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Analysis of phosphor coatings by luminescence thermal degradation
Analytica 0 Elsevier
Scientific
Publishing
Company,
Chimica Acta. 79 (1976)
Amsterdam
-
Printed
330-331
in The Netherlands
Short Communication ANALYSIS OF PHOSPHOR DEGRADATION
SIMON
LARACW
RCA Laboratories, (Received
17th
COATINGS
and JERRY
GERBER
Princeton,
N.J. 08540
March
BY LUMINESCENCE
THERMAL
(U.S.A.)
1975)
Phosphor particles are often coated with thin layers of inorganic materials in order to improve various screening factors. As part of a general study on the effect of coatings on the cathodoluminescence of phosphors, the effect of high-density cathode-ray excitation has been investigated [ 11 . Such studies indicated that there is a luminescence degradation, which is thermal in nature, as a function of time. By plotting this luminescence thermal degradation as a function of coating concentration, obtained by independent analytical means, a method of quantitative analysis was evolved, valid for a particular coating on a particular host phosphor. Experimental Green-emitting ZnCdS: Cu: Al and blue-emitting ZnS:Ag were coated [ 23 with thin layers of various inorganic compounds by conventional techniques. These were then tested in a demountable cathode-ray apparatus [3] . The focussed electron beam was scanned into a small raster, about 2 mm on a side. Power input to the sample was controlled by adjusting the beam current, at a constant 10.kV potential. Currents of 0.1-0.5 @A typically gave a detectable luminescence thermal degradation signal on uncoated samples. Luminescence degradation was observed with a monochromator and an RCA 7266 (520 surface) multiplier phototube. Broad slits were used to obtain a 20.nm band-pass centered about the peak of the emission spectrum. Photomultiplier output was plotted against “beam-on” time on an X-Y recorder. Sample preparation was found to be of importance. The phosphor particles should be in poor thermal contact with each other and with any other surfaces. It was found best to “dust” the phosphor loosely into a depression (l-mm deep) in a multi-sample stainless-steel disk which was placed into the demountable apparatus. Results
and discussion
The technique
calls for exciting
a “blank” (uncoated)
phosphor,
and a
331
UNCOATED
IO
I 20
30 TIME
40
50
(SECONDS
60
1: 0
005
010
0.15
020 SILICA
)
0.25
0.30
COATING,
0.35
040
PERCENT
Fig.1. Feak cathodoluminescence emission intensity as a function of time of excitation several coating concentrations. The near-vertical rises occurred where the sample was rotated to a new beam-landing position. Fig.2. Luminescence phosphors.
thermal
degradation
aa a function
of concentration
for
for silica-coated
coated phosphor, with a high-density cathode-ray beam, and recording the emission intensity as a function of “beam-on” time. Figure 1 is a plot of the recorder traces for a silica-coated, blue-emitting phosphor, operated at 8 kV, 0.30 @A, in a focussed spot of 0.25 mm2, and detecting the emitted photons at 469 nm. It can be seen that the rate of decrease of emission intensity, --dL/dt. is proportional to the concentration of the coating. By plotting (dL/dt)t as a function of coating concentration, a standard cuwe can be derived for any coating on any phosphor, which can then be used to analyze samples quantitatively. Figure 2 illustrates such a curve for silica-coatings on green-emitting phosphor particles. For such “dusted” samples, the degradation reproducibility was 4 5 % as different portions of the sample were positioned under the raster, this being an indication of sample uniformity. The proposed method of analysis has detected coatings as low as 0.026 %, and a detection limit of 50-100 p.p.m. is indicated for silica coatings. The assistance of S.A. Trond, of RCA Lancaster, Pa., is gratefully edged, for his interest and supply of coated phosphors. REFERENCES 1. J. Gerber and S, Larach, to be published. P 9. Larnch and A.E. Wardy, Proc. IEEE, 61 (1973) 916. 3 See, for example, S. Larach, Anal. Chim. Acta, 41 (1968)
189.
acknowl-
045
0.50
Scientific
Publishing
Company,
Chimica Acta. 79 (1976)
Amsterdam
-
Printed
330-331
in The Netherlands
Short Communication ANALYSIS OF PHOSPHOR DEGRADATION
SIMON
LARACW
RCA Laboratories, (Received
17th
COATINGS
and JERRY
GERBER
Princeton,
N.J. 08540
March
BY LUMINESCENCE
THERMAL
(U.S.A.)
1975)
Phosphor particles are often coated with thin layers of inorganic materials in order to improve various screening factors. As part of a general study on the effect of coatings on the cathodoluminescence of phosphors, the effect of high-density cathode-ray excitation has been investigated [ 11 . Such studies indicated that there is a luminescence degradation, which is thermal in nature, as a function of time. By plotting this luminescence thermal degradation as a function of coating concentration, obtained by independent analytical means, a method of quantitative analysis was evolved, valid for a particular coating on a particular host phosphor. Experimental Green-emitting ZnCdS: Cu: Al and blue-emitting ZnS:Ag were coated [ 23 with thin layers of various inorganic compounds by conventional techniques. These were then tested in a demountable cathode-ray apparatus [3] . The focussed electron beam was scanned into a small raster, about 2 mm on a side. Power input to the sample was controlled by adjusting the beam current, at a constant 10.kV potential. Currents of 0.1-0.5 @A typically gave a detectable luminescence thermal degradation signal on uncoated samples. Luminescence degradation was observed with a monochromator and an RCA 7266 (520 surface) multiplier phototube. Broad slits were used to obtain a 20.nm band-pass centered about the peak of the emission spectrum. Photomultiplier output was plotted against “beam-on” time on an X-Y recorder. Sample preparation was found to be of importance. The phosphor particles should be in poor thermal contact with each other and with any other surfaces. It was found best to “dust” the phosphor loosely into a depression (l-mm deep) in a multi-sample stainless-steel disk which was placed into the demountable apparatus. Results
and discussion
The technique
calls for exciting
a “blank” (uncoated)
phosphor,
and a
331
UNCOATED
IO
I 20
30 TIME
40
50
(SECONDS
60
1: 0
005
010
0.15
020 SILICA
)
0.25
0.30
COATING,
0.35
040
PERCENT
Fig.1. Feak cathodoluminescence emission intensity as a function of time of excitation several coating concentrations. The near-vertical rises occurred where the sample was rotated to a new beam-landing position. Fig.2. Luminescence phosphors.
thermal
degradation
aa a function
of concentration
for
for silica-coated
coated phosphor, with a high-density cathode-ray beam, and recording the emission intensity as a function of “beam-on” time. Figure 1 is a plot of the recorder traces for a silica-coated, blue-emitting phosphor, operated at 8 kV, 0.30 @A, in a focussed spot of 0.25 mm2, and detecting the emitted photons at 469 nm. It can be seen that the rate of decrease of emission intensity, --dL/dt. is proportional to the concentration of the coating. By plotting (dL/dt)t as a function of coating concentration, a standard cuwe can be derived for any coating on any phosphor, which can then be used to analyze samples quantitatively. Figure 2 illustrates such a curve for silica-coatings on green-emitting phosphor particles. For such “dusted” samples, the degradation reproducibility was 4 5 % as different portions of the sample were positioned under the raster, this being an indication of sample uniformity. The proposed method of analysis has detected coatings as low as 0.026 %, and a detection limit of 50-100 p.p.m. is indicated for silica coatings. The assistance of S.A. Trond, of RCA Lancaster, Pa., is gratefully edged, for his interest and supply of coated phosphors. REFERENCES 1. J. Gerber and S, Larach, to be published. P 9. Larnch and A.E. Wardy, Proc. IEEE, 61 (1973) 916. 3 See, for example, S. Larach, Anal. Chim. Acta, 41 (1968)
189.
acknowl-
045
0.50