Papers by Keyword: Core-Shell

Abstract: A theoretical study was made of the dependence of the blocking temperature of the core/shell system of nanoparticles on the intensity of their magnetostatic interaction. It is shown that with an increase in the concentration of nanoparticles (intensity of the magnetostatic interaction), the blocking temperature increases. Moreover, the of large nanoparticles changes more significantly.

Abstract: Low temperature oxidized core-shelled magnetite is paramount important in recording geomagnetic field. To characterize the effects of transition zone between the core-shell on the magnetic properties of low temperature oxidation of magnetite, micromagnetic models of hysteresis parameters and microstructures of a multi-layer core-shelled model were systematically investigated by MERRILL (Micromagnetic Earth Related Rapid Interpreted Language Laboratory). Numerical simulations indicate that SD particles (<70 nm) remain highly uniform magnetization, but show decreasing coercivities as oxidation preceeds. For fine SV particles (80 nm to 120 nm), the hysteresis parameters respectively increase and dramatic decrease at the early and late stage of oxidation, and the micromagnetic behaviors vary significantly. Finally the hysteresis parameters of larger SV (>130) particles remain nearly unchanged during oxidation. It indicates that fine SV particles are more sensitive to oxidation, and dominate the dramatic change of experiment observation. Overall, low temperature oxidation of magnetite preferring a multi-layer coupled oxidation process from outside to interior and is capable of recording paleomagnetic signals.

Abstract: Mercury pollution through water causes several dangerous diseases. Various efforts have been made to reduce mercury pollution. One of them is by using sorbent. Many ways to improve absorption efficiency, one of which is using magnetic sorbents. This study focused on the effect of grain size and the concentration of Mn_0.25Fe_2.75O₄@SiO₂ core-shell on mercury absorption efficiency. The synthesis of Mn_0.25Fe_2.75O₄@SiO₂ with 6 and 8 mL of TEOS was carried out through coprecipitation and sol-gel methods. The characterization using XRD, VSM, and FTIR was conducted to determine grain size, properties, and material functional groups proving that SiO₂ was successfully covered on the Fe₃O₄ surface. The percentage of absorption was found by using the AAS instrument. Diffraction data confirmed the presence of Fe₃O₄ and the amorphous SiO₂ phase. According to the Rietveld analysis of all samples demonstrated the particle size of Mn_0.25Fe_2.75O₄ around 11-12 nm. The Mn_0.25Fe_2.75O₄ core had superparamagnetic properties for magnetic separation, and the SiO₂ shell could protect the core of being oxidized or dissolved under acid condition. FTIR results showed the sample had a functional group of the main components of Fe-O and SiO₂ at a wavenumber of 420-507 cm^-1 and 801 cm^-1 (stretching) and 1078 cm^-1 (bending), respectively. The results of the mercury absorption test indicated that the smaller the grain size and the higher the concentration of TEOS, the percentage of mercury uptake would increase. In addition, the absorption percentage increased with the duration of absorption time given.

Abstract: A model of the transition layer between the shell and the core of a ceramic matrix composite from coal waste and clay has been developed. The chemical, granulometric and mineral compositions of the beneficiation of carbonaceous mudstones and clay were studied. The technological and ceramic properties of raw materials for the samples manufacturing were determined. The method of manufacturing multilayer ceramic samples from coal waste, clay and their mixture is given. The number of transition layers in the contact zone between the clay shell and the core from coal wastes is determined. The deformation and swelling phenomena of model samples from coal wastes, clay, and their mixtures were revealed at the firing temperature of more than 1000 °C. The formation of a reducing ambient in the center of the sample with insufficient air flow is shown. The influence of the carbonaceous particles amount and the ferrous form iron oxide in the coal wastes on the processes of expansion of multilayer samples during firing has been established.

Abstract: Core–shell Si/SiC nanostructures appear as promising building blocks for sensing applications, thanks to the high chemical stability of SiC coupled with the semiconducting properties of Si. In order to optimize the fabrication process of such structures, Si nanowires were coated with a thin SiC layer, and integrated as back-gated field-effet transistors. Two approaches for the fabrication of the SiC shell were then investigated. The first approach involves chemical vapor deposition of amorphous SiC on Si nanowires, without the need for masking; the second approach involves carbonization of Si surfaces to produce a thin crystalline SiC layer, but requires a larger thermal budget. The resulting structures were analyzed using high-resolution transmission electron microscopy (HR-TEM), and the devices were characterized electrically. Electrical characterization shows that the carbonization approach induces a dramatic decrease in drain-to-source current associated with gate leakage, whereas the electrical performances were preserved in the case of chemical deposition.

