Awalendra K. Thakur

On the question of percolation threshold in polyvinylidene fluoride/nanocrystalline nickel composites

The dielectric behavior of polyvinylidene fluoride (PVDF), nanocrystalline nickel (nc-Ni) composites has been investigated over a broad frequency range of 40Hz–10MHz. High effective dielectric constant (eeff=2050) and low loss (tanδ=10) at 100Hz have been observed near the percolation threshold. To the best of our knowledge, this is the highest eeff value reported to date among the PVDF based metal-polymer composites. The dielectric properties have been explained by using boundary layer capacitor effect and percolation theory while the dielectric anomalies are attributed to process of fabrication leading to thick insulating layer between the filler particles forming a gap in effective tunneling range of two filler particles and also making a difficulty in probability of higher order tunneling.

Surface and interfacial effect of filler particle on electrical properties of polyvinyledene fluoride/nickel composites

The effect of processing conditions and filler particle size/surface area on the dielectric behavior of polyvinyledene fluoride/nickel composites is reported. Large enhancement of low frequency dielectric constant with reduction in metal particle size in a metal-polymer composite is observed. Enhancement in the dielectric constant has been attributed to increase in interfacial area and consequent interfacial polarization with reduction in metal particle size. The increased interparticle contacts from the nearest neighbors result in enhanced tunneling probability leading to lowering of percolation threshold for nanosized nickel/polyvinyledene fluoride composites as compared to micron nickel/polyvinyledene fluoride composites.

Microstructural studies of (PbLa)(ZrTi)O3 ceramics using complex impedance spectroscopy

Lanthanum modified lead zirconate titanate (PLZT:8∕60∕40) ceramics prepared by a sol-gel route showed a well-defined microstructure comprising of grains separated by boundaries. Complex impedance spectroscopy has provided a convincing evidence for the existence of both grain (bulk) and grain-boundary effects that were separated in the frequency domain in impedance spectrum. The impedance analysis further provided the value of relaxation frequency, which was a characteristic intrinsic property of the material and was independent of sample geometrical factors. Relaxation frequency calculated at different temperatures was used to evaluate bulk dielectric constant (eb), which was compared with the real part of the dielectric constant (e′). The temperature variation of the bulk electrical conductivity (σdc) indicated an evidence of Arrhenius-type thermally activated process showing a linear variation up to a temperature of 713°K and was predominantly governed by grain boundary conduction showing a plateau regi...

Effect of plasticizer on microstructure and electrical properties of a sodium ion conducting composite polymer electrolyte

A plasticized composite polymer electrolyte (PCPE) based on Poly (ethylene oxide) + NaI with Na2SiO3 as the ceramics filler and Poly (ethylene glycol) as the plasticizer has been prepared by solution cast technique. Effect of plasticization on microstrucutre and electrical properties of the materials has been investigated. The changes in the structural and microstructural properties of the material have been investigated by XRD and SEM studies. The electrical conductivity estimated using a. c. impedance spectroscopy was found to be dependent on plasticizer concentration. An enhancement in the ionic conductivity value by three times has been recorded on addition of plasticizer when compared with that of unplasticized composite polymer electrolyte. The temperature dependence of conductivity of the polymer films is found to obey the Arrhenius behavior below and above the melting temperature of PEO. The electrical transport has been found to be a thermally activated process with ions being the predominant charge carrier.

Role of polymer matrix in large enhancement of dielectric constant in polymer-metal composites

Dielectric behavior of polymer (polar/nonpolar)-metal nanocomposites (PMCs) prepared under identical processing conditions have been compared. A high effective dielectric constant (ɛeff>2500) with a moderate loss and a lower ɛeff (74) with low loss was observed, respectively, for polar and nonpolar PMC at their respective percolation thresholds (fc). The results have been explained with the help of percolation theory and dipolar polarization. Similar value of fc observed in both the PMC is attributed to the same order of conductivity of polymer matrices. The dipolar polarization present in the polymer plays a major role in the enhancement of ɛeff.

Polyvinylidene fluoride/nickel composite materials for charge storing, electromagnetic interference absorption, and shielding applications

In this paper, the composites of polyvinylidene fluoride (PVDF)/nickel (Ni) prepared through simple blending and hot-molding process have been investigated for dielectric, electromagnetic shielding, and radar absorbing properties. In order to study complex permittivity of the composites in 40 Hz–20 MHz frequency range, impedance spectroscopy (IS) technique is used. Besides, the complex permittivity and permeability in addition to shielding effectiveness (SE), reflection coefficient (backed by air), and loss factor are calculated using scattering parameters measured in X-band (8.2–12.4 GHz) by waveguide method. Further, in X-band, a theoretical analysis of single layer absorbing structure backed by perfect electrical conductor is then performed. A flanged coaxial holder has also been designed, fabricated, calibrated, and tested for electromagnetic interference SE measurement in the broad frequency range (50 MHz–18 GHz). The IS results indicate large enhancement in dielectric constant as a function of Ni lo...

