Pyroelectric energy harvesting capabilities and electrocaloric effect in lead-free SrxBa1-xNb2O6 ferroelectric ceramics

Journal of Alloys and Compounds 791 (2019) 1038e1045

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Pyroelectric energy harvesting capabilities and electrocaloric effect in lead-free SrxBa1-xNb2O6 ferroelectric ceramics Hui Tang, Xin-Gui Tang*, Ming-Ding Li, Qiu-Xiang Liu, Yan-Ping Jiang School of Physics & Optoelectric Engineering, Guangdong University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, 510006, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 February 2019 Received in revised form 27 March 2019 Accepted 28 March 2019 Available online 29 March 2019

SrxBa1-xNb2O6 ceramics were prepared via the high-temperature solid-state reaction method. Structural, ferroelectric, electrocaloric effect and pyroelectric energy harvesting capabilities of SrxBa1-xNb2O6 ceramics were reported. The main structural phase of tetragonal tungsten bronze with Sr-doped was detected by Xeray diffraction. It was found that the ferroelectric hysteresis loops became slimmer after the temperature higher than Curie temperature of SrxBa1-xNb2O6 ceramics, which was a representative feature of relaxor ferroelectric. In addition, we presented the positive peak values of electrocaloric at vicinity of Curie temperature which was affected by the applied electric field for all samples. It is worth noting that the pyroelectric energy harvesting property is the first time investigated by using Olsen cycle for SrxBa1-xNb2O6 ceramic. The maximum pyroelectric energy harvesting density was 170 kJ m
3 for x ¼ 0.6 in SrxBa1-xNb2O6 with the temperature range from 293 to 433 K. The experimental results indicated that better pyroelectric energy harvesting property and larger electrocaloric effect of SrxBa1-xNb2O6 ceramic benefited from the higher Sr/Ba ratio. Finally, energy-storage capacity for SrxBa1-xNb2O6 ceramic was obtained from the ferroelectric hysteresis loop and the maximum room-temperature energy-storage efficiency was 86.37% with x ¼ 0.6 under 60 kV cm
1. © 2019 Published by Elsevier B.V.

Keywords: Pyroelectric energy harvesting Electrocaloric effect Energy-storage

1. Introduction Traditional ferroelectric ceramics were proverbially used in many areas owing to great dielectric and piezoelectric capabilities, such as PbTiO3 and BaTiO3 based ceramics [1e7]. Ferroelectric ceramics are proverbially applied for refrigeration, converting ambient energy and salvaging waste energy too [8e12], which are thermal byproduct of lots of energy converting devices like household appliances, internal-combustion engines and wearable electronics [13,14]. As we all known, the electrocaloric effect (ECE) and the pyroelectric effect are considered as the alternatives for these energy applications [15,16]. ECE is parameterized by a reversible adiabatic temperature change (DT) or isothermal entropy change (DS) of ferroelectric material when an external electric field is applied or removed [16,17]. Electrocaloric ferroelectric ceramics with large DT are considered as a potential candidate of being used in solid-state refrigerators to replace conventional vapor compression technologies because of effective and eco-friendly [17e19].

* Corresponding author. E-mail address: [email protected] (X.-G. Tang). https://doi.org/10.1016/j.jallcom.2019.03.385 0925-8388/© 2019 Published by Elsevier B.V.

Hence, cooling technology dependent on ECE are widely studied by lots of researchers, a giant ECE is discovered in PZT antiferroelectric ceramics reported by Mischenko et al. [17]. The Olsen cycle that the conception for primary grade thermal energy harvesting dependent on applying ferroelectric hysteresis loop (P-E) in single cycling pattern was firstly introduced to elevate the pyroelectric energy harvesting property in 1981 [20]. Interestingly, it is reported that energy harvesting using the Olsen cycle that dedicated by pyroelectric effects and the changes of electric energy storage capacity with temperature is the order 103 times higher than traditionary pyroelectric function [13,15]. Due to the new approaches for electrical energy harvesting, numerous literature about harvesting ferroelectric energy were springing up. Most of these ferroelectric ceramics are PbTiO3 and PbNbO3 based ceramics, such as the large energy harvesting density of 1.01 kJ cm
3 under 7 kV mm
1 was obtained in 190 mm Pb0.93La0.07(Zr0.65Ti0.35)0.9825O3 thin film [21]. But, considering many detriments caused by lead, it is urgent to find energy applied materials which are not only harmless but also possessing predominantly pyroelectric capabilities. In this direction, Strontium barium niobate (SBN) is a representative lead-free ferroelectric ceramic. Because of superior pyroelectric, piezoelectric and electro-optic capabilities [22,23], SBN ceramics are

H. Tang et al. / Journal of Alloys and Compounds 791 (2019) 1038e1045

anticipated to be promising pyroelectric energy harvesting materials which have not been investigated. SBN polycrystalline widely reported are benefit by its expedient preparation and composition controlled easily [24,25]. In contrast to these previous studies, it is more desirable to adjust the chemical composition of ceramic materials to achieve excellent pyroelectric energy harvesting

Fig. 1. (a) XRD patterns of SrxBa1-xNb2O6 samples; Cross-section SEM images for (b) SBN40, (c) SBN50 and (d) SBN60.

