Role of concentration fluctuations in metallic glass formation

The glass forming composition range in several binary metallic systems is correlated with the composition dependence of S_cc(0), the concentration-concentration fluctuation structure factor at its long wavelength limit. The magnitude of S_cc(0) has been evaluated for the liquid phase from available thermodynamic data. It has been observed that S_cc(0) exhibits a maximum and tends to the ideal value in the glass forming composition range. Significant and systematic negative deviations from ideal values or the tending to zero of S_cc(0) are observed at the stoichiometry corresponding to complexes in the liquid state in the case of compound forming or associated systems. These observations are discussed in terms of the chemical short-range order in the liquid state. It is concluded that while a reasonable degree of order may exist for the compound forming compositions, in the glass forming region itself the liquids are nearly random mixtures of complexes and unassociated component atoms.     

Dynamical structure factor of liquid Li,Na and Al

The dynamical structure factors, S(q, ω) of liquid lithium, sodium and aluminium have been studied near their melting points using Mori’s memory function formalism. The second order memory function was obtained by exploiting non-Markovian nature of the fourth order memory function and assuming that memory functions at their alternate stage have the same functional time dependence. We obtain an analytical expression for the second order memory function, whose form depends on wave number q. Results obtained for dynamical structure factor over a wide range of q values for liquid Na and Al have been found to be in good agreement with recent inelastic X-ray scattering experiments in contrast to the case of liquid Li. The behavior of non-Markovity parameter is also studied as a function of frequency, ω, at different values of q.     

The Debye–Waller factor approaching the glass-transition temperature in phosphate glasses

The temperature dependence of the mean square displacement, 〈u²〉, calculated by elastic neutron scattering for two phosphate glasses, (AgI)_x(AgPO₃)_1−x with x = 0.3, 0.55, is analysed. The studied samples are probed in a wide range of temperatures going from a few tens of K up to the glass transition temperature and well into the undercooled liquid state. In the low temperature regime a solid like behaviour appears, showing a linear temperature dependence of the Debye–Waller factor, while around T_g the onset of a pronounced increase of 〈u²〉 is observed. The temperature dependence of the normalized elastic intensity and the drawn out Debye–Waller factor are fitted and analyzed by the theoretical previsions of Mode Coupling Theory finding a good agreement.     

The effect of cooling rate on the glass transition of an amorphous polymer

Cooling down from the equilibrium state at different rates reveals the dynamic behavior of glass forming materials. In particular, the dependence of the glass transition region on the cooling rate, q is commonly agreed to contain information regarding the activation energy of the relaxation time, τ. In this work experimental and theoretical aspects of such a relationship have been highlighted. Experimentally, the glass transition zone of amorphous polystyrene films has been investigated over two decades of cooling rate (0.5–50 K/min) by using refractive index measurements. The shift of the glass transition temperature and the broadening of the transition zone at increased cooling rate have been characterized. Theoretically, the cooling experiments have been simulated within the integral formulation of the Kovacs–Aklonis–Hutchinson–Ramos (KAHR) model using the Vogel temperature dependence for the relaxation time. The Frenkel–Kobeko–Reiner equation, τq = constant, provided the needed relationship between the experiments and the theory, enabling the evaluation of the relevant parameter of the kinetic model, i.e. the Vogel activation energy and the zero configurational entropy temperature, from the shift of the glass transition temperature with cooling rate.     

Viscosity of (GeS2)x(Sb2S3)1−x supercooled melts

Viscosity of (GeS₂)_x(Sb₂S₃)_1−x melts (x = 0.1–0.9) has been measured by penetration method and parallel-plate method as a function of temperature. The temperature dependence of viscosity can be expressed by a simple equation of Arrhenius type. The kinetic fragility obtained from these plots varies linearly with composition. The Sb₂S₃ component probably affects the network structure and, consequently, the kinetic fragility of a supercooled melt increases. As expected, the heat capacity change at the glass transition, ΔC_p, exhibits similar (though non-linear) compositional dependence.     

