1. Introduction

As a new generation of lighting source, white light emitting diode (wLED) has many advantages and plays a vital role in indoor and outdoor lighting, plant lighting, anti-counterfeiting light sources and other fields [1–4]. At present, the mainstream method for preparing wLED is to combine blue InGaN chip with Ce³⁺:YAG yellow phosphor [5,6]. However, this method suffers from a low color rendering index due to the lack of red light components. Adding red phosphor can make up for the defects of this method. Red commercial phosphors such as SrAlSiN₃:Eu²⁺ and K₂SiF₆:Mn⁴⁺ exhibit efficient red luminescence under the blue LED excitation, and show small thermal quenching above room temperature. However, the preparation conditions of these materials are very demanding [7,8]. Therefore, it is very important to develop red fluorescent materials with simple preparation process, low cost and good chemical and luminescent stability.

Transition metal doped red fluorescent materials have received extensive attention in recent years because of their advantages of non-toxicity and cost effectiveness. Due to the ⁴A₂→⁴T₂ and ⁴A₂→⁴T₁ spin allowed transition, Mn⁴⁺ ions and Cr³⁺ ions show a wide excitation band in the UV-visible region, which match the UV and blue LED chips well. Huang et al. prepared deep red Ca₂YTaO₆:Mn⁴⁺ phosphor by solid state reaction, the color purity reached 98.3% [9]. Hua et al. prepared Y₃Ga₅O₁₂:Cr³⁺ transparent ceramics by solid state reaction, the ceramic phosphor emits red light with a peak position at 709 nm and the corresponding color coordinates are located at (0.7162, 0.2832), which can be a candidate phosphor material for red-emitting LEDs [10]. BaMgAl₁₀O₁₇:Cr³⁺ phosphor and Al₂O₃:Cr³⁺ fluorescent ceramics have also been successfully prepared and demonstrated in the field of plant lighting [11–13].

At present, there are few studies on fluorescent materials based on Ba_0.75Al₁₁O_17.25. In 2006, Li et al. prepared Ba_0.75Al₁₁O_17.25:Mn²⁺, the effect of Mn²⁺ ion concentration on its luminescence properties was studied [14]. In 2009, Zhou et al. prepared Ba_0.75Al₁₁O_17.25–BaMgAl₁₀O₁₇:Eu²⁺, Mn²⁺ solid solution. The addition of Eu²⁺ ions increases the green emission of Mn²⁺ ions [15]. In 2021, Wang et al. prepared Ba_0.75-xAl_11-yO_17.25: xEu²⁺, yMn²⁺ transparent ceramics, which show excellent luminescence properties, especially the luminescence thermal stability [16]. To further explore the possibility of developing promising phosphors based on Ba_0.75Al₁₁O_17.25, in this paper the Ba_0.75Al₁₁O_17.25:Cr³⁺ phosphors were prepared and the luminescence properties have been investigated.

In this paper, Ba_0.75Al₁₁O_17.25 red phosphors with different concentrations of Cr³⁺ ions were prepared by solid state reaction. The crystal structure and luminescence properties of the phosphors were investigated. The phosphor emitted deep red light peaked at about 704 nm. In order to solve the problems of aging and cracking of silicone gel, transparent phosphor ceramics were prepared by spark plasma sintering (SPS). SPS is a powerful method for ceramic sintering which has the advantages of rapid temperature ramping, short sintering time, and easy densification, etc. The thermal properties of transparent phosphor ceramics were studied. Photometric and colorimetric performances of the phosphor-converted LED prototypes which were packaged with the ceramic phosphors were evaluated.

2. Experimental

Ba_0.75Al₁₁O_17.25:Cr³⁺(BAO:Cr) phosphors were prepared by solid state reaction. The raw materials were BaCO₃, Al₂O₃ and Cr₂O₃ (99.99%, Aladdin, China). The raw materials were weighed according to the stoichiometric ratio of Ba_0.75Al_11-xO_17.25:xCr³⁺(x = 1%, 2%, 3% and 4%), and then ball milled in ethanol for 12 h. The obtained slurries were dried in an oven for 12 h, and then screened through a 200-mesh sieve. Ba_0.75Al₁₁O_17.25:Cr³⁺ phosphor powders can be obtained after calcination in a box furnace at 1650 °C for 5 h.

Ba_0.75Al₁₁O_17.25:Cr³⁺ fluorescent ceramics were prepared by SPS. Firstly, the screened powders were sintered at 850 °C in the box furnace for 5 h, then were loaded into the graphite mold and sintered at 1450 °C with 100 MPa uni-axial pressure for 30 min. Finally, the obtained ceramics were thermally treated at 1350 °C for 5 h in a box furnace to remove the carbon pollution introduced in the SPS process.

