J. Eur. Ceram. Soc.pdf

Journal of the European Ceramic Society 37 (2017) 3943–3950

Contents lists available at www.sciencedirect.com

Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc

Enhanced pyroelectric figure of merits of porous BaSn0.05 Ti0.95 O3 ceramics K.S. Srikanth, V.P. Singh, Rahul Vaish ∗ School of Engineering, Indian Institute of Technology Mandi, H.P., 175 005, India

a r t i c l e

i n f o

Article history: Received 25 February 2017 Received in revised form 5 May 2017 Accepted 9 May 2017 Available online 19 May 2017 Keywords: Lead-free Porous Pyroelectric Dielectric

a b s t r a c t Porous BaSn0.05 Ti0.95 O3 (BTS) ceramics were prepared by sintering compacts consisting of BTS and Poly (methyl methacrylate)(PMMA)as pore former. Porous BTS ceramics were systematically characterized for microstructural, ferroelectric, dielectric and pyroelectric properties. Porosity increased from 4% to 22.5% with increasing PMMA content. Dielectric constant decreases and loss increases with porosity. At 22.5% porosity, relative dielectric constant of BTS decreased by 47% (from 2525 to 1335) at 1 MHz/303K. Porosity leads to significant reduction in dielectric constant and volume specific heat capacity, which are of great interest for improving pyroelectric figure of merits (FOMs). Further, FOMs for current responsivity (Fi ), voltage responsivity (Fv ), detectivity (Fd ) and energy harvesting (Fe and Fe * ) are calculated. Compared with dense ceramic, 2% PMMA specimen showed an improvement of Fe by 166% and Fe * by 177%. Fv increased by 77%, Fd by 73% and Fi by 56% at 303K. All of these advancements are favorable for pyroelectric device applications. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction The pyroelectric materials are well-known for their unique thermo-electric conversion ability as these materials have very high sensitivity towards temporal change in temperature and hence have large market for sensors, detectors and thermal imaging applications [1–3]. Many ceramic materials have been documented in literature from past few decades which have proved to be useful in many pyroelectric applications to date [4,5]. Researchers have been still exploring new applications and cost effective materials for efficient usage of these materials. In order to derive best performance out of pyroelectric materials, there should be optimum tradeoff between pyroelectric coefficient, dielectric constant, dielectric loss and specific heat which plays a detrimental role in enhancing their figure of merits (FOMs). The physical or chemical compositional modifications technique is one such method which has been adopted in the past to chemically tune their pyroelectric performance [6–8]. In this direction, porous ceramics have generated scientific interest since they can be promising materials for pyroelectric device applications as density and dielectric constant decrease drastically due to the incorporation of pores which are of scientific interest for many pyroelectric applications. Great

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (R. Vaish). http://dx.doi.org/10.1016/j.jeurceramsoc.2017.05.015 0955-2219/© 2017 Elsevier Ltd. All rights reserved.

efforts have been put in by many academic groups/researchers on innovative processing technologies of porous ceramics, resulting in better control of the porous structures and substantial enhancements of the properties [9]. One such processing technique is by partial sintering route. The partial sintering route manifests many experiments to be conducted to look for an appropriate sintering procedure so as to achieve the desirable porosity [10]. In addition, a dense ceramic matrix is not obtained for the sake of partial sintering. The other innovative and easy method is by adding a pore-forming agent (PFA), usually some pyrolyzable particulates to the ceramic matrix [9,10]. In this method, desirable porosity can be easily obtained when contrasted with the former method, and the PFA-derived porous ceramics also possess built-in sites of pores for some selective applications [9,11]. Literature presents copious instances of dielectric, piezoelectric and pyroelectric properties of porous lead zirconate titanate (PZT) ceramics [12]. Zhang et al. used stearic acid (SA) and Poly(methyl methacrylate)(PMMA) as the pore former to fabricate porous PZT ceramics and demonstrated the difference in both porosity and pore morphology between PMMA and SA derived porous PZT ceramics and evaluated the dependence of electrical properties on these pore microstructures [11]. It has been reported that porous PZT materials have a higher figure of merit which is a method of quantifying the performance of materials for many pyroelectric device applications [13,14]. However, owing to the toxicity of lead (Pb), these materials are facing global restriction and therefore there is an increasing demand to replace

