Pb Melts Research Articles

Surface melting of polycrystals Pb and In within 1 K below melting temperature

Surface melting of polycrystals Pb and In within 1 K below the melting temperature is measured and interpreted based on our model for size dependence of melting temperature. It found that the thickness of liquid skin on the surface of the polycrystals below the melting temperature is dependent on temperature.

Far-infrared absorption in Sb-doped Ge epilayers near the metal–insulator transition

Epitaxial germanium layers doped near the metal–insulator transition were grown by liquid phase epitaxy from a Pb melt. Far-infrared absorption was measured between ∼20 cm−1 and 150 cm−1. Linear optical absorption coefficients were determined for Ge:Sb in the doping range of 9.0×1014 cm−3<ND<6.7×1016 cm−3. The peak absorption was found to increase linearly with dopant concentration as expected. The absorption at ∼50 cm−1 increased superlinearly with Sb concentration as a result of impurity banding. The optimal Sb concentration for extended far-infrared photoconductive response (λ>200 μm) in blocked impurity band detectors is discussed.

Thermodynamic investigation of Pt–Pb binary alloys

The enthalpy of formation of PtPb compound was determined by reaction calorimetry at room temperature. The heat content of the same compound was measured by drop calorimetry in the solid, the liquid and the two-phase states, and its heat capacity was derived. The enthalpy of formation of the Pt 3Pb compound has been deduced. In order to apply the regular associated solution model we calculated the free enthalpy of formation of the liquid from the phase diagram and the thermodynamic functions of the solid intermediate compound and the liquid. The application of the model allowed us to explain the behaviour of the Pt–Pb melt by the existence of two kinds of associates, i.e. Pt 2Pb and PtPb.

Growth of Ga 1− xIn xAs ySb 1− y solid solutions from the five-component Ga–In–As–Sb–Pb melt by liquid phase epitaxy

Gallium antimonide (GaSb) and its solid solutions are widely used in optoelectronic devices for the ecologically important spectral range 2–5 μm. In this paper we discuss a novel approach of using neutral solvents (such as Pb) in GaInAsSb growth process. We employed the excess thermodynamic functions and linear combinations of chemical potentials (EFLCP) thermodynamic model to calculate the melt–solid phase diagrams in the Ga–In–As–Sb–Pb system.

Nano Al 2O 3-Pb AND SiO 2-Pb cermets by sol-gel technique and the phase transformation study of the embedded Pb particles

Sol-gel processing followed by H 2 reduction is used to produce dispersions of nanosized Pb in amorphous SiO 2 and ultrafine γ Al 2O 3 matrices. A depression of 3–5K in Pb melting point is reported. The size and shape of these metastable particles in molten and solid state are discussed in the light of the experimental observations and expectations from the intersection group theory for equilibrium shape.

Reflection high-energy electron-diffraction study of melting and solidification of Pb on graphite

The melting and solidification of Pb thin films on pyrolytic graphite are investigated in situ by reflection high-energy electron diffraction. Thin films with thicknesses of 4--150 monolayers are investigated. The surface morphology of the thin films were studied by scanning electron microscopy. Superheating of the Pb thin films by $4\ifmmode\pm\else\textpm\fi{}2$ to $12\ifmmode\pm\else\textpm\fi{}2$ K is observed from diffraction intensity measurements. Upon cooling the substrate, the Pb on graphite is seen to supercool by $\ensuremath{\sim}69\ifmmode\pm\else\textpm\fi{}4\mathrm{K}.$

Equilibrium interphase interfaces and premelting of the Pb(110) surface.

Equilibrium properties of the solid-liquid and liquid-vapor interphase interfaces of lead and surface melting of Pb(110) were investigated using molecular-dynamics simulations, with the glue model used for a description of the many-body interatomic interactions. The bulk melting temperature was determined to be 619{plus_minus}5 K for the model. Structural, energetic, and transport properties across the solid-liquid interface change gradually from bulk solid to bulk liquid behavior over a transition region of 5--6 (110) layers. The surface region of Pb(110) starts to disorder via generation of vacancies in the temperature range 350 K{lt}{ital T}{lt}400 K. The onset of a quasiliquid surface region is observed around 520 K. The surface disordering process reflects the anisotropy of the (110) surface, being enhanced in the direction parallel to the close-packed rows. Analysis of the results in the framework of the Landau-Ginzburg theory of surface melting shows that the thickness of the interfacial quasiliquid region grows logarithmically for {ital T}{gt}520 K, with a correlation length of 7.7 A, in close agreement with experimental results.

