Luminescence properties of Eu2+, Gd3+ and Pr3+ doped translucent Sialon phosphors

JOURNAL OF RARE EARTHS, Vol. 33, No. 11, Nov. 2015, P. 1142

Luminescence properties of Eu2+, Gd3+ and Pr3+ doped translucent Sialon phosphors Bhupendra Joshi, Soo Wohn Lee* (Research Center for Eco Multi-Functional Nano Materials, Sun Moon University, Chungnam 336-708, Republic of Korea) Received 11 April 2015; revised 21 September 2015

Abstract: Optical properties of hot pressed Sialon ceramics doped with different rare earth oxides (REOs) i.e. Eu2O3, Gd2O3, and Pr2O3 were investigated. The α-Sialon phase was the main phase obtained after sintering as observed by X-ray diffraction (XRD). The transparency of different samples of varying thickness measured from UV to IR region revealed that the samples were translucent in the visible region while transparent in IR region. The thin samples of 150 μm thickness had transmittance as high as 30% in the visible region. The luminescence was observed in transmittance mode to investigate the effect of sample thickness on luminescence intensity. We observed blue, yellow and red emissions in Sialon doped with Gd2O3, Eu2O3, and Pr2O3, respectively. The excitation wavelength for Gd2O3 and Pr2O3 doped samples were in UV region i.e. 280 and 270 nm, respectively, whereas, for Eu2O3 doped samples was in the blue region (460 nm). The Eu2O3 doped Sialon having 300 μm thickness showed better white light extraction as coupled with blue LED. Moreover, the fabricated phosphor samples exhibited high hardness around 20 GPa and fracture toughness above 5 MPa·m1/2. Keywords: Sialon; phosphors; luminescence; rare earth oxides; hot pressing

Recently, transparent/translucent glass and polycrystalline ceramics have been fabricated as remote phosphors for LED[1–3]. The powder phosphor mixed with resin/epoxy has limits for the production of high luminous flux. It is due to the increase in temperature in the LED chip while in operation that causes the deterioration of resin and results in the color shift in the delivered light[3]. On the other hand, transparent/translucent plate type phosphors have high thermal and spectral stability and therefore are potential alternatives in the future for high luminous flux. Also, the mechanical properties are of prime importance because the thin phosphor plates should exhibit high hardness and fracture toughness[4]. Sialon ceramics meet all these requirements. There are some works on the fabrication of transparent/translucent Sialon ceramics[5–7]. Most of these fabricated Sialon ceramics are translucent in visible light region. Transparency depends on the thickness of the polycrystalline ceramics. Increasing the thickness of samples the residual porosity, intragranular phases as well as grain boundaries also increase which reduce the transparency of the polycrystalline ceramics. The translucent phosphors can be used for visible LED purpose. Most of the fabricated plate type glass phosphors are translucent in nature and are few hundred micrometers in thickness[2,8]. However, there are only a few works reported on the fabrication of the transparent/translucent

polycrystalline plate type ceramic phosphors[1,9–11]. It is due to the difficulty in the fabrication process as well as the inclusion of luminescent centers i.e. the presence of rare earth elements as luminescent center deteriorates the transparency of polycrystalline ceramics. Despite the relative difficulty of the fabrication method, the excellent properties of polycrystalline ceramics over glasses make it superior to be applied in advanced applications such in high power LED. Large band gap energy (5.9 eV)[12] and the equiaxed morphology of α-Sialon grains make the material more transparent than the β-Sialon with elongated grains. Although thick α-Sialon samples are translucent in the visible region, the thin samples are transparent as is required for the LED applications. In this paper, we reported on the fabrication of translucent Mg-α/β-Sialon doped with different rare earth oxides (REOs) i.e. Eu2O3, Gd2O3, and Pr2O3. Also, the effect of thickness of the sample on transparency and luminescence was analyzed. Sialon ceramics are the host materials and the doped rare earths (RE) ions act as activators or luminescent centers. Gd2O3 and Pr2O3 doped Sialon ceramics can be excitated in the UV region whereas Eu2O3 in the blue region and emissions are observed in green, red and yellow regions, respectively. Moreover, we studied the mechanical properties such as hardness and fracture toughness of the different REOs doped Sia-

