Partial Phase Transition Research Articles

The Role of Doped Ceria Buffer Layer on Phase Evolution in Pr2NiO4+d cathode and Approached to Suppress Their Reaction in Solid Oxide Fuel Cells

During operation Pr2NiO4+δ cathode undergoes partial phase transitions which results in evolution of PrOx and higher order layered structure (Pr3Ni2O7). Simultaneously, the elemental diffusion occurs where praseodymium migrates towards cathode/buffer interface and reacts with doped ceria deep within its bulk. It is speculated that the presence of ceria may promote the diffusion of Pr into the buffer layer due to a large solubility of Pr in ceria. Recently, we observed that the elemental substitution with Nd on the A-site in Pr2NiO4+δ can slow the diffusion process. With increase in Nd content the formation of PrOx and higher order Pr3Ni2O7 phase is suppressed. In this presentation, we will discuss (1) the role of buffer chemistry on the diffusion or Pr and (2) whether or not the absence of Pr in buffer layer further promotes the phase evolution in Pr2NiO4. Thermal annealing tests were performed at various temperatures and compared to electrochemical measurements in full cells. X-ray diffraction and scanning electron microscopy analyses were used to study the long term phase evolution and reaction with buffer layer.

Rapidly Releasing over 9 wt % of H2 from NH3BH3–Mg or NH3BH3–MgH2 Composites around 85 °C

It is ideal that the hydrogen storage materials for vehicular applications can desorb substantial amounts of hydrogen below 85 °C, the operating temperature of polymer electrolyte membrane (PEM) fuel cells. Ammonia borane (NH3BH3, AB for short), because of its intriguingly high hydrogen density (i.e., 19.6 wt %) and moderate thermal stability, is widely regarded as a promising on-board hydrogen storage medium. However, at this temperature, both its dehydrogenation kinetics and deliverable H-capacity are far from meeting the requirements for practical applications. Here, we report that the Mg- or MgH2-modified AB can deliver over 9 wt % of H2 within 1.5 h at approximately 85 °C. Such pronounced dehydrogenation properties are found to be enabled by the combination of three factors, including partial phase transition of normal AB to its mobile phase AB* in the starting material, adequate sample thermal conductivity, and sufficiently intensive external energy input. A further mechanistic study indicates that ...

Degradation of atrazine in aqueous solution with electrophotocatalytic process using TiO2−x photoanode

The present study investigates the efficiency of a sustainable treatment technology, the electrophotocatalytic (EPC) process using innovative photoanode TiO2−x prepared by a magnetron sputter deposition process to remove the herbicide atrazine (ATZ) from water. The coexistence of anatase and rutile were identified by X-ray diffraction (XRD) and the presence of oxygen vacancies reduce the value of the observed bandgap to 3.0 eV compared to the typical 3.2 eV TiO2, this reduction is concomitant with a partial phase transition which is probably responsible for the increase in photoactivity. The experimental results with an initial concentration of ATZ (100 μg L−1) show that more than 99% of ATZ oxidation was obtained after 30 min of treatment and reaction kinetic constant was about 0.146 min−1. This good efficiency indicates that EPC process is an efficient, simple and green technique for degradation of pesticides such as ATZ in water. The analysis with liquid chromatography technique permits to identify, quantify and see the evolution of ATZ by-products which are generated by dechlorination, dealkylation and alkylic-oxidation mechanisms. Finally, the possible pathways of ATZ degradation by hydroxyl radicals were proposed.

Shock induced phase transition of water: Molecular dynamics investigation

Molecular dynamics simulations were carried out using numerous force potentials to investigate the shock induced phenomenon of pure bulk liquid water. Partial phase transition was observed at single shock velocity of 4.0 km/s without requirement of any external nucleators. Change in thermodynamic variables along with radial distribution function plots and spectral analysis revealed for the first time in the literature, within the context of molecular dynamic simulations, the thermodynamic pathway leading to formation of ice VII from liquid water on shock loading. The study also revealed information for the first time in the literature about the statistical time-frame after passage of shock in which ice VII formation can be observed and variations in degree of crystallinity of the sample over the entire simulation time of 100 ns.

