Method for making moisture insensitive zinc sulfide based luminescent materials

A process is disclosed for coating phosphors with hydrolyzed alkylaluminum. The hydrolyzed alkylaluminum coating renders the phosphors insensitive to atmospheric moisture. the coating process involves vaporizing an aluminum-containing precursor such as trimethylaluminum or triethylaluminum in an inert gas stream and passing this through a fluidized bed containing the phosphor particles. Water vapor is also passed through the fluidized bed and the water and aluminum precursor react on the surface of the phosphor particles to form hydrolyzed trimethylaluminum or other alkylaluminum. The hydrolyzed trimethylaluminum or other alkylaluminum phosphors are particularly useful in electroluminescent lamps.

BACKGROUND OF THE INVENTION 
The present invention describes a process for coating finely divided 
material with hydrolyzed aluminum. More specifically electroluminescent 
phosphors coated with hydrolyzed trimethyl aluminum have been produced 
which are almost completely insensitive to atmospheric moisture. 
Zinc sulfide-based phosphors, typically doped with copper, may be 
stimulated to emit visible light by the absorption of electrical energy in 
a so-called electroluminescent lamp where a layer of the phosphor is 
sandwiched between a front transparent electrode and a back 
non-transparent electrode with a layer of insulating material (typically 
barium titanate) sandwiched between the phosphor layer and the back 
electrode. However, the luminescent efficiency of the phosphor degrades 
much more rapidly if the phosphor is exposed to a moisture-containing 
atmosphere than if it is exposed to a very dry atmosphere. The invention 
which is the subject of this disclosure was motivated by a desire to 
reduce the moisture sensitivity of such zinc sulfide-based phosphors. The 
selected approach was to form a thin yet continuous coating of hydrolyzed 
trimethyl aluminum (TMA) upon the surfaces of the zinc sulfide particles, 
thereby protecting them from the effects of atmospheric moisture. 
The hydrolyzed TMA coatings are formed via chemical vapor deposition with 
the phosphor particles suspended within a gas-fluidized bed. Hydrolyzed 
TMA, presumably consisting mainly of relatively amorphous aluminum 
hydroxide, was selected as a coating material principally because it can 
be formed at relatively low temperatures by the reaction of gaseous TMA 
with gaseous water molecules without the use of oxygen or any other 
coreactant. In this way, the coatings can be formed under conditions that 
are least likely to modify the surface chemical composition of the 
relatively reactive zinc sulfide based phosphor. 
SUMMARY OF THE INVENTION 
The present invention describes a process for forming a coating of 
hydrolyzed trimethyl aluminum on the outer surfaces of phosphor particles. 
Trimethyl aluminum is vaporized in an inert carrier gas and water is 
likewise vaporized in an inert carrier gas. The two carrier gas streams 
are passed through a fluidized bed of phosphor particles wherein the 
trimethyl aluminum reacts with the water on the phosphor particle surfaces 
to form a coating of hydrolyzed trimethyl aluminum. The coated phosphor 
shows an extreme insensitivity to atmospheric moisture. 
In another aspect of the invention phosphor powder particles are coated 
with hydrolyzed trimethyl aluminum and used in electroluminescent lamps. 
The coated phosphor is sandwiched between a transparent electrode and a 
second electrode. The manufactured electroluminescent lamp is resistant to 
atmospheric moisture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The hydrolyzed TMA coatings of the present invention where applied via a 
chemical vapor deposition (CVD) technique. FIG. 1 shows a schematic of the 
system used to carry out the CVD coatings. 
Trimethyl aluminum (Al(CH.sub.3).sub.3) was used as the organo-metallic 
coating precursor. Prior to entering the fluidized bed 16, part or all of 
the inert fluidizing gas or carrier gas was passed through a bubbler 12 
filled with liquid TMA heated to a temperature high enough to produce a 
TMA equilibrium vapor pressure sufficient to yield the desired coating 
rate (typically between 25.degree. and 35.degree. C). The carrier gas flow 
was controlled by valves 54 and 55. The inert fluidizing gas was nitrogen. 
However, argon can also be used. The portion of the fluidizing gas line 13 
located between the bubbler outlet and the fluidized bed inlet 40 was 
heated by a heating tape 30 to a temperature a few degrees above the TMA 
bubbler temperature in order to prevent condensation of the TMA within the 
gas lines. The fluidized bed itself was heated to a temperature of between 
150.degree. and 250.degree. C. by furnace 20 during the coating process. 
Once within the fluidized bed, the TMA is hydrolyzed by reaction with 
gaseous water vapor which is transported into the reactor via a second 
stream of inert gas such as nitrogen or argon through line 23. This inert 
gas stream is passed through a water-filled bubbler 22 before entering the 
fluidized bed via a hollow tube which terminates at a point 19 above but 
not too far from the fluidized bed gas distributor 33. The water bubbler 
temperature and the flow rate of the water-containing inert gas stream are 
adjusted so as to deliver a quantity of water to the fluidized-bed reactor 
16 sufficient to react completely with all of the TMA molecules entering 
the reactor via the fluidizing gas stream. It is best to have a relatively 
large excess of water present within the reactor so as to ensure that all 
of the TMA molecules will be completely hydrolyzed. A five to fifteen fold 
excess is desirable although the reaction will work with just a three fold 
excess. The reaction is allowed to proceed until the desired quantity of 
hydrolyzed TMA coating has been produced. 
Once the desired thickness of the hydrolyzed TMA coating has formed on the 
phosphor particles, the reaction is stopped, the fluidized bed is allowed 
to cool in an inert gas stream and the coated phosphor particles are 
removed from the tubular reactor. 
When the hydrolyzed TMA-coated ZnS phosphors are examined by X-ray 
photoelectron spectroscopy, the virtual absence of emitted electrons 
originating from Zn, S, or Cu atoms indicates that the hydrolyzed TMA 
coatings are continuous, i.e. completely covering the surface of each 
phosphor particle. 
Sylvania Type 723 ZnS:Cu EL phosphor was used in the Examples 1-6. The 
uncoated and hydrolyzed TMA coated phosphors were thoroughly dispersed in 
a polymeric organic binder (a mixture of cyanoethyl cellulose and 
cyanoethyl sucrose). Subsequently, thin layers of the uncoated and coated 
phosphor dispersions were formed upon the surfaces of transparent sheets 
of glass or Mylar that had first been coated with thin transparent 
indium-tin oxide (ITO) which serves as the front transparent electrode for 
the EL lamp. A layer of insulating barium titanate, dispersed in the same 
polymeric organic binder, was subsequently formed on top of each phosphor 
layer. Finally, layers of conductive carbon to serve as the rear lamp 
electrode, were formed upon the surfaces of the barium titanate layers. 
Copper mesh current collectors were attached to each electrode. In some 
cases, the finished lamps were sealed between two sheets of Aclar (a 
special fluorohalocarbon-based, water impermeable transparent plastic 
manufactured by Allied Chemical Corp.) using conventional laminating heat 
sealing equipment. 
All weight percents of the hydrolyzed TMA coatings are based on the 
measured aluminum content which is converted to an equivalent weight 
percent of aluminum hydroxide (Al(OH).sub.3). In the discussion that 
follows, the coating is not pure aluminum hydroxide. 
EXAMPLE 1 
Four 300 gm quantities of the copper-doped zinc sulfide electroluminescent 
phosphor (lot 361S of Type 723 phosphor available from Sylvania) were 
coated using the reaction conditions listed in Table 1. Samples 320-90, 
327-90, 403-90 and 416-90, were coated for 1 1/3 hours, 2 2/3 hours, 5 1/3 
hours, and 5 1/3 hours, respectively. Sample 320-90 was prepared using a 
250.degree. C. reaction temperature, while a 200.degree. C. temperature 
was used to prepare the samples 327-90 and 403-90. Sample 416-90 was 
prepared using a 123.degree. C. coating reaction temperature. The H.sub.2 
O bubbler and TMA bubbler temperatures are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Reaction Condition Summary 
H.sub.2 O 
Sample 
Phosphor Lot/ 
Reaction 
Equivalent 
Bubbler 
N.sub.2 Flow Rate 
TMA Bubbler 
N.sub.2 Flow 
Reaction 
No. Sample Weight 
Temp. 
Al(OH).sub.3 w/o 
Temp. 
Thru H.sub.2 O Bubbler 
Temp. Thru TMA 
Timeler 
__________________________________________________________________________ 
320-90 
361S/300 gm 
250.degree. C. 
1.01 69.degree. C. 
1 1/min 30.degree. C. 
0.5 1/min 
11/3 hr 
327-90 
361S/300 gm 
200.degree. C. 
1.89 68.degree. C. 
1 1/min 30.degree. C. 
0.5 1/min 
22/3 hr 
403-90 
361S/300 gm 
200.degree. C. 
2.96 67.degree. C. 
1 1/min 30.degree. C. 
0.5 1/min 
51/3 hr 
416-90 
398S/300 gm 
123.degree. C. 
3.53 57.degree. C. 
1 1/min 30.degree. C. 
0.25 1/min* 
51/3 hr 
508-90 
361S/300 gm 
200.degree. C. 
6.1 70.degree. C. 
1 1/min 30.degree. C. 
0.5 1/min 
51/3 hr 
514-90 
398S/300 gm 
200.degree. C. 
5.8 70.degree. C. 
1 1/min 30.degree. C. 
0.5 1/min 
51/3 hr 
521-90 
361S/300 gm 
200.degree. C. 
5.4 70.degree. C. 
1 1/min 30.degree. C. 
0.5 1/min 
51/3 hr 
612-90 
361S/300 gm 
200.degree. C. 
70.degree. C. 
1 1/min 30.degree. C. 
