WO2003078543A1

WO2003078543A1 - Novel methods for preparing rare-earth oxysulfide phosphors, and resulting materials

Info

Publication number: WO2003078543A1
Application number: PCT/US2003/007751
Authority: WO
Inventors: Paul H. Holloway; Mostafa Abboudi
Original assignee: University Of Florida
Priority date: 2002-03-13
Filing date: 2003-03-13
Publication date: 2003-09-25
Also published as: US6843938B2; US20030209694A1; AU2003225781A1

Abstract

The present invention provides new and useful methods for preparing rare-earth oxysulfide phosphors. Advantageously, in accordance with the methods of the subject invention, rare-earth oxysulfide phosphors can be prepared with lower reaction temperatures and fewer processing steps than methods currently in use. In a preferred embodiment, the process of the subject invention involves blending two different rare-earth nitrate salts and a sulfidizing agent. Then, the mixture is heated at relatively low temperatures until the rare-earth oxysulfide phosphor is produced.

Description

DESCRIPTION

NOVEL METHODS FOR PREPARING RAPvE-EARTH OXYSULFIDE PHOSPHORS. AND RESULTING MATERIALS

Government Support This invention was made with government support under a grant awarded from D ARP A to Georgia Tech under grant number MD A972-93- 1 -0030 with a subcontract to the University of Florida and administered at the University of Florida under grant number 450947012. The government has certain rights in the invention.

Cross-Reference to Related Applications This application claims the benefit of U.S. Provisional application No. 60/364,838, filed March 13, 2002.

Background of Invention The present invention pertains to advantageous new low temperature processes for synthesizing rare-earth oxysulfide light-emitting phosphors.

Oxysulfide phosphors, which are used in all of the display technologies, are desirable commercial products because of their high luminance efficiency. For example, red, green, and blue emitting phosphors (three different phosphors) are used in every television, computer monitor, and cathode ray tube produced. They are used in electron beams and X-ray digitized image displays. Phosphors are also used in the lighting industry, for example, in fluorescent light bulbs. Current oxysulfide phosphor synthesis methods produce phosphors with sufficient luminosity, but these processes have significant disadvantages in terms of operating costs and processing time. For example, current methods require process temperatures in excess of 500°C. hi one method, process temperatures as high as 1350°C are reached in production of oxysulfide phosphors. Operating and equipment costs associated with reaching and maintaining temperatures of this magnitude create a need for lower temperature methods of preparation. In addition, some methods require inert gases such as nitrogen for the reaction environment. The extra equipment for maintenance of the inert atmosphere in addition to the operating expense of buying a specialty gas creates a need for a process utilizing cheaper accessible reactants. Also, reaction times are frequently an hour or more. Combined with the multiple steps required to prepare, react, and purify the oxysulfide phosphor, the total processing time is often hours. A shorter reaction and processing time is desirable because product may be made more frequently or on short demand time.

Therefore, it is desirable to find a method to produce rare-earth oxysulfide phosphors without the disadvantages of the known methods, thus reducing the operating costs. Because both the display and lighting markets, which deal with consumer items, are very cost competitive, any process that reduces cost will have a huge potential impact.

Brief Summary of the Invention The present invention provides new and useful methods for preparing rare-earth oxysulfide phosphors. Rare-earth oxysulfide phosphors, materials that emit electromagnetic radiation when excited, are useful in many commercial areas including electronics containing cathode ray tubes and medical devices that utilize X-rays.

Advantageously, in accordance with the methods of the subject invention, rare- earth oxysulfide phosphors can be prepared with lower reaction temperatures and fewer processing steps than methods currently in use.

In a preferred embodiment, the process of the subj ect invention involves blending two different rare-earth nitrate salts and a sulfidizing agent. After being thoroughly combined, the mixture can be dried to prevent any hydration of the nitrate salts before the combustion step. Then, the mixture is heated in a furnace to the necessary temperature, preferably around 200°C to 300°C, until the rare-earth oxysulfide phosphor is produced.

If particle sized phosphors are desired, the agglomerated phosphor nodules produced by the process of the subject invention maybe ground without compromising the cathodoluminescent efficiency of the phosphor. In addition, an annealing step to increase the photoluminescence of the phosphor may be added. The annealing step can follow the grinding step. hi a specific example, the subject invention provides new and unique methods for preparing oxysulfide phosphors with the general formula Ln₂O₂S (where Ln is scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium). The phosphors produced according to the subject invnetion comprise a small amount, typically less than about 5%, of a second Lanthanide Group element that is incorporated within the Ln₂O₂S matrix.

