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Microstructure, and luminescence properties of LiBaPO4:Dy3+ phosphors with various Dy3+ concentrations prepared by microwave assisted sintering
Journal of Luminescence 145 (2014) 49–54
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Microstructure, and luminescence properties of LiBaPO4:Dy3+ phosphors with various Dy3+ concentrations prepared by microwave assisted sintering Ru-Yuan Yang n, Hsuan-Lin Lai Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung County 912, Taiwan
art ic l e i nf o
a b s t r a c t
Article history: Received 4 December 2012 Received in revised form 24 May 2013 Accepted 9 July 2013 Available online 16 July 2013
LiBaPO4:Dy3+ phosphors were synthesized by microwave-assisted and conventional sintering. XRD analysis confirmed the phase formation of all the LiBaPO4:Dy3+ phosphors. PL results showed that the optimum concentration of Dy3+ for LiBaPO4:Dy3+ prepared by microwave-assisted sintering is 7 mol%. The phosphors were efficiently excited by the UV–vis light region from 300 to 400 nm, and exhibited blue (483 nm), yellow (576 nm) and red (671 nm) emission corresponding to 4F9/2-6H15/2, 6H13/2, and 6 H11/2 transitions, respectively. The microwave-assisted sintering improves the sintering behavior and provides a more uniform particle morphology of LiBa0.03PO4:0.07Dy3+ phosphors so as to obtain a luminescence intensity greater than that obtained by conventional sintering even at the same sintering temperature. Moreover, all the chromaticity (x, y) of the LiBa0.03PO4:0.07Dy3+ phosphors are located in the white region (0.33, 0.37) even on using microwave-assisted sintering as the heat treatment did not influence the purity of LiBa0.03PO4:0.07Dy3+ phosphors. & 2013 Published by Elsevier B.V.
Keywords: Phosphors Microwave processing Sintering Optical properties
1. Introduction White light-emitting diodes (WLEDs) are widely seen as being the next generation of solid-state lighting, and have attracted considerable attention because of their energy efficiency, high brightness, long lifetime, harmlessness and environment friendliness [1]. The present strategy for generating white light is to combine blue LEDs with yellow luminescence from Y3Al5O12:Ce3+ (YAG) phosphor materials. However, due to the lack of a red light component, white light generated by this method usually renders color poorly. A novel approach has been suggested in which red/ green/blue tricolor phosphors are pumped by near UV-LED chips (350–410 nm) to produce white light [2,3], but there are currently insufficient references on the use of phosphors for converting near UV-LEDs as illumination sources. Therefore, it is important to study phosphors for near UV-LEDs. As an important family of luminescent materials, orthophosphates have attracted intense attention, In particular, researchers have concentrated on the phosphate series of phosphors with an ABPO4 structure, where A is a monovalent cation (Li+, Na+, K+, Rb+, and Cs+) and B is a divalent cation (Mg2+, Ca2+, Sr2+, and Ba2+) due to their large band gap, along with the high absorption of PO43 in UV region, their moderate phonon energy, high thermal and
n
Corresponding author. Tel.: +886 8 7703202x7555; fax: +886 8 7740552. E-mail address: [email protected] (R.-Y. Yang).
0022-2313/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jlumin.2013.07.001
chemical stability, and exceptional optical damage threshold [4–6]. For example, KSrPO4, NaCaPO4, and LiBaPO4, have been reported to act as new phosphor materials for potential applications in WLEDs [4–6]. Recently, Dy3+ ions have attracted considerable attention because of their white light emission. It is well known that a Dy3+ ion with a 4f9 electronic configuration generally has two dominant emission bands. One is the blue band (470–500 nm) due to the 4F9/2-6H15/2 transition. Another is the yellowish band (570–600 nm) due to the 4F9/2-6H13/2 transition [7–9]. White light can be observed not only by mixing the appropriate ratio of the red/green/blue tricolors, but also by creating the appropriate mixture of blue and yellowish emissions. In the past, Dy3+ doped orthophosphates and ABPO4 phosphors have been prepared by solid-state reaction using a conventional sintering furnace [7]. However, the phosphors usually produced agglomerate powders, which would alter the structural characteristics of the powders, and these powders were produced in non-uniform sizes, which would make the properties uncontrollable. It has been reported that, when the phosphors were sintered using the microwave energy as the heating sources, the energy can be absorbed immediately and uniformly compared to results from a conventional solid state sintering process [10]. This technique has been applied recently to prepare various oxide phosphors, such as YInGe2O7:Eu3+ [11], Y2BaZnO5:Eu3+ [12], and Sr2SiO4:Eu3+ [13]. However, to our knowledge, Dy3+ ions doped LiBaPO4 phosphors prepared by microwave-assisted sintering or conventional sintering
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have not yet been reported. Thus, the aim of the present work is to investigate the effects on the microstructure and luminescence properties of LiBaPO4:Dy3+ phosphors prepared by using microwaveassisted sintering.
2. Experimental procedure 2.1. Samples preparation Li2CO3, BaCO3, NH4H2PO4 and Dy2O3 powders, all with a purity of 99.9%, were used as the staring materials for the LiBa1 xPO4: xDy3+ phosphors with different concentrations of Dy3 ions (x ¼0.05, 0.07, 0.1, and 0.15). The powders were mixed in alcohol as a solvent and ball-milled for 1 h with zirconia balls. After drying, the mixed powders were sintered by microwave-assisted sintering in a microwave furnace to form the LiBa1 xPO4:xDy3+ phosphor. A Therm Wave Mod. III microwave furnace with controllable microwave powers up to 1.3 kW at 2.45 GHz was used in this study. Silicon carbide (SiC) was used as a susceptor to provide the indirect heating of the powders because of a very strong heating response to 2.45 GHz microwaves [10]. The material sample was placed on an Al2O3 crucible surrounded by four silicon carbide susceptors and encapsulated by a ceramic fiber insulating material in a microwave cavity. The mixed powders were sintered at 900 1C for 3 h under an air atmosphere with a power of 900 720 W, producing an average heating rate greater than 100 1C/min. For comparison purposes, a sample of LiBa1 xPO4: xDy3+ phosphor (x ¼0.07) was prepared using the same procedures with the conventional sintering method.
