Localized annealing of polysilicon microstructures by inductively heated ferromagnetic films

The monolithic integration of dissimilar microsystems is often limited by conflicts in thermal budget. One of the most prevalent examples is the fabrication of active micro-electromechanical systems (MEMS), as structural films utilized for surface micromachining such as polysilicon typically require processing at temperatures unsuitable for microelectronic circuitry. A localized annealing process could provide for the post-deposition heat treatment of integrated structures without compromising active devices. This dissertation presents a new microfabrication technology based on the inductive heating of ferromagnetic films patterned to define regions for heat treatment. Support is provided through theory, finite-element modeling, and experimentation, concluding with the demonstration of inductive annealing on polysilicon inertial sensing structures. Though still in its infancy, the results confirm the technology to be a viable option for integrated MEMS as well as any microsystem fabrication process requiring a thermal gradient.

Intergranular Corrosion and Stress Corrosion Cracking of Extruded AA6005A

A research program focused on understanding the intergranular corrosion (IGC) and stress corrosion cracking (SCC) behavior of AA6005A aluminum extrusions is presented in this dissertation. The relationship between IGC and SCC susceptibility and the mechanisms of SCC in AA6005A extrusions were studied by examining two primary hypotheses. IGC susceptibility of the elongated grain structure in AA6005A exposed to low pH saltwater was found to depend primarily on the morphology of Cu-containing precipitates adjacent to the grain boundaries in the elongated grain structure. IGC susceptibility was observed when a continuous (or semi-continuous) film of Cu-containing phase was present along the grain boundaries. When this film coarsened to form discrete Cu-rich precipitates, no IGC was observed. The morphology of the Cu-rich phase depended on post-extrusion heat treatment. The rate of IGC penetration in the elongated grain structure of AA6005A-T4 and AA6005A-T6 extrusions was found to be anisotropic with IGC propagating most rapidly along the extrusion direction, and least rapidly along the through thickness direction. A simple 3-dimensional geometric model of the elongated grain structure was accurately described the observed IGC anisotropy, therefore it was concluded that the anisotropic IGC susceptibility in the elongated grain structure was primarily due to geometric elongation of the grains. The velocity of IGC penetration along all directions in AA6005A-T6 decreased with exposure time. Characterization of the local environment within simulated corrosion paths revealed that a pH gradient existed between the tip of the IGC path and the external environment. Knowledge of the local environment within an IGC path allowed development of a simple model based on Fick's first law that considered diffusion of Al3+ away from the tip of the IGC path. The predicted IGC velocity agreed well with the observed IGC velocity, therefore it was determined that diffusion of Al3+ was the primary factor in determining the velocity of IGC penetration. The velocity of crack growth in compact tensile (CT) specimens of AA6005A-T6 extrusion exposed to 3.5% NaCl at pH = 1.5 was nearly constant over a range of applied stress intensities, exposure times, and crack lengths. The crack growth behavior of CT specimens of AA6005A-T6 extrusion exposed to a solution of 3.5% NaCl at pH = 2.0 exhibited similar behavior, but the crack velocity was ~10.5X smaller than that those exposed to a solution at pH =1.5. Analysis of the local stress state and polarization behavior at the crack tip predicted that increasing the pH of the bulk solution from 1.5 to 2.0 would decrease the corrosion current density at the crack tip by approximately 11.8X. This predicted decrease in corrosion current density was in reasonable agreement with the observed decrease in SCC velocity associated with increasing the solution pH from 1.5 to 2.0. The agreement between the predicted and observed SCC velocities suggested that the electrochemical reactions controlling SCC in AA6005A-T6 extrusions are ultimately controlled by the pH gradient that exists between the crack tip and external environment.

Synthesis of novel solid materials from carbon dioxide and carbon monoxide

The increase of atmospheric CO2 has been identified as the primary cause for the observed global warming over the past century. The geological and oceanic sequestration of CO2 has issues, such as cost and leakage as well as effects on sea biota. The ideal solution should be the conversion of CO2 into useful materials. However, most processes require high energy input. Therefore, it is necessary to explore novel processes with low energy demands to convert CO2 to useful solid materials. Amorphous carbon nitride and graphone received much attention due to their unusual structures and properties as well as their potential applications. However, to date there has been no attempt to synthesize those solid materials from CO2. Lithium nitride (Li3N) and lithium imide (Li2NH) are important hydrogen storage materials. However, their optical properties and reactivity has not yet studied. This dissertation research is aimed at the synthesis of carbon nitrides and graphone from CO2 and CO via their reaction with Li3N and Li2NH. The research was focused on (1) the evaluation of Li3N and Li2NH properties, (2) thermodynamic analysis of conversion of carbon dioxide and carbon monoxide into carbon nitride and other solid materials, (3) synthesis of carbon nitride from carbon dioxide, and (4) synthesis of graphone from carbon monoxide. First, the properties of Li3N, Li2NH, and LiNH2 were investigated. The X-ray diffraction measurements revealed that heat-treatment at 500°C introduce a phase transformation of β-Li3N to α-Li3N. Furthermore, the UV-visible absorption evaluation showed that the energy gaps of α-Li3N and β-Li3N are 1.81 and 2.14 eV, respectively. The UV-visible absorption measurements also revealed that energy gaps are 3.92 eV for Li2NH and 3.93 eV for LiNH2. This thermodynamic analysis was performed to predict the reactions. It was demonstrated that the reaction between carbon dioxide and lithium nitride is thermodynamically favorable and exothermic, which can generate carbon nitride and lithium cyanamide. Furthermore, the thermodynamic calculation indicated that the reaction between carbon monoxide and lithium imide can produce graphone and lithium cyanamide along with releasing heat. Based on the above thermodynamic analysis, the experiment of CO2 and Li3N reaction and CO and Li2NH were carried out. It was found that the reaction between CO2 and Li3N is very fast and exothermic. The XRD and element analysis revealed that the products are crystal lithium cyanamide and amorphous carbon nitrides with Li2O and Li2CO3. Furthermore, TEM images showed that carbon nitrides possess layer-structure, namely, it is graphene-structured carbon nitride. It was found that the reaction between Li2NH and CO was also exothermic, which produced graphone instead of carbon nitride. The composition and structures of graphone were evaluated by XRD, element analysis, TEM observation, and Raman spectra.