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.     

大块金属玻璃Zr41Ti14Cu12.5Ni10Be22.5的流变行为研究

采用静态拉伸方法在连续升温条件下动态地测量了大块金属玻璃Zr41 Ti14Cu12.5Ni10Be225(Vitl)的黏度随温度的变化关系.在应变速率与温度的关系曲线中,观测到了与玻璃转变和晶化过程相联系的多个应变速率峰.在玻璃转变温度Tg以上,大块金属玻璃Zr41Ti14Cu125Ni10Be225的过冷液体呈现Newton流体特征,其黏度与温度的关系符合Vogel-Fulcher-Tammann(VFT)关系式,拟合得到脆度D*=36,VFT温度T0=319K,脆度参数m=30,这说明Zr41 Ti14Cu12.5Ni10Be225的过冷液体是一种类似于硅酸盐类的"强"液体.在玻璃转变温度Tg以下,粘度与温度的关系偏离VFT关系,从结构弛豫的观点出发,借助金属玻璃的自由体积模型对其进行了分析和讨论.     

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     

高压作用下相分离液体玻璃转变的分子动力学研究

采用分子动力学模拟方法,研究了二元混合液体在不同外压作用下的相分离与玻璃转变过程,计算了相分离液体在玻璃转变过程中的结构和动力学特征.研究发现,外压会促进相分离的产生,并提高玻璃转变温度,会使β弛豫出现的温度更高、存在的时间更长,导致系统扩散性降低.同时还发现,相分离液体的玻璃转变过程存在微观不均匀现象. 关键词： 相分离 玻璃转变 分子动力学模拟 外压影响     

利用胶体系统研究玻璃态

玻璃态是一种无序结构的固体,组成单元可以是原子、分子、高分子、胶体粒子等等。尽管玻璃态材料在生活中很常见,有广泛的工业应用,但相关理论,尤其是液体到玻璃态转变的理论是物理学中争议最多的领域之一。溶液中的胶体粒子可以形成晶体、液体、玻璃等各种态。在光学显微镜下可以直接看到三维内部单个微米胶体粒子,通过图像处理还可以得到粒子的布朗运动轨迹,从而得到玻璃化转变过程中的微观动力学信息,这是原子分子玻璃系统中难以测量的。文章介绍了胶体作为模型系统对玻璃态的研究,主要包括传统的过冷液体到玻璃态的转变,另外还涉及气相、凝胶、多晶等其他无序或半无序态与玻璃态之间的过渡或转变。     

非晶材料玻璃转变过程中记忆效应的热力学

低温下处于非平衡态的非晶材料升温到玻璃转变以上,要先后发生弛豫和回复最终达到平衡过冷液态,其中弛豫过程中释放的能量在回复过程中以等量的方式获取,表现出明显记忆行为.本文基于氧化物、金属与小分子等多种非晶形成体系,全面探讨了在围绕玻璃转变的一个冷却加热循环过程中的焓弛豫特征,建立了弛豫谱,发现弛豫焓在数值上与熔化焓密切相关.基于弛豫焓与非晶材料动力学Fragility之间的关联,展示了非晶体系在动力学极限(m=175)条件下的玻璃转变热力学基本特征,与热力学二级相变进行了对比.研究深化了对非晶弛豫与玻璃转变热力学的理解.     

非晶氢化硅内的热平衡过程

玻璃内的结构弛豫过程是众所周知的.从熔体快冷经过玻璃转变点T_g之下时,玻璃的高温准热平衡结构被冻结于低温.这种低温非平衡结构是不稳定的,会缓慢地通过弛豫过程转向晶态结构,这是一种热平衡化过程. 非晶硅不能从熔体快冷得到。而是用蒸发、溅射或等离子体分解等原子淀积方法制备.人们认为其结构是远离平衡态的.施东公司的研究人员首次报道非晶硅所达到的热平衡远比人们想象的要充分得多.他们观察到两个温度区域.当样品被冷到温度低干T_E时,其电子和原子结构发生.弛豫时间常数与温度有关.n型a-Si:H样品T_E■130℃.室温下的弛豫时间约…     

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.     

