Kirkendall effect

The Kirkendall effect is that at a sufficiently high temperature at two adjacent solid phases, the volume of one phase is reduced, while increasing the volume of the other phase. The effect is particularly visible when the phase boundary was previously marked, because then observed a displacement of the marking relative to the outer geometry of the sample. The phase boundary migrates itself, but it moves matter between phases and thus the position of the phase boundary relative to the outer sample geometry.

History

The Kirkendall effect was named after Ernest Kirkendall (1914-2005), the observed changes in volume during his doctoral work in the late 1930s, although he was not the correct interpretation. 1942 and 1947 published Kirkendall two papers which describe the scope of discovery. Was convinced was not until 1950, when at a conference one of the opponents of Kirkendalls interpretation (RF flour), this began to assert themselves.

Description

Kirkendall observed the effect of metals or alloys, in particular with copper / brass, which was heated to 780 ° C, for example. With his colleague Alice Smigelskas he could make the shift of the phase boundary with the help of molybdenum wires visible, which had been introduced at the phase boundary; this marking wires move as you do this.

Often arise in the phase, which decreases their volume and characteristic holes near the phase boundary, the Kirkendall holes. This influence, along with the volume changes the stability of metal joints, which is why the Kirkendall effect also has practical importance, for example in reactor technology or in semiconductor technology, if there joints between aluminum and gold are used.

The importance of the Kirkendall effect is, in particular, that has been demonstrated so that diffusion takes place in the solid state via vacancies. ( The alternatives direct exchange of positions or ring exchange of particles have not been observed. ) The Kirkendall effect is created by a flow of vacancies between the phases. This in turn inevitably arises when the mobility of the atoms of the different phases is different, so if the diffusion constants differ.