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Fundamentals of Molecular Dynamics (MD) Simulations and Tools for Examining Nanostructured Materials
Published in Junko Habasaki, Molecular Dynamics of Nanostructures and Nanoionics, 2020
From the dynamical point of view, the coexistence of slow and fast particles is related to that of different length scale motions. Their different temperature dependences resulted in the curvature of the diffusivity against 1/T plot [4]. That is, super-Arrhenius behavior is caused by the coexistence of different length scales. This view is previously obtained from the changes in the dynamics observed in ionic liquid, EMIM- NO3 [46]. In that paper, it was concluded that the large “fragility,” shown by the curvature of diffusion coefficients, can be explained by the changes in the slopes in both temperature regions rather than the suddenly caused rapid decrease of dynamics near T0. Above the inflection point, long-ranged motions with cooperative motions are dominant, while below it, the localized motions become dominant. Then, the acceleration of the dynamics with heterogeneity near the inflection points, which resulted in the gentle slope in the high temperature region, ensures the large changes in the slopes. As found in the above discussions, the fragility of glass-forming liquids is closely related to the concept of heterogeneity of dynamics. Accordingly, it is suggested that multifractality [46–48] due to the coexistence of the different length scales is a good measure of the fragility as well as the heterogeneity. The concept of multifractal can be seen in the Cantor set shown in Fig. 4.4. As depicted in this figure, the mixing of different exponents resulted in the formation of complex heterogeneous structures.
Glass Formation and Structural Modification in Glasses
Published in Abhay Kumar Singh, Tien-Chien Jen, Chalcogenide, 2021
Abhay Kumar Singh, Tien-Chien Jen
Glass-forming liquids fragility is widely controlled from the features of the heat capacity jump throughout the glass transition. Therefore, the shape of the glass transition curve can be analyzed to estimate the fragility. Usually fragile systems have a tendency to undergo large gains in degrees of freedom during the glass transition, this interprets into a large jump in heat capacity within a short temperature range. In contrast to this, strong systems can retain high viscosity over wider temperature ranges, consequently, they exhibit shallower heat capacity gains spread out in temperature. Usually width and height of the glass transition, Δand ΔCp, are used to measure the fragility of the glassy systems [58, 59]. With the help of the DSC measurement, differences between the heat capacity (ΔCp) of the solid and liquid can be achieved. The ΔCp is usually normalized from the melting entropy (ΔSm) to associate variations in the entropy of the corresponding crystalline phase [58]. However, ΔSm is not a frequently available quantity for several chalcogenide glasses, but its value should be more or less constant within the chalcogenide glass system. In a similar way, the width of the glass transition ΔTg can be obtained from the DSC data by measuring the temperature of onset and completion of the glass transition [57, 59]. To get better fragility results, the ΔTg width should be normalized in terms of onset Tg, for the types of glass covering a wide range of transition temperatures [60]. Thus, overall three independent measures of fragility can be defined from simple DSC analysis of glass-forming materials. Generally, these methods are used to describe the effect of average coordination on the physical properties of chalcogenide amorphous networks systems [61, 62].
Phase behaviour and relaxation dynamics of the asymmetric azoxybenzene
Published in Phase Transitions, 2023
Anna Drzewicz, Łukasz Moczkodan, Ewa Juszyńska-Gałązka, Małgorzata Jasiurkowska-Delaporte, Aleksandra Deptuch
A temperature dependence of the relaxation times for some processes may be described by Arrhenius equation: where is the relaxation time at a high temperature limit (typically between 10−19 and 10−12 s [26,27]), corresponds to the activation energy and is the gas constant. If the relaxation time does not vary linearly with temperature, this dependence is denoted by Vogel–Fulcher-Tammann (VFT) equation [28]: where corresponds to the relaxation time for high temperature, is the dimensionless constant used to estimate the fragility parameter [29], and is related to a hypothetical ideal glass transition. The activation plot of relaxation times is presented in Figure 7(a), while the temperature dependence of the dielectric strength of the relaxation processes is shown in Figure 7(b).
Structural characterisation and thermal stability of SnSe\GaSb stacked films
Published in Philosophical Magazine, 2019
F. Sava, C. N. Borca, A. C. Galca, G. Socol, D. Grolimund, C. Mihai, A. Velea
It was shown that memory chips based on phase change materials perform faster than Flash memories [8,9], have a much higher endurance, a lower power consumption [10] and are less expensive [11]. Suitable materials for phase change memories should have short crystallisation time, appropriate crystallisation temperature, depending on the targeted application, i.e. automotive systems require data retention above 125°C, large resistance switching difference between the amorphous and crystalline states and low power consumption. The high-speed re-crystallisation of phase-change chalcogenides is due to their low reduced glass-transition temperature (Tg/Tm) and high fragility of the liquid [12]. Moreover, they can achieve several orders of magnitude in resistance contrast between the amorphous and crystalline state [13]. Thus, they can be rapidly switched between the two states in a few nanoseconds [14] and have more than 10 years of data retention time [15].
Mesomorphic phase transitions of 3F7HPhF studied by complementary methods
Published in Phase Transitions, 2018
Aleksandra Deptuch, Teresa Jaworska-Gołąb, Monika Marzec, Damian Pociecha, Jakub Fitas, Magdalena Żurowska, Marzena Tykarska, James Hooper
Based on the temperature dependence of relaxation time and dielectric increment, the relaxation process that is visible in the SmA* phase (Figure 8(a)) was identified as the soft mode. On the other hand, a Goldstone mode GM (Figure 8(b)) with the largest dielectric increment and relaxation frequency that is independent of temperature was recognized in the SmC* phase. Two relaxation processes, denoted as AFM1 and AFM2, are present in the SmCA* phase (Figure 8(c)). These two processes were not identified so far, but further FDDS measurements in a bias field are planned. The α process, which is visible at the lowest temperatures, is related to the observed glass transition. The temperature dependence of τα can be used to determine the fragility parameter m:where Tg is the glass transition temperature [31,32]. Substances with small m (10–20) are less likely to undergo crystallization than the ones with large m (up to 200) [33]. Relaxation time of the α process in 3F7HPhF can be described by the Vogel–Fuller–Tammann formula [34–36]:which becomes the Arrhenius formula if TVFT = 0. Fitting the formula (4) to the temperature dependence of relaxation time τα measured for 3F7HPhF (inset in Figure 9(b)) gives the following values of the refinement parameters: B = (740 ± 20) K, TVFT = (−76.2 ± 0.7) °C, τ∞ = (8 ± 2)·10−11 s. Combining Equations (3) and (4) leads to the explicit formula for the fragility parameter: