Investigations of magnetic and dielectric properties of cupric oxide nanoparticles

Cupric oxide nanoparticles of ∼8-10 nm width and 40-45 nm length self assembled as large particles ∼1-1.5 μm have been investigated, in the 10-325 K temperature range, using magnetic and dielectric measurements. In magnetic measurements a single broad peak at ∼230 K in a zero field cooled sample has been observed. Coercivity, in magnetization measurements at 10 K, suggests that the nanoparticles are core-shell type particles with an antiferromagnetic core and a ferromagnetic shell. Dielectric measurements, at various frequencies from 3.7 Hz to 949 kHz, exhibit a sharp peak at 284 K followed by weak anomalies around 213 and 230 K.     

Simultaneous measurement of particle size, velocity, andtemperature in thermal plasmas

The in-flight measurement of particle parameters (size, velocity, temperature, and local number density) can prove insight into the plasma processing of solid materials. A measurement technique for simultaneously obtaining the size, velocity, and temperature of particles entrained in high-temperature flow fields is described. Particle size and velocity are obtained from a combination laser-particle-sizing system and laser Doppler velocimeter. The particle temperature is determined by a two-color pyrometry technique and the data rate is a measure of relative particle number density. Typical measured temperatures and velocities for the 5-100 μm particles used in plasma spraying are 1600-3500 K and 100-300 m/s, respectively. Since particle size, velocity, and temperature are measured simultaneously, cold particles (<1600 K) are identified and their relative number density can be quantified. Data from two plasma spray systems, a metal one (Ni-Al) and a metal oxide one (Al₂O₃), are presented and their application to understanding the plasma spray-coating process is illustrated     

Sputtering of condensed noble gases by keV ions

Layers of Kr and Xe condensed at ～ 15 K and 35 K have been bombarded by 3-6 keV Ar+, Kr+ and Xe+ ions.

The sputtered neutrals were detected after a 1.43 m flight path by means of a mass spectrometer. Among the sputtered particles are considerable numbers of Van der Waals molecules, like Kr₂, Kr₃, Xe₂ XeKr and XeKr₂. The energy distribution of the sputtered particles give clues to the sputtering mechanisms involved.     

Evidence for the rare decay B-->K*l+l- and measurement of the B-->Kl+l- branching fraction

We present evidence for the flavor-changing neutral current decay B-->K*l+l- and a measurement of the branching fraction for the related process B-->K l+l-, where l+l- is either an epsilon+epsilon- or a mu+mu- pair. These decays are highly suppressed in the standard model, and they are sensitive to contributions from new particles in the intermediate state. The data sample comprises 123 x 10(6) Upsilon(4S)-->B(-)B decays collected with the BABAR detector at the SLAC PEP-II epsilon+epsilon- storage ring. Averaging over K(*) isospin and lepton flavor, we obtain the branching fractions B(B-->Kl+l-)=(0.65(+0.14)(-0.13)+/-0.04)x10(-6) and B(B-->K*l+l-)=(0.88(+0.33)(-0.29)+/-0.10)x10(-6), where the uncertainties are statistical and systematic, respectively. The significance of the B-->Kl+l- signal is over 8sigma, while for B-->K*l+l- it is 3.3sigma.     

Cems study of corrosion on iron foil at low temperatures

Conversion electron Mössbauer measurements with a proportional counter at 6.3, 78 and 300 K have been done to investigate the corrosion of the surface of iron foils, which consists of small particles of γ-Fe₂O₃. When the average particle sizes of the corroded layer are approximately 4 and 5 nm, magnetic hyperfine splitting was observed in the spectra at 6.3 and 78 K. However, for a corroded layer composed of smaller particles, the splitting was noticed only at 6.3 K. The anisotropy constantK of the small particles was estimated using the magnetic hyperfine fields obtained from the spectra at 6.3 and 78 K.     

Superparamagnetic nanocomposites based on poly(styrene-b-ethylene/butylene-b-styrene)/cobalt ferrite compositions

Magnetic nanoparticles were created in or around the sulfonated (s) polystyrene domains in a phase separated poly[styrene-b-(ethylene-co-butylene)-b-styrene)] block copolymer (BCP) using an in situ inorganic precipitation procedure. The sBCP was neutralized with a mixed iron/cobalt chloride electrolyte and the doped samples were converted to their oxides by reaction with sodium hydroxide and further washing with water. Transmission electron microscopy indicated the presence of nanoparticles in the 5–25 nm size range. The metal oxide particle structures were studied using select area electron diffraction, which revealed that they are of the cobalt iron oxide composition (CoFe₂O₄). These nanocomposites were shown, using a superconducting quantum interference device magnetometer, to be superparamagnetic at 300 K and ferrimagnetic at 5 K. Nanocomposites consisting of smaller particles have a blocking temperature of 70 K, whereas it was 140 K for larger particles.     

