Silver-palladium alloy films

E. Gardeniers, H. V. Jansen, F. C. Gie-lens, M. C. Elwenspoek, Preparation of palladium-silver alloy films by a dual-sputtering technique and its application in hydrogen separation membrane,... 

Electronic Applications. The PGMs have a number of important and diverse appHcations in the electronics industry (30). The most widely used are palladium and mthenium. Palladium or palladium—silver thick-film pastes are used in multilayer ceramic capacitors and conductor inks for hybrid integrated circuits (qv). In multilayer ceramic capacitors, the termination electrodes are silver or a silver-rich Pd—Ag alloy. The internal electrodes use a palladium-rich Pd—Ag alloy. Palladium salts are increasingly used to plate edge connectors and lead frames of semiconductors (qv), as a cost-effective alternative to gold. In 1994, 45% of total mthenium demand was for use in mthenium oxide resistor pastes (see Electrical connectors). 

The palladium-silver alloy membrane system was successfully commercialized in the early 1960s [12], but the reduction of palladium content by the addition of silver would still not be a cost-effective alternative for large-scale processes [42] unless micron-scale films could be prepared, a goal currently being addressed by many researchers. In recent years, the Pd-Cu system has been the most heavily investigated alloy for hydrogen membrane applications due to the high permeability of select alloys [67, 90, 91], enhanced mechanical properties [92] and reported chemical resistance. The elevated permeability identifled for select Pd-Cu alloys is attributed to an increase in both the solubility and diffusivity of the B2 crystalline phase [86-88] as compared to the face-centered-cubic (fee) phase that exhibits permeability values proportional to the Pd-content [89, 91, 93]. 

The membrane module has a plate-type structure 40 mmW x 460 mmL x 8 mmT in size. Figure 12.2 illustrates the configuration of the membrane module, and Fig. 12.3 shows a view of the membrane modules. The membrane modules consist of palladium-rare earth alloy thin film with thickness of less than 20 pm and a porous structural support. The hydrogen permeability of the membrane is several times higher than that of the widely used conventional palladium-silver alloy membrane (Sakamoto, 1992). 

Barbieri et al. [523] prepared palladium membranes by solvated metal atom deposition, a deposition method that created a 0.1-pm thick film of palladium on an alumina tube. Another membrane was prepared from a palladium/silver alloy (21 wt.% silver), which had a thickness of 10 pm. Both membranes suffered from pinholes and cracks and thus did not show infinite hydrogen selectivity. However, conversion exceeding the thermodynamic equilibrium could still be achieved for methane steam reforming at temperatures exceeding 400 °C. 

Palladium has extensive use as a catalyst in hydrogenation and dehydrogenation reactions, due to its capacity of combination with hydrogen. Palladium films are used as electrical contacts in connectors. Palladium-silver and palladium-nickel alloys are used to substitute for gold in jewelry. 

The introduction of other metals to form palladium based alloys has had promising results. In particular doping of the palladium with silver has been shown to improve the stability of the film and increase the solubility of hydrogen. Further, the temperature above which the a palladium hydride occurred was lowered with increasing silver content (Uemiya et al.,1991 Kikuchi Uemiya, 1991). The hydrogen permeability was optimized when the silver content of the alloy was aroimd 23 wt%. Silver occupies interstitial sites in the palladium lattice and so moderates the lattice expansion and contraction due to hydrogen absorption/desorption. 

The diffusion coefficients of palladium in a Pd-Ag alloy and silver in a range of Pd-Ag alloys are known, and the diffusion of palladium and silver atoms in a 20% Pd-Ag alloy was calculated (30) for t = 3600 sec representing the film preparation time. At temperatures of 100°, 200°, 300°, and 400°C, silver atoms would diffuse in this time distances of 3 X 10-4, 0.15, 9, and 150 A, respectively whereas at the corresponding temperatures, palladium atoms would diffuse 26, 460, 3000, and 11,000 A. Palladium atoms can thus penetrate the alloy lattice at moderate temperatures, whereas silver atoms have a probability of diffusing distances equivalent to a few unit cells only when the substrate temperature is greater than 300°-400°C. 

Another important problem is the elimination of the chemical interactions between contacting phases and also of the diffusion of metal atoms into the oxide bulk [487 89], One example is the operation of commonly used indium junctions, which are convenient because films of this soft metal and its alloys can be applied mechanically [490], This fact stimulates the quest for low-temperature techniques for junction fabrication. It is known that silver, gold, and copper, and also probably platinum [202] and palladium [487], are most suitable because of their weak interaction with HTSCs. 

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