- Email: [email protected]
Coexistence of positive and negative electrocaloric effects in lead free perovskite structured ferroelectrics
Accepted Manuscript Coexistence of positive and negative electrocaloric effects in lead free perovskite structured ferroelectrics
Sarir Uddin, Guang-Ping Zheng, Yaseen Iqbal PII:
S1293-2558(19)30614-4
DOI:
10.1016/j.solidstatesciences.2019.105929
Article Number:
105929
Reference:
SSSCIE 105929
To appear in:
Solid State Sciences
Received Date:
25 May 2019
Accepted Date:
05 July 2019
Please cite this article as: Sarir Uddin, Guang-Ping Zheng, Yaseen Iqbal, Coexistence of positive and negative electrocaloric effects in lead free perovskite structured ferroelectrics, Solid State Sciences (2019), doi: 10.1016/j.solidstatesciences.2019.105929
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Coexistence of positive and negative electrocaloric effects in lead free perovskite structured ferroelectrics Sarir Uddin1, 2, 3 *, Guang-Ping Zheng2, Yaseen Iqbal3 1Department
of Physics, Government College Hayatabad, Peshawar 25124, Khyber
Pakhtunkhwa, Pakistan 2Department
of Mechanical engineering, The Hong Kong Polytechnic University, Hung
Hom, Kowloon, Hong Kong 3Materials
Research Laboratory, Department of Physics, University of
Peshawar, Khyber
Pakhtunkhwa, Peshawar 25120, Pakistan *[email protected] Abstracts The coexistence of positive and negative electrocaloric effects (ECE) was observed in the sol-gel derived Pb-free perovskite structured (Bi1/2Na1/2)1-xBaxTiO3 (x = 0, 0.04, 0.05, 0.06, 0.07) (BNBT) ceramics. The maximum electrocaloric temperature change of 2.35 oC has been observed at 100 kV/cm for x = 0.06 at about 70°C before the ferroelectric to antiferroelectric (FE-to-AFE) phase transition temperature. The coexistence of positive and negative ECE at different temperatures may be attributed to two different temperature dependent phase transitions in the ferroelectrics or antiferroelectrics. The sign reversal of ECE in the same material can be utilized to increase the efficiency of the refrigeration cycle and to develop efficient environmentally friendly cooling systems. Keywords: Lead-free, Ferroelectric ceramics, Electrocaloric effect, solid state refrigerants
1
ACCEPTED MANUSCRIPT
1. Introduction Electrocaloric effect (ECE) is the reversible change in temperature (∆T) of a polarizable material due to a changing applied electric field through that material under adiabatic conditions [1]. Small-scale and environmentally friendly refrigerants for microelectronics can be designed using this property of ferroelectric materials. Electrocaloric effect has been investigated in different ceramics and polymers since its discovery in 1930 by Kobeko and Kurchatov in order to find out an alternative to the conventionally hazardous gases based cooling technology [2-5]. ECE is directly proportional to the polarization change (ΔP) and hence the entropy change (ΔS) which is the dipolar disorder produced due to the ionic displacements inside the material by the applied electric field. Large ECE in a material requires large entropy change of the material which may occur near structural or ferroelectric to paraelectric (FE-to-PE) phase transition temperature (Tc) [6]. The ECE can be categorized now into positive or negative depending on the sign of the temperature change (ΔT) [5]. For positive ECE the cooling is produced during the withdrawal of the electric field while for negative ECE the cooling effect is produced by increasing the applied electric field. The negative ECE indicates that the application of electric field increases dipolar disorder and enable the dielectric material to absorb heat and produce cooling effect [7]. Although the works on ECE have been reviewed intensively but no experimental results had been reported about negative ECE [3-5] up to 2011. For the first time Bai et al. [7] reported the negative ECE in the relaxor Bi1/2Na1/2TiO3 ceramics in the temperature range 25-140 oC by using indirect method of measuring ECE at a maximum of 40 kV/cm electric field. Jiang et al. [8] reported coexistence of negative and positive ECE in Bi0.5Na0.5TiO32
