Dimensional Nucleation Research Articles

Hysteresis and the role of nucleation and growth in the hydrogenation of Mg nanolayers

We investigated the hydrogenation of 3 and 10 nm Mg layers sandwiched between Ti using an optical transmission technique (hydrogenography). We observe in situ the two dimensional nucleation and growth of single hydride domains of up to several millimeters in diameter. The low density of nuclei points to preferential nucleation at heterogeneous sites. From an analysis of the growth kinetics we deduce an extremely large edge boundary energy, which we relate to the plastic deformations inherent to the 30% volume expansion of the MgH(2). We find that the nucleation and growth process affects the hysteresis between absorption and desorption. Especially, the absorption branch can be lowered when nucleation barriers are removed. Our results show that when discussing the effect of nano-structuring on hydrogenation it may be quite complex to distinguish the thermodynamic and kinetic effects involved.

A study on electrodeposited zinc oxide nanostructures

Zinc oxide (ZnO) nanostructures prepared by electrochemical deposition method from aqueous zinc nitrate solution at 65 °C onto fluorine doped tin oxide coated glass substrates were investigated. Characterization of ZnO nanostructures was realized using conventional electrochemical techniques, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. Cyclic voltammetry experiments were performed to elucidate the electrodic processes that occurred when potentials were applied and the optimum potential for electrodeposition were determined. From the Mott-Schottky measurements, the flat-band potential and the donor density for the ZnO nanostructure are determined. From single-step potential experiment in the potential ranges from −1.1 to −1.4 V, the formation of ZnO nuclei in the early deposition stages was proceeded according to the three dimensional (3D) instantaneous nucleation followed by diffusion-limited growth rather than a progressive one. SEM images demonstrated that the morphology of ZnO nanostructures depend greatly on the potential depositions. XRD studies revealed that the deposited films were polycrystalline in nature with wurtzite phase.

Crystallization kinetics of Se–Zn–Sb nano composites chalcogenide alloys

Crystallization kinetics could be define the working limits of the material, thereby, this work is described the kinetics of Se96Zn2Sb2 (SZS) chalcogenide alloy and its composites with 0.05% multi walled carbon nano tubes (MWCNT) and 0.05% graphene (GF). The crystallization kinetic parameters have been obtained using the Differential Scanning Calorimetric (DSC) measurement traces at 5, 10, 15 and 20°C/min heating rates, by employing the different approximations at the glass transition, onset crystallization and peak crystallization temperatures. Subsequently, Hruby Hr glass forming ability parameter, thermal stability and nucleation and growth order parameter (n) and dimensional parameter (m) of the materials have also been discussed. Outcomes demonstrate these materials have a variation in kinetic parameters with the incorporation MWCNT and GF in SZS regime. It has been noticed MWCNT composite glass transition activation energy (Eg), onset crystallization temperature (Ec), peak crystallization (Ep) higher than GF composite while lowest for the parent alloy, however, a variation in Hr and thermal stability have been obtained for the composites. The overall two-one dimensional nucleation and growth mechanism have been evaluated for these materials.

Hamilton-Jacobi Equations with Discontinuous Source Terms

We study the initial-value problem for a Hamilton-Jacobi equation whose Hamiltonian is discontinuous with respect to state variables. Our motivation comes from a model describing the two dimensional nucleation in crystal growth phenomena. A typical equation has a semicontinuous source term. We introduce a new notion of viscosity solutions and prove among other results that the initial-value problem admits a unique global-in-time uniformly continuous solution for any bounded uniformly continuous initial data. We also give a representation formula of the solution as a value function by the optimal control theory with a semicontinuous running cost function.

Non—Isothermal Differential Scanning Calorimetry Studies on Nanocrystallization Kinetics of Fe<sub>81</sub>Si<sub>3.5</sub>B<sub>13.5</sub>C<sub>2</sub> Amorphous Alloy

The nanocrystallization kinetics of the Fe81Si3.5B13.5C2amorphous alloy was investigated by differential scanning calorimetry (DSC). The apparent activation energy Ea, as well as the nucleation and growth kinetic parameters has been calculated by Kissinger and Ozawa methods. The changeable activation energy Eawith crystalline fraction α was obtained by the expended Friedman method without assuming the kinetic model function, and the average value of Eawas 364±20 kJ/mol. It was shown that the crystallization mechanism of initial stage (0<α<0.7) of the transformation was bulk crystallization with two and three dimensional nucleation graining growth which was controlled by diffusion. For the middle stage (0.7<α<0.9), the crystallization mechanism is surface crystallization with one dimensional nucleation graining growth at a near-zero nucleation rate. In the final stage(α>0.9)，the local Avrami exponents rose anomalously from 1.4 to about 2.0.

