Facile synthesis of ultra-long α-MnO2 nanowires and their microwave absorption properties

Well crystallized α-MnO₂ nanowires (NWs) with an average diameter of about 40 nm and an average length of about 30 μm were successfully synthesized by hydrothermal method. The complex permittivity and permeability of α-MnO₂ NWs/paraffin composites with 20 vol.% α-MnO₂ NWs were measured in a frequency region from 0.1 to 13 GHz. The value of maximum reflection loss of the composites with 20 vol.% α-MnO₂ NWs is approximately − 35 dB at 3.13 GHz with a thickness of 3.6 mm, and the bandwidth corresponding to reflection loss below − 10 dB is higher than 1.8 GHz with a lower thickness of 1.2 mm.     

Facile synthesis of α-MnO2 nanorods at low temperature and their microwave absorption properties

One-dimensional α-MnO₂ nanorods were fabricated by using low-temperature water-bathing chemical precipitation method at 80 °C. The crystalline structures, morphological evolution process and microwave absorption properties were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and measurement of electromagnetic parameters. The results show that the morphological structures and electromagnetic properties have close relationship with the reaction time. With the prolonging of the treatment time, the as-synthesized products turn from microspheres constituted of tiny dendrites to nanorods with diameters of 20–30 nm and lengths up to 1–2 μm. The electromagnetic characterization shows that the dielectric constants and magnetic permeability values show decreasing trends with the increasing frequency, however, the dielectric and magnetic loss tangents all increase with frequency. The electromagnetic absorption properties of the products have close relationship with the morphologies and thicknesses of the samples. With a thickness of 3 mm, an absorbing peak value of −25 dB was achieved for the sample treated for 24 h. The microwave absorption properties of MnO₂ can be attributed mainly to interfacial polarization, space charge polarization and relaxation phenomena.     

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.     

Hydrothermal synthesis and catalytic properties of α- and β-MnO2 nanorods

One-dimensional α-MnO₂ and β-MnO₂ single-crystalline nanostructures were prepared by hydrothermal process. The products were characterized in detail by multiform techniques: X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Experimental results indicate that the temperature plays important roles in determining produce α-MnO₂ and β-MnO₂ nanorods. In addition, the as-prepared α-MnO₂ and β-MnO₂ nanorods showed excellent catalytic performance in the Fenton-like reaction.     

Constructing γ-MnO2 hollow spheres with tunable microwave absorption properties

Hollow γ-MnO₂ sphere was obtained by annealing the precursor at 400 °C with different holding time. The influences of different holding time on the morphology and crystalline structure of final products have been discussed in detail, and the microwave absorption properties of the as-products were also investigated. The results exhibited that finer crystalline feature of the γ-MnO₂ and larger pore size in the hollow γ-MnO₂ sphere could be obtained with the extended holding time. The MnO₂/paraffin composites (50 wt% loading) present extraordinary microwave absorption performance, and the minimum reflection loss (RL) values is −51.3 dB at 4.9 GHz with the thickness of 3.5 mm. The excellent electromagnetic absorption properties can be ascribed to the hollow structure, perfect impedance matching behavior and the multiple interface polarization effect.     

Fast synthesis and formation mechanism of γ-MnO2 hollow nanospheres for aerobic oxidation of alcohols

γ-MnO₂ hollow nanospheres of about 300-800 nm in size have been synthesized by a fast 1-h 2-step process in the presence of an excess amount of Mn²⁺ in aqueous solution without using any templates, hydrothermal processes and catalytic routes. The evolution of morphologies evidenced that the fast formation mechanism of the γ-MnO₂ hollow nanospheres in the presence of the excess amount of Mn²⁺ in solution followed the “Ostward ripening” process. The as-synthesized γ-MnO₂ hollow nanospheres showed high catalytic activity and selectivity in aerobic oxidation of various alcohols which was attributed to their hollow nature and larger BET specific surface area.     

