Tech_Issue 1 2009_0127_Final:TechToday_012709 ... - Raytheon

onTechnology
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
[Al 2 O 3 ]) 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 (Y 2 O 3 )
and magnesium oxide (MgO) have much
lower strengths, while aluminum oxynitride
(Al 23 O 27 N 5 ) and magnesium aluminate
spinel (MgAl 2 O 4 ) 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 Y 2 O 3 -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
34 2009 ISSUE 1 RAYTHEON TECHNOLOGY TODAY
MATERIALS & STRUCTURES
Figure 1. Scanning electron microscope
image of Raytheon’s recently developed
Y 2 O 3 -MgO optical nanocomposite. In this
back-scatter image, the light- and dark-colored
grains are Y 2 O 3 and MgO, respectively.
With an average grain size of approximately
100 nanometers, this new MWIR
optical material is highly transparent to
radiation with wavelengths of 2-6 micrometers
and is more than twice as strong as
either single-phase Y 2 O 3 or MgO.
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
Figure 2. Raytheon’s optical nanocomposites
appear white and opaque in the visible
spectrum (top), but are transparent in the
mid-wave infrared band (bottom) where
the wavelength is more than 20X larger
than the 100 nanometer grain size.
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.
Rick Gentilman
richard_gentilman@raytheon.com
Contributors: Scott Nordahl, Brian Zelinski
1Phases defined as a discrete part of a material that has a
specific composition and crystalline structure.