Gas-ReductionâNitridation Synthesis of CaAlSiN3:Eu2+ Fine Powder

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Gas-Reduction-Nitridation Synthesis of CaAlSiN3:Eu2+ Fine Powder Phosphors for Solid-State Lighting Takayuki Suehiro, Rong-Jun Xie, and Naoto Hirosaki Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie4038455 • Publication Date (Web): 29 Jan 2014 Downloaded from http://pubs.acs.org on February 5, 2014

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Industrial & Engineering Chemistry Research

Gas-Reduction–Nitridation Synthesis of CaAlSiN3:Eu2+ Fine Powder Phosphors for Solid-State Lighting Takayuki Suehiro,* Rong-Jun Xie, and Naoto Hirosaki SiAlON Unit, Environment and Energy Materials Division, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan *To whom correspondence should be addressed. E-mail: [email protected]

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ABSTRACT: CaAlSiN3 :Eu2+ fine powder phosphors were successfully synthesized from a fully oxidic system, CaO–Al2 O3 –SiO2 by using the gas-reduction–nitridation (GRN) method. The GRN reaction was conducted at low temperatures of ∼1370◦ C to obtain nitrided precursor powders, which could be converted into near single-phase CaAlSiN3 by a moderate heat treatment at 1600◦ C for 4 h in a N2 atmosphere of 0.92 MPa. Highly efficient CaAlSiN3 :Eu2+ red-emitting phosphors were obtained by using a fluoride activator, with the external quantum efficiencies up to 72%.

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1

Introduction

CaAlSiN3 (CASN):Eu2+ has attracted much attention for years as a blue-light converting red-emitting phosphor, which is essential for high-color rendering white light-emitting diodes (LEDs) and liquid crystal display (LCD) backlighting applications.1, 2 CASN:Eu2+ possesses a Wurtzite-related orthorhombic structure with the space group Cmc21 , in which (Ca,Eu) atoms are pseudo-tetrahedrally coordinated by the four nearest nitrogen atoms ˚.2 This characteristic coordination environment with the short bond length of ∼2.4–2.5 A exerts a strong crystal field on Eu2+ , resulting in the deepest red emission (CIE 1931 coordinates up to x=0.68, y=0.31) observed among the nitride phosphors developed so far. A major obstacle to further widespread applications of the CASN phosphor is its high production cost, resulting from the inevitable use of the expensive, air-sensitive oxygen-free raw materials and the consequent complexity of the processing procedures. CASN:Eu2+ phosphors are currently produced by a high-temperature solid-state reaction of the nitride raw materials (Ca3 N2 , AlN, Si3 N4 , EuN) or a direct nitridation of the (Ca,Eu)AlSi alloy precursors. Other low-temperature synthetic routes, such as the nitridation of the (Ca,Eu)AlSi alloy in a supercritical ammonia (100 MPa),3 and the reaction among CaSiF6 , AlF3 , and Li3 N4 have been reported, whereas they could never be industrially applicable, due to the use of extremely high pressure and/or hazardous raw materials, as well as to the inferior photoluminescent properties of the final products. In the current work, we have established a new synthetic route for CASN:Eu2+ fine powder phosphors by applying the GRN method5−15 to the inexpensive, stable CaO– Al2 O3 –SiO2 system, and investigated their practical performance as a red-emitting phos3



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phor for solid-state lighting.

2 2.1

Experimental Section Powder Synthesis by GRN

The CASN powder samples were synthesized by using the GRN method.5−15 The oxide starting materials were prepared by simple wet mixing of CaCO3 (Kojundo Chemical Laboratory, 99.99%, DBET =1.59 µm), α-Al2 O3 (TM-DAR, Taimei Chemicals Co., DBET =0.12 µm), and SiO2 (SP-03B, Fuso Chemical Co., DBET =0.23 µm) powders. The raw powder of ∼0.5 g was loaded in a BN boat (length: 70 mm, depth of the powder bed: ∼5 mm) and set in a horizontal alumina tube furnace (inner diameter of 24 mm). The actual temperature inside the furnace tube was precisely calibrated prior to the experiments. The furnace was subsequently heated to the reaction temperatures of 1300–1400◦ C in an NH3 – 1.5 vol% CH4 gas mixture, introduced at a constant flow rate of 0.65 L/min. After the predetermined reaction time, the sample was furnace cooled in an NH3 atmosphere. The extent of nitridation was estimated by measuring the fraction of observed and theoretical weight loss (∆Wobs /∆Wtheor ), resulting from the reduction–nitridation reaction. The Eu doping of thus-obtained CASN precursor powders was conducted by using the postsynthesis activation (PSA) process.7, 9 The as-prepared CASN powder was dry mixed with an EuF3 powder (Wako Pure Chemical Industries, Ltd., 99.9%), and subsequently heat treated in a gas-pressure sintering furnace (FVPHP-R-5, FRET-18, Fujidempa Kogyo Co.) at 1600◦ C for 4–8 h under a N2 atmosphere of 0.92 MPa.

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2.2

Characterization

Phase assemblage of the synthesized powders was analyzed by X-ray diffractometry (XRD) using Cu Kα1 radiation (SmartLab, Rigaku). Phase purity of the phosphors after the PSA was evaluated by the multiphase Rietveld refinement of the XRD patterns using the program RIETAN-FP.16 The structural parameters (fractional coordinates and isotropic displacement parameters) were fixed to the data for CaSiAlN3 reported by Ottinger.17 Electron micrographs of platinum-coated samples were obtained using a field-emission scanning electron microscope (FESEM; JSM-6340F, JEOL). Specific surface area and the equivalent particle size of the powders were measured by the single-point Brunauer– Emmett–Teller (BET) method (FlowSorb III, Micromeritics). Cationic composition of the nitrided samples was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES; IRIS Advantage, Jarrell Ash). Nitrogen content of the synthesized powders was determined by the Kjeldahl method, and impurity oxygen and carbon contents were analyzed by the selective hot-gas extraction method (TC-436, CS-444LS, LECO Co.). Photoluminescent (PL) properties of the as-prepared samples were evaluated at room temperature using a fluorescence spectrophotometer (F-4500, Hitachi). Quantum efficiencies were measured by a multichannel spectrophotometer (MCPD-7000, Otsuka Electronics). Spectral simulation of the trichromatic white LEDs was carried out by integrating the emission spectra of the synthesized CASN:Eu2+ phosphors, a 450-nm InGaN LED, and a SrSi2 O2 N2 :Eu2+ green-emitting phosphor.18, 19 The luminous efficacy of radiation (LER) values for the synthesized phosphors and the simulated LED systems were

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estimated according to the definition,20 K=

Km

R 780

V (λ)S(λ)dλ 380 S(λ)dλ

380

R 780

where K is LER, Km the luminous coefficient of 683 lm/W, V (λ) the standard luminosity function, and S(λ) the spectral power distribution. The color rendering indices (CRI Ri ) of the simulated LED spectra were calculated according to Japanese Industrial Standard (JIS) Z 8725/8726.

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Results and Discussion

3.1

GRN Synthesis of CASN Precursor Powders

Table 1 shows the phase assemblage and nitridation extent of the samples synthesized from the stoichiometric composition of CaCO3 –0.5Al2 O3 –SiO2 under the various reaction conditions. The nitridation progress at low temperatures of 1300–1360◦ C was sluggish and the synthesized powders were composed of an unindexed compound “2CaO·Si3 N4 ·AlN”21 and AlN (samples ST1–4). The synthesis at 1400◦ C (sample ST9) resulted in the eutectic melting of the reaction system, which retarded further nitridation of the powder. The CaAlSiN3 phase formed at a rather narrow temperature range of 1365–1375◦ C with the soaking time of 4 h (samples ST5, 6 and 8), whereas the obtained powders contained the secondary AlN phase and exhibited the nitridation extents in excess of unity, suggesting the evaporation loss of a constituent during the GRN reaction. To attain a near-stoichiometric composition and improved phase purity of the final CASN powder, we revealed the plausible compositional deviations after the reaction by the ICP-OES analysis, and attempted the syntheses from off-stoichiometric starting compositions. Table 2 summarizes the analyzed cationic ratio of the samples synthesized 6


