Abstract:
A cold isopressing method and mold for compacting a granular ceramic material in which the granular ceramic material is introduced into a cylindrical pressure bearing element of an isopressing mold. The cylindrical pressure bearing element is sufficiently rigid so as to maintain its shape during the introducing of the granular ceramic material. Such element is also sufficiently resilient in a radial direction thereof to deform and bear against the granular ceramic material upon the application of the hydrostatic pressure and to substantially return to its original shape upon the relaxation of the hydrostatic pressure, thereby to allow retraction of the cylindrical pressure bearing element from the granular ceramic material after compaction. In a further aspect, an isopressing method and mold is provided in which the cylindrical pressure bearing element thereof is provided with an enlarged end bore to form an enlarged end section in the finished ceramic tube for sealing purposes.

Description:
This application is a continuation-in-part of Ser. No. 09/668,728 filed Sep. 22, 2000 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a cold isopressing method and mold for compacting a granular ceramic material for use in manufacturing ceramic tubes. More particularly, the present invention relates to such a method and mold in which the ceramic tubes are useful in forming ceramic membrane elements of the type exhibiting infinite hydrogen or oxygen selectivity. In other aspects, the present invention relates to a cold isopressing method and mold in which ceramic tubes are formed having enlarged end portions that are more amenable to sealing than prior art ceramic tubes. 
     BACKGROUND OF THE INVENTION 
     Ceramic tubes have many industrial uses, for instance, insulators, filters, etc. An important recent use involves the employment of such tubes in membrane modules to separate oxygen or hydrogen from a gas stream. The ceramic tubes used in membrane separation applications are manufactured from materials that exhibit infinite oxygen or hydrogen selectivity at high temperatures. In an oxygen-selective membrane, oxygen is ionized at one surface of the membrane to form oxygen ions. The oxygen ions travel through the membrane to the opposite surface thereof where the oxygen ions recombine to form elemental oxygen. In forming the elemental oxygen, electrons are given up from the ions. Depending upon the type of material used in fabricating the membrane, the electrons either flow through the membrane material itself or through separate conductive pathways to initially ionize the oxygen. 
     The traditional method fabricating ceramic tubes involves processes such as slip casting or extrusion. The use of extrusion to form solid-state membrane modules is disclosed in U.S. Pat. No. 5,599,383. 
     Another known method of fabricating ceramic tubes is isopressing. In cold isopressing, a tubular mold is utilized that is formed from a soft neoprene rubber. This mold, known as a bag, is used in conjunction with a mandrel that projects into the bag. The bag is filled with a ceramic material in granular form. The mold is subjected to a hydrostatic pressure within a vessel containing cold or warm water that is sufficient to compact the ceramic material into a green ceramic tube. After compaction, the hydrostatic pressure is relaxed and the green ceramic form is removed from the mold. In this regard, the mandrel is provided with a slight taper to permit separation of the green ceramic tube from the mandrel. The green ceramic tube can then be heated to burn out organic binder materials and the like and to sinter the ceramic. 
     The prior art method of isopressing has found application in the manufacture of short thick tubes. When longer tubes are attempted by this method, defects are found in the fired and sintered tubes. The reason for such defects is that it is impossible to introduce ceramic powder into the mold so that the powder is uniformly distributed. For instance, if there exists a slight wrinkle in the mold, the powder will tend to hang up on the wrinkle to produce a defect in the finished ceramic tube. 
     As will be discussed, the present invention in one aspect provides a cold isopressing method and mold that is particularly applicable to forming long thin, ceramic tubes that can be used in ceramic membrane applications. As will also be discussed, other aspects of the present invention are particularly useful in the fabrication of ceramic tubes having end configurations that are more amenable to sealing than prior art sealing methods. 
     SUMMARY OF THE INVENTION 
