Ga-Bi Alloys Research Articles

The thermophysical properties of Bi-Ga alloys

In this work, an experimental study was conducted to show the effect of Ga addition to Bi-Ga alloys on density, surface tension, and viscosity. The thermophysical properties of the Bi-Ga system in the liquid state were determined using the discharge crucible method in the temperature range of 573–823 K. The experimental results were compared with the literature data and showed a good correlation on changes for studied properties with Ga addition. The isothermal section at 573 K of the investigated properties shows very good agreement with literature data, and with data obtained from models. Segregation in the liquid has a huge effect on the changes in studied properties. A very big reduction of surface tension value with the addition of Bi content was observed. Moreover, increasing viscosity is connected with the miscibility gap region in the Bi-Ga system.

A mechanistic understanding of surface Bi enrichment in dilute GaBi systems.

Experiment has shown that dilute GaBi systems produce a range of self-organised nanostructured patterns at the surface [Tang et al., Nat. Nanotechnol., 2021, 16, 431-439]. Using extensive ab initio molecular dynamics simulations, we elucidate the mechanisms underlying the formation of the Bi surface islands in Bi-doped Ga liquid metals. Here, we show that in order for internal Bi atoms to diffuse to the surface a lateral extension of the Ga surface network is required. Furthermore, the absence of surface Bi patterning perturbs the Ga surface network providing a preferred path for an internal Bi to diffuse. By understanding how and why Bi nucleates at a surface, we increase the ability to control, manipulate and design such systems for use in future electronic devices.

Pseudopotential-based study of electrical transport properties inherent to Bi-Ga alloys

Pseudopotential-based study of electrical transport properties inherent to Bi-Ga alloys

Calculation of the surface tension of liquid Ga-based alloys

As known, Eyring and his collaborators have applied the structure theory to the properties of binary liquid mixtures. In this work, the Eyring model has been extended to calculate the surface tension of liquid Ga–Bi, Ga–Sn and Ga–In binary alloys. It was found that the addition of Sn, In and Bi into Ga leads to significant decrease in the surface tension of the three Ga-based alloy systems, especially for that of Ga–Bi alloys. The calculated surface tension values of these alloys exhibit negative deviation from the corresponding ideal mixing isotherms. Moreover, a comparison between the calculated results and corresponding literature data indicates a good agreement.

Metastable microheterogeneity in liquid monotectic Bi–Ga alloys

The electrical resistivity of liquid Bi–Ga alloys has been measured in both heating and cooling process using the four‐probe method. Anomalous behaviours of resistivity with temperature appear in heating process and are irreversible in subsequent cooling process. Microheterogeneity is then proved to exist in the melt during heating and depicted by the emulsion model. The evolution of heterogeneity in liquid Bi33·3Ga66·7 alloy was investigated through both isothermal and non‐isothermal experiments. The results indicate that Bi33·3Ga66·7 melt experiences a polydisperse–monodisperse–homogeneous state transformation upon successive heating above critical temperature.

Model calculation of the surface tension of liquid Ga–Bi alloy

A convenient model, based on some assumptions, for calculating the composition and temperature dependence of the surface tension of binary liquid alloys is reported. The theoretical calculations of the surface tension of gallium-rich-bismuth alloys are presented. The calculated results are compared with the reported experimental data. A relatively good agreement with experimental behavior of the composition dependence of the surface tension was found, but a disagreement was observed with experimental temperature behavior of the surface tension of these alloys. The calculations were conducted in the temperature range from almost 320K to about 800K. The surface tension was calculated from eutectic composition (xBi=0.0022) to xBi=0.1, and worked out by linear equations. The model calculation and analysis indicate a first order surface phase transition in this system, which is in accord with experimental findings. For this system, γ decreases linearly with increasing temperature at fixed Bi mole fraction xBi, and thus, suggesting a positive surface excess entropy. It is also found that the surface tension isotherms show the linear dependence on the concentration, in the logarithm scale of xBi, in the very narrow concentration range.

