CA1202480A - Phosphor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A blue phosphor suitable for use in a cathode ray tube with high intensity electron beam energization, the phosphor having the general formula:
ZnS ? xZnTe ? yAl2S3 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 mol of ZnS.

Description

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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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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;

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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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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.

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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.

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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.

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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 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 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.

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.

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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.

Claims (9)

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WE CLAIM AS OUR INVENTION:

1. A phosphor having the general formula:

ZnS ? xZnTe ? yAl2S3 wherein x is in the range from 1 x 10-2 to 8 x 10-2, and y in the range from 5 x 10-7 to 5 x 10-4, per mol of ZnS.

2. A phosphor according to claim 1 in which:
y is in the range from 5 x 10-7 to 2,5 x 10-4 mol.

3. A phosphor according to claim 1 in which:
y is approximately 5 x 10-7 mol.

4. A phosphor according to claim 1 in which:
y is approximately 5 x 10-6 mol.

5. A phosphor according to claim 1 in which:
y is approximately 5 x 10-5 mol.

6. A phosphor according to claim 1 in which:
y is approximately 2.5 x 10-4 mol.

7. A phosphor according to claim 1 in which:
y is approximately 5 x 10-4 mol.

8. A screen structure subject to electron excitation in an evacuated cathode ray tube comprising:
a phosphor which emits blue light under electron excitation and forming at least a portion of said screen, said phosphor having the formula:
ZnS ? xZnTe ? yAl2S3 wherein x = 1 x 10-2 to 8 x 10-2 and y = 5 x 10-7 to 5 x 10-4 per mol of ZnS.

9. A screen according to claim 8 in which said screen is part of a video projection tube.