Influence of Grain-Growth Inhibitors on Modified (Ba,Sr)(Sn,Ti)O3 for Electrocaloric Application

The paper reports on effect of grain-growth inhibitors MgO, Y2O3 and MnCO3 as well as Ca modification on the microstructure, dielectric, ferroelectric and electrocaloric (EC) properties of Ba0.82Sr0.18Sn0.065Ti0.935O3 (BSSnT). Furthermore, the effects of the sintering time and temperature on the microstructure and the electrical properties of the most promising material system Ba0.62Ca0.20Sr0.18Sn0.065Ti0.935O3 (BCSSnT-20) are investigated. Additions of MgO (xMgO = 1%), Y2O3 (xY2O3 = 0.25%) and MnCO3 (xMnCO3 = 1%) significantly decreased the mean grain size of BSSnT to 0.4 µm, 0.8 µm and 0.4 µm, respectively. Ba0.62Ca0.20Sr0.18Sn0.065Ti0.935O3 (BCSSnT-20) gained a homogeneous fine-grained microstructure with an average grain size of 1.5 µm, leading to a maximum electrocaloric temperature change |ΔTEC| of 0.49 K at 40 °C with a broad peak of |ΔTEC| > 0.33 K in the temperature range from 10 °C to 75 °C under an electric field change of 5 V µm−1. By increasing the sintering temperature of BCSSnT-20 from 1350 °C to 1425 °C, the grain size increased from 1.5 µm to 7.3 µm and the maximum electrocaloric temperature change |ΔTEC| increased from 0.15 K at 35 °C to 0.37 K at 20 °C under an electric field change of 2 V µm−1. Our results show that under all investigated material systems, BCSSnT-20 is the most promising candidate for future application in multilayer ceramic (MLC) components for EC cooling devices.

Bulk ceramics, thin films and multilayer ceramics (MLC) are typical component structures for EC cooling devices.Among them, MLC components are the most suitable because of their increased cooling capacity compared to thin films and their higher dielectric breakdown strength compared to bulk ceramics.In our previous work, we successfully implemented the lead-based 0.92Pb(Mg 1/3 Nb 2/3 )O 3 -0.08PbTiO 3 (PMN-8PT) material into MLC components to allow for the application of higher electrical loads compared to bulk ceramics and thus achieve higher EC temperature changes [14].However, the fabrication of MLC components based on our developed lead-free material system Ba 0.82 Sr 0.18 Sn 0.065 Ti 0.935 O 3 (BSSnT) [15] turned out to be challenging due to the discontinuous grain growth resulting in a mean grain size of around 40 µm, which causes electrical conductivity paths between the inner electrodes.To inhibit the grain growth, we added MgO to BSSnT in our previous work and achieved a substantial decrease in grain size from 40 µm to 0.4 µm [16].Simultaneously, this reduction in grain size caused a substantial decrease in the EC temperature change.In our current study, we investigate alternative grain-growth inhibitors, which exhibit a gentler effect on grain-growth suppression, so as to avoid the strong degradation of the ECE.It is known that low amounts (≤1 mol%) of Y 2 O 3 addition act as Y 3+ donors and lead to the formation of complex defects in the grain, which hinder abnormal grain growth.Further increasing the Y 2 O 3 amount (>1 mol%) causes acceptor doping [17].Depending on the sintering conditions, an addition of 0.625 mol% up to 1.25 mol% Y 2 O 3 leads to the inhibition of grain growth due to the accumulation of Y 3+ ions in the grain boundaries [17,18].MnCO 3 can also be used to suppress the grain growth through accumulation at the grain boundaries [19,20].In addition, the modification of BSSnT with Ca 2+ ions is also feasible, since an increasing Ca content in Ba 1−x Ca x TiO 3 (0 ≤ x ≤ 0.25) results in a remarkably decreased grain size (from 62 µm to 7 µm) [21].
In the present work, we study the influence of the grain-growth inhibitors MgO, Y 2 O 3 and MnCO 3 as well as Ca modification on the microstructure of BSSnT.For all samples, the resulting dielectric, ferroelectric and EC properties are characterized.Furthermore, the effects of the sintering time and temperature on the grain size and thus the EC properties of the promising material system Ba 0.62 Ca 0.20 Sr 0.18 Sn 0.065 Ti 0.935 O 3  are investigated.The overall aim is to develop a lead-free material system that can be used in MLCs for future EC cooling systems.
Furthermore, we characterized samples via X-ray diffraction (XRD), as well as dielectric, ferroelectric and direct electrocaloric measurements.For details on the methods and equipment see Refs.[16,22].
Materials 2024, 17, 1036 3 of 15 formation of individual coarse grains (>10 µm), which prevents the future fabrication of MLC components.In the BSSnT samples prepared with the addition of MgO or MnCO3, Mg-rich phases (Figure 1d) and Mn-rich phases (Figure 1f) were observed.These secondary phases already occurred at low contents of MgO (Figure 1c) or MnCO3 (Figure 1e).When using Y2O3, Y 3+ ions accumulate at the grain boundary, forming core-shell structures and thus inhibiting further grain growth [24].As shown in Figure 1g, samples prepared by the chemical modification of BSSnT with 10% Ca 2+ on the A-site (BCSSnT-10) showed a bimodal grain size distribution consisting of fine grains (~0.6 µm) and giant grains (~21 µm), which is disadvantageous for MLC components.As shown in Figure 1g, samples prepared by the chemical modification of BSSnT with 10% Ca 2+ on the A-site (BCSSnT-10) showed a bimodal grain size distribution consisting of fine grains (~0.6 µm) and giant grains (~21 µm), which is disadvantageous for MLC components.By increasing the amount of Ca 2+ to 20% (BCSSnT-20), samples exhibited a homogeneous fine-grained microstructure with an average grain size of 1.5 µm (Figure 1h).
For the further investigation of the phase composition and electrical properties, only sintered bulk ceramics with a monomodal distribution of fine grains were selected and are summarized in Table 1 (BSSnT for reference).Their monomodal grain size distribution curves are shown in Figure 2. By increasing the amount of Ca 2+ to 20% (BCSSnT-20), samples exhibited a homogeneous finegrained microstructure with an average grain size of 1.5 µm (Figure 1h).For the further investigation of the phase composition and electrical properties, only sintered bulk ceramics with a monomodal distribution of fine grains were selected and are summarized in Table 1 (BSSnT for reference).Their monomodal grain size distribution curves are shown in Figure 2.

Phase Anaylsis
The XRD patterns shown in Figure 3 indicate that pure perovskite phases were formed within the bulk ceramic samples with no occurrence of secondary phases.According to the Inorganic Crystal Structure Database (ICSD), the XRD pattern of all samples except BCSSnT-20 fit with the comparable material composition of Ba0.6Sr0.39Sn0.26Ti0.74O3(ICSD file no.01-077-9432), indicating a cubic crystal structure (Pm3m, 211 space group, a = 3.985 Å).The XRD pattern of BCSSnT-20 is in accordance with the peaks of the material composition of Ba0.6Sr0.4TiO3(ICSD file no.00-034-0411) with a cubic crystal structure (Pm3m, 211 space group, a = 3.965 Å).The inset of Figure 3 shows the XRD peaks at 2θ values between 42° and 48° for all samples.The modification of BSSnT with calcium caused a shift of the reflections in BCSSnT-20 to higher diffraction angles.These shifts can be explained by the substitution of Ba 2+ ions (ionic radius: 175 pm) by Ca 2+ ions (ionic radius: 148 pm) [25], which leads to a decrease in the lattice parameter and thus to higher diffraction angles.

