Patent Publication Number: US-2002011439-A1

Title: Porous ceramic filter and method for producing same

Description:
REFERENCE TO RELATED APPLICATIONS  
     [0001] This application is a continuation-in-part of application Ser. No. 08/972,540 filed Nov. 18, 1997. 
    
    
     
       TECHNICAL FIELD  
       [0002] The present invention relates generally to a porous ceramic filter containing a porous substrate and at least one porous ceramic layer provided thereon. The filter may take various geometrical forms, including, but not limited to, planar and hollow cylindrical configurations.  
       BACKGROUND OF THE INVENTION  
       [0003] The present invention is directed to a porous ceramic filter that has numerous industrial uses, including use in large-scale water purification systems. Currently, alumina-based asymmetric microfilters having a tubular (i.e., hollow cylindrical) structure are used in such large-scale water purification systems. These prior art filters have a plurality of layers formed on a filter substrate, and require a series of heating cycles, typically carried out at temperatures of at least 1400° C., to sinter the alumina particles of the filter structure. While such filters generally perform well in practical use, they are relatively expensive to manufacture due to the relatively high heating temperatures and number of heating cycles required during manufacture.  
       [0004] Having recognized a need in the art to provide a relatively low cost ceramic filter that performs on a level at least equal to the known filter structures, the present filter and process for producing same have been developed.  
       SUMMARY OF THE INVENTION  
       [0005] The present invention is directed to a ceramic filter having a mechanically strong filter skin wherein, in a preferred embodiment, the filter including the skin displays good corrosion resistance in both acidic and basic solutions. The inventors have found that such filters can be made when using binders containing zirconia precursors to form the filter skins. The invention also includes the formation of ceramic filters using preceramic polymers and their method of production.  
       [0006] The basic concept of the present invention is the use of ceramic precursors in polymeric form to produce a ceramic filter. However, not all preceramic polymeric binders provide compositions that are stable in acidic or basic conditions. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0007] The ceramic filters of the present invention include a porous substrate and at least one porous layer (also called a skin) formed on the porous substrate. The porous layer is formed of ceramic particles bound together by an intergranular phase made up of a ceramic precursor material or a ceramic binder formed by pyrolysis of the ceramic precursor material, as discussed in more detail hereinbelow. In a preferred embodiment, the intergranular phase also contains a source of zirconia.  
     [0008] The term “ceramic precursor” or “ceramic precursor material,” when used in this application, means a soluble and/or meltable (and therefor capable of being fabricated) polymeric or oligomeric compound that possesses an inorganic skeleton that upon heat treatment (pyrolysis) is converted into a ceramic composition.  
     [0009] More particularly, ceramic filters were made by coating a slurry on a round, flat alumina substrate. However, the configuration of the substrate is not limited to a planar disk, but may preferably be formed as a hollow cylindrical tube. The substrate preferably has a porosity within the range of 20-70 vol %, more preferably 30-70% vol %. The substrates used in connection with the working examples herein had a porosity of 35 vol %. Like the porous layer to be formed on the substrate, the substrate has a three-dimensional interconnected network of pores to allow a fluid, such as water, to pass therethrough.  
     [0010] The slurry that was coated on the substrate contained a commercially available alumina powder, RK-1C (manufactured by NGK Insulators, Ltd.) having an average particle size of about 3.2 microns RK-02C (manufactured by NGK Insulators, Ltd.) having an average particle size of 0.7 microns.  
