Ionic and electronic transport in calcium-substituted LaAlO3 perovskites prepared via mechanochemical route

The present work explores mechanosynthesis of lanthanum aluminate-based perovskite ceramics and corresponding effects on ionic-electronic transport properties. La1-xCaxAlO3-δ (x = 0.05-0.20) nanopowders were prepared via one-step high-energy mechanochemical processing. Sintering at 1450°C yielded dense ceramics with submicron grains. As-prepared powders and sintered ceramics were characterized by XRPD, XPS and SEM. Electrochemical studies showed that partial oxygen-ionic conductivity in prepared La1-xCaxAlO3-δ increases with calcium content up to 10 at.% in the lanthanum sublattice and then levels off at ~6×10-3 S/cm at 900°C. La1-xCaxAlO3-δ ceramics are mixed conductors under oxidizing conditions and ionic conductors with negligible contribution of electronic transport in reducing atmospheres. Oxygen-ionic contribution to the total conductivity is 20-68% at 900°C in air and increases with Ca content, with temperature and with reducing p(O2). Impedance spectroscopy results showed however that electrical properties of mechanosynthesized La1-xCaxAlO3-δ ceramics below ~800°C are determined by prevailing grain boundary contribution to the total resistivity.This work was supported by the Slovak Research and Development Agency APVV (contracts SK-PT-18-0039 and 15-0438) and the Slovak Grand Agency (contract No. 2/0055/19). BIAS and AAY would like to acknowledge financial support by the FCT, Portugal (bilateral project Portugal-Slovakia 2019-2020, project CARBOSTEAM (POCI01-0145-FEDER-032295) and project CICECO-Aveiro Institute of Materials (FCT ref. UID/CTM/50011/2019), financed by national funds through the FCT/MCTES and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement). HK thanks to SAIA, n.o. for financial support within National Scholarship Programme of the Slovak republic (NSP).in publicatio

NdBaInO4 based triple (electronic, ionic and protonic) conductor for solid oxide fuel cell applications

Oxide-ion conducting materials have never failed to attract intensive attention due to their potential to be used for the applications of solid oxide fuel cell (SOFC) devices. With the aim of reducing the operating temperatures of SOFC devices to the intermediate high temperature range (500oC-800 oC), the design and synthesis of a new structure family to be used as the electrolyte material could be crucial. In this thesis, the potential of calcium-doped layered perovskite compounds, BaNd1-xCaxInO4-x/2 (where x is the Ca content), as protonic conductors was experimentally investigated. The single phase of monoclinic crystal structure with the P21/c symmetry was confirmed in the as-synthesized BaNd1-xCaxInO4-x/2 solid solutions by XRD characterisations. The acceptor-doped ceramics exhibited improved total conductivities that were 1-2 orders of magnitude higher than those of the parent material, BaNdInO4. The highest total conductivity of 2.6 x 10-3 Scm-1 was obtained for the BaNd0.8Ca0.2InO3.90 sample at a temperature of 750 oC in air. Electrochemical impedance spectroscopy measurements of the x = 0.1 and x = 0.2 substituted samples showed higher total conductivity under humid environments than those measured in a dry environment over a large temperature range (250 oC-750 oC). At 500 oC, the total conductivity of the 20% substituted sample in humid air (~3% H2O) was 1.3 x 10-4 Scm-1. The incorporation of water vapour decreased the activation energies of the bulk conductivity of the BaNd0.8Ca0.2InO3.90 sample from 0.755(2) eV to 0.678(2) eV in air. The saturated BaNd0.8Ca0.2InO3.90 sample contained 2.2 mol% protonic defects, which caused an expansion in the lattice according to the in-situ X-ray diffraction data. Combining studies of the impedance behaviour with 4-probe DC conductivity measurements obtained in humid air which showed a decrease in the resistance of the x=0.2 sample, it could be concluded that experimental evidence indicates that BaNd1-xCaxInO4-x/2 exhibits triple (oxygen-ion, proton and hole) conduction in wet atmospheres. The oxygen and deuterium isotope exchange depth profiling measurements on the BaNd0.8Ca0.2InO3.90 sample were successfully performed where valid diffusion kinetics for oxygen diffusion in this unique layered perovskite were firstly obtained. The obtained surface exchange coefficients for oxygen of the BaNd0.8Ca0.2InO3.90 sample measured under wet atmospheres were significantly higher than those measured in dry 18O2, while the diffusion coefficients in bulk material were decreased in wet atmosphere and a higher the activation energy for oxygen diffusion was achieved according to the Arrhenius plot. The activation energy for oxygen diffusion was increased from 1.08(8) eV to 1.86(11) eV after water incorporation, which implied a hindering effect of protonic defects on the oxygen diffusion process in the bulk material. Besides, fast grain boundary diffusion ‘tail’ was confirmed in the 500 ℃ and 550 ℃ (H218O+O2) wet exchange depth profiles and the grain boundary diffusion products, D_gb∙δ, of 6.64 x 10-14 and 5.31 x 10-13 cm3/s at 500 ℃ and 550 ℃ respectively were calculated. The chemical stability of the obtained phase in the BaNd0.8Ca0.2InO3.90 sample was examined after long duration wet annealing by XRD and STEM-EDS materials characterization methods, where A-site cation exsolution of calcium and barium to the sample surface was confirmed forming corresponding CaCO3 and BaCO3 secondary phases on the surface. In this work, processing high density Calcium substituted BaNdInO4 as an electrolyte was investigated. By means of electrochemical performance characterization methods together with the isotope exchange depth profile measurement, the contribution of different charge carriers in this layered-perovskite structural material was probed. It was shown that BaNd1-xCaxInO4-x/2 exhibits triple (oxygen-ion, proton and hole) conduction in wet atmospheres and the migration of protons are much faster than that of oxide-ions. All these results implied a promising potential of this ceramic, with a unique layered perovskite structure, to be used as the electrolyte material for the intermediate high temperature SOFC devices.Open Acces

