# Presentation: 384927

## M. Meyyappan

- Nanotechnology:
- Aerospace Applications

## 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

## 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

## Benefits of Nanotechnology in

- •	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.

- Composite Development

## 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*

## 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

## 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*

## 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

## 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.

## 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

## 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.

## Nanoparticles

**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

**Source: Wilson et al 2002**

## Nanoparticles (Cont.)

**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

**Source: Wilson et al 2002**

## 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.

## 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

## 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

## 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)

## Multiwall Nanotube Towers

- 	20 nm Al/ 10 nm Fe; 10 minutes

- Grown using ethylene at 750o C

- Delzeit et al., J. Phys. Chem. B, **106**, 5629 (2002)

## •	 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

## 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

## 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?

## 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.

## 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 1012 - 105 /sq
- 		-	Carpeting, floor mats, wrist straps, electronics packaging
- •	EMI Applications: Resistivity should be < 105 /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

## CNT Polymer Composites

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

## CNT Polymer Composites

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

## 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.

## Single-Walled Carbon Nanotubes

- 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

- For Chemical Sensors

- 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)

## SWCNT Sensor Performance

- Detection limit for NO2 is 44 ppb.

- •	Sensor tested for NO2, NH3, 		acetone, benzene, nitrotoulene...
- •	Test condition:
- 	Flow rate: 400 ml/min
- 	Temperature: 23 oC
- 	Purge & carrier gas: N2
- •	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

## 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

## Various Inorganic Nanowires

## 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 |

## 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).

## Vertical Surround-Gate

- A process flow outlining the major
- fabrication steps of a VSG-FET.

- Ng *et al.,* Nano Letters, Vol. 4 (7), p. 1247 (2004)

- Field Effect Transistor

## 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

- 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

- 2-D Thin film PRAM

- 1-D Nanowire PRAM

- Why 1-D Phase-Change Nanowire?

## GeTe Nanowires: TEM, SAED, and EDS

- (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: Melting Experiment 

- 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

## GeTe Nanowires: Melting Point 

- Tm of GeTe nanowires: ~390oC

- 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 Tm is translated into potentially much reduced thermal programming energy of data storage device.

- GeTe Nanowires: Melting Point

## Nanowire Based Thermoelectric Element

***    ****nanowires*
- Conduction electron density of state 
- Seebeck coefficient 
- Structural constraints
-  thermal conductivity 

****PRL 47, 16631 (1993)***

## Future Outlook for Inorganic Nanowires

**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

## 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