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8. A theoretical analysis of local thermal equilibrium in fibrous materials

Author

Tian, Mingwei, Pan, Ning, Qu, Lijun, Guo, Xiaoqing, and Han, Guangting

Subjects
porous fibrous media, local thermal equilibrium, boundary conditions, material property and structure, Chemical Engineering, Mechanical Engineering, Resources Engineering and Extractive Metallurgy, Energy
Abstract

The internal heat exchange between each phase and the local thermal equilibrium scenarios in multi-phase fibrous materials are considered in this paper. Based on the two-phase heat transfer model, a criterion is proposed to evaluate the local thermal equilibrium condition, using derived characteristic parameters. Furthermore, the local thermal equilibrium situations in isothermal/adiabatic boundary cases with two different heat sources (constant heat flux and constant temperature) are assessed as special transient cases to test the proposed criterion system, and the influence of such different cases on their local thermal equilibrium status are elucidated. In addition, it is demonstrated that even the convective boundary problems can be generally estimated using this approach. Finally, effects on local thermal equilibrium of the material properties (thermal conductivity, volumetric heat capacity of each phase, sample porosity, and pore hydraulic radius) are investigated, illustrated, and discussed in our study.

Published
2015

13. Red-emissive carbon quantum dots enable high efficiency luminescent solar concentrators

Author

Liu, Guiju, Zavelani-Rossi, Margherita, Han, Guangting, Zhao, Haiguang, Vomiero, Alberto, Liu, Guiju, Zavelani-Rossi, Margherita, Han, Guangting, Zhao, Haiguang, and Vomiero, Alberto

Abstract

Luminescent solar concentrators (LSCs) are large-area sunlight collectors for efficient solar-to-electricity conversion. The key point for highly efficient LSCs is the choice of fluorophores, which need to have broad absorption, high quantum yield and large Stokes shift. Among various fluorophores, carbon quantum dots (C-dots) hold great promise as eco-friendly alternatives to heavy-metal-containing quantum dots (QDs) due to their adjustable absorption and emission spectra, non-toxicity, low cost and eco-friendly synthetic methods. However, due to the limited absorption band and relatively low quantum yield in the red region, it is a challenge to obtain efficient LSCs based on C-dots. Here, we demonstrated highly efficient LSCs based on red-emissive C-dots. The as-synthesized C-dots have a cubic structure, broad absorption covering 300-600 nm, and red emission (peak located at 595 nm), with a high quantum yield of ∼65% and a large Stokes shift of 0.45 eV. Transient absorption experiments of the C-dots revealed the ultrafast formation of the broad emissive state (1 ps). Based on the excellent optical properties of the C-dots, the as-prepared large-area LSC (10 × 10 × 0.52 cm3) exhibited an optimized external optical efficiency of 4.81% and a power conversion efficiency of 2.41% under natural sun irradiation (70 mW cm−2). Furthermore, a tandem LSC using green-emissive C-dots (top layer) and red-emissive C-dots (bottom layer) as fluorophores exhibited an external optical efficiency as high as 6.78%. These findings demonstrate the possibility of using eco-friendly carbon-based nanomaterials for highly efficient large-area LSCs., Validerad;2023;Nivå 2;2023-07-20 (sofila);Funder: Ca‘ Foscari University of Venice; Shandong Natural Science Funds for Distinguished Young Scholar (ZR2020JQ20); ÅFORSK Foundation; National Key Research and Development Program of China (2019YFE0121600)

Published
2023
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28. The effect of cooling temperature on the skin-core crystalline structure of β-nucleated polypropylene using finite element analysis

Author

Zhang Yuanming, Rong Zhou, Han Guangting, Tingting Sun, and Wei Jiang

Subjects
Polypropylene, Materials science, Nucleation, General Physics and Astronomy, 02 engineering and technology, Atmospheric temperature range, 010402 general chemistry, 021001 nanoscience & nanotechnology, 01 natural sciences, lcsh:QC1-999, 0104 chemical sciences, law.invention, Core (optical fiber), Crystal, chemistry.chemical_compound, chemistry, law, Heat transfer, Crystallization, Composite material, 0210 nano-technology, Layer (electronics), lcsh:Physics
Abstract

Heat transfer during cooling process influences the formation of crystal modification of polypropylene (PP) with nucleating agent (β-nucleated polypropylene). In this study, finite element method (FEM) was applied to simulate the cooling process in PP. Additionally, the skin-core crystalline structure of β-nucleated polypropylene cooled at different temperatures was studied by a wide angle of X-ray diffraction (WAXD). The non-thermal crystallization behaviour of β-nucleated polypropylene was further studied based on the results of FEM. The results indicated that, based on temperature field, FEM was capable of calculating the cooling rate in the skin and core layers of samples. When the cooling temperature increased from 20 °C to 100 °C, the cooling rate decreased from 702.64 °C/min to 79.04 °C/min, and from 119.49 °C/min to 40.00 °C/min in the skin layer and core layer, respectively. The difference of cooling rates from the skin layer to the core layer decreases with the increasing of cooling temperature. Correspondingly, the skin-core crystalline structure is proved to react to the gradient of cooling rate. The non-thermal crystallization also implies that the relative content of β-phase increases with the increasing of temperature, which may attribute to the longer residence time in the temperature range from 105 °C to 140 °C. Keywords: Polypropylene, Finite element method, Skin-core crystalline structure, Non-thermal crystallization behaviour, Cooling rate

Published
2017

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