Print Email Facebook Twitter Modeling of Surface Segregation for Binary Alloys in Vacuum and Gas Environments: Applied to Pd-alloys Title Modeling of Surface Segregation for Binary Alloys in Vacuum and Gas Environments: Applied to Pd-alloys Author Enciso Castillo, D. Contributor Böttger, A.J. (mentor) Faculty Mechanical, Maritime and Materials Engineering Department Department of Materials Science and Engineering Programme Master Materials Science and Engineering Date 2013-11-27 Abstract Computational modeling offers the possibility to design alloys in a cheaper and faster way. A model for surface segregation for binary alloys in vacuum and by adsorption and absorption of gases was developed and applied. The proposed model was used in conjunction with the Miedema semi-empirical model for the calculation of enthalpies of solution and formation of binary alloys and with the cluster variation method (CVM) for the calculation of the ? and ? phase boundaries in the ternary Pd-M-H systems. The models for segregation were applied in a temperature range from 298K to 600K, considering the surface plane (111) of the FCC crystal structure. The model for segregation in vacuum was applied to Pd-Au, Pd-Ag, Pd-Cu, Pd-Ni and Pd-Pt. For this case Pd is segregated for the Pd-Pt alloys only. By increasing the temperature from 298K to 600K, the surface concentrations of Pd decreased for all of the alloys. By varying the plane from (111) to (100) few changes were observed. The calculated results compared to experiments were found to be in good agreement for Pd-Au and Pd-Ag. For the model of surface segregation by adsorption and chemisorption, H2, CO and O2 gases on Pd-Ag, Pd-Cu, Pd-Ni and Pd-Pt were considered. In the presence of adsorbed or chemisorbed hydrogen, segregation of Pd was found for Pd-Ag, Pd-Cu and Pd-Pt alloys. For adsorbed O2 and CO on Pd-Ag and Pd-Cu alloys gases, segregation of Pd was enhanced. The model for segregation by absorption of hydrogen was applied for Pd-Ag, Pd-Cu, Pd-Ni and Pd-Pt alloys, using a hydrogen (bulk) concentration of 0.50 H/M. No Pd segregation was found for Pd-Ag alloys. For Pd-Pt alloys, Pd segregation was observed. For Pd-Cu alloys with large concentrations of Pd, a surface enrichment of Pd was calculated, while for alloys with low concentration of Pd a slight Cu surface enrichment was observed. Enhanced segregation of Ni was found for Pd-Ni alloys. The H/M fractions at the boundaries of the ? and ? phase obtained with the CVM for the Pd85Ag15H system were used as input values to calculate the surface concentration of Ag. As the hydrogen concentration was increased, the surface segregation of Pd also increased. For the surface segregation by adsorption and absorption combined, segregation of Pd was observed for Pd-Ag, Pd-Cu and Pd-Pt alloys. Surface enrichment of Ni was found for Pd-Ni alloys. The proposed model enables to predict surface segregation under various conditions and the results show that it is affected by many parameters. Subject surfacesegregationpalladiumbinary Alloyspalladium membraneshydrogen adsorptionhydrogen absorptionMiedema ModelCluster Variation Method To reference this document use: http://resolver.tudelft.nl/uuid:f936b2de-4107-4e14-991e-cd7a2dace9c6 Part of collection Student theses Document type master thesis Rights (c) 2013 Enciso Castillo, D. Files PDF Master_of_Science_Thesis_ ... ibrary.pdf 23.88 MB Close viewer /islandora/object/uuid:f936b2de-4107-4e14-991e-cd7a2dace9c6/datastream/OBJ/view