【作者】 陈志浩；

【导师】 祖方遒；

【作者基本信息】 合肥工业大学 ， 材料学， 2008， 博士

【摘要】 认知液体结构和性质,涉及诸多领域的科学基础和技术进步。然而,相对于固态和气态,人类对液态的认识仍落后许多。近年来,在高温、高压及深过冷条件下,液态物质的结构多型性(Polyamorphism)现象引起了科技界高度关注,越来越多的证据显示,液态结构及诸种性质随压力或温度的改变而发生异常变化。这些现象打破了人们对液态结构连续渐变的传统认知,为人们更进一步认知液态物质提供了新的突破口。作为一个新的研究领域,温度诱导液-液结构转变(TI-LLST)现象在多种合金体系中得到观察与验证,并在一定程度上探明了其规律及普适性。但到目前为止,人们对研究TI-LLST的努力多着眼于现象学层面,对其过程机理,如动力学特征及转变机制、热力学性质,以及更为微观的电子结构层面等的认识还远远不够。这些方面的不足,正是本文研究的切入点及主要内容。按照Y.Marcus根据相图对合金液态种类的划分,本文选择的研究对象为PbSn、InSn、InPb和CuSn等多种分属不同类别的合金熔体。研究手段包括电阻法、热电势法、DTA热分析和X射线衍射等不同手段。采取不同温度的恒温、不同速率的升温及降温等多种规程进行实验,并运用相变动力学理论、热力学理论以及Fiber-Ziman电子理论等,对其转变特征、机制与本质进行了理论分析与探讨。主要结论及创新点归纳如下:1、探索了合金熔体结构转变过程的动力学现象和特征,并运用相变动力学理论对其动力学行为进行了分析。所揭示的创新性结论为:不同类别合金熔体结构转变过程均属于“形核-长大”类型,且“形核”速率为转变的主导控制因素;但存在两类不同的动力学转变模式—J-M-A转变模式和自动催化模式。文中探讨了转变过程的微观物理机制,并首次对TI-LLST转变速度相对通常固态相变而言较慢的普遍特征给予了合理的解释。2、基于变速升温结果,运用Kissinger方程,计算、比较了不同合金转变的表观激活能E_k,进而分析了不同合金熔体TI-LLST的难易程度,即反映打破旧相原子间束缚力能垒大小,如:InSn80＞InPb80＞PbSn61.9;此外,揭示了TI-LLST的E_k值大小相较一般固体相变而言要小很多。3、通过X衍射手段所得到了InSn20合金熔体结构特征参数,如原子间距,第一配位数、熔体团簇尺度和有序度等,它们在相应转变温度范围内均发生了突变,为液-液结构转变的存在提供了直接证据。利用衍射数据,本文进一步计算得出InSn20合金熔体中偶熵随温度的变化,从热力学角度反映了系统有序度的变化,揭示了TI-LLST为熵驱动的有序-无序非连续的本质。并运用逾渗理论对合金液-液转变过程的非连续性进行了探讨。4、从电子层面探讨了TI-LLST的特征与规律。结合电导率、热电势及结构参数,运用Fiber-Ziman理论,计算了InSn合金转变过程中的费米面电子态密度,揭示了N(E_F)和dN(E_F)/dE在转变过程发生突变及其规律。更多还原

【Abstract】 Compared with solid and gas state, people’s knowledge on liquid structures and properties is superficial. In recent years, many respectable staggered achievements have been obtained and provide abundant phenomenology basis for further exploring the essence of liquid structure theoretically. From press induced liquid-liquid structure transition to temperature induced liquid-liquid structure transition (TI-LLST), more and more evidence suggest the existence of phase transition in liquids. All these present a challenge to our conventional picture of liquids as entities with a continuously varying averaged structure.As a new research field, people’s knowledge to the TI-LLST of liquid alloy is still in phenomenological. There are more works need to be done, such as the essence and mechanism of TI-LLST of liquid alloy, the kinetics and thermodynamics during transition process, and the exploration of the transition effect on electronic level. All these are the subject of our research in this article. We select different alloys used for study, such as PbSn, InSn, PbIn and CuSn, which are belong to different kinds of alloy liquids according to the classification of Y.Marcus. New research methods have been used, such as resistivity, thermoelectricity, DTA and X-ray diffraction. It has been carried out that the isothermal experiments holding at different temperatures and the heating experiments with different heating rate. The characteristic, mechanism and essence of TI-LLST have been studied and discussed using some theories, such as kinetic theory of phase transition, thermodynamics theory and Fiber-Ziman theory. Main conclusions are summarized as follows:1. The kinetic behavior in TI-LLST process of those alloys has been investigated through continuous heating experiments and isothermal experiments. Using kinetic theory of phase transition, we found that the kinetic transition process is nucleation-growth type, and the nucleation rate is the dominant controlling factor of the TI-LLST process. There are two different kinds of transition modes: Johnson-Mehl-Avrami transition mode and self-catalysis transition mode. The microcosmic physics mechanism of the transition process have been discussed in this paper, and for the first time, the characteristic of TI-LLST (the transition speed of TI-LLST is slower than normal solid state phase transitions) have been explained.2. Based on the results of heating experiments, the apparent activation energy E_k of TI-LLST have been calculated using Kissinger equation. E_k is the energy needed to overcome the energy barrier of the structure transition, and could reflect the difficulty level of TI-LLST of different alloy melts, for example InSn80>InPb80 >PbSn61.9. Furthermore, compared with normal solid state phase transition, the E_k value of TI-LLST is much smaller.3. Through X-ray diffraction experiments, we obtain some parameters reflecting atomic structure character, such as mean nearest neighbor atomic distance, the first coordination number, size of ordering domains and the ordering degree. During the change, a sudden drop happened for both the nearest neighbor distance and the coordination number, and the size of short-range orders decreased abruptly. The calculated results of the pair correlation entropy showed ;hat the ordering degree of the melt altered obviously and revealed the discontinuous feature of the TI-LLST. Using percolation theory, the discontinuous feature has been also discussed.4. Based on the results of electronic transport properties and the calculated mean nearest neighbor atomic distance, the dependence of the density of electronic states N(Ef) and its gradient value dN(E_F)/dE on temperature were deduced with the help of Faber-Ziman theory, reflecting that the electronic structure of the melt at Fermi level also changes during the transition.更多还原