Red emitting long decay phosphors

Long decay phosphors are disclosed that are comprised of rare-earth activated divalent titanates. In particular, the long decay phosphors are comprised of EQU (Ca.sub.(1-a-b) Pr.sub.a M.sub.b)O.TiO.sub.2 wherein PA1 0.00001.ltoreq.a.ltoreq.0.1 and PA1 0.0.ltoreq.b.ltoreq.0.3 and M is Zn and/or Mg.

FIELD OF INVENTION 
This invention relates to red emitting long decay phosphors comprised of 
praseodymium-activated calcium titanates that may include the divalent 
metal elements Zn and/or Mg, which partially substitute for the Ca. 
BACKGROUND OF THE INVENTION 
Luminescent materials having long decay periods ranging from a few minutes 
to several hours are known and typically produce their radiation by 
phosphorescence. Such phosphorescent materials have typically been used in 
safety signs or on watch or clock dials. In recent years, technology has 
developed which make it possible to imbed luminescent materials in pressed 
or molded plastic products. Such technology significantly broadens the 
range of long decay phosphor applications. 
Copper-activated zinc sulfide, such as ZnS:Cu,Cl, is frequently used for 
these long decay phosphor applications because copper-activated zinc 
sulfide produces emission in a spectral region having a relatively high 
luminous efficiency. However, the properties of copper-activated zinc 
sulfide are not completely satisfactory since the brightness of the 
phosphor falls off substantially after extended decay periods, such that 
the emission is barely perceptible after about 30 minutes. Furthermore, 
copper-activated zinc sulfide is subject to degradation and deterioration 
when exposed to UV radiation in a moist or humid atmosphere. The body 
color of the material containing the zinc sulfide darkens, possibly due to 
the presence of elemental zinc on the surface. Use of such materials for 
outdoor applications has, therefore, been severely limited. 
Phosphorescent materials having a long decay period may be used in such 
fields as the graphic arts, interior decorating or printing inks. For 
these applications, alkaline earth sulfide phosphors have been used, since 
they can be prepared with a broad gamut of colors ranging throughout the 
visible spectrum from blue to red. These materials, however, are 
hygroscopic and react readily with moisture tending to generate hydrogen 
sulfide, a noxious and toxic substance. These properties severely restrict 
their use in the home. 
Recently, long decay phosphors comprised of rare-earth activated, divalent, 
boron-substituted aluminates have been developed, U.S. Pat. No. 5,376,303. 
These long decay phosphors overcome some of the disadvantages of ZnS:Cu,Cl 
and give long and bright emission in the range from blue to yellow-green. 
This family of phosphors is activated with divalent europium as a 
luminescent center. However, divalent europium has not been used to 
produce orange or red emission in this family of host crystals. 
In the past, (Zn,Cd)S:Cu,Cl or CaS:Eu have been used for orange or red 
emitting long decay phosphors. However, the properties of these materials 
have not been satisfactory because of the above mentioned disadvantages of 
sulfide compounds. In particular, these sulfide materials are subject to 
degradation and deterioration when exposed to UV radiation in the same 
manner as ZnS:Cu,Cl. In addition, these materials contain the toxic 
element cadmium. CaS:Eu provides red emission, but this material readily 
reacts with moisture and tends to generate H.sub.2 S gas into the 
atmosphere. Thus, due to the characteristics of these sulfide materials, 
they are troublesome and difficult to process and undesirable to use in 
finished products. Therefore, chemically stable and non-toxic red emitting 
long decay phosphors are still desired. 
Divalent titanates such as strontium titanates or barium titanates are well 
known as dielectric substances and thermistor materials. The possibility 
of using divalent titanates as the host crystal for a luminescent material 
has also been studied. The emission of various activators in divalent 
titanates are reported in several studies; "Optical spectra of rare earth 
activated BaTiO.sub.3 ", The Journal of Chemical Physics, Vol. 31, No.5, 
1272-1277, November 1959, Seymour P. Keller and George D. Pettit; 
"Luminescence of Sm.sup.3+ in BaTiO.sub.3 Matrix", J. Phys. Chem., Solids 
Pergamon Press, Vol. 23, 749-757, 1962, S. Makishima, K. Hasegawa and S. 
