CA2008542A1 - X-ray intensifying screen, phosphor composition, and process of phosphor preparation - Google Patents

Abstract

X-RAY INTENSIFYING SCREEN, PHOSPHOR COMPOSITION, AND PROCESS OF PHOSPHOR PREPARATION
Abstract of the Disclosure An intensifying screen for producing a latent image in a silver halide radiographic element when imagewise exposed to X-radiation is disclosed. The screen contains a phosphor having a hafnia host containing zirconia in concentrations higher than those found in optical grade hafnia. The phosphor can include as an activator one or a combination of titanium, rare earth, and alkali metal ions. Phosphor preparation processes are also disclosed.

Description

X-RAY INTENSIF~ING SCREEN, PHOSPHOR COMPOSITION, AND PROCESS OF PHOSPHOR PREPARATION
Field of the _n e~on The invention relates to novel X-ray intensiEying screens. More specifically, the invention xelates to fluorescent screens of the type used to absorb an image pattern of X-radiation and to emit a corresponding pat1ern of longer wavelength electromagnetic radiation. The invention additionally relates to certain novel phosphor compositions and to processes for their preparation.
Background Q~ ~h~ Invention A developable latent image is formed in a silver halide emulsion layer of a radiographic element 15 when it is imagewise exposed to X-radiation. Silver halide emulsions, however, more efficiently absorb and consequently are more responsive to longer (300 to 1500 nm) wavelength electromagnetic radiation than to X-radiation. Silver halide possesses native 20 sensitivity to both the near ultraviolet and blue regions of the spectrum and can be sensitized readily to the green, red, and infrared portions of the electromagnetic spectrum.
Consequently it is an accepted practice to employ intensifying screens in combination with silver halide emulsion layers. An intensifying screen contains on a support a fluorescent phosphor layer that absorbs the X-radiation more efficiently than silver halide and emits to the adjacent silver halide 30 emulsion layer longer wavelength electromagnetic radiation in an image pattern corresponding to that of the X-radiation received.
While the phosphor layer and emulsion layer can be integrated into one element, in most instances 35 the adjacent silver halide emul~ion layer i~ coated on a separate support to form a separate radiographic element. In thi~ way, the intensifying screent which s~

is not permanently altered by exposure, can be reused. The most common arrangement for X-radiation exposure is to employ a dual coated radiographic element (an element with silver halide emulsion layers 5 on opposite sides of a support), each emulsion layer being mounted adjacent a separate intensifying screen.
Phosphor~ employed in intensifying screens consist of a host compound, often combined with a small amount of another element that changes the hue 10 and/or improve~ the efficiency of fluorescence. It is generally conceded that the reliable and reproducible creation of phosphors is among the most difficult of chemical investigations. H.W. Leverenz, A~
Introduction to Luminesçençe of Solids, John Wiley &
Sons, Inc., New York9 1950, p. 61 states:
Synthesis and Symbolism of Phosphors Efficient general research on phosphors reguires (1) an exceptionally clean chemical laboratory equipped to synthesize very pure inorganic substances, (2) one or more furnaces capable of attalning at least 1600C with either o~idizing atmospheres, and (3) a physical laboratory having suitable sources of photons and charged material particles and means for controlling and determining the energies and numbers of these particles.
The che _c~l facilities are necessary to prepare luminescence-pure (LP) substances, who approximate degree of purity is indicated in the following series:
approx. 1%
...ore, approx. 90~/O
technically pure, 99.9%
chemically pure (CP), 99.99%
reagent-grade pure (RP), 99.999%
spectroscopically pure ~SP), 99.9999%
luminescence pure (LP), 100~/o ...completely pure.
Leverenz at page 62 and subseguently in his text goes on to suggest that research should be conducted with luminescence pure (LP> materia~s to reduce 10 discrepancies in results. Leverenz is of~ered as an example of the general practice in the phosphor art of employing materials of much lower permissible levels of impurities than in many other fields of ehemistry, ideally impurity levels at or below that of LP
15 materials~
It hag been recognized that the phosphors of highest absorption ePficiencies are those in which the host compound contains at least one element from Period 6 of the Periodic Table of Elements. For example, barium sulfate, lanthanide oxyhalides and oxysulfides, yttrium tantalate, and calcium tungstate, are widely employed phosphor host compound~.
One family of phosphor host compounds that have shown promise in terms of performance, but have been little used are rare earth hafnates. L.H.
Brixner, ~Structural and Luminescent Properties of the Ln2Hf207-type Rare Earth Hafnates", Mat. Res.
Bull., Vol. 19, pp. 143-149, 1984, describes investigations of such phosphor host compounds. Ln is defined to include not only lanthanides, but also scandium and yttrium. A significant practical disadvantage in formulatlng these host phosphor compounds is that ~iring to temperatures in the 1800 to 1900C range is required to obtain a single phase composition. These firing temperatures render rare earth hafnates burdensome to prepare as phosphor host compounds.

One hafnium containing phosphor host compound that has been recognized to possess high efficiency in its absorption of X-radiation, but has enjoyed no practical use is optical grade hafnia. Kelsey U.S.
Patent 4,006,097, issued May 5, 1975, discloses to be useful in the absorption of X-xadiation a phosphor satisfying the formula:
H~O2 Yb with Yb being present in a concentration of 5 X lO 3 to l X lO l, Brixner, cited above, after reporting the properties of Ti~4 as an activator for rare earth hafnates, stated:
We also looked at this same activator in pure HfO2. Under 30kVp Mo radiation x-ray excitation, this composition also emits in a broad band centered around 477 nm as seen in Fig. 5.
This emission has an intensity of about l.6 times that of PAR CaWO4 and could therefore be of interest as an x-ray intensifying screen phosphor, especially in light of the superior absorption of HfO relative to CaWO as seen in Fig. 6.
Unfortunately~ the ~rice of optical grade ~fO is so ~rohibitive ~,,h~ it can,not be used in screen applications. tEmphasi3 added.) Optical grade hafnia contains less than 3 X
lO 4 mole of zirconia per mole of hafnia. It is the difficulty in separating zirconium and hafnium that primarily accounts for the cost of optical grade 30 hafnia.
Zirconium and hafnium are known to be atoms of essentially similar radii, l.454A and l.442R, respectively. Practically all known compounds of zirconium and hafnium correspond to the +4 oxidation state. The chemical properties of the two elements are essentially identical. The elements are found together in nature and can not be entirely separated.

Zirconia and hafnia both exist predominantly in a stable monoclinic crystalline phase at room temperature, with the size of the crystal cell being very similar. As reported by by E. Iwase and S.
Nishiyama, "Luminescence Spectra of Trivalent Rare Earth Ions", Proc. Intern. ~ym. Mol. _ruct. Spectrv., Tokyo, 1962, A-407-1 to 7, the crystal lattice constants of monoclinic hafnia and zirconia are as follows:
Table I
Oxide ~ b-axis c-axis 2 5.11 5.14 5.28 99044l Zr2 5.21 5.26 5.375 9955' Iwase and Nishiyama investigated "high purity Hf and Zr compounds~ for cathodoluminescence - i.e., fluorescence response to electron bombardment.
D.K. Smith and H.W. Newkirk, "The Crystal Structure of Baddeleyite (Monoclinic ZrO2) and its Relation to the Polymorphism of ZrO2", A~ta Crys~~, 18, 1965, pp. 983-991, demonstrates that large single crystals of monoclinic zirconia can be produced by heating ZrO2 with a flux of Li2Mo207 in an oven at 1400C and withdrawing the sample when it reaches 900C.
Chenot et al U.S. Patents 4,068,128 and 4,112,194 disclose a variety of phosphors formed of varied ratio~ of phosphorus, hafnium, oxygen, and, optionally, zirconium. The various phosphor hosts produced by phosphorus in combination with hafnium are, of course, crystallographically dissimilar from hafnia host phosphors and offer no reliable indication of the effect of zirconium on the lumine3cence of monoclinic hafnia crystals.
Summary of the Invention In one aspect, the invention is directed to a screen comprised of a support and a fluorescent layer containing a phosphor capable of absorbing X-radiation and emitting longer wavelength electromagnetic radiation comprised of monoclinic cry~tals of a hafnia phosphor host. The intensifying screen is characterized in that zirconium ions are present in the hafnia phosphor host in concentrations higher than those found in optical grade hafnia.
In another aspect, the invention is directed to a composition containing a phosphor capable of absorbing X-radiation and emitting longer wavelength electromagnetic radiation which is comprised of monoclinic crystals of a hafnia phosphor host consisting essentially of oxygen and hafnium, zirconium, and alkali metal ions satisfying the relationship Hfl-zZrzMy wherein M represents at least one alkali metal;
y is in the range of from 6 X 10 4 to 1.0; and z is in the range of from 4 X 10 4 to 0.3.
In an additional aspect, the invention is directed to a process of preparing a phosphor capable : of absorbing X-radiation and emitting longer wavelength electromagnetic radiation comprised of activated monoclinic crystals of a hafnia phosphor host by heating a hafnium containing compound and an activator containing compound to a temperature sufficient to form and activate the hafnia phosphor host. The process is characterized in that a phosphor exhibiting improved emission properties is obtained by 30 forming a mixture of hafnium, zirconium, and alkali metal compounds containing thermally decomposable ligands chosen to leave residue~ consisting essentially of hafnium, zirconium, and alkali metal oxides on heating, the hafnium, zirconium, and alkali 35 metal ions present prior to heating satisfying the relationship:
~fl_zZrzMm wherein M represents at least one alkali metal;
m is greater than 5 ~ 10 ; and z is in the range of from 4 X lO 4 to 0.3.
5 Description of Preferred Embodiments An essential and novel feature of the intensifying screens of this invention is a hafnia host phosphor which contains higher levels of zirconium than is present in optical grade hafnia. As employed herein the term ~optical grade hafnia" refers to hafnia sold as optical grade or prepared from any optical grade hafnium source.
Optical grade hafnia is the purest form of hafnia commercially available and is therefore sometimes referred to in the literature as "pure hafnia" and in the phosphor art, where high levels of purity are the normal practice, simply as hafnia.
Optical grade hafnia contains less than 3 X 10 4 mole of zirconia per mole of hafnia.
The preferred phosphor host in the intensifying screens of this invention consists essentially of hafnia and zirconia with the hafnium and zirconium ions in the phosphor host sati~fying the relationship (I) Hfl_zZrz where z i9 in the range of from 4 X 10 4 to 0.3, most preferably from 1 X 10 3 to 0.2, and optimally from 2 X 10 3 to 0.1.
Stated another way, the present invention contemplates intbnsifying screens in which the phosphor host is prepared from reagent grade hafnia, - which, referring to relation~hip I, is commercially available with z being slightly less than 2 X 10 2.
Surprisingly, this material ~orms a phosphor having a higher level of luminescence than optical grade hafnia. From the Examples below it is apparent that f~

