Materials & Structures Nanocomposites Enhance Window Durability and Transparency
To improve the strength and transparency of windows used in our products, Raytheon is spearheading an organizationally diverse project to develop a revolutionary new class of highly durable infrared materials. These new materials, called nanocomposite optical ceramics (NCOCs), contain two or more distinct phases that have been combined at the nano scale.1 This project, which is funded by DARPA and monitored by the Office of Naval Research, has a practical goal of replacing sapphire as the material of choice for windows in systems operating in the tactically important mid-wave infrared wavelength range of three to fiive micrometers.
Currently, the transparency of single-phase window materials in the MWIR must be traded against their mechanical durability. The stronger atomic bonds needed for improved strength and hardness also absorb at these wavelengths and limit transparency. Sapphire (single-crystal aluminum oxide [Al2O3]) is the most durable MWIR missile dome material, but it also has the most limited in-band transmittance. Fully transmitting materials such as yttrium oxide (Y2O3) and magnesium oxide (MgO) have much lower strengths, while aluminum oxynitride (Al23O27N5) and magnesium aluminate spinel (MgAl2O4) exhibit intermediate durability and transmittance.
Raytheon is breaking this performance stalemate by creating multiphase, polycrystalline ceramic materials having grain sizes in the nanometer range. By mixing two or more dissimilar compounds to make a multiphase material, we prevent the grain growth that normally occurs during the high-temperature heat treatment needed to eliminate all porosity. Reducing the size of the grains in the material increases its strength and hardness by reducing flaw sizes. Figure 1 shows an electron microscope image of a Y2O3-MgO optical nanocomposite. Note the small size and uniform distribution of the two phases.
Normally, multiphase composites appear opaque because differences in the refractive index between grains scatter the electromagnetic radiation. However, when the size of the phase domains is kept substantially smaller than the wavelength (less than about λ/20), light-scattering is eliminated and transparency is restored. In Figure 2, note that the nanocomposite becomes transparent at the specified wavelength.
Raytheon's approach to fabricating optical nanocomposite MWIR window materials combines newly available nanopowders with aspects of traditional ceramic processing, supplemented by state-of-the-art densification techniques. Nanopowders are produced from carefully controlled reactions of chemical precursors in a flame, plasma torch or liquid bath. Ideal nanoparticles are spherical in shape, less than 50 nanometers in diameter, loosely agglomerated and very pure. The nanopowders are then pressed together in a die or are cast in a mold to form a "green" (un-fired) part of the desired shape, such as a circular disk or a hemispherical dome. The green part — which may contain as much as 50 volume percent void space (porosity) — is then sintered (made dense) by firing at an elevated temperature, which causes individual atoms to diffuse to pores and fill them. In some cases, high pressures and/or electric fields are employed to enhance densification and eliminate porosity. During densification, the part maintains its original shape but shrinks in size by as much as 20 percent. With optimum processing, all porosity is removed, the final grain size is kept under 100 nanometers, and MWIR scattering in the NCOC is eliminated.
The NCOC development team is led by Raytheon Integrated Defense Systems (IDS) and Missile Systems (RMS), and includes leading researchers from Rutgers University, the University of California at Davis, the University of Connecticut and three small companies with unique ceramics capabilities. IDS provides overall project leadership and years of experience in materials development and processing. RMS represents customer needs and also models and characterizes the optical and thermal performance of NCOCs.
By covering the development cycle from need, through innovation, to production, the Raytheon NCOC program is poised to advance the technology and manufacturing readiness levels of this new class of materials and will produce hemispherical domes within a few years. The development of NCOCs will, for the first time in several decades, dramatically expand the collection of materials available for use and may well end the need to trade off optical performance for mechanical durability in MWIR windows applications.
Normally, multiphase composites appear opaque because differences in the refractive index between grains scatter the electromagnetic radiation. However, when the size of the phase domains is kept substantially smaller than the wavelength (less than about λ/20), light-scattering is eliminated and transparency is restored. In Figure 2, note that the nanocomposite becomes transparent at the specified wavelength.
The NCOC development team is led by Raytheon Integrated Defense Systems (IDS) and Missile Systems (RMS), and includes leading researchers from Rutgers University, the University of California at Davis, the University of Connecticut and three small companies with unique ceramics capabilities. IDS provides overall project leadership and years of experience in materials development and processing. RMS represents customer needs and also models and characterizes the optical and thermal performance of NCOCs.
