G. S. Kumar

Experimental determinations of the Lorenz number

The Lorenz number, which was assumed to be a universal factor, was later found to be deviating from its theoretical Sommerfeld value. Our understanding of the physics of the Weidemann-Franz-Lorenz law is based on a large number of investigations. In view of its importance in the understanding of thermal and electrical conductivity (particularly the electronic contribution), an attempt has been made to review the experimental determination of Lorenz number in metals, semi-metals, alloys and degenerate semiconductors.

Impedance spectroscopic studies in SrBi5FeTi4O18

Abstract SrBi5FeTi4O18 belongs to the family of bismuth layer structured ferroelectrics. The samples studied were prepared by the double sintering method. The impedance and dielectric measurements were performed in the temperature range from room temperature to 600°C. From the impedance data, AC conductivity was computed both as a function of frequency, as well as temperature. Complex impedance diagrams are nearly semicircles in the high temperature and high frequency range. The impedance data below 500°C indicated Debye type relaxations. The poled sample indicated a Curie temperature of 580°C. The sample under consideration was found to be a weakly ferromagnetic material at room temperature with a coercive field of 7.08 × 102 G as evidenced by the B—H loop.

Piezoelectric relaxation in polymer and ferroelectric composites

Recently, fairly large numbers of studies have been reported on piezoelectric relaxations of polymers. The piezoelectric constants of polymer and ferroelectric composites also show such relaxations. It has been found that both polymer and polymer and ferroelectric composities exhibit typical Debye-type piezoelectric dispersion. Cole-Cole diagrams for the piezoelectric properties of these materials have been drawn and an attempt has been made to interpret the results.

Impedance analysis and dielectric properties of Ce modified bismuth titanate lead free ceramics synthesized using solution combustion route

Cerium modified bismuth titanate ceramics were studied as potential lead-free ferroelectric materials over a broad temperature range. Polycrystalline samples of Bi4−xCexTi3O12 (xxa0=xa00.2, 0.4, 0.6, 0.8) (BCeT) were prepared using the solution combustion technique. The effect of Ce doping on their crystalline structure, ferroelectric properties and electrical conduction characteristics were explored. An increase in the dielectric constant and decrease in loss tangent is observed due to addition of Ce in bismuth titanate lattice up to a certain threshold doping concentration. The presence of pyrochlore phase decreases the density of ceramics with composition xxa0=xa00.8 which may be due to grain inhibitor property of Ce. The value of remnant polarization increases with increasing doping content up to xxa0=xa00.6 due to decrease in defect sites in the samples, further increase in cerium content induces leakage current which makes the sample lossy. The real–imaginary impedance plots are interpreted using RC model, this explains the contribution of the grain-bulk and the grain boundary resistivities to the total resistivity of the materials. Activation energy increases with increasing Ce concentration indicating a decrease in defect sites due to which the conductivity of the doped ceramics reduces.

Modified Lorentz and Gauss Equations to Describe the Dielectric Behaviour of Sr1-2xNaxNdxBi4Ti4O15 Normal Ferroelectric Compounds

Modified Lorentz and Gauss equation is proposed to describe the temperature dependence of dielectric constant for bismuth layered structure ferroelectric materials i.e. Sr1-2xNaxNdxBi4Ti4O15. The validity of the proposed equation was checked from the regression value obtained. The activation energy for orientation of dipoles, static dielectric constant and infinite dielectric constant obtained from the non linear curve fitting to experimental data by using modified Lorentz and Gauss equation have reasonable values. Modified Lorentz equation is found to be the best fit to the experimental dielectric data. The above modified equations may also be used to describe the dielectric behaviour of other normal ferroelectric materials which show sharp transitions.

Modified Lorentz equation to describe the resonance and anti-resonance behaviour of piezoelectric ceramics

ABSTRACT Modified Lorentz equation is proposed to describe the resonance and anti-resonance behaviour of piezoelectric ceramics. It is applied to Ba(NdxTi1−2xNbx)O3, (Na0.5Bi0.5)(NdyTi1−2yNby)O3 and Sr1−2zNazNdzBi4Ti4O15 piezoelectric ceramics by non linear curve fitting to experimental data to check its validity. The validity is checked from the regression values obtained. The static dielectric constant, infinite dielectric constant, polarizability, piezoelectric charge constant and piezoelectric voltage constant obtained from the non linear curve fitting to experimental data by using modified Lorentz equation have reasonable values. The above modified equation may also be used to describe the resonance and anti-resonance behaviour of other piezoelectric ceramics.

