Mohan L. Verma

Characterization of basic transport properties in a new fast Ag+ ion conducting composite electrolyte system : (1 - x)[0.75AgI :0.25AgCl] :xZrO2

Abstract Studies of some basic ionic transport properties of a new fast Ag+ ion conducting two-phase composite electrolyte system: (1−x)[0.75AgI:0.25AgCl]:xZrO2, where 0≤x≤0.5 (in molar weight fraction), are reported. A ‘quenched/annealed [0.75AgI:0.25AgCl] mixed system/solid solution’ has been used as a first phase host matrix salt as an alternative to the traditional host AgI, while particles (≤5 μm) of the insulating and inert ZrO2 were dispersed as second phase dispersoid. In order to find the ‘optimum conducting composition’ (OCC), different compositions of the two phases were mixed homogeneously adopting various routes of preparation. The phase identification studies revealed the coexistence of separate phases. The temperature-dependent transport property studies were carried out on OCC employing various techniques. The mechanism of ion transport has been explained on the basis of models proposed for two two-phase composite electrolyte systems.

Studies of Polarization/Self-Depolarization and Electret-type Effect in AgI

A novel d.c. polarization/self-depolarization study and electret-type effect in AgI are reported. AgI pellets of varying thicknesses, placed between two blocking (graphite) electrodes, were subjected to an external d.c. potential. A state of complete polarization was attained within ∼10 min, irrespective of the sample thickness. At this state, the potential difference, developed across the sample pellet as a result of polarization/accumulation of mobile Ag+ ions at the bulk/negative electrode interface, was measured experimentally. The potential difference, obtained immediately after the removal of the external d.c. source, has been referred to as ‘instant peak potential (Vp)’. As soon as the external voltage source is switched off, a process of self-depolarization is initiated due to the chemical/self diffusion of polarized mobile Ag+ ions throughout the bulk. ‘Vp’ gives a direct information regarding the extent of mobile ion concentration (n). ‘Vp’ measurements were carried out as a function of temperature and ‘Log Vp vs 1/T’ variation was compared with the ‘Log n vs 1/T’ Arrhenius plot, reported earlier in an entirely independent study. The two variations are almost analogous. This, in turn, supported as an earlier assertion that the abrupt conductivity increase in α-AgI, after β→α-phase transition at ∼147 °C, is predominantly due to the excessive increase in ‘n’. Furthermore, it has also been revealed that the Ag+ ions play another unique role which led to the existence of ‘persistent polarization’ states in AgI. These states are identical to the ‘electret-type effects’, observed in a number of dielectric materials. The polarization state persisted for very long time in ‘thermally stimulated polarized’ sample. A detailed investigation of the persistence/retention of polarization in the thermally-stimulated-polarized sample is reported.

Modeling of ionic diffusion by space charge depolarization

For the study of transport properties of a nano-composite material, an open circuit condition of current is considered, where the sum of diffusion current density, displacement current density, ohmic current density, and excess charge current density is vanished. Including the effects of drift and trapping of ions, drift and trapping diffusion coefficients are modeled respectively and the significance of the type of ionic motion in the study of ionic diffusion is discussed.

Space charge depolarization of wurtzite or zinc blend structured silver iodide: modeling of preliminary studies

Mathematical modeling of a space charge depolarization of AgI in its ionic phase is reported. Initially, cell configuration of AgI sandwiched between two electronic conducting graphite electrodes were polarized by applying an external dc field ∼0.5 V across the cell. The depolarization potential time profile, instantly after the removal of external polarizing field, at various constant temperatures, below the structural phase transition temperature of ∼420 K of AgI is recorded. This is a characteristic temperature, as, the structural, electrical as well as thermodynamic properties of AgI are much different after (i.e. in α-AgI) and before this temperature (i.e. in β-AgI). The recorded profile is compared with the profiles obtained theoretically by considering different isothermal conditions. This study is limited to the isothermal variation of depolarization potential with time only, although this may also be a function of the position of ions inside the sample.

