Particle size effects on magnetic properties of yttrium iron garnets prepared by a sol–gel method

We present detailed measurements of field—and temperature—dependence of magnetization in nanocrystalline YIG (Y₃Fe₅O₁₂) particles. The fine powders were prepared using sol–gel method. Samples with particle sizes ranging from 45 to 450 nm were obtained. We observe that the saturation magnetization decreases as the particle size is reduced due to enhancement of the surface spin effects. Below a critical diameter D_s≅190 nm, the particles become single domains and the coercive forces reaches a maximum at diameters close to the critical value. As the particle size decreases the coercivity diminishes and at D_p≃35 nm diameters the upper limit of superparamagnetic behavior is reached. A quantitative comparison of temperature and particle size dependence of coercivity shows a satisfactory agreement that is expected for an assembly of randomly oriented particles.     

Filtration efficiency of a fibrous filter for nanoparticles

The filtration efficiency for nanoparticles down to 1 nm in size through glass fibrous filters was measured using an improved PSM-CNC system. In addition, the effects of relative humidity and particle charge were investigated for various nanoparticle diameters. The results show that the filtration efficiencies were independent of humidity and affected by particle charge in the case of particles below 100 nm in size. For particles smaller than 2 nm, the particle penetrations increased with decreasing particle size. These results suggest that the thermal rebound phenomena would be operative for nanoparticles with diameters below 2 nm, even though it would depend on the states of both the particles and the filter media. These results are particularly important for experimental investigations of the behavior of nanoparticles on a filter.     

Nanoparticle production by UV irradiation of combustion generated soot particles

Laser ablation of surfaces normally produce high temperature plasmas that are difficult to control. By irradiating small particles in the gas phase, we can better control the size and concentration of the resulting particles when different materials are photofragmented. Here, we irradiate soot with 193 nm light from an ArF excimer laser. Irradiating the original agglomerated particles at fluences ranging from 0.07 to 0.26 J/cm² with repetition rates of 20 and 100 Hz produces a large number of small, unagglomerated particles, and a smaller number of spherical agglomerated particles. Mean particle diameters from 20 to 50 nm are produced from soot originally having a mean electric mobility diameter of 265 nm. We use a non-dimensional parameter, called the photon–atom ratio (PAR), to aid in understanding the photofragmentation process. This parameter is the ratio of the number of photons striking the soot particles to the number of the carbon atoms contained in the soot particles, and is a better metric than the laser fluence for analyzing laser–particle interactions. These results suggest that UV photofragmentation can be effective in controlling particle size and morphology, and can be a useful diagnostic for studying elements of the laser ablation process.     

Nucleation of Polypropylene Crystallization with Gold Nanoparticles. Part 2: Relation between Particle Morphology and Nucleation Activity

The influence of gold nanoparticle morphology on nucleation of isotactic polypropylene (PP) crystallization was investigated. Previous experiments indicated certain nucleation activity of gold nanoparticles, varying with their size. In this work, eight types of gold micro/nanoparticles were used: vacuum-sputtered nanostructures (nanoparticles, nanoislands, and nanolayers), chemically prepared isometric gold nanocrystals (5, 20, and 100 nm diameters), and two types of gold microcrystals with well-developed crystal facets [with (100) and (111) facets, respectively]. To minimize the effect of particle agglomeration, we used our recently introduced sandwich method, in which the nucleating agent was deposited between thin PP films and the nucleation was evaluated by polarized light microscopy (PLM), X-ray scattering (WAXS), and differential scanning calorimetry (DSC). The nucleation activity of Au particles in PP was lower than it might be expected from the previous studies and depended on their morphology. The nucleation activity of Au microcrystals with well-developed facets was higher than the activity of non-faceted Au nanocrystals.     

