Number Of Nucleation Sites Research Articles

A comprehensive model of ordered porosity formation

The model presented in this paper considers heat transfer, gas diffusion and simultaneous growth of solid and gas fractions in an ordered porosity ingot. Special attention is paid to the gas pore nucleation. The diffusion boundary layer ahead of the solid/liquid interface is analysed with respect to the diffusion process and structure formation, both controlled by external gas pressure, solidification velocity and gas fluxes between solid, liquid and pores. The relationships between processing parameters and structure characteristics are described. How the porosity structure is affected by the size of gas nuclei and the number of nucleation sites on the solid/melt interface is analysed.

Effect of Ultra-Fast Cooling after Rolling in Stable Austenite Region on Grain Refinement of C-Mn Steel

Ferrite grain refinement by hot rolling mostly above Ae3 being followed by an ultra-fast cooling has been investigated. An emphasis has been paid on the interval, Δt, between the finish of rolling and the start of water spray cooling of which cooling rate is more than 1000 °C·s-1. When Δt was nearly equal to zero, ultra-fine ferrite of about 1 or 2 μm in grain diameter was obtained for 1.2 to 1.3 mm thick 0.1%C-1%Mn steel sheets near sheet surfaces or in thickness center regions respectively, although the grain size at Δt of 0.5 s was about 3 μm in both regions. The ferrite grains were almost equiaxed and surrounded by high angle boundaries. This grain refinement is likely to be caused by an increased number of nucleation sites for the transformation from austenite to ferrite due to the ultra-fast cooling. Such a grain refinement mechanism is discussed in terms of prior-austenite structures deduced by the misorientation distribution function analysis.

On the coupling between precipitation and plastic deformation in relation with friction stir welding of AA2024 T3 aluminium alloy

We have investigated the influence of plastic deformation on the precipitation kinetics of the S′(S) phase in the AA2024 aluminium alloy, with the objective of understanding the microstructures found in friction stir welds of the same alloy. Different types of thermo-mechanical treatments have been investigated, where plastic deformation has been carried out either before heating in a salt bath, or concurrently with the thermal treatment in a Gleeble compression apparatus. The volume fraction of precipitates has been measured by differential scanning calorimetry and the precipitate morphology has been observed by transmission electron microscopy. It is observed that plastic deformation increases the precipitation kinetics in the temperature range below 300 °C. This can be attributed both to the increase in the number of nucleation sites and to the increase in the effective diffusion coefficient. At higher temperatures the influence of plasticity on the volume fraction is negligible, however, in the presence of deformation during the heat treatment, the precipitate morphology is profoundly modified. The effect of plastic deformation is observed to be similar, whether plastic deformation is carried out before or during the heat treatment. These results are shown to be valuable tools in understanding the microstructures of friction stir welds of the same material. It is shown that taking into account the effect of plastic deformation is not absolutely necessary in the thermo-mechanically affected zone. However, we show that the precipitate microstructure in some parts of the heat-affected zone is strongly influenced by a small amount of plasticity.

A kinetic study on the electrodeposition of cadmium with the presence of organic agents in sulfate solutions

The electrodeposition of cadmium is studied by electrochemical techniques with the presence of the organic agents. The cyclic voltammetry results clearly show that the electrodeposition of cadmium is a diffusion-controlled process associated with a typical nucleation process. With addition of the thiourea, 3-Picolin and benzyl alcohol organic agents simultaneously in sulfate solution, the redox potential of cadmium is shifted to more negative potentials. In this case, the current transients reveal an instantaneous nucleation with a typical three-dimensional (3D) growth mechanism, while it shows progressive nucleation mechanism without the ternary-species organic agents. In addition, the number of nucleation sites is increased with overpotential and cadmium concentration, so the deposition rate is increased. However, the microstructure of the Cd electrodeposits is changed dramatically with the presence of the ternary-species organic agents. In these conditions, the fine-grained deposits with a large number of equally sized hexagonal grains are observed.