ACCEPTED MANUSCRIPT
xKNbO3. Goupil et al. [9] reported the presence of negative ECE in Pb1/3Mg2/3O3-30PbTiO3 single crystal determined by direct and indirect measurements. The simulation of ECE demonstrated on a single crystal of Ba1/2Sr1/2TiO3 system also revealed the coexistence of both positive and negative ECE in the same material reported by Ponomareva and Lisenkov [10]. In this investigation the reversal of sign and coexistence of positive and negative ECE was observed in the sol-gel derived lead-free polycrystalline relaxor (Bi1/2Na1/2)1-xBaxTiO3 (0 ≤ x ≤ 0.07) (BNBT) ceramics in the temperature range from 30 °C to 230 °C. The sign reversal and dual nature of ECE was observed at ferroelectric- to-antiferroelectric (FE-toAFE) and antiferroelectric-to-paraelectric (AFE-to-PE) phase transition temperatures. ECE was observed to depend strongly on ambient temperatures and applied electric field. 2. Experimental Ceramics compositions of (Bi1/2Na1/2)1-xBaxTiO3 (0 ≤ x ≤ 0.07) were fabricated via sol-gel
processing
technique.
Bismuth
acetate
(Bi(CH3COO)3),
sodium
acetate
(Na(CH3COO)), barium acetate (Ba(CH3COO)2) and titanium(IV) butoxide (C16H36O4Ti) were used as precursors. The precursor solutions were mixed and heat treated in order to obtain dried gel powder. The powders were calcined at 800 °C in air. Ceramic pellets having diameter of 10 mm were obtained via coaxial hydrostatic room temperature presser and were sintered at 1200 °C in air. The phase analyses of calcined powders were carried out by Bruker D8 advance difractometer with Cobalt, Kα radiation having wavelength of 1.79021 Å. The apparent bulk densities of sintered ceramics were measured by densitometer using the Archimedes principle. In order to measure dielectric and ferroelectric properties silver paste was painted on the top and bottom faces of the pellets and heat treated at 700 °C for 20 min. 3
ACCEPTED MANUSCRIPT
The temperature dependent relative permittivity (εr) and loss tangent (tan) of ceramics at different frequencies were measured by impedance analyzer (HP 4192A). The temperature dependent polarization was obtained by ferroelectric test system (TF Analyzer 2000E, aixACCT) at 10 Hz frequency after every 10 °C temperature interval. The ECE was calculated by indirect method using Maxwell’s thermodynamic equation. 3. Results and discussion X-ray diffraction (XRD) analysis revealed the formation of single phase perovskite structured Bi1/2Na1/2TiO3 compound with no impurity phase as shown in Fig. 1. It indicates that Ba2+ is incorporated on the 12-fold coordinated A-site of ABO3 perovskite structure to replace Bi3+ or Na+ ion. The phase transitions of BNBT were indirectly investigated via dielectric constant (εr) versus temperature plots and by the dynamic mechanical analysis (DMA) shown in Fig. 2. The broadness and frequency dispersion of the relative permittivity (εr) versus temperature curves (Fig. 2(b)) indicated the relaxor behavior of BNBT and somewhat diffuse phase transitions from FE-to-AFE and then to PE phase [11, 12]. Two anomalies can be observed in εr versus temperature curves which indicated two phase transitions with increasing temperature. The anomaly in the loss tangent (tan δ) curve at depolarization temperature (Td = 150 °C) indicated the FE-to-AFE phase transition also verified by the dynamic mechanical analysis (DMA) shown in Fig. 2(c, d). The peaks of the relative permittivity curves at about 250 °C (Fig. 2(b)) are attributed to the AFE-to-PE phase transition indicating the Curie point (Tc) [13].
