Porosity behaviour of ultrasonic velocities in polycrystalline Y-B-C-O

Longitudinal and shear velocities are measured for five polycrystalline Y-B-C-O samples presenting different porosities. The experimental data are compared with the velocity versus porosity curves calculated according to three different methods and a discussion is presented on the reliability of these methods in estimating the porosity effects, both 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 the single-crystal constants is observed, when the method of Sayers and Smith is used to take porosity into account.