Silver/nickel oxide interface

It is, of course, well known that metal-semiconductor interfaces frequently have rectifier characteristics. It is significant, however, that this characteristic has been confirmed specifically for systems that have been used as inverse supported catalysts, including the system NiO on Ag described above as catalyst for CO-oxidation. In the experimental approach taken, nickel was evaporated onto a silver electrode and then oxidized in oxygen. A space charge-free counter-electrode was then evaporated onto the nickel oxide layer, and the resulting sandwich structure was annealed. The electrical characteristic of this structure is represented in Fig. 8. The abscissa (U) is the applied potential the ordi-... 

There is a whole range of silver-nickel and palladium-based braze fillers of high oxidation and corrosion resistance that have been developed for joining the nickel-rich alloys however, the presence of sulfur, lead, or phosphorus in the base-metal surface or in the filler can be harmful, since quite small amounts can lead to interface embrittlement. 

In the case of so-called active soldering an active solder is used a metallic solder containing interface active additives which make certain that the molten solder wets the ceramics. An example of such a solder is a silver / copper alloy with a titanium or titanium / indium additive which can be used when soldering zirconium (IV) oxide to certain steels, aluminium oxide to nickel / cobalt or iron / nickel alloys and aluminium oxide to a iron / nickel / cobalt alloy. 

A new area of concern for electrical stability arises because of the increasing use of conductive adhesives as replacements for solder. Some conductive adhesives show unstable electrical-contact resistance when used on non-noble metal surfaces such as copper or tin-lead solder. Although stable on gold, palladium, platinum, and silver surfaces, the same adhesives were found to be unstable on tin, tin-lead, copper, and nickel surfaces.The unstable resistance and increase in resistance in temperature-humidity exposures have been attributed to the growth of an oxide layer separating the filler particles from the substrate at the interface, a mechanism similar to that for the loss of backside contact in die-attach materials. 

Volume conductivity measurements indicate that the composition of the electroplated substrates is of prime importance with an increase in resistivity covering 5-6 orders of magnitude. The conductive adhesives have volume resistivities in the range of 6.5 X 10 -1.6 X 10 " fi cm but the values measured for the joined assembhes extend from 9 X 10 " to 1 X 10 fi cm. The general trend observed with all adhesives is that the increase in resistivity is related to the ease of oxidation of the plated substrates, in particular when nickel and aluminium are compared to noble metal plating. Other studies support these results indicating that the best silver-filled adhesives have a volume resistivity of 5 X 10 fl cm approaching the value of solders, typically 2 X 10 fi cm. The contact resistance at the interfaces with the metallic conductors is, however, more important than the bulk conductivity [155-157]. 

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