This dissertation describes the fabrication and operation of porous silicon gas sensors. The first chapter describes the motivation behind gas sensor research and provides the reader with background knowledge of gas sensors including the terminology and a review of various gas sensors. The following two chapters describe both how the porous silicon gas sensors are created and how they have been tested in the laboratory. Chapter 4 describes the steps required to create arrays of gas sensors to provide for a selective device through the application of patented selective coatings. Chapter 5 proposes a physical model that leads to a numerical solution for predicting the operation of the gas sensor. The next chapter builds from this model to analyze and optimize the experimental methods that are used to test both this and other gas sensors. The final chapter of this dissertation describes the prototype gas sensor system that has most recently been created, the company that was formed to further the development of that system, and the future applications of the porous silicon gas sensor.

This dissertation work focuses on the processability improvement of B4C, especially the compaction and sintering improvement of B4C by applying a Ni-B nanolayer coating on individual B4C particles. A modified electroless coating procedure was proposed and employed to coat nanometer Ni-B layer onto micron-sized B4C particles. The thickness was able to be tuned and controlled below 100 nm. Key parameters, including the amount of nickel source, the amount of the surface activation agent (PdCl2), the amount of the complexing agent (C2H8N2), and the addition rate of the reducing agent (NaBH4) were studied. When the targeted thickness was 5 nm, a continuous and uniform nanolayer coating was obtained with the optimal condition of individual parameter combined. Reduction of the as-coated B4C powder in a H2-Ar atmosphere was studied between 400-900C to reduce the surface oxides' Ni2O3 and B2O3. Reduction at 800C in hydrogen atmosphere was found to be the most effective condition to remove oxygen in the coating layer, with Ni2B as the reduction product. Compaction of the as-received, separated and uncoated, and separated with Ni-B coating B4C powders using uniaxial die compaction and combustion driven compaction (CDC) techniques was studied. CDC technique showed the advantage over the traditional uniaxial die compaction by yielding much higher green density and green strength (73% vs. 53.8% green density for the Ni-B coated B4C). Among compacts obtained from the same technique, Ni-B coated B4C compact yielded the densest packing with crack-free compact surface and the highest strength, demonstrating more bonding between B4C particles provided by Ni-B surface coating. Sintering of the Ni-B coated B4C in an Ar atmosphere between 1150 - 1600C with soaking time of 2 hrs and 10 hrs was studied. Liquid phase was found to form during the sintering process. Density measurement showed that the liquid phase Ni-B formed greatly facilitated B4C densification. Considerable density increase and inter-granular connection was achieved when sintered at 1600C for 10 hrs. The density enhancement by Ni-B coating was supported by transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) examination which showed that there was B4C species diffusion into liquid Ni-B phase. This liquid phase enhanced the diffusion of B4C species and formed strong bonding between B4C grains by dissolving small B4C particles and sharp edge and corners of B4C particles. Strength test demonstrated that the Ni-B coating dramatically improved the strength of B4C compacts by yielding a much higher strength of the Ni-B coated samples than the uncoated samples (13.97 vs. 1.79 MPa for the uinaxial die compacted samples, 27.03 vs. 2.21 MPa for the CDC samples). Electrical conductivity Ni-B coated B4C samples was also shown to be improved with the electrical resistivity being reduced from infinite for pure B4C samples to 1.8Ã 10-3 Ω·m for the Ni-B coated samples. This research work has shown that with the Ni-B coating, B4C densification can start at a temperature as low as 1600C via a liquid phase sintering process. / Ph. D.

Phase transformation behavior, micro structural development, mechanical and tribological properties of electroless Ni-B coating was characterized using different characterization techniques. As deposited electroless Ni-B coating containing 94 wt. % of NI and 6 wt. % of B is amorphous. It crystallizes via two exothermic reactions one at 3000C and another at 430˚C. It has been observed that there is also slow evolution of the heat in between this two exothermic reactions. XRD studies display that as deposited coating undergoes multi-stage crystallization events. At the first exothermic peak NI3B phases crystallizes, in between two a phase mixture of Ni and Ni3B and at the second exothermic peak NI2B + Ni3B crystallizes. Evolution of the free Ni in the complete crystalline coating is not predicted by the equilibrium phase diagram of the Ni-B system. Microscopic observation of the as deposited coating displays a novel compositionally modulated microstructure comprises of different length scales ranging from micrometer to nanometer level. In situ TEM study along with composition analysis were carried out in order to track the crystallization pathway and microstructural development. This kind of composition fluctuation of the coating is intrinsic to the deposition process. In best of our knowledge this kind of microstructure is the first time reported example of phase separation in a binary metal-metalloid system without spinoidal decomposition. Effect of this kind of microstructure and phase evolution on the mechanical and tribological properties of the coating is very profound. Increase in the nanocrystalline borides content of the coating increases the hardness value of the coating as well as improved tribological properties of the coating. In the low load regime (5 N and less) wear resistance of the coating is provided by the oxide layer formed on the wear track by preventing the direct contact between the coating and counterface. Local temperature rise due to friction and nancrystalline nature of the coating enhances the tendency of oxide layer formation. Characterization of the oxide layer was carried out using SEM, EPMA, Nanoindenation and Raman Spectroscopy. Whereas in case high load regime (above 5 N) this oxide layer breaks off and direct contact between the coating and counterface is established. This increases the wear rate of the coating. Material is removed from the coating through subsurface crack formation and propagation by low cycle fatigue mechanism. Effect of amorphous phase and free Ni on the tribological properties of the coating is detrimental by promoting a strong adhesion between the coating and steel counter face, whereas nanocrystalline borides shows opposite effect. A nano tribological studies using lateral force microscopy shows that nanocrystalline borides decreases the coefficient of friction of the coating. Phase evolution and microstructural characterization also shows that above 450˚C there is a significant diffusion of the boron from the coating to the steel substrate. This restrict the high temperature tribological studies of the coating up to a temperature range of 450˚C. Wear data along with worn track characterization demonstrate the fact that above 100˚C even in low load regime wear rate is very high. Wear of the coating is mainly governed by the plastic deformation of the coating and breakage of the protective oxide layer. Analytical calculation as well experimental observation shows that during the time of wear the temperature at the local contact region reaches a very high value even up to 1100˚C. This may soften the coating and causes the wear though plastic deformation of the coating.