Search NASA⌕ Search

SEARCH · Search NASA

Results for “SUPERPARAMAGNETISM”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Synthesis of superparamagnetic silica-coated magnetite nanoparticles for biomedical applications

Multifunctional superparamagnetic iron oxide nanoparticles (SPIONs) coated with silica are widely researched for biomedical applications such as magnetic resonance imaging, tissue repair, cell separation, hyperthermia, drug delivery, etc. In this article synthesis of magnetite (Fe{sub 3}O{sub 4}) nanoparticles and their coating with SiO{sub 2} is reported. Fe{sub 3}O{sub 4} nanoparticles were synthesized by chemical co-precipitation and it was coated with silica by hydrolysis and condensation of tetraethylorthosilicate. XRD, FTIR, TEM and VSM techniques were used to characterize bare and coated nanoparticles. Results indicated that the average size of SPIONS was 8.4 nm. X-ray diffraction patterns of silica coated SPIONS were identical to that of SPIONS confirming the inner spinal structure of SPIONS. FTIR results confirmed the binding of silica with the magnetite and the formation of the silica shell around the magnetite core. Magnetic properties of SPIONS and silica coated SPIONS are determined by VSM. They are superparamagnetic. The major conclusion drawn from this study is that the synthesis route yields stable, non-aggregated magnetite-silica core-shell nanostructures with tailored morphology and excellent magnetic properties.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Organophilic, Superparamagnetic, and Reversibly Thermoresponsive Silica-Polypeptide Core–Shell Particles

Particles with a superparamagnetic cobalt inner core, silica outer core, and covalently bound homopolypeptide shell were investigated under thermal and magnetic stimuli. The homopolypeptide was poly(ε-carbobenzyloxy- l -lysine), PCBL, which is known to exhibit a thermoreversible coil ⇔ helix transition when dissolved as a pure polymer in m- cresol. Tethering to a core particle did not prevent PCBL from undergoing this conformational transition, as confirmed by dynamic light scattering and optical rotation, but the transition was broadened compared to that of the untethered polymer. The Co@SiO 2 -PCBL hybrid particles retained the superparamagnetic properties of the cobalt inner nougat. Indeed, some response remains even after aging for >5 years. The aged PCBL shell also preserved its responsiveness to temperature, although differences in the shape of the size vs temperature transition curve were determined compared to the freshly made particles. A reversible coil ⇔ helix transition for a particle-bound polypeptide in a pure organic solvent is rare. Further to providing a convenient tool for characterizing coil ⇔ helix transitions for surface-bound polypeptides without interference from pH or the strong ionic forces that dominate behavior in aqueous systems, the Co@SiO 2 -PCBL/ m -cresol system may prove useful in studies of the effect of shell polymer conformation on colloid interactions. The stability of the magnetic core and polypeptide shell suggest a long shelf life for Co@SiO 2 -PCBL, which can, in principle, be deprotected to yield positively charged Co@SiO 2 -poly( l -lysine) particles for possible transfection or antimicrobial applications or chained magnetically to produce responsive poly(colloids).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Tuning Surface, Phase, and Magnetization of Superparamagnetic Magnetite by Ionic Liquids: Single-Step Microwave-Assisted Synthesis

Achieving colloidal and chemical stability in ferrofluids by surface modification requires multiple steps, including purification, ex situ modification steps, and operation at high temperatures. In this study, a single-step microwave-assisted methodology is developed for iron oxide nanoparticle (IONP) synthesis utilizing a series of imidazolium-based ionic liquids (ILs) with chloride, bis(trifluoromethylsulfonyl)imide, and pyrrolide anions as the reaction media, thus eliminating the use of volatile organics while enabling rapid synthesis at 80 °C as well as in situ surface functionalization. The characterized surface functionality, hydrodynamic particle size, magnetization, and colloidal stability of the IONPs demonstrate a strong dependence on the IL structure, ion coordination strength, reactivity, and hydrophilicity. The IONPs present primarily a magnetite (Fe 3 O 4 ) phase with superparamagnetism with the highest saturation magnetization at 81 and 73 emu/g at 10 and 300 K, respectively. Depending on the IL coating, magnetization and exchange anisotropy decrease by 20 and 2–3 emu/g (at 35 wt % IL), respectively, but still represent the highest magnetization achieved for coated IONPs by a coprecipitation method. Further, the surface-functionalized superparamagnetic magnetite nanoparticles show good dispersibility and colloidal stability in water and dimethyl sulfoxide at 0.1 mg/mL concentration over the examined 3 month period. This study reports on the intermolecular and chemical interactions between the particle surface and the ILs under synthetic conditions as they relate to the magnetic and thermal properties of the resulting IONPs that are well suited for a variety of applications, including separation and catalysis.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Cotunneling enhancement of magnetoresistance in double magnetic tunnel junctions with embedded superparamagnetic NiFe nanoparticles

