1. Introduction

By converting the radiation of blue light emitting diodes into white light emission through yellow phosphors, white light emitting diodes (WLEDs) have shown many advantages compared to traditional filament lamps, such as long life-time, low energy loss and fast response time [1–3]. As a commonly used commercial yellow phosphor with excellent luminous efficacy, Y₃Al₅O₁₂:Ce³⁺ (Ce:YAG), however, is not suitable for indoor illumination due to its CCT, low CRI, and the deficiency of red emission component [4,5].

To improve the fluorescence performance of Ce: YAG, several kinds of approaches have been proposed by researchers, including the supplement of red component by Pr³⁺, Cr³⁺ doping [6,7], the red-shift emission of Ce³⁺ by Mg²⁺, Ba²⁺, Mn²⁺-Si⁴⁺ pairs co-doping [4,8–10], etc. Particularly, the luminescence properties of Ce-doped garnet phosphor can be modified when the crystal field of Ce³⁺ is altered by structural variance. For instance, it has been widely found that the emission wavelength of Ce³⁺ would shift to a longer region with an improved CRI and CCT when using larger rare earth ions like Gd³⁺ and Tb³⁺ to substitute the dodecahedral site of YAG lattices [11–14]. Although the garnet structure is hard to stably exist in Gd₂O₃-Al₂O₃ system (i.e. Gd₃Al₅O₁₂) [15], the emission wavelength of Ce³⁺(∼550 nm) is longer in Tb₃Al₅O₁₂:Ce³⁺ (Ce:TAG) garnet phosphors compared to Ce:YAG [16–18]. Besides, it has been discovered that the emission wavelength of Ce:TAG would be further redshifted when Tb³⁺ ions were partially substituted by the even larger Gd³⁺ ions, making it a promising candidate for producing white light with low color correlated temperature (CCT) and high color rendering index (CRI) [19].

On the other hand, commercial phosphors are normally encapsulated in polymers or glasses with blue light chips to produce white light emission. Due to the low thermal conductivities of those packaging materials, the deterioration of luminous efficacy and alternation of color coordinates under long-term thermal radiation in high power LEDs or laser diodes (LDs) have not been effectively solved. With high thermal conductivities, excellent heat-resistances, and stable chemical and physical properties, transparent phosphor ceramics like Ce:YAG are very promising candidates in the application of high-power density and high brightness illumination [20,21]. Nevertheless, the problem still exists that the white light obtained is too cold and the CRI is too low for indoor illumination. Although high quality white light can be more easily obtained by Ce:TAG for the longer emission wavelength of Ce³⁺, the relevant investigation on Ce:TAG ceramic phosphors is rarely seen. In the report of Li et al. [22], the ceramic quality of Ce:TAG obtained is poor and the detailed characterization on its fluorescence properties remains unclear. And there are no further reports about the investigation and synthesis of Ce:TAG ceramic phosphors, as far as we know.

In this work, (Tb,Gd)₃Al₅O₁₂:Ce³⁺(Ce: TGAG) phosphor ceramics were fabricated for the first time by vacuum solid state reaction. Gd³⁺ ions were introduced to the dodecahedral sites of TAG lattice to investigate its effect on the fluorescence properties of Ce³⁺. The influence of Gd-doping on the crystal structure, microstructure and optical performance of Ce:TAG ceramics was also studied systematically. Based on InGaN blue chips and Ce:TGAG ceramics, the performance of packed LED devices have been investigated to evaluate its possibility in the application of warm white light illumination.

