Nanomaterials: Manufacturing, Processing, and Applications Part 1 (Nanotechnology)

INTRODUCTION

The particles with small size in the range from a few to several tens of nanometers are called quasi zero-dimensional mesoscopic system, quantum dots, quantized or Q-particles, etc.[1] The reason that nanoscale materials and structures are so interesting is that size constraints often produce qualitatively new behavior. Nanotechnology arises from the exploitation of new properties, phenomena, processes, and functionalities that matter exhibits at intermediate sizes between isolated atoms or molecules (— 1 nm) and bulk materials (over 100 nm). As opposed to the microscale, the nanoscale is not just another step toward miniaturization, but is a qualitatively new scale. Hence quantum and size phenomena are allowed to manifest themselves either at a purely quantum level or in a certain admixture of quantum and classical components. At the foundation of nanosystems lie the quantum manifestations of matter that become relevant. Consequently, instead of being a limitation or an elusive frontier, quantum phenomena have become the crucial enabling tool for nanotechnology. Extensive research on semiconductor quantum dots has shown that these particles have properties halfway between macroscopic (bulk) and microscopic (molecular-like) substances and have recently aroused great interest in laser, photochemistry, and nonlinear optics.1-2-4-1 Bawendi et al.[3] have observed a number of discrete electronic transitions and LO-phonon progression which were cleanly resolved for the first time in nanometer-scale cluster in CdSe. Jungnickel and Henneber-ger[5] have described the luminescence properties of semiconductor nanocrystals and the carrier processes that are relevant for the light emission. Their study was concentrated on nanocrystal of size «5 nm, and hence observed strong carrier confinement. A size dependence in the luminescence efficiency of ZnS:Mn nanocrystals has also been observed by Bargava et. al.[6] and stated that the Mn2+ ion d-electron states act as efficient luminescent centers while interacting with s-p electronic states of the host nanocrystals. They showed that this electronic interaction provides an effective energy transfer path and leads to high luminescent efficiencies at room temperature and hence suggested that nanocrystals doped with optically active luminescent centers may create new opportunities in the study and application of nanoscale material structures.


Because nanomaterials possess unique, beneficial chemical, physical, and mechanical properties, they can be used for a wide variety of applications. This review primarily focuses on the synthesis, properties, and applications of nanomaterials. It has been proven that the particles at the nanometer level have improved quality with respect to their potential application that include, but are not limited to, various structural, optical, electrical, mechanical, and catalytic activity, biomedical, next-generation computer chips, kinetic energy (KE) penetrators with enhanced lethality, better insulation materials, low-cost flat-panel displays, elimination of pollutants, tougher and harder cutting tools, high-sensitivity sensors, high-power magnets, future weapon platforms, aerospace, large lasting satellites, longer-lasting medical implants, corrosion resistance, etc. Fig. 1 shows the improvements in the final properties of the nanomaterials.

SYNTHESIS OF NANOMATERIALS

There are several methods that can be used to synthesize solids. Solids can be also prepared in various forms as fibers, films, foams, ceramics, powders, single crystals, and nanoparticles. However, those solids, which are not thermodynamically stable, may be much more difficult to prepare and may require special methods. The oldest and widely used method is the solid-state routing of synthesizing metal oxides. In this traditional technique, the powder reactants are mixed together, pressed into pellets or some other shape, and then heated in a furnace for prolonged periods. However, this method is not very sophisticated because of the following reasons:

• It requires a high temperature to react the reactants.

• Slow diffusion of ions.

• Unhomogenized reaction mixtures.

Applications of nanomaterials.

Fig. 1 Applications of nanomaterials.

• Impure final product because of unreacted reactants.

• Large particle size and bimodal particle size distribution.

• Defects, e.g., points/line/twinning.

• Metal oxides with unusual oxidation states cannot be prepared, e.g., vanadates and tungstates.

• Low surface area.

Solids with nanosize particle size cannot be prepared by traditional method simply because the reactants are not mixed on the atomic scale. All the alternative methods, e.g., hydrothermal, sol-gel, Pechini, CVD, and microwave, described in the rest of this section address this problem by achieving atomic scale mixing of reactants, in gas, liquid, or even solid phases. Most of these are low-temperature methods, although finally firing may be required at high temperatures especially for ceramic-type products. These methods enable the final product with the following characteristics:

• Nanosize particles.

