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Abstract
MgAl2O4:Cr3+ far-red emitting ceramic phosphors were prepared by the solid-state reaction method. The photoluminescence characteristics, especially the luminescence thermal stability, thermal conductivity, and its application in phosphor-converted LEDs were studied. Under 450 nm excitation, the ceramic phosphors exhibited an emission range of 650-750 nm and showed a narrow-band emission peaked at 688 nm. It also shows 708 nm and 718 nm far-red emission, which matches well with the absorption peak of the plant phytochrome PFR. The luminescence thermal stability of MgAl1.99O4:0.01Cr3+ was excellent. The integral intensity of the two emissions peaked at 708 nm and 718 nm at 500 K can maintain 98.27% and 98.24% of the counterparts measured at room temperature, respectively, showing zero thermal quenching behavior up to 500 K. The thermal conductivity of the MgAl1.99O4:0.01Cr3+ ceramic was 10.3 W·m-1·K-1. These results indicate that MgAl2O4:Cr3+ ceramic phosphors can be applied in plant lighting when packaged onto 450 nm blue LED chips.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Light is one of the necessary conditions for plant growth [1,2]. Plant lighting technology can continuously produce crops at any time in one year’s four seasons and accurately regulate the growth cycle of plants [3]. Compared with the traditional fluorescent lamps, LEDs are more efficient, environmentally friendly, have a small volume and longer service life, etc. [4,5]. In recent decades, the blue and near-ultraviolet LED chips to excite red phosphors have been extensively studied for plant lighting. However, far-red light, which also plays a decisive role in plant growth, is often ignored [6]. Most commercial red phosphors are rare earth materials such as Eu2+ ions [7]. However, rare earth materials are expensive [8]. Therefore, transition metal ions are of low price and may also give efficient and desirable emissions when incorporated in suitable host lattices. The research and development of Mn4+ ions luminescence for plant lighting have been extensive in recent years [9,10]. Besides Mn4+, Cr3+ irons generally have two strong and wide absorption bands due to the spin-allowed 4A2→4T1 and 4A2→4T2 transitions [11], respectively. Good luminescence thermal stability, strong red emission, and wide emission make transition metal ions activated phosphor a promising material for red luminescence [12].
As for the host materials, spinel oxides such as MgAl2O4, ZnAl2O4, and ZnGa2O4, etc [13–15] have been studied intensively because of their high mechanical strength, high optical transparency, and good chemical and physical stability. Among the spinel oxides, MgAl2O4 has high thermal conductivity, and the thermal expansion coefficient is small. Its crystal structure belongs to the Fd3m space group and is a face-centered cubic lattice [16,17]. The ion radius of Cr3 + ions (r = 0.615 Å) is similar to that of Al3+ ions (r = 0.535 Å), therefore, the doped Cr3+ ions will enter the Al3+ site in the [AlO6] octahedron [18].
Blum et al. first proposed that red-light and blue-light are the two most influential spectra components for plant growth [19]. Taking the red light, for example, chlorophyll has strong absorption in the wavelength region of 600-700 nm for photosynthesis, and the far-red light of 700-760 nm can affect seed germination. Phytochrome is the most transparent pigment in light receptors, including 600-700 nm red light (PR, L max = 666 nm) and 700-750 nm (PFR, L max = 730 nm). PR and PFR can be transformed by adjusting the ratio of far-red and red light to adjust the height of plants or the time of seed germination [20,21].
In this work, MgAl2O4:Cr3+ far-red emitting phosphors were prepared by the solid-state reaction method. Compared with traditional silicone gel packaging, phosphor ceramics have better thermal conductivity and are more suitable for high power/high brightness pc-LEDs. Therefore, the phosphor powders were densified into ceramics by spark plasma sintering (SPS), which can prepare ceramics with high densification, low sintering temperature and short sintering time. The crystal structure, SEM images, and optical properties of the ceramics were characterized. The crystal field intensity Dq and the value of the Racah B were calculated, and the luminescence thermal stability and the thermal conductivity of the ceramics were measured. Finally, the ceramic was packaged onto blue LED chips to evaluate the photometric parameters.
2. Materials fabrication
The MgAl2-xO4:xCr3+ (x = 0.002, 0.006, 0.01, and 0.014) phosphor powders were prepared by the solid-state reaction method. The raw materials Cr2O3 (99.95%, Aladdin, China), MgO (99.99%, Aladdin, China), and Al2O3 (99.99%, Taimei, Japan) were weighed in stoichiometric proportions, and then ball milled in ethanol for 12 h. The mixtures were put into the oven at 120 °C, drying for 10 h. After grinding and sintering at 850 °C for 5 h, we get the phosphor powders after cooling to room temperature.
The sintered phosphors were used as the raw materials for SPS. 2 g phosphors were put into the graphite die, then put into the chamber of the SPS system (Sinter Land, LABOX-325R, Japan), and then sintered at 1260 °C for 10 min at 100 MPa. After cooling to room temperature, the ceramics were cut and polished for characterization.
