Persistent phosphorescent composite material

Abstract

The invention relates to a persistent phosphorescent ceramic composite material which is a sintered dense body comprising two or more phases, a first phase consisting of at least one metal oxide and a second phase consisting of a metal oxide containing at least one activating element in a reduced oxidation state. The invention furthermore relates to a method for the preparation of a phosphorescent ceramic composite material as defined in any of the previous claims, the method comprising the following steps: preparing a mixture of a metal oxide and a phosphor; fabricating a green body from the mixture; and heat treating the green body in a reducing atmosphere.

Claims

1. A phosphorescent ceramic composite material which is a sintered dense body consisting of: a first phase consisting of zirconia and at least one selected from the group consisting of cerium, magnesium and yttrium, and a second phase consisting of strontium aluminate and at least one dopant selected from the group consisting of europium and dysprosium, the dopant having a reduced oxidation state, wherein the sintered dense body is produced by sintering the first phase and the second phase under oxidizing conditions, and then sintering the first phase and the second phase under reducing conditions.

2. The phosphorescent ceramic composite material according to claim 1, wherein the second phase is a Eu.sup.2+/Dy.sup.3+doped Sr.sub.4Al.sub.14O.sub.25 phase.

3. The phosphorescent ceramic composite material according to claim 1, wherein the amount of the first phase is 40 to 95% by weight and the amount of the second phase is 5 to 60% by weight, relative to the total weight of the two phases.

4. The phosphorescent ceramic composite material according to claim 1, wherein the amount of the first phase is 50 to 95% by weight and the amount of the second phase is 5 to 50% by weight, relative to the total weight of the two phases.

5. The phosphorescent ceramic composite material according to claim 1, wherein the amount of the first phase is 50 to 80% by weight and the amount of the second phase is 20 to 50% by weight, relative to the total weight of the two phases.

6. The phosphorescent ceramic composite material according to claim 2, wherein the amount of the first phase is 40 to 95% by weight and the amount of the second phase is 5 to 60% by weight, relative to the total weight of the two phases.

7. The phosphorescent ceramic composite material according to claim 2, wherein the amount of the first phase is 50 to 95% by weight and the amount of the second phase is 5 to 50% by weight, relative to the total weight of the two phases.

8. The phosphorescent ceramic composite material according to claim 3, wherein the amount of the first phase is 50 to 80% by weight and the amount of the second phase is 20 to 50% by weight, relative to the total weight of the two phases.

9. A sintered and heat treated phosphorescent ceramic composite material consisting of, prior to sintering and heat treating, a blend of: a first phase consisting of zirconia and at least one selected from the group consisting of cerium, magnesium and yttrium, and a second phase consisting of strontium aluminate and at least one dopant selected from the group consisting of europium and dysprosium, the dopant having a reduced oxidation state; wherein the blend is sintered in a first step under oxidizing conditions and then sintered in a second step under reducing conditions.

10. The sintered and heat treated phosphorescent ceramic composite material according to claim 9, wherein the second phase is a Eu.sup.2+/Dy.sup.3+doped Sr.sub.4Al.sub.14O.sub.25 phase.

11. The sintered and heat treated phosphorescent ceramic composite material according to claim 9, wherein the amount of the first phase is 40 to 95% by weight and the amount of the second phase is 5 to 60% by weight, relative to the total weight of the two phases.

12. The sintered and heat treated phosphorescent ceramic composite material according to claim 9, wherein the amount of the first phase is 50 to 95% by weight and the amount of the second phase is 5 to 50% by weight, relative to the total weight of the two phases.

13. The sintered and heat treated phosphorescent ceramic composite material according to claim 9, wherein the amount of the first phase is 50 to 80% by weight and the amount of the second phase is 20 to 50% by weight, relative to the total weight of the two phases.

14. The sintered and heat treated phosphorescent ceramic composite material according to claim 10, wherein the amount of the first phase is 40 to 95% by weight and the amount of the second phase is 5 to 60% by weight, relative to the total weight of the two phases.

15. The sintered and heat treated phosphorescent ceramic composite material according to claim 10, wherein the amount of the first phase is 50 to 95% by weight and the amount of the second phase is 5 to 50% by weight, relative to the total weight of the two phases.

