【作者】 张江伟；

【导师】 徐鑫；

【作者基本信息】 中国科学技术大学 ， 材料学， 2013， 博士

【摘要】 随着全球工业化的快速发展和人口的迅速膨胀,世界各国对淡水资源的需求日益增加,“向海洋要水”已经成为全球的一个共识。而膜蒸馏海水淡化技术作为解决水资源紧缺的一种新兴技术,越来越受到人们的广泛关注。目前,用于膜蒸馏海水淡化应用的膜材料主要分为高分子聚合物膜和陶瓷膜。高分子聚合物膜的热稳定性和化学稳定性差,而且在海水中微生物等作用下易腐蚀、膜寿命短。与高分子膜相比,陶瓷膜具有寿命长、耐高温、抗微生物能力强、化学稳定性好(耐酸碱腐蚀、有机溶剂等)、机械强度高等优点,因此陶瓷膜在海水淡化领域中的应用,近些年来越来越受到人们的重视。目前对于陶瓷膜的研究,主要集中在氧化物,包括氧化铝、氧化锆、氧化钛、二氧化硅等等。为了保证膜在膜蒸馏应用中具有高的渗透通量,膜必须具有高的孔隙率,但高孔隙率的氧化物陶瓷膜强度低,膜在组装和封接的过程中易折断,不易操作,而且传统膜的制膜工艺比较复杂,设备大,制备成本高,也限制了其广泛应用。本文是在湿法纺丝成型的基础上,采用相转化与烧结相结合的方法第一次成功地制备了氮化物陶瓷中空纤维膜。此工艺过程简单,一步成型,简化了多孔膜的制备工艺,降低了制备成本。制备的氮化硅和β-sialon陶瓷中空纤维膜孔隙率大、强度高、渗透性能优异。同时本文是首次将氮化硅和β-sialon中空纤维进行疏水性表面改性并应用于膜蒸馏实验中,这部分工作为非氧化物陶瓷膜在膜蒸馏应用方面的后续研究奠定了基础。第一章主要介绍了陶瓷浆料的分散、流变学特性及影响浆料粘度和流动性的主要因素。其次介绍了几种制备多孔陶瓷的传统方法以及相转化法,重点介绍了膜蒸馏工艺的发展、现状、原理、分类及所用的膜材料,最后提出了本文的主要思路和研究内容。第二章主要介绍了实验原料、制备工艺、膜制备设备以及表征设备等。第三章主要研究了分散剂种类、煅烧处理、分散剂含量、固含量以及球磨时间等对氮化硅陶瓷浆料的影响。通过沉降实验我们最终确定分散剂O-(2-氨丙基)-O’-(2-甲氧基乙基)聚丙二醇(AMPG)对氮化硅粉体的分散效果最好,而且600℃/6h煅烧处理能显著增加粉体表面的氧含量,降低浆料的粘度、提高流动性。浆料的流变特性测试结果表明：浆料的最佳分散剂含量为4wt.%,而且不随固含量变化；浆料球磨16h具有最佳的流动性,通过优化的浆料固含量可以达到50vo1.%,并且浆料仍保持低粘度和好的流动性。第四章是在氮化硅陶瓷浆料稳定分散的基础上,通过相转化与烧结相结合的方法制备了氮化硅中空纤维膜。探讨了陶瓷粉体与粘结剂的比值、浆料的粘度等对氮化硅陶瓷中空纤维膜的形貌、强度、孔隙率、孔径分布以及渗透性能的影响。制备的氮化硅膜管具有典型的非对称结构,靠近内外表面处为指状孔层,中间层为海绵状结构。随着陶瓷粉体与粘结剂比值的增大,浆料的粘度增加,陶瓷纤维外表面处的指状孔逐渐减小,甚至完全消失,而烧结后的膜管外表面逐渐趋向致密化。当陶瓷粉体与粘结剂的比值为1/7时,纤维具有优异的气体和水渗透性能、弯曲强度高(290MPa)、孔隙率大(50%)和孔径分布窄(平均孔径：0.74μm)。此纤维是最适合膜蒸馏应用的理想膜材料。第五章研究了氮化硅陶瓷中空纤维膜的表面改性以及膜蒸馏应用。采用氟硅烷对纤维表面进行改性,改性后的膜管水接触角从56°变为136°,红外光谱在膜表面上也探测到了碳和氟的存在。渗透测试表明膜表面额外的氟硅烷层增加了气体渗透的阻力但并没有严重影响气体渗透,而氟硅烷改性对水渗透有很大的影响,改性后直到气体压力达到3.25bar时膜管才有少量的水透过。上述这些结果都充分的说明了膜管疏水性能的获得。最后,我们采用真空膜蒸馏(VMD)和直接接触式膜蒸馏(DCMD)两种方式对所制备的膜进行了MD脱盐实验。在VMD中,探讨了温度、盐溶液浓度、真空度等因素对MD性能的影响。膜表现了优异的MD性能,当盐溶液浓度为4wt.%、温度为80℃、渗透端真空度保持在0.02bar时,膜的渗透通量达到679L/m2·day,截留率达到99%以上,而且膜具有很好的长期稳定性。而在DCMD中,膜的通量只有VMD的35%左右,这主要是由于DCMD存在温度极化效应,MD驱动力减小,而且膜两端温度不稳定,难以控制。VMD虽然需要额外提供一定的能量,但是优异的MD性能可以弥补这方面的不足。第六章介绍了相转化与烧结法对β-sialon中空纤维膜的制备以及膜蒸馏应用。制备的纤维膜具有典型的结构：靠近内表面为指状孔层,而外表面为海绵状孔层。膜管具有优异的渗透性能和机械强度,1600℃/2h烧结的Z=2的纤维符合膜蒸馏用膜的要求。经氟硅烷表面改性后,膜管具有优异的疏水性能,成功地应用于直接接触式膜蒸馏海水淡化中并取得了优异的膜蒸馏性能。当盐溶液浓度为4wt.%、热料液温度为80℃,渗透端温度为20℃时,膜蒸馏通量达到10.76L/m2·h(258L/m2·day),截留率能维持在99.5%以上,因此β-sialon中空纤维膜在膜蒸馏海水淡化应用中也有很大的潜力。第七章对全文进行了简要的总结,提出了工作中的不足及对未来工作的展望。更多还原

【Abstract】 With the rapid development of the global industrialization and the expansion of the population, the demand for fresh water is increasing all over the world. Obtaining fresh water from the sea has become a global consensus. More and more people pay attention to the membrane distillation desalination technology which is regarded as a new technology to solve the scarce problem of water resources. At present, membrane materials applied in membrane distillation process can be divided into two kinds: polymer membrane and ceramic membrane. As we known, the thermal and chemical stability of the polymer membrane are very bad, what is more, the membrane is very easy to be corroded when in contact with microorganisms, so its life time is very short. Compared with the polymer membrane, ceramic membrane has many advantages, such as longer life time, better high temperature resistant, stronger anti-microbial capability, better chemical stability (acid and alkali corrosion resistant, organic solvent resistant) and high mechanical strength. Therefore, ceramic membrane has attracted more and more attention in the field of seawater desalination in recent years. Now the researches of ceramic membrane mainly focus on the oxide, including alumina, titania, silica, zirconia and so on. In order to ensure that the membrane has a high permeate