Synthesis and characterization of nanocrystalline binary and ternary intermetallic compounds
Intermetallic compounds are among the most important solid-state materials because of their diverse physical properties and widespread use in numerous applications. The possibility of integrating intermetallics with emerging nano-technological applications has generated renewed interest in their synthesis. Current capabilities for synthesizing nanocrystalline materials are well-established for single metals and simple binary phases, but very few processes are capable of reliably producing intermetallic nanoparticles. In this dissertation, I describe several new approaches for synthesizing intermetallic nanocrystals. The first approach involves reducing metal salts in aqueous solution using NaBH4 and precipitating a composite of metal nanoparticles. This nanocomposite can then be annealed and rapidly converted to an intermetallic phase. Using this approach, I successfully synthesized several binary and ternary compounds including known magnetic and superconducting materials. The properties of these materials were found to be comparable or superior to materials synthesized using traditional techniques. The second approach, called the polyol process, utilizes high boiling point polyalcohol solvents to heat metal salts in solution and precipitate nanocrystalline powders. Using this process, I was able to access several binary and ternary intermetallics, including two new phases: AuCuSn2 and AuNiSn2. These compounds were isolated as nanocrystals using low temperature solution synthesis techniques, which had not previously been applied to the synthesis of intermetallic compounds. Further investigation of the AuCuSn2 reaction revealed that it proceeds through a unique four step pathway: (1) galvanic reduction of Au(III) to Au(0) nanoparticles with concurrent oxidation of Sn(II) to Sn(IV) (as a SnO2 shell), (2) formation of NiAs-type AuSn along with Cu and Sn nanoparticles using NaBH4 reduction, (3) aggregation and thermal interdiffusion to form a ternary alloy, and (4) nucleation of the ordered intermetallic compound AuCuSn2. The proposed pathway was confirmed by forming AuCuSn2 via reaction of AuSn nanoparticles with Cu nanoparticles formed ex-situ. Additional investigations into the reactivity and kinetics of chemical transformations involving metal nanoparticles have revealed the idea of orthogonal reactivity in multi-component nanoparticle systems, which would allow phase (or metal) specific reactions to take place sequentially within a system of multiple metal nanoparticles.