properties of mixed oxide superconductors can be characterized. The study of the superconductors at an externally applied magnetic field predicts the existence of two superconducting transitions due to: (i) the one reflecting the intragrain properties and (ii) the other describing intergrain interactions which might be assumed as due to Josephson tunneling. In this abstract, we report our work on the ac susceptibility measurements in PbxBi2−xSr2Ca2Cu3O10 superconductors to study the real part of χ’ as a function of temperature, Pb concentration, and sintering time. The results have been compared with the x‐ray diffraction (XRD) results. The material preparation and the characterization techniques used would be discussed. The XRD and the susceptibility measurements indicate that too high Pb concentrations and too low sintering time does not seem to improve the high Tc phase. The weak link coupling phase determined ...

Ferroelectric properties of (Pb-Bi-Sr-Ca-Cu-O) ceramics in normal and superconducting states

Abstract Ferroelectric hysteresis studies have been carried out on the pellets of Pb0.6Bi1.4Sr2Ca2Cu3O10 superconductor selected out of sixteen samples prepared. The results suggest that the sample behaved like a normal dielectric ceramic between room temperature and 211 K. Below 211 K, its behaviour has been interpreted as that of a relaxor ferroelectric. At 118 K, the (2223) phase is known to become superconducting. At this temperature, a very thin hysteresis loop was observed which has been attributed to low temperature superconducting (2212) phase present in the material. This thin loop also disappeared at 50 K. The studies show that even for high-Tc superconductors, Matthiass suggestion that superconductivity and ferroelectricity are mutually exclusive is valid.