Gold-nickel Alloys Research Articles

Pulse Electroplating of Gold-Nickel Alloys: Morphological and Aesthetic Improvement Compared to DC

The use of pulsed current is well known as a research tool to study the electrodeposition mechanism of metals, however, it was realized that this method could provide a means to improve the properties of deposits. Electrodeposition of gold or its alloys is not only used in the electronics industry due to its conductive and anti-corrosive properties, but also as a final layer due to its aesthetic properties. In this study we focused on a cyanide gold-nickel bath, among the most frequently used in the electroplating industry intended for the decorative sector. The use of modulated currents has proven to be an effective methodology to be able to control and improve the chemical and physical characteristics of metal deposits. Deposits obtained through pulsed currents have a smaller grain size and a more homogeneous surface, both at the level of thickness dispersion on the surface and at the roughness of the samples themselves. A direct consequence of the above results is the improvement of the aesthetic finish of the workpieces with an increase in brightness as well as anti-corrosive properties.

Comparison of Au and Au–Ni Alloys as Contact Materials for MEMS Switches

This paper reports on a comparison of gold and gold-nickel alloys as contact materials for microelectromechanical systems (MEMS) switches. Pure gold is commonly used as the contact material in low-force metal-contact MEMS switches. The top two failure mechanisms of these switches are wear and stiction, which may be related to the material softness and the relatively high surface adhesion, respectively. Alloying gold with another metal introduces new processing options to strengthen the material against wear and reduce surface adhesion. In this paper, the properties of Au-Ni alloys were investigated as the lower contact electrode was controlled by adjusting the nickel content and thermal processing conditions. A unique and efficient switching degradation test was conducted on the alloy samples, using pure gold upper microcontacts. Solid-solution Au-Ni samples showed reduced wear rate but increased contact resistance, while two-phase Au-Ni (20 at.% Ni) showed a substantial improvement of switching reliability with only a small increase of contact resistance. Discussion of the effects of phase separation, surface topography, hardness, and electrical resistivity on contact resistance and switch degradation is also included.

An electrochemical study of Au–Ni alloy electrodeposition from cyanide–citrate electrolytes

Cyclic voltammetry, j– t transients and transient time measurements have been used to characterize the Au–Ni alloy deposition and to obtain the nucleation and growth mechanism. The electrodeposition of gold, nickel and gold–nickel alloys was studied in cyanide–citrate electrolytes. The analysis of the voltammograms and j– t transients clearly shows that deposition of the Au and hence, of Au–Ni alloys, is affected by adsorbed cyanide ligands mainly at low overpotentials and a direct reduction occurs at high overpotentials. In this case, the Au deposition is kinetically favored, while the Ni deposition is controlled by the cyanide–citrate adsorbed adlayer. In an alloy electrolyte, the Au and Ni deposit simultaneously in the whole negative potential region. In addition, the current transients revealed that the nucleation mechanism is progressive for Au–Ni alloy deposition at the lower potentials, and instantaneous at the higher overpotentials with a three-dimensional growth mechanism. The number of nucleation sites increases with overpotential, where the most nucleation sites are concerned with gold deposition. The composition of the alloys is markedly dependent on the potential and varies with polarization behavior. The microstructure of the Au–Ni alloy deposits was investigated by TEM, indicating a gold uniform texture with a single-phase fcc structure containing a few rectangular nuclei of nickel.

Numerical evaluation of the exact phase diagram of an empirical Hamiltonian: Embedded atom model for the Au-Ni system

Molecular-dynamics simulations were used to calculate the Gibbs free energy on the entire compositional range of gold-nickel alloys described with a set of embedded atom potentials available in the literature. Thermodynamic integration and switching Hamiltonian techniques were used to obtain the exact phase diagram (with no approximations), and that corresponding to the regular approximation. Remarkable agreement for some properties, such as the solvus critical point, the congruential melting, the melting points of the pure elements, and the formation entropy of the alloy, contrasts with the poor prediction of the location of the solidus-liquidus lines, reflecting errors in the heat of solution in the liquid phase. The results are compared with recent experimental reassessment of the Au-Ni phase diagram and with ab initio calculations.

Diffusion bonding and brazing of high purity copper for linear collider accelerator structures

Diffusion bonding and brazing of high purity copper were investigated to develop procedures for joining precision machined copper components for the Next Linear Collider (NLC). Diffusion bonds were made over a range of temperatures from 400 \ifmmode^\circ\else\textdegree\fi{}C to 1000 \ifmmode^\circ\else\textdegree\fi{}C, under two different loading conditions [3.45 kPa (0.5 psi) and 3.45 MPa (500 psi)], and on two different diamond machined surface finishes. Brazes were made using pure silver, pure gold, and gold-nickel alloys, and different heating rates produced by both radiation and induction heating. Braze materials were applied by both physical vapor deposition (PVD) and conventional braze alloy shims. Results of the diffusion bonding experiments showed that bond strengths very near that of the copper base metal could be made at bonding temperatures of 700 \ifmmode^\circ\else\textdegree\fi{}C or higher at 3.45 MPa bonding pressure. At lower temperatures, only partial strength diffusion bonds could be made. At low bonding pressures (3.45 kPa), full strength bonds were made at temperatures of 800 \ifmmode^\circ\else\textdegree\fi{}C and higher, while no bonding (zero strength) was observed at temperatures of 700 \ifmmode^\circ\else\textdegree\fi{}C and lower. Observations of the fracture surfaces of the diffusion bonded samples showed the effects of surface finish on the bonding mechanism. These observations clearly indicate that bonding began by point asperity contact, and flatter surfaces resulted in a higher percentage of bonded area under similar bonding conditions. Results of the brazing experiments indicated that pure silver worked very well for brazing under both conventional and high heating rate scenarios. Similarly, pure silver brazed well for both the PVD layers and the braze alloy shims. The gold and gold-containing brazes had problems, mainly due to the high diffusivity of gold in copper. These problems led to the necessity of overdriving the temperature to ensure melting, the presence of porosity in the joint, and very wide braze joints. Based on the overall findings of this study, a two-step joining method is proposed for fabricating the NLC structures. The structure would be assembled with pure silver braze inserts using a self-aligning step joint design, then the assembly would be vacuum diffusion bonded at 700 \ifmmode^\circ\else\textdegree\fi{}C and 3.45 MPa pressure to seal the critical inner portion of the assembly. Finally, during the same furnace cycle, the temperature would be increased to 800 \ifmmode^\circ\else\textdegree\fi{}C in order to react the silver with the copper to form a liquid braze alloy that would join and seal the outer portion of the cells together.

Evaluation of the Compressibility and Thermal Expansivity of Gold-Nickel Alloys

The compressibility and the thermal expansivity of gold-nickel alloys can be estimated in the complete concentration range through the Varotsos and Alexopoulos model, provided that density data are available and the compressibility (or the expansivity) is experimentally known for a single concentration. The results are discussed in relation to the defect compressibility obtained from the elastic properties of each of the end components.

High Voltage Electron Microscope Study of Radiation-induced Precipitation of Gold-Nickel Alloys

Gold-nickel solid solution alloys were irradiated by high voltage electron microscopy with 2.0 MeV electrons. After irradiation, specimens were examined by energy dispersive X-ray microanalysis, quantitative stereo microscopy and high resolution electron microscopy. Defect clusters were formed by irradiation at high temperatures, at which dislocation loops were not formed. β-Ni precipitates were identified, which were octahedral and were surrounded by {111} planes. These precipitates were formed homogeneously and shrank when the irradiation was interrupted. This is radiation-induced homogeneous precipitation, which was observed for the first time in gold-nickel alloys

Thermodynamic Properties of Solid Gold—Nickel Alloys

Thermodynamic Properties of Solid Gold—Nickel Alloys

Heat capacity and magnetisation of gold-nickel alloys

The calorimetric and magnetic properties of gold-nickel alloys have been investigated over the complete concentration range. The parametric region (x or=55% Ni). The paramagnetic and ferromagnetic regions can be discussed within a band model. The critical-concentration region is characterised by peculiar calorimetric and magnetic properties which can be explained by magnetic clustering. A consistent understanding of the onset of ferromagnetism is obtained in terms of cluster interactions.

