Molecular sieve abstract
A method for preparing carbon molecular sieve membrane is invented.
A thin carbon-containing film is first deposited on a porous substrate.
The thin film is then bombarded by high energy ions for surface
modification. The surface modified film is then baked or calcined
at a high temperature. The carbon molecular sieve membrane prepared
according to the present invention can be used for gas separation
as well as liquid separation, ions or solvents, etc., exhibiting
improved permeance and improved selectivity simultaneously in gas
separation. The ion bombardment includes generating plasma and ions
in a gas phase, and applying a negative bias voltage to the substrate.
Molecular sieve claims
What is claimed is:
1. A process for preparing a carbon molecular sieve membrane comprising
the following steps: (a) growing an amorphous carbon-based membrane
on a porous substrate at a low temperature, where said membrane
is able to be pyrolyzed or decomposed at a temperature higher than
the growth temperature; (b) performing a surface modification by
bombarding said membrane with energized gaseous ions; and (c) baking
said surface-modified membrane at a temperature higher than the
growth temperature in Step (a), so that a membrane having a gas
separation effect is formed.
2. The process according to claim 1 wherein said amorphous carbon-based
membrane comprises carbon as a major portion thereof and, optionally,
elements selected from the group consisting of Al, Si, O, N, P and
F.
3. The process according to claim 1 wherein said surface modification
by bombarding the membrane with energized gaseous ions is carried
out with a means selected from means for generating plasma, laser
or ion beams, wherein a negative bias voltage is applied to said
porous substrate, or an ion gun or an ion implantation device is
used to accelerate ions.
4. The process according to claim 3 wherein said surface modification
is carried out by: bombarding said membrane with energized gaseous
ions at a pressure less than 10.sup.5 Pa and with an ion acceleration
bias voltage less than 5000 volts.
5. The process according to claim 1 wherein said baking is carried
out at a pressure of 0.001 Pa-2.times.10.sup.5 Pa, in an environment
of vacuum, N.sub.2 or an inert gas, and for a period of time up
to 100 hours.
6. The process according to claim 1 further comprising repeating
Steps (a), (b) and (c) one or more cycles, thereby further improving
the separation effect of the membrane.
7. The process according to claim 1 wherein said growth of said
amorphous carbon-based membrane in Step (a) comprises using an inductively-coupled-plasma
chemical vapor deposition (ICP CVD) to grow said membrane on said
porous substrate under conditions comprising a gas phase pressure
of 10.sup.-3 -100 torr; a reaction gas mixture comprising 5-100
vol. % of a carbon-containing gas, and 95-0 vol. % of O.sub.2 or
an oxygen-containing gas; a total flowrate of 0.5-10 sccm; RF high
frequency power of 20-1000 W; and a processing time of 0.1-20 hours.
8. The process according to claim 7 wherein said carbon-containing
gas is hexamethyldisiloxane (HMDSO) or methane.
9. The process according to claim 7 wherein said reaction gas
mixture comprises 5-95 vol. % of said carbon-containing gas, and
95-5 vol. % of O.sub.2.
10. The process according to claim 7 wherein said surface modification
by bombarding said membrane with energized gaseous ions in Step
(b) is carried out by means of an inductively-coupled-plasma chemical
vapor deposition and applying a negative bias voltage to said porous
substrate, wherein conditions of generating plasma comprise a gas
pressure of 10.sup.-3 -100 torr; a gas composition comprising 5-100
vol. % of a carbon-containing gas, 95-0 vol. % of O.sub.2 or an
oxygen-containing gas, and 95-0 vol. % of an inert gas; a total
gas flowrate of 0.5-10 sccm; RF high frequency power of 20-1000
W; and a processing time of 1-1000 second.
11. The process according to claim 10 wherein said carbon-containing
gas is hexamethyldisiloxane (HMDSO) or methane.
12. The process according to claim 10 wherein said gas composition
comprises 5-100 vol. % of said carbon-containing gas, and 95-0 vol.
% of O.sub.2.
13. The process according to claim 12 wherein said carbon-containing
gas is hexamethyldisiloxane (HMDSO).
14. The process according to claim 10 wherein said gas composition
comprises 5-100 vol. % of said carbon-containing gas, and 95-0 vol.
% of said inert gas.
15. The process according to claim 14 wherein said carbon-containing
gas is hexamethyldisiloxane (HMDSO).
16. The process according to claim 10 wherein said processing
time is of 3-30 seconds.
17. The process according to claim 16 wherein said negative bias
voltage is of 10-100 volts.
18. The process according to claim 10 wherein said negative bias
voltage is of 10-100 volts.
19. The process according to claim 1 wherein said surface modification
by bombarding said membrane with energized gaseous ions in Step
(b) is carried out by means of an inductively-coupled-plasma chemical
vapor deposition and applying a negative bias voltage to said porous
substrate, wherein conditions of generating plasma comprise a gas
pressure of 10.sup.-3 -100 torr; a gas composition comprising 5-100
vol. % of a carbon-containing gas, 95-0 vol. % of O.sub.2 or an
oxygen-containing gas, and 95-0 vol. % of an inert gas; a total
gas flowrate of 0.5-10 sccm; RF high frequency power of 20-1000
W; and a processing time of 1-1000 second.
20. The process according to claim 19 wherein said carbon-containing
gas is hexamethyldisiloxane (HMDSO) or methane.
21. The process according to claim 19 wherein said gas composition
comprises 5-100 vol. % of said carbon-containing gas, and 95-0 vol.
% of O.sub.2.
22. The process according to claim 21 wherein said carbon-containing
gas is hexamethyldisiloxane (HMDSO).
23. The process according to claim 19 wherein said gas composition
comprises 5-100 vol. % of said carbon-containing gas, and 95-0 vol.
% of said inert gas.
24. The process according to claim 23 wherein said carbon-containing
gas is hexamethyldisiloxane (HMDSO).
25. The process according to claim 19 wherein said processing
time is of 3-30 seconds.
26. The process according to claim 25 wherein said negative bias
voltage is of 10-100 volts.
27. The process according to claim 19 wherein said negative bias
voltage is of 10-100 volts.
Molecular sieve description
FIELD OF THE INVENTION
The present invention is related to a process for preparing a carbon
molecular sieve membrane, and in particular for preparing a supported
carbon molecular sieve membrane.
