Large Electrocaloric Effect in Lead-free Ba(HfxTi1âx)O3 Ferroelectric

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The Large Electrocaloric Effect in Lead-free Ba(HfxTi1x)O3 Ferroelectric Ceramics for Clean Energy Applications Ming-Ding Li, Xin-Gui Tang, Si-Ming Zeng, Qiu-Xiang Liu, Yan-Ping Jiang, Tian-Fu Zhang, and Wen-Hua Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01277 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Sustainable Chemistry & Engineering

The Large Electrocaloric Effect in Lead-free Ba(HfxTi1-x)O3 Ferroelectric Ceramics for Clean Energy Applications

Ming-Ding Li1, Xin-Gui Tang1,*, Si-Ming Zeng2, Qiu-Xiang Liu1, Yan-Ping Jiang1, Tian-Fu Zhang1 and Wen-Hua Li1 1

School of Physics & Optoelectric Engineering, Guangdong University of Technology, No 100 Waihuan Xi Road, Guangzhou 510006, China.

2

Laboratory Teaching Centre, Guangdong University of Technology, No 100 Waihuan Xi Road, Guangzhou 510006, China.

ABSTRACT Ba(HfxTi1-x)O3 ferroelectric ceramics were prepared by conventional solid-state reaction process and the correlation of structure, dielectric, ferroelectric and electrocaloric properties were studied. The consequences indicated that the TC of Ba(HfxTi1-x)O3 ceramics decreased, while TT-O and TO-R increased with the increase of hafnium content. Large electrocaloric effect values of 1.64 K (117 °C) and 1.21 K (76°C) were observed for Ba(HfxTi1-x)O3 with x=0.05 compositions. And corresponding electrocaloric coefficients respectively were 0.33 and 0.24 K·mm·kV-1. In addition, compared with the second-order phase transition region, larger electrocaloric coefficient (∆Tf /∆E ≥ 0.19 K·mm·kV-1) were obtained at first-order phase transition region for all compositions. Meanwhile, the maximum values of electrocaloric coefficients ∆Tf /∆E = 0.38 K·mm·kV-1 was achieved in a smaller electric field for Ba (HfxTi1-x)O3 with x = 0.03 compositions.

KEYWORDS: BHT ceramics, Dielectric, Ferroelectric, Electrocaloric effect, Phase transition

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INTRODUCTION In the recent years, the environmentally friendly electrocaloric (EC) refrigeration technologies has caused serious attentions, particularly in the dielectric materials, because of their potential applications were extensive in solid-state refrigerator for cooling microelectronic devices. On the other hand, compared with vapor-compression refrigeration, the EC refrigeration showed more excellent conversion efficiency. 1, 2 Under the adiabatic conditions, when an electric field was applied or removed from the ferroelectric material, the corresponding change of temperature and entropy were called electrocaloric effect (ECE). 3-5 Interestingly, since Mischenko et al. reported a giant ECE value of 12K in the PZT materials. 6

The study of ECE has newly caused considerable interest and giant ∆T values for lead based

ferroelectric materials like PLZT thick film, Pb0.8Ba0.2ZrO3 and PLZST films were successively acquired under a large breakdown electric field. 7-9 Although the giant ECE was obtained in the thin films, however, compared with ceramics, the heat extraction capacity was always lower and the toxicity of lead-based materials in both the human body and the environment limits it’s used. Therefore, lead-free ferroelectric materials were developed as alternatives to lead-containing materials for EC refrigeration, especially in BaTiO3-based ceramics, such as, BaSnxTi1-xO3

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BZST, 11 BTNY, 12 BCZT, 13 La-doped BNT-6BT, 14 etc. For instance, Qian et al. 15 reported the giant EC effect (∆T = 4.5K at 38 °C) in the BZT bulk ceramics at temperatures close to the first-order phase transition. Moreover, Bai et al.

16

displayd that an EC coefficient of 1.4

K·mm·kV-1 (10 °C) in BaTiO3 ceramics. In particular, the EC values were expected to be remarkably increase by changing the material compositions near the invariant critical point. 17 Based on above and previous research results,

18-21

Ba(HfxTi1-x)O3 (BHT) ceramics were

successfully synthesized in this paper. The corresponding preparation and characterization are showed in supporting information. The structure, dielectric, ferroelectric and electrocaloric properties were investigated. Large ECE values of 1.64 K was observed in BHT with x=0.05 compositions. In addition, the highest EC coefficient ∆Tf /∆E = 0.38 K·mm·kV-1 was achieved under a lower electric field for BHT with x = 0.03 compositions.

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RESULTS AND DISCUSSION

Figure 1. (a) XRD patterns for BHT pellets with x=0.03, 0.05, 0.08, 0.11 at room temperature, (b)-(c) the {200}C and {220}C diffraction patterns of the BHT samples, the label point out the diffraction peak from the orthorhombic phase. Figure 1 shows XRD pattern for the sintered BHT bulk ceramics. From Figure 1a, the single-phase perovskite structures are obtained in the BHT ceramics. It shows that Hf atoms have successfully diffused into the BT lattices, resulting in the formation of homogenous BHT solid solution. In addition, according to the evolution of two pseudocubic {200}C and {220}C diffraction patterns, the evolution in the phase structure of the BHT samples can be evaluated, as shown in Figure 1b,c. The BHT (0.03 ≤ x ≤ 0.05) ceramics presents the coexistence of the tetragonal (TE) and orthorhombic (O) phases, which can be confirmed from its XRD spectrum (Figure 1b,c, the diffraction peak of O phase is labelled by an arrow). There is no doubt that the coexistence of the TE and O phases in BHT (0.03 ≤ x ≤ 0.05) ceramics cause by the intrinsic property of the P4mm-Amm2 transition. 19 Furthermore, the BHTs with x= 0.08 and 0.11 show a rhombohedral (R) phase structure.

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Figure 2. SEM pictures of the surfaces of the BHT bulk ceramics. Figure 2 displays the SEM micrographs of the thermally etched surfaces for BHT bulk ceramics that have been sintered at 1450 °C. On the one hand, all samples exhibit a dense microstructure and a similar transgranular morphology feature. On the other hand, the porosities of ceramics increase with the doped content increases of Hf from 0.03 to 0.11. It is evident that some porosities exist in x=0.11 ceramic sample, indicating that the optimum sintering temperature increase with increase the content of Hf.

Figure 3.Temperature dependence of dielectric properties for BHT ceramics (a) dielectric constant

εγ measured at frequencies from 1 kHz to 100 kHz (b) dielectric loss tanδ measured at 100 kHz. Table 1 Phase transition temperatures (PTTs) of BHT ceramics Phase transition temperatures o

TC ( C) TO-T (oC) TO-R (oC)

x=0.03

x=0.05

x=0.08

x=0.11

εγ – T

tanδ–T

εγ –T

tanδ–T

εγ –T

tanδ–T

εγ –T

tanδ–T

108 42 –

106 38 –

100 54 –

98 52 –

90 66 –

88 66 46

68 – –

66 – –

4


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Figure 3 illustrates temperature (25 °C ≤ T ≤160 °C) dependence of the dielectric constant εγ and loss tangents for BHT ceramics. Table 1 presents the PTTs for the Hf doped BaTiO3 ceramics. It is found that the ferroelectric to paraelectric phase transitions have drifted to low temperature while the temperature is the PTTs from TE to cubic (C) (TC : Curie temperature). In contrast, other two PTTs from O to TE (TO-T) and O to R (TO-R) will move to high temperatures with increasing Hf concentration. This means that the TC of the Hf-doped BT ceramics decrease, while TT-O and TO-R increase with the increase of hafnium content. That is in agreement with the reported previously. 19, 20

Furthermore, the three obvious dielectric peaks (90 °C, 66 °C, 48 °C) are observed in the BHT

with x = 0.08, as shown in Table 1, indicating the phase transitions from C to TE to O to R. In addition, only one broad dielectric peak is acquired for the BHT with x = 0.11, which shows the phase transition from C to R. In addition, TC peaks become broader and broader with increase the content of Hf, indicating the increase of relaxor behavior, which was confirmed in Ba(SnTi)O3 and Ba(ZrTi)O3 ceramic systems. 22, 23

Figure 4. Ferroelectric hysteresis loops at selected temperatures for the BHT bulk ceramics. The inset displays polarization as a function of temperature (P–T) plots under selected electric fields. Figure 4 displays the various temperatures ferroelectric hysteresis loops (P–E) for BHT 5