     The present invention provides a cold isopressing method for compacting a granular ceramic material. As used herein and in the claims, the term “granular ceramic material” means ceramic powder or a mixture comprising a ceramic powder, an organic binder and a plasticizing agent. In accordance with such method, the granular ceramic material is introduced into an isopressing mold having a cylindrical pressure bearing element and at least one mandrel located within the cylindrical pressure bearing element. After the cylindrical pressure bearing element is sealed, the cylindrical pressure bearing element is subjected to a hydrostatic pressure to compact the granular ceramic material. The hydrostatic pressure is relaxed after such compaction. The cylindrical pressure bearing element is sufficiently rigid so as to maintain its shape during the introducing of the granular ceramic material and is also sufficiently resilient in a radial direction thereof to deform and bear against the granular ceramic material upon the application of the hydrostatic pressure and to at least substantially return to its original shape upon the relaxation of the hydrostatic pressure. 
     The resiliency of the cylindrical pressure bearing element allows retraction of the cylindrical pressure bearing element from the granular ceramic material after compaction. In this regard, the use of the term, “at least substantially” with respect to the return of the cylindrical pressure bearing element to its original shape admits to the possibility that after relaxation of hydrostatic pressure, the cylindrical pressure bearing element might be slightly out of round. However, such cylindrical pressure bearing element should not be so out of round that the mold is unable to completely retract from the granular ceramic material. 
     As may be appreciated, the provision of a cylindrical pressure bearing element that will maintain its shape during filling allows for long thin tubes to be fabricated with the ceramic material having a uniform density along the length of the tube. Gross maldistributions of ceramic material that are caused by the wrinkling of the cylindrical pressure bearing element during filling are eliminated. Moreover, the resiliency of the cylindrical pressure bearing element ensures a clean separation of the mold from the compacted ceramic material to also prevent tube defects. 
     The at least one mandrel can be a single mandrel to produce a tube-like structure. Alternatively, multiple, parallel mandrels can be used to form a cylindrical structure having internal passageways. Both types of structures, the tube or the cylindrical structure having internal passageways would be useful in forming ceramic membrane elements. 
     The at least one mandrel can be connected to an enlarged base element that projects into one end of the cylindrical pressure bearing element, thereby to seal the cylindrical pressure bearing element at the one end. The granular ceramic material is introduced into the isopressing mold through the other end of the cylindrical pressure bearing element during filling. A removable end plug is positioned within the other end of the cylindrical pressure bearing element to seal such other end thereof. In this regard, the term “removable” as used herein and in the claims means that such end plug can be removed and is not permanently bonded attached to the cylindrical pressure bearing element. 
     One end of the cylindrical pressure bearing element can be provided with an enlarged, axial end bore positioned so that the compacted granular ceramic material has an enlarged end section. As will be discussed, such an enlarged end section can be used in a sealing arrangement at the connection of a finished ceramic tube to a tubesheet. 
     Preferably, the isopressing mold is vibrated while the granular ceramic material is introduced into the isopressing mold. The vibrations act on the cylindrical pressure bearing element member to help fill the mold and ensure that there is no hang up of the granular ceramic material within the mold. Such transmission of vibrations is possible due to the rigidity of a cylindrical pressure bearing element in accordance with the present invention as opposed to a soft rubber bag of the prior art. As may be appreciated, the use of such a vibration technique during filling is particularly important in the fabrication of long, thin tubes. 
     In case of a single mandrel, the cylindrical pressure bearing element can advantageously be sized such that the granular ceramic material prior to compaction occupies an annular space having a wall thickness no less than about twice that of the granular ceramic material after compaction. 
