Evaluation of Nd0.8−xSr0.2CaxCoO3−δ(x = 0, 0.05, 0.1, 0.15, 0.2) as a cathode material for intermediate-temperature solid oxide fuel cells

A series of Nd_0.8−xSr_0.2Ca_xCoO_3−δ(x = 0, 0.05, 0.1, 0.15, 0.2) cathode materials was synthesized by sol–gel method. The effect of Ca doping amount on the structure was examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermal expansion, and X-ray photoelectron spectroscopy (XPS). Electrochemical properties were evaluated for possible application in solid oxide fuel cell (SOFC) cathodes. Results showed that second phase NdCaCoO_4+δ is generated when the Ca doping amount is higher than 0.1. The increase in Ca limits the electronic compensation capacity of the material, resulting in a decrease in thermal expansion coefficient (TEC). With the increase of Ca content, the conductivity increases at first and then decreases, and the highest value of 443 S cm⁻¹ is at x = 0.1 and T = 800 °C. Nd_0.7Sr_0.2Ca_0.1CoO_3−δ exhibits the lowest area specific resistance of 0.0976 Ω cm² at 800 °C. The maximum power density of Nd_0.7Sr_0.2Ca_0.1CoO_3−δ at 800 °C is 409.31 mW cm⁻². The Ca-doped material maintains good electrochemical properties under the coefficient of thermal expansion (CTE) reduction and thus can be used as an intermediate-temperature SOFC (IT-SOFC) cathode.

The increase in Ca for Nd_0.8−xSr_0.2Ca_xCoO_3−δ limits the electronic compensation capacity, resulting in a decrease in CTE. The Ca-doped material maintains good electrochemical properties under CTE reduction and thus can be used as an IT-SOFC cathode.     

Single crystal growth and structure analysis of type-I (Na/Sr)–(Ga/Si) quaternary clathrates

Single crystals of (Na/Sr)–(Ga/Si) quaternary type-I clathrates, Na_8−ySr_yGa_xSi_46−x, were synthesized by evaporating Na from a mixture of Na–Sr–Ga–Si–Sn in a 6 : 0.5 : 1 : 2 : 1 molar ratio at 773 K for 12 h in an Ar atmosphere. Electron-probe microanalysis and single-crystal X-ray diffraction revealed that three crystals from the same product were Na_8−ySr_yGa_xSi_46−x with x and y values of 7.6, 2.96; 8.4, 3.80; and 9.1, 4.08. It was also shown that increasing the Sr and Ga contents increased the electrical resistivity of the crystal from 0.34 to 1.05 mΩ cm at 300 K.

Single crystals of (Na/Sr)–(Ga/Si) quaternary type-I clathrates, Na_8−ySr_yGa_xSi_46−x, were synthesized by evaporating Na from a mixture of Na–Sr–Ga–Si–Sn in a 6 : 0.5 : 1 : 2 : 1 molar ratio at 773 K for 12 h in an Ar atmosphere.     

Analysis based on scaling relations of critical behaviour at PM–FM phase transition and universal curve of magnetocaloric effect in selected Ag-doped manganites

The critical behaviour of Pr_0.5Sr_0.5−xAg_xMnO₃ (0 ≤ x ≤ 0.2) samples around the paramagnetic–ferromagnetic phase transition is studied based on isothermal magnetization measurements. The assessments based on Banerjee''s criteria reveal the samples undergoing a second-order magnetic phase transition. Various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis were used to determine the values of the ferromagnetic transition temperature T_C, as well as the critical exponents of β, γ and δ. The values of critical exponents, derived from the magnetization data using the Kouvel–Fisher method, are found to be (β = 0.43 ± 0.002, 0.363 ± 0.068 and 0.328 ± 0.012), (γ = 1.296 ± 0.007, 1.33 ± 0.0054 and 1.236 ± 0.012) for x = 0.0, 0.1 and 0.2, respectively. This implies that the Pr_0.5Sr_0.5−xAg_xMnO₃ with 0 ≤ x ≤ 0.2 systems does not belong to a single universality class and indicates that the presence of magnetic disorder in the system must be taken into account to fully describe the microscopic interaction of these manganites. With these values, magnetic-field dependences of magnetization at temperatures around T_C can be well described following a single equation of state for our samples. From magnetic entropy change (ΔS_M), it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott plot (MAP). We used the scaling hypotheses to scale the magnetic entropy change and heat capacity changes to a universal curve respectively for Pr_0.5Sr_0.5−xAg_xMnO₃ samples.

