arXiv subjects

K. B. R. Varma

Publications and source records attributed to K. B. R. Varma.

26 records · Page 2Linked to original sources

arXiv2010

Electrical transport properties of 0.5Li2O-0.5M2O-2B2O3 (M=Li, Na and K) glasses

Transparent glasses in the system 0.5Li2O-0.5M2O-2B2O3 (M=Li, Na and K) were fabricated via the conventional melt quenching technique. Amorphous and glassy nature of the samples were confirmed via the X-ray powder diffraction and the differential scanning calorimetry, respectively. The frequency and temperature dependent characteristics of the dielectric relaxation and the electrical conductivity were investigated in the 100 Hz - 10 MHz frequency range. The imaginary part of the electric modulus spectra was modeled using an approximate solution of Kohrausch-Williams-Watts relation. The stretching exponent, β, was found to be temperature independent for 0.5Li2O-0.5Na2O-2B2O3 (LNBO) glasses. The activation energy associated with DC conductivity was found to be higher (1.25eV) for 0.5Li2O-0.5K2O-2B2O3 (LKBO) glasses than that of the other glass systems under study. This could be attributed to the mixed cationic effect.

cond-mat.mtrl-sci↗

arXiv2010

Dielectric properties of Li2O-3B2O3 glasses

The frequency and temperature dependence of the dielectric constant and the electrical conductivity of the transparent glasses in the composition Li2O-3B2O3 (LBO) were investigated in the 100 Hz- 10 MHz frequency range. The dielectric constant and the loss in the low frequency regime were electrode material dependent. Dielectric and electrical relaxations were respectively analyzed using the Cole-Cole and electric modulus formalisms. The dielectric relaxation mechanism was discussed in the framework of electrode and charge carrier (hopping of the ions) related polarization using generalized Cole-Cole expression. The frequency dependent electrical conductivity was rationalized using Jonscher's power law. The activation energy associated with the dc conductivity was 0.80 \pm 0.02 eV, which was ascribed to the motion of Li+ ions in the glass matrix. The activation energy associated with dielectric relaxation was almost equal to that of the dc conductivity, indicating that the same species took part in both the processes. Temperature dependent behavior of the frequency exponent (n) suggested that the correlated barrier hopping model was the most apposite to rationalize the electrical transport phenomenon in Li2O-3B2O3 glasses. These glasses on heating at 933 K/10h resulted in the known non-linear optical phase LiB3O5.

cond-mat.mtrl-sci↗

arXiv2010

Flat Dielectric Response in 3BaO-3TiO2-B2O3 Glasses

X-ray powder diffraction (XRD) along with differential thermal analysis (DTA) carried out on the as-quenched samples in the 3BaO-3TiO2-B2O3 system confirmed their amorphous and glassy nature, respectively. The dielectric constants in the 1 kHz-1 MHz frequency range were measured as a function of temperature (323-748 K). The dielectric constant and loss were found to be frequency independent in the 323-473 K temperature range. The dielectric behavior was consistent with the universal dielectric response (UDR). The temperature coefficient of dielectric constant was estimated using Havinga's formula and found to be 16 ppm.K-1. The electrical relaxation was rationalized using the electric modulus formalism. The dielectric constant and loss were 17 \pm 0.5 and 0.005 \pm 0.001, respectively at 323 K in the 1 kHz-1 MHz frequency range which may be of considerable interest to capacitor industry.

cond-mat.mtrl-sci↗

arXiv2010

Ion relaxation dynamics in 0.5Li2O-0.5Na2O-2B2O3 (LiNaB4O7) glasses

The frequency and temperature dependence of the dielectric constant, electric modulus and electrical conductivity of the transparent glasses in the composition 0.5Li2O-0.5Na2O-2B2O3 (LNBO) were investigated in the 100 Hz - 10 MHz frequency range. The dielectric data have been analyzed using Cole-Cole equation with an addition of the conductivity term. The electrical conductivity was found to be obeying Jonscher's universal law. The dielectric constant and the loss for the as-quenched glasses increased with increasing temperature, exhibiting anomalies in the vicinity of the glass transition and crystallization temperatures. The bulk DC conductivity at various temperatures was extracted from the electrical relaxation data. Conductivity plots were not found to follow Summerfield scaling due to mixed alkali effect. The imaginary part of electric modulus spectra was modeled using an approximate solution of Kohlrausch-Williams-Watts (KWW) relation. The stretching exponent, β, was found to be 0.56 and temperature independent.

