Formation Of Metastable Phases Research Articles

Exploring metastable phase formation: Swift heavy ion effects on partially stabilized zirconia

Zirconium dioxide shows great potential for use in the nuclear power industry. It exhibits high radiation stability (neutron and heavy ions) and can withstand harsh environments. The paper presents a study on the effects of 710 MeV Bi ion irradiation on polycrystalline partially yttria-stabilized zirconia using the combination of experimental techniques - X-ray diffraction, transmission electron microscopy and Raman spectroscopy. It was revealed that bismuth ion induced tracks retain their crystalline structure and their density tends to saturate at ∼1 × 1012 cm−2. As a result of dense ionization stimulated structural modification a t → t'' phase transformation was observed. The radiation stability of t'' phase against swift heavy ion impact was found to be higher compared to the t phase. No amorphization was detected up to Bi ion fluence of 2 × 1013 cm−2.

Effect of Temperature and Frequency on the Viscoelastic Behavior of Commercial 6082 (Al–Mg–Si) Alloy

<div>The viscoelastic response of pure Al and commercial 6082 and 6082-T6 (Al–Mg–Si) alloys is measured with dynamic–mechanical analyzer as a function of temperature (ranging from 35 to 425°C) and loading frequency (ranging from 0.01 to 100 Hz). The measured data (the storage modulus, loss modulus, and mechanical damping) are compared to available transmission electron microscopy and differential scanning calorimetry data, to ascertain whether unexplained variations of the viscoelastic behavior of the alloys can be correlated to phase transformations. The results suggest that some of these variations may be controlled by the formation and dissolution of metastable phases, such as Guinier–Preston (GP) zones and phases β″, β′, and B′. Indeed, GP zones and phase β″ have been reported to control other mechanical properties. However, due to the high complexity of the aging path of Al–Mg–Si alloys, with formation and dissolution reactions of many precipitate types overlapping along wide temperature intervals, further research is necessary to establish unequivocally the contribution of each individual phase transformation to the overall viscoelastic behavior. Finally, an internal friction peak related to grain boundary sliding is significantly smaller in the alloys compared to pure Al, probably because the precipitates pin the grain boundaries.</div>

INVESTIGATION OF THE INFLUENCE OF PULSED RADIATION GENERATED BY FUNCTIONAL CERAMICS BASED ON THE PRINCIPLE OF PTE ON THE CHARACTERISTICS OF THE CR2O3-SIO2-FE2O3-CAO-AL2O3-MGO-CUO SYSTEM

This work investigates methods for producing ceramic materials based on the Cr2O3-SiO2-Fe2O3-CaO-Al2O3-MgO-CuO system capable of generating modulated pulsed radiation in the far-infrared spectral region. The possibility of synthesizing such ceramics, in addition to helio-technology, using thermomechanical processing and mechanoactivation of the initial carbonates is considered. A comprehensive analysis of the structure and properties of the obtained materials using X-ray structural, electron microscopic analysis, and other methods has been carried out. It has been established that activation by pulsed infrared radiation generated by the principle of pulsed tunneling effect (PTE) leads to changes in the microstructure of the samples, accompanied by the formation of metastable phases at the interfaces and the generation of radiation.

Formation of metastable phases during spark plasma sintering of Fe@Au core–shell particles

ABSTRACT The present work reports the formation of Fe-Au metastable alloys by galvanic replacement, pressureless spark plasma sintering (SPS) and acid treatment. The reaction of partial galvanic replacement between micrometer-sized iron particles and HAuCl4 solution allowed synthesising core–shell Fe@Au particles. When the Fe@Au particles were treated in HCl solution, iron was selectively dissolved and hollow Au particles were formed. Pressureless SPS of the Fe@Au core–shell particles led to the formation of a metastable intermetallic phase, FeAu3, and a supersaturated solid solution, Au(Fe). The latter was resistant to HCl solution; at the same time, the FeAu3 phase decomposed during HCl treatment to form a porous gold structure. This work shows that a combination of galvanic replacement, SPS and acid treatment can be used for the preparation of metastable Fe-Au alloys with different phase compositions and particle morphologies.

Multiscale structural understanding of plasma spraying anti-ablation coating: an example of Ta-Hf-W-C ultrahigh temperature ceramics

The coating formation and structural design of ultra-high temperature ceramics (UHTCs) for aerospace vehicle anti-ablation coatings through plasma spraying remain uncertain despite extensive engineering applications. In this study, (Ta,Hf,W)C/HfC micro-composite, (Ta,Hf,W)C single-phase, and (Ta,Hf,C)/W nanocomposite coatings were successfully prepared using induction plasma spheroidization (IPS), vacuum plasma spraying (VPS), and spark plasma sintering (SPS). Density functional theory (DFT) was employed to capture the electronic structure of (Ta,Hf,W)Cy (y = 1, 0.875, 0.75, 0.5), confirming the influence of carbon vacancies and ceramic metallicity on the plastic forming behavior of sprayed coatings. Additionally, the contribution of HfC to structural stability and WC to the metallicity of carbon-based rock salt-structured UHTCs was elucidated. The pivotal role of carbon vacancies in inducing metastable phase formation and metal phase precipitation for (Ta,Hf,W)C was identified. Ablation behavior variations resulted from differences in multi-scale coating structures, where transverse cracks exhibited more detrimental effects than vertical cracks. Notably, a scale effect of oxidation related to the uniformity of metal elements in the multicomponent ceramic coating was uncovered. It was determined that components capable of forming high melting point oxides should not be uniformly dissolved in the matrix.

Studying the features of complex beryllium-containing silicate phases co-crystallization in zonal samples using the x-ray electron probe microanalysis method

Research subject. Silicate ingots containing four species-forming components Be, Mg, Al, and Si and belonging to the crystallization region of beryllium indialite (with the formula of Mg2BeAl2Si6O18 and a beryl-type structure). Aim. To investigate the fundamental problem of identifying the patterns of matter differentiation and the stable and metastable phase formation in silicate matrices. Methods. The evolution of the phase composition of silicate melts was registered using a temperature gradient method. Results. New data on the features of phase transformations in silicate melts belonging to the region of beryllium indialite were obtained by electron probe microanalysis (EPMA). Co-existing metastable and stable mineral phases were identified, and the similarity of their compositions with different structures was shown. The nature of impurity phases at each stage of crystallization was established. Conclusions. The evolutionary sequence of phase associations ensuring the crystallization of beryllium indialite and metastable phases of a similar composition, the nature of which is determined by the initial ratio of components, was experimentally recorded. The range of possible phase associations that co-crystallize or replace a stable phase with a beryl structure in melts from the region of existence of beryllium indialite in the BeO–MgO–Al2O3–SiO2 system was extended. The selectivity of the coloring element chromium entry into various phases of the studied system is shown depending on the capabilities of their structure. The addition of a chromophore is a reliable criterion for visualizing successive layers, zones, and areas of changing phase associations in the final ingot.

Microstructure and mechanical properties of Al0.5CoCrFeNi HEA prepared via gas atomization and followed by hot-pressing sintering

Pre-alloyed Al0.5CoCrFeNi high-entropy alloy (HEA) powders were prepared via gas atomization and followed by hot-pressing sintering (HPS) at 1200 °C for 1 h to fabricate bulk materials. The phase stability, microstructure evolution and mechanical properties of Al0.5CoCrFeNi HEA were mainly investigated. Results indicated that the atomized powders exhibited a fine-grained FCC + dendritic-like ordered BCC (B2) dual-phase structure. Rapid solidification was favorable to the metastable B2 phase formation during gas-atomization. Partial of the dendritic-like B2 phase dissolved after annealing (>600 °C), resulted in a homogeneous dual-phase structure with irregular B2 phase dispersed in the FCC matrix. The homogeneous structure was “inherited” in the sintered bulks and coarsened after HPS. Tensile test results suggested that the sintered bulks possessed excellent comprehensive mechanical properties with yield strength, ultimate tensile strength and elongation of 587.3 ± 18.7 MPa, 960.2 ± 23.6 MPa and 32.6 ± 5.1%, respectively. The excellent mechanical response during deformation was mainly attributed to the synergistic deformation of the FCC/B2 phase and considerable back stress hardening mechanism.

