A new, noncontact technique is described which entails simultaneous measurements of the surface tension and the dynamic viscosity of molten materials. In this technique, four steps were performed to achieve the results: ͑1͒ a small sample of material was levitated and melted in a high vacuum using a high temperature electrostatic levitator, ͑2͒ the resonant oscillation of the drop was induced by applying a low level ac electric field pulse at the drop of resonance frequency, ͑3͒ the transient signals which followed the pulses were recorded, and ͑4͒ both the surface tension and the viscosity were extracted from the signal. The validity of this technique was demonstrated using a molten tin and a zirconium sample. In zirconium, the measurements could be extended to undercooled states by as much as 300 K. This technique may be used for both molten metallic alloys and semiconductors.
Electrodeposited nickel-molybdenum, nickel-tungsten, cobalt-molybdenum, and cobalt-tungsten were characterized for the hydrogen evolution reaction (HER) in the electrolysis of 30 w/o KOH alkaline water at 25~ The rate-determining step (rds) of the HER was suggested based on the Tafel slope of polarization and the capacitance of electrode-solution interface determined by ac impedance measurement. The HER on the nickel-and cobalt-based codeposits was enhanced significantly compared with that on the electrolytic nickel and cobalt with comparable deposit loadings. The decrease in the HER overpotential was more pronounced on the molybdenum-containing codeposits, particularly on cobalt-molybdenum which also showed a high stability. The enhancement of the HER was attributed to both the synergetic composition and the increased active surface of the codeposits. The real electrocatalytic activity of the electrodes and the effect of their surface increase were distinguished quantitatively. The linear relations between HER overpotential and surface roughness factor of the electrodes on a Y-log(X) plot were obtained experimentally and interpreted based on the Tafel law.
Hybrid electrostatic-aerodynamic levitation furnace for the high-temperature processing of oxide materials on the ground Rev. Sci. Instrum. 72, 2811 (2001); 10.1063/1.1368860Noncontact technique for measuring surface tension and viscosity of molten materials using high temperature electrostatic levitation Rev.
Metallic liquid silicon at 1787K is investigated using x-ray Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Our results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled liquid states. The population of covalent bond pairs in liquid silicon is estimated to be 17% via a maximally-localized Wannier function analysis. Compton scattering is shown to be a sensitive probe of bonding effects in the liquid state. Silicon (Si) presents a fascinating phase diagram as is the case in other systems that form tetrahedrally coordinated networks.[1] Upon melting, Si transforms into a metal accompanied by a density increase of about 10%. The resistivity of liquid Si (l-Si) at the melting temperature T m is 0.75 µΩm, which is comparable to that of simple liquid metals such as l-Al. However, the first neighbor atomic coordination number in l-Si remains 5.5∼6 [2], which is approximately half that of simple liquid metals, hinting that covalent bonds survive even in the metallic state [3]. In fact, molecular dynamics simulations of molten Si at 1800K suggest that approximately 30% of the bonds are covalent and that these covalent bonds possess a highly dynamic nature, forming and breaking up rapidly on a time scale of 20 fs [4]. It is remarkable that two completely different types of bonds−metallic and covalent− can coexist in l-Si. In fact, the coexistence of two forms of liquid in a single component substance has been predicted to undergo a phase transition as a function of temperature and / or pressure [5], and many theoretical studies support the existence of a liquidliquid phase transition (LLPT) [6][7][8]. A recent study reports that l-Si could undergo an LLPT below about 1232K and above about -12kB, separating into a highdensity metallic liquid (HDL) and a low-density semimetallic liquid (LDL) [8]. But, 1232K is far below the melting temperature of 1683K of Si, and as a result the supercooled state has remained inaccessible to current experimental techniques. Very recently, Beye et al. have performed time-resolved x-ray measurements on Si using a femtosecond pulse-laser [9] to reveal liquid polymorphs of Si which could support an LLPT, but these experimental conditions are far from being ideal [6][7][8] so that the experimental confirmation of an LLPT in Si remains an open question.A key requirement for the possibility of an LLPT obviously is that the metallic and covalent bonds coexist in l-Si. Although experimental investigations of the atomic configuration hint at the existence of covalent bonds in l-Si, surprisingly, soft x-ray [10] and magnetic susceptibility measurements [11] of electronic properties so far do not support this viewpoint in that all four valence electrons in l-Si appear to behave like free-electrons. Emissivity and thermal conductivity of l-Si also exhibit a freeelectron like temperature depen...
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