Optical properties, micro-crystallization and etching studies of an organic/inorganic hybrid nonlinear optical crystal: 1-Ethyl-3-methyl imidazolium tribromoplumbate (EMITB)

Single crystals of 1-ethyl-3-methyl imidazolium tribromoplumbate (EMITB), an organic–inorganic nonlinear optical (NLO) material, were grown by the controlled evaporation solution growth method. The grown crystals were characterized by the single-crystal diffraction and X-ray powder diffraction. Second-harmonic generation (SHG) on powder samples has been measured using Kurtz and Perry technique, which indicated that EMITB is a phase-matchable material, and its SHG coefficient is measured to be four times larger than that of KDP. The optical transmittance window and the lower cutoff wavelength of the EMITB have been identified by UV–vis–NIR studies. Its growth morphology was predicted by the Bravais–Friedel–Donnay–Harker (BFDH) model and the influence of pH values on crystal growth using slow solvent evaporation method was also investigated. The results show that both high and low pH values are disadvantageous to the growth of EMITB crystal, and EMITB crystal exhibits a regular crystalline morphology when pH value of the growth solution ranges from 3.8 to 4.0. Etching studies illustrated that the crystal grows in the two dimensional nucleation mechanism.

Non-isothermal crystallization kinetics of FeZrB amorphous alloy

The non-isothermal crystallization kinetics of amorphous Fe 78Zr 7B 15 alloy is investigated to shed light on the crystallization mechanism. The crystallization of amorphous Fe 78Zr 7B 15 alloy exhibits two distinct steps. The apparent activation energy of the first ( E a1 ) and second step crystallization ( E a2 ) are determined by Kissinger and Ozawa equations. The comparative value of E a1 < E a2 implies that the first step crystallization is easier to be occurred. The calculation based on Kissinger–Akahira–Sunose (KAS) model suggests that the local activation energy [ E( x)] decreases with crystallized volume fraction ( x). The Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation is extended to non-isothermal transformation to probe the transformation mechanism. The results show that the crystallization of the first and second step is dominated by low and three dimensional nucleation and growth, respectively. Both nucleation rates of these two steps increase firstly and then reduce.

Electrochemistry of tin in the 1-ethyl-3-methylimidazolium dicyanamide room temperature ionic liquid

The electrochemistry of Sn(II) was studied in the room temperature ionic liquid 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA) on a glassy carbon (GC) and a polycrystalline Pt electrode at 40 °C. The Sn(II) species was introduced into the ionic liquid by either dissolution of SnCl 2 or anodizing a Sn wire. The reduction potential of the Sn(II)/Sn couple produced in these two solutions was found to be different, indicating that different Sn(II) species may be present. The order of the reduction potential of the two Sn(II) species indicates that the Gutmann donor ability of the anions is likely to be DCA − > Cl −. Cyclic voltammetry indicates the stripping efficiency is >90% on the Pt but only 40% on the GC electrode. Analysis of the chronoamperometric transient behavior during electrodeposition suggests that the deposition of Sn on the GC electrode involves a three dimensional progressive nucleation on a finite number of active sites. The diffusion coefficient of SnCl 2 dissolved in the EMI-DCA was found to be 9.8 × 10 −7 cm 2 s −1 which is in the same order of magnitude as those reported for SnCl 2 in several other ionic liquids. Depending on the deposition potentials, potentiostatic electrolysis produced Sn deposits with various unusual morphologies such as hexagonal tubes, spiral nanowires, and dendrite.