Fig. 1. XRD pattern of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave assisted sintering at 900 1C for 3 h in an air atmosphere.
Table 1 Lattice parameters and unit cell volume of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions. Value (x)
Unit cell volume (Å3)
Lattice parameters (Å)
0.05 0.07 0.1 0.15
a
b
c
8.633 8.632 8.630 8.629
8.735 8.735 8.732 8.731
5.211 5.210 5.210 5.208
392.958 392.837 392.611 392.370
2.2. Characterization The crystalline phases of the phosphors were identified using X-ray diffraction (XRD, Bruker D8 Advance) analysis with CuKα radiation of λ ¼1.54 Å using a Ni filter, and with a secondary graphite monochromator. A scan range of 2θ¼10–601 with a step of 0.031 and 0.4 s as a count time per-step were used. Scanning electron microscopy (SEM; HORIBA EX-200) was used to observe particle morphology of the phosphors. The excitation, emission spectra and fluorescence decay time were obtained using photoluminescence measurement (PL, JASCO FP-6000), using a 150 W Xenon lamp as the light source. To ensure measurement accuracy, specimens were measured within the same sample holder to preserve a consistent amount of phosphor materials in all samples.
From the Bragg′s equation (1), the cell parameters for orthorhombic structure can be expressed as 2d sin θ ¼ λ 1 2 dhkl
2
¼
2
ð1Þ 2
h k l þ þ a2 b2 c2
ð2Þ
3. Results and discussion
wherein h, k and l are Miller indices, λ is the wavelength of CuKα radiation radiation (1.54 Å), and θ is the diffraction angle which is determined from the XRD results. Compared with pure LiBaPO4, the lattice constants of LiBa1 xPO4:xDy3+ a little decrease by the introduction of Dy3+ ions as shown in Table 1, indicating that the rare earth ions have doped into the lattices of the LiBaPO4 host. The ion radius of Dy3+ (0.912 Å) is smaller than that of Ba2+ (1.350 Å) so that Ba2+ ion in LiBaPO4 can be replaced by Dy3+ ion to form LiBa1 xPO4:xDy3+ phosphor.
3.1. Structure
3.2. Morphology
It is well known that pure LiBaPO4 has a tetragonal structure with space group P63. Fig. 1 shows the X-ray diffraction patterns of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave-assisted sintering at 900 1C for 3 h in an air atmosphere. All the samples are found to belong to a single phase, which agrees well with the Joint Committee on Powder Diffraction Standards JCPDS (No. 14-0270) except for little shift of diffraction peak positions due to the doping effect. Therefore, the substitution Dy3+ ions were incorporated into the LiBaPO4 host lattice without any significant structural change or observed impurity phase. Additionally, from energy saving, this synthesis temperature is much lower than that of commercial phosphor YGB, BAM, and ZSM. They are usually prepared at temperature above 1000 by the conventional high temperature solid state reaction technique.
The particle size distribution of the phosphor is an important factor for its application in WLEDs. Fig. 2 shows the SEM images of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave-assisted sintering at 900 1C for 3 h in an air atmosphere. The particle morphology of LiBa1 xPO4:xDy3 + phosphors sintered by microwave-assisted sintering are not obviously different, and found to be fine and uniform since the heat energy is generated within the material itself as the materials interact with the microwave power, and are then dispersed uniformly within the material [10–13]. On the other hand, the particle size is seen to gradually decrease from 3 μm to 1 μm as the concentration of Dy3+ ions is increased. This may indicate that the doping ions act as nucleation catalyst, and the nucleation density of the micro-crystallites increased in LiBaPO4 powders as the number of Dy3+ ions increased. It has been reported that the
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Fig. 2. SEM image of LiBa1 xPO4:xDy3+ phosphors with (a) x ¼0.05, (b) x¼ 0.07, (c) x¼ 0.1, and (d) x¼ 0.15 prepared by using microwave assisted sintering at 900 1C for 3 h in an air atmosphere.
existence of doping impurities might affect nucleation and growth rates, since the presence of doping impurities usually lowers the potential barrier for the formation of nuclei and reduces the growth velocities, which were possibly responsible for the decrease in particle size [10]. That is to say, the presence of doping ions may inhibit particle growth following the nucleation process. In general, μm-sized particles of the phosphor should fulfill the demand for practical powder applications. It is also understood that the morphologies and the shapes of the phosphor particles depend on the condition of the starting materials, the reaction temperature, and the preparation process. Lin et al. [14] reported mostly like scurf crystals of LiSrPO4 prepared using a solid-state reaction grown together to a diameter of around 100 μm and aggregate. Those phosphors should be separated again by ballmilling to form package specification for LEDs. Besides, they also proposed that the particle morphology of KSrPO4 phosphors prepared using a solid-state reaction still have smooth-surfaced scurf crystals with a diameter of around 30–50 μm [14]. Han et al. used sol–gel method to prepare high quantum efficiency LiCaPO4: Eu2+ Phosphors. They pointed out that the particles for samples annealed at 800 1C are round with diameters 1–1.5 μm and have a narrow size distribution, the diameter and the width of size distribution increases with increasing the sintering temperature and the morphology changes from spherical to an irregular shape at 900 and 1150 1C [15]. From our result, it is found that using microwave-assisted sintering as the sintering method would lead to a morphological modification for both the shape and size of the final phosphor since the microwave-assisted sintering method could effectively drive the solid grains to complete the densification process. Moreover, microwave assisted sintering could significantly reduce energy consumption and therefore the cost associated with production owning to its rapid heating and cooling rate. The above results also point out that different synthesis methods of phosphors would affect the particles growth mechanism in phosphor. Besides, it is well known that the luminescence
