第七章 固相反应
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3.
Structure tends to perfect, crystal growth Further reaction controlled by the diffusion (by surface, boundary, crystal fissure) of reactant substances through the layer of product
Step 1. The gaseous reagent diffuses from the surroundings onto the catalyst surface.
Step 2.
diffuses into the porous structure.
Step 3.
reacts at the pore walls.
2018/10/13
4
Characteristics
Heterogeneous reaction (2) Reaction interface between the reacting phases, such as nucleus and matrix or crystal and melt (3) Three steps in series Material transport to the interface Reaction at the phase boundary Sometimes, transport of reaction products away from the interface (4) Reaction at the phase boundary liberate or absorb heat, changing the boundary temperature and limiting the rate of the process
Chapter 7 Solid State Reaction
Xu Lingling, Dr. College of Materials Science and Engineering Nanjing University of Technology
2018/10/13
1
Necessary
Popular physicochemistry during high temperature process of solids
2018/10/13
16
Oxidation of metal
On the basic principle of chemical reaction kinetics, the oxidation rate of metal per unit area on the interface VR：
V R KC
9
2018/10/13
7.2 Process of solid reaction
1. Mixing of reactant particles， contact each other
2. New phase(with large amount of defect) formed by chemical reaction on the interface
2018/10/13
4.
10
Possible reaction pathes between two solid grains A and B
A
B
gas phase diffusion
interface diffusion
volume diffusion
2018/10/13
surface diffusion
Classical chemical-reaction homogeneous reactions
kinetics:
The basis for understanding rate phenomena of heterogeneous reaction
2018/10/13
15
Considering the oxidation of metal 1 M ( S ) O2 ( g ) MO ( S ) 2
MgO + Al2O3 MgAl2O4 (Spinell)
Phase 1: formation of seeds
MgO Al2O3
Phase 2: growth of seeds
MgO
Al2O3
x
dx/dt ~ 1/x
2018/10/13
12
Diffusion–reaction in a solid/liquid diffusion–couple
(1)
2018/10/13
5
Characteristics (5) The temperature of solid state reaction takes place is lower than that of melting temperature of reactants or eutectic temperature of system. ~ obvious diffusion ——Tamman Temperature （泰曼温度） / Sintering( 烧 结) Temperature。 metal: Ts = (0.3~0.4)Tm salt: Ts = 0.57 Tm silicate: Ts = (0.8~0.9)Tm (6) An increase in solid – state reaction rate is frequently observed at a polymorphic transition temperature —— Hedvall Effect(海德华定律)。
Step 4. The product,
, diffuses out of the pores.
Step 5. The product returns to surroundings.
2018/10/13
14
7.3 General kinetic limits
The overall reaction rate is determined by the slowest of all series steps.
2018/10/13
3
7.1 Solid State Reactions:
General Aspects
Classical solid state reaction: The starting materials are finely grinded pressed to a pellet and heated to a temperature „near“ the melting temperature. The reaction progress then depends on different parameters 1. Temperature - activation energy of diffusion is lowered with increasing temperature empirical rule: Treaction > 2/3 Tmelting temp. (T[K]) 2. Energetics - classical Solid State reactions are in general exothermic and „thermodynamically controlled“ ! 3. Yield Products are formed in a yield of 100% 4. Grain Structure Porosity, grain size distribution and contact planes between reacting grains are of paramount importance for the reaction progress
Major process of materials manufacture
Control the industrial process
2018/10/13
2
Definition
In general
chemical reaction with solid phase
Strictly
solid +solid solid
2018/10/13
6
Types of solid state reaction
According to the state of reactant solid-solid reaction reaction with liquid reaction with gas
2018/10/13
7
According to the characteristics
11
Solid-Solid Reaction
Model for a classical solid-solid reaction (below melting point !): Interface between two crystals or two grains of a powder possible products: small crystallites (“powder”), single crystals ...
K D
C C0 1 1 K D 0
C0 C D
V D
C0
VD max
—— diffusion-limited
2018/10/13
20
(3) When neither diffusion nor reaction are the rate– controlling factors
• After reacting time t, a layer of product MO with width has been formed on the surface of metal M
• Further
processes:
reaction
controlled
by
the
(1) diffusion of O2 to interface of M-MO; (2) oxidation of metal
Oxidation 氧化反应 Deoxidation 还原反应 Adding 加成反应 Replacement 置换反应 Decomposition 分解反应 ……
2018/10/13
8
According to the reaction mechanism
diffusion-limited 扩散控制 reaction-limited 化学反应速度控制 nucleus-limited 晶核成核速率控制 sublimation-limited 升华控制 ………
On Fick’s First law, the diffusion rate of oxygen VD：
K —— constant of chemical reaction
C —— concentration of oxygen on the
x
dC VD D dX
2018/10/13
interface
1 1 1 V KC 0 DC0
2018/10/13
18
(1) Rate of diffusion >> rate of reaction
K
D
V KC0 VR max
—— chemical reaction-limited
2018/10/13
19
(2) Rate of diffusion << rate of reaction
Liquid
Thin oxide layer as result of solid–liquid reaction at the interface
Interface
Solid particles
Penetrating Liquid
2018/10/13
13
Diffusion–Reaction in a Porous Catalyst
wenku.baidu.comD —— diffusion coefficient of
oxygen in the product layer
17
At stable state of overall reaction
V VR VD
dC KC D dx D
x
C0 C
K C C 0 /(1 ) D
Structure tends to perfect, crystal growth Further reaction controlled by the diffusion (by surface, boundary, crystal fissure) of reactant substances through the layer of product
Step 1. The gaseous reagent diffuses from the surroundings onto the catalyst surface.