Atomistic study of vibrational properties of γ-Al2O3

A study of the vibrational density of states (DOS) of γ-Al₂O₃ is presented. Four structural models from the recent literature are considered: vacant spinel model and three nonspinel models. The vacant spinel and one of the nonspinel models have unit cells with 40 atoms, while the other two models have 160 atoms. The interatomic interactions are computed using classical force fields that include Coulomb and van der Waals attractive interactions, short range repulsive interactions, as well as three-body terms. The oxygen polarizability is included via a core-shell potential. The DOS is compared with ab initio calculations recently published for the vacant spinel model. The classical and ab initio DOS show some differences for frequencies higher than 200 cm⁻¹, the ab initio having more peaks and having a frequency cutoff 100 cm⁻¹ lower than the classical DOS. The DOS of all models present some small differences. While the 160-atoms nonspinel models present a rather structureless DOS, 40-atoms models present peaks and dips relative to the 160-atoms models. The elastic constants of polycrystalline γ-Al₂O₃ are also estimated using several force fields. In general, the classical force field predict higher elastic moduli than the ab initio method. The infrared spectra of the four models are calculated.     

Synthesis and photoluminescence properties of Eu-doped γ-Ca3(PO4)2

Eu³⁺-doped (1% and 3%) γ-Ca₃(PO₄)₂ was synthesized by high-pressure and high-temperature experimental method and the samples were characterized by X-ray diffraction. The luminescence properties of samples were investigated by emission and excitation spectra. The excitation spectra of Eu³⁺-doped γ-Ca₃(PO₄)₂ showed that samples were mainly attributed to Eu³⁺–O²⁻ charge-transfer band at 270 nm, and some sharp lines were also attributed to Eu³⁺ f–f transitions in near-UV regions with the strongest peaks at 395 nm. Under the 395 nm excitation, the intense red emission peak at 611 nm was observed. The strongest line (395 nm) in excitation spectra of those phosphors matched well with the output wavelength of UV InGaN-based light-emitting diodes (LEDs) chip. The luminescent properties suggested that Eu³⁺-doped γ-Ca₃(PO₄)₂ might be regarded as a potential red phosphor candidate for near-UV LEDs.     

Growth mechanism and thermoelectric properties of β-FeSi2 matrix with Si nanowires

A novel composite material consisted of β-FeSi₂ and in situ formed Si nanowires (SiNWs) with the diameter of 50-120 nm, is prepared by mechanical alloying and annealing process. The effects of ball-milling time on the formation of SiNWs are investigated systematically. The growth way of SiNWs is named as autocatalytic growth, in which no catalyst is intentionally added to the system. The possible formation mechanisms of SiNWs are discussed in detail. Due to the increase of boundary scattering, the thermal conductivity of the composite sample with SiNWs decrease significantly. Thus, the composite sample shows excellent thermoelectric performance.     

Synthesis and electrochemical performance of β-MnO2 with semi-tubular morphology

β-MnO₂ with semi-tubular morphology has been prepared in a mixed solution of KMnO₄ and MnCl₂ by a facile hydrothermal route without using templates, catalysts, and organic reagents. The structure of the obtained β-MnO₂ is systematically investigated by XRD, SEM, and TEM. Results show that the as-prepared β-MnO₂ has novel semi-tubular morphology, and its particle shows a diameter of 300–400 nm and length up to 1–4 μm. The prepared β-MnO₂ shows a good electrochemical performance, and delivers a discharge capacity of 195 mAh g⁻¹ after 40 cycles between voltage limit of 1.5 and 4.5 V at a constant current density of 20 mA g⁻¹.     

Solution-combustion synthesis of ε-MnO2 for supercapacitors

ε-MnO₂ nanoparticles were synthesized through a single step solution combustion process without using any template or surfactants. Plate-like ε-MnO₂ materials, 50-150 nm in diameter and 20-25 nm in thickness, were obtained at a higher Mn(NO₃)₂:C₂H₅NO₂ molar ratio (i.e., 2:1), whereas spherical ε-MnO₂ particles (about 60 nm in diameter) were obtained as the Mn(NO₃)₂:C₂H₅NO₂ ratio is lower (e.g., 1:2). Electrochemical performance of the as-prepared ε-MnO₂ nanoparticles was examined. The spherical ε-MnO₂ nanoparticle sample shows a relatively higher specific capacitance of 123 F g^− 1 at the current density of 1 A g^− 1 in 1 M NaSO₄ electrolyte solution, probably due to its porous structures and higher surface areas in comparison with the plate-like counterparties.     