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ceramics at the frequency 10 Hz. All ceramics present characteristic ferroelectric hysteresis loops in room temperature, implying good ferroelectricity. On the other hand, the coercive field, saturation polarization and remanent polarization step by step reduce with the increases of temperatures. In addition, when the temperature reaches above the TC, the ferroelectric hysteresis loops exhibits almost linear characteristics, implying the characteristic of the paraelectric phase. 24 Figure 4 insets show the P–T plots at electric fields of 10, 20, 30, 40 and 50 kV·cm-1 for BHT ceramics. By approaching TO-T and TC, the P–T curves manifest a strong decay that is smeared out with the increasing of electric field. It implies that the order of electric dipoles is gradually devastated by thermal stimulation. The evolution of polarization indicates the corresponding entropy variation. 2 In order to quantify the ECE, we take the indirect method which is taken by a number of scientific communities. 25, 26 i.e. Based on the Maxwell relation (∂P/∂T)E = (∂S/∂E)T, the reversible adiabatic temperature change ∆T and isothermal entropy change ∆S is calculated by the following formulas. 6, 27

∆S = −

∆T = −

1

ρ∫

E2 E1

T CP ρ

∫

 ∂P   ∂T  dE  E E2 E1

 ∂P   ∂T  dE  E

(1)

(2)

Where ρ is the bulk density of the ceramics. CP is the specific heat capacity. The E1 and E2 represent the original and ultimate applied electric fields. In addition, previous studies have demonstrated that E1 is equal to 0 and E is equal to E2 for normal ferroelectric and relaxed ferroelectrics. 28 The ∂P/∂T (pyroelectric coefficient) can be acquired via numeric differentiation of the P–T experimental data (as showed in Figure S1). These data are extracted from the data of the P–E loops, as shown in Figure 4. Furthermore, the CP is measured by DSC. The corresponding results are listed below. The heat capacities CP (J·g-1·K-1) and the density ρ (g·cm-3) are 0.43 and 5.53, 0.42 and 5.58, 0.41 and 5.61, 0.39 and 5.69 for the ceramic with x = 0.03, 0.05, 0.08 and 0.11, respectively.

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Figure 5. The temperature dependence of the EC temperature change (∆T) pictures for BHT ceramics under diverse electric fields. Utilizing the above result, the ∆T and ∆S are calculated using Eq. (1) and Eq. (2), as shown in Figure 5, Figure S2, respectively. Figure 5 displays the ∆T as a function of temperature for BHT ceramics in temperature from 20 °C to 200 °C. For the BHT with x = 0.03, the two peaks of ∆T are observed around in 42 °C and 108 °C, respectively, close to the two phase transition for TO-T and TC respectively, which indicates that excellent EC value can be achieved in the phase transition point. Furthermore, the EC values of ∆T increases with the increasing of electric fields at the same temperature. The ∆T peaks (∆Ts) in the ferroelectric O phase–ferroelectric TE phase switching (second-order phase transition) area shift to higher temperatures in the same way as TO-T when Hf-rate increases. In contrast, the ∆T peaks (∆Tf) in the first-order PTTs area shift to lower temperatures in the same way as TC with the increasing of Hf concentration. Finally, only one ∆T peak could be discovered in the BHT samples with x = 0.11. The same behavior is noticed in isothermal entropy change ∆S for all compositions, as showed in Figure S2. Furthermore, the maximum values of ∆Tf shift to lower temperatures and become broader. A broad EC temperature span is made due to the heterogeneous dispersion and the increase of relaxor behavior with the increasing of Hf concentration in the BHT ceramics. 7



Figure 6.The figure summarizes the EC properties: The EC temperature changes (a) ∆Ts and (b) ∆Tf and the corresponding EC coefficient (c) ∆Ts /∆E and (d) ∆Tf /∆E for BHT ceramics at selected external electric fields (10, 30 and 50 kV·cm-1). Figure 6 shows the EC temperature changes ∆Ts and ∆Tf for BHT ceramics at diverse electric fields. A large ECE values are acquired at larger electric field for all compositions. Figure 5 and Figure 6 displays that the large EC values of ∆Tf under 50 kV·cm-1 are 1.34 K (119 °C), 1.64 K (117 °C), 1.20 K (110 °C) and 1.41 K (92 °C) for BHT with x =0.03, 0.05, 0.08, and 0.11, respectively. Furthermore, the relatively high EC values of ∆Ts = 0.71 K (48 °C), 1.21 K (76 °C) and 0.94 K (80 °C) were achieved at 50 kV·cm-1 due to the ferroelectric O phase–ferroelectric T phase switching for BHT ceramic with x=0.03, 0.05 and 0.08, respectively. Moreover, the above results show that the maximum values of ∆Tf was found, which firstly increases with Hf content and reach to the maximum of 1.64 K for the ceramics BHT with x=0.05 then decreases to ∼1.2K for BHT with x=0.08 composition, as shown in Figure 6b. A corresponding trend is observed for the maximum values of ∆Ts, as Figure 6a showing. On the other hand, in order to establish comparison standard for EC cooling. The EC coefficient ∆Ts /∆E and ∆Tf /∆E are calculated for BHT ceramics, as displayed in Figure 6c,d, respectively. 29, 30 Compared with the second-order phase transition region, the larger electrocaloric coefficient (∆Tf /∆E ≥ 0.19 K·mm·kV-1) were 8


