Visible and IR windows require a combination of high optical transparency and superior thermal and mechanical properties. Materials, fabrication and characterization of transparent ceramics for visible/IR windows are discussed in this review. The transparent polycrystalline Y2O3, Y2O3-MgO nanocomposites and MgAl2O4 spinel ceramics are fabricated by advanced ceramic processing and the use of special sintering technologies. Ceramic processing conditions for achieveing fully densified transparent ceramics are strongly dependent on the initial powder characteristics. In addition, appropriate use of sintering technologies, including vacuum sintering, hot-pressing and spark plasama sintering methods, results in outstanding thermal and mechanical properties as well as high optical transparency of the final products. Specifically, the elimination of light scattering factors, including residual pores, second phases and grain boundaries, is a key technique for improving the characteristics of the transparent ceramics. This paper discusses the current research issues related to synthesis methods and sintering processes for yttria-based transparent ceramics and MgAl2O4 spinel.

Abstract
 1. Introduction
 2. Ceramic Processing
  2.1 Powder preparation and processing
  2.2 Sintering technologies
 3. Materials and Characterization
  3.1 Y2O3 optical ceramics
  3.2 Y2O3-MgO nanocomposite
  3.3 MgAl2O4
 4. Summary and Conclusions
 References

• Wook Ki Jung(Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST))
• Ho Jin Ma(Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST))
• Ha-Neul Kim(Engineering Ceramics Research Group, Korea Institute of Materials Science) Corresponding author
• Do Kyung Kim(Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST)) Corresponding author