Gas phase preparation of nanoparticles Mikko Lassila

Contents General issues Gas phase synthesis of nanoparticles Simulation Reactor technologies: flame, furnice glow, how-wall, plasma and laser reactors Conclusions Sources

General issues Gas-phase (GP) synthesis a well-known chemical manufacturing technique for an extensive variety of nano-sized particles Scientific and commercial interest increased Temperature up to 10 000 K Some problems of liquid phase (LP) prevented In GP by adjusting the conditions the properties of the nanoparticles controlled better Manufacturing process and conditions determine size and morphology of the particles and thus their application properties Manufacturing techniques: flame, hot-wall, plasma and laser evaporation reactors

GP synthesis of nanoparticles Fundamental aspects of particle formation mechanisms occurring once the product species is generated are the same Final characteristics determined by fluid mechanics and particle dynamics within a few milliseconds at the early stages of the process – three major formation mechanisms dominating. Chemical reaction of the precursor leads to the formation of clusters by nucleation or direct inception Surface growth Brownian motion  particles move randomly  coagulation High T, short t  GP reactors problematic

Simulation Numerous models based on particle population balance developed and applied. A simple monodisperse model (Kruis et al. [1]) coupled to fluid dynamics (CFD) by Schild et al. [2] Reveal specific reactor characteristics

Reactors Flame reactor Hot-wall reactor Plasma reactor Laser reactor

Flame reactor A common reactor design for the production of high-purity production of high-purity nanoscale powders in large quantities. Metal oxides: silica, titania, alumina, etc. Powders, liquids and vapors used as precursors The evaporation and chemical reaction E provided by a flame High concentrations can be used 1000 °C < T < 2400 °C; 10 ms < t < 100 ms Primary particle sizes from a few nm up to 500 nm Specific surface areas of powder up to 400 m2/g and higher “Premixed” or “diffusion” flame Three reaction parameters: temperature profile, reactor residence time and reactant concentration

Furnice flow reactor Oven source with T up to 1700 °C A crucible with the source material placed in heated flow of inert carrier gas Advantages: Simplicity of design Disadvantages: Compatibility depends on the vapor pressure Operating T limited by crucible material Impurities from the crucible might be incorporated Very small particles need rapid cooling

Hot-wall reactor Tubular furnace-heated reactors for initiating the synthesis reaction Construction simple and process parameters moderate T = 1700 °C, concentrations variable, gas composition freely selectable, system pressure atmospheric (can also be varied) Precise process control  particle production with specific characteristics Investigated mainly on a lab scale due to high energy requirements Industrial applications: Al-doped TiO2 as a pigment Precursors: metal chlorides and organometallic precursors Mixing of reactants and carrier gas important, premixing avoided Production of oxides, non-oxides, semiconductors and metals in the range from atomic to micrometer dimensions

Hot-wall reactor Advantages: Simplicity of design Pricise control of parameters Flexibility Production of oxides, non-oxides, semiconductors and metals in the range from atomic to μm Disadvantages: Volatile precursors High energy requirements High degree of aggregation at high aerosol concentrations Pro

Plasma reactor Evaporation and reaction energy delivered by a plasma (T = 104 °C) Reactants decomposed into ions and dissociating atoms and radicals Nanoparticles formed upon cooling while exiting the plasma region Electrical methods for producing plasmas: high-intensity arcs and inductively coupled high-frequency discharge Production of nanoparticles by means of thermal plasma a less evaluated field (e.g. carbon black production investigated; Mizuguchi et al. (1994) obtained BaFe12O19 (10 < d < 50 nm)) Vapors quenched by mixing with a cold gas  high cooling rate and nonuniform cooling Residence time < 1 s

Laser reactor Reactant gas heated selectively and rapidly with and IR laser GP decomposition resulting in nanoparticle formation (e.g. Si nanoparticles from SiH4 pyrolysis (Cannon et al. 1982)) High-power intensity of laser  a wide field of solid precursor options (e.g. ceramics and metal oxides) High cooling rate  morphologies differ significantly from typical pyrogenic oxides opening new fields of potential applications Absence of heated walls reduces risks of product contamination

Other methods Spark source and exploding wire Sputtering Inert gas condensation Expansion-cooling Electrospray systems Homogeneous nucleation in aerosol droplets Etc…

Conclusions GP processes generally purer than LP ones Cheaper than vacuum Potential to create complex chemical structures GP synthesis a well known technique for a wide variety of nano-sized particles GP process and product control very good in aerosol processes. An aerosol droplet resembles a very small reactor where chemical segregation minimized Continuous process (GP) versus batch form (LP)

Sources Kruis, F. E.; Kusters, K. A.; Scarlett, B.; Pratsinis, S. E.: A Simple Model for the Evolution of the Characteristics of Aggregate Particles Undergoing Coagulation and Sintering, Aerosol Science and Technology 19 (1993), 514 Schild, A.; Gutsch, A.; Mühlenweg, H.; Pratsinis, S. E.: Simulation of Nanoparticle Production in Premixed Aerosol Flow Reactors by Interfacing Fluid Mechanics and Particle Dynamics, J. Nanoparticle Research 1 (1999), 305 Gutsch, A., Krämer, M., Michael G., Mühlenweg, H., Pridöhl M., Zimmermann, G., Gas-Phase Production of Nanoparticles, KONA 20 (2002), 24-37 Kruis, F. E., Fissan, H., Peled, A. Synthesis of Nanoparticles in the Gas Phase for Electronic, Optical and Magnetic Applications – a Review, J. Aerosol Sci. 29 (1998), 511-535 Wegner, K., Pratsinis, S. E., Gas-phase Synthesis of Nanoparticles: Scale-up and Design of Flame Reactors, Powder Technology 150 (2005), 117-122