NANOPARTICLE DISPERSIONS

Richard Brotzman
Nanophase Technologies Corporation
453 Commerce St.
Burr Ridge, IL 60521

Scientific Drivers

Nanophase Technologies Corporation (NTC) was spun off from Argonne National Laboratory in 1989 and is devoted to the commercialization of nanophase materials. NTC technology is based on gas phase condensation (GPC) techniques for synthesizing inorganic and metallic powders invented by R.W. Siegel et al. (1994; patented by ARCH Development Corporation and assigned to NTC). The basis of GPC is the evolution of a physical vapor from the evaporation of elemental or reacted material followed by immediate condensation and reaction of the vapor into small nanometer particles (Siegel and Eastman 1989). To maintain nanometer-sized particles and weak agglomeration, the aerosol is rapidly cooled and diluted to prevent extensive sintering (to form hard agglomerates) and coalescence growth (aggregates).

Early GPC processes employed natural convection flow current for streaming an elemental reactant into the reactive/condensation chamber. Natural convection of an inert gas at low pressure (e.g., helium at 5 - 50 torr) draws condensed crystallites out of the evaporation/condensation region to the (thermophoretic) collection device. Natural convection is established and limited by a temperature gradient between the hot (e.g., 1500°C) evaporation source and the cold (e.g., -200°C) particle collection device. Although an increase in heating drives natural convection faster, the flow rate is insufficient to overcome the concomitant increase in metal evaporation rate (vapor pressure increases exponentially with temperature). As a result, crystallites become larger and aggregate. In addition to the limitations of natural convection, oxidation of metal crystallites into metal-oxide ceramics must occur in a separate processing step. This type of oxidation process is not conducive to commercial-scale production, and in fact, oxidation is often incomplete (Parker and Siegel 1990).

NTC has developed and patented (Parker et al. 1996) a production system using forced convection flow in order to (1) allow control of the particle/gas stream by eliminating the ill-defined flow pattern of natural convection, (2) speed particle transport from the particle growth region to increase metal vapor generation, and (3) complete reaction of metal crystallites to form oxides and nitrides. The process employs a transferred arc to produce metal vapor in conjunction with forced gas flow; the high cooling rate of the plasma tail flame allows the formation of nanocrystals (typically ca. 8-30 nm) that weakly aggregate into submicron particles. The process produces commercial quantities of nanosized inorganic powders that are spheroidal and have narrow particle size distribution.

Economic Drivers

The NTC process reduced the cost of powder from thousands of dollars per gram to dollars per pound while maintaining the high quality observed in laboratory scale powders. Yet marketing efforts revealed only a modest opportunity for application in industry. The lack of market acceptance of nanocrystalline powders for bulk use is largely attributed to the relatively high expense involved in synthesizing and handling these materials, which must compete with conventional materials at nearly one-tenth the cost.

NTC is today a market-driven company that tries to provide what the buyer wants versus a marketing company than sells what it already makes. Four markets are targeted: electronics, structural ceramics and composites, cosmetics, and industrial catalysts. Table 6.1 shows near- and long-term applications being pursued in collaborative relationships with marquee customers in the identified markets. In many applications, dispersed nanocrystalline powders are required.

Critical Parameters

Although every dispersion application is unique, the powder surface must always be rendered compatible with the dispersing fluid. If a coating is employed, individual powders should be coated with a minimal layer of material without causing aggregation and agglomeration. Additionally, in the cosmetics and skin care market, NTC titania must be chemically passivated and photostabilized by introducing surface impurity traps to force recombination of photogenerated excitons (Solomon and Hawthorne 1983).

Table 6.1
Applications for Nanoparticle Dispersions


NTC developed a coating process that encapsulates nanocrystalline particles with a durable coating not removed by subsequent processing to ensure that the particle size distribution generated by the GPC process is preserved after coating. Until patents are issued, details of coating chemistry and processing remain proprietary. The coating may be engineered to allow dispersion of nanoparticles in organic fluids with dielectric constants ranging from 2 to 20 as well as water, and steric stabilizers or specific chemical groups can be incorporated into the coating to affect greater dispersion stability or specific (targeted) chemical reactivity, respectively.

Bringing GPC Products to the Market

Examples of two dispersion products are given. NTC sells titanium dioxide dispersions for use in the cosmetics and skin care market as physical sunscreen ingredients. Sunscreen formulations are transparent with measured SPF values greater than 15 at 3 wt % titania and 20 at 5 wt % titania.

Abrasion-resistant polymers for oil drilling sensors are in field testing. Laboratory wear testing demonstrates that NTC coated Al2O3 covalently incorporated into an epoxy formulation using targeted chemical reactivity at 30 wt % provides nearly 4 times more wear resistance than 80 - 83 wt % filled epoxy and 19 times more wear resistance than 46.5 wt % filled elastomer-modified epoxy, which are the best commercial materials. The NTC coated Al2O3/epoxy dispersion is also the only material than can be processed by filament winding techniques.

References

Parker, J.C., and R.W. Siegel. 1990. Appl. Phys. Lett. 57(9): 943.

Parker, J.C., et al. 1995. U.S. Patent 5,460,701.

_____. 1996. U.S. Patent 5,514,349.

Siegel, R.W., et al. 1994. U.S. Patents 5,128,081 and 5,320,800.

Siegel, R.W., and J.A. Eastman. 1989. Mat. Res. Soc. Symp. Proc. 132: 3.

Solomon, D.H., and D.G. Hawthorne. 1983. Chemistry of pigments and fillers, Ch. 2. New York: John Wiley & Sons.

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Published: January 1998; WTEC Hyper-Librarian