非晶态物理讲座 第三讲 非晶态金属的结构转变与动力学

在本文中我们将讨论从液态到非晶态(玻璃态)的转变,非晶态金属的结构弛豫、结晶及其动力学. 一、从液态到非晶态的转变 由熔体淬火凝固而形成的玻璃态金属可以认为是一种冻结的液态结构,它的性质和结构与其热历史有关.为了说明液态金属-非晶态金属的转变,图1给出了容易形成玻璃     

低熔点微晶封接玻璃的研究

系统地研究了Ｂ２Ｏ３－ＺｎＯ－ＰｂＯ－ＳｉＯ２系统玻璃的组成与性能，给出玻璃配方与其物化性能测试结果，并就晶核剂以及部分成分对低熔点微晶封接玻璃性能的影响进行了讨论，封接玻璃已广泛地用于微光像增强器，激光器，氙灯，湿敏电阻器等金属与金属，金属与玻璃，玻璃与玻璃，玻璃与陶瓷的封接。     

液态金属的初始状态对凝固微结构影响的模拟研究

采用分子动力学方法对液态金属Ａｌ在不同的初始状态下，以相同急冷速率凝固的过程进行模拟跟踪研究，发现：在玻璃转变温度Ｔｇ以上（即过冷液态）时，系统的微结构组态情况基本一致，相差甚微；但在Ｔｇ以下时，不同的初始液态微结构对其固态微结构有明显的影响；且在Ｔｇ处各种固态微结构之间的差别发生突发性的变化。这一结果对于深入理解液－固微结构之间的转变关系，具有一定的重要意义。     

液态金属结构研究新进展

文章介绍了用内耗方法研究金属液态结构的新进展，发现了随温度变化金属液态结构发生不连续的变化，并经差热分析、X射线衍射等实验证实了这种变化．这对认知金属液态结构提供了新的实验依据。     

非均匀磁场中自由表面液态射流的MHD稳定性分析

基于在液态金属实验回路上的实验，对非均匀磁场中液态金属射流的MHD稳定性进行了研究，建立了一个描述射流性能的简化模型。由此简化模型所得的结果与从实验获得的结果相比较表明，它们相当吻合，并发现在此液态金属射流中存在一个固有稳定性区域。     

YBa2Cu3O7—δ外延薄膜的电流—电压特性

分别测量了不同磁场及其位形下Ｉ－Ｖ曲线随温度的变化关系。实验结果表明，在磁通玻璃转变温度以下，ｌｏｇＩ－ｌｏｇＶ曲线具有正曲率，在磁通玻璃转变温度以下，ｌｏｇＩ－ｌｏｇＶ曲线具有负英率；不同温度下的Ｉ－Ｖ曲线能够很好地被标度到两条曲线上，表明样品中存在磁通液态到玻璃态的转变。     

金属玻璃流变的扩展弹性模型

玻璃-液体转变现象,简称玻璃转变,被诺贝尔物理学奖获得者安德森教授评为最深奥与重要的凝聚态物理问题之一.金属玻璃作为典型的非晶态物质,具有与液体相似的无序原子结构,因此又称为冻结了的液态金属,是研究玻璃转变问题的理想模型材料.当加热至玻璃转变温度,或者加载到力学屈服点附近时,金属玻璃将会发生流动.由于热或应力导致的流动现象对金属玻璃的应用具有重要意义.本文简要回顾了金属玻璃流变现象,综述了流变扩展弹性模型的研究进展和未来发展趋势.     

