Fundamentals of membrane top-layer synthesis and processing

This chapter discusses the fundamentals of membrane top-layer synthesis and processing. Mesoporous, asymmetric multilayer membranes with graded pore diameters can be produced on macroporous alumina or carbon substrates of different shapes. Ceramic membrane production obtains defect-free supported films with homogeneous thickness and a narrow pore-size distribution. The chapter summarizes the basic elements of drying theory relevant to stress formation and related defect formation. To obtain defect-free separation films, the use of support systems of good quality (low roughness, reasonable pore size distribution without too large pores, reproducible wettability) is necessary and multiple dipping procedures are required. The drying process is important and becomes critical with small mesopore systems because of large tensile stresses that build up in the membrane layers. The chapter describes techniques for the measurement of these stresses and includes theoretical and model descriptions for stress and crack development with illustrative examples. Stress levels and membrane cracking can be decreased by applying low controlled drying rates and using organic additions. To obtain membranes on large surface areas and/or on complex support shapes, further optimization of the process is necessary. The chapter discusses the thickness of the formation of a lyogel film with reference to the development of its thickness.