Activation Energy For Structural Relaxation Research Articles

Structural interpretation of the viscous flow and relaxation kinetics in the As-Se and Ge-Se chalcogenide systems

This study investigates the behaviour of As₂₀Se₈₀ and Ge₂₅Se₇₅ chalcogenide glasses near the glass transition temperature using differential scanning calorimetry (DSC) and thermomechanical analysis (TMA), addressing a gap in the literature for these compositions. Precise TMA measurements of viscosity in the range of 10⁶ – 10¹³ Pa·s clarified existing data and improved the quality of fits describing the temperature-dependent viscosity for both compositions. The DSC method enabled a comprehensive analysis of structural relaxation behaviours, which were described using the Tool-Narayanaswamy-Moynihan (TNM) model with the determination of its four parameters. The study discusses the viscosity and structural relaxation behaviours of both glass-formers in relation to their structure, referencing previous results for other binary chalcogenide glasses in the As-Se and Ge-Se systems. The findings indicate significant changes in viscosity and activation energy of structural relaxation with the addition of As or Ge, while the non-linearity and non-exponentiality parameters remain largely unaffected.

Thermo-kinetic and structural characterization of Ce-doped glasses based on Bioglass 45S5

Thermo-kinetic behavior of Ce-doped 45S5 bioactive glass (addition of CeO2 up to 1.5 mol.%) was explored by means of calorimetry and thermomechanical analysis. The apparent activation energy of structural relaxation was estimated to vary in the 540–670 kJ mol−1 range. The relaxation process exhibited broad distribution of relaxation times and the relaxation motions were found to only weakly depend on the material's structure. The activation energy of viscous flow was found to vary in the 585–615 kJ mol−1 range; the viscosity itself showed Arrhenian behavior within 107–1011 Pa s. Thermal stability of the glasses was found to be non-monotonous and generally very low, with the most stable glass composition being 45S5 + 0.5 mol.% CeO2. The cold crystallization was found to behave autocatalytically, with the activation energy decreasing from 360 to 310 kJ mol−1 with the gradual addition of CeO2. While the bulk glasses were crystallization-resistant up to 620 °C, the powdered material with CeO2 content ≥1 mol.% was predicted to crystallize even at temperatures slightly above the glass transition temperature.

Application of advanced thermal analysis for characterization of crystalline and amorphous phases of carvedilol

The thermal behaviour of crystalline and amorphous carvedilol (CAR) phases was studied by advanced thermal analysis using Quantum Design Physical Property Measurement System and Differential Scanning Calorimetry. Theoretical functions describing crystalline carvedilol heat capacity at low temperatures and the Debye-Einstein function for high temperatures were obtained. Based on the experimental heat capacity values, solid and liquid baselines were established, and the state functions (H, S, G) for solid and liquid states were calculated. A comprehensive characterization of melting and glass transition processes was obtained. CAR is easily amorphizable by cooling the liquid. The residual entropy, which quantifies the extent of frozen-in disorder in the amorphous solid, for glassy CAR was estimated as 51 J·mol–1·K–1. The Kauzmann temperature (TK) was estimated based on enthalpy and entropy. Molecular motions in the amorphous phase were also studied. The activation energy for structural relaxation (Ea = 539 kJ·mol–1) and fragility parameter (m = 91) were obtained from the non-isothermal physical ageing. The isothermal physical ageing kinetics of amorphous CAR was studied by applying Kohlrausch-Williams-Watts (KWW) model. The mean molecular relaxation time constant (τKWW = 117 min) and relaxation constant (βKWW = 0.33) were obtained. CAR was classified as a fragile glass-former. Furthermore, τKWW constant for samples aged at 303.15 K is very low, thus, the physical ageing will occur during the short- and long-term storage of amorphous CAR, potentially changing its physicochemical properties during the ageing process. However, the results of molecular mobility studies (high molecular motions) show that the relationship between molecular motions in a glassy solid and its tendency to crystallization does not seem to follow an expected pattern, i.e., no crystallization occurred by thermal treatment of glassy, supercooled liquid and liquid phases of CAR as one would expect. Modern calorimetry and quantitative thermal analysis provided the fundamental kinetic and thermodynamic information about the crystalline and amorphous states of CAR.

