A taper microstructure for impedance matching

The object of this work is to present the concept of a composite material which consists of many closely packed tapered microrods. The material is intended to be used for ultrasonic transducer matching and fabricated using photolithographic methods. We find that for appropriate microrod diameter and taper, relative to the wavelength, the tapered microrod has a smooth continuous transition in its acoustic impedance. Further for the structures that seem most easily fabricated importantly this includes impedance in the range 3 to 10 MRayl, where there is no useful alternative solid. Wave propagation in the microstructure has been studied with PZFlex finite element modeling and this work shows that when an appropriate taper has been selected that energy transferred is dominated by a single mode. The reflection property of the composite has also been investigated and this data is compared with that for single and dual conventional quarter wave matching layers. It is found that the tapered microrod can provide matching over a very broad frequency range. Unlike loaded epoxy implicitly there is no wave scattering.