2. Experimental

High purity commercial Tb₄O₇ (99.999%), Al₂O₃ (99.999%), CeO₂ (99.99%), Gd₂O₃ (99.9%) powders were weighed stoichiometrically according to the formula of Ce_0.001Tb_2.999-xGd_xAl₅O₁₂ (Ce:TGAG) (x = 0, 0.1, 0.2, 0.3). The samples were denoted as G1, G2, G3 and G4 with the increasing amount of Gd³⁺. All powders were mixed and ball milled in ethanol for 24 h. Afterward, the slurry was dried in the temperature of 75 °C and granulated with a 200-mesh sieve. The powder was then pressed into disks under uniaxial compression of 10 MPa and the disks were processed under cold isostatic pressing at 200 MPa for 2 min. The thermolysis procedure was completed in a muffle oven under 600 °C for 3 h with a heating rate of 3 °C /min to remove the organic ingredients in the samples. Samples were vacuum sintered at 1650 °C for 4 hours to obtain transparent ceramics.

The fracture surfaces of the samples were detected with a scanning electron microscope (SEM) (Auriga-39-40, Germany). XRD measurement was taken in an Ultima IV (Rigaku, Japan) diffractometer to confirm its phase structure. The absorption spectra of samples were measured on a V-570-type ultraviolet/visible/near-IR spectrophotometer (JASCO, Japan). The spectral performance, CIE color coordinates, and luminous efficiency of samples were tested under the excitation of a 460 nm blue chip (25 mW pump) using an integrating sphere (Everfine PMS-50 system) at the Shanghai Semiconductor Lighting Engineering Research Center. The photoluminescence (PL), excitation (PLE) spectra were measured using a high-resolution spectrofluorometer (FLS 920, Edinburgh Instruments, UK).

3. Result and discussion

Figure 1(b) displays the photographs of prepared samples G1∼G4 with different concentration of Gd-doping that vacuum sintered at 1650 °C for 4 h. All the samples are 0.2 mm in thickness and 17 mm in diameter. It can be clearly seen that G2∼G4 are bright yellow in appearance but a green-yellow color is observed in G1. The absorption spectra in the 200∼800 nm range of all the samples are shown in Fig. 1(a). In the presented spectra of G1∼G4, it can be seen that the doping of Gd³⁺ hardly affects the absorption characteristics of Ce³⁺ ions in TAG lattice. The dominant absorption bands located in 236 nm($\textrm{E}_\textrm{3}^{1\ -\ 2}$), 252∼ 295 nm ($\textrm{E}_\textrm{2}^{1\ -\ 3}$) and 325 nm (E₁) belong to the spin allowed and spin forbidden f-d transitions of Tb³⁺ cations, respectively. Besides, the absorption band of 352, 359 and 371, 378 nm are related to the f-f transitions of ⁷F₆-⁵D₂, ⁵D₃ of Tb³⁺ ions, which is shown in the enlarged absorption spectra of G1∼G4 at the wavelength between 300∼400 nm. The characterized 4f(²F_5/2)→5d₁ transition of Ce³⁺ ions can also be observed at 463 nm in Fig. 1(a). Generally, the absorption peaks of Ce³⁺ ions in YAG lattice can be detected at 220, 340, and 460 nm corresponding to the 4f-5d₁ transitions of Ce³⁺ [23]. The former two have however not been observed in the absorption spectra of Ce:TAG ceramics in the present work. The absence of these absorption peaks may result from the strong absorption of Tb³⁺ ions at the shorter wavelength region that covers the nearby absorption peaks of Ce³⁺.

 

Fig. 1. (a) Absorption spectra and (b) Photographs of the Ce: TGAG ceramics with the thickness of 0.2 mm;(c) The enlarged absorption spectra of G1∼G4 at the wavelength between 300∼400 nm.

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The XRD patterns of all the samples are presented in Fig. 2(a). It is clear to see that all the diffraction peaks coincide with standard cubic phase TAG (JCPDF No. 76-0111) and no impurity phase can be observed. While with the increasing content of Gd-doping, it is shown in Fig. 2(b) that the 2θ diffraction peaks of Ce:TGAG shift to lower values, and the lattice parameters measured by refinement increase from 12.069 Å to 12.110 Å for the substitution of Tb³⁺(1.04 Å) by the larger Gd³⁺(1.05 Å) ions.

