Enhanced electrocaloric effect at room temperature in Mn2+ doped lead-free (BaSr)TiO3 ceramics via a direct measurement

(Ba1−xSrx)(MnyTi1−y)O3 (BSMT) ceramics with x = 35, 40 mol% and y = 0, 0.1, 0.2, 0.3, 0.4, 0.5 mol% were prepared using a conventional solid-state reaction approach. The dielectric and ferroelectric properties were characterized using impedance analysis and polarization-electric field (P–E) hysteresis loop measurements, respectively. The adiabatic temperature drop was directly measured using a thermocouple when the applied electric field was removed. The results indicate that high permittivity and low dielectric losses were obtained by doping 0.1–0.4 mol% of manganese ions in (BaSr)TiO3 (BST) specimens. A maximum electrocaloric effect (ECE) of 2.75 K in temperature change with electrocaloric strength of 0.55 K·(MV/m)−1 was directly obtained at ∼21 °C and 50 kV/cm in Ba0.6Sr0.4Mn0.001Ti0.999O3 sample, offering a promising ECE material for practical refrigeration devices working at room temperature.


Introduction 
With the fast increasing numbers of transistors in modern integrated circuits entry, thermal accumulation has become one of the crucial factors for chip damage, which causes not only the high electrical consumption, but also the thermal impact on the stability of the electronic components [1,2]. Thus, an urgent demand on the development of novel on-chip cooling modulus traditional vapor-compression technology.
The ECE occurred in polar materials and induced by the variation of electric field. It is denoted by the adiabatic temperature change (ΔT) or isothermal entropy change when an external electric field was applied or removed in certain polar materials [3][4][5]. The earliest reported regarding ECE can be traced back to the report of a finite temperature change (about 0.0036 K) in Rochelle salt by Wiseman and Kuebler in 1963 [6]. However, much effort has been devoted to ECE recently since the giant ECE responses have been reported for Pb(ZrTi)O 3 thin films in 2006 [7] and poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymers and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymers in 2008 [8]. Regarding the thermodynamic theory, the isothermal entropy change ΔS of certain ECE materials can be estimated by 2 where β is the phenomenological coefficient of P 2 in the Gibbs free energy expression of the Landau theory, and P represents the polarization contributed by dipoles' switching in ferroelectric phase, which implies that a large ECE is generally associated with a large polarization change. In other words, a large ECE material should presents a large polarization variation under an electrical stimulation. As discussed above, ferroelectrics, e.g., polyvinylidene fluoride (PVDF)based polymers, ferroelectric (antiferroelectric) ceramics, single crystals, thin films, and liquid crystals, have been considered as the promising option to meet such requirements due to not only its intrinsic large spontaneous polarization, but also the large ferro-to para-electric phase transition heat. Among them, thin films with at least 10 times higher breakdown electric field than bulk ceramic counterparts, which show large ECE responses. Lu et al. [9] reported a giant ΔT = 40 K found in (PbLa)(ZrTi)O 3 (PLZT) (8/65/35 mol%) thin film at 318 K and 1200 kV/cm. Thin film materials, however, are not self-standing, require an indispensable substrate to support their mechanical structure, which are redundant parts for the development of cooling devices besides their extremely weak thermal mass led to a small cooling power. Ferroelectric polymers have also been drawn attention because of their large ECE response introduced by the high breakdown electric field of over 10,000 kV/cm. For instances, Lu et al. [9] reported a large ΔT = 20 K at 306 K and 1600 kV/cm in high-energy electron irradiated P(VDF-TrFE) (68/ 32 mol%) copolymers. Unfortunately, the low thermal conductivity (~0.2 W/(mK)) makes polymer materials a poor thermal transporter while pursuing the thermal exchange between the frigerant and the thermal switch. Compared with two options above, bulk ceramics, e.g., in 2011, Rožič et al. [10] reported a large ΔT ≈ 2 K for PLZT (8/65/35 mol%) bulk ceramics at ~410 K, and a ΔT > 2.5 K for 0.7Pb(Mg 1/3 Nb 2/3 )O 3 -0.3PbTiO 3 (PMN-0.3PT) bulk ceramics at ~410 K with 88 and 90 kV/cm of external fields, respectively, and other numerous ferroelectric bulk materials with good electrocaloric properties have been reported [11][12][13].
Taking both the environmental friendliness characteristic and large ECE response into consideration, lead-free BaTiO 3 (BTO)-based ferroelectric ceramics with a perovskite structure is a promising ECE candidate, due to their first-order phase transition as well. However, for ECE in ferroelectrics, the large polarization change and ECE performance always take place near the ferroelectric-paraelectric phase transition temperature (Curie temperature, T C ), while the T C of BTO is about 120 ℃, which is far from room temperature. Moreover, as a first-order phase transition ferroelectric, the desired ECE performance only occurs within a narrow temperature range. More critically, the dielectric loss due to the oxygen vacancies generated at high temperatures during the sintering process is always an issue in mitigating the breakdown electric field. In this work, these issues will be addressed via ionic doping in both A-and B-site of perovskite structure [14][15][16][17][18][19][20][21][22][23][24]. Here, Sr 2+ ions are used to shift the phase transition towards room temperature. Manganese ions are imported to compensate the oxygen vacancies to enhance the ECE properties and lower the dielectric losses. The Mn 4+ ions will be reduced into Mn 2+ in terms of attracting two electrons from the oxygen vacancy during the sintering process at high temperatures, which will be oxidized into 2 O V  . Since the oxygen vacancies are attracted by the Mn 2+ ions, the movable oxygen vacancies will be greatly mitigated, then the loss tangent will be declined [25].
prepared using a conventional solid-state reaction method. BaCO 3 , SrCO 3 , MnO 2 , and TiO 2 (purity ≥ 99.0%, Aladdin, Co., Ltd., China) were used as raw materials. These raw materials were mixed according to the stoichiometric ratios and ball milled at 250 rpm for 24 h in a planetary miller using ZrO 2 balls and ethanol as the medium. The milled slurry was dried at 65 ℃ for 12 h. Then the dry ceramic powders were ground, sieved, and calcined at 1300 ℃ for 4 h to form BSMT perovskite crystallites. After mixing with 5 wt% polyvinyl butyral, the BSMT crystallite powders were axially pressed into pellets under 6 MPa and further isostatically pressed under 200 MPa. Finally, the ceramic plates were sintered in a temperature range of 1350-1450 ℃ and soaked for 0.5-3 h to obtain dense specimens. Both sides of specimens were polished and sputtered with a gold electrode for electric characterization.

