Manganese activated zinc silicate phosphor

A new and improved manganese activated zinc silicate phosphor is described. The phosphor has cations consisting essentially of zinc, silicon, manganese, and tungsten. The phosphor has an absolute reflectance less than or equal to 13.5% at 275 nm, an absolute reflectance equal to or greater than 80% at 350 nm, and a surface area from about 0.3 m.sup.2 /gm to about 0.4 m.sup.2 gm.

FIELD OF THE INVENTION 
This invention relates to a phosphor. More particularly, this invention 
relates to a manganese activated zinc silicate phosphor. 
BACKGROUND OF THE INVENTION 
This invention is concerned with a manganese-activated zinc orthosilicate 
phosphor, Zn.sub.2 SiO.sub.4 :Mn. The phosphor emits in the green region 
of the visible spectrum and is used, for example, in fluorescent lamps and 
cathode ray tubes. 
Some of the problems associated with Zn.sub.2 SiO.sub.4 :Mn phosphors are 
their relatively poor fluorescent lamp performance and maintenance. Poor 
maintenance means that the light output, or lumens per watt, of the 
phosphor decreases to a greater extent during lamp life than is desirable. 
This invention is concerned with a manganese-activated zinc silicate 
phosphor having an improved reflectivity resulting in an improved 
fluorescence efficiency. 
The importance of high-performance, green-emitting phosphors with low 
depreciation characteristics in fluorescent lamps, has increased in recent 
years with the growing demand for high CRI performance lamps. For that 
reason, in particular, an improved manganese-activated zinc orthosilicate 
phosphor would represent a significant advancement in the art. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, a new and improved 
manganese activated zinc silicate phosphor consists essentially of cations 
of zinc, silicon, manganese, and tungsten. The phosphor has an absolute 
reflectance less than or equal to 13.5% at 275 nm, an absolute reflectance 
equal to or greater than 80% at 350 nm. 
In accordance with another aspect of the present invention a new and 
improved manganese activated zinc silicate phosphor having cations 
consisting essentially of zinc, silicon, manganese, and tungsten is 
prepared by a method which comprises the following steps: 
Step 1--The phosphor powder is heated in a furnace to a temperature of 
about 1000.degree. C. to about 1225.degree. C. in air. 
Step 2--The phosphor powder from Step 1 is cooled. 
Step 3--The phosphor powder from Step 2 is wet milled in an acid solution. 
Step 4--The phosphor powder from Step 3 is separated from the acid 
solution. 
Step 5--The phosphor powder from Step 4 is washed with water. 
Step 6--The phosphor powder from Step 5 is dried to form a manganese 
activated zinc silicate phosphor powder having individual particles, a 350 
nm reflectance equal to or greater than 80%, a 275 nm reflectance equal to 
or less than 13.5%, and a surface area from about 0.3m.sup.2 /gm to about 
0.4m.sup.2 /gm. 
Step 7--The individual particles of the manganese activated zinc silicate 
phosphor powder are coated with a continuous coating of alumina to form a 
manganese activated zinc silicate phosphor powder having a continuous 
coating of alumina on the individual particles. 
Step 8--The manganese activated zinc silicate phosphor powder having a 
continuous coating of alumina on said individual particles is annealed at 
a temperature of about 700.degree. C. to about 850.degree. C. for a period 
of about 15 min. to about 20 hours to form an annealed coated phosphor. 
In accordance with another aspect of the present invention a new and 
improved manganese activated zinc silicate phosphor having cations 
consisting essentially of zinc, silicon, manganese, and tungsten is 
prepared by a method which comprises the following steps: 
Step 1--The phosphor powder is heated in a furnace to a temperature of 
about 1000.degree. C to about 1225.degree. C. in air. 
Step 2--The phosphor powder from Step 1 is cooled. 
Step 3--The phosphor powder from Step 2 is wet milled in an acid solution. 
Step 4--The phosphor powder from Step 3 is separated from the acid 
solution. 
Step 5--The phosphor powder from Step 4 is washed with water. 
Step 6--The phosphor powder from Step 5 is dried. 
