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 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. It is understood 30 tion 89313564.0. An example is the catalytic growth that all processing steps be carried out in an of thin filaments of carbon from gaseous hydrocar- oxygen-rich environment in order to maintain the bon, e.g. ethane at 700 °C using metal alloy cata- desired stoichiometry of such superconducting lysts. When specific metal alloy catalysts are em- phases. After coating the filaments with the desired ployed, as disclosed in the above European ap- thickness of the superconducting phase, the struc- 35 plication and as described hereinbelow, it is in the ture may be infiltrated with a polymer to achieve nature of the growth process that the filaments the desired flexibility and strength for structural intertwine to form a three-dimensional random applications such as filament windings for energy weave (self-woven) network, which has some struc- storage and transmission. tural strength as a free-standing form. By appro- It is clear from these examples that there are 40 priate choice of catalyst particles, filaments as many uses for such novel materials comprised of small as about 0.01 micron in diameter can be interwoven networks of ceramic hollow filaments, produced. whether coated or not. Other applications not listed As disclosed in European patent application can be envisioned which are obvious to those 83313564.0, carbon filaments may be catalytically skilled in the art. 45 grown at temperatures typically less than about For a better understanding of the invention and 1000°C from gas-phase precursors. An example is to show how the same may be carried into effect, the catalytic growth of thin filaments of carbon from reference will now be made, by way of example, to gaseous hydrocarbons, e.g., ethane at 700 C. The the accompanying drawings, wherein :- performance of the process involves rapid catalytic Figure 1 shows a cross-sectional view of a typi- so growth of carbon filaments which eventually ex- cal reactor for forming the present ceramic pand to fill the available space in the shaped mold. micro-tubular material. There is shown a reactor Furthermore, it is in the nature of the growth pro- (1) which may be a metal, glass or ceramic cess that the filaments intertwine to form a three- material of a shape selected to provide the dimensional random weave (self-woven network), desired three-dimensional configuration of the 55 which has some structural integrity as a free-stand- ceramic micro-tubular material containing a ing form. By appropriate choice of catalyst par- catalyst (2) for forming carbon fibers. Conduit 3 ticles, filaments as small as 0.01 micron in diam- is for introducing reactants into the reactor. Hy- eter can be produced. EP 0 443 228 A1 The carbon filaments can be grown catalyt- aspect ratios generally greater than about 200. The ically at elevated temperatures using hydrocarbon actual aspect ratio is not known definitively be- gases and a metal alloy catalyst. The carbon fila- cause it is difficult to find the ends of any one ments range in diameter from 0.01 micron to about particular filament in the intertwined network. 2 microns, may be several hundred microns long 5 Filaments grown from Ni/Cu alloys are further and are grown in a randomly intertwined network at distinguished from those grown from Ni by the a volume density ranging from about 2% to about location of the catalyst particle within the filament. 20%. Rather than being at the tip of the filament, the In more detail, a filamentary carbon network catalyst particle is predominantly found at the mid- can be formed in-situ within a mold by catalytic 70 point of two filaments. Thus a single catalyst par- decomposition of the hydrocarbon feed, utilizing ticle is found to grow two carbon filaments in metal alloy catalyst particles that are applied to the opposite directions. The two filaments are believed walls of the mold. The seeded mold is placed into to grow simultaneously. We have also observed a furnace or heated externally to the desired tem- instances of a single Ni/Cu particle growing as perature and a hydrocarbon gas is allowed to flow 75 many as six carbon filaments, suggestive of the six through the mold. The filamentary network is per- faces of a cube, all active for the growth of a mitted to grow until the mold cavity is filled with the carbon