POROUS CERAMIC STRUCTURES AND METHODS OF MAKING POROUS CERAMIC STRUCTURES

Abstract: The manufacture of porous ceramic structures such as membranes and filters from preceramic polymer precursors such as polyorganoborosilazanes is disclosed. A method comprising the steps of precipitating a preceramic polymer from a casting solution, then pyrolyzing the ceramic structure to form a porous ceramic is used. Structures with a voids volume of greater than 50 % are obtained with the method. Potential uses for the porous structures include ultrafiltration and nanofiltration.

Description: POROUS CERAMIC STRUCTURES AND METHODS OF MAKING POROUS CERAMIC STRUCTURES TECHNICAL FIELD OF THE INVENTION The present invention relates to porous ceramic structures and methods of making a porous ceramic structure . BACKGROUND OF THE INVENTION Organic membranes are used to separate contaminants from fluids. However, organic membranes have limited use because they are normally effective for separating only low temperature, mild fluids . Ceramic membranes offer several advantages over organic membranes. Generally, ceramic membranes are more structurally stable, thermally stable and chemically resistant than organic membranes . The increased structural stability of ceramic membranes permits them to be used in high pressure environments (e.g., to filter a high pressure gas) . Besides being more structurally stable than organic membranes, ceramic membranes can also effectively separate contaminants from high temperature fluids such as molten metal. Ceramic membranes are also more chemically resistant than organic membranes. Organic solvents, chemicals such as chlorine, and in some cases, extremes of pH may degrade organic membranes, but typically do not degrade ceramic membranes. Also, the thermal stability and chemical resistance of ceramic membranes permit them to be sterilized and/or cleaned under harsh conditions (e.g., high heat) or with strong chemicals. Since ceramic membranes can often be reused after adequate cleaning and/or sterilization, they can also be more economical than organic membranes which often cannot be effectively cleaned or sterilized for reuse. Ceramic membranes have conventionally been formed by binding preformed ceramic particles in solution together by sintering. For example, U.S. Patent 5,832,396 teaches a method for making a ceramic filter whereby a slurry including silicon powder, water and methyl cellulose is adsorbed onto a sponge made of a thermal plastic material. The sponge is fired in a heated environment to remove the organic components, and the resulting porous body is sintered to produce a porous ceramic filter. Ceramic membranes have also been made by sol-gel processing techniques. For example, in U.S. Patent 5,104,539, a ceramic membrane is formed by combining a metal alkoxide and water together to form metal oxide particles. The metal oxide particles are then stabilized in a solution by either peptizing the metal oxide particles or adding a surfactant to the solution. The solution containing the metal oxide particles is then dewatered to form a "gel" and is subsequently sintered into a membrane. Conventional ceramic membrane forming processes such as these can be disadvantageous. First, the uniformity of the structure of the pores of the ceramic membranes described in, for example, U.S. Patent 5,832,396, can depend significantly on how uniformly the particles are dispersed in the slurry. During the coating process, the particles in solution can agglomerate or separate thereby possibly disrupting any homogeneous spacing between particles that can have been present in the slurry. Such disruption in the spacing between particles can also decrease the homogeneity of the pores in the ceramic membrane to be formed. Second, dispersing agents and/or particles can shift during the sintering operation. This particle shifting can result in a non- homogenous pore structure in the subsequently formed ceramic membrane. Third, conventional ceramic membrane forming methods cannot be easily adjusted. Since the porosity and pore size of a desired ceramic membrane can largely depend on the size of the particles to be sintered, a large stock of ceramic particles having different sizes can be required if membranes having different pore structures are to be produced. Fourth, the resulting ceramic membranes formed by conventional methods, especially sol-gel methods, can have a low porosity. For example, U.S. Patent 5,104,539 states that the porosity of the membranes formed "have been as high as 39-50%" (column 5, line 57) . Even membranes with a porosity of 50% may not be adequate for some fluid purification applications. Low porosities are disadvantageous, because low porosities often result in high pressure drops across the membrane and a short filtration life in use. SUMMARY OF THE INVENTION The present invention relates to porous ceramic structures and/or methods of making a porous ceramic structure. One aspect of the invention is directed to a method of making a porous ceramic structure comprising: forming a porous preceramic polymer structure by precipitating a preceramic polymer from a casting solution; and pyrolyzing the porous preceramic polymer structure. Another aspect of the invention is directed to a porous ceramic structure derived from a preceramic polymer and having a voids volume (i.e., porosity) of greater than about 50%. Yet another aspect of the invention is directed to a filter medium comprising a silicon nitride ceramic such as silicon boronitride or silicon carbonitride . Methods embodying to the invention permit the formation of porous ceramic structures that can be easily manufactured and that can result in a more homogeneous internal porous ceramic structure. Also, the method according to the invention can result in porous ceramic structures, particularly ceramic filter media, with a greater voids volume, strength, and/or temperature resistance than conventional ceramic filter media. DETAILED DESCRIPTION OF THE INVENTION One aspect of the invention provides for a method of making a porous ceramic structure comprising: forming a porous preceramic polymer structure by precipitating the preceramic polymer from a casting solution; and pyrolyzing the porous preceramic polymer structure . Illustratively, a preceramic polymer (e.g., polyborosilazane pellets) is dissolved in a solvent (e.g., tetrahydrofuran) to form a casting solution. The casting solution can be modified by adding, under controlled conditions, a