Step 7--NH.sub.4 Cl is added to the phosphor powder from Step 6 to form a 
mixture of the NH.sub.4 Cl and the phosphor powder. 
Step 8--The mixture of the NH.sub.4 Cl and the phosphor powder from Step 7 
is heated in a furnace to a temperature of about 1000.degree. C. in air. 
Step 9--The phosphor powder from Step 8 is cooled. 
Step 10--The phosphor powder from Step 9 is wet milled in an acid solution. 
Step 11--The phosphor powder from Step 10 is separated from the acid 
solution. 
Step 12--The phosphor powder from Step 11 is washed with water. 
Step 13--The phosphor powder from Step 12 is dried to form a manganese 
activated zinc silicate phosphor powder having individual particles, a 350 
nm reflectance equal to or greater than 80%, a 275 nm reflectance equal to 
or less than 13.5%, and a surface area from about 0.3m.sup.2 /gm 
0.4m.sup.2 /gm. 
Step 14--The individual particles of the manganese activated zinc silicate 
phosphor powder are coated with a continuous coating of alumina to form a 
manganese activated zinc silicate phosphor powder having a continuous 
coating of alumina on the individual particles. 
Step 15--The manganese activated zinc silicate phosphor powder having a 
continuous coating of alumina on said individual particles is annealed at 
a temperature of about 700.degree. C. to about 850.degree. C. for a period 
of about 15 min. to about 20 hours to form an annealed coated phosphor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A new and improved virgin manganese activated zinc silicate phosphor is 
described. By virtue of the physical and chemical characteristics of the 
new and improved phosphor, it is capable of withstanding the rigors of the 
application of a chemical vapor deposited (CVD) coating of alumina, a 
thermal annealing process necessary to bond the coating to the phosphor 
particles, and the processing into a 40W-T12 lamp and maintaining a 
desirable zero hour lumen output (brightness). The method of preparing 
this new and improved phosphor is not important as long as the product has 
the following attributes: 
(1) no substantial cations other than Zn, Si, Mn, and W; 
(2) an absolute reflectance at 275 nm of equal to or less than 13%; 
(3) an absolute reflectance at 350 nm of equal to or more than 80%; and 
(4) a surface area of 0.3 to 0.4 m.sup.2 /gm as determined by a single 
point BET measurement on a Quantachrome Monosorb surface area instrument. 
The virgin manganese activated zinc silicate phosphor can be defined and 
identified by its cation composition. The cation composition consists 
essentially of: zinc, silicon, manganese, and tungsten. The term "cation" 
as used to describe elements present in the phosphor composition means the 
elements present in the phosphor composition other than oxygen. 
One aspect of the invention provides a simple measurement that determines 
the optical reflection of the virgin (as synthesized) phosphor at 275 nm 
and 350 nm and permits the estimation of the minimum performance level of 
certain alumina coated and annealed manganese activated zinc silicate 
phosphors in 40W-T12 lamps. Moreover, when it is used in conjunction with 
a performance standard such as the zero hour lumen level of the 40W-T12 
lamp, the reflectance measurement permits phosphors that would fall below 
the desired zero hour lumen level to be identified and eliminated before 
the added costs of alumina coating, annealing, and lamp processing are 
incurred. The success of this technique is surprising because it is 
contrary to the commonly accepted view which holds that measurements 
performed on a phosphor outside the lamp envelope are of little value in 
predicting it lamp performance. 
The reflectance measurements were obtained with a Spex Industries Model 
1902 spectrofluorimeter. The emission and excitation monochromators were 
scanned in tandem through the spectral range of interest. The excitation 
source was a 450 Watt Xenon arc lamp. The phosphor powder samples to be 
measured were pressed into anodized aluminum plugs and mounted in the 
sample compartment of the spectro-fluorimeter. The incident beam of the 
spectrofluorimeter impinged the sample at -22.5.degree. from normal and 
the scattered light was collected at 22.5.degree. such that the incident 
and reflected rays subtend a 45.degree. angle bisected by the sample face 
normal. A Kodak reflectance standard powder (#6091) was prepared as 
described above for the powder sample. A "standard" raw reflectance 
spectrum was acquired and stored under computer control. Then the sample 
was measured in an identical fashion. The spectrum of the sample is 
divided by the spectrum of the Kodak standard (taken to be unity) to 
derive a corrected reflectance values shown in FIG. 1 and 2. 
