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M. Meyyappan

Nanotechnology: Aerospace Applications. M. Meyyappan. Nanotechnology Areas of Interest to Aerospace Community. • High Strength Composites (PMCs, CMCs, MMCs…) • Nanostructured materials: nanoparticles, powders, nanotubes… • Multifunctional materials, self-healing materials

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M. Meyyappan

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  1. Nanotechnology: Aerospace Applications M. Meyyappan

  2. 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

  3. 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 two dimensional graphene sheet rolled into a tube. 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 chirality.

  4. 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

  5. CNT Applications: Structural, Mechanical • High strength composites • Cables, tethers, beams • Multifunctional materials • Functionalize and use as polymer back bone - plastics with enhanced properties like “blow molded steel” • Heat exchangers, radiators, thermal barriers, cryotanks • Radiation shielding • Filter membranes, supports • Body armor, space suits Challenges - Control of properties, characterization - Dispersion of CNT homogeneously in host materials - Large scale production - Application development

  6. CNT Applications: Electronics • CNT quantum wire interconnects • Diodes and transistors for computing • Capacitors • Data Storage • Field emitters for instrumentation • Flat panel displays • THz oscillators Challenges • Control of diameter, chirality • Doping, contacts • Novel architectures (not CMOS based!) • Development of inexpensive manufacturing processes

  7. CNT Applications: Sensors, NEMS, Bio Challenges • CNT based microscopy: AFM, STM… • Nanotube sensors: force, pressure, chemical… • Biosensors • Molecular gears, motors, actuators • Batteries, Fuel Cells: H2, Li storage • Nanoscale reactors, ion channels • Controlled growth • Functionalization with probe molecules, robustness • Integration, signal processing • Fabrication techniques

  8. 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

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

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

  11. Ir_Fe Si_ Ni W_ Ni Ta_Ni/Co Cr_Ni Ti_ Ni Plasma Reactor for CNT Growth • Inductively coupled plasma reactor, with an rf-powered bottom electrode, separate heating stage to heat the wafer (in addition to plasma heating) • DC plasma reactor with similar capabilities, but generally lower plasma efficiency and more power consumption Cassell et al., Nanotechnology, 15 (1), 2004 • ICP Operating conditions CH4/H2 : 5 - 20% Total flow : 100 sccm Pressure : 1 - 20 Torr Inductive power : 100-200 W Bottom electrode power : 0 - 100 W

  12. 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

  13. 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?

  14. CNT-based Composites: A Status Update • CNT-Polymer Composites - Conducting polymers, by adding < 1% by weight SWNTs, for electrostatic dissipative (ESD) applications (carpeting, wrist straps, electronics packaging) and electromagnetic interference (EMI) applications (cellular phone parts) - Actuators based on SWNT/Nafion composites demonstrated for artificial muscle applications • CNT-ceramic matrix composites - Early works on MWNT reinforced SiC composites showed 20%  in strength and fracture toughness; processed by conventional ceramic processing techniques - Good interfacial bonding is critical to achieve adequate load transfer across MWNT-matrix interface; colloidal processing, in situ chemical methods may be advantageous to ensure this - MWNTs coated with SiO2 have been developed as microrods reinforcements in brittle inorganic ceramics.

  15. 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.

  16. Hydrogen Storage in CNTs • Impediments to commercialization of fuel cells: safe storage and delivery of hydrogen fuel • Potential solution: adsorption of H2 in a solid support  storage at relatively low pressures and high T • DOE Target: 6.5 wt%, 62 kg H2/m3 • Carbon nanotubes may be attractive for H2 storage - porous structure - low density • Storage mechanisms: physisorption? • To date, several groups have confirmed 1% uptake easily • Higher % claims (5-8%) are not verifiable or reproducible

