Patent Application
20090283721
Embodiments of the present invention are directed to nitridosilicate-based phosphor compounds emitting in the red region of the electromagnetic spectrum. The present compounds exhibit enhanced photoluminescent intensities and longer emission wavelengths than that offered by conventional red nitrides, and thus the present compounds are particularly useful in the white LED lighting industry.
Conventionally, nitridosilicate-based phosphor compounds have contained an alkaline earth metal element (such as Mg, Ca, Sr, and Ba), silicon, nitrogen, and a rare earth element activator such as europium. Examples include Sr2Si5N8, BaSi7N10, and CaSiN2.
As taught in US 2007/0040152 to S. Oshio, a compound such as CaSiN2 becomes a CaSiN2:Eu2+ phosphor emitting red light having an emission peak in the vicinity of 630 nm, where the Eu2+ ions function as the luminescent centers. The excitation spectrum of the compound has a peak around 370 nm, and although the phosphor does not emit red light when excited by 440 to less than 500 nm excitation radiation, it does emit red light with high intensity when excited by 330 to 420 near ultraviolet light.
US 2007/0040152 also elucidated the difficulties in producing a nitridosilicate based compound such as M2Si5N8, MSi7N10, and MSiN2, where M is at least one element selected from Mg, Ca, Sr, and Ba, etc., where the compound contains substantially no oxygen. This may be achieved, it is taught, by using as starting materials the nitrides of the alkaline-earth elements and the rare earth elements, but these nitrides are difficult to obtain, expensive, and difficult to handle. These factors conspire to make nitridosilicate-based phosphors difficult to produce industrially. As stated by the reference: “the conventional nitridosilicate-based compound has the following problems: (1) low purity due to the presence of a large amount of impurity oxygen, (2) low material performance of a phosphor caused by the low purity; (3) high cost; and the like.” The problems include low luminous flux and [low] brightness.
But if the problems inherent in producing nitridosilicate-based phosphors have been explicitly stated, so too have the benefits of a substantially oxygen-free compound. U.S. Pat. No. 7,252,788 to Nagatomi et al. teaches a phosphor having a quaternary host material represented by the general formula M-A-B—N:Z, where M, A, and B are divalent, trivalent, and tetravalent elements, respectively; N is nitrogen, and Z is the activator. As an example, M could be Ca, A aluminum, B silicon, and Z could be Eu, thus forming the compound CaAlSiN3:Eu2+. It is clear from the general formula (and the example) that these phosphors have deliberately excluded oxygen from the constituent elements, and thus these phosphors are in a different class than a conventional phosphor having a sialon group host material (Si—Al—O—N group) and a phosphor having a Si—O—N group host material.
Nagatomi et al. discovered and disclosed in U.S. Pat. No. 7,252,788 that when the oxygen content in the phosphor is large, the emission efficiency decreased (not desirable), and the emission wavelength of the phosphor also tended to shift to a shorter wavelength side. This latter observation is also undesirable because most (if not all) manufacturers are attempting to add a phosphor that is deeper in the red region (i.e., less orange or yellow) for the color rendering benefits a red phosphor offers to the white LED industry. Nagatomi et al. continue: the phosphor they provided includes no oxygen in the host material, with the benefits of exhibiting a higher emission efficiency, and avoiding the shift of the emission wavelength to the shorter wavelength side [of the spectrum].
But this is more easily stated than accomplished. Oxygen contamination was addressed by Nagatomi et al. in US 2006/0017365, where it is taught that the source is believed to be the oxygen adhering to the surface of the raw materials, and thus introduced at the start of the synthesis; oxygen added as a result of oxidation of the surface of the raw materials at the time of preparation for firing, and the actual firing, and the oxygen adsorbed onto the surface of the phosphor particles after firing.
A discussion of oxygen measurements, and an analysis of the possible causes for a discrepancy between measured and calculated values, was also given by Nagatomi et al. in US 2006/0017365. The oxygen content that was measured in their sample was 2.4 percent by weight, to be contrasted with a calculated oxygen concentration of 0.3 percent by weight. The origin of this approximately 2 percent by weight difference between the measured value (with its so-called “excessive oxygen”) versus the calculated amount was attributed to oxygen originally adhering to the surface of the raw materials at the time of preparation of the firing and at the time of firing, and the oxygen adsorbed onto the surface of the phosphor specimen after the firing.
