High-temperature X-ray Diffraction Analysis Research Articles

In Situ High‐Temperature X‐ray Diffraction Study of the Thermal Stability of the Co0.22Cr0.23Fe0.29Ni0.20Ti0.06 High‐Entropy Alloy Produced by High‐Energy Ball Milling

In this report, the effect of Ti addition on the phase evolution of a near‐equiatomic CoCrFeNi‐based high‐entropy alloy (HEA) during annealing in a vacuum is described. The HEA Co0.22Cr0.23Fe0.29Ni0.20Ti0.06 is synthesized through mechanical alloying of constituent elements for a milling duration of 1 h. The as‐prepared HEA is a two‐phase alloy containing body centred cubic (BCC) precipitates in an face centred cubic (FCC) matrix. The thermal stability of the HEA is experimentally studied by subjecting it to 5 h of isothermal annealing at 600, 800, and 1000 °C, using high‐temperature X‐ray diffraction (HTXRD). The HTXRD analysis indicates a noteworthy reduction in the BCC phase content after isothermal annealing at 600 °C. The formation of intermetallic σ‐phases does not occur. At temperatures of 800 and 1000 °C, the alloy matrix exhibits a single‐phase FCC solid solution. At 800 and 1000 °C, HEA undergoes partial oxidation, resulting in the formation of oxide phases on the surface and within the particles of the powder in the form of nanoscale inclusions. This oxidation depletes the metal matrix with titanium, leading to a change in the composition of the HEA. The alloy transforms into a CoCrFeNi solid solution containing 3–4 at% Ti. Thus, the alloy matrix phase remains a stable FCC solid solution.

High-Temperature Oxidation and Phase Stability of AlCrCoFeNi High Entropy Alloy: Insights from In Situ HT-XRD and Thermodynamic Calculations.

The high-temperature oxidation behaviour and phase stability of equi-atomic high entropy AlCrCoFeNi alloy (HEA) were studied using in situ high-temperature X-ray diffraction (HTXRD) combined with ThermoCalc thermodynamic calculation. HTXRD analyses reveal the formation of B2, BCC, Sigma and FCC, phases at different temperatures, with significant phase transitions observed at intermediate temperatures from 600 °C-100 °C. ThermoCalc predicted phase diagram closely matched with in situ HTXRD findings highlighting minor differences in phase transformation temperature. ThermoCalc predictions of oxides provide insights into the formation of stable oxide phases, predominantly spinel-type oxides, at high p(O2), while a lower volume of halite was predicted, and minor increase observed with increasing temperature. The oxidation behaviour was strongly dependent on the environment, with the vacuum condition favouring the formation of a thin, Al2O3 protective layer, while in atmospheric conditions a thick, double-layered oxide scale of Al2O3 and Cr2O3 formed. The formation of oxide scale was determined by selective oxidation of Al and Cr, as further confirmed by EDX analysis. The formation of thick oxide in air environment resulted in a thick layer of Al-depleted FFC phase. This comprehensive study explains the high-temperature phase stability and time-temperature-dependent oxidation mechanisms of AlCrCoFeNi HEA. The interplay between surface phase transformation beneath oxide scale and oxides is also detailed herein, contributing to further development and optimisation of HEA for high temperature applications.

Study of amorphous alumina coatings for next-generation nuclear reactors: High-temperature in-situ and post-mortem Raman spectroscopy and X-ray diffraction

