Solid electret

They produced high performance electrets from thin polymer films metallized so as to yield high capacitance. Both electrical and mechanical properties of these transducers have been remarkable examples of how applications of science of solids, including knowledge of electron traps, conduction processes in insulators and the viscoelastic phenomena of semicrystalline polymers, can be combined.(6) Incidentally, similar ideas have been applied to optimization of the properties of particle microphones, through assemblies of perfectly microspherical polymer carbon systems. These have shown what limits of performance... 

Figure 3 shows one of our photoacoustic cell for X-ray spectroscopy of solid samples The cylindrical cell has a sample chamber at the center with volume of 0.16 cm which has two windows of beryllium (18 mm x 0.5 mm thickness). A microphone cartridge is commercially available electret type (10 mm ) and the electronics of preamplifier for this microphone is detailed elsewhere Figure 4 shows the typical experimental setup for spectroscopic study X-ray was monochromated by channel cut silicon double crystal (111) and ion chamber was set to monitor the beam intensity. Photoacoustic signal intensity was always divided by the ion chamber current for the normalization against the photon flux. X-ray was modulated by a rotating lead plate (1 mm thick) chopper with two blades. 

Thermally stimulated dielectric relaxation of a solid in a polarized (electret) state usually was selected for the following reasons ... 

The electronic charge carriers injected into organic solids from ohmic contacts can build up a stable space charge by trapping in deep traps thus some dyes assume the properties of electrets on application of an electric field in the dark. According to Euler et al. 140>, this effect can be utilized for the storage of electrical energy. 

Solids can also be subdivided by their electrical polarization properties. The preponderant fraction of solids (crystalline or amorphous) are dielectric They have no net electrical polarization. If the individual components (molecules or clusters of ions) do have a net electric dipole moment, and these add nonlinearly, then one has electrets. There are also nanoferroelectrics. 

To investigate the potential of these blends for electret applications at elevated temperatures, the annealing temperature for the ITPD experiment was increased to 120°C and 150°C the effect on the surface potential decay is presented in Fig. 29. The complete potential loss at 120 °C of the 25/75 PPE/PS blend and PS films is due to the fact their glass transition temperatures are below this temperature. At 120°C (Fig. 29 top) the two PPE/PS blends 75/25 and 50/50 are still in the glassy solid state and show similar ITPD trends compared to the curves obtained at 90 °C, but with accelerated decay. The 75/25 PPE/PS blend films still possess a remaining surface potential of 74% after 24h at 120°C. 

Gross, B., Charge Storage in Solid Dielectrics. A Bibliographical Review on the Electret and Related EfiFects. Elsevier, New York, 1964. 

Polymer electrets can be operated as sensors or actuators. Their operation is very similar to that of a piezoelectric material and their direct piezoelectric transducer coefficient (d33) is higher than that of solid PVDF ferroelectric polymers [97]. If a compressive force is apphed to the film, the pores will deform preferentially with respect to the polymer material. Unlike charges within the polymer will be pushed closer together and the potential measured at the contacts will change accordingly. Similarly, the application of a voltage across the electrodes will yield a change in thickness in the material. 

In principle, different types of passive detectors can be used such as thermoluminescent materials, electret devices, and solid-state nuclear track detectors. 

Electriad and dielectric behaviour of polymers reflect macromolecular structure and motion, both in solution and the solid state. Some polymers whidt have special electrical properties may have commaical potential. Mention need ordy be made of polymer electrets, pyro-electrk polymers, photo-conductive polymers as used In ctro-imaging, and conductive poly mas to indicate tR expansion of use over that of insulators. The separation of electrical behaviour into didectric and btdk conductive properties is convenient and has been followed in this review. 

Hsieh WHetal (1997) A micromachinedthin-film Teflon electret microphone. IntConf Solid-State Sensors Actuators 1 425-428... 

Takahashi T (2013) A miniature harvester of vertical vibratory capacitive type achieving several tens microwatt for broad frequency of 20-40 Hz. In 17th international conference solid-state sensors, actuators, and microsystems (transducers 13), Barcelona, pp 1340-1343 Thakur R, Das D, Das A (2013) Electret air filters. Sep Piuif Rev 42(2) 87-129 van Tumhout J et al (1980) Electret filters for high-efficiency air cleaning. J Electrostal 8... 

Vullers R et al (2009) Micropower energy harvesting. Solid-State Electron 53 684-693 Yasuno Y, Riko Y (1999) An approach to integrated electret electroacoustic transducer -Experimental digital microphone, fit Proceedings of the 10th international symposium on electret (ISEIO), Athens, pp 727-730... 

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