The effect of washing process on the cracking of ZrO2 microspheres by internal gelation

ZrO₂ microspheres are widely used as a simulant of UO₂ in the development of nuclear fuel. However, the cracking of ZrO₂ microspheres prepared by internal gelation is still a challenge during drying and sintering processes. To address this issue, we designed and optimized the washing process for obtaining crack-free ZrO₂ microspheres. Through thermogravimetric, infrared, Raman, BET, and SEM analysis, it is shown that the cracking of the microspheres is mainly related to the pores in microspheres. The washing solvent with low surface tension is used to reduce the effect of capillary force on pore shrinkage. Therefore, the optimal washing process was designed as trichloroethylene (TCE)—0.5 M NH₃.H₂O—Propylene glycol methyl ether (PM) and gel microspheres with a high specific surface area of 315.3 m²/g and pore volume of 0.4125 cm³/g were obtained. The characterizations also further showed that when the microspheres were dried and sintered, the water vapor and the decomposition gas of organic matter were completely released from the pores in the microspheres. Our new washing process could be directly extended for preparing crack-free ceramic microspheres by internal gelation.     

Fabrication of CeO2 microspheres by internal gelation process using flow-focusing droplet generator

Sphere-pac fuel is an alternative nuclear fuel technology in which microspheres of two or more sizes are utilized to fill the cladding tube in place of the more conventional single-size fuel pellets. This provides leeway for adjusting the fuel pellet packing density and resulting cladding tube porosity. The current investigation makes use of a flow-focusing droplet generator made from stainless steel (S.S.) 316 L, with a channel internal diameter (I.D.) of 0.5, 0.8, 1, and 3 mm. These microspheres were supposed to be of actinide oxide but here, cerium has been chosen as a surrogate of plutonium. Detailed information about the flow-focusing droplet generator, internal gelation process, and sphere-pac fuel has been provided. The size and size distribution of ceria microspheres were investigated by varying the flow rates of the continuous and dispersed phase. The characterization of ceria microspheres has been conducted using techniques such as scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) analyses. The size of prepared monodisperse microspheres was controlled precisely (within ±2%) in the range of 498–2888 μm using four S.S. 316 L flow-focusing droplet generators with channel I.D. 0.5, 0.8, 1, and 3 mm, respectively, and the coefficient of variation of the size distribution was found to be less than 2%.     

Preparation of UC ceramic nuclear fuel microspheres by combination of an improved microwave-assisted rapid internal gelation with carbothermic reduction process

Uranium carbide (UC) ceramic microspheres filled into a cladding are a potential nuclear fuel format for nuclear reactors. Uniform sized ceramic UC microspheres with a diameter of 675?±?10?µm were successfully prepared by an improved microwave-assisted rapid internal gelation process combined with carbothermic reduction. First of all, the nanoparticle carbon was dispersed into the HMUR stock solution, and the C-UO₃·2H₂O gelled microspheres were prepared using an improved microwave-assisted internal gelation process without cooling the initial stock solutions. Next, the gelled microspheres were subjected to a carbothermic reduction process to obtain ceramic UC microspheres. TG and XRD investigations indicated that the C-UO₃·2H₂O microspheres were firstly reduced into UO₂ at a temperature of 700?°C, and were further converted into UC at 1500?°C in argon atmosphere. Crack-free ceramic UC microspheres with a smooth and metallic shiny surface were obtained at a sintering temperature of 1500?°C for 5?h with an initial C/U molar ratio of 3.5.     

An improvement of microfluidic-assisted internal gelation in the preparation of millimeter-sized ceramic microspheres

The microspheres prepared by the microfluidic-assisted internal gelation process are still challenging to reach the millimeter size due to the large gravity. The gravity would affect the sphericity and size uniformity of microspheres. The improved microfluidic-assisted internal gelation process is designed and optimized to produce large-sized monodisperse ceramic microspheres with good sphericity. The movement mechanism of millimeter-sized sol droplets and gel microspheres in the microchannel is summarized and the state of the dispersed phase entering the measuring cylinder is controlled. The friction between the gel microspheres and the tubing wall is reduced as much as possible, and ZrO₂ ceramic microspheres of 500 ± 5 μm with good sphericity are prepared with the broth at room temperature. The mechanism of sol droplets entering the oil surface is explored to demonstrate the superiority of the improved microfluidic-assisted internal gelation process. The improved microfluidic-assisted internal gelation process could be directly extended to preparing other millimeter-sized monodisperse ceramic microspheres.     

