A geometric and analytic approach to the process of melt vitrification. I. Thermodynamic treatment

By using simple geometric concepts, general thermodynamic dependences governing the process of vitrification are derived. The thermodynamic driving forces of crystallization, ΔG_f,c(T), and stabilization, ΔG_st(T), melts and glasses are considered in the entire temperature interval from temperatures below the glass transition temperature, T_g, to temperatures high above the melting point, T_m. It is show that at large deviations from T_g the temperature dependence of the driving force for stabilization, ΔG_st(T), is non-symmetric and expressions derived by a Taylor expansion become a poor approximation. By using an appropriate thermodynamic model, a more general expression for ΔG_st(T) is obtained and the thermodynamic basis for the existence of superheated glasses is established. The thermodynamic instability of glasses below and above T_g, i.e., of supercooled and superheated glasses is discussed.     

Alkali diffusion and electrical conductivity in sodium borate glasses

The diffusion coefficient of sodium, ²²Na, and silver, ¹¹⁰Ag, and electrical conductivity for sodium borate glasses (4–24 mol% Na₂O) has been measured from 100°C to slightly below the glass transition temperature, T_g. Unlike silicate glasses where the self-diffusion coefficient is much larger than the impurity diffusion coefficient, D_Na and D_Ag had close to the same magnitude in the sodium borate glasses. In some glasses, D_Ag was slightly larger than D_Na. Na₂O-Ag₂O-B₂O₃ glasses show a relatively small mixed alkali effect, despite the significant mass difference between Ag(≈108) and Na(≈23). It is concluded that the mixed alkali effect is more dependent upon the difference in ionic radii rather than differences in mass.     

Formation and structure of titanate glasses

The formation of high titanium oxide (30–60 wt%) containing glasses was studied. Stable titanate glasses with high content of TiO₂ and BaO could be obtained even without other glass-forming oxides. It is demonstrated that the coloration of titanate glasses with high content of TiO₂ is due to oxygen loss during the melting. A systematic study of controlling melting and heat treatment conditions led to the successful decoloration of titanium oxide containing glasses. Infrared spectra and X-ray diffraction studies showed that Ti⁴⁺ in titanate glasses is in sixfold coordination. Phase separation was observed by electron microscopy when the titanate glasses were heat-treated at the temperature above T_g. The crystallization of titanate glasses is generally preceded by phase separation. The obtained crystalline phases are mainly different titanates.     

Water and the glass transition temperature of silicate melts

Literature data on the effect of water on the glass transition in silicate melts are gathered for a broad range of total water content c_w from 3 × 10⁻⁴ to 27 wt%. In terms of a reduced glass transition temperature T_g^*=T_g/T_g^GN, where T_g^GN is T_g of the melt containing c_w≈0.02 wt% total water, a uniform dependence of T_g^* on total water content (c_w) is evident for silicate melts. T_g^* decreases steadily with increasing water content, most strongly at the lowest water content where H₂O is dominantly dissolved as OH. For water-rich melts, the variation of T_g^* is less pronounced, but it does not vanish even at the largest water contents reported (≈27 wt%). T_g^* vs. c_w is fitted by a three-component model. This approach accounts for different transition temperatures of the dry glass, hydroxyl and molecular water predicting T_g^* as a weighted linear combination of these temperatures. The required but mostly unknown water speciation in the glasses was estimated using IR-spectroscopy data for hydrous sodium trisilicate and rhyolite.     

Thermal stability and crystallization kinetics of new As–Ge–Se–Sb glasses

Thermal stability and crystallization kinetics of As₁₄Ge₁₄Se_72−xSb_x (where x = 3, 6, 9, 12 and 15 at.%) glasses are studied by the differential scanning calorimetry. The values of the glass transition temperature (T_g) and the peak temperature of crystallization (T_p) are found to be dependent on heating rate and antimony content. From the heating rate dependence of T_g and T_p the values of the activation energy for glass transition (E_t) and the activation energy for crystallization (E_c) are evaluated and their composition dependence discussed. Crystallization studies have been made under non-isothermal conditions with the samples heated at several uniform rates. Using a recent analysis developed for non-isothermal crystallization studies, information on some aspects of the crystallization process has been obtained. The stability calculations emphasized that the thermal stability decreases with increasing the Sb content.     

