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;


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

fractals

Particle shape


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


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


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

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)][]([)][]([][][

/][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


Spray Drying or Roasting


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


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)


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

Example(1) - AlN


• 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


• 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

che5700 陶瓷粉末處理 gas phase synthesis of ceramic powders characteristics: very fine powders...

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