吸附作用在纳米颗粒悬浮液换热强化中的试验与机理研究

本文通过测量SiO_2纳米颗粒在不同工质、不同浓度的悬浮液在不同温度下的导热系数,结合了液氮冷冻、切割并在断面上复形的技术,并利用电镜观察了纳米颗粒在液体中的分布、团聚和液体与之的亲和性。着重分析了纳米颗粒悬浮液中SiO_2纳米颗粒与液体的亲和性对悬浮液导热性能影响的机理。文中提出了SiO_2纳米颗粒的表面吸附层和在作布朗运动时出现”微对流”而使悬浮液换热强化。     

正电子湮没谱学研究半导体材料微观结构的应用进展

<正>电子湮没谱学技术在研究材料微观缺陷、微观结构方面有着独特的优势,尤其是在针对阳离子空位等负电性空位型缺陷的研究中,可以获取材料内部微观缺陷的种类与分布的关键信息.正电子湮没寿命和多普勒展宽能谱是正电子湮没谱学的最基本的分析方法,在半导体材料的空位形成、演化机理以及分布等研究方面能够发挥独特的作用;此外,慢正电子束流技术在半导体薄膜材料的表面和多层膜材料的界面的微观结构和缺陷的深度分布的研究中有广泛的应用.通过正电子技术所得到的微观结构和缺陷、电子密度和动量分布等信息对研究半导体微观结构、优化半导体材料的工艺和性能等方面有着指导作用.本文综述了正电子湮没谱学技术在半导体材料方面的应用研究进展,主要围绕正电子研究平台在半导体材料微观缺陷研究中对材料的制备工艺、热处理、离子注入和辐照情况下,各种缺陷的微观结构的表征及其演化行为的研究成果展开论述.     

纳米流体介质导热机理初探

纳米流体导热行为具有许多奇异的特性，结合纳米流体的特点和微尺度传热学原理，研究了 热流在纳米颗粒内波动式及非限域的热传导特性、纳米颗粒在悬浮液内的布朗运动、颗粒- 液体界面上液膜层原子的有规则排列、以及纳米颗粒的团簇形成及移动等四方面因素对纳米 流体导热系数的影响. 关键词： 纳米流体 导热     

利用多重光散射技术研究吡虫啉悬浮剂的物理稳定性

Turbiscan Lab稳定性分析仪利用多重光散射技术,能够准确测量乳状液、悬浮液等沉淀层、浮油层或澄清层厚度以及粒子平均粒径随时间的变化情况,定性定量分析体系不稳定性发生的机理和速率。本文利用Turbiscan Lab稳定性分析仪测定了不同萘磺酸盐分散剂GY-D10,NNO,Morwet D-425,TERSPERSE 2425制备的350 g·L-1吡虫啉悬浮剂及其悬浮液背散射光强度随时间的变化情况以及颗粒平均粒径的聚结增大速率,同时利用激光粒度分析仪测定加速试验前后颗粒平均粒径的变化情况,结果表明：添加分散剂GY-D10,用量为4 Wt%时悬浮液的稳定性最好,对应的吡虫啉悬浮剂的稳定性也最好。     

金属催化制备石墨烯的研究进展

石墨烯作为一种新兴的碳素材料,从一出现就引起了众多学者的关注.石墨烯具有许多新奇的特性,使得石墨烯在光电领域及微电子工业等有极大的应用潜力.但是目前难以实现大尺寸、高质量、宏量石墨烯的可控制备,限制了石墨烯的广泛应用.本文分析了各种石墨烯制备方法的利弊,重点从层数控制及大面积制备等方面对金属催化法进行了阐述,固态碳源金属催化法可以实现宏量制备大尺寸、高质量、薄且均匀的石墨烯.综述了金属催化制备石墨烯的相关机理研究,指出了目前研究的局限,并对石墨烯相变机理的下一步研究方向进行了展望.     

热智能材料及其在空间热控中的应用

空间技术等高新领域对智能高效的热控制技术的需求日益提高,而实现智能热控制技术的关键是要实现材料的热物性智能调控,于是热导率可响应外场变化的热智能材料成为了研究的焦点.本文梳理了热智能材料的最新研究进展,从调控机理、调控幅度、应用价值等角度出发,介绍了纳米颗粒悬浮液、相变材料、软物质材料、受电化学调控的层状材料和受特定外场调控的材料等不同种类热智能材料的研究现状,以及以热智能材料为基础的智能热控部件在空间技术等领域的应用.最后,本文对热智能材料未来的研究方向进行了探讨.     

