A class of thermal stable of green-emitting phosphors Ba3Y(PO4)3:Ce3+,Tb3+ (BYP:Ce3+,Tb3+) and red-emitting phosphors Ca3Y(AlO)3(BO3)4:Eu3+ (CYAB:Eu3+) for white-light fluorescent lamps were synthesized by high temperature solid-state reaction. 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 4% for CYAB:0.5Eu3+ (7% for Y2O3:Eu3+, 24% for Y2O2S:Eu3+). The emission intensity of composition-optimized Ba3(Y0.5Ce0.25Tb0.25)(PO4)3 is 70% of that of commercial LaPO4:Ce3+,Tb3+ phosphors, and the CIE chromaticity coordinates are found to be (0.323, 0.534). The emission intensity of Ca3(Y0.5Eu0.5)(AlO)3(BO3)4 is 70% and 83% of those of Y2O3:Eu3+ and Y2O2S:Eu3+ phosphors, respectively, and the CIE chromaticity coordinates are redder (0.652, 0.342) than those of Y2O3:Eu3+ (0.645, 0.347) and Y2O2S:Eu3+ (0.647, 0.343). A white-light fluorescent lamp is fabricated using composition-optimized Ba3(Y0.5Ce0.25Tb0.25)(PO4)3 and Ca3(Y0.5Eu0.5)(AlO)3(BO3)4 phosphors and matching blue-emitting phosphors. The results indicate that the quality of the brightness and color reproduction is suitable for application in shortwave UV fluorescent lamps. The white-light fluorescent lamp displays CIE chromaticity coordinates of x = 0.33, y = 0.35, a warm white light with a correlated color temperature of 5646 K, and a color-rendering index of Ra = 70.
© 2010 OSA
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 . 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 : 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%)  as well as in the longwave UV and visible light regions (weaker mercury emission lines at 365, 405, 436, 546, and 577 nm) . Therefore, the phosphors must also have strong optical absorption in the shortwave UV region of 254 nm.
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)  and Ca3Y(AlO)3(BO3)4 (ICSD:172154) . In addition doping BYP or CYAB with Ce3+ and Tb3+ or Eu3+ does not produce any significant change in the crystalline structure.
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 . 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 , respectively. Results for the CYAB:Eu3+ phosphors reveal that the absorption is mainly attributed to an O2- → 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 → 7FJ (J = 1, 2, 3, 4) transitions . The strong line at 620 nm corresponds to the electric dipole 5D0 → 7F2 transition.
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+ , 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+ , and the emission intensity of CYAB:0.5Eu3+ is 70% and 83% of those of the Y2O3:Eu3+ and Y2O2S:Eu3+ phosphors, respectively.
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.
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.
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.
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(Y0.5Ce0.25Tb0.25)(PO4)3 is 70% that of commercial LaPO4:Ce3+,Tb3+, and the emission intensity for the red-emitting phosphor Ca3(Y0.5Eu0.5)(AlO)3(BO3)4 is 70% and 83% of those of Y2O3:Eu3+ and Y2O2S:Eu3+ phosphors, respectively. The Ba3(Y0.5Ce0.25Tb0.25)(PO4)3 and Ca3(Y0.5Eu0.5)(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.
This research was supported by National Science Council of Taiwan under contract No. NSC98-2113-M-009-005-MY3 (T. M. C.).
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