High Nucleation Rate Research Articles

Water‐blown polyurethane–clay nanocomposite foams from biopolyol—effect of nanoclay on the properties

The present work deals with the development of polyurethane–clay nanocomposite foams by replacing part of the synthetic polyol with castor oil derivative. Hydroxylated castor oil was converted into diethanol amide by transamidation and the resulting polyol was formulated into water‐blown foams. Modified montmorillonite clay was used as nanofiller in different amounts viz. 0.5%, 1.0%, 2.0%, and 5.0% by total weight of the foam formulation. Rheological measurements on the polyol–clay mixtures indicated that up to 1% clay loading there is no significant change in the viscosity with shear rate and beyond 2%, shear thinning occurred. X‐ray diffraction studies further substantiated these results. The effect of the modified clay on the density, mechanical properties such as compression strength, compression modulus, and microstructure of the foams were investigated. The filler thus added had a reinforcing effect on the foam as observed in the density and compression strength measurements. Differential scanning calorimetric studies on Tg and dynamic mechanical analyses on the modulus clearly indicated that 1% clay loading and above led to exfoliation and plasticizing effect. Exfoliated nanocomposites in compositions containing 1% clay and more yielded a much higher nucleation rate than intercalated ones leading to reduced cell size as observed by optical and scanning electron microscopy. Thus, castor oil, which is readily available, relatively inexpensive, and environmentally benign nonedible oil, has been successfully used to prepare filled semirigid foams which can find application in insulation and packing. POLYM. COMPOS. 34:1306–1312, 2013. © 2013 Society of Plastics Engineers

Nucleation and crystallization kinetics of rapidly quenched lithium pyrophosphate glass

66.7Li2O–33.3P2O5 mol% glass prepared by twin-roller rapid quenching has been chosen to investigate the nucleation and crystallization kinetics using differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and high temperature X-ray diffraction (XRD) as the experimental tools. Such studies assume importance in designing materials for solid state ionic devices. The glass exhibits a nucleation rate of 3.1 × 1018 m− 3 s− 1 at a typical temperature of 583 K and crystal growth rate of 2.1 × 10− 2 nm s− 1 at 643 K. Non-isothermal DSC studies reveal that the glass undergoes bulk crystallization with an activation energy of 114 kJ mol− 1. The crystallized phase is identified as lithium pyrophosphate (Li4P2O7). Crystallization kinetic studies reveal that Li4P2O7 crystallizes three dimensionally with increasing number of nuclei. Interestingly, lithium metaphosphate (LiPO3) appears as a secondary phase in the glass matrix, however, the final glass ceramic product obtained after prolonged heat treatment of the glass is single phase Li4P2O7. The secondary phase also grows three dimensionally with an activation energy of 365 kJ mol− 1, but with constant number of nuclei in contrast to Li4P2O7. The high nucleation and growth rate with lower activation energy for crystallization of Li4P2O7 underscore that the glass is extremely prone to nucleation and crystallization.

Damage evolution in TWIP and standard austenitic steel by means of 3D X ray tomography

The evolution of ductile damage of Fe–22Mn–0.6C austenitic TWIP steel by means of 3D X ray tomography in-situ tensile tests is reported for the first time. The comparison with another fully austenitic steel (316 stainless steel) is also carried out. The damage process of TWIP steel involves intense nucleation of small voids combined with the significant growth of the biggest cavities whereas macroscopical triaxiality remains constant. Due to this high nucleation rate, the average cavity diameter remains constant unlike the 316 stainless steel.

Microstructure evolution during novel rheorolling process for producing A356 alloy strip

Microstructure evolution during a novel rheorolling process for producing A356 alloy strip was investigated. Under the high cooling rate caused by the sloping plate and stirring action caused by the vibration and metal flow, high nucleation rate and dendrite breakage happened during the process, which causes the formation of fine spherical or rosette primary grains. The grain size of the primary α-Al phase decreased with the decrease in casting temperature in the cooling slope, but the volume fraction of the primary α-Al phase increased. The change of the primary grain shape is not obvious in the roll gap during the rolling process, and most of the primary grains nearly kept round or rosette shapes. When the casting temperature is 660°C, the strip with a cross-section of 4×160 mm was produced, and homogeneous microstructure was obtained.

