Ultrasonic measurements in fine powders using a photoacoustic pulse-generation technique

In this paper, we demonstrate the applicability of PA pulse generation for the monitoring of ultrasonic properties of powders, for example, uncured carbon-loaded epoxy resin powders, whose average particle diameter is ~15 μm. The acoustic pulse is generated by a short laser pulse (10-nsec or 1-μsec duration) of 1–10-mJ energy and is detected by a piezoelectric detector (polyvinylidene difluoride foil or commercial lead zirconate titanate transducer). This allows ultrasonic measurements over a broad range of porosity and pressure conditions, from loose unconsolidated powers to heat- and pressure-sintered pellets. For loose powders at atmospheric pressures (with a porosity of ~50%), the acoustic velocity is remarkably small (typically 3 × 103 cm/sec), and attenuation and dispersion are large. This appears to be the first demonstration of the use of a PA technique to measure ultrasonic properties in powders.