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Materials with grain sizes in the order of a billionth of a meter are called nanomaterials, or nanocrystalline materials. They exhibit very attractive and useful properties, which can be used for a range of structural and non-structural applications.
Applications
As nanomaterials have exclusive, advantageous physical, chemical, and mechanical properties, they can be applied in a host of applications including, but not limited to, those listed below.
Next-Generation Computer Chips
The microelectronics sector has paid special attention to miniaturization, which involves reducing the size of circuits like transistors, capacitors, and resistors. A considerable reduction in their size enables microprocessors developed using these parts, to operate much faster, thus allowing computations at much greater speeds.
However, there are a number of technical obstacles to achieving these advancements, such as the lack of ultrafine precursors to make these parts, inadequate dissipation of huge amounts of heat generated by these microprocessors because of faster speeds, poor mean time to failures (poor reliability), etc.
Nanomaterials help the industry to overcome these obstacles by offering manufacturers materials with better thermal conductivity, nanocrystalline starting materials, ultra-high-purity materials, and longer-lasting, durable interconnections (connections between different parts in the microprocessors).
Kinetic Energy Penetrators with Improved Lethality
The Department of Defense (DoD) has been using depleted uranium (DU) projectiles (penetrators) for its battle against hardened targets and armored vehicles of enemies. But DU has residual radioactivity; therefore, it is explosive, harmful (carcinogenic), and lethal to personnel who use them. However, some of the key reasons for the sustained use of DU penetrators are that they have an exclusive self-sharpening mechanism on impact with a target, as well as the lack of an appropriate non-explosive, non-toxic alternative for DU.
Nanocrystalline tungsten heavy alloys can be used for such self-sharpening mechanisms due to their exclusive deformation characteristics, for example, grain-boundary sliding. Therefore, nanocrystalline tungsten heavy alloys and composites are being assessed for use as alternative DU penetrators.
Better Insulation Materials
Nanocrystalline materials manufactured by the sol-gel method give rise to foam-like structures known as “aerogels.” In spite of being extremely lightweight and porous, these aerogels can hold loads equal to 100 times their weight. Aerogels are made up of continuous 3D networks of particles with air (or any other fluid, such as a gas) trapped at their interstices.
Since aerogels are porous and include air trapped at the interstices, they are used for insulation in homes, offices, etc. This considerably reduces the cooling and heating bills, thus saving power and decreasing the associated environmental pollution.
They are also being employed as materials for “smart” windows, which darken when the sun is very bright (same as in changeable lenses in sunglasses and prescription spectacles), and lighten when the sun is not shining very brightly.
Phosphors for High-Definition TV
The resolution of a monitor or television is subject to the size of the pixel. These pixels are fundamentally composed of materials known as “phosphors,” which glow when struck by a stream of electrons within the cathode ray tube (CRT). The resolution enhances with a reduction in the pixel size or the phosphors.
Nanocrystalline zinc selenide, cadmium sulfide, zinc sulfide, and lead telluride prepared through the sol-gel methods are potential materials for enhancing the resolution of monitors. The use of nanophosphors is intended to lower the cost of these displays to make personal computers and high-definition televisions (HDTVs) affordable for an average household in the United States.
Low-Cost Flat-Panel Displays
In the laptop (portable) computer industry, the demand for flat-panel displays is high. Japan is leading in this area, mainly due to its R&D efforts on the materials for these displays.
The resolution of these display devices can be significantly improved by synthesizing nanocrystalline phosphors, while considerably bringing down the manufacturing costs. Furthermore, the flat-panel displays manufactured using nanomaterials have far higher contrast and brightness compared to the traditional ones due to their improved magnetic and electrical properties.
Tougher and Harder Cutting Tools
Cutting tools made of nanocrystalline materials like carbides of tantalum, tungsten, and titanium, are a lot harder, much more erosion-resistant and wear-resistant, and last longer than their traditional (large-grained) equivalents. They also allow the manufacturer to machine several materials much faster, thereby boosting productivity and largely minimizing manufacturing costs.
Moreover, miniaturizing microelectronic circuits necessitates microdrills (drill bits having diameters lesser than the thickness of an average human hair [100 µm]) with improved edge retention and much better wear resistance. Nanocrystalline carbides are being used in these micro drills since they are much harder, stronger, and wear-resistant.
Elimination of Pollutants
Nanocrystalline materials have very large grain boundaries corresponding to their grain size. Therefore, they are very active with regards to their physical, chemical, and mechanical properties. Owing to their improved chemical activity, nanomaterials can be employed as catalysts to react with toxic and noxious gases such as nitrogen oxide and carbon monoxide, in power generation equipment and automobile catalytic converters, to avoid environmental pollution caused when gasoline and coal are burnt.
High Energy Density Batteries
Traditional and rechargeable batteries are used in nearly all applications that necessitate electric power. These applications include laptop computers, automobiles, toys, electric vehicles, personal stereos, cordless phones, cellular phones, watches, and next-generation electric vehicles (NGEV) that reduce environmental pollution. The energy density (storage capacity) of these batteries is very low, necessitating frequent recharging. The life of traditional and rechargeable batteries is also low.