     In granular ceramic materials containing a binder, the binder can comprise no more than about 5% by weight of the granular ceramic material. This is to be contrasted with ceramic tubes of the prior art in which binder is 10% or more by weight of the granular ceramic material. The greater the binder content, the greater the time expended in firing and sintering a green ceramic tube formed of compacted, granular ceramic material and also, the greater possibility of tube defects. 
     As stated above, the present invention is particularly applicable to forming membrane units for separating oxygen and hydrogen. As such, the granular ceramic material can be the type that is a capable of conducting one of oxygen ions and hydrogen ions. These membrane elements are preferably in the form of long thin tubes. However, the formation of long thin cylindrical elements in accordance with the present invention are possible. In order such manufacture tubes, a cylindrical pressure bearing element can be provided with a length of no less than about 60 millimeters and a diameter such that the granular ceramic material prior to compaction occupies an annular space having a radial thickness of no more than about 4 millimeters. Unlike the prior art, the single mandrel used in a mold to form a tube need not be tapered and can be of cylindrical configuration. 
     In another aspect, the present invention provides an isopressing mold for compacting a granular ceramic material. The isopressing mold can be provided with a cylindrical pressure bearing element and at least one mandrel located within the cylindrical pressure bearing element. If tubes are to be formed, the at least one mandrel can be a single mandrel. The cylindrical pressure bearing element is sufficiently rigid so as to maintain its shape upon introduction of the granular ceramic material into the cylindrical pressure bearing element and is also sufficiently resilient in a radial direction thereof to deform and bear against the granular ceramic material upon an application of a hydrostatic pressure and to at least substantially return to its original shape upon the relaxation of the hydrostatic pressure. As stated above, this allows the cylindrical pressure bearing element to retract from the granular ceramic material after compaction thereof by the hydrostatic pressure. A means is provided for sealing the cylindrical pressure bearing element at opposite ends thereof. 
     Preferably, the sealing means comprise an enlarged base element connected to the at least one mandrel and positioned within one end of the cylindrical pressure bearing element, thereby to seal the cylindrical pressure bearing element at the one end thereof. A removable end plug is positioned within said other end of said cylindrical pressure bearing element to seal such other end. 
     One end of the cylindrical pressure bearing element can be provided with an enlarged, axial end bore positioned so that the granular ceramic material after having been compacted has an enlarged end section. In case of a single mandrel, the cylindrical pressure bearing element can be sized such that the granular ceramic material prior to compaction occupies an annular space having a wall thickness no less than about twice that of the granular ceramic material after compaction. 
     The isopressing mold can further comprise a rigid support attached to the cylindrical pressure bearing element to ensure that the cylindrical pressure bearing element remains straight during mold filling with the granular ceramic material. Preferably, the material making up the cylindrical pressure bearing element is polyurethane having a hardness of 95A on the durometer scale. 
     As stated above with respect to a single mandrel embodiment of the present invention, the cylindrical pressure bearing element can advantageously have a length of no less than about 60 millimeters and a diameter such that the granular ceramic material prior to compaction occupies an annular space having a radial thickness of no more than about 4 millimeters. 
     Unlike isopressing molds of the prior art in which the mandrel is tapered, an embodiment of the present invention employing a single mandrel can employ such mandrel with a cylindrical configuration throughout. 
     As mentioned previously, short ceramic elements can be readily manufactured by the use of prior art isopressing techniques. Such prior art techniques can be advantageously adapted in accordance with the present invention to produce ceramic tubes having enlarged end sections for sealing purposes. In this regard, further aspects of the present invention involve the manufacture of green ceramic tubular forms having enlarged end sections for sealing purposes. 
     In one of these further aspect, a cold isopressing method is provided for compacting a granular ceramic material into a green ceramic tubular form having an enlarged end section. In accordance with such method, the granular ceramic material is introduced into an isopressing mold having a cylindrical pressure bearing element and at least one mandrel located within the cylindrical pressure bearing element. If tubes are to be formed then the at least one mandrel can be a single mandrel. The cylindrical pressure bearing element is sealed and subjected to a hydrostatic pressure to compact the granular ceramic material into the green ceramic tubular form. Thereafter, the hydrostatic pressure is released. The cylindrical pressure bearing element has an enlarged end bore to produce the enlarged end section of the green ceramic tubular form. 
     In another of the further aspects of the present invention, an isopressing mold is provided for compacting a granular ceramic material into a green ceramic tubular form having an enlarged end section. In accordance with such aspect, the isopressing mold comprises a cylindrical pressure bearing element and at least one mandrel (a single mandrel if tubes are to be formed) located within the cylindrical pressure bearing element. The cylindrical pressure bearing element has an enlarged end bore to produce the enlarged end section within the green ceramic tubular form. A means is provided for sealing the cylindrical pressure bearing element at opposite ends thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims distinctly pointing out the subject matter that applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which: 
     FIG. 1 illustrates an isopressing mold for carrying out a method of the subject invention prior to mold filling with granular ceramic material; 
     FIG. 2 illustrates a filling of the isopressing mold of FIG. 1 with the granular ceramic powder; 
     FIG. 3 illustrates the isopressing mold of FIG. 1 after filling and sealing; 
     FIG. 4 illustrates the isopressing mold of FIG. 1 after relaxation of hydrostatic pressure; 
     FIG. 5 illustrates the isopressing mold of FIG. 1 reinforced by a reinforcing rod; 
     FIG. 6 is a sectional view of FIG. 5 taken along line  6 — 6  of FIG. 5; and 
     FIG. 7 illustrates a ceramic tube fabricated in accordance with the present invention in service as a ceramic membrane element. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, a mold  1  in accordance with the present invention is illustrated. Mold  1  is formed of a cylindrical pressure bearing element  10  and a mandrel  12  coaxially located within cylindrical pressure bearing element  10 . As will be discussed, ceramic material is compacted between cylindrical pressure bearing element  10  and mandrel  12  to form a green ceramic tubular form that can be finished, by firing and sintering in a known manner, to produce a ceramic tube. Although one mandrel  12  is illustrated, multiple mandrels might be employed to fabricate known cylindrical ceramic structures having multiple internal passageways. 
     Cylindrical pressure bearing element  10  is of tubular form and is provided with an inner passageway  14  communicating between ends  16  and  18  thereof. Cylindrical pressure bearing element  10  (for purposes to be discussed hereinafter) can be provided with an enlarged end bore  20 . 
     Mandrel  12  acts to form the inside of the finished ceramic tube. The upper end  24  of mandrel  12  is of hemispherical configuration to form a hemispherical inner surface at a closed end of such finished ceramic form. Further, mandrel  12  is preferably of cylindrical configuration without any taper. An enlarged base element  26  can be connected to mandrel  12  for sealing purposes. In this regard, enlarged base element  26  is configured to be positioned into end  16  of cylindrical pressure bearing element  10 . 
     Although not illustrated, mandrel  12  can be textured to increase the surface area on the inner surface of the finished ceramic tube. Further, in the event that a ceramic tube open at both ends were desired, the hemispherical configuration given upper end  24  of mandrel  12  could be deleted. 
     With reference to FIG. 2, a granular ceramic material  28  is introduced into mold  1  for compaction through end  18  of cylindrical pressure bearing element  10 . For example, in case of an oxygen-selective, ion transport membrane element, ceramic material  28  could can be a mixed conducting ceramic capable of conducting oxygen ions and electrons. Examples of such materials are set forth in the following table. 
     
       
         
               
             
               
               
             
               
               
               
               
             
               
               
             
           
               
                   
               
               
                 EXAMPLES OF MIXED CONDUCTING SOLID ELECTROLYTES 
               
             
          
           
               
                   
                  Material composition 
               
               
                   
               
               
                   1. 
                  (La 1-x Sr x )(Co 1-y Fe y )O 3-δ (0 ≦ y ≦ 1, δ from stoichiometry) 
               
               
                  2. 
                 SrMnO 3-δ   
               
               
                   
                 SrMn 1-x Co x O 3-δ (0 ≦ x ≦ 1, δ from stoichiometry) 
               
               
                   
                 Sr 1-x Na x MnO 3-δ   
               
               
                  3. 
                 BaFe 0.5 Co 0.5 YO 3   
               
               
                   
                 SrCeO 3   
               
               
                   
                 YBa 2 Cu 3 O 7-δ  (0 ≦ δ ≦ 1, δ from stoichiometry) 
               
               
                  4. 
                 La 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 2.6 ; Pr 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 2.6   
               
               
                  5. 
                 A x A′ x′ A″ x″ B y B′ y′ B″ y″ O 3-z   
               
               
                   
                 (x, x′, x″, y, y′, y″ and z all in 0-1 range) 
               
               
                   
                 where: A, A′, A″ = from groups 1, 2, 3 and f-block lanthanides 
               
               
                   
                    B, B′, B″ = from d-block transition metals 
               
             
          
           
               
                   6. 
                  (a) Co—La—Bi type: 
                 Cobalt oxide 
                 15-75 mole % 
               
               
                   
                   
                 Lanthanum oxide 
                 13-45 mole % 
               
               
                   
                   
                 Bismuth oxide 
                 17-50 mole % 
               
               
                   
                 (b) Co—Sr—Ce type: 
                 Cobalt oxide 
                 15-40 mole % 
               
               
                   
                   
                 Strontium oxide 
                 40-55 mole % 
               
               
                   
                   
                 Cerium oxide 
                 15-40 mole % 
               
               
                   
                 (c) Co—Sr—Bi type: 
                 Cobalt oxide 
                 10-40 mole % 
               
               
                   
                   
                 Strontium oxide 
                  5-50 mole % 
               
               
                   
                   
                 Bismuth oxide 
                 35-70 mole % 
               
               
                   
                 (d) Co—La—Ce type: 
                 Cobalt oxide 
                 10-40 mole % 
               
               
                   
                   
                 Lanthanum oxide 
                 10-40 mole % 
               
               
                   
                   
                 Cerium oxide 
                 30-70 mole % 
               
               
                   
                 (e) Co—La—Sr—Bi type: 
                 Cobalt oxide 
                 15-70 mole % 
               
               
                   
                   
                 Lanthanum oxide 
                  1-40 mole % 
               
               
                   
                   
                 Strontium oxide 
                  1-40 mole % 
               
               
                   
                   
                 Bismuth oxide 
                 25-50 mole % 
               
               
                   
                 (f) Co—La—Sr—Ce type: 
                 Cobalt oxide 
                 10-40 mole % 
               
               
                   
                   
                 Lanthanum oxide 
                  1-35 mole % 
               
               
                   
                   
                 Strontium oxide 
                  1-35 mole % 
               
               
                   
                   
                 Cerium oxide 
                 30-70 mole % 
               
             
          
           
               
                   7. 
                  Bi 2-x-y M′ x M y O 3-δ  (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, δ from stoichiometry) 
               
               
                   
                 where: M′ = Er, Y, Tm, Yb, Tb, Lu, Nd, Sm, Dy, Sr, Hf, Th, Ta, Nb, 
               
               
                   
                    Pb, Sn, In, Ca, Sr, La and mixtures thereof 
               
               
                   
                 M = Mn Fe, Co, Ni, Cu and mixtures thereof 
               
               
                  8. 
                 BaCe 1-x Gd x O 3-x /2 where, 
               
               
                   
                 x equals from zero to about 1. 
               
               
                  9. 
                 One of the materials of A s A′ t B u B′ v B″ w O x  family whose 
               
               
                   
                 composition is disclosed in U.S. Pat. No. 5,306,411 (Mazanec et al.) 
               
               
                   
                 as follows: 
               
               
                   
                 A represents a lanthanide or Y, or a mixture thereof; 
               
               
                   
                 A′ represents an alkaline earth metal or a mixture thereof; 
               
               
                   
                 B represents Fe; 
               
               
                   
                 B′ represents Cr or Ti, or a mixture thereof; 
               
               
                   
                 B″ represents Mn, Co, V, Ni or Cu, or a mixture thereof; 
               
               
                   
                 and s, t, u, v, w, and x are numbers such that: 
               
               
                   
                 s/t equals from about 0.01 to about 100; 
               
               
                   
                 u equals from about 0.01 to about 1; 
               
               
                   
                 v equals from zero to about 1; 
               
               
                   
                 w equals from zero to about 1; 
               
               
                   
                 x equals a number that satisfies the valences of the A, A′, B, B′, B″ in 
               
               
                   
                 the formula; and 0.9 &lt; (s + t)/(u + v + w) &lt; 1.1 
               
               
                 10. 
                 One of the materials of Ce 1-x A x O 2-δ  family, where: 
               
               
                   
                 A represents a lanthanide, Ru, or Y; or a mixture thereof; 
               
               
                   
                 x equals from zero to about 1; 
               
               
                   
                 y equals from zero to about 1; 
               
               
                   
                 δ equals a number that satisfies the valences of Ce and A in the 
               
               
                   
                 formula. 
               
