Study on carbon contamination and carboxylate group formation in Y2O3-MgO nanocomposites fabricated by spark plasma sintering

Journal Pre-proof Study on carbon contamination and carboxylate group formation in Y2 O3 -MgO nanocomposites fabricated by spark plasma sintering Seok-Min Yong, Doo Hyun Choi, Kisu Lee, Seok-Young Ko, Dong-Ik Cheong, Young-Jo Park, Shin-Il Go

PII:

S0955-2219(19)30706-X

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2019.10.035

Reference:

JECS 12797

To appear in:

Journal of the European Ceramic Society

Received Date:

16 July 2019

Revised Date:

14 October 2019

Accepted Date:

17 October 2019

Please cite this article as: Yong S-Min, Choi DH, Lee K, Ko S-Young, Cheong D-Ik, Park Y-Jo, Go S-Il, Study on carbon contamination and carboxylate group formation in Y2 O3 -MgO nanocomposites fabricated by spark plasma sintering, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.10.035

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Study on carbon contamination and carboxylate group formation in Y2O3-MgO nanocomposites fabricated by spark plasma sintering

Seok-Min Yonga,*, Doo Hyun Choia, Kisu Leea, Seok-Young Koa, Dong-Ik Cheonga, Young-Jo Parkb, and Shin-Il Gob

Agency for Defense Development (ADD), Yuseong P.O. Box 35, Daejeon 34816,

Republic of Korea b

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Engineering Ceramics Research Group, Korea Institute of Materials Science (KIMS),

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Changwon, Gyeongnam, 51508, Republic of Korea

*Corresponding author: Seok-Min Yong (Ph. D.)

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Tel.: +82-42-821-0924, Fax: +82-42-821-3400 E-mail: [email protected]

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Agency for Defense Development (ADD),

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Yuseong P.O. Box 35, Daejeon 34816, Republic of Korea

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Abstract

For Y2O3-MgO nanocomposites fabricated by spark plasma sintering (SPS), the

strong absorption peaks, which are assigned to the stretching vibrations of carboxylate group (O-C=O), are commonly observed in IR spectra. These absorption peaks can negatively affect military applications such as IR windows or domes. In order to suppress

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the formation of these absorption peaks, therefore, it is important to understand how the carboxylate group in the SPSed Y2O3-MgO nanocomposite originated. In this study, it is demonstrated that the formation of the carboxylate group is related to the carbon contamination during SPS processing. The carbon phase transforms into CO2 gas by annealing and this leads to a reaction with metal oxide phases, resulting in the formation of carbonate phase. This carbonate phase contributes to the absorption peaks of the

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carboxylate group.

Keyword: Y2O3-MgO nanocomposite, Spark plasma sintering, carboxylate group,

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carbon contamination

1. Introduction

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Due to their excellent mid-wavelength infrared (MWIR) transmittance and mechanical properties, Y2O3-MgO nanocomposites have recently received considerable

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attention as promising candidate materials for hypersonic infrared (IR) windows or domes [1-4]. In these nanocomposites, the presence of one phase naturally impedes the grain

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growth of the adjacent phase. This pinning effect is most effective when the phases have comparable volume fractions and are dispersed uniformly in the composites. The reduced

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grain size can increase the optical transparency, especially when the grain size is smaller than the wavelength of incident light (less than approximately λ/20). In addition, a smaller grain size increases the mechanical strength and thermal shock resistance [5, 6]. Therefore, Y2O3-MgO nanocomposites exhibit optical and mechanical properties superior to those of other IR transparent materials, such as Al2O3, MgAl2O4, AlON, and Y2O3 [7].

