Electromagnetic wave absorption in reduced graphene oxide functionalized with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/Fe nanorings

We report the preparation of nanocomposites of reduced graphene oxide with embedded Fe₃O₄/Fe nanorings (FeNR@rGO) by chemical hydrothermal growth. We illustrate the use of these nanocomposites as novel electromagnetic wave absorbing materials. The electromagnetic wave absorption properties of the nanocomposites with different compositions were investigated. The preparation procedure and nanocomposite composition were optimized to achieve the best electromagnetic wave absorption properties. Nanocomposites with a GO:α-Fe₂O₃ mass ratio of 1:1 prepared by annealing in H₂/Ar for 3 h exhibited the best properties. This nanocomposite sample (thickness = 4.0 mm) showed a minimum reflectivity of–23.09 dB at 9.16 GHz. The band range was 7.4–11.3 GHz when the reflectivity was less than–10 dB and the spectrum width was up to 3.9 GHz. These figures of merit are typically of the same order of magnitude when compared to the values shown by traditional ferric oxide materials. However, FeNR@rGO can be readily applied as a microwave absorbing material because the production method we propose is highly compatible with mass production standards.

Open image in new window

     

Novel porous starfish-like Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@nitrogen-doped carbon as an advanced anode for lithium-ion batteries

A Co-based metal-organic framework (Co-MOF) with a unique three-dimensional starfish-like nanostructure was successfully synthesized using a simple ultrasonic method.After subsequent carbonization and oxidation,a nanocomposite of nitrogen-doped carbon with a Co3O4 coating (Co3O4@N-C) with a porous starfish-like nanostructure was obtained.The final hybrid exhibited excellent lithium storage performance when evaluated as an anode material in a lithiumion battery.A remarkable and stable discharge capacity of 795 mAh·g-1 was maintained at 0.5 A·g-1 after 300 cycles.Excellent rate capability was also obtained.In addition,a full Co3O4@N-C/LiFePO4 battery displayed stable capacity retention of 95％ after 100 cycles.This excellent lithium storage performance is attributed to the unique porous starfish-like structure,which effectively buffers the volume expansion that occurs during Li+ insertion/deinsertion.Meanwhile,the nitrogendoped carbon coating enhances the electrical conductivity and provides a buffer layer to accommodate the volume change and accelerate the formation of a stable solid electrolyte interface layer.     

Synthesis,transport and dielectric properties of polyaniline/Co<Subscript>3</Subscript>O<Subscript>4</Subscript> composites

Conducting polyaniline/cobaltous oxide composites have been synthesized using in situ deposition technique by placing fine graded/cobaltous oxide in polymerization mixture of aniline. The a.c. conductivity and dielectric properties are studied by sandwiching the pellets of these composites between the silver electrodes. It is observed that the values of conductivities increase up to 30 wt% of cobaltous oxide in polyaniline and decrease thereafter. Initial increment in conductivity is due to extended chain length of polyaniline where polarons possess sufficient energy to hop between favourable sites. Beyond 30 wt% of cobaltous oxide in polyaniline, blocking of charge carriers takes place reducing the conductivity values. It can be noted that the value of dielectric constant increases up to 10 wt% of cobaltous oxide. Thereafter, it decreases up to 30 wt% of cobaltous oxide and again increases up to 40 wt% of cobaltous oxide and decreases thereafter. The observed behaviour is attributed to the variation of a.c. conductivity. And it is observed that the dielectric loss increases up to 10 wt% of cobaltous oxide in polyaniline, decreases to a lower value of 20 wt% of cobaltous oxide and increases to 35 wt% and thereafter decreases. These values go in accordance with the values of dielectric constant. The results obtained for these composites are of greater scientific and technological importance.     