     [0011] Polyhydridomethylsiloxane (PMHS) and ethoxy-modified PHMS (EtO—PMO) prepared according to procedures and processes described in U.S. Pat. Nos. 5,128,494 and 5,635,250 were first utilized as polymer binders, and were mixed in the slurry. It was found that good slurries for forming filter layers were provided by using the modified EtO—PHMS, because hydrophilic ethoxy groups were substituted in the PHMS polymer chains, thereby making the polymer less hydrophobic. If unmodified PHMS is utilized as the binder, dehydrocoupling to convert the PHMS into a cured preceramic polymer occurs during the post fabrication curing stage by reaction with water. On the other hand, when PHMS is modified with ethoxy groups, dehydrocoupling takes place upon formation of the modified EtO-PHMS by catalytic reaction. The EtO—PHMS serves as the preceramic polymer, which is cured by hydrolysis/condensation rather than by dehydrocoupling. The catalytic dehydrocoupling reaction in connection with EtO-PHMS is provided below:  
     [0012] [CH 3 SiHO] x +CH 3 CH 2 OH Ru 3 (CO) 12  [(CH) 3 SiHO] m [(CH 3 CH 2 ))CH 3 SiO] n    
     [0013] (PHMS) (EtO-PHMS)  
     [0014] The rate of modification depends upon the amount of ruthenium catalyst, Ru 3 (CO) 2 , and alcohol added to the PHMS. About 75% of the Si—H bonds were replaced by Si—EtO after 9 hours by addition of 150 ppm of the ruthenium catalyst to effect dehydrocoupling. More specifically, EtO—PHMS was prepared by forming a solution containing 100 grams of PHMS, 0.02 grams of Ru 3 (CO) 2  (200 ppm based upon the amount of polymer), and 210 grams of ethanol. The solution was refluxed overnight under dry conditions to form the modified EtO—PHMS that was ready for use.  
     [0015] The polymer binder is not limited to those mentioned above. By way of example, HO—PHMS, a PHMS polymer modified with Si—OH groups, may also be utilized. Other high yield precursors to silica may be used, but they may also be significantly more expensive than PENS derivatives.  
     [0016] While the foregoing illustrates a catalytic dehydrocoupling reaction to form a preceramic polymer for carrying out the present invention, U.S. Pat. Nos. 5,405,655, 5,128,494, 5,008,422, 4,952,715, 4,788,309 provide a more comprehensive review of forming preceramic polymers by dehydrocoupling and their use as binders, the subject matter thereof being incorporated herein by reference.  
     [0017] During the course of research on this matter, it was found that when zirconia was present in the binders, it gave mechanically strong filter skins having improved corrosion resistance in both acidic and basic solutions. In this way the filters can be cleaned in either acidic (citric or sulfuric acid) or basic (sodium acetate, sodium hypochlorite or ammonium acetate) solutions without suffering any adverse effects.  
     [0018] Excellent corrosion resistance was demonstrated in a filter made from a binder containing zirconium and silicon in a 1:1 ratio and formed by mixing EtO—PHMS with zirconyl chloride in a water-alcohol solution.  
     [0019] The slurry was formed by mixing alumina powder, EtO—PHMS polymer, and a solvent such as ethanol or a mixture of ethanol and water. An optional pore control agent such as a decomposable polymer may be used. The slurry was ball-milled for one hour to provide a slurry that was white in color, and evenly dispersed and stable at room temperature. If an organic polymer pore control agent is used, it preferably should be mixed with the solvent before mixing with the slurry.  
     [0020] Before deposition of the slurry on the porous substrate, the substrate is preferably soaked in a liquid to prevent quick absorption of the polymer and solvent into the substrate by capillary forces, which would change the desired ratio of powder to polymer in the coated layer. The preferred liquid used to soak the substrate is either water or an ethanol solution (i.e., the solvent material used for the slurry).  
     [0021] After soaking, the substrate was coated with the slurry either by casting or by a path and wash flow technique. During casting, the substrate was placed in a round mold having a depth corresponding to the thickness of the substrate. The slurry was cast over the top of the substrate and excess material was removed by a doctor-blade technique thereby leaving a layer of slurry behind. According to the path and wash flow technique, the substrate was placed in the mold described above, and the slurry was poured and flowed over the surface of the substrate, tilted at an angle of 45° with respect to horizontal. The path and wash flow technique was found to provide particularly uniform coatings, and is thus considered preferable.  
