Noble Metal Systems Research Articles

Measuring the effect of substrate and component finishes on the reliability of isotropic electrically conductive adhesive joints

PurposeAs the transition away from lead‐containing solders gathers momentum, isotropic conducting adhesives (ICAs) are being considered as possible replacements for conventional SnPb solder in a range of applications. Consequently, the reliability of ICA joints is under scrutiny. The purpose of this paper is to report the effect of printed circuit board (PCB) and component finishes on the reliability of ICA joints.Design/methodology/approachPrevious work by the authors identified a suitable test regime to generate relevant reliability data. In the present work, those tests are employed to investigate whether the finishes on the components and/or PCBs have any effect on the reliability of the ICA joints after exposure to damp heat conditions.FindingsThe effect of different finishes is found to be very adhesive material dependent. Two adhesives are studied, and for one material the joint reliability is relatively unaffected by changes in component or PCB surface finish. However, for the second material, and components with a high‐tin content‐plated finish, the joints display a less stable resistance. The surface finish on the PCB is found to have a smaller effect on joint reliability than the component finish, with results dependent on adhesive material type. Performance with one material exhibited little difference in reliability irrespective of the PCB surface finish. For the second material, the joint reliability performance with components having the electroless nickel/immersion gold finish, is not as good as that with components having the immersion tin or silver finishes.Originality/valueThe paper shows that surface finish is an important factor in determining the conductivity of ICA joints during exposure to the 85°C/85%RH environment. Systems containing tin are more prone to lose conductivity and, conversely, noble metal systems are more immune to degradation. This is a major concern as the industry is showing many signs that the component termination of choice will be pure tin.

Nanoclustering of vacancies in thin metal films revealed by x-ray diffuse scattering

The authors report the incorporation of unexpectedly large vacancy clusters into homoepitaxial Ag(001) films. These results, which are for a simple noble metal system, have important implications for understanding the atomic-scale kinetics of surfaces where current models have mostly ignored the role of vacancies. For films grown at 150 K, an average vacancy cluster exhibits a local dilatation volume of 750Å3, which leads to a 1% compressive strain of the film. Vacancy clusters are observed even for films grown near room temperature. These in situ diffuse x-ray scattering experiments measure the local deformation around the cluster and, therefore, provide conclusive evidence of vacancy clusters.

Scattering of surface plasmon polaritons by a nanoparticle with the inclusion of the magnetic dipole contribution

Using the method of the tensor Green’s function of the wave equation, the differential and total cross sections for the scattering of surface optical electromagnetic waves (surface plasmon polaritons) by a spherical nanoparticle into surface plasmon polaritons and light have been obtained with the inclusion of the magnetic dipole contribution. Using the example of noble metal systems, it has been shown that the magnetic dipole contribution may significantly affect the angular dependence of the differential cross sections and increase their anisotropy as the plasmon wavelength increases.

High Throughput Screening of Low Temperature SCR and SCD De-NOxCatalysts in Scanning Mass Spectrometer

High-throughput synthesis and screening methods have been developed for the discovery of highly active lead compounds for the selective catalytic reduction as well as direct decomposition of NO in the temperature range 200–300 °C. The discovery libraries for primary screening consisted of 16 × 16 catalyst arrays on 4 in. square quartz wafers. Catalysts were prepared by robotic liquid dispensing techniques and screened for catalytic activity in Symyx’ scanning mass spectrometer. The scanning mass spectrometer is a fast serial screening tool that uses flat wafer catalyst surfaces, local laser heating, a scanning/sniffing nozzle and a quadrupolar mass spectrometer to compare relative catalytic activities. The feed consisted of NO/NH3 mixtures with optional O2 cofeed and Kr as the internal standard in Ar carrier gas. QMS detection allowed for tracking of H2O, N2, NO, O2, N2O and Kr. Screening protocols for catalytic materials encompassed metal precursors and carriers for supported vanadia systems, extensive doping of V2O5/TiO2, and broad screening of mixed redox metal oxides and supported base and noble metal systems. More than 500 samples could be screened in a single day. Active hits (high NO consumption accompanied by corresponding N2 production) identified in discovery libraries were re-synthesized as focus libraries for lead confirmation and further optimization. These libraries used shallower compositional gradients, for example 56 points (compositions) per ternary, with four 56-point ternaries per 4 in. wafer. Broad screening ternaries were generally 8 or 15 points. The focus libraries more clearly reveal the trends and provide guidelines for secondary screening and scale-up. High conversions were achieved in scanning mass spectrometer so the scalability risk is small for the short contact time reactions.