3944

K.S. Srikanth et al. / Journal of the European Ceramic Society 37 (2017) 3943–3950

Table 1 FOMs of well-known porous ceramics near room temperature. Material

PFA/ method

% porosity

P (␮C/m2 K)

Fd (10−5 Pa−0.5 )

Fv (␮C/m2 K)

Fe (J/m3 K2 )

Fe * (pm3 /J)

Ref.

Ba0.67 Sr0.33 TiO3

(i) PMMA (ii) CNT PMMA Thickness variation (i) High M.W. Polymer (ii) Low M.W. Polymer Freeze Cast (Ice) Heating rate

9.6 9.5 9.6 –

7000 7100 7100 236

26 27.5 26 1.62

– – – –

– – – –

– – – –

[15] [15] [8] [14]

25 21

119 150

– –

0.94 1

– –

– –

[13] [13]

45 – –

269 – –

– – 80–140

– 0.5–3.2 –

1.41 – –

6.57 – –

[31] [32] [33]

Ethyl cellulose

32

420

3.8

–

–

–

[34]

Ba0.67 Sr0.33 TiO3 Pb(Zr0.3 Ti0.7 )O3 (140 nm thick) PbZr0.45 Ti0.55 O3 (thin film) PZT Pb1-x Cax TiO3 (x = 0 0.3) (Thin film) Bi0.5 (Na0.82 K0.18 )0.5 TiO3 (Thick films)

Table 2 Comparison of pyroelectric FOMs for well-known lead-free ceramics with porous BTS ceramics at room temperature. Material

p(10−4 C/m2 K)

Fi (p.m/V)

Fv (m2 /C)

Fe (J/m3 K2 )

Fe * (pm3 /J)

Ref.

BaSn0.05 Ti0.95 O3 (0% PMMA) BaSn0.05 Ti0.95 O3 (2% PMMA) [(K0.5 Na0.5 )0.96 Li0.04 ]Nb0.84 Ta0.1 Sb0.06 O3 PVDF BaTiO3 [Bi0.5 (Na0.95 K0.05 )0.5 ]0.95 Ba0.05 TiO3 Triglycinesulphide (TGS)a Mn:94.6Na0.5 Bi0.5 TiO3 −5.4BaTiO3 [001]a LiNbO3 Ca0.15 (Sr0.5 Ba0.5 )Nb2 O6

4.32 5.57 1.9 0.27 2 3.25 2.8 3.8 0.83 3.61

228 355 42 100 80 112 121 131 35 171

0.01 0.018 0.003 0.147 0.008 0.015 0.362 0.082 0.141 0.02

8.2 22 3 10 4.2 – 263 22 31 17

2.34 6.48 0.1 1.7 0.6 1.7 44.1 2.3 6 3.4

This work This work [3] [35] [36] [3] [35] [28] [36] [37]

a

Single crystal.

such materials for device application. In this direction, few leadfree porous ceramics based on modified BaTiO3 have been recently reported such as Ba0.5 Sr0.5 TiO3 and Ba0.67 Sr0.33 TiO3 for their excellent pyroelectric properties, figure of merits (FOMs) and superior device performance [8,15,16]. Table 1 summarizes data reported for various porous pyroelectric ceramics along with their pyroelectric properties. It is very clear from Table 1 that porous ceramics are not extensively investigated and hence needs further scrutiny (Fv was calculated without considering the effect of specific heat and hence the units are different from them in all the materials reported in Table 2). BaTiO3 undergoes various phase transitions namely, rhombohedral–orthorhombic–tetragonal–cubic with the possibility of shifting either of them close to room temperature by doping. In this context, Sn doped BaTiO3 has already been reported for many dielectric applications with reduction of phase transition temperature towards room temperature [17–19]. For instance, a mere 5% Sn dopant in BaTiO3 not only lowers the phase transition temperature but also leads to enhancement in remnant polarization (Pr ) by about 38% in comparison with pure BaTiO3 which is of interest in improving the pyroelectric coefficient and hence the FOMs [17]. In another study, a mere 11% Sn in BaTiO3 resulted in giant piezoelectric coefficient (d33 ) of 697 pC/N which is highest reported value till date [20]. Such a large scale exploration on Sn doped BT suggests that they could be a promising materials for many pyroelectric applications. We selected BaSn0.05 Ti0.95 O3 (BTS) as it has been reported for improved pyroelectric properties as stated above along with many other promising ferroelectric and piezoelectric properties [17–20]. In order to further explore this material and enhance its properties for device applications, we attempted to fabricate porous BTS ceramics using PMMA as pore forming agent (PFA) which was allowed to burn out during sintering in order to leave pores in the materials. The porous BTS ceramics with varied amount of PMMA were further investigated for their microstructural, dielectric and pyroelectric properties.