Phase Anaylsis
The XRD patterns shown in Figure 3 indicate that pure perovskite phases were formed within the bulk ceramic samples with no occurrence of secondary phases.According to the Inorganic Crystal Structure Database (ICSD), the XRD pattern of all samples except BCSSnT-20 fit with the comparable material composition of Ba 0.6 Sr 0.39 Sn 0.26 Ti 0.74 O 3 (ICSD file no.01-077-9432), indicating a cubic crystal structure (Pm3m, 211 space group, a = 3.985 Å).The XRD pattern of BCSSnT-20 is in accordance with the peaks of the material composition of Ba 0.6 Sr 0.4 TiO 3 (ICSD file no.00-034-0411) with a cubic crystal structure (Pm3m, 211 space group, a = 3.965 Å).The inset of Figure 3 shows the XRD peaks at 2θ values between 42 • and 48 • for all samples.The modification of BSSnT with calcium caused a shift of the reflections in BCSSnT-20 to higher diffraction angles.These shifts can be explained by the substitution of Ba 2+ ions (ionic radius: 175 pm) by Ca 2+ ions (ionic radius: 148 pm) [25], which leads to a decrease in the lattice parameter and thus to higher diffraction angles.

Dielectric Properties
Figure 4 shows the relative permittivity and dielectric loss factor as functions of the temperature measured at 1 kHz for the investigated samples.Compared with BSSnT with a maximum relative permittivity of ε r,m = 25,300 at T m = 27 • C, all grain-growth inhibitors and the Ca modification significantly reduced the relative permittivity and shifted the temperature of the maximum permittivity towards lower temperatures with varying amplitudes.Among them, BCSSnT-20 had a permittivity peak of ε r,m = 6000 at a slightly shifted temperature of T m = 23 • C. In addition, the addition of MnCO 3 resulted in an increased di-electric loss factor.This can be explained by the valence transformation of some manganese ions from Mn 2+ to Mn 3+ after sintering in air.The electrons released by the transformation increase the electrical conductivity and thus contribute to dielectric losses [26].By contrast, the sharply increasing dielectric loss above 50 • C when adding Y 2 O 3 to BSSnT is due to the formation of oxygen vacancies [27].The temperature dependence of relative permittivity and the dielectric loss factor at different frequencies for each investigated composition are shown in Figure S1.

Dielectric Properties
Figure 4 shows the relative permittivity and dielectric loss factor as functions of the temperature measured at 1 kHz for the investigated samples.Compared with BSSnT with a maximum relative permittivity of εr,m = 25,300 at Tm = 27 °C, all grain-growth inhibitors and the Ca modification significantly reduced the relative permittivity and shifted the temperature of the maximum permittivity towards lower temperatures with varying amplitudes.Among them, BCSSnT-20 had a permittivity peak of εr,m = 6000 at a slightly shifted temperature of Tm = 23 °C.In addition, the addition of MnCO3 resulted in an increased dielectric loss factor.This can be explained by the valence transformation of some manganese ions from Mn 2+ to Mn 3+ after sintering in air.The electrons released by the transformation increase the electrical conductivity and thus contribute to dielectric losses [26].By contrast, the sharply increasing dielectric loss above 50 °C when adding Y2O3 to BSSnT is due to the formation of oxygen vacancies [27].The temperature dependence of relative permittivity and the dielectric loss factor at different frequencies for each investigated composition are shown in Figure S1.The diffuseness parameter of the phase transition can be determined using the fol-

Dielectric Properties
Figure 4 shows the relative permittivity and dielectric loss factor as functions of the temperature measured at 1 kHz for the investigated samples.Compared with BSSnT with a maximum relative permittivity of εr,m = 25,300 at Tm = 27 °C, all grain-growth inhibitors and the Ca modification significantly reduced the relative permittivity and shifted the temperature of the maximum permittivity towards lower temperatures with varying amplitudes.Among them, BCSSnT-20 had a permittivity peak of εr,m = 6000 at a slightly shifted temperature of Tm = 23 °C.In addition, the addition of MnCO3 resulted in an increased dielectric loss factor.This can be explained by the valence transformation of some manganese ions from Mn 2+ to Mn 3+ after sintering in air.The electrons released by the transformation increase the electrical conductivity and thus contribute to dielectric losses [26].By contrast, the sharply increasing dielectric loss above 50 °C when adding Y2O3 to BSSnT is due to the formation of oxygen vacancies [27].The temperature dependence of relative permittivity and the dielectric loss factor at different frequencies for each investigated composition are shown in Figure S1.The diffuseness parameter of the phase transition can be determined using the following equation introduced by Uchino and Nomura [28]: The diffuseness parameter of the phase transition can be determined using the following equation introduced by Uchino and Nomura [28]: where C is the Curie-Weiss constant, γ is the diffuseness coefficient with a value between 1 (for an ideal ferroelectric material) and 2 (for an ideal relaxor ferroelectric material).The plots of log(1/ε r − 1/ε r,m ) versus log(T − T m ) and their fitting curves at 1 kHz for the investigated samples are shown in Figure 5.The values of γ are in the range of 1.47-1.90,which corresponds to the diffuse phase transition caused by the existence of different states of polarization and hence different relaxation times in different regions [29].The dielectric data of the investigated samples are summarized in Table 2.The addition of grain-growth inhibitors and Ca modification reduced the dielectric permittivity due to the decreased grain size.With the exception of the Y addition, the investigated grain-growth inhibitors showed no significant influence on the diffuseness of the phase transition.
where C is the Curie-Weiss constant, γ is the diffuseness coefficient with a value between 1 (for an ideal ferroelectric material) and 2 (for an ideal relaxor ferroelectric material).The plots of log 1  ⁄ 1  , ⁄ versus log   and their fitting curves at 1 kHz for the investigated samples are shown in Figure 5.The values of γ are in the range of 1.47-1.90,which corresponds to the diffuse phase transition caused by the existence of different states of polarization and hence different relaxation times in different regions [29].The dielectric data of the investigated samples are summarized in Table 2.The addition of grain-growth inhibitors and Ca modification reduced the dielectric permittivity due to the decreased grain size.With the exception of the Y addition, the investigated grain-growth inhibitors showed no significant influence on the diffuseness of the phase transition.Table 2. Compilation of the temperature of maximum permittivity (Tm), the maximum relative permittivity (εr,m), the loss factor at Tm (tan δm) with standard deviations and the diffuseness coefficient (γ) with standard deviations of BSSnT, BSSnT + Mg, BSSnT + Y, BSSnT + Mn and BCSSnT-20 (measured at 1 kHz).

Sample
Tm

Ferroelectric Properties
Hysteresis loops of polarization in dependence of the electric field varying from −2 to 2 V µm −1 measured at 20 °C and 10 Hz are presented in Figure 6.The samples prepared using grain-growth inhibitors showed a decreased maximum polarization Pm and a decreased remanent polarization Pr compared to pure BSSnT, which is related to their reduced grain size [30].The addition of MgO to BSSnT led to a 55% decrease in the maximum polarization from Pm = 13.4 µC cm −2 to Pm = 6.1 µC cm −2 , whereas the modification of BSSnT with Ca only reduced the maximum polarization by 30% to Pm = 9.5 µC cm −2 .BCSSnT-20 therefore appears to be qualified for future multilayer ceramic fabrication.The ferroelectric characteristics of all studied compositions are summarized in Table 3.

Sample
T m /

Ferroelectric Properties
Hysteresis loops of polarization in dependence of the electric field varying from −2 to 2 V µm −1 measured at 20 • C and 10 Hz are presented in Figure 6.The samples prepared using grain-growth inhibitors showed a decreased maximum polarization P m and a decreased remanent polarization P r compared to pure BSSnT, which is related to their reduced grain size [30].The addition of MgO to BSSnT led to a 55% decrease in the maximum polarization from P m = 13.4 µC cm −2 to P m = 6.1 µC cm −2 , whereas the modification of BSSnT with Ca only reduced the maximum polarization by 30% to P m = 9.5 µC cm −2 .BCSSnT-20 therefore appears to be qualified for future multilayer ceramic fabrication.The ferroelectric characteristics of all studied compositions are summarized in Table 3.  Measurement above 45 °C was not possible for this composition because of the high dielectric losses.BSSnT-Mg, BSSnT-Mn and BCSSnT-20 showed a lower EC effect over a broadened temperature range.However, BSSnT-Mg and BCSSnT-20 can withstand higher electric field changes during EC measurements.Under the application of an electric field change of 5 V µm −1 BSSnT-Mg showed an electrocaloric temperature change of 0.27 K in a broad temperature range from 5 to 50 °C, while BCSSnT-20 showed a maximum electrocaloric temperature change of 0.49 K at 40 °C with a broad peak of |ΔTEC| > 0.33 K in the temperature range from 10 °C to 75 °C (Figure 8).The results indicate that BCSSnT-20 is most suitable for the future fabrication of MLC components, where the application of higher electrical fields and thus higher EC effects are expected.

