High Temperature X-ray Diffractometer Research Articles

Suppressing the oxygen-ionic conductivity and promoting the phase stability of the high-entropy rare earth niobates via Ta substitution

Improving and optimizing the target properties of ceramics via the high entropy strategy has attracted significant attention. Rare earth niobate is a potential thermal barrier coating (TBCs) material, but its poor high-temperature phase stability limits its further application. In this work, four sets of TBCs high-entropy ceramics, (Sm1/5Dy1/5Ho1/5Er1/5Yb1/5)(Nb1/2Ta1/2)O4 (5NbTa), (Sm1/6Dy1/6Ho1/6Er1/6Yb1/6Lu1/6)(Nb1/2Ta1/2)O4 (6NbTa), (Sm1/7Gd1/7Dy1/7Ho1/7Er1/7Yb1/7Lu1/7)(Nb1/2Ta1/2)O4 (7NbTa), (Sm1/8Gd1/8Dy1/8Ho1/8Er1/8Tm1/8Yb1/8Lu1/8) (Nb1/2Ta1/2)O4 (8NbTa) are synthesized using a solid-state reaction method at 1650 °C for 6 h. Firstly, the X-ray diffractometer (XRD) patterns display that the samples are all single-phase solid solution structures (space group C2/c). Differential scanning calorimetry (DSC) and the high-temperature XRD of 8NbTa cross-check that the addition of Ta element in 8HERN increases the phase transition temperature above 1400 °C, which can be attributed to that the Ta/Nb co-doping at B site introduces the fluctuation of the bond strength of Ta-O and Nb-O. Secondly, compared to high-entropy rare-earth niobates, the introduction of Ta atoms at B site substantially reduce thermal conductivity (reduced by 44 %, 800 °C) with the seven components high entropy ceramic as an example. The low thermal conductivity means strong phonon scattering, which may originate from the softening acoustic mode and flattened phonon dispersion in 5–8 principal element high entropy rare earth niobium tantalates (5–8NbTa) revealed by the first-principles calculations. Thirdly, the Ta/Nb co-doping in 5–8NbTa systems can further optimize the insulation performance of oxygen ions. The oxygen-ion conductivity of 8NbTa (3.31 × 10−6 S cm−1, 900 °C) is about 5 times lower than that of 8HERN (15.8 × 10−6 S cm−1, 900 °C) because of the sluggish diffusion effect, providing better oxygen barrier capacity in 5–8NbTa systems to inhibit the overgrowth of the thermal growth oxide (TGO) of TBCs. In addition, influenced by lattice distortion and solid solution strengthening, the samples possess higher hardness (7.51–8.15 GPa) and TECs (9.78 × 10−6 K−1–10.78 × 10−6 K−1, 1500 °C) than the single rare-earth niobates and tantalates. Based on their excellent overall properties, it is considered that 5–8NbTa can be used as auspicious TBCs.

In situ high-temperature X-Ray diffraction study of tantalum metal

An in-situ thermal behavior study was conducted on the metallic tantalum under two conditions. The experimentation was carried out on tantalum pellets which were heated progressively “underwent reaction and under continuous pumping or under controlled monoxide pressure” in a graphite resistance high temperature X-ray diffractometer up to 2300 K. Through the in-situ study, a thermodynamic analysis showed that this involved the formation of Ta2O, Ta2O5 (low temperature modification) and Ta2C likely to be formed between 293 and 2300 K, in agreement with a reaction mechanism that we established to occur in four stages.