• Narrow particle size distribution.

• High surface area.

• Homogenous.

• Pure.

• Improved properties.

Hydrothermal Synthesis

Hydrothermal methods are becoming a popular technique to precipitate mixed metal oxides directly from either homogeneous or heterogeneous solution. Hydrothermal method utilizes water under pressure and at temperatures above its normal boiling point as a means of speeding up the reactions between solids.[7] Water is an excellent solvent because of its high dielectric constant. This decreases with rising temperature and increases with rising pressure, with temperature effect predominating. In addition, the high dielectric constant of water is confirmed to a region of low temperature and high densities (pressure). This property is mainly responsible for increasing the solubility of many sparingly soluble compounds under hydrothermal conditions leading to many useful chemical reactions such as hydrolysis, precipitation, coprecipita-tion, and crystal growth.

Hydrothermal reactions are usually performed in closed vessels. The pressure-temperature relations of water at constant volume are shown in Fig. 2. The reac-tants are either dissolved or suspended in a known amount of water and are transferred to acid digestion reactors or autoclaves (Fig. 3). Under hydrothermal conditions, reactants otherwise difficult to dissolve can go into solution and reprecipitate.

Hydrothermal reaction is a single-step process for preparing several oxides and phosphates.[7-9] Oguri et al.[10] obtained narrow size distribution of spherical submicron titanium hydrous oxide, which could be readily transformed into polycrystalline anhydrous anatase with spherical morphology. Fine particles of ferroelectric lead titanate with high Curie temperature were prepared via hydrother-mal technique.1-11-1 Kutty and Balachandran synthesized lead zirconate titanate (PZT) in better compositional homogeneity and sinterability. This technique was further used for the fabrication of nanocrystalline metal oxides. Sharma et al.[12] have synthesized nanosize a-alumina using hydrothermal method with particle size of 10 nm. Quantum size particles (<10 nm) of Y2O3 could also be achieved by this technique at 170°C using seeds[12] and are shown in Fig. 4. This method was further employed for the fabrication of several other metal oxides, e.g., ZnO, TiO2, and ZrO2, with nanosize particles.[12-15]

Pressure-temperature relations for water at constant volume.

Fig. 2 Pressure-temperature relations for water at constant volume.

Sol-Gel Synthesis

In sol-gel synthesis, the precursors, which are essentially the starting compounds for the preparation of a colloid,consist of a metal or metalloid element surrounded by various links called ligands. These ligands do not include another metal or metalloid atom, but may be inorganic, such as aluminum nitrate [Al(NO3)2], or organic, such as aluminum butoxide [Al(OC4H9)3]. Metal alkoxides are more widely used than any other precursors because metal alkoxides react readily with water. However, for some nonsilicates, especially for transition-metal-oxide gels, inorganic precursors are used. The transition-metal-oxide gels are also used for obtaining thin-film ferroelectric materials such as barium titanate, electrochromic WO3 films, and semiconducting V2O5 films.[16-20] Fig. 5 is the high-resolution transmission electron microscopy (HRTEM) of a-Al2O3 derived by sol-gel method.

Schematic diagram of an autoclave.

Fig. 3 Schematic diagram of an autoclave.

Hydrothermally prepared nanosize yttria.

Fig. 4 Hydrothermally prepared nanosize yttria.

A gel can be classified as aquagel, alcogel, xerogel, and aerogel depending on the nature of the medium that is contained within the gel’s three-dimensional network of particles. An aquagel is a gel wherein water is contained within its interstices. An alcogel is a gel in which the water is replaced by alcohol substitution. When the gel is in as-dried condition, it is called a xerogel. If the gel is supercritically dried (a drying process in which a medium is replaced, by another medium, under controlled conditions so that the gel structure does not collapse), then the resulting gel is termed an aerogel, where the fluid trapped in the gel interstices is air. The techniques used to preserve the gel structure include freeze-drying. This apparatus is called the freeze-dryer and is used commercially to preserve foodstuffs such as instant coffee powders, dry milk powder, and nondairy coffee creamer, has been used to synthesize materials, and is available in large sizes. This method has also been used to synthesize dispersion-strengthened alloy and composite systems.[18] The rationale behind the preservation of the open structure of the aquagel or alcogel is to facilitate the accelerated expulsion of the fluid trapped at the interstices of the gel, which is made of a continuous, three-dimensional network of nanocrystalline particles, during the metal deposition process in a fluidized-bed reactor. The open structure (greater grain boundary area) of the aerogel lends itself to processing at very low temperatures unlike its commercial counterparts. This method is also useful in the hydrogen reduction, carburization, nitrida-tion, and a host of other surface treatment processes. A schematic of the aerogel is depicted in Fig. 6.