               
                 11. 
                 One of the materials of Sr 1-x Bi x FeO 3-δ  family where: 
               
               
                   
                 A represents a lanthanide or Y, or a mixture thereof; 
               
               
                   
                 x equals from zero to about 1; 
               
               
                   
                 y equals from zero to about 1; 
               
               
                   
                 δ equals a number that satisfies the valences of Ce and A in the 
               
               
                   
                 formula. 
               
               
                 12. 
                 One of the materials of Sr x Fe y Co z O w  family where: 
               
               
                   
                 x equals from zero to about 1; 
               
               
                   
                 y equals from zero to about 1; 
               
               
                   
                 z equals from zero to about 1; 
               
               
                   
                 w equals a number that satisfies the valences of Sr, Fe and Co in the 
               
               
                   
                 formula. 
               
               
                 13. 
                 Dual phase mixed conductors (electronic/ionic): 
               
               
                   
                 (Pd) 0.5 /(YSZ) 0.5   
               
               
                   
                 (Pt) 0.5 /(YSZ) 0.5   
               
               
                   
                 (B-MgLaCrO x ) 0.5 (YSZ) 0.5   
               
               
                   
                 (In 90 %Pt 10 %) 0.6 /(YSZ) 0.5   
               
               
                   
                 (In 90 %Pt 10 %) 0.5 /(YSZ) 0.5   
               
               
                   
                 (In 95 %Pr 2.5 %Zr 2.5 %) 0.5 /(YSZ) 0.5   
               
               
                   
                 Any of the materials described in 1-13, to which a high temperature 
               
               
                   
                 metallic phase (e.g., Pd, Pt, Ag, Au, Ti, Ta, W) is added. 
               
               
                 14. 
                 One of the materials of A 2-x A′ x B 2-y B′ y O 5+z  family whose composition 
               
               
                   
                 is disclosed in WO 97/41060 (Schwartz et al.) as follows: 
               
               
                   
                 A represents an alkaline earth metal or a mixture thereof; 
               
               
                   
                 A′ represents a lanthanide or Y, or a mixture thereof; 
               
               
                   
                 B represents a metal ion or mixtures of 3d transition metal ions and 
               
               
                   
                 group 13 metals; 
               
               
                   
                 B′ represents a metal ion or mixtures of 3d transition metal ions and 
               
               
                   
                 group 13 metals, the lanthanides and yttrium; 
               
               
                   
                 0 &lt; x  &lt; 2; 0 &lt; y &lt; 2; z renders the compound charge neutral 
               
               
                 15. 
                 One the matrials of Ln x A′ x Co y Fe y′ Cu y″ O 3-z  family whose 
               
               
                   
                 composition is disclosed in EP 0 732 305 A1 (Dyer et al.) as follows: 
               
               
                   
                 Ln represents a f block lanthanide; 
               
               
                   
                 A′ represents Sr or Ca; 
               
               
                   
                 x &gt; 0, y &gt; 0, x + x′ = 1, y + y′ + y″ = 1, 0 &lt; y ≦ 0.4 
               
               
                   
                 z renders the compound charge neutral 
               
               
                 16. 
                 One of the materials of Ln x A′ x A″ x″ B y B′ y′ B″ y″ O 3-z O 3-z  family whose 
               
               
                   
                 composition is disclosed in EP 0 931 763 A1 (Dyer et al.) as follows: 
               
               
                   
                 Ln represents a f block lanthanide; 
               
               
                   
                 A′ from groups 2; 
               
               
                   
                 A″ from groups 1, 2, 3 and f-block lanthanides 
               
               
                   
                 B, B′ from d-block transition metals excluding Ti and Cr 
               
               
                   
