High-temperature Phase Stability Research Articles

Crystal structure, local structure and magnetic properties of NiCr2-xFexO4 (x = 0.3–0.6) spinel

The crystal and local structures of NiCr2-xFexO4 (x = 0.3 to 0.6) spinels synthesized by a co-precipitation method are examined using x-ray diffraction (XRD) in combination with x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) techniques. Rietveld refinement of XRD patterns illustrates the stabilization of high-temperature cubic phase (space group Fd3‾m) at room temperature accompanied by a distortion of octahedron within the spinel structure. XANES analysis reveals that while Fe and Cr cations have the average oxidation state of +3 with their local environment as tetrahedral (A) and octahedral (B) sites, respectively, Ni cations have the average oxidation state of +2 and they are distributed between the A and B sites. The modeling of EXAFS spectra further quantitatively demonstrates that with increasing x, the occupancy of Ni cations in the A site decreases from almost 70 to 40%. The paramagnetic to long-range ferrimagnetic transition at Curie temperature, TC is found to increase from 253 K for x = 0.3 to above 350 K for x = 0.6 without showing any spin-spiral ordering temperature, TS. The dominance of A-B exchange interaction over B–B exchange interaction contributes to such behaviors which are further confirmed from the variation in bond lengths and bond angles obtained from the structural analysis. Showing an unusually high coercivity, HC of 36 kOe for x = 0.3 compared to other x, an increase in saturation magnetization, MS, and decrease in coercivity, HC with increasing x, have been understood through Néel's two sublattice model proposed for ferrimagnetic structure.

A potential ZrO2 doped scandium tantalite ceramics for thermal/environmental barrier coating material

Despite scandium tantalite (ScTaO4) has great potential application for thermal/environmental barrier coating system, it is necessary to reduce the thermal expansion coefficient difference between them to lower the thermal stress in practical applications. In this paper, (ZrO2)x-(ScTaO4)1-x (x = 0.1, 0.2) material was prepared by doping ZrO2 into scandium tantalite ceramics, and its thermal expansion coefficient and calcium–magnesium–alumino-silicate (CMAS) corrosion behavior were studied. After the CMAS corrosion experiments of (ZrO2)x-(ScTaO4)1-x material, the phase composition and microstructures of samples were detected by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis. The main results show that the alloying effect of ZrO2 doping can reduce the thermal expansion coefficient of ScTaO4 and then alleviate the thermal expansion coefficient difference in thermal/environmental barrier coating system. Besides, Zr4+ ions are separated from (ZrO2)x-(ScTaO4)1-x lattice by molten CMAS under high-temperature corrosion condition and (ZrO2)x-(ScTaO4)1-x material is decomposed into ScTaO4 and t-ZrO2 finally. In addition, only t-ZrO2 is defected in the corrosion products without other crystal type of ZrO2 in the XRD results of the corrosion products, which indicate that ScTaO4 can increase high-temperature phase stability of t-ZrO2 and prevent phase transformation from t-ZrO2 to m-ZrO2.

New class of high-entropy defect fluorite oxides RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Y, Ho, Er, or Yb) as promising thermal barrier coatings

High-entropy ceramics exhibit great application potential as thermal barrier coating (TBC) materials. Herein, a series of novel high-entropy ceramics with RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE2HE2O7, RE = Y, Ho, Er, or Yb) compositions were fabricated via a solid-state reaction. X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) mapping analyses confirmed that RE2HE2O7 formed a single defect fluorite structure with uniform elemental distribution. The thermophysical properties of the RE2HE2O7 ceramics were investigated systematically. The results show that RE2HE2O7 ceramics have excellent high-temperature phase stability, high thermal expansion coefficients (10.3–11.7 × 10−6 K-1, 1200 ℃), and low thermal conductivities (1.10-1.37 W m-1 K-1, 25 ℃). In addition, RE2HE2O7 ceramics have a high Vickers hardness (13.7–15.0 GPa) and relatively low fracture toughness (1.14-1.27 MPa m0.5). The outstanding properties of the RE2HE2O7 ceramics indicate that they could be candidates for the next generation of TBC materials.

