CA1079518A - Single crystalline essentially cubic hafnium, zirconium pyrophosphate luminescent material having improved brightness - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A mixture of starting materials, corresponding to an oxide composition within an area of the ternary diagram for (Hf, Zr)O2, A2O, P2O5 formed by connecting points defined by the molar com-positions: 49 (Hf,Zr)O2 - 1 A2O - 50 P2O5; 15 (Hf,Zr)O2 - 35 A2O - 50 P2O5; and 15 (Hf,Zr)O2 - 1 A2O - 84 P2O5, reacts to form large single crystalline essentially cubic particles of (Hf1-xZrx) P2O7 luminescent material, and an aqueous - soluble second phase removable by washing. The large single crystalline particles show improved emission intensity (brightness) upon x-ray excitation. Use of the material in an x-ray intensifying screen results in improved resolution of an exposed image on x-ray film.

Description

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SINGLE CRYSTALLINE ESSENTIALLY CUBIC
~AFNIUM, ZIRCONIUM PYROPHOSP~TE
LUMINESCENT MATERIAL ~VING
IMPROVED BRIGHTNESS
CROSS REFERENCE TO RELATED APPLICATIONS
Copending U.S. Patent Application S.N. 285,440 and S.N. 290,091 filed concurrently herewith, and assigned to the present assignee, are directed to other hafnium, zirconium phosphate compounds also having utility as luminescent materials, e.g., in x-ray intensifying screens.
S.N. 290,091 includes large single crystalline es-` sentially hexagonal particles of (Hfl xZr~)3 yA4y(P04~4 having strong, about 60 nm bandwidth emission peaking ! at about 350 nm (on an uncorrected basis) with con-, trolled persistence under x-ray excitation. The hexa--I gonal structure has also been found to be an effective host for Eu~2 green emission under ul~raviolet, etc.
l 15 excitation.
i S.N. 285,440 includes a 3:1 (Hfl_xZrx)02:P2O5 luminescent material having broader band emission and heavier absolute density than cubic compound herein, or the hexagonal compound of S.N. 290,091.
BACKGROUND OF THE INVENTION
This invention relates to luminescent materials, and more particularly relates to large single crystal-line essentially cubic particles of hafnium, zirconium i pyrophosphate luminescent materials excitable by x-rays i ~,D-9184 , .:

