Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~241~(~
BACKGP~OUND OF THE INVENTION
Field of the Invention
l'his invention relates to phosphors and more
particularly to a blue phosphor which emits light by electxon
beam excitation.
Description of the Prior Art
. . _ .
As a hlue phosphor which will emit light of high
brightness by electron excitation in a cathode ray tube,
the only phosphor that has been used to any extent is
ZnS:Ag. Such phosphors, however, have poor linearity of
brightness relative to excitation current, so that they show
a brightness saturation characteristic. In a high brightness
cathode ray tube suitable for color image projectors parti-
cularly, ZnS:Ag is usually employed at a high excitation
current. The brightness saturation occurring in the blue
phosphor causes a disorder of color balance ~or ~he o~her
luminous colors, namely, the red and green of the other
phosphors.
Recently, it has become clesirable to employ a
phosphor with a cathode ray tube or a view finder in video
cameras which can provide high brightness at a low accelerating
voltage, for example, 6 kV and which evidences a fast decay
in light emission. Although the phosphor ZnS:Ag provides
relatively high brightness at relatively low accelerating
voltages, the decay of light emission is relatively slow
and it takes 30 microseconds or so for the luminescent level
on the decay curve to decrease to 1/10 of the peak value.
~`
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~2~)2~8~
SU~AR~ OF TIIE INVENTION
The present invention provides a phosphor which
can obviate the defects inherent in the usual prior ar-t
blue phosphor. It provides a phosphor capable of exhibiting
brightness as high as that of the conventional ZnS:Ag by
eleetron beam excitation but is able to withstand stronyer
excitation. It further has the characteristic of evidencing
a fast clecay of light emission.
According to the present invention, there is
provided a phosphor having the general for~ula:
Zns~xz~lTe~yAl2s3
where x is in the range of 1 ~ 10 2 to 8 x 10 2,
and y is 0 or in the ran~e of 5 x 10 7 to
5 x 10 4 per mol of ZnS.
BRIEF DESCRIPTION OF T~E DRA~INGS
... _ ... .
Various aspeets of the present invention are
illustxated in the attaehed sheets of drawings in whieh:
FIG. 1 is a eross-seetional view illustrating
rather sehematieally an apparatus used to produee a phosphor
aeeording to the present invention;
FIG. 2 is a graph of the luminous spectra of
phosphors according to the present invention and those of
the prior art;
FIG. 3 is a table detailing measured results of
the luminous characteristics of the phosphors of the present
invention;
12~
FIG. 4 is a graph plotting luminescence intensity
against excitation current for a phosphor according to the
present inven',-ion and a prior art phosphor;
FIG. 5 is a graph of luminous spectra of phosphors
which preparation is described in various reference examples
from thls invention as compared with a conventional phosphor;
FIG. 6 is a table comparing the lumlnous character-
istics of the materials produced according to this invention
with -the priGr art materials;
FIG. 7 is a graph plotting various luminous
characteristics against iring temperature;
FIG. 8 is a table illustrating the changes in
luminous characteristics of phosphors occurring when the
amounts of sulfur added are changed;
FIGS. 9 and 10 are graphs of luminous spectra of
phosphors in which the added amounts of sulfur are changed;
FIG. 11 is a table setting forth measured results
.
of the relationship between heatins conditions and luminous
characteristics; ~ .
FIG. 12 is a graph of luminous decav characteristics
. possessed by the phosphors of the present invention;
FIG. 13 is a table setting forth measured values
of luminous characteristics possessed by the phosphors of
the present invention;
FIG. 14 is a face view of a projection cathode
ray tube employing the improved phosphor o. the present
invention;
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2~
FIG. 15 is a side elevational view of the tube
shown in FIG. 14, partially broken away to illustrate the
interior construction; and
FIG. 16 is a frag~.entary view of a cathode
ray tube screen employing a three phosphor screen with
which the present invention can be used.
-4a-
8~
.
DESCRIPTIO~ OF THE PREFERRED EMBODIMENTS
-
The present invention is concerned with a phosphor
having the general formula:
ZnS xZnTe yA12S3
where x is in the range of 1 x 10 2 to 8 x 10 2,
and y is 0 or in the range of 5 x 10 7 to
5 x 10 4 per mole of ZnS.
