Polarization Stability of Amorphous Piezoelectric Polyimides
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Table I. Processing parameters and properties of partially-cured, corona poled (13CN)APB/ODPA: cure cycle, T,, degree of imidization (A1780cmln"AI500cm-'), and Pr. Sample Cure cycle (BC) T9 before poling A1780cml/Al500cm"1 Pr 1 hour each, N 2 (BC) (mC/m2) P100 50, 100 97 0.18 N/A P150 50, 150 142 0.69 26 P200 50, 150, 200 166 0.82 9 P240 50,150,200,240 218 1.00 4 Poling The film specimens were poled using either a conventional or a positive corona poling procedure. For the conventional poling, each sample was polarized by the application of a DC electric field (80MV/m) at an elevated temperature (Tg + 5QC) in a silicone oil bath. For corona
poling, a DC field of 20 kV was applied to generate a positive corona using a single tungsten wire for four hours at 223BC and one hour at 212BC. The distance between the corona tip and the specimen was approximately 30mm. An argon gas was maintained during the poling process. For both poling processes, the dipoles were oriented with the applied field at a temperature above Tg, with subsequent cooling to below Tg in the presence of the applied field. The resulting remanent polarization (Pr) is directly proportional to the material s piezoelectric response, and estimated from the following equation [4], Pr =eo Ae Ep
(1)
where co is the permittivity of free space (8.854 pF/m), Ae is the dielectric relaxation strength, and Ep is the poling field. Characterization Degree of imidization. The glass transition temperatures (Tg) of the films were measured by differential scanning calorimetry using a Shimadzu DSC-50 at a heating rate of 20BC/min in air. The Tg was taken as an inflection point of the shift of the baseline of the DSC thermogram. The degree of imidization was determined by a Nicolet FTIR spectrometer in an ATR mode using a Nicolet Contin m IR microscope. The absorption peak at 1780cm"1 (sym. carbonyl stretch) was used to determine the degree of imidization and that at 1500cm1 (ring modes of aromatic moieties) was used as an internal standard [5]. The results are presented in Table I. Thermally stimulated current (TSC) measurement. After poling, Pr was measured as a function of temperature. As the sample was heated through its Tg at a heating rate of 1.5iC/min, the depolarization current was measured using a Keithly 6517 electrometer. The P, is equal to the charge per unit area, which is obtained from the data by integrating the current with respect to time and plotting it as a function of temperature. PiezoelectricMeasurement. The piezoelectric strain coefficient (d 31 ) was measured using a Rheovibron DDV-II-C mechanical analyzer as a function of temperature for a range of frequencies. As the polymer is strained along the direction of applied stress, a charge Q is generated on the surface of the electrodes. A geometric factor is used to produce a geometry independent parameter, namely, surface charge density per unit applied stress. Thermal stability Measurements. Thermal stability of the piezoelectricity in the (03-CN) APB/ODPA system was carried out under both dynamic
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