| Rapid Pulse Electric Current Sintering(PECS) technology has been widely applied in the production of the special ceramics, nano materials, functional gradient materials, ultrafine grain materials and heterogeneous composite materials. This technology can achieve the rapid sintering and densification by a high-intensity pulse current ?owing directly through the indenters and graphite die, or powder material to form the high heating rate(up to 2000 K/min) and high temperature gradient in the powder compact. Compared with the traditional pressureless and hot pressing(HP) sintering, the small grain size, uniform, high density product can be achieved by PECS in short sintering time at low sintering temperature. Although the sintering of powder has been played an important part in modern industrial, the theoretical investigation of the rapid sintering mechanism under strong pulse current has no unified understanding. Especially for non-conductive powder, since there is no current flowing directly through the powder materials, the driving force in the neck growth mechanism becomes a key problem and needs to make progress.As we know, the non-conductive particles compact has remarkable porous structure or non-homogeneous inner structure, obvious non-Fourier heat conduction character will be showed in the process of transient heat conduction on the condition of extra-high heat rate and extra-high temperature gradient. And the relaxation time of Al2O3 compact has been test out through experiment and reported. So the generalized themo-elastic-diffusion coupling theory which including the non-Fourier heat conduction law and non-Fick law is introduced to investigate the mechanism of non-conductive particles compact sintering become necessity and feasibility.In this thesis, the sintering driving force at the initial stage in pulse electric current sintering is investigated for non-conductive Al2O3 powders based on analysis and discussion of thermal-elastic-diffusion coupling transmission under the action of high heating rate and the high temperature gradient with the consideration of non-Fourier heat conduction and non-Fick effect.As the main work and achievement, the sintering mechanism for non-conductive material is proposed and confirmed in the framework of the thermal-elastic-diffusion coupling transmission with the consideration of non-Fourier and non-Fick effect. Specifically.(1) In the framework of the generalized thermoelastic theory with consideration of non-Fourier heat conduction law, local high temperature mechanism for double equal-sized particles model at the contact zone and the interpretation of the sintering neck growth are proposed. Calculations also show that the relaxation time has little influence on the final sintering temperature.(2) As the driving force of neck growth, the free surface curvature on particles neck is proposed to consider the influence of thermal-mechanical coupling effect in PECS. For double equal-sized particles model, the L-S(Lord and Shulman) generalized thermoelastic theory is introduced in order to deduce the expression of vacancy concentration difference at the initial stage of PECS with consideration of the influence of thermal stress. And then the relationship between the activation energy and the vacancy concentration difference is obtained. For the surface diffusion, volume diffusion and coupling diffusions mechanism(comparing with PECS, conventional pressure and HP sintering), numerical calculation shows that, the vacancy concentration difference in PECS raised 3-5 times more than that in pressureless sintering and HP, when considering the influence of the transient thermal stress. It will promote the sintering process since the concentration difference at the neck keeps higher state. On the other hand, the relationship between the activation energy and the vacancy concentration difference shows that the activation energy dropped with the decrease of vacancy concentration difference in the process of neck growth. When the vacancy concentration difference reduced to a certain value, the activation energy will drop quickly in a narrow scope of vacancy concentration difference, but it keep still higher than that in pressureless sintering and HP.(3) On the basis of previous theoretical work, based on the generalized thermoelastic theory with a single relaxation time, the direct finite element method is applied to discrete on the space and time, a finite element program based on GID is developed. The validity of the program is verified by comparing themal wave velocity with the theoretical value.(4) The influence of temperature gradient on the thermal diffusion flux is investigated in this thesis. Thermal diffusion, as a supplement driving force for the surface curvature driving in conventional sintering theory, will have asignificant influence on sintering neck matter migration. For PECS, the influence of high temperature gradient on the thermal diffusion effect will far greater than that of free surface curvature driving. Taking the unequal-sized particles model as an example, the thermal diffusion caused by the temperature gradient provides the dominated driving force in the matter migration at the initial stage.Thermal diffusion flux decreases with the neck growing up, and decreases with the increase of particle radius ratio. It shows that smaller particles have shorter sintering time, which is consistent with the sintering experiment conclusion.(5) The effect of concentration diffusion flux on the sintering driven force has been investigated in the framework of the thermal-elastic-diffusion coupling theory.Based on generalized thermo-elastic-diffusion theory with the consideration of non-Fourier and non-Fick effect, the concentration diffusion flux caused by local concentration gradient acts as extra driving force of initial stage of PECS, as well as thermal diffusion flux. And the calculation for equal-sized particles model show they play the dominated influence on sintering driving force for either volume diffusion mechanism or coupling diffusions mechanism. Especially at the post period of the initial stage of sintering, it can increase the sintering driving force remarkably. |
PDF Full Text Request