Structural Phase Stability Research Articles

A novel low thermal conductivity thermal barrier coating at super high temperature

Novel high temperature-resistant thermal barrier coatings are urgently required as the stringent operating environment. High temperature property studies of zirconia matrix thermal barrier coatings doped with Cr3+, Mg2+, Si4+, and Mn4+ ions prepared by solid phase reaction method are reported. The results of X-ray diffraction measurements prove the coatings prepared by air plasma spray are predominately in a non-transformable cubic structure which is the most stable phase structure at high temperature. Hot corrosion tests of molten salts (at 900 °C) and oxides (at 1400 °C) have been carried out, and the results show that the corrosions lead to the coating surface deterioration. The thermal conductivity of coating doped with Cr3+ is the lowest value until to 1600 °C, and the mechanism has been analyzed. The antioxidant ablation results also proved that the coating doped with Cr3+ showed the best heat-insulating property. All the results show that the coating doped with Cr3+ will be a promising thermal barrier coating.

Explanation of structural differences and similarities between the AT2Al10 phases (where A=actinide, lanthanide or rare earth element and T=transition metal)

Abstract In the current work we have studied the crystallographic relationship between the AT2Al10 phases (where A = actinide, lanthanide and rare earth element and T = transition metal). It is known that with this stoichiometry two structure types exist: tetragonal CaCr2Al10 and orthorhombic YbFe2Al10. It was found that both CaCr2Al10 and YbFe2Al10 types are structural derivatives of the ThMn12 type structure (which has more general formula of AT x Al12− x , with x > 2). CaCr2Al10 structure has a group-subgroup relationship with the ThMn12 structure, while the relationship of the YbFe2Al10 to the ThMn12 was proved applying the strong reflection approach, suggested initially for approximants of quasi-crystals. Proposed here relationship between the studied structures explains the small difference in total energies, calculated using Density Functional Theory. Understanding the connection between these phases allows regarding AT2Al10 composition as somewhat extension of the AT x Al12− x compositional range. Due to the unique magnetic properties of the AT x Al12− x phases, tunable as a function of crystallographic structure, study of structural stability and crystallographic relationships of related phases are of outmost importance.

Structural Optimization and Phase Stability of Cubic Li7La3Zr2O12 for Solid Electrolyte Application

All-solid-state battery is considered as the next-generation batteries due to its high safety and increased energy density, in which the solid electrolyte is one of the key component that determines its performance. The search for stable solid electrolytes with high conductivity is currently an important issue to compete the liquid ones in commercial batteries. The garnet-type structure Li7La3Zr2O12 (LLZO) have attracted great attention, especially the cubic LLZO which is the best conducting Li-oxide garnet reported to date. In this work, the stoichiometric structure of pristine cubic LLZO has been successfully optimized by using the first-principle calculations.This has not been previously reported so far due to the complicated Li disorder. Moreover, the chemical potential diagram is employed to determine the competing phases for phase stability evaluation and is constructed by the thermodynamic conditions of the phase and the constraints imposed by all stable compounds on the Li–La–Zr–O phase diagram. This work is expected to provide a better understanding on the complex cubic LLZO structure as well its phase stability. Figure 1

Raman and X-ray photoelectron spectroscopic characterizations of thermal stability of 3 mol% yttria stabilized zirconia ceramics

Structural variations and phase stability in 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP) subjected to long-term aging at room temperature and thermal treatments in air were investigated by Raman and X-ray photoelectron spectroscopies, and compared with previous cases obtained by hydrothermal treatments. The results revealed a significant difference in the kinetics of zirconia polymorphic transformation in different environments due to the critical roles played by surface and lattice hydroxyl defects. Thermal treatments in air of long-term stored 3Y-TZP revealed a gradual recovery to tetragonal phase due to the ocurrence of dehydroxylation, while thermal treatments of pristine 3Y-TZP caused tetragonal-to-monoclinic polymorphic transformation at low temperature, but then an inverse transformation at higher tempertures due to a competition of hydroxylation and dehydroxylation. Treatments of 3Y-TZP in different environments were found to lead to distinct paths of off-stoichiometric chemistry, including hydroxylation, hydroxyl migration, surface reconstruction, and dehydroxylation during the treatments.

