1. Introduction

In the mid 1970s, fluorescent lamps containing a mixture of triphosphors-the blue-emitting BaMgAl₁₀O₁₇:Eu²⁺ (BAM) [1,2], the red-emitting Y₂O₃:Eu³⁺ [3,4] oxides, and the green-emitting LaPO₄:Ce³⁺,Tb³⁺ [5,6]—became commercially available. These phosphors have been some of the most popular commercialized oxide phosphors. Fluorescent lamps (FLs) employing these oxide phosphors offered luminous output equivalent to that of the lamps employing the common calcium halophosphate phosphor [7]. Although the basics of industrial-scale phosphor synthesis were well-established a decade ago, the development of new phosphors continues because of the importance of phosphor efficiency required for different applications as well as for production cost and hence market share. The following are the basic requirements for an efficient FL phosphor [8]: it should retain its luminescent characteristics at 50 °C, i.e., the operating temperature of the FL; it must be capable of being prepared in a finely divided form (~7 μm) and should retain its luminescent characteristics over long periods of operation in a lamp; and it should be a non-toxic inorganic material of a sufficiently stable nature that will enable it to withstand both processing and operating conditions. A typical FL consists of a glass tube lined on the inside with a coating of phosphor material and filled with a mixture of mercury vapor and argon. As electric current flows through the lamp, the mercury atoms are bombarded by electrons and excited to produce emissions at 254 nm (~65%) and 185 nm (10 ~20%) [9] as well as in the longwave UV and visible light regions (weaker mercury emission lines at 365, 405, 436, 546, and 577 nm) [10]. Therefore, the phosphors must also have strong optical absorption in the shortwave UV region of 254 nm.

2. Experimental

Polycrystalline Ba₃Y(PO₄)₃:Ce³⁺,Tb³⁺ and Ca₃Y(AlO)₃(BO₃)₄:Eu³⁺ phosphors were synthesized by a conventional high-temperature solid-state reaction. For the synthesis of Ba₃Y(PO₄)₃:Ce³⁺,Tb³⁺ phosphors by mixtures of the chemical constituents in stoichiometric molar ratio of BaCO₃: Y₂O₃: (NH₄)₂HPO₄: CeO₂: Tb₄O₇ = 3: (1/2-x-y/4): 3: x: y/4 in an agate mortar, pressed into pellets and calcined at 1400°C for 6 h. The obtained samples were reduced at 1000 °C for 6 h under an reducing atmosphere of 15% H₂ / 85% N₂ in an alumina boat. For Ca₃Y(AlO)₃(BO₃)₄:Eu³⁺ phosphors, CaCO₃: Y₂O₃: Al₂O₃: H₃BO₃: Eu₂O₃ = 3: (1/2-x): 3/2: 4.2: x/2 were calcined at 1100 °C for 8 h, an excess 5 mole % of H₃BO₃ as a flux.

All crystal structure compositions were checked for phase formation by using powder X-ray diffraction analysis with a Bruker AXS D8 advanced automatic diffractometer with Cu Kα radiation. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples were analyzed by using a Spex Fluorolog-3 Spectrofluorometer equipped with a 450-W Xe light source. The Commission International de I’Eclairage (CIE) chromaticity coordinates for all samples were measured by a Laiko DT-101 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan). A white-light FL was fabricated by filling a single-U configuration quartz glass with mercury (~1.1 mg) and argon (~6.1 torr). The inner surface of the quartz glass lamp envelope has a luminescent coating–a blend of green-emitting BYP:0.25Ce³⁺,0.25Tb³⁺, red-emitting CYAB:0.5Eu³⁺ and blue-emitting BAM:Eu²⁺–which emits visible white-light when excited by UV radiation, and a pair of discharge electrodes each arranged at a respective sealed end of the lamp envelope.

3. Results and discussion

Figure 1 illustrates the X-ray powder diffraction patterns for Ba₃Y(PO₄)₃:Ce³⁺,Tb³⁺ (BYP:Ce,Tb) and Ca₃Y(AlO)₃(BO₃)₄:Eu³⁺ (CYAB:Eu) phosphors. The results indicate that all samples of BYP and CYAB match well with the XRD patterns for Ba₃Y(PO₄)₃ (JCPDS:044-0318) [11] and Ca₃Y(AlO)₃(BO₃)₄ (ICSD:172154) [12]. In addition doping BYP or CYAB with Ce³⁺ and Tb³⁺ or Eu³⁺ does not produce any significant change in the crystalline structure.

 

Fig. 1 X-ray powder diffraction patterns for BYP, BYP:Ce³⁺, BYP:Ce³⁺,Tb³⁺ (JCPDS:044-0318) and CYAB, CYAB:Eu³⁺ (ICSD:172154).

