Formation Of Crystalline Nuclei Research Articles

Nucleation Work on Curved Substrates

Nucleation is the initial phase transition process when nuclei of a new phase form within an undercooled or supersaturated parent phase under appropriate conditions. Nucleation most often occurs through a heterogeneous process on active centers on which the probability of nucleus formation is high. In general, the origin of active centers may be difficult to distinguish. In this work, we consider the formation of crystalline nuclei in a melt on various curved substrates. Knowledge of excess free energy plays a key role in understanding the process of formation of clusters and it is not easy to express this quantity in a considered system. Excess free energy is often approximated within the framework of capillarity approximation based on interfacial energy, which depends on interatomic interactions near the interface, as well as the misfit between melts, surface roughness, temperature, composition, etc., near the phase interface. The formation of nuclei requires overcoming a certain energy (nucleation) barrier that is a consequence of balancing the volume and the interfacial free energy. Knowing the nucleation barrier (W) is crucial for understanding this process, as nuclei predetermine the physical properties of a newly formed phase. W is typically expressed as a function of the nucleus radius; however, in nucleation kinetics, one needs to determine (W) as a function of the number of molecules forming the nucleus. We analyze nucleation work on various substrates (flat, convex, and concave) for crystallization from an aluminum melt to show that the formation of nuclei is the most probable on concave substrates. An analytical expression for W can be easily applied to other systems under consideration. We show that under the same conditions, the critical radius of nuclei is identical for various substrate, in contrast with the critical number of molecules forming a nucleus.

Crystallization of Aluminum droplet at low undercooling: homogeneous nucleation approach

Crystallization in an undercooled Aluminum (Al) droplet occurs via the formation of crystalline nuclei by homogeneous or heterogeneous nucleation and successive growth of nuclei. Clusters of a new phase appear in the system due to fluctuations and after reaching a critical size grow up to microscopic size. Crystallization event at low undercooling 6.4°C of 9.91 mg in compact Al sample was detected using isothermal differential scanning calorimetry (DSC) after several tenths of minutes. In this case, standard analysis based on Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is not appropriate as Avrami parameter n obtained by fitting of DSC data is too low (n ≤ 1 and thus the dimensionality of the system d = n – 1 ≤ 0). Al growth rate is extremely high and that is why we presume that the time delay of crystallization, detected by DSC, is slightly higher than nucleation time delay. A homogeneous nucleation model is applied to determine the basic characteristics of nucleation: the size distribution of nuclei, nucleation rate, total number of nuclei and crystallization fraction at the nucleation process. The number of atoms in a liquid Aluminum droplet decreases with time as the formation of a new crystalline phase occurs. As a consequence, a decrease in stationary nucleation occurs. It is shown that decrease in the number of atoms in a liquid Al droplet is predominantly caused by formation of subcritical clusters.

Synthesis of Co-Containing Fe3O4 Particles for Magnetic Themoablation

We developed cobalt-containing Fe3O4 particles whose particle sizes and magnetic properties were precisely controlled so that they could be used as hysteresis-loss heating materials for thermoablation in cancer therapy. The (Co, Fe)3O4 particles were synthesized through the formation of crystalline nuclei from an alkali solution containing Co2+, Fe2+, and Fe3+ ions, and then through crystalline growth using hydrothermal treatment. The coercivity was adjusted in the range from 150 to 490 Oe by changing the Co content. The saturation magnetization was in the range of 76-80 emu/g. The crystalline size obtained from the (311) plane by XRD was almost constant at about 18 nm. The particle size observed in TEM photos was about 20 nm, irrespective of the cobalt content. SiO2 was coated on the surface of the (Co, Fe)3O4 particles to add dispersibility in a water medium. The dispersant of the SiO2-coated (Co, Fe)3O4 particles showed excellent dispersibility, and no precipitation of particles was observed after the solution had been standing for one month.