冷速对液态金属Na凝固过程中微观结构影响的模拟研究

采用分子动力学方法对液态金属Na在四种不同冷速下的快速凝固过程进行了模拟跟踪研究.采用双体分布函数g(r)曲线、Honeycutt-Andersen键型指数法和原子团类型指数法对凝固过程中微观结构的变化进行了分析.结果表明：冷却速率对微结构的转变有决定性影响，当冷速为1.0×10¹⁴和1.0×10¹³K/s时，系统形成以1551和1541键型或以缺陷多面体基本原子团(13 1 10 2)和二十面体基本原子团(12 0 12 0)为主体的非晶态结构；当冷速为1.0×10¹²和1.0×10¹¹K/s时，系统则形成以1441和1661键型或以体心立方基本原子团(14 6 0 8)为主体的晶态结构.同时发现：不同冷速对液态金属Na在液态和过冷态时微观结构的影响甚小；但不同冷速对其固态(非晶态利晶态)时的微观结构有显著的影响，且要在液-固转变点(分别在玻璃转变温度T_g和晶化起始温度T_c)附近或以后才能充分展现出来.根据这一特点，有可能建立另一种确定液态金属T_g和T_c的新方法.原子团类型指数法比键型指数法更有利于研究液态、非晶态等无序体系和一些晶化体系的具体结构特征. 关键词： 液态金属Na 凝固过程 分子动力学模拟 原子团类型指数法     

合金非晶态结构的形成条件

非晶态合金又名“金属玻璃”,其结构类似于一般的玻璃,其原子的三维空间呈拓朴无序状的排列,它没有晶界,堆垛层错等缺陷存在,是一种非常均匀的组织.非晶态合金所表现出的优异性能(机械、物理和化学性能),已引人注目,它是近代极有发展前途的新型合金材料. 目前在制备非晶态合金的方法中,有溅射法、液态轧制法等多种,方法繁多,但按其基本原理归纳起来主要是液态淬火法,即将熔融的合金将其快速喷射到冷衬底上急冷.因此对液态淬火法形成非晶态的机理,是当代金属物理学上广泛研究的一项重要课题.有人[1]曾提出非晶态合金的形成和稳定性问题是决定…     

金属Zn液态结构变化的研究

利用ＴＢ模型给出的原子间相互作用势详细计算了不同温度下Ｚｎ的双体分布函数ｇ（ｒ），结果发现随着温度的不断降低，液态金属Ｚｎ的ｇ（ｒ）第一峰变得高而尖，第二峰由弱变强，说明了液态金属Ｚｎ的有序度随温度降低而不断增强；利用键对分析技术统计了液态金属Ｚｎ在不同温度下的键取向序参数、键对数。键取向序参数及键对数随温度的变化，进一步证明了低温液态的有序度高于高温液态，从而充分说明液态金属在不同温度下有不同的结构形式，而不像人们想象得那样杂乱无章。     

纳米结构非晶合金材料研究进展

迄今为止,人类社会新技术的发展主要是基于各种晶体材料(如金属、半导体等)的应用.晶体材料的性能可以通过改变它们的微观缺陷结构和/或微观化学结构来调控,但这对于当前的非晶材料而言却是难以实现的.新型的纳米结构非晶材料可以通过引入大量的非晶/非晶界面来改变非晶材料的微观缺陷结构和/或微观化学结构,从而实现对其性能的调控.本文主要讨论了目前纳米结构非晶合金材料的研究进展,包括其制备方法、结构特征和新性能.通过利用这些新特性,有可能会开启一个基于非晶材料的新技术时代.     

非晶合金及合金液体的局域五次对称性

简要回顾了从20世纪30年代至今,有关非晶合金及合金液体的局域结构五次对称性的实验、理论和模拟研究.在简单液体的早期研究中,人们已经意识到五次对称性在简单液体的无序结构、过冷和晶化等起着重要作用,二十面体短程序作为五次对称性的典型代表受到了广泛关注.自从Frank提出简单液体中二十面体短程序的结构单元,大量的理论和实验研究已经明确在简单液体、合金液体和金属玻璃中存在局域五次对称性,并且建立了局域五次对称性与合金液体复杂动力学行为、玻璃转变、液体-液体相变以及非晶合金的形变等统一的定量描述和物理图像,表明了局域五次对称性作为结构参量的简单、普遍和有效性.     