Temperature and bias voltage-dependent transport characteristics are presented for double magnetic tunnel junctions (DMTJs) with self-assembled NiFe nanoparticles embedded between insulating alumina barriers. The junctions with embedded nanoparticles are compared to junctions with a single barrier of comparable size and growth conditions. The embedded particles are characterized using x-ray absorption spectroscopy, transmission electron microscopy, and magnetometry techniques, showing that they are unoxidized and remain superparamagnetic to liquid helium temperatures. The tunneling magnetoresistance (TMR) for the DMTJs is lower than the control samples, however, for the DMTJs an enhancement in TMR is seen in the Coulomb blockade region. Fitting the transport data in this region supports the theory that cotunneling is the dominant electron transport process within the Coulomb blockade region, sequential tunneling being suppressed. We therefore see an enhanced TMR attributed to the change in the tunneling process due to the interplay of the Coulomb blockade and spin-dependent tunneling through superparamagnetic nanoparticles, and develop a simple model to quantify the effect, based on the fact that our nanoparticles will appear blocked when measured on femtosecond tunneling time scales.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Gram scale synthesis of Fe/Fe x O y core–shell nanoparticles and their incorporation into matrix-free superparamagnetic nanocomposites

In this paper, significant reductions recently seen in the size of wide-bandgap power electronics have not been accompanied by a relative decrease in the size of the corresponding magnetic components. To achieve this, a new generation of materials with high magnetic saturation and permeability are needed. Here, we develop gram-scale syntheses of superparamagnetic Fe/Fe x O y core–shell nanoparticles and incorporate them as the magnetic component in a strongly magnetic nanocomposite. Nanocomposites are typically formed by the organization of nanoparticles within a polymeric matrix. However, this approach can lead to high organic fractions and phase separation; reducing the performance of the resulting material. Here, we form aminated nanoparticles that are then cross-linked using epoxy chemistry. The result is a magnetic nanoparticle component that is covalently linked and well separated. By using this ‘matrix-free’ approach, we can substantially increase the magnetic nanoparticle fraction, while still maintaining good separation, leading to a superparamagnetic nanocomposite with strong magnetic properties.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Single Domain 10 nm Ferromagnetism Imprinted on Superparamagnetic Nanoparticles Using Chiral Molecules

The rapid growth in demand for data and the emerging applications of Big Data require the increase of memory capacity. Magnetic memory devices are among the leading technologies for meeting this demand; however, they rely on the use of ferromagnets that creates size reduction limitations and poses complex materials requirements. Typically magnetic memory sizes are limited to 30–50 nm. Reducing the size even further, to the ≈10–20 nm scale, destabilizes the magnetization and its magnetic orientation becomes susceptible to thermal fluctuations and stray magnetic fields. In the present work, it is shown that 10 nm single domain ferromagnetism can be achieved. Using asymmetric adsorption of chiral molecules, superparamagnetic iron oxide nanoparticles become ferromagnetic with an average coercive field of ≈80 Oe. The asymmetric adsorption of molecules stabilizes the magnetization direction at room temperature and the orientation is found to depend on the handedness of the chiral molecules. These works point to a novel method for the miniaturization of ferromagnets (down to ≈10 nm) using established synthetic protocols.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tunable Synthesis of Hierarchical Superparamagnetic Fe{sub 3}O{sub 4} Nanospheres by a Surfactant-Free Solvothermal Method

