Originally appeared in Ceramic Industry: October, 2009
Based on dimensions and nanoscale properties, the market for nanomaterials can be broken into several general categories, including nanoparticles, nanofibers and nanolayers (or films). Within each category, definitions encompass many different types of materials that are formed both naturally and synthetically.
For example, nanoparticles include metal-oxide nanopowders, compound semiconductors, metals and alloys. At 65.7%, nanoparticles make up the largest segment of demand globally (see Figure 1). One of the most dynamic markets is the use of metal-oxide nanopowders in the manufacture of structural ceramics.
Quantities of production of metal-oxide nanopowders are expected to increase dramatically over the next five years as manufacturing processes continue to be refined and expanded. However, the major drawback in manufacturing progress continues to be the lack of unified standards and adequate specifications that define the quality, purity and production processes, which continually leads to inconsistent research and application results. Greater research is necessary and will continue to be a strong driver of the market in an effort to develop standards based on performance and manufacturing considerations.
Several mechanically or chemically based methods are currently in use to manufacture nanomaterials, and other methods are being explored on a research level that would increase performance and reduce costs. Major mechanical methods include ball milling, laser ablation, etching, sputtering, sonification and electroexplosion. Major chemical methods include chemical vapor deposition (CVD), sol-gel processing and molecular pyrolysis.
With this in mind, several manufacturing techniques have been researched to determine their cost structures and efficacy in developing superior products. One production technique that has been researched in terms of both cost and performance of materials is electroexplosion of wire (EEW), which has shown promise on the performance side in the manufacture of metal-oxide nanopowders.
The EEW process involves the destruction of metal wire through the application of a dense electrical current within a controlled chamber. The powders produced have a high density of crystal defects, which further intensifies their internal energy. The results are affected by characteristics that include electrical density, length of time during which the current is applied to the wire, temperature of the wire at the time the current is applied, material of the wire, and the ambient medium within the chamber.
A variety of metal powders are currently being produced (in kilogram quantities) in Russia and the U.S. Primary applications are in the area of microelectronics, namely in thick film paste formulations; as additives for propellants and pyrotechnics; for coatings; as sintering aids; and in the self-heating synthesis of high-temperature alloys and compounds.
Electroexploded metal nanopowders have been manufactured in ranges from 5 to 500 times smaller than commercially available metal powders; typical powder sizes average 100 nanometers. Nanopowders, particularly ceramic-based alumina (Al2O3), show increased chemical purity and more consistent grain size when manufactured using EEW. Also, EEW results in much higher chemical and metallurgical reactivity.
The challenges in developing structural ceramics generally have to do with fracture toughness. As is well documented, ceramics are by nature brittle materials. Thus, there is no yield stress-when the ceramic fractures, the system or component usually suffers complete failure. Further, in producing ceramics, another challenge has always been the development of uniform grains in terms of size, shape and density. EEW has been extremely effective in addressing these challenges. Thus, on a performance basis, EEW shows significant promise in the development and production of ceramic nanopowders.
EEW Cost Analysis
The main inhibiting factor in the further development of these nanopowders is cost. When evaluating these costs, primary considerations include:
- The use of machinery that requires a high-voltage power source, typically in the form of a generator or a direct line into the main AC power source for the facility
- Expensive equipment, such as vacuum-based systems, chemical reaction chambers, plasma torches and powder purification units, as well as a limited choice of materials
- High energy costs
Thus, the main application to date for EEW-produced Al2O3 is almost exclusively laboratory research.
The use of EEW-produced nanopowders can provide numerous advantages, including improved combustion for rocket fuels and propellants; enriched lubricants; improved catalysts; improved filtration systems; enriched lithium and nickel-metal-hydride batteries; and improved coatings and treatment for wear and corrosion resistance, as well as conductivity.
Another function of ceramic nanopowders is the formulation of ceramic nanocomposites to improve thermomechanical properties and fracture toughness. Ceramic fibers can be used for fortifying composites where the matrix is a ceramic (CMCs), metal (MMCs) or plastic. Smaller-diameter fibers are thought to have a greater strengthening benefit than larger diameter fibers. Using these fibers in CMCs is difficult, however, because alumina fibers have poor creep resistance. Alumina fibers are also likely to dissolve into the solid matrix.
The next five years will show some of the most dramatic changes in nanopowders since the commercialization of nanomaterials. The most fundamental changes will occur in the standardization and development of consistent processes for production. Yet, any model for production will have to find the correct balance between cost and performance.
This information is based on the report Nanostructured Materials: Developing Markets, Applications & Commercial Opportunities: 2004-2009 Analysis and Forecasts. The report was based on primary and secondary sources including surveying of over 150 scientists, materials suppliers and end-users.
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