Session: 03-05-03: Design, Material Processing, and Applications of Metal and Ceramic Composites

Paper Number: 88559

88559 - Using Freeze-Casting Method to Create Lamellar Copper Structures – An Experimental Study of the Freezing Behavior of Cupric Oxide Colloidal Suspensions 

Composites formed by infiltrating lamellar copper with organic phase change materials (PCMs) offer a  promising solution for efficient thermal management in modern electronics, mainly due to the high anisotropic thermal conductivity in conjunction with a large heat exchange area of the copper matrix. The limitation is to control the pore morphology and structural specifics across length scales in bulk porous materials. Freeze casting is a flexible technique capable of fabricating materials with complex three-dimensional porous structures tailorable for specific applications. In particular, the directional freezing of aqueous colloidal suspensions enables highly anisotropic ice crystals to grow, repelling particles into the space between ice crystals. Removal of ice by sublimation leaves directional pore morphology with lamellar or cellular characteristics. Owing to this unique capability to control structural specifics, freeze casting has been widely used to synthesize ceramic, polymeric, and metallic materials for a wide range of applications. However, only limited research has reported lamellar porous copper by the freeze casting method. To avoid oxidation of copper particles, cupric oxide nanoparticles were used in freeze casting. The cupric oxide green structures were then reduced to copper by sintering in a hydrogen environment. The main challenges are (1) the preparation of well-dispersed and stable suspensions of particles with high solid loading, and (2) the lack of understanding of the solidification behavior and microstructure formation during the freezing stage.

This study[LZ1]  investigates the directional solidification behavior and the effects of suspension composition, freezing temperature, and freezing rate on the formation of porous structure. The aqueous suspension of CuO with 15 vol.% solid is prepared by mixing CuO dry powder (with a nominal size of 80 nm in diameter) in deionized water with polyvinylpyrrolidone (PVP, 1 – 3 wt.% of dry powder[CK2] ) used as a dispersant. Polyvinyl alcohol (PVA) is added as a binder and potassium hydroxide is used to adjust the pH values in the range of 8 – 11. Based on the obtained measurement of the suspension viscosity, the optimal amount of the dispersant and the range of [CH3] pH value is  identified to enable a well dispersed and stable suspension of CuO particles with high solid loading. The freezing curves [CH4] of suspensions with the optimal composition are measured to obtain the freezing temperature of the suspension as well as the effect of the dispersant and the electrolyte on the freezing behavior. The suspensions then undergo directional freezing under various initial freezing temperatures and freezing rates. After sublimation under a near vacuum pressure, the green samples are infiltrated with epoxy to reveal the pore morphology, pore size, and pore density.

We believe that the obtained relationship among the suspension composition, freezing conditions, and the structural specifics of freeze-cast porous CuO, lays the groundwork for freeze-cast porous copper with desirable pore morphology and structures for thermal engineering.

Presenting Author: Ronghui Ma Univ Of Maryland

Presenting Author Biography: Dr. Ma is currently an associate professor in the Department of Mechanical Engineering at University of Maryland Baltimore County

Authors:

Christopher Kasprzak University of Maryland Baltimore County
Christina Hoffman UMBC
Ruey-Hung Chen University of Maryland Baltimore County
Liang Zhu UMBC
Ronghui Ma Univ Of Maryland