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.
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).
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
Non-Patent Document 4: M. Mitomo et al., “Preparation of α-SiAlON Powders by Carbothermal Reduction and Nitridation” (Ceram. Int., 14, 43-48 (1988))
Non-Patent Document 5: J. W. T. van Rutten et al., “Carbothermal Preparation and Characterization of Ca-α-SiAlON” (J. Eur. Ceram. Soc., 15, 599-604 (1995))
Non-Patent Document 6: K. Komeya et al., “Hollow Beads Composed of Nanosize Ca α-SiAlON Grains” (J. Am. Ceram. Soc., 83, 995-997(2000))
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
White LED phosphor is generally obtained by dispersing micron size particles in sealing material such as epoxy resin or silicone resin. In the case of α-type sialon phosphor, the particles are the secondary particles obtained by sintering a plurality of minute primary particles. Although the size and the distribution or the like have been examined, no attention has been paid on the surface texture of the secondary particles.
However, even the devising as disclosed in Patent Documents 8 and 9 only provides the resultant phosphor particles consisting of the secondary particles to which the primary particles having a diameter of about 0.2 to 5 μm are fixed in an irregular manner. This has caused a case where the secondary particles having a complicated surface due to significant concavities and convexes or the interface among the primary particles in the secondary particles has light scattering or absorption, causing the phosphor to have a decreased luminous efficiency.
Furthermore, general raw material powders such as silicon nitride or aluminum nitride have an average diameter of 1 μm or less. When these raw material powders are used as raw material for a conventional method to synthesize nitride or oxynitride phosphor, the resultant fine particles inevitably have a wide particle size distribution. Since the resultant fine particles particularly have a diameter of a few μm or less that strongly scatter the visible light, a problem of a decreased luminous efficiency has been caused.
On the other hand, white LED obtained so far has a lower luminous efficiency than a fluorescence lamp. Thus, LED having a superior luminous efficiency, especially white LED in particular, than a fluorescence lamp has been strongly demanded from the viewpoint of energy conservation in the industry field. Although white LED using oxynitride or nitride phosphor such as sialon phosphor has a higher efficiency than an incandescent lamp, increased applications of the LED including general lighting applications require a further improvement in the luminous efficiency. Thus, an improved luminous efficiency of phosphor has been an important task for the industry field.
In view of the above problems, through various examinations of α-type sialon phosphor, it is the first objective of the present invention to provide a white LED, in particular, a white LED using blue LED or ultraviolet LED as a light source having a superior luminous efficiency that has a peak in a wavelength range from 540 to 600 nm and that has a superior luminous efficiency.
It is the second objective of the present invention to solve the above problem of the conventional technique to provide an LED having a superior luminous efficiency (e.g., a white LED, in particular, a white LED using blue LED or ultraviolet LED as a light source) as well as phosphor having a superior fluorescence characteristic suitable for the LED to the industry field.
Means for Solving Problem
The inventors have examined the phosphor composed of α-type sialon as host material. The inventors have reached the present invention by finding that a specific interstitially solid-soluted element of the α-type sialon can be used to set the crystal lattice size within an appropriate range to provide the flat and smooth surface of the secondary particles. This provides phosphor having a peak in wavelengths in a range from 540 to 600 nm and having superior luminous efficiency. This can be used to provide an illuminator having a superior light emission characteristic.
Furthermore, the present inventors have reached the present invention by finding that the addition of a seed particle as a core of the grain growth into a raw material powder and the use of a dense boron nitride crucible in a synthesis process can improve the flat and smooth surface of the secondary particles.
In order to achieve the first objective, the sialon phosphor of the present invention is characterized in sialon phosphor that includes, as a main component, α-type sialon represented by a general expression: (M1)x(M2)y(Si,Al)12(O,N)16 [wherein M1 is one or more types of elements selected from a group consisting of Li, Mg, Ca, Y, and lanthanide element (whereas except for La and Ce) and M2 is one or more types of elements selected from a group consisting of Ce, Pr, Eu, Tb, Yb, and Er, and 0.3≦X+Y≦1.5 and 0<Y≦0.7 are established], and the sialon phosphor is a powder having a specific surface area of 0.2 to 0.5 m2/g. The above configuration is characterized in that the α-type sialon phosphor has a lattice constant a in a range from 0.780 to 0.788 nm and a lattice constant c in a range from 0.565 to 0.573 nm.