Another aspect of the invention provides a method of phosphor treatment 
that can be applied to sub-standard virgin manganese activated zinc 
silicate phosphors to improve their performance after they have been 
alumina coated, annealed, and processed into lamps. Still further aspects 
and details of the invention are described in the following paragraphs. 
Curve (a) in FIG. 1, shows the optical reflectance in the 250-600 nm region 
of the spectrum of a virgin manganese activated zinc silicate phosphor 
containing no tungsten. The small dips in reflectance in the 330-420 nm 
range are due to Mn.sup.2+ absorptions, while the stronger drop below 330 
nm can be attributed to host lattice absorption, modified by the presence 
of the Mn activator. For virgin zinc silicate phosphors activated with 
approximately 3.3 wt % Mn and containing no tungsten, that are 
substantially free cations other than Zn, Si, and Mn the reflectance at 
275 nm can be used as a guideline to estimate the zero lumen level of the 
alumina coated and annealed manganese activated zinc silicate phosphors 
from which they were derived. Once the 275 nm reflectance of a virgin 
phosphor has been determined, the relationship depicted in FIG. 2 can be 
used to estimate the zero hour lumen level of the phosphor in the alumina 
coated and annealed condition. In FIG. 1 curve (b), we present the 
reflectance of a virgin manganese activated zinc silicate consisting 
essentially of cations of zinc, silicon, manganese, and tungsten. As 
observed in FIG. 1 curve (b), a new absorption in the 300-400 nm range is 
associated with the presence of tungsten in phosphors of this type. This 
absorption has been found to reduce the actual zero hour lumen level of 
the alumina coated and annealed phosphors by as much as 300 lumens below 
that predicted by the 275 nm reflectance measurement because of the body 
color induced during the annealing step. 
Generally speaking, maximum zero hour lumen level (brightness) can be 
obtained from alumina coated and annealed virgin manganese activated zinc 
silicate phosphors, when the 275nm reflectance of the virgin phosphor is 
as low as possible (preferable equal to or less than 13.5%), the BET 
surface area is low (preferably from 0.3 to 0.4 m.sup.2 /gm) and the 350 
nm reflectance is as high as possible (preferably equal to or greater than 
80%). However, virgin phosphors typically exhibit 275 nm reflectances as 
high as 17%, BET surface areas of 0.6 m.sup.2 /gm or higher, and when 
significant amounts of tungsten are present, 350 nm reflectances as low as 
67%. Accordingly, another aspect of the invention provides a method of 
adjusting the reflectances and BET surface area of virgin phosphors that 
do not meet the desired specifications. This tailoring process involves a 
complex combination of thermal treatments, chemical processes, and wet 
milling procedures that are described in more detail in Examples 1 and 2 
and in Tables I and II. 
In Table I, a virgin manganese activated zinc silicate phosphor was 
processed as described in Example 1 at various heating times except for 
Sample #1 which was not subjected to the heat treatment of the other 
Samples. Sample #1 was alumina coated, annealed, and processed into a lamp 
and is the control for comparing the effects of the heat treatment on the 
other parameters measured. 
TABLE I 
__________________________________________________________________________ 
Reflectance and Surface Area and Lumens 
As a Function of Heating Time 
Process 40W-T12 
Heating BET Lamp 
Sample 
Time 275 nm 
350 nm 
Surface Area 
0 Hr Wt % 
Wt % 
No. @ 1225.degree. C. 