The universal curves of magnetic entropy changes and heat capacity changes for Pr_0.5Sr_0.5−xAg_xMnO₃ (0 ≤ x ≤ 0.2) are obtained by using the critical exponents.     

Structural exploration of AuxM− (M = Si,Ge, Sn; x = 9–12) clusters with a revised genetic algorithm

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters. The most interesting finding in the structural study of Au_xSi⁻ (x = 9–12) is the 3D (Au₉Si⁻ and Au₁₀Si⁻) → quasi-planar 2D (Au₁₁Si⁻ and Au₁₂Si⁻) structural evolution of the Si-doped clusters, which reflects the competition of Au–Au interactions (forming a 2D structure) and Au–Si interactions (forming a 3D structure). The Au_xM⁻ (x = 9–12; M = Ge, Sn) clusters have quasi-planar structures, which suggests a lower tendency of sp³ hybridization and a similarity of electronic structure for the Ge or Sn atom. Au₉Si⁻ and Au₁₀Si⁻ have a 3D structure, which can be viewed as being built from Au₈Si⁻ and Au₉Si⁻ with an extra Au atom bonded to a terminal gold atom, respectively. In contrast, the quasi-planar structures of Au_xM⁻ (x = 9–12; M = Ge, Sn) reflect the domination of the Au–Au interactions. Including the spin–orbit (SO) effects is very important to calculate the simulated spectrum (structural fingerprint information) in order to obtain quantitative agreement between theoretical and future experimental PES spectra.

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters.     

Influence of neodymium substitution on structural,magnetic and spectroscopic properties of Ni–Zn–Al nano-ferrites

Ni_0.6Zn_0.4Al_0.5Fe_1.5−xNd_xO₄ ferrite samples, with x = 0.00, 0.05, 0.075 and 0.1, were synthesized using the sol–gel method. The effects of Nd³⁺ doping on the structural, magnetic and spectroscopic properties were investigated. XRD Rietveld refinement carried out using the FULLPROF program shows that the Ni–Zn ferrite retains its pure single phase cubic structure with Fd$\bar{3}$m space group. An increase in lattice constant and porosity happens with increasing Nd³⁺ concentration. FTIR spectra present the two prominent absorption bands in the range of 400 to 600 cm⁻¹ which are the fingerprint region of all ferrites. The change in Raman modes in the synthesized ferrite system were observed with Nd³⁺ substitution. The magnetization curves show a typical transition, at the Curie temperature T_C, from a low temperature ferrimagnetic state to a high temperature paramagnetic state. The saturation magnetization, coercivity and remanence magnetization are found to be decreasing with increasing the Nd³⁺ concentration.

The incorporation of Nd³⁺ in the Ni–Zn–Al ferrite spinel causes an improvement in magnetic parameters. Spectroscopic properties were discussed based on FTIR and Raman measurements and proved the purity and good crystallization of the samples.     

Copper-doped lanthanum manganite La0.65Ce0.05Sr0.3Mn1−xCuxO3 influence on structural,magnetic and magnetocaloric effects

Bulk nanocrystalline samples of La_0.65Ce_0.05Sr_0.3Mn_1−xCu_xO₃ (0 ≤ x ≤ 0.15) manganites are prepared by the sol–gel based Pechini method. The effect of the substitution for Mn with Cu upon the structural and magnetic properties has been investigated by means of X-ray diffraction (XRD), Raman spectroscopy and dc magnetization measurements. The structural parameters obtained using Rietveld refinement of XRD data showed perovskite structures with rhombohedral (R$\bar{3}$c) symmetry without any detectable impurity phase. Raman spectra at room temperature reveal a gradual change in phonon modes with increasing copper concentration. The analysis of the crystallographic data suggested a strong correlation between structure and magnetism, for instance a relationship between a distortion of the MnO₆ octahedron and the reduction in the Curie temperature, T_c. A paramagnetic to ferromagnetic phase transition at T_C is observed. The experimental results confirm that Mn-site substitution with Cu destroys the Mn³⁺–O²⁻–Mn⁴⁺ bridges and weakens the double exchange (DE) interaction between Mn³⁺ and Mn⁴⁺ ions, which shows an obvious suppression of the FM interaction in the La_0.65Ce_0.05Sr_0.3Mn_1−xCu_xO₃ matrix. The maximum magnetic entropy change −ΔS^max_M is found to decrease with increasing Cu content from 4.43 J kg⁻¹ K⁻¹ for x = 0 to 3.03 J kg⁻¹ K⁻¹ for x = 0.15 upon a 5 T applied field change.