2 Characterization
X-ray diffraction (XRD) was used to determine the polycrystalline structure of BSMT ceramics using a Rigaku Ultima IV diffractometer with a Ni-filtered Cu Kα radiation (λ = 1.5406 Å) and a scanning step of 0.02°, from 20° to 80°. The data were further analyzed by the Rietveld refinement using Fullprof program. Hitachi S-3400N scanning electron microscope (SEM) with an energy dispersive X-ray spectroscopy (EDX) analysis was employed to observe the morphology and compositions of BSMT ceramic specimens. The relative density was calculated by the ratio of real density measured using the Archimedes' method and the theoretical density calculated based on XRD patterns. The permittivity as a function of frequency and temperature were obtained using the impendence analyzer (Agilent 4284A). The polarization-electric field (P-E) hysteresis loops as a function of electric field and temperature were procured using Saywer-Tower circuit (RADIANT Multiferroic Test System).
The electrocaloric response in this study was directly obtained using a specially designed calorimeter, including a T-type thermocouple attached to the specimen and a high voltage signal amplifier as the power source. During the test, a step-like voltage was applied to the sample, and the voltage was maintained at a constant value for over 10 s to reach a thermal equilibrium with the surrounding. After that, the voltage was sharply reduced to zero. The typical thermal response time along the sample thickness direction is a few milliseconds. Moreover, a sharply withdrawing electric field was applied to eliminate the influence of joule heat generated in the sample. The detailed procedure can be found in our previous work [26].

1 XRD analyses
XRD patterns procured at room temperature for BSMT ceramics with x = 35, 40 mol% and y = 0, 0.  [19,27,28]. Furthermore, the replacement of Sr 2+ ions to metal ions at A-sites of ABO 3 perovskite structure in BSMT ceramics leads to a lattice distortion and even to a structural/compositional fluctuation for large Sr 2+ ionic contents, forming a BST relaxor ferroelectric. Consequently, the relaxor ferroelectric behaviors will play a major role in both the structural and electrical properties in BSMT ceramics with the large Sr 2+ contents.  Fig. 2, dense and uniform ceramics with distinct grain boundaries and 1-7 μm in grain sizes and over 97% of relative densities indicate that dense ceramics are formed during the sintering process. Furthermore, EDX mapping of Mn 2+ ionic distribution shown in Fig. 3 implies that when the doped Mn 2+ ions are in low content, a small portion of Mn 2+ ions will enter into the ceramic grains and the others are probably to be distributed along the grain boundaries, which can be verified by the shift of XRD peaks and the peak of permittivity as a function of temperature when the Mn 2+ ions are introduced.

