Heterogeneous Surface Nucleation Research Articles

Fabrication of Alkaline Earth Oxide-Containing Glass Adsorbents for Capturing Carbon Dioxide under Mild Conditions

With growing global concern on radiological hazards, there has been strong interest in the adsorbents for the treatment of radioactive carbon dioxide (CO2) containing C-14, which should be removed under mild operating conditions. In this study, we fabricated a new class of alkaline earth oxide-containing glass adsorbents with high CO2 adsorption capacity and good adsorption rate under mild aqueous conditions, which is not feasible with alkaline earth oxide-containing natural minerals. The glass adsorbents were fabricated by incorporating alkaline earth oxides (SrO or CaO) into an amorphous glass framework using network modifiers (Na2O or K2O). The comprehensive characterization of the structure and adsorption and dissolution properties of the fabricated glass adsorbents revealed that CO2 was adsorbed via the dissolution of alkaline earth metal ions and their subsequent carbonation through heterogeneous surface nucleation. Variation in the type of alkaline earth metals and network modifiers significantly affected the CO2 adsorption performance and kinetics of the glass adsorbents by altering the dissolution behavior of the alkaline earth metal ions. The best fabricated glass adsorbent exhibited a high maximum CO2 capacity of 5.56 mmol g−1 and a fast carbonation rate of 5.31 mmol g−1 h−1 in deionized water under ambient conditions, demonstrating its potential application in treating radioactive CO2.

Top-down model of calcium carbonate scale formation in turbulent pipe flows

We investigate calcium carbonate scale formation at high Reynolds numbers in a large pipe rig facility. The calcium carbonate solution is produced from the injection, at a T-joint inlet, of pH-stabilized sodium carbonate and calcium chloride aqueous solutions. A scanning electron microscopy analysis of the deposited mass along the pipe indicates that after an initial transient regime of ion-by-ion crystal growth, calcium carbonate scale is dominated by particulate deposition. While limescale formation in regions that are closer to the pipe’s entrance can be described as the heterogeneous surface nucleation of calcium and carbonate ions driven by turbulent diffusion, we rely upon turbophoresis phenomenology to devise a peculiarly simple kinetic model of deposition at farther downstream regions. Letting Φ and R be the flow rate and the pipe’s radius, respectively, the mass deposition rates per unit time and unit area are predicted to scale as Φα/Rβ (for certain modeled values of the α and β parameters) with suggestive support from our experiments.

Surface Heterogeneous Nucleation-Mediated Release of Beta-Carotene from Porous Silicon.

We demonstrate that the release of a poorly soluble molecule from nanoporous carriers is a complex process that undergoes heterogeneous surface nucleation events even under significantly diluted release conditions, and that those events heavily affect the dynamics of release. Using beta-carotene and porous silicon as loaded molecule and carrier model, respectively, we show that the cargo easily nucleates at the pore surface during the release, forming micro- to macroscopic solid particles at the pores surface. These particles dissolve at a much slower pace, compared to the rate of dissolution of pure beta-carotene in the same solvent, and they negatively affect the reproducibility of the release experiments, possibly because their solubility depends on their size distribution. We propose to exploit this aspect to use release kinetics as a better alternative to the induction time method, and to thereby detect heterogenous nucleation during release experiments. In fact, release dynamics provide much higher sensitivity and reproducibility as they average over the entire sample surface instead of depending on statistical analysis over a small area to find clusters.

Nonisothermal Crystallization-Kinetic Studies on Ag+ Ion-Exchanged Silicate Glasses: Silver Nanocrystals Growth-Kinetics in Glasses

Silver (Ag) nanocrystals (NCs) can be embedded in the glass matrix by simple Na+ to Ag+ ion-exchange (IE) reaction followed by thermal-treatment. We report the nonisothermal crystallization-kinetic studies on the IE silicate glasses by differential thermal analyses (DTA) at different heating rates. Johnson-Mehl-Avrami-Kolomogoroff (JMAK) model was used to examine the crystallization and kinetic behaviour of the NCs grown in glasses. Kissinger and Ozawa models were used to obtain the activation energies for the crystallization. The activation energy obtained from different models are in good agreement. The Avrami kinetic exponent (n) deduced from the kinetic models found to be close to 1. The crystallization-kinetic results indicate the heterogeneous surface nucleation and 1D growth of Ag NCs in the glass matrix. X-ray diffraction, scanning and transmission electron microscopic studies corroborate the crystallization-kinetics results.