Bulk nanocrystalline samples of La_0.65Ce_0.05Sr_0.3Mn_1−xCu_xO₃ (0 ≤ x ≤ 0.15) manganites are prepared by the sol–gel based Pechini method.     

Observation of enhanced magnetic entropy change near room temperature in Sr-site deficient La0.67Sr0.33MnO3 manganite

The effect of Sr-site deficiency on the structural, magnetic and magnetic entropy change of La_0.67Sr_0.33−yMnO_3−δ (y = 0.18 and 0.27) compounds was investigated. The compounds were prepared by the conventional solid-state route and powder X-ray diffraction technique along with Rietveld refinement was carried out to confirm the structure and phase purity. Lattice parameters and unit cell volumes are found to increase with the increase in Sr-deficiency due to the electrostatic repulsion from the neighbouring oxygen ions. A mixed valence state of Mn²⁺/Mn³⁺/Mn⁴⁺ was confirmed using the X-ray photoelectron spectroscopy technique and it was observed that the change of state from Mn³⁺ + Mn³⁺ pairs to Mn²⁺ + Mn⁴⁺ pairs is different for both the studied compounds. A second order ferromagnetic–paramagnetic transition with an enhancement in magnetization in comparison to the pristine compound (La_0.67Sr_0.33MnO₃) was observed due to multiple double exchange interactions. The La_0.67Sr_0.15□_0.18MnO_3−δ compound exhibits a magnetic entropy change (ΔS_M) of 4.61 J kg⁻¹ K⁻¹ at 310 K, and the La_0.67Sr_0.06□_0.27MnO_3−δ compound exhibits a ΔS_M of 4.11 J kg⁻¹ K⁻¹ at 276 K under a field of 50 kOe. In our previous work, we reported a large value of ΔS_M but at higher temperatures, around 350 K. However, in the present case, we have achieved a near room temperature (310 K) MCE with a significant ΔS_M value (4.61 J kg⁻¹ K⁻¹) which is larger than that reported for numerous perovskite manganites. Thus, the studied material could be a potential candidate for room temperature magnetic refrigeration applications.

The effect of Sr-site deficiency on the structural, magnetic and magnetic entropy change of La_0.67Sr_0.33−yMnO_3−δ (y = 0.18 and 0.27) compounds was investigated.     

Effects of A-site composition of perovskite (Sr1−xBaxZrO3) oxides on H atom adsorption,migration, and reaction

Hydrogen (H) atomic migration over a metal oxide is an important surface process in various catalytic reactions. Control of the interaction between H atoms and the oxide surfaces is therefore important for better catalytic performance. For this investigation, we evaluated the adsorption energies of the H atoms over perovskite-type oxides (Sr_1−xBa_xZrO₃; 0.00 ≤ x ≤ 0.50) using DFT (Density Functional Theory) calculations, then clarified the effects of cation-substitution in the A-site of perovskite oxides on H atom adsorption, migration, and reaction. Results indicated local distortion at the oxide surface as a key factor governing H atom adsorption. Subtle Ba²⁺ substitution for Sr²⁺ sites provoked local distortion at the Sr_1−xBa_xZrO₃ oxide surface, which led to a decrement in the H atom adsorption energy. Furthermore, the effect of Sr²⁺/Ba²⁺ ratio on the H atoms'' reactivities was examined experimentally using a catalytic reaction, which was promoted by activated surface H atoms. Results show that the surface H atoms activated by the substitution of Sr²⁺ sites with a small amount of Ba²⁺ (x = 0.125) contributed to enhancement of ammonia synthesis rate in an electric field, which showed good agreement with predictions made using DFT calculations.