Step 2.
diffuses into the porous structure.
Step 3.
reacts at the pore walls.
2018/10/13
4
Characteristics
Heterogeneous reaction (2) Reaction interface between the reacting phases, such as nucleus and matrix or crystal and melt (3) Three steps in series Material transport to the interface Reaction at the phase boundary Sometimes, transport of reaction products away from the interface (4) Reaction at the phase boundary liberate or absorb heat, changing the boundary temperature and limiting the rate of the process
Chapter 7 Solid State Reaction
Xu Lingling, Dr. College of Materials Science and Engineering Nanjing University of Technology
2018/10/13
1
Necessary
Popular physicochemistry during high temperature process of solids
2018/10/13
16
Oxidation of metal
On the basic principle of chemical reaction kinetics, the oxidation rate of metal per unit area on the interface VR：
V R KC
9
2018/10/13
7.2 Process of solid reaction
1. Mixing of reactant particles， contact each other
2. New phase(with large amount of defect) formed by chemical reaction on the interface
2018/10/13
4.
10
Possible reaction pathes between two solid grains A and B
A
B
gas phase diffusion
interface diffusion
volume diffusion
2018/10/13
surface diffusion
Classical chemical-reaction homogeneous reactions
kinetics:
The basis for understanding rate phenomena of heterogeneous reaction
2018/10/13
15
Considering the oxidation of metal 1 M ( S ) O2 ( g ) MO ( S ) 2
MgO + Al2O3 MgAl2O4 (Spinell)
Phase 1: formation of seeds
MgO Al2O3
Phase 2: growth of seeds
MgO
Al2O3
x
dx/dt ~ 1/x
2018/10/13
12
Diffusion–reaction in a solid/liquid diffusion–couple
(1)
2018/10/13
5
Characteristics (5) The temperature of solid state reaction takes place is lower than that of melting temperature of reactants or eutectic temperature of system. ~ obvious diffusion ——Tamman Temperature （泰曼温度） / Sintering( 烧 结) Temperature。 metal: Ts = (0.3~0.4)Tm salt: Ts = 0.57 Tm silicate: Ts = (0.8~0.9)Tm (6) An increase in solid – state reaction rate is frequently observed at a polymorphic transition temperature —— Hedvall Effect(海德华定律)。
Step 4. The product,
, diffuses out of the pores.
Step 5. The product returns to surroundings.
2018/10/13
14
7.3 General kinetic limits
The overall reaction rate is determined by the slowest of all series steps.
2018/10/13
3
7.1 Solid State Reactions:
General Aspects
Classical solid state reaction: The starting materials are finely grinded pressed to a pellet and heated to a temperature „near“ the melting temperature. The reaction progress then depends on different parameters 1. Temperature - activation energy of diffusion is lowered with increasing temperature empirical rule: Treaction > 2/3 Tmelting temp. (T[K]) 2. Energetics - classical Solid State reactions are in general exothermic and „thermodynamically controlled“ ! 3. Yield Products are formed in a yield of 100% 4. Grain Structure Porosity, grain size distribution and contact planes between reacting grains are of paramount importance for the reaction progress
Major process of materials manufacture
Control the industrial process
2018/10/13
2
Definition
In general
chemical reaction with solid phase
Strictly
solid +solid solid
2018/10/13
6
Types of solid state reaction
According to the state of reactant solid-solid reaction reaction with liquid reaction with gas
2018/10/13
7
According to the characteristics
11
Solid-Solid Reaction
Model for a classical solid-solid reaction (below melting point !): Interface between two crystals or two grains of a powder possible products: small crystallites (“powder”), single crystals ...
K D
C C0 1 1 K D 0
C0 C D
V D
C0
VD max
—— diffusion-limited
2018/10/13
20
(3) When neither diffusion nor reaction are the rate– controlling factors
• After reacting time t, a layer of product MO with width has been formed on the surface of metal M
• Further
processes:
reaction
controlled
by
the
(1) diffusion of O2 to interface of M-MO; (2) oxidation of metal
Oxidation 氧化反应 Deoxidation 还原反应 Adding 加成反应 Replacement 置换反应 Decomposition 分解反应 ……
2018/10/13
8
According to the reaction mechanism
diffusion-limited 扩散控制 reaction-limited 化学反应速度控制 nucleus-limited 晶核成核速率控制 sublimation-limited 升华控制 ………
On Fick’s First law, the diffusion rate of oxygen VD：
K —— constant of chemical reaction
C —— concentration of oxygen on the
x
dC VD D dX
2018/10/13
interface
1 1 1 V KC 0 DC0
2018/10/13
18
(1) Rate of diffusion >> rate of reaction
K
D
V KC0 VR max
—— chemical reaction-limited
2018/10/13
19
(2) Rate of diffusion << rate of reaction
Liquid
Thin oxide layer as result of solid–liquid reaction at the interface
Interface
Solid particles
Penetrating Liquid
2018/10/13
13
Diffusion–Reaction in a Porous Catalyst
wenku.baidu.comD —— diffusion coefficient of
oxygen in the product layer
17
At stable state of overall reaction
V VR VD
dC KC D dx D
x
C0 C
K C C 0 /(1 ) D