The shape of diopside glass particles probed by the non-isothermal crystallization kinetics and Differential Scanning Calorimetry

The densification rate of glass powder compacts on heating can be calculated by the Clusters model of sintering, in which the particle shape is considered as a correction factor and obtained by fitting the model itself to experimental data. The crystallization of a diopside glass particle compact was characterized by Differential Scanning Calorimetry (DSC) and the resulting peak compared with the analytically calculated for particles with distinct regular morphologies and constant volume, aiming to access an effective particle shape parameter independently. The crystallization kinetics was calculated for the diopside composition considering heterogeneous surface nucleation and the phase transformation evolving due to a crystallized surface layer, whose thickness grows inward the particles, contracting the residual glass. The crystallization peaks were compared with the experimental DSC peaks for a diopside glass powder with irregular-shaped particles in a narrow size range. The calculated peaks ranged from the same temperature interval of the experimental one, but their overall shape and maximum temperature were greatly influenced by the different particle geometries. The particle shape must thus be taken into account in the analysis of glass powder crystallization by DSC, which is then a promising technique for the characterization of an effective shape factor for glass particles under crystallization.

Effects of B2O3 content and sintering temperature on crystallization and microstructure of CBS glass–ceramic coatings

Borosilicate glass–ceramics precursors with varying compositional ratios in the CaO–SiO2–B2O3 (CBS) system were synthesized by sol–gel method. The precursors were calcined at 1200°C for 2h to form glass powders. The glass–ceramics were prepared by overlaying glass slurries on the substrates before sintering at different temperatures. The as-prepared glasses and glass–ceramics were characterized by differential scanning calorimetry and X-ray diffraction. The crystallization activation energies (Ec) were calculated using the Kissinger method from DSC results. The morphology and crystallization behavior of the glass–ceramics were monitored by scanning electron microscopy. Both glass transition and crystallization temperatures decreased, however, the metastable zone increased. The Ec values of CBS glasses and glass–ceramics were 254.1, 173.2 and 164.4kJ/mol with increasing B2O3 content, whereas that of the calcined G3 glass was 104.9kJ/mol. Finally, the coatings were prepared at a low temperature (700°C). The crystals that grew on the surface of multilayer coatings demonstrated heterogeneous surface nucleation and crystallization after heat-treatment from 700°C to 850°C for 4h.

Heterogeneous surface nucleation of lithium disilicate glass: An isothermal DSC study

The paper aims in correlating isothermal DSC signals to the actual nucleation and crystallization processes in a lithium disilicate glass as determined by optical microscopy. The results show that the DSC signal reflects the change from an initial surface nucleation to a site saturation regime (constant number of crystals) leading to a coast-island microstructure in monolithic samples. An isothermal JMAK analysis revealed a maximum of the instantaneous Avrami coefficient at the onset time of saturation (impingement time). Impingement times were used to approximate heterogeneous surface nucleation rates, which were found to be up to 14 orders of magnitude higher than those of homogeneous volume nucleation and correlate positively with temperature in the range 0.64–0.68 Tm.

Undercooling evolution of pure Sn droplets in various atmospheres based on fast scanning calorimetry

Undercooling of Sn droplets in different atmospheres was studied by fast scanning calorimetry (FSC) at cooling rate of 1,000 K/s. It is found that the undercooling decreased with increasing partial pressure of oxygen. Randomly distributed SnO2 islands were observed to form on the droplet surface, which likely has promoted the heterogeneous surface nucleation. As the partial pressure of oxygen changes, the nucleation rate and growth of SnO2 led to different oxide islands, which resulted in various potential catalytic sites for the nucleation of the molten Sn droplet. The results showed that the nucleation process of the Sn droplets was sensitive to the solidification environment, and therefore the atmosphere should be taken into account in the study of the nucleation behavior of the single Sn droplets.

Surface mineralization of Ti6Al4V substrates with calcium apatites for the retention and local delivery of recombinant human bone morphogenetic protein-2

Titanium alloys are prevalently used as orthopedic prosthetics. Inadequate bone–implant interactions can lead to premature prosthetic loosening and implant failure. Local delivery of osteogenic therapeutics promoting osteointegration of the implant is an attractive strategy to address this clinical challenge. Given the affinity of calcium apatites for bone matrix proteins we hypothesize that titanium alloys surface mineralized with calcium apatites should be explored for the retention and local delivery of osteogenic recombinant human bone morphogenetic protein-2 (rhBMP-2). Using a heterogeneous surface nucleation and growth process driven by the gradual pH elevation of an acidic solution of hydroxyapatite via thermal decomposition of urea, Ti6Al4V substrates were surface mineralized with calcium apatite domains exhibiting good affinity for the substrate. The microstructures, size and surface coverage of the mineral domains as a function of the in vitro mineralization conditions were examined by light and scanning electron microscopy and the surface calcium ion content quantified. An optimal mineralization condition was identified to rapidly (<10 h) achieve surface mineral coverage far superior to those accomplished by week long incubation in simulated body fluids. In vitro retention–release profiles of rhBMP-2 from the mineralized and unmineralized Ti6Al4V, determined by an enzyme-linked immunosorbent assay, supported a higher degree of retention of rhBMP-2 on the mineralized substrate. The rhBMP-2 retained on the mineralized substrate after 24 h incubation in phosphate-buffered saline remained bioactive, as indicated by its ability to induce osteogenic transdifferentiation of C2C12 myoblasts attached to the substrate. This mineralization technique could also be applied to the surface mineralization of calcium apatites on dense tantalum and titanium and porous titanium substrates.

