97Engineering Issues
have a characteristic diameter that is smaller than the mean-free-path of the mol-
ecule
t
o
b
e
s
eparated.
I
f
c
ollisions
b
etween
m
olecules
c
ould
o
ccur
w
ith
s
ignicant
f
requency
w
ithin
t
he
b
arrier,
e
quipartition
o
f
e
nergy
w
ould
r
ender
t
he
b
arrier
i
nef-
fective.
F
inally,
t
he
b
arrier
m
ust
b
e
t
hin
e
nough
t
hat
i
t
h
as
a
dequate
p
ermeability
at
l
ow
p
ressure.
T
here
i
s
l
imited
i
nformation
a
vailable
o
n
d
iffusion
b
arriers
[
11,13–16].
At
least two types of barrier design have been demonstrated to be effective: the
lm-type
b
arrier,
i
n
w
hich
p
ores
a
re
c
reated
i
n
a
n
i
nitially
n
onporous
me
mbrane,
a
nd
t
he
a
ggregate
b
arrier,
i
n
w
hich
t
he
p
ores
c
omprise
s
trings
o
f
v
oids
b
etween
t
he
p
articles
t
hat
a
re
p
ressed
a
nd
s
intered
t
o
f
orm
t
he
b
arrier