If you are like me, ceramic isn’t the first material that pops into my mind when you hear the word “foil.” But, a quartet of researchers from ETH Zurich’s Department of Materials say they have mastered “a set of easy and versatile film deposition techniques” that allow them to create remarkable ultrathin, sheer, free-standing ceramic foils that can be manually handled.
Now, to be sure, ceramic thin “foils” aren’t particularly novel. They are already used in protective films, insulating layers, optical coatings, multilayer capacitors and functional thin films.
The problem with previous foil-making techniques is that the resultant foils are very fragile. In fact, when thicknesses of less than 25 micrometers are needed, existing techniques actually have required that the foils be processed on a supporting substrate and stay attached to at least part of the substrate. Thus, either a porous substrate is required or the substrate must be etched to expose the ceramic material. Although careful etching can reduce the thickness to about 50 nanometers, the active area is limited to few hundred micrometers. Looking at it another way, the largest values of aspect ratio (dividing a dimension, such as length, width or diameter by the thickness of the material) obtain with these current techniques are typically less than 1000.
The new research, however – published in the “Early View” edition of the Journal of the American Ceramic Society (DOI: 10.1111/j.1551-2916.2010.03927.x) – indicates that flexible, free-standing foils composed of most ceramic materials and glass can readily be made as thin as 0.2 micrometers and with high aspect ratios. They also made pieces of 0.5-micrometer-thick foils with areas of several square centimeters. These achievements effectively open a new door to devices and materials that incorporate these foils.
This new method involves creating green films by either spin coating or dip coating onto substrates such as graphite foils, glass carbon and sapphire disks coated with carbon. The choice of carbon-based substrates allows the foils to detach from the substrates fairly easily. The film-coated substrate is then sintered.
The researchers – who include ACerS Fellow Ludwig Gauckler – tested both one-dimensional and three-dimensional sintering, and, in brief, each sintering method had advantages and disadvantages, somewhat depending on the ceramic material, desired properties, wrinkle tolerance, etc. For example, they showed they are able to create Y-TZP foils of various thicknesses (3-14 micrometers) by sintering the green film at relatively low temperatures. The resultant foils are porous, self-supporting and crack-free, and potentially useful for filtering applications,
In general, the alumina and Y-TZP thin foils they created are strong enough to be manipulated with tweezers, are flexible enough to be formed into a roll and are transparent due to the presence of only a few light-scattering grain boundaries (and in some case display interference colors). As can be seen in the photos above, the foil in these experiments can have relatively large areas.
To sum up the mechanical properties of these new foils, the authors write:
“The largest micrometer-thin foils obtained had approximately the same aspect ratio (∼30,000) as a 1-m-thick unsupported bridge spanning the Strait of Dover between France and England would have.”
They also say the best is yet to come. They already are working on tinkering with various sintering parameters and developing green films with more homogeneity, and they say that even thinner, flatter defect-free ceramic foils are within reach.
The group also notes that large-scale fabrication is foreseeable using automated deposition techniques, such as tape casting, electrophoretic deposition or traditional dip coating to deposit the green film.