• Choon-W Nahm

      Articles written in Bulletin of Materials Science

    • Nonlinear electrical properties and aging characteristics of (NiO, MgO, Cr2O3)-doped Zn–Pr–Co–R (R = Y, Er) oxide-based varistors

      Choon-W Nahm

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      The electrical properties and stability of the varistors, which composed of (NiO, MgO, Cr2O3)- doped Zn–Pr–Co–R (Y, Er) oxide-based ceramics, were investigated for different additives. The breakdown voltage of the varistors increased in order of NiO$\rightarrow$undoped$\rightarrow$MgO$\rightarrow$Cr2O3: 1200$\rightarrow$1551$\rightarrow$1691$\rightarrow$1959 V/cm for ZPCY system and undoped$\rightarrow$NiO$\rightarrow$MgO$\rightarrow$Cr2O3: 1024$\rightarrow$1041$\rightarrow$1500$\rightarrow$1668 V/cm for ZPCE system, respectively. The nonlinear coefficient value increased in order of undoped$\rightarrow$NiO$\rightarrow$MgO$\rightarrow$Cr2O3: 21$\rightarrow$25$\rightarrow$38$\rightarrow$50 in ZPCY system and NiO$\rightarrow$undoped$\rightarrow$MgO$\rightarrow$Cr2O3: 27$\rightarrow$32$\rightarrow$35$\rightarrow$38 in ZPCE system, respectively. In ZPCY and ZPCE systems, the Cr2O3-additives most greatly improved the nonlinear properties. In Cr2O3-doped system, ZPCY system exhibited higher nonlinear properties than that of ZPCE system. The stability against d.c. accelerated aging stress was higher in Cr2O3-additives than in NiO- and MgO-additives for ZPCY system and was higher in NiO-additives than in MgO- and Cr2O3-additives for ZPCE system.

    • Effect of aluminum addition on electrical properties, dielectric characteristics, and its stability of (Pr, Co, Cr, Y)-added zinc oxide-based varistors

      Choon-W Nahm

      More Details Abstract Fulltext PDF

      The electrical properties, dielectric characteristics, and its stability against d.c. accelerated aging stress of (Pr, Co, Cr, Y)-added zinc oxide-based varistors were investigated for different aluminum concentrations under a sintering temperature of 1280°C. As the aluminum concentration increased, the average grain size (𝑑) increased in the range of 4.3–5.5 𝜇m and the sintered density increased in the range of 5.63–5.67 g/cm3. As the aluminum concentration increased, the breakdown field decreased in the range of 6327–710 V/cm and the maximum nonlinear coefficient (46.9) was obtained for 0.005 mol% in aluminum concentration, further additions impaired the nonlinear properties. As the aluminum concentration increased, the apparent dielectric constant increased in the range of 500.5–1327.4 and dissipation factor increased in the range of 0.00493–0.0724. The varistor added with 0.001 mol% Al exhibited the highest stability for 𝐸–𝐽 characteristics in which %𝛥 𝐸1\ mA is +1.4% and %𝛥 𝛼 is –5.7%, under stress state of 0.95 𝐸1\ mA/150° C/24 h.

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