Measurement Of Electrocaloric Effect Research Articles

Comprehending the underlying mechanism behind directly/indirectly obtained large electrocaloric response in Bi0.5Na0.5TiO3-based relaxor ferroelectrics

Lead-free ferroelectric electrocaloric (EC) materials are promising candidates for the next generation of cooling materials. Bi0.5Na0.5TiO3 (BNT)-based relaxor ferroelectrics, one of the most representative EC materials, exhibit a superior electrocaloric effect (ECE) near the depolarization temperature (Td). However, there are more and more concerns over the reliability of the widely used indirect measurement method. Here, to comprehend the difference between indirectly and directly obtained ECE results in BNT-based system, the applicability of indirect measurement (based on Maxwell equation) was critically analyzed with respect to the reliable direct measurement (based on heat flow). As evidenced in prototypical BNT-based compositions with different Td values, the dynamic behaviors of local polar structures including ferroelectric domains and polar nano-sized regions (PNRs), can largely affect the measured polarization and thus affect the reliability of indirect measurement. After systematically comparing and analyzing the composition-, temperature-, electric field (E-field)-dependent ECE results obtained from indirect and direct ways, the applicable working regions for the indirect method were determined. This work reveals the underlying mechanism behind direct and indirect ECE measurements, providing the “indirect + direct” experimental strategy for effectively evaluating the ECE performance in BNT-based relaxor ferroelectrics.

T − E phase diagrams and electrocaloric effect in PNN-PT solid solutions

Using the sixth-order free energy expansion of the Landay thermodynamic potential, T − E phase diagrams and the electrocaloric effect (ECE) in solid solutions (1-x)Pb(Ni1∕3Nb2∕3)O3-xPbTiO3 (PNN-PT) were studied under an electric field along the crystallographic directions [001], [101] and [111]. The composition of the samples, temperature range, as well as magnitude and direction of the electric field significantly affect the sequence of phase transitions, stability of various crystalline phases and electrocaloric properties. Direct measurements of the intensive ECE in the 0.8PNN-0.2PT compound were carried out by the method of adiabatic calorimetry at electric field strength E = 6 kV/cm. The large electrocaloric coefficient ΔT∕ΔE = 10 μK ⋅ cm/kV indicates that this compound and other PNN-PT solutions are promising for application as solid-state refrigerants.

Direct measurement of electrocaloric effect of BaTiO<sub>3</sub> using high-speed infrared camera

Direct measurement of electrocaloric effect of BaTiO<sub>3</sub> using high-speed infrared camera

Direct measurement of electrocaloric effect in lead-free (Na0.5Bi0.5)TiO3-based multilayer ceramic capacitors

Multilayer ceramic capacitors (MLCCs) of lead-free NBT-based ceramics are produced and their electrocaloric effect (ECE) is characterised for the first time. Dense MLCCs with 97μm-thick active layers are successfully produced by tape casting. Dielectric permittivity measurements reveal the MLCCs to have properties similar with that reported for the corresponding bulk ceramics, including Td∼50°C and TS∼100°C. Direct ECE measurements also reveal agreement and confirm the previously reported tendency of the high-field ECE peak to shift towards TS. The highest value of ECE, ΔTmax∼1.7K is measured at 90°C under 90kV/cm. A low breakdown strength of 93kV/cm needs to be solved to realise stronger electric fields and achieve commercially viable ECE values.

Quasi-adiabatic calorimeter for direct electrocaloric measurements.

The electrocaloric effect (ECE) in ferroelectric materials is a promising candidate for small, effective, low cost, and environmentally friendly solid state cooling applications. Instead of the commonly used indirect estimates based on Maxwell's relations, direct measurements of the ECE are required to obtain reliable values. In this work, we report on a custom-made quasi-adiabatic calorimeter for direct ECE measurements. The ECE is measured for two promising lead-free materials: Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics. Adiabatic temperature changes of ΔTEC = 0.5 K at 355 K and ΔTEC = 0.3 K at 314 K were achieved under the application of an electric field of 2 kV/mm for the Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 samples, respectively. The quasi-adiabatic ECE measurements reliably match other direct EC measurements using a differential scanning calorimeter or an infrared camera. The data are compared to indirect EC estimations based on Maxwell's relations and show that the indirect measurements typically underestimate the effect to a certain degree.

Modified Differential Scanning Calorimeter for Direct Electrocaloric Measurements

Solid-state refrigeration using the electrocaloric effect (ECE) in ferroelectric materials is a promising alternative to the conventional vapor-compression technology. In spite of growing interest to the investigation of the ECE, direct measurements of the effect are still rare. In this paper, we report on a modification of a differential scanning calorimeter for direct ECE measurements. The importance of proper estimation of the thermal correction factor and use of proper values of the heat capacitance for correct ECE measurements is discussed. The ECE measurements were performed for Ba(Zr0.2Ti0.8)O3 and Ba(Zr0.12Ti0.88)O3 bulk ceramics. Large electrocaloric temperature changes of 0.54 and 0.34 K are achieved under the application of an electric field of 2 kV/mm for the Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 samples, respectively. The relation between the directly measured ECE values and frequently used indirect estimation based on Maxwell's relations is discussed.

