Papers by Author: Ai Fukumori

Abstract: With the goal of fabricating varistors with low varistor voltages, we investigated the effects of adding Ba and Si to BiCoMn-added ZnO varistors on the varistor voltage and the resistance to electrical degradation. Ba₂Mn₃O₈, which reduces the resistance to electrical degradation, was not formed at the grain boundary when Si was added. The resistance to electrical degradation was considerably improved by adding 0.10.15 mol% Si relative to samples to which small amounts of Sb had been added. The varistor voltage increased monotonically with increasing amount of added Si; it was approximately 36 V/mm for 0.1 mol% Si.

Abstract: The effect of simultaneously adding Zr and Y to Bi–Mn–Co–Sb–Si–Cr–Ni-added ZnO varistors (having the same composition as a commercial varistor) on the varistor voltage, leakage current, and resistance to electrical degradation were investigated. Varistor voltage increased with increasing amount of Y for addition of 0–2 mol % Zr. On the other hand, the nonlinear coefficient α prior to electrical degradation changed very little on the addition of both Y and Zr. With the addition of approximately 1 mol% Zr, the leakage current decreased with increasing amount of Y added. A ZnO varistor with a varistor voltage of approximately 600 V/m, a low leakage current, and excellent resistance to electrical degradation was fabricated by adding approximately 2 mol% Y and approximately 1 mol% Zr.

Abstract: With the goal of fabricating varistors with low breakdown voltages (varistor voltages), the effect of adding Sb to Bi-Co-Mn-Ba-added ZnO varistors on the ZnO grain size was investigated. To obtain a uniform ZnO grain size without reducing the grain size, a small amount (e.g., 0.01 mol%) of Sb was added as an additive. This addition suppresses the variation in the ZnO grain size without reducing the grain size. It also improved the resistance to electrical degradation because compounds of Ba and Mn were not formed.

Abstract: With the goal of fabricating low-breakdown-voltage varistors, the effect of adding Ba to ZnO varistors on the ZnO grain size was investigated. Grain growth of ZnO could be markedly promoted by adding both Ba and Bi. The maximum grain size was approximately 150 μm and the minimum varistor voltage was approximately 12 V/mm. However, it had relatively poor tolerance characteristics for electrical degradation. It is speculated that when adding both Ba and Bi to a Mn–Co-added ZnO varistor, it is necessary to form the molten phases of Ba and Bi to promote grain growth of ZnO. It is also conjectured that the growth of ZnO grains is not promoted when Ba and Bi do not coexist in the molten phase because Ba forms compounds with Mn independently with the addition of small amounts of Bi.

Abstract: The effects of the addition of Y or both Y and Zr to Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistors on the varistor voltage and the tolerance characteristics of electrical degradation were investigated. The deterioration of the tolerance characteristics of electrical degradation by the addition of Y was probably caused by an increase in the number of willemite (Zn2SiO4)-type particles or a decrease in the number of spinel (Zn2.33Sb0.67O4)-type particles, but this deterioration was reduced by adding Zr. Moreover, the reduction in the average ZnO grain size due to the addition of Y was a major factor in the increased varistor voltage, and the ZnO grain growth was inhibited by the formation of an un-known compound after adding Y. The varistor voltage of a varistor with 2 mol% added Y increased by approximately 50% compared to a varistor with no Y added.

Abstract: ZnO varistors of the excellent tolerance characteristics for electrical degradation were made by adding Bi2O3-MnO2-Co3O4-Cr2O3-SiO2-Sb2O3-NiO in ZnO. The tolerance characteristics for electrical degradation were evaluated by changing amount of ZrO2-additive. The evaluation methods are voltage-current characteristics, X-ray diffraction, scanning electron microscope, and energy dispersion X-ray spectroscopy. Monoclinic and tetragonal ZrO2 and the compounds originated in Zr were observed at both grain boundaries and triple points. Moreover, the compounds originated in both Zr and Sb improved the tolerance characteristics for electrical degradation. On the other hand, especially monoclinic ZrO2 deteriorated the tolerance characteristics for electrical degradation. It is one key factor of the improvements of the tolerance characteristics for electrical degradation that the mobility of oxide ions or interstitial Zn2+ ions was hindered by forming the compounds contained Zr, Sb, Si, and, Bi atoms.