BACKGROUND OF THE INVENTION
Currently, carbon molecular sieve membranes are prepared by subjecting
a thermoset polymer to pyrolysis or calcination at a high temperature
in an inert gas or vacuum environment, thereby releasing volatile
gases, such as H.sub.2 O, CO, CO.sub.2 CH.sub.4 HCN, N.sub.2 and
H.sub.2 etc., and forming an amorphous carbon membrane having a
pore size of several microns to several angstroms. The pore size
is closely related to the material of the polymer and the conditions
of pyrolysis. Published researches on the carbon molecular sieve
membranes are discussed in the following: (1) Bird and Trimm used
polyfurfuryl alcohol (PFA) to prepare non-supported and supported
carbon molecular sieve membranes. Due to the occurrence of shrinkage
during pyrolysis, a continuous carbon separation membrane could
not be produced [P. L. Trimm et al., Carbon, 21(3), 177 1983].
(2) Koresh and Soffer have done a very systematic research on carbon
molecular sieve membranes [J. E. Koresh and A. Soffer, Sep. Sci.
Technol., 18 (1983) 723; J. E. Koresh and A. Soffer, Sep. Sci. Technol.,
23 (1987) 973]. A hollow fibrous polymer membrane was calcined at
a medium temperature (800-950.degree. C.) in nitrogen or an inert
gas, thereby forming a carbon molecular sieve membrane. The selectivity
of He to O.sub.2 was 8 and the selectivity of He to N.sub.2 was
20. In particular, the permeance of He reached 3.times.10.sup.-7
mol.m.sup.-2.s.sup.-1.Pa.sup.-1 which was tens to hundreds times
higher than that of a polymer membrane. (3) Linkov et al. produced
a three-zoned asymmetrical carbon membrane by subjecting a polyacrylonitrile,
(PAN)-based hollow fibrous precursor to a thermal oxidation stabilization
and a carbonization in an inert atmosphere [V. M. Linkov, R. D.
Sanderson and E. P. Jacobs, J. Membrane Sci., 95 (1994) 93]. The
intermediate zone had longitudinal voids with a length 5-15 .mu.m
and a diameter 3-7 .mu.m. The inner layer had voids with a pore
size of 3-5 .mu.m. The outermost layer is a denser layer of 0.1-0.4
.mu.m. Subsequently, a mixture gas of TiCl.sub.4 and CH.sub.4 was
subjected to a gas phase pyrolysis to grow a TiC membrane on said
hollow carbon fiber, which was then subjected to a high temperature
oxidation in order to reduce the pore size on the outermost layer
to less than 90 nm. In 1994 a combined magnetron sputtering and
ion beam technique was used to coat a diamond-like carbon (DLC)
membrane on the abovementioned fiber. The results indicated that
a low sputtering rate could form a continuous membrane which fully
covered the original voids. However, this composite membrane (without
receiving a further carbonization) still had a Knudsen diffusion
mechanism in transporting gas, and not a molecular sieve mechanism.
(4) Yamada et al. produced a carbon molecular sieve *membrane by
subjecting a polyimide (PI) to a carbonization [Y. Yamada, et al.,
Carbon, 30 719 1992]. The produced membrane had an oxygen-to-nitrogen
separation ratio of 4.6 and had a molecular sieve mechanism. (5)
Damle et al. used various materials and methods to perform various
surface treatments on a commercial carbon membrane having a pore
size of 0.2-1.0 .mu.m: {character pullout} dip-coating a polymer
of polyacrylonitrile (PAN), polyfurfuryl alcohol (PFA), phenol-formaldehyde
resin (PF) or cellulose precursors, on the carbon membrane; {character
pullout} using a plasma polymerization to coat PAN on the carbon
membrane; {character pullout} coating a solution of a PFA resin
monomer on the carbon membrane, and adding a catalyst for an in-situ
polymerization; {character pullout} using high temperature pyrolysis
to decompose propylene into tiny carbon particles to deposit on
the carbon membrane substrate [A. S. Damle at al., Gas Separation
& Purification8(3), 137 1994]. After the abovementioned processing,
said membrane was subjected to high temperature carbonization in
order to improve the properties of the carbon membrane. The results
indicated that the permeance was reduced by all the abovementioned
processing. Besides, except in-situ polymerization, the processing
had no significant improvement in selectivity. Although the in-situ
polymerization process slightly improved the selectivity, the transport
mechanism was in Knudsen diffusion range without involving molecular
sieve effect. (6) Collins and Yin used a DC sputtering technique
to coat a diamond-like carbon (DLC) on a silicon substrate, and
carbonizing the coated substrate by a vacuum baking [Y. Yin and
R. E. Collins, Carbon, 31 (1993) 1333]. A QCM (quartz crystal microbalance)
was used to measure the absorption of benzene and 22-dimethylbutene
in said carbon membrane. It was found that the absorption of benzene
(5.2 .ANG.) was at least ten times greater than the absorption of
22-dimethylbutene (6.0 .ANG.). After the high temperature treatment,
said diamon-like carbon membrane formed extremely fine pores on
the membrane to have very conspicuous molecular sieve functions
and could separate molecules of slightly different sizes. Furthermore,
the porosity and the pore size distribution of said carbon membrane
were affected by the sputtering conditions (e.g. composition of
the gas, bias voltage of the target, etc.) and the conditions of
high temperature treatments of diamond-like membrane. Therefore,
the molecular sieve function could also be varied.
Meanwhile, many people have investigated silica-based molecular
sieve membranes. The published results on this topic are outlined
in the following: (1) Gavalas et al. first successfully used a thermal
CVD to effectively reduce the macropores of a Vycor glass substrate,
by introducing SiH.sub.4 and O.sub.2 separately from both sides
of the substrate in order to carry out reactions within the pores
to form a molecular sieving SiO.sub.2 membrane. The selectivity
of H.sub.2 to N.sub.2 reached 1000; and the permeance of H.sub.2
at 450.degree. C. was 10.sup.-8 mol.m.sup.-2.s.sup.-1.Pa.sup.-1.
They also introduced SiCl.sub.4 and O.sub.2 into the pores from
the same side of said glass substrate for reaction. The reaction
temperature was 600-800.degree. C. (2) Yan et al. used a porous
.alpha.-alumina tube as a substrate, which was first subjected to
a boehimite solution dip-coating, followed by pyrolysis, thereby
forming an .gamma.-alumina membrane on the outside of the tube [S.
Yan, H. Maeda, K. Kusakabe, S. Morooka and Y. Akiyama, Ind. Eng.
Chem. Res., 33 (1994) 2096]. Then, tetraethylorthosilicate (TEOS)
was used as a feed in thermal CVD to deposit SiO.sub.2 membranes.
The permeance of H.sub.2 at 600.degree. C. was 10.sup.-7 mol.m.sup.-2.s.sup.-1.Pa.sup.-1
and the selectivity of H.sub.2 to N.sub.2 reached 1000.