The present work focuses on the investigation of the thermal stability and structural integrity of amorphous alumina coatings intended for use as protective coatings on cladding tubes in Generation IV nuclear reactors, specifically in the Lead-cooled Fast Reactor (LFR) type. High-temperature Raman spectroscopy and high-temperature X-ray diffraction analyses were carried out up to 1050 °C on a 5 µm coating deposited by the pulsed laser deposition (PLD) technique on a 316L steel substrate. The experiments involved the in-situ examination of structural changes in the material under increasing temperature, along with ex-situ Raman imaging of the surface and cross-section of the coating after thermal treatments of different lengths. As it was expected, the presence of α-alumina was detected with the addition of other polymorphs, γ- and θ-Al2O3, found in the material after longer high-temperature exposure. The use of two structural analysis methods and two lasers excitation wavelengths with Raman spectroscopy allowed us to detect all the mentioned phases despite different mode activity. Alumina analysis was based on the emission spectra, while substrate oxidation products were identified through the structural bands. The experiments depicted a dependence of the phase composition of oxidation products and alumina’s degree of crystallization on the length of the treatment. Nevertheless, the observed structural changes did not occur rapidly, and the coating’s integrity remained intact. Moreover, oxidation signs occurred locally at temperatures exceeding the LFR reactor’s working temperature, confirming the material’s great potential as a protective coating in the operational conditions of LFR nuclear reactors.

Oxoborates of the ludwigite group: Natural and mineral-like compounds as prospective materials

Research subject. Natural oxoborates of the ludwigite group, including azoproite, ludwigite, and vonsenite. Their empirical formulas based on five oxygen atoms have the following form: azoproite (Mg1.81Fe2+0.19)∑2.00(Fe3+0.36Ti0.26Mg0.26Al0.12)∑1.00 O2(BO3), ludwigite (Mg1.69Fe2+0.30Mn2+0.01)Σ2.00(Fe3+0.90Al0.07Mg0.02Sn0.01)Σ1.00O2(BO3) and vonsenite (Fe2+1.86Mg0.13)∑1.99 (Fe3+0.92Mn2+0.05Sn4+0.02Al0.02)∑1.01O2(BO3). Aim. To establish the relationship between the composition, crystal structure, and thermal behavior (293–1373 K) of the minerals. Materials and methods. Ludwigite was collected at the Iten’yurginskoe tin skarn deposit; vonsenite was collected at the Titovskoe magnesium-skarn boron deposit; azoproite was collected at magnesian skarns of the Tazheran alkaline massif. The methods of single crystal X-ray diffraction, energy dispersive X-ray spectroscopy, high-temperature X-ray diffraction, Mössbauer spectroscopy, and thermal analysis were used. Results. Low-charge cations (Fe2+, Fe2.5+, Mg2+) tend to occupy the M(1)–M(3) sites, and high-charge cations (Fe3+, Al3+, Ti4+, Sn4+) generally occupy the M(4) site. Azoproite is characterized by the highest melting temperature Tm > 1650 K. Due to the low Fe2+ content, azoproite does not undergo solid-phase decomposition across the investigated temperature range. The melting point of ludwigite exceeds 1582 K, which is due to the high Mg content; as a result of the Fe2+ → Fe3+ oxidation, it gradually decomposes with the formation of hematite, warwickite, and magnetite. The temperatures of oxidation and solid-phase decomposition in the Fe2+-rich vonsenite are approximately 100 K lower than those in ludwigite. The melting point of vonsenite is 1571 K. All the minerals are characterized by a weak degree of thermal expansion anisotropy. The main contribution to the thermal expansion anisotropy is due to the preferred orientation of the [BO3]3– triangles. Conclusions. The thermal properties of the oxoborates depend on their chemical composition. It was established that Tm increases with an increase in the Mg and Ti4+ content, and decreases with an increase in the Fe2+ content. The Fe2+ → Fe3+ oxidation is observed when the FeO component in the minerals exceeds 10 wt %, which leads to the solid-phase decomposition starting at temperatures of about 500–600 K. The values of the 293KαV volume thermal expansion of ludwigite and azoproite are comparable, while the largest values were observed for vonsenite. This is associated with the largest average bond lengths, primarily those of <Fe2+–O>6.

