The elastic properties of a group of optical glasses by the pulse echo set-up

The pulse-echo apparatus, designed and constructed by the author, has been used to reinvestigate the elastic properties of the eighteen optical glasses. The elastic constants are correct to 0·5%. The results are compared with the earlier investigation which utilised the optical method. The possible causes for large discrepancies observed are critically and briefly discussed. A qualitative interpretation of the results has been successfully attempted. The acoustic velocity increases with the decrease in lead and barium oxides and with increase in calcium oxide and boron trioxide components.

Elastic Properties Of Sodium Borovanadate Glasses

The elastic properties of sodium borovanadate glasses have been studied over a wide range of composition using ultrasonic measurements. It is found that variation of different elastic moduli is very similar in any given series of composition. The bulk and shear moduli show a monotonic variation with the covalent bond energy densities calculated from the proposed structural model for these glasses. The bulk moduli also vary as a negative power function of the mean atomic volume. The Debye temperature varies linearly with the glass transition temperature. The implications of the observed behavior have been discussed.

Spectroscopic investigations of manganese ions in microwave-prepared NaPO3-PbO glasses

The EPR spectra of microwave-prepared 70NaPO(3):30PbO glasses containing different weight percentages of manganese ions have been studied. The EPR spectra exhibit a well-resolved hyperfine pattern at g(eff) approximate to 2.0. Optical absorption, fluorescent emission and excitation spectra of the glasses have been examined. The absorption spectrum exhibits a peak near 500 nm and this has been attributed to the spin-allowed E-5(g) --> T-5(2g) transition of Mn3+ ions. The emission spectrum shows a band at 595 nm which has been assigned to the T-4(1g)(G) --> (6)A(1g)(S) spin-forbidden transition of Mn2+ ions in octahedral coordination. Concentration quenching of fluorescence was found to occur above 0.75 wt% of Mn2+ ions. The excitation spectra exhibit four bands characteristic of Mn2+ ions in octahedral coordination. From the observed band positions of the excitation spectra, the crystal field parameter D-q and the Racah interelectronic repulsion parameters, B and C have been calculated. A structural model is proposed based on the IR, Raman and MASNMR studies according to which Mn2+ ions are likely to occupy sites similar to Na+ ions in these glasses.

Structural and lithium ion transport studies in sol-gel-prepared lithium silicophosphate glasses

Lithium silicophosphate glasses have been prepared by a sol-gel route over a wide range of compositions. Their structural and electrical properties have been investigated. Infrared spectroscopic studies show the presence of hydroxyl groups attached to Si and P. MAS NMR investigations provide evidence for the presence of different phosphatic units in the structure. The variations of de conductivities at 423 K and activation energies have been studied as a function of composition, and both exhibit an increasing trend with the ratio of nonbridging oxygen to bridging oxygen in the structure. Ac conductivity behavior shows that the power law exponent, s, is temperature dependent and exhibits a minimum. Relaxation behavior has been examined in detail using an electrical modulus formalism, and modulus data were fitted to Kohlraush-William-Watts stretched exponential function. A structural model has been proposed and the unusual properties exhibited by this unique system of glasses have been rationalized using this model. Ion transport in these glasses appears to be confined to unidimensional conduits defined by modified phosphate chains and interspersed with unmodified silica units.

Structural correlations in GexSe1-x glasses - a neutron diffraction study

Neutron diffraction measurement is carried out on GexSe1-x glasses, where 0.1 less than or equal to x less than or equal to 0.4, in a Q interval of 0.55-13.8 Angstrom(-1). The first sharp diffraction peak (FSDP) in the structure factor, S(Q), shows a systematic increase in the intensity and shifts to a lower Q with increasing Ge concentration. The coherence length of FSDP increases with x and becomes maximum for 0.33 less than or equal to x less than or equal to 0.4. The Monte-Carlo method, due to Soper, is used to generate S(Q) and also the pair correlation function, g(r). The generated S(Q) is in agreement with the experimental data for all x. Analysis of the first four peaks in the total correlation function, T(r), shows that the short range order in GeSe2 glass is due to Ge(Se-1/2)(4) tetrahedra, in agreement with earlier reports. Se-rich glasses contain Se-chains which are cross-linked with Ge(Se-1/2)(4) tetrahedra. Ge-2(Se-1/2)(6) molecular units are the basic structural units in Ge-rich, x = 0.4, glass. For x = 0.2, 0.33 and 0.4 there is evidence for some of the tetrahedra being in an edge-shared configuration. The number of edge-shared tetrahedra in these glasses increase with increasing Ge content.

