A huge amount of research and development activity has been devoted to nano-scale related technologies in recent years. The National Science Foundation projects nanotechnology related products will become a $1 trillion industry by 2015 [1]. Nano-scale technology is defined as any technology that deals with structures or features in the nanometer range or that are less than 100 nanometers, about one-thousandth the diameter of a human hair, and larger than about 1 nm, the scale of the atom or of small molecules. Below about 1 nm, the properties of materials become familiar and predictable, as this is the established domain of chemistry and atomic physics. It should be noted that
nanotechnology is not just one, but many wide ranging technologies in many technical disciplines including but not limited to chemistry, biology, physics, material science, electronics, MEMS and self-assembly. Nano-structures have the ability to generate new features and perform new functions that are more efficient than or cannot be performed by larger structures and machines. Due to the small
dimensions of nano-materials, their physical/chemical properties (e.g. stability, hardness, conductivity, reactivity, optical sensitivity, melting point, etc.) can be manipulated to improve the overall properties of conventional materials. At nanometer scales, the surface properties start becoming more dominant than the bulk material properties, generating unique material attributes and chemical reactions. More fundamentally, the electronic structure of materials becomes size-dependent as the dimensions enter the nanoscale.Delocalized electronic states as in a metal or a semiconductor are altered by the finite dimensions. Hence, the optical properties, including light absorption and emission behavior, will be altered, The fact that nanoscale features are smaller than the wavelength of visible photons also impacts light scattering, enabling the design of nanocrystalline ceramics that are as transparent as glass. Changes in the bonding at the surface of a nanoparticle will affect the electronic structure as well, and the implications for the reactivity of the surface can be significant. Beyond the electronic structure, nanostructuring can also affect transport properties markedly. Nanoscale features that are smaller than or comparable to the wavelengths or mean-free paths of phonons (quanta of lattice vibrations) or electrons permit the design of materials with thermal and electrical conductivity that may be outside the range accessible with ordinary materials. The most significant nano-structures investigated to date are made from single atomistic layers of carbon. These structures include hollow ball shaped “Buckyballs” (Fullerene - C60), carbon nanotubes (CNTs) and graphene sheets which have a very interesting range of mechanical, thermal and electrical properties. It should also be noted that even though the environmental and health effects of nano-scale structures are poorly understood at this time, nano-scale-based technologies are already being used in some industrial applications. A series of nano-materials, including metal nanoparticles and nano-powders, magnetic fluids, nano-adhesives, nanocomposite polymers, and nanocoatings (anti-fog, antireflective, wear and scratch resistant, dirt repellant, biocide, etc.) are being introduced for potential application in the automotive market.
Metal nanoparticles are being considered for potential use in catalytic converters since the catalytic reactivity is significantly enhanced due to the increased surface area and altered electronic structure of the metal nanoparticle. Coolants utilize nanoparticles and nano-powders to increase the efficiency of heat transfer and potentially reduce the size of the automotive cooling equipment. Some manufacturers
are currently using nanomagnetic fluids in shock absorbers to increase vibration control efficiency. Wear-resistant, hardsurface nano-coatings are being investigated for applications in bearings, cylinders, valves, and other highly stressed areas.
Nano-layers of semiconducting materials provide high efficiency electronic components and systems with a longer lifetime. Sensors based on nanolayer structures are used in engine control, airbag, anti-lock brake and electronic stability program systems. Nanoparticles also support the optimization
of conventional components like batteries, catalysts, solar cells or fuel cells.