Thermal stability of surface freezing films in Ga-based alloys: An x-ray photoelectron spectroscopy and scanning tunneling microscopy study

We have investigated the thickness and surface structure of surface freezing films in Ga-Bi and Ga-Pb alloys over a wide temperature range between room temperature and the respective surface freezing transitions by x-ray photoelectron spectroscopy (XPS) and scanning tunnelling microscopy (STM). For the example of a Ga-Bi alloy dilute in Bi, XPS measurements show that the surface freezing film has a nearly constant value of approximately 25 A between the surface freezing temperature of 130 degrees C and room temperature if the sample is cooled slowly (5 Kh). On heating to 130 degrees C the film thickness exhibits a clear hysteresis on melting. On quenching the alloy sample (>100 Kh) the film thickness increases by almost a factor of 10. These observations indicate that the surface freezing films are metastable. The surface structure of the surface freezing films of various Ga-rich Ga-Bi and Ga-Pb alloys has been probed for the first time by STM at different temperatures below and above the bulk eutectic point. Atomically resolved STM images show the surface structures of pure Bi (0001) and Pb (111), respectively, at room temperature. On heating above the eutectic temperature the surface structure of the films does not change significantly as judged from the size and thickness of Pb or Bi terraces. These observations together with the film thickness variation with temperature indicate that the surface freezing films behave like a metastable independent surface phase. These results together with the wetting characteristics of these alloys suggest that surface freezing in these systems is a first order surface phase transition between wetting and metastable surface freezing films. The energy barrier for nucleation is strongly reduced due to a lowering of the interfacial energy if the nucleus is completely immersed in the respective wetting layer.

GaBi alloy liquid metal ion source for microelectronics research

A GaBi alloy liquid metal ion source has been studied. From an analysis of the source mass spectra as a function of emission current, a mechanism is suggested for the production of single- and double-charged ions. There is good agreement with the results of Swanson's investigations of a pure Bi source.

Oscillatory interfacial instabilities in binary metallic fluids

Oscillatory wetting–dewetting instabilities at the liquid/vapour interface ofGa–Pb and, for the first time, of Ga–Bi alloys have been studied employingsecond harmonic generation and kinetic ellipsometry. Alloy samples prepared atx–T conditions inside the respective miscibility gap(Ga0.8Bi0.2: T = 520 K;Ga0.95Pb0.05: T = 595 K) show oscillations on the timescale of typically hours. The surface transforms back andforth from a Ga-rich non-wetting state to a complete wetting state where the Ga-rich bulkis covered with a Bi-rich (or Pb-rich) film. In the case of Ga–Bi the change of the wettingfilm thickness was characterized quantitatively with the aid of the ellipsometricmeasurements. It is consistent with a wetting–dewetting mechanism whereby the wettingfilm varies periodically between macroscopic and microscopic values. The phenomenon canbe understood taking different emissivities of the wet and the nonwet state into account.They cause at otherwise constant conditions different temperatures of the sample, whichhas to adopt its phase compositions according to the respective phase diagram. Thisresults in mass transport to and away from the surface. A model calculation onGa–Bi taking these effects into account recovers in good quantitative agreementthe oscillation period and the oscillations of the temperature as well as of thefilm thickness. Similarities and differences between Ga–Bi and Ga–Pb are finallyconsidered.

The critical behaviour under microgravity of liquid alloys with mutual immiscibility

The process of two-liquid phase separation (TLPS) was studied by the electrical resistivity, ρ, both on the ground and under microgravity obtained on board the sounding rocket. On cooling to the critical temperature, Tc, on the ground, increase of ρ and decrease of the temperature coefficient of ρ, dρ/dT were found in the homogeneous liquid phase of liquid Bi–Ga and Cd–Ga alloys. However, increase of ρ and decrease of dρ/dT were observed for the liquid Hg–Ga system. Both tendencies were explained successfully by the concentration fluctuation model. The rocket experiment for the ρ of liquid Bi–Ga alloys revealed that, under microgravity, the concentration fluctuation grows extensively and the nucleation for TLPS is suppressed.

Comparative thermodynamic investigation of binary Ga-Bi system; Experimental determination of enthalpies of mixing and activity estimation for liquid Ga-Bi alloys

The results of experimental and analytical thermodynamic investigation of binary Ga–Bi system are presented in this paper. Integral molar enthalpies of mixing for liquid Ga–Bi alloys were obtained using quantitative differential thermal analysis. Activities of bismuth in the entire composition range and temperature interval 573–1073 K, were calculated by Zhang–Chou’s method for binary systems having two liquid or solid coexisting phases in combination with other Chou’s method developed for simple eutectic systems. Activities of gallium were derived from Gibbs–Duhem equation. Comparison between experimental, calculated values and existing literature data shows good agreement.