Superconducting and crystallographic properties of Bi,Pb(2212)

Superconducting Bi 2−xPb xSr 2CaCu 2O x (noted Bi,Pb(2212)) has been processed in air at 860°C with a nominal composition of Bi 1.72Pb 0.34Sr 1.83Ca 0.97Cu 2.13O x in contrast to the literature where this phase was produced in argon around 740°C. With the aim of transforming this phase into Bi,Pb(2223) (another Bi-containing superconductor), Bi,Pb(2212) has also been mixed with Ca 2CuO 3 and CuO. A critical temperature T c of 95K was obtained for pure Bi,Pb(2212). On the basis of DTA/TG and X-ray diffraction measurements, we can conclude that pure Bi,Pb(2212) melts at 882°C. If secondary phases are added, Pb leaves the Bi,Pb(2212) crystal structure around 400°C and it is again incorporated above 835°C until the phase melts at 876°C, a temperature somewhat lower due to compositional effects.

The method of volumetric measurements under high pressure. II. Quantitative measurements

Abstract Method of measurements of the jumps of volume during phase transitions at high pressures and temperatures is suggested. Theoretical ground of the method is given. Values of the jumps of volume during melting of Pb and In up to 8 GPa, transition in the melt of Sn (these are first quantitative measurements of the jump of volume in the melt under high pressure), are presented.

Anomalous Temperature Behavior of Impurities in Lead and Silver Hosts

XAFS and Mössbauer effect measurements were made on impurities that lower the bulk melting temperature, Tm, of Pb and Ag. In addition XAFS measurements were made on Tl impurities that raise Tm of Pb. The spectral intensity of the Mössbauer signal in Pb varies as expected from harmonic vibrations of the 1 at.% Sn Mössbauer atoms below a temperature of about 140K. Above this temperature the spectral intensity drops rapidly which is attributed to a localized hopping associated with a high entropy density. In Ag hosts a similar behavior occurs above 900K. The XAFS signal from Hg impurities in Pb, which like Sn lowers Tm, starts deviating from vibrational behavior above 400K where the apparent coordination number decreases. This is also interpreted as a localized hopping. However, Tl, which raises Tm, shows the normal vibrational behavior in XAFS measurements in a Pb host. XAFS measurements on Ga impurities in Ag, which also lower Tm of bulk Ag, show only expected anharmonic behavior up to 900K. All of these results suggest a liquid-like high entropy region within a radius of somewhat more than an atomic spacing around impurities that lower Tm. These high entropy, or “premelted”, regions could act as nucleation sites for three dimensional melting and should dominate over both grain boundaries and free surfaces in nucleating bulk melting in large single crystals.

Influence of Pb impurities on He-bubble formation and growth in Al studied by positron annihilation and transmission electron microscopy.

Positron-lifetime and transmission-electron-microscopy measurements are reported for He (4.5 at. ppm), in pure Al, and Pb-contaminated ({similar to}1 at. ppm) Al:0.5 wt. % Ni alloy. The observed He-bubble growth in pure Al was found to follow at {ital t}{sup 1/5} dependence, i.e., was in good agreement with previously published data. The Pb-contaminated alloy exhibited accelerated He agglomerate and bubble growth. Bubbles observed by transmission electron microscopy in the Pb-contaminated alloy were found to have precipitates attached to them. No precipitates were observed that were not associated with bubbles and no bubbles observed that did not have a precipitate attached. Agglomerate and bubble radii were calculated using the positron-lifetime data. Again the results for pure Al were found to be in good agreement with previously published data. In the Pb-contaminated alloy agglomeration and coalescence occurred at temperatures as low as {similar to}80 {degree}C, while accrual of free volume started at {similar to}180 {degree}C. An increase in bubble growth rate, which occurred between {similar to}350{degree} and {similar to}400 {degree}C, was tentatively associated with the melting of Pb precipitates.

