Phase Stability Research Articles

Influence of Disorder in High-Entropy Alloys on Thermoelectric Properties and Phase Stability

Influence of Disorder in High-Entropy Alloys on Thermoelectric Properties and Phase Stability

Characterization of the microstructural, magnetic, and thermal behaviors of boron-doped Fe–Co–Ni alloys produced via mechanical alloying

FeCoNi alloys, both doped and undoped, were synthesized into nanocrystalline forms using a high-energy mechanical milling technique. The study used the x-ray diffraction technique to examine the microstructure characteristics of powder materials. The identified patterns were examined using Maud software. It was found that mechanical alloying produced solid solutions with BCC and FCC structures and crystallite sizes in the range of 25 nm after 100h of milling. The BCC-Fe(Co,Ni,B) and orthorhombic-Fe3B phases were refined for the alloy doped with amorphous boron, while only the bcc-Fe(Co,Ni,B) phase was identified for the alloy doped with crystalline boron. Meanwhile, the FeCoNi alloy revealed a mixture of FCC-Co and BCC-FeCoNi phases. This sample has a soft ferromagnetic behavior, whereas the doped ones have hard ferromagnetic behaviors. The squareness ratios (Mr/Ms) are typically low. The microstructural variations were associated with magnetic characteristics. The thermal stability of the alloyed powder mixtures was investigated by the DSC technique from 25 to 700 °C for 2h. The alloyed samples were first annealed at selected temperatures and then analyzed using x-ray diffraction. The obtained x-ray diffraction results proved that at temperatures below 230 °C, the initial heavy deformed structure’s structural relaxation followed by the recrystallization of the fcc-Co phase and, at higher temperatures, the recrystallization of new phases, such as α-FeCo, fcc-FeNi, fcc-Ni3Fe, and borides of the (FeNi)23B6, -(FeCo)23B6 types. The stability of the crystalline phases that are formed and their magnetic characteristics can be regulated through a carefully calibrated annealing process.

Mechanical, Thermal Properties, and Extreme Phase Stability of High-Entropy Diborides (V0.2Nb0.2Ta0.2Cr0.2W0.2)B2.

High-entropy diborides (HEDBs) have gained significant attention in industrial applications due to their vast composition space and tunable properties. We propose a solid solution reaction at high temperatures and pressures that successfully synthesized and sintered a novel, dense, and phase-pure HEDB (V0.2Nb0.2Ta0.2Cr0.2W0.2)B2. A high asymptotic Vickers hardness of 26.3 ± 0.6 GPa and a bulk modulus of 320.5 ± 10.6 GPa were obtained. Additionally, we investigated the thermal oxidation process using TG-DSC from room temperature to 1500 °C and explored the phase stability of HEDBs under high-pressure conditions through in situ high-pressure synchrotron radiation X-ray diffraction. We analyzed the formation of lattice distortion, chemical bonding, and band structure in (V0.2Nb0.2Ta0.2Cr0.2W0.2)B2 using first-principles calculations. Surprisingly, we found that the predominant distortion in diborides occurs in the boron layer, supported by ELF. This may be due to uneven electron transfer rather than a straightforward correlation with the atomic radius. These results provide a novel synthesis process and additional experimental data on the mechanical and thermal properties and high-pressure phase stability of HEDBs. Our study offers further insights into the microscopic structure of lattice distortion in HEDBs, which could prove crucial for the selection and design of engineering advanced HEDBs.

Liquid Crystals from Curved Colloidal Rods: Waves, Twists and More.

The curvature of elongated microscopic building blocks plays a crucial role on their self-assembly into orientationally ordered phases. While rod-like molecules form a handful of liquid crystal (LC) phases, curved or banana-shaped molecules show more than fifty phases, with fascinating physical properties, such as chirality or polarity. Despite the fundamental and technological importance of these so-called 'banana-shaped liquid crystals', little is know about their microscopic details at the single-molecule level. Curved colloidal liquid crystals - liquid crystals formed by curved colloidal rods -, are excellent model systems to optically resolve the structure and dynamics of curved building blocks within these condensed phases. Recent advances in the synthesis of curved rod-like particles have unlocked the potential for studying - at the single-particle level - the intimate relationship between shape and phase symmetry, and even confirmed the stability of elusive LC phases. Further developments in this nascent field promise exciting findings, such as the first observation of the colloidal twist-bend nematic phase or the fabrication of functional materials with curvature-dependent properties. In this perspective, we will highlight recent advances in the synthesis and assembly of curved colloidal liquid crystals and discuss the upcoming challenges and opportunities of this field.