Stabilization of aqueous nanoscale zerovalent iron dispersions by anionic polyelectrolytes: adsorbed anionic polyelectrolyte layer properties and their effect on aggregation and sedimentation

Nanoscale zerovalent iron (NZVI) particles are 5–40 nm sized Fe⁰/Fe-oxide particles that rapidly transform many environmental contaminants to benign products and are a promising in situ remediation agent. Rapid aggregation and limited mobility in water-saturated porous media limits the ability to deliver NZVI dispersions in the subsurface. This study prepares stable NZVI dispersions through physisorption of commercially available anionic polyelectrolytes, characterizes the adsorbed polymer layer, and correlates the polymer coating properties with the ability to prevent rapid aggregation and sedimentation of NZVI dispersions. Poly(styrene sulfonate) with molecular weights of 70 k and 1,000 k g/mol (PSS70K and PSS1M), carboxymethyl cellulose with molecular weights of 90 k and 700 k g/mol (CMC90K and CMC700K), and polyaspartate with molecular weights of 2.5 k and 10 k g/mol (PAP2.5K and 10K) were compared. Particle size distributions were determined by dynamic light scattering during aggregation. The order of effectiveness to prevent rapid aggregation and stabilize the dispersions was PSS70K(83%) > ≈PAP10K(82%) > PAP2.5K(72%) > CMC700K(52%), where stability is defined operationally as the volume percent of particles that do not aggregate after 1 h. CMC90K and PSS1M could not stabilize RNIP relative to bare RNIP. A similar trend was observed for their ability to prevent sedimentation, with 40, 34, 32, 20, and 5 wt%, of the PSS70K, PAP10K, PAP2.5K, CMC700K, and CMC90K modified NZVI remaining suspended after 7 h of quiescent settling, respectively. The stable fractions with respect to both aggregation and sedimentation correlate well with the adsorbed polyelectrolyte mass and thickness of the adsorbed polyelectrolyte layers as determined by Oshima’s soft particle theory. A fraction of the particles cannot be stabilized by any modifier and rapidly agglomerates to micron sized aggregates, as is also observed for unmodified NZVI. This non-dispersible fraction is attributed to strong magnetic attractions among the larger particles present in the polydisperse NZVI slurry, as the magnetic attractive forces increase as r⁶.     

Au deposits on graphite: On the nature of high temperature desorption peaks in CO thermal desorption spectra

Due to the discovery of Au as a catalyst for low temperature CO oxidation, the adsorption of CO on Au surfaces has attracted a lot of attention recently. On stepped and rough single crystal surfaces as well as on deposited particles two characteristic desorption states above 100 K have been observed via TPD. We have studied Au deposits on graphite in order to elucidate the nature of these desorption peaks in more detail. For this purpose, Au was deposited at 100 K and 300 K on HOPG as a weakly interacting support. In analogy to other supports, we obtain two desorption states (∼140 K and ∼170 K) whose relative intensities depend strongly on the deposition temperature with the high temperature peak being much more pronounced for the 100 K deposits. After annealing to 600 K, both states drastically lose intensity. XP spectra, on the other hand, show virtually no decrease of the Au 4f intensity as would be expected for desorption or significant changes of the particle morphologies. We conclude that both desorption peaks are defect-related and connected with under-coordinated Au atoms that are lost for the most part upon annealing. These sites could be located at the perimeter of dendritic islands or on small, defect-rich particles in addition to larger particles not adsorbing CO at 100 K. Preliminary STM results are in favour of the second interpretation.     

Ferromagnetic resonance of horse spleen ferritin: core blocking and surface ordering temperatures.

