Compositional dependence of fragility and glass forming ability of calcium aluminosilicate melts

The compositional dependence of glass forming ability (GFA) of two series of calcium aluminosilicate melts is studied by measuring their viscous behavior and crystallization tendency. The first series consists of five compositions on the joining line between the eutectic point of anorthite–wollastonite–tridymite and that of anorthite–wollastonite–gehlenite. The series includes the eutectic compositions as end members. The second series consists of five compositions on a line parallel to the joining line on the alumina rich side. In the present work, GFA is described in terms of glass stability. The Hrubÿ parameter (K_H) [A. Hrubÿ, Czech. J. Phys. B 22 (1972) 1187] obtained from calorimetric data is used as a measure of glass stability. From the viscous behavior of the undercooled melts studied by viscometry the fragility (m) is derived. The results show that m is inversely proportional to K_H for the two series of melts, proposing m as an indirect measure of GFA. However, this proportionality is only valid for comparison of the glasses in the series of systematic substitutions of components. In general, the melts on the joining line between the two eutectic compositions are found to have superior GFA. Within each series GFA increases as a function of the network polymerization.     

Properties and structure of AsTe glasses: (I). Glass-forming ability and related properties

AsTe glasses have been investigated for determining the molecular mechanisms of glass formation and of crystallization. Stable and homogeneous bulk glasses are prepared throughout the range of concentration from 20 to 65 at % As, by using an improved quenching method. GFA (glass-forming ability) is determined with respect to the rate of cooling. Density, microhardness, T_g and ΔC_p at T_g are measured in the whole range of glass formation. Finally a quantitative analysis of the kinetics of crystallization is carried out. It appears that most properties which involve molecular motions exhibit anomalies for As₄₀ Te₆₀ composition (As₂Te₃ is the unique definite compound of the system). So GFA shows a sharp depression for this critical composition; similarly enthalpy and kinetics of crystallization are drastically modified at As₄₀Te₆₀. For Te-rich glasses, crystallization occurs at low temperature by homogeneous nucleation with a small energy of activation. For As-rich glasses, crystallization occurs at much higher temperatures. Above 200°C it is controlled by growth with an high activation energy which indicates deep rearrangements of the relative positions of the atoms by chemical diffusion. A coherent analysis of all the data is developed which shows that important differences of local order exist between amorphous and crystalline states and also between the two well-distinct types of glasses we are dealing with. The aim of paper II is to determine these differences from diffraction data.     

The influence of device geometry on instabilities in current-controlled negative resistors

Experiments have shown that either current-controlled negative differential resistance (CNDR) or threshold switching (TS) devices can be fabricated from a given chalcogenide glass. Device behaviour (CNDR versus TS) is controlled by the geometry of the thin film device, and is interpreted as arising from differences in the thermal boundary conditions between the various geometries. When lateral thermal instabilities are precluded, CNDR behaviour is obtained, and when such instabilities are allowed, TS behaviour is obtained. These results are predicted by consideration of the rate of entropy production in the device, thus confirming that it is device geometry and not choice of the material that governs whether CNDR or TS behaviour will be displayed.     

Glass-forming ability of soda lime borate liquids

We investigate the composition dependence of glass-forming ability (GFA) of a series of iron-containing soda lime borate liquids by substituting Na₂O for B₂O₃. We have characterized GFA by measuring the glass stability against crystallization using a differential scanning calorimeter (DSC). The results show that the GFA decreases when substituting Na₂O for B₂O₃. Moreover, we find that there is no direct link between the kinetic fragility and GFA for the soda lime borate series studied herein. We have also discovered and clarified a striking thermal history dependence of the glass stability against crystallization. In particular, the two glasses containing 20 and 25 Na₂O mol% do not exhibit crystallization exotherms during the second DSC upscan at 10 and 20 K/min following prior slow (10 and 20 K/min) downscans. This indicates that the glass stability of these compositions can be enhanced by cooling their melts to the glassy state slowly, before any reheating. We explain this phenomenon in terms of the thermal history dependence of boron speciation.     

