纳米溶胶及微米线/薄膜热物理特性的理论和实验研究

【作者】 王涛；

【导师】 岑可法； 骆仲泱； 倪明江；

【作者基本信息】 浙江大学 ， 工程热物理， 2008， 博士

【摘要】 本文对纳米溶胶粘度的影响因素进行了实验研究，并利用分子动力学模拟深入分析了溶胶内部应力波以及固液原子间的相互作用势能对纳米溶胶粘度的影响机理；同时，独创性的发展了测量微／纳米线、薄膜材料热物性的实验方法。实验测量了SiO₂，CuO，Al₂O₃等纳米颗粒形成的溶胶系统的粘度。低颗粒体积浓度下，纳米溶胶粘度与体积浓度和颗粒粒径关系的研究表明，纳米溶胶的粘度与颗粒体积浓度基本成线性关系，同时纳米溶胶的粘度非常明显的依赖于纳米颗粒粒径，粒径越小，溶胶粘度的增加越大，尤其对于7nm的SiO₂溶胶，在2％的体积浓度时，其粘度相对基液提高120％。根据温度对纳米溶胶粘度的实验研究表明，纳米颗粒粒径对溶胶粘度的温度特性也没有明显的影响。对溶胶的pH值及其与颗粒粒径关系的研究表明，纳米粉体的添加使溶胶趋于碱性。这可能是在粉体制备过程中带入了碱性硅烷醇基团（Si-OH）的原因。同时，粒径约大，其溶胶越趋向于碱性环境。对纳米溶胶粘度与pH值关系的研究表明，随着pH值向酸性环境的调节，颗粒表面的碱性基团不断被中和，颗粒的表面电势下降，颗粒之间的排斥力变弱，团聚体形状更趋于球形，并导致粘度趋于一固定值；同时，pH值对溶胶粘度的调节受颗粒粒径的影响也较大，当颗粒较小时，pH值的调节会在某个范围内快速的改变溶胶粘度；同时，通过pH值的调节，可以使纳米溶胶的粘度升高或降低，甚至在某些pH值处不同浓度的溶胶具有近似的粘度值，这些特性提高了纳米溶胶在实际应用中的灵活性和多样性。实验结果及数值分析表明，在相同体积浓度下，随着颗粒粒径的减小，颗粒数目也越多，颗粒之间的距离也越小，较强的静电斥力使得颗粒团聚体趋于链状结构，从而使得溶胶粘度迅速增大；同时，由于颗粒粒径越小，其越易于形成团聚体，因而粒径较小的颗粒形成的团聚体内也含有更多的单个颗粒。当颗粒粒径比较大时（大于100 mn），团聚体的长径比趋于1，这意味在静电斥力变弱，更多的球形团聚体形成；在这同时，本征粘度也趋于2．5。利用分形理论，并结合SiO₂纳米溶胶的实验研究得知，团聚体的分形维数分布在2-2．4之间；同时，微观结构分析表明，颗粒团聚体的三维分形维数是大于二维分形维数的。另外一个有趣的现象就是颗粒的分形维数与颗粒粒径存在一定的关系：在低浓度段（<0．005），颗粒粒径越大，分形维数越大；在稍高浓度段，7 nm颗粒的团聚体分形维数会随着浓度的升高而增大，甚至超过20 nm颗粒团聚体的分形维数。利用分子动力学构造了含有纳米颗粒的固液两相系统；并构建了可调节的固体材料；在稳定的温度（143．4K）和压强（1．5-1．6×10⁸Pa）下，对纳米溶胶系统的粘度进行了全面的研究；首次发现纳米溶胶系统的应力张量自关联函数（PTACF）存在着强烈的周期振荡；通过机理分析验证了由于固液界面声学不匹配而造成应力波散射，并进而形成振荡的观点；由于应力波在颗粒中的传播速度较大，因而颗粒密度的减小会提高声学匹配系数，从而在固液界面处形成更佳的声学匹配；声学不匹配的消除，可以减少应力波在固液界面的反射或散射，并进而降低了应力张量自关联函数的振荡。首次获得了应力波在纳米溶胶系统中的传递细节；通过颗粒密度对应力张量自关联函数以及速度自关联函数的影响研究发现：粒径较小或密度较小的颗粒由于布朗运动的加剧，会导致固液界面物理位置振荡的加剧，并直接影响应力波的在界面的反射或散射，从而使得应力波振荡的频率增加；同时，当应力波的波长与颗粒粒径相当时，应力波在颗粒表面发生明显的衍射现象，此时尽管声学匹配系数没发生变化，但更多的能量以“前向散射”的形式传播，“背向散射”也因而减弱，也会造成应力张量自关联函数振荡减弱的表面现象。同时，进一步的合理预测，当颗粒粒径继续减小，比如和分子大小相当时，在固液界面更多的应力波以衍射的方式传播，溶胶的PTACF曲线将不会出现振荡。纳米溶胶粘度值与颗粒体积浓度、颗粒粒径等关系的分子动力学研究表明，纳米溶胶的粘度与颗粒体积浓度基本呈线性关系，但同时，溶胶系统的相对粘度远远超出理论公式预测，并且粘度受纳米颗粒粒径的影响非常大：在相同体积浓度下，随着粒径的增大，纳米溶胶粘度基本呈线性关系增大。成功引入了一种对固液两相系统的势能函数进行拆分分析的方法；通过对纳米溶胶原子间势能关系的分解情况可知，在固液两相系统中，液体原子之间的相互作用会受到纳米颗粒能量场的影响，因而随着颗粒粒径的减小，颗粒之间距离的减小会提高原子间势能关系对粘度的贡献；而同时，应力波衍射的现象又可以缓解这种能量场扭曲的影响，在两种相反作用效果的因素共同制衡下，导致了当纳米溶胶中的颗粒较小时，原子间势能关系对粘度的贡献在总的趋势上是随着粒径增大而增大的（衍射影响变小，能量场扭曲的影响相对加剧），并且原子间势能关系对粘度的贡献开始会小于纯液体，其后慢慢增加，甚至大于纯液体时的粘度。同时，随着颗粒粒径的增大，纳米颗粒表面原子数所占百分比迅速减小，此时固体原子之间可以相互作用的“对数”也将迅速增大。因而，即便在同样体积浓度下，固体原子间势能关系对粘度的贡献将随着粒径的增大而迅速增大。利用径向分布函数，对固液界面液体层的结构特征研究表明，对非极性原子系统，即使在很小粒径的情况下（颗粒间距离也很小），并没有观察到固液界面有序结构的液体层；对极性分子一水分子在电场下的分子动力学模拟表明，在固液界面处，水分子已经开始出现类似晶体的结构特征，并且随着外部电场电荷密度的增大，该趋势增大。利用统计物理理论，成功构建了颗粒之间的电势能这种长程作用关系对溶胶粘度的影响模型。对于球形颗粒，其表面电荷对电粘度的贡献与粒径关系的研究表明，在颗粒表面电荷不变的情况下，溶胶的电粘度随着粒径的减小而迅速增大；同时，在同样的粒径下，电粘度也会随着颗粒体积浓度升高而升高；另外，电粘度对系统总粘度的贡献非常小，基本可忽略不计。对于圆柱形颗粒，其表面电荷对电粘度的贡献与颗粒形状关系的研究表明，其电粘度对总体粘度的贡献并不大，只有千分之一的量级：同时，随着轴径比的增大，在轴径比较小时，电粘度有略微升高，然后随着轴径比的增大而减小，并趋于稳定。利用分子动力学，对纳米颗粒在液体中的自扩散系数与粒径关系的研究表明，颗粒的自扩散系数基本和粒径成反比；同时，计算的结果比Stokes-Einstein公式预测值低6个数量级。利用激光加热结合电信号感应温度变化的原理，发展了瞬态光-电-热（Transient Photo-electro-thermal，TPET）测量技术，该技术可应用于测量导电或非导电的微米／纳米细丝，利用该技术对铂丝，碳纳米管束以及聚合物纤维丝的热扩散系数进行了测量。利用光热方法首次系统测量了具有不同ZrO₂摩尔浓度的有机（PMMA）-无机（SiO₂，ZrO₂）复合薄膜的热物性；系统分析了薄膜结构对其热物性（导热系数，蓄热系数等）的影响；研究结果表明，ZPO做为一种反应原料，其浓度的调节可以达到在不改变基体热物性的基础上对器件光学属性进行调节的目的，因而在光学集成器件的热管理方面具有很大的应用潜力。更多还原

