Compositions: ceramic – Ceramic compositions – Pore-forming
Reexamination Certificate
1999-05-03
2001-05-01
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Pore-forming
C501S084000, C264S086000, C264S087000, C210S500210
Reexamination Certificate
active
06225246
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to ultra and nano filtration, and in particular to a functionality gradient self-supporting ceramic structure which can be used therefor.
FIELD OF THE INVENTION
Separation membranes require some sort of backing material to achieve the necessary mechanical integrity for performance in pressurized modules. In the case of ceramic membranes, the membrane is a thin ultra-fine layer of ceramic material which is normally fused to a coarser grained ceramic substrate, commonly referred to as a support structure. The separation layer of a ceramic membrane lacks sufficient mechanical strength to stand alone, owing to its thinness and brittleness. Typically, commercially available porous ceramics are used as a starting material for subsequent coating processes for preparing fined pored membranes.
At present, there is considerable world-wide interest in the development of ceramic membrane technology. Their feature properties are their abilities to withstand high temperatures, corrosive environments and their ability to be cleaned or de-fouled with strong chemical cleaning agents and/or steam.
Mechanical requirements for ceramic membranes demand a minimum of strength, to withstand stresses arising from thermal variations, often from ambient temperatures through 1200° C.
As with any filtration technology, objectives in membrane formation are to achieve a high product flux along with a high product selectivity. In terms of membrane microstructure, this calls for a structure with high overall porosity, small trans-membrane thickness and many very small and uniform pores at the interface between the membrane and the feed medium. Using conventional forming techniques, these desirable properties are intrinsically at odds with each other, and much research effort is put into seeking workable compromises between these requirements. Alternatively, ways to circumvent the usual relationship between overall porosity, thickness and surface pore size are also sought.
In general then, it can be said that the membrane separation layer must be very fine and defect free. Membranes coated over coarse-pored substrates must be built up to sufficient thickness to “seal” the large substrate pores and ensure a defect free finish.
DESCRIPTION OF THE PRIOR ART
At present, conventional substrate ceramics are formed from mono-disperse powders. A very fine pored substrate normally has a reduced porosity and provides increased resistance (larger trans-substrate pressure drop) to product flux.
Asymmetric structures are an attractive alternative, but at present only the specialized ultra-thin Anotec™/Alcan anodized alumina membranes have this structure [1]. Substrates which are larger and/or tubular however, can not be obtained by the anodizing method used to form such membranes.
Porous ceramics are often formed from green bodies prepared by pressure methods, usually compaction or by isostatic pressing. Tubular forms are made by wet extrusion methods. In general, solid ceramic pieces are desired which are very strong, abrasion resistant and defect free. The raw ceramic powder typically has a very uniform and narrow particle size distribution, so that a very uniform microstructure can be produced in the eventual solid ceramic. High strength is most readily obtained from compacts of uniformly fine grained powder. Often, the objective in imparting the above properties to these generic materials, do not necessarily lend themselves well to membrane making.
The desirable ceramic component (solid piece) microstructure is often somewhat opposite of what might be most useful as a membrane substrate. Here, high porosity is important, and only a minimum of strength is needed. A body fused throughout (yet still quite porous) is sufficient to prevent surface abrasion, and has enough strength to withstand module pressures of up to 500 psi (~3500 kPa). Resistance to shear and torsion are less crucial, as the membranes can be housed in much stronger permeable retainers.
It is not apparent that functionally gradient substrates have been the topic of any research to date. The relative recency of research and development for ceramic membranes is such that there have been many other more immediate problems to solve. In this regard, the concepts proposed here address a new subject area.
Very small, asymmetric ceramic membranes are available commercially from Alcan International (Anotec™) [1]. These have uniform pores (available at 0.02 to 0.2 &mgr;m), formed by the very different method of anodizing sheets of aluminum metal in an electrolyte. There do not seem to be other such anodized ceramic products available. The anodizing process is limited to forming very thin pieces, and thus is not suitable for substrate fabrication. More specifically, U.S. Pat. No. 4,687,551 discloses a thin membrane, with many isolated cylindrical pores which form a finer branch-like structure at one side. A self-supporting substrate could not be made by the anodizing method employed in this reference, as the maximum possible thickness is 100 &mgr;m. These membranes have to be fused to substrates for use in more demanding (pressure, temperature, mechanical stress) environments. Further, this structure is asymmetric in the sense that there are a number of uniform cylindrical pits etched into the one side, from which a number of smaller branch-like capillaries are formed under the anodization treatment. These smaller pores penetrate the opposite surface and produce trans-structure routes. Each pore is isolated, rather than interconnected network. This structure also has distinct layers of pore fineness; a coarse region where the etched pits were made, and a very fine layer where the anodized openings were formed. This is in contrast with a continuously gradient pore size profile.
There are numerous porous ceramic materials available that have been used as membrane supports with homogeneous microstructure. For example, Toto Ltd. (Japan) has porous ceramic tubing with a 0.5 &mgr;m pore size [2].
As noted previously, the porous ceramics used for substrates are not necessarily designed to be membrane supports. Their uniform structure gives no advantage in terms of pore size at the top surface where coatings are applied. The finer the top layer, the thinner the membrane coating layer may be to avoid defects. Additionally, a uniform structure results in a uniform flux resistance across the entire thickness, and in this regard, a uniform structure with fine pores has a high flux resistance.
Uniform porous structures are meant to be machinable to an extent, and then sintered. Normally, what is desired in a ceramic component is a high strength and density, and the porous microstructures (component pre-cursors) are designed, accordingly. In view of this, the design of microstructures for ceramic components is at cross purposes with membrane making. This point is made clearer for cases where uncoated porous ceramic are used as microfiltration (MF) filters (pore sizes above 0.05 &mgr;m) or ultrafiltration (UF) filters (pore sizes between 0.002 and 0.05 &mgr;m). In these cases, the flux resistance remains constant throughout the body, even though the separation was achieved right at the membrane-fluid feed interface.
U.S. Pat. No. 4,737,323 patent actually makes reference to a Norton Ceraflow™ tubular ceramic membrane that is used in a process to extrude liposomes. The ceramic membrane that is mentioned comprises an apparently uniform microstructure ceramic substrate that has a thin membrane on one side of it. This membrane is deposited on the surface by a sol-gel method in a series of successive layers. In this sense its structure is achieved not though a continuous functionally gradient structure, but rather a series of progressively finer discrete layers which constitute the membrane coating. The substrate itself however, is uniform in structure. Often multilayer systems require multiple sinterings, which in general, are detrimental for attaining a high porosity.
SUMMARY OF THE INVENTION
According
Anderson Jiwayne
Marcantoni Paul
National Research Council of Canada
LandOfFree
Functionally gradient ceramic structures does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Functionally gradient ceramic structures, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Functionally gradient ceramic structures will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2476537