H atom adsorption over perovskite (Sr_1−xBa_xZrO₃) was governed by local lattice distortion, which can be tuned by the A-site cation-doping ratio.     

The effects of microstructure,Nb content and secondary Ruddlesden–Popper phase on thermoelectric properties in perovskite CaMn1−xNbxO3 (x = 0–0.10) thin films

CaMn_1−xNb_xO₃ (x = 0, 0.5, 0.6, 0.7 and 0.10) thin films have been grown by a two-step sputtering/annealing method. First, rock-salt-structured (Ca,Mn_1−x,Nb_x)O thin films were deposited on 1$\bar{1}$00 sapphire using reactive RF magnetron co-sputtering from elemental targets of Ca, Mn and Nb. The CaMn_1−xNb_xO₃ films were then obtained by thermally induced phase transformation from rock-salt-structured (Ca,Mn_1−xNb_x)O to orthorhombic during post-deposition annealing at 700 °C for 3 h in oxygen flow. The X-ray diffraction patterns of pure CaMnO₃ showed mixed orientation, while Nb-containing films were epitaxially grown in [101] out of-plane-direction. Scanning transmission electron microscopy showed a Ruddlesden–Popper (R–P) secondary phase in the films, which results in reduction of the electrical and thermal conductivity of CaMn_1−xNb_xO₃. The electrical resistivity and Seebeck coefficient of the pure CaMnO₃ film were measured to 2.7 Ω cm and −270 μV K⁻¹ at room temperature, respectively. The electrical resistivity and Seebeck coefficient were reduced by alloying with Nb and was measured to 0.09 Ω cm and −145 μV K⁻¹ for x = 0.05. Yielding a power factor of 21.5 μW K⁻² m⁻¹ near room temperature, nearly eight times higher than for pure CaMnO₃ (2.8 μW K⁻² m⁻¹). The power factors for alloyed samples are low compared to other studies on phase-pure material. This is due to high electrical resistivity originating from the secondary R–P phase. The thermal conductivity of the CaMn_1−xNb_xO₃ films is low for all samples and is the lowest for x = 0.07 and 0.10, determined to 1.6 W m⁻¹ K⁻¹. The low thermal conductivity is attributed to grain boundary scattering and the secondary R–P phase.

Reduction of thermal conductivity of sputtered CaMn_1−xNb_xO₃ thin films by secondary Ruddlesden–Popper phase and grain size optimization.     

Modification of low temperature magnetic interactions in Dy1−xEuxMnO3

Solid solutions of rare earth ion (Eu³⁺) substituted DyMnO₃, Dy_1−xEu_xMnO₃ (x = 0.0–1.0) have been synthesized by ceramic method. Powder X-ray diffraction revealed single phase nature of the compounds with orthorhombic structure. Contributions from the atomic vibrations to the observation of Raman bands have been established and assigned to symmetry stretching and anti symmetry stretching, bending and tilting modes. Raman band frequencies of tilting, asymmetric stretching and bending modes were found to decrease with increasing europium concentration showing softening. Transport studies revealed that all the compounds show semiconducting nature. While the end compounds display hopping process for electrical conduction, all the substituted compounds showed activated type of conduction, and activated energy was found to reduce with increase in x. Molar susceptibility of the substituted compounds for x = 0.1, 0.3 and 0.5 revealed an antiferromagnetic transition corresponding to Mn ions. The fitted Curie–Weiss temperatures also suggested the existence of antiferromagnetic interactions in all the materials. The magnetic field dependent magnetization at various temperatures revealed paramagnetic nature down to 8 K below which hysteresis loops are observed. The presence of strong ferromagnetic correlations between Dy and Mn spins through apical oxygen ions results in the large coercive fields. For temperatures above the antiferromagnetic temperature of manganese ions (39 K) M–H curves show almost straight lines implying the absence of ferromagnetic interactions in the compounds. Different magnetic transitions: from high temperature paramagnetic state to intermediate temperature antiferromagnetic state to low temperature ferromagnetic states are observed in the M–H data.

Solid solutions of rare earth ion (Eu³⁺) substituted DyMnO₃, Dy_1−xEu_xMnO₃ (x = 0.0–1.0) have been synthesized by ceramic method.     

