D. G. Ivey

Development of an electroplating solution for codepositing Au-Sn alloys

Abstract A relatively stable, weakly acidic, non-cyanide electroplating solution has been developed for deposition of Au–Sn alloys over a range of compositions. The solution consists of Au and Sn chloride salts, as well as ammonium citrate as a buffering agent and sodium sulphite and ascorbic acid as stabilizers. Electrochemical studies have been conducted to examine the effects of the various additives and their concentrations on bath stability and plating behaviour. Preliminary electroplating experiments with the developed solution indicate that uniform, homogeneous deposits can be achieved over a range of compositions, including the technologically important eutectic and near eutectic values.

Microstructural characterization of pulsed electrodeposited Au/Sn alloy thin films

Abstract Based on previous work that identified an electrodeposited composite, multi-layer structure as a viable method of producing eutectic Au/Sn alloys for solder applications, a study of individual phase formation was undertaken. The AuSn phase, because of its higher deposition current (>2.0 mA cm −2 ), has a much faster deposition rate than Au 5 Sn, which is deposited at −2 . AuSn formation is growth controlled, while Au 5 Sn formation is nucleation controlled. The AuSn forms a continuous layer within 60 s with a grain size of 50–75 nm. Because of the high deposition current, the dominant formation mechanism is two-dimensional nucleation, resulting in a relatively rough surface finish. Au 5 Sn, on the other hand, forms a continuous layer within 600 s with an average grain size of 200 nm. Because of the significantly lower deposition current, the dominant formation mechanism is lateral spreading instead of two-dimensional nucleation. The result is a very smooth finish on the deposit surface.

Microstructural and in vitro chemical investigations into plasma-sprayed bioceramic coatings

Hydroxyapatite (HA) coatings plasma sprayed without and with bond coats (titania, zirconia) onto titanium alloy (Ti6A14V) substrates under both atmospheric and low pressure plasma spray conditions were investigated in terms of their microstructure and their resorption resistance during immersion in simulated body fluid (Hanks balanced salt solution). The microstructures of test samples were characterized using SEM on as-sprayed and leached surfaces and on the corresponding cross sections. Selected coating systems were studied by 2-dimensional secondary ion mass spectroscopy imaging to obtain information on plasma spray induced diffusional processes at the coating interfaces, as well as the spatial distribution of minor and trace elements. Coatings consisting of thin (10-15 microm) titania/zirconia (eutectic ratio) and titania bond coats, combined with a 150- to 180-microm thick HA top coat, yielded peel strengths in excess of 32 N/m, as well as sufficient resorption resistance.

Carburisation of interconnect materials in solid oxide fuel cells

Reformed fuel gases utilized in solid oxide fuel cells (SOFCs) contain significant amounts of carbon in the form of CO and CO2. Alloys, such as ferritic stainless steels, which are utilized as interconnect materials in intermediate temperature SOFCs may, therefore, be susceptible to carburisation. Long-term exposure of ferritic stainless steel, with and without a nickel protective layer, to simulated carbon-containing fuel gas environments has been studied. Optical and electron microscopy (both SEM and TEM) techniques were utilized to characterize the microstructures prior to and after carburising.

Hexavalent chromium in tricalcium silicate. Part I quantitative X-ray diffraction analysis of crystalline hydration products

The hydration products of CrVI-doped tricalcium silicate (C3S)⊛ have been investigated. C3S is the main constituent of Portland cement responsible for the strength and stability of hardened Portland cement paste. Chromium trioxide (CrO3), added as a dopant to C3S, simulates hexavalent chromium waste that may be stabilized in ordinary Portland cement. X-ray diffraction was used to monitor the development of the hydration reaction products from the early stages to the late reaction stages. Leaching studies were carried out to evaluate the stability of the CrVI-containing phases in the hydrated C3S matrix.In monolithic waste forms containing hexavalent chromium, CaCrO4·2H2O was found to form within a few minutes of the hydration reaction. With increasing concentration of Ca2+ in the pore solution, Ca2CrO5·3H2O became the stable species. The chromium in this phase was found to be very mobile in a standard acetic acid leaching test.

Hexavalent chromium in tricalcium silicate: Part II Effects of CrVI on the hydration of tricalcium silicate

The composition and structure of hydrated tricalcium silicate (C3S)⊛ pastes admixed with CrVI have been studied. The resultant mixture simulates CrVI waste forms stabilized in ordinary Portland cement. Scanning electron microscopy and transmission electron microscopy were used to identify the microstructural changes accompanying the addition of CrVI solutions to C3S. Energy-dispersive X-ray spectroscopy was used to probe the distribution of chromium in the phases within the hazardous waste forms. Elucidation of the molecular structure of the reaction products was accomplished with Fourier transform infrared and nuclear magnetic resonance spectroscopies.CrVI was found to be contained in the waste form as soluble Ca2CrO5·3H2O and partially chemically bonded within the calcium silicate hydrate (C–S–H) phase. CrVI was also found to increase the condensation of C–S–H and porosity of the waste form.

Thermal stability of Pd/Ge-based ohmic contacts to n-type GaAs

The microstructural changes that occur during high temperature annealing (450–600 °C) of Pt/Ti/Ge/Pd ohmic contacts to n-type GaAs have been studied using transmission electron microscopy (TEM). The metal layers were deposited sequentially by electron beam evaporation onto GaAs doped with Si to a level of ≈5×1018 cm−3. The deposition sequence and metal layer thicknesses were: Pt (50 nm), Ti (30 nm), Ge (90 nm) and Pd (45 nm). The contact microstructure remained uniform up to 550 °C and consisted of a continuous polycrystalline layer of GePd at the semiconductor surface, along with pockets of epitaxially grown Ge. Annealing at 550 °C resulted in the nucleation of a Ge-deficient phase, Ge8Pd21, at the GePd/GaAs interface, which protruded into the GaAs and deteriorated the uniformity of the contact. At higher annealing temperatures, the contact degraded rapidly due to inward diffusion of Ti and Pt and outward diffusion of As, leading to the formation of several binary and ternary phases.

Formation of As2O3 during anodic dissolution of GaAs

The formation of As2O3 particles on GaAs surfaces, which have been anodically polarized at potentials <1 V versus SCE, has been studied using SEM, EDX, AES and XPS. Selective dissolution of GaAs occurs resulting in the formation of an As-rich surface layer. The As layer agglomerates and oxidizes on exposure to air forming As2O3 particles. The particle formation is dependent on test conditions, with p-GaAs forming As2O3 in both dark and daylight conditions; As2O3 only forms on n-GaAs when polarized in daylight. Polarization at corrosion potentials does not lead to particle formation, as GaAs dissolution rates are too low for surface enrichment of As.

Galvanic corrosion behavior of GaAs in acid solutions

The galvanic corrosion behavior of both n-GaAs and p-GaAs coupled to Au, at room temperature, have been examined using the zero resistance ammeter technique (direct measurement), as well as the method of superposition of polarization curves (indirect measurement). Galvanic current densities, which can be converted to corrosion rates, were measured as a function of the cathode-to-anode (C /A) area ratio. In addition, corrosion rates were predicted from superposition of cathodic polarization curves for Au and anodic polarization curves for GaAs. With some exceptions, the current densities increased with increasing C/A ratio, and the agreement between the indirect and direct measurements was quite good. This indicates that the former can be used to determine semiconductor corrosion rates. Sample surfaces, examined using transmission electron microscopy, showed an As-rich surface layer, with a thickness dependent on dopant type and polarization potential.