Clean metal nucleated casting

Nucleated casting is a method of casting metal below its liquidus temperature, casting as a semisolid rather than as a liquid. In nucleated casting, a stream of liquid metal is directed into a gas atomizer, which converts it into a spray and accelerates the spray droplets from the atomization zone toward a collection mold. Because of the high surface-to-volume ratio, high relative velocity, and a large temperature difference between the gas and droplets, the droplets lose heat very efficiently to the surrounding gas. Droplet diameter and process parameters such as gas-to-metal flow ratio, process rate, and spray distance have significant impact on the solid fraction of the metal at the collection zone. Small particles lose all of their heat of fusion and are fully solid when they strike; they will likely remelt in the semi-solid pool. Large particles are fully liquid when they strike. Medium-size particles are partially solid. When the particles strike the surface of the pool, a large number of nucleation sites are available for further solidification. Each nucleation site grows until it strikes an adjacent site and all local liquid is consumed and a fine-grained metallurgical structure results. When operated at high gas-to-metal ratio the semisolid will have small ( 75%) fraction liquid, and a mold is required to contain the metal as it continues to solidify. This process is termed nucleated casting. Alloy 718 metal has been cast in a prototype nucleated casting system for demonstrating the feasibility of the new process. Metallographic examination shows that the as-cast material possesses a uniform, equiaxed 0.075 mm (ASTM 4.5) grain structure. Macrosegregation-related defects were not found in the casting or in forgings that were obtained from the ingots.

Local Self-Order Observed During Chemical Vapor Deposition of Silicon Quantum Dots for Application in Nanocrystal Memories

ABSTRACTA systematic study on the Si dot formation after chemical vapor deposition on silicon oxide substrates is presented. The process has been followed from the early stages of the dot formation up to 25% of coverages. Structural characterization has been performed by means of energy filtered transmission electron microscopy, which allowed us to observe dot sizes down to 0.5 nm in radius. The nanodots are shown to be surrounded by a depleted zone, where no new Si dots are observed to nucleate. This has been attributed to the adatoms capture mechanism by pre-existing dots, during the deposition. The dot radius and the capture size are shown to collapse onto the same curve, thus indicating the scaling behavior of the process. The adatom diffusion process is shown to restrict the number of nucleation sites, the final dot size and the dot position, thus driving the process toward partial self-order.

Effect of TiO 2-doping on the microstructure and the wear properties of laser-clad nickel-based coatings

The effects of TiO 2-doping on the properties of laser clad nickel-based composite coatings on carbon steel substrates have been studied. It is shown that good finish and crack- and porosity-free coatings can be achieved under a proper amount of TiO 2-doping and a suitable laser-scanning speed. Both the cracking sensitivity and the wear resistance are considerably improved at the little expense of a decrease in hardness. This can be attributed to the improvement of the coatings to the refinement of the microstructure both in grain sizes and their homogeneous distribution in the coatings. The microstructural and metallographic analyses by a scanning electron microscopy, energy-dispersive spectrometer and X-ray diffractometer suggest the generation of new phases such as TiB 2 and TiC upon the TiO 2-doping. Thus, it can be concluded that the refinement in microstructure is due to both an increase in number of nucleation sites by the formation of TiB 2 and TiC in the melt pool and an impediment to the growth of larger crystallites by the deposition and insertion of Ti atoms on the grain boundaries.

Modelling the effect of liquid motion on bubble nucleation during beer dispense

Beer dispense involves ejecting supersaturated beer under gas pressure, from a nozzle into a receiving vessel. Bubble nucleation therefore occurs in a flowing liquid. This situation is encountered in other processes, but is not accounted for in current nucleation models. An experimental system was developed to measure bubble production rates and sizes in laboratory scale beer dispense. Experimental results indicate that bubble nucleation is affected by both liquid flow rate and dissolved gas composition. Pre-existing gas nuclei models have been adapted using bubble and droplet detachment models to include the effect of liquid motion and gas composition. The adapted nucleation models were compared to the experimental results. Predicted bubble detachment radii and overall nucleation rates were affected by liquid flow rate, direction of liquid flow, dissolved gas composition, the contact radius and the level of contact angle hysteresis. Accurate predictions were achieved for different surface orientations and liquid flow directions. Accurate predictions occurred at hysteresis levels of 3.5°, 7.5° and 20° for liquid flow rates of 0.6, 2.2 and 3.8 l min −1 , respectively. It is clear that the predicted overall nucleation rate however, also depended on the number of nucleation sites and how many of these were active; although values for these parameters were not experimentally determined in this case. Further understanding of the exact number and size of nucleation sites available and the contact angle for the particular combination of liquid and solid used is required to improve the fit of the model to the experimental data.

