Note: Descriptions are shown in the official language in which they were submitted.
Wo 95l27556 2 1 8 7 3 3 0 ~ ~llU~ r ~ ~c~
PROCESS FOR PRODUCING MF~MRRANF~S
FROM NANOPARTICULATE POWDERS
RA~ K-~uNl~ OF THE INVENTION
Field of the Invention
This invention relates to a process for producing
structures, in particular, ~, having Al~y~LL size
pores. Membranes, in particular, prepared in ac-:uL-la~lce
with the process of this invention are suitable for use in
applications such as high t~ -- aLuL~ gas separation and as
:-uL~LL.,Le materials for the deposition of ultra-thin ceramic
or metal films.
Description of Prior Art
Nembrane t ~Qrhnnlogy is rapidly h ~ ; n~ ~n
~ OLLallL research area in rhQmi~ Qn~inQQring, ~ pQci~lly
in the separation of gases. ~QpQn~in~ on the LLU~:LUL~: and
nature of the materials, LLe~na~oLL of fluids, solutes or
molecules through membranes can occur by one of several
different --h;lnir ~. The transport of any species through
es ~ which is similar to any separation process in
chemical engineering, is driven by the difference in free
energy or rhQmi~ ~l potential of that species across the
membrane. In actual use, the membranes encounter various
combinations of harsh rhQmirll environments and high
temperatures. Thus, it is critical to evaluate the effects
of changes in the thermal rh~mic~l properties and dimension
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st~bility of membrane materials on separation performance
under dif ferent operating conditionfi .
The primary def iciency of the current generation
of ceramic membranes is their poor damage tolerance and
long-term reliability. On the other hand, the main
atlvallLag~s of ceramic materials over conventional metals in
the primary ~Lr u~.Lulcll applications are their superior
~LLe.lYUI and, at high temp~:L~tu~as, good thermal stress
resistance, and ~Yrol 1 ~nt oxidation, corrosion, and erosion
resistance. Unfortunately, the brittleness of ceramics has
restricted their use in these applications where materials
~"~ cs is an; L~I~L criterion. In addition, ceramic
materials are susceptible to thermal ~7L~ abnas and thermal
~hock failure, failures often occurring at t~ ~ c.tu,e:s that
are lower than the service t~ clLu~ ~=s during heating and
cooling.
Nembrane processes have aLL- c.-;Led much attention
from an energy conservation stand-point in industrial gas
separation processes. The separation -- ~ni ~ ~ of gases by
porous solid membranes are conventionally classif ied into
four types: 1) Knudsen diffusion, 2) surface diffusion, 3)
rAri~ ry con~l~ncation with liquid flow, and 4) molecular
sieving. In general, a narrow pore size distribution in a
membrane system is needed in order to obtain a high degree
of separation of mixtures, the re~uired modal size ~ r~n~l i n~
on the type of mixture to be separated.
~YO 95/2~556 ~) 2 1 8 7 3 3 0 rcrn1sss/0~3s~
Conventional preparation of ceramic ~aterials
starts uith powders produced either from synthetic reactions
ithout stric~ chemical process control or by grinding up
naturally occurring minerals To prepare the final
cerzmics, powders zre consolidated into porous compacts,
then sintered into strong, dense ceramics. During these
transformations, the grain size increases, pore shapes
change, and the interior pores become smaller or ~;CApp~Ar
completely .
WO-A-90/00685 describes a process using particle sizes of 50
microns. The membrane is considered to be used as oil bearing. EP-A-
0 ' 580 ' 134 shows a process using particle sizes of 1 to 3 microns and
achieves therefore pore sizes larger than 100 A.
Ceramic membranes having ultra-fine pores are
typically formed by so-called "wet processes, n that is,
DroceSSeS requiring the use of a solvent. Such processes
include slip casting, gel casting, extrusion, and the sol-
gel process. The slip casting and gel casting processes
utilize large amounts of solvents as well as dispersing
agents to form z slurry which is then cast in a mold to form
the desired membrane. Extrusion typically involves the
addition of a solvent along with die lubricants and an
organic polymeric binder to a cera=ic powder to form a
mixture which is then extruded to form, typically, tubular
membranes. In the sol-gel process, a solution of organo-
metallic material is formed and then gelled. The solvent in
the gel is then removed alld the re-- i n i n~ structure heat
treated .
