Influence of (Nd+Y)/Al ratio on sintering behavior and optical features of Y3-xNdxAl5O12 ceramics for laser applications
Introduction
Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) ceramics are attractive materials to use as amplifier media in high energy solid-state lasers. Ikesue et al. [1] has shown in 1995 the feasibility of Nd:YAG ceramics with high level of transmittance and effective laser oscillation, similar to Nd:YAG single-crystals [2,3]. Besides, the high mechanical strength and the high thermal conductivity, the possibility to incorporate high amount of rare-earth ions (Nd3+, Yb3+…) [4], the feasibility to manufacture high dimensions and/or complex architectures (i.e. dopant gradient) with the ceramic route are the main advantages of this material [[5], [6], [7], [8]]. Moreover, Nd:YAG ceramics manufacturing requires elaboration conditions less severe than single-crystal ones [9,10]. Nevertheless, transparent ceramics for lasers are still involved in limited industrial applications. This fact is partially due to the remaining technical issue of ceramics manufacturing without scattering defects. Porosity, impurities and so on are still hard to avoid that decreases laser performances. One type of defects, secondary phases like Al2O3 or YAlO3 in Nd:YAG ceramics, is often due to a control of (Nd+Y)/Al ratio with insufficient accuracy [11].
Optical and laser performances of Nd:YAG ceramics are mainly controlled by residual microstructural defects. Elimination of these defects like pores or secondary phases in ceramic matrix is necessary to reach good optical properties. Light is very effectively scattered because of the different refractive index n between the gas-filled pores, secondary phases and the main matrix ( ≈ 1; = 1.7–1.9; = 1.815). As a consequence, the optical transmittance T is often given by the following expression [12]:where is the extinction coefficient described by the relation: , h is the sample thickness and . This relation is a consequence of Fresnel reflectance for normal incidence and specular reflection (i.e. total reflection loss) [13]. For Nd:YAG, refractive index n = 1.815 at 1 μm, so Eq. (1) gives = 84%. All of defects come from the different steps of the elaboration process: impurities in raw powders, control of Y/Al stoichiometric ratio, or elaboration process with steps like mixing, shaping, sintering, etc. with insufficient accuracy. For example, the influence of sintering on residual porosity content and related optical properties were extensively studied [[13], [14], [15]]. In our previous studies [16], it has been shown that a porosity value lower than 10−4% is necessary to reach laser performance levels similar to single-crystal ones. Such low values remain attainable by simple natural sintering under vacuum for several hours [16] but can be also reached thanks to post-HIP treatment [17]. The influence of secondary phases on Nd:YAG ceramics optical properties have been much less studied as this issue is generally less critical. Moreover, most of these studies report the presence of secondary phases without any discussion of results taking into account their presence.
Y3Al5O12 garnet phase (YAG) is a defined compound as shown on the Y2O3-Al2O3 binary phase diagram (Fig. 1) [18]. YAG phase is thus obtained for a defined Y/Al ratio equal to 0.6000. From this diagram one can see that this material doesn't accept any deviation from stoichiometric composition. For Y/Al ratio smaller than 0.6000, α-Al2O3 secondary phase is in equilibrium with YAG. For Y/Al ratio higher than 0.6000, the secondary phase in equilibrium with YAG is yttrium aluminum perovskite YAlO3 (YAP). These two secondary phases were often reported to be encountered in sintered YAG or Nd:YAG ceramics (see for example [19,20]). Some studies were aimed at finding the influence of stoichiometry deviation on ceramics properties. Patel et al. [21] have highlighted by simulation the different mechanisms and the structural (intrinsic or extrinsic) defects linked to the deviation from stoichiometric Y/Al ratio. Huang et al. [22] were also interested in structural defects created by non-stoichiometry. Liu et al. [23] have shown the influence of aluminum or yttrium excess on crystallographic properties and grain size in YAG-based ceramics. In this study, Y/Al ratio appears to strongly influence the physicochemical properties and microstructure of Nd:YAG ceramics. Especially, grain growth kinetics were investigated but densification behavior was not discussed in regards to the presence of secondary phases, structural defects and sintering aids (i.e. SiO2 and MgO). Sintering kinetics and mechanisms in a given material are well known to be dependent on several parameters including the presence of precipitates, their location (intra- or intergranular), the nature and content of intrinsic and extrinsic defects, etc. [24]. According to the literature, sintering behavior -densification and grain growth-of Nd:YAG ceramics is expected to depend on the Y/Al ratio. Consequently, this work aimed at better understand the influence of its atomic ratio on sintering ability of Nd:YAG ceramics. In this study, densification, grain growth and overall microstructural evolution during sintering with corresponding final optical properties of Nd:YAG ceramics were investigated.
Section snippets
Powder preparation and sintering
Nd:YAG-based ceramics were elaborated by a solid-state reactive sintering process as described elsewhere [25]. According to the flow chart reported in Fig. 2, Nd:YAG ceramics with different amounts of secondary phases (YAlO3 or Al2O3) depending on Y/Al ratio were obtained after sintering. Commercial submicron α-Al2O3 (Ø < 0.5 μm; purity > 99.99%, Baïkowski, France), Y2O3 (Ø < 0.5 μm; purity > 99.99%, Solvay, France) and Nd2O3 (Ø < 1 μm; purity > 99.99%, Alfa Aesar, Germany) were blended
Study of reaction – sintering
This part investigates the reaction sintering behavior of Nd:YAG-based ceramics with various (Nd+Y)/Al ratios. Fig. 3 shows the linear shrinkage obtained by dilatometry of green powder compacts obtained after shaping with various (Nd+Y)/Al ratios compositions (0.5761, 0.6000 and 0.6068). These ratios correspond respectively to Nd:YAG samples with 2 vol% Al2O3, stoichiometric and 2 vol% YAP. During thermal treatment, solid-state reactions occur in the temperature range 1173 K–1800 K according to
Conclusions
This study has investigated the influence of stoichiometry deviation on the sintering behavior and optical properties of Nd:YAG ceramics. The link between Y/Al ratio, sintering behavior, microstructure and optical properties of Nd:YAG ceramics is now better understood. The main conclusions of this work can be summarized below:
- (1)
Y-excess slows-down sintering kinetics and Al-excess acts on the contrary. This result can be explained by the link between (Nd+Y)/Al ratio and intrinsic structural
Acknowledgments
The authors are grateful for Region Limousin for PhD funding and for CILAS Company for laser measurements.
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