Influence of Sintering Time and Temprature on BCSSnT-20 Bulk Ceramics
Since the EC effect is not only affected by the applied electrical field but also determined by the grain size of the samples [31,32], we further studied this correlation on the material BCSSnT-20.Therefore, the microstructure of samples with the same chemical composition was modified by variations of sintering time and temperature.
Our variations of sintering time with a constant sintering temperature of 1350 °C only slightly influenced the microstructure of BCSSnT-20 (shown in Figure 9).With an increase in sintering time from 1 h to 4 h, the grain size increased from 1.5 µm to 1.9 µm (Figure 10), accompanied by an increase in porosity from 0.6% to 1.0%.

Influence of Sintering Time and Temprature on BCSSnT-20 Bulk Ceramics
Since the EC effect is not only affected by the applied electrical field but also determined by the grain size of the samples [31,32], we further studied this correlation on the material BCSSnT-20.Therefore, the microstructure of samples with the same chemical composition was modified by variations of sintering time and temperature.
Our variations of sintering time with a constant sintering temperature of 1350 • C only slightly influenced the microstructure of BCSSnT-20 (shown in Figure 9).With an increase in sintering time from 1 h to 4 h, the grain size increased from 1.5 µm to 1.9 µm (Figure 10), accompanied by an increase in porosity from 0.6% to 1.0%.The electrical properties were marginally improved by increasing the sintering time (Figure 11).The maximum relative permittivity rose negligibly with Tm remaining un- The electrical properties were marginally improved by increasing the sintering time (Figure 11).The maximum relative permittivity rose negligibly with T m remaining unchanged.The slim hysteresis loops indicate that the samples showed relaxor-like behavior, and the temperature-dependent behavior of the remanent polarization indicates that the samples underwent a diffuse phase transition.In addition, the remanent polarization increased faintly with increasing sintering time and so did the electrocaloric temperature change.Results of the characterization are summarized in Table 4 (for the relative permittivity ε r measured at different frequencies, the corresponding diffuseness coefficient γ and the temperature-dependent remanent polarization, see Figures S2-S4.)In conclusion, varying the sintering time affected the grain size slightly and thus only moderately changed the electrocaloric properties.The electrical properties were marginally improved by increasing the sintering time (Figure 11).The maximum relative permittivity rose negligibly with Tm remaining unchanged.The slim hysteresis loops indicate that the samples showed relaxor-like behavior, and the temperature-dependent behavior of the remanent polarization indicates that the samples underwent a diffuse phase transition.In addition, the remanent polarization increased faintly with increasing sintering time and so did the electrocaloric temperature change.Results of the characterization are summarized in Table 4 (for the relative permittivity εr measured at different frequencies, the corresponding diffuseness coefficient γ and the temperature-dependent remanent polarization, see Figures S2-S4.)In conclusion, varying the sintering time affected the grain size slightly and thus only moderately changed the electrocaloric properties.The electrical properties were marginally improved by increasing the sintering time (Figure 11).The maximum relative permittivity rose negligibly with Tm remaining unchanged.The slim hysteresis loops indicate that the samples showed relaxor-like behavior, and the temperature-dependent behavior of the remanent polarization indicates that the samples underwent a diffuse phase transition.In addition, the remanent polarization increased faintly with increasing sintering time and so did the electrocaloric temperature change.Results of the characterization are summarized in Table 4 (for the relative permittivity εr measured at different frequencies, the corresponding diffuseness coefficient γ and the temperature-dependent remanent polarization, see Figures S2-S4.)In conclusion, varying the sintering time affected the grain size slightly and thus only moderately changed the electrocaloric properties.In the second step, we investigated the influence of