Ionic liquids for the synthesis of metal nanoparticles

AbstractIonic liquids (ILs) offer outstanding possibilities as media for manufacturing nanoparticles. Synthesis conditions with high reaction and nucleation rates are achievable leading to the formation of extremely small particles. The IL itself can act as an electronic as well as a steric stabiliser preventing particle growth and particle aggregation. In addition, as highly structured liquids, ILs have a strong effect on the morphology of the particles formed. We have developed two synthesis techniques for the generation of metal nanoparticles that take advantage of the unique properties that ILs offer when compared to conventional volatile organic solvents (VOCs): microwave (MW) synthesis and physical vapour deposition (PVD). The ionic character and high polarisability of the IL renders it highly susceptible to energy uptake via MWs and extreme heating and reaction rates can be achieved. To make full use of the possibilities that ILs offer we have designed a set of reducing ILs which can be used as direct reaction partners for the generation of metal nanoparticles. The negligible vapour pressure of many ILs makes experiments under high vacuum possible and allows for the PVD of metals into ILs. magnified imagePhysical vapour deposition (left) and microwave synthesis of metal nanoparticles in ILs.

The structure of nanoparticle nucleation in planar jets

The effects of large-scale mixing and vapor concentration on homogeneous nucleation rates are investigated via direct numerical simulation of dibutyl-phthalate (DBP) nucleation during cooling and mixing in three-dimensional planar jets. In the simulated cases, a heated jet doped with DBP issues into a co-flow of room-temperature air. As the two streams mix, the DBP vapor becomes highly supersaturated and particles are formed by nucleation. This particle formation takes place in the absence of condensation or coagulation. The simulation results provide a demonstration of how nucleation takes place in narrow regions where molecular mixing of the two streams occurs. When maximum nucleation rates occur in conditions where the nucleation rates are sensitive to ambient conditions, islands of nucleation form. There are two possible nucleation events: initial shear layer nucleation, and later nucleation in coherent structures or eddies generated by the velocity difference between the jet and the co-flow. A scatter plot diagram of observed dilution paths in temperature versus condensable vapor concentration space where nucleation rates are superimposed is shown to be a convenient tool for analyzing nucleation events. Convection by large-scale eddies gradually spreads the range of mixing paths in this space toward higher nucleation rates. The results also show that boundary conditions, including inlet concentration and velocity ratio, have both qualitative and quantitative effects on particle nucleation.

Investigation of the crystallization behavior of isotactic polypropylene polymerized with different Ziegler‐Natta catalysts

AbstractDetailed characterization of the crystallization behavior is important for obtaining better structure property correlations of the isotactic polypropylene (iPP), however, attributed to the complexity in ZN‐iPP polymerization, the relationship between crystallization behavior and the stereo‐defect distribution of iPP is still under debate. In this study, the crystallization kinetics of the primary nucleation, crystal growth and overall crystallization of two iPP samples (PP‐A and PP‐B) with nearly same average isotacticity but different stereo‐defect distribution (the stereo‐defect distribution of PP‐B is more uniform than PP‐A) were investigated. The results of isothermal crystallization kinetics showed that the overall crystallization rate of PP‐A was much higher than that of PP‐B; but the analysis of self‐nucleation isothermal crystallization kinetics and the polarized optical microscopy (POM) observation indicated that the high overall crystallization rate of PP‐A was attributed to the high primary nucleation rate of the resin. The stereo‐defect distribution plays an important role in determining both the nucleation kinetics and crystal grow kinetics, and thus influence the overall crystallization kinetics. A more uniform distribution of stereo‐defects restrains the crystallization rate of iPP, moreover, it has more influence on nucleation kinetics, comparing with the crystal growth. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Extracting Aluminum from Aluminum Alloys in AlCl3-NaCl Molten Salts

AbstractExtracting aluminum from aluminum alloys in AlCl3-NaCl molten salts was investigated in this paper. The influences of experimental parameters such as electrolyte composition, cathodic current density and electrolysis time on the deposits morphology were discussed. The results show that the grain size of the deposits decreases with the increase of AlCl3 content in the electrolyte. Current density has a big effect on the morphology of the deposits. The particle size of deposits increases with the increase of current density, and dendritic morphology forms at high current density. High nucleation rates are achieved at high current densities above the limiting diffusion current density, and will result in a finer grain size. A non-dendritic deposit of aluminum was obtained at 170°C at 50 mA cm−2 cathodic current density for 1 h in the electrolyte having a 1.3 molar ratio of AlCl3/NaCl. The purity of the aluminum deposit is about 99.79% analyzed using inductively coupled plasma.