nonsolvent for the preceramic polymer (e.g., water) to the solvent/preceramic polymer casting solution. Preferably, the nonsolvent, which can be in the form of a pure non-solvent or a non-solvent/solvent mixture, is added to the solvent/preceramic polymer mixture so that the casting solution is at or near the point of incipient precipitation of the preceramic polymer from the casting solution. The casting solution can then be deposited on a substrate (e.g., a polyethylene terephthalate sheet) to form, for example, a film of casting solution. The deposited casting solution, which may or may not later be separated from the substrate, is preferably cooled. Additional nonsolvent can then contact the deposited casting solution (e.g., by immersing the deposited casting solution in a nonsolvent bath) to precipitate the preceramic polymer from the deposited casting solution. Alternatively, the preceramic polymer can be precipitated from the casting solution without the addition of a nonsolvent. For example, the preceramic polymer can be precipitated from the solvent by removing the solvent from the casting solution (e.g., evaporation). The structure precipitated from the deposited casting solution is a porous preceramic polymer structure. After being precipitated, the porous preceramic polymer structure is preferably washed and dried. The porous preceramic polymer structure is then pyrolyzed into a porous ceramic structure (e.g., a porous ceramic membrane). Any suitable technique can be used to pyrolyze the porous preceramic structure. For example, the porous preceramic structure can be heated in an inert gas atmosphere until the porous preceramic polymer structure transforms into a porous ceramic structure. A porous ceramic structure having a uniform pore structure, a high voids volume (e.g, greater than 50%), and small pore size desirably results from the pyrolyzation operation. The preceramic polymer used in the invention is not limited, and includes any polymer which is capable of producing a ceramic when the polymer is subjected to pyrolysis. Preferably, the preceramic polymer is a polymer comprising silicon and/or nitrogen. Advantageously, preceramic polymers having silicon and/or nitrogen can form porous ceramic structures which can withstand temperatures of up to 1400°C without decomposing. Suitable polymers comprising silicon and/or nitrogen include polysilazanes, polysiloxanes and polysilanes . Examples of polysilazanes include polyorganoborosilazane, polyborosilazane, polyphosphosilazane, polyorganometallosilazane, VT50 or ET70 polysilazanes (commercially available from Hoechst) , and NCP 100 or NCP 200 polysilazanes (commercially available from Chisso Corp., Japan) . Examples of polysiloxanes include polyborosiloxane and polyboroorganosiloxane . Examples of polysilanes include polyborosilane . Polyorganoborosilazanes are particularly advantageous, because ceramics formed from polyorganoborosilazanes result in porous ceramic structures which withstand temperatures of up to 2000°C without decomposition. If desired, the preceramic polymers can be blended and/or dissolved with any number of non-preceramic or other preceramic polymers. The preceramic polymers can also be crosslinked if desired. The preceramic polymers can be derived from a variety of preceramic polymer precursors including suitable preceramic or non-preceramic compounds, monomers, and/or oligomers . Suitable preceramic polymer precursors include silanes or boranes . Exemplary silanes include silanes of the formula Si (R,,R 2 , R 3 , R 4 ) wherein R,,R 2 ,R 3 , and R 4 may be independently selected from the group consisting of halogen (preferably Cl) , H, alkyl (preferably C ! -C 6 alkyl), phenyl, vinyl, alkylamino and alkylsilyl. An exemplary borane preceramic precursor is tris t (dichloromethylsilyl) ethyl] borane which is capable of forming a polyborosilazane preceramic polymer. The preceramic polymers and/or preceramic polymer precursors can also include various non-metal and metal atoms. For example, non-metal and metal atoms can be incorporated into the preceramic polymer by reacting a polymer, oligomer or monomer with preceramic polymer precursors such as non-metal or metal compounds (e.g., coordination compounds). Exemplary compounds include, without limitation, dimethylsulfide borane, tris (dimethylamino) borane, tetrakis (dimethylamino) titanium (Ti [N (CH 3 ) 2 ] 4 ) , tris (dimethylamino) phosphine, tris (dimethylamino) aluminum, tetrakis (dimethylamino) aluminum (A1[N(CH 3 ) 2 ] 4 ) , tetrakis (dimethylamino) boron (B [N (CH 3 ) 2 ] 4 ) , tetrakis (dimethylamino) phosphorus (P [N(CH 3 ) 2 ] 4 ) , tetrakis (dimethylamino) zirconium (Zr [N(CH 3 ) 2 ] 4 ) , metal or non- metal alkoxides such as those represented by the formula M(OR 5 ) m (where M is an element selected from the group consisting of elements from groups IIA to VA and groups IIB to VB of the Periodic Table, and wherein each R 5 independently represents a hydrogen atom, an alkyl group or an aryl group provided that at least one R s is not a hydrogen atom, and where m is the valency of M) , or halides (e.g., metal halides) . The same or different preceramic polymer precursors can be used to form the preceramic polymer. Advantageously, the chemical composition of the preceramic polymer, and therefore the subsequently formed porous ceramic structure, can be easily adjusted by selectively choosing the various preceramic polymer precursors used to make the preceramic polymer. For example, the boron content of a preceramic polyborosilazane polymer can be adjusted by increasing or decreasing the amount of boron containing preceramic polymer precursors such as dimethylsulfide borane used to the form the polyborosilazane polymer. Various reagents can be used to help facilitate the formation of preceramic precursors and/or preceramic polymers. Suitable reagents include water, alcohols, ammonia, alkylammonia, etc., preferably in either liquid or gaseous form. The casting solution comprising a preceramic polymer can be formed in any suitable manner. The casting solution can be formed by partially or totally dissolving particles, pellets, fibers, etc. having a preceramic polymer in a solvent. Alternatively, the casting solution can be formed by polymerizing or reacting preceramic polymer precursors (e.g., monomers or oligomers) in a solvent and leaving the subsequently formed, unprecipitated preceramic polymer in the solvent. The preceramic polymer solution can also be induced to dissolve the