Summary Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to measure the concentrations of cobalt, tantalum, titanium, vanadium and chromium in solutions of tungsten carbide. The main advantage of the method described here lies in the speed, convenience and effectiveness of the dissolution procedure. Aliquots of powdered tungsten carbide were dissolved in a solution of 5% aqua regia in 30% hydrogen peroxide. Complete dissolution was usually achieved within 10 min. The accuracy of the method was assessed by the analysis of certified reference materials, secondary reference materials and matrix spiking. The method was successfully applied to commercial type samples with differing compositions. Slightly more emphasis was placed on the measurement of vanadium, since no information on the measurement of this element in solutions of tungsten carbide, by ICP-OES, has been published. Investigation of the interference effects of the elements in the sample matrix on each other was essential for accurate results comparable to other published analytical methods.

Aluminium composites reinforced by particles have received considerable attention because of their superior mechanical properties over monolithic aluminum matrix. Over the last ten years, nanocomposites with nano-sized reinforcements have become a revolutionary progress for composites because they have different strengthening mechanisms as compared to that in composites with micro-sized reinforcements. Consequently novel properties can be expected from the nanometric particulate reinforced composites. The aim of this project was to fabricate SiC (50nm)/7075 aluminium composites via a modified powder metallurgy and extrusion route. Ageing treatment was used to increase the strength of the composites and mechanical tests, including tensile test and abrasive wear test, were performed. The effects of nanometric silicon carbide particulates to the ageing behaviours and mechanical properties of the composites have been studied by optical metallography, scanning electron microscopy and transmission electron microscopy. It was found that the dispersion of nanometric silicon carbide was not homogeneous, but tended to disperse along grain boundaries. Clustering of these nano-reinforcements was also found within the grains. This was particular true when the amount of nano-reinforcement increased to 5%. Compared with the monolithic 7075 alloy, the 1 vol.% SiC (50nm)/7075 aluminium had a higher strength because of effective dislocation pinnings by the reinforcements, while 5% SiC (50nm)/7075 had a much lower strength and ductility because of severe aggregation of nanometric particulates. Nanometric silicon carbide was not as effective as the micro ones in improving abrasive wear resistance of aluminium, this was because of micro-cracking in the aggregation and relatively large abrasive grit. In summary, the addition of a small amount of SiC nanoreinforcements has a high potential to further strengthen 7xxx aluminium alloy. However, the clustering of reinforcements in the matrix will detrimentally affect the strength and ductility of the alloy. The wear resistance of nanometric particulate reinforced composites was inferior to those with micrometric reinforcements. It is suggested that by improving the dispersion of nanometric reinforcements, as well as putting in reinforcememts with different sizes, the mechanical properties and wear resistance can both be increased.

��-SiC is a semiconductor for high temperature devices, which exhibits several outstanding properties such as high thermal stability, good chemical stability and wide band gap. There is a possibility of fabricating a crack-free ultrathin SiC film on silicon wafers by pyrolysis of polymethylsilane (PMS) film. This study looks into the possibility, as the first phase, to modify the surface of silicon and graft PMS onto the surface. A new technique reported in this thesis consists of a surface modification with trimethoxysilylpropene (TSP) followed by the surface attachment of dichloromethylsilane (DMS) in the presence of a platinum catalyst, which acts as the first unit for grafting PMS molecules by the sodium polycondensation of additional DMS monomers. The grafted PMS polymers would serve as the pyrolytic precursor to be converted into thin layers of SiC. Surface analysis of these films on silicon wafers by X-ray photoelectron spectroscopy (XPS) indicated that the silicon surface was successfully modified with TSP, attached with DMS, and finally grafted with PMS. It was also confirmed by powder X-ray diffraction (XRD) that PMS formed simultaneously in the bulk solution was converted into SiC by pyrolysis at temperatures above 1100��C under Ar atmosphere. Extended studies showed that the PMS-derived coatings, formed in an Ar stream containing 1% H��� at 400��C, were significantly oxidized, and further heating to 700��C yielded a Si0��� layer with graphitic carbon. The intensity of the graphite peak decreased with an increase in the pyrolysis temperature. Based on these preliminary studies towards the second phase, i.e. the pyrolysis of PMS to SiC, the need for further research to eliminate the oxidation source(s) is strongly suggested. / Graduation date: 2001

No description available.

The processing of boron carbide powder including sintering optimization, green body optimization and sintering behavior of nano-sized boron carbide was investigated for the development of complex shaped body armor. Pressureless sintered B4C relative densities as high as 96.7% were obtained by optimizing the soak temperature, and holding at that temperature for the minimum time required to reach terminal density. Although the relative densities of pressureless sintered specimens were lower than that of commercially produced hot-pressed B4C, their (Vickers) hardness values were comparable. For 4.45cm diameter and 1.35cm height disk shaped specimens, pressureless sintered to at least 93.0% relative density, post-hot isostatic pressing resulted in vast increases in relative densities (e.g. 100.0%) and hardness values significantly greater than that of commercially produced hot-pressed B4C. The densification behavior of 20-40nm graphite-coated B4C nano-particles was studied using dilatometry, x-ray diffraction and electron microscopy. The higher than expected sintering onset from a nano-scale powder (15008C) was caused by remnant B2O3 not removed by methanol washing, keeping particles separated until volatilization and the carbon coatings, which imposed particle to particle contact of a substance more refractory than B4C. Solid state sintering (1500-18508C) was followed by an arrest in contraction attributed to formation of eutectic liquid droplets of size more than 10X the original nano-particles. These droplets, induced to form well below known B4C-graphite eutectic temperatures by the high surface energy of nano-particles, are interpreted to have quickly solidified to form a vast number of voids in particle packing, which in turn, impeded continued solid state sintering. Starting at 22008C, a permanent liquid phase formed which facilitated a rapid measured contraction by liquid phase sintering and/or compact slumping.

Thesis (M.S.) -- University of Texas at Arlington, 2008.

Thesis (M.S. in M.E.)--School of Mechanical Engineering, Georgia Institute of Technology, 2004. Directed by Peter J. Hesketh. / Includes bibliographical references (leaves 152-160).

Thesis (M.S. Ch. E.)--University of South Florida, 2003. / Title from PDF of title page. Document formatted into pages; contains 70 pages. Includes bibliographical references.

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy