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Alternating differential scanning calorimetric (ADSC) studies have been carried out on bulk Ge17Te83-xTlx glasses over a wide range of compositions (0 ≤ x ≤ 13), to estimate the thermal parameters and also to understand the effect of thallium addition on the non-isothermal crystallization process. It is found that glass transition temperature (Tg) decreases with increasing thallium concentration, indicating decrease in network connectivity with thallium addition. The decrease in Tg is large in the composition range 0 ≤ x ≤ 6 and much less for compositions x ≥ 7. The observed composition dependence of Tg is similar to the variation in switching voltages with composition and is in close agreement with the variation in the average bond energy. Further, two crystallization temperatures (Tc1 and Tc2) have been observed in Ge17Te83-xTlx glasses, in the composition range 2 ≤ x ≤ 10, which indicates that glasses in this range are phase separated. As these glasses are tellurium rich, it is probable that the regions formed by Te- chains crystallize first and the rest of the matrix crystallizes later. The change in enthalpy due to non-reversing part of the heat flow is very small and fairly constant over the entire range of glasses studied. This indicates that structural re-organization is easy in this system which agrees well with the memory type electrical switching observed in these samples.

The variation of electrical resistivity in the system of glasses Ge17Te83-xTlx, with (1

Graphical abstract: - Highlights: • ZnO nanoparticles were prepared by solution combustion method. • PVA–ZnO nanocomposites were synthesized by solution casting method. • Doped and undoped films were characterized using different techniques. • Red shift in optical band gap was observed in Nanocomposite films with respect to nano ZnO. • Photoluminescence intensity was found to be optimum for PVA–10 mol% ZnO nanocomposite film. - Abstract: Nanocomposites of poly vinyl alcohol (PVA) and ZnO have been synthesized using the solution casting technique for different concentrations of nano ZnO powder prepared by low temperature solution combustion method. The formation of polymer nanocomposite and changes in the structural and micro structural properties of the materials were investigated by X-ray diffraction, Energy dispersive X ray spectroscopy and optical microscopy techniques (FTIR and UV–Visible). The surface morphology of PVA–ZnO nanocomposite films were elucidated using Scanning Electron Microscopy. The optical absorption spectrum of nano ZnO shows blue shift in the optical band gap energy with respect to characteristic bulk ZnO at room temperature, whereas PVA–ZnO hybrid films show red shift with respect to nano ZnO. The photoluminescence studies show that the intensity of the blue emission (470 nm) varies with change in concentration of ZnO withmore » an optimum intensity observed at 10 mol% of ZnO.« less

Bulk Ge(17)Te83_,JI glasses (05x.5_13), have been found to exhibit memory type electrical switching. The switching voltages (also known as threshold voltage V-th) of Ge17Te83-xTlx glasses are found to decrease with increasing thallium content. The rate of decrease of Vtry is greater at lower concentrations and \textbackslashid, falls at a slower rate for higher thallium concentrations (x 6). The addition of thallium to the Ge-Te network fragments the covalent network and introduces ionic nature to it; the reduction in network connectivity leads to the decrease in switching voltages with thallium content. The decrease in the glass transition temperatures of Ge17Te83-xTlx glasses with increasing thallium concentration supports the idea of decrease in network connectivity with TI addition. The more metallic nature of TI also contributes to the observed reduction in the switching voltages of Ge17Te83-xTlx glasses with TI content. Further, there is an interesting correlation seen between the threshold voltage V-th and the average bond energy, as a function of TI content. In addition, the switching voltages of Ge17Te83-xTlx glasses have been found to decrease with sample thickness almost linearly. The set-reset studies indicate that the Ge17Te83-xTl2 sample can be switched for more than 10 cycles, whereas other glasses could not be reset beyond two switching cycles. (C) 2010 Elsevier B.V. All rights reserved.