Foundation item: Project supported by Global Research Laboratory (GRL) Program of the National Research Foundation of Korea (NRF) funded by Ministry of Education, Science and Technology (MEST), Republic of Korea (2010-00339) * Corresponding author: Soo Wohn Lee (E-mail: [email protected]; Tel.: +82-41-530-2882) DOI: 10.1016/S1002-0721(14)60538-X

Bhupendra Joshi et al., Luminescence properties of Eu2+, Gd3+ and Pr3+ doped translucent Sialon phosphors

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lon samples.

1 Experimental High-purity powder of α-Si3N4 (SN-E10, UBE Co., Japan) was used as a starting material. The α-Si3N4 powder (87.5 wt.%) was mixed with 3 wt.% MgO (High purity chemicals Co., Ltd., Japan), 9 wt.% AlN (Grade F, Tokuyama Co., Japan) and 0.5 wt.% Eu2O3, Gd2O3 and Pr2O3 (High purity chemicals Co., Ltd., Japan). The samples were named as SN, SN:Eu, SN:Gd and SN:Pr for without REOs, Eu2O3, Gd2O3, and Pr2O3 additions, respectively. The powders were mixed in ethanol with high-purity silicon nitride balls and were ball milled for 24 h. After wet-ball milling, the powders were dried and again dry milled for 12 h. Then, the mixed powders were sieved through a 150-μm aperture. The powder mixtures were hot-press sintered (MVHP, Monocerapia Co., Ltd., Korea) under a uniaxial compression of 30 MPa pressure in 0.1 MPa of N2 at 1850 ºC for 1 h. For the transmittance measurement, the samples were ground to a thickness of 150, 300 and 500 μm and both sides were polished on a diamond plate. After that, the light transmittance was measured using a spectrophotometer (MECASYS Optizen2120) in the range of 200– 1100 nm. The photoluminescence was measured by Eidenburgh instrument FLS920 in transmittance mode to observe the effect of sample thickness on luminescence properties. For luminous efficacy, the Eu2O3 doped samples of various thickness was coupled with blue LED (Model: T56-3BLZ-05, IST, Korea) and spectral distribution curves were also obtained by using an Ocean Optics USB 2000 spectrophotometer. After sintering, crystalline phase was examined by an X-ray diffractometer (XRD, Cu Kα, RIGAKU D/ MAX2200HR diffractometer, Japan). Zwick 3212 hardness tester was used to measure the Vickers hardness and fracture toughness on the basis of indentation and crack length formed under 98 N loads for 15 s on the polished surfaces of the sample.

2 Results and discussion The addition of MgO as a sintering additive gives α/βSialon composite phase in Si3N4-MgO-AlN system[13]. The higher viscosity of MgO during the sintering process causes less Mg2+ ion diffusion in grains. Here Mg2+ cation acts as a stabilizer to α-Sialon phase that compensates charge during the substitution of Si and N in silicon nitride by Al and O, respectively. The addition of a small amount of REOs increases liquid phase during the sintering and more Mg2+ as well as some RE ions can be diffused into the grains to form higher α-Sialon phase. As observed from the XRD patterns in Fig. 1, with the addition of REOs, higher α-Sialon phase formation was

Fig. 1 XRD patterns showing different Sialon phases for different samples; SN (without REOs), SN:Eu (Eu2O3 doped), SN:Gd (Gd2O3 doped), and SN:Pr (Pr2O3 doped)