filament. This extraordinary multi-directional desired volume fraction of filaments. The natural growth is believed to be responsible for the re- tendency of the random weave of filaments uni- markably rapid and space-filling tendency of fila- formly to fill the available space in the mold en- 20 mentary carbon growths from the Ni/Cu systems. sures faithful replication of the internal surface fea- Other combinations of Group VIII-IB solid solu- tures of the mold. tion metal alloys such as Ni/Au, Co/Au, Fe/Au and The catalysts disclosed herein enable a rapid, Co/Cu also show significantly enhanced catalytic voluminous growth to be produced that tends to fill activity over the pure Group VIII metal and the available space. 25 alloy systems produce predominantly bi-directional It has been discovered that two classes of carbon filament growths. As in the case of the metal alloy systems yield such space-filling Ni/Cu system, these alloys are combinations of growths. Both classes are distinguished by metal known catalysts for filamentous carbon formation, combinations that form a series of solid solutions namely Ni, Fe and Co, and a non-catalyst, namely over their whole composition range and are a com- 30 Cu and Au. bination of a transition metal which is known to be A preferred example of the second class of a relatively good catalyst for filamentous carbon metal alloy catalyst is the Ni/Pd system. Ni and Pd growth and one which shows no catalytic activity also form a series of solid solutions over their whatever, or one which is a poor catalyst. whole composition range. Whereas Ni is a rela- The preferred alloy system of the first type is 35 tively active catalyst for filamentary carbon growth, based on the system Ni/Cu, an example of a Group Pd is a relatively poor catalyst. When an alloy of VIII metal, Ni, that is known to be a catalyst for Ni/Pd of about 50/50 wt% is used with ethane or filamentous carbon formation, and a Group IB met- ethylene, an almost "explosive" type of filamentary al, Cu, that is not a catalyst for filamentous carbon carbon growth is obtained. Like the Group VIII-IB formation. Ni and Cu form a series of solid solu- 40 combinations described above, the carbon growth tions over their whole composition range. A pre- tends to fill all available space, but at a much more ferred composition range for the purposes of this rapid rate, in minutes rather than in tens of min- process is from about 20 wt% Ni to about 90 wt% utes. The carbon filaments are also found to be Ni, with the range from about 40 wt% Ni to about predominantly bi-directional, i.e., at least two car- 80 wt% being more preferred. 45 bon filaments grow from a single catalyst particle. When Ni is used as a catalyst for the formation Because of the very rapid space-filling growth with of filamentous carbon from ethane or ethylene, the this catalyst system, the carbon growth tends to be filaments tend to be relatively short with an aspect of a very low volume density, typically about 2 vol ratio of about less than 10 and the Ni catalyst % rather than the more common 5 to about 10 vol particle is found at the tip of the filament. The Ni 50 % within the Ni/Cu system. Another distinguishing tends to become deactivated relatively quickly, feature of this type of growth is that it tends to be most likely by being coated with a layer of carbon "sponge-like" with some resiliency reminding one restricting further access of the hydrocarbon mol- of sponge-rubber. ecules. This results in a relatively inefficient pro- The unexpected beneficial nature of the addi- cess and relatively small yields of carbon per gram 55 tion of Pd to Ni has also been observed with of catalyst and very little intergrowth of the fila- another Group VIII base metal/Pd combination ments. Unexpectedly when Cu is added to the Ni, which forms a solid solution, namely Co/Pd. In this the alloy catalyst forms very long filaments with case also, the alloy system shows significantly EP 0 443 228 A1 higher catalytic activity for carbon growth than the useful as nominally pure Ni/Cu alloys. Ni/Cu sheet non-noble Group VIII metal, the carbon filaments of nominal 55 wt% and 45 wt% Cu composition are predominantly bi-directional and the carbon with minor amounts of Mn and Fe has also been growth has some resiliency and tends to fill avail- found to be an effective catalyst for filamentary able space. 