金纳米流体的电场可调光学性质

利用多次还原法制备了不同粒径的金纳米颗粒,SEM和粒度分析表明其平均粒径分别为11 nm,35 nm和58 nm.进一步通过表面活性剂辅助的液相转移法制备出不同粒径的油基金纳米流体,测试了金纳米流体在电场作用下的光学性质.结果表明金纳米流体在电场作用下表现出明显的双折射现象,且随电场强度的变化双折射具有可调节性.金颗粒粒径和浓度对折射率有明显影响,在实验采用的浓度范围内,折射率随金颗粒浓度和粒径的增加而减小.最后,利用电流变液结构转变机理对金纳米流体的电致双折射进行了分析. 关键词： 纳米流体 双折射 电流变液     

Thermodynamic properties of gold–water nanofluids using molecular dynamics

Conflicts and discrepancies around nanoparticle (NP) size effect on the optical properties of metal NPs of sizes below the mean free path of electron can be traced to the internal damping effect of the hybrid resonance of the inner band (IB) and the conduction band (CB) electrons of the noble metals. We present a scheme to show how alternative mathematical formulation of the physics of interaction between the CB and the IB electrons of NP sizes <50?nm justifies this and resolves the conflicts. While a number of controversies exist between classical and quantum theories over the phenomenological factors to attribute to the NP size effect on the absorption bandwidth, this article shows that the bandwidth behavior can be well predicted from a different treatment of the IB damping effect, without invoking any of the controversial phenomenological factors. It finds that the IB damping effect is mainly frequency dependent and only partly size dependent and shows how its influence on the surface plasmon resonance can be modeled to show the influence of NP size on the absorption properties. Through the model, it is revealed that strong coupling of IB and CB electrons drastically alters the absorption spectra, splitting it into distinctive dipole and quadrupole modes and even introduce a behavioral switch. It finds a strong overlap between the IB and the CB absorptions for Au and Cu but not Ag, which is sensitive to the NP environment. The CB modes shift with the changing refractive index of the medium in a way that can allow their independent excitation, free of influence of the IB electrons. Through a hybrid of parameters, the model further finds that metal NP sizes can be established not only by their spectral absorption peak locations but also from a proper correlation of the peak location and the bandwidth (FWHM).     

表面活性剂对Al_2O_3-H_2O纳米流体热物性特性的影响

本实验通过添加四种不同表面活性剂,分析对Al_2O_3-H_2O纳米流体热物性特性的影响,加入SDBS、SDS和PVP的Al_2O_3-H_2O过冷度分别下降20.17%、6.19%和3.18%,导热系数分别下降2.94%、4.63%和3.71%,相变时间分别延长4.40%、8.60%和6.18%。但添加TEA表面活性剂,反使得Al_2O_3-H_2O纳米流体的过冷度提高12.57%,导热系数下降6.33%,相变时间延长了15.05%。由于四种表面活性剂的影响,使得Al_2O_3-H_2O纳米流体的热导率均下降,相变时间延长。在一定浓度范围内,表面活性剂对相变潜热值的影响基本保持在6%的波动范围内,可认为少量表面活性剂对相变潜热值没有影响,但其相变曲线有较大的差异性。     

AC hot wire measurement of thermophysical properties of nanofluids with 3ω method

We present a new application of a hot wire sensor for simultaneous and independent measurement of thermal conductivity k and diffusivity α of (nano)fluids, based on a hot wire thermal probe with ac excitation and 3 ω lock-in detection. The theoretical modeling of imaginary part of the signal yields the k value while the phase yields the α value. Due to modulated heat flow in cylindrical geometry with a radius comparable to the thermal diffusion length, the necessary sample quantity is kept very low, typically 25μl. In the case of relative measurements, the resolution is 0.1% in k and 0.3% in α. Measurements of water-based Aerosil 200V nanofluids indicate that ultrasound treatment is more efficient than high pressure dispersion method in enhancing their thermal parameters.     

