Si Pb Research Articles

The dissociation energy of the new diatomic molecules SiPb and GePb

The diatomic molecules SiPb and GePb were for the first time identified by producing high temperature vapors of the constituent pure elements in a "double-oven-like" molecular-effusion assembly. The partial pressures of the atomic, heteronuclear, and homonuclear gaseous species observed in the vapor, namely, Si, Ge, Pb, SiPb, GePb, Pb2, Gen, and Sin (n=2-3), were mass-spectrometrically measured in the overall temperature ranges 1753-1961 K (Ge-Pb) and 1992-2314 K (Si-Pb). The dissociation energies of the new species were determined by second- and third-law analyses of both the direct dissociation reactions and isomolecular exchange reactions involving homonuclear molecules. The selected values of the dissociation energies at 0 K (D0 degrees) are 165.1+/-7.3 and 141.6+/-6.9 kJ/mol, respectively, for SiPb and GePb, and the corresponding enthalpies of formation (DeltafH0 degrees) are 476.4+/-7.3 and 419.3+/-6.9 kJ/mol. The ionization efficiency curves of the two species were measured, giving the following values for the first ionization energies: 7.0+/-0.2 eV (SiPb) and 7.1+/-0.2 eV (GePb). A computational study of the species SiPb and GePb was also carried out at the CCSD(T) level of theory using the relativistic electron core potential approach. Molecular parameters, adiabatic ionization energies, adiabatic electron affinities, and dissociation energies of the title species were calculated, as well as the enthalpy changes of the exchange reactions involving the other Pb-containing diatomics of group 14. Finally, a comparison between the experimental and theoretical results is presented, and from a semiempirical correlation the unknown dissociation energies of the SiSn and PbC molecules are predicted as 234+/-7 and 185+/-11 kJ/mol, respectively.

Centrality and system size dependence of multiplicity fluctuations in nuclear collisions at158AGeV

The centrality and system size dependence of multiplicity fluctuations of charged particles produced in nuclear collisions at $158 A$ GeV was studied by the NA49 collaboration. Centrality selected Pb+Pb collisions, semi-central C+C and Si+Si collisions as well as inelastic p+p interactions were analyzed. The number of projectile participants determined on an event-by-event basis was used to characterize the collision centrality. The scaled variance of the multiplicity distribution obtained in the forward rapidity region ($1.1 <y_{c.m.} <2.6$) shows a significant increase towards peripheral collisions. The results are similar for negatively and positively charged particles and about 50% larger for all charged particles. String-hadronic models of nuclear reactions without the fusion process do not reproduce the rise of fluctuations from central towards peripheral collisions. The measured centrality dependence can be reproduced in superposition models with the assumption of contributions from target participants to particle production in the forward hemisphere or in string models with fusion.

Electrochemical deposition of PbTe onto n-Si(1 0 0) wafers

Electrochemical deposition of PbTe from 50 mM Pb(NO 3) 2 + 1 mM TeO 2 + 0.1 M HNO 3 solution onto n-Si(1 0 0) wafers was studied using cyclic voltammetry (CV), chronoamperometry, ex situ SEM, XRD and EDX. Electrochemical behavior of n-Si(1 0 0) electrode in electrolytes 50 mM Pb(NO 3) 2 + 0.1 M HNO 3 and 1 mM TeO 2 + 0.1 M HNO 3 was also studied. No underpotential deposition (UPD) of Pb and Te onto n-Si was observed in the investigated systems indicating weak Pb–Si and Te–Si interactions. Deposition of Pb and Te on n-Si occurred with overvoltage via 3D island growth. Electrosynthesis of PbTe (NaCl-like structure, a = 0.650 nm) takes place due to codeposition of Pb and Te at potentials E > E Pb 2+ /Pb 0 (lead UPD onto tellurium). Cathodic deposition of PbTe onto n-Si(1 0 0) is irreversible – there is no anodic current in the CV curve. Oxidation of PbTe on n-Si is observed only under illumination, when photoelectrons and photoholes are generated in silicon substrate.

