Materials Science Forum Vols. 534-536

Abstract: A study of the different stages of the electroless deposition of copper on carbon nano fibers activated firstly by a chemical treatment of the carbon nano fiber and secondly by a two-step method has been performed from both a chemical and a morphological point of view. The combination of XPS measurements and scanning electron microscopy imaging has allowed optimizing the 2 different stage conditions. On a first hand, the different oxide concentration and treatment time of the carbon nano fibers and on a second hand the different conditions of the sensibilisation (Sn bath), activation (Pd bath) and coating (Cu bath) have ben studied. The control of the homogeneity and thickness of copper thin films on carbon nano fiber can be obtained and further more sintered in order to obtain fully dense materials.

Abstract: The synthesis of silver coated iron base alloy (Sendust : Fe-Si-Al) powder having the both effects of shielding and suppressing of electromagnetic wave was studied. Depending on thickness of silver coating layer, the electromagnetic properties of the dispersed particles complexed with organic binder were examined. EDS-SEM, XRD, ohm meter, network analyzer (PNA8364A) measurements were used to characterize the electromagnetic properties. The silver coated powders have a better microwave absorbance than non coated powder in the low frequency range of below 1.0GHz, which is due to a high dielectric constant and dielectric loss of the silver in microwave frequencies magnetic loss in addition to conduction loss. Reflection loss of the 5% silver containing composite was found to be –6 dB at 1.0GHz with 1.0mm thickness. It is, therefore, successfully proposed that the silver coated sendust flake powders with controlled electrical properties and thickness can be used as thin microwave absorbers in quasi-microwave frequency band.

Abstract: The electrophoretic deposition process of Ni nano-particles in organic suspension was employed for self-repairing of heat exchanger tubes. For this purpose, Ni nano-particles prepared by levitation-gas condensation were dispersed into the solution of ethanol with the addition of dispersant. The pitted Ni alloy specimen was prepared by applying a potential of 0.9 V (vs. Ag/AgCl) in aqueous 0.1 M NaCl solution. For electrophoretic deposition of Ni nano-particles on the specimen, a constant electric field of 100 V cm-1 was applied to the specimen for 180 s in Nidispersed solution. It was found that as the electrophoretic deposition time increased, the size of the pit remarkably decreased due to the agglomeration of Ni nano-particles at the pit with a higher current value rather than the outer surfaces of the specimen with a lower current value. Moreover, the current density increased with electrophoretic deposition time and reached a constant value. From the above, it is concluded that as the electrophoretic deposition proceeds, the pit becomes smaller in size, and hence the nano-particles more extensively aggregate at the pit by lyosphere distortion.

Abstract: A novel sintering process is proposed for bonding of BaTiO₃ and Al₂O₃ layers. Common commercial glass was used and infilterated among filler particles. As the kind of commercial glass, the phenomenon of the infilteration is different. Although Sud-1140 glass forms a glass/filler composite, it is not completely infilterated into the filler particles at 900°C. However as the increase of sintering temperature the infilteration of glass was improved. In this study, GA-1 and GA-12 glasses were infilterated the more than Sud-1140 glass. However, they are reacted by BaTiO₃ layer. The results of the experiment show that constrained sintering and the co-firing of the different materials were possible for glass infilteration using Sud-1140 glass at 1000°C.

Abstract: NiTi shape memory alloy particles have been incorporated inside SnAgCu lead free solder paste in order to improve the mechanical performances of solder paste. However, because of the non-wetting properties of solid NiTi particles by the liquid SnAgCu matrix, the development of fluorine gas treatment of both NiTi and SnAgCu particles has been optimized. Scanning electron microscopy (SEM) micrographs of NiTi/SnAgCu composite materials have shown that “Fluorine treatment” of SnAgCu powders lead to a huge increase of the NiTi powder content inside the liquid SnAgCu matrix where no effect have been observed when NiTi materials are fluorinated. Xray photoelectron spectroscopy analysis of treated SnAgCu powders have been used in order to analyze surface chemistry evolution where wavelength dispersion spectroscopy line profiles, across NiTi-SnAgCu interfaces, have been performed.

