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M. Meyyappan Nanotechnology: Aerospace Applications

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Nanotechnology Areas of Interest to Aerospace Community • High Strength Composites (PMCs, CMCs, MMCs…) • Nanostructured materials: nanoparticles, powders, nanotubes… • Multifunctional materials, self-healing materials • Sensors (physical, chemical, bio…) • Nanoelectromechanical systems • Batteries, fuel cells, power systems • Thermal barrier and wear-resistant coatings • Avionics, satellite, communication and radar technologies • System Integration (nano-micro-macro) • Bottom-up assembly, impact of manufacturing

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Nano-Reinforced Composites • Processing them into various matrices follow earlier composite developments such as - Polymer compounding - Producing filled polymers - Assembly of laminate composites - Polymerizing rigid rod polymers - - • Purpose - Replace existing materials where properties can be superior - Applications where traditionally composites were not a candidate

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• Nanotechnology provides new opportunities for radical changes in composite functionality • Major benefit is to reach percolation threshold at low volumes (< 1%) when mixing nanoparticles in a host matrix • Functionalities can be added when we control the orientation of the nanoscale reinforcement. Benefits of Nanotechnology in Composite Development

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Multifunctionality in Materials • This always implies “structure +” since in most cases the major function of a structure is to carry load or provide shape. Additional functions can be: • Actuation controlling position, shape or load • Electrical either insulate or conduct • Thermal either insulate or conduct • Health monitor, control • Stealth managing electromagnetic or visible signature • Self-healing repair localized damage • Sensing physical, chemical variables NRC Report, 2003

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Multifunctional Materials with Sensing Capability • Building in additional functionalities into load-bearing structures is one key example: - Sensing function * Strain * Pressure * Temperature * Chemical change * Contaminant presence • Miniaturized sensors can be embedded in a distributed fashion to add “smartness” or multifunctionality. This approach is ‘pre-nano’ era. • Nanotechnology, in contrast, is expected to help in assembling materials with such functional capabilities

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Examples of Multifunctional Materials • Possible, in principle, to design any number of composites with multiple levels of functionality (3, 4, 5…) by using both multifunctional matrices and multifunctional reinforcement additives - Add a capsule into the matrix that contains a nanomaterial sensitive to thermal, mechanical, electrical stress; when this breaks, would indicate the area of damage - Another capsule can contain a healant - Microcellular structural foam in the matrix may be radar-absorbing, conducting or light-emitting - Photovoltaic military uniform also containing Kevlar for protection generate power during sunlight for charging the batteries of various devices in the soldier-gear NRC Report, 2003

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Candidates for Multifunctional Composites • Carbon nanotubes, nanofibers • Polymer clay nanocomposites • Polymer cross-linked aerogels • Biomimetic hybrids Expectations: - ‘Designer’ properties, programmable materials - High strength, low weight - Low failure rates - Reduced life cycle costs

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Example of Self-Healing Material ‘Self-healing plastic’ by Prof. Scott White (U. of Illinois) Nature (Feb. 15, 2001) • Plastic components break because of mechanical or thermal fatigue. Small cracks  large cracks  catastrophic failure. ‘Self-healing’ is a way of repairing these cracks without human intervention. • Self-healing plastics have small capsules that release a healing agent when a crack forms. The agent travels to the crack through capillaries similar to blood flow to a wound. • Polymerization is initiated when the agent comes into contact with a catalyst embedded in the plastic. The chemical reaction forms a polymer to repair the broken edges of the plastic. New bond is complete in an hour at room temperature.

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Preparation of Nanoparticles • Plasma processing - Both thermal (plasma arc, plasma torch, plasma spray) and low temperature (cold) plasma are used • Chemical Vapor Deposition - Either on a substrate or in the gas phase (for bulk production) - Metallic oxides and carbides • Electrodeposition • Sol-gel processing • Ball mill or grinding (old fashioned top-down approach) Key Issue: Agglomoration

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Desirable Attributes of Nanoparticles Tremendous increase in surface-to-volume ratio • Increase in solubility • Increase in reactivity • Possible increase in hardness (ex: titanium nitride) Application range is wide as seen in the next two tables.

