Phase Transitions In Na0 Research Articles

Two-phase flow and heat transfer on a cylinder via low-velocity jet impact

Combining the benefits of jet impact and phase-transition heat transfer, jet impact boiling has been widely adopted in industrial scenarios. Notably, owing to limitations of high-velocity jets, such as high impact stress and pumping pressure in liquid circuits, low-velocity jet impacting deserves more attention and explorations. In this paper, under an atmospheric environment with wall heating temperatures ranging from 36 °C to 128 °C, the two-phase flow and heat transfer characteristics (local surface temperature, boiling curves and liquid loss rate) of low-velocity jet impacting on a cylindrical surface are investigated by experiment, followed by the analysis on the effects of jet outlet velocity and impact height. The findings reveal that, with the increase of heat transfer rate, the heat transfer regimes in sequence are non-phase transition (single-phase convection and air-liquid two-phase convection) and nucleate boiling. The influences of jet impact height and outlet velocity on local surface temperatures are pronounced at non-phase transition stage. In impact region, along with average superheat, the growth rates of heat transfer rate and liquid loss rate increase significantly from non-phase transition to nucleate boiling stage, and slow down at anaphase of nucleate boiling stage. Interestingly, the increase in outlet velocity delays the onset of slowdown in growth of liquid loss rate to some extent. Overall, the heat transfer rate and liquid loss rate are significantly enhanced at higher jet outlet velocity of 0.60 m/s, but not affected significantly by impact height.

Transient cooling experiment of laminar jet impacting on cylinder

Liquid jet impact cooling is found in many industrial applications. In transient process of liquid jet impacting on surface, there are correlations among liquid film flow, heat transfer regime and surface temperature when phase transition is involved. Aiming at application scenarios of laminar jet impacting on cylindrical cooling targets, an experiment of laminar jet impacting on cylinder was performed to investigate transient cooling characteristics featured by temperature dropping curves. Liquid film evolutions were acquired by high-speed camera, while surface temperatures were monitored by thermocouples, and effects of jet outlet velocity u (outlet Reynolds number Re) and impact height H were explored. Results reveal that, transient process of jet impact is categorized into non-wetted (film boiling, duration of τ1), developing wetted (mainly transition boiling and nucleate boiling, duration of τ2) and fully wetted stages (mainly non-phase transition convection). Among them, τ1 and τ2 are decreased with rising of Re. Jet exhibits steady, perturbed and fractured states at different H. When u = 0.44 m/s (Re = 680), for steady state jet, τ1 decreases from 119 s to 103 s, while variation of τ2 is insignificant as H increases from 27 mm to 37 mm. When jet is unstable (H = 42–47 mm), τ1 drops remarkably while τ2 rises with increasing H. Effect of H on temperature dropping curves is weakened with increasing Re. Unlike cases of liquid impacting on flat surface, there are clear distinctions in durations of each transient stage, and there is a secondary inflection point for surface temperature variation in present experiment due to secondary mixing of liquid film.

Phase Transitions in Na(Nb0.9Me0.1)O3–δ (Me = Bi, Fe) Solid Solutions

Phase Transitions in Na(Nb0.9Me0.1)O3–δ (Me = Bi, Fe) Solid Solutions

Novel properties of stearic acid / MXene - Graphene oxide shape - Stabilized phase change material: Ascended phase transition temperature and hierarchical transition

