GB1565811A - Luminescent phosphor materials - Google Patents

Description

(54) LUMINESCENT PHOSPHOR MATERIALS (71) We, MINNESOTA MINING AND MANUFACTURING COMPANY, a cor portion organised and existing under the laws of the State of Delaware, - United States of America, of 3M Center, Saint Paul, Minnesota 55101, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to luminescent phosphors and in particular those phosphors which emit ultra-violet radiation when struck by X-rays.

According to the invention there is provided a luminescent phosphor which has the empirical formula: La(l x y z a)GdxCeyTbzThaXO4 in which X represents phosphorus atoms, arsenic atoms or a mixture of phosphorus and arsenic atoms, x is 0.01 to 0.50 and preferably 0.05 to 0.30, y isO or up to 0.50, z is 0 or up to 0.10 and preferably 0 or up to 0.02, a is 0 or up to 0.02, and when X represents phosphorus atoms alone y + z + a is at least 0.01.

As can be seen, the phosphor materials of the invention can be mixed phosphors which are mixtures of phosphates and arsenates in which case in the empirical formula noted above X represents partially a phosphorous atom and partially an arsenic atom, the exact proportion being determined by the relative ratios of the actual number of phosphorus and arsenic atoms, in the overall composition, and in addition y + z + a does not then have to be at least 0.01.

These phosphors of the invention emit strong ultra-violet radiation when irradiated by X-rays and so are useful, for example, in the preparation of intensifying screens, particularly where the photographic material is ultra-violet radiation sensitive as are the materials described in our copending Patent Application No. 37276/75 (Serial No.

1565502). For such a use it is desirable that the phosphors emit the greater part of their total overall emission when stuck by X-rays in the wave-length region of from 250 to 400 nm and preferably of from 300 to 350 nm. Many of the phosphors of the invention do have strong emissions in this preferred wavelength region. Besides use in intensifying screens, the phosphors of the invention have other uses such as in the emission coatings of cathode ray tubes.

The phosphors of the invention also have good X-ray absorption characteristics and a reasonably high efficiency of conversion of the absorbed energy to ultra-violet radiation.

We have found that the phosphors of the invention had advantages in relative emission as compared with prior art phosphors prepared in an analogous fashion from starting materials of the same purity.

The above empirical formula is not intended to represent an exact or necessarily defined chemical structure but merely to indicate the relative proportions of the various elements in the phosphors. However, X-ray diffraction studies have shown that the phosphors of the invention appear to have a 'monazite' crystal structure. When thorium is present it does alter the emission characteristics of the phosphor and so it is belived to be somehow dispersed in the crystal structure, either at lattice sites or interstitial positions.

While pure lanthanum phosphate has a very low ultra-volet radiation emission when struck with X-rays, lanthanum gadolinium phosphate containing at least about 0.5 mole % of gadolinium has a strong emission at about 312 nm when excited. This emission appears to be at a maximum when the gadolinium content is from 5 to 30 mole % and, although at high gadolinium contents, the ultra-violet radiation emitted appears to be less in comparison with the visible light radiation emitted, the higher the gadolinium content, the higher the X-ray absorption. However, gadolinium contents above about 50 mole Wo do not give phosphors which are particularly useful as ultra-violet radiation emitters.

Lanthanum absorbs tungsten X-rays more strongly than, for example, yttrium and tests have indeed confirmed that lanthanum gadolinium phosphates provide a much higher ultra-violet radiation output intensity then the equivalent known yttrium gadolinium phosphates having the same gadolinium content, and the lanthanum present in then phosphors of the invention is belived to give similar advantages.

When some cerium is included in the phosphors this is found to give a broader band of ultra-violet radiation emission, and so a higher total ultra-violet output. The inclusion of terbium as the Tb3+ ion in lanthanum gadolinium phosphates also appears to increase the overall ultra-violet radiation output and give predominantly ultra-violet radiation and blue emission whereas terbium doping of known lanthanum or gadolinium phosphate gives predominantly green emission. The phosphors may include both cerium and terbium but preferably only one or other of cerium and terbium is present.

In the preparation of phosphors of the invention it is important to start from highly pure reagents since only very small quantities of some of the doping elements need to be present and their presence could readily be masked by large amounts of impurities. For example, one should use reagents which are at least 99.0% pure and low in other rare earth elements or which are of Analar grade chemicals.

When the phosphorus is replaced by arsenic, the corresponding arsenates have broadly similar properties. However, the much higher X-ray absorbtion ability of arsenic as compared with phosphorus, 5 times higher for tungsten X-rays and 9 times higher for molbdenum X-rays, generally gives the advantage that the arsenate phosphors of the invention shown a higher absorption for X-rays than the equivalent phosphate phosphors.