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at 1370◦ C with the soaking time of 4 h from the various starting compositions. The deficiency of Ca more than 33% was observed for the stoichiometric starting composition (sample ST6), and the initial content of CaCO3 required to compensate for the deficiency was found to be as much as 2 to 3 times of the stoichiometry (samples EX3 and 4). The observed significant and selective loss of Ca might be partly attributable to the reduction of CaCO3 -derived reactive CaO (SBET ≃ 30 m2 /g), e.g., the reaction CaO + CH4 → Ca(g) + CO + 2H2 , in which the equilibrium pressure of gaseous Ca is expected to reach ∼0.05–0.10 atm at the reaction temperatures of 1600–1700 K.22 The addition of excess amount of Ca also induced the concurrent decrease of Si (samples EX1–4) suggesting the volatilization of some unstable Ca–Si–O phases from the reaction system, though no consistent relationship was found between the decreased contents of Ca and Si (∆Ca/∆Si varied from 2.68 to 8.97). Further optimization was attained by decreasing the content of Al, and the final composition closest to the stoichiometry was obtained from the starting composition with the ratio of Ca, Al, and Si being 2.00 : 0.75 : 1.00 (sample EX5). The XRD pattern of thus-prepared sample EX5 is shown in Figure 1. The sample EX5 consisted mainly of CASN with small amounts of residual CaO (4.8 wt%) and AlN (3.6 wt%), while a weak unidentified diffraction peak at 2θ=30.4◦ was observed. The specific surface area (SBET ) was as high as 2.5 m2 /g with the equivalent particle size (DBET ) of 0.8 µm, which is expected to promote favorable reactivity for the subsequent PSA treatment.

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3.2

Postsynthesis Activation to CASN:Eu2+

Table 3 shows the physicochemical properties of the CASN:Eu2+ powders synthesized by the PSA treatment of the optimally prepared precursor EX5. The nitrogen content (CN ) was found to be in close agreement with the nominal values (30.1/30.4 wt%) in every sample, and the impurity oxygen content (CO ) was in the range of 2.4–3.0 wt%. The impurity carbon content (CC ) of the precursor powder decreased effectively by reacting with the residual oxygen originated from CaO, resulting in sufficiently low CC values of 0.09–0.17 wt% in the final CASN:Eu2+ powders. These results showed that both the powder properties and the anion compositions were scarcely affected by the doping concentration of Eu (CEu ) or by the duration of the PSA treatment. Figure 2 shows the XRD pattern of the synthesized CASN:Eu2+ powder (sample P2a). As expected from the almost ideal nitrogen stoichiometry, the synthesized CASN:Eu2+ powders possessed the high phase purity of ∼96–99% and contained only trace amounts of AlN and CaO (1.4 and 0.7 wt% for the sample P2a). The particle size estimated from the BET analysis increased to ∼3–4 µm by the PSA treatment, which might be ascribed to the transient liquid-phase formation promoted by the EuF3 activator with the melting point of ∼1260◦ C.23 The particle morphologies of the synthesized CASN:Eu2+ powders with the Eu concentration of 2% (sample P2a) and 1% (sample P1a) are shown in Figure 3. Both of the powders consisted of well-defined platelike primary particles of ∼5 µm, as expected from the high crystallinity indicated by the XRD. The results of the Rietveld refinement indicated an appreciable preferred orientation to [010] in every sample, suggesting that the observed platelet morphology reflects the growth of the a–c plane.

