Heterogeneous Nucleation Efficiency Research Articles

The Effect of La, Ce Rare‐Earth Elements on the MnS Inclusion Precipitation and Distribution in U75V Heavy‐Rail Steel

The quality of heavy rail steel is significantly influenced by the distribution and morphology of MnS inclusions. With the help of experimental methods and thermodynamic predictions, the evolution of inclusions in heavy rail steels with different rare‐earth (RE: La + Ce) additions is investigated, and the relationship between RE inclusions and MnS is better interpreted by the classical two‐dimensional mismatch degree theory. The results show that the deoxidizing ability of RE elements added to the steel is greater than the desulfurizing ability. The best data for physical parameters of inclusions in steel are obtained at a RE addition of 122 ppm. The MnS inclusion particles are roughly spherical around the REAlO3 inclusion particles when the aluminum content in the RE inclusion is high. The MnS inclusion particles are better able to fit flatly around the RE2O2S inclusion particles when the sulfur content of the RE inclusion is high. By computing the two‐dimensional mismatch between the interfaces of different inclusions, it is discovered that the heterogeneous nucleation efficiency is comparatively low between MnS and REAlO3, relatively high between MnS and RE2O2S, and highest between MnS and RE2O3.

Significant reduction of grain size and texture intensity in laser powder bed fusion fabricated nickel-based superalloy by increasing constitutional supercooling

Coarse columnar grains are always present in nickel-based superalloys fabricated by laser powder bed fusion (LPBF), which not only causes anisotropy in mechanical properties, but also increases the hot cracking susceptibility of the alloys. Generally, the addition of nucleating agents is a common consideration for grain refinement. However, the intrinsic ultra-high thermal gradient and cooling rate of the LPBF process result in significant differences in grain nucleation and growth compared to conventional casting. The ultra-high thermal gradient of the LPBF process severely inhibits constitutional supercooling, which significantly reduces the heterogeneous nucleation efficiency of nickel-based superalloys. In this work, we introduced Zr into Haynes 230 alloy to improve constitutional supercooling of the LPBF molten pool during solidification and investigated the individual/synergistic effects of nucleating agent (TiC particles) and solute (Zr atoms) addition on the grain refinement of Haynes 230 alloy. It was found that the columnar grain size and texture intensity of LPBF fabricated Haynes 230 alloys were dramatically decreased with Zr and TiC co-addition, which obviously improved the mechanical properties as well as weakened its anisotropy. Our work revealed that inhibition of cell epitaxial growth and promotion of cell nucleation are necessary conditions to refine the grain size of LPBF fabricated nickel-based superalloys. Solutes with high growth restriction factor (Q value), such as Zr in our work, can rapidly generate constitutional supercooling, thereby obviously improving the heterogeneous nucleation efficiency of nucleating agents in nickel-based superalloy during the LPBF process.

Heterogeneous nucleation of M2B-type borides (M = Cr, Fe) attached to TiB2 and Ti(C,N) particles in as-cast high borated steel

Here we for the first time reported the heterogeneous nucleation of orthorhombic M2B-type borides (M = Cr, Fe) attached to TiB2 and Ti(C,N) particles with a specific crystallographic orientation relationship (OR) in as-cast boron-containing steel. The ORs were identified as 0001TiB2∥1¯12¯M2B, 12¯10TiB2∥311¯M2B and 001TiCN∥136¯M2B, 100TiCN~∥31¯0M2B. The transmission electron microscopy characterization and lattice misfit calculation showed that the interfaces of TiB2/M2B and Ti(C,N)/M2B were semi-coherent. The heterogeneous nucleation efficiency was investigated in terms of an interfacial energy calculation based on the misfit dislocation theory.

