Acoustic Properties of Aerogels

Gelation The raw material solution for gelation was prepared by mixing TMOS, ethanol, and ammonia water at predetermined ratios. By maintaining the temperature of the solution at 40°C for 24 h, a hydrophilic silica wet gel with water and ethanol was obtained. 

Silation Next, in order to introduce a hydrophobic methyl group to the silanol group of the gel (Chap. 3), it was soaked in a mixed solution of isopropyl alcohol IPA), dimethoxydimethylsUane (DMDMS), and ammonia water. To remove water from the gel, solvent exchange was carried out in IPA at 40°C for 24 h. 

The most notable feature of silica aerogel was its extremely low acoustic velocity. Even though the silica aerogel is made of an inorganic material with an acoustic velocity of about 5,000 m/s in the case of solid [6], its acoustic velocity is extremely low as a bulk material. 

From these basic experiments, it was found that silica aerogels had very low acoustic impedances compared to those of the current matching layers [7-9], and are therefore very promising as materials for the acoustic matching layers of high-sensitivity airborne ultrasonic transducers. 

The main part of AC-ML is a high-strength porous ceramic filled with an aerogel. 

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As already mentioned, the sound velocity is measured with much better precision than the elastic moduli. Therefore, the generally small influence of some other parameters on the acoustic properties of aerogels is experimentally accessible. It is striking that in aerogels of different materials, the sound velocity was found to be lowered by uniaxial compression [52]. In normal solid ma-... 

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