Growth Kinetics and Properties of Thin Cobalt Films Electrodeposited on n-Si(100)

The cobalt thin films were directly grown on n-Si(100) substrates in a non-aqueous electrolyte of 0.1 M CoCl2 by using pulsed electrodeposition technique. In this paper, the growth, structural and magnetic properties of thin Co films have been investigated. Current transient experiments showed that Co nuclei in the early deposition stage forms through three dimensional (3D) instantaneous nucleation followed by diffusion-limited growth. From the atomic force microscopy (AFM) analysis, it has been found that the film surface consisted of cubic and distorted rectangular-shaped nanocrystallites with the average size of about 140 nm irrespective of the deposition voltage. X-ray diffraction (XRD) measurement indicated that Co was arranged with only a hexagonal close-packed (hcp) structure not involving face-centered cubic (fcc) Co or silicide phases. Vibrating sample magnetometer (VSM) measurements revealed that the easy magnetization axis of Co thin film is parallel to the film plane.

Oscillatory growth of nonfaceted dendrite and faceted plate during unidirectional solidification

Nonfaceted dendrite and faceted plate in succinonitrile-0.7 wt.% salol and camphor-47.4 wt.% and −35 wt.% naphthalene mixtures were in situ observed during unidirectional solidification. Nonfaceted dendrite oscillates in the growth direction during unidirectional solidification, alternatively repeating fast and slow growth. The faceted phase, whose growth is operated by a two dimensional nucleation mode, also shows oscillation of growth velocity. The oscillation in the faceted phase is due to the intrinsic growth nature, while in the nonfaceted phase it is due to experimental artifacts, that is, thermal fluctuations in the cold and hot zones. The implications of the observed dendrite tip fluctuation in relation with the initiation of dendrite sidebranching have been discussed.

Nonequilibrium fluctuations in micro-MHD effects on electrodeposition

In copper electrodeposition under a magnetic field parallel to electrode surface, different roles of two kinds of nonequilibrium fluctuations for micro-magnetohydrodynamic (MHD) effects are discussed; symmetrical fluctuations are accompanied by the suppression of three dimensional (3D) nucleation by micro-MHD flows (the 1st micro-MHD effect), whereas asymmetrical fluctuations controlling 2D nucleation yield secondary nodules by larger micro-MHD flows (the 2nd micro-MHD effect). Though the 3D nucleation with symmetrical fluctuations is always suppressed by the micro-MHD flows, due to the change in the rate-determining step from electron transfer to mass transfer, the 2D nucleation with asymmetrical fluctuations newly turns unstable, generating larger micro-MHD flows. As a result, round semi-spherical deposits, i.e., secondary nodules are yielded. Using computer simulation, the mechanism of the 2nd micro-MHD effect is validated.

Characterization of crystal growth using a spiral nucleation model

Classical concepts of two-dimensional nucleation and spiral growth are used together with recent findings on the dynamics of dislocation spirals to derive a new crystal growth model. Initial growth nuclei result from the organization of adsorbed molecules in spirals around surface dislocations. The energetic barrier for the activation of the spiral nuclei is considerably lower than the admitted by classical two dimensional nucleation models. Stable nuclei evolve into bigger growth hillocks in supersaturated media through the incorporation of adsorbed units into their steps. The displacement velocity of steps during solution and vapour growth is calculated by different kinetic approaches, taking into consideration the distinct role of surface diffusion in each process, and avoiding known limitations of conventional theories. A generalized expression is obtained relating the crystal growth rate with main variables such as supersaturation, temperature, crystal size, surface topology and interfacial properties. At the end of the paper, the crystallization kinetics of sucrose measured at 40 °C is interpreted in the light of the new perspectives resulting from the proposed model. The application example illustrates how to estimate interfacial and topological properties from the experimental crystal growth results.

Growth Kinetics of Fe Films Electrodeposited on n-Si(111)

Iron thin films were grown directly on n-Si(111) substrates by pulsed electrodeposition in a non-aqueous 0.1 M FeCl2 electrolyte solution. In this solution, Fe2+/Si interface showed a good diode behavior by forming a Schottky barrier. From single-step potential experiment in the potential ranges from 1.0 to 1.4 V, the formation of Fe nuclei in the early deposition stages was proceeded according to the three dimensional (3D) instantaneous nucleation followed by diffusion-limited growth rather than a progressive one. When depositing Fe using pulse potentials with the maximum voltage of 1.0 and 1.5 V, it was found that nucleation mechanism of Fe is similar with the results for the case of single-step potential experiment. However, nucleation during depositing Fe at 2.0 V initiated according to the progressive mechanism. Fe thin films, which obtains from pulse potential of 1.4 V with a frequency of 300 Hz, revealed a highly oriented columnar structures perpendicular to the surface of Si(111) and that only the single phase α-Fe(110) was grown on Si(111) substrate.

