Stability Of Metastable Phases Research Articles

Corrosion of Electrochemical Energy Materials: Stability Analyses Beyond Pourbaix Diagrams

Since their first appearance, electrode potential/solution pH charts (Pourbaix diagrams) have served as a general framework for the analysis of material corrosion in aqueous solutions. Their utility in describing electrochemical stability of materials is rooted in the equilibrium thermodynamics based formalism. Electrochemical energy systems, however, do not always operate under thermodynamic equilibrium conditions, and corrosion itself is an inherently irreversible interfacial process. In this Perspective, we highlight a number of representative studies drawn primarily from (photo)electrocatalysis research and aimed to account for the effects of potentiodynamic polarization and transient dissolution, interfacial electrochemical reactions, formation and stability of metastable phases, strain-induced corrosion, as well as the kinetics of dissolution and phase transformations. Some theoretical approaches that can help disentangle corrosion mechanisms under conditions relevant to electrochemical energy conversion applications are briefly discussed. We also advocate for more research efforts directed at the basic science of electrochemical corrosion that has long been deemed as a not “fancy” topic.

ZnO under Pressure: From Nanoparticles to Single Crystals

In the present review, new approaches for the stabilization of metastable phases of zinc oxide and the growth of ZnO single crystals under high pressures and high temperatures are considered. The problems of the stabilization of the cubic modification of ZnO as well as solid solutions on its basis are discussed. A thermodynamic approach to the description of zinc oxide melting at high pressures is described which opens up new possibilities for the growth of both undoped and doped (for example, with elements of group V) single crystals of zinc oxide. The possibilities of using high pressure to vary phase and elemental composition in order to create ZnO-based materials are demonstrated.

Stability of Metastable Phases and Kinetics of Nucleation in a Simple Single-Component System (Molecular Dynamics Simulation) (A Review)

Stability of Metastable Phases and Kinetics of Nucleation in a Simple Single-Component System (Molecular Dynamics Simulation) (A Review)

Kinetically Stable Nonequilibrium Gold-Cobalt Alloy Nanoparticles with Magnetic and Plasmonic Properties Obtained by Laser Ablation in Liquid.

Nonequilibrium nanoalloys are metastable solids obtained at the nanoscale under nonequilibrium conditions that allow the study of kinetically frozen atoms and the discovery of new physical and chemical properties. However, the stabilization of metastable phases in the nanometric size regime is challenging and the synthetic route should be easy and sustainable, for the nonequilibrium nanoalloys to be practically available. Here we report on the one-step laser ablation synthesis in solution (LASiS) of nonequilibrium Au-Co alloy nanoparticles (NPs) and their characterization on ensembles and at the single nanoparticle level. The NPs are obtained as a polycrystalline solid solution stable in air and water, although surface cobalt atoms undergo oxidation to Co(II). Since gold is a renowned plasmonic material and metallic cobalt is ferromagnetic at room temperature, these properties are both found in the NPs. Besides, surface conjugation with thiolated molecules is possible and it was exploited to obtain colloidally stable solutions in water. Taking advantage of these features, an array of magnetic-plasmonic dots was obtained and used for surface-enhanced Raman scattering experiments. Overall, this study confirms that LASiS is an effective method for the formation of kinetically stable nonequilibrium nanoalloys and shows that Au-Co alloy NPs are appealing magnetically responsive plasmonic building blocks for several nanotechnological applications.