Preferential formation of uniform spherical vaterite by harnessing vortex flows and integrated CO2 capture and mineralization

The use of calcium bearing resources to facilitate solvent regeneration and CO2 reuse via carbon mineralization offers a low energy pathway for the production of calcium carbonate. However, a crucial challenge is the lack of specificity in the formation of various calcium carbonate polymorphs during carbon mineralization. One of the less explored but highly effective approaches to tune the morphology and crystal structure of specific carbonate phases involves tuning vortex flows. This approach is an alternative to utilizing chemical reagents that need to be regenerated for tuning the morphologies and crystalline structures to direct the formation of specific carbonate phases. In this study, the efficacy of using homogeneous vortex flows in limiting the agglomeration of carbonate particles and directing the formation of metastable vaterite phases is discussed and contrasted with the influence of inhomogeneous conventional feed flow patterns on precipitated calcium carbonate (PCC). Herein, a Taylor-Couette Carbonate Conversion (TC3) reactor is used to direct the formation of spherical vaterite particles with uniform particle size distribution preferentially over calcite and other phases. The formed vortex patterns inside TC3 reactor provide homogeneous reaction spaces conducive to PCC formation, ensuring uniform mixing throughout the process. By increasing the rotational speed and the residence time, higher purity carbonates with more uniform sizes are obtained. Furthermore, preferential vaterite formation is also observed in leachates obtained from alkaline industrial residues such as construction and demolition waste and steel slag. Thus, the proposed approach is effective in harnessing multiple waste streams such as CO2 emissions and alkaline industrial residues to produce calcium carbonate phases such as vaterite with structural and morphological specificity.

NEW APPROACHES TO THE SYNTHESIS OF FUNCTIONAL MATERIALS WITH SPECIFIED PROPERTIES UNDER THE INFLUENCE OF CONCENTRATED RADIATION AND PULSE TUNNEL EFFECT

This article explores the prospects of synthesizing materials with a complex of specified properties using the pulsed tunneling effect (PTE) and concentrated solar radiation. The mechanisms of formation of metastable phases and the emergence of unique electrical and magnetic properties due to numerous structural distortions are described. The concept of using such materials as “artificial dopants” is proposed. Promising application areas are considered, ranging from the creation of extremely strong materials to the achievement of non-standard superconducting and magnetic effects.

The Beneficial Effect of Iron in Aluminum-Cerium-Based Cast Alloys

Iron (Fe) has been considered a major impurity since it is detrimental to the mechanical properties of many cast aluminum alloys due to the formation of Fe-containing brittle intermetallic phases. Fe is found naturally as an impurity in bauxite ore, resulting in Fe contamination of aluminum alloys with increasing contamination from current recycling practices. The Al–Ce–Fe system was investigated using CALPHAD (CALculation of PHAse Diagrams) modeling and experimental casting techniques. It was found that additions of Fe to the Al–Ce system are beneficial to the strength (slightly) and ductility (significantly) of the ternary alloys, which is attributed to the formation of fine metastable Al8CeFe2 phase with aggregate morphology and equilibrium Al10CeFe2 phase, suppressing coarse proeutectic Al11Ce3 phase in near-eutectic Al–Ce alloys. Heat treatment study showed that the metastable Al8CeFe2 phase transforms to predicted equilibrium Al10CeFe2 phase at 500 °C, with essentially no intermetallic or grain coarsening; thus, the alloy displayed excellent property retention. The Al–Ce–Fe alloy system offers opportunities for sustainable, recyclable alloy development using low-cost Fe and low-cost cerium (a byproduct of rare-earth extraction).

