Band 18, Heft 3

A novel processing method to fabricate AlN-MgO-MgAl 2 O 4 composites has been developed, using non-sintered, porous, aluminum nitride (AlN) preforms infiltrated with a magnesium alloy in two directions, downward and upward, at relatively low temperatures (650°C, 800°C, 950°C). Microstructural, phase, and chemical analyses show that at 950°C and 135 min holding time, a continuous network of ceramic phases can be achieved successfully. Magnesium oxide and spinel phases (MgAl 2 O 4 ) are formed in-situ, when nitrogen gas is used. Due to the formation of a magnesium nitride layer on the surface of the non-sintered aluminum nitride, the infiltration mechanism proceeds effectively. No metallic phases are observed in the samples processed at 800°C and higher: these samples show high electrical resistivity ranging from 6.73×10 8 to 2.10×10 11 Ωcm. Thermal diffusivity, heat capacity and density have been measured using the nano-flash method, differential scanning calorimetry and Archimedes technique, respectively. The effects of residual porosity and holding time on the thermal conductivity have been studied. Maximum thermal conductivity and density at room temperature are 96 W/(mK) and 2.45 g/cm 3 , respectively.

Jetring is a modification of the conventional ring spinning system and based on the usage of an air nozzle and pressurized air during yarn spinning. This study researched the characteristic properties of Jetring yarns and compared Jetring and conventional ring spun yarn properties. For the Jetring yarn production, three different nozzle types having different injector angles and four different air pressure levels were used. In addition to 30° and 45° injector angles, which are widely reported in literature, the effect of an angle of 15° was also studied. At the end of the study, the differences between Jetring and ring spun yarns were determined. It was found that the Jetring spinning system produces yarns having a hybrid character between conventional ring and air-jet yarns. Additionally, the yarn properties of Jetring yarns highly depend on nozzle structural parameters and air pressure level.

This paper deals with frequency optimization of symmetrically laminated skewed open cylindrical shells. The design objective is the maximization of the fundamental frequency and the design variable is the fiber orientations. The first-order shear deformation theory (FSDT) is used for the finite element solution of the shells. The modified feasible direction (MFD) method is used for the optimization routine. For this purpose, a program based on FORTRAN is used. Finally, the numerical analysis is carried out to investigate the effect of skew angles, length to radius ratios, subtended angle and boundary conditions on, the optimal designs and the results are compared.

Ni-Fe metal matrix composites reinforced with TiC have been fabricated by tube furnace sintering at various temperatures. A uniform nickel layer on TiC and Fe powders was deposited using electroless plating technique prior to sintering, allowing closer surface contact than can be achieved using conventional methods, such as mechanical alloying. The reactivity between TiC and Fe powders, to form carbides of Fe, is controlled through Ni layer existing on the starting powders. A composite consisting of quaternary additions, a ceramic phase, TiC, within a matrix of FeNi, has been prepared at the temperature range 1000–1400°C under argon shroud. X-ray diffraction, scanning electron microscope (SEM), compressive testing and hardness measurements were employed to characterize the properties of the specimens. Experimental results carried out at 1200°C suggest that the best properties, such as maximum compression strength (σ max ) and hardness (HB), were obtained at 1200°C and the tube sintering of electroless Ni plated TiC and Fe powders, is a promising technique to produce ceramic reinforced Ni-Fe composites.

The Na 2 SiO 3 -polyacrylamide (PAM) inorganic/organic homo­geneous hybrid was prepared by polymerization of acrylamide monomer in the presence of a water-glass solution. Then, the hybrid was introduced into pores of poplar wood by impregnation treatment, and composite wood was produced. The structure of the controlled and treated wood specimen was investigated by scanning electrode microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The thermal stability of the composite wood was evaluated by thermogravimetric (TG) and oxygen index. The results showed that the treated wood showed a lower weight loss and higher oxygen index when weight percent gains (WPG) of the composite wood reached 31.6%. Also, the composite wood showed improved bending strength, modulus of elasticity in static bending, and hardness, when compared with controlled samples.

Linear formulations are insufficient to take into account the effect of large deflections that occur after initial buckling. This effect can only be considered in the non-linear buckling analysis. In the present work, the non-linear analysis is carried out using the finite element method based on first-order shear deformation shell theories. Two non-linear shell theories, Donnel’s and Sanders’ theories, are used in the analysis. Based on the first-ply failure analysis and non-linear buckling analysis, the critical sizes and parameters of the tapered curved plates that will not fail before global buckling are determined. A parametric study that encompasses the effects of taper angle, critical length-to-height ratio, radius, radius-to-thickness ratio and geometric parameters of the plates is conducted.