Size Controlled Synthesis of Gold Nanoparticles using Photochemically Prepared Seed Particles

Gold nanoparticles having prechosen size ranging from 5 to 110 nm have been prepared in two steps. Firstly, small spherical particles (seed) of average diameters between 5 and 20 nm were prepared by varying the ratio of gold ion concentration to stabilizer/reductant, TX-100 concentration and using UV irradiation. Secondly, 20–110 nm particles were formed by a non-iterative seed-mediated growth where small particles produced by the above irradiation technique were exploited as seeds and fresh Au(III) ions were reduced onto the surface on the seed particles by ascorbic acid. The kinetics of particle formation has also been reported. These methods were fast and showed improved monodispersity sphericity and excellent reproducibility.     

Improved synthesis of aluminium nanoparticles using ultrasound assisted approach and subsequent dispersion studies in di-octyl adipate

The present work reports on an efficient and simple one pot synthetic approach for aluminium nanoflakes and nanoparticles based on the intensification using ultrasound and provides a comparison with the conventional approach to establish the cutting edge process benefits. In situ passivation of aluminium particles with oleic acid was used as the method of synthesis in both the conventional and ultrasound assisted approaches. The aluminium nanoflakes prepared using the ultrasound assisted approach were subsequently dispersed in di-octyl adipate (DOA) and it was demonstrated that a stable dispersion of aluminium nanoflakes into di-octyl adipate (DOA) is achieved. The morphology of the synthesized material was established using the transmission electron microscopy (TEM) analysis and energy dispersive X-ray analysis (EDX) and the obtained results confirmed the metal state and nano size range of the obtained aluminium nanoflakes and particles. The stability of the aluminium nanoflakes obtained using ultrasound assisted approach and nanoparticles using conventional approach were characterized using the zeta potential analysis and the obtained values were in the range of −50 to +50 mV and −100 to +30 mV respectively. The obtained samples from both the approaches were also characterized using X-ray diffraction (XRD) and particle size analysis (PSA) to establish the crystallite size and particle distribution. It was observed that the particle size of the aluminium nanoflakes obtained using ultrasound assisted approach was in the range of 7–11 nm whereas the size of aluminium nanoparticles obtained using conventional approach was much higher in the range of 1000–3000 nm. Overall it was demonstrated that the aluminium nanoflakes obtained using the ultrasound assisted approach showed excellent morphological characteristics and dispersion stability in DOA showing promise for the high energy applications.     

Bipolar diffusion charging of high-aspect ratio aerosols

Recent studies have raised concerns over applicability of the conventional charging theories to non-spherical particles such as soot aggregates and single-walled carbon nanotube aerosols of complex shape and morphology. It is expected that the role of particle structure and shape on particle diffusion charging characteristics may be significant in the submicron size range for carbon nanotubes (CNTs) and nanofibers (CNFs). In this study, we report experimental data on equilibrium charging characteristics of high-aspect ratio aerosol particles such as CNFs and multi-walled CNTs (MWCNTs) when exposed to a bipolar ion atmosphere. A neutral fraction was measured, i.e., the fraction of particles carrying no electrical charge. A differential mobility analyzer (DMA) was used to classify aerosols, leaving a bipolar radioactive charger to infer the bipolar charging characteristics at different mobility diameters in the submicron size range. The measured neutral fractions for CNF aerosol particles were lower than the corresponding Boltzmann values by 24.4%, 42.0%, and 45.8% for mobility diameters of 400 nm, 600 nm, and 700 nm, respectively, while the neutral fractions for measured aerodynamic diameters of 221 nm, 242 nm, and 254 nm were much lower than those expected by Boltzmann charge distribution, by 43.8%, 63.1%, and 67.3%, respectively. Neutral fractions of spherical particles of polystyrene latex (PSL) and diethylhexyl sebacate (DEHS) particles, measured under identical experimental conditions and procedure, agreed well with the Boltzmann charge distribution. The measured neutral fractions for MWCNT aerosol particles were lower than the corresponding Boltzmann values by 22.3%–25.0% for mobility diameters in the size range from 279 nm to 594 nm. Charging-equivalent diameters of CNF particles correlated well with either mobility diameter or equal-area diameter, which were found to be larger than their mobility or equal-area diameters by up to a factor of 5 in the size range of 400 nm–700 nm, while those of MWCNT particles were larger than the corresponding diameters by a factor of 2 in the size range of 279 nm–594 nm.     

