Abstract
High-performance ZnO-Pr6O11 thin-film varistors were fabricated simply by hot-dipping oxygen-deficient zinc oxide thin films in Pr6O11 powder. The films had a composition of ZnO0.81 and a thickness of about 200 nm, which were deposited by radio frequency magnetron sputtering a sintered zinc oxide ceramic target. Special attention was paid on the temperature dependence of the varistors. In 50 min with hot-dipping temperature increased from 300–700 °C, the nonlinear coefficient (α) of the varistors increased, but with higher temperature it decreased again. Correspondingly, the leakage current (IL) decreased first and then increased, owing mainly to the formation and destroying of complete zinc oxide/Pr6O11 grain boundaries. The breakdown field (E1mA) decreased monotonously from 0.02217 to 0.01623 V/nm with increasing temperature (300–800 °C), due to the decreased number of effective grain boundaries in the varistors. The varistors prepared at 700 °C exhibited the optimum nonlinear properties with the highest α = 39.29, lowest IL = 0.02736 mA/cm2, and E1mA = 0.01757 V/nm. And after charge-discharge at room temperature for 1000 times, heating at 100 or 250 °C for up to 100 h, or applying at up to 250 °C, the varistors still performed well. Such nanoscaled thin-film varistors will be very promising in electrical/electronic devices working at low voltage.
Highlights
High-performance ZnO-Pr6O11 thin-film varistors were fabricated by hot-dipping oxygendeficient zinc oxide thin films in Pr6O11 powder
Most of the commercially applied varistors have been ZnO-Bi2O3 based electronic ceramic devices, which are widely used in various electrical circuits, electronic devices and electrical power systems to counteract against dangerous over-voltage surges, due to their highly nonlinear current-voltage (I-V) characteristics and outstanding energy handling capabilities[1,2]
For conventional ZnO-Bi2O3 based ceramic varistors, they are prepared by sintering ZnO powder with a small amount of the so-called varistor forming oxide (VFO) Bi2O3, which is essential for imparting the nonlinearity of ZnO ceramics, while other oxides, such as nickel oxide, manganese oxides, rare metals oxides and so on, are added in small amount to further enhance the non-linearity of the varistors[3,4]
Summary
X-ray photoelectron spectroscopy (XPS) analysis indicated that, only Zn 2p and O 1 s peaks could be detected in the applied oxygen deficient zinc oxide thin films, which had a composition of ZnO0.816,11. From the full spectrum as shown, Zn 2p, Pr 3d and O 1 s peaks could be identified, indicating that some Pr atoms were composite with the zinc oxide film after hot-dipping. The strong peak located at 977.1 ± 0.1 eV is corresponding to Pr4+ oxidation state, while the weak peak at 971.5 ± 0.1 eV can be assigned to Pr3+ oxidation state[16] From this spectrum, the percentages of Pr4+ and Pr3+ oxidation states were further evaluated as 66.7% and 33.3%, respectively. The second one has a binding energy of 531.4 ± 0.1 eV, which is owing to the adsorbed oxygen (such as O2) on the film surface[19] All these results indicate that the as-presented hot-dipped sample is a ZnO-Pr6O11 composite. Considering that the originally applied zinc oxide thin films were oxygen deficient with a composition of ZnO0.81, such results revealed that the zinc oxide thin films were further oxidized during the hot-dipping
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