Glass-forming ability versus stability of silicate glasses. II. Theoretical demonstration

Possible relationships between measures of glass stability (GS) against devitrification on heating (evaluated by the Hrubÿ parameter K_H=(T_c^h−T_g)/(T_m−T_c^h), and the parameter K_w=(T_c^h−T_g)/T_m) and a criterion of glass-forming ability (GFA) – the critical cooling rate – were investigated by computing non-isothermal crystallization for typical values of the main quantities that control crystal nucleation and growth in silicate glasses. We limit these quantities to one thermodynamic parameter – the melting entropy (ΔS_m) and two kinetic parameters that control the viscosity (B and T₀ in the Vogel–Fulcher–Tamman equation or T_g and in Avramov’s equation). The effect of heterogeneous nucleation and, in particular, the possible role of the surface as active substrate is tested. The results presented herein demonstrate that GS and GFA are indeed related concepts.     

Surface nitridation of a phosphate glass

Phosphate glass surfaces were nitrided by reacting them in dry ammonia at temperatures near the glass transition temperature (T_g) for up to 100 h. These treatments have significant effects on surface dependent glass properties. For example, the dissolution rate of a sodium-barium phosphate glass (T_g = 345°C) decreased by over an order of magnitude after 24 h in ammonia at T_g. X-ray photoelectron spectroscopic (XPS) analyses show that nitrogen in chemically incorporated into the glass structure at levels up to 3–5 at.%. Elemental depth profiles, obtained by XPS (for N) and by elastic recoil detection (ERD) analyses (for N and H), indicate that the surface oxynitrile layer extends to about 1 μm in depth. XPS analyses of in situ treated samples reveals the presence of several nitrogen species which affect the surface dependent properties by increasing the structural crosslink density of the glass surface.     

Relationship between T0, Tg and their pressure dependence for supercooled liquids

We show that simple expressions can be derived from the Vogel–Fulcher–Tammann (VFT) law relating the glass transition temperature T_g, the VFT temperature T₀, their pressure derivatives, the steepness index of the ‘Angell plot’ and the strength parameter D of the VFT equation, in good agreement with experimental data. In the same way one can describe the dependence of the dT_g/dP on the relaxation time τ_g chosen to define the temperature T_g. Thus, this procedure allows a consistent rescaling and comparison of pressure dependent parameters obtained from different experiments and simulations.     

Homogeneity of a ZrF4-based glass at the nano-scale

A glass of composition 53ZrF₄–20BaF₂–4LaF₃–3AlF₃–20NaF (T_g=260°C) was prepared by careful crucible melting. High-resolution atomic force microscopy of fracture surfaces displayed the presence of nano-pores with diameters of 20–50 nm, being 4–10 nm deep, in all glasses. It was further found that only glasses without annealing and glasses with an annealing step considerably below T_g showed a distinct pattern, i.e. ripples of ≈20 nm in diameter and an rms roughness of ≈0.6 nm. Glasses annealed either near T_g or at the temperatures of maximum nucleation or maximum crystal growth rates showed both regions with the ripple pattern and regions with nano-hillocks, growing in size with increasing annealing temperature and time. Thus these hillocks nearly reach micro-dimensions of ≈270 nm in diameter and ≈65 nm in height following a 90 min annealing step at 343°C, the temperature of maximum crystal growth. These findings give evidence that the glass system, which is thought to be one of the most suitable for fiber drawing, is much less stable against nucleation and crystallization than anticipated.     

