Number Density Of Nuclei Research Articles

A Thermodynamic Frame for the Kinetics of Polymer Crystallization under Processing Conditions

A retrospective view is given on work, which has been carried out at the Johannes Kepler University during the last two decades on the structure formation in crystallizable polymers during their processing. Emphasis is laid on some useful principles lend from basic physical chemistry: use of the T,S-diagram and of the temperature dependent courses of (effective) surface tensions along coexistence lines of phases. These considerations should be helpful in creating an overview, where a deficiency of direct experimental crystallization kinetics data exists. For a series of industrially important, mostly fast crystallizing, polymers data are given in the present paper for the quiescent melt (temperature dependences of number densities of nuclei and of growth speeds). Critical minimum cooling speeds for bypassing crystallization are given on the basis of the given data. These critical cooling speeds vary by almost six decades from HDPE to i-PS. These results are also helpful in the formulation of a viable classification of materials in the light of their processing conditions. This classification includes metals and glass forming minerals as limiting cases for extremely fast and extremely slowly crystallizing materials.

Growth mechanism of a preferentially oriented mordenite membrane

A preferentially oriented mordenite membrane was successfully prepared on a seeded porous a-alumina support. Characterization results of XRD (X-ray diffractometer) and FE-SEM (field emission scanning electron microscope) revealed that evolutionary selection might predominantly contribute to the formation of the sharply oriented mordenite membrane. The necessary conditions under which evolutionary selection occurs are: (a) the number density of nuclei on the support surface should be high enough at the early stage; (b) the crystals should grow fastest along one direction; and (c) the zeolite layer should proceed via the successive growth of the crystals nucleated on the support surface instead of the accumulation of the crystals formed in the bulk solution.

Kinetics of polymer crystallization under processing conditions: transformation of dormant nuclei by the action of flow

In previous papers [Janeschitz-Kriegl H, Ratajski E, Stadlbauer M. Flow as an effective promotor of nucleation in polymer melts: a quantitative evaluation. Rheol Acta 42 (2003) 355–364; Stadlbauer M, Janeschitz-Kriegl H, Eder G, Ratajski E. New extensional rheometer for creep flow at high tensile stress. Part II. Flow induced nucleation for the crystallization of iPP, J Rheol 48 (2004) 631–639. [1,2]] two types of experiments were carried out: (a) rapid quenches of quiescent melts of i-PP from their state of equilibrium to a series of rather low temperatures and (b) short term shearing or extension of melts at only mild degrees of undercooling. Strong undercooling as well as high mechanical loads cause similar tremendous increases in the number densities of nuclei by many decades. Unexpectedly, an extremely non-linear dependence of the said number densities on the loading times has been found.In the present paper an explanation for this non-linear relation is tried. The assumption is made that in a quiescent melt there is a huge reservoir of badly organized aggregates (local alignments) of chain molecules, which as such can become effective nuclei only at rather low temperatures. These aggregates are assumed to grow by the action of flow after being oriented in the flow direction. In this way a large amount of low quality dormant nuclei can be transformed into nuclei of a better quality, which are active at higher temperatures. Starting at individual aggregates this growth can lead to thread-like precursors initiating shish-kebab structures.Instead of the flow time the specific mechanical work has been found to be a useful universal parameter. Some measurements of the optical retardation are quoted, which strongly support the basic assumption.

Cirrus parametrization and the role of ice nuclei

A parametrization of cirrus clouds formed by homogeneous nucleation is improved so that it can be used more easily in general-circulation models (GCMs) and climate models. The improved parametrization is completely analytical and requires no fitting of parameters to models or measurements; it compares well with full microphysical model results even when monodisperse aerosol particles are used in the parametrization to determine cirrus ice-crystal number densities. However, the presence of ice nuclei in the atmosphere can modify the formation of cirrus clouds. If sufficient ice particles have been generated by heterogeneous nucleation, the saturation ratio of the air parcel will never reach that required for homogeneous nucleation. We calculate the critical number density of ice nuclei, above which homogeneous nucleation will be suppressed. The critical number density depends on the temperature, the updraught velocity, and the supersaturation at which ice nuclei activate. The theory points to key uncertainties in our observations of ice nuclei in the upper troposphere; for ice nuclei that activate at relatively low supersaturations, number density is more important than a precise knowledge of the activation supersaturation. Overall, the theory provides a general framework within which to interpret observations and the results of full microphysical cloud models. The theory can provide analytical test cases as benchmarks for the testing of models in development, and can be implemented itself into larger-scale atmospheric models, such as GCMs. Copyright © 2005 Royal Meteorological Society

