Nano-Materials for Aerospace and Security Applications
L. Boehm
Israel Aircraft Industries
Nanotechnology is "the next big thing". As this phrase becomes common in business and scientific circles throughout the world, the very perception causes concern about the implications and issues surrounding the entire emergence of nanotechnology as well as its impact on so many things affecting our daily lives, and it will affect virtually all aspects of human experience.
Nanotechnology, is more than just a new buzzword;. In fact this phenomenon is best realized in the National Science and Technology Council's recently published "National Nanotechnology Initiative: Research and Development Supporting the Next Industrial Revolution". In this initiative several areas are identified as having the potential to realize significant economic, governmental and societal impact. The areas which are drawing the most attention are the closer to the market:
1. Nano-Crystalline Materials – Nano-Structured materials by design
2. Manufacturing at the Nanoscale – Carbon-nano-tubes (CNT)
3. Chemical-Biological-Radiological-Explosive Detection and Protection
4. Nano-Electronics, -Photonics, and -Magnetics
5. Healthcare, Therapeutics, and Diagnostics
6. Efficient Energy Conversion and Storage – Fuel cells
7. Microcraft and Robotics
Nano-crystalline materials are materials possessing grain sizes on the order of a billionth of a meter. They manifest extremely fascinating and useful properties, which can be exploited for a variety of structural and non-structural applications.
All materials are composed of grains, which in turn comprise many atoms. These grains are usually invisible to the naked eye, depending on their size. Conventional materials have grains varying in size anywhere from 100’s of microns (µm) to millimeters (mm). A micron (µm) is a micrometer or a millionth (10-6) of a meter. An average human hair is about 100 µm in diameter. A nanometer (nm) is even smaller a dimension than a µm, and is a billionth (10-9) of a meter. A nano-crystalline material has grains on the order of 1-100 nm. The average size of an atom is on the order of 1 to 2 angstroms (Å) in radius. 1 nanometer comprises 10 Å, and hence in one nm, there may be 3-5 atoms, depending on the atomic radii. Nano-crystalline materials are exceptionally strong, hard, ductile at high temperatures, wear-resistant, erosion-resistant, corrosion-resistant, and chemically very active. Nano-crystalline materials, or nano-materials, are also much more formable than their conventional, commercially available counterparts.
There are five widely known methods to produce nano-materials, and they are as follows:
· Sol-gel synthesis,
· Inert gas condensation,
· Mechanical alloying or high-energy ball milling,
· Plasma synthesis, and
· Electrodeposition.
All these processes synthesize nano-materials to varying degrees of commercially-viable quantities.
Since nano-materials possess unique, beneficial chemical, physical, and mechanical properties, they can be used for a wide spectrum of aerospace, defense and security technologies and applications.
Aerospace components with enhanced performance characteristics
Drivers for the aerospace industry for exploiting new technologies include:
· Increased safety
· Reduced emissions
· Reduced noise
· Increased capacity
· Increased mobility
Due to the risks involved in flying, aircraft manufacturers strive to make the aerospace components stronger, tougher, and last longer. One of the key properties required of the aircraft components is the fatigue strength, which decreases with the component’s age. By making the components out of stronger materials, the life of the aircraft is greatly increased. The fatigue strength increases with a reduction in the grain size of the material.Nano-materials provide such a significant reduction in the grain size over conventional materials so that the fatigue life is increased by an average of 200-300%. Composite materials with improved fatigue life, damping properties and higher damage tolerance properties due to CNT inclusions, is vastly investigated in the last years.
Nanotubes. Are described as ‘the most important material in nanotechnology today’, These materials have a remarkable tensile strength. Indeed, taking current technical barriers into account, nanotube-based material is anticipated to become 50–100 times stronger than steel at one-sixth of the weight. This development would dwarf the improvements that carbon fibres brought to composites.
A lot of effort is also invested in developing functionalized-carbon-nanotubes (FCNT), as it will provide materials that can enable new technologies in aircraft platforms performance, ballistic protection and conductive fibres.
Also high performance nano-composite materials which are combination of polymers, metals and ceramics, can be used for tribological coatings of aircraft platforms operated at higher temperatures.
Furthermore, components made of nano-structured materials that are perhaps 100x lighter than conventional materials of equivalent strength are possible, so an aircrafts can fly faster and more efficiently (for the same amount of aviation fuel).
Fig. 1 shows the possibility of reducing the weight of aircraft components using composite materials reinforced with carbon-nano-tubes (CNT).
Fig 1. Nanotube-Reinforced Polymer (CNTFRP) and Nanotube-Reinforced Aluminum (CNT/Al) Composites compared to an advanced carbon fiber reinforced polymer (IM7 CFRP) composite:
Longer-lasting satellites
Satellites are being used for both defense and civilian applications. These satellites utilize thruster rockets to remain in or change their orbits due to a variety of factors including the influence of gravitational forces exerted by the earth. Hence, these satellites are repositioned using these thrusters. The life of these satellites, to a large extent, is determined by the amount of fuel they can carry on board. In fact, more than 1/3 of the fuel carried aboard by the satellites is wasted by these repositioning thrusters due to incomplete and inefficient combustion of the fuel, such as hydrazine. The reason for the incomplete and inefficient combustion is that the onboard ignitors wear out quickly and cease to perform effectively. Nano-materials, such as nano-crsytalline tungsten-titanium diboride-copper composite, are potential candidates for enhancing these ignitors’ life and performance characteristics.
