Gas separator

A gas separator includes a porous substrate having fine pores opened on its surface and a metal for separating a gas, the porous substrate having fine pores having an average diameter of 0.1-3.0 .mu.m and a porosity of 25-45%, and the metal for separating a gas being filled into the pores in the porous substrate to close them. A gas separation film seldom exfoliates from the porous substrate, and the gas separator is excellent in durability in comparison with a conventional gas separator.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
The present invention relates to a gas separator for separating a specific 
gas from a mixed gas by diffusion. 
Heretofore, as a technique for obtaining a specific gas from a mixed gas, 
there is known a separation method by the use of an organic or an 
inorganic gas separating film. Among the separating films, examples of a 
hydrogen separating film for use in a film separation method include 
organic polymeric films of polyimide, polysulfone and the like, and 
inorganic compound films of palladium, palladium alloys and the like, and 
examples of an oxygen separating film include films of silver and silver 
alloys. The palladium film and the palladium alloy films have heat 
resistance and can obtain extremely high-purity hydrogen. 
Palladium and the palladium alloys have characteristics which allow 
hydrogen to be dissolved therein and which allow hydrogen to permeate 
therethrough, and by the utilization of the characteristics, a thin film 
comprising palladium or the palladium alloy has been widely used as a gas 
separator for separating hydrogen from a mixed gas containing hydrogen. 
However, the thin film comprising palladium itself is weak in mechanical 
strength, and so, in Japanese Patent Application Laid-open No. 62-273030, 
palladium or the palladium alloy is deposited on the surface of an 
inorganic porous support of a porous glass, porous ceramics, a porous 
aluminum oxide or the like to increase the mechanical strength of the thin 
film comprising palladium or the palladium alloy. 
Japanese Patent Application Laid-open No. 3-146122 discloses a method for 
preparing a hydrogen separator which comprises first forming a palladium 
thin film on the surface of a heat-resistant porous substrate by a 
chemical plating process, and further forming a silver thin film on the 
palladium thin film by the chemical plating process, followed by a heat 
treatment. According to this disclosed method, a hydrogen separator having 
the porous substrate and a palladium alloy thin film covering it can be 
obtained. In this palladium alloy thin film, palladium and silver are 
uniformly distributed by the above-mentioned heat treatment. 
In addition, U.S. Pat. No. 3,359,705 discloses a silver thin film for 
separating oxygen. 
However, these gas separators have a drawback that a material gas to be 
subjected to the gas separation leaks into a purified gas through holes 
(hereinafter referred to as "throughhole-defects") which extend through 
the gas separating film comprising the metal for separating the gas. 
Therefore, the concentration of hydrogen in the purified gas deteriorates 
as much as the leaked material gas. 
In order to remove these throughhole-defects, there is a method of 
thickening the gas separating film comprised of the metal for separating 
the gas, but this method has a problem that a gas permeability of the gas 
separating film deteriorates and hence a gas separation efficiency also 
deteriorates. However, this method prevents a material gas from getting 
mixed in a refined gas, and therefore, a hydrogen gas with high purity can 
be obtained. 
Further, Japanese Patent Application Laid-Open No. 6-277472 discloses that 
pores are closed by filling a metal having a gas separating ability inside 
the pores opened on the surface of a porous substrate. According to this 
method, a leakage of a material gas into a refined gas can be avoided 
without deteriorating an efficiency of separating a gas. 
However, when a thickness of a gas separating film is made thick, the 
method has a problem that adhesive properties between the gas separating 
film and a substrate such as a porous film are weak, and when the hydrogen 
separator obtained by the method is actually used in a gas separation 
process, the gas separating film peels in a short period of time. In 
consequence, such a hydrogen separator cannot be used continuously for a 
long term in order to carry out the gas separation. 
When a hydrogen separating film disclosed in Japanese Patent Application 
Laid-Open No. 6-277472 is used, it is desired that a frequency of an 
exfoliation is decreased though a continuous gas separation for a long 
term in comparison with a separator having a thickened gas separation 
film. 
The present invention is made in light of the background of the problems of 
the above-mentioned conventional techniques, and an object of the present 
invention is to provide a gas separator which can prevent a material gas 
to be subjected to a gas separation from leaking into a purified gas and 
which hardly has an exfoliation of a gas separating film and which has 
excellent durability in comparison with a conventional gas separator. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a gas separator 
comprising a porous substrate having pores opened on its surface and a 
metal for separating a gas, 
the porous substrate having pores having an average diameter of 0.1-3.0 
.mu.m and a porosity of 25-45%, and 
the metal for separating a gas being filled into the pores in the porous 
substrate to close them. 
