Ultrahigh room temperature electrocaloric response in lead-free bulk ceramics via tape casting

Abstract

Solid-state cooling technology based on the electrocaloric effect is attracting increasing attention as an important alternative to traditional cooling systems in microelectronic and integrated electronics. Lead-free bulk ceramics are considered as one of the promising candidates for middle and large-scale electrocaloric cooling because of their environment-friendliness and large heat absorption capacity. However, the room temperature adiabatic temperature change (ΔT) in lead-free bulk ceramics has long been limited by their relatively low dielectric breakdown strength (E_b < 60 kV cm⁻¹), hindering their practical applications. In this work, we propose to use the tape casting technique as a practicable strategy to enhance the densification and decrease the porosity of lead-free bulk ceramics for achieving a high E_b and a large room temperature ΔT. An ultrahigh room temperature ΔT (1.6 K) was realized in a (Ba_0.95Ca_0.05)(Ti_0.94Sn_0.06)O₃ (BCTS) bulk ceramic prepared by the tape casting technique, which is 4 times larger than that of the lead-free bulk ceramics with a similar composition prepared by the conventional ceramic preparing approach. More significantly, unlike the other previously reported results, which only show high ΔT in an extremely narrow temperature range, ΔT of the BCTS bulk ceramic increases from 1.6 K to 2.0 K in the temperature range from 300 K to 345 K, which is comparable with that found in the lead-based bulk counterpart. Most importantly, this work opens up a new avenue to explore lead-free bulk ceramics with a large room temperature ΔT for solid-state refrigeration.

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1. Introduction

Solid-state refrigeration based on the electrocaloric effect (ECE) has recently been drawing remarkable and intense attention as one promising alternative to the conventional cooling technology based on a vapor compression cycle in order to decrease CO₂ emissions.^1–5 Ferroelectric materials are regarded as one of the best candidates for electrocaloric refrigeration because they show large polarization and entropy change near ferroelectric phase transitions through applying/removing an electric field, leading to a large adiabatic temperature change (ΔT).^6–11 Thus, ΔT is often considered as an important parameter in terms of ECE evaluation. Since Mischenko et al. reported a large ΔT of 12 K in PbZr_0.95Ti_0.05O₃ thin films by applying a high electric field (480 kV cm⁻¹) at 495 K in 2006,¹² study of the ECE has been one of the active research issues in the ferroelectric community.^13–17

Compared with polymers,^18,19 single crystals,^20,21 and thick and thin films,^22–26 ferroelectric bulk ceramics have been considered as promising candidate materials for middle and large-scale electrocaloric refrigeration because they possess higher heating/cooling capacity and large volume, which makes them more suitable for commercially cooling devices.^27–32 On the other hand, lead-based ferroelectric materials are strictly regulated by the European Union and other countries owing to the toxicity of lead. Therefore, lead-free bulk ceramics are intensively studied as potential candidate materials for electrocaloric refrigeration.^33–47 Lead-free bulk ceramics usually show a relatively large ΔT near their phase transition temperatures, including the ferroelectric to paraelectric (FTP) phase transition in BaTiO₃ (BT)-based ceramics,^33–44 the ferroelectric polymorphic phase transition in (K_0.5Na_0.5)NbO₃ (KNN)-based ceramics and the morphotropic phase boundary (MPB) in (Bi_0.5Na_0.5)TiO₃ (BNT)-based ceramics.^45–48 For example, Jian and co-workers demonstrated a maximum ΔT of 2.4 K in Ba(Zr_0.05Ti_0.95)O₃ ceramics at 386 K (close to the FTP phase transition temperature).⁹ Goupil showed a maximum ΔT of 0.73 K in BNT-based ceramics at 433 K (close to the MPB temperature) under an electric field of 22 kV cm⁻¹.⁴⁷ Unfortunately, these temperatures with the maximum ΔT are far from room temperature and unsuitable for practical applications. Consequently, up to now, it still is a great challenge to achieve a large room temperature ΔT in lead-free bulk ceramics for practical use.

Among many lead-free bulk ceramics, BaTiO₃ (BT)-based ceramics are regarded as one of the lead-free bulk ceramics with the most potential for electrocaloric refrigeration because of their tunable Curie temperature (FTP phase transition temperature) through intentional composition optimization.^27–44 For example, Luo and co-workers revealed a room temperature ΔT of 0.61 K in Ba(Ti_0.895Sn_0.105)O₃ ceramics under an electric field of 20 kV cm⁻¹.³⁴ Wang et al. reported a room temperature ΔT of 0.69 K in BaHf_0.17Ti_0.83O₃ ceramics at 50 kV cm⁻¹.²⁷ Unfortunately, these BT-based bulk ceramics show low room temperature ΔT (<0.7 K) owing to their relatively low dielectric breakdown strength (E_b < 60 kV cm⁻¹), compared with polymers, and thick and thin films, hindering their practical applications. Despite that, BT-based bulk ceramics still are one of the promising candidates for electrocaloric cooling because of their tunable phase transition near room temperature, which is a cornerstone for achieving a large room temperature ΔT.

Besides ferroelectric phase transitions, the E_b of ferroelectric materials has been shown to be the most critical parameter determining the magnitude of ΔT.⁴⁹ Ultrahigh E_b is a main cause of giant ΔT in polymers and thin films. For example, a large ΔT of 12.5 K was found in a ferroelectric poly-(vinylidene fluoride–trifluoroethylene) because of application of an ultrahigh electric field (3070 kV cm⁻¹) by Neese et al.⁵⁰ Similarly, Peng et al. reported a large ΔT (∼20.7 K) in a Pb_0.97La_0.02(Zr_0.65Sn_0.3Ti_0.05)O₃ thin film under a high electric field (1092 kV cm⁻¹).²² In view of the above, we propose that a large room temperature ΔT would be realized if the E_b of BT-based bulk ceramics could be significantly enhanced. This will probably make a giant step forward for electrocaloric refrigeration.

Generally, thermal breakdown is one of the most important factors leading to the dielectric failure of bulk ceramics. Thermal breakdown in bulk ceramics usually takes place when the Joule heating generated by current flow under a high electric field exceeds the heat conducted away to the surroundings. Therefore, the lower the electrical conductivity, the larger the E_b of lead-free bulk ceramics.⁵¹ Grains, pores and grain boundaries all have impacts on the E_b of bulk polycrystalline ceramics. Among these factors, pores have a stronger effect because higher porosity can result in much higher electrical conductivity and thus low E_b. Consequently, the densification behavior of ferroelectric bulk ceramics has a prominent effect on their E_b,^52,53 and improving the density (i.e., lowering the porosity) can give rise to a higher E_b in bulk ceramics.^54–56 For example, ferroelectric bulk ceramics prepared by hot pressing sintering or spark plasma sintering techniques usually display higher E_b than those prepared by traditional solid-state sintering due to the enhancement of densification with the help of pressure.^57,58 However, one major drawback in the above mentioned methods is the high production cost, which is unfavorable for large-scale industrial production. Compared with conventional ceramic fabrication methods, tape-casting processing will lead to ceramics with much lower porosity and finer grain size, which effectively enhances the E_b of bulk ceramics. Therefore, the tape casting technique is used widely as an advanced fabrication method to prepare high-density ceramics with a thickness from 10 μm (thick film) to 1000 μm (bulk ceramics) without pressure assistance during the sintering process.^59–62 Based on the above discussions, in this work, we propose to use the tape casting technique as a practicable strategy to enhance the densification and decrease the porosity of lead-free bulk ceramics for achieving a high E_b and a large room temperature ΔT. Ba(Sn, Ti)O₃–(Ba, Ca)TiO₃ is a well known ceramic because of its high piezoelectric properties resulting from its tricritical triple point.⁶³ Based on previous study results,^34,44,63 the addition of 5 mol% Ca²⁺ and 6 mol% Sn⁴⁺ in BaTiO₃ can move the phase transition to room temperature and induce a diffuse phase transition simultaneously. Therefore, to achieve a larger room temperature ΔT, Ba_0.95Ca_0.05Ti_0.94Sn_0.06O₃ (BCTS) is selected for obtaining a diffuse phase transition near room temperature in this study, as shown in Fig. 2(b). Fig. 1 is a schematic diagram showing achieving a large room temperature ΔT through using the tape casting technique.

Fig. 1 Schematic illustration of the approach followed in this study.