the sintering temperature T S with a constant sintering time of 1 h. Figure 12 presents the microstructure and Figure 13 the grain size distribution of BCSSnT-20 sintered at sintering temperatures up to 1425 • C. Increasing the sintering temperature led to a grain coarsening from 1.5 µm at 1350 • C to 3.6 µm at 1375 • C, 4.7 µm at 1400 • C and up to 7.3 µm at 1425 • C. Simultaneously, the porosity increased from 0.6% to 2.2%.Along with the increase in the sintering temperature and grain size, the maximum relative permittivity increased significantly with a shift of the temperature of maximum permittivity towards lower temperatures (shown in Figure 14a).Moreover, the peak of relative permittivity became shaper and the diffuseness coefficient γ decreased from 1.90 to 1.72, indicating that the phase transition is no longer suppressed and thus less diffuse in samples with coarse grains [33].The remanent polarization increased with increasing sintering temperature and grain size.With an increasing grain size, the number of low-permittivity grain boundaries is reduced, resulting in less polarization discontinuity on the grain surface and thus improved polarization [34].The maximum electrocaloric temperature change |∆T EC | increased from 0.15 K at 35 • C to 0.37 K at 20 • C under an electric field change of 2 V µm −1 , which can also be explained by the dependency of the EC effect on the grain size [35][36][37].Results of the characterization of the BCSSnT-20 samples in dependence of their sintering temperature are summarized in Table 5 (for the relative permittivity ε r measured at different frequencies, the corresponding diffuseness coefficient γ and the temperature-dependent remanent polarization see Figures S5-S7.)Additionally, the typical values of the directly measured electrocaloric temperature change published for selected BaTiO 3 -based materials are shown in Table 6.The sample of BCSSnT-20 sintered at 1425 • C was in good agreement with previously published results, exhibiting an electrocaloric strength of 0.19 10 −6 K m V −1 .Simultaneously, the maximum of the ECE occurred around room temperature.
µm at 1375 °C, 4.7 µm at 1400 °C and up to 7.3 µm at 1425 °C.Simultaneously, the porosity increased from 0.6% to 2.2%.Along with the increase in the sintering temperature and grain size, the maximum relative permittivity increased significantly with a shift of the temperature of maximum permittivity towards lower temperatures (shown in Figure 14a).Moreover, the peak of relative permittivity became shaper and the diffuseness coefficient γ decreased from 1.90 to 1.72, indicating that the phase transition is no longer suppressed and thus less diffuse in samples with coarse grains [33].The remanent polarization increased with increasing sintering temperature and grain size.With an increasing grain size, the number of low-permittivity grain boundaries is reduced, resulting in less polarization discontinuity on the grain surface and thus improved polarization [34].The maximum electrocaloric temperature change |ΔTEC| increased from 0.15 K at 35 °C to 0.37 K at 20 °C under an electric field change of 2 V µm −1 , which can also be explained by the dependency of the EC effect on the grain size [35][36][37]    In summary, increasing the sintering temperature affected the grain size significantly and thus enhanced the electrocaloric properties of the ceramic samples.However, the dielectric breakdown strength of ceramics decreases with increasing grain size, for example in barium titanate with the relation  ∝  , where E, G and a are the breakdown field strength, the grain size and a constant of approximately 0.5, respectively [38].Therefore, thorough grain-size engineering aiming at high dielectric strength and high EC properties will be necessary for future MLC components.In summary, increasing the sintering temperature affected the grain size significantly and thus enhanced the electrocaloric properties of the ceramic samples.However, the dielectric breakdown strength of ceramics decreases with increasing grain size, for example in barium titanate with the relation E ∝ G −a , where E, G and a are the breakdown field strength, the grain size and a constant of approximately 0.5, respectively [38].Therefore, thorough grain-size engineering aiming at high dielectric strength and high EC properties will be necessary for future MLC components.

