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玻璃转变熵理论的短程结构与动力学统计模型
Alternative TitleA statistical model of entropic scenario on glass transition based on short-range structure and dynamic heterogeneity in metallic glasses
韩懂
Thesis Advisor李晖凌
2019-06-02
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype硕士
Degree Discipline固体力学
Keyword玻璃转变,非晶合金,声子,香农熵,激活能垒
Abstract

玻璃转变是自然界中十分常见的现象,一般是指在快速冷却的过程中,粘性液体不发生晶化而直接转变为无序固体状态的一个过程。一直以来,它都是凝聚态物理学和材料学的重要科学问题之一。在玻璃转变过程中,尽管体系的动力学发生了显著的变化,但是还没有在实验中观测到明显的结构变化。作为一类新型合金,非晶合金的玻璃转变现象也具有类似的特性。此外,由于其无序密堆的结构特征,非晶合金常被用作研究玻璃转变的模型体系。并且,理解非晶合金的玻璃转变机制对于深入认识材料的性能和结构设计也具有十分重要的意义。本文主要采用分子动力学模拟方法,从热力学和动力学两个角度,针对玻璃转变结构变化与动力学演化的两个关键科学问题,即:热玻璃转变过程中过剩熵的起源以及微观结构的变化,动力学不均匀性与玻璃转变的关联,对非晶合金玻璃转变开展了系统研究,提出了玻璃转变熵理论的短程结构和动力学不均匀性统计模型。

首先,我们揭示了热玻璃转变过程中过剩熵的起源以及微观结构的变化。使用分子动力学计算玻璃转变过程中的热力学性质与声子特征演化,获得整个温度范围内的总熵、振动熵和结构熵的变化趋势。证实在除0 K附近低温区域外,玻璃转变过程中熵的变化主要来自于构型熵,而振动熵的贡献很小。结果与非弹性中子散射实验的观测一致,为玻璃转变的Adam–Gibbs熵理论提供了强有力的证据。此外,利用香农信息熵的概念,统计玻璃转变过程中局域Voronoi多面体结构的多样性,发现在玻璃-液体转变之后香农熵的显著变化,与体系的过剩构型熵的显著增加一致,从而为Adam-Gibbs理论提供了坚实的原子结构基础和具体的解释。

然后,我们研究了动力学不均匀性与玻璃转变的关联。使用基于分子动力学的激活弛豫方法(ART),得到玻璃形成体系在不同热力学状态的的激活能谱。我们发现概率分布函数分为两种不同的模式:雪崩弛豫模式和局域弛豫模式。随着温度的逐渐升高,主导体系中动力学的模式由局域弛豫模式转变到雪崩弛豫模式。对激活能垒空间分布的进一步研究表明低能垒的区域是高度局域化的。在玻璃-液体转变后,低能垒区域扩展,高流动性机制趋向于贯穿整个样品发生逾渗,最终动力学不均匀性减少。进一步,我们定义了香农熵和能垒的统计多样性,并发现其在玻璃-液体转变后显著减少,与Kohlrausch-Williams-Watts KWW)方程中的广延指数随约化温度的变化一致。它们均能有效地表征动力学不均匀性的程度,且可以作为为玻璃-液体转变的定量标识。

以上研究创新性地引入香农信息熵概念,对玻璃短程结构和动力学特征进行统计表征。结果为半个世纪以来的Adam-Gibbs玻璃转变熵理论模型提供了原子可信度的证实,澄清了其结构起源。进一步,我们基于香农信息熵使用激活能垒的统计多样性定量表征动力学不均匀程度,并与玻璃转变直接关联。研究结果从玻璃结构出发,从热力学与动力学两个角度为玻璃转变现象提供了新的物理视角。

 

Other Abstract

Glass transition-the ubiquitous phenomenon in nature-is generally regarded as the process in which a viscous liquid circumvents crystallization but evolves continuously into a disordered solid state directly during fast cooling. It is regarded as one of the most key scientific problems in condensed matter physics and materials sciences all the time. In glass transition, an evident change of structures is still not observed in the experiment although dynamics in the system has changed a lot. As a new alloy, glass transition in a metallic glass also has similar features. Furthermore, it is always used for a mode system to study glass transition because of its structural feature, i.e., disorderly close packing. And understanding the mechanism of glass transition is also very significant to gain in-depth understanding of properties and structural design of materials. From thermodynamic and dynamic perspectives, the thesis focused on two key scientific issues on the structural change and dynamic evolution in glass transition by using molecular dynamics simulations, i.e., the origin of excess entropy and the change of microscopic structures in thermal glass transition and the correlation between dynamic heterogeneity and glass transition, and studied glass transition in metallic glasses systematically, then proposed a statistical model of entropic scenario on glass transition based on short-range structure and dynamic heterogeneity in metallic glass.

Initially, we reveal the origin of excess entropy and the change of microscopic structures in thermal glass transition. Through the computation of thermodynamic properties and evolution of phononic features in glass transition via molecular dynamics simulations, total entropy, vibrational entropy and a trend of configurational entropy at the entire temperature range are obtained. Except the temperature range near 0 K, the change of entropy in glass transition is confirmed to be originated from mostly configurational entropy while the vibrational entropy is trivial. The findings are in agreement with recent inelastic neutron scattering experiments and provide a compelling evidence for the Adam–Gibbs entropic scenario on glass transition. Besides, by performing statistics on diversity of local Voronoi polyhedra in glass transition in virtue of Shannon information entropy, it is found that Shannon entropy experiences striking variation during glass transition, which is in accordance with the sudden increase in configurational entropy in the system and lays a solid atomic structure foundation and provide a specific explanation for the Adam–Gibbs entropic scenario.

Afterwards, the correlation between dynamic heterogeneity and glass transition is studied by us. Based on the activation-relaxation technique (ART) , spectrum of activation barrier in glass-forming system with different thermodynamic states can be accquired. It is noticed that the probability distribution function can be decomposed into two distinct modes: cascade relaxation and localized relaxation. As temperature gradually increases, the mode dominating dynamics in the system transforms from localized relaxation to cascade relaxation. Further study on the spatial distribution of activation barriers shows that low-barrier regions are highly localized. After glass-liquid transition, low-barrier regions are extended. The high mobile regimes tend to percolate through the whole sample and lead to the reduction of dynamic heterogeneity in the end. What's more, Shannon entropy and statictical diversity of barriers are defined and decrease a lot after glass-liquid transition, which corresponds to the change of stretching exponent in Kohlrausch-Williams-Watts (KWW) equation as temperature varies. They can both effectively characterize the level of dynamic heterogeneity and are regarded as quantitative signatures for glass transition.

The above research innovatively introduces the concept of Shannon information entropy and statistically characterizes the short-range structure and dynamics of glass. It provides a confirmation of atomic credibility for Adam-Gibbs entropic scenario on glass transition since 1958 and clarifies its structural origin. Furthermore, based on Shannon information entropy, we quantitatively characterize the degree of dynamic heterogeneity by using statistical diversity of activation energy barriers and correlate it with glass transition. From thermodynamic and dynamic perspectives, our study focuses on the structure of glass and then provides a novel physical view for glass transition.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/79106
Collection非线性力学国家重点实验室
Recommended Citation
GB/T 7714
韩懂. 玻璃转变熵理论的短程结构与动力学统计模型[D]. 北京. 中国科学院大学,2019.
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