Multiscale structures and phase transitions in metallic glasses:A scattering perspective

Amorphous materials are ubiquitous and widely used in human society, yet their structures are far from being fully understood. Metallic glasses, a new class of amorphous materials, have attracted a great deal of interests due to their exceptional properties. In recent years, our understanding of metallic glasses increases dramatically, thanks to the development of advanced instrumentation, such as in situ x-ray and neutron scattering. In this article, we provide a brief review of recent progress in study of the structure of metallic glasses. In particular, we will emphasize, from the scattering perspective, the multiscale structures of metallic glasses, i.e., short-to-medium range atomic packing, and phase transitions in the supercooled liquid region, e.g., crystallization and liquid-to-liquid phase transition. We will also discuss, based on the understanding of their structures and phase stability, the mechanical and magnetic properties of metallic glasses.     

Radiophotoluminescence properties of Ag-doped phosphate glasses

The radiophotoluminescence (RPL) of Ag-doped phosphate glass has been used for personal dosimetry for a long time. In this study, we investigated the optical properties of Ag-doped various phosphate glasses. It is revealed that alkali metals in the phosphate are required to achieve the RPL, and that the alkali metals content has an influence on the RPL properties. Also, the ESR signal corresponding to the valence change of Ag was observed only for the phosphate glasses containing alkali metals. These results indicate that the RPL occurs based on the change in Ag valence state, which is consistent with commercial phosphate glasses used for dosimetry. In addition, the decay-time constants of the RPL were on the order of microseconds. Furthermore, initialization of the RPL was observed after heating at 473 K, which indicates that these glasses can be used repeatedly.     

Preparation and characterization of a large mode area liquid-filled photonic crystal fiber: transition from isolated to coupled spatial modes

We describe in detail the manufacturing procedures for selectively closing holes in photonics crystal fibers and their infiltration with different liquids. We apply our method to create a large mode area liquid-filled photonic crystal fiber which consists of 19 liquid strands. By changing the mixing ratio between toluene and ethanol and by varying the temperature, we show continuous tuning from isolated to coupled behavior of the spatial mode profile. This demonstrates the versatility of selectively closed liquid-filled photonic crystal fibers for future photonic devices. Filling with nonlinear liquids, gases, metals, liquid crystals, low melting compound glasses, or quantum dots is possible, and spatial as well as temporal engineering of linear and nonlinear optical properties will become feasible, which should allow the observation of spatiotemporal solitons.     

Structure of liquid metals

A brief review is given of the structure of liquid metals. The structure is examined from the viewpoints of how the pair potential in metals gives rise to various structural properties and how the structure of liquid metals effects other properties. Use is made of the Paskin and Rahman molecular dynamic calculations on liquid sodium to illustrate the insensitive nature of the structure to details in the pair potential. Diffraction data are used to demonstrate that a law of corresponding states exists, at the melting point, for simple liquids, metals and insulators. The molecular dynamic calculations of sodium are also used to demonstrate that some time dependent correlation functions may be more sensitive to the form of the pair potential than the structure factor is to the potential. Some analysis is also made of the role of structure in the electrical resistivity. It is noted that more accurate structure data are needed to answer some of the questions raised in recent attempts to use the structure to extract pair potentials and in detailed correlations of resistivity and structure data.     

Low energy excitations in amorphous metals

The low temperature properties of amorphous metals pertinent to the existence of low energy excitations are reviewed. In an introductory section, the currently accepted model for low energy excitations, i.e. the model of two level systems (TLS), is discussed. The treatment of amorphous metals focuses on the specific heat C, thermal conductivity κ, ultrasonic properties and electrical resistivity ?. Measurements of C and κ are often performed on amorphous superconductors in order to exclude the effect of conduction electrons. The TLS density of states and the TLS-phonon coupling in these materials as determined from C and κ respectively are surprisingly close to values for insulating glasses. Ultrasonic experiments carried out mostly on normal amorphous metals reveal a strongly enhanced TLS relaxation rate with respect to insulators. This can be attributed to TLS-electron coupling. The effect of this coupling on ? is uncertain at present. In order to obtain more insight into the microscopic origin of TLS in metallic and insulating glasses some examples of TLS in crystalline disordered solids are discussed. Finally, some formal analogies are drawn between spin glasses and real glasses.