Formation of ordered arrays of gold particles by nanoindentation templating

Ordered arrays of gold‐rich particles, with diameters ranging from 50 nm to 180 nm, have been formed on a silicon (100) surface through pre‐patterning by nanoindentation. Indentation and gold deposition of the sample is followed by thermal processing, causing the gold to become trapped at the indentation sites. We suggest that gold trapping is via an alloying process with the underlying Si substrate where the native oxide is structurally compromised by the indentation process. The final size for a given particle is directly dependent on the size of the indentation site. It has also been demonstrated that excess gold found on the surface outside of these indentation sites can be readily removed via simple mechanical abrasion without affecting the particles within the indent. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Correlation between the real Co particle size distributions and the magnetic properties of GMR Cu–10Co alloy

The results of transmission electron microscopy (TEM), magnetic and magneto-resistivity investigations of the Cu–10Co (wt%) giant magneto-resistance (GMR) melt-spun ribbons are reported and discussed. To obtain different distributions of ferromagnetic Co particles in a non-magnetic Cu matrix, the alloy was aged at 550 °C for 0.5, 1, 2, 16, and 32 h. Particle size measurements were performed using quantitative TEM metallography methods. Two size classes of Co particles are identified: primary particles (P) precipitated during the melt-spinning process and the secondary particles (S) precipitated during the ageing process. The results of magnetization and coercitivity are correlated with the results of calculations based on the real Co particle distributions determined from TEM micrographs. The behavior of magnetization and coercive force in function of ageing time is explained as related with changes of a mean particle size. It is shown that the GMR effect is not influenced by Co particles distribution for the S particles with the mean size less than 10 nm, whereas for Co distributions with larger mean diameters, the GMR effect is strongly reduced.     

Conceptual limitations and extensions of lung-deposited Nanoparticle Surface Area Monitor (NSAM)

Nanoparticle Surface Area Monitor (NSAM, TSI model 3550 and Aerotrak 9000) is an instrument designed to measure airborne surface area concentrations that would deposit in the alveolar or tracheobronchial region of the lung. It was found that the instrument can only be reliably used for the size range of nanoparticles between 20 and 100 nm. The upper size range can be extended to 400 nm, where the minimum in the deposition curves occurs. While the fraction below 20 nm usually contributes only negligibly to the total surface area and is therefore not critical, a preseparator is needed to remove all particles above 400 nm in cases where the size distribution extends into the larger size range. Besides limitations in the particle size range, potential implications of extreme concentrations up to the coagulation limit, particle material (density and composition) and particle morphology are discussed. While concentration does not seem to pose any major constraints, the effect of different agglomerate shapes still has to be further investigated. Particle material has a noticeable impact neither on particle charging in NSAM nor on the deposition curves within the aforementioned size range, but particle hygroscopicity can cause the lung deposition curves to change significantly which currently cannot be mimicked with the instrument. Besides limitations, possible extensions are also discussed. It was found that the tendencies of the particle deposition curves of a reference worker for alveolar, tracheobronchial, total and nasal depositions share the same tendencies in the 20–400 nm size range and that their ratios are almost constant. This also seems to be the case for different individuals and under different breathing conditions. By means of appropriate calibration factors NSAM can be used to deliver the lung deposited surface area concentrations in all these regions, based on a single measurement.     

Matrix laser cleaning: a new technique for the removal of nanometer sized particles from semiconductors

In this paper a new laser-based technique for the removal of nanoparticles from silicon wafers, called matrix laser cleaning, is introduced. In contrast to the already existing technique dry laser cleaning damages of the substrate can be avoided. Furthermore no liquids are used, avoiding problems that occur, e.g. in steam laser cleaning and other wet cleaning techniques. We show that damage free particle removal of polystyrene particles with diameters of at least down to 50 nm is possible with a cleaning efficiency very close to 100% within a single shot experiment. Furthermore the cleaning threshold is independent of the particle size. PACS 64.70.Hz; 68.43.Vx; 81.65.Cf     