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and also relates to a method for producing such ma-; terials and to x-ray intensifying screens incorporating such materials.
A problem associated with x-ray intensifying screens when used with conventional double emulsion x-ray film packs is so-called "cross-over". Cross-over is a loss of resolution on the exposed film attributed to the phenomenon of a photon in the visible spectrum passing through the emulsion on one side of the ~
pack, through the film base, and exposing the emulsion on the far side. Since the photons are transmitted in all directions from the phosphor on the intensifying screen, the image can become less sharp as a result o~
the far side exposure. Attempts have been made in the past to incorporate dyes into the film base which would render the base opaque` to visible light, thereby eli-minating cross-over. Such attempts have not been completely satisfactory.
Currently, x-ray film packs use plastic supports such as polycarbonates, polystyrenes, polyesters and the like as film bases, which while being opaque to ultraviolet light, are ~ransparent to visible radi-ation. Since the film emulsions are sensitive to ultraviolet light, finding an x-ray phosphor having sufficient brightness (speed) and only emitting in the ultraviolet region would result in a sharp film image by the substan~ial elimination of cross-over.
Self-activated hafnium pyrophosphate, zirconium pyrophosphate and mixed hafnium, zirconium pyrophos-phate luminescent materials, which emit in the lowerultra violet region of the electromagnetic spectrum, are known. For example, United States Patent 2,770,749 issued to A. Bril et al. discloses a zirconium pyrophos-phate phosphor. I. Shidlovsky et al., in "Luminescence of Self-activated Hafnium Pyrophosphate", Abstract #95 of the MayJ 1974 meeting of the Electrochemical Society, ,,:
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disclose the hafnium pyrophosphate emits in the ultra violet region more efficiently than zirconium pyro-phosphate when excited by photoluminescence and catho-doluminescence. In United States Patent 3,941,715, 5 Shidlovsky discloses mixed (Hf,Zr) P207 phosphors which emit in the lower ultraviolet region (below about 3,000 Angstroms) when excited by x-rays, cathode rays or visible radiation.
However, when prepared as taught by Shidlovsky, 10 hafnium pyrophosphate consists of small single crystal-line cubic particles exhibiting mediocre emission in-tensity under x-ray excitation (mediocre x-rays speed) in the ultraviolet region of ~he spectrum. ~' ~Iafnium phosphate luminescent materials having the 15 formula Hf3(P04)~ and containing activator elements such as Cu and Eu are disclosed in United States Patents i 3,905,911 and 3,905,912 to J. E. Mathers, and assigned to the prese`nt assignee. However, upon excitation by x-rays these materials emit only in the visible portion 20 of the spectrum, and are therefore of no utility for x-ray intensifying screen applications requiring ultra-I viole~ emissions.
f SUMMARY OF THE IN_ENTION
In accordance with the invention, it has been dis-25 covered that when (Hf,Zr) P205 is prepared by reacting a mixt~lre of starting materials corresponding on an oxide basis to a composition encompassed by an area of the ternary system for M02, A20 and P205, (where M is selected from the group ~onsisting of hafnium and 30 zirconium, and A is an alkali metal selected from the group consisting of lithium, sodium and potassium), ;
wherein the`area is defined by points having the oxide composition in mole percent as follows:
49 M02 ~ 1 A20 - 5~ P205; 15 M02 - 35 A20 - 50 P205;
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and 15 MO2 - 1 A2O - 84 P2O5, large single crystalline essentially cubic particles result, which when separated from an aqueous-soluble alkali metal-containing second phase, exhibit improved intensity of ultraviolet emissions upon x-ray excitation. When used in x-ray intensifying screens with conventional silver halide emulsion x-ray film, such improved luminescent material results in improved }mage resolution upon the x-ray film by subs~antial elimination o~ crossover.
The inven~ion includes such large particle single crystalline essentially cubic luminescent material of (Hfl xZrx) P2O7, wherein x is within the range o~ from about 0 to 1, having strong, about 6~ nm bandwidth emission peaking at about 340 nm (on an uncorrected basis) with controlled persistence, and also includes a method for producing such luminescent materials, and also includes x-ray intensifying screens incorporating such materials having values of x within the range of about 0.005 to 0.5.
BRI~F DESCRIPTION_OF T~IE DRAWING
Fig. 1 is a photomicrograph of single crystalline essentially cubic particles o~ (Hf,Zr) P207 prepared by a process of the prior art;
Fig. 2 is a photomicrograph o~ single crystalline essentially cubic particles of (Hf,Zr) P207 prepared by reacting starting materials corresponding on an oxide basis to 25 mole percent ~Hfl xZrx) 2~ 20 mole percent Na20 and 55 mole percent P205;
Fig. 3 is a ternary diagram for the M02 ~ A20 ~
P205 oxide system having a shaded portion "A" defining the reaction mixtures of the invention; and Fig. 4 is a schematic diagram of an x-ray intensi-fying screen incorporating the luminescent material of the invention, in cooperative relationship with a silver halide emulsion x-ray film.

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DETAILED DESCRIPTION OF THE INVENTION
For a better understanding of the present in-vention, together with other and further objects, ad-vantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.
Referring now to Fig. 3, there is shown a ternary diagram for M02 - A20 - P205, wherein a boundary line "a" connects points defining 1:1 molar ratio end members of M02:P205 and A20:P205, respectively. Compositions formulated on the P205 side of this boundary line and within the shaded area "A" react ~o a mixture of aqueous-insoluble large single crystalline essentially cubic particles of MP207 and an aqueous-soluble second phase Of alkali phosphates which may be removed from the re-action mixture by a post reaction water wash treatment.
Compositions formulated on the opposite side of the boundary line fire to an aqueous-insoluble es-sentially hexagonal phase described and claimed in copending application S.N. 290,091, filed concurrently herewith, and an aqueous-soluble second phase of alkali phosphatè. Compositions formulated on the boundary line, particularly those in the area about midrange from the end members tend to fire to a mixture of primarily cubic and traces of (less than about 5 percent) hexagonal phases, with an aqueous-soluble alkali phos-phate third phase. Such boundary line compositions are intended to be encompassed within shaded area "A".
Outside the shaded area of the diagram, on the opposite side of line "b", (below 15 mole percent M02) the alkali phosphate second phases tend to mainly in-soluble. On the opposite side of line "c" (below 1 mole percent A20), too little of the aqueous-soluble second phase is formed to allow adequate formation of large single crystalline particles of the insoluble phase.