In spacecrafts, elevated-temperature strength of the material, is crucial for components such as rocket engines, thrusters, and vectoring nozzles. High performance nano-materials can be used for these components as well as for special coatings.
Also, embedding nanoscale electromechanical system components into earth-orbiting satellites, planetary probes, and piloted vehicles potentially could reduce the cost of future space programs. The miniaturised sensing and robotic systems would enhance exploration capabilities at significantly reduced cost.
The vision of NASA for the use on nano-materials for different platforms in presented in Fig. 2
Materials / -Single-wallednanotube fibers / -Nanotube
composites / - Integral
thermal/shape
control / -Smart “skin” materials / -Biomimetric
material
systems
Electronics/
computing / -Low-Power CNT
electronic
components / -Molecular
computing/data
storage / -Fault/radiation
tolerant
electronics / -Nano electronic “brain” for space Exploration / -Biological
computing
Sensors, s/c
components / -In-space
nanoprobes / -Nano flight
system
components / -Quantum
navigation
sensors / -Integrated
nanosensor
systems / -NIEMS flight
systems @ 1mW
Nano-material impact on Security - High-sensitivity sensors
The advance of civil technologies, particularly in the area of IT, electronics and communications, has particular relevance to the design of defence and security systems. In particular, nano-materials will impact on all critical aspects of maintaining a technologically superior capability in these areas . The key drivers of which are:
· Biological and chemical threat detection - nanoBio sensors
· High performance information technology/remote control
· Military uniforms with enhanced features including protection and self-healing
· New vaccines and medical treatments
· Minimising civilian casualties through the use of smart sensors
Sensors employ their sensitivity to the changes in various parameters they are designed to measure. The measured parameters include electrical resistivity, chemical activity, magnetic permeability, thermal conductivity, and capacitance. All of these parameters depend greatly on the microstructure (grain size) of the materials employed in the sensors. A change in the sensor’s environment is manifested by the sensor material’s chemical, physical, or mechanical characteristics, which is exploited for detection. For instance, a carbon monoxide sensor made of zirconium oxide (zirconia) uses its chemical stability to detect the presence of carbon monoxide. In the event of carbon monoxide’s presence, the oxygen atoms in zirconium oxide react with the carbon in carbon monoxide to partially reduce zirconium oxide. This reaction triggers a change in the sensor’s characteristics, such as conductivity (or resistivity) and capacitance. The rate and the extent of this reaction are greatly increased by a decrease in the grain size. Hence, sensors made nano-crystalline materials are extremely sensitive to the change in their environment. Typical applications for sensors made out of nano-crystalline materials are smoke detectors, engine performance sensor, etc.
Another family of nano-materials for security applications are the structural multifunctional nano-materials. The requirements from such materials are: lightweight; mechanically durable; sensing/actuation; radiation protection; electrical conductivity; thermal conductivity. A multifunctional material based on single wall nano-tube (SWNT) combined with polymer nano-composite, was developed at NASA for sensing temperature and pressure. Also a CNT-FET material was developed for sensing gases, such as HCl, NO2 , benzene, acetone and other hazard gasses for environmental monitoring and biothreat detection.
Recently, it was published that Fraunhofer Institutes in Germany developed an Electro-Chemical sensor based on nano-wires (nano-grass) of Au/Sn. This material and its characteristic performance, is shown in Fig. 3
”Nano-grass” for electrochemical Sensors
Nano-wires lead to strongly enlarged surface
To a strongly increased current
Improved Signal/Noise-Ratio
Þ increased Sensitivity
Þ shortened response time
Fig 3. Electro-Chemical sensor, Michael Krausa, Fraunhofer IZT (2005).
Summary
Nanotechnology is primarily about making things. For this reason, most of the activity of R&D is focused on ‘nano-materials’: novel materials whose molecular structure has been engineered at the nanometre scale. It was stated by several people that material science and technology is fundamental to a majority of the applications of nanotechnology. The materials that follow involve either bulk production of conventional compounds that are much smaller (and hence exhibit different properties) or new nano-materials, such as fullerenes and nano-tubes.
The market’s range of nano-materials are considerable.Indeed, it has been estimated that, aided by nanotechnology, novel materials and processes can be expected to have a market impact of over US$340 billion within a decade.
The major nano-materials currently in research and development for Aerospace and Security
· Carbon nano-tubes (CNT): two types of nano-tubes, the single wall carbon nano-tube (SWNT) and the multilayer nano-tubes. The main applications are for space and aircraft manufacture.
· Nano-composite: composite materials are combination of polymers, metals and ceramics. These materials with multifunctional behaviour, might be used for special coatings and high-temperature applications.
· Nano-wires: wires of various metals used as gas sensors, environmental monitoring and nano-actuators.
· Quantum wells: ultra thin layers, a few nanometers thick of semiconductor materials. The main applications are for quantum well lasers for telecommunication and optics.
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