2. Experimental The designated compositions of mixed powders were BTS + y PMMA, where y = 0, 2, 4, 6 and 8 wt.%. The BTS ceramic powder was fabricated using conventional solid state reaction route. High purity reagents of BaCO3 (99% pure), TiO2 (99% pure) andSnCl2 (98.5% pure) powders were used as the starting precursors. Stoichiometric amounts of these powders were ball milled using acetone as the wetting agent to have physical homogeneity. Calcination was done at 1130 ◦ C for 2 h to obtain the BTS compound. PMMA was dissolved in acetone prior to mixing with calcined BTS powder at the designated composition. The mixture was ball billed again to achieve physical homogeneity. The powders were then pressed into a disk shaped pellet with dimensions of 12 mm × 1 mm (diameter × thickness). To begin with, the optimum heating procedure was adopted from literature [16] with an initial heating rate of 2 ◦ C/min upto 240 ◦ C, followed by 1 ◦ C/min upto 420 ◦ C and then 2 ◦ C/min upto 700 ◦ C to assure the complete burnout of PMMA. The compacts were then sintered at 1330 ◦ C for 2 h. The sintered samples were ground to remove the surface layers and coated with silver electrodes prior to electrical measurements. The phase purity of the sintered ceramics was characterized by X-ray diffractometry (XRD) (Philips, Netherlands) with CuK␣ radiation. Scanning electron microscopy (SEM) (FEI-Technai SEMSirion) was used to observe the surface morphology of the samples. The density of the prepared specimens was measured employing Archimedes principle. The polarization-electric field (P-E) hysteresis loops were recorded at 50 Hz at various electric fields and temperatures using a modified Sawyer Tower circuit (Marine India). The dielectric properties of the samples were measured using impedance analyzer (Agilent E4990A, Agilent Technologies Inc., Santa Clara, CA) at a heating rate of 1◦ /min.


3945

Fig. 3. Variation of density with the content of pore former PMMA. Fig. 1. X-ray diffraction pattern of pure BTS ceramic.

3. Results and discussion Fig. 1 shows the X-ray diffraction pattern of pure BTS powder calcined at 1130 ◦ C for 2 h. The BTS system exhibits tetragonal structure (P4 mm space group) [17]. No extra peaks are seen in the XRD data signifying the phase purity of the prepared samples. Changes of microstructure in BaTiO3 with doping are reported in literature and few studies reveal the abnormal grain growth of ceramics [21–23]. In the present study, grains as big as 20 ␮m are observed in addition to some of about 7 ␮m for y = 0. The growth of such large grains will have dramatic consequences on the ferroelectric properties as reported by Upadhyay et al. [17]. Further, we have taken SEM images of all BTS ceramics sintered at 1330 ◦ C/2 h and shown for 0% PMMA and 8% PMMA samples in Fig. 2. It is clear that dense micro sized grains (well interconnected grains without major voids or anomaly) with bimodal grain size distribution (larger grains co-exist with smaller grains) can be observed for y = 0. Further, it is evident from Fig. 2 that the porosity was found to increase when y = 8 at a fixed sintering temperature which is consistent with the measured density of the samples shown in Fig. 3. The relative density of pure BTS (0% PMMA) was 96%. In contrast, the relative density of 8% (by wt. PMMA) ceramic was found to be 77.5%. This shows that the relative density decreases, or in other words porosity increased with PMMA addition. To examine the role of PFA in switching the hysteresis characteristics of BTS ceramics for various dielectric applications, the ferroelectric hysteresis polarization-electric field loops (P-E) have been recorded at room temperature as depicted in Fig. 4(a). The

