Moisture-resistant Electroluminescent Phosphor with High Initial Brightness and Method of Making

Abstract

An electroluminescent (EL) phosphor is described wherein each individual phosphor particle is encapsulated in an inorganic coating, preferably aluminum oxyhydroxide. The encapsulated phosphor shows an extreme insensitivity to atmospheric moisture and suffers only minor loss of initial brightness in lamps. The method of applying the coating is a hybrid process, which involves EL phosphor particles first being coated with a thin inorganic film using an atomic layer deposition (ALD) method in a fluidized bed wherein the precursors are introduced sequentially in repeated cycles, subsequently followed by an additional coating layer applied by a chemical vapor deposition (CVD) method in which the precursors are introduced simultaneously.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of a conventional thick-film EL lamp.

FIG. 2 is a graph of the 100-hour maintenance as a function of aluminum content for conventional CVD-coated EL phosphors operated in an EL lamp at 50° C., 90% rel. humidity.

FIG. 3 is a graph of 100-hour humidity maintenance versus total aluminum coating weight for coated phosphors made by the hybrid ALD/CVD method of this invention.

Claims

1. An electroluminescent phosphor, comprising: individual particles of a zinc sulfide-based electroluminescent phosphor wherein each particle is encapsulated in an inorganic coating, the phosphor exhibiting a retained initial brightness of at least 90% and a 100-hour maintenance of at least 60% when incorporated in an electroluminescent lamp that is operated at 100 V and 400 Hz in a 50° C., 90% relative humidity environment.

2. The phosphor of claim 1 wherein the 100-hour maintenance is at least 75%.

3. The phosphor of claim 1 wherein the electroluminescent phosphor is ZnS:Cu.

4. The phosphor of claim 1 wherein the inorganic coating is aluminum oxyhydroxide.

5. The phosphor of claim 4 wherein the 100-hour maintenance is at least 75%.

6. The phosphor of claim 4 wherein the electroluminescent phosphor is ZnS:Cu.

7. A method for encapsulating an electroluminescent phosphor comprising: (a) forming a fluidized bed of electroluminescent phosphor particles;(b) introducing a first vapor phase precursor into the fluidized bed;(c) purging the fluidized bed;(d) introducing a second vapor phase precursor into the fluidized bed to react with the first precursor and form an inorganic coating on the phosphor particles;(e) purging the fluidized bed;(f) repeating steps (a) through (e) until a desired coating thickness is reached; and(g) introducing the first and second vapor phase precursors simultaneously into the fluidized bed to further increase the coating thickness.

8. The method of claim 7 wherein the first or second vapor phase precursor is either vaporized trimethylaluminum or water vapor and the coating is aluminum oxyhydroxide.

9. The method of claim 8 wherein the desired coating thickness in step (f) is reached after about 100 cycles of repeating steps (a) through (e).

10. The method of claim 7 wherein step (g) is continued until the phosphor exhibits a 100-hour maintenance of at least 60% when incorporated in an electroluminescent lamp that is operated at 100 V and 400 Hz in a 50° C., 90% relative humidity environment.

11. The method of claim 9 wherein step (g) is continued until the aluminum content of the coated phosphor is from about 1 wt. % to about 2.5 wt. %.

12. The method of claim 8 wherein the desired coating thickness in step (f) is about 110 Å.

13. The method of claim 12 wherein step (g) is continued until the aluminum content of the coated phosphor is about 2.2 wt. %.

14. A method for encapsulating an electroluminescent phosphor comprising: (a) forming a fluidized bed of electroluminescent phosphor particles;(b) introducing vaporized trimethylaluminum into the fluidized bed;(c) purging the fluidized bed;(d) introducing water vapor into the fluidized bed to react with the trimethylaluminum and form an aluminum oxyhydroxide coating on the phosphor particles;(e) purging the fluidized bed;(f) repeating steps (a) through (e) until a desired coating thickness is reached; and(g) introducing the vaporized trimethylaluminum and water vapor simultaneously into the fluidized bed to further increase the coating thickness.

15. The method of claim 14 wherein the vaporized trimethylaluminum is introduced in step (d) and the water vapor is introduced in step (b).

16. The method of claim 14 wherein the desired coating thickness in step (f) is reached when the aluminum content of the coated phosphor is about 0.2 weight percent.

17. The method of claim 16 wherein step (g) is continued until the aluminum content of the coated phosphor is about 2.2 wt. %.

18. A coated electroluminescent phosphor, comprising: individual particles of a zinc sulfide-based electroluminescent phosphor wherein each particle is encapsulated in an aluminum oxyhydroxide coating, the phosphor exhibiting a retained initial brightness of at least 90% and having a moisture resistance equivalent to a CVD-coated phosphor having an aluminum oxyhydroxide coating wherein the aluminum content of the CVD-coated phosphor is 3.8 weight percent.

19. The coated phosphor of claim 18 wherein the aluminum content of the coated electroluminescent phosphor is about 2.2 weight percent.