Farid A. Khwaja

Analytical and electrical studies on poly(2-vinylpyridine) and its metal complexes

Abstract Poly (2-vinylpyridine) (P2VP) and its metal-based derivatives have been synthesized and characterized through analytical measurements such as melting point, molar mass and thermal analysis. The d.c. conductivity of these materials was studied as a function of temperature. The conductivity of P2VP is seen to increase by many orders of magnitude on complexation with the transition metals Cu, Co, Pt and Pd.

Electrical conductivity of poly(nickel methacrylate)-I

A Ni-polymethacrylate pellet of diameter 1·4 cm, thickness 0·114 cm, cross-sectional area 1·539 cm2, weighing 470 mg and prepared under a pressure of 8 tons PSI was used for electrical conduction measurements in the temperature range 292–369 K. The electrical behaviour of the sample results from two distinct conduction mechanisms. Ohmic conduction is observed up to 325 K and 300 V. Ionic conduction is operative because of (i) the high value of activation energy in the low (T < 325 K) and high (325 < T < 369 K) temperature regions in contrast to our investigation of Pd-polymethacrylate, (ii) the appearance of reverse current by reversing the polarity of the applied voltage, (iii) current-voltage hysteresis and (iv) a time-relaxation phenomenon. At T < 325 K, the ionic conduction is due to weakly trapped impurity ions, whereas at 325 < T < 369 K, H+ ions are the main source of ionic conduction. The material exhibits semiconducting behaviour in the temperature range under investigation.

Ordering behaviour of Ni-5 at.% Al alloy

Abstract The measurements of tensile strength, lattice parameter, microstructure and texture have been carried out as a function of annealing temperature in Ni-5 at.% Al with a view to understanding ordering behaviour of the material. The non-monotonic variation of the tensile strength in the α-solid solution region of the alloy has been discussed in the light of texture measurements as well as on the basis of appearance of local order due to concentration inhomogeneities.

Concentration dependence of DC conductivity of Pd polyacrylates

Abstract The DC conductivity of several palladium polyacrylates with compositions in the range 5 to 40% w/w Pd has been measured over a wide range of temperatures (293 K to 363 K). Infra-red spectra and X-rays diffractograms have been obtained, making it possible to correlate the structural details with the nature of the conduction mechanism. The temperature and palladium concentration dependence of the DC conductivities are explained in terms of thermal hopping motion of localized charge carriers which are believed to be bipolarons.

Time-dependent delayed radiation-induced conductivity in nickel polymethacrylate—III

Abstract Radiation-induced conduction in nickel polymethacrylate in the dose range 3 × 10 9 to 2·4 × 10 10 α-particles/cm 2 has been studied. The induced current increases slowly with time until it attains a steady-state equilibrium value. The effects of temperature and dose upon the time dependence of the induced current have been investigated and it has been found that the relaxation time, τ, decreases drastically with temperature. Parameters like carrier mobility and number density have been determined for both unirradiated and irradiated samples and found to be two orders of magnitude greater for the former. The exponential increase in the induced current is explained by thermal detrapping from the deep traps. The trap depth is calculated to be 0·42 eV.

Delayed radiation-induced conductivity in nickel polymethacrylate: II

Abstract Generation and transport of charge carriers in nickel polymethacrylate (NiPMA) in the post-irradiation phase have been investigated by means of DC conductivity as a function of temperature, exposure rate and electric field strength. The delayed radiation-induced conductivity due to the temperature- and field-assisted dissociation of electron-holes excitation of the chain segment is appropriately described within the framework of the one-dimensional Onsager theory of geminate-pair dissociation. At high temperatures the conductivity is dominated by the local, liquid-like motion of the heterocharges moving in an amorphous or locally disordered environment. After a certain radiation dose the transition from increasing to decreasing conductivity with increase of exposure rate is due to the radiation-hardening of the material. It is concluded that prolonged irradiation of the material produces new trap-centres which cause a decrease in conductivity.

Concentration and Temperature Dependence of Electrical Conductivity in Thermally Stable Chromium Polyacrylate

Abstract Electrical conductivity of chromium polyacrylate with dopant concentration 30% (w/w), 40% (w/w) and 50% (w/w) of chromium has been measured over a broad range of temperatures (303K to 383K). The electrical conductivity shows dependence on temperature, as well as, level of doping. The conductivity is considered to be due to thermal hopping motion of localized charge carriers, which are believed to be polarons, in the temperature range 303K to 323K and for T > 343K, whereas, it is metal-like in the temperature range 323K to 343K.

MAGNETIZATION AND INTRAGRANULAR CRITICAL CURRENT DENSITY IN Yb1Ba2Cu3O7−x HIGH TEMPERATURE SUPERCONDUCTOR

The Yb1Ba2Cu3O7−x samples have been prepared by the solid state reaction using the appropriate amount of Yb2O3, BaCO3, and CuO powders and characterized them using X-ray diffraction and electrical resistivity techniques. The X-ray diffraction pattern of the sample shows an orthorhombic structure refined in the space group Pmmm. The critical current densities are calculated from magnetization measurements over a temperature range 77 K to 86 K, and in magnetic fields up to 2 KOe. The intragrain critical current density is estimated to range from 5.24 × 106 (A/cm2) at an applied field of 0.2 KOe to 1.01 × 106 (A/cm2) at an applied field of 2 KOe at T = 77 K in this sample.

SYNTHESIS AND DC CONDUCTIVITY OF PALLADIUM POLYACRYLATE

This papers reports the synthesis and the results of the measurements of dc conductivity of heat treated palladium polyacrylate. Infrared spectra and the scanning electron microscopy of the samples before and after heat treatments are compared in order to reveal their structural details. It is conjectured that at T>38°C molecular aggregates are formed or ion pairing takes place in the material due to the breakdown of the polymer chain in it. The semiconductor-like behavior of the electrical conductivity with the increase of temperature from room temperature to 38°C and thereafter, an exponential decrease in conductivity with further increase in temperature exhibiting a metal-like behaviour show that the material undergoes an insulator-metal transition at this temperature.

MONTE CARLO CALCULATION OF ORDER-DISORDER PHASE DIAGRAM OF CU-AU

The problem of order-disorder phase transition and phase diagram of the binary copper-gold alloy is analyzed using Monte-Carlo technique. Unlike traditional approaches, the interaction potentials used in our calculations are obtained from the experimental data of diffuse X-ray scattering for the system. The thermodynamic parameters such as configurational energy, specific heat and the order-disorder transition temperatures are calculated directly from the averages of these quantities over many different configurations after an initial equilbration period. The order-disorder phase diagram is constructed and compared with the experimental one, as well as with the results of some previous theoretical approaches.