5 carbon growth. The preferred form of the catalyst is a fine It has been found that one gram of Ni/Cu alloy powder, although bulk forms such as rolled sheet catalyst can generate at least 100 gm of filamen- or thin films may also be employed. There appears tary carbon before the catalyst particles become to be an appoximate one-to-one correspondence inactive. The actual catalyst loading of a mold can between the size of the catalyst particle and the w therefore be adjusted such that the desired volume diameter of the carbon filament and generally car- density is achieved. The metal alloy particles can bon filaments with diameters less than about 1 at least in part be leached out of the carbon fila- micron are preferred. Surprisingly even when bulk mentary network with acids if so desired. material or powder significantly larger than 1 mi- Hydrocarbon gases may be converted to a cron, up to 40 microns for example, is used, the 75 form of carbon by thermal pyrolysis alone. This alloy disintegrates during the filamentary carbon carbon may be either in particulate form, com- growth process such that particles ranging in size monly known as soot, formed by gas phase from about 0.01 micron to about 2 microns are nucleation and practised in the manufacture of car- generated. The process responsible for this ad- bon black, or in thin film, pyrolytic form when vantageous disintegration is believed to be similar 20 hydrocarbons are decomposed at very high tem- to one known as "metal dusting". peratures, generally above about 900° C. The latter For some applications it may be preferred to is essentially an example of a process more com- manufacture approximately monodisperse carbon monly known as chemical vapor deposition. filaments, i.e., filaments with substantially the same Catalytic filamentary carbon growth requires diameter. In this case it is clearly advantageous to 25 contact between a metal catalyst particle at ele- start with alloy catalyst particles that are all sub- vated temperatures and a carbon-bearing gas. Al- stantially of the same diameter and of a size sub- though various hydrocarbon gases can be used, stantially equal to the desired diameter of the car- ethylene or ethane are preferred. Formation of soot bon filaments. Such alloy powders could be pro- or pyrolytic carbon reduces the overall efficiency of duced by aerosol production from the melt or by 30 the process and may interfere as well with the thermal evaporation at relatively high pressures catalytic activity of the metal catalyst particles, and such that a metal "smoke" is generated, or by is therefore avoided in the practice of this process. pyrolysis of a volatile organo-metallic precursor. It has been proposed that carbon source gases Alloy particles may also be synthesized in-situ that undergo an exothermic decomposition reaction in a mold if desired. Such well-known techniques 35 to elemental carbon are required for filamentary as evaporating an aqueous solution of the salts in carbon growth. Thus gases such as CO, acetylene, Ni and Cu, e.g. nitrates, chlorides, etc., followed by ethylene and butadiene readily form filamentous calcining to the metal oxides and then reducing the carbon, whereas gases such as methane should oxides to the metal alloy by heating in hydrogen, not. Filamentous carbon growth from methane is may also be employed. The concentration of the 40 believed to require the thermal conversion of the metal salts is adjusted such that the desired alloy methane to less stable molecules prior to catalytic composition is obtained. The walls of a mold could conversion to carbon. thus be seeded with the desired metal alloy cata- For the purposes of this process, the hydrocar- lyst by wetting the walls with the starting aqueous bon gases are chosen such that they form insignifi- solution and forming the metal alloy particles in- 45 cant amounts of soot or pyrolytic carbon under the situ. Another technique that may be employed is to processing conditions, i.e., they will decompose to deposit films of the constituents metals onto the carbon only in the presence of the metal alloy walls of the mold by electroplating or electro-less catalyst. Ethylene is preferred in the temperature plating, for example, and then heating the depos- range 550° to 650° C, while ethane is preferred in ited films to form the alloy by interdiffusion. When so the temperature range 650° to 750° C. the hydrocarbon gas in introduced into the mold, Ethane and ethylene, two of the preferred hy- the film will disintegrate during the filamentary car- drocarbon source gases, are readily available and bon growth process. relatively inexpensive. Ethane requires somewhat Minor impurities in the metal catalyst do not higher temperatures for filamentous carbon growth appear to have significant effects. Thus, Monel 55 than does ethylene. The reason for this is believed powder of nominal 70 wt% Ni and 30 wt% Cu to be that the ethane first needs to be converted to compositions with less than about 1 wt% each of ethyiene by pyrolysis before the metal alloy cata- Mn and Fe as impurities has been found to be as lyst can form filamentous carbon. EP 0 443 228 A1 10 Benzene is relatively expensive and carcino- about 500 C to 650 C. For ethane, the tempera- ° ° genic, while CO is also highly toxic. Methane is ture may range from about 650 C to about 800 C. found to require very high reaction temperatures, In the latter case it is believed that the ethane above about 900 °C, for any filamentary carbon needs to be pyrolyzed or thermally converted to growth to occur with the alloy catalysts disclosed 5 ethylene before it can be catalytically converted to herein. Under these conditions, pyrolytic carbon filamentary carbon. Thus, one can envisage deposition also takes place which tends to coat the preheating the ethane to the required temperature catalyst particles with a layer of carbon and render and then letting it contact the catalyst at a lower them inactive relatively quickly. temperature. Acetylene will form filamentary carbon with w Although the carbon filament fabrication pro- Ni/Cu alloy catalysts at temperatures as low as cess disclosed hereinabove has been practised in ° 300 C. Pure acetylene, however, has a tendency an isothermal mode, there may be advantages to to readily form soot due to pyrolysis at tempera- growing the filaments at different temperatures in ° tures as low as about 450 C. order to control how quickly and uniformly the The purity of the feed gases does not appear 75 filamentary network fills up a mold. to be a critical factor. So-called "chemically pure" Flow rates are chosen to optimize the growth grades, approximately 98% purity, of ethane and rate of the carbon filaments and are better defined ethylene have been used successfully without fur- in terms of residence or contact time. A typical ther purification. Although it is likely that some contact time is of the order of 20 sec, although impurities such as hydrogen sulfide may poison 20 shorter as well as longer contact times have been the alloy catalysts, this is now known at the present used successfully. The actual flow rate used will time. Although nominally pure ethane and ethylene depend on the volume of the mold (or reactor) and are preferred, mixtures of these two gases, as well the processing temperature and is adjusted to as mixtures with inert gases such as nitrogen or achieve the desired residence time. Very short argon may also be employed. It has also been 25 residence times result in relatively inefficient use of found that the deliberate addition of hydrogen in the hydrocarbon gas. the range from about 10 vol% to about 90 vol% Although the process has been practised only prolongs the time that a Ni/Cu catalyst remains at ambient atmospheric pressure, the catalytic fila- active. It is speculated that the hydrogen keeps the mentary carbon can be synthesized at pressures catalyst particles relatively free of deposited carbon 30 below and above atmospheric pressure as well. films. Having described in detail different ways of Under the preferred conditions, as much as 80 fabricating the filamentary carbon, in the first step mole% of the ethylene or ethane being fed to the of the present process, a filamentary carbon net- reactor has been converted to carbon in the form work is formed in-situ within a reactor by the cata- of carbon filaments. The by-products of this de- 35 lytic decomposition of a hydrocarbon feed, utilizing composition have been analyzed by gas metal alloy catalyst particles that are applied to the chromatography and found to be primarily hydro- walls of the reactor. The seeded reactor is placed gen, with some methane, as well as smaller into a furnace or heated externally to the desired amounts of simple saturated hydrocarbon mole- temperature and a hydrocarbon gas is allowed to cules such as propane and butane. 