Chemistry of inorganic compounds – Rare earth compound
Reexamination Certificate
1999-01-28
2002-10-15
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Rare earth compound
C423S059000
Reexamination Certificate
active
06464953
ABSTRACT:
The present invention relates to a compound of the LaMO
3
type, M being aluminium, gallium or indium, in powder form or in sintered form, its process of preparation and its use as an oxygen conductor.
At the present time, the fuel-cell field is experiencing a growth in development. However, the existing cells operate at high temperatures, of at least 900° C. At these temperatures, the phenomena of cation diffusion and reduction in the various elements of the cell introduce chemical reactions at the interfaces, followed by a deterioration in the electrical performance. Materials are therefore sought which allow the operating temperatures of these cells to be lowered. With regard to the solid electrolyte part, a compound of the (La,Sr) (Ga,Mg) O
3
type is known which has, for intermediate temperatures lying between 700 and 900° C., a useful ionic conductivity that is higher than that of the yttriated zirconia normally used. However, this type of product can be obtained in sintered form at a suitable density only by pressure sintering (HIP). Furthermore, it has also not been possible hitherto to obtain this type of product with, in addition, a phase purity of greater than 80-90%. Thus, spurious phases appear at the grain boundaries during sintering. These spurious phases block the diffusion of O
2−
ions in the final solid electrolyte system, which entails a reduction in electrical performance.
There is therefore a need for a product which can be obtained in sintered form by the simpler process of pressureless sintering.
There is also a need for a product having an improved phase purity.
One object of the present invention is to provide products which satisfy these needs.
Another object of the invention is to provide precursors for such products.
To this end, the precursor compound of the invention is of the LaMO
3
type, M being aluminium, gallium or indium, and it is characterized in that it is in the form of a powder capable of achieving, by pressureless sintering, a density of at least 93% of the theoretical density and of giving, after sintering, a product substantially free of any electrically active secondary phase at the grain boundaries.
Moreover, the invention relates to a sintered compound of the LaMO
3
type, M being aluminium, gallium or indium, which is characterized in that it is substantially free of any electrically active secondary phase at the grain boundaries and in that it is obtained by pressureless sintering.
In the two compounds described above, the lanthanum and the gallium may be partially substituted.
The invention also relates to a process for the preparation of a precursor compound of the aforementioned type.
According to a first version, the process of preparation of the invention is characterized in that salts of lanthanum and of the element M and, optionally, salts of the substituents for the element M and for the lanthanum are made to react with a base, by means of which. a precipitate is obtained, the precipitate is separated from the reaction mixture and the precipitate is calcined.
According to a second version, the process of preparation is characterized in that a mixture of salts of lanthanum and of the element M and, optionally, of salts of the substituents for the element M and for the lanthanum is formed in a liquid medium; the said mixture is spray-dried and the product obtained is calcined.
According to a third version, the process of preparation is characterized in that a mixture of, on the one hand, a base and, on the other hand, of salts of lanthanum and of the element M and, optionally, of salts of the substituents for the element M and for the lanthanum is formed in a liquid medium; the mixture obtained is sprayed and the product coming from the spraying operation is calcined.
Finally, the invention also relates to a device incorporating an oxygen-conducting solid-electrolyte material in oxide form, of the type such as an oxygen probe, a fuel cell, a membrane-type chemical reactor or an oxygen separation membrane, characterized in that it comprises a sintered compound according to the invention.
Further features, details and advantages of the invention will become even more apparent on reading the description which follows, as well as from the concrete but non-limiting examples intended to illustrate it.
Firstly, the compound of the invention called the precursor compound, i.e. the compound capable of giving-the sintered compound by sintering, will be described.
This compound is in the form of a powder.
In general, this powder has a particle size of at most 10 &mgr;m, more particularly at most 5 &mgr;m and even more particularly at most 2 &mgr;m. The particle size given here corresponds to the average size of the particles forming the powder and measured by the technique of laser scattering, using a particle size analyser of the CILAS HR 850 type (volume distribution).
The precursor compound of the invention is characterized by its sintering behaviour. In fact, it is capable of reaching, by pressureless sintering, a density of at least 93% of the theoretical density. According to a preferred version, it is capable of reaching, under the same conditions, a density of at least 95% of the theoretical density. The density given here is that obtained after sintering in air, at 1500° C., for 6 hours.
It is mentioned here, and with regard to the entire description, that the density is determined by the use of the buoyancy principle on sintered pills immersed in a petroleum medium.
The other specific characteristic of the sintering behaviour of the precursor compound of the invention is the fact that this precursor is also capable of giving, after sintering, and especially after pressureless sintering, a sintered product substantially free of any electrically active secondary phase at the grain boundaries.
The expression “electrically active phase” should be understood to mean any phase capable of having an influence on the electrical properties of the compound in question.
This absence of an electrically active secondary phase at the grain boundaries may be demonstrated by measuring the ionic conductivity of the sintered compound.
The absence of an electrically active secondary phase at the grain boundaries may also be demonstrated by the method of complex impedance spectroscopy carried out in air (as described by J. E. BAUERLE, J. Phys. Chem. Solids, 30 (1969), 2657). This method gives idealized complex impedance plots (Nyquist plots) giving the imaginary part of the impedance as a function of its real part. The plots obtained for the compounds of the invention after sintering demonstrate a lack of response due to the grain boundaries (a semicircle, corresponding to an equivalent electrical circuit whose capacitance would be between 10
−11
and 10
31 8
farads, is not observed).
Finally, it is mentioned here that the expression “substantially free” should be understood to mean that it is impossible to demonstrate the presence of electrically active secondary phases on account of the limits for the detection of such phases using the method that has just been given.
As indicated above, the precursor compound satisfies a formula of the LaMO
3
type, in which M may be aluminium, gallium or indium or else a combination of at least two of these elements.
According to one particular embodiment, the element M is gallium.
According to another particular embodiment, the lanthanum is partially substituted with an alkali metal, an alkaline-earth metal or a rare earth. The expression “rare earth” should be understood to mean the elements of the group consisting of yttrium and the elements of the Periodic Table having an atomic number of between 57 and 71 inclusive. The alkali metal may be, in particular, sodium or potassium. The alkaline-earth metal may more particularly be strontium. Sodium and potassium allow particularly high densities to be obtained after sintering.
The element M, especially gallium, may also be partially substituted with an alkaline-earth metal or with zinc. As an alkaline-earth metal substituting for the element M
Macaudiere Pierre
Seguelong Thierry
Burns Doane Swecker & Mathis L.L.P.
Ildebrando Christina
Rhodia Chimie
Silverman Stanley S.
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