In the present invention, the metal for separating the gas preferably 
covers at least a portion of the surface of the porous substrate to form a 
thin film thereon. 
The porous substrate preferably comprises particles each having a diameter 
being 1.5-6.0 times larger than the average diameter of fine pores. 
The metal for separating the gas preferably covers at least a portion of 
the surface of the porous substrate to form a thin film thereon. 
Further, in the present invention, a depth of the metal for separating the 
gas which penetrates into the porous substrate is preferably in the range 
of 1 to 30 .mu.m from the surface of the porous substrate. 
Furthermore, in the present invention, the metal for separating the gas is 
preferably palladium, an alloy mainly composing palladium, or an alloy 
containing palladium.

DETAILED DESCRIPTION OF THE INVENTION 
A gas separator 1 of the present invention has a porous substrate 2 and a 
metal 3 for separating a gas. The porous substrate 2 is porous, and so it 
has many pores 5, and some of the pores extend to the surface of the 
porous substrate 2 and they are opened thereon. In the present invention, 
the metal 3 for separating the gas is filled into the pores 5 opened on a 
porous substrate surface 2a to close them. In consequence, when a material 
gas to be subjected to a gas separation by the gas separator 1 is passed 
through the pores 5 in the porous substrate 2, a specific gas permeates 
the metal 3 for separating the gas, thereby separating the specific gas 
from the material gas. Furthermore, in the gas separator 1 of the present 
invention, the metal 3 for separating the gas is filled into the pores 5 
to close them, so that the material gas is prevented from leaking into a 
purified gas. Therefore, for example, in the case of the gas separator of 
the present invention in which a palladium alloy is used, a hydrogen gas 
having a purity of 99% or more can be obtained, and usually, the hydrogen 
gas having a purity of 99.9% or more can be obtained. 
As the porous substrate 2, a material which does not react with the 
material gas is preferable. Typical examples of the porous substrate 2 
include alumina, silica, silicaalumina, mullite, cordierite, zirconia, 
carbon, and porous glasses. 
This porous substrate has many three-dimensionally connected fine pores. It 
is important that the average diameter of these pores is in the range of 
0.1 to 3.0 .mu.m. If the average diameter of the pores is less than 0.1 
.mu.m, an anchor effect between the substrate and the plated film is 
small, and therefore, a film is exfoliated from a substrate after being 
used for a long period because of a difference in expansion between the 
substrate and the film caused by a temperature change or because of 
expansion and shrinkage of the film caused by occlusion and release of a 
gas. 
If the average diameter of the pores exceeds 3.0 .mu.m, airtightness of the 
pores cannot be ensured because the pores cannot be closed by a chemical 
plating, and therefore, a gas having an aimed purity cannot be obtained. 
That is, in the present invention, since an average diameter of the pores 
in the porous substrate is specified to the aforementioned size, a gas 
separating film is hardly exfoliated. Such a porous substrate can be 
obtained, for example, by a process described in Japanese Patent 
Application Laid-open No. 62-273030. 
The diameter of the pores in the porous substrate is preferably 
uniformalized, because the uniform diameter permits easy regulation of the 
depth of a solution which penetrates into the porous substrate in the 
activation step or the chemical plating step, and thus permits easily and 
uniformly maintaining the depth of the metal for separating the gas which 
penetrates in the porous substrate. No particular restriction is put on 
the thickness of the porous substrate 2, so long as the porous substrate 2 
can hold a sufficient mechanical strength in a use environment. 
Moreover, the porous substrate 2 preferably has a planar shape, and the 
planar shape include a plane shape and a curved shape. In addition, it 
naturally includes a tubular shape which corresponds to a closed curved 
shape. In the case of the tubular shape, the shape of its section is 
optional, but the tubular substrate having a circular section is easily 
available and preferable. Furthermore, the gas separator or the porous 
substrate 2 may have a plate shape. In this case, it can take an optional 
shape in compliance with its use purpose. 
It is important that the porous substrate 2 has a porosity of 25-45%. This 
is because a gas diffusibility of the porous substrate 2 is bad when the 
porosity is lower than 25%, and mechanical strength is deteriorated when 
the porosity is higher than 45%. 