The above configuration is characterized in that, when powders consisting of the α-type sialon are evaluated based on an X-ray diffraction method, crystal phases other than that of the α-type sialon preferably have diffraction intensities that are all 10% or less to a diffraction line intensity of a face (102) of the α-type sialon.
The above configuration is characterized in that the M1 includes at least Ca, the M2 includes at least Eu and 0<Y≦0.1 is established and, when ultraviolet light or visible light having wavelengths in a range from 250 to 500 nm is emitted as an excitation source to the sialon phosphor, the sialon phosphor shows a light emission characteristic having a peak in a wavelength range from 540 to 600 nm.
The method of manufacturing α-type sialon phosphor of the present invention represented by the above general expression is characterized in that starting raw material includes α-type sialon in an amount of 5 to 30 mass %. In the above configuration, the starting raw material preferably includes the α-type sialon having a specific surface area in a range from 0.5 to 2 m2/g.
Another method of manufacturing the above-described sialon phosphor of the present invention is characterized in that the starting raw material is filled in a boron nitride-made crucible having a density of 1.75 g/cm3 or more and is burned in nitride atmosphere. The crucible preferably consists of pyrolytic boron nitride (P—BN).
An illuminator of the present invention is characterized in being composed of a light source and phosphor and uses sialon phosphor which is at least the above mentioned sialon phosphor or sialon phosphor obtained by the above-mentioned manufacture method.
In order to allow phosphor consisting of sialon (hereinafter simply referred to as sialon phosphor) to have an improved light emission intensity, the inventors have subjected various particles to an analysis regarding the characteristics and compositions thereof. Through an experimental examination, the inventors have found that the control of the particle shape, diameter, composition distribution or the like of the phosphor particles is effective to improve the light emission intensity, thereby reaching the present invention.
In order to achieve the above second objective, the second sialon phosphor of the present invention is characterized in that particles constituting the phosphor has an average circularity degree of 0.75 or more, the phosphor has a particle size distribution D50 of 5 to 30 μm and D10 of 2.0 μm or more.
Another configuration of the sialon phosphor of the present invention is characterized in that the phosphor is composed of particles including light emission-related elements that have a low concentration in the interior of the particle and that have a high concentration at the outer periphery of the particle. In this configuration, the light emission-related element at the outer periphery of the phosphor particle preferably has a concentration 1.2 times or more higher than that of the light emission-related element at the interior of the particle.
In the above configuration, the phosphor is preferably α-type sialon represented by a general expression: (M1)X(M2)Y(Si)12−(m+n)(A1)m+n(O)n(N)16−n [wherein M1 is one or more types of elements selected from a group consisting of Li, Mg, Ca, Sr, Y, and lanthanide metal (whereas except for La and Ce) and M2 is one or more types of elements selected from Ce, Pr, Eu, Tb, Yb, and Er, and 0.3≦X+Y≦1.5, 0<Y≦0.7, 0.6≦m≦3.0, 0≦n≦2.5, X+Y=m/(average valence of M1 and M2)]. Preferably, the M1 is Ca and the M2 is Eu.
A method of manufacturing α-type sialon phosphor of the present invention is characterized in including: a step of mixing silicon-containing material, aluminum-containing material, and raw material including M1 (one or more types of elements selected from a group consisting of Li, Mg, Ca, Sr, Y, and lanthanide metal (except for La and Ce)) and M2 (one or more types of elements selected from Ce, Pr, Eu, Tb, Yb, and Er) to prepare granulated powders; and a step of heating the powders in a nitrogen gas atmosphere at 1500 to 2100 degrees C. to obtain α-type sialon phosphor. Preferably, previously-synthesized α-type sialon phosphor is added to the raw material and mixed with the raw material.
