MICROSCALE THERMAL RELAXATION DURING ACOUSTIC PROPAGATION IN AEROGEL AND OTHER POROUS MEDIA John F. T. Conroy, Bou v ard Hosticka, Scott C. Dav is, Andrew N. Smith, and Pamela M. Norris Department of Mechanical and Aerospace Engineering, Uni versity of Virginia, Charlottes ville, Virginia, USA The longitudinal acoustic velocity in silica aerogel is presented as a function of the interstitial gas type and pressure. This was measured using air-coupled ultrasonic transducers configured for differential pulse transit time measurements. The results are interpreted in terms of the thermal relaxation of the acoustic pulse. The microscale temperature oscillations of the gas and solid phases of the aerogel due to the acoustic pulse are not identical if the rate of heat transfer between the two phases is slow compared to the period of the acoustic oscillation. The energy transferred from the gas to the solid phase is lost to the acoustic propagation and, thus, reduces the amplitude and velocity of the acoustic wave. The gas type and pressure may provide independent v ariables for probing these effects in aerogel. Aerogels are highly porous, nanostructure d solid media commonly formed by the supercritical drying of a liquid-filled gel. A sol-ge l production process involves the hydrolysis and condensation of alkoxy metallates to form a three-dimensional gel. This results in a fractal microstructure composed of nanometer size particles and pores as illustrated in Figure 1. The porosity of aerogels commonly range s between 85% and 97% and may include micro-, meso-, and macropores. Because of proposed applications as acoustic delay lines, impedance matching layers, and acoustic and thermal insulation, the acoustic properties of aerogels have been the w x subject of several recent studies 1 ] 6 . The benefits of utilizing aerogels in such applications include an acoustic velocity less than air and an acoustic impedance that is tunable with production conditions. The theoretical description of acoustic propagation in aeroge ls is not com- plete. At ultrasonic frequencies, the porous aerogel microstructure gives rise to s. unique and anomalous acoustic behavior such as: 1 a nearly linear increase in s . atte nuation with frequency constant Q in the solid skeleton in the ultrasonic wx s. wx frequency range 4; 2 a decrease in acoustic velocity with compressive strain 5; s. wx s. 3 unexplaine d atte nuation bands at distinct ultrasonic frequencies 6 ; and 4 a Received 18 February 1999; accepted 21 April 1999. This work was sponsored by DARPA under contract MDA972-97-C-0020. The authors would also like to thank Dr. Charles Daitch and Veridian-Pacific Sierra Research for their support. Address correspondence to Prof. Pamela M. Norris, Department of Mechanical and Aerospace Engineering, Thornton Hall, University of Virginia, Charlottesville , VA 22903, USA. E-mail: pamela@virginia.edu Microscale Thermophysical Engineering, 3:199±215, 1999 Copyright Q 1999 Taylor & Francis 1089-3954 r 99 $12.00 H .00 199