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17 results on '"Castro, R.H.R."'

6. Effects of concurrent grain boundary and surface segregation on the final stage of sintering: the case of Lanthanum doped yttria-stabilized zirconia

Author

M. M. Gong, Sanchita Dey, Castro R.H.R., Chi-Hsiu Chang, Hui Li, Feng Liu, and L J Wu

Subjects
Materials science, Polymers and Plastics, Dopant, Mechanical Engineering, Metals and Alloys, Sintering, Thermodynamics, 02 engineering and technology, 010402 general chemistry, 021001 nanoscience & nanotechnology, Microstructure, 01 natural sciences, Surface energy, 0104 chemical sciences, Gibbs free energy, symbols.namesake, Crystallography, Grain growth, Mechanics of Materials, Materials Chemistry, Ceramics and Composites, symbols, Grain boundary, 0210 nano-technology, Yttria-stabilized zirconia
Abstract

Dopants play a critical role in tailoring the microstructure during sintering of compacts. These dopants may form solid solution within the bulk, and/or segregate to the grain boundaries (GBs) and the solid-vapor interfaces (free surfaces), each causing a distinct energetic scenario governing mass transports during densification and grain growth. In this work, the forces controlling the dopant distribution, in particular the possibility of concurrent segregation at both surfaces and GBs, are discussed based on the respective enthalpy of segregation. An equation is derived based on the minimum Gibbs energy of the system to determine enthalpy of segregation from experimental interface energy data, and the results applied to depict the role of La as a dopant on the interface energetics of yttria stabilized zirconia during its final stage of sintering. It is shown that La substantially decreases both GB and surface energies (differently) as sintering progresses, dynamically affecting its driving forces, and consequent grain growth and densification in this stage.

Published
2017

17. Experimental study of the structural, microscopy and magnetic properties of Ni-doped SnO2 nanoparticles

Author

Aragón, F.H., Coaquira, J.A.H., Hidalgo, P., Brito, S.L.M., Gouvêa, D., and Castro, R.H.R.

Subjects
*SEMICONDUCTOR doping, *NANOPARTICLES, *NICKEL, *STANNIC oxide, *MAGNETIZATION, *X-ray diffraction, *SOLID solutions
Abstract

Abstract: A polymer precursor method has been used to synthesize Ni-doped SnO2 nanoparticles. X-ray diffraction (XRD) data analyses indicate the exclusive formation of nanosized particles with rutile-type phase (tetragonal SnO2) for Ni contents below 10mol%. In this concentration range, the particle sizes decrease with increasing Ni content and a bulk solid solution limit was determined at ~1mol%. Ni surface enrichment is present at concentrations higher than the solution limit. Only above 10mol% Ni, the formation of a second NiO-related phase has been determined. Magnetization measurements suggest the occurrence of ferromagnetism for samples in the solid solution regime (below ~1mol%). This ferromagnetism is associated with the exchange interaction between electron spins trapped on oxygen vacancies, and is enhanced as the amount of Ni2+ substituting at Sn4+ sites increases. Above the solid solution limit, ferromagnetism is destroyed by the Ni surface enrichment and the system behaves as a paramagnet. [ABSTRACT FROM AUTHOR]

Published
2010
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