Fabrication and characterization of Yb2Si2O7-based composites as novel abradable sealing coatings

Ceramic matrix composites (CMCs) have gradually replaced superalloys for use in hot sections of aero-engines to meet the requirements for increasingly high engine temperatures. However, the abradable sealing coatings (ASCs) commonly applied to superalloys are not suitable for CMCs owing to factors such as the difference in the coefficient of thermal expansion, therefore it is necessary to develop a new system suitable for CMCs. Yb2Si2O7 exhibits good high-temperature phase stability and relatively low hardness and may be a suitable material for ASCs. In the present study, Yb2Si2O7-based composite coatings with different hexagonal boron nitride (hBN) contents (0, 5, 10, and 15 wt%) were deposited by air plasma spray (APS), and the microstructure, mechanical properties, and abradability of the coatings were characterized. The results showed that the system of Yb2Si2O7 combined with hBN exhibits lower hardness, lower friction coefficient, higher wear rate, and smaller IDR value than the pure Yb2Si2O7 coating and is appropriate for use in ASCs. However, excessive hBN caused severe wear of the coatings. The relevant wear mechanisms of the coatings were analysed, and this study may provide guidelines for the development of ASCs suitable for CMCs.

Stabilization of Superionic-Conducting High-Temperature Phase of Li(CB9H10) via Solid Solution Formation with Li2(B12H12)

We report the stabilization of the high-temperature (high-T) phase of lithium carba-closo-decaborate, Li(CB9H10), via the formation of solid solutions in a Li(CB9H10)-Li2(B12H12) quasi-binary system. Li(CB9H10)-based solid solutions in which [CB9H10]− is replaced by [B12H12]2− were obtained at compositions with low x values in the (1−x)Li(CB9H10)−xLi2(B12H12) system. An increase in the extent of [B12H12]2− substitution promoted stabilization of the high-T phase of Li(CB9H10), resulting in an increase in the lithium-ion conductivity. Superionic conductivities of over 10−3 S cm−1 were achieved for the compounds with 0.2 ≤ x ≤ 0.4. In addition, a comparison of the Li(CB9H10)−Li2(B12H12) system and the Li(CB9H10)−Li(CB11H12) system suggests that the valence of the complex anions plays an important role in the ionic conduction. In battery tests, an all-solid-state Li–TiS2 cell employing 0.6Li(CB9H10)−0.4Li2(B12H12) (x = 0.4) as a solid electrolyte presented reversible battery reactions during repeated discharge–charge cycles. The current study offers an insight into strategies to develop complex hydride solid electrolytes.

High temperature phase stability and wear behavior of laser clad Ta reinforced NiCrBSi coating

Ta is added to laser clad NiCrBSi coating to improve high-temperature wear resistance. The influence of heat treatment of 700 °C on Ta reinforced coating is studied. The microstructure is analyzed by XRD, SEM, EDS, TEM and XPS. The tribological performances were investigated on high temperature pin-on-disk tribometer at temperatures up to 700 °C. Ta reinforced coating mainly includes γ-Ni, TaC, Ni3Fe, CrB, M23C6 and Cr2.8Fe1.2B4. Under heat treatment, most of the phases remain stable except that M23C6 is dissolved and coarsened. CrB formed on TaC particle is evolved into Cr2.8Fe1.2B4. The heat treatment has little influence on the hardness of Ta reinforced coating. It is difficult to form an oxide layer on worn surface of NiCrBSi coating at any temperature. On the contrary, an oxide layer form on worn surface of Ta reinforced coating at 700 °C. Ta significantly improves the wear resistance of NiCrBSi coating at high temperatures, which is attributed to Ni matrix strengthened by Ta, TaC with high thermal stability, clean TaC/matrix interface, and compact oxide layer. The main wear mechanism below 600 °C is abrasion and adhesion, and oxidization, which transitions to delamination, abrasion, ploughing and oxidization at 700 °C.

