Method of fabricating ceramic membranes

A method of fabricating a dense membrane by providing a colloidal suspension of a ceramic powder, and providing a polymeric precursor. The precursor is mixed together with the colloidal suspension, and the mixture is applied to a membrane support to form a composite structure. The composite structure is heated to form a dense membrane on the support.

FIELD OF THE INVENTION 
This invention relates to the production of ceramic membranes and more 
particularly to the deposition of thin ion transport membranes over porous 
substrates. 
BACKGROUND OF THE INVENTION 
Solid electrolyte ionic conductors, also referred to as ion transport 
membranes, can be utilized to separate oxygen from gas mixtures containing 
oxygen. Mixed conductors are materials that conduct both oxygen ions and 
electrons and appear to be well suited for oxygen separation since they 
can be operated in a pressure driven mode, in which oxygen transport is 
driven by a difference in oxygen activity, also referred to as oxygen 
partial pressure, on the two sides of the membrane. Perovskites such as 
La.sub.1-x Sr.sub.x CoO.sub.3-y, La.sub.x Sr.sub.1-x FeO.sub.3-y, and 
La.sub.x Sr.sub.1-x Fe.sub.1-y Co.sub.y O.sub.3-z are examples of mixed 
conductors. At elevated temperatures, these materials contain mobile 
oxygen-ion vacancies that provide conduction sites for transport of oxygen 
ions through the material. These materials transport oxygen ions 
selectively, and can thus act as a membrane with an infinite selectivity 
for oxygen. 
Thin electrolyte films are highly desirable because the ideal oxygen flux 
is inversely proportional to the thickness. Thus thinner films could lead 
to higher oxygen fluxes, reduced area, lower operating temperatures and 
smaller oxygen pressure differentials across the electrolyte. 
Solid state gas separation membranes formed by depositing a dense mixed 
conducting oxide layer onto a relatively thick porous mixed conducting 
support were investigated by Teraoka et. al. as disclosed in the Journal 
Ceram. Soc. Japan, International Ed, Vol. 97, No. 5 (1989). The relatively 
thick porous mixed conducting support provides mechanical stability for 
the thin, relatively fragile, dense mixed conducting layers. La.sub.0.6 
Sr.sub.0.4 CoO.sub.3 thin films were deposited onto porous supports of the 
same material by an rf sputtering technique and a liquid suspension spray 
deposition method. Films produced by the sputtering method were cracked 
and porous. Films (approximately 15 .mu.m thick) made by the liquid 
suspension spray followed by sintering at 1400.degree. C. were dense and 
crack-free. 
Teraoka and coworkers expected the oxygen flux to increase by a factor of 
10 for the composite thin film membrane compared to a dense disk. However, 
they obtained an increase of less than a factor of two. 
Pal et al. disclosed an EVD process in a paper entitled "Electrochemical 
Vapor Deposition of Yttria-Stabilized Zirconia Films" wherein a 
yttria-stabilized zirconia ("YSZ") film is deposited onto a porous 
substrate. EVD is a modification of the conventional chemical vapor 
deposition ("CVD") process which utilizes a chemical potential gradient to 
grow thin, gas impervious metal oxide films on porous substrates. The EVD 
process involves contacting a mixture of metal halides on one side of a 
porous substrate and a mixture of hydrogen and water on the opposite side. 
The reactants diffuse into the substrate pores and react to form the 
multi-component metal oxide which is deposited on the pore wall. Continued 
deposition causes pore narrowing until eventually the pores become plugged 
with the multi-component metal oxide. The primary application of EVD to 
date has been in the fabrication of solid electrolyte YSZ, and the 
interconnector material lanthanum chromium oxides as used in solid oxide 
fuel cells ("SOFCs"). 
Richards et al. in U.S. Pat. No. 5,240,480 disclosed an organometallic 
chemical deposition (OMCVD) method to prepare thin films of muti-component 
metallic oxides for use as inorganic membranes. The inorganic membranes 
are formed by reacting organometallic complexes corresponding to each of 
the respective metals and an oxidizing agent under conditions sufficient 
to deposit a thin membrane onto the porous substrate. Both EVD and OMCVD 
process involve expensive and complex equipment and often toxic and 
expensive precursor materials. Furthermore, for multi-component metallic 
oxides (e.g. mixed conducting perovskites), stoichiometry control of the 
oxide film is difficult for these processes. 
