Metastable Hexagonal Phase Research Articles

Strip Casting of Sm2TM17-Type Alloys for Production of the Metastable SmTM7 Phase

Conventional book casting of Sm2TM17-type alloys (where TM = Co, Fe, Cu, Zr) leads to a coarse, highly segregated microstructure, predominantly due to the slow, variable cooling rate from the mould surface towards the centre of the ingot. These cast alloys require a long homogenisation treatment to remove this segregation and develop a super-saturated, metastable SmTM7-type hexagonal phase. This SmTM7 phase is a vital precursor phase required during magnet production to develop the complex cellular structure responsible for high magnetic properties. In this work, strip casting was employed to facilitate rapid solidification to develop thin flakes (<0.5 mm thick) with a columnar grain structure. Rapid cooling has the potential to produce a homogenous microstructure consisting predominantly of the metastable SmTM7 phase. This could remove or significantly reduce the need for the energy-intensive homogenisation treatment usually required in conventional magnet manufacture. This paper investigates the effect of wheel speed (and hence cooling rate) on flake thickness, microstructure, and phase balance of the cast alloys. It was shown that for wheel speeds between 1.1 and 3.0 m/s, the microstructure showed large variation; however, in all cases, evidence of the columnar SmTM7 phase was presented. The adhesion between the melt and the wheel was deemed to be critical for the nucleation and subsequent columnar growth of SmTM7 grains, where the wheel speed controlled both the flow of the alloy onto the wheel and the thickness of the resultant flake. It was determined that in order to achieve a homogenous columnar SmTM7 structure, the maximum flake thickness should be limited to 270 μm to avoid the formation of equiaxed Sm2TM17 grains through insufficient cooling.

The effects of Mn doping on structural, morphological, and optical properties of hydrothermally synthesized h-MoO3 nanorods

Pure and Mn-doped h-MoO3 nanorods with varying doping concentrations (1, 3, 5 M%) have been synthesized via the hydrothermal method. Subsequently, the impacts of Mn incorporation on the microstructural and optical properties of the synthesized nanorods have been examined. The X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) have confirmed the metastable hexagonal phase (h-MoO3) and rod-type morphology, respectively. To explore various crystallographic features, such as crystallite size and lattice strain, the Scherrer formula, Williamson-Hall (W–H), and Halder-Wagner (H–W) techniques were used. In addition, the dislocation density, lattice parameters, and unit cell volume have been evaluated in conjunction with XRD analysis by Rietveld refinement. The optical bandgap was estimated from UV–vis spectroscopy utilizing the Kubelka-Munk (K-M) function and the Tauc relation. The estimated bandgap values ranged from 3.05 to 3.09 eV for the K-M function and 2.92–2.98 eV for the Tauc relation. The optical bandgap increased as the concentration of Mn increased to 3 M% and then decreased. Both approaches showed analogous optical bandgap variation.

Controllable synthesis of hexagonal h-WO3 microflowers for water oxidation reaction

Tailoring the morphology and crystalline phase of metal oxides induces the variation in structure and surficial microenvironment, in turn influencing their catalytic activity. To date, metastable hexagonal phase WO3 is nearly inactive in electrochemical water oxidation reaction owing to the poor conductivity. Here, we report a facile route to precisely fabricate hexagonal WO3 microflowers with the nanosheet building blocks. In comparison to WO3·0.33H2O microflowers, h-WO3 microflowers could significantly catalyze the transformation of H2O to O2 via water oxidation reaction with high activity of 276 mV at 10 mA/cm2. The synthesized h-WO3 microflowers exhibit superior oxygen evolution efficiency than WO3·0.33H2O microflowers and commercial IrO2. Coordinative unsaturated W sites and hexagonal channels enable them to greatly activate and dissociate the O–H bond into O2 in alkaline electrolyte.

Hexagonal Phase Formation and Crystalline Structural Transition in Long-Spaced Aliphatic Polyesters with Side Groups.

Side substitution is an effective method for the chemical modification and functionalization of linear polyesters. The presence of side groups can have a profound effect on the crystalline structure and phase transition of semicrystalline polyesters. Herein, we synthesized the long-spaced polyesters with -OH and -CH3 side groups and various methylene segment lengths and studied the effects of the side groups on the crystal polymorph and phase transition of substituted polyesters. The substituted polyesters grow in the thermally stable phase (form I) at a higher temperature. However, the polyesters crystallize in a metastable hexagonal phase (form II) with trans chain conformation at a lower temperature. The metastable form II transforms into the more stable form I during long-time annealing or upon heating; this phase transition is accompanied by chain tilting and crystal lamellar thickening. This study has elucidated the critical role of side groups in the polymorphic crystallization and phase transition of linear polyesters.