by blending reagent grade hafnium sources with optical grade hafnium sources so that z is about 1 X 10 2 peak phosphor luminescence intensities are realized.
The ~xamples below also demon~trate that by blending optical or reagent grade zirconium compounds with reagent grade hafnium compounds to ~orm zirconium rich hafnia host phosphors, higher luminescence intensities than demonstrated by optical grade hafnia are also realized, until z reaches a level greater than 0.3.
10 The fact that zirconia exhibits a lower luminescence intensity than hafnia when exposed to X-radiation is, of course, to be expected, since zirconium lies in Period 5 of the Periodic Table of Elements. That a limited range of zirconium concentrations in a hafnia host can increase phosphor luminescence has not been realized prior to this invention, The small amounts of other elements found in commercially available reagent grade hafnium and zirconium source compounds are not detrimental to intensifying screen performance. Therefore, other possible impurities of the phosphor host need be given no further consideration.
In the simplest form of the invention monoclinic reagent grade hafnia can be purchased and formed into an intensifying screen. It is also possible to purchase reagent grade hafnium compounds corresponding to the optical grade hafnium compounds employed by Kelsey and Brixner, cited above, and to prepare phosphors according to the invention by those conventional procedures.
To form monoclinic phosphor particles containing a selected ratio o~ hafnium and zirconium, commercially available sources of zirconium and hafnium are intimately intermixed, preferably by being dissolved in a common solvent, followed by coprecipitation. The hafnium and zirconium containing mixture is cho~en so that upon firing only hafnium, zirconium, and oxygen atoms remain as re~idue, any other moieties of the compounds being thermally decomposed or otherwise driven off in firing.
Common sources of hafnium and zirconium include the dioxides, the ba3ic carbonates, the oxychlorides, the oxynitrates, the sulfates, and the tetrachlorides. While the dioxides, the basic carbonates, and the sulfates can be used as purchased to produce phosphors, it i9 advantageous for both 10 handling and phospho-r performance to convert the other sources to less soluble solids that can be fired to give the monoclinic Hfl_zZrz02 phosphor desired. For example, treatment of a~ueous hafnium and zirconium ion containing solutions with base (e.g., alkali or ammonium hydroxide) gives a precipitate which is a mixture of hydrous hafnia and hydrous zirconia, the relative proportions of which depend upon those present in the starting materials.
Other useful solids satisfying Hfl_zZrz requirements can be produced by treating hafnium and zirconium ion containing solutions with organic precipitating agents, since organic materials consisting of carbon, hydrogen, and optionally nitrogen and/or oxygen leave no objectionable residue 25 upon thermal decomposition.
Hafnium and zirconium can be conveniently coprecipitated as carboxylates, such as those containing from about 2 to 20 carbon atomæ. The carboxylate moieties are in one preferred form 30 aliphatic carboxylates containing from about 2 to 10 carbon atoms, including both monocarboxylates and polycarboxylates - particularly dicarboxylates, such as oxalates, succinates, fumarates, etc. Aromatic carboxylates, such as benzoates, phthalates, and their ring substituted homologues, are also convenient to use. A particularly preferred class of carboxylates are a-hydroxycarboxylates containing from 2 to 10 carbon atoms, such as glycolates, lactates, and mandelates. Oxalic acid can be viewed as either a dicarboxylic acid or an ~-hydroxycarboxylic acid.
Oxalates are particularly preferred moieties for forming not only hafnium and zirconium compounds, but also compounds of other metals to be incorporated in forming preferred forms of the phosphor more particularly described below. The carboxylate moieties can form simple carboxylates with the hafnium or zironium or can form hafnium or zirconium carboxylate complexes including additional cations, such as alkali metal or ammonium ions.
The hafnium and zirconium carboxylates can be conveniently formed by reacting in a common solvent the acid, salt, or ester of the carboxylate with hafnium and zirconium containing compounds in the ratios desired in the phosphor. The hafnium and zirconium containing compounds to be reacted can be selected from among compounds such as hafnium tetrachloride, zirconium tetrachloride, hafnium oxychloride, zirconium oxychloride, hafnium basic carbonate, zirconium basic carbonate, hafnium nitrate, zirconium nitrate, zirconium carbonate, hafnium sulfate, zirconium sulfate, and mixtures thereof.
It is also contemplated to employ hafnium and zirconium alkoxides as starting materials. Preferred hafnium and zirconium alkoxideæ are those which satisfy formula II:
(II~
O D(OR)4 where D represents zirconium or hafnium and R represents a hydrocarbon moiety containing from about 1 to 20 (preferably about 1 to 10) carbon atoms.
The hydrocarbon moieties can be chosen from any convenient straight or branched chain or cyclic saturated or unsaturated aliphatic hydrocarbon moiety - e.g., alkyl, cycloalkyl, alkenyl, or alkynyl.
Alternatively the hydrocarbon moiety can be an aromatic moiety e.g.~ benzyl, phenyl, tolyl, xylyl, naphthyl, etc. In a specifically preferred from R is in each instance lower alkyl of from 1 to 4 carbon atom3. Hafnium and zirconium alkoxides are disclosed in U.S. Patents 3,297,414; 3,754,011; 4,525,468; and 4,670,472.
In addition to alkoxide and carboxylate 10 moiety containing hafnium and zirconium compounds various chelates, such as hafnium and zirconium ~-diketones and diaminecarboxylates can be employed.
Exemplary useful hafnium starting materials are set forth under heading III below. All the compounds have otherwise identical zirconium analogues. Further, although water of hydration has been omitted, it is to be understood that under normal ambient conditions most of the compounds exist as hydrates.
(III) Exemplary Hafnium Starting Materials H-l Hafnyl oxalate HfO(C204 H-2 Hafnyl oxalic acid E2[Hfo(c2o4)2]
25 H-3 Dioxalatohafnium Hf(C24)2 H-4 Trioxalatohafnic acid H2[Hf(C204)3]
H-5 Ammonium trioxalatohafnate (NH4)2[~f(C24)3]
H-6 Potassium tetraoxalatohafnate K4[Hf(C2o4)4]
~-7 Sodium tetraoxalatohafnate Na4[Hf (C24)4]
35 H-8 Ammonium ha$nyl oxalate (NH~)2[HfO(C204)2]
H-9 Polyoxalatopolyhafnic acids ~:~0~5~?Y, H-10 Potassium hafnyl tartrate K2[HfO(c4H4o6)2]
H-ll Tetramandelatohafnic acid X4[Hf(02CCHOC6H5)4]
H-12 Triglycolatohafnic acid H3HfOH(OCH2C00)3 H-13 Trilactohafnic acid H3HfOH(OCHCH3C00)3 H-14 Trioxodiha~nium stearate 10Hf203(02C(CH2)l6cH3)2 H-15Trioxodihafnium 2-ethylcaproate Ef2o3(o2cc~c2H5(c~2)3cH3)2 H-16Hafnium acetylacetonate Hf(C5H702)4 15 H-17Potassium bisnitrilotriacetohafnate K2{Hf[N(CH2C02)3~}
H-18Hafnium ethylenediaminetetraacetic acid Ef[(O2ccH2)2NcH2]2 H-19Hafnyl malonate 20HfO(02CCH2C02) H-20 ~afnyl phthalate HfO(02C6H4C02) H-21 Hafnium tetraisopropoxide Hf(OC3H7)4 25 ~-22Hafnium tetra-t-amyloxide Hf(C5H11)4 H-23 Hafnium tetra(phenoxide) Hf(OC6H5)4 H-24 Hafnium di(isopropoxide) bis(2-ethoxyethoxide) Hf(Oc3H7)2(0c2H4oc2H5)2 ~-25 Hafnium tetra(cyclohexoxide) Hf(C6H11~4 H-26 ~afnium di(isopropoxide) bis[2-(2-n-dodecan-oxyethoxy)e~hoxide]
Hf~OC3H7)2(0C2H40c2H40cl2H25)2 Formation of the zirconium rich monoclinic hafnia phosphor host is achieved by heating the 8~

