Thermodynamics and kinetics at the nanoscale: thermal behavior of aluminum nanoparticles and development of nanoporous low-k dielectrics
Nanostructured materials have drawn worldwide attention and have found applications in many fields. Research needs exist in the study of the thermodynamics and kinetics at the nanoscale. Experiments were performed to study the melting behavior of aluminum nanoparticles, to determine the effect of particle size on the oxidation of aluminum nanoparticles, and to develop low-k nanoporous films. Results of differential scanning calorimetry (DSC) experiments indicate that with decreasing particle size, the melting response moves towards lower temperatures. The melting point depression is found to be linear with the reciprocal of particle radius, as predicted by the Gibbs- Thomson equation. The solid-liquid interfacial energy obtained is in good agreement with literature values. In addition, with decreasing particle size, the latent heat of fusion also decreases. The depression of the heat of fusion is larger than expected given the value of the surface tension, presumably because of the presence of defects in the nanosize crystals; the defect energy was found to increase with decreasing particle size. In addition, the derivation of the Gibbs-Thomson equation was discussed and shown to be valid to describe the melting point depression of aluminum nanoparticles. The oxidation reactions of the aluminum nanoparticles with oxygen gas and with molybdenum trioxide, the latter of which makes up a metastable intermolecular composite (MIC), are studied using DSC as a function of the size of the aluminum particles ranging from 30 nm to 160 nm; comparisons are made to the behavior of micron-size aluminum particles. The results show that the fraction of aluminum that reacts prior to aluminum melting for aluminum nanoparticles is considerably larger than that for micron-size aluminum for both AI/O2 and AI/M0O3 reactions. Other methods of characterizing reactivity, such as the onset temperature of the reaction and the maximum rate of heat release, generally also show that the aluminum nanoparticles are more reactive than their micron-size counterparts. However, the heats of reaction are considerably lower than the theoretical values for the nanoparticles. The isoconversional method was used to calculate apparent activation energies, and for AI/O2 reaction, Eg ranges from 200 to 300 kJ/moI, which is in agreement with literature values. In addition, the values of Ea for AI/M0O3 reactions are comparable to those for the AI/O2 reaction, suggesting the same limiting reaction step in these two reactions. A supercritical CO2 (SCCO2) extraction process was performed for the preparation of nanoporous low-k (low dielectric constant) films from two nanohybrid films: i) plasma enhanced chemical vapor deposited (PECVD) composite films of tetravinyltetramethylcyclotetrasiloxane (TVTMCTS) and fluorocarbon (a-C:F) and ii) spin-on deposited (SOD) oly(methylsilsesquioxane)/poly(propyIene glycol) (PMSSQ/PPG) films. After SCCO2 treatment, the TVTMCTS and a-C:F films show a decrease in dielectric constant k of about 10%-14%, whereas no change was found for TVTMCTS film only. Analysis of the Fourier transform infrared (FTIR) spectra of the samples confirms the decrease in the C-F absorption intensity after SCCO2 treatment. SCCO2 extraction of PPG porogen from PMSSQ/PPG films is shown to produce nanoporous, ultralow-A: thin films. Both closed and open cell porous structures were prepared simply by varying the porogen load.