Ultrasonic Evaluation of Polymers and Composites Using Air-Coupled Capacitance Transducres

It is often necessary to evaluate materials using non-contact ultrasonic techniques, for example when the test sample is hot, moving, or highly absorbent to conventional fluid couplants. Several non-contact methods are available, such as various optical techniques [1–3], which are generally expensive and require the sample to have optimized optical characteristics. Electro-magnetic acoustic transducers (EMATs) [4,5] and capacitance devices [6] may be used, but require an electrically conductive sample, and a small stand-off distance of a few millimeters or less. There has been much interest recently in the use of air-coupled transducers [7], which may be piezoelectric, using piezopolymers such as PVDF [8], piezocomposites of PZT and epoxy [9,10], or piezoceramics with impedance matching layers on the transducer face [11]. Another type of device is the electrostatic or capacitance transducer [12,13], which consists of a metallized polymer membrane against a backplate electrode to which a bias voltage is applied. Motion of the membrane causes the charge on the backplate to change, which may be detected using a suitable charge amplifier. These devices in general have a wider bandwidth than their piezoelectric counterparts, and improved sensitivity. The backplates are usually mechanically roughened metal, and it is therefore difficult to manufacture two identical devices. However, using a silicon backplate [14–17] and standard etching techniques, the surface of the backplate may be precisely controlled. Such a device is shown schematically in Figure 1. The backplate consists of a silicon wafer into which pits 40μm in diameter and 80μm apart have been anisotropically etched to a depth of approximately 40μm. A gold electrode is then evaporated onto the backplate, and a thin metallized polymer membrane is then placed next to the plate