Sevi Murugavel

Temperature and frequency dependent conductivity of bismuth zinc vanadate semiconducting glassy system

The ac conductivity of bismuth zinc vanadate glasses with compositions 50V2O5. xBi2O3. (50-x) ZnO has been studied in the frequency range 10−1 Hz to 2 MHz and in temperature range 333.16 K to 533.16 K. The temperature and frequency dependent conductivity is found to obey Jonschers universal power law for all the compositions of bismuth zinc vanadate glass system. The dc conductivity (σdc), crossover frequency (ωH), and frequency exponent (s) have been estimated from the fitting of experimental data of ac conductivity with Jonschers universal power law. Enthalpy to dissociate the cation from its original site next to a charge compensating center (Hf) and enthalpy of migration (Hm) have also been estimated. It has been observed that mobility of charge carriers and ac conductivity in case of zinc vanadate glass system increases with increase in Bi2O3 content. In order to determine the conduction mechanism, the ac conductivity and its frequency exponent have been analyzed in the frame work of various theore...

Structural study on amorphous and crystalline state of phase change material

We report an inelastic (Raman) light scattering study on bulk crystalline GeTe (c-GeTe) and amorphous GeTe (a-GeTe) thin films and found to show pronounced similarities in local structure between the two states. In c-GeTe, the observed Raman modes represent the Ge atoms are in three different environments, namely, tetrahedral, distorted, and defective octahedral sites. On the other hand, in a-GeTe, Raman spectrum reveals Ge sites in tetrahedral and defective octahedral environment. We suggest that the structure of c-GeTe consists of highly distorted as well as defective Ge sites, which leads to the large concentration of intrinsic defects (vacancies). These random defects would act as topological disorder in the lattice and cause the bands to develop tails at the band edges, a continuum of localized levels appearing in the gap. The present study deepens the understanding of the local atomic structure, influence of defects and its close relation to the phase-change mechanism.

Cation diffusion and ionic conductivity in soda-lime silicate glasses

We have studied the mobilities of calcium and sodium ions in silicate glasses of compositions xNa2O (3 - x)CaO x 4SiO2 with x = 0.0, 0.1, 0.3, 1.0 and 3.0 by means of radiotracer diffusion, electrical conductivity measurements, and dynamic mechanical thermal analyses. In glasses containing sodium oxide, the Na+ ions are much more mobile than the Ca2+ ions, and are, therefore, governing the electrical conductivity. In the pure calcium silicate glass, the activation energy of Ca2+ diffusion is higher than the activation energy of the electrical conductivity. This provides strong evidence that the electrical conductivity of this glass is not determined by the migration of Ca2+ ions, but by impurity charge carriers, which are most likely Na+ ions. We sketch the composition-dependent mobilities of Na+ and Ca2+ ions in soda-lime silicate glasses with variable Na2O and CaO content. Our results indicate that the coordination environment of Ca2+ ions remains unchanged when CaO is replaced by Na2O which is consistent with recent results of molecular dynamic simulations. Moreover, our results confirm the formation of dissimilar Na-Ca pairs which lead to a non-random mixing of the cations in the glass. The formation of such pairs was recently deduced from nuclear magnetic resonance spectra of soda-lime silicate glasses.

Hopping conduction in bismuth modified zinc vanadate glasses: An applicability of Mott's model

The dc conductivity measured in a wide range of temperatures (room temperature to 533.16 K) for glass samples of compositions 50V2O5·xBi2O3·(50-x) ZnO; x = 0, 5, 10, 15, and 20, is discussed in this paper. The temperature dependent dc conductivity has been analyzed in the framework of various theoretical models, which describe the hopping conduction in disordered semiconducting systems. It has been observed that Motts model of polaron hopping in transition metals is in good agreement with the experimental data in high as well as intermediate temperature regions. The various polaron hopping parameters have also been deduced. It has been ascertained by these estimated quantities and different approaches that the electrical conduction in present glass system is due to non-adiabatic variable range hopping of small polarons. Moreover, it has been found that Motts and Greaves’ variable range hopping models are in good agreement with the experimental data in the whole studied temperature range in the present i...

Nonlinear ionic conductivity of thin solid electrolyte samples : Comparison between theory and experiment

Nonlinear conductivity effects are studied experimentally and theoretically for thin samples of disordered ionic conductors. Following previous work in this field the {\it experimental nonlinear conductivity} of sodium ion conducting glasses is analyzed in terms of apparent hopping distances. Values up to 43 \AA are obtained. Due to higher-order harmonic current density detection, any undesired effects arising from Joule heating can be excluded. Additionally, the influence of temperature and sample thickness on the nonlinearity is explored. From the {\it theoretical side} the nonlinear conductivity in a disordered hopping model is analyzed numerically. For the 1D case the nonlinearity can be even handled analytically. Surprisingly, for this model the apparent hopping distance scales with the system size. This result shows that in general the nonlinear conductivity cannot be interpreted in terms of apparent hopping distances. Possible extensions of the model are discussed.

Alkali oxide containing mesoporous bioactive glasses: synthesis, characterization and in vitro bioactivity.