Optical studies of dilute CuNi alloys

An optical study of dilute gold-nickel alloys is described where the nickel impurity forms a virtual bound state in the gold hose. The measurements extended from 0.1 eV to 4 eV and the alloys were in the range 1 to 5 at.%. The results are analysed in terms of a consistent model of the virtual bound state dielectric constant which includes s, p and d phase shifts of the impurity potential and changes in the conduction electron effective mass induced by alloying. The results are compared with the recent infrared low temperature absorptance measurements. Theories of the virtual bound state and a calculation of the width and position of this impurity state are considered.

Pulse plating of nickel-gold alloys

The investigations show that during pulse plating with short current pulses and much longer “off” times no significant diffusion polarisation occurs during plating, so that the properties of the deposited gold-nickel alloys from an alkaline cyanide bath are largely independent of the current density. When plating is done with normal direct current under otherwise equivalent conditions the deposits show the normal dependence on the current density.

Optical studies of dilute AuNi alloys

An optical study of dilute gold-nickel alloys is described where the nickel impurity forms a virtual bound state in the gold hose. The measurements extended from 0.1 eV to 4 eV and the alloys were in the range 1 to 5 at.%. The results are analysed in terms of a consistent model of the virtual bound state dielectric constant which includes s, p and d phase shifts of the impurity potential and changes in the conduction electron effective mass induced by alloying. The results are compared with the recent infrared low temperature absorptance measurements. Theories of the virtual bound state and a calculation of the width and position of this impurity state are considered.

Localized modes in Au-V and Au-Ni alloys determined from low-temperature specific-heat measurements

The lattice specific heat as deduced from low-temperature specific-heat measurements from 1–30 K of some gold-vanadium and gold-nickel alloys has been analysed in terms of localized modes including force constant changes.

Properties of gold-nickel alloy brazed joints in high temperature materials

The usefulness of gold-nickel alloys as brazing media in the aeroengine, electronic, nuclear power and spacecraft industries is illustrated by data on the corrosion and oxidation resistance of joints made with these alloys in various heat-resistant materials and on their resistance to fracture under the influence of static, dynamic and impact loads in tests at subzero, normal and elevated temperatures.

Modulated structures in AuNi alloys

A modulated structure has been shown to develop on {100} planes in certain gold-nickel alloys on aging at temperatures in the vicinity of 100–200°C. The critical composition is at 45 at.% Ni and the maximum temperature at which these structures occurs is 220°C. The boundary of the modulated structure region is nearly symmetric, with extreme compositions at approximately 20 and 60 at.% Ni. This structure has been observed by means of electron diffraction and is characterized by the formation of satellite reflections on 〈100〉 axes around each fundamental peak. The modulated structures exhibited a composition amplitude of approximately 5 at. % and characteristic wavelengths which varied from 6.5 Å at 54 at.% Ni to 13 Å at 22 at.% Ni. An activation energy of 13,500 cal. per mol. was calculated for the rate limiting step in the low temperature region, corresponding to the energy of migration for vacancies in this composition range. The satellite reflections disappeared on aging just above the critical temperature ; on reaging at a lower temperature, the satellites did not reappear. This is taken to indicate that the presence of a large concentration of quenched-in excess vacancies is required for the development of the modulated structure. The formation of the modulated structure may be associated with the postulated spinodal decomposition in this system, although the theories of spinodal decomposition do not yet adequately account for the role of vacancies.