To develop novel materials with active photothermal conversion and latent heat storage properties, the shape-stabilized phase change material was fabricated via self-assembly method employing stearic acid (SA) as heat storage medium, graphene oxide and MXene as hybrid matrix (GM) and photothermal conversion agent. The morphology, structure, properties and phase transition behavior were investigated by various techniques. The results shown that SA/GM has sandwich-like structure and SA is confined between matrix nanosheets in multiple crystallographic forms. Due to the superposition of host-guest interaction and intrinsic van der Waals force between nanosheets, the primary melting temperature of SA/GM climbs 27.7 °C approximately than that of SA, as well as the total latent heat decreases by 28.55%–30.85% compared with theoretical values. The confinement effect increases the crystallization activation energy of confined SA, and leads to the formation of non-phase transition layer, transitional confinement layer and proximate bulk layer. However, the above observations are applicable to fatty acids/GM, only the suppressed latent heat occurs in all GM-matrix composites. In addition, the SA/GM7 has excellent photothermal conversion efficiency, as high as 93.34%, and gradient latent heat storage capacity, 45.96 J g −1 at 74.43 °C and 91.19 J g −1 at 92.37 °C. Stearic acid/Mxene - graphene oxide (SA/GM) composite was fabricated though self-assembly method. The confined SA exists in multiple crystal forms and shows abnormal properties, including the ascended phase transition temperature, hierarchical phase transition and the formation of confined layer. The SA/GM presents excellent photothermal conversion efficiency and gradient heat storage/release performances. • SA/GM was self-assembled by the inherent forces of matrix nanosheets and host-guest interaction. • The main melting temperature of SA/GM increased by 27.7 °C approximately. • Non-phase change layer, transitional confinement layer and proximate bulk layer exist in SA/GM. • Ascended transition temperature and stratified phase transition are applicable to fatty acids/GM.

An experimental investigation on performance of microencapsulated phase change material slurry in ground heat exchanger

As a promising alternative to conventional heat transfer fluids, microencapsulated phase change material slurry (MPCS) was applied in ground heat exchanger (GHE) in this paper. A sandbox system embedded with horizontal serpentine GHE was installed and MPCS using N-eicosane (C20H42) as phase change material is prepared. To explore the merits of MPCS, a comparison study between performance of water and MPCS was conducted. In addition, the enhanced heat transfer mechanism of MPCS were studied by comparing performance of MPCS with different inlet temperature and particle concentration. The experimental results indicate that the heat transfer rate and pumping consumption of 25 wt% MPCS with inlet temperature of 30 °C are 10.5 and 2.8 times that of water, respectively. The heat load-to-pumping consumption ratio has an enhancement of 3.73 times in comparison to water. Obviously, considering the phase transition process, MPCS can efficiently improve thermal performance of GHE. However, in the non-phase transition temperature range, the performance of MPCS is even worse than that of water. The comprehensive performance factor of 15 wt% MPCS and 25 wt% MPCS with inlet temperature of 38 °C declined by 45.6% and 36.9% in comparison with pure water.

High dielectric constant, relaxor behavior and phase transitions in Na0.5Bi0.5TiO3 nanorods

We report on relaxor-ferroelectric behavior, ferroelectric to anti-ferroelectric transition and superior dielectric properties of Na0.5Bi0.5TiO3 nanorods synthesized through hydrothermal technique. Na0.5Bi0.5TiO3 nanorods are seen to crystallize in rhombohedral crystal structure. Ferroelectric to anti-ferroelectric transition is observed at 450 K, where the dielectric loss attains its maximum value. Maxwell-Wagner interfacial polarization causes the dielectric constant to be high at room temperature (9 × 105). Dielectric loss at low frequencies is observed to be minimum. Non-Debye relaxation process in Na0.5Bi0.5TiO3 nanorods are clearly seen from the frequency and temperature dependent dielectric studies. Jonscher's relation is employed to analyze the frequency dependent conductivity at various temperatures and the activation energy (Ea) value is observed to be 0.48 ± 0.02 eV. Structural-oxygen vacancy defect-dielectric property correlation is established through a detailed dielectric property analysis in Na0.5Bi0.5TiO3 nanorods.

