The UO2 ceramic microspheres are the most important materials in the spherical fuel elements for high temperature reactor (HTR). A process for preparation of UO2 kernels known as total gelation process of uranium (TGU...The UO2 ceramic microspheres are the most important materials in the spherical fuel elements for high temperature reactor (HTR). A process for preparation of UO2 kernels known as total gelation process of uranium (TGU) was developed as the production process of 10 mW HTR at Tsinghua University. The TGU process is based on the traditional sol-gel process, external gelation process and internal gelation process of uranium (EGU and IGU), which implies that the gelation action is initiated both by ammonia out of the gel particles and hemxamethyl tetra-amine (HMTA) inside the gel particles. The gelation behavior and the properties of uranium microspheres were investigated of the solution with and without HMTA. It is observed that good spherical particles can be obtained without HMTA in the sol, which indicates a more controllable and industrialized route will be set up. Contrasts between this route and the traditional EGU were also listed.展开更多
A novel method was established to investigate the gelation process of polymers. The change of refractive index of a polymer system during gelation was determined in situ in a prismatic cell. It can give reliable infor...A novel method was established to investigate the gelation process of polymers. The change of refractive index of a polymer system during gelation was determined in situ in a prismatic cell. It can give reliable information on the whole gelation process. The apparatus and the execution of this technique are illustrated and the error is also discussed.展开更多
The early stage of polymer crystallization may be viewed as physical gelation process,i.e.,the phase transition of polymer from liquid to solid.Determination of the gel point is of significance in polymer processing.I...The early stage of polymer crystallization may be viewed as physical gelation process,i.e.,the phase transition of polymer from liquid to solid.Determination of the gel point is of significance in polymer processing.In this work,the gelation behavior of poly（butylene succinate）（PBS） at different temperatures has been investigated by rheological method.It was found that during the isothermal crystallization process of PBS,both the storage modulus（G′） and the loss modulus（G″） increase with time,and the rheological response of the system varies from viscous-dominated（G′G″） to elastic-dominated （G′G″）,meaning the phase transition from liquid to solid.The physical gel point was determined by the intersection point of loss tangent curves measured under different frequencies.The gel time（t_c） for PBS was found to increase with increasing crystallization temperature.The relative crystallinity of PBS at the gel point is very low（2.5%-8.5%） and increases with increasing the crystallization temperature.The low crystallinity of PBS at the gel point suggests that only a few junctions are necessary to form a spanning network,indicating that the network is＂loosely＂connected,in another word,the critical gel is soft.Due to the elevated crystallinity at gel point under higher crystallization temperature,the gel strength S_g increases, while the relaxation exponent n decreases with increasing the crystallization temperature.These experimental results suggest that rheological method is an effective tool for verifying the gel point of biodegradable semi-crystalline polymers.展开更多
The kinetic behaviour of aluminum naphthenate/benzene system in the process of gela- tion and degradation wag systematically studied. The results obtained from the kinetic experiments indicated that the gelation stage...The kinetic behaviour of aluminum naphthenate/benzene system in the process of gela- tion and degradation wag systematically studied. The results obtained from the kinetic experiments indicated that the gelation stage was in accordance with a. kinetic model of an auto-catalytic first-order reaction, and the degradation stage was a simple first-order reaction. The rate constants in these two stages at various temperaturs were measured. And the determined apparent activation energies for the gelation stage and for the degradation stage are 27.5±2.0 and 37.7±2.3 kJ/mol, respectively.展开更多
文摘The UO2 ceramic microspheres are the most important materials in the spherical fuel elements for high temperature reactor (HTR). A process for preparation of UO2 kernels known as total gelation process of uranium (TGU) was developed as the production process of 10 mW HTR at Tsinghua University. The TGU process is based on the traditional sol-gel process, external gelation process and internal gelation process of uranium (EGU and IGU), which implies that the gelation action is initiated both by ammonia out of the gel particles and hemxamethyl tetra-amine (HMTA) inside the gel particles. The gelation behavior and the properties of uranium microspheres were investigated of the solution with and without HMTA. It is observed that good spherical particles can be obtained without HMTA in the sol, which indicates a more controllable and industrialized route will be set up. Contrasts between this route and the traditional EGU were also listed.
基金The National Natural Science Foundation of China (Grant No.29774036 and 29904007)
文摘A novel method was established to investigate the gelation process of polymers. The change of refractive index of a polymer system during gelation was determined in situ in a prismatic cell. It can give reliable information on the whole gelation process. The apparatus and the execution of this technique are illustrated and the error is also discussed.
基金supported by the National Natural Science Foundation of China(Nos.50873112,51063004)China National Funds for Distinguished Young Scientists(No.50925313)
文摘The early stage of polymer crystallization may be viewed as physical gelation process,i.e.,the phase transition of polymer from liquid to solid.Determination of the gel point is of significance in polymer processing.In this work,the gelation behavior of poly（butylene succinate）（PBS） at different temperatures has been investigated by rheological method.It was found that during the isothermal crystallization process of PBS,both the storage modulus（G′） and the loss modulus（G″） increase with time,and the rheological response of the system varies from viscous-dominated（G′G″） to elastic-dominated （G′G″）,meaning the phase transition from liquid to solid.The physical gel point was determined by the intersection point of loss tangent curves measured under different frequencies.The gel time（t_c） for PBS was found to increase with increasing crystallization temperature.The relative crystallinity of PBS at the gel point is very low（2.5%-8.5%） and increases with increasing the crystallization temperature.The low crystallinity of PBS at the gel point suggests that only a few junctions are necessary to form a spanning network,indicating that the network is＂loosely＂connected,in another word,the critical gel is soft.Due to the elevated crystallinity at gel point under higher crystallization temperature,the gel strength S_g increases, while the relaxation exponent n decreases with increasing the crystallization temperature.These experimental results suggest that rheological method is an effective tool for verifying the gel point of biodegradable semi-crystalline polymers.
基金This project was supported by the National Natural Science Foundation of China.
文摘The kinetic behaviour of aluminum naphthenate/benzene system in the process of gela- tion and degradation wag systematically studied. The results obtained from the kinetic experiments indicated that the gelation stage was in accordance with a. kinetic model of an auto-catalytic first-order reaction, and the degradation stage was a simple first-order reaction. The rate constants in these two stages at various temperaturs were measured. And the determined apparent activation energies for the gelation stage and for the degradation stage are 27.5±2.0 and 37.7±2.3 kJ/mol, respectively.
