Abstract: Alpha-alumina consisting essentially of solid, spherical, non-aggregated particles, having a diameter from 0.06 to 2 .mu.m, a polydispersion index below 2.0, a total porosity below 0.3 cc/g, a specific surface area from 0.7 to 80 m.sup.2 /g, and an apparent density of the particles from 3.20 to 3.94 g/cm.sup.3. The alpha-alumina is prepared by a process consisting, in that an alpha-alumina precursor, having a water content from 2 to 33% by weight, at starting temperatures ranging from room temperature to 850.degree. C., and consisting essentially of solid spherical, non-aggregated particles, having a particle diameter from 0.07 to 2.3 .mu.m and a polydispersion index of .ltoreq.2.0, is subjected to a heat-treatment at temperatures ranging from 1130.degree. to 1700.degree. C., said precursor being instantaneously brought from the starting temperature to the treatment temperature, and by carrying out the treatment during times, as generally indicated in the accompanying FIG.

Abstract: A process for preparing spheroid particles of oxide compounds, having an average diameter of smaller than 3 microns. An emulsion is prepared of a hydrolyzable liquid oxide compound in a perfluoropolyether, the emulsion is reacted with water, water vapor or with a mixture, in any ratio, of water with a liquid miscible or immiscible with water so as to form an oxide hydrate which is recovered dried and calcined. Oxide compounds include oxides of Ti, Al, Zr, Si or B.

Abstract: This invention relates to a process for stabilizing ferromagnetic chromium dioxide in respect of water and oxidizable organic compounds which it may contact in magnetic tapes. The process is applicable to a starting CrO.sub.2 having good magnetic properties, i.e. a coercive force not lower than about 400 Oersted, a maximum magnetic induction B.sub.m not lower than about 2500 gauss, and a magnetic induction B.sub.r not lower than about 1600 gauss. The CrO.sub.2 is subjected to a heat treatment at a temperature ranging from 300.degree. to 390.degree. C. in the presence of a gas stream not exerting any oxidizing or reducing action on the compounds of trivalent, tetravalent and hexavalent chromium. Suitable gases are, for example, nitrogen, helium and argon.