U.S. patent application number 09/748777 was filed with the patent office on 2001-09-13 for mn2+ activated green emitting sral12 o19 luminescent material.
Invention is credited to Beers, William Winder, Comanzo, Holly Ann, Doughty, Douglas Allen, Srivastava, Alok Mani.
Application Number | 20010020696 09/748777 |
Document ID | / |
Family ID | 23415756 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010020696 |
Kind Code |
A1 |
Srivastava, Alok Mani ; et
al. |
September 13, 2001 |
Mn2+ activated green emitting SrAl12 O19 luminescent material
Abstract
A SrAl.sub.12O.sub.19 green luminescent material is doped with
Mn.sup.2+ activator ions and at least one trivalent rare earth
sensitizer ion species. Preferably, the material contains four rare
earth ions: Ce.sup.3+, Pr.sup.3+, Gd.sup.3+ and Tb.sup.3+.
Optionally, a portion of the aluminum may be substituted with
magnesium. The material may be used as a display device or lamp
phosphor or as an X-ray diagnostic or laser scintillator.
Inventors: |
Srivastava, Alok Mani;
(Niskayuna, NY) ; Comanzo, Holly Ann; (Niskayuna,
NY) ; Doughty, Douglas Allen; (Goffstown, NH)
; Beers, William Winder; (Chesterland, OH) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY
CRD PATENT DOCKET ROOM 4A59
P O BOX 8
BUILDING K 1 SALAMONE
SCHENECTADY
NY
12301
US
|
Family ID: |
23415756 |
Appl. No.: |
09/748777 |
Filed: |
December 27, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09748777 |
Dec 27, 2000 |
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09359900 |
Jul 26, 1999 |
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6210605 |
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Current U.S.
Class: |
252/79.1 |
Current CPC
Class: |
G21K 2004/06 20130101;
C09K 11/7774 20130101; H01J 61/44 20130101; C09K 11/7703 20130101;
C09K 11/7712 20130101; C09K 11/7749 20130101; H01J 29/20 20130101;
C09K 11/7721 20130101; C09K 11/7706 20130101 |
Class at
Publication: |
252/79.1 |
International
Class: |
C30B 015/00; C30B
011/14 |
Claims
What is claimed is:
1. A composition of matter, comprising: AD.sub.12O.sub.19:Mn,R
wherein A comprises at least one of strontium, barium and calcium;
D comprises at least one of aluminum, boron, gallium and magnesium;
and R comprises at least one trivalent rare earth ion.
2. The composition of claim 1, wherein Mn comprises an activator
ion with a 2+ valence state.
3. The composition of claim 1, comprising:
(A.sub.1-xR.sub.x)(D.sub.12-zMn- .sub.z)O.sub.19 wherein
0<x.ltoreq.1; 0<z.ltoreq.0.5.
4. The composition of claim 3, wherein: A comprises strontium; D
comprises at least one of aluminum and magnesium; and R comprises
at least one of cerium, praseodymium, gadolinium and terbium.
5. The composition of claim 4, comprising:
(Sr.sub.1-m-q-r-tCe.sub.mPr.sub-
.qGd.sub.rTb.sub.t)(Al.sub.12-e-gMg.sub.eMn.sub.g)O.sub.19 wherein
0.ltoreq.m.ltoreq.1; 0.ltoreq.q.ltoreq.1; 0.ltoreq.r.ltoreq.0.5;
0.ltoreq.t.ltoreq.0.5; 0.ltoreq.e.ltoreq.0.5; and
0<g.ltoreq.0.5.
6. The composition of claim 4, wherein R comprises: a) cerium,
praseodymium, gadolinium and terbium; or b) at least one of cerium
and praseodymium; or c) cerium and at least one of gadolinium and
terbium; or d) praseodymium and at least one of gadolinium and
terbium.
7. The composition of claim 5, comprising:
(Sr.sub.0.5Ce.sub.0.3Gd.sub.0.1-
Tb.sub.0.1)(Al.sub.11.5Mg.sub.0.25Mn.sub.0.25)O.sub.19.
8. The composition of claim 5, comprising:
(Sr.sub.0.5Ce.sub.0.15Pr.sub.0.-
15Gd.sub.0.1Tb.sub.0.1)(Al.sub.11.5Mg.sub.0.25Mn.sub.0.25)O.sub.19.
9. The composition of claim 5, comprising:
(Sr.sub.0.5Ce.sub.0.15Pr.sub.0.-
15Gd.sub.0.1Tb.sub.0.1)(Al.sub.11.75Mn.sub.0.25)O.sub.19.
10. The composition of claim 1, wherein the composition of matter
comprises a portion of a lamp, a cathode ray tube, a plasma display
device, a liquid crystal display, a laser, an electromagnetic
calorimeter, a gamma ray camera or a computed tomography
scanner.
11. A luminescent device, comprising: a housing; a source of
energetic media contained in said housing; and a luminescent
material contained in the interior of the housing, comprising:
AD.sub.12O.sub.19:Mn,R wherein A comprises at least one of
strontium, barium and calcium; D comprises at least one of
aluminum, boron, gallium and magnesium; and R comprises at least
one trivalent rare earth ion.
12. The device of claim 11, wherein Mn comprises an activator ion
with a 2+ valence state.
13. The device of claim 11, wherein the luminescent material
comprises: (A.sub.1-xR.sub.x)(D.sub.12-zMn.sub.z)O.sub.19 wherein
0<x.ltoreq.1; 0<z.ltoreq.0.5.
14. The device of claim 13, wherein: A comprises strontium; D
comprises at least one of aluminum and magnesium; and R comprises
at least one of cerium, praseodymium, gadolinium and terbium.
15. The device of claim 14, wherein the luminescent material
comprises:
(Sr.sub.1-m-q-r-tCe.sub.mPr.sub.qGd.sub.rTb.sub.t)(Al.sub.12-e-gMg.sub.eM-
n.sub.g)O.sub.19 wherein 0.ltoreq.m.ltoreq.1; 0.ltoreq.q.ltoreq.1;
0.ltoreq.r.ltoreq.0.5; 0.ltoreq.t.ltoreq.0.5;
0.ltoreq.e.ltoreq.0.5; and 0<g.ltoreq.0.5.
