Cold sintering of ZnO-PTFE: Utilizing polymer phase to promote ceramic anisotropic grain growth

Densification of ZnO-PTFE (polytetrafluoroethylene) composites is permitted by the Cold Sintering Process, having no effect on the stability of both materials. Highly dense samples can be obtained by this technique at extremely low temperatures in just a few minutes. Interestingly, the obtained samples show an anisotropy impacting: crystalline, microstructural and electrical properties. While the Wurztite ZnO crystals show a preferential growth along (00l) direction, microstructure observations show a grain growth along the in-plane (perpendicular to pressure application direction) up to 240%. Electrical conductivity is also influenced and is related to microstructure. In this situation, the addition of PTFE insulating phase allows to increase the conductivity in plane compared to the pure cold sintered ZnO sample. A mechanism is proposed to explain this phenomenon which involves PTFE transient distribution competing with the transient liquid driving densification and grain growth associated with cold sintering. This is further confirmed by the observation of a curvature of microstructure direction while approaching die edges. These observations offer a large variety of designs for further orientation driven properties.