Hierarchical Fe{sub 3}O{sub 4} nanospheres with superparamagnetism have been fabricated via a solvothermal method without any surfactants. The effect of reaction parameters like the types of the reagent, the reaction time, and the reagent concentration on the morphology and structure of the resultant samples are investigated. In this method, ferric chloride (FeCl{sub 3} ⋅ 6H{sub 2}O) is used as the iron source, and EG acts as both solvent and reducing agent. The presence of NaHCO{sub 3} or Na{sub 2}CO{sub 3} provides alkine condition and gaseous bubbles in the control over the size of Fe{sub 3}O{sub 4} nanospheres. The formation mechanism for Fe{sub 3}O{sub 4} nanospheres is illustrated by an Ostwald ripening process. The Fe{sub 3}O{sub 4} nanospheres were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and superconducting quantum interference device magnetometer (SQUID-VSM). The Fe{sub 3}O{sub 4} nanospheres exhibited a wider size distribution at the amount of 0.397 mol/L NaHCO{sub 3}, showing a high saturation magnetization close to 84.46 emu/g. The Fe{sub 3}O{sub 4} nanospheres soluble in deionized water is also investigated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Synthesis and Characterisation of Superparamagnetic MgFe{sub 2}O{sub 4} Nanoferrite

Nanocrystalline MgFe{sub 2}O{sub 4} with crystallite size of 10.4 nm was synthesised using combustion method and characterised for structural, electrical, and magnetic properties. The powder X-ray diffraction analysis reveals the single-phase nature of MgFe{sub 2}O{sub 4}. The field emission scanning electron microscopy image shows the nanosized grains. The magnetic study reveals the superparamagnetic behaviour of MgFe{sub 2}O{sub 4} with saturation magnetisation value of 14.57 emu/g. The blocking temperature is determined using low-temperature magnetic measurement which is equal to 139 K. Electrical study shows that the value of AC conductivity at 1 Hz is 1.12 × 10{sup − 8} S/cm at 303 K and increases with frequency as well as temperature. The complex impedance analysis shows the grain and grain boundary resistance equal to 6.23 × 10{sup 6} Ω and 1.27 × 10{sup 7} Ω, respectively. The complex electric modulus analysis confirms the presence of two relaxation processes in nanocrystalline MgFe{sub 2}O{sub 4}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Structural Confinement Assisted a Robust Superparamagnetic State in MgNi{sub 2}O{sub 3} and MgNi{sub 1.5}Co{sub 0.5}O{sub 3} Nanoparticles at Room Temperature

In this study, ideal spherical-shaped cubic metal oxide nanoparticles of < 30 nm in size were prepared by a citric acid sol-gel combustion method. The detailed structural analysis and microscopic techniques clearly support the existing grade of structural confinement effect in MgNi{sub 2}O{sub 3} and MgNi{sub 1.5}Co{sub 0.5}O{sub 3}. Temperature-dependent magnetisation studies at various applied fields revealed a moderate change of T{sub B} near room temperature by distinct particle size. Magnetic crystalline anisotropy and energy barrier density theoretical model results suggested the importance of innermost magnetic particle stability-aided spin orientation with a fascinating magnetic moment in superparamagnetic region exerted by Ni and Co remarkably postured as a significant part of these nanoparticles.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Superparamagnetic Behavior of Zn and Al Substituted Cobalt Nanoferrites

Nanostructured Zn- and Al-doped cobalt spinel ferrites Zn{sub x}Co{sub 1-x}Fe{sub 2-x}Al{sub x}O{sub 4} (0 ≤ x ≤ 1.0) with particle size in the range of 7–13 nm were synthesized by glycol-thermal reaction. XRD analysis confirmed single-phase cubic spinel structure with no impurity phases. The Mössbauer spectra for Zn- and Al-substituted fine powders could be resolved into two quadrupole doublets associated with {sup 57}Fe nuclei in paramagnetic spin phase. An unusual reducing magnetization with increasing particle size has been observed. The field cooled and zero field cooled magnetization measurements show superparamagnetic nanoparticles in the compounds investigated. The blocking temperatures (T{sub B}) are sensitive to particle size. Larger particles are blocked at higher temperatures compared to smaller particles at the same field.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

