The invention relates to seals for gas-carrying lines and t installations which comprise such seals. The invention relates in particular to seals for applications in electrochemical cells and to electrochemical cells which comprise such seals.

In the past, attempts have been made to achieve a good seal using spring constructions and the like. However, seals of this type continue to give problems.

Seals which have to function at high temperatures, such as fuel cells, have always been a major problem. Complex constructions a used in order to absorb expansions which unavoidably take place. Expa sions give rise to problems especially if two adjoining materials do have exactly the same coefficient of thermal expansion.

It has now been found that an improved seal is obtained in t case of gas-carrying lines by fitting an inorganic material which is tough at the operating temperature between at least two parts which mu be able to move relative to one another. Preferably, a material is use which has Bingham or visco-elastic flow characteristics at. temperatu above the glass transition temperature.

An advantageous material for use as a seal is a tough silica glass. More generally, suitable sealing materials are a highly viscous tough alkali metal silicate glass and/or alkaline earth metal silicate glass and/or boron silicate glass and/or lead silicate glass and/or aluminium silicate glass and/or lithium silicate glass. Materials for which the viscosity is 5 x 10 ³ to 5 x 10* P are preferred, in particul (Na ₌ 0) _o . ₁₌ (Ca0) _o . _1o (Si0 ₂ ) _o ., _s which has a viscosity of 10"* P at 1004 (Na ₂ 0) _o . _2O (Ca0) _o . _1o (Si0 _a ) _o .., _o which has a viscosity of 10* P at 924° (Na ₂ 0) _o . _2O (Mg0) _o . _1o (Si0 ₂ ) _o . _vo which has a viscosity of 10* P at 992°

(Na ₂ 0) _o . _2O (K ₂ 0) _o . _3o (Si0 ₂ ) _o . _so which has a viscosity of 10* P at 100 and

(K ₂ 0)o. _ι o ( ^pt >0)o. _ι o (Si0 ₂ )o.βo which has a viscosity of 10 ^* P at 1000° It is surprising that, with the aid a glass, a good seal is obtained which remains in place, does not flow away and is also able t withstand gas pressures without being blown out of place. It is, of course, necessary to use a suitable viscosity of the glass for this purpose.

An example of a glass seal of this type is shown in Fig. .

enlarged detail is given in fig. 1a. For a fuel cell, the gas pipes of the electrodes are passed through an insulating viscous mass 2 into a mixing/splitting chamber 3. The diameters of the bores through which the pipes are passed are larger than the pipe diameter, in general by 1 or 2 mm, so that problems related to thermo-expansion are avoided here. The yield stress and viscosity of the glass at the operating temperature must be sufficiently high to counteract leakage of the glass through the openings as a consequence of its own weight and the pressure gradient.

The viscosity can be controlled by means of the composition of the glass and the temperature of the glass.

The inorganic material used, in particular glass, must have as high as possible a yield stress. When selecting a glass and the components of the glass, a compromise must be sought between constituents of the glass. The glass can contain on the one hand lattice-forming agents (in order to obtain a sufficiently high yield stress) and, on the other hand, additives to lower the melting point. B and Pb have a substantial effect in lowering the melting point (breaking down the lattice). A melting point of 500 to 70C°C is possible. Calcium, sodium and potassium, and also magnesium, give a higher temperature of about 1000° in silicate glasses. In practice, an operating temperature of about 1000°C is used in ceramic fuel cells.

The yield stress of a glass is influenced by the amount of lattice-forming material which is present in the glass. With regard to the viscosity, the processing point for glass industrially is about 10*P because the glass otherwise becomes too tough and is difficult to process.

Provided the yield stress is not too greatly exceeded in an installation, this is generally not serious if a so-called manifold is used. As a result of the capillary action, which is obtained by taking spacings which are not too great between the parts which have to be able to move relative to one another, a capillary action is obtained which counteracts the loss of the tough material. It is also possible, for example in the case of fuel cells, to collect glass which has been lost from the seal at the bottom of the stack and subsequently to return this glass to the desired location. Small losses can therefore easily be offset.

Glasses having Bingham characteristics are known per se and are described, for example, in Handbook of Glass Data, Part C, Ternary Silicate Glasses, Elsevier, Amsterdam, 1987, in particular pages 27, 29-

33 and 182 to 188. The yield stress and the associated phenomena are discussed in The Structure and Mechanical Properties of Inorganic Glass by G.ϋ. Bartenev, Wolters-Noordhoff Publishing, Groningen 1970, pages 14 to 146. A similar description is also given in Rheological Properties of Alkali Borate Glasses by Theodorus Johannes Maria Visser, a thesis published by the Technical University of Eindhoven (1971), pages 6-12, 40-72 and 78-82.

If desired, it is possible to raise the temperature of the glass during a heat cycle of the stack of fuel cells, by which means the pipes are to some extent able to move freely in the glass seal and as a result of which the glass can settle, by which means a gastight seal is obtained. When operating a cell stack, the glass temperature can, however, be lowered somewhat in order to prevent leakage of the glass seal. An additional advantage of this type of gastight seal is the possibility that the sealed elements are to some extent able to move freely relative to one another. Even the replacement of elements is easy In such a case, the glass composition is removed from the holder at somewhat elevated temperature. The element is then removed from the stac and the pipes can be removed from the holder. A new element is placed in position and the glass composition is replaced in the holder at high temperature and the seal is restored.

The gastight seal described here is, of course, also suitable for use in other joins where the sealed elements must be able to move somewhat relative to one another.