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排序方式: 共有1731条查询结果,搜索用时 15 毫秒
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M. Platte 《Ultrasonics》1985,23(3):113-118
A new type of piezoelectric polymer hydrophone for the frequency range above 1 MHz is described, which consists of a needle, coated with a thin layer of polyvinylidene fluoride. The sensitive area is a small polarized zone of the polymeric material around the needle point. Special properties, caused by the constructive design of this ‘needle hydrophone’, are its non-directivity in a wide angular range, and high spatial resolution of the ultrasonic sound field pattern. Construction and performance are compared with other types of piezoelectric polymer hydrophones. Some possible applications are discussed. 相似文献
175.
The performance of transducers used for medical diagnosis depends to a great extent on matching layer, backing impedance and geometry of the active surface. In this paper special attention is given to the element vibrational modes, the optimum matching, the grating lobe and change of the real acoustic impedance into an imaginary impedance as a function of the product of the width and wavelength. Grating lobes on the echo image and the effect of a mismatch of the matching layer are illustrated. Suggestions are made for the design of linear and phased array transducers considering the above-mentioned aspects. 相似文献
176.
Ultrasonic critical angle reflectivity method is used to measure the phase velocity of generalized Lamb waves in cadmium and
zinc platings. In addition to the free propagating modes, leaky-wave modes are also observed. Leaky waves are considered using
principles familiar from the discussion of surface waves in anisotropic media and a relatively good agreement is obtained
between the theoretical computations and measured data. 相似文献
177.
The elastic constants of single crystal galena have been determined from the measured ultrasonic velocities down to liquid
helium temperature. A cryostat incorporating an arrangement to inject the liquid bonding material at low temperature is described.
At 5 K, the values of elastic constants are C11=14.90, C12=3.51 and C44=2.92×1010 N/m2. 相似文献
178.
G. Hayward 《Ultrasonics》1984,22(4):153-162
An analysis of thickness-mode piezoelectric transducer impedance under a variety of mechanical load conditions is presented. A wideband, systems model is used to describe this impedance, which is representative of the electrical input impedance in the transmitting mode and the electrical output impedance for the receiving mode.For a fixed input voltage, the total current flowing through the transducer is considered as the vector sum of two current quantities. Firstly, an input current arising directly from the applied voltage source and secondly, a feedback current, which is generated by secondary piezoelectric action. By treating the problem in this manner, an extremely valuable insight into the nature of the electrical characteristics is provided. Complex variations in the impedance behaviour are readily explained and the physical processes that contribute to such deviations are clearly isolated. 相似文献
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A wide variety of transducer shapes and applications exists in the field of diagnostic ultrasound. As a basic material ferro-electric ceramics are used almost exclusively, but polymer piezoelectric material seems a new alternative. By comparing in a qualitative way the most relevant parameters of these materials, a state of the art judgement may be made about their applicability in diagnostic ultrasound. 相似文献