Che5700 陶瓷粉末處理

Che5700 陶瓷粉末處理
Solid State Reactions
Several possible cases: solid/solid reaction; gas/solid
reactions; solid decomposition reaction; etc.
Difficult to reach uniformity (compared to liquid, gas
Slow reaction rate, require high temperature and long
Reaction starts from surface, often left unreacted cores
May form un-wanted intermediate phases (solid/solid
Need grinding after reaction to get fine particles
May introduce impurity during grinding
Not many steps, the cost may be reasonable
Che5700 陶瓷粉末處理
SiC powder synthesis
• Competitive between different methods
(1) Acheson process: SiO2 sand + C (coke)  electric arc
furnace (> 2000oC)  coarse SiC  grinding,
purification; major method, impurity include: unreacted
Si, Fe, O etc; side reaction SiO2 + C  SiO + CO
(reverse at low temperature, get fine dust)
(2) Gas phase method: SiH4, SiCl4, chlorosilane as raw
material + CH4 (or C2H4)  heating, gas phase reaction
(even by plasma, or laser)  collect product, impurity
from source, or due to incomplete reaction
• High purity light green color (> 99.8%); next dark green
(~99.5%), black (~99%), gray (~90%)
• One of source - petroleum coke: not cheap
•Comparison of come commercial SiC processes; some
may have patent limitations;
•Can use HF to dissolve unreacted SiO2
Che5700 陶瓷粉末處理
Si3N4 powder synthesis
(1) Direct nitridation: Si powder  grinding + catalyst
and binder  kneading  form and dry  high
temperature nitridation  grinding, sieving,
purification  remove un-reacted parts  get final
product (exothermic reaction, may lead to very high
temperature to cause melting of Si)
(2) Gas phase reaction: SiCl4 + NH3  to get first Si(NH)2
+ NH4Cl  calcine to remove NH4Cl, HCl  precursor
powder Si(NH)2  1000oC calcination to get
amorphous Si2N3H (remove NH3)  further heating
1400-1500oC to get crystalline Si3N4
(3) Liquid phase reaction: similar to previous process, use
liquid NH3  filtration and washing to get silicon
imide Si(NH)2  calcination to product
Che5700 陶瓷粉末處理
Si3N4 powder synthesis (2)
• SiCl4 (g) + NH3 (g)  Si(NH)2 + NH4Cl (s) H = 161.5 Kcal/mol …. Exothermic reaction, need
temperature control
(4) SiO2 + C powder  grinding and mixing 
under N2 (may have some hydrogen to minimize
oxidation), heating and reacting  grinding and
sieving  purification  product
 Mostly heterogeneous reactions; some
homogeneous reactions
from 陶
Gas phase
SiCl4 +
liquid phase
•Comparison of some commercial Si3N4 processes and
product characteristics
•Product can be in the form of , , or amorphous form;
beta form: most stable form, difficult to sinter, avoid to
get it
Comparison of costs; numbers will change with time
and place
Taken from Am. Cer. Bull. 70(1), 1991.
So many different raw materials, product characteristics
also different (including cost)
Che5700 陶瓷粉末處理
AlN Powder Synthesis
Gas phase: AlCl3 + 4 NH3  AlN + 3 NH4Cl; 900-1500oK,
>5 hr …high cost, low yield
Organo-metallic precursor: R3Al(l) + NH3  R3AlNH3 
in sequence to get AlN + 3 RH; 400-1000oK (as above, may
get residual carbon)
Alumina + carbon  reduction method: Al2O3 + N2 + 3C
 2 AlN + 3 CO; 1500-2200oK, >5hr; with industrial
Direct nitridation of Al: 2 Al + N2  2 AlN; 1000-1500oK,
>5hr, also with industrial process
Combustion method: new, with potential
Different processes are in competition with each other
Che5700 陶瓷粉末處理
Important parameters of reaction
As shown in SiC process, several important
 Purity of raw materials, size, surface condition,etc.
