Che5700 陶瓷粉末處理 Gas Phase Synthesis of Ceramic Powders Characteristics: Very fine powders...
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Transcript of Che5700 陶瓷粉末處理 Gas Phase Synthesis of Ceramic Powders Characteristics: Very fine powders...
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
Classifications
Che5700 陶瓷粉末處理
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
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
Chemical reaction nucleation, atomistic growth (vapor deposition), particle-particle aggregation
fractals
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. > 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.
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 ~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)
Flame Synthesis of Ceramic Powders
)( productsC
HTT
P
RXNinA
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
Al2O3 Al(C3H7O)3 acetylacetonateSiO2 SiCl4TiO2 TiCl4C alkanes (C: carbon black)Fe2O3, SnO2 metal chlorides
Reaction Kinetics
)exp(
)][]([)][]([][][
/][1
/][1
/][1
1
11
RT
Ekk
BBAAkBAk
dtDdd
dtBdb
dtAda
rate
Ao
ee
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
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(NO3)2.6H2O (metal salt) (105-130 oC) Zn(NO3)2 + 6 H2O ~550 oC ZnO (s) + NOx (g)
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, N2, He are often used Source: Powder Metall. Intern’l, 18(5), 338-340; 18(6),
422-425, 1986.
Various atomization process mostly for metal powder production
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/Si3N4 Polycarrborane siloxane SiC/B4C (sintering
aids)
Che5700 陶瓷粉末處理
•Polymer precursor: at some stage is viscous liquids, appropriate for processing;•Product may not be very dense
Example(1) - AlN
Che5700 陶瓷粉末處理
• 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
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 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