Studies on the Effect of Li2SO4 on the Structure of Lithium Borate Glasses

Thermal and spectroscopic investigations have been carried out on a number of glasses with a wide range of compositions in the pseudoternary glass system, Li2SO4-Li2O-B2O3, to understand the role of sulfate ions in modifying the borate glass structure. Both nuclear magnetic resonance (NMR) and infrared (IR) spectroscopic results indicate that four-coordinate boron atoms are retained in the glass structure to a greater extent in sulfate-containing glasses than in pure lithium borate glasses. There seems to be some evidence for the existence of sulfoborate-type units in Raman spectra in the region of 800-960 cm(-1). These conclusions are supported by the observed behavior of glass transition temperatures and molar volumes. The possibility of formation of sulfoborate-type units is discussed from bonding and thermodynamic points of view.

The molecular dynamics study of lithium ion conduction in phosphate glasses and the role of non-bridging oxygen

Molecular dynamics (MD) simulation of lithium phosphate (Li2O-P2O5) glasses with varying Li2O content has been carried out. Two different P-O distances corresponding to phosphorus coordination with bridging oxygen (BO) and non-bridging oxygen (NBO) were identified in the simulated glasses. NBO-BO interconversion or bond switching was noted, which results in a dynamic equilibration of the tetrahedral phosphate units (P-n, n = 1,3 indicates the number of bridging oxygen atoms in the coordination of phosphorus). The NBO-BO bond switching is mildly activated with an effective activation barrier of 0.03-0.05 eV. Lithium ion jumps do not appear to be strongly coupled to bond switching. But the number of Li+ ions coordinated to an optimum number of NBOs and the number of Li+ ions jumping out of their sites appear to be correlated. Detailed analysis was made of the dynamics of P-n species and new insights have been obtained regarding ion migration in network-modified phosphate glasses.

Thermal crystallization behaviour of Ge-Te-Se glasses

Ge10Te90-xSex (50 less than or equal to x less than or equal to 70) and Ge20Te80-xSex (x = 30, 50) glasses have been prepared by melt-quenching The thermal crystallization behaviour of these samples has been studied by Differential Scanning Calorimetry (DSC), in order to characterise these glasses for memory-threshold switching applications. It is found that Ge10Te90-xSex glasses have higher thermal stability and are more stable against devitrification. These samples may be suitable for threshold switching devices. Ge20Te80-xSex glasses, on the other hand, phase separate on heating and exhibit a double stage crystallization. Based on this, it can be expected that Ge20Te80-xSex samples will show memory behaviour. The activation energy for thermal crystallization of a representative Ge10Te40-xSe50 glass belonging to the Ge10Te90-xSex series has been found by the Kissinger's method to be 0.92 eV. The value of the activation energy obtained also indicates that Ge10Te90-xSex samples are less prone to devitrification and more suitable for threshold behaviour.

Calorimetric studies on 2TeO(2)-V2O5 glasses

Glasses of the composition 2TeO(2)-V2O5 were fabricated via the conventional melt-quenching technique. The amorphous and the glassy nature of the as-quenched samples were confirmed by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC), respectively. The glass transition and crystallization parameters were evaluated under non-isothermal conditions using DSC. X-ray diffraction studies confirmed the presence of partially oriented crystallites in the heat-treated glasses. Kauzmann temperature (lower bound for the kinetically observed glass transition) was deduced from the heating rate dependent glass transition and crystallization temperatures.

Dielectric Relaxation in CaO-Bi2O3-B2O3 Glasses

Glasses in the system CaO-Bi2O3-B2O3 (in molar ratio) have been prepared using melt-quenching route. Ion transport characteristics were investigated for this glass using electric modulus, ac conductivity and impedance measurements. The ac conductivity was rationalized using Almond-West power law. Dielectric relaxation has been analyzed based on the behavior of electric modulus behavior. The activation energy associated with the electrical relaxation determined from the electric modulus spectra was found to be 1.76 eV, close to that the activation energy for dc conductivity (1.71 eV) indicating that the same species took part in both the processes. The stretched exponent beta (0.5-0.6) is invariant with temperature for the present glasses.