Phase composition, microstructure and thermophysical properties of the Srx(Zr0.9Y0.05Yb0.05)O1.95+x ceramics

Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=1.0, 0.9, 0.8, 0.7) ceramics were prepared by solid state reaction sintering. The sintered Sr1.0(Zr0.9Y0.05Yb0.05)O2.95 is a single-phase solid solution while the sintered Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=0.9−0.7) are composites, and a significant grain growth inhibition is observed in the sintered Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=1.0, 0.9). Rare-earth elements distribution in the bulk materials indicates that Yb and Y preferentially substitute Zr-sites in SrZrO3, and the highest solubility of RE2O3 in pure SrZrO3 is ∼0.8 mol%. The sintered Srx(Zr0.9Y0.05Yb0.05)O1.95+x have high thermal expansion coefficients up to ∼11.0×10−6 K-1 (1200°C). Sr0.8(Zr0.9Y0.05Yb0.05)O2.75 has the lowest thermal conductivity of 1.38 W·m-1·K-1 at 800°C. Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=1.0, 0.9, 0.8) show no phase transition from 600 to 1400°C, whereas Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=0.9, 0.8) have excellent high-temperature phase stability over the whole investigated temperature range. Therefore, Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=1.0, 0.9, 0.8) are considered as promising TBCs materials that might be operated at higher temperatures compared to YSZ.

High-entropy intermetallic compound with ultra-high strength and thermal stability

The current research of high-entropy alloys (HEAs) is mainly focused on simple solid-solution alloys or alloys with precipitation strengthening phases. In the present work, we developed a series of high-entropy intermetallic compounds (HEICs). In particular, the multi-principal element Fe0.75Co0.75Ni0.75Cu0.75TiZrHf with B2 ordering shows an ultrahigh compressive yield strength of 2.25 GPa and a fracture strength of 2.52 GPa. The developed HEIC also demonstrates exceptional high-temperature phase stability with retained yield strengths up to 905 MPa and 705 MPa at 800 °C and 900 °C, respectively. The underlying phase formation rule of the HEICs is discussed within the framework of a proposed electronegativity related parameter combined with the atomic size dispersity. Our study broadens the family of high-entropy materials and provides a new perspective for the high-temperature alloy design based on multi-principal element intermetallics.

Fabrication of zirconia ceramics by sintering in a magnetic field

An external magnetic field was applied for activating the process of sintering the ZrO2-based ceramic materials in order to lower high sintering temperatures and improve the quality of the ceramics. The regularities of the formation of the structure of crystalline phases and the strength properties of zirconium ceramics during sintering of compacts from ultradispersed powders in the presence of a constant magnetic field (H = 4.01·1010–7.87·1010 A/m) with the symmetry Fp = C3 have been investigated. Sintering was carried out in the volume of a resistance furnace. The magnetic field inside the furnace volume was created by a toroidal coil with a special symmetrized winding. It has been established that the crystal structure, morphology and shape of grains in ZrO2 ceramics sintered in the constant magnetic field at 1400 °C are characterized by more perfect crystallographic forms when compared with sintering without an applied magnetic field, and the degree of isometricity increases. In this case, the degree of crystallinity increases, on average, by 2.4 times its initial value. The observed formation and stabilization of high-temperature phases (high-temperature modifications of the о* and с phases) of zirconium dioxide when sintering in the presence of the magnetic field are associated with directional changes in the structural characteristics (a decrease in crystalline microdistortions, an increase in the degree of uniformity). The oxide ceramics sintered in the presence of a constant magnetic field (B = 0.02–1 T) with a given symmetry С3 possess improved physical and mechanical characteristics (the strength and density increase by 54% and 15% from their initial values, respectively) and more perfect crystalline forms of microstructures, as compared with sintering without the imposition of an external magnetic field.