Relativistic heavy-atom effects on heavy-atom nuclear shieldings

The principal relativistic heavy-atom effects on the nuclear magnetic resonance (NMR) shielding tensor of the heavy atom itself (HAHA effects) are calculated using ab initio methods at the level of the Breit-Pauli Hamiltonian. This is the first systematic study of the main HAHA effects on nuclear shielding and chemical shift by perturbational relativistic approach. The dependence of the HAHA effects on the chemical environment of the heavy atom is investigated for the closed-shell X(2+), X(4+), XH(2), and XH(3) (-) (X=Si-Pb) as well as X(3+), XH(3), and XF(3) (X=P-Bi) systems. Fully relativistic Dirac-Hartree-Fock calculations are carried out for comparison. It is necessary in the Breit-Pauli approach to include the second-order magnetic-field-dependent spin-orbit (SO) shielding contribution as it is the larger SO term in XH(3) (-), XH(3), and XF(3), and is equally large in XH(2) as the conventional, third-order field-independent spin-orbit contribution. Considering the chemical shift, the third-order SO mechanism contributes two-thirds of the difference of approximately 1500 ppm between BiH(3) and BiF(3). The second-order SO mechanism and the numerically largest relativistic effect, which arises from the cross-term contribution of the Fermi contact hyperfine interaction and the relativistically modified spin-Zeeman interaction (FC/SZ-KE), are isotropic and practically independent of electron correlation effects as well as the chemical environment of the heavy atom. The third-order SO terms depend on these factors and contribute both to heavy-atom shielding anisotropy and NMR chemical shifts. While a qualitative picture of heavy-atom chemical shifts is already obtained at the nonrelativistic level of theory, reliable shifts may be expected after including the third-order SO contributions only, especially when calculations are carried out at correlated level. The FC/SZ-KE contribution to shielding is almost completely produced in the s orbitals of the heavy atom, with values diminishing with the principal quantum number. The relative contributions converge to universal fractions for the core and subvalence ns shells. The valence shell contribution is negligible, which explains the HAHA characteristics of the FC/SZ-KE term. Although the nonrelativistic theory gives correct chemical shift trends in present systems, the third-order SO-I terms are necessary for more reliable predictions. All of the presently considered relativistic corrections provide significant HAHA contributions to absolute shielding in heavy atoms.

An extension of the computer program for dynamical calculations of RHEED intensity oscillations. Heterostructures