Structural-phase transformations of 12% chromium ferritic-martensitic steel EP-823

The features of phase transformations of 12 % chromium ferritic-martensitic steel EP-823 under heating and cooling conditions in the temperature range from 30 to 1100 ℃ were studied by the methods of high-temperature X-ray diffraction analysis (XRD) in situ and differential scanning calorimetry (DSC). According to XRD in situ data, upon heating, the temperatures of the beginning and end of the (α → γ) transformation of ferrite (martensite – austenite) are Ac1 ≈ 880 °C, Ac3 ≈ 1000 °C, respectively. Upon cooling, a diffusion (γ → α) transformation occurs with critical points – Аr1 ≈ 860°С (beginning temperature) and Аr3 ≈ 840 °С (end temperature). According to DSC data, during heating, the critical points of the (α → γ) transformation are Ac1 ≈ 840 °C and Ac3 ≈ 900 °C. During cooling, a martensitic (γ → α) transformation is realized with critical points of the beginning of Ms = 344 ℃ and the end of Mf = 212 ℃ of this transformation. The XRD in situ analysis revealed no precipitation of carbide phases under heating and cooling conditions of steel EP-823. Position of the critical points of phase transformations depends on the research method (XRD in situ or DSC), which is determined by the difference in effective (taking into account the time for shooting in the XRD method) heating-cooling rate. The effect of elemental composition on the position of critical points of phase transformations and the formation of structural-phase states of ferritic-martensitic steels is discussed. It is shown that the increased content of ferrite-stabilizing elements (Cr, Mo, Nb) in composition of EP-823 steel, compared with other steels of the same class, expands the region of existence of the ferrite phase, which can contribute to an increase in the temperature of Ac1 .

Electrophoretic Deposition and Characterization of Thin-Film Membranes Li7La3Zr2O12.

In the presented study, films from tetragonal Li7La3Zr2O12 were obtained by electrophoretic deposition (EPD) for the first time. To obtain a continuous and homogeneous coating on Ni and Ti substrates, iodine was added to the Li7La3Zr2O12 suspension. The EPD regime was developed to carry out the stable process of deposition. The influence of annealing temperature on phase composition, microstructure, and conductivity of membranes obtained was studied. It was established that the phase transition from tetragonal to low-temperature cubic modification of solid electrolyte was observed after its heat treatment at 400 °C. This phase transition was also confirmed by high-temperature X-ray diffraction analysis of Li7La3Zr2O12 powder. Increasing the annealing temperature leads to the formation of additional phases in the form of fibers and their growth from 32 (dried film) to 104 μm (annealed at 500 °C). The formation of this phase occurred due to the chemical reaction of Li7La3Zr2O12 films obtained by electrophoretic deposition with air components during heat treatment. The total conductivity of Li7La3Zr2O12 films obtained has values of ~10-10 and ~10-7 S cm-1 at 100 and 200 °C, respectively. The method of EPD can be used to obtain solid electrolyte membranes based on Li7La3Zr2O12 for all-solid-state batteries.

The quaternary fluoride LiNaCa2Al2F12 in aluminum electrolytes: synthesis, structure, thermal stability

The quaternary fluoride phase LiNaCa2Al2F12 has been obtained by quenching a melt consisting of a stoichiometric mixture of the corresponding simple fluorides at a temperature of 850 ​°C in air. Previously, the phase has been observed by authors as unknown impurity in cooled samples of aluminum production electrolyte. It has been established that the phase does not form solid solution regions with the initial components. The crystal structure has been solved by X-ray powder diffraction technique: a ​= ​5.053(2) Å, c ​= ​17.546(7) Å, V ​= ​448.02 ​Å3, Z ​= ​2, SG. P-421c. Calcium cations and isolated octahedral [AlF6]3- anions form mixed layers, which are separated by flat nets of lithium and sodium cations. Lithium cations occupy positions with a tetrahedral environment of fluorine atoms. The short interatomic distance Li⋯F 1.89 ​Å makes the position unavailable for other cations. Thermal decomposition has been studied by high-temperature X-ray diffraction and thermal analysis. The LiNaCa2Al2F12 phase is stable up to 600 ​°C in air, and then decomposes in the solid state. The final products of decomposition are fluorite and aluminum oxides and aluminates. It has been concluded that the phase was metastable and it was the result of rapid cooling of the melt. The stability at low temperatures has explained by high potential barrier for transformation. The formation of LiNaCa2Al2F12 indicates the presence in the melt with fluorine deficiency of isolated [AlF6]3- anions, the concentration of which increases due to the existence of coordinated Li+-[AlF6]3- pairs in the melt. It is suggested that the presence of [AlF6]3- anions in acidic electrolyte melts ensures the dissolution of alumina.

