Electromagnetic wave absorption properties of multi-walled carbon nanotubes decorated with La-doped BaTiO3 nanocrystals synthesized by a solvothermal method

Ba_1−xLa_xTiO₃/multi-walled carbon nanotube (MWCNT) nanocomposites with different concentrations of La³⁺ doping, were synthesized by a solvothermal process. The prepared nanocomposites had a hybrid microstructure in which Ba_1−xLa_xTiO₃ nanocrystals with diameter of 10–30 nm were firmly immobilized on the MWCNTs sidewalls. Electromagnetic (EM) wave absorption properties of La-doped BaTiO₃/MWCNT nanocomposites were investigated in the 7.5–18 GHz frequency range for an absorber thickness of 1 mm. The reflection loss (RL) calculated from the EM parameters of the samples, moved to low frequencies with increasing La³⁺ doping. The widest absorption bandwidth, with the lowest frequency range, was observed in a nanocomposite doped with 1.5 at% La³⁺. An RL exceeding −5 dB for this sample was obtained in the frequencies ranging from 9.6 to 16.3 GHz, with the optimal RL of −17.4 dB at 10.9 GHz, due to enhanced interfacial polarization resulting in developed ε^″r${{\epsilon }^{″}}_{\text{r}}$. In addition, the RL for the sample shifted to the low frequency region and the peaks became sharper in the 2–18 GHz frequency range with increasing absorber thickness. For BaTiO₃/MWCNT nanocomposites, La³⁺ doping can greatly improve the EM wave absorbing ability in a thin absorber thickness and the donor-doped nanocomposites show promise for application in EM wave shielding materials with broad absorption bandwidths.     

Synthesis of ferrofluid based nanoarchitectured polypyrrole composites and its application for electromagnetic shielding

The monodispersion of magnetic nanoparticles in conducting polymer is the prerequisite to make a high quality composite for tunable electromagnetic interference (EMI) shielding. To meet this challenge, we have designed and synthesized ferrofluid based nanoarchitectured polypyrrole composites containing Fe₃O₄ (8–12 nm) via in situ oxidative polymerization. To tune the microwave signals, polypyrrole composites (PFF) with different monomer/ferrofluid weight ratios have been prepared and characterized in microwave frequency domain. A maximum shielding effectiveness value of SE_A(max) = 20.4 dB (∼99% attenuation) due to the absorption of microwave has been observed in the frequency range of 12.4–18 GHz and attenuation level varied with ferrofluid loading. The electrical conductivity of PFF composite is of the order of 10⁻² S cm⁻¹ order and having superparamagnetic nature with saturation magnetization (M_s) of 5.5 emu g⁻¹. The lightweight PFF composites with high attenuations can provide full control over the atomic structure and are favorable for the practical EMI shielding application for commercial electronic appliances.     

Assembly, characterization and swelling kinetics of Ag nanoparticles in PDMAA-g-PVA hydrogel networks

A series of poly(N,N-dimethylacrylamide)-g-poly(vinyl alcohol) (PDMAA-g-PVA) graft hydrogel networks were designed and prepared via a free radical polymerization route initiated by a PVA-(NH₄)₂Ce(NO₃)₆ redox reaction. Silver nanoparticles with high stability and good distribution behavior have been self-assembled by using these hydrogel networks as a nanoreactor and in situ reducing system. Meanwhile the PDMAA or PVA chains can efficiently act as stabilizing agents for the Ag nanoparticles in that Ag⁺ would form complex via oxygen atom and nitrogen atom, and form weak coordination bonds, thus astricting Ag⁺. The structure of the PDMAA-g-PVA/Ag was characterized by a Fourier transform infrared spectroscope (FTIR). The morphologies of pure PDMAA-g-PVA hydrogels and PDMAA-g-PVA/Ag nanocomposite ones were observed by a scanning electron microscopy (SEM) and transmission electron microscope (TEM). TEM micrographs revealed the presence of nearly spherical and well-separated Ag nanoparticles with diameters ranging from 10 to 20 nm, depending on their reduction routes. XRD results showed all relevant Bragg's reflection for crystal structure of Ag nanoparticles. UV–vis studies apparently showed the characteristic surface plasmon band at 410–440 nm for the existence of Ag nanoparticles within the hydrogel matrix. The swelling kinetics demonstrated that the transport mechanism belongs to non-Fickian mode for the PDMAA-g-PVA hydrogels and PDMAA-g-PVA/Ag nanocomposite ones. With increasing the DMAA proportion, the r₀ and S_∞ are enhanced for each system. The assembly of Ag nanoparticles and the swelling behavior may be controlled and modulated by means of the compositional ratios of PVA to DMAA and reduction systems.     