纳米悬浮液的有效导热系数

文中以有效介质近似理论为基础,考虑了纳米颗粒在基液中强烈的B rown ian运动对强化传热的作用和纳米颗粒的表面吸附液体层、纳米颗粒的粒径和体积分数对纳米悬浮液有效导热系数的影响,建立了预测纳米悬浮液有效导热系数的模型,通过对纳米CuO-去离子水溶液的验证,发现该模型比几种经典模型具有更高的精度,因此具有一定的参考价值。     

温度依变性对纳米颗粒悬浮液导热系数测量影响

本文报告了50 nm氧化铜(体积百分比φ≤0.6%)与去离子水的悬浮液有效导热系数测量结果,讨论了温度依变性对导热系数的影响,运用适用于低浓度悬浮液的能量和质量方程进行分析表明,颗粒空间分布和温度场间相互影响,进而影响导热系数测量结果.考虑低浓度悬浮液中颗粒布朗扩散和热泳作用,讨论了运用准稳态方法测量纳米颗粒悬浮液导热系数的有效性,即需同时保证合适的加热热流密度和液体试样两侧的温差.     

纳米流体导热系数的分子动力学模拟

纳米流体的导热系数相对于基础流体而言有了显著提高,然而这种现象却无法用现有理论进行解释。应用平衡分子动力学方法对纳米流体的导热系数进行了模拟,并研究了纳米颗粒表面类似于固体的液体吸附层的存在和特性以及其对纳米流体导热能力的影响,在此基础上进一步对纳米流体导热性能显著提高的微观机理进行了探讨。模拟结果表明由于固体分子较强的吸引力,一部分液体分子被吸附在固体颗粒表面形成一个薄层,薄层内分子的排列结构不同于纯液体的分子排列结构,导热性能也优于基础流体,从而使得纳米流体的导热系数有了显著提高。最后对纳米颗粒表面液体薄层的厚度进行了定量计算。     

过冷液体钾形核初期微观动力学机理的模拟研究

采用分子动力学方法对液态金属钾凝固过程进行了模拟,根据凝固过程体系平均原子能量、原子成键类型和成团类型,以及均方位移和非Gauss参数等动力学参数的演化特征,对过冷熔体形核初期微观动力学机理进行了研究.结果表明:根据过冷液体钾结晶形核过程热力学、动力学和结构特性的演化规律, 其过冷温度区间可以分为两个明显不同的阶段,潜在结晶核心出现在过冷液体较低温区.过冷熔体钾在形核初期,二十面体团簇结构在α-弛豫阶段逐渐解体,同时具有体心立方(bcc)结构的潜在结晶核心逐步形成,其临界晶核包含约300个原子.     

On the influence of the hydrodynamic interactions on the aggregation rate of magnetic spheres in a dilute suspension

Magnetostatic attraction may lead to formation of aggregates in stable colloidal magnetic suspensions and magneto-rheological suspensions. The aggregation problem of magnetic composites under differential sedimentation is a key problem in the control of the instability of non-Brownian suspensions. Against these attractive forces are the electrostatic repulsion and the hydrodynamic interactions acting as stabilizing effects to the suspension. This work concerns an investigation of the pairwise interaction of magnetic particles in a dilute sedimenting suspension. We focus attention on suspensions where the Péclet number is large (negligible Brownian motion) and where the Reynolds number (negligible inertia) is small. The suspension is composed of magnetic micro-spheres of different radius and density immersed in a Newtonian fluid moving under the action of gravity. The theoretical calculations are based on direct computations of the hydrodynamic and the magnetic interactions among the rigid spheres in the regime of low particle Reynolds number. From the limiting trajectory in which aggregation occurs, we calculate the collision efficiency, representing the dimensionless rate at which aggregates are formed. The numerical results show clear evidence that the hydrodynamic interactions are of fundamental relevance in the process of magnetic particle aggregation. We compare the stabilizing effects between electrostatic repulsion and hydrodynamic interactions.     

Rheology of nanosized hairy grain suspensions

Suspensions of nanosized hairy grains have been prepared by grafting long polydimethylsiloxane chains (molecular weight ) onto silica particles (radius ), dispersed into a good solvent of PDMS. Depending on the particle volume fraction, different rheological behaviors are observed. In the very dilute regime, the suspensions are perfectly stable and the particles behave almost as hard spheres: flow penetration inside the corona is then very weak. When the particle volume fraction goes to the close packing volume fraction, the suspension viscosity does not diverge as for hard spheres due to the increase of flow penetration inside the corona and to corona entanglements. The particles have then the same behavior as polymer stars having an intermediate number of arms (). Finally, in the concentrated regime (), the suspensions form irreversible gels. We shown that this unexpected gelation phenomenon is related to the presence of the silica cores: grafted PDMS chains can adsorb onto different particles and form irreversible bonds between the cores. The viscosity and elastic modulus evolutions during gelation are well described by the scalar percolation model of sol-gel transition. Received 23 March 1998     

Lattice-Boltzmann Simulation of Particle Suspensions in Shear Flow

Inclusion of short-range particle–particle interactions for increased numerical stability in a lattice-Boltzmann code for particle-fluid suspensions, and handling of the particle phase for an effective implementation of the code for parallel computing, are discussed and formulated. In order to better understand the origin of the shear-thickening behavior observed in real suspensions, two simplified cases are considered with the code thus developed. A chain-like cluster of suspended particles is shown to increase the momentum transfer in a shear flow between channel walls, and thereby the effective viscosity of the suspension in comparison with random configurations of particles. A single suspended particle is also shown to increase the effective viscosity under shear flow of this simple suspension for particle Reynolds numbers above unity, due to inertial effects that change the flow configuration around the particle. These mechanisms are expected to carry over to large-scale particle-fluid suspensions.     