Preparation and electrochemical performances of α-MnO2 nanorod for supercapacitor

α-MnO₂ nanorod was prepared by chemical precipitation with surfactant as the structure-directing agent and subsequent heat treatment at 800 °C. The morphology and structure of the prepared α-MnO₂ were investigated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was revealed that the α-MnO₂ nanorod was successfully synthesized without impurities and the diameter of the nanorod was less than 50 nm. In cyclic voltammetry and galvanostatical discharge-charge test, the α-MnO₂ nanorod showed regular capacitive behaviors and good cycling stabilities and delivered a maximum capacitance of 166.2 F/g, which indicated that the α-MnO₂ nanorod was a potential good electrode material for supercapacitor application.     

Low-temperature sintering and microwave dielectric properties of 5Li2O–0.58Nb2O5–3.23TiO2 ceramics

The effect of B₂O₃ addition on the sintering, microstructure and the microwave dielectric properties of the 5Li₂O–0.58Nb₂O₅–3.23TiO₂ (LNT) ceramics have been investigated. It is found that the LNT ceramics could be sintered well at ∼880 °C with low-level doping of B₂O₃ (≤2 wt.%). Only Li₂TiO₃ solid solution (Li₂TiO₃ss) crystal structure could be detected for all the ceramics with various amounts of B₂O₃ addition from the X-ray diffraction (XRD) results. And interestingly, two phases with different color in SEM images are observed in B₂O₃-doped LNT ceramics. EDS results suggest that the two different phases are two Li₂TiO₃ss phases with different amount of Nb. In addition, there is no much degradation in the microwave dielectric properties with the B₂O₃ adding. In the case of 0.5 wt.% B₂O₃-doped samples sintered at 880 °C, good microwave dielectric properties of ?_r = 22, Q × f = 32,000 GHz, τ_f = 9.5 ppm °C⁻¹ are obtained.     

Dielectric properties and microstructure of TiO2 modified (ZnMg)TiO3 microwave ceramics with CaO–B2O3–SiO2

The low-fired (ZnMg)TiO₃–TiO₂ (ZMT–TiO₂) microwave ceramics using low melting point CaO–B₂O₃–SiO₂ as sintering aids have been developed. The influences of Mg substituted fraction on the crystal structure and microwave properties of (Zn_1−x Mg _x )TiO₃ were investigated. The result reveals that the sufficient amount of Mg (x ≥ 0.3) could inhibit the decomposition of ZnTiO₃ effectively, and form the single-phase (ZnMg)TiO₃ at higher sintering temperatures. Due to the compensating effect of rutile TiO₂ (τ_f = 450 ppm/°C), the temperature coefficient of resonant frequency (τ_f) for (Zn_0.65Mg_0.35)TiO₃–0.15TiO₂ with biphasic structure was adjusted to near zero value. Further, CaO–B₂O₃–SiO₂ addition could reduce the sintering temperature from 1150 to 950 °C, and significantly improve the sinterability and microwave properties of ZMT–TiO₂ ceramics, which is attributed to the formation of liquid phases during the sintering process observed by SEM. The (Zn_0.65Mg_0.35)TiO₃–0.15TiO₂ dielectrics with 1 wt% CaO–B₂O₃–SiO₂ sintered at 950 °C exhibited the optimal microwave properties: ε ≈ 25, Q × f ≈ 47,000 GHz, and τ_f ≈ ± 10 ppm/°C.     

The control of the diameter and length of α-MnO2 nanorods by regulation of reaction parameters and their thermogravimetric properties

α-MnO₂-type single-crystal nanorods were synthesized under hydrothermal conditions based on the redox reaction of KMnO₄ in an acidic environment. Several reaction parameters, like the reaction temperature, the reaction time and the concentration of KMnO₄ in the reaction mixture, were varied in order to determine their impact on the structure, the dimensions of the synthesized nanorods, and as well on their thermogravimetric properties. It was found that the reaction time has no significant influence on the diameter, although it has a strong influence on the length of the obtained nanorods. On the other hand, the concentration of KMnO₄ in the reaction mixture has a strong impact on both the diameter and the length. With an increasing concentration of KMnO₄ in the reaction mixture the average lengths and diameters of the isolated MnO₂ nanorods are reduced. The change in dimensions of the synthesized nanorods is reflected in their thermogravimetric properties.     

