Please cite this article in press as: M. Trunec, et al., Transparent alumina ceramics densified by a combinational approach of spark plasma sintering and hot isostatic pressing, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.004 ARTICLE IN PRESS G Model JECS-10699; No. of Pages 5 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 Short communication Transparent alumina ceramics densified by a combinational approach of spark plasma sintering and hot isostatic pressing Martin Trunec a,∗ , Jens Klimke b , Zhijian James Shen c a CEITEC—Central European Institute of Technology, Brno University of Technology, Technicka 10, 616 00 Brno, Czech Republic b Oxide Ceramics, Fraunhofer Institute of Ceramic Technologies and Systems, Winterbergstrasse 28, 01277 Dresden, Germany c Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius vag 16C, 106 91 Stockholm, Sweden a r t i c l e i n f o Article history: Received 20 August 2015 Received in revised form 28 April 2016 Accepted 2 June 2016 Available online xxx Keywords: Alumina Doping Pressure-assisted sintering Grain growth In-line transmission a b s t r a c t In order to increase the in-line transmission of fine transparent alumina in visible light the grain growth during sintering of alumina ceramics was supressed using a combined densification process. This pro- cess combines presintering of a green body by spark plasma sintering with final hot isostatic pressing. The presintering by spark plasma sintering provided bodies with a substantially smaller grain size than pressureless presintering. It is shown that the fine-grained presintered microstructure could be retained during final hot isostatic pressing and alumina ceramics doped with spinel and zirconia nanoparticles in particular could be sintered to full density with only minor grain growth during final hot isostatic press- ing. The novel combined densification process enhanced by the unique nanoparticle doping approach provided fully dense alumina ceramics with an average grain size of 237 nm and an in-line transmission of 76.2% at a wavelength of 632.8 nm and a sample thickness of 0.8 mm. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Alumina (Al 2 O 3 , corundum) is one of the important birefringent transparent ceramics. High hardness, high melting point, excel- lent corrosive resistance, and competitive fracture toughness make transparent alumina ceramics a promising candidate for applica- tions as transparent armors, electromagnetic windows, envelopes of high-pressure halide lamps, etc. [1]. Mechanical properties of submicrometer-grained alumina can exceed the mechanical prop- erties of sapphire (single-crystal alumina) [2,3] and it is believed that such fine-grained alumina could replace sapphire in many optical applications. Up to now, transparent alumina ceramics with a submicrometer-grained structure were prepared exclusively by pressure-assisted sintering processes. Pressureless presintering followed by hot isostatic pressing (PLS/HIP) and spark plasma sintering (SPS) are the two most common densification processes for the production of transparent fine-grained alumina. Recently reported values of real in-line transmission, RIT, (i.e. in-line transmission of light scattered at an angle ≤1 ◦ ) of submicrometer- grained alumina ceramics in visible light are shown in Fig. 1. The ∗ Corresponding author. E-mail addresses: trunec@fme.vutbr.cz, martin.trunec@ceitec.vutbr.cz (M. Trunec). reported values are compared with the theoretical in-line trans- mission of non-absorbing fully dense alumina at a wavelength of 640 nm. The theoretical in-line transmission, T in−line , of an ideally dense fine-grained alumina ceramic only depends on the grain size, d, and was calculated using the model proposed by Apetz and van Bruggen [4]. T in-line = T th exp(-3C sca t/(d 3 )), (1) where T th is the total theoretical transmission limit of a body (including multiple surface reflections and neglecting the grain boundary reflections), C sca is the scattering cross-section of one grain, and t is sample thickness. The scattering cross section, C sca , was calculated using the Mie theory according to the numerical algorithm by Bohren and Huffman [5]. The theoretical transmission limit T th = 0.86, refractive index n = 1.76, and average birefringence n = 0.005 were used for the calculation. As the grain size of the samples measured was represented by a mean value although the grains had a wider grain size distribution, the RIT values could exceed the theoretical limit that was calculated for spherical mono- sized grains. Nevertheless, it is evident from Fig. 1 that alumina samples prepared by pressureless presintering and hot isostatic pressing (PLS/HIP) exhibit RIT near or at the theoretical limit. This means that they reached almost full density and their in-line trans- mission was controlled by the grain size. RIT values from 51 to 64% at a wavelength of 640 nm have been reported by several authors [6–8] for 0.8 mm thick samples with a grain size from 470 to http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.004 0955-2219/© 2016 Elsevier Ltd. All rights reserved.