3 Dielectric properties
The permittivity and dielectric loss as a function of temperature and frequency for BSMT ceramics with Sr 2+ content of 40 mol% are plotted in Fig. 4(a), while the dielectric properties for BSMT ceramics with Sr 2+ content of 35 mol% are plotted in Fig. 4 For other Mn 2+ contents, the shifts are usually larger for BSMT-Sr 2+ 40 mol% than for BSMT-Sr 2+ 35 mol%. In addition, it is observed that the permittivity peak moves towards high temperatures with the increasing frequency, indicating a relaxor ferroelectric characteristic. This is due to the large Sr 2+ ionic content in the BSMT ceramics. It is also noted that, with the increasing manganese ionic content, the permittivity of BSMT specimens increases first and then declines for both the ceramics (35 and 40 mol% of Sr 2+ ). It means that in the course of incorporation of Mn 2+ ions, Mn 2+ ions enter into the lattice, and change the polar states in the ceramics, especially the nanosized polar domains in the relaxor ferroelectric ceramics, which may lead to the enhancement of permittivity near the phase transition [29]. The maximum value of relative dielectric constant of 23,171 is obtained in (Ba 0.6 Sr 0.4 )(Mn 0.001 Ti 0.999 )O 3 sample with a loss tangent of 0.9% at T c = 9.5 ℃ and 1 kHz. It can also be noted that, as the Mn 2+ ionic content increases, the largest loss tangent in the temperature range of (-20)-110 ℃ for all 35 mol% of Sr 2+ doped BSMT decreases (from 5%@Mn 2+ 0 mol%, 1 kHz to 1%@Mn 2+ 0.1 mol%, 1 kHz), and then increases, which implies that a relatively low manganese ionic content (less than 0.4 mol%) can enhance the permittivity. The reason is probably due to the acceptor-type compensation of Mn 2+ ions with oxygen vacancies, i.e., during the sintering process at high temperatures, Mn 4+ in raw material (MnO 2 ) will release two electrons to compensate the oxygen vacancies and to lead to the mitigation of the concentration of oxygen vacancies, thus reducing the loss tangent in the specimens.

4 P-E hysteresis loop
Polarization as a function of applied electric field for BSMT ceramics measured at -20 ℃, 100 ℃, and room temperature and 100 Hz under an external electric field of 50 kV/cm is shown in Fig. 5  which shows the similar tendency as the dielectric properties as a function of Mn 2+ content.

5 ECE
ECE is defined as the isothermal entropy change and ΔT in polar materials when an external electric field is applied or removed. When the applied electric field is removed, the dipolar entropy of material increases in the adiabatic condition. In the meantime, the lattice  entropy decreases, resulting in the temperature drop of material since the total entropy of the material is kept as a constant. Here, except for the ΔS and ΔT, the electrocaloric strength |ΔT/ΔE| is also used to estimate the ECE properties of the material. For BSMT ceramics, the electrocaloric strength (dT/dE) can be calculated using the following equation [30]: where r  is the relative dielectric constant (vacuum permittivity 0  = 8.85 × 10 -12 F/m), T is the temperature, ρ is the density obtained by Archimedes' method, v C is the specific heat capacity which is temperature depended. v C as a function of temperature is presented in Fig. 6. Then calculated dT/dE as a function of temperature is illustrated in Fig. 7, showing the obvious peaks of ECE strength near the T c for all compositions. Comparing the directly measured dT/dE with the calculated ones using Eq. (2), there is still a considerable space for improving the ECE performance.
The ΔT in this work was procured using a direct measurement. The measurement was pursued in the temperature ranging from -30 to 100 ℃ with an   [31]. Furthermore, for both 35 and 40 mol% Sr 2+ doped BSMT specimens, the ΔT max increases and then continuously decreases owing to the higher manganese ionic content resulting in reduction of polarization (Fig. 5). Especially, when a small number of manganese ions are doped in BSMT ceramics, the Mn 2+ ions compensate the oxygen vacancies, reducing the loss tangent. When further increasing the manganese ionic concentration, the excess Mn 2+ ions, behave as pinning centers for switchable domains and acceptortype defect centers, leading to the mitigated polarization as well as ECE properties.
Various ECE materials have been widely investigated in recent years, including ferroelectrics, antiferroelectrics, and multiphase materials in the type of bulk, thin film, multilayer ceramic capacitors (MLCC) inorganic materials, and organic polymers, measured in either direct or indirect approaches [32][33][34][35][36][37][38][39][40][41][42][43]. Moreover, the ECE strength of 0.55 K·(MV/m) -1 in this work is one of the highest values among the lead-free ferroelectric bulk ceramics published in recent years (Table 1). Obviously, low concentration of manganese doping can not only greatly enhance the permittivity, polarization, and ECE performance of BST ceramics, but also broaden the operational temperature range, demonstrating appropriate Mn 2+ doping is an effective way for improving the ECE performance of ECE materials. Based on the results, one can adjust the Mn 2+ and Sr 2+ ionic contents to obtain ferroelectric BSMT which is appropriate for ECE cooling devices, providing a promising ceramic system for ECE refrigeration devices.

Conclusions
In conclusion, BSMT ceramics with x = 35, 40 mol% and y = 0, 0.1, 0.2, 0.3, 0.4, 0.5 mol% were prepared using a conventional solid-state reaction approach. The permittivity and loss as a function of temperature and frequency show a high dielectric constant and low dielectric losses, obtaining in terms of adjusting the concentration of manganese ions from 0.1 to 0.4 mol%.
In addition, polarization response is also enhanced by Mn 2+ doping. Moreover, the maximal electrocaloric response of 2.75 K with electrocaloric strength of 0.55 K·(MV/m) -1 was procured at ~21 ℃ and 50 kV/cm in Ba 0.6 Sr 0.4 Mn 0.001 Ti 0.999 O 3 sample. Furthermore, ΔT > 2.2 K at 5 MV/m over a broad temperature span of 120 ℃ was realized, which provides a promising candidate for practical ECE solid-state cooling devices.