Structural Phase Stability Research Articles

Progress and Prospects in Symmetrical Solid Oxide Fuel Cells with Two Identical Electrodes

Symmetrical solid oxide fuel cells (SOFCs) have attracted increasing attention due to their potential for improved thermomechanical compatibility of the electrolyte and the electrodes, reduced fabrication cost, and enhanced immunity to coking and sulfur poisoning. While the electrode materials of symmetrical SOFCs are initially limited to those with stable phase structures under both reducing and oxidizing atmospheres, many novel electrode materials are currently being developed and investigated that may undergo a beneficial phase transition or reduction in a reducing atmosphere, although the same material may be used initially for the construction of both anode and cathode. Here, the advances made in the development of electrode materials and structures for symmetrical SOFCs are summarized, including single‐phase electrodes, multi‐phase (composite) electrodes, and those that are reducible upon exposure to a reducing atmosphere. The electrical conductivity, thermomechanical properties, and redox behavior of these electrode materials, together with their performance and stability in different SOFCs, are discussed and analyzed. The problems associated with different types of symmetrical SOFCs are outlined and the materials that show promise as symmetrical electrodes are highlighted, offering critical insights and useful guidelines for knowledge‐based rational design of better electrodes for commercially viable symmetrical SOFCs.

The relaxor behavior and dielectric temperature stability of 0.85BaTiO3–0.15Na0.5Bi0.5TiO3–xLiBa2Nb5O15 ceramics

0.85BaTiO3–0.15Na0.5Bi0.5TiO3–xLiBa2Nb5O15 (x=0, 0.003, 0.004, 0.005, 0.006) ceramics were prepared by the solid state method. The effects of LiBa2Nb5O15 content on the phase structure, relaxor behavior and dielectric temperature stability were investigated. With the increasing amount of LBN content, the values of tetragonality decreased and the ceramics had a tendency to be a cubic structure. For x≥0.003, dielectric measurements indicated relaxor behavior. The increasing amount of LBN content improved the dielectric temperature stability of 0.85BT–0.15BT ceramics effectively. It was found that the variation ΔC/C25°C was around ±15% for x=0.004 from 25°C to 150°C. This ceramics with a small temperature coefficient of capacitance and a low dielectric loss in a wide temperature range showed a great potential application for high-temperature applications.

Hydrogen permeability and stability of BaCe0.85Tb0.05Zr0.1O3−δ asymmetric membranes

A mixed proton and electron conductor BaCe0.85Tb0.05Zr0.1O3−δ (BCTZ) has been developed by the partial substitution of Ce with Zr in BaCe0.95Tb0.05O3−δ (BCT) to improve the phase structure stability of BCT. The BCTZ membranes with asymmetric structure have been successfully prepared and evaluated for hydrogen separation. A stable hydrogen permeation flux has been found over 370h operation. The improved stability recommends BCTZ for a potential application in the hydrogen separation.

Comparison of microstructure and hydrogen permeability of perovskite type ACe0.9Y0.1O3-δ (A is Sr, Ba, La, and BaSr) membranes

Hydrogen permeation through ACe0.9Y0.1O3-δ (A = Sr, Ba, La, and BaSr) perovskite-type membranes was studied at high temperatures. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were used to characterize phase structure and phase stability of the membrane. The XRD results showed the formation of a single phase of perovskite structure with configuration orthorhombic for BCY and BSCY membranes. Based on the TGA results, it was found that the phase stability increases in the order of LCY < SCY < BCY < BSCY. The microstructure examinations of the synthesized membranes studied by scanning electron microscopy (SEM) showed the formation of a dense-like grained microstructure membrane, which is applicable for permeation tests. The partial substitution of Sr by Ba in SCY membrane led to an increase of more than 90% in the hydrogen permeation flux; a high permeation flux of about 1.6 ml min−1 cm−2 was achieved at 900 °C with a partial pressure of 0.6 atm. The activation energies for hydrogen permeation increased in the order of BaSr < Ba < Sr < La.

