Compositions – Inorganic luminescent compositions – Compositions containing halogen; e.g. – halides and oxyhalides
Reexamination Certificate
2001-05-18
2003-09-02
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Compositions containing halogen; e.g., halides and oxyhalides
C313S373000, C313S486000, C313S487000, C313S467000
Reexamination Certificate
active
06613248
ABSTRACT:
BACKGROUND OF INVENTION
Field of the Invention
This invention relates to oxide-based materials that have one application as phosphors. More particularly, the phosphors are aluminates or borates doped with Pr
3+
and exhibit quantum splitting when irradiated with vacuum ultraviolet (“VUV”) radiation. This invention also relates to a method of making such quantum-splitting phosphors.
The conversion of a single ultraviolet (“UV”) photon into two visible photons with the result that the quantum efficiency of luminescence exceeds unity is termed quantum splitting. Quantum splitting materials are very desirable for use as phosphors for lighting applications, such as fluorescent lamps. A suitable quantum splitting phosphor can, in principle, produce a significantly brighter fluorescent light source due to higher overall luminous output because it can convert to visible light the part of UV radiation that is not absorbed efficiently by traditional phosphors currently used in commercial fluorescent lamps. Quantum splitting has been demonstrated previously in fluoride- and oxide-based materials. A material comprising 0.1% Pr
3+
in a matrix of YF
3
has been shown to generate more than one visible photon for every absorbed UV photon when excited with radiation having a wavelength of 185 nm. The measured quantum efficiency of this material was 140%, and thus greatly exceeded unity. However, fluoride-based compounds do not have sufficient stability to permit their use as phosphors in fluorescent lamps because they are known to react with mercury vapor that is used in such lamps to provide the UV radiation and form materials that do not exhibit quantum splitting. In addition, producing fluoride-based materials presents a great practical challenge because it involves the use of large quantities of highly reactive and toxic fluorine-based materials.
The applicants recently disclosed oxide-based quantum splitting materials. U.S. Pat. No. 5,552,082 discloses a lanthanum magnesium borate activated with Pr
3+
ion. U.S. Pat. No. 5,571,4151 discloses a strontium magnesium aluminate activated with Pr
3+
ion and charge compensated with Mg
2+
ion. Emission spectra of these materials exhibit a large peak at about 405 nm which is characteristic of quantum splitting. However, these materials still exhibit a considerable emission in the UV wavelength range of less than 350 nm. This part of the emission reduces the overall visible light output that otherwise can be higher. Therefore, it is desirable to provide oxide-based quantum-splitting phosphors that have higher quantum efficiency in the visible range than the prior-art quantum splitting materials. It is also desirable to provide more energy-efficient light sources using quantum-splitting phosphors having higher quantum efficiency. It is further desirable to provide method for making materials having high quantum splitting capability.
SUMMARY OF INVENTION
The present invention provides oxide-based phosphors doped with Pr
3+
ion, which phosphors exhibit quantum splitting when irradiated with VUV radiation. VUV radiation as used herein is radiation having wavelength shorter than about 215 nm. The oxide phosphors of the present invention are oxides of aluminum or boron having positive counterions selected from Group IIA of the Periodic Table. The phosphors of the present invention may be used in mercury vapor discharge lamps to provide energy-efficient light sources.
In one aspect of the present invention, the oxide-based phosphors are strontium or strontium calcium aluminates having the magnetoplumbite crystal structure. The aluminates are doped with Pr
3+
ion. Furthermore, it is advantageous to substitute some of the aluminum ions with magnesium ions for the purpose of charge compensation when Pr
3+
is substituted on the Sr
2+
sites. Such oxide-based phosphors of the present invention have a composition represented by Sr
1−1.5y
Pr
y
Al
12
O
19
, Sr
1−x−1.5y
Ca
x
Pr
y
Al
12
O
19
, or Sr
1−x−z
Ca
x
Mg
z
Al
12−z
Pr
z
O
19
where 0<x<1, y is in the range from about 0.005 to about 0.5, z is in the range from about 0.005 to about 0.5, x+1.5y≦1, and x+z<1.
In another aspect of the present invention, the oxide-based phosphors are calcium or calcium magnesium aluminates activated with Pr
3+
ion having a composition represented by Ca
1−z
Pr
z
Al
12
O
19
, Ca
1−z
Pr
z
MgAl
11.33
O
19
, or Ca
1−z
Pr
z
MgAl
14
O
23
where z is in the range from about 0.005 to about 0.5. In all of these host lattices, the Pr
3+
ion can be charge compensated by the Mg
+
ion or by lattice vacancies.
In another aspect of the present invention, the oxide-based phosphors are strontium borate activated with Pr
3−
having a composition represented by Sr
1+z
Pr
z
B
4
O
7
where z is in the range from about 0.005 to about 0.5.
The present invention also provides a method of making improved quantum-splitting aluminate or borate phosphors. The method comprises the steps of selecting a desired final composition of the phosphor; mixing together materials from the following two groups: (1) at least one oxygen-containing compound of praseodymium and (2) materials selected from the group consisting of oxygen-containing compounds of strontium, calcium, magnesium, aluminum, and boron so to achieve the desired final composition; forming a substantially homogeneous mixture of the selected compounds; and firing the substantially homogeneous mixture in a non-oxidizing atmosphere at a temperature and for a time sufficient to result in the desired composition and to maintain the praseodymium ion in the 3+ valence state.
In another aspect of the present invention, the method further comprises adding at least one compound selected from the group consisting of fluoride salts of aluminum, calcium, and strontium in a quantity sufficient to act as a flux prior to the step of forming the substantially homogeneous mixture. When the oxide-based phosphor is a borate, a quantity of boric acid may be advantageously used, either in place of or in combination with the fluoride salts, as the flux.
Other benefits of this invention may become evident by a perusal of the description and appended claims together with the attached drawings.
REFERENCES:
patent: 5552082 (1996-09-01), Srivastava et al.
patent: 5571451 (1996-11-01), Srivastava et al.
patent: 5788883 (1998-08-01), Srivastava et al.
patent: 6210605 (2001-04-01), Srivastava et al.
R. Pappalardo, “Calculated Quantum Yields for Photon-Cascade Emission (PCE) for Pr3+ and Tm3+ in Fluoride Hosts,” Journal of Luminescence, vol. 14, pp. 159-193 (1976).
W. W. Piper et al., “Cascade Fluorescent Decay in Pr3+-Doped Fluorides: Achievement of a Quantum Yield Greater than Unity for Emission of Visible Light,” Journal of Luminescence, vol. 8, pp. 344-348 (1974).
J. L. Sommerdijk et al., “Two Photon Luminescence with Ultraviolet Excitation of Trivalent Praseodymium,” Journal of Luminescence, vol. 8, pp. 341-343 (1974).
Setlur Anant Achyut
Srivastava Alok Mani
Johnson Noreen C.
Koslow C. Melissa
Vo Toan P.
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