A method of manufacturing β-type sialon phosphor of the present invention is characterized in including: a step of mixing silicon-containing material, aluminum-containing material, and raw material including M3 (one or more types of elements selected from Mn, Ce, and Eu) to prepare granulated powders; and a step of heating the above granulated powders in nitrogen gas atmosphere at a temperature from 1500 to 2100 degrees C. to obtain β-type sialon phosphor. Preferably, previously-synthesized β-type sialon phosphor is added to the raw material and mixed with the raw material.
A luminescent element of the present invention is characterized in including, as constituting elements, the above sialon phosphor and a light-emitting diode having the maximum intensity in light emission wavelengths in a range from 240 to 480 nm.
Effect of the Invention
The first sialon phosphor of the present invention has the large primary particles without changing the secondary particles size and provides particles having a flat and smooth surface in comparison with the conventional one. Thus, particles of the first sialon phosphor can efficiently absorb excitation light to provide a superior light emission characteristic. Furthermore, since the illuminator of the present invention uses the phosphor, a favorable light emission characteristic is obtained.
Since the second sialon phosphor of the present invention has specific particles shape and composition distribution, it has high light emission intensity and luminous efficiency even when the phosphor is individually measured. Furthermore, since the second sialon phosphor is well dispersed in sealing resin, an LED element sealed by the second sialon phosphor is suppressed from causing unnecessary light scattering or light absorption in the sealing resin layer and thus has an improved luminous efficiency.
Since the sialon phosphor of the present invention has the above-mentioned characteristics, it can be preferably used for various LEDs. The sialon phosphor of the present invention can be combined with an LED having the maximum intensity in light emission wavelengths in a range from 240 to 480 nm in particular to provide a white LED.
According to the method for manufacturing the sialon phosphor of the present invention, the phosphor having the above characteristics can be stably provided in a large amount.
THE BEST MODE FOR CARRYING OUT THE CLAIMED INVENTION
The following section will explain the present invention in detail. First, sialon phosphor according to the first embodiment of the present invention will be described.
The α-type sialon has a structure in which the Si—N bonds in the α-type silicon nitride is partially substituted with Al—N bonds and Al—O bonds and electroneutrality is maintained by a specific positive ion ingressed and solid-soluted to the lattice. The α-type sialon is represented by a general expression of Mz(Si,Al)12(O,N)16. In the expression, M represents an element that can be ingressed into the lattice and that is Li, Mg, Ca, Y, and lanthanide metal (except for La and Ce). The solid-solution amount value Z of M is a numeric value determined by a substitutional rate of the Al—N bonds for Si—N bonds.
In order to arise the fluorescence characteristic, a part of M must be an element that can be solid-soluted and that can function as the luminescent center. In order to obtain phosphor emitting visible light, Ce, Pr, Eu, Tb, Yb, and Er are preferably used. Assuming that an element that is among those interstitially solid-soluted into the lattice and that does not contribute to the light emission is M1 and an element functioning as the luminescent center is M2, the general expression can be represented as (M1)x(M2)y(Si,Al)12(O,N)16. In order to arise the fluorescence characteristic, the values X and Y are preferably within the range of 0.3≦X+Y≦1.5 and 0<Y≦0.7.
The α-type sialon is generally obtained by heating mixed powders consisting of silicon nitride, aluminum nitride, aluminum oxide, and an interstitially solid-soluted element in a nitrogen atmosphere at a high temperature to cause the reaction. In a temperature rising process, a part of the constituting components forms a liquid phase through which substances move, thereby generating the α-type sialon solid solution. Thus, the synthesized α-type sialon has a structure in which the plurality of primary particles are sintered to form the secondary particles and thus a lump-shaped body. Thus, the lump-shaped body may be ground for example to form a powder-like formation.
The inventors have reached the present invention based on a finding that the specific surface area of the particles is closely connected to the light emission characteristic as a result of the examination of the relation between the light emission characteristic and the particle property. The specific surface area of particles shows how the particles are flat and smooth and is a sensitive for fine particle.