Thorogood et. al. in U.S. Pat. No. 5,240,480 investigated multi-layer 
composite solid state membranes which are capable of separating oxygen 
from oxygen-containing gaseous mixtures at elevated temperatures. The 
membranes comprise a multi-component metallic oxide porous layer having an 
average pore radius of less than approximately 10 .mu.m and a 
multi-component metallic oxide dense layer having no connected-through 
porosity wherein the porous and dense layers are contiguous and such 
layers conduct electrons and oxygen ions at operating temperatures. 
Carolan et al. in U.S. Pat. No. 5,569,633 investigated surface catalyzed 
multi-layer ion transport membranes consisting of a dense mixed conducting 
multi-component metallic oxide layer, and combinations of porous ion 
conducting and porous mixed ion and electron conducting layers. 
Significant oxygen flux was demonstrated by these prior art ion transport 
membranes in which catalysts were deposited onto the oxidizing surface of 
the composite membrane. Coating on both sides of the membrane did not 
enhance the oxygen flux. 
Anderson et al. in U.S. Pat. No. 5,494,700, which is incorporated herein by 
reference, disclose synthesis of a precipitate-free aqueous solution 
containing a metal ion and a polymerizable organic solvent to fabricate 
dense crack-free thin films (&lt;0.5 .mu.m/coating) on dense/porous 
substrates for solid oxide fuel cell and gas separation applications. 
First, a precipitate-free starting solution is prepared containing cations 
of oxide constituents dissolved in an aqueous mixture comprising a 
polymerizable organic solvent. The precursor film is deposited on the 
substrate by spin-coating technique followed by drying and calcining in 
the presence of oxygen and at the temperature not in excess of 600.degree. 
C. to convert the film of polymeric precursor into the metal oxide film. 
The polymeric precursor method disclosed by Anderson et al. is a 
cost-effective approach and is easy to scale up for manufacturing. 
However, the upper thickness limit for a single coating is typically below 
0.5 .mu.m for this method. Films greater than 0.5 .mu.m usually generate 
cracks during the organics burn-off and sintering due to the large 
shrinkage mismatch between the film and the substrate. Also, the Anderson 
et al. method is mostly confined to producing dense films on planar 
substrates by spin coating technique using a precipitate-free aqueous 
solution. No test results were reported for gas separation applications. 
An additional concern during production of composite membranes, having a 
dense thin film membrane deposited on a porous structured substrate, is 
that such membranes are prone to defects including tiny "pinholes" which 
are produced during the manufacturing operation. In general, the defect 
density tends to increase with higher processing speeds. Such defects are 
highly undesirable because they are non-selective, that is, they 
indiscriminately pass undesired components of a feed fluid. Such defects 
lower the selectivity of the ion transport film and result in diminished 
performance. The elimination of defects is hence essential to develop high 
performance composite films which can be economically produced at high 
processing speed. 
Furthermore, in practice the kinetics of surface exchange processes impose 
resistances additional to ion transport bulk resistance and affect oxygen 
transport across the ion transport membrane. As the film becomes thinner, 
the proportion of the overall resistance due to the ion transport bulk 
resistance decreases while that due to surface exchange increases. As a 
consequence, surface exchange kinetics are likely to become the dominant 
resistance for very thin films (e.g. 5 .mu.m or less). Therefore to get 
the full benefit of the thinner films, it is necessary to de-bottleneck 
the rate limitation imposed by the surface exchange processes. In summary, 
researchers are continuing their search for a cost-effective thin film 
technology for composite ion transport membranes which possess superior 
oxygen flux to enable their use in commercial processes. 
OBJECTS OF THE INVENTION 
It is therefore an object of the invention to provide an improved method of 
depositing a thin ceramic membrane on a porous support. 
It is a further object of this invention to provide such a method which can 
be accomplished economically and in as few as one or two steps to produce 
a membrane having a thickness greater than 0.5 microns. 
A still further object of this invention is to produce a ceramic membrane 
which mitigates limitations imposed by surface kinetics to obtain superior 
oxygen flux without using a surface catalyzed layer. 
Yet another object of this invention is to eliminate defects in ceramic 
membranes to prevent non-ionic permeation through the membranes. 
SUMMARY OF THE INVENTION 
This invention comprises a method of fabricating a ceramic membrane by 
providing a colloidal suspension of a ceramic powder and providing a 
polymeric precursor. The precursor and the colloidal suspension are mixed 
together, and the mixture is applied to a membrane support to form a 
composite structure. The composite structure is heated to form a dense 
membrane on the support. 
