U.S. patent application number 10/336007 was filed with the patent office on 2004-07-08 for fluorescent powder for fabricating high brightness white light emitting diodes.
Invention is credited to Chi, Liang-Sheng, Lin, Yi-Shan, Liu, Ru-Shi, Wang, Chien-Yuan.
Application Number | 20040130264 10/336007 |
Document ID | / |
Family ID | 32322980 |
Filed Date | 2004-07-08 |
United States Patent
Application |
20040130264 |
Kind Code |
A1 |
Liu, Ru-Shi ; et
al. |
July 8, 2004 |
FLUORESCENT POWDER FOR FABRICATING HIGH BRIGHTNESS WHITE LIGHT
EMITTING DIODES
Abstract
A fluorescent material with
formula(Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.sub.1- 2, where x+y=3, x,
y.noteq.0, 0<z<0.5, M is selected from the group consisting
of Tb, Lu, and Yb. The fluorescent material has a host
(Y.sub.xM.sub.y)Al.sub.5O.sub.12 and an activator: Ce. By changing
the composition of metals in the host, the crystal field thereof
can be modulated, thereby altering the interaction force between
the activator and the other components, and changing the wavelength
of emitted colors. The present invention provides a white
light-emitting device comprising a light emitting diode and
above-mentioned fluorescent powder. The LED emits purple-blue or
blue light having a domination wavelength AD range from 430 nm to
500 nm. The fluorescent powder is excited by the purple-blue or
blue light emitted from the LED to generate green-yellow light to
orange-yellow light having a wavelength range from 560 nm to 590 nm
and mixed with the light of LED to form white light.
Inventors: |
Liu, Ru-Shi; (Hsinchu Hsien,
TW) ; Lin, Yi-Shan; (Nan Tou Hsien, TW) ; Chi,
Liang-Sheng; (Yun-Lin Hsien, TW) ; Wang,
Chien-Yuan; (Kaohsiung Hsien, TW) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
32322980 |
Appl. No.: |
10/336007 |
Filed: |
January 3, 2003 |
Current U.S.
Class: |
313/512 |
Current CPC
Class: |
Y02B 20/00 20130101;
Y02B 20/181 20130101; C09K 11/7774 20130101; H05B 33/14 20130101;
H01L 33/502 20130101 |
Class at
Publication: |
313/512 |
International
Class: |
H05B 033/00 |
Claims
What is claimed is:
1. A composition of a fluorescent material having a formula
(Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.sub.12, where x+y=3, x,
y.noteq.0, 0<z<0.5, M is selected from the group consisting
of Tb, Lu, and Yb, wherein the (Y.sub.xM.sub.y)Al.sub.5O.sub.12 is
a host thereof, and Ce is a activator thereof, and wherein by
adjusting the metal component of the
(Y.sub.xM.sub.y)Al.sub.5O.sub.12 host of the fluorescent material,
a crystal field thereof can be modulated, thereby changing
wavelength of light emitted from the fluorescent material.
2. The composition of a fluorescent material as claimed in claim 1
wherein the fluorescent material is synthesized by raw powders
including oxides, nitrates of metal compounds, organic metal
compounds, or salts of metals.
3. The composition of a fluorescent material as claimed in claim 1
wherein the synthesis of the fluorescent material is done by using
solid state sintering or chemical processes.
4. The composition of a fluorescent material as claimed in claim 3
wherein the chemical processes comprises Sol-Gel method and
co-precipitation.
5. A white light emitting device comprising a fluorescent material
having a formula (Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.sub.12 in
combination with a light emitting diode (LED) capable of emitting
light having predetermined wavelength and brightness serving as an
excitation light source, and after packaging the fluorescent
material and the LED and applying current thereon, light generated
by the fluorescent material and light generated by the LED are
mixed to form white light.
6. The white light emitting device as claimed in claim 5 wherein
the formula (Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.sub.12 where x+y=3,
x, y.noteq.0, 0<z<0.5, M is selected from the group
consisting of Tb, Lu, and Yb, wherein the
(Y.sub.xM.sub.y)Al.sub.5O.sub.12 is a host thereof, and Ce is an
activator thereof.
7. The white light emitting device as claimed in claim 5 wherein
the LED emits light having a domination wavelength .lambda.D range
from 430 nm to 500 nm.