A practical computing algorithm working in real time has been developed for calculations of the reflection high-energy electron diffraction from the molecular beam epitaxy growing surface. The calculations are based on a dynamical diffraction theory in which the electrons are scattered on a potential, which is periodic in the direction perpendicular to the surface. New version program summary Title of program:RHEED_v2 Catalogue identifier:ADUY_v1_1 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADUY_v1_1 Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Catalogue identifier of previous version:ADUY Authors of the original program:A. Daniluk Does the new version supersede the original program:Yes Computer for which the new version is designed and others on which it has been tested: Pentium-based PC Operating systems or monitors under which the new version has been tested: Windows 9x, XP, NT, Linux Programming language used:C++ Memory required to execute with typical data:more than 1 MB Number of bits in a word:64 bits Number of processors used:1 Number of bytes in distributed program, including test data, etc.:1 074 131 No. of lines in distributed program, including test data, etc.:3408 Distribution format:tar.gz Nature of physical problem: Reflection high-energy electron diffraction (RHEED) is a very useful technique for studying the growth and the surface analysis of thin epitaxial structures prepared by the molecular beam epitaxy (MBE). RHEED rocking curves recorded from heteroepitaxial layers are used for the non-destructive evaluation of epilayer thickness and composition with a high degree of accuracy. Rocking curves from such heterostructures are often very complex because the thickness fringes from every layer beat together. Simulations based on dynamical diffraction theory are generally used to interpret the rocking curves of such structures from which very small changes in thickness and composition can be obtained. Rocking curves are also used to determine the level of strain and its relaxation mechanism in a lattice-mismatched system. Method of solution: The new version of the program retains the design and structure of the previous one [A. Daniluk, Comput. Phys. Comm. 166 (2005) 123. [1]]. Reasons for the new version: Responding to the user feedback we presented an extension of the RHEED program that enables computing the crystalline potentials for epitaxial heterostructures and corresponding values of the amplitude of the RHEED intensity oscillations. Summary of revisions: (1) In this paper we show how the dynamical approach may be applied to creation of a practical computing algorithm to calculate of the intensity of the specularly reflected RHEED beam during MBE growth of Pb on Si(111). The structural properties of the Pb Si interface have been [Display omitted] [b] [Display omitted] [Display omitted] meticulously studied by Howes and co-workers [P.B. Howes, K.A. Edwards, D.J. Hughes, J.E. Macdonald, T. Hibma, T. Bootsma, M.A. James, Surf. Sci. Lett. 331 (1995) 646; K.A. Edwards, P.B. Howes, J.E. Macdonald, T. Hibma, T. Bootsma, M.A. James, Surf. Sci. 424 (1999) 169. [2,3]], and Lucas and Loretto [C.A. Lucas, D. Loretto, Surf. Sci. Lett. 344 (1995) 1219. [4]] (X-ray diffraction). The new version of the RHEED program has the same design as the previous one [A. Daniluk, Comput. Phys. Comm. 166 (2005) 123. [1]]. To simulate the structural variations of whole crystalline heterostructure along the surface normal direction the substrate and layers are divided into an assembly of n atomic layers. Each of these layers is further divided into an assembly of i thin slices parallel to the surface and each slice is assumed to have a constant potential normal to the surface as shown in Fig. 1. The Fourier component of the scattering potential of the whole crystalline heterostructure can be determined as a sum of contributions coming from all thin slices of n individual atomic layers. To carry out one-dimensional calculations we used the self-consistent thicknessZi_Substrate(), thicknessZi_Layers(), thicknessZn_Substrate(), thicknessZn_Layers(), crystPotUgSubstrate() and crystPotUgLayers() functions. Fig. 2 presents the crystalline potentials (real part) calculated for some Pb layers on a Si(111) substrate at 70 K. Fig. 3 shows a dynamically calculated one-beam rocking curve for Pb/Si(111). [Display omitted] [Display omitted] (2) The presented algorithm is a generalization of the previous one. By attributing 0 to the numberOfLayers and NLayers constant parameters (Fig. 4) and removing appropriate functions from the main program (Fig. 5), we obtain the same results as in the case of monocrystal [A. Daniluk, Comput. Phys. Comm. 166 (2005) 123. [1]]. Typical running time: The typical running time is machine and user-parameters dependent. Unusual features of the program: The program is presented in the form of a basic unit RHEED_v2.cpp. It is not tied to any specific hardware and systems software platform, and could be compiled using C++ compilers, including C++ Builder, VC++ and g++.

Impact of interface relaxation on the nanoscale corrugation in Pb/Si(1 1 1) islands

The nanoscale hexagonal pattern observed in scanning tunneling microscopy (STM) for 3-layer and 4-layer Pb islands on Si(1 1 1) is studied theoretically. We found that besides thickness the atomic rearrangement at the Pb/Si interface plays an important role in determining the STM patterns. Electronic structures of the Pb film on Si(1 1 1) obtained from fully relaxed and unrelaxed Pb films are qualitatively different. Simulated STM images for Pb films with different stacking also show that the corrugation patterns are sensitive to the buried Pb–Si interfacial structure.

Thermomigration in SnPb composite flip chip solder joints

We have used composite flip chip solder joints to study thermomigration. The composite solder joint has 97Pb3Sn on the Si side and eutectic 37Pb63Sn on the substrate side. Redistribution of Sn and Pb occurs in thermomigration, and we found that Sn has moved to the Si side (the hot end) and Pb to the substrate side (the cold end). We estimate the driving force that a temperature gradient of 1000°C∕cm or a temperature difference of 10°C across a solder joint of 100μm in diameter is sufficient to induce the thermomigration.