characteristics of phosphors depend on the morphology of the particles, i.e., size, size distribution, shape, defects, and so on. Optimizing the particle size distribution would lead to a higher packing density for phosphor layers, and thus to a higher resolution as well. Fine particles also give a high packing density and small light-scattering coefficient. From these viewpoints, LiBa1 xPO4:xDy3+ phosphors prepared by microwave-assisted sintering may be suitable in manufacturing WLEDs. 3.3. Excitation and emission spectrum Fig. 3(a) shows the excitation spectrum of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave-assisted sintering at 900 1C for 3 h in an air atmosphere, which comes from the transition of 4F9/2-6H13/2 of Dy3+. Some sharp absorption peaks are found in the wavelength region of 300–400 nm, resulting from the excitation of the f–f shell transitions of Dy3+ ions, and the excitation bands consist of five main peaks located at 325, 338, 350, 365, and 388 nm, with the maximum excitation wavelength at 350 nm. These excitation peaks are respectively attributed to the electronic transitions of 6 H15/2-4K15/2, 6H15/2-4F5/2, 6H15/2-4M15/2, 6H15/2-4P3/2, and 6 H15/2-4M21/2, which are all due to the typical f–f transitions of Dy3+ ions. Fig. 3(b) shows the emission spectrum of LiBa1 xPO4: xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave-assisted sintering at 900 1C for 3 h. The phosphor presents white luminescence when excited with 350 nm wavelength from a xenon lamp. The emission spectrum for the Dy3+ ions in LiBaPO4 shows emission peaks at 483 nm (blue) and 576 nm (yellow), with a small peak at 671 nm (red). These three different emission peaks have the same excitation wavelength, and thus were generated from a single origin. The transitions involved in the blue, yellow and red bands of Dy3+ ions are well known and have been respectively identified as 4F9/26 H15/2, 6H13/2, and 6H11/2 transitions. Fig. 4 shows the schematic of the energy levels of the Dy3+ ion and emission transitions [16].
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Fig. 4. Schematic of the energy levels of Dy3+ ion.
Fig. 3. (a) Excitation spectra of LiBa1 xPO4:xDy3+ phosphor prepared by using microwave assisted sintering at 900 1C for 3 h in an air atmosphere and (b) emission spectra of LiBa1 xPO4:xDy3+ phosphor prepared by using microwave assisted sintering at 900 1C for 3 h in an air atmosphere with λex ¼350 nm.
It is understood that the Dy3+ emission of 4F9/2-6H15/2 transition (483 nm) is magnetic dipole, while the 4F9/2-6H13/2 transition (576 nm) is electric dipole. The relative intensity ratio 4F9/2-6H13/2 to 4F9/2-6H15/2 transitions can be adopted as sensitive parameter for understanding the symmetry around the Dy3+ in the LiBaPO4 host material. (All the fittings were carried out in the range of 450–510 and 535–615 nm for magnetic and electric dipole transitions, respectively.) This parameter is named asymmetric ratio (A21) and defined as R 615 A21 ¼ R535 510 450
I 2 dλ I 1 dλ
ð3Þ
where I1 and I2 stand for the respective integrated intensities of 4 F9/2-6H15/2 and 4F9/2-6H13/2 transitions of Dy3+, respectively. Table 2 shows the asymmetric ratio (A21) value of all samples. A lower symmetry of the crystal environment around Dy3+ leads to a higher value of A21 (41), while increased symmetry would lead to a lower value of A21 (1 4A21 4 0). The asymmetric ratio A21 is found to be 1.44, 1.46, 1.44 and 1.22, for the Dy3+ doping concentration in LiBa1 xPO4 at x ¼0.05, 0.07, 0.1, and 0.15, respectively. In our study, the 4F9/2-6H13/2 transition is dominant only because Dy3+ ions are located at low symmetry sites (i.e. A21 ( 41)) without inversion centers [16]. In other words, the result suggests that there is very little deviation from the inversion symmetry in this matrix of LiBaPO4:Dy3+ phosphor since emission at 576 nm is dominant. However, these results are inconsistent with those reported by Kuang et al. [17]. They pointed out that the 4F9/26 H15/2 transition is more prominent than the 4F9/2-6H13/2 transition in the Dy3+ ions doped SrSiO3 system since the local site
Table 2 The asymmetric ratio (A21), and CIE chromaticity coordinates obtained for LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions. Phosphor
Asymmetric ratio (A21)
CIE coordinates
LiBa1 xPO4:xDy3+ x ¼0.05 x ¼0.07 x ¼0.1 x¼ 0.15
1.44 1.46 1.44 1.22
0.33, 0.33, 0.33, 0.33,
0.37 0.37 0.37 0.37
symmetry around the Dy3+ ions changes. Therefore, the luminescence properties of the material are often affected by either the structure of the matrix or the synthesis technique. Moreover, it is known that the luminescence intensity of phosphors is always dependant on the doping concentration [10]. In this study, the concentration quenching effect was obtained under excitation at 350 nm. As the concentration of Dy3+ ion increased, the probability of energy transfer among Dy3+ ions increased as well. The photoluminescence intensity reached a maximum when the concentration of Dy3+ ions was x ¼0.07, and then decreased as the Dy3+ concentration increased further. 3.4. Comparison To compare to the performance of LiBa1 xPO4:xDy3+ phosphor prepared by using microwave-assisted sintering as the heat treatment method to that prepared using conventional sintering, a sample of LiBa1 xPO4:xDy3+ phosphor (x ¼0.07) was prepared using the same procedures with the conventional sintering method. Fig. 5(a) shows and compares the X-ray diffraction patterns of LiBa0.03PO4:0.07Dy3+ phosphors produced using microwaveassisted sintering and conventional sintering as the heat treatment methods. Both resulting powders are found to be single phase. Moreover, it is found that the full-width at half-maximum
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53
Fig. 6. Comparison of the emission spectrums of LiBa0.03PO4:0.07Dy3+ phosphors via microwave assisted sintering and conventional sintering.