A SiO.sub.2 membrane could only withstand a temperature up to 500.degree.
C., and the separation of H.sub.2 often requires a higher operating
temperature. Furthermore, the SiO.sub.2 membrane could only separate
a mixture of small molecules (such as H.sub.2 He etc.) and other
gases. Therefore, many researchers proposed the membranes containing
Si--C or Si--O--C structure. Such the structure could withstand
a temperature up to 1200.degree. C. and separate gases at a higher
temperature. Furthermore, the pore size could be controlled to separate
a mixture of gases with similar molecule sizes (e.g. O.sub.2 N.sub.2
having a difference of 0.2 .ANG.). (1) Tsay, Dah-Shyang et al. produced
a SiC molecular sieve by adding 5% of alumina into a raw material;
sintering the material into a porous tube; coating the inner wall
of the tube with a SiC having a particle size of 30 nm and containing
2% of alumina; sintering said tube into an asymmetrical tube; filling
the tube with a polydimethylsilane solution; and subjecting the
tube to a thermal treatment, a curing and a pyrolysis. A separation
membrane, which was pyrolyzed at 300.degree. C., had a H.sub.2 permeance
of 10.sup.-7 mol.m.sup.-2.s.sup.-1.Pa.sup.-1 at 200.degree. C.,
and a H.sub.2 /N.sub.2 selectivity of up to 100. A separation membrane,
which was pyrolyzed at 600.degree. C., had a H.sub.2 permeance of
5.times.10.sup.-9 mol.m.sup.-2.s.sup.-1.Pa.sup.-1 at 200.degree.
C., and a H.sub.2 /N.sub.2 selectivity of about 40. (2) Kusakabe
and Morook et al. first used a .gamma.-alumina membrane to reduce
the pore size on an .alpha.-alumina tubular substrate [Z. Li, K.
Kusakabe and S. Morooka, J. Membrane Sci., 87 (1996) 159; Z. Li,
K. Kusakabe and S. Morooka, Sep. Sci. Technol., 32 (1997) 1233].
A polycarbosilane (PC) membrane was coated thereon by pyrolysis
at 350-550.degree. C. The membrane had a H.sub.2 permeance of 5.times.10.sup.-7
mol.m.sup.-2.s.sup.-1.Pa.sup.-1 at 400.degree. C., and a H.sub.2
/N.sub.2 selectivity of about 7.2. Said Si--C--O membrane was repetitively
coated three times and pyrolyzed at 950.degree. C., thereby increasing
the H.sub.2 /N.sub.2 selectivity to 18-63 but the H.sub.2 permeance
was reduced to 10.sup.-9 -10.sup.-8 mol.m.sup.-2.s.sup.-1.Pa.sup.-1
at 500.degree. C.
Therefore, there are two bottlenecks existing in the prior art
methods for preparing carbon molecular sieve membranes: (1) The
carbon membrane is not grown on the porous substrate. As a result,
the mechanical strength of the membrane is not sufficient, and the
carbon membrane is easy to crack. (2) A molecular sieve membrane
grown on a porous substrate always uses a method of reducing the
pore size to increase the selectivity at a cost of greatly reducing
its permeance. Therefore, such a membrane has a limited applications.
SUMMARY OF THE INVENTION
A primary objective of the present invention is to provide a process
for preparing a carbon molecular sieve membrane without the aforesaid
drawbacks in the prior arts. The process of the present invention
uses a novel technique to form a carbon molecular sieve membrane
on a porous substrate, which has a high selectivity while maintaining
a high permeance.
In order to achieve the abovementioned objective, a process for
preparing a carbon molecular sieve membrane according to the present
invention comprises the following steps: (a) growing an amorphous
carbon-based membrane on a porous substrate at a low temperature,
where said membrane is able to be pyrolyzed or decomposed at a temperature
higher than the growth temperature; (b) performing a surface modification
by bombarding said membrane with energized gaseous ions; and (c)
baking said surface-modified membrane at a temperature higher than
the growth temperature in Step (a), so that a membrane having gas
separation ability is formed.
Said amorphous carbon-based membrane comprises carbon as a major
portion thereof and, optionally, other elements selected from the
group consisting of Al, Si, O, N, P and F.
Said surface modification by bombarding the membrane with energized
ions can be carried out with a means selected from means for generating
plasma, laser or ion beams, wherein a negative bias voltage is applied
to said substrate, or an ion gun or an ion implantation device can
be used to accelerate ions. A suitable surface modification condition
includes: bombarding said membrane with energized ions at a pressure
less than 10.sup.5 Pa and with an ion acceleration bias voltage
less than 5000 volts, preferably 10-100 volts.
Said baking of the process of the present invention can be carried
out, for example, at a pressure of 0.001 Pa-2.times.10.sup.5 Pa,
in an environment of vacuum, N.sub.2 or an inert gas such as He
or Ar, and for a period of time up to 100 hours.
Preferably, the process according to the present invention further
comprises repeating Steps (a), (b) and (c) one or more cycles, thereby
further improving the separation effect of the membrane.
In one of the preferred embodiments of the present invention, said
growth of said amorphous carbon-based membrane in Step (a) comprised
using an inductively-coupled-plasma chemical vapor deposition (ICP
CVD) to grow said membrane on said porous substrate. Suitable conditions
for growing said membrane include: a gas phase pressure of 10.sup.-3
-100 torr; a reaction gas mixture comprising 5-100 vol. % of a carbon-containing
gas such as hexamethyldisiloxane (HMDSO) or methane, and 95-0 vol.
% of O.sub.2 or an oxygen-containing gas; preferably 95-5 vol. %
of O.sub.2 a total flowrate of 0.5-10 sccm; RF high frequency power
of 20-1000 W; and a processing time of 0.1-20 hours.
Said surface modification by bombarding said membrane with energized
ions in Step (b) was carried out by means of an inductively-coupled-plasma
chemical vapor deposition and applying a negative bias voltage to
said substrate, wherein conditions of generating plasma include:
a gas pressure of 10.sup.-3 -100 torr; a gas composition comprising
5-100 vol. % of a carbon-containing gas such as hexamethyldisiloxane
(HMDSO) or methane, preferably HMDSO, 95-0 vol. % of O.sub.2 or
an oxygen-containing gas, preferably O.sub.2 and 95-0 vol. % of
an inert gas; a total gas flowrate of 0.5-10 sccm; RF high frequency
power of 20-1000 W; and a processing time of 1-1000 seconds, preferably
3-30 seconds. Preferably, said gas composition comprises 5-100 vol.
