High-temperature X-ray Diffraction Research Articles

Anisotropic thermo-mechanical response of layered hexagonal boron nitride and black phosphorus: application as a simultaneous pressure and temperature sensor.

Hexagonal boron nitride (hBN) and black phosphorus (bP) are crystalline materials that can be seen as ordered stackings of two-dimensional layers, which lead to outstanding anisotropic physical properties. Knowledge of the thermal equations of state of hBN and bP is of great interest in the field of 2D materials for a better understanding of their anisotropic thermo-mechanical properties and exfoliation mechanism towards the preparation of important single-layer materials like hexagonal boron nitride nanosheets and phosphorene. Despite several theoretical and experimental studies, important uncertainties remain in the determination of the thermoelastic parameters of hBN and bP. Here, we report accurate thermal expansion and compressibility measurements along the individual crystallographic axes, using in situ high-temperature and high-pressure high-resolution synchrotron X-ray diffraction. In particular, we have quantitatively determined the subtle variations of the in-plane and volumetric thermal expansion coefficients and compressibility parameters by subjecting these materials to hydrostatic conditions and by collecting a large number of data points in small pressure and temperature increments. In addition, based on the anisotropic behavior of bP, we propose the use of this material as a sensor for the simultaneous determination of pressure and temperature in the range of 0-5 GPa and 298-1700 K, respectively.

Crystal structure, microhardness and thermal expansion of ternary TaIr2B2 boride

The ternary TaIr2B2 boride was prepared by the reaction between iridium and TaB2 powders. It was shown that a choice in favor of any crystal structure of TaIr2B2 boride using only X-ray powder diffraction analysis and single-crystal XRD is rather difficult to make. The DFT calculations were carried out to predict the energetically preferred crystal structure of TaIr2B2. The 11B solid-state NMR spectrum of TaIr2B2 was recorded for the first time. A combination of diffraction and spectroscopic methods, as well as theoretical calculations, allowed us to make clear choice in favor of the triclinic space group P-1 for the crystal structure of TaIr2B2. The Vickers microhardness value for TaIr2B2 was found to be 17.9 ± 2.3 GPa, and the ternary TaIr2B2 boride can be considered a hard material. Therefore, the family of hard ternary boride phases got a new member. The coefficients of thermal expansion of TaIr2B2 measured by in situ high-temperature XRD analysis are independent of temperature along the a and b axes, but the CTE along the c axis deviates noticeably at elevated temperatures. The findings for the new ternary TaIr2B2 boride are of interest for the rational design of complex structural materials containing Ta–Ir–B-based components and prediction of their high-temperature behavior. Further research into this ternary boride as well as other iridium-containing ternary borides will provide a tool to solve the problems faced by high-temperature materials science.

Synthesis and characterization of polymer-derived nanocrystal SiOC powders via high temperature XRD method

In the study, polymer-derived SiOC powders was synthesized by sol-gel method. The resulting composites consists of β-SiO2, SiC and free carbon. Tetraethylorthosilicate (TEOS) and Polydimethylsiloxane (PDMS) were selected as starting materials to obtain organic-inorganic structure. After the gelling process, the powders were heat treated at 1100°C in Argon medium to obtain the desired phases. Scanning Electron Microscopy (SEM), Differential Thermal Analysis (DTA) and Fourier Transform Infrared Spectroscopy (FT-IR) analyses were used for characterization. In addition, instant phase changes were determined by high-temperature XRD in powders subjected to heat treatment up to 1500 °C in a helium environment. The effect of temperatures on the transformation in SiOC synthesis, the transformation temperatures of α-cristobalite to β-cristobalite were sharply determined and the SiC formation temperature was revealed. The effect of temperature on crystal size was also obtained as a result of the study.

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 .

Novel High-Temperature Modification of Belomarinaite KNaSO4: Crystal Structure and Thermal Order–Disorder Phase Transitions

Belomarinaite KNaSO4 (space group P3m1, a = 5.6072(3), c = 7.1781(4) Å and Z = 2) has been studied by high-temperature single crystal X-ray diffraction. The K and Na atoms are disordered, and M1(1c) and M2(1b) sites merge into an M1(2d)(K50%/Na50%) site with increasing temperature, and novel translationsgleiche phase transition of belomarinaite KNaSO4 (P3m1 → P-3m1) was revealed at 123 °C. Then, the temperature increase leads to phase transition of P3¯m1-polymorph of KNaSO4 to α-KNaSO4 (P63/mmc) at 446 °C, and sodium and potassium atoms in K1(1a) and Na1(1b) sites merged into an M4(2a)(K50%/Na50%) site. Crystal structures of KNaSO4 were refined at 300, 500 and 750 °C to R1 = 0.057, 0.056 and 0.048, respectively. The BVS maps and bond-valence energy landscape (BVEL) have been calculated, and the probability of the Na+ migration has been predicted using structural data for belomarinaite at 25 °C. The Na+ ion migration can occur at 2.14 eV in the ab plane and 2.80 eV along the c axis at 25 °C.

