MICROSTRUCTURES OF UNDERCOOLED GERMANIUM

Liquid Ge drops (diameter approximately 7 mm) have been undercooled 60-342 +/- 3-degrees-C below T(m) in dehydrated boron oxide with a cooling rate of 6-degrees-C/min. They always crystallized to the diamond cubic phase and the resulting grain size decreased with increasing undercooling. During crystallization, audible sound was emitted whenever the undercoolings were greater-than-approximately 258-degrees-C. It was determined that cracks were responsible for sound emission while grain refinement gave rise to cracks. Optical and SEM micrographs showed that for undercoolings greater-than-approximately 200-degrees-C, dendrites formed first and they were seeds for the subsequent crystallization. The micrographs also illustrated that grain refinement was due to multiplication of dendrites. During crystallization, the compressive wave (Ge expands on solidification) set up by the rapid dendritic growth recoiled from the liquid germanium-boron oxide interface would be minimized by the surrounding B2O3. It is likely then that dynamic nucleation was not the origin of grain multiplication. The multiplication of grains is therefore attributed to dendritic break-up during solidification. It is further suggested that stress is the origin of dendritic break-up.