Novel approach in analyzing phase transitions in Na0.5Bi0.5TiO3—Comparison with 0.95Na0.5Bi0.5TiO3–0.05CaTiO3

Recently, Na0.5Bi0.5TiO3 and its solid solutions are receiving intensive study as one of the most perspective lead-free ferroelectrics. Not only physical properties, but also the structure and nature of phase transitions of these compositions are of great interest, as their previous studies contain many uncertainties. In the present research, Na0.5Bi0.5TiO3 and 0.95Na0.5Bi0.5TiO3–0.05CaTiO3 solid solutions were thoroughly studied focusing on the elastic and thermal expansion characteristics, accompanying the obtained results by x-ray diffraction, scanning electron microscopy, differential calorimetry, and second harmonic generation measurements. Temperature-frequency dependences of dielectric permittivity were observed to be similar for both compositions. In spite of this, the experimentally obtained temperature dependences of thermal expansion and Young's modulus in Na0.5Bi0.5TiO3 and 0.95Na0.5Bi0.5TiO3–0.05CaTiO3 reveal unambiguous differences in the temperature range of the observed or expected (as in the case of Na0.5Bi0.5TiO3) phase transitions. X-ray diffraction patterns are fitted using Pnma symmetry. This allows us to distinguish the temperature regions with different behaviors of lattice parameters, which correlate with the observed behavior of thermal expansion and Young's modulus. A reduction in the intensity of second optical harmonic was observed upon increasing the temperature in the whole studied temperature range. This encourages us to reconsider the mechanism responsible for the temperature dependence of dielectric permittivity.

Maxwell-Wagner Dielectric Relaxation Induced Superior Dielectric Properties and Phase Transitions in Na0.5bi0.5tio3 Nanorods

Maxwell-Wagner Dielectric Relaxation Induced Superior Dielectric Properties and Phase Transitions in Na0.5bi0.5tio3 Nanorods

Step-controlled growth of MAPbBr3 single crystal and temperature-dependent variations of MA+ cations during nonphase transition

MAPbBr3 perovskite crystals have attracted significant attention due to their outstanding performance in various optoelectronic applications. However, further understanding for the growth mechanism of MAPbBr3 and its temperature-dependent variations of MA+ should be addressed. In this study, MAPbBr3 single crystal was grown using the inverse temperature crystallization method with a crystallization temperature of approximately 60 °C. Optical microscopy and atomic force microscopy were used to illustrate the surface structure of the resultant MAPbBr3. The results show that there is a layered growth mode with concave grooves and 2D nuclei on its surface. In situ variable-temperature Raman measurements were performed on the MAPbBr3 crystal at a temperature range of 80–300 K. The narrowing of Raman bands unrelated to the phase transition of the crystal was observed around 100 K. By combining the variable-temperature X-ray diffractometry results from 80 to 105 K, the reason can be attributed to the lattice distortion and vibration within the crystals.

Dynamics of phase transitions in Na2TiO3 and its possible utilization as a CO2 sorbent: a critical analysis

Thermally-driven structural changes of Na2TiO3 and their effect on its carbonation behaviour.

Numerical and experimental study of effect of paraffin phase change heat storage capsules on the thermal performance of the solar pond

In this paper, the effect of adding the composite PCM (Phase Change Material) heat storage capsules to the heat storage layer of the salt gradient solar pond on the thermal performance of the solar pond was studied numerically and experimentally. Based on the program-controlled temperature simulation of the solar pond experimental platform, the effect of adding the composite PCM (48–50°C and 58–60°C melting point paraffin) heat storage capsules on the solar pond temperature and stability was studied, and a numerical simulation model was established to be compared by the experimental results. The results showed that the experimental temperature was consistent with the simulation results; the solar pond with PCM capsules had a smaller temperature change range than the conventional solar pond during the phase change process, but it did not have such effect in the non-phase transition process; in terms of flow, the addition of the PCM phase change units could reduce the flow rate of the heat storage zone, and the PCM with a larger latent heat had a more obvious suppression effect on the flow. Therefore, within a certain temperature range, adding PCM units to the solar pond had a positive effect on maintaining the stable temperature and stability of solar pond.

