Heat capacities and thermal behavior of alkali borate glasses

The heat capacities of selected glasses in the five alkali borate systems have been measured over a range of high temperatures which includes the respective glass transition regions. The heat capacities per gram atom at 350°K show little variation with composition while those at the low temperature ends of the glass transition, T_g-, show some systematic variations. When compared with the respective 3R values, the heat capacities at T_g- range from about 0.75 for B₂O₃ to values in excess of unity for various alkali borate compositions. The present data on heat capacity have been combined with previous data on the elastic moduli, densities and thermal expansion coefficients to evaluate the Grüneisen constant, γ, for each composition for temperatures around ambient. For B₂O₃, γ has a low value of about 0.25. It decreases with additions of Li₂O and increases with additions of the other alkali oxides.     

Thermoelastic properties of ULE® titanium silicate glass

The elastic moduli, and their temperature and pressure derivatives, for ULE titanium silicate glass, have been measured by the ultrasonic pulse superposition technique. From the extrapolated 0 K values of the elastic moduli and their pressure derivatives, the elastic 0 K Debye temperature and Grüneisen constant have been evaluated, and compared with the thermally measured values. The same anomalies in the dynamic propeties as found in other glasses, i.e. positive temperature and negative pressure derivatives, are also found for ULE glass. The correlation between the changes in the bulk modulus and additive in silica-rich glasses is discussed.     

Determination of the influence of TiO2 on the elastic properties of a mica based glass ceramic by ultrasonic velocity measurements

The influence of small amount (1 or 2 wt%) of TiO₂ additions and crystallization heat treatment on the elastic properties of a mica based glass ceramic have been investigated by ultrasonic velocity measurements. The mica based glass ceramic was prepared through controlled crystallization of a glass in the SiO₂, Al₂O₃, CaO, MgO, K₂O and F system. Evidences of TiO₂ acting as a nucleating agent in this system was demonstrated. The longitudinal and transversal wave velocities of the as-prepared glass and the mica based glass ceramic were measured by using 5 MHz probes at room temperature. Elastic properties namely; longitudinal modulus, Young’s modulus, bulk modulus, and shear modulus were calculated from the ultrasonic velocity values measured and density values obtained experimentally. It has been observed that small amount of TiO₂ additions caused a notable but not significant; however, the crystallization heat treatment had a profound effect on the elastic properties of the glass in the system studied.     

Mössbauer effect studies of alkali borate glasses

Mössbauer studies of a large number of glass samples prepared with alkali oxides in the region of glass formation are reported. Some representative samples are studied in the temperature range 85–500 K. The following glass systems are studied.

Room-temperature isomer shift values decrease gradually with the addition of alkali oxide and the values fall sharply for alkali content higher than 22 mol%. We conclude that iron is in the ferric state and that the oxygen polyhedra of iron changes from octahedral to tetrahedral. The structural change results from the fact that the alkali introduces an extra non-bridging oxygen ion. The size of the alkali cation introduced into the glass has considerable influence on the isomer shift values of iron. In fact, the polarizing power decreases in the order LiNaK, hence the s-character of the FeO bond increases in the order LiNaK. Addition of Al₂O₃ has no effect on the isomer shift values of iron, showing that aluminium ions occupy network-forming positions.For some representative samples the second-order Doppler effect was studied as a function of temperature in the range 85–500 K. The thermal red shifts due to second-order Doppler effect are used to estimate the local specific heats of ferric ion in the glass system. The quadrupole splitting has weak temperature dependence, showing that Fe³⁺ is in a high-spin state.     

Manifestation of thermodynamic glass transition by structure and picosecond dynamics in alkali tellurite glasses

The Raman spectra of the 0.1Cs₂O–0.9TeO₂ melt were measured and analyzed over a broad temperature range including the glassy, supercooled and molten state in an effort to follow the varying structural and dynamical aspects caused by temperature and alkali modifier. The network structure of the glass/melt is formed by mixing TeO₄ trigonal bipyramid and TeO₃ trigonal pyramid units. Changing alkali content and/or temperature results to conversion of the TeO₄ units to TeO₃ units with a varying number of non-bridging oxygen atoms. The low-frequency Raman spectra reveal a well-resolved Boson peak whose frequency also depends on temperature. The variation of the maximum of the Boson peak has been determined and discussed in the framework of current phenomenological models. The short-time dynamics of the system experiences drastic changes when approaching the glass-to-liquid transition. The temperature dependent plot of the correlation times extrapolates to a crossover value, which we assign as spectral evidence of the system's known thermodynamic glass transition temperature. Similar behavior exhibit several spectral features, such as the maximum of the Boson peak, the exponent of the susceptibility and the intensity ratio related to the structural transformation from TeO₄ tbp to TeO₃ tp species occurring in the medium range order structure.     