The study of phase stability and thermal shock resistance of a Scandia–Ceria stabilized zirconia as a new TBC material

The nanostructured Scandia-Ceria stabilized Zirconia (SCSZ) was synthesized via a sol-gel route and the effects of different contents of Scandia stabilizer (3.6–8 mol. %) on phase stability and thermal shock resistance were investigated compared to traditional YSZ. The results revealed that after heat treating at 1400 °C for 25 h, the optimum structure belonged to 4.78SCSZ system due to non-formation of monoclinic and cubic phases in structure and high tetragonal (t′) phase stability. The increased coefficient of thermal expansion of 4.78SCSZ was also confirming the decreased thermal mismatch stresses and improvement in thermal performance compared to traditional YSZs. In this regard, the results of thermal shock resistance showed a promising cyclic lifetime. However, the microstructural investigations indicated that the TGO layer has been formed during thermal treatment and hence, the combined effects of TGO growth and thermal expansion misfit have intensified tensile stresses governing crack growth near and parallel to coating interface.

Non-monotonic variation of structural and tensile properties with Cr content in AlCoCrxFeNi2 high entropy alloys

A series of AlCoCrxFeNi2 (x = 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4) high entropy alloys (HEAs) were designed to investigate the structural evolution and tensile properties variation with Cr content. It was found that all the alloys were composed of face-centered cubic (FCC) phase and disordered body-centered cubic (BCC) plus ordered body-centered cubic (B2) region. However, the volume fraction of FCC phase first increased and then decreased with increasing Cr content, while the valence electron concentration (VEC) values kept decreasing, which contradicts with the universal VEC criterion. Here, the partition ratio of Cr element in FCC/(BCC + B2) region was introduced to describe the structural evolution and phase stability. Our work reveals that Cr is FCC stabilizer at low concentration, and promotes the formation of BCC phase at high concentration. Similarly, the strength and ductility exhibit a non-monotonic variation with increasing Cr content.

Elastic anisotropies and thermal properties of cubic TMIr (TM=Sc, Y, Lu, Ti, Zr and Hf): A DFT calculation

Structural properties, phase stabilities, elastic properties and Debye temperatures of cubic TMIr (TM = Sc, Y, Lu, Ti, Zr and Hf) compounds were investigated by first-principles calculations. The calculated lattice parameters and formation enthalpies agree well with the reported experimental data, indicating the reliability of this work. Polycrystalline elastic modulus, Poisson’s ratio and Debye temperature were calculated based on single-crystal elastic constants. The elastic anisotropy of TMIr was evaluated by elastic anisotropy indexes, surface constructions and projections, and polar plots of elastic modulus. The elastic anisotropy is in a sequence of ZrIr > HfIr > TiIr > LuIr > YIr > ScIr. Furthermore, Debye temperature and the anisotropy of sound velocity of the TMIr were also calculated and discussed.

Zinc substituted MgH2 - a potential material for hydrogen storage applications

Abstract The search for efficient materials for onboard hydrogen storage applications is an emerging research field. Using density functional calculations, we demonstrate Zn substituted MgH2 as a potential material for hydrogen storage. We predicted the ground state crystal structure of ZnH2 which is found to be Pna21 (orthorhombic) structure with meta-stable behavior. The structural phase stability and phase transition of Mg1−xZnxH2 systems have been analyzed. The H site energy of Mg1−xZnxH2 systems is calculated to understand the hydrogen desorption process. Our calculations suggest that Zn substitution reduces the stability of MgH2, thereby it may reduce the decomposition temperature of MgH2. The band structure and density of states calculations reveal that the Mg1−xZnxH2 systems are insulators. The chemical bonding behavior of Mg1−xZnxH2 systems is established as iono-covalent in nature. Moreover, Zn substitution in MgH2 induces disproportionate Mg H bonds which could also contribute the reduction in the decomposition temperature as well as H sorption kinetics.