8. The white light emitting device as claimed in claim 5 wherein
the fluorescent material emits light having a domination wavelength
.lambda.D range from 560 nm to 590 nm.
9. The white light emitting device as claimed in claim 5 wherein
the mixture ratio of the fluorescent material and the wavelength
and the brightness of the LED can be determined by chromaticity.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a fluorescent material for
fabricating high brightness white light emitting diodes. More
particularly, the present invention relates to a fluorescent
material having a formula (Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.sub.12,
in combination with purple-blue light or blue light emitting
diodes, for generating a high brightness white light emitting diode
device.
[0003] 2. Description of the Prior Art
[0004] In 1996, a Japanese company, Nichia Kagaku Kogyo Kabushiki
Kaisha (Tokushima), which is also known as "Nichia Chemical" on the
market, disclosed a method for generating white light by using a
blue light emitting diode (LED) that emits blue light absorbed by a
fluorescent material to emit yellowish light. The yellowish light
is diffused and mixed with blue light to eventually generate high
brightness white light. This new technique has ushered a new era of
white LED illumination and is believed that will soon replace the
conventional fluorescent lamps in the near future. In Taiwan Patent
No. 1,561,77I and also in U.S. Pat. No. 5,998,925, assigned to
Nichia Kagaku Kogyo Kabushiki Kaisha, disclose a yellow light
YAG:Ce fluorescent powder, which has a general formula
(Y.sub.1-p-q-rGd.sub.pCe.sub.qSm.sub.r).sub.3(Al.sub.1-sGa.sub.s).sub.5O.-
sub.12, where 0.ltoreq.p.ltoreq.0.8, 0.003.ltoreq.q.ltoreq.0.2,
0.003.ltoreq.r.ltoreq.0.08, 0.ltoreq.s.ltoreq.1. The blue LED
irradiates a fluorescent material to general yellowish light that
is complementary to the blue light. The blue light and the
yellowish light are then mixed using lens theory to general visible
light that is white color to human eye. This can be further
explained by using a so-called C.I.E. chromaticity diagram. When
wavelength of light emitted by the semiconductor light-emitting
component corresponds to a point on a straight line connecting
point A (blue light) and point B (yellowish light) in a
chromaticity diagram, light of white color can be emitted.
[0005] It is known that the white light is mixed light of different
colors. The white light, which is sensed by human eye as white
color, at least includes two or more colors of light having
different wavelengths. For example, when human eye is stimulated,
at the same time, by the Red, Green, and Blue colors of light, or
by blue light and yellowish light, a white color is sensed.
Accordingly, there have been three major approaches to the
formation of white light for now. The first is using R/G/B LEDs. By
controlling the current passing the LED to generate white light.
The second is using yellow/blue LEDs to generate white light. These
two prior art methods has a common drawback in that when quality of
one of the plural LEDs deteriorates, an accurate white light is no
longer obtained. Furthermore, using plural LEDs is costly. Another
known approach is using InGaN LED, which generates blue light that
can be absorbed by fluorescent dye or powders to emit yellowish
light, that is mixed with blue light to produce white light. This
newly developed has no disadvantage of the former two prior art
methods as described above. Besides, such LED has a simpler driving
circuit and can be made by simple manufacturing process. Further,
such InGaN LED has low power consumption and cost. As a result, the
third approach (InGaN LED) is widely used for various white LED
applications. However, so far, since most commercial InGaN type
blue LED is made by using metal organic chemical vapor deposition
(MOCVD), only blue LED with fixed wavelength can be obtained. There
has been a strong need for providing a series of yellow light
fluorescent powders capable of modulating emitted blue light
wavelengths in a range of from 430 nm to 490 nm.
[0006] As disclosed in Taiwan Patent No. 1,561,77I and also in U.S.
Pat. No. 5,998,925, the light color modulating method used by
Nichia Chemical includes the step of adding a hetero ion. For
example, when the general formula
(Y.sub.1-p-q-rGd.sub.pCe.sub.qSm.sub.r).sub.3(Al.sub.1-sGa.sub.s)-
.sub.5O.sub.12 is (Y.sub.1-qCe.sub.q).sub.3Al.sub.5O.sub.12, 546 nm
yellowish light is obtained. But, after adding Gd into this
formula, the resulting
(Y.sub.1-p-qGd.sub.pCe.sub.q).sub.3Al.sub.5O.sub.12 formula can red
shift the main wavelength to 556 nm.
