Fig. 4: Understanding the prestress-induced nonlinearity.

a FEM simulated internal stress-induced polarization in ND/ZnO composites; the arrows indicate the polarization vectors of grains. b Band structures of a potential barrier for head-to-head configuration under (i) compressive and (ii) tensile stress; the internal stress-induced polarization changes the potential barrier height as well as the negative interfacial charge density (Qi). FEM Simulated the distribution of current density in monolith (c) and composites with different sintering pressures (NZ-100 (d) and NZ-150 (e)) under the same voltage.