Experimental investigation of phased array using tapered matching layers

Although composite piezoelectric materials are effective in increasing the bandwidth of a transducer array, they have one major drawback, i.e. the reduced volume fraction of the ceramic results in the elements having increased electrical impedance, which decreases the transducer array´s sensitivity. The use of tapered matching layers has been proposed as another method of increasing the transducer bandwidth. Tapered matching layers have the potential of realizing a transducer array with a bandwidth comparable to a composite transducer array, but without increasing the electrical impedance. But to the authors´ knowledge, few subsequent publications on this method have appeared in the literature. We have therefore experimentally investigated the use of tapered matching layers to increase the bandwidth of a phased array. These fabricated tapered matching layers have pyramidal structured two-dimensional gratings consisting of high and low acoustic impedance materials. They were used to make a 3.0 MHz phased array. High impedance material was made from epoxy and tungsten powder. Low impedance material was made from epoxy, inorganic micro balloon and polymer powder. We have verified that these tapered matching layers function effectively in both transmit and receive modes. In this paper, the design requirements and the method for making tapered matching layers are presented. Measurements from this phased array are compared with those from a 3.0 MHz phased array with conventional two matching layers.