Characterization of Yb2SiO5-based environmental barrier coating prepared by plasma spray–physical vapor deposition

Due to the high-input power compared to atmospheric plasma spraying (APS), plasma spray-physical vapor deposition (PS-PVD) can primarily achieve a splat-like deposition, allowing for the preparation of high-density environmental barrier coatings (EBCs). In this paper, dense Yb2SiO5-based coatings are prepared by PS-PVD at different substrate temperatures. It was found that the coating deposited at the substrate temperature of 700 °C contained a large amount of silicon-rich amorphous phase. When the substrate temperature increased to 1100 °C and a slow cooling process after deposition was involved, a coating with high crystallinity of ∼77% and low porosity of less than ∼2% was achieved. Phase evolution of the coatings was studied by a semi-in-situ high-temperature X-ray diffractometer. During the heating process, metastable phases X1-Yb2SiO5 and α-Yb2Si2O7 emerged and transformed into stable phases following high-temperature treatment. Furthermore, the effects of long-term thermal aging at 1300 °C on the microstructure, phase composition, thermal conductivity, and hardness of the coating prepared at the substrate temperature of 1100 °C were found to be limited.

High temperature induced “glaze” layer formed in HVOF-sprayed NiCrWMoCuCBFe coating and its wear reduction mechanism

The in-situ formation of oxides on alloy surface induced by high temperature can effectively reduce wear and resist oxidation. In consideration of the solid solution strengthening effect and great oxidation resistance of additional elements at elevated temperature, the NiCrWMoCuCBFe coating was prepared by high velocity oxygen flame (HVOF) spraying technology, and its tribological behavior was scrutinized from 25 to 800 °C. By means of high temperature Vickers hardness tester and high temperature X-ray diffractometer, the mechanical properties and microstructures of NiCrWMoCuCBFe coating were measured. And the effect of the mechanical properties and microstructures of the coating on tribological performance was discussed in detail. The results showed both its friction coefficient (0.37) and wear rate (5.067 × 10−6 mm3·N−1·m−1) at 800 °C were the lowest, which was mainly related to the formation of “glaze” layer on the coating surface at high temperature. The glaze layer consisted of two parts, which were NiCr2O4 oxide film with the ability of interlaminar slip formed in the outer layer and nano-grains existed in the inner layer. Worth mentioning, these nano-grains provided bearing capability while the oxide film was vital to reduce wear rate and friction coefficient. As the ambient temperature increased, many hard oxides were produced on the wear scars, including NiO, Cr2O3, MoO3, and Mo2C. They can improve tribological and mechanical properties of NiCrWMoCuCBFe coating at a wide temperature range.

High-temperature shape memory properties of Cu15Ni35Ti25Hf12.5Zr12.5 high-entropy alloy

The major challenges hindering the application of high-temperature shape memory alloys are their decreased mechanical strength and thermal stability. Herein, we present high-temperature shape memory properties of the Cu15Ni35Ti25Hf12.5Zr12.5 high-entropy alloy with improved mechanical properties and superior shape memory characteristics. Under an immense stress of 1400 MPa, the alloy exhibited a shape recovery ratio of 88.7%. Using an in-situ high-temperature X-ray diffractometer, the alloy was established to undergo phase transformation from B19′ to B2 phase. Additionally, the alloy exhibited a martensitic transformation temperature higher than 100 °C. The fracture strength and ductility were 1670 MPa and 24.7%, respectively, as measured by compression tests. The deformation mechanism and shape recovery phenomenon of this shape memory alloy are also discussed. This study demonstrates the potential of high-entropy shape memory alloys for high-temperature applications.

Structural degradation of tungsten sandwiched in hafnia layers determined by in-situ XRD up to 1520\xa0\xb0C

The high-temperature stability of thermal emitters is one of the critical properties of thermophotovoltaic (TPV) systems to obtain high radiative power and conversion efficiencies. W and HfO2 are ideal due to their high melting points and low vapor pressures. At high temperatures and given vacuum conditions, W is prone to oxidation resulting in instantaneous sublimation of volatile W oxides. Herein, we present a detailed in-situ XRD analysis of the morphological changes of a 3-layer-system: HfO2/W/HfO2 layers, in a high-temperature environment, up to 1520 °C. These samples were annealed between 300 °C and 1520 °C for 6 h, 20 h, and 40 h at a vacuum pressure below 3 × 10–6 mbar using an in-situ high-temperature X-ray diffractometer, which allows investigation of crucial alterations in HfO2 and W layers. HfO2 exhibits polymorphic behavior, phase transformations and anisotropy of thermal expansion leads to formation of voids above 800 °C. These voids serve as transport channels for the residual O2 present in the annealing chamber to access W, react with it and form volatile tungsten oxides. An activation energy of 1.2 eV is calculated. This study clarifies the limits for the operation of W-HfO2 spectrally selective emitters for TPV in high-temperature applications.

