Nanometer size structures are an intermediate form of matter, which fills the gap between atoms/molecules and bulk materials. Often, these types of structures exhibit exotic physical and chemical properties different from those observed in bulk three-dimensional materials. The properties associated with nanoscale particles have been the subject of a number of recent reviews and publications. This interdisciplinary field of mesoscopic and nano-scale systems is important to fundamental physics, as well as for some new technologies. As is well known, some of the metallic nano-particles are thermodynamically unstable and chemically very reactive. For many technological applications, materials with nanoparticles embedded in a matrix are impotant. In this paper for the stabilization of nano-particles the matrices of the "simplest" organic polymers as polyethelene it is used.
A universal method of introduction of metall-containing nano-particles in polymeric matrices has been developed, which allows the fabrication of large amounts (kilogram-scale) of polymer nanoparticle composites. The encapsulation was done by thermal decomposition of metallcontaining compounds (MRn; M = Fe, Co; R = CO, HCOO, CH3COO, C6H5CH2) in solution-melt of polyethylene in mineral oil. The optimum conditions are developed for the decomposition of MCC in order to introduce highly reactive nanoparticles into the polymeric matrix with concentration of 2-40 wt. %.
The samples containing 5 wt. % of Fe, 4 wt. % of Co and 23 wt. % of gamma-Fe2O3 were investigated in detail. For the subsequent characterization we used TEM, small-angle X-ray scattering (SAXS), X-ray RED, EXAFS, X-ray emission and Mossbauer spectroscopy. The morphology, particle size and distribution were investigated using a high-resolution electron microscope (TEM). SAXS studies showed that the particles are regulary dispersed in the polymeric matrix with narrow log-normal size bimodal distribution centered at 3.2±0.3 nm. Mossbauer spectroscopy for Fe-containing samples makes it possible to determine and to control the "phase" constitution of the composites, and also to evaluate the size of the particles.
Magnetization temperature and magnetic field dependencies are measured in the temperature interval from 4.2 to 350 K and in the fields up to 4.5 kOe. It was found that the Fe and Co samples displayed hysteretic behavior already at room temperature with coercive force (Hc) of 980 and 580 Oe, correspondingly. This point to the blocking state of magnetic subsystem of the samples. With cooling the value of Hc increased up to 680 Oe in the Co sample and up to 2500 Oe in the Fe sample at 4.2 K. By extrapolation of coercive force temperature dependence to high temperature region the blocking temperature was found to be 560 K for the Co sample and 680 K for the Fe sample. Blocking temperature of the sample containing 23 wt. % of gamma-Fe2O3 was determined as 80 K from ZFC-FC measurements. Effective magnetic anisotropy constants estimated from blocking temperature value were about 1.5·108 erg/cm3 the Co sample, 3.6·107 erg/cm3 for the Fe sample and 106 erg/cm3 for the gamma-Fe2O3 sample. Magnetization of Co and Fe samples was 1.9 µ /atomCo and about 1.0 µ /atomFe at 4.2 K in the field of 4.5 kOe. The obtained results on magnetic properties are discussed in the framework of core - shell model.
It can be suggested that the materials mentioned above can provide in future increase of the magnetic data storage density at least by an order of magnitude in comparison with the present state of art.
Our experiments demonstrated that we were able to stabilize, isolate and characterize nanoparticles of various composition using a polymer matrix. The matrix, inside which nanoparticles are synthesized, plays an active role in determining their composition and physical properties in addition to providing a means of particle dispersion. This research demonstrates the significant potential of MCC thermodestruction processing for the synthesis of appreciable quantity of nanoparticle containing polymer materials.
This work was supported by the RFBR 02-03-06153, 01-03-32955, 01-03-32783, 02-03-32435 and Complex Programme RAS "Nanomaterials and the supramolecular systems"
N.S.Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences
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