Monomorphs, bimorphs, and multimorphs from polar polymer electrets

Abstract

Mono-, bi-, and multimorphs are poled electrets with + 0, + -, and more complicated alternatingpolarity dipole-orientation patterns, respectively. Because of their unusual macroscopic piezo-and pyroelectrical as well as nonlinear optical properties, such structures are of particular interest for special transducer and optical waveguide applications. In this contribution, the preparation of such dipole-polarization patterns in ferroelectric and amorphous polymer electrets and their nondestructive probing by means of thermal and acoustical techniques are reviewed. I IntroductionAlready in their early studies of piezoelectricity, Jacques and Pierre Curie, who had discovered the effect in 1880, employed three-layer samples both for the direct and the converse piezoelectric e ect 1 . With two inner electrodes between the three individual layers, the + -+ arrangement served mainly two purposes: To increase the sensitivity and to provide shielding of the inner electrodes, while the outer electrodes were always grounded 1 . Today, similar arrangements are still used in order to enhance the sensitivity o f piezoelectric-polymer transducers 2 , but in addition, multilayer structures without inner electrodes are now used as code-speci c transmitters and detectors 2 or as monomorphs, bimorphs and multimorphs see below .Monomorphs, bimorphs, and multimorphs are poled electrets with + 0, + -, and more complicated alternating-polarity dipole-orientation patterns, respectively, across their thickness. They often possess unusual macroscopic piezoelectrical, pyroelectrical, or nonlinear optical properties which make them particularly useful for special transducer or waveguide applications see below . However, the preparation of poled polymer electrets with well-de ned pro les of their internal dipole polarization is not possible without suitable probing techniques.In one of the very early studies on the probing of internal charge and polarization pro les, beeswax electret samples were sliced with a chrome-nickel blade, and the charge on each slice was determined by means of a Faraday cage and an electroscope 3 . This and several later sectioning experiments on electrets are discussed in the still very useful bibliographical review of Gross 4 . It was, however, also shown by Gross and Moraes 5 that the pro le of a true volume polarization can only be reliably obtained by electroding the slices in order to eliminate the surface charge generated in the cutting process and by then determining the total depolarization current of each slice. With this improved sectioning technique, it could be demonstrated that carnauba wax electrets do indeed possess a true volume polarization heterocharge in addition to the injected and trapped space charge homocharge 5 .In the mid-1970s, Peterson et al. 6,7 and Collins 8-10 independently showed that thermal waves or pulses may be employed for the nondestructive probing of charge and polarization pro les in electrets.

Linear and Nonlinear Optical Properties of Photochromic Molecules and Materials

“…Using such a technique, the full width at half-maximum (fwhm), which characterizes the spatial resolution, was 7 μm for a beam diameter of 1.5 μm. Further potentialities of this poling technique are reviewed by Gerhard-Multhaupt and Bauer-Gogonea 26 Photothermal poling setup.…”

“…Further potentialities of this poling technique are reviewed by Gerhard-Multhaupt and Bauer-Gogonea. 218…”

Unbiased iterative reconstruction of polarization and space-charge profiles from thermal-wave experiments

“…To demonstrate the effectiveness of the new iterative scheme, the algorithm was applied to several model polarization distributions g(z), shown as dotted lines in figure 2: (a) A bimorph (two layers of equal thickness with opposite polarization) [28,29]. (b) A poled sheet sandwiched between two unpoled layers [28]. (c) A 'ramp distribution' of the electric field, resulting from a homogeneous space-charge density with compensation charges at both electrodes [30].…”

Heteropolar Charging of Ferroelectrets for Flexible Keyboards and Tactile Sensors

“…Charge patterns in electrets are employed in the preparation of piezo-, pyro-and ferroelectric devices and nonlinear optical polymer waveguide structures [20][21][22]. The concept of a heteropolar cellular PP ferroelectret with regions of different polarizations is simple, but the preparation of neighbouring and closely spaced regions with heteropolar polarization patterns is a nontrivial and difficult process.…”