Polymerization of ionic monomers in polar solvents: kinetics and mechanism of the free radical copolymerization of acrylamide/acrylic acid

A precise data set describing the kinetics of the free radical copolymerization of acrylamide/acrylic acid (AM/AA) in the range of low total monomer concentration as a function of the pH, total monomer concentration, initiator concentration, and comonomer ratio is presented. Strong impact on the reactivity ratios has been identified for the pH and total monomer concentration. Specifically, at constant total monomer concentration of 0.4 mol/l and T=313 K the reactivity ratio of AM increases from 0.54 at pH 1.8 to 3.04 at pH 12. Contrarily, the reactivity ratio of AA decreases from 1.48 to 0.32. The crossover occurs at pH≈4.2. Electrostatic effects due to the variation of the degree of ionization of AA are primarily suggested to influence the kinetics. When the total monomer concentration increases from 0.2 to 0.6 mol/l at constant pH=12, the reactivity ratios of AM and AA decrease from 4.01 to 2.13 and increase from 0.25 to 0.47, respectively. Reduction of electrostatic repulsion between the ionized monomer AA and partially charged growing polymer chain ends due to higher ionic strength at higher total monomer concentration serves as explanation of the effect. The precise data set is the prerequisite for a novel approach to calculate copolymer compositions in case of variable monomer reactivity.     

小麦秸秆接枝丙烯酸制备高吸水性树脂

以N,N'-亚甲基双丙烯酰胺为交联剂,过硫酸钾一硫代硫酸钠为引发剂,采用水溶液聚合法合成了小麦秸秆接枝丙烯酸系列高吸水树脂。研究了单体配比、丙烯酸中和度、引发剂及交联剂用量对吸去离子水的影响。结果表明,该树脂具有良好的吸水性能,吸收去离子水达617 g/g。     

氧化还原引发淀粉与丙烯酸接枝共聚研究

研究了马铃薯淀粉与丙烯酸的接枝共聚反应,以过硫酸钾-亚硫酸氢钠为氧化还原引发体系,考察了不同引发剂浓度、单体浓度、反应温度、反应时间等对单体转化率、接枝率和接枝效率的影响;用红外光谱对共聚物进行了表征。结果表明,采用氧化还原引发体系可使聚合反应低温快速进行,并确定反应较佳条件。     

Preparation and characterization of chitosan-poly(acrylic acid) polymer magnetic microspheres

A modified method to prepare chitosan-poly(acrylic acid)(CS-PAA) polymer magnetic microspheres was reported in this paper. First, via self-assembly of positively charged CS and negatively charged Fe₃O₄ nanoparticles, magnetic CS cores with a large amount of Fe₃O₄ nanoparticles were successfully prepared. Subsequently, the AA monomers were polymerized on the magetic CS cores based on the reaction system of water-soluble polymer-monomer pairs. These polymer magnetic microspheres had a high Fe₃O₄ loading content, and showed unique pH-dependent behaviors on the size and zeta potential. From the magnetometer measurements data, the CS-PAA polymer magnetic microspheres also had superparamagnetic property as well as fast magnetic response. A continuous release of the entrapped ammonium glycyrrhizinate in such polymer magnetic microspheres occurred, which confirmed the potential applications of these microspheres for the targeted delivery of drugs.     

聚乳酸微球的制备

以L-乳酸为原料,锡粒为催化剂,实现了丙交酯的开环聚合反应,红外光谱结果表明,合成了分子量可控的聚乳酸。以二氯甲烷为溶剂,聚乙烯醇为表面活性剂,制备了聚乳酸微球,研究了聚乙烯醇浓度对聚乳酸微球的影响。结果表明,当聚乙烯醇浓度增加时,微球半径变小,但粒径分布均匀度下降,聚乙烯醇浓度为1%时,聚乳酸成球效果较好。     

Cross-linking polymerization of acrylic acid in supercritical carbon dioxide

Cross-linking polymerization of acrylic acid in supercritical carbon dioxide (scCO₂) was studied in a batch reactor at 50 °C and 207 bar with either triallyl pentaerythritol ether or tetraallyl pentaerythritol ether as the cross-linker and with 2,2′-azobis(2,4-dimethyl-valeronitrile) as the free radical initiator. All polymers were white, dry, fine powders. Scanning electron microscopy showed that the morphology of the polymer particles was not affected by cross-linking. As the cross-linker concentration was increased, the polymer glass transition temperature first decreased, then increased. Water-soluble and water-insoluble polymers were synthesized by adjusting the cross-linker concentration. Viscosity measurements showed that the polymer thickening effect strongly depended on the degree of cross-linking. Finally, cross-linking polymerization of acrylic acid in scCO₂ was carried out in a continuous stirred tank reactor. The use of cross-linker decreased the monomer conversion in this system.     

Microfluidic fabrication of ZrO2 microspheres using improved external gelation aided by polyacrylamide networks

Microfluidic generation of emulsion microdroplets, combined with external gelation, is a promising method for the continuous production of high-quality ceramic microspheres. However, the external gelation mode is sensitive to the environmental concentration of sol-gel triggering agent, resulting in gel deformation due to uneven infiltration and reaction within the sol microdroplets. In this work, a polyacrylamide (PAM) gel network was used to prevent the microdroplets from undergoing apparent deformation throughout the gelation process, and dense ZrO₂ microspheres with high monodispersity and sphericity were fabricated successfully. Importantly, owing to the assistance of PAM network, the external gelation process becomes less sensitive and can be treated with a substantially higher triggering agent concentration, thus allowing a faster and more efficient sol-gel production of ZrO₂ ceramic microspheres.     

丙烯酸与透明质酸共聚制备耐盐性凝胶

将丙烯酸与透明质酸钠盐共聚制备了新型聚丙烯酸凝胶,对凝胶性质进行了测定。当丙烯酸添加量为6 mL、交联剂与单体摩尔比为0.015、透明质酸添加量为0.18 g时,凝胶有最好耐盐性及凝胶强度,在0.9%质量分数NaCl溶液中这种接枝共聚物的吸水率为122.695 g/g,在磷酸缓冲液中的吸水率为90.298 g/g;同时凝胶的机械强度也有了明显的改善,达到401.485kPa。添加透明质酸后所得凝胶有更好的耐盐性且透明度好,适于应用于眼科材料。     

双乳化-凝胶法制备单分散海藻酸钙微球及其载BSA研究

采用双乳化-凝胶法制备了单分散的海藻酸钙凝胶微球,并通过正交试验系统考察了海藻酸钠浓度、氯化钙浓度、表面活性剂浓度、搅拌速度和油水比对海藻酸钙凝胶微球粒径及形貌的影响。在优化的条件下,制备出了平均粒径为4μm、单分散和球形度好的海藻酸钙凝胶微球。包埋模型药物牛血清白蛋白(BSA)的过程中,以去离子水作为洗涤液洗涤海藻酸钙微球时,BSA的包封率仅为13%左右;当水洗液的pH值为3.2时,BSA的包封率提高到66%左右,载药率可达16%,这是海藻酸钙pH值响应溶胀和BSA与海藻酸盐之间静电作用的结果。微球中BSA的体外释放曲线表明,该系统具有在模拟胃液中释药速率慢、释药量低、模拟肠液中释药迅速的特性。因此,双乳化-凝胶法制备海藻酸钙微球有望成为制备蛋白类药物控释制剂的一种新方法,以达到靶向快速给药的目的。     

聚甲基丙烯酸接枝炭微球

以乙炔为碳源,采用化学气相沉积法,在氩气气氛下,制备了炭微球(CMSs);并用硫酸和高锰酸钾的混合溶液对CMSs进行氧化处理,使其表面引入含氧官能团;然后用KH-570对氧化CMSs进行修饰并将甲基丙烯酸(MAA)接枝于CMSs表面,制得了接枝微粒PMAA/CMSs.用场发射扫描电子显微镜、红外光谱和热重分析对产物的结构和形貌进行了表征和分析.结果表明:采用逐步接枝法成功地将MAA接枝到CMSs表面,为CMSs在聚合物基体中分散、制备功能材料提供了途径.     

Preparation of monodisperse YSZ ceramic microspheres by high-throughput and instant-mixing droplet microfluidic system

The internal gelation process without cooling at room temperature is a significant challenge for the preparation of yttria-stabilized zirconia (YSZ) ceramic microspheres. In this work, a high-throughput and instant-mixing droplet microfluidic system that can prepare monodisperse YSZ ceramic microspheres was constructed. It mixed two stable solutions at room temperature in an active micromixer at a flow rate of mL/min, and then the mixed unstable precursor solution was quickly dispersed into sol droplets through a coaxial microfluidic device, and the droplets solidified into gel microspheres. The washed and dried gel microspheres could be sintered to obtain ceramic microspheres. Compared with no-micromixing and T-junction micromixers, the system had the best mixing performance and Y and Zr elements in the prepared ceramic microspheres were evenly distributed. Therefore, the system overcame the problem of precursor solution solidification and clogging of the device, and provided a new approach for the internal gelation process without cooling at room temperature.     