The present invention, operating with a simple procedure at reduced costs in terms of energy, time, and starting materials, results in significant cost savings while producing commercial quality rare-earth oxysulfide phosphors. This invention allows mass production of oxysulfide phosphors; the methods of the subject invention have the following very desirable characteristics:

-the short time of reaction and low temperature results in very low energy costs;

-the process is simple in comparison with existing processes, and the reaction can be done in the presence of reasonable quantities of oxygen;

-many, or all, of the reagents are commercially available;

-compounds can be prepared in a few minutes; and

-the compounds obtained according to the subject, invention have a brightness equal to the present commercial products and, in some cases, higher intensity peaks for green or red emission.

Brief Summary of the Figures

Figure 1 is a scanning electron microscopy photomicrograph of Laι.₉₃₆Euo_.o₆₄O₂S.

Figure 2 is an X-ray diffraction spectrum of Laι.₉₃₆Euo.o₆₄O₂S. Figure 3 is a cathodoluminescence spectrum at 2keN of Laι_.936Euo.o₆₄O₂S.

Figure 4 is a cathodoluminescence spectrum at 2keN of Laι_.936Tmo_.o₆ O₂S.

Figure 5 is an X-ray diffraction spectrum of Gd_1.996Tbo.oo₄O₂S.

Figure 6 is a scanning electron microscopy photomicrograph of Gdι_.996Tbo.oo₄θ₂S.

Figure 7 is a cathodoluminescence spectrum at 2keN of Gdι.₉₉₆Tbo_.oo₄θ₂S. Figure 8 is an X-ray diffraction spectrum of Yι.₉₉₆Tbo.oo₄θ₂S. Figure 9A is a scanning electron microscopy photomicrograph of

Figure 10 is a cathodoluminescence spectrum at 2keN of Y₁,_{9 6}Tbo.oo₄O₂S.

Figure 11 A is a plot comparing the peak intensity of ground Laι.₉₃₆Euo_.o₆₄θ₂S and ground and annealed Laι. ₃₆Euo_.o₆₄O S as a function of voltage.

Figure 1 IB is a plot comparing the relative efficiency of ground Lai .₉₃₆Euo.o₆ O₂S and ground and annealed Laι. ₃₆Euo.o₆₄θ₂S as a function of voltage.

Figure 11C is a plot of the ratio of ground Laι.₉₃₆Euo.o₆₄θ₂S intensity to ground and annealed Laι.₉₃₆Euo.o₆₄O₂S intensity as a function of voltage.

Detailed Description of the invention

The present invention provides unique and advantageous processes for manufacturing rare-earth oxysulfide phosphors. Advantageously, the methods or the subj ect invention for synthesizing rare-earth oxysulfide phosphors reduces the number of steps of production, decreases the energy costs by requiring lower temperatures, and uses commercially available components.

In a preferred embodiment, the oxysulfide phosphous produced according to the subject invention have the general formal Ln ₂._xLn _xO₂S, where Ln is a member of the Lanthanide group of elements. The term "Lanthanide" is used herein to denote elements with an atomic number from 57 to 71 {i.e. from lanthanum to lutetium). In a preferred embodiment, the value of x is between about 0.0005 and 0.5. More preferably x is between about 0.001 and 0.1.

A phosphor is a material, which, when excited by electrons, photons, or electric fields, emits electromagnetic radiation. In the present case, the light can be emitted in the visible or infrared regions. These phosphors canbe in the form of a solid solution having a matrix of the rare earth oxysulfide compound detailed above but doped with a small amount of an activator dispersed throughout the matrix. The activator is typically also a rare-earth element.

Advantageously, the luminescent phosphors produced according to the subject invention may be used in anumber of applications. Examples of applications of two of these types of phosphors include: production of the red phosphor used in television screens: Y₂O₂S:Eu and production for an intensifying screen for X-rays: Gd₂O₂S:Tb. hi a preferred embodiment, the rare-earth oxysulfide phosphors are obtained in accordance with the present invention by heating a blend of two different rare-earth nitrate salts and a sulfidizing agent. More specifically, a rare-earth oxysulfide phosphor can be obtained by blending a rare-earth element in its nitrate salt form with a smaller quantity of a different rare-earth nitrate salt. Using techniques known in the art, the two salts are broken into smaller nodules or particles by grinding. An organic sulfidizing agent is blended with the salt mixture followed by additional grinding. Preferably, dithiooxamide is added as an organic sulfidizing agent.

To fabricate the phosphors in accordance with the subject invention, the nitrate and sulfidizing agent blend is heated to temperatures less than 1000°C. Preferably the blend is heated to a temperature between about 200°C and about 300°C. Most preferably, the blend is heated to around 230°C. In one embodiment, to inhibit the hydration of either of the rare-earth nitrate salts, the salt and sulfidizing agent blend is dried before combustion occurs. One embodiment utilizes a desiccator as a preferred technique for drying. The drying time will vary depending upon which rare-earth nitrate salts are used, and can be readily determined by a person skilled in the art. As would be appreciated by the skilled artisan, for certain nitrate salts no drying time would be needed but, for other salts, up to around one week of drying may be employed.

