Low-temperature transverse dielectric and pyroelectric anomalies of uniaxial tungsten bronze crystals
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Low-temperature dielectric and pyroelectric properties were investigated for filled and unfilled tungsten bronze (TB) crystals, i.e., (Sr0.61Ba0.39)5Nb10O30, (K0.5Na0.5)1.0(Sr0.75Ba0.25)4.5Nb10O30, K5.80Li3.82Nb10.12O30, and K5.20Li2.34Nb10.88O30 with the spontaneous polarizations along the c axis at room temperature except for the last one. In all samples, transverse dielectric constant along the a axis showed a universal dielectric dispersion in the low-temperature range resembling a diffuse phase transition. Characteristic relaxation times obtained from the dielectric loss peaks followed the Arrhenius law, signifying thermal activation processes. The distribution of relaxation times seems to be caused by the distribution of activation energy, which has been successfully obtained by scaling the temperature- and frequency-dependent dielectric losses. This broadening may be attributed to the intrinsic randomness due to the charge disorder and quenched random fields caused by the unfilled structure and the off-stoichiometry of the TB compounds. Observed low-temperature dynamics were isotropic in the ab plane, and were insensitive to the poling conditions, degree of disorder, and high-temperature phase transitions. Structural phase transition accompanying a polarization tilt from the c axis toward the [110] axis, which was suggested to be the origin of the low-temperature anomalies, was not confirmed in our study. Some dynamics which are still active in the ferroelectric phase, like the concerted rotations of the oxygen octahedra, have been suggested as one of the possible origins of the observed phenomena.Keywords:
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Abstract This study proposes the idea of reducing the microphony effect and secondary pyroelectric coefficient in pyroelectric detectors by tuning the poling orientation. Mathematically, it has been shown that piezoelectric strain coefficients get altered by changing the poling direction. Eventually, for a couple of materials it has been demonstrated that microphony and secondary pyroelectric coefficient can be diminished by poling them at a given orientation. The poling angle nullifying secondary pyroelectric coefficient was found to be 58.2°, 47.1°, and 78.9° for (PZN‐0.08PT), (PMN‐PT), and (BCT‐0.48BZT) respectively while no such value existed for (PZT‐5A).
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Abstract : A plasma poling device is described which allows essentially instantaneous poling of polymeric films at room temperature at very high fields. Using poly(vinylidene fluoride) both undrawn (comprising essentially pure alpha conformation) and biaxially drawn (both alpha and beta conformation) were polarized in this manner. The polarization of the sample was determined from current integration measurements. The piezoelectric and pyroelectric coefficients were then measured and compared with those estimated from the polarization theory of Mopsik and Broadhurst, which expresses these coefficients as linear functions of polarization. The theory gives an accurate representation of the piezoelectric coefficients, but underestimates the pyroelectric coefficients considerably.
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Vinylidene fluoride trifluoroethylene copolymer VDF-TrFE was found to be pyroelectric when poled by application of dc high field. Temperature dependence of its pyroelectric response and the effects of poling conditions were investigated. It was shown that there existed a critical temperature at 70 °C above which pyroelectricity vanished. The pyroelectric coefficient increased with the increase in poling temperature with a tendency to saturation. The pyroelectric coefficient of these samples poled above 70 °C were constantly 9 nc/cm2 K, which is significantly larger than that of any other polymers.
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By analysis of steady-state current-time-voltage-temperature relationships, it is concluded that a Richardson-Schottky process is dominant at the high fields and temperatures typically employed in poling poly(vinylidene fluoride) to impart pyroelectric and piezoelectric properties. Data on pyroelectric behavior with different layer configurations and with different electrode configurations indicate the primary importance of hole injection.