~ 13-zirconium and hafnium compounds to temperatures up to and including 1400C. Higher firing temperatures can, of course, be undertaken, since the phosphor possesses high thermal stability. However, it i8 a distinct advantage of this invention that firing temperatures above 1400C are not required. Preferred firing temperatures are in the range of from about 900 to 1300C.
Firing is continued until conversion to the 10 monoclinic phase is achieved. For maximum firing temperatures the duration of firing can be less than 1 hour. While extended firing times are possible, once the phosphor has been converted to the monoclinic crystalline form, extending the duration of firing serves no useful purpose. Generally firing times in the range of from 1 to 10 hours, more typically 2 to 5 hours, provide full conversions of the starting materials to the phosphor composition sought.
Since the starting materials are in most instances decomposed at temperatures well below the 900C minimum temperature level contemplated for monoclinic crystal growth, it is generally convenient to heat the ~tarting materials to a temperature above their decomposition temperature, but below 900C, for an initial period to purge volatilizable materials before progressing to the higher crystallization temperatures. Typically, a preliminary heating step in the range of from about 300 to 900C7 preferably in the range of ~rom 400 to 700C, is undertaken.
It is also often convenient to divide firing into two or more consecutive steps with intermediate cooling to permit grinding and/or washing the material. Intermediate grinding can facilitate uniformity while intermediate washing, typically with distilled water, reduces the risk of unwan~ed contaminants~ such as starting material decomposition by-products.

s~

It has been discovered that firing the hafnia phosphor in the presence of a flux of one or a combination of akali metal ions incorporates alkali metal ion in the phosphor and dramatically increases its luminescence intensity. A preferred class of phosphors according to the present invention are those that satisfy the relationship:
(IV) Hfl-zZrzMy 10 where M represents at least one alkali metal;
y is in the range of from 1 X 10 4 to 1 (preferably 0.2); and z is as defined above.
Investigations have revealed that the benefits of alkali metal ion inclusion are fully realized at relatively low concentrations and incorporation of alkali metal ions in concentrations above those required for maximum luminescence enhancement are not detrimental to luminescence.
There is no phosphor performance basis for limiting y to values of 1 or less. Rather it is primarily a phosphor preparation convenience.
Alkali metal ion inclusion in the phosphor can be conveniently accomplished by forming a mixture of the hafnium and zirconium starting materials discussed above and a compound capable of releasing alkali metal ions on heating. The amount of the alkali metal compound employed is chosen to supply alkali metal ion in a concentration in excess of that sought to be incorporated in the phosphor. Th~s, the following is contemplated as a starting material relationship:
(V) Hfl_zZrzMm wherein M represents at least one alkali metal;

m is greater than 3 X 10 2 (preferably from 1 X
10 1 to 6); and z satisfies any of the values noted in connection with relationships I and II.
The alkali metal compounds can be alkali metal analogues of the hafnium and zirconium starting materials discussed above. Preferred alkali metal compound starting materials include alkali metal carbonates, sulfates, oxalates, halides, hydroxides, borates, tungstates, and molybdates. Mixtures of alkali metal starting materials are contemplated, particularly when different alkali metals are being concurrently incorporated in the phosphor. Since in one form the hafnium and æirconium complexes of formula II can contain alkali metal ion, the alkali metal can wholly or in part be provided by these complexes. A convenient preparation approach is to employ alkali metal containing hafnium and zirconium complexes satisfying formula II and to increase the alkali metal content of the starting materials by adding other alkali metal compounds, as indicated above.
In relationship V, m can range of up to 10 or more. Most of the excess of alkali metal is removed ~5 during phosphor preparation. When an excess of alkali metal is incorporated in the phosphor, it is preferred to divide firing into two or more sequential steps with intermediate grinding and washing to remove soluble alkali metal compounds. This reduces the level of alkali metal compounds available for release during heating in a corrosive volatilized form and al~o reduces the possibility of forming less desirable secondary phase~.
Investigation o alkali metal containing zirconium rich hafnia phosphors indicates that they exhibit increased levels of luminescence even after extended washing has reduced the alkali metal content to very low levels, approaching detection limits.
While it is believed that the alkali metal is incorporated into the monoclinic crystals of the phosphor, this has not been conclusively established.
It is possible that the alkali metal content of the phosphor is at least partially a surface remnant of the alkali metal flux on the surface of the monoclinic crystals during their formation during firing.
The highest levels of phosphor luminescence 10 have been obtained by employing lithium as an alkali metal. In a preferred form lithium containing phosphors according to this invention satisfy the relationship:
~VI) Hfl_zZrzLiy wherein y is in the range o~ from 8 X 10 4 to 0.15 and z is selected from any one of the ranges indicated above.
Lithium containing phosphors according to this invention are preferably prepared by selecting starting materials so that the hafnium, zirconium, and lithium ions present prior to heating satisfy the following relationship:
~VII) Hfl_zZrzLim wherein m is in the range of from 4 X 10 2 to 2.0 (optimally from 7 X 10 to 1.5) and z is selected as described above.
When lithium is selected as the alkali metal, it has been observed that, in addition to forming the zirconium rich hafnia phosphor with lithium included, a second phase of lithium hafnate can be formed, depending upon the proportion and selection of lithium compound starting materials. Since lithium hafnate lacks the luminescence intensities of titanium and 5~

lithium con~aining hafnia, a preferred embodiment of the invention, lithium starting materials and their concentrations are selected so that any overall luminescence of the two phases remains higher than that attained in the absence of lithium. As demonstrated in the Examples, increasing levels of lithium carbonate employed as a starting material results first in an increase in overall luminescence eventually followed by a decrease in overall luminescence attributed to the formation of increasingly larger proportions of lithium hafnate.
On the other hand, employing lithium sulfate as a starting ~aterial, increasing proportions result in peak luminescence with still higher proportions of lithium sulfate resulting in a relatively constant high level of luminescence, indicating that the proportion of lithium hafnate ~hich is formed as a second phase is limited at higher lithium sulfate concentrations in the starting materials.
Sodium and potassium compounds employed as ~tartin~, materials in place of lithium compounds also result in markedly increased levels of phosphor luminescence. These alkali metal starting materials, of course, avoid any possibility of forming a lithium hafnate second phase and can therefore be employed well above the preferred maximum concentration levels of lithium startin~, materials without any performance penalty. On the other hand, it has been obsexved that sodium and potassium ions are guite effective at lower coneentrations. Therefore, when M in relationship IV
represents at least one of sodium and potassium, y is preferably in the range of from 6 X 10 4 to 7 X
10 2 (optimally from 8 X 10 4 to 7 X 10 2) The alkali metals cesium and rubidium are also effective to increase phosphor luminescence, but to a lesser extent that than lithium, sodium, and potassium, Combinations of any and all of the alkali S~

metals can be employed in preparing the phosphors of this invention. Particularly useful are combinations of at leaYt two of lithium, sodium, and potassium ions. Lithium and potassium ion combinations have 5 produced particularly high levels of lumine~cence.
It is generally preferred to increase the fluorescence efficiency of the phosphor by blending with the phosphor host before firing a small amount of an activator. Any known activator for optically pure 10 hafnia phosphors can be employed. Titanium (e.g., Ti+4), taught for use in optical grade hafnia by Brixner, cited above, is specifically contemplated for use as an activator. In one preferred form of the invention titanium is incorporated in the phosphor as 15 an activator. Thus, in one preferred form of the phosphor hafnium, zirconium, and titanium are present and satisfy the relationship (VIII) ~fl_zZrzTix where x is the range of from 3 X 10 4 to 1.0 (preferably 0.5 and optimally 0.25) and z satisfies any of the ranges previously indicated.
It is possible to introduce the titanium activator by physically mixing titania with any of the host phosphor forming materials described above. It has been discovered, however, that higher luminescence levels at lower titanium concentrations are possible 30 when the titanium activator in the form of a thermally decomposable compound is physically blended with thermally decomposable hafnium and zirconium compounds. The thermally decomposable moieties of the titanium activator compounds can be selected from among the same compound classes described in connection with hafnium and zirconium. Titanium carboxylates, where the carbo2ylates are chosen as described above, are particularly preferred starting materials for the incorporation of titanium.
The inclusion of titanium in the zi~conium rich hafnia host phosphor not only greatly increases the total luminescence of the phosphor, but also shifts the maximum emission wavelength of the phosphor from the ultraviolet to the blue portion of the spectrum. Emissions in the blue portion o~ the spectrum are more useful for intensifying screen use, since the silver halide emulsions of radiographic elements which are employed in combination with intensifying screens possess native blue sensitivity and/or can be readily spectrally sensitized to these wavelengths while the organic vehicle of the emulsion is transparent in the blue portion of the spectrum.
In a specifically preferred form of the invention the zirconium rich hafnia phosphors include both alkali metal ion and titanium, each introduced as described above. In this form the phosphor satisfies the relationship:
(IX) ~ fl_zZrzMyTix where x, y, and z are as previously defined.
It has been surprisingly discovered that disproportionately large enhancements of luminescence are realized when both alkali metal ion and titanium are incorporated in the phosphor. That is, the luminescence increases imparted by each of the alkali metal ion and titanium alone when added together do 30 not equal or even approach the magnitude of the luminescence increase imparted by a combination of alkali metal ion and titanium employed together in the - phosphor.
Rare earth activators for the phosphors of this invention are also contemplated. As applied to this invention the term "rare earth" is intended to include scandium, yttrium, and the lanthanides. The s~