In a preferred embodiment the colloidal suspension includes a water-based 
solvent such as water or ethylene glycol. The ceramic powder has a mean 
particle size of less than 50 microns in diameter, and more preferably 
substantially all of the ceramic powder has a diameter less than 1 micron. 
More preferably, the colloidal suspension further includes at least one of 
a dispersant and a anti-foaming agent. 
Preferably, providing the polymeric precursor includes preparing a solution 
by dissolving a plurality of metal cation source compounds in an aqueous 
mixture including a polymerizable organic solvent, and heating the 
solution to form the precursor as the polymer containing the metal 
cations. More preferably, the cations are the same as those in the ceramic 
powder in the colloidal suspension. 
The support preferably is porous and has a porosity of at least 10 percent 
by volume, and more preferably has a porosity greater than 30 percent by 
volume. Heating the applied mixture includes drying it, preferably at 
60-100.degree. C. for 5-30 mins, on the support and then annealing it, 
preferably between 600 and 1400.degree. C. for 2-4 hrs, to form the dense 
membrane on the substrate. More preferably, the annealing includes at 
least partially sintering the support. 
In another preferred embodiment, the method of fabricating further includes 
eliminating defects in the formed dense membrane by preparing a second 
solution by dissolving a plurality of metal cation source compounds in an 
aqueous mixture including a polymerizable organic solvent, heating the 
second solution to form a sealing precursor as a polymer containing the 
metal cations, and applying the sealing precursor to the formed dense 
membrane to establish a film thereon. It is preferred that the metal 
cations are the same as those of the ceramic powder which is now part of 
the formed ceramic membrane. The method preferably further includes 
spinning the composite structure after the sealing precursor is applied, 
and then repeating the application and spinning cycle at least twice. The 
preferred size of crystals comprising the film is less than 100 nm, more 
preferably less than 50 nm, and most preferably equal to or less than 20 
nm.

DETAILED DESCRIPTION OF THE INVENTION 
This invention comprises a method of fabricating a ceramic membrane by 
providing a colloidal suspension of a ceramic powder and providing a 
polymeric precursor. The precursor and the colloidal suspension are mixed 
together, and the mixture is applied to a membrane support to form a 
composite structure. The composite structure is heated to form a dense 
membrane on the support. 
Another aspect of the invention involves eliminating defects in a formed 
dense membrane by preparing a solution by dissolving a plurality of metal 
cation source compounds in an aqueous mixture including a polymerizable 
organic solvent, heating the solution to form a sealing precursor as a 
polymer containing the metal cations, and applying the sealing precursor 
to the formed dense membrane. 
One of the main purposes of this invention is to obtain crack-free ion 
transport films with the thickness greater than 0.5 .mu.m using a single 
coating step, or only a few coating steps. In prior art, films greater 
than 0.5 .mu.m usually generate cracks during the organics burn-off and 
sintering process due to the large shrinkage mismatch between the film and 
the substrate. The present invention combines a colloidal suspension of 
ceramic powder into a polymeric precursor to increase the inorganic 
content of the precursor and reduce the shrinkage mismatch between the 
membrane and substrate during sintering. The polymeric precursor may also 
serve as a binder and stabilization agent in the ion transport suspension 
to increase the stability of the system. Preferably, the composition of 
the polymeric precursor is substantially identical to that of the ion 
transport materials in the colloidal suspension to obtain a desired, 
uniform composition of the resulting membrane. By using this liquid 
precursor, a relatively thick (2-5 .mu.m) ceramic membrane can be 
fabricated on a porous support in a single step. 
Another purpose of present invention is to obtain fine grain (&lt;0.5 .mu.m) 
ion transport membranes at low temperatures in a cost-effective manner and 
to mitigate the limitations imposed by surface kinetics. The fine grain 
characteristics, also referred to herein as crystalline size, of the 
overcoat layer or film fabricated by present invention could increase the 
surface area accessible to the oxygen molecules in the gas phase. 
Nanocrystalline films of less than 100 nm, more preferably less than 50 
nm, according to the present invention enhance surface exchange kinetics. 
Superior oxygen flux thus is obtained without using a surface catalyzed 
layer. 
In this invention, the liquid precursor is not restricted to a 
precipitate-free aqueous solution and the substrate is not restricted to 
the planar geometry. Also, the thickness for a single coating can be 
greater than 0.5 .mu.m by introducing the ion transport colloidal 
suspension into the polymeric precursor to increase the inorganic content 
of the system and reduce the shrinkage during sintering. The coating 
methods for applying the suspension include spin coating, dip coating, and 
spray coating. The geometry of the substrate can be tubular or complex 
shapes. 