                 0 ≦ x  &lt; 1, 0 &lt; x′ ≦ 1, 0 &lt; y &lt; 1.1, 0 ≦ y′ &lt; 1.1, x + x′ + x″ = 1.0, 
               
               
                   
                 1.1 &gt; y + y′ &gt; 1.0, z renders the compound charge neutral 
               
               
                   
               
             
          
         
       
     
     Unlike prior art techniques in which the binder content is about 10% by weight, the binder content of ceramic material  28  can preferably be about 5% by weight. As stated previously, this reduces processing times in that there is less organic binder to burn out during the firing of the green ceramic tubular form produced by mold  1 . This also leads to fewer defects in the finished ceramic tube. 
     In order to facilitate the introduction of ceramic material  28  into mold  1 , mold  1  is rested on a vibrating surface  30  provided by a vibrator  32 . Vibrations transmitted through enlarged base portion  26  of mandrel  12  are transmitted to cylindrical pressure bearing element  10 . The use of vibrator  32  is particularly preferred when long ceramic tubes are to be formed in that it prevents hang up of ceramic material  28  without the need to resort to tamping and the like. 
     With reference to FIG. 3, after mold  1  is filled, an end cap  34  is inserted into end  18  of cylindrical pressure bearing element  1 . End cap  34  thereby completes the sealing of mold  1 . As illustrated, end cap  34  is configured to provide a slight clearance between upper end  24  of mandrel  12  to produce a finished ceramic tube having a closed end. The inner surface of end cap  34  is rounded to round the desired closed end of the ceramic tube. As may be appreciated, end cap  34  could be constructed to touch upper end  24  of mandrel  12  so that the finished ceramic tube were open ended. 
     Mold  1  in the state shown in FIG. 3 is then immersed in a liquid such as cold or warm water or other liquid capable of transmitting pressure and subjected to hydrostatic pressure. As such, mold  1  is subjected to a cold isopressing process. The exact pressure and time of course will vary with the actual dimensions of mold  1 . With additional reference to FIG. 4, after the end of the molding a green ceramic tubular form  36  is produced with a closed end  38  and an enlarged end section  40 . Mold  1  is then disassembled by removing end cap  34  (as illustrated) and separating cylindrical pressure bearing element tube  10  from mandrel  12 . Green ceramic tubular form  36  is then removed from mandrel  12  for further processing such as by firing and sintering. 
     With additional reference to FIGS. 5 and 6, mold  1  can be reinforced to ensure cylindrical pressure bearing element  10  remains straight during molding by a steel reinforcing bar  42  of angle stock strapped to cylindrical pressure bearing element  10  by straps  44 , preferably formed from a metal such as aluminum or steel. 
     Cylindrical pressure bearing element  10  is preferably made of a material, that for the given dimensions of cylindrical pressure bearing element  10 , will result in a sufficient rigidity thereof that ceramic material  28  can be introduced into mold  1  (between cylindrical pressure bearing element  10  and mandrel  12 ) while cylindrical pressure bearing element  10  maintains its shape. In this regard, the concern here is to prevent wrinkling of cylindrical pressure bearing element  10  that could produce a hang up of ceramic material within the annular filling space formed between cylindrical pressure bearing element  10  and mandrel  12 . Furthermore, such rigidity ensures that the transverse cross-section of such annular filling space will remain constant along the length of mold  1  so that the finished ceramic tube has a constant thickness. A further material consideration for cylindrical pressure bearing element  10  is that it must be sufficiently resilient to retract or equally pull away from the molded article to allow the finished green ceramic form to be removed from the mold after relaxation of hydrostatic pressure. 
     Preferably, cylindrical pressure bearing element  10  is fabricated from a material such as polyurethane with a hardness of 95A on the durometer scale. Hardnesses of between 75A and 75D on the durometer scale are also useful. Harder materials are preferred over softer materials because it has been found that ceramic materials tends not to adhere to harder materials. 