16. The device of claim 14, wherein R comprises a) cerium,
praseodymium, gadolinium and terbium; or b) at least one of cerium
and praseodymium; or c) cerium and at least one of gadolinium and
terbium; or d) praseodymium and at least one of gadolinium and
terbium.
17. The device of claim 15, wherein the luminescent material
comprises:
(Sr.sub.0.5Ce.sub.0.3Gd.sub.0.1Tb.sub.0.1)(Al.sub.11.5Mg.sub.0.25Mn.sub.0-
.25)O.sub.19.
18. The device of claim 15, wherein the luminescent material
comprises:
(Sr.sub.0.5Ce.sub.0.15Pr.sub.0.15Gd.sub.0.1Tb.sub.0.1)(Al.sub.11.5Mg.sub.-
0.25Mn.sub.0.25)O.sub.19.
19. The device of claim 15, wherein the luminescent material
comprises:
(Sr.sub.0.5Ce.sub.0.15Pr.sub.0.15Gd.sub.0.1Tb.sub.0.1)(Al.sub.11.75Mn.sub-
.0.25)O.sub.19.
20. The device of claim 11, wherein: the device comprises a lamp;
the housing comprises a transparent tube; the energetic media
comprises photons; the source of energetic media comprises a gas in
the transparent tube; and the luminescent material comprises a
phosphor formed on the interior surface of the transparent
tube.
21. The device of claim 20, further comprising a lamp base and at
least one gas discharge electrode.
22. The device of claim 11, wherein: the device comprises a plasma
display; the housing contains a display screen; the energetic media
comprises photons; the source of energetic media comprises a gas
contained inside the housing; and the luminescent material
comprises a phosphor formed on the interior surface of the display
screen.
23. The device of claim 22, further comprising a control circuit
coupled to gas discharge electrodes to excite predetermined
portions of the gas.
24. The device of claim 11, wherein: the device comprises a liquid
crystal display; the housing contains a display screen; the
energetic media comprises photons; the source of energetic media
comprises a lamp contained inside the housing; and the luminescent
material comprises a phosphor formed on a portion of a first
substrate.
25. The device of claim 24, further comprising: a liquid crystal
material comprising a first surface adjacent to the first
substrate; a second substrate adjacent to a second surface of the
liquid crystal material; and thin film transistors and transparent
electrodes on said second substrate.
26. The device of claim 11, wherein: the device comprises a cathode
ray tube; the housing contains a display screen; the energetic
media comprises electrons; the source of energetic media comprises
at least one electron gun or cathode contained inside the housing;
and the luminescent material comprises a phosphor formed inside of
the display screen.
27. The device of claim 26, further comprising: an anode between
the display screen and the electron gun; and at least one electron
deflector between the anode and the electron gun.
28. The device of claim 11, wherein: the device comprises a laser;
the housing contains an emission aperture; the energetic media
comprises photons; the source of energetic media comprises a lamp
or a secondary laser contained inside the housing; and the
luminescent material comprises a single crystal scintillator formed
inside the housing.
29. The device of claim 28, further comprising: a reflecting
mirror; a partially reflecting mirror; at least one electrode in
contact with the single crystal scintillator.
30. The device of claim 11, wherein: the device comprises an
electromagnetic calorimeter, a gamma ray camera or a computed
tomography scanner; the energetic media comprises X-rays or gamma
rays; the source of energetic media comprises an X-ray or a gamma
ray entrance aperture in the housing; the luminescent material
comprises a scintillator formed inside the housing.
31. The device of claim 30, further comprising a photodetector or a
photo sensitive film.
32. A method of making a phosphor, comprising the steps of: mixing
oxide, carbonate, hydroxide, nitrate or oxalate compounds of
strontium, aluminum, manganese, and at least one of gallium,
barium, calcium, boron, magnesium, cerium, praseodymium, gadolinium
and terbium; and heating a resulting mixture to form the
phosphor.
33. The method of claim 32, wherein the step of mixing comprises
mixing oxide or carbonate compounds of strontium, aluminum,
manganese, magnesium, cerium, praseodymium, gadolinium and
terbium.
34. The method of claim 32, wherein the step of heating comprising
heating the resulting mixture in a reducing atmosphere at 1400 to
1600.degree. C. for 8 to 12 hours.
35. The method of claim 32, further comprising adding a flux
comprising at least one fluoride or chloride compound of magnesium,
strontium, aluminum, manganese or ammonium to the step of
mixing.
36. The method of claim 32, further comprising the step of coating
the phosphor on a substrate.
37. The method of claim 36, wherein the substrate comprises a
portion of a cathode ray tube, a liquid crystal display, a plasma
display, a lamp, or a computed tomography detector.
38. A method of making a scintillator, comprising the steps of:
placing a single crystal seed in contact with a melt comprising
strontium, aluminum, oxygen, manganese and at least one of
magnesium, gallium, barium, calcium, boron, cerium, praseodymium,
gadolinium and terbium; moving the seed from a high temperature
zone to a low temperature zone; and forming a single crystal
scintillator in contact with the seed.
39. The method of claim 38, wherein: the method comprises a
Bridgeman-Stockbarger crystal growth method comprising the steps
of: melting at least a portion of a solid material comprising
strontium, aluminum, oxygen, manganese and at least one of
magnesium, cerium, praseodymium, gadolinium and terbium in the high
temperature zone; and moving the seed from the high temperature
zone to the low temperature zone to solidify the melt into the
single crystal scintillator.
40. The method of claim 38, wherein: the method comprises a
Czochralski crystal growth method, comprising the steps of: melting
at least a portion of a solid material comprising strontium,
aluminum, oxygen, manganese and at least one of magnesium, cerium,
praseodymium, gadolinium and terbium in a crucible comprising the
high temperature zone; lowering the seed into the crucible;
rotating the seed about its axis; and lifting the seed from the
crucible into the low temperature zone to pull the single crystal
scintillator boule from the melt.