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MARKET PARTICLES REQUIRED NANOTECHNOLOGY ADVANTAGES Polishing Slurries Aluminum Oxide Faster rate of surface removal reduces operating costs Cerium Oxide Less material required due to small size of particles Tin Oxide Better finishing due to finer particles Capacitors Barium Titanate Less material required for a given level of capacitance Tantalum High capacitance due to reduction in layer thickness and increased surface area resulting from smaller particle size Alumina Thinner layers possible, thus significant potential for device miniaturization Pigments Iron Oxide Lower material costs, as opacity is obtained with smaller particles Zirconium Silicate Better physical-optical properties due to Titanium Dioxide enhanced control over particles Dopants Wide variety of materials Improved compositional uniformity required depending on Reduction in processing temperature reduces application operating and capital costs Nanoparticles Source: Wilson et al 2002

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MARKET PARTICLES REQUIRED NANOTECHNOLOGY ADVANTAGES Structural Ceramics Aluminium Oxide Improved mechanical properties Aluminium Titanate Reduced production costs due to lower sintering temperatures Zirconium Oxide Catalysts Titanium Dioxide Increased activity due to smaller particle size Cerium Oxide Increased wear resistance Alumina Hard Coatings Tungsten Carbide Thin coatings reduce the amount of material required Alumina Conductive Inks Silver Increased conductivity reduces consumption of valuable metals Tungsten Lower processing temperatures Nickel Allows electron lithography Nanoparticles (Cont.) Source: Wilson et al 2002

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Carbon Nanotube CNT is a tubular form of carbon with diameter as small as 1 nm. Length: few nm to microns. CNT is configurationally equivalent to a single or multiple two dimensional graphene sheet(s) rolled into a tube (single wall vs. multiwalled). CNT exhibits extraordinary mechanical properties: Young’s modulus over 1 Tera Pascal, as stiff as diamond, and tensile strength ~ 200 GPa. CNT can be metallic or semiconducting, depending on (m-n)/3 is an integer (metallic) or not (semiconductor). See textbook on Carbon Nanotubes: Science and Applications, M. Meyyappan, CRC Press, 2004.

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CNT Properties • The strongest and most flexible molecular material because of C-C covalent bonding and seamless hexagonal network architecture • Strength to weight ratio 500 time > for Al, steel, titanium; one order of magnitude improvement over graphite/epoxy • Maximum strain ~10% much higher than any material • Thermal conductivity ~ 3000 W/mK in the axial direction with small values in the radial direction • Very high current carrying capacity • Excellent field emitter; high aspect ratio and small tip radius of curvature are ideal for field emission • Can be functionalized

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CNT Synthesis • CNT has been grown by laser ablation (pioneered at Rice) and carbon arc process (NEC, Japan) - early 90s. - SWNT, high purity, purification methods • CVD is ideal for patterned growth (electronics, sensor applications) - Well known technique from microelectronics - Hydrocarbon feedstock - Growth needs catalyst (transition metal) - Growth temperature 500-950° deg. C. - Numerous parameters influence CNT growth

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Surface masked by a 400 mesh TEM grid - Methane, 900° C, 10 nm Al/1.0 nm Fe SWNTs on Patterned Substrates Delzeit et al., Chem. Phys. Lett., 348 , 368 (2001)

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Surface masked by a 400 mesh TEM grid; 20 nm Al/ 10 nm Fe; 10 minutes Grown using ethylene at 750 o C Multiwall Nanotube Towers Delzeit et al., J. Phys. Chem. B, 106 , 5629 (2002)

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• Certain applications such as nanoelectrodes, biosensors would ideally require individual, freestanding, vertical (as opposed to towers or spaghetti-like) nanostructures • The high electric field within the sheath near the substrate in a plasma reactor helps to grow such vertical structures • dc, rf, microwave, inductive plasmas (with a biased substrate) have been used in PECVD of such nanostructures Plasma Reactor for CNT Growth Cassell et al., Nanotechnology, 15 (1), 2004

