Oxygen release–absorption properties and structural stability of Ce0.8Fe0.2O2−x

Oxygen release–absorption properties and structural stability of Ce–Fe mixed oxides (Ce_0.8Fe_0.2O_2?x) with different calcination temperatures (600–1000 °C) were investigated and correlated to their oxygen storage capacity. Iron ions could be incorporated into the CeO₂ lattice to form a solid solution after calcination at low temperatures, but such solid solution was unstable under high-temperature thermal treatments. High-temperature (≥800 °C) calcination resulted in the appearance of exposed Fe₂O₃ phases on the surface of the solid solution, and this structural evolution finally affected the reduction behavior. The Fe³⁺ reduction from the Ce–Fe oxide solid solution was easier than the bulk Fe₂O₃ particles, while the small Fe₂O₃ particles in close contact with CeO₂ could enhance the reducibility of cerium oxides. The strong interaction between the exposed small Fe₂O₃ particles and the solid solution made the Ce–Fe mixed oxides possess good reduction stability and high oxygen storage capacity (OSC) even after repeated redox treatments. Such interactions were absent toward the physically mixed sample. An unusual enhancement on the reducibility of Ce–Fe mixed oxides was observed after a successive redox treatment. Large oxygen evolution appeared at around 600 °C for the recycled samples, and the OSC rose to 1.31 mmol-O₂/g after six redox cycles. The XRD, Raman, and TEM analyses revealed that the material structure of the mixed oxides was stabilized to have an inter-region between the Fe₂O₃ particles and the solid solution after the redox treatment. It was concluded that such microstructural evolutions of composite particle from solid solution under redox conditions brought beneficial property to the OSC of the Ce–Fe mixed oxides.     

Electromagnetic properties and microwave-absorption properties of BaTiO3/BaZn2Fe16O27 composite in 2–18 GHz

BaTiO₃/BaZn₂Fe₁₆O₂₇ composites were prepared by traditional solid state method, BaTiO₃ nano particles were coated around with the BaZn₂Fe₁₆O₂₇ ferrite with hexagonal plate structure. The prepared composite particles were characterized with X-ray diffraction, scanning electron microscopy, and vector network analyzer. With the increase of BaTiO₃ content, the complex permittivity of the composites increases, while the complex permeability decreases; The sharp peaks of the magnetic loss appear at 8.72 and 14.0 GHz, and the peak values are 1.64 and 0.69, respectively, when the mole ratio of BaZn₂Fe₁₆O₂₇/BaTiO₃ is 10:5; The matching thickness of BaTiO₃/BaZn₂Fe₁₆O₂₇ composites is 2.5 mm when the mole ratio of BaZn₂Fe₁₆O₂₇/BaTiO₃ is 10:5, and the minimum reflection loss reaches ?33.49 dB at 8.56 GHz; There are more than one loss peak appeared in the reflection loss cures, and the effective absorption frequency (RL < ?10 dB) ranges from 7.92 to 14.96 GHz with a thickness of 3 mm.     

Sol–gel synthesis,characterization and microwave absorbing properties of nano sized spherical particles of La0.8Sr0.2Mn0.8Fe0.2O3

LSMFO (La_0.8Sr_0.2Mn_0.8Fe_0.2O₃) nano sized powders were synthesized by modified EDTA assisted sol–gel synthesis using EDTA:metal ion = 0.5. Another new synthesis method, sol–gel self combustion using PVA, was also devised for the synthesis of LSMFO and the effects of the PVA mole ratio was investigated. The characterization techniques, XRD, FE-SEM, TG/DTA and ICP, confirmed the formation of the pure phase LSMFO by both the methods. The material attained spherical morphology instead of previously reported rod like structure. Sizes of synthesized powders vary from 14.5 nm to 23 nm. TG/DTA results suggest that LSMFO can be synthesized by the self combustion at temperatures as low as 200 °C. Synthesized LSMFO has excellent microwave absorbing properties in the range of 0.05–18 GHz and appreciable effective bandwidth, 2 GHz, with microwave absorption in excess of 10 dB and peak reflection loss of 25 dB which suggests that the materials can be used as effective microwave absorbers.     