As we expected, a large room temperature ΔT (1.6 K) was realized in BCTS ceramics prepared using the tape casting technique through application of large electric fields (180 kV cm⁻¹), which is ∼4 times larger than that of the lead free bulk ceramics with similar composition prepared by the conventional ceramic preparing approach. Furthermore, a room temperature ΔT of 1.6 K is the highest value reported so far in lead-free bulk ceramics and is comparable with that of the lead-based counterpart. This work demonstrates that the BCTS ceramic prepared by the tape casting technique is a high potential candidate for solid-state electrocaloric refrigeration. We believe that the tape casting technique is a viable strategy to explore a series of lead-free bulk ceramics with a large room temperature ΔT for electrocaloric refrigeration in the future.

2. Experimental procedure

Material preparation

Fine equiaxed (Ba_0.95Ca_0.05)(Ti_0.94Sn_0.06)O₃ powder of ∼200 nm in average size was synthesized by a solid-state reaction using BaCO₃ (99.95%), CaCO₃ (99.99%), nano-TiO₂ (∼5–10 nm, 99.8%), and nano-SnO₂ (50–70 nm, 99.9%). The raw materials were weighed according to the stoichiometry and then ball-milled for 48 h in ethanol. After drying, the mixture was calcined in a covered alumina crucible at 1175 °C for 3 h. A pseudoplastic slurry was prepared by dispersing the calcined powder in a xylene–ethanol solution containing a dispersant (blown menhaden fish oil), an organic binder (polyvinyl butyral), and plasticizers (butyl benzyl phthalate and polyalkylene glycol). The mixture slurry was stirred slowly to de-air for 2–4 h and then tape casted at a rate of 1 cm s⁻¹. Dried tapes were cut, stacked and laminated under 20 MPa and 75 °C to fabricate green compacts. After binder burnout at 600 °C and cold-isostatic pressing at 200 MPa, the samples were heated at 5 °C min⁻¹ to 1400 °C and then held for 4 h. Fig. 2 is a schematic diagram showing the sample preparation process in this study through using the tape casting technique.

Fig. 2 Schematic illustration of the sample preparation process in this study.

Characterization

The phase and morphology of samples were examined using an X-ray diffractometer (Philips X-Pert ProDiffractometer, Almelo, The Netherlands) and the microstructure was examined by field-emission scanning electron microscopy (FE-SEM, Helios NanoLab 600i, FEI, Hillsboro, OR). The temperature dependence of the permittivity was measured from 150 K to 500 K in custom-designed furnaces with a precision LCR meter (E4980A; Agilent, Palo Alto, CA) at 0.1, 1.0, 10, and 100 kHz. The electric displacement–electric field (D–E) loops were measured at 10 Hz using a ferroelectric test system (TF Analyzer 2000; aixACCT, Aachen, Germany). The breakdown strength measurements were performed on a CJ2671 high voltage tester under DC conditions. The samples used for dielectric breakdown measurements were without electrodes. All the samples are about 10 mm in diameter and 0.2 mm in thickness. Different voltages were applied and held for 10 seconds until breakdown occurred. The limited current is 0.5 mA. For each composition, at least 10 specimens were used for the breakdown testing.

3. Results and discussion

Fig. 3(a) shows the X-ray diffraction (XRD) pattern of the BCTS bulk ceramic at room temperature. The sample exhibits a pseudo-cubic phase without any trace of impurities within the precision of the instrument. In addition, the sample exhibits a high relative density of 99.5% and the grain boundaries are clear, and no obvious visible pores can be found. The estimated average grain size is about 1.52 μm, as shown in Fig. 3(b). Fig. 3(c) displays the temperature and frequency dependence of the dielectric permittivity from 150 K to 460 K for the BCTS ceramic. The temperature of the maximum dielectric permittivity (T_m) of the BCTS ceramic is close to room temperature (300 K). This is in agreement with previous reports.⁴⁴ In addition, it can be observed that the BCTS sample shows a diffuse phase transition characteristic (broad peaks of the dielectric permittivity) with frequency dispersion especially at the left side of the dielectric peak [shown in the inset of Fig. 3(c)]. It is believed that the diffuse phase transition and the frequency dispersion of the dielectric permittivity are the two typical characteristics for relaxor ferroelectrics. Fig. 3(d) shows the temperature dependence of the dielectric loss from 140 K to 460 K for the BCTS ceramic. Note that the dielectric loss shows a minimum value at around 305 K, which corresponds to the T_m detected in the dielectric permittivity.⁶⁴Fig. 3(e) shows the inverse dielectric permittivity at 100 kHz as a function of temperature for the BCTS ceramic. It is well known that the dielectric permittivity of a normal ferroelectric should obey the Curie–Weiss law when the temperature is above the Curie temperature. In this work, the dielectric permittivity of the BCTS ceramic obviously deviates from the Curie–Weiss law, which also is a characteristic of relaxor ferroelectrics. Here, T_B is referred to as Burns’ temperature and denotes the temperature above which the dielectric permittivity starts to follow the Curie–Weiss law. For relaxor ferroelectrics, γ is called a diffusion coefficient for characterizing the diffusion degree ranging from 1 (a normal ferroelectric) to 2 (an ideal relaxor ferroelectric). The γ value of the BCTS ceramic is 1.76 [shown in the inset of Fig. 3(e)], suggesting a nature close to a relaxor.

