Synthesis and magnetic properties of cobalt ferrite nanoparticles in polycarbosilane ceramic matrix
Synthesis and magnetic properties of cobalt ferrite nanoparticles in polycarbosilane ceramic matrix. Yurkov G.Y., Shashkeev K.A., Kondrashov S.V., Popkov O.V., Shcherbakova G.I., Zhigalov D.V., Pankratov D.A., Ovchenkov E.A., Koksharov Yu.A. //Journal of Alloys and Compounds. 2016. V.686. P.421–430
Composite materials comprised of a ceramic matrix with metal-containing nanoparticles were prepared by sintering a blend of cobalt ferrite nanoparticles and polycarbosilane. Sintering process was performed either in air or argon, resulting in different material composition. The air-sintering materials consist mainly of oxide phases (silica matrix and cobalt ferrite nanoparticles). The process of sintering in argon leads to partial reduction of oxides and formation of α-Fe, carbide and silicate phases. The prepared samples were characterized by the SEM, TEM, XRD and EMR techniques and the Mössbauer spectroscopy. Static magnetic properties were also studied. All samples were found to be soft magnetic. Sintering, especially in argon, increases remnant magnetization of resulting composite products.
Magnetic nanoparticles based on ferrite spinels, CoFe2O4 composition in particular, have drawn attention due to their interesting physical properties and their prospective application in high density magnetic data storage devices, MRI contrast enhancers, magnetic substances for cell targeting and drug delivery, high-temperature space power systems, and catalysis.
Cobalt ferrite nanoparticles often show high magnetic anisotropy, high saturation magnetization and coercivity, mechanical hardness and chemical stability. There are many methods for preparation of ferrite spinels, e.g. chemical precipitation, sol-gel processing, microemulsion route, sonochemistry, hydrothermal processing, aerosol-vapor methods, and high temperature decomposition of organic precursors.
One of the methods for stabilization of nanoparticles is preparation of a composite comprised of nanoparticles embedded in a ceramic matrix. Typical approaches to preparation of nanocomposites based on silicon ceramics and metal oxides are deposition-precipitation, pore volume impregnation, and sol-gel routes. For example, the sol-gel route was used to prepare CoFe2O4/SiO2 composites; the authors note that a diffraction peak attributable to cobalt ferrite appears after sintering the aforementioned system at temperatures above 700 °С. A similar system, also produced by the sol-gel method, having 50% wt. of the magnetic phase, was found to display two magnetic phenomena: superparamagnetism and spin-glass behavior. A strong magnetic interaction between the nanoparticles was revealed. CoFe2O4/SiO2 aerogels with the needle-like structure were also studied. After sintering at 900 °С, the nanoparticles were superparamagnetic, with their blocking temperatures being 49 K and 303 K in the samples containing 5% and 10% wt. of the magnetic phase, respectively. In view of the structural features of spinels, including CoFe2O4, magnetic properties of the composites depend on the distribution of metal ions among the octahedral and tetrahedral sites in the crystalline lattice.
Prospective precursors for the preparation of the ceramic silicon-containing matrix include polycarbosilanes (PCS). Ceramics-producing polycarbosilanes have a branched structure, can fiberize and provide high yield of ultrafine ceramics (crystallites 10–20 nm in size) upon pyrolysis in an inert medium or vacuum (60–65% wt.). SiC/SiC ceramics (consisting of a SiC matrix and SiC fibers) produced from PCS has the maximum sustained operating temperature of 1200 °С in an oxidizing environment, above which SiC crystallites start to grow leading to structural degradation of the ceramics accompanied by loss of strength. Pyrolysis of PCS in an oxygen-containing medium, in turn, allows producing ceramics based on SiO2.
This work is aimed at preparing composite materials which combine properties of silicon-based ceramics and magnetic properties of iron-cobalt compounds, with potential applications in electromagnetic compatibility solutions and magnetic data storage devices.
The composites synthesized were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Mössbauer spectroscopy (MS), and electron magnetic resonance (EMR) technique. Additionally, static magnetic properties of the composites were studied.
A novel method for the preparation of soft magnetic materials by means of pyrolysis of a cobalt ferrite nanoparticles/PCS blend has been presented. The size of the nanoparticles decreases during sintering as a result of their interaction with the matrix. The product composition depends on the pyrolysis conditions: composites sintered in air mainly contain oxides, including cobalt ferrite and silica, whereas sintering in argon leads to partial reduction of oxides leading to formation of strongly magnetic phases based on α-iron, as well as carbides, silicides and other compounds.
All the samples prepared in this study are soft magnetics. The thermal treatment, especially in argon, increases remnant magnetization of the composites, evidenced by a larger hysteresis loop area and specific EMR spectrum features.
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