Study of the phase stability of La0.6Sr0.4MnO3 – δ oxide with mixed oxygen-electron conductivity

Study of the phase stability of La0.6Sr0.4MnO3 – δ oxide with mixed oxygen-electron conductivity

Computational thermodynamics-guided alloy design and phase stability in CoCrFeMnNi-based medium-and high-entropy alloys: An experimental-theoretical study

A computational thermodynamics approach has been employed to design CoCrFeMnNi-based medium- and high-entropy alloys (M/HEAs) with systematically varied compositions (Co((80-X)/2)Cr((80-X)/2)FeXMn10Ni10 with x = 30, 40, and 50 at.%) and phase stability. Since the formation of sigma phase, usually brittle and undesirable, is a common concern, when this class of alloys is subjected to elevated temperatures (600–1000 °C), predicting its formation becomes essential. Thus, its formation and the phase equilibria were studied using the CALPHAD method, and two empirical methods, namely, valence electron concentration (VEC) and paired sigma-forming element (PSFE). Isothermal aging treatments at 900–1100 °C for 20 h were performed, since CALPHAD and VEC/PSFE predictions diverged. Both prediction methods were compared with experimental characterization by a combination of scanning electron microscopy and high-energy synchrotron X-ray diffraction. The predictions from the VEC/PSFE and CALPHAD calculations (depending on the database used) were shown to be quite accurate.

Phenylethylammonium bromide-assisted solution-phase ligand exchange in CsPbBr3 quantum dot solar cells.

CsPbBr3 quantum dots (QDs) have excellent optical properties and good phase stability, but the long-chain ligands on their surfaces affect the charge transfer between QDs. Here, we propose a simple ligand exchange strategy: solution-phase ligand exchange. By adding an acetone solution of phenylethylammonium bromide to the purification process of CsPbBr3 QDs, the long-chain ligands were effectively replaced and the electric coupling between QDs was improved. As a result, the power conversion efficiency of the solar cell was increased from 1.95% to 2.83%. Meanwhile, the stability of the devices was significantly improved in the unencapsulated case.

Phase stability, piezoelectricity, and ferroelectricity in nitride short-period superlattices

Improving piezoelectric and ferroelectric responses of group III-nitrides is desired for their potential applications in the emerging microelectromechanical-based systems. One possible approach to realize the optimization and control of functionalities is to bring together compounds with different properties to form the ordered multilayer superlattices. In this work, we systematically investigate the phase stability, piezoelectricity, and ferroelectricity in a class of wurtzite-structure-derived nitride superlattices with a periodic alternation of chemically and/or structurally different layers. The structural heterogeneity and phase stability of the ordered wurtzite-structure-derived superlattices are intimately related to the ionic radii mismatch between the substitutional compounds and the parent nitrides. Moreover, the internal structural distortion of the nitride superlattices has a crucial impact on the ferroelectricity and piezoelectricity, namely, piezoelectric and ferroelectric responses become increasingly enhanced as the buckled atomic layers becomes flatter. This work offers fundamental physical insights into the structure–property relationships in nitride superlattices and may propose some material design strategies for achieving high-performance materials with desired responses.

First-principles studies of the structural, mechanical, electronic, magnetic and transport properties of inner transition based RaMO3 perovskites (M = Cm, Bk)

In the quest for new materials, we introduce self-consistent ab initio simulations to investigate the structural and mechanical stability, electronic profile, and transport properties of f-electron-based RaMO3 (M = Cm, Bk) perovskites by employing density functional theory. The energy optimization indicates that these alloys exhibit stability in the ferromagnetic phase. The examination of mechanical parameters demonstrates the ductile nature of these materials. To elucidate the electronic structure of these alloys, we employed a combination of two approximations: Generalized Gradient Approximation (GGA) and GGA + mBJ. Both approximations confirm the alloys’ half-metallic character. The magnetic moments calculated using both approximations were approximately 6 µB for RaCmO3 and approximately 7 µB for RaBkO3. The thermoelectric performance of these materials is evaluated by investigating different transport parameters such as the Seebeck coefficient, electrical and thermal conductivity and power factor. The comprehensive investigation of these two alloys has the potential to expand their applications in spintronics, thermoelectric devices, and radioisotope thermoelectric generators (RTGs).

