Phosphors with long-persistent green phosphorescence

Details Phosphors with long-persistent green phosphorescence Phosphors with long-persistent green phosphorescence

1998-11-06

2001-07-31

Koslow, C. Melissa (Department: 1755)

Compositions

Inorganic luminescent compositions

Compositions containing halogen; e.g., halides and oxyhalides

C117S946000

Reexamination Certificate

active

06267911

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to phosphors, particularly long-persistence green phosphorescence. The invention is also directed to methods of making phosphors in powder, ceramic and crystalline (e.g., single crystalline) form. The phosphors of the invention are Eu
2+
-activated alkaline earth aluminates, particularly strontium aluminates.
BACKGROUND OF THE INVENTION
Persistent phosphorescing materials, such as ZnS:Cu,Co, ZnCdS:Cu and CaSrS:Bi, have been used for many years. Recently a much brighter and longer persistence green phosphor, SrAl
2
O
4
:Eu
2+
:Dy
3+
has been produced.
Strong green luminescence from Eu
2+
-doped SrAl
2
O
4
was reported by H. Lange in Belgian patent 1,347,45 and U.S. Pat. No. 3,294,699. Efficient luminescence in the spectral range 450-520 nm was also reported from Eu
2+
-doped CaAl
2
O
4
, MgAl
2
O
4
, BaAl
2
O
4
, and SrAl
2
O
4
and their counterparts using alkaline earth cationic combinations. (F. C. Palilla, A. K. Levine and M. R. Tomkus (1986), “Fluorescence properties of alkaline earth aluminates of the type MAl
2
O
4
activated by divalent europium,” J. Electrochem. Soc. 115:642).
Long lasting and more efficient phosphorescence has obtained in Eu
2+
doped SrAl
2
O
4
synthesized with excess alumina which results in formation of trapping centers associated with the Sr
2+
vacancy (Abbruscato et al. (1971) J. Electrochem. Soc. 118:930).
Improved long persistence phosphors of certain alkaline earth aluminates were reported by T. Matsuzawa, Y. Aoki, N. Takeuchi and Y. Murayama (1996) J. Electrochem. Soc. 143(8):2670, and in U.S. Pat. No. 5,424,006. The brightness and persistence time of SrAl
2
O
4
:Eu
2+
was improved by co-doping various trivalent rare earth ions to produce appropriate trapping centers. The best result was obtained by co-doping Dy
+3
with EU
2+
into SrAl
2
O
4
and Nd
+3
with Eu
2+
into CaAl
2
O
4
to get long persistent green and purple emission, respectively. U.S. Pat. No. 5,424,006 also reports phosphors in which Mg
2+
is substituted for Sr
2+
in SrAlO
4
:Eu
2+
, Dy
3+
phosphors.
EP published application 765,925 (Moriyama et al.) reports Eu
2+
-activated strontium aluminate phosphors in which part of the Sr
2+
of the host is replaced with Pb
+3
, Dy
+3
or Zn
2+
. The zinc-doped materials are reported to display enhanced brightness and persistence compared to SrAlO
4
:Eu
2+
, Dy
3+
.
EP published application 710,709 (Murayama et al.) reports phosphors of matrix M
1-x
Al
2
O
4-x
where M is at least one metal selected from calcium, strontium, barium and, optionally, magnesium, and x is a number not equal to 0. The matrix comprises europium and a rare earth metal, manganese, tin or bismuth as co-activators.
JP Patent 76031037 (1976, Tokyo Shibaura Electric Co.) reports blue-emitting phosphors containing barium (or calcium or strontium)-potassium (or sodium) aluminates activated with europium and manganese.
JP Patent 94029417 (1994, Matsushita Electronics) reports a strontium aluminate phosphor activated with europium modified by incorporation of yttrium oxide.
JP Patent 94029416 (1994, Matsushita Electronics) reports a europium activated barium aluminate phosphor containing yttrium oxide to enhance phosphorescence.
Zlotnikova et al. (1990) Ukr. Khim Zh. (Russ. Ed.) 56(11):1148-1151 (Chem. Abst. (1991) 115:37798k) reports composition dependence of catho-luminescent properties of a Dy-doped SrAl
2
O
4
-Sr Al
4
O
7
system.
T. R. Kutty et al. (1990) Mater. Res. Bull. 25:1355-1362 report luminescence of Eu
2+
in strontium aluminates prepared by the hydrothermal method. Blue to green luminescent phosphors of general formula Sr
n
Al
2
O
3+n
where n≦1 are reported. The reference also reports the preparation of certain aluminoborates.