16. The sintered and heat treated phosphorescent ceramic composite material according to claim 10, wherein the amount of the first phase is 50 to 80% by weight and the amount of the second phase is 20 to 50% by weight, relative to the total weight of the two phases.

Description

FIGURES

(1) FIG. 1. Intensity of emission Lv as a function of time t of phosphorescent materials according to the present invention comprising different phases of strontium aluminate with rare earth activating elements.

(2) FIG. 2. Microstructures of two samples of inventive ceramic composite material realized with phosphorescent materials of standard and extrafine granulometry, respectively.

(3) FIG. 3. Influence of initial phosphor grain size on luminescent properties of phosphorescent materials according to the present invention.

(4) FIG. 4. Microstructures of two samples of phosphorescent composite material according to the present invention realized with phosphorescent materials without and with a washing step.

(5) FIG. 5. Influence of washing step and treatment temperature on luminescent properties of phosphorescent materials according to the present invention.

(6) FIG. 6. Influence of phosphor concentration on luminescent properties of phosphorescent materials according to the present invention.

(7) FIG. 7. Influence of phosphor concentration on luminescent properties of phosphorescent materials according to the present invention.

(8) FIG. 8. Comparison of luminescent properties of phosphorescent materials according to the present invention with a pure phosphor sample.

EXAMPLES

(9) Next the present invention is described in more detail by referring to the following examples.

(10) Meanwhile the properties of the ceramic composite material were determined by the following methods.

(11) The density is measured following Archimedes' method with absolute ethanol. Each sample is measured three times and the mean value is calculated.

(12) L*a*b* colorimetry measurements are performed after machining and polishing the sample, on the free side (ie the side that was not in contact with the sample holder during heat treatment), with an aperture of 7 mm on three different locations. The equipment is a Minolta CM3610d.

(13) The measurements of the toughness were performed by indentation with a KB250 Prftechnik GmbH equipment. The HV5 indentations were realized under a charge of 5 kg applied during 15 s. The toughness was measured by indentation and evaluated through the formula proposed by K. Niihara (cf Niihara K., A fracture mechanics analysis of indentation induced Palmqvist crack in ceramics, J. Mater. Sci. Lett, 1983, 2, 221-223):
K.sub.Ic=0.018Hva.sup.0.5(E/Hv).sup.0.4.Math.(a/c1).sup.0.5
where E is the elastic (or Young's) modulus (measured value: 220 GPa), Hv is the Vickers hardness in GPa, c is the length of the crack formed following indentation measured from the center of the indentation, and a is the half-length of the diagonal of the indentation.

(14) HV1 microhardness was measured with a LEICA VMHT MOT equipment with a charge of 1 Kg during 15 s. 10 measurements were performed per sample.

(15) The Young's modulus and Poisson ratio were measured by acoustic microscopy (non-destructive control by ultrasounds). The relative measurement uncertainty is 2% for both parameters.

(16) The intensity and decay of the emitted luminescence is measured in a black chamber on up to six samples with a Pritchard PR-880 photometer. The excitation of the phosphor prior to the measurement is done in the chamber with a standard fluorescent tube. The measurement is performed in three stages: (a) the sample is kept in the black chamber during 8 hours prior to charging; (b) the excitation is realized during 20 minutes under a D65 fluorocompact lamp at an excitation intensity of 400 lux; (c) the emitted luminescence is measured during at least 900 minutes with an objective aperture of 3, one of the samples being a reference sample. The sensitivity of the photometer is 0.9 mCd/m.sup.2, to be compared with 0.3 mCd/m.sup.2, which is the lower limit of light perception of the human eye.

(17) The X-ray diffraction measurements are performed in Bragg-Brentano geometry with a Cu anode excited with 45 kV electrons. The different phases are identified on the basis of reference patterns from the literature, and the phase concentrations (given in wt % in the tables below) are estimated with a typical accuracy of 1 wt %.

