Room temperature solvent-free synthesis of monodisperse magnetite nanocrystals

We have successfully demonstrated a facile, solvent-free synthesis of highly crystalline and monodisperse Fe3O4 nanocrystallites at ambient temperature avoiding any heating. Solid state reaction of inorganic Fe(ll) and Fe(ll) salts with NaOH was found to produce highly crystalline Fe3O4 nanoparticles. The reaction, if carried out in the presence of surfactant such as oleic acid-oleylamine adduct, generated monodisperse Fe3O4 nanocrystals extractable directly from the reaction mixture. The extracted nanoparticles were capable of forming self-assembled, two-dimensional and uniform periodic array. The new process utilizes inexpensive and nontoxic starting materials, and does not require a use of high boiling point and toxic solvents, thus is amenable to an environmentally desirable, large-scale synthesis of nanocrystals.     

Synthesis of nanocrystals of gadolinium carbonate by reaction crystallization

The formation of nano-sized crystals of gadolinium carbonate via reaction crystallization was studied in a semi-batch crystallizer using gadolinium chloride and ammonium hydrogen carbonate as the reactants. The gadolinium carbonate crystals were formed by the aggregation of primary particles sized about 5 nm. Thereby, the crystallization parameters acting directly on the aggregation of the primary particles, such as the reactant concentrations, non-stoichiometry of the reactants, solution pH, acoustic energy, and agitation speed, were mechanistically investigated. As such, increasing the reactant concentrations enhanced the crystal size due to higher nucleation of the primary particles for the aggregation. Non-stoichiometric reactant concentrations resulted in a significant reduction of the crystal size, due to the adsorption of the excess species on the primary particles. Similarly, the surface charge of the primary particles depended on the solution pH. Thus, the crystal size was reduced when the pH deviated from the neutral point. The acoustic cavitation of the ultrasound was much more effective than the turbulent fluid motion of the agitation in inhibiting the primary particle aggregation. Thus, the crystal size was remarkably reduced, even at a low acoustic energy of 6 watts.     

First-order metal-insulator transition and infrared identification of shape-controlled magnetite nanocrystals

The first-order metal-insulator transition (MIT) in magnetite has been known for a long time but is still controversial in its nature. In this study, well-defined magnetite nanocrystals (NCs) with controllable size, shape and terminated surface are first employed to elucidate this important issue, and new discoveries such as a highly suppressed phase transition temperature are identified by monitoring the variable-temperature electric resistance and infrared spectroscopy. Significantly, by carefully comparing the infrared vibrational bands of the as-prepared magnetite NCs with octahedral and cubic shapes, respectively, we found that these two forms of magnetite NCs exhibited different transmittance changes and frequency shifts of the infrared characteristics, presumably due to the differences in the lattice distortions on the corresponding {001} and {111} terminal surfaces. This result produced evidence in support of the charge ordering of Fe atoms along the low dimensionality at octahedral B sites undergoing the MIT. Taken together, infrared identification was proposed to be an available characterization strategy for MIT, which can reflect more information on the elusive lattice distortion of crystallographic structure or exposed surfaces.     

Effect of solubility YAG:Nd nanocrystals in glass matrix

The nanocomposites of Y₃Al₅O₁₂:Nd³⁺ (YAG:Nd) incorporated in borate glass were obtained. The single phase of YAG:Nd nanocrystals were obtained by sol-gel method. The borate glass was melted first and ground up then mixed with the nanocrystals. The samples were formed into pellets under pressure and were annealed in temperatures from the range 550-800 °C. The X-ray diffraction patterns show that together with increasing the temperature the contribution of Y₃Al₅O₁₂ phase decreases and the new YBa₃B₉O₁₉ phase is observed. The luminescence measurements indicates that the band structures and distribution of band intensities of glass-YAG:Nd nanocrystal composites depends crucially on annealing temperature.     