Improving of the electrical and magnetic properties of BiFeO3 by doping with yttrium

Bismuth ferrite is one of the most promising multiferroic materials, and the main barriers for exploiting all of its specific properties are difficulties in obtaining pure, high resistive material with nanosized grains. Doping of BiFeO3 with different transition metals and rare earth elements is often used way for overcoming these obstacles. Yttrium doped bismuth ferrite, Bi1−xYxFeO3 (x=0; 0.01; 0.03; 0.05; 0.1), was prepared by auto-combustion method. X-ray diffraction patterns and Raman results showed that partial phase transition from rhombohedral to orthorhombic structure took place at around 10mol% of Y. Effect of Y doping on microstructure was studied from SEM micrographies, showing the reduction of grain size in doped samples. Electrical measurements showed continuous improvement of resistivity with Y doping, whereas the values of saturation and remnant polarizations exhibit maximums at around 5mol% of Y. Yttrium doping also enhanced magnetic properties, leading to weak ferromagnetism.

熱力学モデルに基づく群ロボットの集団移動

This paper proposes a collective movement control method based on both of an internal energy and a phase transition in thermodynamic model for swarm robots, which can move between obstacles with changing an aggregation suitable for arrangement of obstacles in an environment. Swarm robots shape a fixed aggregation by virtual attraction and repulsion forces based on the method, and follow a leader robot with keeping its shape. When the aggregation shape of swarm robots cannot keep to move through narrow spaces surrounded by obstacles, the swarm robots flexibly change the shape according to a shape of the environment based on a partial phase transition provided with directivities, and can continue to move. Simulation results show that swarm robots can move along to the path in an open space, and pass through narrow spaces surrounded by obstacles with changing the aggregation shape according to the environment.

X-ray diffraction investigation of amorphous calcium phosphate and hydroxyapatite under ultra-high hydrostatic pressure

The changes in the crystal structures of synthetically prepared amorphous calcium phosphate (ACP) and hydroxyapatite (HAP) in water (1:1 mass ratio) were studied by synchrotron X-ray diffraction (XRD) under ultra-high hydrostatic pressures as high as 2.34 GPa for ACP and 4 GPa for HAP. At ambient pressure, the XRD patterns of the ACP and HAP samples in capillary tubes and their environmental scanning electron micrographs indicated amorphous and crystalline characteristics for ACP and HAP, respectively. At pressures greater than 0.25 GPa, an additional broad peak was observed in the XRD pattern of the ACP phase, indicating a partial phase transition from an amorphous phase to a new high-pressure amorphous phase. The peak areas and positions of the ACP phase, as obtained through fitting of the experimental data, indicated that the ACP exhibited increased pseudo-crystalline behavior at pressures greater than 0.96 GPa. Conversely, no structural changes were observed for the HAP phase up to the highest applied pressure of 4 GPa. For HAP, a unit-cell reduction during compression was evidenced by a reduction in both refined lattice parameters a and c. Both ACP and HAP reverted to their original structures when the pressure was fully released to ambient pressure.

SiO<sub>2</sub> Free HfO<sub>2</sub> Gate Dielectrics by Physical Vapor Deposition

