Elsevier

Optical Materials

Volume 35, Issue 12, October 2013, Pages 2405-2410
Optical Materials

Improvement of optical properties of Nd:YAG transparent ceramics by post-annealing and post hot isostatic pressing

https://doi.org/10.1016/j.optmat.2013.06.042Get rights and content

Highlights

  • Post-annealing plus post-HIP can improve the optical properties of Nd:YAG ceramics.

  • The post-HIP treatment at 1700 °C can contract or even reduce intergranular pores.

  • Re-annealing at 1350 °C or above, residual intergranular pores expanded.

Abstract

The Nd:YAG transparent ceramics were fabricated by vacuum sintering. The Nd:YAG samples were annealed at 1450 °C for 20 h in air and followed by hot isostatic pressing (HIP) at 1700 °C for 2 h in 200 MPa Ar and then re-annealed at 1250–1450 °C for 10 h in air. The experimental results showed that the optical properties of Nd:YAG samples varied markedly with different post treatments. After air annealing at 1450 °C for 20 h and HIP at 1700 °C for 2 h under 200 MPa of Ar and then air re-annealing at 1250 °C for 10 h, the transmittances of the samples increased from 51.2% to 77.2% (at 400 nm) and 78.4% to 83.6% (at 1064 nm), respectively. The annealing and HIP are effective post treatments to reduce oxygen vacancies and intergranular pores respectively in Nd:YAG transparent ceramics.

Introduction

Neodymium-doped yttrium aluminum garnet (Nd:YAG) ceramics were considered to be an alternative to single crystal as laser material because of low price, ease of manufacture, mass-production, high concentration doping, and better homogeneity of the doping ions [1], [2], [3]. Ikesue and Furusato first demonstrated the possibility of fabricating transparent Nd:YAG ceramics with a required quality for solid-state lasers with a reasonable efficiency [4]. Since then, extensive research works have been done on Nd:YAG ceramics related to the synthesis of high purity and agglomeration-free powders [5], [6], [7], amount and choice of sintering additives [8], [9], improvement of forming techniques [10], [11], and optimization of sintering process [12]. But the optical scattering is still a fatal problem for Nd:YAG ceramics.

Defects such as residual porosity, grain-boundary, and point defects are inevitable in the manufacturing process. However, post-processing, which is an effective way to eliminate the defects of Nd:YAG transparent ceramics, has not been insufficient studied. Our previous studies revealed that post air annealing could compensate oxygen vacancies, but residual porosity cannot be removed [13]. The scattering by pores, as the source of optical losses, must be reduced during manufacturing processing. Hot isostatic pressing (HIP) has proved to be an important method for improvement of ceramic microstructure [14], [15]. HIP, as a post-sintering treatment, also has been applied to fabricate transparent Nd:YAG ceramics [16], [17], [18], [19], [20]. Ikesue and Kamata showed that it could not achieve transparent Nd:YAG by a combination of vacuum sintering at 1750 °C followed by HIP treatment because of the existence of pores and grain-boundary phases [16]. They thought that argon (Ar) penetrated into ceramic body during HIP resulting in pores after HIP. Rabinovitch et al. fabricated transparent Nd:YAG sintering at 1700 °C for 3 h and then with HIP treatment at 1700 °C, 160 MPa with Ar for 90 min [17]. Suárez et al. have successfully prepared transparent Nd:YAG ceramic by post-HIP without the aid of sintering additives [18]. On the other hand, Lee et al. demonstrated that transparent Nd:YAG ceramics could be successfully fabricated by a sinter plus HIP approach with dry-pressed samples or tape-cast samples with 0.02–0.14 wt% SiO2 [19]. The patent literature on Nd:YAG suggests oxygen HIP is one approach to avoid discoloration of Nd:YAG during HIP densification [20]. Meanwhile, post-HIP is used to improve the properties of many transparent ceramics such as Al2O3 [21], Y2O3 [14], [22], MgAl2O4 [23], [24], ZrO2 [15], [25], MgO [26] and Lu2O3 [27]. It is believed that post-HIP with sintered ceramics led to better optical properties. But post-annealing plus post-HIP are used to improve the properties of Nd:YAG ceramics are rarely reported so far. Our previous researches studied the synthesis of high purity and agglomeration-free YAG nano-powders, post-annealing influence on Nd:YAG ceramics, and light scattering loss of Nd:YAG ceramics by pores [5], [6], [13], [28]. Very recently, the post-HIP was adopted to improve transparency. The purpose is to improve the optical properties of Nd:YAG transparent ceramics by the post-annealing plus post-HIP techniques.

In this paper, 2 at.% Nd:YAG nanopowders were synthesized through a modified co-precipitation method, and Nd:YAG transparent ceramics were fabricated by vacuum sintering. The samples were air annealed, subsequently HIP treated and then air re-annealed. The effects of post treatments on the optical properties and microstructure of the Nd:YAG transparent ceramics were investigated. Furthermore, we discussed the detailed mechanism of post treatments.

Section snippets

Experimental

The fabrication details are similar to our previous works [13], [28]. Neodymium nitrate (Nd(NO3)3·6H2O, 99.99%), yttrium nitrate (Y(NO3)3·6H2O, 99.99%) and ammonium aluminum sulfate (NH4Al(SO4)2·12H2O, 99.99%) were mixed together according to the stoichiometric ratio of 2 at.% Nd:YAG (Nd0.06Y2.94Al5O12) in distilled water. The precipitant solution was prepared by dissolving ammonium hydrogen carbonate (NH4HCO3, analytical grade) in mixed solvent of alcohol and distilled water. The mixed solution

Effect of post treatments on phase structures

Fig. 1 shows the XRD patterns of Nd:YAG samples as-prepared and post treated at various conditions. We notice that the phase structures of all the samples are almost the same regardless of different post treatment conditions. All the diffraction peaks of samples are well indexed as the cubic garnet structure of Y3Al5O12 (YAG, JCPDS: 33-0040). No extra peaks are found. Therefore, the post treatments do not produce any relevant effect on the structure and composition of these ceramics, and then,

Conclusions

In summary, the 2 at.% Nd:YAG transparent ceramics were air annealed, subsequently HIP treated and then air re-annealed. The results showed that the optical properties of Nd:YAG transparent ceramics varied markedly with different post treatments. The post-annealing and post-HIP turned out to be an effective post treatment to reduce oxygen vacancies and intergranular pores respectively. After air annealing at 1450 °C, the in-line transmittances of the samples increased, mainly due to the decrease

Acknowledgements

This work was supported by the National Natural Science Foundation of the People’s Republic of China (Grant Nos. 50872083 and 51002098), the Doctoral Program of Higher Education (Grant No. 20090181120092) and the National High Technology Research and Development Program (863).

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