Each of the particles constituting the porous substrate 2 preferably has a 
diameter of 1.5-6.0 times, more preferably 2.0-4.0 times, larger than an 
average diameter of the pores in the porous substrate 2. When the particle 
diameter is smaller than 1.5 times, a packing rate increases, and a 
porosity decreases. When the particle diameter is larger than 6.0 times, a 
porosity increases. 
The kind of metal 3 for separating the gas depends upon the kind of gas to 
be purified. For example, in order to purify a hydrogen gas, palladium, an 
alloy mainly comprising palladium or an alloy containing palladium is 
selected. For the sake of the separation of oxygen, a thin film of silver 
or an alloy mainly comprising silver, or a thin film of an organic 
material is used. 
In the present invention, as shown in FIG. 1, the metal 3 for separating 
this gas is filled into the pores 5 opened on the surface 2a of the porous 
substrate 2 to close these pores 5. In FIG. 1, the metal 3 covers the 
surface 2a of the porous substrate 2 to form the gas separating film 4. 
However, in the gas separator 1 of the present invention, the metal 3 for 
separating the gas present in the porous substrate 2 functions to separate 
the gas, and hence such a gas separating film 4 as shown FIG. 1 is not 
essential. 
However, it is preferable that the metal 3 for separating the gas covers at 
least a part of the surface 2a of the porous substrate 2 to form the gas 
separating film 4 thereon, because the permeation of the gas to be 
purified through the metal 3 for separating the gas can be more assured. 
In this case, the metal 3 may cover a part alone of the porous substrate 
surface 2a, whereby in the covered part, the permeation of the gas to be 
purified through the metal 3 for separating the gas can be more assured. 
The gas separating film 4 preferably covers the porous substrate surface 
2a. The metal 3 for separating the gas, which is filled into the pores 
opened on the surface of the porous substrate to close these pores, is 
preferably continuously connected with the metal for separating the gas 
which constitutes the gas separating film 4, as shown in FIG. 1, whereby 
adhesion between the gas separating film 4 and the porous substrate can be 
improved and the peeling of the gas separating film 4 from the porous 
substrate surface 2a can be sufficiently prevented. 
The thickness of the gas separating film 4 is preferably 50 .mu.m or less, 
more preferably 20 .mu.m or less. If the thickness of the gas separating 
film 4 is in excess of 50 .mu.m, a long time is taken for the material gas 
to diffuse in the gas separating film at the time of gas separation by the 
gas separator, so that a treatment time is prolonged inconveniently. 
The depth of the metal 3 for separating the gas which penetrates into the 
porous substrate 2 is preferably in the range of 1 to 30 .mu.m, more 
preferably 1 to 20 .mu.m, most preferably 1 to 10 .mu.m from the surface 
of the porous substrate. If this depth is less than 1 .mu.m, the closure 
of the pores with the metal 3 for separating the gas is not sufficient, 
and the material gas may leak into the purified gas. In addition, when the 
gas separating film 4 is formed, this gas separating film 4 is liable to 
peel from the porous substrate surface 2a. On the other hand, if this 
depth is more than 30 .mu.m, a long time is taken for the gas to be 
separated to diffuse in the metal 3 for separating the gas at the time of 
gas separation by the gas separator 1, so that a gas separation time is 
prolonged inconveniently. 
In the case that the porous substrate 2 has a tubular shape, the surface 2a 
of the porous substrate having the pores into which the metal 3 for 
separating the gas is filled may be present on an outer side or an inner 
side of the tubular porous substrate. 
In the case that the metal 3 for separating the gas comprises a palladium 
alloy, the content of metals other than palladium is preferably in the 
range of 10 to 30% by weight, as described in "Hydrogen Permeable 
Palladium-Silver Alloy Membrane Supported on Porous Ceramics", Journal of 
Membrane Science, Vol. 56, p. 315-325 (1991) and Japanese Patent 
Application Laid-open No. 63-29540. The main purpose of using palladium in 
the form of the alloy is to prevent the embrittlement of palladium with 
hydrogen and to improve a separation efficiency at a high temperature. It 
is preferable for the prevention of the embrittlement of palladium with 
hydrogen to contain silver as a metal other than palladium. 
A gas separator of the present invention is produced by a method including, 
for example, the following activation step and chemical plating step. 
In the activation step, one surface of the porous substrate is immersed in 
a solution containing an activated metal so that the pressure applied to 
the one surface may be higher than the pressure applied to the other 
opposite surface of the porous substrate, whereby the solution is allowed 
to penetrate into the pores opened on the one surface of the porous 
substrate to which the higher pressure is applied. Owing to the presence 
of such a pressure difference, the activated metal can be deposited not 
only on the surface of the porous substrate but also on the inner surfaces 
of the pores opened on the surface of the porous substrate. On the surface 
on which the activated metal has been deposited, a metal for separating 
the gas will be further deposited by the next chemical plating step. 
In this activation step, the one surface of the porous substrate onto which 
the higher pressure is applied is required to be immersed in the solution, 
but the other opposite surface does not have to be immersed in the 
solution. For example, in the case that the tubular porous substrate is 
used, its outer surface may be immersed in the solution containing the 
activated metal, and the inside portion of the tube can be sucked by a 
vacuum pump. Alternatively, the outer surface of the tubular porous 
substrate may be immersed in the solution containing the activated metal, 
and the pressure may be applied to this solution to maintain the inside 
portion of the tube at a constant pressure. In either case, the outer 
surface and the inner surface of the tube can be inverted, and the inner 
surface of the tube is immersed in the solution and the pressure can be 
changed. 
As the activated metal, a compound containing divalent palladium ions can 
be suitably used. Concretely, the activation step can be achieved by 
alternately immersing the porous substrate in an aqueous hydrochloric acid 
solution of palladium chloride and an aqueous hydrochloric acid solution 
of tin chloride, and while the immersion is done in either solution, the 
predetermined pressure difference is preferably maintained. 
In the next chemical plating step, electroless plating is carried out by 
the use of at least the metal for separating the gas and a plating 
solution containing a reducing agent to deposit the metal for separating 
the gas in the pores of the porous substrate, whereby the metal for 
separating the gas is filled into the pores to close them. In this 
chemical plating step, the one surface of the porous substrate already 
treated in the above-mentioned activation step is treated. For example, 
the chemical plating step can be achieved by replacing the above-mentioned 
solution used in the activation step with the suitable plating solution. 
Also in this chemical plating step, it is preferable that one surface of 
the porous substrate is immersed in the plating solution containing at 
least the metal for separating the gas and the reducing agent so that the 
pressure applied to the one surface may be higher than the pressure 
applied to the other opposite surface of the porous substrate, in the same 
manner as in the above-mentioned activation step. This pressure difference 
makes it easy for the plating solution to permeate into the pores opened 
on the surface of the porous substrate. As described above, the portion on 
which the activated metal has been deposited in the activation step is 
plated in this chemical plating step. 
The depth of the penetrated metal for separating the gas from the surface 
of the porous substrate can be adjusted by controlling an immersion time 
in the chemical plating step, a temperature of the plating solution, a 
difference between the pressures applied to both the surfaces of the 
porous substrate, and the like. 
For the sake of the hydrogen separation, a known chemical plating solution 
containing palladium is used, and for the oxygen separation, a known 
chemical plating solution containing, for example, silver nitrate, EDTA, 
aqueous ammonia and hydrazine is used. 
In the case that the gas separator for separating hydrogen is prepared, it 
is preferable that after the chemical plating of palladium, silver is 
further chemically plated on the palladium-deposited surface, followed by 
a heat treatment, to mutually diffuse palladium and silver and to thereby 
form an alloy of palladium and silver. 
Now, the present invention will be described in detail with reference to 
examples. 
EXAMPLES 1-3 
In the first place, a porous substrate was subjected to an activation 
treatment. A porous .alpha.-alumina cylindrical tube having an outer 
diameter of 10 mm, an inner diameter of 7 mm, a length of 1000 mm and a 
fine pore diameter of 0.2 .mu.m, 0.5 .mu.m, and 1.5 .mu.m were used in 
Examples 1, 2 and 3, respectively. The outer surface of this alumina tube 
was immersed for 1 minute in a 0.1% aqueous hydrochloric acid solution 
containing 0.1% by weight of SnCl.sub.2 .multidot.2H.sub.2 O. A pressure 
inside was kept reduced, and the outer surface of this tube was immersed 
for 1 minute in a 0.1% aqueous hydrochloric acid solution containing 0.01% 
by weight of PdC1.sub.2. This immersion treatment was repeated 10 times in 
each of both the aqueous hydrochloric acid solutions. 
Next, palladium was chemically plated. [Pd(NH.sub.3).sub.4 ]Cl.sub.2 
.cndot.H.sub.2 O (5.4 g), 2Na.cndot.EDTA (67.2 g), aqueous ammonia having 
an ammonia concentration of 28% (651.3 ml) and H.sub.2 NNH.sub.2 
.cndot.H.sub.2 O (0.46 ml) were added to 1 liter of deionized water to 
prepare an aqueous solution, and the outer surface of the porous alumina 
tube which had been subjected to the abovementioned activation treatment 
was immersed in this aqueous solution whose temperature was controlled to 
50.degree. C. This immersion time was changed to adjust a thickness of a 
thin film covered on the surface of the porous substrate and a depth of 
the solution which penetrated into the porous substrate. 
Next, silver was chemically plated. AgNO.sub.3 (3.46 g), 2Na.cndot.EDTA 
(33.6 g), aqueous ammonia having an ammonia concentration of 28% (651.3 
ml) and H.sub.2 NNH.sub.2 .cndot.H.sub.2 O (0.46 ml) were added to 1 liter 
of deionized water to prepare an aqueous solution, and the outer surface 
of the porous alumina tube which had been subjected to the above-mentioned 
activation treatment was immersed in this aqueous solution whose 
temperature was controlled to 50.degree. C. This immersion time was 
changed as shown in Table 1, and silver was then chemically plated so that 
a weight ratio of palladium:silver might be 80:20. 
In the last place, the thus treated porous alumina tube was maintained at 
900.degree. C. for 12 hours to carry out a heat treatment, whereby 
palladium and silver were mutually diffused, and an alloy of palladium and 
silver was formed to obtain a gas separator. 
For the thus obtained gas separator, an airtight test was carried out. An 
argon gas was introduced into an outer peripheral portion of the gas 
separator, and a pressure of 9 kg weight/cm.sup.2 was maintained. At this 
time, an amount of the gas leaked into the gas separator was measured. 
Furthermore, for the gas separator, a hydrogen separation test was carried 
out. A mixed gas comprising 75% by volume of hydrogen and 25% by volume of 
carbon dioxide was used as a material gas. A schematic view of a test 
device is shown in FIG. 2. In the device in FIG. 2, gas seperator 16 is 
sealed in a chamber 7 by O-rings 15. First, chamber 7 was heated up to 
500.degree. C. Next, the above-mentioned mixed gas 17 having a pressure of 
9 kg weight/cm.sup.2 was introduced into the outer peripheral portion of 
gas separator 16 through inlet tube 10 at 2 N liter (i.e., a volume at 
room temperature was 2 liters) per minute. Argon having a pressure of 1 kg 
weight/cm.sup.2 was introduced as a sweep gas 18 into the gas separator 16 
through inlet tube 8 at 0.1 N liter per minute. A purified gas 19 thus 
obtained was quantitatively analyzed by a gas chromatography to inspect a 
gas permeation rate of the purified gas and a hydrogen concentration in 
the purified gas. 
For example, in Example 1, a gas permeation rate per minute in 1 cm.sup.2 
of the palladium film of the gas separator was 55 ml, and a hydrogen 
purity of a purified gas 19 was 99.9% or more. 
Then, the film of the gas separator was measured for an adhesion strength 
with the porous substrate. A metallic material having dimensions of 4 
mm.times.4mm was fixed to a film. The metalic material was pulled 
perpendicularly to the film. A strength when the metallic material is 
exfoliated was defined as an adhesion strength. 
The gas separator was subjected to an evaluation cycle test and a serial 
evaluation test so as to examine durability of the gas separator. In the 
evaluation cycle test was repeated a cycle of heating the gas separator 
from room temperature to 500.degree. C. in Ar gas, exposing it to a mixed 
gas at 500.degree. C., and cooling it to the room temperature in Ar gas. 
The number of steps required for deterioration of airtightness of the gas 
separator was measured. The serial evaluation test was carried out by 
exposing the gas separator to a mixed gas at 500.degree. C., and a time 
required for a deterioration of airtightness of the gas separator was 
measured. 