following rare earths, taught by Iwase and Nishiyama, cited above, for use in high purity hafnia are also contemplated: praseodymium (e.g., Pr+3), samarium (e.g., Smt3), europium (e.g., Eu~3), terbium (e.g., Tb+3), and dysprosium (e.g., Dy+3).
Ytterbium, suggested by Kelsey, cited above, can also be employed as an activator. Gadolinium (e.g., Gd+3) is al.so shown to be an activator in the Examples below. Rare earth activators can enhance the intensity of phosphor luminescence, as demonstrated in the Examples below, and can be employed to shift the emission spectra of the phosphors of the invention.
Rare earth activated zirconium rich hafnia phosphors according to this invention preferably satis~y the relationship:
(X) Hfl_zZrzLw or (XI) Hfl_zZrzMyLw wherein L represents at least one rare earth element;
w is in the range of from 3 X 10 4 to <5 X
10 2, preferably from 1 X 10 3 to 2 X 10 2; and y and z are as previously described.
As illustrated in the examples below, the zirconium can, as a function of its concentration, also permit fine tuning of the peak emission wavelength of the phosphor. Fine tuning to match the peak emission wavelength of the phosphor to the peak absorption wavelength of the silver halide emulsion layer to be exposed can have a significant impact on the efficiency of the overall imaging system. Thus, the zirconium a plays an important role not only in increasing the luminescence of the phosphor while reducing its cost, but also in optimizing its performance. By selection, specific combinations oE

~ 7~ `r)' S~ ~-J 7,'1 ,~

l5>~.~

zirconium concentrations and activator can produce phosphors with peak emission wavelengths that match dye absorption peaks in silver halide emulsion layers o~ radiographic elements.
The zirconium rich hafnia phosphors, once formed to satisfy the composition requirements of this invention, can be employed to form an intensifying screen of any otherwise conventional type. In its preferred construction the intensifying screen is comprised of a support onto which is coated a fluorescent layer containing the zirconium rich hafnia phosphor in particulate form and a binder for the phosphor particles. Zirconium rich hafnia phosphors can be used in the fluorescent layer in any conventional particle size range and distribution. It is generally appreciated that sharper images are realized with smaller mean particle sizes. Preferred mean particle sizes for the zirconium rich hafnia phosphors of this invention are in the range of from from 0.5 ~m to 40 ~m, optimally from 1 ~m to 20 ~m.
It is, of course, recognized that the zirconium rich hafnia phosphor particles can be blended with other, conventional phosphor particles, if desired, to form an intensifying screen having optimum properties for a specific application.
Intensi~ying screen constructions containing more than one phosphor containing layer are also possible, with the zirco~ium rich hafnia phosphor particles being present in one or more of the phosphor containing layers .
The fluorescent layer contains su~ficient ~inder to give structural coherence to the zirconium rich hafnia layer. The binders employed in the fluorescent layers can be identical to those conventionally employed in fluorescent screens. Such binders are generally chosen from organic polymers ~3~5~

-2~-which are transparent to X-radiation and emitted radiation, such as sodium o-sulfobenzaldehyde acetal of poly(vinyl alcohol); chlorosulfonated poly(ethylene); a mixture of macromolecular bisphenol poly(carbonates) and copolymers comprising bisphenol carbonates and poly(alkylene oxides); aqueous ethanol soluble nylons; poly(alkyl acrylates and methacrylates) and copolymers of alkyl acrylates and methacrylates with acrylic and methacrylic acid;
poly(vinyl butyral); and poly(urethane) elastomers.
These and other useful binders are disclosed in U.S.
Patents 2,502,529; 2,887,379; 3,617,285; 3,300,310;

3,300,311, and 3,743,833; and in Research Disclosure, Vol. 154, February 1977, Item 15444, and Vol. 182, June 1979. Particularly preferred intensifying screen binders are poly(urethanes), such as those commercially available under the trademark Estane from Goodrich Chemical Co., the trademark Permuthane from the Permuthane Division of ICI, Ltd., and the trademark Cargill from Cargill, Inc.
The support onto which the fluorescent layer is coated can be of any conventional type. Most commonly, the support is a film support. For highest levels of image sharpness the support is typically chosen to be black or transparent and mounted in a cassette for exposure with a black backing. For the highest attainable speeds a white support, such as a titania or barium sulfate loaded or coated support is employed.
Any one or combination of conventional intensifying screen features, such as overcoats, subbing layers, and the like, compatible with the featureæ described above can, of course, be employed.
Both conventional radiographic element and intensifying screen constructions are disclosed in ~Rsearch Disclosure, Vol. 184, Aug. 1979, Item 18431.
Research Disclosure i8 published by ~enneth Mason ~ 5 Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
In one specifically preferred form of the invention, illustrating intensifying screen3 satisfy;ng the requirements of the invention intended to be employed with a separate silver halide emulsion layer containing radiographic element, the zirconium rich hafnia phosphor can be substi-tuted for any of the conventional phosphors employed in either the front or back intensifying screens of Luckey, Roth et al U.S.
Patent 4,710,637. Similar modification of any of the conventional intensifying screens disclosed in the following patents is also contemplated: DeBoer et al U.S. Patent 4,637,898; Luckey, Cleare et al U.S.
Patent 4,259,588; and Luckey U.S. Patent 4,032,471.
While the zirconium rich hafnia phosphors can be employed for their prompt emission following exposure to X-radiation, they can also be employed as storage phosphors -that is, for their ability to emit electromagnetic radiation in a chosen wavelength range after being exposed to X-radiation and then stimulated by exposure to radiation in a third spectral region.
For example, the phosphors of this invention can be employed in imaging systems of the type disclosed by Luckey U.S. Patent 3,859,527. When employed in such a Yystem the refractive indices of the phosphor and binder are preferably approximately matched, as disclosed by DeBoer et al U.S. Patent 4,637,898.
Examples The invention can be better appreciated by reference to the following specific examples.
E~amples 1-9 Phosphors Containing Varied Ratios of Hafnium and Zirconium (Hfl_zZrz) The purpose of presenting these investigations is to demonstrate that, by varying the zirconium content in a hafnia host phosphor, enhanced phosphor luminescence intensity is achieved over a limited zirconium concent~ation range in which the zirconium content is higher than that found in optical grade hafnium sources, but still only a minor constituent.
Hafnia phosphor samples containing varied amounts of zirconium substituted for hafnium were prepared by the decomposition of the appropriate trilactohafnic and trilactozirconic acid complexes.
The complexes were prepared by the general method 10 described ln W. B. Blumenthal, ~'The Chemical Behavior of Zirconium,~ VanNostrand, Princeton, N. J., 1958, p 333. The varying Hf:Zr ratios are obtained by using the appropriate mixtures of zirconium and hafnium oxychlorides in the precipitation reactions. The oxychlorides were obtained from Teledyne Wah Chang Albany (located at Albany, Oregon) and used as received. The Hf:Zr ratios in the samples were determined from the analytical batch analyses provided by the supplier.
The preparation of trilactohafnic acid for Example 1 was carried out in the following manner:
Optical grade ~Hfl_zZrz, z = 0.000276) hafnium oxychloride ~40 g) and ACS reagent lactic acid ~44 g) from Eastman Kodak Company were each dissolved in about 120 ml of distilled water. The hafnium oxychloride solution was added to the lactic acid solution with rapid stirring to form a precipitate, and the reæulting mixture was heated to 80C with continued stirring for about 0.5 hours. The cooled 30 mixture was filtered, and the collected solid was washed with distilled water. After drying for 15 hour~ at 80C, the ~olid weighed 42 g. (~or C9H1601oHf: theory, C=23.4%, H=3.5%; found, C-22.7%, ~=3.5%).
Approximately 13 g of the trilactohafnic acid was placed in a 50 mL alumina crucible, covered with an alumina lid, heated in air to 700C for one hour in an ashing furnace, then cooled to room temperature.
The solid was transferred to a 20 mL alumina crucible, which was covered with an alumina lid~ The covered 20 mL alumina crucible was placed into a 50 mL alumina crucible, which was thereafter covered with an alumina lid. The crucible assembly wa~ heated to 1000C and maintained at that temperature for 2.5 hours before cooling to room temperature. The resulting solid was ground with an agate mortar and pestle to give a powder that wa~ returned to the 20 mL alumina crucible. The 20 mL crucible was covered with its alumina lid and then heated to 1400C and maintained at that temperature for 1.5 hours before cooling to room temperature. The resultin~ solid was ground with an agate mortar and pestle to give a uniform phosphor powder.
The Example 1 phosphor powder sample was made from optical grade hafnium oxychloride and contained the lowest amount of zirconium. The Example 5 sample 20 wa~ made from reagent grade (designated by the supplier as Reactor Grade Special and subsequently also referred to as R.G.S.) hafnium ~fl zZrz, z -0.019) oxychloride. The Example 2, 3, 4A, and 4B
samples were made by mixing appropriate amounts of the optical grade and reagent grade hafnium oxychlorides.
The Example 6 to 9 samples were made by mixing appropriate amounts of reagent grade hafnium and zirconium oxychloride to obtain a zirconium content indicated in Table I~.
The luminescence response of the phosphor powder was in this and all subsequent Examples measured by placing the phosphor powder sample in aluminum planchets (2mm high x 24 mm diam) at a coverage of about 1.1 g/cm2 and exposing to X-radiation. The X-ray response wa3 obtained u~ing a tungsten target X-ray source in an XRD 6TM
generator. The X-ray tube was operated at 70 kVp and ~3~