 

Fig. 2. (a) XRD patterns of Ce:TGAG ceramics with different concentration of Gd-doping; (b) expanded view of diffraction peaks between 32° and 38°.

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Figure 3 shows the fracture surfaces of Ce:TGAG ceramics sintered at 1650 °C for 4 hours. It is observed that all the samples are fully densified without open pores. A number of sealed pores can be observed in all samples, but the scattering of a small amount of pores is actually recognized to be beneficial to the promotion of the fluorescence efficiency of Ce³⁺ [24]. The grain size distribution of all the samples is uniform and less than 10 µm.

 

Fig. 3. SEM morphologies of the fracture surfaces of (a) G1:Ce:TAG; (b) G2: Ce:(Tb_0.9Gd_0.1)AG; (c) G3: Ce:(Tb_0.8Gd_0.2)AG; (d) G4: Ce:(Tb_0.7Gd_0.3)AG ceramics.

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The PL/PLE spectra of G1∼G4 from 220∼ 750 nm were measured at room temperature and shown in Fig. 4(a). The excitation peaks of all Ce:TGAG ceramics monitored at the characterized emission peak of Ce³⁺ (550 nm) are mostly identical with their absorption bands. Different from other Ce-doped garnet structure phosphors like Ce:YAG or Ce:LuAG, it can be seen in the PLE spectrum that Tb³⁺ cations in TAG can be used as sensitizer itself, and the energy transfer of Tb³⁺→Ce³⁺ can also produce the yellow light emission of Ce³⁺ efficiently. Besides, the emission wavelength of Ce³⁺ shift from 549 nm to 570 nm and its emission intensity is also greatly enhanced with the introduction of Gd³⁺ as shown in the PL spectrum. The emission red shift of Ce³⁺ in Gd-doped TAG ceramics can be attributed to the larger split of 5d energy level of Ce³⁺ ions and the increased stokes shifts with the replacement of dodecahedral Tb³⁺ by the larger Gd³⁺ [19,25,26], which are calculated to be 3524 cm⁻¹ for Ce:TAG and 4195 cm⁻¹ for Ce:TGAG.

 

Fig. 4. (a) Room temperature PL(λex = 460 nm) and PLE(λem = 550 nm) spectra of transparent Ce:TGAG ceramics; (b) PL spectrum of Ce: TGAG ceramics under excitation of 4f⁸ →4f⁷5d¹(E₁) of Tb³⁺ at 327 nm;(c) Schematic representation of the excitation, emission and the energy transfer process of Tb³⁺ and Ce³⁺ observed in this work.

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It is observed in the PL and PLE spectra of all the samples that both the excitation and emission intensity are largely promoted with Gd-doping at the concentration of 10%. Meanwhile a further increase of Gd content until 30% has little influence on the PL/PLE features of Ce:TGAG ceramics. The enhancement of excitation and emission of Ce³⁺ can be ascribed to the improved crystallinity of Gd-doped ceramics that enables a full occupation of Ce³⁺ ions in TAG lattice.

In order to further investigate the effect of Gd-doping on the energy transfer process of Tb³⁺→ Ce³⁺, the PL spectra of Ce:TGAG samples were measured at room temperature under the characteristic 4f⁸ →4f⁷5d¹(E₁) excitation of Tb³⁺ at 327 nm, as shown in Fig. 4(b). It can be found that in addition to the increasing emission intensity of Ce³⁺ with Gd-doping, all the samples have exhibited a typical narrowband ⁵D₄→ ⁷F₅ emission of Tb³⁺ at 543 nm. Notably, the PL intensity of Ce³⁺ under its intrinsic 4f(²F_5/2)→5d₁ excitation at 460 nm remains almost unchanged with the Gd-doping higher than 10%, but the PL intensity under 327 nm excitation reaches its maximum at the Gd content of 10% then decreases with the further increase of Gd-doping, which should results from the inhibited energy transfer of Tb³⁺→ Ce³⁺ with the decreasing amount of Tb³⁺ ions to perform the roles of sensitizer. The schematic representation of the excitation, emission and the energy transfer process can be seen in Fig. 4(c). According to Y. Zorenko et al., the energy is transferred to the 5d excited level of Ce³⁺ after spin allowed and spin-forbidden excitation of the 5d¹4f⁷ states of Tb³⁺ and after relaxation to the ⁵D₀ and ⁵D₃ states of Tb³⁺ [16,27]. Here in the PL spectra in Fig. 4(b), a strong emission of Ce³⁺ and a ⁵D₄→ ⁷F₅ emission of Tb³⁺ at 543 nm was observed in Ce:TGAG at the ⁷F₆→ E₁ excitation of Tb³⁺, which indicates there is energy transfer from the ⁵D₄ level of Tb³⁺ to the 5d₁(E_2g) level of Ce³⁺ as well.

