Increasing Sr Content Research Articles

The Effect of Sr composition on the microstructure and mechanical properties of Al-Si-Zn filler for the brazing of AA6061

The microstructure and properties of Al-Si-Zn-Sr filler metals and brazed 6061-T6 aluminum alloy after heat treatment were studied, and the adsorption energy of Sr element on Si (111) surface was calculated. After Sr was added to Al-6Si-40Zn filler metal, its melting range remained unchanged, and the spreading wettability was improved. The microstructure mainly includes α-Al solid solution, fibrous Si, and η-Zn solid solution. Generally speaking, increasing Sr content could improve the average microhardness of brazed joints. The shear strength of the brazed joint containing Al-6Si-40Zn-0.8Sr filler metal was 134.23 MPa, which was 39.3% higher than that of the brazed joint without Sr The calculation results of adsorption energy showed that the surface of Si (111) mainly adsorbed Sr at the vacancy, which changed the growth mode of Si.

On the incorporation of strontium into the crystal structure of bredigite: structural effects and phase transition

Sr-substitution in the crystal structure of bredigite has been studied in detail. Samples of a hypothetical solid-solution series with nominal composition Ca7-xSrxMg[SiO4]4 (x = 0, 2, …,7) have been prepared from sinter reactions in the temperature range between 1275 and 1325 °C and characterized using powder and single-crystal X-ray diffraction. Synthesis runs between x = 1 and x = 4 resulted in compounds with increasing Sr contents, for which single-crystal diffraction studies revealed the following Sr/(Sr + Ca) atomic ratios: 0.133,0.268, 0.409 and 0.559. They are isostructural to the pure calcium end-member (x = 0) and adopt the orthorhombic space group Pnnm. Evolution of the unit-cell parameters and cell volumes of the solid-solution series are defined by linear or nearly linear trends when plotted against the Sr/(Sr + Ca) atomic ratio. Replacement of calcium with strontium atoms on the different sites shows clear preferences for specific positions. For the experiment with x = 5, formation of bredigite-related single-crystals with Sr/(Sr + Ca) = 0.675 was observed. These samples, however, exhibited a halved c lattice parameter when compared with the corresponding value in the Pnnm structure, pointing to a compositionally induced phase transition somewhere in region between Sr/(Sr + Ca) = 0.559 and 0.675. The crystal structure of this new phase with composition Ca2.32Sr4.82Mg0.86[SiO4]4 was successfully determined in space group Pbam. Basic crystallographic data are as follows: a = 18.869(2) Å, b = 6.9445(8) Å, c = 5.5426(6) Å, V = 726.28(14) Å3, Z = 2. Structure determination was accomplished using charge flipping. Subsequent least-squares refinements resulted in a residual of R(|F|) = 2.70% for 822 independent reflections and 87 parameters. The Pbam- and the Pnnm-structures are in a group-subgroup relationship and topologically related. Both are based on so-called pinwheel-like MSi6O24 clusters consisting of a central magnesium-dominated [MO6]-octahedron as well as six attached [SiO4]-tetrahedra. The clusters are linked into chain-like elements running along [001]. Linkage between the chains is provided by mixed Sr/Ca positions with 6 to 10 oxygen ligands. Differences between the two phases result from changes in Sr-Ca site occupancies in combination with displacements of the atoms and tilts of the tetrahedra. The distortion pattern has been studied using group-theoretical methods including mode analysis. Notably, for the samples with x = 6 and x = 7—the latter corresponding to the hypothetical pure strontium end-member composition—no bredigite-type phases could be identified, indicating that there is an upper limit for the Sr-uptake.