Conclusions
In the present work, we investigate the influence of the grain-growth inhibitors MgO, Y2O3 and MnCO3 as well as Ca modification on the microstructure, electrical properties and electrocaloric characteristics of BSSnT bulk ceramic samples.The addition of MgO ( = 1%), Y2O3 ( = 0.25%) and MnCO3 ( = 1%) significantly decreased the average grain size of BSSnT from 40 µm to 0.4 µm, 0.8 µm and 0.4 µm, respectively.However, the additions of Y2O3 and MnCO3 caused high dielectric losses, especially at elevated temperatures, making the resulting material inappropriate for a future fabrication of EC multilayer ceramic components.The same applies to BSSnT modified with 10% of Ca 2+ (BCSSnT-10), which exhibited a bimodal grain size distribution with coarse grains (~21 µm).In comparison, the modification of BSSnT with 20% Ca 2+ (BCSSnT-20) showed an average grain size of 1.5 µm and a maximum EC temperature change |ΔTEC| of 0.49 K at 40 °C with a broad peak of |ΔTEC| > 0.33 K in the temperature range from 10 °C to 75 °C under an electric field change of 5 V µm −1 .Since the EC effect is also affected by the grain size of the samples, additional sintering experiments were performed on BCSSnT-20.By increasing the sintering temperature from 1350 °C to 1425 °C, the grain size was increased from 1.5 µm to 7.3 µm and the maximum electrocaloric temperature change |ΔTEC| was enhanced from 0.15 K to 0.37 K under an electric field change of 2 V µm −1 .Our results show that among all investigated material systems, BCSSnT-20 is a very promising candidate for application in multilayer ceramic (MLC) components for EC cooling devices.Future work will concentrate on the preparation of BCSSnT-20 MLC components, grain size engineering, and the correlation between the microstructure, dielectric strength, and EC properties.

Conclusions
In the present work, we investigate the influence of the grain-growth inhibitors MgO, Y 2 O 3 and MnCO 3 as well as Ca modification on the microstructure, electrical properties and electrocaloric characteristics of BSSnT bulk ceramic samples.The addition of MgO (x MgO = 1%), Y 2 O 3 (x Y 2 O 3 = 0.25%) and MnCO 3 (x MnCO 3 = 1%) significantly decreased the average grain size of BSSnT from 40 µm to 0.4 µm, 0.8 µm and 0.4 µm, respectively.

Table 2 .
Compilation of the temperature of maximum permittivity (T m ), the maximum relative permittivity (ε r,m ), the loss factor at T m (tan δ m ) with standard deviations and the diffuseness coefficient (γ) with standard deviations of BSSnT, BSSnT + Mg, BSSnT + Y, BSSnT + Mn and BCSSnT-20 (measured at 1 kHz).

Figure 7
Figure 7 shows the electrocaloric temperature change in dependence of the temperature measured at a relatively low electric field change of 2 V µm −1 for all samples.Compared to BSSnT featuring a maximum electrocaloric temperature change of 0.47 K at 35 • C, BSSnT + Y reached its maximum electrocaloric temperature change of 0.31 K at 25 • C. Measurement above 45 • C was not possible for this composition because of the high dielectric losses.BSSnT-Mg, BSSnT-Mn and BCSSnT-20 showed a lower EC effect over a broadened temperature range.However, BSSnT-Mg and BCSSnT-20 can withstand higher electric field changes during EC measurements.Under the application of an electric field change of 5 V µm −1 BSSnT-Mg showed an electrocaloric temperature change of 0.27 K in a broad temperature range from 5 to 50 • C, while BCSSnT-20 showed a maximum electrocaloric temperature change of 0.49 K at 40 • C with a broad peak of |∆T EC | > 0.33 K in the temperature range from 10 • C to 75 • C (Figure 8).The results indicate that BCSSnT-20 is most suitable for the future fabrication of MLC components, where the application of higher electrical fields and thus higher EC effects are expected.Materials 2024, 17, 1036 8 of 15

Figure 11 .
Figure 11.(a) The temperature dependence of the relative permittivity εr and dielectric loss factor tan δ measured at 1 kHz; (b) hysteresis loops of the polarization P in dependence of the electric field

Figure 11 .
Figure 11.(a) The temperature dependence of the relative permittivity εr and dielectric loss factor tan δ measured at 1 kHz; (b) hysteresis loops of the polarization P in dependence of the electric field Figure 11.(a) The temperature dependence of the relative permittivity ε r and dielectric loss factor tan δ measured at 1 kHz; (b) hysteresis loops of the polarization P in dependence of the electric field E measured at 20 • C and 10 Hz; (c) the electrocaloric temperature change |∆T EC | depending on temperature T for BCSSnT-20 sintered at 1350 • C for 1 h (black), 2 h (red) and 4 h (blue).

