DD294346A5 - ALKALIIONENLEITENDER FESTELECTROLYT FOR GAS SENSORS - Google Patents

Abstract

The invention relates to an alkali ion-conducting solid electrolyte for gas sensors. It is made from powders using ceramic methods. The solid electrolyte is used as a gas-tight separation membrane in galvanic cells with oxoanionic solid electrolytes for the potentiometric determination of CO2, SOx or NOx in gases. The invention has for its object to provide the composition of an alkali ion conductor for gas sensors, which can be processed by ceramic methods and at temperatures less than 1 200C dense sintered and relatively large, pure ionic Leitfaehigkeit has. To solve this problem, a solid electrolyte based on the compound M3Ln (PO4) 2 is indicated with M for sodium or potassium ions and Ln for air-redox-stable rare earth or lanthanide cations, which is changed in the composition such that a subspray is at M- Ions in the crystal lattice exists {potentiometric determination; gaseous anhydrides; CO2, SOx and NOx; galvanic cell; oxoanionic solid electrolyte; Alkali ion conductor}

Description

Field of application of the invention

The invention relates to an alkali ion-conducting solid electrolyte for gas. ensoren, which is made of powders by ceramic methods. The solid electrolyte is used as gas-tight separation membrane in galvanic cells with oxoanionic solid electrolyte for the potentiometric determination of CO ₂ , SO _x or NO _x in gases.

Characteristic of the known state of the art

It is known that for the potentiometric determination of CO ₂ , SO _x and NO _x galvanic cells with bodies of a carbonate, sulfate or nitrate with two spatially separated electrode layers, of which one in a reference and the other in one gas-tight separated measuring space is used. Under certain conditions, a voltage can be measured between the electrodes, which, according to NERNST's equation, depends on the gas partial pressures entering the electrochemical equilibrium. Experience has shown that long-term stable arrangements use a multilayer electrolyte containing two half-cells with oxoanionic solid electrolytes and a dense alkali ion-conducting membrane in between. For electrochemical reasons, the same ion must take over the current transport in this membrane as in the oxoanionic solid electrolyte, the potassium salts are preferred due to their higher melting temperature and thus possible higher operating temperature of the sensor compared to the sodium salts. As a potassium ion conductor, there have been proposed P-Al ₂ O ₃ or mullite (EP 0078627) and K ₂ YZr (PO ₄ I _3. A disadvantage is that solid electrolyte bodies produced therefrom can only be densely sintered at relatively high temperatures, for example this temperature is K ₂ YZr (PO ₄ I ₃ 1700 ⁰ C. So far only a few pure potassium ion conductors with high conductivity are known.

Object of the invention

The aim of the invention is an alkali ion-conducting solid electrolyte which is suitable for the potentiometric determination of CO ₂ , SO _x or NO _x as a dense separation membrane and which can be used at comparatively low temperatures as a long-term dense sintered body for gas sensors.

Explanation of the essence of the invention

The object of the invention is to provide the composition of a Alkalionenionenleitors for gas sensors, which can be processed by ceramic methods, at density less than 1200 ° C dense sintered and relatively large, rsin ionic conductivity.

The object is achieved by changing the approach for the preparation of a compound according to the formula M ₃ Ln (PO ₄ I ₂ , in which M potassium or sodium ions and Ln in air redoxstabiles trivalent cation, preferably a rare earth or lanthanide cation is changed so that a deficiency of M-ions in the lattice is formed by either less than or equal to M ₂ O stoichiometry, or by keeping the ratio of M ₃ PO ₄ to LnPO ₄ less than 1 or, instead of one part of M ₂ O, the equivalent amount of MeO where MeO is a redox-stable divalent cation, preferably an alkaline earth cation,

As has been found, one can make the approach that it contains 1 to IOMol-% M ₂ O, that is less than the stoichiometry corresponds or that the molar ratio of M ₃ PO ₄ to LnPO _{4 is} 0.90 to 0.99, or that 1 to 66 mol% M ₂ O are replaced by MeO. For Ln, mixtures of two or more trivalent ions can also be used.