The present invention utilizes the special properties of a liquid precursor 
in several ways. At low temperatures the precursor forms a viscous liquid 
with excellent wetting properties to form a uniform coating on the surface 
of a substrate or an ion transport film on a substrate. The liquid 
precursor will selectively wick into the open pores and effectively plug 
them but yet leave only a very thin layer on the non-porous or 
non-defective area of the substrate or ion transport film, respectively, 
thus the increase in overall resistance due to the overcoat thickness 
should be minimal. The repaired ion transport film can be heated to an 
elevated temperature, either during manufacturing or in-situ in the final 
application, to sinter the polymeric precursor layer. 
Table I summarizes the following examples with specific films and substrate 
materials for different composite film specimens to exemplify the 
invention and should not limit the invention in any way. 
TABLE I 
______________________________________ 
Ex- 
ample Fiim Substrate Results 
______________________________________ 
1 LSC Porous LSC-A i) Fabricated dense 
(25% porosity, 0.5 mm thick) 
LSC film on porous 
substrate 
ii) Defects elimi- 
nation of LSC 
composite film 
2 LSC Porous LSC-A Increased porosity 
(32% porosity, 0.5 mm thick) 
of porous support 
enhanced O.sub.2 
transport 
3 LSC Porous LSC-A Decreased thickness 
(32% porosity, 0.3 mm thick) 
of porous support 
increased O.sub.2 
flux of composite 
film 
4 LSCF Dense LSCF Enhance surface 
(1 mm thick) exchange kinetics 
by a nano- 
crystalline coating 
5 LSFCRM Porous LFCRM Fabricated dense 
(32% porosity, 0.5 mm thick) 
LSFCRM film on 
porous substrate 
6 LSC Porous LSC-A Obtained thicker 
(32% porosity, 0.5 mm thick) 
film (2-5 .mu.m) 
in a single step 
______________________________________ 
EXAMPLE 1 
Composite I 
Fabrication of La.sub.0.05 Sr.sub.0.95 CoO.sub.3 (LSC) Dense Film on Porous 
Substrate 
Preparation of the Polymeric Precursor Solution 
A polymeric precursor solution of La.sub.0.05 Sr.sub.0.95 CoO.sub.3 
containing 0.2 mole of the oxide was prepared as follows: 0.433 g of 
La(NO.sub.3).sub.3.6H.sub.2 O, 4.021 g of Sr(NO.sub.3).sub.2, and 10.135 g 
of a 506.731 g sol/mole Co(NO.sub.3).sub.2 solution were dissolved in 40 
ml deionized water, one at a time, in 250 ml beaker with stirring. 3.0 g 
of glycine and 2.0 g of citric acid and 40 ml of ethylene glycol were then 
added to the solution and dissolved by stirring. The magenta-colored 
precursor was filtered through Fiserbrand medium Q5 filter paper. The 
precursor was heated in a 250 ml beaker at 80.degree. C. to expel the 
water and other volatile matter. After 24 hours, the precursor was 
transferred to a 100 ml beaker and allowed to concentrate at 80.degree. C. 
for 2 more days until the precursor viscosity was 92 centipoise ("cP") at 
room temperature. 
Deposition and Formation of the Dense Film 
A spin coating technique was used to form wet films of the precursor on the 
porous LSC-A substrates. LSC-A substrates were prepared by mixing 
La.sub.0.05 Sr.sub.0.95 CoO.sub.3 powder (by Praxair Specialty Ceramics of 
Praxair Surface Technologies at Woodinville, Wash.) having a powder size 
of 1.2 .mu.m and 20 wt. % of Ag powder (Degussa Corp.) having a size of 
approximately 1 .mu.m, followed by pressing in a 1.5 inch (3.8 cm) die 
under a pressure of 10.4 kpsi and partial sintering at 950.degree. C. to 
obtain the porous substrate with a porosity of about 25% and a thickness 
of 0.5 mm. A few drops of the viscous precursor were then deposited onto 
the polished surface of the substrate which was fixed on a spinning disk. 
A spinning speed of 3500 rpm for 10 seconds was used for film deposition. 
After spin-coating, the as-deposited film of precursor on the substrate 
was dried on a hot plate at 80.degree. C. for 5 minutes, then transferred 
to a hot plate and heated at approximately 300.degree. C. for at least 5 
minutes. The entire spin-coating and drying process was repeated until a 
dense thin film formed on the substrate surface. 
