Phase Change Mechanism Research Articles

A review on numerical modelling of flashing flow with application to nuclear safety analysis

The flashing flow is a relevant multiphase phenomenon in many technical applications including nuclear safety analysis, which has been the subject of intense research for several decades. Numerical studies have evolved from one-dimensional to multi-dimensional. A variety of methods has been proposed, while a broad consensus does not exist yet. The present work aims to present an overview of available models and assess their limitations and perspectives by conducting an extensive literature survey. The final focus was put on recent progresses of computational fluid dynamics simulations. Some consensus on modelling interfacial slip, phase change mechanism and bubble size is identified. Since flashing scenarios often accompanying with high void fraction and broad bubble size range, a poly-disperse two-fluid model is recommended. Thermal phase change model is superior to pressure phase change, relaxation and equilibrium models for practical flashing problems, however incorporation of pressure effects is desirable. Major challenges comprise improving closure models for interphase transfer, bubble dynamics processes, interfacial area as well two-phase turbulence. For this purpose, high-resolution high quality experimental data are important, which are lacking in many cases. Considering that heterogeneous gas structures often exist in flashing flows, multi-field approaches able to handle different shapes of gas-liquid interface and including the shape effect in closure models are recommended for further study.

Plasmonics beyond noble metals: Exploiting phase and compositional changes for manipulating plasmonic performance

Reconfigurable plasmonics constitutes an exciting and challenging new horizon in nanophotonics. This blooming field aims at providing plasmonic nanostructures that present a dynamic and active plasmonic response that can be switched or manipulated by external stimuli to induce a controllable change in the optical properties. Most common plasmonic materials, such as the noble metals gold and silver, cannot deliver this type of behavior. Therefore, significant effort is being invested in developing alternative materials whose optical properties can be controllably modified to provide a reconfigurable plasmonic response. In this perspective, several materials including non-noble metals, transition metal oxides and nitrides, and chalcogenide compounds will be analyzed. The selected materials share interesting characteristics like low cost, good chemical and thermal stabilities, and CMOS compatibility while presenting a reconfigurable plasmonic response governed by different phase-change mechanisms.

Phase-Change Mechanism and Role of Each Element in Ag-In-Sb-Te: Chemical Bond Modulation

Reversible crystallization from the amorphous phase is one of the most promising bases on which to store data for universal electronic memory. However, crystallization mechanism at atomic scale and role of element in Ag-In-Sb-Te are still unsolved, whose memory properties are outstanding. To elucidate them, we studied the bonding topologies through spectroscopic techniques and density functional simulations. It is shown by in-situ X-ray photoelectron spectroscopy that element In dramatically changes its bonding topology on crystallization from Sb-bonds to Te-bonds, which are characterized as that of InSb and AgInTe2, respectively by density functional simulations. Further, the degree of atomic ordering is identified to highly dependent on local environment of In by spectroscopic ellipsometry. The overall results suggest that crystallization of Ag-In-Sb-Te is triggered by the local environment transition of In from InSb-like to AgInTe2-like site. Thereby, Ag provides structural flexibility for local environment transition of In, In disrupts crystallization, and Te assists the other elements to play their respective roles. The present work accounts for unsolved memory properties of Ag-In-Sb-Te and opens the gate to develop enhanced materials for the memory.

High-performance CoSbS-based Na-ion battery anodes

CoSbS and its hard carbon-containing nanocomposite (CoSbS@hard-C) are prepared via simple solid-state reactions, and their potential as Na-ion battery (NIB) anode materials is investigated. The electrochemical phase change mechanism of CoSbS during Na insertion/extraction is thoroughly investigated using various ex situ analytical tools. During Na insertion, the CoSbS undergoes a topotactic transformation owing to the formation of a NaxCoSbS (x ≤ 1.6), and then, it is converted into Na3Sb, Na2S, and Co in the fully Na-inserted state via a conversion reaction. Conversely, during Na extraction, Na3Sb transforms to Sb, which alloys with Co to form CoSb. In the fully Na-extracted state, CoSb and S from Na2S are recombined into CoSbS. Owing to the interesting reaction mechanism of CoSbS, the electrochemical performance of the CoSbS and CoSbS@hard-C anodes is excellent. Specifically, when CoSbS is used as a NIB anode, the topotactic transformation between CoSbS and NaxCoSbS (x ≤ 1.6) leads to stable cycling behavior of the NIB and a volumetric capacity of ~480 mAh cm−3 is retained after 100 cycles. Additionally, the NIB with the CoSbS@hard-C anode presents highly reversible and stable capacity (~570 mAh cm−3 after 150 cycles) and fast rate capability (~500 and ~450 mAh cm−3 at 2C and 5C, respectively), which can be attributed to the presence of uniformly dispersed small (5–8 nm) CoSbS nanocrystallites in the hard carbon matrix. Therefore, CoSbS can be utilized as a new anode material for high-performance NIBs.

