R. P. Tandon

Characterization of thick lead zirconate titanate films fabricated using a new sol gel based process

Lead zirconate titanate (PZT) films 60 μm in thickness have been fabricated using a new sol gel based process. PZT powders are dispersed in a sol gel matrix to form a 0–3 ceramic/ceramic composite. The dielectric properties of these films have been studied as a function of powder concentration, frequency, and temperature. The characteristic Curie point is observed at 420 °C. The ferroelectric behavior measured in terms of the remanant polarization (Pr=35 μC/cm2) and coercive field (Ec=20 kV/cm) was an improvement over values quoted for thin PZT films but lower than that of bulk ceramic. The piezoelectric properties d33 (325 pC/N) and d31 (−80 pC/N) were comparable with those of the bulk ceramic.

Gas and humidity sensors based on iron oxide–polypyrrole nanocomposites

Abstract Nanocomposites of iron oxide and polypyrrole were prepared by simultaneous gelation and polymerization process. This resulted in the formation of mixed iron oxide phase for lower polypyrrole concentration, stabilizing to a single cubic iron oxide phase at higher polypyrrole concentration. The composites in the pellet form were used for humidity and gas sensing investigations. Their sensitivity to humidity was found to increase with increasing concentration of polypyrrole. Gas sensing was performed for CO2, N2 and CH4 gases at varying pressures. The sensors showed a linear relationship between sensitivity and pressures for all the gases studied. The sensors showed highest sensitivity to CO2 gas.

Low‐frequency ac conduction in lightly doped polypyrrole films

The total measured ac conductivity σ(ω)m of lightly doped polypyrrole films has been measured in the frequency range 100 Hz–10 MHz and in the temperature range 77–350 K. In the low‐temperature region the measured ac conductivity is almost independent of temperature but shows a strong dependence on frequency and can be described by a relation σ(ω) = Aωs where the exponent s has been observed to be less than unity. In the high‐temperature region the frequency dependence becomes weak at low frequencies but remains strongly frequency dependent at high frequencies. The weak frequency dependence is due to the contribution of dc conductivity to the measured ac conductivity. A clear Debye‐type loss peak is observed by substracting the contribution of dc conductivity. The frequency dependence of conductivity remains less than quadratic at low frequencies indicating thereby some distribution of relaxation times. This is confirmed by the measurement of dielectric constant as a function of frequency and temperature. ...

A model for the J-V characteristics of P3HT:PCBM solar cells

Current-voltage (J-V) characteristics of an organic bulk heterojunction solar cell have been modeled and compared with the measured characteristics of solar cell based on the blend of poly(3-hexylethiophene) (P3HT) and phenyl [6,6] C61 butyric acid methyl ester (PCBM). In an undoped organic double Schottky junction diode, for V<Vbi the electric field remains constant and is given by (Vbi−V)/d, where Vbi is the built-in voltage, V is the applied voltage, and d is sample thickness. We considered the effect of this constant electric field on the charge carrier transport and solved the drift and diffusion equations to model the J-V characteristics. For V<Vbi the current is found to be dominated by diffusion. A comparison of the theoretical results with the experimental data measured in dark and under different illumination intensities shows good agreement.

A.c. conductivity of poly(N-methylpyrrole)

Abstract The dielectric constant ɛ′(ω) and measured a.c. conductivity σ m (ω) of lightly doped poly ( N -methylpyrrole) films have been investigated in the temperature range 77–350 K and in the frequency range 100 Hz–1 MHz. In the low-temperature region the measured a.c. conductivity can be described by the relation σ(ω) = Aω s , where the exponent s is found to be less than unity. At high temperatures the conductivity remains strongly frequency dependent but the dependence becomes weak at low frequencies. The temperature and frequency dependences of a.c. conductivity σ(ω) can be qualitatively explained by the contributions from two mechanisms, one giving rise to a broad dielectric loss peak having a distribution of relaxation times, and the other giving a linear dependence of conductivity on frequency.

Growth kinetics of polypyrrole, poly (N-methyl pyrrole) and their copolymer, poly (N-methyl pyrrole-pyrrole) : effect of annealing on conductivity and surface structure

Abstract Films of polypyrrole, poly ( N -methyl pyrrole) and their copolymer, poly ( N -methyl pyrrole-pyrrole), are prepared by an electrochemical polymerization technique using a planar electrode configuration. Scanning electron microscopy (SEM) studies reveal a cauliflower morphology for these films which is attributed to the 3D-type nodular growth in these polymers. The effect of annealing on the surface morphology and the conductivity of these films are examined.

Mechanism of charge transport in polypyrrole, poly(N‐methyl pyrrole) and their copolymers

The temperature dependence of the dc conductivity of electrochemically polymerized films of polypyrrole, poly(N‐methyl pyrrole) and their copolymers, poly(N‐methyl pyrrole‐pyrrole), having different percentage of BF−4 ions has been investigated in the temperature range 77–350 K. The observed behavior could be explained in terms of Mott’s variable range hopping model involving a single phonon process. The estimated values of polaron radius yield the realistic values of density of states at the Fermi level which are in good agreement with the values reported earlier for other polyconjugated systems.

DC electrical conduction in tungsten phosphate glasses

Abstract The dc conductivity of semiconducting tungsten phosphate glasses of three different compositions has been measured over a temperature range 100–400 K. The data have been analysed in the light of existing models of polaronic hopping conduction. The high-temperature region can be qualitatively explained by small-polaron hopping between nearest neighbours, while at low temperatures (below 150 K), the behaviour can be explained by Motts variable-range hopping. The value of the electron decay constant α estimated from variable-range hopping is of the order of 2 A −1 , but the estimated disorder energy W D is twice as large as the measured low-temperature activation energy.

Phase change induced by polypyrrole in iron-oxide polypyrrole nanocomposite

Nanocomposites of polypyrrole and iron oxide were prepared using simultaneous gelation and polymerization processes. Varied amounts of pyrrole monomer were added to a solution containing iron nitrate as precursor and 2-methoxy ethanol as solvent. The presence of oxide and polypyrrole was confirmed by using X-ray and FTIR techniques. Some of these nanocomposites exhibited magnetic behaviour. SEM studies of powders indicated presence of nanosized particles. Electrical conductivity studies of powders showed a slight variation in conductivity for lower concentration of pyrrole, with a sudden increase in conductivity at 15% of pyrrole concentration. A transition from a nonmagnetic to magnetic phase was also observed at the same concentration.

LOW FREQUENCY ALTERNATING CURRENT CONDUCTION AND DIELECTRIC RELAXATION IN POLYPYRROLE, POLY(N-METHYL PYRROLE), AND THEIR COPOLYMERS

The alternating current (ac) conductivity [σm(ω)], dielectric constant (e′) and loss (e″) of polypyrrole (PPY), poly(N‐methyl pyrrole) [P(NMPY)] and their copolymers; poly(N‐methyl pyrrole–pyrrole [P(NMPY–PY)] have been measured in the frequency range 102–106 Hz and in the temperature range 77–350 K. At 77 K, the ac conductivity can be expressed by the relation; σac=Aωs where the slope s lies in the range 0.72–0.81 for these three polymers and decreases with increase in temperature. The well‐defined loss peaks, whose magnitude decreases with the increase in frequency, have been observed in the temperature region where the measured ac conductivity approaches the direct current (dc) conductivity. These loss peaks have been associated with the movement of charge carriers in these polymeric films. The dc conductivity has also been measured in the temperature range 77–350 K and an attempt has been made to correlate it with dielectric data.