Preparation of colloidal bismuth particles in polyols
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Nano- and micrometer size spherical bismuth particles were prepared by reducing bismuth salts in simple and mixed polyols at elevated temperatures. In general, the conversion to metallic bismuth was preceded by the formation, at intermediate temperatures, of bismuth-polyol compounds. By changing the type of bismuth salt, the composition of the polyol mixture, and the temperature of the process, these precursors were successfully converted into spherical bismuth particles. It was found that the reduction process proceeds only if the temperature of the polyol exceeds the values at which the intermediate bismuth-polyol are stable, which was determined to be around 220 °C. I. INTRODUCTION
Due to its unique thermal and electrical properties, bismuth represents an important material in many existing and emerging technological applications. For example, this metal is used in large quantities as a nontoxic substitute for lead in brass plumbing fixtures, ceramic glazes, pigments, and lubricants. In electronics, bismuth particles have been used in solder formulations1,2 and to build dry phototools for printed circuit boards.3 More recently, it was suggested that due to the high anisotropic electronic behavior, high intrinsic electron mobility, and low-conduction band effective mass, this metal can be exploited in novel electronic applications.4,5 There are also indications that the transition from semimetal to semiconductor behavior, caused by the decrease in particles size and the related quantum confinement, could have a significant potential in both optical and electrooptical fields.6,7 The very large magnetoresistance effect, observed in Bi single crystals8 and thin films,9,10 has also attracted much interest for various sensing devices. Despite the significant potential of finely dispersed bismuth in all these fields of technology, the number of reported synthetic methods capable of economically generating bismuth particles is rather limited. Previously, such dispersions were prepared by reducing bismuth ions in microemulsions,11,12 electrodeposition,13 vapor flow condensation,14,15 and quenching of liquid bismuth in suitable liquids.16 Although the polyol chemistry is well known as a versatile preparation route for finely divided metals,17–21 it was used only for the generation of submicrometer Bi2O3 particles.22 This work describes a convenient method to produce highly dispersed nanometer and micrometer size bismuth particles by reducing
DOI: 10.1557/JMR.2005.0194 J. Mater. Res., Vol. 20, No. 6, Jun 2005
http://journals.cambridge.org
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bismuth salts at elevated temperatures in simple and mixed polyols. While this paper was processed for publication, Wang and Xia23 reported a similar procedure for the preparation of spherical bismuth particles. The emphasis of the present study, however, is on the elucidation of the mechanisms responsible for the generation of the metallic phase in polyols and the role of the bismuth precursor and the intermediate polyolates in the formation of bismuth spheres
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