Ceria based solid electrolytes

Details Ceria based solid electrolytes Ceria based solid electrolytes

2001-04-27

2004-08-03

Tsang-Foster, Susy (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C252S062200

Reexamination Certificate

active

06770392

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
Electrochemical devices comprising oxygen-ion conducting solid electrolytes have found considerable use in a variety of applications including processes for separating oxygen from oxygen-containing gases and sensors for measuring oxygen concentration in gaseous mixtures. Desirable properties for the solid electrolyte include low resistance to oxygen ion conduction and high strength. Such solid electrolytes are typically made as thin as possible to lower the power requirements for the electrochemical device, but thick enough to provide for sufficient mechanical stability to withstand pressure differences prevailing in the electrochemical device under operating conditions.
Ceria-based electrolytes are known to be useful in the above electrochemical devices. Ceria (CeO
2
) electrolytes have oxygen vacancies which allow for oxygen ion conductivity. Ceria which possesses the fluorite structure exhibits rather low oxygen ionic conductivity under operating conditions. The concentration of oxygen vacancies within the solid electrolyte can be increased by adding a dopant having a different valence from Ce
4+
thereby increasing oxygen conductivity in the solid electrolyte. For example, Japanese laid-open publication H8-169713 teaches that ceria can be doped with alkaline earth metals such as Mg, Ca, Sr, Ba, or transition metals such as Zr, Hf, Nb, or Ta.
While a wide variety of dopants have been shown to be effective in increasing oxygen ionic conductivity in solid electrolytes, yttrium and lanthanides, especially Sm and Gd, and Mg, are considered to be preferred dopants to achieve high ionic conductivity in ceria based electrolytes. Chen & Chen (J. Am. Ceram. Soc. No. 79 (1996) p. 1793)) studied the effect of Mg
2+
, Ca
2+
, Sr
2+
, Sc
3+
, Yb
3+
, Y
3+
, Gd
3+
, La
3+
, Ti
4+
, and Nb
3+
on the sintering behavior of undoped ceria. The researchers found that both titania and magnesia can “markedly enhance grain boundary mobility” which can lead to abnormal grain grown during sintering.
U.S. Pat. No. 3,607,424 discloses a solid electrolyte represented by the formula (CeO
2
)
2
(Gd
2
O
3
)
x
(MgO)
y
. Magnesia and gadolinia are each considered dopants which have been added in considerable amounts to increase oxygen ionic conductivity. Japanese laid-open publication JP 75-012566 discloses a ceria based solid electrolyte, wherein gadolinia and samaria are used as dopants for increasing oxygen ionic conductivity and magnesia is used as a sintering aid.
Japanese laid-open publication JP 62-28316 discloses a perovskite solid electrolyte represented by the formula SrCe
1−x−y
M
x
M′
y
O
3
wherein M is Ti, Zr or Sn, M′ is Y, Sc, Yb, Mg, Nd, Pr or Zn and both x and y are in the range of between 0 and 0.5. Dopants M and M′ are added to increase oxygen ionic conductivity.
Solid electrolytes utilized in oxygen separation devices typically do not possess appreciable connected through porosity, meaning that such solid electrolytes do not contain a network of pores which are capable of transporting gases through the solid electrolyte. Solid electrolytes typically possess densities of above 95% of theoretical density in order to achieve no connected through porosity.
Doped ceria electrolytes formed from submicron powders typically require high compaction pressures (up to 1 GPa) and high sintering temperatures (around 1700° C.) to achieve densities of above 95% of theoretical density. These process conditions increase costs associated with producing such solid electrolytes.
Sintering aids have been used to reduce processing temperatures required to sinter solid electrolytes. Coprecipitation techniques utilizing metal hydroxides have been used to produce powders which readily sinter in the temperature range of 1400 to 1600° C., a desirable range in which to operating process equipment.
Japanese laid-open publication H8-159713 which teaches a coprecipitation method for making doped ceria which can be sintered at temperatures ranging from 1250 to 1600° C. while obtaining a density of above 95% of theoretical density. A coprecipitation method is also used in WO91/09430 which discloses sintering temperatures between 1300 and 1525° C. for a composition containing ceria and two dopants selected from rare earth metals and/or iron, cobalt, and nickel. Coprecipitated powders such as gadolinium doped ceria are commercially available.
Japanese laid-open publication JP 75-012566 discloses magnesia as a sintering aid in ceria doped with gadolinia and samaria. GB-1322959 discloses magnesia as a sintering aid and teaches sintering temperatures of between 1700 to 1850° C., which are beyond preferred temperatures ranges for commercial processes. U.S. Pat. No. 4,465,778 discloses magnesia as a sintering aid for pure zirconia, ceria, and thoria. Baumard and coworkers (J. Less Com. Metals, 127, 125-130 (1987)) showed that sintering can be enhanced by adding small amounts of niobia or titania (0.1 to 0.3 wt %).
The object of the present invention is to provide a ceria based composition of matter which can be formed into a desired shape (referred to as a green body) and sintered to greater than 95% theoretical density at temperatures at or below 1600° C. to form a solid electrolyte.
BRIEF SUMMARY OF THE INVENTION
These objects are solved and other deficiencies of the prior art are overcome by a composition of matter represented by the general formula:
Ln
x
Ln′
x′
A
y
Ti
z
Ce
1−x−x′−y−z
O
2−&dgr;
wherein Ln is selected from the group consisting of Sm, Gd, Y, Ln′ is selected from the group consisting of La, Pr, Nd, Pm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu; A is selected from the group consisting of Mg, Ca, Sr and Ba, and 0.05≦x≦0.25, 0≦x′≦0.25, 0≦y≦0.03, 0.001≦z≦0.03, 0.05≦x+x′≦0.25 and 0.001≦y+z≦0.03, and wherein &dgr; is a number which renders the composition of matter charge neutral.
An alternate embodiment of the above mentioned composition of matter is defined by the general formula wherein Ln is Sm.
An alternate embodiment of the above mentioned composition of matter is defined by the general formula wherein A is Mg.
An alternate embodiment of the above mentioned composition of matter is defined by the general formula wherein 0.1≦x≦0.2.
An alternate embodiment of the above mentioned composition of matter is defined by the general formula wherein y=0.
An alternate embodiment of the above mentioned composition of matter is defined by the general formula wherein x′=0.
Preferred compositions of matter are represented by the formula:
Ln
x
Ti
z
Ce
1−x−z
O
2−&dgr;
wherein Ln is selected from the group consisting of Sm, Gd and Y
wherein 0.05≦x≦0.25, 0.0025≦z≦0.02 and wherein &dgr; is a number which renders the composition of matter charge neutral.
An alternate embodiment of the preferred composition of matter is defined by the general formula wherein Ln is Sm.
An alternate embodiment of the preferred composition of matter is defined by the general formula wherein Ln is Gd.
An alternate embodiment of the preferred composition of matter is defined by the general formula wherein Ln is Y.
The invention also relates to a method of manufacturing a solid electrolyte comprising a composition of matter having a density greater than 95% theoretical density represented by the general formula
Ln
x
Ln′
x′
A
y
Ti
z
Ce
1−x−x′−y−z
O
2−&dgr;
wherein Ln is selected from the group consisting of Sm, Gd, Y, and mixtures thereof; Ln′ is selected from the group consisting of La, Pr, Nd, Pm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu; A is selected from the group consisting of Mg, Ca, Sr and Ba, 0.05≦x≦0.25, 0≦x′≦0.25, 0≦y≦0.03, 0.001≦z≦0.03, 0.05≦x+x′≦0

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