The results of these tests are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Gas separator 
Average pore 
Thickness of 
Particle Airtight test 
film diameter of porous 
diameter 
Porosity 
Leaked gas 
(.mu.m)ubstrate (.mu.m) 
(.mu.m) 
(%) 
(ml/(min .multidot. cm.sup.2)) 
__________________________________________________________________________ 
Example 1 0 
Example 2 0 
Example 3 0 
Comparative Example 1 
0 
Comparative Example 2 
5 
Comparative Example 3 
0 
Comparative Example 4 
0 
Comparative Example 5 
20.0 
10 
__________________________________________________________________________ 
Gas separation test 
Adhesion strength 
Durability 
Purity of 
between film and 
Evaluation 
Serial Evaluation 
porous substrate Refined gas 
cycle test 
Test 
(%) (ml/(min .multidot. cm.sup.2)) 
(kg/cm.sup.2) 
(cycles) 
(hours) 
__________________________________________________________________________ 
Example 1 19.ltoreq. 
6000 
Example 2 94.ltoreq. 
8000.ltoreq.. 
Example 3 160ltoreq. 
6100toreq. 
Comparative Example 1 
39.ltoreq. 
500 
Comparative Example 2 
2000 
Comparative Example 3 
12.ltoreq. 
5500 
Comparative Example 4 
75.ltoreq. 
6000 
Comparative Example 5 
700 
__________________________________________________________________________ 
COMATIVE EXAMPLES 1-5 
Gas separators were subjected to a treatment under the same conditions as 
in the above examples except that a porous substrate having an average 
pore diameter of 0.01 .mu.m, 0.05 .mu.m, 0.2 .mu.m, 0.5 .mu.m and 10 .mu.m 
were used in Comparative Examples 1-5, respectively, and a porosity of 
20%, 28%, 22%, 20%, 45%, respectively, and that a thin film penetrating 
into the porous substrate has a depth of 0.1 .mu.m or less, 0.1 .mu.m or 
less, 2 .mu.m, 7 .mu.m, and more than 30 .mu.m in Comparative Examples 
1-5, respectively. The results are shown in Table 1. 
Each of the gas separators in the Examples had a sufficient adhesion 
strength between a film and a porous substrate and an excellent durability 
of 200 cycles or more of an evaluation cycle test and 6000 hours or more 
of a serial evaluation test. Further, a refined hydrogen gas had a purity 
of 99.9% or more in a hydrogen separation test. 
On the other hand, each of hydrogen separators in Comparative Examples 1 
and 2 had an exfoliation of a hydrogen separation film in an evaluation 
cycle test and a serial evaluation test. A gas separator in Comparative 
Example 2 had a leakage of a refined gas into a material gas in test, and 
the refined hydrogen gas had a low purity of 86%. 
In hydrogen separators of Comparative Examples 3 and 4, a refined gas flow 
rate was decreased to be 38 ml/ (min.cndot.cm.sup.2) and 37 
ml/(min.cndot.cm.sup.2), respectively, in a gas separation test, and 
efficiency of collecting hydrogen is deteriorated. This is because a 
porosity of a porous substrate is so low as 22% and 20%, respectively. 
Accordingly, a gas diffusibility is low, and a gas pressure inside a 
porous substrate is high, thereby reducing a difference of a hydrogen 
pressure which is a driving force of hydrogen transmission. 
Further, in the hydrogen separator of Comparative Example 5, blockage of 
pores by a hydrogen separation film was insufficient, and durability was 
deteriorated in an evaluation cycle test and a serial evaluation test. 
Simultaneously, a refined gas leaked into a material gas in a hydrogen 
separation test, and the refined hydrogen gas had a low purity of 73%. 
Incidentally, in the aforementioned Examples and Comparative Examples, test 
pieces were produced with the same material and in the same condition as a 
porous substrate. An average pore diameter and porosity of the porous 
substrate was measured by a mercury porosimeter and an Archimedes method. 
In the gas separator of the present invention, the metal for separating the 
gas is filled into the pores opened on the surface of the porous substrate 
to close them, whereby the material gas to be subjected to the gas 
separation by the gas separator can be prevented from leaking into the 
purified gas and is free from deterioration of gas separation efficiency. 
For example, according to the gas separator of the present invention using 
a palladium alloy, a hydrogen gas having a purity of 99.9% or more can be 
obtained. 
Further, since an average diameter of pores of a porous substrate is 
specified to a predetermined value, a gas separation film seldom 
exfoliates from the porous substrate, and therefore a gas separator of the 
present invention is excellent in durability in comparison with a 
conventional gas separator.