Intracellular trafficking of superparamagnetic iron oxide nanoparticles conjugated with TAT peptide: 3-dimensional electron tomography analysis

Highlights: Black-Right-Pointing-Pointer We study the intracellular localisation of TAT-SPIONs using 3-D electron tomography. Black-Right-Pointing-Pointer 3-D images of TAT-SPIONs in a cell are clearly shown. Black-Right-Pointing-Pointer Release of TAT-SPIONs from endocytic vesicles into the cytoplasm is clearly shown. -- Abstract: Internalisation of nanoparticles conjugated with cell penetrating peptides is a promising approach to various drug delivery applications. Cell penetrating peptides such as transactivating transcriptional activator (TAT) peptides derived from HIV-1 proteins are effective intracellular delivery vectors for a wide range of nanoparticles and pharmaceutical agents thanks to their amicable ability to enter cells and minimum cytotoxicity. Although different mechanisms of intracellular uptake and localisation have been proposed for TAT conjugated nanoparticles, it is necessary to visualise the particles on a 3-D plane in order to investigate the actual intracellular uptake and localisation. Here, we study the intracellular localisation and trafficking of TAT peptide conjugated superparamagnetic ion oxide nanoparticles (TAT-SPIONs) using 3-D electron tomography. 3-D tomograms clearly show the location of TAT-SPIONs in a cell and their slow release from the endocytic vesicles into the cytoplasm. The present methodology may well be utilised for further investigations of the behaviours of nanoparticles in cells and eventually for the development of nano drug delivery systems.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2012

Superparamagnetic iron oxide nanoparticles as radiosensitizer via enhanced reactive oxygen species formation

Highlights: Black-Right-Pointing-Pointer Ultrasmall citrate-coated SPIONs with {gamma}Fe{sub 2}O{sub 3} and Fe{sub 3}O{sub 4} structure were prepared. Black-Right-Pointing-Pointer SPIONs uptaken by MCF-7 cells increase the ROS production for about 240%. Black-Right-Pointing-Pointer The SPION induced ROS production is due to released iron ions and catalytically active surfaces. Black-Right-Pointing-Pointer Released iron ions and SPION surfaces initiate the Fenton and Haber-Weiss reaction. Black-Right-Pointing-Pointer X-ray irradiation of internalized SPIONs leads to an increase of catalytically active surfaces. -- Abstract: Internalization of citrate-coated and uncoated superparamagnetic iron oxide nanoparticles by human breast cancer (MCF-7) cells was verified by transmission electron microscopy imaging. Cytotoxicity studies employing metabolic and trypan blue assays manifested their excellent biocompatibility. The production of reactive oxygen species in iron oxide nanoparticle loaded MCF-7 cells was explained to originate from both, the release of iron ions and their catalytically active surfaces. Both initiate the Fenton and Haber-Weiss reaction. Additional oxidative stress caused by X-ray irradiation of MCF-7 cells was attributed to the increase of catalytically active iron oxide nanoparticle surfaces.

63 RADIATION, THERMAL, AND OTHER ENVIRON. POLLUTAN↗
DOE OSTI2010

Surface modification of superparamagnetic iron nanoparticles with calcium salt of poly({gamma}-glutamic acid) as coating material

Surface-modified magnetite nanoparticles (MNPs) were synthesized by co-precipitation of aqueous solution of ferrous and ferric salts (molar ratio 1:2) upon adding a base followed by calcium salt of poly({gamma}-glutamic acid) (Ca-{gamma}-PGA) for uniform coating on the surface of MNPs. Both uncoated and Ca-{gamma}-PGA-coated MNPs were characterized using various techniques including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and vibrating sample magnetometric (VSM) studies. Compared with bare MNPs, the IR spectra of coated MNPs showed characteristic peaks of {gamma}-PGA, implying the {gamma}-PGA coating on MNPs did occur. The TEM images depicted an average size of 8-10 nm for bare MNPs and 14 nm for coated MNPs, with their shape being spherical in nature. In the presence of applied magnetic field, a superparamagnetic behavior was observed at room temperature for both bare and Ca-{gamma}-PGA-coated MNPs, with no magnetism left upon magnetic-field removal.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Synthesis of amino-silane modified superparamagnetic Fe{sub 3}O{sub 4} nanoparticles and its application in immobilization of lipase from Pseudomonas fluorescens Lp1