Effect of the B-Site Composition on the Oxygen Permeability and the CO2 Stability of Pr0.6Sr0.4CoxFe1–xO3−δ (0.0 ≤ x ≤ 1.0) Membranes

Dense single-phase perovskite-type Pr0.6Sr0.4CoxFe1–xO3−δ (0.0 ≤ x ≤ 1.0) membranes (0.6 mm thick) were synthesized via EDTA–citric acid complexing route. Subsequently, the effect of various B-site Co/Fe compositions on oxygen permeability, temperature-dependent CO2 stability, microstructure, and electrical properties of the membranes were studied. The crystal structures and the high-temperature phase stability of the perovskite structure in a CO2-containing atmosphere were analyzed using X-ray diffraction. The highest oxygen permeation flux was observed for Pr0.6Sr0.4CoO3−δ with 1.57 cm3(STP) min–1 cm–2 and 1.37 cm3(STP) min–1 cm–2 at 1000 °C under air/He and air/CO2 gradients, respectively. Furthermore, the effect of CO2 as the sweep gas on the temperature-dependent oxygen permeability and stability of the membranes was studied. Basically, the membranes with lower Co contents were found to be less susceptible to CO2 exposure and their microstructures were less affected by CO2. The partial oxidation of methane (POM) to syngas was successfully performed for more than 80 h at 950 °C using a PSCF membrane with a Co content of x = 0.2. The POM reaction shows an average CH4 conversion rate of >98% and a CO selectivity of >95%.

DFT and two-dimensional correlation analysis methods for evaluating the Pu3+–Pu4+ electronic transition of plutonium-doped zircon

Understanding how plutonium (Pu) doping affects the crystalline zircon structure is very important for risk management. However, so far, there have been only a very limited number of reports of the quantitative simulation of the effects of the Pu charge and concentration on the phase transition. In this study, we used density functional theory (DFT), virtual crystal approximation (VCA), and two-dimensional correlation analysis (2D-CA) techniques to calculate the origins of the structural and electronic transitions of Zr1−cPucSiO4 over a wide range of Pu doping concentrations (c=0–10mol%). The calculations indicated that the low-angular-momentum Pu-fxy-shell electron excites an inner-shell O-2s2 orbital to create an oxygen defect (VO-s) below c=2.8mol%. This oxygen defect then captures a low-angular-momentum Zr-5p65s2 electron to form an sp hybrid orbital, which exhibits a stable phase structure. When c>2.8mol%, each accumulated VO-p defect captures a high-angular-momentum Zr-4dz electron and two Si-pz electrons to create delocalized Si4+→Si2+ charge disproportionation. Therefore, we suggest that the optimal amount of Pu cannot exceed 7.5mol% because of the formation of a mixture of ZrO8 polyhedral and SiO4 tetrahedral phases with the orientation (10-1). This study offers new perspective on the development of highly stable zircon-based solid solution materials.

Electronic structure, structural phase stability, optical and thermoelectric properties of Sr2AlM'O6 (M' = Nb and Ta) from first principle calculations

First principle calculations are performed to investigate the electronic structure, structural phase stability, optical properties and thermoelectric properties of double perovskite oxide semiconductors namely Sr2AlM'O6 (M' = Nb and Ta) in the cubic symmetry using WIEN2k. In order to study the ground state properties of these compounds, the total energies are calculated as a function of reduced volumes and fitted with Brich Murnaghan equation. The estimated ground state parameters are comparable with the available experimental data. Calculations of electronic band structure on these compounds have been carried out using generalized gradient approximations and modified Becke-Johanson potential (TB-mBJ). The calculated band gap for Sr2AlNbO6 and Sr2AlTaO6 with GGA and TB-mBJ reveal that these compounds exhibit semiconducting behavior with a direct band gap. To explore the optical transitions in these compounds, the real and imaginary parts of the dielectric function, refractive index, extinction coefficient, reflectivity, optical absorption coefficient, real part of optical conductivity and the energy-loss function are calculated at ambient conditions and analyzed both with GGA and TB-mBJ potentials. Investigations of the thermoelectric properties of these compounds have been carried out by the calculations of transport coefficients based on Boltzmann transport theory in order to analyze the variation of Seebeck's coefficient at different temperatures for various carrier concentrations based on the electronic structure near the valence band maxima.

Doping effect on electronic band structure and magnetic properties of MFeAs(M = Li, Na)

In this paper, detailed examination of the doping effect on electronic band structure, magnetic properties of nonmagnetic (NM) and striped antiferromagnetic (S-AFM) phases of MFeAs ( M = Li , Na ) compounds were carried out using ab initio method. The crystal structure of these compounds is a well known tetragonal structure. Self-consistent calculations were performed by plane wave pseudo potential, density functional based method using PWSCF-Quantum Espresso code. To study the structural phase stability, the total energies of these compounds were calculated as a function of reduced volumes and fitted with Brich Murnaghan equation. 3d valence elements like Mn , Co induce strong local magnetic moments on doping. However, Cu substitution weakens the average local moments. The 3d elements on doping at Fe site directly affect the electrons correlations in the Fe – As layer.