The phosphors produced according to the subject invention can be further processed to optimize the physical properties of the phosphors to meet specific market needs. In one embodiment, the phosphor can be ground using techniques known in the art to fracture the phosphors into individual particles without decreasing cathodoluminescence. i another embodiment, the individual particles can undergo an annealing step after the post-combustion grind to increase the photoluminescence of the particles.

Depending on the host lattice and the activator(s), the conditions of the subject method can vary. With the benefit of the teachings provided herein, optimum conditions can be established, by a person skilled in the art, for each combination of rare earth salts. Factors that can be optimized include:

-the ratio of sulfidizing agent to nitrate (this ratio varies because of the energy and temperature obtained varies for each reaction); and

-drying time (varies from no time up to a week or more) -this parameter varies because of the different tendencies for hydration of the nitrate salts of different rare-earth elements.

Following are examples that illustrate procedures for practicing the invention. These examples should not be construed as limiting.

To prepare an oxysulfide phosphor with the formula Laι, ₃₆Euo.o₆₄θ₂S, which corresponds to doping with 3.2% of europium, the starting compounds shown in Table 1 can be used.

h the first step, the nitrate salts of the rare earths can be put in a mortar and ground, producing a good homogeneity of the nitrates. Then, an organic sulfidizing agent, in this case the dithiooxamide (C₂S₂N₂H ), is added to the mixture of nitrates. The mixture is again thoroughly ground (approximately 15 minutes). This mixture is put in a combustion boat in a desiccator for 24 hours. When the mixture is heated in a furnace at 230 °C in air, a highly energetic reaction takes place in only a few seconds.

The very porous oxysulfide of lanthanum obtained is shown by the scanning electron microscopy (SEM) photomicrograph in Figure 1. The morphology of this powder (Figure 1) is particulate with an average size under one micrometer. These powder particles are agglomerated into nodules of

A post-combustion grinding can be used to break the nodules into individual phosphor particles with a size of, for example, <lμm.

The X-ray diffraction (XRD) spectrum shows only La₂O₂S, without a presence of any secondary phases (Figure 2). Cathodoluminescence emission has been studied at a voltage of 2keN, and Figure 3 shows the emission spectrum. The observed peaks are those expected from phosphors of lanthanum oxysulfide doped with europium, and a red color is observed.

For the preparation of a phosphor with the formula Laι.₉₃₆Tmo.o₆₄θ₂S, which corresponds to doping with 3.2% thulium, the starting compounds shown in Table 2 can be used.

The method of preparation is exactly the same as in Example 1. Figure 4 shows the cathodoluminescent emission spectrum for electrons with an energy of 2keN. The spectrum is that expected from Tm and is dominated by two peaks, one in the blue region at 457 nm, and the second one at 800nm in the near infrared region.

For the preparation of a phosphor with the formula Gdι.₉₉₆Tbo_.oo₄θ₂S, which corresponds to doping with 0.2% terbium, the starting compounds shown in Table 3 can be used.

The same process is used as described above, but in this case, the time of drying in the desiccator is increased (to one week) because of the higher tendency of the gadolinium nitrate to hydrate. With longer drying times, a pure oxysulfide of gadolinium is obtained. As characterized by XRD (Figure 5), only one crystalline phase was formed and it is the expected oxysulfide of gadolinium.

The powder morphology as imaged in the SEM again shows grain sizes less than one micrometer (Figure 6).

The cathodoluminescent spectrum from excitation with a primary electron bean of 2 keN is shown in Figure 7. It is that expected from Tb doped gadolinium oxysulfide, i.e., a very bright green emission.

For the preparation of a phosphor with the formula Yι.₉₉₆Tbo.oo₄θ₂S, which corresponds to doping by 0.2% terbium, the same method is used. The starting compounds are in Table 4.

In this case the molar ratio of organic sulfidizing agent to rare-earth nitrate is equal to 1.8 in order to reduce the organic residue. The XRD pattern from these powders is shown in Figure 8, and only the phase of Y₂O₂S was observed.

The SEM photomicrograph in Figure 9 shows that the powder particles were again typically under one micrometer.

Finally, the 2 keN cathodoluminescent spectrum shown in Figure 10 is that typical for green emission from Tb.

