Analysis Of The Formation Condition Of Lotus - Like Porous Structure In Solid / Gas Eutectic Reaction

Gasar (also known as "lotus type porous material") has been received wide attentions in the last two decades. The most important feature of the lotus type porous material is that the gas phase usually appears as rods of gas pores embedded in the solid matrix. The lotus type porous metals have the characteristic of traditional porous materials such as low density, high strength and large specific surface area, and thus has wide range of potential applications as structural and functional materials.In this study, the solidification rate and bubble rising rate during the Gasar solidification was analyzed by theoretical calculation (thermodynamic and mass transport phenomena). The results of the theoretical calcuation was compared with the experimental results in lotus type Cu, Mg and Ni fabricated by a mould casting technique in hydrogen gas atmosphere.By thermodynamic study, it is found that the relation between pore space l and solidification rate v can be discribed by vl2=A. The velocity of the solid phase is vNiMax=0.506mm/s, vCuMax=1.462mm/s, vMgMax=0.5844mm/s. for nikle, copper and magneisum respectively. It is shown that the pore space l decreases with the rising of pressure P. Solidification rate v decreases with the increasing pore space l.The minimum of bubbles rising rate at P=2atm calculated by application of stokes equation gives VGNi=0.521mm/s, vGCu=0.214mm/s and vGMg=0.024mm/s, for for nikle, copper and magneisum respectively. An ordered gas pore structure can be fabricated when the maximum of solidification rate is lower than the bubbles rising rate.It is concluded that the relations of bubbles rising rate vG, air pressure P and pore space l can be described by -,and the pore spacing (the pores radius) decreases with the increasing of gas pressure P. Bubbles rising rate vG decreases with the increasing of pore spacing.