HfO2 layers, 25-A thick, were grown by cyclic Hf sputter deposition and room temperature oxidation steps on chemically oxidized Si(001). Subsequent in situ annealing and TiN deposition yield a high- $\kappa$ gate-stack for which the original 8-A-thick SiO2 layer is eliminated, as confirmed by transmission electron microscopy. Transistors fabricated with this gate-stack achieve an equivalent oxide thickness in inversion $T_{\rm inv} =9.7$ A, with a gate leakage $J_{g} =0.8$ A/cm2. Devices fabricated without in situ annealing of the HfO2 layer yield a $T_{\rm inv}$ which increases from 10.8 to 11.2 A as the oxidation time during each HfO2 growth cycle increases from 10 to 120 s, also causing a decrease in $J_{g}$ from 0.95 to 0.60 A/cm2, and an increase in the transistor threshold voltage from 272 to 294 mV. The annealing step reduces $T_{\rm inv}$ by 1.5 A (10%) but also increases the gate leakage by 0.1 A/cm2 (30%), and causes a 61 mV reduction in $V_{t}$ . These effects are primarily attributed to the oxygen-deficiency of the as-deposited HfO2, which facilitates both the reduction of an interfacial SiO2 layer and a partial phase transition to a high- $\kappa $ cubic or tetragonal HfO2 phase.

Coordination Site Disorder in Spinel-Type LiMnTiO4

LiMnTiO4 was prepared through solid-state syntheses employing different heating and cooling regimes. Synchrotron X-ray and neutron powder diffraction data found quenched LiMnTiO4 to form as single phase disordered spinel (space group Fd3̅m), whereas slowly cooled LiMnTiO4 underwent partial phase transition from Fd3̅m to P4332. The phase behavior of quenched and slowly cooled LiMnTiO4 was confirmed through variable-temperature synchrotron X-ray and neutron powder diffraction measurements. The distribution of Li between tetrahedral and octahedral sites was determined from diffraction data. Analysis of the Mn/Ti distribution in addition required Mn and Ti K-edge X-ray absorption near-edge structure spectra. These revealed the presence of Mn(3+) in primarily octahedral and Ti(4+) in octahedral and tetrahedral environments, with very slight variations depending on the synthesis conditions. Magnetic measurements indicated the dominance of antiferromagnetic interactions in both the slowly cooled and quenched samples below 4.5 K.

Controllable Thermal Rectification Realized in Binary Phase Change Composites

Phase transition is a natural phenomenon happened around our daily life, represented by the process from ice to water. While melting and solidifying at a certain temperature, a high heat of fusion is accompanied, classified as the latent heat. Phase change material (PCM) has been widely applied to store and release large amount of energy attributed to the distinctive thermal behavior. Here, with the help of nanoporous materials, we introduce a general strategy to achieve the binary eicosane/PEG4000 stuffed reduced graphene oxide aerogels, which has two ends with different melting points. It's successfully demonstrated this binary PCM composites exhibits thermal rectification characteristic. Partial phase transitions within porous networks instantaneously result in one end of the thermal conductivity saltation at a critical temperature, and therefore switch on or off the thermal rectification with the coefficient up to 1.23. This value can be further raised by adjusting the loading content of PCM. The uniqueness of this device lies in its performance as a normal thermal conductor at low temperature, only exhibiting rectification phenomenon when temperature is higher than a critical value. The stated technology has broad applications for thermal energy control in macroscopic scale such as energy-efficiency building or nanodevice thermal management.

Physico-chemical changes of ZnO nanoparticles with different size and surface chemistry under physiological pH conditions

We studied the physico-chemical properties of ZnO nanoparticles under physiological pH conditions (gastric, intestinal and plasma) as functions of their size (20 and 70nm) and surface chemistry (pristine, l-serine, or citrate coating). ZnO nanoparticles were dispersed in phosphate buffered saline under physiological pH conditions and aliquots were collected at specific time points (0.5, 1, 4, 10 and 24h) for further characterization. The pH values of the aqueous ZnO colloids at each condition were in the neutral to slightly basic range and showed different patterns depending on the original size and surface chemistry of the ZnO nanoparticles. The gastric pH condition was found to significantly dissolve ZnO nanoparticles up to 18–30wt%, while the intestinal or plasma pH conditions resulted in much lower dissolution amounts than expected. Based on the X-ray diffraction patterns and X-ray absorption spectra, we identified partial phase transition of the ZnO nanoparticles from wurtzite to Zn(OH)2 under the intestinal and plasma pH conditions. Using scanning electron microscopy, we verified that the overall particle size and morphology of all ZnO nanoparticles were maintained regardless of the pH.