4
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Fig 1. XRD pattern of (Bi1/2Na1/2)1-xBaxTiO3 (0 ≤ x ≤ 0.07) samples, showing single phase perovskite structure.
5
ACCEPTED MANUSCRIPT
Fig 2. (a, b)Temperature and frequency dependence of relative permittivity (εr), loss tangent (tan) for x = 0 and 0.07 respectively (c, d) Frequency and temperature dependence of storage modulus and tanσ of BNBT ceramics for x = 0 and 0.07 respectively.
The
adiabatic
temperature change (∆T) or ECE was calculated by the macroscopic (thermodynamic) approach (Eq. 1) using Maxwell thermodynamic relation [(∂P/∂T)E = (∂S/∂E)T] [14]:
1 𝐸 𝑇∂𝑃
∆𝑇 = ― 𝜌∫𝐸2𝐶∂𝑇𝑑𝐸 1
6
(1)
ACCEPTED MANUSCRIPT
Where T is the ambient temperature, ρ is the density and c is the specific heat capacity of the sample. The values of (∂P/∂T) (Fig. 3(c)) were calculated from the forth order polynomial fit of the Pmax(T) data as shown in Fig. 3(b) (inset). The EC temperature change of the relaxor BNBT shown in Fig. 3(d) indicated strong applied electric field and temperature dependence. The maximum ECE was observed at the FE-to-AFE and AFE-to-PE phase transition temperatures. During FE-to-AFE phase transition, the negative ECE indicated that application of the electric field produce cooling effect and the withdrawal of electric field produce heating while ECE was positive at the AFE-to-PE phase transition. The coexistence of negative and positive ECE in the same material may be attributed to the multiphase transitions [9]. Fig. 3(d) indicates that in the FE state the increase in external applied electric field (E) increase alignment of dipoles and hence increases polarization (P) and decrease entropy (S) thereby producing cooling effect. On the other hand in the AFE state the increase in external applied electric field (E) decrease alignment of dipoles and hence decrease polarization (P) and increase entropy (S) leads to heeting effect as in agreement with the Maxwell’s equation [(∂P/∂T)E = (∂S/∂E)T]. The ECE is positive for most ferroelectrics and cooling is produced during the withdrawal of external electric field [4]. For BNBT the observed negative ECE was the consequence of the increase in polarization with temperature during FE-to-AFE phase transition. When the temperature of a dielectric material is increased in the FE-to-AFE state the thermal excitation may cause some antiparallel dipoles to align with the electric field and hence increase the polarization which may cause negative ECE [7].
7
ACCEPTED MANUSCRIPT
Fig 3. (a) Temperature dependent P-E loops (b) saturation polarization (Pmax) at different electric fields and temperatures (c) Temperature rate of change of polarization (∂P/∂T) versus temperature (d) Electrocaloric temperature changes (∆T) as functions of temperature and applied electric field for x = 0.07. Axelsson et al. [15] used the statistical microscopic model in order to estimate and explain the abnormal negative ECE in the relaxor PMN-PT single crystal material. They found that during the rhombohedral to tetragonal phase transition at 95°C the crystal showed 8
ACCEPTED MANUSCRIPT
negative ECE even at very small applied electric field of 10 kV/cm. According to their thermodynamic statistical model multi phase transitions are necessary for coexistence of positive and negative ECE in the same material at different temperature. The ECE observed in the BNBT is in full agreement with the results obtained by Goupil et al. [9] and Axelsson et al. [15]. According to Ponomareva and Lisenkov [10] the negative ECE is attributed to the noncollinearity of the external applied electric field and the poling direction of the pre-poled sample. A possible refrigeration cycle was proposed to enhance cooling efficiency via utilizing both the negative and positive electrocaloric effects [10]. The dependence of ECE on the Ba2+ doping and applied electric field is shown in Fig 4(a, b). It was observed that the maximum electrocaloric temperature change could be obtained at x = 0.06. The absolute maximum values of the ECE were observed to increase monotonically with increasing applied electric field during the FE-to-AFE and AFE-to-PE phase transition as shown in Fig. 4(b), and could be fitted as |∆T|max α (∆E)b. The value of the exponent b in this power law fit is 1.63 for x = 0.06 which is relatively high compared to that of PZT. The strong electric field dependence of the ECE indicates that the efficiency of the cooling system can be increased if BNBT is used as multi layer thin films component as reported by Bai et al. [16]. The Negative ECE can be of great importance if obtained near room temperature as recently reported in Pb(ZrTi)O3 based, antiferroelectric thin films [17].