Specifically, the phosphor of the present invention preferably has, in addition to the composition, the specific surface area of the phosphor powders in a range from 0.2 to 0.5 m2/g. The specific surface area exceeding 0.5 m2/g causes the light scattering by the particle surface and fine particles to deteriorate the efficiency at which the excitation light is taken into the particles to deteriorate the light emission characteristic, which is not preferred. The specific surface area smaller than 0.2 m2/g is difficultly obtained by isolated primary particles and requires densely-sintered particles. Thus, the specific surface area smaller than 0.2 m2/g inevitably causes very large secondary particles that are deviated from a preferred size of phosphor such as LED and thus is not preferred.
Furthermore, the phosphor of the present invention includes the α-type sialon as a main component and is structured so that the particle interface has an intergranular phase having a different composition and, the crystalline or amorphous secondary phase is easily formed. Thus, entire composition of the phosphor powders does not always correspond to the solid-solution composition of the α-type sialon. In the case of the α-type sialon crystal, an increase in the solid-solution amount of aluminum and oxygen causes an increase in the crystal lattice size. Thus, attention was paid on the lattice constant of the α-type sialon and was examined. The result showed that a favorable light emission characteristic was obtained in the case the lattice constant a was in a range from 0.780 to 0.788 nm and a lattice constant c was in a range from 0.565 to 0.573 nm.
In the present invention, it is desirable that the highly pure α-type sialon crystal phase is maximally included, especially composed of the single-phase if possible, in the phosphor of the present invention from the viewpoint of the fluorescence emission. However, the phosphor of the present invention also may be made of a mixture including a slight amount of indispensable amorphous phase and other crystal phases so long as the characteristic is not deteriorated. According to the result of the examination by the present inventors, preferred phosphor shows, when being evaluated by the powder X-ray diffractometry, the diffraction intensity of the crystal phase other than that of the α-type sialon is 10% or less of the diffraction intensity of the face (102) of the α-type sialon. An existence of a crystal phase having the diffraction intensity exceeding 10% causes a deteriorated light emission characteristic.
When M1 of Ca and M2 of Eu are selected as elements solid-soluted into the crystal lattice of the α-type sialon, ultraviolet light or visible light having a wavelength in a range from 250 to 500 nm can be emitted as an excitation source to the phosphor to provide the phosphor that has a peak in a wavelength range from 540 to 600 nm and that shows light emission from yellow to orange. This phosphor is preferred because this phosphor can provide an illuminator emitting white light such as a white LED by the use of a blue LED as an excitation source for example to provide mixed light of yellow light emitted from the phosphor and the excitation light.
With regards to elements solid-soluted into the crystal lattice of α-type sialon, the atomic weight ratio of Eu functioning as the luminescent center is preferably within a range of 0<Y≦0.1. The value Y exceeding 0.1 causes an interference among solid-soluted Eu ions to cause concentration quenching, thus causing a deteriorated light emission luminance.
As a method for obtaining the sialon phosphor of the present invention, a method of synthesizing the α-type sialon in which Ca and Eu are solid-soluted will be described hereinafter.
Powders of silicon nitride, aluminum nitride, calcium-containing compound, and europium oxide are used as raw material. The method of manufacturing the phosphor of the present invention is characterized in that the mixture of the raw material powders so as to achieve a predetermined composition is caused to include previously-synthesized α-type sialon powders of 5 to 30 mass %.
The α-type sialon powders previously blended in the raw material powders are selectively caused to function as a particle formation origin during a heating processing to promote the growth of the primary particles to cause coarsened primary particles and a flatter and smoother surface. When the α-type sialon powders are previously added to the raw material powders, an effect is provided to suppress the sintering in the synthesis process, thus realizing the generation of sialon that is easily pulverized. This sialon does not require an excessive grinding, can provide powders having a desired particle size by an easy deagglomeration processing, and can suppress the generation of fine particles due to a grind processing that deteriorates the light emission characteristic.
An additive amount of the α-type sialon powders of 5 mass % or more can provide powders having a small specific surface area without causing the promotion of the nucleation, sintering, and grain growth of new α-type sialon particles at a part other than the added α-type sialon particles. An additive amount of the α-type sialon powders of 30 mass % or less does not cause too many grain growth origins which reduce the growth of individual particles. This can prevent a difficulty in obtaining a sufficiently flat and smooth particle surface, which is preferable.