pure BTS (y = 0) ceramic exhibited a well saturated hysteresis loop with maximum polarization of 19.6 ␮C/cm2 at an applied electric field of 32 kV/cm. The resultant porous BTS ceramics with varied amount of porosities also exhibited typical hysteresis loops. However, with an increase in porosity, the saturation polarization reduced (8 ␮C/cm2 for y = 8) when compared at 303K and at 32 kV/cm which can be attributed to their reduced density with PMMA addition. Further, in order to study the temperature dependence of polarization, P-E hysteresis loops are taken exemplarily for all the compositions at a temperature ranging from 303K to 373K. Fig. 4(b) shows the temperature dependence of polarization for y = 0 sample. It is observed that as the temperature increases, the P-E loops shrank and the intensity of polarization reduced. It illustrates that the thermal agitation gradually destroys the dipoles order arrangement. The intensity of polarization decreases with increasing porosity because the volume fraction of ferroelectric BTS phase is decreased with increasing PMMA content (with increasing y value).It suggests that the porous BTS ceramics have proportionally reduced ferroelectric domains compared with dense BTS ferroelectric ceramics [11]. Also there is a decrease in remnant polarization (Pr ) with increasing PMMA content as observed from Fig. 4(a) due to their reducing density. These results are consistent with the sudden decrease in the corresponding dielectric constant of the porous ceramics which are described in subsequent sections. Fig. 5 shows the pyroelectric coefficient (p) of BTS − y PMMA (y=0 to 8%) with different porosities as a function of temperature. Based on the P-E hysteresis loops obtained upon heating, the representative parameters like remnant polarization (Pr ) are extracted to illustrate the effect of PFA in BTS ceramics as shown in the inset of Fig. 5. From the Pr vs T curves, polynomial fitting at two different

Fig. 2. Scanning electron microscopy micrographs of BTS ceramic with (a) BTS-0%PMMA and (b) BTS-8%PMMA.

3946


Fig. 4. Polarization-Electric field (P-E) hysteresis loops of BTS ceramic with different contents of PMMA: (a) comparison at 303K and (b) BTS −0%PMMA at various temperatures.

Fig. 5. Pyroelectric coefficient as a function of temperature of all samples with different wt.% of pore former PMMA and inset shows variation of Pr as a function of temperature.

temperature regime (from 303K-333K and 333K-373K) was done as we could observe two different slopes in Pr vs T curves. The values of pyroelectric coefficient for all investigated compositions were then estimated at two different temperature regimes employing a static method as shown in Eq. (1): p=

dPr dT

(1)

These two plots were then merged into a common temperature window from 303K-373K. The pyroelectric coefficient decreases with increase in porosity at Curie temperature as evident from Fig. 5 for the reasons mentioned above. However, we observe slight increase in pyroelectric coefficient at room temperature for few samples compared with 0% PMMA. Such increments in 2% sample r are due to the larger dP in the temperature regime 303K-333K. dT r 4% sample also had higher dP than 0% sample in the temperature dT regime 303K-333K. However these reduced when compared with 2% sample. Similar kind of anomalous increment in pyroelectric coefficient has been reported by Zhang et al. where pyroelectric coefficient increased from 5000 to 8000 ␮C/m2 K when porosity increased from 1% to 9.6% which accounts to about 60% increment [8].

Further, in order to study the effect of porosity on dielectric constant, Fig. 6(a) shows the relative dielectric constant and dielectric loss plots at room temperature and at 1 MHz frequency as a function of PMMA content. It can be inferred that dielectric constant decreases and tan␦ increases with PMMA addition. This happens because the porous ceramic can be considered as a two-phase system consisting of bulk ceramic material and pores [16]. The increase in porosity can decrease the dielectric constant drastically. The presence of carbon traces as observed in EDX spectra (not shown) could also result in increase in dielectric losses. This suggests that the dielectric constant can be tailored by the degree of porosity for various dielectric applications [24]. Further, Fig. 6 shows the relative dielectric constant and loss as a function of operating temperature at various frequencies for samples containing (b) 0%, (c) 2%, (d) 4%, (e) 6% and (f) 8% PMMA. Two dielectric anomalies can be seen corresponding to phase transitions, viz. TT-O (denoted by T1 in Fig. 6) and TC-T at 290K and 357K respectively which matches with the literature [17]. The frequency dependent variation for both ␧r and tan␦ followed the general trend i.e. both decreases with increase in frequency. This reduction of dielectric constant with increasing frequency can be accredited to the decrease in polarization with increasing frequency. The higher values of dielectric constant at smaller frequencies can be attributed to the presence of all types of polarizations (electronic, ionic, dipolar and interfacial polarizations) in the material. There was almost no change of Curie temperature (Tc ) with increasing porosity, which is also evident from Fig. 6. The pyroelectric coefficient and dielectric constant are essential to estimate the pyroelectric figures of merit (FOMs). There are number of FOMs derived for use in specific pyroelectric applications. The most prevalent FOM relates to pyroelectric sensor application for generating maximum voltage or current for a given power input. The FOM for current responsivity (Fi ) can be mathematically expressed asFi = Cp where cv denotes the specific heat capacity of v the material [25,26]. Since the heat capacity of pores is much lower than that of BTS ceramic, the heat capacity cv of porous ceramics is lower than that of dense ones. cv is determined by the Eq. (2) as shown [15]: cv = cv(dense) ∗ (1 − ˚)

(2)

Where denotes the porosity fraction. Temperature dependent cv (dense) is adopted from literature and used in the present study [17]. The Fi increased from 228 pm/V (0% PMMA) to 355 pm/V (2% PMMA) at 303 K and remains almost constant upto 333 K. These data are consistent with the calculated pyroelectric coefficient


3947

Fig. 6. Temperature dependence of dielectric constant and loss tangent with different porosity: (a) comparison at 303K/1 MHz, (b) BTS −0%PMMA, (c) BTS −2%PMMA, (d) BTS −4%PMMA, (e) BTS −6%PMMA and (f) BTS-8%PMMA.

plots. Although the decrease in pyroelectric coefficient with porosity was dominant over decrease in cv which leads to decrease in Fi (at Curie temperature) with porosity as shown in Fig. 7(a). On similar grounds, to obtain maximum pyroelectric voltage for a given heat input, the FOM for voltage responsivity is written as(Fv ) = p , where ␧0 and ␧ are permittivity of free space and dielecCv εε0 tric constant, respectively [27]. The Fv increased by almost 77% for 2% sample over 0% sample at 303K. This increment in Fv depends on the relationship between specific heat, dielectric constant and pyroelectric coefficient. εr decreased by 14% and p increased by almost 30% upto a definite temperature regime (303K-333K) which resulted in increasing Fv for 2% sample when compared with 0% PMMA. In the temperature regime 333K-373K, the Fv increased

marginally for 2% specimen. This increment was rather marginal compared with the other temperature regime as p decreased drastically in this temperature regime (333K-373K) which impacted more than the decrease of dielectric constant. Moreover, the trend followed by these FOMs depends largely on the values of p and εr and hence the values of FOM depend on them. Furthermore, high detectivity based FOM is written as(Fd ) = √ p [28,29]. Cv

εε0 tanı

In general, these FOMs are used for materials selection and indirect performance evaluation for heat, infrared sensors etc. On the contrary, the thermal-electrical energy harvesting is also a vital application of pyroelectric ceramics. In this direction, energy harvesting FOMs (Fe ) =

p2 εε0 and

(Fe∗ ) =

p2 have εε0 Cv2

been used by many

3948


Fig. 7. Plots of FOM for (a) current responsivity (Fi ), (b) voltage responsivity (Fv ), (c) energy harvesting (Fe ), (d) energy harvesting (Fe * ) and (e) detectivity (Fd ).

