GB2037800A - Phosphor and Process for Preparation Thereof - Google Patents

Abstract

Phosphors have the formula: (Ln1-xAx)2O2S:(X> wherein Ln is at least one of Y, Gd, La and Lu, A is at least one of Pr and Tb, X is at least one of F and Cl, 0.00003</=x</=0.2, and the concentration of X is in the range 2 to 1000 ppm by weight. Such phosphors show a high brightness and luminescence when excited by ultraviolet rays, X-rays and electron beams. The presence of F and/or Cl increases brightness over that of the equivalent halogen-free phosphor. A process for the preparation of such phosphors is also described.

Description

SPECIFICATION Phosphor and Process for Preparation thereof The present invention relates to rare earth oxysulfide phosphors and a process for the preparation thereof.

Rare earth oxysulfide phosphors are known from inter alia U.S. Patent No. 3,502,590. They are excited by ultraviolet rays, X-rays and cathode rays to show luminescence. Particularly, terbiumactivated phosphors are used for X-ray intensifying screens and projection color televisions. However, these known phosphors have insufficient brightness and hue.

With recent improvements in color picture tubes, electron guns and the like, the brightness of the picture screen is improved and, as a result, the current density for exciting the phosphor is increased. In a projection color television, in order to increase brightness the phosphor is excited at a current density at least five times the current density in a conventional television. If a conventional green-emitting phosphor, i.e. a ZnS phosphor, is used for these cathode ray tubes, since the brightness level is saturated by the irradiating electron beam current sufficient brightness cannot be obtained even if the density of the irradiating electron beam current is increased. The disadvantage arises that the hue of the screen changes if the brightness is changed.

Rare earth ion-activated phosphors are used for projection color televisions, because saturation of the brightness level by the irradiating electron beam current occurs considerably less. However, Practivated rare earth oxysulfide phosphors have insufficient brightness though they give excellent green hue. Tb-activated rare earth oxysulfide phosphors have a yellow hue which is considerably different from that of conventional ZnS:Cu, Al phosphors.

U.S. Patent No. 3,520,590 is mentioned to show the state of the art.

According to the present invention there is provided a phosphor having the formula: (Ln-xAx)2o2s :(X) wherein Ln is at least one element of Y, Gd, La and Lu, A is at least one of Pr and Tb, X is at least one of F and CI, 0.000036x10.2, and the concentration of X is in the range 2 to 1000 ppm by weight.

By the present invention it is possible to provide a highly efficient phosphor.

The invention will below be further discussed, and embodiments described by way of example, with reference to the accompanying drawings, in which Figure 1 is a diagram illustrating the luminescence spectrum of a phosphor embodying the present invention, Figures 2, 3 and 4 are diagrams explaining the present invention.

In the phosphor of the present invention, high brightness can be achieved by choosing x within the range given above, and preferably 0.00003 < x < 0.01. When X is F, it is preferred that the amount of X is 2 to 300 ppm by weight, and when X is Cl, the amount of X is preferably 10 to 300 ppm by weight, more preferably 10 to 150 ppm.

The brightness of the phosphor may be a few percent to 30% or more above that of the halogenfree phosphor, while its hue may be the same as that of the halogen-free phosphor.

The phosphor of the present invention may be prepared by mixing the components (i) an oxide composition having the formula (Ln~xAx)203, wherein Ln, A and x are as defined above, or a material capable of being converted on heating into such an oxide, (ii) a material capable of forming an alkali metal sulfide under heating, (iii) a compound containing fluorine or chlorine and, if necessary, (iv) a flux, and heating the mixture.

Component (i) above can be, for example, mixtures containing Ln203 and an oxide of Tb or Pr at a desired ratio and Ln203 compounds in which a part or all of Ln is substituted by a phosphate, oxalate or carbonate.

As component (ii), there can be mentioned, for example, (a) a mixture of an alkali metal carbonate and sulfur, (b) a mixture of alkali metal sulfite and sulfur, (c) a mixture of an alkali metal thiosulfate and sulfur (d) a mixture of an alkali metal thiocyanate and sulfur, and (e) a compound such as an alkali metal thiocyanate. Among these, a mixture of an alkali metal carbonate and sulfur will be used ordinarily.

A compound containing fluorine can be, for example, REF2, RSO3F, R2GeF6, R2TiF6, R2PE6, LiBF4, NaBF4, KBF4 or R'F2. A compound containing chlorine can be, for example, RCI, RbCI, CsCI, R'CI2, SnCI2, PdCl2, R"CI3, RCI03, NaCIO4, Kilo4, CsCI04, RbCI04, NH4CI04, Zn(CíO3)2, Sr(CIO3)2, Ba(CIO3)2, R"'(CIO4)2 or In(CIO4)3. Here, R is Li, Na, K or NH4, R' is Mg, Ca, Sr, Ba, Zn or Cd, R" is Al, Ga, Y, In, La, Gd, Lu, Pr, Tb, Ce or Sc, and R"' is Ca, Sr, Mg or Ba.

The flux (component (iv)) can be, for example, Na4P207 or NaH2PO4.

The temperature to which the mixture of components (i) to (iv) is heated is preferably in the range 900 to 14000Ci more preferably in the range 1000 to 1 3000C.

Figure 1 shows the luminescence spectrum of the phosphor (GdOgggProoo1)2o2s:(F) under excitation by ultraviolet radiation. This spectrum is the same as that of the fluorine-free phosphor but the output of the emission peak is increased.

As described above, in phosphors of the invention, luminescence brightness is improved and the occurrence of saturation of the brightness level is reduced. Thus, with phosphors of the invention, a color picture tube producing a high quality image can be provided, and for a projection color television brightness can be improved remarkably.

For example, a monochromatic projection television screen was made by the conventional method of screening by sedimentation using the phosphor of the invention (Yo 95Tbo os)202S:(F) (the fluorine content being 500 ppm). In the same manner, a monochromatic projection television screen was prepared using the phosphor ZnS:Au, Cu, Al. Under excitation by a current of 500 juA, the brightnesses of the two televisions were compared. That of the television using the phosphor of the invention was higher by about 25% than that of the television using the other phosphor.

For projection color televisions, a phosphor in which A in the general formula is Tb is preferred because its luminescence brightness is generally higher.

As mentioned above, the luminescence hue of a Tb-activated rare earth oxysulfide is yellow, and the luminscence hue of a Tb-activated phosphor of the present invention is also yellow though its brightness is better than that of the conventional phosphor. On the other hand, a Pr-activated phosphor of the present invention has luminescence of an excellent green color, its brightness is better than that of the halogen-free phosphor, but still inferior to that of a conventional phosphor of ZnS:Cu, Au, Al.

When these two phosphors of the invention are mixed, the luminescence hue of the mixture is yellowish compared with that of the Pr-activated phosphor, but brightness is improved. Even such a yellowish green, however, appears green in color contrast. This will readily be understood from the fact that, although the luminescence hue of the conventional phosphor ZnS:Cu, Au, Al is more yellowish than the green signal produced by the NTSC system, this conventional phosphor is oridinarily used as a green-emitting phosphor.

When a mixture of phosphors is employed, the mixed phosphors should be sufficiently similar to each other with respect to their brightness characteristics that the hue does not vary when the exciting current is changed. For example, a mixture comprising 2 parts by weight of a Tb phosphor (Gd0.98Tb002)202S:F and 1 part by weight of a Pr phosphor (YO g9,PrO 003)O2S:F was coated on the inner face of a face plate of a cathode ray tube by a conventional method. The brightness of the green picture screen of the tube under excitation by a current of 500 yA was 11 5 (the brightness of a conventional phosphor ZNS:Au, Cu, Al is taken as 100). The color coordinate of the above mixture (point 3 of Figure 2) is quite similar to that of the conventional phosphor (point 4 of Figure 2).In Figure 2, points 1,2 and 5 indicate the color coordinates of the Pr-activated phosphor alone, the Tb-activated phosphor alone and the green system according to the NTSC system, respectively.

Another example of a mixture of phosphors will now be described. A Pr phosphor (GdO gg,Pr0e0o3)2o2s:F and a Tb phosphor (GBo98Tbobo2)2o2s F are mixed at various mixing ratios as shown in the Table 1 below, and using these mixtures cathode ray tubes are prepared in the manner as described above. The brightness (relative values based on a brightness of ZnS:Au, Cu, Al of 100) and the color coordinates are also shown in Table 1. It will be seen that mixtures of the Pr phosphor (for which X=0.215, Y=0.627) and brightness is 59 and the Tb phosphor (X=0.349, Y=0.555, brightness 158) can be used in a cathode ray tube having a green-emitting picture screen of high brightness.

Table 1 Mixing Ratio Color Coordinates Brightness Tb Pr X Y 0.9 0.1 0.336 0.563 148 0.8 0.2 0.322 0.569 138 0.7 0.3 0.309 0.577 129 0.6 0.4 0.295 0.584 119 0.5 0.5 0.283 0.592 109 0.4 0.6 0.269 0.598 98 0.3 0.7 0.256 0.606 88 0.2 0.8 0.242 0.613 79 0.1 0.9 0.229 0.620 69 Phosphors of the invention can have improved brightness under excitation by X-rays compared with conventional phosphors. For example, the brightness of (Gd0.96Tb0.02)202S:(F) (fluorine content 9400 ppm by weight) under excitation by X-rays of 1 20 KV of a W target is 114 (brightness of the fluorine-free phosphor taken to be 100). The brightness of (Gdo gggPrO 00, )202S:(F) (Fluorine content 9400 ppm by weight) under the same excitation conditions is 127 (Fluorine free phosphor 100). Thus phosphors of the present invention can be effectively used in radioactive ray detectors and X-ray intensifying screens.

Examples of the present invention will now be given.

Examples 10 to 15 Phosphors (Gd0.999Pr0.001)2O2S:(Cl) were prepared in the manner of Example 6, except that KCI was used instead of the starting fluorine compound in the amounts shown in Table 4. In each phosphor of the invention, relative brightness was improved over that of the equivalent phosphor prepared without addition of KCI, but the luminescence spectrum was the same as that of the equivalent phosphor.

Table 4 Chlorine Added Concentration Content of Chlorine (ppm by (ppm by Relative Example No. weight) (Cl/Ln '202) weight) Brightness comparison 0 0 100 10 50 10 106 11 294 14 107 12 2940 36 123 13 8821 67 130 14 17642 90 119 15 196024 158 101 Example 16 Phosphors (Gd1~XTbx)202S:(F) were prepared in the manner of Example 1 except that Tb407 was used instead of Pr6O the Gd/Pr ratio was changed and NH4PF6 was used instead of KCI (the fluorine content was 42 ppm). It was found that the improvement in brightness caused by the halogen depends on the activator ion concentration in the phosphor. This relationship is shown in Figure 3. Curve 6 shows the brightness of the phosphor having a fluorine content of 42 ppm and curve 7 shows the brightness of the equivalent fluorine-free phosphor.In the region where activator ion concentration is high and concentration quenching by the mutual action of the ions is prominent, the effect of the fluorine ion is low. A similar relationship was observed also in the Pr-activated phosphor. When the activator ion concentration is low, the effect of the fluorine ion is high. Even at a Tb concentration of 3x 10-5, the effect of the fluorine ion was observed.

The relationship observed with the phosphor (Gd,~xPrx)202s:(F) prepared by a method similar to that of Example 9 is shown in Figure 4, in which curve 8 shows brightness of this phosphor and curve 9 shows brightness of the equivalent fluorine-free phosphor.

Examples 17 to 24 Gd203 36.143 g Tb407 0.1122 g S 6.830 g Na2CO3 6.836 9 K3PO4 3H20 2.308 g Batches of the above starting materials were mixed with various fluorine compound as shown in Table 5 and the mixture was treated in the manner described in Example 2 to obtain the phosphors (Gd0997Tb0003)202S:(F). The added concentration of fluorine as F/Ln'203 was 9442 ppm by weight. The fluorine content in the phosphor was 42 ppm by weight on average, though it varied to some extent depending on the starting fluorine compound. In each phosphor, relative brightness was improved, compared with that of the fluorine-free phosphor.