The effect of the number of spin-coating cycles on the leak rate of 
nitrogen at room temperature (about 25.degree. C.) is shown in FIG. 1 for 
the composite film specimen. Line S represents nitrogen flow through the 
uncoated substrate, and lines 5, 12 and 18 (which lies along the x-axis) 
represent nitrogen flow after five, twelve and eighteen cycles, 
respectively. The leak rate decreased rather quickly before the fifth 
spin-coating cycle, and more slowly after that. Minimal leak rate was 
detected after 12 spin-coating cycles. A dense gas-tight LSC film (about 2 
.mu.m) was obtained on the porous LSC-A substrate after 18 spin-coating 
cycles, referred to hereinafter as Composite I. Composite I disc was then 
annealed at 600.degree. C. for 2 hours to remove residual organics and 
form the crystalline oxide film for further high temperature permeation 
tests. 
High Temperature Permeation Tests of Composite Film 
The oxygen permeation rate was measured using the Composite I disc specimen 
sealed in an alumina test cell with Ag paste. Permeation tests were 
performed at temperatures of 800-900.degree. C. with He inert gas purge 
and different concentrations of O.sub.2 /N.sub.2 mixtures on the feed 
side. A HP 5890 Gas Chromatograph and oxygen analyzer were used to analyze 
the gas compositions and calculate the oxygen fluxes. With 20% O.sub.2 in 
the feed, measurements were taken using a helium purge of 500 sccm He at 
800, 850 and 900.degree. C. resulting in respective flux values of 0.00, 
0.03, and 3.4 sccm/cm.sup.2. It indicates a phase transformation of LSC 
between 850 and 900.degree. C. from a hexagonal phase to a cubic phase. At 
900.degree. C. the oxygen permeation increases as the oxygen partial 
pressure increases. The oxygen fluxes measured were 3.4, 5.1, 6.8 and 8.2 
sccm/cm.sup.2, respectively, for the feed oxygen partial pressure of 0.2, 
0.4, 0.6 and 0.8 atm as shown in FIG. 2. As compared to 1.0 mm thick LSC 
(curve 122) and 0.5 mm thick (curve 120) LSC-A discs, the composite 
specimen (curve 118) shows much higher oxygen fluxes under identical 
testing conditions. Especially for the feed oxygen partial pressure of 0.8 
atm, a more than five times flux enhancement was demonstrated for the 
composite thin film specimen as compared to 1 mm thick dense disc. 
EXAMPLE 2 
Composite II 
LSC Composite Film on Porous Substrate 
Porosity Effect of Support 
A composite LSC thin film (.about.2 .mu.m) specimen, supported on a porous 
LSC-A substrate was prepared as described in Example 1. LSC-A substrates 
(0.5 mm thick) were prepared by mixing La.sub.0.05 Sr.sub.0.95 CoO.sub.3 
powder (PSC, Woodinville, Wash.) and 20 wt. % of Ag powder (Degussa 
Corp.), followed by pressing in a 1.5" die under a pressure of 10.4 kpsi 
and partial sintering at 900.degree. C. to obtain a substrate with a 
porosity of about 32%. Lowering the sintering temperature increased the 
porosity of the support. 
Oxygen permeation tests were performed at 900.degree. C. under a N.sub.2 
--O.sub.2 /He gradient as a function of feed oxygen partial pressure. The 
oxygen fluxes measured were 3.8, 5.8, 7.4 and 8.7 sccm/cm.sup.2, 
respectively, for the feed oxygen partial pressure of 0.2, 0.4, 0.6 and 
0.8 atm. As compared to composite film specimen with 25% porosity of 
substrate, curve 218, FIG. 3, the oxygen fluxes increased about 6-12% 
under similar testing conditions for the composite film specimen with 32% 
substrate porosity, curve 228. This indicates the oxygen flux of the 
composite film can be improved by increasing the porosity of the porous 
support. 
EXAMPLE 3 
Composite III 
LSC Composite Film on Porous Substrate 
Thickness Effect of Porous Support 
A composite thin film (.about.2 .mu.m) LSC specimen, supported on a porous 
LSC-A substrate was prepared as described in Example 1. LSC-A substrate 
(0.3 mm thick) was prepared by mixing the La.sub.0.05 Sr.sub.0.95 
CoO.sub.3 powder (PSC, Woodinville, Wash.) and 20 wt. % of Ag powders 
(Degussa Corp.), followed by pressing in a 1.5 inch die under a pressure 
of 10.4 kpsi and partial sintering at 900.degree. C. to obtain a substrate 
with a porosity of about 32%. Oxygen permeation tests were performed at 
900.degree. C. under a N.sub.2 --O.sub.2 /He gradient as a function of 
feed O.sub.2 partial pressure. 