Elastic constants and structure of the vitreous system Co3O4P2O5

The elastic moduli of the entire vitreous range of the system CoPO that can be prepared by melting together Co₃O₄ and P₂O₅ oxides in open crucibles, have been measured by ultrasonic techniques at 15 MHz. The bulk, shear, longitudinal and Young's moduli and the Poisson ratio are found to be rather sensitive to the glass composition. It is found from this ultrasonic data, that the glass system can be divided into “three compositional regions”. This behaviour is qualitatively interpreted in terms of the cobalt co-ordination, crosslink densities, interatomic force constants and atomic ring sizes. Also presented is a full discussion of effects of annealing on elastic properties.     

Diffuse X-ray scattering study of the temperature variation of elastic constants in indium arsenide

The elastic constants of indium arsenide have been determined in the temperature range from 80 to 750 K on n-type samples with a carrier concentration of about 8 · 10¹⁷ cm⁻³ by Wooster's method from the measurements of the thermal X-ray diffuse scattering intensities. An average decrease in the elastic constants with increasing temperature over the above mentioned temperature range is found to be about 8%.     

Characterisation of semiconducting V2O5–Bi2O3–TeO2 glasses through ultrasonic measurements

Tellurite containing vanadate (50−x)V₂O₅–xBi₂O₃–50TeO₂ glasses with different bismuth (x=0, 5, 10, 15, 20 and 25 wt%) contents have been prepared by rapid quenching method. Ultrasonic velocities (both longitudinal and shear) and attenuation (for longitudinal waves only) measurements have been made using a transducer operated at the fundamental frequency of 5 MHz in the temperature range from 150 to 480 K. The elastic moduli, Debye temperature, and Poisson’s ratio have been obtained both as a function of temperature and Bi₂O₃ content. The room temperature study on ultrasonic velocities, attenuation, elastic moduli, Poisson’s ratio, Debye temperature and glass transition temperature show the absence of any anomalies with addition of Bi₂O₃ content. The observed results confirm that the addition of Bi₂O₃ modifier changes the rigid formula character of TeO₂ to a matrix of regular TeO₃ and ionic behaviour bonds (NBOs). A monotonic decrease in velocities and elastic moduli, and an increase in attenuation and acoustic loss as a function of temperature in all the glass samples reveal the loose packing structure, which is attributed to the instability of TeO₄ trigonal bipyramid units in the network as temperature increases. It is also inferred that the glasses with low Bi₂O₃ content are more stable than with high Bi₂O₃ content.     

Pressure dependence of elastic properties of low-silica calcium alumino-silicate glasses

The hydrostatic and uni-axial pressure dependence of elastic properties of a low-silica calcium alumino-silicate glass (LSCAS) is determined by ultrasonic pulse-echo techniques at room temperature. The experimental results are used to obtain the third-order elastic constants (TOECs) of these glasses. The pressure dependence of fractal bond connectivity of these glasses is discussed. The normal behavior of positive pressure dependence of ultrasonic velocities was observed for the glass. The pressure dependence of both shear modulus and bulk modulus are positive for these glasses.     

Third-order elastic constants determination in soda–lime–silica glass by Brillouin scattering

Brillouin light scattering allows the measurement of sound velocity and elastic moduli in transparent materials. The ability to select a small scattering volume and to use specific scattering configurations gives important information about the gradient and anisotropy of mechanical properties. Brillouin experiments are often used to measure the second-order elastic constants. When a high accuracy in frequency measurements is achieved, Brillouin scattering may allow the determination of third-order elastic constants in pre-stressed media. Hence, Brillouin scattering provides in principle, a method for the analysis of stress fields in tempered glasses. In order to validate the technique, samples of float soda-lime–silica glass submitted to controlled stresses by four-point flexion were investigated. The results show the expected profile for the velocity of sound waves propagating in directions parallel and perpendicular to the surface of the samples, respectively. They allow the determination of several third-order elastic constants of the investigated glass. This technique was applied to several samples of tempered glass corresponding to different values of the surface stress. The main result is the observation of the expected general trend, namely, through the thickness of the sample, a parabolic variation of the sound velocity whose amplitude increases with the magnitude of the surface stress.     

