Enhanced sintering of Ce0.8Nd0.2O2−δ-La0.8Sr0.2Ga0.8Mg0.2O3−δ using CoO as a sintering aid

Ce_0.8Nd_0.2O_1.9 (NDC) and La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM) electrolytes were prepared using a sol-gel method. NDC-LSGM composite electrolytes were subsequently prepared by adding 5% (w, mass fraction) precalcined LSGM powders to NDC sols. The electrolyte materials of NDC-Co and NDC-LSGM-Co were obtained by adding 1 mol% CoO to NDC sols and NDC-LSGM composite electrolytes, respectively. The microstructure and phase composition of the pellets were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray spectroscopy (EDS). The electrical conductivities of the pellets were measured using alternative current (AC) impedance spectroscopy. The results indicate that a single perovskite phase is observed for the LSGM ceramic, while NDC-Co, NDC-LSGM and NDC-LSGM-Co have a cubic fluorite structure similar to that of NDC. As a sintering aid, CoO can further promote grain growth and increase relative density (＞95%) of the NDC-LSGM composite electrolyte. The enhancement of the total conductivity is primarily attributed to the large increase in the conductivity of the grain boundary. However, the slight decrease of the grain boundary conductivity of the NDC-LSGM-Co electrolyte is caused by the presence of trace amounts of impurity phases in the grain boundaries.     

Electrochemical performance and chemical stability of proton-conducting BaZr0.8−xCexY0.2O3−δ electrolytes

Protonic ceramic fuel cells (PCFCs) using BaZr_0.8−xCe_xY_0.2O_3−δ (BZCY) as electrolyte materials have attracted widespread attention because of their high performance at reduced temperature. However, there are few systematic studies on both the performance and stability of BZCY materials. In this paper, we report our work on the electrochemical performance and chemical stability of BaZr_0.8−xCe_xY_0.2O_3−δ (x = 0, 0.1, 0.3, 0.5, and 0.7) series. The results show that electronic hole conductivity decreases with increasing Ce⁴⁺ content, especially at high temperature. In addition, H₂ atmosphere reduces the conductive activation energy of BZCY. On the contrary, air atmosphere causes serious electronic leakage. These effects are also reflected in the operation of PCFCs, that is, the higher the Ce⁴⁺ content, the higher the open-circuit voltage and output power density. However, low Ce⁴⁺ content may stabilize the materials in CO₂ atmosphere. At 700°C, an anode-supported PCFC based on BaZr_0.1Ce_0.7Y_0.2O_3−δ electrolyte, using humid H₂ fuel, gives a peak power density of 1.0 W cm⁻². At 600°C, BaZr_0.8Y_0.2O_3−δ and BaZr_0.7Ce_0.1Y_0.2O_3−δ show a good stability in CO₂-containing atmosphere.     

Towards the fabrication of La0.98−xSrxCo0.2Fe0.8O3−δ perovskite-type oxygen transport membranes

La_0.98−xSr_xCo_0.2Fe_0.8O_3−δ (LSCF) is a candidate material for use as an oxygen transport membrane (OTM). In this work, fabrication-relevant properties (sintering behaviour, thermal and chemical expansion) of LSCF (x = 0.2, 0.4, 0.6, 0.8) were investigated in order to select the preferred composition for fabricating a thin-film supported membrane able to withstand the thermochemical stresses encountered during manufacturing and operation with simultaneously high oxygen permeation flux.Partial substitution of La by Sr ions in LSCF is beneficial for increasing the oxygen permeation rate, but it causes drawbacks regarding manufacturing and operation. A Sr content of x ≥ 0.6 results in a swelling of the material during sintering, which complicates the manufacturing of thin, leak-free membranes. This swelling is related to oxygen release during heating, combined with the formation of a liquid phase above 1200 °C. Furthermore, an increase in total strain with Sr content is observed. This is caused by the chemical expansion, while there is no significant change in thermal expansion with increasing Sr content.The compositions x = 0.4 and x = 0.6 showed tolerable expansion coefficients as well as adequate sintering behaviour and were therefore selected for the fabrication of thin supported membranes. These supported membranes with a thickness of 30 μm were manufactured by sequential tape casting and characterised regarding microstructure and oxygen flux.     

A new type of Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3−x (Me = Y,Gd) high-entropy ceramics used in thermal barrier coatings

Seeking for new ceramics with excellent thermophysical properties as thermal barrier coatings candidate materials has become a hot research field. In this study, Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x high-entropy ceramic powders were successfully synthesized by the method of solid-state reaction, and the ceramics with single phase were prepared by pressureless sintering at 1600°C. The phase composition, microstructure, element distribution, high-temperature thermal stability, and thermophysical properties of the ceramics were studied. The results showed that Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x ceramics were composed of SrZrO₃ phase and the second phase of AB₂O₄ spinel (i.e., SrY₂O₄ and SrGd₂O₄). The content of the second phase was gradually increased after heat treatment at 1400°C, which significantly improved the thermophysical and mechanical properties of the ceramics. The microhardness and fracture toughness of the ceramics were improved compared with that of SrZrO₃. The thermal conductivities of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 1.30 and 1.28 W m⁻¹ K⁻¹ at 1000°C, which were about 35% and 40% lower than that of SrZrO₃ (1.96 W m⁻¹ K⁻¹) and yttria-stabilized zirconia (2.12 W m⁻¹ K⁻¹), respectively. The thermal expansion coefficients of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 12.8 × 10⁻⁶ and 14.1 × 10⁻⁶ K⁻¹ at 1300°C, respectively, which was more closer to the superalloys compared with SrZrO₃ ceramic (11.0 × 10⁻⁶ K⁻¹).     

A novel series of Ba0.5Sr0.5Al0.2−xMgxFe0.8O3−ξ (x ≤ 0.2) membranes for oxygen permeation application

The pervoskite-type oxides have received attention due to their potential applications in catalysis, fuel cells, sensors, gas separable membranes, and electrolytes. In view of the importance of oxygen separation from air, stable Ba_0.5Sr_0.5Al_0.2−xMg_xFe_0.8O_3−ξ (x = 0–0.2) powders have been synthesized by decomposition of sol–gel derived oxalate at 950 °C for 5 h and characterized with regard to formation, nature of iron species, oxygen permeation, and electrical conductivity. It is shown that magnesium substitution leads to (i) a stable perovskite-type cubic phase with ‘a’ = 3.953–3.978 Å, (ii) weakening of metal–oxygen bond, (iii) reduction of Fe⁴⁺ ions, and (iv) enhancement of oxygen deficiency and electrical conductivity. Their compact discs act as stable oxygen permeable filters with flux density of ∼3.013–3.355 μmol cm⁻² s⁻¹ at 1000 °C. The maximum value corresponds to composition x = 0.2 and hence can be a potential membrane for oxygen separation technology.     

(Ti0.2V0.2Cr0.2Nb0.2Ta0.2)2AlC–(Ti0.2V0.2Cr0.2Nb0.2Ta0.2)C high-entropy ceramics with low thermal conductivity

In recent years, the microstructure and physicochemical properties of high-entropy ceramics have received much interest by the combination of multiple principal elements. Herein, (Ti_0.2V_0.2Cr_0.2Nb_0.2Ta_0.2)₂AlC–(Ti_0.2V_0.2Cr_0.2Nb_0.2Ta_0.2)C high-entropy ceramics (M₂AlC-MC HECs) were prepared by the spark plasma sintering (SPS) technique, attributing to the structural and chemical diversity of MAX phases. The microstructure of M₂AlC-MC HECs was characterized from micron to atomic scales, and the phase composition of M₂AlC-MC HECs was analyzed by a combination of Maud and Rietveld analysis. The results indicate the successful solid solution of Ti, V, Cr, Nb, and Ta atoms in the M-site of the 211-MAX configuration, and all the samples show a classic layered structure. The weight percentage of (Ti_0.2V_0.2Cr_0.2Nb_0.2Ta_0.2)₂AlC in the M₂AlC-MC HECs was more than 90%. Furthermore, the thermoelectric properties of M₂AlC-MC HECs were investigated for the first time in this study, and the electrical conductivity and thermal conductivity of HECs are 3278 S cm⁻¹ and 2.78 W m⁻¹ K⁻¹at 298 K, respectively.     