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obtained at first-order phase transition region for all compositions and maximum ∆Tf /∆E = 0.38 K·mm·kV-1) are obtained at first-order phase transition region for BHT with x=0.03. Table 2 The maximum values (∆Tm) of ECE and corresponding EC coefficient compared for selected ferroelectric materials

TEmax (oC)

∆Tm (K)

E (kV·cm-1)

∆T /∆E (K·mm·kV-1)

Ref

fp

Ba(Hf0.03Ti0.97)O3 Ba(Hf0.03Ti0.97)O3 fp

115 119

0.38 1.34

10 50

0.38 0.27

This work This work

Ba(Hf0.05Ti0.95)O3 fp

Material

117

1.64

50

0.33

This work

Ba(Hf0.05Ti0.95)O3

sp

76

1.21

50

0.24

This work

Ba(Hf0.11Ti0.89)O3

fp

92

1.41

50

0.28

This work

Ba0.94Sm0.04TiO3

76

0.92

30

0.30

2

BaTi0.98Sn0. 02O3

85

0.16

9.84

0.16

10

Ba0.80Sr0.20TiO3

76

0.39

20

0.195

31

BaTi0.885Sn0.105O3

30

0.61

20

0.305

32

Ba(Ti0.944Y0.056)O2.972

75

0.4

45

0.089

33

Ba0.85Ca0.15Ti0.90Hf0.10O3

123

0.74

35

0.21

24

Ba0.98Ca0.02Zr0.085Ti0.915O3

85

0.6

40

0.15

34

Ba0.85Ca0.15Zr0.1Ti0.895Fe0.005O3

72

0.60

33

0.182

13

Gd0.02Na0.5Bi0.48TiO3

97

0.75

90

0.083

35

TEmax, fp and sp indicate the temperature of maximum electrocaloric effect, the electrocaloric performance around first-order and second-order phase transition region, respectively.

Table 2 displays the merit of the EC performance for various ceramics. The ECE of BHT ceramics are beyond nearly all previously studied bulk materials. It means that our study provides outstanding EC materials with larger EC values and coefficients.

CONCLUSIONS In summary, BHT ceramics were synthesized by conventional solid-state reaction process and the correlation of structure, dielectric, ferroelectric and EC properties were analyzed. The consequences indicated that the TC of the Hf-doped BT ceramics decreased, while TT-O and TO-R increased with the increase of hafnium content. All samples exhibited excellent EC property with large ∆T over a broad EC temperature span from room temperature to 150 °C. The large ECE values of 1.64 K (117 °C) and 1.21 K (76°C) were observed for BHT with x=0.05 compositions. And the corresponding EC coefficients were 0.33 and 0.24 K·mm·kV-1, respectively. In addition, compared with the second-order phase transition region, larger electrocaloric coefficient (∆Tf /∆E

9



≥ 0.19 K·mm·kV-1) were obtained at first-order phase transition region for all compositions. Meanwhile, the maximum values of electrocaloric coefficients ∆Tf /∆E = 0.38 K·mm·kV-1 were achieved for BHT with x = 0.03 compositions. This work provided a complete picture for ECE properties around first-order and second-order phase transition region in Hf doped BaTiO3 ceramics, which provided a probability to conceive environmentally friendly EC refrigeration technologies with the characteristic of excellent EC efficiency.

ASSOCIATED CONTENT Supporting Information Experimental section, the ∂P/∂T figures and ∆S figures.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (X.G. Tang) .

ORCID Ming-Ding Li: 0000-0002-3594-0591 Xin-Gui Tang: 0000-0001-6429-2098

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 11574057) and the Science and Technology Program of Guangdong Province of China (Grant Nos. 2016A010104018 and 2017A010104022).

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For Table of Contents Use Only Large electrocaloric effect values of 1.64 K and the highest electrocaloric coefficients ∆Tf /∆E = 0.38 K·mm·kV-1 were achieved in the lead-free Ba(HfxTi1-x)O3 ceramics.

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Large Electrocaloric Effect in Lead-free Ba(HfxTi1âx)O3 Ferroelectric

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Large Electrocaloric Effect in Lead-free Ba(HfxTi1âx)O3 Ferroelectric