Nickel as a modifier of calcium oxalate: an in situ liquid cell TEM investigation of nucleation and growth.

Despite extensive research on the nucleation and growth of calcium oxalate (CaOx) crystals, there are still several challenges and unknowns that remain. In particular, the role of trace metal elements in the promotion or inhibition of CaOx crystals is not well understood. In the present study, in situ graphene liquid cell transmission electron microscopy (in situ GLC TEM) was used to observe real-time, nanoscale transformations of CaOx crystals in the presence of nickel ions (Ni2+). The results showed that Ni2+ form Ni-water complexes, acting as a shape-directing species, generating a unique morphology and altering growth kinetics. Transient adsorption of Ni-water complexes resulted in a metastable phase formation of calcium oxalate trihydrate. Atomistic molecular dynamics simulations confirmed that Ni2+ acts as a weak inhibitor which slows down the CaOx crystallization, elucidating that Ni2+ impacts small-sized CaOx clusters by bringing more water into the clusters. This work highlighted the intricacies behind the effect of Ni2+ on CaOx biomineralization that were made possible to discern using in situ GLC TEM.

Novel Operando Nuclear Magnetic Resonance Approach for Tracking the Electrode State of Charge in Li/Na-Ion Batteries

Lithium and sodium-ion batteries have become the focal point of interest for the energy transition due to their promising applications for electric cars and energy grid storage. Operando characterization of batteries is critical to understand the degradation processes and increasing their electrochemical performance, especially in abusive conditions like during fast charges. Knowledge of the redox state of each electrode is essential to identify the origins of capacity loss in full batteries. Among the methods used for these investigations, operando Nuclear Magnetic Resonance (NMR) methods are promising since, without destroying the battery, they enable monitoring in real-time the electrochemical processes and the formation of short-lived metastable or reactive phases [1,2].Up to now, most operando NMR studies have focused on 7Li NMR spectroscopy to follow the lithiation and degradation processes in the solid electrodes of lithium-ion batteries. However, the large shifts and broadenings of the NMR spectra, which result from the redox-active paramagnetic ions (Ni2/3+, Co4+, Mn3/4+) or from the metallicity of the electrodes [3], complicate operando NMR spectra acquisition and interpretation for full batteries.Herein, we demonstrate that the 1H operando NMR signal of the liquid electrolyte solvent can be used to spy on the positive electrode evolution while benefitting from the high sensitivity of 1H liquid-state NMR compared to 7Li NMR. So far, operando NMR using the liquid electrolyte signal was mainly exploited to follow electrolyte decomposition [4] or to image indirectly dendrite formation [5]. Contrary to the lithium or sodium atoms composing the electrodes, hydrogen atoms in the electrolyte solvent are not a direct probe of the state of charge of the electrode. We demonstrate that the 1H NMR signal of the electrolyte molecules in the battery is, however, highly sensitive to the magnetic susceptibility (and therefore the redox state) of the neighboring particles of positive electrode active material.The state-of-charge (SOC) of the positive electrodes in the charging battery can be tracked indirectly through distortion in the spectrum of the dimethyl carbonate (DMC) solvent near the positive electrode. The effect of the other battery components such as current collectors, separators, or even negative electrodes is shown to be negligible compared to the perturbations induced by the positive electrode. We define specific descriptors to track the changes in the distorted 1H NMR spectrum of the battery and we correlate these changes with the state-of-charge of the positive electrode inside the battery as it is charged and discharged. This approach will enable measuring, operando, the redox state of positive electrodes in batteries, even when their 7Li signals cannot be detected by NMR. Exact knowledge of the redox state will contribute to a better exploitation of the electrochemical curves in full batteries.[1] Bagheri, K.; Deschamps, M.; Salager, E. Nuclear Magnetic Resonance for Interfaces in Rechargeable Batteries. Current Opinion in Colloid & Interface Science 2023, 64, 101675. https://doi.org/10.1016/j.cocis.2022.101675.[2] Liu, X.; Liang, Z.; Xiang, Y.; Lin, M.; Li, Q.; Liu, Z.; Zhong, G.; Fu, R.; Yang, Y. Solid‐State NMR and MRI Spectroscopy for Li/Na Batteries: Materials, Interface, and in Situ Characterization. Advanced Materials 2021, 33 (50), 2005878. https://doi.org/10.1002/adma.202005878.[3] Grey, C. P.; Dupré, N. NMR Studies of Cathode Materials for Lithium-Ion Rechargeable Batteries. Chemical Reviews 2004, 104 (10), 4493–4512. https://doi.org/10.1021/cr020734p.[4] Wiemers-Meyer, S.; Winter, M.; Nowak, S. NMR as a Powerful Tool to Study Lithium Ion Battery Electrolytes. Annual Reports on NMR Spectroscopy 2019, 121–162. https://doi.org/10.1016/bs.arnmr.2018.12.003.[5] Ilott, A. J.; Mohammadi, M.; Chang, H. J.; Grey, C. P.; Jerschow, A. Real-Time 3D Imaging of Microstructure Growth in Battery Cells Using Indirect MRI. Proceedings of the National Academy of Sciences 2016, 113 (39), 10779–10784. https://doi.org/10.1073/pnas.1607903113.

