Metastable Character Research Articles

Mechanical processing and thermal stability of the equiatomic high entropy alloy TiVZrNbHf under vacuum and hydrogen pressure

This study investigates the potential of nanostructuring the equiatomic high entropy alloy TiVZrNbHf by high-pressure torsion to improve its already promising hydrogen absorption properties. The detailed microstructural analysis of the material after processing demonstrates that a homogenous single-phase nanocrystalline structure can be obtained despite shear band development. Due to the metastable character of many high entropy alloys, this analysis was complemented by investigating the thermal stability of the alloy under both vacuum and hydrogen pressure. For the latter, the material was characterized via in situ X-ray diffraction during hydrogen charging at 500 °C, giving a detailed insight into the phase evolution during initial absorption and subsequent cycling. These experiments evidenced the inherent metastability of TiVZrNbHf, which resulted in its decomposition into a bcc, hcp, and C14 Laves phase under both vacuum and hydrogen atmospheres. Despite decomposition, the material retained its nanocrystalline structure under hydrogen pressure, presumably due to hydride formation, while significant grain growth occurred under vacuum. These findings deepen the understanding of the deformation and hydrogen charging behavior of this promising high entropy alloy, suggesting an approach for engineering such alloys for enhanced stability and performance, particularly in solid-state hydrogen storage applications.

Effect of the gelling mechanism on the physical properties of bigels based on whey protein isolate

The effect of the cold-set and heat-set gelling mechanism of whey protein isolate on bigel production was assessed. For this purpose, hydrogel phase was produced with whey protein isolated (10 % w/v) and for oleogel sunflower oil and glycerol monostearate (7.5 % w/v) were used. Bigels were produced by hot emulsification of different hydrogel:oleogel ratios (from 90:10 up to 10:90). For cold-set bigels (CSB) NaCl (200 mM) was added to the aqueous phase prior to the emulsification and the emulsion was cooled to promote the 3D network formation. On the other hand, heat-set bigels (HSB) were produced by heating the emulsion (80 °C, 60 min). Bigels were evaluated through microscopy, FTIR, thermal and texture analyzes. Results showed that depending on the hydrogel:oleogel ratio and gelling mechanism different structures organization were obtained. CSB were more organized, showing that the rate of gelation was the mechanism responsible for the structure. However, for HSB the heat treatment destabilized the emulsion and disorganized structures were observed for high oleogel content. FTIR corroborates the visual observation and showed that the arrangement was purely physical. In addition, the structural arrangement led to different mechanical properties. In general, HSB produced gels with rubber-like behavior, higher elasticity modulus and the presence of a breaking point. In contrast, CSB behaves as squeezing gel, with no breaking point and lower values of elasticity modulus. Moreover, for O/W bigels the dispersed oleogel particles disrupted the WPI network decreasing the gel strength in comparison to pure hydrogels. However, for systems where oleogel was the continuous phase, the gel strength was recovered due to the metastable and dynamic character of these systems. Thus, results showed that the gelling mechanism of the protein exerted an effect on the physical properties of bigels. In addition, the mechanical properties also can be modulated according to the bigel composition, allowing its application in products with different sensorial characteristics.

Experimental evidence for the existence of two neighboring Pr I energy levels for which J is not a good quantum number

We report the discovery of two previously unknown energy levels of neutral praseodymium (Pr I) having odd parity, found during systematic laser spectroscopic recordings of the Pr I spectrum. From unusual intensity ratios of the observed hyperfine structure components we have to conclude that for both levels the quantum number J is not a good quantum number. J is a mixture of values 15/2 and 13/2, and the observed hf patterns of the investigated transitions depend strongly on the combining upper energy level. Moreover, the levels have a metastable character; no transitions to lower lying even energy levels could be found. The intensity distribution within the transition on which the levels were discovered can be roughly explained by a model which assumes transition probability transfer between levels having J = 15/2 and J = 13/2.

