Eco-Materials Processing & Design VI

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Authors: Guan Jun Qiao, Hong Jie Wang, Ji Qiang Gao, Zhi Hao Jin
Abstract: High purity alumina ceramic and Kovar alloy were brazed by Nickel and Titanium foils stacked as Ni/Ti/Ni layer structure. Airtight joints were achieved with shear strength more than 30MPa, after brazing at 995oC for 30min. A sandwich structure was observed in solder, which means an a-Ti solid solution belt at mid part and Ti2Ni intermetallics belts at both sides. The main reaction product at alumina/solder interface was Ni2Ti4O, a complex oxide with structure similar to Ti2Ni, which is the bonding agent and transition from the solder’s metallic crystal lattice to alumina crystal lattice. To simulate the actual serving conditions, a thermal cyclic experiment was undertaken at the 200oC-600oC temperature range. The results showed that shear strength of joint increased dramatically after thermal cycles. This interesting phenomenon is attributed to the annealing effects of thermal cycles, which released residual stress in brazed joints remarkably.
Authors: Hong Quang Nguyen, Mai Van Trinh, Jeung Soo Huh
Abstract: The effect of operating temperature on characteristics of single-walled carbon nanotubes (SWNT) based gas sensor was investigated. SWNT-based sensor was fabricated from SWNT powder (Iljin Nanotech, Korea) by screen-printing method. SWNT powder (30 mg, AP grade) was dispersed into 0.78 gram a-terpineol (Aldrich) by ultrasonic vibration for 1 hour then stirred manually for 1 hour to increase adhesion. From this condensed solution, a thick film of SWNT was printed onto alumina substrates. The film then was sintered at 300oC for 2 hours to remove residual impurities. Upon exposure to some gases such as nitrogen, ammonia or nitric oxide, resistance of the sensor dramatically changes due to gas adsorption. In our experiments, SWNT-based sensor was employed to detect NH3 gas in N2 ambience. After saturated of N2, the sensor exposes to NH3 with various concentrations (from 5 ppm to 100 ppm, diluted by N2 as carrier gas). This sensor exhibits a fast response, high sensitivity but slow recovery at room temperature. By heating at high temperature and increasing the flow-rate of carrier gas, NH3 gas desorbs easily and recovery of the sensor improved. The heating also influenced the characteristics of sensors such as response and reproducibility. Other special changes in electric property of SWNT-based sensor caused by heating are also discussed.
Authors: Masakazu Tane, Soong Keun Hyun, T. Ichitsubo, Hideo Nakajima
Abstract: We studied the plastic behavior of lotus-type porous iron with unidirectional long cylindrical pores. Lotus-type porous iron with different porosities was fabricated by the continuous zone melting method in a pressurized hydrogen and helium atmosphere. To calculate the stress-strain curves for lotus iron, we applied a modified Qiu-Weng’s micromechanical mean-field theory that has recently been proposed by the present authors [J. Mater. Res., in press], and compared the results with those of compression tests. We experimentally found that the deformation resistance and work hardening rate depend on the sample porosity and loading direction. They decrease with an increase in porosity, and their values in the loading along the direction perpendicular to the longitudinal axis of pores are smaller than those in the parallel-direction loading. Our micromechanical calculations reproduce well the stress-strain curves experimentally obtained and express the experimental trends successfully.
Authors: Aya Yamamoto, Hideaki Sano, Guo Bin Zheng, Yasuo Uchiyama
Abstract: Carbon nanofiber/polyimide (CNF/PI) composite films were prepared by dispersing CNFs in polyamic acid, the precursor of polyimide, followed by imidization. Because the as-synthesized CNFs were difficult to disperse, they were treated with ball-milling in water and surfactant solution for 12 or 24 hours. MgO microparticles were also added to enhance the ball-milling effect. As-prepared composite films were approximately 0.05 mm in thickness. The volume fractions of CNFs in composites were 0.1%, 0.2%, 0.5%, 1%, 2%, and 5%, respectively. The effect of treatment of CNFs on the tensile strength and electric resistivity of CNF/PI films were investigated.
Authors: S. Jayalakshmi, Eric Fleury, Yu Chan Kim, Ki Bae Kim
Abstract: Zr50Ni27Nb18Co5 amorphous ribbons were hydrogenated using an electrochemical method. Under a current density of 30 mA/cm2, the thermal stability of the amorphous phase was found to increase with the charging time. Hardness and fracture strength were found to be independent of charging time, indicating that the Zr-Ni-Nb-Co amorphous alloys preserved its mechanical integrity.
