X-RAY INTENSIFYING SCREEN BASED ON A TANTALATE PHOSPHOR AND PROCESS FOR PRODUCING THE PHOSPHOR

Abstract: An X-ray intensifying screen based on a tantalate phos­ phor, said phosphor having the monoclinic M′ structure and selected from the following: (a) YNb x Ta 1-x 0₄ where x is 0 to about 0.15; (b) LuNb x Ta 1-x 0₄ where x is 0 to about 0.2; (c) Y 1-y Tm y Ta0₄ where y is 0 to about 0.03; (d) a solid solution of (a) and (b); (e) a solid solution of (a) and (c); (f) Y 1-y Tb0₄ where y is about 0.001 to about 0.15; (g) Lu 1-y Tb y Ta0₄ where y is about 0.001 to about 0.15; (h) Gd 1-y Tb y Ta0₄ where y is about 0.001 to about 0.15; (i) a solid solution of at least two of (f), (g) and (h); (j) any of (a) to (i) wherein up to 45 mole percent of the yttrium, lutetium of gadolinium is replaced by lanthanum; (k) any of (a) to (i) wherein up to 15 mole percent of the yttrium, lutetium of gadolinium is replaced by ytterbium; and (i) any of (a), (b), (c), (d) and (e) wherein up to 15 mole per cent of the ytterium or lutetium is replaced by gadolinium. The invention also includes a process for producing tanta­ late phosphors which involves mixing the precursor oxides with a flex comprising Li₂SO₄, LiCl, or a eutecitic mixture of BaCl₂/LiCl and firing at a temperature below 1450°C.

Description: X-PAY INTENSI@YING SCREEN BASED ON A TANTALATE PHOSPHOR AND PROCESS FOR PRODUCING PHOSPHOR This invention relates to the preparation of a luminescent material and to the use of the material in X-ray intensifying screens. Background Art X-ray intensifying screens are generally used in conjunction with photographic film and serve to enhance the image formed on the film. Phosphors which are the active components of such screens should be clear, colorless, single phase materials, crystallizing in polyhedr@l shape of well defined habit so as to minimize any scattering of the light generated by the X-rays. Generally, the phosphor particle size should be about 3 to 10 um and the surface area should be about 0.075 to 0.300 m2/g. Moreover, the phosphors should be good absorbers of X-ray radiation of the energy used, they should emit light strongly in the region of the spectrum to which the film is sensitive, and they should provide sharp, undistorted film images. While there are many known matorials which exhibit luminescence, few have the combination of properties necessary to make them useful in X-ray intensitying screens. For example, the most widely used phosphor for X-ray intensifying screens for many years has been calcium. tungstate; it is the standard by which other phosphors are judged. In recent years several other phosphors have been proposed for possible use in X-ray intensifying screens, but the number is smail compared to the total number of phosphors which have been discovered. Blasse and Bril (J. of Luminescence 3, 109 (1970)) describe the cathodo- and photo-luminescent properties of various YTaO4, GdTaO4 and LuTaO4 phosphors, Including those in which some of the tantalum is replaced by niobium or some of the yttrium is replaced by rare earth substituents. These materials all havo the fergusonite M-type structure. It has been recognized that these materials have to polymorphs, a monociinic (12 space group) structure M at low temperatures and a tetragonal (scheelite-type structure with space group I42 ) at high tempera- turves. Transition between these two forms is instantaneous and rextersible. G, M. Molten and A. B. Chase (The American Mineralogist 52, 1536 (1967)) report that this transition occurs at 1325 C in YTaO4. They also disclose a new polymorph of yttrium tantalate and related rare earth tantalates; if the material is crystallizd below the transformation temperature, a different monoelinic strueture (calied M1 phase, with space group P2/a) results, which M1 phase can be converted to the M phase by heating above 1400 C and then cooling to below 1325 C. It is believed that there ns s no prior at on the luminescent properties of the M1-phase materials under ultraviolet or X-ray exaitation. Brief Description of the Drawings Figures la and lb depiet, respectively, M1-type YNb0.05Ta0.95O4 of the instant invention, made as disclosed herein, and the canning electron mierographs of M-type YNb0.05Ta0.95O4, made via conventional ceramic techniques, zor example, as disclosed by Blasse and Bril, loc. cit. Figure 2 shows diffractometer scans of X-ray powder patterns for M-type YTaO4 and M1-type YTaO4. Figures 3a and 3b show the Raman spectra for M-type YTaO4 and M1-type YTaO4 respectively. Figure 4 shows the calculated X-ray absorption of 185 layers of M1-type YTaO4, GdTaO4 and LuTaO4 and of CaWO4. Figure 5 shows the comparison of the luminescent emission spectra under X-ray excitation for M1-type YNb0.05Ta0.95O4 and CaWO4. Figure 6 shows a cross section of a typical X-ray Intensifying screen. Figure 7 snows plots of resolution V3. total phosphor coating weight for X-ray screen pairs of the instant invention and for X-ray screen pairs prepared in a similar manner from CaWO4. Figure 8 shows the calculated X-ray absorption for the screen pairs of the instant inven- tion and -for those of CaWO4, the screen thicknesses being selccted so as to give the same resolution, Figure 9 shows plots of relative speed vs. resolution for X-ray screen pairs of the instant invention and the screen pairs of CaWO4 prepared in a similar manner. Description of the Invention It has been discovered that certain substituted yttrium, gadolinium and lutetium tantalates, which crystallize in a monoclinic M1 structure, show improved X-ray to light conversion efficiencies, compared, to those of the hosts, to those of moncclinic M-structure compositions of the same formula and to those of the commonly used conmercial phosphor CaWO4. Moreover, it has been found that substitution of appropriate amounts of niobium for tantalum significantly enhances the blue fluorescence of M1-structure YTaO4 and LuTaO4; that substitution of appropriate amounts of thulium for yttrium significantly enhances the blue fluorescence of YTaO4; and that substitution of appropriate amounts of terbium for yttrium, lutetium and gadolium in M1-structure YTaO4, LuTaO4 and GdTaO4 results in strong green fluorescence. According to the present invention therefore there is provided an X-ray