High Temperature Effect and Radiation Damage in Yb4Hf3O12 Delta Phase Material

The development of next generation nuclear energy systems such as Gen-IV and fusion reactors need materials that resist radiation damage effects. Therefore, advanced nuclear reactors require the development of new materials that will satisfy their design specifications. A considerable amount of research has been performed on some crystalline oxides with fluorite related structures (CaF2) as potential candidates for inert matrix in nuclear fuel and waste forms. Delta phase, another fluorite-related phase structure is a compound produced from ceramic oxide compositions usually in the form of A4B3O12 or A6B1O12 describe as M7O12 (where M is a metal representing AB and O is oxygen). In this work, we developed a new Yb4Hf3O12 delta phase material using solid-state sintering. The oxide was characterized with an ordered rhombohedral structure called delta phase material The high-temperature XRD result showed that the phase and structure of the composition remain the same up to the high temperature of 1300°C indicating a non-phase transition, but the lattice parameters values revealed a linear thermal expansion in the volume as the temperature increases. The polycrystalline sample was irradiated with 6keV He+ at room temperature to fluences of 7.6 x 1016 ions/cm2 (corresponding to 6.2dpa) to study the radiation tolerance in the compound. The irradiated sample was characterized by transmission electron microscopy (TEM). The result showed no formation of bubble gas but an airy ring was observed in the diffraction pattern indicating that the structure was damaged.

Pressure-induced phase transitions in Na2B12H12, structural investigation on a candidate for solid-state electrolyte

closo-Borates, such as Na2B12H12, are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressure in situ powder X-ray diffraction on Na2B12H12 was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primary-order parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling.

Quantification of charge carriers participating antiferromagnetic to weak ferromagnetic phase transition in Na doped LaFeO3 nano multiferroics

Multiferroic La1-xNaxFeO3 (x = 0.0, 0.05, 0.10, and 0.15) nanoparticles have been synthesized by the citrate sol-gel auto-combustion method. It is uniquely known for the homogeneous distribution of particle size due to the low temperature synthesis condition. The Rietveld refinement of the X-ray diffraction pattern has exhibited the single phase orthorhombic perovskite structure. It is confirmed from micro-Raman analysis that hole doping (Na) produces a structural crossover from the Pnma to Pbnm space group. The red shift observed in UV-visible spectroscopy indicates the shrinkage of bandgap. Photoluminescence analysis supports independently the bandgap shrinkage by the existence of violet-blue emission due to charge disproportion by oxygen related defects. A changeover from semiconductor to metallic nature accompanied by the hopping of charge carriers from Fe3+ to Fe2+/Fe4+ exchange interaction has revealed a considerable increase in conductivity and dielectric constant. Finally, the doping helps to induce transition from an antiferromagnetic to a weak ferromagnetic state due to free carrier concentration. It is found that the hole driven Fe3+ to Fe2+/Fe4+ exchange interactions with change in the Bohr magneton values from 5.9 μB to 4.9/4 μB are responsible for variation in the magnetization values.

Exploration of Iron-Based Mixed Polyanion Cathode Materials for Thin Film Sodium-Ion Batteries