Electron spectroscopy and scanning tunneling microscopy study of quasi-two-dimensional freezing at the liquid/vapor interface of Ga-Bi alloys

We have studied the interfacial characteristics of Ga-rich liquid Ga-Bi alloys at various compositions and temperatures up to 300 degrees C by different electron spectroscopies under ultrahigh vacuum conditions. In particular, the thickness and thickness profiles of Bi-rich wetting and surface freezing films have been measured. It is found that at the surface freezing transition the Bi film thickness jumps from approximately 5 A to macroscopic values of over 100 A. This distinct behavior is discussed in comparison with the results in Ga-Pb. In addition, we report first scanning tunneling microscopy observations of the surface structure of a solidified Ga(0.989)Bi(0.011) alloy after passing a surface freezing transition. The topology at room temperature is characterized by extended, atomically flat Bi terraces covered by nanosized triangular two-dimensional Bi islands.

Theoretical analysis for the determination of surface composition in molten Ga-Bi metal alloys by rare gas scattering

A theoretical model of classical mechanical atom-surface scattering which includes both single and double collisions from a many-body target is applied to the determination of surface composition of molten metal alloys by rare gas scattering. Explicit calculations are compared with recently measured data for the scattering of monoenergetic beams of Ar atoms from the surface of liquid Ga-Bi alloys. These calculations indicate that rare gas scattering can provide a precise measure of surface segregation in the outermost layer of dilute liquid alloys.

Model calculations of surface phase transitions in Ga–Bi alloys: adsorption,wetting, surface freezing and melting

Thermodynamic model calculations of the surface phase transitions inGa–Bi alloys are performed, which account in particular for the finitethickness of the adsorption films. The excess chemical potential of bismuth inthe films is calculated by employing a screened Coulomb potential. Thecalculated dependence of the liquid film thickness predicts adsorption andcomplete wetting transitions, which are in qualitative agreement with theexperimental data. On the basis of surface tension calculations the surfacefreezing–melting transition is modelled and the results coincide with experimentalobservations via second harmonic generation and surface light scattering. Usingan appropriate description of the nucleation mechanism in liquid filmsthe line tension between the liquid and solid films is determined by afit of experimental data. A complete diagram of the surface liquid–solidphase transitions on the vapour/liquid Ga–Bi interface is derived.

Molecular beam epitaxy growth of GaAs1−xBix

GaAs 1−x Bi x epilayers with bismuth concentrations up to x=3.1% were grown on GaAs by molecular beam epitaxy. The Bi content in the films was measured by Rutherford backscattering spectroscopy. X-ray diffraction shows that GaAsBi is pseudomorphically strained to GaAs but that some structural disorder is present in the thick films. The extrapolation of the lattice constant of GaAsBi to the hypothetical zincblende GaBi alloy gives 6.33±0.06 Å. Room-temperature photoluminescence of the GaAsBi epilayers is obtained and a significant redshift in the emission of GaAsBi of ∼84 meV per percent Bi is observed.

Surface freezing and wetting in Ga–Pb alloy: Second harmonic and plasma generation study

The surface freezing and wetting behaviour at the liquid/vapour interface of Ga-rich alloys of Ga and Pb has been studied as a function of temperature at selected compositions both in the homogeneous liquid and the fluid two phase demixing regime. The interfacial phase transitions have been probed by second harmonic generation (SHG) in combination with plasma generation (PG) measurements at different cooling and heating cycles. In the composition range between the eutectic and monotectic points the occurrence of surface freezing is clearly indicated by distinct changes of the SHG and PG intensities, respectively. It is not observed below the eutectic composition for solution very dilute in Pb. Qualitatively, these results agree with the recent observations made in Ga–Bi alloys. A new wetting characteristic is found crossing the miscibility gap at constant composition with varying temperature. A Pb-rich macroscopic wetting film forms which must be concluded from the SHG intensities. This interfacial phase transition causes a clearly detectable drop of the sample temperature which is explained by the corresponding discontinuous change of the optical emissivity at the interface. These interfacial phenomena related with the miscibility gap have not been reported before.

