Phase Redistribution Research Articles

Modeling of phase transition in fabrication of polymer-derived ceramics (PDCs)

Polymer-derived ceramics’ (PDCs) route, which converts preceramic polymers to ceramics through heat treatment, presents a flexible and energy-efficient approach to fabricate ceramic composites with arbitrary geometries and tailorable properties. Due to the huge gaps of thermal and mechanical properties of polymers and ceramics, the current state phase composition and phase distribution largely affect the heat transfer behavior and temperature field evolution that ultimately determine the subsequent polymer decomposition and phase redistribution. In this paper, a computational framework is developed to predict the continuum-level phase transition and its effect on mechanical properties. Molecular dynamics (MD) simulations are carried out first to track the atomic structure evolution and mass loss associated with gas generation. It is found that pyrolysis temperature primarily determines the amount of gases that can be generated. Gas diffusion is triggered as a result of the non-uniform temperature field. Ceramic phase formation depends on the interplay between gas generation and gas diffusion. This computational framework allows real-time temperature field and phase composition map to be explicitly extracted. The phase composition map is incorporated into a finite element model for compression simulation. The effects of heating rate, pyrolysis temperature and pyrolysis time on mechanical response are systematically studied. Conclusions from this study can provide direct guidance for fabricating PDCs with tailored properties.

Dynamics of Lithium Stripping on Graphite Electrodes

As fast charging of lithium-ion batteries becomes a critical barrier to the widespread adoption of electric vehicles, development of facile methods of detection and quantification of lithium plating is a top priority. The voltage plateau observed during the rest or discharge phase ensuing a charge in which plating has occurred is a popular in-operando signature of lithium plating. Recent works have shown that the length of the stripping plateau is not a reliable metric for plating quantification, as was previously thought. An improved analytical method for relative quantification of lithium plating is presented herein. Furthermore, an in-situ method is developed for direct visualization of lithium stripping and graphite phase redistribution during the open circuit rest phase after charge. The dependence of lithium stripping dynamics on temperature and charge rate are comprehensively investigated. These techniques help in understanding the mechanistic underpinnings of the stripping processes occurring in the graphite electrode during relaxation after fast charging and enhance the capability to study lithium plating in a non-destructive manner.

Spectral Instability of Layered Mixed Halide Perovskites Results from Anion Phase Redistribution and Selective Hole Injection.

Despite the ability to precisely tune their bandgap energies, mixed halide perovskites (MHPs) suffer from significant spectral instability, which obstructs their utilization for the rational design of light-emitting diodes. Here, we investigate the origin of the electroluminescence peak shifts in layered MHPs containing bromide and iodide. X-ray diffraction and steady-state absorption measurements prove effective integration of iodide into the cubic lattice and the spatially uniform distribution of halides in the ambient environment. However, the applied electric field during the device operation is found to drive the systematic halide migration. Quantum mechanical density functional theory calculations reveal that the different activation energies required for directional ion hopping lead to the redistribution of anions. In-depth analyses of the electroluminescence spectra indicate that the spectral shifting rate is dependent on the drift velocity of halides. Finally, it is suggested from our study that the dominant red emission is ascribed to the thermodynamically favorable selective hole injection. Our mechanistic study provides insights into the fundamental reason for the spectral instability of devices based on MHPs.

A new physical modeling for two-phase wellbore storage due to phase redistribution

Pressure transient analysis is one of the main dynamic sources for reservoir characterization. The main task in well test interpretation is to understand the reservoir properties using the bottom-hole pressure behavior. Unfortunately, one of the main factors that can affect this task is wellbore storage effects, especially in cases that we have a non-ideal one. In pressure build-up test if pressure is below the bubble point or if gas enters into the wellbore, the phase redistribution can happen during the shut-in period. This phenomenon causes pressure anomalies. Besides, due to pressure increase or existence of under-saturated liquid in the wellbore, the gas starts to dissolve, and it is led to pressure decrease. So the pressure build-up test is dominated by three main factors: firstly, the pressure increase resulted from build-up test. Secondly, the pressure increase due to phase redistribution. Finally, the pressure decrease due to gas dissolution. These effects are known as non-ideal wellbore storage. Most of the studies focused on non-ideal wellbore storage try to propose an empirical equation to simulate the pressure curve more accurately instead of proposing an analytical model, because of the large number of effective parameters in an actual case.This paper is proposing a new analytical model for changing wellbore storage and analysis of gas phase segregation during the pressure build-up test in the existence of the under-saturated liquid. This model is further modified for two-phase flow when pressure falls below the bubble point in the wellhead. When the well is shut in at the start of build-up test, gas bubbles travel upward because of the buoyancy force. This model is presented for the case of dissolving gas pockets while rising in the wellbore. The novelty of this paper in comparison to previous researches is in developing a new physical model for two-phase wellbore storage done by modeling pressure increase due to gas bubbles rise while modeling gas dissolution against time.In this study, first, the basic assumptions for simplifying the model are described and the basic theory behind the proposed model is explained. Then a simple model without gas dissolution, using Laplace transformation, is introduced. Next, the modified model considering gas dissolution effect is proposed. A synthetic case is considered and the corresponding well test type curves are generated using the proposed method. Finally, the comparison between the proposed model and a real build-up case is made. The result is consistent with the typical non-ideal wellbore storage that is seen in the literature.

