Details Of Nucleation Research Articles

Nucleation Kinetics and Molecular Mechanism in Transition-Metal Nanoparticle Formation: The Intriguing, Informative Case of a Bimetallic Precursor, {[(1,5-COD)IrI·HPO4]2}2–

Following a summary of the relevant literature on nucleation and applicable nucleation and growth models and mechanisms, the kinetics and molecular mechanism of nucleation are investigated in detail starting from [(1,5-COD)IrI(NCCH3)2][BF4], 1, which upon addition of HPO42– to 1 forms a neutral, phosphate-bridged species, {[(1,5-COD)IrI(NCCH3)]2·HPO4}0, 2, en route to {[(1,5-COD)IrI·HPO4]2[Bu4N]2}, 3. Post a list of the seven advantages of the {[(1,5-COD)IrI·HPO4]2}2– precatalyst as a novel bimetallic precursor for kinetic and mechanistic studies of nucleation, six important, previously unanswered questions are raised about the ill-understood but exceedingly broad and important topic of nucleation. 1H NMR solution speciation and Signer apparatus solution molecular weight studies establish that in situ prepared {[(1,5-COD)IrI·HPO4]2}2–, 3, exists predominantly in its indicated, dimeric form. The NMR studies also identify {[(1,5-COD)IrI(NCCH3)]2·HPO4}0, 2, as an important, metastable species with one less HPO42–, formed in a dissociative equilibrium of 3 to 2 plus HPO42–. Kinetic studies reveal a first-order dependence of nucleation on the concentration of 3 and hence rule out the higher-order dependence implied by classical nucleation theory. Additional kinetic studies reveal a telling, inverse, quadratic dependence on added HPO42–, results that unveil the previously unavailable insights that a simple bimetallic, Ir2 precursor is sufficient to enable low-molecularity nucleation via {[(1,5-COD)IrI(NCCH3)]2·HPO4}0, 2, as a kinetically competent intermediate, a unique example of a nucleation mechanism known in molecular detail from a precisely defined molecular precursor that also includes spectroscopic detection of a kinetically competent intermediate. The results with {[(1,5-COD)IrI·HPO4]2}2– in comparison to previous results with {[(1,5-COD)IrI·POM]}8– (POM = the polyoxometalate P2W15Nb3O629–) allow insights into the details of nucleation, notably that Ir2 versus Ir3 kinetically effective nuclei are controlled by the different HPO42– and POM8– anion’s surface charge and resultant 2 versus 3 IrI(1,5-COD)+ moieties they are able to bind to achieve surface-charge neutrality. The state-of-the-art nucleation results allow a total of nine insights, conclusions, and two new working hypotheses, insights that promise to help drive a deeper understanding of nucleation, not just in transition-metal nanoparticle formation but hopefully more broadly across nature.

Nucleation and Crystal Formation in Lithium Disilicate‐Apatite Glass‐Ceramic from a Combined Use of X‐Ray Diffraction, Solid‐State NMR, and Microscopy

AbstractGlass‐ceramics are multi‐phase materials that are comprised of one amorphous phase and at least one crystalline phase. Their versatile performance and properties can be engineered by alterations of the three fundamental steps – formulation and production of the amorphous base glass, nucleation, and crystallization. Efforts have been made on syntheses of glass‐ceramics with different components, yet little is known about the details of nucleation and crystallization processes that are essential for tailoring glass‐ceramic properties. Herein, we investigate the nucleation and crystallization mechanisms of a multi‐component, that is SiO2‐Al2O3‐CaO‐Li2O‐K2O‐P2O5‐F, glass‐ceramic system by a combined use of powder X‐ray diffraction (pXRD), solid‐state nuclear magnetic resonance (NMR), and electron microscopic (EM) techniques. The role of P2O5 in the nucleation and crystallization processes is particularly studied. We show that the formation of lithium silicate crystals being independent of the P2O5‐associated crystals, and the separation of P2O5 phases into individual growth domains of lithium orthophosphate and fluorapatite. We also observe the non‐uniform distribution of fluorapatite particles that explains the opalescence effect of this glass‐ceramic.

Kinetics of Hematite to Wüstite by Hydrogen for Chemical Looping Combustion

Kinetics analysis of hematite (Fe2O3) reduction by hydrogen was evaluated by the thermogravimetric analyser (TGA) in the temperature range of 700–950 °C, using continuous streams of 5, 10, and 20% H2 concentrations in N2. A number of kinetic models have been considered, including the single and multi-step models to describe the experimental reduction data. The details of nucleation and growth during the isothermal reduction are described in terms of the local Johnson–Mehl–Avrami (JMA) exponent and local activation energy. The variations of n values (JMA exponent) and activation energies for the reduction conversion indicate the presence of the multi-step reaction process. The reduction was shown to be one-dimensional (1D) growth with a decrease in the nucleation rate.