polymer. For example, the preceramic polymer solution can be agitated (e.g., stirred) or heated to help dissolve preceramic particles or pellets in the solvent. The dissolution of the preceramic polymer can occur at any suitable temperature, depending, for example, upon the solvent chosen. The choice of solvent used to dissolve the preceramic polymer is not limited, and can be determined according to the polymer or polymers to be dissolved. Organic solvents are preferred. For example, exemplary solvents capable of dissolving a polyorganoborosilazane preceramic polymer include n-pentane, n- hexane, n-decane, cyclohexane, toluene, xylene, tetrahydrofuran, di-ethyl ether, chloroform, chlorobenzene, di-benzyl ether, decalin, diethylene glycol, diethyl ether, acetone, benzene, chloroform and combinations thereof. Preferably, the casting solution also includes a nonsolvent for the preceramic polymer. The nonsolvent, which is preferably miscible with the solvent, can be added in a controlled manner to modify a casting solution comprising a preceramic polymer and a solvent . The nonsolvent can be in the form of a pure nonsolvent or a nonsolvent/solvent mixture and can include other chemically compatible substances (e.g., desired solutes). The choice of nonsolvent can be determined in accordance with the solvent and/or preceramic polymer utilized. For example, a nonsolvent for a polyorganoborosilazane polymer can be water. Preferably, the nonsolvent is added in an amount sufficient to bring the casting solution before, at or near the point of incipient precipitation of the preceramic polymer from the casting solution. Once precipitation of the preceramic polymer takes place, the precipitated preceramic polymer can partially or completely redissolve. The casting solution can contain solids, if desired. For example, solids in the casting solution may be used to increase the viscosity of the casting solution or change the properties of the final porous ceramic structure produced. Preferably, however, the casting solution is free of solids. If the preceramic polymer begins to precipitate from the casting solution during the addition of the nonsolvent, the casting solution can be filtered to remove any preceramic polymer precipitates or insoluble impurities that may be in the casting solution. Alternatively, more solvent can be added to redissolve the precipitated preceramic polymer. A solid-free casting solution may be more easily deposited than a casting solution with solids. If desired, the casting solution can also include various pyrolysis aids to assist in the formation of a ceramic when the preceramic polymer is subsequently pyrolyzed. Suitable pyrolysis aids include metal oxides, such as oxides of Al, Mg, Y, Yb, Ti, and/or Si (e.g., Al 2 0 3 , Y 2 0 3 , Si0 2 ) . Other pyrolysis aids can include oxides, nitrates, alcoholates, acetates, acetylacetonates, carbonates, oxalates, and metal halides (e.g. halides of Ti, Zr, Hr, Nb, Ta, Cr) . Combinations of pyrolysis aids can also be included in the casting solution. A substrate may or may not be used in the subsequent processing of the casting solution into the porous ceramic structure . In one embodiment , the porous preceramic polymer structure can be formed without the use of a substrate . Illustratively, the casting solution can be contacted with a nonsolvent before the casting solution ever contacts a substrate. For example, the casting solution can formed into a sheet by extruding the casting solution. Without contacting a substrate, the extruded sheet can be immersed in a bath containing a nonsolvent to precipitate a porous preceramic polymer structure from the extruded sheet of casting solution. The porous preceramic polymer structure can then be pyrolyzed in the manner described below. Preferably, however, the casting solution is deposited on a substrate prior to precipitation of the porous preceramic polymer structure from the casting solution. The casting solution can be deposited on any suitable substrate. The substrate includes any porous or non-porous structure upon which the casting solution can be deposited, including but not limited to work surfaces, molding dies, films, webs, sheets, fabrics made of a polymeric, ceramic, metallic, and/or carbonic material. The shape of the substrate can be chosen according to the desired porous ceramic structure to be formed. For example, if a planar porous ceramic substrate is desired, then the casting solution is preferably deposited on a substrate having a planar surface. Alternatively, if a cylindrical porous ceramic structure is desired, then the casting solution can be deposited on a substrate having a cylindrical surface. Still alternatively, the casting solution can be deposited on and within a foranimous structure, such as a metal or ceramic mesh. The porous ceramic structure to be formed would lie both on and within the openings of the ceramic or metal mesh (i.e., the porous ceramic structure exists at least partially within the pores of the substrate) . The method of depositing the casting solution is not limited and includes any suitable coating, casting, extrusion or molding operation. For example, the casting solution can be coated on a substrate by doctor blade coating, dip coating, spray coating, etc. Alternatively, a film of casting solution can be extruded through a die, and subsequently placed on a work surface or carrier substrate. The thickness of the deposited casting solution typically will be determined by the intended use of the porous ceramic structure to be formed. For example, if the porous ceramic structure to be formed is to be used to filter a high pressure gas, the casting solution coating can be thick in order to form a thick porous ceramic structure, strong enough to withstand the force of differential pressure in a high pressure gas stream. Preferably, the thickness of the deposited casting solution is sufficiently thin so that the deposited casting solution is capable of forming a thin porous preceramic polymer structure which subsequently forms a thin porous ceramic structure. Thin porous ceramic structures can be ideally suited for certain filtration applications due to their high permeabilities. Once deposited, the deposited casting solution is preferably cooled to a temperature lower than the temperature at which the casting solution was formed. This cooling can be accomplished by any suitable means. The deposited casting solution can be cooled by, for example, permitting a previously heated, deposited