Epoxy resin addition on the microstructure, thermal stability and microwave absorption properties of core-shell carbonyl iron@epoxy composites

Abstract In this study, novel CIP@EP composites with a core-shell structure were prepared by using the in-situ polymerization method. The phase structure, morphology and thermal stability were investigated by FTIR, XRD, SEM, TEM and TG. The results indicated that EP were densely covered on the surface of CIP particles, and with the increase of EP contents, the EP shell became thicker, which effectively strengthened the thermal stability of the materials. The magnetic and electromagnetic properties of the composites were systematically investigated. It was found that the content of EP played an important role in improving the magnetic and dielectric properties. High EP content can effectively improve the impedance matching between the dielectric and magnetic loss of the absorbers and enhance the EM wave absorption ability of the composites. This study proves that the CIP@30%EP exhibits an excellent EM wave absorption ability, which has higher value of RL and wider absorption width than other CIP@EP composites. The CIP@30%EP has a strong reflection loss peak of −16.1 dB and wide effective absorption bandwidths of 5.4 GHz at the thickness of 2.2 mm. Furthermore, the method utilized to synthesis the CIP@EP composites can be a suitable and efficient way to prepare other spherical microwave absorption materials.

Structural Properties and Phase Stability of Primary Y Phase (Ti₂SC) in Ti-Stabilized Stainless Steel from Experiments and First Principles.

The morphology and microstructural evaluation of Y phases in AISI 321 (a Ti-stabilized stainless steel) were characterized after hot deformation. The electronic structure and phase stability of titanium carbosulfide were further discussed by first-principle calculations. It was found that Y phases, like curved strips or bones in AISI 321 stainless steel, mostly show a clustered distribution and are approximately arranged in parallel. The width of the Y phase is much less than the length, and the composition of the Y phase is close to that of Ti2SC. Y phases have exceptional thermal stability. The morphology of Y phases changed considerably after forging. During the first calculations, the Ti2SC with hexagonal structure does not spontaneously change into TiS and TiC; however Ti4S2C2 (Z = 2) can spontaneously change into the two phases. The Ti–S bonds are compressed in Ti4S2C2 cells, which leads to poor structural stability for Ti4S2C2. There is a covalent interaction between C/S and Ti, as well as an exchange of electrons between Ti and S/C atoms. Evidently, the mechanical stability of Ti4S2C2 is weak; however, Ti2SC shows high stability. Ti2SC, as a hard brittle phase, does not easily undergo plastic deformation.

Domain evolution and corresponding piezoelectricity of lead-free In2O3-doped K0.5Na0.5NbO3 ceramics together with improved fatigue resistance and temperature stability

In this paper, (1-x)K0.5Na0.5NbO3-x mol% In2O3 (abbreviated as KNN-xIn) lead-free piezoelectric ceramics were prepared using conventional solid sintering method. The effects of In2O3 on the phase structure, microstructure, domain evolution, electrical properties, fatigue behavior and thermal stability of KNN ceramic matrix were systematically studied. XRD results show that all the ceramics possess the orthorhombic phase with domain architectures presented in the PFM. The addition of In2O3 improved the fatigue resistance owing to the enhanced density. The temperature stability of the 1.0 mol% In2O3 doped sample was significantly improved. It was found that the KNN ceramics with 1.0 mol% In2O3 addition exhibited good electrical properties (d33* = 179 pm/V, d33 = 103 pC/N, TC = 388 °C, Pr = 31.52 μC/cm2, εr = 327, tanδ = 3.28%). The results suggested that this material should be an attractive lead-free material for piezoelectric applications.

Investigation of switching behavior of acceptor-doped ferroelectric ceramics

Switching behavior is a general feature in ferroelectrics. The related fatigue effects influenced by defect dipoles in ferroelectrics are still controversial that is focused on the positive and negative effects of oxygen vacancies. Here, we report the polarization switching behavior of acceptor-doped ceramics using the first-order reversal curve (FORC) approach, especially for the abnormal self-rejuvenation effect and the enhanced fatigue endurance in acceptor-doped ceramics. The reversible and irreversible components under electric field in the ceramics were distinguished by the FORC distribution of ideal “hysteron”. The abnormal self-rejuvenation behavior stemmed from dispersive response of hysteron for undoped samples while from the redistribution of defect dipoles for acceptor-doped samples. The self-rejuvenation was induced mainly by the irreversible component. For the fatigue effect, the pinning of domain walls was not the main reason. The re-annealing treatment for a fatigued sample weakened the interactions between the spontaneous polarizations and defect dipoles, but enhanced the dispersion of coercive field. The enhancement of fatigue endurance came from the phase stability of structure in acceptor-doped ceramics, while complex phase evolution existed in undoped ceramic with weak fatigue endurance. Our study shed new light on the interactions between spontaneous polarization and defect dipoles under repetitive AC electric field.

