Pd Si Alloys Research Articles

Short‐Range Atomic Topology of Ab Initio Generated Amorphous PdSi Alloys

AbstractSince the pioneering efforts of Duwez and co‐workers in 1965, when a solid amorphous phase of palladium‐silicon (a‐PdSi) was obtained in the vicinity of the eutectic concentration, much work has been done. However, some points related to the atomic structures remain to be systematized. In this work, 8 amorphous Pd100‐cSic alloys (c = 2.5, 5, 10, 13.34, 15, 17.5, 20, and 22 at%) are generated by molecular dynamics ab initio simulations; the short‐range structure is analyzed using several correlation functions, like Pair Distribution Functions, reduced Pair Distribution Functions, Plane Angle Distribution Functions. Other related properties, like nearest‐neighbors and some Frank–Kasper polyhedra are reported. The generated samples correctly reproduce the scarce experimental pair correlation functions that are reported in the literature. An unexpected outcome is the appearance of structural changes in the neighborhood of Pd86.66Si13.34.

Molecular dynamics simulations of glass formation, structural evolution and diffusivity of the Pd-Si alloys during the rapid solidification process

The atomic structure, glass formation process and diffusion mechanism of Pd100-xSix (x = 10, 20, 30, 40 and 50) alloys during rapid solidification have been investigated by molecular dynamics simulations using embedded atom method potentials. The structure factors, total (or Pd90Si10) and partial pair distribution functions calculated for Pd80Si20 bulk metallic glass are in good agreement with the experimental/other data. Bond angle distribution function, Honeycutt-Andersen index and Voronoi tessellation analysis have revealed that the increasing Si amount caused a decrease in the number of icosahedral-like clusters and an increase in the number of crystal-like clusters. The majority of icosahedral-like clusters in the systems are mostly composed of Pd-centered clusters, suggesting that Pd has an effective role in the glass formation process in Pd-Si systems. It has been observed that the mobility of Si atoms decreases in environments with more Pd atoms. The critical temperature and T0 temperature for Pd-Si liquids have been determined from the self-diffusion coefficients using the mode-coupling theory and the Vogel-Fulcher-Tammann law, respectively. The present findings show us that the Pd70Si30 compound has a critical importance in terms of the glass formation process, which is consistent with experimental observations. We hope that the results will contribute to understanding the Si effect on the atomic local structure of Pd-Si systems and will encourage research on many properties of these systems, such as their mechanical properties.

Pressure dependent evolution of microstructures in Pd80Si20 bulk metallic glass

The effect of the pressure on the local microstructure in the amorphous Pd80Si20 alloy was investigated by coordination analyses and structural calculations using molecular dynamics simulations. We observed that the glass transition temperature increases monotonically as the pressure increases. Also, it was seen that the Pd atoms composing the face centered cubic (fcc)- and icosahedral-like order are the origin of the complexity of the local structure while the short-range order around Si atoms is characterized by a tri-capped trigonal prism. We found that the topological structure and local environment of the system which consists of icosahedral-, hexagonal- and fcc-like structural units, change significantly with the effect of pressure. We revealed that the short-range order of the system develops with pressure due to the increase of icosahedral and some defected icosahedral polyhedra, and Pd atoms are responsible for the formation of the amorphous state of the system.

Universal nature of dynamic heterogeneity in glass-forming liquids: A comparative study of metallic and polymeric glass-forming liquids.

Glass-formation is a ubiquitous phenomenon that is often observed in a broad class of materials ranging from biological matter to commonly encountered synthetic polymer, as well as metallic and inorganic glass-forming (GF) materials. Despite the many regularities in the dynamical properties of GF materials, the structural origin of the universal dynamical properties of these materials has not yet been identified. Recent simulations of coarse-grained polymeric GF liquids have indicated the coexistence of clusters of mobile and immobile particles that appear to be directly linked, respectively, to the rate of molecular diffusion and structural relaxation. The present work examines the extent to which these distinct types of "dynamic heterogeneity" (DH) arise in metallic GF liquids (Cu-Zr, Ni-Nb, and Pd-Si alloys) having a vastly different molecular structure and chemistry. We first identified mobile and immobile particles and their transient clusters and found the DH in the metallic alloys to be remarkably similar in form to polymeric GF liquids, confirming the "universality" of the DH phenomenon. Furthermore, the lifetime of the mobile particle clusters was found to be directly related to the rate of diffusion in these materials, while the lifetime of immobile particles was found to be proportional to the structural relaxation time, providing some insight into the origin of decoupling in GF liquids. An examination of particles having a locally preferred atomic packing, and clusters of such particles, suggests that there is no one-to-one relation between these populations of particles so that an understanding of the origin of DH in terms of static fluid structure remains elusive.