% of said carbon-containing gas, and 95-0 vol. % of O.sub.2. Preferably,
said gas composition comprises 5-100 vol. % of said carbon-containing
gas, and 95-0 vol. % of said inert gas.
After the surface-modified membrane being further subjected to
said high temperature baking, the selectivity and the gas permeance
of the resulting molecular sieve membrane can be greatly improved
simultaneously. The reasons are believed to be: (1) By examining
the gas permeance of a membrane which is baked often being subjected
to surface modification, the increase of H.sub.2 permeance is much
larger than the increase of N.sub.2 permeance. That is the voids
formed by the surface modification followed by the baking only allow
the passage of H.sub.2 in a large quantity while inhibiting the
passage of N.sub.2. (2) The dependence of H.sub.2 permeance on the
permeation temperature is opposite to that of N.sub.2. The activation
energy of H.sub.2 is positive and its permeance increases with an
increase in permeation temperature; while the activation energy
of N.sub.2 is negative and its permeance decreases with an increase
in permeation temperature. Therefore, at a higher temperature, the
selectivity of H.sub.2 /N.sub.2 is greatly increased. Furthermore,
the duration of the surface modification and the magnitude of the
negative bias voltage all need to be optimized. The surface modification
duration can be adjusted in a wider range when a smaller negative
bias voltage is applied. When the duration of surface modification
is too long, the modification effects are not evident. Moreover,
the type of gas used in the surface modification is also influential.
The use of pure Ar or pure O.sub.2 in conducting the surface modification
has a result not as good as that of hexamethyldisiloxane (HMDSO).
However, a membrane which has been subjected to surface modification
has much better separation capability than a membrane which has
not been subjected to surface modification. Therefore, the composition
of the gas used in the surface modification will affect the composition
and the structure of the surface layer of the carbon membrane, and
subsequently affect the separation performance.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic diagram of a device for carrying out a
membrane growth and the surface modification by using an inductively-coupled-plasma
chemical vapor deposition (ICP CVD) according to the present invention;
FIG. 2 shows the relationship between the duration of the surface
modification and the H2 permeance at room temperature;
FIG. 3 shows the relationship between the duration of the surface
modification and the N.sub.2 permeance at room temperature.
Legends: {character pullout} mechanical pump coil {character pullout}
angle valve {character pullout} ENI power supply {character pullout}
RF power supply {character pullout} matching box {character pullout}
RF power introduction {character pullout} substrate {character pullout}
substrate support {character pullout} thermocouple 11. vacuum chamber
12. liquid source isothermal bath 13. baratron pressure gauge 14.
convectron pressure gauge 15. gas source 16. pressure gauge 17.
mass flow controller 18. safety shutoff 19. gas filter
DETAILED DESCRIPTION OF THE INVENTION
The use of a carbon membrane in separating liquids or gases is
one of the inorganic membrane techniques of high potentials. In
comparison with the conventional polymer membrane, a carbon membrane
used for microfiltration or ultra-microfiltration has advantages
such as resistance to high temperatures, resistance to strong acids,
strong bases and organic solvents, and excellent biocompatibility.
A carbon molecular sieve has very tiny pore sizes, which vary from
3 .ANG. from 10 .ANG.. The pore sizes can be controlled to separate
two gaseous molecules with a difference pore size within of 0.2
.ANG.. At present, carbon molecular sieves are mainly used as an
adsorbent in the pressure swing adsorption process. Some carbon
molecular sieves have better separation capability than zeolites.
The application of membrane separation often encounters two problems:
a low permeance and a low selectivity. A low permeance can be improved
by increasing the surface area of a membrane; however, the capital
cost is also increased as a result. A low selectivity can be improved
by recycling or using a multi-stage process; however, the equipment
and operational costs are increased. Therefore, a membrane is practical
and has an industrial value only when the membrane has a high permeance
and a high selectivity at the same time.
The application of carbon molecular sieve membranes has the following
disadvantages: (1) It is difficult to produce an unsupported carbon
membrane that is continuous and free of cracks or voids. This is
because an ordinary unsupported carbon membrane is formed by pyrolyzing
a polymer membrane. Molecules will decompose and escape during pyrolysis,
and the membrane will shrink. When the shrinking of the membrane
is uneven or too much, the resulting stress will cause the formation
of cracks. (2) Furthermore, a carbon membrane is itself fragile,
does not have a sufficient mechanical strength; can not withstand
a high pressure, and ruptures easily. A polymer membrane is usually
much tougher than a carbon membrane.
In order to overcome the abovementioned disadvantages, a carbon
membrane layer with molecular sieving functions can be prepared
on a porous substrate by a pyrolysis process or a membrane deposition
process. The porous substrate per se can provide the strength required
by the carbon molecular sieve. Besides a good film deposition technique
can deposit a continuous film or membrane with a strong adhesion
on the porous substrate. This can solve the problems of being fragile,
easy-to-crack or insufficiently strong.
The separation of ions in a solution or gases is usually carried
out by coating one or more denser membranes on a porous substrate.
The methods of coating a membrane on a porous substrate include:
an aqueous solution precipitation method, a sol-gel method, an electroplating
method, a chemical vapor phase deposition (CVD) method, and a physical
vapor phase deposition (PVD) method, etc. These methods have a common
feature: In order to increase the selectivity of the membrane, the
membrane needs to be denser, thicker, or consisting of multiple
layers. As a result, the permeance will be greatly reduced. For
example, a surface modification by a CVD method is usually used
to decrease the pore sizes of the pores in order to increase the
selectivity. However, the permeance is also greatly decreased since
the pore sizes have become much smaller.
The present invention greatly increases the permeance and the selectivity
of the membrane simultaneously by performing a surface modification
to a membrane grown on a substrate, followed by a high temperature
baking. This achievement can not be accomplished by an ordinary
surface modification of membrane or a multi-layered membrane deposition.
Another feature of the present invention is the use of an additional
inductively coupled plasma. Another power supply can than be used
to independently control the bias voltage of the substrate. Therefore,
that the desired surface modification conditions can be controlled
more accurately.
(1) Growth of Carbon Membrane
(1)-1
An anodic membrane (porous alumina membrane) substrate was cleansed
with tetrachloromethane (CCl.sub.4), and then dried. A porous graphite
substrate was coarsely polished by 3 .mu.m alumina, and then finely
polished by 0.3 .mu.m and 0.05 .mu.m alumina slurries, followed
by washing with water and then dried. The anodic membrane substrate
employed had a porosity of 50%, a diameter of 13 mm, thickness of
77 .mu.m, and an average pore size of 0.02 .mu.m.