Preparation and Performance Analysis of Salix Carboniferous Solid Acid Catalysts

Salix carboniferous solid acid catalyst was prepared by pyrolysis carbonization and sulfonation of Salix psammophila, used in esterification of oleic acid and methanol for biodiesel production. The transformation of pore structure ,peak state of functional groups, phase structure, morphological characteristics,and thermal stability of Salix,Carbonized intermediate and Sulfonated catalyst were characterized by BET Static nitrogen adsorption instrument, Fourier transform infrared spectrometer (FT-IR), Scanning electron microscope(SEM),X-ray diffractometer (XRD). Combined with the analysis of infrared and nitrogen adsorption data, the catalyst made the great esterification rate of oleic acid, which was the result of the synergistic effect of three kinds of functional groups and pore structure. Combined with SEM and XRD data, the amorphous carbon structure of the catalyst is important to the catalytic performance, and the catalyst has good stability. Therefore, the preparation of biomass carbon-based solid acid catalyst have received great attention in many aspects.

The experimental and theoretical study on influence of Al and Cu contents on phase abundance changes in AlxCuyFeCrNiCo HEA system

Two series of AlxCuyFeCrNiCo (0.0≤x≤1.5, 0.5≤y≤2) high entropy alloys were prepared using arc melting and sintering techniques. For all samples, XRD and EDX measurements were performed. Three different structures (two fcc and one bcc) were detected using X-ray diffraction technique, while four chemically different phases were identified by EDX, with presence and stoichiometry dependent on the Al and Cu concentrations. Cu-rich and FeCrNiCo-rich phases were identified, having fcc structures with different lattice parameters. AlNi-rich and Cr-rich phases crystallize in bcc/B2 and bcc structures with similar lattice parameters indistinguishable using standard XRD technique. The preparation method did not influence meaningfully on phase composition and stoichiometry of the analyzed system. The increase of Cu content caused mostly increase of the Cu-rich phase abundance. In turn, the increase of Al led to vanishing of the FeCrNiCo-rich phase and appearance of Cr-rich as well as AlNi-rich phases. The KKR-CPA electronic structure calculations were performed to have information about phase preference and structural stability. The reduction of total energy of multi-phase AlxCuyFeCrNiCo system in comparison to the single-phase one is evidenced. It appears that further lowering of total energy can be achieved when admitting atoms ordering in AlNi-rich phase in frame of B2 crystal structure.

First-principles calculations of structural, elastic and thermodynamic properties of (h, r)-TiAl2

In this work, based on density functional theory (DFT) within generalized gradient approximation (GGA), the first-principles calculations were employed to calculate the structural properties, phase stabilities, electronic, elastic and thermodynamic properties of (h, r)-TiAl2. According to calculated formation enthalpies, r-TiAl2 possesses higher stability. What's more, the values of Cauchy pressures, Poisson's ratio ν and the B/G ratio illustrate that (h, r)-TiAl2 are brittle. The single-crystal and polycrystalline elastic moduli indicate that they take possession of same mechanical properties. Besides, the variations of volume, bulk modulus, heat capacity at constant pressure or volume and Debye temperature as a function of temperature ranging from 0 to 1500 K and pressure ranging from 0 to 50 GPa was also investigated by employing Quasi-harmonic Debye model.

