Showing papers by "Bao Yang published in 2013"

Localized deformation in aluminium foam during middle speed Hopkinson bar impact tests

Abstract: Split Hopkinson pressure bar (SHPB) technique is a widely adopted method to study the dynamic mechanical behaviors of materials. However, the strong localization of the deformation in aluminium foams (near the ends of the specimen) may invalidate the assumption of uniform strain within the specimen granted in the analysis of SHPB results. To appraise how critical this issue could be, the deformation process of aluminium foam under middle speed impact was investigated by combining SHPB technique with high speed photography. Based on the experimental results, finite element analysis was carried out to explore the characteristics of the localized deformation in aluminium foam. The strain distribution was quantitatively analyzed and showed the dependence on the imposed strain rate. An evaluation of the deformation localization by defining maximum loacalized strain and strain rate indicates that the maximum strain and strain rate in the specimen can be more than 100% higher than the average values.

Synthesis and Characterization of Solid-State Phase Change Material Microcapsules for Thermal Management Applications

Abstract: Polyalcohols such as neopentyl glycol (NPG) undergo solid-state crystal transformations that absorb/release sufficient latent heat. These solid-solid phase change materials (PCM) can be used in practical thermal management applications without concerns about liquid leakage and thermal expansion during phase transition. In this paper, microcapsules of NPG encapsulated in silica shell were successfully synthesized with the use of the emulsion technique. The size of the microcapsules was in the range of 0.2–4 μm, and the thickness of the silica shell was about 30 nm. It was found that the endothermic event of the phase change behavior of these NPG-silica microcapsules was initiated at around 39 °C and the latent heat was about 96.0 J/g. A large supercooling of about 43.3 °C was observed in the pure NPG particles without shell. The supercooling of the NPG microcapsules can be reduced to about 14 °C due to the heterogeneous nucleation sites provided by the silica shell. These NPG microcapsules were added into heat transfer fluid PAO to enhance its heat capacity. The effective heat capacity of the fluids can be increased by 56% by adding 20 wt. % NPG-silica microcapsules.Copyright © 2013 by ASME

Nanostructured phase-changeable heat transfer fluids

Abstract: Abstract Cooling is one of the most important technique challenges faced by a range of diverse industries and military needs. There is an urgent need for the innovative heat transfer fluids with improved thermal properties over the currently available. This review paper discusses the concept of using phase-changeable nanoparticles to increase the effective heat capacity and the heat transfer rate of the fluid. A large amount of heat can be absorbed or released when these nanoparticles undergo phase transition from solid to liquid or liquid to gas or vice versa and, thus, enhancing the heat transfer rate. Two types of phase-change fluids are introduced: one contains liquid nanodroplets that will evaporate at elevated temperatures or solidifies at reduced temperatures, called “nanoemulsion fluids”; the other is suspensions of solid-liquid metallic phase-change nanoparticles. The material synthesis and property characterizations of these phase-changeable fluids are two main aspects of this paper. The explosive vaporization of the dispersed nanodroplets would significantly improve the heat transfer in the nanoemulsion fluid. The solid-liquid metallic phase-change nanoparticles will increase the effective heat capacity and thermal conductivity of the base fluids simultaneously. This paper also identifies the several critical issues in the phase-changeable fluids to be solved in the future.

A Nanocomposite Approach: Lowering Thermal Conductivity of Si-Ge Thermoelectric Materials for Power Generation Applications

Abstract: Solid-state thermoelectric power generation devices have many attractive features compared with other methods of power generation, such as long life, no moving parts, no emissions of toxic gases, light weight, low maintenance, and high reliability. The first part of this paper discusses the basic thermoelectric effects and their underlying physics, and the second part introduces nanostructured thermoelectric materials and their characteristics. Especially, the synthesis and characterization of two-component Si-Genanocomposites have been discussed in details. This nanostructured approach is easily scalable and can be used to enhance the dimensionless figure-of-merit ZT of thermoelectric materials resulting in a more efficient thermoelectric devices.