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Poling currents were measured in 24 and 9 μm thick samples of PLZT 8/65/35 under applied fields of up to 150kV/cm, at temperatures from 23 to 170°C. Stable negative currents, flowing for hours in the direction opposite to the applied field, were observed in many samples, at room temperature as well as under heating. The revealed complex character of the currents did not allow the calculation of the pyroelectric coefficient from their instantaneous values. A method was developed to separate the pyroelectric component of the current. These components then yielded for the “biased” pyroelectric coefficient λB (measured under the applied field) values from tens to hundreds of nC/cm2K, depending on the field, temperature, and the poling history of the sample. The largest values of λB were measured close to and well above the Curie temperature (111°C) of the material. Injection of free charge is discussed as possibly responsible for the observed behavior.
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In 0-3 nanocomposites of lead titanate (PT) in polyvinylidenefluoride trifluoroethylene (PVDF-TrFE) it is possible to polarize the inclusions and the matrix separately. The pyroelectric coefficients of both components have the same sign, but the piezoelectric coefficients have the opposite sign. IF matrix and inclusions of a composite of PT in PVDF-TrFE are polarized in the same direction, then the pyroelectric response of both components overlays constructively, the piezoelectric response destructively. By poling in opposite directions a material with reduced pyroelectric but increased piezoelectric sensitivity is produced. Under special poling conditions and for a certain ceramic volume fraction the pyroelectric coefficient vanishes completely for the antiparallel poled case but is piezoelectric with a piezoelectric coefficient of 20 pC/N. Such a material with internally compensated pyroelectricity is of high interest for the application in piezoelectric sensors were the cross sensitivity of piezoelectric sensors to temperature changes is often a large obstacle for their practical use.
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Poling of oriented polytrifluoroethylene (PTrFE) was found to produce pyroelectricity which showed anomalous behavior and formation conditions identical to those in unoriented samples. The pyroelectric activity in oriented PTrFE was greatly enhanced when the poling temperature was low but was reduced as the poling temperature was raised, contrary to the behavior of ordinary thermoelectrets. The enhancement of the pyroelectricity is attributed to a greater increase of the new crystalline region and to the orientation effect, which aligned the dipoles along the electric field. The reduction of the pyroelectricity is attributed to an increase in the degree of chain orientation, which aligns the polar axis parallel to the film plane. This behavior depended strongly on the state of the amorphous region from which the new crystalline region carrying the spontaneous polarization grew.
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Ferroelectric and pyroelectric properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 thin film deposited on Pt/Ti/SiO2/Si substrates using a sol-gel method were investigated.X-ray diffraction results showed that the films were highly(111) oriented.It was found that the films show a stable pyroelectric coefficient(1.58×10-8 Ccm-2K-1) without any poling processes.Nearly constant pyroelectric coefficient was obtained when the applied poling dc bias field increased to greater than three times of its coercive field.The pyroelectric coefficient of the poled 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 thin film capacitor dropped to 96% of its initial value after ten days, which was much more stable than that of PZT samples.Asymmetric pyroelectric currents were found in the 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 thin films with different poling directions.For pos-itive poling, pyroelectric current increased with the poling electric field.In the case of negative poling, pyroelectric current decreased with the poling electric field.These results suggest that the good pyroelectric properties of the 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 thin films could be due to the presence of self-polarization in this material.
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New selective poling schemes for polar semicrystalline polymers are described, and appropriate poling fields and temperatures are specified. In the following investigation of the pyroelectric response and the thermally stimulated depolarization (TSD), poly(vinylidene fluoride) is used as an example for a semicrystalline ferroelectric polymer. It is shown that the amorphous and the crystalline phase, as well as the interface between them contribute to the pyroelectric effect. From dynamic pyroelectrical measurements and TSD experiments, the permanent polarization of the dipoles in the crystalline phase, the frozen-in polarization of the dipoles in the amorphous phase, and the effect of the excess charges of the Maxwell– Wagner interface polarization can be identified. With the proposed selective poling schemes (poling temperatures below and/or above the glass transition temperature, as well as poling fields lower and/or higher than the coercive field), it is possible to clearly separate the respective contributions to the pyroelectric effect.
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