Phase Control in Inorganic Nanocrystals through Finely Tuned Growth at an Ultrathin Scale

Crystalline polymorphs have been considered a prevailing phenomenon in inorganic nanocrystals and provide approaches to modulate fundamental properties and innovative advanced applications. As a basic demand for phase engineering, accessible and controllable synthetic methodologies are indispensable for acquisition of high-quality products in expected phases. Phase stability is also a non-negligible issue that determines continuous gains of functionality and long-term sustainability of characteristic features. Maintaining structural stability of metastable phases provides challenges and opportunities for investigations on fascinating properties and intriguing applications of inorganic nanocrystals. Phase engineering is of great significance to acquire metallic (1T) and semiconducting (2H) Mo- and W-based dichalcogenides for hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR), respectively. The catalysts in1T phase havesuperior electron transfer kinetics and abundant active sites on both basal planes and edges for HER, while ones in2H phase are preferentially deployed for CO2RR to utilize edge sites for catalysis and restrain competitive HER activity. In addition, the photocatalytic performance for HER has been enhanced by combining anatase and rutile phases because electron transfer between the two phases during photocatalysis facilitates the separation of charge carriers and thus impedes the recombination of electron-hole pairs. Although ample effort has been devoted to developing phase engineering, principle understanding at an ultrathin scale remains obscure. In this Account, we provide comprehensive insight into work from our group regarding controllable synthesis of inorganic nanocrystals with phase engineering, critical effects on phase stability, and noteworthy studies on phase-related properties and applications. For bulk materials, phase control and transition have a large energy barrier, so they can only be achieved under rigorous conditions. However, at the initial stage of synthesis, especially for nucleation, there are a small quantity of chemical bonds that contribute to regulate phase and structure with ease. In our work, we mainly modulate nucleation and growth at an ultrathin scale to demonstrate facile approaches for phase engineering. This unique perspective makes for a distinct guidance of controllable synthesis and deliberate stabilization of inorganic nanomaterials with phase engineering. We have developed a series of synthetic strategies for phase engineering to fabricate inorganic nanocrystals in a specific phase with controlled size and composition and adjustable morphologies and surface features. Four sorts of models (MoS2, ZrO2, In2O3, and TiO2) are used for demonstrating finely tuned growth at an ultrathin scale. However, phase engineering has been regarded as immature because only one phase in polymorphs is thermodynamically stable generally. Phase stability of metastable nanocrystals has attracted much interest. Our substantial investigations illustrate several crucial factors on phase stability, leading to inspiration for facilitating persistent emergence of characteristics and functionalities. By full use of the features of a specific phase, we spotlight ligand-induced surface interactions on coverage-dependent electronic structures and chemisorption effects at one-unit thickness of TiO2(B) nanomaterials with phase engineering. Meanwhile, an energy conversion system for overall water splitting (OWS) drives forward steps in function-oriented synthesis of MoS2-based nanomaterials with phase engineering. In the last section, we summarize this theme and highlight several promising directions for future development.

Structure, electrical properties and temperature stability of PIN–PZN–PT piezoelectric ceramics with morphotropic phase boundary compositions

A series of Pb(In[Formula: see text]Nb[Formula: see text]O3–Pb(Zn[Formula: see text]Nb[Formula: see text]O3–PbTiO3 (PIN–PZN–PT) ternary piezoelectric ceramics with compositions at the morphotropic phase boundary (MPB) were prepared. The phase structure, microstructure, electrical properties and temperature stability of PIN–PZN–PT ceramics were investigated systematically. The structural analysis indicated that an MPB phase consisting of rhombohedral and tetragonal phases was formed in all ceramics. For the selected compositions, dielectric measurements demonstrated that the Curie temperature [Formula: see text] changed from 237∘C to 295∘C and showed a strong trend of compositional dependence. All the samples showed good piezoelectic properties. Particularly, the 0.35PIN–0.40PZN–0.25PT sample has the optimum piezoelectric properties, the piezoelectric coefficient [Formula: see text] is 589 pC/N, the electromechanical coupling factors [Formula: see text] and [Formula: see text] are 59% and 49%, respectively. Compared with PZN–PT and PMN–PT ferroelectric crystals, the superior limit of usage temperature of PIN–PZN–PT ceramics was achieved around 230∘C by thermal depoling method. These results make PIN–PZN–PT ceramics a promising material to meet the practical demands of higher temperature and larger electric field operating ranges.