Influence of Ba2+ Ion Substitution on the Structural, Ferroelectric and Electrical Properties of Nano-Structured Na0.5Bi0.5TiO3 Lead Free Piezo Ceramics

Lead free relaxor 0.8NBT-0.2BT nano-structured ceramic composition was processed through sol-gel technique and sintered at 1140°C for 4 h in air. The bulk density exceeded 95% of theoretical density as determined by Archimedes principle. The sintered samples were characterized using X-ray diffraction, field emission scanning electron microscope, energy dispersive spectrometry, Raman, dielectric, AC impedance, piezoelectric and ferroelectric measurements. XRD patterns recorded at room temperature confirmed the rhombohedral phase formation of the samples. Particle size range measured for calcined powder was found to be of 40-50 nm and the average grain size of the final sintered sample was 80 nm. Analysis of Raman spectral data of the sample indicated distortion of NBT. The diffuse phase transition behaviour was investigated from ε versus T curve and the temperature due to structural change coincided with abrupt jump in ε(T) curve. Dielectric maximum and minimum were also evaluated to define phase transitions of 0.8NBT-0.2BT system. NBT-BT has shown normal diffuse type of phase transition. Charge transformational behaviour could be analyzed based on AC and DC conductivity measurements. Ferroelectricity was also confirmed by P-E measurements and piezoelectric constant obtained for the NBT-BT sample was 90 pC/N. GRAPHICAL ABSTRACT

Studies on [Ba(Nd0.1Ti0.8Nb0.1)O3]0.40[Na0.5Bi0.5TiO3]0.50[CaTiO3]0.10 ceramic for transducer application

ABSTRACT [BaNd0.1Ti0.8Nb0.1O3]0.40[Na0.5Bi0.5TiO3]0.50[CaTiO3]0.10ceramic samples were prepared through solid state sintering route. Impedance and dielectric studies were carried out on the prepared ceramic samples. Impedance and dielectric relaxor behaviour was observed. Grain and grain boundary effects are resolved by equivalent circuit modelling of experimental Cole-Cole data. Electromechanical studies are also carried out on the ceramic samples. Broad resonance and anti-resonance peaks have been observed in the modulus data. The values thickness and effective mode of electromechanical coupling factors at 623 K are found to be 0.99983 and 0.99980 respectively. The value of d33 at 673 K and 773 K are found to be 40.32 nm.V−1and 6.44 nm.V−1 respectively.

Dielectric, impedance relaxation and DC resistivity studies on microwave sintered Ba1-xRexTiO3(Re = Nd&Pr) ceramics using 5% SiO2 as sintering aid

ABSTRACT Ba1-xRexTiO3 ceramics (Re = Nd&Pr with x = 0.002 & 0.004) were prepared through microwave sintering route using 5% SiO2 as sintering aid. The dielectric, impedance and DC resistivity studies were carried out over a wide range of temperature. From dielectric studies, it is observed that the transition temperature increased with the substitution of rare earth ions. The room temperature dielectric constant for the Nd (x = 0.002) doped barium titanate is higher when compared to other compositions and the value is 13,600.28. Impedance studies revealed relaxation behaviour. In the present study grain, grain boundary and sample-electrode effects were separated out by using Cole-Cole data. From the DC resistivity studies, it is observed that Pr (x = 0.004) doped barium titanate ceramic showed one order of magnitude PTCR effect.

Electromechanical and AC conductivity studies on Na(0.5-x)KxBi(0.5-x)DyxTiO3 piezoelectric ceramics

ABSTRACT AC conductivity studies were performed on the Na(0.5-x)KxBi(0.5-x)DyxTiO3 ceramics (x = 0.0125, 0.025 & 0.05) prepared through solid state reaction method. The frequency and temperature response of the conductivity is interpreted in terms of the jump relaxation model. Stress and temperature induced modulus resonance and anti-resonance behaviour was observed indicating electromechanical effect in the present ceramic materials. The electromechanical coupling factor have been found to be above 99.95% and is found to increase with increase of x. Value of d33 for x = 0.0125 sample is found to be 23.24 nm.V−1 and it decreases with increase of x.