Comparison of organic ligand effects on morphology control between disc-like zeolite L crystals and zeolite A and W crystals

Thin disc-like zeolite L crystals were hydrothermally synthesized in the presence of organic ligands, such as catechol and acetylacetone. Thus, zeolite L crystals with many channel entrances on the surface were obtained. Although it was speculated that the complex formation of the organic ligands with aluminate ion in the reaction gels influenced the formation of the disc-like zeolite L crystals, bis-tris which can form a complex with aluminate ion did not show the same effect on zeolite L morphology as catechol and acetylacetone. So, the influence of ligands on crystal morphology was compared between some types of zeolites. Both catechol and bis-tris increased crystal size of zeolite A and zeolite W. Thus, the ligand effect on crystal shapes of disc-like zeolite L was different from that of zeolite A and zeolite W. Moreover, the ligand effect was also examined under agitation. For zeolite L synthesis with the addition of catechol and acetylacetone, the obtained crystalline phase was changed to zeolite T by agitation. Hence, other than the complex formation of aluminate ion with ligands such as catechol, the heterogeneous distribution of reactants probably played an important role in the formation of the disc-like zeolite L crystals.     

Effect of preparation methods and optical band gap of ZnO nanomaterials on photodegradation studies

Improving of photo-oxidative efficiency of ZnO has become of importance to meet the requirements of environmental protection. In this research, ZnO nanomaterials prepared by three different methods (thermal decomposition, precipitation and sol-gel-combustion using metal nitrate and different fuels (urea, oxalic acid and citric acid)). Various molar ratios of citric acid to salt used as variable parameter (0.50, 0.75, 1.00, 1.25, and 1.50). These nanomaterials were characterized by studying their structural, morphological, surface and optical properties. The photocatalytic activity was evaluated by photocatalytic degradation of Remazol Red RB-133 (RR) under UV-light irradiation. The obtained results showed that the photocatalytic efficiency was affected by preparation method, type and ratio of fuel to salt. The optimum is a gel precursor containing zinc nitrate and citric acid prepared in the molar ratio of 1. The highly active nanomaterial was applied for photocatalytic degradation of mixtures of two dyes – (RR) and Methylene Blue (MB).     

Fabrication of hierarchical structured superhydrophobic Copper surface by in-situ method with micro/nano scaled particles

Chlorofluoro silane (CFSi) decorated Copper doped SiO₂ coated Copper surface was synthesized by an in-situ method and characterized by various analytical tools like Fourier Transform Infrared (FTIR) Spectroscopy, UV-Visible spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and contact angle measurements. In the present investigation, the bulk Copper plate was served as a resource material for the generation of Copper with simultaneous chemical etching reaction in an aqueous acidic and alcoholic medium. The modified Copper surface has a contact angle of 152°.     

Preparation of graphene nanosheets/SnO2 composites by pre-reduction followed by in-situ reduction and their electrochemical performances

The Graphene nanosheets/SnO₂ composites were synthesized using stannous chloride to restore the semi-reduction graphene oxide (SRGO) under a simple hydrothermal reduction procedure. First graphene oxide was pre-reduced by glucose for a certain time to get SRGO, which keeps the good water-solubility of graphite oxide (GO) and has a good conductivity like graphene nanosheets. The higher electrostatic attraction between SRGO and Sn²⁺ makes SnO₂ nanoparticles tightly anchor on the graphene sheets in the hydrothermal reduction process. The formation mechanism of the composite was investigated by SEM, TEM, XRD, AFM and Raman. Moreover, the electrochemical behaviors of the Graphene nanosheets/SnO₂ nanocomposites were studied by cyclic voltammogram, electrical impedance spectroscopy (EIS) and chronopotentiometry. Results showed that the Graphene nanosheets/SnO₂ composites have excellent supercapacitor performances: the specific capacitance reached 368 F g⁻¹ at a current density of 5 mA cm⁻², and the energy density was much improved to 184 Wh kg⁻¹ with a power density of 16 kW kg⁻¹, and capacity retention was more than 95% after cycling 500 cycles with a constant current density of 50 mA cm⁻². The experimental results and the thorough analysis described in this work not only provide a potential electrode material for supercapacitors but also give us a new way to solve the reunification of the graphene sheets.     