It has further been found that the inclusion of a small amount, up to about 2 mole %, of thorium significantly improves the ultra-violet radiation emission of the phosphors of the invention.

The phosphors of the invention can be prepared in a manner analogous to known materials. Thus, for example, lanthanide oxides can be reacted with phosphoric acid followed by one or more firings at high temperatures, by precipitation of an aqueous mixture or solution of various salts or other soluble compounds of the various elements followed by firing the precipitate, or by reacting lanthanide carbonate with dry ammonium phosphate at high temperatures.

After preparation of the phosphor it will then generally be ground to a fine particle size.

This needs to be small enough to give smooth coatings having good emission definition when struck by X-rays but should not be too small otherwise emission efficiency is greatly reduced. We have found that in general a mean particle size of from 1 to 10 llm gives good results and that a mean particle size of about 5 m is preferred.

The phosphors of the invention can be used in a manner analogous to prior phosphors.

Thus, for example, they can be used in the production of X-ray intensifying screens for photographic films in which case the finely ground phosphor is dispersed in a suitable binder, an example being Pliolite, and spread out as a coating on a support.

The invention will now be illustrated by the following Examples and accompanying drawings which are graphs showing emission spectra of various phosphors. Although these graphs do not show absolute emission intensities, the relative intensities from graph to graph are approximately of the correct order.

In these Examples the raw materials used had the following purities: La203 - ........ .. 99.9% and low in other rare earths Gd(NO3)3.6H2O . .. .. 99.9% and low in other rare earths Ce(NO3)3.6H2O ....... ....... . > 99% and low in other rare earths Tb(NO3)3.6H2O . . . . 99.9% and low in other rare earths ThNO3 . . . ..... > 98No, other rare earths < 0.05Sc H3PO4 . . 85% aqueous solution, Analar grade As203 ....... . ....... . . Analar grade HNO3 . . .. . . ........................ Analar grade Gd2O3 ......... ................................. . made by precipitating solution Gd(NO3)3 with NaOH.

In addition, the crude phosphor after first firing was ground in a pestle and mortar and then given a high temperature firing. It was then milled to a particle size of 1 to 2 11 in a Retsch rotary ball mill, the size being checked from time to time under a microscope.

Dispersion of this phosphor in a binder was accomplished by milling the phosphor, binder and solvent in an Oppermann ball mill.

The binder used in all the Examples was Pliolite S-5B supplied by Goodyear Chemical Co., Pliolite being a copolymer of polystyrene and butadiene. The binder proportion was 10 to 15% by weight of the phosphor. The solvent was toluene.

Example I (comparison) Lanthanum gadolinium phosphate containing 20 mole % gadolinium was prepared by reacting an intimate mixture of lanthanum oxide and gadolinium oxide in the molar ratio of 5:1 with a slight excess of phosphoric acid with continuous mixing until a completely dry mixture was obtained. The mixture was fired at 9500C for one hour, cooled, and the emission spectrum under X-ray irradiation measured. It is shown in Figure 1.

Example 2 Lanthanum gadolinium phosphate doped with cerium was prepared and its emission spectrum measured as above but containing 8 mole % Gd and 2 mole % Ce. Broader ultra-violet radiation emissfaplion was obtained in the presence of cerium as shown in Figure 2.

Example 3 Lanthanum gadolinium phosphate doped with terbium was prepared as in Example 1 above but containing 10 mole % Gd and 0.8 mole % Tb. The emission consisted of a greater number of peaks, mainly in the ultra-violet and blue as shown in Figure 3.

Example 4 Lanthanum gadolinium arsenate, (La,Gd)AsO4 containing 5 mole % Gd was prepared by mixing the oxides La203, Gd203 and As203 with concentrated nitric acid, boiling unti evolution of gases had ceased, and neutralisation of the resultant clear soluion with a 5N solution of sodium hydroxide. The resulting precipitate was filtered off, fired at 950"C for one hour, and identified as (La,Gd)AsO4 by X-ray diffraction. Emission with X-ray irradiation was similar in type and intensity to that of (La. Gd)PO4 as shown in Figure 4.

Example 5 A lanthanum gadolinium phosphate containing 5 mole % gadolinium and doped with 0.05% of thorium was prepared as in Example 1 above. When this was compared with a lanthanum gadolinium phosphate containing 5 mole % gadolinium but not doped with the thorium, its ultra-violet radiation emission was approximately double in intensity.

References to 'analar grade' used herein in connection with the purity of materials refers to the standards in accordance with Analar Standards for Laboratory Chemicals, 7th Edition, 1976, Analar Standard Limited.

The words "Pliolite" and "Analar" are registered Trade Marks.