Address of SiO2-Based Addtives in Bi4Ti3O12 Thin Films

By the crystallographic analysis of BIT, BIT-0.05Bi2O3·SiO2, and BIT-0.10Bi2O3·SiO2 powders, the SiO2-based additives were found to work as a buffer for the Bi composition of BIT; the excess Bi2O3 was incorporated into non-crystalline SiO2 rather than BIT. Thereby, the excess Bi2O3 was suppressed and the stoichiometry in BIT was compensated. In the thin films, the number of nucleation sites, which were at the edge of the amorphous phase, decreased with increasing the additives. The development of dense and fine texture was resulted in the 500°C-crystallized BIT-0.05Bi2O3·SiO2 and BIT-0.10Bi2O3·SiO2 thin films. While the 40 nm-thick BIT films were electrically shortened, the BIT-0.05Bi2O3·SiO2 and BIT-0.10Bi2O3·SiO2 thin films improved electrical properties and exhibited unique ferroelectric behaviors. The SiO2-based additives pinned the leak current path.

Synthesis of Hydroxyapatite – Hyaluronic acid Nano Composite Sol under Well-Controlled Condition

ABSTRACTHydroxyapatite (HAp) – Hyaluronic acid (HYA) nano composite sol was prepared at room temperature, at pH 8, by a coprecipitation method from CaCl2 or Ca(CH3COO)2 and H3PO4. These soluble Ca sources were used in order to increase the number of nucleation sites for HAp on HYA. Starting materials were supplied by two kinds of processes to give solution(s) of only PO43- or, PO43- and Ca2+ simultaneously into HYA solution containing Ca2+. Morphology of the aggregates and crystallite size as well as lattice parameters of the HAp were examined by TEM and XRD data with Rietveld refinement. We observed needle like aggregates of crystallites of HAp. Supplying methods of Ca sources controlled the aspect ratio of the aggregates and HAp crystallite. Lattice parameters of the samples prepared from CaCl2 shifted towards those of chloroapatite. Those from Ca(CH3COO)2, however, correspond to the literature values of single crystal HAp. The results of FT-IR spectra and the changes of rheological property indicate the electrostatic interactions between negatively charged functional groups of HYA and HAp.

Glass sintering with concurrent crystallization

We critically review and discuss the main glass-sintering models: Frenkel, Mackenzie–Shuttleworth, Scherer and the recently developed Clusters model, and focus on the problem of sintering with concurrent crystallization. The Clusters model is tested under various practical conditions. Isothermal tests are carried out on a widely polydispersed alumino-borosilicate (ABS) glass having jagged particles, which is stable against devitrification, and on a soda–lime–silica (SLS) glass with a narrow spherical particle distribution, which crystallizes easily. The algorithm for non-isothermal processes is also tested with two distinct systems: the same ABS glass and a narrow-sized cordierite glass, which is devitrification-prone. In addition to physical parameters such as viscosity, surface tension, particle-size distribution, crystal growth rate and number of nucleation sites, microscopic-particle-packing data are introduced into the model and it is demonstrated that the evolution of both density and pore size distribution can be reasonably predicted. All the results are discussed taking into account the assumptions made in the derivations and other complicating factors, such as irregular particle shape, compositional shifts due to crystallization, temperature gradients and degassing during sintering. Finally, we discuss the physical and processing parameters that determine whether sintering will be favorable over crystallization. We demonstrate that the Clusters model and related algorithm provide a powerful simulation tool to design the isothermal or non-isothermal densification of devitrifying or stable glass compacts with any particle-size distribution, thus minimizing the number of time-consuming laboratory experiments.