Effect of nanoparticles orientation on morphology of polymeric nanocomposite foams: preparation of foamed nanocomposite fibers by supercritical carbon dioxide

ABSTRACT Polymeric foams have received increasing attention in both academic and industrial communities. Using of nanoparticles as heterogeneous nucleation agent has been verified as one of the most valid means to enhance cell nucleation and improve cell morphology. However, few researches have been conducted to investigate the effect of the nanoparticles’ spatial orientation on their nucleation efficiency. In this work, to study the influence of the orientation of nanoparticles on their performance in improving morphology of polymeric foam, thermoplastic polyurethane (TPU) composite fibers with different nanoparticles (carbon nanotubes, graphene and SiO2) were prepared by using different traction speeds. The different traction speeds lead to different orientation state of the nanoparticles which then resulted different nucleation effect. It was found that carbon nanotubes (CNTs) were easily oriented and aligned along the fiber length direction under the high traction speed, while graphene and SiO2 nanoparticles did not show orientation under the traction speed in this study. As a result, the foam of TPU/CNTs composite fibers from high traction speed exhibited a much smaller cell size and higher cell density compared to the foams of the fibers from low traction speeds, while TPU/graphene, and TPU/SiO2 composite fibers with different traction speeds showed almost similar cell size and size density after foaming, indicating that the orientated nanoparticles possessed higher heterogeneous nucleation efficiency. To our best knowledge, this work, for the first time, demonstrated the high nucleation effect of the aligned nanoparticles, which hopefully open a new path for improving the cell morphology of polymeric foam materials.

Effect of ionization of polyamide-66 on its heterogeneous nucleation of poly(ethylene terephthalate) crystallization: An efficient polyamide-66 ionene nucleator promoted by ion‒dipole interactions

A polyamide (PA)-66 ionene successfully was prepared by ≤5.0 mol% of ionization of the amide groups of a PA-66 into ammonium cations via facile, room-temperature solution reaction of the PA-66 with a dilute aqueous HCl, which was confirmed by Fourier transform infrared (FTIR) and 1H nuclear magnetic resonance (NMR) spectroscopies. A small amount (5.0 wt%) of the PA-66 added was found to effectively nucleate both the melt- and solution crystallizations of a poly(ethylene terephthalate) (PET) heterogeneously, as reflected from differential scanning calorimetry (DSC) melt crystallization temperature and -enthalpy increases, a wide-angle X-ray scattering (WAXS) degree of crystallinity (Xc) rise, polarized optical microscopy (POM) schlieren-texture densification and refinement, Mo's method nonisothermal crystallization kinetic a increase and -F(T) decrease of the PET matrix. Compared with the PET/PA-66 (5.0 wt%) system, the same content of the PA-66 ionene incorporated further displayed significantly higher or, say, maximized heterogeneous nucleation efficiency for the PET melt- and solution crystallizations, as verified by similar DSC, WAXS, POM, and Mo's approach observations, to again remarkably expedite the crystallization, enhance the Xc, and refine the crystal size of the PET matrix. Such extraordinarily high efficiency of the PA-66 ionene nucleator arose typically from, at the interphase, the ion‒dipole interactions (IDIs) formed between the amide-carbonyl affected ammonium-chloride ion pairs of the PA-66 ionene and the ester groups of the PET, which considerably were stronger in both covalent and ionic components than the hydrogen bonds present between the PA-66 amide groups and the PET ester groups. The IDIs constituted greatly enhanced interfacial adhesion of the PET/PA-66 ionene (5.0 wt%) to presumably improve its interfacial compatibility, hence refine the size and increase the number density of the PA-66 ionene crystal particles, all of which contributed collectively to the efficiency maximization of the PA-66 ionene heterogeneous nucleator for the PET crystallization. It seems that nucleator ionization offers an effective approach to the enhancement of heterogeneous crystalline nucleation of a polymer by another semicrystalline polymer nucleator.

A new promising nucleating agent for polymer foaming: effects of hollow molecular-sieve particles on polypropylene supercritical CO2 microcellular foaming.

Because polypropylene (PP) foam normally exhibits nonuniform cell size and cracked cellular structure, a narrow cell-size distribution and a well-defined morphology are always the focus of PP foaming technology. In this work, hollow molecular-sieve (MS) particles were applied as a potential nucleating agent in supercritical carbon dioxide (scCO2) foaming of PP. It was observed that the addition of MS particles largely narrowed the cell-size distribution. The resultant PP/MS foams exhibited significant concurrent enhancement in their cell density and mechanical properties: the cell density increased remarkably, by approximately 10 times, and the tensile strength increased from 6.1 MPa to 12.6 MPa. The hollow-structure MS particles resulted in a higher heterogeneous nucleation efficiency in the PP foaming process. We believe that the trapping of CO2 in the hollow holes of MS particles largely increased the solubility CO2 in PP and a number of gas cavities were formed. The existence of gas cavities reduced the energy barrier of heterogeneous nucleation, favoring the formation of a well-defined cellular structure. Additionally, the regular-hexagon shape of the cells might endow the PP foam with better mechanical properties compared with a circular cell shape.