The elastic behaviour of TeO2 glass under uniaxial and hydrostatic pressure

TeO₂ glass of purity exceeding 98.5 mol.% has been made, despite earlier suggestions that some 7.5–10 mol.% of modifier is required to form vitreous telluride networks. It is argued that in view of the high purity of the glass obtained, TeO₂ may well be able to form glass by itself, given an appropriate thermal history in preparation. The hydrostatic and uniaxial pressure dependences of ultrasonic waves propagated in this glass at room temperature have been measured. The results provide the second and third order elastic constants of the glass. The bulk modulus is consistent with a ring diameter averaging about 8 atoms (Te₄O₄ rings) suggesting that the glass is a disordered version of paratellurite; however, if the TeO bending force constant were to be unusually strong, then a larger ring diameter (as in tellurite) would be indicated. Although the anion coordination number is only 4, the pressure derivatives of the second order elastic constants are positive and the third order elastic constants are negative, in marked contrast to the anomalous behaviour of silica-based glasses. These findings suggest that bond bending motions of bridging atoms between the trigonal bypyramidal groups (which are the structural units) do not play an important role in the elastic properties of TeO₂ glass. In consequence the shear and oongitudinal acoustic mode Grüneisen parameters are both positive (γ₁ = +2.14, γ_s = +1.11): the long wavelength acoustic modes stiffen under hydrostatic pressure.     

Adequacy test of the fictive temperatures of silica glasses determined by IR spectroscopy

Adequacy of the fictive temperature determined by the IR spectroscopy method was examined for various silica glasses by measuring the extent of the memory effect. The IR method of fictive temperature measurement of silica glasses relies upon the assumption that a silica glass has a silica structural band with a unique wavenumber when it has a particular fictive temperature. Silica glasses with various impurities or added components such as Cl, F, and OH were given a cross-over heat-treatment and the extent of the resulting deviation of the wavenumber during the second stage heat-treatment at a constant temperature corresponding to the apparent fictive temperature of the sample was determined. Silica glasses containing high concentrations of water or fluorine exhibited distinct memory effects while glasses with low water concentration or Cl did not. These results seem to indicate that the main source of the memory effect in silica glasses is the composition fluctuation rather than density fluctuation. Thus, the IR method of determining the fictive temperature is adequate for high purity silica glasses even when the glass samples have unknown thermal history.     

Thermal conductivity and specific heat of tellerium-based chalcogenide glasses

The specific heat and thermal conductivity of tellerium-based chalcogenide glasses are reported, together with their glass transition and synthesizing temperatures. The thermal conductivity has been measured in the temperature range between 100 and 500 K while the specific heat was determined at temperatures between 373 and 600 K. Below the glass transition temperature, both physical parameters were observed to be temperature independent.     

Brillouin light scattering study of polymeric glassy sulfur

We present here a study of the acoustic dynamics of polymeric glassy sulfur. The data have been collected in the GHz frequency range by means of Brillouin light scattering for different scattering geometries. The choice of the experimental setup allows us to obtain information on the refractive index, n, that comes out to be close to that corresponding to the high temperature polymeric liquid phase. The longitudinal acoustic excitations have been investigated as a function of temperature from deep into the glassy state up to the glass transition temperature. The temperature dependence of the sound velocity is compared to the one measured in the liquid phase. We find that the sound velocity of the glass can be linearly extrapolated from that of the polymeric liquid measured in the THz frequency range with inelastic X-ray scattering.     

DC conductivity in single and mixed alkali vanadophosphate glasses

Three different series of single and mixed alkali vanadium phosphate glasses have been prepared by a melt quench method. DSC studies were carried out on few of these samples and their glass transition temperatures were determined. The glass transition temperatures were found to decrease with alkali content in single alkali systems and increase with second alkali content in mixed alkali systems. The dc electrical conductivity has been measured as a function of temperature. The data has been analyzed in the light of Mott’s small polaron hopping model and activation energies were determined. In one set of single alkali glasses activation energies were found to increase with alkali content and in another set of single alkali systems a transition from predominantly electronic to ionic conduction has been observed above 0.16 mol fraction of alkali content. The mixed alkali glasses have shown higher activation energies and lower conductivities compared to single alkali doped glasses and this has been attributed to a mixed alkali influence on the electrical conduction in these systems.     

