Showing papers by "Hang Zhang published in 2021"

Recent Advances in Solar Energy Full Spectrum Conversion and Utilization

Abstract: Photovoltaic technology is a direct and effective way to utilize solar energy. The mismatch between the absorption band of solar cells and the solar light band restricts solar energy's efficient use. Full-spectrum conversion of solar energy with spectral modification and coupling solar thermal application are reviewed. Additionally, implementing machine learning (ML) methods to improve solar energy utilization is also examined. With the utilization of up-conversion materials for solar cells, the solar-energy-utilization efficiency is enhanced. Combining PV and thermal applications have been validated to be promising for improving efficiency. With the development of computer science and the urgent need to design solar energy materials and forecast PV performances, ML methods in solar energy utilization have been successfully implemented into solar energy utilization.

A Review on Flexible Thermoelectric Technology: Material, Device, and Applications

Abstract: Due to the flexibility and lightness, flexible thermoelectric (TE) technology shows great potential in the field of renewable energy and low-temperature waste heat collection. In recent years, a lot of efforts have been made to improve the efficiency of flexible TE technology, such as synthesizing high-performance flexible TE materials, improving the structure of flexible thermoelectric generators (TEGs), and optimizing the system integration design. This review comprehensively summarizes the flexible TE materials, device types, substrate selection, and fabrication techniques, aiming to reveal the latest research trend of flexible TE technology. The methods used to improve the physical properties of flexible TE materials and device design are discussed, including theoretical analysis, experimental verification, numerical simulation, and especially the potential and challenges of machine learning in flexible TE materials and devices. Besides, we summarized the applications of flexible TE technology in wearable devices, waste heat utilization of industrial heat pipes, medical sensors, Internet of Things, etc. Finally, the current research status of flexible TE technology and the prospect of the potential development of the flexible TE field are discussed.

Enhanced electrocaloric effect within a broad temperature range in lead-free polymer composite films by blending the rare-earth doped BaTiO3 nanopowders

Abstract: This paper investigates the electrocaloric effect (ECE) in polymer nanocomposite films containing ferroelectric poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer matrix and lead-free nanopowders. The nanopowders include pure BaTiO3 and rare-earth substituted Ba0.94R0.04TiO3, where R = La, Nd, Sm, prepared by a modified sol-hydrothermal method. The substitution influences the lattice parameters, but all samples exhibit a perovskite-type tetragonal phase. The dopant enhances the relaxation behavior of the matrix and decreases the current. Using the Maxwell equations, the ECE performance is systematically studied for all samples. As the rare-earth ionic radius decreases, the Curie temperature of the nanocomposite increases and the dielectric constant decreases. The isothermal entropy changes (ΔS), adiabatic temperature changes (ΔT), cooling energy densities (Q), and electrocaloric strengths versus temperature exhibit different dependences on electric field (E). The results demonstrate that rare-earth doping can effectively modify the ECE. Our composites achieve large EC strength values (|ΔT|/|ΔE|) of 7 to 21 μK m kV−1 over a wide temperature range of 25 to 60 °C and electric field range of 750 to 1250 kV cm−1 and provide typical examples of polymer ECE materials containing rare-earth doped nanopowders. The preparation of the polymer nanocomposites and the adiabatic temperature changes and the corresponding electrocaloric strengths of the samples.

Strain engineering on electrocaloric effect in PbTiO 3 and BaTiO 3

Abstract: In isothermal processes, applying the electric field to ferroelectric materials will cause the vibrational entropy change (ΔSvib) along with the corresponding adiabatic temperature change (ΔTvib) induced by the intrinsic structure response, i.e., part of the electrocaloric effect (ECE). Most previous investigations only focused on the total ECE in different materials, but we found that strain engineering can regulate the ECE significantly in the typical ferroelectrics PbTiO3 and BaTiO3. In this paper, ΔSvib and ΔTvib in PbTiO3 and BaTiO3 are extracted using first-principles calculations and the effects of strains on the ECE are then studied. The results show that the isotropic compressive and tensile strains of up to 5% could regulate the thermodynamic properties of these materials effectively. Additionally, we find that compression can cause a positive ECE, while tension can cause a negative ECE, which is further verified by the change of Born effective charge. The calculations are accelerated (> 4×) by graphics processing units (GPUs) using the Compute Unified Device Architecture (CUDA). This method thus provides a new strategy for the regulation of ΔSvib in the ECE.

Effects of Monovacancy on Thermal Properties of Bilayer Graphene Nanoribbons by Molecular Dynamics Simulations

Abstract: The monovacancy defect effect on thermal conductivity of bilayer graphene nanoribbons (BGNs) was investigated using non-equilibrium molecular dynamics (NEMD) simulations in this work. Our results demonstrate that the presence of monovacancy defect in BGNs reduces their thermal transport properties significantly. The major finding of this work shows that the calculated thermal conductivity reduces approximately linearly with the raise of monovacancy concentration. In contrast to the temperature-dependent thermal conductivity in perfect BGNs, the thermal conductivity of defected BGNs first increases and then decreases with the increasing temperature. In addition, when the difference in the monovacancy density between two layers is larger, the thermal conductivity of BGNs is higher. We also calculated the phonon density of states, phonon relaxation time and participation ratio to provide a deeper understanding of the simulation results. Our investigation confirms that the BGNs-based nano-devices could be applied in thermal management by defect engineering.

Achieving anti-sintering of supported platinum nanoparticles using a thermal management strategy

Abstract: Sintering inhibition of a catalyst at high temperatures is a challenge during heterogeneous catalysis. In this paper, we report that hexagonal boron nitride (h-BN) is an optimal material for anti-sintering γ-Al2O3-supported Pt nanoparticles (NPs) originating from the high thermal conductivity of h-BN. The high thermal conductivity of h-BN ensures maximal heat dissipation from Pt NPs to γ-Al2O3, thereby causing both Ostwald ripening and particle coalescence of Pt NPs to be decelerated at elevated temperatures. Inhibition of Pt NP sintering is also shown in the reducible TiO2-supported Pt NPs with the help of h-BN. The proposed anti-sintering strategy using thermal management is universal, providing new insight into the design of anti-sintering materials and structures for a wide range of applications in heterogeneous catalysis.