Effect of antimony addition on the thermal and electrical-switching behavior of bulk Se-Te glasses

The thermal properties and electrical-switching behavior of semiconducting chalcogenide SbxSe55-xTe45 (2 <= x <= 9) glasses have been investigated by alternating differential scanning calorimetry and electrical-switching experiments, respectively. The addition of Sb is found to enhance the glass forming tendency and stability as revealed by the decrease in non-reversing enthalpy Delta H-nr. and an increase in the glass-transition width Delta T-g. Further, the glass-transition temperature of SbxSe55-xTe45 glasses, which is a measure of network connectivity, exhibits a subtle increase, suggesting a meager network growth with the addition of Sb. The crystallization temperature is also observed to increase with Sb content. The SbxSe55-xTe45 glasses (2 <= x <= 9) are found to exhibit memory type of electrical switching, which can be attributed to the polymeric nature of network and high devitrifying ability. The metallicity factor has been found to dominate over the network connectivity and rigidity in the compositional dependence of switching voltage. which shows a profound decrease with the addition of Sb.

Influence of lanthanum doping on the dielectric, ferroelectric and relaxor behaviour of barium bismuth titanate ceramics

Barium lanthanum bismuth titanate (Ba1−(3/2)xLaxBi4Ti4O15, x = 0–0.4) ceramics were fabricated using the powders synthesized via the solid-state reaction route. X-ray powder diffraction analysis confirmed the above compositions to be monophasic and belonged to the m = 4 member of the Aurivillius family of oxides. The effect of the partial presence of La3+ on Ba2+ sites on the microstructure, dielectric and relaxor behaviour of BaBi4Ti4O15 (BBT) ceramics was investigated. For the compositions pertaining to x ≤ 0.1, the dielectric constant at both room temperature and in the vicinity of the temperature of the dielectric maximum (Tm) of the parent phase (BBT) increased significantly with an increase in x while Tm remained almost constant. Tm shifted towards lower temperatures accompanied by a decrease in the magnitude of the dielectric maximum (εm) with an increase in the lanthanum content (0.1 < x ≤ 0.4). The dielectric relaxation was modelled using the Vogel–Fulcher relation and a decrease in the activation energy for frequency dispersion with increasing x was observed. The frequency dispersion of Tm was found to decrease with an increase in lanthanum doping, and for compositions corresponding to x ≥ 0.3, Tm was frequency independent. Well-developed P(polarization)–E(electric field) hysteresis loops were observed at 150 °C for all the samples and the remanent polarization (2Pr) was improved from 6.3 µC cm−2 for pure BBT to 13.4 µC cm−2 for Ba0.7La0.2Bi4Ti4O15 ceramics. Dc conductivities and associated activation energies were evaluated using impedance spectroscopy.

Thermal Crystallization Studies On Ga-Te Glasses

Semieonducting GaxTe~oo-x (17 -< x _< 25) glasses have been prepared by melt quenching method and thermal crystallization studies carried out using differential scanning calorimetry. On heating, virgin GaxTel0o-x glasses exhibit one glass transition and two crystallization reactions.The first crystallization reaction corresponds to the precipitation of hexagonal Te and the second one to the crystallization of the matrix into zinc blende Ga2Te3 phase. If GaxTeloo-x glasses are quenched to ambient temperature from Tcrl and reheated, they exhibit the phenomenon of double glass transition.

Analysis of Raman bandwidths and bandshapes of glasses through their glass-transition temperatures

Raman bandwidths and bandshapes of some molecular and ionic glasses have been investigated through the glass-transition region. Widths of both polarised and depolarised bands exhibit step-like changes during the glass transition. Molecular and ionic glasses differ with respect to the magnitude and the nature of variations in bandwidths and reorientational times. An attempt has been made to understand the changes in bandwidths around the glass-transition temperature.

Dielectric Relaxation in SrLiB9O15 Glasses

Transparent SrLiB9O15 (SLBO) glasses were fabricated via the conventional melt-quenching technique. X-ray powder diffraction and differential thermal analysis carried out on the as-quenched samples confirmed their amorphous and glassy nature, respectively. The dielectric constants in the 100 Hz to 10 MHz frequency range for SLBO glasses were measured as a function of temperature (300–1023 K). The dielectric relaxation characteristics were rationalized using the electric modulus formalism. The electrode polarization effect was subtracted from the low-frequency dielectric constant to have an insight into the intrinsic dielectric behavior of SLBO glasses. The imaginary part of electric modulus spectra was modeled using an approximate solution of Kohlrausch–Williams–Watts relation. The dielectric constant for the as-quenched glass increased with increasing temperature and exhibited anomalies in the vicinity of the glass transition and crystallization temperatures.

X-ray and ultraviolet photoelectron spectroscopy studies of the interaction of carbon monoxide with oxygen adsorbed on silver covered with potassium, caesium or barium

X-ray and He(II) ultraviolet photoelectron spectroscopy studies of the interaction of CO with oxygen on potassium-, caesium- and barium-covered Ag surfaces have shown the formation of carbonate at 300 K. While on a caesium-covered surface only carbonate formation takes place, on the potassium- and barium-covered surfaces molecularly chemisorbed CO is also formed. The variation of the surface concentrations of carbon and oxygen with temperature has been examined and a reaction sequence for the interaction of CO with adsorbed oxygen on potassium-, caesium- and barium-covered Ag surfaces is suggested.

High-Pressure Studies Of SE100-XTEX Glasses

High pressure resistivity measurements on Se100-xTex, glasses for 0≤x≤30 are reported. Two composition regions, where the transport and transformation behaviour are different, are identified. For 0≤x≤6, there is a first-order-like transformation to metallic crystalline states, while for x>6 the transformation appears to be continuous. Glass-transition temperatures also show differences in trends as a function of composition around 6% Te. An attempt is made to explain the composition-dependent trends on the basis of known structural features of selenium glasses and of the nature of tellurium bonding. At concentrations with up to 6% tellurium, Te most likely enters selenium chain terminations, substituting for negatively charged Se1- defects, while at larger concentrations, tellurium probably enters chains and rings by a random substitution.

ESR Studies of Phase Transitions in Double Propionates: Dicalcium Barium Propionate Ca2Ba(C2H5COO)6

Single crystal [(111) and (100) planes], and powder ESR of Mn2+ (substituting for Ca2+) in Ca2Ba(C2H5COO)6 in the temperature range 220°C to -160°C shows (i) a doubling of both the physically and chemically inequivalent sites, and a change in the magnitude (150 G at -6°C to 170 G at -8°C) as well as the orientation of the D tensor across the -6°C transition and (ii) an inflection point in the D vs T plot across the -75°C transition. The oxygen octahedra around the Ca2+ sites are inferred to undergo alternate rotations, showing the participation of the carboxyl oxygens in the -6°C transition. A relation of the type D=D0(1+αT+βT2) seems to fit the D variation satisfactorily.

Effect of pressure on the resistivity of lead oxide-lead halide glasses

Resistivity behaviour of PbO---PbX2 (X=F,Cl) glasses has been investigated as a function of pressure at laboratory temperature. All PbO---PbX2 glasses undergo crystallization under pressure and the resistivities of crystallized samples are lower than the corresponding glasses. Transitions in PbO---PbF2 glasses exhibit a first order behaviour while transitions in PbO---PbCl2 glasses possess features of a continuous transition. The differences in the pressure behaviour of the two glass systems have been attributed to the differences in the ionic sizes of F− and Cl− ions and also to pressure induced modifications of Pb---O bonding.