Ferroelectricity in Pb1+δZrO3 Thin Films

Antiferroelectric PbZrO3 is being considered for a wide range of applications where the competition between centrosymmetric and noncentrosymmetric phases is important to the response. Here, we focus on the epitaxial growth of PbZrO3 thin films and understanding the chemistry–structure coupling in Pb1+δZrO3 (δ = 0, 0.1, 0.2). High-quality, single-phase Pb1+δZrO3 films are synthesized via pulsed-laser deposition. Although no significant lattice parameter change is observed in X-ray studies, electrical characterization reveals that while the PbZrO3 and Pb1.1ZrO3 heterostructures remain intrinsically antiferroelectric, the Pb1.2ZrO3 heterostructures exhibit a hysteresis loop indicative of ferroelectric response. Further X-ray scattering studies reveal strong quarter-order diffraction peaks in PbZrO3 and Pb1.1ZrO3 heterostructures indicative of antiferroelectricity, while no such peaks are observed for Pb1.2ZrO3 heterostructures. Density functional theory calculations suggest the large cation nonstoichiometry is accommodated by incorporation of antisite PbZr defects, which drive the Pb1.2ZrO3 heterostructures to a ferroelectric phase with R3c symmetry. In the end, stabilization of metastable phases in materials via chemical nonstoichiometry and defect engineering enables a novel route to manipulate the energy of the ground state of materials and the corresponding material properties.

Stabilization of metastable phases in hafnia owing to surface energy effects

The recent empirical observation of ferroelectricity in hafnia is rather surprising since all of its known ground-state phases are nonpolar. In this letter, we show that finite size effects, relevant under experimental conditions, may themselves lead to this unexpected phenomenon due to stabilization of metastable polar phase(s). Using surface energies computed from first principles, we determine the thermodynamic stability of a parallelepiped shaped particle constructed from various low energy nonpolar and polar phases of hafnia. We find that at small dimensions, surface effects may stabilize either one of the polar phases or the nonpolar tetragonal phase (the parent phase of the polar phases), suggesting a possible explanation of the ferroelectric behavior observed in pure hafnia films. These results also explain the stabilization of the metastable tetragonal phase in nanoparticles of zirconia, the twin oxide of hafnia. While a comprehensive understanding of the origins of ferroelectricity in hafnia will require inclusion of other relevant factors (such as strain and dopants), this work highlights the importance of finite size effects as a possible key factor.

Stability of metastable phase and soft magnetic properties of bulk Fe-B nano-eutectic alloy prepared by undercooling solidification combined with CU-mold chilling

Bulk Fe83B17 nano-eutectic alloys were prepared by undercooling solidification combined with Cu-mold chilling method. Stable phase Fe2B and metastable phase Fe3B were found to coexist in the as-solidified microstructure. The soft magnetic properties were improved significantly by the nano-lamellar eutectic and the metastable phase and, were increased further by annealing at 1173K for 1.5h after which the metastable phase was decomposed completely.

Quantitative computational screening of Pd-based intermetallic membranes for hydrogen separation

One of the most important processes in producing high purity hydrogen is separating hydrogen from a mixture of gas streams. This work looks at intermetallics, stoichiometric compounds of two or more metals, as prospective membranes for these applications. We describe a systematic computational screening approach based on calculation of key properties like solubility, diffusivity and permeability of hydrogen through these materials using quantitatively accurate Density Functional Theory calculations. Solubility was used as a screening parameter and solubility data has been calculated for 78 Pd-based binary intermetallics. Permeability calculations were performed for the shortlisted candidates. After systematically examining all Pd-based intermetallics, no material was found to have higher hydrogen permeability than pure Pd. To investigate the potential influence of hydrogen on the stability of metastable phases, we examined a number of metastable structures of Pd3Mn. Two metastable Pd3Mn phases are stabilized in the presence of hydrogen, but these phases are predicted to have low permeability. We also explored whether an empirical relationship could be found to rapidly predict H solubility in intermetallics using a QSPR approach. Unfortunately, the values calculated using this empirical approach cannot be used to make useful predictions.

Sol–gel synthesis of tetragonal ZrO2 nanoparticles stabilized by crystallite size and oxygen vacancies

In this letter, we present a facile route to produce metastable tetragonal zirconia (ZrO2) nanoparticles via pH-controlled precipitation of dilute zirconyl nitrate dihydrate [ZrO(NO3)2·2H2O] solution in liquid NH3 under ambient conditions and calcination at 500 °C for 2 h. The phase pure tetragonal ZrO2 nanoparticles are obtained at pH 9. The effect of pH on metastable phase stabilization in precipitated ZrO2 nanoparticles is demonstrated with the help of XRD, SEM/EDX, and X-ray photoelectron spectroscopy techniques. The stability of tetragonal ZrO2 phase is attributed to the smaller crystallite size and greater oxygen deficiency in phase-pure tetragonal ZrO2.