LaNiO3-δ 薄膜中的氧缺位及其对金属输运行为的影响

钙钛矿结构 LaNiO3 因其高电导的金属性输运行为, 有望作为电极材料在今后的氧化物器件工艺中发挥重要作用. 为了细致认识氧缺位对其金属输运的影响, 本文采用溶胶凝胶法在氧气、 空气和真空等气氛下烧结获得具有不同缺氧量的 LaNiO3 -δ 薄膜, 并利用 Mn3 + 的还原性进一步调整其缺氧量δ . 对这些样品的电阻率温度关系测量结果表明, 缺氧量δ > 0. 1 后薄膜的高温电阻率开始超过 Mott-Ioffe-Regel 极限, 变成“坏金属”; 增大δ 至0.2, 薄膜的低温剩余电阻率大幅度上升, 说明氧空位分布无序性导致的载流子局域化有明显作用. 以上样品的金属输运都呈非费米液体行为. 继续增加氧缺位浓度, 样品电阻率在低温a30 K 以下呈绝缘体温度关系, 同时高温金属态转变为费米液体行为, 说明钙钛矿结构镍酸盐的金属-绝缘体转变与费米液体输运之间或存在关联性.     

大块金属玻璃Zr41Ti14Cu12.5Ni10Be22.5的流变行为研究

采用静态拉伸方法在连续升温条件下动态地测量了大块金属玻璃Zr41Ti14Cu12.5Ni10Be22.5（Vit1）的黏度随温度的变化关系.在应变速率与温度的关系曲线中，观测到了与玻璃转变和晶化过程相联系的多个应变速率峰.在玻璃转变温度Tg以上，大块金属玻璃Zr41Ti14Cu12.5Ni10Be22.5的过冷液体呈现Newton流体特征，其黏度与温度的关系符合Vogel Fulcher-Tammann (VFT)关系式，拟合得到脆度D*＝36，VFT温度T0＝319K，脆度参数m＝30，这说明Zr41T 关键词： 大块金属玻璃 应变速率 剪切黏度 自由体积     

Driven injection of a polymer into a spherical cavity: A Langevin dynamics simulation study

The injection of a self-avoiding flexible polymer into a spherical cavity under a driving force is studied by using Langevin dynamics simulation. For given polymer length (N) and driving force (f), the polymer can be completely injected into the cavity only when the radius of the cavity is larger than a transition radius (R_eC). The dependence of R_eC on N and f can be described by a scaling relation R_eC∝N^1/3f^-δ. The value of δ changes from 4/15 in the small f region to 1/6 in the moderate f region due to the screening of the excluded-volume interaction between monomers. We find the complete injection time (τ) decreases monotonously with increasing the cavity radius or decreasing the polymer length. The simulation results are in good agreement with the theoretical predictions from the free energy analysis and a simple kinetic model.     

金属玻璃的键态特征与塑性起源

由于缺乏位错、晶界等典型的晶格缺陷,金属玻璃体系中承载力的形变单元为短程序或中程序原子团簇,键的强度及成键方向是影响原子间协调变形能力主要因素.本文通过与晶态合金对比,指出金属玻璃中原子键合方式与宏观力学性能的潜在关系,综述了金属材料电子结构与力学性能内在关系的最新研究进展,并系统介绍了金属玻璃电子结构特征、表征参量和主要测试手段,使读者对金属玻璃体系中原子间的键态特征有较清晰的认识,对进一步探索本征塑性较好的金属玻璃体系具有一定指导意义.     

Swampland dS conjecture in mimetic f(R,T) gravity

In this paper, we study a theory of gravity called mimetic f(R, T) in the presence of swampland dS conjecture. For this purpose, we introduce several inflation solutions of the Hubble parameter H(N) from f(R, T) = R + δT gravity model, in which R is Ricci scalar, and T denotes the trace of the energy–momentum tensor. Also, δ and N are the free parameter and a number of e-fold, respectively. Then we calculate quantities such as potential, Lagrange multiplier, slow-roll, and some cosmological parameters such as n_s and r. Then we challenge the mentioned inflationary model from the swampland dS conjecture. We discuss the stability of the model and investigate the compatibility or incompatibility of this inflationary scenario with the latest Planck observable data.     

Theory of core-level spectroscopy in f and d electron systems

A review is presented of many body effects in core-level spectroscopy (CLS) of f and d electron systems from a theoretical point of view. Historical developments and the most recent topics in this field are described. The impurity Anderson model (IAM) has been successfully applied to the analysis of X-ray photoemission spectra (XPS) and X-ray absorption spectra (XAS) in f and d electron systems, where the f and d electron states are treated as being on a single atomic site and they are hybridized with valence or conduction electron states. The effect of a core-hole potential in the final state of CLS plays an important role. Typical examples of calculated results for XPS in rare-earth compounds and transition metal compounds are given. Recent developments in the study of resonant X-ray emission spectra (RXES) are also introduced. A theoretical approach beyond the IAM is discussed mainly for the analysis of RXES of transition metal compounds.     