9
ACCEPTED MANUSCRIPT
Fig 4. (a) Maximum negative electrocaloric temperature changes (∆T) and the exponent b as functions of Ba2+ content at T ~ 80 °C for BNBT (b) Absolute values of the maximum electrocaloric temperature changes (∆T) as a functions of applied electric field (∆E) for x = 0.06 (-ECE) at T ~ 80 °C and x = 0.07 (+ECE) at T ~ 230 °C for BNBT. Conclusion The coexistence of negative and positive ECE in the lead-free BNBT ceramics has been observed at FE-to-AFE and AFE-to-PE phase transition temperatures respectively. The maximum ECE of 2.35oC was observed at x = 0.06. The values of ECE are observed to be strongly ambient temperature and applied electric field dependent. The dual nature of ECE in the BNBT can be attributed to the presence of multiphase transitions at different temperatures in the same material. This dual nature of ECE can be utilized to design an efficient solid state environmentally friendly cooling system for microelectronics. The occurrence of relatively large ECE at usable ambient temperature shows the advantage of BNBT over others
10
ACCEPTED MANUSCRIPT
ferroelectric ceramics. The dual nature of ECE in BNBT at different ambient temperatures needs to be verified using the direct measuring technique. Acknowledgment The authors acknowledge the financial support extended by the Higher Education Commission (HEC), Pakistan under the International Research Support Initiative Program (IRSIP). The authors also acknowledge the financial support extended by the National Academy of Sciences (USA), project ID 131, under the PAK-USA S&T Cooperation Program, Award No.0521315 and the Science and Technology Innovation Commission of Shenzhen, China.
References [1].
THACHER P.D., J Appl. Phys., 39 (1968), 1996.
[2].
KOBEKO P., KURCHATOV I., Physik 66 (1930), 192.
[3].
SCOTT J.F., Annu. Rev. Mater. Res. 41 (2011), 229.
[4].
VALANT M., Prog. Mater. Sci. 57 (2012), 980.
[5].
CORREIA T., ZHANG Q., Electrocaloric Materials: New generation of coolers, Springer, London, 2014.
[6].
MISCHENKO A.S., ZHANG Q., SCOTT J.F., WHATMORE W.R., MATHUR N.D., Science 311 (2006), 1270.
[7].
BAI Y., ZHENG G.P., SHI S.Q., Mater. Res. Bull. 46 (2011), 1866.
[8].
JIANG X., LUO L., WANGA B., LI W., CHEN H., Ceram. Inter. 40 (2014), 2627.
[9].
GOUPIL F. L., BERENOV A., AXELSSON A.K., VALANT M., ALFORD M.N., J. Appl. Phys. 111 (2012), 124109.
11
ACCEPTED MANUSCRIPT
[10].
PONOMAREVA I., LISENKOV S., Phys. Rev. Lett. 108 (2012), 167604.
[11].
JO W., SCHAAB S., SAPPER E., SCHMITT L. A., KLEEBE H.J., BELL A. J., RODEL J., J. Appl. Phys. 110 (2011), 074106.
[12].
ZHANG S.T., KOUNGA A.B., AULBACH E., DENG Y., J. Am. Ceram. Soc. 91(12) (2008), 3950.
[13].
UDDIN S., ZHENG G.P., IQBAL Y., UBIC R., CHAN N.Y., CHAN H.W., Mater Res Exp. 1(2014) , 046102.