Li-ion batteries are promising power sources for portable electronics and electric vehicles due to their high energy density. However, its application in grid storage is limited due to high cost and low abundance of Li resources. Next to Li-ion, Na-ion batteries (NIBs) are getting prominent interest due to its low cost, intercalation properties similar to Li and electrochemical potential of Na/Na+ (-2.71 V vs SHE) [1]. Recent studies based on layered NaMnO2 showed high specific capacity of 140 mA h g-1 by six biphasic transitions. However, the layered materials deliver poor cycling performance due to multiple phase transitions in Na insertion/extraction reactions [2]. On the other hand, Polyanion-type compounds are the most promising electrode materials for NIBs due to their stability, safety, and suitable operating voltages. Recently, Na4M3(PO4)2P2O7 (M=Co, Mn, Ni and Fe) are evaluated as electrode materials for energy storage applications due to its Na-ion intercalating properties, low cost and environmental friendliness [3,4]. In this work, the bulk powder of carbon coated Na4Fe3(PO4)2P2O7 material was prepared by solution combustion synthesis technique using ascorbic acid. The crystal structure was identified to be orthorhombic with Pna21 symmetry from Rietveld refinement. It has a 3D intercalating structure for Na-ion with a theoretical capacity of 129 mAh g-1. The Na-ion cell with the carbon coated nano-Na4Fe3(PO4)2P2O7 cathode delivered a discharge capacity of ~110 mAh g-1 at a 0.1C rate (Fig. 1a). The Na4Fe3(PO4)2P2O7 showed excellent rate capability with good cycling stability 100 cycles. Overall the voltage profile shows two plateau regions with the average voltage of ~3.1 V (Fig. 1b). The electrochemical results motivated us to synthesize the Na4Fe3(PO4)2P2O7 thin films for all-solid-state Na-ion batteries, which can be useful to design thin film micro-batteries. Thin films of Na4Fe3(PO4)2P2O7 electrode was deposited on stainless steel substrates using pulsed laser deposition (PLD) using KrF Laser source. Various deposition parameters were optimized to grow uniform thin films on substrates. From X-ray diffraction it was observed that films were crystallized into orthorhombic structure with Pna21 symmetry. Nano size and thickness of Na4Fe3(PO4)2P2O7 thin film grains were identified by field emission electron microscopy (FE-SEM) and atomic force microscopy (AFM) (Fig. 1d). Here, Na-ion intercalating properties of iron-based mixed-polyanion Na4Fe3(PO4)2P2O7 thin films was investigated. The electrochemical properties of both bulk and thin films of Na4Fe3(PO4)2P2O7 were studied. The thin films with thickness of ~200 nm delivered a maximum capacity and similar voltage profiles to the bulk material (Fig. 1d). The work demonstrates the capability of the mixed polyanionic material as high performance thin film Na-ion battery cathode. The electrochemical performances of the bulk and thin films of Na4Fe3(PO4)2P2O7 will be presented. Finally, an all-solid-state thin film micro-battery will be demonstrated using the full cell configuration having Na4Fe3(PO4)2P2O7 as cathode, NASICON Na3Zr2Si2O12 as solid electrolyte and Na3Ti2(PO4)3 as anode [5]. Figure 1. Electrochemical performances of Na4Fe3(PO4)2P2O7 in sodium half-cell architecture. (a) Galvanostatic charge/discharge curves of Na4Fe3(PO4)2P2O7 at a C/10 current rate. (b) Differential capacity vs. voltage (dQ/dV) plots, (c) Rate capability at different C rates and (d) Galvanostatic charge/discharge curves of Na4Fe3(PO4)2P2O7 thin film (inset) AFM and SEM images of the thin films grown by PLD technique [5]. Acknowledgement Author B.S. gratefully acknowledges the DST (SERB), New Delhi, India (PDF/2015/00217) for providing Fellowship. Figure 1

Peculiar properties of phase transitions in Na0.5Bi0.5TiO3−0.06BaTiO3 lead-free relaxor ferroelectrics seen via acoustic emission

Na[Formula: see text]Bi[Formula: see text]TiO3-0.06BaTiO3 relaxor ferroelectrics (RFEs) ceramic samples were investigated by means of dielectric and acoustic emission methods in the temperature range of 70–350[Formula: see text]C and in dependence on frequency. Dielectric curve exhibits two anomalies: strongly dispersive the local maximum, within 130–180[Formula: see text]C and slightly dispersive the smeared high-temperature maximum within 300–310[Formula: see text]C. Acoustic emission exhibits the five groups of bursts: near 123[Formula: see text]C, 150–180[Formula: see text]C, near 225[Formula: see text]C, 300–310[Formula: see text]C and 327[Formula: see text]C. The first and third groups of acoustic emission bursts correspond to phase transitions of R[Formula: see text]c-P[Formula: see text]bm phases coexisting in this temperature, range, while the fourth and fifth groups of acoustic emission bursts correspond to P[Formula: see text]bm-P[Formula: see text] /mbm-Pm[Formula: see text] m phase transitions. The second group of acoustic emission bursts corresponds to strongly dispersive local maximum at which no phase transition has been proved in Na[Formula: see text]Bi[Formula: see text]TiO3-0.06BaTiO3. The existence and the location of the intermediate temperature, [Formula: see text], in Na[Formula: see text]Bi[Formula: see text]TiO3-0.06BaTiO3 compound are discussed.