Deciphering Microstructures and Phases of Gas-Atomised Novel Al-Fe-Si-Cr-Ni Alloys

Rapid solidification techniques, such as gas atomisation, have been widely implemented in metallic alloys/composites to increase solid solubility, avoid or mitigate segregation phenomena, and favour metastable phase formation to enhance performance. Particularly, gas atomisation can enhance the solid solubility of low diffusion coefficient elements like Fe, Ni, Mn, Zr, and Cr in the α-Al matrix, yielding metastable phases. As a result, Al alloys exhibit excellent strength at high temperatures. In this study, the AISI 304L alloy was employed to introduce Fe, Ni, and Cr elements into the AlSi10Mg alloy through gas atomisation, resulting in the formation of two distinct hypereutectic AlFe-based alloys: AlFe9Si8Cr2Ni and AlFe18Si8Cr5Ni2. Gas-atomised alloy powders were separated into different size fractions by sieving and characterised using X-ray diffraction, differential scanning calorimetry, optical microscopy, and scanning electron microscopy. Microstructural analyses revealed dendritic patterns with distinct phases, highlighting the influence of the alloying element content on the solidification processes. Furthermore, a synergic evaluation of the XRD and EDS analysis results allowed the identification of intermetallic phases and their distribution in the two systems.

In-situ observation of single-phase compositionally-complex oxide formation during high-pressure and high-temperature synthesis

Compositionally-complex oxides (CCOs) with equal or non-equal atomic ratios of multi-principal cations are single-phase solid solutions and possess unique structural characteristics and correlated possibilities for tailorable and exceptional functionalities. However, breakdown of single-phase structure and phase separation impair widespread adoption of CCOs under in-service environments. Here, in-situ phase transformation during high-pressure and high-temperature synthesis of an exemplary CCO (CeZrHf)0.852(YbY)0.148O2-δ is for the first time clarified by using synchrotron X-ray energy dispersive diffraction. During synthesizing at ∼4.5 GPa inside a large-volume press, ZrO2 and HfO2 gradually initiate lattice diffusion as monoclinic (022) and (220) peaks collapse and merge completely at 973 K; CeO2 starts diffusing as the dramatic intensity change of cubic (220) peak above 1073 K, causing a metastable phase formation; single-phase solid solution ultimately forms at 1673 K. The results revealed provide in-depth understanding of phase transformation dynamics and insights for novel design paradigm of CCOs.

