Method of making electroluminescent phosphor using sonochemistry
A method of making zinc sulfide based electroluminescent phosphors is described wherein a first-fired zinc sulfide material having a hexagonal β-ZnS crystal structure is at least partially converted to a distorted hexagonal β-ZnS crystal structure by ultrasonic irradiation. The first-fired material is then fired at a lower temperature to form an electroluminescent phosphor.
1. A method of making of zinc sulfide-based electroluminescent phosphors, comprising:
a) firing a mixture of zinc sulfide, at least one activator, and a flux to form a first-fired material having a hexagonal β-ZnS crystal structure;
b) suspending the first-fired material in a liquid medium;
c) at least partially converting the crystal structure of the first-fired material to a distorted hexagonal β-ZnS form by ultrasonic irradiation of the suspension; and
d) firing the at least partially converted first-fired material to form an electroluminescent phosphor.
2. The method of claim 1 wherein said mixture is fired at a temperature from about 1050° to about 1400° C. for about 2 hours to about 8 hours.
3. The method of claim 1 wherein the suspension is ultrasonically irradiated from about 10 minutes to about 90 minutes.
4. The method of claim 2 wherein mixture contains copper and chlorine.
5. The method of claim 4 wherein the first-fired material is blended with a source of copper after the ultrasonic irradiation and prior to the second firing.
6. The method of claim 5 wherein the blended material is fired at a temperature from about 500° C. to about 1000° C. for about 1 to 5 hours.
7. The method of claim 2 wherein the at least partially converted first-fired material is fired at a temperature from about 500° C. to about 1000° C. for about 1 to 5 hours.
8. The method of claim 1 wherein the first-fired material is suspended in water and the ultrasound has a frequency of 20 kHz.
9. The method of claim 1 wherein the suspension containing the first-fired material is circulated through a cell having an ultrasonic transducer.
10. The method of claim 9 wherein at least a portion of the suspension is recirculated through the cell.
11. A method of making of zinc sulfide-based electroluminescent phosphors, comprising:
a) firing a mixture of zinc sulfide, at least one activator, and a flux at a temperature from about 1050° to about 1400° C. for about 2 hours to about 8 hours to form a first-fired material having a hexagonal β-ZnS crystal structure;
b) suspending the first-fired material in a liquid medium;
c) at least partially converting the crystal structure of the first-fired material to a distorted hexagonal β-ZnS form by ultrasonic irradiation of the suspension; and
d) firing the at least partially converted first-fired material at a temperature from about 500° C. to about 1000° C. for about 1 to 5 hours to form a zinc sulfide-based electroluminescent phosphor having a cubic β-ZnS crystal form.
12. The method of claim 11 wherein the phosphor is activated with copper.
13. The method of claim 12 wherein the first-fired material is blended with a source of copper after the ultrasonic irradiation and prior to the second firing.
14. The method of claim 11 the first-fired material is suspended in water and the ultrasound has a frequency of 20 kHz.