Fig. 3 (a) XRD pattern of the BCTS ceramic. (b) SEM images of the thermally etched sample. (c) The temperature and frequency dependence of the dielectric permittivity of the BCTS ceramic. (d) The temperature and frequency dependence of the dielectric loss of the BCTS ceramic. (e) Inverse dielectric permittivity at 100 kHz as a function of temperature for the BCTS ceramic (symbols: experimental data; solid line: fitting to the Curie–Weiss law). The inset shows the diffusion coefficient of the BCTS ceramic.

The characteristic of breakdown strength is analyzed by the two-parameter Weibull distribution function:^61,62

X_i = ln(E_i) (1)

where E_i is the breakdown electric field for the ith sample, and i and n represent the serial number of the samples and the total number of samples, respectively. The obtained breakdown strength is arranged from small to large. According to eqn (1) and (2), the Weibull distribution of the BCTS ceramic is displayed in Fig. 4(a). It is obvious that all data fit well with the Weibull distribution accompanied by a good linear relationship between X_i and Y_i. Weibull shape parameter β, which is known as the shape parameter indicating the dispersion of the experimental data, is 21.86, proving high reliability of the obtained E_b values. The E_b of the BCTS ceramic is 319 kV cm⁻¹, which is the highest value reported so far in lead-free bulk ceramics for electrocaloric refrigeration and comparable with that found in ferroelectric thick films. The high E_b value in this study is ascribed to the fine grains, dense microstructure and low porosity [see Fig. 3(b)].

Fig. 4 (a) Weibull plot of the dielectric breakdown strength for the BCTS ceramic. (b) The electric displacement–electric field (D–E) loops at 180 kV cm⁻¹ for the BCTS ceramic at different temperatures. (c) The current–field (I–E) loops at 180 kV cm⁻¹ for the BCTS ceramic at different temperatures. (d) The electric displacement as a function of temperature in the BCTS ceramic. (e) ΔT of the BCTS ceramic as a function of temperature at different electric fields.

To evaluate the ΔT of the BCTS ceramic using an indirect method, D–E hysteresis loops at different temperatures were recorded at a frequency of 10 Hz. The ferroelectric D–E loops of the BCTS ceramic were measured at 180 kV cm⁻¹ from 300 K to 340 K at an interval of 5 K, and are displayed in Fig. 4(b). Clearly, it is found that the maximum electric displacement (D_max) of the sample decreases as the temperature increases [see Fig. 4(b)].⁶⁵ In addition, the D–E hysteresis loops of the sample became slimmer and slimmer. More importantly, it's worth noting that the applied electric field of 180 kV cm⁻¹ in this study is considerably higher than that previously reported for other lead-free bulk ceramics for electrocaloric refrigeration, such as BT-based,^35–38,46 BNT-based^47,48 and KNN-based ceramics.^32,44,45 In this study, the high applied electric field should be attributed to the uniform and compact microstructure of the sample prepared using the tape casting preparation process. Fig. 4(c) shows the current–electric field (I–E) loops of the BCTS ceramic determined at different temperatures. It can be seen that two current peaks near zero field are observed, suggesting a relaxor ferroelectric nature of the sample.^66,67

