Please cite this article in press as: J.-M. Kim, et al., Microstructure and optical properties of transparent MgAl 2 O 4 prepared by Ca- infiltrated slip-casting and sinter-HIP process, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.02.018 ARTICLE IN PRESS G Model JECS-10526; No. of Pages 8 Journal of the European Ceramic Society xxx (2016) xxx–xxx Contents lists available at www.sciencedirect.com Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc Microstructure and optical properties of transparent MgAl 2 O 4 prepared by Ca-infiltrated slip-casting and sinter-HIP process Jin-Myung Kim, Ha-Neul Kim ∗ , Young-Jo Park, Jae-Woong Ko, Jae-Wook Lee, Hai-Doo Kim Engineering Ceramics Research Group, Korea Institute of Materials Science, 797 Changwondaero, Changwon, Gyeongnam 641-831, Republic of Korea a r t i c l e i n f o Article history: Received 28 October 2015 Accepted 7 February 2016 Available online xxx Keywords: MgAl2O4 Spinel Transparent Slip-casting Calcium aluminate a b s t r a c t In this study, transparent MgAl 2 O 4 , which was fabricated by slip-casting on different porous molds (Al 2 O 3 , gypsum) followed by sinter-HIP process, was investigated. Although the relative packing homogeneity in the green state was slightly improved by using porous alumina molds, rapid densification and retarded grain growth occurred simultaneously for the gypsum-casted specimen due to infiltration of gypsum elements into the MgAl 2 O 4 matrix. The sintering behavior of gypsum-casted specimen was attributed to the formation of liquid phases and solute segregation at grain boundaries. The pre-sintering temperature significantly affected the final-HIPed microstructure so that abnormally grown grains and secondary phases (CaAl 4 O 7 ) appeared in the specimen pre-sintered at 1500 ◦ C followed by post-HIP treatment. For the properly processed specimen, which was pre-sintered at 1450 ◦ C, the light transmittance was significantly improved in near UV wavelength region, and the in-line transmittance was maintained higher than 75% until the wavelength of incident beam decreased to 280 nm. © 2016 Published by Elsevier Ltd. 1. Introduction Magnesium aluminate (MgAl 2 O 4 ) spinel is a representative transparent ceramic material that has large intrinsic band-gap (7.8 eV) and isotropic crystal structure [1,2]. In addition to the optimal light transmittance in the visible-IR range, transparent MgAl 2 O 4 is attractive due to its excellent hardness and ballistic resistance [3], so that it has been regarded as a promising can- didate material for applications such as windows for armor and vehicles, IR domes for IR seekers, high pressure arc lamps, optical lenses and scintillators [4]. Transparent MgAl 2 O 4 can be fabricated as a single crystal via melt growth or a polycrystalline form via sintering of powder compacts; the latter has advantages in manu- facturing cost, the scale-up process and free shaping into complex bodies. To sinter highly transparent polycrystalline MgAl 2 O 4 , the final products should contain less than tens of ppm of resid- ual pores to minimize light scattering [5]. Various consolidation methods (e.g., freeze granulation, gel-casting, slip-casting) and sin- tering techniques (e.g., sinter-HIP, HP-HIP, SPS) have been reported to make homogeneous green compacts and transparent sintered products [6–9]. Moreover, several types of sintering aids have been ∗ Corresponding author. Fax: +82 55 280 3392. E-mail address: skykim@kims.re.kr (H.-N. Kim). examined such as CaO [10–13], B 2 O 3 [14], TiO 2 [12,15], ZrO 2 [15], SiO 2 [12], Y 2 O 3 [16], LiF [17–19], and various types of fluorides and chlorides (e.g., AlF 3 , MgF 3 , NaF, BaF 2 , AlCl 3 , CaCl 2 ) [4,19] because solid state sintering of pure MgAl 2 O 4 requires severe sintering con- ditions to remove residual pores. By means of these sintering aids, the mass transport of MgAl 2 O 4 can be accelerated via a liquid phase or enriched point defects. CaO was the first sintering aid reported for MgAl 2 O 4 [10]. CaO promoted densification at relatively low sintering temperature near 1400 ◦ C, which was explained by liquid phase formation. Recently, Krell et al. revisited CaO doping and showed that 0.1 wt% of CaO addition results in better optical homogeneity for a large spinel panel (D > 100 mm) as well as enhanced in-line transmit- tance [11]. B 2 O 3 has also been found to reduce the sintering temperature below 1300 ◦ C, but abnormally grown grains fre- quently appeared when the sintering temperature or composition was not properly controlled [14]. In the co-doping system of TiO 2 –ZrO 2 , only a hundred tens ppm of co-doped sintering aids gave high transmittance in the near UV wavelength region, sup- press grain growth, and improve the strength [15]. The patent reporting this co-doping process did not provide a detailed mech- anism and reasoning for these improvements. According to Izumi et al. [20] when 0.1–1 mol% of Ti was incorporated into MgAl 2 O 4 single crystals, there was an improvement in visible transmit- tance and an increase in the UV cut-off wavelength. They assumed http://dx.doi.org/10.1016/j.jeurceramsoc.2016.02.018 0955-2219/© 2016 Published by Elsevier Ltd.