The oxygen fluxes measured were 4.6, 8.3, 12.4 and 16.6 sccm/cm.sup.2, 
curve 338, FIG. 4, for the feed oxygen partial pressure of 0.2, 0.4 0.6 
and 0.8 atm, respectively. When purge flow was increased (from 0.5 to 2.8 
lpm) the O.sub.2 flux of 22.1 sccm/cm.sup.2 was obtained as illustrated by 
data point 340. The results indicate that the oxygen flux of the composite 
film is not limited by surface exchange kinetics up to 22.1 sccm/cm.sup.2 
without using a surface catalyzed layer and is improved by reducing the 
thickness of the porous support. Curves 320 and 322 represent dense discs 
of LSC having thicknesses of 0.5 mm and 1.0 mm, respectively. 
EXAMPLE 4 
Effect of Nanocrystalline La.sub.0.2 Sr.sub.0.8 Co.sub.0.8 Fe.sub.0.2 
O.sub.3-x (LSCF) Film on Surface Exchange Kinetics 
Preparation of the Polymeric Precursor Solution 
A polymeric precursor solution of La.sub.0.2 Sr.sub.0.8 Co.sub.0.8 
Fe.sub.0.2 O.sub.3-x containing 0.2 mole of the oxide was prepared as 
follows: 1.732 g of La(NO.sub.3).sub.3.6H.sub.2 O, 3.386 g of 
Sr(NO.sub.3).sub.2, 8.108 g of a 506.731 g sol/mole Co(NO.sub.3).sub.2 
solution and 2.748 g of a 686.979 g sol/mole Fe(NO.sub.3).sub.2 solution 
were dissolved in 60 ml deionized water, one at a time, in a 250 ml beaker 
with stirring. 3.0 g glycine and 2.0 g citric acid and 40 ml ethylene 
glycol were then added to the solution and dissolved by stirring. The 
brown-colored precursor was filtered through Fisherbrand medium Q5 filter 
paper. The precursor was heated in a 250 ml beaker at 80.degree. C. to 
expel the water and other volatile matter. After 24 hours, the precursor 
was transferred to a 100 ml beaker and allowed to concentrate at 
80.degree. C. for 2 more days until the precursor viscosity was 42 cps at 
room temperature. 
Deposition and Formation of the Nanocrystalline LSCF Film on LSCF Substrate 
A spin coating technique was used to form wet films of the precursor on the 
La.sub.0.2 Sr.sub.0.8 Co.sub.0.8 Fe.sub.0.2 O.sub.3-x substrate. LSCF 
substrate (1 mm thick) was prepared by mixing the La.sub.0.2 Sr.sub.0.8 
Co.sub.0.8 Fe.sub.0.2 O.sub.0.3-x powder (PSC of Praxair Surface 
Technologies, Inc., Woodinville, Wash.) and approximately 3 wt. % of PVB 
binder (Butvar of Monsanto), followed by pressing in a 0.5" die under a 
pressure of 10.4 kpsi and sintering at 1250.degree. C. for 4 hrs to obtain 
a dense substrate. A few drops of the viscous precursor were deposited 
onto the polished surface of the dense LSCF substrate which was fixed on a 
spinning disk. A spinning speed of 3500 rpm for 5 seconds was used for 
film deposition. After spin-coating, the as-deposited film of precursor on 
the substrate was dried on a hot plate at 80.degree. C. for 5 minutes, 
then transferred to a ceramic-top hot plate and heated at approximately 
300.degree. C. for at least 5 minutes. The entire spin-coating and drying 
process was repeated 10 cycles until a approximately 1 .mu.m thin film 
formed on the substrate surface, this composite film was then annealed at 
600.degree. C. for 2 hrs to form a LSCF nanocrystalline film (crystalline 
size approximately 20 nm, thickness approximately 1 .mu.m) on a dense LSCF 
substrate. 