Co(OH)F nanorods@KxMnO2 nanosheet core–shell structured arrays for pseudocapacitor application

In this work, Co(OH)F nanorods@K_xMnO₂ nanosheet core–shell nanostructure was assembled on Ni foam by a facile hydrothermal method and incorporated with an electrodeposition process. Benefiting from their core–shell nanostructure and heterogeneous nanocomposites, the arrays present high areal capacitance up to 1046 mF cm⁻² at 1 mA cm⁻² and display a remarkable specific capacitance retention of 118% after 3000 cycles. When the current density increases to 10 mA cm⁻², the capacitance is 821 mF cm⁻² displaying a good rate capability. The excellent electrochemical properties allow them to be used as a promising electrode material for pseudocapacitors and display wide application potential in the field of electrochemical capacitors.

In this work, Co(OH)F nanorods@K_xMnO₂ nanosheet core–shell nanostructure was assembled on Ni foam by a facile hydrothermal method and incorporated with an electrodeposition process.     

Unexpected visible light driven photocatalytic activity without cocatalysts and sacrificial reagents from a (GaN)1–x(ZnO)x solid solution synthesized at high pressure over the entire composition range

Optical and photocatalytic properties were determined for the solid solution series (GaN)_1–x(ZnO)_x synthesized at high pressure over the entire compositional range (x = 0.07 to 0.9). We report for the first time photocatalytic H₂ evolution activity from water for (GaN)_1–x(ZnO)_x without cocatalysts, pH modifiers and sacrificial reagents. Syntheses were carried out by reacting GaN and ZnO in appropriate amounts at temperatures ranging from 1150 to 1200 °C, and at a pressure of 1 GPa. ZnGa₂O₄ was observed as a second phase, with the amount decreasing from 12.8 wt% at x = 0.07 to ∼0.5 wt% at x = 0.9. The smallest band gap of 2.65 eV and the largest average photocatalytic H₂ evolution rate of 2.31 μmol h⁻¹ were observed at x = 0.51. Samples with x = 0.07, 0.24 and 0.76 have band gaps of 2.89 eV, 2.78 eV and 2.83 eV, and average hydrogen evolution rates of 1.8 μmol h⁻¹, 0.55 μmol h⁻¹ and 0.48 μmol h⁻¹, respectively. The sample with x = 0.9 has a band gap of 2.82 eV, but did not evolve hydrogen. An extended photocatalytic test showed considerable reduction of activity over 20 hours.

(GaN)_1–x(ZnO)_x synthesized at high pressure produces H₂ in the presence of visible light without any cocatalysts or sacrificial reagents.     

Influence of Ni content on structural,magnetocaloric and electrical properties in manganite La0.6Ba0.2Sr0.2Mn1−xNixO3 (0 ≤ x ≤ 0.1) type perovskites

We present a detailed study on the physical properties of La_0.6Ba_0.2Sr_0.2Mn_1−xNi_xO₃ samples (x = 0.00, 0.05 and 0.1). The ceramics were fabricated using the sol–gel route. Structural refinement, employing the Rietveld method, disclosed a rhombohedral R$\bar{3}$c phase. The magnetization vs. temperature plots show a paramagnetic–ferromagnetic (PM–FM) transition phase at the T_C (Curie temperature), which decreases from 354 K to 301 K. From the Arrott diagrams M²vs. μ₀H/M, we can conclude the phase transition is of the second order. Based on measurements of the isothermal magnetization around T_C, the magnetocaloric effects (MCEs) have been calculated. The entropy maximum change (−ΔS_M) values are 7.40 J kg⁻¹ K⁻¹, 5.6 J kg⁻¹ K⁻¹ and 4.48 J kg⁻¹ K⁻¹, whereas the relative cooling power (RCP) values are 232 J kg⁻¹, 230 J kg⁻¹ and 156 J kg⁻¹ for x = 0.00, 0.05 and 0.10, respectively, under an external field (μ₀H) of 5 T. Through these results, the La_0.6Ba_0.2Sr_0.2Mn_1−xNi_xO₃ (0 ≤ x ≤ 0.1) samples can be suggested for use in magnetic refrigeration technology above room temperature. The electrical resistivity (ρ) vs. temperature plots exhibit a transition from metallic behavior to semiconductor behavior in the vicinity of T_M–SC. The adiabatic small polaron hopping (ASPH) model is applied in the PM-semiconducting part (T > T_MS). Throughout the temperature range, ρ is adjusted by the percolation model. This model is based on the phase segregation of FM-metal clusters and PM-insulating regions.