Unified approach to the constraint counting theory of glasses

An approach to the constraint counting theory of glasses is applied to many glass systems which include an oxide, chalcohalide, and chalcogenides. In this, shifting of the percolation threshold due to noncovalent bonding interactions in a basically covalent network and other recent extensions of the theory appear natural. This is particularly insightful and reveals that the chemical threshold signifies another structural transition along with the rigidity percolation threshold, thus unifying these two seemingly disparate toplogical concepts. [S0163-1829(99)11441-3].

New transparent crystallized glasses with optical nonlinear LiBGeO4 crystals

Crystallization behaviors of the glass with a composition of 25Li(2)O.25B(2)O(3).50GeO(2) corresponding to lithium borogermanate LiBGeO4 have been examined. It has been confirmed that the LiBGeO4 crystalline phase is formed at the surface of heat-treated glasses. The second harmonic (SH) generation is found from transparent surface crystallized glasses, demonstrating for the first time that the LiBGeO4 phase shows optical nonlinearity. The SH intensity of LiBGeO4 crystallites (powdered state) prepared through crystallization is about ten times as large as that of pulverized alpha-quartz. The SH intensity of transparent crystallized glasses (bulk state) with crystalline layers of 3-4.5 mum thickness increases with increasing heat treatment temperature (540-560degreesC) and time (1-6 h), and the maximum SH intensity among the samples studied is in the order of 1/10 in comparison with that of alpha-quartz single crystal. The transparent crystallized glass obtained by heat treatment at 550alphaC for 3 h exhibits a clear and fine Maker fringe pattern, indicating a highly orientation of LiBGeO4 crystals at the surface.

Characterization of lithium borate–bismuth tungstate glasses and glass-ceramics by impedance spectroscopy

Transparent glasses in the system (1−x)Li2B4O7–xBi2WO6 (0≤x≤0.35) were prepared via melt quenching technique. Differential thermal analysis was employed to characterize the as-quenched glasses. Glass-ceramics with high optical transparency were obtained by controlled heat-treatment of the glasses at 720 K for 6 h. The amorphous nature of the as-quenched glass and crystallinity of glass-ceramics were confirmed by X-ray powder diffraction studies. High resolution transmission electron microscopy (HRTEM) shows the presence of nearly spherical nanocrystallites of Bi2WO6 in Li2B4O7 glass matrix. Capacitance and dielectric loss measurements were carried out as a function of temperature (300–870 K) in the frequency range 100 Hz–40 MHz. Impedance spectroscopy employed to rationalize the electrical behavior of glasses and glass-ceramics suggest the coexistence of electronic and ionic conduction in these materials. The thermal activation energies for the electronic conduction and ionic conduction were also estimated based on the Arrhenius plots.

Electrical switching in chalcogenide glasses - Some newer insights

The recent studies on the switching. behavior of several chalcogenide semiconductors indicate that there exists a close relation between the electrical switching and structural effects in these materials; the two network topological Thresholds, namely the Rigidity Percolation and the Chemical Threshold are found to influence considerably the composition dependence of the switching voltages/fields of many memory and threshold switching glasses. Further, changes in the coordination of constituent atoms are found to effect a change in the switching behavior (memory to threshold), Also, an interesting relation has been established between the type of switching exhibited and the thermal diffusivity of the material.

PL due to Discrete Gap Levels in Some Chalcogenide Glasses- A Configurational Coordinate Diagram Illustration

Photoluminescence (PL) studies were carried out on a-Se and a few Ge20Se80−xBix and Ge20Se70−xBixTe10 bulk glassy semiconductors at 4.2 K with Ar+ laser as excitation source. While a-Se and samples with lesser at% of Bi show fine structured PL with a large Stokes shift, samples with higher at% of Bi did not show any detectable PL. The investigations show at least three radiative recombination transitions. Features extracted by deconvoluting the experimental spectra show that the discrete gap levels associated with the inherent coordination defects are involved in the PL transitions. Absence of PL in samples with higher Bi at% are explained on the basis of nonradiative transition mechanisms. Overall PL mechanism involving gap levels in chalcogenide glasses is illustrated with the help of a configurational coordinate diagram.

