Patent GB-1208154-A (GB)
Title: Improvements in or relating to acoustical transducers
Inventor: MITCHELL RICHARD FRANK
IPC: G01R19/00
Filed: 1968-02-13
Abstract: 1,208,154. Piezo-electric elements. MULLARD Ltd. 6 Jan., 1969 [13 Feb., 1968], No. 7140/68. Heading H1E. [Also in Division H3] An electromechanical transducer comprises a piezo-electric element the nature of the material or the arrangement of the electrodes of which are such that the application of a potential results in the creation of a piezo-electric stress (eE or hD), the magnitude of which as measured in the direction of propagation of the resultant mechanical stress T(x) rises linearly from zero to a maximum and then drops abruptly to zero. The specification sets forth a theory showing that, in such an element, the application of a voltage impulse results in a sharp mechanical impulse which propagates from the point where eE is zero in the direction of increase of this quantity. The linear increase in piezo-electric stress may be obtained by a gradation in the coupling coefficient of the material. Where this is made from a deposited material, e.g. cadmium sulphide, the coefficient depends upon the inclination of the crystalline c-axis of the material and this may be varied, e.g. from 0 to 25 degrees as the material is deposited on the usual substrate. The angle of inclination of the crystals varies with the direction of the vapour beam. Alternatively, in the case of a material such as CdS, the coupling coefficient may be graded by grading the resistivity, e.g. by addition in varying degrees of a dopant such as indium. In the case of a ferroelectric material, the polarization may be graded. As describd, a body of e.g. PZT is held at a temperature just below its Curie point and subjected to a transverse polarizing field which is applied to different portions of the body for different lengths of time depending upon the desired remanent polarization. Alternatively, the body may consist of a series of discs 18 having linearly graded coupling coefficients, the gradation being obtained by the following methods: (a) In the case of deposited materials such as CdS or ZnO, the conditions of vapour deposition are varied from disc to disc. (b) In the case of ferroelectric materials the polarization may be varied from disc to disc, e.g. the same polarization potential may be applied to different areas of the various discs. (c) Alternatively, slices 19 may be cut at a varying angle # from a uniformly polarized cylinder 20 of ferroelectric material. Each such slice may be utilized as a disc 18 or, alternatively, each slice 19 may be out in half along a radial plane, the two halves relatively rotated through 180 degrees and then re-assembled. In the resultant disc, the transverse components of the polarizations cancel each other leaving a longitudinally polarized disc of reduced coupling. Alternatively, the two halves of the slice are reversed with respect to each other in such a manner that the longitudinal components cancel out on reassembly whereby a disc with transverse polarization is obtained. The same result may be obtained without reversal of the halves of the slice by driving the slices in antiphase, but this requires the use of two sets of electrodes in the final assembly. As distinct from methods involving gradation of the material, others using a non-uniform electric field may be used. Where the transducer consists of a single slab, a small central electrode is connected to one major face and an electrode about 10 times larger to the other face. In the case of a transducer comprising a number of discs 18 having uniform properties, different potentials may be applied to the various discs by means of a potentiometer having resistors 16 of graded resistance.