Microstructure of ceramic foams

This paper describes the preparation of ceramic foams by expansion of a ceramic suspension based on a polyurethane system. The microstructure and degree of reticulation of the foamed ceramic were examined and analysed with the help of a simple geometrical model. Like the porous ceramics prepared by the replica processing method, these foamed ceramics possess open cells in a nearly equiaxed shape but the cell size is much finer. The ratio of the window size to the cell size is a useful parameter for characterising the geometry of the foam and is related to the qualitative concept of degree of reticulation. For a face centred cubic array of cells it is related geometrically to the volume fraction of porosity and this relationship is tested using microstructural measurements for a range of ceramic foams.     

Anisotropic grain growth in boria-doped diphasic mullite gels

Densification and anisotropic grain growth were investigated in sol-gel derived, boria-doped diphasic mullite. Boria enhanced viscous flow densification by reducing the viscosity and also produced a fine grain microstructure. Whisker-like mullite grains evolved from the dense, equiaxed microstructure. The onset temperature was ∼1500°C. Chemical leaching was employed to characterize the anisotropic grains. Growth kinetics showed that anisotropic grains followed the empirical equation Gⁿ-G_oⁿ=Kt with n=3 and n=4 for the length and thickness directions, respectively. The activation energies for grain growth were 660 kJ mol⁻¹ for elongation and 800 kJ mol⁻¹ for thickening.     

Elastic analysis of semi-elliptical axial cracks in cylinders under thermal shock using the BS 7910 framework

One of the most common and important situations involving flaws in pressure vessels is the internal thermal shock of a hollow circular cylinder with an axial semi-elliptical internal surface crack. The current approaches available to analyse this specific problem are excessively conservative or cumbersome. This paper considers the internal thermal shock problem and determines the stress-intensity factors for shallowest and deepest points for cylinders with the ratio of thickness to inner radius 0.1 and 0.25. The results of calculations are presented in a similar form to the draft standard BS 7910:1997 and thus permit numerous practical applications. These results are in many cases much less conservative than the draft standard and also reveal some important cases.     

MHD effects and heat transfer analysis in magneto-thermo-fluid-structure coupled field in DCLL blanket

The study on DCLL blanket in ITER is related to magneto-thermo-fluid-structure coupled field issue. A key element in the DCLL concept is the flow channel insert (FCI) which serves as an electrical insulator to reduce the magnetohydrodynamic (MHD) pressure drop, and as a thermal insulator to decouple the high temperature PbLi from the reduced activation ferritic steel (RAFS) structure. In the present work, 16 geometrical models of flow ducts are introduced to study the MHD flow and heat transfer in DCLL blanket in magneto-thermo-fluid-structure coupled physical field. The PISO method on unstructured collocated meshes is applied to simulate the metal liquid flow and heat transfer in the blanket. The consistent and conservative scheme is employed to solve the incompressible Navier-Stokes equations with the Lorentz force included based on the electrical potential formula. The finite element method is used to study thermal mechanical behaviors of FCI. The velocity distribution, MHD pressure drop, electric current stream lines and temperature distribution in liquid blanket, thermal deformations of FCI in various geometrical models under external strong magnetic field are investigated. The pressure drop reduction factor is defined to analyze the influence of FCI structure on the MHD effects in the liquid metal blanket. The nonlinear coupling effects among magnetic field, heat transfer and FCI structure are revealed. The results show that thicker FCI and wider gap would increase the MHD pressure drops in bulk flow; the thicker FCI has smaller displacement but its strain and stress change non-monotonously; the wider gap can enhance the heat transfer performance but lead to larger stress in FCI. The optimal design is essential for the structural safety and high heat efficiency of the system.     

Highly active and stable A-site Pr-doped LaSrCrMnO-based fuel electrode for direct CO2 solid oxide electrolyzer cells

Direct CO₂ electrolysis has been explored as a means to store renewable energy and produce renewable fuels. La chromate-based perovskite oxides have attracted great attention as fuel electrode materials for solid oxide electrolyzer cells. However, the electrochemical catalytic activity of such oxides is relatively low, and their stability has not been confirmed. In this study, Pr is doped into La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ (LSCM) and the applicability of the resulting fuel electrode to direct CO₂ electrolysis is investigated. The polarization resistance of the resulting electrode at 800 °C is decreased by 25%. Distribution function of relaxation times analysis indicates that the observed improvements may be attributed to increased oxygen ion conductivity. A full cell of Pr-doped LSCM-gadolinium-doped ceria (GDC)|scandia-stabilized zirconia|La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-GDC achieves an electrolysis current of 0.5 A cm⁻² at 1.36 V and a Faradaic efficiency close to 100%. Short-term (210 h) stability testing of the cell under an electrolysis current of 0.5 A cm⁻² at 800 °C with pure CO₂ as the feedstock reveals a decrease in applied voltage at a rate of 7 mV kh⁻¹, thereby indicating excellent stability. Thus, given its satisfactory performance and stability, the Pr-doped LSCM electrode may be considered a promising candidate material for direct CO₂ electrolysis.     

Diffusion of boron in alloys

By means of particle tracking autoradiography (PTA), the diffusion coefficients of boron between 900 and 1200°C were measured in 04MnNbB steel, 25MnTiB steel, Ni-B, Fe-30%Ni-B and Fe-3%Si-B alloys, and the frequency factor D₀ and activation energy Q were obtained respectively. The experiment results indicated that there was an obvious difference between the present result and the result obtained by Busby (in 1953). It was found that the boron diffusivity in γ-Fe increased as Ni was added. The diffusivity of boron in Fe-3%Si-B alloy with b.c.c. structure was much slower than one obtained by Busby in α-Fe (1954), which, however, was much faster than the results obtained in γ-Fe (with f.c.c. structure). Based on the present data of boron diffusion coefficients, the mechanism of segregation of boron to grain boundaries is discussed.     

Pressureless consolidation of boron nitride fiber ceramics via a chemical bonding approach

Hexagonal boron nitride (h-BN) materials with excellent physicochemical stability and processibility have been considered as promising engineering materials. However, their practical applications are rather limited by the difficulty in constructing the well-bonded h-BN ceramic. Herein, we present a reaction welding strategy for facile preparation of well-bonded h-BN fibers (BNFs) network by using ethanolamine (ETA) containing alcoholic hydroxyl and primary amine groups as crosslinking agent. The formed >B-O-H₂C-H₂C-H₂N|→B< bond bridges through the esterification and intramolecular coordination (N|→B) reactions between ETA molecules and BNFs containing surface hydroxyls, play an essential role in the construction of the thermodynamically stable >B-N-B< covalent bond between BNFs. Based on this synthetic strategy and a room-temperature molding process followed by a calcination step, a series of resilient BNFs ceramics with high processibility, typical diamagnetism, low dielectric constant (∼ 2.1) and loss, good resistance to temperature different, corrosion, oxidation and abrasion are produced. Besides, they also exhibit excellent strain and damage tolerance, the maximum compression strain and strength can reach up to 38% and ∼ 33 MPa, respectively. Significantly, the deliberately designed BNFs vessel can be successfully used for the alloying of high melting point metals, showing both structural/chemical robustness and good reusability. This ingenious design for well-bonded BNFs network not only is a good model for exploring the welding way for h-BN powders, but also opens up numerous opportunities for practical applications.     

Combined composites layup architecture and mechanical evaluation of type IV pressure vessels: A novel analytical approach

Strict requirements for cost and reliability of compressed gaseous pressure vessels as on-board hydrogen storage necessitates in-depth understanding on the mechanical responses of the composite structure. General numerical methodology to assess this involves a two-step setup: composite layup generation and mechanical property calculation, which becomes highly time and resource-consuming, particularly if multiple designs are scanned for optimization. This study presents a multi-functional analytical tool, combining both aforementioned steps in a single modeling framework. The model enables quick prediction of the strain/stress distribution in correlation with uncomplicated inputs describing the layup design, i.e. stacking sequence with desired winding angles. Additionally, the mechanical evaluation specially focuses on the dome region whose calculation has hardly been inspected analytically in the literature. The results are validated against a three-dimensional finite element calculation implemented in Ansys Workbench, showing qualitative and quantitative agreement. Overall, the study paves the way for composite tank design using analytical approaches, with easy implementation, minimum computational cost and great transparency.     

Degradation behavior under alternating constant-current/constant-voltage discharge for flat-tube solid oxide fuel cells inside stack

An alternating constant-current (CC)/constant-voltage (CV) discharge for the optimal operation mode of solid oxide fuel cells (SOFCs) inside stack is carried out, and the degradation of SOFCs is discussed. The SOFCs are operated alternately in CC/CV mode for 985 h at 750 °C. The results show that the degradation rate of SOFCs first decreases and then increases in the CC/CV alternating operation mode. And the lowest degradation rate is 3.64%/100 h in the CV operation mode at ∼0.85 V, which is about one order of magnitude higher than the average degradation rate (0.36%/100 h) in the corresponding CC operation mode. The EIS analysis indicates that the ohmic resistance increases and the polarization resistance decreases after the CV operation mode, while the impedance change after CC discharge shows the opposite trend.