Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1998-08-25
2001-05-08
Marcantoni, Paul (Department: 1755)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S608000, C423S610000, C501S012000, C501S080000, C501S103000, C501S133000
Reexamination Certificate
active
06228340
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of porous ceramics.
BACKGROUND OF THE INVENTION
Ceramics with controlled porosity find wide applications as catalytic surfaces and supports, adsorbents, chromatographic materials, filters, light weight martials, and thermal and acoustic insulators. In catalytic applications the macropores facilitate material transport to the nanoporous internal regions where reactions can take place. Macroporous silica could be of substantial use as insulating layers in integrated circuits. The low dielectric constant of this material lowers the capacity of the chips which makes them faster. Furthermore, ceramics with regular arrays of pores have unique optical properties such as optical filters which have strong wavelength dependent reflectivity and transmission. They are also candidates for photonic band gap materials. See Yablonovitch, E.
J. Opt. soc. Am.
B 10, 2830295 (1993); and Joannopoulos, J. D., Meade, R. D. & Winn, J. N.
Photonic Crystals: Molding the Flow of Light,
1-137 (Princeton University Press, Princeton, 1995).
Porous materials are commonly categorized according their (average) pore size. Microporous materials have pore diameters ≦2 nm, mesoporous ones have pores in the range of about 2-50 nm, and macroporous materials contain pores ≧50 nm. See IUPAC Manual of Symbols and Terminology, Appendix 2, Part 1, Colloid and Surface Chemistry,
Pure Appl. Chem.
31, 578 (1972). An extension of these materials to the mesoporous regime is provided by the class of materials known as MCM-41. These mesostructures have channel diameters of 2-10 nm and thus are the extension of zeolites, with pore diameters <2 nm. However, control over the size, shape, and ordering of pores larger than 10 nm has remained a challenge. See J. C. Jansen, M. Stöcker, H. G. Karge, and J. Weitkamp (editors), “
Advanced Zeolite Science and Applications”,
Studies in Surface Science, Vol. 85, (Elsevier, Amsterdam, 1994); C. T. Kresge et al.,
Nature
359, 710 (1992); and J. S. Beck et al., J. Am. Chem. Soc. 114, 10834 (1992); J. S. Beck et al., U.S. Pat. No. 5,108,725 (1992). These aluminosilicates are prepared through a liquid crystal mechanism in which a sol-gel process takes place in the interstitial regions of an ordered surfactant phase formed by self-assembly of rodlike micelles acting as templates. See Krauss, T., Song, Y. P., Thomas, S., Wilkinson, C. D .W. & DelaRue, R. M.
Electron. Lett.
30, 1444-1446 (1994). This results in cubically or hexagonally ordered pores, the size of which can be controlled by varying the surfactant and the amount of solubilized additives. Pore sizes are in the range of 1-10 nm. This class of materials has been extended with a number of transition metal oxides. See D. M. Antonielli and J. Y. Ying,
Angew. Chem. Intl. Ed. Engl.
34(18), 2014 (1995); 35(4), 426 (1996).
For larger pore sizes, most notably in the macroporous regime, no method is known to produce ceramics containing periodic pores. It has been possible, however, to produce pores with a well-defined (spherical) shape, namely by the foaming method and by the hollow sphere sintering method. The first method uses a colloidal sol or a powder slurry containing a surfactant which is foamed with a gas and then gelled by a sol-gel reaction. See T. Fujiu, G. T. Messing, and W. Huebner,
J. Am. Ceram. Soc.
73(1), 85 (1990); M. Wu, T. Fijiu, and G. L. Messing,
J. Non-Cryst. Solids
121, 407 (1990). In the second method, hollow microspheres are first blown out of a molten oxide (e.g. glass) which are then sintered together or incorporated into a ceramic matrix. For a review, see R. L. Downs, M. A. Ebner, and W. J. Milner, In: L. C. Klein (Ed.), “
Sol-Gel Technology for Thin Films, Fibers, Performs, Electronics, and Specialty Shapes
”, (Noyes Publications, Park Ridge, N.J., 1988), pp 330-381. In both cases large pores are produced in the range of roughly 10 to 1000 &mgr;m. Pore sizes are generally broadly distributed and do not allow for assembly into regular lattices.
Finally, some colloidal microspheres (most notably silica) can be made sufficiently monodisperse such that they form regular (cubic ) arrays under the right circumstances. These packings can then be sintered to a ceramic. See T. J. Garino and H. K. Bown,
J. Am. Cera. Soc.
70, C311, C315 (1987); A. P. Philipse,
J. Mater. Sci. Lett.
8(12), 1371 (1989). Porosities of these packings are low, however (around 26%), and not easily controlled.
SUMMARY OF THE INVENTION
The present invention enables the production of porous ceramics in which the pore size is controlled and can be varied continuously in, the range of 0.05 &mgr;m to 5 &mgr;m, preferably from one tenth to three micrometers, and the pore size distribution can also be varied as desired.
The invention uses droplets broadly of an aqueous or nonaqueous emulsion as templates around the exterior surface of which material is deposited through a sol-gel process. Subsequent drying and heat treatment yields materials with spherical pores left behind by the emulsion droplets. It is preferred to use nonaqueous emulsions, and as an embodiment of the invention, an oil-immiscible polar liquid was used to replace water using existing surfactants.
By starting with an emulsion of equally-sized droplets (produced through a repeated emulsion fractionation procedure), pores with a uniform and controllable size are obtained. A small amount of a totally insoluble material, such as silicone oil, is added to the oil phase to limit Ostwald ripening. Self-assembly of these droplets into a colloidal crystalline phase leads to a ceramic which contains an ordered array of pores. A narrow size distribution enables the pores to be highly ordered, reflecting the self-assembly of the original monodisperse emulsion droplets into a nearly crystalline array. See Pusey, P. N. & van Megen, W. Phase Behavior of Concentrated Suspension of Hard Colloidal Sphers.
Nature
320, 340-342(1986). Binary crystals (alloy structures) are also possible in the case of a suitably chosen mixture of monodisperse droplets with a different size. The method takes advantage of the fact that the droplets are both highly deformable and easily removable, which prevents the gel from cracking as it becomes stressed during, respectively, aging and drying. Furthermore, emulsions can be made with droplet volume fractions from near zero to as high as 99%. The emulsion fractionation procedure is known and enables one to separate droplets according to their size, thus allowing perfect control over the size and size distribution of the pores in the resulting material.
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Imhof Arnout
Lange Fred F.
Pine David J.
Fulbright & Jaworski
Marcantoni Paul
The Regents of the University of California
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