Natl. Res. Lab. of Metrol., 1-1-4, Umezono, Tsukuba, Ibaraki 305, Japan
Longitudinal sound velocities of thin sheet materials were determined between room temperature and 1000 (degrees)C. The velocities were calculated from sample thickness and round trip echo times. A micrometer or an ellipsometer was used for thickness measurement. A single optical pulse was extracted from a passively mode-locked Nd:YAG laser by using an optical switch. The optical pulse has 30-ps pulse duration and 10-mJ output energy, which is focused on the sample surface to generate ultrasound. Ultrasound is detected with a Michelson interferometer having a frequency response up to 1 GHz. Since this system has no contact with the sample, it is suitable for acoustic measurements at high temperatures. Sound velocities of 304 stainless-steel sheets were measured at various temperatures in the range from RT to 1000 (degrees)C. It was shown that round trip echo time could be measured to an accuracy of 3% for the sample of 20 (mu)m thickness. Furthermore, several kinds of ceramic thin sheets were also examined.