Aerosol

Gas phase powder synthesis

  • Does NOT involve large volumes of liquid (compared to wet process)
  • Timescale is very short
  • High: Purity, yield, throughput
  • Multicomponent + nanophase materials possible
  • Flame hydrolysis:
    • 2H\(_2\) + O\(_2\) -> H\(_2\)O
    • SiCl\(_4\) + 2H\(_2\)O -> SiO\(_2\) + 4HCl

Aerosil

  • Degussa, 1942

Product:

  • 7-40nm particles

  • Surface area: 50-400m^2/g, S = 6/(d*r) (surface, not pores)

  • 200L \(\approx\) 10kg

  • Fumed titania also produced (as pigments)

Process routes

Terms:

  • Coagulation: Attachments of two particles when they collide

  • Coalescence: Fusion (sintering) of two particles

  • Agglomerates: Assemblies of primary particles physically held together by weak interactions (soft agglomerates)

  • Aggregates: Assemblies of primary particles hed together by stronger forces (Hard agglomerates)

  • Precursors often similar to what used in CVD/PVD

Two main processes:

Gas to particle conversion

  • High T reaction between gases
  1. Homogeneous gas phase reactions. Formation of molecular or cluster compounds
  2. Nucleation formed from supersaturated vapour. Homogeneous nucleation from molecules or clusters.
  3. Particle growth, by several mechanisms: Condensation, surface reactions, coagulation. The relative rates of particle collision, coalescence and sintering are important for the shape of the obtained particles.

Spray Pyrolysis

  1. Sol or slurry atomized and pass through heated area
  2. Solvents evaporate, particles pyrolyse or react to form particles.
  • Often hollow spheres are formed
  • Particle size depends on size of droplets and concentration
  • Porosity controlled by concentration and thermal profile

Three types of reactors: Flame Reactor:(Flame pyrolysis) Combustion reaction to heat spray. Short residence time, max use of energy. Risk of contamination by combustion reactants.

Heating Reactor: Spray into hot furnace. Good control of: T + residence time. Growth on walls, formation of hard aggregates.

LASER Reactors: Heated by laser; efficient transfer of heat to gas. Plasma reactors. Precursors: solution from sol-gel -> atomic mixing.

Film deposition

  • Spray pyrolisys (droplet deposition)
  • Evaporation of substrate
  • Aerosol Assisted CVD (AACVD)
    • Up to 5\(\mu\)m/min particle deposition
    • 1-5\(\mu\)m/min typically

Products

Large scale: Titania, silica, Al-powder(From chlorides by flame pyrolisys)

Smaller scale: Bi\(_2\)O\(_3\), Cr\(_2\)O\(_3\), Fe\(_2\)O\(_3\), GeO\(_2\), NiO, MoO\(_2\), SnO\(_2\), V\(_2\)O\(_5\), WO\(_3\), ZrO\(_2\), AlBO\(_3\), Al\(_2\)TiO\(_5\), AlPO\(_4\)

Metal Synthesis: Fumed silica particles: SiCl\(_4\)(g) + 4Na(g) -> Si + 4NaCl.
Niobium metal particles: 2NbCl\(_5\)(g) + 5Mg -> 2Nb + 5MgCl\(_2\)

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