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تسجيل مستخدم جديدLiquid-Liquid Phase Transition in Supercooled Silicon
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We present a review on the study of metastable silicon, primarily focusing mainly on the aspects of liquid-liquid transition, critical point and phase behaviour, structural and dynamic properties of liquid phase as well as crystal nucleation. We begin with an extensive survey of the investigations of liquid silicon pursued over three decades, with salient experimental, theoretical and simulation results. Following which we present various scenarios put forward to rationalize the density and related anomalies often observed in water and other network forming liquids. After which we present the more recent investigations (both simulation and experimental works) of the phase behavior of Silicon. Since a significant part of metastable silicon work is on a classical empirical potential an important question to address is the reliability of this potential in describing the behavior of silicon. To provide a critical assessment of the applicability of classical simulation results to real silicon we present a comparison of the structural, dynamical, and thermodynamic quantities obtained from the SW potential with those from ab initio simulations and with available experimental data. We also discuss the sensitivity of the thermodynamic properties to model parameters.
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A novel liquid-liquid phase transition has been proposed and investigated in a wide variety of pure substances recently, including water, silica and silicon. From computer simulations using the Stillinger-Weber classical empirical potential, Sastry a
nd Angell [1] demonstrated a first order liquid-liquid transition in supercooled silicon, subsequently supported by experimental and simulation studies. Here, we report evidence for a liquid-liquid critical end point at negative pressures, from computer simulations using the SW potential. Compressibilities exhibit a growing maximum upon lowering temperature below 1500 K and isotherms exhibit density discontinuities below 1120 K, at negative pressure. Below 1120 K, isotherms obtained from constant volume-temperature simulations exhibit non-monotonic, van der Waals-like behavior signaling a first order transition. We identify Tc ~ 1120 +/- 12 K, Pc -0.60 +/- 0.15 GPa as the critical temperature and pressure for the liquid-liquid critical point. The structure of the liquid changes dramatically upon decreasing the temperature and pressure. Diffusivities vary over 4 orders of magnitude, and exhibit anomalous pressure dependence near the critical point. A strong relationship between local geometry quantified by the coordination number, and diffusivity, is seen, suggesting that atomic mobility in both low and high density liquids can usefully be analyzed in terms of defects in the tetrahedral network structure. We have constructed the phase diagram of supercooled silicon. We identify the lines of compressibility, density extrema (maxima and minima) and the spinodal which reveal the interconnection between thermodynamic anomalies and the phase behaviour of the system as suggested in previous works [2-9]
A bulk metallic glass forming alloy is subjected to shear flow in its supercooled state by compression of a short rod to produce a flat disc. The resulting material exhibits enhanced crystallization kinetics during isothermal annealing as reflected i
n the decrease of the crystallization time relative to the non-deformed case. The transition from quiescent to shear-accelerated crystallization is linked to strain accumulated during shear flow above a critical shear rate $dotgamma_capprox 0.3$ s$^{-1}$ which corresponds to P{e}clet number, $Pesimmathcal{O}(1)$. The observation of shear accelerated crystallization in an atomic system at modest shear rates is uncommon. It is made possible here by the substantial viscosity of the supercooled liquid which increases strongly with temperature in the approach to the glass transition. We may therefore anticipate the encounter of non-trivial shear-related effects during thermoplastic deformation of similar systems.
Water shows intriguing thermodynamic and dynamic anomalies in the supercooled liquid state. One possible explanation of the origin of these anomalies lies in the existence of a metastable liquid-liquid phase transition (LLPT) between two (high and lo
w density) forms of water. While the anomalies are observed in experiments on bulk and confined water and by computer simulation studies of water-like models, the existence of a LLPT in water is still debated. Unambiguous experimental proof of the existence of a LLPT in bulk supercooled water is hampered by fast ice nucleation which is a precursor of the hypothesized LLPT. Moreover, the hypothesized LLPT, being metastable, in principle cannot exist in the thermodynamic limit (infinite size, infinite time). Therefore, computer simulations of water models are crucial for exploring the possibility of the metastable LLPT and the nature of the anomalies. In this work, we present new simulation results in the NVT ensemble for one of the most accurate classical molecular models of water, TIP4P/2005. To describe the computed properties and explore the possibility of a LLPT we have applied two-structure thermodynamics, viewing water as a non-ideal mixture of two interconvertible local structures (states). The results suggest the presence of a liquid-liquid critical point and a LLPT in this model for the simulated length and time scales. We have compared the behavior of TIP4P/2005 with other popular water-like models, namely mW and ST2, and with real water, all of which are well described by two-state thermodynamics. In view of the current debate involving different studies of TIP4P/2005, we discuss consequences of metastability and finite size in observing the liquid-liquid separation. We also address the relationship between the phenomenological order parameter of two-structure thermodynamics and the microscopic nature of the low-density structure.
We propose a unified model combining the first-order liquid-liquid and the second-order ferroelectric phase transitions models and explaining various features of the $lambda$-point of liquid water within a single theoretical framework. It becomes cle
ar within the proposed model that not only does the long-range dipole-dipole interaction of water molecules yield a large value of dielectric constant $epsilon$ at room temperatures, our analysis shows that the large dipole moment of the water molecules also leads to a ferroelectric phase transition at a temperature close to the lambda-point. Our more refined model suggests that the phase transition occurs only in the low density component of the liquid and is the origin of the singularity of the dielectric constant recently observed in experiments with supercooled liquid water at temperature T~233K. This combined model agrees well with nearly every available set of experiments and explains most of the well-known and even recently obtained results of MD simulations.
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