intensifying screen which includes a phosphor as the active component characterised in that the phosphor comprises a tantalate having the monoclinc M1 structure and having one of the following general formulae: (a) YNbxTal-xO4 where x is 0 to about 0.15; (b) LuNbxTal-xO4 where x is 0 to about 0.2; (c) Yl-yTmyTaO4 where y is 0 to about 0.03; (d) Yl-yTbyTaO4 where y is about 0.001 to about 0.15; (e) Lu1-yTbyTaO4 where y is about 0.001 to about 0.15; (f) Gd1-yTbyTaO4 where y is about 0.001 to about 0.15. Certain of than tantalates defined above may be used in the form of solid solutions as phosphors in X-ray intensifying screens, specifically solid solutions of the tantalates (a) and (b) or (a) and (c) or solid solutions of at least two of the tantalates (d): (e) and (f). As described in more detail hereinafter, according to modification of the invention, in cortian of the above tantalats, a part of the yttrium, lutetium or gadolinium may ba replaced with lanthanium or ytterbium or a part of the yttrium or lutetium may be replaced with gadolinium. This may be advantageous in some circumstances. The blue or green rluorescing luminescentsubstituted phosphor used in this invention has the monoclinic M' structure and is selected froi the aroup consisting of: (a) YNbxTa1-xO4 where x is 0 to about 0.15; (b) LuNbxTa1-xO4 where x is 0 to about 0.2; (c) Y1-yTmyTaO4 where y is 0 te about 0.03; (d) a solid solution of (a) and (b); (e) a solid solution of (a) and (c); (f) Y1-yTbyTaO4 where y is about 0.001 to about 0.15; (g) Lu1-yTbyTaO4 where y is about 0.001 to about 0.15; (h) Gd1-yTbyTaO4 where y is about 0.001 to about 0.15; and (i) a solid solution of at least two of(f), (g) and (h) The luminescent phosphor is useful In X-ray intensifying screens. The blue-fluorescing phosphor used in this invention includes: (a) the Nb-containing yttrium tantalate of the formula YNbxTa1-xO4 where x is 0 to about 0.15, preferably about 0.001 to about 0.15. more preferably about 0.02 to about 0.10, most pieferably about 0.05; (b) the Nb-containing lutetium tantalate of the forniula LuNbxTa1-xO4 where x is 0 to about 0.2, pre ferably about 0.005 to about 0.2, more preferably about 0.02 to about 0.05; (c) the Tm-containing yttrium tantalate of the formula Y1-yTmyTaO4 where y is 0 to about 0.03, pre ferably about 0.001 to about 0.03, more preferably about 0,002 to about 0.01, most preferably 0.005; (d) a solid solution of (a) and (b); ; and (e) a solid solution of (a) and (c). The green-fluorescing phosphor used in this invention includes: (a) the Tb-containing yttrium tantalate of the formula Y1-yTbyTaO4 where y is about 0.001 to about 0. 15, preferably about 0.005; (b) the Tb-containing lutetium tantalate of the formula Lu1-yTbTaO4 where y is about 0.001 to about 0.15, preferably about 0.005; (c) the Tb-containing gadolinium tantalate of the formula Gd1-yTbyTaO4 where y is about 0.001 to about 0.15, preferably about 0.03 to about 0.035; and (d) a solid solution of at least two of (a), (b) ajd (c). The phosphor used in this invention has the monoclinic M1 structure' wich P2/a space group symmetry, and exhibits greater luminescence than related materials having the monoclinic M structure with I2 space group symmetry. Differences in physical characteristics of M- and M1-phase crystals make the two materials easily discernible. The Mphase crystals made by the aforesaid previously described, known solid state procedure grow in irregularly-shaped pieces, whereas the M1-phase crystals made by the p@ocess of this invention grow n regular, nearly polyhedral particles or narrow si@ distribution. This difference can be seen in the scanning electron mlcrographs of the two types of materials (depicted in Figures la and lb). The M material (Figurelb) has a surface area of 1,3 m2/g and t particle size of 0.5 m. The M' material (Figure la) has a surface area of G.l9 m2/g and a particle size of 4.2 m. The surface areas reported herein were reasured via krypton or nitroe gas absorption in a "Macromuratics" surface pore volume analyzer. The particle sizes, unless otherwise noted, were calculated from the measured surface areas using the formula EMI7.1 <tb> Particle <SEP> size <SEP> (pt <SEP> = <SEP> Ensitp <SEP> 6 <tb> <SEP> Surface <SEP> area <SEP> \mi <SEP> x <SEP> ) <tb> Unit cell parameters obtained from single crystal X-ray diffractometer measurements show that the b-axis and unit cell of the M1 phase are approximately half those of the M phase. X-ray powder diffraction patterns sho marked differences, as seen from the comparison of the patterns shown in Figure 2 for M-type YTaO4 and M1-type YTaO4. Differences in the crystal structure of the two phases also affect the Raman spectra as shown in Figures 3a and 3b depicting M-typc YTaO4 and M1-type YTaO4, respectively. The invention process provides the M1-type lumines@ent material; defined above, uniform, colorless, single-phase particles having a surface area of about 0.75 to 0.300 m2/g a particle size of about 3 to 10 m, which material can be directly incorporated into an X-ray intensifying screen without additional processing. The invention process consists of first blending stoichiometric quantities of appropriate oxides from the group Ta2O5, Y2O3, Gd2O3, Lu2O3, Nb2O5, Tm2O3, Tb2O3 and Tb4O7, in a Spex# oscillating-type shaker, for about 1G to 20 minutes. This mixture, in a container which is inert to the ingredients at process conditions, i Is then prefired in air at about 1000 C to about 1200 C for about 10 to 14 hours. The container can be of a noble metal. but alumina is preferred because of its lower cost. The prefired material is then mixed with a flux comprising Li2SO4, pure LiCl or a BaCl2/LiCl eutectic (see C. Sandomini, gazz. Chim. ital., 44I, 383 (1914)), with the weight of the flux equalling 1/5 to 1/2 of the total weight nf the miyture The resultant mixture is then fired for at least about 3 hours s to about 24 hours or more at a temperature of about 1100 C to less than about 1450 C, such as about 1100 C to about 1300 C. The luminescent product is recovered by water leaching, filtering and washing and then dried. The partIcle sze and uniformity of the phcsphor, made according to this process, is shots in the scanning electron micrograph depicted in Figure la, Alternatively to the above, the prefiring step can be omitted, particularly when Li2S04 is used as the flux, and the appropriate oxides which a.re b;' ended and mixed directly with the flux can be fired as indicated above. Without prefiring, longer firing times, for example, at least about 4 hours to about 14 hours or more, are preferred and advantageous. The luminescent product is formed in the M1 phase by crystallizing the mixture below about 1450 C. Therefore, care must be token to fire the mixture below this temperature. As indicated above, a firing temperature of about 1100 C to about 1300 C is used, with a temperature of about 1200 C being preferred. The invention herein provides X-ray intensifying sereens wherein the phosphor of the screen is as defined above. Especially surprising is the performance of the phosphor with the M1 structure when used in ar. X-ray intensifying screen. It shows an emission efficiency which is superior to compositions of the same formula but of theM structure; it also is superior to CaWO4. When used in an X-ray intensifying screen the phosphor converts X-radiation into photons which are more readily captured by a photographic emulsion to provide a permanent record. The first step in this conversion prccess is the absorption of X-rays by the phosphor. Figure 4 shows the calculated fraction of X-rays absorbed b 185 m layers of M1-type YTaO4, GdTaO4 and LuTaO4 and of CaWO4 (the latter employed as a standard for comparison herein) over the X-ray energy range commonly used in radiography. It ray be seen thpt or this thickness, which 15 typical of thephosphor content of pairs of screens commonly used, the tantalate screens of this invention offer improved X-ray absorption. The second step of this cnonversion process is the emission of light (fluorescence). In many applications it is desirable to have higher efficiency of emission than that shown by CaWO4 in order to reduce the X-ray exposure required to form a radiogram. The efficiency of light emission is indicated by the area under curves such as shown in Figure 5 where the luminescent emission spectrum (A) of a preferred composition YNb0.05Ta0.95O4, under X-ray excitation, is shown along with, for comparison, the luminescent emission spectrum (B) of a commercial CaWO4 X-ray phosphor, under the same conditions of excitation and measurement. The phosphor used in this invention silows a more intense emission pattern. The emitted light must be captured on the film. Photographic emulsions most commonly used in medical X-ray applications show a generally uniform response to radiation in the range 350 to 400 nm. Beyond 400 nm sensitivity becomes progressively lower; it virtually disappears beyond about 470 nm. YTaO4 and the Nb- and Tm-containing compositions herein show strong blue band emission and peaks around 400 nm and are m ore effective, when used with conventional film, than CaWO4. LuTaO4 and the Nb-containing compositions also show strong blue band emission. Tb-containing compositions of YTaO4, GdTaO4, LuTaO4 and solid solutions thereof show strong green emission around 500 nm, thus making them effective with X-ray film which is sensitive to such radiation. In a typical X-ray intensifying screen the powdered phosphor composition, as a thin layer, is adhered to a flexible support, such as cardboard or a polyester film, by means of a polymeric binder. The phosphor-binder composite can contain about 85% to about 96% phosphor, by weight. The phosphor layer is typically coated onto the support at a wet thickness of about O.005 Inch (0.0127 cm) to about 0.050 inch (0.127 em). The phosphor layer composition can be prepared, using common dispersion techniques, in any conventional binder. This is usually accomplished by ball-milling the phosphor, binder, solvent and other adjuvants for an appropriate period of time. The procedures and useful binding agents are fully described in the pricr art, for example, U.S. Patents 2,648,013; 2,819,183; 2,907,882, 3,043,710; and 3,895,157. Optionally, it is frequently desi@able to overcoa@ the phosphor layer with a thin, transparent F@o@@@tive layer, to permit @lsaning. üseful protective layers are also described in the prior art, @uch as the aforesaid United States patents. It is sometimes also desirable first to coat the flexible support (that is, beneath the phosphor) with a light reflective layer, such as a white pigment, to further intensify the light output of the screen. Reflective layers are well-known in the X-ray screen art. Those such as discloced in U.S. Patents 3,043,710 and 3,895,157 are particularly @eful. In actual use the phosphor-coated sheets re usually pressed against both sides of a photographic film with emulsion on each side. Such a pack with phosphor coatings adjacent to photographic emulsion coatings is held in a film cassette for X-ray exposure. Figure 6, a cross section of o-t anX-ray intensifying screen of this invention, shows the support 1, the reflective layer 2, the phosphor layer 3 and the protective coating t. Conventional supports which can be used include cardboard, suit-bly sized or coated, for example, with saryta, cellulose, acetate propionate, cellulose acetate, cellulose propionate, cellulose acetate butyrate. poly(vinyl chloride or vinyl acetate), polyamide, metal sheeting, for example, aluminum, and poly (ethylene terephthalate), the latter being a preferred support. For us@ in an X-ray intensifying screen, the support must be perme- able to X-rays. The support may have, for example, n thickness of about 0.00025 inch (0.00064 cm) to about 0.30 inch (0.76 cm), with films having a thickuess up to about 0.02 inch (0.05 cm) being prefcrred. The choice of screen thickness ganerally involves a trade-off between variables such as resolution and image noise. Thin screens have the advantage of high resolution, but suffer from high Image noise which is due in part to tho fact that' they absorb a smaller fraction of the incident X-rays. Figure 7 shows the relationship between resolution and total phosphor coating weight for CaWO4 and for YNb0.03Ta0.97O4 when a pair of screens is used to image blue-sensitive, Cronex# 4 X-ray Film with a conventional X-ray unit operating at 80 KVp, 2 mAs. Surprisingly, the YNb0.03Ta0.97O4 screen pair provides a superior result since, for the same resolution, a substantially higher coating weight can be used. This results in much higher X-ray absorption, and hence lower image noise, for the same resolution. For example, a pair of Cronex# Par intensifying screens made with CaWO4 has a phosphor content equivalent to pure CaWO4 having a thickness of about 84 m. Commercial-type screens made with YNb0.05Ta0.95O4, and having the same resolution, contain the equivalent of phosphor having a thickness of about 124 m. Figure 8 shows the calculated X-ray absorption or both materjals, the latter being shown on the figure as YNaO4:Nb. Over the range of importance in most; general-purpose radiography, 40-100 kiloelectron volts (KEV), the YTaO4:Nb screen has almost twice as much X-ray absorption as CaWO4 of equivalent thickness and, thus, Provides the possibility of reducing 'the X-ray exposure to almost half without increasing the image noise. One measure of performance of and X-ray intensifying screen is the speed (analogous to pbotographic speed), that is, the optical density achieved after X-ray exposure of a film in coutact with the screen. This has been determined by measuring the density of blackening of the X-ray film after exposure of such film in contact with the screen in acccrdance with the procedure carried out as set forth below. @@ Sample Freparation The phosphor is sieved through a 400 mesh sieve (U.S. Sieve Series) and the following components are introduced into a 15 cc-vial containing eight 4 m- diameter glass beads (see U.S. Patent 3,043,710): 6.19 g of YNb0.05Ta0.95O4 phosphor (the amount will vary directly with the density of the phosphor) 1.00 ml of butyl acetate 2.54 g of polyvinyl butyral binder. The resultant mixture is shaken on a Spex# oscillatory-type shaker for 15 minutes, then immediately coated on a pigment board using a Goodner mechanical coater and a 10 mil (254 m) draw-down knife. The resultant coating is dried in air for at least 15 minutes to give a dry coating of 5 # 1 nil (130 m 1 25 um) in thickness A 1 inch X 1-1/2 inch (2.54 x 3.81 cm) sample is cut from the coated board and mounted on a pigment board with other samples and standards for exposure tests. B. Sample Testing The pigment board with the screen samples attached is inserted into an 8 inch x 10 inch (20.3 cm x 25.4 cm) cassette containing a sheet of high speed medical X-ray film, in this instance Du Pont Cronex# 4 film. An X-ray exposure of 1.0 mAs (at 80 KVp) is made at á distance of 25 inches (63.5 cm) from a tungsten source filtered by 1/4 inch (6.35 mm) thick aluminum. The film then removed and processed in an automatic rapid proceasser which is operated at 33 C and contains the standard p-N-methylamine hydrcsulfate/hydroquinone de@eloper, with a total processing time of 90 seconds (to develop, fix, wash and dry). The optical density of the exposcd and develop@d film is determined using a MaeBeth digital densitometer, recorded as the "speed" and quoted relative to the density of an identical exposure rrade with a commercial Crone-4 Par intensifying screen made with CaWO4. As a result of impioved X-rey to light conversion efficiency, the speed of the X-ray intensifying screen made with the aforesaid M1- structure phosphor is higher than that r..ade with an M-structure phosphor of the same formula and of a similar coating thickness. Screens made with the M1structure phosphor containing an appropriate amount of the activator o intensifier ffo, Tm or Tb are faster than those made with similar phosphor coating which does not contain the activator or intensifier and they are faster than the commonly used CaWO4. This increased speed is important for applications in which lower X-ray exposure is desired. It is sometimes desirable to adjust the speed of a screen in order to better match the sensitivity of a particular film or to achievc a particular radiological objective. Since it is difficult to adjust the speed of a phosphor such as CaWO4, screen adjustments, for example, slower speeds, are achieved by adding a dye to the screen; however, this can result in image noIse being produced. There- fore, it is far more desirable to adjust the speed of the phosphor. The speed of the instant phocphor can be adjusted, over the range of intermediate spceds; between the highest speed measured and reported herein for the instant phosphor and the speed of CaWO4, or even lower, by changing the concentration of the activator or intensifier or by making an appropriate addition of certain other rare earth elements. The dependence of speed on activator concentration is moderately strong and control of speed in whis man@er rcquires very careful control of activator concentration. The dependence of speed on the concentration of the certain other rare earth elements is considerably weaker and the use cf such elements provides a convenient way of making phosphors with a substantially continuous range of speeds. The speed of tne instant phosphor can bc slowed by replacing up to about 15 mole percent of the yttrium, lutetium or gadolinium with ytterbium. For example, the addition of Yb to the preferred Nb-containing yttrium tantalate results in the phosphor of the composition Y1-yYbNb0.05Ta0.95O4 where y is 0 to about 0.15. If y is 0 in this formula, that is, if no Yb is present, it t can be seen that the resultant compound represents an embodiment of the previously described YNbxTa1-xO4. As y is increased, the speed of the phosphor decreases until, when y is about 0.1, the speed is about equal to that of the CaWO4 screen. Further increase in y results in further decrease in speed. The speed of the instant phosphor can also be slowed by replacing up to about 45 mole percent of the yttrium, lutetium or gadolinium with lanthanum. For example, the addition of La to the preferred Nb-containing yttrium tantalate results in the phosphor of the composition Y1-yLaNb0.05Ta.095O4 where y' is 0 to about 0.45. If y is O in this formula, that is If no Da is present. it. can be seen that the resultant compound represents an embodiment of the previously deseribed YNbxTa1-xO4, As y is increased, the steed of the phosphor S decreased and when y reaches about 0.4, the speed is about equal to that of the CaWO4 screen. Further increase in y results in further decrease in speed. The speed of the instant blue-emitting phosphor can be slowed by replacing up to about 15 mole percent of the yttrium or lutetium with gadolinium. For example, the addition of Gd to the preferred Nb- containing yttrium tantalate results in the pnosphor o the composition Y1-yGdyNb0.05Ta0.95O4 where y 0 to about 0.15. If y is O in this formula, that is, if no Gd is present, it can be seen that the resultant compound represents en embodiment of the previously described YNbxTa1-xO4. As y is increased, the speed of the phosphor decreases; when y Is about 0.08, the speed is about equal to that of the CaWO4 screen. Further increase in y results in further decrease in speed. Some lumineseent materials exhibit an undesirable delayed fluoreseence (sometimes called "afterglow", "phosphorescence" or "lag") that results from unexplained, but much slower, secondary emission. Delayed fluorescence becomes a problem only then the phosphor screen is used ill rapid succession for several films. Afterglow which persists for times longer than that taken for the film change will then impose a prior image as a ghost on the succeeding film. Net lag (that is, lag minus background) is absent in screens rade with the more preferred YNbxTa1-xO4 where x is about 0.05. hag is determined by exposing the screen sample to 400 mAs (80 KVp) X-rays from the same tungsten source described above, but without the aluminum filter, and then, after 15 seconds, contacting the screen sample with a fresh piece of film for 10 minutes. The film is then developed and the optical donsity is measured as described above. The optical density so recorded is the lag. Background is determined by exposing a piece of the same X-ray film employed to dctermine lag to the lag exposure in the absence of any test screcn. u diference between the hackground and lag is recorded as the net lag. Best Hode The preferred phosphor which emits blue light is YNb0.05Ta0.95O4. Stoichiometric amounts of Ta2O5, Nb2O5 and Y2O3 are well blended and th preflred in an alumina container in air at about 1000 C to about 12000C for about 10 to 14 hours. This material is then mixed with an equal amount, by weight, of flux. In the following examples of this composition LiCl was used as the flux; however, Li2SO4 is the preferred flux material. The resultant mixture is fired for 4 to 14 hours at 1200 C. The YNb0.05Ta0.95O4 M1 structure product in the form of a whIte @e uniform powder is recovered by water leaching, filtering and washing and then dried. In the following examples and experiments parts are by weight unless otherwise noted. X-ray excited speeds were determined by the procedure aescribed above and are reported relative to the speed of Cronex# Par intensifying screen made with CaWO4, the latter being 1.0 on the relative basis. Since relative intensities may vary with the time of measurement, the relative intensities reported in any of the following tables for compositions being compared therein (in any one table) were determined at substantially the same time. Example 1 YNb0.05Ta0.95O4 A. 10.000 g of Ta2O5, 0.317 g of Nb2O5 and 5.318 g of Y2O3 were wcighed to the nearest mg and blended for 30 minutes in a Spex# oscillating-type shaker. This mixture was prefired in an AL2O3 container in air at 1000 C for 14 hours. The prefired material was then mixed with LiCl flux (1 part of prefired material to 1 part of flux) and fired for 4 hours at 1200 C. The product was recovered by water leaching, filtering and washing and then dried .The product (Figure a depicts the electron scanning micrograph) was recovered as a white, uniform powder, which fluoresced bright blue under UV or X-ray excitation, having an average particle size of 4.2 m (as determined with a Firsher# subsieve sizer) and a surface area of 0.19 m2/g (as determined by gas absorption). X-ray diffraction measurements showed that it crystallized as a single phase with the M1 structure and the following lattice constants: a = 5.2975 A b = 5.4486 A c = 5.1103 A ss = 96.453 d@grees p (X-ray density) = 7.46 g ml-1. The X-ray excited spectrum is similar to that of CaWO4 but peaks at higher energy (409 vs 428 nm; See Figure 5). The X-ray excited speed is 1.86 times that of a Croncx# Par intensifying screen made with CaWO4. B. YNb0.05Ta0.95O4 was prepared in a manner identical to that used for pert A above except that the e firing at 18 hours was carried out at 1600 C, that is, above the transition tempcrature. The particulate product was recovered as colorless single-phase YNb0.05Ta0.95O4 with the M structure, as determined by X-ray diffraction, having a particle size of 5.3 m and a surface area 0.15 m2/g. The lattice parameters were: a = 5.3271 A b = 10.933 A c= 5.054 A ss = 95.52 degrees p (X-ray) -7.56 g ml-1. The X-ray excited speed was 0.84 relative to the Cronex# Par intensifying screen (CaWO4). This is to be compared with 1.86, found for the M1-structure material of the same formula as described in part A above, demonstrating the superior performance of the M1-structure material relative to the M-structure material. Differences in the X-ray diffraction and the Raman spectra of the M and M1 materials are shown in Figures 2 and The procedure of this part B was repeated, except that the flux of part A was absent and the firing temperature was 1500 C. Such procedure is representative of known solid state procedures. The electron scanning micrograph of the resultant product is shown in Figure lb. C. YNb0.05Ta0.95O4 with the M1 structure, made in the manner described in part A above, was converted to M-structure material by heating to a temperature of 1600 C for 2 hours. The X-ray excited speed for this converted material was 0.95, again demonstrating the superior performance of the M1structure material relative to the M-structure material. Examples 2-6 YNbxTa1-xO4 Compositions with various amounts of Nb were prepared according to the procedure used in Example 1. The quantities of the oxides used in each example are shown in Table I. All products were single phase with the M1 structure. Particlc sizes and surface areas of the phosphor particles are shown along with the X-ray excited speeds. These Examplas domonstrate that the instant prccess yields phosphors of particle size between about 3 m and 10 m and surface areas of about 0.075 m2/g to about 0.3 m2/g. Table I Grams