Phase Evolution and Thermodynamics of Al-Doped Cubic LLZO Studied by High-Temperature X-Ray Diffraction

Li7La3Zr2O12 (LLZO) is a promising solid-state electrolyte for advanced Li-ion batteries due to its high ionic conductivity, non-flammability and high electrochemical stability against Li anodes and high voltage cathodes. However, synthesis of phase pure LLZO is challenging due to easy formation of secondary phases, such as the pyrochlore La2Zr2O7, that are detrimental to the performance of a LLZO electrolyte. Although many studies have been performed on the electrochemical performance of LLZO, the understanding of the thermodynamics of the synthesis of LLZO is still lacking.Here we report on high-temperature X-ray diffraction (HTXRD) following the crystallization of a gel of Al-doped LLZO. Gels with 0, 10 and 20 mol% excess Li were prepared and calcined at 500 °C to remove the volatile components before HTXRD. The powders calcined at 500 °C consisted primarily of La2Zr2O7 with trace amounts of Li2CO3 and La2O2CO3 and were used as starting materials to study the formation of the cubic LLZO phase. During HTXRD the samples were heated from 500 °C to 700 °C in steps of 20 °C, and further to 1000 °C in steps of 50 °C. Upon heating the pyrochlore phase gradually transforms to cubic LLZO. In all samples, cubic LLZO first appears around 600 °C and the pyrochlore phase disappears around 750 °C in the samples with 0 % and 20 % excess Li. We performed two analyses of the sample with 10 % excess Li by HTXRD, one with normal deposition thickness (Figure a) and another with thinner deposition thickness (Figure b), see attached Figures. In the thin sample, cubic LLZO appears at a lower temperature of 560 °C compared to the other experiments. The pyrochlore phase does not fully disappear but instead reappears and crystallizes at the highest temperatures investigated, indicating extensive Li loss from the thin sample. These observations suggest that atmospheric exposure and the relative surface area of powder are important factors for LLZO formation.To understand the thermodynamics of LLZO formation, we propose the following reaction to take place based on the identified reactants and products:6.25 Li2CO3 + 0.25 Al2O3 + 2 La2Zr2O7 + La2O2CO3 → 2 Li6.25−xAl0.25La3Zr2O12−0.5x + 7.25 CO2(g) + x Li2O(g).The enthalpy, entropy and Gibbs energy of the reaction were calculated from literature values and DFT calculations and agree with the experimental observations. Finally, we discuss the significance of the highly disordered Li sublattice for the entropy of cubic LLZO. We believe our findings will contribute to improved understanding of LLZO formation and facilitate the synthesis of high-performance phase pure LLZO electrolytes.Figure captions:HTXRD patterns of powder with 10 % Li excess from 500 to 1000 °C. (a) Normal deposition thickness. (b) Deposited as a thin layer on the Pt strip. The red asterisk and green square denote La2Zr2O7 and La2O3 (PDF no. 04-005-6788) impurity phases, respectively. The blue triangle denotes a Pt peak, visible due to the thin deposition. The XRD patterns obtained at 30 °C after the HTXRD are shown at the top of the plots. Figure 1

Mechanism of intermediate temperature abnormal plasticity in Co–Al–W-base superalloy