In nature, ferritin, an iron-storage molecule, is found in species ranging from bacteria to man. In the past 50 years its chemical, physical, and magnetic properties have been studied, searching to relate function and structure. Horse spleen ferritin has been investigated by EPR at temperatures between 7 and 290 K. These spectra change from an isotropic line at 290 K to an anisotropic one at 19 K, with a behavior consistent with a system of particles that undergoes superparamagnetic relaxation. A blocking temperature of (116+/-9) K is obtained. A new temperature-dependent signal is observed in the low field region at temperatures higher than 80 K. At 7 K no EPR signal appears, suggesting (14+/-5) K as the Néel temperature of surface spins. Analysis of the temperature dependence of the distance between EPR lines extrema, under the view of two theoretical models, allowed the evaluation of magnetic parameters. These parameters are 2K/M=2.7 x 10(3) Oe and MV=1.9 x 10(-17) emu or K/M=1.3 x 10(3) Oe and MV=2.0 x 10(-17) emu, where K is the anisotropy energy per unit volume, M is the sample magnetization, and V is the superparamagnetic core volume. The results are also discussed, and some structural models in the literature are considered.     

The effect of preadsorbed K on the size distribution of Au nanoparticles on TiO2(1 1 0) surface

The effect of K on the morphology of Au nanoparticles deposited on TiO₂(1 1 0) surface is investigated by STM-STS and AES methods. For comparison, the enhanced concentration of oxygen defect sites generated by Ar⁺ bombardment was also studied. It was found that both the K additive and the oxygen defect sites induce a pronounced decrease in the average size of the Au nanoparticles evolved at 320 K. On the clean TiO₂(1 1 0) the average size of Au particles is 4.3 nm at approximately monolayer coverage of gold, while in the presence of K or oxygen vacancies this value decreased to 2.5 nm. In spite of the reduced average diameter detected at room temperature, the mean size of the Au nanoparticles increased significantly from 2.5 nm up to 7 nm on the effect of annealing at 500-700 K for K precoverages of 0.3-1 ML. For the clean and the Ar⁺ pretreated TiO₂(1 1 0) surfaces the mean size of the Au particles changed only slightly on the effect of the same thermal treatments.     

Cathodic needle growth from Mo(CO)6 vapors at higher electric fields

It is shown that metallic needle crystals grow on electron emitting areas of field emission cathodes working in Mo(CO)₆ vapor. The growth is driven by ions impinging from a field emission arc plasma, and the products are classified into two categories depending on the thermal energy supplied externally to the substrate: 1) Mo particles are agglomerated in dendritic shapes at 300-1300 K, and 2) whisker-like crystals with an unknown structure are obtained at 1300-1500 K. The high-temperature needles are capped with Taylor cones during the growth, and it is shown that they are a metastable phase of Mo.     

Synthesis of nanocrystalline YFeO3 and its magnetic properties

Single phase nanocrystalline YFeO₃ has been synthesized by a simple solution method. The average particle diameter is 42.2 nm. The particles exhibit ferromagnetic behaviour in the temperature range 10-300 K with a coercivity of 23 kOe. The magnetization versus temperature over the temperature range 2-300 K obeys Bloch equation with a Bloch constant value 9.98×10⁻⁶ K^−3/2. Ferromagnetic hysteresis loops have been observed up to a temperature of 300 K. At 10 K a field-cooled sample shows an exchange bias field.     

连续激光二极管端面抽运Nd:YAG激光器噪声研究

以连续激光二极管端面抽运NdYAG激光器为实验模型,研究了该激光器中输出激光强度噪声、频率噪声和场分布噪声的特性,并根据实验数据推算出NdYAG晶体的粒子上能级寿命随温度变化为1.83μsK,发射截面随温度变化为-0.146×10-19cm2K。     

Magnetic birefringence of iron oxyhydroxide nanoparticles stabilised by sucrose

Magnetically induced optical birefringence is used to investigate pharmaceutically important iron-sucrose aqueous suspensions. XRD and TEM measurements of the system of oxyhydroxide particles stabilised by sucrose have shown that this system contains iron oxyhydroxide in the form of 2-5 nm particles. The mineral form of the iron-core is suggested to be akaganeite. Anisotropy of the optical polarizability and magnetic susceptibility of akaganeite nanoparticles are calculated. The permanent dipole moment obtained for the nanoparticles studied was found to be negligible, in agreement with the characteristic superparamagnetic behaviour of the magnetic nanoparticles observed at room temperature. The Neel temperature of these nanoparticles is estimated as below 276 K. The results obtained are discussed against a background of the earlier studies of similar nanoscale systems.     