The most promising automotive applications of nanotechnology include the following:
• Improved materials with CNTs, graphene and other nanoparticles/structures
• Improved mechanical, thermal, and appearance properties for plastics
• Coatings & encapsulants for wear and corrosion resistance,permeation barriers, and appearance
• Cooling fluids with improved thermal performance
• Joining interfaces for improved thermal cycle and crack resistance
• Metal alloys with greater mechanical strength
• Metal matrix and ceramics with improved mechanical properties
• Solder materials with crack resistance or lower processing temperature
• Displays with lower cost and higher performance
• Batteries for electric vehicles and fuel cells with improved energy capacity
• Automotive sensors with nano-sensing elements, nanostructures and nano-machines
• Hybrid electric vehicles using electrical interconnects for high-frequency and high-power applications
• Electrical switching including CNT transistors, quantum transistors, nano-electro-mechanical switches, electronemission amplification, and more efficient solar cells
• Self-assembly using fluid carriers
NANOPARTICLE THERMAL MATERIALS
In spite of advances in efficiency of vehicle powertrain systems and electronics, the removal of waste heat continues to be an important challenge. With increasing focus on reduced component size and mass, the traditional approach of increasing the area available for heat exchange with a cooling fluid (air, water/ethylene glycol) to manage higher heat loads is not acceptable. Increasing thermal power densities requires innovations in new coolants and thermal coupling materials.
The concept of using nano-fluids as a means of improving coolant performance was proposed over a decade ago [2].
Reports of up to 100% increase in liquid thermal conductivity with the addition of nanometer scale particles motivated a large amount of scientific/technical inquiry in the ensuing years [3]. Nano-fluids are a solid-liquid composite containing nanoparticles with sizes in the 1-100 nm range dispersed and suspended in a liquid. A variety of nanoparticle solids have been used as additives, including metals such as copper and gold, alumina, SiC and CuO ceramics and carbon nano-tubes. The surprisingly large increases in liquid thermal conductivity have been reported for relatively small particle loadings (<10% by volume). In addition, there have also been reports of higher critical heat flux (dry-out) for nano-fluids used in liquid-vapor phase cooling applications. These observations have been made for a number of liquids, including water/ethylene glycol, alcohols and oils. The results defy conventional experience which requires much higher volume loading of larger particles to produce slurries with comparable increases in effective liquid thermal conductivity. These observations have stimulated numerous theories attempting to understand and describe the phenomena, but the nature of the thermal enhancement mechanism still remains controversial. This situation is further aggravated by inconsistent results from different laboratories, and some claims that if carefully measured, the enhancements are smaller and explained by established theories. Nevertheless, the potential for significantly improved coolants may provide impetus for further improvements in engine efficiency and reduced size and weight of cooling system components. In addition, there are efforts to examine improvements in the thermal and rheological properties of lubricants with the addition of nano-scale particles [4].In automotive electronics, the use of thermal interface materials (TIM) to thermally couple electronic devices to heat sinks for waste heat removal is common practice. Although the thermal resistance of TIM has been reduced over the years, these materials still represent a major bottleneck in the thermal stack-up between semiconductor die and the cooling medium. As a result, components capable of handling higher power densities often operate at de-rated performance levels to mitigate high temperatures and to compensate for the harsh automotive environment. This problem is especially critical in hybrid electric vehicle power control systems, where switching transistors can operate at power densities in excess of 300 W/cm2.
Spurred by enhanced nano-fluid thermal properties, investigators translated the nano-composite ideas into the realm of TIM. It is common practice to boost the thermal conductivity of silicone oils, polymer gels, phase-change materials and thermoplastics by the addition of solid particles of micrometer scale size. Research has shown that optimal particle loading achieves improved thermal conductivity and low modulus (to accommodate thermal expansion mismatch of components) with a variety of materials and particle shapes/sizes [5]. Mixtures of nano- and micro- scale particles add another dimension for controlling thermal, rheological and mechanical properties [6]. Of particular interest is the use of carbon nano-tubes for TIM applications. The CNT is essentially a single atomic layer of graphite (graphene) which is rolled up onto itself. There are single- and multi-walled versions of CNT which can exhibit thermal conductivity in excess of 1000 Watts/meter ° Kelvin (for comparison, Cu = 400W/mK) and high tensile strength along the axis of the tube. Applications to TIM have involved
two basic approaches:
1. Simple addition of CNT to the TIM matrix (grease, gel,etc.)
2. Growth of vertically aligned CNT ‘carpets’ on the heatsink or device package.
In the former approach, CNT loading is increased until percolation of fibers provides a thermal path from mating surfaces. In the latter growth method, the individual CNT provide a direct high-conduction path between surfaces [6, 7]. In this case, tantalizing reports of low thermal impedance (∼
0.05 cm2C/W) have motivated continuing development of growth methods more amenable to high volume, low cost electronics production. At this point, efficient growth of high quality CNT is still time consuming and requires temperatures in excess of 500°C on catalyzed surfaces. In spite of the prospect that nano-composite materials offer improved thermal conduction, several issues need to be
resolved. Dispersing nanoparticles to avoid aggregation can be crucial to improving performance. In many cases the dispersions are not stable and over time lead to degraded thermal performance. In the case of liquids, maintaining a time-stable suspension can be problematic, since many candidate particle materials are denser than the liquid and tend to settle out. As it turns out, it is the nanometer-sized particles that can mitigate this problem. The intrinsic Brownian motion of liquid molecules surrounding the particles can maintain a dispersion/suspension.
Although the thermal properties of CNT are impressive, the performance gains in CNT composites are not as large as anticipated. High-interface thermal resistance in both CNT fillers and vertically aligned CNT tips severely impedes coupling between the CNT and the matrix or mating surface. Work continues on materials and methods to functionalize the CNT surface to improve the thermal coupling.
As composite technology progresses, we would expect to see the eventual penetration of nanoparticles into the realm of thermal management materials. The final issues to be confronted will be the value of performance gains achievable in a high volume, low cost automotive market.
DISPLAYS USING NANOTECHNOLOGY
Displays with improved performance and unique features are made possible by nanotechnology. Additionally, lower cost light emission sources, such as lasers are possible in the near future. Display technology, under rapid development for consumer electronic devices and home entertainment systems, is also being pursued for automotive applications. Improved performance, longer life, higher energy efficiency, unique presentation features, reduced package size and innovation become the value proposition for implementing this new technology. Automotive displays are expected to directly utilize nanotechnology in a variety of ways. Light emitting devices, such as LEDs, OLEDs (Organic Light Emitting Diode), fluorescent or field-emissive displays, electro-luminescent and perhaps lasers, are utilizing nano-phosphors and nanolayers to improve their performance. For example, silver nanoparticles on the cathode surface allow surface plasmon localization. This provides a strong oscillator decay channel that generates a two-fold increase of intensity for flexible OLED displays. Optical thin films, non-linear holographic reflectors, micro-lenses, and light conversion films are examples of materials that modulate or redirect electromagnetic radiation. Light projection systems, flatpanel displays, including cameras and other optical detectors that provide the input signals are all expected to benefit from nanotechnology developments. One particular area of interest is nano-phosphors, since these materials possess strikingly different absorption and emission characteristics while operating with better efficiencies and life times than their related bulk phosphors. Since the particle size determines the band-gap energy, coupling nanophosphors with new semiconductor materials (with and without doping) means that a wide variety of designed phosphors and new devices will likely be developed.
Although many materials under consideration are somewhat exotic and expensive, inexpensive materials, such as zinc oxide, and titanium dioxide are also used in the nano-world. Considerable work is being done but much of it is in the realm of industrial secrecy. Most first generation nano-phosphors, Q-dots included, are based on toxic elements such as cadmium and lead. Alternative materials (manganese or copper-doped zinc sulfide, D-dots) are coming onto the market. Although these materials are still relatively expensive, the cost will reduce as applications are identified and escalate the demand for material. Today nano-phosphors have many applications in display devices and more are being discovered. Photonic properties of these materials are indicative of their electrical properties. The arrangement of the electrons, dictated by energy states, sets the rules for how a material will interact with incident photons. In this regard, conductors, insulators, and semiconductors each have unique valance and conduction electron energy band arrangements. A dielectric or insulator material will absorb a photon when a valence band electron can be excited (interband) to a higher conduction band, the energy being greater than the band gap of the material. Most dielectrics are transparent to visible light since the energy of photons at these wavelengths are insufficient to promote the electrons. A conductive material is opaque since it will either absorb or reflect photons due to the many energy bands available for electrons to be promoted within the conduction band (intraband). It is the semiconductor materials (especially with doping) that allow controllable interaction with incident photons due to free electrons in the partially filled conductive band and the energy states available in the “adjustable” band gap energy. Coupling these electrical properties with the dimensional size of the material, we now have the ability to break up the energy bands into discrete levels; that is, we can widen the band gap by controlling the physical size of the particle.