For Me, mixtures of two or more divalent ions can also be used. As an aid for improving mechanical or electrical properties, up to 30% by mass of M ₂ SO ₄ or Y-Al ₂ O _{3 may be} added. It is advantageous that the masses, which are prepared according to the instructions, can be processed relatively well ceramic to gas-tight bodies and in contrast to the pure alkali phosphaton M ₃ PO _{4 are} not hygroscopic. For the composition K ₂₁₉₇ La (PO ₄ I) ₁₈₉ , the electrical conductivity at 813 ⁰ C is 2.75 χ 10 " ⁴ S / cm. The highest conductivities are achieved when part of the alkali is replaced by alkaline earth metal phosphate the conductivity at 813 ⁰ C for K _{2 0} .88Sr, œLa (P0 _{4) 2} 1.2 × 10 ^-2 S / cm.

Ausführungsbolsplel

The invention is explained in more detail below on preparation examples:

example 1

For 5g K ₂₉₇ La PO ₍₄ Ii ₁₉₉ are 3.341 g KHCO _3, 1,830g La ₂ O _3, and 2,967g (NH ₄ I ₂ HPO ₄ 5h dry-mixed in the ball mill together and calcined at 600 ⁰ C for 24 h.

The resulting sintered product is mechanically comminuted and 96h at 1050 ⁰ C reacted in a solid state reaction in platinum crucible. After repeated mechanical comminution can be produced from the resulting powders by known ceramic methods moldings which can be sintered gas tight at 1050 ⁰ C.

Example 2

To prepare 10 g of K ₂ , ₄ Sr ₀ , ₃ La (PO ₄ ) ₂ , 5.352 g of KHCO ₃ , 3.628 g of La ₂ O ₃ , 0.986 g SrCO ₃ and 5.883 g (NH ₄ I ₂ HPO _{4 are} thoroughly premixed in an agate mill and brought during a first 24-hour annealing at 600 ° C in a platinum crucible to the reaction. the sintered cake is crushed and subjected at 1050 ° C 96h a second annealing. After mechanical comminution can be prepared from powders tablets which can be sealed internally at 1050 ⁰ C ,

Example 3

The preparation of 10 g of a sodium ion conductor of the composition Na ₂ , 4 Baao, ₃ La (P0 ₄ ) ₂ by means of a solid state reaction by weighing 2.9912 g of Na ₂ CO ₃ , 1.3923 g BaCO ₃ , 3.8310 g La ₂ O ₃ and 5.4101 g of NH ₄ H ₂ PO ₄ and intimate mixing. After a first annealing for 24 h at 600 ⁰ C, the sinter cake is crushed and annealed again at 105O ⁰ C 96h. A three-hour dry grinding gives a fine-grained powder, which presses after addition of 5% by weight of Y-Al ₂ O ₃ as a sintering aid and at 1050 ⁰ C and one hour sintering gas-tight moldings.

Claims (4)

1. Alkaline ion conducting solid electrolyte for gas sensors, which can be produced from powders by ceramic methods, characterized in that for these powder, the solid electrolyte having the chemical composition M ₃ Ln (P0, () ₂ is changed so that a deficiency of M ions in Crystal lattice is formed, in which either less M2O, between 1 and 10Mol% corresponds to the stoichiometry, used or the ratio M3PO4 to LnPÜ4 between 0.9 and 0.99 held or instead of a part M ₂ O the equivalent amount of MeO, between Where M is potassium or sodium ions, Ln is redox-stable trivalent cations, rare earth or lanthanide cations, and redox-stable divalent cations, alkaline earth cations. 2. Alkali ion-conducting solid electrolyte claim 1, characterized in that for Ln mixtures of two or more trivalent ions are used. 3. alkali ion-conducting solid electrolyte according to claim 1 and 2, characterized in that mixtures of two or more divalent ions are used for Me. 4. alkali ion-conducting solid electrolyte according to claim 1 to 3, characterized in that M ₂ SO ₄ or Y-Al ₂ O _{3 are added} up to 30Ma .-% as an aid for improving mechanical and electrical properties.