researchers for pyroelectric energy harvesting materials selection [27,30]. The temperature dependent FOMs are plotted in Fig. 7. However, it can be summarized from the nature of FOMs that for greater performance, high pyroelectric coefficient (p) and low dielectric constant (ε) is primary requirement of an ideal material. Further, we have also plotted all FOMs at room temperature to have an insight into the potential of porous ceramics over pure ceramics for pyroelectric device applications as depicted in Fig. 8. It is seen that all the calculated FOMs increased for 2% PMMA content

and thereafter decreased as the decrease in p impacted more than the decrease in dielectric constant. In order to compare the performance of porous BTS with other well-known lead free pyroelectric material, we have collected the data from literature and presented in Table 2. It can be concluded from such studies that pyroelectric FOMs can be enhanced for device applications by suitably selecting the% of porosity in dense ceramics.


3949

Fig. 8. All FOMs at room temperature.

4. Conclusions Porous BTS ceramics were successfully fabricated using BTS as a base ceramic material with varied amount of PMMA (2, 4, 6 and 8%) as a pore-forming agent. The density of the specimens decreased with PMMA which resulted in an increase in porosity upto 22.5% for 8% by wt. PMMA addition. The dielectric constant decreased and dielectric loss increased with increasing PMMA content at fixed frequency. When porosity level reached 22.5%, the relative dielectric constant of the BTS specimens decreased more than 47% (from 2525 to 1335) at 1 MHz and at 303K. The dielectric loss increased slightly but still remained less than 0.1%. These results are encouraging to obtain high pyroelectric figures of merit for device applications. The figure of merit for voltage responsivity (Fv ) showed 77% improvement over dense ceramic for 2% PMMA sample. Similarly, the maximum figure-of-merit for energy harvesting (Fe )increased by 166% and (Fe ∗ ) by 177% for 2% PMMA compared with dense BTS.Fd increased by 73% and Fi by 56% at 303K.From this study it can be concluded that porosity has a detrimental effect on ceramic materials for pyroelectric device applications. Acknowledgements One of the author (Rahul Vaish) acknowledges the support from Indian National Science Academy (INSA), New Delhi, through a grant by the Department of Science and Technology (DST), New Delhi, India under the INSPIRE faculty award-2011 (ENG-01) and INSA Young Scientists Project. References [1] R.W. Whatmore, Pyroelectric Devices, Rep. Materials, Prog. Phys. 49 (1986) 1335–1386. [2] S. Bauer, B. Ploss, A method for the measurement of the thermal, dielectric, and pyroelectric properties of thin pyroelectric films and their applications for integrated heat sensors, J. Appl. Phys. 68 (1990) 6361–6367. [3] S.T. Lau, C.H. Cheng, S.H. Choy, D.M. Lin, K.W. Kwok, H.L.W. Chan, Lead-free ceramics for pyroelectric applications, J. Appl. Phys. 103 (2008). [4] M. Vaish, M. Sharma, R. Vaish, V.S. Chauhan, Electrical energy generation from Hot/ColdAir using pyroelectric ceramics, Integr. Ferroelectr. 167 (2015) 90–97. [5] B. Charlot, D. Coudouel, F. Very, P. Combette, A. Giani, Droplet generation for thermal transient stimulation of pyroelectric PZT element, Sensor Actuat. A-Phys. 225 (2015) 103–110. [6] S. Patel, A. Chauhan, S. Kundu, N.A. Madhar, B. Ilahi, R. Vaish, K.B.R. Varma, Tuning of dielectric, pyroelectric and ferroelectric properties of 0.715Bi0.5 Na0.5 TiO3 -0.065BaTiO3 -0.22SrTiO3 ceramic by internal clamping, AIP Adv. 5 (2015).

