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Abstract
MgAl2O4:Mn2+ transparent ceramics were fabricated by reactive spark plasma sintering (SPS). The ceramic samples show narrow-band green emission under the 450 nm blue light excitation, which is corresponding to 4T1(4 G)-6A1(6S) transition of Mn2+ in the tetrahedral site. The emission peak of the Mg0.93Al2O4:0.07Mn2+ ceramic sample was located at 525 nm with the full-width at half-maximum (FWHM) value of 36 nm. The internal quantum yield (IQY) of Mg0.93Al2O4:0.07Mn2+ reached 63%. The emission intensity remained ∼98% at 150 °C compared to its initial value at room temperature, showing excellent thermal quenching performance. The results indicated that MgAl2O4:Mn2+ ceramic phosphor is a promising candidate for high brightness, wide gamut display backlight applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
During the past two decades, white light-emitting diodes (wLEDs) have been blooming in the fields of solid-state lighting and liquid crystal display (LCD) backlights due to its high luminous efficacy, long lifetime, compact size, eco-friendliness, and cost-effective manufacturing process [1–3]. So far, the mainstream commercial white lighting and LCD backlights is to combine blue InGaN chip with Ce3+:YAG yellow phosphors, a proper amount of the transmitted blue light is mixed with the emitted yellow light to obtain white light [4–7]. But for this methodology, the lack of a red component in Ce3+:YAG's PL spectra causes the low CRI value. To solve this problem, many red emitting phosphors, such as (Sr,Ca)2Si5N8:Eu2+ and CaAlSiN3:Eu2+, as well as green emitting phosphors to which human eyes are sensitive, such as Ba [Li2(Al2Si2)N6]:Eu2+ and β-SiAlON:Eu2+, have been developed and investigated intensively [8–11. Whereas, the large FWHM of YAG:Ce3+ (>100 nm), (Sr,Ca)2Si5N8:Eu2+ (>90 nm), Ba [Li2(Al2Si2)N6]:Eu2+ (∼57 nm) and β-SiAlON:Eu2 + (∼55 nm) partly limits their application as backlights owing to the unsatisfying color gamut [8,12,13]. What’s more, the low diffusion coefficient, high saturated vapor pressure, and difficulty in removing the contamination (for instance, carbon) of nitrides make its ceramic fabrication demanding and costly. In recent years, some green phosphors with comparatively smaller FWHM (<50 nm) have been synthesized by researchers, for example, SrGa2S4:Eu2+ (47 nm) and RbLi(Li3SiO4)2:Eu2+ (42 nm), but unfortunately both sulfide and silicate are chemically and thermally unstable [14,15].
Compared with the rare earth Eu2+, obtaining the narrow FWHM emissions via the d-d electron transition of transition metal Mn2+ seems more feasible. So far, γ-AlON:Mn2+ (44 nm), Zn2SiO4:Mn2+ (42 nm), Sr2MgAl22O36:Mn2+ (26 nm) and ZnAl2O4:Mn2+ (18 nm), etc. have been reported [16–19]. The d-d electron transition of Mn2+-doped phosphors is parity-forbidden and largely determined by the crystal field of host materials. Consequently, the quantum efficiency (QE) of Mn2+-doped phosphors is usually lower than the current commercial phosphor products and the excitation and emission spectra vary a lot in different host materials. Boosting the QE and looking for proper host materials with expected PL properties, which in particular can perfectly match the blue InGaN LED/LD chips, are the main concerns.