confirmed. In absence of REOs and only with MgO, distinct β-Sialon phase can be observed[14]. In Fig. 1, the β-Sialon phase was observed in the sample. Ramesh et al.[15] reported that Eu2O3 has less viscosity than other REOs, and with Eu2O3 addition, higher α-Sialon formation was observed around 96%. The lower viscosity of Eu2O3 helps to form more liquid phase and therefore more cations will diffuse in the grains to form higher α-Sialon phase. The volume fractions (vol.%) of α and β-Sialon phases were calculated from the intensities of XRD peaks for each sintered samples as listed in Table 1. In sintered Sialon ceramics, it is difficult to obtain single α-Sialon phase because of the different additives used. The other phases that exist in sintered Sialon ceramics are β-Sialon, AlN polytypoid and oxynitride glass. However, more than 90 vol.% of α-Sialon can be obtained by controlled sintering conditions. The liquid phase obtained while sintered with MgO has a higher melting point and viscosity, which results in the lower density after sintering[14]. The addition of small amount of REOs having a lower melting point and viscosity can enhance the densification. Pores in ceramics degrade the mechanical and optical properties of the materials and therefore formation of excess liquid phase is needed to eliminate the pores during sintering. The addition of REOs not only enhanced the densification process but also increased the formation of α-Sialon phase. Table 1 Phase composition, mechanical properties and relative densities of different samples Phase Samples

composition/vol.%

Hardness/ GPa

Fracture toughness/ (MPa·m1/2)

Relative density/%

α-Sialon

β-Sialon

SN

64.50

35.50

18.96±0.36

5.27±0.09

99.05

SN:Eu

96.20

3.80

19.80±0.41

5.76±0.11

99.11

SN:Gd

95.39

4.61

19.40±0.23

5.14±0.60

99.23

SN:Pr

93.64

6.36

19.16±0.58

5.20±0.30

99.16

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The relative densities of all samples were above 99% as shown in Table 1. Thus, it can be reasonably supposed that the complete densification took place. The hardness and fracture toughness values of different samples are also shown in Table 1. High hardness and fracture toughness were observed for different REOs added samples. High fracture toughness of silicon nitride based ceramics makes them less brittle and can be made plates as thin as few hundred micrometers. In silicon nitride based ceramics, there exist intergranular glassy phase and these intergranular phases with microcracks, bridging, crack deflection and grain pullout improved the fracture toughness of Sialon ceramics. Although, intergranular glassy phase exists; the mechanical properties of Sialon are better as compared to other ceramics like YAG, Y2O3 and other transparent ceramics[16,17]. Fig. 2 shows the SEM micrographs of fracture surfaces of REOs doped Sialon ceramics. The observed grains were anisotropic in a structure having equiaxed grains with minor elongated grains. For SN:Eu (Fig. 2(a)), small grains were observed as compared to SN:Gd (Fig. 2(b)) and SN:Pr (Fig. 2(c)). Ramesh et al.[15] reported that the Sialon glass formed by Eu2+ had low viscosity as compared to other rare earth formed Sialon glasses and produced higher liquid phase during sintering at a higher temperature. Higher liquid phase assisted the diffusion of metal cations to stabilize the α-Sialon phase. We also observed the fine small grains rich in α-Sialon phase with Eu2O3 addition. However, with the addition of Gd2O3 and Pr2O3 grains were large and elongated in structure. However, the grain boundary phases should be

higher in Eu2O3 added Sialon ceramics because of excess liquid phase and small grains. Also, from the micrograph it is clearly seen that transgranular fracture was more prominent in SN:Gd and SN:Pr. But due to the low viscosity of Eu2O3, thick intergranular glassy phase was formed due to which intergranular fracture occurred and higher fracture toughness was observed as shown in Table 1. In the polycrystalline ceramics, the thick samples lose transparency due to light scattering by pores, different phases and grain boundaries. As shown in Fig. 3, thin samples of 150 μm thickness show better transparency in visible spectrum. SN:Eu shows less transparency than the SN:Gd and SN:Pr samples due to the presence of excess liquid phase which increased the intergranular glassy phase causing more scattering of light. The mismatch between the refractive indices of grains and grain boundary phases causes scattering of light[6,18]. The addition of rare earth oxides was minimized to 0.5 wt.% to acquire partial transparency in the samples. Excess of additives generate more liquid phase giving rise to higher intergranular glassy phase during the cooling process in sintering and the transparency of the material is reduced. Therefore, to retain the partial transparency on the material in the visible light spectrum, the addition of additive i.e. REOs was taken into small amount and the resultant material was found to be translucent in visible light spectrum. The emission spectra were taken in transmission mode. The emission intensities were increased with decreasing thickness. This is obvious because in the polycrystalline