Heat capacity of oxide glasses measured by AC calorimetry

Heat capacities of vitreous silica, and some binary and ternary silicate, borate, and phosphate glasses were measured in the temperature range from 300 to 840 K by AC calorimetry. In this method, the conditions of measurement, especially AC frequency and sample thickness, must be examined in order to measure the heat capacity with high accuracy, and thus, an optimum condition for measuring the heat capacity was investigated. The relationship between heat capacity and chemical composition was discussed in terms of the Debye model, which can be used for calculation of the heat capacity of atomic solids. We found that heat capacities of all the samples studied here scaled with the molar heat capacity at the Debye temperature have a similar magnitude and temperature dependence, from 300 K to the glass transition points. Based on this observation, we propose an empirical equation which is composed of separate contributions of the compositional and temperature dependence of the heat capacity.     

Dielectric behaviour in a vanadium phosphate glass

The dielectric constant and conductivity of 80% V₂O₅: 20% P₂O₅ glass has been measured in the frequency range 10² to 10⁹Hz and in the temperature range 80 to 350°K. It is shown that the dielectric behaviour over these ranges is described by a Debye type relaxation process with distribution of relaxation times. A method is proposed to determine the width of distribution from the data at fixed frequencies and different temperatures. The width of distribution increases at frequencies ω > 10/τ, which leads to an a.c. conductivity at these frequencies almost linearly proportional to frequency and independent of temperature. The estimated value of the static dielectric constant of about 30 was found to decrease with temperature while the infinite frequency dielectric constant of 10 was independent of temperature. The carrier concentration calculated from the dielectric relaxation time and the d.c. conductivity through a thermal diffusion model shows reasonable agreement with direct measurement using electron paramagnetic resonance.     

Preparation of Nd2O3-doped calcium aluminosilicate glasses and thermo-optical and mechanical characterization

The calcium aluminosilicate glass (CAS) is an important class of optical materials due to the many applications envisaged, including its use as active media for glass lasers. In order to study how Nd₂O₃ doping affects the mechanical and the thermo-optical properties of CAS glass, two series of CAS glass, doped with Nd₂O₃ up to 5 wt%, were prepared in a vacuum atmosphere. The rare earth changes the physical properties, and this influence of doping ion content is discussed for both the series of samples in terms of mechanical, thermal, and thermo-optical properties. The study analyzed hardness and elastic moduli, glass transition temperature, crystallization temperature, thermal diffusivity, specific heat, density, thermal conductivity, refractive index, and thermo-optical properties, like temperature coefficient of the optical path length (dS/dT). The results presented provide information about the sample’s structure, and show that for Nd₂O₃ concentration up to 5 wt% there were no significant changes in the glass host material.     

Modeling and measurement of residual stresses in a bulk metallic glass plate

The recent advent of multi-component alloys with exceptional glass forming ability has allowed the processing of large metallic specimens with amorphous structure. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was investigated using two models: (i) instant freezing model, and (ii) viscoelastic model. The first one assumed a sudden transition between liquid and elastic solid at the glass transition temperature. The second model considered the equilibrium viscosity of BMG. Both models yielded similar results although from vastly different approaches. It was shown that convective cooling of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 plates with high heat transfer coefficients could potentially generate significant compressive stresses on the surfaces balanced with mid-plane tension. The crack compliance (slitting) method was then employed to measure the stress profiles in a BMG plate that was cast in a copper mold. These profiles were roughly parabolic suggesting that thermal tempering was indeed the dominant residual stress generation mechanism. However, the magnitude of the measured stresses (with peak values of only about 1.5% of the yield strength) was significantly lower than the modeling predictions. Possible reasons for this discrepancy are described in relation to the actual casting process and material properties. The extremely low residual stresses measured in these BMG specimens, combined with their high strength and toughness, serve to further increase the advantages of BMGs over their crystalline metal counterparts.