Ga-rich Liquid Research Articles

Microstructural evolution of a focused ion beam fabricated Mg nanopillar at high temperatures: Defect annihilation and sublimation

Microstructural evolution of focused ion beam machined Mg nanopillars was investigated by in situ heating experiments in a transmission electron microscope. The dislocation loops generated by a Ga+ ion beam were annihilated at around half of the melting point of Mg. At a higher temperature (673 K), the sublimation of Mg occurred due to the reduced stability of Mg under the vacuum environment. In the course of sublimation, the Ga-rich liquid was formed and stabilized the structural instability of moving solid–vapor interface.

Critical evaluation and thermodynamic optimization of Mg–Ga system and effect of low pressure on phase equilibria

A complete literature review and critical evaluation of the Mg–Ga binary system are presented. All stable phases known in this system were thermodynamically modeled in the framework of CALPHAD. Liquid phase was modeled using the Modified Quasichemical Model in the pair approximation which takes into account short-range ordering. α-(Mg) solid solution of hcp structure was modeled as a substitutional solution. All intermetallic phases Mg5Ga2, Mg2Ga, MgGa, MgGa2, Mg2Ga5, as well as Ga-rich phase, were treated as stoichiometric compounds. Gas phase was assumed to behave as an ideal solution. Thermodynamic optimization was carried out by evaluating enthalpic and entropic contributions to the Gibbs free energy of all phases independently. It showed that the optimized model parameters along with the model equations could reproduce the available and reliable experimental data in the Mg–Ga system. Representation of partial excess Gibbs free energy of Mg in Ga-rich liquid was improved compared to the previous thermodynamic modeling. Low pressure phase equilibria in this system were analyzed using the developed thermodynamic model, and it was compared with a recent observation of an Mg nanopillar fabricated by Ga+ ion beam in a focused ion beam (FIB), followed by in-situ heating in a transmission electron microscope (TEM). Reported surface melting of Mg nanopillar in the TEM column pressure (~10−10bar) is attributed to the fact that decreasing pressure enhanced sublimation of Mg from the Mg nanopillar significantly, leaving Ga-rich liquid.

Liquid gallium-lead mixture phase diagram, surface tension near the critical mixing point, and prewetting transition

Quite recently, we reported a semianalytical equation of state (EOS) for the Ga-Pb alloy [Phys. Rev. B 78, 024205 (2008)], which was based on the first-order perturbation theory of fluid mixtures, within the simplified random phase approximation, in conjunction with the Grosdidier et al. model pair potentials for Ga-Ga and Pb-Pb with a suitable nonadditive pair potential between Ga-Pb unlike pairs. In the present work, we employ the present EOS to calculate the Ga-Pb phase diagram along the immiscibility gap region. The accuracy of the EOS is tested by consulting the empirical binodal curve. A statistical-mechanical-based theory for the surface tension is employed to obtain an analytical expression for the alloy surface tension. We calculated the surface tension along the bimodal curve and at extreme conditions of temperatures and pressures. The surface tension exhibits reasonably well the prewetting transition of Pb atoms at the surface of the Ga-rich liquid alloy and could qualitatively explain the prewetting phenomena occurring in the Ga-rich side of the phase diagram. The predicted prewetting line and wetting temperature qualitatively agree with the empirical measurements.

Calculation of surface tension and surface phase transition line in binary Ga–Tl system

The surface phase transition (SPT) line has been calculated at the Ga-rich side of the binary Ga–Tl system. The liquid alloys at a given temperature can be divided into the following four different regions as a function of Tl-content of the alloy: (i) at very low Tl-content one bulk Ga-rich alloy is obtained not covered by any nano-layer, (ii) at a somewhat higher Tl-content one bulk Ga-rich liquid phase is obtained, covered by a Tl-rich nano-layer, (iii) at medium Tl-content two bulk liquids are obtained, (iv) at high Tl-content one Tl-rich liquid is obtained with no nano-layer on it. An improved version of the Ga–Tl phase diagram is offered, showing the equilibrium SPT line and the region of the Ga-rich bulk liquid, covered by a Tl-rich nano-layer. The presence of the latter changes some physical properties of the system. Particularly, the temperature coefficient of the surface tension changes from a low negative value to a high positive value when increasing the Tl-composition of the alloy and the SPT line is crossed. The position of the SPT line is essential to control Marangoni convection in liquid monotectic alloys.