Abstract: The frequency profile of complex permeability (μr) and permittivity (εr) and their relationship with microwave absorbing properties are investigated in soft magnetic metal-polymer composite materials. The Fe-Si-Al alloy powder was forged by an attrition mill to get the flaky shape. The composite sheet was fabricated in which the flaky powder was dispersed in the polymer and aligned in the direction perpendicular to the electromagnetic wave propagation. We found that dielectric loss (εr") spectra for composite sheets can be controlled by combining the aspect ratio of the flaky powder and the contents of the filler. The permittivity (εr). of magnetic composite was increased with attrition milling time. The maximum attenuation peak of reflection loss is shifted to lower frequency ranges as the aspect ratio increased, while the value of the maximum attenuation peak gets smaller gradually. From these results, we could design a noise absorber sheet (t=1.0 ㎜) for a quasi-microwave band, which is impedance matched at 1.8 ㎓ with respect to -7.7 ㏈ reflection loss.

Abstract: The research conducted was focused primarily on the development of a process for obtaining silver-coated tungsten powders for applications related to electrical-conducting devices. Particles of high strength material when coated with silver offer a means of obtaining desirable electrical properties and high strength. The coating process employed aqueous ammoniacal silvernitrate electrolytes with a formaldehyde solution as the reductant. Modifying additives were also applied. The reduction and subsequent deposition of silver occurred selectively on the surface of the tungsten particles. The morphologies of the coated particles were assessed by SEM imaging. The silver was uniformed coated on tungsten powder and its thickness was estimated to be approximately 100nm on the basis of a mass account.

Abstract: Lithium manganese oxide (LiMn2O4) powders for lithium ion batteries were synthesized from two separate raw material pairs, LiOH/MnO and LiOH/MnO2. The prepared powders and their electrochemical properties were investigated. Powders calcined at 780°C were composed of a single-phase spinel structure but those treated at 850°C showed a higher intensity ratio of I400 to I311, a slightly larger lattice parameter, and an increased discharge capacity by 10% under 3.0~4.3V voltage range. The XPS study on the oxidation states of manganese repealed that powders made from LiOH/MnO had less Mn3+ ion and gave better battery performances than those from LiOH/MnO2.

Abstract: To co-fire with commercial LTCC (Low Temperature Co-fired Ceramic) materials at 850oC ~ 880 oC, different contents of B2O3 were added to the Bi2O3-ZnO-Nb2O5 (BZN) ceramics. The dielectric properties of BZN ceramics sintered at low temperatures were studied. According to the test results, the cubic phase of BZN was transformed into orthorhombic in all the test materials. A BiNbO4 phase was formed in test materials with 2 ~ 5 wt% of B2O3 addition. The BiNbO4 phase was inhibited by extra ZnO addition. The phase transformation of cubic BZN was controlled during the synthesis process of cubic and orthorhombic ZnO-Nb2O5 phase with excess ZnO content. The Cubic and orthorhombic phases of BZN could coexist and be sintered densely at 850 oC/2hr.

Abstract: Sr2Ta2O7, a layered perovskite compound, has been reported to possess most excellent photocatalytic properties among the layered perovskite materials. Recently, we have successfully demonstrated that Ba5Ta4O15 that was prepared under Ta rich atmosphere has high photocatalytic performance as well as Sr2Ta2O7. In this study, several amount of Ba was doped into Sr2Ta2O7, and the photocatalyst samples with a mol ratio of Sr: Ba: Ta = (1-x): x: 1 were prepared by the polymerized complex method
to investigate the effect of Ba substitution for the Sr site on photocatalytic activity. The maximum photocatalytic performance was obtained for x = 0.2 near the solid solubility limit, which is three times as high as that of undoped Sr2Ta2O7 and is in the highest level in a series of tantalum photocatalysts reported so far. The increase of photocatalytic activity would be caused by crystal distortion due to doping of Ba ion.