Previously, Mn2+ activated MgAl2O4 phosphors, in the forms of powders and ceramics, have been investigated by some pioneering work, which disclosed that this material is promising for the applications in LCD backlights or dosimetry [20,21]. With respect to the various kinds of phosphors forms, including powders, single crystals, glass, ceramics and glass ceramics, etc., intensive efforts have been made on ceramic materials which exhibit much higher physicochemical thermal stability and higher heat release capability than the counterparts of the transparent silicone encapsulated phosphors, which suffers from severe thermal degradation under high-power/high brightness light excitation [22–26]. The cubic MgAl2O4 spinel structure is feasible to be densified into transparent ceramics and has high thermal conductivity and low thermal expansion. In this work, the Mn doped magnesium aluminate (MgAl2O4) spinel transparent ceramic phosphors were fabricated via SPS. The phase composition, microstructures and luminescence and thermal properties of the ceramic phosphors were investigated and discussed.
2. Experimental
2.1 Preparations of phosphor powders and transparent ceramics
Phosphor powders Mg(1-x)Al2O4:xMn2+ (x = 0.03, 0.05, 0.07, and 0.1) were synthesized via solid-state reaction. The raw materials including MgO (99.99%), Al2O3 (99.99%) and MnO2 (99.99%) were weighed and then ball milled. The slurries were then dried and thoroughly ground in an agate mortar. When the powders were cooled down to room temperature naturally, they were fabricated into ceramic samples by SPS under 100 MPawith the holding temperatures in the range of 1160 °C-1310 °C. After sintering, the ceramic samples were annealed in air at 950 °C for 5 hours to remove the carbon contamination during the SPS process. Then the samples were double-side polished for further characterization.
2.2 Characterization
The crystalline phases of the prepared ceramics with different compositions were tested by X-ray diffraction (XRD, Model MiniFlex 600, Rigaku, Japan) in the 2θ range from 10° to 90° with a step of 0.02°. Microstructures of the ceramics were observed by the scanning electron microscopy (SEM, MIRA3, Tescan, Czech). The optical transmittance was measured by a UV-VIS-NIR spectrometer (Lambda 1050, Perkin Elmer, USA). The thermal conductivity was tested by the flash lamp method (LFA-457, Netzch, Germany). Photoluminescence (PL) spectra, photoluminescence excitation (PLE) spectra and the fluorescence decay time, were measured by a fluorescent spectrophotometer (FLS-1000, Edinburgh Instruments, U. K.) equipped with a 450 W Xe lamp as the excitation source. Temperature-dependent spectra ranging from 30 °C to 240 °C with an interval of 30 °C were measured by the same equipment with a cryostat (Oxford Instruments, UK). The internal quantum efficiency (IQE) values of the transparent ceramic phosphors were measured within an integrated sphere attachment equipped with the fluorescence spectrometer (FLS-1000, Edinburgh Instruments, UK). IQE was obtained by calculating the ratio of the number of emitted photons to the number of absorbed photons in our experiment. [27]
The brief procedure of the IQE measurement process is briefly provided as follows,
3. Results and discussion
Figure 1 shows the SEM images of the raw material powders and the mixed powders after ball milling. The raw materials of Al2O3 and MgO were mostly in the form of clusters, with serious aggregation. The profiles and particle size of the MnO2 powders are not quite uniform. After ball milling, the mixture powders were homogeneously mixed, composed of particles with an average size of about 1 µm. Table 1 presents the names of Mg0.97Al2O4:0.03Mn2+ ceramics under different sintering holding temperatures.
Fig. 1. SEM images of the raw material powders: Al2O3 (a), MgO (b), MnO2(c) and the mixture powders after ball milling (d).
Table 1. Mg0.97Al2O4:0.03Mn2+ ceramics under different sintering temperatures
Figure 2 shows the XRD θ-2θ scanning patterns of the Mg0.97Al2O4:0.03Mn2+ ceramics sintered at temperatures in the range of 1160 °C-1310 °C and the standard JCPDS card of MgAl2O4 (PDF#21-1125). All the ceramic samples are of the pure spinel MgAl2O4 phase, which belongs to a cubic structure (space group Fd3m).
Fig. 2. XRD θ-2θ scanning patterns of Mg0.97Al2O4:0.03Mn2+ ceramic samples sintered at 1160 °C (S1), 1185 °C (S2), 1210 °C (S3), 1235 °C (S4), 1260 °C (S5), 1285 °C (S6), 1310 °C (S7) and standard JCPDS card (No. 21-1125) of spinel MgAl2O4 is provided for comparison.