Refl. % 
Refl. % 
m.sup.2 /gm 
Lumens 
W Mn 
__________________________________________________________________________ 
#1 0 hrs 16.0 73.0 0.54 4362 0.057 
3.3 
#2 4 hrs 13.5 89.4 0.44 4687 0.04 
2.7 
#3 6 hrs 13.2 83.8 0.42 4722 
#4 7 hrs 11.7 84.1 0.34 4810 
#5 8 hrs 11.1 80.9 0.35 4849 0.04 
2.81 
#6 16 hrs 
10.0 75.0 0.28 4782 0.03 
2.53 
#7.sub.(a) 
16 hrs 
10.7 82.9 0.28 4949 
2 hrs 
__________________________________________________________________________ 
.sub.(a) two step firing; 
1st: 1225.degree. C. 16 hrs. in air 
2nd: 1000.degree. C. 2 hrs. in air with 2 wt % NH.sub.4 Cl 
EXAMPLE 1 
Approximately 0.5 Kg. of a virgin manganese activated zinc silicate 
phosphor having cations consisting essentially of zinc, silicon, 
manganese, and tungsten was placed in a quartz boat and heated at 
20.degree. C./min to a temperature of 1225.degree. C., in an ambient of 
static air. The phosphor was held at that temperature for a predetermined 
period, ranging from 4 to 16 Hrs., and was then cooled at 20.degree. 
C./min to 900.degree. C. and removed from the furnace. 
After cooling to room temperature, the partially sintered phosphor cake was 
roll crushed to a powder and sieved through a 60 mesh screen. The phosphor 
was subsequently loaded into a 2 liter (4.5"dia..times.9"ht.) polyethylene 
mill jar containing 600 ml of citric acid solution (0.48w/o citric acid/gm 
phosphor) and 0.5 Kg. of Burundum (Tradename of U.S. Stonewear) grinding 
(0.5"dia..times.0.5"ht.) cylinders and rolled at 91 rpm for 30 min. Upon 
completion of this mill-wash process, the phosphor was separated from the 
citric acid solution and re-dispersed in 1.0 liter of distilled water. The 
phosphor was allowed to settle for 15 minutes and most of the liquid was 
decanted off, together with some of the fine phosphor particles that 
remained in suspension. The water dispersion and decanting process was 
then repeated a second time and was followed by a final dispersion in 
water and subsequent washing and suction drying on a buchner funnel. The 
phosphor received a final drying process in a 100.degree. C. oven for at 
least 3 hours, and was ready for use after sieving through a 400 mesh 
screen. 
A chemical vapor deposited (CVD) alumina coating was applied to the heat 
treated phosphor particles utilizing a fluid bed coating technique. As 
shown in Table II, Samples No. 2-7, a blend of approximately 300 gms to 
375 gms of the heat treated phosphor and approximately 0.05% by weight of 
a fluidization aid such as Aluminum Oxide C, available from Degussa, Inc., 
was loaded into a fluid bed column comprising a 40 millimeter ID quartz 
tube having a quartz frit fused to the bottom acting as a distributor 
plate. A 32 millimeter quartz agitator disc was positioned inside the 
quartz tube. The agitator disc was attached to a vibromixer agitator. 
Approximately 50 millimeters from the base of the agitator a series of 
five holes of approximately 20 mils in diameter (0.5 millimeter) are 
circumferentially located on the agitator shaft. The agitator disc itself 
was located approximately 25 millimeters above the quartz distributor. A 
series of approximately six copper coil windings of 1/4" tubing were 
located immediately around the frit located at the bottom of the quartz 
tube such that one coil was below the distributor and the remaining five 
coils were above the distributor plate. The total length of the coil 
assembly was approximately 55 millimeters of which approximately 45 
millimeters were above the distributor plate. In addition, there was a 
copper foil of approximately 70 millimeters which was sandwiched between 
the cooling coil and the quartz tube to provide improved heat transfer. In 
addition there was insulation located above the copper coil windings to 
further reduce heat transfer between the heated and the unheated portions 
of the tube. This insulation comprised approximately 50 millimeters of 1" 
wide by 1/2" thick Fiberfax roll insulation. The edge of the Fiberfax 
insulation matched exactly the level between the unheated and heated part 
of the 3-Zone Lindberg furnace, that is, between the bottom and center 
zones of the furnace. The furnace zones were 6", 12", and 6"in length, and 
a spike thermocouple was located at the midpoint of each zone. The 
operating conditions for the fluid bed CVD alumina coating of the phosphor 
are show in Table II. The height of the phosphor bed was from 300 mm to 
400 mm. The amount of phosphor charged into the column was from 300 gm to 
375 gm. The external thermocouple was located against the outside wall of 
the quartz column level with the 5 holes in the agitator shaft where the 
oxygen/inert gas mixture enters the fluidized phosphor bed. The 
temperatures recorded by the external thermocouple during the coating 
operation ranged from 342.degree. C. to 435.degree. C. as shown in Table 
II. The first two furnace zones were set for 500.degree. C. and the last 
zone was shut off. The flow rates shown in Table II ranging from 250 
cc/min to 350 cc/min of an inert gas such as nitrogen, argon, helium, 
neon, or mixtures thereof for the bubbler and from 300 cc/min to 400 
cc/min of the inert gas for the carrier were the flow rates that pass 
through the distributor plate at the bottom of the quartz column. A 
fluidized phosphor bed was formed by passing the inert gas upwardly 
through the phosphor particles in order to suspend the particles in the 
inert gas stream. In addition to supporting the phosphor particles in a 
fluidized bed, the inert gas functions as a carrier gas for the vaporized 
trimethyl aluminum. The inert gas was passed through the bubbler 
containing liquid trimethyl aluminum at approximately 30.degree. C. and 
the liquid trimethyl aluminum was vaporized into the inert gas before 
passing through the distributor plate into the fluidized phosphor bed. 