Effect of Nanocrystalline LSCF Coating on Surface Exchange Kinetics 
In order to study the effect of nanocrystalline coating on surface exchange 
kinetics of oxygen transport membranes, .sup.18 O isotope exchange 
experiments were performed on LSCF discs with and without surface 
modification. A mass spectrometer was used to measure the change in the 
gas phase .sup.18 O concentration with time at the University of Twente, 
Netherlands. From these data a surface oxygen exchange coefficient and an 
oxygen tracer diffusion coefficient was derived. FIG. 5a shows the result 
of an .sup.18 O isotope exchange of LSCF disc coated with a 
nanocrystalline LSCF film (crystalline size approximately 20 nm, thickness 
approximately 1 .mu.m) as described in last section. The rate of isotope 
exchange of the coated sample is clearly higher than that of the uncoated 
1 mm thick dense LSCF sample (FIG. 5b). The surface oxygen exchange 
coefficient (k), tracer diffusion coefficient (D*) and critical thickness 
(D*/k) for the results are derived and shown in Table II. Coated LSCF 
membrane resulted in a 4-5 fold increase of the surface oxygen isotope 
exchange rate at 500.degree. C. This indicates nanocrystalline coating can 
enhance surface exchange kinetics for the composite OTM applications. 
Table II Oxygen tracer diffusion coefficient and surface exchange 
coefficient from .sup.18 O isotope exchange and gas phase analysis for 
LSCF membranes with/without surface coating. 
______________________________________ 
Temperature/ 
Sample 
.degree. C. 
PO.sub.2 /bar 
D* [cm.sup.2 /s] 
k [cm/s] 
D*/k [cm] 
______________________________________ 
LSCF 500.000 0.210 1.88*10.sup.-8 
8.63*10.sup.-8 
0.220 
LSCF + 
500.000 0.210 1.88*10.sup.-8 
4.60*10.sup.-7 
0.040 
coating 
______________________________________ 
EXAMPLE 5 
Fabrication of La.sub.0.19 Sr.sub.0.80 Fe.sub.0.69 Cr.sub.0.20 Co.sub.0.20 
Mg.sub.0.01 O.sub.3 (LSFRCM) Composite Film 
Preparation of the Polymeric Precursor Solution 
A polymeric precursor solution of LSFCRM containing 0.2 mole of the oxide 
was prepared as follows: 1.646 g of La(NO.sub.3).sub.3.6H.sub.2 O, 3.386 g 
of Sr(NO.sub.3).sub.2, 9.480 g of a 686.979 g sol/mole Fe(NO.sub.3).sub.2 
solution, 1.601 g of Cr(NO.sub.3).sub.3.9H.sub.2 O, 1.013 g of a 506.731 g 
sol/mole Co(NO.sub.3).sub.2 solution and 0.051 g of 
Mg(NO.sub.3).sub.2.6H.sub.2 O were dissolved in deionized water, one at a 
time, in a 250 ml beaker with stirring. 3.0 g of glycine and 2.0 g of 
citric acid and 40 ml of ethylene glycol were then added to the solution 
and dissolved by stirring. The brown-colored precursor was filtered 
through Fisherbrand medium Q5 filter paper. The precursor was heated in a 
250 ml beaker at 80.degree. C. to expel the water and other volatile 
matter. After 40 hours, the precursor was transferred to a 100 ml beaker 
and allowed to concentrate at 80.degree. C. for 1 more day until the 
precursor viscosity of about 82 cP at room temperature. 
Preparation of the Substrate 
LSFCRM substrate (1 mm thick) was prepared by mixing LSFCRM powder (PSC, 
Woodinville, Wash.) and approximately 3 wt. % of 9VB binder (Butvar of 
Monsanto), followed by pressing in a 1.5" die under a pressure of 10.4 
kpsi and partial sintering at 1000.degree. C. to obtain a substrate with a 
porosity of approximately 32%. 
Deposition and Formation of the Dense Film 
A spin coating technique was used to form wet films of the precursor on the 
LSFCRM porous substrate. A few drops of the viscous precursor were 
deposited onto the polished surface of the substrate which was fixed on a 
spinning disk. A spinning speed of 3500 rpm for 5 seconds was used for 
film deposition. After spin-coating, the as-deposited film of precursor on 
the substrate was dried on a hot plate at 80.degree. C. for 5 minutes, the 
transferred to a ion transport-top hot plate and heated at approximately 
300.degree. C. for at least 5 minutes. The entire spin-coating and drying 
process was repeated for thirty cycles until a dense thin film formed on 
the substrate surface. 
EXAMPLE 6 
Fabrication of La.sub.0.05 Sr.sub.0.95 CoO.sub.3 Film by a Polymeric 
Precursor With Colloidal Suspension 
In this Example, the polymeric precursor is introduced into the colloidal 
suspension system to increase the inorganic content of the system and 
reduce the shrinkage mismatch between film and substrate during sintering. 