Fig. 5. (a) Comparison of X-ray diffraction patterns of LiBa0.03PO4:0.07Dy3+ phosphors via microwave assisted sintering and conventional sintering and (b) SEM image of LiBa0.03PO4:0.07Dy3+ phosphor by using conventional sintering at 900 1C for 3 h in an air atmosphere.
(FWHM) of the peaks of LiBa0.03PO4:0.07Dy3+ phosphor prepared by microwave-assisted sintering is less than that by obtained by conventional sintering, which means the crystallization of LiBa0.03PO4:0.07Dy3+ phosphor prepared by microwave-assisted sintering is better than that by conventional sintering. In addition, microwave-assisted sintering was conducted at a power level of 900 W, corresponding to a heating rate about 100 1C/min, while conventional sintering used a heating rate of 5 1C/min. The result indicates that a better degree of crystallization can be obtained with a rapid heat generation rate since part of the heat generation takes place within the material itself in microwave-assisted sintering [11–13]. Fig. 5(b) shows the SEM image of the LiBa0.03 PO4:0.07Dy3+ phosphor produced by conventional sintering at 900 1C for 3 h in an air atmosphere. It can be clearly seen that the morphology of conventionally sintered powders is remarkably big, agglomerate, and non-uniform. On the contrary, Fig. 2 shows the particle size of the microwave-assisted sintered powders to small, refined, and uniform. Since the particle surface is activated by plasma, the driving force for the growth of the grains by microwave-assisted sintering is enhanced [11–13]. Additionally, lattice vibration could be induced by microwave-assisted sintering to accelerate diffusion, thus quickly obtaining highly-dense powders with a fine microstructure [11–13]. Fig. 6 compares the emission spectrum of LiBa0.03PO4:0.07Dy3+ phosphors produced using microwave-assisted sintering and conventional sintering as the method of heat treatment. The intensity of the emission spectra of LiBa0.03PO4:0.07Dy3+ phosphor by microwave-assisted sintering is seen to be much stronger than that by conventional sintering, possibly due to the non-uniform grain size distribution in the conventionally-sintered phosphor. Moreover, the strong and narrow emission feature is indicated by the presence of Dy3+ ions in the tetragonal structure of LiBaPO4. Accordingly, microwave sintering for 3 h not only yields homogeneous phosphor powder, but also gives itself to substituting Dy3+ for Ba2+ ions.
Fig. 7. Decay profiles of the luminescence of 4F9/2 level of LiBa0.03PO4:0.07Dy3+ phosphors via (a) microwave assisted sintering and (b) conventional sintering, recorded under excitation at 350 nm and emission at 576 nm.
Fig. 7(a) and (b) compares the decay profiles of the luminescence of the 4F9/2 level of LiBa0.03PO4:0.07Dy3+ phosphors via microwave-assisted and conventional sintering recorded under excitation at 350 nm and emission at 576 nm. The decay behavior is expressed as [10] t I ¼ I 0 exp ð4Þ τ where I and I0 are respectively the luminescence intensities at times 0 and t, and τ is the luminescence lifetime of the excited states in LiBaPO4. According to this equation, the lifetime value of microwave-assisted sintered phosphor is 1.25 ms, which is slightly longer than the 1.08 ms from conventionally-sintered phosphor.
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4. Conclusion Novel white emitting LiBa1 xPO4:xDy3+ phosphors were prepared by both microwave-assisted and conventional sintering to investigate their luminescent properties. Microwave-assisted sintering is found to improve sintering behaviors and provides a more uniform particle morphology of the LiBa0.03PO4:0.07Dy3+ phosphors. The PL results show that, upon 350 nm excitation, the phosphors exhibited three intense emission bands centering at 483 nm (blue) and 576 nm (yellow), with a small peak at 671 nm (red). Additionally, the microwave-assisted sintered phosphor has a higher luminous intensity than that of the conventional sintered phosphor even at the same sintering temperature, due to the higher degree of crystallization and particle uniformity. The results above suggest it could be used as an efficient phosphor material for solid-state lighting.
Fig. 8. CIE1931 chromaticity diagram of LiBa0.03PO4:0.07Dy3+ phosphors via microwave assisted sintering and conventional sintering at 900 1C for 3 h in an air atmosphere.
3+
4
The lifetime values of Dy ( F9/2) in the present study are in the range of milliseconds due to the forbidden nature of the f–f transition, while the lifetime of the electric-dipole-allowed nature of the f–d transition is usually not longer than 1 μs [10]. The 4F9/2 lifetime increased with microwave-assisted sintering due to the decreased non-radiative rate of the energy transfer from the excited states of Dy3+ ions to the surface defect states. As seen in the SEM results, the agglomerate and non-uniform morphology of conventionally-sintered powders might become a non-radiative path so as to reduce phosphor lifetime. The above result indicates that the choice of heating mechanism would affect the luminescence properties in phosphor [10]. 3.5. Commission International de l’Eclairage (CIE) 1931 chromaticity In general, color is expressed by means of color coordinates disclosed by the Commission International de l’Eclairage (CIE) 1931, a two-dimensional graphical representation of any color perceptible by the human eye on an x–y plot. Fig. 8 shows all results calculated from Fig. 6 drawn in the Commission International de l’Eclairage (CIE) 1931 chromaticity diagram. The chromaticity (x, y) coordinates of the prepared LiBa0.03PO4:0.07Dy3+ phosphors are located in the white region (0.33, 0.37) due to excellent stability of LiBa0.03PO4:0.07Dy3+ phosphors, produced with either microwave-assisted or conventional sintering. The above results indicate that microwave-assisted sintered LiBaPO4: Dy3+ phosphor is a promising material which could be used in WLEDs.