Crystallization Kinetics of the Cu47.5Zr47.5Al5 Bulk Metallic Glass under Continuous and Iso-Thermal Heating

The crystallization kinetics of Cu47.5Zr47.5Al5 BMG was studied by differential scanning calorimetry (DSC) using the mode of continuous heating and isothermal annealing. It is found that Tg, Tx, and Tp, display a dependence on the heating rate in the case of continuous heating. The activation energies, Eg, Ex and Tp determined by the Kissinger method, yield 445, 264 and 285 kJ/mol, respectively. The local activation energy, E(x), was determined by the Doyle-Ozawa method, which gives the average activation energy 204 kJ/mol. On the other hand, the isothermal kinetics was modeled by the Johnson-Mehl-Avrami (JMA) equation, the Avrami exponent versus crystallization fraction was calculated at different temperatures. Details of nucleation and growth behaviors are discussed in terms of the local Avrami exponent and local activation energy during the isothermal crystallization. X-ray shows that the quenched BMG only includes the glass single phase. The BMG heated to 873 K has the precipitation of the body-center cubic (BCC) CuZr.

Crystallization kinetics of (Ni0.75Fe0.25)78Si10B12 amorphous alloy

Amorphous ribbon specimen of (Ni0.75Fe0.25)78Si10B12 has been prepared by a single roller melt-spinning technique in the air atmosphere. The crystallization kinetics of the alloy has been investigated using different thermal analysis by means of continuous heating and isothermal heating. The activation energy of the alloy has been calculated by using Kissinger plot method and Ozawa plot method based on differential thermal analysis data, respectively. The products of crystallization have been analyzed by X-ray diffraction. A single phase of γ-(Fe, Ni) solid solution with grain size of about 10.3 and 18.5nm precipitates in the amorphous matrix after annealing at temperatures 715 and 745K, respectively. The crystallized phases are γ-(Fe, Ni) solid solution, Fe2Si, Ni2Si, Fe3B and unidentified phase after annealing at 765K. The details of nucleation and growth during the isothermal crystallization are expatiated in terms of local Avrami exponent and local activation energy.

Evolution of crystal size distributions in a CMSMPR—the effect of temperature, flow rate, and mixing speed with time

A continuous mixed-suspension, mixed-product removal (CMSMPR) crystallizer with automatic computer control and data acquisition was constructed. Operating this CMSMPR at rpm’s greater than 100 rpm assures well-mixed conditions. The crystallization of NaCl, an impurity in a concentrated CaCl2 solution, was performed by cooling under well-mixed conditions. Atomic absorption spectrometry (AAS) and inductively coupled plasma-atomic emission spectrometry (ICP-AES) have been used to measure solution concentrations. Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), light microscopy and particle size analyzer with advanced laser light scattering have been used to characterize the crystals produced by the CMSMPR crystallizer. Octahedral NaCl crystals are observed. Steady state crystal size distribution (CSD) is observed at nine to ten times the mean residence time for the crystallizer. The CSD has been analyzed to look at the details of nucleation, growth, and aggregation rate as a function of crystallization temperature, rpm, and feed flow rate with the application of a Hounslow’s constant aggregation rate equation.

Nanocrystallization kinetics of Ni-based bulk amorphous alloy

The bulk amorphous alloy Ni 45Ti 23Zr 15Si 5Pd 12 has been prepared by copper mold casting. The crystallization kinetics of the alloy has been investigated using differential scanning calorimetry by means of continuous heating and isothermal annealing, and the products of crystallization have been analyzed by transmission electron microscopy and X-ray diffraction. The isothermal activation energy obtained from Arrhenius equation increases in the process of crystallization transformation. The details of nucleation and growth during the isothermal crystallization are expatiated in terms of local Avrami exponent and local activation energy. Meanwhile, the origins of nanocrystal formation during the annealing process have also been discussed.

Acoustic deinking of paper: Pulse effectiveness in particle detachment

Deinking remains an important step in environmentally conscious manufacturing of paper. Removing xerographic ink from a print sample using acoustic microcavitation provides an easy method to observe context for studying cavitation-induced erosion at surfaces. Experiments indicate that microcavitation evolves microimplosions which are effective in causing deinking preferentially at ink sites. Acoustic microcavitation is brought about by low megahertz acoustic fields giving rise to micron-size bubbles that live a few microseconds. In exposing a surface to continuous waves for a defined duration, one could obtain cavitation effects in an average, overall sense; the details of nucleation, evolution of inertial events, and the precise interplay of field parameters in effecting cavitation, however, get glossed over. Studying pulsed cavitation using tone bursts at low duty cycles, instead of CW insonification, reveals interesting details of the initiation and evolution of acoustic microcavitation. Specifically, it is found that short-pulse deinking is more effective than long-pulse deinking. This fact seems to raise an anomalous question of how the sample knows it is being visited by a short pulse or a long pulse. Recent experiments operationally demonstrate why short-pulse deinking is more effective. [Work supported by NSF.]

