Mn4&#x002B; activated Al2O3 red-emitting ceramic phosphor with excellent thermal conductivity

Cheng Tian; Hui Lin; Hui Lin; Dawei Zhang; Peiping Zhang; Ruijin Hong; Zhaoxia Han; Xinglu Qian; Jun Zou; Jun Zou

doi:10.1364/OE.27.032666

1. Introduction

White light-emitting diodes (wLEDs) have been widely used in lighting and displays because of its low energy cost, long lifetime, high luminance and environmental friendliness to replace the incandescent lamps, fluorescent lamps, and high intensity discharge (H.I.D.) lamps [1–4]. Till now, combining blue InGaN chips with Ce:YAG yellow phosphors has been the most mature and successful commercial method to fabricate wLEDs [5,6]. However, this route lacks red-light component, in result leading to poor color rendering index (CRI) [7,8]. This deficiency can be compensated by adding red-emitting phosphors [9]. Therefore, great efforts have been made to develop red-emitting phosphors, which were mainly based on Eu²⁺ and Mn⁴⁺ or Mn²⁺ ions. For instance, Qiao et al. reported a kind of blue-light-excited red emission phosphor Rb₃YSi₂O₇:Eu²⁺, which showed broad-band red emission due to selective occupation of Eu²⁺ in the polyhedra with small coordination numbers. Zhang et al. reported a kind of Mn²⁺ based β-Ca₃(PO₄)₂-type compound with the nominal chemical formula of Sr₉MnLi(PO₄)₇ red emitting phosphor. The emission peak was located at about 615 nm. And the intensity of Mn²⁺ emission had been increased by 159 times with the introduction of Eu²⁺ [37,38]. The most widely used materials are Eu²⁺ activated nitride compounds, for instance, the commercial phosphor (Ca, Sr)AlSiN₃:Eu²⁺ [10]. The drawbacks of this material are the high cost of the raw materials and the demanding synthesis conditions. What`s more, this phosphor also absorbs green light which will lower the light conversion efficiency of the LED [11]. So numerous work has been devoted to exploring new red phosphors, for instance, the Mn⁴⁺ activated phosphors. Due to the spin- and parity-forbidden ²E_g→⁴A_2g transition, Mn⁴⁺ can exhibit red luminescence with the emission wavelength ranging from 600 nm to 750 nm [11,12]. Mn⁴⁺ doped ﬂuorides such as K₂SiF₆:Mn⁴⁺, which has already become a commercial phosphor, are also a research focus. Nevertheless, the preparation process of Mn⁴⁺ doped ﬂuorides is complicated, and the HF used in the preparation process is toxic [13]. Mn⁴⁺ doped oxides, such as CaMg₂Al₁₆O₂₇:Mn⁴⁺, KMgLaTeO₆:Mn⁴⁺, and Mg₂Al₄Si₅O₁₈:Mn⁴⁺ have also been developed [14–16]. However, the crystal structures of these powders are intricate, which makes it difficult to study the factors affecting Mn⁴⁺ luminescence. Another problem to be issued for the Mn⁴⁺ activated red phosphors is the luminescence thermal stability. The current commercial packaging materials of phosphor-converted (PC) wLEDs are transparent silicone, which has low thermal conductivity (0.1∼0.4 W·m⁻¹·K⁻¹) and unsatisfying heat resistance. As the operation time increases, the energy loss in the high energy→low energy light converting process and the heat from the blue LED chip will lead to the temperature rise of the device. Aging of silicone and thermal quenching of the phosphors are inevitable, especially for high-power wLEDs [17,18]. Above all, developing Mn⁴⁺ activated red-emitting phosphors, which own simple structure, good luminescence thermal stability and excellent heat removal capability, and can be easily synthesized with non-toxic raw materials, is urgent. In recent years, all inorganic phosphor alternatives in various forms, such as glasses, ceramics, single crystals and glass ceramics, have been developed [36]. Ceramic phosphors, owing to the high thermal conductivity, high temperature durability and high luminous efficacy, have become a hot spot in the field of high power LEDs [19,20]. Xie et al. reported a promising translucent CaAlSiN₃:Eu²⁺ red-emitting ceramic with a relative density as high as 99%. The phosphor ceramic showed translucency, better thermal stability than powder (15% increase at 300 °C) and a relatively high thermal conductivity (4 W·m⁻¹·K⁻¹) [21]. Zhang et al. reported a red-emitting Lu₃Al₅O₁₂:Mn⁴⁺ ceramic phosphor. Under excitation of 460 nm light, the quantum yield (QY) of the ceramic was 47.8% [24].

As one of the most versatile ceramics in structural and optical applications, Al₂O₃ has been chosen as the host material in this study because of its unique characteristics, such as the high hardness and the good thermal conductivity (30 W·m⁻¹·K⁻¹) [22]. Since the Mn element usually exhibits a variety of valence states (+2, +3, +4 and + 7 are the most common). It is difficult to control the valence state of Mn ions [23]. To keep Mn⁴⁺ to be the majority existence among the various valent Mn as high as possible, suitable charge compensators should be considered. Xu at el. reported the influence of different con-dopant ions such as Li⁺, Mg²⁺, Na⁺, Si⁴⁺ and Ge⁴⁺ on the luminescent properties of α-Al₂O₃:Mn⁴⁺. They finally found that Mg²⁺ had the best promoting effect on the luminescence of α-Al₂O₃:Mn⁴⁺ and the optimal Mn/Mg ratio is about one [35]. In this paper, Mg²⁺ was chosen as the co-doping ions. The introduction of Mg²⁺ was expected to beneﬁt the charge balance in Al₂O₃:Mn⁴⁺, and Mn⁴⁺–Mg²⁺ pairs may enlarge the average distance between Mn⁴⁺ ions, therefore, the adverse resonant energy migration among Mn⁴⁺ ions is expected to be suppressed [24].

In this paper, translucent Al₂O₃:Mn⁴⁺, Mg²⁺ phosphor ceramics were fabricated by high temperature solid-state reaction method. The structural and optical properties of the fabricated ceramics were characterized. The crystal-strength parameter Dq, Racah parameters B and C, and the nephelauxetic ratio β1 were calculated to investigate the influence of crystal field strength and nephelauxetic effect on the emission of Mn⁴⁺ in the Al₂O₃ host. The valence states of Mn ions were investigated by electron spin resonance (ESR). The thermal-quenching activation energy ΔE was calculated to characterize the fluorescence thermal stability of the fabricated ceramics. The thermal conductivity of the Al₂O₃:Mn⁴⁺, Mg²⁺ phosphor ceramics was also measured to evaluate its capability of heat removal.

2. Experimental

2.1 Material preparation

Al₂O₃:Mn⁴⁺, Mg²⁺ ceramics were synthesized via the high temperature solid-state reaction method. The raw materials were MnO₂, Al₂O₃, and MgO, which were purchased from Aladdin Biochemical Technology Co., Ltd, China. The raw materials were weighed according to the designed composition, that is Al_(2-x-y)O₃:xMn⁴⁺, yMg²⁺(x = y = 0.1‰, 0.5‰, 1‰, 5‰, and 1%). Each batch of the mixtures was ball-milled in ethanol for 24 h. Then, the mixtures were dried at 100 °C, the powders were uni-axially pressed into plates at 10 MPa and then iso-statically pressed at 200 MPa. The green bodies were placed in an alumina boat and sintered at different temperatures from 1200 °C to 1750 °C for 5 h under the flowing oxygen and air atmosphere. The as-sintered Mn⁴⁺ activated Al₂O₃ ceramic samples were obtained after natural cooling to room temperature. Then the ceramic samples were sliced into thin sheets of different thickness.

2.2 Characterization

The crystal structures of samples were analyzed by X-ray diffraction (XRD) using a Rigaku MiniFlex 600 system, with the Cu K_α radiation (λ=0.15408 nm), in the 2θ range from 20 ° to 90 ° with a scanning step of 0.02 °. The microstructures of the samples were characterized by scanning electron microscope (SEM, Carl Zeiss, Merilin Compact, Germany). The photoluminescence (PL) emission and excitation (PLE) spectra, quantum yield, temperature-dependent emission spectra from 298 K to 473 K and luminescence decay curves were measured by a fluorescence spectrometer (Edinburg Instrument, FLS1000, U. K.). The optical reflectance spectra of samples were tested by using an UV-Vis-NIR spectrophotometer (Lambda 1050, Perkins Elmer, U. S. A.). Electron paramagnetic resonance (ESR, Bruker, E500, Germany) was measured to investigate the valence states of manganese. Thermal conductivity of the phosphor ceramic samples was measured by the flash lamp method (Netzch, LFA-457, Germany). The CIE coordinates of the 44.55 W blue LED chips packaged with ceramic phosphor sheets of different thickness were measured in an integrating sphere, which was connected to a CCD detector with an optical fiber.

3. Results and discussion

The XRD patterns of Al_2(1-x-y)O₃:xMn⁴⁺, yMg²⁺ (x = y = 0.1‰, 0.5‰, 1‰, 5‰, and 1%) and the standard powder XRD data of α-Al₂O₃ (PDF#10-0173) are shown in Fig. 1. Since the ionic radius of Mg⁴⁺ (0.54 Å) is very close to the ionic radius of Al³⁺ (0.53 Å), the XRD patterns of the as-prepared ceramics matched well with the standard card. The XRD patterns indicated the samples with equal Mn and Mg contents up to 5‰ were of the α-Al₂O₃ single phase. When the Mn (Mg) content reached to 1%, there were some other weak diffraction peaks, which belong to another type of Al₂O₃ (PDF#10-0414).

Fig. 1. XRD patterns of Al_2(1-x-y)O₃ (x = y = 0.1‰, 0.5‰, 1‰, 5‰, and 1%) samples sintered at 1700 °C in oxygen and the standard XRD card of α-Al₂O₃ (PDF#10-0173).