Authors: Yoon Ho Kim, Tohru Sekino, Hirokazu Kawaoka, Takafumi Kusunose, Tadachika Nakayama, Koichi Niihara
Abstract: The electrical conductivity was provided to structural ceramics by controlling the grain boundary phase. We focused on the grain boundary phase of Si3N4 ceramics, which can be considered as an infinite network for conducting paths. In this study, we investigated the correlationship of the microstructure, mechanical properties, and electrical conductivity of Si3N4 ceramics with V2O5 based glasses. The Si3N4 ceramic with V2O5 based glasses were successfully fabricated by controlling the composition of grain boundary phase. Fabricated materials by a PECS method indicated a very fine microstructure. The mechanical properties of Si3N4 ceramics with V2O5 based glasses were not good compared to those of conventional Si3N4. However, the values for the SNVB and the SNVBA were four or six orders of magnitude higher at room temperature and had excellent mechanical properties compared to pure V2O5 based glasses.
Authors: Yu Jin Kim, Seong Jin Hwang, Hyung Sun Kim
Abstract: The desirable characteristics of the microwave materials include excellent dielectric constant εr, low dielectric loss (high quality factor, Q*f0 value), and small temperature coefficient (~0ppm/°C) of resonant frequency (Tf). The advantages of using a glass-ceramic include shape stability during the sintering stage, improved dielectric properties, mechanical strength, and controlled thermal expansivity. In the present study, glass-ceramic compositions consisting of a glass system (BaO-TiO2-SiO2-Al2O3) and a ceramic filer (BaNd2Ti5O14) were prepared. The glass and glass-ceramic compositions were evaluated for sintering behavior such as shrinkage, densities, thermal properties, and microstructures. In addition, the microwave dielectric properties (≈ 15~20 for εr, > 6000 GHz for Q*f0) were determined by a network analyzer at 6-8 GHz. These results suggest that the compositions can be a candidate for LTCC dielectric materials.
Authors: Ju Hyeon Lee, Byoung Sung Kim, Jae Chun Lee, Sung Park
Abstract: Nanometer size zinc oxide (ZnO) powder was prepared by a novel “solution-combustion method (SCM)”, and it was used as a semiconductor photocatalyst to evaluate its photoreduction properties. Aqueous solution of heavy metal ions (Cu-EDTA) was used for the photocatalysis reaction under UV illumination. The result was then compared with other semiconductor photocatalyst powders such as titanium dioxide (TiO2) powder (P25; Degussa) and TiO2 powder prepared by a homogeneous precipitation process at low temperature (HPPLT). The zinc oxide nanopowder showed the highest removal rate of the Cu++ ions from the solution among the photocatalyst powders compared. The superior photoreduction ability of the ZnO nanopowder appears to be due to its excellent UV absorption capacity.
Authors: Yeon Kyu Park, Seung Mo Kang, Daekyu Kim, Chong Mu Lee
Abstract: This paper investigates the reactive sputter deposition of tungsten carbide (WCx) films for replacing hexavalent chromium which was found to cause cancer in human body. The deposition rate of the films was proportional to rf-power and inversely proportional to the CH4 content in the sputtering gas. The hardness of the WCx coatings increased as the rf-power increased. The highest hardness was obtained at the CH4 concentration of 10 vol.% in the sputtering gas. The hardness of the WCx film deposited under optimal conditions was far higher than that of the electroplated chromium film although the corrosion-resistance of the former was slightly inferior to that of the latter. Considering all these experimental results, it may be concluded that the reactive sputterdeposition of WCx coatings can be effectively applied to the parts with complex geometries as an alternative to chromium electroplating.
Authors: D.W. Lee, Tae Suk Jang, Dae Hoon Lee, Byoung Kee Kim
Abstract: Iron and its nitride (e-Fe3N) nanoparticles were fabricated by the CVC using Fe(CO)5 precursor without the aid of LN2 chiller. The iron particles synthesized at 400 oC were a mixture of amorphous and crystalline a-Fe. Fully crystallized iron particles were then obtained above 600 oC. Iron-nitride particles that were easily formed at 500 oC at 1 atm., however, were not fully developed in vacuum unless the reaction temperature reached 850 oC. Nevertheless, the work chamber needed to be maintained in vacuum to obtain finer iron-nitride particles. The synthesized particles possessing the core-shell type structure were all nearly spherical and enclosed with Fe3O4 or Fe3O4-related amorphous layer. The iron nanoparticles (~20 nm) synthesized at 600 oC at 760 torr exhibited iHc ~ 1.0 kOe and Ms ~ 170 emu/g, whereas the iron-nitride particles (~20 nm) synthesized at 850 oC at 0.01 torr exhibited iHc ~ 0.45 kOe and Ms ~ 115 emu/g.

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