ARTICLE IN PRESS alternative method in order to obtain fine powders. at 1500C through the following reaction: simplify the process,and reduce the cost. d of synthesizing SiaN4 SrO+Eu203+C+N2 Sr2SisNs Eu +CO2. The residual carbon in powders prepared by CRN can be containing the oxide precursor powder loaded inside an -free ng th aumina/quartz tube through which NH,or NH-CH gas (1300-00 NH.C high reducing and nitriding agents.For example,Suehiro et a ianiaasoroyanisminikhytoshrhne ialon:Eu 00 nm a temperature thar As shown in the previous section,oxynitride and nitride phosphors emit visible blue,green,yellow,and red light powder in the Cao-siolo processing parameters such as heating rate,flow rate. phosphors in white LEDs.The fabrication of both 3 as bichromatic and white LEDshas s bee CaSi+2NH3 CaSiN2 +2H2 6975.Table I shows a summary of the optical propertie of white LEDs prepared by combining silicon-based 4.3.Carbothermal reduction and nitridation(CRN) CRN is also thesizing silico based oxynitride and nitride compounds6 It differs Phci8970ihgyTabicauedeahiteLEDycontnimg o( phosphor (e whit NH 4 rat 750 R mixture of ode.nitride and carbon powders.Zhang et al was produced.Furthermore.Sakuma et a.69 showed [67]prepared a-sialon:Eu (Ca Eu,)pow hat the chromaticity coordin rom(0.5 es of white LEDs using Ca AGo0509. 1681 cantly from (0.393.0.461)to (0.383.0.433)when they wer measured at 200C.The high stability of the chromaticity SisNa+CaO+AlO+Eu:Os+C+N: →,N6:Eu+C0 coordinate due to ow the quenching en sitio coworkers 139.401 demonstrated that white-to-dayligh white LEDs could also be realized using a single short- o ng a mix and osphor ynitride and nitride phosphors (K) Reference 63-74 0-6 20 20d 0 The luminous eficacy was measured at (35mA)Other data were measured at20mA Please cite this article as:R.Xie,N.Hirosaki,Sci.Technol.Adv.Mater.(2007),do:0..00alternative method in order to obtain fine powders, simplify the process, and reduce the cost. GRN is an effective and cheap method of synthesizing oxynitride and nitride phosphors. In this approach, the reaction is generally performed in an alumina boat containing the oxide precursor powder loaded inside an alumina/quartz tube through which NH3 or NH3–CH4 gas flows at appropriate rates at high temperatures (1300–1600 1C). The NH3 or NH3–CH4 gas acts as both reducing and nitriding agents. For example, Suehiro et al. [64] used GRN to prepare submicron-sized a-sialon:Eu2+ powders (300 nm) at a temperature 200 1C lower than that used in the solid-state reaction. The precursor was a powder in the CaO–SiO2–Al2O3 system. The effects of processing parameters such as heating rate, flow rate, volume ratio of gases, temperature, and holding time were investigated later by Li et al. [65]. Gal et al. [63] synthesized CaSiN2 using CaSi as the precursor powder and NH3 as the nitriding agent through the following reaction [63]: CaSi þ 2NH3 ! CaSiN2 þ 2H2: 4.3. Carbothermal reduction and nitridation (CRN) CRN is also a cheap method of synthesizing silicon￾based oxynitride and nitride compounds [66]. It differs from GRN in that (i) carbon powder is applied as the reducing agent and (ii) N2 instead of NH3 or NH3–CH4 is used as the nitriding agent. The precursor for CRN is a mixture of oxide, nitride and carbon powders. Zhang et al. [67] prepared a-sialon:Eu2+ (Ca1xEuxSi10Al2N16) pow￾ders