Abstract: A protective element for an electrochemical accumulator is made of granular varistor material, the mean diameter of the grains of the varistor material being less than 1 .mu.m, in particular less than 250 nm and, for more than 60% of the grains of the varistor material, the grain size deviating by at most 40% from its statistical average. For the preparation of such grains, a dispersion is made up to form biochemical hollow bodies which have an internal diameter in accordance with the above requirements. The dispersion is admixed with salts which dissolve in the dispersion with the formation of intermediate ions of a substance forming an intermediate for a varistor material, the intermediate ion of a substance forming an intermediate for a varistor material being an intermediate ion of an element required for the varistor material or of a corresponding compound.

Abstract: The invention provides a method of making a magnesium/aluminium spinel compound having the formula Mg.sub.(1+x) Al.sub.(2+y) O.sub.4 in which 0.ltoreq.x.ltoreq.0.2 and 0.ltoreq.y.ltoreq.0.35. Particulate aluminium metal and a magnesium-containing component selected from magnesium oxide and precursors thereof are mixed together and heated to 800.degree.-1150.degree. C. in an oxidising environment to cause oxidation of at least part of the aluminium. Further heating then takes place to a temperature of 1150.degree.-1350.degree. C. in the oxidizing environment to cause aluminium oxide and magnesium oxide in the admixture to react together.

Abstract: A method of making a .beta.-alumina compound which is a polyaluminate of the general formula M.sub.y O.multidot.xAl.sub.2 O.sub.3 in which M is a metal selected from monovalent metals and divalent metals, y=2 when M is a monovalent metal, y=1 when M is a divalent metal and x=4-12 comprises forming a green precursor of the 62 -alumina compound by mixing together particulate aluminium metal and a reagent compound comprising an oxide of the metal M or a precursor thereof. The mixture is heated to 800.degree.-1150.degree. C. in an oxidizing environment to cause oxidation of at least part of the aluminium. Further heating then takes place to 1150.degree.-1350.degree. C. in said environment to cause the oxidized aluminium and oxide of the metal M to react to form the .beta.-alumina compound.

Abstract: A superstoichiometric MgAl.sub.2 0.sub.4 spinel, which can be used, for example, as material for a high temperature crucible, is synthesized by, producing a precursor mixture comprising intimately mixed oxides of the corresponding metals in the spinel. Magnesium is added in a superstoichiometric amount to the precursor mixture. Prior to spinel formation, from one to five percent by weight of a reactive salt containing fluorine and/or boron, based on the metal oxides of the spinel, are added to the precursor mixture, and the doped precursor mixture is mixed for from 0.5 to 3 hours. Finally, the mixed precursor mixture is heated for between 0.5 and 4 hours at a temperature between 1150.degree. C. and 1400.degree. C. to synthesize the Mg.sub.(1+x) Al.sub.2 0.sub.4 spinel which is superstoichiometric with respect to magnesium.

Abstract: The present invention provides a process for the production of a K- or Rb-.beta."- or -.beta.-aluminum oxide powder preparation, wherein a cubic densely packed aluminum oxide (.gamma.-aluminum oxide) or a precursor thereof is subjected to attrition together with a doping material or a precursor thereof, the so obtained mixture is calcined in an oxygen-containing atmosphere, potassium oxide or rubidium oxide or a precursor of these compounds is added to the calcination product obtained, the mixture is subjected to attrition and then calcined in an oxygen-containing atmosphere at a temperature greater than 900.degree. C. The present invention also provides a process for the production of a ceramic formed body in which a powder obtained by the above process is converted by attrition, isostatic pressing and sintering into a ceramic formed body. Such a ceramic formed body can be used as an ion conductor.

Abstract: The invention provides a method of making .beta."-alumina by heating aluminium oxide in the presence of Na.sub.2 O. A metal oxide dopant in the form of Li.sub.2 O, MgO, ZnO, CoO, NiO, FeO or mixtures thereof, or a precursor of the dopant, is dispersed in a cubic-close packed aluminium oxide or a precursor thereof to form a starting mixture which is calcined by heating to 250.degree.-1100.degree. C. in an oxygen-containing atmosphere. Na.sub.2 O is then dispersed in the calcined starting mixture to form a final mixture, and the final mixture is heated in an oxygen-containing atmosphere to above 1100.degree. C. to produce .beta."-alumina.

Abstract: A method of making beta"-alumina comprises dispersing in a precursor of aluminum oxide a member of the group comprising sodium oxide and its precursors, and dispersing in the precursor of aluminum oxide a spinel stabilizer which is a member of the group comprising spinel-forming oxides and the precursors thereof, to form a mixture. The mixture is then heated to a temperature at which at least some of the precursor of aluminum oxide is converted to beta"-alumina. The precursor of aluminum oxide is one which, when calcined in air by itself, has a calcination product which, when subjected to X-ray diffraction, displays a predetermined X-ray diffraction trace. The X-ray diffraction trace is one whose peak with the highest intensity in the 2 (theta) range of 44.degree.-48.degree. and whose peak with the highest intensity in the 2 (theta) range of 63.degree.-69.degree. respectively have maximum intensities and integrated intensities which comply with equations (I) and (II):A/S is greater than 0.