1. Che5700 陶瓷粉末處理 Gas Phase Synthesis of Ceramic Powders 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

2. Classifications Che5700 陶瓷粉末處理 Combustion (flame method): SiCl 4 + O 2  SiO 2 + 2 Cl 2 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

3. Classification(2) 6. Aerosol : similar to spray drying; various sources; since in solution, hence difficult for non- oxides CVD: mostly for growing thin films; Che5700 陶瓷粉末處理

4. Chemical reaction  nucleation, atomistic growth (vapor deposition), particle-particle aggregation

6. fractals

7. Particle shape Che5700 陶瓷粉末處理 Shape influenced by: reaction temperature, cooling rate, e.g. if T higher than melting point of product  spherical particles, fast cooling (e.g. > 10 6 o C/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.

8. Process Characteristics Che5700 陶瓷粉末處理 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 ~10 3 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)

9. Flame Synthesis of Ceramic Powders Adiabatic flame temperature: exist for exothermic reactions Che5700 陶瓷粉末處理 Different precursors can be used: May need extra source of fuels Al 2 O 3 Al(C 3 H 7 O) 3 acetylacetonate SiO 2 SiCl 4 TiO 2 TiCl 4 C alkanes (C: carbon black) Fe 2 O 3, SnO 2 metal chlorides

10. Reaction Kinetics  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 Che5700 陶瓷粉末處理

11. Spray Drying or Roasting Che5700 陶瓷粉末處理 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(NO 3 ) 2.6H 2 O (metal salt)  (105-130 o C)  Zn(NO 3 ) 2 + 6 H 2 O  ~550 o C  ZnO (s) + NO x (g)

12. Ultrasonic Gas Atomization Che5700 陶瓷粉末處理 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, N 2, He are often used Source: Powder Metall. Intern’l, 18(5), 338-340; 18(6), 422-425, 1986.

13. Various atomization process mostly for metal powder production

17. Pyrolysis of Polymers 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/Si 3 N 4  Polycarrborane siloxane  SiC/B 4 C (sintering aids) Che5700 陶瓷粉末處理

18. Polymer precursor: at some stage is viscous liquids, appropriate for processing; Product may not be very dense

19. Example(1) - AlN Che5700 陶瓷粉末處理 Primary amine (RN) + acetonitrile CH 3 CN (as solvent)  electrolyte tetraalkylammonium salt R 4 NX + Al as cathode  Al dissolve into solvent, to form polyaminoalane precursor  Heated to 750 – 1150 o C to obtain polymer, remove solvent and excess amine  Heat under vacuum, from viscous state into polymer powder  Calcination at above 750 o C, under NH 3, to get AlN powder, amorphous, crystal size about 35 nm

20. Example (2) – Pechini process Che5700 陶瓷粉末處理 Mixed salt solution + hydroxycarboxylic acid (such as citric acid 檸檬酸 ) + poly (hydroxy alcohol), e.g. ethylene glycol  mixing, heating to 80 – 120 o C, to get clear solution  Continue to heat, 150 – 250 o C, condensation reaction  To get resin particles, containing proper amount of metal  Heat to 400 o C, to get char  Heat to 500 – 900 o C to get oxide particles Can get uniform (in composition) powders Metal salt + citric acid  viscous liquid  droplet  calcine  oxide powder