B. Smets et al. (1989) J. Electrochem. Soc. 136(7):2119-2123 reports blue-emitting phosphors: 2SrO.3Al
2
O
3
:Eu
2+
and 1.29 (Ba, Ca)O, 6 Al
2
O
3
:Eu
2+
. In the background section of the reference the authors refer to an earlier report of blue-green emitting phosphors 4SrO.7Al
2
O
3
:Eu
2+
and BaO4Al
2
O
3
:Eu
2+
which could be synthesized only in the presence of small amounts of B
2
O
3
.
Chemekova et al. (1977) Terzisy Dokl. Uses. Soveshch. Rostu. Krist. 5th 2:184-185 (Chem. Abst. (1980) 93:85423h) reports synthesis of single crystals in the calcium oxide-alumina system. Addition of Europium is said to produce phosphors.
SUMMARY OF THE INVENTION
This invention provides improved green phosphors of alkaline earth aluminates, particularly improved phosphors based on SrAl
2
O
4
:Eu
2+
,Dy
3+
. Improvements in brightness and phosphorescence persistence are achieved by various co-doping schemes and/or by selective synthesis of the alkaline earth aluminate host using a quenching step which is believed to generate the host containing &bgr;-phase (high temperature phase) aluminate.
Improvements to brightness and persistence are achieved by doping divalent ions, Mg
2+
or Zn
2+
into the Eu
2+
-activated, trivalent metal ion-doped phosphor to replace Al
3+
in the host. Preferably the amount of Mg
2+
or Zn
2+
introduced in the phosphor is substantially equivalent to the molar amount of trivalent rare earth metal (e.g., Dy
3+
). It is believed that this type of substitution reduces the charge defects induced by substitution of the trivalent rare earth ion sites into alkaline earth metal ion sites (e.g., Sr
2+
sites) in the aluminate.
Improvements in phosphor properties are also achieved by doping monovalent alkali metal ions: Li
+
, Na
+
, K
+
, Cs
+
, and Rb
+
into the Eu
2+
-activated, trivalent metal ion-co-doped phosphor to replace alkaline earth metal ion (e.g., Sr
2+
) in the host. Preferred alkali metal ions are Na
+
and K
+
. Preferably the amount of monovalent ion introduced into the phosphor is substantially equivalent to the molar amount of trivalent rare earth metal ion (e.g., Dy
3+
). It is believed that this type of substitution also reduces the charge defects induced by substitution of the trivalent rare earth ion sites into alkaline earth metal ion sites (e.g., Sr
2+
sites) in the aluminate.
Improvements in phosphor properties, particularly phosphor brightness, are further achieved by codoping Y
3+
, La
+3
, Lu
−3
, Gd
+3
and Bi
3+
into the Eu
2−
-activated, trivalent metal ion-doped phosphor to replace alkaline earth metal ion (e.g., Sr
2+
) in the host. Co-doping with Y
+3
is more preferred. The amount of Y
+3
that can be introduced to replace the alkaline earth metal ion is up to three times the molar amount of other trivalent rare earth metal ions (eg.,Dy
3+
).
A separate aspect of this invention is the discovery that application of a quenching step after sintering during the synthesis of the alkaline earth aluminate phosphors results in phosphors with significantly improved properties. The phosphors produced using this quenching step exhibit improved brightness and longer persistence than analogous phosphors prepared without use of the quenching step (non-quenched phosphors.)
Without wishing to be bound by any particular theory, it is presently believed that application of the quenching step to the phosphors of this invention after sintering results in the formation of alkaline earth aluminate phosphor at least in part in the high temperature phase (D-phase). Phosphor material prepared without such a quenching step is believed to be substantially in the lower temperature phase (&agr;-phase). Application of the quenching step in the synthesis of SrAl
2
O
4
matrix phosphors is believed to generate phosphors containing the high temperature phase, &bgr;-SrAl
2
O
4
. Non-quenched SrAl
2
O
4
is believed to be substantially (50% by weight or more) in the &agr;-phase. The specific quenching of SrAl
2
O
4
is accomplished by rapid cooling of the sintered mat

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