High Efficiency of Noble Metal and Metal Oxide Catalyst Systems for the Selective Reduction of NO with Propene in Lean Exhaust Gas

An investigation was conducted of noble metal and metal oxide catalysts deposited on Al2O3. The noble metals Pt, Pd, Rh the metal oxides CuO, SnO2, CoO, Ag2O, In2O3, catalysts were examined. Also investigated were noble metal Pt, Pd, Rh-doped In2O3/Al2O3 catalysts prepared by single sol–gel method. Both were studied for their capability to reduce NO by propene under lean conditions. In order to improve the catalytic activity and the temperature window, the intermediate addition propene between a Pt/Al2O3 oxidation and metal oxide combined catalyst system was also studied. Pt/Al2O3 and In2O3/Al2O3 combined catalyst showed high NO reduction activity in a wider temperature window, and more than 60% NO conversion was observed in the temperature range of 300–550 °C.

DFT Calculations for Cu-, Ag-, and Au-Containing Molecules

The physicochemical properties of a set of molecules containing Cu, Ag, or Au atoms were calculated using the GAUSSIAN program suite, with the purpose of investigating the various density functional theory (DFT) approaches for subsequent application in cluster calculations. The test set comprised the copper-based molecules CuH, CuO, CuS, Cu2, CuCl2-, CuCH3, CuC2H2, Cu2(HCO2)4(H2O)2, and Cu6H6(PH3)6 and the silver and gold diatomics AgH, AgO, AgS, Ag2, AgCu, AuH, AuO, AuS, Au2, AuCu, and AuAg. The DFT methods used were SVWN, BLYP, and BPW91, together with the DFT hybrids B3LYP and B3PW91. The calculations were carried out with the basis sets LANL2MB, LANL2DZ, 3-21G, and 6-311G (when available). The properties calculated were the molecular geometry, vibrational frequencies, and dissociation energies. It was found that all the DFT-based methods, particularly when allied with the LANL2DZ basis set, produced results which are significantly closer to experimental values than those of the traditional Moller−Plesset (MP2) electron correlation method and which are also applicable to considerably larger molecules. Over the whole test set of molecules, the RMS errors of the four “BX” methods, in conjunction with LANL2DZ, were typically 3−4% for geometries, 6−8% for frequencies, and 10−16% for dissociation energies, with BPW91 and the popular B3LYP at the lower and upper ends of these ranges, respectively, and with the errors being overestimates and underestimates, respectively. The corresponding values for SVWN and MP2 were 2% and 6%, 12% and 12%, and 33% and 42%, with these errors typically being ± and +, + and −, and + and −, with + and − representing overestimates and underestimates, respectively. While the best bond lengths are produced by SVWN (a local spin density approximation), which is not uncommmon, this advantage over the gradient corrected (BX) methods is only slight, and the latter are markedly superior for frequencies and especially dissociation energies. Not supported by the present results are the notions that (all) pure DFT methods underestimate metal−ligand bond lengths and overestimate bond strengths and that hybrid methods are superior (and neither that DFT methods are overcorrelated). Testing on a subset of molecules with BPW91/LANL2DZ revealed no benefit in supplementing this basis set by the addition of diffuse functions, nor of the counterpoise correction. There appear to be specific incompatibilities with some method/basis set combinations, and even the continuing availability of 3-21G for these metals is questionable. Because of its accuracy and reliability, the combination BPW91/LANL2DZ is recommended for these noble-metal systems (and to extensions such as the cluster-model approach to adsorption of a molecule on a metal surface).

