Che5700 陶瓷粉末處理

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Che5700 陶瓷粉末處理
Solid State Reactions
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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?
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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
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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
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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
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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
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CO + ½ O2  CO2 K = PCO2/[PCO x PO2]
Solid-Solid Inter-diffusion
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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
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