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The crystal structure of the host materials can influence the luminescence properties. The crystal system of α-Al₂O₃ is classified as trigonal system (space group R-3c (167)). The oxygen ions form approximately hexagonal packing, and Al³⁺ ions were filled in the center of the octahedra. The lattice parameters of α-Al₂O₃ are a = b = 4.7588 Å, and c = 12.991 Å, respectively. Figure 2(a) and 2(b) shows the crystal structure of Al₂O₃, in which Al³⁺ is coordinated with six O^2-, constituting the octahedron. It can be clearly seen that Al³⁺ has only one site in the α-Al₂O₃ host. And it is obvious that when doped into the α-Al₂O_3, the Mn⁴⁺ ions can only replace the Al³⁺ ions in the AlO₆ octahedra.

Fig. 2. Schematic diagram of α-Al₂O₃`s crystalline structure (a); A single AlO₆ octahedron. The Al³⁺ cation, which can be replaced by Mn⁴⁺, is surrounded by six O^2- ions, forming an octahedron (b).

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The diffuse reflection spectra of Al₂O₃ doping with different contents of Mn⁴⁺ and Mg²⁺ are exhibited in Fig. 3. Two obvious absorption peak located at ∼320 nm (31250 cm⁻¹) and ∼480 nm (20833.3 cm⁻¹) are attributed to the spin-allowed transitions of Mn⁴⁺: ⁴A_2g→⁴T_1g and Mn⁴⁺: ⁴A_2g→⁴T_2g, respectively. A weak absorption peaked at ∼389 nm (25706.9 cm⁻¹) is attributed to the spin-forbidden transition of Mn⁴⁺: ⁴A_2g→²T_2g. The absorption band at ∼250 nm is due to Mn⁴⁺-O^2- charge transfer transition [14,28]. It can also be seen that there is a weak absorption peaked at ∼540 nm, and the intensity of this absorption peak increases as the Mn⁴⁺ and Mg²⁺ contents increase. It might be caused by metal to metal charge transfer (MMCT) transition between the Mn²⁺, Mn³⁺, and Mn⁴⁺ ions [25].

Fig. 3. Optical reflectance spectra of Al_2(1-x-y)O₃:xMn⁴⁺, yMg²⁺ (x = y = 0.1‰, 0.5‰, 1‰, and 5‰) phosphor ceramics sintered at 1700 °C in oxygen.

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The SEM images of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ceramics sintered in oxygen at different temperature are shown in Fig. 4. When the sample was sintered at 1200 °C, it was not dense enough, which can be seen from Fig. 4(a). The microstructure of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ sintered at 1200 °C still consisted of tiny spherical particles. As the sintering temperature increased from 1300 °C to 1700 °C, the microstructure of the ceramics became denser and denser. There were almost no pores in these five samples. When the sintering temperature is 1750 °C, some pores began to appear. The average size gradually grew up with the sintering temperature increasing. The SEM morphology of the thermally etched fine surface of the Al₂O₃:0.5‰Mn⁴⁺, 0.5‰Mg²⁺ ceramic sintered at 1700 °C in oxygen has been provided in Fig. 4(h). The average grain size of the ceramic is about 10 µm.

Fig. 4. SEM images of Al₂O₃:0.5‰ Mn⁴⁺, 0.5‰ Mg²⁺ ceramics sintered at different temperatures (a) 1200 °C, (b) 1300 °C, (c) 1400 °C, (d) 1500 °C, (e) 1600 °C, (f) 1700 °C, (g) 1750 °C in oxygen, (h) the thermally etched fine surface of the Al₂O₃:0.5‰Mn⁴⁺, 0.5‰Mg²⁺ ceramic sintered at 1700 °C in oxygen

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Figure 5 shows the PLE and PL spectra of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ ceramic sintered at 1700 °C in oxygen (AMMO:0.5‰-1700°C) at room temperature. The PLE spectrum exhibited a wide excitation band ranging from 260 nm to 550 nm, which can be fitted by three Gaussian curves [26,27]. The ﬁtted peaks were located at 316 nm, 385 nm and 477 nm, respectively, which were similar to the results of the diffuse reflection spectra. Under the 395 nm excitation, the ceramics showed deep red emission from 650 nm to 800 nm with two peaks located at 678 nm and 694 nm.

Fig. 5. The room temperature PLE (λ_em=678 nm) and PL (λ_ex=395 nm) spectra of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ ceramic sintered at 1700 °C in oxygen. The PLE spectrum was fitted by three Gaussian curves.

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The PL spectra of Al_2(1-x-y)O₃:xMn⁴⁺, yMg²⁺ (x = y = 0.1‰, 0.5‰, 1‰, and 5‰) and Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ sintered at different temperatures from 1200 °C to 1750 °C under the 460 nm excitation were measured, as shown in Fig. 6(a) and 6(b), and the strongest emission was located at 678 nm. Obviously, the emission intensity firstly increased with the increase of Mn and Mg contents and sintering temperature; however, further increasing the Mn and Mg content and sintering temperature led to the emission intensity drop. The emission intensity reached the maximum when the Mn (Mg) content was 0.5at‰ and the sintering temperature was 1300 °C. The emission intensity of the ceramic sintered at 1700 °C in oxygen is about 1.3 times higher than that in air. The PLE spectra monitored at 678 nm of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ ceramic samples sintered at different temperatures from 1200 °C to 1750 °C exhibited broad excitation bands ranging from 250 nm to 550 nm, as shown in Fig. 6(c). The strongest band was located at about 330 nm when Mn and Mg contents were both 0.5at‰ and the sintering temperature was 1300 °C. The fluorescence decay curves are also studied, as illustrated in Fig. 6(d). The decay curves were fitted by the single exponential function model by the following formula:

(1)$$I (t )={A_\textrm{1}}\cdot \exp \left( {-\frac{t}{\tau }} \right) + {A_2}$$

Where I(t) refers to the emission intensity at time t, A₁ and A₂ are constants, τ is the emission decay. The emission lifetime values of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ sintered at different temperature from 1200 °C to 1750 °C were calculated to be 725.5 µs, 685 µs, 676.2 µs, 669.6 µs, 668.3 µs, 660.6 µs and 659.9 µs, respectively. With the sintering temperature raised, the decay time of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ showed a slight decrease. Although Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ sintered at 1300 °C exhibited the strongest emission intensity, considering the need of transparency for LED packaging, the optical transmission of the sample should be taken into account. The optical transmittance of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ ceramic samples sintered at different temperatures from 1200 °C to 1750 °C were shown in Fig. 7.

Fig. 6. The PL spectra of Al_2(1-x-y)O₃:xMn⁴⁺, yMg²⁺ (x = y = 0.1‰, 0.5‰, 1‰, 5‰) (a); the PL (b) and PLE (c) spectra of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ sintered at different temperatures from 1200 °C to 1750 °C in oxygen; the fluorescence decay curves of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ sintered at different temperature from 1200 °C to 1750 °C in oxygen (d).

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Fig. 7. The optical transmittance spectra of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ ceramic samples sintered at different temperatures and atmosphere (thickness:2 mm).

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For the optical transmittance measurement, the ceramic samples were cut to be 2 mm thick. As shown in Fig. 7, the optical transmittance of Al₂O₃:0.5at‰Mn⁴⁺, 0.5at‰Mg²⁺ in the range of 370∼470 nm and 600∼800 nm increased first and then decreased with the increase of the sintering temperature. AMMO:0.5‰-1700 °C has the highest transmittance, and the ceramic sintered in oxygen has higher transmission than the ceramic sintered in air. So AMMO:0.5‰-1700 °C was selected for further investigation. The quantum yield (QY) of the sample was measured to be 46% under the 395 nm excitation, and QY was reduced to be 28.7% under the 460 nm excitation.

The crystal field intensity and coordination environment can influence the luminescence of Mn⁴⁺. The effect of crystal-field strength on the energy levels of Mn⁴⁺ was analyzed by Tanabe-Sugano diagram, as shown in Fig. 8. The crystal-field strength (Dq) can be calculated by the peak at 477 nm (20964.4 cm⁻¹) which refers to the ⁴A_2g→²T_2g transition of Mn⁴⁺:

(2)$$D{\kern 1pt} q=\frac{{E({{}^\textrm{4}{A_{\textrm{2g}}} \to {}^4{T_{2g}}} )}}{{10}}$$

The difference (10681.2 cm⁻¹) between ⁴A_2g→⁴T_2g and ⁴A_2g→⁴T_1g transitions can be used to evaluate the Racah parameter B by the following formula:

(3)$$\frac{{D{\kern 1pt} q}}{B}=\frac{{\textrm{15(}x\textrm{ - 8)}}}{{{x^2} - 10x}}$$

Fig. 8. Tanabe-Sugano energy-level diagram of Mn⁴⁺ in the octahedral crystal field of Al₂O₃:Mn⁴⁺, Mg²⁺.

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where the parameter x is defined as:

(4)$$x = \frac{{E({{}^4{A_{2g}} \to {}^4{T_{1g}}} )- E({{}^4{A_{2g}} \to {}^4{T_{2g}}} )}}{{Dq}}$$

The Racah parameter C can also be calculated according to the peak energy of Mn⁴⁺: ²E_g→⁴A_2g (14749.3 cm⁻¹) derived from emission spectra with the following expression.

