scholarly journals Nucleation and Post-Nucleation Growth in Diffusion-Controlled and Hydrodynamic Theory of Solidification

Crystals ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 437
Author(s):  
Frigyes Podmaniczky ◽  
László Gránásy
Keyword(s):  
Bond Order ◽  
Single Mode ◽  
Amorphous Structure ◽  
Hydrodynamic Theory ◽  
Cubic Phase ◽  
Diffusion Controlled ◽  
Amorphous Medium ◽  
Diffusive Dynamics ◽  
Crystal Model ◽  
Average Bond

Two-step nucleation and subsequent growth processes were investigated in the framework of the single mode phase-field crystal model combined with diffusive dynamics (corresponding to colloid suspensions) and hydrodynamical density relaxation (simple liquids). It is found that independently of dynamics, nucleation starts with the formation of solid precursor clusters that consist of domains with noncrystalline ordering (ringlike projections are seen from certain angles), and regions that have amorphous structure. Using the average bond order parameter q¯6, we distinguished amorphous, medium range crystallike order (MRCO), and crystalline local orders. We show that crystallization to the stable body-centered cubic phase is preceded by the formation of a mixture of amorphous and MRCO structures. We have determined the time dependence of the phase composition of the forming solid state. We also investigated the time/size dependence of the growth rate for solidification. The bond order analysis indicates similar structural transitions during solidification in the case of diffusive and hydrodynamic density relaxation.

Kinetics of hexagonal–cubic phase transformation of zinc sulfide in vacuo, in zinc vapor, and in sulfur vapor

1968 ◽  
Vol 46 (18) ◽  
pp. 2881-2886 ◽  
Author(s):  
Tamas Bansagi ◽  
E. A. Secco ◽  
O. K. Srivastava ◽  
Ronald R. Martin
Keyword(s):  
Phase Transformation ◽  
Zinc Sulfide ◽  
Solid Phase ◽  
Cubic Phase ◽  
Transformation Rate ◽  
Zinc Vapor ◽  
Sulfur Vapor ◽  
Diffusion Controlled ◽  
Transformation Reaction ◽  
Kinetics Of

The kinetics of the hexagonal–cubic phase transformation of zinc sulfide have been studied in vacuo, in zinc vapor, and in sulfur vapor in the temperature range 800–900 °C by a powder X-ray diffraction technique. The transformation rate is enhanced in the presence of sulfur and zinc vapor. In vacuo and in sulfur vapor the reaction is nucleation-controlled with activation energies of 95.0 and 98.5 ± 5 kcal, respectively. In zinc vapor the reaction is diffusion-controlled with the diffusion coefficient expressed as[Formula: see text]The modes of catalytic action by sulfur and zinc vapor on the solid phase transformation reaction are discussed.

scholarly journals The Kinetic Peculiarities of the Nanocrystallization of Amorphous Alloys Fe84Nb2B14, which are Doped by Rare Earth Metals

2017 ◽  
Vol 18 (1) ◽  
pp. 122-128 ◽  
Author(s):  
L. Boichyshyn ◽  
M.-O. Danyliak ◽  
B. Kotur ◽  
T. Mika
Keyword(s):  
Amorphous Alloys ◽  
Rare Earth Metals ◽  
Amorphous Structure ◽  
Differential Scanning Calorimetric ◽  
X Ray Diffraction ◽  
X Ray ◽  
Dimensional Diffusion ◽  
Diffusion Controlled ◽  
Dsc Method ◽  
Xrd Method

The thermal stability and crystallization of the Fe82Nb2B14RE2 (RE = Y, Gd, Tb, Dy) amorphous alloys were investigated by differential scanning calorimetric (DSC) method. By X-ray diffraction (XRD) method has been established that the initial AMA have amorphous structure. The RE alloying of Fe82Nb2B14RE2 amorphous alloys increase the nanocrystallization temperatures for ~ 110 K and activation energies of crystallization for ~ 330 kJ/mol. The Avrami constant was found to be 1.86 for Fe84Nb2B14 at 703 K, 1.17 for Fe82Nb2B14Y2 at 813 K, 1.36 for Fe82Nb2B14Gd2 at 808 K, 1.76 for Fe82Nb2B14Tb2 at 808 K and 1.92 for Fe82Nb2B14Dy2  at 808 K. Two-dimensional diffusion controlled growth mechanism with decreasing nucleation rate was observed in the alloys.