A phosphor according to the present inventi.on
can be manufactured using the following method.
A raw material for composing the phosphor defined
by the above-identified general formula i.s combined with
sulfur powder in an amount of 0.5 to 2 weight percent for
the raw material, which is then filled into the bottom of
a furnace tube 2, such as a quartz tube, vertically inserted
into a furnace 1 as shown in FIG. 1. ~ layer of carbon 4,
for example granular activated charcoal or carbon,is placed
over the layex 3 of raw material so as to isolate the
material 3 from the air. A lid 5 made, for example, of
quartz is mounted on the quartz tube 2 at its upper open
end to close the quartz tube 2 by its deaa weight. This
provides a predetermined space 6 formed within the upper
portion of the quartz tube 2, extending between the layer
of carbon 4 and the lid 5. The partially filled tube,
at least as to its bottom portion, is inserted into the
vertical type furnace 1 which is kept in a predetermined
heated state or, after the quartz tube 2 is inserted into
the furnace, the furnace 1 is heated to fire the raw
material 3 at a temperature in the ranae from 830~C to
1200C, and preferably from 830C to 1030C. The especi.all~
preferred firing range is 830C to 930C. In FIG. 1,
reference numeral 7 denotes aenerally a heating mean.s for
the vertical tvpe furnace 1, the heatin~ m~ans 7 presenting
such a temperature distribution that the portion of a
quartz tube 2 which is filled with raw material 3 is kept
at its highest temperature in the direction of the axis
of the furnace tube. As indicated, the upper portion of
the quartz tube 2 can project above the furnace 1.
If the raw material ZnS contains excess sulfur as
compared to the required stoichiometric amount, it can be
considered equivalent to the raw material which has been
supplemented with sulfur powder. Thus, in this case the
addition of the sulfur powder can be omitted.
The phosphor according to the present invention
will now be described in detail. For ease in understanding,
a phosphor havina a fundamental composltion according to
this invention has a composition ZnS xZnTe, that is,
y is equal to O.and x is in the range from 1 x 10 2 to
8 x 10 2. The luminous characteristics of this basic
phosphor will be described in conjunc~ion with the method
of its manufacture, and examples of its luminous characteristics.
Reference Example l
Zinc telluride (ZnTe) of 99.99% purity in an amount
corresponding to the factor x in the general formula was
added to 32 grams of conventional zinc sulfide of high purity
(luminescence grade), mixed well in a mor-tar and then put
into a con-tainer of 100 ml volume made of polyethylene.
Balls composed of agate measuring 5 mm in diameter and pure
water were added to the mixture in the container in amounts
of three times and twice as much as the ZnS, respec-tively.
The material was subjected to a ball mill treatment for
ex-tended periods of time, for example, 24 hours. The raw
material thus made was subjected to a suction filter treat-
ment by a vacuum pump or subjected to a forced filter
treatment and then dried at 120C for five hours. After
drying, it was combined with 320 mg oF 99 ~ 999~ pure sulfur
powder and then mixed sufficiently in the mortar. This
mixture was then fired. The firing treatment was performed
in a vertical type furnace 1 as shown in FIG. 1. In this
example, the mixture was filled in the bottom of the closed
end quartz tube of 30 mm diameter and 50 mm in length.
About lOg of the granular activated charcoal or carbon 4
was piled on the raw material 3 so as to isolate the raw
material 3 from air or oxygen. A lid 5 made of quartz glass
.
was mounted on the quartz tube 2 at its upper open end.
This quartz -tube was inserted into the vertical furnace 1
and kept at 930C.
The temperature in the furnace 1 was lowered to
about 700C momentarily by the insertion of the quartz
tube but was restored to 930C after five minutes or so.
After a three-hour baking at 930C, the quartz tube 2 was
pulled out from the furnace 1 and put into water to be cooled.
Thereafter, the material 3 was pulled out from the quartz
tube 2 and non-reactive material adherin~ to the surface
of the material 3 was washed out and removed. ~bout 30g
of phosphor was pro~uced.
