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.     

Study of structural,dielectric, electric,magnetic and magnetoelectric properties of K0.5Na0.5NbO3 − Ni0.2Co0.8Fe2O4 composites

Magnetoelectric composites with general formula (x) Ni_0.2Co_0.8Fe₂O₄- (1- x) (K_0.5Na_0.5)NbO₃ (x = 0, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.0) have been synthesized by solid state reaction method. X-ray diffraction pattern asserts the existence of both the constituent phases in the synthesized composites. FESEM micrographs are used to investigate the microstructure and for calculation of average grain size of the composites. Temperature dependent dielectric properties are investigated as a function of temperature and found to enhance with addition of ferrite in the composites. P-E hysteresis loops obtained for individual (K_0.5Na_0.5)NbO₃ (KNN) phase and composites indicate the ferroelectric ordering in the composites. Saturation and remnant magnetization show increasing trend with increase in NCFO concentration. FC-ZFC magnetization curves indicate charge ordering, metal-insulator transition (Verwey transition) in NCFO and composites. Impedance spectroscopy shows that bulk resistance reduces with increase in temperature, thereby indicating negative temperature coefficient of resistance (NTCR) behaviour of the composites. The magnetoelectric effect is confirmed by measuring magnetoelectric voltage coefficient, α_ME and the maximum value of α_ME is 5.389 mV/cm-Oe for 20% NCFO-80% KNN composite.     

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.     

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.     

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).     

Sintering of high-entropy nanoparticles obtained by polyol process: A case study of (La0.2Y0.2Nd0.2Sm0.2Gd0.2)2Ce2O7-δ

High-entropy (HE) ceramics nanoparticles have received much attention due to their interesting properties. However, very limited studies have been conducted on their sintering. Here, we report the sintering behavior of HE A₂B₂O₇ type rare earth oxide nanoparticles obtained by polyol process. HE cerate (HECe) (La_0.2Y_0.2Nd_0.2Sm_0.2Gd_0.2)₂Ce₂O_7-δ is chosen as an exemplary case, which is considered as a good candidate for thermal insulation. HECe nanoparticles with size of 2.6–7.1 nm can be synthesized through polyol process followed by annealing in air at 300–700 °C. HECe nanoparticle compact can be densified by directly sintering at 1500 °C. The sintering temperature could be further decreased using a two-step sintering process, i.e., 1500 °C 5 min-1300 °C 5 h. Our results show that fine particle and abundant oxygen vacancies probably dominate the densification process. By controlling the sintering regime, we can tune the microstructure of HECe ceramics and thermal conductive properties accordingly.     

Synthesis,characterization and catalytic properties of La2−x Sr x NiO4−δ

Various compositionsx in the catalyst system La_2–x Sr _x NiO_4– have been prepared by conventional techniques and characterized by X-ray powder diffraction, electron microscopy and BET surface measurements. The catalytic properties of these catalysts have been tested in the propylene oxidation reaction. The catalytic activity can be correlated with the oxygen content and with the strontium substitution.     

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.     

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.     

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.     

Comparison of Mg–Al layered double hydroxides intercalated with OH− and CO32− for the removal of HCl,SO2, and NO2

Journal of Porous Materials - Two different Mg–Al layered double hydroxides (LDHs), OH?Mg–Al LDH and CO3?Mg–Al LDH, are prepared and utilized for the efficient removal...     

Influence of Al-substitution on structures,resistivity and magnetic properties of LaxSr(2−x)Fe(1+y)Mo(1−y)O6 (x=3y,y=0.05, 0.1, 0.15, 0.2)

Two series of single-phased La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ and La_xSr_(2−x)Fe_(1+0.5y)Al_0.5yMo_(1−y)O₆ (x=3y, y=0.05, 0.1, 0.15 and 0.2) double perovskites were prepared by solid-state reaction. The effects of Al-substitution on the structures, resistivity and magnetic properties of La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ were investigated. Although Al-replacement exhibits a negligible influence of on the B-site ordering degree, it results in the suppression of magnetisation caused by non-magnetic Al³⁺ ions. Reduction of grain sizes leads to increased resistivity, thus an optimised magnetoresistance (MR) behaviour is observed. The greatest MR extent improvement can be obtained when y is 0.15 and the MR% of the Al-doped ceramics reaches −10.5% (10 K, 1 T), which is 2 times greater than that of the undoped ceramics (−4.6%, 10 K, 1 T). Interestingly, the Curie temperature (Tc) of both Al-doped and undoped samples maintained relatively constant values of approximately 420 K and 405 K, respectively, which were different results from the data obtained for similar electron-doping systems in the literature.     

