Contents lists available at ScienceDirect Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc Original Article Fabrication and microstructural characterization of the novel optical ceramic consisting of α-Al 2 O 3 @amorphous alumina nanocomposite core/ shell structure A. Eftekhari a , B. Movahedi a, ⁎ , G. Dini a , M. Milani b a Department of Nanotechnology Engineering, Faculty of Advanced Sciences and Technologies, University of Isfahan, Isfahan, 81746-73441, Iran b Department of Advanced Materials and Renewable Energies, Iranian Research Organization for Science and Technology, Tehran 33131-93685, Iran ARTICLE INFO Keywords: Core/Shell Sintering Nanocomposite Optical properties Al 2 O 3 ABSTRACT In this study, α-Al 2 O 3 @amorphous alumina nanocomposite core-shell structure was synthesized from AlCl 3 and the commercial α-Al 2 O 3 nanoparticles as the starting materials via a wet chemical route. The results indicated that the shell material mainly comprised of ammonium chloride and boehmite phases. Boehmite was trans- formed to the amorphous and γ-Al 2 O 3 phases after the calcination process and the shell material was completely converted to γ-Al 2 O 3 at 1000 °C. However, for the α-Al 2 O 3 @amorphous alumina core-shell nanoparticles were completely converted to α-Al 2 O 3 at 1000 °C. It can be concluded that α-Al 2 O 3 core particles, as the seed crys- talline, help to transforming of γ-Al 2 O 3 phase as the shell material directly without forming transitional phases to α-Al 2 O 3 . The optical polycrystalline alumina was fabricated using spark plasma sintering of α-Al 2 O 3 @amor- phous alumina core-shell nanocomposite. The body sintered has a final density of ∼99.8% and the in-line transmittance value is ∼80% within the IR range. 1. Introduction Alumina or aluminum oxide (Al 2 O 3 ) is one of the ceramic materials and is used in a wide range of applications such as adsorbent, catalyst, transparent armor for ballistic instrument, laser, discharge lamp, in- frared (IR) airborne sensor [1–5]. Alumina exists in different structures (allotropic forms), but the most common form of crystalline alumina is known as corundum, α-Al 2 O 3 , which is the thermodynamically stable. α-Al 2 O 3 can form from several paths (e.g., η-Al 2 O 3 → θ-Al 2 O 3 → α- Al 2 O 3 or boehmit → γ-Al 2 O 3 → σ-Al 2 O 3 → θ-Al 2 O 3 → α-Al 2 O 3 ). Para- meters such as particle size, heating rate, pH, impurity and atmosphere have significant effects on the phase transformation [6–9]. Over the past decade with development of synthesis processes, forming and sintering techniques, transparent polycrystalline ceramics have been produced at temperatures near to 1400 °C and even less as well as in short time. The main challenge during the sintering process of these ceramics is the grain growth [10,11]. Consequently, in some studies the effects of co-sintering materials such as La 2 O 3 [12–15], Y 2 O 3 [16,17], CeO 2 [18,19], MgO [20,21], and ZrO 2 [22], on the grain growth have been investigated. Co-sintering materials help to speed up the sintering process. On the other hand, some researchers have been concentrated on the use of α-Al 2 O 3 particles as a seed in the sintering process of other alumina structures [10,23–26]. These particles also are useful to control the grain size, porosity and distribution in the final structure of sintered materials. As a result, α-Al 2 O 3 nanostructure ceramics with 98% relative density could be obtained during pres- sureless sintering process [27–30]. Recently, Ghanizadeh et al. [31] utilized α-Al 2 O 3 as a seed to synthesis alumina nanoparticles and then using spark plasma sintering (SPS) process to sinter α-Al 2 O 3 nano- particles without any co-sintering or dopant materials. They reported that this ceramic offered 99.9% relative density with in-line transmit- tance values of up to ∼80% within the IR range. Additionally, in order to prevent the abnormal grain growth during phase transformation from θ to α- Al 2 O 3 , Cheng et al. [32], utilized the sol-gel method to coat the boehmite phase on the surface of θ-Al 2 O 3 particles. Kafili et al. [33,34], synthesized alumina@yttria core-shell nanocomposite powders via a wet chemical rout and produced the yt- trium-aluminum-garnet (YAG) transparent ceramic during subsequent spark plasma sintering (SPS) process. Jayasancar et al. [35] synthesized the alumina@aluminum titanate composite via the core-shell method. They reported that this method is an effective technique to control the grain size, formation temperature and sintering of aluminum titanate. Furthermore, they found that the formation temperature of aluminum titanate strongly reduced by the coating of TiO 2 on the alumina https://doi.org/10.1016/j.jeurceramsoc.2018.02.038 Received 31 July 2017; Received in revised form 6 February 2018; Accepted 28 February 2018 ⁎ Corresponding author. E-mail address: b.movahedi@ast.ui.ac.ir (B. Movahedi). Journal of the European Ceramic Society xxx (xxxx) xxx–xxx 0955-2219/ © 2018 Elsevier Ltd. All rights reserved. Please cite this article as: Eftekhari, A., Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.02.038