A near-net-shape process for the production of mullite matrix ceramic composites below 1300 °C has been achieved by reaction bonding AI2O3, silicon, mullite seeds and a eutectic of A12O3-SiO2-mixed rare earth oxide. The roles of the transient liquid phase from the eutectic and the mullite seeds are examined. The approximate eutectic composition was estimated from the A12O3-SiO2-Y2O3 phase diagram as 22 wt % AI2O3/46 wt% S102/3 2 wt% Y2O3. The fusion temperature of the mixed-rare-earth-based eutectic composition was lower (1175 °C vs. those of pure, rare earth oxides based eutectic composition; Pr6On/A12O3/SiO2: 1224 °C, EU2O3/AI2O3/S1O2: 1259 °C and Y2O3/Al2O3/SiO2: 1345 °C). The densification characteristics of the reaction-bonded mullite (RBM) mixture were investigated. Density increased with eutectic, and decreased with mullite seed contents. Oxidation and volume expansions due to Si and mullite formation are examined by thermogravimetric analysis (TGA) and dilatometric measurement. TThe measured weight gain and maximum volume expansion were lower than theoretical values due to preoxidation of the Si powder. Dilatometric curves indicated sintering shrinkage is compensated by the oxidation-induced volume-expansion. AI2O3 + SiO2 mixtures of the mullite composition exhibited shrinkage exclusively. X-ray diffraction of the RBM sinters display major mullite peaks and minor residual a-A12O3. Mullite develops with low residual AI2O3 when 7.5 wt% mixed-rare-earth-oxide eutectic and 5 wt% mullite seeds are incorporated into the mix. The final sinter is > 90 % theoretical density, > 90 % mullite, and suffers 2.2 % sintering shrinkage. Transmission-electron-microscopy (TEM) and Energy dispersive X-ray spectra (EDX) were employed to follow mullite evolution. Model samples were utilised to study diffusion-, and reaction-, rates. The highest reaction rates at the lowest temperature occur when the eutectic penetrates an AI2O3/S1 layer. Bimodal pellets with and without eutectic (or with and without mullite seeds) directly illustrate their roles. Mullite seeds promote mullite formation, but the transient liquid accelerates Si oxidation, mullite formation and densification. / Thesis / Master of Engineering (ME)

Solid-state NMR measurement at cryogenic temperature shows significant potential for biological analysis due to its advantages for sample stability and detection sensitivity. However, at low temperature lineshape broadening and low spectral resolution are commonly observed and limit the applications for complex protein systems. Here, we explored the hypotheses for the underlying mechanisms of broad linewidths at low temperatures by studying E. coli Dihydrofolate reductase (DHFR). Our results support the hypothesis that conformational heterogeneity is a major source of linebroadening. We measured the protein backbone torsion angle (Ψ) at 105 K. In a selectively enriched protein sample with only one amide 13 C’- 15 N correlation expected, we identified three different conformations with distinct N chemical shift values accounting for the dramatic broadening observed in low temperature NMR spectra. We presume that backbone torsion angle fluctuates among the conformers on picosecond timescale at room temperature and are ‘frozen out’ giving rise to static heterogeneity at cryogenic temperatures. MD simulations support this hypothesis. QM/MM predicted chemical shifts based on snapshots from a MD simulation show excellent agreement with our data in that the average agrees well with the room temperature shift and the distribution agrees well with the low temperature spectral lineshape. On the other hand, our data suggest that there is no relationship between the μs - ms motions at room temperature and the lineshape broadening at low temperature. Resonance assignments of the apoenzyme in solution and associated liganded states were accomplished to identify the conformational transition in chemical exchange. We analyzed the 15 N relaxation dispersion profile of each residue at room temperature in solution; the rates appear to be organized in functional groups that exchange in a concerted fashion, with shift differences related to ligated-vs-unligated changes. The chemical shift changes associated with μs - ms exchange phenomena (and with ligation) are about an order of magnitude too small to explain the low temperature lineshapes, and also have no correlation with the low temperature lineshapes.