A New Capacitance Sensor for Measuring the Void Fraction of Two-Phase Flow Through Tube Bundles.

Evaluating the two-phase flow parameters across tube bundles is crucial to the analysis of vibration excitation mechanisms. These parameters include the temporal and local variation of void fraction and phase redistribution. Understanding these two-phase parameters is essential to evaluating the stability threshold of tube bundle configurations. In this work, capacitance sensor probes were designed using finite element analysis to ensure high sensor sensitivity and optimum response. A simulation-based approach was used to calibrate and increase the accuracy of the void fraction measurement. The simulation results were used to scale the normalized capacitance and minimize the sensor uncertainty to ±5%. The sensor and required conditioning circuits were fabricated and tested for measuring the instantaneous void fraction in a horizontal triangular tube bundle array under both static and dynamic two-phase flow conditions. The static calibration of the sensor was able to reduce the uncertainty to ±3% while the sensor conditioning circuit was able to capture instantaneous void fraction signals with frequencies up to 2.5 kHz.

Soil Properties Control Microbial Carbon Assimilation and Its Mean Residence Time

Microbial assimilation and stabilization of soil organic carbon (SOC) is an important process in global carbon cycling. For an improved understanding of climate-induced changes in ecosystem C dynamics, it is important to know the group-specific turnover of microbial C. Consequently, we wanted to answer the questions if fungi store newly assimilated C longer than bacteria and if climatic and edaphic properties of different regions affect microbial C assimilation and its subsequent release. This study presents results from a 112-day long field experiment where endogenous soil microorganisms were labelled with 13C labelled glucose to follow the dynamics of newly assimilated C in two study regions (Kraichgau and Swabian Alb). Whereas microbial assimilation of newly added C was higher in Kraichgau than in Swabian Alb, the opposite result was obtained for the mean residence time (MRT) of microbial biomass C (76 days in Kraichgau and 93 days in Swabian Alb). The accelerated turnover rates of microbial C in the warmer soils of Kraichgau with lower clay content might be an important mechanism explaining the differences in SOC content between both regions. Gram-positive bacteria assimilated more 13C-glucose into their biomass than fungi and the MRT of C was higher in bacteria as compared to fungi in both regions. Beside these dynamic substrate utilization strategies, we could proof cross feeding by gram-negative bacteria. Carbon MRT in fungi was region specific and was best represented by a two-pool model; the initial MRT ranged between 5 to 39 days and was, in the end, higher than 4 years. This provides evidence of a possible shift in fungal community composition; fast growing fungi dominant in the early phase and internal redistribution of C in the second phase of decomposition. Our study identified microbial group and region specific MRT of freshly assimilated C as an important parameter, which might help to explain the commonly found variation in soil respiration and SOC stabilization.

Ultrasonic-Induced Phase Redistribution and Acoustic Hardening for Rotary Ultrasonic Roller Burnished Ti-6Al-4V

Ultrasonic energy can promote the dislocation motion in ultrasonic-assisted machining. The phase redistribution of Ti-6Al-4V alloy will occur in ultrasonic-assisted machining with appropriate ultrasonic powers. The variations of the volume fractions of β-Ti and α-Ti influence the microhardness on the processed surface. In the present paper, various rotary ultrasonic roller burnishing experiments were conducted to investigate the ultrasonic-induced phase redistribution of Ti-6Al-4V at different ultrasonic powers. The volume fractions of β-Ti and α-Ti were measured by X-ray diffraction. SEM images of the processed surfaces were used to analyze the microstructure evolution and grain refinement of Ti-6Al-4V at various ultrasonic powers. Then, the distributions of local misorientation, texture and grain boundary were observed by EBSD. The results indicated that the microhardness approached the maximum value on the processed surface while the volume fraction of β-Ti reached its maximum value. Finally, the modified phenomenological model was applied to elucidate the relationship among the microhardness, the phase volume fractions and the grain refinement. The fitting degree for microhardness between the modified phenomenological model prediction and experimental measurement was 92.2 pct.

