GB832608A - Improvements in or relating to piezo-electric transducers - Google Patents

Abstract

832,608. Piezo-electric elements. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 7, 1956 [Feb. 7, 1955], No. 3598/55. Class 40 (8). A piezo-electric torsional transducer comprises an elongated, longitudinally-polarized, piezo-electric body of constant cross-section which is weakened in torsion by being thinned in the region of, and as measured along, a minor axis, four electrodes being arranged symmetrically about the minor axis. If the electrodes 5, 6, 7, 8 shown in the cross-section of Fig. 1 are connected in the manner illustrated the body will vibrate torsionally in response to signal voltages. The body may be thinned by the provision of grooves 1, 2 which may have the configuration shown or may have straight sides which include an angle of up to 90 degrees. Polarization is achieved by applying voltages of 10 to 150 kilovolts per cm. to temporary electrodes on the end faces of the body. The body shown in Fig. 1 could be made by cementing two grooved elements back to back. In the modification of Fig. 6 two identical bars 12, each having grooves 13 are secured together face to face after they have been polarized and provided with electrodes 14, 15, 16, 17. A glass spacer 18 interposed between the bars 12 provides insulation between the electrodes. The spacer is terminated short of one end to provide room for electrode connections in the form of strips of silver foil. If the bars 12 are polarized in the same direction common connections are made to pairs of electrodes lying diagonally opposite each other as shown. If the bars are arranged with their polarizations in opposite directions, common connections are made to immediately opposite pairs of electrodes. In the latter case the transducer can be provided with a closed polarization circuit by replacing portions of the glass spacer at the ends of the bar by a material having a high dielectric constant, This might be the piezo-electric material itself, in which case an integral step of half the thickness of the glass may be provided at each end of the bar. Alternatively a spacer of barium strontium titanate may be used. In these constructions the glass spacer may be omitted entirely. Suitable materials for the transducer are titanates, zirconates and niobates or mixtures of these substances. A suitable mixture consists of barium titanate with 4% of lead titanate or with about 8% of both lead and calcium titanate.