Atomistic modeling of anisotropic mechanical properties of lanthanum zirconate nanocystal

Lanthanum zirconate (La2Zr2O7, or LZ) has been widely recognized as a promising candidate material for thermal barrier coating (TBC) applications since it has low thermal conductivity, high-temperature phase stability, and low sintering activity. However, the mechanical properties of LZ crystal have not been fully understood, which hinders it from future applications. In this work, atomistic simulations were performed to study the anisotropic mechanical properties of LZ crystal. Using both the first principles and molecular dynamics (MD) calculations, uniaxial tensile tests of LZ crystal in [001], [011], and [111] directions were simulated. The stress-strain curves of the tensile tests were calculated, and the ultimate tensile strength and toughness were derived. The Young's moduli in [001], [011], and [111] directions were calculated using both the stress-strain curves and an analytical method for small deformation. Additionally, shear stress-strain curves in {111}<110> and {111}<112‾> directions were investigated using both the first principles calculations and the MD method. Results show that Young's modulus of LZ crystal is highly anisotropic. The crystal has the highest Young's modulus in [111] direction, and {111}<112‾> direction is the favorable slip system during shear deformations.

Investigation of the High-Temperature Oxidation Behavior of the Al0.5CoCrFeNi High Entropy Alloy

As a new class of materials, High Entropy Alloys (HEAs), have been recently used for various usages such as high-temperature applications. High entropy alloys have such unique properties as high strength, high-temperature phase stability, good fracture resistance, and excellent softening resistance at high temperatures. Studies on the oxidation behavior of as-cast high entropy alloys are, however, currently limited. The oxidation behavior of Al0.5CrCoFeNi HEA was investigated at three different temperatures of 800, 900, and 1000°C, and in the air. The results showed that the oxidation kinetics of this alloy followed the parabolic rate law. However, at 1000°C, two-step oxidation kinetics was found: in the first step, it followed the almost linear oxidation rate law (n=0.97); in the second one, the predominant mechanism followed the parabolic rate law (n=0.56). Analysis of the morphology and phase composition of the oxide layers was done using the Scanning Electron Microscopy (SEM) and X-ray diffraction. At all temperatures, the results showed that the oxide layers consisting of Cr2O3/Al2O3 and spinel of Ni, Co, and Fe were formed in the Al depleted zone.

High-entropy (Nd0.2Sm0.2Eu0.2Y0.2Yb0.2)4Al2O9 with good high temperature stability, low thermal conductivity, and anisotropic thermal expansivity

The critical requirements for the environmental barrier coating (EBC) materials of silicon-based ceramic matrix composites (CMCs) include good tolerance to harsh environments, thermal expansion matches with the interlayer mullite, good high-temperature phase stability, and low thermal conductivity. Cuspidine-structured rare-earth aluminates RE4Al2O9 have been considered as candidates of EBCs for their superior mechanical and thermal properties, but the phase transition at high temperatures is a notable drawback of these materials. To suppress the phase transition and improve the phase stability, a novel cuspidine-structured rare-earth aluminate solid solution (Nd0.2Sm0.2Eu0.2Y0.2Yb0.2)4Al2O9 was designed and successfully synthesized inspired by entropy stabilization effect of high-entropy ceramics (HECs). The as-synthesized HE (Nd0.2Sm0.2Eu0.2Y0.2Yb0.2)4Al2O9 exhibits a close thermal expansion coefficient (6.96×10-6 K-1 at 300–1473 K) to that of mullite, good phase stability from 300 to 1473 K, and low thermal conductivity (1.50 W·m–1·K–1 at room temperature). In addition, strong anisotropic thermal expansion has been observed compared to Y4Al2O9 and Yb4Al2O9. The mechanism for low thermal conductivity is attributed to the lattice distortion and mass difference of the constituent atoms, and the anisotropic thermal expansion is due to the anisotropic chemical bonding enhanced by the large size rare-earth cations.