The crystal structure of all samples was analyzed by Cu K_α radiation X-ray diffractometer (40 kV, 15 mA) (Rigaku, Model Mini Flex 600, Japan). The microstructural morphology of the samples was characterized by scanning electron microscopes (SEM) (FEI, Quanta FEG 250, U. S. A.). The excitation spectra, emission spectra, fluorescence decay curve and temperature dependent emission spectra of the samples were detected by a fluorescence spectrophotometer (Edinburgh Instruments, FLS-1000, U. K.). The photometric and colorimetric performances of the BAO:Cr ceramic phosphors packaged phosphor-converted LED prototypes were evaluated by an LED opto-electronic analyzer (Everfine, ATA-500, China).

3. Results and discussion

XRD θ-2θ scanning patterns of the Ba_0.75-xAl₁₁O_17.25:xCr³⁺ (x = 1%, 2%, 3%, 4%) phosphors are shown in Fig. 1. It can be clearly seen that the diffraction peaks of all the phosphor powders samples match the standard card (#ICSD 29441) of Ba_0.75Al₁₁O_17.25 well, indicating that the Ba_0.75Al₁₁O_17.25 phase is successfully obtained, and the doping of Cr³⁺ ions did not introduce any impurity phase.

 

Fig. 1. XRD patterns of Ba_0.75-xAl₁₁O_17.25:xCr³⁺ (x = 1%, 2%, 3%, and 4%) phosphors.

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Figure 2 shows the structure diagram of Ba_0.75Al₁₁O_17.25 host lattice, in which Ba²⁺ ion has only one coordination with 9 O^2- ions, while Al³⁺ ion has five coordination structures in forms of tetrahedrons and octahedrons, and Cr³⁺ ions may replace Al1 (in brown polyhedron) or Al3 (in blue polyhedron) [16]. The Al1 site is coordinated with six equivalent O4, the six bond lengths are all 1.898 Å, the nearest neighboring cations are six completely equivalent Al2, the next neighbor cations are six equivalent Al3, and the octahedron centered on Al1 is a centrosymmetric octahedron. According to the selection rules, the d-d transition of Cr³⁺ ions in centrosymmetric complexes is prohibited [17,18]. Therefore, in Ba_0.75Al₁₁O_17.25 Al3 is the only site for Cr³⁺ which can give luminescence.

 

Fig. 2. The structure diagram of Ba_0.75Al₁₁O_17.25 unit cell.

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Excitation and emission spectra of Ba_0.75Al₁₁O_17.25 phosphors doped with different Cr³⁺ ion concentrations are given in Fig. 3. Under the bluish violet light excitation of 405 nm, the phosphor emits red light peaked at 704 nm. When the Cr³⁺ ion doping concentration is 3%, the emission intensity of the phosphor is the highest. According to Tanabe-Sugano energy level diagram (Fig. 3(b)), when the ²E energy level is the lowest excited state, there is little possibility of coupling with the lattice, which will lead to narrow-band luminescence. Therefore, the narrow-band emission at 704 nm can be attributed to the energy level transition of Cr³⁺ ion ²E → ⁴A₂ [12]. The emission bands on the left side of the strongest peak are the anti-Stokes side bands, which are generated by the vibration of 3d³ level when Cr³⁺ ions are located in octahedral polyhedron. With the 704 nm red emission peak monitored, the peak values of the excitation band are 408 nm and 550 nm, respectively, corresponding to the ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ transitions of Cr³⁺ ions.

 

Fig. 3. (a) excitation spectra and emission spectra of the Ba_0.75-xAl₁₁O_17.25:xCr³⁺ (x = 1%, 2%, 3%, and 4%) phosphors. (b) Tanabe-Sugano energy level diagram of Cr³⁺. (c) excitation spectra of the Ba_0.72Al₁₁O_17.25: 3%Cr³⁺ is fitted by Gaussian curves.

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In order to further verify the crystal field intensity of Cr³⁺ ion, the excitation spectrum is fitted into multiple Gaussian peaks (see Fig. 3(c)), and Dq/B is calculated according to the following formulae [19,20]:

Figure 4 shows the fluorescence decay curves of the Ba_0.75-xAl₁₁O_17.25:xCr³⁺ (x = 1%, 2%, 3%, and 4%) phosphor powders emitting peaked at 704 nm under the 405 nm excitation. The decay curves can be well fitted with a single exponential function and can be expressed by the following equation:

 

Fig. 4. Fluorescence decay curves of the Ba_0.75-xAl₁₁O_17.25:xCr³⁺ (x = 1%, 2%, 3%, and 4%) phosphors’ red emission at 704 nm.

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To study the luminescence thermal quenching behavior of the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ phosphors, temperature dependent emission spectra of the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ phosphor were measured, as shown in Fig. 5. With the temperature increased, the narrow-band emission at 704 nm gradually decreases. When the temperature reaches 205 °C, the peak emission intensity is about 50% of that at room temperature. However, due to the increase of the vibrational side band energy, the luminescence intensity of the anti-Stokes shift emission gradually increases. In addition, the FWHM values of the red emission also gradually increased. Therefore, when the temperature reaches 205 °C, the integrated red emission intensity can still maintain 88% of the counterpart at room temperature, indicating that the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ phosphor has good luminescence thermal stability.