Acknowledgments The authors would like to thank Mr. Yu-Ming Peng for sample preparing, the National Science Council and Bureau of Energy, Ministry of Economic Affairs of Taiwan, R.O.C. for the financial support under Contract nos. NSC 100-2221-E-006-040-MY2, NSC 101-2628-E-020-002-MY3 and 100-D0204-6 and the LED Lighting Research Center of NCKU, and National Nano Device Laboratories, and the Precision Instrument Center of National Pingtung University of Science and Technology for supporting with the experimental equipment.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Microstructure, and luminescence properties of LiBaPO4:Dy3+ phosphors with various Dy3+ concentrations prepared by microwave assisted sintering Ru-Yuan Yang n, Hsuan-Lin Lai Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung County 912, Taiwan
art ic l e i nf o
a b s t r a c t
Article history: Received 4 December 2012 Received in revised form 24 May 2013 Accepted 9 July 2013 Available online 16 July 2013
LiBaPO4:Dy3+ phosphors were synthesized by microwave-assisted and conventional sintering. XRD analysis confirmed the phase formation of all the LiBaPO4:Dy3+ phosphors. PL results showed that the optimum concentration of Dy3+ for LiBaPO4:Dy3+ prepared by microwave-assisted sintering is 7 mol%. The phosphors were efficiently excited by the UV–vis light region from 300 to 400 nm, and exhibited blue (483 nm), yellow (576 nm) and red (671 nm) emission corresponding to 4F9/2-6H15/2, 6H13/2, and 6 H11/2 transitions, respectively. The microwave-assisted sintering improves the sintering behavior and provides a more uniform particle morphology of LiBa0.03PO4:0.07Dy3+ phosphors so as to obtain a luminescence intensity greater than that obtained by conventional sintering even at the same sintering temperature. Moreover, all the chromaticity (x, y) of the LiBa0.03PO4:0.07Dy3+ phosphors are located in the white region (0.33, 0.37) even on using microwave-assisted sintering as the heat treatment did not influence the purity of LiBa0.03PO4:0.07Dy3+ phosphors. & 2013 Published by Elsevier B.V.
Keywords: Phosphors Microwave processing Sintering Optical properties
1. Introduction White light-emitting diodes (WLEDs) are widely seen as being the next generation of solid-state lighting, and have attracted considerable attention because of their energy efficiency, high brightness, long lifetime, harmlessness and environment friendliness [1]. The present strategy for generating white light is to combine blue LEDs with yellow luminescence from Y3Al5O12:Ce3+ (YAG) phosphor materials. However, due to the lack of a red light component, white light generated by this method usually renders color poorly. A novel approach has been suggested in which red/ green/blue tricolor phosphors are pumped by near UV-LED chips (350–410 nm) to produce white light [2,3], but there are currently insufficient references on the use of phosphors for converting near UV-LEDs as illumination sources. Therefore, it is important to study phosphors for near UV-LEDs. As an important family of luminescent materials, orthophosphates have attracted intense attention, In particular, researchers have concentrated on the phosphate series of phosphors with an ABPO4 structure, where A is a monovalent cation (Li+, Na+, K+, Rb+, and Cs+) and B is a divalent cation (Mg2+, Ca2+, Sr2+, and Ba2+) due to their large band gap, along with the high absorption of PO43 in UV region, their moderate phonon energy, high thermal and
n
Corresponding author. Tel.: +886 8 7703202x7555; fax: +886 8 7740552. E-mail address: [email protected] (R.-Y. Yang).
0022-2313/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jlumin.2013.07.001
chemical stability, and exceptional optical damage threshold [4–6]. For example, KSrPO4, NaCaPO4, and LiBaPO4, have been reported to act as new phosphor materials for potential applications in WLEDs [4–6]. Recently, Dy3+ ions have attracted considerable attention because of their white light emission. It is well known that a Dy3+ ion with a 4f9 electronic configuration generally has two dominant emission bands. One is the blue band (470–500 nm) due to the 4F9/2-6H15/2 transition. Another is the yellowish band (570–600 nm) due to the 4F9/2-6H13/2 transition [7–9]. White light can be observed not only by mixing the appropriate ratio of the red/green/blue tricolors, but also by creating the appropriate mixture of blue and yellowish emissions. In the past, Dy3+ doped orthophosphates and ABPO4 phosphors have been prepared by solid-state reaction using a conventional sintering furnace [7]. However, the phosphors usually produced agglomerate powders, which would alter the structural characteristics of the powders, and these powders were produced in non-uniform sizes, which would make the properties uncontrollable. It has been reported that, when the phosphors were sintered using the microwave energy as the heating sources, the energy can be absorbed immediately and uniformly compared to results from a conventional solid state sintering process [10]. This technique has been applied recently to prepare various oxide phosphors, such as YInGe2O7:Eu3+ [11], Y2BaZnO5:Eu3+ [12], and Sr2SiO4:Eu3+ [13]. However, to our knowledge, Dy3+ ions doped LiBaPO4 phosphors prepared by microwave-assisted sintering or conventional sintering
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have not yet been reported. Thus, the aim of the present work is to investigate the effects on the microstructure and luminescence properties of LiBaPO4:Dy3+ phosphors prepared by using microwaveassisted sintering.
2. Experimental procedure 2.1. Samples preparation Li2CO3, BaCO3, NH4H2PO4 and Dy2O3 powders, all with a purity of 99.9%, were used as the staring materials for the LiBa1 xPO4: xDy3+ phosphors with different concentrations of Dy3 ions (x ¼0.05, 0.07, 0.1, and 0.15). The powders were mixed in alcohol as a solvent and ball-milled for 1 h with zirconia balls. After drying, the mixed powders were sintered by microwave-assisted sintering in a microwave furnace to form the LiBa1 xPO4:xDy3+ phosphor. A Therm Wave Mod. III microwave furnace with controllable microwave powers up to 1.3 kW at 2.45 GHz was used in this study. Silicon carbide (SiC) was used as a susceptor to provide the indirect heating of the powders because of a very strong heating response to 2.45 GHz microwaves [10]. The material sample was placed on an Al2O3 crucible surrounded by four silicon carbide susceptors and encapsulated by a ceramic fiber insulating material in a microwave cavity. The mixed powders were sintered at 900 1C for 3 h under an air atmosphere with a power of 900 720 W, producing an average heating rate greater than 100 1C/min. For comparison purposes, a sample of LiBa1 xPO4: xDy3+ phosphor (x ¼0.07) was prepared using the same procedures with the conventional sintering method.