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3.3

Photoluminescent Properties

The PL excitation and emission spectra of the synthesized CASN:Eu2+ powders measured under the excitation at 450 nm are shown in Figure 4, and the relevant spectral parameters and the quantum efficiencies are listed in Table 4. The synthesized CASN:Eu2+ samples showed the broad excitation band suitable for the conversion of both near-ultraviolet and blue LEDs, and the emission band peaking at 648–655 nm with the full width at half-maximum (fwhm) of around 96 nm, as reported earlier for the conventionally prepared samples.2 The CASN:Eu2+ powders synthesized with the activator concentration of 2% (samples P2a and b) exhibited highly saturated deep red emission with the dominant wavelength (λd ) of 611 nm and the corresponding chromaticity coordinates of (0.67, 0.33). The P2 samples also exhibited the practically high absorption (Abs) and internal quantum efficiency (IQE) of >80%, and the resulting external quantum efficiency (EQE) reached as high as 70–72%. The blue-shifting of the emission peak wavelength (λpeak ) from ∼655 to 648 nm and the consequent improvement of the LER by ∼30% were attained for the P1 samples, whereas the Abs and IQE values appreciably decreased, due to the deviation of the activator concentration from the optimal value,2 considering the comparable physicochemical properties observed irrespectively of the Eu concentration (Table 3). Despite the lower QEs of the P1 samples, the resulting CIE Y values (relative luminance) were more than 80% of those for the P2 samples, owing to the aforementioned improvement of the LER. The prolonged PSA treatment up to 8 h led to no appreciable improvement of the QE values, while the PL properties have been precisely controlled by tuning the activator concentration, demonstrating the high reproducibility of the GRN–PSA process.

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3.4

Application to White LEDs

Practical performance of the synthesized CASN:Eu2+ phosphors in solid-state lighting applications was evaluated by the spectral simulation of trichromatic warm-white LEDs with the correlated color temperature (CCT) of 3500 K. The emission spectra of the simulated white LEDs are shown in Figure 5, and the relevant color rendering properties and the LER values are summarized in Table 5. The results for the system using the previously reported (Sr,Ca)2 Si5 N8 :Eu2+ -based phosphor19 are also shown for comparison. The LED systems using the synthesized CASN:Eu2+ phosphors can attain the high general CRI Ra value of 84, as well as the special CRIs R9 (strong red) of 75–95, R13 (skin tone of European women) of 88–91, and R15 (skin tone of Asian women) of 95–97. These results were satisfactory to meet the requirements for high-color rendering general lighting, e.g., defined by JIS Z 9112 (type AA, class WW-SDL, Ra > 82, R9 > 64, R15 > 82). The theoretical LER values for the LED systems using the CASN:Eu2+ phosphors with the activator concentration of 2% and 1% were 243 and 268 lm/W respectively, reflecting the LER of the CASN:Eu2+ phosphors themselves. While the system using the synthesized CASN:1% Eu2+ phosphor showed the CRIs comparable to those attained by the (Sr,Ca)2 Si5 N8 :Eu2+ phosphor, further improvement of the LER was achieved in the current system, owing to the optimally blue-shifted emission wavelength (648 nm vs 661 nm) and the narrower fwhm of the emission band (96 nm vs 114 nm).19

4

Conclusions

We have established a facile and industrially applicable new synthesis route for CASN:Eu2+ phosphors by using the GRN method. A fine CASN precursor powder with the DBET of 10


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0.8 µm was synthesized from the system CaO–Al2 O3 –SiO2 by optimizing the starting composition, at a low processing temperature of 1370◦ C. Highly crystalline CASN:Eu2+ powders with the phase purity of ≥96% were obtained by the moderate heat treatment of the GRN-derived precursor at 1600◦ C using an EuF3 activator. The tunable red emission with λd of 605–611 nm and the practically high EQE of ∼50–70% were achieved by the developed GRN–PSA process with reproducibility. The results of the spectral simulation of trichromatic warm-white LEDs have demonstrated that the systems using the synthesized CASN:Eu2+ phosphors can attain the high CRI Ra value of 84, along with the special CRIs of R9 =75–95, R13 =88–91, and R15 =95–97, showing the promising applicability to the general illumination with excellent color-rendering properties.