41. The method of claim 38, wherein the single crystal scintillator
comprises a laser light emission source, an electromagnetic
calorimeter detector, a computed tomography detector or a gamma
radiation detector.
42. The method of claim 38, wherein the single crystal scintillator
comprises: AD.sub.12O.sub.19:Mn,R wherein A comprises at least one
of strontium, barium and calcium, D comprises at least one of
aluminum and magnesium; and R comprises at least one of cerium,
praseodymium, gadolinium and terbium.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to a luminescent material
doped with various ions, and more particularly to a
SrAl.sub.12O.sub.19 material doped with Mn.sup.2+ Ce.sup.3+,
Pr.sup.3+, Gd.sup.3+, Tb.sup.3+ and/or Mg.sup.2+ and used as a lamp
phosphor, a display phosphor or as a laser crystal.
BACKGROUND OF THE INVENTION
[0002] A luminescent material absorbs energy in one portion of the
electromagnetic spectrum and emits energy in another portion of the
electromagnetic spectrum. A luminescent material in powder form is
commonly called a phosphor, while a luminescent material in the
form of a transparent solid body is commonly called a
scintillator.
[0003] Most useful phosphors and scintillators emit radiation in
the visible portion of the spectrum in response to the absorption
of radiation which is outside the visible portion of the spectrum.
Thus, the phosphor performs the function of converting
electromagnetic radiation to which the human eye is not sensitive
into electromagnetic radiation to which the human eye is sensitive.
Most phosphors are responsive to more energetic portions of the
electromagnetic spectrum than the visible portion of the spectrum.
Thus, there are phosphors and scintillators which are responsive to
ultraviolet light (as in fluorescent lamps), electrons (as in
cathode ray tubes) and x-rays (as in radiography).
[0004] Two broad classes of luminescent materials are recognized.
These are self-activated luminescent materials and
impurity-activated luminescent materials.
[0005] A self-activated luminescent material is one in which the
pure crystalline host material upon absorption of a high energy
photon elevates electrons to an excited state from which they
return to a lower energy state by emitting a photon. Self-activated
luminescent materials normally have a broad spectrum emission
pattern because of the relatively wide range of energies which the
electron may have in either the excited or the lower energy states.
Thus, any given excited electron may emit a fairly wide range of
energy during its transition from its excited to its lower energy
state, depending on the particular energies it has before and after
its emissive transition.
[0006] An impurity activated luminescent material is normally one
in which a non-luminescent host material has been modified by
inclusion of an activator species which is present in the host
material in a relatively low concentration, such as in the range
from about 200 parts per million to 1,000 parts per million.
However, some materials require several mole or atomic percent of
activator ions for optimized light output. With an impurity
activated luminescent material, the activator ions may directly
absorb the incident photons or the lattice may absorb the incident
photons and transfer the absorbed photon energy to the activator
ions.
[0007] The photon absorbed by the lattice may create mobile
migrating electrons and holes in the lattice. Due to favorable
charge configurations, the migrating electrons and holes are
trapped at the activator ions, where they recombine and emit a
photon of luminescent light.
[0008] Alternatively, if the photon is absorbed directly by the
activator ion, the photon raises one or more electrons of the
activator ion to a more excited state. These electrons, in
returning to their less excited state, emit a photon of luminescent
light.
[0009] In many commonly employed impurity activated luminescent
materials, the electrons which emit the luminescent light are d or
f shell electrons whose energy levels may be significantly affected
or relatively unaffected, respectively, by the surrounding crystal
field. In those situations where the activator ion is not much
affected by the local crystal field, the emitted luminescent light
is substantially characteristic of the activator ions rather than
the host material and the luminescent spectrum comprises one or
more relatively narrow emission peaks. This contrasts with a
self-activated luminescent material's much broader emission
spectrum.
[0010] When a host lattice absorbs the incident photon (i.e. the
excitation energy) and transfers it to the activator ion, the host
lattice acts as a sensitizer. The host lattice may also be doped
with sensitizer atoms. The sensitizer atoms absorb the incident
photon either directly, or from the host lattice, and transfer it
to the activator ion.
[0011] One prior art green light emitting phosphor is
Zn.sub.2SiO.sub.4:Mn.sup.2+. This phosphor is used in display
devices, such as plasma displays and cathode ray tubes (CRT), and
in various fluorescent lamps. The phosphor absorbs the emitted UV
radiation from the lamp or plasma display or electrons in a CRT and
emits radiation in the green spectral range.
[0012] It is generally advantageous for a phosphor to be resistant
to radiation damage and exhibit a high lumen maintenance. Radiation
damage is the characteristic of a luminescent material in which the
quantity of light emitted by the luminescent material in response
to a given intensity of stimulating radiation decreases after the
material has been exposed to a high radiation dose. Lumen
maintenance is the ability of a luminescent material to resist
radiation damage over time. Luminescent materials with a high
resistance to radiation damage over time have a high lumen
maintenance.
[0013] However, the Zn.sub.2SiO.sub.4:Mn.sup.2+ phosphor has shown
a significant decrease in light output after several hundred hours
of bombardment by energetic UV radiation or electrons. Therefore,
the phosphor suffers from poor lumen maintenance.
[0014] Two of the current inventors recently proposed a new
Sr.sub.1-xPr.sub.xAl.sub.12-yMg.sub.yO.sub.19 phosphor in U.S. Pat.
No. 5,571,451. This phosphor emits light in the blue spectral range
due to emission from the Pr.sup.3+ activator. Furthermore, this
phosphor exhibits a high quantum efficiency in the blue spectral
range due to a Pr quantum splitting effect. However, this phosphor
does not exhibit luminescence in the green spectral range.
[0015] In view of the foregoing, it would be desirable to provide a
green emitting phosphor or scintillator material that exhibits an
adequate lumen maintenance. It would also be desirable to provide a
method of making such a phosphor or scintillator.