[14].
LINES M.E., GLASS A.M., Principles and Applications of Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1977.
[15].
AXELSSON A.K., GOUPIL F.L., DUNNE L.J., MANOS G., VALANT M., ALFORD N.M., Appl. Phy. Lett. 102 (2013), 102902.
[16]. BAI Y., ZHENG G.P., SHI S.Q., J. Appl. Phys. 108 (2010), 104102. [17]. GENG W, LIU Y, MENG X, BELLAICHE L, SCOTT J.F, DKHIL B, JIANG A., Adv Mater. 27 (2015) 3165.
12
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Highlights: 1. The Negative ECE can be of great importance if obtained near room temperature as recently reported in Pb(ZrTi)O3 based, antiferroelectric thin films [17]. (Reference added) 2. The adiabatic temperature change (∆T) or ECE was calculated by the macroscopic (thermodynamic) approach (Eq. 1) using Maxwell thermodynamic relation [(∂P/∂T)E = (∂S/∂E)T] [14].
(Citation is corrected)
Sarir Uddin, Guang-Ping Zheng, Yaseen Iqbal PII:
S1293-2558(19)30614-4
DOI:
10.1016/j.solidstatesciences.2019.105929
Article Number:
105929
Reference:
SSSCIE 105929
To appear in:
Solid State Sciences
Received Date:
25 May 2019
Accepted Date:
05 July 2019
Please cite this article as: Sarir Uddin, Guang-Ping Zheng, Yaseen Iqbal, Coexistence of positive and negative electrocaloric effects in lead free perovskite structured ferroelectrics, Solid State Sciences (2019), doi: 10.1016/j.solidstatesciences.2019.105929
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Coexistence of positive and negative electrocaloric effects in lead free perovskite structured ferroelectrics Sarir Uddin1, 2, 3 *, Guang-Ping Zheng2, Yaseen Iqbal3 1Department
of Physics, Government College Hayatabad, Peshawar 25124, Khyber
Pakhtunkhwa, Pakistan 2Department
of Mechanical engineering, The Hong Kong Polytechnic University, Hung
Hom, Kowloon, Hong Kong 3Materials
Research Laboratory, Department of Physics, University of
Peshawar, Khyber
Pakhtunkhwa, Peshawar 25120, Pakistan *[email protected] Abstracts The coexistence of positive and negative electrocaloric effects (ECE) was observed in the sol-gel derived Pb-free perovskite structured (Bi1/2Na1/2)1-xBaxTiO3 (x = 0, 0.04, 0.05, 0.06, 0.07) (BNBT) ceramics. The maximum electrocaloric temperature change of 2.35 oC has been observed at 100 kV/cm for x = 0.06 at about 70°C before the ferroelectric to antiferroelectric (FE-to-AFE) phase transition temperature. The coexistence of positive and negative ECE at different temperatures may be attributed to two different temperature dependent phase transitions in the ferroelectrics or antiferroelectrics. The sign reversal of ECE in the same material can be utilized to increase the efficiency of the refrigeration cycle and to develop efficient environmentally friendly cooling systems. Keywords: Lead-free, Ferroelectric ceramics, Electrocaloric effect, solid state refrigerants
1
ACCEPTED MANUSCRIPT
1. Introduction Electrocaloric effect (ECE) is the reversible change in temperature (∆T) of a polarizable material due to a changing applied electric field through that material under adiabatic conditions [1]. Small-scale and environmentally friendly refrigerants for microelectronics can be designed using this property of ferroelectric materials. Electrocaloric effect has been investigated in different ceramics and polymers since its discovery in 1930 by Kobeko and Kurchatov in order to find out an alternative to the conventionally hazardous gases based cooling technology [2-5]. ECE is directly proportional to the polarization change (ΔP) and hence the entropy change (ΔS) which is the dipolar disorder produced due to the ionic displacements inside the material by the applied electric field. Large ECE in a material requires large entropy change of the material which may occur near structural or ferroelectric to paraelectric (FE-to-PE) phase transition temperature (Tc) [6]. The ECE can be categorized now into positive or negative depending on the sign of the temperature change (ΔT) [5]. For positive ECE the cooling is produced during the withdrawal of the electric field while for negative ECE the cooling effect is produced by increasing the applied electric field. The negative ECE indicates that the application of electric field increases dipolar disorder and enable the dielectric material to absorb heat and produce cooling effect [7]. Although the works on ECE have been reviewed intensively but no experimental results had been reported about negative ECE [3-5] up to 2011. For the first time Bai et al. [7] reported the negative ECE in the relaxor Bi1/2Na1/2TiO3 ceramics in the temperature range 25-140 oC by using indirect method of measuring ECE at a maximum of 40 kV/cm electric field. Jiang et al. [8] reported coexistence of negative and positive ECE in Bi0.5Na0.5TiO32
