Yu-Ping Sun

Vapor-compression refrigeration, though ubiquitous, relies on gaseous refrigerants with substantial climate footprints. Solid-state caloric (SSC) cooling offers a green alternative, yet its performance is constrained by field hysteresis and parasitic losses from secondary‑fluid heat exchange. Here we demonstrate the liquid-state barocaloric effect in molecular liquids, achieving isothermal entropy changes of 230 J·kg ^-1 K ^-1 and adiabatic temperature changes of 32 K. Without requiring a liquid-solid transition, the liquid-state BC effect is robust over a temperature span exceeding 100 K. Together with the colossal entropy change, this leads to a relative cooling power three times that of SSC materials. Crucially, the liquid serves simultaneously as refrigerant, heat-transfer medium, and pressure-transmitting fluid, eliminating interfacial thermal resistance and enabling a Carnot-like cycle with a coefficient of performance far surpassing that of SSC systems. Molecular dynamics simulations and quasi-elastic neutron scattering reveal that the colossal thermal response originates from pressure-suppressed translational and rotational molecular motions, primarily governed by molecular polarity and intermolecular interactions. Our work establishes liquid-state barocaloric cooling as a sustainable paradigm that circumvents the longstanding efficiency bottlenecks of SSC refrigeration.

Transparent lead‐free piezoelectric ceramics are promising for multifunctional optoelectronic devices. However, the simultaneous realization of high optical transparency, piezoelectricity, and light‐tunable luminescence remains challenging. In this study, Nd ³⁺ ‐modified 0.94(K _0.5 Na _0.5 ) _{1−3 x} Nd _x NbO ₃ –0.06Sr(Zn _1/3 Nb _2/3 )O ₃ ceramics were designed to regulate phase structure, microstructure, defect chemistry, and optical−electrical coupling. Nd ³⁺ doping has shown to drive the structural evolution from a tetragonal‐dominated phase toward a tetragonal and cubic coexist phase, while regulating grain growth and enhancing densification. The optimized composition with x = 0.01 exhibits a high relative density of 99.7%, refined grains, an optical transmittance of 67.26% at 780 nm, and an enlarged optical bandgap of 3.02 eV. Meanwhile, characteristic Nd ³⁺ upconversion emissions are observed under 808 nm excitation, and both transmittance and upconversion photoluminescence can be modulated by 365 nm light irradiation. This photochromic‐induced luminescence quenching is attributed to the presence of light‐triggered color centers, which are often associated with various types of defects including alkali vacancies, oxygen vacancies, and dopant‐related defects. Benefiting from the balanced pseudo‐cubic structure, dense microstructure, enhanced dielectric response, and retained ferroelectric polarization, the x = 0.01 ceramic achieves a maximum piezoelectric coefficient of 136 pC/N. This work provides an effective strategy for integrating transparency, piezoelectricity, upconversion photoluminescence, and photochromic modulation in lead‐free KNN‐based ceramics.

Two-dimensional (2D) MXenes possess advantages for electromagnetic wave absorption owing to high electrical conductivity, abundant surface functional groups, and defects, but their excessive conductivity often causes impedance mismatch. High-entropy (HE) engineering can reduce the conductivity of MXenes and thereby improve impedance matching, while the conductivity loss will also decrease. Here, NiCo2S4 (NCS) with relatively high electrical conductivity was coupled with HE MXene (Ti1/4V1/4Cr1/4Mo1/4)3C2Tx (HEM) to balance dielectric attenuation and impedance matching. The NCS/HEM achieves an optimal minimum reflection loss of −57.2 dB (thickness of 1.31 mm) and an optimal effective absorption bandwidth of 4.2 GHz (13.6–17.8 GHz, thickness of 1.4 mm). The enhanced electromagnetic wave absorption performance originates from the synergistic effects of optimized impedance matching, improved dielectric attenuation, and facilitated multiple reflections and scattering from heterostructures. This work provides a feasible strategy for designing high-performance MXene-based absorbers through synergistic effects by constructing heterostructures.

Bismuth-based pyrochlore phases, as linear dielectrics, offer a compelling combination of ultralow loss, moderate permittivity, and high breakdown strength, rendering them promising for dielectric energy storage applications. However, the advancement of related research is impeded by the inherent thermodynamic instability in the Bi2O3–TiO2 phase diagram, which makes the synthesis of phase-pure Bi2Ti2O7 particularly challenging. In this work, we demonstrate that the stability of Bi2Ti2O7-based phase is significantly enhanced through careful control of the thermal processing temperature. The resulting materials, therefore, show ultralow dielectric loss and reduced polarization switching hysteresis as well as improved breakdown field. Consequently, a high energy density of 65.5 J/cm3 and an excellent energy efficiency of 84.3% are achieved concurrently. The findings reported herein help to elucidate the relationship between pyrochlore structure stabilization and thermal treat process, thereby providing an effective way for improving the energy storage performance of Bi2Ti2O7-based thin films.

High energy density and high efficiency have long been the pursuit in the development of dielectric capacitors. In this work, energy density and efficiency are synergistically improved by non-stoichiometric Ti-excess design in BaBi4Ti4O15 thin films. On the one hand, excessive Ti promotes grain refinement and the formation of an amorphous phase, significantly inhibits leakage current, and enhances the breakdown field strength. On the other hand, relatively high polarization can be maintained despite the introduction of an amorphous phase by excessive Ti. Ultimately, a high energy density of 135.8 J cm−3, a high efficiency of 90.7%, and a remarkable energy storage figure of merit of 1460 J cm−3 were achieved in BaBi4Ti4.2Mn0.05O15 thin film. The thin film exhibits excellent stability and reliability regarding frequency (0.8–50 kHz) and temperature (20–230 °C), as well as cycling (106) performances. These results demonstrate that Ti-excess designed BaBi4Ti4O15 lead-free thin films are promising candidates for high-performance dielectric capacitor applications.