We report, for the first time, the synthesis of sodium oxide containing mesoporous bioactive quaternary glasses and compared with two different mesoporous ternary silicate systems by modified sol-gel process. With the aid of three different glass systems, a systematic analysis has been made on phosphorous-bearing (P-bearing) and phosphorous-free (P-free) mesoporous bioactive glasses to investigate the role of phosphorus on in vitro bioactivity of various silicate glasses with constant alkali oxide content. The combined use of multiple analytical techniques XRD, FTIR, SEM, nitrogen adsorption/desorption analysis before and after soaking in the SBF solution allowed us to establish strong correlation between composition, pore structure and bioactivity. We find that the P-bearing mesoporous glasses show the rapid hydroxycarbonate apatite (HCA) crystallization than P-free mesoporous glasses independent of calcium content. The present study reveals that the presence of phosphorous jointly with calcium in the bioactive glass system significantly enhances the rate of apatite formation as well as crystallization of apatite phase. Additionally, we find that a glass with sodium orthophosphate rich phase enhances the solubility when immersed in SBF and further accelerate the kinetics of apatite formation. The influences of the chemical composition and their superior textural properties on bioactivity are explained in terms of the unique structure of mesoporous bioactive glasses.

Diffusion of calcium and barium in alkali alkaline-earth silicate glasses

Diffusion of the radioactive isotopes 45Ca and 133Ba in alkali alkaline-earth silicate glasses of compositions 2O·2CaO·4SiO2 with =Li, K, Cs and K2O·2BaO·4SiO2 was investigated at temperatures below the respective glass-transition temperatures. The diffusion profiles of the radiotracers were recorded by means of an ion-beam sputtering technique in conjunction with activity measurements. Analysis of the Ca and Ba profiles is based on the thin-film solution of the diffusion equation and a contribution which takes into account the instrumental broadening of the profiles caused by sputtering effects. The diffusion coefficients of the alkaline-earth ions were determined down to values of as low as 10−21 m2 s−1. Our diffusion data enables us to determine accurate values for the activation enthalpy of diffusion and the pre-exponential factor. The activation enthalpies reveal a distinct correlation with the radius ratio of the alkaline-earth ions to the alkali ions and show that the alkaline-earth ions are most mobile when the radii of the alkaline-earth and alkali ions are similar. The diffusion study strongly supports recent results from mechanical loss spectroscopy. A simple model is proposed which qualitatively explains the correlation of the alkaline-earth mobility with the radius ratio of the cations.

Bio-inspired synthesis of microporous bioactive glass-ceramic using CT-DNA as a template

We report, for the first time, bio-inspired synthesis of a bioactive glass-ceramic with superior textural properties in atmospheric conditions using CT-DNA as template. The phase composition, structure, morphology, and textural properties of the bioactive glass sample were evaluated with various analytical techniques before and after in vitro tests. The BET surface area analysis of the obtained glass-ceramic sample reveals that it possesses a high surface area with a range of (micro- to meso-) pore sizes. The TEM analysis of the glass-ceramic phase indicates that the amorphous phase consists of spherical particles, whereas the crystalline phase is found to have needle-like shape. In the glass-ceramic, we find a new type of crystalline phase (Na0.11Ca0.89)(P0.11Si0.89)O3, which is different from the earlier observation on 45S5® glass-ceramic sample. The accelerated in vitro bioactivity of the glass-ceramic is evidenced based on the hydroxyl carbonate apatite (HCA) layer formation on the glass-ceramic surface after immersing the bioglass sample in simulated body fluid (SBF), by FTIR, SEM and EDX analysis. Additionally, the ion release kinetics of the bioglass sample in SBF is followed by ICP-AES with simultaneous pH measurements. The in vitro cytotoxicity experiments on the glass-ceramic sample using osteosarcoma cells by following the MTT assay method indicate that the sample has good biocompatibility and may serve as an effective biomaterial for bone tissue engineering.

Bulk and Interfacial Ionic Conduction in LiAlSiO4 Glass Ceramics Containing Nano- and Microcrystallites

Abstract The lithium ion transport in LiAlSiO4 glass and glass ceramics with different degrees of crystallinity, χ, is studied by means of conductivity spectroscopy. In glass ceramics with low degrees of crystallinity, χ < 0.42, the lithium ion conductivity is higher than in the glass and increases with increasing χ. We ascribe this conductivity enhancement to fast ionic conduction at the interfaces between the crystallites and the glassy phase. The dependence of the conductivity on the degree of crystallinity is fitted by using a three-phase continuum percolation model, the phases being the ion conducting glassy phase, the insulating crystallites and the highly conducting interfaces. In the framework of this model, we have to make the assumption that the width of the interfaces is about 1/3 of the crystallite radii. Possible origins of these anomalous interfacial widths are discussed.

Probing ion transport at the nanoscale: Time-domain electrostatic force spectroscopy on glassy electrolytes

We have carried out time-domain electrostatic force spectroscopy on two different ion-conducting glasses using an atomic force microscope. We compare the electrostatic force spectroscopic data obtained at different temperatures with macroscopic electrical data of the glasses. The overall consistency of the data shows that electrostatic force spectroscopy is capable of probing the ion dynamics and transport in nanoscopic subvolumes of the samples.