cond-mat.mtrl-sci↗

arXiv2009

The glass transition and crystallization kinetic studies on BaNaB9O15 glasses

Transparent glasses of BaNaB9O15 (BNBO) were fabricated via the conventional melt-quenching technique. The amorphous and the glassy nature of the as-quenched samples were respectively, confirmed by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC). The glass transition and crystallization parameters were evaluated under non-isothermal conditions using DSC. The correlation between the heating rate dependent glass transition and the crystallization temperatures was discussed and deduced the Kauzmann temperature for BNBO glass-plates and powdered samples. The values of the Kauzmann temperature for the plates and powdered samples were 776 K and 768 K, respectively. Approximation-free method was used to evaluate the crystallization kinetic parameters for the BNBO glass samples. The effect of the sample thickness on the crystallization kinetics of BNBO glasses was also investigated.

cond-mat.mtrl-sci↗

arXiv2008

Enhancement of nonvolatile polarization and pyroelectric sensitivity in Lithium tantalate (LT)/ Poly (vinylidene fluoride) (PVDF) nano composite

For pyroelectric sensor application, materials having large ferroelectric polarization at low applied field and high pyroelectric sensitivity are required. PVDF is one of the ferroelectric polymers, which is being used for pyroelectric detector application. Following properties of PVDF such as low density, low acoustic impedance, wide bandwidth, flexibility, toughness, and ease of fabrication into complex patterns and arrays make it suitable for large area detector applications. But the PVDF has low value of pyroelectric coefficient (p). High field (above 1200kV/cm) is required to pole the PVDF film. To reduce poling field and to get high value of ferroelectric polarization at low poling field, nano composite films of LT/PVDF have been prepared. Since LT is ferroelectric (max. poling field ~ 200kV/cm and (p) ~ 2.3 * 10-8 C cm-2/ K) in nature, the nonvolatile polarization of composite increased from 0.014microC/cm2 to 2.06microC/cm2 at an applied field of 150kV/cm as the volume fraction of LT (fLT) nano particle increases from 0.0 to 0.17. Similarly the increase of fLT from 0.0 to 0.17 results in increase of pyroelectric voltage sensitivity from 3.93 V/J to 18.5V/J.

cond-mat.mtrl-sci↗

arXiv2008

Crystallization of Beta-phase Poly (vinylidene fluoride) films using dimethyl sulfoxide (DMSO) solvent and at suitable annealing condition

In literature it has been reported that the gamma-phase PVDF film is formed from dimethyl sulfoxide (DMSO) solvents regardless of preparation temperature. In this report the crystallization of both alpha and gamma phase from DMSO solvent by varying preparation temperature has been described. This paper also describes the conversion of gamma-phase to beta- phase and alpha-phase at different annealing condition. When thin films were annealed at 90C for 5 hours, then maximum beta-phase content (greater than 95%) is present in PVDF thin film. The PVDF films completely converted to alpha-phase, when they were annealed at 160C for 5 hours. From (X-ray diffraction) XRD, Fourier Transform Infrared Spectrum (FTIR), (Differential thermal analysis) DTA and Raman studies, it is confirmed that the PVDF thin films, cast from DMSO solution and annealed at 90C for 5 hours, have maximum percentage of beta-phase(greater than 95%).

cond-mat.mtrl-sci↗

arXiv2008

Study on dielectric behavior of Lithium Tantalate(LT) nano particle filled poly (vinylidene fluoride) (PVDF) nano composite

For pyroelectric detector application materials should have low dielectric constant, high pyroelectric coefficient, large non volatile polarization at small applied electric field and low specific heat. Large field (greater than 1200kV/cm) is need to pole ferroelectric polymer poly (vinylidene fluoride) (PVDF) and it has low sensitivity compared to other pyroelectric materials. To increase non volatile polarization at low poling field and to increase pyroelectric coefficient, LiTaO3 (LT) nano particles were added to PVDF matrix to make LT/PVDF composite. It is important to study the dielectric properties of the composite (to be used in detector application) because dielectric constant varies with volume fraction of filler and with frequency. Nano composite films of LT/PVDF with different volume fraction (i. e fLT = 0.047, 0.09 and 0.17) of LT were prepared by dispersing LT nano particles in solution of PVDF. The dielectric properties of LT/PVDF composite were studied by varying the volume fraction of LT. The dielectric permittivity of LT/ PVDF composites increased compared to PVDF as the volume fraction of LT increases but the loss tangent is almost constant at higher frequency. In low frequency region, for fLT = 0.17 the dielectric permittivity of composite is greater than PVDF and LT. The dielectric loss tangent is also increased from 0.04 to 0.175 as fLT increases from 0 to 0.17 at 1 kHz. The dielectric permittivity behavior of composite has been explained using percolation model and space charge polarization model.

cond-mat.mtrl-sci↗