Note: Descriptions are shown in the official language in which they were submitted.
7~3
II~ROVED CE-DOPED YTTRIUM ALUMINUM
GARNET AND DEVICES INCORPORATING SAME
TECHNICAL FIELD
This invention relates to Ce-doped Yttrium Aluminum
Garnet phosphors, and more particularly relates to such
phosphors having improved emission characteristics, and
also relates to devices incorporating ~hem.
BACKGROUND ART
The Ce-doped yttrium aluminum garnet, having the
nominal Composition ~Y(l-x)cex)3Al5t)l2~
referred to by the acronym "YAG:Ce", was developed for
its emission characteristics under cathode ray excit-
ation. See G. Blasse and A. ~ril, "A New Phosphor for
Flying-Spot Cathode-Ray Tubes for Color Television:
Yellow-Emitting Y3A15012:Ce," Applied Physics Letters,
Vol. II, No. 2, July 15, 1967, p.53.
More recently, the phosphor with Ce in the quad- I
rivalent state has b en shown to have utility in fluo-
rescent lamps and high pressure mercury vapor discharge
lamps. See British Patent 1,371,207, issued to Thorn
Electrical Industries Limited on Oct. 23, 1974. The
2~ patent states that the phosphor is useful in high
pressure mercury vapor discharge lamps to improve
visible light emission by absorption of the blue Hg
lines and re-emission at longer visible wavelengths.
The patent states that the phosphor is also useful in
fluorescent lamps where careful control of the light
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emission is required, for example, for photocopying or
for special color rendering purposes.
Still more recently, the phosphor has been used
in combination with a red-emitting europium activated
yttrium vanadate or vanadate phosphate in order to
improve the color rendering index of high pressure mer-
cury vapor discharge lamps. See U.S. patent 4,034,257,
issued to Mary V. Hoffman on July 5, 1977, and assigned
to the General Electric Company. The patent states
that the YAG:Ce phosphor is efficiently excited in the
420-460 nanometer absorption band, which is necessary
to improve the color rendering index, and that emission
in the yellow-green spectrum corresponds to the maximum !
eye~sensitivity region~ thereby contributing to the
lumen output of the lamp.
DISCLOSURE OF INVENTION
; It has now been discovered that YAG:Ce phosphors,
r having the approximate composition expressed by the molar
formula:
3-(S +x) a XAl5oy:ce3
where: ~ = 0 to O.Q3
y = 12-1.5X
3 = o. ol to 0.10
a= cation vacancies, and
Ce is substantially in the trivalent state,
exhibit: increased brightness at room temperature and
elevated temperatures; better thermal stability; and
better maintainance in fluorescent and high pressure
- mercury vapor discharge lamps (herein high intensity
discharge or HID lamps) than the YAG:Ce phosphors of
the prior art.
This improved YAG:Ce phosphor is produced by a
method comprising:
(a) mixing startin~ components which upon ~iring
will yield the phosphor composition; ~nd
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(b) firing the mixture at a temperature within the
range of about 1200C to 1700C,
characterized in that the firing is at least
concluded in a strong reducing atmosphere for several
hours, whereby quadrivalent Cerium is substantially re-
duced to trivalent Cerium.
The improved phosphor is useful in fluorescent
lamps and HID lamps, both singly and in combination
with one or more other phosphors, for example, to
improve the color rendering index of the lamp.
While Ce may range from S of 0.01 to 0.10, it is
preferred for optimum luminescence to maintain Ce
within the range of about 0.05 to 0.07.
While cation vacancies are not essential to the
achievement of an improved phosphor, such vacancies
noticeably improve efficiency of luminescence. Such
vacancies may be obtained by omission of controlled
- amounts of Y203 during phosphor synthesis.
Starting components are, of course, any materials
which upon firing will yield the phosphor composition,
including for example, oxides and materials which upon
heating decompose to oxides, such as hydroxides, car
bonates, sulfates, citrates, formates, oxalates, etc.
Firing may be carried out in a single step, wherein
the atmosphere is first neutral or weakly reducing for
a time sufficient to form the phosphor compolsition,
and then strongly reducing for a time sufficient to
substantially reduce quadrivalent Ce to the trivalent
state, typically at least 2-3 hours.
However, it is preferred to carry out firing in two
steps, a first firing to form the phosphor, and refiring
in a strongly reducing atmosphere, since such procedure
permits the use of an open hearth gas furnace for firing,
which provides a slightly reducing atmosphere early
in the firing cycle, followed by a longer (usually 6
to 7 hours) period of near neutral atmosphere. A typical
firing would include heating from about 1200C to 1600C
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in about 30 minutes in a slightly reducing atmosphere,
and holding at 1600~C for the remainder of the period in
a near-neutral or neutral atmosphere. ;
Refiring is preferably carried out at about 1400C
to 1600C for at least about 4 to 6 hours. A strongly
reducing atmosphere is herein defined as a gas or gas i
mixture capable of rendering a controlled reduction
of the valence state of multi valent ions during a heat
treatment. Examples of such atmospheres are H2, dis-
sociated ammonia (about 75 volume percent H2 and 25
volume percent N2), and carbon monoxide (CO).
BRIEF DESCRIPTION OF THE DRAWING
Fig. l is a graph of Excitation Efficiency
(arbitrary Units) versus wavelength of emission in nano- I
15 meters of a YAG:Ce phosphor with and without refiring i 1
in a strongly reducing atmosphere;
Fig. 2 is a graph of absorption expressed as re-
mission function k/S=(l-R)2/2R versus wavelength of
incident radiation in nanometers of the phosphors of
Fig. l;
Fig. 3 is a front elevation view of a fluorescent
lamp containing the YAG:Ce phosphor of the invention;
Fig. 4 is a front elevation view of an HID lamp
containing the YAG:Ce phosphor of the invention; and
~5 Fig. 5 is a graph of the difference in Spectral
Power Distribution (SPD), watts per nanometer versus
nanometers, for a 400 watt HID lamp incorporating the
YAG:Ce of the invention and the SPD of a 400 watt HID
lamp without phosphor.
BEST MODE FOR CARRYING OUT THE INVENTION
;
For a better understanding of the presen~ in-
vention, together with oth~r and further objects,
advantages and capabilities thereof~ reference is made 1,
to the following disclosure and appended claims in
connection with the above-described drawings.
1.
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The phosphor powder preferably has an average
particle size by Coulte~ counter of less than 7,
micrometers which may be obtained, for example, by ball
milling starting components to achieve a fine, well-
blended mixture. Typical starting materials are Y203,Al~OH)3 and CeO2 powders having average particle sizes
of less than 4 micrometers by Coulter counter.
Following firing, the phosphor is preferably screened,
for example, through 325 mesh, to break up agglomerates.
EXAMPLE I
The following example describes preparation of a
phosphor for use in a fluorescent lamp application.
Yttrium oxide (about 99.9% pure), aluminum hydroxide
(about 99.8% pure) and ceric oxide (about 99.9 %pure) in
the form of fine powders are dry-blended in the mole-
cular proportions:
Y203 1.455
Al(OH)3 5.00
CeO2 0.06
For each five po1m ds of this blend, sufficient water
~about 1 liter) is added to produce a thick slurry in
a one-gallon ball mill. The slurry is milled for eight
hours, then dried in a convection drying oven at 150C
for at least 3 to 4 hours.
The dried powder is flrst fired in high density
alumina crucibles (150 cm3) in a high-temperature,
open-hearth gas furnace according to the following
procedure:
- a. Crucibles containing the powder are placed
in furnace at 1200C.
b. Furnace is force-heated to 1600C at a rate
of 15 to 20 per minute.
c. Furnace is maintained at 1600C for six hours
(neutral flame).
d. Crucibles are removed immediately from the
furnace and air quenched.
e. Upon cooling to room temperature the phosphor
is pulverized.
*Trade Mark
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The pulverized phosphor is repacted in alumina
crucible-lined molybdenum boats and then refired in
a molybdenum-element high-temperature muffle furnace
according to the procedure:
a. Boats are stoked into the hot zone of the
furnace at a rate of about 400 500~C per hour.
b. Phosphor is refired for 4-6 hours at 1500C in
a dissociated ammonia atmosphere.
c. Boats are stoked out of the hot zone of the
furnace at a rate of approximately 400-500C
per hour into a cooling zone of the furnace.
d. Upon cooling to near room temperatures
the phosphor is repulverized. !
e. l'he phosphor is washed in water at room
temperature, (about two liters of water per
pound of phosphor).
f. The phosphor is filtered and dried in a drying
oven at 150C for at least 3 to 4 hours.
g. The phosphor is screened through 325-mesh.
The phosphor obtained from this process will
possess a fairly narrow particle size distribution
with an average particle size of 6 to 7 ~m (Coulter
counter).
Referring now to Fig. 1, there is shown excitation
spectra for the phosphor prepared as in Example I,
both with the refiring step and without it. The
spectra were obtained by monitoring the visible emission
at about 535 nanometers resulting from excitation bands
with peaks at 230, 340 and 460 nanometers, respectively.