0.5 1/min 
51/3 hr 
__________________________________________________________________________ 
*Diluted with an additional 0.25 1/min flow of purified nitrogen 
Electroluminescent lamps were subsequently fabricated with ITO-coated plate 
glass serving as the transparent support/transparent front electrode. 
Additional lamps were similarly fabricated using the virgin (uncoated) 
phosphor, rather than one of the coated phosphor samples. All of these 
lamps, without being packaged or in any way protected from ambient 
laboratory temperature and humidity, were continuously driven using a 120 
V a.c. power supply operated at 400 Hz. At least two lamps made from each 
coated phosphor were tested against a lamp made from the uncoated 
reference material. The lamp brightnesses were recorded as a function of 
time using a calibrated photodiode detector. The maximum brightnesses 
measured with the uncoated and coated-phosphor lamps, the ratio of the 
maximum brightnesses (coated versus uncoated), and the ratio of the 
half-life values obtained with the uncoated and coated phosphor-containing 
lamps are all listed in Table 2 for each of the coated-phosphor samples 
(320-90, 327-90, 403-90 and 416-90). The half-life of a lamp is the time 
required for the measured brightness to drop to exactly half of its 
initial maximum value. 
As shown in Table 2, the maximum brightnesses measured with the lamps 
containing the four coated materials were all within 10% of that measured 
with the corresponding uncoated-phosphor control lamps. Also, the 
half-lives of the coated phosphor-containing lamps were all more than an 
order of magnitude greater than were those of the corresponding lamps 
fabricated using the uncoated phosphor. Moreover, the greater the mass of 
the coating i.e., the greater the coating thickness, the larger was the 
half-life relative to that of the corresponding control lamp. 
The measured brightness versus time data obtained with the lamps containing 
coated phosphor number 320-90, along with the data obtained with the 
corresponding uncoated-phosphor control lamp are plotted in FIG. 2. The 
corresponding data obtained with the lamps fabricated from sample number 
327-90 and its corresponding control lamp are similarly compared in FIG. 
3. The data for sample 403-90 are likewise compared in FIG. 4. FIG. 5 
shows the data for lamps fabricated using sample 416-90. 
As shown in FIGS. 2-5, the initial brightnesses of the lamps are nearly 
identical. However, while the brightness of the lamps fabricated with the 
uncoated phosphor decreased to half of their initial brightness in less 
than four hours of operation, the lamps fabricated with the coated 
phosphor from 416-90 was operating above half brightness after more than 
200 hours of continuous operation. The maximum brightnesses measured with 
the uncoated and coated-phosphor lamps, the ratio of the maximum 
brightnesses (coated versus uncoated) and the ratio of the half-life 
values obtained with the uncoated and coated phosphor-containing lamps are 
all listed in Table 2, along with the comparable data for samples 320-90, 
327-90, and 403-90. 
TABLE 2 
__________________________________________________________________________ 
Test Result Summary for Unpackaged EL Lamps Supported 
Upon ITO-Coated Glass 
Equivalent Maximum Brightness (fl) 
B(coated 
t.sub.1/2 (coated) 
Sample No. 
w/oAl(OH).sub.3 
Uncoated 
Coated 
B(uncoated) 
t.sub.1/2 (uncoated) 
__________________________________________________________________________ 
320-90 
1.01 40.0 41.2 1.03 15 
327-90 
1.89 42.4 42.4 1.00 32 
403-90 
2.96 45.4 40.8 0.90 40 
416-90 
3.53 39.3 38.2 0.97 70 
__________________________________________________________________________ 
As shown, the half-life of a coated-phosphor lamp, relative to that of an 
uncoated-phosphor lamp, increases with increasing coating weight i.e., 
with increasing coating thickness. Sample 416-90 was also examined using 
the XPS surface analysis technique so as to gauge the extent to which the 
ZnS:Cu phosphor particles are completely encapsulated by the coating. 
Samples 320-90, 327-90, and 403-90, as well as the uncoated phosphor (Lot 
398), were similarly examined. The results of these five analyses are 
summarized in Table 3 (where the surface concentrations of Zn, S, Al, 0, 
and Cu detected by the XPS technique are expressed on an atomic percent 
basis). 
TABLE 3 
______________________________________ 
XPS Surface Analysis Results for Uncoated and Coated 
Type 723 ZnS:Cu EL Phosphor 
Sample Zn S Al O Cu 
______________________________________ 
Uncoated 33.35 39.13 n.d. 12.18 
0.34 
(Lot 398) 
320-90 0.27 n.d. 42.21 55.95 
n.d. 
327-90 0.20 n.d. 40.89 53.22 
n.d. 
403-90 n.d. n.d. 41.89 53.39 
n.d. 
416-90 0.04 n.d. 40.92 54.36 
n.d. 
______________________________________ 
As shown, no sulfur or copper was detected near the surface of any coated 
material. Further, only a very small amount of zinc could be detected with 
any of the coated materials. Thus, these materials appear to be virtually 
completely encapsulated in a coating composed of only two elements 
detectable by XPS analysis: aluminum and oxygen. 
EXAMPLE 2 
Using three coated phosphor materials, Sample 320-90, 327-90 and 403-90 
along with the virgin (uncoated) phosphor, additional electroluminescent 
lamps were fabricated with sheets of flexible ITO-coated Mylar serving as 
the transparent support/transparent front electrode. In contrast to 
Example 1, these lamps were all heat-sealed between two sheets of 7.5 mil 
thick water-impermeable Aclar. As before, each lamp was continuously 
operated using a 120 V a.c. power supply driven at 400 Hz. The resulting 
brightness and half-life values (obtained from the recorded brightness 
versus time data for each lamp) are all listed in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Test Result Summary for Aclar-Packaged EL Lamps Supported Upon ITO-Coated 
Mylar 
Equivalent w/oAl(OH).sub.3 
Maximum Brightness (fl) 
B(coated) 
t.sub.1/2 (coated) 
Sample No. 
(Approx) Uncoated 
Coated 
B(uncoated) 
t.sub.1/2 (uncoated) 
__________________________________________________________________________ 
320-90 
1 35.2 31.2 0.89 2.9 
327-90 
2 40.8 41.6 1.02 2.3 
403-90 
3.0 38.4 41.2 1.07 &gt;2 
__________________________________________________________________________ 
As shown in Table 4, the maximum brightnesses measured with the lamps 
containing the three coated materials were all within approximately 10% of 
those measured with the corresponding uncoated-phosphor control lamps. 
Further, the thicker the coating, the brighter the coated 
phosphor-containing lamps relative to the uncoated-phosphor control lamp. 
Most surprisingly, despite the fact that all of the lamps were well sealed 
between sheets of the water-impermeable polymeric packaging material, the 
half-lives of the lamps containing the coated phosphors were all at least 
twice that of the corresponding uncoated-phosphor control lamp. 
The averaged brightness versus time data obtained with the lamps containing 
coated phosphor number 320-90, along with the data obtained with the 
corresponding uncoated-phosphor control lamp, are plotted in FIG. 6. The 
corresponding data obtained with the lamps fabricated from sample number 
327-90 and its corresponding control lamp are similarly compared in FIG. 
7. The data for sample 403-90 are likewise compared in FIG. 8. In each 
case, the packaged coated phosphor-containing lamps ran considerably 
brighter than did the corresponding lamp containing the standard uncoated 
phosphor. Moreover, the longer the lamps were burned, the greater the 
relative difference between the brightnesses of the lamps containing the 
coated and uncoated phosphors. This trend is particularly noteworthy in 
the case of the lamps fabricated from sample 403-90 (FIG. 8). In this case 
the coated phosphor-containing lamps yielded brightnesses 50% greater than 
those obtained with the uncoated phosphor after only about 150 hours of 
continuous operation. These same lamps were about 75% brighter than the 
control lamp after burning for about 300 hours. 
EXAMPLE 3 
In addition to the Aclar-packaged flexible lamps fabricated using the 
uncoated phosphor and coated phosphor materials described in Example 2, 
completely unpackaged i.e., unprotected, lamps were fabricated using the 
uncoated phosphor and coated phosphor number 403-90. These unpackaged 
lamps were burned alongside and at the same time as were the corresponding 
Aclar-packaged lamps containing the uncoated and coated phosphor. The 
average brightness versus time data obtained with the packaged and 
unpackaged lamps fabricated from coated phosphor number 403-90 are 
compared in FIG. 9. The corresponding brightness versus time data obtained 
with the packaged and unpackaged lamps fabricated from the standard, 
uncoated phosphor are similarly compared in FIG. 10. 
As shown in FIG. 9, the completely unprotected lamps containing the coated 
phosphor yielded brightnesses that were always within 10% of those 
obtained with the Aclar-packaged lamps. In contrast, the brightnesses 
obtained with the unpackaged lamps fabricated from the uncoated phosphor 
decreased rapidly with time. As a result, the unpackaged uncoated 
phosphor-containing lamp brightnesses were, on average, only about 15% of 
those obtained with the Aclar-packaged lamp containing the same uncoated 
phosphor. These results are illustrated in FIG. 11 in which the 
unpackaged-to-packaged lamp brightness ratios obtained with both the 
uncoated and coated phosphors are compared. 
Thus, whereas the severe moisture sensitivity of the uncoated phosphor 
completely prevents its use in unpackaged electroluminescent lamps, the 
application of the hydrolyzed TMA coatings formed via the fluidized bed 
CVD process outlined in this invention render the phosphor practically 
moisture insensitive. As a result, at least for some EL lamp applications, 
it appears that it may be possible to completely eliminate the costly 
water-impermeable polymeric packaging materials that are absolutely 
necessary in all EL lamps marketed today. On the other hand, the use of 
highly moisture-insensitive ZnS phosphors prepared according to the 
present invention used in conjunction with such water-impermeable 
packaging materials perhaps also including thin layers of special 
desiccating substances to further prevent moisture from reaching the 
luminescent material, would result in EL lamps with lifetimes exceeding 
those of any similar device produced today. 
