English

• 1. INTRODUCTION

  Light-emitting diode (LED) lighting has many advantages such as high luminous efficiency, low energy consumption, environmental protection and long service life. Now it has been widely used in various lighting fields^[1-7]. Phosphor-converted white LEDs (WLEDs) are the mainstream of applications^[8], wherein yellow powder phosphor (mainly YAG: Ce³⁺) combined with blue LED chip (InGaN, peak around 460 nm) can yield white light^[9-11]. This lighting is only composed of blue and yellow component resulting in cold white lighting, which is harmful to the health of human eyes. This deficiency can be compensated by the introduction of red light component. For classical phosphor-converted WLEDs, aging of organic resin and heat quenching of phosphors are inevitable, which significantly affect related service life and lighting quality^[12-15]. Replacing resin-mixed phosphors materials with crystalline phosphor ceramics can effectively solve the above problems, especially for high-power WLEDs. Gel-casting as a near-net forming method own various advantages for fabricating ideal LED phosphor ceramics with complicated shapes in short forming time^[16-18]. The luminescent properties of Mn²⁺ activated phosphors are governed by the covalent character of the respective bonds between the activator and the adjacent anions. It was reported that Mn²⁺ ion in a weak crystal field could emit a green light, while could give rise to a red light in a strong crystal field^[8]. Bi³⁺ doping can significantly enhance the emission intensity of YAG: Ce³⁺ system^[19-22]. Thus, Bi³⁺ was introduced into YAG: Ce³⁺, Mn²⁺ ceramic system and expected to reduce the doping amount of Mn²⁺. After systematical characterizations, the effect of Bi³⁺ upon the luminescence property of YAG: Ce³⁺, Mn²⁺ was studied, involving the performance of the corresponding WLED module.

  2. EXPERIMENTAL

  2.1 Syntheses of the materials

  Cerium nitrate (Ce(NO₃)₃·6H₂O, 99.95%), manganese monoxide (MnO, 99.99%), bismuth oxide (Bi₂O₃, 99.99%), alumina (Al₂O₃, 99.99%), yttrium oxide (Y₂O₃, 99.99%) and silica (SiO₂, A. R.) were weighed according to the stoichiometric ratio of Y_2.994-zCe_0.006BizAl_5-2yMn_ySi_yO₁₂ and mixed by a ball milling in ethanol with 0.5 wt% tetraethyl orthosilicate (TEOS, 99%) and 0.05 wt% magnesium oxide (MgO, 99.9%) as the sintering additives followed by drying and sieving^[23].

  The powders were mixed and transferred into a slurry having a solid loading of 55 vol% (relative to the total volume of powder and water) deionized water. Simultaneously, 15 wt% (relative to water) acrylamide (A. R. 99%) was added as the monomer, 1.2 wt% (relative to water) N, N΄-methylene diacrylamide (99%) and 2.5 wt% (relative to powder) poly(acrylate ammonium) (PAA, 40%+) was used as gelling and dispersant agent, respectively. The slurry was degassed in a vacuum mixer to reduce trapped air bubbles and then N, N, N΄, N΄-tetramethylethylenediamine (TEMED, 99%) was added as a catalyst and ammonlum persulfate (APS, 99.99%) as an initiator. Then, the slurry was cast into the metal mold and gelled in constant temperature and humidity chamber at 65 ℃ for 1 h. After gelation and demolding, the green body was dried for 24 h at room temperature and then presintered at 1000 ℃ for 10 h to burn out organic additives. Final sintering was performed at 1750 ℃ for 10 h in vacuum. The obtained ceramic was annealed at 1350 ℃ for 8 h in air and then ground and polished on both sides.

  2.2 Material characterization

  The phase structures of specimens were identified by X-ray diffractometer with Cu-Kα tube (XRD, Model MiniFlex600, Rigaku, Japan, 10 < 2θ < 80°, 10 °/min) and were refined by Rietveld method. The fracture surfaces of the green bodies and sintered ceramics were observed by scanning electron microscopy (SEM, Model Nova NanoSEM230, FEI, USA). Mirror-polished specimens on both surfaces were used to measure optical transmittance (UV-visible near-infrared spectrophotometer, Lambda 950, Perkin Elmer, USA). The densities of ceramics were measured by the Archimedes method. Photoluminescence (PL) and photoluminescence excitation (PLE) were recorded by a spectrophotometer (FLS920, Edinburgh Instruments, Britain) equipped with a 450 W xenon lamp as the light source. In order to study potential application in WLEDs, these ceramics were assembled with commercial blue LED chip in the COB package mode with the help from Fujian CAS-Ceramic Optoelectronics Technology Co., Ltd.