Determination of Particle Size Distribution by Par-Tec® 100: Modeling and Experimental Results

Some particle size analyzers, such as the Par-Tec® 100 (Laser Sensor Technology, Redmond, WA, USA), measure the so-called cord length distribution (CLD) as the laser beam emitted from the sensor randomly crosses two edges of a particle (a cord length). The objectives of this study were to develop a model that can predict the response of the Par-Tec® 100 in measuring the CLD of a suspension for spherical and ellipsoidal particles and to infer the actual particle size distribution (PSD) using the measured CLD output. The model showed that the measured CLD is reasonably accurate for the spherical particles. However, this measurement progressively deteriorates as the shape of particles changes from spherical to ellipsoidal with large ratios of major to minor diameters. Experimental results obtained with spherical particles having a normal and a non-normal PSD indicated that the Par-Tec® 100 measurements deteriorate as the PSD deviates from a normal distribution. The information obtained from these experiments also showed that the model can reasonably predict the Par-Tec® response. Use of the inferred PSD rather than the measured CLD made a major improvement in estimating the actual PSD. Mean particle size analysis revealed that the Par-Tec® 100 volume-weighted mean particle size is closest to the unweighted mean particle size measured by sieve analysis.     

Fractal Character of Dynamic Light Scattering of Particles

Dynamic light scattering signals from particles, exhibit fractal characteristics. This feature can be used to determine the particle size. The use of the fractal dimension, as a quantitative method to analyze the properties of dynamic light scattering signals from submicron particles, is presented. The analysis is performed directly on the time‐resolved scattered intensity, and the Box Dimensions of light scattering signals of particles with diameters 100, 200, 500 and 1000 nm. The experimental results show that the fractal dimensions of light scattering signals correlate well with particle size. In the submicron size range, the smaller the particles, the larger their fractal dimensions. Compared with the PCS technique, only several hundreds of samples are required in the fractal method. Therefore, the data processing is easily accomplished. However, this method only provides the mean particle size, but not the particle size distribution.     

Effects of size and interactions on the magnetic behaviour of elliptical (0 0 1)Fe nanoparticles

Arrays of elliptical particles with aspect ratio 1:3 and short axes 50, 100 and 150 nm were prepared by electron-beam lithography and ion-beam milling of epitaxial (0 0 1)Fe films of thicknesses 10 and 20 nm. The domain state of an individual particle imaged by magnetic force microscopy in zero field after demagnetization was observed to change from being bi-domain or multidomain (MD) to stable single domains (SD) as the lateral size and film thickness were decreased. The critical size for SD formation was found to be close to the actual lateral sizes of 100 nm×300 nm and 150 nm×450 nm for the thicknesses of 20 and 10 nm, respectively. Only in the 10 nm thick ellipses of lateral size 100 nm×300 nm, the magnetization reversal may take place through coherent rotation. For all other investigated samples, the experimental switching field is lower than what would be required for this process.     

Interfacial Reactions and Cubic Neodymium Oxide Formation in Low Dispersed Nd2O3-SiO2 System by Wet Chemical Method

Neodymium (binary oxide) powders are synthesized by a solgel technique. Prepared powders are heat treated under different temperature for different time duration and obtained nanostructure of Nd. Metal particle have diameters in the range 7.8-21.6nm. It is found that the heat treatment plays an important role to produce different structure of Nd-doped silica matrix. The peak position shifts to lower angle as the size of the nano metal oxide particles size increases.     

Enhanced near field mediated nanohole fabrication on silicon substrate by femtosecond laser pulse