Figure 5(a) and (b) have displayed the emission spectra of G1 and G2 with a combination of a 460 nm InGaN blue LED to confirm the fluorescence properties of Ce:TGAG ceramics in the application of white light illumination. It is obvious that the red component in fluorescence spectrum was promoted in G2 with the introduction of Gd³⁺ ions. With a more efficient conversion of blue light to longer wavelength light, the CCT of samples can be adjusted from 7000 K to 3700 K through the control of Gd-doping concentration as shown in the CIE color space chromaticity diagram in Fig. 5(c), which proves the great potential of Ce: TGAG ceramics to obtain “warm” white light.

 

Fig. 5. The spectral performance of (a) Ce:TAG and (b) Ce:(Tb_0.9Gd_0.1)AG ceramics under 460 nm wavelength with the thickness of 0.2 mm and (c) color coordinate of Ce:(Tb,Gd)AG ceramics based LED in CIE-1931 color space chromaticity diagram.

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A detailed comparison of fluorescent properties including the CRI, CCT, the full width at half maximum (FWHM), and CIE coordinates of Ce:YAG ceramics(in [11]) and Ce:TGAG ceramics is presented in Table 1. It has been shown that a much higher CRI of 72.2 was obtained in Ce:TAG (G1) compared to Ce:YAG ceramic at approximate CCT. Furthermore, the CCT values can be remarkably reduced from 5053 K to 3681 K with even higher CRIs in Ce:TGAG ceramics and the FWHM is also widening with the introduction of Gd³⁺. The lower CCT in G2∼4 can be attributed to the red-shifted emission wavelength of Ce³⁺, and also the higher emission intensity with higher absorption of the blue light. However, it is worth to mention that the luminous efficacy of Ce:TGAG phosphor ceramics are relatively lower than Ce:YAG ceramics, which indicates the necessity to compensate its luminous intensity such as to introduce composite phase in ceramic matrix [28].

Table 1. Comparison of the CRI, CCT, LE, FWHM, and CIE coordinates of Ce:YAG ceramics [28] and Ce:TGAG ceramics.

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4. Conclusions

Ce:TGAG transparent phosphor ceramics have been fabricated for the first time through solid-state vacuum sintering for the application of warm white light illumination. The maximum emission peak of Ce³⁺ ions under 460 nm excitation was found to shift from 550 nm to 570 nm through Gd-doping and the emission intensity was greatly promoted. A similar result of the red-shift and enhanced emission of Ce³⁺ was also obtained under the 4f(²F_5/2)→ 5d₁(E₁) excitation of Tb³⁺ at 327 nm by the energy transfer of Tb³⁺→Ce³⁺. Packed LED devices based on Ce:TGAG ceramics and blue light chips have shown that the red component in the output light significantly increased with Gd-doping, and the CCT can be reduced from 5053 K to 3681 K with a high CRI around 73. Therefore, Ce: TGAG phosphor ceramics are very promising candidates for the application of warm white light illumination. A further investigation on the enhancement of luminous efficacy and the thermal quenching behaviour of Ce:TGAG is still going on.