Extensive attention has been paid to consolidate nanoparticles into nanocrystalline components that possess better properties than their coarse-grained counterparts. Nanocrystalline monolithic tungsten (W) has been envisaged to possess better properties than coarse-grained tungsten and to improve the performance of many military components. Commercially available nano-W powders were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and Brunauer, Emmett, and Teller (BET) measurement. While the bulk of nano-W powders consisted of bcc-W as confirmed by XRD and TEM, much of their surface consisted of WO3 with traces of WO2 and WC. Despite the irregular morphology and agglomerates greater than 1 ïm in size, the diameter of individual nano-W powders ranged from 30 to 100 nm with a surface area of 10.4 m2/g. To obtain green bodies of higher densities and more homogeneous microstructures after consolidation, W nanopowders were de-agglomerated in water and slip cast in plaster molds. De-agglomeration in water was conducted by repeated ultrasonication, washing, centrifuge and pH adjustment. The change in particle size and morphology was examined via SEM. After the initial surface oxide was removed by repeated washing, the reactivity of W nanoparticles to water was somewhat inhibited. Increasing the number of cycles for ultrasonication and washing increased the pH, the degree of de-agglomeration and the stability of W suspension. The zeta potential was more negative with increasing pH and most negative at pH values close to 5. Viscosity also decreased with increasing pH and reached a minimum at a pH 5. To obtain the highest solid loading with the lowest viscosity, the pH value of W suspension was adjusted to 5 using aqueous tetramethylammonium hydroxide solutions. The relative density of the slip cast increased with longer ultrasonic time, increasing slurry pH up to 5, and consequent increase in solids loading. Smaller particles were separated from larger ones by ultrasonication, washing with water and centrifugation. At a 27.8 vol.% solids loading, the size-separated fine W slurry was slip cast into pellets with relative green densities up to 41.3 % and approximate particle sizes of 100 nm. W powders were also ultrasonicated in aqueous poly (ethyleneimine) (PEI) solutions with various concentrations. SEM examinations of particle sizes showed that 1 wt.% PEI led to the optimum dispersion and ultrasonication for longer time with a low power resulted in better dispersion. 0.5 g of W powders were ultrasonicated in 10 ml aqueous poly (allylamine hydrochloride) (PAH) solutions with molar concentrations ranging from 0.01 to 0.05 M. W suspensions with 0.03 M and 0.04 M PAH after two washing cycles showed improved dispersion. Cold isostatic pressing can further increase the green density following slip casting. Sintered slip casts made from de-agglomerated nanoparticle W showed a lower density, more uniform microstructure, smaller grains and smaller pores than the sintered dry pressed pellets.
| Identifer | oai:union.ndltd.org:ucf.edu/oai:stars.library.ucf.edu:etd-5094 |
| Date | 01 January 2009 |
| Creators | Yang, Zhengtao |
| Publisher | STARS |
| Source Sets | University of Central Florida |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Electronic Theses and Dissertations |
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