scholarly journals Diffusion in generalized hydrodynamics and quasiparticle scattering

2019 ◽  
Vol 6 (4) ◽  
Author(s):  
Jacopo De Nardis ◽  
Denis Bernard ◽  
Benjamin Doyon
Keyword(s):  
Steady State ◽  
Correlation Functions ◽  
Spin Diffusion ◽  
Diffusion Constant ◽  
Hydrodynamic Theory ◽  
State Diffusion ◽  
Diffusive Dynamics ◽  
Interacting Systems ◽  
Quasiparticle Scattering ◽  
Definition Of

We extend beyond the Euler scales the hydrodynamic theory for quantum and classical integrable models developed in recent years, accounting for diffusive dynamics and local entropy production. We review how the diffusive scale can be reached via a gradient expansion of the expectation values of the conserved fields and how the coefficients of the expansion can be computed via integrated steady-state two-point correlation functions, emphasising that {\mathcal PT}𝒫T-symmetry can fully fix the inherent ambiguity in the definition of conserved fields at the diffusive scale. We develop a form factor expansion to compute such correlation functions and we show that, while the dynamics at the Euler scale is completely determined by the density of single quasiparticle excitations on top of the local steady state, diffusion is due to scattering processes among quasiparticles, which are only present in truly interacting systems. We then show that only two-quasiparticle scattering processes contribute to the diffusive dynamics. Finally we employ the theory to compute the exact spin diffusion constant of a gapped XXZ spin-1/2−1/2 chain at finite temperature and half-filling, where we show that spin transport is purely diffusive.

Synthesis of Green-Emitting Phosphors by Bicontinuous Cubic Phase Process

2008 ◽  
Vol 47-50 ◽  
pp. 658-661
Author(s):  
Wen Chen Chien ◽  
Yang Yen Yu ◽  
Chien Yu Chien
Keyword(s):  
Yttrium Oxide ◽  
Spherical Shape ◽  
Amorphous Structure ◽  
Precursor Powder ◽  
Cubic Phase ◽  
Body Centered Cubic ◽  
Yttrium Hydroxide ◽  
Eds Analysis ◽  
Novel Approach ◽  
Bicontinuous Cubic

In this study, a novel approach for preparation of terbium-doped yttrium oxide phosphors (Y2O3: Tb) by using the bicontinuous cubic phase (BCP) process was reported. EDS analysis revealed the homogeneous precipitation occurred in the BCP structure. The experimental results showed that the prepared precursor powder was yttrium hydroxide with a spherical shape, primary size 30-50nm, and amorphous structure. High crystallinity phosphors with body-centered cubic structure were obtained after heat treatment above 700oC for 4h. However, the primary size of phosphors grew to 100-200nm and the dense agglomerates with a size below 1µm were formed during the calcination. The photoluminescence analysis showed that the obtained Y2O3: Tb phosphor had a strong green emitting at 544nm. The strongest emitting intensity occurred at Tb concentration of 1mol%. This study indicated that the BCP process could be used to prepare the highly efficient yttrium oxide-based green emitting phosphors.

scholarly journals Diffusion-Controlled Anisotropic Growth of Stable and Metastable Crystal Polymorphs in the Phase-Field Crystal Model

2009 ◽  
Vol 103 (3) ◽  
Author(s):  
G. Tegze ◽  
L. Gránásy ◽  
G. I. Tóth ◽  
F. Podmaniczky ◽  
A. Jaatinen ◽  
...  
Keyword(s):  
Phase Field ◽  
Anisotropic Growth ◽  
Diffusion Controlled ◽  
Crystal Model ◽  
Phase Field Crystal

scholarly journals Fe-doped effects on phase transition and electronic structure of CeO2 under compressed conditions from ab initio calculations