  3. RESULTS AND DISCUSSION

  3.1 Structure determination

  All of the Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (0≤z≤0.01) ceramics are in light orange color (Fig. 1), which is different from yellow color of YAG: Ce ceramics. Fig. 2 shows the XRD patterns of YAG: Ce³⁺, Mn²⁺, Bi³⁺ ceramics with different Bi³⁺ contents. All diffraction peaks are well indexed as cubic garnet structures of Y₃Al₅O₁₂ (YAG No 00-071-0255) without any impurities. The lattice parameters of YAG doped Ce³⁺ and Mn²⁺ are larger than that of pure YAG (12.01068 Å vs. 12.0060 Å, Table 1). The lattice parameter increases gradually with the increasing amount of Bi³⁺ doping, implying a partial substitution of Ce³⁺ ions (radius 1.143 Å, 8-CN) and Bi³⁺ ions (radius 1.17 Å, 8-CN) for Y³⁺ sites (radius 1.019 Å, 8-CN), Mn²⁺ ions (radius 0.66 Å, 4-CN; radius 0.67 Å, 6-CN) for Al³⁺ sites (radius 0.39 Å, 4-CN; radius 0.535 Å, 6-CN) and Si⁴⁺ ion (radius 0.40 Å, 4-CN) for Al³⁺ sites (radius 0.535 Å, 4-CN) in the garnet lattice^{[21, 24]}.

  Figure 1

  

  Figure 1.  Photograph of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (0≤z≤0.01) ceramics

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  Figure 2

  

  Figure 2.  XRD patterns of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (0≤z≤0.01) ceramics

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  Table 1

  

  Table 1.  Lattice Parameters of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (0≤z≤0.01) Ceramics

  DownLoad: CSV

  Sample	z = 0	z = 0.00005	z = 0.0001	z = 0.0025	z = 0.0075	z = 0.01
  α (Å)	12.0107	12.0116	12.0129	12.0154	12.0293	12.0455

  3.2 Fluorescent properties

  As shown in Fig. 3, the excitation spectra of the two specimens display strong broad absorption peaks at 340 and 458 nm, which can be assigned to the ²F_5/2→ 5d¹ and ²F_7/2→ 5d¹ energy level transition of Ce³⁺ ion^[22]. This indicates that YAG: Ce³⁺, Mn²⁺, Bi³⁺ can effectively absorb the blue light emitted by the blue LED chip. In addition, YAG: Ce³⁺, Mn²⁺, Bi³⁺ ceramics owns a higher absorption intensity than YAG: Ce³⁺, Mn²⁺ ceramics, demonstrating that Bi³⁺ ions can enhance the absorption resulting in a higher emission intensity. There is no difference in the excitation spectra between the samples with Bi³⁺ doping and without Bi³⁺ doping (Fig. 4), indicating that the introduction of Bi³⁺ does not change coordination environment around the activators^[20]. As shown in Fig. 5, the emission intensity increases first and then decreases with increasing Bi³⁺ ions. When the doping concentration of Bi³⁺ is 0.0001 mol, the emission intensity reaches the maximum, which is 58% higher than that of no Bi³-doping.

  Figure 3

  

  Figure 3.  Absorption spectra of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (z = 0, z = 0.0001) ceramics

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  Figure 4

  

  Figure 4.  PLE spectra of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (z = 0, z = 0.0001) ceramics (monitored at 533 nm)

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  Figure 5

  

  Figure 5.  PL spectra of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (0≤z≤0.01) ceramics