Viscosity of germanium sulfide melts

Viscosities of Ge_xS_1−x melts (0.30⩽x⩽0.44) have been measured by penetration viscometry from 10⁷ to 10¹³ Pa s. The temperature dependence of equilibrium viscosities in this range can be expressed approximately by a simple Arrhenius equation. Both the heat capacity change at the glass transition and the activation energy of viscous flow monotonously increase with germanium content as predicted by the Adam–Gibbs theory. Therefore, the connectivity of the germanium–sulfur network is reduced due to decreasing concentration of sulfur which causes increasing fragility of the undercooled liquid. The glass transition temperature exhibits a maximum near the GeS₂ composition where heteropolar bonds are predominantly formed.     

The structural properties of the liquid CdTe based on Reverse Monte Carlo modelling

The Reverse Monte Carlo (RMC) method only provides information on structure, not on dynamics, and so has restricted applicability. We have only attempted to describe the trends in structural terms. The static structure of liquid Cadmium Telluride is performed using RMC modelling technique fitting to neutron diffraction data. In order to retain its tetrahedral environment with the coordination number about four in its liquid state, we have used some plausible physical constraints. In this way, it is possible to derive static structure of liquid CdTe with analysing the experimental data. We have shown the results for the partial static structure factors S_ij(q) and the partial pair distribution functions g_ij(r). We have also determined the distribution of the number of neighbors within the first coordination shell and calculated the bond angle distribution. A good agreement with the experiment is observed for the structural properties of liquid CdTe.     

Kinetics of relaxation and crystallization of sodium metaphosphate glass

Kinetics of structural relaxation and crystallization of NaPO₃ glassforming melt is studied by means of thermal analyses. It is demonstrated that activation energy depends strongly on fictive temperature T_f. The dependence of the onset temperature T_o of the glass transition interval on the heating rate q⁺ is investigated for samples that were previously cooled down at a rate q⁻ of about 850 K/min. The dimensionless fragility F is a measure of the dependence of the activation energy for spatial rearrangement on the changes of structure. According to the present results F is large for NaPO₃ (i.e. phosphates are ‘fragile’ substances).     

Dielectric study of the segmental relaxation of low and high molecular weight polystyrenes under hydrostatic pressure

In this work we analyze by means of dielectric spectroscopy the dynamics of the α-relaxation process of low and high molecular weight polystyrene over a wide range of pressures and temperatures. The results are interpreted in terms of a recently proposed equation which describes the behavior of the structural relaxation time, τ(T, P), as a function of both pressure and temperature. This equation has been derived from the Adam–Gibbs (AG) theory by writing the configurational entropy, S_c, in terms of the excess thermal heat capacity and of the excess thermal expansion. Consequently, the molecular dynamic of glass-forming liquids can be linked to its thermodynamic properties. The pressure dependence of the segmental dynamics for both polymers is here measured and analyzed in the AG framework for the first time. τ(T, P) was found to be very well described using the extended AG equation. Additionally, the pressure dependence of the fragility and glass transition temperature (T_g) is analyzed and discussed in terms of the role of chain length and end groups.     

Alloying effect on the viscous flow in metallic glasses

The temperature dependence of viscosities near the glass transition is measured from the rates of thermal transformation for metallic glasses PtNiP, PdNiP, NiPBA1 and (Fe, Co)PBA1. Alloying among metallic elements which lowers the glass transition temperature T_g lowers the ideal glass transition temperature T₀, but raises the residual configurational entropy S_g and the activation energies for “diffusive” rearrangement, $\text{Δμ}{}^1$, of the alloying glasses, while compositional ordering associated with the addition of metalloids raises the T_g and T₀ and lowers the S_g and $\text{Δμ}{}^1$. Results are correlated to the atomic ordering and stability of the glasses. The extracted free volume and the critical diffusive volume are much smaller, by a factor of 4, for metallic glasses than for many other glasses.     