Table 4 .
Compilation of the average grain size (d 50 ) and porosity (Φ); the temperature of maximum permittivity (T m ), maximum relative permittivity (ε r,m ), loss factor at T m (tan δ m ) with standard deviations and diffuseness coefficient (γ) with standard deviations measured at 1 kHz; the maximum polarization (P m ), remanent polarization (P r ) and coercive field E c measured at 20 • C and 10 Hz; the maximum electrocaloric temperature change (|∆T EC |)measured at an electric field change of 2 V µm −1 ; and the corresponding temperature (T) for BCSSnT-20 sintered at 1350 • C with sintering times (t s ) of 1 h, 2 h and 4 h.
. Results of the characterization of the BCSSnT-20 samples in dependence of their sintering temperature are summarized in Table 5 (for the relative permittivity εr measured at different frequencies, the corresponding diffuseness coefficient γ and the temperature-dependent remanent polarization see Figures S5-S7.)Additionally, the typical values of the directly measured electrocaloric temperature change published for selected BaTiO3-based materials are shown in Table6.The sample of BCSSnT-20 sintered at 1425 °C was in good agreement with previously published results, exhibiting an electrocaloric strength of 0.19 10 −6 K m V −1 .Simultaneously, the maximum of the ECE occurred around room temperature.

Figure 14 .
Figure 14.(a) Temperature dependence of the relative permittivity εr and dielectric loss factor tan δ measured at 1 kHz; (b) hysteresis loops of the polarization P in dependence of the electric field E measured at 20 °C and 10 Hz; (c) the electrocaloric temperature change |ΔTEC| depending on the temperature T for BCSSnT-20 sintered for 1 h at 1350 °C (black), 1375 °C (red), 1400 °C (blue) and 1425 °C (green).

Figure 14 .
Figure 14.(a) Temperature dependence of the relative permittivity ε r and dielectric loss factor tan δ measured at 1 kHz; (b) hysteresis loops of the polarization P in dependence of the electric field E measured at 20 • C and 10 Hz; (c) the electrocaloric temperature change |∆T EC | depending on the temperature T for BCSSnT-20 sintered for 1 h at 1350 • C (black), 1375 • C (red), 1400 • C (blue) and 1425 • C (green).

Table 5 . 2 V
Compilation of the average grain size (d 50 ) and porosity (Φ); the temperature of maximum permittivity (T m ), maximum relative permittivity (ε r,m ), loss factor at T m (tan δ m )with standard deviations and diffuseness coefficient (γ) with standard deviations measured at 1 kHz; the maximum polarization (P m ), remanent polarization (P r ) and coercive field (E c ) measured at 20 • C and 10 Hz; the maximum electrocaloric temperature change (|∆T EC |) measured at an electric field change of 2 V µm −1 ; and the corresponding temperature (T) for BCSSnT-20 sintered for 1 h at sintering temperatures (T s ) of 1350 • C, 1375 • C, 1400 • C and 1425 • C. µm −1

Table 1 .
Compilation of samples prepared with different mole fractions of additives (x additive ).Sintering temperature (T S ) and average grain size (d 50 ) refer to emphasized concentrations (bold font), which were also used for further investigations.

Table 1 .
Compilation of samples prepared with different mole fractions of additives (xadditive).

Table 6 .
Compilation of the directly measured electrocaloric temperature change (|ΔTEC|) at different electric field changes (ΔE), as well as the corresponding temperature (T) and electrocaloric strength (ΔT/ΔE) for the selected BaTiO3-based materials.

Table 6 .
Compilation of the directly measured electrocaloric temperature change (|∆T EC |) at different electric field changes (∆E), as well as the corresponding temperature (T) and electrocaloric strength (∆T/∆E) for the selected BaTiO 3 -based materials.