casting solution to remain undisturbed so that the heat from the deposited casting solution dissipates from the deposited casting solution over time. Alternatively, the deposited casting solution can be cooled with a cooling device such as an air cooler or a chill roll. If the solvent and/or the nonsolvent of the casting solution have a relatively high surface tension, the substrate surface on which the casting solution is deposited is preferably wettable (i.e., have zero or substantially zero angle of contact) when contacted by the casting solution. Wettable substrates can induce the formation of more even casting solution coatings, which can result in more even porous ceramic structures. If a substrate material is not properly wettable, the substrate can be rendered wettable by the application of an appropriate surface treatment. For example, a substrate surface can be modified by subjecting the surface thereof to either a plasma discharge (e.g., corona discharge) or a chemical treatment. If an unsupported porous ceramic structure is the desired product, the casting solution can be contacted directly with a nonsolvent (e.g., without the use of a substrate) or the casting solution can be deposited on a temporary substrate or support surface. Examples of temporary substrates include sheets, films, and work surfaces (e.g., endless belts or drums) made of polymeric, metallic and/or ceramic materials. A preferred temporary substrate is a polyethylene terephthalate film. The temporary substrate is preferably easily separated from the deposited casting solution, or the subsequently formed porous preceramic polymer structure or porous ceramic structure . Preferably, the temporary substrate has a smooth nonporous surface from which the deposited casting solution, the subsequently formed porous preceramic polymer structure or the subsequently formed porous ceramic structure can be easily separated (e.g., by stripping). After a porous preceramic polymer structure has been formed, the temporary substrate can be separated either before, during or after pyrolyzation of the porous preceramic polymer structure. For example, a porous preceramic polymeric structure can be separated from a substrate such as a thin Mylar sheet or a flat working surface. Alternatively, the substrate can be removed from the porous polymer structure during the pyrolysis operation. For example, a porous preceramic polymeric structure/substrate combination can be placed in a furnace. During the pyrolyzation process, the porous polymeric structure is pyrolyzed into a ceramic while the temporary substrate decomposes. The temporary substrate can even be removed prior to the formation of any porous structure. For example, the casting solution can be deposited on a substrate, partially solidified, separated from the substrate, and subsequently formed into a porous preceramic polymer structure. If a composite ceramic structure comprising a porous ceramic structure is desired, the substrate (i.e., a permanent substrate) upon which the casting solution is deposited may become permanently attached to the porous ceramic structure to be formed. It may be desirable to join a substrate to the porous ceramic structure to be formed if the porous ceramic structure needs enhanced structural support and/or a prefilter. Permanent substrates are preferably made of a material which can withstand the high temperatures of pyrolysis without being decomposed. For example, metal, glass, ceramic, and graphite normally do not degrade when subjected to high temperatures and can be used as a permanent substrate for the porous ceramic structure to be formed. Preferably, the permanent substrate is porous and/or wettable. The porous substrate is preferably wetted by the casting solution, so that the casting solution will penetrate the openings in the porous substrate when the casting solution is deposited thereon. During the precipitation of the porous preceramic polymer structure from the deposited casting solution, the porous substrate can become firmly attached to the porous preceramic polymer structure. It is not essential, however, that the substrate be wetted. If the substrate is not wetted, the deposited casting solution can be largely confined to the outer (or nominal) surface of the substrate (as opposed to at least a portion of the surface of the pores) , but is nonetheless adherent thereto. The adhesion of the deposited casting solution or the subsequently precipitated porous preceramic polymer structure to the substrate can be caused by normal adhesive forces and/or mechanical interlocking between the casting solution or the precipitated porous preceramic polymer structure, and a surface structure of the substrate. Non-limiting examples of permanent substrates include nonwoven or woven fabrics, meshes, porous plates, porous grids, etc. For example, the casting solution can be cast as a film onto a non-wettable fibrous web. Since the casting solution does not wet the non-wettable fibers of the web, the casting solution is carried on the surface of the substrate. If the permanent substrate is porous and is wettable by the casting solution, the substrate can have a sufficiently high critical surface tension so that the casting solution can completely permeate the porous substrate. The resulting porous preceramic polymer structure precipitates in and around the substrate (e.g., such that a significant portion of the entire surface of the substrate is coated, including the surface of the pores throughout the substrate) , and is permanently supported thereby. Suitable wetted substrates that can serve as permanent supports for the porous ceramic structure include nonwoven or woven fibrous webs, or open meshes. For example, a wettable metal or ceramic fabric can be immersed in the casting solution so that the casting solution impregnates and coats the fabric. The coated and impregnated casting solution is then precipitated into a porous preceramic polymer structure which becomes attached to the fabric. The preceramic polymer can be precipitated from the casting solution in any suitable manner. One preferred method of precipitating the preceramic polymer from the casting solution is by contacting the deposited casting solution with a nonsolvent. The nonsolvent used to contact the deposited casting solution can be any combination of materials provided that the resultant nonsolvent is capable of precipitating the preceramic polymer from the casting solution. The nonsolvent can comprise