We present a detailed study on the physical properties of La_0.6Ba_0.2Sr_0.2Mn_1−xNi_xO₃ samples (x = 0.00, 0.05 and 0.1).     

Improved electrical transport properties of polycrystalline La0.8(Ca0.12Sr0.08)MnO3 ceramics by Ag2O doping

Polycrystalline La_0.8(Ca_0.12Sr_0.08)MnO₃:mol%Ag_x (LCSMO:Ag_x, x = 0, 0.1, 0.2, 0.3 and 0.4) ceramics were synthesized by the sol–gel technique. Structural, electrical and magnetic properties of the LCSMO:Ag_x ceramics were investigated in detail. X-ray diffraction (XRD) data analyses revealed that all the samples were crystalized in the orthorhombic structure with space group of Pnma. With the increase in Ag doping (x), the grain sizes of the LCSMO:Ag_x samples increased and the amount of grain boundaries (GBs) decreased accordingly. At the same time, the Mn–O bond distance and the Mn–O–Mn bond angles changed correspondingly, leading to the slight increase in the lattice constants (a, b and c) and slight expansion of cell volume (V). For the LCSMO:Ag_x sample with x = 0.3, the optimal values of temperature coefficient of resistivity (TCR) and magnetoresistance (MR) reached 16.22% K⁻¹ (265.1 K) and 42.07% K⁻¹ (270.48 K), respectively. In addition, the fitting analysis of ρ–T curves showed that the experimental data were consistent with the theoretical calculation data. In the T < T_MI (metal-insulator transition temperature) region, the electrical conduction mechanism of LCSMO:Ag_x was clarified by electron-magnon, electron–electron and electron-phonon scattering. In the T > T_MI region, the resistivity data were interpreted by using the adiabatic small-polaron hopping model. Furthermore, in the entire temperature range, the phenomenological equation called the percolation model was used to explain the resistivity data and the phase-separation mechanism of ferromagnetic metallic (FM) and paramagnetic insulating (PI) phases. All the obtained results indicated that the improvement in the electrical properties of the LCSMO:Ag_x samples was attributed to the doping of Ag, which changed the A-site (La, Ca and Sr ions) average ion radius, the Mn–O–Mn bond angles and the Mn–O bond distance. In addition, the grain size increased, which led to improvement in the Mn⁴⁺ ion concentration and the GBs connectivity in the LCSMO:Ag_x polycrystalline ceramics.

Polycrystalline La_0.8(Ca_0.12Sr_0.08)MnO₃:mol%Ag_x (LCSMO:Ag_x, x = 0, 0.1, 0.2, 0.3 and 0.4) ceramics were synthesized by the sol–gel technique.     

Strain-effected physical properties of ferromagnetic insulating La0.88Sr0.12MnO3 thin films

The functional perovskite La_1−xSr_xMnO₃ (LSMO) possesses various exotic phases owing to competing physical parameters and internal degrees of freedom. In particular, the nature of the ferromagnetic insulating phase (FMI) has not been adequately explored, resulting in a limited understanding of the relationship between crystal structure and magnetism. To investigate this structure–property relationship, epitaxial La_0.88Sr_0.12MnO₃ thin films were grown on two different substrates, (001) SrTiO₃ and (001) (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7, by pulsed laser deposition. Element-specific and surface-sensitive techniques were applied in conjunction with bulk magnetometry to investigate the inextricable link between the structures and magnetic properties of the films and the effects of tuning the strain. The results unambiguously demonstrate that structure–property relationship of a FMI LSMO tuned by strain has a crucial role for manipulating the properties in the FMI regime.

The functional perovskite La_1−xSr_xMnO₃ (LSMO) possesses various exotic phases owing to competing physical parameters and internal degrees of freedom.     

