High temperature ac conductivity relaxations in dielectric ceramics: grain boundary/intergranular phase effects
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High temperature ac conductivity relaxations in dielectric ceramics: grain boundary/intergranular phase effects Xuetong Zhao1 · Yupeng Li1 · Lulu Ren1 · Chao Xu1,2 · Jianjie Sun1 · Lijun Yang1 · Ruijin Liao1 Received: 27 June 2020 / Accepted: 6 August 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The electric polarization and dc conductivity as two main factors cause electric relaxation in dielectric ceramic, which are difficult to be distinguished from each other at high temperatures. In this work, it is found that the two key factors can be separated via conjoint analysis of various complex planes such as complex dielectric permittivity, the impedance, the electric modulus, and the ac conductivity planes. Taking ZnO ceramics as a typical example, the ac conductivity relaxations caused by the long range and short-range migration of charge carriers are discussed as a function of frequency at high temperatures (433–473 K). Under the applied ac electric field, the migration of charge carriers within the ZnO ceramic can be restricted by two high-resistance barriers from grain boundaries and intergranular phases. These barriers result in two dispersion processes in conductivity response, which exhibit two relaxation peaks with activation energies of 0.75 eV and 0.89 eV. It was proposed that, in high temperature region, the ac conductivity relaxations of ZnO ceramic are the result of carrier migration localized between grain boundaries, and carrier migration localized between intergranular phases.
1 Introduction The measurement of the ac response of dielectric ceramics is one of the most powerful techniques to provide deep insight into the underlying physics that may consist of various phenomena such as electronic or molecular polarization, hopping charge transport, ferroelectric relaxation, magnetocapacitance [1]. Electric relaxation phenomenon as one of the ac responses is often observed in dielectric ceramics via several complex planes, e.g., the permittivity (ε*), the conductivity (σ*), the impedance (Z*) and the electric modulus (M*) [2]. All the complex planes are alternative when characterizing the same macroscopic relaxation process and can be easily transformed into each other according to the complex plane equations [2, 3]:
* Xuetong Zhao [email protected] 1
State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Shapingba District, 400044 Chongqing, People’s Republic of China
State Grid Zhejiang Electric Power Co, Jiaxing Power Supply Company, 314033 Jiaxing, People’s Republic of China
2
𝜀∗ (𝜔) =
1 M ∗ (𝜔)
=
𝜎 ∗ (𝜔) 1 = j𝜔𝜀0 j𝜔C0 Z ∗ (𝜔)
(1)
where ω = 2πf is the angular frequency, ε0 = 8.85 pF/m is the permittivity of vacuum, C0 = Sε0/l is the capacitance of the empty measuring cell, S is the electrode area, l is the thickness of the sample. Different complex planes exhibit different features, leading to varied applications for analysis. For example, the permittivity and conductivity are mostly
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