Local atomic structures of amorphous Pd80Si20 alloys and their configuration heredity in the rapid solidification

ABSTRACTThe local atomic structures of amorphous Pd80Si20 alloys and their configuration heredity in the rapid solidification are investigated by a molecular dynamics simulation with the help of cluster-type index method based on Honeycutt–Anderson bond-type index and an inversely tracking technique of atomic trajectories. Their short-range orders are found to be various Kasper clusters as well as their distorted configurations, and among which (10 2/1441 8/1551) bi-capped square Archimedean anti-prism (BSAP) clusters are dominated, e.g. Si-centred Pd10Si1 clusters. These Kasper clusters mainly exist in the form of isolated basic clusters. Few medium-range orders can be detected, especially for Si-centred Kasper clusters. Similarly to icosahedrons of Cu–Zr amorphous alloys, their sustainable configuration heredity also occurs firstly in the super-cooled liquid region, and BSAP clusters have higher onset temperature Tonset and bigger descendible fraction F than other Kasper clusters in the rapid solidification of Pd80Si20 alloys.

Molecular-dynamics simulations of binary Pd-Si metal alloys: Glass formation, crystallisation and cluster properties

Using Quantum Sutton-Chen potentials, Molecular Dynamics was used to obtain an atomistic description of melting, glass formation and crystallisation processes for palladium metal, metallic silicon and their alloys, from PdSi to Pd5Si. Glass formation and crystallisation were investigated using different cooling rates, by analysing the radial distribution function, enthalpy, density, volume, and diffusion coefficient. As expected, glass forms at high cooling rates, while crystallisation occurs at slower rates. The glass transition temperature was estimated from the Wendt-Abraham parameter vs temperature curves and the splitting of peaks in the radial distribution function. The effect of the Pd/Si ratio on the glass transition was studied by analysing the volume change during cooling of the Pd-Si alloys. The glass forming ability was found to increase with increasing Pd concentration in the alloys. Overall, our results reveal that Quantum Sutton-Chen potentials allow good descriptions of the melting, glass transition, and crystallisation during the heating and cooling processes.

Effect of copper addition on the glass forming ability in Pd-Si binary amorphous alloying system

The effect of copper addition on the glass forming ability in Pd-Si amorphous alloys was systematically studied by X-ray diffraction and differential scanning calorimeter with a binary alloy series of Pd81+xSi19-x (x=0, 1, 1.5, 2, 2.5, 3, 4) and a ternary alloy series of Pd83.5-xCuxSi16.5 (x=0, 2, 4, 6, 8, 10, 12). It has been found that the critical size of the Pd-Si alloys could be improved significantly from 8mm to 15mm and ΔTx could be enhanced from 18.7 K to 54.7 K with the addition of copper. The determined activation energy demonstrates that Cu additions improve the thermal stability of the system significantly. The different results of the measured criteria to evaluate the GFA for the Pd-Si and the Pd-Cu-Si alloys may be linked to a different crystallization behavior. The enhanced GFA is linked to a more thermodynamically stable liquid and a reduction of atomic diffusion in the ternary alloy by Cu addition.

Amorphous phase stability and the interplay between electronic structure and topology

It is well-known that the stability of Pd-Si amorphous alloys can be improved substantially by minor additions of alloying elements such as Cu and Ag. Such improvement in stability is explained herein, whereby microscopic models based on efficient atomic packing and electronic structure were applied to the results of first principles simulations of Pd82Si18, Pd77.5Si16.5Cu8 and Pd75Si15Cu7Ag3 metallic glasses. It was revealed that while the atomic packing model fails to unequivocally explain the stabilizing effect of the binary Pd-Si alloy due to minor additions of Cu and/or Ag, the contribution of electronic states with lower energies to the stability of the amorphous structure is increased markedly. Further, the observed enhancement in the compressive ductility as a result of the addition of Ag to the Pd77.5Si16.5Cu8 alloy can be correlated to the combined effects of an increased heterogeneity in the local topology, weakened covalency and, hence, reduced directionality of Pd-Pd bonds as well as enhanced metallicity in the Pd75Si15Cu7Ag3 amorphous alloy. The analysis was further expanded to amorphous alloys where their characteristic binary prototypes are synthesised from late transition metals and non-metals as well as those comprising early transition metals and late transition metals. The findings shed light on the effects of minor alloying on the cooperative and competitive relationship between the topological and electronic structure of amorphous alloys.

