• Volume 18, Issue 2

      April 1995,   pages  99-194

    • A new phenomenon: the transient metastable graphitization of alloyed white iron

      G I Silman A A Zhukov

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      In previous papers the case of stable-mottled irons has been analyzed. Hereunder a different case is looked into, when the mottled structure is a transient metastable state. Then the sequence of transformations during isothermal graphitizing annealing of some alloyed white irons can be summed up as follows: {ie99-1}

      In the transient mottled state the alloys have a much lower hardness and can be machined before the final heat treatment.

      A simplified theoretical approach to this phenomenon is being offered.

    • Spectroscopic characterization of polysilicon solar cells

      S B Manamohanan R K Garg S Parthasarathi M M Pradhan

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      Polysilicon ingots were made by float zone, using solar grade silicon (SOG-Si) prepared by acid leaching of metallurgical grade silicon (MG-Si). Impurity contents of the MG-Si and SOG-Si were analysed by emission spectroscopy and Fourier transform infrared spectroscopy. The main metal impurities Ca, Al, Fe, Cu, Mn, Mg, V, Ti and Zn and non-metal impurities C, O and B present in the two types of samples were determined. Elemental analysis for a knowledge of the impurity content is important to correlate its effect on the spectral response and diffusion length of minority carrier, and to determine solar cell process reliability. Spectral response and diffusion length for float zone silicon solar cell have been determined.

    • Thermoluminescence of cement

      R K Gartia S Ingotombi S Dorendrajit Singh

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      Cement is found to be a thermoluminescent material. The glow curves of ordinary portland cement obtained from two different sources are found to be qualitatively similar. White cement (J K Brand) exhibits intense thermoluminescence (TL) compared to Birla white brand. The emission of J K white cement seems to be mainly due to Mn impurities which enters the product through limestone, the major ingredient in the preparation of cement. The possible use of TL as a spectroscopic technique to characterize cement is discussed.

    • The rate of corrosion of concrete reinforcement and possibilities of its mathematical modelling

      V Živica

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      The paper describes a method for the mathematical modelling of steel reinforcement corrosion rate. This method is based exclusively on experimental results and expression of the influence of significant corrosion factors in the form of functional relations. The method takes into account the reality of the effects of corrosion factors, their contigency and complexity, and various circumstances occurring in practice. It represents one way towards the development of methods for the prediction of service life of reinforced concrete and structures.

    • EPR studies on molybdenum phosphates Mo2P4O15, NaMo3P3O16 and BaMo2P4O16 in the temperature range 300–4·2K

      B B Das G Constentin M M Borel A Grandin A Leclaire B Raveau

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      EPR studies of molybdenum phosphates, Mo2P4O15, NaMo3P3O16 and BaMo2P4O16 in the range 300–4·2 K was carried out. Delocalization of 4d1 electron on a Mo5+ site in the MoO6 octahedra occurs due to tunneling process. This gives rise to unresolved EPR lineshapes in all the cases except in Mo2P4O15. The slight indication of hyperfine structure in Mo2P4O15 is due to less feasibility of delocalization of the 4d1 electron, as MoO6 octahedron is connected through a PO4 group to only three octahedra as compared with higher numbers in the case of others. The negative g-shift is observed in all the cases due to positive sign of the spin-orbit coupling constant. The decreasing trend in giso-values in Mo2P4O15 and NaMo3P3O16 with decreasing temperature is observed, whereas in BaMo2P4O16 the giso-value increases with decreasing temperature.

    • Thermoelectric power in Gd3+-substituted Cu-Cd ferrites

      C B Kolekar P N Kamble A S Vaingankar

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      Polycrystalline CdxCu1−xFe2−yGdyO4 ferrites fory=0·0 and 0·1 were prepared by ceramic technique. X-ray diffractograms of powder samples show cubic symmetry withx⩾0·2 fory=0·0 and 0·1, while compositions withx=0·0 fory = 0·0 and 0·1 are tetragonal. The thermopower measurements for Gd3+-undoped ferrites in the temperature range 300 K to 788 K shown-type conductivity forx⩾0·2. The substitution of Gd3+ changedn-type conductivity of the compositions top-type. The mobilities calculated show decreasing trend on Gd3+ substitution. The values of activation energy ΔE and drift mobilityEd suggest polaron formation in substituted samples. The conduction mechanism is explained on the basis of localized model and formation of Gd3++Fe2+ stable pairs at B site and Cu1++Fe3+ at A site.

