Transparent ceramics have extensive applications in military and civilian industries because of its intrinsic optical, mechanical properties, and the market size is expected to increase further. Also infrared-transparent ceramics recently drew much attention as infrared window or dome materials due to the outbreak of modern local war. When this infrared-transparent material is applied to a high-speed missiles or an aircraft, factors such as the mechanical damage from friction with air, and the heat arising from friction or an aircraft can severely deteriorate the optical properties and function that the material protects the IR sensor. For the material to exhibit reliable performance in such harsh environments, high mechanical properties, thermal shock resistance, and low emissivity are required in addition to high transmittance. Infrared-transparent ceramics such as Y2O3, MgO, Y3Al5O12 (YAG), MgAl2O4 (Spinel), AlON have cubic structures, so they are optically isotropic. Thus they have high transmittance. Among them, Y2O3 has longer wavelength cut off than other ceramics mentioned above, and has transmittance above 80 % in the infrared wavelength range. Meanwhile Y2O3 and MgO do not mix each other below 2110 ℃ according to Y2O3-MgO binary phase diagram. So when Y2O3 and MgO are sintered together in the volume ratio 50:50, they inhibit the mutual grain growth maximally. The difference in refractive indices is large between Y2O3 (1.843) and MgO (1.642) at the 4.85 μm, so it is important to minimize the light scattering through small grain size. According to a study, Y2O3-MgO nanocomposites showed transmittance above 80 % in the mid-infrared wavelength range when they have mean grain size of 200 nm. Y2O3-MgO nanocomposites have higher mechanical properties and thermal shock resistance, and lower emissivity than most of mid-infrared transparent ceramics. So many studies about Y2O3-MgO nanocomposites were done.Many efforts were made to synthesize the Y2O3-MgO composite nanopowder: flame spray pyrolysis, wet-precipitation, hydrothermal method, and sol-gel method. Especially in the sol-gel method, the processing time is short, cost is low, and it is easy to control composition causing fine and homogeneous distribution in the molecular level. So sol-gel method is widely used.To fabricate synthesized Y2O3-MgO composite nanopowder, many sintering methods were taken: conventional sintering, microwave sintering, hot pressing, hot isostatic pressing, and spark plasma sintering. Especially in the spark plasma sintering grain size is largely reduced because sintering time is very short unlike other sintering methods, and relative density close 100 % can be achieved. However the carbon contamination coming graphite mold, sheet, punch which are used in the spark plasma sintering, deteriorate optical propety because the carbon work as aborption factor. Until now, studies improve the transmittance through post-annealing for long hours from which carbon atoms can be deleted. Two-step sintering is a promising method which is used to realize high relative density and small grain size. In a study, one-step or two-step conventional sintering method was compared. When holding in the second low temperature after initial higher temperature in the two-step sintering condition, the grain growth stopped and relative density increased. Mean grain size of 200 nm and relative density close to 100 % were achieved, whereas the one-step sintering brought about mean grain size of 400 nm. Until now there is no example that Y2O3-MgO composite nanopowder was spark plasma sintered through two-step.In this work, cubic phase 50:50 vol. % Y2O3-MgO composite nanopowder which is fine and homogeneous was successfully synthesized through sol-gel method. The effect of calcination temperature on the nanoparticle growth and occurrence of absorption peaks in the range of infrared wavelength was analyzed. The synthesized nanopowder was spark plasma sintered through one-step and two-step methods. All the bulks showed relative densities close to 100 %. After sintering the effect of remaining absorption factors in the nanopowder on the transmittance of bulks was analyzed. In the one-step sintering condition through elements analysis, it was clarified that tungsten punch inhibited carbon contamination 30 % compared to graphite punch. In the condition of one-step sintering using tungsten puch the mean grain size was 175 nm and in the IR-range maximum transmittance of 81 % was achieved without post-annealing. This result was more higher than the transmittance of previous studies using graphite punch without post-annealing. Also in a variety of two-step sintering conditions using tungsten punch, the grain sizes were 113-129 nm. The grain growth was effectively inhibited as much as 26-35 % compared to one-step sintering. Especially in the two-step sintering condition, mean grain size of 129 nm and realtive density of 100 % were achieved. Without post-annealing transmittance above 80 % was maintained in the whole infrared wavelength range, maximally increasing to 86 %. This result was more higher than the transmittance measured by one-step sintering in this work. Comparing to previous studies, this transmittance was much more higher than them without post-annealing. The small grain size caused large grain boundary areas. So in the same two-step sintering condition the hardness increased as much as 15 % compared to one-step sintering condition, resulting in 12.26±0.41 GPa. By largely reducing grain size, this result was more higher than that of most of the present studies which sintered Y2O3-MgO nanocomposite by one-step. Totally, in the two-step spark plasma sintering of Y2O3-MgO nanocomposite using BN-coated tungsten punch the infrared transmittance and hardness was more higher compared to the values reported by previous studies. Also the mean grain size of Y2O3-MgO nanocomposite was much more small than Y2O3 and MgO, respectively. Through this, we figured out that Y2O3 and MgO inhibit the mutual grain growth below 2110 ℃, not reacting each other. And considering both mid-infrared transmittance and hardness, Y2O3-MgO nanocomposite showed superiority than Y2O3, MgO and MgAl2O4.
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