Formation and Mechanical Properties of Pd-Si Binary Bulk Metallic Glasses

Glassy spherical samples in the diameters up to 10 mm were produced in a binary Pd-Si alloy system. These Pd-Si bulk metallic glasses (BMGs) combine high strength of about 1600 MPa and superplasticity of over 70% together. In addition to abundant micrometer-scale shear bands, 10–20 nanometer-sized shear bands were also observed on the side surface of the deformed sample. The excellent ductility shown by the Pd-Si BMGs is suggested to arise from the nanoscale structural inhomogeneity.

The viscosity of eutectic Pd-Si alloys

The viscosity of eutectic PdSi alloys has been measured by different techniques. The conventional oscillating cup viscosimetry was used in the ground-based laboratory, while the oscillating drop technique was applied in a microgravity experiment. Different working equations were utilised to derive the viscosity from the damping of the oscillations, and the results thus obtained were compared to each other and available literature data. Whereas systematic deviations exist, an overall satisfactory agreement was found.

The viscosity of eutectic Pd – Si alloys – a case study

Abstract The viscosity of eutectic PdSi alloys has been measured using different techniques. Conventional oscillating cup viscosimetry was used in the ground-based laboratory, while the oscillating drop technique was applied in a microgravity experiment. Different working equations were utilised to derive the viscosity from the damping of the oscillations, and the results thus obtained were compared to each other and available literature data. An overall satisfactory agreement was found, while, at lower temperatures, an increasing difference between the viscosities obtained using the two models was observed.

Short Order and Hydrogen Transport in Amorphous Palladium Materials

Molecular dynamics simulation was used for investigating hydrogen migration in Pd-Si alloy at a temperature Т = 300 K. The strong affect of hydrogen dynamics and its defects creation to structure of palladium matrix is stated. The partial radial distribution function calculation for silicon specifies a preferable arrangement of silicon atoms relative to each other in the second coordination sphere. Model calculations have shown that not only silicon atoms can affect hydrogen mobility. Hydrogen itself also can significantly change the diffusion of the other components in the alloy.

Microstructural features of phase transformation in a binary Pd–Si metallic glass

Glassy ribbons of Pd–Si alloys were prepared by a combination of melt spinning and flux treatment. The crystallization behaviour of a Pd81Si19 glassy alloy was studied through isothermal annealing at temperatures ranging lower than the glass-transition temperature T g to around the onset of crystallization. The evolution of microstructures arising from isothermal annealing was investigated by X-ray diffraction (XRD) and (high-resolution) transmission electron microscopy ((HR)TEM). XRD spectra showed that, after the sample was annealed at a sub-T g temperature, its first diffraction peak was split into two overlapping broad peaks. TEM analysis revealed the formation of a spherical, particle-like glassy phase embedded in the glassy matrix together with a finely connected network morphology within both. Combining these observations with compositional analysis suggested that phase separation had taken place during sub-T g annealing. When the glassy alloy was annealed at temperatures higher than T g, nanocrystalline structures, composed of Pd3Si and Pd phases plus a Pd9Si2 phase with a lamellar structure, was formed.

Study of Pd–Si alloys density by means of penetrating γ-radiation

Temperature dependences of the density of Pd–Si alloys containing from 11 up to 33 at.% Si have been studied by means of the absolute method of penetrating γ- radiation in an interval from room temperature up to 1900 K. The accuracy of the measurement is ±0.6% and its resolution is within 0.2–0.3%. Liquidus and solidus temperatures of the investigated alloys ( T L and T S, respectively), density changes during melting ( Δρ) and pre-crystallization overcooling (Δ T) have been determined on the basis of the obtained data. T L and T S values are in good agreement with the known phase diagram of the system. Density and molar volume isotherms and also Δ ρ and Δ T concentration dependences have been plotted and demonstrate some anomalies near the stoichiometric composition of Pd 3Si and in the concentration interval of easy amorphization of Pd–Si alloys.

The bulk properties of Pd-Si alloys at temperatures from room temperature to 1600 °C

The absolute method of penetrating gamma radiation is used to measure the temperature dependences of density of Pd-Si alloys containing 11 to 33 at.% Si in the range from room temperature to 1600 °C. The resultant data are used for determining the liquidus and solidus temperatures of investigated alloys, the variation of their density in melting, and pre-crystallization cooling which corresponds to the cooling rate of 2 °C/min.