The temperature variation of the nuclear quadrupole resonance frequency in the two compounds 3,4-dichloro nitrobenzene and 4-chloro 2 nitrophenol were analyzed in the light of the Kushida and Brown models. The internal torsional frequencies at each temperature was calculated using Bayer’s model with Tatsuzaki correction under a two-mode approximation to obtain the temperature variation of the torsional frequencies in both compounds. The torsional frequencies are seen to lie between 25 cm i1 and 85 cm i1 in the first compound and between 30 cm i1 and 42 cm i1 in the case of the second compound. The temperature dependence of the torsional frequencies is linear above 150 K in the first compound and is linear throughout the range 77 K to 350 K in the second. The smaller temperature dependence in the second compound may be the eect of hydrogen bonding in this compound. PACS numbers: 76.60 Gv

The dielectric properties of polyvinyl alcohol (PVA) - zinc oxide (ZnO) hybrid nanocomposite films for different concentrations of nano ZnO (5 mol%-20 mol%) have been studied at ambient temperature in the frequency range 10 Hz-10 MHz. These studies indicate that the dielectric constant is maximum for 10 mol% ZnO-PVA hybrid film. An unusual low intensity loss peak is observed around 2.5-5 MHz in all the samples and this peak is more pronounced in the 10 mol% ZnO film. Impedance and modulus spectroscopy have been used to understand the dielectric and conductivity behaviour of these samples. These samples exhibit high bulk resistance and the Cole-Cole plot shows only the onset of the semicircles at room temperature. AC conductivity is found to obey Jonscher's power law and the enhancement in the conductivity is found to be highest for 10 mol% ZnO doped PVA nanocomposite film suggesting its suitability for use in sensing applications. The gas sensing performance of the hybrid films for detection of various gases such as NO2, H2S, NH3, LPG, and C2H5OH was studied and compared with that of pure PVA film. It was found that the hybrid sensor can complement the drawbacks of pure PVA as porous surface morphology and surface area of interface of nano-ZnO plays a vital role in sensing applications. At ambient temperature, 100 ppm concentration of both NO2 gas and H2S gas exhibit maximum response of 7.1% and 4.1% respectively for the 10 mol% hybrid sensor, whereas at 50 ppm concentration of H2S gas the 20 mol% ZnO-PVA sensor was found to be more sensitive with a reasonable response of 7%.

Photo-thermal Deflection (PTD) technique is used to investigate the thermal diffusivity (alpha) of Ge17Te83 - xTlx (0

Dielectric and conductivity behaviors of nano ZnO doped polyvinyl alcohol (PVA) composites for various concentrations of dopant were investigated using impedance spectroscopy for a wide range of temperatures (303 K–423 K) and frequencies (5 Hz–30 MHZ). The dielectric properties of host polymer matrix have been improved by the addition of nano ZnO and are found to be highly temperature dependent. Anomalous dielectric behavior was observed in the frequency range of 2.5 MHz–5 MHz. Increase in dielectric permittivity and dielectric loss was observed with respect to temperature. The Cole-Cole plot could be modeled by low resistance regions in a high resistance matrix and the lowest resistance was observed for the 10 mol. % films. The imaginary part of the electric modulus showed asymmetric peaks with the relaxation following Debye nature below and non-Debye nature above the peaks. The ac conductivity is found to obey Jonscher's power law, whereas the variation of dc conductivity with temperature was found to f...

The chlorine-35 NQR frequencies and their temperature variation in 2,4,6-, 2,3,6- and 2,3,4-trichloro anisoles have been studied and compared with the corresponding chlorophenols with a view to studying the effect of hydrogen bonding. The observed frequencies have been assigned to the various chlorines with the help of the additive model of the substituent effect. The temperature dependence has been analysed in terms of the Bayer—Kushida—Brown models. The torsional frequencies and their temperature dependence have been calculated numerically under a two mode approximation. A comparison of the trichloro anisoles with the corresponding trichloro phenols has shown that the resonance frequency decreases due to hydrogen bonding while the torsional frequencies are not affected.