【作者】 张立明；

【导师】 黄勇；

【作者基本信息】 清华大学 ， 材料科学与工程， 2005， 博士

【摘要】 本论文以抑制和消除胶态成型陶瓷坯体的气孔、裂纹等缺陷为目的,系统研究了氧化铝悬浮液的流变特性及其影响因素;提出了丙烯酸聚合改性氮化硅粉体表面的方法,并通过超声诱导实现了氮化硅悬浮液的可控、均匀固化;详细研究了胶态成型陶瓷坯体中气孔、开裂等缺陷的成因,并提出了抑制这些缺陷的工艺方案。从悬浮液中颗粒间作用势变化的角度出发,分析了静电稳定的悬浮液剪切变稀和剪切稠化的结构原因,认为悬浮液的剪切变稀是热力学颗粒簇分解的结果,而剪切稠化则起因于悬浮液中水力学颗粒簇的形成。系统研究了柠檬酸铵加入比例、悬浮液pH值、固相体积分数、粉体粒径等因素对氧化铝悬浮液流变特性的影响规律,提出静电稳定的悬浮液在高、低剪切速率下的粘度分别取决于颗粒表面的Stern电位和Zeta电位,并确定了粉体粒径与悬浮液剪切稠化临界剪切速率之间的定量关系。提出通过丙烯酸聚合实现氮化硅粉体表面改性从而提高氮化硅悬浮液固相体积分数的方法。以超声波处理为手段,实现了凝胶注模氮化硅悬浮液的可控、均匀固化,有效提高了坯体强度,并研究证实超声对凝胶注模成型固化过程的促进作用起因于超声对悬浮液中自由基“笼蔽效应”的破坏。分析了胶态成型坯体中气孔的成因,提出在真空脱气的同时对悬浮液进行磁力搅拌的物理脱气方法,有效提高了脱气效果,降低了坯体中气孔的数量和孔径,提高了坯体和烧结体的力学性能。综合分析失重、差热等物理现象和化学反应产物,系统研究了凝胶注模氧化铝陶瓷坯体热氧化脱脂过程,发现了凝胶注模陶瓷坯体脱脂开裂的根本原因在于有机物热氧化气体产物的集中释放。在保证坯体强度的前提下,提出了降低坯体有机物含量,避免凝胶注模陶瓷坯体脱脂开裂的系统工艺方法。提出纯铝酸钙水泥水解辅助固化氧化铝悬浮液的新方法。通过水泥水解形成的Ca²⁺使海藻酸钠交联形成凝胶,同时提高悬浮液中的离子强度,降低了颗粒表面的Zeta电位,综合了Gelcasting和DCC的特点,有效提高了坯体强度。更多还原

【Abstract】 This research was deveolped aiming to eliminate or inhibit the flaws suchas pores and cracks in ceramic green bodies formed by colloidal processing,and further to obtain perfect green bodies with high strength. In particular,rheological behaviors of alumina suspensions, a novel method of preparingconcentrated silicon nitride suspension, and the controllable and uniformsolidification of silicon nitride suspension were studied. The reasons for thegeneration of pores and cracks in colloidal formed green bodies were foundout, and some applicable solutions to suppress these flaws were broughtforward.The rheological behaviors of alumina suspension were studied. Thestructural reasons for the shear thinning and shear thickening behaviors ofelectro-stabilized suspensions were analyzed from the view of the change ofinteractive potential between particles in the suspension. The effects oftri-ammonium citrate ratio, the pH and the solid volume fraction of thesuspension and the diameter of suspended particles on the rheologicalbehaviors of alumina suspension were studied. It is believed that shearthinning is the result of decomposition of thermodynamic particle clusters,while shear thickening can be attributed to the formation of hydrodynamicparticle clusters in the suspension. The viscosities of alumina suspension athigh and low shear rate are respectively determined by Stern and Zetapotential near the particle surface, and the critical shear rates (the onset ofshear thickening) dependence on the particle size was quantified with a simpleshear model.A surface modification method for Si3N4 particles, by the adsorption ofgrowing polymer chains on the particles during the polymerization of acrylicacid in thin Si3N4 suspension, was proposed to increase the solid volumefraction of Si3N4 suspension. The suspension, containing more than 50vol%surface modified Si3N4 powder, was solidified by gelcasting controllably anduniformly under a ultrasonic wave aid, which accelerated the solidification ofSi3N4 suspension because it destroyed the cage effect of free radicals, and thestrength of green body was increased obviously.The genesis of the pores in colloidal formed green bodies was alsostudied in this research, and a method of degassing the suspension withmagnetic stirring under vacuum was put forward. As a result, the degassingeffect of the suspension is improved, then the pores volume and diameter inthe solidified green body are reduced, resulting in the increment of thestrength and density of the sintered body.Further, the debinding of gelcasting green bodies was comprehensivelystudied with TG/DTA and FTIR methods. The fact that polyacrylamide inceramic green bodies decomposed and oxidized in a narrow temperature rangewas found to be the major reason for the occurring of cracks during greenbody debinding. A systematic approach was put forward to avoid suchcracks during debinding, which could guarantees the strength of green bodywith less organic content.Additionally, pure calcium aluminate cement was applied to solidifyAl2O3 suspension for the first time. During hydrolysis, the cement yields Ca2+,then the molecular chains of sodium alginate in Al2O3 suspension arecrosslinked, and the Zeta potential near Al2O3 surface is reduced because ofthe increased ionic strength in the suspension. As a result, the strength of thegreen body from the solidified suspension is increased.更多还原