基础材料和有机材料 (37).pdf
Diffusion QUESTIONS 1.If the diffusion of Ca2+in CaO is intrinsic diffusion until the melting point of CaO(2600 oC),try to calculate the concentration of impurity trivalent ion.(Known that GS of CaO is 6 eV.)The diffusion of Ca2+in CaO is vacancy mechanism,and the diffusion coefficient .Therefore,if the diffusion of Ca2+in CaO is intrinsic diffusion until the melting point of CaO(2600 oC),the induced by M3+should be much higher than the vacancy concentration induced by thermal defect.VCa CaV CaOCaD/2+Analysis：CaCaCaOVMM +22 M3+25.4410-61.08810-5 62319Ca10)2873101.382101.6026exp(-)exp(-E/2kTV5.44 Solution：At 2600 oC,concentration of Schottky defect is：Defect reaction equation of M3+dissolving in CaO is:Therefore,the concentration of M3+should be much higher than the vacancy concentration induced by thermal defect.2.Try to discuss the influence of ZnCl2 addition(10-4 mol%)on the diffusion abilities of Na+and Cl-from room temperature to the melting temperature.Analysis：When there is ZnCl2 dissolving in NaCl,the diffusion coefficient of Na+is determined by thermal defect concentration(Schottky defect),together with impurity point defect concentration.At low temperatures,the influence of impurity point defect concentration on the diffusion coefficient of Na+dominates.At higher temperatures,intrinsic diffusion of Na+would dominate.For Cl-,intrinsic diffusion dominates over the entire temperature range.)(10)(102662molmolVClZnZnClNaClNaNaCl +)2exp(KTENn=966K)108.6162.32exp(1056ccTT =Solution：Known that GS of NaCl is 2.3 eV.If Frenkel defect concentration of NaCl is 10-6,the temperature should be higher than Tc calculated by the following equation:When the temperature is higher than Tc,intrinsic diffusion of Na+dominates.3.The activation energy for diffusion of C,N,and H in bcc iron is 84 kJ/mol,75 kJ/mol and 13 kJ/mol,respectively.Try to analyze and explain the differences.With the decrease of atomic radius,activation energy decreases.element C N H atomic radius（）0.77 0.71 0.46 For C,N and H,the diffusions in bcc iron occur by a vacancy mechanism.Therefore,the main factor affecting their activation energy for diffusion is the atomic radius.Solution：4.(1)Speculate the oxygen pressure dependence of Fe2+diffusion coefficient in iron-poor Fe3O4.(2)Speculate the oxygen pressure dependence of Fe2+diffusion coefficient in iron-rich Fe2O3.T)T)exp(-G/Rexp(-G/RP P FeFeV V K K 2Fe2FeV V(g)O(g)OO O2 21 12Fe2Fe2 21 1O O2 2FeFeFeFe0 0FeFeFeFeo o2 2FeFe2 2 +(1)Iron-poor Fe3O4 could be considered as FeO+Fe2O3，and the chemical formula can be written as Fe1-xO.The defect reaction equation:)/exp(422132RTGPVVhOFeFe =Q61/Fe61312122Dr)3/exp()41(OOFeOFePoRTGPVx =+Solution：(2)Iron-rich Fe2O3 could be considered as Fe2O3+FeO,and the chemical formula can be written as Fe2+xO3.The defect reaction equation:)RTGexp(-eVK2)(21Or 2)(21232212O2O22 =+OOOFeOOFePVegOoOgOeFVOFe61/61312322Dr)3/exp()41(=+OOFeOOOPoRTGPVx)/exp(.422132RTGPVVeOOO =Q 5.The diffusion coefficients of positive ions in soda-lime silicate glass are shown in the figure.(1)Explain the diffusivity of Na+and Ca2+are larger than that of Si4+.(2)Explain the nonlinearity of Na+diffusion coefficient curve.(3)For tempered glass,how will the diffusion coefficient curves change?(4)What is the activation energy for diffusion of Na+in liquid glass?0.5 1.0 1.5 1000/T-5 -6 -7 -8 -9 -10 Si Ca Na logD(cm2/sec)(1)According to the random network theory,Na+and Ca2+ions are network modifying ions and easy to diffuse among the vacancies of the network.Experimental results show that the activation energy for diffusion is about 25 kcal/mol.Si4+ion is network forming ion,the viscous flow activation energy associated with the migration of Si4+is approximately 100 kcal/mol.(2)The nonlinearity of Na+diffusion coefficient curve represents the transition of glass,and the nonlinearity is caused by the rapid change of the glass structure.(3)For tempered glass,the diffusion coefficient will increase.That is because the network structure of tempered glass is more open and Na+is easy to diffuse.(4)The activation energy for diffusion of Na+could be determined by calculating the slope of the curve at high temperatures.Solution：6.The activation energy for diffusion of Fe2+in FeO is very low,which is 23 kcal/mol.However,the activation energy for diffusion of Mg2 in MgO is much higher,which is 83 kcal/mol.Try to explain the difference.FeVFeFe +2132Solution：FeO and MgO have the same structure which is similar with NaCl and the similar positive ionic radius.However,nonstoichiometry may occur for iron oxide(FeO),for the iron can be present in both Fe2+and Fe3+states.The formation of Fe3+ions must be offset by formation of Fe2+vacancies.With the increase of oxygen pressure,the vacancy concentration increases,and the diffusion coefficient of Fe2+in FeO further increases.