Each of the slip casting, gel casting, extrusion
and sol-gel processes utilize solvents and most of these
processes utilize organic additives which must later be
A~IJiJ'., ~n~T
Woss/2755~ 2 1 87330 F~l~ o~c7
removed. This greatly limits the minimum size o~ the pores,
typically submicron size, which can be formed in the
resulting :7LL~I~ LUL~ due to the requirement that the removal
of solvents or organics requires that the pore size in the
I~LU. LUL2 be larger than the molecules being removed.
In addition, the removal of solvents produces
CArillAry stresses in the D~Lu~:LuL~: which increase as the
pore size of the structure decreases. To avoid cracks in
the submicron pore size D~Lu LUL~8, elaborate and expensive
drying schemes are re~uired. When n~nnci~e or AnYDLLI size
pores are desired, the problem become8 e-~cDnt;:~lly
;n Lable due to the l L~ ' capillary stresses
encountered. See Hsieh, H.P. et al., ~Mi~:LuuuLuus Ceramic
Nembranes", Polvmer Journ~l. Volume 23, No. 5, pages 407-415
(1991? which teaches ConvPntinn~1 ceramic forming t~Drhn;5~uDc
such as extrusion, ession and injection molding which
can be used to produce ceramic membranes with Dy ~ ic
DLLLl~iLUIt:S and large pores from particles of well controlled
size distributions. See also Chan K. et al., "Ceramic
Membranes-Growth FLu~ua. LD and 0,UUUL Lu..ities'', Ceramic
Bullet;n, Volume 70, No. 4, (1991) which teaches the use of
the sol-gel prûcess for producing membranes having submicron
pore sizes; Zievers, J. F. et al., "Porous Ceramic# For Gas
Filtration", CerAm;r Bullet;n. Volume 70, No. 1, pages 108-
111, (1991) which teaches the use of layered porous ceramic
filter elements for gas filtration; and Breck, D. W. et al.,
"Nolecular Sieves", Scientific American (1959) which teaches
~ v~O95fl7556 2187330 ~crf~Sg~0435l
the use of molecular sieves for separating very similar
mo l ecul es .
Zeolites are a group of minerals, both naturally
occurring and synthetically prepared, whose crystal
structures contain pores on the order of about 3 to 20
AnyaLLI ~ in size. However, the preparation of monolithic
discs or sheets of material using zeolite with only 3 to 20
Anu,~LLu_ size rnnnPc~p~ pores is not possible because the
resulting micron size powder would contain crystals of
zeolite which form shapes containing micron size pores with
A~ .LL~ size pores within the crystals.
EP-A-0 ~ 426 ' 546 and FR-A-2150390 disclose processes for producing
membranes using additives or solvents The membranes achieved by
these processes have pores sizes larger than 100 A
SUM~L~RY 0~ ~E IN~rENTION
Accordingly, it is an objection of this invention
to provide a process for producing a monolithic structure
having An~:,LL, size and nanosize pores.
It is another object of this invention to produce
ceramic and/or metal membranes having n;~nnsi 7e and Angstrom-
size pores.
It is yet another obj ect of this invention to
provide a process for producing ceramic andJor metal
membranes which requires no solvents or dispersants which
can require elaborate and expensive drying schemes to avoid
cracks in the resulting submicron ~L~u-,LuLe:.
It is yet another obj ect of this invention to
provide a process for producing ceramic and/or metal
membranes which avoids the use of organic additives or
solvents which must be removed during the manufacturing
-tr
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process and, thus, limit the minimum pore ~ize obtainable to
the size of the molecules being removed from the final
product .
These and other objects of this invention are
achieved by a process f or producing a membrane having a
plurality of Allg~L size pores comprising the steps of
forming a loose powder layer of at least one of a metal
powder and a ceramic powder comprising a plurality of
subst~ntjAlly all nanometer-size particles and compacting
said loose powder layer of said at least one of said metal
powder and said ceramic powder to form a con~ol 1~9Ated powder
porous membrane. By "subs~An~;Ally all r-- ~r-size
particles, " we mean a powder having greater than about 959
nanometer-size particles. A critical feature of this
process is the requirement that nanometer-size ceramic
powders be utilized. In a preferred '-';- L of the
process of this invention, compacting of the nanometer-size
particles is carried out by cold-isostatic pressing.
