Interface Engineering in Mixed Ionic Electronic Conductor Thin Films for Solid State Devices

Author

Chiabrera, Francesco Maria

Director

Tarancón Rubio, Albert

Garbayo Senosiain, Iñigo

Tutor

Morante i Lleonart, Joan Ramon

Date of defense

2019-06-28

Pages

283 p.



Department/Institute

Universitat de Barcelona. Facultat de Física

Abstract

Interface-dominated materials such as nanocrystalline thin films have emerged as an enthralling class of materials able to engineer functional properties of transition metal oxides widely used in energy and information technologies. In this direction, it has been recently proved that grain boundaries (GBs) in the perovskite La1-xSrxMnO3±δ (manganite) deeply impact its functional properties, boosting the oxygen mass transport while abating the electronic and magnetic order. The impact of grain boundary in nanocrystalline thin films is so relevant to radically change the behaviour of the material, transforming an electronic conductor into a mixed ionic-electronic conductor functional for redox-based solid state devices. Based on these preliminary studies, it became crucial to understand the origin of this enhancement, in order to gain engineering capabilities and potentially extend it to other functional perovskite materials. Following this approach, this thesis focuses in analysing the remarkable properties of GBs in manganites and, ultimately, investigating the possibility of engineering these interfaces. First, the structural and chemical characterization of the LSM thin films deposited by pulsed laser deposition (PLD) is presented. The compositional analysis of the layers revealed a severe Mn deficiency, ascribed to the plasma-background interactions during the deposition. The analysis of the GBs of these Mn-deficient thin films revealed a remarkable local modification of ionic composition, consisting in a Mn and O depletion along with a La and Sr enrichment (viz. GBdef). Then, through a PLD combinatorial approach, Mn was progressively inserted in the perovskite structure, altering the overall cationic ratio of the thin films (Mn/(La+Sr)). The variation of cationic chemical potential of the thin films was observed to significantly affect the GB composition, which passed from Mn depletion (La-enrichment) to Mn enrichment (La-depletion), while maintaining an O deficiency character (viz. GBrich). This behaviour suggests that through the tuning of the overall cationic concentration in the thin films the GB composition can be altered, offering an innovative way for engineering chemical defects in strained interfaces. The effect of these different GBs on the electrical conductivity and the oxygen mass transport properties of LSM thin films with different Mn content was then measured. It was found that in the layers characterized by GBdef, the lack of Mn hinders the low temperature metal insulator transition and, in its place, a variable range hopping mechanism occurs, where electrons tunnels across the GBs for reaching distant Mn atoms. Moreover, a simultaneous decrease of activation energies of both GB oxygen diffusivity and GB oxygen surface exchange coefficient was observed further decreasing the Mn concentration in these thin films, indicating a strong interdependence between the two phenomena. The results suggest that the GB accumulation of oxygen vacancies is at the origin of the large improvement of both oxygen mass transport parameters observed in LSM polycrystalline thin films. In LSM thin films characterized by GBrich, the low temperature metallic behaviour is progressively restored and an increase of electronic conductivity is observed in the entire temperature range. Additionally, in these layers relative changes of Mn do not give rise to a variation of the oxygen diffusivity, meaning that the GBs oxygen vacancy concentration is not altered anymore. Overall, the results demonstrate the possibility of engineering the functional properties of LSM polycrystalline thin films by modifying the GB cationic composition. In the third part of the thesis, the effect of Co substitution on LSMC functional properties was investigated. The LSMC thin films were produced by combinatorial PLD, which allow a direct measure of real-continuous spread LSMC system. The oxygen mass transport properties of bulk and GB were evaluated by finite element model fitting of 18O exchange profiles. The results revealed that GBs enhance the transport properties of the whole material in the range of composition under study, although for high Co concentration the GB effect is concealed by the high bulk diffusion.

Keywords

Electroquímica; Electrochemistry; Química física; Physical and theoretical chemistry; Piles de combustible; Pilas de combustible; Fuel cells; Electròlits; Electrolitos; Electrolytes

Subjects

544 - Physical chemistry

Knowledge Area

Ciències Experimentals i Matemàtiques

Note

Programa de Doctorat en Nanociències / Tesi realitzada a l'Institut de Recerca en Energia de Catalunya (IREC)

Documents

CHIABRERA_PhD_THESIS.pdf

46.61Mb

 

Rights

ADVERTIMENT. L'accés als continguts d'aquesta tesi doctoral i la seva utilització ha de respectar els drets de la persona autora. Pot ser utilitzada per a consulta o estudi personal, així com en activitats o materials d'investigació i docència en els termes establerts a l'art. 32 del Text Refós de la Llei de Propietat Intel·lectual (RDL 1/1996). Per altres utilitzacions es requereix l'autorització prèvia i expressa de la persona autora. En qualsevol cas, en la utilització dels seus continguts caldrà indicar de forma clara el nom i cognoms de la persona autora i el títol de la tesi doctoral. No s'autoritza la seva reproducció o altres formes d'explotació efectuades amb finalitats de lucre ni la seva comunicació pública des d'un lloc aliè al servei TDX. Tampoc s'autoritza la presentació del seu contingut en una finestra o marc aliè a TDX (framing). Aquesta reserva de drets afecta tant als continguts de la tesi com als seus resums i índexs.

This item appears in the following Collection(s)