Papers by Keyword: Glass Transition

Abstract: Modeling of Glass transition is attempted based on the Cluster Variation Method. Free energy functional of an L10 ordered phase is employed to describe the first order nature of the transition. Free energy contour surface calculated as a function of temperature and an order parameter which simulates an amount of defects provides a generalized stability diagram in which the ideal glass transition temperature is identified as a critical point. Transition kinetics is investigated by Path Probability Method which is the kinetics version of the CVM to time domain. Continuous cooling behavior is calculated by explicitly incorporating the temperature dependent viscosity term based on VFT (Vogel-Fulcher-Tamman) formula. The glass transition is realized as the freezing of the order parameter due to the enhanced viscosity. The extension of the present theoretical scheme to non-Bravais lattice is attempted by Continuous Cluster Variation Method.

Abstract: The free volume relaxation just under the glass transition region was investigated by the high-resolution density measurement using the bulk metallic glasses with the compositions of Pd40Ni40P20 and Zr55Cu30Ni5Al10. The relaxation process was well described by a stretched exponential function with Kohlrausch exponent values less than unity. The reduced free volumes in an as-quenched state were estimated as 0.0117 and 0.0353 for Pd40Ni40P20 glass and Zr55Cu30Ni5Al10 glass, respectively. The specific heat curves Cp(T) for Pd42.5Cu30Ni7.5P20 alloy were obtained for the supercooled liquid, the equilibrium liquid and the crystallized alloy. The isenthalpic Kauzmann temperature TKH and isentropic Kauzmann temperature TKS were estimated as 471 K and 522 K, respectively, from the specific heat data.

Abstract: This study investigated the mixed alkali effect in a series of phosphate based glasses. These glasses were of the composition 0.5P2O5-0.2CaO-0.3-xNa2O-xK2O where x=0 to 0.3 in steps of 0.05. This study considered density measurements using Archimedes’s principle, thermal characterisation using differential scanning calorimetry, phase analysis following crystallisation using X-ray powder diffraction (XRD), and degradation studies combined with ion release. The results showed that these mixed alkali glasses showed a linear decrease in density, with the ternary single alkali glass with 0.3mol K2O showing a 3% reduction in density as compared to that with 0.3mol Na2O which correlated well with the difference in ionic diameter and atomic weight of both cations. These glasses also showed intermediate glass transition temperature (Tg) values, compared to those of the ternary single alkali glasses having the same alkali oxide content, and the minimum Tg value was recorded for equimolar amounts of both alkali oxides. However, they did not show any significant change in the degradation rate compared to the glass with 0.3mol Na2O with the exception of the 0.25mol K2O glass. The single alkali glass with 0.3mol K2O showed a significant increase in the degradation rate by an approximate one order of magnitude. For the mixed alkali glasses with low molar concentration of K2O, only sodium phosphate-rich phases [NaCa(PO3)3 and Na4Ca(PO3)6] were detected from XRD; at high molar concentrations however, potassium phosphate-rich phases [KCa(PO3)3 and KPO3] were detected. At equimolar concentration of both alkali cations, KCa(PO3)3 and Na4Ca(PO3)6 were identified. K+, Ca2+, and P3O9 3- release followed the degradation behaviour where the highly degrading glasses with 0.25 and 0.3mol K2O released the highest amount of these ions; however, there was no definite trend in the remaining glass compositions.

Abstract: A set of the techniques, including large- and small-angle X-ray scattering, differential scanning calorimetry, electrical resistance and microhardness measurements was used to study the changes in structure of the well-known Fe40Ni40P14B6 metallic glass under continuous heating up to the crystallization onset temperature. The measurements performed in situ and after rapid cooling from different temperatures revealed that structural relaxation is a multi-stage process involving variations of the short-range order, relief of quenched-in stresses, changes of the free volume concentration and enhancement of the concentration inhomogeneities. The temperature ranges of each process have been established. Using proposed approximate equation describing the scattering particles growth at a constant heating rate, it has been shown that the enhancement of phase separation in the glass investigated is governed by diffusion-limited growth mechanism.

Abstract: Preparation of monodispersed inorganic-organic hybrid particles has become one of the most attractive research topics in recent years. In this study, phenylsilsesquioxane (PhSiO3/2) particles were synthesized from phenyltriethoxysilane (PhSi(OEt)3), ethanol (EtOH), hydrochloric acid for hydrolysis and ammonia water for polycondensation by the sol-gel method. Spherical PhSiO3/2 particles were obtainable by varying the mole ratios of EtOH, H2O, HCl and NH3, to PhSi(OEt)3. The particle size was changed from 0.1 to 5.0 .m with the mole ratio. Under the optimum conditions, monodispersed particles with 1.50 .m in diameter and 0.05 of normalized standard deviation were successfully prepared.

Abstract: A molecular dynamic model for instantaneously undercooled iron melt has been applied to model the isothermal annealing of the stable percolation clusters. They contain mutual penetrating and contacting icosahedra with atoms at the vertices and centers. Such stable clusters form only below the critical temperature Tg ~1180 K. We identify Tg as the glass transition temperature. It is established that the time when a homogeneous nucleation starts is minimal for Tg. The stable icosahedral percolation clusters do not form above Tg. The proposed quantitative model describes the atomic mobility in metal glasses, where root-mean-square atomic displacement is the sum of contributions from both linear (Einsteinian) and logarithmic terms, associated to irreversible structure relaxation. The activation parameters of the model change abruptly at the icosahedral percolation transition.

Abstract: This paper studies the phase transitions of Cu and Ni alloys as they cool from melting temperature to room temperature under high-pressure conditions. The interatomic forces acting between the atoms are modeled by the tight-binding potential. Control over the environmental pressure and the cooling temperature is maintained by a canonical ensemble (N, P, T) system. The numerical results confirm that the metal phase transition is influenced significantly by the pressure conditions, even in the case of pure Cu and Ni metals. Three specific transition pathways are identified for the Cu and Ni alloys as they cool from melting temperature to room temperature, namely a transition at the melting temperature to a crystalline structure, a transition at the glass transition temperature to a glass (amorphous) structure, and finally solidification at the melting temperature followed by a subsequent transition at the glass transition temperature. The results reveal that glass transition generally occurs at lower pressures in alloys with higher Cu compositions, while glass transition following prior solidification tends to takes place at higher pressures in alloys with higher Ni compositions.

Abstract: This study investigates the influence of viscoelastic properties of a series of ethylene-vinyl acetate copolymer on impact noise and vibration damping of wood/polymer/wood sandwich composites. It was found that the impact noise and vibration damping of composites are very sensitive to the state of molecular motion of polymer. The noise and vibration damping of composites was better when the polymer was in the glass transition state (vinyl acetate 55~75%) at the test-temperature, and it was worse when the polymer was in rubbery state (vinyl acetate 47~20%) or in glassy state (vinyl acetate100~87%). The impact noise decreased by about 6-12dB when the glass transition state of polymer was sandwiched between wood panels.