Investigation of the process of nanohole formation on silicon surface mediated with near electromagnetic field enhancement in vicinity of gold particles is described. Gold nanospheres with diameters of 40, 80 and 200 nm are used. Irradiation of the samples with laser pulse at fluences below the ablation threshold for native Si surface, results in a nanosized surface modification. The nanostructure formation is investigated for the fundamental (λ = 800 nm, 100 fs) and the second harmonic (λ = 400 nm, 250 fs) of the laser radiation generated by ultrashort Ti:sapphire laser system. The near electric field distribution is analyzed by an Finite Difference Time Domain (FDTD) simulation code. The properties of the produced morphological changes on the Si surface are found to depend strongly on the polarization and the wavelength of the laser irradiation. When the laser pulse is linearly polarized the produced nanohole shape is elongated in the E-direction of the polarization. The shape of the hole becomes symmetrical when the laser radiation is circularly polarized. The size of the ablated holes depends on the size of the gold particles, as the smallest holes are produced with the smallest particles. The variation of the laser fluence and the particle size gives possibility of fabricating structures with lateral dimensions ranging from 200 nm to below 40 nm. Explanation of the obtained results is given on the basis simulations of the near field properties using FDTD model and Mie's theory.     

Size-dependent nanoparticle reaction enthalpy: Oxidation of aluminum nanoparticles

Here we present a model describing the particle size dependence of the oxidation enthalpy of aluminum nanoparticles. The model includes the size dependence of the cohesive energy of the reactant particles, the size dependence of the product lattice energy, extent of product agglomeration, and surface capping effects. The strongest effects on aluminum nanoparticle energy release occur for particle diameters below 10 nm, with enhanced energy release for agglomerated oxide products and decreased energy release for nanoscale oxide products. An unusual effect is observed with all nanoparticle reaction enthalpies converging to the bulk value when agglomeration of the products approaches the transition between nanoparticle→nanoparticle and nanoparticle→bulk energetics. Optimal energy output for Al NP oxidation should occur for sub-10-nm particles reacting with significant agglomeration.     

Production of Monodisperse Nanoparticles and Application of Discrete-Monodisperse Model in Plasma Reactors

The particle growth in plasma reactor were investigated by using the discrete-monodisperse (D-M) model for various process conditions. The monodisperse large sized particle distribution predicted by the D-M model are in good agreement with the large sized particles by the discrete-sectional model and also in the experiments by Shiratani et al. (1996). Some fractions of the small size particles are in a neutral state or even charged positively, but most of the large sized monodisperse particles are charged negatively. As the mass generation rate of monomers increases, the large sized particles grow more quickly and the production rate of nanoparticles of 100nm by plasma reactor increases. As the initial electron concentration or the monomer diameter increases, it takes longer time for the large sized particles to grow up to 100nm, but the large sized particle concentration of 100nm increases and the resulting production rate of large sized particles of 100nm increases. As the residence time increases, the time for the large sized particles to grow up to 100nm decreases and the large sized particle concentration of 100nm increases and, as a result, the production rate of large sized particles of 100nm increases. We propose that the plasma reactor can be a good candidate to produce monodisperse nanoparticles.     

Continuous Size Classification of Nanoparticles Utilizing Brownian Motion in Microchannel Size Exclusion Chromatography

This study presents a new method for classifying the sizes of colloidal nanoparticles of below 100 nm in diameter in liquid dispersion using a microchannel size exclusion chromatography (SEC) chip. This chip can classify polydisperse colloidal nanoparticles containing a mix of two monodisperse nanoparticles into several monodisperse particle populations. The particles classified by the SEC chip are then sequentially analyzed by a photon correlation spectroscopy (PCS) method in combination with a flow cell. Two different pillar patterns of such SEC chips were used in experiments to investigate the effects of these patterns on the nanoparticle classification performance. The results obtained were compared with those from a numerical simulation. Standard polystyrene latex particles with diameters of 20 nm and 100 nm were used in this study. The usefulness of this methodology was verified since the simulation and measurement results were in good agreement with each other.     

高浓度超细颗粒的后向光子相关光谱测量技术研究

针对光子相关光谱颗粒粒径测量法不能在线测量高浓度超细颗粒的问题,通过分析溶液浓度对光子相关光谱测量法的影响,设计了一种后向散射光路,提出了后向光子相关光谱测量法.实验采用50 nm、100 nm以及500 nm三种标准乳胶球颗粒,在不同的浓度下分别用两种方法进行了测试.结果证明,后向光子相关光谱测量法能有效抑制多重散射的影响,适用于高浓度超细颗粒粒径的在线测量.