To form membranes having highly uniform nanometer-
size pores, it is generally desired that the nanoparticulate
powder be relatively uniform in size. In addition, the mean
pore size of the membranes produced in accordance with the
process of this invention can be controlled based upon the
mean particle size of the powder being pressed. That is,
the smaller the mean particle size of the powder, the
smaller will be the mean pore size of the resulting
.e. Membranes ~Luduc~d in accordance with this
~ wossn~ss6 2 ~ ~ 7330 : Pcr~llss~lo~l~7
process have a higher porosity than those produced }~y other
known processes for producing membranes, in particular,
ceramic membranes.
D~SCRIPTION OF ~ u ~M~ODTlrFNTS
In accordance with a preferred ~hoA;-- ~ of this
invention, c.nes having a plurality o~ AnyaL~ ~ size
pores are produced by compacting at least one of a metal
powder and a ceramic powder comprising substantially all
n:-~ t~r-size particles to form ~ roncnl i~Ated porous layer
of powder, that is, a cnncol ;A~ted powder porous membrane,
the compacting being carried out by cold-isostatic pressing.
To eliminate large pores f rom within the resulting
~L .,~ Lu,æ, that is, pores greater than about three t3) times
the particle size employed, compaction ~Le:aa~LæS between
abou~(l5,000 psi) and about~,~300,000~si~ are preferred.
"'03 To produce a membrane having uniform pore sizes in
accordance with the process of this invention, nanometer
size particles having a narrow particle size distribution
are desirable. In particular, it is preferred that the
metal and/or ceramic powder comprise at least about 98
n;l- ~t~r-Size particles an~ that at least 95% of the
nanometer-size particles be less than about 30 nanometers.
In a particularly preferred ~"hoAir L, the particle size of
the nanometer-size particles is in the range of about 2
nanometers to about 30 n;~- ' F.rS.
The consolidated powder porous membranes produced
in accordance with this process are strong, the particles
WOgS/27556 2 l 8 7 3 3 0 r~l" ~c~c7
being bonded ~c a result of cold welding and electrostatic
forces. In accordance with another preferred ~ t of
this invention, the ~LLt~ Lil of the membrane can be
in-;L-ased by fast-firing the ~ nncnl i~l~ted porous layer of
powder. However, there are two; L~.l.L heating conditions
which must be observed - a low sintering t~ cltULe: and a
short hold time. A low sintering t~ cltuLe: minimi7~c the
amount of d~ncif;c~tion taking place and, thu5, r-~nt~;rc
the large porosity present in the membrane. A short hold
time m1n;m;7Ac the amount of particle growth and, thus,
reduces the amount of pore growth in the resulting - c-l.e.
For ceramic ~ ' anes, sintering ~ -tuL~s
required by the process of this invention are typically a
few hundred degrees lower than the temperatures reguired to
densify the ceramic. For example, alumina can be _ l~t~ly
densiried at 1550-C, but membranes produced in accordance
with this process by compacting a ceramic powder comprising
n ~r-size particles of alumina may be fired at lOoO-C
to strengthen it. In a preferred: ~ ir L of the process
of this invention, the concol;~l~ted porous layer of ceramic
material resulting from compaction of the ceramic powder i5
fired at a temperature between about 800-C and about 2000-C.
In accordance with a preferred: ir L of this
invention, the hold time for the membrane within the firing
process is less than 30 minutes and, preferably less than 5
minutes. CULL~ >~ in~ly~ a heating rate of about
0.5-C/minute to about 2000-C/minute is preferred. Upon
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completion of the iring process, the resulting =e~rane is
cooled, preferably as quickly as possible without causing
da~age to the membrane.
