Generation of Elastic Waves by Transient Surface Heating

When the surface of a body is subjected to transient heating (e.g., by electron bombardment or rf absorption) elastic waves are produced as a result of surface motion due to thermal expansion. This process is analyzed, with particular emphasis on the case of an input heat flux varying harmonically with time, to relate the elastic wave amplitude to the characteristics of the input flux and the thermal and elastic properties of the body. Experiments performed with both electron impact and rf absorption verify the proportionality of the stress wave amplitude and the absorbed power density, and correlate well with the thermal and elastic properties of the heated medium. Comparison of the elastic wave stress amplitude with radiation pressure shows that the former may be much greater than the latter, as experiments have demonstrated. When a barium titanate crystal was used to detect the elastic waves produced, heating by a single 2-μsec pulse of electrons or microwave radiation produced easily detectible signals at power levels down to 2 W/cm2, corresponding to a computed peak surface temperature rise of about 0.001°C, and produced piezoelectric crystal voltages ranging from about one to more than 60 mV/kW/cm2 absorbed power density.