Table 5 Fluorine Added Relative Example No. Compound Amount (g) Brightness comparison not added 0 100 17 (NH4)2GeF6 0.668 156 18 Li2GeF6 0.601 155 19 Na2GeF6 0.698 152 20 K2GeF6 0.794 152 21 LiPF6 0.478 145 22 NaPF6 0.504 158 23 KPF6 0.552 155 24 NH4PF6 0.489 164 Example 25 Phosphors (Gd0999P001)2O2S::(Cl) were prepared in the manner of Example 10 except that BaCI2 . 2H20, ZnCl2, AICI3. 6H20, NH4CI, KCI03 and NaCI were respectively used instead of KCI at a Example 1 Gd203 33.318 g GdPO4.0 904H2O 4.294 g Pr6O,1 0.0341 g S 9.561 g K3P04. 3H2O 3.233 g Na4P207 0.984 g Na2CO3 9.572 g KCI 0.671 g The above starting materials were charged into a 200 ml bottle composed of acrylic resin and mixed by rolling. The mixture was then charged into a 50 ml alumina crucible on which an alumina lid was placed. The mixture was fired in air at 11 800C for 3 hours. The yellowish product was removed from the crucible and immediately tipped into water, which was then agitated with a stirrer.Since Na2Sx dissolved out, the solution became yellowish. Washing with water was repeated until the solution became colorless, and the washed solid was passed through a 325 mesh sieve. The supernatant was discarded by decantation, and 500 ml of a 0.15 N aqueous solution of HCI was added to the residue and the mixture was agitated for 1 hour. After the HCI treatment, the phosphor was washed with pure water until the conductivity of the washing liquor was reduced below 10 ohm-cm-. The phosphor was sedimented and the supernatant was thrown away, and the phosphor recovered was dried at 1 400C. The phosphor so obtained had the composition (Gd0g69Pr01)202S:Cl, in which chlorine content was 67 ppm by weight.The brightness of this phosphor under excitation of 254 nm was 130 (comparative value for the phosphor prepared in the same way but without addition of KCI taken as 100).

Examples 2 to 5 Gd203 35.160 g Tb407 1.122 g S 6.830 g Na2CO3 6.836 g K3PO4 - 3H2O 2.308 g Batches of the above starting materials were mixed with NH4PF6 in amounts to provide the fluorine contents given in Table 2 (in which Ln' is the sum of elements Ln, Pr and Tb), and the resulting mixture was treated in the manner described in Example 1 to obtain the phosphors {Gd097Tb003)202S:(F). As shown in Table 2 the relative brightness of the phosphors of the invention was increased over that of the phosphor containing no fluorine. The luminescence of the phosphors was the same as that of the fluorine-free phosphor.

Table 2 Fluorine Added Concentration Content of fluorine (ppm by tppm by Relative Example No. weight) (fiLn'2O2) weight) Brightness comparison 0 0 100 2 31 2 108 3 314 4 117 4 1571 10 120 5 9427 42 123 comparison 62844 5490 55 Examples 6 to 9 Phosphors (Gdo gggPrO 00,)202S:(F) were prepared in the manner described in Example 1, except that NH4PF6 was used instead of KCI. The added concentrations of NH4PF6, the fluorine contents and the relative brightnesses are shown in Table 3. In each phosphor of the invention, relative brightness is improved over that of the fluorine-free phosphor and the luminescence spectrum is the same as that of the fluorine-free phosphor.

Table 3 Fluorine Added Concentration Content of fluorine tppm by tppm by Relative Example No. weight) (F/LN'2O2) weight) Brightness comparison 0 0 100 6 1259 9 129 7 3147 35 128 8 9442 91 134 9 31474 119 123 concentration of 8821 ppm by weight (as Cl/Ln'203). In these phosphors, the Cl content was 67 ppm on average, though the Cl content varied to some extent depending on the starting chlorine compound. The relative brightness values of these phosphors were 121, 130, 127, 128, 131 and 131, respectively (that of the equivalent chlorine-free phosphor being 100).

Example 26 Gd2O3 33.3189 GdPO4 4.032 9 Pr6O11 0.0341 9 S 9.5619 K3PO4.3H20 3.233 9 Na4P207 0.984 g Na2CO3 9.572 9 NaHF2 0.185g A mixture of the above starting materials was treated in the manner of Example 1 to obtain (Gd0.999Pr0.001)2O2S:(F) (fluorine content 21 ppm). The relative brightness of this phosphor was 118 (brightness of the equivalent fluorine-free phosphor 1 00).

Example 27 A phosphor was prepared as described in Example 26 except that 0.3512 9 of NH4SO3F was used instead of Na HF2. The fluorine content in this phosphor was 10 ppm and its relative brightness was 105 (equivalent fluorine-free phosphor 100).

Example 28 A phosphor was prepared as described in Example 26 except that 0.3294 9 of NaBF4 was used instead of Na HF2. The fluorine content in the phosphor was 33 ppm and its relative brightness was 124 (equivalent fluorine-free phosphor 100).