Surface behaviour and undercooling in the liquid Ga-Bi binary system detected by optical second harmonic generation

We employ an optical second harmonic generation(SHG) technique to investigate the surface behaviours at the liquid(solid)/vapour interface of the Ga-Bi binary metallic system. In a heating and cooling cycle between 280°C and room temperature, there is no change of the SH-intensity in the heating process, whereas there exists an abrupt and abnormal change of the SH-intensity in the cooling process. It is interesting to find that a macroscopic Bi-rich solid layer is floating on the surface of the Ga-rich liquid phase just below the monotectic temperature (222°C±2°C) in the cooling process, in spite of the Bi-rich phase being heavier than the Ga-rich phase. On the other hand, different undercooling behaviours are observed at the surface and in the bulk. The behaviours of surface solidification and surface melting are different from those in the bulk.

Phase relations and crystal structures in the Ag–Ga system

Abstract The Ag–Ga phase diagram and the ζ-Ag2Ga↔ζ′-Ag2Ga transformation have been investigated by use of optical, scanning and transmission electron microscopy combined with X-ray powder diffraction and thermal differential analysis. On the basis of the present study a new equilibrium phase diagram for the Ag–Ga system is proposed. Only two intermetallic phases are found in the Ag–Ga system: ζ′ and ζ. The compositional change of Ga in ζ′ is described by replacing Ga atoms in 1b position in space group P 6 2m with Ag atoms. In alloys having 25–40 wt.% Ga, the high temperature ζ-Ag2Ga is found to decompose into a Ga-rich liquid and solid ζ′-Ag2Ga at 302°C. In Ga-poor alloys (15 wt.%), the high temperature ζ-Ag2Ga is found to decompose into α-Ag and liquid at 611°C and into α-Ag and ζ′-Ag2Ga at 395°C.

Experimental evidence for a wetting transition in liquid GaPb alloys

The surface composition of Ga-rich liquid in equilibrium with Pb-rich solid has been investigated between 508 K and the monotectic temperature (586 K) of the GaPb system using Auger spectroscopy. A Pb-rich layer is shown to thicken up to 9 equivalent monolayers of Pb at the surface of the liquid as the temperature increases. The thickness of the surface layer diverges logarithmically as the composition of the liquid approaches the monotectic composition. This is the signature of the existence of complete wetting in the GaPb system occurring in the metastable region of coexistence of the two liquids.

Observations of anisotropic wetting in solid-liquid PbGa alloys

The equilibrium wetting of Pb-rich solid by Ga-rich liquid has been investigated between 560 K and the monotectic temperature (586 K) of the GaPb system. The distribution of liquid on the surface of Pb-rich crystals, 10 to 20 μm in size, has been studied. At temperatures below 570 K (i.e. some 16 K or more below the monotectic temperature) the Ga-rich liquid is distributed in droplets in a three-fold pattern around the [lcub]111[rcub] facets on the Pb crystal. The wetted orientations are close to [lcub]210[rcub], which have the highest surface free energy of the crystal. Between 570 K and the monotectic temperature, the Ga drops move away from the [lcub]210[rcub] poles. This is hypothesized to be connected with surface premelting phenomena.

Thermodynamic model for the annealing process of Si-implanted GaAs

The authors theoretically investigate the annealing process of Si-implanted GaAs. Four different annealing methods are considered, and the calculation of point defect concentrations is carried out, assuming that the conditions of the phase equilibrium for GaAs are different depending on the annealing method. In an anneal with As overpressure, GaAs is in equilibrium with a gas phase. When an SiO2 cap is used, GaAs will be in equilibrium with an As-rich liquid phase. In the case of an anneal with an Si3N4 cap, equilibrium with the outside phases is not established. Without any surface protection, GaAs is in equilibrium with a Ga-rich liquid. On the basis of the results, influences of the various annealing methods are discussed, and a new annealing method is proposed.