10 mA, and the X-radiation from the tube was filtered through 0.5 mm Cu and 1 mm Al filters before reaching the sample. The luminescent response was measured using an IP-28TM photomultiplier tube at 500 V
bias. The voltage from the photomultiplier was measured wi~h a KeithleyTM high impedance electrometer and is proportional to the total light output of the sample.
The major luminescence peak of the phosphor samples was centered at about 280 nm. This value was obtained by taking the prompt emission spectrum of the powder uslng the unfiltered X-ray source described above. The tube w~s operated at 70 kVp and 30 mA.
The qpectrum was acquired with an Instruments S.A.
Model HR 320TM grating spectrograph equipped with a Princeton Applied Research Model 1422/OlTM
intensified linear diode array detector. The data acquisition and processing was controlled by a Princeton Applied Research Model 1460 OMA IIITM
optical multichannel analyzer. The spectrum was corrected for the spectral response of the detector-spectrograph combination.
The relative luminescence intensity of the phosphor powder samples as a function of their zirconium conten-t is set out in Table II.
_able II
Hfl_zZrz EXAMPLE N0. Zr CONTENT (z)RELATIVE INTENSITY
1 (Control) 0.000276 100 2 0.00040 231 3 0.0010 238 4A 0.01 710 4B 0.01 743 0.019 365 6 0.10 350 7 0.20 155 8 0~30 224 9 (Control~ 0.50 80 2~

The data of Table II demonstrate that there is an enhancement in hafnia phosphor performance when the zirconium level increased over that found in optical grade hafnium sources (repre~ented by the Control 1). Ranges of z of from 4 X 10 4 (0.0004) to 0.3 are demonstrated -to exhibit higher luminescence intensities than optical grade hafnia. Best results are demonstrated when z is in the range of from 1 X
(0.001) to 0.2, optimally in the range of from 5 X 10-3 (0.005) to 0.1.
Examples 10-14 Preparation of Phosphors in the Presence of an Alkali Metal Ion (Hfl-zZrzMm~
The purpose of presenting these investigations is to demonstrate that the performance of hafnia host phosphors with an elevated zirconium level shown to be effective in Examples 1-9 can be further dramatically improved by preparing the hafnia phosphor in the presence of an alkali metal ion.
In each example a sample consi~ting of 14.72 gram3 of trilactohafnic acid (prepared as described in Examples 1-9 from RGS hafnium oxychloride, z -- 0.019) was thoroughly ground with an agate mortar and pestle with K2C03 or Li2C03 (Alfa Products; Ultra 25 Pure ~rade). The mole percent of the alkali carbonate flux, based on hafnium, was chosen as indicated below in Table III. The mixtures prepared were heated as described above in Examples 1-9, except for the addition of a wa~hing step after firing to lOOO~C.
30 This step involved washing the charge with 159 mL of distilled water for 1 hour. The solid was collected and dried for 5 minute intervals at 20, 35 and 50%
power in a 500W CEM model MDS-81TM microwave oven.
The procedure described above in Examples 1-9 was then completed.
X-ray diffraction analysis of the samples confirmed the presence of monoclinic hafnia. The presence of alkali metal ion in the phosphor powder samples prepared in the presence of alkali carbonate flux was confirmed by atomic absorption analysis.

Hfl_zZrzMm E_~mEl~ _ M m_ Intens~(Ex. 1_= lOQ~

K 0.2 520 11 K 0.5 510 10 12 K 2.0 545 13 K 4.0 1005 14 Li 0.14 1005 A 140 to 275 percent increase in luminescence intensity relative to Example 5 is seen in the above examples containing alkali metal ion.
Referring back to Example 1, it is apparent that the hafnia phosphor samples containing both zirconium in higher levels than found in optical grade hafnium sources and alkali metal ion exhibit luminescence intensities ranging from >5 to >10 times those demonstrated by the hafnia phosphor prepared ~rom an optical grade hafnium source.
~xa~21Q~ 18 Titanium Activated Phosphors (Hf l_zZrzTix) The purpose of presenting these investigations is to demonstrate the utility of titanium as an activator ~or the hafnia phosphors of this invention containing higher than optical grade concentrations o zirconia. The titanium also shifts 30 the maximum spectral emission band of the phosphor to visible wavelengths in the blue portion of the spectrum.
In each example a sample consisting of 14.72 gram~ of trilactohafnic acid (prepared as described above in Example~ 1-9, z = 0.019) was thoroughly ground with varying portions of ammonium bis(o~alato)-oxotitanium (IV), (NH4)2TiO(C204)22H20, from Johnson Matthey (99.998%). The mole percent titanium, based on hafnium, i~ indicated below in Table IV. The mixtures were heated and further examined as in Examples 1-9.
X-ray diffraction analyses of Examples 17 and 18 each showed traces of unreacted TiO2. A small amount of hafnium titanate was detected as an i~purity phase in Example 18.
The relative luminescence outputs of Examples 5 and 15-18 are set out in Table IV. Not only were the luminescence outputs greatly increased in Examples 15-18, but the luminescence band maximum shifted to 475 nm, thereby providing increased emissions of visible spectrum wavelengths more advantageous fo~
intensifying screen applications.
Table IV
Hfl_zZrzTix Exam~ xIntensity (Ex. 1 = 100) 20 15 0.02 5330 16 0.05 4000 17 0.10 2730 18 0.~5 1680 From Table IV it is apparent that the inclusion of titanium in the hafnia phosphor samples containing higher than optical grade zirconium concentrations resulted in large increases in luminescence intensities. Thus, the titanium acted as an activator for the phosphor samples. 0 Examples 19-33 Preparation of Titanium Activated Phosphors in the Presence of Lithium Carbonate (Hfl_zZrzTixLim) The purpose of presenting these investigation~ is to demonstrate that the performance of hafnia host phosphors with an elevated zirconium level (z = 0.19) and containing titanium as an activator can be further improved by preparing the hafnia phosphor in the presence of an alkali metal ion.
A sample consisting of 12.26 g of trllactohafnic acid (prepared as in Examples 1-9) was 5 thoroughly ground with 0.1 g (5 mole percent, x =
0.05~ of TiO2 (EM Chemicals; Optipur grade) and a selected amount of Li2C03 ~Alfa Pxoducts;
Ultrapure grade). The mixtures were processed and teeted similarly as in Examples 10-14. In Examples 10 21-23 the size of the trilactohafnic acid sample was 13.00 grams with the titania increased to 0.106 g to maintain the titanium at 5 mole percent (x = 0.05).
The relative intensity of the titanium activated phosphor samples as a function of the alkali 15 metal flux employed is given in Table V.
Table V
Hfl_zZrzTixMm Example _ mIntensitv (Ex. 1=100) 0.01 2210 21 0.02 1000 22 0.06 3380 23 0.10 6370 24 0.10 5960 0.20 13500 26 0.20 14000 27 0.40 13700 28 0.50 13300 29 0.50 13500 1.0 8695 31 1.5 5610 3~ 2.0 3155 33 4.0 735 Samples in which more than 10 mole percent (m 35 = 0.20) Li2C03 was added revealed the presence of lithium hafnate in the X-ray powder patterns. The amount of lithium hafnate formed in the samples increased with the Li2C03 amount. At 200 mole percent (m = 4.0) Li2C03 added, lithium hafnate is the primary phase.
From Table V it can be appreciated that values of m of from about 4 X lO 2 (o 04) to 2.0 gave significantly impro~ed results, with values of m of from about 1 X 10 1 (0.10) to 1.5 providing the highest luminescence intensitie~ observed in these comparisons.
In these comparisons it should be noted that 10 Example 19 did not provide luminescence intensity as high as that reported in Table IV for Example 16, even though both contained 5 mole percent -titanium (x =
0.05) and neither was prepared in the presence of an alkali metal flux. This difference is attributed to the less efficient incorporation of the titanium activator in Example 19 resulting from employing titania rather than a titanium carboxylate salt as a starting material.
Examples 34-43 Preparation of Titanium Activated Phosphors in the Presence of Lithium Sulfate (Hfl_zZrzTixLim) The purpose of presenting these investigations is to demonstrate that the proportions of lithium hafnate formed as a second phase can be controlled and reduced by substituting another lithium salt for lithi~m carbonate.
The same procedures were employed as in ~xamples 19-33, except that for Li2C03 there was substituted Li2S04 ~Aldrich anhydrous: 99.99%).
The relative intensity of the titanium activatived phosphor samples as a function of the lithium sulfate flux employed is given in Table VI.
In Table VI the performance data from Table V is also represented for samples prepared using lithium carbonate at the concentration levels as the lithium sulfate.