2~8~
Curves 11 to 15 in the graph of FIG. 2 show the
luminous spectra from the phosphors due to electron beam
excitation o:E 15 kV and ~ nA when the added amounts of
ZnTe to the phosphors were varied in the range from 1 ~ lO ~
to ~ x 10 2 ~.ol~ Curves 11 through 15 indicate the luminous
spectra generated from the phosphors whose added amount
of ZnTe (the value x) was 1 x lO 2 mol, 2 x 10 2 mol,
3 x 1.0 2 mol, 4 ~ 19 2 mol anc' 8 ~ 10 2 mol, respectivGly.
A curve 10 indicates the luminous spectrum of the conventional
phosphor ZnS:Ag for comparison.
Fig. 3 is a table indicating measured results of
luminous characteristics (lumen e~uivalent, energy conversion
efficiency, relative brightness, emission peak wavelength
and color coordinates x and y) of these phosphors in which the
relative brightness and the energy conversion efficiency are
..
stated as a relative value if the prior art ZnS:Ag is assumed
to be lOO.
As will be clear from FIGS. 2 and 3, with an increase
of tAe added amount of ZnTe, the emission peak wavelengths
of the luminous spectra of these phosphors are gradually
moved to the long wavelength side as the added lmount of
ZnTe is increased. Thus, when the amount added is 1 x 10 2
mol, the wavelength is 4140.0 A. ~Ihen the the added amount
of ZnTe is 8 x 10 2 mol~ the wavelength is 49~0.0 A. The
values of the color coordinate y in the table of FIG. 3
are increased with a result tha-t the brightness for the
viewer's visual sense is increased In this case, although
the energy conversion efficiency is lower, -~he phosphors with
48( 1
amounts of ZnTe in the range from 1 x 10 2 to 8 x 10
can be used in practice. From FIGS. 2 and 3 it may be
supposed that the added amounts of ZnTe existing in the
range from 1 Y~ 10 2 to 2 x 10 2 mol would have the same
emission pea]c wavelength as those of the conventional
phosphor ZnS:Ag. If the amount of ZnTe is added in this
range, the brightness thereof cannot be made as hiyh or
higher than the conventional phosphor. The curve ~ in
FIG. 4 shows the measured results of brightness versus
excitation current of the phosphor having an added amount
of Zn~e of 2 x 10 2 mol and formed by the above-mentioned
manufacturing method. In the graph of FIG. 4, the abscissa
indicate a relative value of excitation current and the
ordinates indicate the luminous intensity resulting from
converting a light emitted from each of the measured samples
into an electromotive force. In this graph, the curve 9
shows the results obtained from the conventional phosphor
ZnS-Ag for comparison. In the graph of FIG. 4, the initi.al
value for brightness of each phosphor wa~-selected to be
the same. As will be clear from this graph, the new phosphors
have better linearity of briqhtness associated with an
increase in excitation current than shown by the conventional
phosphor ZnS:Ag.
Reference Exa~.ple 2
A manufacturing method similar to that of Reference
Example 1 was used, but the firing temperature and the duration
of firing were changed to 1200C and one hour, respectively.
48(~
FIG. 5 is a graph indicatiny the luminous spectra
exhibited by phosphors where the added amount of ZnTe was
varied in the range from l x lO 2 to 8 Y~ lO 2 mol due to
electron bea~ excitation of lA kV, at 4 nA. In the yraph
of FIG. 5, curves 21 through 28 indicate the luminous spectra
made from phosphors having added arnounts of ZnTe of
l x 10 2 mol; 2 x lO 2 mol; 3 x 10 2 mol; 4 x lO 2 mol;
5 x 10 2 mol; 6 x 10 2 mol; 7 x 10 2 mol; and 8 ~ 10 2 mol,
respectively. The curve 20 indicates the properties of the
conventional phosphor ZnS:Ag for comparison.
FIG. 6 is a table showing the measured results of
luminous characteristics of these phosphors. The relative
brightness and the energy conversion efficiency are indicated
by a relative value based upon the conventional phosphor
ZnS:Ag which is assumed to be lO0. As will be clear from
FIGS. 5 and 6, as in the case of FIGS. 3 and 4, as the
amount of ZnTe increases, the emission pea~ wavelength,
the value of the color coordinate y and the relative bright-
ness are increased.