Nanocrystalline materials produced using sol-gel methods have a foam-like (aerogel) structure that can store significantly more energy than their traditional equivalents. Hence, they are highly suitable for separator plates in batteries. Moreover, nickel-metal hydride (Ni-MH) batteries made of nanocrystalline nickel and metal hydrides have been predicted to necessitate much lesser recharging and to last considerably longer because of their large grain boundary (surface) area and improved chemical, physical, and mechanical properties.
High-Power Magnets
A magnet’s strength is measured in terms of saturation magnetization and coercivity values. These values will increase when there is 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 demonstrated that magnets made of nanocrystalline yttrium-samarium-cobalt grains have highly uncommon magnetic properties because of their extremely large surface area.
Common applications for these high-power rare-earth magnets include ultra-sensitive analytical instruments, quieter submarines, land-based power generators, automobile alternators, motors for ships, and magnetic resonance imaging (MRI) in medical diagnostics.
High-Sensitivity Sensors
Sensors use their sensitivity to detect the variations in different parameters they are programmed to measure. The parameters include chemical activity, thermal conductivity, electrical resistivity, magnetic permeability, and capacitance. All of these parameters depend a lot on the microstructure (grain size) of the materials used in the sensors.
A variation in the sensor’s environment is revealed by the sensor material’s physical, chemical, or mechanical characteristics, which is leveraged for detection. For example, a carbon monoxide sensor made of zirconium oxide (zirconia) applies its chemical stability to identify whether carbon monoxide is present.
When carbon monoxide is present, the oxygen atoms in zirconium oxide react with the carbon in carbon monoxide to reduce zirconium oxide partially. This reaction activates a modification in the sensor’s characteristics, such as capacitance and conductivity (or resistivity).
The rate and the degree of this reaction are significantly increased by a decrease in the grain size. Therefore, sensors made of nanocrystalline materials are highly sensitive to variations in their environment. Common applications for sensors made using nanocrystalline materials are ice detectors on aircraft wings, smoke detectors, automobile engine performance sensors, etc.
Automobiles with Greater Fuel Efficiency
Existing automobile engines waste substantial amounts of gasoline, thus adding to environmental pollution by burning the fuel incompletely. A traditional spark plug is not made to burn the gasoline totally and efficiently. This problem is amplified by faulty, or worn-out, spark plug electrodes.
Since nanomaterials are harder, stronger, and considerably more erosion-resistant and wear-resistant, they are currently being proposed for use as spark plugs. These electrodes extend the service life of the spark plugs and help burn fuel far more efficiently and fully. A totally new spark plug design known as the “railplug” is also in the prototype stage.
This railplug applies the technology derived from the “railgun”—a spin-off of the famous Star Wars defense program. However, these railplugs produce much stronger sparks (with an energy density of almost 1 kJ/mm2). Hence, traditional materials erode and corrode very quickly, and quite often are not of any practical use in automobiles.
By contrast, railplugs made of nanomaterials are much more long-lasting than even the traditional spark plugs. Furthermore, automobiles waste substantial amounts of energy by losing the thermal energy produced by the engine. This is particularly true with diesel engines. Hence, plans have been proposed to coat the engine cylinders (liners) with nanocrystalline ceramics, such as alumina and zirconia, so that they preserve heat in a more efficient manner, thus ensuring full and efficient fuel combustion.
Aerospace Components with Enhanced Performance Characteristics
Owing to the hazards involved in flying, aircraft manufacturers aim to make the aerospace components tougher, stronger, and last longer. One of the main properties needed in aircraft components is fatigue strength, which declines as the age of the component increases. By manufacturing the components using more robust materials, the aircraft’s life can be significantly increased.
The fatigue strength increases with the decrease in the grain size of the material. Nanomaterials offer such a considerable reduction in the grain size over traditional materials that fatigue life is increased by an average of 200%–300%. Moreover, components made using nanomaterials are stronger and can work at higher temperatures, enabling aircraft to fly faster and more efficiently (using the same quantity of aviation fuel).
In spacecraft, higher-temperature strength of the material is critical as the components (for example, thrusters, rocket engines, and vectoring nozzles) work at much higher temperatures than aircraft and at greater speeds. Nanomaterials are ideal contenders for spacecraft applications, as well.
Better and Future Weapons Platforms
Traditional guns, such as 155 mm howitzers, cannons, and multiple-launch rocket systems (MLRS), use the chemical energy derived by burning a charge of chemicals (gun powder). The penetrator can be propelled at a maximum velocity of about 1.5–2.0 km/second.
Conversely, electromagnetic launchers (EML guns), or railguns, utilize electrical energy, as well as the concomitant magnetic field (energy), for propelling the penetrators/projectiles at velocities of up to 10 km/second. Such an increase in velocity causes greater kinetic energy for the same penetrator mass. The amount of energy is directly proportional to the damage imparted to the target. Therefore, the DoD (particularly the U.S. Army) has undertaken wide-ranging research on the railguns.
