Oxygen transport,thermal and electrochemical properties of NdBa0.5Sr0.5Co2O5+δ cathode for SOFCs

In this study, the crystal structure, thermal, oxygen transport, electrical conductivity and electrochemical properties of the perovskite NdBa_0.5Sr_0.5Co₂O_5+δ (NBSC55) are investigated. In the temperature range of 250 °C–350 °C, the weight loss upon heating was due to a partial loss of lattice oxygen and along with a reduction of Co⁴⁺ to Co³⁺. The tend of weight-loss slows down as temperature increased above 350 °C indicating a reduction of Co³⁺ to Co²⁺ during this stage. The oxygen migration is dominated by surface exchange process at high temperature range (650-800 °C); however, the bulk diffusion process prevails at low temperature range (500–600 °C). For long-term testing, the polarization resistance of NBSC55 increases gradually form 3.13 Ω cm² for 2 h to 3.34 Ω cm² for 96 h at 600 °C and an increasing-rate for polarization resistance is around 0.22% h^?1. The power density of the single cell with NBSC55 cathode reached 341 mW cm^?2 at 800 °C.     

Influence of domain switching process on the electrical fatigue behavior of ferroelectrics

We report an experimental approach, designed based on the recent findings that domain switching in ferroelectric ceramics can be separated into three regimes during antiparallel electric field loading, to investigate the influence of domain switching process on the electrical fatigue behavior of ferroelectrics. Uniaxial compressive stress (−2 MPã -100 MPa) and thermal loading (20 °C–150 °C) were used to tune the domain switching process. Under the same loading condition, the bipolar electrical fatigue behavior of soft lead zirconate titanate ceramics was systematically characterized. The amplitude and frequency of the applied electric field are 2 kV/mm and 10 Hz, respectively. By analyzing the evolution of the domain switching process, combined with the measured polarization and strain response, as well as the cracks observed on the surface of the specimen, it is found that the fatigue of ferroelectric ceramics was mainly related to the domain switching process near the coercive electric field: the regime 2 defined in this paper. The underlying mechanism was further discussed by considering the interplay between the domain switching process with the main factors affecting the electrical fatigue of ferroelectrics, namely defect redistribution, charge carrier injection, and crack initiation.     

Simulation of thermal barrier coating spallation induced by the initiation/growth/coalescence of internal crack and interfacial crack based on a real image model

In the furnace cycle test, the growth of oxide film leads to the propagation and coalescence of multiple cracks near the interface, which should be responsible for the spallation of thermal barrier coatings (TBCs). A TBC model with real interface morphology is created, and the near-interface large pore is retained. The purpose of this work is to clarify the mechanism of TBC spallation caused by successive initiation, propagation, and linkage of cracks near the interface during thermal cycle. The dynamic growth of thermally grown oxide (TGO) is carried out by applying a stress-free strain. The crack nucleation and arbitrary path propagation in YSZ and TGO are simulated by the extended finite element method (XFEM). The debonding along the YSZ/TGO/BC interface is evaluated using a surface-based cohesive behavior. The large-scale pore in YSZ near the interface can initiate a new crack. The ceramic crack can propagate to the YSZ/TGO interface, which will accelerate the interfacial damage and debonding. For the TGO/BC interface, the normal compressive stress and small shear stress at the valley hinder the further crack propagation. The growth of YSZ crack and the formation of through-TGO crack are the main causes of TBC delamination. The accelerated BC oxidation increases the lateral growth strain of TGO, which will promote crack propagation and coalescence. The optimization design proposed in this work can provide another option for developing TBC with high durability.     

Ca2+ doping effects on the structural and electrical properties of Na0.5Bi4.5Ti4O15 piezoceramics

Bismuth layer structured Na_0.5Bi_4.5Ti₄O₁₅ (NBT) ferroelectric is one of the most promising materials for potential applications at high temperature. However, it is challenged to achieve a balance between high Curie temperature piezoelectric coefficient and excellent thermal stability for NBT piezoceramics. Here, through chemical modification at the A site of NBT with Ca²⁺, novel (Na_0.5Bi_0.5)_1-xCa_xBi₄Ti₄O₁₅ piezoceramics with excellent properties fabricated by solid state reaction were studied. After doping of Ca²⁺, the Curie temperature T_C increased from 648 °C to 662 °C while the piezoelectric coefficient d₃₃ increased from 14 pC/N to 22 pC/N which can be attributed to the intrinsic contribution of TiO₆ octahedral lattice distortion (tilting and rotation) and the extrinsic contribution of the increased density of domain walls. The composition of (Na_0.5Bi_0.5)_0.95Ca_0.05Bi₄Ti₄O₁₅ ceramics with x = 0.05 has the optimal performance with high T_C of 655 °C, large d₃₃ of 22 pC/N, high electrical resistivity ρ close to 10⁷ Ω cm at 500 °C and especially excellent thermal stability of d₃₃ only about 5% reduction after being annealed at 625 °C. The work effectively reveals the great potential of CNBT-5 ceramics for high-temperature piezoelectric applications.     