a pure nonsolvent or a nonsolvent/solvent mixture, and can include the same nonsolvent present in the casting solution. However, if the nonsolvent used to precipitate the preceramic polymer from the casting solution is a nonsolvent/solvent mixture, then the ratio of solvent to nonsolvent is preferably lower than the ratio of solvent to nonsolvent in the casting solution. Moreover, solutes such as salt can be used to control the properties of the nonsolvent. For example, a non-solvent may comprise water and salt. The nonsolvent can contact the deposited casting solution in any suitable manner. For example, a substrate coated with a casting solution can be immersed in a bath of nonsolvent. Alternatively, the nonsolvent can contact the deposited casting solution by being dispensed onto the deposited casting solution. For example, a roller coater or spray coater can be used to dispense a nonsolvent onto a casting solution coating. Another method of precipitating the preceramic polymer from the deposited casting solution is to remove solvent from the casting solution. For example, the solvent may be removed from the casting solution by evaporating the solvent, thereafter leaving a precipitated preceramic porous structure and possibly a nonsolvent. Evaporation of the solvent can be induced by heating the casting solution, or subjecting the casting solution to vacuum. The precipitation of the preceramic polymer from the casting solution is preferably controlled to obtain a porous preceramic polymer structure having desired flow characteristics, pore sizes and pore structures . The variables that may be controlled include, without limitation: the choice and/or quantity of preceramic polymer, solvent and nonsolvent; the ratio of nonsolvent to solvent; the rate of cooling; the concentration of the preceramic polymer in the solution; the temperature of the casting solution; and/or the mode of adding of the nonsolvent to the casting solution (e.g., the type of process used, such as spraying; the rate of adding the nonsolvent, intensity of any mixing during the addition of the nonsolvent, and/or the geometry of any apparatus used in adding the nonsolvent) . Parameters such as these can be controlled and adjusted to obtain porous preceramic polymer structures with predetermined characteristics. For example, the porous preceramic polymer structures can be precipitated from the casting solution in a controlled manner so that the resulting porous preceramic polymer structure can have a uniform, tapered or graded pore structure. The method according to the invention provides for a number of advantages. First, the substantially solid-free casting solution can be deposited on a substrate without the problem of possible nonuniform particle migration during the deposition process. Consequently, the porous ceramic structure made according to the invention can have a more homogeneous internal pore structure than conventional ceramic filter media. Second, the method of the invention can induce the formation of porous structures having greater porosities than have been produced by conventional ceramic filter medium forming methods. Third, the method according to the invention is easily adjustable. By controlling the precipitation process, the pore structure, and porosity of the resulting porous structures can be tailored to predetermined requirements. Stocks of different sized ceramic particles need not be maintained in order to form porous structures with varying pore sizes. Also, by controlling the precipitation process, filter elements having multiple porous ceramic layers with differing characteristics, such as different porosities, can be formed. For example, two porous preceramic polymer layers having two different porosities can be precipitated from casting solution layers by adjusting the precipitation parameters of each layer. The two porous preceramic polymer layers can then be combined and pyrolyzed to form a multiple layer ceramic filter element with a graded pore structure. A multiple layer filter element having a graded pore structure can be advantageous. For example, a porous ceramic layer with a large pore size can be used as a prefilter or a support structure for a porous ceramic layer with a smaller pore size. The formation of a porous preceramic polymer structure from a casting solution can be carried out as an intermittent or batch operation or as a continuous or semicontinuous process. A small scale operation can be most conveniently carried out as a batch operation, while at high production rates, a continuous or semicontinuous operation may be more convenient. In a continuous process, the substrate can be in the form of, for example, an endless belt or a drum, which circulates the casting solution through the entire porous preceramic polymer structure forming operation. For example, the deposition of the casting solution, the passage of the deposited casting solution through a cooling region, the precipitation of the preceramic polymer from the deposited casting solution by a nonsolvent liquid, the drying and the washing operations can take place in a continuous process. Following precipitation, the porous preceramic polymer structure can be washed to remove any remaining solvent . Any suitable washing method or washing fluid can be used. Water is a preferred washing fluid, but any volatile liquid in which the solvent is soluble and which can be subsequently removed (e.g., during drying) can be used as the washing liquid. One or several washes or baths can be used as required to reduce the solvent content of the porous preceramic structure. In the continuous process, the flow of wash liquid may be countercurrent to the path of the porous preceramic polymer structure, which can, for example, be passed through a series of shallow washing liquid baths in the washing stage . Once the porous preceramic polymeric structure has been formed, the porous preceramic polymeric structure can then be pyrolyzed to form a porous ceramic structure. Pyrolysis of the porous preceramic polymer structure can occur in any suitable manner. For example, pyrolysis can occur in any suitable atmosphere. The properties and/or composition of the pyrolyzed porous ceramic structure can be altered depending on the atmosphere in which pyrolysis takes place. If it is desired that the composition of the porous ceramic structure substantially corresponds to the composition of the porous preceramic polymer structure, pyrolysis can take place in a non-reactive atmosphere, such as an atmosphere having an inert gas or a vacuum. Examples of inert gases include nitrogen and argon. Pyrolysis can also take place in an atmosphere having a reactive gas. Reactive gases include air, oxygen, ammonia, hydrogen peroxide, water, hydrogen, alkylamine, or mixtures of reactive gases. If pyrolysis is carried out in a reactive atmosphere, the composition of the porous ceramic structure can be different from the composition of the precursor porous preceramic polymer structure. Depending upon the reactive gas used in the pyrolysis operation, atomic constituents can be added and/or removed from the porous preceramic polymer structure when pyrolyzing the porous preceramic polymer structure in a reactive gas atmosphere. For example, the pyrolysis of a porous preceramic polymer structure in a reactive amine atmosphere can result in more nitrogen in the porous ceramic structure than is present in the porous preceramic polymer structure . In an alternative example, pyrolysis of a carbon-containing porous preceramic polymer structure in a reactive oxygen atmosphere can result in less carbon than is present in the carbon-containing porous preceramic polymer structure prior to pyrolysis. For example, carbon in a carbon-containing porous preceramic polymer structure can react with the gaseous oxygen during the pyrolysis operation thereby forming gaseous carbon oxides (e.g., CO) which do not remain in the formed porous ceramic structure . Pyrolysis of the porous preceramic polymer structure can be performed by any suitable means or with any suitable parameters . For example, the porous preceramic polymer structure can be heated at a temperature between about 200 to about 2000 °C, by a kiln, furnace or other heating device. Alternatively, a radiation device such as a microwave or gamma ray device can also pyrolyze the porous preceramic polymer structure into a porous ceramic structure. The final composition of the porous ceramic structure is not limited and can vary depending upon the starting materials and/or reacting conditions used to make the porous ceramic structure. The porous ceramic structure can include, for example, a silicon nitride (e.g., silicon oxynitride) , a silicon carbide, a metal silicate, or a ceramic such as a Si-N, Si-C-N, Si-O-N, Si-O-C-N, Si-M-N, Si-M-O-N, Si-M-O, Si-M-C-N, Si-M-C, or Si-M-O-C-N system ceramic, wherein M is at least one element selected from the group consisting of metallic elements (e.g., alkali earth metals) and nonmetals from groups IIIB to VIB of the Periodic Table. Preferably, the porous ceramic structure includes a ceramic comprising silicon or nitrogen. More preferably, the porous ceramic structure includes a ceramic comprising silicon and nitrogen. Examples of ceramics comprising silicon and nitrogen include silicon boronitride, silicoboron carbonitride, silicon metallonitride or silicon carbonitride . Silicon nitride based porous structures are preferred because they can be subjected to temperatures of up to 1400 °C without decomposing. Also, porous ceramic structures including silicoboron carbonitride (e.g., Si 30 B 10 C 43 N 20 ) are also preferred because such ceramic structures can be thermally stable up to 2000 °C. The resulting porous ceramic structure can have any suitable voids volume and pore rating (pore size) , preferably a high voids volume and a small pore size. The porous ceramic structures preferably has a voids volume of at least about 50%. More preferably, the porous ceramic structures include a voids volume of at least about 60%. Even more preferably, the voids volume of the porous ceramic structure is at least about 80%. The porous ceramic structures embodying the invention can have pore sizes and/or characteristics suitable for use in a wide variety of processes, including microfiltration, ultrafiltration, and/or nanofiltration processes. For example, some porous ceramic structures can have a nominal pore size of about 10 microns or more, in the range from about 1 micron to about 10 microns, in the range from about 0.04 microns to about 10 microns, or in the range from about .01 microns or less to about 1 micron. Other porous ceramic structures embodying the invention can have the nominal pore dimensions in the range from about 10 angstroms or less to about 200 angstroms or more. Still other porous ceramic structures embodying the invention can have a molecular weight cutoff rating of about 200 daltons or less, in the range from about 200 daltons to about 1000 daltons, in the range from about 1000 daltons to about 500,000 daltons, or greater than about 500,000 daltons. The porous ceramic structures can have predetermined removal rating requirements and/or predetermined pore structures. The porous ceramic structures can have any suitable removal rating. For example, for microfiltration processes the porous ceramic structures can have an absolute removal rating within the range from about 0.04 μM to about 20 μM. The porous ceramic structures can also have any suitable thickness, including a thickness within the range from about 0.01 mm to about 0.4 mm or more. The porous ceramic structures can also have pores extending from surface to surface in a relatively uniform structure, or in a tapered or graded pore structure . The resulting porous ceramic structure can take a variety of forms. Preferably, the porous ceramic structure is a flat, thin, porous ceramic structure such as a ceramic membrane . The porous ceramic structure can also have a corrugated, pleated, or curved form or can be in the form of disks, cylinders, plates etc. The resulting porous ceramic structure can take a variety of forms, because the casting solution and/or the porous preceramic polymer structure can be shaped in any desired manner prior to pyrolysis. For example, the casting solution and/or the porous preceramic polymer structure can be corrugated or molded into a corrugated shape with or without other elements (e.g., support and/or drainage layers) prior to pyrolysis. Subsequently, the corrugated casting solution or corrugated porous preceramic polymer structure may be processed into a porous ceramic structure having a corrugated form by subsequent precipitation and/or pyrolysis (previously described in detail) . The porous ceramic structures of the invention preferably form part of a filter element. A variety of filter elements can be formed according to the invention. For example, as mentioned above, a porous preceramic polymer film can be attached to a porous