On the role of liquid phase stability and GFA parameters

Role of liquid phase stability on the glass forming ability (GFA) has been reviewed and the alloy systems have been analyzed by introducing a contribution factor (w_g) to the characteristic temperature T_g, in the γ parameter. The kinetics of glass formation for various alloy systems has been found to vary with liquid phase stability in metastable state. The GFA of a fragile liquid is found to be more responsive to the contribution of metastable stability compared to strong liquid.     

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.     

Thermodynamic characteristics of Ti-based glass-forming alloys

Based on thermodynamic characteristics of the stable metallic liquid at melting temperature and the supercooled liquid, the present work calculated the mixing enthalpy ΔH^mix, the mixing entropy ΔS^mix and the Gibbs free energy difference between the supercooled liquid and the resulting crystalline phases ΔG of typical Ti-based amorphous alloys. The results show that for the case of larger ΔS^mix, moderate ΔH^mix for the stable liquid and smaller ΔG for the supercooled liquid, Ti-based alloys tend to achieve high glass-forming ability (GFA). A new parameter, β, defined as (T_g ? T_k)/(T_l ? T_g), has been introduced to evaluate the GFA of Ti-based bulk amorphous alloys (wherein T_g, T_l, and T_k represent the glass transition temperature, the liquidus temperature, and the Kauzmann temperature, respectively). Experimental data imply that the larger the β, the better the GFA for Ti-based amorphous alloys.     

Zero-point entropy of glasses as physical reality

This paper presents an analysis of the physical nature of the author’s results obtained since 1995 in the field of thermodynamics of the vitreous state (classic approximation). Excess entropy and free energy at 0 K are considered as the measures of non-equilibrium extent of glass with respect to crystal. Experimental data for melts and glasses of various chemical natures are generalized. A general form of the relations between reversible changes in free energy, enthalpy and entropy and their frozen values at the formation of any glass is shown. The problem of functional dissimilarity of the heat capacity of glass and that of crystal has been solved within the framework of the principle of free energy minimization as a result of self-organization of the vibration spectrum according to the frozen structure. The complete stabilization of glass structure at Kauzmann’s temperature or below it is forbidden by topologic reasons and (or) by thermodynamics. The effects of pressure on the thermodynamic properties of glasses are shown. Possible particular applications of the analysis of self-organization in chaotic systems are exemplified by the modeling of the abilities of neural systems.     

Phase competition of Cu64Zr36 and its effect on glass forming ability of the alloy

In the present paper, crystallization behavior and glass forming ability (GFA) of Cu₆₄Zr₃₆ bulk metallic glass (BMG) were studied based on the crystal phase competition. Electrical resistivity and X‐ray diffraction results indicate that Cu₆₄Zr₃₆ glass underwent a two‐stage crystallization process, during which Cu₁₀Zr₇ and Cu₅₁Zr₁₄ crystals precipitate at first and then there are only Cu₁₀Zr₇ and Cu₈Zr₃ phases at the end of the second stage. Intriguingly, it was found that it is the competing phase Cu₁₀Zr₇ that weakens GFA of the Cu₆₄Zr₃₆ alloy, because the Cu₁₀Zr₇ precipitate is fully restrained in the cast rod when substituting Zr with 7.5at%Ti and thereby its diameter with fully amorphous structure is enlarged from 1 mm to 2 mm.     

Effect of free electron concentration on glass-forming ability of Ca–Mg–Cu system

Intensive studies show that the best glass-forming ability (GFA) is presented as the Fermi surface made contact with pseudo-Brillouin boundaries. Via surveying the GFA of electronic simple Ca–Mg–Cu metallic glass system, we find that the best GFA is presented as the free electrons per atom (e/a) equals to 1.75, which is closed to the value of 1.8 proposed for the ideal amorphous state. The pseudo-gap or minimum in the density of states at the Fermi surface proposed by nearly free electron theory has not been observed in the Ca–Mg–Cu system. Our results indicate that the free electron concentration is an important but not the sole factor for influences in the glass formation.     

Influence of TM and RE elements on glass formation of the ternary Al-TM-RE systems

The effects of TM and RE elements on glass formation ability (GFA) of the Al-TM-RE systems are studied systematically. The TM elements show distinct differences: critical sizes of the Al-TM-Ce systems are in the order of Ni > Co > Fe > Cu. However, the RE elements show similar effect on glass formation. These results are discussed in terms of the GFA-related factors, i.e., reduced glass transition temperature (T_rg = T_g (T_x)/T_L, where T_g, T_x and T_L are the glass transition temperature, onset crystallization temperature and liquidus temperature, respectively) and the mixing enthalpy ΔH_Al-TM. A new parameter of T_L − T_g (T_x) is potentially proposed and used to evaluate the GFA of the Al-based alloys, as well as bulk glass-formers.     

EAM calculations of the thermodynamics of amorphous copper

The structure and thermodynamics of pure amorphous Cu is studied with molecular dynamics, Monte Carlo and quasi-harmonic calculations using the embedded atom method. The pair distribution function, atomic volume, thermal expansion, enthalpy, entropy, and Gibbs free energy are calculated for the cyrstalline, liquid and amorphous phases. The glass transition temperature of the amorphous phase is found to be approximately 400 K ( 0.3 T_m). The free energy difference between the amorphous and supercooled liquid phases was determined to be at most 0.01 eV per atom. The free energy of the liquid and crystalline phases in our calculation is found to agree within 0.4% of experiment over the temperature range 400–2200 K. The free energy of the glass is found to be fairly well described by the simple Turnbull model. This is the first atomic-level calculation of the free energy of a metallic glass.     

The influence of Ag substitution for Cu on glass-forming ability and thermal properties of Mg-based bulk metallic glasses

A group of pseudo-ternary Mg–(Cu–Ag)–Dy bulk metallic glasses (BMGs) was developed by copper mold casting. The glass-forming ability (GFA) is significantly improved by the coexistence of similar elements of Ag and Cu. The critical diameter for glass formation increases from 10 mm for ternary Mg_56.5Cu₃₂Dy_11.5 alloy to 18 mm for pseudo-ternary Mg_56.5Cu₂₇Ag₅Dy_11.5 alloy. Thermal stability, crystallization and melting behaviors of the Mg-based BMGs were evaluated. The decrease of Gibbs free energy difference between undercooled liquid and crystalline phases caused by similar element substitution with optimal amount can be responsible for the increase in GFA of the resulting alloys.     

On the glass transition in vitreous silica by differential thermal analysis measurements

Scanning calorimetry measurements of the glass transition in vitreous SiO₂ (about 120 wt ppm. OH groups) are reported. Data were obtained upon heating after controlled cooling through the glass transition, and after annealing at temperatures between 990 and 1292 K. The onset of the glass transition is at 1247 K, and the supercooled liquid state is reached at 1475 K. The step in the specific heat is (2.9 ± 0.7) J mol⁻¹ K⁻¹. This value, lower than the results of drop calorimetry experiments, agrees with the calculated value from viscosity data. The glass transition is nearly twice as wide as expected from the temperature dependence of the viscosity. Annealing reduces the enthalpy of glasses as usually expected, and the corresponding entropy decrease is in agreement with results for other network glasses. In vitreous silica, depending on the annealing temperature, both exothermic and endothermic processes take place. Based on Davis’ and Tomozawa’s results, endothermic processes upon annealing are attributed to the diffusion of the OH groups.     

Glass forming ability criteria for La–Al–(Cu, Ni) alloys

Criteria for evaluating the glass forming ability (GFA) of La–Al–(Cu, Ni) alloys were studied. The GFA criteria can be categorized into two types, according to whether the liquidus temperature T_l is included in the formula or not. The T_l inclusive criteria generally predict a maximum in GFA at the eutectic composition at which T_l is a minimum. However, for La–Al–(Cu, Ni) alloys, the best GFA does not correspond to the eutectic. The paper discusses the suggestion that the solidification temperature T_s should be used instead of T_l for evaluating the GFA of La–Al–(Cu, Ni) alloys.     

Continuous cooling approximation for the formation of a glass

Non-isothermal equations describing the liquid-crystal transformation are derived using the isothermal Avrami equations. A theoretical expression for the critical cooling rate for the formation of a glass is found. Calculations based on this expression are in better agreement with experimental values than those derived from TTT (time-temperature-transformation) curves. A study performed on typical glass forming materials enables the glass forming ability (GFA) to be determined by experimentally measuring crystallization temperatures at different cooling rates which are easily accessible with commonly available technology. The behaviour of the rate constant for crystallization is also obtained from the same data in the experimental range considered. In both cases no previous knowledge of the parameters involved is needed.With some assumptions the values of the viscosity in the crystallization temperature range can be estimated.Although the study was performed for an Avrami index of 4 an extension to other values of n is made under some restricted conditions and a more general treatment is outlined.     

Crystallisation kinetics,glass forming ability and thermal stability in glassy Se100 − xInx chalcogenide alloys

Differential scanning calorimetry (DSC) studies have been done under non-isothermal conditions at different heating rates for glassy Se_100 ? xIn_x (5 ≤ x ≤ 20) alloys. DSC traces with well-defined endothermic and exothermic troughs and peaks at glass transition (T_g), crystallisation (T_c) and melting (T_m) temperatures were observed. The crystallisation kinetics parameters, Avrami index (n), activation energy for crystallisation (E_c) and frequency factor (K_o), have been calculated on the basis of the classical Johnson–Mehl–Avrami (JMA) model and related methods derived by Kissinger, Augis–Bennett and Mahedevan. Activation energy for glass transformation (E_t) has been evaluated on the usual two different non-isothermal methods developed by Moynihan and Kissinger. An extension of the Augis–Bennett method well known for evaluating E_c to calculate E_t has been explored with satisfactory results. Results obtained from these methods are in close agreement with each other. Close correlation between E_t, E_c and heating rate (β) was observed. The glass forming ability (GFA) and thermal stability parameters have been calculated for each glass system. It was found that the proportion of indium additive changed significantly the values of glass/crystal transformation, GFA and thermal stability of the studied system.     

Unusual Glassy Smectic-G Liquid Crystal: Heat Capacity of N-p-n-Pentyloxybenzylidene-p'-n-butylaniline between 11 and 393 K

Abstract

The heat capacities of the title compound (C₃H₁₁,O—C₆H₄,- CH=N—C₆H₄,—C₄H₉, abbreviation 5O ? 4) with a purity of 99.92 mole percent have been measured with an adiabatic-type calorimeter between 11 and 393 K. The transition temperature and the enthalpy and entropy of phase transition for stable crystal → S_G, S_G → N and N → isotropic liquid were T _c = 299.69 K/ΔH = 22.68 kJ mol^?1/ΔS = 75.70 JK^?1 mol^?1, 325.72/7.11/21.79 and 342.48/1.78/5.22, respectively. The crystal which melts at 285.5 K is a metastable modification. The S_A phase hitherto reported in between S_G and N does not exist. The glassy So state was realized by rapid cooling of the specimen from the So phase. The molar enthalpy of the glassy S_G state at 0 K was by (10.1±0.1) kJ mol^?1 higher than that of the stable crystalline state and the residual entropy of the glassy state was (9.40±0.83) JK^?1 mol^?1. The relaxational heat-capacity anomaly was observed from as low as 100 K and double glass transition phenomenon occurred around 200 K; a quite unusual phenomenon which has never been observed for the glassy states of nematic and cholesteric liquid crystals. The present results give a fair evidence that the unusual glass transition phenomenon previously found for the S_G state of 6O?4 (a homologous compound) is not exceptional at all but common to the smectic glasses; at least common to the glassy S_G states. Two possible origins responsible for the double glass transitions have been discussed.     

Vibrational entropy of substitutional solid solutions

Positive deviations of the entropy of mixing from that of the ideal case have been established basing upon the experimental study of ionic substitutional solid solutions, carried out with the help of various techniques. These deviations are attached to the vibrational entropy, corresponding to the negative deviations of Debye temperatures of solid solution from additivity. The existence of linear correlation between the enthalpy of mixing and vibrational contribution to entropy is confirmed. Theoretical basis of this correlation, established empirically for the alkali halogenides, is given. The question is discussed, dealing with the universality of the relations of that kind and independence of vibrational entropy on temperature.