Syntactic foam for air-equivalent solid backing by natural origin Shirasu-microballoon

Low density, solid, spurious-echo-free backing materials are necessary component for high efficiency, air-equivalent backed transducers, typically, for imaging and CW Doppler systems including 2D FPA (focal plane array) configuration. Syntactic foams are known useful material for this purpose; however, physical nature, size and size-distribution are the issues for material choice of microballoons or microcapsules used therein. Industrial origin microballoons such as of silica, glass, carbon or plastic materials are available for choice. Higher density of microballoon is desirable to simulate air-backing with solid, stiff material, adequately wide size-distribution of microballoon filling material is desired, which by industrial origin materials is somewhat tricky requirement because they are size controlled for general or specified application. In Kagoshima Japan, and other districts having similar volcanic activity for millions of year, ash has piled up to make soil have glassy particles having naturally pressurized gas inclusion, called "Shirasu" in volcanology term, which by simple high temperature process can make a variety of natural origin microballoons, called Shirasu-microballoon. Using this relatively wide size-distribution microballoon, epoxy based syntactic foam are made for evaluation. It was author\´s consistent finding that the Shirasu microballoon based syntactic foam can easily make air-equivalent backing material whose acoustic impedance is less than 0.2 Mrail but yet sufficiently "solid" to support fragile PZT substrate or array. Another designation can yield matched-to-water material good for lens and/or echo-free water vessel wall. Its cost advantage also benefits when a quite a large size component is required. However, careful filtering out of larger balloons is required if 7 to 10 MHz or higher frequency is concerned