Fig. 1. XRD pattern of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave assisted sintering at 900 1C for 3 h in an air atmosphere.
Table 1 Lattice parameters and unit cell volume of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions. Value (x)
Unit cell volume (Å3)
Lattice parameters (Å)
0.05 0.07 0.1 0.15
a
b
c
8.633 8.632 8.630 8.629
8.735 8.735 8.732 8.731
5.211 5.210 5.210 5.208
392.958 392.837 392.611 392.370
2.2. Characterization The crystalline phases of the phosphors were identified using X-ray diffraction (XRD, Bruker D8 Advance) analysis with CuKα radiation of λ ¼1.54 Å using a Ni filter, and with a secondary graphite monochromator. A scan range of 2θ¼10–601 with a step of 0.031 and 0.4 s as a count time per-step were used. Scanning electron microscopy (SEM; HORIBA EX-200) was used to observe particle morphology of the phosphors. The excitation, emission spectra and fluorescence decay time were obtained using photoluminescence measurement (PL, JASCO FP-6000), using a 150 W Xenon lamp as the light source. To ensure measurement accuracy, specimens were measured within the same sample holder to preserve a consistent amount of phosphor materials in all samples.
From the Bragg′s equation (1), the cell parameters for orthorhombic structure can be expressed as 2d sin θ ¼ λ 1 2 dhkl
2
¼
2
ð1Þ 2
h k l þ þ a2 b2 c2
ð2Þ
3. Results and discussion
wherein h, k and l are Miller indices, λ is the wavelength of CuKα radiation radiation (1.54 Å), and θ is the diffraction angle which is determined from the XRD results. Compared with pure LiBaPO4, the lattice constants of LiBa1 xPO4:xDy3+ a little decrease by the introduction of Dy3+ ions as shown in Table 1, indicating that the rare earth ions have doped into the lattices of the LiBaPO4 host. The ion radius of Dy3+ (0.912 Å) is smaller than that of Ba2+ (1.350 Å) so that Ba2+ ion in LiBaPO4 can be replaced by Dy3+ ion to form LiBa1 xPO4:xDy3+ phosphor.
3.1. Structure
3.2. Morphology
It is well known that pure LiBaPO4 has a tetragonal structure with space group P63. Fig. 1 shows the X-ray diffraction patterns of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave-assisted sintering at 900 1C for 3 h in an air atmosphere. All the samples are found to belong to a single phase, which agrees well with the Joint Committee on Powder Diffraction Standards JCPDS (No. 14-0270) except for little shift of diffraction peak positions due to the doping effect. Therefore, the substitution Dy3+ ions were incorporated into the LiBaPO4 host lattice without any significant structural change or observed impurity phase. Additionally, from energy saving, this synthesis temperature is much lower than that of commercial phosphor YGB, BAM, and ZSM. They are usually prepared at temperature above 1000 by the conventional high temperature solid state reaction technique.
The particle size distribution of the phosphor is an important factor for its application in WLEDs. Fig. 2 shows the SEM images of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave-assisted sintering at 900 1C for 3 h in an air atmosphere. The particle morphology of LiBa1 xPO4:xDy3 + phosphors sintered by microwave-assisted sintering are not obviously different, and found to be fine and uniform since the heat energy is generated within the material itself as the materials interact with the microwave power, and are then dispersed uniformly within the material [10–13]. On the other hand, the particle size is seen to gradually decrease from 3 μm to 1 μm as the concentration of Dy3+ ions is increased. This may indicate that the doping ions act as nucleation catalyst, and the nucleation density of the micro-crystallites increased in LiBaPO4 powders as the number of Dy3+ ions increased. It has been reported that the
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Fig. 2. SEM image of LiBa1 xPO4:xDy3+ phosphors with (a) x ¼0.05, (b) x¼ 0.07, (c) x¼ 0.1, and (d) x¼ 0.15 prepared by using microwave assisted sintering at 900 1C for 3 h in an air atmosphere.
existence of doping impurities might affect nucleation and growth rates, since the presence of doping impurities usually lowers the potential barrier for the formation of nuclei and reduces the growth velocities, which were possibly responsible for the decrease in particle size [10]. That is to say, the presence of doping ions may inhibit particle growth following the nucleation process. In general, μm-sized particles of the phosphor should fulfill the demand for practical powder applications. It is also understood that the morphologies and the shapes of the phosphor particles depend on the condition of the starting materials, the reaction temperature, and the preparation process. Lin et al. [14] reported mostly like scurf crystals of LiSrPO4 prepared using a solid-state reaction grown together to a diameter of around 100 μm and aggregate. Those phosphors should be separated again by ballmilling to form package specification for LEDs. Besides, they also proposed that the particle morphology of KSrPO4 phosphors prepared using a solid-state reaction still have smooth-surfaced scurf crystals with a diameter of around 30–50 μm [14]. Han et al. used sol–gel method to prepare high quantum efficiency LiCaPO4: Eu2+ Phosphors. They pointed out that the particles for samples annealed at 800 1C are round with diameters 1–1.5 μm and have a narrow size distribution, the diameter and the width of size distribution increases with increasing the sintering temperature and the morphology changes from spherical to an irregular shape at 900 and 1150 1C [15]. From our result, it is found that using microwave-assisted sintering as the sintering method would lead to a morphological modification for both the shape and size of the final phosphor since the microwave-assisted sintering method could effectively drive the solid grains to complete the densification process. Moreover, microwave assisted sintering could significantly reduce energy consumption and therefore the cost associated with production owning to its rapid heating and cooling rate. The above results also point out that different synthesis methods of phosphors would affect the particles growth mechanism in phosphor. Besides, it is well known that the luminescence
characteristics of phosphors depend on the morphology of the particles, i.e., size, size distribution, shape, defects, and so on. Optimizing the particle size distribution would lead to a higher packing density for phosphor layers, and thus to a higher resolution as well. Fine particles also give a high packing density and small light-scattering coefficient. From these viewpoints, LiBa1 xPO4:xDy3+ phosphors prepared by microwave-assisted sintering may be suitable in manufacturing WLEDs. 3.3. Excitation and emission spectrum Fig. 3(a) shows the excitation spectrum of LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave-assisted sintering at 900 1C for 3 h in an air atmosphere, which comes from the transition of 4F9/2-6H13/2 of Dy3+. Some sharp absorption peaks are found in the wavelength region of 300–400 nm, resulting from the excitation of the f–f shell transitions of Dy3+ ions, and the excitation bands consist of five main peaks located at 325, 338, 350, 365, and 388 nm, with the maximum excitation wavelength at 350 nm. These excitation peaks are respectively attributed to the electronic transitions of 6 H15/2-4K15/2, 6H15/2-4F5/2, 6H15/2-4M15/2, 6H15/2-4P3/2, and 6 H15/2-4M21/2, which are all due to the typical f–f transitions of Dy3+ ions. Fig. 3(b) shows the emission spectrum of LiBa1 xPO4: xDy3+ phosphors with various concentrations of Dy3+ ions prepared by microwave-assisted sintering at 900 1C for 3 h. The phosphor presents white luminescence when excited with 350 nm wavelength from a xenon lamp. The emission spectrum for the Dy3+ ions in LiBaPO4 shows emission peaks at 483 nm (blue) and 576 nm (yellow), with a small peak at 671 nm (red). These three different emission peaks have the same excitation wavelength, and thus were generated from a single origin. The transitions involved in the blue, yellow and red bands of Dy3+ ions are well known and have been respectively identified as 4F9/26 H15/2, 6H13/2, and 6H11/2 transitions. Fig. 4 shows the schematic of the energy levels of the Dy3+ ion and emission transitions [16].