ZrxCo0.8−xNi0.2−xFe2O4-graphene nanocomposite for enhanced structural,dielectric and visible light photocatalytic applications

Zirconium doped nickel cobalt ferrite (Zr_xCo_0.8−xNi_0.2−xFe₂O₄) nanoparticles and Zr_xCo_0.8−xNi_0.2−xFe₂O₄-graphene nanocomposites were synthesized by a cheap and facile co-precipitation method. Annealing was done at 750 °C for 6.5 h. Spinel cubic structure of prepared nanoparticles was confirmed by X-ray powder diffraction (XRD) technique. Crystalline size of nanoparticles was observed in the range of 18–27 nm. Graphene was synthesized by Hummer's method. Formation of rGO was confirmed by UV-visible spectroscopy (UV-vis) and XRD. Zr_xCo_0.8−xNi_0.2−xFe₂O₄-graphene nanocomposites were prepared by ultra-sonication route. Grain size of nanoparticles and dispersion of nanoparticles between rGO layers was determined by Scanning electron microscopy (SEM). In application studies of nanoparticles and their nanocomposites, photocatalytic efficiency of nanoparticles under visible light irradiation was observed by degradation of methylene blue. Charge transfer resistance was measured by electrochemical impedance spectroscopy (EIS) and the variation in dc electrical resistivity was analyzed by room temperature current voltage characteristics (I-V). Dielectric constant was also evaluated in frequency range from 1 MHz to 3 GHz. All these investigations confirmed the possible utilization of these materials for a variety of applications such as visible light photocatalysis, high frequency devices fabrication etc.     

High-temperature deformation of a BaCe0.8Y0.2O3−y+Ni composite

Steady-state compressive creep experiments have been performed on a 60 vol.% BaCe_0.8Y_0.2O_3−y/40 vol.% Ni composite in the temperature range of 1100–1370 °C at stresses of 2–150 MPa. The addition of Ni to BaCe_0.8Y_0.2O_3−y increases the creep rate compared to BaCe_0.8Y_0.2O_3−y without Ni. The composite creep response is modeled on the basis of hard grains (BaCe_0.8Y_0.2O_3−y) surrounded by the soft Ni phase.     

Solution combustion synthesis of La1−xSrxFe1−yCuyO3±w (x=0, 0.2; y=0, 0.2) perovskite nanoparticles: Conventional vs. microwaves ignition

La_1−xSr_xFe_1−yCu_yO_3±w (x=0, 0.2; y=0, 0.2) nanoparticles have been prepared by solution combustion synthesis exploiting both conventional and microwave heating in the ignition of the self-sustaining reactions. Interaction of microwaves with the reaction mixture allowed significant reduction of the ignition time according to the dielectric properties of the precursor gels, which have been measured at room temperature in the 0.5–3 GHz frequency range. Both the ignition strategies led to the preparation of crystalline single-phase products without affecting particles morphology. The ignition technique influenced only the average particles size with those prepared by microwaves-ignition, possessing typically larger dimension, as a probable consequence of the higher temperatures reached due to microwave absorbing products. Perfectly crystallised nanoparticles were obtained after combustion syntheses and calcination at 600 °C for 3 h in the particle size range between 20 and 80 nm dependently upon the heating source and the dopant level.     

Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3−δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3−δ layers

The oxygen permeation performance of a number of La_0.1Sr_0.9Co_0.8Fe_0.2O_3−δ (LSCF1982)-based membranes, consisting of dense LSCF1982 layer with/without porous LSCF1982 layer, was analyzed on the basis of the thickness of the dense layer and catalytic effect of the porous layer. A 0.27 mm thick dense membrane gives oxygen permeation flux ($J_{O_2}$) of 2.33 sccm min⁻¹ cm⁻² at 900 °C, which is increased to 3.55 sccm min⁻¹ cm⁻² on applying a porous layer of LSCF1982 onto the dense membrane. The membrane gives a stable flux for 300 h. The flux was further improved by reducing the thickness of the dense LSCF1982 layer and at 950 °C a flux of 4.47 sccm min⁻¹ cm⁻² is obtained with 0.012 mm thick membrane.     

