EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

In ZnO varistors, understanding and controlling the microstructure plays a fundamental role, since the number and character of grain boundaries determines the current-voltage characteristics. In this context the so-called inversion boundaries (IBs), a special class of grain boundaries, are a key to microstructure design. IBs are formed by inverting the ZnO-stacking in the polar [0001]-axis during grain growth. If inversion takes place at a Zn layer, so-called head-to-head IBs are created. As an important consequence, grains with IBs grow exaggeratedly fast and eventually determine the microstructure. Moreover, IB formation induces O terminated grain surfaces, which are especially favorable for varistor behavior. Experimentally it is known that IBs never occur in pure ZnO, but only upon addition of specific dopants like Sb, Sn or Fe. In this work we use ab-initio calculations to determine the energetics of the inversion boundary as a function of doping. To do so, we calculate the total energy of the pure and doped IBs with different coverage and the surface energies of the polar (0001) surface of pristine ZnO, as well as the reference structures needed to obtain the chemical potential of the dopants. Combining these results with thermodynamic data for oxygen as a function of pressure and concentration, we predict the Gibb’s free energy for IB formation as a function of process conditions for a series of dopants. We show that at sintering conditions, IBs covered with 1/3 monolayer of Sb exhibit a very low Gibb’s free energy, triggering the strong tendency for IB formation observed experimentally. Also for ½ coverage of Sn and full coverage of Fe, low IB formation energies are found, in agreement with the experimental observation that they are also able to lead to IB formation, but to a smaller extent.