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High Volume Production of CNTs • Needed for composites, hydrogen storage, other applications which need bulk material • Floating catalysts (instead of supported catalysts) • Carbon source (CO, hydrocarbons) • Floating catalyst source (Iron pentacarbonyl, ferrocene…) • Typically, a carrier gas picks up the catalyst source and goes through first stage furnace (~200° C) • Precursor injected directly into the 2nd stage furnace • Decomposition of catalyst source, source gas pysolysis, catalyzed reactions all occur in the 2nd stage • Products: Nanotubes, catalyst particles, impurities

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CNT-Based Composites • Carbon nanotubes viewed as the “ultimate” nanofibers ever made • Carbon fibers have been already used as reinforcement in high strength, light weight, high performace composites: - Expensive tennis rackets, air-craft body parts… • Nanotubes are expected to be even better reinforcement - C-C covalent bonds are one of the strongest in nature - Young’s modulus ~ 1 TPa  the in-plane value for defect-free graphite • Problems - Creating good interface between CNTs and polymer matrix necessary for effective load transfer  CNTs are atomically smooth; h/d ~ same as for polymer chains  CNTs are largely in aggregates  behave differently from individuals • Solutions - Breakup aggregates, disperse or cross-link to avoid slippage - Chemical modification of the surface to obtain strong interface with surrounding polymer chains WHY?

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General Issues in Making CNT Composites • Polymer matrix composites - Nanotube dispersion - Untangling - Alignment - Bonding - Molecular Distribution - Retention of neat-CNT properties • Metal and Ceramic Matrix Composites - High temperature stability - Reactivity - Suitable processing techniques - Choice of chemistries to provide stabilization and bonding to the matrix.

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Conducting Polymers Based on Carbon Nanotubes • High aspect ratio allows percolation at lower compositions than spherical fillers (less than 1% by weight) • Neat polymer properties such as elongation to failure and optical transparency are not decreased. • ESD Materials: Surface resistivity should be 10 12 - 10 5 /sq - Carpeting, floor mats, wrist straps, electronics packaging • EMI Applications: Resistivity should be < 10 5 /sq - Cellular phone parts - Frequency shielding coatings for electronics • High Conducting Materials: Weight saving replacement for metals - Automotive industry: body panels, bumpers (ease of painting without a conducting primer) - Interconnects in various systems where weight saving is critical

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CNT Polymer Composites E.V. Barrera, Rice University in Carbon Nanotubes: Science and Applications: M. Meyyappan, CRC Press, 2004

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CNT Polymer Composites E.V. Barrera, Rice University in Carbon Nanotubes: Science and Applications: M. Meyyappan, CRC Press, 2004

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Nanotubes: EMI Shielding • More & more components are packaged in smaller spaces where electromagnetic interference can become a problem - Ex: Digital electronics coupled with high power transmitters as in many microwave systems or even cellular phone systems • Growing need for thin coatings which can help isolate critical components from other components of the system and external world • Carbon nanofibers have been tested for EMI shielding; nanotubes have potential as well - Act as absorber/scatterer of radar and microwave radiation - High aspect ratio is advantageous - Efficiency is boosted by small diameter. Large d will have material too deep inside to affect the process. At sub-100 nm, all of the material participate in the absorption - Carbon fibers and nanotubes (< 2 g/cc) have better specific conductivity than metal fillers, sometimes used as radar absorbing materials.

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Single Wall Carbon Nanotube Every atom in a single-walled nanotube (SWNT) is on the surface and exposed to environment Charge transfer or small changes in the charge-environment of a nanotube can cause drastic changes to its electrical properties Single-Walled Carbon Nanotubes For Chemical Sensors

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Sensor fabrication: SWCNT dispersions-- Nice dispersion of CNT in DMF 2. Device fabrication-- (see the interdigitated electrodes below) 3. SWCNT deposition— Casting, or in-situ growth SWCNT Chemiresistor Jing Li et al., Nano Lett., 3 , 929 (2003)

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Detection limit for NO 2 is 44 ppb. SWCNT Sensor Performance • Sensor tested for NO 2 , NH 3 , acetone, benzene, nitrotoulene… • Test condition: Flow rate: 400 ml/min Temperature: 23 o C Purge & carrier gas: N 2 • Sensitivity in the ppb range • Selectivity through (1) doping, (2) coating CNTs with polymers, (3) multiplexing with signal processing • Need more work to speedup recovery to baseline