Energetic Systematics of Metal-Organic Frameworks: A Case Study of Al(III)-Trimesate MOF Isomers.

Thermal stability and thermodynamic properties of aluminum(III)-1,3,5-benzenetricarboxylate (Al-BTC) metal-organic frameworks (MOFs), including MIL-96, MIL-100, and MIL-110, have been investigated through a suite of calorimetric and X-ray techniques. In situ high-temperature X-ray diffraction (HT-XRD) and thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC) revealed that these MOFs undergo thermal amorphization prior to ligand combustion. Thermal stabilities of Al-BTC MOFs follow the increasing order MIL-110 < MIL-96 < MIL-100, based on estimated amorphization temperatures. Their thermodynamic stabilities were directly measured by high-temperature drop combustion calorimetry. Normalized (per mole of Al) enthalpies of formation (ΔH*f) of MIL-96, MIL-100, and MIL-110 from Al2O3, H3BTC, and H2O (only Al2O3 and H3BTC for MIL-100) were determined to be -56.9 ± 13.7, -36.2 ± 17.9, and 62.8 ± 11.6 kJ/mol·Al, respectively. Our results demonstrate that MIL-96 and MIL-100 are thermodynamically favorable, while MIL-110 is metastable, in agreement with thermal and hydrothermal stability trends. The enthalpic preferences of MIL-96 and MIL-100 may be attributed to their shared trinuclear μ3-oxo-bridged (Al3(μ3-O)) secondary building units (SBUs) promoting stabilization of Al polyhedra by the ligands within these frameworks, in comparison to the sterically strained Al8 octamer cluster cores formed in MIL-110. Furthermore, similar ΔH*f of MIL-96 and MIL-100 explain their concurrent formation as physical mixtures often encountered during synthesis, implying the importance of kinetic factors that may facilitate the formation of Al-BTC framework isomers. More importantly, the normalized formation enthalpies of Al-BTC MOF isomers follow a negative correlation with the ratio of charged coordinated substituents to linkers (normalized per mole of Al within the MOF formula unit), with enthalpic preference given to systems with smaller (O2- + OH-)/ligand ratios. This trend has been successfully extended to the previously measured ΔH*f of several Zn4O-based frameworks (e.g., MOF-5, MOF-5(DEF), MOF-177, UMCM-1), all of which have been found to be metastable with respect to their dense phases (ZnO, H2O, and ligands). The result suggests that carboxylate MOFs with higher metal coordination environments attain more enthalpic stabilization from the coordinated ligands. Thus, the formation of some lanthanide/actinide, transition metal, and main group carboxylate frameworks may be energetically more favored, which, however, requires further studies.

Understanding the initial stage oxidation and microstructural evolution of detonation sprayed NiCoCrAlY bond coat using in-situ high-temperature X-ray diffraction

This study focuses on in-situ high-temperature X-ray diffraction (HT-XRD) investigation to understand the oxidation, microstructural evolution, and recrystallization of the NiCoCrAlY bond coat. HT-XRD analysis was carried out at a constant temperature of 1423 K for 3 h in a 10−4 Pa vacuum. The formation of α-Al2O3, Cr2O3, Co3O4, and NiCr2O4 oxides and microstructural evolution, such as homogenization of phases (γ and β) during in-situ HT-XRD, has been analyzed with FE-SEM (EDS), Density Functional Theory calculations and Rietveld refinement's support. β-(Ni,Co)Al phase depletion (by 77%) was found at 1423 K during the HT-XRD; however, no TGO formation was noticed.