SUMMARY OF THE INVENTION
[0007] The main objective of the invention is to provide an
alternative approach to produce red shift. This is achieved by
adjusting magnitude of crystal field of the host of a fluorescent
material. The electron configuration of three valance Ce is
[Xe]4f.sup.1, wherein the 4f orbital is split by spin-orbitalal
coupling into .sup.2F.sub.5/2 and .sup.2F.sub.7/2, and wherein the
5d orbital is split due to crystal field interactions. As the
crystal field increases, the level of the split 5d orbital also
increases. This results in the reduced energy difference between
the lowest energy level of the 5d orbital and the 4f orbital of the
excited Ce.sup.3+, thereby generating red shift of light.
[0008] In accordance with the present invention, a composition of a
fluorescent material having a formula
(Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.s- ub.12 is provided, wherein
x+y=3, x, y.noteq.0, 0<z<0.5, M is selected from the group
consisting of Tb, Lu, and Yb, wherein the
(Y.sub.xM.sub.y)Al.sub.5O.sub.12 is a host thereof, and Ce is an
activator thereof, and wherein by adjusting the metal component of
the (Y.sub.xM.sub.y)Al.sub.5O.sub.12 host of the fluorescent
material, a crystal field thereof can be modulated, thereby
changing wavelength of light emitted from the fluorescent
material.
[0009] Other objects, advantages and novel features of the
invention will become more clearly and readily apparent from the
following detailed description when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 illustrates respective excitation spectrum (on the
left) and emission spectrum (on the right) of the synthesized
powder (Y.sub.1.80Tb.sub.1.20Ce.sub.0.05)Al.sub.5O.sub.12 of the
preferred embodiment 1, where spectrum A corresponds to 470 nm blue
light excitation, spectrum B corresponds to 460 nm blue light
excitation.
[0011] FIG. 2 is emission spectrum where spectrum C is the emission
spectrum corresponding to the (Y.sub.3Ce.sub.0.05)Al.sub.5O.sub.12
reference powder of the reference example, the spectrum D
represents the emission spectrum of
(Y.sub.2.375Tb.sub.0.625Ce.sub.0.05)Al.sub.5O.sub.12 fluorescent
powder of the second preferred embodiment, and spectrum E
corresponds to the
(Y.sub.1.80Tb.sub.1.20Ce.sub.0.05)Al.sub.5O.sub.12 fluorescent
powder of the first preferred embodiment.
[0012] FIG. 3 illustrates red shift of the spectrum when increasing
weight of Tb, where the F point presents the spectrum C in FIG. 2,
the G point presents the D spectrum in FIG. 2, and the H point
presents the E spectrum in FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] The present invention pertains to a method of fabricating
high brightness white light emitting diodes with fluorescent
powders. The fluorescent powder in accordance with one preferred
embodiment has a formula (Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.sub.12
where x+y=3, x, y.noteq.0, 0<z<0.5, M is selected from the
group consisting of Tb, Lu, and Yb. According to this invention, a
purple-blue light emitting diode or a blue light emitting diode is
incorporated with a fluorescent powder having the above-mentioned
formula (Y.sub.xM.sub.yCe.sub.z)Al.sub.- 5O.sub.12, where x+y=3, x,
y.noteq.0, 0<z<0.5, M is selected from the group consisting
of Tb, Lu, and Yb, whereby generating high brightness white color
light.
[0014] Now, several preferred examples embodying the present
invention for preparing such fluorescent powder having the formula
(Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.sub.12 and one reference example
will be disclosed in details.
[0015] Preferred Embodiment 1:
[0016] 1. A mixture for forming a composition having a
stoichiometry of (Y.sub.1.80Tb.sub.1.20Ce.sub.0.05)Al.sub.5O.sub.12
is prepared by mixing and grinding 3.1750 grams of
Y(NO.sub.3).sub.3 6H.sub.2O, 8.6400 grams of Al(NO.sub.3).sub.3
9H.sub.2O, 0.1000 grams of Ce(NO.sub.3).sub.3 6H.sub.2O, and 8.6400
grams of Tb.sub.4O.sub.7.
[0017] 2. The mixture of Step 1 is placed in a crucible and is
calcined in air at 1000.degree. C. with a heating rate of 5.degree.
