Patent GB-641469-A (GB)
Title: Improved piezoelectric crystals
IPC: H03H9/17
Filed: 1946-11-21
Abstract: 641,469. Piezo-electric crystals. WESTERN ELECTRIC CO., Inc. Nov. 21, 1946, No. 34524. Convention. date, Aug. 9, 1943. [Class 40 (viii)] Plates having rectangular major faces are formed from one of the primary phosphates or arsenates of potassium or ammonium with an orientation with respect to the crystal axes (Fig. 1) permitting operation in the face longitudinal or face shear modes of motion. The face-longitudinal plates have their width lying in the plane of any two of the axes and the plane containing their length and the third axis at a selected angle to one of the two axes. The face shear elements have their major faces perpendicular to the X or Y axes or at an angle of not greater than 45 degrees to the Z axis. The longitudinal mode elements 8, 9 and 10 (Fig. 3) have their length at 45 degrees to the two axes to give a maximum of motion along their length when excited by electrodes on their major faces. In the case of the Z-cut element 10, the 45 degrees orientation also corresponds with the largest piezo-electric constant and the least coupling with the face shear mode, the corresponding angle for the X and Y cut elements being 24.5 degrees with respect to the Z axis. Elements 8, 9 and 10 may be rotated about their width and a graph (Fig. 3A, not shown) illustrates that in the case of a typical Z-cut element 10 a decrease of temperature coefficient is obtained, but at the cost of electromechanical coupling, a favourable balance being obtained at 60 degrees. The width to length ratio is preferably less than 0.5 to avoid spurious modes of operation, but simultaneous operation in other modes may, however, be desirable for example in the width mode to increase the frequency range of supersonic generators. Details are given of two typical X-cut elements 8 with their length at 22.5 degrees and 67.5 degrees to the Z axis and two Z-cut elements 10 with the length at 22.5 degrees to the X axis. The face shear elements are shown at 5, 6 and 7 (Fig. 3) and these when excited by electrodes on their major faces have all of their motion in the face shear mode and none in the face longitudinal mode. The Z- cut element 7 is of most interest since it has a large value of piezo-electric constant which furthermore is maintained if the element is rotated about its length and/or width. The electrodes, for all the above elements, may be of gold, platinum, aluminium or silver and may be deposited on part or the whole of the major faces. In addition nickel plating may be applied to adjust the frequency of operation. Parallels are drawn with the operation of the present elements and those formed from other substances and described in Specifications 437,294, 554,788, 554,789 and 555,400. Specification 569,285 also is referred to. The Specification as open to inspection under Sect. 91 states that a number of elements such as that shown at 10 (Fig. 3) cemented together at their major faces may be cemented to a ceramic wafer and a steel resonator to form a supersonic vibrator. Much additional information about the constants of the crystalline materials and the equations by which the performance of elements may be calculated is given. The subject-matter of Specifications 641,470 and 641,471 also is included. This subject-matter does not, however, appear in the Specification as accepted.