(5)$$\frac{{E({{}^\textrm{2}{E_\textrm{g}} \to {}^4{A_{2g}}} )}}{B} = \frac{{3.05{\kern 1pt} C}}{B} + 7.9 - \frac{{1.8{\kern 1pt} B}}{{D{\kern 1pt} q}}$$

At last, the calculated values of Dq, B, and C of Al₂O₃: Mn⁴⁺, Mg²⁺ are 2096.4 cm⁻¹, 1202.3 cm⁻¹, and 2128.6 cm⁻¹, respectively. Then, the radio of Dq to B was determined to be ∼1.74, which is lower than 2.2. This indicated that the Mn⁴⁺ ions in the α-Al₂O₃ host were in a weak crystal field environment [34]. In this case, according to the Tanabe-Sugano energy level diagram, the ²E_g level in the 3d³ electronic configuration is independent of the crystal field, and its energy does not change significantly with the crystal field strength. So the emission energy of Mn⁴⁺ mainly depends on the nephelauxetic effect. Meanwhile, the nephelauxetic effect highly originates from the covalent bonds between Mn⁴⁺ and the ligands. The nephelauxetic ratio β₁, established by Brik et al, which can reflect the degree of the nephelauxetic effect, can be calculated via the following expression:

(6)$${\beta _1} = \sqrt {{{\left( {\frac{B}{{{B_0}}}} \right)}^2} + {{\left( {\frac{C}{{{C_0}}}} \right)}^2}}$$

B₀ and C₀ are the Racah parameters of free Mn⁴⁺ ions, and the values of B₀ and C₀ are 1160 cm⁻¹ and 4303 cm⁻¹, respectively. Based on the Racah parameters of the Mn⁴⁺ in the α-Al₂O₃ host, β₁ was calculated to be 1.15. The calculation result was not in line with the model established by Brik et al. The main reason may be that the AlO₆ octahedron in α-Al₂O₃ was close to the ideal octahedral structure. When the Mn⁴⁺ ions replace the Al³⁺ ions, the new Mn⁴⁺-O²⁻Al³⁺ bond angle is close to 180 °, the overlap of wave function between Mn⁴⁺ and ligands reaches the maximum value, resulting the increase of the probability density of electrons and the reduction of the ²E_g energy. The value of the Racah parameters is significantly decreased. This may result in the deep red emission of Mn⁴⁺ in Al₂O₃ host, and it also explains why nephelauxetic ratio β₁ of Al₂O₃:Mn⁴⁺, Mg²⁺ cannot meet the model [29,30].

Since the content of Mn⁴⁺ is very low, X-ray photoelectron spectroscopy (XPS) is unable to detect the presence of Mn⁴⁺. In order to confirm the existence of Mn⁴⁺ and the valence states of Mn, ESR spectrum was measured. The ESR spectrum of AMMO:0.5‰-1700 °C at room temperature is shown in Fig. 9. The ESR signal of Mn (I = 5/2) located at 1500G to 2500G (g≈3.5205) can be clearly observed, which indicated the existence of Mn⁴⁺. A broad ESR signal ranging from 2500 G to 4500 G (g≈2.0858) proves that Mn²⁺ also exists in the ceramic sample. However, the ESR signal of Mn²⁺ didn’t show the typical six-fold hyperfine lines of Mn (I = 5/2), which may be caused by an uneven distribution of Mn²⁺ in Al₂O₃ or drowned by the interactions between Mn²⁺ ions [31,32]. It can also be seen that there was a weak signal ranging from 300 G to 1000 G, which might be related to Mn²⁺-Mn³⁺ or Mn³⁺-Mn⁴⁺ combinations [32].

Fig. 9. Room temperature ESR spectrum of AMMO:0.5‰-1700 °C.

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The temperature-dependent PL spectra under the 395 nm excitation are exhibited in Fig. 10(a). As the temperature increased, the luminescence intensity decreased. The thermal quenching of luminescence intensity I(T) with temperature T can be described by using the following equation:

(7)$$I(T )=\frac{{{I_0}}}{{\textrm{1 + }A\cdot \exp \left( {-\frac{{\Delta E}}{{\textrm{k}T}}} \right)}}$$

Where I₀ is the initial intensity at 25 °C, I(T) is the emission intensity at temperature T, A is a constant, k is the Boltzmann constant, ΔE is the activation energy. By fitting the data with a linear function, the ΔE value of AMMO:0.5‰-1700 °C was calculated to be 0.52 eV [33,34].

Fig. 10. Temperature-dependent PL spectra of AMMO:0.5‰-1700 °C measured from 20 ℃ to 200 ℃ (a); a plot of the dependence of ln(I₀/I(T)-1) on 1/kT according Eq. 7 (b).

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The integrated emission intensity of AMMO: 0.5‰-1700 °C at 120 °C was decreased to 43% of that at room temperature. The thermal conductivity has been measured and presented in Fig. 11. At room temperature, the thermal conductivity of the sample was 26.27 W·m⁻¹·K⁻¹. As the temperature was increased to 150 °C, the thermal conductivity was reduced to 18.25 W·m⁻¹·K⁻¹, and the thermal conductivity further decreased to 12.6 W·m⁻¹·K⁻¹ at 300 °C. This indicated that AMMO: 0.5‰-1700 °C ceramic retained the high thermal conductivity of dense bulk α-Al₂O₃ (The thermal conductivity of Al₂O₃ is about 30 W·m⁻¹·K⁻¹ at 0 °C).

Fig. 11. Thermal conductivity measured by the laser flash method as a function of temperature of the AMMO: 0.5‰-1700 °C sample.

