Solid-solid Phase Transformations Research Articles

Conformation-assisted solid-solid phase transition of LiTFSI electrolyte salt and the lithium ion coordination

Single crystal growth and characterization of the lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI), the most common electrolyte salt for lithium-ion batteries, have been performed and succeeded in unraveling the atomic structures of its different crystalline phases. The structures of two crystalline phases (phase I: orthorhombic, Pccn; phase II: monoclinic, P21/c) have been determined through temperature-dependent X-ray crystallography of the LiTFSI single crystal on heating, and the solid-solid phase transformation between phase I and phase II has been dictated. Interestingly, a conformational change of TFSI⁻ from transoid to cisoid has been discovered during the transition from phase I to phase II, which has been further confirmed by the temperature-dependent Raman spectroscopy. The coordination of Li⁺ with the TFSI⁻ ions of different conformations has been also elucidated in the polymorphic crystalline structures. The solid-solid phase transformation of the first-order leads to the cracking of the LiTFSI crystal, probably along the lithium-ion or the fluorine-rich layer in phase II. In the molten state, the coexistence of the transoid conformation and the cisoid conformation is found in the TFSI⁻ ions, affirming the recent observation in the concentrated non-crystalline state. This work is anticipated to shed light on the (de)solvation and the transport of lithium ions in complex fluids encompassing LiTFSI electrolyte solutions from the structural aspects.

β-phase formation of poly(vinylidene fluoride) foam based on the porous morphology control via supercritical carbon dioxide

The increase of electroactive β phase of poly(vinylidene fluoride) (PVDF) is important in energy harvesting applications. In this work, a green method of supercritical CO2 physical foaming was applied to obtain the PVDF foams with high content of β-crystal by utilizing the biaxial stretching when the cells grew, which were applied in the field of piezoelectric sensors. By changing the foaming condition, the PVDF foams with various structures were obtained, and a comprehensive study on the crystal transformation of them was carried out. The PVDF foam with increased β crystals content was more accessible to obtain at a high saturation pressure, corresponding to the large expansion ratio and small cell size. The acquisition of β crystals mainly came from the α-β solid-solid phase transformation during decompression, rather than the annealing process. Due to the introduction of cell structure, the piezoelectric output performance of the sample was effectively amplified. The produced PVDF foam with the β-crystal content of 30%, exhibited the low density of 0.11 g/cm3 and the highest piezoelectric output of 4.1 V (corresponding to a specific voltage of 35 V‧cm3/g). This foaming strategy provides guidance for lightweight design and high-performance fabrication of piezoelectric materials.

Complexity of the KNa2ZrF7, KNa2HfF7, and KNa2(Hf,Zr)F7 compounds

New mixed-cation fluorides KNa2HfF7 (Pmn21, a = 8.213(2) Å, b = 6.126(2) Å, c = 5.979(2) Å, Z = 2) and KNa2(Hf,Zr)F7 (refined composition KNa2Hf0.44(1)Zr0.56(1)F7; Pmn21, a = 8.2210(8) Å, b = 6.1288(6) Å, c = 5.979(1) Å, Z = 2) together with isostructural KNa2ZrF7 (Pmn21, a = 8.221(2) Å, b = 6.118(2) Å, c = 5.994(2) Å, Z = 2) have been synthesised in aqueous solutions and structurally characterised at room temperature by powder and single-crystal X-ray diffraction. The solid-solid phase transformations have been revealed during the differential scanning calorimetry experiments and further confirmed by temperature-dependent X-ray powder diffraction. It has been found that mixed-cation phases KNa2ZrF7, KNa2HfF7 and KNa2(Hf,Zr)F7 undergo irreversible phase transformations, which can be explained by their decomposition over 673 K. XPS analysis confirmed the presence of exclusively terminated fluorines around the transition metals and the absence of oxygen impurities in the crystal structures of compounds.

Accelerating metal nanoparticle exsolution by exploiting tolerance factor of perovskite stannate.

The octahedral symmetry in ionic crystals can play a critical role in atomic nucleation and migration during solid-solid phase transformation. Similarly, octahedron distortion, which is characterized by Goldschmidt tolerance factor, strongly influences the exsolution kinetics in the perovskite lattice framework during high-temperature annealing. However, a fundamental study on manipulating the exsolution process by octahedron distortion is still lacking. In this study, we accelerate Ni metal exsolution on the surface of perovskite stannates by increasing the [BO6] octahedron distortion in the lattices. Decreasing the A-site ionic radius (rBa2+ = 161 pm → rSr2+ = 144 pm → rCa2+ = 134 pm) increased the density of exsolved Ni nanoparticles by up to 640% (i.e., 47 particles μm-2 of Ba(Sn, Ni)O3 → 304 particles μm-2 of Ca(Sn, Ni)O3) after the identical exsolution process. Based on the theoretical calculation and experimental characterization, the decrease in crystal symmetry by octahedral distortion promoted the Ni exsolution owing to the boosted Ni migration by weakening the bond strength and generating domain boundaries. The findings highlight the importance of octahedral distortion to control atomic migration through the perovskite lattice framework and provide a strategy to tailor the density of uniformly populated nanoparticles in nanocomposite oxides for multifunctional material design.

Multiple structural phase transitions in single crystal silicon subjected to dynamic loading

Single crystal Silicon (SC Si), as one of the most extensively used elements, has a series of complex phase changes, however, it is not fully understood under dynamic loading conditions. In this letter, both the shock induced multiple solid-solid phase transformations and solid-liquid transition are systematically investigated using large-scale molecular dynamics (MD) simulations. With the increase of particle velocities up to 2.8 km/s, the SC Si is revealed to undergo a continuous structural phase transitions from cubic diamond (cd) to β-tin, bct5, near-simple cubic (sc), and body centered cubic (bcc) structure. Furthermore, the liquid Si is distinguished and shows solid-liquid coexistence under intensive shocks. Our work dissects the continues multiple phase transitions in shocked Si for the first time in MD simulations, which provides new understandings of phase transitions of Si under dynamic loadings, and strongly supports and complements the relevant reported experimental researches.

The use of polymorphic state modifiers in solid lipid microparticles: The role of structural modifications on drug release performance

This study investigates the correlation between the structural and release properties of solid lipid microparticles (MPs) of tristearin containing 5 % w/w of four different liquid additives used as crystal modifiers: isopropyl myristate (IM), ethyl oleate (EO), oleic acid (OA) and medium chain triglycerides (MCT). All additives accelerated the conversion of the unstable α-form of tristearin, formed after the MPs manufacturing, to the stable β-polymorph and the transformation was completed within 24 h (for IM and EO) or 48 h (for OA and MCT). The kinetic of polymorphic transition at 25 °C was investigated by simultaneous synchrotron SAXS/WAXS and DSC analysis after melting and subsequent cooling of the lipid mixture. After crystallization in the α-phase, additives accelerate the solid-solid phase transformation to β-tristearin. SAXS data showed that two types of structural modifications occurred on MPs during storage: compaction of the crystal packing (slight decrease in lamellar thickness) and crystal growth (increased number of stacked lipid lamellae). The release behavior of a model hydrophilic drug (caffeine) at two different amounts (15 % and 30 %) from MPs was studied in water and biorelevant media simulated the gastric and intestinal environment. It was particularly significant that the introduction of IM, EO and MCT were able to prolong the drug release in water, passing from a diffusion-based Higuchi kinetics to a perfect zero-order kinetic. Moreover, the overall release profiles were higher in biorelevant media, where erosion/digestion of MPs was observed. After 6 months, a moderate but statistically significant change in release profile was observed for the MPs with IM and EO, which can be correlated with the time-dependent structural alterations (i.e. larger average crystallite size) of these formulations; while MPs with OA or MCT displayed stable release profiles. These findings help to understand the correlation between release behavior, polymorphism and supramolecular‐level structural modification of lipid formulations containing crystal modifiers.

Study on polycyclic macromolecular drug solid stability: A case exploration of methylcobalamin

As one of derivatives of Vitamin B12, methylcobalamin (MeCbl) is an indispensable “Life Element” and plays an essential role in maintaining human normal physiology function and clinical medicine application. Because of the intricate molecular structure, strong hygroscopicity and optical instability, maintaining its solid stability is a great challenge in pharmaceutical preparation. Based on the structure features of MeCbl hydrates, this study explored the drug solid stability by designing solid-solid phase transformation (SSPT) experiments. Three hydrate powders of MeCbl that had special structure with isolated site and channel water molecules were discovered. It was found that drying condition and surrounding humidity were controlling factors influencing the final solid form. The inter-conversion relations relevant to heating-induced and humidity-induced structure changes were established among the three hydrate powders. Powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, high performance liquid chromatography and dynamic vapor sorption were used to characterize the differences and related properties of stably prepared MeCbl hydrate powders. The particle size of product could be regulated and controlled by optimizing operating conditions of crystallization process, where ultrasound-assisted and seeding-introduced were applied as promising strategies to enhance solution crystallization process. This study opens up the possibility for the stable preparation and large-scale production of polycyclic macromolecular bulk drugs like methylcobalamin.

