Che5700 陶瓷粉末處理
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Transcript 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.
characteristics:
Difficult to reach uniformity (compared to liquid, gas
phases)
Slow reaction rate, require high temperature and long
time
Reaction starts from surface, often left unreacted cores
May form un-wanted intermediate phases (solid/solid
system)
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
Taken
from 陶
業雜誌
L
Carbothermal
reaction
Gas phase
SiCl4 +
liquid phase
NH3
reaction
system
•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
process
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
parameters:
Purity of raw materials, size, surface condition,etc.
Degree of mixing between raw materials (distance for
diffusion)
Any carrier (solvent, or carrier gas)? Its purity and
effect
Reaction temperature and time
Catalyst or not? (some impurity may have catalytic
effect)
Reaction path (mechanism), any intermediates?
Shrinking core & shrinking sphere
models
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
layer.
Most often: mass transfer of the solid phase.
•
Control mechanism
may change with
temperature
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
kinetics:
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
固相擴散的因素就是影響上述諸項目的因素
= chemical potential; = electrical potential; Z
= valence of species; F = Faraday constant;
d i
J i Ci Bi
dx
[1 ( Z 1) X B ]
2/3
i i Zi F
( Z 1)(1 X B )
2/3
2K
Z (1 Z ) 2 t
R
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
direction);
• diffusion couple; often
controlled by the slower
(moving) one
Impurity (Fe) effect: form - whisker;
T
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
Mechanism
dissolution of gaseous reactants
into
nanosized liquid droplets of a
catalyst
metal product in alloy liquid
product concentration keeps
increasing
crystallization of product to form
a
liquid-solid interface
growth of solid region in confined
direction
nanorods nanowires
JACS,2001,123,3165
呂世源教授提供
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)
JACS,2001,123,3165
呂世源教授提供