Advanced Materials Research Vols. 875-877

Abstract: In previous work, ZnO nanorods and nanoflowers were synthesized on indium-tin-oxide (ITO) substrates by hydrothermal growth at low temperature ,using the different concentrations of equimolar (1:1) zinc nitrate Zn (NO₃)₂6H₂O and methenamine (C₆H₁₂N₄) mixed as precursors solution, and adding ammonia solution to control the pH levels. In this paper, for comparison, the same experiment without ammonia solution was also discussed. SEM, XRD were utilized to characterize morphologies and crystal structures of ZnO. It was indicated that the pH of precursor solution leads to the significantly changes in the nanostructured ZnO. Besides, a hierarchical structure of some of the micro/nanotubes was obtained .The possible growth mechanism is discussed in this work.

Abstract: Magnesium titanate based dielectric materials are used for producing type-I capacitors. A common way of obtaining this material is a solid-state reaction. The process of sintering can be enhanced if mechanical activation preceedes. In this work starting powders of magnesium carbonate (MgCO₃) and titanium dioxide (TiO₂) with a rutile crystal modification were weighed to attain a 1:1 molar MgCO₃:TiO₂. Mechanical activation of the starting mixture was performed by high energy ball milling using ZrO balls and vessels with ball to powder mass ratio 40:1. The observed grinding times were 15, 30, 60 and 120 minutes. The isothermal sintering of compacted powders was conducted at 1100°C during 30, 60 and 180 minutes. For specimens synthesized in such a manner, microwave dielectric properties were measured, quality factor Q and the dielectric constant (ε_r) in function of frequency. The measurements of electrical resistivity, capacitance and loss tangent of samples were measured in the frequency range from 500 Hz to 5 MHz frequencies with a HIOKI 3532-50 LCR HiTESTER device at a constant voltage mode (amplitude 0.5 V of sinusoidal signal applied to the specimens).

Abstract: The main focus of the research presented in this paper is the investigation of the ability of various polyolefin resins to be converted into integral-skin cellular composites by using the rotational foam molding process. Integral-skin foamed rotational moldings are formally denoted as cellular composites ideally having a clearly distinct surface layer of solid skin of uniform thickness that is encapsulating a seamlessly coupled fine-celled foamed core or layer of uniform cell density and distribution. A systematic comparative material characterization study that attempts to derive practical guidelines about determining the roto-foamability of polyolefins that would be useful for rotomolding processors is presented. The study included two experimental methods of characterization, a melt rheology-based and a rotational foam molding processing-based. The experimental results from both implemented characterization methods revealed good agreement. A comprehensive insight into the key polyolefin material characteristics that would ensure satisfactory results if processed using the rotational foam molding technology have been provided. The experimental results revealed that high quality polyethylene (PE) based cellular morphologies can be obtained from both dry blended and melt compounded foamable compositions for both 6-fold and 3-fold expanded foams. Unlike PE resins, it was observed that successful foaming of polypropylene (PP) resins in rotational foam molding can only be successfully accomplished over a very narrow range of melt temperatures that are close to the melting point of the polymer and by using PP grades with a quite limited range of Melt Flow Rates (MFR).

Abstract: In this paper, successful structural transformation of carbon black (CB) into nanotubes and nanoonion like structures at relatively low temperatures in the presence of transition metal catalyst is reported. This study focuses also on the influence of the temperature on the structural transformation of CB into nanostructures. The experiments were carried out at 700°C and 1000°C in a horizontal tube furnace under N₂ atmosphere. The obtained samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). It was found that increase in the synthesis temperature from 700°C to 1000°C influences the morphology of the produced nanotubes significantly and the degree of crystallinity also increased with the temperature..

Abstract: The stress concentration of the circular hole within the mechanical mart can easily derived by the photo-elastic technique if the testing specimen is sufficiently developed. The testing specimen using the characteristic of the photo-elastic coefficient, with actual form like the mechanical part by rapid prototyping forming is thus used in this paper. The rapid mold for prototype is made by the RTV-2 silicone gel associated with the vacuum chamber to remove the air within it. The testing scheme is by way of the simply supported beam where the load is imposed on the middle of the specimen. Results show that the smaller diameter of hole on specimen provides a larger stress concentration effect. When the diameter of the hole is small, the stress concentration effects caused by the machined hole and the forming hole have almost no difference. However, the stress concentration effects of the larger diameter of the machined hole and of the formed hole give significant difference, the stress concentration caused by the machined hole is much greater than one of the formed hole. In this paper, the actual shape of the specimen which is as same as the original mechanical components can be quickly obtained by the rapid prototyping technology for the photoelastic technique to reveal the experimental mechanics of real applications.

Abstract: The most expensive and principal step in the process of converting cellulosic biomass into biorenewables and biofuel is the pretreatment during which lignin and cellulose crystalline structure is broken down. In this study, TiO₂ nanoparticles were utilized as photocatalyst in addition to alkaline (ammonia and sodium hydrate) treatment of corn stover biomass to improve delignification ability and reduce the energy and chemical loadings to make the process more environmentally friendly. Raman Spectroscopic imaging, as an accurate and rapid chemical characterization method, was utilized to investigate the molecular compositional and structural changes occurring during the pretreatment process in the corn stover biomass qualitatively. Lignin characteristic peaks at 1,600, 1,620 and 1,690 cm^-1 as well as cellulose characteristic peaks at 1,059 and 2,888 cm^-1 were identified through Raman Spectroscopic measurement as indicators of the effects of the pretreatment. Real-time quantitative analysis of these functional peaks revealed the molecular structural changes during delignification, potentially may lead to optimization of the processing parameters to reduce the processing cost in the future.

Abstract: Al-0.6%Cu alloy processed by equal channel angular pressing (ECAP) was investigated in this study. The samples processed with 1, 2 and 4 ECAP passes were annealed at different temperatures. And the microstructures of the Al-0.63%Cu alloy samples were observed. The results show that the grains of the alloy are refined to sub-micron level after multipass ECAP. Moreover, it is found that the recrystallization temperature of the Al-0.6%Cu alloy samples decreases with increasing the number of ECAP passes. Based on the study above, effect of cold rolling reduction and ECAP passes on recrystallization temperature of materials should be further investigated.

Abstract: Activated carbons were prepared from coconut shell by chemical activation method and utilized as electrode materials for electrochemical double layer capacitor (EDLC). A preliminary characteristic of activated carbon from coconut shell includes the Brunnaeur Emmett Teller (BET) analysis and cyclic voltammetry measurements. The BET surface area is not affected by the variation of activation temperature as both of the samples showed BET surface area of about 850-870 m2g-1. The N2 adsorption–desorption isotherms showed that the sample exhibited type I characteristics according to IUPAC classification, which confirms its micro-porosity. Compared with the un-activated carbon samples, the activated ones exhibited the better electrochemical properties with a specific capacitance of 150 F g−1 at a scan rate of 2 mV s−1. The good performance of activated carbon is attributed to the enhancement of surface area due to the KOH pretreatment which can open new pores accessible for the ionic transport

Abstract: Similar amount of carbon with three different types of morphologies, i.e. smooth pyrolytic carbon (SPC), granular pyrolytic carbon (GPC) and carbon nanofiber (CNF) was coated on the natural graphite (NG) spheres in a fluidized bed by chemical vapor deposition (CVD) from acetylene. The effect of carbon-coating morphologies on the electrochemical performance of NG spheres were studied, the results show that: PC is more effective in improving the first coulombic efficiency (CE); both GPC and CNF can improve the cyclability, however, GPC improves the first CE whereas CNF decreases the first CE.

Abstract: Welding is an efficient material joining process. But the localised temperature rise up to the melting point in the vicinity of arc causes non uniform expansion and contraction resulting in residual stress and distortions. Post weld, Pre weld and in-process methods have been studied to reduce these unwanted effects of welding. A trailing heat sink is one such method. This consists of a cooling agent traversing at a constant distance behind the weld arc. Introduction of a cooling agent is reported to reduce the distortion. A liquid nitrogen jet is used as the cooling agent in the present study. A finite element model developed using Sysweld is used to study the effect of liquid nitrogen heat sink on weld distortions and residual stresses. The finite element model developed is used to make a parametric study on the effect of cooling jet radius, cooling strength and the distance between the jet and weld arc on out of plane distortions and stresses. Comparative study of the effect of cooling jet is made for mild steel (MS) and stainless steel (SS). It is observed that the out of plane distortions reduce with increasing cooling strength, jet radius. Reduction in distance between the jet and weld arc also causes a reduction in distortions.