The structural relaxation and glass transition of La---Al---Ni and Zr---Al---Cu amorphous alloys with a significant supercooled liquid region

An amorphous phase with a wide supercooled liquid region, > 50 K, was found to form over wide composition ranges in the La---Al---Ni and Zr---Al---Cu systems. The largest values for the temperature span between the crystallization temperature, T_x, and the glass transition temperature, T_g, ΔT_x(-T_x−T_g), are 69 K for La₅₅ Al₂₅Ni₂₀ and 88 K for Zr₆₅Al_7.5Cu_27.5. The structural relaxation behavior on annealing was examined for the two amorphous alloys with the largest ΔT_x values. The magnitude of the structural relaxation increases gradually with increasing annealing temperature, T_a, and then rapidly in the T_a range slightly below T_g and decreases significantly on annealing T_g. The rapid increase in the magnitude of the structural relaxation on annealing near T_g is due to the glass transition. The single-stage structural relaxation indicates that there is no distinct difference in relaxation times (atomic bonding forces) between the constituent atoms in the two metal-metal-type amorphous alloys. The existence of an optimum bonding state is thought to cause the wide supercooled liquid region for the two amorphous alloys.     

Reduced glass transition temperature and glass forming ability of bulk glass forming alloys

Onset temperature (solidus) T_m and offset temperature (liquidus) T_l of melting of a series of bulk glass forming alloys based on Zr, La, Mg, Pd and rare-earth elements have been measured by studying systematically the melting behaviour of these alloys using DTA or DSC. Bulk metallic glass formation has been found to be most effective at or near their eutectic points and less effective for off-eutectic alloys. Reduced glass transition temperature T_rg given by T_g/T_l is found to show a stronger correlation with critical cooling rate or critical section thickness for glass formation than T_rg given by T_g/T_m.     

EXAFS and XANES joint analyses for semiconducting vanadium phosphate glasses

Vanadium EXAFS spectra of 50V₂O₅-50P₂O₅ glasses with different C = V⁺⁴/V_tot have been measured. The V-O distances increase by ΔR₁ = (0.03 ± 0.02) Å to ΔR₂ = (0.07 ± 0.03) Å going from a glass with C = 0.64 to C = 0.84 and from C = 0.51 to C = 0.84, respectively. The EXAFS data show a basically similar structure of the vanadium sites for both the V⁴⁺ and V⁵⁺ ionic states. The density of the glasses increases with C from the value d₁ = 2.81 g/cm³ for C = 0.51 to d₂ = 2.92 g/cm³ for C = 0.84 indicating a more random packing of glass units.     

SIMS study of the concentration of dopants in LPCVD silicon thin films

Measurements of solid phase dopant concentration (S) of LPCVD Si thin films as a function of substrate temperature (T_s = 500−640 ° C) and gas phase doping ratio (R = 1 × 10⁻⁵ −4 × 10⁻²) by SIMS indicate different behaviors of P and B in the films. A linear relation S = b(T)R is observed for B-doped film with b(T) varying from 4 to 50 depending on T_s. Boron-doped microcrystalline film has a higher doping efficiency than that of P-doped ones.     

Cation site occupation by Ag/Na ion-exchange in R2O–Al2O3–SiO2 glasses

Ag⁺/Na⁺ ion-exchanged R₂O–Al₂O₃–SiO₂ glasses with uniform concentration profile of Ag⁺ and Na⁺ were prepared by heat treatment in molten silver salt followed by holding at the same temperature in an ambient atmosphere. Their glass transition temperature (T_g) and thermal expansion coefficient (TEC) were measured and structures were investigated using ²⁹Si-MAS NMR, ²⁷Al-MAS NMR, IR and Raman spectroscopies. Both T_g and TEC decreased with increase of the exchange ratio, but T_g was still above the ion-exchange temperature of 400°C even for the fully exchanged sample. The ²⁹Si- and ²⁷Al-MAS NMR spectra were mostly unchanged and no sign of the structural alteration of the glass network was observed. On the other hand, the vibrational spectra showed remarkable peak shifts depending on the exchange ratio. From these structural results, it was found that when the exchange ratio was low, the introduced Ag⁺ ions were stabilized at the non-bridging oxygen (NBO) site, and then Na⁺ ions in AlØ₄ site were exchanged by Ag⁺ ions after full replacement of NBO sites, where Ø represents the bridging oxygen.     

