1. GENERAL INTRODUCTION
Although nanoscience and nanotechnology are topics of recent origin, nanomaterials have been
known for a long time. The term nanotechnology is commonly used now-a-days by people and press.
The subject of nanoscience and technology has gained great importance because of the potential
applications in various areas such as chemical and textile industries, materials industry, medical
diagnostics, drug and gene delivery and electronics.
There are three important "nano" terms to consider when a beginner trying to understand the
increasing news coverage and scientific developments in the field of nanotechnology. These terms
are: Nanoscale, Nanoscience and Nanotechnology. Nanoscience and nanotechnology refer to the
control and manipulation of matter at nanometer dimensions.
1.1. WHAT IS NANO?
The word "nano" originates from the Greek word "nanos" which means "dwarf" (i.e. an
abnormally short person). However, in scientific language it is a prefix which has a value equal to
"one billionth, i.e. 10−9". Therefore, one nanometer is one billionth of a meter (1 nm = 10 −9 m).
Following examples helps to understand a sense of nano scaled objects and figure 1 gives
qualitative idea.
1) Diameter of a hydrogen atom is about 0.1 nm. If ten hydrogen atoms are aligned in a line, then the
resulting length would be approximately 1 nm. 1 nm is approximately 5 silicon atoms aligned in a line.
2) A single strand of human hair is around 20,000 nm in diameter.
3) Size of a DNA molecule is about 2.5 nm.
Fig. 1: Size comparison from macroscale objects to nanoscale objects.
At the nanometer length scale, the laws of physics operate somewhat differently; the classical
mechanics that we encounter in everyday life give way to quantum mechanics. At the nanoscale,
for example, a tablespoon is not smooth, but instead composed of discrete atoms and molecules.
Note: Metric prefixes: There are twenty prefixes officially specified by International system of Units (SI).
Prefix Symbol Factor
Decimal
Alternate name
Yotta
Y
1024
1 000 000 000 000 000 000 000 000
Septillion
Zetta
Z
1021
1 000 000 000 000 000 000 000
Sextillion
Exa
E
1018
1 000 000 000 000 000 000
Quintillion
Peta
P
1015
1 000 000 000 000 000
Quadrillion
Tera
T
1012
1000000 000 000
Trillion
Giga
G
109
1000000000
Billion
Mega
M
106
1000000
Million
Kilo
k
103
1000
Thousand
Hecto
h
102
100
Hundred
Deca
da
101
10
Ten
100
1
One
Deci
d
10−1
0.1
Centi
c
10−2
0.01
Hundredth
Milli
m
10−3
0.001
Thousandth
Micro
µ
10−6
0.000001
Millionth
Nano
n
10−9
0.000000001
Billionth
Pico
p
10−12
0.000000000001
Trillionth
Femto
f
10−15
0.000000000000001
Quadrillionth
Atto
a
10−18
0.000000000000000001
Quintillionth
Zepto
z
10−21
0.000000000000000000001
Sextillionth
Yocto
y
10−24
0.000000000000000000000001
Septillionth
Tenth
1.2. NANOSCIENCE
The word itself is a combination of "nano", from the Greek "nanos" (or Latin "nanus"),
meaning "dwarf", and the word "science". The word nano also refers to the 10−9 or one billionth.
The study of phenomena, properties and manipulation of structures having nanometer
dimension (1-100 nanometers) is referred to as nanoscience. Nanoscience is concerned with
materials and systems whose structures and components exhibit novel and significantly improved
physical, chemical and biological properties, phenomena and processes, because of their small
nanoscale size.
1.3. NANOTECHNOLOGY
The words nanoscience and nanotechnology both stem from the term nanometer, which is just a
scale of measurement. Nanotechnology is not just a new field of science and engineering, but a new
way of looking at and studying materials at nano scale. A scientific and technical revolution has
begun, which is based upon the ability to systematically organize and manipulate matter at the atomic
or molecular level, i.e. on the nanometer scale.
What is nanotechnology?
