• K L Chopra

      Articles written in Bulletin of Materials Science

    • Melt-spinning technique for preparation of metallic glasses

      R C Budhani T C Goel K L Chopra

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      Rapid solidification of metallic melts at a rate ≳105 °K/sec has gained considerable interest for casting non-equlibrium crystalline structures in general and metallic glasses in particular. Of the variety of techniques used for rapid solidification, melt-spinning and melt-extraction are widely used. This paper describes the design and development of a melt-spinning technique in our laboratory. Optimization of spinning parameters for smooth and continuous ribbons is discussed. Glassy ribhons of Cu-Zr and Pd-Ge alloys have been made under optimized conditions. A brief account of some of the structural and electronic properties of these glasses is illustrated.

    • Photon, electron and ion beam induced physical and optical densification in chalcogenide films

      K L Chopra L K Malhotra K Solomon Harshavardhan S Rajagopalan

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      Irradiation of 80° deposited Se-Ge films with band gap photons, 8 keV electrons and 50 keV He+ ions produces, primarily irreversible red shift of the absorption edge and major changes in doping and electrochemical adsorption processes. These changes have been attributed predominantly to the radiation-induced thickness and hence volume contraction resulting due to the physical collapse of the low density columnar microstructure of the obliquely deposited films. The farir and Urbach tail studies reveal an enhanced strength of electron-phonon coupling, in obliquely deposited films having a very porous columnar microstructure, compared to normally deposited films. It has been shown that this enhanced electron phonon coupling is conducive to large thickness contraction and associated changes. These changes have been used in reprographic and lithographic applications. A correlation has been established between the thickness contraction and lithographic parameters. The possibility of generating both positive and negative relief patterns on the same film by controlling the etching time has been demonstrated.

    • a-Si:H/CuInS2 heterojunctions for photovoltaic conversion

      Satyendra Kumar A N Tiwari O S Sastry D K Pandya K L Chopra

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      Heterojunctions of hydrogenated a-Si films prepared by r.f. sputtering with spraypyrolyzed CuInS2 films have been studied. Capacitance-voltage measurements establish the formation of abrupt heterojunction. The barrier height varies from 0·26 to 0·55 V as the resistivity of CuInS2 film decrease from 1·5 × 103 to 65 Θm. These junctions exhibit photovoltaic behaviour withVoc = 220 mV andIsc = 0·20 mA/cm2.

    • Flash-evaporated thin films of CuInSe2

      R D Pachori A Banerjee K L Chopra

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      Flash-evaporated technique has been developed for deposition of CuInSe2 thin films. A control over the stoichiometry and chemical composition of the films has been obtained by varying the deposition parameters. Single phase chalcopyrite structure films with optical gap ∼ 1·15 eV have been obtained. The electronic properties of the films have been tailored for solar cell applications.

    • R.F. magnetron sputtered tungsten carbide thin films

      P K Srivastava V D Vankar K L Chopra

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      Thin films of tungsten carbide have been deposited on stainless steel substrates held at 500°C by r.f. reactive magnetron sputtering in two different modes of introducing argon and acetylene gases called normal and high rate mode. A single phase fcc-WC is formed in the normal mode whereas a mixture of A-15-W3C, hexagonal-WC and graphitic- and diamond-carbon is found in the high rate mode. A microhardness value as high as 3200 kgf/mm2 (as compared to the bulk value of 1800 kgf/mm2) is obtained in the film deposited by normal mode.

    • Cobalt oxide-iron oxide selective coatings for high temperature applications

      C S Uma L K Malhotra K L Chopra

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      Cobalt oxide-iron oxide coatings on stainless steel have been prepared by spray pyrolysis technique. These coatings have absorptance (α) = 0·94 and emittance (ε100) = 0·20 which are much better values than for cobalt oxide or iron oxide alone on stainless steel substrates. The coatings have been found to be stable for temperatures upto 400°C.

    • Dielectric behaviour of ZnO-based ceramic semiconductors

      Subhash C Kashyap K L Chopra Bharat Bhushan

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      The effect of composition, sintering parameters, frequency and temperature on the dielectric parameters of ZnO-based ceramic semiconductors (cersems) having small amounts of Bi2O3, Sb2O3, CoO, MnO2, La2O3 and/or Cr2O3 has been investigated. The unusually high dielectric constant of these composites, arising due to a two-phase microstructure has been explained on the basis of a depletion layer model. The agreement in values of barrier height and donor concentration calculated fromC−2V plot, Hall measurement and I-V characteristics of these cersems supports the validity of barrier and depletion layer models. The depletion and inter-granular layers are estimated to be nearly 102 nm and 1–2 nm respectively. The observed variation of dielectric constant/capacitance with sintering parameters and temperature of measurement has also been explained on the basis of simplified microstructure and depletion layers. The loss peak (fmax) observed at 300 kHz remains practically unaltered with change in composition and sintering parameters. The observed dielectric dispersion in the range 102−106 Hz, exhibiting multiple relaxation times and activation energy of relaxation process as 0·36 eV, has been explained on the basis of Debye-type relaxation process originating due to trapping/detrapping and possibly due to scattering of carriers in the depletion regions.