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  3.3 Mechanism hypothesis

  Fig. 6 is the schematic energy level diagrams of Bi³⁺-Ce³⁺ions. Earlier studies of the luminescence of Bi³⁺ ions in oxides have shown that the excitation band belongs to the ¹S₀ → ³P₁ transition and the emission band belongs to the ³P₁ → ¹S₀ transition. The excitation spectrum of Ce³⁺ ions overlaps with the emission spectrum of Bi³⁺ ions^[25]. The Bi³⁺ ions emission through ³P₁ → ¹S₀ transition can be absorbed by the ground state of Ce³⁺ ions, and the additional energy can excite Ce³⁺ ions^{[21, 22]}. Therefore, the increased strength of PL is mainly attributed to the reabsorption of energy from the emission band of Bi³⁺ ions by Ce³⁺ ions. However, when the doping Bi³⁺ is too much, a plurality of Bi³⁺ ions tends to be agglomerate. These agglomerates serve as a capture center to generate a non-radiative transition to the ground state, and the corresponding energy transfer to the Ce³⁺ ions is reduced. Thus, the energy transfer from Bi³⁺ to Ce³+ is weakened and further lower luminescence intensity of Ce³⁺ when the doping Bi³⁺ exceeds the critical concentration (z = 0.0001).

  Figure 6

  

  Figure 6.  Schematic energy level diagrams of Bi³⁺-Ce³⁺ ions

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  3.4 Microstructure

  Due to the sensitization of Bi³⁺ ions, the emission intensity of Ce³⁺ ions becomes higher and the energy transfer from Ce³⁺ to Mn²⁺ is further enhanced. Therefore, the optimal concentration of Mn²⁺ ions is bound to decrease. Based on the XRD patterns of YAG: Ce³⁺, Mn²⁺, Bi³⁺ ceramics with different Mn²⁺ contents (Fig. 7), all the diffraction peaks are well indexed as cubic garnet structure of Y₃Al₅O₁₂ (YAG No 00-071-0255) without any impurity. As shown in Fig. 8a, the green body is well-balanced and has almost no reunion. The particles in the green body adhere to the polymer gel, which can offer enough strength to support the weight of the green body (Fig. 8a). The green body remains without distortion after being calcined at 1000 ℃ for 10 h to completely remove the polymer gel (Fig. 8b). Consequently, obtained ceramics are highly-dense (relative density 99.95%) and pore-free with average grain size about 17 µm and clean grain boundaries (Fig. 8c, 8d).

  Figure 7

  

  Figure 7.  XRD patterns of Y_2.9939Ce_0.006Bi_0.0001Al_5-2yMn_ySi_yO₁₂ (0.01≤y≤0.04) ceramics

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  Figure 8

  

  Figure 8.  SEM images of green body as-prepared (a), after calcining (b), and thermal-etched surface of Y_2.9939Ce_0.006Bi_0.0001Al_4.96Mn_0.02Si_0.02O₁₂ ceramic using slurry with 55 vol% solid (c, d)

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  3.5 Application evaluation

  The emission band peaked at 533 nm is broad and can be attributed to the electronic transition of Ce³⁺ ions 5d → 4f (Fig. 9), and that of 590 nm can be attributed to the ⁴T_1g → ⁶A_1g electronic transition of Mn²⁺ ion^[26-29]. By the introduction of Mn²⁺ from 0.01 to 0.04, the emission peak is significantly red-shifted from 533 to 590 nm. With increasing Mn²⁺ doping, the emission peaked at 533 nm gradually weakens, and the emission peaked around 590 nm firstly increases and then decreases. When y = 0.01, the emission peaked at 590 nm reaches the maximum, which indicates that the energy transfer of Ce³⁺ → Mn²⁺ is enhanced in comparison with no Bi³⁺-doped YAG: Ce³⁺, Mn²⁺. The in-line transmi-ttance of 1 mm thick Y_2.9939Ce_0.006Bi_0.0001Al_4.96Mn_0.02Si_0.02O₁₂ ceramic is up to 81.6% (at 1100 nm), implying high transparency and no significant defects (Fig. 10). The transmittance decreases gradually with increasing the Mn²⁺ doping contents. Two board absorption bands centered at 340 and 458 nm in the spectra are assigned to the 4f → 5d transitions of Ce³⁺ ion.