Example 29 Y203 19.7389 Y(PO)i,ocg. .1.04H2O 3.223 9 Tb407 1.8693 g S 9.561 9 K3PO4. 3H2O 3.233 9 Na4P207 0.984 9 Na2CO3 9.572 9 NH4PF6 0.4889 9 A mixture of the above starting materials was treated as described in Example 1 to obtain (Y0.95TB0.05)2O2S:(F) (fluorine content 60 ppm. The luminescence spectrum of this phosphor was the same as that of the equivalent fluorine-free phosphor. The relative brightness was 160 (equivalent fluorine-free phosphor 100) Example 30 Y203 20.767 9 Y(PO4)1099. 1.04H2O 3.391 9 Pr6O11 0.0341 g S 9.5619 K3PO4. 3H20 3.233 g Na4P2O, 0.984 9 Na2CO3 9.572 g KCI 0.6710g A mixture of the above starting materials was treated as described in Example 1 to obtain (Y0.999Pr0.001)2O2S:(Cl) (chlorine content 75 ppm), the relative brightness was 132 (brightness of the equivalent chlorine-free phosphor 100).

Example 31 La203 32.486 9 Tb407 0.1122 9 S 6.830 9 Na2CO3 6.836 9 K3P04 . 3H20 2.308 g (NH4)2GeF6 0.1 11 9 A mixture of the above starting materials was treated as described in Example 1 to obtain (La0997Tb0003)202S:(F) (fluorine content 11 ppm). The luminescence spectrum of the phosphor was the same as that of the equivalent fluorine free phosphor. The relative brightness was 110 (equivalent fluorine-free phosphor 1 00).

Example 32 Lu203 39.798 g Pr6O,1 0.03419 S 6.830 g Na2CO3 6.836 g K3PO4. 3H2O 2.308 g NH4PF6 0.489 g A mixture of the above starting materials was treated as described in Example 1 to obtain (LuO gggPrO 001)202S:(F) (fluorine content 35 ppm). The luminescence spectrum of the phosphor was the same as that of the equivalent fluorine-free phosphor. The relative brightness was 11 5 (equivalent fluorine-free phosphor 100).

Example 33 Gd2O3 32.627 g Y203 1.1299 Tb407 1.8699 S 6.830g Na2CO3 6.836 9 K3PO4.3H2O 2.308 g NH4PF6 0.489g A mixture of the above starting materials was treated as described in Example 1 to obtain (Gdo gYo 05Tbo o5)202S (F) (fluorine content 87 ppm). The relative brightness of the phosphor was 132 (equivalent fluorine-free phosphor 1 00).

Claims (11)

Claims

1. A phosphor having the formula: (Ln 1 ~xAx)2o2S :(X) wherein Ln is at least one element of Y, Gd, La and Lu, A is at least one of Pr and Tb, X is at least one of F and Cl,0.00003~x < 0.2, and the concentration of X is in the range 2 to 1000 ppm by weight

2. A phosphor according to claim 1 wherein 0.00003~x~0.1.

3. A phosphor according to claim 1 or claim 2 where X is F and the concentration of X is in the range 2 to 300 ppm by weight.

4. A phosphor according to claim 1 or claim 2 wherein X is Cl and the concentration of X is in the range 10 to 300 ppm by weight.

5. A phosphor according to any one of claims 1 to 4 wherein Ln is Gd.

6. A phosphor according to any one of claims 1 to 4 wherein Ln is Y.

7. A phosphor substantially as any herein described with reference to the accompanying drawings or in the Examples (excluding those described for comparative purposes).

8. A process for the preparation of a phosphor according to any one of the preceding claims comprising mixing (i) an oxide composition having the formula: (Ln1-xAx)2o3 wherein Ln, A and X are as in claim 1, or a material capable of being converted on heating into such an oxide, (ii) a material capable of forming an alkali metal sulfide on heating and (iii) a compound containing at least one fluorine and chlorine, and heating the mixture.

9. A process according to claim 8 wherein said mixture is heated to a temperature in the range 900 to 14000C.

10. A process according to claim 8 or claim 9 wherein the material capable of forming an alkali metal sulfide on heating is at least one of a mixture of an alkali metal carbonate and sulfur, a mixture of an alkali metal sulfite and sulfur, a mixture of an alkali metal thiosulfate and sulfur, a mixture of an alkali metal thiocyanate and sulfur and an alkali metal thiocyanate.

11. A process of preparing a phosphor substantially as any herein described in the Examples.

1 2. A fluorescent screen having a phosphor according to any one of claims 1 to 7 or prepared by a process according to any one of claims 8 to 11.