The gd-pb (gadolinium-lead) system

[69Dem] studied the Gd-Pb system by means of differential thermal analysis (DTA), optical metallography, and X-ray methods. [69Dem] prepared their Gd by Ca reduction of the fuoride, followed with distillation. Four different batches were used; their analyses are reported in Table 2. The Pb contained Mg, Si, Ca, Fe, Cu, Ag, and Bi as impurities, but the amount of each did not exceed 1 ppm (weight?). The samples were prepared in sealed tantalum crucibles by direct melting of Gd and Pb. The alloys were maintained in the liquid state for 30 min, cooled, inverted, and remelted at least twice. Micrographic examination showed the samples to be homogeneous. DTA was used to determine the liquidus curves and reaction isotherms of the system (heating and cooling rates not stated). Liquidus temperatures were obtained from cooling arrest data while both heating and cooling data were used to establish the reaction horizontals. Temperatures for eutectic and peritectic equilibria were reproducible to + 2 *C below 1140 *C and to  7 *C above 1140 *C, Liquidus temperatures were accurate to  10 *C. The liquidus curve in the region GdsPb 4 to GdPb2 of the assessed diagram has been drawn to fulfdl thermodynamic requirements. Due to their reactivity in air, the specimens for X-ray diffraction (XRD) were prepared by crushing or filing the alloys in an inert gas atmosphere. Also, metaUographic specimens were prepared and protected from air under kerosene. Addition of Pb to Gd gives rise to a catatectic equilibrium and to two solid solutions whose extension [69Dem] determined from photomicrographs of several alloys quenched from 1080 *C (ctGd) or from thermal data ([3Gd). Despite the very good melting point of the Gd used, the ct ,-~ 13 transformation was 23 *C higher than the accepted value listed in [86Gsc]. In such instances, correction of both the transformation temperature of the pure metal and the catatectic equilibrium sometimes is made to compensate for the difference. However, in the absence of reliable experimental data, we prefer to retain the original catatectic equilibrium at the reported temperature. The Pb melting point was raised about 1 *C by Gd additions, yielding a periteetic reaction that was verified by DTAwith a pure Pb sample as reference against an alloy at 97.44 at.% Pb. XRD experiments showed the solubility of Gd in (Pb) to be less than 0.5 at.%.

Melting behaviour of In and Pb particles embedded in an Al matrix

Microstructures of melt spun hypomonotectic Al−7wt%In, hypermonotectic Al−5wt%Pb and near monotectic Al−2wt%Pb alloys have been examined by transmission electron microscopy and consist of 10–150 nm diameter faceted In particles and 5–150 nm faceted Pb particles homogeneously distributed in an Al matrix. As-melt spun In and Pb particles exhibit near cube-cube and cube-cube orientation relationships with the Al matrix respectively, and truncated octahedral shapes bounded by {111} and {100} facets. The melting behaviour of In and Pb particles in as-melt spun Al−7wt%In, Al−5wt%Pb and Al−2wt%Pb alloys has been investigated by heating and cooling experiments in a differential scanning calorimeter and in situ heating experiments in a transmission electron microscope. In and Pb particles embedded within the Al matrix grains melt at superheatings in the range 0–40 K above the bulk equilibrium In and Pb melting points. Superheating of In and Pb particle melting within the Al matrix grains is caused by a kinetic difficulty of nucleating melting which increases with decreasing In and Pb particle size. In and Pb particles along the grain boundaries of the Al matrix melt at undercoolings in the range 0–7 K below the bulk equilibrium In and Pb melting points.

Surface-induced melting and freezing I. Medium-energy ion scattering investigation of the melting of Pb{hkl} crystal faces

Ion-shadowing and -blocking experiments on cylindrically shaped single crystals of Pb reveal a strongly orientation-dependent disordering (melting) of the surface as the temperature T increases toward the bulk melting point T m. Most of the crystal faces become strongly disordered, but not the faces with orientations around the {111} and {100} planes. On the disordered surfaces the melted layer thickness is found to diverge logarithmically with increasing temperature. The disordering is shown to be driven by the orientation-dependent interface free energy which the ordered solid surface has in excess of a surface completely wetted with a liquid layer.