Microstructure evolution and solidification behaviors of A2017 alloy during cooling/stirring and rolling process

A novel semisolid rheo-rolling process of A2017 alloy was achieved by combining the shape rolling mill with the vibrating sloping plate device. The microstructure evolution and solidification behaviors during the process were investigated. The high cooling rate caused by the sloping plate and stirring action caused by the vibration and metal flow lead to a high nucleation rate as well as two primary grain growth patterns, direct globular growth as well as dendrite growth and subsequent breakage, which causes the formation of fine spherical or rosette primary grains. During the rolling process, the grains of the strip were elongated. The primary grain size of A2017 alloy strip increases with the increment of casting temperature. When the casting temperature was between 650 °C and 660 °C, A2017 alloy strip with good quality was produced by the proposed process. The microstructures of the strip are mainly composed of spherical or rosette grains.

Unsaturated Emulsifier-Mediated Modification of the Mechanical Strength and Oil Binding Capacity of a Model Edible Fat Crystallized under Shear

The effects of processing using a scraped surface heat exchanger (SSHE) before and after adding unsaturated monoglyceride (UM) on blends of fully hydrogenated soybean oil (FHSO) and soybean oil (SO) were studied. Mixtures of 40:60 and 45:55 FHSO:SO were melted at 80 °C for 30 min and crystallized statically or in the SSHE (shear rate of 25 s(-1)) at a cooling rate of 9 °C/min. Upon shearing and UM addition, polymorphic transformations toward more (β) or less (β') stable forms were governed by the combination between system concentration, composition, and crystallization conditions, as determined by differential scanning calorimetry and powder X-ray diffraction. Nuclear magnetic resonance was used to measure the solid fat content (SFC) development which showed to increase with processing conditions due to the high nucleation rate induced. Processing conditions greatly affected the nano- and microcrystalline structures which were characterized by polarized light microscopy (PLM), cryogenic transmission electron microscopy (Cryo- TEM), and Scherrer analysis of the powder X-ray diffraction data. Crystallization under shear promoted the longitudinal growth of the nanoplatelets; nevertheless, meso structural elements showed a decrease in their dimensions under the same crystallization conditions. The relative oil loss determined gravimetrically was inversely related to the elastic modulus and yield stress of the sheared fat blends, and values were closer to the desirable usability ranges for bakery applications. Our results suggest that fully hydrogenated fats can be functionalized by crystallization in a SSHE and/or by judicious addition of an unsaturated emulsifier.

A Novel Process in Semi-Solid Metal Casting

In this research a new process for semi-solid casting of ductile iron based on the high nucleation rate combined with locally mechanical stirring is presented. In this process at first fully liquid ductile iron was poured on the peripheral surface of a wheel rotating against pouring direction. At this stage, the solid crystals nucleated at the chilling surface were pushed to the melt by a heat resistance steel cutter and finally the semi-solid slurry was generated. Reheating treatment was done on the samples to achieve more efficiency of semi-solid casting process. The effects of the travelling distance of solid particles during casting, the reheating time and temperature were examined. The results showed that the process effectively changes the dendrite structure to globular one.

Effect of propane/silane ratio on the growth of 3C-SiC thin films on Si(1 0 0) substrates by APCVD

3C-SiC thin films were grown on Si(100) substrates at 1200°C by atmospheric pressure chemical vapor deposition. We performed an in-depth study on the effect of propane/silane ratio (it is expressed by C/Si which means the ratio of C atom to Si atom in propane and silane) on crystalline quality and microstructure of 3C-SiC thin films. The 3C-SiC thin films exhibit epitaxial nature with pyramid-like morphology or polycrystalline columnar grains with rounded shape, which are dependent on C/Si ratio. The growth mechanism of 3C-SiC film with different C/Si ratios is discussed based on the cross-sectional transmission electron microscopy characterization. The changes in crystalline and microstructure with increasing C/Si ratio are attributed to high secondary nucleation rate.

