Abstract
Herein, the effect of nanocrystal percentage in bulk-ZnO varistors was studied. The structure of ZnO nanocrystals was examined using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). The XRD studies showed that the nanocrystals were indexed with the hexagonal wurtzite structure of ZnO nanostructures. The average crystallite size deduced from XRD analysis ranged between 135 and 273 nm, eight-fold lower than that of the nanoparticles observed in FE-SEM micrographs (1151–2214 nm). The percentage of nanocrystals added into the ZnO varistor was increased from 0 to 100%. Electrical measurements (I–V profiles) showed that the non-linear region, breakdown field, and activation energy were found to decrease with the addition of ZnO fine crystals up to 10% and then increased upon a further increase in fine crystals. However, the electrical conductivity measured at room temperature was improved, and the highest value of 2.11 × 10−5 was observed for 10% fine crystals and then decreased upon a further increase in the fine crystal concentration in bulk ZnO. The breakdown field decreased with the increase in the percentage of ZnO nanostructures in the varistor up to 10% and then increased upon the further addition of ZnO nanostructures. The nonlinear coefficient (α) was decreased from 18.6 for bulk ZnO and remained close to unity for the samples that contained fine crystals. The electrical conductivity was generally improved with the increase in the concentration of the ZnO fine crystals. The activation energy was found to be 128, 374, and 815 meV for the bulk samples and 164, 369, and 811 meV for the samples that contained 100% fine crystals for the three temperature regions of 300–420, 420–580, and 580–620 K, respectively. These results will provide a pathway toward the determination of a correlation between the electrical and microstructural properties of ZnO-based varistors for future device applications.
Highlights
Over the past decades, ZnO has been used in various applications including varistors, transparent electrodes, gas sensors, and photocatalysis [1,2,3,4,5,6,7,8]
Electrical measurements (I–V profiles) showed that the non-linear region, breakdown field, and activation energy were found to decrease with the addition of ZnO fine crystals up to 10% and increased upon a further increase in fine crystals
The electrical conductivity measured at room temperature was improved, and the highest value of 2.11 × 10−5 was observed for 10% fine crystals and decreased upon a further increase in the fine crystal concentration in bulk ZnO
Summary
ZnO has been used in various applications including varistors, transparent electrodes, gas sensors, and photocatalysis [1,2,3,4,5,6,7,8]. The formation of the nonlinearity, in parallel with high current densities and breakdown fields, is the most significant property of varistors. The increase in current is more rapid than that in voltage; the I–V curve shows an upturn region at current densities beyond 103 A/cm and breakdown fields (≤5000 V/cm) This upturn region indicates that the voltage drops at the grains and controls ZnO for varistor applications. In very small amounts, aluminum is a potential dopant for increasing the conductivity of the ZnO grains and enhancing the varistor’s performance at high currents [31,32,33,34]. In this work, we studied the effect of increasing the fine crystal amount in the bulk-ZnO varistor on the electrical properties. The structure and morphology were characterized by using XRD and FE-SEM techniques
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