Influence of component manipulation on the structural,mechanical, thermophysical and electrical properties of La0·2Ce0.2Nd0.2(ZrxY1−x)0.4O2−δ high-entropy ceramics

A series of high-entropy ceramics (HECs) with compositions of La_0·2Ce_0.2Nd_0.2(Zr_xY_1−x)_0.4O_2−δ (x = 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0, the corresponding names being HEC(Zr_0·5/Y_0.5, Zr_0·6/Y_0.4, Zr_0·7/Y_0.3, Zr_0·8/Y_0.2, Zr_0·9/Y_0.1, Zr_1·0/Y₀)) were sintered in air at 1600 ^°C for 10 h. When x is in the range of 0.5–0.7, a fluorite phase is formed. Then, as x exceeds 0.7, a second pyrochlore-structured phase appears, and its content gradually increases with the increasing x. The grain growth of the samples is inhibited by increasing in the relative Zr content. The grain refinement and the formation of second phase reduce the thermal conductivity and reinforce the mechanical properties of the samples. HEC(Zr_0.9/Y_0.1) has the lowest thermal conductivity (50–500 °C) and brittleness index, as well as the highest fracture toughness among all samples. In addition, La_0·2Ce_0.2Nd_0.2(Zr_xY_1−x)_0.4O_2−δ ceramics have excellent thermal stability under Ar atmosphere in 50–1400 °C. The thermal expansion coefficients of the samples marginally change regardless of the variation in x. All samples show higher oxygen barrier property than Y₂O₃-stabilized ZrO₂.     

Electrochemical and microstructural characteristics of nanoperovskite oxides Ba0.2Sr0.8Co0.8Fe0.2O3−δ (BSCF) for solid oxide fuel cells

Nanoperovskite oxides, Ba_0.2Sr_0.8Co_0.8Fe_0.2O_3?δ (BSCF), were synthesized via the co-precipitation method using Ba, Sr, Co, and Fe nitrates as precursors. Next, half cells were fabricated by painting BSCF thin film on Sm_0.2Ce_0.8O_x (samarium doped ceria, SDC) electrolyte pellets. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS) measurements were carried out on the BSCF powders and pellets obtained after sintering at 900 °C. Investigations revealed that single-phase perovskites with cubic structure was obtained in this study. The impedance spectra for BSCF/SDC/BSCF cells were measured to obtain the interfacial area specific resistances (ASR) at several operating temperatures. The lowest values of ASR were found to be 0.19 Ω cm², 0.14 Ω cm² 0.10 cm², 0.09 Ω cm² and 0.07 Ω cm² at operating temperatures of 600 °C, 650 °C, 700 °C, 750 °C and 800 °C, respectively. The highest conductivity was found for cells sintered at 900 °C with an electrical conductivity of 153 S cm^?1 in air at operating temperature of 700 °C.     

Oxyacetylene ablation of (Hf0.2Ti0.2Zr0.2Ta0.2Nb0.2)C at 1350 − 2050 °C

Ablation resistance of a multi-component carbide (Hf_0.2Ti_0.2Zr_0.2Ta_0.2Nb_0.2)C (HTZTNC) was investigated using an oxyacetylene flame apparatus. When the surface temperature of the HTZTNC was below 1800 °C, (Nb, Ta)₂O₅, (Hf, Zr)TiO₄, and (Hf, Zr)O₂ were found to be the main oxidation products, while at higher temperature, formation of (Hf, Zr, Ti, Ta, Nb)O_x was favored and its content gradually increased with the increase in ablation temperature. Based on the ablation results and thermodynamic simulation analysis, a possible ablation mechanism of HTZTNC was proposed. Active oxidation of TiC and outward diffusion of TiO were demonstrated to occur during the ablation process, which constitute the critical steps for the ablation of HTZTNC. These results can contribute to the design of ablation resistant ultra-high-temperature ceramics.     

Porous (Ce0.2Zr0.2Ti0.2Sn0.2Ca0.2)O2-δ high-entropy ceramics with both high strength and low thermal conductivity

Single-phase (Ce_0.2Zr_0.2Ti_0.2Sn_0.2Ca_0.2)O_2-δ porous high-entropy ceramics have been in-situ fabricated by foam-gelcasting-freeze drying method at different temperatures. The microstructure, phase composition, and properties of the obtained ceramics were investigated. The results indicate that compared with other porous ceramics reported in the literatures, this type of ceramics exhibits excellent performance. The sample prepared at 1350 °C shows high porosity (88.6 %), low thermal conductivity (0.023 W m^-1 K^-1), and high compressive strength (1.48 MPa). The current study suggests that porous (Ce_0.2Zr_0.2Ti_0.2Sn_0.2Ca_0.2)O_2-δ high entropy ceramics are promising candidates for thermal insulation applications.     