High-resolution transmission electron microscopy of a-alumina synthesized by sol-gel process.

Fig. 5 High-resolution transmission electron microscopy of a-alumina synthesized by sol-gel process.

The specific advantages of the sol-gel synthesis technique are as follows:

• Sol-gel synthesis is a very viable alternative method to produce nanocrystalline elemental, alloy, and composite powders in an efficient and cost-effective manner.

• Almost any combination of materials could be synthesized at very low temperatures.

• Greater control of material chemistry and homogeneity is possible.

• Sol-gel synthesized powders could be processed, such as for coating, carburization, and nitridation, at substantially lower temperatures.

• Nanocrystalline powders could be consolidated at much lower pressures and temperatures.

• Enhanced densification of high-temperature materials without the low-temperature binders, which are detrimental to their performance under extreme conditions, is also possible via sol-gel synthesis of nano-crystalline materials.

• Thermomechanical processing of the components could be accomplished at significantly lower processing conditions.

• Processes, such as infiltration, could be carried out uniformly because of the continuous, three-dimensional network of nanocrystalline particles.

However, there are several factors that affect the sol-gel chemistry, but among them, the pH of the aqueous solution plays important roles in the particle morphology, stability, and the particle size of the final reaction products. During the polymerization process, the three-dimensional networks of particles serve as nuclei for further growth. This growth proceeds by a mechanism called Ostwald ripening whereby particles dissolve and repreci-pitate on larger, less-soluble nuclei. Ostwald ripening ceases to exist when the difference in solubility between the smallest and largest particles becomes negligible. Nevertheless, this growth continues to larger sizes at higher temperatures. Europium-doped ytrrium oxide (Eu:Y2O3) was synthesized by a sol-gel method in the presence of Tween-80 and e-caprolactam in pH range 4 -10. It has been observed that the variation in surface area, pore size, and pore volume of the final product was strongly dependent on the initial pH of the solution. The powder with a large surface area 230 m2/g) and low pore diameter 16 nm) was obtained when the powder was processed at pH ~ 4. The crystallite sizes of the powders processed at pH ~ 4 and 10 were found to be 35 and 198 nm, respectively.

A schematic of an aerogel structure.

Fig. 6 A schematic of an aerogel structure.

At low pH, the reaction rate of the hydrolysis is governed by the hydronium ion in solution (H2O+H+ ! H3O+) and is also observed by Sakka and Kamiya[21] (described below). In this reaction, the amount of water is small because of the rapid formation of H3O+. Cagle and Keefer have stated that the hydrolysis/condensation in low pH condition is relatively controlled and selective, thus generating relatively more linear polymers of metal.[22-24] Hydrolysis can be represented by the following equation

tmp187118_thumb

Condensation can take place by any of the following two equations:

tmp187119_thumb

The linear polymerization can be explained by a simple steric argument: monomers (II) are more readily hydrolyzed than dimers (III or IV), which are, in turn, more readily hydrolyzed than middle groups in chains. Therefore the reaction polymerization at low pH is expected to be a linear chain (III or IV) with low cross-links and is also suggested by Pope and Mackenzie.[25]