Synthesis of Ba<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript>−Sr<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript> solid solution and their dielectric properties

Oxides of the type, Ba_3-xSr_xZnNb₂O₉ (0 ≤x ≤3), were synthesized by the solid state route. Oxides calcined at 1000°C show single cubic phase for all the compositions. The cubic lattice parameter (a) decreases with increase in Sr concentration from 4.0938(2) forx = 0 to 4.0067(2) forx = 3. Scanning electron micrographs show maximum grain size for thex = 1 composition (∼ 2 μm) at 1200°C. Disks sintered at 1200°C show dielectric constant variation between 28 and 40 (at 500 kHz) for different values of x with the maximum dielectric constant atx = 1.     

Synthesis and microwave absorption properties of sandwich-type CNTs/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/RGO composite with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> as a bridge

A novel sandwich-type CNTs/Fe₃O₄/RGO composite with Fe₃O₄ as a bridge was successfully prepared through a simple solvent-thermal and ultrasonic method. The structure and morphology of the composite have been characterized by Fourier-transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. This new structure can effectively prevent the agglomeration of GO and the combination of CNTs/Fe₃O₄ and RGO shows a strong reflection loss (R_L) (?50 dB) at 8.7 GHz with absorber thickness of 2.5 mm. Moreover, compared with CNTs/Fe₃O₄/GO composite, it is found that the thermal treating process is beneficial to enhance the microwave absorption properties, which may be attributed to high conductivity of RGO. On this basis, the microwave absorbing mechanism is systematically discussed. All the data show that the CNTs/Fe₃O₄/RGO composite exhibits excellent microwave absorption properties with light density and is expected to have potential applications in microwave absorption.     

Comparative study of dielectric properties of MgNb<Subscript>2</Subscript>O<Subscript>6</Subscript> prepared by molten salt and ceramic method

Magnesium niobate (MgNb₂O₆) powder was synthesized by the conventional ceramic route as well as by the molten salt route using a eutectic mixture of NaCl-KCl as the salt and Mg(NO₃)₂-6H₂O and TiO₂ as the starting materials. Pure phase of MgNb₂O₆ could be obtained by the molten salt method at 1100°C. However, in ceramic method the pure phase of MgNb₂O₆ was obtained by heating at 1025°C for 20 h. On sintering at 1100°C the dielectric constant and dielectric loss of MgNb₂O₆ obtained by the molten salt method was found to be 19.5 and 0.004 at 100 kHz at room temperature. Lower values were obtained for these oxides prepared by the ceramic route, 16.6 and 0.000518, respectively. In both cases the dielectric constant was quite stable with frequency.     

An efficientfficient,controllable and facile two-step synthesis strategy: Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@RGO composites with various Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles and their supercapacitance properties

An efficient,controllable,and facile two-step synthetic strategy to prepare graphene-based nanocomposites is proposed.A series of Fe3O4-decorated reduced graphene oxide (Fe3O4@RGO) nanocomposites incorporating Fe3O4 nanocrystals of various sizes were prepared by an ethanothermal method using graphene oxide (GO) and monodisperse Fe3O4 nanocrystals with diameters ranging from 4 to 10 nm.The morphologies and microstructures of the as-prepared composites were characterized by X-ray diffraction,Raman spectroscopy,nitrogen adsorption measurements,and transmission electron microscopy.The results show that GO can be reduced to graphene during the ethanothermal process,and that the Fe3O4 nanocrystals are well dispersed on the graphene sheets generated in the process.The analysis of the electrochemical properties of the Fe3O4@RGO materials shows that nanocomposites prepared with Fe3O4 nanocrystals of different sizes exhibit different electrochemical performances.Among all samples,Fe3O4@RGO prepared with Fe3O4 nanocrystals of 6 nm diameter possessed the highest specific capacitance of 481 F/g at 1 A/g,highlighting the excellent capability of this material.This work illustrates a promising route to develop graphene-based nanocomposite materials with a wide range of potential applications.     

