Huijun Wu

Engineering thermal and mechanical properties of flexible fiber-reinforced aerogel composites

Flexible aerogel was successfully supported onto 4-layer-aligned glass fibers through impregnation, followed by drying at atmospheric pressure. The prepared nanocomposites can achieve a low thermal conductivity of 0.026xa0W/(m·K), which holds great promising for their use in thermal insulation applications. By choosing different designs of the four fiber layers, in the terms of LLLL, LTLL, LLTT, LTLT, LTTL and LTTT, the laminated structures of aerogel composites can be accordingly controlled. Further microstructure characterization of the composites revealed the homogeneous dispersion of nanoporous structure through the aerogel matrix, as well as highly-aligned fibers to reinforce the structure. Detailed investigation on the thermal and mechanical properties of the prepared fiber-aerogel composites showed that their performances were highly related to the laminated design of fiber layers. The introduction of fibers as the backbone of the aerogel and the suitable design of their laminated structure can greatly improve the mechanical properties of the aerogel composites while maintaining a low thermal conductivity.

Engineering Thermal and Mechanical Properties of Multilayer Aligned Fiber-Reinforced Aerogel Composites

This paper demonstrates a controllable way to fabricate multilayer aligned fiber-reinforced aerogel composites with low thermal conductivity (e.g., 0.022–0.028 W m−1 K−1) and greatly improved bending and compressive strengths compared to pure aerogels. Four-layer aligned fibers reinforced aerogel composites with six laminated structures were successfully synthesized by impregnating four layers of aligned glass fibers in silica aerogels via sol–gel technique and drying at ambient pressure. The fiber alignments and the laminated structures greatly affected the compressive and bending strengths of the aerogel composites. For instance, as the orthogonal number of the fiber layers was increased, the bending strength and stability of the aerogel composites were improved while the compressive strength of the aerogel composites was degraded. A heat transfer model based on the unit cell of surface contact hollow cubic structure and the parallel law of equivalent thermal resistance concerning the as-prepared aerogel composites was proposed to explore the heat transfer mechanism at three directions in the aerogel composites. The calculated thermal conductivities were validated with the experimental results. The findings open a possible way to controlling the thermal and mechanical properties of the fiber-reinforced aerogel composites by regulating the fiber alignments and the laminated structures.

Modeling and analysis on the cooling energy efficiency of sun-shading of external windows in hot summer and warm winter zone

In hot summer and warm winter zone, decreasing the inferred transmittance of building glass is an effective way to reduce the shading coefficient of external windows and increase the cooling energy efficient rates. Based on an office building model in Guangzhou, the DeST-c code was used to study the effects of the inferred transmittance of building glass on the cooling energy efficient rates. The results show that the cooling energy efficient rates could be significantly increased while the indoor natural lighting could not be degraded by reducing the inferred transmittance of the building glass. As the inferred transmittance of building glass decreases from 1 to 0 and the visible transmittance keeps at 0.7, the cooling need of the office building reduces by 23.3%. Moreover, the annual cooling energy efficient rate increases by form 0.67% to 18.3% compared with the cooling space load of the office building with the shading coefficient of 1.0. By making hourly analysis on the variations laws of the cooling energy efficient rates at all hours in need of cooling load in a year, it can be found that the cause of the significant increase of annual cooling energy efficient rates is that as the inferred transmittance decreases, the hourly cooling energy efficient rates increase significantly for the hours with higher cooling load. The research results can provide references for the optimization of aerogel glass sun-shading performance and the building cooling energy efficiency in South China.

Simulation and Evaluation of an Open-Type Thermal Storage System Using Composite Sorbents for Building Efficiency

A family of composite sorbents has been acknowledged as promising thermal storage materials for low grade thermal energy storage owing to its high specific storage capacity and low regenerating temperature. This paper reports a simplified numerical model aiming to determine the dynamic characteristics of the composite sorbents and evaluate the specific capacity and COP for thermal energy storage. The computational results were validated with the experimental measurements carried out on an open-type thermal storage set-up for the composite sorbents. By using the simple model, the dynamic characteristics of the composite sorbents with different CaCl2 content were determined. The effects of CaCl2 content in the composite sorbents and the operating parameters on the specific capacity and the coefficient of performance of the open-type sorption system were evaluated for low grade heat storage and building efficiency applications.

Engineering Surface and Optical Properties of TiO2-Coated Electrospun PVDF Nanofibers Via Controllable Self-Assembly

Understanding the effect of a porous TiO2 nanolayer on the optical scattering and absorption through electrospun fibers is of great importance for the design and development of advanced optical extinction materials. Based on electrospinning and controllable self-assembly techniques, pure electrospun poly(vinylidene fluoride) (PVDF) fibers and TiO2-coated ones with different self-assembly cycles were prepared. The effect of TiO2 self-assembly cycles on surface parameters, e.g., thickness, assembled content, and porosity of the TiO2 nanolayer were determined by scanning electron microscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy. With an increase in the self-assembly cycles, the TiO2-coated electrospun PVDF fibers presented rougher surfaces and greater average diameters. According to the characterized surface parameters, the effects of the controllable self-assembly on the optical refractive index, absorption index, and infrared extinction were investigated to increase the optical properties of electrospun PVDF fibers. The results indicated that an increase of almost 120–130 cm−1 in infrared extinction could be achieved through the controllable self-assembly with only 5.7 wt. % assembled TiO2 content. This is highly efficient when compared with other coating modes. We believe that this study could give some positive guidance in the design of TiO2-coated electrospun fibers for improving their surface and optical properties.

Self-Assembly Synthesis of Silver Nanowires/Graphene Nanocomposite and Its Effects on the Performance of Electrically Conductive Adhesive

Among recent advances in electronic packaging technologies, electrically conductive adhesives (ECAs) attract most researchers’ attention, as they are environment-friendly and simple to apply. ECAs also have a lower operating temperature and volume resistivity compared with conventional electronic conductive adhesives. In ECAs, the conducting fillers play a significant role in improving conductivity and strength. In this work, as filler additives, the silver nanowires/graphene nanocomposites (AgNWs-GNs) were successfully fabricated via a facile self-assembly method. The characteristics of the as-prepared nanocomposites were evaluated by FTIR (Fourier Transform infrared spectroscopy), XRD (X-ray Diffraction), XPS (X-ray photoelectron spectroscopy), TEM (Transmission electron microscope) and Raman tests, demonstrating a successful synthesis process. Different amounts of AgNWs-GNs were used as additives in micron flake silver filler, and the effects of AgNWs-GNs on the properties of ECAs were studied. The results suggested that the as-synthesized composites can significantly improve the electrical conductivity and shear strength of ECAs. With 0.8% AgNWs/GNs (AgNWs to GO (Graphite oxide) mass ratio is 4:1), the ECAs have the lowest volume resistivity of 9.31 × 10−5 Ω·cm (95.4% lower than the blank sample without fillers), while with 0.6% AgNWs/GNs (AgNWs to GO mass ratio is 6:1), the ECAs reach the highest shear strength of 14.3 MPa (68.2% higher than the blank sample).

Effect of Thermal Properties of Building Glass on Cooling Energy Consumption of Buildings

Taking an office building in Guangzhou, China as example, the building environment simulation software DeST-c is used to study the effect of thermal properties of building glass on the cooling energy consumption. The results show that decreasing the shading coefficient of building glass can significantly reduce the cooling energy consumption. However, the heat transfer coefficient of building glass has little effect on the cooling energy consumption. The mechanism of the reduction of cooling energy consumption caused by the decrease of the shading coefficient is also discussed through hourly analysis on the cooling energy efficient rates all the year round.