Semiconductor particles at the size and scale where this is possible are known as quantum dots, and the smaller the quantum dot, the larger its corresponding band gap. Quantum dots can absorb photons over a broad wavelength interval. Conversely quantum dots emit photons over a very narrow, temperature insensitive wavelength band, since the quantum confinement of the energy states in three dimensions approximates that of an atom having discrete atomic levels.
Quantum dots are also called artificial atoms. In general, the area of nano-optics operates on different
principles than bulk optics. Nano-optic elements consist of numerous nano-scale structures created in regular patterns on or in a material. Depending upon the optical function, they can be created with metals, dielectrics, non-metals and semiconductors, epitaxially grown crystals, glass and plastics. In whatever form, creating the nano-structured material istransformative. Nano-optic devices can perform their optical functions in very thin layers, often less than a micron in thickness. The optical effects can be achieved in a shorter focal length compared to bulk optics because the subwavelength-size structures of nano-patterns interact with light locally, involving quantum effects as well as classical optical performance. This feature of nano-optics allows for very compact form factors. The ability to understand how a material will interact with photons for generating a display or display element is primarily dependent upon the energy states of the electrons. The nano-scale interaction of photons and materials, termed nano-photonics, is a field still in its infancy with plenty of room to grow. This term encompasses a very broad field of materials, processes, and potential applications. For example, a new emerging roadmap targeting development of concepts, technologies, and devices has been released within the framework of the Photonics21 strategic research agenda. This roadmap is promoted by the EU Network of Excellence on nano-photonics (PhOREMOST), composed of 34 partners and over 300 researchers.
The majority of the developing technologies referenced by PhOREMOST are not directly applicable to future automotive emissive optical displays, projection systems, or imagers. Many anticipated nano-photonic materials will be coupled with silicon-based wafer processing to generate digital information processing and communication lightbased features (plasmonics) to increase processing speed while greatly reducing the power dissipation associated with today's electron-based metal and semiconductor materials.
However, other processing developments such as material processing using sols and self-assembly techniques are expected to indirectly advance display technology as they provide the means to create these new properties economically. Nanotechnology is engineering and it is all about practical applications of physics, chemistry, and materials science. Nano-photonics is that specialized region of study where the effects of light interacting with matter on a very small scale will be the engine to generate new products and features almost unimaginable today.
NANO-COMPOSITES
Nano-composites are materials that incorporate nano-sized particles into a matrix of standard material such as polymers. Adding nanoparticles can generate a drastic improvement in properties that include mechanical strength, toughness and electrical or thermal conductivity. The effectiveness of the nanoparticles is such that the amount of material added is normally only 0.5-5.0% by weight. They have properties that are superior to conventional microscale composites and can be synthesized using simple and inexpensive techniques. [8] A few nano-composites have already reached the marketplace, while a few others are on the verge, and many continue to remain in the laboratories of various research institutions and companies. The global nano-composites market is projected to reach 989 million pounds by the end of the 2010, as stated in a report published by Global Industry Analysts, Inc. Nano-composites comprising nanoparticles such as Nanoclays (70% of volume) or nano-carbon fillers, carbon nanotubes, carbon nano-fibers and graphite platelets are expected to be a major growth segment for the plastics industry.
HOW NANO-COMPOSITES WORK
Nanoparticles have an extremely high surface-to-volume ratio which dramatically changes their properties when compared with their bulk sized equivalents. It also changes the way in which the nanoparticles bond with the bulk material. The result is that the composite can be many times improved with respect to the component parts.
WHY NANO-COMPOSITES?
Polymers reinforced with as little as 2% to 6% of these nanoparticles via melt compounding or in-situ polymerization exhibit dramatic improvements in properties such as thermomechanical, light weight, dimensional stability, barrier properties, flame retardancy, heat resistance and electrical conductivity.