A Cu-Y-Al glassy alloy with strong beta relaxation and low activation energies for structural relaxation and crystallization

Structural relaxation, beta relaxation and crystallization processes of the Cu53Y42Al5 metallic glass were studied in the present work. X-ray diffractometry was used to study the structure of the samples. Phase transformations were studied by differential scanning calorimetry by using a power compensated type differential calorimeter. Cu-mould casting resulted in the formation of crystalline samples while fully glassy Cu53Y42Al5 alloy samples were produced by melt spinning at a high cooling rate of about 105-106 K/s. This glassy alloy is characterized by strong beta relaxation and low activation energy values for crystallization and relaxation in comparison with other metallic glasses studied earlier.

Correlation between the activation energies of structural relaxation and viscous flow for BaO–P2O5–Al2O3 glasses

Viscosity and structural relaxation were studied by thermomechanical analysis for six Al2O3-doped BaO–P2O5 glasses. The low-temperature viscosity data (107– 1011 Pa•s) were described and modeled by the Vogel-Fulcher-Tammann, Avramov-Milchev and Mauro-Yue-Ellison-Gupta-Allan equations. Temperature dependence of the activation energy of viscous flow was determined from the tangent of the extrapolated (in terms of the three viscosity equations) viscosity-temperature data-curves. Values of the activation energy of viscous flow were compared to the activation energies of volume relaxation determined from the exact physico-chemical description of the cyclic thermomechanical measurements performed over the glass transition region. In addition, differential scanning calorimetry was used to determine the activation energies of enthalpy relaxation. Good correspondence between the relaxation and viscosity activation energies was found in the respective temperature ranges. Good agreement was also confirmed for the viscosity and enthalpy fragilities, as well as for the indicator of the thermodynamic fragility.

Dynamic Fragility of α‐Olefin Molecular Bottlebrushes

ABSTRACTThe glass transition temperature, dynamic fragilities, and flow activation energy of a series of well‐entangled poly(α‐olefin) (PαO) molecular bottlebrushes were measured as a function of side chain length (Nsc). The PαO bottlebrushes studied here have side chain lengths, Nsc, ranging from 4 (poly(1‐hexene)) to 10 (poly(1‐dodecene). A linear polyolefin (polypropylene), with Nsc = 1, was included in this study as a reference. The observed glassy dynamics behavior in the PαO bottlebrushes is opposite to that observed in linear polymers, namely, the glass transition temperature, the dynamic fragility, and the activation energy of structural relaxation are decreasing functions of the backbone rigidity. This anomalous behavior is due to a decrease in correlation between adjacent backbones, which is directly related to their cooperativity in the α‐relaxation, as Nsc and the concomitant distance between backbones increase. This change in conformation is also manifested as an increase in free volume and the consequent decrease in monomeric friction coefficient. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1293–1299

Kinetic Processes in Amorphous Materials Revealed by Thermal Analysis: Application to Glassy Selenium

It is expected that viscous flow is affecting the kinetic processes in a supercooled liquid, such as the structural relaxation and the crystallization kinetics. These processes significantly influence the behavior of glass being prepared by quenching. In this paper, the activation energy of viscous flow is discussed with respect to the activation energy of crystal growth and the structural relaxation of glassy selenium. Differential scanning calorimetry (DSC), thermomechanical analysis (TMA) and hot-stage infrared microscopy were used. It is shown that the activation energy of structural relaxation corresponds to that of the viscous flow at the lowest value of the glass transition temperature obtained within the commonly achievable time scale. The temperature-dependent activation energy of crystal growth, data obtained by isothermal and non-isothermal DSC and TMA experiments, as well as direct microscopic measurements, follows nearly the same dependence as the activation energy of viscous flow, taking into account viscosity and crystal growth rate decoupling due to the departure from Stokes–Einstein behavior.

New method for determination of hidden supercooled liquid region of TiZr-based amorphous alloys

TiZr-based metallic glass has a unique glass transition point. It is located at the top of the first exothermic peak instead of at the typical endothermic transformation reaction before the supercooled liquid region (∆Tx). It is hard to identify the glass transition temperature (Tg) and ∆Tx of these TiZr-based amorphous alloys by an exothermic-exothermic reaction. In this study, based on the physical characteristics of the free volume reduction, KWW-exponent β, the activation energy for structural relaxation (Qrelax), and the incubation time by isothermal annealing in DSC analyses, the exact Tg point can be precisely identified. As a result, the Ti42Zr42Ta3Si10Sn3 amorphous alloy presents an extra-wide supercooled liquid region (∆Tx), up to 149 K.