Electrical conductivities of mixed cation glasses

The electrical conductivities of (1−x) Li₂O · x BaO · 2 SiO₂, (1−x) Na₂O · x MgO ·2 SiO₂, (1−x) Na₂O · x CaO · SiO₂ and (1−x) Na₂O · x BaO · 2SiO₂ glasses were measured at temperature ranging from room temperature to 450°C. The transport numbers for Na⁺ ion in (1−x) Na₂O · x BaO · 2 SiO₂ glasses were measured. It was found that the alkali ion carried a significant part of the current in these glasses except one that had no alkali ions, and the conductivity decreased markedly as the alkali oxide was substituted by an alkaline earth oxide. The results of conductivity measurements combined with the data hitherto reported on mixed alkali glasses led to the proposal that the so-called mixed alkali effect could be explained on the basis of the independent path model in which it is assumed that cations can move only through vacant sites left by those of the same type.     

TiO2-SiO2 monolithic glass formation from sol-gel

This study demonstrates that hydrolysis should be carried out in a step manner in gel synthesis. The key to the increase in the amount of water added is the control of the hydrolysis rate of Ti(OC₄H₉)₄. The hydrolysis of Si(OC₂H₅)₄ can be carried out at about 75°C. The amount of added water (γ_WI), which varied with TiO₂ content (in mol%), was about 64–88% of the total amount of added water. The hydrolysis reaction should be performed at room temperature while Ti(OC₄H₉)₄ is added. The total amount of added water (γ_W) is related to the amount of solvent (R). For example, if TiO₂ is 40mol%, γ_W will vary from 3.2 to 8.0 when R varies from 0.8 to 2.0. The amount of added water was affected by the distribution of solvent in the metal alkoxides. The amount of added water can be increased when R_Si(OC2H5)4 = 1, R_Ti(OC4H9)4 > 1. The rate of rise in temperature of the thermolysis of the dried gel should be less than 10°C per hour, and the heat treatment temperatyre is related to the TiO₂ content (in mol%). Gel glasses without devitrification can only be obtained by thermolysis at 600°C from the gel with no less than 20 mol% TiO₂.     

Structure and properties of the pure and Pr-doped Ge25Ga5Se70 and Ge30Ga5Se65 glasses

