A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Dynamic Modeling for the Design and Cyclic Operation of an Atomic Layer Deposition (ALD) Reactor(MDPI, 2013-08-19) Travis, Curtisha D.; Adomaitis, Raymond A.A laboratory-scale atomic layer deposition (ALD) reactor system model is derived for alumina deposition using trimethylaluminum and water as precursors. Model components describing the precursor thermophysical properties, reactor-scale gas-phase dynamics and surface reaction kinetics derived from absolute reaction rate theory are integrated to simulate the complete reactor system. Limit-cycle solutions defining continuous cyclic ALD reactor operation are computed with a fixed point algorithm based on collocation discretization in time, resulting in an unambiguous definition of film growth-per-cycle (𝑔𝑝𝑐). A key finding of this study is that unintended chemical vapor deposition conditions can mask regions of operation that would otherwise correspond to ideal saturating ALD operation. The use of the simulator for assisting in process design decisions is presented.Item Model-based Analysis of Atomic Layer Deposition Growth Kinetics and Multiscale Process Dynamics(2014) Travis, Curtisha Denise; Adomaitis, Raymond A.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A first principles model describing the reaction kinetics and surface species dynamics for the trimethylaluminum (TMA) and water half-reactions of alumina atomic layer deposition (ALD) is coupled with a dynamic film growth model and reactor-scale species transport model. The reaction kinetics model is based on reported enthalpies and transition state structures from published quantum-chemical computational studies; these data are used to determine kinetic parameters using statistical thermodynamics and absolute reaction rate theory. Several TMA half-reactions were modeled to account for TMA adsorption and subsequent reaction on a range of growth surfaces spanning bare to fully hydroxylated states. Several water reactions were also considered. By coupling the reaction rate models with surface species conservation equations, a dynamic model is created which is useful for examining the relative rates of competing surface reactions. To describe the continuous cyclic operation of the deposition reaction system, a numerical procedure to discretize limit-cycle solutions is developed and used to distinguish saturating growth per cycle from non-saturating conditions. The transition between the two regimes is studied as a function of precursor partial pressure, exposure times, and temperature. Finally, a cross-flow tubular ALD reactor system model is derived with components describing the precursor thermophysical properties, precursor delivery system, reactor-scale gas-phase dynamics, and surface reaction kinetics derived from absolute reaction rate theory. These model components are integrated to simulate the complete multiscale ALD process. Limit-cycle solutions defining continuous cyclic ALD reactor operation are computed with a fixed point algorithm based on temporal and spatial discretization within the reactor, resulting in an unambiguous definition of film growth per cycle. The use of the simulator for assisting in process design decisions and optimization frameworks is presented.Item High elastic modulus nanopowder reinforced resin composites for dental applications(2007-08-27) Wang, Yijun; Lloyd, Isabel K.; Greer, Sandra C.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dental restorations account for more than $3 billion dollars a year on the market. Among them, all-ceramic dental crowns draw more and more attention and their popularity has risen because of their superior aesthetics and biocompatibility. However, their relatively high failure rate and labor-intensive fabrication procedure still limit their application. In this thesis, a new family of high elastic modulus nanopowder reinforced resin composites and their mechanical properties are studied. Materials with higher elastic modulus, such as alumina and diamond, are used to replace the routine filler material, silica, in dental resin composites to achieve the desired properties. This class of composites is developed to serve (1) as a high stiffness support to all-ceramic crowns and (2) as a means of joining independently fabricated crown core and veneer layers. Most of the work focuses on nano-sized Al2O3 (average particle size 47 nm) reinforcement in a polymeric matrix with 50:50 Bisphenol A glycidyl methacrylate (Bis-GMA): triethylene glycol dimethacrylate (TEGDMA) monomers. Surfactants, silanizing agents and primers are examined to obtain higher filler levels and enhance the bonding between filler and matrix. Silane agents work best. The elastic modulus of a 57.5 vol% alumina/resin composite is 31.5 GPa compared to current commercial resin composites with elastic modulus <15 GPa. Chemical additives can also effectively raise the hardness to as much as 1.34 GPa. Besides>alumina, diamond/resin composites are studied. An elastic modulus of about 45 GPa is obtained for a 57 vol% diamond/resin composite. Our results indicate that with a generally monodispersed nano-sized high modulus filler, relatively high elastic modulus resin-based composite cements are possible. Time-dependent behavior of our resin composites is also investigated. This is valuable for understanding the behavior of our material and possible fatigue testing in the future. Our results indicate that with effective coupling agents and higher filler loading, viscous flow can be greatly decreased due to the attenuation of mobility of polymer chains. Complementary studies indicate that our resin composites are promising for the proposed applications as a stiff support to all-ceramic crowns.