     [0022] The thus coated substrate was cured to effect crosslinking of the polymer material. Curing can take place at temperatures below 200° C. After curing, the filter may be used “as is.” However, to improve strength, chemical durability and wetting characteristics, the filter was pyrolyzed at a higher temperature, within a range of [500] 450° C. to 900° C., more preferably 500° 0  C. to 700° C., to convert the now cured polymer (i.e., preceramic polymer binder) to a ceramic product, particularly, amorphous silica. Accordingly, the final structure of the porous layer includes alumina particles bound together by an amorphous silica intergranular phase.  
     [0023] In more detail, the substrate coated with the slurry layer was heated at 5° C./min to 150-200° C. and held at 150-200° C. for two hours. Thereafter, the coated substrate was heated at 5-10° C./min. to 500° C. and held for 5 hours or less at 500° C. By pyrolyzing the coated substrate at a temperature above 450° C., the coating provided on the substrate was converted from a hydrophobic state to a hydrophilic state.  
     [0024] Pyrolysis at a temperature below 450° C. may be effected when utilizing HO-PHMS to convert the coating to a completely inorganic hydrophilic state. Additionally, those embodiments containing an organic polymer pore control agent (such as polyamides) required pyrolysis above 450° C. to burn out the additive. However, lower temperatures may be utilized for other contemplated pore control agents such as polyethers, polyacetates, and polyvinylalcohols.  
     [0025] After pyrolysis, the filter was evaluated for corrosion resistance. Particularly, the filtration property permeance was evaluated by measuring the amount of water that passes through the filter per unit area of the filter per KPa over the course of a day. The permeance measurements were taken at 1 to 45 KPa. The properties of numerous embodiments of the filter formed according to the present invention are recorded below in Tables 1A and 13B.  
     [0026] In Tables 1A and 1B, sample nos. 16-5, 17-3, 18-3 and 19-3 had dual layer structures, including two porous layers provided on the porous substrate. Further, reference examples REF 2 and REF 3 are embodiments of prior art, sintered filters, provided for comparative purposes.  
               TABLE 1A                          PREPARATION, FORMULATION AND       FILTRATION PROPERTIES                                         Type           Polyamide   Permeance           of   Coating   Binder   Wt %/   (m 3 /m 2  · day ·       Sample   Al 2 O 3     Method   Wt. %   Solvent   Kpa)                                             1   RK-02C   Casting   10               2   RK-02C   Casting   20       0.13       3   RK-1C    Casting   8       4   RK-1C    Casting   10       5   RK-1C    Casting   15       6   RK-1C    Casting   5       7   RK-1C    Casting   8       8   RK-1C    Casting   10       9   RK-02C   Casting   10    8/EtOH       10    RK-02C   Casting   10    5/PrOH       11    RK-02C   Casting   10   10/PrOH       12    RK-02C   Casting   10   15/PrOH       13    RK-02C   Casting   20   10/PrOH       14    RK-02C   Casting   20    5/PrOH       15-1   RK-02C   Casting   15    5/PrOH       15-1   RK-02C   Path and   15    5/PrOH   0.22               wash       16-1   RK-02C   Casting   15   10/PrOH       16-2   RK-02C   Casting   15   10/PrOH       16-3   RK-02C   Path and   15   10/PrOH   1.2               wash       16-4       16-5   RK-02C   Path and   15   10/PrOH   0.17       2nd layer       wash       on 16-3           17-1   RK-1C    Path and   15       0.96               wash                  
 
     [0027]               TABLE 1B                          PREPARATION, FORMULATION AND       FILTRATION PROPERTIES                                         Type           Polyamide   Permeance           of   Coating   Binder   Wt %/   (m 3 /m 2  · day ·       Sample   Al 2 O 3     Method   Wt. %   Solvent   KPa)               17-2   RK-1C   Path and   15                       wash       17-3   RK-1C   Path and   15       1.06       2nd layer        wash       on 17-1       18-1   RK-1C   Path and   20       0.54               wash       18-2   RK-1C   Path and   20               wash       18-3   RK-1C   Path and   20       2nd layer       wash       on 18-1       19-1   RK-1C   Path and   10       1.05               wash       19-2   RK-1C   Path and   10               wash       19-3   RK-1C   Path and   10       0.89       2nd layer        wash       on 19-1       21-1    RK-02C   Path and   15               wash       21-2    RK-02C   Path and   15               wash       REF 2   1           layer            RK-02C           2           layers           RK-1C       REF 3   1           layer            RK-02C           layers           2           RK-1C                    
     [0028] As shown in Tables 1A and 1B, filters based on the larger alumina particle size (RK-1C) demonstrated higher permeability that the smaller particle size (RK-02C) based filter for the same polymer/powder ratio. Further, the permeability of the RK-1C based filters decreased by addition from 15% to 20%.  