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It is difficult, however, to obtain excellent optical and mechanical properties with Y2O3-MgO nanocomposites when using the conventional pressureless sintering process. Kear et al. [5] reported that a sample sintered in air at 1600 oC for 5 h showed a density level of less than 95 %. After re-heating sample to 1700 oC in air for 5 h, the density increased to nearly 100 %, but the grain size increased to approximately 10 μm. In general, therefore, densification of transparent nanocomposites is conducted using special

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sintering techniques, such as hot pressing (HP), hot isostatic pressing (HIP), and spark plasma sintering (SPS). The SPS technique has significant advantages over HP and HIP because it can complete the densification within a short sintering time owing to the

application of a high electric field (typical heating rate > 50 oC/min) [8]. A few studies of

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SPS of Y2O3-MgO nanocomposites have been reported. Xu et al. [9] and Jiang et al. [10]

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studied the influence of SPS and the post-sinter annealing conditions on the optical transmittance. Huang et al. [11] investigated the effect of SPS temperature on the

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mechanical properties. Xie et al. [4] evaluated the influence of moisture absorption on the manufacturing process and optical properties.

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According to previous works, for SPSed Y2O3-MgO nanocomposites, strong absorption peaks at around 7 μm are commonly observed in the IR spectra. These

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absorption peaks, which are assigned to asymmetrical and symmetrical stretching vibrations of the carboxylate group (O-C=O) [2, 3, 9, 12], can limit or negatively affect

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the military applications such as IR windows or domes. To suppress the formation of these absorption peaks, therefore, it is important to understand how the carboxylate group in the SPSed Y2O3-MgO nanocomposite originated. Most researchers have considered that not only unremoved residual carbonate after the calcination and annealing but also carbon contamination from the graphite die and paper contribute to these absorption peaks [1-4,

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9, 12]. To the best of our knowledge, however, detailed study on the origin of the carboxylate group in the SPSed Y2O3-MgO nanocomposites has not yet been conducted. In this work, the formation mechanism of the carboxylate group in the SPSed Y2O3MgO nanocomposite was studied in relation to carbon contamination during SPS processing.

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2. Experimental Commercial Y2O3 (> 99.9 %, < 50 nm, Guangdong Rare Earth Industry, China) and MgO (> 99.99 %, < 100 nm, Sukgyung AT, Republic of Korea) nanopowders were used

to prepare Y2O3-MgO composite nanopowder. Before the mixing process, the Y2O3 and

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MgO nanopowders were calcined at 1000 oC for 4 h in air. Then, the Y2O3 and MgO

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powders (volume ratio of 50:50) were mixed by planetary mill in an anhydrous alcohol using ZrO2 jar and balls at a rotation speed of 300 rpm for 12 h. After milling, the slurry

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was dried using a rotary evaporator, and sieved through a 200-mesh screen. Densification of the prepared composite nanopowder was carried out using a SPS

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system under vacuum (~ 6 Pa). The composite nanopowder was loaded into a graphite die with 22 mm inner diameter, and heated from room temperature to 1300 and 1400 oC

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at a heating rate of 50 oC/min under uniaxial pressure of 50 MPa. It was held for 5 min before turning off the power. The temperature was controlled by an optical pyrometer

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focused on the outer surface of the graphite die. Post-sinter annealing was conducted at 1100 oC for 20 h in air. The annealed samples were then mirror-polished on both surfaces for further characterization and investigation. The crystallite phase of the starting powder was characterized by X-ray diffractometer (XRD, D8 Discover, Bruker, USA) with Cu Kα radiation. The fracture surfaces of the

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sintered samples were examined by scanning electron microscopy (FE-SEM, Quanta 650, FEI, Netherlands). Fourier transform infrared spectroscopy (FTIR, Nicolet iS50, Thermo Scientific, USA) was used to measure the transmittance of the starting powder and sintered samples in a wavelength range of 2 ~ 10 μm. To analyze the carbon phase in the starting powder and the sintered samples, Raman spectroscopy (LabRAM HR UVVisible-NIR, Horiba Jobin Yvon, France) was performed. Energy-dispersive X-ray

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spectroscopy (EDS) was performed using a transmission electron microscope (TEM, Titan G2 60-300, FEI, Netherlands).

3. Results and discussion

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XRD pattern (Fig. 1(a)) shows that the starting powder is composed of cubic Y2O3

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(JCPDS No. 72-0927) and MgO (JCPDS No. 74-1225) phases. Other crystalline phases were not detected. SEM image (Fig. 1(b)) shows that the starting powder has micrometer-

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sized soft agglomerates composed of small crystals with an average size of 77.6 nm. Fig. 1(c) shows the FTIR spectrum of the starting powder. The spectrum shows an absorption

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peak around 2.9 μm, which is characteristic of the stretching vibration of the hydroxyl group (O-H) and is attributed to water absorbed from the air [1]. Strong absorption peaks

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at 7 μm, which are assigned to asymmetrical and symmetrical stretching vibrations of the

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carboxylate group (O-C=O) [1, 13], are also observed.

