Sintered solid electrolyte having a high oxygen-ion conductivity

Details Sintered solid electrolyte having a high oxygen-ion conductivity Sintered solid electrolyte having a high oxygen-ion conductivity

1996-02-06

1998-01-20

Bell, Bruce F.

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204295, 205344, 429 33, 429 40, 429 41, 429 44, 429193, 501103, 501152, G01N 2726

Patent

active

057097860

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This application is a 371 of PCT/DE93/01009 filed Oct. 23, 1993.
Metal oxides having oxygen-ion conductivity are known to be used, inter alia, in sensors by means of which the composition of gas mixtures, such as exhaust gases from internal combustion engines, is determined. However, the conductivity of the metal oxides such as zirconium(IV) oxide, hafnium(IV) oxide or cerium(IV) oxide which are customary for this purpose and are stabilized, for example, with magnesium oxide or calcium oxide is unsatisfactory.
It is known that the oxygen-ion conductivity of these metal oxides can be increased by addition of certain oxides of trivalent and/or divalent metals. In the case of zirconium(IV) oxide, for example, scandium(III) oxide, yttrium(III) oxide and the oxides of the heavy rare earths, such as erbium, dysprosium and ytterbium, are effective for this purpose. These additives are also known as stabilizers, because they stabilize the cubic phase of the zirconium(IV) oxide at low temperatures and/or maintain the tetragonal phase in a metastable state at low temperatures. Both phases have a pronounced oxygen-ion conductivity. The relationship between the type and amount of the stabilizer on the one hand and ion conductivity on the other hand is described, for example, by D. W. Strickler and W. G. Carlson in "Electrical Conductivity in the ZrO.sub.2 -Rich Region of Several M.sub.2 O.sub.3 --ZrO.sub.2 Systems", J. Am. Ceram. Soc., Vol. 48 (1974), 286, 288.
There is an optimum for the molar ratio of the stabilizer to zirconium(IV) oxide (cf. FIG. 3 in the cited work). One then speaks of full stabilization. However, partially stabilized zirconium(IV) oxide having a lower stabilizer content has also been used in gas sensor technology (DE-A2-28 52 638).


SUMMARY OF THE INVENTION

The sintered solid electrolytes of the invention which have a high oxygen-ion conductivity can be used alone, i.e. in thin solid electrolyte layers, or else in combination with other functional elements, for example with metal particles in the form of cermet electrodes. These are produced, as is known, by cosintering of metallic and ceramic particles and have a cohesive, electron-conducting layer of metal particles which is embedded in a likewise cohesive support framework of ceramic particles having oxygen-ion conductivity.
The solid electrolytes of the invention have, over wide temperature ranges, a significantly higher ion conductivity than solid electrolytes made from starting powders having a coarser particle structure. Gas sensors using these solid electrolytes, for example the .lambda. sensors customary for internal combustion engines, therefore operate reliably and reproducibly even at temperatures below 300.degree. C. for example at from 150.degree. to 200.degree. C. This is important in the phase directly after starting the engine, when the usual high exhaust gas temperature has not yet been reached.
Furthermore, the solid electrolytes of the invention have a lower temperature dependence of the conductivity than comparable electrolytes made of starting powders having a coarser crystal structure. This is due to the increased proportion of the oxygen-ion transport in the grain boundaries of the nanocrystalline zirconium(IV) oxide with the reduced activation energy Q.sub.GB of grain boundary diffusion (Q.sub.GB =0.5 Q.sub.lattice).
Owing to the increased conductivity of the solid electrolytes and/or a reduced polarization of the cermet electrodes having a support framework of the sintered solid electrolytes of the invention, .lambda. sensors or gas sensors and also fuel cells, high-temperature electrolysis cells or oxygen pump cells using the solid electrolytes of the invention are able to be more heavily loaded. The sinter activity of the mixed oxides, i.e. their ability to sinter together to give mechanically strong bodies, is very good as a result of the nanocrystalline structure of the starting materials and can be increased further by the addition of certain neutral oxides, such as aluminum oxide, wi

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S.D. Ramamurthi et al.: "Nanometer-Sized ZrO.sub.2 Particles Prepared by a Sol-Emulsion-Gel Method". In: J.Am. Ceram. Soc. 73, 1990, pp. 2760-2763, No month available.
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