基础材料和有机材料 (47).pdf

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1、 Object:sintering of single powder Main mass transfer modes:9.3 Solid State Sintering Evaporation-Condensation Diffusion r x P0ln2PPMdRTrPPMdRTrr012ln(11)1.Evaporation-Condensation Mass Transfer Model The surface tension can make the vapor pressure P of concave and convex surfaces lower than and hig

2、her than that of plane surfaces(P0),respectively,and can be expressed by Kelvin formula:For spherical surfaces:(1)For non-spherical surfaces:(2)For the solid particles with uneven surface,the convex part shows positive pressure and the concave part is negative pressure,so there is a mass migration f

3、rom the convex part to the concave part.Definition:Because of the different curvature of the solid particle surface,there must be different vapor pressure in different parts of the system at high temperature.The particles will evaporate from the convex particle surface with high vapor pressure and c

4、ondense to the concave neck with low vapor pressure through gas phase transfer,so that the neck is gradually filled and sintering is promoted.Cause of mass transfer:different curvature causes pressure difference P Restrictive condition:particles are small enough,r C0Cc 1C 2C)Vacancy diffusion:Prefer

5、ably,from the neck surface the contact point;Then,from the neck surface the inside surface diffusion interfacial diffusion volume diffusion.Pathway of diffusion mass transfer Atomic diffusion:opposite to the direction of vacancy diffusion;diffusion end point:neck.Diffusion path:(1)Surface diffusion

6、from the surface of the particles to the neck(2)Grain boundary diffusion from grain boundary to the neck(3)Volume diffusion from the surface of the particles to the neck(4)Volume diffusion from grain boundary to the neck(5)Volume diffusion from the interior of particles to the neck(6)Evaporation-con

7、densation mass transfer from the surface of particles to the neck(7)Dissolution-precipitation mass transfer from the surface of particles to the neck (the temperature at which surface diffusion occurs volume diffusion temperature)Example:for Al2O3,Ts330 Tv900.Initial stage of sintering Kinetic model

8、 for diffusion mass transfer According to the sintering temperature and diffusion degree,the diffusion mass transfer process can be divided into:Initial stage of sintering middle stage of sintering final stage of sintering Characteristics:mainly surface diffusion;the porosity of the green body is la

9、rge,and the shrinkage is only about 1%Reason:the surface diffusion promotes the smooth surface of the pores and the spheroidization of the pores,but has no obvious influence on the disappearance of the pores and the shrinkage of the sintered body.xrDkTrt160*1 53 51 5VVLLDkTrt33 5160*2 56 52 5See the

10、 book for details Under the effect of vacancy concentration difference,there is a relationship between the neck growth rate and the vacancy diffusion rate:In diffusion mass transfer process,the central distance of particles is shortened,and the shrinkage is shown as follows:Kinetic model of the init

11、ial stage of diffusion mass transfer a.Sintering time t b.Particle radius r For large particles:Not fully sintered for a long time，x/r0.1 For small particles:high densification rate,x/r 0.4 Discussion:The main technological factors to be controlled in sintering are as follows:xrt15LLt25xrrLLr 3565t

12、the neck is enlarged the curvature the driving force C time(min)Extension of time does not work For example,in the sintering experiment of Al2O3 and NaF,the densification can not be achieved by prolonging the sintering time for diffusion mass transfer,so it is proper to hold a shorter time for such

13、sintering.The relationship between neck growth rate and particle size for Al2O3 sintered at 1600 for 100h c.Temperature T-the decisive factor Y：sintering shrinkage L/L K：sintering rate constant t：sintering time LLDkTrt(5)*2/56525YP KtFormula deformation:keep the temperature and particle size are con

14、stant log1log YPtK ln/KA Q RTThe self-diffusion coefficient D*increases with the increase of temperature Fig.9-11 Shrinkage of Al2O3 at the initial stage of sintering(logarithmic coordinate)the value of sintering activation energy Q .Middle stage of sintering Characteristic:Grain boundary diffusion

15、and lattice diffusion dominate.The porosity is reduced to 5%and the shrinkage is up to 80%-90%.Reason:The particles are bonded,the neck is enlarged,the shape of pores change from irregular to cylindrical,and they connected with each other;the grain boundary begins to move;the grains grow normally.Be

16、cause the growth of the neck makes the spherical particles become polyhedron gradually,the distribution of the grains and the way of spatial accumulation are very complex at this time.Coble proposed a polyhedral model.10*3cfPDKTLttConclusion:Pc t,so the densification rate is faster in the middle sta

17、ge of sintering.LThe length of a cylindrical space；tsintering time；tfsintering time at the middle stage Tetrakaidecahedron dodecahedron 62*3cfPDKTLttCharacteristic:The pores are completely isolated and the grains have grown obviously.The shrinkage of green body is 90-100%.Final stage of sintering In

18、 the final stage of sintering,the relationship between porosity(PC)and sintering time(t)is as follows:There is no obvious difference between this relation and that in the middle stage of sintering.When T and r are constant,PC decreases linearly with t.1.0 0.9 0.8 0.7 0 10 100 1000 t(min)Relative density