Processing of mayenite electride and its composites in spark plasma sintering

2019 Summer.Includes bibliographical references.Mayenite electride, as the first inorganic room temperature stable electride, has attracted intensive research interests since the early 2000s due to its great potential in various applications such as catalysts, conductive oxides and thermionic emission materials. Mayenite electride is developed from mayenite, a stoichiometric compound of CaO and Al2O3 (12CaOˑ7Al2O3, referred to as C12A7 hereafter) that has a cubic unit cell with a positively charged lattice framework [Ca24Al28O64]4+ of twelve crystallographic subnano-cages per unit and O2- anions clathrated in the cages to maintain charge neutrality. When mayenite is heat treated in a reducing environment, electrons replace O2- ions clathrated in the cages. The electrons can migrate through the inter-cage framework, leading to the formation of electride (C12A7:e-), an electrically conductive form of C12A7. A variety of methods to make C12A7:e- powder and bulk materials have been investigated in the literature, all of which involve multiple steps and long-time (days to weeks) of heat treatment at high temperatures (>1100 ˚C). Although fundamental knowledge of the structure and functionality of C12A7:e- is advancing in the field, the formation of other calcium aluminate phases during the synthesis of mayenite or its electride has been overlooked. Most of the previous studies also lack detailed microstructure characterization. In addition, monolithic C12A7:e- does not provide continuous ohmic contact due to the destruction of the surface cages during processing, which limits its direct use in thermionic emission devices. To address the aforementioned practical issues and to fill in the fundamental knowledge gap, we investigated the effect of adding different reinforcing particles, including carbon black (CB), Ti, and TiB2, on the formation of C12A7:e- via spark plasma sintering (SPS), with attention particularly paid to address phase formation during the processing. Specifically, preformed C12A7 powder was synthesized via a solid-state reaction and used as the precursor base in SPS to study the effect of additives. In addition, a novel approach using in-situ reaction in SPS was proposed in the present work to significantly reduce the processing time. My research revealed that both Ti and TiB2 effectively reduced C12A7 to its electride phase, C12A7:e-. However, addition of Ti and TiB2 also led to partial decomposition of C12A7 into secondary calcium aluminate phases, primarily Al2O3-rich calcium monoaluminate (CA) and CaO-rich tricalcium aluminate (C3A). Although CB did not effectively reduce C12A7 to C12A7:e-. it did not result in the formation of any secondary calcium aluminate phases. Using Ti foils on the top and bottom of the preformed C12A7 powder in SPS created C12A7:e- with a near-theoretical maximum electron concentration ~ 10^21/cm^3. For the in-situ reaction approach, the chemical homogeneity and size distribution of precursor powders are critical to forming C12A7:e- in the typical processing time frame of SPS (5-15 minutes). The fast heating rate and C-rich environment in SPS increased the CaCO3 decomposition temperature to above 930°C, which is consequential to the calcium aluminate formation reaction. Adding Ti powder lowered the CaCO3 decomposition temperature in SPS and allowed for the formation of C12A7:e- via in-situ reaction sintering. The work function of a 50-50wt% C12A7:e- -Ti composite in this study is ~ 2.6 eV

Novel ceramic membranes for gas separation in enabling the clean energy delivery

Ceramic membranes have wide applications in clean energy delivery and greener chemical synthesis. However, the prerequisite to realize such potentials is the membranes to possess sufficiently high CO2-resistance. In this thesis, a series of novel oxygen selective ceramic ion-conducting membranes with high stability have been theoretically and experimentally investigated via the strategies including element doping, short circuit design and membrane surface decoration. The developed membranes display very high stability in CO2-containing atmosphere at high temperatures

Biomaterials

This book provides an overview of biomaterials science with a focus on health and medical applications that can be improved with new biomaterials with non-allergenic elements. These materials are designed to meet functional requirements and overcome the disadvantages of classical alloys used as biomaterials in human tissue. Over seven chapters, this volume explains the problems created by classical alloys and examines how the new generation of biomaterials helps both doctors and patients. It is designed for students, doctors, patients, and researchers worldwide

Impact of Alkaline Doping and Reducing Conditions on LaFeO\u3csub\u3e3\u3c/sub\u3e