Structural characterization and magnetic properties of the spinel compound CoIn0.5Cr1.5S4

Single crystals of the magnetic semiconductor CoIn_0.5Cr_1.5S₄, belong to the system CoIn_(2‐2X)Cr_(2X)S₄ with x = 0.75, was grown by the chemical transport method. X‐ray powder diffraction characterization by the Rietveld method indicated that CoIn_0.5Cr_1.5S₄ crystallizes in the space group Fd‐3m, Z = 8, with a = 10.0700(6) Å and V = 1021.2(1) ų, in a normal spinel structure. The temperature dependence of the DC magnetization suggests that the studied compound presents a ferromagnetic behavior with a Curie temperature Tc = 220 K. Sharp spin‐glass like behavior was found also. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relation between configurational entropy and relaxation dynamics of glass-forming systems under volume and temperature reduction

The structural relaxation dynamics of two molecular glass-forming systems have been analyzed by means of dielectric spectroscopy, under cooling and compression conditions. The relation of the dynamic slowing down with the reduction of the configurational entropy, S_C, as predicted by Adam and Gibbs (AG), was also investigated. As S_C is not directly accessible by experiments, it was estimated, following a common procedure in literature, from the excess entropy S_exc of the supercooled liquid with respect to the crystal, determined from calorimetric and expansivity measurements over the same T–P range of dynamics investigation. The AG relation, predicting linear dependence between the logarithmic of structural relaxation time and the reciprocal of the product of temperature with configurational entropy, was successfully tested. Actually, a bilinear relation between S_exc and S_C was found, with different proportionality factors in isothermal and isobaric conditions. Using such results, we derived an equation for predicting the pressure dependence of the glass transition temperature, in good accordance with the experimental values in literature.     

Structural variation of expanded fluid mercury during M–NM transition: A Reverse Monte Carlo study

The structure of fluid Hg during the metal–non-metal transition has been modelled by Reverse Monte Carlo simulation in terms of the structural factors S(Q) measured by X-ray diffraction at SPring-8. The full region of S(Q) including S(0) was employed in the simulation so as to take the effect of fluctuation phenomenon into account. The modelling reveals that as the M–NM transition is approached, the distribution of coordination numbers, P(N), of the first-neighbor shell broadens obviously and shifts to lower values, supporting the speculation of Franz’s model. Furthermore, the coordination number P_S(N), denoted for atoms with a shorter bond, changes markedly in width and average value, while much smaller is found for that of longer bond as the density decreases. The variation of P_S(N) of the atoms with the shorter bond are directly related to the M–NM transition of fluid Hg.     

Glassy state and structure of Sn–Sb–Se chalcogenide alloy

The effect of Sn addition on the glass transition and structure of c-Sb₂₀Se₈₀ chalcogenide alloy have been studied by X-ray diffraction and differential scanning calorimetric studies. The increase in the glass forming region and the glass transition temperature with the addition of Sn is discussed by considering the formation of [SnSe₄] tetrahedra, another type of network former, which inhibits the crystallization. The differential scanning calorimetric studies on Sn_xSb₂₀Se_80−x (8 ⩽ x ⩽ 18) glassy samples reveal a single glass transition temperature for all values of x while a single crystallization peak was obtained only for 10 ⩽ x < 12. The X-ray diffraction studies reveal that the glass crystallizes to Sb₂Se₃ and SnSe₂ phases upon annealing. The glass formation and composition dependence of glass transition temperature in the Sn–Sb–Se chalcogenide alloy could be understood by considering the topological phase transitions and a chemically ordered network model.     

Thermal expansion of glasses in the As2Se3–AsI3 system

Thermal expansion coefficients (α) of glasses in the As₂Se₃–AsI₃ system are measured in the glass transition region and temperature dependence of the fictive temperature is calculated on the basis of relaxation model. It is found that the increase of AsI₃ content results in: an increase of α, decrease of the glass transition temperature (T_g), increase of the α change at T_g, an effect of quenching rate on α, and also changes in the structural relaxation times spectrum. The data are discussed within the framework of the assumption that the addition of AsI₃ to As₂Se₃ results in: (1) destruction of the As₂Se₃ glass network, (2) structural inhomogeneity of the glasses increase, (3) the temperature dependence of chemical–structural equilibria occurring in the liquid state increases.     

Wide and small angle X-ray scattering measurements for expanded fluid Se accompanying the semiconductor–metal and metal–nonmetal transition

Wide and small angle X-ray scattering measurements for supercritical fluid Se in the wide density region from liquid to dense vapor have been carried out using synchrotron radiation at SPring-8. The large background for the small angle spectra with the previous high-pressure vessel was much suppressed using a new vessel with a large diamond window. We carefully measured wide angle X-ray scattering near the critical density and obtained the structure factor, S(k), and the pair distribution functions, g(r). By small angle X-ray scattering measurements, we succeeded in observing a medium-range fluctuation in the semiconductor–metal transition in expanded fluid Se.     

Dynamical structure of fluid mercury: Molecular-dynamics simulations

We have carried out molecular-dynamics simulations for nonmetallic fluid mercury in liquid and vapor phases using a Lennard–Jones type effective potential and shown that the structure factors S(Q) and the dynamic structure factors S(Q, ω) of nonmetallic fluid mercury obtained by our MD simulations are in good agreement with recent X-ray diffraction and inelastic X-ray scattering experiments. We conclude from these results that, though the fluid mercury which shows a metal–nonmetal transition is a ‘complex’ fluid, the nonmetallic fluid mercury is a relatively ‘simple’ liquid, which can be well described by the single density-independent Lennard–Jones type potential.     

Glass transition,thermal expansion and relaxation in B2O3 glass measured by time-resolved X-ray diffraction

In situ heating experiments using high-energy, high-intensity synchrotron radiation, can be successfully designed to study structural evolution with temperature of glassy materials. Coherent diffraction from glassy materials forms a succession of halos or diffraction maxima in reciprocal space and the variation with temperature, of the wave-vector Q_max or angular position of the first diffracted intensity I(Q_max) maximum below T_g can be used to determine the iso-structural volume expansion. In the present work we have obtained synchrotron X-ray diffraction patterns in transmission during in situ heating of a B₂O₃ glass. Samples were obtained by melting the B₂O₃ glass rods which were then air-cooled or liquid nitrogen-cooled. The evolution with temperature (and time) of the position of the first diffraction maximum of the diffraction pattern accurately reflected the thermal expansion coefficient and the relaxation behavior of the B₂O₃ glass. Such results allowed determination by diffraction of the glass transition temperature, T_g, at 580 K, as well as information on the structural relaxation during thermal annealing. The total volume changes due to relaxation were measured to be about 1.5 vol.% and 2.5 vol.%, for the air-cooled and the liquid nitrogen-cooled B₂O₃ glass, respectively.     

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.     

Raman line shape analysis as a mean characterizing molecular glass-forming liquids

Raman scattering spectra in glass forming toluene were studied in the temperature range 50–323 K with the goal of extracting information about homogeneous, inhomogeneous and orientational broadening. It was found that the temperature dependence of inhomogeneous line width allows one to depict two peculiar temperatures: T_A and T_g, where T_g is the glass transition temperature and T_A is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the α-relaxation time dependence on temperature, τ_α(T). Temperature dependence of the orientational phase loss time τ_OPL was found to correspond well to τ_α at T > T_A and continues approximately Arrhenius behavior for lower temperature in contrast to τ_α(T). Also, a comparative analysis of homogeneous broadening of polarized and depolarized lines was done, which provided an estimation of the orientational broadening γ_NL(T). The found γ_NL(T) decreases linearly as the temperature decreases and goes to zero at T ~ T_c, where T_c is the critical temperature in framework of the mode-coupling theory (note that T_c is close to other peculiar temperatures T_B and T_β, but we did not intent to distinguish among them in the present work). Thus, it was shown that the Raman line shape analysis in molecular glass forming materials allows one to extract peculiar temperatures: T_A, T_g, and, probably, T_c.The test of the possibility to use a probe molecule for the Raman line shape analysis has revealed that the extracted data for probe molecule lines do not characterize the host matrices, at least in the low-viscous state (T > T_A).