(1)-2
The porous alumina membrane substrate or the porous graphite substrate
were mounted in a vacuum chamber, with a vacuum of <1.times.10.sup.-3
torr generated in the vacuum chamber.
(1)-3
The substrates were cleansed with Ar+ ions with the following cleaning
High Pressure frequency power source Bias voltage of substrate
Time 0.05 torr 50 W 200 V 10 min
(1)-4 Conditions for the Deposition of Membranes:
After introducing the reaction gases, the gas composition was adjusted
by controlling gas partial pressure and the total pressure was adjusted
by an evacuation valve. After the pressure had stablized, the deposition
of the membrane was started by fixing the power of the high frequency
power source.
(1)-5 Growth of Membrane:
During the deposition of the membrane, the variation of the plasma
power was constantly monitored.
(2) Performing Surface Modification by Ion Bombardment:
There were two situations:
2-(a) The modification conditions were the same as the membrane
growth conditions except the additional substrate bias during surface
modification:
The bias voltage of the substrate and the operation time were set
in advance. Then immediately after the completion of the membrane
growth, the power supply for generating a bias voltage on the substrate
was started.
2-(b) In addition to applying a bias voltage, the modification
conditions were different from those of the membrane growth:
Upon completion of the growth of the membrane, the conditions for
depositing the membrane were changed to the surface modification
conditions.
(3) High Temperature Baking:
After the surface modification, the membrane was sent into a high
temperature furnace to bake at 500.degree. C. or 600.degree. C.
at 0.007 torr for 4 hours. The furnace temperature was raised at
2.4.degree. C./min from room temperature.
The main difference between the present invention and the conventional
multi-layer membrane approach or the conventional CVD surface modification
lies in:
For an ordinary membrane-deposition method to increase the membrane
selectivity, the membrane needs to be denser, thicker or having
more layers, thereby usually reducing the permeance. For example,
for an ordinary CVD method to perform a surface modification, the
sizes of the pores are decreased to increase the selectivity. However,
since the pore sizes have become smaller, the permeance is also
decreased significantly.
A plasma surface modification method according to the present invention
can dramatically increase the permeance and the selectivity simultaneously.
This has a great difference with the ordinary composite membrane
concept or pore-size-reducing concept, and thus has novelty and
non-obviousness. The present invention performs a surface modification
to a grown membrane, followed by a high temperature baking. As a
result, the H.sub.2 permeance of a membrane prepared according to
the present invention is far higher than that of a membrane without
being subjected to the surface modification before pyrolysis. Therefore,
the surface modification by an ion bombardment according to the
present invention, unlike the ordinary modification method for increasing
the selectivity where the permeance is sacrificed, can greatly increase
the permeance and the selectivity of the membrane simultaneously.
The permeance of H.sub.2 can reach (1-5).times.10.sup.-6 mol/m.sup.2.s.Pa
and the selectivity can reach 81. At present, the highest H.sub.2
permeance of a molecular sieve published in the literature is about
10.sup.-7 mol/m.sup.2.s.Pa. The results of the present invention
are ten times higher than the highest value published in the literature.
The present invention performs a surface modification by an ion
bombardment first, and then a high temperature baking. In other
words, the present invention does not perform a surface modification
(depositing a second layer of membrane) on an established carbon
molecular sieve (which has been subjected to a high temperature
pyrolysis), but performs a surface modification prior to the high
temperature pyrolysis. The objective of the high temperature baking
according to the present invention is not for forming a second layer
of membrane by the surface modification. That is the present invention
does not use a high temperature pyrolysis to reduce the pore sizes
in order to increase the selectivity. Instead, the high temperature
baking is to pyrolyse a carbon precursor membrane formed by the
low temperature growth and the ion bombardment. The surface microstructure
so obtained is greatly different from that of a membrane which was
directly subjected to the high temperature baking without receiving
the surface modification first. Furthermore, a change of such a
microstructure requires an ion bombardment with a suitable energy.
For an ordinary CVD method, the components in the gas phase will
not form energized ions, and therefore will not have the abovementioned
effects.
Furthermore, the source of gas phase components used in the ion
bombardment according to the present invention does not necessarily
contain a component such as carbon, silicon, or aluminum, etc. that
can form a membrane. An ion bombardment by Ar, O.sub.2 etc. still
has the effect of increasing the separation effect. Therefore, the
present invention is greatly different from the CVD surface modification
method where only a feed capable of deposition is used, or followed
by the additional pyrolysis of the deposition layer.
The main differences between the present invention and the conventional
plasma-enhanced CVD method or the thermal pyrolysis method in preparing
a molecular sieve membrane are:
(1) Using a plasma-enhanced CVD method to deposit a membrane, followed
by a surface modification by applying a bias voltage for an appropriate
period of time: The surface modification of the membrane comprises
bombarding the surface of the membrane with ions for a suitable
period of time. The surface modification of the membrane by an ion
bombardment can be done by generating a plasma and ions in a gas
phase by various methods, and by applying a negative bias voltage
to a substrate attached to the membrane for accelerating the ions
in bombarding the surface of the membrane.
(2) The membrane must receive a surface modification followed by
a high temperature baking in order to greatly increase the selectivity
and the permeance while separating a mixture gas: The membrane with
a surface microstructure formed by an ion bombardment modification
needs to be subjected to a high temperature baking in order to form
a membrane having a high permeance and a high selectivity simultaneously.
The present invention can be better understood through the following
examples and drawings.
FIG. 1 shows a schematic diagram of a device suitable for performing
a membrane growth and a surface modification by an inductively-coupled-plasma
CVD method, which mainly comprises:
{character pullout} mechanical pump {character pullout} coil {character
pullout} angle valve {character pullout} ENI power supply {character
pullout} RF power supply {character pullout} matching box {character
pullout} RF power introduction {character pullout} substrate {character
pullout} substrate support {character pullout} thermocouple 11.
vacuum chamber 12. liquid source isothermal bath 13. baratron pressure
gauge 14. convectron pressure gauge 15. gas source 16. pressure
gauge 17. mass flow controller 18. safety shutoff 19. gas filter
A detailed description of some parts of 11 vacuum chamber; 9 substrate
support; vacuum pressure gauge; reaction gas transport system; evacuation
system; power supply system follows:
11. Vacuum Chamber:
Cylindrical: diameter 32 cm, height 32 cm, thickness 0.6 cm, made
of 316 stainless steel.