Preparation and properties of novel epoxy/graphene oxide nanosheets (GON) composites functionalized with flame retardant containing phosphorus and silicon

2-(Diphenylphosphino)ethyltriethoxy silane (DPPES) was grafted onto the surface of graphene oxide nanosheets (GON) via a condensation reaction. X-ray photoelectron spectroscopy, X-ray diffractometry, Fourier transform infrared spectroscopy and Raman spectroscopy verify that DPPES did not only covalently bond to GON as a functionalization moiety, but partly restored its conjugated structure as a reducing agent. DPPES on graphene sheets oxide was observed by transmission electron microscopy, and contributed to the favorable dispersion of DPPES-GON in nonpolar toluene. Additionally, the flame retardancy and thermal stability of epoxy/DPPES-GON nanocomposites that contain various weight fractions of DPPES-GON were studied using the limiting oxygen index test, UL-94 test and by thermogravimetric analysis in nitrogen. The composites containing 10 wt% DPPES-GON can pass V-0 rating in UL-94 test. Adding 10 wt% DPPES-GON in epoxy greatly increased the char yield and LOI by 42% and 80%, respectively. Epoxy/DPPES-GON nanocomposites with phosphorus, silicon and graphene layer structures were found to exhibit much greater flame retardancy than neat epoxy. The synergistic effects among silicon, phosphorus and GON can improve the flame retardancy of epoxy resin.     

Synthesis,characterization and dielectric behavior of (ES)-form polyaniline/cerium(III)-nitrate-hexahydrate composites

In this study, morphological, thermal and dielectric properties of chemically synthesized polyaniline (PANI) and its cerium(III)-nitrate-hexahydrate (Ce(NO₃)₃·6H₂O) doped composites with various doping levels were investigated. Characteristic bands of emeraldine salt (ES)-form of PANI was clearly observed in UV–visible (UV–vis) and Fourier transform infrared (FTIR) spectroscopies. Thermal analyses carried out by Differential scanning calorimetry (DSC) indicated that there was a glass transition at about 70 °C and the doping process increased thermal stability of the polymer. In the surface morphologies examined by scanning electron microscopy (SEM), various microstructures depending on the doping level were observed. Real dielectric constant exhibited a significant decrease due to the increase in the doping level especially at higher frequencies. Conductivity mechanisms of PANI and its composites were investigated by universal power law as conventional models could not provide a complete picture for all the samples. Dramatic increases up to 45, 35 and 30 times in the conductivity of 10% doped PANI were observed for 300, 350 and 400 K, respectively.     

Silica-coated iron nanoparticles: Shape-controlled synthesis,magnetism and microwave absorption properties

Body-centered cubic (bcc) phase iron nanocrystals with granular, rod-like and flaky shapes were prepared through a simple surfactant-controlled chemical reduction route. In view of extra stability and enhanced manipulative ability, thus-prepared iron nanoparticles were morphology-retained modified with a thin silica shell through a Stöber process. A serial of techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectrometer (XPS), thermogravimetry (TG), vibrating sample magnetometer (VSM) and scalar network analyzer (SNA) were used to characterize the iron particles before and after silica coating. Results showed that the surface silica coating could effectively improve the oxidation resistance and microwave absorption performance of iron particles, while slightly influenced their magnetic properties. Furthermore, the flaky Fe@SiO₂ nanocapsules particles exhibited better microwave absorption performance than that of the granular and rod-like counterparts, which could be ascribed to the shape effect.     

Sand flower layered double hydroxides synthesized by co-precipitation for CO2 capture: Morphology evolution mechanism,agitation effect and stability

Magnesium–aluminum layered double oxides (Mg–Al-LDO) derived from calcination of layered double hydroxides (LDH) is one of the most capable candidates for CO₂ capture. LDHs with sand flower and dense layered morphology are prepared by co-precipitation method at pH = 10 under different condition with and without stirring. The sand flower LDH prepared with stirring has better CO₂ adsorption performance. Additionally, the sand flower morphology under calcination can be preserved very well, while the morphology can only remain stable below 100 °C after hydrothermal treatment and reconstruction. The formation process of the sand flower LDH is investigated in detail and the related morphology evolution mechanism is proposed. This study can provide insight into the effect of mechanical interaction on chemical reaction and give a new way to control the morphology of layered materials.     