The mechanism of intermediate temperature abnormal plasticity of Co–Al–W-base superalloy was investigated. The interrupted tensile specimens were systematically characterized through high-temperature XRD, SEM, as well as TEM. The superalloy exhibits a low elongation (approximately 5.2 %) at 700 °C for two reasons. On the one hand, the γ/γ′ lattice mismatch decreases with the increasing temperature (0.36 % at 600 °C, 0.25 % at 700 °C, and 0.19 % at 800 °C). The low γ/γ′ lattice mismatch facilitates a/2<110> type dislocation to cut into the γ′ phase in the form of paired a/2<110> type dislocation. When the paired a/2<110> type dislocation slips out from the γ′ phase and it will be hindered by MC carbide, increasing the stress concentration around MC carbide, so that it leads to early MC carbide breaking and microvoid forming. On the other hand, the stacking fault energy decreases with the increasing temperature. The a/2<110> type dislocation easily resolves into two a/6<112> type partial dislocations in the γ′ phase at 800 °C. The a/6<112> type partial dislocation suppresses the surrounding a/2<110> type dislocations to cut into the γ′ phase, relaxing the stress concentration around MC carbide. Therefore, the abnormal plasticity of Co–Al–W-base superalloy occurs at 700 °C.

Preparation of novel silver borates by soft hydrothermal synthesis in sealed tubes: New representatives of larderellite and veatchite families

New silver-based representatives of the larderellite and veatchite families, Ag2B10O14(OH)4∙H2O (1) and Ag2B5O8(OH)∙H2O (2) respectively, have been prepared using a soft hydrothermal synthetic approach inside a sealed silica tube. Both structures are built of the pentaborate groups yet exhibit various dimensionality: 2D for 1 and 1D for 2. The silver atoms in 1 exhibit noticeable anharmonicity approximated using Gram–Charlie approach. 1 was additionally characterized by thermal analysis, high-temperature X-ray powder diffraction and IR spectroscopy. Thermal expansion of 1 is strongly anisotropic (α11 = 38, α22 = −13, α33 = 14 × 10−6 °С−1) which is related with the hinge mechanism of the larderellite chains from corner-sharing pentaborate groups. Topology and complexity analysis of these compounds within the larderellite and veatchite families is presented.

Dielectric and structural properties of double-sintered (Na1-xKx)NbO3, (0.460 ≤ x ≤ 0.485) ceramics

Ceramic samples of Na1-xKxNbO3, (0.460 ≤ x ≤ 0.485) were prepared by using the double sintering solid-state reaction method. The grain size and porosity were found to vary with composition. The dielectric constant of the material phase was obtained from the measured dielectric constant of the prepared ceramic pellets by applying porosity correction. Among the prepared compositions, the dielectric constant was observed to be minimum for x = 0.475. The temperature variation of the dielectric constant and lattice parameters was measured for the samples with x = 0.475, in the range from room temperature (RT) to 500 °C. Three distinct phase transitions, monoclinic to tetragonal, at about 205 °C; tetragonal to tetragonal, at about 410 °C; and tetragonal to cubic, at 460 °C, were observed from the dielectric constant and high-temperature X-ray diffraction (HT-XRD) measurements of the prepared compositions.

In situ анализ кристаллической решетки нитридных однокомпонентных и многослойных покрытий ZrN/CrN в процессе термоциклирования

Introduction. Thermal expansion is an important thermal and physical characteristic of materials, showing its expansion when heated. Knowing this property is important both from a scientific point of view and for practical applications. Materials with low thermal expansion are widely used in electronics, thermal barrier coatings and other applications. Mismatch in thermal expansion between different materials can lead to thermal stress on contact surfaces. The in-situ synchrotron X-ray diffraction method can detect this mismatch. Thermal stress requires an analysis of the coefficient of thermal expansion. Bulk expansion behavior is observed in thermally sprayed coatings. The CTE is important for designing and predicting coating performance under thermal stresses. Changes in the KTE can cause cracking and degradation of the coating. In-situ X-ray diffraction analysis helps to understand thermal expansion, crystallite size and stress and strain variation with temperature change. The aim of this work is to interpret and use in-situ high temperature X-ray diffraction as an effective tool to study the thermal mismatch behavior of a W-Co alloy substrate (8 % w/w Co, WC — matrix) with CrN, ZrN and CrZrN multilayer coatings and the characteristic differences between single component coatings and its combination in a multilayer coating. Research Methodology. In this work, specimens of chromium and zirconium nitride coatings deposited on W-Co hard alloy substrates were investigated. The fundamental method in this work is in-situ analysis using synchrotron radiation. The lattice parameter as a function of cycling temperature, the coefficient of thermal expansion during heating and cooling, and the thermal expansion mismatch between the substrate-coating pair and the coating layers in the multilayer coating were evaluated. Results and discussion. The lattice parameters and thermal expansion of the coatings are investigated. The lattice parameter of all coatings decreased during thermal cycling, indicating nitrogen evaporation. The multilayer coating has the least change in the parameter, possibly due to diffusion barriers. Lattice distortions do not differ between single and multilayer coatings. All coatings exhibit thermal expansion similar to the substrate. The multilayer coating creates conditions for compressive stresses in one phase and tensile stresses in the other phase, so the lifetime of multilayer coatings is expected to be high.

