Papers by Keyword: Maghemite

Abstract: Reasoning of particular mechanism of anomalous thermal transport behaviours are not identified yet for the nanofluids. In this study, iron oxide (Maghemite: MH) and graphene (Gr) flake dispersed deionized water (DW) hybrid nanofluid system were developed for the first time to evaluate the thermal conductivity (TC) enhancements along with the analysis of anomalous TC behavior implementing modified effective medium theories (EMTs). A solvo-thermal two-step method was used to develop the MH nanoparticle and exfoliated Gr flake dispersed hybrid nanofluids with different compositions. Stability of as-prepared hybrid nanofluids were monitored using Ultraviolet-Visible (UV-Vis) spectroscopy. The maximum sedimentation rate was observed ~ 8.4 % after 600 hours. The results showed an overall maximum TC enhancement of ~ 43 % at 25 °C. EMTs were modified with the consideration of flat geometry of Gr flake. It is found that, modified EMTs with the crumpled factor (due to the non-flatness or crumple of Gr flake) of ~ 0.205 the predicted effective TC enhancements are agreed with the experimental TC’s of Gr-NMP/MH-DW hybrid nanofluids samples. The estimated crumple factor value of exfoliated Gr flakes using images analysis was also found nearly similar (~ 0.232). This agreement exposed that, Gr flake’s with negligible thickness compared to its extremely wide basal plane dimensions and its non-flatness or crumpled geometry in the nanofluids have the leading impacts on the effective TC properties of the Gr flake dispersed nanofluids. This modified model opens the new doors to analyse the insight of the thermophysical properties of various types of nanofluids by introducing potential other parameters.

Abstract: Reverse co-precipitation (RCP) in ambient atmosphere is one of the strategies to produce magnetite nanoparticles in a rapid, simple, and cost-effective synthesis route without applying temperature surfactants or inert gases. However, RCP of ferrous/ferric blended salt in sodium hydroxide (NaOH) solution in an oxidizing medium produced of maghemite as a dominant phase rather than magnetite because of the oxidation of Fe²⁺ to Fe³⁺ happened. Based on this background, an oil membrane layer-assisted reverse co-precipitation approach has been examined to synthesis of magnetite in ambient atmosphere at room temperature. The result showed that although addition of benzene as an oil membrane layer was effective to prevent oxidation of magnetite to maghemite, but the magnetite particle size for the samples from the oil membrane layer-assisted reverse co-precipitation method was much larger than that from a reverse co-precipitation method without addition of oil membrane layer.

Abstract: We prepared hydroxyapatite (HA) capsules encapsulating maghemite particles. In order to evaluate enzyme immobilization behavior of the HA capsules under alkaline condition, we immobilized five kinds of enzymes with different isoelectric point in carbonate/bicarbonate buffer (CBB, pH 10.0). When the enzymes in CBB were moderately charged, immobilization efficiency on the HA capsules showed the highest value. It was suggested that immobilization efficiency was affected according to both pI of enzyme and pH of the surrounding solution and that enzyme immobilized on the HA capsules by not only electrical double layer interactions but also ion interaction and other interactions.

Abstract: Magnetite’s abilities rely on the quantitative phases present in the sample. Magnetite quality can strongly influence several physical properties, such as magnetism, catalytic performance, and Verwey transition. However, differentiation of magnetite and maghemite through the conventional X-ray diffractogram comparison are not relevant for the intermediate phases. In this study, the deviation from the ideal stoichiometric magnetite and the relative quantification of both phases were mathematically achievable through a new XRD technique. Various synthesis conditions were applied to obtain different crystallite sizes, in the range of 9 to 30 nm. Generally, the stoichiometric deviation and maghemite content would be significantly influenced by the final size, whereas system conditions (temperature of solution, agitation rate, and pH of solution) would only have minor significance. In this study, iron oxide nanoparticles prepared using the co-precipitation method was calculated to contain 100% magnetite for particles of 30.26 nm in size, while 100% maghemite was calculated for particles at 9.64 nm.

Abstract: It is well-known that apatite has high affinity to many kinds of biomolecules. In this study, the authors aimed to develop the apatite microcapsules with both enzyme immobilization property by the bioaffinity of apatite and magnetism of core materials. The authors encapsulated maghemite particles, possesing ferrimagnetism, with apatite by attaching Apatite Nuclei on the surfaces of maghemite particles and subsequently immersing them in SBF. To evaluate performance of the microcapsules as enzyme immobilization carriers, the authors investigated enzyme immobilization property of the microcapsules fabricated by biomimetic method using Apatite Nuclei and SBF from the viewpoint of the difference of isoelectric point of the three-types of enzymes.