2021 ◽  
Vol 127 (10) ◽  
Author(s):  
Karnchana Sathupun ◽  
Komsilp Kotmool ◽  
Prutthipong Tsuppayakorn-aek ◽  
Prayoonsak Pluengphon ◽  
Arnab Majumdar ◽  
...  
Keyword(s):  
Phase Transition ◽  
High Pressure ◽  
Electronic Structure ◽  
Ab Initio ◽  
Orthorhombic Phase ◽  
Cubic Phase ◽  
Fe Doping ◽  
Lattice Constants ◽  
High Pressure Phase Transition ◽  
Average Bond

AbstractAb initio study of high-pressure phase transition and electronic structure of Fe-doped CeO2 with Fe concentrations of 3.125, 6.25, and 12.5 at% has been reported. At a constant-pressure consideration, the lattice constants and the volume of the supercell were decreased with an increasing concentration of Fe. The average bond length of Fe–O is lower than that of Ce–O. As a result, Fe doping induces the reduced volume of the cell, which is in good agreement with previous experiments. At high pressure (~ 30 GPa), it was found that the transition pressure from the fluorite to the cotunnite orthorhombic phase decreases at a higher concentration of Fe, indicating that the formation energy of the compound is induced by Fe-doping. Furthermore, compression leads to interesting electronic properties too. Under higher pressures, the bandgap increases in the cubic structure under compression and then suddenly plummets after the transition to the orthorhombic phase. The 3d states of Fe mainly induced the impurity states in the bandgap. In both the undoped and Fe-doped systems, the bandgap increased in the cubic phase at high pressure, while the gap and p-d hybridization decrease in the orthorhombic phase.

Observations on the Structure of Ta2O5

1972 ◽  
Vol 30 ◽  
pp. 540-541
Author(s):  
P. S. Kotval ◽  
C. J. Dewit
Keyword(s):  
Electron Microscope ◽  
Structural Features ◽  
Amorphous Structure ◽  
Polished Surface ◽  
Distilled Water ◽  
Anodic Films ◽  
Amorphous Films ◽  
Beam Heating ◽  
Diffraction Patterns ◽  
Hexagonal Arrays

The structure of Ta2O5 has been described in the literature in several different crystallographic forms with varying unit cell lattice parameters. Earlier studies on films of Ta2O5 produced by anodization of tantalum have revealed structural features which are not consistent with the parameters of “bulk” Ta2O5 crystalsFilms of Ta2O5 were prepared by anodizing a well-polished surface of pure tantalum sheet. The anodic films were floated off in distilled water, collected on grids, dried and directly examined in the electron microscope. In all cases the films were found to exhibit diffraction patterns representative of an amorphous structure. Using beam heating in the electron microscope, recrystallization of the amorphous films can be accomplished as shown in Fig. 1. As suggested by earlier work, the recrystallized regions exhibit diffraction patterns which consist of hexagonal arrays of main spots together with subsidiary rows of super lattice spots which develop as recrystallization progresses (Figs. 2a and b).

A TEM investigation of a hyper-eutectic lead-tin alloy

1988 ◽  
Vol 46 ◽  
pp. 562-563
Author(s):  
John A. Sutliff
Keyword(s):  
Volume Diffusion ◽  
Eutectic Mixture ◽  
Precipitation Reaction ◽  
Directionally Solidified ◽  
X Ray Diffraction ◽  
X Ray ◽  
Transmission Electron ◽  
Diffusion Controlled ◽  
Discontinuous Precipitation Reaction ◽  
Controlled Precipitation

