Effect of temperature on particle size for vapor-phase synthesis of ultrafine iron particles

For the vapor-phase synthesis of iron particles from FeCl₂ at temperatures ranging from 800 to 950‡C the reason is sought why the model based on the classical nucleation theory brought an increase of particle size with temperature increase, in reverse to experimental observation. The nucleation rate according to the classical theory should decrease with a temperature increase, due to the decrease of super-saturation ratio resulting from the increase of vapor pressure. The decrease of nucleation rate ultimately leads to an increase of particle size. Yang and Qiu’s nucleation theory was applied in place of the classical theory. However, the same result as with the classical theory was obtained : the nucleation rate decreased with the temperature increase. Finally, an Arrhenius-type nucleation rate equation was introduced. The preexponential factor and the activation energy for nucleation were determined to be 1348.2 sec_-1 and 159.1 KJ/mol, respectively. With these values put into Park et al.’s model, good agreement was obtained in temperature dependence of particle size between model prediction and experimental data.     

Bi-/multi-modal pore formation of PLGA/hydroxyapatite composite scaffolds by heterogeneous nucleation in supercritical CO2 foaming

Scaffolds with multimodal pore structure are essential to cells differentiation and proliferation in bone tissue engineering.Bi-/multi-modal porous PLGA/hydroxyapatite composite scaffolds were prepared by supercritical CO_2 foaming in which hydroxyapatite acted as heterogeneous nucleation agent.Bimodal porous scaffolds were prepared under certain conditions,i.e.hydroxyapatite addition of 5%,depressurization rate of 0.3 MPa·min~(-1),soaking temperature of 55℃,and pressure of 9 MPa.And scaffolds presented specific structure of small pores(122 μm±66 μm)in the cellular walls of large pores(552μm±127μm).Furthermore,multimodal porous PLGA scaffolds with micro-pores(37 μm±11 μm)were obtained at low soaking pressure of 7.5 MPa.The interconnected porosity of scaffolds ranged from(52.53±2.69)% to(83.08±2.42)%by adjusting depressurization rate,while compression modulus satisfied the requirement of bone tissue engineering.Solvent-free CO_2 foaming method is promising to fabricate bi-/multi-modal porous scaffolds in one step,and bioactive particles for osteogenesis could serve as nucleation agents.     

Morphological changes in poly(?-caprolactone) in dense carbon dioxide

Morphological changes that take place in poly(?-caprolactone) upon exposure to carbon dioxide at high pressures have been explored as a function of pressure and temperature. SEM and DSC results point to a competition between CO₂-modulated crystallization and pressure-induced phase separation which leads to unique morphologies. At 293 K, exposure to CO₂ at pressures up to 45 MPa leads to recrystallization resulting in higher level of crystallinity and higher melting temperatures. Highest crystallinity levels along with distinct crystal morphology were observed after exposure to CO₂ at 308 K and 21 MPa. At a higher pressure at this temperature (308 K/34 MPa) polymer undergoes melting, and foaming is achieved during depressurization prior to solidification. At 323 K, the polymer is found to display unique crystal morphology with concave crystal geometry as well as porous domains. The results are discussed in terms of the crystallization and phase separation paths that are followed during exposure to CO₂ and the depressurization stages.     

Effects of compressed carbon dioxide treatment on the specificity of oxidase enzymatic complexes from mate tea leaves

This work evaluates the enzymatic activity of peroxidase (POD) and polyphenoloxidase (PPO) present in the crude extract of mate tea leaves (Ilex paraguariensis St. Hill) submitted to compressed CO₂. The effects of temperature, exposure time, solvent reduced density, pressure, and depressurization rate on the activity of peroxidase and polyphenoloxidase were evaluated through a fractionated factorial experimental planning. Results show that temperature of 30 °C, pressure of 70.5 bar, exposure time of 1 h, depressurization rate of 10 kg m⁻³ min⁻¹ and carbon dioxide reduced density of 0.60 led to an enhancement of around 25% in the peroxidase activity and a polyphenoloxidase activity loss of 50%. Using this experimental condition, thermal stability at low temperature (−4 °C) and the influence of successive pressurization/depressurization cycles were determined. Results suggest that it is possible to increase the specificity of the enzymatic extract towards enhancing POD or PPO activity depending on the experimental condition employed, and that the processing of enzymatic complexes with compressed CO₂ may be a promising route to increase the specificity of enzymatic extracts.     

