This application is a divisional of a pending application, U.S. Ser. No. 12/300,127 filed on Nov. 10, 2008, which is the National Stage Application of PCT International Application No. PCT/JP2007/059527, both of which are hereby incorporated by reference in their entireties.
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The present invention relates to sialon phosphor excited by ultraviolet light or blue light to emit visible light and the manufacture method thereof as well as an illuminator and a luminescent element using the same. In particular, the present invention relates to phosphor that can be used for a blue light-emitting diode (blue LED) or an ultraviolet light-emitting diode (ultraviolet LED) and the manufacture method thereof as well as an illuminator and a luminescent element using the same, in particular, a white light-emitting diode (white LED).
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Phosphor is well-known that uses silicate, phosphate, aluminate, and sulfide as host material and that uses transition metal or rare-earth metal for the luminescent center.
On the other hand, such white LED has attracted attention that is excited by an excitation source having high energy (e.g., ultraviolet light or blue light) to emit visible light and has been developed. However, when the above-described conventional phosphor is used, the exposure to the excitation source disadvantageously causes a decreased phosphor luminance.
As phosphor having a small decrease in the luminance, nitride or oxynitride phosphor has attracted attention recently as material that has a stable crystal structure and that can shift the excitation light and the light emission toward the long wavelength.
As nitride and oxynitride phosphor, α-type sialon (Si—Al—O—N) for which a specific rare-earth element is activated has been known as having a useful fluorescence characteristic and has been examined for the use to white LED or the like (see Patent Documents 1 to 5 and Non-Patent Documents 1 and 2).
The α-type sialon has a structure in which the Si—N bonds of α-type silicon nitride crystal is partially substituted with Al—N bonds and Al—O bonds and electroneutrality is maintained by a specific element (Ca, Li, Mg, and Y or lanthanide metal except for La and Ce) interstitially solid-soluted to a crystal lattice. A part of the interstitially solid-soluted element is a rare-earth element functioning as the luminescent center to cause the fluorescence characteristic.
The α-type sialon is obtained by burning mixed powders consisting of silicon nitride, aluminum nitride, optionally aluminum oxide, and the oxide of an interstitially solid-soluted element or the like in nitrogen at a high temperature. The proportion between silicon nitride and aluminum compound, the type of an interstitially solid-soluted element, and the ratio of an element functioning as the luminescent center for example can provide various fluorescence characteristics. In particular, α-type sialon, which is obtained by solid-soluting Ca functioning as an interstitially solid-soluted element and Eu functioning as the luminescent center, is efficiently excited in a wide wavelength range from a ultraviolet region to a blue region and emits light in a range from yellow to orange. Thus, the development of a combination of this α-type sialon with an LED emitting blue light (which is a complementary color to yellow to orange) has been expected for white LED.
Ca2(Si, Al)5N8, CaSiAlN3 or β-type sialon obtained by activating a rare-earth element also has been found to have the similar fluorescence characteristic (see Patent Documents 6 and 7 and Non-Patent Documents 2 and 3).
In addition, nitrides (e.g., aluminum nitride, silicon nitride magnesium, silicon nitride calcium, silicon nitride barium, gallium nitride, silicon nitride zinc) and oxynitride phosphor (hereinafter also referred to as nitride phosphor and oxynitride phosphor in this order) have been suggested.
In the case of the α-type sialon powders for example, the reduction-nitridation method has been known as a synthesis method of these phosphors. According to the reduction-nitridation method, mixed powders of aluminum oxide (Al2O3), silicon oxide (SiO2), oxide of metal or element that can be solid-soluted into the lattice or the like is subjected to a heating processing in nitrogen atmosphere under the existence of carbon (see Non-Patent Documents 4 to 6).
Although the methods reported in Non-Patent Documents 4 to 6 are characterized in that raw material powders are low-cost and can be synthesized at a relatively low temperature of about 1500 degrees C., a plurality of intermediates are caused in the synthesis and gas components such as SiO and CO are generated to difficultly provide the single-phase one, causing a difficulty in the strict control of the composition and the control of the particle size.
Sialon powders are also obtained by burning the mixture of silicon nitride, aluminum nitride, and the oxide of metal or an element or the like solid-soluted into the lattice at a high temperature to grind the resultant sintered compact. However, this has caused a problem of a decreased light emission intensity of phosphor due to a grinding operation.
As described above, in the conventional technique, nitride including a constituting element and a compound including an activating element are merely mixed and heated or the mixture of oxides of the constituting elements is merely subjected to reduction-nitridation by carbon or the like. This conventional technique cannot provide nitride phosphor or oxynitride phosphor having a sufficient characteristic.
In the case of sialon phosphor in particular, when a manufacture method is used in which oxides including the constituting elements of solid-soluted elements (e.g., calcium or yttrium) or the activating element (e.g., cerium, europium) are used as raw material, a burning process causes a liquid-phase sintering to cause a stronger binding among particles. This has caused a case where a grind processing under severe conditions may be required in order to obtain powders having a target particle size. In this case, the grind processing under severe conditions cause an increase in the contamination and introduce defects into the surfaces of the respective particles, thereby disadvantageously causing a deteriorated light emission characteristic.
In order to solve this problem, the present inventors have suggested a manufacture method rarely requiring a grind processing by using raw material not including oxygen (e.g., raw material such as calcium fluoride or calcium cyanamide) and by devising a method for mixing raw materials for a burning process for example. Thus, the present inventors could improve the light emission intensity (see Patent Documents 8 and 9).
In order to realize white, a combination of a plurality of colors different from a monochromatic light is required. A general white LED is composed of a combination of ultraviolet LED or blue LED and phosphor that uses the light from the LED as an excitation source and that emits visible light (see Patent Documents 10 and 11 for example). Thus, in order to improve the white LED efficiency, it is required to improve the luminous efficiency of the ultraviolet LED or the blue LED itself and to improve the efficiency of phosphor in the LED. It is also required to improve an efficiency at which emitted light is taken out to the outside. In order to increase use of white LED including a general lighting use, all of these efficiencies must be improved.
Patent Document 1: Japanese Patent No. 3668770
Patent Document 2: Japanese Patent No. 2003-336059 A
Patent Document 3: Japanese Patent No. 2003-124527 A
Patent Document 4: Japanese Patent No. 2003-206481 A
Patent Document 5: Japanese Patent No. 2004-186278 A
Patent Document 6: Japanese Patent No. 2004-244560 A
Patent Document 7: Japanese Patent No. 2005-255895 A
Patent Document 8: Japanese Patent No. 2008-45271 A
Patent Document 9: Published Japanese translation of a PCT application No. 2005-123876
Patent Document 10: Japanese Patent No. H5-152609 A
Patent Document 11: Japanese Patent No. H7-099345 A
Non-Patent Document 1: J. W. H van Krebel “On New Rare-Earth Doped M—Si—Al——O—N Materials”, T U Eindhoven, The Netherlands, 145-161 (1998)
Non-Patent Document 2: Dai 52 kai Ouyou Butsurigaku Kankei Rengou Kouenkai Kouen Yokousyu (March 2005, Saitama University) P. 1614-1615
Non-Patent Document 3: Dai 65 kai Ouyou Butsurigakkai Gakujyutsu Kouenkai Kouen Yokousyu (September 2004, TOHOKU GAKUIN UNIVERSITY) No. 3, p. 1282-1284