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The behavior of intrinsic and extrinsic defect relaxations in ZnO ceramics has been systematically investigated. The defect relaxations are characterized by various parameters, and electric modulus is proposed to be more sensitive to the defect relaxations. Four defect relaxations with various energy levels are found at different temperatures. Two defect relaxations appearing below 243 K with energy levels (L1, L2) about 0.24 eV and 0.35 eV are suggested to be intrinsic defects relating to interstitial Zn and vacancy oxygen, which are not affected by additives and impulse current degradation. Another two defect relaxations appearing above 353 K are proposed to be extrinsic defects associated with energy level (L3) at intergranular phase and energy level (L4) at grain-boundary interfaces, respectively. The extrinsic defect relaxations are closely related with additives and electrical performance. Only L4 was reduced from 0.84 eV to 0.76 eV by impulse current degradation while other defect energy levels keep unchanged. It is further proposed that L4 is mainly responsible for the electrical properties and stability of ZnO ceramics, and it mainly contributes to the grain boundary resistance of ZnO ceramics. A schematic energy band diagram is demonstrated for interpreting the intrinsic and extrinsic defect relaxations in ZnO ceramics.