Rheo-diecasting of Al–Si–Pb immiscible alloys

Immiscible Al–Si–Pb alloys are successfully produced using the newly developed rheo-diecasting process (RDC). The microstructure of RDC Al–Si–Pb alloys is characterised by spherical and fine Pb particles dispersed uniformly in the Al–Si alloy matrix. The discussion focuses on the mechanism of the RDC process for immiscible alloys.

Formation of nanocrystalline and amorphous phase of Al–Pb–Si–Sn–Cu powder during mechanical alloying

Al–15%Pb–4%Si–1%Sn–1.5%Cu alloys (mass fraction, %) were prepared by mechanical alloying (MA). Phase transformation and microstructure characteristics of the alloy powders were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the nanocrystalline supersaturated solid solutions and amorphous phase in the powders are obtained during MA. The effect of ball milling is more evident to lead than to aluminum. During MA, the mixture powders are firstly fined, alloyed, nanocrystallized and then the nanocrystalline partly transforms to amorphous phase. A thermodynamic model is developed based on semi-experimental theory of Miedema to calculate the driving force for phase evolution. The thermodynamic analysis shows that there is no chemical driving force to form a crystalline solid solution from the elemental components. But for the amorphous phase, the Gibbs free energy is higher than 0 for the alloy with lead content in the ranges of 0–86.8 at.% and 98.4–100 at.% and lower than 0 in range of 86.8–98.4 at.%. For the Al–2.25 at.%Pb (Al–15%Pb, mass fraction, %), the driving force for formation of amorphization and nanocrystalline supersaturated solid solutions are provided not by the negative heat of mixing but by mechanical work.

Order–disorder phase transition on the Pb-adsorbed Si(1 1 0) surface

A new reversible temperature-dependent phase transition on Pb/Si(1 1 0) was found by photoemission spectroscopy and low energy electron diffraction. The room temperature 1 × 1 surface was found to transit into low temperature 4 × 2 with a transition temperature of roughly 200 K. The phase transition does not induce a temperature-dependent variation in both the energy bands and the Si 2p and Pb 5d spectra. This indicates that the phase transition is close to an order–disorder one.

Influence of high current pulsed electron beam treatment on the tribological properties of Al–Si–Pb alloy

The tribological properties of Al–Si–Pb alloys irradiated by high current electron beam (HCPEB) were investigated under dry sliding conditions using a pin-on-disc type wear testing machine. The HCPEB parameters used to treat the samples are: electron energy: 21–25 keV; energy density: 1.5–2.5 J/cm 2; pulse duration: 1.5 μs; number of pulses: 15. The results show that the microstructure and mechanical properties can be greatly improved. These factors contribute to great increase in wear resistance of HCPEB irradiated Al–Si–Pb alloys at 2.0 J/cm 2 and 2.5 J/cm 2. Optical observation and X-ray photoelectron spectroscopy (XPS) analysis reveal that the low wear rate and lowest level in coefficient of friction at high load levels for Al–Si–Pb alloys HCPEB irradiated with more than 1.5 J/cm 2 are due to a film of lubricant covering almost the entire worn surface. This film is a mixture of different constituents containing Al, Fe, Si, O and Pb, in which Pb exists in the form of Pb 4SiO 6. With the applied load increased, the dominant wear modes exhibit successively the oxidative wear, film spalling, and adhesive wear.

System size dependence of hadron production in A+A collisions at 40 A GeV beam energy

Rapidity and transverse mass spectra of charged kaons and negative pions measured in C+C, Si+Si and minimum bias Pb+Pb collisions at 40 A GeV are reported. A monotonous increase of the inverse slope parameter with system size is observed. The ⟨K+⟩/⟨K−⟩ ratio rises with ⟨NW⟩ from the light ion collisions to peripheral Pb+Pb and stays constant from there on. A steep rise of the ⟨K+⟩/⟨π+⟩ ratio is observed for the small systems with the ratio in Si+Si collisions being slightly higher than in peripheral Pb+Pb collisions. The measurement of the ⟨K−⟩/⟨π−⟩ ratio in Si+Si collisions is significantly higher than the ratio in peripheral Pb+Pb collisions; it is rather close to the observed ⟨K−⟩/⟨π−⟩ ratio in central Pb+Pb collisions.