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.     

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.     

Structural,magnetic and magnetocaloric study of La0.7−xEuxSr0.3MnO3 (x=0.1, 0.2 and 0.3) manganites

This paper reports the structural, magnetic and magnetocaloric properties of La_0.7−xEu_xSr_0.3MnO₃ (x=0.1, 0.2 and 0.3) polycrystalline manganites elaborated using the solid-state reaction at high temperatures. The X-ray powder diffraction shows that all the prepared compounds are single phase. La_0.6Eu_0.1Sr_0.3MnO₃ is crystallized in the rhombohedral symmetry, whereas a structural transition towards orthorhombic system is observed for x≥0.2. Eu doping was found to induce a decrease of the Curie temperature T_C from 343 K (x=0.1) to 272 K (x=0.3). All compounds undergo a large magnetocaloric effect and have consequently potential applications in magnetic refrigeration domain around room temperature.     

Corrosion of HIPed β-Si6−zAlzOzN8−z (z =0, 1, 2, 3) ceramics by NaCl vapor

Tests for the corrosion of β-Si_6−zAl_zO_zN_8−z (z =0, 1, 2, 3) (β-Si₃N₄ and β-SiAlON) ceramics were carried out at 1300 °C for 3–24 h in NaCl vapor of various concentrations (1.67 × 10⁻², 3.33 × 10⁻², 5.0 × 10⁻² g l⁻¹), which was carried by flowing Ar gas. The densified Si₃N₄ and SiAlON ceramics were fabricated by HIPing under N₂ of 150 MPa at 1700 °C. The corroded surface was observed by optical and scanning electron microscopy (OM and SEM). The phases produced during corrosion were identified by X-ray diffraction. The thickness of the corroded scale was determined by cross sectional SEM observation. The Si₃N₄ ceramics were hardly corroded by NaCl vapor, while the z=1 and 2 SiAlONs were slightly corroded with the formation of bubbles on the surface; the z=3 ceramics were severely corroded with the formation of Al₂O₃ needle crystals and fine mullite crystals, depending on the NaCl vapor concentration. Quantitative X-ray microanalysis showed that 2 at.% Na is contained in all the scales of the SiAlONs. The severe corrosion of the z=3 SiAlON was explained on the basis of the kinetic results for the thickness of the scale.     

(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.     

New Trends and Future Opportunities in the Enzymatic Formation of C−C,C−N,and C−O bonds

Organic chemistry provides society with fundamental products we use daily. Concerns about the impact that the chemical industry has over the environment is propelling major changes in the way we manufacture chemicals. Biocatalysis offers an alternative to other synthetic approaches as it employs enzymes, Nature's catalysts, to carry out chemical transformations. Enzymes are biodegradable, come from renewable sources, operate under mild reaction conditions, and display high selectivities in the processes they catalyse. As a highly multidisciplinary field, biocatalysis benefits from advances in different areas, and developments in the fields of molecular biology, bioinformatics, and chemical engineering have accelerated the extension of the range of available transformations (E. L. Bell et al., Nat. Rev. Meth. Prim. 2021 , 1, 1–21). Recently, we surveyed advances in the expansion of the scope of biocatalysis via enzyme discovery and protein engineering (J. R. Marshall et al., Tetrahedron 2021 , 82, 131926). Herein, we focus on novel enzymes currently available to the broad synthetic community for the construction of new C−C, C−N and C−O bonds, with the purpose of providing the non-specialist with new and alternative tools for chiral and sustainable chemical synthesis.     

Catalytic activity of nanometer La1−xSrxCoO3 (x = 0, 0.2) perovskites towards VOCs combustion

Combustive oxidation of volatile organic compounds (VOCs), such as propyl alcohol, toluene and cyclohexane, were studied. The combustion was catalyzed by nanoparticles of La_1−xSr_xCoO₃ (x = 0, 0.2) perovskites prepared by a co-precipitation method. The results showed high activities of the perovskite catalysts. Compared to LaCoO₃, in particular, La_0.8Sr_0.2CoO₃ was much higher in catalytic ability. The total oxidation of VOCs followed the increasing order: cyclohexane < toluene < propyl alcohol. The T_99% of cyclohexane was 40 °C lower than that of toluene, which appeared to be determined by the bond strengths of the weakest C–H and C–C bonds. The 100-h stability experiments showed that La_1−xSr_xCoO₃ (x = 0, 0.2) perovskite was highly stable.