Papers by Author: Ruben L. Menchavez

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Authors: Minoru Takahashi, Koichiro Adachi, Ruben L. Menchavez, Masayoshi Fuji
Abstract: In this study, we propose a new process to fabricate electrically conductive alumina by gelcasting and reduction sintering. The process used the conventional gelcasting method except for varying amounts of monomer at 2.8, 5.5, and 8.0 wt.% relative to the weight of the slurry. In the plastic mould, the slurry was under in situ solidification for 3 hrs at 25oC to achieve gelation. The freshly gelled bodies were demolded, carefully dried, and then sintered at 1100oC, 1300oC, and 1550oC in nitrogen atmosphere. The holding times at 1100oC and 1300oC was 2 hours, while at 1550oC were 2, 4, and 6 hrs. The sintered alumina body was characterized by electrical property, X-ray diffraction, and scanning electron microscopy. Results showed that monomer additions and sintering schedule significantly affect in lowering electrical resistance. The low value was 3.6×106 +cm at 8.0wt.% monomer addition and sintering at 1550oC for 2 hrs. The effect of physical properties on electrical conductivity and the corresponding reaction mechanism were discussed in details.
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Authors: Ruben L. Menchavez, Masayoshi Fuji, Tomohiro Yamakawa, Takeshi Endo, Minoru Takahashi
Abstract: This study investigates the phase compositions in gelcast dense alumina and porous alumina that were sintered in an inert atmosphere. Both materials were gelcasted using identical slurry composition except for adding surfactant to slurry intended for porous shaping. The resulting gelcasted samples were dried and then sintered in argon gas at 1700oC. The X-ray photoelectron analysis was used for identifying the phase compositions in sintered samples, which was supported with TG/DTA and electrical measurements. Results showed that, in addition to alumina and carbon phases, the dense sample yielded Al4C3 and Al, while porous sample yielded aluminum oxycarbides. The formation of these phases reduced the carbon content through different chemical reactions to a limited extent because they formed a protective layer on alumina grains, which prevented the direct contact between alumina and carbon. Subsequent measured electrical resistivities of dense and porous alumina were, respectively, 0.20 and 0.32 ,-cm, indicating that the carbon networks are preserved in both materials.
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Authors: Ruben L. Menchavez, Koichiro Adachi, Masayoshi Fuji, Minoru Takahashi
Abstract: This work demonstrated an in-situ pyrolysis of gelcast alumina under reduction sintering to make alumina and carbon composite in providing semi-electrical conductivity. To increase the carbon content, the monomer was varied in the premix solution with reduction sintering in nitrogen gas. Two-probe method was used to measure electrical resistance of the sintered samples. The results revealed that the increase of monomer addition and sintering treatment were effective in reducing electrical resistance. The lowest value was 3.6×106-cm, which is a potential candidate for electrostatic shielding application. The reduction-sintered sample was re-sintered in an air in order to gain insight on the conductive path due to carbon network. Further tests such as XRD, TGA/DTA, and scanning electron microscopywere used to explain the semi-conductive property of the material.
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