Abstract:
A luminescent composition excited by ultraviolet or selected visible wavelength radiations consisting essentially of aluminum oxide-germanium dioxide matrix activated by chromium. The composition provides strong line emissions in the deep red.

Description:
BACKGROUND OF THE INVENTION 
     The luminescent composition consisting essentially of aluminum oxide activated by chromium is well known. This composition gives a line emission in the deep red. Line-emitting phosphors have been found useful in a number of applications. In U.S. Pat. No. 3,772,099, dated Nov. 13, 1973, issued to Ryan et al and assigned to the present assignee is disclosed the use of line-emitting phosphors for providing a distinctive information label for identifying explosives. A predetermined combination of line-emitting phosphors can be used as &#34;coding&#34; phosphors which provide a very individualistic emission when excited by predetermined energy such as ultraviolet radiation. These coding phosphors can provide post-explosion information as to the origin, date of manufacture, and type of explosive used. 
     SUMMARY OF THE INVENTION 
     There is provided a luminescent composition excited by ultraviolet or selected visible wavelength radiations consisting essentially of aluminum oxide-germanium dioxide matrix activated by chromium. This composition has line emissions in the deep red which are much stronger than the line emissions of the chromium-activated aluminum oxide. This composition consists essentially of Al 2  O 3 .sup.. xGeO 2  :yCr wherein &#34;x&#34; is from about 0.012 to about 0.20 and &#34;y&#34; is from about 0.006 to about 0.035. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference may be had to the accompanying drawings, in which: 
     FIG. 1 is a graph showing the effect of GeO 2  content on the brightness or output of Al 2  O 3 .sup.. GeO 2  :Cr; and 
     FIG. 2 is a graph showing the effect of chromium content on the brightness or output of Al 2  O 3 .sup.. GeO 2  :Cr. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention relates to a luminescent composition which can be excited to luminescence by ultraviolet or visible wavelength radiations of deep purple or green. X-ray diffraction tests show that this luminescent composition consists essentially of aluminum oxide-germanium dioxide matrix activated by chromium and the formulation can be expressed as Al 2  O 3 .sup.. xGeO 2  :yCr. The graph in FIG. 1 shows the effect of GeO 2  content on the output of the luminescent composition. The values of &#34;x&#34; are shown on the abscissa versus the relative brightness or output of the composition compared to a similarly prepared phosphor without GeO 2  matrix modification. As can be seen from the graph, values of &#34;x&#34; from about 0.012 to about 0.20 show a marked increase in relative brightness of the composition compared to the composition where &#34;x&#34; is zero. The optimum range of &#34;x&#34; as shown in FIG. 1 is from 0.05 to 0.10. 
     The graph of FIG. 2 shows the values of &#34;y&#34; plotted along the abscissa versus the relative brightness or output of the luminescent composition compared to similarly prepared phosphor without the GeO 2  matrix modification. The values of &#34;y&#34; from about 0.006 to about 0.035 are shown from the graph to have good relative brightness. The optimum values of &#34;y&#34; as shown by the graph of FIG. 2 are from 0.015 to 0.025. 
     This luminescent composition has strong absorption bands at 5600 A (yellowish green) and 4100 A (deep violet). The main emission at room temperature is due to two lines at 6942 A and 6927 A which are deep red. These characteristics make this composition attractive for use as a &#34;coder&#34; in a materials labeling phosphor system such as the one described in the aforesaid Ryan U.S. Pat. No. 3,772,099. When used as a &#34;coder&#34; the present phosphor can be excited by a 4080 A wavelength not common to the vast majority of phosphors, thus simplifying its detection. In addition, because of its sharp, narrow emission lines, its identification among nine or 10 other &#34;coders&#34; is a relatively simple matter. 
     A typical example of the formulation of the luminescent composition is as follows: 1.00 mole of Al 2  O 3  ; 0.08 mole of GeO 2  ; 0.01 mole of Cr 2  O 3 . These compounds are mixed and fired at 1350° C for 4 hours in air. The resulting phosphor has 3.3 times more output than a similarly prepared phosphor without the GeO 2  modification of the matrix. Higher firing temperatures may further improve this phosphor&#39;s performance, but difficulties may be encountered with crucibles cracking or furnaces not operating properly at such elevated temperatures.