{character pullout} Substrate Support:
A disk on the top of the substrate support is used for mounting
the substrate. In order to avoid contaminating the membrane, the
disk is made of graphite (because the membrane to be deposited is
a carbon membrane). The inside of the disk is inserted with a thermocouple
{character pullout} (K-type Chromel/Alumel type, non-grounding type)
for measuring the temperature of the substrate. The thermocouple
is wrapped with a quartz pipe in order to isolate itself from the
graphite disk.
Vacuum Pressure Gauge:
The membrane deposition system uses two types of pressure gauges
to measure the pressure inside the chamber. They are a baratron
pressure gauge 13 and a convectron pressure gauge 14:
(1) Baratron pressure gauge: Its reading is independent of the
type of the gas, and has a measurement range of 1-10.sup.-4 torr.
This pressure gauge is used to accurately measure the pressure during
the ICP CVD of membrane in the present system.
(2) Convectron pressure gauge: This is a thermocouple type vacuum
gauge, and is used to measure the amount of heat carried away by
the collision between the gas molecules and the heated body in the
system and convert the measurement into a pressure in the system.
Its reading will vary depending on the type of gas. An actual pressure
value can be obtained by using a conversion curve provided by the
operation manual. This pressure gauge can measure a pressure of
1000.about.10.sup.-4 torr.
Reaction Gas Transport System:
The transport path of the reaction gas is shown in FIG. 1. Its
flowrate is accurately controlled by a mass flowrate controller
17. The vapor pressure is controlled by an isothermal bath 12. A
needle valve is used to control the flowrate. The magnitude of the
actual flowrate needs to be calibrated.
Evacuation System:
A mechanical pump {character pullout} and a Root's pump are used.
The terminal pressure of the mechanical pump {character pullout}
can be lower than 1.times.10.sup.-3 torr. In the system, this pump
is used for a primary pumping and as an upstream pump for the Root's
pump. The pressure of the Root's pump can be lower than 1.times.10.sup.-3
torr. The Root's pump is used to increase the evacuation rate in
the system.
Power Supply System:
Both power supply devices supply a.c. power. An RF high frequency
power supply {character pullout} is made by the HUTTINGER Co. of
Germany. It has a fixed frequency of 13.56 MHz and is used as the
power of the inductively-coupled plasma (ICP) in the system. Another
is an ENI power supply {character pullout} which is made by the
ENI Co. of the U.S.A. with an adjustable frequency (50.about.230
kHz) and a maximum output power of 5 kW (800V.times.6.25A), and
is used to apply a bias voltage to the substrate in the system.
Examples 1-3
Different Modification Durations
Using an inductively-coupled-plasma chemical vapor deposition (ICP
CVD) to deposit a membrane on a porous alumina membrane substrate,
the growth conditions for the membranes were: a total pressure of
0.05 torr, a vapor phase composition of 50% HMDSO+50% O.sub.2 (based
on the partial pressure ratio), a total flow rate of 3.42.about.5.20
sccm, an RF power of 100W, and a period of one hour. The surface
modification conditions were the same as the membrane growth conditions,
except a negative bias voltage of 50V was applied to the substrate.
After the surface modification, the substrate was baked at a high
temperature furnace at a pressure of 0.007 torr and 600.degree.
C. for 4 hrs, with a temperature rising rate of 2.4.degree. C./min
from room temperature. Table 1 shows the gas permeances of the membranes
subjected to surface modification at a bias voltage of 50V for various
durations. The permeances of H.sub.2 N.sub.2 O.sub.2 at several
permeation temperatures and the selectivity of H.sub.2 /N.sub.2
O.sub.2 /N2 are listed. The durations of the bias voltage were from
5 to 30 seconds.
TABLE 1 The dependence of gas separations on the duration of surface
modifications at -50 V. Example 1 2 3 Conditions Duration (sec)
5 10 30 permeance H.sub.2 Room temp. 15.58 6.24 2.36 100.degree.
C. 19.81 9.83 3.32 150.degree. C. 23.43 11.15 4.24 N.sub.2 Room
temp. 0.47 0.34 0.34 100.degree. C. 0.59 0.30 0.24 150.degree. C.
0.81 0.26 0.21 O.sub.2 Room temp. 1.70 0.43 0.35 100.degree. C.
2.04 0.43 0.25 150.degree. C. 2.35 0.43 0.23 selectivity H.sub.2
/N.sub.2 Room temp. 33 19 7 100.degree. C. 34 32 14 150.degree.
C. 29 43 20 O.sub.2 /N.sub.2 Room temp. 3.62 1.27 1.03 100.degree.
C. 3.47 1.42 1.06 150.degree. C. 2.91 1.61 1.11 The unit of gas
permeance is 10.sup.-7 mol/m.sup.2 .multidot. s .multidot. Pa.
Comparative Example 1-4
All membrane deposition conditions and surface modification conditions
for the Comparative Examples 1-4 were the same as those of Examples
1-3 except that a negative bias voltage was not used in Comparative
Example 1. The modification durations for Comparative Examples 2-4
were 60-300 seconds which were longer than those used in Examples
1-3. The separation results are shown in Table 2.
As shown in FIG. 2 the permeance of H.sub.2 at room temperature
of the membrane of Example 1 is 15.58.times.10.sup.-7 mol/m.sup.2.s.Pa,
wherein a negative bias voltage was applied for 5 second, which
is far larger than the permeance of H.sub.2 of the membrane of Comparative
Example 1 (4.42.times.10.sup.-7 mol/m.sup.2.s.Pa), wherein a bias
voltage was not applied. Even considering that the plasma in these
two cases might be slightly different, it still can be sure that
the permeance of the membrane after being surface-modified to a
negative bias voltage for 5 seconds is larger than the permeance
of the membrane without being modified. This result is contrary
to the assumption that a new layer is formed on top of the ICP CVD
layer during the surface modification, because the coverage of the
new layer will reduce the permeance of the resulting membrane instead
of increasing the permeance thereof. It is deemed that the application
of a negative bias voltage for 5 seconds should be interpreted as
some kind of surface modification, instead of depositing an additional
layer or blocking the pores on the surface. After the surface-modified
membrane has been subjected to a high temperature baking, the atomic
movements, the breakage or formation of chemical bonds on the surface
thereof and the shrinkage of the membrane might be completely different
from the membrane that has not been subjected to a surface modification
prior to the high temperature baking. Therefore, in comparison with
the membrane without the surface modification, the pores on the
surface of the surface modified membrane can effectively inhibit
the permeance of N.sub.2 (FIG. 3) and conspicuously increase the
permeance of H.sub.2 (FIG. 2).