40 flow through the reactor. The filamentary network is Any unreacted feed gas may be partially re- permitted to grow until the reactor cavity is filled cycled, i.e., mixed with pure feed gas to make the with the desired volume fraction of filaments. After process even more efficient. The effluent gas growth, the carbon filaments are converted to tubu- stream from the reactor may also be used as the lar ceramic filaments by first coating them with a source of hydrogen if that is desired during the 45 ceramic or ceramic forming material by chemical filamentary growth. vapor deposition, electro deposition and electro- The temperature range available for the fab- less deposition and then exposing them to an oxi- rication of the carbon filaments with the preferred dizing atmosphere at elevated temperature such catalyst systems, i.e., Ni/Cu or Ni/Pd, is from about that the carbon filaments are oxidized and removed 300° C to about 800° C and is determined primarily so as carbon monoxide or carbon dioxide. This pro- by the hydrocarbon gas used. At lower tempera- cess may be carried out within the same reactor tures the rates of carbon growth are not sufficiently described above without any need for handling the rapid, while at higher temperatures, the catalyst filamentary network. Thus the carbon network may particles tend to become coated with a carbon be coated with a layer of SiO2, for example, ap- coating rendering them inactive. 55 plied by CVD using tetraethoxysilane which results It has been shown that, for Ni/Cu and acety- in a shape conformal coating, covering all exposed lene, temperatures as low as 300° C may be used. surfaces of the carbon filaments. Hexamethyl- For ethylene the temperature may range from disilazane (HMDS) may be used to lay down a 11 EP 0 443 228 A1 12 SiC, 2. A method according to claim 1, wherein said metallic catalyst is in finely divided form and the catalyst is contacted with the hydrocarbon- (s) in said mold at a temperature from the 25 dissociation temperature of the hydrocarbon to about 900° C, and wherein the conformal ce- ramic coating is deposited to a thickness of from about 0.01 micron to about 0.5 micron on the carbon fibers. 30 3. A method according to claim 1 or 2, wherein the conformal coating is deposited by chemical vapor deposition, electroplating or electro-less deposition. 35 4. A method according to claim 1 , 2 or 3, wherein a second conformal coating is deposited on the ceramic tubes after volatilizing the carbon. 40 5. A method according to claim 4, wherein the second conformal coating is carbon, a metal, a ceramic compound or mixture thereof. 6. A method according to any preceding claim, 45 wherein a liquid matrix material is introduced to the mold and converted to a solid after the volatilization step. 7. A method according to claim 6, wherein the 50 matrix material is a polymer, elastomer, epoxy resin, metal, alloy or ceramic. 8. A method according to claim 6 or 7, wherein the resulting structure has at least one section 55 with a thickness of less than one millimeter. 9. A method according to any preceding claim, EP 0 443 228 A1 -8 FIG. I EP 0 443 228 A1 FIG. 2 ij-* FIG. 3 10 Patent Application Number J) European EUROPEAN SEARCH REPORT Office EP 90 30 1777 DOCUMENTS CONSIDERED TO BE RELEVANT Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE Category of relevant passages to claim APPLICATION Ont. C1.5) EP-A-0 198 558 (DOW CHEMICAL) 1-3,6,7 C 04 B 30/02 * claims 1,16 * ,10 Abstract; C 04 B 14/46 C 08 K 7/04 CHEMICAL ABSTRACTS, vol. 107, no. 10, 1-3,6,7 C 04 B 35/80 1987, 308, abstract no. ,10 September page C 22 C 1/09 82733f, Columbus, Ohio, US; & JP-A-62 124 909 (MITSUI ENGINEERING AND SHIPBUILDING CO., LTD) 06-06-1987 EP-A-0 325 236 (YAZAKI CORP.) 1,9 * Abstract; claim 6; page 2, line 53 - page 3, line 2 * CHEMICAL ABSTRACTS, vol. 106, no. 18, May 1987, page 292, abstract no. 142876w, Columbus, Ohio, US; & JP-A-62 04 997 (MITSUI ENGINEERING AND SHIPBUILDING CO., LTD) 10-01-1987 DE-A-3 421 196 (DORNIER SYSTEM) 1,6,7 * Claims 1,6 * TECHNICAL FIELDS SEARCHED am. C1.5) C 04 B 30 C 04 B 38 C 04 B 35 C 04 B 14 C 08 K 7 D 01 F 9 The present search report has been drawn up for all claims Place of search Dale of completion of the search THE HAGUE 08-10-1990 DAELEMAN P.C.A. CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention 3 E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document