Example 1

(18) A sample 1 containing 20 wt % of phosphor has been prepared as follows: Mixing 80.0 g of zirconia powder containing 3 mol % of yttria (TZ-3YS obtained from TOSOH Corporation) and 20.0 g of Sr.sub.4Al.sub.14O.sub.25: Eu,Dy powder with 3.0 g of organic binder composed of 1.2 g (40%) PVA and 1.8 g (60%) PEG 20 000 in solution at 50% in water, with 200 ml distilled water and 1 kg of zirconia balls; Attrition/milling at 400 U/min during 30 min in a zirconia bowl; Filtering of the suspension, rinsing of the balls and bowl with 450 ml IPA, spray-drying of the filtered suspension and rinsing liquid.

(19) 7 g of powder were then pressed in a 40 mm mould. During a first heat treatment, debinding and sintering were performed in one step in a furnace under ambient atmosphere, at 1475 C., with a soak-time of 2 h with 21 h ramp-up time and 11 h cooling time (total treatment time of 34 h).

(20) The obtained pellets were machined and polished. The typical density as measured by the Archimedes method was 5.371 g.Math.cm.sup.3. Typical colorimetry was L*(D65)=97.01, a*(D65)=1.81; b*(D65)=2.21. Phase analysis by X-ray diffraction indicated that the phase ratios of the zirconia (tetragonal to cubic) were not modified with respect to a phosphor-free sample, and that the phosphor remained in the Sr.sub.4Al.sub.14O.sub.25 phase. At this stage, the phosphor was not functional and no persistent luminescence was detected.

(21) The second heat treatment was performed in reducing atmosphere, at 1450 C. during 4 h with a ramp-up rate of 150 C..Math.h.sup.1, under Ar/H.sub.2 atmosphere. After this treatment, the samples showed persistent luminescence. The density after treatment was 5.37 g.Math.cm.sup.3 and the hardness of the pellet was about 1250 Hv with a toughness of about 5.1 MPa.Math.m.sup.0.5. The colorimetry was L*(D65)=92.86, a*(D65)=1.31, b*(D65)=2.53, very close to the colour before sintering

Example 2

Effect of the Strontium Aluminate Phase

(22) The potential of two different strontium aluminates with rare-earth (RE) dopants to obtain a persistent phosphorescent ceramic material that is suitable, e.g., for watch applications was investigated.

(23) Two phases showed suitable performances for such applications: the Eu.sup.2+/Dy.sup.3+ doped SrAl.sub.2O.sub.4 phase which emits around 520 nm (green) and the less used Eu.sup.2+/Dy.sup.3+ doped Sr.sub.4Al.sub.14O.sub.25 phase which emits around 495 nm (blue). Although the green-emitting phase is most widely used, the blue-emitting material shows very interesting properties in terms of persistence and perceived intensity.

(24) Two samples with 20% by weight of active SrAlO material were prepared in the manner as described in example 1, with a pre-sintering performed at 900 C. under air and a sintering in reducing atmosphere at 1450 C. for 3 h (sample 2.1 incorporating the green-emission SrAl.sub.2O.sub.4 material and sample 2.2 incorporating the blue-emission Sr.sub.4Al.sub.14O.sub.25 material). The results are given in the following table 1 and in FIG. 1.

(25) TABLE-US-00001 TABLE 1 Sr.sub.xAl.sub.yO.sub.z phases ZrO.sub.2 phases (Sr.sub.4Al.sub.14O.sub.25/ (tetragonal/ Pre- Colour Density SrAl.sub.2O.sub.4/ cubic/ sample sintering sintering (LAB) (g .Math. cm3) SrAl.sub.12O.sub.19) monoclinic) 2.1 900 C. in 1450 C. in 93.9/5.9/ 5.33 0/18/0 60/21/1 air Ar/H.sub.2, 3 h 9.2 2.2 900 C. in 1450 C. in 95.7/3.5/ 5.33 18/0/0 60/21/1 air Ar/H.sub.2, 3 h 6.3

(26) The data prove that the sample with Sr.sub.4Al.sub.14O.sub.25 showed an emitted intensity that is 10 times higher than for the green emitting material. Although SrAl.sub.2O.sub.4 can be functionally incorporated in a zirconia matrix, it is clearly preferable to use Sr.sub.4Al.sub.14O.sub.25. However, the low performances of the SrAl.sub.2O.sub.4 containing samples could be due to some process steps. For example, as SrAl.sub.2O.sub.4 is water-soluble, it could be preferable not to use water-based methods for atomisation.