BRIEF SUMMARY OF THE INVENTION
[0016] One embodiment of the present invention provides a
composition of matter, comprising AD.sub.12O.sub.19:Mn,R where A
comprises at least one of strontium, barium and calcium, D
comprises at least one of aluminum, gallium, boron and magnesium
and R comprises at least one trivalent rare earth ion.
[0017] Another embodiment of the present invention provides a
luminescent device, comprising a housing, a source of energetic
media contained in the housing and a luminescent material contained
in the interior of the housing. The luminescent material comprises
AD.sub.12O.sub.19:Mn,R where A comprises at least one of strontium,
barium and calcium, D comprises at least one of aluminum, gallium,
boron and magnesium and R comprises at least one trivalent rare
earth ion.
[0018] Furthermore, an embodiment of the present invention provides
a method of making a phosphor, comprising the steps of mixing
oxide, carbonate, hydroxide, nitrate or oxalate compounds of
strontium, aluminum, manganese and at least one of gallium,
magnesium, boron, calcium, barium, cerium, praseodymium, gadolinium
and terbium, and heating a resulting mixture to form the phosphor.
An embodiment of the present invention also provides a method of
making a scintillator, comprising the steps of placing a single
crystal seed in contact with a melt comprising strontium, aluminum,
oxygen, manganese and at least one of gallium, magnesium, boron,
calcium, barium, cerium, praseodymium, gadolinium and terbium,
moving the seed from a high temperature zone to a low temperature
zone and forming a single crystal scintillator in contact with the
seed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a perspective view of a magnetoplumbite crystal
structure.
[0020] FIG. 2 is side cross sectional view of a fluorescent lamp
coated with a phosphor in accordance with an exemplary embodiment
of the present invention.
[0021] FIGS. 3 and 4 are side cross sectional view of cathode ray
tubes coated with a phosphor in accordance with an exemplary
embodiment of the present invention.
[0022] FIG. 5 is a side cross sectional view of a liquid crystal
display device coated with a phosphor in accordance with an
exemplary embodiment of the present invention.
[0023] FIG. 6 is a side cross sectional view of a plasma display
device coated with a phosphor in accordance with an exemplary
embodiment of the present invention.
[0024] FIG. 7 is a side cross sectional view of an X-ray detection
device containing a scintillator in accordance with an exemplary
embodiment of the present invention.
[0025] FIG. 8 a side cross sectional view of a laser containing a
scintillator in accordance with an exemplary embodiment of the
present invention.
[0026] FIG. 9 is a schematic of one method of making a scintillator
in accordance with an exemplary embodiment of the present
invention.
[0027] FIG. 10 is a schematic of another method of making a
scintillator in accordance with an exemplary embodiment of the
present invention.
[0028] FIG. 11 is comparison of emission spectra of the phosphor in
accordance with an exemplary embodiment of the present invention
and of a prior art phosphor under 254 nm incident radiation.
[0029] FIG. 12 is comparison of chromaticity color coordinates of
the phosphor of an exemplary embodiment of the present invention
and of a prior art phosphor.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present inventors have discovered that SrAl12O.sub.19 is
a luminescent material in the green spectral range when it is doped
with Mn.sup.2+ activator ions. Furthermore, trivalent rare earth
ions, such as Ce, Pr, Gd and Tb act as sensitizers in
SrAl.sub.12O.sub.19:Mn.sup.2+. This material has a more saturated
green luminescence (i.e., a sharp emission peak with a maximum
wavelength in the green spectral region) and an equivalent absolute
quantum efficiency compared to the prior art
Zn.sub.2SiO.sub.4:Mn.sup.2+ phosphor, having a broad emission peak
in the green-yellow spectral range. The
SrAl.sub.12O.sub.19:Mn.sup.2+ phosphor is also superior to the
Zn.sub.2SiO.sub.4:Mn.sup.2+ phosphor with respect to high energy
radiation damage resistance and lumen maintenance because of an
inherent stability of its magnetoplumbite lattice structure.
[0031] While the present inventors do not wish to be bound by any
particular theory as to why Mn and trivalent rare earth ion doping
produces green emission from the SrAl.sub.12O.sub.19, the present
inventors believe the following.
[0032] The SrA.sub.12O.sub.19 material crystallizes in a
magnetoplumbite structure, as shown in FIG. 1. The Mn.sup.2+ dopant
ions substitute Al ions on the Al tetrahedral cation sites not
occupying the mirror plane. Therefore, the Mn ions have a
tetrahedral coordination because each Mn ion has four bonds.
Tetrahedrally coordinated Mn.sup.2+ ions are subject to a weak
crystal field. Therefore, SrAl.sub.12O.sub.19:Mn.sup.2+ emits
radiation in the green spectral range because the emitted
luminescent light is substantially characteristic of the Mn.sup.2+
activator ions rather than of the host material.
[0033] The strontium ions occupy cation sites inside the
magnetoplumbite lattice mirror plane. Therefore, these ions expand
the lattice mirror plane and are a source of crystal field effects
in the lattice. The trivalent rare earth dopant ions occupy the Sr
lattice sites. Since the rare earth ions are larger than the Sr
ions, the rare earth ions may cause a greater amount of mirror
plane expansion than the Sr ions and act as sensitizers for the
Mn.sup.2+ activator ions.
[0034] The preferred trivalent rare earth ions are cerium (Ce),
praseodymium (Pr), gadolinium (Gd) and terbium (Tb). However, other
trivalent rare earth ions may be used. The present inventors
believe that each rare earth ion has a different sensitizer
function. Pr acts as a sensitizer for 185 nm incident radiation.
Therefore, if the luminescent material is exposed to 185 nm
radiation, the Pr ions on the Sr lattice sites absorb the incident
radiation and transfer the energy generated by the incident
radiation to the Mn.sup.2+ activator ions on the Al sites.
Therefore, if SrAl.sub.12O.sub.19:Mn.sup.2+ is used as a green
emitting phosphor for a UV gas discharge lamp that emits at 185 nm,
then the phosphor should be doped with Pr activator ions.
