S. P. Gubin

Magnetic molecular clusters as promising materials for refrigeration in low-temperature regions

A wide class of magnetic molecular materials - molecular clusters with high magnetic moment containing 3d transition metals (such as `Fe8, `Mn12ac, etc) - have been considered from the point of view of their use as refrigerants in low-temperature regions. The consideration was made in the framework of the model based on the Langevin theory of a superparamagnet. The magnetic entropy change caused by a change in an external magnetic field was calculated for various magnetic clusters. The estimations made show that the magnetic molecular clusters could be promising materials for magnetic refrigeration in low-temperature regions (below about 20 K).

Magnetic and structural properties of Co nanoparticles in a polymeric matrix

Abstract Magnetic, magnetic resonance and structural properties of Co nanoparticles in polyethylene matrix have been investigated. The materials were prepared by a method of thermal decomposition of cobalt formate in the polyethylene melt in a mineral oil and contained 4xa0wt% of Co. Transmission electron microscopy data showed that the particles diameter was 4xa0nm. According to EXAFS studies the particles can be presented as consisting of metallic core and a surface shell interacting with the surrounding matrix. Magnetic and magnetic resonance studies showed that the material has high blocking temperature (about 600xa0K) and the magnetic anisotropy constant of the nanoparticles is of an order of magnitude higher than that in a bulk cobalt. The origin of the high magnetic anisotropy of the Co nanoparticles is related to the surface effects. The material has relatively high hysteresis at room temperature (590xa0Oe), which makes it promising for magnetic recording applications.

Reduced graphene oxide

This paper describes a novel approach to the preparation of reduced graphene oxide and its dispersions in organic solvents. Graphite oxide, graphene oxide, and reduced graphene oxide have been prepared and characterized by a variety of physicochemical techniques.

Graphene: A Novel Carbon Nanomaterial

This review analyzes what the term graphene means today, and examines graphene preparation and identification methods and its chemical properties. The applications of this novel carbon nanomaterial are briefly discussed.

Low temperature electron paramagnetic resonance anomalies in Fe-based nanoparticles

A study of the electron paramagnetic resonance of Fe-based nanoparticles embedded in polyethylene matrix was performed as a function of temperature ranging from 3.5 to 500 K. Nanoparticles with a narrow size distribution were prepared by the high-velocity thermodestruction of iron-containing compounds. A temperature-driven transition from superparamagnetic to ferromagnetic resonance was observed for samples with different Fe content. The unusual behavior of the spectra at about 25 K is considered evidence of a spin-glass state in iron oxide nanoparticles.

Nanomaterials based on metal-containing nanoparticles in polyethylene and other carbon-chain polymers

This paper provides a convenient and simple method for fabricating of large amounts (kilogram-scale) of nanomaterials containing nanoparticles in a polymeric matrix. The encapsulation was done by thermal decomposition of metal containing compounds (MRn; M=Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Mn, Mo, Nd, Ni, Pt, Re, Ti, Pb, Sr, Sm, W, Zn; R=CO, HCOO, CH3COO; C2O4, C6H5CH2 and so on) in high temperature solutions of polymer (polyethylene, polypropylene, polyamide). By using physical methods (Mossbauer spectroscopy, X-ray RED, X-ray emission, SAXS, EXAFS, STM, AFM), the composition and structure of nanoparticles, and their interaction with the polymer matrix are determined is some cases. Particle size measurement (2-30 nm) and magnetic properties showed that the material really contained nanoparticles, which retained their unique properties.

Electron paramagnetic resonance of ferrite nanoparticles

Three types of iron-based oxide nanoparticles (weight compositions Fe2O3, BaFe2O4, and BaFe12O19) embedded in a polyethylene matrix are studied using the electron paramagnetic resonance technique. All nanoparticles are found to be multiphase. Thermal variations of electron paramagnetic resonance spectra reveal the presence of two phases in the Fe2O3 nanoparticles. One such phase undergoes an antiferromagnetic-like transition near 6 K. Nanoparticles of BaFe2O4 demonstrate a resonance anomaly near 125 K that could indicate the presence of a magnetic phase. Reduced magnetic anisotropy in BaFe12O19 nanoparticles may be related to either structural imperfection or particle smallness (effective diameter of less than 10 nm). Our data clearly show that low temperature experiments are desirable for the correct identification of nanoparticles by means of the electron paramagnetic resonance technique.

Optical properties of cadmium sulfide nanoparticles on the surface of polytetrafluoroethylene nanogranules

The spectral characteristics in the visible range of synthesized nanocomposites based on cadmium sulfide and ultradispersed polytetrafluoroethylene are investigated experimentally. Local perturbations are found in the long-wavelength regions of the reflection and absorption spectra, which are related to the optical transitions between the critical points of the valence and conduction bands of the nanocomposite. The dispersion relations for the refractive index and the absorption coefficient are obtained for undoped and managanesedoped nanocomposites. The following important parameters of the nanocomposites are obtained from the spectral measurements: the fundamental optical absorption edge (the band gap), the refractive index (ω → 0), and the absorption coefficient. It is shown that doping with managanese affects changes in these parameters.

Optical and photoluminescent properties of nanomaterials based on cadmium sulfide nanoparticles and polyethylene

The spectral characteristics of absorption and photoluminescence of synthetic composite materials based on cadmium sulfide nanoparticles and polyethylene are studied in the visible and near-IR wavelength ranges. It is shown that the quantum efficiency of a material with CdS nanoparticles about 6 nm in size in a matrix of high-pressure polyethylene is the highest. The estimation of the photoluminescence linewidth shows that the linewidth of this material is the smallest, amounting to 0.55 eV, which indicates that the structure of these nanoparticles is more perfect.

Nanometallization of Ultradispersed Polytetrafluoroethylene

The high reactivity of nanoparticles and their tendency toward spontaneous compaction accompanied by deterioration of basic physical properties make stabilization a major challenge in fabrication of materials based on metal nanoparticles. The best-developed method of stabilization is embedding nanoparticles in polymer matrices. The known “friability” of the structures of most partially crystalline carbon-chain polymers forms the basis for the method of introducing metal-containing nanoparticles into solution melts of polymers in hydrocarbon oils [5]. However, this method is inapplicable to “hard” polymer matrices, such as polytetrafluoroethylene. At the same time, creation of materials that combine opposite properties of the components at the nanolevel is one of the rapidly developing strategies of modern materials science. For example, it is expedient to augment the high thermal and chemical stability of polytetrafluoroethylene with electrical conductivity on a semiconductor scale, magnetization by means of introducing metal nanoparticles, or optoelectronic properties by introducing so-called quantum dots— nanoparticles of some metal sulfides and selenides. vents hinder the introduction of different nanoparticles into this matrix and their uniform distribution over its bulk. In searching for the solution to this problem, we have focused our attention on ultradispersed polytetrafluoroethylene (UPTFE). UPTFE was fabricated by a thermal gas dynamic method suitable for commercial production of UPTFE [6]. The essence of the method is in formation of a finely dispersed UPTFE powder upon the thermodestruction of the block polymer. The method of production and the trademark UPTFE-FORUM are covered by patents [6‐8].