EXAMPLE 4 
Electroluminescent lamps were fabricated upon sheets of ITO-coated 
polyester (Sierracin Intrex-100) and subsequently heat-sealed between two 
sheets of either a relatively water-impermeable plastic (Aclar, a product 
of Allied Signal Corp.) or a relatively water-permeable polyester material 
(obtained from General Binding Corp.). Identical lamps were fabricated 
using the as-received phosphor and the phosphor coated following the 
reaction conditions outlined above in Table 1, Sample No. 508-90. The 
lamps were all evaluated using a 120 VAC power supply operated at 400 Hz. 
The relative humidity in the test environment was maintained in the 50-60% 
range, and the ambient temperature ranged between 20.degree. and 
23.degree. C. Lamp brightness was measured as a function of time using a 
calibrated optometer. 
The brightness versus time data obtained with the lamps fabricated with the 
uncoated and coated phosphor (508-90) packaged in Aclar are compared in 
FIG. 12. Whereas the initial brightness of the lamp containing the coated 
phosphor was somewhat lower than that of the lamp containing the uncoated 
phosphor, equal brightnesses were measured after about 24 hours of 
operation. Thereafter, the brightness of the lamp containing the coated 
phosphor exceeded that of the lamp containing the uncoated phosphor, the 
difference in brightness increasing with time of operation. 
The comparable brightness versus time data obtained with the lamps 
fabricated with the uncoated and coated phosphor (508-90) packaged between 
sheets of the polyester material are similarly compared in FIG. 13. 
Whereas the brightness of the coated phosphor lamp decreases very 
gradually over hundreds of hours of operation, the brightness of the 
uncoated phosphor lamp decreases very rapidly, dropping to about 25% of 
its initial value within the first 24 hours of operation. 
The brightness versus time data obtained with the uncoated-phosphor lamps 
packaged in Aclar and in the water-permeable polyester material are 
compared in FIG. 14. The brightness versus time data obtained with the 
coated-phosphor (508-90) lamps packaged in Aclar and in the polyester 
material are similarly compared in FIG. 15. Whereas the coated phosphor 
performs equally well in either packaging material, the standard uncoated 
phosphor exhibits acceptable performance only in an essentially 
hermetically packaged lamp using a packaging material having an extremely 
low water permeability. 
EXAMPLE 5 
This example compares the performance of two different lots of coated 
phosphor in identically constructed electro-luminescent lamps. Two 
different lots of Type 723 ZnS:Cu phosphor (lots 361S and 398S) were 
employed. As in Example 4, these two phosphor lots were coated using the 
conditions outlined in Table 1. The coated phosphors were assigned sample 
numbers 514-90 and 521-90, respectively. The EL lamps were fabricated upon 
sheets of the same ITO-coated polyester material. Lamps containing each of 
the two coated-phosphor lots were packaged between sheets of 
water-impermeable Aclar and between sheets of a relatively water-permeable 
polyester material. As in previous Examples, the lamps were all tested 
using a 120 VAC/400 Hz power supply, the lamp brightness being recorded as 
a function of operating time. 
The brightness versus time data obtained with the two coated phosphors in 
Aclar-packaged lamps are compared in FIG. 16. The data obtained with the 
two coated phosphors packaged in polyester are likewise compared in FIG. 
17. As shown, very similar lamp performance was measured with the two 
coated phosphors, independent of whether water-permeable or 
water-impermeable packaging was used. Thus, these data demonstrate both 
the moisture insensitivity of the coated phosphor and the reproducibility 
of the phosphor coating process itself. 
EXAMPLE 6 
A quantity of Type 723 ZnS:Cu phosphor (Lot RB361S) was fractionated by 
means of a 325 mesh sieve. The small-particle fraction was then coated, 
again following the coating conditions outlined in Table 1 for Samples 
508-90, 521-90 and 514-90. EL lamps were fabricated using the 
large-particle (&gt;325 mesh) uncoated phosphor, the small-particle (&lt;325 
mesh) uncoated phosphor, and the coated small-particle phosphors (612-90). 
Lamps containing each of the three phosphors were packaged in 
water-impermeable Aclar and in a water-permeable polyester material. All 
of the lamps were operated using a 120 VAC/400 Hz power supply. 
The brightness versus time data recorded with the lamps packaged in the 
water-permeable material are compared in FIG. 18. Whereas all three lamps 
had nearly identical initial brightnesses, the lamps containing the 
large-particle and small-particle uncoated phosphor suffered very rapid 
brightness reductions due to the extreme moisture sensitivity of the 
uncoated ZnS-based phosphor. In contrast, the lamp containing the coated 
small-particle phosphor exhibited only a very gradual brightness loss 
during the 200 hour measurement period, demonstrating the relative 
moisture insensitivity of the coated phosphor. 