     As stated previously, the present invention is particularly useful in forming long thin ceramic tubes having lengths greater than about 60 mm and wall thicknesses in the order of about 1 mm. In order to form such ceramic tubes, cylindrical pressure bearing element  10  is provided with a length greater than bout 60 mm and mandrel  12  is sized slightly shorter to accommodate end cap  34 . The inner diameter of cylindrical pressure bearing element  10  and the outer diameter of bar-like portion  22  of mandrel  12  are sized to produce a radial thickness within the resultant annular filling area of about 4 mm. As a general rule, such radial thickness should be no less than twice the desired thickness of compacted ceramic material. 
     By way of example, for a ceramic tube length greater than about 60 mm., an outer diameter of about 20 mm and a wall thickness of about 1 mm, formed of a ceramic material given above, mold  1  will be subjected to a pressure of about 275 Mpa (40 ksi) for about 5 minutes to produce the green ceramic tubular form such as 36. 
     As stated above, the present invention in further aspects concerns the formation of tubular structures with enlarged end sections for sealing purposes. The use of such a structure is shown in FIG. 7 which illustrates a joint assembly  2 . Joint assembly  2  is designed to join a ceramic membrane  3  to a tube sheet  4 . Ceramic membrane  3 , produced by firing and sintering green ceramic tubular form  36 , is provided with a closed end  46  and an enlarged end section  48 . Joint assembly  2  consists of a fixture  50  connected to tube sheet  4  and a follower  52  having a hex-like head  54 . 
     Follower  52  and its hex-like head  54  are provided with internal bores  56  and  58 , respectively, that form an inner passage to allow a permeate stream produced within ceramic membrane  3  to flow through follower  52  and therefore joint assembly  2 . 
     Follower  52  fits within a passageway  60  of fixture  50  and enlarged end section  48  of ceramic membrane  3   m  fits within a cylindrical pressure bearing element cavity  62  of follower  52  with the remainder of ceramic membrane  3  projecting from fixture  50 . A threaded engagement  64  is provided between follower  52  and fixture  50  so that follower  52  is able to exert pressure against a rope seal  66  wrapped around enlarged head section  17  and against enlarged head section  48 . Enlarged head section  48  in turn exerts pressure on a packing  68  situated between the undersurface of enlarged head section  48  and the end of passageway  60 . In this regard, hex-like head  54  of follower  52  facilitates the exertion of torque upon follower  52  and therefore the pressure against rope seals  66  and  68  with a wrench or a like tool. 
     The pressure exerted upon rope seal  66  and packing  68  creates a seal between the lateral surface and undersurface of enlarged head section  48  and follower  52  to seal ceramic membrane  3  to fixture  50  and therefore tube sheet  4 . 
     It is to be noted that fixture  50  and follower  52  can be fabricated from HAYNES 230 alloy. HAYNES 214 and INCOLOY 800 are other possible materials. The rope seals  48  and  50  can be formed of plied and twisted filaments of alumina-boriasilicate. Such filaments are sold commercially as NEXTEL 312 ceramic fibers as a product of 3M Ceramics Materials Department, 3M Center, St. Paul, Minn., 55144, United States. The particular rope-like material, described above, can be obtained in finished form from Coltronics Corp., Brooklyn, N.Y., United States, as item# CT301. 
     It should be further noted that although enlarged head section  17  is of cylindrical pressure bearing element configuration, other configurations are possible, for instance, flaired and other conical shapes. Further, although the seal is described with reference to a ceramic tube closed at one end, such sealing could be effectuated at opposite ends of a ceramic tube open at opposite ends. 
     An additional point is that although enlarged head section  48  is illustrated as having been formed in an elongated, thin-walled ceramic tube, the present invention also covers the formation of such enlarged head sections in shorter ceramic tubes such as can be formed by prior art isopressing techniques. In such case, the cylindrical pressure bearing element of the prior art mold is modified with an enlarged end section of the present invention. While such a cylindrical pressure bearing element is resilient in a radial direction, it is formed of a material that is not so rigid as to maintain its shape during filling and thus, long, thin tube-like elements are not possible. 
     While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions may be made without departing from the spirit and scope of the present invention.