Phosphor based on a lanthanum cerium terbium phosphate with stabilized brightness, preparation process and use in a luminescent device

Abstract

The phosphor of the invention is based on a lanthunum cerium terbium phosphate, and it is characterized in that the phosphate consists of particles having a mean size of at most 4 μm, in that it has a lithium content of at most 30 ppm, a boton content of at most 30 ppm and in that it has a variation of brightness between the brightness measured on the phosphor at 25° C. and that measured on the same phosphor at 200° C. of at most 4%.

Claims

1. A process for preparing a phosphor, the process comprising: a) introducing, continuously and with stirring, a first solution comprising soluble salts of the elements lanthanum, cerium and terbium into a second solution comprising phosphate ions and having an initial pH of less than 2, thereby forming a precipitate, and, during the introduction of the first solution into the second solution, maintaining the pH of the precipitation medium at a constant value of less than 2; b) recovering the resulting precipitate; c) calcining the precipitate at a temperature of not more than 1000° C.; d) heat-treating the calcined precipitate under a reducing atmosphere, in the presence of lithium tetraborate (Li.sub.2B.sub.4O.sub.7) in a quantity by mass of not more than 0.2%, at a temperature of between 1050° C. and 1150° C. and over a time of between 2 hours and 4 hours, wherein the lanthanum cerium terbium phosphate consists of particles having an average size of not more than 4 μm, wherein the phosphor has a lithium content of not more than 30 ppm and a boron content of not more than 30 ppm, and wherein the brightness of the phosphor at 25° C. does not vary by more than 4% from the brightness of the same phosphor at 200° C.

2. The process as claimed in claim 1, wherein the pH of the precipitation medium is held constant at a value of between 1 and 2.

3. The process as claimed in claim 1, wherein the pH of the precipitation medium is maintained by addition of a basic compound.

4. The process as claimed in claim 3, wherein the pH of the precipitation medium is maintained by addition of ammonium hydroxide.

5. The process as claimed in claim 1, wherein the phosphate ions of the second solution are in the form of a phosphoric acid solution.

6. The process as claimed in claim 1, wherein the quantity of lithium tetraborate is between 0.1% and 0.2%.

7. The process as claimed in claim 1, wherein the product obtained from step d) is redispersed in water at a temperature of at least 30° C., to give a suspension, then held in suspension and lastly separated from the liquid medium of said suspension.

8. A phosphor based on a lanthanum cerium terbium phosphate heat treated in the presence of lithium tetraborate (Li.sub.2B.sub.4O.sub.7), wherein the phosphate consists of particles having an average size of not more than 4 μm, wherein the phosphor has a lithium content of not more than 30 ppm and a boron content of not more than 30 ppm, and wherein the brightness of the phosphor at 25° C. does not vary by more than 4% from the brightness of the same phosphor at 200° C.

9. The phosphor as claimed in claim 8, wherein the brightness of the phosphor at 25° C. does not vary by more than 2% from the brightness of the same phosphor at 200° C.

10. The phosphor as claimed in claim 8, wherein the phosphate consists of particles having an average size of not more than 3.5 μm.

11. The phosphor as claimed in claim 8, wherein the phosphate consists of particles having an average size of between 2.5 μm and 4 μm.

12. The phosphor as claimed in claim 11, wherein the phosphate consists of particles having an average size of between 2.5 μm and 3.5 μm.

13. The phosphor as claimed in claim 8, wherein the phosphate consists of particles having a dispersion index of not more than 0.7.

14. The phosphor as claimed in claim 8, wherein the lanthanum cerium terbium phosphate conforms to the formula (La.sub.xCe.sub.yTb.sub.z)PO.sub.4 in which x, y and z satisfy the following formulae:
x+y+z=1
0.2≦y≦0.45; and
0.1≦z≦0.2.

15. A method for forming a UV excitation device, the method comprising depositing a phosphor of claim 8 onto a support by screen printing, electrophoresis, or sedimentation, such that a UV excitation device is formed.

16. A luminescent device comprising a phosphor as claimed in claim 8 as green luminescence source.

17. The luminescent device as claimed in claim 16, characterized in that it is a UV excitation device.

18. The luminescent device as claimed in claim 16, wherein the device is selected from a trichromatic lamp, a mercury vapor lamps, a lamp for backlighting liquid-crystal systems, a plasma screen, a xenon excitation lamp, a light-emitting diode excitation device and a UV excitation marking system.

Description

EXAMPLE 1

(1) This example relates to the preparation of a precursor of a phosphor according to the invention.

(2) In a 1 liter beaker, a solution of rare earth nitrates (solution A) is prepared as follows: 171.5 g of a 3.0 M La(NO.sub.3).sub.3 solution, 179.8 g of a 2.88 M Ce(NO.sub.3).sub.3 solution, 54.5 g of a 2.57 M Tb(NO.sub.3).sub.3 solution, and 107.7 mL of deionized water are mixed, giving a total of 0.7 mol of rare earth nitrates, corresponding to a composition (La.sub.0.44Ce.sub.0.43Tb.sub.0.13) (NO.sub.3).sub.3.

(3) In a 1 liter reactor, 436 ml of deionized water, admixed with 29.4 g of 85% Normapur H.sub.3PO.sub.4 and then 28% aqueous ammonia NH.sub.4OH, to give a pH of 1.7, are introduced (solution B). The solution is heated to 60° C. Solution A prepared above is added to the mixture, slowly and with stirring, at temperature (60° C.) and with the pH regulated to 1.7. The resulting mixture is aged at 60° C. for 15 minutes. At the end of aging, the solution has a milky white appearance. It is left to cool to 30° C. It is filtered on a frit and washed with two liters of water, after which it is dried and calcined in air at 900° C. for 2 hours.

(4) This gives a monazite-phase rare earth phosphate, (La,Ce,Tb)PO.sub.4. The particles have a D.sub.50 of 3.0 μm, with a dispersion index of 0.5.

EXAMPLE 2

(5) This example relates to the preparation of a phosphor according to the invention.

(6) The precursor obtained in example 1 is calcined at 1100° C. for 4 hours under a reducing atmosphere (Ar/H.sub.2) in the presence of 0.1% by weight of lithium borate, Li.sub.2B.sub.4O.sub.7 relative to the amount of (La,Ce,Tb)PO.sub.4 precursor. The resulting product is then redispersed in hot water at 60° C., filtered on a Buchner filter, and then washed with 1 L of water.

(7) It is subsequently dried in an oven at 60° C. overnight. Subsequently it is deagglomerated by a 30-minute ball mill treatment, and then screened.

(8) The particles have a D.sub.50 of 3.2 μm, with a dispersion index of 0.6.

(9) The luminescence yield is set at 100% as a reference.

(10) The boron content noted is 19 ppm; the lithium content measured is 12 ppm.

COMPARATIVE EXAMPLE 3

(11) This example relates to the preparation of a phosphor, using a flux different from that used to prepare the phosphor according to the invention.

(12) The precursor obtained in example 1 is calcined at 1000° C. for 3 hours under a reducing atmosphere (Ar/H.sub.2) in the presence of 1% by weight of lithium fluoride, LiF, relative to the amount of (La,Ce,Tb)PO.sub.4 precursor. To facilitate the disintegration of the resulting product, it is subsequently redispersed in a 5% nitric acid solution, filtered on a Büchner filter and subsequently washed with 5 L of water. It is subsequently dried in an oven at 60° C. overnight. Thereafter it is deagglomerated by a 1-hour ball mill treatment, and then screened.

(13) The particles have a D.sub.50 of 4.5 μm, with a dispersion index of 0.5.

(14) The luminescence yield measured at 25° C. is 99% relative to the product from example 2.

COMPARATIVE EXAMPLE 4

(15) This example relates to the preparation of a phosphor, using a flux different from that used to prepare the phosphor according to the invention.

(16) The precursor obtained in example 1 is calcined at 1000° C. for 3 hours under a reducing atmosphere (Ar/H.sub.2) in the presence of 52.3% by weight of boric acid and 0.85% of Li.sub.2CO.sub.3, relative to the amount of (La,Ce,Tb)PO.sub.4 precursor. To facilitate the disintegration of the resulting product, it is subsequently redispersed in a 5% nitric acid solution, filtered on a Buchner filter and subsequently washed with 3 L of water, then redispersed in a 5% potassium hydroxide solution, filtered and washed with 3 L of water.

(17) It is subsequently dried in an oven at 60° C. overnight, and then screened.

(18) The particles have a D.sub.50 of 7.3 μm, with a dispersion index of 1.2.

(19) The luminescence yield measured at 25° C. is 98% relative to the product from example 2.

(20) The boron content is 530 ppm, and the lithium content 44 ppm.

(21) The variations in brightness are given in the table below.

(22) TABLE-US-00001 B.sub.25 B.sub.200 Variation Example 2 100 98 2% Example 3 99 80 19%  Example 4 98 91 7%

(23) The brightness at 200° C. is measured on a phosphor which is held at 200° C. on a hotplate (temperature raised in stages of 20° C., with 15 minutes per stage).