Therefore, comparing the separation results of Comparative Example
1 with Example 1 one can realize that the process according to
the present invention is not based on the concept of ordinary composite
membranes, and also is not based on the an ordinary CVD process
to modify the surface of the membrane. An ordinary CVD process undergoes
surface modification to decrease the pore sizes, so that the selectivity
of the membrane is increased with the permeance of the membrane
being greatly reduced at the same time. The method of surface modification
by plasma according to the present invention can greatly increase
the permeance and the selectivity simultaneously, and is dramatically
different from the ordinary concepts. The membrane prepared according
to the process of the present invention has a H.sub.2 permeance
which is more than ten times higher than the highest (about 10.sup.-7
mol/m.sup.2.s.Pa) of the carbon molecular sieve membrane disclosed
in the literature.
Comparing the selectivities of Comparative Examples 2-4 with those
of Examples 1-3 the former ones are conspicuously lower. This is
due to the fact that the duration of surface modification is too
long. Then, the pores of the membrane after high temperature baking
are too small to allow a high permeance of H.sub.2. Therefore, the
duration of the surface modification needs to be optimized.
TABLE 2 The dependence of gas separation on the duration of surface
modifications at -50 V. Comparative Example 1-4 1 2 3 4 Conditions
Duration (sec) 0 60 120 300 permeance H.sub.2 Room temp. 4.42 0.71
0.71 0.63 100.degree. C. 12.05 0.87 0.74 0.55 150.degree. C. 26.52
1.07 0.84 0.58 N.sub.2 Room temp. 1.02 0.20 0.17 0.17 100.degree.
C. 3.60 0.15 0.13 0.13 150.degree. C. 6.68 0.14 0.12 0.10 O.sub.2
Room temp. 1.31 0.17 0.18 0.21 100.degree. C. 3.75 0.10 0.12 0.16
150.degree. C. 7.52 0.10 0.11 0.15 selectivity H.sub.2 /N.sub.2
Room temp. 4.34 3.64 4.30 3.72 100.degree. C. 3.35 6.06 5.72 3.97
150.degree. C. 3.97 7.54 7.11 5.70 O.sub.2 /N.sub.2 Room temp. 1.28
0.87 1.06 1.21 100.degree. C. 1.04 0.70 0.94 1.22 150.degree. C.
1.13 0.66 0.94 1.43 The unit of gas permeance is 10.sup.-7 mol/m.sup.2
.multidot. s .multidot. Pa.
Examples 4-6
Different Bias Voltages for Surface Modifications
All membrane deposition conditions and surface modification conditions
were the same for Examples 4-6 and Examples 1-3 except the duration
of bias voltage applied (10 seconds) and the negative bias voltages
(10-40 V). The negative bias voltage and the separation results
are shown in Table 3.
TABLE 3 Example 4 5 6 Conditions Bias (V) 10 30 40 permeance H.sub.2
Room temp. 18.73 10.39 8.93 100.degree. C. 22.72 15.53 10.74 150.degree.
C. 25.06 17.60 11.65 N.sub.2 Room temp. 0.33 0.27 0.24 100.degree.
C. 0.40 0.29 0.21 150.degree. C. 0.48 0.29 0.21 O.sub.2 Room temp.
-- -- -- 100.degree. C. -- -- -- 150.degree. C. -- -- -- selectivity
H.sub.2 /N.sub.2 Room temp. 56 38 36 100.degree. C. 57 54 52 150.degree.
C. 52 61 57 O.sub.2 /N.sub.2 Room temp. -- -- -- 100.degree. C.
-- -- -- 150.degree. C. -- -- -- The unit of gas permeance is 10.sup.-7
mol/m.sup.2 .multidot. s .multidot. Pa.
Examples 7-8
Different Gas Phase Compositions
All membrane growth conditions and surface modification conditions
were the same for Examples 7-8 and Example 6 except the gas phase
composition for membrane growth. The gas phase composition separation
and the results are shown in Table 4.
TABLE 4 Examples 7 8 Gas phase 40% HMDSO + 80% HMDSO + Conditions
composition 60% O.sub.2 20% O.sub.2 permeance H.sub.2 Room temp.
3.61 31.13 100.degree. C. 5.11 48.08 150.degree. C. 6.58 59.72 N.sub.2
Room temp. 0.35 1.58 100.degree. C. 0.29 1.87 150.degree. C. 0.29
2.36 O.sub.2 Room temp. 0.40 5.79 100.degree. C. 0.37 6.57 150.degree.
C. 0.35 6.94 selectivity H.sub.2 /N.sub.2 Room temp. 11 20 100.degree.
C. 18 26 150.degree. C. 23 25 O.sub.2 Room temp. 1.18 3.67 100.degree.
C. 1.27 3.51 150.degree. C. 1.23 2.94 The unit of gas permeance
is 10.sup.-7 mol/m.sup.2 .multidot. s .multidot. Pa.
Example 9
All membrane growth conditions and surface modification conditions
were the same for Examples 9 and Example 6 except the duration
for membrane growth before surface modification one hour with respect
to 30 minutes. The results are shown in Table 5.
TABLE 5 Example Duration for 9 Conditions deposition 30 min permeance
H.sub.2 Room temperature 3.84 100.degree. C. 7.49 150.degree. C.
11.56 N.sub.2 Room temperature 0.45 100.degree. C. 0.39 150.degree.
C. 0.38 O.sub.2 Room temperature 0.50 100.degree. C. 0.49 150.degree.
C. 0.51 selectivity H.sub.2 /N.sub.2 Room temperature 9 100.degree.
C. 19 150.degree. C. 30 O.sub.2 /N.sub.2 Room temperature 1.11 100.degree.
C. 1.26 150.degree. C. 1.36 The unit of gas permeance is 10.sup.-7
mol/m.sup.2 .multidot. s .multidot. Pa.
Examples 10-12
Change of Gas Phase Composition in Surface Modification
All membrane growth conditions and surface modification conditions
were the same for Examples 10-12 and Example 1-3 except the gas
phase composition in surface modification. The separation results
are shown in Table 6.
TABLE 6 Examples 10 11 12 Gas phase 80% HMDSO + 100% 80% HMDSO
+ Conditions composition 20% Ar O.sub.2 20% O.sub.2 permeance H.sub.2
Room temp. 26.89 2.91 12.84 100.degree. C. 32.04 3.32 15.34 150.degree.
C. 32.95 3.89 17.19 N.sub.2 Room temp. 0.66 0.31 0.15 100.degree.
C. 0.85 0.27 0.21 150.degree. C. 1.07 0.25 0.25 O.sub.2 Room temp.
2.45 0.36 0.59 100.degree. C. 3.04 0.31 0.87 150.degree. C. 3.83
0.30 1.01 selectivity H.sub.2 /N.sub.2 Room temp. 40 9 81 100.degree.
