Effect of BaO-CaO-SiO2 addition on dielectric and electrocaloric properties of lead-free 0.2Ba(Ti0.9Sn0.1)O3-0.8Ba(Zr0.18Ti0.82)O3 ceramics

: In recent years, compared with traditional refrigeration technology, the electrocaloric refrigeration technology has been applied in variety of green cooling devices due to its high efficiency and environmentally friendly. Recent findings reveal that the ceramics system with additives (such as, LiBiO 2 , PbO) can create large ECE under low electric field. In this work, the ternary glass BaO - CaO - SiO 2 (BCS) was used as a sintering aid to enhance the electrocaloric (EC) response of 0.2Ba(Ti 0.9 Sn 0.1 )O 3 - 0.8Ba(Zr 0.18 Ti 0.82 )O 3 (BZSnT20) bulk ceramics. The EC properties of BZSnT20 could be also improved via controlling sintering temperature and the content of BCS sintering aid. It was found that BZSnT20 with 0.5wt% BCS show the maximum EC response of 3.2K under 7MV/m with sintering temperature of 1262℃. These results demonstrated that the ECE of BZSnT20 can be boosted by using appropriate additives, which provides an effective route to realize large ECE under low electric fields.


Introduction
New solid refrigeration technology has attracted growing attention in recent years. Much progress has been made on electrocaloric refrigeration [1], magnetic card refrigeration [2], thermoelectric refrigeration [3] and semiconductor refrigeration [4] etc. However, high cost, bulky device and the requirements of powerful magnetic field have limited the wide application of magnetic card cooling technology. Besides, the efficiency of thermoelectric refrigeration and semiconductor refrigeration technology are very low (≤ 10%). Compared with these cooling technologies, electrocaloric refrigeration has many advantages, including high refrigeration efficiency, simple devices, low-cost and ease of miniaturization. Electrocaloric refrigeration uses electrocaloric effect (ECE) to achieve cooling effect. The term 'ECE' has been used to describe the polarization state of the electric dipole in the dielectric. By applying external electric field on EC materials, the polarization state can be changed thus resulting in isothermal entropy change S  and adiabatic temperature change T  [5]. The mechanism of electrocaloric refrigeration technology determines that it is a high efficiency and environmentally friendly technology. Therefore, it has a broad prospect of application.
In recent years, in order to obtain higher electrocaloric effect at lower voltage, multi-layered thick membrane structure devices has been designed [6]. To fulfill the demand of low-temperature co-firing of the dielectric layer and the metal electrode, the sintering temperature of the electrocaloric material should be lowered [7]. Since the sintering temperature of ceramics are typically higher than the melting temperature of metal electrode materials. Fortunately, sintering aids can decrease the sintering temperature of the ceramic film, so that the metal electrode can be completely maintained after the sintering process [8]. BaO-CaO-SiO2(BCS) has the advantages of high chemical stability and high thermal stability. Up to now, BZT20 system is one of the most abundant EC materials in the electrocaloric refrigeration technology area, however, BCS has not been widely used as sintering aid in BZT20 system to achieve high ECE .
Besides, the mechanisms of action of BZSnT20 at different sintering temperatures and BCS sintering aid contents were also analyzed, thereby defining the optimum content and sintering temperature of BCS glass sintering aid. This work indicated that environmentally friendly electrocaloric refrigeration devices can be achieved via controlling sintering temperature and the content of BCS sintering aid in BZSnT20 system, and large ECE under low electric fields at room temperature can also be realized.

Experimental section
The BZSnT20 bulk ceramics were fabricated by the solid-state reaction method The dielectric properties of BZSnT20 were characterized using a SR715 LCR meter (Stanford Research Systems). The thermal analysis of the sample was performed on the differential scanning calorimetry (DSC) (TA Q100). The heat absorb and release of the EC material were directly measured by a self-made T-type thermocouple, then the entropy variation (ΔS) was calculated according to the Figure 1 shows the morphology of BZSnT20 with different contents of sintering aid and sintering temperatures. It can be seen from figure 1 that the surface of all BZSnT20 are relatively smooth with few plaques. Interestingly, as the content of the BCS glass sintering aid is 0wt%, with the sintering temperature. However, with the same sintering temperature, the color of BZSnT20 gradually deepened (from yellow to brownish dark) with the contents of BCS sintering aid increases from 0.4wt% to 2wt%. While the contents of BCS sintering aid is fixed, the color of BZSnT20 will be deepened with the increase of sintering temperature from 1233 ℃ to 1284 ℃. As a result that the addition of BCS sintering aid determine the color of BZSnT20 bulk ceramics.