Unexpected spinodal decomposition in as-cast eutectic high entropy alloy Al30Co10Cr30Fe15Ni15

By integrating calculation of phase diagram (CALPHAD) simulations, ab initio molecular dynamics (AIMD) and experimental validation, we shed light on the unexpected spinodal decomposition in as-cast Al30Co10Cr30Fe15Ni15 alloy, as opposed to the eutectic BCC/B2 microstructure predicted by CALPHAD. We showed that the eutectic high entropy alloy (EHEA) and spinodal decomposition (SD) may be correlated with each other. A shift from as-cast EHEA to SD can be triggered by the strong short-range ordering in the EHEA liquid phase and high configuration entropy of the metastable phase. CALPHAD showed that the low eutectic temperature of Al30Co10Cr30Fe15Ni15 alloy is stabilized by the short-range and topological ordering (SRO), rendering a small undercooling ΔT for partitionless solidification of the metastable B2 phase. AIMD calculations showed that the particular metastable-like liquid preordering might reduce interface energy between the eutectic liquid and saturated B2, facilitating metastable saturated phase formation even with a small undercooling and suppressing the eutectic reaction. The single B2 phase might have solidified in the spinodal region of the phase diagram resulting in a shift from as-cast EHEA to SD. The interplay between SD and EHEAs presents an alternative route to explore spinodal HEAs by simple as-casted samples without lengthy heat treatments for homogenization.

In situ observation of the phase transformation kinetics of bismuth during shock release

A time-resolved x-ray diffraction technique is employed to monitor the structural transformation of laser-shocked bismuth. Results reveal a retarded transformation from the shock-induced Bi-V phase to a metastable Bi-IV phase during the shock release, instead of the thermodynamically stable Bi-III phase. The emergence of the metastable Bi-IV phase is understood by the competitive interplay between two transformation pathways towards the Bi-IV and Bi-III, respectively. The former is more rapid than the latter because the Bi-V to B-IV transformation is driven by interaction between the closest atoms while the Bi-V to B-III transformation requires interaction between the second-closest atoms. The nucleation time for the Bi-V to Bi-IV transformation is determined to be 5.1±0.9 ns according to a classical nucleation model. This observation demonstrates the importance of the formation of the transient metastable phases, which can change the phase transformation pathway in a dynamic process.

Silicon Poisoning and Effects of Tantalum on AlSi Alloys

Grain refinement in Al-Si alloys is crucial for enhancing material castability and mechanical properties. Industrial practice involves adding inoculants, composed of TiB2 particles coated with metastable Al3Ti via adsorption, to the melt. This introduces essential free titanium for metastable phase formation and subsequent growth restriction. The superstoichiometric grain refiner Al-5Ti-1B, with 2.2 wt.% free titanium, is applied for this purpose. A peritectic reaction forms α-aluminium from this layer. However, when silicon content exceeds 3.5 wt.%, grain coarsening occurs due to silicon’s detrimental effect. This study quantified silicon poisoning in an Al-10Si alloy using stoichiometric and superstoichiometric grain refiners through ASTM-standardized TP1 tests. Adding 0.02 wt.% tantalum acted as an antidote to the τ1 phase, resulting in a finer microstructure. This was attributed to the formation of a Ta-rich layer on TiB2 particles, which causes α-aluminium formation via a peritectic reaction without generating ternary phases with Ti or Si. Correlating to the increasing particle size curves from the TP1 tests, phases were collected in the filter cake with the help of a PoDFA apparatus. These could be examined more closely on the SEM and identified as needle- or plate-shaped. By using an EDS unit, the phases found were assigned to the poisoning phase and further investigated. After the addition of tantalum, a solubility of tantalum could be detected in former poisoning phases. In combination with the gradients of the grain size, it can thus be assumed that tantalum is both an antidote for silicon poisoning in the Al-Si-Ti system and can itself have a grain-refining effect in this system.