Currently there are a lot of different opinions about what is nanotechnology? In general,
nanotechnology (sometimes shortened to "nanotech") can be understood as a technology of design,
fabrication and applications of nanostructures and nanomaterials. Nanostructures and nano materials are
those with at least one dimension falling in nanometer scale ( 1-100 nm), and include nanoparticles
(including quantum dots, when exhibiting quantum effects), nanorods and nanowires, thin films, and
bulk materials made of nanoscale building blocks.
Nanotechnology also includes fundamental understanding of physical properties and
phenomena of nanomaterials and nanostructures. In the United States, nanotechnology has been
defined as being "concerned with materials and systems whose structures and components exhibit
novel and significantly improved physical, chemical and biological properties, phenomena &
processes due to their nanoscale size".
Nanotechnology is an interdisciplinary which needs physics, chemistry, engineering, biology
etc., so that it's full potential can be exploited for the advantage of mankind. What has been
achieved in nanotechnology so far is only the tip of the iceberg. However, to fully explore the
potential of nanotechnology it is essential to know: What are nanomaterials? How and why do they
differ from other materials? How to synthesize/analyze the nanomaterials? Organize them and
understand some already proven applications areas.
Nanotechnology is a key technology for the future and hence governments have invested
billions of dollars in its future. Through its National Nanotechnology Initiative, the USA has
invested 3.7 billion dollars, European Union has invested 1.2 billion and Japan 750 million dollars.
In order to explore novel physical properties and phenomena and realize potential applications of
nanostructures and nanomaterials, the ability to fabricate and process nanomaterials and
nanostructures is the first corner stone in nanotechnology.
What are the benefits of studying the behavior and interactions of nanoscale materials?
New behavior at the nanoscale is not necessarily predictable from that observed at large size
scales. Important changes in behavior are caused not by the order of magnitude size reduction, but
also by new phenomena such as size confinement, predominance of interfacial phenomena, and
quantum mechanics. It is notable that, all relevant phenomena at the nanoscale are caused by the
tiny size of the organized structure as compared to molecular scale, and by the interactions at their
predominant and complex interfaces.
What are the aims of nanotechnology?
Once we are able to control feature size, we can enhance material properties and device
functions beyond those that we currently know or even imagine. Nanotechnology aims to gain
control of structures and devices at the atomic, molecular and supramolecular levels, and to learn
how to efficiently manufacture and use these devices.
Why is the study of nanoscience different than the same problems on a larger scale?
Physics is different on the nanometer scale. Properties not seen on a macroscopic scale now
become important - such as quantum mechanical and thermodynamic properties. Rather than
working with bulk materials, one works with individual atoms and molecules. By learning about an
individual molecule’s properties, we can put them together in very well-defined ways to produce
new materials with new and amazing characteristics.
Why is nanotechnology suddenly becoming such a big field?
There are multiple reasons for this. One is availability of new instruments able to "see" and
"touch" at nano scale. In the early 1980's the Scanning Tunneling Microscope (STM) was invented at
IBM-Zurich in Switzerland. This was the first instrument that was able to "see" atoms. A few years
later, the Atomic Force Microscope (AFM) was invented, expanding the capabilities and types of
materials that could be investigated. Hence, Scanning Probe Microscopy (SPM) was born, and since
then multiple similar techniques have evolved from these instruments to "see" different properties at
the nanometer scale. In addition, "older" techniques such as electron microscopy have continued to
evolve as well, and now can image in the nanometer range. Currently, there are a large number of
complementary instruments that help scientists in the nano realm and hence, it is possible to image
individual molecules, and even single atoms.
In addition to the enabling technologies, scientists have realized the future potential of this
research. By convincing politicians and leaders around the world, countries have instituted initiatives to
promote nanoscience and nanotechnology in their universities and labs. With the recent increase in
funding, many scientists are pursuing this research and the rate of discovery has increased
dramatically.
What are the important aspects of nanotechnology?
Nanotechnology has three important aspects: size, structure, and resulting novel properties.
Size: About 3-10 atoms along a line results the length of a nanometer. In comparison, the diameter of
a human hair is about 20,000 nanometers and a smoke particle is about 1,000 nanometers.
