Abstract:
An test cell is adapted for both making and testing samples. The cell includes a bottom plate and a top plate having concentric apertures defining a central test cavity. A post attached to the bottom plate closes off the bottom of the test cavity. A slide closes off the top of the test cavity. The top of the post is spaced slightly from the underside of the slide to define a test cavity of substantially uniform thickness. The test cavity is filled with phosphor suspended in uncured resin, closed, and the resin is cured. Once cured, the sample is stable, although delicate, and can be re-measured several times with reproducible results. The measurement takes place in the cell, using a thin film of oil for wetting surfaces.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for testing phosphor and, in particular, is an improvement on the “oil cell” method used for almost fifty years. 
     The method and apparatus are described in the context of phosphor for thick film electroluminescent (EL) lamp. Any kind of phosphor can be tested with the new apparatus and method. 
     An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include phosphor particles or there can be a separate layer of phosphor particles adjacent the dielectric layer. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current. 
     EL phosphor particles are typically zinc sulfide-based materials, including one or more compounds such as copper sulfide (Cu 2 S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors typically contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. The color of the emitted light is determined by the doping levels. Although understood in principle, the luminance of an EL phosphor particle is not understood in detail. The luminance of the phosphor degrades with time and usage, more so if the phosphor is exposed to moisture or high frequency (greater than 1,000 hertz) alternating current. 
     The “oil cell” method for testing phosphor typically relies on castor oil as a liquid dielectric in which phosphor particles are dispersed. Suitable electrodes are provided and the phosphor is driven to luminance. Up to a point, the oil cell method provides a quick and convenient way to evaluate new phosphor formulations. The mobility of particles in an electric field in a liquid dielectric is well known; see Lehmann, “Dielectric Behavior of Electroluminescent Zinc Sulfides,”  Journal of the Electrochemical Society , Vol. 103, No. 1, pgs. 24–29, January 1956. This mobility is believed to be one of the causes of unstable and non-reproducible measurements of luminance from an oil cell. The oil cell test method has been found to have almost a ±10% variance in measured luminance even under the most controlled conditions with highly experienced operators. 
     The variability means that life testing is virtually impossible because the luminance changes over time for reasons having nothing to do with the life of the phosphor. (“Life” is generally accepted to mean the time for luminance to decay to half of initial luminance.) 
     The variability also means that performing a plurality of tests on a sample is essentially pointless. For example, a series of tests on a single sample is essentially the same as a series of tests on a plurality of samples. Any variation in luminance due to a particular parameter is lost in variations from other causes. 
     Thin, thick film layers of phosphor are known in the art. As used herein, and as understood by those of skill in the art, “thick-film” refers to one type of EL lamp and “thin-film” refers to another type of EL lamp. The terms only broadly relate to actual thickness and actually identify distinct disciplines. In general, thin film EL lamps are made by vacuum deposition of the various layers, usually on a glass substrate or on a preceding layer. Thick-film EL lamps are generally made by depositing layers of inks on a substrate, e.g. by roll coating, spraying, or various printing techniques. A thin, thick-film EL lamp is not a contradiction in terms and such a lamp is considerably thicker than a thin film EL lamp. Other distinctions between the two types of lamps are described in the report  Electroluminescent Material for Flat Panel Display , Final Report for CRADA No. ORNL95-0371, October 2000. 
     U.S. Pat. No. 4,513,023 (Wary) discloses phosphor in a UV curable dielectric layer having a thickness of 0.2–1.2 mils (5.1–30.5 μm). Although phosphor particles in a solid dielectric have less mobility than in a liquid dielectric, the cured layer has a variable thickness across the area thereof, which makes measurements of luminance inconsistent. The option to date, taking phosphor samples and making complete EL lamps, does not guarantee reproducible results and costs considerable time and money. 
     Thus, the oil cell method and apparatus have been accepted as a quick indication of proof of concept rather than as an analytical tool. 
     In view of the foregoing, it is therefore an object of the invention to provide a method and apparatus for reproducibly measuring the optical and electrical characteristics of a phosphor. 