     [0029] The porosity and the skeletal density of the porous layer of several embodiments of the present filter are summarized below in Table 2.  
                                   TABLE 2                                           Porosity   Density           Porous Layer   Type of Al 2 O 3     (vol %)   (g/cm 3 )                          10% EtO-PHMS   RK-1C   0.458   3.879           20% EtO-PHMS   RK-1C   0.471   3.915           15% EtO-PHMS*   RK-1C   0.536   4.036                                  
 
     [0030] As shown, the porosity of the porous layer fell within the target range of 20-70% vol % and the preferable target range of 30-70 vol %. Porosity of the porous layer may be modified by altering the particular type of polymer binder utilized, particle size/particle size distribution of the alumina power, ratio between the polymer and binder, amount of solvent, and inclusion of pore size control agents such as polyamide.  
     [0031] In addition, mercury porisometry showed that the porous layer had fairly narrow pore size distribution, generally within a range of 0.7 to 1.1 μm.  
     [0032] The microstructure of several embodiments of the present invention was analyzed by using scanning electron microscopy. Particularly, the microstructure of the filter porous layer, the bonding of the porous layer to the alumina substrate, and the bonding of the different layers were investigated.  
     [0033] The porous layer adhered very well to the substrate and conformed well to the substrate surface. Powder particles of the porous layer penetrated into interparticle spaces along the substrate surface. Further, the porous layer was found to be homogeneous and defect free.  
     [0034] Various embodiments were subjected to a four-point bend test to evaluate the mechanical behavior of the final filter structure. It was found that the porous layer provided on the substrate did not degrade the strength of the substrate, and in some cases improved the base strength of ,-the substrate.  
     [0035] The polymer binder wt % (based upon 100% ceramic powder) was then evaluated in terms of the resulting filtration properties and mechanical [strength] integrity of the skin filters. A summary of the results is provided below in Table 3:  
                       TABLE 3                       Polymer (wt %) (based   Filtration   Mechanical       on ceramic powder)   Properties   Integrity                                            5   Good   Very Poor       8   Good   Poor       10   Good   OK       15   Good   Good       20   OK   Good                  
 
     [0036] As shown in Table 3, an increase in content of the polymer relative to the alumina powder is effective to enhance the strength of the porous layer. However, an increase in polymer percentage generally reduced the filtration properties of the filter It was found that polymer percentages above 20% significantly reduced the filtration properties of the filter. Accordingly, the polymer percentages preferably not greater than 20 wt %, more preferably 4-15 wt %, based upon the alumina powder.  
     [0037] In an attempt to improve the corrosion resistance (chemical stability) of the skins in acidic and especially in basic conditions used for cleaning the filters, slurry formulations including precursors to Zro 2  were developed. The ZrO 2 -derived binder demonstrated excellent corrosion resistance. Formulation of binders made of Si:Zr ratios of 1:1 and ZrO 2  precursors alone. It was found that a mixture of Si:Zr≦1 preferred to obtain sufficient resistivity against corrosion in basic conditions. The miscibility of the two components is also very important to obtain improved corrosion resistance.  