Structure: Nanotechnology is not just about the size of looking at very small things, it is about
structure, or how things are put together, arranged, or assembled. It is the ability to work - observe,
manipulate, and build - at the atomic or molecular level.
Novel properties: Nanotechnology produces materials and systems that exhibit novel and
significantly changed physical, chemical, and biological properties because of their size and
structure. When a substance consists only of clusters of a few hundred atoms, the laws of quantum
mechanics influence dramatic changes in its mechanical, optical, and electronic properties. These
properties include improved catalysts, tunable photoactivity, increased strength etc.
What are the future implications of nanotechnology?
Scientists debate the future implications of nanotechnology. Nanotechnology may be able to
create many new materials and devices with a vast range of applications, such as in medicine,
electronics, biomaterials and energy production. On the other hand, nanotechnology raises many of
the same issues as any new technology, including concerns about the toxicity and environmental
impact of nanomaterials, and their potential effects on global economics, as well as speculation
about various doomsday scenarios. These concerns have led to a debate among advocacy groups
and governments on whether special regulation of nanotechnology is warranted.
1.4. ORIGIN (or EMERGENCE) OF NANOTECHNOLOGY
Nanotechnology is a new word, but it is not an entirely new field and research on nanometer
scale is not new at all. The study of biological systems and the engineering of many materials such
as colloidal dispersions, metallic quantum dots, and catalysts have been in the nanometer regime
for centuries. For example, Chinese are known to use gold (Au) nanoparticles as an inorganic dye
to introduce red color into their ceramic porcelains more than thousand years ago. Use of colloidal
gold has a long history, though a comprehensive study on the preparation and properties of
colloidal gold was first published in the middle of the 19 th century. Colloidal dispersion of gold
prepared by Michael Faraday in 1857, his gold samples are still in British museum in UK showing
beautiful magenta-red colour (not golden!) solution. Medical applications of colloidal gold present
another example. Colloidal gold was, and is still, used for treatment of arthritis. A number of
diseases were diagnosed by the interaction of colloidal gold with spinal fluids obtained from the
patient. What has changed recently is an explosion in our ability to image, engineer and manipulate
systems in the nanometer scale. What is really new about nanotechnology is the combination of
our ability to see and manipulate matter on the nanoscale and our understanding of atomic scale
interactions.
How the idea of nanotechnology has started?
The ideas and concepts behind nanoscience and nanotechnology started with a talk entitled
"There's plenty of room at the bottom" by Nobel Laureate Richard P. Feynman, a physicist, at the
annual meeting of the American Physical Society at the California Institute of Technology
(CalTech) on December 29, 1959, long before the term nanotechnology was used
[http://www.zyvex.com/nanotech/feynman.html]. This historical talk lead to call him as father of
nanotechnology. He talked about the problem of manipulating and controlling things on a small
scale and building nano-objects atom by atom or molecule by molecule. In this talk, he stated,
"The principles of physics, as far as I can see, do not speak against the possibility of maneuvering
things atom by atom (i.e. it is possible to manipulate and control individual atoms and molecules).
It is not an attempt to violate any laws; it is something, in principle, that can be done; but in practice, it
has not been done because we are too big".
Richard P. Feynman, in a way, suggested the bottom-up approach in his famous. In 1959 talk,
he also stated that "it is interesting that it would be, in principle, possible (I think) for a physicist to
synthesize any chemical substance that the chemist writes down. Give the orders and the physicist
synthesizes it. How? Put the atoms down where the chemist says, and so you make the substance.
The problems of chemistry and biology can be greatly helped if our ability to see what we are
doing, and to do things on an atomic level, is ultimately developed − a development which I think
cannot be avoided". However, the world had to wait a long time to put down atoms at the required
place.
