Positive and Negative Electrocaloric Effect in Lead-Free Silver Niobate Antiferroelectric Ceramic Depending on Affluent Phase Transition

Abstract

We prepared a dense AgNbO₃ ceramic using a conventional solid-state reaction method. The phase structure, electrical properties and electrocaloric effect (ECE) were systematically investigated. Large negative and positive ECEs (−4.38 °C at 65 °C and 2.3 °C at 210 °C) under an external electric field of 180 kV·cm⁻¹ were obtained in the eco-friendly AgNbO₃ antiferroelectric (AFE) ceramic due to affluent phase transition and a high electric field. The large positive and negative ECEs originated from the phase transition between ferrielectric (FIE) phases (the orthorhombic space group (Pmc2₁) and AFE phases (Pbcm) tuned by an applied external field. Additionally, a probable mechanistic model was proposed to illustrate the generation of positive and negative ECEs. This study may provide guidelines for the design of high-efficiency solid-state cooling devices.

1. Introduction

Conventional vapor-compression refrigeration technology emits hazardous gas, causing inevitable environmental problems. Hence, eco-friendly cooling technologies with high conversion efficiency and which are carbon-neutral are important for developing new types of refrigeration devices [1]. Cooling devices based on the electrocaloric-effect (ECE) are expected to replace the currently dominant conventional vapor-compression refrigeration devices due to their capacity for reversible temperature change (non-polluting) under the stimulation of an external electric field [2]. The ECE refers to the isothermal entropy (ΔS) or adiabatic temperature change (ΔT) of a dielectric material upon application or withdrawal of an external electric field [3,4,5,6,7]. Recent theoretical and experimental studies have shown that there are two types of ECEs in various ferroelectric (FE)/antiferroelectric (AFE) bulk and film systems, including positive and negative types [8,9,10,11]. Usually, a large magnitude of |ΔT|, for either the positive ECE or its negative counterpart, is expected and required for commercial solid-state cooling devices that can be widely used in biological refrigeration, on-chip cooling and temperature regulation [12,13]. Some studies have focused on the coexistence of positive and negative ECEs, such as BaZr_0.2Ti_0.8O₃ (BZT) relaxor ferroelectric (RFE) films [14], (Pb_0.97La_0.02)(Zr_xSn_0.94−xTi_0.06)O₃ (PLZST) antiferroelectric ceramics [15], and (Pb_0.97La_0.02)(Zr_0.95Ti_0.05)_1+yO₃ antiferroelectric ceramics [16]. However, Pb-based materials suffer from the harmful effects of Pb elements on the environment and human beings and, as a result, their use will be seriously limited in the future. Nonetheless, the ECE of lead-free RFE is lower than that of Pb-based materials due to its strong dependence on a high breakdown electric field, resulting in a failure to meet market demand for commercial devices.

Silver niobate (AgNbO₃) was synthesized in the 1950s [17]. Subsequent studies mainly focused on analyzing its structure, phase transitions, its piezoelectric, as well as dielectric, properties and its energy-storage behavior [18,19,20,21,22,23,24,25,26,27,28,29,30,31]. AgNbO₃ exhibits various phase transformations and shows AFE-like electrical polarization under a high applied electric field. A series of phase transitions take place in AgNbO₃ materials with increasing temperature: $\mathrm{orthorhombic}({\mathrm{M}}_1)\stackrel{~70 {}^{°}\mathrm{C}}{\leftrightarrow }\mathrm{orthorhombic}({\mathrm{M}}_2)\stackrel{~270{}^{°}\mathrm{C}}{\leftrightarrow }\mathrm{orthorhombic}({\mathrm{M}}_3)\stackrel{~350{}^{°}\mathrm{C}}{\leftrightarrow }\mathrm{orthorhombic}\left(\mathrm{O}\right)\stackrel{~380{}^{°}\mathrm{C}}{\leftrightarrow }\mathrm{tetragonal}\left(\mathrm{T}\right)\stackrel{~580{}^{°}\mathrm{C}}{\leftrightarrow }\mathrm{cubic}\left(\mathrm{C}\right)$. M₁ is the ferrielectric (FIE) phase with weak ferroelectricity, while M₂ and M₃ are disordered AFE phases. The phase transitions for M₁–M₂ and M₂–M₃ are associated with cation (Nb and Ag) displacements [22,32]. Therefore, based on the abundant phase transition properties of AgNbO₃, it is predicted to obtain an efficient ECE.