Example 3

Influence of Sr4Al14O25 Grain Size and Sintering Conditions

(27) The influence of the grain size of the initial phosphor material on the obtained performances was studied for two different sintering conditions.

(28) The images in FIG. 2 show the microstructures of the samples with standard granulometry (D.sub.V10=1.2 m; D.sub.V50=2.5 m; D.sub.V90=6.4 m, as in the samples 3.1 and 3.2, at left) and so-called extra-fine granulometry (D.sub.V10=0.1 m; D.sub.V50=1.4 m, D.sub.V90=4.7 m, as in the samples 3.3 and 3.4, at right).

(29) The behaviour of the four samples is displayed in table 2 and FIG. 3.

(30) TABLE-US-00002 TABLE 2 Sr.sub.xAl.sub.yO.sub.z phases ZrO.sub.2 phases (Sr.sub.4Al.sub.14O.sub.25/ (tetragonal/ Pre- Colour Density SrAl.sub.2O.sub.4/ cubic/ sample sintering sintering (LAB) (g .Math. cm.sup.3) SrAl.sub.12O.sub.19) monoclinic) 3.1 1475 C. in 1450 C. in 94.8/2.5/ 5.33 20/0/0 56/23/1 air N.sub.2/H.sub.2, 4 h 4.1 3.2 900 C. in 1450 C. in 95.7/3.5/ 5.33 20/0/0 56/23/1 air N.sub.2/H.sub.2, 3 h 6.3 3.3 1475 C. in 1450 C. in 94.6/2.2/ 5.35 18/0/0 60/21/1 air N.sub.2/H.sub.2, 4 h 3.2 3.4 900 C. in 1450 C. in 95/3/5 5.35 18/0/0 60/21/1 air N.sub.2/H.sub.2, 3 h

(31) Although all four samples showed persistent luminescence, it is preferable in this case to use a strontium aluminate powder with standard grain size, as the samples with small powder grain size showed systematically a lower emitted intensity. Furthermore, it appears that pre-sintering at 900 C. is more favourable than at 1475 C. for the persistence. Samples with pre-sintering at 1500 C. were comparable to samples pre-sintered at 1475 C., and samples with sintering in reducing atmosphere at 1500 C. were comparable to samples sintered at 1450 C. (not shown here).

Example 4

Influence of Sr4Al14O25 Pre-Treatment

(32) The influence of a pre-treatment of the phosphor powder before incorporation into the zirconia slurry was studied for different sintering conditions. This pre-treatment consists in washing the powder in an aqueous acidic solution, such as, for example, a diluted solution of acetic acid (at a concentration of for instance 10% by mass) at a temperature of 70 C. for a few hours. It is known that the washing step leads to the removal of an amorphous phase from the powder preparation.

(33) The images in FIG. 4 show the microstructures of the samples without washing (samples 3.1 and 3.2 of example 3, at left) and with an additional washing step (samples 4.1 and 4.2, at right).

(34) The presentation in FIG. 5 summarizes the behaviour of the two types of samples, obtained each under two different conditions. In this figure, the two samples which have not been washed are the samples 3.1 and 3.2 described in the example 3.

(35) The properties of two washed samples 4.1 and 4.2 are provided in the following table 3.

(36) TABLE-US-00003 TABLE 3 Sr.sub.xAl.sub.yO.sub.z phases ZrO.sub.2 phases (Sr.sub.4Al.sub.14O.sub.25/ (tetragonal/ Pre- Colour Density SrAl.sub.2O.sub.4/ cubic/ sample sintering sintering (LAB) (g .Math. cm.sup.3) SrAl.sub.12O.sub.19) monoclinic) 4.1 1475 C. in 1450 C. in 88/1.3/ 5.38 19/0/0 60/20/1 air N.sub.2/H.sub.2, 4 h 1.2 4.2 900 C. in 1450 C. in 94.3/3.1/ 5.38 19/0/0 60/20/1 air N.sub.2/H.sub.2, 3 h 4.6

(37) Again, all samples show persistent luminescence, but a pre-treatment of the phosphor material leads to lower emitted intensities. This effect is not fully understood and could have several origins (difference in grain size, for example).