[0035] Ce acts as a sensitizer for 254 nm incident radiation.
Therefore, if the luminescent material is exposed to 254 nm
radiation, the Ce ions on the Sr lattice sites absorb the incident
radiation and transfer the energy generated by the incident
radiation to the Mn.sup.2+ activator ions on the Al sites.
Therefore, if SrAl.sub.12O.sub.19:Mn.sup.2+ is used as a green
emitting phosphor for a UV gas discharge lamp that emits at 254 nm,
then the phosphor should be doped with Ce activator ions. If the
SrAl.sub.12O.sub.19:Mn.sup.2+ is used as a green emitting phosphor
for a lamp or another radiation source that emits at both 185 nm
and 254 nm, then both Ce and Pr sensitizers should be used. It
should be noted that Pr and Ce act as sensitizers for ranges of
different wavelengths extending to about 300 nm, and not just for
185 and 254 nm wavelengths.
[0036] The current inventors believe the following possible
mechanism of energy transfer utilizing Gd ions. The Gd ions reside
on adjacent Sr sites in the magnetoplumbite SrAl.sub.12O.sub.19
lattice to form a Gd ion sublattice. The Pr or Ce sensitizers
absorb the incident radiation and transfer the energy to at least
one Gd ion in the sublattice. The Gd ions then transfer the energy
to other Gd ions in the sublattice, until the energy reaches a Gd
ion adjacent to a Mn.sup.2+ activator. The energy is then
transferred from the sublattice to the activator. Thus, the Gd ion
sublattice facilitates energy transfer to the activator ions.
Therefore, Gd may be added to SrAl.sub.12O.sub.19:Mn.sup.2+ in
addition to Pr and/or Ce.
[0037] Tb acts as a quantum efficiency enhancer in
SrAl.sub.12O.sub.19:Mn.- sup.2+. The present inventors discovered
that when SrAl.sub.12O.sub.19:Mn.- sup.2+ is doped with Tb ions,
the green Mn.sup.2+ quantum efficiency is improved compared to
SrAl.sub.12O.sub.19:Mn.sup.2+ that is not doped with Tb ions.
Therefore, SrAl.sub.12O.sub.19:Mn.sup.2+ may be doped with Tb ions
in addition to being doped with Pr, Ce and/or Gd ions. The current
inventors believe that a complex, multistage energy transfer
probably occurs when SrAl.sub.12O.sub.19:Mn.sup.2+ is doped with Tb
ions as well as other trivalent rare earth ions to achieve an
improved quantum efficiency. The current inventors determined that
the green light is emitted mainly from the Mn.sup.2+ ions, and not
from the Ce.sup.3+ or Tb.sup.3+ ions in
SrAl.sub.12O.sub.19:Mn.sup.2+, Tb.sup.3+, Ce.sup.3+.
[0038] For at least the reasons described above, the
SrAl.sub.12O.sub.19:Mn.sup.2+ material may comprise any combination
of one, two, three or four trivalent rare earth ion dopant species,
depending on the required use of the material. Preferably, the
dopant species comprise Pr, Ce, Gd and Tb.
[0039] Furthermore, a portion of the Al ions may be replaced by
gallium, boron or magnesium ion dopant species. A preferred dopant
species that substitutes the Al ions are the Mg ions. Mg ions act
as charge compensating ions when the Sr.sup.2+ lattice sites are
filled by trivalent rare earth ions. A portion of the Sr ions may
also be replaced by calcium or barium ion dopant species, if
desired.
[0040] Therefore, the luminescent material according to an
exemplary embodiment of the present invention may be characterized
by the following generic formula: AD.sub.12O.sub.19:Mn,R, where A
comprises at least one of strontium, calcium and barium, D
comprises at least one of aluminum, gallium, boron and magnesium
and R comprises at least one trivalent rare earth ion. Mn comprises
an activator ion with a 2+ valence state.
[0041] Mn ion concentration may range from greater than zero to 50
mole or atomic percent or less of {fraction (1/12)} of the D
cationic species. The preferred Mn ion concentration range is 20 to
30 atomic percent and the preferred Mn ion concentration is 25
atomic percent of {fraction (1/12)} of the D cationic species. In
other words, there may be 0-0.5 moles or ions of Mn out of a total
12 moles or ions of the D cationic species. The remaining 11.5 to
12 moles of the D cationic species may comprise Al or a combination
of Al and Mg.
[0042] The magnesium ion concentration may range from zero to 50
mole or atomic percent of {fraction (1/12)} of the total Al ion
concentration. The preferred Mg ion concentration range is 20 to 30
atomic percent and the preferred Mg ion concentration is 25 atomic
percent of {fraction (1/12)} of the Al cationic species. In other
words, there may be 0-0.5 moles or ions of Mg out of a total 12
moles or ions of the D cationic species.
[0043] R ion concentration may range from greater than zero to 100
mole or atomic percent of the A cationic species (i.e., the
strontium cationic species, which may be partially substituted by
calcium or barium cationic species, if desired). The preferred R
ion concentration range is 25 to 75 atomic percent and the
preferred R ion concentration is 50 atomic percent of the A
cationic species.
[0044] The concentrations of Mn and R ions may be expressed by the
following formula: (A.sub.1-xR.sub.x)(D.sub.12-zMn.sub.z)O.sub.19
where 0<x.ltoreq.1; 0<z.ltoreq.0.5. In one preferred material
of the present invention, A comprises strontium, D comprises at
least one of aluminum and magnesium and R comprises at least one of
cerium, praseodymium, gadolinium and terbium.