Electrochemical characterization of the Ni–Fe alloy electrodeposition from chloride–citrate–glycolic acid solutions

Cyclic voltammetry and transient time ( τ) measurements have been used to characterize the system, while potentiostatic I– t transients have been recorded to obtain the nucleation and growth mechanism. The electrodeposition of nickel, iron and nickel–iron alloys on the platinum was studied by electrochemical techniques in the presence of complexing compounds. The cyclic voltammetry results clearly show that the electrodeposition of nickel, iron and nickel–iron alloys is a diffusion-controlled process with a typical nucleation mechanism. The redox potentials of the Ni and Fe are shifted to more cathodic potentials in the complexing electrolytes. In addition, the current transients reveal that nucleation mechanism is instantaneous with a typical three-dimensional (3D) nucleation and growth process. The number of nucleation sites is increased with the overpotential and the nickel concentration. It is found that with addition of the glycolic acid to sodium citrate solution, the number of nucleation sites and so the nucleation rate is increased for the Ni–Fe alloy deposition. However, the microstructure of the Ni–Fe alloy electrodeposits is changed dramatically in the presence of the glycolic acid. Therefore, a large number of equally sized spherical grains are observed in spherical clusters. The spherical particles become finer and the surface roughness is decreased.

Formation of titanium silicide on narrow gates using laser thermal processing

One of the crucial issues that must be faced when using titanium silicide in advanced IC structures is the difficulty encountered in transforming the silicide from its high to its low resistivity phase. As gate dimensions are reduced, there is a reduction in the number of nucleation sites available to initiate transformation on laterally and vertically confined films. In this paper we demonstrate a novel technique, using a pulsed excimer laser, to produce thicker silicides over the gate than over the source and drain regions. This is difficult to achieve using conventional thermal processing. The increased thickness of silicide over the gate region assists in alleviating the phase transformation constraints. In this paper, we demonstrate fabrication of low resistivity salicide on gate lengths as small as 0.07 /spl mu/m. We also demonstrate, through the use of two-dimensional thermal simulations, the use of amorphization as a method for overcoming the barriers to integration of laser thermal processing in conventional MOSFET fabrication.

Investigation of the stability of glassy state in the Zr- and Hf-based glassy alloys correlated with their transformation behavior

The transformation behavior from glassy state was investigated in Zr- and Hf-based glassy alloys. The primary phases are metastable face-centered-cubic (fcc) Zr2Ni and fcc Hf2Ni phases in the Zr65Al7.5Ni10Cu17.5 and Hf65Al7.5Ni10Cu17.5 glassy alloys, respectively. By substitution of 5 at.% Pd for Cu, the primary phase changes to an icosahedral quasicrystalline phase in both alloys. It is found that the addition of elements, which have a positive or weak chemical affinity with one of the constitutional elements in the Zr–Al–Ni–Cu and Hf–Al–Ni–Cu glassy alloys, is effective for the precipitation of the icosahedral phase. It is suggested that Pd plays a dominant role in an increase in the number of nucleation sites. Since an icosahedron is contained as a structure unit in the icosahedral, fcc Zr2Ni and fcc Hf2Ni phases, it is implied that these phases are correlated with the local icosahedral order. The high-resolution transmission electron microscopy images of the as-spun Zr65Al7.5Ni10Cu7.5Pd10 and Hf65Al7.5Ni10Cu12.5Pd5 alloys reveal a possibility of the existence of the icosahedral ordered regions. It is therefore, concluded that the icosahedral short- or medium-range order exists and it stabilizes the glassy state in the Zr- and Hf-based multicomponent alloys.