Influence of partial substitution of Cu atoms by Zn and Cd atoms on polymorphic transformation in the Cu1.75Te crystal

The [Formula: see text], [Formula: see text] and [Formula: see text] compounds have been synthesized and low-temperature modification single crystals obtained from the high temperature modification by polymorphic transformation. The method of high temperature X-ray diffractometer has been used to study [Formula: see text] (x = 0.05 Zn and Cd) layered single crystal. It was established that such a substitution has a significant impact on the number and temperature of polymorphic transformations. Crystallgraphic parameters were determined for each phase. Temperature dependence of lattice parameters were obtained and determined change mechanism of lattice parameters by influence of temperature.

Liquid structure evolution of molten iron in blast furnace hearth

The iron-carbon interfacial reaction between molten iron and carbon brick was carried out to simulate the working condition of blast furnace (BF) hearth. The carbon content in molten iron after the reaction was detected to be 5.0% which was almost saturated. XRD and SEM-EDS were conducted on the surface of polished rectangle iron before and after iron-carbon interfacial reaction. Fine striped graphite was observed in iron before iron-carbon interfacial reaction, a large amount of flake-like graphite was observed in iron after iron-carbon interfacial reaction. As a structure-sensitive physical property, the viscosity of molten iron was the macroscopic expression of its liquid structure. The liquid structure of molten iron (Fe-4.5%C, Fe-5.0%C) was measured through a high temperature X-ray diffractometer. The X-ray original diffraction intensity, the structure factor, the pair distribution function, the radial distribution function, and the main parameters of molten iron were obtained through the calculation. The presence of pre-peak in the structure factor indicated that there was a medium-range order in molten iron, some compounds or cluster of atoms might exist in molten iron, the structure model of atoms in liquid Fe-4.5%C was predicted through the structure parameters. The increase of carbon content after iron-carbon interfacial reaction was the essential reason for liquid structure evolution of molten iron in hearth.

Liquid-liquid structure transition and its effect on the solidification behaviors and microstructure of Sn75Bi25 alloy

This paper studied the liquid–liquid structure transition (L-LST) of Sn75Bi25 alloy with atom percentage via two direct methods such as a high temperature X-ray diffractometer and a modified high temperature Hall effect instrument as well as several indirect methods including resistivity, density, viscosity and thermal analysis methods. The results indicated that the reversible first-order liquid–liquid structure transition did occur in the range from 990 to 1068 K during heating and occur in the range from 865 to 945 K during cooling. From the atomic and molecular scale, the nature of the L-LST was the breaking of the residual covalent bonds and the cooperative rearrangement of atoms during heating. In addition, the breaking of the residual covalent bonds occurred not only during the L-LST but also before and after the L-LST. The L-LST reflected a qualitative change of melt structure. From the view of the electronic structure, the nature of the L-LST was the releasement of free electrons and the localized-extended transition of electron. Besides, a novel strategy combining atomic structure and electronic structure was developed to reveal the nature of the L-LST. The influences of the L-LST on the solidification behaviors and morphologies of Sn75Bi25 alloy also were investigated. The results showed that the undercooling of the eutectic phase increased after the L-LST, which was responsible for the drastic decrease in the spacing of the eutectic phase.