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performance [26]. In this work, DT and DS were calculated by using Maxwell relations to study ECE of samples. Furthermore, this present work is focused on pyroelectric energy harvesting performances with the increasing of the Sr content in SBN ceramics. 2. Experiment SrxBa1-xNb2O6 (x ¼ 0.4, 0.5 and 0.6, abbreviated as SBN40, SBN50 and SBN60, respectively) samples were fabricated from powders synthesized by solid-state reaction process. Analytical reagent grade including SrCO3, BaCO3, and Nb2O5 were selected as raw materials. The yttrium-doped zirconia balls were used as an abrasive, and ethanol was used as a grinding medium. The powders were mixed and placed in a nylon tank, and then milled for 16 h. Then dried mixture was calcined in air at 1473 K for 5 h. Next, making the powders remilled, dried, added suitable adhesive, then dry pressing to pills. Finally the ceramic were fabricated by sintering at 1623 K for 2 h. The crystal structures were identified by X-ray diffractometer (XRD, D8 ADVANCE, Bruker). The microstructure of ceramic crosssection was examined by a Hitachi S-3400N-II scanning electron microscope (SEM). And pill-like samples were 10 mm diameter with 0.60 mm thickness. Temperature-dependent ferroelectric hysteresis loops were measured using Radiant Technologies Precision premier II (Albuquerque, NM, USA) with a sinusoidal type drive field with frequency of 10 Hz in a temperature range between 295 and 423 K.

Fig. 2. The P-E hysteresis for SBN ceramics: (a) SBN40, (b) SBN50, (c) SBN60 measured at 10 Hz, 50 kV cm
1 and different temperature. (d) the remanent polarization (Pr) as a function of temperature for SBN ceramics.

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H. Tang et al. / Journal of Alloys and Compounds 791 (2019) 1038e1045

3. Results and discussion 3.1. Structural analysis Fig. 1(a) displayed XRD patterns for the different ratio SrxBa1polycrystalline samples. The diffraction peaks are indexed in accordance with JCPDS 39-0265 [27]. It is recorded that the main

xNb2O6

phase is tetragonal tungsten bronze in SBN [28]. Fig. 1 displays three respectively peak which is marked with red color. It can be noticed that intensities for three marked diffraction peaks (320), (410), and (311) become stronger with the Sr ion increasing. It also can be found that positions of the peaks are stable with the mild change of Sr content. The SEM micrographs of SBN40, SBN50 and SBN60 samples

Fig. 3. Variation of polarization (P) as a function of temperature (T) at different electric field for (a) SBN40, (b) SBN50 and (c) SBN60. Pyroelectric coefficient (vP/vT) as a function of temperature at different electric field for (d) SBN40, (e) SBN50 and (f) SBN60.

H. Tang et al. / Journal of Alloys and Compounds 791 (2019) 1038e1045

cross-section are displayed in Fig. 1(bed). It demonstrates that both SBN ceramics showed dense microstructures, furthermore, almost of grains are granular crystals. And it can be observed that grain size are about 0.91e3.40 mm, 1.30e4.67 mm and 1.31e4.81 mm in length for SBN40, SBN50 and SBN60 ceramic, respectively. Moreover, that revealed the grain sizes are slightly bigger with the Sr content added more for the three samples.

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3.2. Ferroelectric properties In Fig. 2(aec), it shows that P-E hysteresis for SBN ceramic at 10 Hz, 50 kV cm
1 and different temperature. And spontaneous polarization (Ps) reduces with the temperature increasing. After the temperature reached the temperature of ferroelectric-paraelectric phase transition which called as Curie point (TC), the P-E

Fig. 4. Entropy change (DS) values at various electric fields for (a) SBN40, (b) SBN50 and (c) SBN60. The adiabatic temperature change (DT) value under the applied electric field for (d) SBN40, (e) SBN50 and (f) SBN60.