Surface freezing transitions in liquid binary alloys

In this paper recent results on surface phase transitions at the interface of Ga–Bi alloys are reviewed. The wetting transition in the liquid–liquid demixing region is briefly summarized. Then, the surface freezing transition, which occurs in the concentration range between the eutectic ( x Bi=0.0022) and the monotectic ( x Bi=0.085) compositions, is described. This was discovered recently using second harmonic generation and plasma generation methods. The surface freezing line was found approximately 20 K above the eutectic point; it approaches the liquidus line with increasing Bi mole fraction and presumably merges at the monotectic point. Arguments for the thermodynamic stability of the surface freezing Bi-rich films and a possible correlation between the wetting transition and the surface freezing transition are discussed. Finally, we present preliminary experimental results on a study of surface freezing in Ga–Pb.

Light scattering study of surface freezing and surface viscoelasticity in a eutectic liquid Ga–Bi alloy

Surface freezing in a metallic liquid, the eutectic Ga–Bi alloy, has been studied for the first time by capillary wave spectroscopy at various wave numbers and temperatures. The surface freezing transition is marked by a sharp drop of the surface tension σ by 25 mJ m −2 and a hysteresis behaviour indicating a first-order interfacial phase transition. Analysis of the capillary wave spectra yields information on the surface excess viscoelastic modulus governing shear normal to the surface plane which is found to be consistent with a metallic solid-like surface freezing film on top of the eutectic Ga–Bi alloy.

Wetting transition in liquid Ga–Bi alloys: light scattering study of surface energy and entropy

We have studied the surface energy, surface entropy and, partly, have received an estimate of the viscosity of liquid Ga–Bi alloys at compositions mainly in the Ga-rich phase region and at temperatures up to 620 K. Measurements have been performed by the noninvasive method of capillary wave spectroscopy which has been applied for the first time to investigate the surface properties of a liquid alloy at high temperatures. Analysis of the concentration dependence of the surface energy according to the Gibbs adsorption equation yields that in the composition range of 10 −2≲ x Bi≲10 −1 and at temperatures of 450 K≲ T≲500 K a surface excess of Γ Bi∼1.36×10 −5 mol m −2 resides at the liquid–vapour interface corresponding to pure Bi. At lower x Bi a reduction of Γ Bi occurs which is indicative of a prewetting transition. The variations of surface energy and entropy with composition are not consistent with a description by a simple monolayer model as was deduced from X-ray reflectivity results. Instead it is found that a multilayer model qualitatively accounts for the characteristic change of the surface quantities. The thickness of the multilayer interfacial region is estimated from the change of the relative surface entropy. This yields values between 10 and 20 Å with a trend for an increasing number of surface layers towards the complete wetting transition at the monotectic point. All isopleths of the surface energy exhibit clear kinks at temperatures 10–20 K above the liquidus curve for compositions below the monotectic point. The corresponding discontinuity of the surface entropy is consistent with a first order transition of surface freezing reported recently for the Ga–Bi system.

Surface freezing in liquid Ga–Bi alloys: optical second harmonic and plasma generation study

The phenomenon of surface freezing has been studied by second harmonic and plasma generation measurements in the entire composition range of liquid Ga–Bi alloys. Surface freezing was observed for the first time for all alloys with compositions between the eutectic (xBi = 0.0022, Teut = 29.48 °C) and monotectic point (xBi = 0.085, Tmono = 222 °C). On cooling of such alloys a Bi-rich solid-like film forms on top of the bulk liquid phase at temperatures well above the liquidus line. On melting of these films a clear hysteresis behaviour is found, characterizing this type of transition as a first order phase transition. The maximum difference of 20 K between the liquidus temperature and the melting temperature of the surface freezing films was observed for the eutectic alloy. The line of surface freezing temperatures merges with the liquidus approaching the monotectic point. An estimate of the thickness of the Bi-rich surface freezing films from the interfacial free energies yields a value of the order of 10 nm. The correlation of the surface freezing and wetting transition occurring in the Ga–Bi system as well as the thermodynamics of surface freezing are qualitatively discussed.