Preparation and characterization of mesoporous Co<Subscript>3</Subscript>O<Subscript>4</Subscript> electrode material

Mesoporous electrode materials of spinel Co₃O₄ were synthesized by hydrothermal method using polyethylene glycol-6000 (PEG-6000) as dispersant and subsequent calcination at different temperatures in air. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nitrogen adsorption and electrochemical measurements. The results showed that the dispersant PEG-6000 had a distinct effect to control the porosity, particle size and homogeneity of Co₃O₄; the calcination temperature had a significant influence on the crystal structure, surface area, porosity and morphology, and indeed electrochemical performance. The resultant Co₃O₄ sample calcinated at 350° C possessed a narrow mesopore distribution around 4 nm and exhibited excellent electrochemical performance. It had the specific capacitance as high as 348.7 F/g, increased by 21.5% over that of pure Co₃O₄ and showed good cyclical charge–discharge stability.     

Ionic liquid-derived Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/carbon nano-onions composite and its enhanced performance as anode for lithium-ion batteries

In this work, a novel composite of Co₃O₄ nanoparticle and carbon nano-onions (CNOs) is synthesized by using ionic liquid as carbon and nitrogen source through a facile carbothermic reduction followed by low-temperature oxidation method. The SEM and HRTEM images reveal that the Co₃O₄ particles are homogenously embedded in the CNOs. Due to the unique nano-structure, the electrolyte contacts well with the active materials, leading to a better transfer of lithium ions. Moreover, the unique nano-structure not only buffers the volume changes but also facilitates the shuttling of electrons during the cycling process. As a result, the electrode made up of Co₃O₄/CNOs composite delivers favorable cycling performance (676 mAh g^?1 after 200 cycles) and rate capability (557 mAh g^?1 at the current of 1 C), showing a promising prospect for lithium-ion batteries as anode materials.     

A simple approach for the synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocrystals

By using Co²⁺ and Co³⁺ salts, and freshly extracted ovalbumin, Co₃O₄ nanocrystals have been synthesized successfully. The pH of the solution was self-regulated for the hydrolysis of metal ions as the ovalbumin-water mixture was highly basic. Water soluble ovalbumin proteins served as a perfect matrix for entrapment of Co²⁺ and Co³⁺ ions thus forming a gel. Upon heat treatment, the dried gel precursor decomposed into nanocrystalline Co₃O₄. The crystallite size obtained by XRD line profile fitting was 45 ± 8 nm and particle size estimated from the SEM was in the range 20 nm-2 μm. EPR results show a very good fit to literature reports for nanocrystals in the size range of 8–17 nm. Even though the overall particle size is quite large and its distribution is quite wide EPR results confirm nanocrystalline nature of the particles obtained. Presented route is simple, cost effective, and environmentally friendly.     

The synergetic electromagnetic properties and enhanced microwave absorption of BiFeO<Subscript>3</Subscript>/BaFe<Subscript>7</Subscript>(MnTi)<Subscript>2.5</Subscript>O<Subscript>19</Subscript> composite

The effective microwave absorption materials could contribute to alleviating the electromagnetic wave pollution. However, conventional microwave absorption materials usually suffer from insufficient absorption intensity and the narrow effective absorption bandwidth. Herein, BiFeO₃/BaFe₇(MnTi)_2.5O₁₉ composites are proposed to address these issues through offering synergetic electromagnetic properties and proper electromagnetic properties. BiFeO₃ combined with BaFe₇(MnTi)_2.5O₁₉ exhibits dielectric multiple relaxation behaviors, strong ferromagnetic resonance and electromagnetic matching, ensuring increased multi-band microwave absorption. Accordingly, the minimum reflection loss (RL) of the composite with volume ratio of 1.5:1 reaches ??48 dB, and the bandwidth less than ??10 dB covers multi-frequencies at C, X and Ku band. These results suggest that BiFeO₃/BaFe₇(MnTi)_2.5O₁₉ composite could be a promising microwave absorption material in imaging, healthcare, information safety and military fields.     

