Tez özetleri Astronomi ve Uzay Bilimleri Anabilim Dalı

Effect of AL₂O₃ Addıtıon on the Electrıcal Conductıvıty and

Mıcrostructure of Zno
From past to present various scientific studies have been carried out about ZnO ceramics which have an important place in today's technological applications. ZnO is a nonstoichiometric n-type semiconductor compound. The elements which are called as donor and used most widely to increase the electrical conductivity are Al, Ga and In. The advantages of aluminum is that it can be easily maintained and its economical. The electrical properties of ZnO ceramics depend on sintering conditions, microstructure, the concentration of intrinsic defects in its crystalline form at high sintering temperatures and the type and the proportion of donor addition.
In this study, the effect of 0 - 2 mol % Al₂O₃ additions on the microstructure and the electrical conductivity of ZnO ceramics were investigated. Samples were sintered at 1200, 1300 and 1400 ^oC for 1, 3 and 5 hours. The bulk densities of the samples were calculated by their mass/volume measurement. X-ray diffraction analysis (XRD) was conducted in order to identify the phases of the sintered samples. The microstructures of the samples were investigated by an optical microscopy and a scanning electron microscopy (SEM), the electrical conductivities were calculated by measuring the electrical resistances at room temperature. To investigate the electrical behavior of the samples with increasing temperature, the activation energies were calculated from the measurements taken within the range from room temperature to 150 °C.
It was determined by XRD that the ZnAl₂O₄ phase was formed in the samples of 0.5 - 2 mol % Al₂O₃ doped mixtures. This phase was not detected in the Al₂O₃-added compositions below 0.5 mol %, which indicated a limited solid solution of Al₂O₃ within the ZnO crystal structure.
It was observed that the bulk densities of all samples increased with increasing temperature. However, the bulk densities of zinc oxide samples were decreased with decreasing the Al₂O₃ addition.
As a result of the microstructural investigations, it was found that the grain size and the number of coarse grain were increased with increasing sintering temperature and time. In addition, the increase in the sintering temperature and time contributed to the decrease of the samples porosity values. It was observed that Al₂O₃ addition above the solid solubility limit decreased the grain size and the number of coarse grains. Furthermore, the increase in Al₂O₃ additions applied to ZnO increased the pore distribution of the samples.
It was determined that the resistivity values of Al₂O₃ doped samples which were below the solid solubility limit were lower than those of ZnO. However, it was observed the resistivity of the samples containing more than 0.5 mol % Al₂O₃ had an increasing trend due to the insulating properties of ZnAl₂O₄.
The electrical resistance values of the 0 - 0.5 mol % Al₂O₃ doped samples were measured with 1 ^oC intervals from room temperature to 150 ^oC. The activation energy values of the samples were calculated by the slope of the lines in the graph plotted with the electrical conductivity - temperature. The lattice defects in the pure zinc oxide samples supported the fact that electrical conductivity was increased with the increase of temperature. As a result, it is determined that the electrical resistance and the activation energy values of the pure zinc oxide samples ​​decreased. The Al₂O₃ contribution made below the solid solubility limit increased the free carrier charge concentration of the sample. In this way the electrical resistance of the samples decreased.