Heterogeneous‐Structured Refractory High‐Entropy Alloys: A Review of State‐of‐the‐Art Developments and Trends

AbstractRefractory high‐entropy alloys (RHEAs) inspire the development of novel high‐temperature structural materials due to their outstanding resistance to softening and phase stability at elevated temperatures. However, they struggle to simultaneously achieve high‐temperature strength and room‐temperature ductility, while exhibiting insufficient room‐temperature strain hardening capability. Heterogeneous structure strengthening possesses a unique plastic self‐coordinated ability, which can effectively maintain strain hardening rate to achieve an excellent combination of strength and ductility. Benefiting from slow atomic diffusion, severe lattice distortion, and broad compositional design space, RHEAs with heterogeneous structures can be prepared from both chemical composition and interface structure perspectives. Chemical composition heterogeneity primarily focuses on fluctuations of alloying elements at the nanoscale, along with the formation of heterogeneous precipitates and unique lamellar eutectic structures. While, interface structure heterogeneity manifests in the activation of phase transformation and twin boundaries within grains, along with the formation of grains of vastly different sizes. The trend in RHEAs development is toward structural‐functional integration. Heterogeneous structures can also optimize functional properties, such as irradiation resistance, biomedical properties, and high‐temperature softening resistance of RHEAs. Finally, a brief outlook is provided on the future development direction of heterogeneous structure RHEAs.

Phase stability and transition of CrTaVW high-entropy alloy

Refractory high-entropy alloys (RHEAs) are susceptible to phase transition at elevated temperatures, inducing deviation in the phase constitution from the intended design hence affecting the performance significantly. Accurate prediction of phase constitution and modulation of phase stability over a broad temperature range is crucial to develop RHEAs with superior properties. In this study, thermodynamic and first-principles calculations were integrated to investigate the temperature-dependent phase constitution in the CrTaVW alloy system. The effect of V and W atoms occupation sites on the stability of TaCr2 Laves phase was evaluated. Both V or W atoms tend to occupy Cr sites in TaCr2, and V exhibits stronger alloying ability than W due to the reduced electronic density at the Fermi level. Moreover, with increasing the doping content, the V and W exhibit opposite influencing trends on the formation and structural stability of TaCr2 Laves phase. The Gibbs free energies of the Laves phase and solid solutions with variable compositions were obtained, enabling predictions for the critical temperatures of the phase transitions. The predicted behaviors of phase transitions were validated by experiments. This study provides guidance for modulating phase constitution and achieving desirable phase stability in RHEAs.

Influence of Long Milling Time on the Electrical Resistivity of Nanocrystalline Ni2MnSn Heusler Alloy Obtained by Mechanosynthesis.

A Ni2MnSn Heusler alloy was obtained as a single B2 phase after 12 h of mechanical milling. The influence of prolonged milling on the phase stability was analysed for milling times up to 50 h, related to mean crystallite size, lattice strain, and electrical resistivity. The nature of the powders in the milled range was found to be nanocrystalline, with a mean crystallite size of about 33 ± 2 nm. An evaluation of the internal stresses induced by milling was performed, a linear behaviour was found, and a coefficient of the internal stress increase with milling time was proposed. Particle size distributions of milled samples were analysed, and the morphology of the powders was visualised by scanning electron microscopy. The elemental distribution of milled samples was quantified by energy-dispersive X-ray spectroscopy. Electrical resistivity measurements were performed on compacted samples, and their behaviour with milling time was analysed.

Carbamic acid and its dimer: A computational study.

A recent work by Marks et al. on the formation of carbamic acid in NH -CO interstellar ices pointed out its stability in the gas phase and the concomitant production of its dimer. Prompted by these results and the lack of information on these species, we have performed an accurate structural, energetic and spectroscopic investigation of carbamic acid and its dimer. For the former, the structural and spectroscopic characterization employed composite schemes based on coupled cluster (CC) calculations that account for the extrapolation to the complete basis set limit and core correlation effects. A first important outcome is the definitive confirmation of the nonplanarity of carbamic acid, then followed by an accurate estimate of its rotational and vibrational spectroscopy parameters. As far as the carbamic acid dimer is concerned, the investigation started from the identification of its most stable forms. For them, structure and vibrational properties have been evaluated using density functional theory, while a composite scheme rooted in CC theory has been employed for the energetic characterization. Our results allowed us to provide a better interpretation of the feature observed in the recent experiment mentioned above.