Evolution of Cu-In Catalyst Nanoparticles under Hydrogen Plasma Treatment and Silicon Nanowire Growth Conditions.

We report silicon nanowire (SiNW) growth with a novel Cu-In bimetallic catalyst using a plasma-enhanced chemical vapor deposition (PECVD) method. We study the structure of the catalyst nanoparticles (NPs) throughout a two-step process that includes a hydrogen plasma pre-treatment at 200 °C and the SiNW growth itself in a hydrogen-silane plasma at 420 °C. We show that the H2-plasma induces a coalescence of the Cu-rich cores of as-deposited thermally evaporated NPs that does not occur when the same annealing is applied without plasma. The SiNW growth process at 420 °C induces a phase transformation of the catalyst cores to Cu7In3; while a hydrogen plasma treatment at 420 °C without silane can lead to the formation of the Cu11In9 phase. In situ transmission electron microscopy experiments show that the SiNWs synthesis with Cu-In bimetallic catalyst NPs follows an essentially vapor-solid-solid process. By adjusting the catalyst composition, we manage to obtain small-diameter SiNWs-below 10 nm-among which we observe the metastable hexagonal diamond phase of Si, which is predicted to have a direct bandgap.

Metastable Hexagonal Phase SnO2 Nanoribbons with Active Edge Sites for Efficient Hydrogen Peroxide Electrosynthesis in Neutral Media.

Electrochemical two-electron oxygen reduction reaction (2 e- ORR) to produce hydrogen peroxide (H2 O2 ) is a promising alternative to the energetically intensive anthraquinone process. However, there remain challenges in designing 2 e- ORR catalysts that meet the application criteria. Here, we successfully adopt a microwave-assisted mechanochemical-thermal approach to synthesize hexagonal phase SnO2 (h-SnO2 ) nanoribbons with largely exposed edge structures. In 0.1 M Na2 SO4 electrolyte, the h-SnO2 catalysts achieve the excellent H2 O2 selectivity of 99.99 %. Moreover, when employed as the catalyst in flow cell devices, they exhibit a high yield of 3885.26 mmol g-1 h-1 . The enhanced catalytic performance is attributed to the special crystal structure and morphology, resulting in abundantly exposed edge active sites to convert O2 to H2 O2 , which is confirmed by density functional theory calculations.

Metastable Hexagonal Phase SnO2 Nanoribbons with Active Edge Sites for Efficient Hydrogen Peroxide Electrosynthesis in Neutral Media

AbstractElectrochemical two‐electron oxygen reduction reaction (2 e− ORR) to produce hydrogen peroxide (H2O2) is a promising alternative to the energetically intensive anthraquinone process. However, there remain challenges in designing 2 e− ORR catalysts that meet the application criteria. Here, we successfully adopt a microwave‐assisted mechanochemical‐thermal approach to synthesize hexagonal phase SnO2 (h‐SnO2) nanoribbons with largely exposed edge structures. In 0.1 M Na2SO4 electrolyte, the h‐SnO2 catalysts achieve the excellent H2O2 selectivity of 99.99 %. Moreover, when employed as the catalyst in flow cell devices, they exhibit a high yield of 3885.26 mmol g−1 h−1. The enhanced catalytic performance is attributed to the special crystal structure and morphology, resulting in abundantly exposed edge active sites to convert O2 to H2O2, which is confirmed by density functional theory calculations.

Wurtzite InAs Nanocrystals with Short-Wavelength Infrared Emission Synthesized through the Cation Exchange of Cu3As Nanocrystals

Colloidal InAs nanocrystals (NCs) are among the most promising light emitters in the short-wavelength infrared (SWIR) range. These InAs NCs are eligible to crystalize into two distinct phases, i.e., cubic zinc-blende phase and metastable hexagonal wurtzite phase. However, InAs NCs directly produced through molecular precursors unanimously exhibited zinc-blende structure, and the synthesis of their wurtzite counterparts has never been achieved. Herein, we report the first successful synthesis of high-quality wurtzite InAs NCs, which show bright SWIR photoluminescence with a tunable emission peak and high quantum yield of up to ∼37%. This is achieved by the cation exchange of Cu3As NCs with controllable size and morphology, followed by the growth of inorganic passivation layers to eliminate the possible surface trap centers. We further expand the synthesis route to wurtzite ternary InAs0.5P0.5 NCs with quasi-one dimensional confinement, which exhibit polarized SWIR emission, exploring the potential of these anisotropic NCs as efficient polarized light emitters.