The brightness versus time performance data obtained with the lamps 
packaged in the water-impermeable material are similarly compared in FIG. 
19. In contrast to the polyester-packaged lamp data, all of the lamps 
exhibited relatively gradual reductions in brightness over the 600 hour 
measurement period. However, the brightness of the lamp containing the 
uncoated small-particle phosphor decreased more rapidly than did that of 
either the lamp containing the uncoated large-particle phosphor or that 
containing the coated small-particle material. The poorer maintenance of 
the lamp containing the uncoated small-particle phosphor is consistent 
with earlier observations concerning the relationships between phosphor 
particle size and lamp performance. However, the fact that the coated 
small-particle phosphor exhibits a maintenance curve nearly identical to 
that of the uncoated large-particle phosphor is not anticipated by earlier 
results. 
In general, the smaller the average particle size of the phosphor, the more 
uniform is the appearance of the electro-luminescent lamp (by virtue of 
the greater particle density and improved thickness uniformity of the 
layer of phosphor particles which constitutes the basic lamp element). 
Thus, these data show that, by the use of coated small-particle ZnS-based 
phosphors produced via the particle-coating process described herein, an 
EL lamp manufacturer will be able to fabricate lamps of improved 
definition and uniformity with performance characteristics comparable to 
those typically obtained using the standard, larger-particle uncoated 
phosphor. 
The phosphor used in Examples 1-6 cited was copper activated zinc sulfide 
EL phosphor. This phosphor was specifically developed for use in 
alternating current (A.C.) electroluminescent devices such as those 
described in the Examples. It is probable that all sulfide based EL 
phosphors suitable for use in A.C. EL devices may be made moisture 
insensitive by application a hydrolyzed trimethyl aluminum coating. It is 
also considered likely that sulfide-based EL phosphors developed for use 
in direct current electroluminescent devices may be rendered moisture 
insensitive by the application of the coatings described in the present 
invention. The following examples demonstrate the applicability of the 
process to other EL phosphors. 
Examples 7, 8 and 9 described below demonstrate the effectiveness of the 
coating process on three additional ZnS-based electroluminescent materials 
currently available from Sylvania. Example 10 demonstrates a different 
kind of moisture insensitivity and Example 11 attempts to further 
characterize the hydrolyzed TMA coatings. The coating reaction conditions 
for Examples 7-11 are listed below. 
Phosphor Coating Conditions 
Phosphor Weight: 300 gm 
Carrier Gas: Purified N.sub.2 
Water Bubbler Temp./N.sub.2 Flow Rate: 70.degree. C./1.0 1/min. 
TMA Bubbler Temp./N.sub.2 Flow Rate: 30.degree. C./0.5 1 min. 
Coating Time/Temp.: 5 1/3 hr./200.degree. C. 
EXAMPLE 7 
Electroluminescent lamps were fabricated upon sheets of ITO-coated 
polyester and subsequently heat-sealed between two sheets of a relatively 
water-permeable polyester material (obtained from General Binding Corp.) 
The electro-luminescent phosphor used was Sylvania Type 523 (Lot ELB357), 
a ZnS-based material co-doped with both copper and manganese. Identical 
lamps were fabricated using the as-received phosphor and the phosphor 
coated following the reaction conditions outlined above (Sample No. 80390 
with a 6.51% equivalent (Al(OH).sub.3 loading). The lamps were evaluated 
using a 120 VAC power supply operated at 400 Hz. The relative humidity in 
the test environment ranged between 50 and 60%, and the ambient 
temperature ranged between 21 and 24.degree. C. Lamp brightnesses were 
measured as a function of time using a calibrated optometer. 
The brightness versus time data obtained with the lamps fabricated with the 
uncoated and coated phosphor are compared in FIG. 20. As shown, a somewhat 
higher initial brightness was measured with the lamp containing the 
uncoated phosphor. However, after several hours of operation, the 
brightness of the lamp containing the coated phosphor exceeded that of the 
lamp containing the uncoated phosphor, the difference in brightness 
increasing with time of operation. These data demonstrate the relative 
moisture insensitivity of the coated manganese-doped phosphor and, 
conversely, the extreme moisture sensitivity of the standard, uncoated 
phosphor. 