The increased excitation efficiency of the phosphor
resulting from the refiring is shown by the increased
strength of the excitation peaks, by a factor of about
four times at 230 nanometers, 3.3 times at 340 nano-
meters, and 2 times at 460 nanometers.
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TAsLE I
U, N, O
U Open hearth gas flame with lean gas
mixture, giving rise to slightly oxi-
dizing conditions.
N Open hearth gas flame with balanced gas
mixture, giving rise to near neutral
conditions.
O Open hearth gas flame with enriched gas
mixture, giving rise to slightly reducing 1
conditions.
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Referring now to Fig. 2, there i8 shown reflectance
spectra (expresseld as remission function) for the
phosphor of the Example, both with and without refiring.
The spectra were obtained by using two monochromators,
one providing the probe beam, the other detecting the
reflected beam. This arrangement effectively avoided
- interference from luminescence of the sample. The
obtained spectra show a similarity to the excitation
spectra of Fig. 1 in the visible and long uv regions.
Around 350 nanometers, a continuous absorption band
begins, increasing in intensity toward the short uv. j
This broad continuum does not significantly contribute
to the visible emission. Refiring the phosphor reduces !
this absorption band roughly by a factor of two, and
increases the absorption band in the blue region.
The refiring produces a conversion of Ce+4 to
Ce~3 resulting in an increase in the typical Ce~3
absorption peaks at 230, 340 and 460 nanometers and a
lowering of the typical Ce+4 continuous absorption
in the uv.
The twofold efficiency increase for excitation in
the blue band is simply due to an increase in the
concentration of Ce+3 centers after the reducing
treatment. The higher enhancement factors in the uv
(3.3 at 340 nm and 4.0 at 228 nm) result both from the
1~
increased concentration of Ce' J and the diminished
competition for absorption of exciting radiation by
the inactive Ce+4 centers.
EXAMPLE II
Various samples of YAG:Ce phosphors were pre-
pared as in Example I, except that the conditions of
the first firing were varied to include "over-reduced,"
"normal-reduced" and "under-reduced" conditions,
(designated "O", "N" and "U", respectively). These
conditions are as listed in Table I.
I
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g
Some of th~ "N" samples were refired as in Example
I (designated "R"). Brightness under 254 nm and 365
nm excitation was measured on plaques prepared by
setting the phosphor from acetone slurry. Results
are shown on Table II, together with results for
YV04:Dy and "cool white", standard fluorescent lamp
phosphor of the alkaline earth halophosphate type
having the formula (Ca,Cd)5(F,Cl~ (P04)3:Sb:Mn.
TABLE II
Normalized Brightness Values ~'
Sam~le365 nm excitation 254 nm excitation ¦
N2 96 87.5 ¦
N 100 100 ¦
03 162 112
04 120 85
U 71 65
R5 131 150
R6 276 465
YV04Dy 282 3375
Cool White 5 5250
1 sample with optimum CRT response.
2 sample with optimum CRT response, different lot.
3 average particle size greater than 6 micrometers
4 average particle size about 4 micrometers
5 refiring at 1500C for 2 hours.
6 refiring at 1500C for 4 hours.
As may be seen from Table II, at 365 nm excitation,
the phosphors of this invention, R5 and R6 exhibit in-
creased brightness over the N, U and Cool White samples.
In addition R6 exhibits dramatically increased brightness
over all samples, except YV04:Dy, whose brightness is
comparable. At 254 nm excitation, R and R exhibit in-
creased brightness over the N, 0 and U samples, with R6again showing a dramatic increase.
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EXAMPLE III
This example demonstrates that the YAG:Ce phosphor
of the invention shows improved efficiency of blue-to
yellow conversion over the prior art YA~:Ce phosphors.
Samples of Example II were excited at 450 nanometers
and spectrally integrated visible emission intensity
was measured. Results are shown in Table III.
TABLE III
SampleNormalized Room Temperature Intensity
10 O3 100
N2 51 ,
-U 44
R4 165
*Obtained from area under emission spectra, given
in units of Kl millivolt nanometers, where K~ is an
instrumental constant inclusive of excitation flux at
450 nm.
EXAMPLE IV
This Example demonstrates the improved temperature
dependence of emission of the YAG:Ce phosphors of the
invention. Table IV shows the effect of temperature UpOIl
emission intensity from various YAG:Ce samples, for
excitation at 450-456 nanometers and 340-345 nanometers.