Ex. Product No. Composition Ta2O5 Y2O3 Nb2O5 2 YTaO4 10 5.112 -0 3 YNb0.005Ta0.995O4 10 5.133 0.030 4 YNb0.02Ta0.98O4 10 5.216 0.123 5 YNb0.01Ta0.090O4 10 5.680 0.568 6 YNb0.15Ta0.85O4 10 6.014 1.062 Phosphor Particles Surface Ex. Size Area X-ray No. ( m) m2/g Speed 2 5.4 0.15 1.34 3 8.0 0.10 1.68 4 7.3 0.11 1.76 5 8.9 0.09 1.84 6 d.9 0.09 1.75 Example 7 To further demonstrate the higher speed of M1-structure material over M material, samples of M1 YTaO4 were prepared according to the procedure of Example 1 and then heated in air for 4 hours at the temperatures indicated in Table II in order to determine the temperature of conversion to M. After being cooled the structure of each sample was determined by X-ray diffaction and jts speed as an X-ray phosphor was @easured. As shown in Table II the speed drops markedly upon conversion of the phosphor to the M structure. Additional experiments which were carried out showed that the transition temperature for M1 to M conv@rsion is 1455 # 5 C. Table II Temperature to Which Heated Structure X-ray Speed 1200 C M1 1.08 1300 C M1 1.05 1400 C M1 1.08 1500 C M 0.63 Examplles 8-11 and Experiments A and B Y1-yTmyTaO4 Compositions with the M1 structure were prepared in a manner identical to that used for Example 1. The amouts of oxides used for each Preparation are shown in Table III. All products were single phase with the M1 structure. The X-ray excited speeds are also shown in Table III. For comparison Experiments A and B show that as y increases significantly above about 0.02, that is, above about 0.03, the speed decreases markedly. The phosphors of Experiments A and B are not useful in the invention. Table IIII Grams Ex. Product X-ray No. Composition Ta2O5 Tm2O3 Y2O3 Speed 8 Y0.998Tm0.002TaO4 10 0.017 5.102 1.72 9 Y0.995Tm0.005TaO4 10 0.044 5.086 1.78 10 Y0.99Tm0.01TaO4 10 0.087 5.061 1.68 11 Y0.98Tm0.02TaO4 10 0.175 5.010 1.39 Expt. No. A Y0.95Tm0.05TaO4 10 0.437 4.856 0.48 B Y0.90Tm0.10TaO4 10 0.873 4.601 0.19 Examples 12-16 LuNbxTa1-xO4 Compositions with various amounts of Nb were prepared according to the procedure used in Example 1. The quantities of the oxides used in each example are shown in Table IV. All products were single phase with the M1 structure. The X-ray excited speeds are also shown. Table IV Ex. Product Grams No. Composition Lu2O3 Ta2O5 Nb2O5 Speed 12 LuNb0.005Ta0.995O4 10 11.046 0.033 1.2 13 LuNb0.02Ta0.98O4 10 10.879 0.134 1.5 14 LuNb0.05Ta0.95O4 10 10.546 0.334 1.5 15 LuNb0.1Ta0.9O4 10 9.991 0.668 1.4 16 LuNb0.2Ta0.8O4 10 8.881 1.336 1.1 Examples 17-21 and Experiments C and D Y1-yTbyTaO4 Compositions with the M1 structure were prepared in a manner identical to that usen for Example 1. The amounts of oxides used fof each preparation are shown in Table V. All products were single phase with the M1 structure. For these compositions X-ray performance was measured by exposing a thick 1/2@ (1.27 cm) diameter pellet of phosphor to 20 KVp No radiation and dctecting the light output from the phosphor with an EMI-Geneom Ine. 9635Q photomultiplier. The Intensity of emission is compared to that from an equal area of a Du Pont Cronex# Lo-Dose/2 Mammography Screen made with CaWO4 (reported in the table as Relative Intensity). Considering the reduced sensitivity o@ the photomultiplier to green light versus blue and the inereased X-ray absorption relative to CaWO4 realizable with an 80 KVp X-ray beam, the effcctive speed is expscted to be higher than the relative intensity as measured herein. For a Cronex# 4 X-ray film properly sensitized for green light, the effective speed relative to the Cronex# Par/ Cronex# 4 screen/film comoination is expected to be about twice the relative intensity. Effective Speed is included in Table V along with the other results. For comparison Experiment C shows that the emission decreases as y increases above about 0.15 and Experiment D shows that emission decreases upon going from the t" structure te the -A structure. The l;i structure was obtained by heating M1-structure material (Y0.995Tb0.005TaO4 made the same as in Example 18) to 1600 C for 4 hours. The phosphors of Experiments C and D are not useful in the invention. Table V Effec Ex. Product Grams Relative tive No. Composition Y2O3 Ta2O5 Tb2O3 Intensity Speed 17 Y0.938Tb0.002TaO4 10 19.602 0.033 1.30 2.60 18 Y0.995Tb0.005TaO4 10 19.661 0.082 1.67 3.24 19 Y0.98Tb0.02TaO4 10 19.962 0.331 1.32 2.64 20 Y0.95Tb0.05TaO4 10 20.592 0.855 1.00 2.00 21 Y0.90Tb0.10TaO4 10 21.736 1.805 0.94 1.88 Expt. No. CY0.80Tb0.20TaO4 10 24.453 4.061 0.42 0.84 D Y0.995Tb0.005TaO4 10 19.661 0.82 0.43 0.86 (M structure) Examples 22-30 and Experiment E Gd1-yTbyTaO4 Compositions with the M1 structure were prepared in a manner identical to that used for Example 1. The amounts of oxide used for each preparation are shown in Table VI. All products were single phase with the M1 strueture. For these compositions X-ray performance was measured by the method used for Examples 17-21. The results are shown in Table VI. Experiment E shows that emission decreases as y increases above about 0.15. The phosphor of Experiment E is not useful in the invention. Table VI Effec Ex. Product Grams Relative tive No. Composition Ta2O5 Tb4O7 Gd2O3 Iutensity Spced 22 Gd0.998Tb0.002TaO4 10 0.017 8.189 0.81 1.62 23 Gd0.99Tb0.01TaO4 10 0.085 8.042 0.90 1.80 24 Gd0.98Tb0.02TaO4 10 0.169 3.042 1.06 2.12 25 Gd0.975Tb0.025TaO4 10 0.212 8.001 1.12 2.24 26 Gd0.97Tb0.03TaO4 10 0.254 7.960 1.15 2.30 27 Gd0.965Tb0.035TaO4 10 0.296 7.919 1.18 2.36 28 Gd0.96Tb0.04TaO4 10 0.338 7.878 1.10 2.20 29 Gd0.94Tb0.06TaO4 10 0.508 7.713 0.82 1.64 30 Gd0.5Tb0.1TaO4 10 0.846 7.385 0.96 1.92 Extp. No. E Gd0.8Tb0.2TaO4 10 1.692 6.565 0.43 0.86 Exampla 31 Gd0.97Tb0.03TaO4 A mixture of 54.9 g of Ta2O5, 43.70 g of Gd2O4; 1.40 g of Tb4O7 and 100.00 g of Li2SO4 was placed in an alumina container and fired at 1200 C for 4 hours. The product was recovered by leaching and washing and then dried. X-ray diffraction measurements cn the white product showed that it, was single phaso and had the M1 structure. The relative intensity of emission, measured by the method used for Examples 17-21, was 1.36. This result may be compared to that of Example 26 which was carried out according to the procedure of Example 1 which utilized both a prefining step and a firing step. Example 32 Gd0.97Tb0.03TaO4 A phosphor was prepared using ingredients and procedure identical to that of Example 31 except that the mixture s fired for 24 hours. The single phase M1 product gave a relative intensity of emission of 1.59, measured by the method used for Examples 17-21. Examples 33-36 Y0.5Gd0.5-yTbyTaO4 Solid solution compositions with the M1 structure were prepared in a meanner identical to th used for Example 1. The amounts of oxide used for each preparation are shown in Table VII. All products were single phase with the M1 structure. For these compositions X-ray performance was measured by the method used for Examples 17-21. The results are shown in Table VII. Table VII Effec EX. Product Grams Relarive tive No. Composition Y2O3 GdO3 Tb2O3 Ta2O5 Intensity Speed 33 Y0.5Gd0.493- 2.556 4.062 0.041 10 1.45 2.90 Tb0.005TaO4 34 Y0.5Gd0.49- 2.556 4.021 0.083 10 1.40 2.80 Tb0.01TaO4 35 Y0.5Gd0.475- 2.556 3.898 0.207 10 1.02 2.04 Tb0.025TaO4 36 Y0.5Gd0.45- 2.556 3.693 0.414 10 1.10 2.20 Tb0.05TaO4 Example 37 Lu0.995Tb0.005TaO4 This composition was prepared i@ a manner identical to that used for Example 1, except that 10.000 g of Ta2O5, 8.963 g of Lu2O3 and 0.041 g of Th2O3 were used. The product eas single phase wi@@ the M1 structure. X-ray performance, that is, the intensity of emission, measured by the method used for Examples 17-21, was 1.0. Examples 38-40 and Experiment F Y1-yYoyNb0.05Ta0.95O4 In order to demonstrate that the intrinsic speed of the phosphor can be adjusted by appropriate substitutions of elements, various amounts of Yb were substituted for Y in the phosphor with the highest speed measured and reperted herein, namely, YNb0.05 Ta0.95O4, an embodiment of the previously described TNbxTa1-xO4. The compositions with the M1 structure were prepared in a manner identical to that used for Example i. The amounts of oxide used for eacn preparation are shown in Table VIII. All products were single phase with the M1 structure. For these compositions X-ray performance was measured by the method used for Examples 17-21. The results are shown in Table VIII along with those for the YNb0.05Ta0.95O4 of Example 1 for comparison. The relative intensities of all compositions in the table were measured at substantially the same time. The phospher of Experimont F is not nesful in this invention. Table VIII Ex. Product Grams Relative No. Composition Yb2O3 Y2O3 Nb2O5 Ta2O5 Intensity 1 YNb0.05- 0 5.381 0.317 10 1.8 Ta0.95O4 38 Y0.995Yb0.005- 0.047 5.354 0.317 10 1.7 Nb0.05Ta0.95O4 39 Y0.965Yb0.035- 0.329 5.193 0.317 10 1.4 Nb0.05Ya0.95O4 40 Y0.9Yb0.1- 0.939 4.843 0.317 10 0.95 Nb0.05Ta0.95O4 Expt. No. F Y0.8Yb0.2- 1.878 4.305 0.317 10 0.31 Nb0.05Ta0.95O4 Examples 41-43 and Experiment G Y1-yGdyNb0.05Ta0.95O4 Gd can also be substituted in the instant composition to adjust the phosphor speed. In order to demonstrate this, varous amounts of Gd were substituted for Y in the phosphor with the highest speed measured and reported herein, namely, YNb0.05Ta0.95O4, an embodiment of the previously described YNxTa1-xO4. The compositions with the M1 structure were prepared in a manner identical to that used for Example 1. The amounts of oxide used for each preparation are shown in Table IX. All products were single phase with the M1 structure. For these compostions X-ray performance was measured by the method used for Examples 17-21. The results are shown in Table IX along with the results for the YNb0.05Ta0.95O4 of Example 1 for comparison. The relative intensities of all compositions in the table were measured at substantially the same time. The phosphor of Experiment G is not useful in this invention. Table 1X Ex. Product Grams Relative No. Coposition Gd2O3 Y2O3 Nb2O5 Ta2O5 Intensity 1 YNb0.05Ta0.95O4 0 5.381 0.317 10 1.9 41 Y0.99Gd0.01- 0.086 5.329 0.317 10 1.80 Nb0.05Ta0.95O4 42Y0.95Gd0.05- 0.432 5.114 0.317 10 1.26 Nb0.05Ta0.95O4 43 Y0.9Gd0.1- 0.864 4.845 0.317 10 0.83 Nb0.05Ta0.95O4 Expt. No. G Y0.8Gd0.2- 1.728 4.307 0.317 10 0.52 Nb0.05Ta0.95O4 Examples 44-47 and Experiments H and I 1-yLayNb0.05Ta0.95O4 Thosphor speed can also be adjusted by incorporating La into the phosphor. In order to demonstrate this, various amounts of La were substituted for Y in the phosphor with the highest speed measured and reported herain, namely, YNb0.05Ta0.095O4, an ombodiment of the previously described YNbxTa1-xO4. The compositions with the M1 structure were prepared in a manner identical to that used for Example 1. The amounts of oxide used for each preparation are shown in Table X. All products were single phase with the M1 structure. For these compositions X-ray performance was measured by the method used for Examples 17-21 The results are shown in Table X along with those for the YNb0.05Ta0.95O4 of Example 1 for comparison. The relative intensities of all compositions in the table were measured at substantially the same time. The phosphors of Exporiments H and I are not useful in this invention. Table X Ex. Product Grams Relative No. Composition Y2O3 La2O3 Nb2O5 Ta2O5 Intensity 1 YNb0.05Ta0.95O4 5.381 0 0.317 10 1.8 44 Y0.9La0.1- 4.843 0.776 0.317 10 1.6 Nb0.05Ta0.95O4 45 Y0.8La0.2- 4.305 1.553 0.317 10 1.4 Nb0.05Ta0.95O4 46 Y0.7La0.3- 3.767 2.229 0.317 10 1.1 Nb0.05Ta0.95O4 47 Y0.6La0.4- 3.229 3.106 0.317 10 0.9 Nb0.05Ta0.95O4 Expt. No. H Y0.5La0.5- 2.690 3.882 0.317 10 0.6 Nb0.05Ta0.95O4 I Y0.4La0.6- 2.152 4.658 0.317 10 0.3 Nb0.905Ta0.95O4 Example 48 A. A reflective. suspension was prepared by milling the following ingredients in a ball mill for a period of about 20 hours: Titantium Dioxide 100 g Chlorosulfonated polyethylene 40 g p-Butyl Acetate 105 g Mixed petroleum naphtha (Initial Boiling Point 247 F API grade of 59-61 at 60 F and specific gravity of 0.7385) 105 g Dioctyl ester of sodium sulfosuccinic acid 2 g. The milled suspension was filtered, coated on a 0.010 inch (0.0254 cm) thick sheet of biaxiallyomiented polyethylene terephthalate, to a wet thickness of 0.010 inch (0.0254 cm), and dried. coating procedure was repented several timos. B. A phosphor suspension was prepared by Milling the following ingredients in a ball mill for about 10 hours : M1-type Yttrium Niobium Tantalate Phosphor (YNb0.03Ta0.95O4) : 425 g Polyvinyl Butyral (PVB) Binder Solution 172.4 g. The PVB solution was comprised of the following