  17. Lithium Storage in CNTs • Rechargeable lithium batteries: work by intercalation and de-intercalation of lithium between two electrodes - Transition metal oxide cathode and graphite anode • Production improvement: high energy capacity, fast charging time, long cycle time • How do you get high energy capacity? - Determined by the saturation Li concentration of the electrode material • For graphite, this concentration is LiC6 yields a capacity of 372 mA h/g • For nanotubes  inner cores, inter-tube channels, interstitial sites (inter-shell van der Waals spaces) all are available for Li intercalation • To date, a reversible capacity of 1000 mA h/g has been demonstrated • Exact locations of Li ions still unknown

  18. Field Emission • When subjected to high E field, electrons near the Fermi level can overcome the energy barrier to escape to the vacuum level • Common tips: Mo, Si, diamond • Applications: - Cathode ray lighting elements - Flat panel displays - Gas discharge tubes in telecom networks - Electron guns in electron microscopy - Microwave amplifiers • Fowler - Nordheim equation:  is work function,  is field enhancement factor • Plot of ln (I/V2) vs. (1/V) should be linear • At low emission levels, linearity seen; in the high field region, current saturates • Critical: low threshold E field, high current density, high emission site density (for high resolution displays)

  19. Field Emission Test Apparatus • Cathode and anode enclosed in an evacuated cell at a vacuum of 10-9 - 10-8 Torr • Cathode: glass or polytetrafluoroethylene substrate with metal- patterned lines - nanotube film tranferred to substrate or grown directly on it • Anode located 20-500 µm from cathode • Turn-on field: electric-field required to generate 1 nA - should be small • Threshold field: electric field required to yield 10 mA/cm2

  20. • Needs - For displays, 1-10 mA/cm2 - For microwave amplifiers, > 500 mA/cm2 • To obtain low threshold field - Low work function () - Large field enhancement factor ()  depends on geometry of the emitter;  ~ 1/5r • Threshold field values (in V/µm) for 10 mA/cm2 - Mo - 50-100 - Si - 50-100 - P-type diamond - 130 - Graphite Powder - 17 - Carbon nanotubes - 1-3 (stable at 1 A/cm2) _ Field Emission Needs

  21. Flat Panel Displays • Working full color flat panel displays and CRT-lighting elements have been demonstrated in Japan and Korea • Display - Working anode, a glass substrate with phosphor coated ITO stripes - Anode and cathode perpedicular to each other to form pixels at the intersection - Phosphors such as Y2O2S: Eu (red), Zns: Cu, Al (green), ZnS: Ag, Cl (blue) - 38” prototype display showing a uniform and stable image • Lighting Element - Phophor screen printed on the inner surface of the glass and backed by a thin Al film (~100 nm) to give electrical conductivity - Lifetime testing of the lighting element shows a lifespan over 1000 hrs.

  22. CNT in Microscopy Atomic Force Microscopy is a powerful technique for imaging, nanomanipulation, as platform for sensor work, nanolithography... Conventional silicon or tungsten tips wear out quickly. CNT tip is robust, offers amazing resolution. Simulated Mars dust 2 nm thick Au on Mica Nguyen et al., Nanotechnology, 12, 363 (2001)

  23. MWNT Scanning Probe Nguyen et al., Appl. Phys. Lett. 81 (5), 901 (2002)

  24. Single-Walled Carbon Nanotubes For Chemical Sensors 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

  25. SWCNT Chemiresistor • 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 Jing Li et al., Nano Lett., 3, 929 (2003)

  26. SWNT Sensor Response to NO2 with UV Light Aiding Recovery Detection limit for NO2 is 44 ppb.

  27. Boron Nitride Nanotubes • Electronic properties are independent of helicity and the number of layers • Applications: Nanoelectronic devices, composites • Techniques: Arc discharge, laser ablation • Also: B2O3 + C (CNT) + N2 2 BN (nanotubes) + 3 CO

  28. Various Inorganic Nanowires V.S. Vavilov (1994)

  29. (0001) Vertically-Aligned Nanowires for Device Fabrication

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

  31. 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

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