Europaisches Patentamt J European Patent Office 00 Publication number: 0 443 228 A1 Office europeen des brevets EUROPEAN PATENT APPLICATION © Application number: 90301777.0 © int. CIA C04B 30/02, C04B 14/46, C08K 7/04, C04B 35/80, @ Date of filing: 19.02.90 C22C 1/09 Date of publication of application: @ Applicant: EXXON RESEARCH AND 28.08.91 Bulletin 91/35 ENGINEERING COMPANY P.O.Box 390, 180 Park Avenue Designated Contracting States: Fiorham Park, New Jersey 07932(US) BE DE FR GB IT NL SE @ Inventor: Witzke, Horst 8 Fawn Drive Flemington, New Jersey 08822(US) Inventor: Kear Bernard Henry RD No. 10 Campbell's Brook Road Whitehouse Station, New Jersey(US) Representative: Mitchell, Alan et al ESSO Engineering (Europe) Ltd. Patents & Licences Mailpoint 72 Esso House Ermyn Way Leatherhead, Surrey KT22 8XE(GB) Producing a three-dimentional random weave of thin ceramic tubes. © An entirely fluid-phase method is disclosed for producing interwoven ceramic filamentary micro-tu- bular materials. In the process, a three-dimensional random weave of ceramic tubes is generated, with diameters in the range of about .01 to 2.0 microns, by forming carbon filaments by catalytic decomposi- tion of a hydrocarbon feed, coating the filaments with a ceramic coating and then oxidizing the coated filaments to remove the carbon core leaving behind hollow ceramic micro-tubular filaments (A). The ce- 00 ramic micro-tubular materials may be free-standing M porous structures and may have a variety of uses as <M thermal insulators, catalyst supports, superconductor CO supports, filters or as reinforcements for composites. 5 FIG. 2 a. u Rank Xerox (UK) Business Services EP 0 443 228 A1 This invention relates to producing a three- within a shaped mold, formed in the shape of a dimensional random weave of thin ceramic tubes. shuttle tile, coating the filaments with a Si contain- Although many filamentary materials are known ing deposit by chemical vapor deposition (CVD), for their utility as thermal insulators, there is still followed by oxidation to eliminate the carbon fila- need for higher efficiency systems and lower cost 5 ment core. The volume fraction as well as the fabrication procedures. Today's tiles used in the bridging of the filaments is easily controlled by this American Space Shuttle, for example, are pro- method as discussed in copending European pat- duced in a costly process from melt-spun filaments ent application 89313564.0, filed 22nd December, of SiO2. To produce the desired tile with controlled 1989. An added advantage of this process, in addi- volume fraction, the SiO2 filaments are chopped 10 tion to its low-cost, is the ability to produce ultra- up, randomized, placed in a mold and partially fine, less than 1 micron in diameter hollow fila- sintered under pressure at elevated temperatures. ments, with controlled diameter and wall thickness. The sintering process is designed to achieve a For the same volume fraction, such hollow struc- specific volume fraction of interwoven filaments tures should be even more effective in impeding that are firmly bonded at points of contact in a 75 heat transfer, while sacrificing little in structural random array. Such an open structure has high strength. thermal impedance coupled with modest structural The novel material to be described in more strength, which makes it ideal for shuttle tiles and detail hereinbelow may also find utility as a poten- other insulating purposes where thermal insulation tial replacement for environmentally hazardous as- for a transitory period is required. 20 bestos in insulating materials employed in the con- According to the present invention there is struction industry. The micro-tubular material mim- provided a method for producing a three-dimen- ics the structure of asbestos, i.e., it is in the form of sional random weave of thin ceramic tubes, which thin-walled hollow tubular filaments. method comprises: contacting a metallic catalyst Catalyst supports are generally ceramic materi- for growing multi-directional carbon fibers with one 25 als such as silicon dioxide or aluminum oxide pre- or more gaseous hydrocarbons in a mold at a pared with high surface areas in the form of pellets. temperature sufficient to form filamentary carbon The novel material disclosed herein, if grown in the and insufficient to cause the pyrolytic deposition of shape of a brick as for the shuttle tile, can also be carbon, depositing a conformal coating consisting employed to construct a very high and controllable of a ceramic or ceramic forming material on the 30 surface-area catalyst support bed of predetermined carbon fibers, and heating the coated fibers in an dimensions by stacking the bricks. Such a porous oxygen containing atmosphere for a time sufficient body, now of macroscopic dimensions, can now be to volatilize, by oxidation, all or substantially all of loaded with catalyst particles by known methods, the carbon, to form the ceramic tubes. for example, liquid infiltration. The method disclosed herein results