Effect of vehicles' changing lanes in the Biham–Middleton–Levine traffic flow model

This paper studies the effect of vehicles' changing lanes in the original Biham–Middleton–Levine traffic flow model. According to local vehicular information, the dynamics allows for vehicles changing their rows or columns. Simulation results show that the intermediate stable phase identified in the original model can be observed and maintained in a wide range of the vehicle density in our model. Some new kinds of space configurations have been first presented and discussed. One also notes that the geometric structure of such intermediate stable phases is highly regular for square and rectangular aspect rations.

Structural, anisotropic elastic and thermal properties of MB (M=Ti, Zr and Hf) monoborides

To better clarify and understand the applications of the transition-metal monoborides, first principles calculations were performed to investigate the structural properties, phase stability, elastic properties, and thermal conductivity of NaCl-type, FeB-type and CrB-type MB (M=Ti, Zr and Hf) monoborides. The results of equilibrium structures, cohesive energies and formation enthalpies are in good agreement with available experimental and other theoretical data. Based on the elastic constants, the polycrystalline elastic properties including bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio, are obtained by Voigt–Reuss–Hill approximation. All these monoborides are mechanically stable. The elastic anisotropies of monoborides are investigated via the various anisotropic indexes and the 3D surface constructions, and the order of elastic anisotropy is NaCl-type>CrB-type>FeB-type. By using the Cahill’s model, the minimum thermal conductivities of these monoborides were also investigated, and the results indicate that the minimum thermal conductivities show a dependence on direction.

Oxygen permeability and CO2-tolerance of Ce0.8Gd0.2O2−δ-LnBaCo2O5+δ dual-phase membranes

A series of oxygen permeable dual-phase composite oxides 60 wt% Ce0.8Gd0.2O2–δ-40 wt% LnBaCo2O5+δ (CGO-LBCO, Ln = La, Pr, Nd, Sm, Gd and Y) were synthesized through a sol-gel route and effects of the Ln3+ cations on their phase structure, oxygen permeability and chemical stability against CO2 were investigated systemically by XRD, SEM, TG-DSC and oxygen permeation experiments. XRD patterns reveal that the larger Ln3+ cations (La3+, Pr3+ and Nd3+) successfully stabilized the double-layered perovskite structure of sintered LBCO, while the smaller ones (Sm3+, Gd3+, and Y3+) resulted in the partial decomposition of LBCO with some impurities formed. CGO-PBCO yields the highest oxygen permeation flux, reaching 2.8×10−7 mol·s−1·cm−2 at 925 °C with 1 mm thickness under air/He gradient. The TG-DSC profiles in 20 mol% CO2/N2 and oxygen permeability experiments with CO2 as sweep gas show that CGO-YBCO demonstrates the best chemical stability against CO2, possibly due to its minimum basicity. The stable oxygen permeation flux of CGO-YBCO under CO2 atmosphere reveals its potential application in the oxy-fuel combustion route for CO2 capture.

The relationship between bond ionicity, lattice energy, coefficient of thermal expansion and microwave dielectric properties of Nd(Nb(1-x)Sb(x))O4 ceramics.

The crystalline structure refinement, chemical bond ionicity, lattice energy and coefficient of thermal expansion were carried out for Nd(Nb(1-x)Sb(x))O4 ceramics with a monoclinic fergusonite structure to investigate the correlations between the crystalline structure, phase stability, bond ionicity, lattice energy, coefficient of thermal expansion, and microwave dielectric properties. The bond ionicity, lattice energy, and coefficient of thermal expansion of Nd(Nb(1-x)Sb(x))O4 ceramics were calculated using a semiempirical method based on the complex bond theory. The phase structure stability varied with the lattice energy which was resulted by the substitution constant of Sb(5+). With the increasing of the Sb(5+) contents, the decrease of Nb/Sb-O bond ionicity was observed, which could be contributed to the electric polarization. The ε(r) had a close relationship with the Nb/Sb-O bond ionicity. The increase of the Q×f and |τ(f)| values could be attributed to the lattice energy and the coefficient of thermal expansion. The microwave dielectric properties of Nd(Nb(1-x)Sb(x))O4 ceramics with the monoclinic fergusonite structure were strongly dependent on the chemical bond ionicity, lattice energy and coefficient of thermal expansion.

