Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2002-07-03
2003-12-02
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S483000, C313S485000, C313S486000
Reexamination Certificate
active
06657379
ABSTRACT:
TECHNICAL FIELD
The invention is based on an illumination unit having at least one LED as light source in accordance with the preamble of claim
1
. This is in particular an LED which emits in the visible or white region and is based on an LED which emits primarily UV/blue.
BACKGROUND ART
An illumination unit having at least one LED as light source, which emits, for example, white light, is currently obtained predominantly by combining a Ga(In)N-LED, which emits in the blue at approximately 460 nm, and a yellow-emitting YAG:Ce
3+
phosphor (U.S. Pat. No. 5,998,925 and WO 98/12757). For good color rendering, two different yellow phosphors are often used, as described in WO 01/08453. A problem in this case is that the two phosphors often have different temperature characteristics, even if their structures are similar. A known example is the yellow-luminescent Ce-doped Y garnet (YAG:Ce) and the (Y,Gd) garnet which, by comparison, is luminescent at a longer wavelength. This leads to fluctuations in the color locus and changes in the color rendering at different operating temperatures.
The publication “On new rare-earth doped M—Si—Al—O—N materials” by van Krevel, TU Eindhoven 2000, ISBN 90-386-2711-4, Chapter 11 has disclosed a class of phosphor materials which are known as sialons (&agr;-sialons), which represents a contraction of their structure. An emission in the range from 560 to 590 nm with excitation at 365 nm or 254 nm is achieved by means of doping with Eu.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide an illumination unit having at least one LED as light source, the LED emitting primary radiation in the range from 300 to 485 nm, this radiation being partially or completely converted into longer-wave radiation by phosphors which are exposed to the primary radiation of the LED, which is distinguished by a high level of constancy at fluctuating operating temperatures. A further object is to provide an illumination unit which emits white light and in particular has a good color rendering and a high output.
This object is achieved by the following features: An illumination unit having at least one LED as light source, the LED emitting primary radiation in the range from 300 to 485 nm, this radiation being partially or completely converted into longer-wave radiation by phosphors which are exposed to the primary radiation of the LED, wherein conversion takes place at least with the aid of a phosphor which emits yellow-orange with a peak emission wavelength at 540 to 620 nm and originates from the class of the Eu-activated sialons, the sialon corresponding to the formula M
p/2
Si
12−p−q
Al
p+q
O
q
N
16−q
:Eu
2+
, where M=Ca individually or in combination with at least one of the metals Sr or Mg, where q=0 to 2.5 and p=0.5 to 3.
Particularly advantageous configurations are given in the dependent claims.
According to the invention, the phosphor used for the LED-based illumination unit is a sialon which emits yellow-orange and originates from the class of the
Eu-activated sialons, the sialon corresponding to the formula M
p/2
Si
12−p−q
Al
p+q
O
q
N
16−q
:Eu
2+
, where M=Ca individually or in combination with Sr and Mg, where q=0 to 2.5 and p=0.5 to 3. It is preferable to select a high value for p, specifically p=2 to 3, and a relatively low value for q, specifically q=0 to 1. Instead of pure Al, it is possible in particular to use a mixture of Al and Ga, with the Ga forming up to 20 mol %.
The Eu content, which replaces some of the cation M, should be 0.5 to 15%, preferably 1 to 10%, of the M cation, so that the emission wavelength can be selected particularly accurately and the light efficiency can also be optimized. An increase in Eu content generally shifts the peak emission toward longer wavelengths. Surprisingly, it has been found that a changing concentration of the cation M also shifts the peak emission wavelength. At a low concentration of the M cation, good absorption by the Eu ion can be obtained by selecting the amount of the Eu ion to be over 10% of the M cation.
Particular advantages of this phosphor in connection with an LED-based illumination unit are its high efficiency, its excellent thermal stability (no sensitivity to changes in the operating temperature) and a surprisingly high luminescence extinction temperature, as well as the high color rendering which can be achieved thereby, in particular in combination with at least one further phosphor. The extinction temperature, i.e. the temperature at which the luminescence is destroyed on account of the heat supplied, is even so high as to lie outside the preselected measurement range (maximum 140° C.).
A further advantage of this class of phosphors is that the starting material (in particular Si
3
N
4
) is already present in extremely finely dispersed form.
Consequently, it is not necessary to mill the phosphor. By contrast, conventional phosphors, such as YAG:Ce have to be milled, so that they remain dispersed in the casting resin and do not sink to the bottom. This milling operation often leads to loss of efficiency. Despite having a fine grain size of the starting material, the phosphor according to the invention has a surprisingly high absorption. This phosphor therefore no longer has to be milled, with the result that one operation is saved and there are no efficiency losses. Typical mean grain sizes of the phosphor are 0.5 to 5 &mgr;M.
In addition to the production of a colored light source by excitation by means of UV radiation or blue primary emission from an LED, in particular the generation of white light with the aid of this phosphor offers advantages. This is achieved either with an UV-emitting LED as primary light source, by using at least two and preferably three phosphors. An alternative is to use a blue-emitting LED and one or two phosphors. Excellent results are achieved with a mixture of thermally stable garnet phosphor, preferably YAG:Ce, and an Eu-doped sialon.
White light with good color rendering is also achieved by the combination of a blue LED (e.g. primary emission at 450 to 485 nm), a green phosphor (emission between 490-525 nm) and a yellow-orange (YO) emitting phosphor (emission: 540-620 nm).
The YO phosphor used is M
p/2
Si
12−p−q
Al
p+q
O
q
N
16−q
:Eu
2+
. In this formula, M is Ca individually or in combination with Sr and/or Mg. This YO phosphor has an excellent thermal stability and very good luminescence at relatively high temperatures which are typical of LEDs: up to 80° C., it did not show any drop in luminescence within the scope of the measurement accuracy. By contrast, the conventional yellow phosphors have a significantly measurable drop in luminescence at 80° C.: this drop is 5% for YAG and 10-20% for (Y,Gd)AG.
When a blue LED is used as the primary light source, in particular mixtures of an Eu-doped sialon with a chlorosilicate (Eu-doped or Eu,Mn-doped) or with SrAl
2
O
4
:Eu
2+
achieve good color renderings of over Ra=75. Compared to a mixture of YAG and (Y,Gd)AG, the color rendering is almost identical; the efficiency is even slightly higher and the thermal extinction is considerably improved. If necessary, the color rendering in the red can be improved still further by the addition of a red phosphor, e.g. Sr
2
Si
5
N
8
:Eu
2+
or SrS:Eu
2+
.
A white mixture can also be produced on the basis of an UV-emitting LED by means of this Eu-doped sialon together with a blue phosphor, such as for example BaMgAl
10
O
17
:Eu
2+
(BAM) or (Ca,Sr,Ba)
5
(PO
4
)
3
Cl:Eu
2+
(SCAP). If necessary, the color rendering can be improved still further by adding a green phosphor (for example Eu-doped thiogallates or Sr aluminate) and a red phosphor (for example Eu-doped Sr nitride or Sr sulfide). A further possibility is to use the Eu-doped sialon as the only phosphor with excitation by a blue-emitting LED (peak emission of approximately 470 to 485 nm).
Depending on the Eu
2+
content
Ellens Andries
Huber Günter
Kummer Franz
Frishauf Holtz Goodman & Chick P.C.
Harper Holly
Patel Nimeshkumar D.
Patent-Treuhand-Gesellschaft fur Elektrische Gluehlampen mbH
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