The firing temperatures were 930C and 1200C
in Reference Examples l and 2, respectively. Comparison
of FIGS, 2 and 3 with FIGS. 5 and 6 shows that Reference
Examples l and 2 have the same added amount of ZnTe but
have different luminous characteristics. The reason for this
is that the process of Reference Example l forms a cubic
system ZnS which is stable at low temperature, while the
process of Reference Example 2 forms a hexagonal syster
ZnS which is stable at high temperatures. Since the added
--10--
ZnTe does not form a hexagona] system but a cubic system
by a high te~.perature firing system, the added ZnTe of a
cubic lattice system is dif~i.cult to assimilate in-to the
ZnS of a hexagonal lattice system formed by the hiyh
temperature firing treatment.
Reference Example 3
The phosphors were prepared by methods similar to
those of Reference Example 1, but using an amount of ZnTe
fixed at 0.025 mol and the firing temperature was varied
from $30C to 1250C. Curves 30, 3i and 32 in the graph
of FIG. 7 indicate measured results of lumen equivalentJ
energy conversion efficiency, and the relative brightness
possessed by the phosphors treated at the various firing
temperatures. The energy conversion efficiency and the
relative brightness were determined as a relative value in
which each value possessed by the phosphor at a firing
temperaturQ of 830C was tak~en as 100. ~s shown in the
graph of FIG. 7, the lumen.e~uivalent exhibits the highest
value when the firing temperature is about 8gOC. Thereafter,
it decreases rapidly as the firing temperature reaches about
1030C, at which temperature the ZnS host in the phosphor
is changed from a cubic system to a hexagonal system.
The lumen e~uivalent changes only slightly at firing
temperatures ranging from 1050C to 1150C and decreases
significantly as the firing temperature exceeds 1200C.
The increase and/or decrease of the lumen equivalent depends
on the crystal structure of ZnS as a host in the phosphor.
In the low temperature region where the system is cubic,
~3~
the value of lumen e~uivalent changes substantially while
in the region of high temperature where the amount of
hexagonal system is high, the value of lumen equivalent
does not change substan-tially.
The energy conversion efficiency has peak values
in both the low and high temperature regions. The maximum
value achievecl occurs near the range from 1050C to
1100C in the high temperature reyion. The produc-t of
lumen ecuivalent and the energy conversion efficiency which
constilu-tes the relative brightness exhibi,ts its maximum
value at a temperature near 880C.
The graph of FIG. 7 shows that if the firing
temperature is in the range frorn 830C to 1200C, a rela-tive:Ly
high energy conversion efficiency can be obtained. From the
point of view of the lumen equivalent and the relative
brightness, it is desirable that the firing temperature be
in the range from 830C to 1030C, and preferably from
~30C ~o 930C. It was confirmed that a phosphor formed
at a firing temperature ranging from 830C to 930C has
high relative brightness as compared with the conventional
phosphor ZnS:Ag. When the firing te~.peratures were 980C
and 880C, the relative brightness was increased up to
120Do and 182gD, respectively.
Reference Example A
The phosphors were formed by a method sim.ilar to
that of Reference Example 1, but the addea amount of ZnTe
was held at 0.02 mol and the temperature and duration of
firing were fixed ~t 950C in three hours, respectively.
The amount of sulfur in Reference Example 1 was changed to
0~8~
a range of 0.5 to 5 weight percent for the raw material of
the phosphor. FIG. ~ is a table indicating the measured
results of luminous characteristics of the phosphors,
namely, the lumen e~uivalent, the energy eonversion
efficiency, the relative brightness, the emission peak
wavelength, the color coordinates and the relative brightness
as compared with the conventional ZnS:Ag. These values
were obtained at amounts of sulfur ranging from O to 5.5
weight percent. In the table of FIG. ~, the energy conversion
efficiency and the relative brightness are relative values
each of which is assumed to be 100 when the added amount of
sulfur powder is at one percent by weight.
FIGS. 9 and 10 are graphs indicating the luminous
spectra of thè phosphors produced according to this example.
In FIG. 9, the curve 40 indicates the luminous spectrum
of the phosphor where no sulfur powder was added into the
phosphor raw material. Curves 41, 42 and 43 indicate the
luminous spectra of phosphors when 1~0 weight percent,
2.5 weight percent, and 5 weight percent of sulfur powder,
respectively, were mixed into the phosphor raw material and
fired.