Solid phase reaction of Ti with Si−Ge layers prepared by Ge-implantation

Titanium germanosilicide films from thin Ti films (∼27.5 nm) are formed by solid phase reaction of Ti/Si0.62Ge0.38 bilayer at different annealing temperatures ranging from 600°C to 800°C. The effect of crystallographic state of Si−Ge alloy film on the reaction, phase formations, and polymorphic phase transformations, stability of germanosilicides have been investigated by x-ray diffraction, atomic force microscope, and sheet resistance measurements. Both amorphous and relaxed epitaxial Si0.62Ge0.38 films are prepared by Ge-multiple implantations into Si wafers with appropriate dose and energy followed by different post-implantation RTA schemes comprising alternative implantation and annealing in on case, and single final annealing in another one. XRD results indicate that the reaction sequence in both cases is found to be Ti/Si like with the formation of C49-Ti(Si−Ge)2 as a precursor to the low resistivity C54-Ti(Si−Ge)2. The films formed on amorphous alloy layer exhibit lower polymorphic transition temperature (∼750°C), smoother surface, lower sheet resistance and less agglomeration as compared to those on c:Si−Ge films. These characteristics are due to enhanced nucleation of C54 phase as a result of greater number of nucleation sites in the reaction with amorphous films. The formation of Ti(Si−Ge)2 films is, however, accompanied by the decrease of Ge content in Ti(Si−Ge)2 films formed on both amorphous and crystalline alloy films and indicates possible segregation/diffusion effects during the germanosilicidation.

Effects of Ag and Pd on the nucleation and growth of the nano-icosahedral phase in Zr65Al7.5Ni10Cu7.5M10 (M = Ag or Pd) metallic glasses

The nucleation and growth of a nano-icosahedral phase from a supercooled liquid region of Zr65Al7.5Ni10Cu7.5M10 (M = Ag or Pd) glasses have been examined by differential scanning calorimetry and transmission electron microscopy. The growth rate of the icosahedral phase is nearly constant at the initial stage and much slower than that of the Zr2Ni phase in the Zr65Al7.5Ni10Cu17.5 metallic glass. The homogeneous nucleation rate has a maximum value of 4.4 x 1020 m-3 s-1 at 695 K in the Zr65Al7.5Ni10Cu7.5Ag10 glass, which is approximately 102 times higher than that for the formation of quasicrystalline phase in the Zr69.5Al7.5Ni11Cu12 glass and 104 times higher than that of the Zr2Ni phase in the Zr65Al7.5Ni10Cu17.5 glass. With increasing Pd content, the nucleation rate of the primary phase increases significantly and the growth rate decreases at the crystallization temperature. Thus, the addition of Ag and Pd is effective for an increase in the number of nucleation sites and the suppression of grain growth, which is the main reason for the formation of icosahedral nanoparticles. The significant increase in the nucleation rate is due to an increase in the number of nucleation sites resulting from the short-range ordering consisting of Zr-(Ag or Pd) strong pairs. It is implied that the strong pair Zr-(Ag or Pd) also contributes to the restraint of the long-range rearrangements of the constitutional elements. The formation of the nanoicosahedral phase suggests that icosahedral short-range order exists in the glassy state in the present alloys.

Investigation of Crystallization Behavior of Sputter-Deposited Nitrogen-Doped Amorphous Ge2Sb2Te5 Thin Films

The crystallization behavior of nitrogen-doped amorphous Ge2Sb2Te5-(N) phase-change thin films was studied by utilizing differential scanning calorimetry, in situ ellipsometry and in situ transmission electron microscopy. The combined analysis of in situ ellipsometry isotherms of amorphous Ge2Sb2Te5-(N) films and the Johnson-Mehl-Avrami equation revealed that the crystallization process of amorphous Ge2Sb2Te5-(N) films changes depending on the nitrogen content. The crystallization behavior of Ge2Sb2Te5 film revealed a two-step process that includes spherical-nucleation and disc-shaped grain growth. In contrast, nitrogen-doping into Ge2Sb2Te5 thin films suppresses the second step and the crystallization of Ge2Sb2Te5-(N) becomes a one-step process that is the primary nucleation process. The number of nucleation sites during the crystallization of amorphous Ge2Sb2Te5-(N) films, increased markedly with the annealing temperature in the spherically shaped nuclei and eventually saturated. The effective crystallinity of Ge2Sb2Te5-(N) alloy films decreased with the increase in nitrogen content, mainly due to the grain-size refinement.

