Surface induced order in liquid metals
Surface induced order in liquid metals
复制标题
液态金属中的表面诱导有序
DOI：
10.1080/08940889908260987
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发表时间：
1999
影响因子：
--
通讯作者：
H. Tostmann
中科院分区：
文献类型：
--
作者：
E. DiMasi;H. Tostmann
液体的表面可以表现出与主体非常不同的结构顺序。也许最熟悉的例子是定义明确的曲面本身的存在。长期以来，人们一直认识到，表面张力会产生一种障碍，限制在整体中自由运动的颗粒的流动性。最近，对液体表面结构的微观观察发现了各种现象，例如液态烷烃中的表面冻结(见本期M.Deutsch的文章)，以及可能在二元液体中发生的表面分离和表面相变[11.在本文中，我们想要提请注意液体金属(LM)中的表面诱导有序化，以及通过同步辐射在理解LM和合金的体属性和表面结构之间的相互作用方面的最新进展。出于几个原因，液态金属-蒸气界面是一种特殊的表面。液体的表面张力跨越很大的范围，但当归一化到熔化温度时，发现非金属液体的表面张力具有类似的量级--在0.1至0.25×N/MK的范围内(表I)。这表明表面张力(通常在温度中线性)和熔点以一种简单的方式与非金属液体中吸引人的相互作用的标度。金属液体的归一化表面张力通常较高，且跨度较大(0.2-2.4~1.0-~N/mK)，这使得这种简单的描述不适用。此外，由于表面张力相对较高，相对于非金属液体和晶体表面(缺陷和台阶边缘非常重要)，光刻表面非常平坦。这使得LM成为研究吸附、表面分离和化学反应的理想底物，特别是如果人们对无序但均匀的底物的电位中是什么诱导吸附有序的问题感兴趣的话[2]。大块Lm的微观结构是由原子间相互作用驱动的，这种相互作用强烈地依赖于电子结构。我们还可以问，这些相互作用是如何在表面的结构中表现出来的。Lm由带电离子核(一种经典的硬球流体)组成，其强烈的相互作用被传导电子海(量子流体)屏蔽。在汽液界面，这种屏蔽的库仑势让位于蒸汽中普遍存在的较弱的范德华相互作用。由于界面上的电势变化如此之大，电势梯度很高，产生了一种作用力，作用于液体表面的离子，就像它们被包装在硬壁上一样。分析计算和分子动力学模拟预测，如图1a所示，LM表面的原子在平行于界面[3]的层中分层。相反，对于非金属液体的汽相界面，预测了单调的密度分布(图1b)。
The surface of a liquid can exhibit structural order that is very different from that of the bulk. Perhaps the most familiar example is the existence of the well-defined surface itself. It has long been recognized that the surface tension creates a barrier that limits the mobility of particles that move freely in the bulk. More recently, microscopic observations of the surface structure of liquids have found a variety of phenomena, such as surface freezing in liquid alkanes (see article by M. Deutsch in this issue), as well as the surface segregation and surface phase transitions that may occur in binary liquids [11. In this article, we want to draw attention to surface induced ordering in liquid metals (LM), and to recent progress enabled by synchrotron radiation towards understanding the interplay between bulk properties and surface structure in LM and alloys. The liquid metal-vapor interface is a special surface for several reasons. The surface tensions of liquids span a large range, but when normalized to the melting temperature, they are found to be of comparable magnitude for nonmetallic liquids- in the range of 0.1 to 0.25 x N/mK (Table I). This suggests that the surface tension (typically linear in temperature) and the melting point scale in a simple way with the attractive interactions in the nonmetallic liquid. The normalized surface tensions of metallic liquids are typically higher and also span a much larger range (0.2-2.4~ 1 0-~ N/m K), making this kind of simple description inapplicable. In addition, due to the comparatively high surface tension, the LM surface is extremely flat relative to nonmetallic liquids and also to crystalline surfaces where defects and step edges are important. This makes the LM an ideal substrate to study adsorption, surface segregation, and chemical reactions, especially if one is interested in the question of what induces adsorbates to order in the potential of a disordered but uniform substrate [2]. The microscopic structure of the bulk LM is driven by the interatomic interactions, which are strongly dependent upon the electronic structure. We can also ask how these interactions manifest themselves in structure at the surface. The LM is comprised of charged ion cores (a classical hard sphere fluid) whose strong interactions are screened by a conduction electron sea (a quantum fluid). At the liquid-vapor interface, this screened Coulomb potential gives way to the weaker van der Waals interactions that prevail in the vapor. Since the potential changes so substantially across the interface, the potential gradient is high, producing a force that acts on the ions at the liquid surface as though they were packed against a hard wall. Analytic calculations and molecular dynamics simulations predict that atoms at the LM surface are stratified in layers parallel to the interface [3], as indicated schematically in Figure la. By contrast, a monotonic density profile is predicted for the vapor interface of a nonmetallic liquid (Figure lb).