CURRENT APPLICATIONS OF NANOCOMPOSITES
Applications of nano-composite plastics are diversified such as thin-film capacitors for computer chips; solid polymer electrolytes for batteries, automotive engine parts and fuel tanks; impellers and blades, oxygen and gas barriers, food packaging etc. with automotive and packaging accounting for a majority of the consumption. [9] The automotive segment is projected to generate the fastest demand for nano-composites if the cost/performance ratio is acceptable. Some automotive production examples of nano-composites include the following: Step assist - First commercial application on the 2002 GMC Safari and Chevrolet Astro van; Body Side Molding of the 2004 Chevrolet Impala (7% weight savings per vehicle and improved surface quality compared with TPO and improved mar/scuff resistance); Cargo bed for GM's 2005 Hummer H2 (seven pounds of molded-in-color nanocomposites); Fuel tanks (Increased resistance to permeation); under-hood (timing gage cover (Toyota) and engine cover (Mitsubishi).
KEY CHALLENGES FOR NANOCOMPOSITES FOR FASTER COMMERCIALIZATION
• Develop low cost and high production volume to meet fast to market needs.
• Develop fast, low cost analytical methods with small quantity of samples which can provide a degree of exfoliation and degree of orientation, (TEM, XRD, Rheology considered too expensive and time consuming) for example, IR can detect silicon-oxygen bond in clay, which can help to evaluate degree of clay dispersion.
• Develop in-line testing of nano-composites.
• Develop alternative nano-clay treatments for better adhesion of nano-filler to polymer.
• Improve understanding the effect on performance by blending nano-fillers with conventional reinforcements such as glass fiber.
• Prediction of orientation / flow modeling.
• Understand the rheology and chemo-rheology of the polymer composites.
• Cost/performance ratio to substitute HIPS (High impact polystyrene), PC/ABS (Polycarbonate/AcrylonitirileButadiene-Styrene) and PC (Polycarbonate) with TPO
(Thermoplastics Polyolefins).
• Fine dispersion, full exfoliation and interfacial adhesion.
• High stiffness without affecting impact properties.
OPPORTUNITIES AND FUTURE TRENDS FOR NANO-COMPOSITES
Nano-fillers are expensive compared to conventional fillers, so one must use them wisely depending on the final part performance requirements. In many cases, it may be cost effective to use nano-filler where it is needed such as on the top layer of a part surface or middle layer of thickness or localized areas of the part (nano-composite pre-molded inserts). New nanotechnology applications are being demonstrated by R&D engineers, but the commercial officers balk at increased costs. The nano-clays cost about $3/lb and are used in loadings of 3-4 percent. The conventional competitor material is talc, which costs 30 cents/lb and is used at loadings of 10-15 percent. Another issue: Widespread replacement with nano-composites may require extensive re-tooling because of differences in shrinkage rates. Recent news of an innovative method of growing carbon nano-tubes may revolutionize the implementation of nanotechnology. Use of Nano-polypropylene (PP) for value added substitution such as high cost engineering plastics or development of molded-in-color nano-composites to replace glass-filled, painted PP for interior applications such as instrument panels will see major growth. Functional nanocomposites development is underway such as functionalized clays which add properties to clay including anti-static and moisture repellent characteristics and selective chemical barriers. Ultraviolet-curable nano-composites (electronics) and foaming and nucleating effect of nano-fillers (improve properties, desirable cell size and density, use of microcellular processes such as MuCell) will be commercialized soon. There is potential for body panels and large moldings to substitute for steel, aluminum, magnesium and Sheet-Molding Compound (SMC), where thermoplastics are currently excluded due to inadequate physical or mechanical performance.
There is a need to develop a low cost, carbon nano-tube based composite for high-end engineered plastics. For designers, there is a need to develop flow simulation software with or without a hybrid fiber-filled system (including orientation effect and warpage) so output can be used directly for structural analysis.
There are also many opportunities for development of new fillers and improvements such as nano-composites of a new nano-ceramic fiber, titanium dioxide (TiO2), magnetic particles, carbon nano-tubes and other molecularly reinforced polymers. Mixtures of different nano-materials or combinations of nano-materials with traditional additives are increasingly being considered.
