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Smart Mater. Struct. 21 (2012) 025021 dipoles form and give rise to large dielectric and piezoelectric properties [39]. The phase transition between ergodic relaxor and ferroelectric is field-dependent and occurs at the Curie temperature TCurie. The latter can be determined from the maximum of the pyroelectric current (∂ D/∂ T )σ,E versus temperature T curve measured under a specified electric field E. The material can revert phase from ferroelectric to ergodic relaxor by heating it above TCurie and/or depoling below a critical electric field Ecr(T). The Curie temperature TCurie is expected to become nearly frequency-independent in the vicinity of the ergodic-relaxor–ferroelectric phase transition [40]. This is due to the reduction in dipole thermal fluctuations as the material is cooled below TCurie [40–42]. In 8/65/35 PLZT, the ferroelectric phase cannot be established upon cooling under zero electric field [43]. However, the ferroelectric phase can be induced from the ergodic relaxor phase by applying an external electric field E greater than Ecr(T) [44–47]. Unfortunately, TCurie and Ecr(T) for the PLZT composition 8/65/35 have not been reported in the literature. Therefore, we can only speculate upon the ferroelectric–relaxor phase transitions in 8/65/35 PLZT based on data reported for 9/65/35 PLZT [47, 48]. Note also that the remnant polarization Pr(T) of ferroelectric relaxors does not vanish above TCurie due to the persistence of nanopolar domains interacting with each other at temperatures between TCurie and TB [42]. However, the remnant polarization is negligibly small in the relaxor state with respect to its value in the ferroelectric state [42]. The converse of the pyroelectric effect is the elec- trocaloric effect. It is the change in temperature caused by a change in applied electric field under adiabatic conditions [49]. Lu et al [50] recently demonstrated a large electrocaloric effect on 8/65/35 PLZT thin films. The electrocaloric temperature change of T = 40 K was observed for 0.45 μm thick 8/65/35 PLZT at about 45 ◦ C under an electric field of 125 MV m−1. In addition, Sebald et al [51] have shown theoretically that materials exhibiting large electrocaloric activity are of interest in pyroelectric energy harvesting. Consequently, the present study aims to experimentally assess the pyroelectric energy harvesting capabilities of 8/65/35 PLZT. 2.4. Physical modeling of the Olsen cycle Recently, Kandilian et al [19] developed a physical model predicting the amount of energy harvested by relaxor ferroelectric materials undergoing the Olsen cycle. The model accounted for temperature-dependent properties of the material. The energy density ND was expressed as [19] (4) F Y Lee et al Table 1. Thickness and cross-sectional area of the different 8/65/35 PLZT samples and their electrodes investigated in this study. Sample Thickness Electrode size Sample size # (μm) (cm×cm) (cm×cm) 1 490 2 720 3 600 4 650 5 370 6 370 7 290 0.81 × 0.90 0.815 × 0.95 0.90 × 0.95 0.979 × 0.838 0.845 × 0.653 0.833 × 0.644 0.414 × 0.429 0.81 × 0.90 0.815 × 0.95 0.90 × 0.95 0.979 × 0.838 0.991 × 0.843 0.993 × 0.839 0.876 × 0.852 ND(EL, EH, Tcold, Thot) = (EH − EL) ×ε0[ε(T )−ε(T)](E+E) hot operating temperatures Tcold and Thot, respectively. The saturation polarizations of the material at Tcold and Thot are denoted by Ps(Tcold) and Ps(Thot), respectively, and expressed in C m−2. Note that Kandilian et al [19] erroneously called Ps (T ) the spontaneous polarization instead of saturation polarization, as sometimes found in the literature [52]. The piezoelectric coefficient d33 is expressed in C N−1, s33 is the elastic compliance (in m2 N−1 ) and x3 = α3 (Thot − Tcold ), where α3 is the linear thermal expansion coefficient (in K−1). Note that this model was based on the assumption that the dielectric contribution to the primary pyroelectric coefficient was negligible compared with the dipole contribution (see equation (8) in [19]). The model successfully predicted the energy density harvested by PMN-32PT [19] and PZN-5.5PT [20]. Here also, the model predictions will be compared with the energy density experimentally measured with 8/65/35 PLZT. 3. Experiments 3.1. Samples Hot isostatically pressed 8/65/35 PLZT ceramics with grain diameter around 5 μm were acquired from Aura Ceramics, Inc. (New Hope, MN, USA). The specimen was cut into seven samples with thicknesses ranging from 290 to 720 μm using a diamond abrasive saw. Rectangular gold electrodes were sputtered on opposite faces. Electrical wires were bonded to the electrodes using conductive silver epoxy. Table 1 lists the thickness and the cross-sectional dimensions of each sample used in this study. Note that the electrodes in samples 5–7 did not fully cover the faces. This electrode configuration was chosen in order to minimize electrical conduction (leakage current) around the samples’ edges. 3.2. Experimental set-up Two thermally insulated beakers containing Dow Corning 100 cSt silicone oil were maintained at temperatures Tcold and Thot thanks to temperature-controlled hot plates [19, 23]. J-type thermocouples were immersed in each bath to monitor and control their temperature. The samples were alternately dipped between the cold and hot baths to create the temporal temperature oscillations required in the Olsen cycle. Sufficient time (∼20–60 s) was given for the sample’s 2 r cold r hot H L d33x3 + Ps(Tcold) − Ps(Thot) + s 33 where εr (Tcold ) and εr (Thot ) are the low permittivities of the pyroelectric material at the cold and frequency relative 3PDF Image | Pyroelectric waste heat harvesting using relaxor ferroelectric
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