  Figure 9

  

  Figure 9.  PL spectra of Y_2.9939Ce_0.006Bi_0.0001Al_5-2yMn_ySi_yO₁₂ (0.01≤y≤0.04) ceramics

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  Figure 10

  

  Figure 10.  Photograph (up) and UV-visible spectra (down) of Y_2.9939Ce_0.006Bi_0.0001Al_5-2yMn_ySi_yO₁₂ (0.01≤y≤0.04) ceramics

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  In order to evaluate the potential practical application in WLEDs, these ceramics were assembled with commercial blue LED chip in the COB package mode. With increasing the amount of Mn²⁺ ions, the corresponding color coordinates move from cold white to warm white region (Fig. 11). For Y_2.9939Ce_0.006Bi_0.0001Al_4.96Mn_0.02Si_0.02O₁₂ ceramic (thickness 0.6 mm), related WLED module exhibits correlated color temperature (CCT) 3960 K and luminous efficiency (LE) 92 lm/W.

  Figure 11

  

  Figure 11.  CIE chromaticity diagram of Y_2.9939Ce_0.006Bi_0.0001Al_5-2yMn_ySi_yO₁₂ (0.01≤y≤0.04) ceramics

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  4. CONCLUSION

  The Bi³⁺ doped YAG: Ce³⁺, Mn²⁺ ceramics were synthesized by gel-casting method. The doping effect and related mechanism of Bi³⁺ on the luminescence intensity of YAG: Ce³⁺, Mn²⁺ were studied. The luminescence intensity firstly increases with increasing Bi³⁺ doping, approaches the maximum as Bi³⁺ being 0.0001 and then decreases. The energy transfer from Bi³⁺ to Ce³⁺ leads to the improvement of emission intensity up to 58% and a red-shifted peak at 590 nm. The obtained ceramics exhibits clean grain boundary, high relative density (99.95%) and in-line transmittance as high as 81.6% at 1100 nm. The COB-packaged LED module assembled from Y_2.9939Ce_0.006Bi_0.0001Al_4.96Mn_0.02Si_0.02O₁₂ phosphor ceramic displays CCT 3960 K and LE 92 lm/W, implying that the YAG: Ce³⁺, Mn²⁺, Bi³⁺ ceramic can serve as a promising phosphor material for warm WLEDs application.

  
  
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• 

  Figure 1  Photograph of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (0≤z≤0.01) ceramics

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  Figure 2  XRD patterns of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (0≤z≤0.01) ceramics

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  Figure 3  Absorption spectra of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (z = 0, z = 0.0001) ceramics

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  Figure 4  PLE spectra of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (z = 0, z = 0.0001) ceramics (monitored at 533 nm)

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  Figure 5  PL spectra of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (0≤z≤0.01) ceramics

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  Figure 6  Schematic energy level diagrams of Bi³⁺-Ce³⁺ ions

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  Figure 7  XRD patterns of Y_2.9939Ce_0.006Bi_0.0001Al_5-2yMn_ySi_yO₁₂ (0.01≤y≤0.04) ceramics

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  Figure 8  SEM images of green body as-prepared (a), after calcining (b), and thermal-etched surface of Y_2.9939Ce_0.006Bi_0.0001Al_4.96Mn_0.02Si_0.02O₁₂ ceramic using slurry with 55 vol% solid (c, d)

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  Figure 9  PL spectra of Y_2.9939Ce_0.006Bi_0.0001Al_5-2yMn_ySi_yO₁₂ (0.01≤y≤0.04) ceramics

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  Figure 10  Photograph (up) and UV-visible spectra (down) of Y_2.9939Ce_0.006Bi_0.0001Al_5-2yMn_ySi_yO₁₂ (0.01≤y≤0.04) ceramics

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  Figure 11  CIE chromaticity diagram of Y_2.9939Ce_0.006Bi_0.0001Al_5-2yMn_ySi_yO₁₂ (0.01≤y≤0.04) ceramics

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  Table 1.  Lattice Parameters of Y_2.994-zCe_0.006Bi_zAl_4.9Mn_0.05Si_0.05O₁₂ (0≤z≤0.01) Ceramics

  Sample	z = 0	z = 0.00005	z = 0.0001	z = 0.0025	z = 0.0075	z = 0.01
  α (Å)	12.0107	12.0116	12.0129	12.0154	12.0293	12.0455

  下载: 导出CSV
•