Crystallisation kinetics of Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG alloy during heating

The crystallisation behaviour of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass (BMG) alloy during heating from the amorphous state has been investigated. Upon heating, the nucleation process was not consistent with classical nucleation theory. This problem can be overcome by substituting the nucleation density with the number density of nuclei, N, for growth of the crystalline phase, which is estimated from the length scale of the microstructure. The growth of crystalline phase is the governing process for crystallisation of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy upon heating. The growth process is more important than the nucleation process for analysing the crystallisation mechanisms of the amorphous state. Non-isothermal heating DSC data have been analysed with a modified crystallisation kinetics model based on the non-isothermal transformation theory. Using this model, the onset crystallisation temperatures at various heating rates have been calculated. The calculated critical heating rate to avoid crystallisation is 190 K s−1, which matches well with the experimental value. The onset crystallisation times for isothermal annealing conditions have also been calculated and compared with the isothermal DSC data. Time temperature transformation (TTT) and continuous heating transformation (CHT) diagrams have been constructed quantitatively. The crystallisation kinetics of BMG alloys can be more reliably analysed with our modified crystallisation kinetics model than with several nucleation models.

Nucleation kinetics of polymer formation on nucleation agent

The initial stadium of crystallization of polymers (nucleation process) on a nucleation agent is studied within the standard nucleation model. A nucleation agent is added to the melt to increase the nucleation rate. It is supposed that nucleation agent forms nucleation centers, or active centers, on which the formation of crystalline polymer starts. The basic kinetic equations describing such a process are solved numerically to determine the basic characteristics of nucleation process—the number density of nuclei, nucleation rate and the total number of nuclei as a function of time. The results of numerical computation are in good agreement with the experimental data obtained by small angle X-ray scattering (SAXS) method for polyethylene. The rate of formation of nuclei of various sizes reaches the same limit at long times. This limit is lower than the corresponding stationary nucleation rate due to exhaustion of active centers during polymer formation process, which was taken into account.

Nucleation kinetics of folded chain crystals of polyethylene on active centers.

Kinetic equations describing formation of nuclei of a new phase on active centers from supercooled melt taking into account exhaustion of active centers are solved numerically. Basic characteristics of nucleation process (total number of nuclei and nucleation rate) of folded chain crystals of polyethylene at low supercooling are determined and compared with measured data. Our model gives good coincidence with experimental measurements of the total number density of nuclei. Nucleation rate reaches some quasistationary limit at sufficiently long time, which is approximately 20% of the stationary nucleation rate determined by the standard way.

Influence of surface additives iodine and indium on the initial growth in copper chemical vapor deposition

The influence of surface additives, iodine and indium, on copper chemical vapor deposition (CVD) in its initial growth period is studied. The Cu CVD has been carried out on TaN/SiO2 and Cu(sputtered)/TaN/SiO2 substrates using Cu–hexafluoroacetylacetonate–vinyltrimethylsilane (Cu(hfac)(vtms)) precursor. Addition of iodine enhances the horizontal growth and reduces the vertical growth of Cu nuclei on the TaN surface. Addition of indium increases the number density of copper nuclei significantly on TaN. Both effects accelerate the initial growth rate and shorten the initial period. The X-ray photoelectron spectra confirm that iodine floats out to the growing copper surface, but indium does not. Since iodine is floating on the copper surface, it is a surfactant and its influences persist throughout the deposition. On the other hand, the influences of extranucleation sites provided by indium cease at the end of initial growth. Hence, Cu CVD on the In-added TaN surface develops a continuous film quickly, but its growth afterwards is similar to that without indium. On the Cu(sputtered)/TaN/SiO2 substrate, the surface is featured with faceted and protruded Cu grains in the initial growth. The iodine adsorption on sputtered copper worsens the initial morphology and leads to a porous and less conductive film. After 1-h deposition at 145°C, the sheet resistivity of Cu film on iodine-adsorbed TaN/SiO2 substrate is 1.8 ± 0.1μΩ-cm.

