Abstract:
A phosphor expressed fundamentally as YA10 3  :Ce is produced through baking under a usual condition of a mixture of Y 2  O 3 , Al 2  O 3 , CeO 2  and a controlled amount of barium compound exemplified by BaCO 3 , BaO, Ba(OH) 2  and BaF 2 . The presence of the barium compound in the mixture causes intensification of ultraviolet emission and/or suppression of the formation of blue-emitting Y 3  Al 5  O 12  :Ce. The use of BaF 2  as the barium compound or partial replacement of at least one of Y 2  O 3  and Al 2  O 3  respectively by YF 3  and AlF 3  is favorable to the intensification of the ultraviolet emission.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 691,784, filed June 1, 1976, now U.S. Pat. No. 4,070,301. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of producing a cerium-activated yttrium orthoaluminate (YAlO 3 ) phosphor which emits ultraviolet radiation and has a short decay time. 
     Cerium-activated phosphors generally exhibit a very short emission decay time τ 1/e , less than 100 nanoseconds, and accordingly have attracted much attention as favorable to flying-spot scanners and beam-indexing tubes. An emission spectrum extending over the entire range of visible part is required of a phosphor for flying-spot-scanners. For beam-indexing tubes, it is required of a phosphor that the peak wavelength λ max  of its emission spectrum is within the range of about 340-400 nm. 
     Examples of commercial phosphors which meet, or nearly meet, these requirements are a Ca 2  MgSi 2  O 7  :Ce phosphor (designated as P16) whose emission spectrum has a peak λ max  approximately at 370 nm, a Y 3  Al 5  O 12  :Ce phosphor (P46) whose λ max  is approximately 540 nm, a Y 2  SiO 5  :Ce phosphor (P47) with λ max  at approximately at 410 nm and P48 which is a 70/30 mixture of P46 and P47. 
     Y 2  O 3  -Al 2  O 3  system can present three different phases; Y 3  Al 5  O 12 , YAlO 3  and Y 4  Al 2  O 9 . Among phosphors given by activation of these yttriumaluminates by cerium, the most greatest interest has been attached to Y 3  Al 5  O 12  :Ce(P46) because Y 3  Al 5  O 12  is the most stable among the three phases and can be obtained readily as a single phase through a simple solid state reaction. 
     Weber reported in J. of Appl. Phys., 44, 3205-3208 (1973) that the photoemission band of a single crystal of YAlO 3  :Ce prepared by the Czochralski method had a decay time τ 1/e  of 16 nsec and a peak wavelength in the emission spectrum λ max  of 370 nm. These values suggest that YAlO 3  :Ce phosphor is quite attractive as a beam-indexing phosphor. Some patent applications, e.g. Japanese Patent Application Disclosure No. 48(1973)-26686, have also disclosed YAlO 3  :Ce phosphor. Unlike Y 3  Al 5  O 12 , however, YAlO 3  is a quasi-stable phase and requires some consideration of preparation techniques. Naka et al reported in Kogyo Kagaku Zasshi (Japan), 69, No. 6,1112-1116(1966) that, in the preparation of YAlO 3  from Y 2  O 3  and Al 2  O 3 , there occurred coexistence of Y 3  Al 5  O 12  and Y 4  Al 2  O 9  even by the employment of a very severe baking condition such as 1500° C.-25 hr. The above-referred Japanese Pat. Appln. too shows the necessity of employing a baking temperature higher than 1550° C. for obtaining a YAlO 3  :Ce phosphor of a practicable performance. According to the results of our experiments on the production of YAlO 3  :Ce phosphor by solid state reaction, the coexisting Y 3  Al 5  O 12  :Ce phase lessens as the baking temperature is raised but does not disappear completely even through baking for 4 hr at 1600° C. 
     The coexistence of Y 3  Al 5  O 12  :Ce in YAlO 3  :Ce phosphor as an ultraviolet-emitting phosphor is particularly and significantly disadvantageous from the following reasons. First, Y 3  Al 5  O 12  :Ce emits visible light. Second, the excitation band of Y 3  Al 5  O 12  :Ce somewhat overlaps the emission band of YAlO 3  :Ce with the result that part of the ultraviolet radiation emitted by YAlO 3  :Ce is absorbed in Y 3  Al 5  O 12  :Ce. 
     Concerning the production of YAlO 3  :Ce phosphor using Y 2  O 3 , Al 2  O 3  and CeF 3  as starting materials, our above-referred application Ser. No. 691,784 has disclosed that a phosphor of improved luminous intensity can be obtained through baking at about 1000°-1400° C. by the addition of a controlled amount of barium compound, e.g. BaCO 3 , BaO or BaF 2 , to the starting materials. 
     Japanses Patent Application Disclosure No. 49(1974)-113784, relating to Y 2  SiO 5  :Ce phosphor, shows that the addition of BaF 2  to the starting materials for this phosphor causes an increase in the luminous intensity due to an intensification of radiation approximately at 450 nm while there occurs no enhancement of a main peak (approximately at 420 nm) of Y 2  SiO 5  :Ce. This means a change in the emission spectrum of Y 2  SiO 5  :Ce by the use of BaF 2 . For YAlO 3  :Ce, we have confirmed that the use of a barium compound causes enhancement of its main peak (370 nm) without causing any change in the emission spectrum. 
     Japanese Patent Application Disclosure No. 50(1975)-45790 relating to Y 2  Si 2  O 7  :Ce shows an increase in the luminous intensity caused by the use of BaF 2 . As is known, Y 2  Si 2  O 7  has four crystallographic modifications; α, β, γ and δ. This Japanese application describes that the use of BaF 2  is effective for obtaining a single phase of β-Y 2  Si 2  O 7  :Ce which exhibits the most intense emission. 
     Since YAlO 3  has only one crystallographic structure, the effect of BaF 2  on YAlO 3  :Ce phosphor shown in our application Ser. No. 691,784 differs in nature or principle from that on Y 2  Si 2  O 7  :Ce phosphor. 
     For Y 3  Al 5  O 12  :Ce, we have recognized through later studies that the addition of a barium compound to commonly used starting materials produces a suppressive effect on the formation of Y 3  Al 5  O 12  :Ce by baking. When no barium compound was added, pure Y 3  Al 5  O 12  :Ce phase was obtained at baking temperatures above about 1300° C. and baking at lower temperatures resulted in the coexistence of YAlO 3  :Ce phase. When a few mole% barium compound was added to the same starting materials, the coexistence of YAlO 3  :Ce phase did not expire even at baking temperatures somewhat above 1300° C., so that it was necessary to raise the baking temperature at least to 1400° C. for obtaining pure or nearly pure Y 3  Al 5  O 12  :Ce phase. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of producing YAlO 3  :Ce phosphor, which method is effective for suppressing the formation of Y 3  Al 5  O 12  :Ce phase and gives a phosphor of an improved emission characteristic. 
     It is another object of the invention to provide a method of producing YAlO 3  :Ce phosphor which features an intensified ultraviolet emission. 
     According to a method of the invention, an improved YAlO 3  :Ce phosphor is produced fundamentally by the steps of (a) preparing a substantially stoichiometric mixture of powdered Y 2  O 3 , Al 2  O 3 , and CeO 2 , (b) adding a controlled amount of a powdered barium compound to the above mixture, and (c) baking the resultant mixture at a temperature in the range from about 1000° to about 1400° C. 
     In practice, the steps (a) and (b) may be accomplished simultaneously. Alternatively, the mixture prepared at step (a) may be baked at a temperature below about 1150° C. and then pulverized prior to step (b). 
     Barium compounds useful in a method of the invention are BaO, BaCO 3 , BaF 2 , BaCl 2 , and barium salts such as Ba(OH) 2 , Ba(NO 3 ) 2  and BaC 2  O 4  which give BaO at temperatures below 1400° C. The amount of the barium compound in step (b) is preferably made to range from 0.1 to 8 mole% of the stoichiometric mixture in step (a). Usually a range from about 0.5 to about 5 mole% is the most advantageous. However, it is permissible to increase the amount of the barium compound up to 20 mole% of the Y 2  O 3  -Al 2  O 3  -CeO 2  mixture accompanied by an additional step of washing the product of step (c) with an acid, preferably a dilute nitric acid. 
     The amount of CeO 2  is made such that the Ce concentration in the produced phosphor ranges from 0.1 to 5 mole% of the host material, YAlO 3 , as in conventional YAlO 3  :Ce phosphors. 
     Judging from X-ray diffraction pattern of the product of a method of the invention, the product is not always a perfectly single phase of YAlO 3  :Ce. Sometimes the coexistence of BaAl 2  O 4  together with Y 2  O 3  and/or Y 4  Al 2  O 9  is observed other than Y 3  Al 5  O 12  :Ce. However, the use of a barium compound is quite effective for suppressing the formation of Y 3  Al 5  O 12  :Ce which is very detrimental to an ultraviolet-emitting phosphor is explained hereinbefore. The coexisting phases other than YAlO 3  :Ce and Y 3  Al 5  O 12  :Ce can readily be removed by washing the product with an acid such as a dilute nitric acid. It is necessary that barium is mixed with the essential materials (Y 2  O 3 , Al 2  O 3  and CeO 2 ) in the form of one of the above listed compounds. Substantially no improvement on the emission characteristic of the product can be accomplished if barium is introduced in the form of a barium aluminate such as BaAl 2  O 4 . 
     The YAlO 3  :Ce phosphor produced according to the invention includes less Y 3  Al 5  O 12  :Ce than analogous phosphors produced by conventional methods and hence features a remarkable lowering in the luminous intensity of radiation in the visible part of the emission spectrum. Furthermore, there occurs in many cases a considerable intensification of the ultraviolet emission attributable to YAlO 3  :Ce. In regard of the intensification of the ultraviolet emission, it is preferable that the mixture prepared at step (b) contains a fluoride. This is realized by the use of BaF 2  in an amount as specified hereinbefore as the barium compound. Alternatively (or at the same time), a portion of Y 2  O 3  may be replaced by YF 3  and/or a portion of Al 2  O 3  may be replaced by AlF 3 . In this case, it is preferable that either of the molar ratios 2YF 3  /Y 2  O 3  and 2AlF 3  /Al 2  O 3  is within the range from 0.001 to 0.10. When both YF 3  and AlF 3  are used, the sum of the molar ratios 2YF 3  /Y 2  O 3  and 2AlF 3  /Al 2  O 3  is made to range from 0.001 to 0.10. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the dependence of the intensities of ultraviolet emission and yellow emission of a cerium-activated yttriumaluminate phosphor on the amount of a barium compound added to the starting materials of the phosphor; 
     FIG. 2 is a graph showing the same as FIG. 1 except for some changes in the raw materials; 
     FIG. 3 is a graph showing the dependence of the intensity of ultraviolet emission of a cerium-activated yttriumaluminate phosphor on the baking temperature in the production of the phosphor by a method of the invention in comparison with the same resulting from a conventional production method; and 
     FIG. 4 is a graph similar to FIG. 3 with respect to yellow emission of the same phosphor. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     The following compounds were employed as the starting materials of a cerium-activated yttriumaluminate phosphor and weighed in the following proportion with correction for ignition losses. 
     
         ______________________________________Y.sub.2 O.sub.3 (99.99% purity)                0.4975    molesAl.sub.2 O.sub.3 (99.99%)                0.500     molesCeO.sub.2 (99.99%)   0.0050    molesBaCO.sub.3 (special class reagent grade)                ×   moles______________________________________ 
    
     The value for x was taken as 0 and 0.03 to produce two kinds of phosphors. 
     The weighed materials were mixed in ethanol for about 20 hr by means of a ball mill. After drying, the mixture was fired at a temperature of 1300° C. in a platinum cruicible (in air) for 4 hr to give a phosphor. 
     Each phosphor thus produced had an emission in the yellow part of the spectrum attributable to Y 3  Al 5  O 12  :Ce other than ultraviolet emission attributable to YAlO 3  :Ce. To examine luminous intensities of these two types of emissions individually, a HOYA U360 filter which cuts off visible light was interposed between an aluminum plate coated with each phosphor and the photosensitive surface of a photomultiplier tube for measurement of the ultraviolet emission and a TOSHIBA V-Y360 filter which cuts off ultraviolet for measurement of the yellow emission. The aluminum plate was irradiated with a 10 kV electron beam in a demountable irradiation apparatus. 
     The phosphor obtained when BaCO 3  was not contained in the starting materials (x = 0) was taken as a standard, and the luminous intensity of this phosphor was given the value of 100 for either of the ultraviolet and yellow emissions. Then, the intensity of the ultraviolet emission of the other phosphor (obtained when x = 0.03, meaning the addition of 3 mole% BaCO 3  to the essential materials of the cerium-activated phosphor) was 99 and the yellow emission intensity of this phosphor was 66. Thus, the presence of BaCO 3  in the materials subjected to baking caused a manifest lowering in the yellow emission attributable to Y 3  Al 5  O 12  :Ce substantially without affecting the ultraviolet emission intensity of YAlO 3  :Ce, so that the resultant phosphor was of an improved performance as an ultraviolet-emitting phosphor. 
     EXAMPLE 2 
     This example is generally similar to Example 1 except for the use of a high purity BaO in place of BaCO 3 . A phosphor obtained by the incorporation of 3 mole% BaO (x = 0.03 according to the definition in Example 1) gave a relative value 102 on the same basis as in Example 1 for the intensity of the emission attributable to YAlO 3  :Ce and a considerably small value, 53, for that attributable to Y 3  Al 5  O 12  :Ce. 
     EXAMPLE 3 
     This example too is generally similar to Example 1 except that BaCO 3  was replaced by the same amount (3 mole%) of Ba(OH) 2 . The relative intensity of the ultraviolet emission (of YAlO 3  :Ce) was 100 and that of the yellow emission (of Y 3  Al 5  O 12  :Ce) was 95 for a phosphor obtained by the use of Ba(OH) 2 . 
     EXAMPLE 4 
     Use was made of Ba(NO 3 ) 2  in place of BaCO 3  in Example 1. When the starting materials contained 3 mole% Ba(NO 3 ) 2 , meaning x = 0.03, the resultant phosphor gave a relative value 104 for the intensity of the ultraviolet emission and 80 for the yellow emission. 
     EXAMPLE 5 
     This example is generally similar to Example 1, but BaF 2  was used in place of BaCO 3  and the value for x (moles BaF 2 ) was made 0.001, 0.030, 0.080 and 0.20. The luminous intensities of the resultant four kinds of phosphors were as follows on the same basis as in Example 1. 
     
         ______________________________________× = 0.001 (0.1 mole% BaF.sub.2) :                105 (ultraviolet                emission) and                85 (Yellow emission)× = 0.030 (3 mole% BaF.sub.2) :                161 (ultraviolet) and                42 (yellow)× = 0.080 (8 mole% BaF.sub.2) :                104 (ultraviolet) and                22 (yellow)× = 0.020 (20 mole% BaF.sub.2) :                57 (ultraviolet) and                11 (yellow)______________________________________ 
    
     These values are graphically presented in FIG. 1. 
     When the phosphor obtained by the use of 20 mole% BaF 2  was washed with a dilute nitric acid (5 N), the luminous intensity values increased to 98 (ultraviolet) and 18 (yellow). 
     EXAMPLE 6 
     A portion of Y 2  O 3  in the starting materials of Example 1 was replaced by YF 3  (99.99% purity) to provide the following mixing ratio. 
     
         ______________________________________Y.sub.2 O.sub.3 0.485     molesYF.sub.3        0.020     molesAl.sub.2 O.sub.3           0.500     molesCeO.sub.2       0.010     molesBaCO.sub.3      ×   moles______________________________________ 
    
     Four kinds of phosphors were produced through the procedures of Example 1 by setting the value for x at 0, 0.001, 0.03 and 0.10. Evaluating the luminous intensity of the phosphor obtained without using BaCO 3  (x = 0) as 100 for either of the ultraviolet emission attributable to YAlO 3  :Ce and the yellow emission attributable to Y 3  Al 5  O 12  :Ce, the luminous intensities of the other three kinds of phosphors were as follows. 
     
         ______________________________________× = 0.001 (0.1 mole% BaCO.sub.3) :                100 (ultraviolet) and                93 (yellow)× = 0.030 (3 mole% BaCO.sub.3) :                131 (ultraviolet) and                70 (yellow)× = 0.10 (10 mole% BaCO.sub.3) :                84 (ultraviolet) and                37 (yellow)FIG. 2 presents these date in graphic form.______________________________________ 
    
     EXAMPLE 7 
     Using the YF 3  -containing mixture of Example 6 with a fixed value 0.030 for x (3 mole% BaCO 3 ), the baking temperature was varied within the range of 1000°-1400° C. to examine the influence of the baking temperature on the luminous intensity of the product. The baking time was constantly 4 hr at every temperature. The same experiment was conducted also in the case of x = 0. 
     The experimental results are graphically presented in FIGS. 3 and 4 for the ultraviolet emission (YAlO 3  :Ce) and the yellow emission (Y 3  Al 5  O 12  :Ce), respectively. 
     FIGS. 3 and 4 show that there is a difference between YAlO 3  :Ce and Y 3  Al 5  O 12  :Ce in the dependence of their luminous intensity on the baking temperature, that both intensification of the ultraviolet emission of YAlO 3  :Ce and suppression of the yellow emission of Y 3  Al 5  O 12  :Ce caused by the presence of BaCO 3  in the starting materials exhibit variations in scale as the baking temperature is varied and that, in this case, the merit of using BaCO 3  is appreciable at baking temperatures above 1100° C. As is known, the influence of the baking temperature on the performance of yttriumaluminate phosphor must be considered in correlation with the baking time. In many cases, a phosohor produced through baking for relatively long period of time at a relatively low temperature and another phosphor of the same composition produced through baking for a shorter period of time but at a higher temperature have analogous characteristics. When, for example, each composition of Example 7 was baked at 1100° C. for 10 hr, the luminous intensities of the resulting phosphors for the emissions attributable to YAlO 3  :Ce and Y 3  Al 5  O 12  :Ce were approximately equal to the values found on the curves of FIGS. 3 and 4, respectively, at a baking temperature of 1150° C. 
     EXAMPLE 8 
     Referring to Example 6, the proportion of BaCO 3  was fixed at 0.030 moles (3 mole%) and the amount of YF 3  was varied as follows. 
     
         ______________________________________Y.sub.2 O.sub.3         0.495 - α/2                        molesYF.sub.3      α        molesAl.sub.2 O.sub.3         0.50           molesCeO.sub.2     0.010          molesBaCO.sub.3    0.030          moles______________________________________ 
    
     The value for α was made 0, 0.001, 0.005, 0.010, 0.10 and 0.20 to produce six kinds of phosphors. The baking of each mixture was performed for 4 hr at 1300° C. 
     The luminous intensities of these phosphors were measured in accordance with Example 1, and the value 100 was given to the luminous intensity of the phosphor obtained without using YF 3  (α = 0) for either of the emission of YAlO 3  :Ce and that of Y 3  Al 5  O 12  :Ce. Then the relative values for the luminous intensities of the other phosphors were as follows. 
     