ACCEPTED MANUSCRIPT
xKNbO3. Goupil et al. [9] reported the presence of negative ECE in Pb1/3Mg2/3O3-30PbTiO3 single crystal determined by direct and indirect measurements. The simulation of ECE demonstrated on a single crystal of Ba1/2Sr1/2TiO3 system also revealed the coexistence of both positive and negative ECE in the same material reported by Ponomareva and Lisenkov [10]. In this investigation the reversal of sign and coexistence of positive and negative ECE was observed in the sol-gel derived lead-free polycrystalline relaxor (Bi1/2Na1/2)1-xBaxTiO3 (0 ≤ x ≤ 0.07) (BNBT) ceramics in the temperature range from 30 °C to 230 °C. The sign reversal and dual nature of ECE was observed at ferroelectric- to-antiferroelectric (FE-toAFE) and antiferroelectric-to-paraelectric (AFE-to-PE) phase transition temperatures. ECE was observed to depend strongly on ambient temperatures and applied electric field. 2. Experimental Ceramics compositions of (Bi1/2Na1/2)1-xBaxTiO3 (0 ≤ x ≤ 0.07) were fabricated via sol-gel
processing
technique.
Bismuth
acetate
(Bi(CH3COO)3),
sodium
acetate
(Na(CH3COO)), barium acetate (Ba(CH3COO)2) and titanium(IV) butoxide (C16H36O4Ti) were used as precursors. The precursor solutions were mixed and heat treated in order to obtain dried gel powder. The powders were calcined at 800 °C in air. Ceramic pellets having diameter of 10 mm were obtained via coaxial hydrostatic room temperature presser and were sintered at 1200 °C in air. The phase analyses of calcined powders were carried out by Bruker D8 advance difractometer with Cobalt, Kα radiation having wavelength of 1.79021 Å. The apparent bulk densities of sintered ceramics were measured by densitometer using the Archimedes principle. In order to measure dielectric and ferroelectric properties silver paste was painted on the top and bottom faces of the pellets and heat treated at 700 °C for 20 min. 3
ACCEPTED MANUSCRIPT
The temperature dependent relative permittivity (εr) and loss tangent (tan) of ceramics at different frequencies were measured by impedance analyzer (HP 4192A). The temperature dependent polarization was obtained by ferroelectric test system (TF Analyzer 2000E, aixACCT) at 10 Hz frequency after every 10 °C temperature interval. The ECE was calculated by indirect method using Maxwell’s thermodynamic equation. 3. Results and discussion X-ray diffraction (XRD) analysis revealed the formation of single phase perovskite structured Bi1/2Na1/2TiO3 compound with no impurity phase as shown in Fig. 1. It indicates that Ba2+ is incorporated on the 12-fold coordinated A-site of ABO3 perovskite structure to replace Bi3+ or Na+ ion. The phase transitions of BNBT were indirectly investigated via dielectric constant (εr) versus temperature plots and by the dynamic mechanical analysis (DMA) shown in Fig. 2. The broadness and frequency dispersion of the relative permittivity (εr) versus temperature curves (Fig. 2(b)) indicated the relaxor behavior of BNBT and somewhat diffuse phase transitions from FE-to-AFE and then to PE phase [11, 12]. Two anomalies can be observed in εr versus temperature curves which indicated two phase transitions with increasing temperature. The anomaly in the loss tangent (tan δ) curve at depolarization temperature (Td = 150 °C) indicated the FE-to-AFE phase transition also verified by the dynamic mechanical analysis (DMA) shown in Fig. 2(c, d). The peaks of the relative permittivity curves at about 250 °C (Fig. 2(b)) are attributed to the AFE-to-PE phase transition indicating the Curie point (Tc) [13].