Electropulsing strengthened 2GPa boron steel with good ductility

For the purpose of further enhancing the mechanical properties of a boron steel used in the automotive industry, the electropulsing is applied to strengthen the steel. The results show that the steel exhibits unexpected high strength and ductility. The engineering ultimate-tensile-strength and fracture strain are about 2022MPa and 0.246, respectively. The microstructure examinations indicate that the martensitic was refined greatly during the electropulsing strengthening process. The fine martensitic with high-density dislocation makes the steel with good mechanical properties. The amazing results can be attributed to the thermal and athermal effects of the electropulsing. Due to the effect of rapid heating during electropulsing, a large overheating could be obtained, which could result in the high nucleation rate of austenite. Moreover, the pulse current itself can increase the austenite nucleation rate by decreasing the thermodynamic barrier. Finally, the fine lath-martensitic with high-density dislocation formed during the subsequent quenching process.

Evaluation of the guided random parameterization method for critical cooling rate calculations

We focus on a recently suggested approach to the calculation of critical cooling rates for glass formation. It is a “random parameterization” method that is guided by a limited number of isothermal scanning calorimetry experiments. However, several assumptions have been made in its derivation that may not mirror the actual crystallization behavior of most supercooled liquids, which may jeopardize the estimation of glass forming ability. We evaluate those assumptions and the applicability of the method is tested for lithium disilicate glass (which displays moderate internal nucleation rates) and dibarium titanium silicate glass (which displays very high internal nucleation rates, similar to those of metallic glasses). Both glasses nucleate homogeneously and exhibit polymorphic crystallization. Our calculations show that some overlooked variables, such as the sample geometry, nucleation induction-times, surface crystallization and the breakdown of the Stokes–Einstein/Eyring equation, have significant roles on the calculated time–temperature–transformation curves during heating experiments. We demonstrate that the proposed random parameterization method can only be used when a glass forming liquid that undergoes internal crystallization is cooled from above its liquidus to various test temperatures. If the sample undergoes predominant surface crystallization or if it is heated to the test temperature several corrections must be made.

Experimental determination of the solid–liquid equilibrium, metastable zone, and nucleation parameters of the flunixin meglumine–ethanol system

Measurements of the metastable zone and solubility for flunixin meglumine–ethanol system were obtained. The solubility was measured within the temperature range from 288.15 to 328.15K. The mole fraction solubility was correlated satisfactorily with the temperature by the equation: xeq=2.35×10−12e0.07121T. The value of enthalpy of dissolution, enthalpy of fusion and enthalpy of mixing were determined to be 49.04, 64.03 and −14.99kJmol−1 respectively. The metastable zone width of flunixin meglumine was measured by an electric conductivity method. A comparison of the nucleation temperatures from electric conductivity measurement and from focused beam reflectance measurement (FBRM) shows that both detection techniques give almost the same results for flunixin meglumine. The nucleation parameters of flunixin meglumine in ethanol were determined from the metastable zone data. Over the equilibrium temperature range from 312.28 to 325.55K, the nucleation rate constant was varied from 0.00001 to 0.00120#/m2min, whereas the nucleation order was varied from 2.23022 to 3.39299. The obtained high values of nucleation order indicated a high rate of nucleation.

Nanocrystalline diamond coating on carbon fibers produced at different temperatures: Morphological, structural and electrochemical study

In this paper, the morphological, structural and electrochemical properties of nanocrystalline diamond (NCD) films grown on carbon fibers (CF) were investigated. The CF substrates were produced at three different heat treatment temperatures (HTT):1000, 1500 and 2000°C. The HTT variation promoted different organization indexes on the CF structures. Consequently, the NCD coating formation was strongly affected by the substrate HTT. The changes in the properties of the diamond films were discussed as a function of the film morphology evolution using CH4 flow rate of 0.25, 0.5 and 1.0sccm in the feed gas. The X-ray diffraction measurements for the CF and NCD/CF composites were determinant to characterize the crystallinity of the NCD films as a function of the CF HTT and of the CH4 addition. Based on the diffractograms, the Scherrer's equation was applied to the (111) NCD peak, resulting in grain size values varying from 11.0 to 5.0nm depending on the CH4 flow rate and on the CF HTT. The scanning electron microscopy images confirmed the deposition of a continuous NCD coating with high nucleation rate covering the whole CF, while their quality was analyzed by Raman spectroscopy measurements. The NCD grain agglomerates increased as a function of the increase in the CH4 flow rate from 0.25 to 1.0sccm, showing similar film morphology to that of the unfaceted diamond balls obtained by chemical vapor deposition. This behavior confirmed the expected tendency by decreasing the diamond quality with the CH4 addition, especially for the films grown on CF treated at 1500 and 2000°C. This performance was also corroborated by the cyclic voltammetry measurements concerning the electrode potential window and their responses in a redox couple.

