Temperature dependence of elastic moduli of lanthanum gallogermanate glasses

Temperature dependence of sound velocities of a series of Lanthanum Gallogermanate glasses have been determined from ultrasonic pulse-echo and Brillouin scattering measurements ranging from room temperature up to and through the glass transition temperature. Both longitudinal and transverse velocities of these glasses are composition dependent. The density and index of refraction of the samples were also studied. The experimental results are used to obtain elastic moduli. The correlation of elastic stiffness, the crosslink density, and the fractal bond connectivity of the glass are discussed. The normal behavior of negative temperature dependence of elastic properties is observed in these glasses. A possible explanation of the observed discrepancy of high temperature sound velocity of these glasses from two different measurements is given. (C) 2002 Kluwer Academic Publishers

Elastic properties of lanthanum gallogermanate glasses

Room temperature ultrasonic velocities of eight lanthanum gallogermanate glasses were determined by pulse-echo technique. The results indicate that both longitudinal and transverse velocities of these glasses are composition dependent. The density and index of refraction of the samples were also studied. The experimental results were used to obtain elastic constants. The measured values of Young's modulus and bulk modulus for our glasses show good agreement with the theoretically calculated results based on the model of Makishima and Mackenzie. (C) 1999 Kluwer Academic Publishers

Impedance study of 33.3Na(2)O-(66.7-x)B2O3-xSiO(2) glasses

Glasses with composition 33.3Na(2)O-(66.7-x)B2O3-XSiO2 are studied using ac impedance. The Koblrausch-Williams-Watts function, phi(t)=exp[-(t/tau(0))(1-n)], and the coupling model are utilized for analyzing electric modulus. Conductivities increase with SiO2 contents and may differ as much as 200 times. In addition, n decrease with increasing SiO2 contents as well as temperatures. Glasses with higher SiO2 contents have smaller primitive energies (E-a). Further analyses show that the conductivities increase linearly with the ratios of the fraction of non-bridging oxygen (NBO) to the fraction of the four-coordinated boron units (N-4). The increase of conductivities with SiO2 content is attributed to the smaller primitive energies for the sodium ions associated with NBO and smaller coupling interaction between ions. (c) 2006 Elsevier B.V. All rights reserved

Mixed alkali effect in galliogermanate glasses

The relaxation of alkali ions in 20(Li(2)O-Cs(2)O)-15Ga(2)O(3)-65GeO(2) glasses is investigated in both the ac conductivity and the electric modulus formalisms. The power law exponents that are obtained from ac conductivity are close to one at relatively low temperatures and decrease with increasing of temperatures. The coupling parameters obtained from the electric modulus decrease with increasing temperatures. However, for samples with pronounced mixed alkali effect, the coupling parameters are less sensitive with temperature changes. In those samples, the ion-ion coupling and the coupling between ion and structural-relaxation are both important for ionic relaxation. The near constant loss found at low temperatures is originated from the vibrational relaxation of ions within their cages. This may also be the origin of the ion and structural-relaxation coupling. The dc conductivities indicate that the mixed alkali effect is most pronounced in the sample containing 8 mol% of Li(2)O. (C) 2008 Elsevier Ltd. All rights reserved

Sol-gel-derived percolative copper film

CU-SiO2 films were prepared by the sol-gel method. Two-dimensional fractal copper films were formed after the films were thermally treated in reducing atmosphere. dc resistances of the films decrease 12 orders of magnitude as the content of copper increases from 70 to 80 mol%. During the resistance measurement under argon atmosphere, samples showed a sharp increase or decrease of resistance at a transition temperature which is ascribed to the oxidation of Cu into CuO. The oxidation was also observed in the in situ high temperature X-ray diffraction under vacuum condition. The evolution of the morphology of the films was studied by scanning electron microscopy. As the content of copper increases, the forms of copper particles change from discrete to aggregate then to interconnecting. The coverage coefficients of the copper range from 23 to 55% and the fractal dimensions range from 1.65 to 1.77. The percolation thresholds for the coverage coefficient and the fractal dimension are about 33% and 1.71, respectively, which corresponds to the sample containing 72.5 mol% of Cu. (C) 2007 Elsevier Ltd. All fights reserved