The correlations among bond ionicity, lattice energy and microwave dielectric properties of (Nd(1-x)La(x))NbO4 ceramics.

(Nd1-xLax)NbO4 ceramics were prepared via a conventional solid-state reaction route and the correlations among bond ionicity, lattice energy, phase stability and microwave dielectric properties were investigated. The diffraction patterns showed that the (Nd1-xLax)NbO4 ceramics possessed a monoclinic fergusonite structure. The chemical bond ionicity, bond covalency and lattice energy were calculated using the empirical method. The phase structure stability varied with the lattice energy which resulted due to the substitution content of La(3+) ions. With the increase of La(3+) ion contents, the decrease of Nd/La-O bond ionicity was observed, which could be attributed to the electric polarization. εr has a close relationship with the Nd/La-O bond covalency. The increase of the Q × f values and τf values could be attributed to the change in the lattice energy. The microwave dielectric properties of (Nd1-xLax)NbO4 ceramics with a monoclinic fergusonite structure were strongly dependent on the chemical bond ionicity, bond covalency and lattice energy.

ChemInform Abstract: Electronic Structure, Elastic Anisotropy, Thermal Conductivity and Optical Properties of Calcium Apatite Ca5(PO4)3X (X: F, Cl or Br).

AbstractThe structural properties, phase stability, electronic structure, elastic properties, and optical properties of Ca5(PO3)4F (FA), Ca5(PO3)4Cl (ClA) and Ca5(PO3)4Br (BrA) are studied by DFT calculations with the generalized gradient approximation.

Structural phase transition, elastic and thermal properties of boron arsenide: Pressure-induced effects

The phase transition of boron arsenide (BAs) has been studied by means of a density-functional theory calculation. Features such as structural phase stability, elastic properties, sound velocity, Debye temperature and melting temperature have been obtained at zero and high pressures. The transition pressure (Pt) of the material of interest from zinc-blende to NaCl phase has been determined and found to agree well with experiment. At pressures lower than Pt the zinc-blende phase is found to be thermodynamically and mechanically more stable than the NaCl phase. The mechanical behavior has been studied in terms of ductility and brittleness by means of different methods and found to differ only on the exact border between the two types of mechanical behaviors. The behavior of the longitudinal sound velocity under pressure indicated the softening of its corresponding phonons.

DFT investigation on CO sensing characteristics of hexagonal and orthorhombic WO3 nanostructures

The structural stability, electronic properties and adsorption characteristics of CO on pure and Mo substituted WO3 are optimized and studied using density functional theory with B3LYP/LanL2DZ basis set. Structural stability of both hexagonal and orthorhombic phases of WO3 nanostructures is discussed with calculated energy. The electronic properties of WO3 are analyzed in terms of HOMO–LUMO gap, ionization potential and electron affinity. Dipole moment and point group of pure and Mo substituted WO3 nanostructures are also studied. The significance of the present work is to give clear insights into the adsorption characteristics of CO on orthorhombic and hexagonal WO3 nanostructures. The adsorption characteristics of CO can be enhanced with the substitution of Mo impurity in the WO3 base material. The adsorption characteristics CO on WO3 are reported in terms of adsorbed energy, average energy gap variation, Mulliken population analysis and density of states spectrum. The best adsorption sites of CO onto hexagonal and orthorhombic phases of WO3 nanostructures are identified and reported.

First-principles investigation of the structural stability and electronic properties of Pd doped monoclinic Cu6Sn5 intermetallic compounds

Abstract Tri-layer Au/Pd/Ni(P) films have been widely used as surface finish over the Cu pads in high-end packaging applications. It was found that a thin (Cu,Pd)6Sn5 IMC layer was beneficial in effective reducing inter-diffusion between a Cu substrate and a solder, and therefore the growth of the IMC layer and the EM (electromigration) processes. In this study, the structural properties and phase stability of monoclinic Cu6Sn5-based structures with Pd substitutions were studied by using the first-principles method. The (Cu,Pd)6Sn5 structure with the 4e site substituted by Pd has the lowest heat of formation and is the most stable among (Cu,Pd)6Sn5 structures. Hybridization of Pd-d and Sn-p states is a dominant factor for stability improvement. Moreover, Pd atoms concentration corresponding to the most stable structure of (Cu,Pd)6Sn5 was found to be 1.69 %, which is consistent with the experimental results.