     [0038] The procedure for making the zirconia-based aspect of the invention included the following steps:  
     [0039] 1. Precursor synthesis or modification (if necessary).  
     [0040] 2. Binder solution preparation.  
     [0041] 3. Slurry preparation by mixing binder solutions with alumina powder (RK-02C) and solvent, as necessary.  
     [0042] 4. Filter skin fabrication by wash coating.  
     [0043] 5. Standard heat treatment: 5° C./min to 200° C./2 h, 10° C./min to ˜500° C./5 h.  
     [0044] 6. Corrosion resistance testing according to the following procedure:  
     [0045] Immersing filters in 2% citric acid, 5000 ppm H 2 SO 4 , and pH 12 NaOCl (5000 ppm Cl) solutions (separately) for 3 days or until degradation is observed in solution.  
     [0046] If the results in the first test were sufficient, further testing was performed by sequential immersing of filter sin 2% citric acid, followed by 5000 ppm H 2 SO 4  and pH 12 NaOCl for 3 days each.  
     [0047] 7. Samples showing good integrity after corrosion testing were evaluated by scanning electron microscopy (SEM), both before and after testing, and for mechanical integrity.  
     [0048] The following examples show the percentage of binder, quantities of alumina, EtO-modified PENS, zirconia source water, and other solvent components; quantity of ammonium-acetate (when used), and the viscosity, flux, microstructure, and evaluation of corrosion resistance observed.  
                                                                                       Example       Al 2 O 3     PHMS 1     ZrOCl 2     H 2 O   EG   EtOH   PrOH   NH 4 Oac   Vis. 2         Micro   Corro.           No.   Binder %   (g)   (g)   (g)   (g)   (g)   (g)   (g)   (g)   (cps)   Flux 7     Struc.   Resist.   Comments                  1   5   6   0.45   0.81   2       5       0.2                           2   8   6   0.72   1.3   1       3               1.7   Bub.   Good                                                   0.4       3   8   6   0.72   1.3   5   4           0.31       0.6   Bub   Good       4   5   6   0.45   0.81   4   4           0.2       1   Bub.   Good       5   10   6   0.9   1.61   5   5           0.39           Bub,   Good       6   12   6   1.09   1.93   5   5           0.45       Bub.   Good       7   8   6   0.72   1.3   4   4           0.31       0.02   Bub.   Good       8   10   6   0.91   1.61   5   5           0.39       0.03   Bub.   Good       9   8   6   0.72   1.3       4   4       0.31       0.1   Good   Good       10   10   6   0.91   1.61       5   5       0.39       0.35   Good   Good                                                     11   5   6   0.45   0.81   1.5   5   0.2   Top layer cracks           12   8   6   0.72   1.3   2   6       Top layer cracks       13   8   6   0.72   1.3   2   6   0.31   Top layer cracks       14   8   6   0.72   1.3   0.8   6       Top layer cracks                                                                         15   8   10   1.21   2.14   8.3   8.3           0.51       28.5/                                                               33       16   10   10   1.51   2.68   8.3   8.3           0.64       37.5/                                                   39       17   8   10   1.21   2.14       8.3   8.3       0.51       47/                                                   48       18   10   10   1.51   2.68       8.3   8.3       0.64       54/58       19   8   8 3     0.97   1.71       5.3   5.3       0.41   12               Powder                                                               penetration       20   8   8   0.97   1.71       8   8       0.41   33.5/       Good   Good                                               28.5       21   8   8   0.97   1.71   9.3       4       0.41   too                                               low       22   8   8   0.97   1.17   9.3       4       0.41   22.5/       Bubble                                               25       23   8   8 1     0.97   1.71       2.7   2.7       0.41   17.5               Polymer                                                               separation       24   8   8 1     0.97   1.71       4   4       0.41   26.5               Polymer                                                               separation       26   8   8 1     0.97   1.71       5.3           0.41   38/89               Polymer                                                               separation       26   8   8 1     0.97   1.71       8           0.41   61/59               Polymer                                                               separation       27   8(SiZr   8   0.65   2.29   9.3               0.55                   Polymer           *½)                                                   separation       28   8   8   0.97   1.71   9.3   4           0.41   19/20       Bubble   Good   Quick                                                               sedimentation       29   8   8   0.97   1.71   9.3           4   