How do mineral coatings affect dissolution rates? An experimental study of coupled CaCO 3 dissolution—CdCO 3 precipitation

Coupled CaCO 3 dissolution-otavite (CdCO 3) precipitation experiments have been performed to 1) quantify the effect of mineral coatings on dissolution rates, and 2) to explore the possible application of this coupled process to the remediation of polluted waters. All experiments were performed at 25°C in mixed-flow reactors. Various CaCO 3 solids were used in the experiments including calcite, aragonite, and ground clam, mussel, and cockle shells. Precipitation was induced by the presence of Cd(NO 3) 2 in the inlet solution, which combined with aqueous carbonate liberated by CaCO 3 dissolution to supersaturate otavite. The precipitation of an otavite layer of less than 0.01 μm in thickness on calcite surfaces decreases its dissolution rate by close to two orders of magnitude. This decrease in calcite dissolution rates lowers aqueous carbonate concentrations in the reactor such that the mixed-flow reactor experiments attain a steady-state where the reactive fluid is approximately in equilibrium with otavite, arresting its precipitation. In contrast, otavite coatings are far less efficient in lowering aragonite, and ground clam, mussel, and cockle shell dissolution rates, which are comprised primarily of aragonite. A steady-state is only attained after the precipitation of an otavite layer of 3–10 μm thick; the steady state CaCO 3 dissolution rate is 1–2 orders of magnitude lower than that in the absence of otavite coatings. The difference in behavior is interpreted to stem from the relative crystallographic structures of the dissolving and precipitating minerals. As otavite is isostructural with respect to calcite, it precipitates by epitaxial growth directly on the calcite, efficiently slowing dissolution. In contrast, otavite’s structure is appreciably different from that of aragonite. Thus, it will precipitate by random three dimensional heterogeneous nucleation, leaving some pore space at the otavite-aragonite interface. This pore space allows aragonite dissolution to continue relatively unaffected by thin layers of precipitated otavite. Due to the inefficiency of otavite coatings to slow aragonite and ground aragonite shell dissolution, aragonite appears to be a far better Cd scavenging material for cleaning polluted waste waters.

PREPARATION OF SnO2 NANOWIRES SYNTHESIZED BY VAPOR-SOLID MODE AND ITS GROWTH MECHANISM

Single crystal SnO2 nanowires with diameters of 20 ~ 100nm have fabricated by the reaction of vapor SnO with the oxygen. Vapor-Solid (VS) mechanism is applied to explain to the growth process. Scan electron microscopy (SEM), Transmission electron microscopy (TEM), Selected area diffraction (SAED) and High resolution transmission electron microscopy (HRTEM) studies show that the SnO2 nanowires grow along [100] or [010] orientation, which is not in agreement with the symmetry of SnO2 crystal structure. We prove that the low vapor saturation is the key growth condition of the SnO2 nanowires which prevents the two dimensional nucleation on the side faces and causes one dimensional large aspect ratio.

On the adsorption and condensed film formation of binary mixtures of dodecyl-, tetradecyl-, hexadecyl-, and octadecyl-trimethylammonium bromides at the mercury–electrolyte interface

The adsorption and condensed film formation on mercury at the negative potential region for binary mixtures of dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB), octadecyltrimethylammonium bromide (OTAB) is studied in KBr at various temperatures from 5 to 45 °C. The formation of the CTAB condensed film is hindered with the addition of DTAB and TTAB. There are interactions between unlike hydrophobic chains. The strong interactions between the CTAB molecules do not take place when DTAB or TTAB is present above a certain concentration. This hindering is more pronounced in the case of TTAB compared to the same DTAB concentration, i.e. the increase of the chain length hinders the film formation. The initially adsorbed molecules play a templating role in the kinetics of the film formation and in the self-assembling of the molecules. The initial induction time strongly depends on the temperature. The less surface active CTAB can hinder the OTAB film formation in binary mixtures. Also, increased interaction between OTAB and CTAB can be observed, indicating synergy effects in the film formation in some cases. The temperature range that the film is formed can be changed using mixtures of surfactants. Thus, the development of the film can become impossible, more difficult or even easier. Hysteresis phenomena are observed. The capacity versus time curves in the case that condensed film is formed are treated with the Avrami plot formulation, giving values between 1.5 and 2 indicating a progressive one dimensional nucleation with constant growth rate or a decrease of the nucleation rate during the overall film formation. There is generally a marked effect of the chain length of the alkyl chain on the film formation.

