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 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
Flame reactor
Hot-wall reactor
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-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

10. 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

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 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),
Wegner, K., Pratsinis, S. E., Gas-phase Synthesis of Nanoparticles: Scale-up and Design of Flame Reactors, Powder Technology 150 (2005),