Electrocaloric Responsivity Research Articles

Large electrocaloric responses in [Bi1/2(Na,K)1/2]TiO3‐based ceramics with giant electro‐strains

AbstractThe electrocaloric effect (ECE) is investigated through indirect measurement in two lead‐free [Bi1/2(Na,K)1/2]TiO3‐based ceramics that were previously reported to display giant electro‐strains. In the Nb‐doped ceramic, denoted as BNKT‐2.5Nb, a decent temperature change of ΔT=1.85 K and an electrocaloric responsivity of ΔT/ΔE=0.37 (10−6Km V−1) are found around room temperature (32°C). While in the Ta‐doped ceramic, BNKT‐1.5Ta, a wide operation temperature range (Tspan ~55 K) is observed near room temperature. Additional electrical measurements, as well as transmission electron microscopy experiments, are performed to identify the mechanisms of the ECE in both ceramics.

Lead free Ba0.8Ca0.2TexTi1−xO3 ferroelectric ceramics exhibiting high electrocaloric properties

Direct and indirect electrocaloric measurements were performed on the new Tellurium (Te) doped Ba0.8Ca0.2TiO3 (BCT) ceramics. The effects of Te addition on structural, electrical, and electrocaloric properties of BCT ceramics were investigated. The incorporation of the Te element in the BCT induced the decrease of the Curie temperature and the enhancement of the electrocaloric effect. The significant electrocaloric temperature change ΔT = 1.237 K determined by the direct method was obtained at the relatively moderate field of ∼25 kV/cm in Ba0.8Ca0.2Ti(1−x)TexO3 with x = 0.02. The corresponding electrocaloric responsivity ΔT/ΔE = 0.495 × 10−6 K m V−1 is higher than that observed in pure BCT ceramics and is one of the highest reported so far in lead-free ferroelectric materials. The material's coefficient of performance was determined at the phase transition with a maximal value of 14.7.

Structural, dielectric and electrocaloric properties in lead-free Zr-doped Ba0.8Ca0.2TiO3 solid solution

Abstract We investigate in the present work the additional data points ( x =0.02, 0.06 and 0.08) to the previous reported paper (Asbani et al., 2015) [21] that sufficiently enrich to understand the EC properties and underlying phase diagram of the lead-free Ba 0.8 Ca 0.2 Ti (1− x ) Zr x O 3 ( x BCTZ) system. X-ray diffraction analysis performed at room temperature, confirms a continuous tetragonal solid solution with P 4 mm (No. 99) space group that evolved to pseudo-cubic symmetry for the high Zr-content compounds. Ferroelectric and paraelectric behaviors were highlighted using P–E hysteresis data versus temperature. Phase transition was confirmed by dielectric permittivity measurements versus temperature showing a decrease of the Curie temperature when Zr-content increases. From P–E hysteresis recording the electrocaloric temperature change (Δ T ) was calculated that ranged in between 0.12 and 0.27 K and the electrocaloric responsivity ( ξ ) in 0.15 to 0.34×10 −6 K·m/V under 7.95 kV/cm applied electric field. The zero-field entropy is compared to electrocaloric isothermal entropy to estimate the extent the EC is being under-driven.

Polarization rotation associated critical phenomena in epitaxial PbTiO3 thin films near room temperature

Strain-driven and temperature-driven monoclinic-orthorhombic phase transition in epitaxial PbTiO3 exhibit similar behavior under electric field, i.e., polarization discontinuity is reduced at the first-order ferroelectric-ferroelectric transition whose latent heat vanishes at a critical point. Due to critical phenomena the energy barrier for polarization rotation significantly diminishes, and hence thermodynamic response functions tend to diverge in the induced monoclinic states. Phenomenological calculations show that dielectric and piezoelectric properties are highly tunable by in-plane strain and electric field, and large electromechanical response may occur in epitaxial PbTiO3 thin films at room temperature. Phenomenological calculations show that large electrocaloric responsivity can also be expected at room temperature by manipulating the phase transition.

Indirect and direct electrocaloric measurements of (Ba1−xCax)(Zr0.1Ti0.9)O3 ceramics (x = 0.05, x = 0.20)

Electrocaloric effect in lead-free (Ba1−xCax) (Zr0.1Ti0.9)O3 ceramics (with x = 0.05 and x = 0.20) has been investigated by using two different indirect methods. In the first classic approach, the electrocaloric temperature change (ΔT) was calculated from the Maxwell relation based on measured P–E hysteresis loops recorded at different temperatures. In the second approach, the ΔT was calculated from the Maxwell relation based on pyroelectric current measurements. The advantage of such approach is that it enables direct determination of the pyroelectric coefficient, which should be otherwise calculated from the hysteresis loops in the classic approach. Direct method was finally used for verification of the obtained results. Good agreement between methods was found near the cubic-to-tetragonal phase transition temperature with a significant electrocaloric responsivity of ΔT/ΔE = 0.30 K mm/kV for the composition x = 0.05 obtained by direct method.