Pd–Si Binary Bulk Metallic Glass Prepared at Low Cooling Rate

We report that binary bulk metallic glasses can be made up to 6 mm indiameter in a Pd–Si alloy system by air cooling at slow cooling rate(about 8 K/s). The high stability of the undercooled liquid and thelarge glass-forming ability (GFA) of the binary alloy are contributedto the removing of heterogeneous impurities in the alloy melt byemploying the fluxing technique. It has been found that decreasingcooling rate can increase the supercooled liquid region and thermalstability of the glassy alloy. After fluxing, a wider supercooled liquidregion (ΔT = 58 K) and higher glass-forming ability have beenobtained in a Pd81Si19 binary glassy alloy prepared byslow cooling rate.

Electron diffraction and high-resolution electron microscopy study of an amorphousPd82Si18alloy with nanoscale phase separation

A structure model of nanoscale phase separation has been proposed for a structure of an amorphous Pd-Si alloy with the eutectic composition on the basis of electron diffraction and high-resolution electron microscopy (HREM). A sputter-deposited amorphous ${\mathrm{Pd}}_{82}{\mathrm{Si}}_{18}$ alloy film with a thickness of about 10 nm was specially used in order to minimize the effect of multiple scattering. A local atomic ordering with sizes of 1--2 nm was observed by HREM, similar to that observed in liquid-quenched ribbon specimens. From HREM images and nanodiffraction analysis, the atomic structure of the ordering regions was determined to be of the fcc-Pd type. Atomic pair distribution function (PDF) analysis for the thin-film alloy was performed by precise measurement of haloelectron-diffraction intensity using the imaging-plate technique. In the diffraction intensity analysis, the inelastic part of intensity was removed by an intensity correction using electron energy-loss spectroscopy. In order to explain the results of HREM, nanodiffraction, and PDF studies comprehensively, a nanoscale phase-separation structure model has been constructed with the help of reverse Monte Carlo calculation. A structure model with fcc-cluster regions embedded in a dense-random-packing structure of Pd and Si was finally obtained. The model proposed here is thought to be typical for amorphous alloys with near-eutectic compositions cooled under limited cooling rates.

Structural modeling of Pd–Si and Fe–Zr–B amorphous alloys based on the microphase separation model

“Microphase separation” structure models were constructed for amorphous Pd 82Si 18 and Fe 90Zr 7B 3 alloys using the reverse Monte Carlo χ 2 calculation. The input experimental data were atomic pair distributions obtained from these alloys by electron diffraction. The calculations were based on two initial structure models: (1) with local α-phase atomic clusters embedded in a dense-random-packing (DRP)-matrix and (2) with an entirely DRP structure. It was concluded that fcc-clusters as small as 1–2 nm in size with a volume fraction less than 20% and bcc-clusters less than 1 nm with a larger volume fraction (about 40%) are distributed in the DRP-like matrices in the Pd–Si and Fe–Zr–B alloys, respectively. The matrix structure in the Pd–Si alloy is mainly composed of the trigonal prisms surrounding Si atoms.

Formation of a palladium carbide with a palladium-silicide-like structure in fullerene-C 60/Pd multilayers

We present structural studies of C 60/Pd multilayer films. A new phase of a layered structure with a sixfold (or threefold) axis, which is similar to the metastable phase in Pd–Si alloys, has been observed by means of transmission electron microscope and X-ray diffraction in the C 60/Pd multilayer films after annealed at 470 K 60 min. This new phase, which is believed to be one metastable phase of Pd–C, is a hexagonal structure with the lattice constant a = 0.438 nm. The reciprocal lattice points become continuous spikes in the direction c∗ of the reciprocal lattice, which indicates the Pd–C phase consists of layers.

Amorphous palladium-silicon alloys for the oxidation of formic acid and formaldehyde. A voltammetric investigation

The electrocatalytic oxidation of formic acid and formaldehyde on Pd and on amorphous Pd(Si) was studied by cyclic voltammetry and the results compared with the literature for similar systems. The oxidation of HCOOH on Pd occurs through direct catalytic dehydrogenation via (:C(OH)2)ads while on Pd(Si) this intermediate does not appear to be formed. This is a consequence of the presence of inert Si on the surface that diminishes the probability of adjacent free sites. At high HCOOH concentrations, that intermediate undergoes dehydration on the Pd surface and COads oxidation peak is observed. For HCHO, the oxidation mechanism on both electrode materials appears similar to that previously proposed for Pt. However, the oxides formed on the amorphous Pd(Si) alloy are more reactive than those on Pd thus affecting the overall kinetics of the process for both organic molecules, a fact revealed by the increase in anodic currents observed in the voltammograms.