Thermally processed phase change behavior on the stoichiometric efficacy of quaternary Se77.5-X Te20 Sn2.5 AgX (X = 2.5, 5, 7.5) thin films

Thin films of the quaternary metal chalcogenide have recently gained attention due to their improved electrical, optical, thermal, and structural properties. The structural, thermal, morphological, optical, and electrical properties of Ag-modified novel quaternary Se–Te–Sn–Ag thin film systems are investigated in this paper. Temperature-dependent structural, electrical, and thermoelectric properties are given special attention. From Differential Thermal Analysis/Thermogravimetric Analysis (DTA/TGA), the Ag content in this composition induces thermal stability and crystalline phase as well as glass phase transitions. High-temperature X-ray diffraction (HTXRD) patterns were used to investigate the temperature-dependent structural phase transition, grain size, lattice strain, and dislocation density. The morphological observations show that Ag concentration promotes the nucleation and growth processes of nanorod structures in as-deposited thin films. Optical characteristics reveal higher IR transparency in thin films and high refractive indexes in the range of 3.22–3.29. The as-deposited thin films resistivity decreases with increasing temperature and silver content. The Hall measurement corroborates the results obtained from the Seebeck measurements, indicating that the samples are n-type. The phase change switching mechanism of phase change devices is still unclear; there are two possible mechanisms: thermal and electronic models. This work proposes thermally processed phase transition behaviors in structural and electrical studies. The result of the above analyses shows that the quaternary Se–Te–Sn–Ag thin film will be a potential candidate for phase change device applications.

Unlocking structural and photoelectric properties of lead-free double perovskites at high temperature

The lead-free double perovskite, increasingly sought for optoelectronic pursuits, is distinguished by its notable stability, optical properties, and non-toxic characteristic. An extensive inquiry into the structural and photoelectric properties of Cs2NaBiCl6 at heightened temperatures was undertaken using experimental evaluations and theoretical computations. The compound Cs2NaBiCl6, synthesized using a hydrothermal technique, was confirmed via various analytical methods. Advanced techniques such as high-temperature X-ray diffraction, X-ray fluorescence, X-ray photoelectron spectroscopy, and selected area electron diffraction were employed for the investigation of its crystal structure. Further investigation of microstructure was undertaken using electron microscopes. High-temperature luminescence stability disclosed a high exciton binding energy of 326 meV. The Huang-Rhys factor (6.4) denotes a robust electron-phonon coupling effect and facile formation of self-trapped excitons (700 nm). Applied first principles render valuable insight into structural and photoelectric nuances, notably at escalated temperatures. Cs2NaBiCl6 as an indirect bandgap semiconductor and details the experimental and theoretical bandgap, which exhibits an increasing trend before subsequently declining at elevated temperatures. These results strongly support the material's high-temperature stability, emphasizing its applicability in applications such as rare-earth-doped phosphors. The dynamic stability of the material, evidenced by its mechanical performance, further underscores its potential for application in photoelectric devices.

Structural transformation behavior in a high Nb–TiAl alloy additively manufactured by laser powder bed fusion

High Nb–TiAl alloys fabricated using laser powder bed fusion (LPBF) exhibit a metastable α2 phase due to the rapid cooling rate of the LPBF process. However, the understanding of the structural transformation behavior in LPBF-fabricated high Nb–TiAl alloys remains limited. In this work, a high Nb–TiAl alloy with a nominal composition of Ti–48Al–2Cr–8Nb (at. %) was additively manufactured using LPBF. The structural transformation from metastable α2 phase to γ phase was then studied by the in-situ high-temperature x-ray diffraction (XRD), in-situ heating transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Furthermore, the phase composition and microstructure of the samples annealed at different temperatures were determined by XRD and electron backscatter diffraction (EBSD). Our findings demonstrate that the α2 phase in LPBF-fabricated high Nb–TiAl alloy remains stable up to 873 K, after which it decomposes and forms fine γ lamellar colonies along with a disordered γ′ phase as an intermediate phase. Notably, the transition from α2 to γ′ phase is reversible. In contrast, the formation of the γ phase is irreversible and persists down to room temperature. It is also found that the changes in grain size and phase composition of the LPBF-fabricated high Nb–TiAl alloy have a significant impact on its hardness. This study provides valuable insights into the structural transformation behavior of additively manufactured high Nb–TiAl alloys by LPBF, offering guidance for regulating their structure and properties.