Silica-Assisted Nucleation of Polymer Foam Cells with Nanoscopic Dimensions: Impact of Particle Size, Line Tension, and Surface Functionality

Core–shell nanoparticles consisting of silica as core and surface-grafted poly(dimethylsiloxane) (PDMS) as shell with different diameters were prepared and used as heterogeneous nucleation agents to obtain CO2-blown poly(methyl methacrylate) (PMMA) nanocomposite foams. PDMS was selected as the shell material as it possesses a low surface energy and high CO2-philicity. The successful synthesis of core–shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The cell size and cell density of the PMMA micro- and nanocellular materials were determined by scanning electron microscopy. The cell nucleation efficiency using core–shell nanoparticles was significantly enhanced when compared to that of unmodified silica. The highest nucleation efficiency observed had a value of ∼0.5 for nanoparticles with a core diameter of 80 nm. The particle size dependence of cell nucleation efficiency is discussed taking into account line tension effects. Complete engulfment by the polymer matrix of particles with a core diameter below 40 nm at the cell wall interface was observed corresponding to line tension values of approximately 0.42 nN. This line tension significantly increases the energy barrier of heterogeneous nucleation and thus reduces the nucleation efficiency. The increase of the CO2 saturation pressure to 300 bar prior to batch foaming resulted in an increased line tension length. We observed a decrease of the heterogeneous nucleation efficiency for foaming after saturation with CO2 at 300 bar, which we attribute to homogenous nucleation becoming more favorable at the expense of heterogeneous nucleation in this case. Overall, it is shown that the contribution of line tension to the free energy barrier of heterogeneous foam cell nucleation must be considered to understand foaming of viscoelastic materials. This finding emphasizes the need for new strategies including the use of designer nucleating particles to enhance the foam cell nucleation efficiency.

Facile preparation of open-cellular porous poly (l-lactic acid) scaffold by supercritical carbon dioxide foaming for potential tissue engineering applications

Porous poly (l-lactic acid) (PLLA) scaffold has been widely used as substitutes for tissue engineering. However, PLLA is intrinsically difficult to be produced for low-density and high porosity by physical foaming procedure, due to its weak melt strength, slow crystallization kinetic, and low heterogeneous nucleation efficiency in bulk. For the sake of enhancing the PLLA’s crystallization and heterogeneous nucleation in supercritical CO2 (ScCO2) foaming, a pressure-induced flow (PIF) method was applied on neat PLLA. As a result, the PLLA’s crystallinity and its spherulite size increased from original 15.3% and 5.0nm to 42.1% and 28.5nm, respectively. High-strength and low-density PLLA foams with uniform and controlled cellular morphology were feasibly produced, varying the foaming temperature from 100 to 140°C in a ScCO2 batch foaming process. The closed cellular structure was transformed to high-interconnected porous structure, when reducing the foaming temperature. The open-cellular PLLA scaffolds foams, with a relatively high open pore content of 77.3%, prepared using this approach can achieve a high porosity up to 92.5%, and exhibit excellent compressive stress. To evaluate its tissue engineering application, long-term culture of mouse embryonic fibroblast cells (MEFs) demonstrated that the selected open-cellular PLLA scaffold provided prominent advantages ranging from enhanced cell adhesion and proliferation to facilitated nutrient transport. The combination of PIF and CO2 foaming process can produce a great promising PLLA scaffold for tissue engineering with desirable open-cellular structure, high-strength, low-cost and easy production capacity.