Electron doping induced stable ferromagnetism in two-dimensional GdI3 monolayer

As a two-dimensional material with a hollow hexatomic ring structure, Néel-type anti-ferromagnetic (AFM) GdI₃ can be used as a theoretical model to study the effect of electron doping. Based on first-principles calculations, we find that the Fermi surface nesting occurs when more than 1/3 electron per Gd is doped, resulting in the failure to obtain a stable ferromagnetic (FM) state. More interestingly, GdI₃ with appropriate Mg/Ca doping (1/6 Mg/Ca per Gd) turns to be half-metallic FM state. This AFM−FM transition results from the transfer of doped electrons to the spatially expanded Gd-5d orbital, which leads to the FM coupling of local half-full Gd-4f electrons through 5d−4f hybridization. Moreover, the shortened Gd−Gd length is the key to the formation of the stable ferromagnetic coupling. Our method provides new insights into obtaining stable FM materials from AFM materials.     

Tensile plasticity in metallic glasses with pronounced β relaxations

Metallic glasses are commonly brittle, as they generally fail catastrophically under uniaxial tension. Here we show pronounced macroscopic tensile plasticity achieved in a La-based metallic glass which possesses strong β relaxations and nanoscale heterogeneous structures. We demonstrate that the β relaxation is closely correlated with the activation of the structural units of plastic deformations and global plasticity, and the transition from brittle to ductile in tension and the activation of the β relaxations follow a similar time-temperature scaling relationship. The results have implications for understanding the mechanisms of plastic deformation and structural origin of β relaxations as well as for solving the brittleness in metallic glasses.     

Redshift drift constraints on f(T) gravity

We explore the impact of the Sandage−Loeb (SL) test on the precision of cosmological constraints for f(T) gravity theories. The SL test is an important supplement to current cosmological observations because it measures the redshift drift in the Lyman-α forest in the spectra of distant quasars, covering the “redshift desert” of 2≲z≲5. To avoid data inconsistency, we use the best-fit models based on current combined observational data as fiducial models to simulate 30 mock SL test data. We quantify the impact of these SL test data on parameter estimation for f(T) gravity theories. Two typical f(T) models are considered, the power-law model f(T)_PL and the exponential-form model f(T)_EXP. The results show that the SL test can effectively break the existing strong degeneracy between the present-day matter density Ω_m and the Hubble constant H₀ in other cosmological observations. For the considered f(T) models, a 30-year observation of the SL test can improve the constraint precision of Ω_m and H₀ enormously but cannot effectively improve the constraint precision of the model parameters.     

On the Physical Meaning of Disperse Parameters of Frequency Dependence of Dielectric Permittivity in the Havriliak–Negami Model

This paper is devoted to an analysis of the frequency dispersion of dielectric permittivity in order to establish the physical meaning of the dispersion parameters α and β in the Havriliak–Negami model. It is found that parameter α is associated with the spectrum broadening of relaxation time τ. Examples with the appearance of a dispersion with parameter α of about 0.4 and 0.7 are presented. A conclusion is drawn that disorder creates a broadening of the relaxation times. Dispersion parameters α and β are introduced to preserve the unified relaxation time. The possibility of the analysis of experimental results on the basis of the Debye model in order to obtain the relaxation time distribution and determine parameters α and β according to the log(τ_HN/τ) chart is shown.     

A simplified cavity analysis for estimating energy coupling during laser ablation and drilling of solids — experiment

The theory for a general departure function, f, for laser-irradiated cavities was previously developed to estimate laser energy coupling to an opaque solid target as a function of heat transfer and the cavity shape and size. In this article, a specific form of f is calculated for ultraviolet (UV) laser ablation of copper (Cu) and aluminum (Al) targets. Methods are also given for calculating the geometric factor, a, and experimentally determining the heat transfer parameter, ν, which is shown for this form of f to be the intensity-dependent effective reflectivity of the material. Experimental results for different gauges of laser energy coupling with a solid target are given and compared to calculations of net absorbed energy based on f and the incident laser energy. Using the simplified cavity analysis, the results demonstrate that the experimental values for f fall within the limits predicted by the theory, and that energy coupling can be predicted to within a mean of 2% of experimental gauges. Neglecting the factors in f from calculations of energy coupling can lead to large errors for laser-irradiated cavities, establishing that both cavity shape and heat transfer should be simultaneously considered. In addition, a first-order sensitivity analysis based on f shows that the initial rate of change in material removal strongly increases with reflectivity, which can lead to runaway cavity formation for highly reflective materials.