Manufacturing of Porous Surfaces with Micro-Scale Features for Advanced Heat Transfer.

Abstract

Several recent heat transfer studies indicated that porous surfaces with nano/micro-scale features lead to heat transfer efficiency improvements up to 180-300%. Mass production of such surfaces, however, remains a challenge when robustness, cost-effectiveness and high productivity requirements are considered. In addition, a desire to use the existing production facilities and processes imposes another constraint onto the selection and development of the manufacturing processes to fabricate such advanced heat transfer surfaces. Powder metallurgy combined with conventional metal forming processes has been considered a good candidate for the fabrication of porous surface with micro-features. A cold compaction and incomplete sintering process for the fabrication of porous micro-features has been developed and investigated in terms of its capabilities of producing porous micro-features. This study has identified important parameters and their effects on the porous micro-feature forming process to assess the potentials for application in mass production. A finite element model for the cold compaction of powder into micro-features was developed and used to study the density distribution during compaction. The major limitation of this process is the low achievable aspect ratio. Considering the limitation of the above method, a hot compaction process for the fabrication of porous micro-feature was developed and investigated. The major advantages of this process have been found to be high achievable aspect ratio and low taper angle, which is favorable for heat transfer application. This study has also systematically studied the effects and temperature and force on porosity and strength of the formed micro-features. In addition, the possibility of producing strong and low thermal-resistant bonding between porous layer and substrate has been demonstrated. For the accurate analysis and design of the hot compaction process, proper modeling of the material diffusion behavior at the neck is necessary. A multi-particle model has been developed for the pressure assisted sintering process with boundary conditions, which simulates the pressure assisted sintering process as a function of force, temperature and time. The results have been validated with experiments.