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H. Tang et al. / Journal of Alloys and Compounds 791 (2019) 1038e1045

hysteresis loops become slimmer [29]. It can clearly imply a typical characteristic of relaxor ferroelectric [30]. Temperature-dependent remanent polarizations (Pr) for SBN ceramics are shown in Fig. 2(d). And it is observed that Pr almost decrease regularly with the rising temperature, further, Pr tend to be relatively stable after phasetransition temperatures. The TC of SBN40, SBN50 and SBN60 are 328 K, 355 K, and 407 K, which are gotten by extrapolation from Fig. 2(d), respectively. Due to the influence of space charges and others, Pr are not zero when the temperature shift to TC, the similar phenomenon can be found in other ceramics [31].

3.3. Electrocaloric effect To calculate the electrocaloric effect, the polarization was gotten by using the upper parts of P-E hysteresis loops for all the samples at various electric fields and temperatures. As shown in Fig. 3(aec), the general tends of P are decrease with the temperature gradually increased. Moreover, it is found that obviously decreases emerge near phase transition temperatures that are received from the extrapolation in Fig. 3(d). The analogous phenomenon was reported by Liu et al. for Sr0.61Ba0.39Nb2O6 ceramic [32]. Hence, the significant reduced positions are mostly consistent for the same samples. Based on Maxwell relations (vP/vT)E¼(vS/vE)T the DS and DT under the applied electric field with the range from E1 to E2 are expressed as [17]:

1

DS ¼


r

Eð2

E1

T Cp r

DT ¼




vP vT

Eð2

E1



 dE

(1)

E

vP vT

 dE

(2)

E

Where Cp is the special heat capacity, Cp ¼ 532.00, 507.45, 446.24 J g
1 K
1 for SBN40, SBN50, SBN60, respectively. The measured results of density r are 5.70, 5.66, 5.78 g cm
3 for SBN40, SBN50, SBN60, respectively. And the curves of (vP/vT)E for all ceramics displayed in Fig. 3(cef) were received from the polynomial fits of the P-T data. As displayed in Fig. 3(cef), the negative vP/vT values reveal the extent of the P transform with the increasing temperature. Consistent with predictions, the negative extreme points of vP/vT happened at the phase transition temperatures for all the three samples, which will cause a positive ECE. However, the weakly shift of peaks of vP/vT with the applied electric field cannot clearly be seen in these curves. The value of DS and DT for all the compositions under various electric fields are deduced from Eq. (1) and Eq. (2) and illustrate in Fig. 4. Regularly, the positive extreme points of DS and DT are at vicinity of Curie temperature for all samples. That similar regular as the above can be found in BaTiO3-based materials reported by Luo

Fig. 5. The energy density calculated by Olsen cycle at various temperature range for (a) SBN40, (b) SBN50, (c) SBN60 and (d) all the samples at 60 kV/cm.

H. Tang et al. / Journal of Alloys and Compounds 791 (2019) 1038e1045

et al. [33]. It is easy to find that the peak position of DS and DT for the three compositions shift towards higher temperature with the raise applied electric fields, indicating Curie temperatures of three compositions shift to higher temperature at the same time. The same phenomenon was also observed in many ferroelectric ceramic, such as Ce-doped BaCexTi1-xO3 ceramics [34]. As anticipated, the extreme values of DS (DSmax) and DT (DTmax) are remarkably larger under the larger electric field at the same temperature. Most significantly, the values of DS and DT are increase with the Sr content added more under the same condition. The DSmax and DTmax are 0.39 J kg
1 K
1 and 0.32 K for the SBN60 under 60 kV cm
1, respectively. In addition, SBN60 showed a relatively better property on the aspect of ECE than the others. 3.4. Pyroelectric energy harvesting and energy storage The pyroelectric energy harvesting performances for SBN ceramics can be received by the Olsen cycle. The corresponding principle was presented in our previous report [35]. Area 1 / 2/3 / 4 is the valid thermal energy harvesting between two hysteresis loops operated between the high applied electric field (EH) and the low applied electric field (EL), taken at a high (TH) temperature and low temperature (TL). The first isothermal route (1 / 2) is EL raise to EH at temperature TL (293 K in this work). The

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second process (2 / 3) is composed of heating the sample from TL to TH (DTH-L ¼ TH - TL) under unchanged electric field EH. The third route (3 / 4) is related to a decline in the electric field from EH to EL at 293 K. At last, the cycle return to the initial state (4 / 1) by cooling the sample from TH to TL under EL. The pyroelectric energy harvesting density ND that defined as the electrical energy harvesting per complete cycle every-unit volume of sample can be computed by Refs. [9,10]:

I ND ¼

EdP

(3)