Structural,optical and dielectric properties of tellurium-based double perovskite Sr2Ni1−xZnxTeO6

In this paper, Sr₂Ni_1−xZn_xTeO₆ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) double perovskite compounds were synthesised by the conventional solid-state method, and the structural, optical and dielectric properties were investigated. The Rietveld refinement of X-ray diffraction data shows that all compounds were crystallised in monoclinic symmetry with the I2/m space group. Morphological scanning electron microscopy reported that the grain sizes decreased as the dopant increased. The UV-vis diffuse reflectance spectroscopy conducted for all samples found that the optical band gap energy, E_g, increased from 3.71 eV to 4.14 eV. The dielectric permittivity ε′ values increased for the highest Zn-doped composition, Sr₂Ni_0.2Zn_0.8TeO₆, being ∼1000 and ∼60 in the low- and high-frequency range, respectively. All samples exhibited low dielectric loss (tan δ ≤ 0.20) in the range of 10⁴–10⁵ Hz frequency. Impedance measurement revealed that grain resistance decreased with enhancement in Zn content in the Sr₂NiTeO₆ crystal lattice.

In this paper, Sr₂Ni_1−xZn_xTeO₆ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) double perovskite compounds were synthesised by the conventional solid-state method, and the structural, optical and dielectric properties were investigated.     

A-site deficient semiconductor electrolyte Sr1−xCoxFeO3−δ for low-temperature (450–550 °C) solid oxide fuel cells

Fast ionic conduction at low operating temperatures is a key factor for the high electrochemical performance of solid oxide fuel cells (SOFCs). Here an A-site deficient semiconductor electrolyte Sr_1−xCo_xFeO_3−δ is proposed for low-temperature solid oxide fuel cells (LT-SOFCs). A fuel cell with a structure of Ni/NCAL-Sr_0.7Co_0.3FeO_3−δ–NCAL/Ni reached a promising performance of 771 mW cm⁻² at 550 °C. Moreover, appropriate doping of cobalt at the A-site resulted in enhanced charge carrier transportation yielding an ionic conductivity of >0.1 S cm⁻¹ at 550 °C. A high OCV of 1.05 V confirmed that neither short-circuiting nor power loss occurred during the operation of the prepared SOFC device. A modified composition of Sr_0.5Co_0.5FeO_3−δ and Sr_0.3Co_0.7FeO_3−δ also reached good fuel cell performance of 542 and 345 mW cm⁻², respectively. The energy bandgap analysis confirmed optimal cobalt doping into the A-site of the prepared perovskite structure improved the charge transportation effect. Moreover, XPS spectra showed how the Co-doping into the A-site enhanced O-vacancies, which improve the transport of oxide ions. The present work shows that Sr_0.7Co_0.3FeO_3−δ is a promising electrolyte for LT-SOFCs. Its performance can be boosted with Co-doping to tune the energy band structure.

Fast ionic conduction at low operating temperatures is a key factor for the high electrochemical performance of solid oxide fuel cells (SOFCs).     

Improvement of electric field-induced strain and energy storage density properties in lead-free BNKT-based ceramics modified by BFT doping

In this research, the effects of Ba(Fe_0.5Ta_0.5)O₃ (BFT) additive on the phase evolution, the dielectric, ferroelectric, piezoelectric, electric field-induced strain responses, and energy storage density of the Bi_0.5(Na_0.80K_0.20)_0.5TiO₃–0.03(Ba_0.70Sr_0.03)TiO₃ (BNKT–0.03BSrT) ceramics have been systematically investigated. The ceramics have been prepared by a solid-state reaction method accompanied by two calcination steps. X-ray diffraction indicates that all ceramics coexist between rhombohedral and tetragonal phases, where the tetragonal phase becomes dominant at higher BFT contents. The addition of BFT also promotes the diffuse phase transition in this system. A significant enhancement of electric field-induced strain response (S_max = 0.42% and = 840 pm V⁻¹) is noted for the x = 0.01 ceramic. Furthermore, the giant electrostrictive coefficient (Q₃₃ = 0.0404 m⁴ C⁻²) with a giant normalized electrostrictive coefficient (Q₃₃/E = 8.08 × 10⁻⁹ m⁵ C⁻² V⁻¹) are also observed for this composition (x = 0.01). In addition, the x = 0.03 ceramic shows good energy storage properties, i.e. it has a high energy storage density (W = 0.65 J cm⁻³ @ 120 °C) with very high normalized storage energy density (W/E = 0.13 μC mm⁻²), and good energy storage efficiency (η = 90.4% @ 120 °C). Overall, these results indicate that these ceramics are one of the promising candidate piezoelectric materials for further development for actuator and high electric power pulse energy storage applications.

