Threshold switching in niobate glass — thick-film devices

Thick-film, niobate-glass threshold switches have been made in reducing gas atmospheres at high temperatures (1100–1400°C) using SiO₂ and B₂O₃ as glass formers. Their temperature characteristics are far superior to those of similar vanadate switches previously reported [1,2], and they are stable at low ON currents (< mA) if they are protected from atmospheric oxidation. They do not require forming.The process consists in pre-firing the printed, powdered glass in air and then reducing in argon/hydrogen gas. While the reducing conditions are not critical and virtually full reduction to tetragonal NbO₂ occurs, the pre-firing conditions in air are critical if high-resistance devices are to be obtained. Many devices can be made on a single alumina chip and the results of a statistical analysis are given.The electrical characteristics — measured, chiefly, using a double-pulse technique to examine the change in delay time with pulse separation, ON current, pulse width, etc. — are similar to those previously reported for the vanadate switches [2]. These measurements, together with the results of a microscope and electron-microprobe examination, indicate that a thermal mechanism of switching with filament or channel formation is the most likely. A steady-state computer simulation based on the thermal model indicates the temperatures to be expected during switching, and these are calculated to be less than the NbO₂ semiconductor-metal transition temperature of roughly 800°C, for all the cases examined.     

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.     

Effects of solute–solute avoidance on metallic glass formation

An ideal cluster-distribution configuration is proposed for transition metal–metalloid metallic glasses with high glass-forming ability, in which full solute–solute avoidance and multispecies clusters increase the stability of alloy systems. The liquid structure of four typical Fe-based glass former alloys were investigated by ab initio molecular dynamics simulations, and the simulation results and experimental phenomena confirmed the validity of the proposed configuration.     

Evolution of glass properties during a substitution of S by Se in Ge28Sb12S60 −xSex glass network

In this paper, a detailed study to examine the influence of chalcogen S/Se mole % in the Ge₂₈Sb₁₂S_60 ?xSe_x glass system, with x = 0, 15, 30, 45 and 60, is presented that provides insight into the effect of chalcogen content on the glass network and properties. Specifically, we report results of a systematic study to evaluate the relationship between compositional variation, glass properties and dominant bonding configurations. These materials are important to applications in optics manufacturing where correlation of physical and optical properties is required to predict fabrication behavior and ultimate material performance. It has been found that the dominant bonds in the glass system change upon reaching a specific molar ratio (percentage, %) of chalcogen substitution, between 30 < x < 45 mol%, changing from Ge―Se to Sb―Se bonds as the dominant bond type. This singularity has been observed using micro-Raman spectroscopy and X-ray photoelectron spectroscopy. This effect of the dominant bond configurational change was also shown to impart changes in important physical properties including micro-hardness, thermal properties, and the glass' viscometric behavior. Results indicate that the observed dominant bond change was responsible for a constant value in the evolution of both the micro-hardness and calorimetric glass transition temperature. The viscosity was also affected by the change in dominant bond type, breaking the monotony of the viscosity evolution during the S substitution, due to the total strength of the vitreous system which does not linearly increase.     

New large grain,highly crystalline,transparent glass–ceramics

Two main reasons assure the transparency in the visible of some glass–ceramics (TGC): their crystal sizes are much smaller than the wavelength of light or the difference between the refractive index of glass matrix and crystal phase is small. The majority of traditional TGC have nano-size crystals and small to moderate crystallized volume fraction, usually between 3% and 70%. In this article we present a new type of transparent glass–ceramics having large (micrometric) grain size and very high crystallized volume fraction, which reaches up to 97%. Their high transparency mainly results from simultaneous variations of the glass matrix and crystal compositions during crystallization, which considerably decreases the difference between the respective refractive indexes, and this factor prevails, regardless of crystal size. Preliminary tests of their optical properties indicate that this new family of TGC can be further developed by doping with transition metals and rare-earths.     