Fluctuations of Step Edges: Revelations About Atomic Processes Underlying Surface Mass Transport

AbstractExperimental advances in recent years make possible quantitative observations of step-edge fluctuations. By applying a capillary-wave analysis to these fluctuations, one can extract characteristic times, from which one learns about the mass-transport mechanisms that underlie the motion as well as the associated kinetic coefficients [1-3]. The latter do not require a priori insight about the microscopic energy barriers and can be applied to situations away from equilibrium. We have studied a large number of limiting cases and, by means of a unified formalism, the crossover between many of these cases[4]. Monte Carlo simulations have been used to corroborate these ideas. We have considered both isolated steps and vicinal surfaces; illustrations will be drawn from noble-metal systems, though semiconductors have also been studied. Attachment asymmetries associated with Ehrlich-Schwoebel barriers play a role in this behavior. We have adapted the formalism for nearly straight steps to nearly circular steps in order to describe the Brownian motion of single-layer clusters of adatoms or vacancies on metal surfaces, again in concert with active experimental activity [3,5]. We are investigating the role of external influences, particularly electromigration, on the fluctuations.

Estimating the metal-ceramic van der Waals adhesion energy

Abstract We invoke the Lifshitz theory of van der Waals forces to derive a simple, analytical expression for the adhesion across a planar metal-ceramic interface. The van der Waals energy is conveniently expressed as a function of readily accessible bulk properties of the respective materials. Application to several non-reactive metal-sapphire systems shows the predicted van der Waals energy to provide a good lower-bound estimate of the measured adhesion energy. Apart from platinum and palladium, where strong metal-aluminium interactions have been predicted, adhesion energies in noble-metal systems are well accounted for by van der Waals interactions alone. Not surprisingly, adhesion in the more chemically active metals exceeds the van der Waals energy.

Adlayer Core-Level Shifts of Random Metal Overlayers on Transition-Metal Substrates

We calculate the difference of the ionization energies of a core-electron of a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using density-functional-theory. We analyze the initial-state contributions and the screening effects induced by the core hole, and study the influence of the alloy composition for a number of noble metal-transition metal systems. Data are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x) Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our analysis clearly indicates the importance of final-state screening effects for the interpretation of measured core-level shifts. Calculated deviations from the initial-state trends are explained in terms of the change of inter- and intra-atomic screening upon alloying. A possible role of alloying on the chemical reactivity of metal surfaces is discussed.

Preparation and behaviour of acetylene black press-electrodes as adsorbents and metal catalysts supports

Acetylene black press-electrodes were prepared through stirring acetylene black in distilled water, mixing with a Teflon-emulsion and pressing the obtained compact mass on a nickel lattice. The above electrodes exhibit a large surface and show strong adsorptive properties, which were studied in the case of nitro compounds. The adsorptive properties of the acetylene black material become much more pronounced after its electrochemical pretreatment through application of alternated oxidative and reductive potentiostatic pulses. Through this procedure cation-exchangeable groups are created on the carbon surface, while simultaneously the effective area of the material becomes larger. Considerable amounts of nitro compounds can be attached to the electrooxidized material through a procedure of successive adsorption of the nitro compound and its electroreduction.Through ion-exchange of the electrooxidized material in aqueous solutions containing noble metal salts, followed by electrochemical reduction of the exchanged metal ions to the metallic state, catalytically active noble metal systems are obtained, Ag-modified electrodes were found to catalyze the electroreduction of aliphatic nitrocompounds, while Pd-modified electrodes showed catalytic activity in the electroreduction of nitrobenzene and the electrooxidation of formic acid. Moreover, Pd-modified systems proved to be very suitable for the catalysis of chemical reactions, eg, for the hydrogenation of benzoyl chloride and oleic acid.