Assessment of (La1-xSrx)MnO3±δ perovskites as oxygen- carrier materials in chemical-looping processes

(La1-xSrx)MnO3±δ (x = 0, 0.3, 0.5, 0.7, 1) perovskites prepared by co-precipitation of nitrate salts have been evaluated for a chemical looping process with fuel CH4 and CO2 or O2 as oxidants. The CH4 conversion is initially, at small deficiencies, high and is directed to total oxidation products. Subsequently, and as the available oxygen is being reduced, the CH4 conversion decreases and the main reaction products become CO and H2. The higher the Sr content the higher the CH4 conversion and the concentration of created oxygen vacancies. When the CO2 splitting reaction is used for the oxidation of the reduced solid, only a part of the created deficiency is used up, which decreases as Sr content increases. SrMnO3 oxidates CH4 almost exclusively to CO2 and, even when strongly oxygen deficient, does not react with CO2. The oxygen transport capacity with CO2 as oxidant decrease as Sr content increases and attain average values of approximately 1 wt% for LaMnO3 and 0 wt% for SrMnO3. Instead, the oxygen transport capacity with O2 as oxidant increase as Sr content increase and attain average values of approximately 1 wt% for LaMnO3 and 6.5 wt% for SrMnO3. The experimental results have been explained on basis of proposed defect models for (La1-xSrx)MnO3±δ according to which the Mn4+ → Mn3+ reduction creates oxygen vacancies the concentration of which increases with Sr content and do not participate in the CO2 splitting reaction. On the other hand, the subsequent Mn3+ → Mn2+ reduction creates oxygen vacancies the concentration of which increases as Sr content decreases and their elimination, upon oxidation, participates in the CO2 splitting reaction.

Effects on Microstructure and Ionic Conductivity of the Co-Doping with Strontium and Samarium of Ceria with Constant Oxygen Vacancy Concentration

Partially substituted cerias are attractive materials for use as electrolytes in intermediate-temperature solid oxide fuel cells (SOFCs). Ceria doped with Sm or Gd has been found to have high ionic conductivities. However, there is interest in whether doping with multiple elements could lead to materials with higher ionic conductivities. The present study looks at the effects of co-doping Sr and Sm in ceria. A compositional series, Ce0.8+xSm0.2−2xSrxO2−δ (with x = 0–0.08), designed to have a constant oxygen vacancy concentration, was successfully prepared using the citrate–nitrate complexation method. A solubility limit of ~5 cation% Sr was found to impact material structure and conductivity. For phase-pure materials, with increasing Sr content, sinterability increased slightly and intrinsic conductivity decreased roughly linearly. The grain boundaries of phase-pure materials showed only a very small blocking effect, linked to the high-purity synthesis method employed, while at high %Sr, they became more blocking due to the presence of a SrCeO3 impurity. Grain capacitances were found to be 50–60 pF and grain boundary capacitances, 5–50 nF. The variation in the bulk capacitance with Sr content was small, and the variation in grain boundary capacitance could be explained by the variation in grain size. Slight deviations at high %Sr were attributed to the SrCeO3 impurity. In summary, in the absence of deleterious effects due to poor microstructure or impurities, such as Si, there is no improvement in conductivity on co-doping with Sr and Sm.

Photovoltaic effect in Sr-doped BiFeO3 thin films

We present the structural and photovoltaic properties of Sr-doped polycrystalline BiFeO3 thin films prepared by sol-gel method. The result shows that all the BiFeO3 thin films doped with 0 ∼ 20 at.% Sr are rhombohedral distortion perovskite structure and the lattice constant decreases with increasing Sr amount. The addition of Sr can effectively inhibit the grain growth and enhance the uniformity of BFO films, besides, the band gap can be tuned availably as well. As Sr content increases, both the short circuit photocurrent density and the open circuit photovoltage begins to increase and then decrease, the maximum values achieved when 15 at.% Sr-doped. The power conversion efficiency accordingly increases first and then drops with Sr content increases, reaching 0.011% when the Sr-doped content is 15%, which results from the decreased band gap and oxygen vacancies. Highlights All the BiFeO3 thin films are rhombohedral distortion perovskite structure. Addition of Sr decreases the band gap of BFO thin films. The power conversion efficiency increases as Sr content increases. The maximal power conversion efficiency is 0.011%

Mechanical and corrosion properties of extruded Mg–Zr–Sr alloys for biodegradable implant applications