Dielectric properties
Change of the permittivity and loss tangent (at room temperature) for ferroelectric capacitors with BCS sintering aid at different sintering temperatures are shown in Fig. 2. Interestingly, as the content of the BCS glass sintering aid in samples is 0wt%, the permittivity of BZSnT20 increases with the increase of sintering temperature. However, the sintering temperature did not show any significant effect on the color of samples (Fig. 1). Because the sintering temperature of BZSnT20 is increased from 1233 ℃ to 1284 ℃, which not achieving the optimal sintering temperature of BZSnT20 (1400 ℃), leading to the insufficient sintering of BZSnT20 [13]. The insufficient sintering of BZSnT20 leads to a light color and a low dielectric constant.
With increasing of the content for BCS glass sintering aid in samples, the permittivity of BZSnT20 shows a tendency of increasing first and then decreasing.
And the color of BZSnT20 is gradually deepened (from yellow to brownish) (Fig.1).
As the content for the BCS glass sintering aid in samples is up to 0.5wt%, the dielectric constant shows a peak ,which is a 2.5-fold increase compared to BZSnT20 of no BCS glass sintering aid. The loss tangent as a function of BCS glass content is shown in Figure 2 [14][15][16][17][18][19] .At the same time, the BCS liquid phase can get into the grain boundaries of BZSnT20 bulk ceramics during the sintering process, which reduces the defects in the ceramics grain, along with loss tangent decreasing further [20].
However, with the excessive addition of BCS glass sintering aid (1,1.5 and 2wt%), the dielectric constant of BZSnT20 is gradually decrease and the color is gradually darkens (Fig. 1) with the lattice volume expansion [21,22]. It is all known that Zr 4+ or Ti 4+ is partially substituted by Ca 2+ , which is accompanied by the transition of electrons between ions, producing a part of oxygen vacancies [23]. The presence of oxygen vacancies can reduce the space of oxygen octahedrons, resulting in the decrease of the permittivity of bulk ceramics. The BZSnT20 bulk ceramics, sintered with the 0.5wt% BCS exhibit the permittivity maximum up to 11000 at 1284℃ sintering temperature. But BZSnT20 shows a relatively small slope at 1284℃ sintering temperature compared to the samples at 1262℃ sintering temperature, and the increase of the permittivity is rather limited from 1262℃ to 1284℃. Meanwhile, it can also be seen (Fig. 2b) that the loss tangent of all sample are relatively low at the sintering temperature of 1262℃.
The combination of relatively high permittivity and relatively low loss tangent in BZSnT20 bulk ceramics is highly desirable for strengthening the electrocaloric effect.
Therefore, the electrocaloric effect of BZSnT20 bulk ceramics at the sintering temperature of 1262℃ are discussed below.

Electrocaloric performance
According to the formula 00 (1  Figure 3 displays the directly recorded EC signal of BZSnT20 bulk ceramics (300um) under 5MV/m at room temperature. Applying an electric field on the BZSnT20 bulk ceramics, the dipoles of the ceramic, it is the change from a disordered arrangement to an ordered arrangement leading to an exothermic process, which is one of the source of the ECE. As the applied electric is removed, the endothermic peak is obtained around room temperature by the dipole changes from order to disorder, displayed in the peak in Fig. 3(a). In general, the heat Q can be estimated by the integral area of the exothermic and endothermic peak. The isotherm entropy where E c is the specific heat capacity of the ceramic at room temperature. Thus, the values obtained from exothermic and endothermic peaks are nearly the same during both EC heating and cooling process [25].
As shown in Fig.3(b), electric field-induced ECE increases linearly with the electric field for all these BZSnT20 bulk ceramics. With increasing of the content for BCS glass sintering aid in samples, from 0wt% to 1.0wt%, the ECE of BZSnT20 bulk ceramics presents a trend of first increasing and then decreasing. Hou et al. reported that the entropy change reaches 4.8 under an electric field of 10MV/m [26]. It is noted that under an low electric field of 7MV/m, a large ECE( T  =3.2K , ) is achieved using 0.5wt% BCS addition in this work, which data are summarized in Table 1. To gain more detailed information on enhancement ECE on BaTiO3 ceramics system, we note that the modification of BaTiO3 ceramics system by the elements of Zr and Sn, may be formed the four phases (paraelectric cubic, ferroelectric tetragonal, orthogonal and rhombohedral), and converged into a multi-phase point transition (critical point ICP) [27][28].The fact that multi-phase coexistence can decrease the electric field required and thus enhance the ECE [29]. In addition, the Ca 2+ and Si 4+ in BCS glass sintering aids could shift to the samples matrix, then partially replace A and B sites in perovskite structure, so that improving a huge ECE substantially at room temperature. However, with the excessive addition of BCS glass sintering aid (1wt.%), Ca 2+ and Si 4+ can enter the band gap position of the crystal, forming more impurity phases, and the ECE may be weakened partly. It is worth noting that the BZSnT20 with 1.0 wt.% BCS sintering aid is easy to be ruptured even at a low electric field of 5MV/m. This might be mainly caused by the field-driven strain of the dielectric layers and structural defects in BCS samples with 1.0 wt% BCS sintering aid. More importantly, it indicates that a larger EC response could exist in BCS-added BZSnT20 bulk ceramics with 0.5wt% BCS, so that ECE and breakdown electric field could be modified through employing tiny additives.
[Insert Fig. 3] The ambient temperature dependence of ECE for BZSnT20 using BCS as additives is shown in Figure 4. With the increase of ambient temperature, from 0 ℃ to 60 ℃, the ECE gradually increase and finally tend to a constant. This was primarily attributed to the different orientations of the dipoles in BZSnT20 at various ambient temperatures, resulting in the change of ECE [30]. Therefore, the diagram is divided in two areas that it is separated the low temperature region (I) and the higher temperature region (Ⅱ). In region Ⅰ, the ECE increases with the ambient temperature increasing from 0 ℃ to 20 ℃. In region Ⅱ, as the ambient temperature increases from 20 ℃ to 60 ℃, the ECE keeps basically at the same level. Therefore, the electrocaloric performance of BZSnT20 with 0.5% BCS is relatively stable near room temperature, and it can produce a huge ECE over a wide operating temperature (from 20℃ to 60℃), even under a relatively lower electric field (5MV/m). Table 1 Figure 1 The sintered morphology of BZSnT20 ferroelectric capacitors fabricated with different BCS glass sintering aid contents at different sintering temperature.   and as functions of ambient temperature for BZSnT20 ferroelectric capacitors with 0.5% BCS sintering aids under electric elds of 5MV/m and 1262 sintering temperature.