Cyclic Phase Transitions Research Articles

Superplasticity induced by cyclic phase transitions in nanosystems: An atomic study

Three samples, namely, iron (Fe) bulk, nanowire (NW) and nanotube (NT) with the initial body centered cubic (bcc) structure were designed and constructed. Using molecular dynamics simulation, tensile experiments were performed on the three samples. The results show that the NT exhibits the highest plasticity followed by the NW and bulk. Cyclical phase transitions were observed in the NW and NT, releasing the accumulated stress and allowing the samples to achieve the superplasticity. Focusing on nucleation and growth, the phase transitions were analyzed in detail and it could be concluded that the higher surface-volume ratio and the surface roughness induced by grain reorientation are the main reasons for the highest plasticity in the NT. In addition, the dislocation behaviors in the three samples were investigated. The results of this work might be helpful to understand the mechanism of phase transition induced superplasticity and provide some new ideas for the material design with high plasticity requirement.

Back Cover

The cover picture simulates a chemical system that can be driven by chemical fuels and pre‐programmed to complete the phase transitions. The lying and standing toy bears represent the sol and gel states of the system, respectively. Sodium hydroxide and 1,3‐propanesulfonate together form the “chemical hand” that drives a pH clock reaction for controling the transient phase behaviors of a polymer system containing polyvinyl alcohol and boric acid to complete the successive sol‐to‐gel‐to‐sol transition. Moreover, the system can achieve the cyclic phase transitions by refueling. More details are discussed in the article by Wang et al. on page 2794–2800.image

Transient Polymer Hydrogels Based on Dynamic Covalent Borate Ester Bonds

Comprehensive SummaryIn nature, biological systems maintain their unique structures and functions by using nonequilibrium processes driven by chemical fuels. Inspired by natural systems, transient hydrogel systems based on chemical reaction networks have been developed that are in thermodynamically nonequilibrium states. The formation of dynamic covalent bonds is an effective tool for designing analogous systems. Herein, we design a transient polymer hydrogel based on fuel‐mediated covalent borate ester bonds. The pH‐sensitive covalent borate ester bond is formed by the reaction between polyvinyl alcohol (PVA) and boric acid (B(OH)3). Sodium hydroxide (NaOH) and 1,3‐propanesulfonate (PrS) are used together as the chemical fuels to temporally control the pH of the system. Meanwhile, the lifetime of the transient hydrogel can be simply controlled by adjusting the composition of the chemical fuel, and the cyclic phase transitions can also be achieved. These programmable transient hydrogels have potential applications in the fields of information transmission and fluid guidance.

Diurnal Transcriptome and Gene Network Represented through Sparse Modeling in Brachypodium distachyon.

We report the comprehensive identification of periodic genes and their network inference, based on a gene co-expression analysis and an Auto-Regressive eXogenous (ARX) model with a group smoothly clipped absolute deviation (SCAD) method using a time-series transcriptome dataset in a model grass, Brachypodium distachyon. To reveal the diurnal changes in the transcriptome in B. distachyon, we performed RNA-seq analysis of its leaves sampled through a diurnal cycle of over 48 h at 4 h intervals using three biological replications, and identified 3,621 periodic genes through our wavelet analysis. The expression data are feasible to infer network sparsity based on ARX models. We found that genes involved in biological processes such as transcriptional regulation, protein degradation, and post-transcriptional modification and photosynthesis are significantly enriched in the periodic genes, suggesting that these processes might be regulated by circadian rhythm in B. distachyon. On the basis of the time-series expression patterns of the periodic genes, we constructed a chronological gene co-expression network and identified putative transcription factors encoding genes that might be involved in the time-specific regulatory transcriptional network. Moreover, we inferred a transcriptional network composed of the periodic genes in B. distachyon, aiming to identify genes associated with other genes through variable selection by grouping time points for each gene. Based on the ARX model with the group SCAD regularization using our time-series expression datasets of the periodic genes, we constructed gene networks and found that the networks represent typical scale-free structure. Our findings demonstrate that the diurnal changes in the transcriptome in B. distachyon leaves have a sparse network structure, demonstrating the spatiotemporal gene regulatory network over the cyclic phase transitions in B. distachyon diurnal growth.

New insights of solid-state alloying and amorphous-nanocrystalline cyclic phase transitions during Cr-40wt.%Mo powder milling

In this paper, the solid-state alloying of Cr-40wt.%Mo powder by high-energy milling and corresponding amorphous-nanocrystalline cyclic phase transitions were investigated. XRD, TEM and DSC analyses were utilized for phase, microstructure and thermodynamics stability characterization of the milled powders, respectively. The results demonstrate high-energy milling can induce severe lattice distortion and further amorphization of milled powder. Free volume/flow unit widely exists within the amorphous structure, greatly increasing the storage energy of milled powder and subsequently decreasing the thermodynamic barrier of atomic migration. For solid-state alloying of Cr-40wt.%Mo powder, there are two paths of atomic diffusion: diffusion reaction induced by crystal defects and diffusion reaction induced by free volume within the amorphous structure. Coexistence of Cr(Mo)0, Cr(Mo) and Mo(Cr)0 multi-phases is also found in this experiments. Furthermore, the cyclic phase transitions between amorphous alloy phase and nanocrystalline alloy phase are presented and discussed in details. It is concluded that the cyclic phase transitions mainly depend on the evolution of microstructure and storage energy, which are originally influenced by the reciprocating shear deformations during high energy milling.

Superior surface hardening by cyclic-phase-change-diffusion (CPCD): Supersaturation of hard boride in titanium subsurface layer

This research demonstrates a novel concept of surface hardening, termed “cyclic-phase-change-diffusion (CPCD),” where cyclic phase transitions were induced at the surface by thermal cycling across the phase transition temperature along with diffusion of surface hardening atoms. Specifically, boron was diffused through the surface of titanium during thermal cycling across the α-β phase transition temperature. The CPCD method is shown to result in an unusually deeper (~125μm) and richer boride layer, on titanium, compared that obtained by isothermal diffusion either at the equivalent temperature or at a higher temperature. This concept is quite promising for superior surface hardening of titanium and its alloys.

Electrochemical Lithiation Cycles of Gold Anodes Observed by In Situ High-Energy X-ray Diffraction

Significant developments of Li-ion batteries will be necessary to cope with the growing demands in electromobility or home storage of (sustainable) electrical energy. A detailed knowledge on the microscopic processes during battery cycling will be increasingly crucial for improvements. Involved phase changes at ambient temperature often involve metastable intermediate states, making both experimental observation and theoretical prediction of process pathways difficult. Here we describe an in situ high energy X-ray diffraction study following the initial alloying and dealloying of Li with an Au thin-film model anode using ionic liquid electrolyte. Six different crystalline alloy phases were observed to be involved in the cyclic phase transitions. Apart from the highest lithiated phase determined in this study, Li3Au, none of the observed phases could be related to known, thermodynamically stable Li–Au phases. Structural search calculations following the minima hopping method (MHM) allowed the assignment of...

Flexible Infrared Responsive Multi-Walled Carbon Nanotube/Form-Stable Phase Change Material Nanocomposites.

Flexible infrared (IR)-responsive materials, such as polymer nanocomposites, that exhibit high levels of IR responses and short response times are highly desirable for various IR sensing applications. However, the IR-induced photoresponses of carbon nanotube (CNT)/polymer nanocomposites are typically limited to 25%. Herein, we report on a family of unique nanocomposite films consisting of multi-walled carbon nanotubes (MWCNTs) uniformly distributed in a form-stable phase change material (PCM) that exhibited rapid, dramatic, reversible, and cyclic IR-regulated responses in air. The 3 wt % MWCNT/PCM nanocomposite films demonstrated cyclic, IR-regulated on/off electrical conductivity ratios of 11.6 ± 0.6 and 570.0 ± 70.5 times at IR powers of 7.3 and 23.6 mW/mm(2), respectively. The excellent performances exhibited by the MWCNT/PCM nanocomposite films were largely attributed to the IR-regulated cyclic and reversible form-stable phase transitions occurring in the PCM matrix due to MWCNT's excellent photoabsorption and thermal conversion capabilities, which subsequently affected the thickness of the interfacial PCM between adjacent conductive MWCNTs and thus the electron tunneling efficiency between the MWCNTs. Our findings suggest that these unique MWCNT/PCM nanocomposites offer promising new options for high-performance and flexible optoelectronic devices, including thermal imaging, IR sensing, and optical communication.

Deformation-induced cyclic phase transitions of dissolution-precipitation of nitrides in surface layers of Fe-Cr-(Ni)-N alloys

Methods of Mossbauer spectroscopy, transmission electron microscopy, and X-ray diffraction have been used to study structural and phase transformations in surface layers of iron and Fe-Cr-(Ni) alloys subjected to ion-plasma nitriding and subsequent severe cold plastic deformation by shear under pressure in Bridgman anvils. It has been shown that the cyclic phase transformations of dissolution-precipitation of nitrides in the alloys result in the formation of nitrogen-supersaturated solid solutions, precipitation of secondary nitrides, and the nanostructurization of the metallic matrix. It has been established that the introduction of chromium into iron alloys accelerates the formation of nitrides upon nitriding and makes it possible to obtain solid solutions strongly supersaturated with nitrogen (more than 10 at % N in the fcc lattice) during subsequent deformation.

Atomistic mechanism of cyclic phase transitions in Nd–Fe–B based intermetallics

Mechanical milling induced atomic disorder and the mechanism of the cyclic crystal → amorphous → crystal phase transitions in a Nd 2Fe 14B intermetallic – based Nd 10Fe 85B 5 alloy were investigated. Least square first shell fitting of Extended X-ray Absorption Fine-Structure (EXAFS) data of mechanically milled samples revealed that phase evolution during mechanical milling of Nd 10Fe 85B 5 alloy occurred in two stages as a result of two dynamical effects: milling induced forced atomic mixing and enhanced diffusion due to continuous production of point defects. In the first stage of milling, atomic mixing of Nd and Fe atoms dominated, resulting in extensive chemical and structural disorder in the initially ordered intermetallic, followed by amorphization. The second stage of milling was characterized by a remarkable increase in point defect density. Enhanced diffusion resulted in redistribution of alloy atoms and annihilation of these point defects leading to the precipitation of α-Fe nanocrystals in the amorphous matrix. Thus the time evolution of defect structures formed during mechanical milling and their atomic scale interactions led to the cyclic phase transitions in the Nd 10Fe 85B 5 alloy. These milling induced changes in atomic order were found to strongly influence magnetic properties. Crystallization of mechanically milled alloy and effect of annealing parameters on magnetic properties was also analyzed.

Influence of cyclic phase transitions on some properties of the ferroelectric perovskites

Abstract The results of investigations of the influence of cyclic phase transitions on certain properties of the ferroelectric perovskites are presented. The materials were PZT-type ceramics obtained either by hot-pressing or by classical ceramic technology. It has been found that a close relationship exists between the concentration of linear defects and the stability of the resonant frequency as a function of temperature and time. The investigated samples can be divided into two groups: samples with high initial stability of resonance frequency and samples with low initial temperature stability. Our investigations show that in this two cases it is possible to distinguish three ranges of numbers of thermocycles, but the dependencies in the two cases differ one from the other. The change of concentration of defects and their distribution inside the crystallites lead to changes of temperature stability of resonant frequency. After 5–6 thermocycles the stability increases, but if the initial stability of the resonant frequency is high the changes are too small for practical applications.

Temperature- and cyclic nucleotide-induced phase transitions of Histoplasma capsulatum.

The transition from yeast to mycelia of Histoplasma capsulatum could be accomplished by shifting the temperature of incubation from 37 to 25 degrees C. It was accompanied by many changes in cellular metabolism, including changes in respiration, intracellular cyclic adenosine 3',5'-monophosphate (cAMP) levels, and activities of two enzymes specific for the yeast phase, cystine reductase (EC 1.6.4.1) and cysteine oxidase (EC 1.13.11.20). Even at 37 degrees C, the yeast to mycelial transition could be induced by cAMP and agents which raise the intracellular levels of cAMP (theophylline, acetylsalicylic acid, prostaglandin E1, and nerve growth factor). During this morphogenesis the same pattern of changes occurred as in the temperature-induced transition. Therefore, these changes were not simply dependent on a shift in temperature, but rather were part of the process of the phase transition.

Solvation of oxygen in lecithin bilayers

The solubility of oxygen in dipalmitoyllecithin (DPL) and paraffin C 19 has been investigated by measurement of the enhanced proton relaxation rates under the influence of oxygen pressure. The paraffin shows a noticeable effect in the rotator phase, but not so in the crystalline phase. In contrast to paraffins, both phases of DPL-bilayers dissolve oxygen, but the solubility in the liquid-crystalline phase is greater than in the crystalline state by a factor ≈13. Furthermore, the experiments indicate a distribution of electron relaxation times in the crystalline phase in contrast to the liquid-crystalline phase. A possible explanation of this behaviour is a multiphase structure of the “crystalline” lamellae. The biological relevance of these results could be a triggering of the gas-transport by the alveolar lining of lungs, if cyclic phase transitions occur during the breathing-cycle.