Fluidized state gelation of ceramic microspheres: Feasibility study of a continuous process and analysis of the chemical dehydration phenomena

A continuous gelation process for producing aluminum and zirconium hydroxide microparticles was conceived and tested in a lab‐scale apparatus. Fluidized state was devised as an appropriate hydrodynamic configuration to maintain the reacting sol microdrops suspended: stable non‐coalescing spherical gel particles in a size range quite below 100 μm were produced via gelation. The microspheres produced by a subsequent calcination step showed a diameter ranging between 2 μm and 20 μm. This powder, without further size classification, was an optimal product to manufacture test specimens by sintering. Measurements on the mechanical properties, such as toughness, flexural strength and hardness, classified the test bars as a very promising ceramic material. In order to enlighten the chemical drying phenomenon and devise the main governing mechanisms acting in the gelation reaction, an extremely simplified experimental system was set. A single drop of sol was deposited into a given amount of the dehydrating liquid medium and observed by an optical microscope; image analysis permitted us to identify the gel formation, the size reduction kinetics, as well as secondary phenomena.     

Preparation of uniform calcium alginate gel beads by membrane emulsification coupled with internal gelation

With the objective of making calcium alginate gel beads with small and uniform size, membrane emulsification coupled with internal gelation was proposed. Spherical gel beads with mean size of about 50 μm, and even smaller ones in water, and with narrow size distribution were successfully obtained. Experimental studies focusing mainly on the effect of process parameters on bead properties were performed. The size of the beads was mainly dependent on the diameter of the membrane pores. High transmembrane pressure made for large gel beads with wide size distribution. Low sodium alginate concentration produced nonspherical beads, whereas a high concentration was unsuitable for the production of small beads with narrow distribution. Thus 1.5% w/v was enough. A high surfactant concentration favored the formation of small beads, but the adverse effect on mass transfer should be considered in this novel process. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 848–852, 2003     

Concentration of mixed acid by electrodialysis for the intensification of absorption process in acrylic acid production

The absorption process in acrylic acid production was water-intensive. The concentration of acrylic acid before distillation process was low, which induced to large amount of wastewater and enormous energy consumption.In this work, a new method was proposed to concentrate the side stream of absorption column and thus increase the concentration in bottom product by electrodialysis. The influence of operating conditions on concentration rate and specific energy consumption were investigated by a laboratory-scale device. When the voltage drop was 1 V·cP~(-1)(1 cP=10~(-3) Pa·s), flow velocity was 3 cm·s~(-1) and the temperature was 35 °C, the concentration rates of acrylic acid and acetic acid could be 203.3% and 156.6% in the continual-ED process. Based on the experimental data, the absorption process combined with ED was simulated, in which the diluted solution from ED process was used as spray water and the concentrated solution was feed back to the absorption column. The results shown that the flow rate of spray water was decreased by 37.1%, and the acrylic acid concentration at the bottom of the tower was increased by 4.56%. The ions exchange membranes before and after use 1200 h were tested by membrane surface morphology(scanning electron microscope), membrane chemical groups(infrared spectra), ion exchange capacity, and membrane area resistance, which indicated the membrane were stable in the acid system. This method provides new method for energy conservation and emission reduction in the traditional chemical industry.     

K2S2O8-Na2S2O3引发淀粉与丙烯酸接枝产物吸水性能的主要影响因素

以玉米淀粉和丙烯酸为主要原料,采用溶液聚合的方法,制备了淀粉与丙烯酸接枝共聚物,用FTIR等方法对共聚产物结构进行了表征。研究了在过硫酸钾-硫代硫酸钠组成的氧化-还原引发体系中,引发剂、交联剂不同浓度及丙烯酸不同中和度等因素对共聚产物吸水性能的影响。实验结果表明采用K2S2O8-Na2S2O3引发体系,通过控制引发剂、交联剂浓度,接枝产物吸去离子水可达920 g/g;采用玉米粉代替玉米淀粉与丙烯酸接枝共聚,产物吸去离子水可达870 g/g;这些实验结果进一步提高了吸水树脂的吸水能力。     

CeO2-TiO2负载型复合光催化剂的制备和性能研究

以玻片为载体，采用溶胶-凝胶法制备了CeO₂-TiO₂复合光催化剂。采用XRD等对催化剂进行了表征。通过可溶性染料酸性媒介红B的微波辅助光催化氧化反应，考察了其光催化活性；并研究了复合催化剂的煅烧温度、铈掺杂量和涂覆次数等对催化活性的影响。实验结果表明，x（CeO₂）＝0.03％，涂覆次数为2次，在400 ℃焙烧2 ｈ的CeO₂-TiO₂负载型光催化剂的活性最高。