In the present study, we report a large negative ECE (−4.38 °C at 65 °C) and a positive ECE (2.3 °C at 210 °C) under an external electric field of 180 kV·cm⁻¹ in the eco-friendly AgNbO₃ AFE ceramic. The simultaneously achieved positive and negative ECE in AgNbO₃ results from phase transitions between FIE and AFE with a high electric field. The coexistence of positive and negative ECEs represents a novel technology for the development of solid-state cooling devices.

2. Experimental Procedure

2.1. Fabrication

AgNbO₃ ceramics were synthesized by a conventional solid-state reaction method in a tube furnace, using Ag₂O (≥99.7%) and Nb₂O₅ (≥99.99%) as the starting materials. All starting materials were obtained from the Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. Stoichiometric amounts of Ag₂O and Nb₂O₅ were mixed in absolute ethyl alcohol for 24 h using a planetary ball mill. After drying at 110 °C, the mixture was calcined at 850 °C for 6 h in an O₂ atmosphere. Subsequently, the calcined powders were milled and dried again. The powders were then pressed into disks with a diameter of 10 mm using a 5 wt% solution of polyvinyl alcohol (PVA) as a binder, followed by cold isostatic pressing under 200 MPa for 1.5 min. After burning out the PVA binder, the sintering process was carried out at 1080 °C for 6 h in an O₂ atmosphere to avoid the possible decomposition of the silver oxide at high temperature.

2.2. Characterization

The densities of the ceramic samples were measured in alcohol using the Archimedes method. The microstructure of AgNbO₃ ceramics was examined by scanning electron microscopy (SEM, JSM-5510, JEOL Ltd., Tokyo, Japan). The phase structure was determined using an X-ray diffractometer (XRD, Bruker D8 Advanced Diffractometer, Bruker AXS, Ettlingen, German) with monochromatic Cu Kα radiation (λ = 1.5405 Å). For the electric property measurements, the AgNbO₃ ceramics were polished to 150 μm in thickness and were sputtered with Au top electrodes of 2 mm in diameter by a DC sputtering method. The temperature-dependent dielectric properties were assessed by an inductance capacitance resistance analyzer (LCR, TH2828, Tonghui Co., Ltd., China) with an automated temperature controller at a rate of 2 °C·min⁻¹. Polarization-electric field (P-E) hysteresis loops were measured using a ferroelectric tester (Radiant Technologies, Inc., Albuquerque, NM, USA) with a temperature-controlled probe station. Differential scanning calorimetry (DSC, DSC-Q2000, TA Instruments, New Castle, DE, USA) was used to measure the heat flow and specific heat capacity (C_p) on crushed unpoled ceramics in air at a scan rate of 10 °C·min⁻¹.

3. Results and Discussion

The XRD pattern of the sintered AgNbO₃ ceramic in the range of 20~70° is shown in Figure 1. A single phase assigned to an orthorhombic perovskite structure was achieved (JCPDS Card No.70-4738), which was consistent with results reported previously [33]. It is worth noting that an antipolar M₁ (Pbcm symmetry/AFE) phase and a polar M₁ (Pmc2₁ non-centrosymmetry/FE) phase coexist at room temperature [34,35,36]. SEM images of the AgNbO₃ ceramic are presented in the inset of Figure 1. The grains of the ceramics developed satisfactorily, resulting in a dense and cubic microstructure. The grain size was in the range of 2~4 μm and the density of the ceramic was 6.565 g·cm⁻³ representing 96% of the theoretical density. The results indicate that a high breakdown strength (BDS) was achieved, contributing to the large obtained polarization value.

