Lijie Qiao

Optimized electrocaloric refrigeration capacity in lead-free (1−x)BaZr0.2Ti0.8O3-xBa0.7Ca0.3TiO3 ceramics

This paper demonstrates the impact of composition on the electrocaloric effect in lead-free (1−x)BaZr0.2Ti0.8O3−xBa0.7Ca0.3TiO3 ceramics, especially on the morphotropic phase boundary. The samples exhibited typical diffused ferroelectric-paraelectric phase transition, which was diffused the most around the tricritical point of rhombohedral, tetragonal, and cubic phases at x = 0.3. The x = 0.3 sample had the highest ΔT max of 0.30 K under 20 kV/cm field and the widest cooling peak within a wide composition range, which endowed it with the best refrigeration capacity. Although the x = 0.7 sample had even higher ΔT max , less diffused phase transition narrowed the cooling peak, which reduced the refrigeration capacity.

Combined effects of diffuse phase transition and microstructure on the electrocaloric effect in Ba1-xSrxTiO3 ceramics

This paper demonstrates the combined effects of diffuse phase transition and microstructure on the electrocaloric effect (ECE) in Ba1−xSrxTiO3 (x = 0.2–0.4) ferroelectric ceramics. The ECE was characterized by direct heat flow measurement and an indirect thermodynamic method, which both provided consistent results. With increasing amount of Sr, the phase transition diffused and the grains grew larger. Since larger grains favor a high ΔT max while a diffuse phase transition does not, the combination of these two effects induced nonmonotonic variation of the ECE. The x = 0.35 sample showed the best performance, with an ECE strength of 0.23 K·m/MV near room temperature.

Both High Reliability and Giant Electrocaloric Strength in BaTiO3 Ceramics

BaTiO3 has a giant electrocaloric strength, |ΔT|/|ΔE|, because of a large latent heat and a sharp phase transition. The electrocaloric strength of a new single crystal, as giant as 0.48 K·cm/kV, is twice larger than the previous best result, but it remarkably decreased to 0.18 K·cm/kV after several times of thermal cycles accompanied by alternating electric fields, because the field-induced phase transition and domain switching resulted in numerous defects such as microcracks. The ceramics prepared from nano-sized powders showed a high electrocaloric strength of 0.14 K·cm/kV, comparable to the single crystals experienced electrocaloric cycles, because of its unique microstructure after proper sintering process. Moreover, its properties did not change under the combined effects of thermal cycles and alternating electric fields, i.e. it has both large electrocaloric effect and good reliability, which are desirable for practical applications.

Hydrogen accumulation and hydrogen-induced cracking of API C90 tubular steel

Abstract A study was undertaken of hydrogen accumulation on grain boundaries, at inclusions, and near a kneaded notch tip; the susceptibility for blistering; and hydrogen-induced cracking (HIC) of ...

Direct observation of two 90° steps of 180° domain switching in BaTiO3 single crystal under an antiparallel electric field

Domain-switching in BaTiO3 single crystal under an antiparallel electric field was investigated in this paper by polarized light microscope. It was found that 180° domain-switching consisted of two 90° switching steps. The polarization reversal of domains took place when an antiparallel electric field was applied to the BaTiO3 single crystal. The domain-switching process was different under different field intensities. Under a low field, all domains switched 180° through two 90° domain-switching steps. However, under a high field, only some 90° domains were nucleated in the initial period and subsequently all domains switched to the direction of the external field quickly.

A systematic modification of the large electrocaloric effect within a broad temperature range in rare-earth doped BaTiO3 ceramics

This paper presents a systematic exploration of modifying the electrocaloric effect (ECE) in BaTiO3 ceramics by rare-earth substitution (Ba0.94R0.04TiO3, R = La, Ce, Nd, Sm, Eu, Gd, Dy, Er). All samples exhibit a dense microstructure after sintering in the temperature range of 1350–1450 °C, and they exhibit a high resistivity of ∼1011 Ω cm except for Er doped samples due to the amphoteric incorporation of Er. The rare-earth doping changes the lattice symmetry where the tetragonal distortion enhances with the decrease in the rare-earth ionic radius. Accordingly, the ferroelectric and ECE properties are modified and the first-order phase transition is diffused. The Curie temperature and latent heat increases, and the polarization intensity is strengthened with the decrease of the doping ionic radius, while the peak of ECE ΔT widens and shifts to a higher temperature. The samples show a large ECE value of ∼0.35 K m MV−1 over a wide temperature range from room temperature to 140 °C, which provides a series of top-level ECE materials.

