High Efficiency Refrigeration and Pyroelectric Energy Harvesting Applications of Lead Titanate Based Relaxer Ceramic


 Relaxer ferroelectrics are highly attractive in electrocaloric due to large breakdown electric field and wide phase transition temperatures, which has great potential applications in solid-state cooling. In this work, relaxer ferroelectrics are synthesized by a traditional solid state reaction method. Phase transition and relaxer characteristics are studied by temperature dependent dielectric spectrums. Temperature dependent ferroelectric properties are also studied to calculate electrocaloric effect. Maximum value of refrigeration efficiency η ( ΔT/ΔE ) is about 0.079 K·cm/kV, our result lays a foundation and provides a reference for studying high efficiency solid-state electrocaloric refrigeration. Pyroelectric energy harvesting are studied, maximum pyroelectric energy harvesting is 279 kJ/cm 3 .


Introduction
Refrigeration is indispensable in our daily life, such as: food preservation, air conditioning, medical aspects (organ refrigeration, organ transplantation), microelectronic refrigeration, and etc [1,2]. The refrigeration method of the common compressor has almost reached its limit, the organic gas it discharged directly destroys the olfactory oxygen layer causing the global greenhouse effect, environmental damage appeals more and more attention recently. As a result, nding a new way of cooling method becomes an urgent task.
The unique properties and great variety of relaxer ferroelectrics make them highly attractive [3]. During the past decades, intensive research efforts have been conducted to develop solid-state cooling technologies [4,5]. Electrocaloric effect (ECE) is the isothermal entropy change (ΔS) and adiabatic temperature change (ΔT) of polar materials during application and removal of electric eld, which is more environmental friendly and satis es the demand of realizing next-generation solid-state cooling devices for various applications [4], the operation schematics of electrocaloric cooling technology is shown in Figure 1.
Recently, relaxer ferroelectrics for future solid-state refrigeration technologies become very hot [6,7], high energy conversion e ciency, easy miniaturization, wide temperature region of phase transitions and large breakdown electric eld indicate the potential applications in solid-state refrigeration technologies.
It is a well-known fact that thin lms have advantages in small solid state cooling devices, but bulk materials play an important role on larger scale devices [8]. As a result, ECE of bulk materials are desired, we should pay more attentions to ECE of bulk materials. Bulk materials including multilayer capacitors, ceramics and single crystals have been reported a lot, such as: PMN-PT single crystal [9], PLZT multilayer capacitors [10], Ba(Ti,Ce)O 3 -(Ba,Ca)TiO 3 ceramics [11], PLZT ceramics [12], and etc. Compared to multilayer capacitors and single crystals, ceramics have the advantages of low-cost and easier fabrications [13]. As a result, ECE of ceramics are important. are achieved, maximum value of ΔT is about 1.96 K, maximum value of refrigeration e ciency is η (ΔT/ ΔE) is about 0.079 K·cm/kV, the huge refrigeration e ciency η and large reversible adiabatic temperature change ΔT indicate that applications in future solid-state refrigeration devices.

Experimental
Ceramic samples of PLHT and PLZT were prepared by a conventional solid state reaction method using high-purity raw materials Pb 3 O 4 , La 2 O 3 , ZrO 2 , HfO 2 , and TiO 2 (AR, Macklin Biochemical Co., Ltd., China).
The powders were weighed based on nominal composition and mixed in ethanol using zirconia balls for 12 h. The mixture was ball-milled for 24 h after calcination at 900 o C for 3 h, the green body was sintered at 1300 o C for 5 h. The sintered sample discs were polished into a thickness of 0.5 mm, and then the two parallel surfaces were covered with silver paste and nally red at 600 o C for 30 min as electrodes. Temperature dependent dielectric permittivity γ and loss tanδ of PLHT and PLZT ceramics are shown in Figs. 3a-b. In lower temperature region ( 150 o C), both loss tanδ and permittivity γ exhibit single independent peak clearly for PLZT and PLHT ceramics, which are due to the phase transition of ferroelectric→paraelectric phase, similar results are also reported [14]. Maximum permittivity γ appears around 405.15 K, and 355.15 K respectively for PLZT and PLHT ceramics. From Fig. 3, T m (temperature of maximum γ ) shift to higher temperatures with increasing frequencies, but maximum values of dielectric permittivity γ decrease with increasing frequencies, this phenomenon indicates typical relaxer behaviors.
Dielectric relaxation usually denotes reorientational processes in condensed matter that can be detected by dielectric spectroscopy [15]. Generally speaking, while temperature is above Curie temperature, then permittivity γ of a normal ferroelectric usually is suitable for the Curie-Weiss law, and dielectric In order to calculate ECE of PBLZST ceramics, P-E loops and I-E curves of PLZT and PLHT ceramics under various temperatures are shown in Figs. 4a-b, with increasing measured temperatures, P-E loops appear to be typical relaxer ferroelectric loops, which indicates the relaxer nature, saturation polarization decreases sharply with increasing temperatures. I-E curves provide detailed picture of phase switching process, a single sharp current peak is observed in the I-E curves, which can be attributed to the domain switching. Flat I-E curves with two split current peaks can be observed in PLHT ceramics, indicating the higher composition of nonpolar phase. The transition from short-range nonergodic relaxer ferroelectric to long-range metastable ferroelectric phase can be disassembled by two parts: the generation of microsized domains from PNRs that embedded in the easily polarized matrix, and the alignment of the microsized domains driven by high electric eld. As a result, the two current peaks in the I-E curves can be ascribed to the formation of micro-sized domains and the domain switching under further applied electric eld [19].
Ferroelectrics have been widely studied in energy harvesting applications due to the outstanding pyroelectric characteristics which are promising to convert heat uctuations into electrical energy.
Pyroelectric effect re ects the variation of spontaneous polarization in pyroelectric material with changing temperatures [27]. Signi cant variations of polarization under different temperatures induce giant pyroelectric energy harvesting density (N D ), which can be estimated by using Olsen cycle between two hysteresis loops [28]. A complete Olsen cycle (A→B→C→D→A) consists of two isothermal (A→B and C→D) and two isoelectric (B→C and D→A) process in Fig.5a. Energy conversion in these processes have been described in elsewhere [29]. The output electrical energy density N D per cycle in Fig.5a is calculated by the following formula [27]: , where E and P are electric eld and polarization respectively, it is numerically equal to green shaded area in Fig.5a. In this work, the maximum pyroelectric energy harvesting are 279 and 234 kJ/cm 3 respectively for PLZT and PLHT ceramic with ΔT = 90°C (shown in Fig. 5b). Our results shows higher values than previous reports in PZT ceramic [30], BZT-50BCT ceramic [31], SrBaNb 2 O 6 ferroelectric ceramics [32], and etc.

Conclusion
In this work, PLZT and PLHT ceramics are synthesized by a traditional solid state reaction method, and pure phase is obtained from XRD spectrum of all sintered samples. Phase transition behaviors and relaxer characteristics are studied. Maximum reversible adiabatic temperature change ΔT are 1.96 K and 1.24 K respectively for PLZT and PLHT ceramics, maximum value of refrigeration e ciency is η (ΔT/ΔE) is about 0.079 K·cm/kV, the current progresses achieved in this work show potential applications for solid-state refrigeration devices. Maximum pyroelectric energy harvesting values are 279 and 234 kJ/cm 3 respectively for PLZT and PLHT ceramic.