Surface hierarchical porosity in poly (ɛ-caprolactone) membranes with potential applications in tissue engineering prepared by foaming in supercritical carbon dioxide

This article describes the preparation of porous poly (ɛ-caprolactone), PCL, membranes by supercritical CO₂ (SCCO₂) foaming, displaying surface hierarchical macroporosity which could be tailored by careful control of the pressure, in the range of 150–250 bar, and depressurization processes in several steps, showing also pore interconnectivity between both membrane faces. The membranes exhibited two distinct types of surface macroporosity, the larger with diameter sizes of 300–500 μm were surrounded by and also composed of smaller pores of 15–50 μm (same size as inner pores). Membranes were prepared by solvent casting and submitted to different SCCO₂ foaming. Parameters such as membrane thickness, CO₂ flow, foaming time, pressure, temperature and the depressurization processes (rate and profiles), were varied to determine their influence on final porosity and to decipher which parameters were the most critical ones in terms of surface hierarchical pore organization. No remarkable changes in PCL crystallinity were found when membranes were processed under SCCO₂. Finally, biological evaluation of the porous membranes was achieved by seeding human skin fibroblasts on the prepared membranes. The results, in terms of cell adhesion, spreading, proliferation and metabolic activity indicate that these membranes could hold promise for the fabrication of meshes with controlled porosity for tissue engineering applications.     

Fabrication and cell morphology of a microcellular poly(ether imide)–carbon nanotube composite foam with a three-dimensional shape

The preparation of microcellular poly(ether imide) (PEI) based foams with three-dimensional geometry remains a great challenge worldwide. In this study, we fabricated microcellular PEI–carbon nanotube (CNT) bead foams with a batch rapid depressurization method in a self-designed mold with supercritical carbon dioxide (scCO₂) as a blowing agent. The effects of the saturation time, foaming temperature, foaming pressure, and depressurization rate on the microcellular structures of the PEI foam were analyzed by the Taguchi approach to determine the optimum foaming conditions, and the influence of the CNT content on the cell structure was analyzed. The results show that the depressurization rate and foaming temperature were the key factors influencing the cell size and cell density (N _f); that is, the high depressurization rate and low foaming temperature favored a small cell size and high N _f. The foaming temperature also influenced the foaming ratio (ϕ), and a high ϕ was obtained at a high foaming temperature. Under optimal foaming conditions, PEI with 2.0 wt % CNTs presented the best cell structure; N _f, cell size, and ϕ were 6.14 × 10¹⁰ cell/cm³, 2.43 μm, and 2.08, respectively. The mechanical properties of the final parts were related more to the foaming time and CNT concentration, and the maximum tensile and compression strength were reached at 3 h foaming time and 2.0 wt % CNT, that is, at 2.75 and 15.1 MPa (10% strain), respectively. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47501.     

A novel method for preparing Si3N4 ceramics with unidirectional oriented pores from silicon aqueous slurries

We describe a low-cost and simple method to prepare porous silicon nitride (Si₃N₄) ceramics with unidirectional oriented pores. Using Si powders as raw material, porous ceramics were fabricated via solidification of the aqueous slurries followed by the nitridation and post-sintering. The formation mechanism is similar to the ordered porous metals that are fabricated under high temperature and high pressure. Differently, the green bodies with oriented pores were obtained at room temperature and atmospheric pressure. The nucleation and growth of the H₂ bubbles generated from hydrolyzed Si powders contributed to the highly oriented pores along the solidification direction of the aqueous slurries. Additionally, the pore microstructure was closely associated with the viscosity of the slurries, which can be well controlled by varying the concentration of organic additive and solid loading.     