Efficient and reliable materials for gas separation, syngas production, and hybrid nuclear power plants must be capable of reliably operating at a high-temperature range of 700-1000°C and under exposure to highly oxidizing and reducing conditions. Candidate materials for these applications include alkaline metal doped lanthanum ferrite. In the first study, the impact of A site substitution by different alkaline metals on lanthanum ferrite (LMF, M=Ca, Sr, and Ba) was investigated. The study focused on thermal expansion near the Néel transition temperature and a magneto-elastic contribution to thermal expansion was identified for each sample. Iron oxidation, Fe3+ to Fe4+, was identified as a preferred charge-compensation mechanism for Ca substitution while a mix of iron oxidation and oxygen-vacancy formation was identified for Sr and Ba substituted samples. The second study focused only on calcium substituted lanthanum ferrite but with a comparison between stoichiometric and sub-stoichiometric quantities of iron on the B site. The samples were heat treated in oxidizing (air), mildly reducing (Ar), and very reducing (5% H2-N2) atmospheres to compare the impact of iron sub-stoichiometry and PO2 on the Néel transition and orthorhombic-to-rhombohedral transition temperatures. Treatment in reducing conditions caused the Néel transition temperature to increase for all samples. The orthorhombic-to-rhombohedral transition temperature was determined to decrease for samples treated in Ar and to occur gradually over a broad temperature range when treated in 5% H2-N2. Iron deficiency during preparation was determined to cause a decrease in calcium actual content and a general increase in both phase transition temperatures in all samples. Iron vacancy formation was also determined to be unlikely due to the high energy of the defect and the samples compensated for iron sub-stoichiometry by rejecting calcium on the A site in favor of lanthanum

In Brain Multi-Photon Imaging of Vaterite Drug Delivery Cargoes loaded with Carbon Dots

Biocompatible fluorescent agents, such as phenylenediamine carbon dots (CDs), are key contributors to the theragnostic paradigm, enabling real-time in vivo imaging of drug delivery cargoes. This study explores the optical properties of these CDs, demonstrating their potential for two-photon fluorescence imaging in brain vessels. Using an open aperture z-scan technique, we measured the wavelength-dependent nonlinear absorption cross-section of the CDs, achieving a peak value near 50 GM. This suggests the potential use of phenylenediamine CDs for efficient multiphoton excitation in the 775 - 895 nm spectral range. Mesoporous vaterite nanoparticles were loaded with fluorescent CDs to examine the possibility of a simultaneous imaging and drug delivery platform. Efficient one and two-photon imaging of the CD-vaterite composites, uptaken by macrophage and genetically engineered C6-Glioma cells, was demonstrated. For an in vivo scenario, vaterite nanoparticles loaded with CDs were directly injected into the brain of a living mouse, and their flow was monitored in real-time within the blood vessels. The facile synthesis of phenylenediamine carbon dots, their significant nonlinear responses, and biological compatibility show a viable route for implementing drug tracking and sensing platforms in living systems

Mixed Ionic-Electronic Conducting Membranes (MIEC) for Their Application in Membrane Reactors: A Review

Mixed ionic-electronic conducting membranes have seen significant progress over the last 25 years as efficient ways to obtain oxygen separation from air and for their integration in chemical production systems where pure oxygen in small amounts is needed. Perovskite materials are the most employed materials for membrane preparation. However, they have poor phase stability and are prone to poisoning when subjected to CO2 and SO2, which limits their industrial application. To solve this, the so-called dual-phase membranes are attracting greater attention. In this review, recent advances on self-supported and supported oxygen membranes and factors that affect the oxygen permeation and membrane stability are presented. Possible ways for further improvements that can be pursued to increase the oxygen permeation rate are also indicated. Lastly, an overview of the most relevant examples of membrane reactors in which oxygen membranes have been integrated are provided.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 679933. The present publication reflects only the author’s views and the European Union is not liable for any use that may be made of the information contained therein

Ca L2,3 edge XANES and Sr K edge EXAFS study of hydroxyapatite and fossil bone apatite

Upon burial, the organic and inorganic components of hard tissues such as bone, teeth, and tusks are subjected to various alterations as a result of interactions with the chemical milieu of soil, groundwater, and presence of microorganisms. In this study, simulation of the Ca L$_{2,3}$-edge X-ray absorption near edge structure (XANES) spectrum of hydroxyapatite, using the CTM4XAS code, reveals that the different symmetry of the two nonequivalent Ca$_{(1)}$ and Ca${(2)}$ sites in the unit cell gives rise to specific spectral features. Moreover, Ca L$_{2,3}$-edge XANES spectroscopy is applied in order to assess variations in fossil bone apatite crystallinity due to heavy bacterial alteration and catastrophic mineral dissolution, compared to well-preserved fossil apatite, fresh bone, and geologic apatite reference samples. Fossilization-induced chemical alterations are investigated by means of Ca L$_{2,3}$-edge XANES and scanning electron microscopy (SEM) and are related to histological evaluation using optical microscopy images. Finally, the variations in the bonding environment of Sr and its preference for substitution in the Ca$_{(1)}$ or Ca$_{(2)}$ sites upon increasing the Sr/Ca ratio is assessed by Sr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy