U.S. Philips Corporation

Thomas Rink

Cornelis Reinder Ronda

Michael Bredol

Ulrich H Kynast

Guido Hutten

The invention relates to a method of coating red-luminescing phosphor particles with hematite (.alpha.--Fe.sub.2 O.sub.3), whereby first Fe(OH).sub.3 is deposited on the phosphor particles and subsequently converted into .alpha.--Fe.sub.2 O.sub.3 at a raised temperature. A good adhesion of the hematite pigments to the phosphor particles is achieved in that first an aqueous solution of an iron(III) salt and a complexing agent is prepared, in which the complexing agent and its concentration are so chosen that in combination with Fe.sup.3+ ions complex ions are formed which on the one hand do not yet precipitate at a pH-value which is at least 0.5 higher than the value at which the phosphor particles just fail to be chemically attacked anymore, and which are dissociable at a further increase in the pH-value so that Fe(OH).sub.3 is formed, in that subsequently, if the pH-value is not yet high enough, a basic substance is added to the mixture, in that the pH-value is increased to a value which is at least 0.5 below the value at which Fe(OH).sub.3 starts to precipitate and at least 0.5 higher than the value at which the phosphor particles just fail to be chemically attacked anymore, in that subsequently the phosphor particles are added while stirring, in that at any moment, as required, a substance is added which dissociates upon an increase in temperature and which behaves as a base upon dissociation, and in that then the temperature of the suspension is slowly increased, whereby the phosphor particles are coated with Fe(OH).sub.3 through homogeneous precipitation.

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Munich, Georgia

Thomas Rink