Acknowledgment.

T.S. is grateful to Mr. Y. Yajima of National Institute for

Materials Science for the chemical analyses.

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References (1) Sakuma, K.; Hirosaki, N.; Kimura, N.; Ohashi, M.; Xie, R.-J.; Yamamoto, Y.; Suehiro, T.; Asano, K.; Tanaka, D. White Light-Emitting Diode Lamps Using Oxynitride and Nitride Phosphor Materials. IEICE Trans. Electron. 2005, E88-C, 2057. (2) Uheda, K.; Hirosaki, N.; Yamamoto, Y.; Naito, A.; Nakajima, T.; Yamamoto, H. Luminescence Properties of a Red Phosphor, CaAlSiN3 :Eu2+ , for White Light-Emitting Diodes. Electrochem. Solid State Lett. 2006, 9, H22. (3) Li, J.; Watanabe, T.; Sakamoto, N.; Wada, H.; Setoyama, T.; Yoshimura, M. Synthesis of a Multinary Nitride, Eu-Doped CaAlSiN3 , from Alloy at Low Temperatures. Chem. Mater. 2008, 20, 2095. (4) Kubus, M.; Meyer, H.-J. A Low-Temperature Synthesis Route for CaAlSiN3 Doped with Eu2+ . Z. Anorg. Allg. Chem. 2013, 639, 669. (5) Suehiro, T.; Hirosaki, N.; Komeya, K. Synthesis and sintering properties of aluminium nitride nanopowder prepared by the gas-reduction–nitridation method. Nanotechnology 2003, 14, 487. (6) Suehiro, T.; Hirosaki, N.; Xie, R.-J.; Mitomo, M. Powder Synthesis of Ca-α′ -SiAlON as a Host Material for Phosphors. Chem. Mater. 2005, 17, 308. (7) Suehiro, T.; Hirosaki, N.; Xie, R.-J.; Sakuma, K.; Mitomo, M.; Ibukiyama, M.; Yamada, S. One-step preparation of Ca-α-SiAlON:Eu2+ fine powder phosphors for white light-emitting diodes. Appl. Phys. Lett. 2008, 92, 191904.

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(8) Suehiro, T.; Hirosaki, N.; Xie, R.-J.; Sato, T. Blue-emitting LaSi3 N5 :Ce3+ fine powder phosphor for UV-converting white light-emitting diodes. Appl. Phys. Lett. 2009, 95, 051903. (9) Suehiro, T.; Onuma, H.; Hirosaki, N.; Xie, R.-J.; Sato, T.; Miyamoto, A. Powder Synthesis of Y-α-SiAlON and Its Potential as a Phosphor Host. J. Phys. Chem. C 2010, 114, 1337. (10) Suehiro, T.; Hirosaki, N.; Xie, R.-J. Synthesis and Photoluminescent Properties of (La,Ca)3 Si6 N11 :Ce3+ Fine Powder Phosphors for Solid-State Lighting. ACS Appl. Mater. Interfaces 2011, 3, 811. (11) Hirosaki, N.; Suehiro, T. Jpn. Patent 4581120, 2010. (12) Hirosaki, N.; Suehiro, T. Jpn. Patent 5212691, 2013. (13) Hirosaki, N.; Suehiro, T. U.S. Patent 7598194 B2, 2009. (14) Hirosaki, N.; Suehiro, T. Chin. Patent ZL 2005800090952, 2009. (15) Hirosaki, N.; Suehiro, T. Korean Patent 1012063310000, 2012. (16) Izumi, F.; Momma, K. Three-dimensional visualization in powder diffraction. Solid State Phenom. 2007, 130, 15. (17) Ottinger, F. Ph.D. Thesis, ETH Z¨ urich, 2004. (18) Yaguchi, A.; Suehiro, T.; Sato, T.; Hirosaki, N. One-Step Preparation of BlueEmitting (La,Ca)Si3 (O,N)5 :Ce3+ Phosphors for High-Color Rendering White LightEmitting Diodes. Appl. Phys. Express 2011, 4, 022101. 13