In the graph of FIG. 10, curves 44, 45, 46 and 47
indicate the luminous spectra of the phosphors when 0.5
weight percent, 1~0 weight percent, 1~5 weight percent,
and 2.0 weight percent, respectively, of sulfur powder were
mixed into the phosphor raw material and then fired~ As
shown by curve ~0, when no sulfur powder is added, the
substituted a~ount of Te in ZnTe for sulfur in ZnS as the
48C~
host material is increased, the impurity contained in the
added ZnTe causes luminescence on the red side thereby
decreasing the color purity of the blue phosphor.
On the other hand, if the amount of sulfur powder
is increased, the substituted amount of Te in ZnTe for
S in ZnS is decreased thereby shifting the luminous spectrum
to the shorter wavelength side of the blue spectrl~n. The
curves 40 'o 47 show that the amount of sul~ur added for
producing the blue phosphor should be in the range from
about 0.5 to 2 weight percent. In this case, when the
ZnS used as the raw material contains excess sulfur as
compared with the stoichiometric amount, if the amount of
excess sulfur is in the range of 0.5 to weight percent,
the addition of the sulfur powder can be omitted.
Reference Example 5
Raw material of the same composition was fired at
930C Eor three hours as in Reference Example 1. The firing
was performed on the basis of a quick-heating method by
which the raw material was rapidly heated up to 930C
and a slow-heating method wherein the raw material was
gradually heated from room temperature at temperature increases
of 10C per minute. FIG. 11 is a table showing measured
values of luminous characteristics of the phosphors obtained
according to the quick-heating method and the slow-hea-ting
method, respectively. The table of FIG. 11 shows that the
phosphor produced according to the slow-heating method exhibits
a relatively large lumen e~uivalent and energy conversion
efficiency as compared with the phosphors produced according
-14-
to the auick-heating method, but the difference is not large.
Consequently, the manner of heating the raw material 50 as
to reach the firing tempexature is not considered important.
FIG. 12 is a graph illustrating the luminous
attenuation characteristic, namely, l~uninescent decay of a
cubic-based phosphor sample of ZnS 0.02 ZnTe produced according
to the method of Reference Example 1. The e].ec~ron beam
excitation was created by a pulse of one microsecond, and
the frequency of 1 kHz. In the graph of FIG. 12, the
abscissa is graduated in microseconds. As shown from graph 12,
it takes three microseconds for luminescence to decay to
1/10 of the peak height of luminescent intensity. Comparing
the decay time of 30 microseconds in the case of the con-
ventional ZnS:Ag, the decay time according to the phosphor
produced in Reference Example 5 was reduced to about 1/10.
Inithe aforementioned manufacturing method, if
the fired phosphor is washed in sodium hydroxide or potassium
hydroxide, the relative brightness could be improved further.
The improvements of the present invention provide
a phosphor which does not cause brigh-tness saturation easily,
and retains the fundamental improvements of the system
ZnS xZnTe where x is in the range from 1 x 10 2 to 8 x 10 2
mol, namely, a higher brightness and a much faster decay
characteristic as compared with the conventional blue phosphor
ZnS:Ag. In particular, the improved phosphor evidences an
energy conversion efficiency without deteriorating the
aforementioned characteristics substantially.
The improved phosphors of the present invention
will be illustrated with reference to an embodiment.
Embodimen t
Zinc telluride of 99.99% purity and aluminum sulfate
in amounts corresponding to 0.025 mol as x and 5 x 10 7 -to
5 x 10 4 mol as y in the aforementioned general formula
were added to 32q of conventional zinc sulfide of high purity
(luminescence grade) mixed well in a mortar, and then put
into a 100 ml container made of polyethylene. Balls of
agate of 5 I~L in diameter and pure water were added to the
mixture in the container with ratios of three times and twice
as much as ZnS, respectively. These materials were subjected
to a ball mill treatment for a long period of time, such
as 24 hours. The raw material thus made was subjected to a
suction filter treatment by a vacuum pump or subjected to
a forced filter treatment and then dried at 120C for five
hours. -After drying, 320 mg of 99.g99~ purity sulfur were
added and mixed sufficiently in the mortar. This mixture
was then fired. The firing was performed in the vertical
type furnace shown in FIG. 1. The mixture was filled in
the bottom of a single end closed quartz tube of 30 mm in
diameter and 50 mm in length. About lOg of granular
activated charcoal or carbon was piled on the filled material
so as to isolate the material from air or oxygen. The lid
made of quartz glass was mounted on the quartz tube at its
upper open end. The quar~z tube was inserted into the
ver~ical furnace maintained at 930C.
