Areas of critical importance in dispersions and coatings for nanoparticle systems include (1) preparation of the dispersions; (2) stabilization of the dispersed phase; (3) scaleup and control of the process; and (4) preparation of subsequent nanoparticle coatings. U.S. involvement in the area of dispersion preparation includes work in lasers, single electron transistors, photoresists, oxide systems, photographic emulsions, inks, magnetic recording materials, fillers, and paper coatings.
Preparing materials in the nanosize range normally makes use of liquid phase precipitation and sol-gel formation as wet chemical methods. Hybrid chemical/physical methods include both spray pyrolysis and flame hydrolysis. There are also numerous physical methods such as mechanical size reduction (which includes media mills and high shear agitation in presence of media) and physical vapor phase separation.
Described within this chapter is work at Lucent Technologies (Bell Labs) by P. Wiltzius. He discusses sol-gel processing where a concentrated colloidal dispersion is chemically converted into a gel. After drying such a dispersion, a glass or ceramic product is obtained. Such a process, when properly controlled, can lead to a variety of shapes -- particles, fibers, thin films, tubes as plates -- that are stable. Wiltzius describes a silica casting process yielding tubes of one meter or more in length. He also describes polymer dispersed liquid crystals for cellular phones and hand-held computers, and chemical mechanical polishing for semiconductor applications.
Another preparation described within this chapter is physical vapor synthesis. Such work, reported by R. Brotzman from Nanophase Technologies, utilizes vaporized metal particles in the presence of a reactive gas to yield 25 nm nanocrystalline metal oxides. These resulting powders have high purity, spherical morphology, and controlled aggregation. Applications are numerous, including assembled ceramic composites, coatings, and dispersed systems for cosmetics, inks, and specialty applications in electronics and industrial catalysts.
Integral to the success of dispersion preparation is the scaleup and control of the process. In Chapter 4 S.K. Friedlander of UCLA describes in detail various aspects of aerosol reactors. Commercial and pilot scale activities in these areas include flame reactors for the production of pyrogenic silica where worldwide production in 1991 reached 100,000 tons. Friedlander describes pyrolysis reactors for preparing carbon black particles in the 20 to 300 nm range. With evaporation/condensation aerosol generators, a metal is evaporated into an inert gas, and then the vapor is systematically cooled to produce 3-100 nm particles. Such a process, although batch size limited, has the benefits of producing fairly pure materials.
B. Kear of Rutgers University describes in Chapter 5 a three-step process for preparing tungsten/ carbon/cobalt powders. The first step is where tungsten and cobalt are prepared as aqueous solutions; secondly, spray drying of these solutions produces powders; then these powders are reduced in a fluidized bed to nanophase material. Such a process has good scalabilty and compositional range.
In all the cited processes for production/dispersion of nanoparticles there remains a need to
- control nucleation and grain growth
- establish conditions for controlling the process for high reproducibility
- determine process/product relationships that lead to continuous uniformity
- characterize the interfacial properties between the continuous and dispersed phases, regardless of composition
- develop process models to scale systems effectively and shorten cycle time to manufacturing
In this chapter, M. Gell of the University of Connecticut describes issues in coating nanostructural materials. Availability and reproducibility of starting materials are critical. The deposition process for creating the coated layer requires careful control of the process parameters so as to achieve critical nanostructured properties of increased hardness, adherence, and strength for systems such as cobalt/tungsten/carbon coatings and erosion-resistant layers with polymer matrix composites. High temperature, crack-resistant coatings require oxygen-impermeable layers. Also, advanced thermal barrier coatings will need nanostructures with reduced conductivity.
Accordingly, preparation of nanoparticle dispersions and subsequent deposition in coatings will require careful attention to raw material uniformity, process control for generating the particles, and stabilization of the nanosize dispersion to prevent agglomeration. These will be a few of the challenges facing the technical community in the future.
Brotzman, R., 1997. Nanoparticle dispersions. Presentation at the WTEC workshop on R&D status and trends in nanoparticles, nanostructured materials, and nanodevices in the United States, May 8-9, Rosslyn, VA.
Friedlander, S.K. 1993. Controlled synthesis of nanosized particles by aerosol processes. Aerosol Sci. Tech. 19, 527.
Gell, M. 1997. Nanostructured coatings. Presentation at the WTEC workshop on R&D status and trends in nanoparticles, nanostructured materials, and nanodevices in the United States, May 8-9, Rosslyn, VA.
Kear, B. 1997. Nanostructured bulk materials: Synthesis, processing, properties, and performance. Presentation at the WTEC workshop on R&D status and trends in nanoparticles, nanostructured materials, and nanodevices in the United States, May 8-9, Rosslyn, VA.
Siegel, R.W. et al. 1994. U.S. Patents 5128081 and 5320800.
Siegel, R.W. and J.A. Eastman. 1989. Mat. Res.Soc., Symp. Proc. 132, 3.
Wiltzius, P. 1997. Nature, 385, 321.