Computational Study of Surface Deposition and Gas Phase Powder Formation during Spinel Chemical Vapor Deposition Processes

Chemical vapor deposition (CVD) processes are often employed to produce high quality materials. In some applications, a relatively fast deposition rate is required to produce thick pieces of material, e.g., on the order of centimeter, in an economical manner. However, in some processes the formation of solid nuclei or powder in the gas phase can be a major obstacle to increasing the deposition rate. The homogeneous powder formation reaction can compete with the surface deposition reaction and consume the gas phase reactants, thus reducing the available material to grow on the substrate surface. Understanding the thermodynamics as well as kinetics of these two competing reactions helps elucidate the reaction conditions that best reduce powder formation and increase deposition rate. As an example where gas phase powder formation can significantly limit the surface deposition rate CVD growth of spinel (MgAl2O4) is investigated. In this CVD process, reverse water gas shift (rWGS) is employed to generate water, which reacts with either chlorides or fluorides of magnesium and aluminum to produce spinel. Density functional theory (DFT) was used to study the thermodynamics of surface reactions of spinel deposition. This study shows that the surface reaction to form spinel is thermodynamically favorable. However, the spinel deposition could be limited by its surface kinetics and/or competition for reactants with gas phase powder formation. We have employed molecular dynamics (MD) with a reactive force field (ReaxFF) to simulate gas phase rWGS and powder formation. These simulations show that the reactants’ residence time should be minimized to reduce powder formation. A comparison of magnesium and aluminum chlorides with their corresponding fluorides indicates that fluorides are better in reducing powder formation

OCPortugal consortium: activities and challenges

The newly-constituted Ocean Colour Portugal consortium (OCPortugal) aims at promoting the use of remote sensing (RS) products in the Atlantic off Portugal. In close collaboration with the European Space Agency (ESA), OCPortugal currently links the following institutions: Centre of Oceanography (IO), Faculty of Sciences of the University of Lisbon; CENTRIA and CITI from Faculty of Science and Technology , New University of Lisbon (FCTUNL); and Sagremarisco-Viveiros de Marisco Lda. Through a coordinated set of actions undertaken by these research groups, OCPortugal brings together expertise in different research areas such as marine biology, applied optics, satellite imagery, and parallel computing