Particle detection properties of charge coupled devices

A detailed quantitative analysis of particle events observed in Charge Coupled Devices (CCDs) requires the knowledge of particle detection properties of the employed CCDs such as the detection threshold for energy deposition, efficiency for different kinds of radiation and the relation between the signal and the deposited energy inside the sensitive volume. For this purpose the CCD-chip was described by a simple geometrical model containing the effective thicknesses of the p-n semiconductor junctions dj and a covering dead layer d_T above them as parameters. The corresponding geometrical quantities were experimentally estimated to be 1.3 μm and 0.9 μm respectively by using different kinds of radiation.     

Dust agglomeration

Dust agglomeration plays an important role in astrophysics and atmospheric sciences as well as in industrial processes. This article reviews the current knowledge of the physical interactions that lead to particle sticking in gaseous environments as well as the morphologies of the resulting dust aggregates. With this basic knowledge of dust–dust interactions, the development of ensembles of interacting dust particles can be treated using Smoluchowski's equation. Considering analytical solutions and simplified physical conditions, the temporal evolution and the mass distribution functions of dust aggregates are discussed. Based on this, general dust aggregation phenomena can be modelled and introduced into more complex scenarios.     

Review of particle properties: Particle data group

This review of the properties of leptons, mesons, and baryons is an updating of Review of Particle Properties, Particle Data Group [Rev. Mod. Phys. 48 (1976) No. 2, Part II; and Supplement, Phys. Lett. 68B (1977) 1]. Data are evaluated, listed, averaged, and summarized in tables. Numerous tables, figures, and formulae of interest to particle physicists are also included. A data booklet is available.     

Effects of the agglomeration state on the Raman properties of Co3O4 nanoparticles

The features of the Raman spectra of Co₃O₄ 30‐nm nanoparticles depend strongly on their agglomeration state. When measured at low incident laser power, the spectrum of isolated nanoparticles corresponds to that found in bulk materials, whereas the agglomerated nanoparticles present a clear red‐shift and broadening of the Raman bands. On the other hand, when measured at even lower power, both agglomerated and isolated nanoparticles show the same spectrum of microscopic particles. These effects have been studied by variations of the 532‐nm laser power and the environmental temperature. The thermal dependence of Raman spectra of agglomerated nanoparticles is different to that of isolated nanoparticles but is comparable to the one of bulk material. The different behaviour of the nanoparticles at different agglomeration state is associated to the transmission of phonons among the particles. On the other hand, an increase of the laser power causes a larger number of acoustic phonons, producing a variation of the vibration anharmonicity of the nanoparticles. This increase is more pronounced in the agglomerated nanoparticles, due to the transmission of phonons, causing a much intense modification of the Raman spectrum produced by the laser power. These results clearly indicate that the agglomeration state of the nanoparticles affects their Raman properties. Copyright © 2012 John Wiley & Sons, Ltd.     

碳纳米流体强化传热研究

在去离子水中分别添加了单壁、多壁碳纳米管材料,通过配比一定质量的亲水性分散剂,经超声波振荡试制了水基碳纳米流体。测试分析了不同碳纳米管材料质量分数下的纳米流体热导率和动力粘度等重要热物性参数。测试结果表明:碳纳米管粒子能够强化基液工质导热性能,随着其质量分数的增加,水基单壁、多壁碳纳米流体热导率均明显提高;单壁碳纳米流体粘度显著增加,多壁碳纳米流体粘度无明显变化,多壁碳管更适用于纳米流体强化传热。     

Magnetic and ultrasonic investigations on magnetite nanofluids

Magnetite nanofluids of various concentrations have been prepared through co-precipitation method. The structural and magnetic properties of the magnetic nanofluids have been analyzed which respectively revealed their face centered cubic crystal structure and super paramagnetic behavior. Ultrasonic investigations have been made for the nanofluids at different temperatures and magnetic fields. Open- and close-packed water structure is considered to explain the temperature effects. The inter particle interactions of surface modified nanomagnetite particle and the cluster formation are realized through the variations in ultrasonic parameters.     

Migration and agglomeration of heliumatoms in copper

The magration and agglomeration of Helium atoms in He irradiated Cu was studied between 10 K and 900 K by perturbed γγ angular correlation (PAC) measurements using the radioactive probe atom¹¹¹In. Trapping of interstitial and substitutional He atoms is observed already after irradiation at 10 K. Substitutional He (He-yacancy pair) is bound in the nearest neighbourhood to the¹¹¹In probe atom and seems to be stable up to 725 K (E _He1V1 ^b ≥2.1 eV). The onset of vacancy assisted He agglomeration in Cu is observed at 250 K.     