Visible upconversion emission of Pr3+ doped gadolinium gallium garnet nanocrystals

The luminescence properties of a Pr3+-doped gadolinium gallium garnet (GGG, Gd3Ga5O12) nanocrystalline host were investigated. Dominant blue/green emission was observed emanating from the 3P0 --> 3H4 transition after excitation using a wavelength of 457.9 nm. Continuous wave excitation into the 1D2 level of the Pr3+ ion at 606.9 nm transition produced blue upconversion luminescence spectra, ascribed to emission from the 3P1 --> 3H4 and 3P0 --> 3H4 transitions. The increase in the decay times of the observed transitions following excitation with 606.9 nm is indicative of the dominance of an energy transfer upconversion (ETU) mechanism relative to excited state absorption (ESA). Furthermore, blue, green and red upconversion emission was observed from the 3P0, 3P1 and 1D2 states following excitation into the 1G4 energy level with 980 nm. No change in the decay times of the emitting states was observed following excitation with a wavelength of 980 or 457.9 nm; hence, upconversion was determined to primarily occur through ESA. The luminescence properties of the nanocrystals are compared to a single crystal of GGG:Pr3+ (bulk) with an identical Pr3+ concentration (1%).     

Magnetometric diagnostics of ferritin in living matter

Investigations in a broad sample set of living matter, including plants and silty soil fractions from various climate and soil zones, microorganisms, and muscle tissues of birds and mammals (a total of about 200 samples) show that iron in native substances occurs in the form of nanodimensional particles of hydroxide (ferrihydrite). Based on the data of Mössbauer spectroscopy and magnetic measurements, these nanoparticles can be identified as nuclei of the globular protein ferritin. Most measurements were performed at 290 K (i.e., without using cryogenic techniques).     

Microstructural effect of gadolinium oxide nanocrystals upon annealing on electrical properties of memory devices

The microstructure evolution of sputtered gadolinium oxide nanocrystal (NC) memory devices upon annealing has been characterized in detail by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). TEM results indicate that the as-deposited film is composed of metallic Gd clusters embedded in an amorphous Gd_xO_y matrix. The Gd clusters undergo phase transformation to oxide NCs upon annealing, reaching a maximum density of 7.9-9.1 × 10¹¹ cm^− 2 at 850 °C, which is consistent with the largest memory window width. Upon annealing at even higher temperature, TEM diffraction patterns and XPS composition profiles indicate apparent Si diffusion into the NC layer, probably from the SiO₂ tunneling oxide or the Si substrate, leading to the formation of gadolinium silicate NCs. The presence of silicate NCs gradually deteriorates the device performance due to the reduction of barrier confinement for stored charges, although the dot density is only marginally decreased. The results suggest that the optimum memory device performance is dominated by not only the most considered size and density of NCs, but also the composition and phase inside.     

Synthesis of pyramidal,cubical and truncated octahedral magnetite nanocrystals by controlling reaction heating rate

Pyramidal, cubical and truncated octahedral magnetite nanocrystals have been synthesized by thermal de-composition of iron (III) acetylacetone (Fe(acac)₃) in the presence of oleic acid under various reaction rate controlled by heating rate. The magnetite nanocrystals were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). High-resolution transmission electron microscopy (HRTEM) was applied to reveal the structural information of single magnetite (Fe₃O₄) nanocrystals. Magnetization curves of the three types of magnetite nanocrystals show that the pyramidal crystals exhibit a slight hysteresis compared to the other two despite of the similar size range. The results suggest that in addition to the surfactant selective capping and varying reaction temperature, the reaction rate variation is also an effective means for controlling the morphology and functions of the magnetite nanocrystals.     

Benchtop MRI for pharmacokinetic evaluation of two aqueous-based nano-scaled formulations of oleic acid stabilized magnetite nanocrystals

Background: The interplay between numerous factors, including the size, shape, coating, surface charge and composition of particles is known to affect the pharmacokinetics and biodistribution of superparamagnetic iron oxides (SPIOs). This makes understanding the role of each factor independently quite challenging.

Methods: In the present study, the in vivo magnetic resonance imaging (MRI), biodistribution and hepatic clearance evaluations of two SPIOs Formulations A and B developed from ～13.5?nm hydrophobic oleic acid stabilized monodisperse magnetite nanocrystals core and lipid-based amphiphilic stabilizers were performed using a prototype benchtop MR imager (22?MHz) and pulsed nuclear magnetic resonance (NMR) system (20?MHz), respectively. Formulation A was composed of mPEG-2000-DSPE and Formulation B was composed of Phospholipon-100H, sucrose ester M-1695 and Cremophor RH-40.

Results: The in vivo MRI investigations showed that both formulations were safe and effective as potential liver MR contrast agents with sustained liver contrast for at least seven days. In addition, ex vivo relaxometric investigations revealed that the formulations predominantly distribute to the liver and spleen following I.V. injection. The hepatic clearance kinetics determined based on the relaxometric quantification method indicated that both formulations exhibited a biphasic clearance process with a slow terminal clearance half-life of 11.5 and 12.7 days, respectively, for Formulations A and B.

Conclusions: The results of this study showed the potential biomedical applications of the investigated magnetopharmaceutical formulations as MRI contrast agents.     

不同尺度下金属钆的磁热效应研究

制备了微米级金属钆粉,并用氢电弧等离子装置制备了纳米级的钆粉,同时用扫描电镜(SEM)以及透射电镜(TEM)对不同粒度的金属钆进行观察分析,在振动样品磁强计(VSM)上测量了样品的磁性能,并计算了样品的磁熵变.结果表明,粒度对钆的磁热效应有较大的影响,而对居里温度基本上没有什么影响.粒度为100～154μm钆的在外场变化为1591.55kA/m的最大磁熵变为1.8813 J/kg·K,而纳米级的钆的磁热效应远小于微米粉末以及块状钆的磁热效应.     

Magnetometric determination of the angular position of an object

A method is described for finding the angular position of an object by measuring at two selected points in space the components of an alternating magnetic field set up by two sources. The angular position of the object and its coordinates can be measured even if the object is moved or rotated about any axis through 0 to 360° in the presence of magnetic interference. Translated from Izmeritel'naya Tekhnika, No. 12, pp. 34–37, December, 1996.     

Precise Magnetometric Diagnostics of Critical-Current Inhomogeneities in High-Temperature Semiconductor Tapes

Technical Physics Letters - We present a new precise method of contactless magnetometric determination of the local critical current in high-temperature superconductor (HTS) tapes, which involves...     

Recording Defects in Rotating Systems with a Magnetometric Hall Detector

A system is considered for recording defects in rotary systems. The detectors are provided by Hall-effect sensors. Synchronous accumulation is used to eliminate noise. Numerical simulation results are given.     

An experimental study of hydrogen solubility in liquid aluminium

Hydrogen solubility in molten aluminium at different temperatures from 973–1123 K, has been measured using Sieverts' method. Inert gas (helium or argon) was used as a reference gas to calibrate the measurement system of the Sieverts' apparatus. The measured hydrogen solubility was found to vary with the reference gases. Helium was detected to be soluble in liquid aluminium. When helium is used as the reference gas, its solubility resulted in lower measured hydrogen solubility than that when argon was used to calibrate the measurement system of the apparatus. Argon gas was therefore considered as an appropriate reference gas when Sieverts' method is used to measure the hydrogen solubility in liquid aluminium. The hydrogen solubility, S, in liquid aluminium as a function of melt temperature, T, determined in the present investigation is expressed as log S = (−2980/T) + 3.07.     

