Synthesis of Porous Ceramics Through Directional Solidification and Freeze-Drying
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Synthesis of Porous Ceramics Through Directional Solidification and FreezeDrying Predrag Kisa, Patrick Fisher, Al Olszewski, Ian Nettleship, Nicholas G. Eror; Materials Science and Engineering Department, 848 Benedum Hall, University of Pittsburgh, Pittsburgh PA 15213 USA ABSTRACT This study investigated the microstructural characteristics of directionally solidified freeze-dried silica sols. Porous structures were formed by depositing silica sol on silicon (100) single crystals. The deposited sols were unidirectionaly solidified by placing the silicon substrate on a copper block immersed in liquid nitrogen and then subsequently freeze-dried. Freeze drying removal of ice crystals created three-dimensional pore channels ranging from 3 to10 micrometers in diameter aggregated in grain like colonies 50-100 micrometers in diameter. Pore size, spacing, colony size and microstructure were determined using optical microscopy (OM) and scanning electron microscopy (SEM) while the structure of the amorphous SiO2 was characterized by X-ray diffraction (XRD). The microstructure results are compared and contrasted with silica aerogel obtained through conventional processing using supercritical CO2. INTRODUCTION Recent studies have shown that directional freezing of aqueous sols, suspensions and gels coupled with freeze drying can be used to develop unique directional pore structures in ceramics materials. The initial investigation of pattern formation by freezing aqueous colloid systems was performed by Luyet [1]. Though his primary interest was behavior of ice crystals in biological samples his research gave the first systematic analysis of ice patterns formation in colloids. Since then, freeze-gelation has been used in the processing of porous ceramics [2]. Examples involving silica include the casting of an aqueous silica sol containing finely dispersed alumina ceramic fillers, utilizing growth of fine equiaxed ice crystals [3]. Low-k porous silica thin films have also been processed from sols by coupling freeze drying and directional solidification [5-6]. Freeze-drying generally helped avoid the shrinkage associated with drying and reduce the problem of cracking. Magnesium sulfate aqueous solution freeze-drying had been used for the formation of catalyst supports [4]. Unfortunately, salt solutions suffer from back melting of salt rich ice as the evaporating surface recedes during sublimation. This study investigated the controlled solidification of ice through a nanoparticulate colloid as a viable way of making highly porous ceramics with aligned pores and high surface area. An aqueous silica sol with an average particle size of 20 nm was chosen so that the effect of solidification on pore morphology could be studied. Additionally only one concentration of sol – 40vol% was used throughout the study.
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EXPERIMENTAL DETAILS Two sets of experiments were performed; in the first set of experiments silica sol (Ludox – Grace Davison) droplets were placed on a silicon (100) single crystal wafer and frozen on the coppe
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