Shionoya; and "Photoluminescence of Cr doped CaTiO.sub.3 ", Physical 
Review B, Vol.2, No.11, 4351-4353, December 1970, L. Grabener and S. E. 
Stokowski. None of these studies disclose red emitting long decay 
phosphors using a divalent titanate as the host material. 
ADVANTAGES AND SUMMARY OF THE INVENTION 
The subject invention is directed to providing red emitting long decay 
phosphors with high chemical stabilities that overcome the disadvantages 
of prior art phosphors. 
More specifically, the subject invention is directed to producing 
phosphorescent emission by praseodymium activation in a chemically stable 
calcium titanate host, wherein part of the Ca in the host material may be 
replaced by the divalent metal elements Zn and/or Mg. 
In particular, the subject invention is achieved with a long decay phosphor 
comprising a composition of 
EQU (Ca.sub.(1-a-b) Pr.sub.a M.sub.b)O.TiO.sub.2 
wherein 
0.00001.ltoreq.a.ltoreq.0.1 and O.ltoreq.b.ltoreq.0.3 
and M is a divalent metal element selected from the group consisting of Mg, 
Zn and a combination thereof. 
In addition, the subject invention is directed to a method for producing 
such long decay phosphors. 
Further objects and advantages of the subject invention will be apparent to 
those skilled in the art from the following detailed description of the 
disclosed praseodymium-activated calcium titanates that may include the 
divalent metal elements Zn and/or Mg.

DETAILED DESCRIPTION OF TEE PREFERRED EMBODIMENTS 
The subject invention will now be described in detail for specific 
preferred embodiments of the invention, it being understood that these 
embodiments are intended as illustrative examples and the invention is not 
to be limited thereto. 
The red emitting long decay phosphors of the subject invention are 
comprised of praseodymium-activated calcium titanates that may include the 
divalent metal elements Zn and/or Mg as partially substituting for the Ca, 
as represented by the following composition: 
EQU (Ca.sub.(1-a-b) Pr.sub.a M.sub.b)O.TiO.sub.2 (I) 
wherein 
0.00001.ltoreq.a.ltoreq.0.1 and 0.ltoreq.b.ltoreq.0.3 
and M is a divalent metal element selected from the group consisting of Mg, 
Zn and a combination thereof. 
It is to be understood that the above formula (I) and similar formulae as 
disclosed herein are intended to represent the ratio of elemental 
constituents present in the long decay phosphor without necessarily 
suggesting or representing the molecular composition of the individual 
crystal phases present in the subject long decay phosphors. 
Long decay phosphorescence is herein understood to refer to spectral 
emission that can be visually perceived for periods of at least several 
minutes and, preferably, for several hours after removal of the excitation 
source. The excitation source may produce excitation by electromagnetic 
radiation, such as x-rays, sunlight or the radiation from artificial light 
sources. In addition, the excitation may be provided by electron beams. 
The concentration for both the luminescent center and the trapping site may 
be adjusted to yield the optimum decay characteristics for a given long 
decay phosphor. The concentration of the praseodymium that functions as a 
luminescent center is preferably in the range from about 0.001 to about 
10.0 atom % and, more preferably, in the range from about 0.01 to about 
1.0 atom % of the total calcium, praseodymium, zinc and magnesium present. 
The divalent metal elements zinc and/or magnesium serve as a substitute 
for the divalent metal element calcium and are preferably present in the 
range from about 0 to about 30.0 atom % of the calcium. More preferably, 
the zinc and/or magnesium divalent metal elements are present in the range 
from about 0.05 to about 20.0 atom % of the calcium. 
The raw materials that are used in the preparation of the long decay 
phosphors of the subject invention are readily available high purity 
materials which preferably have a purity of at least 99.9%. More 
preferably, the purity is greater than 99.99%. 
The raw materials that are mixed and fired to prepare the long decay 
phosphors of the subject invention include a component comprising calcium, 
a component comprising praseodymium and a component comprising titanium. 
The mixture of raw materials may also include a component or components 
comprising the divalent metal elements in and/or Mg. Typically, whenever 
Zn and Mg are both to be present in the long decay phosphor, a component 
comprising Zn and a component comprising Mg are both included in the 
mixture of raw materials. 