Magnetometric Demagnetizing Factors for Various Shapes of Rotational Symmetry

We have investigated the magnetometric demagnetizing factors of rotation-symmetrical magnetic bodies of relative permeability $mu$ placed in a uniform magnetic field parallel to the axis of rotation. The bodies we considered include a cylinder, a sphere, a bicone, and a rotating astroid. We numerically calculated the magnetometric demagnetizing factors from the surface magnetic charge densities $sigma$ obtained by the surface magnetic charge simulation method. We show that the magnetometric demagnetizing factors for various shapes of rotation-symmetrical bodies are significantly different from those of a sphere. We have determined the shape producing the largest demagnetizing field in ferromagnetic bodies with sufficiently large relative permeability $mu$ . We report the magnetometric demagnetizing factors for uniformly magnetized bodies ($mu=1$ , i.e., susceptibility $chi=0$ ) of various shapes.      

Extracellular synthesis of magnetite and metal-substituted magnetite nanoparticles

We have developed a novel microbial process that exploits the ability of Fe(III)-reducing microorganisms to produce copious amounts of extracellular magentites and metal-substituted magnetite nanoparticles. The Fe(III)-reducing bacteria (Theroanaerobacter ethanolicus and Shewanella sp.) have the ability to reduce Fe(III) and various metals in aqueous media and form various sized magnetite and metal-substituted magnetite nano-crystals. The Fe(III)-reducing bacteria formed metalsubstituted magnetites using iron oxide plus metals (e.g., Co, Cr, Mn, Ni) under conditions of relatively low temperature (<70 degrees C), ambient pressure, and pH values near neutral to slightly basic (pH = 6.5 to 9). Precise biological control over activation and regulation of the biosolid-state processes can produce magnetite particles of well-defined size (typically tens of nanometers) and crystallographic morphology, containing selected dopant metals into the magnetite (Fe(3-y)XyO4) structure (where X = Co, Cr, Mn, Ni). Magnetite yields of up to 20 g/L per day have been observed in 20-L vessels. Water-based ferrofluids were formed with the nanometer sized, magnetite, and metal-substituted biomagnetite particles.     

二氧化钛纳米晶的制备及其光致发光的研究

以Ti（SO4）2为前驱体,采用沉淀法制备了二氧化钛纳米晶,通过X射线衍射（XRD）、透射电子显微镜（TEM）和光致发光（PL）光谱对微晶进行表征。研究表明,制备的TiO2纳米晶呈类球形颗粒且分散性好,平均粒径最小约为4nm,热处理温度升高到750℃时,样品仍为单一的锐钛矿相;PL谱分析表明,室温下,用高于带隙的能量激发,在370～550nm范围内纳米TiO2粒子呈现出强而宽的发光带,分别对应于价带和导带间的电子跃迁以及表面态的发光;另外发现随焙烧温度升高,粒径增大的同时,发光峰的强度呈无规则变化,分析可能与表面态和晶粒内部缺陷的数量有关。     

Preparation and spectroscopy study of monodispersed metal oxide nanocrystals

SnO₂ nanocrystals with an average particle size of 3.5 nm are obtained by heating the mixture of hydrous SnO₂ nanoparticles and SrCO₃ particles. SrCO₃ can be removed from the product by washing with diluted HNO₃ acid. A series of SnO₂ nanocrystal samples with different particle size are prepared by heating at different temperature. All the samples obtained are characterized by XPS and Raman spectra. XPS result shows that the binding energy of Sn3d_5/2 decreases as the particle size increases. There is a surface-related Raman peak observed, which increases in intensity when the SnO₂ particles size decreases. The size-related change of the surface structure is responsible for the properties of nanocrystals.     

GISAXS study of Si nanocrystals formation in SiO2 thin films

We present a study on amorphous SiO/SiO₂ superlattice using grazing incidence small-angle X-ray scattering (GISAXS). Amorphous SiO/SiO₂ superlattices were prepared by high vacuum evaporation of 3 nm thin films of SiO and SiO₂ (10 layers each) on Si(100) substrate. After the evaporation, samples were annealed at 1100 °C for 1 h in vacuum, yielding Si nanocrystals formation. Using a Guinier approximation, the shape and the size of the crystals were obtained.