Structural, electronic and transport properties of X3SnC (X=Cr/Mn/Cu) electrodes—first principle approach

The density functional theory is approached for the comparative study of electronic, transport and magnetic properties of SnC electrode and X3SnC (X = Cu/Mn/Cr) electrodes. The cohesive energy analysis exhibits the highest stability of SnC which reduces gradually by dispersing of elements Cr/Mn/Cu, bulk modulus shows highest compressibility of Cu3SnC electrode. Enthalpy of SnC reveals its strongest forming ability and alloying nature. The energy band diagrams exhibit the semiconducting nature of SnC electrode and conducting nature of Cu3SnC/Mn3SnC/Cr3SnC electrodes. The spin-up and spin-down density of states explains the magnetic nature of Mn3SnC and Cr3SnC electrodes. The ionicity factor reveals the purely covalent and partially ionic nature of inter atomic bonds of Sn-X/C-X. The current voltage characteristics also reveal the semiconducting nature of SnC, purely metallic nature of Cu3SnC electrode, and the negative resistance features of Mn3SnC and Cr3SnC electrodes. Transmission curves also support the current voltage characteristics.

PEO nanocomposite polymer electrolyte for solid state symmetric capacitors

Physical and electrochemical properties of polyethylene oxide (PEO)-based nanocomposite solid polymer electrolytes (NPEs) were investigated for symmetric capacitor applications. Nanosize fillers, i.e., Al2O3 and SiO2 incorporated polymer electrolyte exhibited higher ionic conductivity than those with filler-free composites. The composites have been synthesized by the completely dry (solution-free) hot-press method. The addition of filler in fractional amount to the solid polymer matrix at room temperature further enhances the ionic conductivity. Nature of the NPEs were studied using X-ray diffraction and energy-dispersive spectra analyses. Thermal stability of the resulting electrolyte was analysed by thermogravimetric analysis and differential scanning calorimetric studies. Morphology changes occurred during the addition of fillers was evidenced by scanning electronic microscope images. Solid polymer electrolytes exhibiting these parameters was found to be suitable for solid state capacitors. The results obtained from the electrolytes with an optimum compositions (PEO70AgI30)93(Al2O3)7 and (PEO70AgI30)95(SiO2)5 used in the (PEO70AgI30)70(AC)30 electrodes for symmetric capacitor applications and their performances were analysed by impedance spectroscopic, Bode plot, cyclic voltammetry, discharge characteristics and leakage current profile.

Optical properties of Sr2SiO4:Eu2+, Dy3+phosphors prepared by combustion method

Abstract In the present paper, TL and PL study of Dy3+ doped Sr2SiO4:Eu2+ phosphor is reported. A polycrystalline sample of Sr2SiO4:Eu2+, Dy3+ was prepared by combustion method. The obtained phosphor was characterized by powder X-ray diffraction, scanning electron microscopy, UV-Vis spectroscopy, PL and thermoluminescence. The results of the XRD studies obtained for Sr2SiO4:Eu2+, Dy3+ phosphor revealed its monoclinic structure. The average crystallite size was calculated as 12.77 nm. Thermoluminescence study was carried out for the phosphor using UV irradiation and a single glow peak was found. The thermoluminescence glow curves of the samples were measured at various concentrations of co-dopant. The kinetic parameter has been calculated using Chen’s glow curve method. In this paper, the photoluminescence and afterglow behavior of these phosphors are reported.

Modeling of space charge dielectric constant

The space charge polarization/depolarization is considered, and to relate the geometrical arrangement of constitutive phases to the dielectric response, three space charge dielectric constants, viz., trapped space charge dielectric constant εt, drift space charge dielectric constantεd and dielectric constant ε are modeled. It is observed that the order of εt is same for AgI and 0.9[0.75AgI:0.25AgCl]:0.1SiO2 nanocomposite in β− and α−phases. In β− phase, εd and ε are 10 times larger and 10 times smaller in the second system of respective β− and α−phases. In β− phase, the ionic concentration provides the main contribution to the dielectric response of both systems, and in α− phase, the ionic mobility has the main contribution to the dielectric nature.