Identification of Expanded Austenite in Nitrogen-Implanted Ferritic Steel through In Situ Synchrotron X-ray Diffraction Analyses

The existence and formation of expanded austenite in ferritic stainless steels remains a subject of debate. This research article aims to provide comprehensive insights into the formation and decomposition of expanded austenite through in situ structure analyses during thermal treatments of ferritic steels. To achieve this objective, we employed the Plasma Immersion Ion Implantation (PIII) technique for nitriding in conjunction with in situ synchrotron X-ray diffraction (ISS-XRD) for microstructural analyses during the thermal treatment of the samples. The PIII was carried out at a low temperature (300–400 °C) to promote the formation of metastable phases. The ISS-XRD analyses were carried out at 450 °C, which is in the working temperature range of the ferritic steel UNS S44400, which has applications, for instance, in the coating of petroleum distillation towers. Nitrogen-expanded ferrite (αN) and nitrogen-expanded austenite (γN) metastable phases were formed by nitriding in the modified layers. The production of the αN or γN phase in a ferritic matrix during nitriding has a direct relationship with the nitrogen concentration attained on the treated surfaces, which depends on the ion fluence imposed during the PIII treatment. During the thermal evolution of crystallographic phase analyses by ISS-XRD, after nitriding, structure evolution occurs mainly by nitrogen diffusion. In the nitrided samples prepared under the highest ion fluences—longer treatment times and frequencies (PIII 300 °C 6 h and PIII 400 °C 3 h) containing a significant amount of γN—a transition from the γN phase to the α and CrN phases and the formation of oxides occurred.

Size-induced metal–insulator transition temperature decrease of VO2 nanoparticles obtained by laser irradiation in water

Vanadium dioxide (VO2) is a highly attractive and multifunctional material for technological applications due to its metal–insulator transition, which is accompanied by several orders of magnitude changes in electrical resistivity and optical transmissivity. In this work, we report on the metal–insulator transition of VO2 nanoparticles which are produced by a simple, efficient, and low-cost two-step preparation method based on infrared laser irradiation of V2O5 powder dispersed in water and subsequent stoichiometric stabilization by vacuum annealing at 773 K. VO2 nanoparticles have an average size of 36 nm with facet-like contours and undergo a structural phase transition from a monoclinic to a tetragonal phase, accompanied by a metal–insulator transition occurring around 323 K, which is about 17 K lower than bulk VO2. No evidence of the formation of intermediary metastable phases is observed. Our findings reveal that the size of VO2 nanoparticles is a key parameter to moving the transition closer to room temperature, which is important for many applications.

Development of Electrochemical Anion Doping Technique for Expansion of Functional Material Exploration

AbstractInstead of conventional cation doping strategy, anion doping is a promising new strategy for advances of energy conversion and storage technologies such as batteries, catalysts, electrolysis, and fuel cells. To synthesize mixed‐anion compounds, novel synthesis techniques such as topochemical reaction, high‐pressure reaction, solvothermal reaction have been developed. Despite these excellent synthesis techniques, synthesizable mixed‐anion compounds are still limited. For further expansion of the material exploration of mixed‐anion compounds, herein, an electrochemical anion doping technique is developed, which can flexibly control a species of anion, the doping rate and the degree of anion doping. The concept of the new synthesis technique is verified by F doping to the perovskite oxide La0.5Sr0.5CoO3−δ. Quantitative control of F in the perovskite host material is succeeded by using an electrochemical reactor composed of La0.5Sr0.5CoO3−δ‐BaF2|BaF2|PbF2‐Pb, and phase‐pure F‐doped La0.5Sr0.5CoO3−δ powder is obtained. Moreover, nano‐size crystalline domains with amorphous phase are formed on the particle surface under the high‐rate F doping, suggesting that tuning the anion doping rate enables the control of the formation of metastable phase. As demonstrated, the electrochemical anion doping technique opens up new possibilities for advances of energy materials by utilizing function of anionic species.