Large room-temperature electrocaloric effect in lead-free BaHfxTi1−xO3 ceramics under low electric field

Ferroelectrics are promising candidate materials for electrocaloric refrigeration. Materials with a large electrocaloric effect (ECE) near room temperature and a broad working temperature range are getting closer to practical applications. However, the enhanced ECE is always achieved under high electric field, which limits their wide cooling applications. In this paper, the phase diagram of lead-free BaHfxTi1−xO3 (BHT) ferroelectric ceramics was established. A large ECE under relatively low electric field (ΔE = 10 kV/cm) is firstly reported in BHT ferroelectric ceramics. The direct temperature change (ΔT = 0.35 °C under 10 kV/cm) in BHT ceramics is comparable with those reported in the literature under high electric fields. Meanwhile, the electrocaloric efficiency (ΔT/ΔE = 0.35 K mm kV−1 under 10 kV/cm) is thirteen percent higher than the best value reported previously under high electric field (ΔE = 145 kV/cm). We demonstrate that the ECE can be greatly enhanced by tuning the composition of the lead-free BHT ceramics to its first-order phase transition (FPT), invariant critical point (ICP) or diffuse phase transition (DPT). It is shown that the enhancement in ECE is strongly dependent on the nature of structural phase transition and electric field coupling effect, which has been confirmed by both the indirect and direct ECE measurements. A phenomenological explanation based on Landau model was also proposed to understand this phenomenon. Our findings in this work may provide a better understanding and design methodology for developing more practically useful electrocaloric materials.

Tuning the electrocaloric enhancement near the morphotropic phase boundary in lead-free ceramics.

The need for more energy-efficient and environmentally-friendly alternatives in the refrigeration industry to meet global emission targets has driven efforts towards materials with a potential for solid state cooling. Adiabatic depolarisation cooling, based on the electrocaloric effect (ECE), is a significant contender for efficient new solid state refrigeration techniques. Some of the highest ECE performances reported are found in compounds close to the morphotropic phase boundary (MPB). This relationship between performance and the MPB makes the ability to tune the position of the MPB an important challenge in electrocaloric research. Here, we report direct ECE measurements performed on MPB tuned NBT-06BT bulk ceramics with a combination of A-site substitutions. We successfully shift the MPB of these lead-free ceramics closer to room temperature, as required for solid state refrigeration, without loss of the criticality of the system and the associated ECE enhancement.

Interpretation of the Electrocaloric Effect in Na1/2Bi1/2TiO3-SrTiO3-PbTiO3 Solid Solutions

Results of direct measurements of electrocaloric effect and polarization are mutually compared in solid solutions 0.4Na1/2Bi1/2TiO3-(0.6-x)SrTiO3-xPbTiO3. Three temperature regions are distinguished in respect of a phase transition temperature. Above the phase transition, electrocaloric effect is determined by orientation of polar nanoregions by electric field. In wide temperature regions above and below the phase transition temperature, the difference between results, obtained from direct electrocaloric effect measurements and from polarization hysteresis loops, is explained by temperature dependence of concentration of polar nanoregions, oriented by electric field. Existence of a common ΔT(P) curve for all temperatures in the ferroelectric state indicates that polarization can be treated as a parameter, which solely determines electrocaloric effect in this state.

Direct and indirect electrocaloric measurements on 〈001〉-PbMg1/3Nb2/3O3-30PbTiO3 single crystals

A direct electrocaloric effect (ECE) measurement system, based on a modified-differential scanning calorimeter (DSC), allowing the acquisition of both thermal (ECE, heat capacity) and electrical (P-E loops, leakage current) information simultaneously, was used to analyze 〈001〉-oriented PbMg1/3Nb2/3O3-30PbTiO3 single crystals. Different electric-field-induced phase transitions were identified on direct ECE measurements and confirmed by dielectric measurements. The strongest ECE (ΔTEC = 0.65 K) was measured for an applied electric field E = 10 kV/cm just above the temperature of depolarization. The direct ECE measurements were compared with indirect measurements obtained from dielectric polarization measurements versus electric field and temperature and a very good agreement was found. A region with negative ΔTEC was identified by both direct and indirect measurements. This phenomenon was attributed to the formation of a reversible field-induced phase transition towards a state with a different polar direction.

Estimate of Thermodynamic Indirect Measurement on the Electrocaloric Effect

The thermodynamics indirect measurement based on Maxwell relation is one of the most popular methods for the studies on electrocaloric effect of ferroelectric material, including bulk ceramics, single crystals and thin films.However,recent reports indicated that the results obtained through this method did not agree with those from the direct measurements in many cases, such as nearTcor under high electric field. This paper discussed the applicability of thermodynamics indirect measurement and analyzed the reason why its calculated result may be different from the true value of electrocaloric effect. Because the thermodynamics indirect characterization depends on the measurement of polarization at different temperatures, the testing frequency ofP-Eloops, leakage current of sample and temperature resolution of measurement play the important roles in the accuracy measurement of electrocaloric effect.

Direct Measurements of the Electrocaloric Effect in Bulk PbMg1/3Nb2/3O3 (PMN) Ceramics

Electrocaloric effect (ECE), i.e., a reversibly change in the temperature of an electrocaloric material due to the adiabatically applied electric field, is very attractive phenomenon due to its applications, for example in electric refrigeration. The first observations of the giant ECE were based on indirect measurements of the electric polarization and not on direct measurements of the ECE itself. By using direct measurements of ECE we show existence of the giant ECE in PMN ceramics near the electric field-induced ferroelectric phase transition.