| Owing to the universality and practicability of constrained sintering problems, the study on the constrained sintering behavior has exhibited great application values and scientific significance. As an effective solution for the prediction and modelling of the constrained sintering behavior, continuum mechanics of sintering has drawn increasing attention. In recent years, LTCC thick films have become the most widely applied materials in the industry of electronics and integrated circuits. However, there still lacks systematically theoretical research on the constrained sintering behavior of LTCC thick films.In this thesis, the constrained densification behaviors of LTCC thick films under different constraints were studied by utilizing a lab-made optical dilatometer. On the basis of the continuum mechanics of sintering, isotropic and anisotropic sintering parameters were determined for the LTCC thick films through a typical vertical sintering method. Then, the obtained sintering parameters were successfully applied in the prediction and modelling of the constrained sintering behavior. The microstructure evolution laws for the studied constrained films were also acquired through detailed quantitative analysis. The main contents were outlined below:(1) After the tape casting process, LTCC thick films with good quality were successfully prepared by using YF microwave ceramic powder and HP ferrite powder. Moreover, films with different thickness could be obtained by changing the processing parameters. An in-situ optical dilatometer was built and it successfully recorded the densification behavior of various constrained LTCC films by utilizing a lab-made vertical sintering platform and a lab-made quartz rocking arm.(2) The uniaxial viscosities were measured for several LTCC films like Dupont 951 tape (DU), Ferro A6M tape (FE), YF and HP films by adopting the vertical sintering method. For the vertically aligned LTCC films, it was found that the sintering behavior in the vertical direction was slightly affected by the gravity of their own, however, the microstructure remained isotropic throughout the sintering process. It could be seen that the uniaxial viscosity for the LTCC films increased in a non-linear way, exhibiting a slight increase in the low-density region but a fast rise in the high-density region, which keeps a good consistency with the Raj’s model. The activation energy Qη of the uniaxial viscosity for the DU films was computed from the Arrehenius plots between the ln(Ep) and 1/T, with a calculated value of 290±47kJ/mol.(3) A uniaxially loaded sintering model was designed to determine the anisotropic sintering parameters for the LTCC films. Uniaxial loads were applied to the vertically aligned FE film and a uniaxially constrained sintering behavior could be obtained by adjusting the uniaxial loads. As observed in the research, the uniaxial sintering behavior was obviously altered by uniaxial loads, but the overall densification behavior was only slightly changed. By carefully checking the microstructure of the uniaxially loaded FE films, pores were found to align parallel to the uniaxial loading direction and the observed pore orientation was enhanced with the increase of the uniaxial load. According to the boundary conditions for the uniaxially constrained sintered FE films, anisotropic constitutive equations of continuum mechanics were simplified and equations for determining the anisotropic sintering parameters were obtained. Uniaxial viscosity and viscous poisson’s ratio were obtained for the uniaxially constrained film by adopting corresponding data. By comparing with isotropic sintering parameters, it was found that the uniaxial viscosity of the uniaxially constrained film increased apparently and the viscous poisson’s ratio decreased gradually with the increase of density.(4) The densification behavior of FE films constrained by rigid substrate was systematically studied. Owing to the constraining stress from the substrate, the film shrinkage in the in-plane direction was hindered and occurred only in the thickness direction, leading to a lower density than that of the freely sintered film. Anisotropic microstructure evolution was observed in the rigid substrate constrained sintered film with pores orientation aligning parallel to the substrate. The sintering activation energies for the free and constrained films were determined through Arrhenius and Master Sintering Curve (MSC) methods. On the basis of the Arrhenius method, the activation for freely sintered films was 530±30 kJ/mol, but a varying value was obtained for the constrained film, which decreased from 640 kJ/mol to 359 kJ/mol with the increase of density. According to the MSC method, the determined apparent activation energy was 510 kJ/mol and 440 kJ/mol for. the free and constrained films, respectively. It could be clearly seen that the activation energy of the constrained sintered film was lower than that of freely sintered films. Prediction and modelling of the constrained densification behavior was achieved though the isotropic and anisotropic continuum mechanics constitutive equations. By comparing the measured and theoretically predicted densification behavior, it could be seen that the anisotropic constitutive equations were able to well predict the densification behavior of constrained films within the region of high density (>75%).(5) An upward warpage could be observed for the HP film constrained by a flexible substrate. Compared with the films constrained by rigid substrates, the constraining stress was released to some extent by the film warpage and the hindrance to the densification process was thus lowered, leading to a higher final density for the flexible substrate constrained film. A prediction for the densification behavior was achieved through the isotropic constitutive equations and a good consistency with the actual densification rate was observed in the low density region. However, a discrepancy occurred and developed at elevated densities. The microstructure was characterized for the flexible substrate constrained HP film and a porosity gradient was observed, where the porosity decreased with increasing the distance away from the interface. The constraining stress was determined through the bending degree of the flexible substrate and it was higher than that calculated through the isotropic constitutive equations, which could be assigned to the anisotropic microstructure. |
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