Coating processes – Electrical product produced
Reexamination Certificate
2001-01-05
2003-11-11
Chen, Bret (Department: 1775)
Coating processes
Electrical product produced
C427S115000, C427S126300
Reexamination Certificate
active
06645548
ABSTRACT:
FIELD OF APPLICATION OF THE INVENTION
The invention relates to the way of producing a reference electrode for a galvanic cell having e.g. a lithium-, sodium- or potassium-ion-conducting solid electrolyte and being of the type as used for potentiometric sensors for detecting CO
2
, SO
x
and NO
x
.
Characteristics of the Solutions Known in the Art
Already known is a series of reference electrodes for sodium-ion-conducting solid electrolytes whose chemical potential results from the equilibrium between an Na
2
O-containing double oxide in the form Na
x
Me
y
O
z
and, on the other hand, (i) the respective oxide Me
m
O
n
or (ii) a double oxide Na
a
Me
b
O
c
with a lower Na
2
O content:
(i)
Na
+
solid electrolyte
| Na
x
Me
y
O
z
, Me
m
O
n
| O
2
, noble metal
(ii)
Na
+
solid electrolyte
| Na
x
Me
y
O
z
, Na
a
Me
b
O
c
| O
2
, noble metal
II
I
In these systems, the sodium potential establishes itself as a result of the following relationships:
(
i
)
Na
x
Me
y
O
z
≡
x
Na
+
x
4
O
2
+
q
Me
m
O
n
(
y
=
qm
,
z
=
cn
+
x
/
2
)
(
ii
)
Na
x
Me
y
O
z
≡
(
x
-
a
)
Na
+
x
-
a
4
O
2
+
Na
a
Me
b
O
c
(
y
=
b
,
z
=
c
+
(
x
-
a
)
/
2
)
On this basis, the Na potential, assuming that the oxides involved in the equilibrium coexist in pure form in each case, is defined as follows:
(
i
)
ln
a
Na
=
Δ
f
G
Na
x
Me
y
O
z
o
-
q
·
Δ
f
G
Me
m
O
n
o
x
RT
-
1
4
ln
PO
2
(1a)
(
ii
)
ln
a
Na
=
Δ
f
G
Na
x
Me
y
O
z
o
-
c
·
Δ
f
G
Na
a
Me
b
O
c
o
(
x
-
a
)
RT
-
1
4
ln
PO
2
(1b)
where &Dgr;
f
G*
Na
x
Me
y
O
z
, &Dgr;
f
G*
Na
a
Me
b
O
c
and &Dgr;
f
G*
Me
m
O
n
are the free standard enthalpies of formation of the oxides involved. The sodium potential is therefore at all times a defined function of just the ambient temperature and the ambient oxygen partial pressure. As the measuring electrodes in a series of gas sensors are of similar structure and therefore likewise are oxygen-pressure-dependent, the output signal of the complete sensor ultimately becomes independent of the oxygen potential in the ambient gas atmosphere.
Examples of said cases are inter alia: (i) NaAl
11
O
17
/Al
2
O
3
[1], Na
2
Si
2
O
5
/SiO
2
[3]; (ii) NaAl
5
O
8
and NaAl
11
O
17
[1, 2].
Customarily, the oxides present in pure form as solids are pulverized, optionally mixed with an inert electron conductor, e.g. Au, pressure-molded to form a pellet, and, as a compact molding or sintered body, brought into contact with the solid electrolyte. The electrode contact area (phase boundary II) is always that of two solids, which has the usual disadvantages that even with polished surfaces, only punctate contact will be established. Furthermore, the minimum achievable layer thickness, i.e. the spacing between the phase boundaries I and II is subject to a lower limit, as a result of which the oxygen potential at phase boundary I may differ from the oxygen potential at phase boundary II. However, the identity of the two potentials is a precondition for the validity of the equations (1a) and (1b).
REFERENCES:
patent: 5114803 (1992-05-01), Ishihara et al.
patent: 5552086 (1996-09-01), Siiman et al.
patent: 5707763 (1998-01-01), Shimizu et al.
patent: 6156453 (2000-12-01), Shimizu et al.
patent: 195 03 783 (1996-08-01), None
patent: 2 046 502 (1980-11-01), None
Aldinger Fritz
Gollhofer Stephanie
Näfe Helfried
Chen Bret
Fulbright & Jaworski L.L.P.
Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V.
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