Unveiling the Re effect on long-term coarsening behaviors of γ′ precipitates in Ni-based single crystal superalloys

In response to the increased demand for very long-term service reliability of industrial gas turbine (IGT) blades, the microstructural stability of two single crystal (SX) superalloys with different Re addition (0Re and 2Re in wt.%, named as alloy 0Re5Ta and 2Re5Ta) were investigated during long-term thermal exposure (>5000 h) at 900 °C, with the help of atom probe tomography (APT) and high-temperature X-ray diffraction (XRD) analysis. During long-term thermal exposure, the γ′ precipitates coarsened gradually in both alloys. At the same time, the γ′ precipitates became more cuboidal for alloy 0Re5Ta and nearly spherical for alloy 2Re5Ta due to the increased positive lattice misfit for alloy 0Re5Ta and the decreased negative lattice misfit for alloy 2Re5Ta. Such lattice misfit evolution was mainly attributed to the inversion of the W partitioning from γ matrix to γ′ precipitates over time. The addition of Re can effectively decrease the coarsening rate during thermal exposure. Further investigations on the coarsening kinetics confirmed the Re effect on reducing the interfacial energy and enhancing the rate-limiting effect of Cr. Additional particle size distributions (PSDs) showed a gradual transition from the initial matrix-diffusion coarsening-controlled mechanism to the interface-diffusion coarsening-controlled mechanism at prolonged time, wherein Re triggered the interface-diffusion mechanism earlier by leading to sharper interfaces. Although Re increases the magnitude of the lattice misfit in the initial microstructure, it can significantly enhance the microstructural stability of SX superalloys for IGT application.

Enhanced negative thermal expansion property in (KMg)xSc2-x(WO4)3 ceramics synthesized by solid–state reaction

Negative thermally expanded (NTE) (KMg)xSc2-x (WO4)3 (0 = x ≤ 1.5) ceramics have been prepared via solid-state reaction. X-ray diffraction (XRD) analysis reveals that (KMg)xSc2-x (WO4)3 undergoes a composition-induced phase transition as the (KMg)3+-substitution content increases. For x = 0.25, both hexagonal (KMg)xSc2-x (WO4)3 and orthorhombic Sc2(WO4)3 coexisted in the sample. For 0.25 < x ≤ 1, a single-phase, hexagonal (KMg)xSc2-x (WO4)3, was obtained. When x is further increased to 1.25, monoclinic MgWO4 impurity was detected in the sample. Meanwhile, as x increases, the grain size of (KMg)xSc2-x (WO4)3 ceramics grows up. Hexagonal (KMg)xSc2-x (WO4)3 ceramics show high densification and strong NTE property. Thermomechanical analysis indicates that the linear coefficients of thermal expansion (CTEs) of hexagonal (KMg)xSc2-x (WO4)3 ceramics decrease from −7.48 × 10−6 °C −1 to −14.06 × 10−6 °C −1 with the increase of x. High-temperature XRD analysis reveals that hexagonal (KMg)0.5Sc1.5(WO4)3 show an anisotropic NTE. The intrinsic linear CTEs are αa = αb = −6.74 × 10−6 °C −1, αc = 11.03 × 10−6 °C −1 and the volumetric CTE is αv = −2.52 × 10−6 °C −1.

Structural and Chemical Compatibilities of Li1- x Ni0.5 Co0.2 Mn0.3 O2 Cathode Material with Garnet-Type Solid Electrolyte for All-Solid-State Batteries.

All-solid-state batteries (ASSBs) based on ceramic materials are considered a key technology for automobiles and energy storage systems owing to their high safety and stability. However, contact issues between the electrode and solid-electrolyte materials and undesired chemical reaction occurring at interfaces have hindered their development. Herein, the chemical compatibility and structural stability of composite mixtures of the layered cathode materials Li1- x Ni0.5 Co0.2 Mn0.3 O2 (NCM523) with the garnet-type solid electrolyte Li6.25 Ga0.25 La3 Zr2 O12 (LLZO-Ga) during high-temperature co-sintering under various gas flowing conditions are investigated. In situ high-temperature X-ray diffraction analysis of the composite materials reveals that Li diffusion from LLZO-Ga to NCM523 occurs at high temperature under synthetic air atmosphere, resulting in the decomposition of LLZO-Ga into La2 Zr2 O7 and the recovery of charged NCM523 to the as-prepared state. The structural stability of the composite mixture at high temperature is further investigated under N2 atmosphere, revealing that Li diffuses toward the opposite direction and involves the phase transition of LLZO-Ga from a cubic to tetragonal structure and the reduction of the NCM523 cathode to Ni metal. These findings provide insight into the structural stability of layered cathode and garnet-type solid-electrolyte composite materials and the design of stable interfaces between them via co-sintering for ASSBs.