Thermophysical study of structural phase transitions in Na0.95Li0.05NbO3 solid solution

Temperature dependences of specific heat C p(T) and coefficient of thermal expansion ;(T) for Na0.95Li0.05NbO3 sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The C p(T) and α(T) anomalies at T 3 = 310 ± 3 K, T 2 = 630 ± 8 K, and T 1 = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ↔ Q ↔ S(R) ↔ T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition Q → R/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the C p(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed.

Mode-crystallography analysis of the crystal structures and the low- and high-temperature phase transitions in Na0.5K0.5NbO3

Na0.5K0.5NbO3has been synthesized by the conventional solid-state reaction process. The crystal structures and phase transitions, at low and high temperature, determined from the Rietveld refinements of X-ray and neutron powder diffraction data are reported. The structure evolution of Na0.5K0.5NbO3in the temperature range from 2 to 875 K shows the presence of three phase transitions. The first one, at ∼135 K, is discontinuous from the rhombohedralR3c(No. 161) space group to the room-temperature orthorhombicAmm2 (No. 38) space group; the second is discontinuous from the orthorhombic to the tetragonalP4mmspace group (No. 99) at ∼465 K, and the third is continuous from the tetragonal to the cubic Pm\overline{3}m space group (No. 221) at ∼700 K. The obtained phase-transition sequence isR3c→Amm2 →P4mm→Pm\overline{3}m. No previous studies at low temperature have been carried out on the material with composition Na0.5K0.5NbO3. In the course of the determination of the three experimentally found phases, a novel method of refinement is presented. This is a step forward in the use of the symmetry-adapted modes as degrees of freedom in the refinement process: the parameterization of a direction in the internal space of the, in this case, sole irreducible representation, GM4−, responsible for the symmetry breaking from the parent cubic space group to the polar distorted low-symmetry phases. Eventually, this procedure enables the calculation of the spontaneous polarization.

Topological tuning in three-dimensional dirac semimetals.

We study with first-principles methods the interplay between bulk and surface Dirac fermions in three dimensional Dirac semimetals. By combining density functional theory with the coherent potential approximation, we reveal a topological phase transition in Na_{3}Bi_{1-x}Sb_{x} and Cd_{3}[As_{1-x}P_{x}]_{2} alloys, where the material goes from a Dirac semimetal to a trivial insulator upon changing Sb or P concentrations. Tuning the composition allows us to engineer the position of the bulk Dirac points in reciprocal space. Interestingly, the phase transition coincides with the reversal of the band ordering between the conduction and valence bands.

Microstructure and Mechanical Properties of Friction Stir Welded Q345 Steel

The Q345 steel sheet of 2.2mm thickness was welded by friction stir welding, and the microstructure and mechanical properties of the joints were analyzed. The experimental results demonstrate that a perfect joint, without internal defects, can be obtained using a stir-pin rotation speed of 800rpm, a weld speed of 50mm·min ¹1 , and water-sprayed cooling. The macroscopic morphology of the cross-section of the weld seam is bowl-like. Phase transition occurred in part of the heat-affected zone, and austenite grains turned to fine ferrite and pearlite grains during cooling: these co-existed with the non-phase transition base metal. The strongest dynamic recrystallisation occurred in the stir zone, where proeutectoidferrite exhibited slightly overheated characteristics and the matrix was an acicular distribution of sorbite. The hardness of the weld zone was evenly distributed and 12% greater than the base material. The tensile strength of the welded joints was higher than that of the base material. The enhanced mechanical properties were mainly due to the fine grain size in the heat-affected zone and the sorbite matrix in its stir zone. [doi:10.2320/matertrans.M2013332]