C./min for 24 hours and followed by cooling down at a cooling rate
of 5.degree. C./min to form intermediate powders.
[0018] 3. The intermediate powders of Step 2 is again placed in a
crucible and is sintered in air at 1500.degree. C. with a heating
rate of 5.degree. C./min for 24 hours.
[0019] 4. The sintered powders of Step 3 is reduced in a
H.sub.2/N.sub.2 (5%/95%) reductive ambient at 1500.degree. C. for
12 hours. This reduces Ce.sup.4+ to Ce.sup.3+. It is noted that
this step, which can improve light brightness, is optional.
[0020] Preferred Embodiment 2:
[0021] 1. A mixture for forming a composition having a
stoichiometry of
(Y.sub.2.375Tb.sub.0.625Ce.sub.0.05)Al.sub.5O.sub.12 is prepared by
mixing and grinding 4.1897 grams of Y(NO.sub.3).sub.3 6H.sub.2O,
8.6400 grams of Al(NO.sub.3).sub.3 9H.sub.2O, 0.1000 grams of
Ce(NO.sub.3).sub.3 6H.sub.2O, and 0.2836 grams of
Tb.sub.4O.sub.7.
[0022] 2. The mixture of Step 1 is placed in a crucible and is
calcined in air at 1000.degree. C. with a heating rate of 5.degree.
C./min for 24 hours and followed by cooling down at a cooling rate
of 5.degree. C./min to form intermediate powders.
[0023] 3. The intermediate powders of Step 2 is again placed in a
crucible and is sintered in air at 1500.degree. C. with a heating
rate of 5.degree. C./min for 24 hours.
[0024] 4. The sintered powders of Step 3 is reduced in a
H.sub.2/N.sub.2 (5%/95%) reductive ambient at 1500.degree. C. for
12 hours.
[0025] Reference Example:
[0026] 1. A mixture for forming a reference composition having a
stoichiometry of (Y.sub.3Ce.sub.0.05)Al.sub.5O.sub.12 is prepared
by mixing and grinding 5.2923 grams of Y(NO.sub.3).sub.3 6H.sub.2O,
8.6400 grams of Al(NO.sub.3).sub.3 9H.sub.2O, 0.1000 grams of
Ce(NO.sub.3).sub.3 6H.sub.2O
[0027] 2. The mixture of Step 1 is placed in a crucible and is
calcined in air at 1000.degree. C. with a heating rate of 5.degree.
C./min for 24 hours and followed by cooling down at a cooling rate
of 5.degree. C./min to form intermediate powders.
[0028] 3. The intermediate powders of Step 2 is again placed in a
crucible and is sintered in air at 1500.degree. C. with a heating
rate of 5.degree. C./min for 24 hours.
[0029] 4. The sintered powders of Step 3 is reduced in a
H.sub.2/N.sub.2 (5%/95%) reductive ambient at 1500.degree. C. for
12 hours.
[0030] The fluorescent powders of the above three examples are all
cooled down to room temperature and are ground, respectively, with
mortars and pestles. The light emitting properties of these powders
are analyzed by using a spectrophotometer known in the art.
[0031] Please refer to FIG. 1 to FIG. 3. FIG. 1 illustrates
respective excitation spectrum (on the left) and emission spectrum
(on the right) of the synthesized powder
(Y.sub.1.80Tb.sub.1.20Ce.sub.0.05)Al.sub.5O.sub.12 of the preferred
embodiment 1, where spectrum A corresponds to 470 nm blue light
excitation, spectrum B corresponds to 460 nm blue light excitation.
As shown in FIG. 11, the light efficiency of the spectrum A (470 nm
light excitation) is higher than that of the spectrum B (460 nm
light excitation). It is unexpectedly found that the fluorescent
powder according to this invention has improved light efficiency
when excited by blue light having a relatively longer
wavelength.
[0032] In FIG. 2, spectrum C is the emission spectrum corresponding
to the (Y.sub.3Ce.sub.0.05)Al.sub.5O.sub.12 reference powder of the
reference example, the spectrum D represents the emission spectrum
of (Y.sub.2.375Tb.sub.0.625Ce.sub.0.05)Al.sub.5O.sub.12 fluorescent
powder of the second preferred embodiment, and spectrum E
corresponds to the
(Y.sub.1.80Tb.sub.1.20Ce.sub.0.05)Al.sub.5O.sub.12 fluorescent
powder of the first preferred embodiment. As shown in FIG. 2, the
reference example, (Y.sub.3Ce.sub.0.05)Al.sub.5O.sub.12 excited by
470 nm blue light, has an emission spectrum peak at about 546 nm.
The second preferred embodiment,
(Y.sub.2.375Tb.sub.0.625Ce.sub.0.05)Al.sub.5O.sub.1- 2, has an
emission spectrum peak at about 548 nm. The first preferred
embodiment, (Y.sub.1.80Tb.sub.1.20Ce.sub.0.05)Al.sub.5O.sub.12, has
an emission spectrum peak at about 552 nm. FIG. 3 illustrates red
shift of the spectrum when increasing weight of Tb. It is verified
from these figures that changes of the metal components of the host
of the fluorescent powder can modulate crystal field of the
compound and change interaction force relating to the activator,
thereby resulting in changes of wavelengths.
[0033] In FIG. 3, the chromaticity diagram plotted by different
Tb/Y ratio is depicted, wherein the F point presents the spectrum C
in FIG. 2, the G point presents the D spectrum in FIG. 2, and the H
point presents the E spectrum in FIG. 2. According to the CIE
chromaticity diagram of FIG. 3, as the Tb increases, shift to long
wavelength is observed.
[0034] To sum up, the present invention provides a fluorescent
material (Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.sub.12, where x+y=3, x,
y.noteq.0, 0<z<0.5, M is selected from the group consisting
of Tb, Lu, and Yb. The host
(Y.sub.xM.sub.yCe.sub.z)Al.sub.5O.sub.12 has an activator: Ce. By
changing the composition of metals in the host
(Y.sub.xM.sub.yCe.sub.z- )Al.sub.5O.sub.12, it is possible to
modulate the crystal field thereof, thereby altering the
interaction force between the activator and the other components.
This causes the different split 5d atomic orbitals of Ce. When the
Ce ion is excited from its ground state (4f) to its excitation
state (5d), the change of the crystal field results in the change
of wavelength of the emitted photons and colors thereof.
[0035] The present invention is also directed to a white
light-emitting device comprising a light emitting diode and the
above-mentioned fluorescent powder. The white light-emitting device
can be an excitation light source, which emits purple-blue or blue
light having a domination wavelength .lambda.D range from 430 nm to
500 nm, in combination with suitable fluorescent powder that can be
excited by the purple-blue or blue light emitted from the LED to
generate green-yellow light to orange-yellow light having a
wavelength range from 560 nm to 590 nm. The light generated by the
LED and the light generated by the fluorescent powder are mixed to
produce white light.
[0036] According to the method disclosed in this application, the
purple-blue or blue light is generated by low power consumption
light emitting diodes in combination with a suitable fluorescent
material. After packaging, a high brightness white LED with good
light properties operated at very low voltage is obtained.
[0037] The fluorescent powder of this invention features its
different wavelengths of the emitted light that can be obtained by
simply adjusting the components of the host. The fabrication
processes of the fluorescent material of this invention is simple
and is thus suited for mass production.
[0038] In contrast to the prior art fluorescent material used in
conventional white LED devices, the present invention has the
following advantages:
[0039] 1. Since the fabrication process of the short-wavelength
blue LED is much more difficult than the fabrication process of
long-wave-length blue LED, the fluorescent powder of this invention
can be collocated with blue LED whose emitting light is modulated
to longer wavelength. Also, according to the excitation spectrum,
the light efficiency using 470 nm blue light is higher than that
using 460 nm short wavelength blue light.
[0040] 2. The present invention has higher reliability. According
to this invention, the color of light is changed by adjusting
component of host crystal. Compared to the prior art method that
uses changes of hetero ion weight. This might cause undesired light
color shift error introduced from the step of measuring the weight
of raw materials.
[0041] It is to be understood, however, that even though numerous
characteristics and advantages of the present invention have been
set forth in the foregoing description, together with details of
the structure and function of the invention, the disclosure is
illustrative only, and changes may be made in detail, especially in
matters of shape, size, and arrangement of parts within the
principles of the invention to the full extent indicated by the
broad general meaning of the terms in which the appended claims are
expressed.
* * * * *