1. NANOMATERIALS
1.1.1. Definitions: nanoscience, nanotechnology and nanomaterials It is the ability to design and characterize materials at the nanoscale that distinguishes modern nanotechnology from previous activities in materials science and chemistry. Although nanotechnology is widely talked about, there is little consensus about where the nanodomain begins. In the recent report [Nanoscience and nanotechnologies: opportunities and uncertainties, The Royal Society and the Royal Academy of Engineering, London, 2004], the definitions of nanosciene and nanotechnology are presented without the use of dimensions. Nanoscience is the study of phenomena and manipulation of materials at atomic, molecular and macromolecular scales, where properties differ significantly from those at a large scale. Nanotechnologies are the design, characterization, production, and application of structures, devices, and systems by controlling shape and size on the nanoscale. Nanomaterials is the interdisciplinary discipline crossing the boundary between nanoscience and nanotechnology and link the two areas together. It is recognized that the size range that provides the greatest potential and, hence, the greatest interest is that below 100 nm (equivalent to approximately 500 atom diameters), however, there are still many applications for which larger particles can provide properties of great interest. Nanoparticles can come in a wide range of morphologies from spheres, through flakes and platelets, to dendritic structures, tubes, and rods. Furthermore, nanomaterials cover a hugely diverse range of materials: polymers, metals and ceramics.
1.1.2. Critical dimensions Not all materials that can be made into very small particles mean they have practical use. However, the fact that these materials can be made at nanoscale gives them the potential to have some very interesting properties which sharply differ from ones characteristic for massive materials. It is important that materials at the nanoscale between 1 nm and 100 nm lie between the quantum effects of atoms and molecules and the bulk properties of materials. It is this scale where many properties of materials are controlled by phenomena that have their critical dimensions at the nanoscale. By being able to fabricate and control the structure of nanoparticles, the designers can influence the resulting properties and, ultimately, design materials to give desired properties. It is widely known, the electronic properties can be controlled at this scale. This phenomena forms the basis for modern electronics industry. Recent investigations have shown that the range of applications where the physical size of the particle can provide enhanced properties that are of benefit are extremely wide.
1.1.3. Nanoparticles fabrication Manufacturing nanoparticles can be achieved through a wide variety of different routes. In essence, there are four generic routes to make nanoparticles: wet chemical, mechanical, form-in-place, and gas-phase synthesis. Wet chemical processes include chemistry, hydrothermal methods, sol-gels, and other precipitation processes. Essentially, solutions of different ions are mixed in well-defined quantities and under controlled conditions of heat, temperature and pressure to promote the formation of insoluble compounds, which precipitate out of solution. These precipitates are then collected through filtering and/or spray drying to produce a dry powder. Mechanical processes include grinding, milling, and mechanical alloying techniques. Provided that there is a coarse powder, this coarse powder mechanically is transformed into finer and finer powder. The most common processes are either planetary or rotating ball mills. The advantages of these techniques are that they are very simple, require low cost equipment and, provided that a coarse feedstock powder and be made, the powder can be processed. However, there are difficulties such as agglomeration of particles, broad particle size distribution, contamination from the process equipment, and often difficulty in getting to the very fine particle sizes with viable yield. It is commonly used for metals and inorganics. Gas phase synthesis includes flame pyrolysis, electro-explosion, laser ablation, and plasma synthesis techniques. The production of fullerenes and carbon nanotubes is a specific subset of gas-phase synthesis techniques. Form-in-place processes vacuum deposition processes such as physical vapour deposition (PVD) and chemical vapour deposition (CVD), and spray coating. These processes are focused to the production of nanostructured layers and coatings with enhanced properties for different applications. These processes are quite inefficient for the fabrication of powders. Powders can be manufactured by scraping the deposits from the collector. The recently gained knowledge on the atomic and molecular level about macroscopic phenomena (adsorption, bonding, catalysis, oxidation and other surface reactions, diffusion, desorption, melting and other phase transformations, nucleation, growth, friction, hardness and lubrication and etc.) has given new understanding about processes in materials used for different applications in extreme environment conditions. Materials used for specified applications can be divided into two groups: (i) those that utilize external surfaces, and (ii) nanostructured materials where most of surfaces of nanoparticles composing material resides at interfaces in the volume of material.
