Gas Phase Synthesis of Ceramic Powders

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Gas Phase Synthesis of Ceramic Powders
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• Characteristics
• Very fine powders (smallest concentration of
  precursor)
• Additives can be uniformly added (easy mixing)
• Can be used for non-oxides
• Often single step process, short reaction time
• Powder recovery and waste gas treatment may be
  problems
• Cost of precursor may be expensive
• High temperature and reactive gas may cause
  corrosion problem
• Can use laser of plasma to activate reaction
• Often high purity product, unless incomplete
  reaction with residual reactants

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Classifications
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• Combustion (flame method) SiCl4 O2 ? SiO2 2
  Cl2 to fabricate carbosil, aerosil etc already
  commercialized, easy to get chain-like aggregates
• Spray drying(decomposition by heating) can
  obtain uniform powder, associated with
  granulation, if operated improperly, may get hard
  agglomerates
• Laser method high purity, submicron products,
  expensive, difficult to scale up
• Conventional heating simple, broad application,
  mostly for single component system
• Plasma method high purity, fine powder used for
  high energy barrier low pressure system, low
  productivity, often amorphous, still require
  calcination

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Classification(2)
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• 6. Aerosol similar to spray drying various
  sources since in solution, hence difficult for
  non-oxides
• CVD mostly for growing thin films

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Chemical reaction ? nucleation, atomistic growth
(vapor deposition), particle-particle aggregation
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fractals
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Particle shape
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• Shape influenced by reaction temperature,
  cooling rate, e.g. if T higher than melting point
  of product ? spherical particles, fast cooling
  (e.g. gt 106 oC/sec) ? often amorphous products.
• If flame temperature slightly higher than melting
  point, often become sticky particles ? easy to
  form aggregates (fractals)
• Cluster-cluster aggregation may happen high
  temperature ? sintering is possible to change
  particle shape. Particles obtained at low
  temperature, tend to maintain its shape and
  crystallinity.

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Process Characteristics
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• Flame method principal method for silica and
  titania
• Plasma method (a) thermal e.g. DC arc, RF
  induction to get plasma (b) low temperature use
  glow discharge due to effect of electron and
  ions in plasma ? create lots of active species
  (may be radicals) ? fast reaction, for some
  difficult synthesis reactions. Important
  parameter E/P (electric field/pressure) cold
  plasma E/P 103 V/cm-Pa thermal type E/P 10-4
  V/cm-Pa
• Laser method some laser function as heater, some
  can activate molecules (special wavelength)

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Flame Synthesis of Ceramic Powders
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• Different precursors can be used
• May need extra source of fuels
• Al2O3 Al(C3H7O)3 acetylacetonate
• SiO2 SiCl4
• TiO2 TiCl4
• C alkanes (C carbon black)
• Fe2O3, SnO2 metal chlorides

Adiabatic flame temperature exist for exothermic
reactions
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Reaction Kinetics
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• For reaction like this a A b B ? d D the
  rate equation is as follows
• True kinetics depend on reaction conditions,
  e.g. plasma condition must be different from
  conventional heating ? different mechanism

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Spray Drying or Roasting
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• Precursor solution was sprayed from orifice
  (under high pressure), being heated to decompose
  at the same time. Also used for granulation.
• Drying and roasting the difference is in
  temperature, higher for roasting, salt molecules
  require higher temperature to decompose or react
• E.g. Zn(NO3)2.6H2O (metal salt)? (105-130 oC) ?
  Zn(NO3)2 6 H2O ? 550 oC ? ZnO (s) NOx (g)

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Ultrasonic Gas Atomization
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• Gas nozzle that generates shock waves, in
  association with ultrasound, to disintegrate melt
  stream into droplets to get quickly solidified
  metal particles.
• This method relies on stable melt flow, fast
  enough gas flow rate to disperse liquid into
  small droplets
• When liquid temperature is not high, often get
  coarse particles (probably due to high viscosity,
  difficult to disperse) high gas/melt ratio ?
  fine particles
• Ar, N2, He are often used
• Source Powder Metall. Internl, 18(5), 338-340
  18(6), 422-425, 1986.

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Various atomization process mostly for metal
powder production
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Pyrolysis of Polymers
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• Similar to metal-organic decomposition.
• Polymer pyrolysis often used for non-oxide
  powders, Inside polymer, must have M-C or M-N
  bonds
• Can be used for coating (film), making fiber,
  bulk, or binder for nonoxide powders
• Examples
• Polycarbosilane ? SiC
• Polyaminoalane ? AlN
• Alkalene trisilazan ? SiC/Si3N4
• Polycarrborane siloxane ? SiC/B4C (sintering
  aids)

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• Polymer precursor at some stage is viscous
  liquids, appropriate for processing
• Product may not be very dense

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Example(1) - AlN
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• Primary amine (RN) acetonitrile CH3CN (as
  solvent) ? electrolyte tetraalkylammonium salt
  R4NX Al as cathode
• Al dissolve into solvent, to form polyaminoalane
  precursor
• Heated to 750 1150 oC to obtain polymer, remove
  solvent and excess amine
• Heat under vacuum, from viscous state into
  polymer powder
• Calcination at above 750oC, under NH3, to get AlN
  powder, amorphous, crystal size about 35 nm

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Example (2) Pechini process
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• Mixed salt solution hydroxycarboxylic acid
  (such as citric acid ???) poly (hydroxy
  alcohol), e.g. ethylene glycol
• mixing, heating to 80 120 oC, to get clear
  solution
• Continue to heat, 150 250 oC, condensation
  reaction
• To get resin particles, containing proper amount
  of metal
• Heat to 400 oC, to get char
• Heat to 500 900 oC to get oxide particles
• Can get uniform (in composition) powders
• Metal salt citric acid ? viscous liquid ?
  droplet ? calcine ? oxide powder