Magnesium (Mg) alloys have been extensively studied for their potential application as biodegradable implants. Zirconium (Zr) and strontium (Sr) are some of the few elements that are considered both biocompatible and biofunctional. In this study, extrusion was performed on Mg–Zr–Sr alloys in order to further improve their mechanical and corrosion properties and to clarify the effects of Zr and Sr additions on the materials properties of Mg alloys. Results indicated that in the extruded Mg–Zr–Sr alloys, the intermetallic Mg17Sr2 phases were accumulated at the grain boundaries, which triggered particle-stimulated nucleation, leading to reduced grain size and deformation twining. Increasing Zr content from 0.5 wt% to 3 wt% in the extruded Mg–xZr–1Sr (x = 0.5–3 wt%) alloys resulted in an increase of 25.4% in elongation and 5.9% in ultimate tensile strength. On the other hand, increasing Sr content from 1 wt% to 3 wt% in Mg–0.5Zr–xSr (x = 1–3 wt%) alloys improved only the tensile strength by 19.3% and the highest ultimate strength of 302 MPa was observed in extruded Mg–0.5Zr–3Sr. The tensile yield strength of extruded Mg–xZr–ySr (x = 0.035–3 wt%; y = 0.2–3 wt%) alloys ranged from 210 to 275 MPa. Compressive strength and strain of extruded Mg–Zr–Sr alloys ranged from 289 to 368 MPa and from 11.0% to 18.9%, respectively. The corrosion rates of the extruded Mg–Zr–Sr alloys ranged from 4.6 to 10.7 mm y−1 from potentiodynamic polarization tests. Overall, the extruded Mg–0.5Zr–3Sr showed optimum mechanical and corrosion properties and can be considered a promising biodegradable implant material.

Liquid-Phase Cyclohexene Oxidation with O2 over Spray-Flame-Synthesized La1-x Srx CoO3 Perovskite Nanoparticles.

La1−x Sr x CoO3 (x=0, 0.1, 0.2, 0.3, 0.4) nanoparticles were prepared by spray‐flame synthesis and applied in the liquid‐phase oxidation of cyclohexene with molecular O2 as oxidant under mild conditions. The catalysts were systematically characterized by state‐of‐the‐art techniques. With increasing Sr content, the concentration of surface oxygen vacancy defects increases, which is beneficial for cyclohexene oxidation, but the surface concentration of less active Co2+ was also increased. However, Co2+ cations have a superior activity towards peroxide decomposition, which also plays an important role in cyclohexene oxidation. A Sr doping of 20 at. % was found to be the optimum in terms of activity and product selectivity. The catalyst also showed excellent reusability over three catalytic runs; this can be attributed to its highly stable particle size and morphology. Kinetic investigations revealed first‐order reaction kinetics for temperatures between 60 and 100 °C and an apparent activation energy of 68 kJ mol−1 for cyclohexene oxidation. Moreover, the reaction was not affected by the applied O2 pressure in the range from 10 to 20 bar. In situ attenuated total reflection infrared spectroscopy was used to monitor the conversion of cyclohexene and the formation of reaction products including the key intermediate cyclohex‐2‐ene‐1‐hydroperoxide; spin trap electron paramagnetic resonance spectroscopy provided strong evidence for a radical reaction pathway by identifying the cyclohexenyl alkoxyl radical.

Tourmaline as an indicator for late-magmatic to hydrothermal fluid evolution of the Neoproterozoic Baotan tin deposit, South China

Neoproterozoic tin mineralization in South China was closely related to the highly-evolved peraluminous granites in the southeastern and western margins of the Yangtze Block. The Baotan (23 Mt @ 0.43 % Sn) is a largest tin deposit in the Jiuwandashan–Yuanbaoshan tin district, northern Guangxi, and the orebodies are mainly hosted in the Lower Neoproterozoic Sibao Group metasedimentary rocks and Early Neoproterozoic mafic rocks intruding the Sibao Group as well as the apex of the Pinying granitic pluton. This deposit shows widely important tourmaline alteration. Based on petrographic observations, four generations and eight occurrences of tourmalines are identified in the Baotan tin deposit. The first generation is late-magmatic tourmalines and occurs as disseminated (Tur1a) and tourmaline–quartz nodule (Tur1b) in the Pingying biotite granite. The pre-ore hydrothermal tourmalines (Tur2) occur as tourmaline–quartz veinlet in the biotite granite and can be subdivided into both the early Tur2a (second generation) and the late Tur2b (third generation) according to microscopic observation and BSE image. The fourth generation is syn-ore hydrothermal tourmalines (Tur3), and they occur as disseminated in mafic- (Tur3a) and metasedimentary (Tur3c) host rocks, or as vein-type tourmaline–quartz–cassiterite–chlorite ores in mafic- (Tur3b) and metasedimentary (Tur3d) host rocks.The different generations of tourmalines have both similarities and distinct differences in their compositions. Both the late-magmatic Tur1a and Tur1b show similar chemical compositions, with high Fe/(Fe + Mg) (0.73 ∼ 0.96, avg. 0.89) and Na/(Na + Ca) (0.87 ∼ 0.98, avg. 0.95) ratios, and Li, Zn, Ga, Nb and Ta contents, and belong to alkali group and schorl tourmaline. Based on mineral textural and compositional features and Mössbauer spectroscopic analyses, it can be speculated that the Tur1b in the tourmaline–quartz nodules and Tur1a are mainly sourced from a late-magmatic immiscible B–Na–Fe-rich hydrous melt in a reduced environment. The pre-ore Tur2a has similar SiO2, Al2O3, TiO2, Na2O, Li, Be, V, Sn and Zr contents, Fe/(Fe + Mg) and Na/(Na + Ca) ratios and total Al (apfu) with the late-magmatic Tur1a and Tur1b, indicating that it also originates from the Pingying granitic magma. The lower compatible elements (e.g., Fe, Mn, Co, Ni and Zn) and higher LILEs (e.g., Sr, Rb, Ba and Cs) in the pre-ore Tur2a and Tur2b suggest that these tourmalines precipitate from the early evolved magmatic hydrothermal fluids. The wide variations of Fe/(Fe + Mg) and Na/(Na + Ca) ranges, elevated Sr and V contents for the syn-ore Tu3a to Tur3d are attributed to the compositional differences of the host rocks and different degrees of interaction between the ore-forming fluids and the host rocks. The increasing Sr content from the late-magmatic Tur1 and pre-ore Tur2 to the syn-ore Tur3 suggests an involvement of the meteoric water. The Mössbauer spectral features of tourmalines from the late-magmatic Tur1 and syn-ore Tur3 confirm that they form under the different redox environments. The early fluids are relatively reduced magma-derived fluids. The addition of recycled meteoric water causes the fluids at the Tur3 stage to become relatively oxidized and also to be Sr-rich. The significant increase of Sn content from the late-magmatic Tur1 and pre-ore Tur2 to the syn-ore Tur3 also reflect a change from the early reduced fluid to the late oxidized fluid. The textural and compositional changes of Tur1 to Tur3 reflect the evolution of the ore-forming fluids. Fluid mixing between an acidic, reduced, Sn-rich magmatic hydrothermal fluid and cooler, oxidizing meteoric water and as well intensive water–rock reaction are considered as the main processes that cause extensive cassiterite precipitation.

Electrocaloric temperature changes in epitaxial Ba1−xSrxTiO3 films

The basic aim of our study is to investigate the correlation between structural parameters and the electrocaloric effect in lead-free epitaxial Ba1−xSrxTiO3 (BSTO) based thin film architectures. Therefore, BSTO thin films with Sr contents of x = 0 to x = 0.3 were grown on SrRuO3 buffered SrTiO3 single crystalline substrates by pulsed laser deposition. Structural characterization verified an epitaxial growth for all Sr contents with an additional tetragonal distortion compared to bulk material. Temperature and frequency-dependent measurements of dielectric properties revealed increased permittivity values for thicker films with broad maxima indicating a diffuse phase transition. The temperature of maximum permittivity decreases with increasing Sr content, whereas polarization measurements indicate a relaxor-like behaviour in particular above room temperature. Adiabatic temperature changes were determined with the indirect method resulting in |ΔT| values of up to 2.9 K for a 680 nm thick BSTO layer with x = 0.3 at an applied electric field of 750 kV cm−1.

Charge doping effects on magnetic properties of single-crystal La1−xSrx(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 (0≤x≤0.5) high-entropy perovskite oxides

The influence of hole doping on magnetic properties is mapped for the compositionally complex high-entropy oxide ${AB\mathrm{O}}_{3}$ perovskite ${\mathrm{La}}_{1\text{\ensuremath{-}}x}{\mathrm{Sr}}_{x}({\mathrm{Cr}}_{0.2}{\mathrm{Mn}}_{0.2}{\mathrm{Fe}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Ni}}_{0.2}){\mathrm{O}}_{3}$ $(0\ensuremath{\le}x\ensuremath{\le}0.5)$. It is found that aliovalent A-site substitution is a viable means to manipulate the magnetically active B-site sublattice. A series of single-crystal films are synthesized and show a general trend toward stronger ferromagnetic response and a shift in magnetic anisotropy as the Sr concentration increases. Magnetometry demonstrates complex and nonuniform responses similar to rigid and uncoupled composites at intermediate dopings. This behavior points to the presence of locally inhomogeneous magnetic phase competition, where ferromagnetic and antiferromagnetic magnetic contributions create a frustrated matrix containing uncompensated spins at the boundaries between these regions. The observations are discussed in the context of known responses to hole doping in the less complex ternary $\mathrm{La}{T}_{M}{\mathrm{O}}_{3}$ $({T}_{M}=\mathrm{Cr},\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni})$ oxides, and they are found to be different from a simple sum of the doped parents. The results are summarized in a preliminary magnetic phase diagram.

Effect of Sr doped the YFeO3 rare earth ortho-ferrite on structure, magnetic properties, and microwave absorption performance

Given the remarkable performances of rare earth multiferroic ortho-ferrites with magnetic optical and dielectric properties, the Y1-xSrxFeO3 (x = 0, 0.05, 0.1, 0.15) perovskite structure microwave absorbing ferrite materials was successfully synthesized by Sr2+ ions A-site doping based on sol-gel technology in this paper. The XRD of all samples was refined with FullProf software, which confirmed the formation of the orthogonal perovskite structure (SG: Pnma). The SEM and TEM results display the average particles size of the samples is distributed between 110 and 160 nm. The increase of Sr doping concentration leads to the increase of particles size, which may be related to the growth of preferred orientation and incomplete substitution. The XPS analysis shows that Fe3+ was accompanied by the presence of Fe2+ with the doping of Sr2+ ions and oxygen vacancies increased significantly. The samples change from weak ferromagnetic state to paramagnetic state with the increase of Sr content. The minimum reflection loss (RL) of the Y0.95Sr0.05FeO3 samples at 12.2 GHz reached −30.87 dB with thickness of 2.2 mm, where its effective absorption bandwidth (EAB, RL ≤ −10 dB) reached 2.4 GHz (11.3–13.7 GHz). Moreover, the EAB of the Y0.85Sr0.15FeO3 samples reached 2.64 GHz, and the corresponding range is 9.0–11.6 GHz (X-band).

First-principle study of Sr-doping effect in Nd1-x SrxNiO2

Sr-doped infinite layer nickelate was recently reported as a superconductor analogous to the layered copper oxides. Previous works revealed that two bands of mainly Nd 5d and Ni 3d characters cross the Fermi level in , whose behaviors under Sr doping are still unclear. We perform density functional theory calculations of supercell to systematically study the evolution of its electronic structure at different doping levels. Our results show that a relative shift occurs between the Nd 5d and Ni 3d bands with Sr doping, deviating significantly from the commonly adopted rigid-band approximation for studying doped compounds. In addition, the relative positions of O-p bands rise with respect to the Fermi level when the Sr concentration increases, resulting in a reduced charge transfer gap. We further estimate the hole concentrations on each band and Hall coefficients, and discuss their relevance to recent experiments.

Significantly enhanced room-temperature TCR and MR of La0.67K0.33-xSrxMnO3 ceramics by adjusting Sr content

Strontium (Sr)-doped La0.67K0.33-xSrxMnO3 (LKSMO) ceramics were obtained by traditional sol-gel method. With increasing Sr content, full width at half maximum (FWHM) for temperature-dependent resistivity curves increased, resistivity decreased, and peak resistivity temperature (TP) shifted toward high temperature. Microstructure of LKSMO ceramics was characterized by X-ray diffraction, scanning electron microscopy, X-ray photoemission spectroscopy, energy dispersive spectrometry, and Fourier transform infrared spectroscopy, etc. Connection between microstructure and properties of ceramics was studied by combining double exchange mechanism, Jahn-Teller effect, and phenomenological percolation model. Electrical transport mechanism of ferromagnetic metal (FMM) and paramagnetic insulator (PMI) regions was employed to explain temperature coefficient of resistivity (TCR) and magnetoresistance (MR) behaviors of as-obtained samples. Various scattering factors, effective mass of itinerant electron, and energy difference between FMM and PMI phases affected FWHM of ρ-T curves in Sr-doped LKSMO ceramics. In sum, these findings provide physical mechanism and experimental guidance for synthesis of high-TCR and MR polycrystalline ceramics at room temperature.

Investigation of La2-XSrxNi0.8Co0.2O4+d (0.0 ≤ x ≤ 0.2) Materials as Oxygen Electrodes for Solid Oxide Cells

Lanthanide Nickelate Ln2NiO4+δ (Ln = La, Pr or Nd) has drawn significant attention as an oxygen electrode for solid oxide cells (SOCs) due to its mixed ionic and electronic conductivity (MIEC) property. They exhibit very good electro-catalytic activity as well as high oxygen diffusion coefficient (D*) and surface exchange coefficient (k*) values [1]. Earlier reported results [2, 3] show that the partial substitution of Co2+ at B-site in La2Ni1−xCoxO4+δ (LNCO) lead to an enhancement in the transport and electrochemical properties of the material. The present study aims to investigate the effect of Sr2+ substitution at A-site in LNCO i.e. La2-xSrxNi0.8Co0.2O4+δ (LSNCO) in order to further investigate the structural, physico-chemical and electrochemical properties of the materials. Three different compositions i.e. LSNCO5, LSNCO10 and LSNCO20 were studied with x = 0.05, 0.10 and 0.20 respectively. First the materials were characterized by XRD, TGA and SEM. Then, the electrochemical characterizations were carried out by fabricating the symmetrical half-cells (8YSZ//GDC//LSNCO) as well as single cells (NiO-YSZ//8YSZ//GDC//LSNCO) using Electrochemical Impedance Spectroscopy (EIS) in 700 – 900 °C temperature range.All these materials exist with orthorhombic structure and a decrease in the cell volume as well as oxygen content (4+δ) is observed with Sr-substitution. The TGA analyses as a function of temperature reveals that these materials show reversible oxygen exchange and are always over-stoichiometric in 25-1000 °C temperature range. However, the loss of oxygen with increasing temperature is decreased with increasing Sr-content. The EIS data analysis of the symmetrical half-cells shows an increase in polarization resistance (Rp) with increasing the Sr-content. For instance, the Rp values of 0.047 and 0.88 Ω.cm2 are observed for LSNCO5 and LSNC20 respectively at 800 °C under air at OCV conditions. The EIS measurements under different partial pressure of oxygen reveals that four electrode processes (as fitted by 4R//CPE along with a series resistance Rs) are involved in the oxygen reduction/evolution mechanism. The observed electrode processes are; ionic transfer of oxide ion through electrode electrolyte interface (R1), charge transfer process (R2), adsorption/desorption of atomic species (R3) and molecular gas diffusion process (R4). The impedance data analysis under polarization (-0.2 to +0.2 V) reveals a decrease in the exchange current density with the increase of Sr-content. For instance, the io value is 0.040 A.cm-2 for LSNCO5 which is decreased to 0.003 A.cm-2 for LSNCO20 at 800 °C. Current-Voltage characteristics obtained from the single cell measurements show a decrease in the current density with increasing Sr-content.The detailed structural, physico-chemical and electrochemical properties of these materials will be presented and discuss in detail.

Structural, electrical, and magnetic properties of bulk Nd1-xSrxNiO3 (x = 0–0.3)

The single-phase Nd1-xSrxNiO3 (x = 0–0.3) polycrystalline samples were successfully synthesized, and the structure, resistivity, and magnetic property were investigated. All of samples were well indexed to an orthorhombic structure with Pnma space group. With increasing Sr concentration x, the lattice parameter a decreased, the lattice parameter b and c increased, the obtained unit-cell volume systematically increased due to the larger ion radius of Sr2+ than Nd3+. Sr partial substitution for Nd suppressed the metal-insulator and antiferromagnetic transition until x = 0.2. However, as the Sr content increased to 0.3, some abnormal phenomena were observed. The present work shows that physical properties are correlated with the evolution of perovskite tolerance factor in Nd1-xSrxNiO3 system.

Isothermal redox cycling of A‐ and B‐site substituted manganite‐based perovskites for CO2 conversion

AbstractConversion of CO2 into fuels and valuable chemicals can simultaneously address two major global issues: climate change and the increasing energy demand. Most of the previous studies regarding the thermochemical splitting of CO2 into CO have been carried out at elevated temperatures of up to 1400°C. In this study, under isothermal reaction conditions, manganite‐based perovskites are used to convert CO2 to CO in a thermogravimetric analyzer (TGA). In addition, the use of CH4 as a reducing agent has lowered the reaction temperatures (≤900°C). This study primarily focuses on the influence of A‐ and B‐site substitution on the redox performances of the manganite‐based perovskites. Based on the redox performances of four samples (CaMnO3, La0.5Ca0.5MnO3, La0.5Sr0.5MnO3, and Y0.5Sr0.5MnO3) with different A‐site compositions, La0.5Sr0.5MnO3 displays the best stability. Similarly, in the La1‐xSrxMnO3 perovskite family (x = 0.5, 0.25, and 0.10), the performance of La0.5Sr0.5MnO3 outshines the other two compositions. It is observed that as the Sr content increases, the activation energy for reduction decreases. Substituting Mn with Al and Fe in the B‐site of the La1‐xSrxMnO3 perovskite, did not enhance the performance. The perovskite materials investigated have shown decrease in activity after the first cycle, possibly due to sintering and powder densification. However, after the first cycle, the perovskites illustrated good stability for 10 redox cycles. In this paper, size variance and metal‐oxygen bond strength provide the best explanation for the trends observed during the reduction of the perovskites.

Enhancement of piezocatalytic activity at the ferro-paraelectric phase transition of Ba1-xSrxTiO3 nanopowders

Nanocrystalline Ba1-xSrxTiO3 (x = 0–0.3) powders were investigated with special interest in their piezocatalytic performance for degradation of organic pollutants. A great improvement of piezocatalytic activity was obtained for the compositions x = 0.2, in which the polarization shows a large change under external stress. Furthermore, with the increase of working environment temperature, the piezocatalytic performance of Ba0.8Sr0.2TiO3 increases firstly and then decreases, giving the best piezocatalytic performance near its Curie temperature. For a better understanding of the piezocatalytic mechanism, the dynamics of charge carriers transfer on solid/liquid interface was discussed with respect to activation energy and overpotential of catalyst. It was suggested that the enhanced piezocatalytic activity of Ba1-xSrxTiO3 comes from two aspects: (1) the overpotential provided by the piezoelectric effect decreases with the increasing Sr content, (2) the activation energy for the piezocatalytic reaction is greatly reduced at ferro-paraelectric phase transition. Combination of these two aspects gives rise to the nonlinear dependence of piezocatalytic activity on Sr content.

A Novel Fast-Setting Strontium-Containing Hydroxyapatite Bone Cement With a Simple Binary Powder System.

In recent years, strontium-substituted calcium phosphate bone cement (Sr-CPC) has attracted more and more attentions in the field of bone tissue repair due to its comprehensive advantages of both traditional CPC and Sr ions. In this study, a crucial Sr-containing α-Ca3–xSrx(PO4)2 salt has been synthesized using a simplified one-step method at lower synthesis temperature. A novel Sr-CPC has been developed based on the simple binary Sr-containing α-Ca3–xSrx(PO4)2/Ca4(PO4)2O cement powder. The physicochemical properties and hydration mechanism of this Sr-CPC at various Sr contents were intensively investigated. The setting product of this Sr-CPC after a set for 72 h is a single-phase Sr-containing hydroxyapatite, and its compressive strength slightly decreased and its setting time extended with the increase of Sr content. The hydration process included the initial formation of the medium product CaHPO4⋅2H2O (30 min∼1 h), the following complete hydration of Ca4(PO4)2O and the initially formed CaHPO4⋅2H2O (2∼6 h), and the final self-setting of α-Ca3–xSrx(PO4)2 (6 h∼). The compressive strength of Sr-CPC, which was closely related to the transformation rate of Sr-containing hydroxyapatite, tended to increase with the extension of hydration time. In addition, Sr-CPC possessed favorable cytocompatibility and the effect of Sr ions on cytocompatibility of Sr-CPC was not obvious at low Sr contents. The present study suggests α-Ca3–xSrx(PO4)2 is a kind of vital Sr-containing salt source which is useful to develop some novel Sr-containing biomaterials. In addition, the new Sr-containing cement system based on this simple binary α-Ca3–xSrx(PO4)2/Ca4(PO4)2O cement powder displayed an attractive clinical application potential in orthopedics.

Temperature-dependent broadband dielectric and ferroelectric properties of Ba(1−x)SrxTiO3 ceramics for energy storage capacitor applications

In the recent past, high energy storage and fast discharge capacitors have attracted considerable attention among the scientific community. In this context, a series of lead-free barium titanate-based ceramics with composition Ba(1−x)SrxTiO3 (x = 0.00–0.50) are synthesized using a solid-state reaction method to study their storage and discharge characteristics. The ceramics were sintered at different temperatures, and the ceramics sintered at 1300 °C/2 h showed the maximum relative density with tetragonal symmetry for x ≤ 0.3 and cubic symmetry for x ≥ 0.4. The uniformity in grain sizes for the ceramics improved with an increase in Sr content till x = 0.3. The pristine BaTiO3 (BTO) displayed three humps at − 60, 20, and 125 °C corresponding to rhombohedral-tetragonal-cubic phase transitions, respectively, and Tc shifted to room temperature with an increase in Sr concentration. The recoverable energy density for x = 0.3 goes around 29 mJ/cm3 with an energy efficiency of 56.1%. For pure BTO, the recoverable energy density obtained is 31.2 mJ/cm3 with an energy efficiency of 38% at an electric field of 15 kV/cm. The samples' diffusive coefficient is obtained by fitting the modified Curie–Weiss law and found that all the samples exhibited ferroelectric behavior as the γ value is around one. This study focused on the structural, microstructural, broadband dielectric spectroscopy and the ferroelectric properties of Sr-modified BaTiO3 ceramics. Such studies form an extensive evaluation of the properties of the synthesized ceramics for energy storage applications.

Enhanced aging and thermal shock performance of Mn1.95\u2212xCO0.21Ni0.84SrxO4 NTC ceramics

The Mn1.95−xCo0.21Ni0.84SrxO4 (MCNS) (0 ≼ x ≼ 0.15) based negative temperature coefficient (NTC) materials are prepared by co-precipitation method. The replacement of Mn by Sr plays a critical role in controlling the lattice parameter, relative density, microstructure, and electrical properties. The lattice parameter and relative density increase with the increase of Sr content. A small amount of Sr restrains the grain growth and increases the bulk density. Moreover, the room resistivity ρ25, material constant B25/50, activation energy Ea, and temperature coefficient α values of MCNS ceramics are influenced by the Sr content and ranged in 1535.0–2053.6 Ω·cm, 3654–3709 K, 0.3149–0.3197 eV, and (−4.173%)–(−4.111%), respectively. The X-ray photoelectron spectroscopy (XPS) results explain the transformation of MCNS ceramics from n- to p-type semiconductors. The conduction could arise from the hopping polaron between Mn3+/Mn4+ and Co2+/Co3+ in the octahedral sites. The impedance data analysis also discusses the conduction mechanism of the MCNS ceramic, whereas grain resistance dominates the whole resistance of the samples. Furthermore, the aging coefficient (ΔR/R) of MCNS ceramics is found to be < 0.2%, which indicates the stable distribution of cations in the spinel. Finally, the MCNS ceramics demonstrate excellent thermal durability with < 1.3% of resistance shift after100 thermal shock cycles.