Hongmei Zhong

Thermoplastic polyurethane (TPU) foams are widely used because of their unique properties, while the crystallization behavior and crystal structure of TPU are critical to determining foam cellular structure and final performance. However, the role of high‐pressure CO ₂ in the isothermal crystallization of TPU remains unclear. In this work, the isothermal crystallization of TPU under high‐pressure CO ₂ at different temperatures was investigated using both high‐pressure and regular differential scanning calorimetry (DSC). The results showed that the effect of CO ₂ pressure on TPU crystallization strongly depended on isothermal temperature. At low temperatures, the crystallinity first increased and then decreased with increasing CO ₂ pressure, whereas at high temperatures it generally increased with pressure. These changes were governed by the combined effects of CO ₂ plasticization and supercooling degree. The synergistic effects of temperature and CO ₂ pressure on the crystallization of hard segment domains were further clarified for TPUs with different initial crystallinity. More importantly, the results indicate that the CO ₂ ‐controlled crystallization behavior can alter the perfection, packing state, and growth mode of hard‐segment crystals, which is expected to further affect cell nucleation, cell growth, and foam stability during TPU foaming. Therefore, this study provides a useful basis for understanding and regulating foam structure and properties through crystallization control under high‐pressure CO ₂ .

Passive radiative cooling has garnered significant interest as a clean and sustainable cooling strategy. However, addressing the issue of overcooling at low temperatures remains a challenge. In this study, we designed a single‐layer thermochromic film for adaptive thermal management, which consists of a PDMS matrix, SiO ₂ microparticles, TiO ₂ nanoparticles, and thermochromic microcapsules. Thermochromic microcapsules enable passive switching between solar heating and radiative cooling by modulating their visible spectra in response to temperature fluctuations. SiO ₂ microparticles are used to enhance thermal emission within the atmospheric transparent window, whereas TiO ₂ nanoparticles are chosen for strong solar reflection. After comparing several samples, the optimal mass ratio of PDMS:TiO ₂ :SiO ₂ :thermochromic microcapsules is set as 1:0.8:0.1:0.1. Dynamic spectra measurements verify that the thermochromic film modulates visible light reflectivity from 0.94 to 0.81 as temperature decreases from 35 to 20 °C. Outdoor testing indicates that the film provides a 1 °C increase in cold conditions and a 2 °C decrease in hot conditions.

Passive radiative cooling has been extensively explored as a sustainable alternative to traditional cooling systems. However, relying solely on this cooling mode restricts their application in temperature-fluctuating environments and may cause overcooling on cold days. To address this challenge, researchers are increasingly developing dual-mode devices that integrate radiative cooling with solar heating, utilizing the Sun’s continuous heat source for passive heating. In this perspective, we summarize the latest advancements in this field and highlight specific strategies for switchable dual-mode devices. These strategies generally fall into two categories: Janus design and all-in-one design. Finally, we discuss future challenges and opportunities in on-demand radiative cooling and solar heating, intending to advance this technology toward practical applications.

Passive radiative cooling technology serves as an energy-free alternative to traditional cooling systems. Porous polymer structures are frequently employed for radiative cooling by leveraging the refractive index mismatch between the polymer and the pores, enabling the scattering of incoming sunlight. Recently, water-soluble and readily available Sodium chloride (NaCl) particles have been utilized as sacrificial templates for sustainable pore creation. Nevertheless, optimizing NaCl particle size, and thus the polymer pore size to enhance scattering capabilities remains a challenge. Here, we report a simple, scalable, and sustainable approach to creating an optimized porous polydimethylsiloxane (PDMS) film. This approach utilizes ultrafine NaCl powders as sacrificial templates, which were synthesized via ultrasonic precipitation to ensure their small size. The ultrafine NaCl particles have a size distribution centered around 6-8 μm, and the as-fabricated porous PDMS film achieves a high thermal emissivity of 0.95 within the atmospheric window (8-13 μm) and exhibits a reflectivity of 0.95 within the visible range (0.4-0.78 μm). Due to the desired dual-spectrum properties, the porous PDMS film exhibits a superior subambient cooling capacity over that fabricated with typically larger NaCl particles under strong sunlight. This study offers a scalable and practical radiative cooling solution for sustainable thermal management.

Multilayer piezoelectric stacks, which are multiple layers of piezoelectric materials placed on top of each other, are widely used to achieve precise linear movement and high-force generation. In this paper, a dynamic stiffness (DS) method for the dynamic vibration analysis of multilayer piezoelectric stacks is presented. First, the general solutions for all physical quantities of the three vibration contributions (i.e., pure vibration, symmetrically coupled vibration, and anti-symmetrically coupled vibration) are derived from the governing equations of motion. Then, the DS matrices of each layer of the piezoelectric stack are obtained, and they are assembled to form a global DS matrix. The electrical impedances and the mode shapes of a piezoelectric stack consisting of two piezoelectric disks connected in series and in parallel are calculated using our method as well as by the finite element method. The comparison shows good agreement. Finally, the effect of the number of layers on the dynamic responses of piezoelectric stacks is investigated. The DS method developed here provides an efficient and accurate analytical tool for the parametric and optimization analysis of the coupled vibrations of multilayer piezoelectric structures.

Given the strong coupling relationship between the Seebeck coefficient and electrical conductivity, achieving a significant increase in the thermoelectric power factor (PF) remains a formidable challenge. Previous studies have demonstrated that the ErAs:InGaAs system has the potential to leverage the energy filtering (EF) effect to enhance the Seebeck coefficient while maintaining high electron concentrations. Here, we present a method that significantly enhances the thermoelectric PF of InGaAs through the dual effects of quantum confinement and EF effects. Density Functional Theory calculations further demonstrate that the alterations in the ErAs band structure induce the EF effect. The results show that the ErAs quantum dots:In0.53Ga0.47As composite films exhibit high electrical conductivity and high Seebeck coefficient in the intermediate temperature range, resulting in a fivefold increase in the PF at 515 K. Our research presents a prospective approach to leveraging the combined effects for the advancement of high-performance thermoelectric materials.