NOVELTY - A layer of metal oxide material is made, introducing metal particles into it. The metal particles are subsequently oxidized. The solid electrolyte (16) produced, is conductive to oxygen ions and insulating with respect to electron conduction. The particles are oxidized exothermically. This occurs during operation of the high temperature fuel cell. The particles are a metal from the third- or fourth sub-group of the periodic table. They are Sc, Y, La, Ti, Zr or Hf. They are alternatively the same material as the metal in the metal-oxidic material.    USE - To make a gas-tight solid electrolyte layer for a high temperature fuel cell.    ADVANTAGE - In prior art, defects such as pores and micro-cracks allow unwanted hydrogen diffusion between anode and cathode. This limits the cell open-circuit voltage and reduces cell efficiency. The new process is carried out easily and produces an extremely gas-tight, solid electrolyte layer. On oxidation of the metal particles in the layer, post-compression arises within it. This reduces its gas permeability and hence limits hydrogen diffusion. Imperfections such as pores and micro-cracks are sealed effectively, by the increased volume resulting from metal oxidation. The exothermic nature of the metal oxidation stage, further assists pore- and crack closure. All the metal is converted to oxide. Suitable selection of the metal particles, ensures that their introduction has no negative effects on high temperature fuel cell operation. The layer can be made thinner, saving materials and reducing manufacturing time. Greater electrochemical efficiency results from its reduced electrical resistance. The layer can be a few tens of microns thick, and no additional sintering stage is required. A mechanism entitled splat formation, enhances oxygen ion conductivity. Use of a precursor is at least as economical and reliable for introduction, as spraying metal particles. Further practical points and advantages are discussed.    DETAILED DESCRIPTION - To introduce the particles, a metal precursor is used, especially a metal hydride. The mass ratio of metal particles is in the range 1%-10%, with respect to the remaining material of the solid electrolyte layer. The metal particles are introduced in a defined distribution. Thermal spraying is the technique used to make the solid electrolyte layer. Metal particles and/or metal precursor particles and particles of metal-oxidic material, are sprayed onto a substrate. The parameters are so adjusted and/or the metal precursor particles are so selected, that from the metal precursor particles, the metal particles are formed in-flight. The solid electrolyte layer is made by plasma-spraying. It is made by vacuum-plasma-spraying. It comprises zirconium oxide. Zirconium hydride is the precursor used to form the metal particles. The solid electrolyte layer is a ceramic layer. AN INDEPENDENT CLAIM IS INCLUDED FOR the corresponding solid electrolyte layer.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic partial cross section of a high temperature fuel cell.    high temperature solid oxide fuel cell (SOFC) (10)    anode (14)    solid electrolyte layer (16)    cathode (18)    anode-electrolyte-cathode unit (20)