Potential thermal barrier coating materials: RE2FeTaO7 (RE = Y, Eu, Gd, Dy) compounds

In this paper, the A2B2O7-type compounds RE2FeTaO7 synthesized via a solid-state reaction were researched for high-temperature applications, and their thermophysical properties were documented for the first time. The relationships between thermophysical properties and structures of RE2FeTaO7 were established, the results show that the RE2FeTaO7 compounds have lower thermal conductivity (1.5–2.8 W m−1 K−1) than 8YSZ (2.4–3.0 W m−1 K−1) and La2Zr2O7 (2.0–2.8 W m−1 K−1) at temperature of 100–900 °C, implying their prominent thermal insulation performance. The thermal expansion coefficients (TECs, ∼9.2 × 10−6 K−1, 1200 °C) of RE2FeTaO7 are equivalent to La2Zr2O7 (∼9.0 × 10−6 K−1, 1200 °C). Furthermore, the excellent high-temperature phase stability, high hardness (∼10.2 GPa) and fracture toughness (∼1.7 MPa m1/2) jointly indicate that RE2FeTaO7 compounds are potential thermal barrier coating materials.

Crystallography, thermodynamics and phase transitions in refractory binary alloys

We investigate phase stability in all binary alloys comprised of elements from groups 4 (Ti, Zr, Hf), 5 (V, Nb, Ta) and 6 (Cr, Mo, W) of the periodic table. First-principles calculations of the energy landscapes along crystallographic pathways that connect bcc to hcp and bcc to ω show that group 4 elements are very distinct from group 5 and 6 elements. While group 5 and 6 elements are stable in bcc, group 4 elements favor hcp and ω and are predicted to be dynamically unstable in bcc. A comprehensive first-principles investigation of the 36 refractory binary systems using statistical mechanics techniques reveals six distinct classes of alloys, each with a unique phase diagram topology. The predictions of this study are in excellent agreement with previous experimental work. One exception is a class of refractory alloys with high temperature miscibility gaps that are not predicted with the methods used in this work. Our calculations predict the stability of a low-temperature Laves phase in the Nb-V binary that has yet to be observed experimentally. The relationships between alloy chemistry and high-temperature phase stability revealed in this study provide a basis for the systematic design of multicomponent disordered refractory alloys.

Stabilization of high-temperature Ag2Se phase at room temperature during the crystallization of an amorphous film

Amorphous films of Ag2Se were prepared by laser ablation. After being heated up to 270 °C, the films form oriented crystalline structure of orthorhombic β-modification upon further cooling at temperatures of 110 – 90 °С. The films undergo reversible β↔α phase transitions during the following temperature cycling, while the oriented crystalline structure stays the same. X-ray analysis showed no impurities reflections, excluding the one with very low intensity attributed to an elemental silver. The silver nanocrystals apparently form due to the selenium evaporation from the film surface during heating. The temperature dependence of the film's conductivity coincides with the one for bulk stoichiometric Ag2Se samples.It was shown that a considerable amount of α-Ag2Se is preserved in the film at room temperature even at low cooling rate. This phase is subjected to a significant mechanical stress caused by a density difference between α and β modifications. This circumstance points to the possibility of stabilization of high-temperature superionic modification of Ag2Se at room temperature in a nanolayered structure, in which some alternating layers create a similar stress on the Ag2Se layers.

Phase stability and conductivity of rare earth co-doped nanocrystalline zirconia electrolytes for solid oxide fuel cells

Abstract Solid oxide fuel cell (SOFC) is a green energy technology that directly coverts chemical energy into electricity. Scandia stabilized zirconia (SSZ) shows the highest conductivity among zirconia based electrolytes for SOFCs. However, the stability of the cubic phase, which is the desirable phase for high conductivity, can be an issue in SSZ electrolytes due to its transformation to other low-conducting phases at higher temperatures. In the present investigation, SSZ electrolyte was co-doped with ytterbia, gadolinia and ceria with an objective of improving the high-temperature phase stability. Both the doped and co-doped compositions exhibited a single cubic phase in the as-processed condition. The phase stability at high temperature was studied by aging the sintered pellets at 900 °C for 500 h in air. X-ray diffraction and transmission electron microscopy analysis revealed formation of small amount of the low-conducting tetragonal phase in 1 mol % ytterbia and gadolinia co-doped compositions on ageing which resulted in conductivity degradation. Increasing the doping level to 2 mol% prevented the formation of the tetragonal phase. The ceria co-doped composition (1 mol%), on the other hand, was clean without any sign of the secondary phases even after high-temperature ageing. The rhombohedral ‘β’ phase formed in the binary composition (SSZ) after sintering but was absent in all the co-doped compositions. The conductivity of the co-doped samples was higher than the binary SSZ. Thus, it can be said that rare earth co-doping is an effective way of improving the phase stability and conductivity of SSZ electrolytes.