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While ~ values determining the molar ratios of hafnium and zirconium may range from 0 to 1, and thus -encompass both the hafnium pyrophosphate and zirconium pyrophospha~e end members for x-ray intensifying screen useJ it is preferred to maintain the values of g be-tween about 0.005 and 0.5. It ~as been found that as the ratio of zirconium to hafnium increases, relati~e x-ray persistence desirably decreases, but the absolute density of the luminescent material and accordingly the - 10 x-ray stopping power of the material undesirably decrease. Based upon these considerations, it is particularly pre~erred to maintain x within the range of about 0.~5 to 0.2 for x-ray intensifying screen applications.
Starting materials as the oxides or as precursors thereof may be employed. As used herein, the term "precursor" means any compound which upon heating to the reaction temperature or below, decomposes, hydro-lyzes, or otherwise converts to the desired oxide, such as carbonates, nitrates, sulphates, formates oxylates, , halides, etc. Preferred starting materials are hafnium oxychloride hafnium oxide, zirconyl nitrate, dibasic ammonium hydrogen phosphate and sodium pyrophosphate.
general procedure for preparation of the lumi-nescent material will now be described. The formulated mixture of starting material is reacted at a firing teTnperature within the range of about 800C to 1400C
for about 1 to 16 hours, to form a fùsed reaction mass.
This mass is then washed with water to substantially remove the aqueous soluble second phase and any soluble contaminant material to leave a powder of the sub-stantially single crystalline particles ~f the lunii-nescent material. The powder may then be dried. Pre-ferably, the mixture is heated at least once prior to firing to a temperature within the range of about 260C
to 400C for about 1 to 16 hours, in order to insure .~ . .

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the remo~ral o~ at least a portion of contained vola-~ 'e material, and in some cases to partially decompose al~or pre-react precursor Inaterials~ This prefiring heat tl~atm~nt is then followed by reducing the mixture to a reLati~re]y uniform powder mixture, such as by pulverizi.ng or mortaring with a mortar and pestle~ In addition, it is pre~erred to carry out washing by first forming an a~ueous slurry of th~!
fused reaction mass, followed by agitatirg the slurry at a -- moderate temperature, e.g., wi.thin the range of about 80~C
to 105C~ until the crystalline po~der appears, and then washing the powder with copious amoun's of water to remove the soluble second pllase and any soluble contarl,inant mateLia].s ~ he in~7ention is nct limited to the procedure cited above. For example, optimum final fixing temperature ls contingent upon such factors as hafnium to zirccnium ratio, particular alkali ~lux chosen, etc. Ila~nium and zirconium phosphate salts cou].c~ be precipitated from solution and ~ired with approp-iate alkali phosphate flux m.ixtuxes, or the desired ~ xZrxP207 matrix material could be ~ormed and ZO ~hen re~ired with appropriate alkali phosphate flux to ~orm large sin.~le crystals, without departing fro~l the spirit o~:
scope of this invention.
Table ] shows data collected on 1~27 lumi]-escen' m,aterial s~-:mpl~s ohtaine~. from reacti.on mixtures formulated ; 25 to ~ali bo~ itll~n ~he s~lacled area "~" Gn Figu-~e 3, to give an ~se~ti~ cubic s'~ruc~ure (s~n~le nu~be.rs 1, 2, 3 and 5), and at about midrange on the cubic-hexagonal boundary line "a" to give ma;.nl~ cubic structuresj with lleXclgOllal mate1-i.al al50 present (sample numbers 4, 6 and 7).
A11 saI~lples were ]?repared rom starting mate.ria's of reacto.r g~ade hafni~ o~rchloride (H~OC12.8~I2O) conta.ini.ng about 3.1 weight percent zirconium on an ele~ental basls, corres-ponding to an ~. ~alue of 0.057, æirconyl nitrate~ dibasic arnmonium hy~roy~n phosphate (NH~H2P0~) and sodi~m 3S pyrophosphate (Na~P2O7.1OH2O). : ;

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Median particle diameter was measured by Coulter Counter; these measurements indicate that 50 percent of the particles are at least about 6.0 micrometers in diameter and preferably at least about 7.0 micro-meters in diameter. Relative powder cell x-ray speed was measured by the following technique. A powder sample is poured into a brass cup, lightly tapped and smoothed with a spatula, to give a 1 inch diameter by 1/16 inch deep sample. The cup is placed 8 inches from a tungsten target x-ray tube opera~ed at 80 kilovolts and 25 miliamperes. The emissions from the sample are scanned with a grating spectrometer having a photo-multiplier tube with spectral sensitivity of 250 to 700 nanometer, to give a first order spectral energy distri-bution from 300 to 700 nanometers.