Sintering behaviour of nano-crystalline γ-Al2O3 powder without additives at 2-7 GPa

Al₂O₃ ceramics were fabricated without additives under high pressure (2-7 GPa) at different temperatures (600-1200 °C) using nanocrystalline alumina powder with metastable γ-Al₂O₃ phase as the starting material.It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreases and nano-scale grains in the sintered structure can be achieved.On the other hand the sintered samples at 7 GPa and high temperature (1000 °C) have shown micron-scale large grain sizes compared to those sintered at lower pressures, for the same temperature and sintering time. This could be attributed to the higher input energy in the system at high pressure and high temperature conditions, thereby reaching the final stage in sintering more quickly.In this work, the best combination of grain size (∼200 nm) and density (98.0% TD) was obtained under the sintering condition of 1000 °C at 7 GPa with a holding time of 1 min.Thus for high pressure/high temperature conditions, the sintering time should be reduced to prevent grain growth.     

The nucleation kinetic aspects of gypsum nanofiltration membrane scaling

Gypsum NF-200 (Filmtec) membrane scaling was examined with respect to the nucleation theory. First the gypsum nucleation kinetic parameters — nucleation rate constant k_N and heterogeneous nucleation factor f(θ) — were calculated, with the model based on classical nucleation theory using the nonlinear (quasi-Newtonian) estimation method. Subsequently, the method of the maximum permissible water recovery bound (here expressed as maximum NF retentate supersaturation, S_max) prediction was proposed. When comparing the predicted and experimental data, it was found that predicted S_max values were lower than the measured ones; however, most of the results were within about a 10% confidence interval. The difference in the measured and predicted S_max values was identified to be the result of mass gypsum nucleation and the nuclei growth mechanism. Furthermore, when considering that the presence of the solid phase inside the NF module retentate channel is not recommended, we concluded that the proposed S_max prediction procedure better describes maximum permissible gypsum retentate supersaturation than the experimental one. Powder X-ray diffraction data show that the only solid phase present under our measurements conditions is gypsum.     

Effect of die temperature on the morphology of microcellular foams

A study on the extrusion of microcellular polystyrene foams at different foaming temperatures was carried out using CO₂ as the foaming agent. The contraction flow in the extrusion die was simulated with FLUENT computational fluid dynamics code at two temperatures (150°C and 175°C) to predict pressure and temperature profiles in the die. The location of nucleation onset was determined based on the pressure profile and equilibrium solubility. The relative importance of pressure and temperature in determining the nucleation rate was compared using calculations based on classical homogeneous nucleation theory. Experimentally, the effects of die temperature (i.e., the foaming temperature) on the pressure profile in the die, cell size, cell density, and cell morphology were investigated at different screw rotation speeds (10 ～ 30 rpm). Experimental results were compared with simulations to gain insight into the foaming process. Although the foaming temperature was found to be less significant than the pressure drop or the pressure drop rate in deciding the cell size and cell density, it affects the cell morphology dramatically. Open and closed cell structures can be generated by changing the foaming temperature. Microcellular foams of PS (with cell sizes smaller than 10 μm and cell densities greater than 10 cells/cm³) are created experimentally when the die temperature is 160°C, the pressure drop through the die is greater than 16 MPa, and the pressure drop rate is higher than 10⁹ Pa/sec.     