The optimum elastic wave band gaps in three dimensional phononic crystals with local resonance

Using multiple-scattering theory, we investigate the optimization of the elastic wave band gaps of three dimensional three-component phononic crystals with local resonance. The optimum gaps of two systems including Au spheres coated with Pb embedded in Si matrix and Pb spheres coated with plastic embedded in Si matrix are obtained by tuning the ratio of the inner and the outer radii of the coating layers. It also shows that the elastic wave band gaps for the two systems versus the filling fractions and the radius ratio display different features.

Growth of highly (100) oriented lead zirconate titanate films on silicon and glass substrates using lanthanum nitrate as a buffer layer

Highly oriented lead zirconate titanate [Pb(Zr,Ti)O3; PZT] thin films were deposited on Pt∕Ti∕SiO2∕Si and glass substrates by the sol-gel method using lanthanum nitrate as a buffer layer. When the lanthanum nitrate buffer layer was annealed at temperatures between 450 and 550°C, the PZT layer coated onto this buffer layer showed strong (100) orientation. The film deposited on the buffer layer had this orientation, regardless of the other deposition conditions, such as the pyrolysis temperature, pyrolysis time, annealing temperature, and heating rate. The lanthanum nitrate buffer layer also acted as a very effective diffusion barrier against Pb–Si interdiffusion, thus allowing for the direct deposition of PZT films on Si, SiO2∕Si, and glass substrates. Using this buffer layer, highly oriented PZT film was fabricated stably and reproducibly, regardless of substrate material and the coating conditions. The nature of the lanthanum nitrate buffer layer and its role in the growth of the highly (100) oriented PZT films were investigated.

Electronic structure of a two-dimensional alloy: Sn–Pb–Si on Si(111)

Scanning tunnelling microscopy and core level photoemission spectroscopy experimentsdemonstrate that an ordered surface reconstruction of Pb, Sn and Si adatoms(1/3 monolayer overall coverage) can be formed with symmetry on the Si(111) substrate. Pb, Sn and Si atomically intermix, occupyingT4 sites as in the case ofthe well known pure 1/3 ML Sn or Pb reconstructions. The electronic structure of this surface, investigated withscanning tunnelling spectroscopy and valence band photoemission, shows asemiconducting behaviour (0.4 eV bandgap), and two surface bands centred at ± 0.7 eV with respect to the Fermi level. These features are significantly different from those ofthe electronic structure of the binary Pb–Si/Si(111) and Sn–Si/Si(111) 2D alloys.This occurrence demonstrates that tailoring the electronic structure of a surfacealloy is viable just by mixing adatoms with the same valence and different sizes.

Spray forming of Al–Si–Pb alloys and their wear characteristics

A modified spray forming process is described to synthesize liquid immiscible alloys based on Al–Si–Pb system. The atomization of the melt was carried out at a gas pressure of 1.0 MPa and the nozzle to substrate distance was varied from 0.35 to 0.55 m. The microstructure and wear characteristics of the spray-deposits were investigated. The results invariably exhibited an equiaxed grain morphology of the primary α-phase with variation in grain size from 10 to 25 μm in addition to a uniform dispersion of ultra-fine particles of lead and globular silicon particles in Al-matrix. The size of Si particles varied from 0.5 to 5 μm and that of Pb particles from 0.1 to 25 μm with variation in deposition distance. The microstructural variation was also observed in different regions of the deposit. The atomized powder particles revealed two different types of microstructures, the one showing a cellular-dendritic morphology of the primary α-phase whereas the other exhibited fine spherical Al-particles dispersed in the matrix of Pb-rich phase. The wear rate and the coefficient of friction of Al–Si–Pb alloy were observed to be lower than that of Al–Si alloy at loads varying from 10 to 90 N at a sliding velocity of 1.0 ms−1. The possible reasons for this behaviour are discussed in the light of microstructural features of the alloy and the nature of the worn out surfaces of the wear test specimens.

