Key Engineering Materials Vols. 345-346

Abstract: High aspect ratios maghemite of iron oxide nanoparticles were prepared using water in oil microemulsions. A four component microemulsion system with cationic HTAB, n-butanol, noctane, and salt solution was used. Precipitations of iron oxide were initiated by mixing a microemulsion system containing Fe2+ with another microemulsion system containing OH-. The resulting particles were characterized using XRD, AGM and TEM. The XRD result showed the formation of maghemite. TEM showed that the average length of needle shaped particles increased from 28 nm up to 42 nm as the aging time was increased from 4 to 24 hours while the average diameter spherical particle remained at around 8 nm. The AGM confirmed that the particles are superparamagnetic.

Abstract: A newly developed resonator in combination with a UDS 200 rheometer allows performing different kinds of measurements. The characteristics of the resonator are its resonant frequency slightly above conventional rheometers, the high sensitivity allowing measurement of fluids with viscosities as low as that of water and the very small oscillation amplitudes. The nonlinear behavior of viscoelastic liquids can be analyzed using parallel superposition of shear. It is known that the parallel complex shear modulus differs from the complex modulus of a fluid at rest. First measurements have shown the ability of the instrument to quantify this effect. At the same time, normal force, shear rate viscosity and complex viscosity can be measured on a sample.

Abstract: High purity MgO is a common ceramic material used as a protecting layer in an alternating current plasma display panel (AC-PDP). In this work, pure MgO was added by CaO for a better protecting layer. The effect of CaO addition was discussed according to the changes in the microstructure, mechanical strength and the ability of the sample to withstand surface breakdown (flashover) with various CaO addition in the powder mixture. In addition, it was found that 2.5 wt% CaO-addition shows a better electrical discharge. Since secondary electron emission coefficient contributes in the electrical discharge, it could be predicted that the 2.5 wt% CaO added MgO has a higher secondary electron emission constant and becomes a promising candidate for a protecting layer in AC-PDP.

Abstract: The stress relaxation phenomena in magnesium alloys, containing Al, Sr and Ca solutes have been investigated. The samples were deformed at a constant initial strain rate at various temperatures between room temperature and 300 °C. Stress relaxation, i.e. a decrease in the stress with time, was measured at various stresses and at various temperatures. The stress relaxation curves were analysed as a function of the internal stress at the beginning of the relaxation and the test temperature.

Abstract: In this study, interactions between a main crack and a surrounding layer of crazing patterns are considered. Analysis of the stress field distribution induced during these interactions is based on the resolution of some differential equations along with appropriate boundary conditions and the use of a numerical approach. These equations are established according to Mohr’s criteria since the craze growth occurs along directions parallel to the minor principal stress axis. Because this damage can constitute an important toughening mechanism, the mode I stress intensity factor (SIF) is employed to quantify the effects on a crack of the damage consisting of crazing patterns. It is proven, herein, that crazes closer to the main crack dominate the resulting interaction effect and reflect an antishielding of the damage while a reduction constitutes a material toughness.

Abstract: Relatively poor creep resistance is considered as a deficiency of thermoplastic materials in general. In order to comprehensively and deeply study the effect of nanoparticles incorporated into a polyamide matrix a systematic investigation on various kinds of nanofillers under differed temperatures and stress levels have been performed. It was found that the creep resistance of polyamide 66 can be significantly enhanced due to the incorporation of nanofillers and the behavior of the composites can be easily described by the model of Findley.

Abstract: The tensile tests and the three-point bending tests have been conducted to investigate the crack initiation and propagation and the fracture behavior of 91W-6.3Ni-2.7Fe with three kinds grain sizes of 1~3μm, 10~15μm and 30~40μm. SEM was introduced to analyze the grain sizes, the micro-defects, the deformations and the fracture behaviors of tungsten alloys. The test results show that under the same loading conditions, the crack initiation and the crack propagation are not only related to grain size, but also related to the contiguity of tungsten grains and the interface between the tungsten grains and the matrix.