The crystal structure of MgAl2O4:Cr3+ samples was characterized by X-ray diffraction (XRD) on an X-ray diffractometer (Rigaku, Model Mini Flex 600, Japan) equipped with a Cu Kα radiation (λ = 0.15418 nm, 40 kV, 15 mA). A scanning step of 0.02 ° was used in the 2θ range of 10 °–90 °. The field emission scanning electron microscopy (FE-SEM, Carl Zeiss, Merilin compact, Germany) was used to investigate the morphology of the ceramic. The photoluminescence (PL), photoluminescence excitation (PLE), and the temperature-dependent PL spectra (300 K-575 K) were measured by a fluorescence spectrophotometer (Edinburgh Instruments, FLS1000, U. K.). The flashlamp method (Netzch, LFA-457, Germany) was used to measure the thermal conductivity of ceramics. The CIE chromaticity coordinates and electroluminescence (EL) spectra of pc-LEDs were measured with an integrating sphere (Everfine, ATA-500, China).
3. Results and discussion
MgAl2O4 is usually called spinel and belongs to the Fd3m space group. Mg2+ ions and Al3+ ions occupy the oxygen tetrahedron gap and the oxygen octahedron gap left in the crystal cell [22]. Because the six-coordination tendency of Cr3+ ions (r = 0.615 Å, CN = 6) and the ion radius of Al3+ ions (r = 0.535 Å, CN = 6) are similar, the doped Cr3+ ions will enter the Al3+ site in the [AlO6] octahedron (Fig. 1).
Fig. 1. (a) The crystal structure of MgAl2O4. (b) and (c) is the coordination polyhedrons of Al3+ and Mg2+.
The XRD patterns of MgAl2-xO4: xCr3+ (x = 0.002, 0.006, 0.01 and 0.014) ceramics are presented in Fig. 2. All diffraction peaks of the ceramics match well with the standard data of MgAl2O4 (PDF#99-0098), and no secondary phase is detected, which indicates the doping of Cr3+ ions will not change the structure of MgAl2O4 spinel.
The fracture surface SEM image of the MgAl1.99O4:0.01Cr3+ ceramic is shown in Fig. 3. The average size is in the range of 200-400 nm, and the grain size is uniform. The microstructure of the ceramic sample is relatively dense with no pores.
When Dq/B is less than 2.3, it is in a weak crystal field, showing broadband emission. It shows strong and narrow peak emission through 2E→4A2 transitions when Dq/B is greater than 2.3 [26]. Cr3+ ions belong to a 3d electronic structure [27]. As can be seen in Fig. 4, we can use the following equations to figure out the value of crystal field Dq and Racah parameter B [28].
By calculation, the lattice parameters are determined to be: Dq = 1856.2 cm-1, x = 3.8092, and Dq/B was calculated to be 2.67, indicating the energy of the 2E state is lower than that of 4T2. These results demonstrate that it is in a strong crystal field, exhibiting the 688 nm sharp 2E→2A2 emission.
The excitation of MgAl2O4:Cr3+ ceramic was shown in Fig. 5 (a). By monitoring the emission at 688 nm, the excitation spectrum was measured. In the 300-650 nm wavelength range, there are two peaks at 393 nm and 545 nm, which are attributed to transitions of 4A2→4T1 and 4A2→4T2, respectively. Figure 5(b) shows the PL emission of MgAl2O4:Cr3+ ceramics with different doping concentrations. In the wavelength region of 650 nm-750 nm, the most intensive emission is very sharp, which belongs to the 2E→4A2 spin-forbidden transition to its full width at half maximum (FWHM) is only 7.7 nm. A total of 5 peaks can be observed in the emission spectra.
Fig. 5. (a) PLE spectrum of MgAl1.99O4:0.01Cr3+. (b) PL spectra of MgAl2-xO4: xCr3+ (x = 0.002, 0.006, 0.01 and 0.014) (λ ex= 450 nm).
In the MgAl2O4 spinel structure, Cr3+ ions tend to occupy the octahedron Al3+ sites, 2E energy level is below the 4T2 energy level. It exhibits a sharp red emission because of the strong crystal field, showing zero phonon lines centered at 688 nm and multi-photon sidebands of transition R-lines at 676 nm, 698 nm, 708 nm, and 718 nm which are attributed to the 2E→4A2 transitions [29]. When the Cr3+ impurity concentration gradually increases, the emission peak at 688 nm does not shift, but the emission peak intensity increases. When the Cr3+ concentration was increased to 0.01, it reached the highest luminescence intensity [30–32]. However, after this point, the emission peak intensity decreased sharply when the Cr3+ concentration was further increased. The excitation spectrum of MgAl2O4:Cr3+ shows that it can be combined with ordinary blue LED chips, and its PL spectra show that MgAl2O4:Cr3+ can be used for plant growth LEDs.
Luminescence thermal stability is one of the essential characteristics of phosphor materials. The LED chips will inevitably reach up to 150 °C or even higher the operation. Therefore, we measured the PL spectra of MgAl1.99O4:0.01Cr3+ ceramic from 300 K to 575 K (Fig. 6). Obviously, with the increase in temperature, the integral intensity of this narrow-band emission gradually decreased due to the thermal coupling of 2E→4A2, while the other emission peaks show good thermal quenching performance. The emission intensities of 708 nm and 718 nm at 500 K can also maintain 98.27% and 98.24% of the counterparts measured at room temperature, respectively. It shows that the MgAl1.99O4:0.01Cr3+ ceramic has good luminescence thermal stability. It is promising to be used in pc-LED applications.