Since a railgun works on electrical energy, the rails have to be excellent conductors of electricity. Furthermore, they need to be so strong and inflexible that the railgun does not sag while firing and collapse due to its own weight. Copper is the apparent choice when it comes to high electrical conductivity.
However, railguns made of copper wear out considerably faster because of the erosion of the rails by the hypervelocity projectiles. Moreover, they lack high-temperature strength. The erosion and wear of copper rails call for very frequent barrel replacements.
To fulfill these necessities, a nanocrystalline composite material made of copper, tungsten, and titanium diboride is being assessed as a potential candidate. This nanocomposite exhibits the necessary electrical conductivity, satisfactory thermal conductivity, outstanding high strength, hardness, high rigidity, and wear/erosion resistance.
This results in erosion-resistant and wear-resistant railguns that last longer and can be fired more often than their traditional equivalents.
Longer-Lasting Satellites
Satellites are being employed for both civilian and defense applications. These satellites make use of thruster rockets to stay in or alter their orbits because of various factors, such as the impact of gravitational forces applied by the earth. Hence, thrusters are needed to reposition the satellites.
To a great extent, the life of these satellites is governed by the amount of fuel they can take on board. In reality, repositioning thrusters waste over one-third of the fuel carried aboard by the satellites, caused by partial and inefficient burning of the fuel, such as hydrazine. The partial and inefficient combustion occurs due to rapid wearing out of onboard ignitors that stop performing effectively as a result.
Nanomaterials like nanocrystalline tungsten-titanium diboride-copper composite are promising options for improving the performance features and life of these ignitors.
Longer-Lasting Medical Implants
In general, medical implants like heart valves and orthopedic implants are made of stainless steel and titanium alloys. These alloys are mainly used in humans as they are biocompatible, that is, they do not adversely react with human tissue. These materials are comparatively non-porous when used in orthopedic implants (artificial bones for hip, etc.).
If an implant must mimic a natural human bone in an effective way, the adjacent tissue must penetrate the implants, thus offering the implant the necessary strength. Since these materials are comparatively impermeable, human tissue does not penetrate the implants, thus minimizing their effectiveness.
Moreover, these metal alloys wear out fast, requiring frequent and very expensive surgeries. But nanocrystalline zirconia (zirconium oxide) ceramic is hard, corrosion-resistant (biological fluids are corrosive), wear-resistant, and biocompatible.
It is also possible to make nanoceramics porous as aerogels (aerogels can endure up to 100 times their weight) if they are produced using sol-gel methods. This would lead to much lesser implant replacements, and thus a substantial reduction in surgical expenses. Nanocrystalline silicon carbide (SiC) is a potential material for artificial heart valves mainly because of its low weight, wear resistance, extreme hardness, high strength, corrosion resistance, and inertness (SiC does not react with biological fluids).
Ductile, Machinable Ceramics
As such, ceramics are extremely hard, brittle, and tough to machine. These properties of ceramics have dissuaded prospective users from leveraging their advantageous properties. However, these ceramics have been used more and more with reduced grain size. Zirconia, a hard, brittle ceramic, has even been made a superplastic, that is, it can be deformed to greater lengths (up to 300% of its initial length). However, these ceramics must have nanocrystalline grains to be superplastic.
Actually, nanocrystalline ceramics like silicon carbide (SiC) and silicon nitride (Si3N4) have been used in automotive applications such as ball bearings, high-strength springs, and valve lifters. This is because they have good machinability and formability, together with superior physical, mechanical, and chemical properties. They are also used as components in high-temperature furnaces.
It is possible to press and sinter nanocrystalline ceramics into different shapes at considerably lower temperatures. By contrast, it would be very hard, if not impracticable, to press and sinter traditional ceramics even at high temperatures.
Large Electrochromic Display Devices
An electrochromic device comprises materials wherein an optical absorption band can be added, or a current band can be modified by passing current through the materials, or by applying an electric field.
Nanocrystalline materials like tungstic oxide (WO3⋅xH2O) gel are used in huge electrochromic display devices. The reaction controlling electrochromism (a reversible coloration process that is influenced by an electric field) is the double-injection of ions (or protons, H+) and electrons, which form tungsten bronze by combining with the nanocrystalline tungstic acid. These devices are mainly used in ticker boards and public billboards to send information.
Electrochromic devices are analogous to liquid-crystal displays (LCD) generally used in watches and calculators. But electrochromic devices display information by changing in color in response to an applied voltage. The color gets bleached upon reversing the polarity. The resolution, contrast, and brightness of these devices largely rely on the tungstic acid gel’s grain size. Therefore, nanomaterials are being investigated for this purpose.
Summary
From the examples provided in the article, it is quite clear that nanocrystalline materials produced by the sol-gel method can be used in an extensive range of unique, new, and existing applications. It is also obvious that nanomaterials outclass their traditional counterparts by virtue of their excellent physical, chemical, and mechanical properties, and by their outstanding formability.