Red-shift and improved afterglow of Sr3Al2-xBxO5Cl2:Eu2+, Dy3+ phosphors via a cation substitution strategy

Sr₃Al_2-xB_xO₅Cl₂:Eu²⁺, Dy³⁺ (x = 0, 0.2, 0.4) long persistent phosphors were prepared via solid-state process. The pristine Sr₃Al₂O₅Cl₂:Eu²⁺, Dy³⁺ phosphor exhibits orange/red broad band emission around 609 nm, which can be attributed to the electric radiation transitions 4f⁶5 d¹→4f⁷ of Eu²⁺. Upon the same excitation, the B³⁺-doped Sr₃Al_2-xB_xO₅Cl₂:Eu²⁺, Dy³⁺ phosphors display red-shift from 609 nm to 625 nm with increasing B³⁺ concentrations. The XRD patterns show that Al³⁺ can be replaced by B³⁺ in the host lattice at the tetrahedral site, which causes lattice contraction and crystal field enhancement, and thereafter achieves the red-shift on the emission spectrum. The XPS investigation provides direct evidence of the dominant 2-valent europium in the phosphor, which can be ascribed for the broad band emission of the prepared phosphors. The afterglow of all phosphors show standard double exponential decay behavior, and the afterglow of Sr₃Al₂O₅Cl₂:Eu²⁺, Dy³⁺is rather weak, while the sample co-doped with B³⁺shows longer and stronger afterglow, as confirmed after the curve simulation. The analysis of thermally stimulated luminescence showed that, when B³⁺ is introduced, a much deeper trap is created, and the density of the electron trap is also significantly increased. As a result, B³⁺ ions caused redshift and enhanced afterglow for the Sr₃Al_2-xB_xO₅Cl₂:Eu²⁺, Dy³⁺ phosphor.     

Preparation of a CeO2–ZrO2 based nano-composite with enhanced thermal stability by a novel chelating precipitation method

In this study, a chelating agent is introduced to prepare CeO₂–ZrO₂ nano-composite through a precipitation process. The physicochemical properties of the oxide precursors, nano composite materials are strongly dependent on the preparation method and whether a chelating agent is used. Adding an appropriate quantity of chelating agent SO₄²⁻ can facilitate thermal stability and phase structure uniformity of CeO₂–ZrO₂ mixed oxides. The calculation results showed that the Gibbs free energy of chelating complex of [ZrSO₄]²⁺ (ΔG = −127.2469 kJ/mol) is higher than the [Ce(III)SO₄]⁺ (ΔG = -29.8279 kJ/mol). The precipitation chemical potential of Zr⁴⁺ moves close to the precipitation chemical potential of Ce³⁺. The novel and low-cost chelating precipitation method can modify the homogeneity of the compounds at the atomic scale, which can offer a powerful opportunity for, and provide direction in, the design of materials with exceptional properties.     

The effect of synthesis conditions on the physicochemical properties of magnesium aluminate materials

Magnesium aluminate-based materials were prepared by applying different methods: (i) mechanochemical milling of the initial mixture of magnesium and aluminium nitrate powders (in appropriate stoichiometric amounts) followed by heat treatment at temperatures of 650 °C and 850 °C and (ii) melting of the mixture of nitrate precursors at 240 °C followed by thermal treatment at 650 °C, 750 °C and 850 °C. The effect of synthesis method on the structure and morphology of the obtained solids was studied by using various techniques such as: nitrogen adsorption-desorption isotherms, powder XRD, IR spectroscopy and SEM. It was shown that the mechanochemical milling performed before calcination procedure leads to obtaining of nanocrystalline magnesium aluminate spinel phase at lower temperature of 650 °C in comparison with the method using thermal treatment only (at 750 °C). The obtained nanomaterials exhibit mesoporous structure.     

Wetting,infiltration and interaction behavior of CMAS towards columnar YSZ coatings deposited by plasma spray physical vapor

Thermal barrier coatings (TBCs) produced by electron beam physical vapor deposition (EB-PVD) or plasma spray (PS) usually suffer from molten calcium-magnesium-alumino-silicate (CMAS) attack. In this study, columnar structured YSZ coatings were fabricated by plasma spray physical vapor deposition (PS-PVD). The coatings were CMAS-infiltrated at 1250?°C for short terms (1, 5, 30?min). The wetting and spreading dynamics of CMAS melt on the coating surface was in-situ investigated using a heating microscope. The results indicate that the spreading evolution of CMAS melt can be described in terms of two stages with varied time intervals and spreading velocities. Besides, the PS-PVD columnar coating (～100?μm thick) was fully penetrated by CMAS melt within 1?min. After the CMAS attack for 30?min, the original feathered-YSZ grains (tetragonal phase) in both PS-PVD and EB-PVD coatings were replaced by globular shaped monoclinic ZrO₂ grains in the interaction regions.     

The potential of energy flexibility of space heating and cooling in Portugal

This study assesses the potential of energy flexibility of space heating and cooling for a typical household under different geographical conditions in Portugal. The proposed approach modifies the demand through the optimization of the thermostat settings using a genetic algorithm to reduce either operational costs or interaction with the grid. The results show that the used energy flexibility indicator expresses the available potential and that flexibility depends on several factors, namely: i) thermal inertia of the archetypical household; ii) the time of use electricity tariffs; iii) users’ comfort boundaries; and iv) the geographical location of the houses.     

FE simulation of the curing behavior of the epoxy adhesive Hysol EA-9321

Cold-curing adhesives, characterized by an unsteady curing degree, present various advantages for assembling large scale structures set up under outdoor conditions. Thus various applications can be found in aerospace and automotive industries where structures are affected by thermal and mechanical loads. Hence, the curing state of the adhesive must be known to evaluate the lifetime of such bonded structures. The evolution of the polymerization of the adhesive Hysol EA-9321 during the curing process was examined in this paper. To that end, the curing degree of the adhesive was experimentally and analytically investigated for different curing cycles with a view to a potential application in the aerospace domain, where structures are assembled at low temperatures. Existing dynamic and isothermal curing models were applied to simulate the curing behavior of the adhesive. Then, an FEM model was developed to simulate the process of adhesive curing by taking into account a thermo-kinetic coupling.