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The measured chromaticity and EL spectrum of the LEDs driven by a 44.55 W electrical power, was shown in Fig. 12(a) and 12(b). The LEDs emitted blue-purple violetlight. The CIE coordinate (x = 0.1805, y = 0.0319) of AMMO:0.5‰-1700 °C (thickness:0.3 mm) was in the blue-purple region. As the thickness of the ceramic increased, the CIE coordinates gradually moved toward (0.1805, 0.0319). If blue chips of lower power are available, the CIE coordinates can be moved to purple-red region.

Fig. 12. CIE chromaticity diagram of LEDs fabricated by combining AMMO:0.5‰-1700 °C (The thickness of sample is 0.3 mm, 0.5 mm, 1 mm and 1.5 mm, respectly) with blue chips (a). EL spectrum of LEDs (The thickness of ceramic is 1.5 mm) (b).The electrical driven power of LEDs is 44.55 W.

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4. Conclusions

In conclusion, the Al₂O₃:Mn⁴⁺, Mg²⁺ red-emitting ceramics have been successfully sintered in air and oxygen atmosphere by high temperature solid-state reaction. The XRD and PL spectra showed Mn was successfully doped into the Al₂O₃ crystal structure without any impurity phase formation when the Mn contents are in the range of 0.1at‰ to 1at‰. The PLE spectrum demonstrated that Al₂O₃:Mn⁴⁺, Mg²⁺ ceramics had two broad excitation peaks in the 270-400 nm region and the 430-500 nm region. The ceramic exhibited deep red emission peaked at 678 nm under the 460 nm excitation. The optimized Mn⁴⁺ and Mg²⁺ doping concentration is 0.5at‰. Under the 395 nm excitation, the emission quantum yield of Al₂O₃:0.5at‰ Mn⁴⁺, 0.5at‰ Mg²⁺ sintered at 1700 °C was measured to be 46%. The ESR measurement confirmed that the valence states of Mn were mainly + 4 and + 2, accompanied by a small amount of Mn³⁺. The thermal conductivity at room temperature was measured to be 26.27 W·m⁻¹·K⁻¹@30 °C. The LED combining blue chips and the as-prepared ceramic can emit blue-purple light. These results show that the Al₂O₃:Mn⁴⁺, Mg²⁺ red-emitting ceramic is a type of high-performance red-emitting material, which has promising application in high-power PC-LEDs.

Funding

Ministry of Science and Technology of the People's Republic of China (2016YFB1102303); Science and Technology Commission of Shanghai Municipality (18PJ1408800); Scientific and Innovative Action Plan of Shanghai (17142202700).

Acknowledgments

This work was sponsored by Shanghai Pujiang Program (18PJ1408800), the National Key Research and Development Program of China (2016YFB1102303) and Shanghai Science and Technology Innovation Action Plan (17142202700).

Disclosures

The authors declare no conflicts of interest.

References

1. E. F. Schubert and J. K. Kim, “Solid-State Light Sources Getting Smart,” Science 308(5726), 1274–1278 (2005). [CrossRef]

2. J. J. Wierer, J. Y. Taso, and D. S. Sizov, “Comparison between blue lasers and light-emitting diodes for future solid-state lighting,” Laser Photonics Rev. 7(6), 963–993 (2013). [CrossRef]

3. D. F. Feezell, J. S. Speck, S. P. Denbaars, and S. Nakamura, “InGaN/GaN Light-Emitting Diodes for High-Efficiency Solid-State Lighting,” J. Disp. Technol. 9(4), 190–198 (2013). [CrossRef]

4. R. Zhang, H. Lin, Y. Yu, D. Chen, J. Xu, and Y. Wang, “A new-generation color converter for high-power white LED: transparent Ce³⁺:YAG phosphor-in-glass,” Laser Photonics Rev. 8(1), 158–164 (2014). [CrossRef]

5. L. Lv, X. Jiang, S. Huang, X. chen, and Y. Pan, “The formation mechanism, improved photoluminescence and LED applications of red phosphor K2SiF6:Mn⁴⁺,” J. Mater. Chem. C 2(20), 3879–3884 (2014). [CrossRef]

6. C. Ma, Y. Cao, X. Shen, Z. Wen, R. Ma, J. Long, and X. Yuan, “High reliable and chromaticity-tunable flip-chip w-LEDs with Ce:YAG glass-ceramics phosphor for long-lifetime automotive headlights applications,” Opt. Mater. 69, 105–114 (2017). [CrossRef]

7. A. A. Setlur, W. J. Heward, Y. Gao, A. M. Srivastava, R. G. Chandran, and M. V. Shankar, “Crystal chemistry and luminescence of Ce³⁺-doped Lu₂CaMg₂(Si, Ge)₃O₁₂ and its use in LED based lighting,” Chem. Mater. 18(14), 3314–3322 (2006). [CrossRef]

8. H. Zhu, C. Lin, W. Luo, S. Shu, Z. Liu, Y. Liu, J. Kong, E. Ma, Y. Cao, R. Liu, and X. Chen, “Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes,” Nat. Commun. 5(1), 4312 (2014). [CrossRef]