In 1974, Norio Taniguchi used the term "nanotechnology" to describe materials whose
dimensions are less than a micrometer. In 1981, Scanning Tunneling Microscope (STM) was
developed by Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratory that could
"see" individual atoms and later a number of tools collectively called scanning probe microscopes
of various types [e.g. atomic force microscopy (AFM)] were developed. The team associated with
these developments got the Nobel Prize for physics in 1986. The tools they developed can help to
see and place atoms and molecules wherever needed. Combining with other well-developed
characterization and measurement techniques such as transmission electron microscopy (TEM), it
was possible to study and manipulate the nanostructures and nanomaterials to a great detail and
often down to the atomic level. In 1985, Richard E. Smalley and his co-workers synthesized a sixty
atom carbon molecule, later named as fullerenes, and awarded Nobel Prize in chemistry in 1996.
Around the same time, Eric Drexler has given widespread propaganda in popularizing the
concept of nanotechnology and founded the field of molecular nanotechnology. In his book,
Engines of Creation (1986), Drexler introduced a world of tiny machines or assemblers that can
construct new structures with atomic level precision. He also first published the term "grey goo" to
describe what might happen if a hypothetical self-replicating molecular nanotechnology went out
of control. Drexler's vision of nanotechnology is often called "Molecular nanotechnology" (MNT).
Drexler at one point proposed the term "zettatech" which never became popular. Later in 1991,
Sumo Iijima discovered carbon nanotubes, while he was working for NEC Carporation, Japan.
In the early 2000's, the field was subject to growing public awareness and controversy, with
prominent debates about both its potential implications, exemplified by the Royal Society's report
on nanotechnology, as well as the feasibility of the applications envisioned by advocates of
molecular nanotechnology, which culminated in the public debate between Eric Drexler and
Richard Smalley in 2001 and 2003. Governments moved to promote and fund research into
nanotechnology with programs such as the National Nanotechnology Initiative.
The early 2000's also saw the beginnings of commercial applications of nanotechnology,
although these were limited to bulk applications of nanomaterials, such as the Silver Nano platform
for using silver nanoparticles as an antibacterial agent, nanoparticle-based transparent sunscreens,
and carbon nanotubes for stain-resistant textiles. Now a day, nanotechnology is already all around us
if you know where to look. This technology is not new, it is the combination of existing
technologies and our new found ability to observe and manipulate at the atomic scale, this makes
nanotechnology so compelling from scientific, business and political viewpoints.
1.5. NANOSCALE SIZE EFFECT / SIGNIFICANCE (IMPORTANCE) OF NANOSCALE
[What happens when the dimensions of structures are reduced to nanoscale levels?]
At the nanometer length scale, the laws of physics operate somewhat differently; the classical
mechanics that we encounter in everyday life give way to quantum mechanics. At the nanoscale, for
example, a tablespoon is not smooth, but instead composed of discrete atoms and molecules.
Nanotechnology is not only a simple continuation of miniaturization from micron meter scale
down to nanometer scale. Materials in the micrometer scale mostly exhibit physical properties the
same as that of bulk form; however, materials in the nanometer scale may exhibit physical
properties distinctively different from that of bulk.
When at least one of the dimensions of any type of material is reduced below ~ 100 nm, then
it's mechanical, thermal, optical, magnetic and other properties change at some size characteristics
of that material. Thus within the same material one can get a range of properties. For example,
consider a semiconductor like CdSe, which is normally reddish in colour. If one brings down the
particle size (i.e. diameter) of CdSe to say ~ 10 nm, its powder still has red colour. But below
about 5 nm size, a dramatic change occurs in the optical properties of CdSe. As illustrated in figure
the colour of ~ 4.5 nm size particles is yellow, ~ 3.8 nm size particles is light green and that of ~
2.8 nm size particles is green, and ~ 2 nm size particles is blue. Not only the visual appearance but
also other properties change dramatically. The melting point of the solid changes when we make
tiny particles of material, for example, small particles of gold melt at a much lower temperature than
a big piece of gold.
Blue
Green
Light green
Yellow
Red
Fig. 2: Colour change with particle size in CdSe nanoparticles (colour figure in soft copy only).
The main feature of nanomaterials is that, the size determines their properties. When we are
dealing with the objects in the real world (bulk objects), properties do not change much with size.
But in nano world, the change in size affects properties enormously.