LNAPL saturation distribution under the influence of water table fluctuations using simplified image analysis method

Laboratory-scale column experiments were conducted to investigate the influence of water table fluctuation on light non-aqueous phase liquid (LNAPL) migration and redistribution in porous media. A simplified image analysis method (SIAM) was used to evaluate the saturation distributions of the LNAPL and water in the entire domain under dynamic conditions. Two LNAPL volumes differentiated as low and high volumes were exposed to two patterns of water table fluctuations. High-resolution SIAM images of the soil column during LNAPL migration and subsequent water table fluctuations were collected and analyzed. Results show that when the water table was lowered in the experiment of low LNAPL volume by 10 cm (from 28 to 18 cm) and then raised back to its initial location (28 cm), the water saturation dropped from 100% initially to around 30% while the LNAPL saturation increased and remained at 7% to 25% at 36 cm. By doubling the LNAPL volume to 50 ml, the initial LNAPL saturation above the water table increased and the capillary fringe height increased from 36 to 38 cm. Comparison between the two LNAPL volumes showed that the LNAPL covered a larger area for the case of high LNAPL. The LNAPL saturation extended up to around 53 cm while for the case of low LNAPL, the LNAPL extended up to height of 50 cm. Moreover, water saturation below was lesser for the case of high LNAPL. This was due to the increasing volume of LNAPL which caused the water to be pushed downward. However, similar downward migration of water was observed in both sets of experiments. This indicates that doubling the volume of LNAPL while maintaining the same groundwater fluctuation pattern did not affect the general LNAPL behavior. Overall, this study demonstrates that the SIAM technique is an accurate and cost-effective tool for the visualization of the time-dependent NAPL/water movement in laboratory-scale experiments and dynamic changes in fluid saturation in porous media.

Permeability regain and aqueous phase migration during hydraulic fracturing shut-ins

Hydraulic fracturing has become a key technology to economically extract oil and gas from unconventional reservoirs. During hydraulic fracturing, fluid loss and water invasion into formation can cause serious permeability reduction near fracture face. At the same time, field practice also showed that well shut-ins after hydraulic fracturing could significantly increase hydrocarbon outputs, whereas the inner mechanism still remains unknown. In this paper, firstly, we studied permeability reduction after water invasion and permeability enhancement after well shut-ins using a core flooding system. Then, to investigate the inner mechanism, we studied aqueous phase migration during shut-ins using nuclear magnetic resonance (NMR) method. Results showed that fluids invasion reduce matrix permeability while well shut-ins can improve permeability and this improvement depends on the length of shut-ins time. NMR results showed that aqueous phases mainly distribute in macropores and mesopores after water invasion, while in shut-ins period, these invaded aqueous phases redistribute and migrate from larger pore spaces to smaller ones. Aqueous phase redistribution and migration during shut-ins period can remove near fracture water-block, reduce capillary discontinuity and increase the relative permeability of hydrocarbon phase, and this is the reason for permeability enhancement and hydrocarbon output increase after well shut-ins.

Transformation and migration in secondary zinc–air batteries studied by in situ synchrotron X-ray diffraction and X-ray tomography

Unique information about transformation and phase redistribution in a Zn–air battery anode was obtained using high energy synchrotron X-ray diffraction.

Thermal Effect on the Structural and Phase Condition and Mechanical Properties of Ultrafine-Grained Titanium Alloy VT16 in the Temperature Range of 293 – 923 K

The effect of pre-recrystallization annealing on the evolution of the structural and phase state and the deformation and fracture behavior of ultrafine-grained titanium alloy VT16 obtained by pressing with change of the deformation axis and gradual lowering of the temperature within 1023 – 723 K is studied. It is shown that the process of pre-recrystallization annealing is accompanied by retrogression of the strained structure, which lowers the strength characteristics, and by a β → α phase transformation and redistribution of the alloying elements, which keep the strength properties at a high level.

A mechanistic approach to developing two phase flow pattern transition maps for two-phase dielectric fluids subject to high voltage polarization

High voltage electric fields can be used as an active heat transfer enhancement technique for convective boiling or condensing dielectric fluids, with the major component of this enhancement being attributed to phase redistribution and mixing in the system due to electric body forces. Many performance prediction tools for convective boiling and condensation rely on flow pattern maps to identify the flow regime for use with flow pattern specific heat transfer and pressure drop correlations [1,2]. In this paper, a mechanistic approach to developing two phase flow pattern maps for two-phase dielectric fluids subject to high voltage fields using a fully contained, equation-based electric Froude number approach is presented and compared against the experimental flow pattern data for EHD convective boiling & condensation available in the literature and from the current experimental study. Despite the development of newer diabatic, semi-mechanistic flow pattern models for free-field heat exchanger performance analysis, it is recommended that the Steiner equations for flow pattern transition be used as a basis for the mapping when additional physics such as EHD is coupled, as the empirical fits in newer semi-mechanistic maps are unreliable when additional physics is present.

Phase separation in amorphous hydrophobically modified starch–sucrose blends: Glass transition, matrix dynamics and phase behavior

The phase behavior and matrix dynamics of amorphous blends of octenyl succinic anhydride (OSA) modified starch and sucrose was studied as function of blend composition and water content. Phase separation into two amorphous phases, one enriched in OSA starch and the other in sucrose, was confirmed by differential scanning calorimetry (DSC). DSC and 1H solid-state NMR show that the phase separation is only partial. The glass transition temperature (Tg) of the OSA starch-rich phase was found to be ∼30–100 K higher than the Tg of the sucrose-rich phase, depending on blend composition and water content. A novel type of coupling between changes in physical state of the sucrose-rich phase and plasticizer redistribution is proposed, leading to an unexpected increase of the glass transition temperature of the modified starch-rich phase at higher matrix water contents. A quantitative model for the phase separation of the anhydrous blends into two amorphous phases is presented. The model predicts that, with increasing blend sucrose content, the weight fraction of the sucrose-rich phase decreases, while the sucrose content of both the OSA starch-rich phase and the sucrose-rich phase increases. This novel phenomenon is relevant in the understanding of the stability and performance of multiphase food and pharmaceutical components.

Phase Transformation Temperatures and Solute Redistribution in a Quaternary Zirconium Alloy

This study investigates the phase stability and redistribution of solute during heating and cooling of a quaternary zirconium alloy, Excel (Zr-3.2Sn-0.8Mo-0.8Nb). Time-of-flight neutron diffraction data are analyzed using a novel Vegard’s law-based approach to determine the phase fractions and location of substitutional solute atoms in situ during heating from room temperature up to 1050 °C. It is seen that this alloy exhibits direct nucleation of the βZr phase from martensite during tempering, and stable retention of the βZr phase to high temperatures, unlike other two-phase zirconium alloys. The transformation strains resulting from the $$\alpha \leftrightarrow \beta $$ transformation are shown to have a direct impact on the development of microstructure and crystallographic texture.

Experimental study on phase separation of refrigerant at horizontal T-junction

In this paper, the phase redistribution of refrigerant two-phase flow at a horizontal branch T-junction was investigated experimentally. The T-junction had equal-sized inlet and outlet tubes (internal diameter 8.0 mm). During the experiments, three refrigerants, namely R134a, R600a and R245fa were considered. The inlet mass flux and quality were varied from 200 to 500 kg m−2 s−1 and from 0.1 to 0.9, respectively. Meanwhile, the mass flux of the branch was regulated by keeping the mass flow ratios to 0.3, 0.5 and 0.7. Under these experimental conditions, intermittent and annular flows at the T-junction inlet were identified and predicted. After analyzing the experimental data, it was found that the vapor is preferred to flow into the branch. As the inlet quality increases, the fraction of vapor extracted into the branch decreases, whereas the mass flux has little influence on the phase redistribution of refrigerant under the same flow pattern. Furthermore, a higher vapor fraction can be obtained by a larger mass flow ratio. For the effect of refrigerant on the phase separation, the vapor faction of the branch satisfies the following order: R245fa > R600a > R134a. The product of momentum ratio and viscosity ratio between vapor and liquid is defined to characterize the phase separation degree of refrigerant qualitatively. In order to test the applicability of the published models in the literature, the present data are compared with the predicted values. The results show that the models for air-water are not appropriate to predict the phase redistribution of refrigerant two-phase flow at the T-junction. A separation model for refrigerant should be proposed by coupling with the pressure drop.

Broadband absorption with gradient metasurfaces

A metasurface with appropriately designed transverse spatial inhomogeneities can provide the desired phase redistribution in response to an incident wave with arbitrary incident angle. This property of gradient metasurfaces has been used to modify light propagation in unusual manners, to transform the impinging optical wavefront with large flexibility. In this work, we show how gradient metasurfaces can be tailored to offer high absorption in thin absorptive layers, and how to design realistic metasurfaces for this purpose using dielectric materials.

Finite size effect on the structural and magnetic properties of MnAs/GaAs(001) patterned microstructures thin films

MnAs epitaxial thin films on GaAs(001) single crystalline substrates crystallize at room temperature (RT) in a mixture of two crystalline phases with distinct magnetic properties, organized as stripes along the MnAs [0001] direction. This particular morphology is driven by anisotropic epitaxial strain. We elucidate here the physical mechanisms at the origin of size reduction effect on the MnAs crystalline phase transition. We investigated the structural and magnetic changes in MnAs patterned microstructures (confined geometry) when the lateral dimension is reduced to values close to the periodicity and width of the stripes observed in continuous films. The effects of the microstructure’s lateral size, shape and orientation (with respect to the MnAs mathrm{[11}bar{2}mathrm{0]} direction) were characterized by local probe synchrotron X-ray diffraction (μ-XRD) using a focused X-ray beam, X-ray Magnetic Circular Dichroïsm - Photo Emission Electron Microscopy (XMCD-PEEM) and Low Energy Electron Microscopy (LEEM). Changes in the transition temperature and the crystalline phase distribution inside the microstructures are evidenced and quantitatively measured. The effect of finite size and strain relaxation on the magnetic domain structure is also discussed. Counter-intuitively, we demonstrate here that below a critical microstructure size, bulk MnAs structural and magnetic properties are restored. To support our observations we developed, tested and validated a model based on the size-dependence of the elastic energy and strain relaxation to explain this phase re-distribution in laterally confined geometry.

Role of dams in the phase transfer of antibiotics in an urban river receiving wastewater treatment plant effluent

Dams are universally constructed to change hydrodynamic forces and impound water in urban rivers, but few efforts have been made to expound the impact of dams on the transport of antibiotics in rivers. In this study, the main object and focus was to elucidate the intercept effect of rubber dam and the gate dam on the sediment-water phase redistribution of eight typical antibiotics along an urban river with the water replenished from wastewater treatment plants (WWTPs). The results show that the eight antibiotics were widely detected in the whole river segment, and the WWTPs were found to be the main sources. Among the eight antibiotics, fluoroquinolones had the highest concentrations in the surface water, while tetracyclines constituted the dominant composition in the sediments. Due to the retention effect, the rubber dam encouraged the enrichment and degradation of the antibiotics in the river. However, the role of gate dams depended mainly on the opening and closing of the sluice gate. For all eight antibiotics, each dam or WWTP exhibited a generally consistent impact on the changes (e.g., an increase or decrease) of the sediment-water partitioning coefficients, although obvious differences existed between effect degree. Moreover, the changes in the coefficients caused by the rubber dam were completely opposite to those affected by gate dams. Contrary to the other six antibiotics, the two sulfonamides showed significant relationships with the dissolved organic matter in the surface water but had no significant relationships with the organic matter in the sediments.

A study of well test data interpretation model for waterbearing reservoirs with phase redistribution

In China, plentiful marine reservoirs exist. Net pay thickness in individual gas reservoirs where partial penetration was performed can be hundreds of meters. Due to the influence of condensate water and formation, water phase separation phenomenon, where gas rose up and liquid moved down, and a morsel of water production emerged in some gas wells, which makes the build-up curves distorted and thus unable to be interpreted. On the basis of seepage theory and Laplace transformation, a seepage mathematical model and a well test interpretation model for gas wells with phase separation considered are developed to analyze the impact of such various elements as phase separation and partial penetration on the pressure and pressure derivative log-log plot. With practical data of well test in Xihu Sag, reliability analysis of the mathematical model mentioned above was demonstrated. Theoretical research results proposed in our study substantially improved the accuracy of well test interpretation for thick water-bearing gas reservoirs and laid a technical foundation of development of the similar oil & gas reservoirs.

Different diffusion mechanisms of oxygen in ReBa2Cu3O7−x (Re = Y, Ho) single crystals

In the present study, the time dependencies of the resistivity of ReBa2Cu3O7−x (Re=Y, Ho) single crystals with varying degree of deviation from oxygen stoichiometry was investigated. It was shown that the accelerated transport of oxygen ions in the initial stage of the process can be carried out along the one-dimensional non-stoichiometric vacancies’ accumulation by the single file diffusion mechanism. The final phase redistribution of oxygen ions in ReBa2Cu3O7−x (Re=Y, Ho) is described by the classical mechanism of volume diffusion. The substitution of yttrium by holmium significantly affects the charge redistribution and changes the efficient interaction of ions in CuO-planes, thereby modifying the mass transfer diffusion mechanisms in the oxygen sublattice. Thus, a significant length change of the time intervals occurs, corresponding to the oxygen ions’ single file and volume diffusion mechanisms.