Residual Stress in NiO-YSZ Composites and Its Relationship to Microstructure

From the view point of mechanical reliability, the thermal expansion coefficient of anodes is adjusted to add proper amount of YSZ into NiO to prevent large thermal stress between anodes and electrolytes and keep the stress conditions as compressive from R.T. to operation temperatures for protect the electrolyte from tensile destruction and buckling destruction.

 

By contrast, Nickel oxide and YSZ, which thermal expansion coefficient is 14.1 and 10.8 [10^-6 K^-1], were contacted directly in anode. This suggests that large thermal strain and stress exist in anode at R.T. caused by the difference of large thermal shrinkage from sintering temperature e.g. 1400 ^oC to R.T. Moreover, these thermal strain and stress are probably strongly influenced by the difference of microstructure such as porosity, volumetric ratio, average diameter, uniformity because the introduction of them occurs at contact area between NiO and YSZ. However, stress conditions in anode, i.e. NiO-YSZ composites, were never studied in author’s knowledge.

 

In this study, we evaluated residual stress and strain in NiO-YSZ composites numerically and experimentally.

 

Numerical simulation was performed by Finite element method (FEM) using MARC. Residual stress and strain were evaluated by X-ray diffraction method. Both results suggested that stress conditions in NiO and YSZ are tensile and compressive, agreed with the expected states from the difference of thermal expansion coefficient. By contrast, average strain and stress were not so high. In the case of uniform NiO-YSZ (50:50 vol%), average stress is ca. 100 MPa and ca. -100 MPa (minus was compressive) for NiO and YSZ, respectively. These phenomena were explained by stress and strain relaxation because contact area between nickel oxide and YSZ was spatially distributed and introduced stress and strain reduced each other.

 

However, stress and strain distributions also suggested that stress and strain widely distributed and its FWHM, i.e. dispersion, was over 200 MPa. In addition, distribution analysis based on the simulation results of various uniformity NiO-YSZ revealed that the increment of ununiformity caused the increment of stress dispersion. These results indicated that bending moment introduced and stress concentration in the microstructure of NiO-YSZ composites causes these widely dispersion.