[7] S. Patel, A. Chauhan, R. Vaish, Large pyroelectric figure of merits for Sr-modified Ba0.85 Ca0.15 Zr0.1 Ti0.9 O3 ceramics, Solid State Sci. 52 (2016) 10–18. [8] G.Z. Zhang, S.L. Jiang, Y.K. Zeng, Y.Y. Zhang, Q.F. Zhang, Y. Yu, High, pyroelectric properties of porous Ba0.67 Sr0.33 TiO3 for uncooled infrared detectors, J. Am. Ceram. Soc. 92 (2009) 3132–3134. [9] T. Ohji, M. Fukushima, Macro-porous ceramics: processing and properties, Int. Mater. Rev. 57 (2012) 115–131. [10] J. Roscow, Y. Zhang, J. Taylor, C.R. Bowen, Porous ferroelectrics for energy harvesting applications, Eur. Phys. J-Spec. Top. 224 (2015) 2949–2966. [11] H.L. Zhang, J.F. Li, B.P. Zhang, Microstructure and electrical properties of porous PZT ceramics derived from different pore-forming agents, Acta Mater. 55 (2007) 171–181. [12] T. Zeng, X.L. Dong, C.L. Mao, Z.Y. Zhou, H. Yang, Effects of pore shape and porosity on the properties of porous PZT 95/5 ceramics, J. Eur. Ceram. Soc. 27 (2007) 2025–2029. [13] G. Suyal, N. Setter, Enhanced performance of pyroelectric microsensors through the introduction of nanoporosity, J. Eur. Ceram. Soc. 24 (2004) 247–251. [14] Q. Zhang, S. Corkovic, C.P. Shaw, Z. Huang, R.W. Whatmore, Effect of porosity on the ferroelectric properties of sol-gel prepared lead zirconate titanate thin films, Thin Solid Films 488 (2005) 258–264. [15] S.L. Jiang, P. Liu, X.S. Zhang, L. Zhang, Q. Li, J.L. Yao, Y.K. Zeng, Q. Wang, G.Z. Zhang, Enhanced pyroelectric properties of porous Ba0.67 Sr0.33 TiO3 ceramics fabricated with carbon nanotubes, J. Alloy Compd. 636 (2015) 93–96. [16] Y.Y. Zhang, G.S. Wang, T. Zeng, R.H. Liang, X.L. Dong, Electric field-dependent dielectric properties and high tunability of porous Ba0.5 Sr0.5 TiO3 ceramics, J. Am. Ceram. Soc. 90 (2007) 1327–1330. [17] Sanjay Kumar Upadhyay, V. Raghavendra Reddy, Pallab Bag, R. Rawat, S.M. Gupta, Ajay Gupta, Electro-caloric effect in lead-free Sn doped BaTiO3 ceramics at room temperature and low applied fields, Appl. Phys. Lett. 105 (2014). [18] L. Xie, Y.L. Li, R. Yu, Z.Y. Cheng, X.Y. Wei, X. Yao, C.L. Jia, K. Urban, A.A. Bokov, Z.-G. Ye, J. Zhu, Static and dynamic polar nanoregions in relaxor ferroelectric Ba(Ti1-x Sn x ) O3 system at high temperature, Phys. Rev. B 85 (2012) 014118. [19] C. Lei, A. Bokov, Z. Ye, Ferroelectric to relaxor crossover and dielectric phase diagram in the BaTiO3 -BaSnO3 system, J. Appl. Phys. 101 (2007) (84105-84105). [20] Y. Yao, C. Zhou, D. Lv, D. Wang, H. Wu, Y. Yang, X. Ren, Large piezoelectricity and dielectric permittivity in BaTiO3 -xBaSnO3 system: the role of phase coexisting, Europhys. Lett. 98 (2012) 27008. [21] W. Cai, C.L. Fu, J.C. Gao, C.X. Zhao, Dielectric properties and microstructure of Mg doped barium titanate ceramics, Adv. Appl. Ceram. 110 (2011) 181–185. [22] A.C. Caballero, J.F. Fernández, C. Moure, P. Durán, Y.M. Chiang, Grain growth control and dopant distribution in ZnO-Doped BaTiO3 , J. Am. Ceram. Soc. 81 (1998) 939–944. [23] Y. Bai, X. Han, K. Ding, L.J. Qiao, Combined effects of diffuse phase transition and microstructure on the electrocaloric effect in Ba1-x Srx TiO3 ceramics, Appl. Phys. Lett. 103 (2013) 162902. [24] H. Banno, Effects of shape and volume fraction of closed pores on dielectric elastic, and electromechanical properties of dielectric and piezoelectric ceramics-a theoretical approach, Am. Ceram. Soc. Bull. 66 (1987) 1332–1337. [25] P. Yu, Y.D. Ji, N. Neumann, S.G. Lee, H. Luo, M. Es-Souni, Application of single-Crystalline PMN-PT and PIN-PMN-PT in high-Performance pyroelectric detectors, IEEE T. Ultrason. Ferr. 59 (2012) 1983–1989. [26] Y.X. Tang, H.S. Luo, Investigation of the electrical properties of (1-x)Pb(Mg1/3 Nb2/3 )O3 -xPbTiO3 single crystals with special reference to pyroelectric detection, J. Phys. D: Appl. Phys. 42 (2009).

