The performance of maintenance is

considerably improved as a result of annealing ZnS-Cu

Eu

Br ACEL powder at a higher temperature in a shorter time. When treated the particles of phosphor must be coated with excessive copper sulphide. Otherwise

the luminescence performance would be spoilt. If excessive copper sulphide has been washed by a dilute solution of potassium cyanide (or sodium cyanide)

the particles must be coated again with copper sulphide and baked

thus brightness of electroluminescence can be partially restored and the deteriorated performance improved. The reason for the improvement of the phosphor maintenance is attributed to the improvement of the condition of Cu

2

S phase

but not that the of luminescent center.