~ 5f-~2 Table VI
Hfl_zZrzTixMm Li2C3 Li2S4 Example _m _,ntensi~y E am~l~ m Intensity 0.0~ 2210 34 0.01 1545 21 0.02 1000 35 0.02 1545 36 0.04 2105 22 0.06 3380 37 0.06 3605 23 0.10 6370 38 o.10 7645 ~4 0.10 5960 0.20 13500 39 0.20 9115 26 0.20 14000 28 0.50 13300 40 0.50 12400 1.0 8695 41 1.0 9820 32 2.0 3155 42 2.0 9330 33 4.0' 735 43 4.0 9185 The most important advantage of employing lithium sulfate as a flux as compared to lithium carbonate is that a reduced amount of the lithium hafnate phase is produced. This results in significant improvements in phosphor luminescence when higher proportions of the lithium flux are employed during phosphor formation. At lower, preferred flux concentrations the lithium carbonate flux yields higher luminescence.
Examples 44-47 Preparation of Phosphors in the Presence of Varied Alkali Metal Ions The purpose of presenting these investigations is to demonstrate that all of the alkali metals significantly enhance phosphor luminescence.
Example 25 was repeated, except that 10 mole percent (m = 0.2) of another alkali metal carbonate was substituted for lithium carbonate: Na2C03 (0.265 g; EM Chemicals Suprapur Reagent), K2C03 (0.346 g; Alfa Products Ultrapure grade), Rb2C03 (0.5774 g; A~SAR 99.9%), or Cs2C03 (0.8146 g;

AESAR 9 9 . 9~/s ) .
The luminescence intensitie~ measured ~or the resulting samples are set out in Table VII.
Table VII
5 Example Carbonate source I e sity (Ex. 1 = 100) 19 None 2520 Li2C3 13500 44 Na2C03 10400 46 Rb2C3 3645 47 Cs2C03 4840 From Table VII it is apparent that all of the alkali metals are effective to increa~e the luminescence of the hafnia phosphors prepared from sources having higher zirconium contents than found in optical grade sources of hafnium. From Table VII it is observed that the lower the atomic number alkali metals lithium, sodium, and potassium offer a significant performance advantage over the heavier alkali metals rubidium and cesium when equal starting concentration are employed.
Examples 48-51 Preparation of Phosphors Using Varied Alkali Metal Compounds The purpose of presenting these investigations is to demonstrate the utility of alkali metal compounds completed by moieties other than sulfate and carbonate.
Example 25 was repeated, except that one of the following lithium sources was ~ubstituted for lithium carbonate: 0.2548 g Li2C204 (10 mole percent, m = 0.2, Alfa Products reagent grade)l 0.212 g LiCl (20 mole percent, m = 0.2, Alfa Products anhydrous Ultrapure grade), 0.4343g LiBr (20 mole perce~t, m = 0.2, MC~ anhydrou~) or 0.21 g LiOH-H20 (20 mole percent, m = 0.2, MCB reagent).
The luminescence intensities are given in Table VIII.

2 ~ ~5 -3~-Table_VIII
Example ~Li~hl~m Cmpd.ntensity (Ex 1 = lOQ) 19 None 2520 48 Li2C2o4 12695 49 LiCl 6730 LiBr 9400 51 LiOH:H20 13185 From Table VIII it is apparent that all of the lithium compounds improve the luminescence o~ the 10 phosphor. Whi.le both lithium hydroxide and lithium oxalate produced significantly higher levels of luminescence than the lithium halides, alkali carboxylates are clearly more convenient to handle than alkali hydroxides.
15 Example~ 52-54 Enhancement of Phosphor Luminescence by a Combination of Titanium and Alkali Metal Ion The purpose of presenting these investigations is to demonstrate the synergistic improvement of luminescence produced by the combination of an alkali metal ion and the titanium activator.
Example 52 : A ~ample consisting of 13.475 g o~
25 trilactohafnic acid (prepared as described in Examples 1-9) was thoroughly ground in an agate mortar and pestle with 0.2032 g Li2C03 (10 mole percent, m =
0.2, Alfa Products Ultrapure grade) and processed as in Examples 10-14.
30 E~ample 53 Example 15 was repeated, except that 13.475 g of trilactohafnic acid was used with 0.44 g of TiO2 (2 mole percent, x - 0.02, EM chemicals Optipur grade).
Example_~
Example 53 was repeated, except for the addition of 0.2032 g Li2C03 (10 mole percent, m =
0.2, Alfa Products Ultrapure grade) in the starting mixture.

The luminescence performances of Examples 5 and 52-54 are compared in Table IX.
T~le IX
~m~ Litions Intensity (Ex. 1 = 100 none 365 52 10 mole % Li2C03 1120 53 2 mole % TiO2 5690 54 10 mole % Li2C03+ 14600 2 mole % TiO2 From Table IX it is apparent that a disproportionately large increase in luminescence was realized by employing both the titanium activator and the alkali metal ion. While each of the titanium and alkali metal alone enhanced luminescence, a larger increase in luminescence was attained when titanium and alkali metal ion were employed together than could have been predicted assuming the separate enhancements of luminescence to be fully additive. 0 Examples 55-62 Phosphors Containing 5 Mole Percent or Less Titanium The purpose of presenting these investigations i~ to demonstrate the enhancements in luminescence produced by the use as starting materials of titanium at concentrations of 5 mole percent ( x =
0.05) and less, thereby pre~enting a better performance definition of the lower ranges of titanium concentrations.
Potassium tetraoxalatohafnate (IV) 5-hydrate 30 was prepared as described in Inorg. Syn., VIII, 42 (1966) using R.G.S. hafnium oxychloride 8-hydrate (z =
0.019). Upon drying at 70-90C for 1-16 hours in a convection oven, the product analyzed at closer to a 3-hydrate composition and all ~ubsequent use of this 35 material was calculated as the 3-hydrate. Fifteen grams of the material was thoroughly ground in an agate mortar and pestle with 0.03-5 mole percent of potassium bis(oxalato)oxotitanate (IV) 2-hydrate (Alfa 2~

Product~, recrystallized from ethanol). The mixtures were placed in 20 mL alumina crucibles, covered with alumina lids, and then placed in 100 mL alumina crucibles, which were covered wi~h alumina lids. The samples were heated in air to 1000C for 2.5 hours, then cooled to room temperature. The resulting solids were removed ~rom the crucibles, broken into small pieces with an alumina mortar and pestle and washed by stirring in 50 mL of distilled water. The solids were 10 then collected and dried in a convection oven at 80~C. The charges were placed in 10 mL alumina crucibles with alumina lids and heated in air to 1300C for 2 hours, followed by cooling to room temperature.
The luminescence intensities of the samples are set out in Table X.
Table X
Example MolQ percent Ti Intensity (Ex. 1 = 100) None 365 56 0.3 6128 From Table X it is apparent that even at the lowest concentrations of titanium (Hfl_zZrzTix 30 where x = 3 X 10 4, Example 55) much higher levels o~ luminescence are observed than in Example 5, which lacked titanium. While some of the enhancement in luminescence as compared to Example 5 can be attributed to the presence of potassium, comparing lumine~cence values from Table III, in which potassium was introduced without titanium being present, it is apparent that a part o~ the luminescence enhancement must be attributed to additional presence of the titanium.
Exam~les 6~=8 Varied Levels of Zirconium in Pho~phors Prepared in the Presence of Alkali Metal Ion The purpose of presenting these investigations is to demonstrate the effect of varied levels of zirconium in the hafnia host phosphor when the hafnia phosphor was prepared in the presence of alkali metal ion.
Two grades of potassium tetraoxalatohafnate (IV~ 3-hydrate were prepared as in Example 55 from optical grade hafnium oxychloride 8-hydrate and R.G.S.
hafnium oxychloride 8-hydrate. Potassium tetraoxalatozirconate 3-hydrate was prepared as in 15 Example 55 from R.G.S. zirconium oxychloride 8-hydrate. A series of Hfl_zZrz02 samples in which z was varied from 2.76 x 10 to 6.84 x 10 2 were prepared from mixtures of the above precursors. The powders were combined and ground in 20 an agate mortar and pestle. The procedures of Examples 55-62 were employed, with the addition of 10 mole percent K2C03 (Alfa Product~ Ultrapure grade) to each sample.
Luminescence intensities as a function of zirconium levels (z) are given in Table XI.
Table XI
E~am~le zIntensity (Ex. 1 = 100 63(Control)2.8 x 10 4 380 64 4.3 x 10 4 165 9.6 x 10-3 770 66 1.9 x 10-2 520 67 4.0 x 10 2 595 68 6.0 x 10-2 610 Note that Example 66 was identical to Example 10, 35 except for employing a different final firing temperature, and the luminescence measured was identical.

