Axial convection enhanced laser CVD of carbon rods

A novel Axial Convection Enhanced Laser Chemical Vapor Deposition (LCVD) setup facilitated the growth of carbon rods in chamber-free open-air conditions. The axial convection configuration paved the discovery of a real-time in-situ rod growth rate and diameter measurement technique. The technique utilizes the 656 nm hydrogen-alpha emission during precursor pyrolysis to track the growth rate and diameter of the rod along its axis. Real-time measurement of rod growth rate along its axis points to transient growth rates that appear to vary greatly inside the growth region. An in-depth analysis of variational properties of the observed transient growth rate yielded a method to identify the focal plane intrinsic to the process and characterize its linear and molar energetic efficiency. Also, this work marks the development of a first-of-its-kind closed-loop automated LCVD rod growth system yielding the longest LCVD carbon rods to date.{09} Precise control afforded by the closed-loop system can help in investigating the effect of the position of the rod growth front within the laser-induced growth region on the obtained microstructure and mechanical properties of the rods. Integration and communication between various LCVD sub-systems aided in realizing a turn-key hands-free LCVD system.; Over the years, LCVD has been used to deposit a wide variety of materials as rods and more complex 3D-structures ranging in size from millimeters to sub-microns. However, the high sensitivity of the process to deposition temperature has caused control problems in obtaining structures with reproducible microstructural characteristics. The results of this work (1) helped gain a better understanding of the process behavior which has been (2) put to use by the implementation of a closed-loop controlled growth algorithm.; Axial convection enhancement offered by the developed LCVD system's coaxial tube design was found to have a positive impact on the growth rate of the produced carbon rods. In fact, growth rates observed are the highest to date at an operating pressure of 1 bar. Furthermore, axial convection enhancement seems to eliminate growth rate saturation witnessed in prior growth rate variation plots as a function of laser power.; Growth rate---a temperature dependent process characteristic which has been found to be influential in determining the resulting deposit microstructure---has predominantly been measured either online via optical means using human intervention or offline by deposit length measurements causing uncertainties in reported growth rate values. This being said, it is crucial to obtain accurate growth rate values in-situ and in real-time to form a reliable growth rate-microstructure correlation. In the first part of this work (ARTICLE 1: Position dependence of growth rate in convection enhanced laser CVD of carbon rods), carbon rods were grown using a two-tube axial flow configuration that allowed the precursor (ethylene) to be delivered coaxially to the growing rod in the inner tube. Argon was delivered through the outer tube as a protective shroud to encapsulate the reaction zone from the ambient. The 656 nm hydrogen-alpha emission produced during precursor pyrolysis allowed accurate in-situ real-time monitoring of the position, growth rate, and diameter of the produced rod along its axis. Obtaining the growth rate along the rod axis essentially provides a "growth rate map" throughout the laser-induced growth region.; Work done with a three-tube coaxial flow system (ARTICLE 2: On the importance of focus tracking in LCVD growth of carbon rods) eliminated flow mixing issues encountered in the two-tube system and further bolstered the existence of an unsteady transient growth rate phenomenon witnessed in the growth region. The absence of a steady-state growth region has brought to light the importance of the need for a control system to maintain the rod growth front at a particular position in the growth region to obtain desired...