Synthesis and microwave absorption property of ternary composites: polyaniline/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/Ba<Subscript>3</Subscript>Co<Subscript>2</Subscript>Fe<Subscript>24</Subscript>O<Subscript>41</Subscript>

Polyaniline (PANI)/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite was prepared by an in-situ polymerization method. The phase structure, morphology and magnetic properties of the as-prepared PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite were characterized by XRD, FT-IR, SEM, TEM, and VSM, respectively. The microwave absorption properties of the composite were investigated by using a vector network analyzer in the 2–18 GHz frequency range. The results show that the maximum reflection loss value of the PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite reaches ?30.5 dB at 10.5 GHz with a thickness of 3 mm and the bandwidth of reflection loss below ?10 dB reaches up to 1.2 GHz. The excellent microwave absorption properties of the as-prepared PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite due to the enhanced impedance match between dielectric loss and magnetic loss.     

Thermoelectric properties of WO<Subscript>3</Subscript>-based ceramics doped with Co<Subscript>2</Subscript>O<Subscript>3</Subscript>

Tungsten trioxide (WO₃) doped with cobalt sesquioxide (Co₂O₃) was prepared by a conventional mixed oxide processing route and the thermoelectric properties were studied from 300 up to 1,000 K. The addition of Co₂O₃ to WO₃ resulted in an increase in both the grain size and porosity, indicating that Co₂O₃ promotes the grain grown of WO₃. The magnitude of the electrical conductivity (σ) and the absolute value of the Seebeck coefficient (|S|) depended strongly on the Co₂O₃ content. As for the power factor (σS ² ), the 5.0 mol% sample has the maximum value of the power factor which is 0.12 μWm⁻¹K⁻² at 873 K.     

Synthesis of well-defined Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanorods/N-doped graphene for lithium-ion batteries

The geometric size and distribution of magnetic nanoparticles are critical to the morphology of graphene (GN) nanocomposites, and thus they can affect the capacity and cycling performance when these composites are used as anode materials in lithium-ion batteries (LiBs). In this work, Fe₃O₄ nanorods were deposited onto fully extended nitrogen-doped GN sheets from a binary precursor in two steps, a hydrothermal process and an annealing process. This route effectively tuned the Fe₃O₄ nanorod size distribution and prevented their aggregation. The transformation of the binary precursor was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). XPS analysis indicated the presence of N-doped GN sheets, and that the magnetic nanocrystals were anchored and uniformly distributed on the surface of the flattened N-doped GN sheets. As a high performance anode material, the structure was beneficial for electron transport and exchange, resulting in a large reversible capacity of 929 mA·h·g^–1, high-rate capability, improved cycling stability, and higher electrical conductivity. Not only does the result provide a strategy for extending GN composites for use as LiB anode materials, but it also offers a route for the preparation of other oxide nanorods from binary precursors.

Open image in new window

     

Light weight RGO/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocomposite for efficient electromagnetic absorption coating in X-band

Reduced graphene oxide (RGO)/magnetite (Fe₃O₄) nanocomposite has been synthesized by an in-situ facile hydrothermal method. The XRD pattern reveals the development of nanocomposite in which both phases are coexistent. Raman Spectroscopy shows the main characteristics peaks of D and G bands at 1349 cm^?1 and 1595 cm^?1 for graphitic structures. The intensity ratio (I_D/I_G) is also calculated, which indicate the degree of defects in the material. This ratio (I_D/I_G), increases from 0.84 for GO to 0.91 for RGO/Fe₃O₄ nanocomposite and promotes the defects which are beneficial for electromagnetic (EM) absorption. The SEM image depicts that, Fe₃O₄ spherical nanoparticles are dispersed over the surface of graphene sheets and provide a thermal conducting path for heat dissipation between different layers of graphene. The EM absorption properties have been analyzed at 2–18 GHz of RGO and RGO/Fe₃O₄. The addition of proper content of Fe₃O₄ magnetic nanoparticles in RGO sheets improved the Reflection Loss (RL) from ??13.5 dB to ??20 dB at a frequency of 9.5 GHz. Moreover, due to magnetic loss and interfacial polarization, the effective bandwidth increases from 2.5 GHz to 3.8 GHz at a coating thickness of 1.5 mm. Hence this light weight nanocomposite is an excellent material for strong EM absorption in X-band.     