 

Fig. 5. (a) Temperature-dependent emission spectra of Ba_0.72Al₁₁O_17.25:3%Cr^3 +phosphors under excitation at 405 nm. (b) The line graph of the emission intensity at 704 nm and the integrated intensity of the red emission spectra as a function of temperature.

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In order to avoid the problems of low thermal conductivity and thermal degradation of the silicone gel which supports the phosphor powders for phosphor-converted LED package, the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ phosphor powders were densified into Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramics by SPS, and the luminescence and thermal properties of Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramics were investigated.

The XRD pattern of the ceramic is given in Fig. 6 (a). The diffraction peaks match the standard card (#ICSD 29441) well, indicating that the pure phase Ba_0.75Al₁₁O_17.25 is obtained. The excitation and emission spectra of the powder and ceramic samples are given in Fig. 6(b), showing the same spectral profiles.

 

Fig. 6. (a) XRD patterns of Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramic. (b) Comparison of emission spectra (λ_ex= 405 nm) and excitation spectra (λ_em= 704 nm) of Ba_0.72Al₁₁O_17.25:3%Cr³⁺ phosphor powders and ceramics. (c) The CIE chromaticity diagram of Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramic’s emission.

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The CIE chromaticity diagram of Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramic’s emission is shown in Fig. 6 (c). Under the 405 nm excitation, the color coordinates of the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramic emission are close to the spectral locus, showing the deep red emission is of high color purity. For further verification, the color purity of the red emission was calculated according to the following equation [21],

The optical transmittance of the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ phosphor ceramic is shown in Fig. 7. The two absorption bands peaked at 400 nm and 500 nm correspond to the Cr³⁺: ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ transitions, respectively, which are in accordance with the excitation peaks. The optical transmittance of the sample at 800 nm is 46%. A picture of the ceramic sample is given in the inset of Fig. 7(a), and the text behind the ceramic can be clearly seen. The sample shows strong absorptions at 450 nm (blue light) and 530 nm (yellow light), which can be combined with Ce³⁺:YAG phosphor and blue LED chip together to generate white light. Figure 7(b) shows the SEM image of the cross-sectional fracture surface of the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ phosphor ceramic. The grain size of the ceramic is uniform, the structure is dense, and there are no pores observed inside the grains or on the grain boundaries.

 

Fig. 7. (a) The optical transmittance spectrum of the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramic. Inset: a photograph of Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramic. (b) SEM image of the cross-sectional fracture surface of Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramic.

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Fig. 8. Thermal conductivity of the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramic at different temperatures.

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Thermal conductivity is always concerned since it is vital to the heat release for the ceramic phosphors. At room temperature, the thermal conductivity of the ceramic is 4.87 W/(m·K) (see Fig. 8), which is much higher than that of the silicone gel (0.1-0.4 W/(m·K)) and good for the heat dissipation of the ceramic phosphor during LED operation, reducing the luminescence thermal quenching.

Finally, the Ba_0.72Al₁₁O_17.25:3%Cr³⁺ ceramic and YAG:0.3%Ce³⁺ ceramic were packaged onto a 450 nm InGaN blue LED chip and the schematic diagram is shown in Fig. 9 (c). The color coordinates (0.2995,0.3101) of the phosphor-converted LED prototype’s EL spectrum of are marked in the CIE chromaticity diagram, which are on the blackbody radiation curve, showing that the Ba_0.72Al₁₁O_17.25:Cr³⁺ ceramic phosphor is suitable for the application in w-LED lighting.

 

Fig. 9. (a) a schematic diagram to illustrate the structure of the ceramic phosphor converted LED prototype. (b) EL spectrum and (c) CIE chromaticity diagram of the 3% BAO:Cr/YAG:0.3%Ce³⁺ceramic phosphors packaged phosphor-converted LED prototype.

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4. Conclusions

In this paper, Ba_0.75Al₁₁O_17.25:Cr³⁺ red phosphors were synthesized by solid state reaction. The phosphors show a narrow peak deep red emission peaked at 704 nm under the 405 nm excitation. Ba_0.75Al₁₁O_17.25:Cr³⁺ phosphor has excellent luminescence thermal stability, when the sample temperature rises to 205 °C, the integrated intensity of the red emission can maintain 88% of the counterpart at room temperature. In order to avoid the problems of low thermal conductivity and degradation of silicone gel for supporting the phosphor powders, Ba_0.72Al₁₁O_17.25:3%Cr³⁺ phosphor powders were densified into ceramics by SPS. The color purity of the phosphor ceramic’s red emission can reach 97%. At room temperature, the thermal conductivity of ceramic is 4.87 W/(m·K), which is much higher than that of silicone gel. With a low thermal resistance package, the phosphor ceramic can keep a low temperature during the phosphor conversion process, which is beneficial to high power/high brightness phosphor converted solid state light sources.