POROSITY BEHAVIOR OF ULTRASONIC VELOCITIES IN POLYCRYSTALLINE Y-B-C-O

Longitudinal and shear velocities are measured for five polycrystallin e Y-B-C-O samples presenting different porosities. The experimental da ta are compared with the velocity versus porosity curves calculated ac cording to three different methods and a discussion is presented on th e reliability of these methods in estimating the porosity effects, bot h at low and at high void concentrations. For the calculation several sets of the nine elastic constants of Y-B-C-O monocrystals are used. A remarkable good agreement between our data and those obtained from th e single-crystal constants is observed, when the method of Sayers and Smith is used to take porosity into account.