Method to assess the homogeneity of partially crystallized glasses: Application to a photo-thermo-refractive glass

We describe a new method for the study of both optical and crystallization homogeneity of partially crystallized glasses or glass–ceramics at different spatial scales (from 100 mm to 1 μm). The method relies on the association of different techniques, such as interferometry, optical microscopy and differential scanning calorimetry. The method was tested to probe the optical and crystallization homogeneity of both as-made and UV-exposed, and pre-nucleated samples of a photo-thermo-refractive (PTR) glass. This study demonstrates that pure UV-exposure did not lead to an improvement of the crystallization homogeneity of the glass. However, the benefit of associating UV-exposure and nucleation thermal treatment was clear. These combined treatments permit to homogenize the crystal distribution in PTR glass at millimeter and micron scale. This result is of major commercial interest.     

Influence of particle size on the crystallization kinetics of glasses produced from waste materials

The crystallization behavior and kinetics of glasses produced from coal fly ashes, red mud and silica fume were investigated by using differential thermal analysis, X-ray diffraction and scanning electron microscopy techniques. The kinetic parameters of the glass-crystallization transformation were estimated under non-isothermal conditions applying three different equations, namely, Kissinger, Matusuta-Sakka and Ozawa. Non-isothermal differential thermal analysis curves were obtained using both coarse and fine glass samples. The crystallization activation energies of coarse glasses are in the range of 233-439 kJ/mol while the activation energies of fine glasses change in the range of 369-450 kJ/mol. Avrami exponent, n, values of coarse glasses indicated the three-dimensional bulk crystallization. This result is in well agreement with the cross-sectional scanning electron microscopy investigations. The values of the n obtained experimentally are in the range of 1.24-1.36 for fine glasses which show the one-dimensional surface crystallization. The crystallized phase of the glass-ceramic samples produced from waste glasses by applying the controlled heat treatment process was identified as diopside by X-ray diffraction analysis.     

Influence of alkali and alkali-earth metal oxide substitutions on the properties of lithium-iron-phosphate glasses

The influences of different alkali and alkali-earth oxide substitutions on the properties of lithium-iron-phosphate (LIP) glasses have been studied. Na₂O, K₂O, MgO, CaO and BaO were used to substitute Li₂O to prepare LIP glasses with molar compositions of (20 − x)Li₂O − xR₂O(RO) − 30Fe₂O₃ − 50P₂O₅ (x = 2.4, 4, 5.6 and 7.2). The glass transition temperature (T_g) was determined by the differential thermal analysis technique. The density and chemical durability of the prepared glasses were measured based on the Archimedes principle and the weight losses after the glasses were boiled in water. The results show that T_g decreases with the initial substitutions, whereas the density and chemical durability increase. The diminution of the aggregation effect of Li⁺ ions on the glass structure due to the decrease in Li⁺ concentration, the larger molecule weights of the substitutes, the mixed-alkali and depressing effects as well the slower mobility of substitute ions mainly contribute to the initial changes in T_g, density and chemical durability of the LIP glasses, respectively. Further increasing the amounts of substitutes brings about increasing diminution of the aggregation effect of Li⁺ ions and breakage of the glass network on the one hand and increasing amounts of substitutes with larger molecule weights and ion radii on the other hand. Both aspects influence the glass properties oppositely and consequently non-monotonic variations in the properties of LIP glasses with the substitutions are observed.     

Influence of phosphate precursors on the structures of sodium aluminophosphate sols,gels and glasses

Transparent and homogeneous aluminophosphate gels and glasses have been widely synthesized through an aqueous sol–gel route, extending significantly the glass-forming range compared to that accessible via the melt-cooling route. Different phosphorus precursors, sodium polyphosphate (NaPO₃) and orthophosphate species (NaH₂PO₄ and/or H₃PO₄) were compared with regard to the macroscopic properties and the microscopic structure of the resultant gels and glasses as characterized by extensive high-resolution liquid- and solid-state NMR. Sodium polyphosphate solution results in a substantially wider composition range of homogenous gel formation than orthophosphate solutions, and the two routes produce significant structural differences in the sol and xerogel states. Nevertheless, the structures of the glasses obtained upon gel annealing above 400 °C are independent of the P-precursors used.     

On the dependence of the properties of glasses on cooling and heating rates II: Prigogine-Defay ratio, fictive temperature and fictive pressure

In a previous analysis (J. Chem. Phys. 125 (2006) 094505) it was shown by us that - in contrast to earlier believe - a satisfactory theoretical interpretation of the experimentally measured values of the so-called Prigogine-Defay ratio, Π, can be given employing only one structural order parameter. According to this analysis, the value of this ratio has to be, in accordance with experimental findings, larger than one (Π > 1). This analysis is extended here and, in particular, the dependence of the value of Π on cooling and heating rates is studied. Finally, employing the general model-independent definition of internal (fictive) pressure and fictive temperature, developed by us (J. Non-Crystalline Solids 355 (2009) 653), it is shown how these parameters behave in dependence on temperature for different sets of cooling and heating rates. Some further consequences and possible extensions are discussed briefly.     

Influence of high temperature deformation on the fracture behavior of a bulk Zr-based metallic glass

Nanocrystalline/amorphous matrix composites obtained by isothermal compression at high temperatures and low strain rates were characterized using transmission electron microscopy. To study the influence of high temperature deformation on the fracture behavior and room temperature plasticity, compression tests with a constant strain rate of 1 × 10⁻⁴ s⁻¹ were applied to the deformed samples. Fracture features of as-cast alloy and deformed samples were analyzed using scanning electron microscopy. Compared with the as-cast alloy, the room temperature plasticity of deformed sample is not destroyed both in the range of 370-395 °C at 1 × 10⁻³ s⁻¹ and at 395 °C in 1 × 10⁻² to 1 × 10⁻³ s⁻¹, and deteriorated at higher temperatures and lower strain rates. Corresponding to the TEM images, the homogenously dispersed nanocrystals with small size contribute to the compressive plasticity, and the aggregated large nanoparticles destroy the plasticity of the sample after high temperature deformation.     

Structure and dynamics of oxide melts and glasses: A view from multinuclear and high temperature NMR

From a presentation of the various nuclear magnetic resonance (NMR) experiments that allows to characterize the local structure and dynamics of oxide glasses and melts, we show that it becomes possible to evidence not only the details of the coordination state of the constituting atoms but also the nature of polyatomic molecular motifs extending over several chemical bonds.     

Effect of Ni-addition on the crystallization behavior and the oxidation resistance of Zr-based metallic glasses below the crystallization temperature

The crystallization behavior of Zr_65.0Al_7.5Ni_10.0Cu_17.5 metallic glasses by addition of Ni with 753 K annealing treatment and its effect on the oxidation resistance around the supercooled liquid region at 663 K were studied. By annealing at 753 K, the nanocrystalline phase of bct-Zr₂Cu precipitates was observed in the Zr_65.0Al_7.5Cu_27.5 specimen, while microstructures consisting of finer nanocrystalline bct-Zr₂Cu, fcc-Zr₂Ni and Zr₆Al₂Ni formed in the Zr_65.0Al_7.5Ni_10.0Cu_17.5 specimen. The oxidation resistance of the melt-spun Zr_65.0Al_7.5Ni_10.0Cu_17.5 specimen was improved by addition of Ni, which is evidenced by less mass gain and thin oxide scale. The microstructural refinement by the formation of numerous nanocrystalline phases of bct-Zr₂Cu, fcc-Zr₂Ni and Zr₆Al₂Ni from the matrix resulted in an improvement of the oxidation resistance, whereas a relative coarse nanocrystalline phase consisting of bct-Zr₂Cu exhibited fast oxidation along grain boundaries. Although the oxide species for both specimens were composed of a large amount of CuO/Cu₂O, some tetragonal and monoclinic-ZrO₂ as well as a minor amount of the oxide state of Cu³⁺, the amount of oxides especially for ZrO₂ in the Zr_65.0Al_7.5Ni_10.0Cu_17.5 specimen was lower, which was probably due to suppressed oxygen diffusion in ZrO₂.     

纳米SiC对Si3N4/SiC复相陶瓷性能及显微结构的影响

本研究通过采用纳米SiC粉体及有机前驱体两种途径,制备了Si3N4/SiC粒子(Si3 N4/纳米SiC)复相陶瓷,研究了纳米SiC对Si3 N4/SiC复相陶瓷性能及显微结构的影响,讨论了材料强化的机制与显微结构的关系.     

The review of possible interrelations between ionic conductivity, internal friction and the viscosity of glasses and glass forming melts within the framework of Maxwell equations

The review contains the results of application of Maxwell equations for mechanical relaxation and electrical conductivity, to the systematization of large amount of experimental data related to mechanical, viscous, and electrical properties of inorganic glasses and glass forming melts. The generalization of internal friction results shows the existence of characteristic values for the ratios of temperatures, responsible for α-, β- and ionic relaxations; they are independent on the frequency and chemical composition. This is the evidence for the main role of elastic deformations at various corpuscular processes and the existence of characteristic scale of activation barriers predetermined by local volumes of deformation. It is shown the possibility of very precise calculation of the temperature of “ionic” internal friction maximum for one-alkali oxide glasses directly from Maxwell equation and d.c. conductivity experimental data. The volumes of particles overcoming the potential barrier at viscous flow practically coincide with the results of direct structural determinations. The existence of universal relation between d.c. conductivity and viscosity for the extremely wide temperature intervals (Littleton relation) is proved for silicate and phosphate melts. The theory of this dependence is proposed. The results show the effectiveness of the attempts to unite the continual and discrete approaches within the framework of Maxwell equations to obtain the simplest understanding the mechanisms of different types of relaxation. The review comprises many Russian papers unknown in English scientific literature.     

On the dependence of the properties of glasses on cooling and heating rates: I. Entropy, entropy production, and glass transition temperature

The glass transition is theoretically described in terms of a generic non-equilibrium thermodynamics approach employing De Donder's structural order parameter method, appropriate expressions for the relaxation behavior of glass-forming systems and a simplified but qualitative correct model of glass-forming melts with one order parameter related to the free volume of the system. Employing this approach the behavior of a variety of thermodynamic quantities describing glass-forming systems in vitrification and devitrification processes is interpreted theoretically. The present paper is devoted to the computation of the entropy, the entropy production and the glass transition temperature in dependence on the cooling and heating rates, varying latter parameter in a broad interval. A comparison with experimental results is given and some further consequences and possible extensions are discussed briefly.     

高温高压退火对碳化硅性能的影响

以CVD生产的碳化硅为原料在高温高压下进行退火处理,探究退火温度对碳化硅性能的影响.实验结果表明:随着退火温度的增加,碳化硅的颜色由浅绿色变为无色;体积有明显的缩小、密度增加;退火后碳化硅的抗压强度显著增强,机械强度有所增加;退火后的SiC结构并没有发生变化,15R-SiC在798.6 cm-1处拉曼峰强度减少,4H-SiC在970 cm-1拉曼峰强度有所增强.     

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 Sb_xSe_55−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 ΔH_nr, and an increase in the glass-transition width ΔT_g. Further, the glass-transition temperature of Sb_xSe_55−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 Sb_xSe_55−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.     

Effect of silver doping on the structure and phase separation of sulfur-rich As–S glasses: Raman and SEM studies

We report on the structural details and microphase separation of the bulk glasses Ag_x·(As₃₃S₆₇)_100-x for 0x25. Glass–glass phase separation occurs over a wide range of Ag content, i.e. 4x20. An off-resonant polarized Raman spectroscopic study has been carried out to elucidate structural aspects at the short- and medium-range structural order of the glasses. Analysis of Raman spectra revealed quantitative changes of the sulfur-rich microenvironments that reduce upon adding Ag. Scanning electron microscopy combined with X-rays microanalysis have been utilized to examine the type and extent of phase separation, and to provide quantitative details on the atomic concentrations in the Ag-poor and Ag-rich phases. It has been shown that at 7 at.% Ag the Ag-rich phase percolates through the structure; this effect can be associated with an ionic-to-superionic behavior of these glasses in accordance with similar studies on the stoichiometric arsenic sulfide glass; although the phase separation observed in the present glasses is qualitatively different.     

Structural analysis and devitrification of glasses based on the CaO-MgO-SiO2 system with B2O3, Na2O, CaF2 and P2O5 additives

Glasses, whose basic composition was based on the CaO-MgO-SiO₂ system and doped with B₂O₃, P₂O₅, Na₂O, and CaF₂, were prepared by melting at 1400 °C for 1 h. Raman and infrared (IR) spectroscopy revealed that the main structural units in the glass network were predominantly Q¹ and Q² silicate species. The presence of phosphate and borate units in the structure of the glasses was also evident in these spectra. X-ray analysis showed that the investigated glasses devitrified at 750 °C and higher temperatures. The crystalline phases of diopside and wollastonite dominated, but weak peaks, assigned to akermanite and fluorapatite, were also registered in the diffractograms. The presence of B₂O₃, Na₂O, and CaF₂ had a negligible influence on the assemblage of the crystallized phases, but it caused a reduction of crystallization temperature, comparing to similar glasses of the CaO-MgO-SiO₂ system.     

Investigation of the growth processes from vapor phase of silicon carbide epitaxial layers: Thermodynamic analysis of the high temperature processes in the system Si‐C‐H‐Cl

In the present work are presented the results of the thermodynamic analysis of the interaction processes in the system Si‐C‐H‐Cl in the temperature interval 1000‐3000 K. The equilibrium pressures of the components in the system Si‐C‐H‐Cl with taking account the formation of the condensed phases C, Si and SiC have been determined. The optimal conditions giving the maximum yield of silicon carbide by pyrolysis of mixture of volatile compounds of carbon and silicon have been defined. The thermodynamic analysis of the examined system showed that the increasing of the content of hydrogen in the initial mixture allows to decrease the optimal temperature for obtaining of silicon carbide by the method of pyrolysis and essentially to increase its maximum possible yield. (© 2008 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of fictive temperature on the structure of E-glass: A high resolution,multinuclear NMR study

The variation of the structure of E-glass with fictive temperature has been examined with high resolution, multinuclear (¹¹B, ²⁷Al, ²⁹Si, ¹⁷O, and ¹⁹F) MAS NMR. The previously observed decrease in 4-coordinated boron with increasing fictive temperature is confirmed and, for the first time, is directly correlated with an increase in non-bridging oxygens. An increase in 5-coordinated Al is also observed, and is linked to an increase in association of F with Al at higher fictive temperature.     

Nonlinear susceptibility measurements in a supercooled liquid close to Tg: Growth of the correlation length and possible critical behavior

The existence of a growing correlation length associated with the strong increase of relaxation times of glass-formers close to the glass transition is still a major open question in glass physics. It has been recently proposed that the ac nonlinear susceptibility of a supercooled liquid close to the glass transition temperature T_g would be a probe of dynamical correlations. As for spin glasses, where the nonlinear susceptibility diverges at the transition, this quantity is tailored to reveal the possible “hidden” critical behavior of the glass transition. We have developed a high sensitivity experimental device to measure the nonlinear dielectric susceptibility of an insulating material at finite frequency. It measures the third harmonics of the current flowing out of a capacitor with the supercooled liquid as the dielectric layer. It is based on a bridge with two capacitors, and reaches a sensitivity better than 10^− 7 (ratio of third to first harmonics). Our first results on supercooled glycerol are presented. They clearly reveal the growing of the correlations close to the glass transition.     

Influence of substrate temperature on the structure of ZnO:Al thin films

ZnO: Al films were prepared using low cost spray pyrolysis technique. The dependence of the physical properties on the substrate temperature was studied. The best films obtained at 500°C substrate temperature with preferred [002] orientation. The sheet resistance decreases with increased substrate temperature, and values as low as R_sh = 207 Ω/cm² are reached for substrate temperature of 500°C. The optical transmittance of films increased by increasing the substrate temperature and received to 75% at 500°C. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)