Reactivity of metal-containing monomers 70. Preparation and magnetic properties of metal-containing nanocomposites

Nanocomposites containing magnetically active nanoparticles stabilized by the carbon-containing matrix formed in parallel were obtained by the polymerization—destruction synthesis. The composition, structure, and magnetic properties of nanocomposites synthesized by the thermal decomposition of unsaturated metal carboxylates and transition metal (Co^II, Ni^II, Fe^III) acrylamide complexes were studied by powder X-ray diffraction, scanning and transmission electron microscopy, and Mössbauer spectroscopy. The key macro stages and kinetic features of thermolysis of the metal-containing monomers were identified. The variation of the conditions of thermal transformations allows one to control the magnetic properties of nanocomposites from ferromagnetic to superparamagnetic behavior.

     

Improvement of the magnetic properties of low-neodymium magnets by minor addition of titanium

Rapidly solidified nanocomposite Nd₉Fe_77−xB₁₄Ti_x alloys, consisting of magnetic Nd₂Fe₁₄B phase and soft magnetic phases, were investigated. The effect of titanium addition on the structure and magnetic properties was studied. It was found that 2–4 at% Ti addition leads to substantial increase of the coercivity and maximum energy product, maintaining the remanence unchanged. The highest properties: J_r=0.81 T, _JH_c=907 kA/m, (BH)_max=99 kJ/m³, were achieved for the Nd₉Fe₇₃B₁₄Ti₄ alloy. This effect we attribute to the formation of fine and homogeneous grain structure and a change of the phase morphology in the Ti-containing alloys. The initial magnetization curve indicates a change of the coercivity mechanisms giving rise to pinning of domain walls, which is caused by reduction of the crystallite size.     

Structure determination of CaMnO3 and CaMnO2.5 by X-ray and neutron methods

Low-temperature synthesis of the oxygen-deficient compound CaMnO_2.5 from polycrystalline CaMnO₃ preserves the existing structural framework of the oxidized precursor. The crystal structure of CaMnO_2.5 was determined using neutron powder diffraction data analyzed by the Reitveld profile refinement method. The structure of the reduced phase can be described by the orthorhombic distortion $\text{(a}{}_0\text{+}\text{b}{}_0\text{2}\text{)}\text{2}\text{= a}{}_{\mathrm{<mtext>c</mtext>}}\text{2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, where a_c is the simple cubic distance (～3.7 Å) characteristic of the Mn⁴⁺O^2?Mn⁴⁺ framework in CaMnO₃. The unique features of the structure are five-coordinate Mn³⁺ cations with nearly square pyramidal (～C_4v) coordination and ordered oxygen vacancies. The preparation, structure refinement, and noncubic distortions of single crystals of CaMnO₃ are also described. Attempts to transform single crystals of CaMnO₃ into CaMnO_2.5 by well-ordered topotactic changes have not been successful.     

Metal-containing nanoparticles with core-polymer shell structure

Metal-polymer nanocomposites, which comprise nanoparticles of metals and/or their oxides and carbides uniformly distributed in stabilizing polymer matrices, are prepared through solid-phase polymerization of metal-containing monomers followed by controlled thermolysis of synthesized metal-containing polymers. Using X-ray diffraction, electron microscopy, ferromagnetic resonance, and IR spectroscopy, it is shown that nanoparticles present in these systems have a characteristic core-shell structure that comprises a metal-containing core and a surface layer, i.e., a polymer shell. Parameters of the components are estimated.     

Formation,Structure and Magnetic Properties of Polymer Matrix Nanocomposites Processed by Thermal Decomposition of the Fe(III)Co(II) Acrylate Complex

Structure and magnetic properties of polymer matrix nanocomposites, processed by pyrolysis of the Fe(III)Co(II) acrylate complex, were investigated. It was shown that thermal transformation of the complex studied consisted of three macrostages: dehydration, solid state polymerisation and decarboxylation of a metallopolymer form. The main products of the decomposition were nanoparticles stabilised by polymeric matrix. The crystalline phases, which were found in the fully processed material, were Fe₃O₄, CoFe₂O₄ and CoO. The mean crystallite size was 10nm. In the intermediate stages of the thermolysis iron was present in the forms of Fe^III (trivalent low – spin iron), Fe²⁺(divalent high – spin iron) and Fe₃O₄. The hysteresis loops measured at temperatures below 200K were opened and shifted towards negative field. The coercivity and remanence showed room temperature values of 0.18T and 15.5mT, respectively.     