by firing a mixture of Si3N4, CaCO3, Al2O3, Eu2O3, and C powders at 1600 1C in flowing N2. a-Sialon:Eu2+ was formed via the following chemical reaction [68]: Si3N4 þ CaO þ Al2O3 þ Eu2O3 þ C þ N2 ! CaSi10Al2N16 : Eu þ CO2: The a-sialon:Eu2+ phosphor emitted yellow emission with a band centered at 585–605 nm, which is consistent with that prepared by the solid-state reaction. CRN was also utilized to prepare the Sr2Si5N8:Eu2+ phosphor by heating a mixture of Si3N4, SrCO3, Eu2O3, and C powders at 1500 1C through the following reaction: Si3N4 þ SrO þ Eu2O3 þ C þ N2 ! Sr2Si5N8 : Eu þ CO2: The residual carbon in powders prepared by CRN can be removed by post-annealing the phosphor powders in a carbon-free atmosphere (e.g., N2) at 1600 1C. 5. Applications of oxynitride and nitride phosphors in white LEDs As shown in the previous section, oxynitride and nitride phosphors emit visible blue, green, yellow, and red light efficiently under UV and/or visible-light irradiation. This closely matches the emission wavelengths of UV-, NUV-, or blue-LED chips, enabling their use as downconversion phosphors in white LEDs. The fabrication of both bichromatic and multichromatic white LEDs has been attempted by combining silicon-based oxynitride and nitride phosphors with a blue-LED chip [11,12,39,40, 69–75]. Table 1 shows a summary of the optical properties of white LEDs prepared by combining silicon-based oxynitride and nitride phosphor(s) with a blue-LED chip. The first bichromatic white LED using an oxynitride phosphor was reported by Sakuma and coworkers [11,12,69,70]. They fabricated a white LED by combining an orangish-yellow a-sialon:Eu2+ phosphor (lem=586 nm) with a blue-LED chip (lem=450 nm). A warm white LED with a correlated color temperature (CCT) of about 2750 K was produced. Furthermore, Sakuma et al. [69] showed that the chromaticity coordinates of white LEDs using Ca￾a-sialon:Eu2+ varied from (0.503, 0.463) to (0.509, 0.464), whereas those of LEDs using YAG:Ce3+ shifted signifi- cantly from (0.393, 0.461) to (0.383, 0.433) when they were measured at 200 1C. The high stability of the chromaticity coordinates is due to the low thermal quenching of a-sialon:Eu2+. By tuning the emission wavelength of a-sialon:Eu2+ through tailoring the composition, Xie and coworkers [39,40] demonstrated that white-to-daylight white LEDs could also be realized using a single short￾wavelength Li-a-sialon:Eu2+ phosphor. The luminous efficacy of these white LEDs was 40–55 lm/W, about ARTICLE IN PRESS Table 1 Examples of white LEDs utilizing silicon-based oxynitride and nitride phosphors Blue LEDs+phosphors Color temperature (K) Average color￾rendering index Luminous efficacy (lm/W) Reference Yellow Green Red Ca-a-sialon:Eu – – 2600–3100 57 26, 42, 51, 55 [11,12,69,70] Li-a-sialon:Eu – – 3000–6150 63–74 40–44, 46–55 [39,40] Ca-a-sialon:Eu b-sialon:Eu CaAlSiN3:Eu 2780–6850 84–90 26–35 [11,12,70] – Ca-a-sialon:Yb Sr2Si5N8:Eu 2700–6700 82–83 17–23 [72] – SrSi2O2N2:Eu Sr2Si5N8:Eu 3200 90 25a [73] – SrSi2O2N2:Eu CaSiN2:Eu 5206 90.5 30 [74] a The luminous efficacy was measured at 1W input (35 mA). Other data were measured at 20 mA. R.-J. Xie, N. Hirosaki / Science and Technology of Advanced Materials ] (]]]]) ]]]–]]] 11 Please cite this article as: R.-J. Xie, N. Hirosaki, Sci. Technol. Adv. Mater. (2007), doi:10.1016/j.stam.2007.08.005