Structural and microwave absorption properties of nanostructured Fe–Co alloys

We analyzed nanostructured Fe₆₀Co₄₀ alloy obtained by mechanical alloying using a planetary ball mill. The prepared powders were characterized using X-ray Diffraction (XRD), Laser particle-measurement, scanning electron microscopy (SEM), X band waveguide and cavity resonator associated with Network analyzer. Obtained results are discussed according to milling time.XRD patterns show after 12 h of milling the formation of a disordered solid solution having body-centerd cubic (bcc) structure. After 36 h milling, morphological studies indicated that the average crystallites size is around 13 nm and the particles average diameter is about 3.6 μm. The microwave absorbing characteristic was enhanced between 0 and 54 h of milling (from ?0.8 to ?13.807 dB) with decreasing in the relative dielectric permittivity ε_r.     

Preparation and microwave absorbing properties of hollow glass microspheres/Fe3O4/Ag composites with core–shell structure

The low-density, conductive and magnetic hollow glass microspheres (HGM)/Fe₃O₄/Ag composites have been successfully synthesized via co-precipitation and chemical plating method. The morphology, composition, microstructure, magnetic and microwave absorbing properties of the composites were investigated based on the analyses of the results using scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction, vibrating sample magnetometer and vector network analyzer. The results showed that the HGM/Fe₃O₄ composites were successfully prepared, and the coating layers on the surface of HGM are compact and continuous. Moreover, the final composites were completely covered with Ag nanoparticles. With the addition of Ag nanoparticles, the saturation magnetization of the HGM/Fe₃O₄ composites reduces from 32.08 to 14.77 emu/g, whereas its conductivity increases to 0.48 S/cm. The reflection loss (R) of HGM/Fe₃O₄/Ag composites is lower than ?10 dB at 8.2–8.7, 9.6–10.8 and 11.4–11.9 GHz, and the minimum loss value is ?19.1 dB at 9.9 GHz.     

Synthesis and microwave absorbing properties of Mn–Zn nanoferrite produced by microwave assisted ball milling

In this paper, well dispersed spinel Mn_xZn_1?xFe₂O₄ (x = 0.3,0.5 and 0.7) were obtained by microwave assisted ball milling at 2.45 GHz through only one step. The synthesized products were characterized by X-ray diffraction, high resolution transmission electron microscope, vibration sample magnetometer, and vector network analysis. Synthesized Mn–Zn nanoferrite showed the saturation magnetization reached 84.91emu/g when the x was 0.7 and the largest magnetic loss tangent at the frequency of 2.45 GHz. Microwave absorbing properties of these composites were studied at the frequency range of 2–18 GHz. Two microwave reflection loss peaks appeared for all the spinel ferrite. When x was 0.5, the minimum reflection loss appeared at the highest frequency. When x was 0.7, these two minimum reflection loss peaks, ?17.36 and ?48.13 dB, were calculated with the ?10 dB bandwidth at the frequency ranges of 2.24–5.04 and 13.28–14.88 GHz, respectively. Resonance reflection loss peaks shifted to lower frequencies when the matching thickness increased.     

Microwave dielectric properties and low-temperature sintering of (Zn0.8Mg0.2)2SiO4–TiO2 ceramics with Li2O–B2O3

Effects of Li₂O–B₂O₃ on the sintering behavior and the microwave dielectric properties of (Zn_0.8Mg_0.2)₂SiO₄–TiO₂ ceramics were investigated as a function of Li₂O–B₂O₃ content and sintering temperature. The Li₂O–B₂O₃ combined additives successfully reduced the sintering temperature of (Zn_0.8Mg_0.2)₂SiO₄–TiO₂ ceramics from 1,250 °C to 900 °C. With the increase of Li₂O–B₂O₃ content, the TiO₂ phase decreased and the unknown second phase increased, which led to the dielectric constant (ε _r ) and the maximum Q × f value decrease, and the temperature coefficient of resonant frequency (τ _f ) shift to a negative value. The specimens with 3 wt%Li₂O–B₂O₃ sintered at 900 °C for 2 h showed ε _r of 8.84, Q × f value of 15,500 GHz, and τ _f of 17.8 ppm/°C. And the material was compatible with Ag electrodes, which made it a promising ceramic for low temperature co-fired ceramics technology application.     

Preparation and excellent microwave absorption properties of silver/strontium ferrite/graphite nanosheet composites via sol–gel method

In this experiment, excellent microwave absorption properties of Ag/SrFe₁₂O₁₉/NanoG were prepared via a two-step reaction. First, the SrFe₁₂O₁₉ was deposited on the surface of NanoG by sol–gel method. Then, the Ag/SrFe₁₂O₁₉/NanoG was perpared via electroless plating. The obtained ternary composites were analyzed by SEM, XRD, VSM and a vector network analyzer. The results indicated that the NanoG is covered by SrFe₁₂O₁₉ and SrFe₁₂O₁₉/NanoG is completely wrapped by silver layer. Ag/SrFe₁₂O₁₉/NanoG’s conductivity and Ms are 2.2 S/cm and 15.4 emu/g, respectively. Measurement of the vector network analyzer suggests that the microwave absorbing properties of Ag/SrFe₁₂O₁₉/NanoG are better than those of NanoG and SrFe₁₂O₁₉/NanoG. Its maximum reflection loss value can reach ?29 dB at 10.9 GHz when the thickness is 2.0 mm, and it’s the best match thickness of the composites in the X band.     

M?ssbauer and Magnetic Studies of Co0.5Mn0.5Fe2O4 and Mn0.1Mg0.2Co0.7 Fe2O4 Nanoferrites

We report the magnetic properties of Mn_0.5Co_0.5Fe₂O₄ and Mn_0.1Mg_0.2Co_0.7Fe₂O₄ nanoferrites. The compounds were synthesized by a glycol-thermal method with average particle sizes of about 13?nm and 8?nm, respectively. The M?ssbauer measurements were done at 300?K. The distribution of cations between tetrahedral?(A) and octahedral?(B) sites is investigated. The M?ssbauer spectra indicate ferrimagnetic behavior of the compound. Field cooled (FC) and zero field cooled (ZFC) magnetizations were performed by a Superconducting Quantum Interference Device (SQUID) magnetometer from 4?C380?K. Variation of the magnetizations with the applied fields (up to 50 kOe) were recorded at isothermal temperatures 4, 50, 100, 200 and 300?K. An increase in FC magnetization is observed with increasing applied field. This is explained based on superparamagnetic behavior of the particles.     

Manganese dependence of microstructure and mechanical properties in Fe–Mn alloy

Two Fe–Mn alloys with relatively low Mn content were designed. The microstructure characteristics and resultant mechanical properties were investigated in detail by means of electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction. The results show that the formation of α′-martensite is effectively suppressed and the yield strength and total elongation are significantly enhanced by increasing Mn content from 12 to 13 mass%. A great amount of α′-martensite can effectively enhance strain hardening rate, but they deteriorate ductility. The austenite grain is always divided by multiple-variant ?-martensite plate. In addition, the prior austenite grain boundaries and austenite/?-martensite interfaces can act as obstacle to suppress the growth of ?-martensite plates.     

Preparation,characterization and microwave absorption properties of bamboo-like β-SiC nanowhiskers by molten-salt synthesis

Bamboo-like and cubic single-crystalline silicon carbide nanowhiskers (SiC_NWs) were synthesized using multiwalled carbon nanotube via a process of calcination in the molten-salt circumstance. The system was heated to 1,250 °C and maintained for 6 h in argon atmosphere, and obtained the sample. The as-prepared sample was characterized by a series of techniques. Especially, the microwave absorption properties of SiC_NWs/paraffin composites (30 wt%) were investigated over 2–14 GHz. The result shows the optimal reflection loss can reach ?48.1 dB at 13.52 GHz when the thickness of the match is only 1.9 mm. The excellent microwave absorption properties of the SiC_NWs/paraffin composites due to the dielectric loss would make it as a promising candidate for the application of absorbing materials. In addition, a possible growth mechanism of SiC_NWs was also discussed.     

Preparation and properties of electroless Ni–Zn–P alloy films

Electroless deposition of Ni–Zn–P layers was studied on steel electrodes by varying the bath temperature (40–90°C), pH and chemical composition. The deposition parameters were optimized. Alloys containing 70–86 wt % Ni, 6–20 wt % Zn and 6–10 wt % P are obtained at 20 m h^–1 and 85°C. Corrosion measurements were performed in aerated 5% sodium chloride solution, the corrosion potential and current density are, respectively, –0.49 V/SCE and 2.6 A cm^–2.     