    • Lanthanum oxide doping effect on dc electrical conductivity of ferroelectrics sodium vanadate and rubidium vanadate

      D V Pawar T S Magdum S H Chavan

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      The temperature dependence of dc electrical conductivity was measured by two-probe technique in the vicinity of phase transition point for ferroelectrics sodium vanadate and rubidium vanadate doped with different concentrations of La2O3. These materials show a sharp change in conductivity at their phase transition temperatures. The results were found to obey the conventional exponential law and the activation energies were calculated for ferroelectric and paraelectric states. It was found that activation energy in ferroelectric phase is smaller than in the paraelectric phase. The activation energy increases slowly with increase in doping concentration of La2O3 up to 0·1 mol%, however, it decreases with further increase in doping concentration, in both ferro and para states. The dc electrical conductivity below the Curie temperature is of mixed type (ionic-electronic) while it is electronic type above the Curie temperature.

    • An analysis of the electrical conductivity of the Ag2SO4-K2SO4 binary system

      K Singh S D Wachasunder S S Bhoga

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      Electrical conductivity of the Ag2SO4-K2SO4 binary system shows three maxima at 20, 70 and 90 mole% of K2SO4 added to Ag2SO4. The first and the third maxima have been explained in the light of intragrain percolation due to lattice distortion, whereas, the second maximum by the surface percolation. The limit of solid solubility has been set at 20 mole% on the basis of evidences obtained from XRD, DTA and SEM techniques.

    • Resistivity studies of melt spun and annealed Al-Mn-Fe-Si multiply twinned samples

      Kulvir Singh D Bahadur GVS Sastry

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      The quaternary Al75Mn10Fe5Si10 alloy was synthesized by induction melting, melt spinning and subsequent annealing. The melt spun sample shows the multiply twinned nodular-type microstructure. These plates surround an icosahedral seed of the nodule. Annealing induces relaxation/transformation of the seeds due to diffusion process. We studied the electrical properties of all the three states (as-cast, quenched and annealed) of Al75Mn10Fe5Si10 alloy. The results are discussed in the framework of Ziman-Faber theory.

    • Room-temperature low-cycle fatigue behaviour of Ti-27Al-15Nb alloy

      P N Singh C Ramachandra B K Singh V Singh

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      Low-cycle fatigue (LCF) behaviour of the alloy Ti-27Al-15Nb, in (α2+B2) heat-treated condition was studied in total axial strain control mode at different total strain amplitudes (Δɛ1/2) from ±0·65 to ±1·0% and room temperature. While there was little hardening of the material at the lowest strain amplitude (Δɛt/2: ±0·65%), pronounced hardening was observed at the higher strain amplitudes (Δɛt/2⩾0·83%). The cyclic stress, at high strain amplitudes, continuously increased from the beginning till fracture of the specimens. The LCF resistance of the material was found to be low and this was due to its poor ductility at room temperature. Dual slope was observed in the Coffin-Manson plot, with less slope of the upper segment than that of the lower one, as observed in several alloys. The fracture behaviour pointed to brittleness and faceted features were observed.

    • Oxidation and corrosion behaviour of laser-alloyed mild steel with chromium

      A K Pattanaik A S Khanna C S Harandranath K Wissenbach G L Goswami

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      The oxidation and corrosion of mild steel coated with chromium powder followed by laser treatment has been investigated. Chromium powder was deposited on mild steel surface using atmospheric plasma spraying. The chromium-coated surface was irradiated separately using a pulsed Nd:YAG laser and a continuous CO2 laser. The oxidation tests were carried out in air at 700°C and anodic polarization tests in 1N H2SO4 solution. There was significant difference in the morphologies of the surface alloys formed using CO2 laser and Nd:YAG laser. Samples treated with CO2 laser showed better oxidation resistance whereas the surface alloys formed using Nd:YAG laser showed poor oxidation resistance. Anodic polarization tests carried out on samples laser-treated with CO2 laser showed improved corrosion resistance.

    • Structure of vacuum deposited SnSe films

      T Subbarao B K Samantaray A K Chaudhari

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      Tin selenide (SnSe) thin films prepared on mica and glass substrates by vacuum sublimation technique and examined by scanning electron microscopy and transmission electron diffraction techniques showed epitaxial and polycrystalline nature respectively irrespective of substrate temperature. Grain size of the films deposited on glass substrate increased with increase in substrate temperature.

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