V. A. Zhuravlev

Structural and Physical Properties of MWNT/Polyolefine Composites

Composite materials containing multi-walled carbon nanotubes (MWNT) in polyolefine (polyethylene and polypropylene) matrix were prepared by coagulation precipitation. The morphology and structure of produced composites was investigated by scanning and transmission electron microscopy. Thermal analysis (oxidation in air) was performed for composites in temperature range 20–500°C. It was shown that the addition of MWNT increases the thermal stability of composites. Electrophysical properties of MWNT/PSt composites were investigated using a four-probe technique. Observed electrical percolation threshold of composite materials is near to 3–3.5wt%. Electromagnetic (EM) response of prepared materials was investigated in broadband region (26–37 GHz and 250–500 GHz). It was found that the EM response of MWNT/polyolefine depends on their electrophysical properties with percolation threshold near 2–3wt%.

Electrophysical and Electromagnetic Properties of Pure MWNTs and MWNT/PMMA Composite Materials Depending on Their Structure

Three types of carbon multi-walled nanotubes (MWNT) with different morphology and mean diameter (7.5, 10.5, 22 nm) were used to produce polymethylmetacrylate-based composites (MWNT/PMMA) with variable MWNT loading (0.5–5wt%). The electrophysical properties of produced composites and electromagnetic response in frequency range of 2–12 GHz were investigated. Electrical conductivity of composites depends on the MWNT type increasing with lowering of the nanotube diameter and loading in composite. Reflectance and transmittance coefficients were calculated from dielectric permittivity data and found to correlate with electrical conductivity. The highest level of EM reflection was obtained for 7.5 nm MWNT-based composites and makes the value of 0.65–0.7, whereas 22 nm MWNT/PMMA composites are more transparent for EM radiation with reflection coefficient 0.3–0.4 for the highest MWNT loading. Thus MWNT/PMMA composites demonstrate high EM shielding properties.

Structure and Properties of Multiwall Carbon Nanotubes/Polystyrene Composites Prepared via Coagulation Precipitation Technique

Coagulation technique was applied for preparation of multiwall carbon nanotube- (MWNT-)containing polystyrene (PSt) composite materials with different MWNT loading (0.5–10 wt.%). Scanning and transmission electron microscopies were used for investigation of the morphology and structure of produced composites. It was shown that synthesis of MWNT/PSt composites using coagulation technique allows one to obtain high dispersion degree of MWNT in the polymer matrix. According to microscopy data, composite powder consists of the polystyrene matrix forming spherical particles with diameter ca. 100–200 nm, and the surface of MWNT is strongly wetted by the polymer forming thin layer with 5–10 nm thickness. Electrical conductivity of MWNT/PSt composites was investigated using a four-probe technique. Observed electrical percolation threshold of composite materials is near to 10 wt.%, mainly due to the insulating polymer layer deposited on the surface of nanotubes. Electromagnetic response of prepared materials was investigated in broadband region (0.01–4 and 26–36 GHz). It was found that MWNT/PSt composites are almost radiotransparent for low frequency region and possess high absorbance of EM radiation at higher frequencies.

Radiation-thermal synthesis of W-type hexaferrites

The results of investigations of the phase composition, structural parameters, static and dynamic magnetic properties of BaCo0.7Zn1.3Fe16O27 hexaferrites obtained by the method of self-propagation high-temperature synthesis in combination with mechanochemical activation and radiation-thermal post-sintering are presented. The prospects of the proposed energy-saving approach for the production of ferrite ceramics with a hexagonal structure is shown.

Structural and magnetic properties of SHS-produced multiphase W-Type hexaferrites: Influence of radiation-thermal treatment

Characterized were phase composition, structural parameters, magnetic parameters, and microwave absorption properties of BaCo0.7Zn1.3Fe16O27 hexaferrites produced by a method combining SHS, mechanical activation, and radiation-thermal treatment. The suggested process takes advantage of its lower time and energy consumption.

Magnetic properties and microstructure of SHS-Produced co-containing hexaferrites of the Me2W system

BaCo2 − xZnxFe16O27 hexaferrite powders were prepared by mechanoactivated SHS. The ferrites were characterized by XRD and phase analyses. The values of specific saturation magnetization, field of magnetocrystalline anisotropy, spin reorientation transition, and Curie’s temperatures were derived from static and dynamic measurements of respective magnetic properties. The spectra of magnetic permeability and dielectric permittivity were measured within the microwave range.

Choice of a dielectric relaxation model of a substance by hypothesis testing

A procedure for processing of measured complex permittivity spectra of a substance for the main Debye, Cole-Cole, Cole-Davidson, and Havriliak-Negami relaxation models is described. The most probable model is chosen by hypothesis testing. The results of processing of experimental permittivity spectra for methyl alcohol and water mixtures are presented.

Iron-filled multi-walled carbon nanotubes for terahertz applications: effects of interfacial polarization, screening and anisotropy

Interface interactions in multicomponent nanoparticles can affect electromagnetic properties of an absorbing system. In this work, we investigate the electromagnetic response of multi-walled carbon nanotubes (MWCNTs) filled with iron-containing nanoparticles (ICNs) in the terahertz frequency range. MWCNTs with different iron content have been synthesized by aerosol-assisted catalytic chemical vapour deposition method from toluene containing a certain quantity of ferrocene used as a catalyst. According to the x-ray diffraction analysis, encapsulated ICNs were mainly in the form of iron carbide. Thin composite films were prepared from the iron-filled MWCNTs and polymethylmethacrylate (PMMA) by casting and stretching methods. The composites showed an enhanced permittivity and anisotropy in the transmittance spectra when iron content increased. This behaviour was related to the mechanism based on electrical conductivity and polarization of ICNs and ICN/MWCNT interfaces. Since terahertz field penetrates inside MWCNTs, the filling of their cavities can be a way of varying the electromagnetic properties of MWCNT-containing composites.

Mechanisms of the self-propagating high-temperature synthesis, phase composition, and magnetic properties of complex oxide ferrimagnets with M structure

Mechanisms of the combustion of reagent mixtures that form complex oxide ferrimagnets (hexaferrites with M structure) are investigated. The phase composition of the end product is found, depending on technological conditions that include self-propagating high-temperature synthesis with preliminary mechanical activation and/or subsequent ferritization. The basic static and dynamic magnetic properties of an end product synthesized in the optimum mode are determined. An energy-saving technique for manufacturing hexaferrites with M structure, making it possible to lower production costs substantially, is proposed.

Effect of heat treatment on the structural parameters and magnetic properties of copper ferrite nanopowders obtained by the sol-gel combustion*

Phase composition, structure parameters and basic magnetic characteristics obtained by the sol-gel combustion nanopowders of ferrospinel CuFe2O4 are investigated. A comparison of the properties of synthesized materials: first sample - immediately after the combustion of the gel and second sample - after annealing at 1073 K for 4 hours are performed. Annealing leads to an increase in the concentration of the phase with tetragonal crystal structure. Particle sizes and the value of anisotropy field of this phase also increased.