Low temperature treatment of the (001) ZnTe substrate surface with the assist of atomic hydrogen

Recent success in the fabrication of the bulk ZnTe has enabled the realization of large area single crystal substrates. The growth and application of ZnTe homoepitaxial layers by molecular beam epitaxy should involve the pretreatment of the substrate surface prior to the nucleation. An atomic hydrogen treatment was performed to remove the surface oxide of ZnTe. The oxide could be removed at around 100 °C with the assist of the atomic hydrogen beam. The reconstructed surface could be achieved by annealing at around 230 °C. The two dimensional nucleation was also confirmed. The homoepitaxial layer has shown such a high quality that interface, or defects could not be confirmed by the transmission electron microscopy.

Electrochemical behavior of lanthanum and yttrium ions in two molten chlorides with different oxoacidic properties: The eutectic LiCl-KCl and the equimolar mixture CaCl2-NaCl

The electrochemical behavior of LaCl3 and YCl3 was studied in two molten chloride mixtures with different oxoacidic properties, the eutectic LiCl-KCl and the equimolar CaCl2-NaCl melt at different temperatures. The stable oxidation states of both elements have been found to be (III) and (0) in both melts, and it was found that both La(III) and Y(III) cations were less solvated by the chloride ions in the calcium-based melt, which was explained by the stability of CaCl4 2- ions in that melt. Transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry and chronoamperometry were used in order to study the reaction mechanism and the transport parameters of electro active species at a tungsten electrode. The results showed that in the eutectic LiCl-KCl, the electro crystallization of lanthanum and yttrium seems to be the controlling electrochemical step while in CaCl2-NaCl this phenomenon has not been observed. That was explained in terms of the differences in the physicochemical properties of the systems, especially interfacial tensions. In the eutectic LiCl-KCl chronoamperometric studies indicated instantaneous and three dimensional nucleation and crystal growth of lanthanum and yttrium whatever the applied over potential of the rare earth metal is, whereas in the equimolar mixture CaCl2-NaCl, the corresponding electrochemical exchanges were found to be quasi-reversible, and the values of the kinetic parameters, K0 and ?,were obtained for both reactions. Mass transport towards the electrode is a simple diffusion process, and the diffusion coefficients have been calculated. The validity of the Arrhenius law was also verified by plotting the variation of the logarithm of the diffusion coefficient versus 1/T.

Arsenic flux dependence of island nucleation on InAs(001).

The initial stages of InAs(001) homoepitaxial growth are investigated using a combination of kinetic Monte Carlo simulations based on ab initio density functional theory and scanning tunneling microscopy. In the two dimensional island nucleation mode investigated, the island number density is found to decrease with increasing As. This behavior is explained by a suppression of the effective In-adatom density leading to a reduction in island nucleation. The relevant microscopic processes responsible for this reduction are identified.

Kinetics of Electrocrystallization of PbO2 on Glassy Carbon Electrodes. Influence of the Electrode Rotation

Electrodeposition of PbO2 on glassy carbon electrodes was studied in the stationary conditions and on the rotating disk electrode using chronoamperometric technique. The kinetics of electrodeposition was significantly retarded by the hydrodynamic mass transport flow. This observation is in agreement with the mechanism involving formation of soluble Pb(III) or Pb(IV) species, which may, subsequently, form PbO2. The kinetics of the nucleation and crystal growth was determined by the analysis of the initial part of the transients and of the whole curves. This process proceeds through the 3 dimensional progressive nucleation and crystal growth mechanism with an induction time. The growth rate constant is strongly affected by convection while the nucleation process and the induction time are much less influenced. * Corresponding author. Tel.: +34965903628; fax:+34965903537;e-mail:vicente.montiel@ua.es