EXA~PLE I
Approximately 4 grams of nanoparticulate 8 =ol
percent Y2 O3-doped ZrO2 (YSZ) powder having a mean diameter
of about 20 nanometers was die-pressed to form a disc of
about~,(2 . 25"~ in ~ or, The ceramic disc was then cold-
S7 rn~
isostatically pres5ed a~(55, 000 psi~. Pore-size distribution
3,~ Io8 ~
analysis of the pressed YSZ disc indicated that it was about
50S porous with a uniform distribution of pores. The mean
pore radius of th~ membrane was det~rm;n~ to be about 27
An ~:.LL~ . In a gas separation test, the membrane prepared
in accordance with this example was found to be effective in
the separation of an H2/C02 gas mixture. The membrane was
found to be at least four times more F -~hl e to H2 than to
CO2 .
It will be apparent to those skilled in the art
that different membrane shapes can be formed in accordance
with the process of this invention including discs and
tubes .
To improve the r~ n;c~ Le~l~Lh~ the membranes
can be heat treated by fast-firing to preserve the
uniformity of the pore size distribution. ~embranes
produced in accordance with the process of this invention
have a porosity of about 30% to 55%, that is, a~out 30% to
about 55% porous. The mean pore radius of the membranes
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'~
produced in accordance with the process of thi6 invention i6
between about V5 to V20 of the mean particle diameter of
the powder used. In other words, if a powder with a mean
particle diameter of 10 n~- ~r~6 is used, a membrane with
a mean pore radius of about 5 AnyaLL~ - will be obtained.
If ~ ,l,le support or multilayers of r- '_I~es
are desired, powders of different particulate size can be
pressed together to form r ' c~ne layers of different mean
pore sizes. In particular, in accordance with one
of the process of this invention for producing
~ultilayer membranes, the loose powder layer of nAr te~-
size particles of metal powder and/or ceramic powder is
rormed on a coarse particle layer of metal and/or ceramic
powder particles where the coarse particle layer comprises a
plurality of particles, aul/a~ Lially all larger than
n~- tr~r-size. In accordance with one I ';--nt of the
process of this invention, the loose powder layer and the
coarse particle layer are simult~nQollcly compacted together,
forming a multilayer c~7ncol ir~ted powder porous I,e,
In accordance with another: -'i- L of the process of this
invention, the coarse particle layer is compacted and the
loose powder layer is formed on the compacted coarse
particle layer and s~lh6r-~r~r~ntly compacted onto the compacted
coarse particle layer to form a multilayer ~ nncol ~ ted
powder porous membrane.
wo ssn7ss6 2 1 8 7 3 3 0 PCI/llS9~s/11~35i ~
EXi~PLE I I
This example demonstrates a method for makiny a
ceramic membrane having a two-layer structure.
Approximately 4 grams of submicron size 8 mol
percent Yz03-doped ZrO2 (YSZ) powder having a mean 1'9iA ' -r
of about 0 . 3 microns were die-pressed to form a disc of
;,(2.25") in diameter. Before removal of the YSZ disc from the
s~in1~cs steel die, approximately 0.2 g of nanoparticluate
Al203 powder having a mean diameter of about lO nanometers
were spread evenly on the top surface of the YSZ disc, and
die-pressed once again to form a two-layer porous ~LLU~_~U' ~.
The two-layer ceramic structure was them cold-isostatically
pressed at~ 58,000 psi~. Accordingly, the YSZ powder, in this
/o8~
case, was used as the supporting :7LLuL~LuL~= for the thin
Al203 membrane.
In a gas separation test, the membrzne prep2red in
this example was found to be effective in the separation of
H2/C02 mixture. The membrane was found to be at least four
times more p~ ~hle to X2 than to C02. The gas transfusing
rate across the membrane was ~ign;f;c:lntly Pnh~nl-~d in the
two-layer membrane structure compared to that of Example I.
~ hile in the f oregoing specif ication this
invention has been described in relation to certain
preferred ~ LS thereof, and many details have been
set forth for purpose of illustration, it will be apparent
to those skilled in the art that the invention is
susceptible to additional ~mho~l;r LS and that certain of
11 A'1~',L;~ .r, ~J
wo ss/27ss6 2 1 8 7 3 3 ~ Q4~C~
the details described herein can be varied c~n~ rably
without departing from the basic prinl-;pl~ of the
invention.