As shown in Fig. 3, it can be observed that sample S1 and S2 are opaque. As the sintering temperature rises to 1210 °C, the optical transmittance of the S3-S5 around 520 nm can reach up to 50% (see Fig. 4). After the sintering holding temperature continues to rise, sample S6 and S7 shows the color of black brown, which may be caused by severe carbon contamination at higher sintering holding temperatures. Considering the optical transmittance, the optimal sintering holding temperature for the transparent ceramic samples is 1210 °C within our experimental conditions.
Figure 5 shows the thermally etched surface SEM images of the sample S1-S7. For sample S1, there are many open pores as well as the insufficient shrinkage existing inside the ceramic caused by the inadequate densification, leading to the low transparency. With the increase of the sintering holding temperature, the microstructure of the SPSed ceramic samples grow denser. When the sintering holding temperature rises to 1210-1260 °C, the grain size of samples S3-S5 is relatively regular and uniform, which exhibited dense microstructure with an average grain size of approximately 0.3-1 µm. As the sintering holding temperature continues to rise above 1260 °C, the microstructures of the samples S6 and S7 become ununiform, some exaggeratedly grown grains appeared in S7.
Fig. 5. SEM image of surface morphology of sample S1 (a) and (b), S2 (c), S3 (d), S4 (e), S5 (f), S6 (g) and S7 (h).
Figure 6 shows the thermal conductivity of sample S3 measured at several temperatures in the range of 25-250 °C. The thermal conductivity at room temperature is 11.8 W·m-1·K-1 and remains 8.3 W·m-1·K-1 at 250 °C, which is beneficial to heat release under high-brightness light excitation.
Fig. 6. Thermal conductivity of sample S3 measured at different temperatures in the range of 25 °C-250 °C.
The optimal sintering temperature was obtained within our experimental conditions, and then a group of MgAl2O4:Mn2+ ceramics with different doping concentrations of Mn2+ were prepared at the sintering holding temperature of 1210 °C, as listed in Table 2. Seen from Fig. 7, it can be observed that all the samples showed a dense structure and good transparency. With the Mn2+ doping concentration increasing, the color turns from blackish green to olive green, then to grass green, finally becomes yellow green, and the color changes from dark to light. Seen from the XRD θ-2θ scanning patterns shown in Fig. 8, the ceramic samples D1, D2, D3 and D4 are all of the spinel MgAl2O4 structure, indicating the Mn2+ were all dissolved in the lattice even the Mn2+ doping concentration is as high as 10%. The Mn2+ ions are quite possibly to replace Mg2+ due to the similar ionic radii (rMn2+ = 0.66 Å; rMg2+ = 0.57 Å), the same valence states, and the same tetrahedral four O2--coordination.
Table 2. Mg(1-x)Al2O4:xMn2+ ceramics with different doping concentrations of Mn2+
The optical transmittance was presented in Fig. 9. For all the samples, the optical transmittance increased with the Mn2+ doping concentration changing from 3at% to 7at%, reaching the maximal for sample D3 then decreased when the Mn2+ doping concentration is 10at%. It is noted that sample D3 shows the highest optical transmittance. There are two small absorption bands at 426 nm and 451 nm, which coincide with the excitation bands at 426 nm and 451 nm in the PLE spectrum. The MgAl2O4:Mn2+ ceramic phosphor of high transparency with suitable thickness increases the possibility for adequate absorption of the blue light by Mn2+ ions. Sample D2 with the lowest optical transmittance may be due to the low Mn2+ doping concentration induced higher melting point of the chemical composition. Thus, the densification process is relatively slow and more carbon may be introduced during the sintering. For samples D1 and D2, there is strong additional absorption band in the visible range overlapping with the samples’ green emission. This could be related to the absorption the electronic transition from 5Eg to 5T2g of the non-luminescent Mn3+ [28]. During the sintering process, most of the Mn4+ ions have been reduced to Mn2+, and a small part of the Mn4+ ions are reduced to Mn3+. The formation of Mn3+ may be caused by charge compensation, due to the chare imbalance brought by the remaining Mn4+ in the MgAl2O4 spinel host lattice. Mn3+ ion tends to be located in the octahedron of the MgAl2O4 [29].