Highlights: ► Magnetic nanoparticles were synthesized by chemical co-precipitation method. ► Surface was functionalized with amino-silane and used for lipase immobilization. ► Characterized through TEM, SEM, XRD, FT-IR and VSM analysis. ► The functionalization and immobilization did not affect the magnetite properties. ► The immobilized lipase showed greater functional property than free lipase. - Abstract: Superparamagnetic nanoparticles (Fe{sub 3}O{sub 4}–magnetite) were prepared by chemical co-precipitation method and their surface was functionalized with 3-aminopropyltriethoxysilane via silanization reaction to obtain amino functionalized magnetic nanoparticles. The purified lipase from Pseudomonas fluorescens Lp1 was immobilized onto functionalized magnetite using glutaraldehyde as the coupling agent. The characterization of the nanoparticles was done by scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, vibrating sample magnetometry and Fourier transformed infrared spectroscopy. The size of the magnetite was measured about 10–30 nm. The results of characterization study revealed the successful immobilization of lipase on to functionalized magnetite. The saturation magnetization of magnetic nanoparticles was found to be 28.34 emu/g whereas the immobilized magnetic nanoparticle was 17.074 emu/g. The immobilized lipase had greater activity at 50 °C and thermal stability upto 70 °C. It exhibited excellent reusability for 4 cycles and storage stability upto 15 days by retaining 75% of its initial activity.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Preparation and characterization of bifunctional dendrimer modified Fe{sub 3}O{sub 4}/CdTe nanoparticles with both luminescent and superparamagnetic properties

Highlights: • The fluorescent superparamagnetic dendrimeric Fe{sub 3}O{sub 4}/CdTe nanoparticles are synthesized in this paper. • The synthesized nanocomposites maintain excellent magnetic properties. • The synthesized nanocomposites maintain highly luminescent markers with narrow emission bands. - Abstract: Magnetic nanoparticles Fe{sub 3}O{sub 4} were prepared by hydrothermal coprecipitation of ferric and ferrous ions using NaOH. The surface modification of Fe{sub 3}O{sub 4} nanoparticle by dendrimers has rendered the nanoparticle surface with enriched amine groups which facilitated the adsorption and conjugation of thioglycolic acid (TGA) modified CdTe quantum dots to form a stable hybrid nanostructure. Three generations (first generation: G0F, second generation: G1F, third generation: G3F) of bifunctional dendrimeric Fe{sub 3}O{sub 4}/CdTe nanoparticles were successfully prepared using this technique and characterized by microscopy. The optical and magnetic properties of the dendrimeric Fe{sub 3}O{sub 4}/CdTe nanoparticle were also investigated. The microscopic study reveals 3 different sizes for 3 generations, 16 nm (G0F), 31 nm (G1F) and 47 nm (G3F). Among three generations of nanoparticles, the G1F has the best optical property with a luminescent quantum yield of 25.6% and the G0F has the best magnetic property with a saturation magnetization of 19.3 emμ/g.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Unusual superparamagnetic behavior in bulk Ba 0.198 La 0.784 Ti 0.096 Fe 0.8 O 3-δ

Here, we report the observation of superparamgnetic behavior induced by the substitution La 3+ by Ba 2+ at A-sites and Fe +3 by Ti 4+ at B-sites. Ba 0.198 La 0.784 Ti 0.096 Fe 0.8 O 3-δ was synthesized via solid state reaction. The Rietveld refinements revealed an orthorhombic (Pbnm). The low-mag STEM results showed that the particles were found to exhibit square like shape and formed in single crystal with high degree of crystallinity. The particle size ranges of 50 300 nm. Element mapping demonstrates a complementary distribution between Ba and La, while homogenous distribution is found for Fe and O. The optical band gap determined from UV-vis analysis is equal to 1.93 eV which is smaller than the reported one for LaFeO 3 and BaTiO 3 . The hysteresis loops at 300 K and 10 K show superparamagnetic behavior, which deviate extremely from the antiferromagnetic nature of LaFeO 3 perovskite. The temperature dependent magnetization demonstrates that the material kept a net magnetization of 12.29 (emu/g) at very low temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Superparamagnetic iron oxide polyacrylic acid coated γ-Fe{sub 2}O{sub 3} nanoparticles do not affect kidney function but cause acute effect on the cardiovascular function in healthy mice

