Electrolytes for ceramic oxide fuel cells

The main objective of this dissertation is the development and processing of novel ionic conducting ceramic materials for use as electrolytes in proton or oxide-ion conducting solid oxide fuel cells. The research aims to develop new processing routes and/or materials offering superior electrochemical behavior, based on nanometric ceramic oxide powders prepared by mechanochemical processes. Protonic ceramic fuel cells (PCFCs) require electrolyte materials with high proton conductivity at intermediate temperatures, 500-700ºC, such as reported for perovskite zirconate oxides containing alkaline earth metal cations. In the current work, BaZrO3 containing 15 mol% of Y (BZY) was chosen as the base material for further study. Despite offering high bulk proton conductivity the widespread application of this material is limited by its poor sinterability and grain growth. Thus, minor additions of oxides of zinc, phosphorous and boron were studied as possible sintering additives. The introduction of ZnO can produce substantially enhanced densification, compared to the un-doped material, lowering the sintering temperature from 1600ºC to 1300ºC. Thus, the current work discusses the best solid solution mechanism to accommodate this sintering additive. Maximum proton conductivity was shown to be obtained in materials where the Zn additive is intentionally adopted into the base perovskite composition. P2O5 additions were shown to be less effective as a sintering additive. The presence of P2O5 was shown to impair grain growth, despite improving densification of BZY for intermediate concentrations in the range 4 – 8 mol%. Interreaction of BZY with P was also shown to have a highly detrimental effect on its electrical transport properties, decreasing both bulk and grain boundary conductivities. The densification behavior of H3BO3 added BaZrO3 (BZO) shows boron to be a very effective sintering aid. Nonetheless, in the yttrium containing analogue, BaZr0.85Y0.15O3- (BZY) the densification behavior with boron additives was shown to be less successful, yielding impaired levels of densification compared to the plain BZY. This phenomenon was shown to be related to the undesirable formation of barium borate compositions of high melting temperatures. In the last section of the work, the emerging oxide-ion conducting materials, (Ba,Sr)GeO3 doped with K, were studied. Work assessed if these materials could be formed by mechanochemical process and the role of the ionic radius of the alkaline earth metal cation on the crystallographic structure, compositional homogeneity and ionic transport. An abrupt jump in oxide-ion conductivity was shown on increasing operation temperature in both the Sr and Ba analogues.

Development and characterization of Poly(L-lactic acid) (PLLA) platforms for bone tissue engineering

The development of scaffolds based on biomaterials is a promising strategy for Tissue Engineering and cellular regeneration. This work focuses on Bone Tissue Engineering, the aim is to develop electrically tailored biomaterials with different crystalline and electric features, and study their impacts onto cell biological behavior, so as to predict the materials output in the enhancement of bone tissue regeneration. It is accepted that bone exhibits piezoelectricity, a property that has been proved to be involved in bone growth/repair mechanism regulation. In addition electrical stimulations have been proved to influence bone growth and repair. Piezoelectric materials are therefore widely investigated for a potential use in bone tissue engineering. The main goal is the development of novel strategies to produce and employ piezoelectric biomaterials, with detailed knowledge of mechanisms involved in cell-material interaction. In the current work, poly (L-lactic) acid (PLLA), a synthetic semi-crystalline polymer, exhibiting biodegradibility, biocompatibility and piezoelectricity is studied and proposed as a promoter of enhanced tissue regeneration. PLLA has already been approved for implantation in human body by the Food and Drug Administration (FDA), and at the moment it is being used in several clinical strategies. The present study consists of first preparing films with different degrees of crystallinity and characterizing these PLLA films, in terms of surface and structural properties, and subsequently assessing the behavior of cells in terms of viability, proliferation, morphology and mineralization for each PLLA configuration. PLLA films were prepared using the solvent cast technique and submitted to different thermal treatments in order to obtain different degrees of crystallinity. Those platforms were then electrically poled, positively and negatively, by corona discharge in order to tailor their electrical properties. The cellular assays were conducted by using two different osteoblast cell lines grown directly onto the PLLA films:Human osteoblast Hob, a primary cell culture and Human osteosarcoma MG-63 cell line. This thesis gives also a comprehensive introduction to the area of Bone Tissue Engineering and provides a review of the work done in this field in the past until today, in that same field, including the one related with bone’s piezoelectricity. Then the experimental part deals with the effects of the crystallinity degrees and of the polarization in terms of surface properties and cellular bio assays. Three different degrees of crystallinity, and three different polarization conditions were prepared; which results in 9 different configurations under investigation.