Phase Transformation Mechanism Research Articles

Hierarchical Architecture Engineering of Branch-Leaf-Shaped Cobalt Phosphosulfide Quantum Dots: Enabling Multi-Dimensional Ion-Transport Channels for High-Efficiency Sodium Storage.

New-fashioned electrode hosts for sodium-ion batteries (SIBs) are elaborately engineered to involve multifunctional active components that can synergistically conquer the critical issues of severe volume deformation and sluggish reaction kinetics of electrodes toward immensely enhanced battery performance. Herein, it is first reported that single-phase CoPS, a new metal phosphosulfide for SIBs, in the form of quantum dots, is successfully introduced into a leaf-shaped conductive carbon nanosheet, which can be further in situ anchored on a 3D interconnected branch-like N-doped carbon nanofiber (N-CNF) to construct a hierarchical branch-leaf-shaped CoPS@C@N-CNF architecture. Both double carbon decorations and ultrafine crystal of the CoPS in-this exquisite architecture hold many significant superiorities, such as favorable train-relaxation, fast interfacial ion-migration, multi-directional migration pathways, and sufficiently exposed Na+ -storage sites. In consequence, the CoPS@C@N-CNF affords remarkable long-cycle durability over 10000 cycles at 20.0Ag-1 and superior rate capability. Meanwhile, the CoPS@C@N-CNF-based sodium-ion full cell renders the potential proof-of-feasibility for practical applications in consideration of its high durability over a long-term cyclic lifespan with remarkable reversible capacity. Moreover, the phase transformation mechanism of the CoPS@C@N-CNF and fundamental springhead of the enhanced performance are disclosed by in situ X-ray diffraction, ex situ high-resolution TEM, and theoretical calculations.

Inducing Crystal Phase Transformation of Titanium Oxide by Pulse Laser Irradiation

To selectively pattern titanium oxide with desired properties, we report inducing crystal phase transformation from rutile TiO2 to γ/λ-Ti3O5 and corundum Ti2O3 by pulse laser irradiation. The crystal phase of the produced titanium oxide could be systematically controlled by laser fluence. This crystal phase transformation simultaneously increased the electrical conductivity with changing the electrical conduction mechanism from band conduction to variable-range hopping conduction. This report also proposes the mechanism of the crystal phase transformation. Comparing thermodynamic calculations and finite element simulations of the temperature rise due to laser irradiation showed that this phase transformation can be interpreted as a reduction reaction caused by laser heating. Our findings indicate that laser irradiation can spatially pattern various crystal phases of titanium oxides on a substrate, and this technique will enable the regioselective, fine, and highly productive fabrication process of titanium oxide-based electronic devices.

In-Depth Investigation of the Mechanism of Dehydration-Induced Phase Transformation from Nb3O7(OH) to H-Nb2O5: A Theoretical and Experimental Approach.

H-Nb2O5 is a promising energy material, which can be typically obtained from any other polymorph after conducting high temperature calcination (∼1273 K). Recently, a low-temperature dehydration from Nb3O7(OH) was employed to prepare H-Nb2O5 at 723 K for 2 h, and yet the transformation mechanism has remained unclear in the literature. Here, the dehydration kinetic and phase transformation mechanism of the Nb3O7(OH) were investigated for the first time by experiments, density functional theory, and molecular dynamics calculations. After dehydration, the orthorhombic Nb3O7(OH) initially transformed into an intermediate Nb-O compound with dislocations, preserving parent structure, and subsequently transformed into monoclinic H-Nb2O5. The activation energy for the transformation from Nb3O7(OH) to H-Nb2O5 was as low as 1.35 eV, compared to that of T-Nb2O5 to H-Nb2O5 (3.60 eV). Furthermore, the defect-rich H-Nb2O5 obtained from Nb3O7(OH), does not exhibit pristine bound exciton state due to severe recombination of photogenerated carriers, resulting in poor photocatalytic activity.

In situ insight into temperature-dependent microstructure evolution of carbon doped phase change materials

Carbon-doped Ge2Sb2Te5 (CGST) is a potential candidate in phase change random access memory (PCRAM) with superb thermal stability and ultrahigh cycle endurance. Direct observation of the microstructure evolution of CGST is desirable to uncover the phase transformation mechanism on the relationship of nucleation/crystalline behaviors of the crystalline phase at elevated temperatures and the pristine amorphous phase at room temperature. Here, we investigate the structural evolution of CGST using combined in situ techniques. Our in situ x-ray diffraction and ellipsometry results demonstrate that CGST exhibits a much higher phase transition temperature than undoped one. Temperature-dependent in situ transmission electron microscopy observations further reveal that carbon doping plays a critical role in tailoring the properties of GST by tuning the stochasticity of nucleation/crystallization, stabilizing amorphous and crystalline GST via isolating and refining the grain size at room temperature and elevated temperature. Our work provides detailed information for understanding the microscopic origin of crystallization kinetics of carbon-doped phase change materials toward high-performance PCRAM.

Reliability improvement of SnAgCu interconnections under extreme temperature condition by TiO2 nanoparticles doping: Experiments and first principles calculations

Electronic components of the spacecraft during their service may endure intricate and extreme temperature environments. In this study, the interfacial microstructure evolution and reliability of Cu/Sn-3.0Ag-0.5Cu (SAC305)/Ni and Cu/Sn-3.0Ag-0.5Cu-0.05TiO2 (SAC305–0.05TiO2)/Ni interconnections under thermal shock from −196 °C to 150 °C are investigated. Short-column-type (Cu,Ni)6Sn5 intermetallic compounds (IMCs) are developed at the solder/Ni interface for both interconnections after the reflow process, and their morphology changes to long-rod type after 200 cycles. Extremely thin and discontinuous (Ni,Cu)3Sn4 phases are generated between (Cu,Ni)6Sn5 and Ni only in SAC305 interconnections after 200 cycles. The rate of interfacial IMCs growth in SAC305–0.05TiO2 interconnections is slower than that in the SAC305 interconnections. Cracks are formed in the IMC layer of the SAC305 interconnection after 200 cycles, which propagate through the whole IMC layer after 250 cycles. While for the SAC305–0.05TiO2 interconnection, no crack is observed until after 250 cycles. TiO2 nanoparticles doping could suppress the interfacial IMC growth, leading to an enhancement in the interconnection reliability under thermal shock. First-principles calculations are performed to further understand the mechanism for phase transformation of Ni-Cu-Sn IMCs and the reliability improvement of interconnections.

Constitutive modeling hot tensile behavior of Ti6Al4V alloy by considering phase transformation and damage mechanisms

The hot tensile and fracture behavior, as well as damage mechanisms of Ti6Al4V alloy are investigated by the hot tension tests and microstructure observation experiments. For comprehensively characterizing the hot tensile behaviors, a developed dislocation density-based constitutive model was established by coupling the flow softening, phase transformation and damage mechanisms. The results show that the flow stresses under the relatively high strain rates (0.01–1 s−1) easily peak at low strains and then decrease with the further straining. However, under the low strain rate (0.001 s−1), the flow stress tends to a continuously increasing trend until fractured. After hot deformation, the fraction of β phase greatly increases compared to that under the initial state. Meanwhile, β-phase fraction increases with the strain rate decreasing or the deformation temperature increasing. The great input thermal energy by the high temperature and the long deformation period under low strain rates promote the α-β phase transformation. Furthermore, the necking diffusion caused by dislocation motion enhances the homogeneous deformation under low strain rate or high temperature. The cavitations preferentially nucleate at the α/β phase interface due to the inconsistent strain. The comparison of the measured and calculated results confirms the excellent prediction ability of the established model in reproducing the hot tensile behavior, β-phase fraction and damage evolution under different processing parameters.

Microstructure evolution and phase transformation mechanism of Ti–46Al–8Nb–2.5V alloy based on phase transformation geometric models

The microstructure of conventionally solidified and rapidly solidified Ti–46Al–8Nb-2.5V alloys was analyzed with phase transformation geometric models, revealing the phase transformation mechanisms. The β→α→γ phase transformation processes follow Burgers and Blackburn orientation relationships (ORs), respectively, from which the calculated transformation between the β/β0 and γ phases follow K–S OR. Whlie, the direct β/β0→γ phase transformation can be directly observed in the microstructure, which follows a variety of ORs, mainly following the K–S OR but also following the N–W or Bain ORs, most of which deviate from the strict ORs. In cases where N–W and Bain ORs are followed, there is a large dislocation spacing between the habit planes of the β/β0 and γ phases, resulting in low static energy at the interface and narrow elongated morphology of precipitated phases. Conversely, when following K–S OR for β/β0 → γ phase transformation, interfacial migration energy is relatively lower and grain precipitated phases is larger. The rapidly solidified microstructure of Ti–46Al–8Nb-2.5V alloy consists of the massive α2 phase, massive γ phase, and basket-weave β0 phase closely resembling those found in β-type titanium alloys. After annealing at service temperature, the rapidly solidified alloy forms a microstructure highly similar to that of conventionally solidified TiAl alloy. Various metastable phases, such as the Ti2Al, L12 and Ti1.4Al phases, precipitate in the blocky γ phase, which provides a theoretical basis for studying the phase transformations of TiAl alloys in service.

Alloying strategies for additive manufacturing of Ti6Al4V based alloys, composites and functionally graded materials: Microstructure and phase evolution of intra and inter-layer

The further developments of additive manufacturing (AM) provides a possibility for Ti6Al4V and its functionally graded materials (FGMs), but limits further applications due to its low ductility, low microhardness and poor wear resistance. The key lies in the presence of microstructures such as epitaxial columnar β-grain orientation and heterogeneous phase distribution in AM Ti6Al4V, as well as the interfacial bonding problems (intermetallic phases, delamination, cracking) in AM Ti6Al4V FGMs. In order to address these fundamental intra and inter-layer problems, firstly the microstructure and phase evolution process of AM Ti6Al4V columnar-to-equiaxed transformation (CET) promoted by some alloying strategies was summarized in this paper according to alloying elements and reinforcing particle types. Subsequently, controlled changes in the composition and microstructure of AM Ti6Al4V FGMs were achieved by adding interlayers/introducing gradient interfaces, and the metallurgical reactions and phase transformation mechanisms that inhibit the formation of intermetallic compounds and interfacial defects were explored. Finally, the difficulties and future research directions for alloying strategies to achieve high strength and ductility of AM Ti6Al4V were presented. This review aims to improve the reliability of AM Ti6Al4V and its FGMs for applications in critical load-bearing structures through in-situ alloying and multi-material for the microstructure design of AM Ti6Al4V.

Phase transformation and incidental effects of metastable crystalline TiAlN on the material removal mechanism

Spinodal decomposition of Ti1−xAlxN crystal structure significantly impacts their physical properties, particularly at the atomic scale. In this study, we employed systematic three-dimensional molecular dynamics simulations to understand the process of material removal in TiAlN. Orthogonal simulations were conducted with cutting depth and Al content as variables to analyze their impact on the phase transformation mechanism and the resulting surface finish. The simulations reveal that the aluminum content and depth of cut significantly influence the phase transformation process of supersaturated TiAlN crystals through spinodal decomposition. It is observed that the aluminum content has a maximum stable solubility of approximately 0.6 in TiAlN crystals, above which the system undergoes significant spinodal decomposition during cutting to produce hexagonal phases with lower hardness. Increasing the aluminum content leads to wider phase change and deeper sub-surface damage in the system. On the other hand, the cutting depth will promote phase transformation by affecting the stress distribution, thereby increasing the root mean square roughness of the height profile.

Regularities of Changes in the Structure of Different Phases of Deformed Zirconium Alloys as a Result of Raising the Annealing Temperature According to Texture Analysis Data

Based on the data of synchrotron and electron microscopic studies of deformed and annealed Russian zirconium alloys, the possibility of analyzing the structural-phase state and crystallographic texture of individual phases has been demonstrated. A qualitative and quantitative phase analysis of deformed and annealed tubes made of Zr-Nb-(Sn-Fe-O) alloys was carried out using diffraction patterns obtained with synchrotron radiation. The main α-Zr phase and the following additional phases: β-Nb, β-Zr, and the Laves phase (intermetallic compound Zr(Nb,Fe)2), were found in the alloys. According to the results of texture analysis of all phases present in the alloy, the mechanisms of plastic deformation, recrystallization, and phase transformations of the main and additional phases were established. It is shown that during plastic deformation of the Zr-1%Nb alloy, a dynamic phase transformation β-Nb→α-Zr→β-Zr is observed. It is established that during recrystallization, larger grains of α-Zr are misoriented relative to the deformed matrix by rotating the prismatic axes around the basal axes by 30°, while fine grains are improved by polygonization and maintain the orientation of the deformed matrix. Processes for changing the orientation of grains of additional phases as a result of high-temperature annealing are also considered.

Effect of organic acids on the solid-state polymorphic phase transformation of piracetam

Metastable polymorphs are frequently used in oral solid dosage forms to enhance the absorption of poorly water-soluble drug compounds. However, the solid phase transformation from the metastable polymorph to the thermodynamically stable polymorph during manufacturing or storage poses a major challenge for product development and quality control. Here, we report that low-content organic acids can exhibit distinct effects on the solid-state polymorphic phase transformation of piracetam (PCM), a nootropic drug used for memory enhancement. The addition of 1 mol% citric acid (CA) and tricarballylic acid (TA) can significantly inhibit the phase transformation of PCM Form I to Form II, while glutaric acid (GA) and adipic acid (AA) produce a minor effect. A molecular simulation shows that organic acid molecules can adsorb on the crystal surface of PCM Form I, thus slowing the movement of molecules from the metastable form to the stable form. Our study provides deeper insights into the mechanisms of solid-state polymorphic phase transformation of drugs in the presence of additives and facilitates opportunities for controlling the stability of metastable pharmaceuticals.

Elastic architected mechanical metamaterials with negative stiffness effect for high energy dissipation and low frequency vibration suppression

In this research, mechanical metamaterials with negative stiffness (NS) effect were architected and fabricated for high energy dissipation and low frequency vibration suppression. Periodic cellular metamaterials formed by connecting multiple NS elements in series exhibited hysteresis and lengthy sawtooth loading and unloading plateaus, which made them desirable for use in energy dissipation applications. And the key point was that the deformation of these metamaterials was completely reversible. To determine the underlying cause of hysteresis, the mechanical behavior with phase-transition characteristics was analyzed. Then, a finite element model was built, and the numerical simulation results were compared to the experimental test findings of the metamaterials, and the two were in good agreement, verifying the validity of the model. Furthermore, the influences of structural parameters on mechanical characteristics were systematically investigated and discussed utilizing the numerical models that had undergone experimental validation. The energy dissipation effect resulting from the phase transformation mechanisms could also be applied to the field of vibration control. Corresponding hammer tests were performed to explore the dynamic behavior of the NS metamaterials. The results show that the proposed metamaterials exhibit excellent vibration suppression performance, especially for low frequency vibration mitigation.

The gradient high entropy alloy coating prepared in-situ by bidirectional diffusion of Mo/Nb and FeCoCrNi

Based on the influence of Mo and Nb content on the eutectic degree of FeCoCrNi high-entropy alloy (HEA), this study prepared in-situ HEA composite coatings with a gradient eutectic degree using the principle of element diffusion in the molten pool under a laser beam, namely Mo-CC and Nb-CC. Both of the two coatings were composed of the face-centered cubic (FCC) phase and Laves phase. Within the coating, there was a gradual decrease in the Laves phase and an increase in the FCC phase from top to bottom. The Laves phase in Mo-CC was rich in Mo (21.36 at.%) element and exhibited an orientation relationship with the FCC phase of [01¯1]Laves//[01¯1]FCC. The phase interface was a non-coherent crystal plane. The Laves phase in Nb-CC is enriched with Nb (36.15 at.%) element and demonstrated an orientation relationship with the FCC phase of [1¯2¯1]Laves//[001]FCC. The phase interface was a semi-coherent crystal plane. Furthermore, the process and mechanism of FCC phase transformation into FCC + Laves phase were elucidated by examining the mixing enthalpy, lattice distortion, and atomic radius difference. In terms of properties, the hardness of Mo-CC and Nb-CC from the top to the bottom section gradually decreased from the outer to inner section. Nb-CC possessed a higher surface average hardness (773.66 HV0.3) and superior tribological properties compared to Mo-CC. The gradient control method for HEA and the phase structure formation mechanism presented in this study provided valuable insights for the research and development of new HEA coatings.

Ideal plasticity and shape memory of nanolamellar high-entropy alloys.

Understanding the relationship among elemental compositions, nanolamellar microstructures, and mechanical properties enables the rational design of high-entropy alloys (HEAs). Here, we construct nanolamellar AlxCoCuFeNi HEAs with alternating high- and low-Al concentration layers and explore their mechanical properties using a combination of molecular dynamic simulation and density functional theory calculation. Our results show that the HEAs with nanolamellar structures exhibit ideal plastic behavior during uniaxial tensile loading, a feature not observed in homogeneous HEAs. This remarkable ideal plasticity is attributed to the unique deformation mechanisms of phase transformation coupled with dislocation nucleation and propagation in the high-Al concentration layers and the confinement and slip-blocking effect of the low-Al concentration layers. Unexpectedly, this ideal plasticity is fully reversible upon unloading, leading to a remarkable shape memory effect. Our work highlights the importance of nanolamellar structures in controlling the mechanical and functional properties of HEAs and presents a fascinating route for the design of HEAs for both functional and structural applications.

Phase transformation kinetics of high-carbon steel during continuous heating

Effects of heating temperature, heating rate, and holding time on the austenitization process of near-eutectoid high-carbon steel have been investigated. The dilatometric tests were carried out at different heating rates of 1–100 °C/Sec. The Ac1 and Ac3 critical temperatures, the volume fractions of the parent and produced austenite phases, and consequently, the coefficient of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation were obtained from dilatometric diagrams. The activation energy for the transformation was estimated using the Kissinger equation. The optical microscope was used for the microstructural characterization of samples austenitized at different temperatures and durations. The grain growth kinetics of austenite during processing was also characterized. Cellular Automata simulation was used to investigate the nucleation and growth of austenite from the initial full pearlitic structure. Based on the results, the activation energy of 242.6 kJ/mol at low heating rates (1 °C/Sec to 12 °C/Sec) and 436.94 kJ/mol at high heating rates (25 °C/Sec to 100 °C/Sec) were obtained, which indicated the different phase transformation mechanisms. This was attributed to the presence of boron, which postpones nucleation at grain boundaries and helps to activate the shear mechanism at high heating rates. Finally, a continuous heating transformation (CHT) diagram was drawn, which is an indispensable tool to consider the effect of heating rate on the austenitization process and austenite grain size.

Chemical reaction and phase transformation mechanism of electrospun iron (III) acetylacetonate-polyacrylonitrile fibers during pre-oxidation process

Low-temperature pre-oxidation of electrospun metal-compound-incorporated carbon fibers can involve phase transitions and chemical reactions. However, their mechanics remain unclear. In this work, a mixture of polyacrylonitrile (PAN) and iron acetylacetonate is subjected to electrospinning followed by a pre-oxidation process at 230 °C, producing PAN fibers loaded with iron compounds. Our detailed investigations shed light on the mechanisms of phase transitions and associated chemical reactions during the pre-oxidation process. Our findings reveal that PAN undergoes cyclization during pre-oxidation, and that crystalline iron acetylacetonate within PAN fibers transforms into amorphous FeO and Fe2O3.

CO2-Triggered Hierarchical-Pore UiO-66-Based Pickering Emulsions for Efficient and Recyclable Suzuki-Miyaura Cross-Coupling in Biphasic Systems.

Hierarchical-pore metal-organic frameworks (H-MOFs) are considered to be emerging stabilizers for Pickering emulsion formation because of their hierarchically arranged pores, tailorable structures, and ultrahigh surface areas. However, stimulus-triggered Pickering emulsions built by H-MOFs have been seldom presented to date despite their great significance in diverse applications. Herein, by grafting Pd(OAc)2 on the hierarchical-pore zirconium MOF UiO-66, namely, H-UiO-66, with the aid of 1-alkyl-3-methylimidazolium 2-cyanopyrrolide salts ([CnMIM][2-CN-Pyr], n = 4, 6, and 8), a series of Pd(OAc)2-[CnMIM][2-CN-Pyr]@H-UiO-66 have been developed and utilized as emulsifiers for constructing CO2-switching Pickering emulsions. It was found that Pd(OAc)2-[CnMIM][2-CN-Pyr]@H-UiO-66 was able to stabilize the n-hexane-water mixture to form a Pickering emulsion even at an amount of 0.5 wt %. Upon alternate addition of CO2 and N2 at normal pressure, Pickering emulsions could be smartly converted between demulsification and re-emulsification. Through combining varieties of spectroscopic techniques, the mechanism of the switchable phase transformation lay in the acid-base reaction of ionic liquids with CO2 on H-UiO-66 and the creation of more hydrophilic salts, which reduced the wettability of the emulsifier and destabilized the emulsion. As an example of application, the stimulus-triggered Pickering emulsion was employed as a palladium-catalyzed Suzuki-Miyaura cross-coupling microreactor to achieve the combination of chemical reactions, isolation of products, and recovery of catalysts.

An in-situ self-etching enabled high-power electrode for aqueous zinc-ion batteries

Sluggish storage kinetics is considered as the main bottleneck of cathode materials for fast-charging aqueous zinc-ion batteries (AZIBs). In this report, we propose a novel in-situ self-etching strategy to unlock the Palm tree-like vanadium oxide/carbon nanofiber membrane (P-VO/C) as a robust free-standing electrode. Comprehensive investigations including the finite element simulation, in-situ X-ray diffraction, and in-situ electrochemical impedance spectroscopy disclosed it an electrochemically induced phase transformation mechanism from VO to layered ZnxV2O5⋅nH2O, as well as superior storage kinetics with ultrahigh pseudocapacitive contribution. As demonstrated, such electrode can remain a specific capacity of 285 mA h g−1 after 100 cycles at 1 A g−1, 144.4 mA h g−1 after 1500 cycles at 30 A g−1, and even 97 mA h g−1 after 3000 cycles at 60 A g−1, respectively. Unexpectedly, an impressive power density of 78.9 kW kg−1 at the super-high current density of 100 A g−1 also can be achieved. Such design concept of in-situ self-etching free-standing electrode can provide a brand-new insight into extending the pseudocapacitive storage limit, so as to promote the development of high-power energy storage devices including but not limited to AZIBs.

Reversible Amorphous-Crystalline Phase Transformation in an Ultrathin van der Waals FeTe System.

Searching for new phase-change materials for memory and neuromorphic device applications and further understanding the phase transformation mechanism are attracting wide attention. Phase transformation from the amorphous phase to the crystal phase has been unraveled in iron telluride (FeTe) bulk film deposited by pulsed laser deposition, recently. However, the van der Waals-layered feature of FeTe in the crystal form was not noted, which will benefit the scaling of the memory devices and shine light on phase-change heterostructures or interfacial phase-change materials. Moreover, the demonstration of advanced memory or neuromorphic device applications is lacking. Here, we investigate the phase transformation of FeTe starting from mechanically exfoliated van der Waals layers from a bulk single crystal. Surficial amorphization is revealed at the surface layers of FeTe flakes after exfoliation under ambient conditions, which could be transformed back to the crystalline phase with laser irradiation or heating. The conductance drop of the flake devices near 400 K verifies the phase transformation electrically. Memristor behavior of the amorphous surface in FeTe has been further demonstrated, proving the reversibility of the phase transformation and shining light on the possible applications of neuromorphic devices.

Effect of scaled ionic charges on the freezing point depression of aqueous CaCl2 solution

We examine the performance of two force fields, OPLS-AA (Optimized Potentials for Liquid Simulations - All Atom) using full ionic charges, and Madrid-2019 using scaled ionic charges, in predicting the freezing point depression of aqueous CaCl2 solutions across concentrations ranging from 5 to 30 wt%. The direct coexistence method is employed to characterize the phase transformation mechanism. Our predictions corroborate that the Madrid-2019 force field exhibits predictive consistency with experimental measurements relative to OPLS-AA. The dissimilarities in the equilibrium properties, such as diffusion coefficients, hydration structures, and densities, underscore the advantages in the ion-water and ion-ion interaction mechanisms described by Madrid-2019 force field.