Fig. 4(d) shows the variation of D with respect to temperature for the BCTS ceramic, which was extracted from the upper branch of the D(E) loops shown in Fig. 4(b). For all the samples, D decreases smoothly with the increase of temperature from 300 K to 345 K. According to the Maxwell relation , the ΔT for dielectric materials, where the electric field changes from E₁ (initial external electric field = 180 kV cm⁻¹) to E₂ (final external electric field = 0 kV cm⁻¹) is expressed by:

where ρ is the mass density. The values of (∂D/∂T)_E were obtained from the derivatives of D(T) curves. C_p is the heat capacity, and the value of 440 J kg⁻¹ K⁻¹ was obtained from ref. 35. Fig. 4(e) displays ΔT as a function of temperature for the BCTS ceramic at different electric fields. The ΔT at room temperature (300 K) of the BCTS ceramics is 1.6 K, which is the highest value reported so far in lead-free bulk ceramics and is comparable with that found in lead-based relaxor ferroelectric materials [see Fig. 5(a)]. It should be pointed out that there is usually a peak in the directly measured and indirectly estimated ΔT for lead-free and lead-based relaxor ferroelectric materials under a low applied electric field (<60 kV cm⁻¹), and the temperature of the maximum ΔT was found to coincide with T_m. However, this peak cannot be found near room temperature (300 K), which is close to T_m [see Fig. 3(b)], because it shifts to higher temperature under the electric field of 180 kV cm⁻¹ in this study. Similar results also can be found in other ferroelectric materials, including polymers,⁵⁰ thin films,²⁶ and bulk ceramics.³² The reason can be attributed to an electric field induced phase transition in relaxor ferroelectric materials. This phenomenon should be noted and considered in designing and exploring new relaxor ferroelectric materials for electrocaloric refrigeration in the future; that is to say, the T_m of new relaxor ferroelectric materials should be below 300 K (depends on the applied electric field) to achieve an ultrahigh room temperature ΔT under a high applied electric field.

Fig. 5 (a) Comparison of the room temperature (300 K) ΔT achieved in this work and other previously reported lead-free and lead-based bulk ceramics. (b) Comparison of the temperature stability of ΔT for this work and representative lead-free and lead-based bulk ceramics.

Fig. 5(a) shows a comparison of the room temperature (300 K) ΔT and applied electric field for the BCTS bulk ceramic and other representative lead-free and lead-based bulk ceramics. It can be seen that an ultrahigh room temperature ΔT was achieved in the BCTS ceramic, which is almost 4 times higher than that of other lead-free bulk ceramics due to the high applied electric field. In comparison with the widely used lead based ceramics, this room temperature ΔT value is even superior to that of PMN–PT (0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃, a well-known relaxor ferroelectric, denoted as PMN–PT) bulk ceramics. Fig. 5(b) shows a comparison of ΔT and the operating temperature range for the BCTS ceramic and representative lead-free and lead-based bulk ceramics. Unlike the other previously reported results, which only show high ΔT in an extremely narrow temperature range, ΔT of the BCTS bulk ceramic increases from 1.6 K to 2.0 K with the increase of temperature from 300 K to 345 K in this study, which is much higher than that of previously reported lead-free bulk ceramics and lead-based bulk ceramics.^{27,32,40,45,68–70} This result demonstrates that the BCTS ceramic prepared using the tape casting technique is a very promising candidate for solid-state refrigeration. The results confirm that the tape casting technique as a practicable strategy is feasible and effective to improve the room temperature ΔT of lead-free bulk ceramics for electrocaloric refrigeration.

4. Conclusions

In this work, we successfully synthesized a high-density BCTS ceramic using the tape casting technique. A large room temperature ΔT (1.6 K) was realized in the BCTS bulk ceramic by application of a high electric field of 180 kV cm⁻¹, which is the highest reported room temperature ΔT in lead-free bulk ceramics. Encouragingly, the ΔT of the BCTS bulk ceramic increases from 1.6 K to 2.0 K in the temperature range from room temperature to 345 K, which is even superior to the lead-based bulk counterpart. This work offers a viable strategy for the exploration of lead-free bulk ceramics with a large room temperature ΔT for electrocaloric refrigeration. We believe the findings in this study represent a significant step toward the practicality of electrocaloric refrigeration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (Grant No. 51772239 and 11572103) and scientific research initiation funds for the doctoral program of Xi’an International University (Grant No. XAIU2019001). Florian Weyland and Nikola Novak gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft under NO 1221/2-1 and from the Slovenian Research Agency under program P1-0125, respectively.

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