At high pH, the reaction is governed by the hydroxyl ions (OH). Although the initial growth leads to linear chains, because of the high concentration of OH ions, it results in the cyclization because the probability of in-termolecular reaction is higher than intramolecular reac-tion.[26] At high pH value, hydrolysis/condensation is uncontrolled and unselective, which leads to highly branched polymers. It also generates larger interconnected particles.[26-28] The polymeric chain at high pH is larger than the one at low pH. At high pH, the most probable metal-oxygen polymeric network formed in the chain is the structure V as shown in Fig. 7. Nevertheless, the larger interstices at pH>7 result in larger grains, as shown in Fig. 7. Thus the crystallite size of the powder at pH>7 (198 nm at pH ~ 10) was smaller than the powder at pH < 7 (35 nm at pH~4). At pH~ 10, a cube-like morphology of the particles is seen in Fig. 8. In contrast, the morphology at pH ~ 4 has totally changed into polygonal shape with size of 40 nm (shown in Fig. 8).

tmp187120_thumbSchematic diagram of polymeric network in different

Fig. 7 Schematic diagram of polymeric network in different

Modified sol-gel synthesis: Microemulsions as microreactors

Microemulsion-based sol-gel synthesis is a versatile technique to prepare materials with novel microstructures, in particular, ultrafine (nanosize) powders, e.g., TiO2, Al2O3, ZrO2, etc.[30-32] A microemulsion may be defined as a thermodynamically stable, optically isotropic solution of two immiscible liquids (e.g., water and oil) consisting of microdomains of one or both liquids stabilized by an interfacial film of surfactants.[33,34] The surfactant molecule generally has a polar (hydrophilic) head group and a long chained aliphatic (hydrophobic) tail. Such molecules optimize their interactions by residing at the oil / water interface, thereby considerably reducing the interfacial tension. In water-in-oil microemulsion, the aqueous phase is dispersed as microdroplets (typically 10-25 nm in size) surrounded by a monolayer of surfactant molecules in the continuous hydrocarbon phase. The aqueous cores of microemulsions containing soluble metal salts are used as microreactors for the synthesis of nanoparticles. Because of the dynamic nature of the microdroplets, the exchange mechanism involves coalescence and fusion of the droplets upon collision, which then disintegrate into droplets, and this process occurs continuously in the microemulsion.[35] If two reactants, A and B, are dissolved in the aqueous core of two identical water-in-oil microemulsions, upon mixing, they will form a precipitate, AB. The growth of these particles in microemulsion is suggested to involve interdrop exchange and nuclei aggregation.[36,37] Recently, this method has been applied for the fabrication of cubic BaTiO3 (please refer to Fig. 9).

High-resolution transmission electron microscopy of Eu-doped yttria at pH (a) 4 and (b) 10.

Fig. 8 High-resolution transmission electron microscopy of Eu-doped yttria at pH (a) 4 and (b) 10.

Nanosize metal oxides synthesized by microemulsion-mediated sol-gel.

Fig. 9 Nanosize metal oxides synthesized by microemulsion-mediated sol-gel.

Polymerized Complex Method

Wet chemical method using polymeric precursor based on the Pechini process has been employed to prepare a wide variety of ceramics oxides.[38] The process offers several advantages for processing ceramic powders such as direct and precise control of stoichiometry, uniform mixing of multicomponents on a molecular scale, and homogeneity. In this process, an alpha hydroxycarboxylic acid, preferentially citric acid, is used to chelate various cations by forming a polybasic acid. In the presence of a polyhy-droxy alcohol, normally ethylene glycol, these chelates react with the alcohol to form ester and water by-products. When the mixture is heated, polyesterification occurs in the liquid solution and results in a homogenous sol, in which metal ions are uniformly distributed throughout the organic polymeric matrix. When excess solvents are removed, an intermediate resin is formed. This resin gives metal oxides on burning. All the organic matter removes on heat treatment.

In polymerized complex method, several metal ions in a solution could be first chelated to form metal complexes and then polymerized to form a gel, which seems to be one of the most suitable among several other chemical solution processes of nanocrystalline particles because rigidly fixed cations are homogeneously dispersed in the polymer network and have few chances to segregate even during pyrolysis. This method has been already successfully applied to prepared highly pure samples of various double oxides such as BaTiO3,[39] Y6WO12,[40] mixed-cation oxides,[41] and even for various superconductors[42] with multiple cationic compositions.