It will be seen from Table IV, for example, that
percent Variation from Normalized Room Temperature
Visible Emission at 450~456 nm Excitation decreases
to 32 percent at 246C for Sample O3, but has de-
creased to only 11 percent at 220C for Sample R4.
For 340-345 nm Excitation, comparable percents are
-15 for O3, but +9.7 for R4.
Table Y shows similar data for 254 nm Excitation.
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TABLE V
Normalized Room
Temperature Visible Percent
Samplè Temp.C Emission Intensity Variation
; 5 O3 25 100 o
81 -2
197 -2
N2 25 125 0
; 92 0
204
289
; U 25 . 75 o
112 3
227 -4
293 -2
R4
0
120 +~ 5
220 ~7 7
320 -2
Although the variations are smaller for 254 nm
Excitation than for excitations of longer wavelength, it
will be seen that R4 actually shows increased emission
of ~8.5% and +7.7~/O at 120C and 220C, respectively,
versus 2 percent at 197C for 03; +4% at 204C for
N and -4% at 227C for U.
EXAMPLE V
Maintenance losses (decrease in emission intensity
with time) fo~ high load fluorescent lamps containing
the YAG:Ce phosphor of Example I, refired, were deter-
mined for a 24 inch tube operating at 2.0 Amps discharge
current with a power dissipation varying from about 110
: to 140 watts. Maintenance loss after 820 hours of
operation was only 13%, compared to 60~/~ loss after only
312 hours for a similar lamp containing YVO4 EU~3 and
compared to about 13% loss after only 500 hours for2-envelope commercial lamps of the high pressure
mercury vapor type containing YV04Eu+3.
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EXA~LE VI
ThP inside surface of the outer envelope of three
400 watts, high-pressure mercury-vapor (HPMV) lamps was
coated with the YAG:Ce phosphor in question, using
standard techniques for phosphor coatings of lamps.
The energy output, and the lumen output over a
portion of the operating life of the lamp, are given
in Table VI and are contrasted with the performance
data of a clear (uncoated) 400 watts HPMV lamp.
The presence of the phosphor increases luminosity
from the 20,500 lumens for the clear lamp up to 22,000
lumens for the coated lamps.
The maintenance loss of the coated lamps at 500
hours of operation averages 4%, as can be inferred from
the Table. This should be contrasted with typical
10% maintenance losses at 500 hours for the currently-
used high-temperature phosphor YV04:Eu+3;Tb~3.
Two conclusions can be drawn from inspection of
Figure 5. Figure 5 also gives the differenc~ in spectral
power distribution (SPD) between the Bl lamp and the
clear lamp listed in the Table. The effect of the
phosphor is to provide an emission band centered at
550 nm, even at the high temperatures (300C) of the
outer lamp-envelope. In addition, the 436 nm emission
line of mercury is seen to be strongly absorbed by the
phosphor. This absorption of blue light and conversion
to yellow-green emission centered at 550 nm is ad-
vantageous in providing a light source with a reduced
- color-temperature.
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-15-
In the accompanying drawings Figs. 3 and 4 re-
spectively shows views of a fluorescent lamp and a
mercury vapour discharge lamp embodying the invention.
Referring now to Fig. 3, a typical fluorescent lamp
containing mercury vapour is shown including a glass
envelope 6 having end caps 7 and 8 with electrical lead-
in wires 9 and connected to cathodes (not shown). A
layer 10 of the phosphor of the present invention is
coated upon the internal surface of the glass envelope
10 6. In Fig. 4 the high pressure mercury lamp or arc ~'
discharge lamp is shown which includes the conventional
arrangement of an arc tube (not shown) supported upon a
metal harness 4. Surrounding the arc tube and harness
is a bulbous envelope 12, with a coating 5 of the
15 phosphor of this invention disposed upon the internal ',
surfaces thereof.
While there has been shown and described what are
at present considered the preferred embodiments of the
invention, it will be obvioue to those skilled in the
art that various changes and modifications may be made
therein without departing from the scope of the in-
vention as defined by the appended claims.
INDUSTRIAL APPLI CAB ILITY
.
Ce-doped Yttrium Aluminum Garnet phosphors are
- 25 improved regarding brightness, maintenance, thermal
stability and blue-to-yellow conversion over prior art
phosphors by refiring in a strongly reducing atmosphere
to reduce quadrivalent Ce substantially to trivalent
Ce. Such phosphors are useful in fluorescent lamps
and particularly useful as color correctors in high
pressure mercury vapor discharge lamps.
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