Morphology and magnetic properties of ultrafine ZnFe2O4 particles

Well-crystallized ultrafine ZnFe₂O₄ particles of several nanometers in size have been prepared by the coprecipitation method, and their particle morphology and magnetic properties, especially at low temperatures, examined. Room-temperature X-ray diffraction, transmission electron microscopy, magnetization measurements at various temperatures from 300 K to 4.2 K, and Mössbauer spectroscopy at various temperatures from 300 K to 4.2 K, and at 4.2 K with a longitudinal magnetic field of 16.4 kOe applied have been employed. The formation of short-range and long-range magnetic order in small ZnFe₂O₄ particles above and below approximately 30 K is discussed. Below 30 K, the appearance of spontaneous magnetization and its hysteretic property is confirmed for small ZnFe₂O₄ particles.     

Magnetic properties in silica-coated CoFe2O4 nanoparticles: Quantitative test for a theory of single-domain particles with cubic anisotropy

We report magnetic properties of silica-coated CoFe₂O₄ nanoparticles (about 32 nm) in a wide temperature range (4-700 K). Coating CoFe₂O₄ nanoparticles with amorphous silica yields an assembly of noninteracting single-domain particles with cubic magnetocrystalline anisotropy. We find that the reduced remanence is about 0.81-0.82 at low temperatures (?100 K), very close to the theoretical prediction (0.83) for cubic anisotropy. The room-temperature anisotropy constant K₁ inferred from the data is also in quantitative agreement with the bulk value (1.8×¹⁰⁶ erg/cm³). The current work thus provides a quantitative test for a theory of noninteracting single-domain particles with cubic anisotropy.     

Single and double electron capture associated with target K-shell ionization for F7 + ,8 + ,9++Ar

Ratios for target Ar K-shell ionization associated with single and double electron capture, as well as the ratios corresponding to total capture and the projectile K x rays, were determined for 1.8- to 2.2-MeV/u F^{7 + ,8 + ,9+} projectiles. This work was performed at Western Michigan University with the tandem Van de Graaff accelerator. Coincidences between emitted K-shell X-rays (both target and projectile) and the corresponding charge-changed particles were observed. The F⁹⁺ Ar K X-ray coincidence ratios for double to single capture are found to well exceed unity over the limited energy range of the measurements. Possible explanations for this anomalous behavior are discussed.     

炸药爆轰法制备纳米石墨粉及其在高压合成金刚石中的应用

介绍了一种制备纳米石墨粉的新方法——负氧平衡炸药爆轰法. 对合成的黑粉产物进行x射线衍射分析，确认其为石墨结构，平均晶粒度为2.58 nm. 透射电子显微分析的结果表明，炸药爆轰法制备的黑粉为六方结构的纳米石墨粉，颗粒呈球形或椭球形. 用小角x射线散射法测定纳米石墨粉的粒度分布在1—50 nm，有92.6wt%的粉末粒度小于16 nm. 平均粒径为8.9 nm. 纳米石墨粉的比表面积约为500—650 m²/g. 在六面顶压机中用纳米石墨粉在Fe粉触媒的作用下进行金刚石的高压合成实验 关键词： 纳米石墨粉 爆轰 金刚石 合成     

α-Fe_2O_3纳米粒子Morin相变的Raman光谱研究

本文首次用Raman光谱法研究了粒子尺寸和Co2 +包附对α-Fe2 O3纳米粒子Morin相变温度TM的影响机制。测量结果显示纳米粒子α -Fe2 O3的振动模相对大块样品发生了红移和宽化 ,粒子愈小红移和宽化愈显著。表明表面Fe3+离子与配位氧离子的键距增大 ,键长呈从体相Fe-O键长过渡到表面Fe -O键长的某种分布。这种小尺寸引起的键长改变导致α -Fe2 O3粒子的单离子各向异性能减小 ,因此降低了样品的TM。分析显示Co2 +包附导致α -Fe2 O3粒子TM 的大幅下降可能是由单离子各向异性的减小和Co2 +与Fe3+之间的磁偶极子相互作用各向异性共同作用的结果     

不同温度下团簇生长的Monte Carlo模拟

引入基底表面束缚能、最近邻粒子间的耦合能以及应力场,对粒子扩散势垒进行修正,采用Monte Carlo方法对不同温度下薄膜生长过程进行模拟研究.结果表明,当400 K≤T ≤ 480 K,所得团簇的平均分支宽度基本保持不变,其值近似为单粒子直径.当500 K≤T ≤ 680 K,团簇分支宽度随着温度的升高而逐渐增大至约4个粒子.随着温度的继续升高,由于粒子较高的活跃性而无法凝聚形成大团簇,团簇包含粒子数的平均值小于2.还研究了不同温度下团簇在生长过程中的形貌演化过程以及团簇数变化规律.