The effects of Ba(Fe_0.5Ta_0.5)O₃ additive on phase, dielectric, ferroelectric, piezoelectric, electric field-induced strain, and energy storage density of the Bi_0.5(Na_0.80K_0.20)_0.5TiO₃–0.03(Ba_0.70Sr_0.03)TiO₃ ceramics have been investigated.     

3D printed interdigitated supercapacitor using reduced graphene oxide-MnOx/Mn3O4 based electrodes

In this study hybrid nanocomposites (HNCs) based on manganese oxides (MnO_x/Mn₃O₄) and reduced graphene oxide (rGO) are synthesized as active electrodes for energy storage devices. Comprehensive structural characterizations demonstrate that the active material is composed of MnO_x/Mn₃O₄ nanorods and nanoparticles embedded in rGO nanosheets. The development of such novel structures is facilitated by the extreme synthesis conditions (high temperatures and pressures) of the liquid-confined plasma plume present in the Laser Ablation Synthesis in Solution (LASiS) technique. Specifically, functional characterizations demonstrate that the performance of the active layer is highly correlated with the MnO_x/Mn₃O₄ to rGO ratio and the morphology of MnO_x/Mn₃O₄ nanostructures in HNCs. To that end, active layer inks comprising HNC samples prepared under optimal laser ablation time windows, when interfaced with a percolated conductive network of electronic grade graphene and carbon nanofibers (CNFs) mixture, indicate superior supercapacitance for functional electrodes fabricated via sequential inkjet printing of the substrate, current collector layer, active material layer, and gel polymer electrolyte layer. Electrochemical characterizations unequivocally reveal that the electrode with the LASiS synthesized MnO_x/Mn₃O₄–rGO composite exhibits significantly higher specific capacitance compared to the ones produced with commercially available Mn₃O₄–graphene NCs. Moreover, the galvanostatic charge–discharge (GCD) experiments with the LASiS synthesized HNCs show a significantly larger charge storage capacity (325 F g⁻¹) in comparison to NCs synthesized with commercially available Mn₃O₄–graphene (189 F g⁻¹). Overall, this study has paved the way for use of LASiS-based synthesized functional material in combination with additive manufacturing techniques for all-printed electronics with superior performance.

LASiS-based HNCs of nanostructured MnO_x/Mn₃O₄.     

Site-selective doping effect,phase separation,and structure evolution in 1:1:1 triple-cation B-site ordered perovskites Ca4−xSrxGaNbO8

Oxygen-deficient perovskites are a family of important materials that may exhibit oxide ionic conductivities. We attempted to introduce oxygen-vacancy disordering in perovskite Ca₄GaNbO₈ (Ca₄-type) by substituting Ca²⁺ with larger Sr²⁺. Sr²⁺-to-Ca²⁺ substitution did not lead to oxygen-vacancy ordering–disordering transition but an interesting Ca₄-to-Sr₄ type structure transition. Rietveld refinements revealed that the two-type structures exhibit similar oxygen-vacancy ordering and identical 1:1:1 triple-cation B-site ordering. Close inspection of the two-type structures revealed the subtle structure difference lies in the orientations of GaO₄ tetrahedra, which is the origin of the formation of the narrow two-phase region (0.3 ≤ x < 0.65) in Ca_4−xSr_xGaNbO₈. More importantly, the A- and B-site cavities with large differences in size for both structures resulted in a site-selective doping behaviour for Sr²⁺ in Ca_4−xSr_xGaNbO₈. These structural changes found in Ca_4−xSr_xGaNbO₈ will provide a broad route approaching new oxygen-deficient phases with oxide ionic conductivities.

Sr²⁺-to-Ca²⁺ substitution resulted in new 1:1:1 triple-cation B-site ordered perovskites, where the structure difference lies in the orientations of GaO₄-tetrahedra.