Preface: Special issue dedicated to Jürgen Sprekels on the occasion of his 65th birthday

This special volume is dedicated to Jurgen Sprekels on the occasion of his 65th birthday, in tribute to his important achievements in several theoretical and applied problems, especially in the fields of Partial Differential Equations, Optimal Control, Hysteresis, Thermodynamics and Phase Transitions, Mechanics of Solids. For more information please click the “Full Text” above.

Magnetic hysteresis of cerium doped bismuth ferrite thin films

Abstract The influence of Cerium doping on the structural and magnetic properties of BiFeO 3 thin films have been investigated. Rietveld refinement of X-ray diffraction data and successive de-convolution of Raman scattering spectra of Bi 1− x Ce x FeO 3 (BCFO) thin films with x =0–0.20 reflect the single phase rhombohedral (R3c) formation for x x ≥0.08. All low wavenumber Raman modes ( −1 ) showed a noticeable shift towards higher wavenumber with increase in doping concentration, except Raman E-1 mode (71 cm −1 ), shows a minor shift. Sudden evolution of Raman mode at 668 cm −1 , manifested as A 1 -tetragonal mode, accompanied by the shift to higher wavenumber with increase in doping concentration ( x ) affirm partial structural phase transition. Anomalous wasp waist shaped ( M–H ) hysteresis curves with improved saturation magnetization ( M s ) for BCFO thin films is attributed to antiferromagnetic interaction/hybridization between Ce 4f and Fe 3d electronic states. The contribution of both hard and soft phase to the total coercivity is calculated. Polycrystalline Bi 0.88 Ce 0.12 FeO 3 thin film found to exhibit better magnetic properties with M s =15.9 emu/g without any impure phase.

Analysis of the transformation behaviors of a Chinese coal ash using in‐/ex‐situ XRD and SEM‐EXD

AbstractCrystal and phase transitions of a Chinese Datong coal ash were evaluated using room temperature X‐ray diffraction (RT‐XRD), in‐situ high temperature (HT)‐XRD, and scanning electron microscope combining energy dispersive X‐ray analysis (SEM‐EDX) as temperature varied from 300 °C to 1600 °C. The transforming behaviors of ash were related to its melting properties. Results revealed that the transforming behavior of minerals from in‐situ HT‐XRD was different with that from routine RT‐XRD. Low‐temperature Datong ash contained quartz and clays (such as kaolinite); the changes in minerals at low‐temperature region (300–1000 °C) were mainly due to the minerals decomposition and partial phase transition. Significant transition of quartz to cristobalite occurred at ca. 1100 °C; the continuous transition and reaction of silica and alumina at ca. 1200 °C accelerated the softening of the ash (ST ≈ 1290 °C). An X‐ray amorphous liquid phase was formed mainly by the eutectic solution of silica and mullite at ca. 1400 °C, which obviously affected the hemisphere temperature (HT ≈ 1490 °C) of ash; meanwhile, ultra fine alumina solid particles, which were probably derived from kaolinite, were found in the melted ash. Virtually, the fusing (FT > 1500 °C) of Datong ash was considered to be determined by the formation of solid particles and eutectic solution. Eventually, the semi‐quantitative evaluation of transformations of ash was successfully obtained through the combined methods. © 2014 Curtin University of Technology and John Wiley & Sons, Ltd.