3950


[27] C.R. Bowen, J. Taylor, E. LeBoulbar, D. Zabek, A. Chauhan, R. Vaish, Pyroelectric materials and devices for energy harvesting applications, Energy Environ. Sci. 7 (2014) 3836–3856. [28] R. Sun, J. Wang, F. Wang, T. Feng, Y. Li, Z. Chi, X. Zhao, H. Luo, Pyroelectric properties of Mn-doped 94.6Na0.5 Bi0.5 TiO3 -5.4BaTiO3 lead-free single crystals, J. Appl. Phys. 115 (2014) 074101. [29] L. Xing, C. Zhihui, W. Dun, F. Bijun, D. Jianning, Z. Xiangyong, X. Haiqing, L. Haosu, Enhancing pyroelectric properties of Li-doped (Ba0.85 Ca0.15 )(Zr0.1 Ti0.9 )O3 lead-free ceramics by optimizing calcination temperature, Jpn. J. Appl. Phys. 54 (2015) 071501. [30] C.R. Bowen, J. Taylor, E. Le Boulbar, D. Zabek, V.Y. Topolov, A modified figure of merit for pyroelectric energy harvesting, Mater. Lett. 138 (2015) 243–246. [31] Y. Zhang, Y. Bao, D. Zhang, C.R. Bowen, Porous PZT ceramics with aligned pore channels for energy harvesting applications, J. Am. Ceram. Soc. 98 (2015) 2980–2983. [32] A. Seifert, P. Muralt, N. Setter, High figure-of-merit porous Pb1-x Cax TiO3 thin films for pyroelectric applications, Appl. Phys. Lett. 72 (1998) 2409.

[33] A. Seifert, L. Sagalowicz, P. Muralt, N. Setter, Microstructural evolution of dense and porous pyroelectric Pb1- x Cax TiO3 thin films, J. Mater. Res. 14 (1999) 2012–2022. [34] H. Zhang, S. Jiang, K. Kajiyoshi, Enhanced pyroelectric and; piezoelectric figure of merit of porous Bi0.5 (Na0.82 K0.18 )0.5 TiO3 lead-Free ferroelectric thick films, J. Am. Ceram. Soc. 93 (2010) 1957–1964. [35] S.B. Lang, D. Das-Gupta, Pyroelectricity: fundamentals and applications Handbook of Advanced Electronic and Photonic Materials and Devices, vol. 4, 2000, pp. 22–50. [36] S.B. Lang, Pyroelectricity: from ancient curiosity to modern imaging tool, Phys. Today 58 (2005) 31. [37] J. Zhang, X. Dong, F. Cao, S. Guo, G. Wang, Enhanced pyroelectric properties of Cax (Sr0.5 Ba0.5 )1-x Nb2 O6 lead-free ceramics, Appl. Phys. Lett. 102 (2013) 102908.