Regular Article A robust approach for highly transparent Y 2 O 3 ceramics by stabilizing oxygen defects Wook Ki Jung, Ho Jin Ma, Youngtae Park, Do Kyung Kim ⁎ Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea abstract article info Article history: Received 11 March 2017 Accepted 24 April 2017 Available online xxxx Highly transparent Y 2 O 3 ceramics were fabricated suppressing the formation of oxygen vacancies during vacuum sintering for the first time. Oxygen stabilization in transparent Y 2 O 3 ceramics was successfully enabled by addi- tional ZrO 2 oxygen source considering thermodynamics. The oxygen-stabilized ceramics approached nearly the- oretical transmittance without further post-annealing and hot isostatic presssing. This new approach is potentially applicable for the fabrication of other transparent ceramics. © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Transparent Y 2 O 3 ceramics Oxygen vacancy Thermodynamic stabilization Y 2 O 3 /ZrO 2 dual powder bed One-step vacuum sintering Transparent polycrystalline ceramics have been investigated for in- frared (IR) windows, solid-state laser media, scintillators and bio-appli- cations in order to substitute for the single crystals [1–9]. From the perspective of Mie scattering theory, high optical performance of poly- crystalline ceramics can be achieved by fabricating pore-free and high purity ceramics [10]. Special sintering technologies such as vacuum sintering, vacuum hot pressing and spark plasma sintering (SPS) are used in order to obtain fully dense ceramics. Sintering in a vacuum con- dition is widely preferred to remove the nitrogen gas from the pellets [11]. While full density can be obtained via sintering in a vacuum, the final products show black color, or so-called discoloration [12,13]. It is known that the discoloration is due to the oxygen vacancy in the ce- ramics resulted from the reducing condition in the vacuum. The oxygen vacancies give rise to the formation of F and F + centers in the wide band gap and these absorption bands can contribute to the transmission per- formance below 300 nm [10,14]. It is still unclear what the exact conse- quences of oxygen vacancies are for larger wavelengths and discoloration. The oxygen vacancies in the sintered ceramics can be removed con- ventionally by post-annealing in air or in an oxygen atmosphere [15,16]. However, the post-annealing is insufficient to improve the optical trans- mittance due to the pore development by evaporation of residual car- bon or sulfur particles in the bulk [17,18]. It is difficult to eliminate the oxygen vacancies perfectly preventing the pore development during the post-annealing and the optical performances are inevitably deterio- rated. The pore development during the post-annealing inevitably dete- riorates the optical performance and therefore, it is difficult to obtain highly transparent ceramics by post-annealing. To solve this problem, the transparent ceramics fabricated via sintering in a vacuum need post-annealing in air for a long time and then further a hot isostatic pressing (HIP) procedure [13,19]. The further HIP process is disadvanta- geous in the aspect of cost-effectiveness. There is demand to fabricate transparent ceramics via the most simple route possible for the commercialization. The transparent yttria ceramics has been fabricated recently by one- step vacuum hot pressing employing tantalum foil [20,21]. The green body was wrapped by the tantalum foil to prevent carbon contamina- tion from the graphite mold. The formation of oxygen vacancies also could be restricted to some degree resulting from the low sintering tem- perature of the hot pressing. The as-sintered samples showed high opti- cal transmittance, but the use of tantalum still has limitations in commercialization considering cost-effectiveness. Wrapping the Y 2 O 3 green body with tantalum foil also can cause deformation of the final sintered body including wrinkles. Herein, we fabricated transparent Y 2 O 3 ceramics preventing the formation of oxygen vacancies during the vacuum sintering step. The development of oxygen vacancies was suppressed by thermody- namic stabilization of yttria phase in a vacuum. At elevated temper- atures in a vacuum condition, the decrease in oxygen of the yttria powder bed induces oxygen ion diffusion from the pellets to the bed, thereby yielding an undesired oxygen-deficient phase. It was possible to prevent loss of oxygen ions from the pellets using a suit- able oxygen supplier considering the thermodynamics. We applied this strategy and successfully fabricated transparent Y 2 O 3 ceramics by one-step vacuum sintering for the first time. The as-sintered ce- ramics showed transmittance that was close to the theoretical value from the visible to the IR range. Scripta Materialia 137 (2017) 1–4 ⁎ Corresponding author. E-mail address: dkkim@kaist.ac.kr (D.K. Kim). http://dx.doi.org/10.1016/j.scriptamat.2017.04.036 1359-6462/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat