• P K Khare

      Articles written in Pramana – Journal of Physics

    • Transient currents in discharge mode in cellulose acetate: polyvinyl acetate blend films

      P K Khare R S Chandok A P Srivastava

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      The transient currents measured in discharge mode with cellulose acetate (CA): polyvinyl acetate (PVAc) blend films (≈ 20µm thick) as a function of charging field [(1.5–4.5)×104 V/cm], temperatures (323–373 K) and polymer weight ratio (90:10 and 75:25) have been found to follow Curie-von Schweidler law, characterized with two slopes in short and long time regions. Isochronals characteristics (i.e. current/temperature plots at constant times) constructed from these data seemed to reveal a broad peak observed at 363 K. Values of activation energy increase with PAVc content and also with time of observation. Space charge due to trapping of injected charge carriers in energetically distributed traps and induced dipoles created because of the piling up of charge carriers at the phase boundary of heterogeneous structure of blend are considered to account for the observed currents.

    • Transient and thermally stimulated depolarization currents in pure and iodine doped polyvinyl formal (PVF) films

      P K Khare

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      Transient currents, measured with pure and iodine doped polyvinyl formal (PVF) films as a function of poling field (15–100 kV/cm) and temperature (30–95°C), have been found to follow Curie-von Schweidler law characterized with two slopes in short and long time regions. The isochronals (i.e. current/temperature plots at constant times) have been found to give rise to a peak located at 75°C. The order of current has been found to increase with increase in poling field, temperature and iodine mixing. The comparative studies of the isochronals with the thermally stimulated discharge current (TSDC) indicated the strong resemblance between the two studies. It is suggested that both the dipolar orientation due to molecular mechanism of motions with the side chains and space charge due to trapping of injected charge carriers in energetically distributed traps may be responsible for the observed currents. The dependence of current and activation energy on iodine mixing is explained on the basis of a charge transfer type of interaction.

    • Field emission theory of dislocation-sensitized photo-stimulated exo-electron emission from coloured alkali halide crystals

      B P Chandra R S Chandok P K Khare

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      A new field emission theory of dislocation-sensitized photo-stimulated exo-electron emission (DSPEE) is proposed, which shows that the increase in the intensity of photo emission fromF-centres during plastic deformation is caused by the appearance of an electric field which draws excited electrons out of the deeper layer and, therefore, increases the number of electrons which reach the surface. The theory of DSPEE shows that the variation of DSPEE flux intensity should obey the following relation$$\frac{{\Delta J_e \left( \varepsilon \right)}}{{J_e \left( o \right)}} = \left[ {\frac{{Y_s }}{{d_F }}\exp \left( {\frac{\chi }{{kT}}} \right) - 1} \right]$$. The theory of DSPEE is able to explain several experimental observations like linear increase of DSPEE intensityJe with the strain at low deformation, occurrence of the saturation inJe at higher deformation, temperature dependence ofJe, linear dependence ofJe on the electric field strength, the order of the critical strain at which saturation occurs inJe, and the ratio of the PEE intensity of deformed and undeformed crystals. At lower values of the strain, some of the excited electrons are captured by surface traps, where the deformation generated electric field is not able to cause the exo-emission. At larger deformation (in between 2% and 3%) of the crystal, the deformation-generated electric field becomes sufficient to cause an additional exo-electron emission of the electrons trapped in surface traps, and therefore,t here appears a hump in theJe versusε curves of the crystals.

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