Tangyuan Li

History-dependent selection of primary cellular/dendritic spacing during unidirectional solidification in aluminum alloys

History-dependent selection of primary cellular/dendritic spacing is investigated systematically during unidirectional solidification of a series of aluminum alloys. A single crystal is formed in the sample before each experimental run, so that the influence of grain boundary on the primary spacing is avoided. The experimental results are compared with those of the two-dimensional crystal growth in the same alloy system and transparent model alloys. It is found that the primary cellular/dendritic spacing is remarkably history dependent. The average primary spacing is dependent not only on the current growth conditions, but also remarkably on the way those conditions were achieved. There exists a wide allowable range of primary spacings for a given growth condition. Experimental results are also compared with the Hunt-Lu model, which shows excellent fit between them, especially on the selection of cellular spacing. By comparing the three-dimensional experiments with the two-dimensional ones, it is also found that the allowable range of the primary spacing for the three-dimensional growth is wider than that for the two-dimensional growth.

Large strain and strain memory effect in bismuth ferrite lead-free ceramics

An electric-field-induced strain in ferroelectric materials has extensive applications in actuators and sensors. BiFeO3 (BFO) ceramic is a promising candidate for high-temperature electromechanical coupling applications due to its exceptionally high Curie temperature (Tc ∼ 825 °C). Here, we reported an enhanced electric-field-induced strain in defective BFO ceramics. These defective BFO ceramics exhibit highly asymmetric S–E loops along with a strain memory effect. A large peak-to-peak strain (Sp–p) of 0.66% is achieved in the Bi2O3-deficient sample with x = −0.15, which is the largest strain value ever obtained in BFO-based ceramics. A strain shape memory of 0.24% is observed in samples with x = 0.09 due to the presence of defects and the orientation of the defect-dipoles. The combination of large strain and high TC would make the defective BiFeO3-based ceramics one of the most important candidates for high-temperature electromechanical coupling applications.

Large electrocaloric strength and broad electrocaloric temperature span in lead-free Ba0.85Ca0.15Ti1−xHfxO3ceramics

The electrocaloric (EC) effect of Ba0.85Ca0.15Ti1−xHfxO3 (abbreviated as: BCTHx) ceramics has been systematically investigated using an indirect method. Our results indicate that the BCTH0.06 ceramic exhibits a sharp ferroelectric–paraelectric phase transition, around which a large EC value of 1.03 K is obtained under the electric field of 35 kV cm−1, associated with a relatively broad electrocaloric temperature span. The giant EC strength of the BCTH0.06 ceramic is 0.29–0.31 K mm kV−1 under an external electric field of 9 to 35 kV cm−1, which is larger than most previous values. The temperature of the EC peak shifts to higher temperature as the electric field increases, which implies that electric field may be another powerful tool to obtain a large EC value and modulate the EC peak. The ceramics with higher Hf contents display a strong diffusive character. The BCTH0.15 ceramic shows a relatively small EC strength (0.10–0.13 K mm kV−1) with a broader EC temperature span. The diffusive feature of the BCTHx ceramics with high Hf concentrations broaden the electrocaloric temperature span and meanwhile decrease the EC strength considerably. Our results indicate that compositional modification in BaTiO3-based ceramics offers a promising route for designing potential dielectric cooling materials with excellent EC properties.

Large electrocaloric efficiency over a broad temperature span in lead-free BaTiO3-based ceramics near room temperature

We report a large electrocaloric efficiency of 0.029 K cm kV−1 at 303 K and in a wide operating temperature range of 293 K to 313 K in a lead-free Ba0.9Sr0.1(Ti0.9Zr0.1)0.95Sn0.05O3 ceramic by using direct electrocaloric effect (ECE) measurements. Sn4+ doping in Ba0.9Sr0.1Ti0.9Zr0.1O3 not only tunes the rhombohedral-to-paraelectric phase transition temperature to room temperature but also slightly widens the phase transition region, by slightly strengthening the diffuse character and maintaining its good ferroelectricity. Also, polar nanoregions embedded in the matrix facilitate polarization rotation because of a flat energy landscape associated with the relaxor-to-ferroelectric phase transition, inducing enhanced entropy changes and consequently excellent ECE performance.