Synthesis of Ultrafine, Multicomponent Particles Using Phospholipid Vesicles
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SYNTHESIS OF ULTRAFINE, MULTICOMPONENT PARTICLES USING PHOSPHOLIPID VESICLES H. LIU, G. L. GRAFF, M. HYDE, M. SARIKAYA, and I. A. AKSAY Department of Materials Science and Engineering, and Advanced Materials Technology Center, Washington Technology Center, University of Washington, Seattle, WA 98195 Because of their unique propertiesof self-assembly and selective ion permeability across the lipid bilayers, phospholipid vesicles were used as reaction vessels for the synthesis of ultrafine, multicomponent ceramicparticles containing Y, Ba, Cu, and Ag. Chemical inhomogeneities in the system were limited to the individual particle size (< 50 nun), which was a considerable improvement over particles prepared using bulk precipitation routes. The consistent barium deficiency was a serious problem that arose when attempting to control the stoichiometry of the multicomponent system. Our experimental evidence suggests that chemical interactionsbetween the barium cations and the vesicleforming phospholipidmay inhibit the precipitation of barium salts. In a parallel study, we performed consolidation studies on vesicle-precipitated Ag 2 0 particles before and after the removal of the phospholipidmolecules. Particlepacking was greatly improved in the surfactant coatedparticles. This demonstrates the multifunctionality of this biomimetic system in which the vesicle membrane simultaneously acts as: (i) a reaction cell for particleprecipitation, (ii) an ion selective membrane that affects precipitation kinetics, (iii) a barrier to prevent spontaneous agglomeration of the ultrafine particles,and (iv) a lubricant/dispersantthatfacilitatesparticle rearrangementduring consolidation.
INTRODUCTION Recent trends in powder processing have placed emphasis on the use of ever-smaller particles as starting materials. With the use of gas-phase precipitation routes such as CVD and the gas-condensation method, remarkable properties such as ductility in ceramic materials, and significant increases in strength and hardness have been observed in nanophase materials.1-4 Particle size reduction into the nanometer 5 range can also result in substantially altered chemical, electrical, optical, and magnetic properties. Although the improved properties of these nanophase systems are enticing, the synthesis, dispersion, and consolidation of these ultrafine particles is extremely challenging. 6 Gas phase particle synthesis typically requires sophisticated equipment and high vacuum. Once formed, nanometer-sized particles are susceptible to rapid agglomeration due to van der Waals attractions and can fuse at contact points due to the high surface energies of the particles, thus making dispersion into primary particles impossible. One possible solution is to utilize colloidal techniques to produce ultrafine particles and regulate particle interactions. Surfactants or polymeric additives are commonly added to colloidal dispersions to control interparticle forces and stabilize the individual particles. Another level of sophistication is to form particles within
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