On the stability of scale-invariant black holes

Quadratic scale-invariant gravity non minimally coupled to a scalar field provides a competitive model for inflation, characterized by the transition from an unstable to a stable fixed point, both characterized by constant scalar field configurations. We provide a complementary analysis of the same model in the static, spherically symmetric setting, obtaining two Schwarzschild-de Sitter solutions, which corresponds to the two fixed points existing in the cosmological scenario. The stability of such solutions is thoroughly investigated from two different perspectives. First, we study the system at the classical level by the analysis of linear perturbations. In particular, we provide both analytical and numerical results for the late-time behavior of the perturbations, proving the stable and unstable character of the two solutions. Then we perform a semi-classical, non-linear analysis based on the Euclidean path integral formulation. By studying the difference between the Euclidean on-shell actions evaluated on both solutions, we prove that the unstable one has a meta-stable character and is spontaneously decaying into the stable fixed point which is always favoured.

Band parameters of group III–V semiconductors in wurtzite structure

The properties of most III–V semiconductor materials in the wurtzite structure are not known because of their metastable character. However, recent advances in the growth of III–V wurtzite nanorods open new perspectives for applications. In this work, we present a systematic computational study of bulk wurtzite III–V semiconductors, using predictive ab initio methods, to provide a necessary base knowledge for studying the nanostructures. The most important physical properties of bulk systems, i.e., lattice constants, elasticity, spontaneous polarization, piezoelectricity, band structures, deformation potentials, and band offsets, have been studied. Comparison with the available experimental and theoretical data shows the high credibility of our results. Moreover, we provide a complete set of parameters for a six-band k⋅p model, which is widely used for simulating devices based on semiconductor heterostructures.

Polymorphism of Na2CaPO4F: crystal structures, thermal stability and structural complexity

The compound Na2CaPO4F has at least three polymorphic modifications: monoclinic α (also known as the natural mineral nacaphite), orthorhombic β and rhombohedral γ. All three polymorphs have antiperovskite-type structures belonging to either 2H (α and β) or 15R (γ) polytypes. The β-phase was synthesized using CaF2, NaF and Na3PO4 as initial reagents at 800 ​°C. Its crystal structure is orthorhombic (Pnma, a ​= ​5.3542(1), b ​= ​7.0878(2), c ​= ​12.2560(3) Å, V ​= ​465.11(3) Å3, Z ​= ​4) and based upon the chains of fluorine-centered face-sharing octahedra running along [100]. Upon heating, the β form is stable up to 640 ​°C, when it melts and, in the temperature range of 640–800 ​°C, the γ form crystallizes. Its crystal structure (rhombohedral, R3¯m, a ​= ​7.0272(3), c ​= ​40.609(2) Å, V ​= ​1736.66(18) Å3, Z ​= ​15) consists of framework based upon pentamers of face-sharing [F(Na,Ca)6] octahedra connected to each other through common Na vertices. The strongest thermal expansion for both modifications is parallel to the modules of face-sharing anion-centered octahedra, whereas it is almost isotropic within the plane perpendicular to the modules. The information-theoretic structural complexity analysis points out to the possible metastable character of the β-polymorph. The proposed stability row of the Na2CaPO4F phases under ambient conditions corresponds to the following sequence: α ​> ​γ ​> ​β. This agrees well with the relations among their structural complexities, degrees of order, physical and information densities. The sequence of phase transitions α → β → γ proceeds via transitional metastable β-phase. The α → β transition is reversible and of the order-disorder type with the conservation of structural topology, whereas the β → γ transition is reconstructive and irreversible. The latter transition is associated with the transformation of the antiperovskite 2H polytype into the 15R polytype.

