CROSS-REFERENCES TO RELATED APPLICATION

This application is related to U.S. application Ser. No. 08/637032, filed concurrently herewith, which is incorporated herein by reference.

CROSS-REFERENCES TO RELATED APPLICATION

This application is related to U.S. application Ser. No. 08/637032, filed concurrently herewith, which is incorporated herein by reference.

TECHNICAL FIELD

This invention relates to ZnS:Cu electroluminescent phosphors. More particularly, it relates to a ZnS:Cu electroluminescent phosphor having improved halflife and brightness characteristics.

BACKGROUND ART

Electroluminescent (EL) phosphors are incorporated into thick film AC electroluminescent devices used for backlighting liquid crystal displays (LCD), for automotive dashboard and control switch illumination, and for emergency egress lighting. Some important characteristics of these EL devices include brightness, color and halflife. Such characteristics depend inherently on the phosphors which are incorporated into the EL devices.

One particularly important class of EL phosphors are the copper-activated zinc sulfide phosphors, ZnS:Cu, which have commercially desirable brightness and color characteristics. Such phosphors may be blue, blue-green, green or yellow-orange (co-activated with manganese) emitting. U.S. Pat. No. 4,859,361 to Reilly et al., which is hereby incorporated by reference, describes generally how to make copper-activated zinc sulfide phosphors. First, the phosphor precursor materials, ZnS, a copper source and a chloride flux are mixed together and heated to form a hexagonal ZnS material containing copper and chlorine ions. Next, the hexagonal ZnS is subjected to low intensity milling to convert some of the hexagonal ZnS to its cubic crystalline form. And finally, the milled material is blended with zinc sulfate and copper sulfate and refired at a lower temperature to form an electroluminescent phosphor.

JP 4-270780 discloses improving the halflife of a copper-activated zinc sulfide phosphor to about 410 hours by incorporating small amounts of gold into the phosphor. U.S. Pat. No. 5,110,499 to Reilly further improves the halflife of copper-activated zinc sulfide phosphors to about 450 hours by increasing the average particle size of the phosphor. WO 91/16722 to Faria discloses that increasing the mechanical stressing of the hexagonal ZnS material between the first and second firing steps increases the halflife of the phosphor but at the expense of brightness.

Since many of the applications for EL phosphors are now requiring halflife performances on the order of 1000 hours without sacrificing brightness, it would be a significant advantage to produce a copper-activated zinc sulfide EL phosphor having a longer halflife while maintaining or improving the desirable brightness and color characteristics.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of the prior art.

It is another object of the invention to provide an electroluminescent phosphor having an improved halflife without sacrificing brightness or emission color.

It is a further object of the invention to provide a method for increasing the halflife of a ZnS:Cu phosphor without substantially affecting its brightness or emission color.

In accordance with one aspect the invention, there is provided an electroluminescent phosphor having the general formula ZnS:Cu,Cl,Au, a halflife of at least about 900 hours, and x and y color coordinates where the x color coordinate is from about 0.156 to about 0.196 and the y color coordinate is from about 0.370 to about 0.430.

In accordance with another aspect the invention, there is provided a method for increasing the halflife of a ZnS:Cu electroluminescent phosphor comprising:

blending amounts of zinc sulfide, a copper source, a gold source and a chloride flux to form a mixture;

firing the mixture at a temperature from about 900° C. to about 1400° C. for about 2 hours to about 8 hours to form a zinc sulfide material containing copper, chlorine and gold;

subjecting the zinc sulfide material to a low intensity milling for between about 190 minutes to about 280 minutes to form a milled material;

blending the milled material with an amount of zinc sulfate and an amount of copper sulfate; and

refiring the milled material at a temperature from about 500° C. to about 1000° C. for about 1 to 5 hours to form an electroluminescent phosphor.