The unidirectional carbon fiber (CF) reinforced aluminum (Al) composites have been fabricated by the low pressure infiltration (LPI) of molten Al into porous CF preform. Prior to the fabrication of the unidirectional CF/Al composites, the unidirectional CF preform was prepared by sintering of CFs and copper (Cu) particles under the spark plasma sintering (SPS). The compression strength of CF preform was examined to determine the infiltration pressure of molten Al into CF preform. The effects of the different infiltration pressures and sizes of Cu particles on the densification of unidirectional CF/Al composites have also been investigated. The compression strength of CF preform increased with increasing of the contact area between CFs and Cu particles. The density of CF/Al composites improved with the increase of the infiltration pressure. The CF/Al composites, into which the bimodal Cu particles are added, have average particle sizes of 2.55 μm and 11.79 μm, with a high relative density of about 95% with the nearly homogeneous fiber distribution.

In this study, the non-isothermal crystallization kinetics of polypropylene (PP) in the presence of nanoclay particles were investigated using differential scanning calorimetry (DSC) with various cooling rates varying from 0.5°C/min to 80°C/min. Such kinetics were compared with those obtained for the pure PP matrix. The modified Avrami analysis was used to describe the non-isothermal crystallization process. The results obtained indicate that the presence of nanoclay significantly affected the crystallization rate of the PP resin, since an increase of the crystallization temperature as the nanoclay content increased was observed. This was attributed to the nucleating ability of these particles. Moreover, it was seen that the increase of both the cooling rate and the nanoclay content decreased the Avrami exponent n, suggesting a change in the obtained crystalline shape. For the nanocomposite materials, as well as for neat PP, the mechanism of crystallization was found to undergo two transitions, at about 5°C/min and 40°C/min. This suggests that the surface-induced nucleation at the clay particles follows the same mechanisms as those of the complete spherulitic structure. However, a lower value of activation energy for crystallization was obtained as the clay content increased, confirming the nucleating effect of clay particles.

This paper deals with the influence of admixtures – silica fume, superplasticizer and air entraining agent – on the improvement of the bond strength between bundled carbon filaments and cement mortars. Pull-out tests were carried out to examine the bond between carbon-fiber yarn and various cement matrices. An overall microstructural analysis was conducted and correlated with pull-out data. Experimental results showed that all the applied admixtures contributed to improved bonding. The air entraining agent was found to be the most effective in improving the bond strength of the studied carbon-fiber reinforced cement mortar. Air bubbles formed inside the mortar mix as a result of added air entraining agent and the ones formed in the vicinity of the outer filaments of the yarn may provide a mechanical barrier at the interface, leading to a significant increase in interface friction.

A simple technique to prepare plastic color-filters based on the surface plasmon resonance (SPR) of silver nanoparticles has been developed. In particular, silver/polystyrene nanocomposites have been prepared by dissolving silver acetylacetonate in amorphous polystyrene and heating the obtained solid solution at a convenient temperature (ca. 200°C). When the metal precursor decomposes, the produced silver atoms diffuse into the polymeric matrix and clusterize, leading to the formation of a contact-free dispersion of hyperfine silver phase. The obtained films have been characterized by UV-VIS spectroscopy and the optical extinction spectra have been fitted by the Mie’s theory, introducing the limitation of the mean free path due to the collisions of conduction electrons with the boundary of the nanoparticles.

Radiation limits above permission limit have harmful effects on living bodies (i.e., carcinogenic, etc.). Heavyweight concrete is used in facilities such as nuclear power plant and radiotherapy room where radioactive impermeability is required. Aggregate and cement play an essential role in both the shielding performance and the structural behavior of concrete. Concretes with 0% (control), 15%, 30%, 45% and 60% of conventional aggregate replaced by hematite aggregate for each control concrete, were fabricated with 300, 350, 400 and 450 kg/m 3 cement contents. Water-cement ratio was kept constant at 0.40. In addition to investigation of the mineralogical structure of hematite, physical and mechanical experiments were carried out on the concrete produced. The results show that slump and compressive strength, like other mechanical properties, increase as the hematite aggregate replacement volume increase.