in the 35 Exactly the same concept can also be ex- production of ceramic filamentary micro-tubular ploited to fabricate a fixed bed for simple filtration materials. The method is an entirely fluid-phase purposes, for example to remove dust particles method for producing such interwoven ceramic fila- from the air, or for more sophisticated chemical mentary tubular materials. The process depends separations when the surfaces of the filamentary for its success on the ability to generate a three- 40 networks have been appropriately pretreated, e.g., dimensional random weave of ceramic tubes, for to give a chemically absorptive surface. example with diameters in the range of about .01 to By the fabrication method disclosed herein it is 2.0 microns, by forming carbon filaments by the also possible to entirely fabricate ultra-fine com- catalytic decomposition of the hydrocarbon feed, posites that are reinforced with a three-dimension- before coating the filaments with the ceramic coat- 45 al, tubular network. This has very broad implica- ing and then oxidizing the coated filaments to re- tions for the design of advanced high specific move the carbon core, leaving behind hollow ce- strength composite structures. In the design of ramic micro-tubular filaments. The ceramic micro- such composites for structural applications, thin- tubular materials may be free-standing porous walled tubes are preferred reinforcing, or load-bear- structures and may have a variety of uses as so ing elements. This is because tubes make better thermal insulators, catalyst supports, superconduc- use of the intrinsic structural strength of materials tor supports, filters or as reinforcements for com- than rods of the same dimensions. In engineering posites. practice, tubular elements are frequently linked to- A preferred way of putting the present inven- gether to form a three-dimensional structure of tion into effect to produce an insulating material 55 great strength and flexibility, e.g., as in a geodesic comparable with that mentioned above for produc- dome. In nature, similar engineering principles are ing today's Shuttle tiles would involve growing an exploited, but with the added complication that the interwoven network of carbon substrate filaments hollow, or cellular structures are themselves com- EP 0 443 228 A1 posites of intricate design. Networks of cellulose drocarbon for forming the carbon fibers is intro- fibers provide much of the reinforcement in natural duced through conduit 4, a volatile ceramic pre- composites, e.g., trees, grasses, bamboos. Al- cursor is introduced through conduit 5 and oxy- though many attempts have been made to mimic gen or oxygen containing gas is introduced such natural composite designs, so far these efforts 5 through conduit 6. A matrix material may be have met with little success, primarily because of added through conduit 7. Gases may be re- the difficulty of making hollow filaments with appro- moved from the reactor through conduit 8. The priately small dimensions. conduits are fitted with valves (9) to control or By the present method, almost any desired stop the flow of the various materials into or filament-filler matrix combination can be produced io from the reactor. by utilizing chemical vapor deposition (CVD) to Figure 2 is a photomicrograph of a ceramic modify the surface properties of the filamentary micro-tubular material as shown by scanning micro-tubular material. Infiltration of filler matrix ma- electron microscopy. The Figure shows an en- terials can be achieved by adaptation of existing tangled weave of ceramic tubes. The letter A materials technologies. 75 indicates one such tube. Yet another application for these porous struc- Figure 3 is a photomicrograph of a ceramic tures is as substrates for the recently discovered micro-tubular material at a higher magnification thin superconducting oxide layers. These could be than Figure 2. The letter A indicates an end of a applied by soi-gei techniques, i.e., dipping and ceramic tube having an outside diameter of draining, or by more advanced techniques such as 20 about 1 micron and a wall thickness of about 0.1 by CVD from multiple sources. After deposition on micron. the surface of the filaments either an annealing The present invention provides for novel ce- treatment below the melting point, or a brief melt- ramic filamentary micro-tubular materials and a ing operation may be necessary to coarsen the versatile process for making such materials entirely grain structure of the superconducting phase. It is 25 from the gas phase with minimal handling. The known that an alumina substrate is ideal for this process relies on the rapid catalytic growth of in that good wetting between the alumina carbon filaments at temperatures typically less than purpose, ° and the high Technitium (Tc) superconducting ox- about 1 000 C from gas-phase precursors as de- ides occurs, without significant chemical degrada- scribed in the aforesaid European patent applica- tion if the exposure time is brief.