【Abstract】 The viscosities of nancolloidal dispersions wre studied experimentally in this thesis. And molecular dynamics simulations were employed to further study the effect of stress wave and potential between solid atoms and liquid atoms on the viscosity. At the same time, new measurement methods were developed to characterize the thermophysical properties of micro/nano wires/filmsThe viscosities of dilute nanocolloidal dispersions with SiO₂, CuO, Al₂O₃ nanoparticles were measured respectively. The viscosities increase with volume fraction of particles linearly and the size effect is very obvious. The viscosity increases fast with the decreasing particle size. For the nanocolloidal dispersions with 7 nm SiO₂ - water at a volume fraction of 2%, there is an increase of 120% for the viscosity. And no obvious temperature effect for viscosity is observed.The study on the pH value of nanocolloidal dispersions shows that the pH values increase with the addition of nanopowder, which maybe related with the silanol groups on the particle surfaces. At the same volume fraction, the pH value with smaller particles is bigger than that with bigger particles. The study on the relationship between viscosity and pH value,ζ-potential shows that with decreasing pH value, the viscosity decreases and tends to be a fixed value. During the adjustment of pH value, the silanol groups are considered to be neutralized and the electrostatic potential on the particle surface becomes weak. With the decrease of interparticle repulsive force, the particle conglomerations tend to be spherical and then the viscosity tends to be a fixed value according to Einstein’s relationship. At the same time, the size effect is obvious during the adjustment of viscosity by weak acid. For smaller particles, the addition of weak acid can change the viscosity more quickly at a certain scale of pH value. And the interesting phenomenon is the viscosities of dispersions with different particle volume fraction can have the same value during the adjustment of pH value, which will increase the potential for the application of nanocolloidal dispersions.The experiments and numerical analysis show that the structure of particle conglomerations is considered to play an important role in the viscosity. For nanocolloidal dispersions at constant volume fraction, because the average interparticle distance decreases with decease of particle size, making the attractive van der Waals force more important, the stronger electrical repulsive force will elongate the cluster to be a chain, which will increase the viscosity. At the same time, the probability of aggregation increases with decreasing particle size, and the clusters will contain more individual particles. The axial ratio of the clusters tends to be 1 with increasing particle size （> 100 nm）, which confirms the electrical repulsive force become weaker and more spherical clusters are formed, and the intrinsic viscosity also tends to be 2.5.The viscosities of nanocolloidal dispersions were analyzed numerically with fractal theory. The results show that, for dilute dispersion, the fractal dimension of the aggregates ranges from 2 to 2.4. And the fractal dimensions of space distribution of clusters are bigger than that for section area