Thermal self-reduction of metal hydroxide acrylate monolayer nanoparticles leads formation of nanoparticulate and porous structured alloys.

Chemical and physical designs of alloy nanomaterials have attracted considerable attention for the development of highly functional materials. Although polyol processes using ionic precursors are widely used to synthesise alloy nanoparticles, the reduction potential of polyols limits their chemical composition, making it difficult to obtain 3d transition metals. In this study, we employed pre-synthesized metal hydroxide salt monolayer nanoparticles as precursors to obtain alloy nanoparticles. Simultaneous dehydroxylation of the hydroxide moiety and decomposition of the organic moiety allowed the formation of stable face-centred cubic metals passing through the metal carbide and metastable hexagonal close-packed metal phases. This self-reduction process enabled the formation of nanoparticulate bimetallic alloys and macroporous/mesoporous-structured bimetallic alloys by compositing hard/soft templates with pre-synthesized metal hydroxide salt nanoparticles. We believe that the strategy presented in this study can be used to design nanostructures and chemical compositions of multimetallic alloy nanoparticles as well as bimetallic systems.

Epitaxial growth of hexagonal GdFeO3 thin films with magnetic order by pulsed laser deposition

• Hexagonal GdFeO 3 films were prepared using pulsed laser deposition. • The films were obtained on indium tin oxide (ITO(111)) layer. • The hexagonal GdFeO 3 film showed weak ferromagnetism. Hexagonal rare-earth iron oxides, h - R FeO 3 , are promising multiferroic materials. The ferroic properties of h - R FeO 3 can be controlled by varying R . However, h - R FeO 3 exists in a metastable phase, resulting in a limit of available range of R . In this study, we fabricated h -GdFeO 3 phase via epitaxial stabilization. The h -GdFeO 3 epitaxial film with c -axis orientation was deposited on an indium tin oxide (ITO(111)) layer as the bottom electrode via pulsed laser deposition. The metastable hexagonal phase was obtained in a narrow range of deposition parameters. High-crystalline h -GdFeO 3 film was obtained at substrate temperature of 780 °C and oxygen pressure of 13 Pa. The a -axis of the h -GdFeO 3 phase is parallel to the [11–2] directions of ITO(111) layer. The h -GdFeO 3 film exhibited weak ferromagnetism with a Néel temperature of 43 K, which was lower than that of the previously reported h - R FeO 3 films ( R = Dy–Lu, Y and Sc) owing to the longer a -axis.

Revealing Surface Restraint-Induced Hexagonal Pd Nanocrystals via In Situ Transmission Electron Microscopy.

Achieving metal nanocrystals with metastable phase draws much attention due to their anticipated fascinating properties, wheras it is still challenging because their polymorphism nature and phase transition mechanism remain elusive. Here, phase stability of face-centered cubic (fcc) Pd nanocrystals was studied via in situ spherical aberration (Cs)-corrected transmission electron microscopy (TEM). By constructing a well-defined Pd/C composite structure, Pd nanocrystals encapsulated by graphite, the dispersion process of fcc Pd was observed through a nucleation and growth process. Interestingly, Cs-corrected scanning TEM analysis demonstrated that the newly formed Pd nanocrystals could adopt a metastable hexagonal phase, which was considered challenging to obtain. Accordingly, formation mechanism of the hexagonal Pd nanocrystals was proposed, which involved the combined effect of two factors: (1) templating of graphite and (2) size effect. This work is expected to offer new insight into the polymorphism of Pd nanocrystals and pave the way for the future design of metastable metal nanomaterials.

Phase transformations due to isochronal annealing of Mg – rare earth – Sc–Mn squeeze cast alloys

AbstractMagnesium alloys containing rare earths (Gd, Y, Ce, ≈3– 10 wt.%), Sc (< 1 wt.%) and Mn (≈1 wt.%) were squeeze cast. Electrical resistivity and hardness responses to isochronal annealing were measured in the range 20 –580 °C and phase changes responsible were identified by transmission electron microscopy. In alloys containing Gd and Y simultaneous precipitation of three phases takes place, namely the metastable c-base-centred orthorhombic phase (Mg–Y, Mg–Gd) in the form of prismatic plates, the thermally stable hexagonal Mn2Sc phase in the form of fine basal discs and a metastable hexagonal coherent phase, containing Mn and Y or Gd, in the form of very thin basal plates (missing in the Mg alloy with 10 wt.% Gd). In the Ce-containing alloy the Mg12Ce tetragonal phase, present after casting in various forms, transforms into a rod-like form. Simultaneously, fine Mn2Sc discs precipitate on basal planes and the grain boundary eutectic transforms to the equilibrium phases.

Structural and phase features of zinc selenide nanoparticles obtained by laser evaporation

The first production of weakly agglomerated zinc selenide nanopowders by evaporation of a target of the required chemical composition in argon using a fiber ytterbium laser is reported. Nanoparticles have the shape of polyhedrons and, more rarely, spheres with an average size of 18 nm. According to XRD data, the obtained nanopowders contain two phases: a stable cubic phase (sphalerite, 40 wt.%) and a metastable hexagonal phase (wurtzite, 60 wt.%). An analysis of HRTEM showed that most of the nanoparticles have structural defects. Due to superstoichiometry, defects in the crystal structure of the crystallographic shift type are formed in nanoparticles. Magneli phases, Wadsley defects and various kinds of heterogeneity inside the particles were found. Keywords: Laser evaporation, zinc selenide, nanoparticles, high resolution transmission electron microscopy (HRTEM), defects, phase composition, crystallographic shift, Magnelli phase, Wadsley defects.

Структурные и фазовые особенности наночастиц селенида цинка, полученных лазерным испарением

The first production of weakly agglomerated zinc selenide nanopowders by evaporation of a target of the required chemical composition in argon using a fiber ytterbium laser is reported. Nanoparticles have the shape of polyhedrons and, more rarely, spheres with an average size of 18 nm. According to XRD data, the obtained nanopowders contain two phases: a stable cubic phase (sphalerite, 40 wt. %) and a metastable hexagonal phase (wurtzite, 60 wt. %). An analysis of HRTEM showed that most of the nanoparticles have structural defects. Due to superstoichiometry, defects in the crystal structure of the crystallographic shift type are formed in nanoparticles. Magneli phases, Wadsley defects and various kinds of heterogeneity inside the particles were found.

Structural investigations on Ge2Sb2Te5 thin films using polarised Raman studies

Ge-Sb-Te (GST) based materials are the main source of ingredient for Phase change memory (PCM) applications. This ternary compound has the capacity to shift faster between crystalline as well as amorphous state when a suitable electrical or optical pulse is applied on it. Its crystalline phase has two states viz. face centered cubic (FCC) which is actually the metastable state and hexagonal phase which is primarily more stable state as compared to the FCC state. However, only FCC phase is suitable for PCM applications, but this study emphasizes the study of stable hexagonal phase also. The need to understand the stable hexagonal phase arises to further understand the useful metastable FCC stage. In this work, local anatomical studies have been carried out using X-ray diffraction technique (XRD) along with Raman spectroscopy that includes polarization studies on Ge2Sb2Te5 thin films in amorphous, FCC and hexagonal states. This work provides information about the symmetry of vibrational states using inelastic Raman scattering studies in polarization mode having vertical and horizontal geometries showing different Raman scattering intensities corresponding to parallel (VV-vertical vertical) and perpendicular (VH-vertical horizontal) scattering configurations. It is expected that interpretations from these studies will be helpful to understand the molecular structure and orientation of various flawed structural states such as coordination and orientation defects.

Influence of vacancy defect on the structural and optical properties of hexagonal barium titanate ceramics: Experimental and theoretical analysis

This study aims to investigate the properties of the metastable hexagonal barium titanate and its corresponding phase evolution with annealing treatment. Specifically, the metastable hexagonal (h) phase, its defect structure, and the tetragonal (t) one were investigated from both the first-principles computational and the experimental perspectives. The powders prepared using self-propagating high-temperature synthesis were consolidated by spark plasma sintering (SPS) under vacuum conditions. The ceramics were subjected to an annealing treatment at 800 and 1200℃ to restore the oxygen stoichiometry and initiate phase transformation. The unannealed sample had about 80 wt% of the h-BaTiO3 phase and 8.4% oxygen defect concentration. Our first-principles calculations showed that oxygen vacancy induced spin-polarized solution with half-metallic behavior.