Composition tunable memory and threshold switching in Al20As xTe80− x semiconducting glasses

I-V studies indicate a composition dependent switching behavior (Memory or Threshold) in bulk Al20AsxTe80−x glasses, which is determined by the coordination and composition of aluminum. Investigations on temperature and thickness dependence of switching and structural studies on switched samples suggest thermal and electronic mechanisms of switching for the memory and threshold samples, respectively. The present results also show that these samples have a wider composition range of threshold behavior with lower threshold voltages compared to other threshold samples.

Effect of Dilatation on the Elasto-Plastic Response of Bulk Metallic Glasses Under Indentation

Elasto-plastic response of bulk metallic glasses (BMGs) follows closely the response of granular materials through pressure dependent (or normal stress) yield locus and shear stress induced material dilatation. On a micro-structural level, material dilatation is responsible for stress softening and formation of localized shear band, however its influence on the macro-scale flow and deformation is largely unknown. In this work, we systematically analyze the effect of material dilatation on the gross indentation response of Zr-based BMG via finite element simulation. The strengthening/softening effect on the load-depth response and corresponding stress-strain profiles are presented in light of differences in elastic-plastic regimes under common indenters. Through comparison with existing experimental results, we draw conclusions regarding selection of suitable dilatation parameters for accurately predicting the gross response of BMGs

Electrical switching studies on AS(40)Te(60-x)Se(x) and As35Te65-xSex glasses

The I-V characteristics of bulk As40Te60-xSex and As35Te65-xSex glasses have been studied with a current sweep of 0-18 mA-0, over a wide range of compositions (4 less than or equal to x less than or equal to 22). All the glasses studied showed a threshold electrical switching behaviour. The number of switching cycles withstood by the samples has been found to depend on the ON-state current. It is seen that the switching voltages increase with increase in selenium content. Further, the switching voltages are found to be almost independent of the thickness of the sample (d), in the range 0.18-0.3 mm. Also, the switching voltages and the number of switching cycles withstood by the samples are found to decrease with temperature.

The effect of host glass on optical absorption and fluorescence of Nd3+ in xNa(2)O-(30-x)K2O-70B(2)O(3) glasses

The effect of host glass composition on the optical absorption and fluorescence spectra of Nd3+ has been studied in mixed alkali borate glasses of the type xNa(2)O-(30-x)K2O-69.5B(2)O(3)-0.5Nd(2)O(3) (X = 5,10,15,20 and 25). Various spectroscopic parameters such as Racah (E-1, E-2 and E-3), spin-orbit (xi(4f)) and configuration interaction (alpha, beta) parameters have been calculated. The Judd-Ofelt intensity parameters (Omega(lambda)) have been calculated and the radiative transition probabilities (A(rad)), radiative lifetimes (tau(r)), branching ratios (beta) and integrated absorption cross sections (Sigma) have been obtained for certain excited states of the Nd3+, ion and are discussed with respect to x. From the fluorescence spectra, the effective fluorescence line widths (Deltalambda(eff)) and stimulated emission cross sections (sigma(p)) have been obtained for the three transitions F-4(3/2) --> I-4(9/2), F-4(3/2) --> I-4(11/2) and F-4(3/2) --> I-4(13/2) of Nd3+. The stimulated emission cross section (sigma(p)) values are found to be in the range (2.0-4.8) x 10(-2)0 cm(2) and they are large enough to indicate that the mixed alkali borate glasses could be potential laser host materials.

The role of defects in carrier type reversal in bismuth doped Ge-Se glasses by photoluminescence spectroscopy

The role played by defects in bringing out n-type conduction in Ge20Se80-xBix and Ge20Se70-xBixTe10 glasses is using investigated photoluminescence (PL) spectroscopy. It was found that for both the systems, the compositions at lower Bi content exhibit luminescence with fine features associated while the compositions that show n-type conduction do not exhibit luminescence. The identification of the associated fine features, carried out by deconvoluting the experimental spectra, reveals that Bi addition brings out a relative diminishing in D+ defects as compared to D- ones. The study gives an overall indication for the role played by native defects in bringing out n-type conduction in Bi-doped glasses.