Chemical Vapor Deposition

Chemical vapor deposition (CVD) may be defined as the deposition of a solid on a heated surface from a chemical reaction in the vapor phase. It is a versatile process suitable for the manufacturing of coatings, powders, fibers, and monolithic components. It is possible to produce most metals, metal oxides, and nonmetallic elements such as carbon and silicon as a large number of compounds including carbides, nitrides, oxides, intermetallics, and many others. The main advantage of CVD is that the deposition rate is high and thick coatings or nanoparticles can be readily obtained. The process is generally competitive and, in some cases, more economical than the physical vapor deposition (PVD). Additionally, it is not restricted to a line of sight deposition, which is a general characteristic of sputtering, evaporation, and other PVD processes. However, two major areas of applications of CVD have rapidly developed the last 20 years or so, namely, in the semiconductor industry and in the metallurgical coating industry which includes cutting tool fabrication. Very recently, the CVD process has been given enormous attention owing to the possibility of mass production of monodisperse nanoscale powders; however, the mechanism of powder synthesis kinetics is still not clear.[43-46] Kim et al.[46] have synthesized nanosize TiO2 powders using CVD. Carbon nanotubes have also synthesized by CVD method using Fe-Mo nanoparticles.[47]

Microwave Synthesis

Recently, there has been a growing interest in heating and sintering of ceramics by the microwave energy.[48,49] The interest in the use of microwave processing spans a number of fields from food processing to medical applications to chemical applications. A major area of research in microwave processing of ceramics includes microwave material interaction, dielectric measurement, microwave equipment design, new material development, sintering, joining, and modeling. Therefore the microwave processing of ceramics has emerged as a successful alternative to conventional processing. Nevertheless, microwave method not only offers the advantages of a uniform heating at lower temperature and time than the conventional method, but also provides an economic method of processing. The microwave energy has been already successively utilized in the fabrication of ceramics as well as carbon fibers at low temperature and time. Varadan et al.[50] and Sharma et al.[51] have synthesized various electroceramics such as barium strontium titanate (BST) and lead zirconate tita-nate (PZT) by microwave. Fig. 10 shows the schematic diagram of a typical domestic microwave unit used by Sharma et al. These materials are observed to have improved mechanical, electrical, and electronic properties. Until recently, microcoiled carbon fibers with large surface area have also been fabricated by using microwave aid.[52]

Fig. 11 shows a schematic diagram of a microwave chemical deposition unit used for the fabrication of carbon nanotubes and coils. It consists of microwave magnetron, circulator, four-stub tuner, waveguide, cavity, etc. The microwave power can be adjusted from 0 to 3000 W at a frequency of 2.45 GHz. The function of circulator was to prevent power reflected by the load, thus preventing overheating of the magnetron. The forward and reflected powers were determined by a power meter that is helpful in determining impedance matching. The four-stub tuner, consisting four threaded stubs spaced at 3/8 wavelength apart, was another part to optimize impedance matching.

Schematic diagram of microwave used for the powder.

Fig. 10 Schematic diagram of microwave used for the powder.

Schematic diagram of the microwave chemical vapor deposition of carbon nanotubes.

Fig. 11 Schematic diagram of the microwave chemical vapor deposition of carbon nanotubes.

These stubs were adjusted properly, and the four-stub tuner became a matching network which maximized the power transmitted to the load by matching the source impedance to that of the load. As an important part of cavity, sliding short was used to adjust the length of the cavity such that it could resonate at 2.45 GHz. High field intensities could be attained when the cavity resonates. A quartz tube, which was the reaction chamber, passed through the cavity. Reaction gases were introduced from one end of the quartz tube and exhausted at the other end. The flow rates were controlled by a set of flow controller. In this microwave CVD system, SiC was chosen as substrate because of its high loss tangent; thus it could absorb microwave energy effectively. A fibrous morphology with a hollow tube inside was obtained. The diameter of these Multi-wall nano tubes (MWNT) ranges from 20 to 30 nm as shown in Figs. 12-14.

Transmission electron microscopy of the CNTs obtained from microwave.

Fig. 12 Transmission electron microscopy of the CNTs obtained from microwave.

High-resolution transmission electron microscopy of MWCNT from microwave CVD.

Fig. 13 High-resolution transmission electron microscopy of MWCNT from microwave CVD.

High-resolution transmission electron microscopy of MWNTs with Encapsulated Co.

Fig. 14 High-resolution transmission electron microscopy of MWNTs with Encapsulated Co.

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