Thesis (Ph. D.)--University of Texas at Austin, 2005. / Supervisor: Thomas F. Edgar. Vita. Includes bibliographical references.

Applications such as device interconnects require thick, patterned films that are currently produced by screen printing pastes consisting of metallic particles and subsequently sintering the films. For Ag films, achieving adequate electrical conductivity requires sintering temperatures in excess of 700˚C. New applications require highly conductive films that can be processed at lower processing temperatures. Although sintering temperatures have been reduced by utilizing finer nanoparticles (NPs) in place of conventional micron-size particles (MPs), realization of theoretically achievable sintering kinetics is yet to be achieved. The major factors that inhibit NP sintering are 1) the presence of organic molecules on the NP surfaces, 2) the dominance of the non-densifying surface diffusion over grain boundary or lattice diffusion 3) agglomeration of NPs, and 4) low initial density of the NPs. Here, we report a film fabrication technique that is capable of eliminating these deleterious factors and produces near fully dense Ag films that exhibit an order of magnitude higher conductivity when compared to other film fabrication techniques at processing temperatures of 150 – 250 °C. The observed results establish the benefits of NP diffusion kinetics to be far more profound when the deleterious factors to sintering are eliminated. The sintering behavior exhibits two distinct temperature regimes – one above 150 ᵒC where grain boundary diffusion-dominated densification is dominant and one below 100 ᵒC where surface diffusion-dominated coarsening is dominant. An analytical model is developed by fitting the experimental data to the existing models of simultaneous densification and grain growth, and combining this model with existing models of the dependence of conductivity on grain boundary scattering and pore scattering. The combined model successfully describes the evolution of density, grain size and conductivity of nanoparticulate films as a function of annealing treatment, with reasonable accuracy. The model was also used to evaluate the effect of initial NP size and initial relative density of films on the final sintered properties and conductivity of films. / text

The overall objective of the project, on which this thesis is based, is to develop a novel hydrocarbon-based proton exchange membrane (PEM) material that can produce a proton conductivity of 0.1 S/cm at the operating conditions of 50 % relative humidity and 120 oC, which is the performance target set by the U.S. DOE for automotive application. As a part of this project, our efforts have been focused on the investigation of the effects of solution-casting conditions on the final morphology and properties of disulfonated poly(arylene ether sulfone) multiblock copolymer films from the viewpoint of phase separation of block copolymers. Of equal importance to this work, is a possibility of utilizing a rheological technique for monitoring the transformation and kinetics of block copolymers during solvent removal process, which was initially examined in order to provide fundamental quantitative understanding and practical information on the solvent removal process. Our results demonstrated that solvent selectivity and drying temperature as well as the block length had considerable effects on the final morphology and properties. The proton conductivity could be significantly increased by simply utilizing a selective solvent, dimethylacetamide (DMAC), which is good and marginal for the sulfonated and unsulfonated blocks, respectively, rather than N-methyl-2-pyrrolidone (NMP), a neutral solvent for both blocks. The drying temperature was also observed to have considerable effects on the final properties, being coupled with the effects of solvent selectivity. Also, it was shown that the multiblock copolymer consisting of longer blocks was more sensitive to the processing conditions. From the morphological study using transmission electron microscopy and small-angle X-ray scattering, evidences for the above observations were obtained. In the second part of this dissertation, the evolution of GÎ and GË of the solutions of a styrene-butadiene-styrene (SBS) triblock copolymer in toluene was obtained as a function of concentration using a modified parallel-plate device and a rheology test scheme developed in this study in an effort to quantify the phase separation kinetics. Then, the information on the phase transformation and kinetics of the SBS block copolymer in the solution was obtained by analyzing the GÎ and GË data with the Avrami equation. The Avrami exponent was found to be approximately 1, which indicates that the phase transformation occurred by a one-dimensional growth mechanism. The rate constant showed a strong concentration-dependence. After the initial increase up to 45 vol %, the rate constant drastically decreased and, finally, converged to 0 at 70 vol %. It is believed that, at the concentration range below 45 vol %, the phase separation became more intense as the polymer molecules had more chances to interact owing to the concentration increase. However, above 45 vol %, the phase transformation became weaker due to the limited mobility of the polymer molecules, which finally led to a “kinetically frozen-in” structure, in which the polymer molecules could not move any longer. Thus, it can be concluded that the solvent removal rate is one of the dominant factors that decide the final microstructures of solution-cast block copolymer films. / Ph. D.

Thesis (M.A.Sc.)--Naval Postgraduate School, 2001 / Includes bibliographical references (p. 163-166). Also available in electronic format via the Defense Technical Information Center website.