Small-diameter Nanoparticles Research Articles

Incidence of the Brownian Relaxation Process on the Magnetic Properties of Ferrofluids.

Ferrofluids containing magnetic nanoparticles represent a special class of magnetic materials due to the added freedom of particle tumbling in the fluids. We studied this process, known as Brownian relaxation, and its effect on the magnetic properties of ferrofluids with controlled magnetite nanoparticle sizes. For small nanoparticles (below 10 nm diameter), the Néel process is expected to dominate the magnetic response, whereas for larger particles, Brownian relaxation becomes important. Temperature- and magnetic-field-dependent magnetization studies, differential scanning calorimetry, and AC susceptibility measurements were carried out for 6 and 13.5 nm diameter magnetite nanoparticles suspended in water. We identify clear fingerprints of Brownian relaxation for the sample of large-diameter nanoparticles as both magnetic and thermal hysteresis develop at the water freezing temperature, whereas the samples of small-diameter nanoparticles remain hysteresis-free down to the magnetic blocking temperature. This is supported by the temperature-dependent AC susceptibility measurements: above 273 K, the data show a low-frequency Debye peak, which is characteristic of Brownian relaxation. This peak vanishes below 273 K.

The Influence of the Calcination Time on Synthesis of Nanomaterials with Small Size, High Crystalline Nature and Photocatalytic Activity in the TiO2 Nanoparticles Calcined at 500 °C

The development of new materials with diverse applications that fit well in the context of the current economy, where energy issues abound, is paramount. The goal of this study was to generate materials with high photocatalytic properties, at low cost and with less energy, and without health and ecological risks. Such materials would allow for a form of sustainable development that respects nature. This study investigated the influence of calcination time on titanium dioxide nanoparticles (TiO2 NPs) produced by green synthesis using Aloe vera leaf extract under a constant temperature of 500 °C. The interaction between synthesis conditions like calcination time and the size of nanoparticles produced in relation to changes in photocatalytic activity were analyzed and discussed. The results showed that when calcination was increased at 500 °C, the synthesis of small-diameter nanoparticles was promoted. TiO2 were 23 ± 2 nm (D1) and 83 ± 5 nm (D2) after 5 h and 1 h of calcination, respectively. Moreover, the calcination duration promoted an increase in crystalline nature. In the same way, the level of reduction of azo dye Remazol Red Brilliant F3B (RR180) increased when calcination time increased, and therefore, changed the optic and photo-catalytic properties of the TiO2 nanomaterial. In addition, TiO2 nanopowders (size 23 ± 2 nm) had the higher efficiency in photodegradation (100%) of dye RR180 under visible light irradiation for 60 min for up to one hour duration, but TiO2 NPs (83 ± 5 nm) had the higher efficiency (100%) for up to two hours duration.

Heat Transfer Enhancement From Inline and Staggered Arrays of Cylinders in a Heat Exchanger Using Alumina–Water Nanofluid

Abstract The flow of alumina–water nanofluid across heated circular tubes arranged in inline and staggered arrays in a heat exchanger has been studied numerically using the finite volume method (FVM). For calculating the nanofluid’s thermophysical properties such as effective thermal conductivity and effective viscosity, Corcione’s correlations are utilized. Corcione’s correlations consider nanoparticles size, their Brownian motion, and operating temperature while calculating these effective properties of nanofluids. The impact of three parameters on heat transfer characteristics across inline and staggered arrays of heated circular cylinders has been examined. These parameters are nanoparticle diameter dp, which is varied between 10 nm and 50 nm, nanoparticle volume fraction ɸ varying from 0.01 to 0.05, and Reynolds number Re ranging from 10 to 200. It is observed that heat transfer augmentation across both inline and staggered arrays occurs when nanoparticle concentration is increased and smaller diameter nanoparticles are used. Mean Nusselt number NuM is increased by 31% when ɸ is increased from 0.01 to 0.05 at Re = 200 and dp = 10 nm in an inline array and by 25% in a staggered array. NuM is enhanced by 20% for the inline array and 16% for the staggering array when dp decreases from 50 nm to 10 nm at Re = 200 and ɸ = 0.05. At any given value of dp, ɸ, and Re, the mean Nusselt number is always higher for staggered array in comparison with the inline array. The results reported in the present study can be utilized for the optimal design of various heat exchange systems under the given operating conditions. The present results are extensively validated with the available experimental/numerical studies.

Small onion-like BN leads to ultrafine-twinned cubic BN

Nanotwinned cubic boron nitride (nt-cBN) with remarkable hardness, toughness, and stability has attracted widespread attention due to its distinct scientific and industrial importance. The key for nt-cBN synthesis is to adopt an onion-like BN (oBN) nano-precursor and induce phase transition under high pressure. Here, we found that the size change of oBN used greatly affected the mechanical performance of products. With the precursor size decreasing from ~320 to 90 nm, the Vickers hardness of nanostructured products improved from 61 to 108 GPa, due to the fact that large oBN nanoparticles possessed more flattened, orderly and graphite-like shell layers, in sharp contrast to the highly wrinkled and imperfect layers in small-diameter nanoparticles, thus resulting in the apparent reduction of ultrafine-twin substructure in the synthetic products. This study reveals that only small oBN precursor could produce complete ultrafine nt-cBN with outstanding performance. A practical route was proposed to further improve the performance of this important material.