Heterogeneous Nucleation and Solidification Prediction ofSn-5wt.%Pb Droplets

We develop a model for heterogeneous nucleation catalyzed by oxidationon the droplet surface and the internal nucleation similar to surfaceoxidation. The fraction covered by oxide on the surface of the dropletis calculated as a function of time and temperature based on thechemical reaction dynamic and transition state theory, which is areasonable expression for the oxidation behaviour of Sn-5 wt.%Pbdroplets. The continuous-cooling-transformation curves were computedusing the above experimental results under the heterogeneous surfacenucleation and the internal nucleation of droplets. Also, we predict thesolidification behaviour of the droplet in gas-atomized spraying processusing this model. The same model can be applied to predict thenucleation behaviour for any type of cooling schedule.

Nucleation in undercooled Zr41Ti14Cu12Ni10Be23 melts

The undercooling and solidification of bulk glass forming Zr41Ti14Cu12Ni10Be23 liquids is investigated by processing in high purity graphite crucibles. Isothermal nucleation studies are performed between the liquidus and the glass transition temperature. The resulting time–temperature–transformation diagram is similar to the diagram obtained from containerlessly processed samples. Heterogeneous surface nucleation at the container walls does not effect the crystallization. The solidification products of these differently processed specimens are investigated by scanning electron microscopy as well as by electron microprobe. The investigations reveal the morphology and typical length scale of the microstructure after crystallization at different degrees of undercooling. The number density of nuclei during primary crystallization is estimated from the microstructure.

Crystallization of Supercooled Zr41Ti14Cu12Ni10B23 Melts During Continuous Heating and Cooling

AbstractThe crystallization behavior of the bulk glass forming Zr41Ti14Cu12Ni10Be23 liquid was studied under different heating and cooling rates. Investigations were performed in high purity graphite crucibles since heterogeneous surface nucleation at the container walls does not effect the crystallization of the bulk sample. A rate of about 1 K/s is sufficient to circumvent crystallization of the melt while cooling from the equilibrium melt. In contrast, upon heating a rate of more than 150 K/s is necessary to avoid crystallization of Zr41Ti14Cu12Ni10Be23 samples. The difference between the critical heating and cooling rate is discussed within classical nucleation theory and diffusion limited crystal growth. The calculated difference of the critical heating and cooling rate can be explained by the fact that nuclei formed during cooling and heating are expose to different growth rates.

The “Buffer” Layer in the Cvd Growth of β-SiC on (001) Silicon

ABSTRACTThe concept of a “buffer” layer in the epitaxial growth of compound semiconductors on (001) silicon substrate is discussed on the basis of homogeneous and heterogeneous surface nucleation. Experimental results on the nucleation of β-SiC on (001) Si by Chemical Vapor Deposition (CVD) are presented and they are discussed in terms of the model for the growth of the buffer layer.

Nucleation in bulk undercooled nickel-base alloys

The undercooling behaviour of binary Cu 1− x Ni x bulk melts was investigated over the entire concentration range 0 ⩽ × ⩽ 1 using an electromagnetic levitation technique in an ultraclean environment. The elimination of container-wall induced nucleation has led to undercooling levels of bulk melts up to 30 K larger than the highest values obtained so far by droplet dispersion. The analysis of the results within the framework of a nucleation theory for binary systems indicates that, despite the large undercoolings, the limit of homogeneous nucleation is not achieved. Microstructural investigations of the samples reveal heterogeneous surface nucleation as the prevailing nucleation mechanism. The role of surface oxidation as the responsible catalyst for nucleation is discussed with respect to the different undercooling behaviour of the samples in helium and HeH 2 atmospheres.

Containerless undercooling of bulk Fe-Ni melts

Bulk samples of Fe1−xNix, with x covering the whole concentration range 0≤x≤1, have been undercooled in a containerless state by electromagnetic levitation, yielding undercooling levels not achieved to date by any other technique. Comparison with a theoretical approximation indicates that, despite the large undercoolings attained, the limit of homogeneous nucleation is not yet reached. The solidification of the undercooled melts is initiated rather by heterogeneous surface nucleation originating from surface oxidation processes.

Crystal Nucleation in Pd-Si Alloys

ABSTRACTWe report here a study of the crystal phase nucleation in undercooled droplets of Pd-Si alloys with composition near the Pd84.5Si15.5 eutectic composition. Molten droplets are released at the top of a drop tube and solidify (to either a crystalline or glassy state) during descent. This provides a containerless (and nearly gravity free) environment so that nucleation due to container walls or vibrations is eliminated. We found that crystallization, due to homogeneous nucleation, is bypassed in droplets of 1 mm diameter when cooled at 760 K/sec. From this an upper limit of the homogeneous nucleation rate is estimated. Results are compared with our previously published study of nucleation in 0.06mm to 0.33 mm diameter droplets, which indicated that nucleation results from heterogeneous surface nucleation and that the number of these nuclei is dependent on the atmosphere in the drop tube.