Facile green synthesis of 2D hexagonal MoO3 for selective detection of ammonia at room temperature

A facile green synthesis of 2D MoO3 has been demonstrated for detection of ammonia at room temperature. The bulk MoO3 was exfoliated in water via ultrasonication assisted liquid exfoliation technique. Interestingly, the resulting MoO3 nanosheets exhibited metastable hexagonal phase unlike the base material (orthorhombic (α) MoO3), as confirmed by different characterization techniques. The phase stability has been confirmed by annealing studies and Thermo-gravimetric analysis (TGA) and the complete transformation back to α-MoO3 was observed at 300 °C. The phase transformation has been attributed to the intercalation of water molecules in the 2D layers of MoO3. The dispersion in organic solvents like ethanol further proves this argument as no phase transformation was observed in the later. The exfoliated hexagonal MoO3 based devices responded fairly well towards ammonia in the concentration range of 0.3 ppm to 5 ppm at room temperature and also superior to the ones prepared in Ethanol medium.

Antimicrobial and electrical properties of ce‐ and ni‐doped zns nanoparticles obtained by a sonochemical method

AbstractIn this work, cerium‐ and nickel‐codoped ZnS nanoparticles were obtained by a sonochemical method for 20 min. The nanoparticles were characterized by X‐ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), and microscopy electronic transmission (MET). The electrical properties are estimated through I‐V curves and the antimicrobial activity was analyzed against E. coli (gram‐negative) and S. aureus (gram‐positive) bacteria using the diffusion disk methodology. The diffractograms indicate the obtaining of cubic structure ZnS for the pure sample, whereas the doped samples present the cubic and hexagonal structures of the ZnS. The bandgap reduced from 3.60 to 3.52 eV, for pure and codoped samples. I‐V curves indicate an increase in resistivity with doping, being more evident for samples containing cerium. Antimicrobial activity increased as doping occurred, where the codoped sample showed the best results. Even for the low amount of dopant (1 mol%), the antimicrobial activity can be increased by about 50% for the codoped sample, compared to the pure ZnS. Thus, it is known that the doping of ZnS with cerium and nickel provides the stabilization of the hexagonal metastable phase, which acts to restrict electronic mobility and, consequently, improves the antimicrobial response of the material.

Tuned structure of europium-doped Al2O3-ytrium luminescent composites and their spectroscopic behavior

This work proposes a smart way to produce lanthanide-doped alumina and yttrium spherical compounds. We were able to tune the structure of the Al2O3-yttrium binary material prepared herein by controlling the Y3+/Al3+ ratio. Small ratio (Y0·1/Al1 mol/mol) afforded the metastable hexagonal phase (YAlO3), which remained stable in the final product even after thermal treatment at 1100 °C. On the basis of XRD data, increasing Y3+/Al3+ ratio led to the formation of the crystalline YAG phase after thermal treatment. The luminescence data showed that Eu3+ ions doped into the binary material were highly sensitive to changes in the environment around the Eu3+ ion, and the emission spectra corroborated that the latter ion migrated to yttrium sites in the YAG structure. The luminescence confirmed the presence of a secondary Eu3+ ion coordination site in the material, but at higher Y3+/Al3+ ratio, the YAG structure has predominated.

Nanolath martensite-austenite structures engineered through DQ&P processing for developing tough, ultrahigh strength steels

A novel processing route comprising thermomechanical rolling followed by direct quenching and partitioning (DQ&P) was designed for developing tough, ductile, ultrahigh strength steels using 0.4 wt% carbon steels. A preliminary characterization of a laboratory-rolled, high-silicon DQ&P steel revealed an excellent combination of mechanical properties comprising high yield and tensile strengths (~1025 and ~2137 MPa, respectively), besides reasonable elongation (≈ 12%) and good impact toughness (T28J temperature ≈ −10 °C). The high mechanical properties are attributed to the formation of desired nanolath martensite-retained austenite (≈ 15%) structures. Features such as nano-twinned martensite, interlath austenite etc. were comprehensively characterized through high-resolution transmission electron microscopy. Attempts have been made to detect possible existence of metastable hexagonal omega phase within the boundaries of nano-twinned martensite and understand its nature of formation.