Nucleation and Grain Growth Kinetics of Nano Icosahedral Quasicrystalline Phase in Zr<SUB>65</SUB>Al<SUB>7.5</SUB>Ni<SUB>10</SUB>Cu<SUB>17.5−<I>x</I></SUB>Pd<I><SUB>x</SUB></I> (<I>x</I>=5, 10 and 17.5) Glassy Alloys

We examined the nucleation and grain growth kinetics of a nano icosahedral phase formed as a primary phase in the Zr 65 Al 7.5 Ni 10 Cu 17.5-x Pd x (x = 5, 10 and 17.5) glassy alloys by differential scanning calorimetry and transmission electron microscopy. The nano icosahedral grains are distributed homogeneously, indicating the homogeneous nucleation mode. The grain growth rate of the icosahedral phase is nearly constant at the initial stage and much lower than that of the cubic Zr 2 Ni phase in the Zr 65 Al 7.5 Ni 10 Cu 17.5 glassy alloy. The growth rate decreases with increasing Pd content. It is (4.5 ± 0.4) x 10 -9 m s -1 at x = 5 and decreases to (8.3 ± 0.8) x 10 -10 m s -1 at x = 17.5. The homogeneous nucleation rate at crystallization temperature, T x , changes drastically with Pd content, and is (1.1 ± 0.3) x 10 20 m -3 s -1 in the Zr 65 Al 7.5 Ni 10 Cu 7.5 Pd 10 glass, which is approximately 10 2 times higher than that in the Zr 65 Al 7.5 Ni 10 Cu 12.5 Pd 5 glass and 10 4 times higher than that of the Zr 2 Ni phase in the Zr 65 Al 7.5 Ni 10 Cu 17.5 glass. Thus, it is clarified that with increasing the Pd content, the nucleation rate of the primary phase increases significantly and its growth rate decreases. The addition of Pd is effective for the increase in the number of nucleation sites and the suppression of grain growth. The formation of the nano icosahedral phase by the addition of Pd implies the existence of icosahedral short-range order in the glassy state of the Zr-Al-Ni-Cu alloy.

Crystallization kinetics and thermal degradation behavior of low-density polyethylene blended with poly(bispropoxyphosphazene)

Low-density polyethylene (LDPE) was melt blended in a Brabender with different amounts of poly(bispropoxyphosphazene) flame retardant, MFR. The crystallization kinetics and thermal degradation behavior of the blends were investigated. It was found that MFR was incompatible with LDPE, where MFR particles dispersed in LDPE matrix with very weak interfacial bonding. In the blends, polyethylene crystals were still in an orthorhombic form and spherulitic structure was observed. Under isothermal crystallization, the crystallization rate and crystallinity were both increased with the addition of MFR up to 20 phr, probably due to the decrease of viscosity and the increase in the number of nucleation sites. The Avrami parameter n was in the range of 2.2 to 2.7 for the blends. In thermal degradation behavior, the addition of MFR increased the maximum-rate degradation temperature ( T max), char yield at 650°C and the limiting oxygen index (LOI) of LDPE, indicating that MFR additive was beneficial in flame retardancy. Yet, it deteriorated the tensile mechanical properties of LDPE.

Effect of Modified Metal/Passivation Interfaces on Stress Voiding in Interconnects+

AbstractStress and electromigration voids are believed to nucleate at the metal/passivation interface. In this study, modified interfaces were synthesized to experimentally elucidate the role of the interface on stress voiding for comparison with theoretical predictions. Interfaces were modified by depositing photoresist in a controlled manner on Ti/Algfi lines prior to passivation deposition. Stress voiding susceptibility was assessed in as-received lines as well as those subjected to storage anneals for 1, 5, and 25 days at 190°C to accelerate void formation. The global textures of all the lines were <111> fiber and were not significantly affected by the interface modification. Differences in stress voiding were attributed to the different interfaces. Higher ratios of voids in modified regions to voids in unmodified regions were observed both in as-received lines and those given a 1 day storage anneal due to the heterogeneous nucleation sites provided by the photoresist contamination in the modified regions. However, the void ratio appears to decrease exponentially with longer time anneals. Lowered driving force for void formation in the modified regions leads to less voiding with time, while higher driving force and a limited number of nucleation sites in the unmodified regions leads to more voiding with time. Local textures at a limited number of voided sites in modified and unmodified regions were identical to the global texture of the line. In our limited sampling, voids did not form in local regions of weaker <111> texture.