When particles are of a reasonable size, say 50 nm and above, their properties will be similar
to those of the bulk. When they are a little smaller, say 10-50 nm, their properties may vary
linearly with size. However, when the size of becomes very small (less than 10 nm), we get some
unusual, new properties. Here we see the quantum effects. In this size, electrons confined in a
small volume or box exhibit energies which depend on the length of box. For example: (a) if the
size of the particles in the materials like iron (or any other material which is magnetic) decreases,
then the properties will change. They lose their magnetism when the size if very small; (b) Gold
shines as metal. However when very tiny particles of gold of 1 nm diameter are made they are no
longer metallic. They do not shine like bulk gold. Bulk gold is not chemically reactive (that’s why
we make jewellery of gold), but small particles of gold however are chemically reactive.
Properties of materials can be different at the nanoscale for two main reasons:
1. nanomaterials have a relatively larger surface area or surface area to volume ratio when
compared to the same mass of material produced in a bulk form. This can make materials more
chemically reactive (in some cases materials that are inert in their bulk form become reactive
when produced in their nanoscale form), and affect their strength or electrical properties.
For example, gold is chemically inert at macroscopic scale. However, gold becomes
reactive and catalytic at nanoscale, and even melts at lower temperature. The larger
surface area permits simultaneous interaction of chemicals with catalysts, which makes the
catalyst more effective.
For example: Consider a cube of side 2 cm length having a surface area of 24 cm 2. If it is
divided into 8 small cubes, its length reduces to 1 cm. But, the total surface area of these
8 cubes becomes 48 cm2. If each of these cubes is further divided in to 1021 cubes, their
side length reduces to 1 nm. Then the total surface area of 8 × 1021 cubes = 8 × 6 × 1021
nm2 = 48 × 107 cm2.
2. Quantum mechanical effects which become significant at nano regime - affecting the
optical,
electrical and magnetic behaviour of materials. One example is the
"quantum size effect" where the electronic properties of solids are altered with great
reductions in particle size. This effect does not come into play by going from macro
to micro dimensions. However, it
becomes pronounced when the nanometer size
range is reached.
Applications of Nano Materials
1. Next-Generation Computer Chips
2. Kinetic Energy (KE) Penetrators with Enhanced Lethality
3. Better Insulation Materials
4. Phosphors for High-Definition TV
5. Low-Cost Flat-Panel Displays
6. Tougher and Harder Cutting Tools
7. Elimination of Pollutants
8. High Energy Density Batteries
9. High-Power Magnets
10. High-Sensitivity Sensors
11. Automobiles with Greater Fuel Efficiency
12. Aerospace Components with Enhanced Performance Characteristics
13. Better and Future Weapons Platforms
14. Longer-Lasting Satellites
15. Longer-Lasting Medical Implants
16. Ductile, Machinable Ceramics
17. Large Electrochromic Display Devices
Nanomaterials (nanocrystalline 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.
Applications
Since nanomaterials possess unique, beneficial chemical, physical, and mechanical properties,
they can be used for a wide variety of applications. These applications include, but are not
limited to, the following:
Next-Generation Computer Chips
The microelectronics industry has been emphasising miniaturisation, whereby the circuits, such
as transistors, resistors, and capacitors, are reduced in size. By achieving a significant reduction
in their size, the microprocessors, which contain these components, can run much faster, thereby
enabling computations at far greater speeds. However, there are several technological
impediments to these advancements, including lack of the ultrafine precursors to manufacture
these components; poor dissipation of tremendous amount of heat generated by these
microprocessors due to faster speeds; short mean time to failures (poor reliability), etc.
Nanomaterials help the industry break these barriers down by providing the manufacturers with
nanocrystalline starting materials, ultra-high purity materials, materials with better thermal
conductivity, and longer-lasting, durable interconnections (connections between various
components in the microprocessors).