A simple solvothermal preparation of Mg-doped anatase TiO2 and its self-cleaning application

Self-cleaning materials have attracted considerable attention for their unique properties and practical applications in energy and environmental areas. Various commercial ceramic products developed based on photo-induced hydrophilic conversion of TiO2 surfaces have also been provided. However, UV light induced and metastable character of anatase TiO2 in medium/high temperature ceramic glaze limited practical applications for self-cleaning photocatalytic materials, which needs further development to improve their efficiency and utilities. In this work, Mg-doped anatase TiO2 was synthesized via a template-free and surfactant-free solvothermal method. The results indicated that MgCl2·6H2O content played key roles in the formation of anatase TiO2. Compared to TiO2-undoped sample, Mg-doped TiO2 sample showed higher photodegradation rate of 99.5 % in 80 min exposed to visible light irradiation. Owing to the introduction of the catalyst, the ceramic glaze showed super-hydrophilicity (water contact angle around 7°) and exhibited excellent stain resistance (e. g. the dried Marker ink on the glaze can immediately float once meeting water after 10 cycles). The glaze after more times recycles could restore super-hydrophilicity is irradiated by visible light for 1 h. This indicates that the introduction of Mg-doped anatase TiO2 demonstrated great practical applications for self-cleaning ceramics toward sunlight.

Strength of diamond - silicon carbide interfaces in silicon carbide bonded diamond materials containing graphitic interlayers

The properties of composite materials strongly depend on the bonding between the reinforcement particles and the matrix phase. High strengths of the diamond/SiC interface have recently been reported for silicon carbide bonded diamond materials. Due to the metastable character of diamond under manufacturing conditions, graphite interlayers can form within the diamond/SiC interface. In order to investigate their influence on the strength of the interface, a material with intentionally generated graphitic interlayers was prepared and micro-cantilevers were cut by focused ion beam milling (FIB) at the SiC/graphite-interlayer/diamond interface. By in-situ bending experiments inside a scanning electron microscope (SEM), a mean value of 7.0 ± 1.2 GPa was obtained as interface strength, which is only slightly lower than the value of graphite-free SiC/diamond interfaces. These values are indicative of direct covalent bonding of the graphitic layers to the diamond reinforcement particles. The results show that thin graphitic interlayers have only little effect on the properties of SiC bonded diamond materials.

Antimony nitride discovered by theoretical structure prediction, rapid thermal annealing, and in situ X-ray diffraction

Nitride materials have promising properties for optoelectronics, but synthesis is challenging because of their metastable character. Therefore, even some simple binary nitrides have not been reported, and their properties remain unknown. Here, we report a metastable Sb–N phase. Unconstrained first-principle crystal-structure predictions suggest that a semiconducting layered SbN phase could be synthesized under activated nitrogen conditions. Experimentally, we approach the synthesis by rapid thermal annealing of an amorphous thin-film precursor. In situ X-ray diffraction monitoring reveals a transient phase appearing at 500°C and lasting for 17 s. With molecular dynamics simulations, we conclude that the phase observed is related to the predicted ground-state structure but that it contains stacking variations. This research highlights the synergy between computational materials discovery and in situ structural characterization during synthesis and processing, enabling the identification and synthesizability of new materials at the frontier of thermodynamically metastable regions.

Gadolinium trisilicide − a paramagnetic representative of the YbSi3 type series

Abstract Binary gadolinium trisilicide GdSi3 was prepared by high-pressure high-temperature synthesis at typically 9.5(5) GPa and 870 K before quenching to ambient conditions. At ambient pressure, GdSi3 exhibits an exothermic decomposition at 647(10) K into the thermodynamically stable phases GdSi2–x and Si, indicating its metastable character. Powder X-ray diffraction data is consistent with the YbSi3-type crystal structure comprising slabs of condensed Si2 dumbbells, which enclose layered arrangements of gadolinium cations. Quantum chemical analysis of the chemical bonding shows, that the framework is formed by silicon dumbbells with homopolar bonds. The magnetic moment of 8.13(8) µ B is consistent with Gd4f 7 (Gd3+ state) and antiferromagnetic ordering is observed below 64(1) K.