-16-
The temperature in the furnace was lowered to
about 700C momentarily by the insertion of the quartz tube
but was restored to 930C after five minutes or so. ~fter
a three-hour baking at 930C, the auartz tube was pulled
out from the furnace and put into water to be coole~.
The filled material was then pulled GUt ~rom the quartz tube
and non--reactive material which had adhered to the surface
of the filled material was washed out and removed. About
30g of phosphor were produced.
FIG. 13 is a table indicating measured results of
the energy conversion efficiency and the relative brishtness
as compared with the conventional Zn~:Ag, as well as phosphors
obtained by firing raw material containing no aluminum
sulfate. In this case, the energy conversion efficiency was
measured using the phosphor without the aluminum sulfate
as a reference (100~). As will be apparent from the table
o FIG. 13, the addition of the alumi.n~ sulfate improves
the energy conversion efficiency, particularly when y = 5 x 10 7
to 2.5 x 10 4. As the amount of a~uminum sulfate is increased,
the emission peak wavelength of the luminous spectrum is
shifted to the short wavelength side. hus, although the
phosphors with ZnTe alone tend to decrease the lumen e~ui-
valent, that is, the relative brightness, phosphors which
have been supplemented with aluminum sulfate of less than
5 x 10 4 mol exhibit sufficiently high relative brightness
when compared with the conventional ZnS:As.
4~3~
As described above, this invention provides a
blue phosphor which is difficult to saturate, exhibits hiyh
brightness characteristics at high electron beam excitation,
and fast decay characteristics. It further has hiyh energy
conversion efficiency.
In the manufacture of the phosphor accordin.y to
this invention, since the phosphor is fired in a vertical
~ype furnace 1, and he '-2W ma erial within the furnace tvbe
can be isolated from air by placiny activated charcoal or
carbon 4 thereon and merely mountiny a lid 5 onto the upper
open end, the handling can be made quite easy~ More spe-
cifically, since the raw material 3 within the quartz tube 2
is covered with activated carbon 4, air in the auartz tube
is absorbed by the activated carbon and never reaches the
raw material 3. Furthermore, since the ~uartz tube 2 is
. . .
arranged to have a space 6 over the activated carbon 4,
upon firing the space 6 is filled with a gas containing
sulfur mixed into the raw material 3. Thus, when firiny,
the sulfur of the ZnS host material can be prevented from
escaping therefrom and the sulfur can be prevented from
dropping below the stoichiometric amount. Therefore, it is
possible to manufacture the phosphors having excellent
luminous characteristics with excellent reproducibility.
FIGS. 14 through 16 illustrate structures in whi.ch
the improved phosphors o' the present invention find their
greatest utility. FIGS. 1~ and 15 illustrate a video
projection tube of the type used in projection systems wherein
separate tubes are provided for blue, red, and green emitti.ng
48~
phosphors. The signals from each projection tube are passed
through suitable lenses and focused on the projection screen
in the proper synchronization.
In FIG. 1~, reference numeral 51 indicates generally
a projection tube having a face 52. The tube itself includes
a cylindrical neck portion 53 which merges into a conical
section 5A terminating in the face 52. A phosphor layer 55
of the type described in tne present application is applieci
to the inner surface of the face 52. It receives energization
by electron excitation from an electron gun 56 located in
the neck portion 53.
The blue phosphor of the present invention is also
useful in -television type tubes including three types of
phosphors, regardless of the geometric arrangement of the
phosphors. The particular structure shown in FIG. 16 includes
stripes 57, 58 and 59 of red, blue and green phosphors, with
guard bands 60 of substantially equal width as the stripes
57 to 59 disposed therebetween. In this instance, the
blue phosphor strip is composed of the improved material
of the present invention.
he above description is based on representative
embodiments of the invention, but it will be apparent that many
modifications and variations can be effected by one skillecL
in the art without departing from the spirit or scope of
the present invention and the scope of the invention shoulcl
be determined by the appended claims only.
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