substrate (e.g., ceramic fabric) prior to pyrolysis. After pyrolysis, the porous ceramic structure formed from the porous preceramic polymer film can then be attached to the substrate to form a filter element. If desired, the filter element can also include additional constituents, including, but not limited to at least one layer to provide support and/or better drainage. Exemplary supports and/or drainage components include porous metal or ceramic grids or fabrics . The substrate of the filter element can be of any desired material, can be in any suitable form and can be attached to the porous ceramic structure in any suitable manner. The porous substrate used in the filter element can be any type of substrate including, a porous preceramic polymer structure, a green porous structure, or a porous ceramic or metallic structure (e.g., a metal mesh or a ceramic fabric) . Other exemplary substrates can include the permanent substrates described above. Further, the porous ceramic structure can be attached to the substrate in any number of ways. For example, the porous ceramic structure can be impregnated in a porous substrate or can be coated as a layer on a porous substrate. Filter elements according to the invention can include any suitable number of porous ceramic structures and/or supporting substrates. For example, two, three, four, etc., porous ceramic structures and/or supporting substrates having the same or different properties (e.g., materials or voids volume) can be laminated together or spaced apart to form a filter element according to the invention. The components of the filter element can be formed separately or together. For example, a laminated structure comprising a green porous ceramic substrate and a porous preceramic polymer structure can be pyrolyzed together to form a composite filter element. Alternatively, a plurality of porous ceramic structures having the same or different characteristics (e.g., porosity) can be separately formed by separate precipitation and/or pyrolyzation processes. The separately produced porous ceramic structures can then be bonded together (e.g., by sintering) to form a multilayer filter element. A filter element according to the invention provides for favorable advantages. For example, filter elements having a substrate supported porous ceramic structure can have more structural stability than an unsupported porous ceramic structure. Additionally, the filter elements according to the invention can provide for better overall filtration properties than unsupported porous ceramic structures. For example, a porous metallic or ceramic substrate can serve as a prefilter for a porous ceramic structure with a smaller pore rating than the porous metallic or ceramic substrate. The porous ceramic structures and/or filter elements of the invention are well suited for use as a filter medium or as a liquid/liquid separation medium in cartridge form. Such cartridges can be self-contained separation elements, provided with a porous ceramic structure or filter element according to the invention in tubular form, capped off by end caps at each end. Either or both end caps can have a through opening for fluid circulation through the porous ceramic structure in either direction. End caps can be attached to the ends of the porous ceramic structures or filter elements by, for example, sintering. Such cartridges take many forms, including simple cylinders, corrugated cylinders, stacked discs, etc. To further illustrate the method according to the invention, the following example describes in detail how an unsupported silicoboron carbonitride porous ceramic structure can be formed. EXAMPLE A tris [ (dichloromethylsilyl) ethyl] borane monomer can be synthesized by the dropwise addition of 0.5 mol dimethylsulphide borane, (CH 3 ) 2 SBH 3 (in 250 ml toluene solution), to 1.5 mol dichloromethylvinylsilane dissolved in 300 ml toluene at 0°C in an argon atmosphere. After stirring the reaction mixture for 36 hours at room temperature and subsequent evaporation of the solvent, a viscous liquid, compound 1, can be isolated in 94% yield. The organosilylborane of compound 1 can be subsequently polycondensed at 0° C in tetrahydrofuran (THF) by ammonolysis . After separation from the by-product, ammonium chloride, and distillation of the solvent at room temperature, the polyorganoborosilazane in formula 2 can be isolated in 80% yield as a white solid powder. R= -CH(CH 3 ) -Si(CH 3 )Cl 2 R'= -CH(CH 3 ) -Si(CH 3 ) -NH- To form a casting solution, the white solid powder can then be dissolved in a solvent such as THF. Once the white solid powder is dissolved, the casting solution can be modified by adding water, a nonsolvent for the polyborosilazane, to the casting solution comprising THF and polyborosilazane. The water is added in an amount sufficient to bring the modified casting solution to the point of incipient precipitation of the polyborosilazane from the modified casting solution. If desired, the modified casting solution can be filtered to remove any insoluble particulates or contaminants. The casting solution can then spread as a film on a polyethylene terephthalate substrate . To precipitate the polyorganoborosilazane polymer from the casting solution, the casting solution coated polyethylene terephthalate substrate can be immersed into a bath of nonsolvent to precipitate the polyborosilazane from the coated casting solution. The precipitated polyborosilazane forms a thin polyborosilazane membrane which may be washed and dried. The polyethylene terephthalate substrate can be stripped from the polyborosilazane membrane . The polyborosilazane membrane is then placed in a furnace having an argon atmosphere where the polyborosilazane membrane undergoes pyrolysis into a ceramic membrane. The resulting ceramic membrane can include a ceramic having the composition Si 30 B 10 C 43 N 20 and can have a voids volume of greater than 50%. While the invention has been described in some detail by way of illustration and example, it is understood that the invention is not restricted to the specifically described embodiments in the specification. Rather, the invention includes all modifications, equivalents and alternatives falling within the spirit and the scope of the invention. For example, while porous ceramic structures for use in filtration have been described in some detail, the porous ceramic structures according to the invention can be used for other purposes as well (e.g., as a catalytic support for a catalyst) .