of clusters which can be obtained from the microscopic photos of clusters in suspensions. Another interesting phenomenon is that there is a size effect for the fractal dimension of the aggregates. At lower particle volume fraction （for SiO₂ nanocolloidal dispersions, < 0.005）, the fractal dimension increases with particle size, and with increasing particle volume fraction, the fractal dimension of clusters with smaller particle will increase. At the volume fraction of 2%, the fractal dimension of clusters with 7 nm particles is bigger than that with 20 nm particles.Extensive equilibrium molecular dynamics （MD） simulations were conducted to study the nanocolloidal dispersions with different model solid material. The model systems have environment with a constant temperature of 143.4 K and a constant pressure of 1.5-1.6×10⁸ Pa. The oscillations of pressure tensor autocorrelation function （PTACF） of nanocolloidal dispersions are first observed. The simulation results show that the intensity of the oscillation decreases with the decreasing of particle size and density, while the frequency of the oscillation increases with the decreasing of particle size and density. Careful analysis of the relationship between the oscillation and nanoparticle characteristics proposes that the stress wave scattering/reflecting at the particle-liquid interface plays a critical role in PTACF oscillation. Our modeling shows that it is practical to eliminate the PTACF oscillation though suppressing the acoustic mismatch at the solid-liquid interface by designing special nanoparticle materials.The details of propagation of stress wave were first revealed. The density/size effect of PTACF and velocity autocorrelation function （VACF） shows that the decreasing particle density or size will induce more Brownian motion/vibration of solid particles, and also increase the oscillation frequency of PTACF. It is also found when the particle size is comparable with the wavelength of the stress wave, diffraction of stress wave happens at the interface. This means more ’forward scattering’ and less ’back scattering’ although the acoustic impedance is not changed. Such effect substantially suppresses the PTACF oscillation and improves the stability of viscosity calculation. It can be predicted that with the decreasing particle size （-atom） and more ’forward scattering’ of stress wave, the PTACF oscillation will be eliminated.The MD simulation results also show that the viscosities of nanocolloidal dispersions increase linearly with particle volume fraction and values are bigger than that predicted by Einstein’s relationship. At the same particle volume fraction, the viscosity increase almost linearly with increasing particle size.A new analytical method with decomposing stress tensor was successfully introduced in this thesis. The simulation results show that the viscosity contributed by potential between liquid atoms increases with decreasing particle size. Two mechanisms are revealed to explain our results. First, the potential between liquid atoms are supposed to be affected by the solid atoms, especially with decreasing distance