The oxygen content in Nagatomi et al.'s samples of U.S. Pat. No. 7,252,788 similarly show a 2 plus percent by weight value: 2.2, 2.2, and 2.1 in Tables 1 and 3.
Tabling the oxygen discussion for the moment, and turning to a different topic of background, the present inventors have disclosed and patented phosphor compositions having a halogen content, and have enumerated the benefits thereof. Their compositions and synthesis techniques have been used in several types of host crystal lattices, and in phosphors that emit in several regions of the electromagnetic spectrum. For example, an aluminate based, blue emitting phosphor with halogen has been described in Published Application No. US 2006/0027786; a yellow-green emitting silicate-based phosphor with halogen has been described in U.S. Pat. N. 7,311,858, and an orange emitting silicate-based phosphor with halogen has been described in Published Application No. US 2007/0029526. These three examples have been chosen in particular to show that the blue through orange region of the spectrum has been addressed, but what is missing is a phosphor emitting in the red, with the same enhanced attributes including photoluminescent intensity demonstrated by the other members of the series.
The present inventors have shown that halogen inclusion is beneficial in nitridosilicate-based red phosphors, and equally unexpected is that in achieving that goal, the oxygen content was simultaneously reduced to a level below 2 percent by weight, with the attendant advantages outlined above.
Embodiments of the present invention are directed to the fluorescence of a nitride-based deep red phosphor having at least one of the following novel features: 1) an oxygen content less than about 2 percent by weight, and 2) a halogen content. Such phosphors are particularly useful in the white light illumination industry, which utilizes the so-called “white LED.” The selection and use of a rare earth halide as a raw material source of not only the activator for the phosphor, but also the halogen, is a key feature of the present embodiments. The present phosphors have the general formula MaMbBc(N,D)3:Eu2+ where Ma is a divalent alkaline earth metal such as Mg, Ca, Sr, Ba; Mb is a trivalent metal such as Al, Ga, Bi, Y, La, and Sm; and Mc is a tetravalent element such as Si, Ge, P, and B; N is nitrogen, and D is a halogen such as F, Cl, or Br. An exemplary compound is CaAlSi(N1-xFx)3: Eu2+. The present phosphors have a chemically stable structure, and are configured to emit visible light having a peak emission greater than about 620 nm with a high emission efficiency.
Embodiments of the present invention are directed to the fluorescence of a nitride-based deep-red phosphor having at least one of the following novel features: 1) an oxygen content less than about 2 percent by weight, and 2) a halogen content of virtually any amount. Such phosphors are particularly useful in the white light illumination industry, which utilizes the so-called “white LED.” The selection and use of a rare earth halide as a raw material source of not only the rare earth activator for the phosphor, but also the halogen, is a key feature of the present embodiments. While not wishing to be bound by any particular theory, it is believed the halogen may play a dual role in enhancing the properties of these phosphors: by reducing the oxygen content in addition to causing an increase in photoluminescent intensity and spectral emission.
There are several ways to describe the formula of the present phosphors. In one embodiment, the present phosphors have the form M-A-B—(N,D):Z, where M, A, and B are three cationic metals and/or semimetals with divalent, trivalent, and tetravalent valences, respectively; N is nitrogen (a trivalent element), and D is a monovalent halogen that along with the nitrogen contributes to the anionic charge balance. Thus, these compounds may be thought of as halogen-containing nitrides. The element Z is an activator in the host crystal, providing the photoluminescent centers. Z may be a rare earth or transition metal element.
The present nitride-based red phosphors may be described in a slightly different format, to emphasize the approximate ratios of the constituent elements. This formula takes the form MmMaMb(N,D)n:Zz, where the stoichiometry of the constituent elements (m+z):a:b:n follows the general ratios 1:1:1:3, although deviations from these integer values are contemplated. It is noted the formula shows that the activator Z substitutes for the divalent metal Mm in the host crystal, and that the host material of the phosphor contains substantially no oxygen (or at least, less than about 2 percent by weight).