Thermally pressure-induced partial structural phase transitions in core–shell InSb-SiO2 nanoballs/microballs: characterization, size and interface effect

Core–shell InSb-SiO2 nanoballs/microballs were synthesized on a Si substrate by carbonthermal reactions at a temperature of 900 °C. High-resolution transmission microscopy (HRTEM) images revealed that the surfaces of the InSb nanoballs/microballs were covered by amorphous SiO2 layers. On the basis of our theoretical calculation, the thermal expansion coefficient (TEC) of the InSb crystals is ten times higher than that of the SiO2 shell. Therefore, the SiO2 serves as a constraining shell for the InSb core so that the compressive stress of ∼−94 MPa can accumulate in the InSb core while a tensile stress of 196 MPa forms in the SiO2 shell. The thermal excitation accumulated compressive stress in the InSb core, causing a partial structural phase transition from a cubic zinc-blende structure to a hexagonal wurtzite structure. Many lattice defects, such as stacking faults and Moiré fringes, have been observed on the surface of the InSb core. In situ temperature-dependent XRD patterns showed that a reversible InSb hexagonal (002) peak appeared and disappeared as the temperature increased and decreased at a transit point of 200 °C, respectively. As the temperature increased, the XRD diffraction peaks of the InSb wurtzite phase shifted significantly to lower angles because of the formation of compressive stress in the InSb nanoballs. The pressure-induced partial structural phase transitions of the nanostructured InSb occurred at −94 MPa of the compressive stress. This is the first report of this value, which is the lowest value in the pressure-induced phase transition of the nanostructure InSb from the cubic zinc-blende structure to the hexagonal wurtzite structure.

Design and Observation of Biphase TiO2 Crystal with Perfect Junction.

Bicrystalline materials have wide applications from silicon chips to photocatalysis, but the controlled synthesis of nanocrytals with ordered phase junction has been challenging, in particular via chemical synesthetic routes. Here, we propose a general strategy to design biphase crystals formed via partial solid-to-solid phase transition with perfect phase junction, e.g., being atomically sharp and built of two particular sets of epitaxially joined planes of the two component phases, and present such an example by designing, synthesizing, and characterizing the interface of two TiO2 phases, namely, TiO2-B/anatase biphase nanocrystals that are obtained conveniently via one-pot chemical synthesis. Our design strategy classifies the common solid-to-solid phase transition into three types that are distinguishable by using the newly developed stochastic surface walking (SSW) method for unbiased pathway sampling. Only Type-I crystal is predicted to possess perfect phase junction, where the phase transition involves one and only one propagation direction featuring single pathway phase transition containing only one elementary kinetic step. The method is applicable for the understanding and the design of heterophase materials via partial phase transition in general.

Unusual crystal structures in the system (SnSe)nBi2Se3

As intermediates between layered tellurides and chain-like sulfides, selenides exhibit a huge variety of structures and are promising candidates for thermoelectrics. The compounds (SnSe)nBi2Se3 cover the whole range of compound classes. (SnSe)0.5Bi2Se3 exhibits the 21R-GeSb2Te4 structure type (R-3m, a = 4.172Å, c = 38.86 Å, R1(obs) = 0.0246) [1] instead of the 12P stacking expected for this stoichiometry, HRTEM indicates the presence of stacking disorder. This phase coexists with "phase X" [2]. From such samples, e.g. (SnSe)~2Bi2Se3, single crystals of new compounds could be retrieved. Their structures resemble those of minerals of the lillianite series. Slowly cooled samples near the dystectic composition [2] in the phase diagram yielded lillianite-type (SnxBi1-x)5Se6 (x ≍ 2, Cmcm, a = 4.196 Å, b = 13.83 Å, c = 21.19 Å, R1(obs) = 0.0347) mixed with (SnxBi1-x)11Se13 (x ≍ 3, C2/m, a = 13.85 Å, b = 4.205 Å, c = 23.33 Å, β = 98.70, R1(obs) = 0.0594), which is isotypic to KSn5Bi5Se13 [3]. Further a new phase (SnxBi1-x)8Se9 with heyrovskite-type structure (x ≍ 3, Cmcm, a = 4.193 Å, b = 13.87Å, c = 32.01 Å, R1(obs) = 0.0392) was obtained. These new structures consist of tropochemically twinned slabs that represent distorted cutouts of the rocksalt type. They are interconnected via cations in trigonal prisms. They are members of a homologous series that differ concerning the thickness of the slabs. The cubic high-temperature phase (SnSe)nBi2Se3 (~2.5 < n < 4) yields a metastable pseudocubic disordered rocksalt-type material (Fm-3m, a = 5.936 Å, R1(obs) = 0.0473) upon quenching. For n = 4, it contains homogeneously distributed nanoscale SnSe precipitates. Such a heterostructure might explain the low thermal conductivity (0.83 Wm-1K-1 at room temperature). The rather low electrical conductivity might be further tuned by substitution, which promises an intriguing approach for new nanostructured thermoelectrics easily accessible by partial phase transitions and exsolution effects.