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Fig. 4. Schematic of the energy levels of Dy3+ ion.
Fig. 3. (a) Excitation spectra of LiBa1 xPO4:xDy3+ phosphor prepared by using microwave assisted sintering at 900 1C for 3 h in an air atmosphere and (b) emission spectra of LiBa1 xPO4:xDy3+ phosphor prepared by using microwave assisted sintering at 900 1C for 3 h in an air atmosphere with λex ¼350 nm.
It is understood that the Dy3+ emission of 4F9/2-6H15/2 transition (483 nm) is magnetic dipole, while the 4F9/2-6H13/2 transition (576 nm) is electric dipole. The relative intensity ratio 4F9/2-6H13/2 to 4F9/2-6H15/2 transitions can be adopted as sensitive parameter for understanding the symmetry around the Dy3+ in the LiBaPO4 host material. (All the fittings were carried out in the range of 450–510 and 535–615 nm for magnetic and electric dipole transitions, respectively.) This parameter is named asymmetric ratio (A21) and defined as R 615 A21 ¼ R535 510 450
I 2 dλ I 1 dλ
ð3Þ
where I1 and I2 stand for the respective integrated intensities of 4 F9/2-6H15/2 and 4F9/2-6H13/2 transitions of Dy3+, respectively. Table 2 shows the asymmetric ratio (A21) value of all samples. A lower symmetry of the crystal environment around Dy3+ leads to a higher value of A21 (41), while increased symmetry would lead to a lower value of A21 (1 4A21 4 0). The asymmetric ratio A21 is found to be 1.44, 1.46, 1.44 and 1.22, for the Dy3+ doping concentration in LiBa1 xPO4 at x ¼0.05, 0.07, 0.1, and 0.15, respectively. In our study, the 4F9/2-6H13/2 transition is dominant only because Dy3+ ions are located at low symmetry sites (i.e. A21 ( 41)) without inversion centers [16]. In other words, the result suggests that there is very little deviation from the inversion symmetry in this matrix of LiBaPO4:Dy3+ phosphor since emission at 576 nm is dominant. However, these results are inconsistent with those reported by Kuang et al. [17]. They pointed out that the 4F9/26 H15/2 transition is more prominent than the 4F9/2-6H13/2 transition in the Dy3+ ions doped SrSiO3 system since the local site
Table 2 The asymmetric ratio (A21), and CIE chromaticity coordinates obtained for LiBa1 xPO4:xDy3+ phosphors with various concentrations of Dy3+ ions. Phosphor
Asymmetric ratio (A21)
CIE coordinates
LiBa1 xPO4:xDy3+ x ¼0.05 x ¼0.07 x ¼0.1 x¼ 0.15
1.44 1.46 1.44 1.22
0.33, 0.33, 0.33, 0.33,
0.37 0.37 0.37 0.37
symmetry around the Dy3+ ions changes. Therefore, the luminescence properties of the material are often affected by either the structure of the matrix or the synthesis technique. Moreover, it is known that the luminescence intensity of phosphors is always dependant on the doping concentration [10]. In this study, the concentration quenching effect was obtained under excitation at 350 nm. As the concentration of Dy3+ ion increased, the probability of energy transfer among Dy3+ ions increased as well. The photoluminescence intensity reached a maximum when the concentration of Dy3+ ions was x ¼0.07, and then decreased as the Dy3+ concentration increased further. 3.4. Comparison To compare to the performance of LiBa1 xPO4:xDy3+ phosphor prepared by using microwave-assisted sintering as the heat treatment method to that prepared using conventional sintering, a sample of LiBa1 xPO4:xDy3+ phosphor (x ¼0.07) was prepared using the same procedures with the conventional sintering method. Fig. 5(a) shows and compares the X-ray diffraction patterns of LiBa0.03PO4:0.07Dy3+ phosphors produced using microwaveassisted sintering and conventional sintering as the heat treatment methods. Both resulting powders are found to be single phase. Moreover, it is found that the full-width at half-maximum
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Fig. 6. Comparison of the emission spectrums of LiBa0.03PO4:0.07Dy3+ phosphors via microwave assisted sintering and conventional sintering.
Fig. 5. (a) Comparison of X-ray diffraction patterns of LiBa0.03PO4:0.07Dy3+ phosphors via microwave assisted sintering and conventional sintering and (b) SEM image of LiBa0.03PO4:0.07Dy3+ phosphor by using conventional sintering at 900 1C for 3 h in an air atmosphere.