New extensional rheometer for creep flow at high tensile stress. Part II. Flow induced nucleation for the crystallization of iPP

A novel extensional rheometer for creep experiments has been presented in part I. The device can be used also for an investigation of the influence of short term elongational flow on the structure development in crystalline polymers. It enables the application of defined portions of mechanical work to the polymer sample in its state of undercooled melt. For the purpose short-term elongational creep is used. With increasing mechanical work the number density of nuclei in iPP increases by decades. Results, as previously obtained with shear flow, are introduced for comparison. Prospects of a theoretical description are discussed.

A study of the initial oxidation of copper in 0.1 MPa oxygen and the effect of purity by metallographic methods

To clarify the initial oxidation mechanism of copper, the oxidation was carried out at 400 °C in 0.1 MPa oxygen using 99.9999% (6 N) and 99.5% (2 N) pure specimens. Oxidation of 6 N copper after 60 s showed that the number density of the oxide nuclei varied with the face of copper crystals, while the nucleation occurred preferentially at the grain boundaries. A metallographic examination indicated that the products of initial oxidation consist of both CuO and Cu 2O. CuO is firstly formed as a thin uniform film on the copper surface, and then Cu 2O nucleates and grows beneath the CuO film. This result is different from the conclusion reached in the literature that CuO does not appear until the laterally growing Cu 2O nuclei have covered the whole surface using other methods. In contrast to 6 N copper, nucleation of Cu 2O was much delayed for 2 N copper, though a thin CuO film was similarly formed on 2 N copper surface. Impurities in 2 N copper should be responsible for slow nucleation of Cu 2O and slow growth of nuclei.

The influence of solute distribution on the high nucleation density of Al crystals in amorphous aluminum alloys

An extension of a previous model that describes the role of solute atoms on glass formability leads to the conclusion that solute distribution plays an important role in the formation and stability of amorphous metals. A random distribution of solutes is shown to produce local solute-depleted regions (on the size scale of the mean inter-solute spacing) that provide preferred sites for the formation of crystalline nuclei. The possibility that these solute-depleted regions are responsible for the exceptionally high number density of critical Al nuclei is explored for three Al–Y binary alloys using a computer simulation. Up to 10 7 Y atoms were placed at random locations in the system, and the number of solute-free regions were counted as a function of the size of these regions. The experimentally observed number density of critical nuclei (∼3 × 10 21 m −3) is reproduced for a critical nucleus about 5 Al atoms in diameter, containing ∼60 Al atoms in an fcc array. Good agreement with previous suggestions of the size of a critical nucleus (about 6 atoms in diameter, containing about 100 atoms) support the conclusion that the current model provides a reasonable physical explanation for the quenched-in features responsible for the exceptionally high nucleation density in some amorphous Al alloys.

Effects of Oxygen Partial Pressure on the Nucleation Behavior and Morphology of Chemically‐Vapor‐Deposited Zirconia on Hi‐Nicalon Fiber and Si

A ZrO2 coating was prepared on Hi‐Nicalon fiber and single‐crystal Si by chemical vapor deposition (CVD) using ZrCl4, CO2, and H2 as precursors at 1050°C. The effects of oxygen partial pressure on the nucleation behavior of the CVD‐ZrO2 coating were systematically studied by intentionally varying the controlled amount of O2 into the CVD chamber. Characterization results suggested that the number density of tetragonal ZrO2 nuclei apparently decreased with increasing the oxygen partial pressure from 4 × 10−3 to 1.6 Pa. Also, the coating layer became more columnar and contained larger monoclinic ZrO2 grains. The observed relationships between the oxygen partial pressure and the nucleation and morphologic characteristics of the ZrO2 coating were attributed to the grain size and oxygen deficiency effects, which have been previously reported to cause the stabilization of the tetragonal ZrO2 phase in bulk ZrO2 specimens.