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-LO-TABLE II
d I hkl o Sample 7* 4.73 A 59 111 4.10 100 200 3.664 22 210 3.346 24 211

2.900 62 220 2.470 81 311 2.366 24 222 2.050 13 400 1.881 40 331 1.835 51 420 ' 1.677 35 ~22 1.580 44 333,511 1.452 -- ~40 1.387 -- 531 1.369 -- 442,600 1.298 -- 620 1.253 __ 533 1.238 -- 622 1.150 -- 551,711 1.139 -- 640 1.099 -- 642 1.070 -- 553,731 0.968 -- 822,660 0.949 -- 751,555 0.~261 -- 933,771,755 0.8057 -- 1020,862 0.794~ -- 951,773 o *Sample is Cubic with aO = 8.219 A (obtained using extra-polation function) From Debye Sherrer camera film d = interplanar distance I = relative intensity hkl = Miller indices . . .

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Relative persistence was measured by a technique similar to that outlined under Federal Specification for X-ray Intensifying -GG-S-00176b.
Several metal coins or other roentgenopaque test objects were placed on the front of a cassette containing screens but no film, and exposed under specified conditions. The cassette was then taken to the dark room and a set time after the termination of the roentgen exposure, the cassette was opened and an ~mexposed piece of x-ray film was quickly inserted between the screens and the cassette closed. After remaining in the closed cassette for a set time, the film was removed, processed, and examined for evidence of shadows of the test objects.
A medium to weak persistence is generally desired for x-ray in~ensifying screen applications. Results shown in Table I reveal that samples, 3, 5, and 7 exhibited medium to weak persistence, making such com-positions particularly desirable for x-rày intensifying screen use. Note that sample 7 having 14.8 mole percent of zirconium, had the lowest persistence of all samples tested. Samples 1 and 2, containing essentially no hafnium, were not measured for persistence due to their low relative x-ray speeds.
EXAMPLE I
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To better understand the invention, the detailed procedure employed for the synthesis of the samples of Table I is set forth in respect to the specific com-position of sample 7.
Table III sets forth the starting materials, their molar ratios, their formula weights, their gram ratios based upon 1 mole, and the batch ratio in grams based upon a 2 mole batch.

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T A B L E III
_ Batch Gram Ratio Mol Formula Ratio (2 mol) Ratio Weight (1 mol) (Grams) HfOCl~.8H20* 0.225 440.3499.08 198.15 (assay 48.4%
(Hf,Zr)02) Zirconyl 0.025 262.17 6.55 13.10 Nitrate ~ 10 (assay 47%

:~ ZrO2) '' NH4H2PO4 0.750 117.38 88.03 176.05 (assay 98%) Na4P207 10H20 0.125 446.06 55.76 111.50 *containing 3.1 weight percent elemental Zr per unit weight of zirconium plus hafnium.

These starting materials were weighed in accordance with the batch ratio weights shown in the last column of Table III5 and blended in a polyethylene bag by inter- -mittently rolling the bag with a rolling pin and shaking the bag. The thus roughly blended material was then divided between two 500 ml volume alumina crucibles, covered, placed in a furnace at 260C for 1 hour, removed from the furnace and allowed to cool to room temperature.

The material was then mortared with a porcelain mortar and pestle, recombined into a single batch, redivided between the two crucibles and reheated for 2 hours at 260C. The cooling, mortaring, blending and reheating procedure was then repeated. However, during reheating the temperature was raised from 260C to 370C during a time of about 1 hour and then reduced gradually to about 200C and held at this temperature for approximately 16 hours, after which time the crucibles were removed from the furnace and allowed to cool to room temperature. The material was then recom~ined and mixed using the bag rolling and shaking procedure described above. The now homogeneous 5~