Self-diffusion in the chalcogenide glass system SeGeAs

Se, As and Ge self-diffusion were investigated in three different glasses of the chalcogenide system SeGeAs by means of the radioactive tracers ⁷⁵Se, ⁷³As and ⁷¹Ge. All D values (Se between 200 and 290°C, As between 240 and 290°C and Ge between 280 and 295°C) lay between 10^?14 and 5 × 10^?16 cm² s^?1. The diffusion profiles were analyzed using a chemical micro-etching technique. Roles of glass structure and possible diffusion mechanism are discussed.     

Mössbauer study of structural relaxation at the glass transition

Mössbauer spectra of very dilute solutions of ⁵⁷Fe²⁺ ions in propane-(1,2)diol show distinct anomalies in the quadrupole splitting, the linewidth and the recoilfree fraction near the glass transition temperature. The observed time and temperature dependence of these anomalies can be correlated with structural relaxation processes which occur in a glass upon stabilization into the supercooled liquid phase.     

Corrosion of electrocast AZS refractories by CAS glass–ceramics melting

Extensive corrosion experiments on electrocast alumina–zirconia–silica (AZS) refractories by molten CaO–Al₂O₃–SiO₂ (CAS) and Na₂O–CaO–SiO₂ (NCS) glasses were carried out at various temperatures under static condition. The features and mechanism of the corrosion were compared and analyzed. The changes of microstructure and phase composition of refractories in the course of the melt corrosion were also studied. X-ray diffraction (XRD), scanning electron microscope (SEM) and chemical analysis were used to characterize the corroded refractory materials and reacted melts. The reasons of alumina–zirconia–silica bricks corroded are the meltdown of their own composition, penetration or permeation of alkali oxide in the glass melt and scouring of the glass melt. The results show that the refractories resistance against corrosion of the oxides like Na₂O, K₂O or CaO is weak, and that the corrosion mechanism of NCS/AZS is different from that of CAS/AZS. In a static condition, CaO–Al₂O₃–SiO₂ melts corroded alumina–zirconia–silica brick more severely than Na₂O–CaO–SiO₂. The result provides useful reference to a prospective selection of refractory materials in glass and glass–ceramics manufacture.     

Formation of glass in the GeSiS system

The formation of glass in the GeSiS system was investigated. After synthesis of material with the general formula Ge_1?xSi_xS_y, where x was chosen to be 0.05, 0.1, 0.2, 0.3 and y was in the range 1.28–3.6, cylindrical samples were prepared and used for the characterization of glass by means of DTA. It was found that the substitution of germanium with silicon does not lead to any expressive change of the glass transition temperature, crystallization and the onset of melting.     

Nucleation and growth of needle-like fluorapatite crystals in bioactive glass–ceramics

The nucleation behaviors of glass–ceramics with different Ca–mica (Ca_0.5Mg₃AlSi₃O₁₀F₂)/fluorapatite ratios were investigated. By using differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscope with an energy dispersive spectrometer (SEM/EDS), the effect of CaO and P₂O₅ addition on the nucleation behaviors was studied. Results showed that the addition of CaO and P₂O₅ promoted nucleation process and led to the formation of different nucleation phases. After further heat treatment, Ca₅(PO₄)₃F crystals were of needle-like morphology, instead of particle-like reported in previous studies. This can be attributed to the one-dimensional rapid growth of fluorapatite along the c-axis. The values of the Avrami parameter, n, and the dimensionality of crystal growth, m, are found to be 2 and 1, respectively, which indicated that the bulk nucleation is the dominant mechanism in crystallization, and one-dimensional growth of fluorapatite is preferred. Since needle-like fluorapatite crystals are of the same morphology to hydroxyapatites in human bones, the glass–ceramics thus prepared show excellent bioactivity in vivo.     

Electrical and dielectric properties of glass system NaPO3–KHSO4

Glass samples have been prepared in the NaPO₃–KHSO₄ binary system with the classical melting, casting and annealing steps. Electrical and dielectrical properties of glass samples were studied. Measurements of DC and AC conductivity and complex electrical permittivity of xNaPO₃–(100 ? x)KHSO₄ glass system were carried out at temperatures ranging from room temperature to temperature located 15 °C below glass transition temperature T_g. Results showed that changes of NaPO₃ concentration considerably affect values of observed parameters. DC conductivity of glass increases as NaPO₃ concentration grows until concentration x = 60. However, beyond this value a sharp decrease of DC conductivity was observed. In addition relaxation times showed abrupt changes at concentration x = 60, corresponding to the lowest relaxation times at the temperature 90 °C.     

Investigation of glass forming ability in Ce–Al–Ni alloys

The compositional dependence of the glass forming ability (GFA), the correlation between their GFA and the GFA related parameters, and the thermal stability of the Ce–Al–Ni alloys were investigated. Rapidly quenched Ce₆₅Al_xNi_35 ? x (x = 2, 5, 10, 17, 20) and Ce₇₀Al_xNi_30 ? x (x = 2, 5, 10, 15, 20) ribbons were prepared by melt spinning, and their phase transformations were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The experimental results indicated that the GFA of Ce₆₅Al_xNi_35 ? x (x = 2, 5, 10, 17, 20) and Ce₇₀Al_xNi_30 ? x (x = 2, 5, 10, 15, 20) alloys increased firstly and then decreased with the increasing of the Al content up to 20 at.%, respectively. It was found that only one parameter, F₁, in evaluated currently available empirical GFA parameters searching for metallic glasses with a good GFA, can reflect the GFA of the Ce–Al–Ni alloys. It was indicated that the thermal stability of alloy with fully amorphous maybe lower than that of alloy with partial amorphous.     

Multiferroic behavior in silicate glass nanocomposite having a core–shell microstructure

Nanosized iron core and barium titanate shell microstructure was generated within a silicate glass of composition 23.1 Na₂O, 23.1 BaO, 23.0 TiO₂, 7.6 B₂O₃, 5.8 Fe₂O₃, 17.4 SiO₂ by first reducing it at 893 K for ½ h and then subjecting it to heat treatment at 759 K for 4 h. Transmission electron microscopy showed the composite particles to have a mean diameter of 3.9 nm. The nanocomposite exhibited both ferroelectric and ferromagnetic behavior. The dielectric constant peak was not prominent because of a small thickness of the barium titanate phase. The magnetic hysteresis loop showed an asymmetric behavior giving rise to a small exchange bias field. This is believed to arise due to exchange interaction between the ferromagnetic iron core and the thin layer of Fe₃O₄ on the core surface with a spin glass-like behavior. The magnetization under zero-field cooled (ZFC) and field cooled (FC) conditions indicated superparamagnetic behavior at temperatures higher than 300 K. The optical absorption spectra exhibited a peak at around 325 nm. This was analyzed satisfactorily on the basis of a metal core–oxide shell nanoconfiguration. The extracted values of metal core conductivity showed a metal insulator transition for iron core diameters less than 2.4 nm. The present synthesis approach will lead to newer multiferroic nanocomposites and glasses with multifunctionalities.     

Mathematical analysis of the reaction–diffusion of water in glass tubes

The diffusion and reaction of water with glass tubes are mathematically analyzed. A general mathematical model is developed that takes into account the absorption of molecular water from the vapour phase on to the glass tube surface, its subsequent diffusion, and both the forward and backward reactions of formation of the silanol groups. The general solutions are obtained numerically while analytical solutions are obtained for special cases of small time or fast reaction. An approximate integral solution is obtained for the latter and it is shown that the equilibrium model with fast reaction can be cast into a simple yet accurate algebraic form which is easy to implement. The models are applied to few cases of practical importance to optical fiber processing.     