Fig. 2 SEM micrographs of fractured surface morphologies of different Sialon compositions (a) SN:Eu; (b) SN:Gd; (c) SN:Pr

Fig. 3 Transmission spectra with different thicknesses (150, 350 and 500 μm) of different Sialon samples with inset optical images of samples having thickness of 150 μm (a) SN:Eu; (b) SN:Gd; (c) SN:Pr

Bhupendra Joshi et al., Luminescence properties of Eu2+, Gd3+ and Pr3+ doped translucent Sialon phosphors

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Fig. 4 Photoluminescence spectra of different Sialon ceramics phosphors having different thicknesses (150, 350 and 500 μm) (a) SN:Eu; (b) SN:Gd; (c) SN:Pr

ceramics, thicker samples offer more light absorption and scattering than the thinner samples. The samples with different REOs show different emission spectra and are shown in Fig. 4. As shown in Fig. 4(a), SN:Eu is excited by 460 nm wavelength light that shows emission in yellow region, i.e. 570 nm wavelength. This type of excitation and emission is important in the field of lightening. Most of the LED uses phosphors having yellow emissions for the generation of white light. The commercial white LEDs available are YAG:Ce phosphor powders coupled with blue LED. Herein, the fabricated SN:Eu plates are also excited by blue light and have emission in yellow region. The main excitation wavelength in blue region corresponds to the 4f7→4f65d1 transition absorption of Eu2+. The broad emission was observed from 500–650 nm due to the allowed 4f65d1→ 4f7 transition of Eu2+. The presence of Eu3+ in this system is negligible, otherwise sharp emission peaks would be observed at 560 and 630 nm[19]. In Eu2+, the 5d-4f transition is observed and the emission depends upon the host material. Whereas, the transition in Eu3+ is 4f-4f and in such case the emission is dominated by activator or called luminescence center (Rare earth cations) and very small change can be observed with different hosts. For more clear view we provided the energy level scheme of Eu2+ in Fig. 5. The Eu3+ reduces to Eu2+ in reducing carbon monoxide gas in a nitrogen atmosphere due to the graphite dies used for sintering as suggested in many literatures[20–22]. Also higher amount of AlN is added as

Fig. 5 Configurational coordinate model showing the relationship between Eu2+ and host lattice (α-Sialon)

compared to MgO and Eu2O3 in the system which gives higher content of nitrogen for reducing environment. Therefore, the luminescence was due to the Eu2+ state in the SN:Eu samples. The broad emission can be attributed to the higher charge of N3– with respect to O2– and the nephelauxetic effect that causes the ligand splitting of 5d levels to be larger and the center of gravity of the 5d states to occur at longer wavelength than with oxygen environment[23]. SN:Gd and SN:Pr samples with various thicknesses were excited by UV light of wavelengths 280 and 270 nm, respectively. The emission for SN:Gd is in a blue region with a maximum emission peak at 470 nm wavelength as shown in Fig. 4(b). The broad emission peak in SN:Gd from 400–600 nm is due to the charge transfer (CT) process via the excitation of an electron from the oxygen 2p state to a Gd3+ 4f state[24]. Similarly, the main emission for SN:Pr is in a red region with a maximum emission peak at 618 nm, and there was also small emission peak at 507 nm as shown in Fig. 4(c). The broad emission from 500–680 nm was also observed for SN:Pr and can be attributed to the f-f transition in Pr3+ ions. Moreover, the emissions bands in SN:Pr composed of five different bands which are assigned as: 3P0→3H4 (507 nm), 3P0→3H5 (547 nm), 1D2→3H4 (618 nm), 3P0→3H6 (635 nm) and 3P0→3F2 (670 nm). This type of emission was also reported by Xie et al.[25]. Based on the literatures[25,26], the energy diagram for Pr3+ is shown in Fig. 6. The dominant emission peak often depends upon the host material. For example Gd2O2S:Pr3+ has been reported to have dominant green emission[27]. However, in SN:Pr3+, the red emission was dominant. The luminescence intensity was found to vary with thickness, and thin samples had higher luminescence intensity. In trivalent electronic structure of RE element, the 4f subshell is partially filled and completely filled outer 5s2 and 5p6 subshell. With increasing the nuclear charge, electrons enter into the underlying 4f subshell rather than external 5d subshell[28]. Because the filled 5s2 and 5p6 subshells screen the 4f electrons, the RE3+ luminescence arises from intraconfigurational, 4f-electron transitions and is typically sharp and well resolved as seen with Pr3+ in our study.