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Fig. 1. (a) XRD pattern, (b) SEM image, and (c) FTIR spectrum of the starting powder.

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SEM micrographs of the fracture surfaces of the fabricated Y2O3-MgO

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nanocomposites are shown in Fig. 2. The average grain sizes of the samples sintered at 1300 and 1400 oC are 150.8 and 226.2 nm, respectively. For the sample sintered at 1300 o

C, residual pores of < 80 nm are occasionally observed at the grain boundary junction

(yellow circles in Fig. 2(a)). On the other hand, it is difficult to observe residual pores in the sample sintered at 1400 oC, which indicates that densification is almost completed. This result agrees well with the measured relative densities (97.8 and 99.5 % for the -6-

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samples sintered at 1300 and 1400 oC, respectively).

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Fig. 2. SEM micrographs of fracture surfaces of Y2O3-MgO nanocomposites fabricated

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at sintering temperatures of (a) 1300 and (b) 1400 oC.

Fig. 3 shows IR transmittance spectra of the Y2O3-MgO nanocomposites fabricated

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at the sintering temperatures of 1300 and 1400 oC. In the IR transmittance spectra of the

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as-sintered samples, absorption peaks of the carboxylate group were not observed. It is considered that the carboxylate group pre-existing in the starting powder thermally decomposed into carbon phases during SPS processing and remained in the samples as glassy carbon and/or GO phases [14].

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Fig. 3. IR transmittance spectra of Y2O3-MgO nanocomposites fabricated at sintering

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temperatures of 1300 and 1400 oC.

Before annealing, it is noteworthy that an absorption peak at 4.9 μm, which is ascribed

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to CO gas [14], was clearly observed in the sample sintered at 1300 oC. This result can be

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explained by carbon contamination during SPS processing. At the high temperature of > 500 oC and vacuum in graphite atmosphere, CO2 and/or CO gas is present in the vacuum

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chamber. The CO gas was able to go deep into the sample through open pore channels and encapsulate itself in the closed pores as densification proceeded. With a further

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increase in the density, the CO gas precipitated as solid carbon phases (2CO(g)↑ → CO2(g)↑+ C(s)) on the pore surface due to the high pressure and/or low temperature [16].

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This is the reason that no absorption peak of CO gas was detected in the sample sintered at 1400 oC.

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Raman spectroscopy was conducted to confirm the transformation of the carboxylate

group in the starting powder and/or CO gas in the as-sintered sample into solid carbon phases. Fig. 4 shows the Raman spectra of the starting powder and of the Y2O3-MgO nanocomposites sintered at 1400 oC. In the range of 1000 ~ 2000 cm-1, the starting powder shows many Raman peaks, which can be assigned to various bonds between C and O/H/N.

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The as-sintered and annealed samples exhibited almost the same Raman characteristics in that wavenumber range. This suggests that impurities in the starting powder were not fully removed through SPS and annealing. Although these peaks make it difficult to find the D- and G-bands (~1350 and 1600 cm-1) of the solid carbon phases, the 2D-band (~ 2700 cm-1) which represents the degree of the stacking order of the carbon phase [14, 16] is detected in the as-sintered samples. In contrast, no 2D-band is detected in the starting

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powder or annealed sample. This result demonstrates that the carboxylate group in the starting powder and/or the CO gas trapped in the closed pores precipitated as solid carbon phase. Also, it is supposed that during annealing, the solid carbon phases react with

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oxygen and transform into CO2 gas.

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Fig. 4. Raman spectra of starting powder and Y2O3-MgO nanocomposites sintered at

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1400 oC.

In Fig. 3, it can be seen that annealing brings about two changes in the transmittance.

