| 摘要 |
953,340. Piezoelectric elements. SOUTH AFRICAN COUNCIL FOR SCIENTIFIC & INDUSTRIAL RESEARCH. May 24, 1960 [June 3, 1959], No. 18363/60. Heading H1E. [Also in Division H4] An electromechanical transducer comprises a powder capable of being electrically polarized to impart piezo-electric properties, mixed with an insulating liquid which wets the powder. The two constituents are mixed to form a paste consisting e.g. of 70% barium titanate in ethoxylene resin or polyisobutylene. The mixture is made at 100‹ C. in order to eliminate moisture. The volume percentage of the powder may range from 10 to 90 and the size of particles from 50 to 500 microns. If the transducer is intended for use at a frequency corresponding to a wavelength # the size of the particles is made equal to or less than #/2. The pastes comprising finer powders may be polarized at room temperatures using fields of about 8 kv./cm. for about 1 min. For the coarser powders, a temperature of 135‹ C. is used for polarization. The paste 1 may be confined within a soapstone ring 2 upon the surface of an object 3 which is to be subjected to ultrasonic testing, the polarizing potential being applied between the object 3 and an electrode 4b. The bottom of the electrode may be coated with a silicone paste or with polyethylene. The electrode may have a backing 4a of energy absorbing material. After polarization, the transducer may be used to produce ultrasonic pulses for probing of the object 3. For optimum efficiency the thickness of the paste should be of the same order as the particle size, i.e. about half a wavelength. In order to obtain thin layers, paste 17, Fig. 5, may be smeared over the surface of the test object 18. An electrode 14 resiliently mounted in an insulating housing 15 may then be applied to any part of the smeared area.
|