Stabilization of metastable phases in Mg–Li alloys by high-pressure torsion

In this work we demonstrate that pure compression of the single phase bcc Mg–12 wt.% Li and Mg–20 wt.% Li alloys in a high-pressure torsion system induces the precipitation of hexagonal close-packed (hcp) Mg and hcp Li-rich phases, which remain stable under ambient conditions. This transformation is enhanced with increasing pressure. Shearing under pressure of the multiphase microstructure results in the dissolution of the metastable hcp phases and the subsequent growth of the bcc matrix grains.

Comparative analysis of cadmium doped magnesium ferrite Mg(1−x) Cdx Fe2O4 (x = 0.0, 0.2, 0.4, 0.6) nanoparticles

Oxalyl dihydrazide–metal nitrate combustion route was employed to synthesize Mg(1−x) Cdx Fe2O4 (x=0.0, 0.2, 0.4, 0.6) nanoparticles (NPs). Ferrite NPs were analyzed by various physico-chemical techniques viz. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Vibrating sample magnetometer (VSM) was used to study effect of doping on the magnetic parameters of ferrite. Combustion method proved a low temperature route for preparation of mono disperse ferrite nanoparticles with average particle diameter of 22–34nm. In the present study saturation magnetization and remnant magnetization increased with cadmium content up to x=0.4, Mg0.6Cd0.4Fe2O4 showed promising magnetic and micro structural properties exploring its potentiality as soft magnetic material. The temperature of ferrite formation (300°C) was much lower than the reported value (700°C) for co-precipitation method. This can be attributed to the fact that intimate mixing of cations and exothermic decomposition of combustion mixture facilitates solid state reaction and stabilization of metastable phases thus lowering the external temperature required for ferrite formation. Another advantage of combustion approach is that it does not involve sintering and milling at elevated temperature (as required in conventional ceramic method) which introduces lattice defects, strains and causes coarsening of ferrite. In the present case high purity products with desired stoichiometry and promising magnetic properties were obtained.

Metastable Phase in Undercooled Fe-Co Alloy

Metastable phase was observed in as-solidified microstructure of undercooled Fe-Co alloy, provided that the initial undercooling (T) of the melt exceeds the critical value. On this basis, the forming mechanism and stability of metastable phase were investigated. The forming mechanism of metastable phase in undercooled Fe-Co alloy was concluded as cooperative effect of “competitive nucleation—remelting—extensive growth—incomplete solid-state transformation” in this work. With the increase of annealing time, the number and dimension of metastable phase decreased at the same time. Moreover, metastable phase was transformed to stable phase completely when annealing time increased to three hours.

Kinetic Approach to Biomineralization: Interactions of Synthetic Polypeptides with Calcium Carbonate Polymorphs