NIR luminescent Er/Yb co-doped SiO2–ZrO2 nanostructured planar and channel waveguides: Optical and structural properties

Optical and structural properties of planar and channel waveguides based on sol–gel Er³⁺ and Yb³⁺ co-doped SiO₂–ZrO₂ are reported. Microstructured channels with high homogeneous surface profile were written onto the surface of multilayered densified films deposited on SiO₂/Si substrates by a femtosecond laser etching technique. The densification of the planar waveguides was evaluated from changes in the refractive index and thickness, with full densification being achieved at 900 °C after annealing from 23 up to 500 min, depending on the ZrO₂ content. Crystal nucleation and growth took place together with densification, thereby producing transparent glass ceramic planar waveguides containing rare earth-doped ZrO₂ nanocrystals dispersed in a silica-based glassy host. Low roughness and crack-free surface as well as high confinement coefficient were achieved for all the compositions. Enhanced NIR luminescence of the Er³⁺ ions was observed for the Yb³⁺-codoped planar waveguides, denoting an efficient energy transfer from the Yb³⁺ to the Er³⁺ ion.     

Facile and template-free preparation of α-MnO2 nanostructures and their enhanced adsorbability

In this paper, nanostructured MnO₂ materials were successfully prepared through a simple and template-free hydrothermal method. X-ray diffraction pattern indicates that the as-prepared nanomaterials are α-MnO₂. Transmission Electron Microscopy and Scanning Electron Microscopy images demonstrate that nanostructured rod-clusters α-MnO₂ could be evolved from the nanorods. Brunauer-Emmett-Teller (BET) surface area measurement was employed to characterize the surface property. Moreover, the as-obtained (α-MnO₂) nanomaterials could act as an efficient adsorbent to remove Congo Red and Methlylene Blue. More significantly, the nanomaterials are nontoxic and environmentally friendly via biological methylthiazolyldiphenyltetrazoliumbromide assay experiments. Its nontoxic and enhanced adsorbability properties guarantee their safe applications in environmental protection and industrial aspects.     

Microwave-assisted synthesis and humidity sensing of nanostructured α-Fe2O3

Nanocrystalline α-Fe₂O₃ has been prepared on a large-scale by a facile microwave-assisted hydrothermal route from a solution of Fe(NO₃)₃·9H₂O and pentaerythritol. A systematic study of the morphology, crystallinity and oxidation state of Fe using different characterization techniques, such as transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy was performed. It reveals that nanostructured α-Fe₂O₃ comprises bundles of nanorods with a rhombohedral crystalline structure. The individual nanorod has 8-10 nm diameter and ∼50 nm length. The as-prepared nanostructured α-Fe₂O₃ (sensor) gives selective response towards humidity. The sensor shows high sensitivity, fast linear response to change in the humidity with almost 100% reproducibility. The sensor works at room temperature and rejuvenates without heat treatment. The as-prepared nanostructured α-Fe₂O₃ appears to be a promising humidity sensing material with the potential for commercialization.     

Low-temperature hydrothermal synthesis of α-MnO2 three-dimensional nanostructures

Single-crystalline α-MnO₂ three-dimensional nanostructures were synthesized via a novel redox reaction of KMnO₄ and Cr(NO₃)₃ under hydrothermal conditions. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDS), and high-resolution transmission electron microscopy (HRTEM). The addition of HNO₃ into the reaction has a significant effect on the morphologies of the final products. The α-MnO₂ three-dimensional nanostructures were obtained under the acidic condition, while α-MnO₂ nanowires were obtained without the addition of HNO₃. A mechanism for the growth of α-MnO₂ three-dimensional nanostructures was proposed.     

Synthesis of single-crystalline α-MnO2 nanotubes and structural characterization by HRTEM

Single-crystalline α-MnO₂ nanotubes were synthesized by hydrothermal method. The growth of α-MnO₂ nanotubes is through the formation of the core (γ-MnO₂)-shell (α-MnO₂) nanofibers, and then through the formation of the cavity by the dissolution of the core. The outer and the inner diameters of as-synthesized nanotubes are in the range from 13.3 to 39.2 nm, and from 2.0 to 10.8 nm, respectively. The lattice images on the wall and in the center correspond to the (2 2 0), and the (2 1 1) interplanar spacing of the tetragonal-structure α-MnO₂.