Active suspensions and their nonlinear models

Active suspensions, such as suspensions of self-propelled microorganisms and related synthetic microswimmers, are known to undergo complex dynamics and pattern formation as a result of hydrodynamic interactions. In this review, we summarize recent efforts to model these systems using continuum kinetic theories. We first derive a basic kinetic model for a suspension of self-propelled rodlike particles and discuss its stability and nonlinear dynamics. We then present extensions of this model to analyze the effective rheology of active suspensions in external flows, the effect of steric interactions in concentrated systems, and the dynamics of chemotactically responsive suspensions in chemical fields.     

Mixtures of anisotropic and spherical colloids: Phase behavior, confinement, percolation phenomena and kinetics

Purely entropic systems such as suspensions of hard rods, platelets and spheres show rich phase behavior. Rods and platelets have successfully been used as models to predict the equilibrium properties of liquid crystals for several decades. Over the past years hard particle models have also been studied in the context of non-equilibrium statistical mechanics, in particular regarding the glass transition, jamming, sedimentation and crystallization. Recently suspensions of hard anisotropic particles also moved into the focus of materials scientists who work on conducting soft matter composites. An insulating polymer resin that is mixed with conductive filler particles becomes conductive when the filler percolates. In this context the mathematical topic of connectivity percolation finds an application in modern nano-technology. In this article, we briefly review recent work on the phase behavior, confinement effects, percolation transition and phase transition kinetics in hard particle models. In the first part, we discuss the effects that particle anisotropy and depletion have on the percolation transition. In the second part, we present results on the kinetics of the liquid-to-crystal transition in suspensions of spheres and of ellipsoids.     

Direct visualization of pH-dependent evolution of structure and dynamics in microgel suspensions

We use 3D confocal microscopy combined with image analysis and particle tracking techniques to study the structure and dynamics of aqueous suspensions of fluorescently labelled p(NIPAm-co-AAc) microgel particles. By adjusting the pH we can tune the interactions between the microgel particles from purely repulsive near neutral pH, to weakly attractive at low pH. This change in the interaction potential has a pronounced effect on the manner in which the suspensions solidify. We directly follow the evolution of the system after a quench from the liquid state to obtain detailed information on the route to kinetic arrest. At low pH and low concentration, dynamic arrest results mainly from crystallization driven by the attraction between particles; crystal nucleation occurs homogeneously throughout the sample and does not appear to be localized to geometric boundaries. Moreover, the growth of crystals is characterized by nucleation-limited kinetics where a rapid growth of crystal domains takes place after a long concentration-dependent lag time. At low pH and high concentration, relaxation of the suspension is constrained and it evolves only slightly, resulting in a disordered solid. At neutral pH, the dynamics are a function of the particle number concentration only; a high concentration leads to the formation of a disordered soft glassy solid.     

Can secondary nucleation exist in ice banding of freezing colloidal suspensions?

The formation mechanism of ice banding in the system of freezing colloidal suspensions, which is of significance in frost heaving, ice-templating porous materials and biological materials, is still a mystery. Recently, the theory of secondary nucleation and growth of ice has been proposed to explain the emergence of a new ice lens. However, this theory has not been quantitatively examined. Here, we quantitatively measure the initial interfacial undercooling of a new ice lens and the nucleation undercoolings of suspensions. We find that the interfacial undercooling cannot satisfy the nucleation undercooling of ice and hence disprove the secondary nucleation mechanism for ice banding.     

Settling Behavior Characterization of Submicron Particles in Dilute and Concentrated Suspensions

In separation processes, the knowledge of particle size and density arc often not enough to describe the settling behaviour in a concentrated suspension. Therefore, a direct method for the characterization of the settling behavior of submicron particles in concentrated suspensions is introduced in a centrifugal field by a manometric sedimentation analysis. By means of this cumulative method in a homogeneous suspension, the analyses of both the interfacial settling rate and the settling rate of the particles within the concentrated suspension are possible. This permits a differential examination of settling processes in a broad concentration range. First, the influence of the solid concentration on the settling rate at the interface and within a monodisperse suspension with a range from 0.01 to 30 vol.% is represented. The relationship between the increase in settling rate through particles settling in a cluster and a concentration decrease in the suspension is also represented. Consideration of the possibilities of the analysis of polydisperse suspensions demonstrates the field of applications for this method.     

Characterization of suspensions of particles in water by an ultrasonic resonant cell

This paper presents a resonant technique to accurately measure phase-velocity and attenuation of longitudinal acoustic waves in suspensions of solid particles in water. The technique is based on exciting thickness resonances of a layer of fluid and analyzing its spectrum. To this end, a resonant cell to contain the fluid is described and used. Two different type of water suspensions are studied: titanium dioxide and alumina particles; particle volume fraction is in the range 0–0.18. Simultaneous determination of particle size distribution in the suspension by an optical method are also carried out. Finally, the experimental results are compared with theoretical predictions obtained from three different approaches.