Abstract
Freeze foaming is a method to manufacture cellular ceramic scaffolds with a hierarchical porous structure. These so-called freeze foams are predestined for the use as bone replacement material because of their internal bone-like structure and biocompatibility. On the one hand, they consist of macrostructural foam cells which are formed by the expansion of gas inside the starting suspension. On the other hand, a porous microstructure inside the foam struts is formed during freezing and subsequent freeze drying of the foamed suspension. The aim of this work is to investigate for the first time the formation of macrostructure and microstructure separately depending on the composition of the suspension and the pressure reduction rate, by means of appropriate characterization methods for the different pore size ranges. Moreover, the foaming behavior itself was characterized by in-situ radiographical and computed tomography (CT) evaluation. As a result, it could be shown that it is possible to tune the macro- and microstructure separately with porosities of 49–74% related to the foam cells and 10–37% inside the struts.
Highlights
Ceramic foams cover a wide range of applications, including as a support material for catalysts [1,2,3], pore burners [4], and thermal insulators [5], as well for waste water treatment [6], metal filtration [7], and in scaffolds for bone substitute [8,9,10]
This freeze foaming process does not require the use of organic templates and pore formers and is important for the production of ceramic foam structures
The aim of this study was to determine the influence of the suspension composition and its resulting rheological behavior on the hierarchical porous foam structure consisting of foam cells and strut pores
Summary
Ceramic foams cover a wide range of applications, including as a support material for catalysts [1,2,3], pore burners [4], and thermal insulators [5], as well for waste water treatment [6], metal filtration [7], and in scaffolds for bone substitute [8,9,10] They can be manufactured, e.g., by the polymeric sponge method [11], direct foaming methods [12,13], or freeze casting [14,15,16]. Possible applications cover a wide range from biomedical uses, e.g., artificial bones, support material for catalysts, pharmaceutical products, as well as thermal insulators [17,18,19] The diversity of these applications results from the range of initial materials (ceramics, metals, metalorganic frameworks), variable starting suspensions, and the resulting foam structure properties. The latter is characterized, e.g., by the cell geometry, the cell size distribution, the proportions of open and closed cells, and the type of cell struts, which are formed either by pore-forming gases (air or steam) or by ice crystals
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