"Molecular dynamics simulations of materials with non-linear electrical properties"

Materials with non-linear electrical properties present an intense non-linearity at their characteristic current-voltage (I-V) curve and are known as variable resistors, varistors. Particularly, under normal operation voltages, varistor operates as a resistor but under higher voltages than the normal ones, it operates as a conductor. The transition from resistors’ to conductors’ phase is determined to each varistor by its breakdown voltage value (V_br). Varistors find application in protection of electronic and electrical system arrays that are fed by the power network at 220V against unwanted pulses of overvoltage or overcurrent to the grid. The main representative of the new generation of varistors is the Zinc Oxide (ZnO), which constitutes the base of ceramic polycrystalline materials suitably modified with supplementary oxides like Bi₂O₃, MnO, Co₃O₄. 

The suggested dissertation aims to study materials that are characterized by non-linear electrical properties by the aid of molecular dynamics simulation. Molecular dynamics constitutes a powerful tool to study structural and dynamical material properties.

 Primarily, the source code which executes simulations at constant temperature, volume and pressure will be built, and will run under appropriate periodic conditions. Subsequently, the source code will be enriched by the application of an external electrical field in a specific direction, so that changes concerning structural and dynamical material properties will be evaluated under the field influence.

Afterwards, this study will be focused on the crystal ZnO as a standard material known for its non-linear electrical properties. This structural study will be based on the investigation of Radial Distribution Functions (RDF). The dynamic study of ions diffusion will be based on the evaluation of Mean Square Displacement (MSD) functions, while a hybrid method which combines classical molecular dynamics with alternative charge transfer calculations will be used for electrons’ diffusion.

After the evaluation of all these methods application to ZnO crystal, the study will be focused on polycrystalline ZnO, a material characterized by a different crystallographic orientation among neighbouring grains of ZnO, by investigating interfaces properties. These interfaces play a significant role as the non-linear behaviour of varistors is attributed to the development of potential barriers among them. Every interface at the ceramics polycrystalline ZnO varistors is well known that is characterized by a breakdown voltage equal to 3V. The attributes mapping of interfaces will permit the study of the developed dynamic barriers as well as their dependence on the applicable external electrical field.

Finally, simulations and theoretical calculations will extend to composite materials consisting of polycrystalline ZnO, modified with additional oxides. An effort regarding the simulation of complex glass crystal materials will be made, where glassy layer interferes among single crystals ZnO. These specific materials have been successfully tested in protection systems arrays on low state voltage, given the fact they appear a characteristic breakdown voltage that is equal to 3V.