EXAMPLE 8 
This example demonstrates the effectiveness of the coating when applied to 
another copper-doped ZnS-based electroluminescent material, Type 728 
ZnS:Cu, manufactured and marketed by Sylvania. Electroluminescent lamps 
were fabricated upon sheets of ITO-coated polyester and subsequently 
heat-sealed between two sheets of a relatively water-permeable polyester 
material (obtained from General Binding Corp.). Two ZnS:Cu 
electroluminescent phosphors were used: Sylvania Type 723 (Lot ELB398) and 
Sylvania Type 728 (Lot ELB418). Identical lamps were fabricated using the 
as-received phosphors and the phosphors coated following the reaction 
conditions outlined above. The coated version of Type 723, Sample 72090, 
contained an equivalent Al(OH).sub.3 loading of 5.70% while the coated 
version of Type 728, Sample No. 80290, contained a 6.87% equivalent 
Al(OH).sub.3 loading. All of the lamps were evaluated using a 120 VAC 
power supply operated at 400 Hz. Lamp brightnesses were measured as a 
function of time using a calibrated optometer. 
The brightness versus time data obtained with the lamps fabricated with the 
uncoated and coated phosphors are compared in FIG. 21. As shown, somewhat 
higher initial brightnesses were measured with the lamps containing the 
uncoated phosphors. However, after several hours of operation, the 
brightnesses of the lamps containing both coated phosphors exceeded those 
of the lamps containing both uncoated phosphors. The difference in 
brightness between the coated and uncoated phosphor-containing lamps 
increases with time of operation. Thus, both copper-doped phosphors Type 
723 and Type 728 are rendered moisture-insensitive via the application of 
a hydrolyzed TMA coating. Further, the brightnesses of non-hermetically 
packaged electroluminescent lamps containing Sylvania Type 728 ZnS:Cu 
phosphor coated with hydrolyzed TMA as described herein are substantially 
greater than are obtained with identically constructed and packaged lamps 
containing Sylvania Type 723 ZnS:Cu phosphor similarly coated with 
hydrolyzed TMA. 
EXAMPLE 9 
This example demonstrates the effectiveness of the coating when applied to 
another copper-doped ZnS-based electroluminescent material, Type 729 
ZnS:Cu, manufactured and marketed by Sylvania. Electroluminescent lamps 
were fabricated upon sheets of ITO-coated polyester using two ZnS:Cu 
electroluminescent phosphors: Sylvania Type 728 (Lot ELB418) and Sylvania 
Type 729 (Lot ELB396). Identical lamps were fabricated using the 
as-received phosphors and the phosphors coated following the reaction 
conditions outlined above. The coated versions of the Type 728 and Type 
729 phosphors, Samples 82290 and 82390, respectively, each contained an 
equivalent Al(OH).sub.3 loading of approximately 6.5%. The lamps 
containing the as-received uncoated phosphors were heat-sealed between two 
sheets of a relatively water-impermeable plastic (Aclar, a product of 
Allied Signal Corp.) The lamps containing the coated phosphors, on the 
other hand, were packaged between sheets of a relatively water-permeable 
polyester material (obtained from General Binding Corp.) All of the lamps 
were evaluated using a 120 VAC power supply operated at 400 Hz. Lamp 
brightnesses were measured as a function of time using a calibrated 
optometer. 
The brightness versus time data obtained with the lamps fabricated with the 
uncoated and coated phosphors are compared in FIG. 22. As shown, somewhat 
higher initial brightnesses were measured with the Aclar-packaged lamps 
containing the uncoated phosphors. However, after several hours of 
operation, the brightnesses of the polyester-packaged lamps containing the 
coated phosphors exceeded those of the lamps containing the uncoated 
phosphors. These data clearly demonstrate the remarkable moisture 
insensitivity of both phosphor types after the application of the 
hydrolyzed TMA coating. 
EXAMPLE 10 
This example demonstrates the EL-lamp performance of a ZnS-based 
electroluminescent material is unaffected by water washing either before 
or after the application of a hydrolyzed TMA coating formed as described 
above. Electro-luminescent lamps were fabricated upon sheets of ITO-coated 
polyester using Sylvania Type 729 ZnS:Cu EL phosphor (Lot ELB396). 
Identical lamps were fabricated using the uncoated phosphor, either as 
received or after a water wash followed by vacuum drying, and the phosphor 
coated following the reaction conditions outlined above, either as-coated 
or after a water wash followed by vacuum drying. The coated version of the 
phosphor, Sample 82390, contained an equivalent Al(OH).sub.3 loading of 
approximately 6.5%. The lamps containing the uncoated phosphor were 
heat-sealed between two sheets of a relatively water-impermeable plastic 
(Aclar, a product of Allied Signal Corp.). The lamps containing the coated 
phosphor, on the other hand, were packaged between sheets of a relatively 
water-permeable polyester material (obtained from General Binding Corp.). 
All of the lamps were evaluated using a 120 VAC power supply operated at 
400 Hz. Lamp brightnesses were measured as a function of time using a 
calibrated optometer. 
The brightness versus time data obtained with the untreated uncoated 
phosphor in Aclar-packaged lamps and with the untreated coated phosphor in 
polyester-packaged lamps are compared in FIG. 23. The data obtained with 
the water-washed uncoated phosphor in Aclar packaged lamps and with the 
water-washed coated phosphor in polyester-packaged lamps are similarly 
compared in FIG. 24. These data demonstrate not only that the coated 
phosphor is insensitive to the effects of moisture in an operating 
electroluminescent lamp, but that the effectiveness of the coating is 
undiminished by immersion in water (followed by drying) prior to lamp 
fabrication. 