Effect of the Annealing Atmosphere on Crystal Phase and Thermoelectric Properties of Copper Sulfide.

Cu2-xS has become one of the most promising thermoelectric materials for application in the middle-high temperature range. Its advantages include the abundance, low cost, and safety of its elements and a high performance at relatively elevated temperatures. However, stability issues limit its operation current and temperature, thus calling for the optimization of the material performance in the middle temperature range. Here, we present a synthetic protocol for large scale production of covellite CuS nanoparticles at ambient temperature and atmosphere, and using water as a solvent. The crystal phase and stoichiometry of the particles are afterward tuned through an annealing process at a moderate temperature under inert or reducing atmosphere. While annealing under argon results in Cu1.8S nanopowder with a rhombohedral crystal phase, annealing in an atmosphere containing hydrogen leads to tetragonal Cu1.96S. High temperature X-ray diffraction analysis shows the material annealed in argon to transform to the cubic phase at ca. 400 K, while the material annealed in the presence of hydrogen undergoes two phase transitions, first to hexagonal and then to the cubic structure. The annealing atmosphere, temperature, and time allow adjustment of the density of copper vacancies and thus tuning of the charge carrier concentration and material transport properties. In this direction, the material annealed under Ar is characterized by higher electrical conductivities but lower Seebeck coefficients than the material annealed in the presence of hydrogen. By optimizing the charge carrier concentration through the annealing time, Cu2-xS with record figures of merit in the middle temperature range, up to 1.41 at 710 K, is obtained. We finally demonstrate that this strategy, based on a low-cost and scalable solution synthesis process, is also suitable for the production of high performance Cu2-xS layers using high throughput and cost-effective printing technologies.

Complex evolution of phase during the thermal investigation of Brushite-type calcium phosphate CaHPO4•2H2O

This study focused on a detailed examination of the thermal behavior of Brushite-based calcium phosphate (CaHPO4•2H2O, DCPD) to identify and characterize the intermediate phases which have been the subject of previous several controversies. For that, in situ high-temperature X-ray diffraction (HT-XRD) supported by Fourier transform infrared spectroscopy (FTIR), simultaneous thermal gravimetry and differential thermal analysis (TG/DTA), and scanning electron microscopy (SEM) analysis were used and the combination of results showed that the progressive thermal stress of DCPD in air resulted in a complex heterogeneous formulation consisting of dibasic calcium phosphate anhydrous (CaHPO4, DCPA) and an amorphous calcium phosphate, which appears at low temperatures (~160 °C) and persists up to 375 °C. This amorphous phase was identified as a disordered calcium pyrophosphate (Ca2P2O7, CPP) by the appearance in FTIR spectra of a characteristic band δ(P—O—P) of pyrophosphate groups, located at 740 cm−1. This band is shifted to low frequencies (725 cm−1) as the temperature is increased, indicating the crystallization of the amorphous CPP material into γ-CPP. The high temperature treatment (≥ 375 °C) leads to β−CPP polymorph. The quantification of the amorphous CPP phase by HT-XRD analysis depends on the heating rate of the decomposition of DCPD and presents a maximum at 300 °C for 10 °C min−1. According to the present characterization results, obtaining pure DCPA from the thermal dehydration of DCPD is not effective and leads to biphasic materials including a disordered calcium pyrophosphate phase.

