Impact response of germanium over 300-1143 K temperature range

Impact response of < 111 > oriented germanium single crystals and polycrystalline samples obtained by high-pressure spark plasma sintering of pure germanium powder was studied in two series of planar impact tests performed at 300 and 1143 K with samples of different thicknesses and in a series of tests with 2 mm single crystals preheated up to the temperatures 300-1143 K. In all the tests, the samples were shock-loaded by tungsten impactors having velocity 980 +/- 40 m/s, while the velocity of the interface between the germanium sample and the fused silica window was continuously monitored by velocity interferometer. Under compression, the cubic diamond (cd) germanium transforms into its high-pressure (beta-Sn or liquid) modification. The stress corresponding to the upper bound of the existence of impact loaded cd germanium was found to depart upward from that obtained in the static experiments. At temperatures greater than 900 K, this departure increases due to the initiation of melting in the shock-loaded material. Part of the velocity histories recorded with either single or polycrystalline samples was characterized by a four-wave (instead of the expected three-wave) structure. This "surplus" wave seems to be caused by a short-term existence of an intermediate (nonequilibrium) germanium phase which, however, does not affect the principal germanium Hugoniot.