0.41   26/29       Good   Good   Quick                                                               sedimentation       30   8   8   0.97   1.71       9.3   9.3       0.41   25/23   0.99   Good   Good       31   8   8   0.97   1.71   10.6           6.67   0.41   27/27   0.46   Good   Good       32   8   8   0.97   1.71   10.6   5.3           0.41   15/20   1.14   Bubble       33   8   6   0.72   1.3       6.5   6.3       0.31   29   0.33       34   8   6   0.72   1.3   7           6   0.31   30.5       35   8   6   0.72   1.3   8   4           0.31   15       36   8   6   0.72   1.3                   0.31   13/14   0.67   Good   Good       37   8(Si:Zr =   6   0.48   1.71   10               0.41           ½)               (0.2 g                           PA)       38   8   8   0.97   1.71   11.7                       0.4                           (0.4 g                           PA)       39   8   8   0.64   2.29   11.7               0.54   23   0.58   Bubble                           (0.4 g                           PA)       40   8   8   0.97   1.71   11.7               0.41   24/22   Poor   Good       Quick                           (0.3 g                                   Sedimentation                           PA)       41   8   8   0.64   2.29   11.7               0.54   31/27   Poor   Good       Quick                           (0.3 g                                   sedimentation                           PA)       42   5   8   0   2.15   12.5               0.81   23/29   Poor   Good   Good       43   8   8   0   3.43   13               1.21   27/25   Poor   Good   Good   Small                                                               cracking       44   8   6   0.72   1.29   12.5   15           0.31   47               Powder                                                               penetration       45   6 5     8   0   3.2 4     9.3                   8               Poor bonding       46   8 5     8   0   4.3 4     8.2                   17               Poor bonding       47   10 5     8   0   5.3 4     7.2                   35               Poor bonding       48   8 5     8   0   1.68   12.5               0.6   17/20   0.56   Good       pH = 2.22       49   10 5     8   0   2.09   12.5               0.75   23/26   0.38   Good       pH = 2.25       50   8 5     8   0   4.4 4     8.1                   14/18               Poor bonding       51   10 5     8   0   5.5 4     7                   21/27               Poor bonding       52   20 5     8   0   11 4     1.5                   22               Poor bonding       53   15 5     8   0   8 1     6.5               0.2   23               Poor bonding       54   15 5     8   0   8 1     9               0.1   23               Poor bonding       55   10.2 5     8   0   2.13   11.5               0.51   23/28   0.5           pH = 0.92       56   16.4 5     8   0   3.43   12.5               0.82   27.32   0.29           pH = 0.89       57   8   6   0.72   1.3       6       6       48.3   0.27       58   8   6   0.72   1.3       7       7       35.5   0.29       59   10.2 5     8   0   2.13   13               0.51   17/   0.34   Crack-       pH = 0.81                                               17.5       ing       60   16.4 5     8   0   3.43   14               0.82   21/   0.44   Crack-       pH = 0.56                                               21.5       ing       61   5   6   0.45   0.805       7       7       34.5/   1.16   Some                                               35.5   0.64   holes       62   8   6   0.72   1.3       8       8       31.5/   0.37                                               31.5       63   10.2 5     8   0   2.13   13               0.63       0.18   Good       pH = 1,61,                                                               pH = 1.25                                                               One day       64   10.2 5     8   0   2.13   13               0.63       0.74           Slurry after                                                               one day, pH =                                                               1.25, bonding                                                               is very good,                                                               coupon is on                                                               glass.       