Impedance study of V2O5-TeO2-BaO glasses

Five V2O5-TeO2-BaO glasses were studied using ac impedance. The measurements revealed that dc conductivities increase with V2O5 contents. For glasses having the same V2O5 content, the glass containing less BaO content has higher conductivity. The electric modulus was computer simulated using a stretched exponential relaxation function, Phi(t) = exp[-(t/tau(0))(1-n)]. The coupling parameter, n, is temperature and composition dependent. The variation of coupling parameter is interpreted in terms of Ngai's coupling model. The dielectric constants at high frequency limit are insensitive to the temperature but increase with V,05 contents. The activation energies obtained from dc conductivity and relaxation time To are about the same. Both decrease with increasing V2O5 contents. (c) 2005 Elsevier B.V. All rights reserved

Optical and physical properties of lanthanum gallogermanate glasses

Optical properties of lanthanum gallogermanate glasses were investigated by infrared (IR) transmission and IR reflectivity measurements. Several physical properties of these glasses, including density, index of refraction and ultrasonic velocities, were also measured. The experimental results are used to obtain elastic constants. These glasses are transparent over frequencies ranging from the near ultraviolet to the mid-IR (similar to 8 mu m) and have ionic bond properties in their structure. (C) 1998 Elsevier Science B.V. All rights reserved

Polarized Raman scattering in lanthanum gallogermanate glasses

Polarized Raman scattering has been used to characterize the structure of two series of lanthanum gallogermanate glasses with the composition xLa(2)O(3)-GeO2-0.25Ga(2)O(3) and 0.33La(2)O(3)-GeO2-yGa(2)O(3), where x, y = 0.2, 0.25, 0.33, and 0.5, We assigned the vibrational modes to different types of atomic motions in the glass network. No direct evidence of the six-fold coordinated (octahedral) germanium or gallium ions in the glass structure has been observed. The nature of the low frequency Raman peak - the so called Boson peak - is discussed in terms of the existing theories. (C) 1999 Elsevier Science B.V. All rights reserved

Structure of properties of lanthanum galliogermanate glasses

Lanthanum galliogermanate glasses were prepared. Raman spectra, molar volumes, glass transition temperatures acid activation energies for glass transition and crystallization were obtained. For glasses having the same La2O3/GeO2 ratio, the molar volumes increase with the Ga2O3 content, and the glass transition temperatures, activation energies for glass transition and crystallization, increase initially then decrease as the ratio of Ga2O3/GeO2 increases. For glasses having the same Ga2O3/GeO2 ratio, the molar volumes increase with La2O3 content, and the glass transition temperatures increase as the ratio of La2O3/GeO2 increases. The change of glass structure and its properties with composition is correlated with the concentration of lanthanum ion. (C) 1998 Elsevier Science B.V. All rights reserved

Elastic properties of lanthanum aluminosilicate glasses

The elastic properties of two series of lanthanum aluminosilicate glasses (15La(2)O(3)-xAl(2)O(3)-(85 - X)SiO2 and 25La(2)O(3)-yAl(2)O(3)-(75 - y)SiO2, where x, y = 15, 20, 25, 30, 35 mol%), were obtained by the ultrasonic pulse-echo technique, at room temperature. The correlation of elastic stiffness, the cross-link density, and the fractal bond connectivity of these glasses are discussed. The derived experimental values of Young's modulus, bulk modulus, shear modulus and Poisson's ratio for our glasses are compared with those theoretically calculated values in terms of the Makishima-Mackenzie model. (C) 2003 Elsevier Ltd. All rights reserved