ingredients : n-Butyl acetate 164.1 g n-Propanol 164.1 g Polymeric organic silicone fluid (2% by wt in toluene) 8.1 g Potassium salt of monoethylphenyl phenolmonosulfonic acid 2.2 g Glycerol monolaurate 13.5 g Polyvinyl butyral (granular, intrinsic viscosity 0.81) 54.0 g. This phosphor suspension was then coated over reflective elements which had been prepared as des cribed above in part A. The phosphor layer coating weights were varied from a wet thickness of 0.01 to 0.03 inch (0.0254 to 0.0762 cm). The screens prepared in this manner were suitable for use as X-ray screens. Each of these screen was tested by exposure, in conjunction with a portion of conventional X-ray film, to an X-ray unit at 80 KVP, 2 mAs through a 2 mm aiuminum filter. A step-wedge and a resolving power target were also placed between the X-ray source and the film/sereen (in direct contact therewith). Eacn film was then developed, fixed and wasned in a conventional X-ray developing system and the speed and resolution were determined for each sample. @nese measurements were also made on coptrol samples watch utilized conventional calcium tungstate phosphere, the osntrol samples bein@ propared andcoated, at similar coating weights, over similar reflective layers. For all comparable samples the YNbxTa1-xO4type phosphor screens show improved performance over the CaWO4 screens. This can be seen in Figure 7, were reslution is plotted against the phosphor coating weight, and in Figure 9, where the speed of the screen relative to Par CaWO4 = 1.0 is plotted aga'nst the resolution obtained. Example 49 A phosphor suspension was prepared by milling the following ingredients in a ball mill for about 6 hours : Yttrium Nibium Tantalate Phosphor (YNb0.05Ta0.95O4) : 4400 g Polyvinyl Butyral (PVB) Binder Solution 2977 g. The PVB solution was comprised of the following ingredients (in weight %) : n-Butyl acetate 40.4 n-Propanol 40.4 Polymeric organic silicone fluid (2% by wt in toluene) 2.0 Potassium salt of monoethylphenyl phenolmonosulfonic aced 0.6 Glyeerol monolaurate 3.3 Polyvinyl butyral (granular, intrinsic viseosity 0.81) 13.3. The phosphor suspension was then coateu over a reflective layer which had been preparad as deseribed in part A of Example 48 and which hed alrcady been coated on respective polyethylene tere@hthalate film supports as deseribed therein. Several indiviqual coatings were made, varying the wet thiakn@ss of the phosphor from about 0.02 to 0.045 inch (0.0508 to 0.1143 cm). After drying, each of the phosphor layers was ove@@@@@@@ @@th a thin prot@ctive layer of a cellulose ester polymer and the screens were then baked overnight at about 70 C. The final acreens prepared in this manner were eminently suitable for use as X-ray screens and were tested as described in Example 48. When compared to conventional CaWO4 screens they produced superior results in speed and image sharpness. Utility The luminescent material described above is useful a@ a phosphor in X-ray intensifying screens.

Claims: CLAIMS: 1. An X-ray intensifying screen which includes a phosphor as the active component characterised ill that the phosphor comprises a tantalate having the monoclinic M1 structure and having one of the following general formulae: (a) YNbxTa1-xO4 where x is 0 tp about 0.15; (b) LuNbxTa1-yO4 where x is O to about 3.2; (c) Yl-yTmyTaO4 where y is 0 to about 0.03; (d) Yl-yTbyTaO@ where y is about 0.001 to about 0.15; (e) Lu1-yTbyTaO4 where y is about 0.001 to about 0.15; (f) Gd1-yTbyTaO4 where y is about 0.001 to bout 0.15. 2. A screen according to claim 1 wherein the phosphor is a solid solution of the tantalates (a) and (b) or of (a) and (c) or is a solid solution of at least two o r the tantalates (d) , (e) and (f) . 3. A modification of the invention claimed in claim 1 or claim 2 whierein the phosphor comprises one of the tantalates (a) to (f) or (a! to (e) modified as follows. (g) up to 45 mole per cent of the yttrium, lutetium or gadolinium in any of the tantalates (a) to (f) is replaced with lanthanum; (h) up to 15 mole per cent of the yttrium, lutetium or gadolinium in any of the tantalates (a) to (f) is replaced with ytterbium; or (i) up to 15 mole per cent of the ytterium or lutetium in any of the tantaiales (a) to (c) is replaced with gadolinium. 4. A screen according to claim 1 wherein the phosphor is the tantalate of general formula YNbyTa1-xO4 wherein X is 0 to 0.15. 5. A screcn according to claim 4 wherein x is 0.02 to 0.10. 6. A screen according to claimm 5 wherein the phosphor is YNb0.05Ta0.95O4. 7. A screen according to any one of the preceding claims wherein the phosphor comprises particles of particle size from 3 to 10 m and surface area from 0.075 to 0.300m2/gram 8. A procese for preparing a phosphor for use in the screen claimed in any one of Claims 1 to 7 which process comprise the st sips: (a) intimately mixing stoichiometric quantities of appropriate precursor oxides; (b) prefiring the resultant mixture in air in an inert container at about 1000 C to about 1200 C for about 10 to about 14 hours; (c) mixing the resultant prefied mixture with a flux selccted from Li2SO4, LiCl and BaCl2/LiCl eutectie mixture, the weight of mac :: :iux being 1/5 to 1/2 of the total weight of the mixture; (d) firing the flux-containing mixture in an inert container at about 1100 C to less than aobut 1450 C or at least about 3 hours; end (e) recovering the phosphor. 9. A process according to claim 8 wherein the firing of the flux-containing mixture is carried out at about 1100 C to a@out 1300 C. 10. A process for pr@paring a phosphor for use in the screen claimed in any one of claims 1 to 7 which process comprises the s@eps: (a) intimately mixing stoichiometric quantities of app@opriate pred@rsor oxides and a flux selected from Li2SO4, Li@@ an@ a BaCl2/LiCl cutectic mixture; (b) firing tie resultant mixture in an inert container at about 1100 C to less than about 1450 C for at least about 4 hours; and (c) recovering the phosphor. 11. A process according to claim 1C wherein the firing is carricd out at about 1100 C to about 1300 C. 12. A process according to Claim 10 or Claim 11 wherein the flux is Li2SO4.

Inventors: BRIXNER, LOTHAR HEINRICH
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1979
Main classification: G21K4
IPC codes: C09K11, G21K4