Porous LSCF/Dense 3YSZ Interface Fracture Toughness Measured by Single Cantilever Beam Wedge Test

Sandwich specimens were prepared by firing a thin inter-layer of porous La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) to bond a thin tetragonal yttria-stabilised zirconia (YSZ) beam to a thick YSZ substrate. Fracture of the joint was evaluated by introducing a wedge between the two YSZ adherands so that the stored energy in the thin YSZ cantilever beam drives a stable crack in the adhesive bond and allows the critical energy release rate for crack extension (fracture toughness) to be measured. The crack path in most specimens showed a mixture of adhesive failure (at the YSZ-LSCF interface) and cohesive failure (within the LSCF). It was found that the extent of adhesive fracture increased with firing temperature and decreased with LSCF layer thickness. The adhesive failures were mainly at the interface between the LSCF and the thin YSZ beam and FEM modelling revealed that this is due to asymmetric stresses in the LSCF. Within the firing temperature range of 1000-1150C, the bonding fracture toughness appears to have a strong dependence on firing temperature. However, the intrinsic adhesive fracture toughness of the LSCF/YSZ interface was estimated to be 11 Jm2 and was not firing temperature dependent within the temperature range investigated.Comment: 13 figures, 1 table, journal paper publishe

Mechanical Properties of La0.6Sr0.4Co0.2Fe0.8O3 Fuel Cell Electrodes

Mixed ionic-electronic conductive (MIEC) perovskite material La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF6428) is a promising candidate for the cathode in intermediate temperature solid oxide fuel cells (IT-SOFCs). Understanding the three dimensional (3D) microstructural characteristics of such a material is crucial to its application because they predominately determine the performance and durability of the porous cathodes and hence of the SOFCs. They affect the overall cathode kinetics and thus the electrochemical reaction efficiency, as well as the mechanical properties, which dominate the lifetime of SOFCs. It is necessary to balance the trade-off between the electrochemical performance, which is improved by high porosity and minimal sintering, and the ability to withstand mechanical constraints, which is improved by the opposite. To date LSCF6428 has been widely investigated on subjects of microstructure-related electrochemical performance, while little work has been reported on the mechanical properties and their correlation with the 3D microstructures. The main purpose of this research was to study the mechanical properties (i.e. elastic modulus, hardness and fracture toughness) of LSCF6428 cathode films and bulk samples fabricated by high temperature sintering, and to evaluate the effect of 3D microstructural parameters on elastic modulus, and the Poisson’s ratio where applicable, by means of both experimental and numerical methods. Room-temperature mechanical properties were investigated by nanoindentation of porous bulk samples and porous films sintered at temperatures from 900 to 1200 °C. A spherical indenter was used so that the contact area was much greater than the scale of the porous microstructure. The elastic modulus of the bulk samples was found to increase from 33.8 to 174.3 GPa and hardness from 0.64 to 5.32 GPa as the porosity decreased from 45 to 5 vol% after sintering at 900 to 1200 °C. Densification under the indenter was found to have little influence on the measured elastic modulus. The residual porosity in the nominally dense sample was found to account for the discrepancy between the elastic moduli measured by nanoindentation and by impulse excitation. Based on the optimisation of a commercial LSCF6428 ink formulation, crack-free films of acceptable surface roughness for indentation were also prepared by sintering at 900 to 1200 °C. It was shown that reliable measurements of the true properties of the films were obtained by data extrapolation provided that the effects from both surface roughness and substrate were minimised to neglected levels within a certain range of indentation depth to film thickness ratio (which was 0.1 to 0.2 in this study).The elastic moduli of the films and bulk materials were approximately equal for a given porosity. Based on the crack length measurements for Berkovich-indented samples, the fracture toughnesses of bulk LSCF6428 were determined to increase from 0.51 to 0.99 MPa·m1/2, after sintering at 900 to 1200 °C. The microstructures of films before and after indentation were characterised using FIB/SEM slice and view technique and the actual 3D microstructure models of the porous films were reconstructed based on the tomographic data obtained. Finite element modelling of the elastic modulus of the resulting microstructures showed excellent agreement with the nanoindentation results. The 3D microstructures were numerically modified at constant porosity by applying a cellular automaton algorithm based method, so that the influence on elastic modulus of factors other than porosity could be evaluated. It was found that the heterogeneity of the pore structure has a significant influence on the elastic properties computed using mechanical simulation.Open Acces

Analyses of microstructural and elastic properties of porous SOFC cathodes based on focused ion beam tomography

Mechanical properties of porous SOFC electrodes are largely determined by their microstructures. Measurements of the elastic properties and microstructural parameters can be achieved by modelling of the digitally reconstructed 3D volumes based on the real electrode microstructures. However, the reliability of such measurements is greatly dependent on the processing of raw images acquired for reconstruction. In this work, the actual microstructures of La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) cathodes sintered at an elevated temperature were reconstructed based on dual-beam FIB/SEM tomography. Key microstructural and elastic parameters were estimated and correlated. Analyses of their sensitivity to the grayscale threshold value applied in the image segmentation were performed. The important microstructural parameters included porosity, tortuosity, specific surface area, particle and pore size distributions, and inter-particle neck size distribution, which may have varying extent of effect on the elastic properties simulated from the microstructures using FEM. Results showed that different threshold value range would result in different degree of sensitivity for a specific parameter. The estimated porosity and tortuosity were more sensitive than surface area to volume ratio. Pore and neck size were found to be less sensitive than particle size. Results also showed that the modulus was essentially sensitive to the porosity which was largely controlled by the threshold value.Comment: 21 pages, 14 figures, 2 tables, journal paper published in Journal of Power Source

Elevated serum miR-133a predicts patients at risk of periprocedural myocardial injury after elective percutaneous coronary intervention

Background: Periprocedural myocardial injury (PMI) is a frequent complication of percutaneous coronary intervention (PCI) associated with poor prognosis. However, no effective method has been found to identify patients at risk of PMI before the procedure. MicroRNA-133a (miR-133a) has been reported as a novel biomarker in various cardiovascular diseases. Herein, it was sought to determine whether circulating miR-133a could predict PMI before the procedure. Methods: Eighty patients with negative preoperative values of cardiac troponin T (cTnT) receiving elective PCI for stable coronary artery disease (CAD) were recruited. Venous serum samples were collected on admission and within 16–24 hours post-PCI for miRNA measurements. PMI was defined as a cTnT value above the 99% upper reference limit (URL) after the procedure. The association between miR-133a and PMI was further assessed. Results: Periprocedural myocardial injury occurred in 48 patients. The circulating level of miR-133a was significantly higher in patients with PMI before and after the procedure (both p < 0.001). Receiver operating characteristic curve analysis of the preoperative miR-133a level revealed an area under the curve (AUC) of 0.891, with a sensitivity of 93.8% and a specificity of 71.9% to predict PMI. Additionally, a decrease was found in fibroblast growth factor receptor 1 (FGFR1) in parallel with an increase in miR-133a levels in patients with PMI. Conclusions: This study demonstrates for the first time that serum miR-133a can be used as a novel biomarker for early identification of stable CAD patients at risk of PMI undergoing elective PCI. The miR-133a-FGFR1 axis may be involved in the pathogenesis of PMI

Distribution, sources and health risk assessment of polycyclic aromatic hydrocarbons in urban soils under different landform conditions of Taiyuan, China

Public concern about polycyclic aromatic hydrocarbons (PAHs) is rising due to their potential carcinogenic, teratogenic, and mutagenic effects. This study assessed PAHs in Taiyuan City’s plain and mountain soil, investigating concentrations, distribution, sources, and carcinogenic risk. Σ21PAHs concentrations in plain topsoil ranged from 133.2 to 6,410.6 ng/g (mean 1,444.7 ng/g), and in mountain soil from 66.5 to 2,250.2 ng/g (mean 585.5 ng/g). Approximately 55.1% of plain and 19.0% of mountain soil samples had contamination levels exceeding 600 ng/g. In plain soil, 4-ring and 5-ring PAHs dominated, while 2-ring and 3-ring PAHs were prevalent in mountain soil. Polluted areas in Taiyuan were primarily centered in the central-north, with higher content closer to industrial or business districts. PAH isomer ratios and principal component analysis/multiple linear regression (PCA/MLR) indicated coal combustion as the main PAH source, followed by coke production, vehicle emissions, and biomass combustion. Incremental lifetime cancer risks (ILCRs) showed Taiyuan’s PAH-related cancer risks were generally low, though heavily contaminated areas exhibited moderate risks. Plain regions had three times higher cancer risk than mountains, with children facing higher risk than adults. These findings highlight the need to consider PAH pollution while enhancing urban environmental quality