Temperature sensitivity and transition kinetics of uniform corrosion of zirconium alloys in superheated steam.

Corrosion transition during uniform corrosion of zirconium alloys receives much attention since it is the major degradation procedure. However, predicting the time and oxide thickness at transition has been hindered by the lack of knowledge about transition kinetics and how it responds to varied temperatures. Current study investigated the temperature-sensitivity of corrosion kinetics, transition behavior and microstructures of various zirconium alloys corroded in superheated steam ranging from 390 °C/10.3 MPa to 455 °C/10.3 MPa by autoclave experiment and microscopy analyses. Transition time was found to follow Arrhenius-type relationship with temperature for the first time. Both the transition oxide thickness and metastable oxide thickness increased with temperature, which was theoretically deduced and experimentally confirmed. In Zr-4 oxides, a transition thickness varying from 3.3 µm at 390 °C to 4.2 µm at 455 °C was observed. Microstructure results presented rather large HCP-ZrO particles (200∼400 nm) at O/M interface and they were even larger at the protruded positions. An intense sub-stoichiometric atmosphere was identified at O/M interface, promoting the growth of metastable oxides. The activation energy of transition kinetics was 86∼114 kJ/mol, which is close to diffusion activation energy of oxygen in tetragonal zirconia. A new model based on parabolic-law empirical relationship was thus proposed to predict transition kinetics. Predictions regarding the time to oxidation breakaway at 900-1000 °C were reported, and the results were in good agreement with the experimental data.

Dynamic Structures and Fast Transition Kinetics of Oxidized G-Quadruplexes.

8-oxoguanines (8-oxoG) in cells form compromised G-quadruplexes (GQs), which may vary GQ mediated gene regulations. By mimicking molecularly crowded cellular environment using 40% DMSO or sucrose, here it is found that oxidized human telomeric GQs have stabilities close to the wild-type (WT) GQs. Surprisingly, while WT GQs show negative formation cooperativity between a Pt(II) binder and molecularly crowded environment, positive cooperativity is observed for oxidized GQ formation. Single-molecule mechanical unfolding reveals that 8-oxoG sequence formed more diverse and flexible structures with faster folding/unfolding transition kinetics, which facilitates the Pt(II) ligand to bind the best-fit structures with positive cooperativity. These findings offer new understanding on structures and properties of oxidized G-rich species in crowded environments. They also provide insights into the design of better ligands to target oxidized G-rich structures formed under oxidative cell stress.

A Phase Field Study of the Influence of External Loading on the Dynamics of Martensitic Phase Transformation.

An elastoplastic phase field model was employed for simulations to investigate the influence of external loading on the martensitic phase transformation kinetics in steel. The phase field model incorporates external loading and plastic deformation. During the simulation process, the authenticity of the phase field model is ensured by introducing the relevant physical parameters and comparing them with experimental data. During the calculations, loads of various magnitudes and loading conditions were considered. An analysis and discussion were conducted concerning the volume fraction and phase transition temperature during the phase transformation process. The simulation results prominently illustrate the preferential orientation of variants under different loading conditions. This model can be applied to the qualitative phase transition evolution of Fe-Ni alloys, and the crystallographic parameters adhere to the volume expansion effect. It is concluded that uniaxial loading promotes martensitic phase transformation, while triaxial compressive loading inhibits it. From a dynamic perspective, it is demonstrated that external uniaxial loading accelerates the kinetics of martensitic phase transformation, with uniaxial compression being more effective in accelerating the phase transformation process than uniaxial tension. When compared to experimental data, the simulation results provide evidence that under the influence of external loading, the martensitic phase transformation is significantly influenced by the applied load, with the impact of external loading being more significant than that of plastic effects.

Co-amorphous formulation of dipyridamole with p-hydroxybenzoic acid: Underlying molecular mechanisms, physical stability, dissolution behavior and pharmacokinetic study.

Coamorphization has been proven to be an effective approach to improve bioavailability of poorly soluble active pharmaceutical ingredients (APIs) by virtue of solubilization, and also contributes to overcome limitation of physical stability associated with amorphous drug alone. In current work, a co-amorphous formulation of dipyridamole (DPM), a poor solubility drug, with p-hydroxybenzoic acid (HBA) was prepared and investigated. At a molar ratio of 1:2, DPM and HBA were melted result in the formation of a binary co-amorphous system. The DPM-HBA co-amorphous was structurally characterized by powder X-ray diffraction (PXRD), temperature modulated differential scanning calorimetry (mDSC), high performance liquid chromatography (HPLC) and solution state 1H nuclear magnetic resonance (1H NMR). The molecular mechanisms in the co-amorphous were further analysed via Fourier-transform infrared (FTIR) and Raman spectroscopies, as well as density functional theory (DFT) calculation. All the results consistently revealed the presence of hydrogen bonding interactions between -OH of DPM and -COOH on HBA. Accelerated test and glass transition kinetics showed excellent physical stability of DPM-HBA co-amorphous compared with amorphous DPM along with glass transition temperatures (Tg). The phase-solubility study indicated that complexation occurred between DPM and HBA in solution, which contributed to the solubility and dissolution enhancement of DPM in co-amorphous system. Pharmacokinetic study of co-amorphous DPM-HBA in mouse plasma revealed that the DPM exhibited 1.78-fold and 2.64-fold improvement in AUC0­∞ value compared with crystalline and amorphous DPM, respectively. This current study revealed coamorphization is an effective approach for DPM to improve the solubility and biopharmaceutical performance.

