• Volume 2, Issue 4

      November 1980,   pages  221-294

    • Physics of silver halides and their applications

      D N Bose P A Govindacharyulu

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      Silver halides are notable for exhibiting both ionic and electronic conductivity which is responsible for their widespread use in photography. Due to differences in ionicity and bonding, their properties vary from the highly ionic silver fluoride to the more covalent silver iodide. These fundamental properties such as defect formation energies, carrier mobilities and band-structures, particularly of silver iodide, have only recently been determined. One interesting finding is the presence of high surface fields, caused by differences between interstitial and vacancy formation energies, which results in separation of photo-generated electron hole pairs. This throws new light on the photographic process and on the suitability of silver bromide as a photographic material.a-AgI is also the first fast ion conductor to be discovered. Structural instability, low defect formation energy and optimum cation size are responsible for this phenomenon but existing models fail to explain all the experimental results. Recent microwave conductivity and neutron scattering experiments which provide fresh insight are discussed.

    • Development of a roller quenching apparatus for the production of amorphous phases

      S T Lakshmikumar R M Mallya E S R Gopal

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      The details of an apparatus designed to produce amorphous phases by rapid quenching from the melt are described. A drop of molten material is squeezed between two copper rollers rotating against each other at 5000 RPM and a thin foil of the material is produced. The system produces cooling rates of the order of 105 K/sec. Details of the development and construction are mentioned.

    • Structural and dielectric studies of some niobate-titanate systems

      A M Varaprasad A L Shashi Mohan D K Chakrabarty A B Biswas

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      Ferroelectric ceramics of the general formulax (Li1/2 La1/2 TiO3) (1−x) (LiNbO3) have been found to have tungsten bronze (TTB) and ilmenite type structure in the ranges 0·8≤x≤1·0 and 0≤x≤0·2 respectively. The system is biphasic in the other ranges. All single phase materials exhibit dielectric anomalies in the temperature range 300–600 K. The high dielectric constant of the TTB phase is explained as due to the existence of localised rotational excitons arising from a large number of defect structural units and their coupling with one of the low frequency lattice modes. Information about the lattice modes to which the rotational excitons are coupled has been obtained from Raman spectra.

    • Studies on thick films of photoconducting cadmium sulphide

      D P Amalnerkar M S Setty N R Pavaskar A P B Sinha

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      Highly photosensitive films of CdS have been prepared using the thick film technique. The films obtained from the composition containing CdS-100, CdCl2−10 and CuCl2−0·05 parts by weight (reacted at 500° C) are found to give the best photosensitivity on firing at 600° C. The ratio of light to dark current ∼108–109 which is considerably higher than what is reported for thin films, single crystals and sintered layers. A strong chemisorption of oxygen is found to be responsible for high photosensitivity. The spectral response for doped CdS film is similar to that obtained for thin films, single crystals and sintered layers and also shows a red shift with increasing Cu concentration. However, the undoped CdS has a broad spectral response at room temperature ranging fromλ=550 to 690 nm; unlike the thin films and single crystals which give a sharp peak atλ=510 to 520 nm. A probable explanation has been suggested for this type of behaviour.

    • Effect of an electric field on the oxidation of aluminium at room temperature

      K Bhavani V K Vaidyan

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      Effect of an applied electric field on the oxidation rate of vacuum deposited aluminium thin films were studied at room temperature in the laboratory atmosphere. Above the critical field, a negative potential applied to the film enhanced the rate of oxidation, while a positive field retarded the rate. This suggests an oxidation mechanism involving cationic diffusion. The observed change in the rate of oxidation for an applied field agrees with the Mott-Cabrera theory.

    • Charge-transfer satellites in the x-ray photoelectron spectra of 3d° and 4d° metal oxides

      M S Hegde

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      O(2p)-metal (d) charge-transfer satellites in core level spectra of 3d° metal ions in CaO, Sc2O3, TiO2 and V2O5 and 4d° ions in SrO, Y2O3, ZrO2 and Nb2O5 have been reported. A systematic increase in the satellite separation is noticed going from CaO to V2O5 and from SrO to Nb2O5.

    • Self-diffusion considerations inω-phase of hard HCP metals

      M N Vijayakumar

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      Hard hcp metals like Zr and Ti undergoω-phase transformation under high pressures. Ample experimental evidence to suggest that this phase can be retained under ambient conditions of temperature and pressure is also available. The structural implications of the phase, consisting of two inequivalent lattices with different co-ordination numbers and nearest neighbour distances, impose certain restrictions on atomic migrationvia lattice vacancies. The present paper describes the configurational problems associated with vacancies, and also gives a calculation for correlation factor for self-diffusion taking into account the lattice anisotropy of theω-structure.

    • Optical constants of WSe2 single crystals

      M K Agarwal P D Patel O Vijayan

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      The optical constantsn andk for WSe2 single crystals have been determined from the reflectivity measurements on basal plane of the crystal at wavelengths 5460, 4360 and 4050 Å. From the variation ofk with frequency of radiation, the optical energy band gapEo of the crystal has been evaluated.

    • Electroless deposition of Alpha-PbO2 and Tl2O3

      R N Bhattacharya P Pramanik

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      A chemical method for electroless deposition of thin film ofa-PbO2 and Tl2O3 has been developed. The deposition has been performed by ammonia, persulfate ion and metal ions at a higher temperature. The electrical resistance, mobility and carrier concentration have been measured with variation of thickness of the films. Optical absorption spectra reveal the band edges which are 1·7 eV and 1·95 eV ofa-PbO2 and Tl2O3 respectively.

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