Abstract: In this work, core-shell ZnO@SiO₂ nanoparticles (NPs) were infiltrated into a macro/meso-porous silicon (PS) structure, to study its luminescent properties. The core-shell ZnO@SiO₂ NPs were obtained by colloidal synthesis. The core-shell ZnO@SiO₂ NP was 5 nm in diameter. The macro/meso-PS structure was made in two steps: we obtained the macroporous silicon (macro-PS) layer fist and the mesoporous silicon (meso-PS) layer second. This process was conducted using different electrolyte solutions, and the change of electrolyte led to a decrease in the special charge region over the wall macro-PS layer; this allowed the building of the meso-PS layers on the walls and the bottom of the macro-PS layer. The SEM results show the cross-section of the macro/meso-PS structure with and without core-shell ZnO@SiO₂ NPs. These SEM images show that the core-shell ZnO@SiO₂ NPs that infiltrated into macro/meso-PS structure were more efficiently bonded over all the porous walls. The core-shell ZnO@SiO₂ PL interacted with the macro/meso-PS structure, modifying its PL intensity and controlling a shift toward a lower wavelength.

Abstract: The p-Cu_xO core/n-ZnO shell heterostructure nanowire (NW) arrays were fabricated by thermal decomposition. Based upon the core/shell nanowire-based all oxide p-n junctions. The samples were analyzed by XRD, SEM, EDS and TEM. X-ray diffraction (XRD) analysis showed that the p-Cu_xO core/n-ZnO shell NW consisted of phase of p-Cu_xO and wurtzite phase of n-ZnO. The morphology analysis showed average diameter and length of nanowires of ̴ 50 to 200 nm and ̴ 10 to 30 µm, respectively. The EDS spectrum confirmed the presence of required elements in the p-Cu_xO core /n-ZnO shell NWs. It was found that Zn, O and Cu are distributed over the wire areas according to a ratio of 1:2 by atomic% ratio of Cu:Zn to get good core/shell structure. The TEM characterizations showed that the n-ZnO shell nanoparticles were comprised of n-ZnO polycrystalline nanoparticles (NPs) on the surface of p-Cu₂O core NWs. The H₂S gas sensing properties of the p-Cu_xO/n-ZnO NWs were evaluated in air containing dilute H₂S gas at sensing temperatures (T) of 350°C. The response of 20.6 for p-Cu_xO/n-ZnO NW sensor to H₂S gas was enhanced compared to that of the n-ZnO NW. The enhanced response of p-Cu_xO/n-ZnO NW sensor is due to increasing surface area, the increased amount of chemisorbed oxygen species on NP surface and the increased conductivity.

Abstract: Graphene has been proved to be an excellent enhancer in metal matrix composites. Core-shell structured SiC nanoparticles and graphene nanosheets (GNSs) were fabricated and incorporated into aluminum matrix using ball milling in the current study. Graphite powder was exfoliated into thin GNSs, which are flexible to wrap SiC nanoparticles. The ductile aluminum particles were firstly flattened and then repeatedly welded and fractured into equalized particles during the ball milling of Al alloy-SiC-GNSs composite powder, which were observed using scanning electron microscopy and X-Ray diffraction. SiC-GNSs were embedded and dispersed into the aluminum matrix during the milling process.

Abstract: In this research core-shell Cu@Si nanoparticles were obtained through evaporation of elemental precursors by a high-powered electron beam. The structures of the particles were investigated in order to elucidate their mechanisms of formation. The thermal stability of the particles was studied with the help of molecular dynamics calculations. The parameters of the thermal stability of the composite nanoparticles Cu@Si with different size were determined. It was concluded that with the temperature increasing the diffusion of copper atoms on the surface begins, leading to a reversal of the structure and the formation of particles having a particle type Si@Cu.

Abstract: Well-aligned NiSi/SiC core-shell nanowires were grown on Ni-coated p-type crystal Si (100) substrates by using hot-wires chemical vapor deposition (HWCVD) technique. The growth of the nanowires was performed at a substrate temperature of 450°C and facilitated by a hot-filament at a temperature above 1800°C. Electron microscopy characterizations were employed to investigate the morphology, and microstructure properties of the nanowires. A high-resolution transmission electron microscopy (TEM) images indicate that the nanowires were structured by single crystalline NiSi and amorphous SiC as the core and shell respectively. Moreover, the TEM images showed presence of 3C-SiC nano-crystallites embedded within an amorphous matrix in the shell.