Densified Eu2+- doped SrAl2O4 by Pulsed Electric Current Sintering

Makoto Nanko, Shinya Taniguchi and Koji Matsumaru

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AZojomo (ISSN 1833-122X) Volume 4 July 2008

Topics Covered

Abstract
Introduction
Experimental Procedure
Results and Discussion
Conclusions
Acknowledgements
References
Contact Details

Abstract

Eu²⁺-doped SrAl₂O₄ (SrAl₂O₄:Eu²⁺) was synthesized by a conventional solid-state reaction process and consolidated by pulsed electric current sintering. It was found that highly densified bodies were obtained at die temperature of 1600ºC for 5 min under 57 MPa in vacuum. The sintered SrAl₂O₄: Eu²⁺ showed good photoluminescence by UV light irradiation and mechanoluminescence by elastic deformation. Physical properties of sintered SrAl₂O₄:Eu²⁺ were also evaluated such as Young’s modulus, Vicker’s hardness, Poison’s ratio and thermal conductivity at room temperature.

Keywords
SrAl₂O₄: Eu²⁺, Pulsed Electric Current Sintering (PECS), Densification, Physical Properties, Luminesence.

Introduction

SrAl₂O₄-based ceramics, such as SrAl₂O₄:Eu²⁺ and SrAl₂O₄:Eu²⁺,Dy³⁺, are excellent phosphor oxides with strong photoluminescence at green visible region [1-4] and exhibits mechanoluminesence phenomenon [5-7], which is a light emission caused by mechanical stress such as collision. However this phosphor oxide is difficult to obtain densified bodies [7]. In order to develop new applications of this phosphor oxide, densification technique should be established.

Pulsed electric current sintering (PECS) technique [8-14] is the latest pressure-sintering process, and is consisted by applying pulsed electric current into conductive die and /or sample in the die to rise temperature. This method is sometimes called as Spark Plasma Sintering (SPS). Rapid heating (a few minuets to 2000ºC!) can be achieved in the PECS technique. PECS is also useful to consolidate advanced materials which are difficult to densify in conventional sintering techniques, such as nano-composites, ceramics and intermetallic compounds [14-17].

Hasezaki et al. reported sintering and phosphorescent properties of SrAl₂O₄ ceramics by PECS [18]. They sintered the powder mixture of SrCO₃, Al₂O₃, Eu₂O₃ and Dy₂O₃ by PECS and successfully sintered SrAl₂O₄ co-doped with Dy³⁺ and Eu²⁺. However they did not mentioned the densification. In the present study, the PECS process was applied to SrAl₂O₄: Eu²⁺ powder synthesized by the solid-state reaction. Physical properties of sintered SrAl₂O₄: Eu²⁺ were also evaluated such as Young’s modulus, Poisson’s ratio, Vicker’s hardness, heat capacity and thermal conductivity at room temperature.

Experimental Procedure

SrCO₃, Al₂O₃ and Eu(NO₃)₃ were used as the starting materials. These materials were weighed to be (Sr0.99, Eu0.01)0.985Al₂O₄-δ. The powder mixture was prepared by mixing in ethanol with a mortar. The reacted powder was prepared with the conventional solid-state reaction process at 1400ºC for 12 h in air. The reacted powder was reduced at 1500ºC for 3 h in a stream of Ar-1% H2 gas mixture. Figure 1 shows X-ray diffraction patterns of the prepared powder. The SrAl₂O₄ phase was obtained after the reduction process. The powder was sintered at temperature ranging from 1400 to 1600ºC for 5 min in vacuum. The uni-axial pressure during PECS was 28 and 57 MPa. Atmospheric sintering was conducted at temperature ranging from 1300 to 1500ºC in a stream of Ar-1%H2 gas mixture. Green bodies for the atmospheric sintering were produced by cold-isostatic pressing under 200 MPa for 5 min.

Figure 1. X-ray diffraction patterns of SrAl₂O₄: Eu²⁺ prepared by a conventional solid-state reaction processing.