Engineering the dimension and crystal structure of bismuth molybdate photocatalysts via a molten salt-assisted assembly approach

In this work, a facile molten salt-assisted assembly strategy was studied to achieve controlled synthesis of bismuth molybdate photocatalysts including Bi2MoO6, Bi6Mo2O15, Bi4MoO9 and Bi14MoO24. The transition of bismuth molybdates from Aurivillius-type to Fluorite-type could be selectively and efficiently achieved by adjusting the molten salt synthesis time. It was observed that the bismuth molybdate samples exhibited an obvious redshift in the absorption band with increasing of Bi/Mo molar ratio (from 3 to 14) in the materials, indicating a reduced band-gap. As a result, the photocatalytic activity of the Fluorite-type Bi14MoO24 sample for TC removal was higher than that of Bi2MoO6, Bi6Mo2O15 and Bi4MoO9 samples under a visible light irradiation. Furthermore, the mechanisms of phase change between Aurivillius-type and Fluorite-type as well as their band structures of the bismuth molybdates were systematically investigated in this work. This study not only provides a new strategy for the controlled synthesis of bismuth molybdates with different dimensionalities and structures, but also opens a new pathway to the design and optimization of advanced bismuth molybdates-based photocatalysts for applications in energy harvesting and environmental remediation.

Controlled 3D nanoparticle deposition by drying of colloidal suspension in designed thin micro-porous architectures

ABSTRACT Nanoparticle deposition by drying of colloidal suspension in thin micro-porous architectures has attracted a lot of attention in scientific research as well as industrial applications. However, the underlying mechanisms of such three-dimensional (3D) deposition are not yet fully revealed due to the complexity of the co-occurring processes of two-phase fluid flows, phase change and mass transport. Consequently, the control of 3D nanoparticle deposition remains a challenge. We use a combined experimental and numerical approach to achieve controlled 3D nanoparticle deposition by drying of colloidal suspension in two pillar-based thin micro-porous architectures. By the design of pillar layout, rectangular-spiral and circular-spiral deposition configurations are obtained globally. By varying the surface wettability, vertically symmetric and sloped nanoparticle depositions can be achieved locally. While the numerical modeling reveals the mechanisms of liquid internal flow, as well as the impact of local drying rate on nanoparticle transport, accumulation and final deposition, the experimental results of deposition configurations validate the controlling strategies. This combined experimental and numerical work provides a framework to achieve desired 3D nanoparticle deposition in thin micro-porous architectures, with a thorough understanding of the underlying mechanisms of two-phase fluid flows, phase change and mass transport.

Bio-Based Phase Change Materials Incorporated in Lignocellulose Matrix for Energy Storage in Buildings—A Review

Due to growing consciousness regarding the environmental impact of fossil-based and non-sustainable materials in construction and building applications, there have been an increasing interest in bio-based and degradable materials in this industry. Due to their excellent chemical and thermo-physical properties for thermal energy storage, bio-based phase change materials (BPCMs) have started to attract attention worldwide for low to medium temperature applications. The ready availability, renewability, and low carbon footprint of BPCMs make them suitable for a large spectrum of applications. Up to now, most of the BPCMs have been incorporated into inorganic matrices with only a few attempts to set the BPCMs into bio-matrices. The current paper is the first comprehensive review on BPCMs incorporation in wood and wood-based materials, as renewable and sustainable materials in buildings, to enhance the thermal mass in the environmentally-friendly buildings. In the paper, the aspects of choosing BPCMs, bio-based matrices, phase change mechanisms and their combination, interpretation of life cycle analyses, and the eventual challenges of using these materials are presented and discussed.

In situ TEM revealing pretreatment and interface effects in Ge2Sb2Te5

Random access memory devices that rely on phase changes are primarily limited by the speed of crystallization. However, imaging structural dynamics in working devices with nanoscale resolution remains elusive. Using an ultrafast liquid-quenching system in transmission electron microscope, we obtain a melt-quenched amorphous phase-change material Ge2Sb2Te5 nanosheet. We find that the applied incubation field (pretreatment) leads to nonuniform nucleus formation and growth in the amorphous Ge2Sb2Te5, accompanied by a slight change in electric properties. Moreover, the phase-change mechanism changes from the material inherent crystallization mechanism (nucleation-dominated) to the heterocrystallization mechanism, indicating the incubation period of nucleation might be bypassed.