The electrocaloric effect around the orthorhombic- tetragonal first-order phase transition in BaTiO3

This paper demonstrates the electrocaloric effect (ECE) around BaTiO3s orthorhombic-tetragonal first-order phase transition. By manipulating a field-induced transition of a metastable phase in the thermal hysteresis zone, a huge exothermic or endothermic peak appears after first applying or removing electric fields because of the energy change of lattice structure. A large ECE of ΔT/E = 1.4K·m/MV, equaling to latent heat, is achieved under 10kV/cm at 10°C. The entropy change for polarization ordering alone induces an ECE two orders of magnitude lower under the same condition. It confirms the dominant factor to ECE of the energy flow due to the structural phase transition.

Cobalt doped LaSrTiO3−δ as an anode catalyst: effect of Co nanoparticle precipitation on SOFCs operating on H2S-containing hydrogen

This article compares the effects of Co doping on phase structures and stability of lanthanum strontium titanate (LST) anodes and electrochemical measurements in solid oxide fuel cells (SOFCs) employing H2S-containing H2 as fuel. The Co-doped LST (LSCT) with a perovskite structure was synthesized via a solid state approach, achieving excellent phase purity and refined particle size. The catalytic activity and electrochemical performance are significantly improved by introducing Co. A maximum power density of 300 mW cm−2 was achieved at 900 °C with 5000 ppm H2S–H2 in a fuel cell having a 300 μm thick YSZ electrolyte. Trace amounts of metallic Co nanoparticles with sizes typically no larger than 10 nm in diameter were detected on the LSCT surface after reduction in H2 at 900 °C. The nano-sized Co clusters could reduce the anode polarization resistance, as well as improve the cell performance, compared with undoped LST anodes. The LSCT anode catalyst was electrochemically stable in 5000 ppm H2S–H2 during the test time at high operating temperature. The LSCT anode catalyst also had relatively high redox stability in reversible oxidation–reduction cycles.

Zero thermal expansion in (1-x)PbTiO3-xBi(Mg,Ti)1/2O3 piezoceramics

The compounds (1−x)PbTiO3–xBi(Mg,Ti)1/2O3 (x = 0–0.7) were prepared in order to hunt for low thermal expansion piezoceramics, in particular for zero thermal expansion over a wide temperature range. The negative thermal expansion of PbTiO3 was much weakened by forming a solid solution with Bi(Mg,Ti)1/2O3. Solutions of (1−x)PbTiO3–xBi(Mg,Ti)1/2O3 with x = 0.2 and x = 0.4 exhibited zero thermal expansion covering a wide temperature range (from RT to about 500 °C). The (1−x)PbTiO3–xBi(Mg,Ti)1/2O3 ceramics had high density and their mechanical performances were satisfactory with high fracture toughness. The promising applications of the zero thermal expansion materials (1−x)PbTiO3–xBi(Mg,Ti)1/2O3 are due to its high piezoelectric d33, controllable thermal expansion, and high thermal stability.

Delayed crack propagation in barium titanate single crystals in humid air

Domain witching of ferroelectrics under mechanical or electric load in vacuum or dry air has been intensively studied. However, the effects of environments on the domain switching in ferroelectrics have not been well understood. Here, we demonstrate that domain configurations in BaTiO3 single crystal under sustained load can be significantly affected by the humidity due to the decrease in surface energy and electrostatic energy upon adsorption of polar water molecules. Consequently, the crack propagation behaviors of the ferroelectrics under sustained load can be remarkably altered.