WHAT WE CLAIM IS: 1. A luminescent phosphor which has the empirical formula: Laixvz.a GdCeTb2ThXO4 in which X represents phosphorus atoms, arsenic atoms or a mixture of phosphorus and arsenic atoms, xis 0.01 to 0.50, y is 0 or up to 0.50, z is 0 or up to 0.10, a is 0 or up to 0.02, and when X represents phosphorus atoms alone y + z + a is at least 0.01.

2. A luminescent phosphor as claimed in Claim 1 in which X represents an arsenic atom or a mixture of phosphorus and arsenic atoms.

3. A luminescent phosphor as claimed in Claim 1 or Claim 2 in which x is 0.05 to 0.30.

4. A luminescent phosphor as claimed in any preceding claim in which z is 0 or up to 0.02.

5. A Luminescent phosphor as claimed in any preceding claim in which one or other but not both of y and z are zero.

6. A luminescent phosphor substantially as herein described in any of Examples 2 to 5.

7. A luminescent phosphor composition comprising a phosphor as claimed in any preceding claim in the form of fine particles dispersed in a binder.

8. An X-ray intensifying screen comprising a layer of a finely ground phosphor as claimed in any of claims 1 to 6 dispersed in a binder, the layer having been coated on a support.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. Dispersion of this phosphor in a binder was accomplished by milling the phosphor, binder and solvent in an Oppermann ball mill. The binder used in all the Examples was Pliolite S-5B supplied by Goodyear Chemical Co., Pliolite being a copolymer of polystyrene and butadiene. The binder proportion was 10 to 15% by weight of the phosphor. The solvent was toluene. Example I (comparison) Lanthanum gadolinium phosphate containing 20 mole % gadolinium was prepared by reacting an intimate mixture of lanthanum oxide and gadolinium oxide in the molar ratio of 5:1 with a slight excess of phosphoric acid with continuous mixing until a completely dry mixture was obtained. The mixture was fired at 9500C for one hour, cooled, and the emission spectrum under X-ray irradiation measured. It is shown in Figure 1. Example 2 Lanthanum gadolinium phosphate doped with cerium was prepared and its emission spectrum measured as above but containing 8 mole % Gd and 2 mole % Ce. Broader ultra-violet radiation emissfaplion was obtained in the presence of cerium as shown in Figure 2. Example 3 Lanthanum gadolinium phosphate doped with terbium was prepared as in Example 1 above but containing 10 mole % Gd and 0.8 mole % Tb. The emission consisted of a greater number of peaks, mainly in the ultra-violet and blue as shown in Figure 3. Example 4 Lanthanum gadolinium arsenate, (La,Gd)AsO4 containing 5 mole % Gd was prepared by mixing the oxides La203, Gd203 and As203 with concentrated nitric acid, boiling unti evolution of gases had ceased, and neutralisation of the resultant clear soluion with a 5N solution of sodium hydroxide. The resulting precipitate was filtered off, fired at 950"C for one hour, and identified as (La,Gd)AsO4 by X-ray diffraction. Emission with X-ray irradiation was similar in type and intensity to that of (La. Gd)PO4 as shown in Figure 4. Example 5 A lanthanum gadolinium phosphate containing 5 mole % gadolinium and doped with 0.05% of thorium was prepared as in Example 1 above. When this was compared with a lanthanum gadolinium phosphate containing 5 mole % gadolinium but not doped with the thorium, its ultra-violet radiation emission was approximately double in intensity. References to 'analar grade' used herein in connection with the purity of materials refers to the standards in accordance with Analar Standards for Laboratory Chemicals, 7th Edition, 1976, Analar Standard Limited. The words "Pliolite" and "Analar" are registered Trade Marks. WHAT WE CLAIM IS:

1. A luminescent phosphor which has the empirical formula: Laixvz.a GdCeTb2ThXO4 in which X represents phosphorus atoms, arsenic atoms or a mixture of phosphorus and arsenic atoms, xis 0.01 to 0.50, y is 0 or up to 0.50, z is 0 or up to 0.10, a is 0 or up to 0.02, and when X represents phosphorus atoms alone y + z + a is at least 0.01.

2. A luminescent phosphor as claimed in Claim 1 in which X represents an arsenic atom or a mixture of phosphorus and arsenic atoms.

3. A luminescent phosphor as claimed in Claim 1 or Claim 2 in which x is 0.05 to 0.30.

4. A luminescent phosphor as claimed in any preceding claim in which z is 0 or up to 0.02.

5. A Luminescent phosphor as claimed in any preceding claim in which one or other but not both of y and z are zero.

6. A luminescent phosphor substantially as herein described in any of Examples 2 to 5.

7. A luminescent phosphor composition comprising a phosphor as claimed in any preceding claim in the form of fine particles dispersed in a binder.

8. An X-ray intensifying screen comprising a layer of a finely ground phosphor as claimed in any of claims 1 to 6 dispersed in a binder, the layer having been coated on a support.