Probing structure–heterogeneous nucleation efficiency relationship of mesoporous particles in polylactic acid microcellular foaming by supercritical carbon dioxide

Heterogeneous nucleating agents that can improve cell morphology of polymer foams have been studied extensively, however, the exact relationship among particle structure, surface modification and nucleating efficiency still remain elusive. Previously, we demonstrated that mesoporous structure benefited nucleating efficiency by comparing solid silica particles and mesoporous particles (MCM-41). Herein, the feasibility of using another type of mesoporous particles, namely, SBA-15 as a nucleating agent for polymer foaming was investigated, but surface modification issue was emphasized. Results reveal that SBA-15 particles show excellent nucleation performance on polylactic acid foaming, and such nucleating effect are dependent on the surface modification. The surface modification using fluorinated silane significantly decreases nucleation energy barrier, and thus shows the highest nucleation efficiency. This work provides a comprehensive insight into structure–nucleating efficiency relationship for mesoporous particles, and highlights the importance of surface modification as a structural factor to optimize the design of effective nucleating agents for polymer foaming.

Insight into the nucleating and reinforcing efficiencies of carbon nanofillers in poly(vinylidene fluoride): a comparison between carbon nanotubes and carbon black

Poly(vinylidene fluoride) (PVDF) nanocomposites containing homogeneously dispersed multi-walled carbon nanotubes (MWCNTs) and carbon black (CB) were fabricated by a small melt mixer. The uniform dispersion of the nanofillers in PVDF was confirmed by both scanning electron microscopy and transmission electron microscopy. Both the heterogeneous nucleation efficiency and crystallization half-time show that MWCNTs exhibit higher nucleation efficiency than CB for the crystallization of PVDF. Meanwhile, MWCNTs show greater contribution to the reinforcement of the storage modulus of PVDF as revealed by dynamic mechanical analysis, especially at low temperatures. However, the enhancement of the storage modulus in the melt state is reversed due to the network formed by serious agglomeration of CB. This study provides some insights into the nucleating and reinforcing efficiency of MWCNTs and CB in polymers.

Multiwalled carbon nanotubes grafted with polyamidoamine (PAMAM) dendrimers and their influence on polystyrene supercritical carbon dioxide foaming

In this study, two types of multiwalled carbon nanotubes (MCNTs) are used for the preparation of polystyrene (PS)/MCNT composites. For each type of MCNT, two surface modifications, i.e., grafted with PS brushes (MCNTs-PS) and grafted with polyamidoamine (PAMAM) dendrimers and subsequent PS brushes (MCNTs-PAMAM-PS), are applied. The effects of aspect ratio and surface modification of the MCNTs on their dispersion in the PS matrix, as well as the dynamic mechanical properties and supercritical carbon dioxide (scCO2) foaming of the composites, are investigated. The results indicate that the MCNTs-PS have a more uniform dispersion and a higher dynamic mechanical property reinforcement compared to MCNTs-NH2 and MCNTS-PAMAM-PS. For the heterogeneous nucleation of MCNTs in PS scCO2 microcellular foaming, long MCNTs show a small nucleation effect, while the nucleation effect of the short MCNTs is much more significant. Furthermore, the introduction of PAMAM dendrimers on the surface of MCNTs can improve the heterogeneous nucleation efficiency.

Synthesis of silica particles grafted with poly(ionic liquid) and their nucleation effect on microcellular foaming of polystyrene using supercritical carbon dioxide

We present the synthesis and characterization of silica particles modified by a poly(ionic liquid), namely, poly[2-(methacryloyloxy)ethyl]trimethylammonium tetrafluoroborate (P[MATMA][BF4]), which has a strong carbon dioxide absorption, and its application as a nucleation agent in polymer foaming. Silica particles with different particle sizes were fabricated by the sol–gel method and then grafted with (P[MATMA][BF4]) brushes by surface-initiated atom-transfer radical polymerization. The modified silica particles (SiO2P[MATMA][BF4]) were introduced into a polystyrene matrix by solution blending. The (P[MATMA][BF4]) brushes grafted onto the silica particles were able to slightly increase the carbon dioxide solubility, and consequently, SiO2P[MATMA][BF4] showed a higher heterogeneous nucleation efficiency in the foaming process compared to amino-functionalized silica particles (SiO2NH2) used as control.