1、 (1) Volume diffusion in metal crystals (2) Diffusion in ionic crystals (3) Diffusion in covalent crystals 6.5 Diffusion in solid 1. Diffusion of atoms in various types of lattices (1) Volume diffusion in metal crystals Vacancy diffusion is the main diffusion mechanism in metal crystals. G Gm+ Gf/2
2、For different metalsthe melting point G D For example： The diffusion of C, N and O in most metals is interstitial diffusion. The diffusion of C in Fe crystals (steel iron) Exception: when the interstitial atoms are relatively small to a certain degree or the lattice structure is relatively open, the
3、 interstitial diffusion mechanism dominates. Characteristics: it diffuses very fast a lot of interstitial voids, and G Gm Only for a few crystals with large interstitial void, such as F 、 O2 in CaF2、 UO2 (2) Diffusion in ionic crystals Two diffusion mechanisms Vacancy diffusion: Interstitial diffusi
4、on: For most ionic crystals, such as MgO、 NaCl、 FeO、 CoO Application: CaF2 can reduce the melting point and sintering temperature of glass, and it can also act as a clarifying agent. The content of feldspar should not exceed 50%, otherwise 2% CaF2 must be added. Example: CaCl2 was introduced into KC
5、l and the diffusion of K+ can be analyzed. The matrix was KCl. LnD 1/T Hm+ Hf/2 R 1 2 Hm R For intrinsic diffusion: For nonintrinsic diffusion: In the ionic crystals, the defects affecting diffusion come from two forms: Intrinsic point defect: It needs high energy, significant diffusion can only occ
6、ur at higher temperatures. Doping point defect: The diffusion often occurs at moderate temperature, a small amount of impurities can greatly accelerate the diffusion. From the relationship of LnD 1/T, the right figure can be obtained: Line ： At high temperature, intrinsic diffusion plays a leading r
7、ole. RTLnD LnDHHRTDHHRTD a N v a vGGmfmfVfm.1/2exp( )/2exp( )/2000 0 022It can be analyzed from the figure: LnD 1/T H m R H m+ H f/2 R 1 2 RHfHm /2the slope = V CaClK2addition RTLnD LnDHRTDHRTa v CaCl S RHRTD a N v v CaClGmmmmVm.1exp( ) exp( / ).exp( ) exp( )000 0 220 0 222LnD 1/T H m R H m+ H f/2 R
8、 1 2 Line ： At low temperature, the Schottky defect concentration is so small that it can be ignored, so the nonintrinsic diffusion plays a leading role. RHmthe slope LnD 1/T Hm+ Hf/2 R 1 2 Hm R Analysis: With the increase of impurity content, which direction does the turning point of nonintrinsic d
9、iffusion to intrinsic diffusion move? For intrinsic diffusion: For nonintrinsic diffusion: Discussion: With the increase of impurity content, the turning point of nonintrinsic diffusion to intrinsic diffusion moves to high temperature. (3) Diffusion in covalent crystals Covalent crystals have open c
10、rystals structure, and the lattice space is larger than metal and ionic crystals. Diffusion Mechanism: Vacancy diffusion mechanism Reason: the directionality and saturation of the chemical bonds of covalent crystals From the energy point of view: Interstitial diffusion is not conducive to bonding, a
11、nd is also not conducive to energy reduction. Example: For diamond, the size of interstitial void is about to the size of atom it is often diffused as vacancy diffusion mechanism. Due to the directionality and saturation of the chemical bonds the self-diffusion activation energy of a covalent crysta
12、l the activation energy of a metal with a similar melting point D Example: Ag Ge have similar melting point 184KJ/mol 289KJ/mol Diffusion activation energy It shows that the directivity and saturation of the covalent bond have a strong influence on the migration of the vacancy. (2) Anion vacancy typ
13、e: ZrO2 (1) Cationic vacancy type: FeO、 NiO、 MnO Non-stoichiometric compounds: (such as FeO、 NiO、 CoO、 MnO etc.) 2. Diffusion of atoms in nonstoichiometric compounds Due to the vacancy caused by the change of atmosphere, the diffusion coefficient is obviously dependent on the ambient atmosphere. Fe
14、FeFe V K G RT0 0 2 exp( / ) FeOFe OG RT VPV P GRT22031213 16 0exp( / ) 4 (14) . .exp( 3 )(1) Cationic vacancy type: FeO、 NiO、 MnO Due to the existed valence cations, there are 5 15 VFe/ in Fe1-xO The equilibrium constant Fe V FemOmOm mOmD a N v v VGRTa v PGRTGRTa v PS SRH HRTD PH HRT22202 200201316
15、00201316 0 0016 0 .exp( )(14) . .exp(3).exp( )(14) .exp(/ 3).exp(/ 3). .exp(/ 3) LnD 1/T / 2m fH HR 0 / 3mH HRDiscussion ： Keep the temperatureT constant, plotting as LnDLnPO2, the slope of the line is K=1/6; Keep the oxygen partial pressure PO2 constant, plotting as LnD1/T, the slope of the line is
16、 LnDFe K=1/6 LnPO2 Influence of oxygen partial pressure on diffusion coefficient of Fe (DFe) The reduction of the partial pressure of oxygen at high temperature will result in the following defect reactions: Oe2O (g) V1OO22 O OO OO OK P V eP V K GRTV P GRT22212 212 30013 16 0. 4 . exp( ) (14) . .exp
17、( 3 )氧 OmOm mOmD a v VGRTa v PS SRH HRTD PH HRT220200201316 0 0016 0 exp( )(14) . .exp(/ 3).exp(/ 3). .exp(/ 3)(2) Anion vacancy type: ZrO2 LnD 1/T / 2m fH HR 0 / 3mH HRK= -1/6 LnPO2LnDFe Discussion ： Keep the temperatureT constant, plotting as LnDLnPO2, the slope of the line is K=-1/6; Keep the oxy
18、gen partial pressure PO2 constant, plotting as LnD1/T, the slope of the line is Influence of oxygen partial pressure on diffusion coefficient of Fe (DFe) The relationship between D and T in hypoxic oxides Whether its the metal ion or the oxygen ion, the relationship between diffusion coefficient and
19、 temperature is linear (shown in the LnD 1/T plot), and the slope are the same. Conclusions： For the transition metal non-stoichiometric oxides, the increase of oxygen partial pressure will be beneficial to the diffusion of metal ions, but unfavorable to the diffusion of oxygen ions. E F LnD 1/T 0 /
20、 3mH HR mHR / 2m fH HR (Intrinsic diffusion) (Diffusion in non-stoichiometric oxides) (Nonintrinsic diffusion or impurity diffusion) In the non-stoichiometric oxides, if the contributions of intrinsic defect vacancy, impurity defect vacancy and non-stoichiometric vacancy caused by the change of atmosphere to the diffusion coefficient are all considered, there are two turning points in the LnD 1/T plot.