Table XI demonstrate3 that hafnia prepared from optical grade sources as in Control Example 63 yields inferior luminescence as compared to samples in which the zirconium content z i9 equal to at least 1 X
10 2. Comparing Tables II and XI, it is apparent that the presence of potassium ion is responsible for a significant increase in luminescence at zirconium levels equal to that in R.G.S. hafnia (z = 0.019) and above.
Examp,les 69-72 Determinations of Alkali Metal Ion Incorporation in Phosphors Differing in Zirconium Levels The purpose of presenting these : investigations is to provide quantitative determinations of alkali ion incorpora-tion levels (y) in several phosphors satisfying the general relationship Hfl_zZrzTixMy and having differing zirconium levels (z) satisfying the requirements of the invention.
Samples were prepared as in Examples 63-68, except for the further addition of 0.2151 g of recrystallized potassium bis(oxalato)oxotitanate (IV) 2-hydrate (Alfa Products) to satisfy the ratio x =
0.03.
~ 25 Proportions of zirconium, titanium, and : potassium ion in the completed phosphor samples were determined by atomic absorption analysis and inductively coupled plasma spectrometry. The lumine~cence of the phosphors together with their alkali ion content observed on analysis, y(obs), are reported in Table XII. The amounts of zirconium and titanium present in the starting materials, z~calc) ~:~ and x(calc), are compared in Table XII to the amounts of zirconium and titanium found on analysis, z(obs) and ~(obs).

2~

Table XII
Hfl_zZrzTixMy Int-ensit~
Ex. ~ 1=100) -c~a-l~- z(obs) _(calc~ x(obs) y(obs~
5 69 9820 4.3 x 10 4 4.31 x 10 4 0.03 0.022 0.022 9820 9.6 x 10 4 8.79 x 10 4 0.03 0.026 0.019 71 9~20 1.9 x 10-2 1.78 x 10-2 0.03 0.031 0.025 72 9820 4.0 ~ 10 2 3.87 x 10 2 0 03 0.027 0.023 Although all samples exhibited similar luminescence, when a corresponding phosphor was formed from optical grade hafnium starting materials [z(obs~=
2.91 X 10 4], a significantly lower luminescence was observed, 15 Example 73-77 Rare Earth Activated Phosphors The purpose of presenting these investigations is to demonstrate enhanced performance of rare earth activatored zirconium rich hafnia phosphors -~fl_zZrzLw, where L represents a rare 20 earth, z = 0.019.
Example 73 A sample consisting of 15.00 g of potassium tetraoxalatohafnate (IV) 3-hydrate prepared as in Example 55 wa~ combined and ground thoroughly with 25 0.0861 g (x = 0.03) of Eu(N03)3.6H20 (Johnson Matthey REacton grade, 99.99%). The mixture was fired in a double alumina crucible setup (described in Example 1) to 1000C in air for 2 hours. The recovered ingot was broken into small chunks and 30 washed in 70-150 mL of distilled water. The relative luminescence intensity was 770.
~: When Example 73 was repeated with increased levels of zirconium, small shifts in emission maxima were observed, indicating that zirconium content can 35 be usefully varied in preparing these phosphorg for use in inten~ifying screens. In intensifying screens intended to be employed with spectrally sensitized silver halide radiographic elements optimum imaging ~ ~3 e~ficiency occurs when the emission peaX of the phosphor corresponds with the absorption peak o~ the spectral sensitizing dye or dye combination present in the radiographic element.
5 Example 74 A sample consi~ting of 14.82 g of potassium tetraoxalatohafnate (IV) 3-hydrate prepared from R.G.S. hafnium oxychloride 8-hydrate was ground with 0.0042 g <x - 0.001) SmF3 (Johnson Matthey REacton grade) and processed as in Example 73. The relati~e luminescence output was 735.
Examp1~ 75 Example 74 was repeated, however, 0.083 g (x = 0.02) of SmF3 waæ u~ed. The relative luminescence output was 400.
E ample 76 A sample consisting of 15.00 g of potassium tetraoxalotohafnate (IV) 3-hydrate prepared from R.G.S. hafnium oxychloride 8-hydrate was ground with 20 0.11 g (x = 0.015) of Gd203 (Rhone Poulenc 99.99%). The mixtures were fired as in Example 73, and then reheated in air at 1300C for 2.5 hours. The relative luminescence output was 430.
Example 77 Example 76 was repeated, except that 0.22 g (x = 0.03) of Gd203 was used. The relative luminescence output was 490.
Exam~le 78 Intensifying Screen The purpose of presenting these 30 investigations is to demonstrate enhanced performance of intensifying screens containing zirconium rich hafnia phosphors.
~ afnyl oxalic acid 3-hydrate was prepared for use as a ~tarting material in the manner described in 35 Zhurnal Neoorganich~eskQi Khimi, Vol. II, p. 980 ~1957) using R.G.S. hafnium oxychloxide, z = 0.019. Two samples each con~isting of 55.29 g of hafnyl oxalic acid 3-hydrate ware thoroughly ground with 1.11 g of Li2C03 (Aldrich; reagent grade), 4.36 g of K2S04 (J.T. Baker; reagent grade) and 0.50 g of TiO2 ~EM Chemicals; Optipur grade~.
The samples were placed in 250 mL alumina crucibles, covered with alumina lids, and heated in air to 1000C for 2.5 hours, then cooled to room temperature. The resulting solids were removed from the crucibles, broken into small pieces with a mortar and pestle, and washed by stirring in 500 mL of distilled water. Each sample was then collected and dried in a convection oven at 80C. The charges were placed in 50 mL alumina crucibles with alumina lids and heated in air to 1400C for 1.5 hours, followed by cooling to room temperature. The two samples (combined weight, 53.1 g) were ground together and shaken through a nylon sieve with 60 ~m openings to give 28.6 g of product phosphor.
The phosphor was mixed with 13%
PermuthaneTM polyurethane solution in a methylene chloride and methanol mixture to produce a dispersion with 25 parts of phosphor and 1 part of binder by weight. The dispersion was coated on a blue tinted transparent poly(ethylene terephthalate) film support to produce a coating with about 6.25 g/dm2 of the phosphor. This coating when excited with unfiltered X-rays from a tungsten target tube operated at 70 kVp and 10 mA gives a speed that is about 3.5 times larger than that obtained from a commercial CaW04 (PAR
screen) when the emission is compared using an IP-28TM photomultiplier tube.
The invention has been described in detail with particular reference to preferred embodiments thereo~, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims (67)

1. A screen comprised of a support and a fluorescent layer containing a phosphor capable of absorbing X-radiation and emitting longer wavelength electromagnetic radiation comprised of monoclinic crystals of a hafnia phosphor host, characterized in that zirconium ions are present in the hafnia phosphor host in concentrations higher than those found in optical grade hafnia.

2. A screen according to claim 1 further characterized in that hafnium and zirconium ions in the phosphor host satisfy the relationship Hf1zZrz wherein z is in the range of from 4 X 10-4 to 0.3.

3. A screen according to claim 2 further characterized in that z is in the range of Prom 1 X
10-3 to 0.2.

4. A screen according to claim 3 further characterized in that z is in the range of from 2 X
10-3 to 0.1.

5. A screen according to claim 1 further characterized in that said phosphor additionally includes an activating amount of alkali metal ions.

6. A screen according to claim S further characterized in that the hafnium, zirconium, and alkali metal ions satisfy the relationship Hf1-zZrzMy wherein M represents ions of at least one alkali metal;
y i in the range of from 1 X 10 4 to 1; and z is in the range of from 4 X 10 4 to 0.3.

7. A screen according to claim 6 further characterized in that y is in the range of from 1 X 10-4 to 0.2

8. A screen according to claim 5 further characterized in that said alkali metal ions include lithium ions.

9. A screen according to claim 8 further characterized in that the hafnium, zirconium, and lithium ions satisfy the relationship Hf1-zZrzLiy wherein y is in the range of from 8 X 10 4 to 0.15 and z is in the range of from 4 X 10 4 to 0.3.

10. A screen according to claim 9 further characterized in that z is in the range of from 1 X 10 3 to 0.2.

11. A screen according to claim 8 further characterized in that said fluorescent layer additionally includes lithium hafnate as a second phosphor.

12. A screen according to claim 5 further characterized in that said alkali metal ions include at least one of sodium and potassium ions.

13. A screen according to claim 12 further characterized in that the hafnium, zirconium, and the alkali metal ions satisfy the relationship Hf1-zZrzMy wherein M represents at least one of sodium and potassium, y is in the range of from 8 X 10 4 to 7 X 10 2 and z is in the range of from 4 X 10 4 to 0.3.

14. A screen according to claim 13 further characterized in that z is in the range of from 1 X 10 3 to 0.2.

15. A screen according to claim 5 further characterized in that said alkali metals include at least one of cesium and rubidium ions.

16. A screen according to claim 5 further characterized in that said phosphor includes at least two alkali metal activators chosen from among lithium, sodium, and potassium.

17. A screen according to claim 16 further characterized in that said alkali metal activators include lithium and potassium ions.