(FWHM) of the peaks of LiBa0.03PO4:0.07Dy3+ phosphor prepared by microwave-assisted sintering is less than that by obtained by conventional sintering, which means the crystallization of LiBa0.03PO4:0.07Dy3+ phosphor prepared by microwave-assisted sintering is better than that by conventional sintering. In addition, microwave-assisted sintering was conducted at a power level of 900 W, corresponding to a heating rate about 100 1C/min, while conventional sintering used a heating rate of 5 1C/min. The result indicates that a better degree of crystallization can be obtained with a rapid heat generation rate since part of the heat generation takes place within the material itself in microwave-assisted sintering [11–13]. Fig. 5(b) shows the SEM image of the LiBa0.03 PO4:0.07Dy3+ phosphor produced by conventional sintering at 900 1C for 3 h in an air atmosphere. It can be clearly seen that the morphology of conventionally sintered powders is remarkably big, agglomerate, and non-uniform. On the contrary, Fig. 2 shows the particle size of the microwave-assisted sintered powders to small, refined, and uniform. Since the particle surface is activated by plasma, the driving force for the growth of the grains by microwave-assisted sintering is enhanced [11–13]. Additionally, lattice vibration could be induced by microwave-assisted sintering to accelerate diffusion, thus quickly obtaining highly-dense powders with a fine microstructure [11–13]. Fig. 6 compares the emission spectrum of LiBa0.03PO4:0.07Dy3+ phosphors produced using microwave-assisted sintering and conventional sintering as the method of heat treatment. The intensity of the emission spectra of LiBa0.03PO4:0.07Dy3+ phosphor by microwave-assisted sintering is seen to be much stronger than that by conventional sintering, possibly due to the non-uniform grain size distribution in the conventionally-sintered phosphor. Moreover, the strong and narrow emission feature is indicated by the presence of Dy3+ ions in the tetragonal structure of LiBaPO4. Accordingly, microwave sintering for 3 h not only yields homogeneous phosphor powder, but also gives itself to substituting Dy3+ for Ba2+ ions.
Fig. 7. Decay profiles of the luminescence of 4F9/2 level of LiBa0.03PO4:0.07Dy3+ phosphors via (a) microwave assisted sintering and (b) conventional sintering, recorded under excitation at 350 nm and emission at 576 nm.
Fig. 7(a) and (b) compares the decay profiles of the luminescence of the 4F9/2 level of LiBa0.03PO4:0.07Dy3+ phosphors via microwave-assisted and conventional sintering recorded under excitation at 350 nm and emission at 576 nm. The decay behavior is expressed as [10] t I ¼ I 0 exp ð4Þ τ where I and I0 are respectively the luminescence intensities at times 0 and t, and τ is the luminescence lifetime of the excited states in LiBaPO4. According to this equation, the lifetime value of microwave-assisted sintered phosphor is 1.25 ms, which is slightly longer than the 1.08 ms from conventionally-sintered phosphor.
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4. Conclusion Novel white emitting LiBa1 xPO4:xDy3+ phosphors were prepared by both microwave-assisted and conventional sintering to investigate their luminescent properties. Microwave-assisted sintering is found to improve sintering behaviors and provides a more uniform particle morphology of the LiBa0.03PO4:0.07Dy3+ phosphors. The PL results show that, upon 350 nm excitation, the phosphors exhibited three intense emission bands centering at 483 nm (blue) and 576 nm (yellow), with a small peak at 671 nm (red). Additionally, the microwave-assisted sintered phosphor has a higher luminous intensity than that of the conventional sintered phosphor even at the same sintering temperature, due to the higher degree of crystallization and particle uniformity. The results above suggest it could be used as an efficient phosphor material for solid-state lighting.
Fig. 8. CIE1931 chromaticity diagram of LiBa0.03PO4:0.07Dy3+ phosphors via microwave assisted sintering and conventional sintering at 900 1C for 3 h in an air atmosphere.
3+
4
The lifetime values of Dy ( F9/2) in the present study are in the range of milliseconds due to the forbidden nature of the f–f transition, while the lifetime of the electric-dipole-allowed nature of the f–d transition is usually not longer than 1 μs [10]. The 4F9/2 lifetime increased with microwave-assisted sintering due to the decreased non-radiative rate of the energy transfer from the excited states of Dy3+ ions to the surface defect states. As seen in the SEM results, the agglomerate and non-uniform morphology of conventionally-sintered powders might become a non-radiative path so as to reduce phosphor lifetime. The above result indicates that the choice of heating mechanism would affect the luminescence properties in phosphor [10]. 3.5. Commission International de l’Eclairage (CIE) 1931 chromaticity In general, color is expressed by means of color coordinates disclosed by the Commission International de l’Eclairage (CIE) 1931, a two-dimensional graphical representation of any color perceptible by the human eye on an x–y plot. Fig. 8 shows all results calculated from Fig. 6 drawn in the Commission International de l’Eclairage (CIE) 1931 chromaticity diagram. The chromaticity (x, y) coordinates of the prepared LiBa0.03PO4:0.07Dy3+ phosphors are located in the white region (0.33, 0.37) due to excellent stability of LiBa0.03PO4:0.07Dy3+ phosphors, produced with either microwave-assisted or conventional sintering. The above results indicate that microwave-assisted sintered LiBaPO4: Dy3+ phosphor is a promising material which could be used in WLEDs.
Acknowledgments The authors would like to thank Mr. Yu-Ming Peng for sample preparing, the National Science Council and Bureau of Energy, Ministry of Economic Affairs of Taiwan, R.O.C. for the financial support under Contract nos. NSC 100-2221-E-006-040-MY2, NSC 101-2628-E-020-002-MY3 and 100-D0204-6 and the LED Lighting Research Center of NCKU, and National Nano Device Laboratories, and the Precision Instrument Center of National Pingtung University of Science and Technology for supporting with the experimental equipment.
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