Biomineralization processes are the subject of numerous investigations. This article gives a review of the study on interactions between the charged polypeptides and the mineral surfaces involved in biomineralization, with an additional kinetic approach. The influence of polypeptides on two types of precipitation processes is discussed: the spontaneous precipitation from supersaturated solution and the growth kinetics of calcite seed crystals. In the first case the phenomenon of the formation and stabilization of metastable phases was found while in the second case the influence of the applied polypeptides on the kinetics and mechanisms of calcite crystal growth was investigated. Calcium carbonate polymorphs, calcite and vaterite, were used as biomineral substrates and acidic polypeptides, poly-L-aspartic (pAsp) acid and poly-L-glutamic (pGlu) acid, as simplified models of naturally occurring soluble acidic proteins. A basic polypeptide, poly-L-lysine (pLys), was also used in experiments in order to find out whether conformity between the crystal surface and the adsorbed polypeptide, or just the electrostatic interactions, have a decisive role in these processes. The addition of a particular polypeptide into the precipitation system caused a significant inhibition of nucleation and growth of vaterite, the extent of inhibition being in the order Inh(pAsp) > Inh(pGlu) >> Inh(pLys). In addition to the inhibition of precipitation, the change of the polymorphic composition and the crystal morphology of the precipitate were also achieved. The explanation of such acidic polypeptide behaviour is a consequence of kinetic constraints through the diverse efficiency of inhibition of both calcite nucleation and vaterite growth caused by adsorption of acidic polypeptides. The acidic polypeptides also caused the inhibition of calcite crystal growth, the effect being pAsp > pGlu, and changed the observed mechanism of growth controlled by the integration of ions into the spiral steps, as found for the model systems, to the surface nucleation rate-determining mechanism. Nonselective, weak and electrostatic adsorption of pLys at the crystal surface was probably responsible for increasing the calcite crystal growth rate when pLys was present at low concentrations and for inhibiting it at pLys higher concentrations. The strongest interactions between the crystal surfaces and the polypeptides, observed for the calcite/pAsp system, can account for coordinative interactions between the side chain carboxylic groups of the predominantly planar arrangement of the pAsp structure (beta-pleated sheet) and Ca2+ ions from the crystal surface.

Regularities of formation of nanocrystalline particles in titanium subgroup dioxides

Trends in formation and evolution of nanocrystalline phases in TiO2, ZrO2, and HfO2 powders were studied. The stability of metastable phases increases in the order of HfO2 > ZrO2 > TiO2, and the probability of formation of thermodynamically stable phases increases in the order of TiO2 > ZrO2 > HfO2. The enthalpies of formation were determined for TiO2, ZrO2, and HfO2 nanocrystallites, as well as the activation energies of their subsequent growth. For TiO2 and ZrO2, a phenomenological model was advanced to describe nanocrystallite formation and subsequent polymorphic transformations.

Stabilization of metastable phases in spatially restricted fields: the case of the Fe2O3 polymorphs

In this work we show how the confinement of particles in a silica matrix with pores, acting as nano-vessels, plays an important role in the formation of pure E-Fe2O3 nanoparticles and their thermal stability. In particular, a HRTEM study of a series of Fe2O3-SiO2 xerogels annealed at different temperatures reveals that, at low temperatures, gamma-Fe2O3 nanoparticles are formed and only transform to F-Fe2O3 after subsequent annealing at higher temperatures. These data are complemented by measurements of the SiO2 matrix porosity as well as with calorimetric and structural analysis of the nanoparticles after matrix removal. The gathered data indicate that the SiO2 matrix acts as a barrier, hindering thermal diffusion and particle growth even at the high temperatures where the formation of epsilon-Fe2O3 appears to be favoured, thereby preventing its transformation to alpha-Fe2O3.

Formation and Stability of Metastable Phases in Mechanochemical Synthesis

Some conditions of formation of solid solutions and intermetallic compounds in the course of mechanochemical synthesis are considered. The main factor among those determining the thermodynamic possibility of formation of metastable phase in mechanochemical synthesis is substantial increase in the internal energy of the initial material due to accumulation of stored strain energy and appearance of nanostructural (excited) state. Competition (choice) between phases possible in the given system is determined by kinetic factors. As a rule, the first to appear in metallic systems are regions of solid solution phase based on a relatively low-mobility component. Energy barriers in generation and growth of excess phase (especially for phases with low energy of formation) are an additional factor of stability of the metastable state of supersaturated solid solution in the process of mechanochemical synthesis due to scarceness of mobile vacancies.

Structural Properties and Stability of Metastable Phases in the Zr-Nb System

The lattice parameters of the bcc (b) and omega (W) phases occurring metastably in a series of Zr-rich Zr-Nb alloys have been determined at and above room temperature (TR) using neutron diffraction techniques. In the first place, the effect of temperature changes upon the lattice parameters of the b and W phases in alloys with 10 and 18 at.% Nb was monitored using neutron thermodiffraction. A method of analysis is applied to the data which involves a confrontation between the observed structural properties and an idealised -or ``reference'- behaviour (RB) which admits a simple mathematical description. A generalised form of Vegard's law is adopted as RB for the b phase, whereas a specific RB is proposed for the W structure. The experimental data are well accounted for by this interpretation scheme, leading to a picture of the isothermal reactions occurring at high temperature which involves the transfer of Nb from the W to the b phase. Finally, the neutron diffraction data on the W phase are combined with an electron microscopy study for the alloy with 10 at.% Nb aged at 773 K, which provides information on the composition of this phase and its evolution towards thermodynamic equilibrium.

