Copper whisker growth from inside sulfur-doped YBa2Cu307-X pellets

We report on the growth of Cu whiskers (10 mu m in linear transverse size) at the surface of a YBCO pellet, in a sulfur-rich atmosphere at a high synthesis temperature. Growth arises from inside the pellet since there is no Cu in the surrounding atmosphere. We present the experimental conditions and electron microscopic observations. We briefly propose a mechanism based on chemical considerations, in particular, a Cu reductive nucleation of the CuS phase at the surface of YBCO. Natural phenomena, laboratory findings, and computer simulation work (based on an Eden model) support this interpretation.