Banca de QUALIFICAÇÃO: RAPHAEL ANACLETO MARTINS PIRES DE OLIVEIRA

Uma banca de QUALIFICAÇÃO de DOUTORADO foi cadastrada pelo programa.
STUDENT : RAPHAEL ANACLETO MARTINS PIRES DE OLIVEIRA
DATE: 13/12/2022
TIME: 14:00
LOCAL: Remoto
TITLE:

Effects of La Content in Ceria-Lanthana Thin Films prepared by Pulsed Laser Deposition

PAGES: 39
BIG AREA: Engenharias
AREA: Engenharia de Materiais e Metalúrgica
SUBÁREA: Materiais Não-Metálicos
SPECIALTY: Cerâmicos
SUMMARY:

Solid oxide electrochemical reactors are a promising alternative for hard to tackle chemical reactions such as oxidative coupling of methane (OCM), in which methane is converted to C₂₊ products. They contain oxygen-conducting membranes that deliver oxygen species in a highly controllable way, avoiding undesired complete oxidation, thus enhancing selectivity. Cerium oxide is one the main studied oxide membrane material because, when doped 3+ rare earth cations such as Gd, Sm or La, it gains high ionic conductivity in the ideal temperature range for conversion reactions (600-900°C). Interestingly, a recent work has shown that La₂Ce₂­O₇ is a promising catalyst for OCM.  The presented work investigates the structural properties La_xCe_(1-x)O_2(1-x/2) thin films covering a wide La content range from x = 0 to 0.7. The films were prepared by pulsed laser deposition on Si, quartz and glass substrates, using La_xCe_(1-x)O_2(1-x/2) targets with different stoichiometries and oxygen partial pressure of a range of 1x10^-1_, 2x10^-2 and 5x10^-3 mbar. The techniques used was x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) to study the effect of doping on the structural properties of the films. XRD results indicate that the film growth is textured. For low La content (x≤0.5), the films growth preferably in the [100] direction of the cubic crystalline fluorite, whereas for x>0.5 film crystalline orientation is random. La doping induces an expansion of the fluorite structure reflecting an increase in lattice parameter (from 5.42 Å, for x=0, to 5.69 Å, for x=0.7) that follows the same trend that is observed for the powders from which the PLD targets were made. The MEV images also show that the films have a different texture, having a more tubular form with a increase in La content, these images was corroborated by AFM images. The doping effects in Ce-La thin films were studied using Raman spectroscopy by following the changes in the main fluorite F_2g band at ~464 cm⁻¹. La doping induces a significant broadening of this peak. Such effect is usually attributed to reduced size in pure ceria nanoparticles, but in this case, it is probably associated to cationic (or anionic) disorder that exists in the so- disordered fluorite structure that is characteristic of ceria-lanthana solid solutions. The disorder results in a reduced phonon correlation length that induces the broadening. For SE results the films prepared in 1x10^-1mbar presented a range of thickness of 475-625 nm, and roughness range of 26-60 nm. These results will pave the way for the application of Ce-La oxides thin films as catalysts for OCM in electrochemical reactors based on ceria.

BANKING MEMBERS:
Presidente - Interno ao Programa - 2604128 - ALEXANDRE JOSE DE CASTRO LANFREDI
Membro Titular - Examinador(a) Externo à Instituição - ANDRÉ LUIZ DA SILVA - USP
Membro Titular - Examinador(a) Externo à Instituição - EVERTON BONTURIM - UPM
Membro Titular - Examinador(a) Externo à Instituição - JOSE HUMBERTO DIAS DA SILVA - UNESP
Membro Suplente - Examinador(a) Interno ao Programa - 1734912 - ROOSEVELT DROPPA JUNIOR
Membro Suplente - Examinador(a) Interno ao Programa - 1282172 - ROBERTO GOMES DE AGUIAR VEIGA
Membro Suplente - Examinador(a) Interno ao Programa - 1545089 - DANIEL ZANETTI DE FLORIO
Membro Suplente - Examinador(a) Externo à Instituição - VICTOR BURATTO TINTI - DTU