Numerical study of melting and solidification in a wavy double-pipe latent heat thermal energy storage system

The objective of this paper is to develop the influences of channel waviness on the performance of a latent heat storage system during phase change mechanism. The heat exchanger is a vertically oriented double pipe where the heat transfers to/from the PCM in the annulus by moving the water in the inner tube. Various wavelengths, as well as wave amplitudes, are examined at various fluid Re and water temperatures (Tin) to find the effects of channel waviness on different aspects of melting/solidification time, pressure drop, pumping power and exchanged heat rate. Increasing Re, Tin and amplitude of wavy wall improves the system performance during melting and solidification mechanisms. Besides, it is found that there is an optimum dimensionless wavelength of 0.2 for achieving the minimum melting and solidification times as a result of maximum heat exchanged between the water and PCM. Furthermore, the waviness has an almost negligible effect on the pumping power which is reduced for the dimensionless wavelengths higher than 2.0. In the best scenario, the required time to melt and solidify the PCM reduces by almost 28.6% and 57.63%, respectively, using wavy channels compared with the smooth wall case.

Photoelectrochemical activity of copper vanadate nanostructures

Producing clean energy by using low cost and efficient photoelectrodes via hydrothermal method was achieved. Bare CuV2O6 monoclinic structure and structure/phase changing mechanism of Bi dopant in CuV2O6 and conversion mechanism of Cu3(VO4)2 anorthic phase from CuV2O6 monoclinic was revealed by X-ray diffraction (XRD). Band gap variations of two different phases and dopant, morphological effect on band structure of product were examined. Photoelectrochemical performance of fabricated electrodes was analyzed by linear sweep voltammogram (LSV), electrochemical impedance spectra (EIS), and chronoamperometry (CA) studies under illumination. Synergistic effect of nanoflower morphology and Bi dopant effect on Cu3(VO4)2 phase showed higher current density of 1.29 mA/cm2 in LSV with high electronic conductivity. Long time stability of same photoelectrode for 6 h of illumination was reported. Phase and morphological changing mechanism of Bi dopant in copper vanadate were proposed and proficient electrode was suggested for efficient photoelectrochemical water splitting.

Perspective on liquid metal enabled space science and technology

With the rapid development of deep space exploration and commercial flight, a series of tough scientific and technological challenges were raised, which urgently require ever advanced technologies to tackle with. Recently, liquid metals, as a kind of newly emerging functional material, are attracting various attention and many breakthroughs have been made on earth. Such a scientific trend also suggests promising approaches for solving those extreme challenges in space environment. To fulfill the increasing needs thus involved, this article is dedicated to systematically introducing liquid metal material and its related disciplines into space science and technology. Firstly, existing application of liquid metal cooling for space nuclear power was summarized. Then, some potential space practices were tentatively put forward, such as liquid metal thermal interface medium, liquid metal phase change material, liquid metal convection cooling, metal alloy thermal storage, liquid metal electromagnetic shielding and liquid metal electronic printing. Fundamental as well as practical issues that would differ with earth were interpreted. Finally, potential liquid metal space experiments were proposed to investigate the liquid metal hydrodynamic characteristic, wettability and phase change mechanism in space physical environment. Overall, liquid metal enabled space science and technology investigation will not only help efficiently solve the current and future space technological problems, but also aid to stimulate the advancement of liquid metal space material science.

Experimental study on the oscillation characteristics of thin liquid film in a microchannel

Characterization of thin liquid film is important for comprehensive understanding the phase-change heat transfer mechanisms in an oscillating slug flow. In this study, the thin film thickness of oscillating slug flow in a square tube was experimentally investigated using a nano-particle tracking velocimetry technique. Square tubes with inner dimension of 3.0 × 3.0, 1.6 × 1.6, 0.5 × 0.5, and 0.3 × 0.3 mm2 were used for test tubes, water was used as test fluids. The results show asymmetric velocity distribution in the thin liquid film. An obvious decrease of response time was observed for the thin liquid film when the oscillation frequency increases. The measured thin film thickness fits well with Taylor’s law when the capillary number is small. Deviation from Taylor’s law was found for larger capillary number, due to the prominent effect of bubble deceleration on the thin film thickness when the oscillation frequency increases. The relationship between acceleration process and deceleration process in different oscillation frequency is also studied.