         ______________________________________α = 0.001 (0.1 mole% YF.sub.3) :             105 (ultraviolet emission)             and 55 (yellow emission)α = 0.005 (0.5 mole% YF.sub.3) :             121 (ultraviolet) and             83 (yellow)α = 0.010 (1 mole% YF.sub.3) :             131 (ultraviolet) and             100 (yellow)α = 0.10 (10 mole% YF.sub.3) :             133 (ultraviolet) and             114 (yellow)α = 0.20 (20 mole% YF.sub.3) :             80 (ultraviolet) and             154 (yellow)______________________________________ 
    
     As seen from these data, the use of an increasing amount of YF 3  brings about a progressive intensification of both the ultraviolet and yellow emissions, but the ultraviolet emission alone is significantly suppressed when the amount of YF 3  exceeds about 10 mole% of Y 2  O 3 . A great intensification of the yellow emission by the use of a large amount of YF 3  was confirmed by an experimental result that a phosphor obtained by excluding BaCO 3  from the above composition and setting α at 0.20 exhibited luminous intensity of 90 for the ultraviolet emission and 389 for the yellow emission. This example demonstrates the preferableness of using YF 3  in an amount of about 0.1  to about 10 mole% of Y 2  O 3 . 
     EXAMPLE 9 
     A composition of the following mixing ratio was employed in this example to examine the replacement of a portion (about 4 mole%) of Al 2  O 3  by AlF 3 . 
     
         ______________________________________Y.sub.2 O.sub.3 0.495     molesAl.sub.2 O.sub.3           0.490     molesAlF.sub.3       0.020     molesCeO.sub.2       0.10      molesBaCO.sub.3      ×   moles______________________________________ 
    
     The value for x was taken as 0 and 0.03 to produce two kinds of phosphors through baking at 1300° C. for 4 hr. 
     When x = 0.03 (3 mole% BaCO 3 ), the luminous intensities of the resultant phosphor were 124 for the emission of YAlO 3  :Ce and 49 for the emission of Y 3  Al 5  O 12  :Ce, evaluating the luminous intensities of the other phosphor (x = 0) as 100. 
     EXAMPLE 10 
     Y 2  O 3 , Al 2  O 3  and CeO 2  were weighed in the proportion given in Example 1 (excluding BaCO 3 ) and mixed in ethanol according to Example 1. This mixture was baked in a platinum crucible at 1000° C. for 4 hr and then cooled to room temperature, followed by pulverizing. BaF 2  was added to the heat-treated and powdered mixture in four different amounts as employed in Example 5 (0.1 to 20 mole%), and each of the resultant mixtures was baked for 4 hr at 1300° C. The emission characteristics of the thus produced phosphors were substantially identical with those of the corresponding phosphors produced in Example 5. 
     EXAMPLE 11 
     This example illustrates a joint use of YF 3  and AlF 3  in combination with Y 2  O 3  and Al 2  O 3 . The following materials were mixed in the indicated proportion by the procedure of Example 1 and baked for 4 hr at 1300° C. 
     
         ______________________________________Y.sub.2 O.sub.3 0.480        molesYF.sub.3        0.030        molesAl.sub.2 O.sub.3           0.485        molesAlF.sub.3       0.030        molesCeO.sub.2       0.010        molesBaCO.sub.3      0 or 0.03    moles______________________________________ 
    
     Taking the luminous intensities of the phosphor produced without using BaCO 3  as 100, the luminous intensities of the phosphor obtained by the use of BaCO 3  were 132 for the ultraviolet emission and 17 for the yellow emission. The replacement of BaCO 3  in this example by the same quantity (0.03 moles) of BaF 2  produced little change in the emission characteristic of the product. In this case the luminous intensity for the ultraviolet emission was 139 and that for the yellow emission was 20.