Showing papers by "Mao-Sheng Cao published in 2009"

Porous Fe3O4/Carbon Core/Shell Nanorods: Synthesis and Electromagnetic Properties

Abstract: The porous Fe3O4/carbon core/shell nanorods were fabricated via a three-step process. α-Fe2O3 nanorods were first obtained, and α-Fe2O3/carbon core/shell nanorods were subsequently fabricated using glucose as a carbon source by a hydrothermal method, in which the thickness of the carbon coating was about 3.5 nm. Fe3O4/carbon core/shell nanorods were synthesized after an annealing treatment of the product above under a mixture of Ar/H2 flow. After the H2 deoxidation process, the Fe3O4 core exhibited a character of porosity; the thickness of the carbon shell was decreased to about 2.5 nm, and its degree of graphitization was enhanced. The interesting core/shell nanostructures are ferromagnetic at room temperature, and the Verwey temperature was about 120 K. Electromagnetic properties of the core/shell nanorod–wax composite were investigated in detail. The maximum reflection loss was about −27.9 dB at 14.96 GHz for the composite with a thickness of 2.0 mm, and the absorption bandwidth with the reflection los...

High dielectric loss and its monotonic dependence of conducting-dominated multiwalled carbon nanotubes/silica nanocomposite on temperature ranging from 373 to 873 K in X-band

Abstract: The dielectric properties of multiwalled carbon nanotubes/silica (MWNTs/SiO2) nanocomposite with 10 wt % MWNTs are investigated in the temperature range of 373–873 K at frequencies between 8.2 and 12.4 GHz (X-band). MWNTs/SiO2 exhibits a high dielectric loss and a positive temperature coefficient (PTC) of dielectric effect that complex permittivity increases monotonically with increasing temperature. The PTC effect on the dielectric constant is ascribed to the decreased relaxation time of interface charge polarization, and the PTC effect on the dielectric loss is mainly attributed to the increasing electrical conductivity. The loss tangent strongly supports the dominating contribution of conductance to the dielectric loss.

Dual nonlinear dielectric resonance and nesting microwave absorption peaks of hollow cobalt nanochains composites with negative permeability

Abstract: The permittivity and permeability behaviors of the hollow cobalt nanochains composites have been investigated in 2–18 GHz. The permittivity presents two dielectric resonance peaks at about 12.3 and 14.5 GHz, respectively, which mainly results from the cooperative consequence of the hollow structure and the one-dimensional structure of the as-synthesized Co nanochains. The negative permeability behavior within 12.3–18 GHz is attributed to radiation of the magnetic energy according to the as-established equivalent circuit model. Two strong absorption peaks of the composites nest at the resonance frequencies due to the effect of the dual nonlinear dielectric resonance and the negative permeability behavior.

Synthesis and enhanced ethanol sensing properties of α-Fe2O3/ZnO heteronanostructures

Abstract: α-Fe 2 O 3 /ZnO heteronanostructures were synthesized via a three-step process. XRD, SEM, TEM and EDS analyses indicated that their diameters and lengths were ∼40 nm and 0.35–1.2 μm, respectively, in which the largest thickness of ZnO shells was about 10 nm. The heteronanostructures exhibited a dramatic improvement in ethanol sensing characteristics compared to the pure α-Fe 2 O 3 nanorods. Based on the space-charge layer model, such enhanced sensing properties were attributed to small thickness of ZnO shell. Our results demonstrate that One-dimensional (1D) metal oxide heteronanostructures, whose shell thickness is comparable to Debye length, are very promising materials for fabricating gas sensors with good performances.

Synthesis, magnetic and electromagnetic wave absorption properties of porous Fe3O4/Fe/SiO2 core/shell nanorods

Abstract: Porous Fe3O4/Fe/SiO2 core/shell nanorods were fabricated, in which the diameter of the pores was 5–30 nm. The magnetic and electromagnetic properties were investigated. The temperature dependent magnetic measurements showed that these nanorods were ferromagnetic with a Verwey temperature of 129 K. The electromagnetic data indicated that effective complementarities between the dielectric loss and the magnetic loss were realized, suggesting that they have excellent electromagnetic wave absorption properties. Thus the porous core/shell nanorods could be used as a kind of candidate absorber.

Synthesis and enhanced ethanol sensing characteristics of alpha-Fe2O3/SnO2 core-shell nanorods.

Abstract: ?-Fe2O3/SnO2 core?shell nanorods are synthesized via a three-step process. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses reveal that their diameters and lengths are respectively in the ranges 35?120?nm and 0.35?1.2??m, and the thickness of the shell composed of 3.5?nm SnO2 nanoparticles is about 10?nm. The core?shell nanostructures exhibit a dramatic improvement in ethanol sensing characteristics compared to pure ?-Fe2O3 nanorods. The sensor response is up to 19.6 under 10?ppm ethanol exposure at 220??C. Both the response time and the recovery time of the core?shell structures are less than 30?s. Based on the space?charge layer model and semiconductor heterojunction theory, the small thickness of the SnO2 shell and the formation of heterojunctions contribute to the enhanced ethanol sensing characteristics. Our results demonstrate that one-dimensional metal oxide core?shell nanostructures whose shell thickness is smaller than the Debye length are very promising materials for fabricating gas sensors with good performances.

High-temperature conductance loss dominated defect level in h-BN: Experiments and first principles calculations

Abstract: The dielectric properties of hexagonal boron nitride are investigated in detail. The permittivities hold extremely low values ranging from room temperature to 1500 °C, however, the dielectric loss tangents increase rapidly above 1000 °C. At 1500 °C, the dielectric loss tangent is 20 times more than that at room temperature. The first principles calculations show that the boron vacancy (VB) that gives an acceptor energy level near the valence band presents the lowest ionization energy in the investigated defects, and the calculated VB ionization energy agrees with the experimental value. It indicates that the rapid increase in dielectric loss tangents at high temperature is contributed by electrical conductivity produced by VB ionization under thermal excitation.

Low dielectric loss and non-Debye relaxation of gamma-Y2Si2O7 ceramic at elevated temperature in X-band

Abstract: Bulk single-phase gamma-Y2Si2O7 ceramic has been synthesized from a mixture of Y2O3 powder and SiO2 nanopowder at 1400 °C. The dielectric properties are reported at the temperature ranging from room temperature to 1400 °C in X-band. The results show that gamma-Y2Si2O7 exhibits low dielectric loss and non–Debye relaxation behavior different from that of SiO2. The peculiar relaxation peak is attributed to the structural relaxation polarization caused by thermal-excitation structural defects, which implies that no ionic conductance exists in this material. Such low dielectric loss will draw much attention for potential dielectric applications at high temperature.

Structural and electrical properties of Nd ion modified lead zirconate titanate nanopowders and ceramics

Abstract: A modified sol-gel method is used for synthesizing Nd ion doped lead zirconate titanate nanopowders Pb1–3x/2NdxZr0.52Ti0.48O3 (PNZT) in an ethylene glycol system with zirconium nitrate as zirconium source. The results show that it is critical to add lead acetate after the reaction of zirconium nitrate with tetrabutyl titanate in the ethylene glycol system for preparing PNZT with an exact fraction of titanium content. It has been observed that the dopant of excess Nd ions can effectively improve the sintered densification and activity of the PNZT ceramics. Piezoelectric, dielectric and ferroelectric properties of the PNZT ceramics are remarkably enhanced as compared with those of monolithic lead zirconate titanate (PZT). Especially, the supreme values of piezoelectric constant (d33) and dielectric constant () for the PNZT are both about two times that of the monolithic PZT and moreover, the remnant polarization (Pr) also increases by 30%. According to the analysis of the structures and properties, we attribute the improvement in electrical properties to the lead vacancies caused by the doping of Nd ions.

Synthesis and magnetic properties of CdS/α-Fe2O3 hierarchical nanostructures

Abstract: CdS/α-Fe2O3 hierarchical nanostructures, where the CdS nanorods grow irregularly on the side surface of α-Fe2O3 nanorods, were synthesized via a three-step process. The diameters and lengths of CdS nanorods can be tuned by changing the ethylenediamine (EDA) and Cd ion concentrations. The magnetic investigations by superconducting quantum interference device indicate that the hierarchical nanostructures have an Morin transition at lower temperature (230 K) than that of the single bulk α-Fe2O3 materials (263 K). Importantly, the hierarchical nanostructures exhibit weakly ferromagnetic characteristics at 300 K. A sharp peak assigned to the surface trap induced emission are observed in room temperature PL spectra. Combining with the optoelectronic properties of CdS, the CdS/α-Fe2O3 hierarchical nanostructures may be used as multi-functional materials for optoelectronic and magnetic devices.