Introduction

A luminescent material absorbs various types of energy and emits thermal and electromagnetic radiation in the ultraviolet (UV), infrared, and visible ranges with the last being most common. In particular, inorganic luminescent materials have been studied for more than 100 years and are now used everyday in display technologies, such as fluorescent lamps, cathode ray tubes (CRTs), X-ray plates, advertisement signboards, plasma display panels (PDP), and light emitting diodes (LEDs), as well as in applications in fields such as advanced medicine and security, all of which are areas where people require information.

Nanocrystalline and amorphous alloys of a desired composition can be synthesized from pure crystalline powder using a mechanical alloying (MA) method [1-4]. Koch [5] applied MA to Ni₆₀Nb₄₀ production, and Schwarz et al. [6] have used it to examine the thermodynamics of amorphous materials.

Although there have been advancements in luminescent materials since the commercialization of GaN blue LEDs in 1966, such as the development of the white LED, their still requires improvement. Red luminescent materials are not efficient because the difference between their excitation and emission energies is greater than those in the case any other luminescent material. Therefore, highly efficient red luminescent materials, rather than green or blue luminescent materials, for blue LEDs and near-UV LEDs are needed.

Chong et al. [7] examined the photoluminescent (PL) properties of Y₂O₃:Eu³⁺ phosphor films fabricated with a sol-gel process. Dhoble et al. [8] prepared the same by using combustion synthesis and precipitation techniques. PL characteristics and X-ray excitation of films fabricated by using the two different methods were compared. Wang et al. [9] synthesized Y_0.95-xM_xBO₃:0.05Eu³⁺ (M:Bi³⁺, La³⁺, Sc³⁺, 0 ≤ x ≤ 0.1) for a PDP by using a solid-state reaction and examined it under ultraviolet light. Chen et al. [10] reported the preparation and PL properties of YBO₃:Eu³⁺ powders synthesized by using a solvothermal process with isopropanol used as a solvent.

This study examined the PL properties of Eu³⁺-doped Y₂O₃-H₃BO₃ powders synthesized by using MA. X-ray diffractometry (XRD) and luminescence spectrophotome- try were used to measure the concentrations of Eu³⁺ as a function of the annealing time, annealing temperatures, and milling time (t_m).

Experimental

Y₂O₃-H₃BO₃:Eu³⁺ mixtures were prepared by mixing yttrium oxide (Y₂O₃) and boric acid (H₃BO₃) at a molar ratio of 1:1, followed by the addition of 1, 2, 4, 6, 8, and 10 mol% europium(III) nitrate hexahydrate (Eu(NO₃)₃·6H₂O) (FW 446.07). The mixtures were then milled using a planetary ball mill at room temperature under the following conditions: 2.68 g powder and 13 zirconia balls, 15 mm in diameter at sample-to-balls weight ratio of 50:1. These were rotated in an 80 mL zirconia jar at 500 rpm. For accurate analysis, a same amount of the mixture was milled under similar conditions, for t_m of 0, 15, 30, 60, 120, and 300 min. The grinding operation was stopped for 30 min, which was followed by grinding for 15 min to avoid an excessive increase in temperature inside the mill pot. The as-milled powders were annealed at 700, 800, 900, 1000, 1100, and 1200ºC for 1, 2, and 3 h, respectively.

XRD (PANalytical, X'pert PRO MPD) was performed at room temperature with Cu Ka (λ = 1.54060 Å) radiation. The acceleration voltage and current were 40 kV and 30 mA, respectively. The irradiation rate was 1 sec/ 0.03 step over the 2θ measurement range of 10-100°. The PL of the Eu doped Y₂O₃-H₃BO₃ mixtures was measured between 520 nm and 700 nm as a function of the annealing temperature, annealing time, and t_m. A xenon discharge lamp (20 kW) was used under the following conditions: excitation at 240 nm, excitation slit at 10.0 nm, emission slit at 8.0 nm excitation, and scan speeds of 300 nm/min.

Results and Discussion

Fig. 1 shows the XRD patterns of the Y₂O₃-H₃BO₃:Eu³⁺ mixtures synthesized by MA as a function of t_m. The H₃BO₃ peak at approximately 2θ = 28.0° disappeared in the case of 15 min milling, indicating the formation of a solid solution of H₃BO₃ in the Y₂O₃. After milling for 120 min, the XRD intensity began to decrease, and all elemental lines broadened slightly [11]. This peak broadening reflects not only a decrease in crystallite size, but also strain induced by the ball-milling. After milling for 300 min, some Y₂O₃ peaks and amorphous formations were observed[12].

Fig. 1

X-ray diffraction patterns of Y₂O₃-H₃BO₃:Eu³⁺ powders fabricated by planetary ball milling as a function of different t_m.