Study of different effect on foaming process of biodegradable bionolle in supercritical carbon dioxide

Microcellular foaming of biodegradable Bionolle in supercritical CO₂ has been produced. The effects of a series of variable factors, such as saturation temperature, saturation pressure, and depressurization time and step on the foam structures and density, were studied through measurement of density and SEM observation. The experimental results show that higher saturation temperatures lead to an increase in bulk densities; and different depressurization time and step result in different product cell morphology. In addition, at some saturation temperature, the orientation of the cells can be found in the product morphology. XRD experimental results show that the foaming treatment with SC CO₂ increased the crystallinity of Bionolle. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2901–2906, 2006     

Influence of Na2O content on the nucleation kinetics in glasses of compositions close to the Na2O · 2CaO · 3SiO2 stoichiometry

The kinetics of nucleation and growth of Na₂O · 2CaO · 3SiO₂ crystals in glasses with small deviations from the stoichiometric composition is studied. The stationary nucleation rate, induction period, and crystal growth rate as functions of the temperature and Na₂O content in the glass are measured. It is found that relatively small variations in the glass composition significantly affect the crystal nucleation rate. The experimental data are analyzed within the framework of the classical nucleation theory. It is shown that an increase in the Na₂O content in the glass brings about a decrease in the kinetic and thermodynamic barriers of nucleation. Deceased.     

A high-pressure polar light microscopy to study the melt crystallization of myristic acid and ibuprofen in CO2

A high-pressure polar light microscopy approach was proposed and developed to study the melt crystallization behaviors of myristic acid and ibuprofen respectively in CO₂ at different pressures and crystallization temperatures. The crystallization kinetics was analyzed by the Avrami equation. Results revealed that the crystallization rates of both myristic acid and ibuprofen increased with the CO₂ pressure, while the crystallization activation energy of ibuprofen decreased (more negative) with the increase of CO₂ pressure. On the other hand, the crystallization rate of ibuprofen decreased with the increase of the crystallization temperature at fixed pressure. However, the presence of CO₂ did not change the nucleation or growth patterns of myristic acid and ibuprofen, as indicated by the analyzed results of the Avrami equation. The X-ray diffraction (XRD) analysis further confirmed that CO₂ had no influence on the crystal form of myristic acid or ibuprofen. This study revealed that the crystallization behaviors of myristic acid and ibuprofen were evidently different from those of polymers in CO₂ reported in the literature.     

Kinetics of surface reactions in carbon deposition from light hydrocarbons

Carbon deposition from ethene, ethine and propene as a function of pressure was studied at various temperatures and two different surface area/volume ratios. Deposition rates as a function of pressure of all hydrocarbons indicate Langmuir-Hinshelwood kinetics which suggests that the deposition process is controlled by the heterogeneous surface reactions (growth mechanism). These kinetics are favored at decreasing reactivity (C₃H₆>C₂H₂>C₂H₄), decreasing temperature and residence time as well as increasing surface area/volume ratio. A linear rate increase at high pressures suggests that carbon is additionally or preferentially deposited by aromatic condensation reactions between polycyclic aromatic hydrocarbons large enough to be physisorbed or condensed on the substrate surface (nucleation mechanism). The results completely agree with earlier results obtained with methane.     

A parametric study into the morphology of polystyrene-co-methyl methacrylate foams using supercritical carbon dioxide as a blowing agent

In this study the foaming of poly(styrene-co-methyl methacrylate) (SMMA) using supercritical carbon dioxide is investigated. The effect of different foaming parameters such as temperature and pressure is studied in a quantitative and systematic way, with the aim to control and predict the resulting foam morphology. It is shown that once the polymer properties, such as the glass transition temperature and the solubility of CO₂ are known, full control of the desired foam morphology can be obtained by a proper selection of temperature, pressure and depressurization rate.     

Adsorption of Cu(II) from Aqueous Solution by Porous Mn3[Co(CN)6]2 · nH2O Nanospheres

Prussian blue analogue of porous Mn₃[Co(CN)₆]₂ · nH₂O nanospheres with a large surface area were prepared by simple mixing K₃[Co(CN)₆]₂ and manganous nitrate solution at room temperature. The morphology and structure of the prepared products were characterized by XRD, FE-SEM, TEM, and BET. The results indicated that the product was composed of nanospheres with the diameter of ～250 nm, which was of porous structure with the pore diameter in the 2.5–4 nm range. The adsorption behavior of Cu(II) ions from aqueous solution onto porous nanospheres was investigated as a function of parameters, such as the equilibrium time, the pH, the initial concentration, and the temperature. A maximum adsorption capacity of 140.85 mg g^?1 of Cu(II) was achieved. Due to the simple synthetic method and its high adsorption capacity, the porous nanospheres had the potential to be utilized as an effective adsorbent for Cu(II) removal.     

The effect of propylene–ethylene copolymers with different comonomer content on melting and crystallization behavior of polypropylene

The effect of propylene–ethylene copolymers (PEc) with different ethylene‐unit contents on melting and crystallization behaviors of isotactic‐polypropylene (iPP) were investigated by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The results show that the addition of PEc decreases significantly crystallization temperature (T_c) of iPP, but slightly affects melting temperature (T_m). With increasing the ethylene‐unit content of the propylene–ethylene copolymers, the decrease in crystallization temperature of iPP is smaller. The PLM results show that the spherulite growth rate decreases with increasing crystallization temperature for iPP and iPP/PEc blends. The higher the ethylene‐unit content of the copolymers is, the lower the spherulite growth rate (G) of iPP/PEc blends is. The influence of the PEc on nucleation rate constant (K_g) and fold surface energy (σ_e) of iPP was examined by nucleation theory of Hoffman and Lauritzen. The results show that both K_g and σ_e of iPP/PE20(80/20) and iPP/PE23(80/20) blends are higher than those of iPP, demonstrating that the overall crystallization rate of iPP/PEc blends decreased as compared to that of iPP, resulting from the decrease of the nucleation rate and the spherulite growth rate of iPP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Preparation of porous PLGA/HA/collagen scaffolds with supercritical CO2 and application in osteoblast cell culture

Biocompatible three-dimensional scaffolds for cell culturing may facilitate methods for the repair of damaged human tissues. A novel hybrid porous scaffold of poly(lactic-co-glycolic acid), hydroxyapatite and collagen was prepared using a supercritical CO₂ saturation technique. Expansion factors of scaffolds with different compositions were studied after supercritical CO₂ treatment to choose the optimal composition for three-dimensional culture. The supercritical CO₂ process conditions, such as saturation temperature, saturation time and saturation pressure were varied to evaluate their influence on pore structure. The results showed that the pore size and porosity of the scaffold could be controlled by manipulating these process conditions. The porous samples were characterized by environmental scanning electron microscopy, energy-dispersive X-ray spectroscope, Fourier transform infrared spectroscopy and X-ray diffractometry. Finally, MG-63 cells were successfully cultured on the porous scaffold as assessed by electron and confocal microscopy, confirming the biocompatibility of this new hybrid porous scaffold.     

Experimental study of CO2 hydrate formation in porous media with different particle sizes

To investigate the effect of the particle size of porous media on CO₂ hydrate formation, the formation experiments of CO₂ hydrate in porous media with three particle sizes were performed. Three kinds of porous media with mean particle diameters of 2.30 μm (clay level), 5.54 μm (silty sand level), and 229.90 μm (fine sand level) were used in the experiments. In the experiments, the formation temperature range was 277.15–281.15 K and the initial formation pressure range was 3.4–4.8 MPa. The final gas consumption increases with the increase in the initial pressure and the decrease in the formation temperature. The hydrate formation at the initial formation pressure of 4.8 MPa in 229.90 μm porous media is much slower than that at the lower formation pressure and displays multistage. In the experiments with different formation temperatures, the gas consumption rate at the temperature of 279.15 K is the lowest. In 2.30 and 5.54 μm porous media, the hydrate formation rates are similar and faster than those in 229.90 μm porous media. The particle size of the porous media does not affect the final gas consumption. The gas consumption rate per mol of water and the final water conversion increase with the decrease in the water content. The induction time in 5.54 μm porous media is longer than that in 2.30 and 229.90 μm porous media, and the presence of NaCl significantly increases the induction time and decreases the final conversion of water to hydrate.