Grain Refinement by Authigenic Inoculation Inherited from the Medium-Range Order Structure of Ni-Cr-W Superalloy

The combination of liquidus casting and thermal control solidification furnace was applied to obtain a fine-grained ingot. A rapid quenching method and x-ray diffraction measurement were used to investigate the effect of authigenic inoculation on grain refinement. The structure factor S(Q) of liquid Ni-Cr-W superalloy at 1400 °C (Liquidus temperature) and bright-field image of the microstructures quenched from 1400 °C have been measured by the high-temperature x-ray diffractometer and the transmission electron microscopy (TEM), respectively. The results show that a pre-peak exists on a S(Q) curve at the liquidus temperature. The clusters of atom in rapidly quenched microstructures obtained by isothermal heat treatment at 1400 °C were studied using TEM. Meanwhile, the effect of isothermal different temperatures on rapidly quenched microstructures was studied. The results also show that there are only the globular, equiaxed grains distributed in the solidification structure. These particles are inherited from the medium-range order structure, which is beneficial for grain refinement. The normalized work-hardening rate-strain curve indicates the work-hardening rate of fine grain is higher than that of conventional grain at the same temperature and the same deformation.

Combined experimental and ab initio based determination of the thermal expansion of La0.5Sr0.5Co0.25Fe0.75O3

AbstractThe thermal expansion of La0.5Sr0.5Co0.25Fe0.75O3 (LSCF55) is investigated both by first principles phonon calculations combined with the quasi‐harmonic approximation (QHA) and by experimental approaches. Within the framework of the QHA, the volumetric thermal expansion coefficient of rhombohedral LSCF55 is calculated as αV,GGA = 50.34 × 10−6 K−1. For comparison, the lattice expansion and the volume expansion of LSCF55 grain are measured by in situ high‐temperature X‐ray diffractometer (HT‐XRD). An anisotropic thermal expansion of rhombohedral LSCF55 with αa,hex = 10.89 × 10−6 K−1 and αc,hex = 21.18 × 10−6 K−1 is obtained. The volumetric thermal expansion coefficient is measured as αV,HT‐XRD = 43.17 × 10−6 K−1. In addition, the effectively isotropic expansion coefficients of a polycrystalline LSCF55 bar specimen are measured using a vertical high‐performance thermo‐mechanical analyzer and yield αl,bar specimen = 17.37 × 10−6 K−1 and αV,bar specimen = 52.11 × 10−6 K−1.

Effects of PdO modification on the performance of La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes for solid oxide fuel cells: A first principle study

Effects of palladium (Pd) impregnation on the performance of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathodes are investigated with density functional theory plus U (DFT + U) and experimental methods. In-situ high temperature X-ray diffractometer results show that the impregnated Pd species exist at states of palladium oxide (PdO) at 700 °C. The measured electrochemical impedance spectroscopy at 700 °C indicates PdO modification promotes the catalytic activity of LSCF cathodes. The modification structure of PdO on LSCF surfaces and effects of PdO modification on the performance of LSCF cathodes are investigated with DFT + U methods. The results show that B-8 with PdO molecule modification by a parallel posture on LSCF surface is the most stable structure. O2 prefers to be adsorbed on AO-terminated surfaces rather than that on BO2-terminated ones. The oxygen surface adsorption activity of LSCF surface is improved by PdO modification. The calculated partial densities of states (PDOS) and Fermi level of O2 adsorption on LSCF surfaces imply that the charge transfer is easier with PdO modification than that without PdO modification because PdO acts as a metal-like modification. The PdO modification on LSCF surface leads to a better oxygen surface adsorption activity of LSCF cathodes.

Structure and properties of WC–Co coatings obtained on steel substrates by liquid state sintering in vacuum

In this paper, the authors report the structure and mechanical properties of WC–Co coatings produced on low carbon steel substrates by liquid state sintering in vacuum. Special attention is paid to the research of structure evolution during sintering and of complex carbide formation. Material structure was investigated by optical microscopy and scanning electron microscopy. The coating phase composition was researched within the temperature range of 600–1200°C using high temperature X-ray diffractometer. Brittle complex carbides appeared within the temperature range of 600–800°C. The result was confirmed by differential scanning calorimetry. Complex carbide formation increases when the carbon content is low. The results of the research were used to develop a method to suppress the formation of brittle complex carbides by changing the chemical composition of the substrate surface.

