W. Jack Lackey

Forced flow-thermal gradient chemical vapor infiltration (FCVI) for fabrication of carbon/ carbon

Abstract Carbon/carbon composites with porosities as low as 7% were fabricated within 8–12 hours using the forced flow-thermal gradient chemical vapor infiltration (FCVI) process. Preforms consisting of 40 layers of T-300 plain weave carbon cloth were infiltrated with a feed containing a carbon source and a diluent. The carbon sources studied in the present work included propylene, propane, and methane and the diluent was hydrogen. Shorter processing times were obtained when propylene and propane were used as compared to methane. The highest deposition rate obtained in the present study was ~ 3 μm/h which is more than an order of magnitude faster than the typical value of 0.1–0.25 μm/h for the isothermal infiltration process. In the infiltrated composites it was observed that the tows in a cloth were appreciably infiltrated, independent of their position in the preform. Whereas, the coating thickness between the tows and cloth layers strongly depended on the temperature, i.e. position within the preform.

Mass transfer and kinetics of the chemical vapor deposition of SiC onto fibers

An internally consistent set of data was generated for the chemical vapor deposition (CVD) of SiC from methyltrichlorosilane (MTS) and H 2 at atmospheric pressure. A moving fiber tow was used as the substrate. Coating rates between 0.3 and 3.7 µm/min and deposition efficiencies between 24 and 48% were obtained for MTS and H 2 flow rates in the range 30 to 200 cm 3 /min and 300 to 2000 cm 3 /min, respectively. The data were analyzed and found to be best fit under a mass transfer regime. Based on this fit, a value of the constant in the Chilton–Colburn j factor expression for a moving fiber tow was estimated to be 2.74 × 10 −6 with a standard deviation of 3.2 × 10 −7 . The efficiency of the reaction was found to decrease with increases in the total flow rate, indicating that the effect of the decreased residence time of reagents in the reactor was larger than the increase in the mass transfer coefficient. Finally, a comparison between the efficiencies for a stationary and a moving tow revealed that the moving tow had a higher efficiency, possibly due to a disruption of the boundary layer by the tow motion or due to the decrease in the canning of the moving tow.

Multi‐material and advanced geometry deposition via laser chemical vapor deposition

Laser chemical vapor deposition (LCVD) as a manufacturing process holds the potential to build compositionally and geometrically unique objects. Georgia Techs LCVD system has been used in the past to create three‐dimensional and laminate structures out of carbon. Recently molybdenum and boron nitride were successfully deposited and upgrades to the system have allowed for higher spatial resolutions and more varied geometric capabilities. Upgrades include the addition of a fourth linear stage and implementation of an argon ion laser. Detailed thermal and fluid modeling have provided more insight as to the important parameters and characteristics of the LCVD process.

Temperature controlled laser chemical vapor deposition (LCVD) using thermal imaging

Purpose – To describe the thermal imaging control system used to deposit lines of graphite in a laser chemical vapor deposition (LCVD) system.Design/methodology/approach – A thermal imaging‐based control system is applied to the LCVD process to deposit layered carbon lines of uniform height and width. A 100 W CO2 laser focused to a 200 μm diameter spot size is used to provide the heat source for the carbon deposition. A high resolution thermal imaging camera is used to monitor and control the average deposition temperature.Findings – Carbon lines are grown with heights of 250 μm and widths of 170 μm consisting of 20 layers. Laser spot temperatures are in excess of 2,170°C, and the total pressure used is 1 atm with a 75 percent methane concentration and the remainder hydrogen. The length of the lines is 3.3 mm, and the scan speed is 5 mm/min. The volumetric deposition rate is 0.648 mm3/h.Research limitations/implications – The temperature process control resulted in uniform geometry at the center of the li...

Large grain polycrystalline silicon via chemical vapor deposition

Large grain polycrystalline Si films were grown by chemical vapor deposition (CVD) onto TiB 2 substrates using the SiCl 4 –H 2 reagent system. A statistically designed processing study was used to correlate the film growth rate, crystallographic orientation, and grain size with deposition temperature, the SiCl 4 : H 2 ratio, and the level of B doping. Each process variable influenced grain size with temperature having the dominant effect. Grains as large as 15 to 20 μ m were achieved for a coating thickness of about 50 μ m.