65   10.2 5     8   0.24   2.13   13               0.51       0.18   Good,       pH = 0.95                                                       few       one day                                                       bubbles                                                       inside       66   8.16   8   0   1.71   12               0.53           Very       pH = 1.75                                                       good,                                                       no                                                       crack-                                                       ing       67   10.2 5     8   0   2.13   13               0.65           Very       pH = 1.75                                                       good       68   10.2   8(RK-       2.13   13               0.65           Small       pH = 1.75               1C)                                       crack-                                                       ing       69   10.2   8 RK-       2.13   13               0.65           Good       pH = 1.75,               02C                                       surface,       Coating over                                                       al-       81-1 without                                                       though       heat                                                       some       treatment                                                       crack-                                                       ing       70   10.2   8 RK-       2.13   13               0.65           Good       pH = 1.75,               02C                                       surface       Coating over                                                       al-       81-1 which                                                       though       was heated to                                                       some       550° C.                                                       crack-                                                       ing       71   8.16   8 RK-       1.71   16           0.53               Very       Coupons were               1C                                       good,       first coated                                                       no       by RK-1C,                                                       crack-       then, by                                                       ing       RK-02C           10.2   8 RK-       2.13   13           0.65               and no       immediately               02                                       big                                                       holes       83   8 RK-   0.6   1.08       8   8       0.15           Very           The same way           1C                                       good,           to prepare                                                   no           coupons as                                                   crack           above                                                   ing           8 RK-   0.6   1.08       7   7       0.15           and           02                                       no                                                   big                                                   holes                                                                          
 
     [0049] In the case of a mixed Si:Zr binder, when ethanol (rich) water is used as the solvent, cracking and top layer is always observed, which can be improved by reducing the binder percentage. However, total elimination of top layer and cracking is very difficult if not impossible since a thin top layer comprising a polymer-derived ceramic layer whose particles when in a slurry subsequently precipitate after deposition is still observed for the samples prepared using 5% binder. Top layer and cracking are caused by (1) poor solubility of the partially polymerized ZrOCl 2  in ethanol, which may cause some extent of agglomeration, and (2) quick evaporation of ethanol after the coating is made, which makes it impossible for the solvent to “filter” down any extra amount of the binder.  
     [0050] Bubbles are always observed for those systems in which PHMS—OEt is not well dissolved with the solvent. These systems include ethylene glycol/water, n-PrOH/water (1:1), and water bubbles are caused by fine micelles (5-10 μm) of PHMS—OEt in the slurries. After coating is made, the solvent penetrates through the substrates first, then through the fine polymer particles, which can be clearly observed during coating preparation. When PHMS—OEt mixes well in the solution, no bubbles are observed.-  
     [0051] The best systems identified for Si:Zr=1:1 use EG/EtOH or EG/PrOH as the solvent. Both ZrOCl 2  and PHMS—OEt can dissolve in the solvent and good slurries are obtained.  
     [0052] Another potential problem is if a dispersion of the fine powder particles in the slurries is too good, which causes some penetration of the slurries into substrates resulting in pore clogging and subsequent poor flux.  
     [0053] Because RK-02 has a size of 0.7 μm, which is smaller than the holes in the substrates, too good a dispersion of slurries will make coating impossible due to penetration to the substrates. For example, no coating can be obtained by using slurries with surfactant-treated powder. No coating can be obtained by using ethylene glycol as a solvent. The best slurries should have somewhat good dispersion but a certain degree of fine agglomeration is required and easy sedimentation in 10-30 minutes. Filtration properties are greatly affected by penetration of slurries to the subsurface of the substrates.  