9. B. Wang, J. R. Ling, Y. F. Zhou, W. T. Xu, H. Lin, S. Lu, Z. X. Qin, and M. C. Hong, “Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes,” J. Lumin. 213, 421–426 (2019). [CrossRef]

10. C. Bulloni, A. Garcia-Fuente, W. Urland, and C. Daul, “Effect of Ca²⁺ codoping on the Eu²⁺ luminescence properties in the Sr₂Si₅N₈ host lattice: a theoretical approach,” Phys. Chem. Chem. Phys. 17(38), 24925–24930 (2015). [CrossRef]

11. K. Li, H. Lian, and R. V. Deun, “Site occupancy and photoluminescence properties of a novel deep-red-emitting phosphor NaMgGdTeO₆: Mn⁴⁺ with perovskite structure for w-LEDs,” J. Lumin. 198, 155–162 (2018). [CrossRef]

12. J. Jiang, Y. Cheng, W. Chen, Z. Liu, T. Xu, M. He, L. Zhou, R. Yuan, W. Xiang, and X. Liang, “Mn⁴⁺/Zn²⁺:YAG glass ceramic for light emitting devices,” Mater. Res. Bull. 105, 277–285 (2018). [CrossRef]

13. C. Liao, R. Cao, Z. Ma, Y. Li, G. Dong, K. N. Sharafudeen, and J. Qiu, “Synthesis of K₂SiF₆:Mn⁴⁺ phosphor from SiO₂ powders via redox reaction in HF/KMnO₄ solution and their application in warm-white LED,” J. Am. Ceram. Soc. 96(11), 3552–3556 (2013). [CrossRef]

14. B. Wang, H. Lin, J. Xu, H. Chen, and Y. Sheng, “CaMg₂Al₁₆O₂₇:Mn⁴⁺-based Red Phosphor: A Potential Color Converter for High-Powered Warm W-LED,” ACS Appl. Mater. Interfaces 6(24), 22905–22913 (2014). [CrossRef]

15. K. Li, H. Lian, and R. V. Deun, “A novel deep red-emitting phosphor KMgLaTeO₆:Mn⁴⁺ with high thermal stability and quantum yield for w-LEDs: structure, site occupancy and photoluminescence properties,” Dalton Trans. 47(8), 2501–2505 (2018). [CrossRef]

16. A. Fu, L. Zhou, S. Wang, and Y. Li, “Preparation, structural and optical characteristics of a deep red-emitting Mg₂Al₄Si₅O₁₈: Mn⁴⁺ phosphor for warm w-LEDs,” Dyes Pigm. 148, 9–15 (2018). [CrossRef]

17. Q. Zhang, R. Zheng, J. Ding, and W. Wei, “Excellent luminous efficiency and high thermal stability of glass-in-LuAG ceramic for laser-diode-pumped green-emitting phosphor,” Opt. Lett. 43(15), 3566–3569 (2018). [CrossRef]

18. H. Su, Y. Nie, H. Yang, D. Tang, K. Chen, and T. Zhang, “Improving the thermal stability of phosphor in a white light-emitting diode (LED) by glass-ceramics: Effect of Al₂O₃ dopant,” J. Eur. Ceram. Soc. 38(4), 2005–2009 (2018). [CrossRef]

19. A. Wagner, B. Ratzker, S. Kalabukhov, and N. Frage, “Enhanced external luminescence quantum efficiency of ceramic phosphors by surface roughening,” J. Lumin. 213, 454–458 (2019). [CrossRef]

20. B. Wang, J. Ling, Y. Zhou, W. Xu, H. Lin, S. Lu, Z. Qin, and M. Hong, “YAG:Ce³⁺, Mn²⁺ transparent ceramics prepared by gel-casting for warm white LEDs,” J. Lumin. 213, 421–426 (2019). [CrossRef]

21. S. Li, Q. Zhu, L. Wang, D. Tang, Y. Cho, X. Liu, N. Hirosaki, T. Nishimura, T. Sekiguchi, Z. Huang, and R. J. Xie, “CaAlSiN3:Eu²⁺ translucent ceramic: A promising robust and efficient red color converter for solid state laser displays and lighting,” J. Mater. Chem. C 4(35), 8197–8205 (2016). [CrossRef]

22. J. Xu, J. Wang, Y. Gong, X. Ruan, Z. Liu, B. Hu, B. Liu, H. Li, X. Wang, and B. Du, “Investigation of an LuAG:Ce translucent ceramic synthesized via spark plasma sintering: Towards a facile synthetic route, robust thermal performance, and high-power solid state laser lighting,” J. Eur. Ceram. Soc. 38(1), 343–347 (2018). [CrossRef]

23. T. Arai and S. Adachi, “Mn-activated Na₂SiF₆ red and yellowish-green phosphors: A comparative study,” J. Appl. Phys. 110(6), 063514 (2011). [CrossRef]