This study describes the distribution of intravenously injected polyacrylic acid (PAA) coated γ-Fe{sub 2}O{sub 3} NPs (10 mg kg{sup −1}) at the organ, cellular and subcellular levels in healthy BALB/cJ mice and in parallel addresses the effects of NP injection on kidney function, blood pressure and vascular contractility. Magnetic resonance imaging (MRI) and transmission electron microscopy (TEM) showed accumulation of NPs in the liver within 1 h after intravenous infusion, accommodated by intracellular uptake in endothelial and Kupffer cells with subsequent intracellular uptake in renal cells, particularly the cytoplasm of the proximal tubule, in podocytes and mesangial cells. The renofunctional effects of NPs were evaluated by arterial acid–base status and measurements of glomerular filtration rate (GFR) after instrumentation with chronically indwelling catheters. Arterial pH was 7.46 ± 0.02 and 7.41 ± 0.02 in mice 0.5 h after injections of saline or NP, and did not change over the next 12 h. In addition, the injections of NP did not affect arterial PCO{sub 2} or [HCO{sub 3}{sup −}] either. Twenty-four and 96 h after NP injections, the GFR averaged 0.35 ± 0.04 and 0.35 ± 0.01 ml min{sup −1} g{sup −1}, respectively, values which were statistically comparable with controls (0.29 ± 0.02 and 0.33 ± 0.1 ml{sup –1} min{sup –1} 25 g{sup –1}). Mean arterial blood pressure (MAP) decreased 12–24 h after NP injections (111.1 ± 11.5 vs 123.0 ± 6.1 min{sup −1}) associated with a decreased contractility of small mesenteric arteries revealed by myography to characterize endothelial function. In conclusion, our study demonstrates that accumulation of superparamagnetic iron oxide nanoparticles does not affect kidney function in healthy mice but temporarily decreases blood pressure. -- Highlights: ► PAA coated γ-Fe{sub 2}O{sub 3} nanoparticles were injected intravenously into healthy mice. ► We examine the distribution and physiological effects of nanoparticle accumulation. ► No effects of accumulation in kidney on acid-base status or GFR. ► Acute hypotension associated with decreased contractility of small vessels. ► We demonstrate physiological effects of PAA γ-Fe{sub 2}O{sub 3} NP for medical purposes.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

Aqueous Superparamagnetic Magnetite Dispersions with Ultrahigh Initial Magnetic Susceptibilities

Superparamagnetic nanoparticles with a high initial magneti susceptibility χo are of great interest in a wide variety of chemical, biomedical, electronic, and subsurface energy applications. In order to achieve the theoretically predicted increase in χ o with the cube of the magnetic diameter, new synthetic techniques are needed to control the crystal structure, particularly for magnetite nanoparticles larger than 10 nm. Aqueous magnetite dispersions (Fe 3 O 4 ) with a χ o of 3.3 (dimensionless SI units) at 1.9 vol %, over 3- to 5-fold greater than those reported previously, were produced in a one-pot synthesis at 210 °C and ambient pressure via thermal decomposition of Fe(II) acetate in triethylene glycol (TEG). The rapid nucleation and focused growth with an unusually high precursor-to-solvent molar ratio of 1:12 led to primary particles with a volume average diameter of 16 nm and low polydispersity according to TEM. The morphology was a mixture of stoichiometric and substoichiometric magnetite according to X-ray diffraction (XRD) and Mössbauer spectroscopy. Finally, the increase in χ o with the cube of magnetic diameter as well as a saturation magnetization approaching the theoretical limit may be attributed to the highly crystalline structure and very small nonmagnetic layer (~1 nm) with disordered spin orientation on the surface.

77 NANOSCIENCE AND NANOTECHNOLOGY↗