Ceramic electrets

Electret. An electrical analogue of a permanent magnet a material that is permanently electrified and exhibits electrical charges of opposite sign at its extremities. In order to retain their charge for a long period (days or weeks) ceramic electrets must be polarized at high temperature materials that have been treated in this way include the titanate dielectrics. 

Gubkin and Skanavi [6S] prepared stable ceramic ciectrets from various titanate materials, such as strontium-bismulb titanate, barium titaiute, strontium titanate, calcium titaiute, etc. They showed that ceramic electrets retain the majority of their charge without the need of shorting them with metal fbilt. Thomas A. Dickinson (66,67) showed... 

Quartz and piezoelectric ceramic crystals have more temperature independent constants than PVDF, so they are used for force and acceleration transducers. However, PVDF films can be used for large area flexible transducers. Their sensitivity to stress or strain allows the construction of pressure sensors (using the J33 coefficient), and accelerometers by mounting a seismic mass on the film. PVDF electrets are particularly suited for large area hydrophones (Fig. 12.21) that detect underwater signals. Their... 

Dias, C. J., and Das Gupta, D. K., Inorganic ceramic/polymer ferroelectric composite electrets, IEEE Trans. Dielectr. Electr. Insul., 3, 706-734 (1996). 

Until the late sixties the only known ferroelectrics, piezoelectrics, and pyroelectrics were certain inorganic monocrystals, or polycrystalline ceramics like lead titanate zirconate perovskites. Other known materials with macroscopic polarization were electrets, (for example mixmres of beeswax and rosin) in which the polarization was produced by application of the electric field in the melted state and then by cooling and the solidification of the polarized material. 

Detaint J, Philippot E, Jumas JC, Schwartzel J, /.arka A, Capelle B, Doukhan JC (1985) Crystals growth, physical characterization and BAW devices appUcations of berUnite. Proceedings of 39th Annual Frequency Control Symposium, pp 234—246 Devonshire AF (1949) Theory of Barium Titanate Part I. Phil Mag 40, Serie 7, 309 1040-1063 Devonshire AF (1951) Theory of Barium Titanate Part II. Phil Mag 42, Serie 7, 333 1065-1079 Dias CJ, Das-Gupta DK (1996) Inorganic ceramic/polymer ferroelectric composite electrets. IEEE Trans Dielectr Electr Insul 3 706 734... 

Looking beyond ceramic materials to electrets, polymer and elastomeric piezoelectric materials, and so-called electroactive materials, a wide-open field of research in high-strain piezoelectric materials is appearing. Using the same technology to manipulate the chemical and physi-... 

The physical properties of low-melting-point organic materials are inadequate for certain types of applications. Hans Jaffe 27,28 showed that ceramic materials can be poled in the desired manner alter they are fired and cooled if they are properly compounded and processed. This important work opened up a new field for electret researdi and applications. 

Wb will try to suggest the answer to the question, What causes the piezo- and pyroelectricity in electrets—dipoles (or spontaneous polarization), charges, or both " The exact answer to this question was not found during the Special Discussion Sesskm at ISE 7 in Berlin, 1991. The investigation of relaxation processes by means of the TSD method seem to lead to the answer to the rtrave question conceming polymer-ceramic compodtes. it will be shown by the example of the electret and piezoelectric properties of some multilayer systems [17-19]. 

In previous papers (9,31-33) we have investigated the dielectric and electret properties of the composite pdy(methyl methacrylate) (PMMAVBaTiO, ceramic system. The 0-3 type composites used in the studies were prepared by polymerization of methyl methacrylate (MMA) in which fine particles of ferroelectric ceramics BaHO, were dispersed. The BaTiOt ceramics were ofarained by the known method of triple sintering of BaCO) and TK), in stoichioaietiic ratio. The final process was conducted at about 1620 K for 3 h. resulting in a ceramic density of 5.57 X 10 kg/m This was pulverized and sifted... 

Previously, the dielectric constants c and c /c. were measured by Funikawa ei a). [12] for other 0-3 composites, Le., a PVA/PZT system with 8 vol% fraction of PZT. In accordance with opinkm of Furukawa et al. [12], the monotonous increase of c with decreasing frequency suggests that the DC conductivity presumably due to ionk impurities is considerably high at these temperatures (Fig. 20). The first increase of c observed at low temperature is ascribed to the accumulation of ionic impurities at the interfroe of PVA polymer and PZT ceramk powder [12]. It is important for electret behavior of the polymer-ceramic composite. The second increase of c at higher tcnqieraiures is ascribed to the electrode polarization [12]. 

Today, the term electret is associated first of ail with a thin polyethylene (PE) or polytetrafluoroethylene (FTFE) (119], containing an implanted and highly persistent space charge. Sudb electrets have wide and diversified applications (118,119] however, tteir piezoelectric parameters are low. With regard to piezoelectric properties of electrets, different polymer-ferroelectric ceramic materiab have been made (4,12,16]. 

The work described in this diapter lead us to the oonduskm that the study of electrets in polymer-ferroelectric ceramic composiles is a fasdiuting field of basic and applied research. 

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