Effect of different solvents in the synthesis of LaCoO3 nanopowders prepared by the co-precipitation method

This paper describes the obtention of LaCoO₃ nanopowders by the co-precipitate method with ammonium hydroxide from solvent such as water, ethyl alcohol and ethylene glycol. The crystalline structure and average particle size are dependent of type-solvent. The XRD patterns indicated that the LaCoO₃ nanopowders prepared with water and ethyl alcohol exhibit a pure perovskite-type LaCoO₃ in the rhombohedra structure. The average diameter of the particles prepared with ethyl alcohol and water are 27 ± 4.49 nm and 64.4 ± 12.92 nm respectively. High resolution transmission electron microcopy revealed an oriented attachment mechanism for the growth of aggregated LaCoO₃ nanocrystals. Room temperature magnetization results of the heat treated LaCoO₃ nanopowders exhibited a paramagnetic behavior. The average particle size and formation temperature of LaCoO₃ obtained in this study is comparatively lower than those reported in the literature.     

A comprehensive study of soft magnetic materials based on FeSi spheres and polymeric resin modified by silica nanorods

A novel soft magnetic composite (SMC) based on spherical FeSi particles precisely covered by hybrid phenolic resin was designed. The hybrid resin including silica nano-rods chemically incorporated into the phenolic polymer matrix was prepared by the modified sol–gel method. A chemical bridge connecting silica nano-rods with the base polymeric net was verified by FTIR, ¹³C and ²⁹Si NMR spectroscopy, whereas the shape and size of silica nano-rods were determined by TEM. It is shown that the modification of polymeric resin by silica nano-rods generally leads to the improved thermal and mechanical properties of the final samples. The hybrid resin serves as a perfect insulating coating deposited on FeSi particles and the core–shell particles can be further compacted by standard powder metallurgy methods in order to prepare final samples for mechanical, electric and magnetic testing. SEM images evidence negligible porosity, uniform distribution of the hybrid resin around FeSi particles, as well as, dimensional shape stability of the final samples after thermal treatment. The hardness, flexural strength and density of the final samples are comparable to the sintered SMCs, but they simultaneously exhibit much higher specific resistivity along with only slightly lower coercivity and permeability.     

Preparation and properties of poly(urea–formaldehyde) microcapsules filled with epoxy resins

The poly(urea–formaldehyde) (PUF) microcapsules filled with epoxy resins have potential for self-healing or toughening polymeric composites. A series of PUF microcapsules containing epoxy resins were synthesized by selecting different process parameters including surfactant type, surfactant concentration, adjusting time for pH value and heating rate. The effects of process parameters on the size and surface morphology of microcapsules were discussed. The storage stability, solvent resistance and the mechanical strength of microcapsules were investigated. The morphology of microcapsules was observed using scanning electron microscopy (SEM) and optical microscopy (OM). The results indicate that the formation of microcapsules is affected by the surfactant type. The size of microcapsules can be controlled by the surfactant concentration. The surface morphology of microcapsules can be adjusted by the surfactant concentration, the adjusting time for pH and the heating rate. The microcapsules prepared by using surfactant sodium dodecylbenzene sulfonate (SDBS) show good storage stability, excellent solvent resistance and appropriate mechanical strength.     

Ultrafine PZT based ceramics synthesized by auto-ignition of metal–polymer gel: Enhanced sinterablity and higher remnant polarization

PbZr_0.53Ti_0.47O₃ (PZT) nanoparticles within the size range 1–8 nm were synthesized by auto-ignition of metal–polymer gel in single step without any calcination. The auto ignitable gel was obtained by adjustment of nitrate/polymer molar ratio. The amount of the carbonaceous residue in the as-ignited powder was negligible. The as-ignited ultrafine PZT powders showed excellent sinterability and were sintered directly to about 96% of the theoretical density at 1000 °C without any further calcination. Dielectric loss of the sintered PZT sample at room temperature was very low and increased very slowly up to 300 °C, which is advantageous for dielectric application point of view. The measured value of remnant polarization of the 1100 °C-sintered sample was 54.2 μC/cm², which is considerably higher than the value reported in the literature for the same composition.     

Investigation of size dependent structural and optical properties of thin films of CdSe quantum dots

Cadmium selenide (CdSe) quantum dots were grown on indium tin oxide substrate using wet chemical technique for possible application as light emitting devices. The structural, morphological and luminescence properties of the as deposited thin films of CdSe Q-dot have been investigated, using X-ray diffraction, transmission electron microscopy, atomic force microscopy and optical and luminescence spectroscopy. The quantum dots have been shown to deposit in an organized array on ITO/glass substrate. The as grown Q-dots exhibited size dependent blue shift in the absorption edge. The effect of quantum confinement also manifested as a blue shift of photoluminescence emission. It is shown that the nanocrystalline CdSe exhibits intense photoluminescence as compared to the large grained polycrystalline CdSe films.     