Carbon-13 NMR Spectroscopy Study of L-Zeolite- and Silica-Supported Platinum Catalysts

NMR studies of CO adsorbed on small Pt particles show evidence of changes in the metallic nature of these particles with size. Large particles on silica or the exterior of zeolite crystallites have conduction-band electrons that cause a Knight shift for adsorbed CO. Small particles in zeolite cavities are diamagnetic clusters, and yield spectra for linear and bridging carbonyls similar to those of transition-metal cluster compounds. 13C NMR of CO offers a simple probe of metal dispersion and particle size for these Pt catalysts and other noble metal systems.

Controlled electronic coupling between two quantum wells: A photoemission study of noble-metal systems.

Double quantum wells are grown on Au(111) and Cu(111) substrates using Ag as the well material and either Cu or Au as a barrier material. The barrier thickness is varied to change the coupling between the two wells, and the resulting double-quantum-well states are probed with photoemission. In the limit of a very thick barrier, the coupling is eliminated, and the allowed states become those of two independent wells. When the barrier thickness is reduced to zero, the two quantum wells combine to form a single well. Between these two limits, the observed evolution of the binding energies and intensities is compared with the predictions of a simple theoretical model based upon Bloch wave functions. The effects of smoothing the barrier potential are studied by annealing a Au barrier sample, allowing atomic interdiffusion between the Au barrier and the Ag wells.

Controlled spatial distribution of noble metal catalysts on electrochemically activated carbon fibre supports

The preparation of electrocatalytic noble metal systems on carbon fibre supports is investigated in the present study. This is achieved through electrochemical activation of the carbon fibres, followed by ion exchange of the cationic components of complex noble metal salts and chemical reduction with NaBH 4, without and after thermolysis, with H 2 stream, with triethoxysilane or dimethyl-ethoxysilane, or with BuLi (CH 3CH 2CH 2CH 2Li). The best results referring to the catalytic activity of the produced systems on model electrochemical reactions are observed in the case of reduction with NaBH 4 after thermolysis. The results are correlated with those obtained by galvanostatic pulse reduction of the exchanged complex noble metal ions. The catalytic efficiency of the noble metal depositions obtained by galvanostatic pulse reduction approaches that observed on noble metal depositions received by chemical reduction with NaBH 4 after thermolysis. Chemical reduction has the advantage of the higher electrocatalytic activity, but electrochemical reduction offers the possibility of the formation of more stable catalytic layers, as well as of multiple noble metal layers of considerable thickness.

Electronic and Optical Properties of AuAl2 and PtAl2

AbstractElectronic energy bands of two isostructural intermetallic compounds, AuAl2 and PtAl2, are calculated using the composite wave variational version of the APW method. The crystal potential is constructed by superposition of the atomic potential of Herman and Skillman. A comparative study between the 9d noble metal system (PtAl2) and the 10d noble metal system (AuAl2) is attempted. From the band structure data, density of states, joint density of states, ϵ2(ω) spectra and the Fermi surfaces are worked out. Results are interpreted in the light of band structure data and compared and correlated with available experimental results.

Thermodynamic Calculation on the Vaporization of Fission-Produced Noble Metal System in Vacuum and Oxidative Atmosphere at High Temperatures

The vaporization behavior of fission-produced noble metal system (Mo-Ru-Pd, Mo-Ru-Rh and Mo-Tc-Pd) in vacuum and oxidative atmosphere at high temperatures was investigated by the thermodynamic calculation on the basis of the regular solution model and the available experimental data. The palladium and rhodium in the allyoys, and molybdenum and ruthenium in the alloys vaporize preferentially at high temperature in vacuum and in ari, respectively. At low temperature below 385K in air, ruthenium and technetium in the alloys vaporize predominantly by oxidation.