Electronic structure and crystal phase stability of palladium hydrides

The results of electronic structure calculations for a variety of palladium hydrides are presented. The calculations are based on density functional theory and used different local and semilocal approximations. The thermodynamic stability of all structures as well as the electronic and chemical bonding properties are addressed. For the monohydride, taking into account the zero-point energy is important to identify the octahedral Pd-H arrangement with its larger voids and, hence, softer hydrogen vibrational modes as favorable over the tetrahedral arrangement as found in the zincblende and wurtzite structures. Stabilization of the rocksalt structure is due to strong bonding of the 4d and 1s orbitals, which form a characteristic split-off band separated from the main d-band group. Increased filling of the formerly pure d states of the metal causes strong reduction of the density of states at the Fermi energy, which undermines possible long-range ferromagnetic order otherwise favored by strong magnetovolume effects. For the dihydride, octahedral Pd-H arrangement as realized, e.g., in the pyrite structure turns out to be unstable against tetrahedral arrangement as found in the fluorite structure. Yet, from both heat of formation and chemical bonding considerations, the dihydride turns out to be less favorable than the monohydride. Finally, the vacancy ordered defect phase Pd3H4 follows the general trend of favoring the octahedral arrangement of the rocksalt structure for Pd:H ratios less or equal to one.

Structural stability and thermodynamics of CrN magnetic phases fromab initiocalculations and experiment

The dynamical and thermodynamic phase stabilities of the stoichiometric compound CrN including different structural and magnetic configurations are comprehensively investigated using a first-principles density-functional-theory (DFT) plus U approach in conjunction with experimental measurements of the thermal expansion. Comparing DFT and DFT+U results with experimental data reveals that the treatment of electron correlations using methods beyond standard DFT is crucial. The non-magnetic face-centered cubic B1-CrN phase is both, elastically and dynamically unstable, even under high pressure, while CrN phases with non-zero local magnetic moments are predicted to be dynamically stable within the framework of the DFT+U scheme. Furthermore, the impact of different treatments for the exchange-correlation (xc)-functional is investigated by carrying out all computations employing the local density approximation and generalized gradient approximation. To address finite-temperature properties, both, magnetic and vibrational contributions to the free energy have been computed employing our recently developed spin-space averaging method. The calculated phase transition temperature between low-temperature antiferromagnetic and high-temperature paramagnetic (PM) CrN variants is in excellent agreement with experimental values and reveals the strong impact of the choice of the xc-functional. The temperature-dependent linear thermal expansion coefficient of CrN is experimentally determined by the wafer curvature method from a reactive magnetron sputter deposited single-phase B1-CrN thin film with dense film morphology. A good agreement is found between experimental and ab initio calculated linear thermal expansion coefficients of PM B1-CrN. Other thermodynamic properties, such as the specific heat capacity, have been computed as well and compared to previous experimental data.

Theoretical Study of Structural, Elastic Properties and Phase Transitions of Cu&lt;sub&gt;2&lt;/sub&gt;ZnSnS&lt;sub&gt;4&lt;/sub&gt;

The total energy, the electronic properties, phase transitions, and elastic properties of Cu2ZnSnS4(CZTS) in the three structures are investigated by first-principles calculations based on density functional theory. Results show that the total energies of stannite (ST) and primitive-mixed CuAu (PMCA) structures are higher than that of kesterite-type (KS), and the KS is the ground state structure. Relationships between enthalpy and pressure of the KS, ST and PMCA structure of CZTS are also investigated at 0 K, since the pressure can have profound impacts on the electronic structure, possible phase transitions and structure stability. And results also show that KS structure is always the most stable; ST is the second; and the PMCA structure is the most unstable; phase transitions of three structures could not occur in high pressure. The high ratios of shear modulus to bulk modulus (G/B) indicate that CZTS compounds in three types have ductile behaviors. The Poisson ratios for the three structures are from 0.27 to 0.31, which again proves that all structures of CZTS have better plasticity. The results can increase more hints about further research directions, and these effects can play an important role in future experimental preparation technology and theoretical work of CZTS materials.