Grain growth kinetics, microstructure and magnetic properties of mechanically activated Nd1−xCaxMnO3±δ manganites

The nanograin polycrystalline Nd1−xCaxMnO3±δ (x = 0; 0.15; 0.25) with orthorhombic structure were obtained by mechanical activation. X-ray diffraction analysis and transmission electron micrographs showed that the average crystallite size is in the range of 10–30 nm. Grain growth kinetics was studied using three different models using data from high temperature X-ray diffraction analysis. Two models – normal grain growth and grain growth in a two-phase system – were compatible with experimental data up to higher temperatures. Grain growth was found to be kinetically hindered in Ca-doped compositions despite the lower activation energy. That was accounted for by the surface modification involving formation of oxide phases trapping grain boundaries and limiting their mobility. It was found that the Neel temperature in NdMnO3+δ monotonically decreased with decreasing grain size, while the Curie temperature in Nd0.85Ca0.15MnO3-δ varied nonmonotonically. The spin-reorientation transition was found as strongly manifested in NdMnO3+δ and completely suppressed in Nd0.85Ca0.15MnO3-δ. The magnitude of the spin-reorientation effect decreased with decreasing grain size. The peculiarities of the magnetic behaviour were accounted for by the reduction of Jahn-Teller distortion with decreasing grain size, the emergence and increase of local structural and magnetic inhomogeneities, competing d-d superexchange, double-exchange and f–d exchange.

Cobalt-free perovskite Ba1-xNdxFeO3-δ air electrode materials for reversible solid oxide cells

We report on Ba1-xNdxFeO3-δ, a cobalt-free perovskite material, with a view to its use as a next-generation air electrode material in reversible solid oxide cells (RSOCs). BaFeO3-δ (BFO) has long been considered a potential candidate cathode material due to its high oxygen vacancy concentration and electrical conductivity; however, it is difficult to synthesize in a single phase. To overcome this problem, Nd3+ is doped into Ba-sites to reduce impurity phases and create a single perovskite phase. In-situ high temperature and room temperature X-ray diffraction analyses are carried out to investigate Nd3+-doped BFO. We find that Ba0.97Nd0.03FeO3-δ shows the highest electronic conductivity and lowest TEC value among the various doping concentrations tested, making this material most suitable for application in RSOCs. In addition, the polarization resistance of Ba0.97Nd0.03FeO3-δ has the lowest value in yttria-stabilized zirconia symmetric cells. To determine the reasons for the high catalytic activity of Ba0.97Nd0.03FeO3-δ, X-ray photoelectron spectroscopy and iodometric titration are carried out, the results demonstrate that the lower doping concentration of Nd3+ results in an advantage in terms of the number of additional oxygen vacancies created. Moreover, electrical conductivity relaxation measurements show that the Ba0.97Nd0.03FeO3-δ has a fast bulk diffusion coefficient and fast surface exchange coefficient. Hence, the solid oxide fuel cell and electrolysis mode performances when using Ba0.97Nd0.03FeO3-δ are excellent and a high power output of 1.2 W cm−2 at 800 °C can be achieved.

Thermochemistry and phase stability of the polymorphs of yttrium tantalate, YTaO4

Thermodynamic parameters of the three fergusonite-related polymorphs M’, M and T of YTaO4 and their phase stabilities were investigated experimentally. The debate on the relative stability of the M and M’ phases was resolved, and it was shown that the compound does not transform to a cubic polymorph prior to melting. The enthalpies of formation of M and M’ were determined using high temperature oxide melt solution calorimetry, and the heat capacities and heat contents were measured. The enthalpy of the M’ to T phase transition was measured by differential thermal analysis. The stability of the T phase up to its melting at 2090 °C was demonstrated using high temperature X-ray diffraction and thermal analysis. Its enthalpy of fusion was determined using drop-and-catch calorimetry. The thermodynamic properties of YTaO4 assessed in this study enable the thermodynamic modeling of its polymorphs and related materials systems of technological importance.

X-Ray Diffraction Analysis of the Amorphous–Crystalline Phase Transition in Ni

Systematic data on the amorphous–crystalline transition in Ni have been obtained by high-temperature X-ray diffraction analysis. It is established that the amorphous structure of Ni nanoparticles is stable up to 200°C. Ni nanocrystals, which have coherent-scattering regions (CSRs) 5–15 nm in size (depending on the isothermal annealing temperature) are formed in the temperature range of 300–600°C. The activation energy of nanocrystal growth has been estimated to be 67.3 kJ/mol. The dependence of the unit-cell parameter of nanocrystalline Ni on the CSR size is determined. An increase in the lattice constant is observed with an increase in CSR in nanocrystalline Ni particles.

