AMPLITUDE DEPENDENT INTERNAL-FRICTION AND MAGNETOACOUSTIC EFFECTS IN YBCO CERAMICS

The paper is concerned with effects in damping and Young's modulus obs erved in YBaCuO ceramic superconductors vibrating ultrasonically eithe r in or without magnetic field. The amplitude and time dependencies wh ich arise as a result of high amplitude vibrational excitation are pre sented for coarse-grained and fine-grained specimens. Weak magneto-aco ustic effects were observed, and found to be strongly influenced by vi bration amplitude and by thermal pre-history of the sample. Analysis o f the results shows that, in the vicinity of liquid helium temperature , high amplitude ultrasonic stresses induce structural modifications i n these YBaCuO ceramics. Dislocation point defect interaction mechanis ms are proposed for explaining the micro-plastic behaviour of the YBaC uO samples observed in the ultrasonic experiments. The magneto-acousti c effects are discussed in the framework of flux line pinning models.