Bifunctional behavior of Er³⁺ ions as the sintering additive and the fluorescent agent in Er³⁺ single doped γ-AlON transparent ceramics

Abstract

We report on the sintering promoting and fluorescent activator roles of Er³⁺ in AlON:Er³⁺ transparent ceramics prepared by pressureless sintering with Er₂O₃ and AlON powder. There shows that the transparency of samples varied with the content of Er₂O₃ additive. The AlON:Er³⁺ ceramics showed upconversion luminescence peaking at 546 nm, 662 nm and 840 nm under the 980 nm excitation due to transition of ⁴S_3/2/²H_11/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and ⁴S_3/2/²H_11/2→⁴I_13/2 of Er³⁺ ions, respectively. The infrared spectra exhibited strong emission at 1534 nm corresponds to ⁴I_13/2→⁴I_15/2 transition. The mechanism of the IR and visible emission bands in AlON:Er³⁺ ceramics are discussed, which suggest it should be attractive for lighting and display devices applications.

Introduction

SPINEL-STRUCTURE aluminum oxynitride (AlON) is an important single-phase and stable solid solution material in the Al₂O₃–AlN system [1], [2]. Fully dense AlON transparent ceramics shows promising and attractive optical and excellent mechanical properties which makes it a suitable material for use in military aircraft, IR windows, visible window, missile domes, transparent armor, and scanner POS windows [1], [2], [3]. As a non-oxide material, the sintering process for AlON usually includes elevated temperatures and long sintering durations, even with high pressure to achieve full density and optical transparency [3], [4]. Some additives such as Y₂O₃, La₂O₃, BN and MgO are often introduced to accelerate the densification process and lower the sintering temperature [4], [5], [6], [7]. Previous reports suggest that the sintering additive impacts AlON sintering from those aspects as grain growth inhibitors, ionic diffusion, liquid-phase-sintering and so on [4], [5], [6], [7], [8]. Usually, a suitable amount of rare earth oxide such as Y₂O₃, La₂O₃ can be added to the AlON and promote sintering either as grain growth ionic diffusion or liquid-phase-sintering, and Zhang et al. also suggest that glass phases liquid containing Eu³⁺ in grain boundaries may form in the grain boundaries at a comparatively lower temperature to impact sintering [9]. Based on the above facts, Er³⁺ may also can be used as a potential sintering aid in fabrication of transparent γ-AlON ceramics.

Besides on the sintering promotion of rare earth ions, recently, rare-earth (RE) ion doped AlON was also the focus in numerous investigations owing to their unique luminescence properties and potential applications in lighting and display devices [9], [10], [11], [12], [13], [14], [15]. Many AlON-based phosphors have been prepared recently by different methods and their luminescence properties have been investigated, e.g. phosphors of AlON doped with Eu²⁺, Er³⁺, or co-doped with Mn²⁺–Mg²⁺, Eu²⁺–Mg²⁺, Yb³⁺–Tm³⁺, Ce³⁺–Tb³⁺, etc. [9], [10], [11], [12], [13], [14], [15]. Among RE ions, Er³⁺ is an attractive optical activator which offers favorable energy level schemes with resonant spacings and long lived excited states and being accessible under near-infrared radiation, suitable to convert infrared to visible light [16]. Zhang et al. have studied the preparations and the upconversion luminescence properties of Er³⁺-doped and Er³⁺, Mg²⁺ co-doped AlON powder phosphors and concluded that the doping of Mg²⁺ might enhance red emission as a result of introducing Er³⁺ into the AlON lattice and lowering the crystal symmetry of lattice [13], [14], [15] However, the Er³⁺ single doped AlON transparent ceramics have never been reported to our best of knowledge. In addition, other issues concerns with Er³⁺ in AlON, such as sintering promotion of Er ions in AlON and the infrared emission properties of Er³⁺ in AlON still remains to be further studied.

In this study, transparent AlON:Er³⁺ ceramics were fabricated with Er₂O₃ and AlON powder synthesized by carbothermal reduction and nitridation method. The roles of Er³⁺ concentration in densification and the luminescence properties including the infrared emission and upconversion emission of the ceramics were investigated in detail.