Fig. 6. Temperature-dependent PL spectra of MgAl1.99O4:0.01Cr3+ ceramic under 450 nm excitation. Inset: the PL spectra at 300 K and 500 K.
As shown in Fig. 7, with the temperature increased to 525 K, the total integral area of PL spectra keeps an upward trend. The emission intensity can be maintained at 575 K. This is because, with the increase in temperature, the Cr3+ ions vibrate more violently around the equilibrium position in the host lattice, the vibration amplitude and emission bandwidth increase [33], and the vibration amplitude increases, increasing the emission bandwidth [33]. Figure 8 shows that the FWHM values of MgAl1.99O4:0.01Cr3+ emission spectrum increase from 7.7 nm at room temperature to 90.8 nm at 575 K. As we all know, the emission efficiency is determined by the radiation and nonradiative transitions [34,35]. The probability of nonradiative transition increases with the temperature increasing, and the emission efficiency decreases. High activation energy can ensure a low probability of nonradiative transition at high temperatures, which benefits excellent luminescence thermal stability. The following formula can describe the dependence of luminescence on temperature:
where I0 is the initial emission intensity at room temperature, I(T) is the luminescence intensity at a certain temperature, K is the Boltzmann constant, and Eα can be obtained by calculating the slope [36]. It can be found in Fig. 9 that the activation energy of 688 nm is 0.25 eV, and that of 708 nm is 0.74 eV, which is higher than the counterparts of many materials (Table 1). Rigid [AlO6] units are formed in the MgAl2O4 host via edge-sharing with adjacent [AlO6] octahedrons and corner-sharing with [MgO4] tetrahedrons [37]. The rigid coordination structure of Cr3+ ions enables the strong heat resistance to thermal quenching of luminescence.Fig. 7. Normalized temperature-dependent integral and peak intensity of MgAl1.99O4:0.01Cr3+ ceramic excited at 450 nm.
Table 1. Emission peak wavelengths and activation energies of several typical transition metal ion-doped materials.
The traditional LED package is to disperse the fluorescent powder in the transparent silica gel. Due to the low thermal conductivity of the transparent silica gel, the heat is difficult to conduct, so the heat generation is serious. In the process of EL, part of the heat is generated during the conversion of (high frequency) light to (low frequency) light when the fluorescent material is excited by the light emitted by the blue chips. Prolonged heat will accelerate the aging of the chip and packaging silicone gel, make the chip invalid, and affect the service life and emission performance of LED. Therefore, good thermal conductivity is significant. As shown in Fig. 10, the thermal conductivity of MgAl1.99O4:0.01Cr3+ ceramic from 293 K to 573 K was measured. With the temperature increasing, the thermal conductivity decreases. It has a much higher thermal conductivity of 10.3 W·m-1·K-1 than silicone gel (0.1–0.4 W·m-1·K-1) [44] at room temperature. The thermal conductivity at 573 K is 8.5 W·m-1·K-1, which is beneficial to practical pc-LED applications.
The pc-LED was fabricated by packaging MgAl1.99O4:0.01Cr3+ ceramic phosphors onto 450 nm blue LED chips. The chromaticity and luminescence spectra of the phosphor converted LED prototype driven by 300 mA @ 8.593 V are shown in Fig. 11 (a). The EL spectra mainly include two emission bands: the emission of 450 nm blue chip and the far-red emission of MgAl2O4:Cr3+. Its far-red emission region matches well with the optical absorption of phytochrome PFR [45], which shows a good prospect of application as a pc-LED artificial light source for indoor plant growth. Figure 11 (b) shows the color coordinate of the pc-LEDs (x = 0.2221, y = 0.0583).
Fig. 11. (a) Absorption curves of PR and PFR phytochrome [41] and EL spectrum of pc-LED (thickness: 1.2 mm). (b) CIE chromaticity coordinates of the pc-LED.
4. Conclusions
In summary, the MgAl2O4:Cr3+ ceramic phosphors with different doping concentrations of Cr3+ have been prepared by solid-state reaction. Its luminescent properties and luminescence thermal stability were studied. MgAl2O4:Cr3+ can be excited by blue light and emit both red and far-red light, which is consistent with the absorption peak of plant phytochrome. It is found that this kind of phosphor has high activation energy and can maintain a low nonradiative transition probability at high temperatures. The thermal conductivity of MgAl1.99O4:0.01Cr3+ ceramic is measured to be 10.3 W·m-1·K-1 at room temperature, which is beneficial to heat removal. These results show that MgAl2O4:Cr3+ ceramic phosphors have application prospects in plant lighting LEDs.
Funding
Shanghai Science and Technology Innovation Program (19511104600); Shanghai Pujiang Program (18PJ1408800).
Acknowledgments
This work was sponsored by Shanghai Pujiang Program (18PJ1408800), and Shanghai Science and Technology Innovation Program (19511104600).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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