Solidification of Pb particles embedded in Al

Hypermonotectic alloys of Al-5 wt% Pb and Al-5 wt% Pb-0.5 wt% X where X = Mn, Cu, Zn, Fe and Si have been manufactured by chill-casting and melt-spinning. The resulting microstructures have been examined by a combination of optical microscopy, scanning and transmission electron microscopy, and electron probe microanalysis. The as-solidified hypermonotectic alloys exhibit a homogeneous bimodal distribution of faceted Pb particles embedded in a matrix of Al, with chill-cast Pb particle sizes of 1–2 μm and 5–50 μm, and melt-spun Pb particle sizes of 5–10 nm and 50–100 nm. The larger Pb particles are formed during cooling through the region of liquid immiscibility while the smaller Pb particles are formed during monotectic solidification of the Al matrix. The Pb particles exhibit a cube-cube orientation relationship with the Al matrix, and a truncated octahedral shape with {111} and {100} facets. The as-solidified Pb particle distributions are resistant to coarsening during post-solidification heat treatment. The equilibrium Pb particle shape and therefore the anisotropy of solid Al-solid Pb and solid Al-liquid Pb surface energies have been monitored by in situ heating in the transmission electron microscope over the temperature range between room temperature and 550°C. The anisotropy of solid Al-solid Pb surface energy is constant between room temperature and the Pb melting point, with the {100} surface energy 14% greater than the {111} surface energy, in good agreement with geometric near-neighbour bond energy calculations. The {100} facets disappear when the Pb particles melt, and the anisotropy of solid Al-liquid Pb surface energy decreases gradually with increasing temperature above the Pb melting point, until the Pb particles become spherical at about 550°C. The kinetics of Pb particle solidification have been examined by heating and cooling experiments in a differential scanning calorimeter. Pb particle solidification is nucleated catalytically by the Al matrix on the {111} facet surfaces, with an undercooling of 22K and a contact angle of 21°C. Ternary additions of Mn, Cu, Zn and Fe do not influence the Pb particle solidification behaviour, but Si is a potent catalyst and stimulates the Pb particles to solidify close to the equilibrium Pb melting point.

Correlation length of a dense liquid and surface melting.

We report an excellent agreement between the calculated value of the correlation length for solidlike order in a simple liquid close to freezing and the recent experimental data of Pluis et al. [Phys. Rev. B 40, 1353 (1989)] on surface melting of Pb(110).

Surface melting of Pb(110) studied by x-ray photoelectron diffraction.

The temperature-dependent forward-scattering intensity of 4${\mathit{f}}_{7/2}$ core-level photoelectrons at 1344.7 eV kinetic energy was studied for a Pb(110) crystal along the [001] and [11\ifmmode\bar\else\textasciimacron\fi{}0] azimuths. The forward-scattering enhancement peaks, as a measure of crystalline order near the surface, decrease slowly up to about 500 K. Above this temperature a much more pronounced decrease is observed, indicative of a disordering of the Pb(110) surface. A change in the growth law of the disordered layer is seen near 575 K.

Melting and possible amorphization of Sn and Pb in Ge/Sn and Ge/Pb mechanically milled powders

Mixtures of Ge–Sn and Ge–Pb powders were ball-milled to form a fine dispersion. After 32 h of milling the average diameter of the hard Ge particles embedded in the Sn (or Pb) matrix was about 10 nm. As the Ge concentration was increased in each system, the melting point,TM, and the enthalpy of fusion, ΔHM, of Sn (or Pb) decreased. Only small changes in ΔTMand ΔHMwere observed after heating cycles in the DSC to above the melting point. The melting endotherm measured by DSC disappeared for Ge-rich compositions (88 and 95 vol.% Ge for Ge–Sn; 93.5 vol.% Ge for Ge–Pb). It is suggested that atomic disorder/melting is nucleated at the Ge/Sn (or Ge/Pb) interfaces and the melting point and enthalpy of fusion decrease as the interfacial area increases. When the Ge volume reaches a value where essentially all the Sn (or Pb) atoms are adjacent to the Ge particle surfaces, the Sn is in a disordered–perhaps amorphous–state such that no melting transition is observed.

Melting of the top Pb(110)-surface layers: Experimental evidence for a discontinuous phase transition

We have used Spin Polarized Low Energy Electron Diffraction (SPLEED) to probe the short range order at a Pb(110)-surface during melting. We find that the short range order in the top two surface layers changes discontinuously at T S = 524 ± 1 K, i.e. the top layers melt via a first order phase transition.

Surface melting of Pb(110): A compilation of experimental results

Recently, the first direct experimental observations have been made of a depression in melting point at the surface of a three-dimensional crystal. Since then, numerous experiments with various techniques have been devoted to surface melting. The most intensively studied surface in this respect, so far, is Pb(110). This paper attempts to compare and combine the experimental information which is presently available for this surface. A detailed picture of surface melting of Pb(110) emerges.