Figure 2a shows the temperature dependence of the dielectric constants on heating and cooling cycles at 100 kHz with 3 °C per minute of the heating/cooling rate for dielectric measurements. Three obvious dielectric anomalies were detected in the heating curve. The first dielectric anomaly was located at about 70 °C (mark as T₁), corresponding to the phase transition from the M₁ to the M₂ phase. The second dielectric anomaly was located at about 270 °C (mark as T₂), corresponding to the phase transition from the M₂ to the M₃ phase. The third dielectric anomaly was caused by the phase transition from the M₃ to the O₁ phase (at about 380 °C), which corresponds to the Curie temperature (T_c) of pure AgNbO₃. It is worth noting that a weak dielectric anomaly was found near 170 °C on the cooling curve, which could be assigned to the freezing temperature (T_f) of the antipolar alignment of Nb⁵⁺ ions in the lattice [24]. The phase transition temperature was further demonstrated by the DSC curves, as shown in Figure 2b. A strong endothermic event was observed on heating at ~350 °C, with a much weaker endothermic event at ~380 °C. The corresponding exotherms on cooling occurred at ~330 °C and ~370 °C, respectively. The results of the DSC curves are consistent with the temperature dependence of the dielectric constant. Characteristic P-E and I-E loops of AgNbO₃ antiferroelectric ceramics at room temperature were observed, as shown in Fig 2c. Double P-E loops indicated large saturated polarizations (P_s) and non-zero remanent polarizations (P_r). Two strong current peaks, E_F (the electric field from antiferroelectric to ferroelectric phase transition) and E_A (the electric field from ferroelectric to antiferroelectric phase transition) could be observed in the I-E loops.

The temperature dependence of the P-E loops collected at 180 kV cm⁻¹ was evident during the heating process, as shown in Figure 3a–c. In order to unambiguously reveal the variation in the P-E loops at different temperature stages, the results were divided into three temperature segments. For the temperature stage from 30 °C to 110 °C, the P_max value monotonically decreased with increase in temperature, while the E_F showed an upward trend. For the temperature stage from 110 °C to 170 °C, P_max monotonically increased with temperature increase, while E_F reduced. When the temperature was above 170 °C (>T_f), the P_max value monotonically decreased again with increase in temperature, while the E_F again showed an upward trend compared with that for P-E at 170 °C. It was observed that there was an opposite trend in the changes between P_max and E_F at different temperature stages, which further reinforced the complexity of the structure in the AgNbO₃ AFE ceramics.

In order to evaluate the electrocaloric effect of AgNbO₃ AFE ceramics, the P_max values under different electric-fields and temperatures were extracted from Figure 3, as presented in Figure 4a. Below the E_F (140 kV cm⁻¹), the P_max values monotonically increased with increase in temperature (dP/dT > 0, Figure 4b), while, above this field (140 kV cm⁻¹), the situation was more complex, with polarization occurring non-monotonically with a peak near T₂. For T < T₂, the P_max values obviously increased with increase in temperature (dP/dT > 0, Figure 4b), whereas above T₂, it decreased rapidly (dP/dT < 0, Figure 4b). With increase in the electric-field intensity and temperature, the AFE phase gradually transformed to ferroelectric, so the P_max value gradually increased. When the electric-field reached the E_F, the polarization state of the AFE phase rapidly transformed to the FE state. With increase in temperature, this AFE-FE transformation process was accentuated, bringing about a rapid increase in the polarization value. However, when the temperature exceeded T₂, under the action of the high temperature and electric field, the FE state became dominant, so the polarization value rapidly decreased with further increase in temperature, which reflected the intrinsic characteristics of the ferroelectric state.

The ECE performance of this system was measured using an indirect method based on the P_max-T data. The well-known Maxwell relation ${\left(\frac{\partial P}{\partial T}\right)}_E={\left(\frac{\partial S}{\partial E}\right)}_T$, the adiabatic temperature change (ΔT) and the adiabatic entropy change (ΔS) are given by the equations [37,38]:

$$\Delta \mathrm{T}=-\frac{1}{\rho ·C_P}{{\displaystyle \int }}_{{\mathrm{E}}_1}^{{\mathrm{E}}_2}\mathrm{T}{\left(\frac{\partial \mathrm{P}}{\partial \mathrm{T}}\right)}_{\mathrm{E}}\mathrm{dE}$$

(1)

$$\Delta \mathrm{S}=-\frac{1}{\mathsf{\rho }}{{\displaystyle \int }}_{{\mathrm{E}}_1}^{{\mathrm{E}}_2}{\left(\frac{\partial \mathrm{P}}{\partial \mathrm{T}}\right)}_{\mathrm{E}}\mathrm{dE}$$