Structure transformation of liquid aluminium modified by various electropulse parameters

In this paper, the structural transformations of liquid aluminium modified by different electropulse technique parameters were examined using high-temperature X-ray diffractometer and differential scanning calorimetry apparatus. The results showed that the structure changes depend on the modifying time and temperature. A decrease of the modifying time could result in an obviously weaker principal peak in the structure factor curve compared to the optimal electric pulse technique parameters, but slight increases of coordination number (Ns), correlation radius (rc) and average atom number per cluster (Nat) were still observed. On the other hand, the structure factor curve of the electropulse-modified liquid aluminium at the high modifying temperature of 850°C tended to be overlapped with that of the unmodified during the principal peak range. In this case, the competition result between the effect of electropulse-induced high degree of order and the destruction of superheating determined the final microstructure of the electropulse-modified liquid aluminium.

Heredity of medium-range order structure from melts to the microstructure of Ni–Cr–W superalloy

The structure factor S(Q), intensities and pair distribution function g(r) of liquid Ni–Cr–W superalloy at different temperatures have been measured by a high-temperature X-ray diffractometer. Coordination N min, correlation radius r c, the nearest atomic distance r 1, solidification microstructure and compression performance have been studied. The results show that a pre-peak exists on the structure factor curve at the liquidus temperature, and a fine structure of equiaxed, globular and non-dendritic primary grains can be achieved by casting the alloy at liquidus temperature. Liquid structure feature of Ni–Cr–W superalloy is found to depend on temperature. During the solidification, some structural information carried by the medium-range order (MRO) structure is inherited from the melt to the microstructure, which is beneficial for grain refinement. The maximum yield strength measured from typical microstructure of the equiaxed and non-dendritic grains at 1400 °C is 543 MPa. The results show that refinement and non-dendritic grain is beneficial to the improvement of the yield strength.

Heat-induced spinodal decomposition of Ag-Cu nanoparticles.

Solvothermal synthesis was used for Ag-Cu nanoparticle (NP) preparation from metallo-organic precursors. The detailed NP characterization was performed to obtain information about nanoparticle microstructure and both phase and chemical compositions. The resulting nanoparticles exhibited chemical composition inside a FCC_Ag + FCC_Cu two-phase region. The microstructure study was performed by various methods of electron microscopy including high-resolution transmission electron microscopy (HRTEM) at an atomic scale. The HRTEM and X-ray diffraction studies showed that the prepared nanoparticles form the face centred cubic (FCC) crystal lattice where the silver atoms are randomly mixed with copper. The CALPHAD approach was used for predicting the phase diagram of the Ag-Cu system in both macro- and nano-scales. The predicted spinodal decomposition of the metastable Ag-Cu nanoparticles was experimentally induced by heating on an X-ray powder diffractometer (HT XRD). The nucleation of the Cu-rich phase was detected and its growth was studied. Changes in the Ag-rich phase were observed in situ by X-ray diffraction under vacuum. The heat treatment was conducted at different maximum temperatures up to 450 °C and the resulting particle product was analysed. The experiments were complemented by differential scanning calorimetry (DSC) measurements up to liquidus temperature. The start temperatures of the spinodal phase transformation and particle aggregation were evaluated.

In-Situ High Temperature XRD on U0.54Pu0.46O2-x A Study of the Miscibility Gap

ABSTRACTIt has been shown in previous studies that a miscibility gap exists in the hypo-stoichiometric region UO2-PuO2-Pu2O3 with one phase poor in oxygen, and the other with an O/M (Oxygen to Metal ratio) close to 2.00. Data on the evolution of this region in temperature, especially in the vicinity of the oxygen content corresponding to the highest temperature at which the gap can be observed, is scarce. A high temperature X-ray diffractometer with a dedicated gas control setup was used to study the described region in-situ. We have observed reflections of the two cubic phases, with one increasing and the other decreasing in intensity during the thermal plateaus lasting up to 20 h. We compare the calculated lattice parameters with literature. We estimated the O/M evolution of our samples from a comparison of phase fractions values obtained by Rietveld refinement and calculations using the Calphad method.