Nanoindentation-induced deformation, microfracture, and phase transformation in crystalline materials investigated in situ by acoustic emission

Abstract

Multiple phase transitions in Sc doped Sb2Te3 amorphous nanocomposites under high pressure

Subnanosecond switching speed from an amorphous state with stable crystal precursors to the crystalline state was recently achieved in amorphous Sc-doped Sb2Te3 (a-SST) phase change materials (PCMs), which is about two orders of magnitude faster than that in the well-studied Ge2Sb2Te5 and Ge1Sb2Te4 PCMs. However, the phase change mechanism and phase stability of a-SST remain unknown. Here, we prepared amorphous Sc0.3Sb2Te3 nanocomposites within a minute amount of face-centered-cubic (FCC) type nanograins embedded in the amorphous matrix. Using in situ high-pressure synchrotron X-ray diffraction, we found that nanograins were frustrated under high pressure and gradually dissolved into the matrix around 11.0 GPa. Beyond 11.0 GPa, the a-SST matrix transformed into a uniform high density metallic like amorphous state with a five orders of magnitude drop in electric resistivity compared to the pristine materials. When further compressed to 23.97 GPa, the high density amorphous (HDA) phase switched into a body-centered-cubic (BCC) high-pressure structure, a different phase from the ambient pressure crystalline structure. Upon decompression back to ambient pressure, a pure amorphous phase was sustained. The present study provides additional insight into the phase change mechanism of amorphous nanocomposites.

Nitrogen doping-induced local structure change in a Cr2Ge2Te6 inverse resistance phase-change material

This work investigated the phase change mechanism of a nitrogen-doped Cr2Ge2Te6 phase change material via Raman scattering.

On the Numerical Prediction of Process-Dependent Properties of Current High-Performance Materials with Material Model *MAT_254 in LS-DYNA

Over the last years, the forecast quality in production and especially (cold) forming simulations has reached a high level at least for standard materials. It has become state-of-the-art to take the results of these process simulations into account e.g. for crashworthiness analyses. For most of the currently used high-performance materials, however, the manufacturing processes do not only result in plastic deformation and thinning of the parts. There usually is a thermal component to these processes, which significantly influences the microstructure of the material and results in a spatial distribution of material properties within the final part. In order to predict the later behavior of the part with the same quality as for standard materials and standard processes, the virtual process chain has to be closed and material models have to be used that are able to simulate the microstructure evolution during the process. The press-hardening of the manganese-boron steel grade 22MnB5 is, for example, a standard in automotive industry. For this material FEM software for process simulation tools like LS-DYNA were extended in the past to take the microstructure evolution into account. The implemented material models are tailored for this certain alloy and process. To ensure the usability of the simulation tool the material models were generalized. In LS-DYNA, material model *MAT_GENERALIZED_PHASECHANGE (*MAT_254) has been devised to serve this purpose. Core of the model is a phase evolution table that takes into account up to 24 individual phases and consequently defines up to 552 possible phase transformations, not all of which have to be defined. The appropriate transformation law for each of the phase changes can be chosen from a list of generic phase change mechanisms. Due to its relatively flexible input structure and generality of the implementation, the model has been successfully used for different processes such as press-hardening, welding, additive manufacturing, and heat treatment, and for different materials, such as ultra-high strength steels, aluminum 6xxx alloys and titanium. This contribution will introduce the material formulation in detail and show the recent extensions based on the microstructure evolution of titanium.

Local- and Intermediate-Range Structures on Ordinary and Exotic Phase-Change Materials by Anomalous X-ray Scattering.

Local- and intermediate-range atomic structures were investigated on amorphous phases of an ordinary phase-change material, Ge2Sb2Te5 (GST), and an exotic one, Cu2GeTe3 (CGT), by using anomalous X-ray scattering close to K absorption edges of each element to find a fast amorphous-crystalline phase-change mechanism. The obtained data were analyzed by using reverse Monte Carlo modeling to obtain partial structure factors, partial pair distribution functions, and three-dimensional atomic configurations. Ring statistics were carefully examined to clarify the similarity and difference compared with the corresponding crystal structures, and it was found that amorphous GST has a number of four-membered rings indicating fragments of crystal structure, and amorphous CGT has a remarkable number of three-membered rings showing a collapse of crystal structures composed of purely six-membered rings. A persistent homology analysis was carried out and long-range ring structures of the constituent elements were observed in the amorphous phase, which may originate from fragments of crystal structures with a long-range periodicity.

