Method of making composite piezoelectric transducer arrays

DETAILED DESCRIPTION The present invention is directed to a new composite structure providing thin wafers of piezoceramic which are electroded on opposing major first and second surfaces and polarized along the thickness between the electrodes.
The wafers are spaced apart by a layer of passive polymer.
An element includes at least two electroded ceramic wafers and an intermediate passive polymer layer.
The piezoelectric wafers are driven electrically along the wafer thickness to generate acoustic vibration in the perpendicular direction (see FIG.
7).
This mode of vibration is referred to as transverse `31` mode.
By providing a large electrode area on the major surfaces of the ceramic wafer, and a relatively small wafer thickness, the resulting capacitance for each element is increased and the corresponding electrical impedance is reduced.
The elements can be provided in a 2--2 or 3-1 composite structure.
The number of wafers in a given element can be selected to provide a desired impedance match.
In a linear or phased array, there may be hundreds of wafers provided in a strip.
In a 2-D array, a plurality of such elements can be provided in a square lattice in two dimensions.
The ceramic wafers interspersed with polymer layers results in a low acoustic impedance composite structure.
Thus, both electrical impedance and acoustic impedance are optimized for tiny array elements.
Another aspect of the present invention are various methods of making the composite structure.
In a first method, wafers of piezoelectric ceramic are electroded along their major surfaces and polarized along the thickness.
The wafers are stacked with spacers along one edge to build a stack of tens or hundreds of wafers, depending on the desired application.
The stack is then cast in an epoxy polymer matrix.
The area containing the spacers is trimmed away to provide the composite structure.
If the original surface electrodes extend to the edges of the element,.
channels are cut into the composite at regular intervals along opposite edges and the channels are then filled with polymer