Composition and Pressure Dependence of the Elastic Constants of Gold-Nickel Alloys

Measurements of the elastic constants of single crystals of gold and gold-nickel alloys have been made by the ultrasonic pulse-superposition method at room temperature, with pressures varying from atmospheric to 10 Kbar. The values of the adiabatic elastic stiffnesses of gold obtained at atmospheric pressure are $C=4.200$, ${C}^{\ensuremath{'}}=1.473$, and ${B}_{S}=17.28$, in units of ${10}^{11}$ dyne/${\mathrm{cm}}^{2}$. The pressure derivatives of the elastic constants of gold are $\frac{\mathrm{dC}}{\mathrm{dP}}=1.84$, $\frac{d{C}^{\ensuremath{'}}}{\mathrm{dP}}=0.438$, and $\frac{d{B}_{S}}{\mathrm{dP}}=6.15$. The shear constants $C$ and ${C}^{\ensuremath{'}}$ increase nonlinearly as a function of nickel concentration, whereas the bulk modulus, ${B}_{S}$, decreases initially on alloying. The experimental results for dilute solutions were interpreted in terms of a model which assumes that the dominant contribution to $C$, ${C}^{\ensuremath{'}}$, and ${B}_{S}$ arises from gold-gold ion-core repulsive interactions. Using an exponential form for the ion-core potential, it was also possible to calculate the variation with composition of the shear constants for concentrated solutions. The ion-core contribution to the heat of formation was also calculated, and this appears to be in good agreement with the thermodynamic data. These results have led to an interpretation of the strain-energy term in alloy formation as arising from the changes in the short-range repulsive interaction between neighboring ions.

The catalytic hydrogenation of ethylene on nickel-copper and nickel-gold alloys

The rate of ethylene hydrogenation on copper-nickel alloy films in the temperature range 0 ° to 21 °C has been measured. The films were prepared by successive deposition of copper and nickel by the usual film-forming procedures followed by sintering the films in 5 cm of hydrogen at either 300 ° or 500 °C. The activity per unit area of film increases to a value 7 to 15 times as great for a catalyst containing 10% to 20% Cu as for pure nickel. Similarly, catalysts containing 5% to 20% nickel are as much as eight times as active as pure nickel. A minimum of activity with composition is found in the 25% to 35% nickel region, where the activity is about the same as on pure nickel. X-Ray data show that films homogenized by heating to 500 °C in a few cm of hydrogen form an alloy containing about 60% copper; the excess of nickel or copper present gives a diffraction pattern for the pure metal. Gold-nickel alloys containing 15% and 55% gold, respectively, have rates only about one-tenth as great as that of pure nickel. The apparent energies of activation for the reaction over the copper-nickel alloys vary with composition in the range 9 to 12 kcal/mole; those for the two gold-nickel alloys are about 4 kcal. The reaction rate is first order with respect to hydrogen and independent of ethylene for the copper-nickel alloys and for pure nickel; it is first order with respect to both hydrogen and ethylene for copper.

Electron Transfer in Dilute Gold Alloys

Isomeric chemical shifts were observed in the Mössbauer spectra of Au197 atoms in dilute solutions in 19 metals and semiconductors at 4.2°K. The shifts were interpreted as arising from transfer of (conduction) electrons in the 6s shell of gold, probably toward the gold atom, in the alloys. A correlation was found between the shifts and the host electronegativities. The relationship between the 6s electron of gold and the nickel d bands in gold—nickel alloys is discussed. It is shown that the ground and first excited nuclear states of Au197 have radial charge moments sufficiently different (the excited state is probably ``larger'') to require collective nuclear excitation. The isomeric shift in Al2Au does not directly confirm the electroneutrality hypothesis for intermetallic compounds.

Determination of nickel in gold-nickel alloys

An ion exchange separation based on the difference in adsorption from a chloride solution combined with an EDTA titration is suggested as a rapid and accurate method for the determination of nickel in nickel-gold alloys.

Resistance and magneto-resistance of dilute alloys of copper and gold with nickel at low temperatures

Synopsis In continuation of a systematic investigation on the electrical properties of alloys of noble metals and transition metals at low concentration, electric resistance measurements on dilute copper-nickel and gold-nickel alloys are reported. Besides an observable minimum, the resistance temperature relation does not show any irregularity down to 1.2°K. Deviations from the Kohler diagram and a decrease of the resistance in a magnetic field for the higher nickel concentrations in copper (2.2 at. %) indicate the existence of anomalies of the same character as found for the alloys with chromium and manganese. The effects are too small, however, to allow for an application of the suggestions made by Korringa and Gerritsen for the case of the alloys with manganese.