The influence of Pb on the friction and wear behavior of Al–Si–Pb alloys

The tribological behavior of hot-extruded Al–Si–Pb alloys have been investigated under dry sliding at various temperatures. It is seen that Al–Si–Pb alloys containing 20 and 25 wt.% lead have better characteristics of friction and wear than those in the base alloy without lead at room temperature. Under high applied loads, the stable wear rate with the lowest level of coefficient of friction is found to be controlled by the formation of a tribo-layer covering most of the worn surfaces. The tribo-layer consists of compounds including Al2O3, Pb4SiO6 and Fe2O3. A tribo-layer consisting of mixture of Pb4Al2(SiO3)7, SiO2, Al2O3 and Fe2O3 is also found to be formed on the worn surface of an (Al–Si–Cu)–15Pb alloy at 100 °C, but no tribo-layer was observed on the worn surface of this alloy at 200 °C.

Pb/Si(1 0 0)2 × 1 surface studied by high resolution core-level photoemission spectroscopy

A Pb/Si(1 0 0)2 × 1 surface has been studied at T=120 K by high resolution core-level spectroscopy using a third generation synchrotron light source. A strong component (S), could be identified at a binding energy (BE) of −0.20 ± 0.01 eV with respect to the bulk peak. This is indicative of some reorganization of the topmost silicon atoms after Pb adsorption with the formation of Si–Pb bonds and Pb–Pb symmetric dimers. However, this shift is in the opposite site respect to the bulk peak, as compared with Sb/Si(1 0 0)2 × 1, thus suggesting the interplay of final states screening effects and charge transfer in the core-level position. Another component (smaller than S) is present at +0.15 eV BE, and might be due to contribution from subsurface silicon atoms, as on the clean surface, and/or to surface or interface defects. On the Si 2p core-level taken in bulk sensitive mode, we found a very narrow bulk component with a total full width half maximum (FWHM) of 160 meV at T=120 K indicative of an unreacted Si–Pb interface on top of an ideal Si bulk termination. The Pb 5d core-level spectrum is well represented by one doublet, thus suggesting that each Pb atom is adsorbed in a unique environment, i.e., there is no multisite adsorption.

Dry sliding wear behavior of hot extruded Al–Si–Pb alloys in the temperature range 25–200 °C

The wear behavior of hot extruded Al–Si–Pb alloys was investigated under dry conditions in a temperature range 25–200 °C using a pin-on-disc type wear testing machine. The results show that the microstructure and mechanical properties can be greatly improved and porosity can be significantly decreased by hot extrusion. These factors contribute to great increase in wear resistance of hot extruded Al–Si–Pb alloys. Optical observation and X-ray photoelectron spectroscopy (XPS) analysis reveal that at room temperature the almost constant wear rate and lowest level in coefficient of friction at high load levels for Al–Si–Pb alloys with more than 15 wt.% lead are due to a film of lubricant covering almost the entire worn surface. This film is a mixture of different constituents containing Al, Fe, Si, O and Pb, in which Pb exists in the form of Pb 4SiO 6. In the case of Al–Pb–Si alloy with 15% lead worn at 100 °C, X-ray analysis reveal that constant wear rate and lowest level in coefficient of friction are caused by a film of lubricant with a main constituent of Pb 4Al 2(SiO 3) 7. At 200 °C, no film of lubricant was formed on the worn surface of Al–Pb–Si alloy with 15% lead.

Role of hydrogen in stabilizing new hydride phases or altering old ones

The role of hydrogen impurities in many of the more active metals in the formation of ternary compounds has often not been considered or recognized when these reactants do not contain significant metal hydride phases. This is a particular problem because most phase identifications or structure determinations have traditionally depended on X-ray diffraction means. Extended investigations have established that the alkaline-earth metals, plus some other divalent metals, have evidently often contributed substantial amounts of hydrogen, and this feature has led to numerous erroneous reports of supposed binary phases that form with the main-group (p) elements. Methods of detection and some ready means to avoid these problems will be described. The particularly serious examples that occur among phases reported with β-Yb5Sb3, Mn5Si3, and Cr5B3-type binary structures for pnictogen (As–Bi) and tetrel (Si–Pb) elements are detailed for 32 examples. Fluoride has proven to be useful as a stand-in for hydride in a variety of these systems.