Enhancing the activation of the Ni2+/Ni3+ redox couple by gradient-tantalum doping for high specific capacity and outstanding interfacial stability

P2-type transition metal layered oxides (TMOs) have gathered significant interest by virtue of their low-cost and excellent cycling stability. However, the extensive practical application of TMOs is constrained by inherent factors, such as inferior rate performance and deficient interfacial stability. Herein, the gradient Ta-doped Na0.78(Ni0.33Mn0.67)1-xTaxO2 (NMT) cathode material was synthesized via a one-step solid-state reaction method to overcome these obstacles. The Ta modification augments the Ni2+/Ni3+ redox couple activity, in accordance with the principle of charge conservation, thus bolstering the reversible capacity of the material. Furthermore, the gradient Ta-doping not only preserves the stability of the bulk phase of TMOs but also serves as a potent suppression against the emergence of deleterious interfacial side reactions. As a result, the NMT cathode delivers an improving initial discharge specific capacity of 105 mAh g−1 at 0.1 C and sustains an excellent capacity retention of 91.4 % after 500 cycles at 5 C within the voltage range of 2.0–4.0 V. Benefiting from the robust Ta–O bond, the highly reversible structure evolution of the gradient Ta-doped material during the Na+ extraction/insertion process is clearly elucidated through in-situ XRD. This research provides a straightforward and efficacious approach to elevate the electrochemical performance of TMOs for sodium-ion batteries (SIBs) in the future.

Ti4+ doped high-entropy fluorite oxide ceramics ((ZrHfCeYEr)(1-x)/5Tix)O2-δ: Thermal, mechanical and cyclic thermal shock resistance properties studies

High-entropy fluorite oxide ceramics are promising candidates for spacecraft applications due to their excellent high-temperature thermal-mechanical properties. In this work, four kinds of high-entropy fluorite oxide ceramics ((ZrHfCeYEr)(1-x)/5Tix)O2-δ (HEFOs, x = 0, 0.025, 0.05, 0.075) with different Ti4+ contents were successfully synthesized through solid-state reaction method with conventional sintering. They all exhibited phase stability when annealed at 1500 ℃ for 20 h. When the content of Ti4+ was increased to x = 0.075, the thermal conductivity reduced to1.34 W·m−1·K−1 at 1100 ℃, and the hardness and elastic modulus increased to 14.01 GPa and 205.70 GPa, respectively. Furthermore, the cyclic thermal shock resistance of HEFOs at different temperatures was investigated. The initial flexural strength reached 219.03 MPa when the content of Ti4+ was increased to x = 0.075. After 60 thermal shock cycles at 1200 ℃ and 1500 ℃, the residual strength ratios were 97.56 % and 83.36 %, respectively. The results of this study establish the foundation for future applications of high-entropy fluorite oxide ceramics in spacecraft hot-end components.

Enhanced thermoelectric performance of Cr-doped ZnSb through lattice thermal conductivity reduction below the phonon glass limit

Zinc antimonide (ZnSb) is a promising thermoelectric material due to its economic viability, elemental abundance, and superior phase stability compared to other Zn-Sb compounds. However, its widespread application is hindered by a low figure-of-merit (zT). Extensive research has explored doping, alloying, and nanostructuring to improve zT. This study investigates the impact of Cr doping in CryZn1-ySb (y = 0.0 – 0.03). Cr doping with y = 0.01 significantly reduces lattice thermal conductivity below the phonon glass limit. Debye-Callaway model calculations suggest a combined effect of point defects and decreased grain size caused by Cr incorporation. This reduction translates to enhanced thermoelectric performance, with the y = 0.01 sample exhibiting a 70 % improvement in zT at 673 K, reaching ∼0.67. Exceeding the Cr substitution limit leads to the formation of a detrimental secondary CrSb2 phase, increasing thermal conductivity. The Single Parabolic Band model calculations predict further zT enhancement in y = 0.01 sample to ∼0.2 at 300 K through optimized carrier concentration (307 % improvement in zT). These findings demonstrate the potential for significant zT improvement in ZnSb via defect engineering and carrier concentration tuning.

