Zinc oxide has a semiconducting nature. Zinc oxide (ZnO) is an n-type semiconductor. An n-type semiconductor is a material in which loosely bound electrons are available. However, zinc oxide (ZnO) is not like conventional semiconductors such as germanium or silicon. In germanium or silicon semiconductors, these extra electrons come from dopants. But in ZnO, no dopants are required to be added externally to contribute extra electrons. Instead, defects in the crystal lattice are responsible for the n-type behavior of zinc oxide. This means that even when zinc oxide is intrinsic (or pure), without intentional doping, the extra electrons primarily result from defects in the crystal structure.
Zinc oxide has a hexagonal wurtzite structure. Ideally, every Zn²⁺ ion is coordinated by O²⁻ ions in a fixed geometry. However, in practice, the lattice has imperfections, and these defects generate free electrons. These imperfections can be of two types in a ZnO crystal. These are oxygen vacancies and zinc interstitials.
Oxygen Vacancies
In ZnO crystal some oxygen atoms may be missing from its lattice site. Each missing oxygen atom leaves behind two unpaired electrons. These electrons are not tightly bound and can easily move through the lattice, acting as free carriers.
Zinc Interstitials
A zinc atom occupies a position between normal lattice points. These are called interstitial Zn atoms. Zn atoms have a +2 charge (Zn²⁺) and bring extra electrons. These interstitial Zn atoms act as shallow donors, providing additional free electrons. Like oxygen vacancies, Znᵢ defects contribute to n-type conduction.
Role of Grain Boundaries in ZnO Blocks
Grain boundaries play a crucial role in zinc oxide (ZnO) blocks in contributing to their non-linear electrical behavior. There are a large number of tiny sintered grains in a ZnO crystal. The narrow spaces between these grains are known as grain boundaries. These grain boundaries are not empty; they are filled with bismuth oxide (Bi₂O₃), cobalt oxide (CoO), and manganese dioxide (MnO₂). These oxides exhibit p-type behavior. Since ZnO has the property of an n-type semiconductor, a depletion zone is formed at the junction of the n-type ZnO and the p-type additives at the boundaries. These act like Schottky barriers at the grain interfaces.
At the grain boundaries, free electrons from the ZnO grains diffuse into the p-type additive layer. This creates depletion regions (p-n-p junctions) across the boundaries. These grain boundaries act as blocking barriers under normal conditions, allowing only a very tiny leakage current to flow. As a result, the grain boundaries offer very high resistance under normal conditions.
During a surge, when the voltage exceeds the breakdown threshold, these barriers collapse, and current starts flowing easily. After the surge is over and the voltage drops to normal, the barriers reform automatically, restoring the high-resistance state.
Role of Additive Oxides in ZnO Blocks
As discussed already, in a ZnO gapless lightning arrester, the zinc oxide blocks are not made of pure ZnO. There is 4–6% by weight of various additive oxides, like Bi₂O₃, CoO, MnO₂, etc. These additives play the main role in the non-linear resistance behavior of the blocks.
Bismuth Oxide (Bi₂O₃) does not have any direct contribution to influencing the electrical behavior of the blocks. It facilitates densification during sintering, promoting good grain boundary formation. It also contributes to a uniform microstructure.
Cobalt Oxide (CoO or Co₃O₄) and Manganese Dioxide (MnO₂) act as p-type substances. Therefore, they accept the loosely bound electrons of nearby n-type ZnO grains. Thereby, they help in turning the grain boundaries into depletion regions.
There are some other optional oxides which may also be added to enhance grain boundary properties. These are:
- Antimony Oxide (Sb₂O₃): Enhances grain boundary structure and stability.
- Chromium Oxide (Cr₂O₃): Sometimes used for fine-tuning grain boundary properties.
- Nickel Oxide (NiO): Helps control electrical degradation.
With additives, grain boundaries behave like non-linear barriers for both directions of current. If we imagine only grain boundaries in a block, we will find that there is a dense network of p-n-p junctions. So, under normal conditions, the barriers resist the current in both the positive and negative directions. Similarly, for a surge voltage waveform in both directions, the boundaries offer low resistance. This is why grain boundaries offer non-linearity to alternating current. Also, a ZnO block achieves fast response, high energy absorption, and self-recovery.