The improved pyroelectric properties, high depolarization temperature and excellent thermal stability in lead-free (1−x)(Bi0.5Na0.5)TiO3-xBa(Ni0.5Nb0.5)O3 ceramics

(1−x)(Bi0.5Na0.5)TiO3-xBa(Ni0.5Nb0.5)O3 (x = 0–0.04) lead-free ceramics (abbreviated as (1−x)BNT-xBNN) were synthesized by a conventional solid-state reaction method. The effect of BNN contents on phase structure, microstructure, ferroelectric, dielectric, pyroelectric properties and thermal stability of (1−x)BNT-xBNN ceramics were systematically investigated. The pyroelectric coefficient (p) at room temperature increased gradually from 3.01 × 10−8 C cm−2K−1 at x = 0 to 5.94 × 10−8 C cm−2K−1 at x = 0.04. The dielectric loss (tan δ) and dielectric constant (εr) decreased first with the small amount of BNN content, then increased slightly with further BNN addition, acquiring the minimum tan δ and εr at x = 0.02. When x = 0.02, the optimal over-all properties were observed with pyroelectric figure of merit (FOMs) of Fv = 3.82 × 10−2 m2 C−1 and figure of merit of Fd = 2.74 × 10−5 P a −1/2. More importantly, the depolarization temperature (Td) of 0.98BNT-0.02BNN ceramics is up to ∼195 °C which is higher than most other reported BNT-based ferroelectric ceramics. In addition, it can be exposed to temperature up to ∼145 °C with negligible deterioration of pyroelectric properties, showing excellent thermal stability. The overall properties reveal that 0.98BNT-0.02BNN ceramics is a promising pyroelectric material for infrared detectors.

Microstructure, sulfate retention, and aqueous stability of barite-borosilicate glass-ceramics

In order to increase the solubility of sulfate in glass matrix, barite−borosilicate glass-ceramics were proposed and prepared by melt quenching method. The effects of SO3 addition (in the form of BaSO4, 0–8 wt%, abbreviated as S0S8) on microstructure, phase structure, and aqueous stability of barite–borosilicate glass-ceramics were studied. The sulfate retention of the glass-ceramics was evaluated. The results show that the samples with addition of 0–4 wt% SO3 possess the amorphous structure. Barite phase appears when the addition SO3 content increases to 5 wt%, and the amount of barite crystals increases with further increasing SO3 content. Furthermore, BaSO4 in borosilicate glass melts is more stable than that of Na2SO4. The sample S4 possesses SiO2 and BaSO4 phases in temperature range of 800–1150 °C. The SO3 content in the sample S4 remains almost unchanged when the temperature increases from 800 to 1050 °C, and decreases rapidly with further increasing temperature. About 2.17 wt% of SO3 can be retained in the sample S6 after melting at 1200 °C. Moreover, the normalized leaching rates of Na and B of the samples S4, S6, and S8 after 28 days are one order of magnitude higher than that of the sample S0. The sample S6 has a lower normalized leaching rate of SO42− (∼1.28 × 10−2 g m−2 d−1) after 28 days.

Investigation of Switching Behavior of Acceptor-Doped Ferroelectric Ceramics

Switching behavior is a general feature in ferroelectrics. The related fatigue effects influenced by defect dipoles in ferroelectrics are still controversial that is focused on the positive and negative effect of oxygen vacancies. Here, we report the polarization switching behavior of acceptor-doped ceramics using the first-order reversal curve (FORC) approach, especially for the abnormal self-rejuvenation effect and the enhanced fatigue endurance in acceptor-doped ceramics. The reversible and irreversible components under electric field in the ceramics were distinguished by the FORC distribution of ideal “hysteron”. The abnormal self-rejuvenation behavior stemmed from dispersed response of hysteron for undoped samples while from the redistribution of defect dipoles for acceptor-doped samples. The self-rejuvenation was induced only by the irreversible component. For the fatigue effect, the pinning of domain walls was not the main reason. The re-annealing treatment for a fatigued sample weakened the interactions between the spontaneous polarizations and the defect dipoles, but enhanced the dispersion of coercive field. The structure decomposition was another reason for fatigue effect. The enhancement of fatigue endurance comes from the phase stability of structure in acceptor-doped ceramics, while complex phase evolution exists in undoped ceramic with weak fatigue endurance. Our study shed new light on the interactions between spontaneous polarization and defect dipoles under repetitive AC electric field.