Theoretical and experimental studies of the nano-sol -micron line / film thermophysical properties
|Keywords||nanocolloidal dispersion viscosity molecular dynamics micro/nano material measurement of thermophysical properties|
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.