第七章 固相反应

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
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interface
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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

Classical chemical-reaction homogeneous reactions
kinetics:

The basis for understanding rate phenomena of heterogeneous reaction
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Considering the oxidation of metal 1 M ( S ) O2 ( g ) MO ( S ) 2
Chapter 7 Solid State Reaction
Xu Lingling, Dr. College of Materials Science and Engineering Nanjing University of Technology
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Necessary

Popular physicochemistry during high temperature process of solids
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 4. The product,
, diffuses out of the pores.
Step 5. The product returns to surroundings.
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7.3 General kinetic limits

The overall reaction rate is determined by the slowest of all series steps.
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.
(1)
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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)。
1 1 1 V KC 0 DC0
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(1) Rate of diffusion >> rate of reaction
K
D

V KC0 VR max
—— chemical reaction-limited
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(2) Rate of diffusion << rate of reaction
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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
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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
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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
D —— diffusion coefficient of
oxygen in the product layer
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At stable state of overall reacdx D
x
C0 C

K C C 0 /(1 ) D


Major process of materials manufacture
Control the industrial process
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Definition

In general
chemical reaction with solid phase

Strictly
solid +solid solid
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Types of solid state reaction
According to the state of reactant solid-solid reaction reaction with liquid reaction with gas
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According to the characteristics
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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 ...
Liquid
Thin oxide layer as result of solid–liquid reaction at the interface