Preparation,mechanical properties and in vitro degradability of wollastonite/tricalcium phosphate macroporous scaffolds from nanocomposite powders

A new class of scaffolds with a gain size of 200 nm was prepared from wollastonite/tricalcium phosphate (WT) nanocomposite powders (termed “nano-sintered scaffolds”) through a two-step chemical precipitation and porogen burnout techniques. For a comparison, WT scaffolds with a grain size of 2 μm were also fabricated from submicron composite powders (termed “submicron-sintered scaffolds”) under the same condition. The resultant scaffolds showed porosities between 50 ± 1.0% and 65 ± 1.0% with a pore size ranging from 100 μm to 300 μm. The WT nano-sintered scaffolds exhibited compressive strength and elastic modulus values that were about twice that of their submicron-sintered counterparts. The in vitro degradation tests demonstrated that the degradability could be regulated by the grain size of bioceramics. The decreased specific surface area of pores in the nano-sintered scaffolds led to their reduced degradation rate. The mechanical properties of the nano-sintered scaffolds exhibited less strength loss during the degradation process. The WT macroporous nano-sintered scaffolds are a promising and potential candidate for bone reconstruction applications.     

Preparation and electromagnetic wave absorption properties of CoNi@SiO<Subscript>2</Subscript> microspheres decorated graphene–polyaniline nanosheets

In this work, we successfully parepared the quaternary composites of CoNi@SiO₂@graphene@PANI via a four-step method. The structures, chemical composition and morphologies of obtained composites are analyzed in detail. The electron microscopy results show spherical CoNi@SiO₂ particles evenly dispersed into the surface of graphene@polyaniline nanosheets. The electromagnetic parameters indicate that CoNi@SiO₂@graphene@PANI exhibits enhanced electromagnetic absorption properties compared to CoNi@SiO₂, which can be mainly attributed to the improved impedance matching and multi-interfacial polarization. The maximum reflection loss of CoNi@SiO₂@graphene@PANI can reach ??43 dB at 15.4 GHz and the absorption bandwidth with the reflection loss exceeding ??10 dB is 5.7 GHz (from 12.3 to 18 GHz) with the thickness of 2 mm. Our results demonstrate the quaternary composites composed of CoNi@SiO₂ microparticles and rGO–PANI nanocomposites can serve as light weight and high-performance EM absorbing material.     

Growth process and microwave absorption properties of nanostructured γ-MnO2 urchins

A low-temperature hydrothermal method was developed to synthesize urchinlike γ-MnO₂ nanostructures. Time-dependent evolutions of morphology and crystallinity were investigated to explore the growth mechanism of the γ-MnO₂ urchins. The results show that the growth process of the γ-MnO₂ urchins occurs in two main stages, which are the generation of γ-MnO₂ microspheres and the following epitaxial growth of γ-MnO₂ nanoneedles on the surface of the initial microspheres. Microwave absorption properties of the urchinlike γ-MnO₂ nanostructures were studied in terms of complex permittivity and permeability. An effective absorption bandwidth (reflection loss lower than −10 dB) of 8.8 GHz was achieved from the γ-MnO₂/paraffin wax composite.     

High Dielectric Permittivity and Maxwell–Wagner Polarization in Magnetic Ni0.5Cu0.3Zn0.2Fe2O4 Ceramics

Nanocrystalline Ni_0.5Cu_0.3Zn_0.2Fe₂O₄ (NCZFO) powder was fabricated by a modified sol?Cgel method and then the compacted powder of NCZFO was sintered at 950, 1000, and 1100?^°C for 6 h. The dielectric and electrical properties of sintered samples were investigated as functions of frequency and temperature. All of the NCZFO samples exhibit the high dielectric response behavior and show the Debye-like relaxation, which is attributed to the Maxwell?CWagner polarization and thermally activated mechanisms. The impedance spectroscopy analysis reveals that the NCZFO ceramics are electrically heterogeneous. The sintering temperature has significant influence on the dielectric dispersion behavior of the NCZFO samples, which should be mainly attributed to the large variation of the grain conduction activation energies.