Fig. 2 shows the PL intensity of a 300-min-milled Y₂O₃-H₃BO₃:Eu³⁺ (Eu = 8 mol%) powder mixture, milled for 300 min, that had been annealed for 1 h at every temperature increase of 100ºC from 700ºC to 1200ºC, and subjected to 240 nm excitation. The intensity was highest when the mixture was annealed at 800°C, and decreased for annealing temperatures above 800°C. The samples showed a more broad emission, and a shift of the peaks with increasing annealing temperature above 800°C has been recorded. This phenomena of the peaks can be attributed to the changes in the crystal structure [13]. Further investigation on the mechanism of above phenomena and more detailed measurement of Y₂O₃- H₃BO₃:Eu³⁺ system are need. The PL intensity at 613 nm for the mixture of 800°C was highest for the mixture annealed at 800°C, and whereas the intensity at 628 nm was similar regardless of annealing temperature. The overall shape of the luminescence PL intensity differed a little with from that of a Eu = 4 mol% doped mixture [12]. When the mixtures were subjected to 240 nm excitation, the PL intensities were observed at approximately 592 nm, 613 nm, 628 nm, and 650 nm. This might be due to the transition of free Eu³⁺ ions from the ⁵D_o level to the ⁷F_j (j = 1-3) level. The orange luminescence peak at approximately 592 nm might be due to the ⁵D_o → ⁷F₁ magnetic dipole transition of the Eu³⁺ ions. The red luminescence peaks at approximately 613 nm and 628 nm were assigned to the ⁵D_o → ⁷F₂ forced electric-dipole transition [14,15]. The luminescence peak at approximately 650 nm was attributed to a ⁵D_o → ⁷F₃ transition [16]. We compared the PL emission spectra of Y₂O₃-H₃BO₃:Eu³⁺mixture by MA with that of YBO₃:Eu³⁺ samples by electrodeposition and ionic liquid-assisted hydrothermal process [17,18]. While the intensities and the shapes of the PL spectra differed slightly, the peak locations are very similar.

Fig. 2

PL emission spectra for Y₂O₃-H₃BO₃:Eu³⁺ (Eu: 8 mol%) mixtures milled for 300 min annealed at the indicated temperatures.

Fig. 3 shows the variations in PL intensity for Y₂O₃- H₃BO₃:Eu³⁺ powder mixtures due to differing concentrations of Eu and differing annealing temperatures (1 h annealing time). The PL intensity of the powder mixture was highest after annealing at 800°C with the addition of Eu = 8.0 mol% for Y₂O₃-H₃BO₃ powder mixture. The PL intensity gradually decreased with increasing annealing temperatures. This phenomenon was different from a Eu = 4.0 mol% doped Y₂O₃-H₃BO₃ mixture, with the highest intensity occurring after annealing at 900°C [12].

Fig. 3

The variations in the PL emission spectra for Y₂O₃- H₃BO₃:Eu³⁺ mixtures (t_m = 300 min) as a function of different temperatures and Eu³⁺concentrations.

Fig. 4 shows the PL intensity of Y₂O₃-H₃BO₃:Eu³⁺ (Eu: 8 mol%) powder mixture milled for 300 min and annealed for 1, 2, and 3 h at 800ºC. Based on the PL intensity dependence annealing time, the highest luminescence intensity was obtained at wavelengths of 613 nm for the 1 h annealed mixture, as it was the case for the mixtures milled for 120 min, regardless of Eu mol%, but the luminescence intensity decreased with increasing annealing time.

Fig. 4

PL emission spectra for Y₂O₃-H₃BO₃:Eu³⁺ (Eu:8 mol%) mixtures milled for 300 min and annealed 800ºC for the indicated time.

Fig. 5 compares the variation in PL intensities of Y₂O₃- H₃BO₃:Eu³⁺ (Eu:4 mol%, Eu:8 mol%) powder mixtures milled for 120 min and 300 min and then annealed for 1 h at every 100ºC from 700ºC to 1200ºC. The luminescence intensities for powder mixtures milled for 120 min were low, but in general, more than those for powder mixtures milled for 300 min. For the Y₂O₃-H₃BO₃:Eu³⁺ powder mixtures milled for 120 min, the highest PL intensity was observed for annealing temperatures of 1000°C at Eu = 4.0 mol%. The other luminescence intensities of the powder mixture were similar.

Fig. 5

The variations in the PL emission spectra for Y₂O₃- H₃BO₃:Eu³⁺ (Eu:4 mol% and 8 mol%) for milling time of 120 min and 300 min.

Fig. 6 shows the XRD pattern of the Y₂O₃-H₃BO₃:Eu³⁺ mixture milled for 300 min and annealed at 800ºC for 1 h with Eu = 8 mol%. Peaks corresponding to YBO₃ with Y₂O₃ are observed.

Fig. 6

X-ray diffraction patterns for Y₂O₃-H₃BO₃:Eu³⁺ (Eu: 8 mol%) mixtures annealed at 800ºC for 1 h.

Conclusions

Y₂O₃-H₃BO₃:Eu³⁺ powders were synthesized at room temperature using a MA method. In the case of milling for 300 min, some PL peaks due to Y₂O₃ ([222], [440], [622]) and amorphous formations were observed. The 300-min-milled mixture that was annealed at 800°C for 1 h with Eu = 8 mol% had the strongest PL intensity for each annealing temperature investigated (increasing from 700ºC to 1200ºC at increments of 100ºC). The PL of the powder mixture, excited by a xenon discharge lamp (20 kW) at 240 nm, was detected at approximately 592 nm (orange color, ⁵D_o → ⁷F₁), 613 nm, 628 nm (red color, ⁵D_o → ⁷F₂), and 650 nm. The highest luminescence intensity was obtained for mixtures annealed for 1 h, but the luminescence intensity decreased with increasing annealing time. The luminescence intensities of powder mixtures milled for 120 min were low in general, but more than those of powder mixtures milled for 300 min, other conditions being equal, therefore we could know that Y₂O₃- H₃BO₃:Eu³⁺ powders must be milled for more than 120 min. The PL peaks due to the presence of YBO₃ and Y₂O₃ were observed after annealing at 800ºC for 1 h for a 300- min-milled Y₂O₃-H₃BO₃ with Eu = 8 mol%.

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