Density of the sintered bodies was evaluated by a liquid-replacement technique with toluene. Microstructure of the sintered bodies was observed by scanning electron microscopy. Young’s modulus and Poisson’s ratio of the densified Eu²⁺-doped SrAl₂O₄ were evaluated by using the ultrasonic technique with 5 MHz in frequency [19]. Vicker’s hardness was measured under 245 mN for 15 s. Thermal conductivity measurement was conducted by using a leaser flush method with heat capacity measured by the differential scanning calorimetry.

Results and Discussion

Figure 2 shows relative density of sintered body as a function of sintering temperature in atmospheric sintering in a stream of Ar-1%H2 gas mixture. Relative density increases with increasing sintering temperature and reaches to approximately 60%. Figure 3 shows SEM photographs of fractured surfaces of sintered SrAl₂O₄:Eu²⁺ by the atmospheric sintering at 1300 and 1500ºC. Even the sample sintered at 1500ºC (Figure 3 (b)) is in the initial or intermediate stage in sintering. As Zheng described previously[7] , SrAl₂O₄: Eu²⁺ has low sinterability in pressureless sintering.

Figure 2. Relative density as a function of sintering temperature on atmospheric sintering of SrAl₂O₄: Eu²⁺ in a stream of Ar-1%H2 gas mixture.

Figure 3. SEM photographs of fractured surfaces of sintered SrAl₂O₄:Eu²⁺ by the atmospheric sintering in a stream of gas mixture of Ar-1% H2. (a) and (b) show the fractured surfaces of the samples sintered at 1300 and 1500ºC and , respectively.

Figure 4 shows relative density of sintered body as a function of sintering temperature (die surface temperature) via PECS process. SrAl₂O₄:Eu²⁺ can be densified into over 99% by PECS at 1600ºC for 5 min under 57 MPa in the uni-axial pressure.

Figure 4. Relative density of sintered body as a function of sintering temperature (die surface temperature) via PECS process.

Figure 5 shows SEM photographs of fractured surfaces of sintered SrAl₂O₄:Eu²⁺ by PECS process. The samples densified by PECS at 1500ºC include only closed pores. In the sintered body consolidated by PECS at 1600ºC under 57 MPa, closed pores are not observed on the fractured surface. In order to densify SrAl₂O₄ ceramics, it is useful to apply high pressure during sintering.

Figure 5.SEM photographs of fractured surfaces of sintered SrAl₂O₄:Eu²⁺ by PECS process.

The dense SrAl₂O₄:Eu²⁺ bodies showed good photoluminescence by UV irradiation (Figure 6) and mechnoluminescence by plastic deformation (Figure 7). Brightness of their luminescence is easily observed by the necked eyes. Table 1 shows the some physical properties of the sintered SrAl₂O₄:Eu²⁺at room temperature. Young’s modulus and hardness of SrAl₂O₄:Eu²⁺ are comparable silica glass and lower than structure ceramics such as mullite and Al₂O₃.

Figure 6. Photoluminascence of sintered SrAl₂O₄:Eu²⁺ by PECS process.

Figure 7. Mechanoluminascence of sintered SrAl₂O₄:Eu²⁺ by PECS process.

Table 1. Physical properties of sintered SrAl₂O₄: Eu²⁺

Conclusions

Highly densified SrAl₂O₄:Eu²⁺ can be obtained by PECS processing with the powder synthesized by the conventional solid-state reaction process. The sintered SrAl₂O₄:Eu²⁺shows good photoluminescence and mechnoluminescence with high brightness which is easily visible by the necked eyes. Physical properties of sintered SrAl₂O₄:Eu²⁺ were also evaluated at room temperature. Young’s modulus, hardness and thermal conductivity of SrAl₂O₄:Eu²⁺ were approximately comparable with silica glass.

Acknowledgements

The authors thank Associate Professor I. Ihara for evaluating the elastic properties, Associate Professor, S. Nagasawa for evaluating the Vicker’s hardness and Associate Professor M. Takeda for evaluates thermal conductivity of sintered SrAl₂O₄:Eu²⁺. The authors also wish to express their gratitude to the Japanese government for partially supporting the present work through the 21st Century Center of Excellence (COE) Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT).

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Contact Details

This paper will be also published in “Advances in Technology of Materials and Materials Processing Journal, 9[2] (2007) 125-130”.