Claims: What is claimed is: 1. A method of making a porous ceramic structure comprising: forming a porous preceramic polymer structure by precipitating a preceramic polymer from a casting solution; and pyrolyzing the porous preceramic polymer structure to form a porous ceramic structure . 2. The method of claim 1, wherein precipitating the preceramic polymer includes contacting the casting solution with a nonsolvent . 3. The method of claim 2, wherein the nonsolvent comprises a nonsolvent/solvent mixture. 4. The method of claim 1, wherein precipitating the preceramic polymer includes removing a solvent from the casting solution. 5. The method of claim 4, wherein removing a solvent includes evaporating the solvent. 6. The method of claim 1 further comprising forming the casting solution. 7. The method of claim 6, wherein forming the casting solution comprises dissolving a preceramic polymer in a solvent. 8. The method of claim 7, wherein forming the casting solution further comprises adding a nonsolvent to a solution comprising the preceramic polymer and the solvent. 9. The method claim 1 further comprising depositing the casting solution on a substrate prior to precipitating the preceramic polymer from the casting solution. 10. The method of claim 9, wherein depositing the casting solution on a substrate comprises coating the casting solution on the substrate. 11. The method of claim 9, wherein depositing the casting solution on a substrate comprises impregnating the substrate with the casting solution. 12. The method of claim 1, wherein the preceramic polymer comprises polyorganoborosilazane. 13. The method of claim 1, wherein pyrolyzing the porous preceramic polymer structure includes heating the porous preceramic polymer structure in an inert gas atmosphere. 14. The method of claim 9, wherein the substrate comprises a ceramic . 15. The method of claim 1, wherein the porous ceramic structure is a filter medium. 16. The method of claim 1, wherein the porous ceramic structure comprises a voids volume of at least about 50%. 17. The method of claim 1, further comprising filtering the casting solution prior to precipitating the preceramic polymer therefrom. 18. The method of claim 1, wherein the porous ceramic structure is a microporous membrane. 19. The method of claim 1, wherein pyrolyzing the porous preceramic polymer structure includes heating the porous preceramic polymer structure to a temperature between about 200 and about 1500 °C. 20. The method of claim 1 wherein pyrolyzing the porous preceramic structure comprises heating the porous preceramic polymer structure in an atmosphere comprising a reactive gas. 21. A porous ceramic structure derived from a preceramic polymer and having a voids volume of greater than 50%. 22. The porous ceramic structure of claim 21, wherein the preceramic polymer comprises silicon and nitrogen. 23. The porous ceramic structure of claim 21, wherein the porous ceramic structure comprises a silicoboron carbonitride ceramic. 24. The porous ceramic structure of claim 21, wherein the porous ceramic structure comprises a Si 30 B, 0 C 43 N 20 ceramic . 25. The porous ceramic structure of claim 21, further comprising pores having an average pore size of about 0.04 to about 10 microns . 26. The porous ceramic structure of claim 21, wherein the porous ceramic structure is a membrane. 27. The porous ceramic structure of claim 21, wherein the porous ceramic structure is a filter medium. 28. The porous ceramic structure of claim 21, wherein the preceramic polymer comprises polyorganoborosilazane. 29. The porous ceramic structure of claim 21, comprising a tapered pore structure . 30. The porous ceramic structure of claim 21, wherein the porous ceramic structure is flat. 31. A filter medium comprising a porous ceramic structure having a ceramic selected from the group consisting of silicon boronitride, silicon oxynitride, and silicon carbonitride. s 32. The filter medium of claim 31, wherein the silicon boronitride comprises silicoboron carbonitride. 33. The filter medium of claim 32, wherein the silicoboron carbonitride comprises Si 30 B 10 C 43 N 20 . 0 34. The filter medium of claim 31, having a voids volume of at least about 50%. 35. The filter medium of claim 31, wherein the filter medium s comprises micropores . 36. The filter medium of claim 31 comprising pores having an average pore size between about 0.04 to about 10 microns. 0 37. A filter element comprising: the filter medium of claim 31; and a substrate adjacent to the filter medium. 38. The filter element of claim 37, wherein the substrate is 5 porous. 39. The filter element of claim 38, wherein the substrate is a nonwoven fabric, a woven fabric, or a mesh. 0 40. The filter element of claim 38, wherein the filter medium exists at least partially within the pores of the substrate. 41. The filter element of claim 37, wherein the substrate comprises a ceramic or a metal . 42. A filter element comprising: the porous ceramic structure of claim 21; and a substrate adjacent to the porous ceramic structure. 43. The filter element of claim 42, wherein the substrate is porous . 44. The filter element of claim 43, wherein the substrate is a nonwoven fabric, a woven fabric, or a mesh. 45. The filter element of claim 43, wherein the porous ceramic structure exists at least partially within the pores of the substrate . 46. The filter element of claim 42, wherein the substrate comprises a ceramic or a metal. 47. The filter element of claim 42 wherein the porous ceramic structure comprises a molecular weight cutoff rating from about 1000 daltons to about 500,000 daltons. 48. The filter element of claim 42 wherein the porous ceramic structure is capable of use in an ultrafiltration process. 49. The filter element of claim 42 wherein the porous ceramic structure is capable of use in a nanofiltration process. 50. The porous ceramic structure of claim 21 wherein the porous ceramic structure comprises a molecular weight cutoff rating from about 1000 daltons to about 500,000 daltons. 51. The porous ceramic structure of claim 21 wherein the porous ceramic structure is capable of use in an ultrafiltration process . 52. The porous ceramic structure of claim 21 wherein the porous ceramic structure is capable of use in a nanofiltration process.

Inventors: BHANOT ASHOK K, GEIBEL STEPHEN A, BHANOT, ASHOK, K., GEIBEL, STEPHEN, A.
Applicants: PALL CORP, PALL CORPORATION
Application year: 1998
Main classification: C04B35
IPC codes: B01D39, B01D71, C04B35, C04B38, C04B41