Particle size effect on phase and magnetic properties of polymer-coated magnetic nanoparticles

Polymer-coated magnetic nanoparticles are hi-tech materials with ample applications in the field of biomedicine for the treatment of cancer and targeted drug delivery. In this study, magnetic nanoparticles were synthesized by chemical reduction of FeCl₂ solution with sodium borohydride and coated with amine-terminated polyethylene glycol (aPEG). By varying the concentration of the reactants, the particle size and the crystallinity of the particles were varied. The particle size was found to increase from 6 to 20 nm and the structure becomes amorphous-like with increase in the molar concentration of the reactant. The magnetization at 1 T field (M_1T) for all samples is > 45 emu/g while the coercivity is in the range of 100-350 Oe. When the ethanol-suspended particles are subjected to an alternating magnetic field of 4 Oe at 500 kHz, the temperature is increased to a maximum normalized temperature (3.8 °C/mg) with decreasing particle size.     

碳纳米流体强化传热研究

在去离子水中分别添加了单壁、多壁碳纳米管材料,通过配比一定质量的亲水性分散剂,经超声波振荡试制了水基碳纳米流体。测试分析了不同碳纳米管材料质量分数下的纳米流体热导率和动力粘度等重要热物性参数。测试结果表明:碳纳米管粒子能够强化基液工质导热性能,随着其质量分数的增加,水基单壁、多壁碳纳米流体热导率均明显提高;单壁碳纳米流体粘度显著增加,多壁碳纳米流体粘度无明显变化,多壁碳管更适用于纳米流体强化传热。     

Nanoparticle transport effect on magnetohydrodynamic mixed convection of electrically conductive nanofluids in micro-annuli with temperature-dependent thermophysical properties

This is a numerical investigation of nanoparticle transport effect on magnetohydrodynamic mixed convective heat transfer of electrically conductive nanofluids in micro-annuli with temperature-dependent thermophysical properties. The modified Buongiorno's non-homogeneous model is applied for the nanoparticle-fluid suspension to simulate the migration of nanoparticles into the base fluid, originating from the thermophoresis (nanoparticle migration because of temperature gradient) and Brownian motion (nanoparticle slip velocity because of concentration gradient). Due to surface roughness at the solid–fluid interface in micro-annuli, the wall surfaces are subjected to a linear slip condition to assess the non-equilibrium region near the interface. The fluid flow has been assumed to be fully developed, and the governing equations including continuity, momentum, energy, and nanoparticle transport equation are reduced to a system of ordinary differential equations, before they have been solved numerically. The results are presented with and without considering the dependency of thermophysical properties upon the temperature. It is indicated that ignoring the temperature dependency of thermophysical properties does not significantly affect the flow fields and heat transfer behavior of nanofluids, but it changes the relative magnitudes. Furthermore, in the presence of magnetic field, smaller nanoparticles are more appropriate than larger ones.     

Fingering instabilities in dewetting nanofluids

The growth of fingering patterns in dewetting nanofluids (colloidal solutions of thiol-passivated gold nanoparticles) has been followed in real time using contrast-enhanced video microscopy. The fingering instability on which we focus here arises from evaporatively driven nucleation and growth in a nanoscopically thin precursor solvent film behind the macroscopic contact line. We find that well-developed isotropic fingering structures only form for a narrow range of experimental parameters. Numerical simulations, based on a modification of the Monte Carlo approach introduced by Rabani et al. [Nature (London) 426, 271 (2003)10.1038/nature02087], reproduce the patterns we observe experimentally.     

Particle size polydispersity of the rheological properties in magnetorheological fluids

Dispersion of metal particles in fluids can be used to manufacture magnetorheologic fluids(MRF).Properties of these dispersion systems are mainly determined by the arrangements and contacts among particles.In this paper,particles with smaller sizes than those in the target dispersion system are added using iron particles dispersed in silicon oil as a model to control the arrangements and contacts.The result suggests that these small-sized particles have a significant effect on the viscoelastic properties of...