Chemical and physical properties of bulk aerosols within four sectors observed during TRACE‐P

Chemical and physical aerosol data collected on the DC‐8 during TRACE‐P were grouped into four sectors based on back trajectories. The four sectors represent long‐range transport from the west (WSW), regional circulation over the western Pacific and Southeast Asia (SE Asia), polluted transport from northern Asia with substantial sea salt at low altitudes (NNW) and a substantial amount of dust (Channel). WSW has generally low mixing ratios at both middle and high altitudes, with the bulk of the aerosol mass due to non‐sea‐salt water‐soluble inorganic species. Low altitude SE Asia also has low mean mixing ratios in general, with the majority of the aerosol mass comprised of non‐sea‐salts, however, soot is also relatively important in this region. NNW had the highest mean sea salt mixing ratios, with the aerosol mass at low altitudes (<2 km) evenly divided between sea salts, non‐sea‐salts, and dust. The highest mean mixing ratios of water‐soluble ions and soot were observed at the lowest altitudes (<2 km) in the Channel sector. The bulk of the aerosol mass exported from Asia emanates from Channel at both low and midaltitudes, due to the prevalence of dust compared to other sectors. Number densities show enhanced fine particles for Channel and NNW, while their volume distributions are enhanced due to sea salt and dust. Low‐altitude Channel exhibits the highest condensation nuclei (CN) number densities along with enhanced scattering coefficients, compared to the other sectors. At midaltitudes (2–7 km), low mean CN number densities coupled with a high proportion of nonvolatile particles (≥65%) observed in polluted sectors (Channel and NNW) are attributed to wet scavenging which removes hygroscopic CN particles. Low single scatter albedo in SE Asia reflects enhanced soot.

Shape, size, and number density of InAs quantum dots grown on the GaAs(1¯1¯3¯)B surface at various temperatures

InAs quantum dots (QD's) were grown on the GaAs(1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}3\ifmmode\bar\else\textasciimacron\fi{})B surface by molecular-beam epitaxy at temperatures between 435 and $490\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ Their shape, size, and number density were investigated by in-situ scanning tunnelling microscopy. The shape of the QD's is given for the most part by {110}, (1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{})B, and vicinal (001\ifmmode\bar\else\textasciimacron\fi{}) bounding facets, and does not change significantly with growth temperature. The diameter at the base and the height of the QD's increase monotonously from 25 to 54 nm, and from 3.5 to 9.8 nm, respectively, whereas the number density decreases as temperature increases. This is explained by assuming a slight decrease of the number density of critical growth nuclei with increasing temperature. The size distribution is bimodal: besides the coherent QD's, some larger and probably incoherent islands are observed that are extended along [332\ifmmode\bar\else\textasciimacron\fi{}]. Post annealing increases the diameter and the height by about 30% and decreases the number density, but does not change the shape significantly.

Effects of 3-Thiothiol-1-propane-sulphonic acid sodium salt and Sulphuric Acid on the Initial Stages of Copper Electrodeposition

SUMMARYInitial stages of copper electrodeposition on a glassy carbon electrode from acid sulphate solutions are studied using cyclic voltammetry and chronoamperometry. The electrolyte consisted of 0.05 mol dm3 copper sulphate and 1.4 ∼ 1.8 mol dm 3 sulphuric acid containing 2 × 102 g dm 3 3-thiothiol-l-propane-sulphonic acid sodium salt (NaO3S (CH2)3 SSH). The results show that the additive produces an initial inhibition of copper deposition, probably related to the adsorption whereas sulphuric acid promotes copper electrodeposition. The initial deposition kinetics corresponds to a model including instantaneous nucleation and diffusion controlled growth. Addition of 3-thiothiol-l-propane-sulphonic acid sodium salt and sulphuric acid does not change the initial nucleation of copper electrocrystallisation but it increases the nucleation rate and the number density of nuclei at the surface.