<p> Low-temperature, lead-free solders mitigate heating-induced warpage caused by the  differences in coefficient of thermal expansion between printed circuit boards (PCBs), substrates,  and dies during package assembly. Eutectic and near-eutectic Sn-Bi solders are promising low  temperature candidates because they show high reliability at low strain rates during thermal  cycling. However, Sn-Bi low temperature solder (LTS) has poor performance at high strain rates  during drop-shock testing. Alloying additions such as Ag, Cu, and Sb have been shown to increase  the ductility and strength of eutectic Sn-Bi and therefore improve the overall reliability during both  thermal cycling and drop-shock. Small Sb additions to Sn-Bi LTS are of particular interest because  these additions significantly increase ductility while maintaining the tensile strength. This increase  in ductility was previously attributed to small SnSb intermetallic particles that form within the Sn  phase on the interface of Sn and Bi in 1.0wt% Sb containing samples. Despite the fact the no SnSb  intermetallic compound (IMC) particles have been found in 0.5Sb-42Sn-Bi samples in any  previous studies or in our own studies, it was thought that the SnSb IMC particles were responsible  for the improved reliability and ductility of Sn-Bi.  This work encloses our efforts to understand how small Sb additions to eutectic Sn-Bi  impact the solder microstructure and the resulting mechanical properties of the solder alloy. We  began by studying possible solidification pathways through phase diagram analysis in Thermo?Calc to understand how the microstructure is predicted to develop and compared these models to  the literature data. Next, we analyzed the microstructures of our custom Sb-containing alloys  through a combination of scanning electron microscopy (SEM), energy dispersive spectroscopy  (EDS), and electron probe microanalyzer-wavelength dispersive spectroscopy (EPMA-WDS) and  determined that no SnSb IMC particles were found in the 0.5Sb-42Sn-Bi alloy and at 0.5 wt% the  Sb remained in solid solution with Sn. Nanoindentation was then used to evaluate the strain rate  sensitivity of Sn-Bi LTS with Sb additions and we found that, while the alloy hardness remains  sensitive to different strain rates, the Sb in solid solution with Sn altered the deformation behavior  of the alloy and decreased the amount of planar slip during indentation. To study the stability of  the microstructure and the alloy behavior in use, shear testing was performed before and after  isothermal aging. Our results suggest that Sb in solid solution with the Sn-rich phase contributes  significantly to the changes in the eutectic microstructure and the mechanical properties. </p>

New halo-carbon precursor, CH3Cl, is used in this work to replace the traditional C3H8 gas as a carbon precursor for the homoepitaxial growth of 4H-SiC. The traditional SiH4-C3H8-H2 systems require high growth temperatures to enable the desirable steplow growth for high-quality epilayers. A well known problem of the regular-temperature growth is the homogeneous gas-phase nucleation caused by SiH4 decomposition. However, the degree of Si cluster formation in the gas phase and its influence on our low-temperature epitaxial growth was unknown prior to this work. Growth at temperatures below 1400°C was demonstrated previously only for a limited range of substrate surface orientations and with poor quality. Mirror-like epilayer surface without foreign polytype inclusions and with rare surface defects was demonstrated at temperatures down to 1280-1300°C for our halo-carbon growth. Quantitatively different growth-rate dependences on the carbon-precursor flow rate suggested different precursor decomposition kinetics and different surface reactions in CH3Cl and C3H8 systems. Photoluminescence measurement indicated the high quality of the epilayers grown at 1300°C. A mirror-like surface morphology with rare surface defects was demonstrated for the growth on low off-axis substrates at 1380°C. The most critical growth-rate limiting mechanism during the low-temperature epitaxial growth is the formation of Si clusters, which depleted the Si supply to the growth surface, in the gas phase. Presence of chlorine in the CH3Cl precursor significantly reduces but does not completely eliminate this problem. The addition of HCl during growths improved the growth rate and surface morphology drastically but also brought up some complex results, suggesting more complex mechanisms of HCl interaction with the gas-phase clusters. These complicated results were explained partly by an additional mechanism of precursor depletion enhanced in presence of HCl. Complex changes in the effective silicon to carbon ratio in the growth zone indicated that the supply of carbon species may also be enhanced at least at low HCl flow rates. This fact allowed us to suggest that the gas-phase clusters may contain a significant amount of carbon. The new model assuming coexistence of the silicon and carbon in the gas-phase clusters enabled the explanation of the complex experimental trends reported in this work.

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