C. 38 12 73 150.degree. C. 31 16 69 O.sub.2 /N.sub.2 Room temp.
3.67 1.17 3.70 100.degree. C. 3.56 1.15 4.16 150.degree. C. 3.57
1.20 4.05 The unit of gas permeance is 10.sup.-7 mol/m.sup.2 .multidot.
s .multidot. Pa.
Examples 13-16
Change of Gas Phase Composition in Surface Modification
All membrane growth conditions and surface modification conditions
were the same for Examples 13-16 and Example 1-3 except the gas
phase composition and the bias voltage during surface modification.
The separation results are shown in Table 7.
TABLE 7 Example 13 14 15 16 Gas phase comp. Ar CH.sub.4 Conditions
ENI bias voltage 50 V 100 V 30 V 50 V permeance H.sub.2 Room temp.
2.55 1.28 2.53 1.65 100.degree. C. 4.69 1.81 6.28 2.48 150.degree.
C. 8.35 2.63 10.54 4.54 N.sub.2 Room temp. 0.40 0.24 0.17 0.36 100.degree.
C. 0.40 0.21 0.19 0.32 150.degree. C. 0.42 0.19 0.23 0.31 O.sub.2
Room temp. 0.46 0.23 0.36 0.36 100.degree. C. 0.63 0.19 0.45 0.32
150.degree. C. 0.74 0.18 0.60 0.34 selectivity H.sub.2 /N.sub.2
Room temp. 6 5 15 5 100.degree. C. 12 9 33 8 150.degree. C. 20 14
46 15 O.sub.2 /N.sub.2 Room temp. 1.15 0.96 2.13 1.00 100.degree.
C. 1.59 0.92 2.30 0.98 150.degree. C. 1.77 0.97 2.65 1.10 The unit
of gas permeance is 10.sup.-7 mol/m.sup.2 .multidot. s .multidot.
Pa.
Examples 17-18
All membrane growth conditions and surface modification conditions
were the same for Examples 17-18 and Example 1-3 except that the
gas phase composition was 100% CH.sub.4. After surface modification,
the membranes were baked in a high temperature furnace under 0.007
torr and at 500.degree. C. for 4 hrs with a temperature rise rate
of 2.4.degree. C./min. The separation results are shown in Table
8.
TABLE 8 Examples 17 18 Conditions ENI bias voltage 10 V 30 V permeance
H.sub.2 Room temperature 4.15 1.99 100.degree. C. 6.88 2.25 150.degree.
C. 9.65 3.02 N.sub.2 Room temperature 0.22 0.17 100.degree. C. 0.26
0.12 150.degree. C. 0.27 0.11 O.sub.2 Room temperature 0.52 0.38
100.degree. C. 0.70 0.27 150.degree. C. 0.81 0.25 selectivity H.sub.2
/N.sub.2 Room temperature 19 12 100.degree. C. 26 19 150.degree.
C. 36 27 O.sub.2 /N.sub.2 Room temperature 2.36 2.24 100.degree.
C. 2.69 2.25 150.degree. C. 2.99 2.27 The unit of gas permeance
is 10.sup.-7 mol/m.sup.2 .multidot. s .multidot. Pa.
Examples 19-20
All membrane growth conditions and surface modification conditions
were the same for Examples 19-20 and Example 1-3 except the gas
phase composition (90% CH.sub.4 +10% aluminium tri-sec-butanolate
for the former). After the surface modification, the membranes were
baked in a high temperature furnace under 0.007 torr and at 500.degree.
C. for 4 hrs with a temperature rise rate of 2.4.degree. C./min.
The surface modification conditions and separation results are shown
in Table 9.
TABLE 9 (The unit of gas permeance is 10.sup.7 mol/m.sup.2 .multidot.
s .multidot. Pa) Example 19 90% CH.sub.4 + 20 Gas phase 10% Aluminium
100% Aluminium composition tri-sec-butanolate tri-sec-butanolate
Conditions ENI bias voltage 30 V 50 V permeance H.sub.2 Room temperature
1.74 3.57 100.degree. C. 2.97 4.83 150.degree. C. 4.29 6.21 N.sub.2
Room temperature 0.13 0.19 100.degree. C. 0.11 0.14 150.degree.
C. 0.10 0.12 O.sub.2 Room temperature 0.34 0.56 100.degree. C. 0.29
0.42 150.degree. C. 0.29 0.39 selectivity H.sub.2 /N.sub.2 Room
temperature 13 19 100.degree. C. 27 35 150.degree. C. 43 52 O.sub.2
Room temperature 2.62 2.95 100.degree. C. 2.64 3.01 150.degree.
C. 2.90 3.25
Examples 21-22
Growth of a Membrane on a Porous Graphite Substrate
Using an inductively-coupled-plasma chemical vapor deposition (ICP
CVD) to deposit a membrane on a porous graphite substrate, the growth
conditions were: a total pressure of 0.05 torr, a vapor phase composition
of 50% HMDSO+50% O.sub.2 (based on the partial pressure ratio),
an RF power of 100W, and a deposition time of 3 hours. The surface
modification conditions were the same as the membrane growth conditions,
except that a negative bias voltage of 50V was applied to the substrate.
After the surface modification, the substrate was baked in a high
temperature furnace under a pressure of 0.007 torr, and at 600.degree.
C. for 4 hours with a temperature increase rate of 2.4.degree. C./min.
Table 10 shows various durations of the bias voltage and the results
of gas separation.
TABLE 10 Examples 21 22 Conditions Duration (s) 10 30 permeance
H.sub.2 Room temperature 2.58 1.15 100.degree. C. 3.69 2.46 150.degree.
C. 4.58 3.17 N.sub.2 Room temperature 0.23 0.13 100.degree. C. 0.15
0.19 150.degree. C. 0.14 0.18 O.sub.2 Room temperature 0.28 0.15
100.degree. C. 0.20 0.21 150.degree. C. 0.20 0.20 selectivity H.sub.2
/N.sub.2 Room temperature 11 9 100.degree. C. 25 13 150.degree.
C. 33 18 O.sub.2 /N.sub.2 Room temperature 1.22 1.15 100.degree.
C. 1.33 1.11 150.degree. C. 1.43 1.11 The unit of gas permeance
is 10.sup.-7 mol/m.sup.2 .multidot. s .multidot. Pa.
Although the present invention has been described with reference
to specific details of certain embodiments thereof, it is not intended
that such details should be regarded as limitations upon the scope
of the invention except as and to the extent that they are included
in the accompanying claims. Many modifications and variations are
possible in light of the above disclosure. |