Oxygen as a inert gas/oxygen mixture was also introduced into the 
fluidized bed through holes circumferentially located on the shaft of the 
vibrating mixer above the vibrating disc. A continuous protective alumina 
coating was formed on the surface of the individual phosphor particles 
when the vaporized trimethyl aluminum was exposed to the oxygen at a 
temperature sufficient for a reaction between the oxygen and the vaporized 
trimethyl aluminum to occur. The coating times ranged from 5.67 hours to 
about 7.5 hours as indicated in Table II. Once the phosphor particles were 
coated with a continuous coating of alumina, they were transferred into a 
quartz boat and annealed at 768.degree. C. in static air and held at that 
temperature for 4 hours in a Rapid-Temp furnace. There is approximately a 
one hour linear ramp and cool down time programmed into the annealing 
heating cycle. The annealing temperature can be from about 700.degree. C. 
to about 850.degree. C. and the period of time held at temperature can be 
from about 15 minutes to about 20 hours. After the annealing step, at 
least one layer of the phosphor was coated in a 40W-T12 lamp using a 
conventional water base suspension system. The coated 40W-T12 lamp was 
then processed into a finished 40W-T12 lamp and zero hour lumen data was 
obtained, shown in Table I. 
In addition, at least one layer of a blend of phosphors, one of which being 
the phosphor of this invention, can be coated in a 40W-T12 lamp. 
The control, Sample No. 1, was processed through a large fluid bed column 
rather than the 40 millimeters column described above to deposit a 
continuous coating on the phosphor particles of the control Sample No. 1. 
As shown in Table II, a blend of approximately 1500 gms of the control 
phosphor, Sample #1, and approximately 0.05% by weight of a fluidization 
aid such as Aluminum Oxide C, available from Degussa, Inc., was loaded 
into a fluid bed column comprising a 80 millimeter ID quartz tube having a 
quartz frit fused to the bottom acting as a distributor plate. A 65 
millimeter stainless steel agitator disc was positioned inside the quartz 
tube. The agitator disc was attached to a vibromixer agitator. 
