Showing papers by "Bao Yang published in 2014"

Highly Thermally Conductive Papers with Percolative Layered Boron Nitride Nanosheets

Abstract: In this work, we report a dielectric nanocomposite paper with layered boron nitride (BN) nanosheets wired by one-dimensional (1D) nanofibrillated cellulose (NFC) that has superior thermal and mechanical properties. These nanocomposite papers are fabricated from a filtration of BN and NFC suspensions, in which NFC is used as a stabilizer to stabilize BN nanosheets. In these nanocomposite papers, two-dimensional (2D) nanosheets form a thermally conductive network, while 1D NFC provides mechanical strength. A high thermal conductivity has been achieved along the BN paper surface (up to 145.7 W/m K for 50 wt % of BN), which is an order of magnitude higher than that in randomly distributed BN nanosheet composites and is even comparable to the thermal conductivity of aluminum alloys. Such a high thermal conductivity is mainly attributed to the structural alignment within the BN nanosheet papers; the effects of the interfacial thermal contact resistance are minimized by the fact that the heat transfer is in the ...

Investigation on the reaction of iron powder mixture as a portable heat source for thermoelectric power generators

Abstract: This paper reports our investigation on the thermal behavior and ignition characteristics of iron powder and mixtures of iron with other materials such as activated carbon and sodium chloride in which iron is the main ingredient used as fuel. Thermal analysis techniques such as differential scanning calorimetry (DSC) and thermogravimetric analysis were used to characterize the materials and for further understanding of reaction kinetics of the pyrophoric iron mixtures. The experimental results demonstrated that iron micron particles react exothermically to the oxygen in atmosphere and produced iron oxide with ignition temperature of 427.87 °C and heat generation of 4,844 J g−1. However, in this study, the pyrophoric iron mixture acts as a heat source for the thermoelectric power generators, the final mixture composition is determined to compose of iron powder, activated carbon, and sodium chloride with the mass ratio of approximately 5/1/1. The mixture generated two exothermic peaks DSC curves that showed ignition temperature of 431.53 and 554.85 °C and with a higher heat generation of 9,366 J g−1 at higher temperature. The effects of test pan materials and heating rate on the ignition were also examined by DSC method. Kinetic data such as the activation energy (Ea), the entropy of activation (ΔS#), enthalpy of activation (ΔH#), and Gibbs energy of activation (ΔG#) on the ignition processes was also derived from the DSC analysis. From the ignition temperature, heat generation, and kinetics test data, the mass ratio of 5/1/1 proved to generate the most amount of heat with high temperatures for the standalone thermoelectric power generators.

The Deformation Measurement and Analysis on Meso-Structure of Aluminum Foams During SHPB Test

Abstract: The split Hopkinson pressure bar (SHPB) is most widely used to measure the dynamic mechanical properties of materials. Such a testing methodology implies the assumption of uniform deformation during an impact test. However, the experimental verification of this assumption for aluminum foams has, as yet, been unreported. In this paper, a test system combining the SHPB with a high-speed digital camera was designed and constructed to study the problem. In the system, the synchronization between SHPB and the high-speed digital camera, the lighting, and the surface treatment of specimen are established. The deformation of meso-structure of aluminum foams during the SHPB impact was observed successfully; furthermore, the localized strains along the specimens were measured quantitatively. Experimental results show that the deformation is non-uniform; that means the assumption of uniform deformation for aluminum foam is not well satisfied. Therefore, a need exists for some modifications to characterize the dynamic mechanical properties of aluminum foams by SHPB.

Phase Change Material Particles and Their Application in Heat Transfer Fluids

Abstract: Phase change materials (PCMs) have received considerable attention for the application of thermal energy storage and transfer. This chapter discusses synthesis and characterization of several types of PCM particles, as well as the use of PCMs to enhance the performance of heat transfer fluids. Two different PCM microcapsules are introduced first: one comprises solid–liquid PCM paraffin encapsulated in polymer shell; the other involves solid–solid PCM neopentyl glycol (NPG) core and silica shell. Then the synthesis of low-melting metallic nanoparticles and NPG nanoparticles without shells are discussed. The last part of this chapter is dedicated to a new type of phase-changeable fluids, nanoemulsion fluids, in which the dispersed nanodroplets can be liquid–vapor PCM or liquid–solid PCM, depending on the PCM properties and the operating temperature. Material synthesis and property characterizations of these phase-changeable fluids are two main aspects of this chapter.

Experimental Study of Phase-Changeable Water/Polyalphaolefin Nanoemulsion Fluids

Abstract: In this work, thermal properties especially phase change heat transfer properties of one new type of nanostructured heat transfer fluid: Water/Polyalphaolefin (PAO) nanoemulsion fluid are investigated. Water is added into PAO fluid to form nanoemulsion fluids in which dispersed water nanodroplets are formed by self-assembly.The liquid-to-vapor phase change results, expressed in terms of surface heat flux and heater temperature, have shown that the presence of water nanodroplets has a drastic impact on the liquid-to-vapor phase change behavior of the nanoemulsion fluid studied: the water nanodroplet formed inside can enhance its heat transfer coefficient by over 300% after the incipience of its phase change. In addition to that, the vaporization of the water nanodroplet inside is found to be different depending on the concentration of water inside, which happens to coincide with the structure change with different water concentrations as observed in SANS measurement. On the other hand, the effective specific heat is also found to increase with higher water concentration until reaching a maximum value which also happens to coincide with the structure transition from spherical to cylinder shape with the increasing of water concentrations as observed from SANS measurement. More study is still needed to understand the mechanism behind these phenomena.Copyright © 2014 by ASME