Elaboration of superparamagnetic cobalt–ferrite nanocomposites from films of chitosan chelates
Marco A. Garza-NavarroVirgilio Ángel González GonzálezAlejandro TorresMoisés Hinojosa RiveraAntonio Francisco García LoeraMiguel José‐Yacamán
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Abstract Cobalt–ferrite nanocomposites were synthesized from polymeric films of chelates of Co(II) and Fe(III) ions within a chitosan matrix by a solid‐state coprecipitation reaction with weight content ratios of chitosan to cobalt–ferrite of 50/50 and 25/75 w/w. Morphological and crystalline studies of the composites were performed by high‐resolution transmission electron microscopy, X‐ray diffraction, and selected area electron diffraction with a nanobeam diffraction probe. The results show nanoparticles around 4 nm with a spinel structure, consistent with the cobalt–ferrite phase. The magnetic behavior was evaluated with curves of the applied‐field‐dependent magnetization [ M ( H )] and the temperature‐dependent magnetization [ M ( T )]. Both the M ( H ) and M ( T ) curves showed typical superparamagnetic behavior, depicting an absence of hysteretic characteristics and the characteristic peak at blocking temperature in the zero‐field‐cooled curve. There was also evidence of strong interparticle and intraparticle interactions, which suggested magnetic frustration in the particle magnetic moment alignment with the applied field. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010Keywords:
Superparamagnetism
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An experiment was carried out to investigate the manufacturing condition for chemical coprecipitation method, and the effect of Li2O3 substitution on the magnetic and physical properties of Co-Ni spinel ferrite fine particles. As one of the examinations, alkaline aqueous solution inflowed into the metal mixture solution on a constant speed. As a result, this method yeilded fine particles with more stable properties than the pH control method (the conventional method). As regards the (CoO)0.5(NiO)0.5-x(Li2O3)0.5x·Fe2O3 (x = 0—0.5) composition, the new method showed a good properties in (CoO)0.5(NiO)0.3(Li2O3)0.1·Fe2O3 composition, with magnetic properties of σs = 64.3 x 10-6 Wb·m/kg (51.2 emu/g), HcJ = 406.0 kA/m (5.12 kOe). K1 = 10.0 x 104 J/m3, K2 = -39.6 x 104 J/m3. The average particle size was about 34 nm. The rotational hysteresis integral (Rh), which was related to the magnetization mechanism of these fine particles was 1.66, and it was found that the magnetization mechanism was an incoherent rotation one.
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In this paper the coprecipitation was improved(called spraying coprecipitation)to pre-pare nano gas sensitive materials such as ZnGa 2 O 4 and ZnFe 2 O 4 .The structural analysis was done by XRD,SEM and TEM.The result shows that the size and the uniform of the material pre-pared by the spraying coprecipitation is better than ever.Transport and reaction process of spray-ing coprecipitation was analysed by hydrodynamics theory.Some experiments were explained.The principle of spraying coprecipitation was also discussed.
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Coprecipitation is the method most frequently applied to prepare Layered Double Hydroxides (LDHs). Two variations of this method can be used, depending on the pH control conditions during the precipitation step. In one case the pH values are allowed to vary while in the other they are kept constant throughout coprecipitation. Although research groups have their preferences, no systematic comparison of the two variations of the coprecipitation method is available in the literature. On this basis, the objective of the present study was to compare the properties of LDHs prepared using the two forms of pH control in the coprecipitation method. The results showed that even though coprecipitation is easier to perform under conditions of variable pH values, materials with more interesting properties, from the point of view of technological applications, are obtained at constant pH. Higher crystallinity, smaller particle size, higher specific surface area and higher average pore diameter were found for materials obtained by coprecipitation at constant pH, when compared to the materials obtained at variable pH.
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ZnGa2O4 nano crystals were prepared by an improved coprecipitation method, which we call ‘spray coprecipitation’. XRD results shows the resulting crystal size using the new method is under 10nm, whereas the powder prepared by ordinary coprecipitation is about 30nm. XRD results also shows ZnO peaks exists in ZnGa2O4 powder prepared by traditional coprecipitation, but disappears in ZnGa2O4 nano crystal prepared by spraying coprecipitation. SEM and TEM were used to analysis the structural characteristics of ZnGa2O4 nano crystals. The gas sensitive characteristics of ZnGa2O4 nano crystals are reported.
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Ni-Zn spinel ferrite and Cu-doped spinel ferrite were prepared by a conventional ceramic processing method. Microwave absorption, complex permittivity and permeability of the (Ni0.5Zn0.5)Fe2O4 and (Ni0.4Cu0.2Zn0.4)Fe2O4 spinel ferrites within the frequency range of 0.5-18 GHz were investigated. The reflection loss calculation results show that the Ni-Zn spinel ferrite and Cu-doped Ni-Zn spinel ferrite are good electromagnetic wave absorbers in the microwave range. The single layer (Ni0.4Cu0.2Zn0.4)Fe2O4 spinel ferrite absorber with a thickness of 9.2 mm achieved a reflection loss below -10 dB (90% absorption) at 0.5-2.3 GHz, and the minimum value is -35.63 dB at 1.1 GHz. When the first layer and second layer are (Ni0.5Zn0.5)Fe2O4 and (Ni0.4Cu0.2Zn0.4)Fe2O4 spinel ferrites respectively, the laminated absorbers with double spinel ferrite layers with a thickness of 3 mm achieved a reflection loss below -10 dB at 9.9-12.3 GHz, and the minimum value is -35.3 dB at 11.7 GHz.
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The chemical coprecipitation process for synthesizing BaCoTiFe_(10)O_(19) ultrafine powders has been investigated by means of corrosion versus pH plot (E-pH plot) for metal element, thermodynamic calculation and co-dump coprecipitation experimentation. The results show that the theoretical pH values of complete coprecipitation of all Fe~(3+), Ti~(4+), Co~(2+) and Ba~(2+) cations were in 9≤pH≤12.2 obtained by E-pH plot and pH≥7.9 obtained by thermodynamic calculation, respectively. The co-dump coprecipitation experiments indicate that the minimum pH value necessary to the formation of single-phase BaCoTiFe_(10)O_(19) under 900℃ for 2h was pH=8.5. It was suggested that three different results and their dependency originate from both synergetic coprecipitation effect of cations and coordination effect of Cl~- anions.
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