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RESEARCH

The effects of multiwall carbon nanotubes on the electrical characteristics of ZnO‑based composites N. Asaadi1 · M. Parhizkar1 · H. Bidadi1 · S. Mohammadi Aref1 · M. Ghafouri2 Received: 9 February 2020 / Accepted: 8 August 2020 © Islamic Azad University 2020

Abstract In this experimental work, the effects of multiwall carbon nanotubes (MWCNTs) on electrical characteristics of zinc oxide– MWCNT–high-density polyethylene composite varistors have been investigated. All the samples were made at the temperature of 130 °C and pressure of 60 MPa by the hot-press method. Results show that increasing zinc oxide content in the mixture increases breakdown voltage up to 170 V, where the highest nonlinear coefficient (α ~ 13) corresponds to the samples with 95 wt% of ZnO. Results with regard to the effects of MWCNT as an additive reveal that increasing its content from 1 to 2.5% in the composites, the breakdown voltage decreases to 50 V, but the highest nonlinear coefficient (~ 14) corresponds to the sample with 1.5% of MWCNT content. It is also revealed that, heat treatment of the sample at a constant temperature of 135 °C and different time intervals from 2 to 10 h, the sample with 6 h annealing time shows maximum breakdown voltages (Vb = 140 V) with the highest nonlinear coefficient (~ 14). Investigation of the potential barrier height of samples shows a complete consistency with the breakdown voltage variations. The results have been justified regarding XRD patterns and SEM micrographs of samples. Keywords  Multiwall carbon nanotube · ZnO · Composite varistor · Nonlinear coefficient

Introduction Zinc oxide with its excellent electrical properties has played a fundamental role in different branches of physics and electronics [1, 2]. Zinc oxide has a direct bandgap of 3.37 eV at room temperature and large exciton binding energy of 60 meV which makes it suitable for a wide range of applications such as gas sensors, solar cells, photocatalysis and surge protectors [3–9]. One of the most widespread applications of ZnO is in the field of surge protector voltagedependent devices, called varistors. A varistor is used to protect electronic circuits against excessive transient energies or overvoltages [10]. At low voltages, a varistor acts as an insulator with high resistance and linear (I–V) characteristic. When the applied voltage approaches a value known * M. Parhizkar [email protected] 1



Department of Condensed Matter Physics, Faculty of Physics, University of Tabriz, 29 Bahman Blvd., Imam St., Tabriz, Iran



Department of Physics, Faculty of Physics, Shabestar Branch, Islamic Azad University, Shabestar, Iran

2

as breakdown voltage, (I–V) characteristic of the varistor becomes nonlinear and it acts like a conductor which connects electrical current to the ground [11, 12]. The quality of a varistor is specified by a parameter called nonlinear coefficient which is infinite for an ideal varistor. The bestreported value for nonlinear coefficients is ~ 50 (for ceramic ZnO-based high voltage varistors) [13]. Z