First-order phase transformation and structural studies in Se85In15−xZnx chalcogenide glasses

Present research work describes the crystallization kinetics and structural studies in Se85In15−xZnx chalcogenide glasses. Bulk alloys of Se85In15−xZnx were synthesized by melt-quenching procedure. High resolution X-ray diffraction (HRXRD) was used to confirm the amorphous nature of synthesized samples. Non-isothermal differential scanning calorimetry (DSC) measurements were performed at 5, 10, 15, 20 and 25 K min−1 heating rates to study kinetics of crystallization in Se85In15−xZnx. Various crystallization parameters such as glass transition (Tg), onset crystalline (Tc), peak crystallization (Tp) and melting temperature (Tm) were calculated from DSC curves. The activation energies of structural relaxation (ΔEt) and crystallization (ΔEc) were determined by using Kissinger, Moynihan and Ozawa approaches. ΔEt is found to be the lowest for Se85In6Zn9 sample which shows this sample has the highest probability of escape to a state of lower configurational energy and has greater stability. Thermal stability of various compositions was studied and found to vary with Zn content. Further, HRXRD and field emission scanning electron microscope were used for the study of first phase transformation in Se85In15−xZnx samples.

Relaxation dynamics of Fe55Cr10Mo14C15B6 metallic glass explored by mechanical spectroscopy and calorimetry measurements

In this work, the mechanical relaxation dynamics of Fe55Cr10Mo14C15B6 metallic glass is explored by mechanical spectroscopy. The temperature-dependent loss modulus E″(T) shows the features of β relaxation well below glass transition temperature T g. This β relaxation can be well described in the framework of anelastic theory by a thermal activated process with activation energy of 165 kJ mol−1. Structural relaxation, also known as physical aging, has a large effect on the glass properties. The activation energy spectrum of structural relaxation is characterized by differential scanning calorimetry measuring the heat flow difference between as-quenched and relaxed states. The obtained energy spectrum is well described by a lognormal distribution with maximum probability activation energy of 176 kJ mol−1. The obtained activation energy of structural relaxation is similar to that of β relaxation observed from mechanical spectroscopy. Both values are also close to the Johari–Goldstein β relaxation estimated by the empirical rule E β = 26RT g.

Qualitative change in structural dynamics of some glass-forming systems.

Analysis of the temperature dependence of the structural relaxation time τ(α)(T) in supercooled liquids revealed a qualitatively distinct feature-a sharp, cusplike maximum in the second derivative of logτ(α)(T)at some T(max). It suggests that the super-Arrhenius temperature dependence of τ(α)(T) in glass-forming liquids eventually crosses over to an Arrhenius behavior at T<T(max), and there is no divergence of τ(α)(T) at nonzero T. T(max) can be above or below T(g), depending on the sensitivity of τ(T) to a change in the liquid's density quantified by the exponent γ in the scaling τ(α)(T)∼exp(A/Tρ(-γ)). These results might turn the discussion of the glass transition in a different direction-toward the origin of the limiting activation energy for structural relaxation at low T.

Heat capacities of crystalline and glassy lithium metaphosphate up to the transition region

Heat capacity measurements have been conducted by means of DSC on both crystalline and glassy lithium metaphosphate, from room temperature up to the melting region. The heat capacity of the glass is slightly higher than that of the crystal. Contrary to the crystal, in the neighborhood of Tg, Cp increases rapidly by 10 J mol−1 K−1 conferring to this glass a “fragile character.” Nevertheless, the passage through Tm does not show any discontinuity and the values of the glass and of the crystal are identical. The Debye model appears to be realistic to describe the glass heat capacity to temperature dependence. The Debye temperature and frequency were determined by minimizing the Rp and χ2 parameters of the Cv fitting curve. From the calculation of the entropy of the liquid at T > Tm, the excess entropy of the glass at Tg was determined. Using the dependence of the glass transition on the heating rate, we calculated the values of the activation energy for structural relaxation (Erelax) and of the lower limit of the glass transition temperature (\( {T_{\text{g}}^{{^\circ }} } \)) which is a thermodynamic parameter, contrary to Tg which is a kinetic parameter.

