Papers by Keyword: Structural Relaxation

Abstract: Glass molding press is an efficient and promising process for glass lens manufacturing. Molding temperature and velocity are crucial factors for pressing, and the annealing rate directly influences the residual stress and volume change of the final lens. The research contributes to studying the key factors that influence the quality of the molded lens. In the paper, by incorporating stress relaxation and structural relaxation of the viscoelastic material, numerical simulation is undertaken to determine the proper process parameters. Furthermore, the whole process of a spherical lens molding is simulated to predict the final residual stress and volume change, also the influence of different annealing rates is estimated.

Abstract: Microstructural changes and relaxation processes were examined in Fe-Zr based rapidly solidified samples. These phenomena occur far below crystallization temperature in the course of heat treatment, or even at room temperature, induced by absorbed hydrogen.

Abstract: A research program has been completed in order to analyze structural changes during heating of amorphous alloys belonging to Fe-Ni-P system. Special attention has been given to thermodynamics and mechanism of crystallization, to determine some aspects of development for crystalline phases. Experimental material used to determine characteristics of crystallization consisted in long ribbons, 30 thick and 18 mm wide, fabricated by mean of “Planar Flow casting” as amorphous Fe₄₂Ni₃₈P₁₆B₄ alloy. Differential Thermal Analysis (DTA) and X-rays diffraction have been used to determine crystallization temperature of this alloy. Curves of differential thermal analysis for heating rates ranging between 1°C/minute and 20 °C/minute have been used to determine activation energy of crystallization.

Abstract: The increased optical fiber development for a variety of physical and chemical measurements using smart structures and sensors leads to investigate the mechanical and chemical reliability of standard commercial acrylate polymer coated fibers. A comparison of mean failure strength of as-aged fibres to the water influence for similar exposure duration has revealed the highest sensitivity to dimethyl sulfoxide reagent (DMSO). SEM investigation revealed different damage levels in function of the aging conditions.

Abstract: Zr-based bulk glassy (BG) alloys show high tensile strength and a high Charpy impact value. In this study, the free volume changes for the hypoeutectic Zr60Cu30Al10 BG alloy during isothermal annealing below glass transition temperature (Tg) have been investigated by positron annihilation lifetime and coincidence Doppler broadening (CDB) measurements. The positron lifetime of hypoeutectic and eutectic BG alloys is almost the same, although the atomic volume ratio, estimated by the density for the eutectic BG alloy is different from that for the hypoeutectic BG alloy. The CDB spectra show a marked difference between eutectic and hypoeutectic BG alloys; that is, the spectrum of the hypoeutectic BG alloy is closes to that of Zr metal than that of the eutectic BG alloy. This result exhibits that the elemental fraction of Zr atoms around free volume in hypoeutectic BG alloy is greater than that in eutectic BG alloy. The CDB ratio profiles for the hypoeutectic BG alloy during annealing shows no appreciable change. The same trend was observed in the eutectic BG alloy.

Abstract: Structural relaxation process in the Zr-Cu metallic glasses is investigated by using molecular dynamics simulations. The enthalpy change in isothermal annealing of the glassy state cannot be fitted by a simple exponential function but obeys a stretched exponential function, which indicates that the relaxation in glassy phase is not a single Debye type process. A close examination of individual atomic motion reveals that the enthalpy relaxation is related to a string-like cooperative motion of atoms. The analysis of the local symmetry around each atom shows that a network of the icosahedral clusters grows in the glassy phases during annealing and it closely relates to the free-volume annihilation in the structural relaxation.

Abstract: The kinetics of structural relaxation in fragile glass former, Pd46Cu35.5P18.5 BMG, and strong glass former, Zr50Cu40Al10 BMG, was investigated by volume relaxation. The former exhibited a relaxation phenomenon that is well understood by the local topological instability model, while the latter showed monotonous relaxation behavior over a wide range down to Tg-60 K. The discrepancy may be closely related to the difference in the fragility of both glasses.

Abstract: Consistent model including structural relaxation is necessary for correct glass stress calculation in numerical computations of glass forming processes. Calculation of glass relaxation phenomena is often done by combining independent empirical formula on stress relaxation (for a simple temperature regime) and independent model of time-temperature dependence on Tool fictive temperature, Tf. Another approach was developed and verified here. Tool-Narayanaswamy and Moynihan/Mazurin relaxation model was adopted. Relaxation was obtained not from empirical model, but from calculated time-temperature dependence of Tool fictive temperature (Tf), dynamic viscosity, heat capacity and coefficient of thermal expansion. Heat conduction in a glass probe of known temperature history, structural relaxation and stress calculation were used in one computation scheme using Matlab. The stress of glass probe was measured using polarized light (Senarmont method). Rather high stress computation accuracy was obtained in comparison to stress experimental results. The applied model approach is being to be extended for application in commercial finite element codes for modeling of glass forming processes.

Abstract: Thin layers of the OLED related polyphenylene-vinylene (PPV) deposited by a precursor on micro-fabricated Si cantilevers were studied by applying the vibrating-reed technique during repeated temperature cycling between 100 and 520 K. By means of the Langmuir-Blodgett method for film production, the dependence of damping and elastic modulus on well defined values of film thickness (16 to 69 nm) was determined. Simultaneous measurements of these quantities showed four damping peaks during heating around 130 K (called γ), 250 K (β), 350 K (β’), and 400 K (called C). Three of them (γ, β’, C) disappeared after heating to the highest temperature (520 K) indicating their presence in the precursor only. The activation parameters of the relaxation peaks (γ, β, β’) were estimated and assigned to specific atomic movements in the molecule. Peak C occurs during the conversion process of precursor to polymer. Earlier results are essentially substantiated, indicating only slight differences to those for layers produced previously by spin coating. The observed thickness dependence of damping for the γ and β peaks suggests a weaker contribution of molecules in the surface region than of those in the bulk, while the β’ peak is supposed to result from molecules in the interface region between layer and substrate.

Abstract: The kinetics of structural relaxation in a Pd40Ni40P20 bulk metallic glass was investigated by the volume relaxation due to density experiments and the enthalpy relaxation due to specific heat experiments. A two-step relaxation process was found in the volume relaxation, while the enthalpy relaxation seemed to be one-step relaxation process with a spectrum of relaxation times. First-step volume relaxation only in as-quenched glass was the process with a spread of relaxation times at lower relaxing temperature, while a Debye-type relaxation behavior was observed at higher temperature near Tg and in pre-annealed glass. The comparison of the kinetics of volume and enthalpy relaxations at the same temperature showed a disagreement with the prediction of free volume theory.