Solid-to-solid polymorphic transitions in amino acid crystals

The properties of crystals are related to their structure and therefore they can be different for different polymorphs. For example, the pharmaceutical characteristics bioavailability and shelf life can depend on the formed polymorph and its stability. We aim to understand transitions between polymorphs that occur in the solid phase, with the ultimate goal to induce or inhibit these transitions. We apply Molecular Dynamics (MD) simulations as a computational microscope to study these processes at the molecular level. Our used model systems for polymorphic behavior of molecular crystals in a pharmaceutical context are amino-acid crystals, in particular DL-norleucine. The polymorphs of DL-norleucine consist of tightly packed hydrogen-bonded bilayers with straight side chains that are weakly bound through Van-der-Waals interactions. When DL-norleucine undergoes a polymorphic transition, the bilayers shift with respect to each other. In the simulations of the enantiotropically-related beta and alpha polymorph, we observe that the transformed lattice parameters and molecular properties behave identically with temperature for both polymorphs. Consequently, the polymorphs only differ in the orientation of the molecular bilayers in the a'c' and the b'c'-plane, which explains the ease of transitions between them. Moreover, in simulations of the beta polymorph at 350 K we observe partial phase transitions which we could follow with the help of specifically designed order parameters. The transitions are exclusively occurring in the b'c'-plane. This indicates a possible transformation mechanism in which first shifts of bilayers occur in this plane, followed by shifts in the a'c'-plane. Interestingly, the region of highest flexibility of the molecule shifts from the middle to the end of the carbon chain at the highest studied temperature.

Structural, optical and thermal characterization of nanostructured CdSe obtained by mechanical alloying

A CdSe nanostructured alloy was produced by high energy Mechanical Alloying (MA). The existence of three crystalline phases, two CdSe polymorphs and a small fraction of CdO phase, was revealed through X-ray powder diffraction. With continued MA process, the CdO phase was dispersed between the nanostructures of zinc blende (ZB) and wurtzite (WZ). Differential Scanning Calorimetry (DSC) measurements indicated sublimation at 122°C confirmed by thermal treatment. Raman spectroscopy revealed that the sublimated material was composed by a trigonal Se. After the thermal treatment, a partial phase transition from ZB to WZ was observed, which is in agreement with that observed by DSC, as well as by nucleation of the CdSeO3.

Dynamics of the α and β Polymorphs of dl-Norleucine at Different Temperatures: Sliding to a Partial Phase Transition

The enantiotropically related α and β polymorphs of dl-norleucine are an interesting probe for polymorphism in molecular crystals. We present the results of quantitative molecular dynamics simulations of these two polymorphs at both stable and metastable temperatures. Because of a fully flexible force field, we can judge the differences in crystal and molecular properties between the polymorphs. In simulations of the β polymorph at 350 K, we observe partial phase transitions which we could follow with the help of specifically designed order parameters. The transitions are exclusively occurring along b′. This indicates a possible transformation mechanism in which first shifts of bilayers occur in this direction, followed by shifts along a′. The transformed lattice parameters and molecular properties behave identically with temperature for the two polymorphs. Consequently, the polymorphs only differ in the orientation of the molecular bilayers, which explains the ease of transitions between them. The partial transitions consist of different types which differ in the number of interfaces involved and in the amount of distortion of the crystal. At the time scale of the simulations, we did not observe a full polymorphic phase transition.