Size control of magnetite nanoparticles in hydrothermal synthesis by coexistence of lactate and sulfate ions

The size of magnetite nanoparticles was controlled by means of coexistence effects of anions in the starting solution of organic solvent-free hydrothermal synthesis via coprecipitation. As the coexisting anions, lactate ion, which forms relatively stable complex with ferric ion, and sulfate ion were used. The hydrothermal synthesis was carried out at various concentrations of lactate and sulfate ions. The obtained magnetite nanoparticles were characterized by a scanning electron microscope, transmission electron microscope, powder X-ray diffractometer, dynamic light scattering particle size analyzer and superconducting quantum interference device. The formation mechanism of magnetite nanoparticles in this system was investigated based on the experimental results. The coexisting anions remarkably influenced both formation of crystalline nuclei and dispersion stabilization of formed precipitates. The particle size depended strongly on the concentration of lactate and sulfate ions. It has been clarified that lactate ion has the effect to decrease the particle size and sulfate ion promotes the particle growth. By adjusting the concentration of lactate and sulfate ions properly as the operating factor, we could control successfully the particle size from 9.5 to 38.6 nm in median size.

Polytetrahedral Nature of the Dense Disordered Packings of Hard Spheres

We study the structure of numerically simulated hard sphere packings at different densities by investigating local tetrahedral configurations of the spheres. Clusters of tetrahedra adjacent by faces present relatively dense aggregates of spheres atypical for crystals. The number of spheres participating in such polytetrahedral configurations increases with densification of the packing, and at the Bernal's limiting density (the packing fraction around 0.64) all spheres of the packing become involved in such tetrahedra. Thus the polytetrahedral packing cannot provide further increase in the density, and alternative structural change (formation of crystalline nuclei) begins henceforth.

Spinodal for the solution-to-crystal phase transformation

The formation of crystalline nuclei from solution has been shown for many systems to occur in two steps: the formation of quasidroplets of a disordered intermediate, followed by the nucleation of ordered crystalline embryos within these droplets. The rate of each step depends on a respective free-energy barrier and on the growth rate of its near-critical clusters. We address experimentally the relative significance of the free-energy barriers and the kinetic factors for the nucleation of crystals from solution using a model protein system. We show that crystal nucleation is 8-10 orders of magnitude slower than the nucleation of dense liquid droplets, i.e., the second step is rate determining. We show that at supersaturations of three or four k(B)T units, crystal nuclei of five, four, or three molecules transform into single-molecule nuclei, i.e., the significant nucleation barrier vanishes below the thermal energy of the molecules. We show that the main factor, which determines the rate of crystal nucleation, is the slow growth of the near-critical ordered clusters within the quasidroplets of the disordered intermediate. Analogous to the spinodal in supersaturated fluids, we define a solution-to-crystal spinodal from the transition to single-molecule crystalline nuclei. We show that heterogeneous nucleation centers accelerate nucleation not only because of the wettinglike effects that lower the nucleation barrier, as envisioned by classical theory, but by helping the kinetics of growth of the ordered crystalline embryos.

On the control of microstructure in rapidly solidified Nd–Fe–B alloys through melt treatment

Rapidly solidified (RS) Nd 2Fe 14B alloys were prepared by melt-spinning under different melt treatment conditions i.e., the melt temperature was varied prior to ejection onto the quenching wheel. X-ray diffraction, transmission electron microscopy, thermal analysis and magnetic measurement were conducted on the as-quenched alloys to investigate their structural and magnetic characteristics. RS Nd 2Fe 14B alloys may display a variety of microstructures depending upon the thermal history of the melt before ejection: it was possible to synthesize an entirely amorphous structure, a partially amorphous structure containing nuclei and/or nanophases and a nanocrystalline structure. The relationship between the formation of crystalline nuclei or nanophases and the thermal history of the melt was studied. A lower melt ejection temperature produced a nanocrystalline microstructure, while higher melt ejection temperatures ( T>1723 K) largely eliminated the presence of nuclei and associated nanophases and produced an amorphous product. The experimental results indicated that optimization of the melt treatment conditions will produce rapidly solidified Nd–Fe–B alloys with a more uniform microstructure.

A characteristic feature of crystalline thin-film growth of Si on a 7 × 7 superlattice surface of Si(111)

A Si film has been grown on a 7 × 7 superlattice surface of a Si(111) substrate at 250 and 380°C. Reflection high-energy electron diffraction patterns of the Si film at 250°C consist of a diffuse 2 × 2 structure and a weak 7 × 7 structure at 380°C. The experimental results indicate that the initial growth of crystalline nuclei is caused by the crystallization of amorphous-like islands at 250°C and by the formation of an island composed of three 5 × 5 structural units with a corner hole at their center at 380°C. The formation of crystalline nuclei is discussed.