(2)

where C_p is the heat capacity, ρ is the density of the samples, T is the operating temperature, P is the maximum polarization at an applied electric field E, and E₁ and E₂ are the initial and final applied electric fields. The values of ${\left(\frac{\partial P}{\partial T}\right)}_E$ were obtained from sixth-order polynomial fits to the raw P_max-T data extracted from the upper branches of the P-E loops for E > 0. ρ is the density of the AgNbO₃ ceramics (6.565 g·cm⁻³). The heat capacity C_p was selected to be 0.33 J·g⁻¹·K⁻¹ at room temperature. The profiles of ΔT and ΔS versus temperature (ΔT-T and ΔS-T) under different electric fields were calculated according to Equations (1) and (2), as shown in Figure 4c,d. It was found that a positive and negative ECE were achieved over a broad temperature range of 30~230 °C. In particular, a large negative ECE (−4.38 °C under 180 kV cm⁻¹ at 65 °C) was obtained in the ambient temperature range with a broad temperature span, which contributed to a large dP/dT (>0) and large entropy change (|ΔS|~4.4 J mol⁻¹ K⁻¹ in Figure 4d) in this temperature range. In addition, a positive ECE (2.3 °C under 180 kV cm⁻¹ at 210 °C) was also obtained under a high electric-field (>E_F) in the temperature range from 170 °C to 230 °C. As previously mentioned with respect to Figure 3, it should be noted that the ferroelectric state dominated the system at this temperature stage when E_F > 140 kV cm⁻¹; therefore, the decrease in the polarization value with increase in temperature caused the appearance of the positive ECE owing to negative dP/dT, which was consistent with normal ECEs observed in FE materials [39,40]. Overall, a positive and negative ECE in the lead-free silver niobate AFE ceramic was obtained. However, the deeper ECE mechanisms of AgNbO₃ lead-free AFE ceramics need to be explored in the future.

Based on the structure, phase transition and measurement results for AgNbO₃, we consider that the generation of positive and negative ECE derives from the synergistic effect of the temperature (T) and the electric field (E) [33]. To better illustrate this synergistic effect, a mechanistic model is proposed as shown in Figure 5. At T < T₁, the FE phase decreases while the AFE phase gradually increases with increasing temperature in the FIE phase of the AgNbO₃ ceramics (Figure 5a). This transition from an FE phase to an AFE phase will cause a decay in polarization. The inversion of the ferroelectric dipoles plays a role in enhancing the polarization. It is well-known that the entropy reduces by aligning dipoles in the ferroelectric state when an electric field is applied, resulting in a positive ECE according to Equation (1) in this temperature segment. When the temperature rises to T₁, a phase transition occurs from a weak FE phase M₁ to an AFE phase M₂. Figure 5b displays the process of the negative ECE in the AFE phase induced by an electric field in the temperature range of T₁ to T_f. At this stage, the M₂ AFE phase is dominant. The AFE phase is relatively stable because the movement of Nb⁵⁺ is partially frozen within this temperature range. When a moderate electric field (E < E_F) is applied in the AFE phase, the entropy will generally increase by misaligning the dipoles, therefore generating the negative ECE [41]. It is generally accepted that E_F in AFE ceramics decreases with increase in temperature, which means it is easier for the dipoles to deflect under the external electric field. Therefore, a higher saturated polarization can be obtained at higher temperature under the same E. As the temperature rises from T₁ to T_f, the increasing saturated polarization leads to a positive ${\left(\frac{\partial P}{\partial T}\right)}_E$, causing a negative ECE. The generation of ECEs at T > T_f is exhibited in Figure 5c. When the temperature increases from T_f to T_c, the AFE polar phase gradually decreases, with the appearance of a paraelectric non-polar phase, and then the polarization weakens. At a lower electric field (E < E_F), the dipoles will be more easily inversed and the entropy will generally increase by misaligning the dipoles at the higher temperature, leading to the negative ECE. However, the dipoles are completely aligned along the orientation of the electric field under E > E_F, but the polarization values reduce because of the decrease in dipoles at higher temperatures, which leads to a negative ${\left(\frac{\partial P}{\partial T}\right)}_E$, resulting in a positive ECE.

4. Conclusions

In summary, a AgNbO₃ lead-free AFE ceramic was prepared by a conventional solid-state reaction method. The AgNbO₃ ceramic showed the coexistence of positive and negative ECEs over a wide temperature range (from 40 °C to 230 °C). A large negative ECE (−4.38 °C under 180 kV cm⁻¹ at 65 °C) and a positive ECE (2.3 °C under 180 kV cm⁻¹ at 210 °C) were obtained. This original study not only broadens the field of investigation of AgNbO₃ in refrigeration, but also highlights the advantages of application of this component with regard to the electrocaloric effect.