18. A screen according to claim 1 further characterized in that said phosphor includes an activating amount of titanium ions.

19. A screen according to claim 18 further characterized in that said fluorescent layer additionally includes a second phase comprised of hafnium titanate.

20. A screen according to claim 18 further characterized in that the hafnium, zirconium, and titanium ions satisfy the relationship Hfl-zZrzTix wherein x is in the range of from 3 X 10 4 to 1.0 and z is in the range of from 4 X 10 4 to 0.3.

21. A screen according to claim 20 further characterized in that x is in the range of from 3 X 10 4 to 0.5.

22. A screen according to claim 18 further characterized in that the hafnium, zirconium, and titanium ions satisfy the relationship Hfl-zZrzTix wherein x is in the range of from 3 X 10 4 to 0.25 and z is in the range of from 4 X 10 4 to 0.3.

23. A screen according to claim 22 further characterized in that z is in the range of from 1 X 10 3 to 0.2.

24. A screen according to claim 1 further characterized in that said phosphor includes an activating amount of rare earth ions.

25. A screen according to claim 24 characterized in that said rare earth ions include at least one of praseodymium, samarium, europium, gadolinium, terbium, dysprosium, and ytterbium ions.

26. A screen according to claim 24 further characterized in that the hafnium, zirconium, and rare earth ions satisfy the relationship Hfl-zZrzLw wherein L represents at least one rare earth element;
w is in the range of from 3 X 10 4 to <5 X
10-2; and z is in the range of from 4 X 10 4 to 0.3.

27. A screen according to claim 18 further characterized in that said titanium activated phosphor includes a further activating amount of alkali metal ions.

28. A screen according to claim 27 further characterized in that the hafnium, zirconium, titanium, and alkali metal ions satisfy the relationship Hfl-zZrzMyTix wherein M represents at least one alkali metal;
x is in the range of from 3 X 10 4 to 1.0;
y is in the range of from 6 X 10 4 to 1.0; and z is in the range of from 4 X 10 4 to 0.3.

29. A screen according to claim 28 further characterized in that x is in the range of from 3 X 10 4 to 0.5;
y is in the range of from 8 X 10 4 to 0.2; and z is in the range of from 1 X 10 3 to 0.2.

30. A composition containing a phosphor capable of absorbing X-radiation and emitting longer wavelength electromagnetic radiation which is comprised of monoclinic crystals of a hafnia phosphor host consisting essentially of oxygen and hafnium, zirconium, and alkali metal ions satisfying the relationship Hfl-zZrzMy wherein M represents at least one alkali metal;
y is in the range of from 6 X 10-4 to 1.0; and z is in the range of from 4 X 10-4 to 0.3.

31. A composition according to claim 30 in which y is in the range of from 8 X 10-4 to 0.2

32. A composition according to claim 30 in which the said alkali metal ions include an activating amount of lithium ions.

33. A composition according to claim 32 in which the hafnium, zirconium, and lithium ions satisfy the relationship Hfl-zZrzLiy wherein y is in the range of from 8 X 10-4 to 0.2 and z is in the range of from 1 X 10-3 to 0.2.

34. A composition according to claim 33 in which z is in the range of from 5 X 10-3 to 0.1.

35. A composition according to claim 32 which additionally includes lithium hafnate as a particulate second phase.

36. A composition according to claim 30 further characterized in that the alkali metal ions include an activating amount of at least one of sodium and potassium ions.

37. A composition according to claim 36 in which the hafnium, zirconium, and alkali metal ions satisfy the relationship Hfl-zZrzMy wherein M represents at least one of sodium and potassium ions, y is in the range of from 6 X 10-4 to 7 X 10-2 and z is in the range of from 1 X 10-3 to 0.2.

38. A composition according to claim 37 in which y is in the range of from 8 X 10-4 to 7 X 10-2 and z is in the range of from 5 X 10-3 to 0.1.

39. A composition acording to claim 30 in which the alkali metal ions include an activating amount of at least one of cesium and rubidium ions.

40. A composition according to claim 30 in which the phosphor includes an activating amount of ions of a combination of at least two of lithium, sodium, and potassium ions.

41. A composition according to claim 40 in which the phosphor includes an activating amount of a combination of lithium and potassium ions.

42. A composition according to claim 30 in which the phosphor includes a further activating amount of titanium ions.

43. A composition according to claim 42 in which the hafnium, zirconium, titanium, and alkali metal ions satisfy the relationship Hfl-zZrzMyTix wherein M represents alkali metal ions;
x is in the range of from 3 X 10-4 to 0.1;
y is in the range of from 6 X 10-4 to 0.2; and z is in the range of from 4 X 10-4 to 0.3.

44. A composition according to claim 43 in which y is in the range of from 8 X 10 4 to 0.2 and z is in the range of from 1 X 10 3 to 0.2.

45. A composition according to claim 44 in which z is in the range of from 5 X 10 3 to 0.1.

46. A process of preparing a phosphor capable of absorbing X-radiation and emitting longer wavelength electromagnetic radiation comprised of activated monoclinic crystals of a hafnia phosphor host by heating a hafnium containing compound and an activator containing compound to a temperature sufficient to form and activate the hafnia phosphor host characterized in that a phosphor exhibiting improved emission properties is obtained by forming a mixture of hafnium, zirconium, and alkali metal compounds containing thermally decomposable ligands chosen to leave residues consisting essentially of hafnium, zirconium, and alkali metal oxides on heating, the hafnium, zirconium, and alkali metal ions present prior to heating satisfying the relationship:
Hfl-zZrzMm wherein M represents at least one alkali metal;
m is greater than 3 X 10-2; and z is in the range of from 4 X 10-4 to 0.3.

47. A process according to claim 46 further characterized in that heating is restricted to temperatures of 1400°C or less.

48. A process according to claim 47 further characterized in that maximum heating temperatures in the range of from 900°C to 1300°C are employed.

49. A process according to claim 46 further characterized in that after heating to a temperature sufficient to form the monoclinic phosphor crystals the composition is washed with water and then reheated to a temperature in excess of 900°C.

50. A process according to claim 46 further characterized in that the hafnium and zirconium compounds include hydrous hafnia and hydrous zirconia.

51. A process according to claim 46 further characterized in that at least the hafnium and zirconium ligand containing compounds are carboxylates containing carboxylate moieties of from 2 to 20 carbon atoms.

52. A process according to claim 51 further characterized in that the carboxylates are .alpha.-hydroxycarboxylates containing from 2 to 10 carbon atoms.

53. A process according to claim 523 further characterized in that the .alpha.-hydroxycarboxylates contain from 2 to 6 carbon atoms.

54. A process according to claim 53 further characterized in that the hafnium, zirconium, and alkali metal ligand containing compounds are oxalates.

55. A process according to claim 51 further characterized in that the ligand containing compounds include alkoxide moieties of from 1 to 20 carbon atoms.

56. A process according to claim 51 further characterized in that the hafnium and zirconium .alpha.-hydroxycarboxylates are formed by reacting with the acid, salt, or ester of the a-hydroxycarboxylate in a common solvent a hafnium or zirconium containing compound chosen from the class consisting of hafnium tetrachloride, zirconium tetrachloride, ha~nium oxychloride, zirconium oxychloride, hafnium basic carbonate, zirconium basic carbonate, hafnium nitrate, zirconium nitrate, zirconium carbonate, hafnium sulfate, zirconium sulfate, and mixtures thereof.

57. A process according to claim 46 further characterized in that the alkali metal compounds are chosen from the group consisting of alkai metal carbonates, sulfates, oxalates, halides, hydroxides, borates, tungstates, molybdates, and mixtures thereof.

58. A process according to claim 46 further characterized in that m is from 1 X 10 1 to 6 and z is in the range of from 1 X 10 3 to 0.2.

59. A process according to claim 58 further characterized in that the alkali metal ions are chosen from the class consisting of sodium, potassium, cesium, rubidium, and mixtures thereof.

60. A process according to claim 59 further characterized in that at least a portion of the alkali metal ions are removed by washing.

61. A process according to claim 59 further characterized in that the alkali metal ions are comprised of lithium ions.

62. A process according to claim 61 further characterized in that the hafnium, zirconium, and lithium ions present prior to heating satisfy the relationship:
Hfl-zZrzLim wherein m is in the range of from 4 X 10 2 to 2.0 and z is in the range of from 4 X 10 4 to 0.3.

63. A process according to claim 62 further characterized in that m is in the range of from 7 X 10 2 to 1.5.

64. A process according to claim 61 further characterized in that the thermally decomposable lithium compound is lithium carbonate and lithium ions in excess of those forming the phosphor form a second phosphor comprised of lithium hafnate.

65. A process according to claim 46 further characterized in that an activating amount of a titanium ion providing compound is mixed with the hafnium, zirconium, and alkali metal containing compounds prior to heating.

66. A process according to claim 65 further characterized in that the titanium ion providing compound is chosen from the class consisting of particulate titania and at least one thermally decomposable compound of titanium chosen to leave residues consisting essentially of titania on heating.

67. A process according to claim 46 further characterized in that an activating amount of at least one rare earth ion providing compound is mixed with the hafnium, zirconium, and alkali metal containing compounds prior to heating.