[0045] Cerium and praseodymium ion concentration may range from
zero to 100 mole or atomic percent each of the total Sr ion
concentration. It should be understood that any combination of Ce
and Pr ion concentrations cannot exceed 100 mole or atomic percent
of the Sr ion concentration. The preferred Ce and Pr ion
concentration range is 10 to 40 atomic percent each of the Sr
cationic species. If Ce ions are added without adding Pr ions, then
the preferred Ce ion concentration is 30 atomic percent of the Sr
cationic species. If Pr ions are added without adding Ce ions, then
the preferred Pr ion concentration is 30 atomic percent of the Sr
cationic species. If both Ce and Pr ions are added, then preferred
Ce and Pr ion concentration is 15 atomic percent each of the Sr
cationic species.
[0046] Gd and Tb ion concentration may range from zero to 50 mole
or atomic percent each of the total Sr ion concentration. The
preferred Gd and Tb ion concentration range is 5 to 15 atomic
percent each and the preferred Gd and Tb ion concentration is 10
atomic percent each of the Sr cationic species.
[0047] The concentrations of Mg, Ce, Pr, Gd and Tb ions in the
preferred material of the present invention may be expressed by the
following formula:
(Sr.sub.1-m-q-r-tCe.sub.mPr.sub.qGd.sub.rTb.sub.t)(Al.sub.12-e-g-
Mg.sub.eMn.sub.g)O.sub.19 where 0.ltoreq.m.ltoreq.1;
0.ltoreq.q.ltoreq.1; 0.ltoreq.r.ltoreq.0.5; 0.ltoreq.t.ltoreq.0.5;
0.ltoreq.e.ltoreq.0.5; and 0<g.ltoreq.0.5. One preferred
material of the present invention contains non-zero concentrations
of Mg, Ce, Pr, Gd and Tb ions. Another preferred material of the
present invention contains non-zero concentrations of at least one
of Ce and Pr ions. Another preferred material of the present
invention contains non-zero concentrations of Ce and at least one
of Gd and Tb ions. Yet another preferred material of the present
invention contains non-zero concentrations of Pr and at least one
of Gd and Tb ions. Three specific preferred materials of the
present invention are:
[0048] 1)
(Sr.sub.0.5Ce.sub.0.3Gd.sub.0.1Tb.sub.0.1)(Al.sub.11.5Mg.sub.0.2-
5Mn.sub.0.25)O.sub.19,
[0049] 2)
(Sr.sub.0.5Ce.sub.0.15Pr.sub.0.15Gd.sub.0.1Tb.sub.0.1)(Al.sub.11-
.5Mg.sub.0.25Mn.sub.0.25)O.sub.19, and
[0050] 3)
(Sr.sub.0.5Ce.sub.0.15Pr.sub.0.15Gd.sub.0.1Tb.sub.0.1)(Al.sub.11-
.75Mn.sub.0.25)O.sub.19.
[0051] The luminescent material described above may be used in many
different applications. For example, the material may be used as a
phosphor in lamp, in a cathode ray tube, in a plasma display device
or in a liquid crystal display. The material may also be used as a
scintillator in an electromagnetic calorimeter, in a gamma ray
camera, in a computed tomography scanner or in a laser. These uses
are meant to be merely exemplary and not exhaustive.
[0052] The AD.sub.12O.sub.19:Mn,R phosphor may be used in a lamp.
For example, the phosphor may be used in a linear fluorescent lamp,
as shown, for example in FIG. 2. A fluorescent lamp comprises a
bulb 1 filled with a gas, the phosphor 2 formed on the interior
surface of the bulb 1, plural cathodes or gas discharge electrodes
3 and a lamp cap or base 4. Alternatively, the phosphor 2 may be
coated on the outside surface of the bulb 1, or on a separate
envelope containing the gas. The bulb 1 is preferably made of
glass. Other appropriate transparent materials may also be used.
The gas, such a mercury, emits radiation (i.e. ultraviolet
radiation) when a potential is applied to the cathode 3 through the
base 4. The phosphor 2 absorbs the incident UV radiation from the
gas and emits green light.
[0053] The AD.sub.12O.sub.19:Mn,R phosphor may be used in a cathode
ray tube (CRT). For example, the phosphor may be used in a CRT
adapted for use in a television set, as shown, for example in FIG.
3. The CRT contains at least one, and preferably three electron
gun(s) 5, at least one electron beam deflector 6, an anode 7, a
display screen 10 and the phosphor 2 coated on the inside of
screen. The CRT operates by emitting an electron beam 8 from the
gun 5. The beam 8 is attracted to the phosphor 2 by the anode 7.
The deflectors 6 control the position of the beam 8 on the phosphor
2. The portion of the phosphor 2 that absorbs the incident electron
beam 8 emits green light 9 through the screen 10. Alternatively,
the CRT may comprise a cathode array as shown, for example in FIG.
4. The CRT comprises an array of Spindt cathodes 11 (only one
cathode is shown for clarity). When a control circuit (not shown)
applies a potential to a particular cathode 11, it emits an
electron beam 8 toward the phosphor 2. The phosphor 2 coverts the
electron beam 8 into an emission of green light 9.
[0054] The AD.sub.12O.sub.19:Mn,R phosphor may be used in a liquid
crystal display (LCD), such as the one shown, for example, in FIG.
5. The LCD comprises a transparent substrate 12, a light source 13,
an array of plural control transistors 14, 15, such as thin film
transistors (only two are shown for clarity), a transparent
electrode 16, 17 in electrical contact with each transistor, liquid
crystal material 18, a transparent counter substrate 19, the green
emitting AD.sub.12O.sub.19:Mn,R phosphor 2 formed on the counter
substrate 19 above electrode 16, another phosphor 20 formed on the
counter substrate 19 above electrode 17, a transparent display
screen 21, and an opaque housing 22. When transistor 14 is switched
on, the electrode 16 applies a potential to the liquid crystal
material 18 directly above the electrode 16. The applied potential
forces the liquid crystal material 18 to become transparent above
electrode 16. The liquid crystal material remains opaque above
electrode 17 if no potential is applied to electrode 17 from
transistor 15. The light from lamp 13 may now pass through the
transparent portion of the liquid crystal material 18 to reach the
phosphor 2. Phosphor 2 absorbs the light from lamp 13 and emits
green light through screen 21. An image may be formed on the screen
21 by controlling the transmission of light from lamp 13 to various
colored phosphors 2, 20 through the liquid crystal material. The
phosphor 2 may alternatively be formed above counter substrate 19
or on the inside surface of screen 21.