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Fig. 8 Luminous efficacy of different thicknesses Eu2O3 doped Sialon phosphors under 455 nm LED excitation Fig. 6 Energy level diagram of the Pr3+ ion

The spectral distribution curves for SN:Eu samples, as coupled with the blue LEDs are shown in Fig. 7. Thin sample SN:Eu (150 μm) had higher transparency, leading to higher transmission of the blue light from the sample. The luminous efficacy is higher in the transparent samples due to the higher intensity of visible light spectra as shown in Fig. 7. The luminous efficacies of SN:Eu samples with different thicknesses are shown in Fig. 8. The luminous efficacy was calculated on the basis of power applied to the blue LED. The current and voltage applied to the LED were 20 mA and 3.2 V, respectively. The highest luminous efficacy of 50.78 lm/W was observed for 150 μm thick SN:Eu sample. However, increasing the thickness of the sample, the opacity was also increased and the transmission of blue light was decreased, therefore the luminous efficacy was lower in thicker samples. Moreover, the CIE (Commission Internationale de L’Eclairage) co-ordinates were calculated for SN:Eu samples of different thicknesses on the basis of spectra from Fig. 7, and are presented in Table 2. The SN:Eu sample shows yellow luminescence and is suitable for the extraction of white light as coupled with blue LED.

Fig. 7 Photon distribution spectra of different-thicknesses Eu2O3 doped Sialon phosphors showing visible spectrum of light

The CIE co-ordinates for different thicknesses of SN:Eu samples are shown in Fig. 9. Due to the higher blue light component in 150 μm thick sample, the cool white light was produced with color temperature (CCT) of 14228 K. The sample having a thickness of 300 μm shows color coordinate in a white region having CCT of 7405 K. As observed from the spectral distribution curve in Fig. 7, the intensity of blue light and yellow light are comparable and thus the combination of blue and yellow light gives white light. But the 500 μm thick sample showed yellow luminescence rather than white because of the poor transparency of the sample, resulting from blocking most of the blue light. Hence, due the lower blue light Table 2 Chromaticity co-ordinate and color temperature of SN:Eu phosphors with different thicknesses Thickness/μm

CIEx

CIEy

CCT

150

0.289

0.238

14228

300

0.311

0.271

7405

500

0.380

0.355

3781

Fig. 9 CIE chromaticity diagram showing the color co-ordinates of SN:Eu having different thicknesses (Inset optical image of SN:Eu showing white light when excited by blue light)

Bhupendra Joshi et al., Luminescence properties of Eu2+, Gd3+ and Pr3+ doped translucent Sialon phosphors

component, the sample produced warm white light having CCT of 3781 K.

3 Conclusions The transparent/translucent Sialon phosphor ceramics were fabricated by hot press sintering and their microstructure, mechanical and optical properties were studied. The fabricated Sialon phosphor ceramics exhibited high hardness and fracture toughness that made it superior to other transparent polycrystalline ceramics and glass ceramics. The thin samples showed better luminescence intensity as well as good transparency. Eu2O3 doped sample had yellow luminescence that could be used as remote phosphor for white LED. Moreover, the 300 μm thick SN:Eu sample showed good extraction of white light as compared to thin (150 μm) or thick (500 μm) samples. The fabricated Sialon phosphors showed different emission wavelengths with respect to the addition of different REOs.

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