4
ACCEPTED MANUSCRIPT
Fig 1. XRD pattern of (Bi1/2Na1/2)1-xBaxTiO3 (0 ≤ x ≤ 0.07) samples, showing single phase perovskite structure.
5
ACCEPTED MANUSCRIPT
Fig 2. (a, b)Temperature and frequency dependence of relative permittivity (εr), loss tangent (tan) for x = 0 and 0.07 respectively (c, d) Frequency and temperature dependence of storage modulus and tanσ of BNBT ceramics for x = 0 and 0.07 respectively.
The
adiabatic
temperature change (∆T) or ECE was calculated by the macroscopic (thermodynamic) approach (Eq. 1) using Maxwell thermodynamic relation [(∂P/∂T)E = (∂S/∂E)T] [14]:
1 𝐸 𝑇∂𝑃
∆𝑇 = ― 𝜌∫𝐸2𝐶∂𝑇𝑑𝐸 1
6
(1)
ACCEPTED MANUSCRIPT
Where T is the ambient temperature, ρ is the density and c is the specific heat capacity of the sample. The values of (∂P/∂T) (Fig. 3(c)) were calculated from the forth order polynomial fit of the Pmax(T) data as shown in Fig. 3(b) (inset). The EC temperature change of the relaxor BNBT shown in Fig. 3(d) indicated strong applied electric field and temperature dependence. The maximum ECE was observed at the FE-to-AFE and AFE-to-PE phase transition temperatures. During FE-to-AFE phase transition, the negative ECE indicated that application of the electric field produce cooling effect and the withdrawal of electric field produce heating while ECE was positive at the AFE-to-PE phase transition. The coexistence of negative and positive ECE in the same material may be attributed to the multiphase transitions [9]. Fig. 3(d) indicates that in the FE state the increase in external applied electric field (E) increase alignment of dipoles and hence increases polarization (P) and decrease entropy (S) thereby producing cooling effect. On the other hand in the AFE state the increase in external applied electric field (E) decrease alignment of dipoles and hence decrease polarization (P) and increase entropy (S) leads to heeting effect as in agreement with the Maxwell’s equation [(∂P/∂T)E = (∂S/∂E)T]. The ECE is positive for most ferroelectrics and cooling is produced during the withdrawal of external electric field [4]. For BNBT the observed negative ECE was the consequence of the increase in polarization with temperature during FE-to-AFE phase transition. When the temperature of a dielectric material is increased in the FE-to-AFE state the thermal excitation may cause some antiparallel dipoles to align with the electric field and hence increase the polarization which may cause negative ECE [7].