[0055] The AD.sub.12O.sub.19:Mn,R phosphor may be used in a plasma
display device, such as the one as shown, for example, in FIG. 6.
The plasma display device comprises a transparent display screen
21, an opaque housing 22, a gas envelope 23, an array of gas
discharge electrodes 24 (only one electrode is shown for clarity)
and a control device 25, such as a transistor. The phosphor 2 may
be formed on the interior or exterior surface of the gas envelope
23 or on the interior surface of the screen 21. When the control
device 25 applies a potential to electrode 24, the electrode 24
creates a localized plasma discharge in the gas contained in the
envelope 23. The localized plasma emits UV radiation that is
absorbed by an adjacent portion the phosphor 2. The irradiated
portion of the phosphor 2 then emits green light through the screen
21. An image may be formed on the screen 21 by controlling the
application of the potential to different electrodes 24 of the
electrode array.
[0056] The scintillator of the present invention may be used in a
computed tomography (CT) scanning system, as shown for example in
FIG. 7. The CT scanning system is used to obtain cross sectional
images of the human body. In a CT scanning system, an X-ray source,
such as an X-ray tube 41 rotates in a circle about the patient 43.
An X-ray detector 42 is placed on the opposite side of the patient
43. The detector 42 rotates synchronously with the X-ray source
about the perimeter of the circle. The detector comprises the
AD.sub.12O.sub.19:Mn,R scintillator optically coupled to a
photodiode or another type of photodetector. Alternatively, the
detector 42 may comprise a AD.sub.12O.sub.19:Mn,R phosphor coated
on a transparent substrate and optically coupled to a photodiode or
another type of photodetector.
[0057] Alternatively, the AD.sub.12O.sub.19:Mn,R scintillator may
comprise a laser crystal, as shown for example in FIG. 8. The laser
comprises a housing 51, the scintillator crystal 52 and a light
source, such as a lamp 53. A potential is applied to the crystal 52
thorough electrodes from a voltage source 54. The crystal 52 emits
coherent green radiation through aperture 55 while the crystal is
irradiated by the light source 53 and a potential is applied from
the voltage source 54. The laser may optionally contain a full
mirror 56 and a half mirror 57 for amplification of the coherent
light amplitude by back and forth reflection of the light between
the mirrors. Alternatively, the laser crystal 52 may be cleaved
and/or processed to form a full mirror surface on the back of the
crystal and a partial mirror surface on the front surface of the
crystal 52 facing the aperture 55.
[0058] The AD.sub.12O.sub.19:Mn,R scintillator may also be used as
a gamma ray camera or an electromagnetic calorimeter. In a gamma
ray camera, the scintillator absorbs gamma rays and emits green
light to expose a film. In an electromagnetic calorimeter, the
scintillator absorbs high energy incident radiation, such as gamma
rays collected by a telescope or positrons emitted by a positron
source, and emits green light. Incident radiation from a distal
radiation source enters the housing through an aperture in the
housing. In these applications, this aperture may be considered as
the source of incident radiation for the scintillator. Of course
the AD.sub.12O.sub.19:Mn,R phosphor and scintillator may be used in
applications other than those described above.
[0059] The AD.sub.12O.sub.19:Mn,R phosphor may be made by any
ceramic powder method, such as a liquid phase (flux) method or a
solid state method. Preferably, the method of making the phosphor
comprises the following steps. First, compounds of the phosphor
material are mixed in a crucible or another suitable container,
such as a ball mill. For example, the starting materials may be
blended using a ball mill with ZrO.sub.2 or yttrium toughened
zirconia milling media. The preferred starting phosphor compounds
comprise oxides, carbonates, hydroxides, nitrates or oxalates of
the metal constituents. For example, to form
(Sr.sub.1-m-q-r-tCe.sub.m-
Pr.sub.qGd.sub.rTb.sub.t)(Al.sub.12-e-gMg.sub.eMn.sub.g)O.sub.19,
strontium carbonate (SrCO.sub.3), aluminum oxide (alumina,
Al.sub.2O.sub.3) or aluminum hydroxide (Al(OH).sub.3), praseodymium
oxide (Pr.sub.6O.sub.11), cerium oxide (Ce.sub.2O), gadolinium
oxide (Gd.sub.2O.sub.3), terbium oxide (Tb.sub.4O.sub.7), magnesium
carbonate (MgCO.sub.3) or magnesium oxide (MgO), and manganese
oxide or carbonate (MnCO.sub.3) may be mixed in the crucible or
ball mill.
[0060] The blended materials are then fired in a reducing
atmosphere for 5-15 hours at 1,400 to 1600.degree. C., preferably
for 10 hours at 1500.degree. C. to sinter the material. The
reducing atmosphere may comprise forming gas (2 percent hydrogen
and 98 percent nitrogen). Preferably, the starting materials
also
[0061] contain a flux that promotes the reaction of the starting
materials during the firing step to form the ceramic phosphor.
Preferably, the flux comprises a halogen compound, such as a
fluoride or a chloride compound. The preferred halogen compounds
comprise magnesium, aluminum or strontium fluoride or magnesium,
strontium, manganese or ammonium chloride. However, the phosphor
may be fired without adding a flux. The fired mixture is then
coated onto the substrate, such as a display screen or a lamp bulb.
Preferably, a suspension of the mixture particles and a liquid is
used to coat the substrate.
[0062] The AD.sub.12O.sub.19:Mn,R scintillator may be made by any
crystal growth method. Preferably, the scintillator is made by
either the Bridgeman-Stockbarger method or the Czochralski method.
A schematic of the Bridgeman-Stockbarger crystal growth method is
shown in FIG. 9. A solid AD.sub.12O.sub.19:Mn,R material is placed
in contact with a single crystal seed 61 in a housing or container.
