Microscale transprot in the thermal processing of new and emerging advanced materials

Abstract

This paper discusses microscale transport processes that arise in the fabrication of advanced materials. In many cases, the dimensions of the device being fabricated is in the microscale range and, in others, underlying transformations that determine product quality and characteristics are at micro or nanoscale levels. The basic considerations in these transport phenomena are outlined. Three important materials processing circumstances are considered in detail. These include the fabrication of multilayer and hollow optical fibers, as well as those where microscale dopants are added to achieve desired optical characteristics, thin film fabrication by chemical vapor deposition and microscale coating of fibers and devices. It is shown that major challenges are posed by the simulation as well as experimentation over microscale dimensions. These include accurate simulation to capture large gradients and variations over relatively small dimensions, simulating high pressures and viscious dissipation effects in microchannels, modeling effects such as surface tension that become dominant at microscale dimensions, and coupling micro and nanoscale mechanisms with boundary conditions imposed at the macroscale. Similarly, measurements over microscale dimensions are much more involved that those over macro or industrial scales because of access to the regions of interest, small tension effects and other mechanisms that are difficult to measure and that can make the process infeasible, and difficulty in achieving desired accuracy for validating the mathematical and numerical models. The paper presents some of the approaches that may be adopted to overcome these difficulties. Comparisons between experimental and numerical results are included to show fairly good agreement, indicating the validity of the modeling of transport.