Study on Kinetics of Transformation in Medium Carbon Steel Bainite at Different Isothermal Temperatures.

Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.

Phase Transition Kinetics in Austempered Ductile Iron (ADI) with Regard to Mo Content.

The phase transformation to ausferrite during austempered ductile iron (ADI) heat treatment can be significantly influenced by the alloying element Mo. Utilizing neutron diffraction, the phase transformation from austenite to ausferrite was monitored in-situ during the heat treatment. In addition to the phase volume fractions, the carbon enrichment of retained austenite was investigated. The results from neutron diffraction were compared to the macroscopic length change from dilatometer measurements. They show that the dilatometer data are only of limited use for the investigation of ausferrite formation. However, they allow deriving the time of maximum carbon accumulation in the retained austenite. In addition, the transformation of austenite during ausferritization was investigated using metallographic methods. Finally, the distribution of the alloying elements in the vicinity of the austenite/ferrite interface zone was shown by atom probe tomography (APT) measurements. C and Mn were enriched within the interface, while Si concentration was reduced. The Mo concentration in ferrite, interface and austentite stayed at the same level. The delay of austenite decay during Stage II reaction caused by Mo was studied in detail at 400 °C for the initial material as well as for 0.25 mass % and 0.50 mass % Mo additions.

What can we still learn from the electrochromic band-shifts in Photosystem II?

Electrochromic band-shifts have been investigated in Photosystem II (PSII) from Thermosynechoccocus elongatus. Firstly, by using Mn-depleted PsbA1-PSII and PsbA3-PSII in which the QX absorption of PheD1 differs, a band-shift in the QX region of PheD2 centered at ~ 544 nm has been identified upon the oxidation, at pH 8.6, of TyrD. In contrast, a band-shift due to the formation of either QA•- or TyrZ• is observed in PsbA3-PSII at ~ 546 nm, as expected with E130 H-bonded to PheD1 and at ~ 544 nm as expected with Q130 H-bonded to PheD1. Secondly, electrochromic band-shifts in the Chla Soret region have been measured in O2-evolving PSII in PsbA3-PSII, in the PsbA3/H198Q mutant in which the Soret band of PD1 is blue shifted and in the PsbA3/T179H mutant. Upon TyrZ•QA•- formation the Soret band of PD1 is red shifted and the Soret band of ChlD1 is blue shifted. In contrast, only PD1 undergoes a detectable S-state dependent electrochromism. Thirdly, the time resolved S-state dependent electrochromism attributed to PD1 is biphasic for all the S-state transitions except for S1 to S2, and shows that: i) the proton release in S0 to S1 occurs after the electron transfer and ii) the proton release and the electron transfer kinetics in S2 to S3, in T. elongatus, are significantly faster than often considered. The nature of S2TyrZ• is discussed in view of the models in the literature involving intermediate states in the S2 to S3 transition.

Microscopic identification of the order parameter governing liquid-liquid transition in a molecular liquid.

A liquid-liquid transition (LLT) in a single-component substance is an unconventional phase transition from one liquid to another. LLT has recently attracted considerable attention because of its fundamental importance in our understanding of the liquid state. To access the order parameter governing LLT from a microscopic viewpoint, here we follow the structural evolution during the LLT of an organic molecular liquid, triphenyl phosphite (TPP), by time-resolved small- and wide-angle X-ray scattering measurements. We find that locally favored clusters, whose characteristic size is a few nanometers, are spontaneously formed and their number density monotonically increases during LLT. This strongly suggests that the order parameter of LLT is the number density of locally favored structures and of nonconserved nature. We also show that the locally favored structures are distinct from the crystal structure and these two types of orderings compete with each other. Thus, our study not only experimentally identifies the structural order parameter governing LLT, but also may settle a long-standing debate on the nature of the transition in TPP, i.e., whether the transition is LLT or merely microcrystal formation.

The effect of excipients on the stability and phase transition rate of xylazine hydrochloride and zopiclone.

The compatibility of thermodynamically unstable polymorph of two active pharmaceutical compounds (xylazine hydrochloride form X and zopiclone form C) with different excipients was investigated. The effects of the excipient and its amount in the sample on the thermal properties and possible chemical interactions were studied. The most commonly used excipients in the pharmaceutical industry - calcium carbonate, lactose hydrate, cellulose, magnesium stearate hydrate and calcium stearate hydrate were selected for this study. The dependence of the phase transition rate from an unstable to a more stable polymorph on the excipients and their amounts in the initial sample was analysed at 80°C, and the corresponding phase transition rate constants were calculated.