The present nitride-based red phosphors may be described in yet another manner, this format emphasizing the stiochiometric relationship between the amounts of the metals and halogen(s) present relative to the amount of nitrogen present in the nitride host. This representation has the form MmMaMbD3wN[(2/3)(m+z)+a+(4/3)b-w]Zz. The parameters m, a, b, w, and z fall within the following ranges: 0.01≦m≦1.5; 0.01≦a≦1.5; 0.01≦b1.5; 0.0001≦w≦0.6, and 0.0001≦m≦0.5.
The metal Mm may be an alkaline earth or otherwise divalent metal such as Be, Mg, Ca, Sr, Ba, Zn, Cd, and/or Hg. Different combinations are possible, and Mm may be a single one of these elements, or a mixture of any or all of them. In one embodiment, the metal Mm is Ca.
Ma is a trivalent metal (or semimetal) such as B, Al, Ga, In, Y, Sc, P, As, La, Sm, Sb, and Bi. Again, different combinations and contents of these metals/semimetals are possible, and in one embodiment, the metal Ma is Al.
Mb is a tetravalent element such as C, Si, Ge, Sn, Ni, Hf, Mo, W, Cr, Pb, Ti, and Zr. In one embodiment, the tetravalent element Mb is Si.
The element D is a halogen such as F, Cl, or Br in this nitride-based compound, and may be contained within the crystal in any of a number of configurations: for example, it may be present in a substitutional role (substituting for nitrogen) in the crystalline host; it may be present interstitially in the crystal, and/or perhaps within grain boundaries that separate crystalline grains, regions, and/or phases.
Z is an activator comprising at least one or more of the rare earth elements and/or transition metal elements, and include Eu, Ce, Mn, Tb, and Sm. In one embodiment the activator Z is europium. According to one embodiment of the present invention the activator is divalent, and substitutes for the divalent metal Mm in the crystal. The relative amounts of the activator and the divalent metal Mm may be described by the molar relationship z/(m+z), which falls within the range of about 0.0001 to about 0.5. Keeping the amount of the activator within this range may substantially avoid the so-called quenching effect manifested by a decrease in emission intensity caused by an excessive concentration of the activator. The desired amount of the activator may change with the particular choice of activator.
An exemplary compound according to the present embodiments is CaAlSi(N1-xFx)3: Eu2+. Other halogens including chlorine may be used instead of the fluorine, or in combination with the fluorine. This compound emits in the deep-red region of the spectrum with a photoluminescent intensity greater than that demonstrated by the nitrides of the prior art, where the amount of halogen inclusion influences the degree to which the peak emission wavelength shift toward longer wavelengths (deeper into the red).
Prior art starting materials have typically consisted of the nitrides and oxides of the metals. For example, to produce the phosphor CaAlSiN3:Eu2+ in U.S. Pat. No. 7,252,788, it is taught that the nitride starting materials for the calcium, aluminum, and silicon sources may be Ca3N2, AlN, and Si3N4, respectively. The source of the europium in this disclosure was the oxide Eu2O3. In contrast, the sources of the metals in the present phosphors may be at least in part the halides of the metals, and typical examples include MgF, CaF, SrF, BaF, AlF, GaF, BF, InF, and (NH4)2SiF6. The europium may be supplied by either of the two fluorides EuF2 and EuF3. The use of halides of the divalent, trivalent, and tetravalent metals is not the only way to supply the halogen to the phosphor: an alternative method is to use a flux such as NH4F or LiF.
Specifically, compounds of the divalent metal Mm appropriate as raw materials in the synthesis of the present phosphors include nitrides, oxides, and halides; e.g., Mm3N2, MmO, MmD2, where again D is F, Cl, Br, and/or I. Analogous raw material compounds of the trivalent metal Ma are MaN, Ma2O3, and MaD3. The tetravalent metal starting compounds include Mb3N4, and (NH4)2MbF6. Compounds of the halide anion D include NH4D and AeD, where Ae is an alkaline metal such as Li, Na, and MD2, where Me is an alkaline earth metal such as Mg, Ca, etc.