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Figure 2 shows the concentration effect of BYP:0.15Ce³⁺,xTb³⁺ and CYAB:xEu³⁺ with various concentrations of Tb³⁺ and Eu³⁺. For BYP:0.15Ce³⁺,xTb³⁺ phosphors, the photoluminescence (PL) intensity increases with Tb³⁺ content until a maximum x value of about 0.25 mol, after which it decreases due to interactions between Tb³⁺ ions. For CYAB:xEu³⁺ phosphors, the optimal doping concentration was observed at 0.50 mol, and the PL intensity was found to decline dramatically as the content of Eu³⁺ exceeded 0.50 mol due to concentration quenching. The PL spectra for BYP:Ce³⁺ phosphors show the parity- and spin-allowed 5d ¹ → 4f ¹ band emission of Ce³⁺ centered at 367 nm [13]. The BYP co-doped Ce³⁺, Tb³⁺ phosphor shows the broad band emission of Ce³⁺ and sharp characteristic emissions of Tb³⁺, i.e., several bands centered at 485, 540, 580, and 619 nm corresponding to transitions from the ⁵D₄ level to the ⁷F₆, ⁷F₅, ⁷F₄, and ⁷F₃ levels [14], respectively. Results for the CYAB:Eu³⁺ phosphors reveal that the absorption is mainly attributed to an O^2- → Eu³⁺ charge transfer at about 254 nm, and that the emission spectrum under 254 nm excitation consists of four groups of emission lines located at about 596, 620, 645 and 691 nm. These four groups are due to the ⁵D₀ → ⁷F_J (J = 1, 2, 3, 4) transitions [15]. The strong line at 620 nm corresponds to the electric dipole ⁵D₀ → ⁷F₂ transition.

 

Fig. 2 Concentration dependence of excitation and emission intensities for (a) BYP:0.25Ce³⁺,xTb³⁺ phosphors and (b) CYAB:xEu³⁺ phosphors (λ_ex = 254 nm).

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Figure 3 shows the optimal-emission intensity of the green-emitting phosphor BYP:0.25Ce³⁺,0.25Tb³⁺ compared to that of a commercial LaPO₄:Ce³⁺,Tb³⁺ phosphor, and the optimal-emission intensity of the red-emitting phosphor CYAB:0.5Eu³⁺ compared to those of phosphors Y₂O₃:Eu³⁺ and Y₂O₂S:Eu³⁺ under excitation at 254 nm. The PL spectrum of the BYP:Ce³⁺, Tb³⁺ phosphor shows sharp, characteristic green emission bands centered at 540 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ [14], and the emission intensity of BYP:0.25Ce³⁺,0.25Tb³⁺ is 70% of that of a commercial LaPO₄:Ce³⁺,Tb³⁺ phosphor. The CYAB:Eu³⁺ phosphor shows sharp red emissions centered at 620 nm attributed to the electric dipole ⁵D₀ → ⁷F₂ transitions of Eu³⁺ [15], and the emission intensity of CYAB:0.5Eu³⁺ is 70% and 83% of those of the Y₂O₃:Eu³⁺ and Y₂O₂S:Eu³⁺ phosphors, respectively.

 

Fig. 3 Relative emission intensities of (a) BYP:0.25Ce³⁺,0.25Tb³⁺ and commercial LaPO₄:Ce³⁺,Tb³⁺ phosphor; (b) CYAB:0.5Eu³⁺ and commercial Y₂O₃:Eu³⁺ or Y₂O₃:Eu³⁺ phosphors excited at 254 nm.

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Temperature dependence is highly important for FL applications. The temperature-dependent relative emission intensities of BYP:0.25Ce³⁺,0.25Tb³⁺ and CYAB:0.5Eu³⁺ phosphors and of commercial phosphors under excitation at 254 nm are compared in Fig. 4 . As seen from the insets, the relative emission intensity decreases as the temperature increases. We observed a decay of only 3% at 150 °C for BYP:0.25Ce³⁺,0.25Tb³⁺ (3% for LaPO₄:Ce³⁺,Tb³⁺), and a decay of 6% at 200 °C for CYAB:0.5Eu³⁺ (10% for Y₂O₃:Eu³⁺, 35% for Y₂O₂S:Eu³⁺). The results indicated that the thermal stability of BYP:0.25Ce³⁺,0.25Tb³⁺ above 150 °C was higher than that of the commercially available LaPO₄:Ce³⁺,Tb³⁺, and that CYAB:0.5Eu³⁺ above 200 °C showed less serious thermal quenching than Y₂O₃:Eu³⁺ or Y₂O₂S:Eu³⁺, respectively.