In accordance with yet another aspect the invention, there is provided an electroluminescent lamp comprising:

a first electrode, a first layer of dielectric material adjacent to the first electrode, a second layer of a dielectric material adjacent to the first layer;

the second layer containing an electroluminescent phosphor having a general formula ZnS:Cu,Cl,Au, a halflife of at least about 900 hours, and x and y color coordinates where the x color coordinate is from about 0.156 to about 0.196 and the y color coordinate is from about 0.370 to about 0.430; and

a second electrode adjacent to the second layer of dielectric material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the structure of an electroluminescent lamp.

PREFERRED EMBODIMENTS

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

FIG. 1 is a schematic representation of the structure of an electroluminescent lamp 60. A conductive substrate material, such as aluminum or graphite, forms a first electrode 62 of the lamp 60, while a transparent conductive film, such as indium tin oxide, forms a second electrode 64. Sandwiched between the two conductive electrodes 62 and 64 are two additional layers of dielectric material 70 which can be, for example, cyanoethyl cellulose or cyanoethyl starch. Adjacent to the first electrode 62 is a layer of dielectric material 70 in which may be embedded particles of a ferroelectric material 72 such as barium titanate. Adjacent to the second electrode 64 is a layer of dielectric material 70 in which may be embedded particles of the electroluminescent phosphor 74 of this invention.

The halflife of an electroluminescent lamp as used herein is defined as the time it takes for the brightness of the EL lamp to reach half its initial value. The initial brightness of the lamp is typically measured after a 24 hour burn-in period. The burn-in period is used to stabilize the brightness of the lamp and increase the reproducibility of the measurement. Brightness is typically measured in footlamberts (fL) at conditions of 100 volts and 400 hertz.

A typical copper-activated zinc sulfide phosphor is ZnS:Cu,Cl which is available from OSRAM SYLVANIA INC. of Towanda, PA. For example, OSRAM SYLVANIA Type 723 ZnS:Cu,Cl has CIE color coordinates where x is 0.176 ±0.02 and y is 0.400 ±0.03. The typical halflife of this phosphor is about 400 hours. The halflife of this phosphor can be improved to between 600-800 hours by increasing copper content. However, this causes an undesirable color shift in the emission spectrum of phosphor resulting in a green emission color where the y color coordinate is increased to about 0.480 to 0.490.

We have found that the halflife of ZnS:Cu phosphors can be significantly improved by doping the phosphors with minor amounts of gold and increasing the amount of low intensity milling between the first and second firings. The resultant ZnS:Cu phosphor has a halflife of at least 900 hours and exhibits an average increase in brightness while only experiencing a slight shift in its average y color coordinate of less than or equal to about 0.03.

In a general method, the phosphor is made by combining zinc sulfide, a copper source, a gold source and a chloride flux. Other components such as sulfur and ZnO may also be added. The mixture is then fired at a temperature from about 900° C. to about 1400° C. for about 2 to about 8 hours to produce hexagonal ZnS containing copper, chlorine and gold. Preferred first step firing conditions are 1100° C. to 1300° C. for 4 to 6.5 hours. Water washing is used to remove any residual chloride flux.

At this point, the ZnS is not yet substantially electroluminescent. In order to make it electroluminescent, the ZnS is subjected to a low intensity milling to induce the transformation of the ZnS to its cubic crystalline form. The amount of low intensity milling used in this method is substantially greater than the amount indicated by the prior art. For example, U.S. Pat. No. 4,859,361 discloses milling for 90 minutes between firings and WO 91/16722 teaches mechanically stressing the phosphor precursor for up to about 120 minutes. In the instant invention, the preferred amount of low intensity milling is at least 50% longer than the prior art, between about 190 to about 280 minutes.

After milling, the ZnS is blended with amounts of copper sulfate and zinc sulfate and fired at a temperature from about 500° C. to about 1000° C. for about 1 to about 5 hours to form the electroluminescent phosphor. Preferred second step firing conditions are 650° C. to 850° C. for 1.5 to 3 hours. After cooling, the phosphor is given additional washes including washing with acetic acid to remove copper oxide and unreacted ZnO and washing with a KCN solution to remove superficial copper. Final finishing steps include water washing, filtering, drying and sieving to -325 mesh.

The following non-limiting examples are presented to enable those skilled in the art to more clearly understand and practice the present invention. These examples should not be considered as a limitation upon the scope of the present invention, but merely as being illustrative and representative thereof.

Luminescent grade S-10 ZnS from OSRAM SYLVANIA containing 1 wt. % chloride was doped with 0.5 wt. % gold using a solution of gold chloride. The S-10 ZnS is slurried with deionized (DI) water and the gold solution is added. The gold doped slurry is then dried. The gold doped S-10 was mixed in varying proportions with amounts of S-10 ZnS containing 1 wt. % chloride to produce ZnS mixtures containing different amounts of gold. The relative proportions of the two S-10 blends are given in Table 1. To each of these mixtures, 23.87 g CuSO₄, 1270.06 g S, 79.45 g ZnO, 476.27 g BaCl₂, 476.27 MgCl₂ and 317.51 g NaCl were added. The latter three components, BaCl₂, MgCl₂ and NaCl, are flux materials.

The mixtures were fired in covered crucibles at 1205° C. for 5 1/4 hours. The fired cakes were removed from the furnace and washed to remove the flux material. Approximately three hot DI water washes (0.7 gal./lb) were needed to remove the flux. (The chloride level in the supernatant liquid can be tested by either measuring the conductivity of the liquid or precipitating AgCl with AgNO₃.) The washed material was subjected to slight milling using a Simpson Mix-Muller in 35 lb. quantities for different time periods to induce the transformation of hexagonal ZnS to cubic ZnS. The milled material was blended with ZnSO₄.7H₂ O and CuSO₄ in the ratio of 1159 g of ZnSO₄.7H₂ O and 113.5 g of CuSO₄ per every 10 lbs. of milled material and fired in an electric furnace for about 2 1/4 hours at 730° C. After firing, the fired cakes were removed from the furnace, cooled to room temperature, blended and washed with DI water. After two or three DI water washes, the material was washed with acetic acid (1 gal. glacial acetic acid/5 gal. DI water) to remove copper oxide and unreacted ZnO. After subsequently washing at least twice with DI water, the material was washed with KCN (2 lbs. KCN/5 gal. DI water) to remove most of the superficial copper from the phosphor. The KCN wash changes the body color of the material from a dark gray to almost colorless. The phosphor was then washed with DI water, filtered, dried and sieved to -325 mesh. The phosphors were incorporated into conventional thick film EL devices and evaluated for brightness, halflife and color. The results of which are given in Table 2. A Type 723 control sample made without using the gold dopant or the longer milling period is provided for comparison.

              TABLE 1______________________________________                  S-10 with  Au-doped S-10 with                  1 wt. % Cl                           Milling TimeNo.    1 wt. % Cl (g)  (g)      (min)______________________________________1      142.88          15732.85 2002      95.25           15780.48 2253      142.88          15732.85 2504      47.63           15828.10 2005      95.25           15780.48 2256      95.25           15780.48 2607      162.38          15713.35 2258      95.25           15780.48 2259      95.25           15780.48 22510     47.63           15828.10 25011     95.25           15780.48 19012     95.25           15780.48 22513     28.12           15847.61 225______________________________________

