Synthesis and characterization of $Y_2O_3-MgO$ nanocomposite for infrared transparent ceramics

Abstract

Recently, transparent ceramics are widely applied for a variety of field such as laser ceramics, armor ceramics, scintillators, and optoelectronic devices. Especially, infrared transparent materials have been researched even for applying in the next generation infrared-guided missiles, infrared radiation thermometer and missile domes. These materials are occasionally used in severe external environment. Accordingly, the infrared transparent ceramics should meet superior mechanical, chemical, optical properties and thermal shock resistance. $Y_2O_3$ is the representative material of transparent ceramics because it has high transmittance in the wide wavelength region from ultraviolet (UV) to infrared-ray. On the other hands, because of the weak mechanical strength, it is not suitable for application in harsh environment. To improve this intrinsic shortcomings of $Y_2O_3$, $Y_2O_3-MgO$ nanocomposite material has been researched since 2005. By the Zener pinning effect, two phases are able to inhibit a grain growth effectually at the grain boundaries during the consolidation process. As grain size decreases, mechanical properties of nanocomposite can be improved and high transmittance in mid-IR region can be maintained, though the transmission in visible and UV region is lost. However, with decreasing grain size transparent region could be extended to visible-ray due to negligible grain boundaries scattering which resulted from the large difference of refractive index. Therefore, it is important to fabricate the fine grains with high relative density. The size and distribution of synthesized powder play a significant role in determining the sintering ability and material properties. The fine and homogenous powder enhances the sinterability and accelerates densification rate. Although a few researchers have investigated synthesis and sintering of this material, concurrently achieving fine domain size and full density are currently difficult as ever. In this thesis, I investigated the synthesis and characterization of $Y_2O_3-MgO$ nanocomposite. Firstly, to obtain the fine and highly homogenous nanoparticles, Glycine Nitrate Process (GNP) method was used under the various synthesis conditions. The ratio of nitrate-to-glycine, as the fuel and the complex agent respectively, was adjusted from 0.25 to 1.5 in order to diminish both the initial particle size and tendency of agglomeration whereby optimized condition was defined to enhance the sinterability and characteristics. By decreasing the ratio of nitrate-to-glycine to 0.5, the crystallite size decreased to 11.70 nm with cubic $Y_2O_3$ and MgO phase. The synthesized powder was sintered by hot-press method to densify the $Y_2O_3-MgO$ nanocomposite while minimizing the unintended grain coarsening during the sintering process. The maximum hardness of 10.59 GPa was achieved at the 0.75 ratio. The transmittance was close to the theoretical value at the mid-infrared region. In the glycine deficient system, large and clustered grains with many large pores led to low transmittance and hardness. The hardness dropped to 8.0 GPa due to the residual pores. In addition, sintering temperature was changed from 1150 to $1350^\circ C$ without changing other variables. As the sintering temperature increased, the relative density increased with removing the residual pores. The high relative density led to the improved transmittance at 82.3 %. However, the large grain size did not result in the high hardness at the high temperature. Lastly, dispersant (EPE8400) and surfactant (PEG300) were added in the precursor to modify the size and morphology of synthesized nanoparticle during the combustion reaction. The dispersants including organic constitute affected the particle coalescence and agglomeration. In addition, PEG affected the morphology of particles to become rod-like shape. Synthesized particles with dispersant and surfactant showed outstanding sinterability, which increased the transmittance closed to the theoretical value of 82.3 %.