24. Y. Zhang, S. Hu, Y. Liu, Z. Wang, W. Ying, G. Zhou, and S. Wang, “Preparation, crystal structure and luminescence properties of red-emitting Lu₃Al₅O₁₂:Mn⁴⁺ ceramic phosphor,” J. Eur. Ceram. Soc. 39(2-3), 584–591 (2019). [CrossRef]

25. S. Dey, R. A. Ricciardo, H. L. Cuthbert, and P. M. Woodward, “Metal-to-Metal Charge Transfer in AWO4 (A = Mg, Mn, Co, Ni, Cu or Zn) Compounds with the Wolframite Structure,” Inorg. Chem. 53(9), 4394–4399 (2014). [CrossRef]

26. X. Huang and H. Guo, “Finding a novel highly efficient Mn⁴⁺-activated Ca₃La₂W₂O₁₂ far-red emitting phosphor with excellent responsiveness to phytochrome PFR: Towards indoor plant cultivation application,” Dyes Pigm. 152, 36–42 (2018). [CrossRef]

27. X. Zhang, J. Nie, S. Liu, Y. Li, and J. Qiu, “Deep-Red Photoluminescence and Long Persistent Luminescence in Double Perovstkite-type La₂MgGeO₆:Mn⁴⁺,” J. Am. Ceram. Soc. 101(4), 1576–1584 (2018). [CrossRef]

28. A. M. Srivastava and W. W. Beers, “Luminescence of Mn⁴⁺ in the Distorted Perovskite Gd₂MgTiO₆,” J. Electrochem. Soc. 143(9), L203–L205 (1996). [CrossRef]

29. M. G. Brik, S. J. Camardello, and A. M. Srivastava, “Influence of Covalency on Mn⁴⁺²E_g→⁴A_2g Emission Energy in Crystals,” ECS J. Solid. State. Sc. 4(3), R39–R43 (2014).

30. M. G. Brik, S. J. Camardello, A. M. Srivastava, N. M. Avram, and A. Suchocki, “Spin-Forbidden Transitions in the Spectra of Transition Metal Ions and Nephelauxetic Effect,” ECS J. Solid State Sci. Technol. 5(1), R3067–R3077 (2016). [CrossRef]

31. Y. Zhang, S. Hu, Y. Liu, Z. Wang, W. Ying, G. Zhou, and S. Wang, “Preparation, crystal structure and luminescence properties of red-emitting Lu₃Al₅O₁₂:Mn⁴⁺ ceramic phosphor,” J. Eur. Ceram. Soc. 39(2-3), 584–591 (2019). [CrossRef]

32. Y. Zhang, S. Hu, Y. Liu, Z. Wang, G. Zhou, and S. Wang, “Red-emitting Lu₃Al₅O₁₂: Mn transparent ceramic phosphors: valence state evolution studies of Mn ions,” Ceram. Int. 44(18), 23259–23262 (2018). [CrossRef]

33. L. Wang, R. J. Xie, T. Suehiro, T. Takeda, and N. Hirosaki, “Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives,” Chem. Rev. 118(4), 1951–2009 (2018). [CrossRef]

34. W. Chen, Y. Cheng, L. Shen, C. Shen, X. Liang, and W. Xiang, “Red-emitting Sr₂MgGe₂O₇:Mn⁴⁺, phosphors: Structure, luminescence properties, and application in warm white light emitting diodes,” J. Alloys Compd. 762, 688–696 (2018). [CrossRef]

35. Y. Xu, L. Wang, B. Qu, D. Li, J. Lu, and R. Zhou, “The role of co-dopants on the luminescent properties of α-Al₂O₃:Mn⁴⁺ and BaMgA_l1O₇:Mn⁴⁺,” J. Am. Ceram. Soc. 102(5), 2737–2744 (2018). [CrossRef]

36. S. Li, L. Wang, N. Hirosaki, and R. J. Xie, “Color Conversion Materials for High-Brightness Laser-Driven Solid-State Lighting,” Laser Photonics Rev. 12(12), 1800173 (2018). [CrossRef]

37. Z. Zhang, C. Ma, R. Gautier, M. S. Molokeev, Q. Liu, and Z. Guo, “Structural Confinement toward Giant Enhancement of Red Emission in Mn²⁺-Based Phosphors,” Adv. Funct. Mater. 28(41), 1804150 (2018). [CrossRef]

38. J. Qiao, L. Ning, M. S. Molokeev, Y. Chuang, Q. Zhang, K. R. Poeppelmeier, and Z. Xia, “Site-Selective Occupancy of Eu²⁺ Toward Blue-Light-Excited Red Emission in a Rb3YSi2O7:Eu Phosphor,” Angew. Chem., Int. Ed. 58(33), 11521–11526 (2019). [CrossRef]

Mn⁴⁺ activated Al₂O₃ red-emitting ceramic phosphor with excellent thermal conductivity

Abstract

1. Introduction

2. Experimental

2.1 Material preparation

2.2 Characterization

3. Results and discussion

4. Conclusions

Funding

Acknowledgments

Disclosures

References

Cited By

Figures (12)

Equations (7)

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