Thermophysical properties of thorium–praseodymium mixed oxides

Thorium–Praseodymium mixed oxide solid solutions (Th1−yPry)O2−x (y = 0.15, 0.25, 0.4, 0.55) were prepared by co-precipitation method. These mixed oxides form single-phase fluorite solid solutions (fm3m). Heat capacity (Cp) measurements and lattice thermal expansion characteristics of these solid solutions were determined with differential scanning calorimeter in the temperature range of 298–800 K and high temperature X-ray diffractometer in the temperature range of 298–2,000 K, respectively. The Cp,298 of (Th 1−yPry)O2−x pertaining to the solid solutions with the compositions, y = 0.15, 0.25, 0.4, and 0.55, were found to be 65.2, 62.4, 60.1, and 57.1 J K−1 mol−1, respectively. The coefficients of lattice thermal expansion in the temperature range of 298–2,000 K of (Th1−yPry)O2−x for these solid solutions with the compositions y = 0.15, 0.25, 0.4, and 0.55 were found to be 16.97, 20.43, 25.63 and 30.82 × 10−6 K−1, respectively.

Structure transitions near liquidus and the nucleation of undercooled melt of Ni–Cr–W superalloy

The liquid structure of Ni–Cr–W superalloy at 1405°C and 1420°C is characterized by high temperature X-ray diffractometer, and the effect of liquid structure on exothermic reaction and solidification microstructure has been investigated. The effect of melt structure on the nucleation is used for elucidating the mechanism of grain refinement. It demonstrates that the formation of atomic clusters near liquidus of the Ni–Cr–W alloy can significantly refine the microstructures of the alloys and decrease the degree of undercooling. A pre-peak in front of the main peak of the structure factor S(Q) curve is observed when the melt is cooled down to 1405°C, which is closely related to atomic clusters. The size of the atomic clusters in the melt decreased from 11.05Å to 8.8Å as the temperature increased from 1405°C to 1420°C. Meanwhile, compared with the melt directly cooled down, the degree of undercooling of the melt held at 1405°C becomes smaller. Fine equiaxed and non-dendritic grains with an average size of 60μm and the circularity shape factor (CSF) values of 20 are obtained when the melt is kept at 1405°C for 20min before solidification. The formation of equiaxed and non-dendritic grains is due to the formation of clusters and the high rate of nucleation in the entire undercooled melt.

Thermal expansion behaviour of a versatile monazite phase with simulated HLW: A high temperature x-ray diffraction study

Abstract Synthetic analogues of monazite are considered as an alternate ceramic host for the immobilization of high level waste (HLW) generated in the nuclear fuel cycle. The Ca doped CePO 4 (Ce 0.8 Ca 0.2 PO 4 ) is expected to be a versatile monazite phase for this purpose due to the mixed valence state of cerium in this compound which assists in accommodating cations of any valence by initiating the internal redox reaction, viz., Ce 3+ ⇌ Ce 4+ + e − , as required by the incoming cation for charge neutrality of the lattice. The thermal expansion of this versatile monazite phase with 20 wt.% simulated HLW was measured using high temperature x-ray diffractometer (HTXRD) in the temperature range of 298–973 K and are reported here. The thermal expansion behaviour is found to be similar for CePO 4 , Ce 0.8 Ca 0.2 PO 4 and Ce 0.8 Ca 0.2 PO 4 with 20 wt.% simulated HLW with the average volume expansion coefficient of 27.4, 26.5 and 24.1 × 10 −6 K −1 respectively in the temperature range of 298–973 K.