The constituting elements and composition of the α-type sialon powders previously included in the raw material are not limited. The reason is the excitation of ultraviolet light to blue light causes the fluorescence characteristic to be arised mainly at a region close to the powder surface. However, the use of the α-type sialon powders including a different luminescent center element or including an impurity element, e.g., iron, that inhibits the light emission has a significant influence on the characteristic of the α-type sialon phosphor layer formed on the surface and thus is not preferred.
In the present invention, the α-type sialon powders previously added preferably has a specific surface area of 0.5 to 2 m2/g. The specific surface area of 2 m2/g or less sufficiently achieves the effect to the grain growth. On the other hand, the specific surface area of 0.5 m2/g or more does not cause a remarkably-large diameter of the secondary particles of the synthesis powders and consequently require no grind processing for example to easily achieve the target specific surface area of 0.2 to 0.5 m2/g, which is preferred.
The method for mixing the respective raw materials including the above-described α-type sialon may be a dry blending method or a method for carrying out a wet blending in inactive solvent substantially not reacting with the respective components of the raw material to subsequently remove the solvent for example. Preferred mixing apparatuses include a V-type mixing machine, a rocking mixer, a ball mill, a vibrational mill or the like.
Powders obtained by being mixed to achieve a desired composition (hereinafter referred to as raw material powders) are filled in a container such as a crucible in which at least a face having a contact with the raw material powders is made of boron nitride. Then, the powders are heated in a nitrogen atmosphere in a temperature range from 1600 to 1800 degrees C. for a predetermined time to obtain α-type sialon. The reason why boron nitride is used for the material of the container is that boron nitride has a very low reactivity to the respective components of the raw material. The inventors have found that an increased density of boron nitride used to constitute the crucible advantageously increases the primary particles as in a case where the α-type sialon powders are added to the raw material powders to allow the surface to be flat and smooth.
The boron nitride ceramics to use as the crucible preferably has a density of 1.75 g/cm3 or more. The density of 1.75 g/cm3 or less easily causes gas permeation in the crucible to promote the volatilization of the components included in the raw material powders filled in the crucible. This not only causes a change in the composition but also causes carbon monoxide gas, cyan gas or the like existing in the furnace to enter the crucible to cause the reaction with the raw material powders and the inhibition of the grain growth, which is not preferred. Thus, the density of the crucible preferably maximized. Pyrolytic boron nitride (P—BN) manufactured by the gas phase method in particular is very dense and thus is preferably used.
The maximum bulk of the raw material powders is preferably filled in the container because it can suppress the interparticle sintering during heating. Specifically, in order to synthesize α-type sialon for phosphor efficiently, the bulk of the raw material powders are preferably filled in the synthesis container with a bulk density of 1.0 g/cm3 or less.
The temperature in the heating processing of 1600 degrees C. or more prevents the existence of many unreacted products or an insufficient growth of the primary particles. The temperature in the heating processing of 1800 degrees C. or less prevents remarkable interparticle sintering.
A heating time for the heating processing may be selected within a range to prevent inconveniences such as the existence of many unreacted products, an insufficient growth of the primary particles, or an interparticle sintering. According to the examination by the present inventors, the heating time is preferably within a range from 2 to 24 hours.
Since the α-type sialon obtained by the above-described operation has a lump-shaped form, this α-type sialon may be subjected to a deagglomeration processing, a grinding processing, and optionally a classification processing to have a powder-like form having a predetermined size and can be used as powder-like phosphor applied to various uses.
In order to favorably use of the α-type sialon as white LED phosphor, the secondary particles may have an average diameter of 3 to 30 μm. The secondary particles having an average diameter of 3 μm or more prevents a decreased light emission intensity. The secondary particles having an average diameter of 30 μm or less allows the resultant α-type sialon to be uniformly dispersed into the resin for sealing LED and prevents variation in the light emission intensity and color tone and can be practically used.
The lump-shaped body consisting of the α-type sialon obtained by the above-described manufacture method has a relatively-high easy pulverizability and can be easily ground by a mortar or the like to have a predetermined particle size. However, this lump-shaped body also may be ground by general grinding machines such as a ball mill, a vibrational mill, or a jet mill.