Kinetic Energy (KE) Penetrators with Enhanced Lethality
The Department of Defense (DoD) is currently using depleted-uranium (DU) projectiles
(penetrators) for its lethality against hardened targets and enemy armoured vehicles. However,
DU has residual radioactivity, and hence, it is toxic (carcinogenic), explosive, and lethal to the
personnel who use them. However, some of the important reasons for the continued use of DU
penetrators are that they possess a unique self-sharpening mechanism on impact with a target,
and the lack of suitable non-explosive, non-hazardous replacement for DU. Nanocrystalline
tungsten heavy alloys lend themselves to such a self-sharpening mechanisms because of their
unique deformation characteristics, such as grain-boundary sliding. Hence, nanocrystalline
tungsten heavy alloys and composites are being evaluated as potential candidates to replace
DU penetrators.
Better Insulation Materials
Nanocrystalline materials synthesised by the sol-gel technique result in foam like structures
called "aerogels." These aerogels are porous and extremely lightweight; yet, they can loads
equivalent to 100 times their weight. Aerogels are composed of three-dimensional, continuous
networks of particles with air (or any other fluid, such as a gas) trapped at their interstices. Since
they are porous and air is trapped at the interstices, aerogels are currently being used for
insulation in offices, homes, etc. By using aerogels for insulation, heating and cooling bills are
drastically reduced, thereby saving power and reducing the attendant environmental pollution.
They are also being used as materials for "smart " windows, which darken when the sun is too
bright (just as in changeable lenses in prescription spectacles and sunglasses) and they lighten
themselves, when the sun is not shining too brightly.
Phosphors for High-Definition TV
The resolution of a television, or a monitor, depends greatly on the size of the pixel. These pixels
are essentially made of materials called "phosphors," which glow when struck by a stream of
electrons inside the cathode ray tube (CRT). The resolution improves with a reduction in the
size of the pixel, or the phosphors. Nanocrystalline zinc selenide, zinc sulfide, cadmium sulfide,
and lead telluride synthesised by the sol-gel techniques are candidates for improving the
resolution of monitors. The use of nanophosphors is envisioned to reduce the cost of these
displays so as to render high-definition televisions (HDTVs) and personal computers affordable
to be purchased by an average household in the U. S.
Low-Cost Flat-Panel Displays
Flat-panel displays represent a huge market in the laptop (portable) computers industry.
However, Japan is leading this market, primarily because of its research and development efforts
on the materials for such displays. By synthesising nanocrystalline phosphors, the resolution of
these display devices can be greatly enhanced, and the manufacturing costs can be significantly
reduced. Also, the flat-panel displays constructed out of nanomaterials possess much higher
brightness and contrast than the conventional ones owing to their enhanced electrical and
magnetic properties.
Tougher and Harder Cutting Tools
Cutting tools made of nanocrystalline materials, such as tungsten carbide, tantalum carbide, and
titanium carbide, are much harder, much more wear-resistant, erosion-resistant, and last longer
than their conventional (large-grained) counterparts. They also enable the manufacturer to
machine various materials much faster, thereby increasing productivity and significantly
reducing manufacturing costs. Also, for the miniaturisation of microelectronic circuits, the
industry needs microdrills (drill bits with diameter less than the thickness of an average human
hair or 100 µm) with enhanced edge retention and far better wear resistance. Since
nanocrystalline carbides are much stronger, harder, and wear-resistant, they are currently being
used in these microdrills.
Elimination of Pollutants
Nanocrystalline materials possess extremely large grain boundaries relative to their grain size.
Hence, nanomaterials are very active in terms of their of chemical, physical, and mechanical
properties. Due to their enhanced chemical activity, nanomaterials can be used as catalysts to
react with such noxious and toxic gases as carbon monoxide and nitrogen oxide in automobile
catalytic converters and power generation equipment to prevent environmental pollution arising
from burning gasoline and coal.
High Energy Density Batteries
Conventional and rechargeable batteries are used in almost all applications that require electric
power. These applications include automobiles, laptop computers, electric vehicles, nextgeneration electric vehicles (NGEV) to reduce environmental pollution, personal stereos,
cellular phones, cordless phones, toys, and watches. The energy density (storage capacity) of
these batteries is quite low requiring frequent recharging. The life of conventional and
rechargeable batteries is also low. Nanocrystalline materials synthesised by sol-gel techniques
are candidates for separator plates in batteries because of their foam-like (aerogel) structure,
which can hold considerably more energy than their conventional counterparts. Furthermore,
nickel-metal hydride (Ni-MH) batteries made of nanocrystalline nickel and metal hydrides
are envisioned to require far less frequent recharging and to last much longer because of their
large grain boundary (surface) area and enhanced physical, chemical, and mechanical properties.