CaMnO2.5 and Ca2MnO3.5: New oxygen-defect perovskite-type oxides

We found that when the precursors CaMnO₃ and Ca₂MnO₄ are reduced with any one of a variety of inorganic (H₂, NH₃) or organic (C₂H₄, C₃H₆) reducing agents between 300 and 500°C, topotactic reaction occurs to produce the ordered oxygen-defect phases CaMnO_2.5 and Ca₂MnO_3.5, respectively. Orthorhombic cell constants for CaMnO_2.5 are a = 5.43(1), b = 10.24(1), and c = 3.74(1) Å, and for Ca₂MnO_3.5 are a = 5.30(1), b = 10.05(1), and c = 12.24(1)Å. These compounds were characterized by powder X-ray diffraction, thermogravimetric analysis, magnetic susceptibility, and infrared spectroscopy. The reduced compounds reversibly oxidize to their respective precursor in oxygen at low temperatures.     

Ferromagnetic resonance of cobalt nanoparticles in the polymer shell

The magnetic properties of cobalt spherical nanoparticles (～ 5–9 nm in size) in a polymer shell are investigated using ferromagnetic resonance (FMR) spectroscopy. The metal-polymer complex is prepared through the frontal polymerization of the cobalt acrylamide (CoAAm) complex, followed by the thermolysis at a temperature of 643 K. Analysis of the ferromagnetic resonance spectra demonstrates that the material has a high blocking temperature of ～700 K. The anisotropy constant equal to 0.5 erg/cm³ is somewhat larger than the anisotropy constants characteristic of cobalt macrostructures. This difference is associated with the predominance of the surface anisotropy of nanoparticles. The surface anisotropy constant is calculated to be 0.17 erg/cm², and the anisotropy field is determined to be ～350 Oe. It is revealed that the polymer shell affects the magnetic properties of nanoparticles.     

Thermal and electrical properties of Te80Si20−xPbx glasses

Results of calorimetric, electrical resistivity and X-ray measurements of Te₈₀Si_20?xPb_x glasses are reported. Experimental findings concerning correlations between transformation temperatures obtained from DSC and resistivity measurements are presented. Conduction activation energy is discussed in terms of changes of randomness of amorphous structure caused by concentration changes.     

Vanadyl hydrogenphosphate hydrates: VO(HPO4) · 4H2O and VO(HPO4) · 0.5H2O

Two vanadyl(IV) monohydrogenphosphate hydrates have been crystallized from aqueous media and their structures determined by single-crystal X-ray diffraction. The first, a tetrahydrate, VO(HPO₄) · 4H₂O, is triclinic, $\text{P}\text{1}$, with a = 6.379(2), b = 8.921(2), c = 13.462(3) Å, α = 79.95(2), β = 76.33(3), γ = 71.03(3)°. Final residuals of R₁ = 0.058 and R₂ = 0.065 were obtained using 1250 unique data and 140 parameters. The second was found to be the hemihydrate, VO(HPO₄) · 0.5H₂O, with orthorhombic symmetry, Pmmn. Complete structure solution and refinement using data from a 2.7 × 10⁵ μm³ crystal gave atomic parameters in close agreement with those recently reported in a parallel study (C. C. Torardi and J. C. Calabrese, Inorg. Chem.23, 1308, 1984). Final residuals R₁ = 0.041 and R₂ = 0.042 were obtained on optimizing the 45 structural variables using 458 observed intensities. The structures of these two hydrates and that of the pyrophosphate, (VO)₂P₂O₇, show a close correspondence. The degree of condensation of the vanadyl octahedra and phosphate tetrahedra, and the amount of water of crystallization in these materials are closely coupled and depend on the formation temperature.