Studies on polycrystalline layered ceramic oxide: LiFeVO4

Abstract A new polycrystalline layered ceramic oxide, LiFeVO4, has been prepared by a standard solid state reaction technique. The preparation conditions were optimised using thermogravimmetric analysis (TGA) technique. Material formation under the reported conditions was confirmed by X-ray diffraction studies. A preliminary structural analysis indicated that the crystal structure was orthorhombic with lattice parameters: a=4·3368 Å, b=13·1119 Å and c=16·3426 Å. The phase morphology and surface property were studied by scanning electron microscopy. Complex impedance analysis of the sample indicated bulk contribution to electrical properties at T≤125°C, grain boundary effects at the temperatures ≥125°C, negative temperature coefficient of resistance (NTCR) effect and evidence of temperature dependent electrical relaxation phenomena in the sample. The dc conductivity σdc shows typical Arrhenius behaviour when observed as a function of temperature. The activation energy value was estimated to be 0·24 eV. The value of σdc, evaluated from complex impedance spectrum, shows a jump of nearly two orders of magnitude at higher temperature (∼1·24 × 10−5 S cm−1 at 350°C) when compared with that of σdc (1·14 × 10−6 S cm−1 at 50°C). Alternating current conductivity spectrum obeys Jonschers universal power law. The results of σac v. temperature are also discussed.

Design and Optimization of Multilayered Electromagnetic Shield Using a Real-Coded Genetic Algorithm

We report optimized design of multilayered electromag- netic shield using real coded genetic algorithm. It is observed that the shielding efiectiveness in multilayer design is higher than single layered counterpart of equal thickness. An efiort has been made to develop an alternative approach to achieve speciflc objective of identifying the design characteristics of each layer in the multilayered shielding con- flguration. The proposed approach incorporates interrelated factors, such as absorption and re∞ection in the design optimization as per speciflc shielding requirements. The design problem has been solved using shielding efiectiveness theory based on transmission line (TL) modeling and real-coded genetic algorithm (GA) with simulated bi- nary crossover (SBX) and parameter-based mutation. The advantage of real-coded GA lies in e-cient solution for electromagnetic interfer- ence (EMI) shielding design due to its strength in solving constraint optimization problems of continuous variables with many parameters without any gradient information. Additionally, the role of material parameters, such as permittivity and permeability on re∞ection char- acteristics and shielding efiectiveness, has also been investigated and optimized using the proposed models and real-coded GA. Theoretical optimization of electromagnetic parameters has been carried out for SE » 40dB for many industrial/commercial applications and SE » 80dB for military applications.

Relaxation behavior in clay-reinforced polymer nanocomposites

The effect of clay reinforcement on dielectric, conductivity, and mechanical relaxation behavior of a polymer clay nanocomposite film is reported. Polymer nanocomposite is composed of three component polymers (polyacrylonitrile) as a host matrix, salt (LiPF6) as conducting species, and clay (sodium montmorillonite) as intercalant. The macroscopic parameters like polymer glass transition temperature and available free mobile charge carriers have been analyzed properly using dynamic mechanical analysis and dielectric analysis. Dielectric analysis indicated distribution of relaxation time as a function of clay concentration, whereas conductivity spectrum exhibited dispersion at lower frequency followed by saturation region at intermediate frequency. The dispersion behavior is related to the electrode polarization attributed to faster ion dynamics. The dielectric and conductivity relaxation are in excellent correlation with mechanical relaxation owing to the changes in glass transition temperature due to polymer-ion-clay interaction. The proposed mechanism is a sequel to the experimental results.

ON THE DESIGN AND RELIABILITY ANALYSIS OF ELECTROMAGNETIC ABSORBERS USING REAL- CODED GENETIC ALGORITHM AND MONTE CARLO SIMULATION

In this paper, we propose an approach for designing and quantitatively assessing the performance of the multilayered radar- absorbing structure. In our proposed approach, a flve layered radar- absorbing materials design is optimized from the predeflned materials database. But to determine the optimal choice of the material and thickness of each layer, a combined binary and real-coded genetic algorithm (GA) is used to handle the integer and real variables involved in such designs. Further, the proposed approach employs the Latin hypercube sampling with Monte Carlo Simulation to carry out the performance based reliability analysis of the design. Absorber synthesized results are compared with the published work using other algorithms. The outcomes of our approach show that the combined GA works quite well, and most prominently the reliability analysis provides the decision maker a means to select among the several design alternatives available before him.