In recent years, deterioration of luminescence efficiency of blue-emitting ZnS:Ag,Al powder phosphors has been investigated for high-voltage (e.g., 6-10 kV) field-emission displays (FEDs). It has been found by cross-sectional transmission electron microscopy that the degree of crystallinity of ZnS host crystal within the electron penetration depth is one of the important factors in order to obtain ZnS:Ag,Al phosphors having a longer lifetime of luminescence efficiency. ZnS:Ag,Al phosphors with lower defect density N-0 as compared with a P22B ZnS:Ag,Al phosphor for color-TV use have been developed by using a suitable firing method with subsequent annealing and surface treatments. As a result, ZnS:Ag,Al phosphors with lower N-0 were successfully prepared, and also FED tubes having a longer lifetime in blue luminescence were made by using ZnS:Ag,Al phosphors with lower N-0. In addition, it was revealed that both the initial luminescence efficiency and the thermal quenching point of ZnS:Ag,Al phosphors with lower N-0 are considerably higher than those of a P22B ZnS:Ag,Al phosphor. (C) 2003 American Vacuum Society.