High temperature mechanical and thermal properties of CaxBa1-xZrO3 solid solutions

The excellent high temperature phase stability enables the perovskite BaZrO3 to be a promising candidate for thermal barrier coatings (TBCs). Further improvement of its thermo-physical properties is necessary to fulfill the increasing requirements for TBCs. In this work, CaxBa1-xZrO3 (x = 0.05, 0.10, 0.15, 0.20, 0.25) solid solutions are fabricated and the influence of the Ca2+ content on their mechanical/thermal properties is studied. They have moderate mechanical properties and their residual Young's moduli at 1473 K are around 80% of the values at room temperature. Compared with BaZrO3, CaxBa1-xZrO3 solid solutions have decreased thermal conductivities which are comparable to those of YSZ and other potential TBCs materials. Among these solid solutions, Ca0.25Ba0.75ZrO3 exhibits the lowest thermal conductivity that is in the range from 2.06 W⋅m-1⋅K-1 to 2.52 W⋅m-1⋅K-1. Meanwhile, CaxBa1-xZrO3 solid solutions have higher thermal expansion coefficients than that of BaZrO3. These good high temperature mechanical and thermal properties reveal their promising applications as high temperature structural ceramics including TBCs.

Microstructure and thermophysical properties of CeO2-doped SmTaO4 ceramics for thermal barrier coatings

Abstract

High-pressure, high-temperature phase stability of iron-poor dolomite and the structures of dolomite-IIIc and dolomite-V

The stability of Fe-poor dolomite, CaMg0.98Fe0.02(CO3)2, was studied by Raman spectroscopy and single-crystal X-ray diffraction at high pressures (P < 60 GPa) and high temperatures (T < 2300 K). Density functional theory calculations were employed to complement the experimental study. Between 40 and 60 GPa and 1800–2300 K, we observed the formation of a high P,T phase, “dolomite-V”, which, after quenching to ambient temperature, remained stable down to 12 GPa. The dolomite-V phase crystallizes in the space group C2/c with Z = 4 formula units. The structure of the high pressure polymorph “dolomite-IIIc” was solved, which crystallizes in the space group P1¯ with Z = 8 formula units. The combined experimental and theoretical findings show that at high pressures and low to moderate temperatures Dol-IIIc is formed, while at high pressures and high temperatures Dol-V becomes stable. Assuming that thermodynamic equilibrium is obtained at high-pressure, high-temperature conditions, the current study extends our understanding of the phase stability of Fe-poor dolomite polymorphs at upper and lower mantle conditions.

Iron-rich carbonates stabilized by magnetic entropy at lower mantle conditions

Constraining the flux of carbon in and out of the interior of the Earth due to long-term geological processes is important, because of the influence that it has on climate change. On timescales of billions of years, host minerals such as carbonate phases could play a significant role in the global carbon cycle, transporting carbon into the lower mantle as a component of subducting slabs. We use density functional theory based calculations to study the high-pressure, high-temperature phase stability of Mg1-xFexCO3. Our results show that iron-rich phases, where carbon is in tetrahedral coordination, are only stable at lower mantle conditions due to their magnetic entropy, which is also responsible for the unusual shape of their phase boundary. Low-pressure carbonate phases are found to be highly anisotropic, but high-pressure carbonate phases are not, which has important implications for their seismic detectability. Our work confirms that future discussions of the global carbon cycle should include the deep Earth.