Ai Fukumori

Effect of Ba Addition on Electrical Characteristics of Bi-Based ZnO Varistors

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.

Effect of Zr-Addition on Electric Degradation Characteristics of ZnO Varistors

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.

Effect of Si and Ba Addition on ZnO Grain Growth and Electrical Characteristics of Bi-Based ZnO Varistors

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.

Effects of Sb addition on ZnO grain growth and the electrical characteristics of Ba-added-Bi-based ZnO varistors

The varistor voltage increases as the number of ZnO grain boundaries between electrodes increases. Therefore, in order to fabricate varistors with low varistor voltages, it is necessary to reduce the number of ZnO grain boundaries between electrodes. The present study uses this method for increasing the ZnO grain size. With the goal of fabricating varistors with low varistor voltages, the effect on the ZnO grain size of adding Sb to Bi- Ba-Co-Mn-added ZnO varistors was investigated. Ba was added in order to increase the ZnO grain size, and Sb was added in order to achieve a uniform ZnO grain size without reducing the grain size. The addition of a small amount of Sb reduced the formation of ZnO grains having a smaller grain size, and the addition of 0.02 mol% Sb to the 0.5-mol%-Bi- and 0.5 mol% Ba added ZnO varistor exhibited a low varistor voltage of approximately 31 V/mm and the highest resistance to electrical degradation, because compounds containing both Ba and Mn do not form at grain boundaries between ZnO grains.

Effect of Sb Addition on ZnO Grain Growth and Electrical Characteristics of Bi–Co–Mn–Ba-Added ZnO Varistors

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.

Effect of Simultaneous Addition of Zr and Y on Electrical Characteristics of ZnO Varistors

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.

Control of Varistor Voltage by Grain-size Control of Bi-added ZnO Varistors

Relationship between the grain size of ZnO and the amounts of Bi and Sb for Bi-Ba-Sb-Co-Mn-added ZnO was investigated. The amount of Bi at which the grain size of ZnO became the maximum shifted to the notably larger one by adding small amount of Sb such as 0.01 mol%. The maximum value of the grain size when adding Sb was approximately 1.3 times larger than that when not adding Sb. Because Ba3Mn2O8 was not formed when Sb is added and the much amount of Ba forms the coexistence phase with Bi at grain boundaries of ZnO, the grain size of ZnO becomes large. The tolerance characteristic to the electrical degradation was improved by addition of Sb, because Ba3Mn2O8 was not formed and Mn distributed more uniformly at grain boundaries.