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Smart Mater. Struct. 21 (2012) 025021 F Y Lee et al Figure 5. Experimentally measured energy density harvested by 8/65/35 PLZT (sample 4) as a function of high electric field. The hot source temperature Thot was equal to (a) 100 ◦ C, (b) 120 ◦ C, (c) 130 ◦ C and (d) 160 ◦ C. The high electric field EH ranged from 0.4 to 2.5 MV m−1 . The cold source temperature Tcold and low electric field EL were set as 65 ◦ C and 0.2 MV m−1 , respectively. The relative error between the model predictions and experimental data is denoted by δ. Figure 7 shows that the D–E paths of the Olsen cycles were not closed since points 4 and 4′ did not coincide. The off- set was caused by leakage current across the PLZT ceramic at high temperatures and/or large electric fields [10, 23, 64, 65]. The loss in energy density associated with the leakage current was estimated to be 15–20%. Moreover, note that the Olsen cycles did not follow a smooth path between EL and EH during isothermal processes 1–2 and 3–4 in the D–E diagram. It indicates that these processes were not performed under quasiequilibrium conditions [20]. This can be attributed to the inhomogeneity of the sample caused by the microcracks. Indeed, microcracks may have propagated along the grain boundaries of the sample while the Olsen cycle was performed under high electric fields and/or high temperatures [61]. These fractures introduced spatial variation in the local electric field near the crack front [62]. 4.8. Discussion The maximum energy density of 888 J l−1/cycle produced by 8/65/35 PLZT should be compared with those achieved by other pyroelectric materials. In fact, table 2 compares the maximum energy density generated from the Olsen cycle for different materials, temperature ranges and operating electric fields. Note that a maximum energy density of 900 J l−1/cycle using 60/40 P(VDF–TrFE) was reported by Olsen et al [4] for temperatures between 25 and 120 ◦ C and electric fields cycled between 20 and 50 MV m−1. However, it is unclear whether these experimental results were averaged over multiple cycles and if they were repeatable. In fact, Navid et al [23] produced 204 J l−1/cycle averaged over five cycles for the same material, temperature range and operating electric fields. To the best of our knowledge, the maximum energy density reported in the present study for 8/65/35 PLZT is the largest energy density experimentally measured repeatably over multiple cycles. It could be further increased by increasing the electrical breakdown strength of the material. For example, samples can be (i) pre-stressed such as in a thin layer unimorph ferroelectric driver and sensor actuators to increase sample durability [66] or (ii) fabricated into single or multilayer thin film capacitors with thicknesses of the order of nanometers to increase the applied electric field without sample failure [67]. The use of thin film PLZT would substantially reduce the applied voltage delivered to the electrical circuit during the Olsen cycle. For example, only 7.5 V applied across a 10 μm thin PLZT film would be 7PDF Image | Pyroelectric waste heat harvesting using relaxor ferroelectric
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