Magnetic Nanoparticles Obtained by Homogeneous Coprecipitation Sonochemically Assisted

The present work demonstrates preparation of magnetic nanoparticles by a novel method. Magnetic nanoparticles with potential for biomedical and environmental applications were obtained by homogeneous coprecipitation method sonochemically assisted. The effect of ultrasonic cavitation on changing the reaction environment was studied. The chemical reaction media used here was very similar to those used for Sono-Fenton process. The ultrasonic energy has driven the growth of particles; smaller diameter nanoparticles were obtained by applying a higher power. For the first time, it was demonstrated the iron oxide nanoparticles synthesis at pH lower than 6.

Plasmonic nanocrystal solar cells utilizing strongly confined radiation.

The ability of metal nanoparticles to concentrate light via the plasmon resonance represents a unique opportunity for funneling the solar energy in photovoltaic devices. The absorption enhancement in plasmonic solar cells is predicted to be particularly prominent when the size of metal features falls below 20 nm, causing the strong confinement of radiation modes. Unfortunately, the ultrashort lifetime of such near-field radiation makes harvesting the plasmon energy in small-diameter nanoparticles a challenging task. Here, we develop plasmonic solar cells that harness the near-field emission of 5 nm Au nanoparticles by transferring the plasmon energy to band gap transitions of PbS semiconductor nanocrystals. The interfaces of Au and PbS domains were designed to support a rapid energy transfer at rates that outpace the thermal dephasing of plasmon modes. We demonstrate that central to the device operation is the inorganic passivation of Au nanoparticles with a wide gap semiconductor, which reduces carrier scattering and simultaneously improves the stability of heat-prone plasmonic films. The contribution of the Au near-field emission toward the charge carrier generation was manifested through the observation of an enhanced short circuit current and improved power conversion efficiency of mixed (Au, PbS) solar cells, as measured relative to PbS-only devices.

Synthesis and characterization of Fe–B nanoparticles for potential magnetic applications

The crystallization and structure of Fe-B nanoparticles (NPs) of different sizes formed in a single process by gas aggregation from Fe80B20 targets were analyzed by transmission electron microscopy. It is con- cluded that all NPs are covered by an amorphous Fe-B shell while the crystal structure of the NPs core depends on their size. Large NPs with diameters C30 nm are mono- crystalline tetragonal Fe3B, small diameter NPs (B20 nm) are completely amorphous whereas in middle size NPs, with diameters between 20 and 30 nm, difference Fe-B phases (tetragonal Fe3B and orthorhombic FeB) together with defaulted areas are observed. This work opens new possibilities to produce Fe-B NPs tailoring their magnetic properties by controlling their size and composition.

Influence of Scaffold Size on Bactericidal Activity of Nitric Oxide-Releasing Silica Nanoparticles

A reverse microemulsion synthesis was used to prepare amine-functionalized silica nanoparticles of three distinct sizes (i.e., 50, 100, and 200 nm) with similar amine content. The resulting hybrid nanoparticles, consisting of N-(6-aminohexyl)aminopropyltrimethoxysilane and tetraethoxysilane, were highly monodisperse in size. N-Diazeniumdiolate nitric oxide (NO) donors were subsequently formed on secondary amines while controlling reaction conditions to keep the total amount of NO released constant for each particle size. The bactericidal efficacy of the NO-releasing nanoparticles against Pseudomonas aeruginosa increased with decreasing particle size. Additionally, smaller diameter nanoparticles were found to associate with the bacteria at a faster rate and to a greater extent than larger particles. Neither control (non-NO-releasing) nor NO-releasing particles exhibited toxicity toward L929 mouse fibroblasts at concentrations above their respective minimum bactericidal concentrations. This study represents the first investigation of the bactericidal efficacy of NO-releasing silica nanoparticles as a function of particle size.

Numerical study of turbulent flow and heat transfer characteristics of nanofluids considering variable properties

Turbulent flow and heat transfer of three different nanofluids (CuO, Al 2O 3 and SiO 2) in an ethylene glycol and water mixture flowing through a circular tube under constant heat flux condition have been numerically analyzed. New correlations for viscosity up to 10% volume concentration for these nanofluids as a function of volume concentration and temperature are developed from the experiments and are summarized in the present paper. In our numerical study, all the thermophysical properties of nanofluids are temperature dependent. Computed results are validated with existing well established correlations. Nusselt number prediction for nanofluids agrees well with Gnielinski correlation. It is found that nanofluids containing smaller diameter nanoparticles have higher viscosity and Nusselt number. Comparison of convective heat transfer coefficient of CuO, Al 2O 3 and SiO 2 nanofluids have been presented. At a constant Reynolds number, Nusselt number increases by 35% for 6% CuO nanofluids over the base fluid.