More than 10 years ago, as capabilities of measuring particles below 100 nanometers (nm) were developed, significant research focused on "ultrafine" particles resulting from vehicle emissions and combustion-related manufacturing processes such as welding Since that initial research into nanoparticles as byproducts, interest in engineered nanoparticles has grown The breadth of processes creating and utilizing nanoscale materials raises more challenges . Engineered nanomaterials are being created via multiple methods, for example, arc discharge, laser ablation, CVD, gas-phase synthesis, sol-gel synthesis, and high-energy ball milling These processes can begin from the "bottom up," assembling nanomaterials from their components, for example by chemical synthesis or phase change processes Other manufacturing methods begin with bulk materials, reducing their size via mass change processes to create nanomaterials from the "top down "
The bottom-up synthesis routes are, by far, the most widely used for nanoparti-cles While engineered nanoparticles often are thought of as precursors or raw materials to be incorporated into higher value-added products via one of the five families of processes described previously in this chapter, the initial step of synthesizing nanoparticles most closely fits within the family of "phase change processes," which includes processes such as CVD . The use of top-down methods such as high-energy ball milling is limited to larger diameter particles with less stringent monodispersity and purity requirements Ball milling is essentially a grinding process that would fit within the machining processes of the "mass change processes "
As with the other manufacturing processes, the process-structure-property interrelationships are significant For example, the manufacturing process can affect the atomic structure of carbon nanotubes, which in turn affects many properties, such as the electrical conductivity (e g , metallic vs semiconducting), thermal conductivity, strength, and stiffness One relatively coarse difference is the production of singlewalled nanotubes (SWNTs) versus multi-walled nanotubes (MWNTs) . Single-walled nanotubes have better conductivity and strength properties but are much less reactive and therefore more difficult to functionalize (i e , to create compatibility with other materials for bonding) In general, the properties of nanoparticles are governed by process-induced factors such as the size and size distribution, degree of porosity, and surface reactivity In synthesis processes, size and structure can be controlled through the use of catalyst particles, template materials (e g , to control nucleation and precipitation behavior), and controlled-size droplets or aerosols
The six nanomaterials that are the focus of this book — carbon black, carbon nanotubes, fullerenes (also known as C60 or buckyballs), nano silver, nano titanium dioxide, and nano zero-valent iron — can all be fabricated using many methods, and with the interest in nanomaterials, new methods are being discovered rapidly A quick search in the U. S . Patent and Trademark Office database  brings up roughly 50 patents issued in the past two years with "nanoparticle" in the title . These patents include methods of making nanoparticles, modifying nanoparticles, and products incorporating nanoparticles Table 3 1 provides a few examples of manufacturing techniques for the six target materials
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