solid state reactions
DESCRIPTION
Solid State Reactions. Che5700 陶瓷粉末處理. 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 - PowerPoint PPT PresentationTRANSCRIPT
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
SiC powder synthesis
Che5700 陶瓷粉末處理
• 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
Si3N4 powder synthesis
Che5700 陶瓷粉末處理
(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
Si3N4 powder synthesis (2)
Che5700 陶瓷粉末處理
• 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
Carbothermal reaction
L
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)
AlN Powder Synthesis
Gas phase: AlCl3 + 4 NH3 AlN + 3 NH4Cl; 900-1500oK, >5 hr …high cost, low yieldOrgano-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 processDirect nitridation of Al: 2 Al + N2 2 AlN; 1000-1500oK, >5hr, also with industrial processCombustion method: new, with potentialDifferent processes are in competition with each other
Che5700 陶瓷粉末處理
Important parameters of reaction
Che5700 陶瓷粉末處理
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 + H2OCaCO3 + heat CaO + CO2
Thermodynamics and Kinetics
Che5700 陶瓷粉末處理
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;
KRTGG orxn ln
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
Che5700 陶瓷粉末處理
Taken from TA Ring, 1996, different temperature, different controlling mechanism
Shrinking Sphere Model
Che5700 陶瓷粉末處理
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);
Solid-Solid Reactions
Che5700 陶瓷粉末處理
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
dx
dBCJ iiii
FZ iii
tR
KZZXZXZ BB 2
3/23/2 2)1()1)(1(])1(1[
Carter eq. For solid reaction kinetics
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;
•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;
Schematic for mechanism: nucleation of Si3N4 on Si growth + CVD Si3N4 (whisker form) will stop further reaction between Si & N2
T
dissolution of gaseous reactants into nanosized liquid droplets of a catalyst metal product in alloy liquid
product concentration keeps increasingcrystallization of product to form a liquid-solid interface growth of solid region in confined direction nanorods nanowires
JACS,2001,123,3165
Vapor-Liquid-Solid (VLS) Growth Mechanism
呂世源教授提供
VLS Examples
birth of a Ge nanowire on a Au nanocluster
Ge nanowires with Au as catalystSingle crystal nanowireAu clusters remain as the tip of nanowires (dark dots)
JACS,2001,123,3165 呂世源教授提供