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(19) Suehiro, T.; Xie, R.-J.; Hirosaki, N. Facile Synthesis of (Sr,Ca)2 Si5 N8 :Eu2+ -Based Red-Emitting Phosphor for Solid-State Lighting. Ind. Eng. Chem. Res. 2013, 52, 7453. (20) Ohno, Y. Color Rendering and Luminous Efficacy of White LED Spectra. Proc. of SPIE 2004, 5530, 88. (21) Huang, Z.-K.; Sun, W.-Y.; Yan, D.-S. Phase relations of the Si3 N4 –AlN–CaO system. J. Mater. Sci. Lett. 1985, 4, 255. (22) Chase, M. W. Jr. NIST-JANAF Thermochemical Tables Fourth Edition; National Institute of Standards and Technology: Gaithersburg, 1998. (23) Sobolev, B. P.; Tkachenko, N. L. Phase diagrams of BaF2 –(Y,Ln)F3 systems. J. Less-Common Met. 1982, 85, 155.

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Table 1. Phase Assemblage and Nitridation Extent of the Powders Synthesized under the Various GRN Conditions

sample

reaction conditions

phase assemblage

∆Wobs /∆Wtheor

ST1

1300◦ C, 2 h

“2CaO·Si3 N4 ·AlN”, AlN

0.84

ST2

1350◦ C, 2 h


0.83

ST3

1350◦ C, 4 h


0.86

ST4

1360◦ C, 4 h


0.88

ST5

1365◦ C, 4 h

CaAlSiN3 , AlN

1.12

ST6

1370◦ C, 4 h

CaAlSiN3 , AlN

1.18

ST7

1375◦ C, 2 h


0.88

ST8

1375◦ C, 4 h

CaAlSiN3 , AlN

1.15

ST9

1400◦ C, 2 h

N/A (molten)

0.69

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Table 2. Analyzed Cationic Ratios of the Samples Synthesized by the GRN (1370◦ C, 4 h) from the Starting Compositions with Excess Amounts of Ca

sample

a

final compositiona Ca Al Si

initial composition Ca Al Si

ST6

1.00

1.00

1.00

0.666(3)

1.036(5)

0.964(5)

EX1

1.20

1.00

1.00

0.761(2)

1.106(9)

0.894(6)

EX2

1.50

1.00

1.00

0.805(2)

1.094(5)

0.906(5)

EX3

2.00

1.00

1.00

0.872(2)

1.132(6)

0.868(6)

EX4

3.00

1.00

1.00

1.192(3)

1.121(7)

0.879(7)

EX5

2.00

0.75

1.00

1.083(3)

0.924(6)

1.076(6)

Normalized against the total Al + Si = 2.

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Table 3. Physicochemical Properties of the CASN:Eu2+ Powders Synthesized by the GRN–PSA Process

sample

CEu (mol %)

PSA conditions

puritya (wt %)

EX5b

a b

SBET (m2 /g)

DBET (µm)

CN (wt %)

CO (wt %)

CC (wt %)

2.50

0.8

27.2(1)

4.6(1)

1.5(1)

P2a

2.0

1600◦ C, 4 h

98

0.54

3.6

29.5(1)

2.4(1)

0.14(1)

P2b

2.0

1600◦ C, 8 h

99

0.41

4.6

29.6(1)

2.8(1)

0.09(1)

P1a

1.0

1600◦ C, 4 h

96

0.71

2.7

29.6(1)

2.9(1)

0.17(1)

P1b

1.0

1600◦ C, 8 h

96

0.66

2.9

29.4(1)

3.0(1)

0.10(1)

Phase purity determined by the XRD. Before the PSA treatment.