     Another object of the invention is to improve the oil cell test to provide an analytical tool for evaluating phosphor. 
     A further object of the invention is to provide an oil cell in which the same sample can be tested a plurality of times. 
     Another object of the invention is to provide an oil cell that can be operated continuously for long periods in order to provide an indication of the operating life of a phosphor. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved in this invention in which an test cell is adapted for both making and testing samples. The cell includes a bottom plate and a top plate having concentric apertures defining a central test cavity. A post attached to the bottom plate closes off the bottom of the test cavity. A slide closes off the top of the test cavity. The top of the post is spaced slightly from the underside of the slide to define a test cavity of substantially uniform thickness. The test cavity is filled with phosphor suspended in uncured resin, closed, and the resin is cured. Once cured, the sample is stable, although delicate, and can be re-measured several times with reproducible results. The measurement takes place in the cell, using a thin film of oil for wetting surfaces to improve optical coupling and to eliminate air/dielectric interfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an oil cell constructed in accordance with a preferred embodiment of the invention; 
         FIG. 2  is a cross-section of an oil cell constructed in accordance with a preferred embodiment of the invention; 
         FIG. 3  is a chart illustrating the repeatability of measurements with a test cell constructed in accordance with the prior art; and 
         FIG. 4  is a chart illustrating the repeatability of measurements with a test cell constructed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a perspective view of an oil cell constructed in accordance with a preferred embodiment of the invention. Cell  10  includes top plate  11  made from a suitable plastic such as Plexiglass® acrylate (poly(methyl methacrylate)) or Delrin® acetal resin. Bottom plate  12  is preferably made, from Macor® machinable glass ceramic. Other dimensionally stable, rigid materials could be used instead for either plate. The two plates define central cavity  14  wherein samples are made and tests are performed. 
     Window  15  is located in one side of top plate  11  and extends to cavity  14 . Glass slide  16  fits within window  15  and is inserted into the cell to intersect cavity  14 , approximately bisecting the cavity and closing off the upper portion of the cavity. Within cell  10 , slide  16  is held in place by set screws, such as set screw  17 , which is preferably made from nylon to avoid cracking slide  16 . Top plate  11  is fastened to bottom plate  12  by bolts, such as bolt  18 . 
       FIG. 2  is a cross-section of an oil cell constructed in accordance with a preferred embodiment of the invention and provides a better view of the cavity within cell  10 . Top plate  11  is machined or cast in such a way that window  15  ( FIG. 1 ) defines shoulder  21  around cavity  14  for clearing glass slide  16 . The portion of cavity  14  in bottom plate  12  is dimensioned to define shoulder  23  for engaging and supporting glass slide  16  when the slide is pushed downward by set screw  17 . 
     The portion of cavity  14  in bottom plate  12  encloses post  25 , which is preferably made from stainless steel. The depth of cavity  14  in bottom plate  12  is slightly greater than the height of post  25 , leaving gap  26 . The area between the upper surface of post  25  and the lower surface of slide  16  has a substantially constant thickness; i.e. the surfaces are substantially parallel. 
     In one embodiment of the invention, gap  26  was 0.004±0.0001 inches (101.6±2.5 μm). The tolerance figure relates to flatness, not to the size of the gap. A gap in the range of at least 2–8 mils (51–203 μm.) could be used. If gap  26  is larger than a given value, a higher voltage is required for good, measurable, light output. If gap  26  is smaller than a given value, manipulation of the film (described below) becomes more difficult. 
     Post  25  is held against the bottom of cavity  14  by bolt  27  and washer-nut  28 . 
     The following non-limiting example is presented to illustrate the use of the apparatus. In general, one prepares a thin sample of phosphor in a thick film dielectric resin. Phosphor suspended in uncured resin is placed in the measurement cell and cured in gap  26  to assure uniform thickness of the sample. The test cell acts as a mold for making the sample. The amount of resin and phosphor is chosen to fill the volume above post  26 . Once cured, the sample is stable, although delicate, and can be re-measured several times with reproducible results. The measurement takes place in the cell, using a thin film of oil for wetting surfaces to improve optical coupling and to eliminate air/dielectric interfaces. A suitable sensor is placed in the upper portion of cavity  14  for measuring luminosity, color, or other optical characteristics. 
     Sample Preparation 
     