Effect of the size of nuclei on crystallization behavior of Li2O · 2SiO2 glass

Crystallization peak temperature was measured for Li2O · 2SiO2 glass with various number densities of nuclei induced by long heat-treatment (∼405 h) at 460°C. Its crystallization process of Li2O · 2SiO2 glass was simulated numerically in order to examine the effect of the initial size of nuclei on crystallization behavior, and the relationship between crystallization peak temperature and the number density of nuclei in the glass was considered. It was found that the effect of nuclei size is not significant when the heat-treatment temperature is lower than 460°C. By considering the effect of nuclei size, number density of nuclus was evaluated by DTA method accurately.

定温ＤＴＡによる結晶成長速度の評価

Isothermal DTA was performed at various temperatures for nucleated Li2O·2SiO2 glasses and the time neccesary to obtain crystallization peak, t0, was measured. The number density of nuclei in the glass was evaluated from crystallization peak temperature, TC, obtained by DTA with the heating rate of 10°C/min. The crystal growth rate of Li2O·2SiO2 glass was calculated from t0 and TC. By this method, crystal growth rate was measured more conveniently than by the traditional method based on the observation by micsroscope. The accuracy of crystal growth rate obtained was discussed, and it was found that good accuracy is attainable by using the glass samples with proper number density of nuclei.

The Effect of NH[sub 3] Plasma Treatment on the Electroless Copper Deposition on TaN[sub x] (x = 0=1) Diffusion Barriers

The effect of NH 3 plasma treatment of the surface of the TaN x barrier films on electroless copper deposition on TaN x films was investigated to enhance the quality of electroless Cu with the low electrical resistivity of TaN x barrier films. Electrical resistivities of TaN x barrier films were increased by less than 10% by plasma treatment. Auger electron spectroscopy depth profiles results showed that only a small fraction of the TaN x layer depth was affected by the plasma treatment. Field-emission scanning electron microscope micrographs of palladium activated TaN x films showed the number density of palladium nuclei on TaN x barriers was remarkably increased by the NH 3 plasma treatment. Electroless copper deposition on TaN x barriers was improved significantly by the NH 3 plasma treatment without a large increase in electrical resistivity.

Nucleation and Growth of Electroless Palladium Deposition on Polycrystalline TiN Barrier films for Electroless Copper Deposition

A mechanism of the nucleation and growth of palladium by electrochemical deposition on dc magnetron sputtered polycrystalline TiN thin films was proposed and effects of the microstructure of substrates were investigated. Field-emission scanning electron microscopy results showed that the nucleation of palladium and the number density of palladium nuclei are determined in the early stage of electrochemical deposition of palladium on TiN films. Plan-view transmission electron microscopy images of electrochemically nucleated palladium on TiN films for 5 s showed that the palladium nuclei were mainly formed on the grain boundaries of polycrystalline TiN thin films. These results mean that the microstructure of the substrate plays an important role in the electrochemical deposition of palladium, and the grain refinement of the substrate is thought to be an attractive method of acquiring high-quality films in electrochemical deposition. © 2002 The Electrochemical Society. All rights reserved.

Analysis of the initial stages of electrocrystallization of Fe, Co and Fe-Co alloys in chloride solutions

The electrodeposition of Fe, Co and Fe-Co alloys onto Pt has been investigated from chloride solutions at pH 2.0. The Fe-Co alloys were deposited from solutions with molar ratio of 1:10, 1:1 and 10:1. Electrocrystallization of metals was studied by potentiostatic steps. The Scharifker and Hills model was used to analyze current transients. A progressive nucleation mechanism was found to fit the experimental results for both Co and Fe-Co alloys deposited from 1:10 solution. In the case of pure Fe and Fe-Co alloy, deposited from 1:1 and 10:1 solutions, the nucleation process occurred by the instantaneous mechanism. The nuclei number densities and rate of nucleation were calculated and their values were 2.6 to 19.1 x 103 cm-2 for Fe and 0.10 to 15.8 x 106 cm-2 s-1 for Co deposition.