Firstly, the annealing in air significantly improved the IR transmittances of the samples, which is attributed to restoration of oxygen deficiency caused by the reducing environment in the SPS chamber [17]. After the annealing, the samples show good IR -9-

transmittance (79.7 and 82.6 % at 5 μm for the samples sintered at 1300 and 1400 oC, respectively), which is close to the theoretical value of 85 % [1]. The transmittance of the sample sintered at 1300 oC is slightly lower than that of the sample sintered at 1400 oC, which is mainly due to the residual pores. Secondly, strong absorption peaks of the carboxylate group at around 7 μm appeared again after annealing. To determine the relationship between carbon contamination and the formation of the carboxylate group, the influence of the annealing temperature on the IR transmittance of the samples sintered

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at 1300 oC was investigated, with results shown is Fig. 5. Below 800 oC of annealing temperature, there was no significant change in the IR transmittance, whereas raising the

annealing temperature to 1000 oC resulted in a decrease of the IR transmittance over entire

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IR wavelength range. Generally, for SPSed Y2O3-MgO nanocomposites, it is well known

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that as-sintered samples are gray in color, and become white after annealing in air. This indicates that oxygen deficiency caused by reducing environment in SPS chamber has

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been restored by annealing in air [17]. The cross-section of the sample annealed at 1000 C (inset in Fig. 5) shows that oxygen deficiency in the inner region of the sample has not

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yet been restored. This can cause a difference of the refractive index between the outer and inner regions of the sample [18-20], which is probably the reason why annealing at

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1000 oC resulted in a decrease of the IR transmittance.

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Fig. 5. IR transmittance spectra of Y2O3-MgO nanocomposites (sintered at 1300 oC) annealed at various annealing temperatures.

Also, after annealing at 1000 oC, absorption by CO gas decreased and absorption due to the carboxylate group appeared. With increasing of the annealing temperature to 1100 o

C, the absorption by the carboxylate group increased, while absorption by CO gas

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disappeared. Based on this, the formation mechanism of the carboxylate group in the SPSed Y2O3-MgO nanocomposite can be suggested to be as follows. Trapped CO gas and/or precipitated carbon phase transform into CO2 gas by annealing in air due to the

reaction with oxygen [16]. Then, the CO2 gas reacts with metal oxide phases (Y2O3 and

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MgO), resulting in the formation of carbonate phase [21, 22]. To confirm whether or not

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the carbonate phase exists at the grain boundary, EDS was conducted. Fig. 6 shows results of EDS mapping and line scan across the MgO and Y2O3 grains of the Y2O3-MgO

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nanocomposite sintered at 1400 oC. The dark grains are MgO phase and the bright grains are the Y2O3 phase. EDS results demonstrate that a small amount of carbonate phase is

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uniformly distributed through the entire region of the sample regardless of the grains and grain boundaries. This carbonate phase contributes considerably to the absorption peaks

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of the carboxylate group. In the case of SPSed MgAl2O4 spinel, it has been reported that carbon phases formed during SPS processing transform into high pressure CO/CO2 gases

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by annealing in air and lead to pore formation along the grain junctions [14]. In the case of the SPSed Y2O3-MgO nanocomposite, however, no pores were observed at the grain junctions after annealing. This result supports the idea of the formation of carbonate phase through annealing.

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Y2O3-MgO nanocomposite sintered at 1400 oC.

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Fig. 6. EDS mapping (top) and line scan (bottom) across the MgO and Y2O3 grains of the

With the increase of the annealing temperature to 1300 and 1400 oC, the IR

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transmittance dramatically decreases; this is attributed to the increase of the average grain size by grain growth. The sample annealed at 1400 oC shows the absorption peak at 4.24

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μm, which can be associated with CO2 in the environment [12, 14]. Also, the absorption by the carboxylate group slightly decreases with increasing annealing temperature, which

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can be explained by the small portion of CO2 gas that escapes from the sample due to the

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increased temperature, decreasing the formation of carbonate phase. This means that if carbonate phase is allowed to form in the SPSed Y2O3-MgO nanocomposites, it will be very difficult to remove the residual carbonate from the consolidated Y2O3-MgO nanocomposite.