The raw material that is preferably used for providing the divalent metal 
element in the praseodymium-activated calcium titanate is a carbonate salt 
or an oxide of the divalent metal element. Preferably, whenever Zn is 
included, the zinc salts are used since they are relatively easy to use 
because of their reactivity, ease of handling and commercial availability 
in high purity. High purity TiO.sub.2 may be used as the TiO.sub.2 source 
for substantially the same reasons. Either form of titanium oxide, anatase 
or rutile, may be used. Anatase is the preferred source of the TiO.sub.2. 
Although rutile gives a slight improvement in the initial brightness, the 
samples made using anatase give significantly higher phosphorescent 
emission at longer decay times as shown in is FIG. 5. Praseodymium oxide 
or oxalate salts are preferably used as the source of the rare earth 
activator. 
The raw materials may be weighed and mixed by either a dry or wet mixing 
process in order to get a well blended and uniform raw material mixture. 
The wet mixture may be dried in an oven and sieved before firing. A small 
amount of flux may be added to the raw material mixture. While there are 
many fluxes known to one skilled in the art that may be used while still 
remaining within the scope of the subject invention, boric acid, calcium 
fluoride and ammonium tetrafluoroborate are herein disclosed to provide 
acceptable results. The boric acid may also be provided in the form of 
boric oxide. The fluxes tend to promote better and more complete reaction 
and improve the texture and morphology of the final phosphors. The boric 
acid used is preferably in the range of 3-10 mol % as compared to the 
total amount of calcium titanate produced. The use of boric acid may have 
a beneficial effect on brightness or decay depending on the presence of 
zinc in the composition. The calcium fluoride is also preferably used in 
the same range. The ammonium tetrafluoroborate may be used to shift the 
absorption edge of the phosphor so that it is sensitized by ambient light. 
The mixture may be fired in a single step process in air or under mildly 
reducing conditions at 1100.degree.-1400.degree. C. for about 1-8 hours. 
The mixture may also be fired in a 2-step process wherein the first step 
is carried out under mildly reducing conditions and then the pre-fired 
product is re-fired a second time under oxidizing conditions. The mildly 
reducing conditions may be provided, for example, by firing in the 
presence of activated carbon. More specifically, the first step in the 
2-step process may be carried out for about 1 to about 8 hours at about 
1100.degree. to about 1400.degree. C. Preferably, the first step is 
carried out for about one hour at about 1350.degree. C. The second step 
may be carried out for about 1 to about 4 hours at about 700.degree. to 
about 1300.degree. C. Preferably, the second step is carried out for about 
one hour at about 1000.degree. C. The time schedule of the firing steps 
may be adjusted so as to optimize the decay properties. After firing, the 
fired material may then be pulverized and sieved and submitted for 
phosphorescent measurement using known methods. 
Bismuth oxide may also be used as a suitable additive in the firing 
mixture. Although this material may adversely affect the long decay 
characteristics, it can give an initial improvement in brightness. 
It is believed that the main crystal habit of the CaTiO.sub.3 long decay 
phosphor is one having a perovskite structure. Preferably, the CaO and 
TiO.sub.2 are present in a molar ratio of about 1:1. With increasing zinc 
concentration, peaks of Zn.sub.2 TiO.sub.4 and Ca.sub.2 Zn.sub.4 Ti.sub.15 
O.sub.36 are observed. Accordingly, with higher concentrations of zinc, 
the resulting materials consist of a mixture of calcium titanate, zinc 
titanate and calcium zinc titanate. It is not clear if these complex 
phases have any effect on the brightness or decay characteristic of these 
phosphors. 
These praseodymium-activated calcium titanates that may include the 
divalent metal elements Zn and/or Mg have red emission at about 614 nm and 
produce this red phosphorescence for at least about 10 minutes after 
removal of the excitation source of UV radiation at 365 nm. The length of 
time that the long decay phosphor is exposed to the excitation source may 
affect the relative phosphorescent brightness. FIG. 6 shows that the 
relative phosphorescent brightness time reached a maximum in less than 
about 30 seconds of excitation for the phosphor produced in Example 1. 