     [0054] An inappropriate slurry is obtained by using water as a solvent when PHMS—OEt is used. Addition of polyacrylic acid as a surfactant improves the properties of such slurries. But, PHMS—OEt still is not mixed with water; instead it forms small spherical micelles particles in slurries (oil/in water emulsion), which causes bubbles to be formed.  
     [0055] When ZrOCl 2  is used as the only binder component, water can be used as a solvent. Properties of the binder and consequent strength of the skin filter strongly depend on pH of the solution. When the pH of the solution is raised, zirconium oxychloride exists as a polymeric material in the solution and the binding ability of zirconium material is reduced as a result. For example, at pH=2.2, the polymeric zirconium material gives good binding for the freshly prepared slurries, but poor binding for the slurries aged for one day. It is assumed that at this pH, the developed ZrO 1 Cl b OH c  is too polymerized and there are not enough free Zr—OH sites for bonding to the alumina surface. When pH=0.9, 5% zirconium binder gives good binding for the slurries freshly prepared or prepared for one day. However, cracking is observed. More severe cracking is observed for pH=0.56 for those using 8% of the binder. At this stage it is believed that the binder is a monomeric or oligomeric zirconyl chloride that is not polymerized enough to serve as a ceramic filter. When pH was adjusted to 1.6 (1.3 after one day), no significant cracking is observed (very few cracks). Bonding is very good even for solution aged one day. Therefore, a pH of about 1.5 should be good for the slurries. At this pH, both good bonding to the substrate and good microstructure can be obtained.  
     [0056] When using zirconium carbonate instead of zirconyl chloride as the ZrO 2  source, the binding ability of the zirconium material is very poor. Poor binding is caused by the polymeric properties of the binders, formed after dissolution of zirconium carbonate in acetic acid. Unless a strong acid was used to break down the zirconium polymeric structure, it was impossible to use zirconium carbonate as a binder.  
     [0057] When NH 4 OAc is used in Zr:Si=1:1 formulations to increase the pH of the solution, corrosion resistance of the binder materials was reduced. It is suggested that less Si—O—Zr bonding occurs under these conditions as the result of the polymerization of Zr—O—Zr in the solutions.  
     [0058] When RK-1C is used as an intermediate layer using 5% ZrOCl 2  as a binder (Example 68), cracking is observed. However, the cracks are well covered by the second RK-02 coating and a very smooth surface can be obtained even though some cracking is still observed.  
     [0059] The second fine layer (with RK-02) can be obtained in two ways. It can be prepared over the first RK-1C coating immediately after the coating is prepared and is still wet. It can be prepared after the first layer coating is heated at 550° C. Both give similar results. However, no good coating (adequate top layers) can be obtained when the RK-02C coating is prepared over the first one which was heated at 150° C., to cure the zirconyl chloride binders. It seems that this temperature is not high enough to cure the binder which is then dissolved when redispersed in the top layers water-based slurry.  
     [0060] SEM (scanning electron microscopy) pictures for Examples 71 to 74 show that cracking has been eliminated in the skin and all big holes have been eliminated. More dilute solutions are used for the intermediate layers in Examples 82 and 83 in an attempt to form only a very thin intermediate layer. The intermediate layer only covers big holes at the surface of the porous alumina substrates. Therefore, only very thin layer will play the role as confirmed by the results.  
     [0061] While the examples described herein show the use of silica and zirconium as ceramic precursors, other candidates for such precursors include polysiloxanes, polycarbosilanes, partially hydrolyzed sol gel derivatives of metallic oxides such as zirconium oxide, titanium oxide, and aluminum oxide and metal phosphate binders based on Al, Ca, Mg, Zn, Zr, Sn, and mixtures thereof.  
     [0062] While the foregoing description provides a detailed review of particular embodiments formed according to the present invention, various changes and modifications may be made to the present invention by one of ordinary skill in the art and still fall within the scope of the -present claims.