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Boron Nitride Nanotubes • Electronic properties are independent of helicity and the number of layers • Applications: Nanoelectronic devices, composites • Techniques: Arc discharge, laser ablation • Also: B 2 O 3 + C (CNT) + N 2  2 BN (nanotubes) + 3 CO

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Various Inorganic Nanowires V.S. Vavilov (1994) Motivation • One-dimensional quantum confinement • Bandgap varies with wire diameter • Single crystal with well-defined surface structural properties • Tunable electronic properties by doping • Truly bottom-up integration possible

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Application Summary for Nanowires MATERIAL APPLICATION Silicon Electronics, sensors Germanium Electronics, IR detectors Tin Oxide Chemical sensors Indium Oxide Chemical sensors, biosensors Indium Tin Oxide Transparent conductive film in display electrodes, solar cells, organic light emitting diodes Zinc Oxide UV laser, field emission device, chemical sensor Copper Oxide Field emission device Wide Bandgap Nitrides (GaN) High temperature electronics, UV detectors and lasers, automotive electronics and sensors Boron Nitride Insulator Indium Phosphide Electronics, optoelectronics Zinc Selenide Photonics (Q-switch, blue-green laser diode, blue-UV photodetector) Copper, Tungsten Electrical interconnects

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Vertically-Aligned Nanowires for Device Fabrication ZnO Nanowires Germanium Nanowires P. Nguyen et al., Advanced Materials, Vol. 17, p. 549 (2005). H.T. Ng et al., Science, Vol. 300, p. 2149 (2003).

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A process flow outlining the major fabrication steps of a VSG-FET. Ng et al., Nano Letters, Vol. 4 (7), p. 1247 (2004) Vertical Surround-Gate Field Effect Transistor

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Low Thermal Energy for Programming Reduced melting point at 1-D Reduced programmable element volume Reduced activation energy at 1-D Device Scalability Ultra-low current / voltage / power operation Reduced thermal interference between neighboring memory cells Top electrode Bottom electrode PCM layer PCM nanowire 2-D Thin film PRAM 1-D Nanowire PRAM Why 1-D Phase-Change Nanowire?

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(a) TEM image of an individual GeTe nanowire with a diameter of about 40 nm. The inset shows an SAED pattern of fcc cubic lattice structure. (b) EDS spectrum of the same GeTe nanowire. 40 nm <110> Ge:Te ≈1:1 GeTe Nanowires: TEM, SAED, and EDS

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In-situ Tm measurement of GeTe nanowire under TEM image monitoring (a) The GeTe nanowire is under room temperature. (b) The GeTe nanowire is heated up to 400  C when the nanowire is molten and its mass is gradually lost through evaporation. The remaining oxide shell can be seen from the image. Liquid GeTe GeTe Nanowires: Melting Experiment and In-Situ Monitoring by TEM

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T m of bulk GeTe: 725 o C T m of GeTe nanowires: ~ 390 o C 46% reduction! The melting temperature of the nanowire is identified as the point at which the electron diffraction pattern disappears and the nanowire starts to be evaporated. Lower T m is translated into potentially much reduced thermal programming energy of data storage device. GeTe Nanowires: Melting Point

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Low dimensional systems nanowires Conduction electron density of state  Seebeck coefficient  Structural constraints thermal conductivity  0 5 10 15 20 25 0 10 20 30 wire width (nm) figure of merit, ZT n-doped p-doped *PRL 47, 16631 (1993) Nanowire Based Thermoelectric Element

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Future Outlook for Inorganic Nanowires Nanowire-based Radiation-harden Central Processing Unit Nanowire-based Detector Sensory Systems Nanowire-based Hybrid Energy Conversion/Storage Unit Nanowire-based Ultra-high Density Data Storage Nanowire-based Peripheral Optical Interconnect/ Transmitter

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Summary • Nanotechnology is an enabling technology that will impact the aerospace sector through composites, advances in electronics, sensors, instrumentation, materials, manufacturing processes, etc. • The field is interdisciplinary but everything starts with material science. Challenges include: - Novel synthesis techniques - Characterization of nanoscale properties - Large scale production of materials - Application development • Opportunities and rewards are great and hence, there is a tremendous worldwide interest