Prior art references have disclosed the oxide of europium, Eu2O3, as the source of the europium activator, as this material is a readily available commercial compound. The present inventors have discovered, however, that the oxygen in this compound has a deleterious effect on the photoluminescent properties of the phosphor. One way of eliminating this problem is to use a europium source that does not contain oxygen, such as the substantially pure Eu metal, but this is a very expensive approach that is difficult to implement. One embodiment of the present invention is to use a Eu halide, such as EuF3 and/or EuCl3 as europium-containing starting materials. The present inventors have found that when a europium halide such as EuF3 is used as the europium source, the emission efficiency of the phosphor increases, and the emission wavelength of the phosphor shifts to a longer wavelength. Thus one embodiment of the present invention is to use a europium compound EuD3(D=F, Cl, Br, I), and not Eu2O3, as the europium source. These concepts will be illustrated, and discussed more fully, in conjunction with the accompanying figures.
Not only do the EuF3 generated samples emit at longer wavelengths than Eu2O3 based samples having the same europium content, but the EuF3 generated samples are brighter as well. This is illustrated in
Data from an experiment comparing the optical properties of CaAlSiN3 type samples made with 1) Eu2O3, 2) EuF2, 3) EuF3, and 4) Eu2O3 with a 3% NH4F flux is shown in
The effect of doping the present nitrides with alkaline earth metals is investigated in
The halogen may be introduced as a salt of the alkaline earth metal component. This data is shown in
Alternatively, the halogen may be introduced as a salt of the trivalent component, which may be the transition metal element aluminum. Use of AlF3 as a raw material substituting for AlN at a 5 atomic percent level in a CaAlSiN3:Eu2+ type phosphor is shown in
Alternatively, the halogen may be introduced as a salt of the tetravalent metal, semi-metal, or semiconducting element, which may be silicon. An experiment similar to the
The halogen may also be supplied in the form of a flux for these nitride-based red phosphors. The effect of adding an NH4F flux to the starting materials is investigated in
Fluxes other than NH4F may be used, of course, such as LiF and B2O3. LiF and B2O3 were compared to NH4F, each at 2 atomic percent in
But does the nature of the halogen in the flux matter? In other words, what is the effectiveness of a chlorinated flux versus a fluorinated flux? This question was investigated in
The effect of NH4F addition to a CaAlSiN3:Eu2+ phosphor made with Eu2O3, in other words, a non-halogenated red nitride phosphor because the europium source was an oxide rather than a salt of a halogen, is shown in
The effect of an NH4F flux addition on the CIE x and y values of the luminescence are shown in
Phosphor Synthesis Processes (with Emphasis on Oxygen Elimination)
The present phosphor synthesis processes will be described using the exemplary compound CaAlSi(N,F)3:Eu2+. The raw materials are weighed and mixed according to the stoichiometric ratios needed to produce the desired phosphor. Nitrides of the elements Mm, Ma, and Mb are commercially available as raw materials. Halides of the divalent metal Mm, and various ammonium halide fluxes, are also commercially available. Raw material sources of europium include its oxide, but this is a viable option primarily when a halogen containing flux is also used. The mixing may be performed using any general mixing method of which typical ones are mortar or ball mill.
In a specific example, the particular raw materials are Ca3N2, AlN, Si3N4, and EuF2. In this example, the europium fluoride is being used specifically as a replacement for the traditionally used europium oxide, to utilize the benefits of a reduced oxygen content. One embodiment further reduces the oxygen content by weighing and mixing the raw materials in a glove box under an inert atmosphere, which may comprise nitrogen or argon.
The raw materials are thoroughly blended, and the mixture then heated in an inert atmosphere to a temperature of about 1400° C. to 1600° C. In one embodiment, a heating rate of about 10° C. per minute is used, and maintained at this temperature for about 2 to 10 hours. The product of this sintering reaction is cooled to room temperature, and pulverized using any number of means known in the art, such as a motar, ball mill, and the like, to make a powder with the desired composition.