between particle surfaces, which will affect the energy dissipation in colloidal system. Second, the diffraction of stress wave （especially for smaller particles） can relax the distortion of energy filed near particle surface. For smaller particles, the diffraction effect tends to be weaker with increasing particle size and the effect of potential will domain the viscosity. At the same time, the pair of solid atoms which can be interacted will increase with increasing particle size. Considering the effect of potential, the viscosity contributed by potential between solid atoms will increase with increasing particle size.During the MD simulation, the radius distribution function was employed to study the structure of liquid layer adhere to solid particle. For dispersions with non-polar molecules, no obvious special structure can be observed. For dispersions with polar molecules （H₂O）, an additional electric filed is introduced into colloidal system. The simulation results show that the liquid layer adheres to solid material tends to form a regular structure, especially with the increase of electric filed intensity.Statistical mechanical theory of transport processes was employed to study the interparticle electrostatic contribution （long range potential） to the viscosity of aqueous suspensions. For spherical particles, the numerical calculation shows that, with the same charges on particle surface, the electrostatic contribution to the viscosity increases quickly with deceasing particle size. And the contribution will increase with increasing particle volume fraction. For cylindrical particles, the electrostatic contribution to the viscosity will first increase and then decrease to be a constant value with increasing axial ratio of particle. For the two cases, the value of the contribution is very small, which can be neglected compared with viscosity of base fluid.The MD simulation was also used to study the self-diffusion of nanoparticles. The simulation results show that the relationship between self-diffusion and the reciprocal of particle diameter is linear, but the value is -6 orders smaller than that predicted by Stokes-Einstein relationship.A transient photon-electro-thermal （TPET） technique based on step laser heating and electrical thermal sensing was developed to characterize the thermophysical properties of one-dimensional micro/nanoscale conductive and nonconductive wires. Applying the TPET technique, the thermal diffusivities of conductive Pt wire, single-wall carbon nanotube （SWCNT） bundles and nonconductive cloth fibers were measured.A photothermal experiment was designed and conducted to characterize the thermophysical properties of organic [Polymethyl Methacrylate （PMMA）]/inorganic hybrid material films. The effect of film structure on the thermophysical properties （thermal conductivity, thermal effusivity） of hybrid films was studied. The measurement results indicate that the dose of ZPO can adjust the optic property of films without changing their thermophysical properties significantly, which will provide valuable guidance on the thermal management during device packaging.更多还原