Near-eutectic Pb-Sn alloys are important solders used by the electronics industry. In these solders, the eutectic mixture, which solidifies last, is the important microstructural consituent. The orientation relation (OR) between the eutectic phases has previously been determined for directionally solidified (DS) eutectic alloys using x-ray diffraction or electron chanelling techniques. In the present investigation the microstructure of a conventionally cast, hyper-eutectic Pb-Sn alloy was examined by transmission electron microscopy (TEM) and the OR between the eutectic phases was determined by electron diffraction. Precipitates of Sn in Pb were also observed and the OR determined. The same OR was found in both the eutectic and precipitation reacted materials. While the precipitation of Sn in Pb was previously shown to occur by a discontinuous precipitation reaction,3 the present work confirms a recent finding that volume diffusion controlled precipitation can also occur.Samples that are representative of the solder's cast microstructure are difficult to prepare for TEM because the alloy is multiphase and the phases are soft.

Spherulite Structures in a Melt Spun Fe-Ni Austenitic Steel

1985 ◽  
Vol 43 ◽  
pp. 52-53
Author(s):  
G. M. Michal ◽  
T. K. Glasgow ◽  
T. J. Moore
Keyword(s):  
Austenitic Steel ◽  
Room Temperature ◽  
Elevated Temperatures ◽  
Large Range ◽  
Amorphous Structure ◽  
Nucleation And Growth ◽  
Ni Alloys ◽  
Melt Spun ◽  
Crystal Fibers ◽  
Rapidly Solidified

Large additions of B to Fe-Ni alloys can lead to the formation of an amorphous structure, if the alloy is rapidly cooled from the liquid state to room temperature. Isothermal aging of such structures at elevated temperatures causes crystallization to occur. Commonly such crystallization pro ceeds by the nucleation and growth of spherulites which are spherical crystalline bodies of radiating crystal fibers. Spherulite features were found in the present study in a rapidly solidified alloy that was fully crysstalline as-cast. This alloy was part of a program to develop an austenitic steel for elevated temperature applications by strengthening it with TiB2. The alloy contained a relatively large percentage of B, not to induce an amorphous structure, but only as a consequence of trying to obtain a large volume fracture of TiB2 in the completely processed alloy. The observation of spherulitic features in this alloy is described herein. Utilization of the large range of useful magnifications obtainable in a modern TEM, when a suitably thinned foil is available, was a key element in this analysis.

Use of fractals to describe microstructures of porous thick-film platinum electrodes

1992 ◽  
Vol 50 (1) ◽  
pp. 314-315
Author(s):  
Steven D. Toteda
Keyword(s):  
Fractal Dimension ◽  
Power Plants ◽  
Electrical Response ◽  
Cubic Phase ◽  
Platinum Electrodes ◽  
Fine Grained ◽  
Impedance Response ◽  
Platinum Particles ◽  
Energy Surfaces ◽  
Highly Porous

Zirconia oxygen sensors, in such applications as power plants and automobiles, generally utilize platinum electrodes for the catalytic reaction of dissociating O2 at the surface. The microstructure of the platinum electrode defines the resulting electrical response. The electrode must be porous enough to allow the oxygen to reach the zirconia surface while still remaining electrically continuous. At low sintering temperatures, the platinum is highly porous and fine grained. The platinum particles sinter together as the firing temperatures are increased. As the sintering temperatures are raised even further, the surface of the platinum begins to facet with lower energy surfaces. These microstructural changes can be seen in Figures 1 and 2, but the goal of the work is to characterize the microstructure by its fractal dimension and then relate the fractal dimension to the electrical response. The sensors were fabricated from zirconia powder stabilized in the cubic phase with 8 mol% percent yttria. Each substrate was sintered for 14 hours at 1200°C. The resulting zirconia pellets, 13mm in diameter and 2mm in thickness, were roughly 97 to 98 percent of theoretical density. The Engelhard #6082 platinum paste was applied to the zirconia disks after they were mechanically polished ( diamond). The electrodes were then sintered at temperatures ranging from 600°C to 1000°C. Each sensor was tested to determine the impedance response from 1Hz to 5,000Hz. These frequencies correspond to the electrode at the test temperature of 600°C.

Sign in / Sign up

Export Citation Format

Share Document