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Table 4. PL Properties of the Synthesized CASN:Eu2+ Phosphors under the Excitation at 450 nm

CIE coordinates x y

λd (nm)

λpeak (nm)

FWHM (nm)

P2a

0.668

0.330

611.4

654

96

0.83

0.86

P2b

0.666

0.331

610.9

655

94

0.84

P1a

0.644

0.352

604.8

648

96

P1b

0.644

0.352

604.9

648

97

sample

quantum efficiencies Abs IQE EQE

LER (lm/W)

CIE Y

0.72

99

1.00

0.83

0.70

101

1.02

0.74

0.62

0.46

133

0.83

0.75

0.65

0.49

131

0.81

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Table 5. CRI and LER Values for the Simulated Trichromatic White LED Systems Using the Synthesized CASN:Eu2+ Red-Emitting Phosphors

Ra

CRIs R9 R13

R15

LER (lm/W)

CASN:2% Eu2+

84

75

91

97

243

CASN:1% Eu2+

84

95

88

95

268

84

96

88

95

246

red phosphor used

(Sr,Ca)2 Si5 N8 :Eu2+ a

a

Ref. 19.

19



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Figure captions Figure 1. XRD pattern of the sample EX5 synthesized by the GRN process. Figure 2. X-ray Rietveld refinement pattern of the synthesized CASN:Eu2+ powder (sample P2a). Figure 3. FESEM micrograph of the CASN:Eu2+ powders synthesized by the GRN–PSA process: (a) sample P2a, and (b) sample P1a. Figure 4. PL excitation and emission spectra of the CASN:Eu2+ powders synthesized by the GRN–PSA process (samples P2a and P1a). The sharp lines in the excitation spectra are artifacts due to the monitoring wavelength. Figure 5. Simulated emission spectra of trichromatic white LED systems using the synthesized CASN:Eu2+ red-emitting phosphors (samples P2a and P1a). The broken line indicates the Planck curve with 3500 K.

20


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6000 CaAlSiN3 CaO AlN

5000

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


4000 3000 2000 1000 0 10

20

30

40

50

60

70

80

2θ (degree) Figure 1. XRD pattern of the sample EX5 synthesized by the GRN process.


90


Rwp = 13.37% S = 1.52

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

CASN CaO AlN

2θ (degree) Figure 2. X-ray Rietveld refinement pattern of the synthesized CASN:Eu2+ powder (sample P2a). ACS Paragon Plus Environment

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(a)

(b)

Figure 3. FESEM micrograph of the CASN:Eu2+ powders synthesized by the GRN—PSA process: (a) sample P2a, and (b) sample P1a. ACS Paragon Plus Environment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Normalized PL intensity


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CASN:2% Eu2+ CASN:1% Eu2+

1

0.5

0

300

400

500

600

700

800

Wavelength (nm) Figure 4. PL excitation and emission spectra of the CASN:Eu2+ powders synthesized by the GRN—PSA process (samples P2a and P1a). The sharp lines in the excitation spectra are artifacts due to the monitoring wavelength. ACS Paragon Plus Environment

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Intensity

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CCT=3500 K (0.405, 0.391) Red phosphor used: CASN:2% Eu2+ CASN:1% Eu2+ (Sr,Ca)2Si5N8:Eu2+

400

500

600

700

Wavelength (nm) Figure 5. Simulated emission spectra of trichromatic white LED systems using the synthesized CASN:Eu2+ red-emitting phosphors (samples P2a and P1a). The broken line indicates the Planck curve with 3500 K. ACS Paragon Plus Environment


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Table of Contents graphic


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Gas-ReductionâNitridation Synthesis of CaAlSiN3:Eu2+ Fine Powder

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