Microstructure evolution and properties of Mg–3Sn–1Mn (wt%) alloy strip processed by semisolid rheo-rolling

The high cooling rate caused by the sloping plate and stirring action caused by the vibration and metal flow lead to a high nucleation rate as well as two primary grain growth patterns, direct globular growth as well as dendrite growth and subsequent breakage, which causes the formation of fine spherical or rosette primary grains. The change of the primary grain shape is not obvious in the roll gap. However, solid fraction increases from the entrance to the exit of the roll gap. The spherical or rosette grains were remained eventually. When the casting temperature is 670°C, and the vibrating amplitude is 1.5mm, the strip with a cross section of 4mm×160mm was produced. Homogeneous microstructure was obtained. Mechanical properties of the present product were higher than that of Mg–3Sn–1Mn (wt%) alloy casting with the addition of 0.87wt% Ce.

Preparation of ZSM-5 zeolite coatings within capillary microchannels

A method is described to incorporate a zeolite ZSM-5 layer as a potential catalytic coating or membrane into tubular supports by the in situ crystallization of a previously deposited silica layer. The silica layer simultaneously provides nucleation sites and functions as a nutrient source giving coatings with excellent uniformity and controllable coating thickness. The ability to control the simultaneous dissolution of the silica layer in conjunction with nucleation and growth processes was found important in controlling the morphology of the zeolite crystals contained in the final zeolite coating. The formation of a ±4 μm coating containing ±300 nm crystals was obtained at high silica dissolution and nucleation rates. A continuous intergrown zeolite layer of ±7.6 μm was obtainable at high silica dissolution rates in the presence of sodium.

Grain refinement of Al–Cu alloy in low voltage pulsed magnetic field

The effect of a low voltage pulsed magnetic field (LVPMF) on grain refinement of Al–Cu alloy was investigated at different solidification stages. The cooling curve was also studied to investigate the grain refinement mechanism of LVPMF. The fine grains are obtained by applying the LVPMF during the nucleation stage. The LVPMF has no obvious influence on the solidification structure when it is applied during liquid phase stage or crystal growth stage. Application of LVPMF increases the nucleation temperature of the isomorphous transformation, and also decreases the recalescence magnitude of the alloy. The refining mechanism was proposed that the LVPMF provides extra energy for nucleation, which decreases the energy barrier and the critical radius for nucleation, leading to high nucleation rate and grain refinement.

Crystallisation kinetics of palm stearin, palm kernel olein and their blends

The kinetics of isothermal crystallisation of palm stearin, palm kernel olein and their blends (20–80g/100g palm stearin with 20g/100g increment) were determined using differential scanning calorimetry (DSC). The mechanism of crystallisation (n) was calculated by applying the Avrami equation using the crystallisation curves obtained at 10, 15 and 20°C. The DSC isothermal crystallisation data at 10, 15 and 20°C fitted well into the Avrami equation over the entire fractional crystallisation with correlation coefficient always greater than 0.98. Based on the Avrami exponent obtained, the palm stearin and palm kernel olein have different nucleation and growth mechanisms. The suggested mechanism for palm kernel olein was high nucleation rate at the beginning of crystallisation, which decreased with time, and plate-like growth (n=2). On the other hand, the mechanism for palm stearin was instantaneous heterogeneous nucleation followed by spherulitic growth (n=3). For blends of palm stearin and palm kernel olein, the mechanisms of crystallisation were 2 and 3 depending on the composition of the blends and crystallisation temperatures.