Growth, Characterization, and Elastic Properties of Bismuth Based High Temperature Superconducting Whiskers.

Whiskers of bismuth based high temperature superconductors have been grown by two different techniques. Splat quenched glass, and disks of sintered material have been used. Good quality single crystals have been obtained from both techniques. Electron diffraction shows the structure to be rm Bi_2Sr_2CaCu_2O_{x} for whiskers grown by both techniques. Transport and SQUID magnetometer measurements indicate that the transition temperature for whiskers grown from the sintered material is between rm T_{c}~ 75 K and rm T_{c} ~ 90 K. Whiskers grown from the glass have shown transition temperatures near 107 K indicating the presence of rm Bi_2Sr_2Ca _2Cu_3O_{x}. Young's modulus has been measured using a device that applies uniaxial stress and simultaneously measures the elongation (strain) of the whisker. Whiskers with rm T_{c}~ 75 K grown from sintered disks have displayed anomalous elastic properties. Above 270 K, the stress-strain response is hysteretic. Young's modulus is soft, with Y ~ 20 GPa at 270 K and decreases sharply with increasing temperature. Below 270 K, hysteretic behavior is not observed and Y does not vary as quickly with temperature. This suggests that a structural phase transition may occur (ferroelastic). Other whiskers grown from sintered disks show a stiffer Young's modulus with Y ~ 80 GPa. No hysteresis is observed in the stress-strain response of these whiskers nor is Y strongly temperature dependent. Whiskers grown from a glass also show a stiff Young's modulus of Y ~ 80 GPa without hysteresis. Again, very little temperature dependence is observed.