    • Preparation and degradation behaviour of Bi-cuprate superconductors

      Balvinder Gogia Subhash C Kashyap D K Pandya K L Chopra

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      The role of preparation conditions and the effect of addition of Pb in Bi-Sr-Ca-Cu-O (BSCCO) superconductor, on theTc,Jc and grain orientation have been studied. Calcination at a temperature higher than the melting point of Bi2O3 after the prereaction at 800°C leads to formation ofc-axis oriented nearly single phase material. The presence of lead yields a nearly highTc phase (2223) exhibiting a maximum transition temperatureTc=110 K and ΔTc=2 K. The addition of lead and grain orientation together result in an increase inJc by nearly two orders of magnitude. The degradation behaviour was studied in terms of changes inTc,Jc and structure on exposure to atmosphere up to 250 days. Samples with 0.6 Pb were found to be superconducting withTc=97 K even after 250 days. The highTc phase was stable against degradation. Our studies indicate that the degradation of leaded BSCCO is a surface phenomenon rather than bulk phenomenon.

    • Effect of laser irradiation on the superconducting properties of high-Tc SmBa2Cu3Ox

      B K Roul A K Pradhan V V Rao P Bhattacharya P Pramanic V R Kalvey D N Bose K L Chopra

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      Studies of the effect of high power laser (Q-switched Ruby laser, 694 nm, 30 ns) irradiation on the critical current density (Jc) and magnetic hysteresis at 77K and temperature variation of microwave induced d.c. voltage on SmBa2Cu3Ox ceramic samples have been performed. Irradiation did not substantially changeTc but caused a strong increase inJc and magnetic hysteresis at 77K. The microwave-induced d.c. voltage at 77K showed appreciable decrease after irradiation. SEM studies showed grain growth due to sintering which improves the interconnectivity among the superconducting grains. These are attributed to physical densification and consequent reduction in the number of weak links. The increase of magnetic hysteresis after laser irradiation is presumably connected with the creation of defects which act as pinning centres. Thermal modelling suggests that on irradiation the surface melts up to a depth of 1µ and laser-induced evaporation occurs at energy density of 2·5 J/cm2.

    • Creation of nanostructures on nickel thin films by STM

      V Srinivas M V H Rao B K Mathur K L Chopra

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      A scanning tunneling microscope (STM) has been used to create nanostructures on nickel thin films. A systematic procedure for the creation of such structures has been developed. Various possible mechanisms involved in such creations have been discussed. Based on our observations, a field-induced suction of the plastically deformed surface has been proposed.

    • My experiments with thin films—the nanostate of matter

      K L Chopra

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      The black art of thin films exhibiting whimsical properties and thus called ‘the fourth state of matter’ enticed the author to a challenge to unravel the mysteries of matter createdab-initio on a nanoscale. Nucleation and growth studies of evaporated and sputtered films (both polycrystalline and epitaxial) and the effects of kinetic energy, angle of incidence, electric field, substrate agitation, formation of metastable and amorphous structures, etc led to exciting discoveries. The fields of amorphous metal alloy and semiconducting films were thus born. Electron and optical transport investigations on pure and doped metal, semiconducting, oxide and polymer films established a variety of new phenomena of which the mean free path effects and specular scattering of conduction electrons in epitaxial metal films, giant thermopower in quenched metal films, sputter quenching, giant photocontraction effect in amorphous Ge-chalcogenide films, enhancement of superconducting transition temperature in transition metal films, nanosize multi layer coating (the first known superlattices) are noteworthy. A variety of new deposition processes such as solution growth of polymer films, solution growth of chalcogenide and oxide films, spray pyrolysis, coprecipitation of multicomponent oxides (for thick films), activated reactive evaporation/magnetron sputtering were innovated. This lecture will share the excitement of the contributions by the author, his 60 Ph.D students and a number of post doctoral fellows and faculty members at the Thin Film Laboratory, IIT Delhi and the Microscience Laboratory at IIT Kharagpur.

    • Surface structure of silver thin films on In2O3:Sn and Al2O3

      S Kasiviswanathan P S Asoka Kumar B K Mathur K L Chopra

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      Surface structure of thin silver films (200 Å) on two technologically important films, indium tin oxide (ITO) and aluminium oxide, has been studied using scanning tunneling microscope. ITO films were prepared by reactive electron beam evaporation. Aluminium oxide films were prepared by oxidizing 2000 Å thick aluminium films evaporated on to H2 terminated single crystal silicon substrates. The surface structure of silver on ITO and aluminium oxide appeared to be same and was characteristic of Stranski-Krastanov type. The observed asymmetry in the island shape was attributed to the anisotropic nature of the strain fields surrounding the nucleation centres.

    • Detection of surface lattice defects using line scan method

      M V H Rao B K Mathur K L Chopra

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      The presence of different kinds of surface lattice defects such as missing atom, interstitial atom, line defects, in graphite single crystal have been identified by using scanning tunneling microscope. These defects cause displacement of atoms from their mean position and lattice strain is introduced. By measuring the displacement of atoms from their mean position. lattice strain has been calculated. It is found that among single point defects, vacancies cause maximum lattice strain.

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