A colloidal system consists of a dispersed phase (or discontinuous phase) 
distributed uniformly in a finely divided state in a dispersion medium (or 
continuous phase). Classical colloidal systems involve dispersions for 
which at least one dimension of dispersed phase lies in the range of 
1-1000 .mu.m, i.e. between 10 .ANG. and 1 .mu.m. In applied colloidal 
systems the upper size limit is commonly extended to at least 10,000 to 
100,000 .mu.m. In colloidal systems according to the present invention, 
the dispersed powder size is preferably less than 50 .mu.m, more 
preferably less than 1 .mu.m, and most preferably less than 0.1 .mu.m. The 
polymeric precursor is used as a binder and stabilization agent in the ion 
transport suspension to increase the stability of the system. Usually 
composition of the polymeric precursor is identical to that of ion 
transport in the liquid suspension to obtain the desired composition of 
films. By using this liquid precursor, crack-free ION TRANSPORT films can 
be achieved with the thickness greater than 0.5 .mu.m for a single coating 
step. Table II illustrates the formulation to prepare the liquid precursor 
of LSC for the composite ION TRANSPORT film fabrication. 
TABLE III 
______________________________________ 
Formulation Used for Preparing LSC Liquid Precursor 
Component Quantity Function 
______________________________________ 
Part A 
LSC 20 g ION TRANSPORT fine 
powder 
Polypropylene Glycol 
1.67 g Anti-foaming 
Darvn C 0.80 g Dispersant 
Water (or Ethylene Glycol) 
50 g Solvent 
Part B 
Polymeric Precursor of LSC 
40.5 g Binder 
______________________________________ 
The liquid precursor is prepared by adding Part A into a polyethylene 
bottle and ball-milling with YSZ balls for 18-24 hours. If the ion 
transport powder is sensitive to water, an ethylene glycol can be a 
substitute for the solvent. Component for part B is prepared by the 
process described in Example 1. Part B is then added to Part A and the 
mixture ball-milled for additional 4 hours. This liquid precursor then is 
ready for the film fabrication by dip coating or spray coating techniques. 
It can be used to fabricate the ion transport films with the thickness in 
the range of 2-5 .mu.m for the tubular or complex composite elements. 
The invention disclosed herein is especially useful for fabricating thin 
films of mixed conducting oxides presented by the structure: A.sub.r 
A'.sub.s A".sub.t B.sub.u B'.sub.v B".sub.w O.sub.x where A, A', A" are 
chosen from the groups 1, 2, 3 and the F block lanthanides; and B, B', B" 
are chosen from the D block transition metals according to the Periodic 
Table of the Elements adopted by the IU wherein 0&lt;r.ltoreq.1, 
0.ltoreq.s.ltoreq.1, 0.ltoreq.t.ltoreq.1, 0.ltoreq.u.ltoreq.1, 
0.ltoreq.v.ltoreq.1, 0.ltoreq.w.ltoreq.1 and x is a number which renders 
the compound charge neutral. Preferably, A, A', A" of the enumerated 
structure is a Group 2 metal consisting of magnesium, calcium, strontium 
and barium. Preferred mixed conducting oxides are presented by the formula 
A'.sub.s A".sub.t B.sub.u B'.sub.v B".sub.w O.sub.x where A represents a 
lanthanide, Y, or mixture thereof, A' represents an alkaline earth metal 
or mixture thereof; B represents Fe; B' represents Cr, Ti, or mixture 
thereof and B" represents Mn, Co, V, Ni, Cu or mixture thereof and s, t, 
u, v, and w each represents a number from 0 to about 1. The present 
invention is also useful for other thin film mixed conductors disclosed in 
the following U.S. Patents, all of which are incorporated herein by 
reference: U.S. Pat. No. 5,702,999 (Mazanec et al.); U.S. Pat. No. 
5,712,220 (Carolan et al.) and U.S. Pat. No. 5,733,435 (Prasad et al.). 
The present invention encompasses methods of coating deployment on the 
different substrates including: 
1) spin coating of liquid precursors; 
2) dip coating of liquid precursors; 
3) spray coating of liquid precursors; 
4) slip casting of liquid precursors; 
5) thermal spray, plasma spray; and 
6) combinations thereof. 
The term "comprising" is used herein as meaning "including but not limited 
to", that is, as specifying the presence of stated features, integers, 
steps or components as referred to in the claims, but not precluding the 
presence or addition of one or more other features, integers, steps, 
components, or groups thereof. Alternative embodiments will be recognized 
by those skilled in the art and are intended to be included within the 
scope of the claims.