Susceptibility of silicon wafers to acoustic microcavitation damage

Acoustic methods are being increasingly used in semiconductor processing. Given the increase in circuit densities being implemented on semiconductor chips the requirements for particulate contamination tolerance are becoming more stringent. The semiconductor industries road-mapped specifications for 1998, 0.25 μm feature size require control of all particulate contaminants larger than 0.08 μm. Fine cleaning of post CMP wafers is currently being done by using the megasonics process. The cleaning effectiveness depends on the strength of the acoustic fields used. In using high-intensity, high-frequency acoustic fields it is crucial to ensure that the surface of the silicon wafer does not suffer any damage due to cavitation. This paper assesses the damage potential of semiconductor wafers to acoustic microcavitation. Acoustic microcavitation is brought about by low megahertz acoustic fields giving rise to micron size bubbles that live a few microseconds. In exposing a surface to continuous waves one could obtain cavitation effects in an average, overall sense; the details of nucleation, evolution of inertial events, however, get glossed over. Both, pulsed and cw insonification were studied. Experimental measurements of the thresholds for cavitation damage of semiconductor wafers will be presented.

Acoustic microcavitation at surfaces: New results in deinking of paper

Deinking remains an important step in environmentally conscious manufacturing of paper. A novel method based on acoustic microcavitation is used for deinking xerographic ink from paper. Microcavitation evolves microimplosions, which are effective in causing deinking preferentially at ink sites. Acoustic microcavitation is brought about by low megahertz acoustic fields giving rise to micron-sized bubbles that live a few microseconds. In exposing a surface to continuous waves for a defined duration one could obtain cavitation effects in an average, overall sense; the details of nucleation, evolution of inertial events, and the precise interplay of field parameters in effecting cavitation, however, get glossed over. Studying pulsed cavitation using tone bursts at low duty cycles, instead of CW insonification, reveals interesting details of the initiation and evolution of acoustic microcavitation. Experiments indicate that the 2-D context of how a xerographic print is deinked might be useful in inferring some attributes of 3-D pulp deinking. Assuringly, the results indicate that paper fibers are entirely undamaged, and the ink separation leaves them immaculately white, and that acoustic methods for deinking are a viable, chemical free, environmentally responsible means of recycling paper.

Triangular Polymer Single Crystals: Stereocomplexes, Twins, and Frustrated Structures

The crystallographic origins leading to the formation of polymer single crystals with highly unusual triangular shapes are analyzed. Triangular crystals are obtained: (a) for stereocomplexes of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) crystallizing in a trigonal unit cell with R3c (or R-3c) symmetry. In this case, the triangular morphology results from the different molecular characteristics (mainly molecular weight) of the stereoisomer polymers used to form the complex, which in turn introduce different rates of deposition on opposite sides of the (110) crystallographic growth plane and therefore to the triangular morphology. A contrario, when molecularly (near) identical stereoisomers are used in equal concentrations, or when an (achiral) homopolymer crystallizing in the same crystal structure is used, the growth rates are equal and hexagonal crystals are formed (b) for some multiple twins, in particular triple growth twins of PLLA in its α form (c) for frustrated trigonal polymer crystal structures, in which different nucleation sites exist on opposite sides of a given growth plane. This difference in growth rates results from a genuine lack of symmetry of the crystal structure: triangular crystals are a morphological marker of the frustrated character of the packing in the unit cell. Further consequences of this frustration, such as differences in lamellar thicknesses for nominally equivalent growth sectors, shed new light on the details of nucleation and growth theories of polymer single crystals.

Condensation of evaporated Cr on NaCl

A study of Cr deposited at very low incidence rates (0.01 Å/sec) on rock salt preheated in ultra-high vacuum prior to evaporation at different temperatures indicates that the nucleation sites consist almost entirely of surface defects. Isolated particles of Cr which are defect nucleated have been used to estimate the effective migration distance of Cr atoms on NaCl. When the number of nuclei depends on the number of surface defects the nuclie density is independent of the adatom flux rate although the growth rate is proportional to the flux. This would indicate that the details of nucleation should be deduced from the growth rate of isolated particles rather than the density of nuclei per se. A growth law for defect-nucleated isolated particles is given, and the results are in accord with observations. If the evaporation time and rate are known, E a– E d, the difference between the energies for absorption and surface diffusion may be estimated from the observed size of isolated particles. For Cr on NaCl this value is 0.23 eV. Cr nucleated at cleavage steps and gross structural irregularities does not ordinarily grow epitaxially; however isolated particles show epitaxial overgrowth, and there are indications of growth anisotropy along preferential substrate directions.

Dilute Solution Theory of Polymer Crystal Growth: Fractionation Effects.

A nonequilibrium (kinetic) theory of polymer molecular weight (MW) fractionation is formulated and applied to binary and multicomponent polyethylenes crystallized from unstirred xylene solutions. High MW components crystallize more readily than do low MW components. This fractionation effect is enhanced as the crystallization temperature is raised. At low crystallization temperatures (high undercoolings) the polymer tends to fractionate according to the volume fraction distribution of its MW components and thus, the number average MW of the crystal that is formed is approximately equal to the weight average MW of the polymer in solution. It is shown that MW fractionation does not depend on the details of nucleation. but rather on the rates for post-nucleation crystal growth. The effects of MW polydispersity on crystalline properties is considered and in particular it is shown that polydispersity tends to mask the intrinsic dependence of crystal thickness on MW. The variables which govern and influence fractionation are also discussed.