Solid Solutions Rb0.95NbxMo2-xO6.475-0.5x (x = 1.31-1.625) with Orthorhombic β-Pyrochlore Structure: Thermal Behavior and Electronic Structure of β-Pyrochlores Compounds Based on [Nb(Ta)/Mo] Octahedral Framework.

Solid solutions Rb0.95NbxMo2-xO6.475-0.5x (x = 1.31-1.625) having a β-pyrochlore structure with an orthorhombic system were synthesized by solid-state reaction. The elemental composition was confirmed by X-ray microanalysis. The Rb0.95Nb1.375Mo0.625O5.79 structure refinement was performed using the Rietveld method. The crystal structure consists of ordered O-Mo-O chains partly occupied by Nb atoms. The oxygen vacancies are necessary to save the electroneutrality of the unit cell. It predominantly appears between Mo atoms that lead to form two disconnected defect octahedra [MoO5□···MoO5□]. The structural defects cause the low thermal stability; the compounds obtained decompose in the 748-758 °C temperature range. The high-temperature phase transition of the CsNbMoO6 and CsTaMoO6 nonlinear optical β-pyrochlores has been studied by differential thermal analysis, differential scanning calorimetric analysis, high-temperature X-ray diffraction analysis, and second harmonic generation analysis. At room temperature the compounds possess the cubic noncentrosymmetric F4̅3m cell. Under heating to 437 °C and 401 °C for CsNbMoO6 and CsTaMoO6, respectively, they undergo transition into centrosymmetric Fd3̅m modification. This is accompanied by the SHG signal disappearing, as well as the 402 reflection, which is characteristic of the F4̅3m space group. The positions of the valence and conduction bands were determined by reflectance spectra and XPS analysis for structure-related β-pyrochlores CsNbMoO6, CsTaMoO6, and Rb0.95Nb1.375Mo0.625O5.79.

Lattice Expansion of BaCe&lt;sub&gt;0.54&lt;/sub&gt;Zr&lt;sub&gt;0.36&lt;/sub&gt;Y&lt;sub&gt;0.1&lt;/sub&gt;O&lt;sub&gt;3-δ &lt;/sub&gt;Ceramic Electrolyte

Abstract. Solid oxide fuel cell (SOFC) is an electrochemical conversion device that undergoes a thermal cycling at various operating temperature where lead to the degradation of its mechanical properties. Electrolyte among the main component in SOFC plays a crucial part in defined the overall performance which facing a lattice expansion event when exposed to heating. Thus, in this paper BaCe0.54Zr0.36Y0.1O3-δ (BCZY) was selected as potential electrolyte for intermediate temperature solid oxide fuel cell (IT-SOFC) to investigate its lattice expansion as a function of temperature. The sample was prepared via a sol gel method and calcined at 1100°C for 10 hours to form a powder and then pressed to become a pellet. To ensure a good densification in such pellet, two-steps sintering processes was indicated at 1500°C and ground to a powder form prior to the lattice expansion measurements. High temperature X-ray diffraction (HT-XRD) was used to study the lattice expansion of sample in the temperature range of 25°C to 700°C with interval 100°C under air atmosphere. HT-XRD analysis was done using X’pert Highscore Plus software and Visual for Electronic and Structural Analysis (VESTA) software was used to observe the crystal structure. Phase and structural analysis of BCZY electrolyte materials were discussed. Apparently, the BCZY shows an average of 97% phase purity from room temperature to 700°C. Rietveld refinement analysis revealed that the BaCe0.54Zr0.36Y0.1O3-δ exhibits cubic symmetrical structure with unit cell, a=b=c that varied from 4.3440Å - 4.3731Å for all the temperature studied. Thus, the expansion percentage for the lattice expansion from room temperature to 700°C was about 12.6 %.