Ab initio calculation of vibrational frequencies of GexSe1−x glass

We have used the density functional theory to make the models of Ge_xSe_1?x glass for which the energy is a minimum. The clusters, Ge₂Se₂, Ge₂Se₃, Ge₃Se, Ge₃Se₂, Ge₄Se, GeSe₃, GeSe₄, chain mode zig-zag Ge₄Se₃, corner sharing GeSe₄, and edge sharing Ge₂Se₆, have been made successfully and their vibrational spectra have been calculated from the first principles. We are able to optimize the bond distances as well as the bond angles. The calculated values of the frequencies of vibrations of the various clusters have been compared with those obtained from the experimental Raman spectra of actual glasses, Ge_xSe_1?x(0 < x < 0.3). The local concentration, x within 0.25 nm is nonuniform in the amorphous material. When the same cluster occurs in two stable configurations, low frequency vibrations of frequency, ν < 100 cm^?1, are found. The corner sharing GeSe₄ has low frequency modes at 54 cm^?1 and 93 cm^?1 whereas these modes disappear in the pyramidal configuration. The low frequency modes are therefore associated with the breaking of C₄ symmetry of the pyramidal configuration. The computed vibrational frequencies of clusters Ge₃, Ge₄Se₃, Ge₂Se₃, GeSe₃ and Ge₃Se₂ are actually present in the Raman spectra of the glass, Ge_xSe_1?x(0 < x < 0.3).     

Melting enthalpy ΔHm for describing glass forming ability of bulk metallic glasses

A new melting enthalpy ΔH_m criterion for the prediction of glass forming ability (GFA) of alloys is proposed and five Zr–Al–Ni–Cu bulk metallic glasses (BMG) with critical dimension Z_max up to ? 7.5 mm are also developed by us in the light of the optimum ΔH_m of Zr–Al–Ni–Cu alloy system. And then, we researched the relationships between ΔH_m and two GFA parameters (critical cooling rate R_c and Z_max) of five bulk metallic glass (BMG) systems, such as Mg–Ni–Nd, Pd–Cu–Si, La–Al–Ni–Cu, Zr–Al–Ni–Cu and Zr–Ti–Ni–Cu–Be, respectively. The results show that the relationships between ΔH_m and R_c are all concave upward parabolas, and the optimum ΔH_ms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.3960 kJ mol^?1, 21.2202 kJ mol^?1, 19.7146 kJ mol^?1, 18.1455 kJ mol^?1 and 13.1558 kJ mol^?1, respectively. On the contrary, the relationships between ΔH_m and Z_max are all concave downward parabolas, and the optimum ΔH_ms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.5530 kJ mol^?1, 21.0830 kJ mol^?1, 19.6603 kJ mol^?1, 19.7231 kJ mol^?1 and 13.1173 kJ mol^?1, respectively. Furthermore, other BMGs’ R_cs or Z_maxs predicted by above-mentioned relationships satisfactorily agree with the tested results, which indicates that these relationships are reliable. However, the predicted results are reliable only if the main components are similar with the fitted BMGs or the additive is sparkle enough that the alloy’s character does not change. On the whole, the ΔH_m can act as a criterion for quickly predicting the alloy’s GFA and be helpful for the development of new BMGs.     

Preparation of lithium ion conducting glasses and glass–ceramics for all-solid-state batteries

Highly lithium ion conducting glasses and glass–ceramics were prepared by a mechanical milling technique in the Li₂S-based sulfide and oxysulfide systems. The Li₂S–P₂S₅ glass–ceramics showed ionic conductivity as high as 3.2 × 10^?3 S cm^?1 at room temperature. All-solid-state batteries using these sulfide-based materials as a solid electrolyte showed excellent charge–discharge performance with high capacity and high cycleability. The cells with the combination of the SnS–P₂S₅ glassy electrode and the Li₂S–P₂S₅ glass–ceramic electrolyte worked as a secondary battery, which was a first step of glassy monolithic cells with a common glass network.