Soft-metal whiskers

Need oxygen to grow, according to researchers at Drexel University in Philadelphia. Over a period of days or weeks at room temperature, tiny protrusions can sprout spontaneously like hair from soft metallic surfaces of, for example, tin or indium. The phenomenon has resisted interpretation for more than 50 years and poses a reliability problem for semiconductor manufacturers; unexpected whisker-induced short circuits have led to failures of heart pacemakers, avionic relays, and satellites. The Drexel scientists placed soft-metal compounds in an evacuated glass tube and in air. Those in air were also partially coated with fingernail polish. After three months, only the samples directly exposed to air grew whiskers (top photo). The researchers think that oxygen diffusion at grain boundaries creates soft-metal oxides with a larger volume than the pristine metal. The resulting stress field then pushes the whiskers out. The incipient whiskers seen on the evacuated-tube sample (bottom) were attributed to trace amounts of oxygen in the tube or adsorbed on the surface. (M. W. Barsoum et al., Phys. Rev. Lett. 93, 206104, 2004 http://dx.doi.org/10.1103/PhysRevLett.93.206104 .)