 

Fig. 4 Temperature dependence of relative emission intensities for (a) BYP:0.25Ce³⁺,0.25Tb³⁺; (b) CYAB:0.5Eu³⁺ (λ_ex = 254 nm). The insets show comparisons of thermal stability between (a) BYP:Ce³⁺,Tb³⁺ and LaPO₄:Ce³⁺,Tb³⁺; (b) CYAB:Eu³⁺, Y₂O₃:Eu³⁺ and Y₂O₂S:Eu³⁺ (λ_ex = 254 nm).

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Figure 5 illustrates the CIE chromaticity diagram of BYP:0.25Ce³⁺,0.25Tb³⁺ and CYAB:0.5Eu³⁺ phosphors mixed at different weight ratios of 1:0, 4:1, 3:1, 3:2, 3:3, 2:3, 1:3, and 0:1. The CIE chromaticity diagram is found to be tunable from green through yellow to red in the visible spectral region, with chromaticity coordinates (x, y) correspondingly varying from (0.323, 0.534) to (0.652, 0.342), by controlling the weights of the BYP:0.25Ce³⁺,0.25Tb³⁺ and CYAB:0.5Eu³⁺ phosphors. White-light phosphors were fabricated by using a phosphor blend of composition-optimized green-emitting BYP:0.25Ce³⁺,0.25Tb³⁺、red-emitting CYAB:0.5Eu³⁺ and blue-emitting BaMgAl₁₀O₁₇:Eu²⁺, which the CIE color coordinates x = 0.324 and y = 0.317.

 

Fig. 5 CIE chromaticity diagram of BYP:0.25Ce³⁺,0.25Tb³⁺ and CYAB:0.5Eu³⁺ phosphors mixed with different weight ratios and excited at 254 nm. (1) 1:0; (2) 4:1; (3) 3:1; (4) 3:2; (5) 3:3; (6) 2:3; (7) 1:3; (8) 0:1; and commercial phosphors (9) Y₂O₂S:Eu³⁺; (10) Y₂O₃:Eu³⁺; (11) LaPO₄:Ce³⁺,Tb³⁺; (12) BaMgAl₁₀O₁₇:Eu²⁺ ; (13) white-light phosphors (mixing of BYP:0.25Ce³⁺,0.25Tb³⁺、CYAB:0.5Eu³⁺、BaMgAl₁₀O₁₇:Eu²⁺); (14) white-light Fluorescent Lamp.

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Figure 6 is the EL spectrum of a white-light FL under a power of 8 watts. The radiation emitted by the gas discharge is mostly in the UV region of the spectrum, with only a small portion in the visible spectrum. The excited fluorescent layer efficiently emits a white light having three main peaks at around the 445 nm emission of BAM:Eu²⁺, the 540 nm emission of BYP:0.25Ce³⁺,0.25Tb³⁺ and the 620 nm emission of CYAB:0.5Eu³⁺. The white-light FL shows CIE chromaticity coordinates of x = 0.33, y = 0.35, a warm white light with a correlated color temperature (CCT) of 5646 K, and an average color-rendering index (CRI) of Ra = 70.

 

Fig. 6 PL spectrum of a fluorescent lamp fabricated using mercury vapor and mixture of BAM:Eu²⁺, BYP:0.25Ce³⁺,0.25Tb³⁺, and CYAB:0.5Eu³⁺ phosphors. The insets show the 8 W white-light fluorescent lamps.

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4. Conclusion

In summary, we have analyzed and demonstrated the creation of a warm white-light FL. The emission intensity for the composition-optimized green-emitting phosphor Ba₃(Y_0.5Ce_0.25Tb_0.25)(PO₄)₃ is 70% that of commercial LaPO₄:Ce³⁺,Tb³⁺, and the emission intensity for the red-emitting phosphor Ca₃(Y_0.5Eu_0.5)(AlO)₃(BO₃)₄ is 70% and 83% of those of Y₂O₃:Eu³⁺ and Y₂O₂S:Eu³⁺ phosphors, respectively. The Ba₃(Y_0.5Ce_0.25Tb_0.25)(PO₄)₃ and Ca₃(Y_0.5Eu_0.5)(AlO)₃(BO₃)₄ phosphors show high thermal stability compared to commercial LaPO₄:Ce³⁺,Tb³⁺, Y₂O₃:Eu³⁺ and Y₂O₂S:Eu³⁺, and matching blue-emitting phosphors show that the quality of the brightness and color reproduction is suitable for application to short UV FLs. These results indicate that the white-light FL has CIE = (0.33, 0.35), CCT = 5646 K, and CRI = 70.