Metastable FeCN2@nitrogen-doped carbon with high pseudocapacitance as an anode material for sodium ion batteries.

Pseudocapacitive materials are good candidates for fast charging anodes of sodium ion batteries (SIB). However, pseudocapacitive materials with a high surface area face the severe problem of low initial coulombic efficiency. In this work, micro-sized nitrogen-doped carbon (NC) coated and supported polyhedron FeCN2 networks are designed and synthesized by a facile in situ gel-swelling technique. Impressively, FeCN2@NC as an SIB anode exhibits excellent rate performance with highly reversible rate capacities of 466 and 303 mA h g-1 at 0.2 and 10.0 A g-1, respectively. Furthermore, the FeCN2@NC anode shows a high initial coulombic efficiency (ICE) of 86% due to a low surface area. Electrochemical tests and density functional theory (DFT) calculation indicate that the metastable character enables the low intercalation/conversion reaction energy for FeCN2 and further greatly promotes the fast pseudocapacitive storage mechanism for FeCN2@NC. This work provides evidence that FeCN2 is a new type of metastability induced pseudocapacitive material with high initial coulombic efficiency.

Phase Transition toward a Thermodynamically Less Stable Phase: Cross-Nucleation due to Thin Film Growth of a Benzothieno-benzothiophene Derivative.

The molecule 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene is an organic semiconductor, with outstanding properties in terms of molecular packing and its use in organic electronics. The asymmetric shape of the molecule causes a double layer crystal structure at room temperature. In this work we report its thin film growth by physical vapor deposition starting from the monolayer regime up to thick films. The films are studied in terms of their morphology, crystallographic properties, and thermal stability by atomic force microscopy and X-ray diffraction methods. It is found that the bulk molecular packing of the bilayer is formed at the initial thin film growth stage. After a thickness of one double layer, a transition into a new polymorph is observed which is of metastable character. The new phase represents a single layer phase; the crystal structure could be solved by a combination of X-ray diffraction and molecular dynamics simulations. The observed thin film growth is outstanding in terms of surface crystallization: the formation of a metastable phase is not associated with the initial thin film growth, since the first growth stage represents rather the bulk crystal structure of this molecule. Its formation is associated with cross-nucleation of one polymorph by another, which explains why a metastable phase can be formed on top of a thermodynamically more stable phase.

Temperature and time effect of thermal aging treatment on microstructure and corrosion resistance of UNS S31803 duplex stainless steel

Duplex stainless steels correspond to a class of steel in which the microstructure is composed basically by the phases ferrite and austenite. Given the metastable character of ferrite, associated phase transformations can occur during thermal treatments, which can lead to a reduction in corrosion resistance. Therefore, the aim of this work was to evaluate the influence of the solution-treated at 1100°C for 30min and aging at 500 and 600°C for 1, 3 and 12h, on the microstructure and corrosion resistance of DSS UNS S31803. The steels were characterized by SEM, EDS, XRD, hardness, microhardness and thermal analysis. The steels aged at 500°C for 1h presented the phase alpha line dispersed in the ferrite, and the steels aged at 600°C for 12h presented the phases alpha line, sigma and chi. In the assays of double cycle potentiodynamic reactivation in solution of 0.5M H2SO4 and 0.01M KSNC, it was observed that no sensitization occurred. Furthermore, in the tests of cyclic potentiodynamic polarization, in 3.5% NaCl solution, it was verified that the precipitation of the alpha line phase did not alter corrosion resistance; nevertheless, the sigma phase reduced corrosion resistance.

Catalytic Asymmetric [4+1] Spiroannulation of α-Bromo-β-Naphthols with Azoalkenes by an Electrophilic Dearomatization/S RN 1-Debromination Approach

Catalytic Asymmetric [4+1] Spiroannulation of α-Bromo-β-Naphthols with Azoalkenes by an Electrophilic Dearomatization/S _RN 1-Debromination Approach

Photocatalytic Properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) Pyrochlores: Hybrid DFT Calculations and Experimental Study.

The photocatalytic properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) pyrochlores are examined via ab initio calculations and experiments. A coprecipitation method is applied for the synthesis of nanopowder pyrochlores. The pyrochlore phase formation starts at 500 °C (Bi2Ti2O7) and 550 °C (Bi1.5Ti2O6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi1.5Ti2O6.25 phase in comparison with the Bi2Ti2O7 phase. The estimated crystallite size is 30-40 nm with noticeable agglomerates of about 100-300 nm according to scanning electron microscopy (SEM) and with the formation of particles (510-580 nm) in the aqueous medium. The isoelectric points of the nanopowders seem to be shifted to the strongly acidic region, resulting in the formation of negative surface particle charges of -33 mV (Bi2Ti2O7) and -27 mV (Bi1.5Ti2O6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m2/g (Bi2Ti2O7) and 12.00 m2/g (Bi1.5Ti2O6.25). The use of the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional allows achieving an excellent agreement between theoretical and experimental structural parameters. The screened Coulomb hybrid HSE03 functional is the most appropriate for describing the optoelectronic properties. Bismuth titanate pyrochlores are wide-gap semiconductors with strong abilities to be active photocatalysts under visible irradiation. The optical Eg values for direct/indirect transition according to the experiment, 3.19/2.94 eV (x = 0) and 3.24/3.03 eV (x = 0.5), and the DFT/HSE03 calculations, 2.92/2.87 (x = 0) and 3.42/- eV (x = 0.5), are in the visible light region and are close. The calculated low effective masses of the charge carriers and suitable band edge positions confirm the ability of the pyrochlores to act as photocatalysts. The photocatalytic activity has been evaluated through the decomposition of rhodamine B under visible irradiation. Bi2Ti2O7 shows the highest activity in comparison with Bi1.5Ti2O6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.

The untypical high-pressure Zintl phase SrGe6

Abstract The binary strontium germanide SrGe6 was synthesized at high-pressure high-temperature conditions of approximately 10 GPa and typically 1400 K before quenching to ambient conditions. At ambient pressure, SrGe6 decomposes in a monotropic fashion at T = 680(10) K into SrGe2 and Ge, indicating its metastable character. Single-crystal X-ray diffraction data indicate that the compound SrGe6 adopts a new monoclinic structure type comprising a unique three-dimensional framework of germanium atoms with unusual cages hosting the strontium cations. Quantum chemical analysis of the chemical bonding shows that the framework consists of three- and four- bonded germanium atoms yielding the precise electron count Sr[(4bGe0]4[(3b)Ge−]2 in accordance with the 8 − N rule and the Zintl concept. Conflicting with that, a pseudo-gap in the electronic density of states appears clearly below the Fermi level, and elaborate bonding analysis reveals additional Sr–Ge interactions in the concave coordination polyhedron of the strontium atoms.

A porothermoelastic wellbore stability model for riserless drilling through gas hydrate-bearing sediments in the Shenhu area of the South China Sea

Wellbore instability during the gas hydrate drilling process frequently occurs due to the metastable character of sediment-hosted hydrates and weakly consolidated sediments. An accurate prediction of wellbore stability in gas hydrate drilling is very important. In this paper, a transient heat transfer model between the wellbore and sediment in a gas hydrate reservoir is proposed to simulate temperature profiles inside the drilling string and annulus. Using these temperature profiles, a porothermoelastic model is proposed to compute the temperature and pore pressure field in the entire formation during riserless drilling. Then, the thermodynamic stability of the gas hydrate and wellbore stability are analysed. The findings indicate that it is not necessary to use low-temperature drilling mud during riserless drilling. The risk of in situ hydrate dissociation is very low with a drilling mud density of 1.1 g/cm3. Then, a safe mud density window is obtained for future riserless drilling in the gas hydrate reservoirs in the Shenhu area.

Speciation of reactive sulfur species and their reactions with alkylating agents: do we have any clue about what is present inside the cell?

Buffered aqueous solutions of inorganic, organic, cysteine, GSH and GAPDH polysulfide species were used. Additional experiments in human plasma and serum revealed that monobromobimane can extract sulfide from the endogenous sulfur pool by shifting speciation equilibria, suggesting caution should be exercised when interpreting experimental results using this tool. In the majority of cases, the speciation of alkylated polysulfide/thiol derivatives depended on the experimental conditions. Alkylation perturbed sulfur speciation in both a concentration- and time-dependent manner and strong alkylating agents cleaved polysulfur chains. Moreover, the labelling of sulfenic acids with dimedone also affected cysteine speciation, suggesting that part of the endogenous pool of products previously believed to represent sulfenic acid species may represent polysulfides. We highlight methodological caveats potentially arising from these pitfalls and conclude that current derivatization strategies often fail to adequately capture physiological speciation of sulfur species. This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Structural complexity of uranophane and uranophane-β: implications for their formation and occurrence

Information-based parameters of structural complexity for two polymorphs of Ca[(UO 2 ) 2 (SiO 3 OH) 2 ] · 5H 2 O, uranophane and uranophane-β, were calculated in order to assess the impact of the structural complexity on their stability and formation. Calculations proved that uranophane-β is far more complex ( I G,total = 691.90 bits/cell) than uranophane ( I G,total = 345.95 bits/cell), which indirectly suggests that uranophane may form as a metastable phase during crystallization process. This suggestion is further supported by the lower density of uranophane (3.834 g/cm 3 ) compared to that of uranophane-β (3.983 g/cm 3 ). The hypothesis of a metastable character of uranophane is in agreement with Goldsmith’s simplexity principle and the Ostwald–Volmer rule and is well supported by the frequency of occurrences of both dimorphic minerals in Nature.

Wellbore stability analysis during drilling through marine gas hydrate-bearing sediments in Shenhu area: A case study

Because of the metastable character of natural gas hydrate-bearing sediments (GHBS), they can be easily affected by drilling operations, thereby inducing the wellbore instability risk. This issue is studied in this paper by means of numerical simulations based on the first exploration well performed in the Shenhu area of the South China Sea in 2007. The corresponding simulated results were compared against the field caliper logging curve. Besides this analysis, the effects of both drilling mud properties (i.e., temperature, density, and salinity) and initial reservoir conditions (i.e., permeability, and saturation) on the wellbore stability were further investigated under the condition of drilling mud invasion into the GHBS. It is anticipated that the coupled multiphysics phenomena triggered under operation conditions can lead to wellbore instability and may also trigger the formation of fractures. The simulation results show that the predicted yield region around the borehole of well SH2 conforms to the borehole enlargement observed in the field. The sensitivity analyses indicate that a local increase in the pore pressure is likely to be generated in the wellbore vicinity, triggered by the large amount of free gas to be released during hydrate dissociation, which is in turn induced by heating (i.e., coming from the drilling-friction effects) and the high salinity of the drilling muds typically used. This study shows that by engineering the salinity of the drilling mud, the amount of free gas to be produced from marine GHBS can be controlled, preventing in this way further wellbore instability issues. This research also shows that an increase in the drilling-mud density not only impact on the development of pore pressures near the borehole, but it also affects the extent of drilling-mud invasion. This effect will extend the hydrate dissociation to a wider area and will also reduce the resistance of the formation, leading to further deformation and potential failure. The analyses also show that the increase in the initial hydrate saturation is associated with a stronger sediment and a more stable wellbore. The model also predicts that GHBS with higher absolute permeability will dissipate faster the excess of pore pressure in the surrounding sediment, contributing to wellbore stability; however, the extent of the hydrate dissociation zone will expand and thereby will increase the yield region.