To explain both microstructure and

To explain both microstructure and electrical observations, the following hypothesis is proposed. As depicted on figure 4, for pure ZnO sintering, particles are first placed in the die and get surrounded by liquid (here it is water with 2M acetic acid). When pressure and temperature is raised, dissolution-precipitation occurs, and nearly isotropic grain growth is observed. However, uniaxial pressure is applied and slight deviation from an isotropic growth is observed both in microstructural and electrical observations. When PTFE is present in the sample, the mechanism is more aggressively affected. First stage is similar with a particle distribution and the liquid surrounding them. The application of pressure and temperature first affects the PTFE, which is mechanically much softer than ZnO ceramic grains. Therefore, PTFE deforms perpendicular to pressure axis and fills the ZnO intergranular spaces. Such as in the pure ZnO case, liquid phase must escape the die due to temperature increase. It is slowly moving from the center to the edges of the system, but PTFE, due to its deformation prevents fluid flowing along the pressure axis (out-of-plane). Then, fluids flows mostly along the in-plane direction to escape the die. An important grain growth occurs for ZnO but it needs liquid phase for mass transport. Thus, grain growth is oriented along the in plane axis, following fluid path. The presence of acetic acid even enhances this phenomenon since it is a good solvent for catalyzing ZnO growth [16]. This sintering mechanism leads to a highly anisotropic microstructure, as shown on figure 2. We have recently demonstrated from molecular dynamic modelling [17] that the surface diffusion is enhanced by 103 times relative to the activity with no liquid under cold sintering conditions. This would bias the in morphology and account for the anisotropic grain growth. This in turn will also impact the electrical properties shown in figure 3 where the resistivity increase along the out-of-plane axis along with PTFE content, but also with the resistivity decrease. Since grains grown along the in plane direction, there is less grain boundaries in the growing direction (grains reach about 15-20 µm) when PTFE is present. ZnO resistivity is governed by grain boundaries impedance component in the composites. Therefore, their reduction in the serial grain boundary interfaces leads to an increased conductivity.