Accelerating Formulation Design via Machine Learning: Generating a High-throughput Shampoo Formulations Dataset

Liquid formulations are ubiquitous yet have lengthy product development cycles owing to the complex physical interactions between ingredients making it difficult to tune formulations to customer-defined property targets. Interpolative ML models can accelerate liquid formulations design but are typically trained on limited sets of ingredients and without any structural information, which limits their out-of-training predictive capacity. To address this challenge, we selected eighteen formulation ingredients covering a diverse chemical space to prepare an open experimental dataset for training ML models for rinse-off formulations development. The resulting design space has an over 50-fold increase in dimensionality compared to our previous work. Here, we present a dataset of 812 formulations, including 294 stable samples, which cover the entire design space, with phase stability, turbidity, and high-fidelity rheology measurements generated on our semi-automated, ML-driven liquid formulations workflow. Our dataset has the unique attribute of sample-specific uncertainty measurements to train predictive surrogate models.

Influence of oxygen pressure on the ferroelectricity of pulsed laser deposition fabricated epitaxial Y-doped HfO2

Ferroelectric properties of hafnium-based thin films have gained significant interest, yet the fundamental mechanisms responsible for the emergence of the ferroelectric phase continue to be inadequately investigated. In contrast with polycrystalline films fabricated by atomic layer deposition or sputter methods, which possess uncertainty in polarization orientation, epitaxial ferroelectric HfO2-based materials are less investigated, especially for factors such as electric field and oxygen vacancy, which are proposed and examined for their potential impacts on phase stability. In this study, Y-doped hafnium oxide (HYO) ferroelectric epitaxial films were fabricated using pulsed laser deposition, with variations in oxygen pressure during the deposition process. Structural and electrical analyses of HYO epitaxial ferroelectric films prepared under differing oxygen pressures revealed a correlation between the ferroelectric properties of the films and the oxygen content. An optimal selection of oxygen pressure was found to be conducive to the formation of HYO epitaxial ferroelectric films, presenting a promising avenue for future ferroelectric memory applications.

Electric-Field Manipulation of Magnetic Chirality in a Homo-Ferro-Rotational Helimagnet.

Ferro-rotational (FR) materials, renowned for their distinctive material functionalities, present challenges in the growth of homo-FR crystals (i.e., single FR domain). This study explores a cost-effective approach to growing homo-FR helimagnetic RbFe(SO4)2 (RFSO) crystals by lowering the crystal growth temperature below the TFR threshold using the high-pressure hydrothermal method. Through polarized neutron diffraction experiments, it is observed that nearly 86% of RFSO crystals consist of a homo-FR domain. Notably, RFSO displays remarkable stability in the FR phase, with an exceptionally high TFR of ≈573K. Furthermore, RFSO exhibits a chiral helical magnetic structure with switchable ferroelectric polarization below 4K. Importantly, external electric fields can induce a single magnetic domain state and manipulate its magnetic chirality. The findings suggest that the search for new FR magnets with outstanding material properties should consider magnetic sulfates as promising candidates.

Mixed Proton and Oxide Ion Conduction, Phase Stability, and Conducting Mechanisms in the Sr2CeO4-based Materials

Sr2CeO4 possesses structural characteristic of one-dimensional (1D) edge-sharing CeO6 octahedral chains, exhibits superior phase and chemical stability compared to the perovskite SrCeO3. There have been previous attempts to improve electrical conductivity through proton conduction by acceptor-doping of Yb on the Ce site. However, its electrical conductivity remained low, for instance, at 500 °C, the reported value was 3.16×10−7 S/cm, hence limiting its practical applications. In this paper, we synthesized four new rare earth doped Sr2Ce0.95M0.05O3.975 (where M = Y, Dy, Er, Tb) by the solid state reaction method. We thoroughly investigated their phases, electrical properties, and stability. Additionally, for the first time, we investigated the proton and oxide ion conduction mechanisms in Sr2CeO4-based materials using a bond-valence-based method. This analysis revealed a clear 1D conduction along the c-axis within the CeO6 tetrahedral chains and 2D conduction within the a-c plane, respectively. The findings revealed that all doped materials showed obvious proton conduction, alongside partial oxide ion conduction. Notably, the Er-, Dy-, and Y-doped samples demonstrated significantly improved conductivity under wet condition (with a water partial pressure of ∼ 0.03 atm) at 500 °C, reaching approximately ∼ 10−4 S/cm. This is nearly three orders of magnitude higher than the previously reported Yb-doped Sr2CeO4.