On the Determination of Enhancement Factors of Solvent Diffusion

AbstractIn dilute binary f.c.c. alloys of the substitutional type a nonlinear increase of the solvent diffusion coefficient D by alloying solutes with positive excess charge is frequently observed. The formation of vacancy–impurity complexes leads to this increase, which is usually described in terms of the enhancement factors bi in the following manner: D(NB) = D(0) (1 + b1NB + b2NB2 + + b3NB3 + …). The determination of enhancement factors by using this polynomial is critically discussed. From the complex model according to Dorn and Mitchell and more generally according to Bérces and Kovács and its application to solvent diffusion by Hehenkamp et al. it becomes reasonable, that the dependence of the solvent diffusion coefficient on the solute concentration is approximately given by: D(NB) = D(0) exp (b1NB). This relation, which is already known in the literature for a long time, yields an alternative method for the detemination of enhancement factors. Results are presented for many noble metal systems and discussed in the frame of two different estimation methods of the Gibbs free energy of binding δg1 of a vacancy–impurity pair calculated from the enhancement factors b1 and b2.

Spectroscopic characterisation and photochemical behaviour of a titanium hydroxyperoxo compound

In order to obtain more information about the generation of peroxide species during water photo-cleavage using M/TiO2(M = noble metal) systems in which an O2 evolution is normally not observed, a titanium hydroxyperoxo compound has been prepared by the reaction of TiCl4 with H2O2. The structure and the thermal behaviour of this compound have been examined using TG-MS, XRD and TPD techniques. XPS, i.r., e.s.r. and u.v.-visible reflectance spectroscopy have been used to characterise the types of peroxo species present in this sample. The photochemical decomposition of this compound, which leads to O2 evolution, has been studied compared to a TiO2(Degussa P25) sample. A model is proposed to explain the lack of O2 evolution during water photo-cleavage on M/TiO2 systems where peroxo species, similar to those observed in the titanium hydroxyperoxo compound, become stabilised against photodecomposition.

Thermodynamic calculation on the vaporization of fission-produced noble metal system in vacuum and oxidative atmosphere at high temperatures.

The vaporization behavior of fission-produced noble metal system (Mo-Ru-Pd, Mo-Ru-Rh and Mo-Tc-Pd) in vacuum and oxidative atmosphere at high temperatures was investigated by the thermodynamic calculation on the basis of the regular solution model and the available experimental data. The palladium and rhodium in the allyoys, and molybdenum and ruthenium in the alloys vaporize preferentially at high temperature in vacuum and in ari, respectively. At low temperature below 385K in air, ruthenium and technetium in the alloys vaporize predominantly by oxidation.

Catalytic oxidation of polychlorinated biphenyls in a monolithic reactor system

Air containing 200-1000 ppm of Aroclor 1254 vapors (a mixture of polychlorinated biphenyls (PCBs)) was passed through a 22.86 cm long heated section of 0.64 cm i.d. monolithic {alpha}-alumina support, which had been previously treated with one or more catalytic agents. At 600{degree}C, with a residence time of about 9.4 s, results showed that the overall PCB destruction efficiency ranged from a high of 97.3% for supported CuO to a low of 69.3% for Cr{sub 2}O{sub 3}. Supported catalysts of Co{sub 3}O{sub 4}, CuCr{sub 2}O{sub 4}, and Pt-Pd were intermediate in activity. Relative activity for this group of p-type catalysts in deep oxidation was in accordance with Sabatier's principle, decreasing with increasing heats of O{sub 2} chemisorption. Selectivity toward oxidation of the more toxic high-chlorine-containing PCBs was found to be greater for catalysts made from transition metal oxides than for noble metal systems or homogeneous (thermal) decomposition.

Thermodynamic investigations of solid α-silver-arsenic and -bismuth alloys

The vapor pressure of the impurity component above solid α-Ag-As and α-Ag-Bi alloys has been measured by means of the Knudsen method. Alloys with an impurity concentration as low as 0.1 at.% Bi could be studied. Values of the chemical excess potential are calculated. The excess potential is positive and increases monotonically in case of α-Ag-As, whereas in α-Ag-Bi a flat minimum is observed. These results, which are in good agreement with thermodynamic data from other noble metal systems with electropositve solutes, are explained theoretically. In addition the solubility curve of Bi in Ag has been redetermined. Deviations up to 50 K from the literature are found.