Thermophysical Properties of Lanthanum and Samarium Zirconate—Hafnates

The synthesis and identification of lanthanum and samarium zirconate–hafnates of the pyrochlore structure type have been reported. The heat capacity of the samples in the temperature range 310–1380 K was measured by the differential scanning calorimetry method. The temperature dependences of the cubic lattice parameters were determined, and the thermal expansion coefficients were evaluated in the range 298–1273 K using high-temperature X-ray powder diffraction. The thermal diffusivity of the samples was measured by the laser flash method, and the temperature dependences of the thermal conductivity were calculated considering the porosity of the samples.

Critical Evaluation and Calorimetric Study of the Thermodynamic Properties of Na2CrO4, K2CrO4, Na2MoO4, K2MoO4, Na2WO4, and K2WO4

This paper evaluates crystallographic data and thermodynamic properties for sodium chromate, potassium chromate, sodium molybdate, potassium molybdate (K2MoO4), sodium tungstate, and potassium tungstate collected from the literature. A thorough literature review was carried out to obtain a good understanding of the available data, and a critical evaluation has been performed from room temperature to above the melting temperatures. Also, the solid–solid transition and melting properties of the six pure salts were measured by differential scanning calorimetry, and high-temperature x-ray powder diffraction measurements were performed to determine the crystal structures and space groups associated with the phases of K2MoO4. This work is the first step towards the development of a thermodynamic model for the Na+, K+//Cl−, SO42−, CO32−, CrO42−, Cr2O72−, MoO42−, Mo2O72−, WO42−, W2O72−, O2− system that is relevant for high temperature corrosion in atmospheres containing O–H–S–C–Cl and alkali salts.

Experimental measurement and modeling of lattice thermal expansion and specific heat capacity of C15-Cr2Nb Laves phase intermetallic compound

In order to determine the lattice thermal expansion and specific heat capacity (Cp) of the C15-Cr2Nb Laves phase, an experimental investigation and modeling were carried out in the present paper by employing high-temperature X-ray diffraction (HTXRD) and differential scanning calorimetry (DSC). A thorough analytical model was utilized to predict the temperature-dependent variations of enthalpy and specific heat, and the predicted findings were compared to the experimentally measured data. Arc melting was employed to synthesize Cr2Nb samples with distinct hexagonal (C14/C36) and cubic (C15) Laves phase structures, which were then subjected to the appropriate heat treatment. Room temperature XRD patterns, SEM, and EDAX experiments on homogenized samples at 1273 K for 72 h validated the homogeneity and C15 (Fd-3 m) crystal structure of Cr2Nb. Rietveld refinement was used to determine the lattice parameters and relative phase fractions of the constituent phases. The average thermal expansion coefficient was determined to be αai = 0.659 × 10−5 and αVi = 1.977 × 10−5 K−1 respectively. The Cp of the C15-Cr2Nb Laves phase intermetallic compound was measured employing heat flux DSC, utilizing conventional “three-step” procedures, and being reported for the first time. The measured Cp and enthalpy increment data (HT – H298.15) with varying temperatures reported in the present investigation are being correlated utilizing the theoretical model applying the quasi-harmonic Debye Grüneisen model in the temperature range 0–860 K and illustrated different contributions to the total heat capacity i.e., vibrational, anharmonic, and electronic.

Effect of Temperature on the Radial Distribution Function of Atoms in the Silicate Glass 2sio2·PBO

In this article, results of investigation of the effect of temperature on radial distribution function of atoms in lead-silicate glass are reported. Radial distribution functions of atoms in the lead-silicate glass 2SiO2∙PbO were calculated via Mathematica 5.1 Wolfram Research from high-temperature X-ray diffraction patterns. It turned out that in the range of 773-973 K mainly variations of the location of atoms in the third coordination sphere have observed, while in the range of 973-1123 K, changes in the first and second coordination spheres take place.

