The Application of High Energy Density Transducer Materials to Smart Systems
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Mat. Res. Soc. Symp. Proc. Vol. 459 01997 Materials Research Society
HIGH POWER PROJECTOR MATERIALS High power projector materials have traditionally focused on the lead zirconate titanate compositions known by their copyrighted (by Clevite) names as PZT-4 and PZT-8. With the exception of a few barium titanate holdouts, all production underwater projectors in use in the U.S. Navy are of one of these two compositions. Recent trends have been to develop and introduce new high energy density drive materials into the Fleet and to tailor each material to its specific need or application. New drive materials include Terfenol-D [1-3], low temperature magnetostrictive materials [4], lead magnesium niobate (PMN) [5], and piezoelectric polymers [6]. Each of these materials has a potential use or niche and each has advantages and disadvantages. A useful way to rank these materials among themselves and to the PZT benchmark, is to calculate the field-limited energy density of the basic material.
This metric is defined as one-half the product of the Young's modulus of the material times the square of the maximum strain obtainable at the electric or magnetic field limit of the material. This permits a comparison of materials on a per unit volume basis. Table 1 is an abridged summary of materials currently of interest. The field limit used for the PZT-8 benchmark is 10 V/mil which is the industry standard for reliability and acceptable losses. Lead magnesium niobate (PMN) is an electrostrictive ceramic which promises high strain while all of its other properties (notably cost) resemble that of PZT. The values given for PMN are representative of its current state of development. The energy density of polyvinylidene fluoride (PVDF) voided homopolymer is considerably less than that of PZT but its pc makes for a very broadband device. Its copolymer, poly(vinylidene fluoridetrifluoroethylene) is also suited for broadband operation but is stiffer and at its full field limit becomes a true competitor to PZT if one can conquer the engineering challenge of reliably applying the field of 750 V/mil. The drive levels associated with the Terfenol-D tabulation are for the general case of limiting harmonic distortion to approximately 15% to permit broadband transducer operation and for the higher drive case in which it is presumed that operation will only be around resonance where the projector will act as a bandpass filter and suppress radiated harmonics. Terbium-dysprosium, used at cryogenic temperatures, yields the highest known magnetostriction of any material. The material cited is Tb0. 6Dy 0.4 at 770 K and represents data collected at a relatively low magnetic field. The sonar system designer desires a transducer element which uses a completely characterized and tractable driver material which, if possible, possess electrical properties similar to existing materials so that an improved transducer can easily be incorporated as a simple upgrade to the system. At first look the incorporation of a high energy density relaxor material a
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