Ferroelectric phase changes and electrocaloric effects in Ba(Zr0.1Ti0.9)1−x Sn x O3 ceramics solid solution

The effect of tin (Sn) incorporation in lead-free Ba(Zr0.1Ti0.9)O3 ceramics was investigated. Single phases with perovskite structure were identified using X-ray diffraction analysis. Dielectric permittivity and pyroelectric current measurements carried out versus temperature revealed ferroelectric-to-paraelectric structural phase transitions showing intermediate ferroelectric-to-ferroelectric sequence anomalies for low Sn-content compounds. Electrocaloric effects were highlighted by recording P–E hysteresis loops as function of temperature and the electrocaloric temperature change ∆T = 0.2 K was evaluated under 8.7 kV cm−1 applied electric field. Electrocaloric responsivity of 0.23 K mm kV−1 near Curie temperature remained almost constant until x = 0.075 before decreasing for compounds with x ≥ 0.1. Basing on pyroelectric measurement, we highlight the presence of one quadruple point in this system where the simultaneous presence of Zr and Sn in BaTiO3 matrix seems to stabilize ferroelectric and electrocaloric behaviors instead of Sn element alone.

Electrocaloric effect in (1 − x)PIN-xPT relaxor ferroelectrics

The electrocaloric effect (ECE) is investigated in both relaxor as well as morphotropic composition of (1 − x)Pb(In1/2Nb1/2)O3-xPbTiO3 system. The ECE is analyzed by evaluating the electrocaloric temperature change (ΔT) using thermodynamic Maxwell equation. The relaxor phase (x = 0.15) show an anomalous broadening in ECE and the maximum of ΔT is found to shift from the proximity of depolarization temperature (Td) to around T′m with increase in electric field. However, for morphotropic composition (x = 0.33) a region with negative ΔT is observed below Td along with anomalous broadening in positive ΔT in the proximity of TRT. The maximum value of ΔT obtained for relaxor phase is 0.37 K whereas 0.12 K is observed for morphotropic composition at 225C. The electrocaloric responsivity ξmax is 0.21 mmK/kV for relaxor phase whereas 0.06 mmK/kV is obtained for the morphotropic composition at 18 kV/cm electric field.

Electrocaloric effect in Ba0.2Ca0.8Ti0.95Ge0.05O3 determined by a new pyroelectric method

The present letter explores the electrocaloric effect (ECE) in the lead-free oxide Ba0.8Ca0.2Ti0.95Ge0.05O3 ceramics (BCTG). The electrocaloric responsivity was determined by two different methods using the Maxwell relationship . In a first well-known indirect method, hysteresis loops were measured in a wide temperature range from which the pyroelectric coefficient and thus ξ were determined by derivation of data. In the second novel method the pyroelectric coefficient pE and consequently the electrocaloric responsivity ξ were determined by direct measurements of the pyroelectric currents under different applied electric fields. Within the experimental error, good agreement was obtained between two methods with at about 410 K.

Electro-caloric effect in lead-free ferroelectric Ba1−xCax(Zr0.1Ti0.9)0.925 Sn0.075O3 ceramics

The influence of the incorporation of Calcium in lead-free ferroelectric Ba(Zr0.1Ti0.9)0.925Sn0.075O3 perovskite on its structural phase transition, dielectric properties, ferroelectric behavior, pyroelectric current, and electrocaloric effect was investigated. X-ray diffraction study allowed the identification of a continuous solid solution in the composition range 0≤x≤20mol%. Dielectric and pyroelectric measurements permitted to determine the exits phase transition. The adiabatic temperature change ∆T was calculated by the indirect method from the thermal variation of P–E hysteresis loops using the Maxwell relation. The incorporation of Calcium was found to increase the electrocaloric responsivity within a broad range of temperature with an optimum of ΔT/ΔE=0.27Kmm/kV observed for x=15mol%.

Lead-free Ba0.8Ca0.2(ZrxTi1−x)O3 ceramics with large electrocaloric effect

The electrocaloric effect was investigated in lead-free Zr doped Ba0.8Ca0.2(ZrxTi1−x)O3 (BCTZ) ceramics synthesized by a conventional sintering process. Room-temperature x-ray diffraction analysis showed that the tetragonal structure is obtained in BCTZ for x ≤ 0.08 and a pseudo cubic phase for x > 0.08. The dielectric spectroscopy and calorimetry revealed that the Curie temperature decreases as a consequence of Zr doping and that the BCTZ exhibits a first order ferroelectric phase transition. The electrocaloric effect was determined by the calculation of the electrocaloric change of temperature (ΔT) using the Maxwell relation based on the P–E hysteresis loops measured at different temperatures. A large electrocaloric responsivity ΔT/ΔE = 0.34 × 10−6 Km/V was found for x = 0.04, which significantly exceeds of values found so far in other lead-free electrocaloric materials.

Impact of critical point on piezoelectric and electrocaloric response in barium titanate

The prediction that the piezoelectric tensor is diverging at the critical point is verified for BaTiO${}_{3}$ (BTO), which is the oldest perovskite structured ferroelectric material with an extremely long and eventful research history. Here we investigate experimentally by dielectric and calorimetric measurements the existence and the position of the critical point in the electric-field-temperature ($E$-$T$) phase diagram of BTO in the vicinity of the paraelectric to ferroelectric phase transition. Measurements of the piezoelectric coefficient ${d}_{31}$ as a function of the temperature and the electric field applied along the [001] direction show a critical enhancement of the piezoelectric response in the vicinity of the critical point, in agreement with recent calculations by Porta et al. [J. Phys.: Condens. Matter 22, 345902 (2010)]. The electrocaloric responsivity is found to be enhanced due to the latent heat on the paraelectric to ferroelectric transition locus below the critical point.