7
ACCEPTED MANUSCRIPT
Fig 3. (a) Temperature dependent P-E loops (b) saturation polarization (Pmax) at different electric fields and temperatures (c) Temperature rate of change of polarization (∂P/∂T) versus temperature (d) Electrocaloric temperature changes (∆T) as functions of temperature and applied electric field for x = 0.07. Axelsson et al. [15] used the statistical microscopic model in order to estimate and explain the abnormal negative ECE in the relaxor PMN-PT single crystal material. They found that during the rhombohedral to tetragonal phase transition at 95°C the crystal showed 8
ACCEPTED MANUSCRIPT
negative ECE even at very small applied electric field of 10 kV/cm. According to their thermodynamic statistical model multi phase transitions are necessary for coexistence of positive and negative ECE in the same material at different temperature. The ECE observed in the BNBT is in full agreement with the results obtained by Goupil et al. [9] and Axelsson et al. [15]. According to Ponomareva and Lisenkov [10] the negative ECE is attributed to the noncollinearity of the external applied electric field and the poling direction of the pre-poled sample. A possible refrigeration cycle was proposed to enhance cooling efficiency via utilizing both the negative and positive electrocaloric effects [10]. The dependence of ECE on the Ba2+ doping and applied electric field is shown in Fig 4(a, b). It was observed that the maximum electrocaloric temperature change could be obtained at x = 0.06. The absolute maximum values of the ECE were observed to increase monotonically with increasing applied electric field during the FE-to-AFE and AFE-to-PE phase transition as shown in Fig. 4(b), and could be fitted as |∆T|max α (∆E)b. The value of the exponent b in this power law fit is 1.63 for x = 0.06 which is relatively high compared to that of PZT. The strong electric field dependence of the ECE indicates that the efficiency of the cooling system can be increased if BNBT is used as multi layer thin films component as reported by Bai et al. [16]. The Negative ECE can be of great importance if obtained near room temperature as recently reported in Pb(ZrTi)O3 based, antiferroelectric thin films [17].
9
ACCEPTED MANUSCRIPT
Fig 4. (a) Maximum negative electrocaloric temperature changes (∆T) and the exponent b as functions of Ba2+ content at T ~ 80 °C for BNBT (b) Absolute values of the maximum electrocaloric temperature changes (∆T) as a functions of applied electric field (∆E) for x = 0.06 (-ECE) at T ~ 80 °C and x = 0.07 (+ECE) at T ~ 230 °C for BNBT. Conclusion The coexistence of negative and positive ECE in the lead-free BNBT ceramics has been observed at FE-to-AFE and AFE-to-PE phase transition temperatures respectively. The maximum ECE of 2.35oC was observed at x = 0.06. The values of ECE are observed to be strongly ambient temperature and applied electric field dependent. The dual nature of ECE in the BNBT can be attributed to the presence of multiphase transitions at different temperatures in the same material. This dual nature of ECE can be utilized to design an efficient solid state environmentally friendly cooling system for microelectronics. The occurrence of relatively large ECE at usable ambient temperature shows the advantage of BNBT over others
10
ACCEPTED MANUSCRIPT
ferroelectric ceramics. The dual nature of ECE in BNBT at different ambient temperatures needs to be verified using the direct measuring technique. Acknowledgment The authors acknowledge the financial support extended by the Higher Education Commission (HEC), Pakistan under the International Research Support Initiative Program (IRSIP). The authors also acknowledge the financial support extended by the National Academy of Sciences (USA), project ID 131, under the PAK-USA S&T Cooperation Program, Award No.0521315 and the Science and Technology Innovation Commission of Shenzhen, China.
References [1].
THACHER P.D., J Appl. Phys., 39 (1968), 1996.
[2].
KOBEKO P., KURCHATOV I., Physik 66 (1930), 192.
[3].
SCOTT J.F., Annu. Rev. Mater. Res. 41 (2011), 229.
[4].
VALANT M., Prog. Mater. Sci. 57 (2012), 980.
[5].
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Highlights: 1. The Negative ECE can be of great importance if obtained near room temperature as recently reported in Pb(ZrTi)O3 based, antiferroelectric thin films [17]. (Reference added) 2. The adiabatic temperature change (∆T) or ECE was calculated by the macroscopic (thermodynamic) approach (Eq. 1) using Maxwell thermodynamic relation [(∂P/∂T)E = (∂S/∂E)T] [14].
(Citation is corrected)