The E is the electric field and P is the polarization. And the values of ND for all the samples under different electric field for the different temperature range DTH-L (40 K, 60 K, 80 K, 100 K, 120 K and 140 K) are calculated by Eq. (3), showing in Fig. 5(aec). In Fig. 5(c), the extreme value of energy density is 170.0 kJ m
3 for SBN60 estimated with DTH-L ¼ 140 K. This pyroelectric energy harvesting density result is better than many lead-based and lead-free materials, like the soft and hard Pb(ZrxTi1
x)O3 [36], BCTZ20 and BCTZ50 [37]. In Fig. 5(aec), due to the change of temperature-dependent pyroelectric coefficient, the pyroelectric energy harvesting density for the wide temperature range is much higher than the narrow temperature range. With the Sr increase, the energy density increase sharply for same temperature range and electric field,

Fig. 6. P-E hysteresis for SBN ceramic: (a) SBN40, (b) SBN50, (c) SBN60, measured at 10 Hz, room-temperature and different electric fields. (d) Energy-storage density calculated from P-E hysteresis loops of SBN60 ceramics, the blue area and the gray area show the energy-storage density and energy-loss density, respectively. The inset of (d) shows the energy-storage density, energy-loss density and energy-storage efficiency of all SBN ceramic. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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H. Tang et al. / Journal of Alloys and Compounds 791 (2019) 1038e1045

showed in Fig. 5(d). For the result that is worth to note the contributions made by the Sr content for enhance pyroelectric energy harvesting performance. As expected, it is obviously that the pyroelectric energy harvesting density under a high electric field is higher than a lower one for all samples. The pyroelectric energy harvesting density of SBN60 are 98.0, 135.0, 170.0 kJ m
3 under 40, 50, 60 kV cm
1 with DTH-L ¼ 140 K, respectively. The same as ECE, it is considered that pyroelectric energy harvesting character is beneficially affected by the lower TC, the similar result is reported by Ando et al. [37]. All in all, designing the composition of the ferroelectric material and applying a safe and higher electric field are effective ways to obtain a predominant pyroelectric energy harvesting performance. Fig. 6 revealed the sundry of P-E hysteresis loops for the SBN compositions. It is discovered that the Ps increases sharply while the Pr increases weakly with the increasing electric field. Furthermore, with the Sr2þ increasing, the P-E hysteresis loops become slimmer. Moreover, it is characterized the energy-storage density W for SBN60 at room temperature in Fig. 6(d). The energy-storage characteristic was also reported by many researchers [19,29,38e40]. Large energy storage capacitors with short discharge times cater to the desires for the advance electronic devices, like electric vehicle motors. These particular properties of SBN ceramic are able to obtain from the P-E hysteresis loops. The definition for energy-storage density (W) is the energy stored per unit volume in ferroelectric material, W can be given by W ¼ ʃPdE [39]. From the equation, the W and the energy-loss density (Wloss) can be received by numerical integration of the area shown in Fig. 6(d). The results reveals that W is calculated to be 0.19 J cm
3 the Wloss value is 0.03 J cm
3 for the SBN60 ceramic under 60 kV cm
1. The energy-storage efficiency (h) is computed by the formula h ¼ W/(W þ Wloss). Additionally, the maximum roomtemperature energy-storage efficiency in SBN was 86.37% with x ¼ 0.6 under 60 kV cm
1. In the inset of Fig. 6(d), it can be clearly found that the W, Wloss and h increase with the Sr content increase. Designing the content of the raw material is also the significant detail for improving the energy-storage property. 4. Conclusions (SrxBa1-x)Nb2O6 were prepared via the solid-state reaction method. The electric-field-dependent and temperature-dependent P-E hysteresis loops was measured to study the ferroelectric properties of ceramics. The phase-transition temperatures are obtained by extrapolation from curve of Pr at different temperature. The slim P-E hysteresis loops occurring at T > TC in ceramics also reflected a relaxor behavior. The positive DSmax and DTmax occurred at Curie temperatures were 0.39 J kg
1 K
1 and 0.32 K for the SBN60 under the 60 kV cm
1, respectively. Moreover, we calculated the pyroelectric energy harvesting density by using Olsen cycle to discuss pyroelectric energy harvesting performance for SrxBa1xNb2O6 ceramic. The maximum pyroelectric energy harvesting density was estimated to be 170 kJ m
3 for SBN60 with temperature range from 293 to 433 K. And the energy-storage density and efficiency are 0.19 J cm
3 and 86.37% for SBN60 at room temperature, respectively. These above results of SrxBa1-xNb2O6 ceramic show a great potential in ambient energy conversion and scavenging waste energy area. Acknowledgements This work was supported by the National Natural Science Foundation of China [Grant No. 11574057]; the Guangdong Provincial Natural Science Foundation of China [Grant No. 2016A030313718]; and the Science and Technology Program of

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