Kirkendall effect
The effect can be observed, for example, by placing insoluble markers at the interface between a pure metal and an alloy containing that metal, and heating to a temperature where atomic diffusion is reasonable for the given timescale; the boundary will move relative to the markers.
This process was named after Ernest Kirkendall (1914–2005), assistant professor of chemical engineering at Wayne State University from 1941 to 1946.
One of these is the prevention or suppression of voids formed at the boundary interface in various kinds of alloy-to-metal bonding.
His second paper revealed that zinc diffused more quickly than copper in alpha-brass, which led to the research producing his revolutionary theory.
Until this point, substitutional and ring methods were the dominant ideas for diffusional motion.
At the time it was submitted, the paper and Kirkendall's ideas were rejected from publication by Robert Franklin Mehl, director of the Metals Research Laboratory at Carnegie Institute of Technology (now Carnegie Mellon University).
[2] A bar of brass (70% Cu, 30% Zn) was used as a core, with molybdenum wires stretched along its length, and then coated in a layer of pure copper.
Diffusion was allowed to take place at 785 °C over the course of 56 days, with cross-sections being taken at six times throughout the span of the experiment.
Over time, it was observed that the wire markers moved closer together as the zinc diffused out of the brass and into the copper.
Compositional change of the material from diffusion was confirmed by x-ray diffraction.
Relative to the molybdenum markers, the copper–brass interface moves toward the brass at an experimentally measurable rate.
One consequence of this equation is that the movement of an interface varies linearly with the square root of time, which is exactly the experimental relationship discovered by Smigelskas and Kirkendall.
[1] Darken also developed a second equation that defines a combined chemical diffusion coefficient
One important consideration deriving from Kirkendall's work is the presence of pores formed during diffusion.
These voids act as sinks for vacancies, and when enough accumulate, they can become substantial and expand in an attempt to restore equilibrium.
Porosity occurs due to the difference in diffusion rate of the two species.
[5] Pores in metals have ramifications for mechanical, thermal, and electrical properties, and thus control over their formation is often desired.
is a concentration difference between components, has proven to be an effective model for mitigating Kirkendall porosity.
Controlling annealing temperature is another method of reducing or eliminating porosity.
[7] In 1972, C. W. Horsting of the RCA Corporation published a paper which reported test results on the reliability of semiconductor devices in which the connections were made using aluminium wires bonded ultrasonically to gold-plated posts.
His paper demonstrated the importance of the Kirkendall effect in wire bonding technology, but also showed the significant contribution of any impurities present to the rate at which precipitation occurred at the wire bonds.
Both Kirkendall voids and Horsting voids are known causes of wire-bond fractures, though historically this cause is often confused with the purple-colored appearance of one of the five different gold–aluminium intermetallics, commonly referred to as "purple plague" and less often "white plague".
