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Monolithic 3 YTZP and 3 YTZP containing 2.5 vol% of single-walled carbon nanotubes ( SWCNT) were fabricated by Spark Plasma Sintering ( SPS) at 1250°C. Microstructural characterization of the as-fabricated 3 YTZP/ SWCNTs composite shows a homogeneous CNTs dispersion throughout the ceramic matrix. The specimens have been crept at temperatures between 1100°C and 1200°C in order to investigate the influence of the SWCNTs addition on high-temperature deformation mechanisms in zirconia. Slightly higher stress exponent values are found for 3 YTZP/ SWCNTs nanocomposites ( n~2.5) compared to monolithic 3 YTZP ( n~2.0). However, the activation energy in 3 YTZP ( Q = 715 ± 60 kJ/mol) experiences a reduction of about 25% by the addition of 2.5 vol% of SWCNTs ( Q = 540 ± 40 kJ/mol). Scanning electron microscopy studies indicate that there is no microstructural evolution in crept specimens, and Raman spectroscopy measurements show that SWCNTs preserved their integrity during the creep tests. All these results seem to indicate that the high-temperature deformation mechanism is grain-boundary sliding ( GBS) accommodated by grain-boundary diffusion, which is influenced by yttrium segregation and the presence of SWCNTs at the grain boundary. [ABSTRACT FROM AUTHOR]

This paper investigates the effective steady-state creep response of porous Ni-YSZ composites used in solid oxide fuel cell applications by numerical homogenization based on three-dimensional microstructural reconstructions and steady-state creep properties of the constituent phases. The Ni phase is found to carry insignificant stress in the composite and has a negligible role in the effective creep behavior. Thus, when determining effective creep, porous Ni-YSZ composites can be regarded as porous YSZ in which the Ni phase is counted as additional porosity. The stress exponents of porous YSZ are the same as that of dense YSZ, but the effective creep rate increases by a factor of 8-10 due to porosity. The relationship of creep rate and volume fraction of YSZ computed by numerical homogenization is underestimated by most existing analytical models. The Ramakrishnan-Arunchalam creep model provides the closest approximation among all analytical models. [ABSTRACT FROM AUTHOR]

Abstract: The sintering stress is a driving force for morphological evolution of pores in sintering, and can be determined from knowledge of the microstructure. The sintering stress of non-equilibrium, non-uniform and non-isotropic porous glass films cast and sintered on rigid substrates was computed from synchrotron X-ray microtomography data. This method was able to show how the inhomogeneous distribution of local density at the particle scale led to a difference in local sintering stress. The anisotropic microstructure in the film was correlated to the deviatoric components of the sintering stress, which were defined by surface energy tensor. The topological evolution of pore structure was characterized by genus and Euler characteristic. [Copyright &y& Elsevier]

Abstract: Macroscopic shrinkage in sintering is described as a linear function of the sintering stress tensor and the viscosity tensor, which is determined by taking into account the grain boundary diffusion mechanism and the anisotropy in the microstructure. For a simple orthorhombic structure in equilibrium, the anisotropic shrinkage rate is dominated by the deviatoric component of the viscosity tensor, which is approximately proportional to the logarithm of the aspect ratio of a volume element, and acts so as to deform the elongated structure to be more isotropic. [Copyright &y& Elsevier]

Abstract: The sintering stress is originally defined for equilibrium states, where the mechanical stress just balances the internal surface tension forces so that the porous material does not shrink. The sintering stress for a given porous structure in equilibrium was calculated by three methods theoretically; the energy difference method, the curvature method, and the force balance method. The sintering stresses by three different methods were exactly the same for idealized porous materials in equilibrium, in which the pore surface had a constant curvature at any point. The grain boundary energy contributed to the sintering stress, and it affected the thermal stability of the porous structure. [Copyright &y& Elsevier]

The diffusion of atoms changes the morphology of particles in sintering. When the attachment kinetics of adatoms is slower than the diffusion of atoms, the sintering is controlled by the local processes taking place at the interface. This interface-controlled sintering, for example, the sintering by evaporation–condensation, is described as the sintering through the surface motion by the difference between mean curvature and the average of mean curvature. The sintering of two identical spherical particles has been studied by the Surface Evolver program until they reach the final equilibrium shape. Although the evaporation–condensation is a “non-densifying mechanism”, the shrinkage occurred due to the motion of mass centers of particles in a long time. The shrinkage rate was approximately proportional to the sintering force. [Copyright &y& Elsevier]

The maintenance of the equiaxed shape of grains after a large deformation is a common feature of superplasticity in polycrystalline solids. This feature is similar to that seen in the deformation of soap froth. The topological transformation of the grains was analyzed by using the Surface Evolver which simulated the three-dimensional simple shearing flow of soap froth. In this model the equiaxed shape of the grains could be maintained in a disordered system of N grains when both face-creation switching and face-elimination switching occurred αγN times in shear strain γ. The simulation gave α-values from 4.9 to 4.5 in the monodispersed structure and α=4.7 in the polydispersed structure. The strain, which is associated with grain switching, is represented by γ″=ΔF/αN, where ΔF is the mean cumulative number of grain switchings. The intragranular strain, which is related to the aspect ratio of the grains, is represented by γ′=γ−γ″. The back stress by boundary tension was proportional to γ′. It can act to restore the equiaxed shape, and is one of the origins of anelasticity. [Copyright &y& Elsevier]