Example 5 — Emission Intensities For Selected Rare-earth Oxysulfide Phosphors Powders of Ln₂O₂S (in this case, Ln=La,Gd,Y) containing luminescent activation centers of either Eu⁺³, Tb⁺³, or Tm⁺³ have been prepared by combustion reactions using metal nitrates and dithiooxoamide ((CSΝH₂)₂). Their cathodoluminescent properties have been measured versus activator type and concentration, current density, and beam energy. Measurements of the relative peak intensity for D₂→ F₄ (534nm) and ⁵Dι→⁷F₃(589nm) to ⁵D₀→⁷F₂(627nm) in La₂O₂S:Eu⁺³ phosphor have shown that at high activator concentration, emissions from high energy levels, (⁵D₂ and ⁵Dι), are more suppressed than from the low energy level, (⁵Do). The ⁵D₂/⁵Do and ⁵Dι/⁵D₀ ratios have also been decreased when current density and beam energy were increased. In addition, matrix effects can lead to quenching of Eu emission from ⁵D₂, ⁵Dι, and ⁵Do levels and suppression of high energy level emission from Tb oxysulfides.

Figures 11 A-l 1C show data resulting from grinding the powder to micrometer size particles. This data shows that such grinding does not reduce significantly the cathodoluminescent intensity. This is based on the fact that the intensity and relative efficiency is similar for both ground (G) phosphor and phosphor that has been both ground and annealed (G-A) at 900 °C for 90 minutes (in air).

The emission intensity from the ⁵D₂ ^→7F (534nm) and ⁵Dι→⁷F₃(589nm) to ⁵Do^→7F₂(627nm) luminescent transitions in Eu⁺³ doped Ln O₂S (Ln=La,Gd,Y) powder phosphors prepared by combustion reaction have been studied. It has been shown that emissions from high energy levels (⁵D₂ and ⁵Dι) are suppressed relative to those from low energy level (⁵Do) at higher current density (e.g. 30 vs,200uA/cm²) and beam energy (e.g.

2 vs. 5 keN). It has also been shown that the decay of the emitted intensity during electron-beam radiation is greater from ⁵D₂ ^→7F₄ (537nm) and ⁵Dι→⁷F₃(586nm) transitions vs. ⁵D₀ ^→7F₂(624nm) at a current density of 200uA/cm2. However, no decay was observed for these transitions at a current density of 30uA/cm . These results can be interpreted in terms of perturbations to the energy transfer process (e.g. electrostatic dipole and magnetic dipole interactions) by internal electric fields those are induced by charging, and due to thermal quenching caused by electron-beam heating. In both cases, quenching of emission from higher excited states can be expected at higher current density and beam energy.

It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.

Claims

Claims We claim:

1. A method for synthesizing a rare-earth oxysulfide phosphor wherein said method comprises:

(a) combining a first rare-earth nitrate salt and a second rare-earth nitrate salt;

(b) adding a sulfidizing agent to said first and second rare-earth nitrate salts; and

(c) heating.

2. The method, according to claim 1 , wherein the rare-earth oxysulfide phosphor

1 9 1 9 has the chemical formula Ln ₂-χLn _xO₂S where Ln and Ln are different elements each having an atomic number from 57 to 71.

3. The method, according to claim 1 , wherein the rare earth nitrate salt is selected from the group consisting of nitrate salts of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

4. The method, according to claim 1, wherein the sulfidizing agent is dithiooxamide (C₂S₂N₂H₄).

5. The method, according to claim 1 , wherein heating is conducted in a furnace in the presence of air.

6. The method, according to claim 1 , wherein the heating temperature is less than 1000°C.

7. The method, according to claim 6, wherein the heating temperature is between about 200°C and about 300°C.

8. The method, according to claim 7, wherein the heating temperature is about 230°C.

9. The method, according to claim 2, wherein the value of x is between about 0.001 and about 0.1.

10. The method, according to claim 1, further comprising a period of drying before heating the sulfidizing agent and nitrate salt blend.

11. The method, according to claim 10, wherein the period of drying is sufficient to prevent hydration of the rare-earth nitrate salts.

12. The method, according to claim 1, further comprising grinding the rare-earth oxysulfide phosphor following heating of the sulfidizing agent and the nitrate salt blend.

13. The method, according to claim 12, further comprising annealing the ground rare-earth oxysulfide phosphor.

14. The method, according to claim 1 , wherein the amount of said first nitrate salt is sufficiently greater than the amount of said second nitrate salt such that a solid solution is formed wherein the rare earth contributed by said first rare-earth nitrate salt forms a lattice and the rare earth contributed by said second rare-earth nitrate salt is dispersed throughout the lattice.

15. The method, according to claim 2, wherein Ln¹ is selected from the group consisting of La, Gd and Y; and Ln² is selected from the group consisting of Ev, Tm and Jb.

16. The method, according to claim 1, used to produce a compound of the formula La₂._x Eu_x O₂S.

17. The method, according to claim 1, used to produce a compound of the formula La₂-_X Tm_x O₂S.

18. The method, according to claim 1, used to produce a compound of the formula La₂._x Tb_x O₂S.

19. A rare-earth oxysulfide phosphor produced by a process comprising:

(c) heating.