Thermochemical expansion of mixed-conducting (Ba,Sr)Co0.8Fe0.2O3−δ ceramics

The thermal and chemical expansion of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and SrCo_0.8Fe_0.2O_3−δ (SCF) mixed ionic-electronic conductors were studied in combination with oxygen nonstoichiometry (δ) at 298–1223 K and p(O₂) = 10⁻⁴ to 1.00 atm. In order to minimize the effects of phase separation or oxygen-vacancy ordering processes, the data were collected in dynamic cooling mode using dense ceramic samples. The procedure was justified by a very fast equilibration at given p(O₂) in high-temperature range demonstrated for ceramics samples with different specific surface area. The difference in nonstoichiometry of BSCF and SCF at temperatures ≥973 K was found to be ≤0.03 oxygen atoms per formula unit. BSCF demonstrates favorably smaller chemical expansion compared to SCF and many other mixed conductors, originating from smaller δ variations and larger unit cell less sensitive to temperature and nonstoichiometry changes. Excessive thermochemical expansion impedes however the use of BSCF in single-phase fuel cell cathodes and planar mixed-conducting membranes.     

One-step thermal processing of BaCe0.8Y0.2O3−δ hydrogen permeable multichannel hollow fiber membrane

BaCe_0.8Y_0.2O_3−δ (BCY) is the most widely studied proton-conducting material and is frequently fabricated as a dense membrane for hydrogen separation. However, the limitation of preparing dense BCY membranes is the extremely high sintering temperature (>1500°C). Herein, the BCY 7-channel hollow fiber membrane was prepared by a one-step thermal processing (OSTP) with Co₂O₃ as a sintering aid. The results showed that the addition of 1 wt.% Co₂O₃ at a reduced temperature of 1350°C was the optimum composition and sintering condition for densification and forming a single perovskite-phase structure. The hydrogen permeation flux of the BCY hollow fiber membrane reached up to 0.34 ml min⁻¹ cm⁻² at 900°C. The long-term stability test was conducted for 300 h. The successful attempt of such a strategy provides a green and straightforward path for the preparation of dense ceramic proton-conducting membranes on a large scale. This promotes its industrial application in high-temperature hydrogen separation.     

高熵稳定双钙钛矿阴极材料SmBa(Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)2O5+δ的制备及性能优化

通过改进的自蔓延燃烧法合成制备高熵双钙钛矿SmBa(Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)2O5+δ(HE-SBC)阴极材料,并复合10％(摩尔分数)Gd2O3掺杂CeO2(GDC)以优化性能.结果表明:通过B位高熵的方法可以显著减小Co离子由价态变化而引起的热膨胀,从而降低SmBaCo2O5+δ的热膨胀...     

High performance BaCe0.8Y0.2O3−a (BCY) hollow fibre membranes for hydrogen permeation

In this work, BaCe_0.8Y_0.2O_3−α (BCY) perovskite hollow fibre membranes were fabricated by a phase inversion and sintering method. BCY powder was prepared by the sol–gel technique using ethylenediaminetetraacetic acid (EDTA) and citric acid as the complexing agents. Gel calcination was carried out at high temperature to form the desired crystal structure. The qualified BCY hollow fibre membranes could not be achieved even the sintering was carried out at temperatures up to 1550^oC due to the poor densification behavior of the BCY material. The addition of sintering aid (1 wt% Co₂O₃) inside BCY powder as the membrane starting material significantly improved the densification process, leading to the formation of gas-tight BCY hollow fibres. The optimum sintering temperature of BCY hollow fibre membrane was 1400 °C to achieve the best mechanical strength. H₂ permeation through the BCY hollow fibre membranes was carried out between 700 and 1050 °C using 25% H₂–He mixture as feed gas and N₂ as sweep gas, respectively. For comparison purpose, the disk-shaped BCY membrane with a thickness of 1 mm was also prepared. The measured H₂ permeation flux through the BCY hollow fibres reached up to 0.38 mL cm⁻² min⁻¹ at 1050 °C strikingly contrasting to the low values of less than 0.01 mL cm⁻² min⁻¹ from the disk-shaped membrane. After the permeation test, the microstructure of BCY hollow fibre membrane was still maintained well without signals of membrane disintegration or peeling off.