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What are nanomaterials?

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Introduction

What are nanomaterials 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. All materials are composed of grains, which in turn comprise many atoms. These grains are usually invisible to the naked eye, depending on their size. Conventional materials have grains varying in size anywhere from 100's of microns (�m) to millimeters (mm). A micron (�m) is a micrometer or a millionth (10-6) of a meter. An average human hair is about 100 �m in diameter. A nanometer (nm) is even smaller a dimension than a �m, and is a billionth (10-9) of a meter. A nanocrystalline material has grains on the order of 1-100 nm. The average size of an atom is on the order of 1 to 2 angstroms (�) in radius. 1 nanometer comprises 10 �, and hence in one nm, there may be 3-5 atoms, depending on the atomic radii. Nanocrystalline materials are exceptionally strong, hard, ductile at high temperatures, wear-resistant, erosion-resistant, corrosion-resistant, and chemically very active. Nanocrystalline materials, or nanomaterials, are also much more formable than their conventional, commercially available counterparts. Nanomaterials research literally exploded in mid-1980's in the U. S. There are five widely known methods to produce nanomaterials, and they are as follows: * Sol-gel synthesis, * Inert gas condensation, * Mechanical alloying or high-energy ball milling, * Plasma synthesis, and * Electrodeposition. All these processes synthesize nanomaterials to varying degrees of commercially-viable quantities. To date, of all the above process, only sol-gel synthesis can * produce materials (both metals and ceramics) at ultra-low temperatures (around 150-600 �F vis-�-vis 2500-6500 �F for conventional techniques), * large quantities (to be commercially viable) relatively cheaply, * synthesize almost any material, * co-synthesize two or more materials simultaneously, * coat one or more materials onto other materials (metal or ceramic particulates, and three-dimensional objects), * produce extremely homogeneous alloys and composites, * synthesize ultra-high purity (99.9999%) ...read more.

Middle

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 contribute to environmental pollution by not completely combusting the gas. 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. ...read more.

Conclusion

Zirconia, a hard, brittle ceramic, has even been rendered superplastic, i. e., it can 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 color when a voltage is applied. When the polarity is reversed, the color 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. From the above examples, it is quite evident that nanocrystalline materials, synthesized by the sol-gel technique, can be used in a wide variety of new, unique and existing applications. It is also very evident that nanomaterials outperform their conventional counterparts because of their superior chemical, physical, and mechanical properties and of their exceptional formability. ...read more.

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