Effect of citrate on poly(vinyl pyrrolidone)-stabilized gold nanoparticles formed by PVP reduction in microwave (MW) synthesis

Colloidal PVP (poly(vinyl pyrrolidone))–stabilized gold nanoparticles (PVP–AuNPs) are synthesized in aqueous solution with PVP as a reducing and stabilizing agent using a short microwave (MW) heating duration of 1 min. The size and uniformity of the synthesized PVP–AuNPs can be varied by modifying the concentration of sodium citrate (Na₃Ct), which acts predominantly as mediator of the stability of PVP–AuNP formation during the rapid synthesis. Due to the increase in the Na₃Ct concentration, the number of citrate ions adsorbed on the growing surface of AuNPs increase, and less reactive gold solute complexes are formed, leading to slow stable reactions that produce small, uniform colloidal PVP–AuNPs. We therefore demonstrate that by adjusting the Na₃Ct concentration used in the PVP reduction, the diameter of PVP–AuNPs was varied from 19.47 ± 3.97 nm to 7.94 ± 0.14 nm when using constant concentrations of chloroauric acid (HAuCl₄) and PVP.     

Preparation and structure development of NiO/YSZ nanocomposite powders by urea hydrolysis

NiO/YSZ composite powders, with various NiO contents, have been prepared by the urea hydrolysis method. The crystallization behavior and microstructure of composite powders has been studied in detail, using differential scanning calorimetry analysis, X-ray diffraction, and transmission electron microscope. The results indicated that the actual NiO content of the NiO/YSZ powders largely deviated from the nominal value, and finally reached a saturated value. The NiO addition would retard the crystallization of NiO/YSZ composite. When the calcination temperature was increased, the NiO crystallites first precipitated at around 500 °C, and then the YSZ phase presented at about 600 °C. The calcined powders consist of NiO/YSZ nanocomposite particles, which are comprised of nano-sized NiO and YSZ crystals. In addition, with the aid of H₂ plasma treatment, it is easier to distinguish the Ni and YSZ phases of Ni/YSZ cermets after sintering and subsequent reduction. This could reveal that such Ni/YSZ cermets exhibited a uniform microstructure that has fine Ni particles homogeneously dispersed within the YSZ matrix. As the NiO content was increased, the size and density number of the Ni phase within an YSZ matrix was increased.     

Morphology control and luminescence properties of YAG:Eu phosphors prepared by spray pyrolysis

Starting from nitrate aqueous solutions with citric acid and polyethylene glycol (PEG) as additives, Y₃Al₅O₁₂:Eu (YAG:Eu) phosphors were prepared by a two-step spray pyrolysis (SP) method. The obtained YAG:Eu phosphor particles have spherical shape, submicron size and smooth surface. The effects of process conditions of the spray pyrolysis on the crystallinity, morphology and luminescence properties of phosphor particles were investigated. The emission intensity of the phosphors increased with increasing of sintering temperature and solution concentration due to the increase of the crystallinity and particles size, respectively. Adequate amount of PEG was necessary for obtaining spherical particles, and the optimum emission intensity could be obtained when the concentration of PEG was 0.10 g/ml in the precursor solution. Compared with the YAG:Eu phosphor prepared by citrate-gel (CG) method with non-spherical morphology, spherical YAG:Eu phosphor particles showed a higher emission intensity.     

Dielectric properties of CaCu3Ti4−xCoxO12 (x = 0.10, 0.20, and 0.30) synthesized by semi-wet route

Undoped CaCu₃Ti₄O₁₂ and cobalt doped CaCu₃Ti_4−xCo_xO₁₂ (CCTCO) with x = 0.10, 0.20 and 0.30 were prepared by semi-wet route for the first time using solid TiO₂ powder and metal nitrate solutions. XRD analysis confirmed the formation of single-phase material in the samples sintered at 900 °C for 6 h. Structure does not change on doping with cobalt and it remains cubic in all the three compositions studied. Lattice parameter increases slightly with cobalt doping. SEM micrographs of the CaCu₃Ti_4−xCo_xO₁₂ samples show that the grain size is in the range of 2–8 μm for these samples. EDX studies show the percentage of elements along with the grain and grain boundary, which confirm the rich phase of copper oxide at grain boundaries. Dielectric properties have been measured as a function of temperature and frequency.