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Fast firing of Bi2O3-based ZnO varistor materials, which includes zero minutes soaking at 1100°C with 120°C/min heating and 145°C/min cooling rate, was made possible using millimeter-wave sintering (mS) technique. The overall sintering time of the process is less than 18 minutes, and the varistor characteristics obtained are α = 38, JL = 5.55 × 10−6 A/cm2 and Vbk = 600 V/mm, whereas the intrinsic parameters of the materials are φb = 2.84 eV, Nd = 1.85 × 1024 m−3 and Ns = 7.02 × 1011 cm−2. By contrast, conventional sintering (cS) process needs higher sintering temperature (1200°C), longer soaking time (60 min) and slower ramping rate (30°C/min) to obtain ZnO materials with the same marvelous nonlinear properties as those prepared by mS-process. Moreover, millimeter-wave sintering (24 GHz, mS) process enhances the densification kinetics and grain growth behavior more efficiently than the microwave sintering (2.45 GHz, μS) process, resulting in better varistor characteristics for ZnO materials. However, sintering by millimeter-wave for too long period induces overfiring of the samples, which results in a density reversion phenomenon. Such a phenomenon leads to the decrease in surface state (Ns) and the potential barrier height (φb), which are presumed to be the mechanism leading to the degradation of ZnO materials' nonlinear properties.