Approximately 50 millimeters from the base of the agitator a two micron 
stainless steel filter element was welded in line and functioned as the 
diffuser of the oxygen mixture. The agitator disc itself was located 
approximately 25 millimeters above the quartz distributor. A series of 
approximately eleven copper coil windings of 1/4" tubing were located 
immediately around the frit located at the bottom of the quartz tube such 
that one coil was below the distributor and the remaining ten coils were 
above the distributor plate. In addition, there was a copper foil of 
approximately 120 millimeters which was sandwiched between the cooling 
coil and the quartz tube to provide improved heat transfer. In addition 
there was insulation located above the copper coil windings to further 
reduce heat transfer between the heated and the unheated portions of the 
tube. This insulation comprised approximately 50 millimeters of 1" wide by 
1/2" thick Fiberfax roll insulation. The edge of the Fiberfax insulation 
matched exactly the level between the unheated and heated part of the 
3-Zone Lindberg furnace, that is, between the bottom and center zones of 
the furnace. The furnace zones were 6", 12", and 6" in length, and a spike 
thermocouple was located at the midpoint of each zone. The operating 
conditions for the fluid bed CVD alumina coating of the phosphor control, 
Sample No. 1, are shown in Table II. The height of the phosphor bed was 
approximately 500 mm. The amount of phosphor charged into the column was 
1500 gm. The external thermocouple was located against the outside wall of 
the quartz column level with the two micron diffuser where the 
oxygen/inert gas mixture enters the fluidized phosphor bed. The 
temperatures recorded by the external thermocouple during the coating 
operation ranged from 325.degree. C. to 379.degree. C. as shown in Table 
II. The first two furnace zones were set for 500.degree. C. and the last 
zone was shut off. The fluid bed temperature was recorded by a 
thermocouple located within the fluid bed half-way between the distributor 
plate and the top of the expanded bed. The mid-bed temperature was from 
385.degree. C. to 410.degree. C. during the 71/2 hour run. The gasses that 
pass through the distributor plate at the bottom of the quartz column have 
flow rates as shown in Table II. For the bubbler, 1200 cc/min of an inert 
gas such as nitrogen, argon, helium, neon, or mixtures thereof and for the 
carrier, 1000 cc/min of the inert gas. A fluidized phosphor bed was formed 
by passing the inert gas upwardly through the phosphor particles in order 
to suspend the particles in the inert gas stream. In addition to 
supporting the phosphor particles in a fluidized bed, the inert gas 
functions as a carrier gas for the vaporized trimethyl aluminum. The inert 
gas was passed through the bubbler containing liquid trimethyl aluminum at 
approximately 30.degree. C. and the liquid trimethyl aluminum was 
vaporized into the inert gas before passing through the distributor plate 
into the fluidized phosphor bed. A continuous protective alumina coating 
was formed on the surface of the individual phosphor particles when the 
vaporized trimethyl aluminum was exposed to the oxygen at a temperature 
sufficient for a reaction between the oxygen and the vaporized trimethyl 
aluminum to occur. The oxygen as an oxygen/inert gas mixture was 
introduced at 1500 cc/min. for 02 and 50 cc/min. for N.sub.2 into the 
fluidized bed through the two micron filter elements located on the shaft 
of the vibrating mixer above the vibrating disc. The coating time was 7.5 
hours as indicated in Table II. Once the phosphor particles were coated 
with a continuous coating of alumina, they were transferred into a quartz 
boat and annealed at 768.degree. C. in static air and held at that 
temperature for 4 hours in a Rapid-Temp furnace. There is approximately a 
one hour linear ramp and cool down time programmed into the annealing 
heating cycle. The annealing temperature can be from about 700.degree. C. 
to about 850.degree. C. and the period of time held at temperature can be 
from about 15 minutes to about 20 hours. After the annealing step, at 
least one layer of the phosphor was coated in a 40W-T12 lamp using a 
conventional water base suspension system. The coated 40W-T12 lamp was 
then processed into a finished 40W-T12 lamp and zero hour lumen data was 
obtained, shown in Table I. 