Figure 10 exhibits the PLE and PL spectra of samples D1-D4. Seen from the PLE spectra of D1-D4, the excitation bands peaked at 278 nm, 365 nm, 385 nm, 426 nm and 451 nm are associated with Mn2+ transition from the 6A1(S) ground state to the excited states, which is 6A1(S)→4A2(4F), 4E(D),4T2(4D), [4A1(4 G),4E(4 G)], and4T2(G), respectively. The excitation band around 450 nm overlaps well with the output wavelengths of the commercial blue LEDs/LDs.
Under the 450 nm excitation, green emission can be achieved in all samples D1-D4 of the room temperature PL spectra, and all the emission spectra comprise a narrow emission band peaking around 525 nm, corresponding to the 4T1(4 G)-6A1(6S) transitions of tetrahedral Mn2+. With the Mn2+ doping concentration increase from 3at% to 10at%, the emission wavelength shifted from 520 nm to 530 nm, meanwhile the FWHM of the emission band gradually increasing from 33 nm to 38 nm, which is attributed to higher Mn2+ doping and the subsequent inhomogeneous broadening [30]. The FWHM of emission band can be expressed as follows [31]
where W0 is the FWHM at 0 K and hw is the energy of lattice vibration that interacts with electronic transitions. The higher concentration of Mn2+ doping will lead to large lattice distortion and enhancement of the phonon energy, thus making FWHM broader. The red-shift of Mn2+ emission was possibly ascribed to the formation of Mn2+–Mn2+ pairs in the MgAl2O4 host lattice [32,33]. The FWHM of Mg0.93Al2O4:0.07Mn2+ (36 nm) is apparently smaller than that of commercial β-SiAlON:Eu2+ (54 nm), showing that MgAl2O4:Mn2+ would be a promising phosphor for wide gamut display backlight application [13].The concentration dependence of the emission intensity from D1 to D4 under 450 nm excitation are presented in Fig. 10. With the Mn2+ concentration gradually increases, the emission intensity increases initially, reaches the maximum for D3, and then decreases in D4, which may be due to the concentration quenching phenomenon. The IQE variation is consistent well with the florescence intensity, which increases in sequence for D1, D2 and D3 and drops for D4. The IQE of sample D3 is 63%, which is higher than the result (45%) reported in Ref. [20] and both of the IQE values are still lower than that of the β-SiAlON:Eu2+ commercial phosphor [13,34]. It is pointed out that the IQE of the MgAl2O4:Mn2+ powder phosphor is possibly to be improved by fine controlling the fabrication parameters of the phosphor particles [20].
For the Mn2+ activated phosphor in which the Mn2+ ions are located in the highly rigid and isolated tetrahedrons (not sharing facet or edge with the neighboring tetrahedron), usually narrow band green emission with high thermal quenching resistance and high IQE are shown due to the effective suppression of non-radiative transitions and the concentration quenching [35,36].
For sample D4, there is a weak red emission peaked at 686 nm. According to many studies, it is preliminarily determined that the red emission peaked at 686 nm does not originate from Mn4+ ions [21,37]. Cr3+ ion is an impurity ion that is difficult to remove in alumina raw material, and Cr3+ ion will emit red light with a peak wavelength of 686 nm in the MgAl2O4, so it is speculated that the red emission peaked at 686 nm is probably from the emission of the impurity Cr3+ ion [38].
Figure 11 shows the decay curves of D1, D2, D3 and D4 under the 450 nm excitation, with the 525 nm emission monitored at room temperature. As the Mn2+ concentration increased from 3at% to 7at%, slight decrease of emission lifetime was shown and all the luminescence decay curves can be well fitted by a single-exponential function. But for the high doping sample D4, non-radiative transition probability boosts and concentration quenching happens, in result the emission lifetime descends to 3.28 ms. These results are in good agreement with the variation of both the emission intensity and the quantum yield by changing the Mn2+ doping concentration.
Fig. 11. Luminescence decay curves and the corresponding emission lifetime of D1-D4 at room temperature.
As shown in Fig. 12, the emission peak position of the Mg0.93Al2O4:0.07Mn2+ ceramic slightly shifts from 530 nm (30 °C) to 525 nm (240 °C), owing to the crystal lattice distortion. With increasing temperature, meanwhile the FWHM value is increased from 36 nm to 40 nm, which is still sharp enough for achieving wide gamut display in backlights application, indicating its decent luminescence property of high temperature resistance. It is worth noting that the emission intensity of Mg0.03Al2O4:0.07Mn2+ ceramic also descended with increasing temperature. The integrated emission intensity at 150 °C can retain 98% of the initial intensity at 30 °C and it declines to 74% of initial intensity when the temperature was further raised to 240 °C. The excellent luminescence thermal stability can be attributed to the high symmetry, especially the high rigidity and corresponding high Debye temperature (880 K) of the MgAl2O4 crystal lattice [20]. Furthermore, the good thermal stability is also assured by the impurity energy levels of Mn2+ being well positioned between the valence band and conduction band, diminishing the unexpected non-radiative transition process [39].
As for phosphor application, especially under the LD excitation, the slow decay rate of Mn2+ luminescence is not ideal since it may lead to excitation bleaching. In this work, the emission spectra of sample D3 under different laser power was tested with an integrating sphere (see Fig. 13). With the increase of laser power, the luminescence intensity of the sample increased continuously, and the luminescence saturation was not reached when the laser current was turned on to the maximum, which indicated that ceramics have bright application prospects in the field of laser lighting.
Fig. 13. Emission spectra of sample D3 excited by a 450 nm laser diode under different driven currents.
To further explore the relationship between the light output of the sample and the laser power density, we plotted the scatter plots of the luminescence intensity at 530 nm as a function of the laser power density, as shown in Fig. 14. The light output of the sample still did not show obvious saturation when the laser power density reached 28 W/mm2, indicating that the MgAl2O4:Mn2+ ceramic phosphor may get a chance to serve as a phosphor converter driven by LD or high optical density LEDs.
Fig. 14. The light output of sample D3 under the 450 nm laser diode excitation as a function of the laser power density.
4. Conclusions
Green-emitting Mn2+ doped spinel MgAl2O4 ceramic phosphors were prepared by SPS. Under the 450 nm blue excitation, narrow-band green emission with high thermal resistance due to 4T1(4 G)-6A1(6S) transition of Mn2+ located in the tetrahedra sites was shown. The thermal conductivity of the Mg0.97Al2O4:0.03Mn2+ ceramic phosphor remains as high as 8.3 W·m-1·K-1 at 250 °C, which is beneficial to heat release under high power/high brightness light excitation. The Mg0.93Al2O4:0.07Mn2+ ceramic phosphor showed narrow emission bandwidth (FWHM = 36 nm) and the emission intensity at 150 °C can still be kept at 98% of that at room temperature. All of these results suggested that the MgAl2O4:Mn2+ phosphor ceramic is quite promising for high-brightness, wide color gamut display backlights applications.
Funding
Shanghai Pujiang Program (18PJ1408800); Shanghai Science and Technology Innovation Program (19511104600); Natural Science Foundation of Inner Mongolia (2019MS01009).
Acknowledgments
This work has received funding from the Science and Technology Commission of Shanghai Municipality and Science and Technology Department of Inner Mongolia Autonomous Region.
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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