1.1.4. External surface The concentration of atoms or molecules at the surface of a solid or liquid can be estimated from the bulk density. For a bulk density of 1 g/cm3 (such as ice or water), the molecular density ρ – in units of molecules per cm3 is ≈ 5⋅1022. The
surface concentration of molecules σ (molecules/cm2) is proportional to σ2/3, assuming cube like packing, and on the order of 1015 molecules/cm2. Because the densities of most solids or liquids are within a factor of 10 or so of each other, 1015 molecules/cm2 is a good order-of-magnitude estimate of the surface concentration of atoms or molecules fro most solids or liquids. Of course, surface atom concentration of crystalline solids may vary by a factor of two or three, depending on the type of packing of atoms at a particular crystal face. Thin films are of great importance to many real-world problems. Their material costs are very little as compared to the bulk material, and they perform the same function in surface processes. For example, a monolayer of rhodium, a very expensive metal, which contains only about 1015 metal atoms per cm2, can catalyze the reduction of NO to N2 by its reaction with CO in the catalytic converter of an automobile, or it can catalyze the conversion of methanol to acetic acid by the insertion of a CO molecule.
1.1.5. Dispersed materials
Dispersion D
Dispersed materials are constructed from clusters and small particles. Their properties depend on the size of clusters. Let us introduce quantitative characteristic of individual clusters and particles.
Number of atoms in the cluster n
Fig. 1. Clusters of atoms with cubic packing having 8, 27, 64, 125, and 216 atoms. Whereas in an eight atom cluster all of the atoms are on the surface, the dispersion rapidly declines with increasing cluster size, as shown in the lower part of the figure [1]
All of the atoms in a three- or four-atom cluster are by necessity “surface atoms”. As a cluster grows in size, some atoms may become completely surrounded by neighboring atoms and are thus no longer on the “surface” (Fig. 1). A particle of
finite size is frequently described by its dispersion D, where D is the ratio of the number of surface atoms to the total number of atoms
D=
number of surface atoms total number of atoms
For very small particles, D is unity. As the particle grows and some atoms become surrounded by their neighbors, the dispersion decreases (Fig. 1). Of course, D also depends somewhat on the shape of the particles and how the atoms are packed. The dispersion is already as low as 10-3 for particles of 10 nm (100 Å) radius. Many chemical reactions are facilitated by surface atoms. Consider a monolayer of gold atoms (a layer of gold one-atom thick) deposited on iron. This film has a dispersion of unity since all the gold atoms are on the surface. About 50 layers of gold atoms (D = 1/50) are needed to obtain the optical properties that impart the familiar yellow color characteristic of bulk gold. Often the surface of a material is roughened deliberately. Automobile brake pads are designed to optimize the desired mechanical properties of surfaces in this way, as is the corrugated design of rubber soles of tennis shoes. The large number of folds of the human brain helps to maximize the number of surface sites, which also facilitate charge transport of molecules. These are some of the examples that show how external surfaces are frequently used in nature. External surfaces are a key element of technology, ranging from catalysts and passivating coatings, to computerintegrated circuitry and the storage and retrieval of information.
1.1.6. Internal surface
Nanostructured thin films-materials full of nanocrystallites (nanoparticles) have very large internal interface surface areas. Many thin films that are nanostructured accommodate atoms and molecules at the interface between nanograins where atoms and molecules can adsorb or undergo chemical reactions. These materials adsorb preferentially certain atoms and molecules according to their size and/or polarizability. This properties is of great commercial importance and may be used to store gases in thin film materials, to separate mixtures of gases (selective membranes), or to carry out selective chemical reactions. The properties of the internal surfaces in nanostructured materials are under intensive investigations. These studies will result in rapid developments in molecular studies of phenomena at the buried interfaces including electrochemistry, tribology, and biology and will lead to rapid developments of new nanotechnologies. In the following Chapters, the basic knowledge about thin film growth kinetics using physical vapour deposition technology, and the evolution of film microstructure is presented.
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