              TABLE 2______________________________________         Milling InitialSample wt. %   Time    Brightness                        x color                               y color                                      HalflifeNo.   gold    (min)   (fL)   coordinate                               coordinate                                      (h)______________________________________Control 0.00     85     26.5   0.176  0.392   4371     0.0045  200     26.6   0.186  0.422  10512     0.003   225     28.8   0.186  0.423  10203     0.0045  250     27.9   0.184  0.416   9434     0.0015  200     28.9   0.182  0.408   9675     0.003   225     29.4   0.182  0.411   9236     0.003   260     30.2   0.184  0.423  11347     0.00512 225     28.9   0.187  0.434  12548     0.003   225     29.4   0.186  0.426  11599     0.003   225     30.1   0.186  0.431  116010    0.0015  250     27.3   0.185  0.422  142111    0.003   190     25.1   0.185  0.425  163412    0.003   225     29.2   0.183  0.419  112213    0.0088  225     28.5   0.184  0.426  1183______________________________________

The results given in Table 2 show a dramatic increase in the halflife of the EL phosphor and an average increase in its brightness as compared to the control sample. The average halflife of the Type 723 phosphors made by this method was 1100 ±200 hours which is about 2.5 times the halflife of the control sample and the average brightness value was 28 ±1 fL which represents an average increase of about 6% over the control. With regard to emission color, the average x color coordinate value for the phosphors made by this method was 0.185 ±0.002 which is an increase of only about 0.009 over the x value for the control and the average y color coordinate value for these phosphors was 0.422 ±0.007 which is an increase of about 0.030 over the y value for the control. Both color coordinates are still within the acceptable limits for Type 723 phosphors. Thus, the halflife of this phosphor has been dramatically increased while maintaining emission color and increasing the average brightness.

Chemical analysis of the phosphor samples found that for all samples the chlorine concentration was about 0.03 wt. % and the copper concentration was about 0.06 wt. %. Unexpectedly, the amount of gold detected in the phosphors was considerably less than the amount added to the initial mixture before firing. The amount of gold detected in the phosphors ranged from 0.27 to 0.87 ppm whereas the amount of gold added initially ranged from 15 to 88 ppm. The average gold concentration in the samples was about 0.5 ±0.2 ppm.

Another typical ZnS:Cu,Cl phosphor is green emitting OSRAM SYLVANIA Type 728 phosphor. Type 728 has a higher copper content than Type 723 which causes it to have a greener emission color. The typical color coordinates for this phosphor are x=0.182 ±0.20 and y=0.455 ±0.30 and the average halflife is 550 hours. Two samples of this phosphor type were prepared according to the above method except that for each sample 158.54 g gold doped S-10 ZnS (1 wt. % Cl), 15717.20 g S-10 ZnS (0.1 wt. % Cl), and 33.88 g CuSO₄ were used in the initial mixtures. The amount of low intensity milling used between firings was 280 minutes. Both samples were incorporated into lamps and their emission characteristics were measured. The results of the measurements are given Table 3. Average values for a Type 728 control phosphor are provided for comparison.

              TABLE 3______________________________________         Milling InitialSample wt. %   Time    Brightness                        x color                               y color                                      HalflifeNo.   gold    (min)   (fL)   coordinate                               coordinate                                      (h)______________________________________Con-  0.00     85     27.5   0.183  0.447   547trol*14    0.005   280     29.6   0.194  0.460  142515    0.005   280     30.1   0.197  0.482  1212______________________________________ *Average values

For the first Type 728 sample, No. 14, the halflife is at least about 1200 hours and, for the second sample, No. 15, the halflife is at least about 1400 hours. Both Type 728 samples exhibit a halflife more than twice the average value for the control sample. There has also been an increase in brightness of at least about 8% over the control. Moreover, for each sample, the x and y color coordinates are within the specified range for Type 728 phosphors, i.e., an x value from about 0.162 to about 0.202 and a y value from about 0.425 to about 0.485.

The incorporation of minor amounts of gold and the substantially increased low intensity milling between the first and second firing is expected to produce similar benefits for other types of ZnS:Cu EL phosphors. In particular, the halflife of yellow-orange emitting ZnS:Cu,Mn phosphors should be increased without exhibiting unacceptable changes in their emission color or brightness.

While there has been shown and described what are at the 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.