Gas phase preparation of nanoparticles

1. Gas phase preparation of nanoparticles Mikko Lassila

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

3. 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

4. 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

5. 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

6. Reactors • Flamereactor • Hot-wallreactor • Plasma reactor • Laser reactor

7. 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

8. 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

9. Hot-wallreactor • 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 TiO2as 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

10. Hot-wallreactor • Advantages: • Simplicity of design • Pricisecontrol of parameters • Flexibility • Production of oxides, non-oxides, semiconductors and metals in the rangefromatomic to μm • Disadvantages: • Volatileprecursors • Highenergyrequirements • Highdegree of aggregation at highaerosolconcentrations • Pro

11. 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

12. 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

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

14. 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)

15. 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 Technology19 (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, KONA20 (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