EXAMPLE 11 
The purpose of this example is to illustrate the fact that the properties 
of the hydrolyzed TMA coating formed according to the teachings of this 
invention are substantially different from those that would be expected by 
one skilled in the chemical art. The reaction between TMA and water at 
temperatures no higher than a few hundred degrees Centigrade should result 
in the formation of aluminum hydroxide and methane: 
EQU Al(CH.sub.3).sub.3 +3CH.sub.4 +Al(OH).sub.3 
The Al(OH).sub.3 produced by this low temperature reaction is expected to 
be substantially amorphous. Electron diffraction analyses of a number of 
ZnS-based phosphors coated via the TMA hydrolysis reaction carried out in 
a gas-fluidized bed have revealed no evidence of crystallinity, in 
agreement with this expectation. However, XPS surface analyses of several 
of the coated phosphor materials have yielded O/Al atomic ratios of 
approximately 1.3, far from the 3.0 value expected for Al(OH).sub.3 but 
close to the 1.5 value expected for Al.sub.2 O.sub.3 as shown in Table 3. 
Thermogravimetric analyses have also been performed with a number of coated 
phosphor samples. Typical TGA data obtained with a sample of uncoated Type 
723 phosphor heated to above 800.degree. C. at a 10.degree. C./min. ramp 
rate are shown in FIG. 25. As expected, the percent weight change versus 
temperature plot is essentially flat, indicating that the sample did not 
gain or lose weight significantly during the analysis. Comparable data 
obtained with a sample of Type 723 phosphor coated via the TMA hydrolysis 
reaction (sample 40390 with a coating containing an amount of aluminum 
equivalent to 3.0% Al(OH).sub.3) are similarly plotted in FIG. 26. In this 
case, an approximate 0.1% weight loss is detected occurring at 
temperatures between 350.degree. and 600.degree. C. Indeed, this is the 
temperature range over which amorphous Al(OH).sub.3 would be expected to 
transform to Al.sub.2 O.sub.3 when so heated. However, for a material 
containing 3.0% Al(OH).sub.3, the complete conversion of the amorphous 
hydroxide coating to an oxide phase by the elimination of water vapor 
would result in an approximate 1.0% weight loss, an order of magnitude 
greater than is observed. Similar results have been obtained in a number 
of other TGA experiments with ZnS-based materials coated via the TMA 
hydrolysis reaction. Thus, the TGA data suggest that the coating behaves 
more like an oxide than a hydroxide, contrary to our expectations. 
Finally, there is the fact that the effectiveness of the coating formed at 
200.degree. C. is completely unaffected by water washing as described in 
Example 10 above. If the coating were an oxide formed at much higher 
temperatures, perhaps via the high temperature heat treatment of an 
amorphous hydroxide, this result would not be surprising. However, it is 
surprising indeed that a relatively thin amorphous aluminum hydroxide 
coating formed at 200.degree. C. should be apparently unaffected by this 
procedure. 
Thus, whereas one skilled in chemistry would expect that a coating formed 
via the reaction of TMA with water vapor at a temperature in the vicinity 
of 200.degree. C. would consist essentially of amorphous aluminum 
hydroxide, all of the evidence accumulated to date conflicts with that 
expectation. Rather, the accumulated evidence suggests that the coating so 
formed upon the surfaces of ZnS-based phosphor particles suspended within 
a gas-fluidized bed (as described in this disclosure) consists 
substantially of some unidentified compound of aluminum and oxygen. 
It is considered likely that coatings formed via a gas-phase reaction 
between TMA and H.sub.2 O might also be effective in protecting so-called 
thin-film EL devices from moisture attack. Such thin-film EL devices 
typically contain layers of conducting, dielectric, and luminescent 
materials that may be formed via gas-phase reactions not at all unlike the 
TMA/H.sub.2 O reaction used to form the moisture-protective barriers 
described above. Thus, at least in principle, it would be relatively 
simple to incorporate such TMA/H.sub.2 O reactions into existing 
manufacturing processes to form one or more thin moisture-protective 
barrier layers that become integral parts of such EL devices. The 
TMA/H.sub.2 O reaction carried out at a temperature between 100.degree. C. 
and 300.degree. C. might be used to coat the thin polycrystalline phosphor 
film before applying the final insulating and conductive electrode layers. 
It may even be possible to entirely eliminate the insulating layer from 
such devices since, to the best of our knowledge, the coatings formed via 
the TMA/H.sub.2 O reaction are themselves electrically insulating. 
While there has been shown and described what are at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the scope of the invention as defined by 
the appended claims, specifically the use of other alkylaluminums such as 
triethylaluminum or the use of a stirred-bed rather than a fluidized-bed 
reactor.