The surface energies of solid molybdenum, niobium, tantalum and tungsten

Experimental determination of surface energy for high-energy surface: A review

The experimental determination of surface free energy is of great importance for engineering applications. However, there is no universal, reliable, and convenient measurement means to achieve experimental determination of solid surface energy for high-energy surface. In this work, the existing techniques for experimental determination of surface energy of solids, including indirect (4 kinds) and direct methods (3 kinds), were critically reviewed. In the indirect methods: the explicit interfacial bonding characteristics are required for the multiphase equilibrium technique and for the determination method from crystal equilibrium shape; the critical surface energy technique does not satisfy Zisman's hypothesis that the solid-liquid interface tension is zero (or close to zero), and the parameters fitted by empirical equations cannot have definite physical meaning; the derivation based on the surface tension in the liquid state can only obtain the surface energy of the solid phase near the melting point, and is limited to the prediction of the surface energy of elemental metal. Among the direct determination methods, except for the zero-creep method, are based on generalized determination technologies. All determined results are strongly influenced by the accuracy of the particle (grain) size, scale effect, the atmosphere, etc. This leads to the fact that the errors are still huge regardless of the determination method used, and it is difficult to achieve uniformity of the data. All methods rely to some extent on specific assumptions or theoretical models.

Morphogenesis of Liquid Indium Microdroplets on Solid Molybdenum Surfaces during Solidification at Normal Pressure and under Vacuum Conditions

Electrical and optical applications based on micro- and nanoparticles have specific demands on their interfacial properties. These properties are strongly related to atmospheric conditions to which the particles were exposed during their formation. In this study, metallic In microparticles are synthesized by solidification of In droplets on an amorphous Mo substrate at normal pressure and under vacuum conditions. The influence of ambient pressure on the interface and surface shape is investigated. While solidification at atmospheric pressure leads to collapsed particles with undisturbed contact to the substrate, low pressures result in smooth spherical particles but with cavities inside. Numerical simulations with COMSOL Multiphysics reveal different temperature profiles and heat flux in particles during solidification for both cases. This indicates different starting conditions of the solidification, which leads to the described phenomenon eventually. The investigation of the varying process conditions on the particle shape in combination with the calculated and measured temperature curves over time gives valuable insights into new approaches to synthesize micro- and nanoparticles with defined interfacial properties. Both ambient pressure and cooling rate provide well-controllable and reliable parameters for the realization of different interfacial shapes.

Proven Anti-Wetting Properties of Molybdenum Tested for High-Temperature Corrosion-Resistance with Potential Application in the Aluminum Industry

The behavior of Mo in contact with molten Al was modelled by classical molecular dynamics (CMD) simulation of a pure Mo solid in contact with molten Al at 1200 K using the Materials Studio®. Results showed that no reaction or cross diffusion of atoms occurs at the Mo(s)-Al(l) interface, and that molten Al atoms exhibit an epitaxial alignment with the exposed solid Mo crystal morphology. Furthermore, the two phases {Mo(s) and Al(l)} are predicted to interact with weak van der Waals forces and give interfacial energy of about 203 mJ/m2. Surface energy measurements by the sessile drop experiment using the van Oss-Chaudhury-Good (VCG) theory established a Mo(s)-Al(l) interface energy equivalent to 54 mJ/m2, which supports the weak van der Waals interaction. The corrosion resistance of a high purity (99.97%) Mo block was then tested in a molten alloy of 5% Mg mixed in Al (Al-5 wt.%Mg) at 1123 K for 96 h, using the ALCAN's standard "immersion" test, and the results are presented. No Mo was found to be dissolved in the molten Al-Mg alloy. However, a 20% mass loss in the Mo block was due to intergranular corrosion scissoring the Mo block in the ALCAN test, but not as a result of the reaction of pure Mo with the molten Al-Mg alloy. It was observed that the Al-Mg alloy did not stick to the Mo block.

Thermal degradation of refractory layered metamaterial for thermophotovoltaic emitter under high vacuum condition

Emissivity-tunable metamaterials of layered refractory metal and dielectric have great potentials as a simple thermophotovoltaic (TPV) selective emitter due to its near-omnidirectional, polarization-independent, and broadband selective emissivity. However, it is known that the stability of the layered structure is limited by the oxidation of metals. While there still exists ambiguity concerning the main source of oxygen between adjacent oxide layers and external residual oxygen, most reports focus on the adjacent layers. In this report, thermal stability of a tungsten-based layered metamaterial is investigated under a high-vacuum environment with great care to reduce residual oxygen. The results show unprecedented thermal stability up to 1200 °C for 3 h without any measurable oxidation of metal. This implies that the interlayer diffusion of oxygen from adjacent oxide layers is not exclusively responsible for the oxidation of metal. At such a high temperature, the layered metamaterial theoretically yields a high convertible radiative power density of 3.04 W/cm² with comparable spectral efficiency of 40.2%. Finally, after performing series of thermal tests under higher thermal loads, we propose a novel high-temperature degradation model for layered metamaterials, the stability of which is ultimately limited by the agglomeration of thin metal layers.

Universal binding energy relation for cleaved and structurally relaxed surfaces

The universal binding energy relation (UBER), derived earlier to describe the cohesion between two rigid atomic planes, does not accurately capture the cohesive properties when the cleaved surfaces are allowed to relax. We suggest a modified functional form of UBER that is analytical and at the same time accurately models the properties of surfaces relaxed during cleavage. We demonstrate the generality as well as the validity of this modified UBER through first-principles density functional theory calculations of cleavage in a number of crystal systems. Our results show that the total energies of all the relaxed surfaces lie on a single (universal) energy surface, that is given by the proposed functional form which contains an additional length-scale associated with structural relaxation. This functional form could be used in modelling the cohesive zones in crack growth simulation studies. We find that the cohesive law (stress-displacement relation) differs significantly in the case where cracked surfaces are allowed to relax, with lower peak stresses occurring at higher displacements.