EXAMPLE 2 
Approximately 0.5 Kg. of the same lot of the virgin manganese activated 
zinc silicate phosphor powder used in Example 1 was placed in a quartz 
boat and heated at 20.degree. C./min to a temperature of 1225.degree. C., 
in an ambient of static air. The phosphor was held at that temperature for 
approximately 16 Hrs., and was then cooled at 20.degree. C./min to 
900.degree. C. and removed from the furnace. After cooling to room 
temperature, the partially sintered phosphor cake was roll crushed to a 
powder and sieved through a 60 mesh screen. The phosphor was subsequently 
loaded into a 2 liter (4.5"dia..times.9"ht.) polyethylene mill jar 
containing 600 ml of citric acid solution (0.48w/o citric acid/gm 
phosphor) and 0.5 Kg. of burundum grinding cylinders (1/2".times.1/2") and 
rolled at 91 rpm for 30 min. Upon completion of this wet milling process, 
the phosphor was separated from the citric acid solution by filtering and 
the phosphor was dispersed in 1.0 liter of distilled water. The phosphor 
was allowed to settle for 15 minutes and most of the water was decanted 
off, together with some of the fine phosphor particles that remained in 
suspension. The water dispersion and decanting process was then repeated a 
second time and was followed by a final dispersion in water and subsequent 
washing and suction drying on a buchner funnel. The phosphor received a 
final drying process in a 100.degree. C. oven for at least 3 hours. Two 
weight percent of ammonium chloride was added to approximately 350 gm of 
the dried phosphor and blended. The blended mixture was then fired at a 
temperature of 1000.degree. C. for 2 hours in air and cooled to room 
temperature. The partially sintered phosphor cake was roll crushed to a 
powder and sieved through a 60 mesh screen. The phosphor was subsequently 
loaded into a 2 liter (4.5"dia..times.9"ht.) polyethylene mill jar 
containing 600 ml of citric acid solution (0.48w/o citric acid/gm 
phosphor) and 0.5 Kg. of burundum grinding cylinders (1/2".times.1/2") 
and rolled at 91 rpm for 30 min. Upon completion of this wet milling 
process, the phosphor was separated from the citric acid solution by 
filtering and the phosphor was dispersed in 1.0 liter of distilled water. 
The phosphor was allowed to settle for 15 minutes and most of the water 
was decanted off, together with some of the fine phosphor particles that 
remained in suspension. The water dispersion and decanting process was 
then repeated a second time and was followed by a final dispersion in 
water and subsequent washing and suction drying on a buchner funnel. The 
phosphor received a final drying process in a 100.degree. C. oven for at 
least 3 hours, and was ready for use after sieving through a 400 mesh 
screen. The reflectance and surface area data are shown in Table I, Sample 
No. 7. The phosphor, Sample No. 7, was then coated with alumina, annealed 
and incorporated into 40W-T12 fluorescent lamps. The zero hour lumen data 
for Sample No. 7 are also shown in Table I. 
TABLE II 
__________________________________________________________________________ 
Process Conditions for CVD Alumina Coating Phosphor Particles 
Bed Height Bed Flow rates (cc/min) 
Coating 
Sample 
at Temp. 
Charge 
T.C. 
Ex. T.C. 
To Distributor 
To Agitator 
Time 
No mm gm .degree.C. 
.degree.C. 
Bubbler 
Carrier 
O.sub.2 /N.sub.2 
Hr. 
__________________________________________________________________________ 
#1 500 1500 
385/ 
325-379 
1250 1000 
1500/50 
7.5 
410 
#2 300 300 N.A. 
370-380 
250 400 500/50 7.5 
#3 320 300 N.A. 
380-413 
300 350 500/50 6.25 
#4 320 315 N.A. 
400-413 
300 350 500/50 6.58 
#5 300-350 
315 N.A. 
365-405 
350 300 500/50 5.67 
#6 330 300 N.A. 
342-356 
300 350 500/50 6.25 
#7 400 375 N.A. 
390-435 
350 300 500/50 6.75 
__________________________________________________________________________ 
Shown in FIG. 4 is a fluorescent lamp 10, such as a 40W-T12 fluorescent 
lamp. Lamp 10 comprises an elongated sealed glass envelope 12 of circular 
cross section containing electrodes 14 and 16 at each end supported by 
lead-in wires 18, 20 and 22, 24, respectively, which extend through glass 
presses 26, 28 in mount stems 30, 32 to the contacts in bases 34, 36 
affixed to the ends of the lamp 10. 
Envelope 12 is filled with an inert gas such as argon or a mixture of argon 
and neon at a low pressure, for example, two torr, and a small quantity of 
mercury, at least enough to provide a low vapor pressure of about six 
microns during operation. 
The interior of envelope 12 is coated with a layer of phosphor 38, such as 
the manganese activated zinc silicate phosphor of Example 1 in accordance 
with the present invention. 
A phosphor coating suspension was prepared by dispersing the phosphor 
particles in a water base system employing polyethylene oxide as the 
binder with water as the solvent. 
