Thermal and electrical switching studies on Ge20Se80-xBix (1 <= x <= 13) ternary chalcogenide glassy system

Alternating Differential Scanning Calorimetric (ADSC) and electrical switching studies have been undertaken on Ge20Se80-xBix glasses (1 <= x <= 13), to understand the effect of topological thresholds on thermal properties and electrical switching behavior. It is found that the compositional dependence of glass transition temperature (Tg), crystallization temperature (T-c1) and thermal stability (AT) of Ge20Se80-xBix glasses show anomalies at a composition x= 5, the rigidity percolation/stiffness threshold of the system. Further, unusual variations are also observed in different thermal properties, such as T-g, T-c1, Delta T, Delta C-p and Delta H-NR, at the composition x= 10, which indicates the occurrence of chemical threshold in these glasses at this composition. Electrical switching studies indicate that Ge20Se8o_RBig glasses with 5 11 exhibit threshold switching behavior and those with x = 12 and 13 show memory switching. A sharp decrease has been noticed in the switching voltages with bismuth concentration, which is due to the more metallic nature of bismuth and the presence of Bi+ ions. Further, a saturation is seen in the decrease in V-T around x = 6, which is related to bismuth phase percolation at higher concentrations of Bi. (C) 2010 Elsevier B.V. All rights reserved.

Chemical toughening of epoxies. II. Mechanical, thermal, and microscopic studies of epoxies toughened with hydroxyl-terminated poly(butadiene-co-acrylonitrile)

Diglycidyl ether–bisphenol-A-based epoxies toughened with various levels (0–12%) of chemically reacted liquid rubber, hydroxyl-terminated poly(butadiene-co-acrylonitrile) (HTBN) were studied for some of the mechanical and thermal properties. Although the ultimate tensile strength showed a continuous decrease with increasing rubber content, the toughness as measured by the area under the stress-vs.-strain curve and flexural strength reach a maximum around an optimum rubber concentration of 3% before decreasing. Tensile modulus was found to increase for concentrations below 6%. The glass transition temperature Tg as measured by DTA showed no variation for the toughened formulations. The TGA showed no variations in the pattern of decomposition. The weight losses for the toughened epoxies at elevated temperatures compare well with that of the neat epoxy. Scanning electron microscopy revealed the presence of a dual phase morphology with the spherical rubber particles precipitating out in the cured resin with diameter varying between 0.33 and 6.3 μm. In contrast, a physically blended rubber–epoxy showed much less effect towards toughening with the precipitated rubber particles of much bigger diameter (0.6–21.3 μm).

Studies on glass transition and starch re-crystallization in wheat bread during staling using electrical impedance spectroscopy

Bread staling is a very complex phenomenon that is not yet completely understood. The present work explains how the electrical impedance spectroscopy technique can be utilized to investigate the effect of staling on the physicochemical properties of wheat bread during storage. An instrument based on electrical impedance spectroscopy technique is developed to study the electrical properties of wheat bread both at its crumb and crust with the help of designed multi-channel ring electrodes. Electrical impedance behavior, mainly capacitance and resistance, of wheat bread at crust and crumb during storage (up to 120 h) is investigated. The variation in capacitance showed the glass transition phenomenon at room temperature in bread crust after 96 h of storage with 18% of moisture in it. The resistance changes at bread crumb showed the starch recrystallization during staling.

Pressure sensitive flow and constraint factor in amorphous materials below glass transition

The constraint factor, C (given by the hardness-yield strength ratio H/Y in the fully lastic regime of indentation), in metallic glasses, is greater than three, a reflection of the sensitivity of their plastic flow to pressure. Furthermore, C increases with increasing temperature. In this work, we examine if this is true in amorphous polymers as well, through experiments on amorphous poly(methyl methacrylate) (PMMA). Uniaxial compression as well as spherical indentation tests were conducted in the 248-348 K range to construct H/Y versus indentation strain plots at each temperature and obtain the C-values. Results show that C increases with temperature in PMMA as well. Good correlation between the loss factors, measured using a dynamic mechanical analyzer, and C, suggest that the enhanced sensitivity to pressure is possibly due to beta-relaxation. We offer possible mechanistic reasons for the observed trends in amorphous materials in terms of relaxation processes.

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 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 studied and the Kauzmann temperature was deduced 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. An 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.

Effect of strain rate and temperature on the plastic deformation behaviour of a bulk metallic glass composite

The composites consisting of amorphous matrix reinforced with crystalline dendrites offer extraordinary combinations of strength, stiffness, and toughness and can be processed in bulk. Hence, they have been receiving intense research interest, with a primary focus to study their mechanical properties. In this paper, the temperature and strain rate effects on the uniaxial compression response of a tailored bulk metallic glass (BMG) composite has been investigated. Experimental results show that at temperatures ranging between ambient to 500 K and at all strain rates; the onset of plastic deformation in the composite is controlled by that in the dendrites. As the temperature is increased to the glass transition temperature of the matrix and beyond, flow in the amorphous matrix occurs readily and hence it dictates the composite's response. The role of the constituent phases in controlling the deformation mechanism of the composite has been verified by assessing the strain rate sensitivity and the activation volume for deformation. The composite is rate sensitive at room temperature with values of strain rate sensitivity and activation volume being similar to that of the dendrites. At test temperatures near to the glass transition temperature, the composite however becomes rate-insensitive corresponding to that of the matrix phase. At low strain rates, serrated flow akin to that of dynamic strain ageing in crystalline alloys was observed and the serration magnitude decreases with increasing temperature. Initiation of the shear bands at the dendrite/matrix interface and propagation of them through the matrix ligaments until their arrest at another interface is the responsible mechanism for this. (C) 2011 Elsevier B.V. All rights reserved.