4. Conclusions - 12 -

In this work, the formation mechanism of the carboxylate group in SPSed Y2O3-MgO nanocomposite was investigated in relation to carbon contamination during SPS processing. During SPS processing, carbon contamination was caused by evaporation of CO gas from graphite dies. The carboxylate group in the starting powder and/or the CO gas trapped in the closed pores precipitated as solid carbon phases. The trapped CO gas and/or precipitated carbon phase transform by annealing into CO2 gas. Then, CO2 gas

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reacts with metal oxide phases (Y2O3 and MgO), resulting in the formation of carbonate phase. This carbonate phase contributes to the absorption peaks of the carboxylate group. Also, if carbonate phase forms in SPSed Y2O3-MgO nanocomposites, it is very difficult to remove the residual carbonate from the consolidated Y2O3-MgO nanocomposite.

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Therefore, in order to remove the absorption peaks due to the carboxylate group, it is

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important to use a starting powder containing no carboxylate group and to suppress

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carbon contamination during SPS processing.

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Declarations of interest: none

Declaration of interests

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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgements This work was supported by DAPA and ADD.

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References [1] J. Wang, D. Chen, E. H. Jordan, M. Gell, Infrared-transparent Y2O3-MgO nanocomposites using sol-gel combustion synthesized powder, J. Am. Ceram. Soc. 93 (2010) 3535-3538. [2] S. Xu, J. Li, C. Li, Y. Pan, J. Guo, Hot pressing of infrared-transparent Y2O3-MgO nanocomposites using sol-gel combustion synthesized powders, J. Am. Ceram. Soc.

ro of

98 (2015) 1019-1026. [3] S. Xu, J. Li, C. Li, Y. Pan, J. Guo, Infrared-transparent Y2O3-MgO nanocomposites fabricated by the glucose sol-gel combustion and hot-pressing technique, J. Am. Ceram. Soc. 98 (2015) 2796-2802.

-p

[4] J. Xie, X. Mao, X. Li, B. Jiang, L. Zhang, Influence of moisture absorption on the

re

synthesis and properties of Y2O3-MgO nanocomposites, Ceram. Int. 43 (2017) 40-44. [5] B. H. Kear, R. Sadangi, V. Shukla, T. Stefanik, R. Gentilman, Submicron-grained

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transparent yttria composites, Proc. of SPIE 5786 (2005) 227-233. [6] J. Wang, L. Zhang, D. Chen, E. H. Jordan, M. Gell, Y2O3-MgO-ZrO2 infrared

na

transparent ceramic nanocomposites, J. Am. Ceram. Soc. 95 (2012) 1033-1037. [7] D. C. Harris, L. R. Cambrea, L. F. Johnson, R. T. Seaver, M. Baronowski, R.

ur

Gentilman, C. S. Nordahl, T. Gattuso, S. Silberstein, P. Rogan, T. Hartnett, B. Zelinski, W. Sunne, E. Fest, W. H. Poisl, C. B. Willingham, G. Turri, C. Warren, M. Bass, D.

Jo

E. Zelmon, S. M. Goodrich, Properties of an infrared-transparent MgO:Y2O3 nanocomposite, J. Am. Ceram. Soc. 96 (2013) 3828-3835.

[8] K. Morita, B. -N. Kim, K. Hiraga, H. Yoshida, Fabrication of transparent MgAl2O3 spinel polycrystal by spark plasma sintering processing, Scr. Mater. 58 (2008) 11141117.

- 15 -

[9] S. Xu, J. Li, H. Kou, Y. Shi, Y. Pan, J. Guo, Spark plasma sintering of Y2O3-MgO composite nanopowder synthesized by the esterification sol-gel route, Ceram. Int. 41 (2015) 3312-3317. [10] D. T. Jiang, A. K. Mukherjee, Spark plasma sintering of an infrared-transparent Y2O3-MgO nanocomposite. J. Am. Ceram. Soc. 93 (2010) 769-773.

ro of

[11] L. Huang, W. Yao, J. Liu, A. K. Mukherjee, J. M. Schoenung, Spark plasma sintering and mechanical behavior of magnesia-yttria (50:50 vol.%) nanocomposites, Scr. Mater. 75 (2014) 18-21.