Aerosol Dynamics Modeling With Chemkin-Pro Surface-Kinetics User-Routines

Abstract The Chemkin-Pro Application Programming Interface (API) was used to implement surface-kinetics user-routines to expand current aerosol dynamics models. Phase change mechanisms were expanded to include homogeneous nucleation in supersaturated environments, and particle size-dependent vapor condensation and evaporation. Homogeneous nucleation of water droplets was modeled with classical nucleation theory (CNT) and a modified form of nucleation theory published by Dillmann, A., and Meier, G. E. A. (1991, “A Refined Droplet Approach to the Problem of Homogeneous Nucleation From the Vapor-Phase,” J. Chem. Phys., 94(5), pp. 3872–3884). The Chemkin-Pro homogeneous nucleation module, developed in this work, was validated against published data for nucleation fluxes at varying pressures, temperatures, and vapor concentrations. A newly released feature in Chemkin-Pro enabled particle size-dependent surface reaction rates. A Chemkin-Pro vapor condensation and evaporation module was written and verified with the formulation published in Hinds, W. C. (1999, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, Wiley, New York). Lastly, Chemkin-Pro results for coagulation in the transition regime were verified with the semi-implicit method developed by Jacobson, M. Z. (1999, Fundamentals of Atmospheric Modeling, Cambridge University Press, New York, NY). Good performance was observed for all three Chemkin-Pro modules. This work illustrates the utility of the Chemkin-Pro API, and the flexibility with which models can be developed using surface-kinetics user-routines. This paper illustrates that Chemkin-Pro can be developed to include more physically representative aerosol dynamics processes where rates are defined based on physical and chemical parameters rather than Arrhenius rates. The methods and modules developed in this work can be applied to industrial problems like material synthesis (e.g., powder production), processes involving phase change like heat exchangers, as well as more fundamental scientific processes like cloud physics.

Cr-Triggered Local Structural Change in Cr2Ge2Te6 Phase Change Material.

Cr2Ge2Te6 (CrGT) is a phase change material with higher resistivity in the crystalline phase than in the amorphous phase. CrGT exhibits an ultralow operation energy for amorphization. In this study, the origin of the increased resistance in crystalline CrGT compared to amorphous CrGT and the underlying phase change mechanism were investigated in terms of both local structural change and associated change in electronic state. The density of states at the Fermi level in crystalline CrGT decreased with increasing annealing temperature and became negligible upon annealing at 380 °C. Simultaneously, the Fermi level shifted from the vicinity of the valence band to the band gap center, leading to an increase in resistance. The phase change from amorphous to crystalline CrGT occurred through a metastable crystalline phase with a local structure similar to that of the amorphous phase. Cr nanoclusters were confirmed to exist in both the amorphous and crystalline phases. The presence of Cr nanoclusters induced Cr vacancies in the crystalline phase. These Cr vacancies generated hole carriers, leading to p-type conduction. Photoelectron spectroscopy of the Cr 2s core level clearly indicated a decrease in the fraction of Cr-Cr bonds and an increase in the fraction of Cr-Te bonds in crystalline CrGT upon annealing. Meanwhile, the coordination number of the Cr nanoclusters decreased as the number of Cr-Cr bonds was reduced. Together, these results imply that the origin of the increased resistance in crystalline CrGT is the filling of Cr vacancies by Cr atoms diffusing from Cr nanoclusters.

Phase change, micro-structure and reaction mechanism during high temperature roasting of high grade rare earth concentrate

The deposit of Bayan Obo in Inner Mongolia is the world's largest rare earth element (abbreviated as REE) resource. The exploration of the theory of mineral formation of Bayan Obo is an important foundation for mineralogical research, and is the scientific basis for mining, industrial beneficiation, smelting and extraction, and processing and utilization. With the rapid development of science and technology, the demand for the utilization of rare earth elements is increasing, and the separation process between rare earth elements needs to be developed. The purpose of this paper is to provide high temperature experimental information for the formation and application of rare earth minerals. To this end, the mineral evolution of high-grade rare earth concentrates with increasing temperature and the migration of rare earths at different stages and their reaction mechanisms were studied. According to thermogravimetric analysis and differential scanning calorimetry (TG-DSC), calcination was carried out at different temperature ranges, and the calcined products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope and energy dispersive spectrometer (SEM-EDS) and other analytical techniques. The results are shown in this process, the rare earth phase is first converted into rare earth oxide and rare earth oxyfluoride. As the temperature increases, Ca5(PO4)3F and a large number of self-shaped spherical Ca-RE-OF and Ca-RE-PO4 particles are formed, and the separation of La and Ce elements is discovered. According to the phase diagram analysis, the production of Ca5(PO4)3F is due to the reaction of monazite and fluorite, and the phases CeF2 and CeF3 are formed during the reaction. When it reaches 1500 °C, barium ferrite is produced and a new substance containing Ba2+ is formed.