Dielectric properties of B<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Sm(Co<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics at microwave frequency

The microwave dielectric properties and the microstructures of Sm(Co_1/2Ti_1/2)O₃ ceramics with B₂O₃ additions (0.25 and 0.5 wt%) prepared by conventional solid-state route have been investigated. The prepared Sm(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. Doping with B₂O₃ (up to 0.5 wt%) can effectively promote the densification of Sm(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. It is found that Sm(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1,260 °C due to the grain boundary phase effect of B₂O₃ addition. At 1,290 °C, Sm(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% B₂O₃ addition possess a dielectric constant (ε _r) of 27.7, a Q × f value of 33,600 (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of −11.4 ppm/ °C. The B₂O₃-doped Sm(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba(Co<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>

Polycrystalline nonstoichiometric Ba(Co_1/3Nb_2/3)O₃ (BCN) materials have been synthesized and investigated. Deviations from stoichiometry have been shown to lead to the formation of crystalline Ba₆CoNb₉O₃₀ (barium deficiency) and Ba₈CoNb₆O₂₄ (cobalt deficiency). The effect of phase composition on the microwave dielectric properties of BCN has been studied. The results demonstrate that the dielectric properties of BCN-based materials can be tuned by varying cobalt content. The materials obtained are high-Q microwave dielectrics with temperature-stable properties.     

Electrochemical capacitance study on Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowires for super capacitors application

One-dimensional Co₃O₄ nanowires have been prepared by utilizing the ordered mesoporous silica material SBA-15 as template. The results of transmission electron microscope (TEM) and N₂ adsorption–desorption characterizations show that the Co₃O₄ nanowires possess a uniform size and a large Brunauer-Emmett-Teller (BET) surface area. Its electrochemical performance was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques in various concentration of KOH solution. This nanomaterial shows a small resistance, a high specific capacitance (SC), and a strong cyclic stability. The maximal SC value of 373 F·g⁻¹ was obtained in 6 M electrolyte under the scan speed of 3 mV·s⁻¹ at the first CV cycle. After 500 CV cycles, the SC value is about 90% of the original value. It is considered that the short path of ion transfer given by nanomaterial brought on the great pseudo capacitance performance.     

Mechanical properties and microstructure of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/mullite composite

An Al₂O₃/5 vol.% mullite composite was synthesized by using reaction sintering of Al₂O₃/0.78 wt.% SiC at 1,600 °C for 2 h in air. The phase analysis of the Al₂O₃/mullite composite was carried out using X-ray diffraction (XRD). There were two kinds of mullite in alumina/mullite composite, namely, 3Al₂O₃·2SiO₂ and Al_5.65Si_0.35O_9.175. The microstructure of the Al₂O₃/mullite composite was investigated using scanning electron microscope (SEM) and transmission electron microscope (TEM). The mechanical properties such as Young’s modulus, Poisson’s ratio, hardness, toughness and strength of the Al₂O₃/mullite composite were investigated. The influence of mullite on the composite is discussed.     

Facile synthesis and characterization of polypyrrole/Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocomposites with adjustable intrinsic electroconductivity

Polypyrrole (PPY)/Co₃O₄ nanocomposites (NCs) were synthesized by a facile in situ polymerization of pyrrole in the presence of Co₃O₄ nanoparticles which were obtained by a rheological phase reaction method. The structure and morphology of the as-prepared PPY/Co₃O₄ NCs were investigated by X-ray diffraction, Fourier transform infrared spectra, Raman spectra, scanning electron microscopy and transmission electron microscopy, which confirmed the formation of the nanocomposites and indicated some interactions between PPY chains and Co₃O₄ nanoparticles. Different PPY/Co₃O₄ ratios were selected in order to study conductive properties. The electrical conductivity measurements indicated that ac conductivity tended to remain constant up to about 10⁷ Hz for all samples, and thereafter increased with frequency. The desired electrical properties of PPY/Co₃O₄ NCs can be modulated simply by controlling the contents of Co₃O₄ nanoparticles.