Separation of fission products from high-level waste salt systems by partial crystallization: CsCl-NaCl-LiCl-KCl study

Pyroprocessing refers to a series of high-temperature molten-salt processes used to manage and recycle nuclear materials to reduce the waste volume and hazardous environmental impact of the final waste products. Fission products with high halide affinity, including cesium (Cs-137) with a half-life of 30 years, are dissolved into a molten-salt bath. The separation and recovery of cesium can benefit waste-stream minimization because Cs-137—a strong gamma-ray emitter—is a potential source of environmental contamination. This work proposes the recovery of Cs+ from a chloride-salt matrix using a thermally controlled solid-liquid separation. Thermodynamic calculations for the CsCl-NaCl-LiCl-KCl system were performed to predict the partitioning of Cs+ into liquid and solid phases as functions of salt composition and temperature. The selected composition simulated the formulation of high-level salt waste from used fuel treatment. These calculations showed that once the solidus temperature was reached, Cs+ partitions to the liquid phase. According to the thermodynamic calculations, a thermally controlled liquid-solid separation holds the potential to recover up to 92.8 % of the initial cesium chloride. The phase transition temperatures of the sample, including the solidus and liquidus temperatures, were determined using differential scanning calorimetry. The phase composition of the system was studied at various temperatures using high-temperature X-ray diffraction. A laboratory-scale crystallization apparatus was used to melt the sample partially, allowing the collection of the liquid fraction for elemental analysis. All results demonstrated that Cs+ in this system concentrates in the liquid phase and confirmed the viability of recovering CsCl through thermally controlled solid-liquid separation.

Trigger of the Highly Resistive Layer Formation at the Cathode-Electrolyte Interface in All-Solid-State Lithium Batteries Using a Garnet-Type Lithium-Ion Conductor.

Interfacial materials design is critical in the development of all-solid-state lithium batteries. We must develop an electrode-electrolyte interface with low resistance and effectively utilize the energy stored in the battery system. Here, we investigated the highly resistive layer formation process at the interface of a layered cathode: LiCoO2, and a garnet-type solid-state electrolyte: Li6.4La3Zr1.4Ta0.6O12, during the cosintering process using in situ/ex situ high-temperature X-ray diffraction. The onset temperature of the reaction between a lithium-deficient LixCoO2 and Li6.4La3Zr1.4Ta0.6O12 is 60 °C, while a stoichiometric LiCoO2 does not show any reaction up to 900 °C. The chemical potential gap of lithium first triggers the lithium migration from the garnet phase to the LixCoO2 below 200 °C. The lithium-extracted garnet gradually decomposes around 200 °C and mostly disappears at 500 °C. Since the interdiffusion of the transition metal is not observed below 500 °C, the early-stage reaction product is the decomposed lithium-deficient garnet phase. Electrochemical impedance spectroscopy results showed that the highly resistive layer is formed even below 200 °C. The present work offers that the origin of the highly resistive layer formation is triggered by lithium migration at the solid-solid interface and decomposition of the lithium-deficient garnet phase. We must prevent spontaneous lithium migration at the cathode-electrolyte interface to avoid a highly resistive layer formation. Our results show that the lithium chemical potential gap should be the critical parameter for designing an ideal solid-solid interface for all-solid-state battery applications.

Tailoring the electrochemical reduction kinetics of dual-phase BaCe0.5Fe0.5O cathode via incorporating Mo for IT-SOFCs

The BaCe0.5Fe0.5O3-δ (BCF) cathode consists of the ion-electron mixed conducting phase BaCe0.15Fe0.85O3-δ (BCF1585) and the proton-conducting phase BaCe0.85Fe0.15O3-δ (BCF8515). In this paper, the electrochemical performance is improved by incorporating the high valence element Mo into the BCF and applied to intermediate-temperature solid oxide fuel cells (IT-SOFCs). High-temperature X-ray diffraction (HT-XRD) and O2-temperature programmed desorption (O2-TPD) results show that Mo doping enhances the structural stability of BCF. The X-ray photoelectron spectroscopy (XPS) results suggest that the introduction of Mo increases the amount of adsorbed oxygen and thus the oxygen reduction reaction (ORR) catalytic activity. Compared to BCF, the polarization impedance of BaCe0.5Fe0.45Mo0.05O3-δ (BCFM) at 800°C is 0.154 Ω·cm2, a reduction of 22%. Meanwhile, the BCFM output power at 800°C is 778.01 mW·cm-2, an improvement of 32.17%, and maintains a stable current density after 250 h at 0.7 V. The results demonstrate that Mo doping is an effective strategy to enhance the electrochemical performance of BCF.