flux in the application of membrane distillation, the membrane must have high porosity. But the mechanical strength of the oxide ceramics is very low at high porosity, which means that the membranes are easy to break in the process of membrane assembling and sealing. What is more, the preparation process of traditional membrane is more complex and the equipment is very big, which leads a high cost and limited application scope.This thesis is based on the process of wet spinning, non-oxide nitride hollow fiber membranes have been successfully prepared by a combined phase-inversion and sintering method for the first time. This process is very simple, one-step forming, and it can simplify the process in preparation of porous ceramic membranes. Therefore, the preparation cost can be greatly reduced. The prepared silicon nitride and β-sialon hollow fibers have large porosity, high strength and excellent permeate performance. At the same time, this study is the first time to fulfill the surface modification of silicon nitride and (3-sialon hollow fibers from hydrophilicity to hydrophobicity and the application of membrane distillation. This work lays a foundation for further researches of non-oxide ceramic membranes in membrane distillation applications.The first chapter mainly introduces the dispersion of the ceramic suspension, rheological properties and the main factors influencing the suspension viscosity and fluidity. Secondly, it simply describes several traditional methods and phase-inversion process in the preparation of porous ceramic, and detailedly introduces the development, present situation, principle, classification of membrane distillation process and the use of membrane materials in this process. Finally, the the main ideas and research content of this thesis were proposed.The second chapter mainly introduces the experimental materials, preparation process and the equipments in preparation and characterization of the membrane.The third chapter mainly studies the effect of the dispersant category, calcination treatment, dispersant content, solids content of the powders and the milling time on the silicon nitride ceramic suspension. O-(2-aminopropyl)-O’-(2-methoxyethyl)-polypropylene glycol (AMPG) was selected as the best dispersant for the dispersion of silicon nitride powders by sedimentation tests. Calcination treatment at600℃for6h in air can significantly increase the oxygen amount of the powders surface, reduce the viscosity and improve the fluidity of the suspension. The test results of the rheological properties indicate that the optimal dispersant amount is4wt%, which is independent of the solids volume content of the suspension. Milling process can effectively improve the fluidity of the suspension. The optimal milling time is16h. The solids volume content of the suspension can reach50vol%with a low viscosity and good fluidity.The forth chapter is on the preparation of the silicon nitride hollow fibers by a combined phase-inversion and sintering method based on the stable dispersion of silicon nitride ceramic suspension, and the influence of ceramic powders/binder ratio, the viscosity of the suspension on the morphology, structure, strength, porosity, pore size distribution and permeability were discussed. The prepared membrane has typical asymmetric structure with a long finger-like layer near the inner and outer surface and a sponge-like region in the middle of the fiber. With increasing of the powders/PESf ratio, the viscosity of the suspension increases and the finger-like voids extending from the outer surface is greatly reduced, even disappears completely. In addition, the outer surface of the fiber is tended to densification after sintering. When the ceramic powders/binder ratio is fixed to7/1, the obtained fiber shows a good combination of gas and water flux, bending strength (290MPa), porosity (50%) and average pore size (0.74μm), which are the most suitable ceramic membranes for membrane distillation applications.The fifth chapter studies the surface modification and membrane distillation (MD) applications of the silicon nitride hollow fiber. The surface of the fiber was grafted by fluoroalkysilane. The water contact angles were changed from56°to130°, carbon and fluorine were detected on the surface of the grafted membrane by infrared spectroscopy test. Permeability test indicates that the additional FAS coating increases the membrane resistance to gas permeation but does not sacrifice too much permeability. However, the FAS grafting has a great effect on the permeance of water. The liquid water permeation was not detected for the grafted membrane until the pressure up to3.25bar. These results adequately demonstrate the hydrophobicity of the modified membrane. After surface modification, the water desalination performance of the prepared fiber was tested using two MD methods:vacuum membrane distillation (VMD) and direct contact membrane distillation (DCMD). In VMD, the influence of temperature, salt concentration, vacuum degree on the MD performance are studied. The membrane exhibits satisfactory membrane distillation performance with a high flux of679L/m2·day, a rejection rate over99%and good long-term stability for desalination of4wt.%NaCl solution at80℃when the permeate side is under a vacuum pressure of0.02bar. In DCMD, the membrane flux is only about35%compared with the flux in VMD. This is mainly due to the existence of temperature polarization effect which leads to the decrease of the driving force. What is more, the DCMD process is difficult to control due to the presence of temperature instability at both sides of the membrane. Though VMD needs provide some extra energy, the excellent MD performance can make up this deficiency.The sixth chapter introduced the preparation of P-sialon hollow fibers by a combined phase-inversion and sintering method and their application in membrane distillation process. The prepared membrane also has a typical structure with a long finger-like layer near the inner surface and a sponge-like layer near the outer surface. The prepared membrane has excellent permeability performance and mechanical strength. The fibers (Z=2) which sintered at1600℃for2h can meet all the requirements of the membrane used in membrane distillation process. The membrane has excellent hydrophobic property after grafting by FAS. The grafted fibers were successfully applied to desalination by direct contact membrane distillation process. The membrane exhibits excellent membrane distillation performance with a high flux of10.76L/m2·h (258L/m2·day) and a rejection rate over99.5%for desalination of4wt.%NaCl solution at80℃when the temperature of the permeate side was fixed at20℃. Therefore, the β-sialon hollow fiber membrane also has great potential in the application of membrane distillation for desalination.In Chapter7, a short conclusion of this dissertation has been made. The lack of this work and the direction of our future work have been discussed.更多还原