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تسجيل مستخدم جديدSurface Induced Order in Liquid Metals and Binary Alloys
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Measurements of the surface x-ray scattering from several pure liquid metals (Hg, Ga, and In) and from three alloys (Ga-Bi, Bi-In, and K-Na) with different heteroatomic chemical interactions in the bulk phase are reviewed. Surface-induced layering is found for each elemental liquid metal. The surface structure of the K-Na alloy resembles that of an elemental liquid metal. Bi-In displays pair formation at the surface. Surface segregation and a wetting film are found for Ga-Bi.
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Surface sensitive x-ray scattering techniques with atomic scale resolution are employed to investigate the microscopic structure of the surface of three classes of liquid binary alloys: (i) Surface segregation in partly miscible binary alloys as pred
icted by the Gibbs adsorption rule is investigated for Ga-In. The first layer consists of a supercooled In monolayer and the bulk composition is reached after about two atomic diameters. (ii) The Ga-Bi system displays a wetting transition at a characteristic temperature T_w~220 C. The transition from a Bi monolayer on Ga below T_w to a thick Bi-rich wetting film above T_w is studied. (iii) The effect of attractive interactions between the two components of a binary alloy on the surface structure is investigated for two Hg-Au alloys.
X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au_{82}Si_{18}, at temperatures above the alloys melting point. Surface-induced atomic layering, the hallmark of liquid metals, is also found below the crystallin
e monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.
We analyze, by means of an RPA calculation, the conditions under which a mixture of oppositely charged polyelectrolytes can micro-segregate in the neighborhood of a charged surface creating a layered structure. A number of stable layers can be formed
if the surface is sufficiently strongly charged even at temperatures at which the bulk of the mixture is homogeneous.
Auxetic materials have the counter-intuitive property of expanding rather than contracting perpendicular to an applied stretch, formally they have negative Poissons Ratios (PRs).[1,2] This results in properties such as enhanced energy absorption and
indentation resistance, which means that auxetics have potential for applications in areas from aerospace to biomedical industries.[3,4] Existing synthetic auxetics are all created by carefully structuring porous geometries from positive PR materials. Crucially, their geometry causes the auxeticity.[3,4] The necessary porosity weakens the material compared to the bulk and the structure must be engineered, for example, by using resource-intensive additive manufacturing processes.[1,5] A longstanding goal for researchers has been the development of a synthetic material that has intrinsic auxetic behaviour. Such molecular auxetics would avoid porosity-weakening and their very existence implies chemical tuneability.[1,4-9] However molecular auxeticity has never previously been proven for a synthetic material.[6,7] Here we present a synthetic molecular auxetic based on a monodomain liquid crystal elastomer (LCE). When stressed perpendicular to the alignment direction, the LCE becomes auxetic at strains greater than approximately 0.8 with a minimum PR of -0.8. The critical strain for auxeticity coincides with the occurrence of a negative liquid crystal order parameter (LCOP). We show the auxeticity agrees with theoretical predictions derived from the Warner and Terentjev theory of LCEs.[10] This demonstration of a synthetic molecular auxetic represents the origin of a new approach to producing molecular auxetics with a range of physical properties and functional behaviours. Further, it demonstrates a novel feature of LCEs and a route for realisation of the molecular auxetic technologies that have been proposed over the years.
We report an x-ray scattering study of the microscopic structure of the surface of a liquid alkali metal. The bulk liquid structure factor of the eutectic K67Na33 alloy is characteristic of an ideal mixture, and so shares the properties of an element
al liquid alkali metal. Analysis of off-specular diffuse scattering and specular x-ray reflectivity shows that the surface roughness of the K-Na alloy follows simple capillary wave behavior with a surface structure factor indicative of surface induced layering. Comparison of thelow-angle tail of the K67Na33 surface structure factor with the one measured for liquid Ga and In previously suggests that layering is less pronounced in alkali metals. Controlled exposure of the liquid to H2 and O2 gas does not affect the surface structure, indicating that oxide and hydride are not stable at the liquid surface under these experimental conditions.
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