The Ge₂₅Ga₅Se₇₀ and Ge₃₀Ga₅Se₆₅ pure and Pr³⁺-doped glasses were prepared by direct synthesis from elements and PrCl₃. It was found that up to 1 mol% PrCl₃ can be introduced in the Ge₂₅Ga₅Se₇₀ and Ge₃₀Ga₅Se₆₅ glasses. Both types of glasses with overstoichiometric and substoichiometric content of Se were homogeneous and of black color. The optical energy gap is E^opt_g=2.10 eV, and the glass transition temperature is T_g=543 K for Ge₂₅Ga₅Se₇₀ and T_g=633 K for Ge₃₀Ga₅Se₆₅. The long-wavelength absorption edge is near 14 μm and it corresponds to multiphonon processes. Doping by Pr³⁺ ions creates absorption bands in transmission spectra, which can be assigned to the electron transitions from the ground ³H₄ level to the higher energy levels of Pr³⁺ ions ³H₅, ³H₆, ³F₂, ³F₃ and ³F₄, respectively. By excitation with YAG:Nd laser line (1064 nm), two intense luminescence bands (1343 and 1601 nm) were excited. The first band can be ascribed to electron transitions between ¹G₄ and ³H₅ energy levels of Pr³⁺ ions. Full width at half of maximum (FWHM) of the intensity of luminescence was found to be 70 nm for (Ge₂₅Ga₅Se₇₀)_1 − x(PrCl₃)_x and (Ge₃₀Ga₅Se₆₅)_1 − x(PrCl₃)_x glasses. The FWHM in selenide glasses is lower than in halide and sulphide glasses. The second luminescence band (1601 nm) can be probably ascribed to the transitions between ³F₃ and ³H₄ energy levels of Pr³⁺ ions. The absorption and luminescence spectra of Pr³⁺ ions in studied glasses are slightly influenced by stoichiometry of glassy matrix. The Raman spectra of studied glasses were deconvoluted and assignment of Raman bands to individual vibration modes of basic structural units was suggested. The structure of studied glasses is mainly formed by corner-sharing and edge-sharing GeSe₄ tetrahedra. The vibration modes of Ga-containing structural units were not found, they are apparently overlapping with Ge-containing structural units due to small difference between atomic weights of Ge and Ga. In the glasses with substoichiometry of Se, the Ge–Ge bonds of Ge₂Se₆ structural units were found. In Se-rich glasses the Se–Se vibration modes were found. In all studied glasses also ‘wrong' bonds between like atoms were found in small amounts. Maximum phonon energy of studied glasses is 320 cm⁻¹.     

d-f and f-f transition bands of praseodymium and samarium ions in silica glasses

d-f and f-f transition bands of Pr and Sm ions in silica glasses have been studied by the absorption, excitation and emission spectra. In particular, Sm-doped silica glasses were treated with different atmospheres around the T_g temperature. The change of the valency state and the transition band shown after the treatments are discussed.     

Structural distribution and rigidity of the glass network determined by EPR of Cu

The linewidth-broadening of the EPR spectra of Cu²⁺ in silicate, borate and phosphate glasses was analyzed in terms of the distribution of g_| and A_| (δ_| and δA_|) and related to the distribution of the rigidity of the network structure. X- and K-band spectra were measured for the glasses doped with ⁶³Cu²⁺ (93% abundance). The linewidth of the HFS shoulders with parallel orientation to H increased linearly with increasing m or microwave frequency. δg_| and δA_| showed a marked dependence on glass composition. For example, in Na₂O---B₂O₃ glasses, on going from x (mol% of Na₂O) being small through intermediate to large, δg_| varied from small through large to negligibly small. In contrast to these glasses δg_| was extremely large for 75PbO · 25B₂O₃ glass. The large δg_| for the Na₂O---B₂O₃ glassesof intermediate x was attributed to the coexistence of various borate groups competitively coordinating to Cu²⁺. Negligibly small δg_| for 70Na₂O · 30B₂O₃ glass and extremely large δg_| for 75PbO ·25B₂O₃ glass, both with a narrower structural distribution, reflect regidity of the glass network. The Pb---O bonding is strong enough to distort the coordination of Cu²⁺-complex. The situation is the reverse in Na₂O---B₂O₃ glasses.     

Sub-Tg mechanical relaxation of a La55Al25Ni20 amorphous alloy

Dynamic mechanical properties of an amorphous La₅₅Al₂₅Ni₂₀ alloy were measured by a forced oscillation method in the temperature range from room temperature to 453 K, just below the glass transition temperature (T_g = 481 K). The internal friction at a constant frequency 62.8 rad/s of this alloy showed a peak at about 400 K and the peak position shifts with frequency. Structural relaxation reduces the magnitude of the relaxation peak but has little affect on the peak position. Using the time-temperature superposition process, master curves for storage E′ (ln ω) and loss E″ (ln ω) moduli are constructed. Activation energy of the relaxation obtained from shift factors is low, 100 kJ/mol, which is close to that for diffusion of the Al in Al and Ni in Al. The relaxation spectra are very broad with a half width of 2 3 decades.