The seed 61 may comprise AD.sub.12O.sub.19:Mn,R or another material
with a magnetoplumbite crystal structure. The solid material is
then placed into a high temperature zone 62. The high temperature
zone may comprise a resistance or lamp heater. The heater may have
a bar or strip shape that melts only a portion of the solid
material or it may be a furnace that melts the entire solid
material to form a melt region 64. The seed 61 and the melt region
64 are then moved into a low temperature zone 63. If the high
temperature zone 62 comprises a bar shaped heater, then the low
temperature zone 63 comprises the area away from the heater. If the
high temperature zone 62 comprises a furnace, then the low
temperature zone 63 may be an area outside the furnace or a second
furnace set to a lower temperature than the first furnace. The two
furnaces are preferably separated by an insulating material 65.
When the melt region 64 reaches the low temperature zone, it
solidifies as a single crystal 66 that has the same lattice and
orientation as the seed 61. The seed and the solid material may be
moved relative to a stationary heater or furnace. Alternatively,
the heater or furnace may be moved relative to a stationary seed
61. The relative movement may be vertical, horizontal or in any
other direction.
[0063] A schematic of the Czochralski crystal growth method is
shown in FIG. 10. The starting materials, comprising strontium,
aluminum, oxygen, and at least one of magnesium, manganese,
gallium, boron, barium, calcium, cerium, praseodymium, gadolinium
and terbium are placed in a crucible 71 and heated to form a
reactant melt 72. The crucible is located in a housing, such as a
quartz tube 73, and heated by r.f. or resistance heaters 74. The
melt temperature is determined by a thermocouple 75. A single
crystal seed 76 attached to a seed holder 77 is lowered into the
melt (the melt is the high temperature zone). As the seed 76 is
rotated about its axis and lifted from the melt 72, a single
crystal scintillator boule 78 forms below the seed. The size of the
crystal boule 78 increases as the seed 76 is lifted further away
from the melt 72 toward the low temperature zone above the heaters
74. The boule is then sliced and polished into scintillator
crystals.
SPECIFIC EXAMPLE
[0064] A
(Sr.sub.0.5Ce.sub.0.3Gd.sub.0.1Tb.sub.0.1)(Al.sub.11.5Mg.sub.0.25-
Mn.sub.0.25)O.sub.19 phosphor was made by the following method.
Stoichiometric amounts of oxide and carbonate starting materials
(SrCO.sub.3, Gd.sub.2O.sub.3, CeO.sub.2, Tb.sub.4O.sub.7,
Al.sub.2O.sub.3, MnCO.sub.3 and MgO) were well blended and fired at
1000.degree. C. for five hours under a slightly reducing atmosphere
(97% N.sub.2 and 2% H.sub.2 forming gas). The partially reacted
material was reground after cooling to room temperature under the
same reducing atmosphere. The reground material was reheated to
1550.degree. C. under the same atmosphere. The resultant phosphor
luminesced bright green under short wavelength UV excitation.
Aluminum or magnesium fluoride fluxes may also be added to the
starting materials to promote the reaction between the starting
materials.
[0065] The (Sr.sub.0.5Ce.sub.0.3Gd.sub.0.1Tb.sub.0.1
)(Al.sub.11.5Mg.sub.0.25Mn.sub.0.25)O.sub.19 phosphor of the
present invention and a Zn.sub.2SiO.sub.4:Mn.sup.2+ prior art
phosphor were irradiated with 254 nm incident radiation, and their
emission spectra were measured with a spectrometer. The spectra are
shown in FIG. 11. The
(Sr.sub.0.5Ce.sub.0.3Gd.sub.0.1Tb.sub.0.1)(Al.sub.11.5Mg.sub.0.25Mn.sub.0-
.25)O.sub.19 phosphor exhibited a maximum emission wavelength of
517 nm. This wavelength in the green spectral range is only 6 nm
away from the Zn.sub.2SiO.sub.4:Mn.sup.2+ maximum emission
wavelength of 523 nm.
[0066] The color coordinates of the
(Sr.sub.0.5Ce.sub.0.3Gd.sub.0.1Tb.sub.-
0.1)(Al.sub.11.5Mg.sub.0.25Mn.sub.0.25)O.sub.19 phosphor and the
Zn.sub.2SiO.sub.4:Mn.sup.2+ prior art phosphor are described in the
table below and are shown graphically on the CIE chromaticity
diagram in FIG. 12.
1 TABLE PHOSPHOR x y
(Sr.sub.0.5Ce.sub.0.3Gd.sub.0.1Tb.sub.0.1)(Al.sub.11.5Mg.sub.0.25Mn.sub.0-
.25)O.sub.19 0.173 0.736 Zn.sub.2SiO.sub.4:Mn.sup.2+ 0.249
0.694
[0067] The chromaticity color coordinates x and y are known in the
phosphor art, and are defined for example in a textbook by K. H.
Butler, "Fluorescent Lamp Phosphors, Technology and Theory" (Penn.
State U. Press 1980), pages 98-107. The solid curve in the diagram
shows the monochromatic emission wavelength corresponding to
certain x and y coordinates. As seen in FIG. 12, the
(Sr.sub.0.5Ce.sub.0.3Gd.sub.0.1Tb.su-
b.0.1)(Al.sub.11.5Mg.sub.0.25Mn.sub.0.25)O.sub.19 phosphor
(filled-in circle 81) which has a peak emission wavelength in the
green range, has a more saturated green luminescence than the prior
art Zn.sub.2SiO.sub.4:Mn.sup.2+ phosphor (open circle 82) which has
a peak emission wavelength in the green-yellow range. Furthermore,
the two phosphors have an equivalent absolute quantum efficiency.
Therefore, the AD.sub.12O.sub.19:Mn,R phosphor may replace the
Zn.sub.2SiO.sub.4:Mn.sup.- 2+ phosphor in virtually all
applications. It should be understood that the example above is
meant to merely illustrate the present invention and should not be
deemed as limiting the scope of the claims.
[0068] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope of the
invention.
* * * * *