Tortuosity analysis of porous powder compacts

Abstract

Advancement of consumer electronic devices and electric vehicle urges a need for batteries with higher power. Increasing the amount of active material is found to be ineffective as thicker electrodes may add limitations on transport characteristics. Thus, microstructure enhancement and analysis is a crucial step in the development of fluid transportation property of batteries. Therefore, the main objective of this work is to study the effect of porous structure parameters on the tortuosity of ternary powder compacts. This thesis mainly reviews existing approaches in tortuosity evaluation of porous structures and presents results from DEM simulation and Voronoi graph assisted analysis of ternary powder compacts. Distribution of tortuosity factor was found for the ternary compacts applying standard Dijkstra‘s algorithm on the constructed Voronoi diagram. Comparison of tortuosity distribution curves of the ternary packing structures in terms of three different size ratios (rsmall:rmedium:rlarge 1 cm: 2 cm: 4 cm, 1 cm: 2cm: 6cm and 1 cm: 2 cm: 8cm) of particles and different volume fractions (fsmall:fmedium:flarge 5%: 5%: 90%, 15%: 15%: 70%, 25%: 25%: 50%, 35%: 35%: 30% and 45%: 45%: 10%) of particles and different coordinate directions have been conducted. The results demonstrated a trend for tortuosity distribution peak to rise from fraction 5:5:90 to 45:45:10. Thus mixtures with higher fraction of small particles give narrower range of tortuosity factor values, while samples of higher proportion of large particles have a wider tortuosity distribution. This disposition becomes well defined for the samples, with the growth of large particles in size (1:2:6 and 1:2:8), resulting to more asymmetric and positively skewed tortuosity factor distribution. Another point is a lack of a distinct inclination of tortuosity factor distribution to change in a certain way with the alteration of coordinate axis direction. However, there are some deviations with respect to the z-axis, which can be associated either with difference in boundary conditions of walls orthogonal to z-direction from that of x- and y-directions or unevenness of particle arrangement as a consequence of the action of gravitational force during packing. The same tendency is observed in Voronoi cell edge lengths and face areas as in tortuosity distribution with respect to particle volume fraction, where the parameter distribution peaks reduce in height and extend towards larger values with the increase in large particle volume fraction. On the contrary, with respect to particle size ratio, the face areas are observed to show the opposite tendency of peaks to become narrower and higher in fractions 15:15:70 and 25:25:50, and to shift to the in fraction 5:5:90, with the increases in size of a large particle. However, it can be claimed that appearance of higher values of the face areas and their increase in quantity can compensate the difference in tendency. Therefore, the relation between tortuosity and Voronoi parameters can be revealed, which states that with the increase in the distribution of Voronoi cell edges and face areas towards larger values, the distribution of tortuosity also increases towards large values and vice versa by enlarging the portion of smaller Voronoi cell edges and faces tortuosity factor reaches smaller and more uniform values. Due to inverse relation between the diffusivity and tortuosity, smaller Voronoi cell edges and faces contributes to a better diffusivity.