Table I shows the color coordinates and brightness of some of the cited 
examples under UV radiation at 365 nm. 
TABLE I 
______________________________________ 
Relative 
Intensity (%) 
Color with no 
Coordinates Boric 
Boric 
Item x/y Acid Acid 
______________________________________ 
(Ca.sub.0.9 Pr.sub..001 Zn.sub..009)O .multidot. TiO.sub.2 
(Ex. 1) 0.671/0.325 
16.0 19.7 
(Ca.sub.0.9 Pr.sub..001 Mg.sub..009)O .multidot. TiO.sub.2 
(Ex. 2) 0.670/0.327 
15.6 5.0 
(Ca.sub.0.999 Pr.sub..001)O .multidot. TiO.sub.2 
(Ex. 3) 0.670/0.327 
16.2 9.7 
(Y.sub.0.964 Eu.sub..036).sub.2 O.sub.2 S 
0.652/0.339 
100.0 
______________________________________ 
The thermostability and the stability in water were also tested. A sample 
of the phosphor of Example 1 was baked in air for 1 hour at 700.degree. C. 
for testing thermostability. Another sample of the phosphor of Example 1 
was stirred in water for 48 hours at room temperature and then dissolved 
material was measured. These tests showed these materials have relatively 
stable chemical compositions. 
This invention will now be described in detail with respect to the specific 
preferred embodiments thereof, the materials and the process steps being 
understood as examples that are intended to be illustrative only. In 
particular, the invention is not intended to be limited to the materials, 
conditions, process parameters and the like recited herein. 
EXAMPLES OF THE PREFERRED EMBODIMENTS 
Example 1 
A red emitting long decay phosphor having a composition 
EQU (Ca.sub.0.9 Pr.sub.0.001 Zn.sub.0.099)O.TiO.sub.2 
was prepared by starting with a mixture of: 
______________________________________ 
CaCO.sub.3 90.08 g 
TiO.sub.2 79.90 g 
Pr.sub.6 O.sub.11 
0.17 g 
ZnO 8.06 g 
H.sub.3 BO.sub.3 
3.09 g. 
______________________________________ 
The mixture was fired in a furnace for 2 hours in air at 1300.degree. C. 
After cooling, the product obtained was ground and sieved for evaluation. 
The particle size, emission spectrum, color coordinates and decay 
characteristics under photoexcitation were measured using methods known to 
one skilled in the art. The particle size of this materials was about 24 
.mu.m and the emission peak was at about 614 nm. 
Example 2 
A red emitting long decay phosphor having a composition 
EQU Ca.sub.0.9 Pr.sub.0.001 Mg.sub.0.099)O.TiO.sub.2 
was prepared by starting with a mixture of: 
______________________________________ 
CaCO.sub.3 90.08 g 
TiO.sub.2 79.90 g 
Pr.sub.6 O.sub.11 
0.17 g 
MgO 3.99 g 
H.sub.3 BO.sub.3 
3.09 g. 
______________________________________ 
The mixture was fired in a furnace for 2 hours in air at 1300.degree. C. in 
a manner similar to that described in example 1. The particle size of this 
material was about 25 .mu.m and the emission peak was observed at about 
614 nm. 
Example 3 
A red emitting long decay phosphor having a composition 
EQU (Ca.sub.0.999 Pr.sub.0.001)O.TiO.sub.2 
was prepared in a manner similar to that described in Example 1 by starting 
with a mixture of: 
______________________________________ 
CaCO3 99.999 g 
TiO.sub.2 79.90 g 
Pr.sub.6 O.sub.11 
0.17 g 
H.sub.3 BO.sub.3 
3.09 g. 
______________________________________ 
Example 4 
A red emitting long decay phosphor having the composition of Example 3 was 
prepared using a 2-step firing process in which the mixture was fired in 
the presence of activated carbon for 4 hours at a temperature of about 
1350.degree. C. to produce a pre-fired product and then the pre-fired 
product was re-fired in air for 8 hours at a temperature of about 
1000.degree. C. The 2-step process used in this example are not 
necessarily optimum, but is disclosed to show that the long decay 
phosphors made according to this 2-step process were observed to show 
improvement with respect to the single-fired product.