Similar production methods may be used to phosphors where Mm, Ma, and Mb are other than Ca, Al, and Si, respectively. In this case, compounding amounts of the constituent raw materials may vary.
The present inventors have shown that by using europium halide instead of europium oxide, the oxygen content in the phosphor product may be reduced to less than 2 percent by weight. In a specific example, substituting the halide for the oxide resulted in an oxygen reduction of from about 4.2 percent to about 0.9 percent. In one study performed by the present inventors, the residual 0.9 percent was attributed to the act of weighing and mixing the raw materials in air, rather than in an inert atmosphere.
In air, Ca3N2 decomposes to give ammonia and calcium hydroxide:
Ca3N2+6H2O→3Ca(OH)2+2NH3,
and ammonia has been observed escaping from the raw material mixture when the starting materials were mixed in air. The surface of the mixture gradually becomes white when the raw materials are kept in air for a period of time, even if only a few minutes. Thus, it is necessary to innovate procedures in which oxygen is deliberately excluded and/or removed from the reaction system. The following procedure has been implemented by the present inventors.
The raw materials Ca3N2, AlN, Si3N4, and EuF2 are sealed within an inert atmosphere such as nitrogen and/or argon, and maintained in such a state using a glove box. The raw materials are then weighed within the inert atmosphere, usually in a glove box, and then mixed using ordinary methods known in the art, including mixing with either a mortar or ball mill. The resulting mixture is placed in a crucible, which is then transferred to a tube furnace connected directly to the glove box. This is so that exposure of the mixed raw materials to an inert atmosphere is maintained. In the tube furnace, the mixed raw materials are heated to a temperature of about 1400° C.-1600° C. using a heating rate of about 10° C. per minute, and maintained at that temperature for a time anywhere from about 2 to 10 hours. The sintered product is cooled to room temperature, and pulverized using known methods, including mortar, ball mill, and the like, to produce a powder with the desired composition.
The oxygen, fluorine, and chlorine contents of about 7 exemplary phosphors was measured by EDS, and the results are shown in
In
The apparent ability (or evidence for the possibility) of a halogen in the europium salt to getter oxygen during the synthesis is shown in
That a halogen may be incorporated into the host lattice of the present nitride-based red phosphors by either a halogen containing flux or halogen containing europium source is shown by the data in
In summary, then, the exemplary phosphors Ca0.97AlSiN3Eu0.03Cl0.15 and Ca0.97AlSiN3Eu0.03F0.15 have an oxygen content less than about 2 weight percent, and are brighter than their non-halogen containing counterparts. The emission spectra of these exemplary nitride-based red phosphors is shown in
The present nitride-based red phosphors are capable of being excited at wavelengths ranging from about 300 nm to about 610 nm, as shown in
Normalized excitation spectra for phosphors having the generalized formula Ca1-xAlSiN3Eux are shown in
According to further embodiments of the present invention, the present red phosphors may be used in white light illumination systems, commonly known as “white LEDs.” Such white light illumination systems comprise a radiation source configured to emit radiation having a wavelength greater than about 280 nm; and a halide anion-doped red nitride phosphor configured to absorb at least a portion of the radiation from the radiation source, and emit light with a peak intensity in a wavelength range greater than about 640 nm. Exemplary spectra of light intensity versus wavelength emitted by these warm white luminescent systems are shown in
An example of a high CRI, warm-white lighting system made available to the industry as a result of the present red contribution is shown in
A second example of a high CRI, warm-white lighting system is shown in
A third example of a high CRI, warm-white lighting system is shown in
The success of the present nitride-based red phosphors at providing solutions to the warm white light industry may be seen in the context of
The white light illumination system in
The present application claims priority to U.S. Patent application 61/054,399 titled “Nitridosilicate-based red phosphors,” by Liu et al., filed May 19, 2008. U.S. provisional 61/054,399 is herein incorporated in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61054399 | May 2008 | US |