         
         
           
             1. Tare watch glass 
             2. Weigh out 0.030 g of clear release liner UV resin (e.g. Clear Coat release liner C-2, mfg. by Kolorcure) 
             3. Weigh out 0.060 g of phosphor powder to make a 2:1 phosphor: UV resin ratio 
             4. Mix sample thoroughly 
             5. Place 0.030 g of this mixture onto the cell post 
             6. Place a clear glass slide onto the cell covering the phosphor mixture 
             7. Attach the cell cover using 14 in./oz. of torque 
             8. Place the cell fixture into a UV curing oven using a lamp intensity of 0.4–0.5 Watts/cm 2  for 15 seconds 
             9. Run the sample through the UV oven two times to ensure good curing through the glass slide 
             10. Disassemble the fixture and carefully remove the glass slide 
             11. Using a razor blade scraper, carefully scrape the phosphor film off of the glass slide 
             12. Place the film onto a watch glass and pass through the UV oven two more times at the same conditions as above to post-cure the film 
             13. Wipe the slide and fixture with acetone to remove any residue before casting the next film 
             14. Repeat steps 1–12 to cast a second film from the same phosphor mixture 
             15. Repeat steps 1–13 for all phosphor powder samples to be measured
 
Sample Measurement
 
             1. Place a fraction of a drop of castor oil onto the post 
             2. Place the phosphor film to be measured onto the post on top of the oil 
             3. Place a fraction of a drop of castor oil on top of the phosphor film 
             4. Place a conductive glass slide on top of the phosphor film on the cell 
             5. Attach the cell cover by first tightening the four wing nuts underneath the cell, then the two set screws on top of the cell measurement cover (use only a light finger tight pressure) 
             6. Attach the assembled cell to an AC sine wave power source at 380 Vrms/400 Hz 
             7. Measure the luminance, x-color and y-color of the phosphor sample 
             8. Detach the measured cell from the power source 
             9. Disassemble the cell 
             10. Carefully remove the phosphor film sample from the cell 
             11. Carefully wipe all oil off of the film, slide and the cell 
             12. Repeat steps 1–11 for each phosphor film samples to be measured until all samples have been measured three times in a random order 
             13. Document the three measurements of film  1  and three measurements of film  2  for each phosphor sample mixture 
             14. Average the median measurements from each of the two films. 
             15. The average of the two medians is the luminance value that should be reported.
 
Test Results
 
           
         
       
    
     The following table lists the data from measuring the luminance of thirty samples twice in random order using a test cell constructed in accordance with the prior art. The data is plotted in the chart shown in  FIG. 3 . Test 1 provides the x coordinate of each data point and test 2 provides the y coordinate of each data point. In a perfect world, x would equal y for each sample and the points would form a straight line along the dashed line shown. As readily seen from  FIG. 3 , there is a considerable amount of scatter in the data. 
                                                   Sample   Test 1   Test 2                                1   15.8   17.1       2   14.0   16.6       3   15.4   16.3       4   15.2   15.6       5   15.6   16.3       6   15.2   17.3       7   15.0   17.2       8   15.5   15.0       9   15.9   18.0       10   15.7   15.7       11   15.2   17.7       12   15.9   16.3       13   15.4   15.6       14   16.1   18.1       15   13.7   17.1       16   15.9   16.7       17   15.8   16.3       18   15.3   17.1       19   15.0   17.5       20   15.9   18.2       21   15.9   17.5       22   15.6   17.1       23   15.1   18.4       24   15.0   17.5       25   13.7   18.7       26   15.1   18.7       27   15.1   15.2       28   12.7   17.6       29   15.1   18.3       30   15.0   18.9                    
The following table lists the data from measuring the luminance of thirty samples twice in random order using a test cell constructed in accordance with the invention. The data is plotted in the chart shown in  FIG. 4 . Test 1 provides the x coordinate of each data point and test 2 provides the y coordinate of each data point. Obviously, the data clusters much more closely around the dashed line, indicating a much more accurate measurement.
 
                                                   Sample   Test 1   Test 2                                1   20.22   18.72       2   19.45   19.11       3   18.02   18.06       4   19.79   19.54       5   20.59   19.79       6   18.98   19.30       7   16.63   16.96       8   17.74   17.35       9   16.46   16.78       10   17.60   17.23       11   17.98   17.87       12   17.14   16.81       13   20.45   19.72       14   19.94   19.69       15   20.02   19.46       16   20.41   20.14       17   20.17   19.74       18   19.65   18.45       19   18.37   18.67       20   18.82   18.05       21   18.79   18.65       22   18.48   17.94       23   19.82   19.65       24   20.50   19.60       25   20.28   19.40       26   19.19   19.38       27   19.59   19.48       28   19.39   19.46       29   18.84   18.77       30   19.72   18.96                    
The following table lists the data from measuring the luminance of phosphor that was first tested in November, 2001, using an old test cell of the prior art. That is, the following is old data using the old test cell.
 
                                                                       Batch #1   Batch #3   Batch #4                                            13.50   16.77   15.53               14.05   15.88   14.87               14.47   16.85   15.81               14.26   16.96   14.61               14.35   15.81   15.63               13.92   15.91   15.68               15.02   16.65   16.62               14.42   16.85   15.61               14.82   15.89   16.22               14.27   15.75   15.57           mean   14.31   16.33   15.62           range   1.52   1.21   2.01           range as % of mean   10.6%   7.4%   12.9%                        
The statistic “range as percent of mean” is used instead of standard deviation because the percent is relative to magnitude whereas standard deviation is affected by frequency of occurrence. The standard deviations for the three columns are 0.43, 0.52, and 0.58, respectively. Note that the column with the smallest standard deviation (Batch #1) does not have the smallest range as a percent of mean (Batch #3). That is, the two statistics are not linearly related, if related at all. The average range as a percent of mean is 10.3 percent.
 
     The same phosphor lot was located and re-measured using a test cell constructed and operated in accordance with the invention. In these tests, the brightness of each film was measured three times. Nine films were made. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 range as 
               
               
                   
                 test 1 
                 test 2 
                 test 3 
                 mean 
                 range 
                 % of mean 
               
               
                   
                   
               
             
             
               
                   
                 10.53 
                 10.46 
                 10.13 
                 10.37 
                 0.40 
                 3.9% 
               
               
                   
                 10.73 
                 11.15 
                 10.34 
                 10.74 
                 0.81 
                 7.5% 
               
               
                   
                 11.79 
                 12.25 
                 12.57 
                 12.20 
                 0.78 
                 6.4% 
               
               
                   
                 11.86 
                 11.27 
                 11.32 
                 11.48 
                 0.59 
                 5.1% 
               
               
                   
                 11.93 
                 11.97 
                 11.09 
                 11.66 
                 0.88 
                 7.5% 
               
               
                   
                 12.94 
                 12.10 
                 12.17 
                 12.40 
                 0.84 
                 6.8% 
               
               
                   
                 10.54 
                 11.03 
                 11.21 
                 10.93 
                 0.67 
                 6.1% 
               
               
                   
                 10.85 
                 11.26 
                 11.22 
                 11.11 
                 0.41 
                 3.7% 
               
               
                   
                 11.81 
                 12.96 
                 12.23 
                 12.33 
                 1.15 
                 9.3% 
               
               
                   
                   
                   
                   
                   
                 AVG 
                 6.3% 
               
               
                   
                   
               
             
          
         
       
     
     The invention thus provides a method and apparatus for reproducibly characterizing a phosphor and enabling one to test a sample a plurality of times. The cell can be operated continuously for long periods in order to provide an indication of the operating life of a phosphor. The cell is much easier to operate and is less prone to error, e.g. by variations in torque on the attaching bolts. 
     Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, a fitted disk could be used instead of a slide but this would make the apparatus less easy to use. Other UV resins can be used instead of the disclosed resins. Curing by other mechanisms, e.g. heat or e-beam, can be used instead of UV curing. Other oil can be used instead of castor oil. Other clamping and fastening methods can be used, e.g. a cam type of lock. Other phosphor to resin ratios and voltages can be used within the limit of having sufficient brightness for accurate measurement. It is possible to measure the film that has been cured in the cell without removing it as well; however, re-measurement of the film after removal is not possible without adding at least some oil, which changes luminance.