The phosphor suspension was applied in the usual manner of causing the 
suspension to flow down the inner surface of envelope 12 and allowing the 
water to evaporate, leaving the binder and phosphor particles adhered to 
the envelope 12 wall. The phosphor coated envelope 12 was then heated in a 
lehr to volatilize the organic components, the phosphor layer 38 remaining 
on the envelope 12 wall. 
Envelope 12 is processed into a fluorescent lamp by conventional lamp 
manufacturing techniques. 
While we recognize that alternative explanations may be equally valid, we 
believe that the following theory best explains the operation of our 
invention. Not to be bound by theory, we hypothesize that during the 
thermal annealing process, wherein the Al.sub.2 O.sub.3 coating is bonded 
to the phosphor particle, there occurs an interdiffusion of the chemical 
species making up the coating and the phosphor substrate. We further 
hypothesize that this interdiffusion causes a substantial reduction in the 
luminescence efficiency of the effected zone of the substrate. Moreover, 
because particle volume is directly proportional to the cube of the 
particle diameter, the diffusion effected zone will occupy a larger 
proportion of the total volume of small particles. As a consequence, the 
light output of small phosphor particles will be drastically reduced by 
interdiffusion, while its effect on the light output of the larger 
particles will be almost negligible. Based on this theory, we conclude 
that in order to maximize brightness after coating and annealing, virgin 
phosphors should have a minimum of smaller diameter particles and that 
both the reflectance measured at 275 nm and the BET surface area are 
affectations of the particle size distribution necessary to achieving 
optimum zero hour lumens (brightness) in the alumina coated and annealed 
condition. This conclusion is not unreasonable since the optical 
scattering coefficient is known to be particle size related. In Table I, 
it can be seen that the process of Example 1, which is responsible for a 
decrease in 275 nm reflectance from 16-10%, is also responsible for a 
substantial increase in particle size. The decreasing values of BET 
surface area listed in Table I reflects an increase in the particle size 
of phosphor particles as seen by SEM photomicrographs. The manganese 
activated zinc silicate particles are not spherical, rather they are 
porous, highly aggregated, serpentine or coral-like particles and that 
when they are measured by conventional particle sizing methods, 
unrealistic particle diameters result. This is not unexpected since most 
particle sizing instruments (Coulter Counter, Microtrak, etc.) assume a 
spherical particle morphology in deriving a particle diameter from some 
type of physical optical or electrical measurement. The morphology of 
manganese activated zinc silicate phosphor particles explains why a 
correlation between virgin phosphor particle size, as determined by one of 
the more conventional techniques, and the zero hour lumen output 
(brightness) of its alumina coated and annealed derivative, isn't obvious. 
The absorption in the 300-400 nm range (see FIG. 1 curve (b)) as explained 
earlier, is associated with the use of tungsten as a phosphor ingredient; 
tungsten-free manganese activated zinc silicate phosphors do not absorb in 
this region. Moreover, this absorption may be associated with tungsten as 
a second phase or as a surface coating rather than just bulk 
(incorporated) tungsten. We hypothesize that tungsten and manganese can be 
left on the surface of the virgin (as synthesized) phosphor, as in Sample 
#1, Table I. The 4 hour heat treatment (Sample #2 - Table I), causes 
additional tungsten and manganese to diffuse to the surface of the 
phosphor where they are subsequently removed by the mill-wash and 
decantation process of Example 1, thereby eliminating most of the 300-400 
nm absorption. If the heat treatment of Example 1 is extended for too long 
a period, as in Samples #3 through 6, the nature of the surface species 
are either chemically altered or build up to the point that the mill-wash 
is no longer capable of removing them, consequently increasing the 350 nm 
absorption. Both surface and bulk analyses support this hypothesis. 
Surface Analysis using ESCA (Electron Spectroscopy for Chemical Analysis) 
shows that the surface becomes richer in tungsten and manganese as a 
consequence of the 1225.degree. C. heat treatment and that the surface 
concentration of both species are returned to their original values by the 
citric acid mill-wash. Furthermore, the tungsten assays show that tungsten 
is removed from the phosphor by the method of Example 1. 
While there has been shown and described what is at present considered the 
preferred embodiment 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.