For fabricating a porous ceramic, the freezing affords several advantages over traditional tape casting:
By use of the freezing bed, the water in the slip is frozen, from the bottom up through the thickness, immediately after the tape has been cast. (Temperature profile is indicated for an aqueous system) An apparatus for freeze tape casting (see figure) is basically a traditional tapecasting apparatus augmented with a freezing bed. The frozen tape is then freeze-dried before sintering. In Freeze Tape Casting, a slip is cast into a tape as in traditional tape casting, but then, unlike traditional tape casting, the water and any other solvents in the tapeare frozen. Freeze tape casting expands this range to greater than 3 mm, thus eliminating the need for lamination steps in traditional processing. Traditional tape casting has been used to make ceramic sheets, for electronic and structural applications, ranging in thickness from 5 μm to 1,000 μm.
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The slip may also contain one or more organic solvent(s) plus a significant quantity of organic binders that make the tape strong and flexible after evaporation of the solvent( s). Freeze tape casting offers similar benefits for fabrication of optimally porous ceramics for use as catalysts, gas sensors, and filters.įreeze tape casting includes, and goes beyond, traditional tape casting, in which an aqueous ceramic slip (ceramic and/or ceramic precursor particles suspended in water) is cast onto a poly(ethylene terephthalate) or poly(tetrafluoroethylene) carrier film by use of a doctor- blade assembly. Freeze tape casting was developed to enable optimization of the microstructures of porous ceramic components for use as solid oxide electrodes in fuel cells: Through alignment and grading of pores, one can tailor surface areas and diffusion channels for flows of gas and liquid species involved in fuel-cell reactions.
Glenn Research Center, Cleveland, Ohioįreeze tape casting is a means of making preforms of ceramic sheets that, upon subsequent completion of fabrication processing, can have anisotropic and/or functionally graded properties that notably include aligned and graded porosity.
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