powder mixture was charged into a single 500 ml alumina crucible, the crucible covered and placed in an ele-ctrical furnace at 500C. The temperature was then in-creased to about 1300C over a period of about 1.5 hours and held at this temperature for about 6 hours.
The crucible was then removed from the furnace and allowed to cool to room temperature. A white body-colored fused reaction mass resulted.
The fused mass was agitated in a pot of boiling water for 1 hour. The materlal, now dispersed, was allowed to settle, and the supernatent liquid discarded.
The settled slurry was then agitated in hot deionized water (short of the boilîng point) for about one half hour. The material was again allowed to settle and the supernatent liquid discarded. The hot deionized water wash was repeated again twice and the resulting slurry filtered, oven dried, and passed through a 60-mesh sieve. The sieved white body-colored, free flowing powder had large single crystalline particulate char-acter and strong emission, (about 60 nm in bandwidthpeaking at about 340 nm on an uncorrected basis), with weak persistence, under x-ray excitation.
Referring now to Figs. 1 and 2, photomicrographs at 2000 x magnificatiGn of crystalline powders of MP207 prepared by a prior art process and by the process of the invention, r~spectively, are shown. The prior art material of Fig. 1 was formulated from a reaction mix-ture of starting materials corresponding on an oxide basis to 47 mole percent (Hfl_Xzrx)02 and 53 mole per cent P205, and fired at a temperature within the range o about 1,000C to 1,200C for a few hours.
The material of Fig. 2 was formulated from a reaction mixture of starting materials corresponding on an oxide basis to 25 mole percent (Hfl xZrx)02, 55 mole percent P205 and 20 mole percent Na2o, falling within ~-9184 ~7~35~3 the shaded are~ "~" of Flg. 3, placed in a furnace at 500C, heated to 1,300C in ~.5 hours and fi~ed at 1,300C for 5.5 hours. As can be seen ~rom a com?arison of ~he photomicro-graphs, the particle diameters of ~he Figure 2 material are orders of magnitude larger than the p~rticle diameters of the n~aterial of Figure 1. In addition, c~mparison of the relative intensities of ultraviolet emissions ~f the two materials upon x-ray excitation indicates that the m~terial of Figure 2 would have a superior x-ra~ speed while the material o~ Figure 1 would exhibit a relatively inferior x-ray speed.
Referring now to Fig. 4, there is shown one embodiment of an x-ray intensifying screen 10 in~orporating a base layer 11 substantially transparent to x-rays, but substantially opaque to visible and ultraviolet radiation. Such base layer could be of a plastic material. A layer of an x-ray excitable luminescent layer 12 is secured to base layer 11, for example, by dispersing the material in an organic binder matrix and coating the matrix on the base layer. Incoming x-rays, upon striking luminescent layer 12, stimulate ultraviolet emissions in all directions.
However, the opaqueness of the base layer 11 to such ultraviolet radiation results in the substantial redirection of such radiation away from the base layer and toward an x-ray film 13 of base layer 13b and emulsion layer 13a. Since the film is more sensitive to the ultraviolet radi~tion than to x-rays, the image produced upon the film by the exposure of the film to such radiation is thereby intensified.
While in its broadest aspects the invention essentially is directed to the production of large s:Lngle crystalline lumines-cent material, it is contemplated that activator elements or compounds may be added to the material. However, it should be recognized that such activators, depending upon their characteristics, may significantly influence the emission spectra o~ the material upon excitation by x-rays or ~'` , .
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other means. Accordingly, the addition of such acti-vators is not intended for the particular application of x-ray intensifying screen luminescent materials contemplated herein as a preferred embodiment of the invention.
While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

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Claims (2)

CLAIMS:
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An x-ray intensifying screen for use with conventional silver halide emulsion x-ray film to intensity an exposed image on the film, the screen com-prising:
(a) a base layer of a material substantially transparent to x-rays, but substantially opaque to visible and ultraviolet radiation; and (b) a layer of an x-ray excitable luminescent material secured to the base layer, characterized in that the luminescent material consists essentially of large single crystalline cubic particles of (Hf1-xZrx)P2O7, wherein at least 50% of the particles are at least 6.0 micrometers in diameter, as measured by coulter counter, and further character-ized in that x is in the range of 0.05 to 0.2, whereby resolution of the image is improved by the substantial elimination of crossover.

2. The x-ray intensifying screen of Claim 1 wherein at least 50 percent of the particles are at least about 7.0 micrometers in diameter, as measured by Coulter Counter.