 Degree of mixing between raw materials (distance for
 Any carrier (solvent, or carrier gas)? Its purity and
 Reaction temperature and time
 Catalyst or not? (some impurity may have catalytic
 Reaction path (mechanism), any intermediates?
Shrinking core & shrinking sphere
Examples of Shrinking Core Reactions
FeO + H2  Fe + H2O
CaCO3 + heat  CaO + CO2
Che5700 陶瓷粉末處理
Thermodynamics and Kinetics
To show whether the reaction is a spontaneous
reaction; G negative, then spontaneous,
unless limited by kinetics or mass transfer
effect (most likely).
Reactions can be divided into: decomposition,
oxidation, reduction, etc.; may be multiple;
Items to show effect on thermodynamics: gas
phase: partial pressure, total pressure,
moisture, or even CO2;
Grxn  Go  RT ln K
Che5700 陶瓷粉末處理
Solid state diffusion
In theory, gas/solid reaction, rate control steps may
include: (a) surface reaction; (b) mass transfer around
particle; (c) diffusion inside product layer; (d) heat
transfer around particle; (e) heat transfer inside product
Most often: mass transfer of the solid phase.
Control mechanism
may change with
Taken from TA Ring, 1996, different temperature,
different controlling mechanism
Che5700 陶瓷粉末處理
Shrinking Sphere Model
If product flakes off the original particle  shrinking
sphere model, e.g. CaCO3 decomposition reaction
Steps included in this model: (a) mass transfer of A to
particle; (b) surface reaction; (c) mass transfer of product
away from particle; (d) heat transfer
Another type of model: nucleation and growth model –
e.g. 7 C + 2 B2O3(l)  B4C (s) + 6 CO (g) ; where
nucleation and growth of B4C – major mechanism; Avrami
ln( 1 – XB) = - (k t)m (general form);
Che5700 陶瓷粉末處理
Solid-Solid Reactions
A major type, many examples, e.g.
 NiO + Al2O3  NiAl2O4
 ZnO + Al2O3  ZnAl2O4
 BaCO3 + TiO2  BaTiO3 + CO2 (g)
 4 B + C  B4C
 SiO2 + C  SiC + CO2 carbothermal reaction
 In addition to solid state diffusion, at sufficient high
temperature, may change to gas phase reaction
mechanism, e.g. SiO2 + C  SiO (g) + CO; SiO + 2C
 SiC + CO (free energy change of former reaction
less than zero at >1900oK)
 Partial pressure of oxygen  competitive between
formation of oxide or carbide
CO + ½ O2  CO2 K = PCO2/[PCO x PO2]
Solid-Solid Inter-diffusion
Diffusion flux ~ (concentration) Ci, ion mobility Bi
and electrochemical potential gradient, 所以影響
 = chemical potential;  = electrical potential; Z
= valence of species; F = Faraday constant;
d i
J i  Ci Bi
[1  ( Z  1) X B ]
i  i  Zi F
 ( Z  1)(1  X B )
 Z  (1  Z ) 2 t
Carter eq. For solid reaction kinetics
•Taken from TA Ring, 1996;
•Several different mechanisms
•Charge balance should be
maintained, if form space
charge  electrical field, affect
ion mobility (in opposite
• diffusion couple; often
controlled by the slower
(moving) one
Impurity (Fe) effect:  form - whisker;
Schematic for mechanism: nucleation of Si3N4 on Si 
growth + CVD Si3N4 (whisker form)  will stop further
reaction between Si & N2
Vapor-Liquid-Solid (VLS) Growth
dissolution of gaseous reactants
nanosized liquid droplets of a
metal  product in alloy liquid
product concentration keeps
crystallization of product to form
liquid-solid interface
growth of solid region in confined
 nanorods  nanowires
VLS Examples
birth of a Ge nanowire on a Au nanocluster
Ge nanowires with Au as catalyst
Single crystal nanowire
Au clusters remain as the tip of nanowires (
dark dots)
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