How to determine activation energy of glass transition

The article critically reviews the current methodologies for determination of apparent activation energy of structural relaxation, ∆h*, in the glass transition range. Tool–Narayanaswamy–Moynihan phenomenological model was used to simulate data for all major types of relaxation behavior, which were consequently evaluated in terms of the tested methodologies (curve-fitting, evaluation of ∆h* from intrinsic cycles and evaluation of ∆h* from constant heating rate cycles). Advantages and disadvantages of particular methodologies are demonstrated and thoroughly discussed. In addition, effects of various data-distortive effects influencing determination of glass transition activation energy are demonstrated and described. The discussed data-distortive effects include presence thermal gradients, improperly designed temperature programs, incorrectly applied subtractions of the thermokinetic background, or inability of the DSC instrument to perform high cooling/heating rates. Detailed guide for correct determination of ∆h* from DSC measurements is introduced.

Kinetic characterization of barium titanate–bismuth oxide–vanadium pentoxide glasses

The glasses with the composition (80 − x)V2O5·20Bi2O3·xBaTiO3 with x = 2.5, 5, 7.5 and 10 mol % were prepared by a melting technique. The crystallization behavior and the microstructure of the glasses were investigated by using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mean value of the activation energy of structural relaxation (〈Eg〉) decreased from 395 ± 3 to 369 ± 1.83 kJ/mol when BaTiO3 increased from 2.5 to 10 mol %. The activation energies obtained by the methods Kissinger and Ozawa were in the range from 213 ± 0.65 to 256 ± 1.23 kJ/mol. Different analysis methods were used to estimate the Avrami exponents. Their values range from 4.26 ± 0.6 to 2.62 ± 0.11 for the exothermic peak of the prepared glasses. Moreover, synthesized glasses-ceramic containing BaTi4O9 and Ba3TiV4O15 were estimated by using XRD.

Glass transition kinetics in ball milled amorphous GaxTe100‐x nanoparticles

We report the glass transition kinetics in ball milled amorphous GaxTe100‐x (x=3, 6, 9 and 12) nanoparticles. From field emission scanning electron microscopic investigation, it is observed that ball milled samples of a-GaxTe100‐x contain high yield of nanoparticles with their diameter in the range of 20-40nm. X-ray diffraction patterns of these samples suggest that these nanoparticles are amorphous in nature. Glass transition kinetics of these amorphous nanoparticles shows that the values of glass transition temperature increase with the increase in composition and heating rate. The activation energy for structural relaxation calculated by two methods shows good agreement with each other and is found to increase with increasing gallium concentration, which can be explained in terms of change in chemical bonding at higher concentrations.

Relaxation of rapidly quenched metallic glasses: Effect of the relaxation state on the slow low temperature dynamics

The relaxation spectrum of rapidly quenched Mg65Cu25Y10 metallic glass ribbons is studied by mechanical spectroscopy at temperatures below and around the glass transition. The comparison between hyper-quenched and relaxed samples is used to examine the origin of the low temperature “excess wing” of internal friction commonly observed in mechanical spectroscopy of metallic glasses. The results show that the excess wing can be attributed to access of the system to the broad α-relaxation process while evidence of secondary relaxations is not found. This suggests that in this glassy system the activation energies of structural relaxation and low temperature deformation are directly related to the activation energy of the main relaxation process of the glassy state.

Fragile-strong behavior in the AsxSe1−xglass forming system in relation to structural dimensionality

