Vasilis Papavassiliou

Creating Conductive Copper–Silver Bimetallic Nanostructured Coatings Using a High Temperature Reducing Jet Aerosol Reactor

We report production of bimetallic nanostructured copper– silver coatings by in situ deposition and sintering of metal nanoparticles produced as an aerosol. The metal nanoparticles themselves have potential applications in printed electronics, catalysis, antibacterial coatings, and heat transfer fluids. In many applications, nanoparticles are dispersed in an ink, which is then printed or coated onto a substrate and converted into a nanostructured thin film. Direct deposition from the aerosol allows us to produce nanostructured thin films without first dispersing the particles in a solvent. The high temperature reducing jet process allows formation of these metal nanoparticles from low-cost metal salt precursors in the gas phase. In this method, a fuel-rich hydrogen flame provides a low-cost source of energy to drive nanoparticle formation in a reducing environment. The aqueous precursor solution is delivered into the hot combustion product gases within a converging–diverging nozzle. The high-speed gas flow atomizes the precursor and provides exceptionally rapid mixing of the precursor with the hot gases. Here, particles are formed, then immediately quenched and deposited on a glass substrate. The effect of the silver content of the mixed copper–silver films on their electrical conductivity was studied systematically, revealing an abrupt transition from low conductivity to high conductivity between 30 wt.% and 40 wt.% silver. Copyright 2013 American Association for Aerosol Research

Flame-driven Aerosol Synthesis of Copper–Nickel Nanopowders and Conductive Nanoparticle Films

We report the continuous one-step synthesis of bimetallic copper-nickel nanostructured coatings by deposition and sintering of metal nanoparticles produced as an aerosol using a flame driven high temperature reducing jet (HTRJ) process. The HTRJ process allows gas-phase (aerosol) formation of metal nanoparticles from low-cost metal salt precursors. These can be collected as discrete powders for subsequent use in formulating conductive inks or for other applications. However, direct deposition of nanoparticles to form coatings allows measurements of electrical conductivity of films of deposited nanoparticles as a function of composition and sintering temperature, without actually formulating and printing inks. This is the approach taken here for the purpose of screening nanoparticle compositions quickly. We characterized the microstructure and composition of both nanopowders and films and found that their composition consistently matched the ratio of metals in the precursor solution. The electrical conductivity was highest (∼10(4) S/m) for films with 60:40 and 40:60 copper-to-nickel mass ratios. These films maintained their conductivity during extended storage (1 month) under ambient conditions. The oxidation resistance and high conductivity observed here suggest that 60:40 and 40:60 Cu:Ni nanoparticles have promise as lower cost replacements for silver nanoparticles in conductive ink formulations.

A High-Temperature Reducing Jet Reactor for Flame-Based Metal Nanoparticle Production

We present a new flame-based aerosol reactor configuration that combines thermal decomposition and hydrogen reduction to produce metal nanoparticles. This approach uses a fuel-rich hydrogen flame as a source of low-cost energy to initiate particle synthesis, but separates the flame chemistry from the particle formation chemistry. Hot combustion products pass through a nozzle to produce a high-temperature reducing jet. A liquid precursor solution is rapidly atomized, evaporated, and decomposed by the expanding jet, initiating particle formation. In particular, here we have produced carbon-coated copper nanoparticles from an aqueous copper formate precursor solution and characterized them by aerosol mobility distribution measurements, electron microscopy, and x-ray diffraction. Copper serves here as a prototype for non-oxide materials that are generally difficult to produce in flame-based reactors. This work demonstrates that such materials can be produced in substantial quantities with particle diameters below 50 nm in this new process.

Ballast gas for heat transfer control in fixed-bed reactors

Controlled oxidation reactions of hydrocarbons are practiced today in fixed-bed, fluid bed or transport bed reactors with oxygen or air as the oxidant. In fixed-bed reactors, due to the exothermicity of the oxidation reactions, heat removal and temperature control are critical in achieving safe and optimum reaction conditions that maximize conversion and selectivity. Industrial reactors often control hot spot formation by using an appropriate diluent (ballast) gas. Steam, nitrogen, methane and carbon dioxide are the most commonly used ballast gases. The scope of this paper is to elucidate the benefits of ballast gas use as a tool to optimize heat transfer and reduce the hot spot effect in controlled oxidations with fixed-bed reactors. Ballast gas does not participate in the reaction, but it is used to control heat removal and flammability. A homogeneous one-dimensional reactor model was used to study and compare two oxidation processes: (1) ethylene oxidation to ethylene oxide and (2) o-xylene oxidation to phthalic anhydride. For both processes hot spot temperature and conversion decreased as methane replaced nitrogen in the process ballast gas. Selectivity increased in the ethylene oxide case but decreased in the phthalic anhydride case, indicating that ballast gas effect on selectivity depends on the reaction mechanism. Thus, there is an optimum ballast gas composition that will optimize hot spot temperature and reactor yield.

Combustion-Driven Synthesis of Non-Oxide Nanoparticles in a High Temperature Reducing Jet

Copper nanoparticles were synthesized using a new technique that combines thermal decomposition and hydrogen reduction. The technique uses a flame as a source of low-cost energy to initiate particle synthesis. Molar concentration of the aqueous precursor solution, reactor pressure, and flow rate of hydrogen, oxygen, and nitrogen, were varied to control particle size, size distribution, and morphology. Copper serves as a prototype for non-oxide materials that are generally difficult to produce in flame-based reactors. This work demonstrates that such materials can be produced in substantial quantities (~0.3 g/hr) with particle diameters below 50 nm using this approach.