Detailed experiments involving extensive high resolution transmission electron microscopy (TEM) revealed significant microstructural differences between Cu sulfides formed at low and high relative humidity (RH). It was known from prior experiments that the sulfide grows linearly with time at low RH up to a sulfide thickness approaching or exceeding one micron, while the sulfide initially grows linearly with time at high RH then becomes sub-linear at a sulfide thickness less than about 0.2 microns, with the sulfidation rate eventually approaching zero. TEM measurements of the Cu2S morphology revealed that the Cu2S formed at low RH has large sized grains (75 to greater than 150 nm) that are columnar in structure with sharp, abrupt grain boundaries. In contrast, the Cu2S formed at high RH has small equiaxed grains of 20 to 50 nm in size. Importantly, the small grains formed at high RH have highly disordered grain boundaries with a high concentration of nano-voids. Two-dimensional diffusion modeling was performed to determine whether the existence of localized source terms at the Cu/Cu2S interface could be responsible for the suppression of Cu sulfidation at long times at high RH. The models indicated that the existence of static localized source terms would not predict the complete suppression of growth that was observed. Instead, the models suggest that the diffusion of Cu through Cu2S becomes restricted during Cu2S formation at high RH. The leading speculation is that the extensive voiding that exists at grain boundaries in this material greatly reduces the flux of Cu between grains, leading to a reduction in the rate of sulfide film formation. These experiments provide an approach for adding microstructural information to Cu sulfidation rate computer models. In addition to the microstructural studies, new micro-patterned test structures were developed in this LDRD to offer insight into the point defect structure of Cu2S and to permit measurement of surface reaction rates during Cu sulfidation. The surface reaction rate was measured by creating micropatterned Cu lines of widths ranging from 5 microns to 100 microns. When sulfidized, the edges of the Cu lines show greater sulfidation than the center, an effect known as microloading. Measurement of the sulfidation profile enables an estimate of the ratio of the diffusivity of H2S in the gas phase to the surface reaction rate constant, k. Our measurements indicated that the gas phase diffusivity exceeds k by more than 10, but less than 100. This is consistent with computer simulations of the sulfidation process. Other electrical test structures were developed to measure the electrical conductivity of Cu2S that forms on Cu. This information can be used to determine relative vacancy concentrations in the Cu2S layer as a function of RH. The test structures involved micropatterned Cu disks and thin films, and the initial measurements showed that the electrical approach is feasible for point defect studies in Cu2S.
We report our conclusions in support of the FY 2003 Science and Technology Milestone ST03-3.5. The goal of the milestone was to develop a research plan for expanding Sandia's capabilities in materials modeling and simulation. From inquiries and discussion with technical staff during FY 2003 we conclude that it is premature to formulate the envisioned coordinated research plan. The more appropriate goal is to develop a set of computational tools for making scale transitions and accumulate experience with applying these tools to real test cases so as to enable us to attack each new problem with higher confidence of success.
High-purity aluminum samples were implanted with 35 keV Cl{sup +} then polarized in both Cl{sup -}-containing and Cl{sup -}-free electrolytes in order to ascertain corrosion behavior as a function of Cl{sup -} content in the oxide. Implant fluence between 5 x 10{sup 15} and 2 x 10{sup 16} Cl{sup +} cm{sup -2} resulted in little or no localized attack. Implant fluences of 3 x 10{sup 16} and 5 x 10{sup 16} Cl{sup +} cm{sup -2} resulted in significant pitting in a Cl{sup -}-free electrolyte with the severity scaling as a function of implant fluence. The low variability in the pitting behavior of the 5 x 10{sup 16} Cl{sup +} cm{sup -2} sample suggests that this implant dosage results in a critical Cl{sup -} concentration in the oxide for pit nucleation. The passive current density (i{sub pass}) decreased with increasing implant fluence. A space-charge effect is proposed to account for this phenomenon, although effects from defect interactions and possible oxide thickening are still under consideration.