Lowering of relaxor transition temperature in Lead-based perovskites under pressure

The dielectric constants of lead iron niobate (PFN) and 40% lead zinc niobate (PZN) added to lead iron niobate (PFN0.6-PZN(0.4)) have been measured as a function of pressure up to 6 GPa under isothermal conditions between room temperature and 348 K. The relaxer transition temperature measured at 1 kHz excitation frequency varies at a rate -24.5 K/GPa for PFN and at a rate of - 28.8 K/GPa for the PFN0.6-PZN(0.4) composition.

Cluster model of the glass transition

A cluster model of the glass transition has been developed, treating the relative size of the cluster as an order parameter. The model accounts for some of the features of the glass transition.

ESR studies of non-silicate inorganic glasses through their glass transformation ranges

ESR spectra of three inorganic glasses doped with Mn2+ and Fe3+ have been studied through their glass transition temperatures (Tg). Spectral features in each case have been discussed with reference to site symmetries. The intensity of the ESR signal has been bound to decrease in the region of Tg. An attempt has been made to explain this interesting feature on the basis of a two-state model.

Role of free volume in strain softening of as-cast and annealed bulk metallic glass

Plasticity in amorphous alloys is associated with strain softening, induced by the creation of additional free volume during deformation. In this paper, the role of free volume, which was a priori in the material, on work softening was investigated. For this, an as-cast Zr-based bulk metallic glass (BMG) was systematically annealed below its glass transition temperature, so as to reduce the free volume content. The bonded-interface indentation technique is used to generate extensively deformed and well defined plastic zones. Nanoindentation was utilized to estimate the hardness of the deformed as well as undeformed regions. The results show that the structural relaxation annealing enhances the hardness and that both the subsurface shear band number density and the plastic zone size decrease with annealing time. The serrations in the nanoindentation load-displacement curves become smoother with structural relaxation. Regardless of the annealing condition, the nanohardness of the deformed regions is similar to 12-15% lower, implying that the prior free volume only changes the yield stress (or hardness) but not the relative flow stress (or the extent of strain softening). Statistical distributions of the nanohardness obtained from deformed and undeformed regions have no overlap, suggesting that shear band number density has no influence on the plastic characteristics of the deformed region.

Optical Properties of Nanocrystalline Potassium Lithium Niobate in the Glass System (100-x) TeO2-x(1.5K(2)O-Li2O-2.5Nb(2)O(5))

Optically clear glasses of various compositions in the system (100-x) TeO2-x(1.5K(2)O-Li2O-2.5Nb(2)O(5)) (2 <= x <= 12, in molar ratio) were prepared by the melt-quenching technique. The glassy nature of the as-quenched samples was established via differential scanning calorimetry (DSC). The amorphous and the crystalline nature of the as-quenched and heat-treated samples were confirmed by the X-ray powder diffraction and transmission electron microscopic (TEM) studies. Transparent glasses comprising potassium lithium niobate (K3Li2Nb5O15) microcrystallites on the surface and nanocrystallites within the glass were obtained by controlled heat-treatment of the as-quenched glasses just above the glass transition temperature (T-g). The optical transmission spectra of these glasses and glass-crystal composites of various compositions were recorded in the 200-2500 nm wavelength range. Various optical parameters such as optical band gap, Urbach energy, refractive index were determined. Second order optical non-linearity was established in the heat-treated samples by employing the Maker-Fringe method.

Toughness of as-cast and partially crystallized composites of a bulk metallic glass

The deformation and fracture response of a bulk metallic glass (BMG) post-annealing above the glass transition temperature is examined. The toughness of the glass-matrix composite exhibits a sharp transition beyond a critical volume fraction of crystallization to values as low as that of brittle silicate glass. Instrumented indentation tests supplemented by impact tests were used to study this ductile to brittle transition exhibited by the partially crystallized samples. Indentation on the anneal-embrittled specimens shows lateral cracks in addition to cracks along the corners. The applicability of the Poisson's ratio-toughness correlation with respect to partially crystallized samples is also investigated.

Low temperature Properties of glasses-unsolved problems

Glasses show very interesting behavior well below the glass transition temperature. Inspite of various experimental observations, even simple quantitative explanations relating these relaxation phenomena to structural properties are absent. In this paper we have tried to point out a phenomenological approach to this problem by identifying certain parameters which we think can be used to characterize these relaxations.

Glass-Transition In Isopentane - A Monte-Carlo Study

Monte Carlo simulations with realistic interaction potentials have been carried out on isopentane to investigate the glass transition. Intermolecular pair-correlation functions of the glass show distinct differences from those of the liquid, the CH-CH pair-correlation function being uniquely different from the other pair-correlation functions. The coordination number of the glass is higher than that of the liquid, and the packing in the glass seems to be mainly governed by the geometrical constraints of the molecule. Annealing affects the properties of the glass significantly.

Ultrasonic velocities and thermal expansion coefficients of amorphous Se80Te20 and Se90Te10 alloys near glass transitions

Precise measurements of the ultrasonic velocities and thermal expansivities of amorphous Se80Te20 and Se90Te10 alloys are reported near the glass transition. The samples are produced by liquid quenching. The longitudinal and transverse velocities are measured at 10 MHz frequency using the McSkimin pulse superposition technique. The thermal expansivities,agr, are measured using a three-terminal capacitance bridge. Theagr-values show a sharp maximum near the glass transition temperature,T g. The ultrasonic velocities also show a large temperature derivative, dV/dT nearT g. The data are discussed in terms of existing theories of the glass transition. The continuous change inagr shows that the glass transition is not a first-order transition, as suggested by some theories. The samples are found to be deformed by small loads nearT g. The ultrasonic velocities and dV/dT have contributions arising from this deformation.