Effect of Mo Addition on Microstructure and Properties of Nb/Nb<sub>5</sub>Si<sub>3</sub> In-Situ Composite

Nb/Nb5Si3 in-situ composites are very attractive structural materials for these materials perform a good balance in mechanical properties, including a high strength at high temperature (>1373K) and reasonably high fracture toughness at room temperature. Metastable phase Nb3Si plays an important role in the properties of Nb/Nb5Si3 composites by affecting microstructure and volume fracture of ductile phase. In this paper, Nb-10Si-xMo and Nb-18Si-xMo (x=0,5,15) are prepared by arc melting and annealed at 1473K for 100h. Single edge-notched bending (SENB) test was used to study the fracture toughness of Nb-Si-Mo alloys. The stability of metastable phase is analyzed by XRD. The room temperature fracture toughness of Nb-10Si is 10.47MPa(m)1/2 and higher than that of binary Nb-18Si alloys at near-eutectic compositions. The addition of Mo improves the fracture toughness of as cast Nb-Si alloys from 4.1 MPa(m)1/2 to 9.9MPa(m)1/2 at near-eutectic compositions and decreases it from 10.47 MPa(m)1/2 to 8.8MPa(m)1/2 at hypoeutectic compositions.

Structural properties and stability of metastable phases in the Zr-Nb system: Part II. Aging of bcc (β) alloys and assessment of β-Zr

The lattice parameters (LPs) of the bcc (β) phase occurring metastably in a series of Zr-rich Zr-Nb alloys have been determined at and above room temperature (T R ) using neutron diffraction techniques. LP values at T = T R were determined in alloys with Nb contents in the range 8.4 ≤ at. pct Nb ≤ 23 using both standard and high-resolution neutron diffractometry. These results support the applicability of Vegard’s law at T R . In addition, a neutron thermodiffractometric technique is used to establish the LP vs T relation of the β phase in two Zr-Nb alloys with 18 and 23 at. pct Nb up to 700 K. Such high-temperature data are used in a generalized Vegard’s law approach to gain insight into the LP vs T relation of the β phase in pure Zr, which is metastable in this temperature range. With these new LP values, an assessment is performed of the thermal expansion coefficient of β-Zr covering both stable and metastable states. As a direct application of the present findings, the results of an aging experiment are discussed, which provide information on the composition of the β phase taking part in the bcc-hcp equilibrium in the Zr-Nb phase diagram at 900 K. Such composition has been the subject of contradictory reports in recent literature. The composition value arrived at in the present analysis agrees well with the results of a thermodynamic calculation of the Zr-Nb phase diagram (A. Fernandez Guillermet: Z. Metallkd., 1991, vol. 82, p. 478)

Structural properties and stability of metastable phases in the Zr-Nb system: Part I. Systematics of quenching-and-aging experiments

An experimental study is presented of the effect of an isothermal treatment (“aging”) at 773 K upon the structural properties of three metastable phases formed by quenching Zr-Nb alloys from 1273 K, viz. hcp (α), bcc (β), and Ω. Using neutron-diffraction (ND) measurements and Rietveld analysis, the lattice parameters (LPs) and the constitution of Zr-Nb alloys with up to 18 at. pct Nb are determined. By combining these data with the LP vs at. pct Nb relations previously determined by us on quenched alloys, an analysis is performed of the composition changes and phase reactions occuring upon aging. The present results open up the possibility of using quenching-and-aging experiments to gain insight into the α+β metastable equilibrium in the Zr-rich side of the Zr-Nb system.