High-Power Magnets
The strength of a magnet is measured in terms of coercivity and saturation magnetisation values.
These values increase with a decrease in the grain size and an increase in the specific surface
area (surface area per unit volume of the grains) of the grains. It has been shown that magnets
made of nanocrystalline yttrium-samarium-cobalt grains possess very unusual magnetic
properties due to their extremely large surface area. Typical applications for these high-power
rare-earth magnets include quieter submarines, automobile alternators, land-based power
generators, motors for ships, ultra-sensitive analytical instruments, and magnetic resonance
imaging (MRI) in medical diagnostics.
High-Sensitivity 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 nanocrystalline materials are extremely sensitive to the change
in their environment. Typical applications for sensors made out of nanocrystalline materials are
smoke detectors, ice detectors on aircraft wings, automobile engine performance sensor, etc.
Automobiles with Greater Fuel Efficiency
Currently, automobile engines waste considerable amounts of gasoline, thereby contributing to
environmental pollution by not completely combusting the fuel. A conventional spark plug is
not designed to burn the gasoline completely and efficiently. This problem is compounded by
defective, or worn-out, spark plug electrodes. Since nanomaterials are stronger, harder, and
much more wear-resistant and erosion-resistant, they are presently being envisioned to be used
as spark plugs. These electrodes render the spark plugs longer-lasting and combust fuel far more
efficiently and completely. A radically new spark plug design called the "railplug" is also in the
prototype stages. This railplug uses the technology derived from the "railgun," which is a spinoff of the popular Star Wars defense program. However, these railplugs generate much more
powerful sparks (with an energy density of approximately 1 kJ/mm2). Hence, conventional
materials erode and corrode too soon and quite frequently to be of any practical use in
automobiles. Nevertheless, railplugs made of nanomaterials last much longer even the
conventional spark plugs. Also, automobiles waste significant amounts of energy by losing the
thermal energy generated by the engine. This is especially true in the case of diesel engines.
Hence, the engine cylinders (liners) are currently being envisioned to be coated with
nanocrystalline ceramics, such as zirconia and alumina, so that they retain heat much more
efficiently and result in complete and efficient combustion of the fuel.
Aerospace Components with Enhanced Performance Characteristics
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. Nanomaterials provide such
a significant reduction in the grain size over conventional materials that the fatigue life is
increased by an average of 200-300%. Furthermore, components made of nanomaterials are
stronger and can operate at higher temperatures, aircrafts can fly faster and more efficiently (for
the same amount of aviation fuel). In spacecrafts, elevated-temperature strength of the material
is crucial because the components (such as rocket engines, thrusters, and vectoring nozzles)
operate at much higher temperatures than aircrafts and higher speeds. Nanomaterials are perfect
candidates for spacecraft applications, as well.
Better and Future Weapons Platforms
Conventional guns, such as cannons, 155 mm howitzers, and multiple-launch rocket system
(MLRS), utilise the chemical energy derived by igniting a charge of chemicals (gun powder).
The maximum velocity at which the penetrator can be propelled is approximately 1.5-2.0
km/sec. On the other hand, electromagnetic launchers (EML guns), or railguns, use the electrical
energy, and the concomitant magnetic field (energy), to propel the penetrators/projectiles at
velocities up to 10 km/sec. This increase in velocity results in greater kinetic energy for the same
penetrator mass. The greater the energy, the greater is the damage inflicted on the target. For
this and other reasons, the DoD (especially, the U. S. Army) has conducted extensive research
into the railguns. Since a railgun operates on electrical energy, the rails need to be very good
conductors of electricity. Also, they need to be so strong and rigid that the railgun does not sag
while firing and buckle under its own weight. The obvious choice for high electrical conductivity
is copper. However, the railguns made out of copper wear out much too quickly due to the
erosion of the rails by the hypervelocity projectiles and they lack high-temperature strength. The
wear and erosion of copper rails necessitate inordinately frequent barrel replacements. In order
to satisfy these requirements, a nanocrystalline composite material made of tungsten, copper,
and titanium diboride is being evaluated as a potential candidate. This nanocomposite
possesses the requisite electrical conductivity, adequate thermal conductivity, excellent high
strength, high rigidity, hardness, and wear/erosion resistance. This results in longer-lasting,
wear-resistant, and erosion-resistant railguns, which can be fired more frequently and often than
their conventional counterparts.
Longer-Lasting Satellites
Satellites are being used for both defence and civilian applications. These satellites utilise
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. Nanomaterials, such as
nanocrsytalline tungsten-titanium diboride-copper composite, are potential candidates for
enhancing these ignitors’ life and performance characteristics.
Longer-Lasting Medical Implants
Currently, medical implants, such as orthopaedic implants and heart valves, are made of titanium
and stainless steel alloys. These alloys are primarily used in humans because they are
biocompatible, i.e., they do not adversely react with human tissue. In the case of orthopaedic
implants (artificial bones for hip, etc.), these materials are relatively non-porous. For an implant
to effectively mimic a natural human bone, the surrounding tissue must penetrate the implants,
thereby affording the implant with the required strength. Since these materials are relatively
impervious, human tissue does not penetrate the implants, thereby reducing their effectiveness.
Furthermore, these metal alloys wear out quickly necessitating frequent, and often very
expensive, surgeries. However, nanocrystalline zirconia (zirconium oxide) ceramic is hard,
wear-resistant, corrosion-resistant (biological fluids are corrosive), and biocompatible.
Nanoceramics can also be made porous into aerogels (aerogels can withstand up to 100 times
their weight), if they are synthesized by sol-gel techniques. This results in far less frequent
implant replacements, and hence, a significant reduction in surgical expenses. Nanocrystalline
silicon carbide (SiC) is a candidate material for artificial heart valves primarily due to its low
weight, high strength, extreme hardness, wear resistance, inertness (SiC does not react with
biological fluids), and corrosion resistance.
Ductile, Machinable Ceramics
Ceramics, per se, are very hard, brittle, and hard to machine. These characteristics of ceramics
have discouraged the potential users from exploiting their beneficial properties. However, with
a reduction in grain size, these ceramics have increasingly been used. Zirconia, a hard, brittle
ceramic, has even been rendered superplastic, i.e., it can be deformed to great lengths (up to
300% of its original length). However, these ceramics must possess nanocrystalline grains to be
superplastic. In fact, nanocrystalline ceramics, such as silicon nitride (Si3N4) and silicon
carbide (SiC), have been used in such automotive applications as high-strength springs, ball
bearings, and valve lifters, because they possess good formability and machinabilty combined
with excellent physical, chemical, and mechanical properties. They are also used as components
in high-temperature furnaces. Nanocrystalline ceramics can be pressed and sintered into various
shapes at significantly lower temperatures, whereas it would be very difficult, if not impossible,
to press and sinter conventional ceramics even at high temperatures.
Large Electrochromic Display Devices
An electrochromic device consists of materials in which an optical absorption band can be
introduced, or an existing band can be altered by the passage of current through the materials,
or by the application of an electric field. Nanocrystalline materials, such as tungstic oxide
(WO3.xH2O) gel, are used in very large electrochromic display devices. The reaction governing
electrochromism (a reversible coloration process under the influence of an electric field) is the
double-injection of ions (or protons, H+) and electrons, which combine with the nanocrystalline
tungstic acid to form a tungsten bronze. These devices are primarily used in public billboards
and ticker boards to convey information. Electrochromic devices are similar to liquid-crystal
displays (LCD) commonly used in calculators and watches. However, electrochromic devices
display information by changing colour when a voltage is applied. When the polarity is reversed,
the colour is bleached. The resolution, brightness, and contrast of these devices greatly depend
on the tungstic acid gel’s grain size. Hence, nanomaterials are being explored for this purpose.
superior chemical, physical, and mechanical properties and of their exceptional formability.