[12] H. J. Ma, W. K. Jung, C. Baek, D. K. Kim, Influence of microstructure control on and

mechanical

properties

of

infrared

transparent

-p

optical

Y2O3-MgO

re

nanocomposite, J. Eur. Ceram. Soc. 37 (2017) 4902-4911.

[13] R. K. Pati, J. C. Ray, P. Pramanik, Synthesis of nanocrystalline alumina powder using

lP

triethanolamine, J. Am. Ceram. Soc. 84 (2001) 2849-2852. [14] K. Morita, B. -N. Kim, H. Yoshida, K. Hiraga, Y. Sakka, Distribution of carbon

na

contamination in MgAl2O4 spinel occurring during spark-plasma-sintering (SPS) processing: I – Effect of heating rate and post-annealing, J. Eur. Ceram. Soc. 38

ur

(2018) 2588-2595.

[15] K. Morita, B. -N. Kim, H. Yoshida, K. Hiraga, Y. Sakka, Spectroscopic study of the

Jo

discoloration of transparent MgAl2O4 spinel fabricated by spark-plasma-sintering (SPS) processing, Acta Mater. 84 (2015) 9-19.

[16] K. Morita, B. -N. Kim, H. Yoshida, K. Hiraga, Y. Sakka, Distribution of carbon contamination in MgAl2O4 spinel occurring during spark-plasma-sintering (SPS) processing: I – Effect of SPS and loading temperatures, J. Eur. Ceram. Soc. 38 (2018)

- 16 -

2596-2604. [17] D. T. Jiang, A. K. Mukherjee, Synthesis of Y2O3-MgO nanopowder and infrared transmission of the sintered nanocomposite, Proc. of SPIE 7030 (2008) 703007. [18] M. Bellotto, A. Caridi, E. Cereda, G. Gabetta, M. Scagliotti, G. M. B. Marcazzan, Influence of the oxygen stoichiometry on the structural and optical properties of reactively evaporated ZrOx films, Appl. Phys. Lett. 63 (1993) 2056-2058.

ro of

[19] J. -Y. Choi, C. -H. Choi, K. -H. Cho, T. -G. Seong, S. Nahm, C. -Y. Kang, S. -J. Yoon, J. -H. Kim, Effect of oxygen vacancy and Mn-doping on electrical properties of Bi4Ti3O12 thin film grown by pulsed laser deposition, Acta Mater. 57 (2009) 24542460.

-p

[20] L. Dong, R. Jia, B. Xin, B. Peng, Y. Zhang, Effects of oxygen vacancies on the

re

structural and optical properties of β-Ga2O3, Sci. Rep. 7 (2017) 40160. [21] E. -M. Köck, M. Kogler, T. Bielz, B. Klötzer, S. Penner, In-situ FTIR spectroscopic

lP

study of CO2 and CO adsorption on Y2O3, ZrO2, and yttria-stabilized ZrO2, J. Phys. Chem. C 117 (2013) 17666-17673.

na

[22] S. -I. Jo, Y. -I. An, K. -Y. Kim, S. -Y. Choi, J. -S. Kwak, K. -R. Oh, Y. -U. Kwon, Mechanism of absorption and desorption of CO2 by molten NaNO3-promoted MgO,

Jo

ur

Phys. Chem. Chem. Phys. 19 (2017) 6224-6232.

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Figure captions Fig. 1. (a) XRD pattern, (b) SEM image, and (c) FTIR spectrum of the starting powder. Fig. 2. SEM micrographs of fracture surfaces of Y2O3-MgO nanocomposites fabricated at sintering temperatures of (a) 1300 and (b) 1400 oC. Fig. 3. IR transmittance spectra of Y2O3-MgO nanocomposites fabricated at sintering temperatures of 1300 and 1400 oC. Fig. 4. Raman spectra of starting powder and Y2O3-MgO nanocomposites sintered at

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1400 oC.

Fig. 5. IR transmittance spectra of Y2O3-MgO nanocomposites (sintered at 1300 oC) annealed at various annealing temperatures.

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Y2O3-MgO nanocomposite sintered at 1400 oC

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Fig. 6. EDS mapping (top) and line scan (bottom) across the MgO and Y2O3 grains of the

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