(38) The results also confirm that pre-sintering at 900 C. is more favourable than an initial treatment at 1475 C. for the persistence. Samples with initial treatment sintered at 1500 C. were comparable to samples heated at 1475 C., and samples with sintering in reducing atmosphere at 1500 C. were comparable to samples sintered at 1450 C.

Example 5

Effect of the Sr4Al14O25 Concentration

(39) The influence of the concentration of Sr.sub.4Al.sub.14O.sub.25 in the composite material was studied, with samples comprising 20% by weight, 30% by weight and 50% by weight of phosphor material.

(40) The results of these experiments are displayed in the FIGS. 6 and 7. In the FIG. 6, the data for the sample with 20% by weight of phosphor material correspond to the data for the sample 3.1 in example 3. In the FIG. 7 the data for the sample with 20% of phosphor material correspond to the data for the sample 3.2 in example 3.

(41) The properties of samples 5.1 and 5.2 with 30% by weight of phosphor material and samples 5.3 and 5.4 with 50% by weight of phosphor material are provided in the following table 4.

(42) TABLE-US-00004 TABLE 4 Sr.sub.xAl.sub.yO.sub.z phases ZrO.sub.2 phases (Sr.sub.4Al.sub.14O.sub.25/ (tetragonal/ Pre- Colour Density SrAl.sub.2O.sub.4/ cubic/ sample sintering sintering (LAB) (g .Math. cm.sup.3) SrAl.sub.12O.sub.19) monoclinic) 5.1 1475 C. in 1450 C. in 96.3/3.1/ 5.02 30/0/0 49/20/1 air N.sub.2/H.sub.2, 4 h 5.4 5.2 900 C. in 1450 C. in 96.2/3.9/ 5.02 Not Not air N.sub.2/H.sub.2, 3 h 7.4 measured measured 5.3 1475 C. in 1450 C. in 93.7/3.1/ 4.49 49/0/0 28/16/7 air N.sub.2/H.sub.2, 4 h 6.3 5.4 900 C. in 1450 C. in 95.5/5.0/ 4.48 50/0/0 33/15/2 air N.sub.2/H.sub.2, 3 h 8.8

(43) All samples showed persistent luminescence. A higher phosphor concentration led to a marked increase of the emitted light intensity. Again, pre-sintering at 900 C. is more favourable than an initial treatment at 1475 C. for the persistence. Samples with initial treatment sintered at 1500 C. were comparable to samples heated at 1475 C., and samples with sintering in reducing atmosphere at 1500 C. were comparable to samples sintered at 1450 C. Sintering times of 3 h, 6 h and 9 h also yielded comparable results in terms of emitted luminescence.

(44) The elastic (Young's) modulus decreases with increasing phosphor content, from 216 GPa for pure zirconia to 182 GPa for the sample with 50 weight % phosphor.

(45) The Poisson ratio also tended to decrease with increasing phosphor content. The toughness was measured at 5.9 MPa.Math.m.sup.0.5 and 3.9 MPa.Math.m.sup.0.5 for 20% and 50% in weight of Sr.sub.4Al.sub.14O.sub.25, respectively.

(46) Finally, the FIG. 8 displays the emitted luminescence of the 20% and 50% phosphor-zirconia composites treated at 900 C. in air and then at 1450 C. in reducing atmosphere, in comparison with the emitted luminescence of a pure phosphor sample of the same type as used in example 1 and the further samples of the present application (Sr.sub.4Al.sub.14O.sub.25 film of 160 m thickness). Remarkably, the intensity is comparable at the outset, and is even higher after 200 minutes and more for the zirconia-phosphor sample than for the pure phosphor. This is an unexpected result and shows the tremendous potential of the approach of the inventors: a tough technical ceramic is obtained, with high tenacity and high elastic modulus, with luminescent properties that are equivalent to those of the pure phosphor powder.

(47) It may be further noted that the measured luminescence is comparable on samples of 0.6 mm and 2 mm thicknesses.