The improvement of the electrocaloric effect (ECE) in (1−x)(Na0.5Bi0.5)TiO3−xSrTiO3 (NBT-ST) lead-free relaxor ferroelectric ceramics was determined by indirect measurements method and is reported here. The phase transition temperature can be reduced to room temperature by tuning the compositions of the (1−x)NBT-xST material. A large ECE (ΔTmax = 1.64 K, ΔSmax = 2.52 K and ΔT/ΔE = 0.33 K mm kV−1 at 50 kV cm−1) was obtained at 60 °C when x = 0.25. A suitable response over a broad temperature range from 30 °C to 70 °C can be obtained with x = 0.26 with very high cooling values (ΔT > 1 K). In addition, the 0.7NBT-0.3ST AFE-like bulk ceramic exhibited excellent temperature stability in its energy-storage properties from room temperature to 120 °C. The maximum value of the recoverable energy density was 0.65 J/cm3 obtained at 65 kV/cm. Taken together, these properties signified that (1−x)NBT-xST is a promising material for applications in cooling systems and energy-storage in the room temperature range.

Large electrocaloric response and high energy-storage properties over a broad temperature range in lead-free NBT-ST ceramics

The electrocaloric effect was investigated in lead-free Zr doped Ba08Ca02(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  ≤ 008 and a pseudo cubic phase for x > 008 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 = 034 × 10−6 Km/V was found for x = 004, which significantly exceeds of values found so far in other lead-free electrocaloric materials

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

The electrocaloric effect (ECE), i.e., the adiabatic temperature change ΔTEC, of the lead-free relaxor ferroelectric 0.85K0.5Na0.5NbO3-0.15SrTiO3 (KNN-STO) ceramics is investigated. The ECE data obtained by a direct method show the existence of a large ECE near the temperature of the dielectric permittivity maximum. Due to the high break-down electric field, a large ΔTEC exceeding 1.2 K at 300 K and 1.9 K at 340 K was observed at 159 kV/cm in a broad temperature range of 80 K. Such a high ECE response near the room temperature is comparable to that found in lead-based ceramic materials, thus making KNN-STO a strong candidate to replace lead-based materials in future electrocaloric applications.

Large electrocaloric effect in lead-free K0.5Na0.5NbO3-SrTiO3 ceramics

The electrocaloric effects (ECEs) of the morphotropic phase boundary (MPB) composition 0.82(Na0.5Bi0.5)TiO3-0.18(K0.5Bi0.5)TiO3 (NBT-18KBT) are studied by direct measurements. The maximum ECE ΔTmax = 0.73 K is measured at 160 °C under 22 kV/cm. This corresponds to an ECE responsivity (ΔT/ΔE) of 0.33 × 10−6 K m/V, which is comparable with the best reported values for lead-free ceramics. A comparison between the direct and indirect ECE measurements shows significant discrepancies. The direct measurement of both positive and negative electrocaloric effect confirms the presence of numerous polar phases near the MPB of NBT-based materials and highlights their potential for solid-state cooling based on high field-induced entropy changes.

/pdf/electrocaloric-enhancement-near-the-morphotropic-phase-4f3l2sdqkl.pdf

Electrocaloric enhancement near the morphotropic phase boundary in lead-free NBT-KBT ceramics

Phase structure and properties of sodium bismuth titanate lead-free piezoelectric ceramics

Polycrystalline lead-free Sodium Bismuth Titanate (NBT) ferroelectric ceramics doped with rare earth (RE) element are prepared using solid state reaction method. Optical, ferroelectric, and electrocaloric properties were investigated. The introduction of RE3+ ions in the NBT host lattice shows different light emissions over the wavelength range from visible to near infrared region. The ferroelectric P-E hysteresis loops exhibit an antiferroelectric-like character near room temperature indicating possible existence of a morphotropic phase boundary. The enhanced electrocaloric response was observed in a broad temperature range due to nearly merged phase transitions. Coexistence of optical and electrocaloric properties is very promising for photonics or optoelectronic device applications.

B. Asbani

Papers

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

Electrocaloric effect and luminescence properties of lanthanide doped (Na1/2Bi1/2)TiO3 lead free materials

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

Dielectric permittivity enhancement and large electrocaloric effect in the lead free (Ba0.8Ca0.2)1-xLa2x/3TiO3 ferroelectric ceramics

Intrinsic dead layer effects in relaxed epitaxial BaTiO3 thin film grown by pulsed laser deposition