Abstract: Magnetic nanoparticles were prepared from a low crystalline ferrihydrite (Fe₅O₇(OH)·4H₂O) precursor by a chemically-induced transformation method using mixed FeCl₂/NaOH solution. The products obtained were characterized using a vibrating sample magnetometer, X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. Experimental results show that while the concentration of NaOH was 0.17 mol/L, and the FeCl₂ concentration was increased from 0.09 mol/L to 0.42 mol/L in the mixed solution, the as-prepared products were binary nanoparticle systems comprised of ferrimagnetic γ-Fe₂O₃ sphere-type particles and antiferromagnetic α-FeO(OH) rod-type particles. The resulting particles were highly crystalline. The volume ratio of γ-Fe₂O₃ and α-FeO(OH) particles was estimated from magnetization data, which showed that the volume of γ-Fe₂O₃ particles decreased and α-FeO(OH) particles increased with increasing FeCl₂ concentration. Such nanoparticle systems could be suitable for synthesis of binary ferrofluids, which have different behavior to conventional ferrofluids.

Abstract: PEGylated, monodispersed, superparamagnetic, iron oxide nanoparticles (Fe₃O₄ / γ-Fe₂O₃) were synthesized by using a novel metal-organic approach in three steps. Ferric nitrate nonahydrate, Fe (NO₃)₃^.9H₂O, was used as iron source, which was sterically hindered among the interstices and / or in the cavities of β-Cyclodextrin (β-CD) molecules, following a modified complexation procedure. Via a polyol process the obtained complex system was first dispersed in polyethylene glycol (PEG) and under mild thermal treatment and in the presence of 1,12 dodecanediol, a new complex system of carboxylate type was formed, between ferric nitrate and PEG, denoted Fe (NO₃)₃^.9H₂O-PEG. This metal-organic precursor was thermally decomposed, forming the iron oxide nanoparticles. The obtained particles were characterized by X-ray diffraction spectroscopy (XRD), Fourier Transform Infrared spectroscopy (FTIR) and transmission electron microscopy (TEM).

Abstract: In the course of this work, two iron oxide nanopowder samples (a mixture of FeO / γ-Fe₂Ο₃-Fe₃O₄) were composed, implementing the methodology. The synthesis used is a simple thermal decomposition route of organometallic precursors. The organometallic precursor used was the iron acetylacetonate (Fe (acac)₃) which underwent reductive thermal degradation. The shape of the nanoparticles was examined and determined by the reaction time and the ratio of the used surfactants, oleic acid (OA) and oleylamine (OA_m). The first sample underwent controlled oxidation in the air to transform the non-magnetic FeO phase to a mixture of magnetic phases, while the second sample underwent thermal reduction in a hydrogen atmosphere to produce a composite nanomaterial, with α-Fe, Fe₃O₄, γ-Fe₂Ο₃, being the dominant phases.

Abstract: Magnetic iron oxide nanoparticles synthesized via co precipitation and thermal decomposition yielded largely monodisperse size distributions. Both methods produced a mixture of magnetite and maghemite. However CP NPs were indexed as magnetite-rich while TD yielded largely maghemite NPs. XRD-and TEM-measured diameters of the co-precipitated particles were approximately between 9 to 15 nm, while thermally decomposed diameters were in the range of 8 to 10 nm. FTIR spectra revealed no distinct differences in the bulk structure of the two systems. Based on the Density functional theory calculations and on HOMO-LUMO gap energies, we propose that ferric Fe is the state of preference by the surfactant in bidentate linkage.

Abstract: A new magnetitediatomite compositions was synthesized by co-precipitation diatomite with Fe²⁺ and Fe³⁺ in the presence of diatomite. Magnetic compositions of diatomite were investigated using XRD and SEM. Comprehensive analysis for SEM and XRD data shows that the most favorable magnetic particle including to the diatomite pores in DM-15 and DM-30. Increasing of content of maghemite in compositions causes reduction of the proportion of smectite and mica, which is associated with the destruction of the crystal lattice. Determined by the interaction between the diatomite surface and including particles of magnetite and maghemite can be hydrogen bonds between the SiOH and SiOH₂⁺-groups of the diatomite surface and the iron (II) and iron (III) oxides.