Enhancing plasticity of ‘self-sharpening’ tungsten high-entropy alloy via tailoring μ-precipitation

In our recently published work (Acta Materialia 186 (2020) 257–266), we have designed a new equimolar tungsten high-entropy alloy with excellent penetration ability to satisfy the highly desirable of ‘self-sharping’ in wide range of engineering application. This alloy has outstanding dynamic compressive properties and superior penetration performance than that of 93 W alloys. In this work, the tension properties of the tungsten high-entropy alloy were significantly improved by μ phase precipitation design strategy to tailor the morphology and distribution of μ phase. Through controlling the phase transformation process, the μ phase changes from liquid-solid phase transformation to solid-solid precipitation phase transformation. This can effectively impede the brittleness caused by the μ phase segregation at the grain boundary and phase boundary. Moreover, the Orowan effect caused by nano-sized μ-phase particles improves the tensile strength effectively (enhancing ∼150%) and ensure the ductility. This material design strategy significantly improves the tension ductility of the alloy and provides a new paradigm to solve the similar problem of material brittleness.

Catalytic-assembly of programmable atom equivalents

Escape from metastable states in self-assembly of colloids is an intractable problem. Unlike the commonly adopted approach of thermal annealing, the recently developed enthalpy-mediated strategy provided a different option to address this dilemma in a dynamically controllable manner at room temperature. However, it required a complex catalytic-assembly DNA strand-displacement circuitry to mediate interaction between multiple components. In this work, we present a simple but effective way to achieve catalytic-assembly of DNA-functionalized colloidal nanoparticles, i.e., programmable atom equivalents, in a far-from-equilibrium system. A removable molecule named "catassembler" that acts as a catalyst was employed to rectify imperfect linkages and help the system escape from metastability without affecting the assembled framework. Notably, catalytic efficiency of the catassembler can be effectively improved by changing the seesaw catassembler in toehold length design or numbers of the repeat units. Leveraging this tractable catalytic-assembly approach, different ordered architectures were easily produced by directly mixing all reactants, as in chemical reactions. By switching bonding identities, solid-solid phase transformations between different colloidal crystals were achieved. This work opens up an avenue for programming colloid assembly in a far-from-equilibrium system.

Large barocaloric effects in two novel ferroelectric molecular plastic crystals

A new type of materials combining a high-temperature plastic phase and a low-temperature ferroelectric phase, the ferroelectric plastic crystals, are promising for application in refrigeration systems based on solid-state caloric effects.

Oriented loading and enduring confinement endow long-life-span Se cathode based on chemical affinity

Reversible electrochemical conversion between selenium (Se) and lithium selenide (Li2Se) can be achieved in the carbonate-based electrolytes through a solid-solid phase transformation, avoiding the fatal shuttle effect of intermediate products in the ethter-based electrolytes. However, the solid-solid conversion puts forward more stringent requirements on the initial distribution state of active Se, which cannot be satisfied by the existing physical methods for Se loading. Herein, a novel in-situ oxidization strategy is developed to construct an ideal Se/dual-carbon composite (marked as Gr@Se⊂HC). In the process, under the guidance of the N/O heteroatoms, Se molecules oxidized from the pre-embedded ZnSe are orientedly deposited inside of the hollow carbon (HC) boxes that are implanted on the 3D graphene scaffold. More impressively, the durable chemical confinement effect from the N/O functional groups during the overall lithiation/delithiation process and the fast electron transfer path provided by the delicate graphene (Gr) framework contribute to the suppressive micro-structure degradation and rapid interfacial charge transfer. Thereby, Gr@Se⊂HC delivers the ultra-long cycle life and the outstanding high-rate capability. This work proposes an original strategy for the synthesis of Se cathode and opens a new route for propelling the development of the long-life-span Li-Se batteries.

Phase-transforming metamaterial with magnetic interactions

Solid-solid phase transformations can affect energy transduction and change material properties (e.g., superelasticity in shape memory alloys and soft elasticity in liquid crystal elastomers). Traditionally, phase-transforming materials are based on atomic- or molecular-level thermodynamic and kinetic mechanisms. Here, we develop elasto-magnetic metamaterials that display phase transformation behaviors due to nonlinear interactions between internal elastic structures and embedded, macroscale magnetic domains. These phase transitions, similar to those in shape memory alloys and liquid crystal elastomers, have beneficial changes in strain state and mechanical properties that can drive actuations and manage overall energy transduction. The constitutive response of the elasto-magnetic metamaterial changes as the phase transitions occur, resulting in a nonmonotonic stress-strain relation that can be harnessed to enhance or mitigate energy storage and release under high-strain-rate events, such as impulsive recoil and impact. Using a Landau free energy-based predictive model, we develop a quantitative phase map that relates the geometry and magnetic interactions to the phase transformation. Our work demonstrates how controllable phase transitions in metamaterials offer performance capabilities in energy management and programmable material properties for high-rate applications.

Solid-Solid Phase Transformations and Their Kinetics in Ti-Al-Nb Alloys

The application of light-weight intermetallic materials to address the growing interest and necessity for reduction of CO2 emissions and environmental concerns has led to intensive research into TiAl-based alloy systems. However, the knowledge about phase relations and transformations is still very incomplete. Therefore, the results presented here from systematic thermal analyses of phase transformations in 12 ternary Ti-Al-Nb alloys and one binary Ti-Al measured with 4–5 different heating rates (0.8 to 10 °C/min) give insights in the kinetics of the second-order type reaction of ordered (βTi)o to disordered (βTi) as well as the three first-order type transformations from Ti3Al to (αTi), ωo (Ti4NbAl3) to (βTi)o, and O (Ti2NbAl) to (βTi)o. The sometimes-strong heating rate dependence of the transformation temperatures is found to vary systematically in dependence on the complexity of the transformations. The dependence on heating rate is nonlinear in all cases and can be well described by a model for solid-solid phase transformations reported in the literature, which allows the determination of the equilibrium transformation temperatures.

Effect of crystallite size on the low-temperature solid-solid phase transformations in the WO3 system

Ferroelectric nanostructured ε phase tungsten trioxide (ε-WO3) is emerging as an important functional material with unique gas sensing and optoelectronic properties. However, ε-WO3 phase is typically unstable in the bulk at room temperature (RT) and it has only been produced at ambient conditions by rapid solidification processes, such as Flame Spray Pyrolysis (FSP). This work aims to assess the overall stability of the ε phase at the nanoscale by studying its transformation from the stable γ phase to upon cooling. In-situ Raman spectroscopy has explored the effect of crystallite size on the solid-solid phase transformation from the symmetric γ (monoclinic) phase into the acentric ε (monoclinic) phase. The phase transformation onset temperature (Tonset) ranges from −43 °C for bulk state to −150 °C for crystallites of 68 nm in size and varies linearly with the inverse of the crystallite size radius (1r).

Engineering reversible isomerization at the nanoscale via intermolecular interactions

Engineering reversible isomerization at the nanoscale via intermolecular interactions

Interface migration in aluminum bicrystals via premelting

We investigate solid-solid phase transformation and migration of interface between ambient and strained crystal of Al using molecular dynamics simulation by means of premelting. In agreement with previous observations both Al(100) and Al(110) exhibit premelting near melting temperature and strain enhance the thickness of melt irrespective to sign. In bicrystal of Al, if one of crystal is subjected to strain then internal stresses completely relax during premelting and additionally provides thermodynamic driving force. This melt recrystallise into ambient crystal in pico seconds. The propagating interface velocity as function of temperature and strain both are quantitatively measured. Two types of interface propagation is observed, for thin interfacial melt (two times of lattice constant or less) interface migration is not continuous it is stickslip and stochastic, and for thick premelted layer interface migration is barrier less. The results obtained are applicable to many structural changes like grain evolution, twinning, fracture and dislocations, phase transformation such as melting-solidification and solid-solid.

Spherical pores as ‘microstructural informants’: Understanding compositional, thermal, and mechanical gyrations in additively manufactured Ti-6Al-4V

Detailed analysis of defects such as spherical porosity can act as informants, providing some information regarding the complex and often hidden physics associated with additive manufacturing. Variation in the presence and nature of these defects can shed new insights into the AM process. In this paper, the compositional, crystallographic, microstructural, and morphological characteristics surrounding gas pores in Electron Beam Melted Ti-6Al-4V have been assessed and correlated with different scanning strategies (raster and two point melting ones, Dehoff and random). The large spherical pores (>25μm), exclusively present in raster scan, exhibit perturbations normal to the vertical sidewalls of the pores that are likely the result of elastic instabilities resulting from chemical and crystallographic variations and initiated by vertical compression caused by thermal stresses related to the cyclic process – effectively a form of microbuckling. Electron backscatter diffraction maps support the theory that these perturbations occur at elevated temperatures and prior to the final solid-solid phase transformation.

In Situ Constructing the Kinetic Roadmap of Octahedral Nanocrystal Assembly Toward Controlled Superlattice Fabrication.

Crystallization and growth of anisotropic nanocrystals (NCs) into distinct superlattices were studied in real time, yielding kinetic details and designer parameters for scale-up fabrication of functional materials. Using octahedral PbS NC blocks, we discovered that NC assembly involves a primary lamellar ordering of NC-detached Pb(OA)2 molecules on the front-spreading solvent surfaces. Upon a spontaneous increase of NC concentration during solvent processing, PbS NCs preferentially self-assembled into an orientation-disordered face-centered cubic (fcc) superlattice, which subsequently transformed into a body-centered cubic (bcc) superlattice with single NC-orientational ordering across individual domains. Unlike the deformation-based transformation route claimed previously, this solid-solid phase transformation involved a hidden intermediate formation of a lamellar-confined liquid interface at cost of the disassembly (melting) of small fcc grains. Such highly condensed and liquidized NCs recrystallized into the stable bcc phase with an energy reduction of 1.16 kBT. This energy-favorable and high NC-fraction-driven bcc phase grew as a 2D film at a propagation rate of 0.74 μm/min, smaller than the 1.23 μm/min observed in the early nucleated fcc phase under a dilute NC environment. Taking such insights and defined parameters, we designed experiments to manipulate the NC assembly pathway and achieved scalable fabrication of a large/single bcc supercrystal with coherent ordering of NC translation and atomic plane orientation. This study not only provides a design avenue for controllable fabrication of a large supercrystal with desired superlattices for application but also sheds new light on the nature of crystal nucleation/growth and phase transformation by extending the lengths from the nanoscale into the atomic scale, molecular scale, and microscale levels.

Influence of Nanoscale Parameters on Solid–Solid Phase Transformation in Octogen Crystal: Multiple Solution and Temperature Effect

In this study, the influence of two nanoscale parameters (e.g., ratios of energy and width of two different interfaces) and temperature have been investigated which drastically affects the formation of interfacial melt during solid-solid phase transformations in energetic Octogen crystal. For different critical values of these parameters and depending on the energy barrier of the solid-melt interface, the appearance of propagating interfacial melt can be either continuous-reversible without the hysteresis or jump-like first-order discontinuous transformation with hysteresis. It is found that these nanoscale parameters significantly affect the phase transformation mechanism, induces different scale effects, and changes the nanoscale behavior of octogen crystal.

The formation of secondary phase in sub-rapid solidification process of Al-Mg-Si alloys

The formation of secondary phases (Mg2Si and (Si)) in Al-Mg-Si hypoeutectic alloys during the sub-rapid solidification was investigated via the CALPHAD-coupled pseudo-front tracking (PFT) model [Du Q, Jacot A. Acta Mater 2005; 53:3479–93] and V-shaped solidification experiment. The calculated and experimental results show that the amount of secondary phases initially increases and then decreases, as the cooling rate increases. We found from the calculated results that the local equilibrium at liquid-solid interface exhibits two different modes within the range of cooling rate of 0.2–300 K/s. It starts as the Scheil local equilibrium (Scheil-LE) and gradually transforms into a mode like the Non-partition Local Equilibrium (NPLE) encountered in solid-solid phase transformation. The NPLE-like mode at the solid/liquid interface can lead to the decrease of the volume fraction of secondary phases. At lower cooling rate, the solute diffusion in the liquid is more uniform, there is Scheil-LE mode without concentration peak at the front of solid/liquid interface, and the volume fraction of secondary phases increases with the increase of cooling rate. At higher cooling rate, the effect of NPLE-like is more significant and the volume fraction of secondary phases decreases with the increase of cooling rate. The results indicate that the finite diffusion of solute in the liquid plays a key role in solute distribution and formation of secondary phases during the solidification process.