Atomic Layer Deposition for Engineering Carbon Hollow Fiber Membrane Pore Structure

Abstract

Alumina (Al2O3) films, fabricated using atomic layer deposition (ALD) are useful components of composite materials, providing corrosion resistance, transport barriers, or dielectric properties. The process dynamics of alumina ALD in and around pores are examined in order to support the fabrication of membranes with planar and hollow-fiber geometries. In this study carbon molecular sieve (CMS) membranes are created from pyrolysis of polymer precursors which have properties based on the geometry of and pretreatment applied to the precursor before inert-gas pyrolysis. Modeling and simulation are performed for gaseous reactant transport inside arbitrarily porous networks common to such membranes using analytic and numerical methods to identify potential mass transfer limitations. Associated experimental work used two ALD reactor systems to deposit alumina on the precursor fibers and completed membranes. When the requisite ALD parameters fell outside of the operating conditions of the commercial system, a second system was designed and constructed to support long exposures and simplify masking of undesired deposition surfaces. Characterization of the coated precursors and final CMS membranes was conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and several performance metrics including constant-pressure permeation and pure water flux measurements. This research details study is the successful fabrication of several desirable membrane geometries using ALD, along with transport insight and a proposed reaction mechanism for coating nucleation on the polymer surface.