A series of physical properties have been measured throughout and above the glass transition for the whole As${}_{x}$Se${}_{1\ensuremath{-}x}$ system, including the activation for viscous flow ${E}_{\ensuremath{\eta}}$, the activation energy for enthalpy relaxation ${E}_{H}$, and the activation energy for structural relaxation ${E}_{a}$ obtained by specific heat spectroscopy. All properties show a double minimum at an average coordination number $\ensuremath{\langle}r\ensuremath{\rangle}$ $=$ 2.3 and $\ensuremath{\langle}r\ensuremath{\rangle}$ $=$ 2.5 with a local maximum at $\ensuremath{\langle}r\ensuremath{\rangle}$ $=$ 2.4. This is in stark contrast to the physical properties previously measured on the same samples at room temperature and which instead show a single minimum centered at $\ensuremath{\langle}r\ensuremath{\rangle}$ $=$ 2.4. The observed trend is consistent with the dimensionality of the network derived from structural data obtained by nuclear magnetic resonance. An analysis of the complex heat capacity also reveals a bimodal relaxation process in As-rich glasses, which explains why they are kinetically fragile but appear thermodynamically strong. Finally, these results demonstrate that previous observations of an ``intermediate phase'' in As${}_{x}$Se${}_{1\ensuremath{-}x}$ glasses near $\ensuremath{\langle}r\ensuremath{\rangle}$ $=$ 2.3 is associated with the high temperature behavior of the glassy network and should be interpreted in terms of the temperature dependence of structural constraints rather than the number of constraints in the room-temperature glass.

Crystallization kinetics in as-synthesis high yield of a-Se100−xTex nanorods

Abstract The paper reports the synthesis of high yield of a-Se 100− x Te x ( x = 3, 6, 9 and 12) nanorods using one of the simplest approaches i.e. melt quenching technique. The morphology and microstructure of as-prepared alloys is studied using scanning Electron Microscopy (SEM) and transmission electron microscopy (TEM). From SEM investigation, it is observed that these powder samples of a-Se x Te 100− x contain high yield of nanorods and their diameter is of the order of several hundred nanometers. The XRD patterns of these samples suggest that these nanorods are amorphous in nature. Crystallization kinetics in these nanorods of amorphous Se x Te 100− x glasses are studied at different heating rates (5, 10, 15 and 20 K min −1 ) under non-isothermal condition using differential scanning calorimetry. It is observed that the value of glass transition temperature and crystallization temperature varies with the composition and heating rate. From the heating rate dependence of glass transition temperature and crystallization temperature, the activation energy for structural relaxation (Δ E t ), the activation energy of crystallization (Δ E c ) and the order parameter ( n ) have been calculated. The composition dependence of the activation energy for thermal relaxation and activation energy for crystallization is discussed in terms of the structure of Se–Te glassy system.

Threshold voltage drift in phase change memories: scaling and modeling

ABSTRACTThe phase-change-memory (PCM) relies on the ability of a chalcogenide material, usually Ge2Sb2Te5 (GST), to switch from the amorphous phase with a high electrical resistance (~MΩ) to a crystalline phase with a low electrical resistance (∼kΩ). The structural stability of the amorphous phase is critically affected by temperature-activated crystallization and resistance drift due to structural relaxation (SR) [1]. While amorphous chalcogenides are relatively stable with respect to crystallization, thanks to a high activation energy above 2 eV [2], the lower activation energy of SR can strongly affect the PCM electrical properties of the amorphous phase, including resistance, activation energy for conduction and, most importantly, the threshold voltage VT. The latter marks the boundary between read and programming operations in PCM, thus VT instability must be carefully predicted to minimize read disturbs within the array. To this purpose, understanding the switching mechanisms and modeling of VT is of utmost importance for PCM device development and scaling.

Non-Isothermal Crystallization in Amorphous GaxSe100-x Nanorods

Thermal properties of amorphous GaxSe100-x (a-GaxSe100-x) nanorods were studied at different heating rates (5, 10, 15, and 20 K/min) using differential scanning calorimetry. It has been observed that the values of glass transition temperature and crystallization temperature vary with composition and heating rate. The activation energy for structural relaxation (ΔEt), the activation energy of crystallization (ΔEc) and the order parameter (n) were calculated using the heating rate dependence of glass transition temperature and crystallization temperature. Several parameters such as thermal stability parameter (S), the crystallization enthalpy (ΔHc), entropy (ΔS) changes during crystallization process and crystallization rate factor (Kp) were calculated to study the thermal stability of these glasses. Whereas the parameters such as Hruby number (HR) and relaxation time (τTg) were estimated to understand the glass forming ability (GFA). The fragility index (Fi) for the present glasses was also calculated to ascertain whether these glasses were obtained from kinetically strong-glass-forming (KS) or kinetically fragile-glass forming (KF) liquids.