Robert E. Barletta

Deposition and Modification of Tantalum Carbide Coatings on Graphite by Laser Interactions

Graphite surfaces can be hardened and protected from erosion by hydrogen at high temperatures by refractory metal carbide coatings, which are usually prepared by chemical vapor deposition (CVD) or chemical vapor reaction (CVR) methods. These techniques rely on heating the substrate to a temperature where a volatile metal halide decomposes and reacts with either a hydrocarbon gas or with carbon from the substrate. For CVR techniques, deposition temperatures must be in excess of 2000{degrees}C in order to achieve favorable deposition kinetics. In an effort to lower the bulk substrate deposition temperature, the use of laser interactions with both the substrate and the metal halide deposition gas has been employed. Initial testing, involved the use of a CO{sub 2} laser to heat the surface of a graphite substrate and a KrF excimer laser to accomplish a photodecomposition of TaCl{sub 5} gas near the substrate. Results of preliminary experiments using these techniques are described.

The development of CVR coatings for PBR fuels

Particle bed reactors (PBRs) are being developed for both space power and propulsion applications. These reactors operate with exhaust gas temperatures of 2500 to 3000 K and fuel temperatures hundreds of degrees higher. One fuel design for these reactors consists of uranium carbide encapsulated in either carbon or graphite. This fuel kernel must be protected from the coolant gas, usually H{sub 2}, both to prevent attack of the kernel and to limit fission product release. Refractory carbide coatings have been proposed for this purpose. The typical coating process used for this is a chemical vapor deposition. Testing of other components have indicated the superiority of refractory carbide coatings applied using a chemical vapor reaction (CVR) process, however technology to apply these coatings to large numbers of fuel particles with diameters on the order of 500 pm were not readily available. A process to deposit these CVR coatings on surrogate fuel consisting of graphite particles is described. Several types of coatings have been applied to the graphite substrate: NbC in various thicknesses and a bilayer coating consisting of NbC and TaC with a intermediate layer of pyrolytic graphite. These coated particles have been characterized prior to test; results are presented.

Biodegradation Testing of Bitumen

An estimate of the rate of biodegradation of bituminous material is necessary to predict the long-term stability of low- and intermediate- level waste solidified using bitumen. Data from a series of scoping experiments have been analyzed to determine the rate of degradation of blown bitumen samples under a variety of conditions. Among the variables investigated were the effect of soil type, moisture, sample surface area and microbial strain. The rate of degradation was measured by monitoring metabolic CO 2 release. Using this data it was found that, for degradation in soil, a mean rate of 5.5 × 10 −4 cm/yr represented all data to within a factor of about two. This mean rate is nearly that for distilled bitumen samples measured by other workers.

Preparation, Characterization and Performance of CVR Coatings for PBR Fuels

As a part of the US Space Nuclear Thermal Propulsion Program, a process to deposit refractory carbide coatings using a fluidized bed chemical vapor reaction (CVR) process has been developed. Several types of coating have been applied to the graphite substrate which served as a surrogate fuel kernel. The coatings include NbC in various thicknesses and a bilayer coating consisting of NbC and TaC with an intermediate layer of pyrolytic graphite(PG). They were applied to a surrogate fuel kernel consisting of a PG‐coated, graphite particle. The particles were characterized prior to test for coating thickness, grain size, stoichiometry (NbC only), free carbon and surface area.The initial screening tests for these coatings consisted of heating in flowing hot hydrogen at one atmosphere. The carbon loss from these particles was measured as a function of time. Exposure temperatures ranging from 2500 to 3000 K were used and samples were exposed for up to 14 minutes in a cyclical fashion, cooling to room temperature ...

RESONANCE RAMAN SPECTRUM OF CARBON TETRACHLORIDE

Carbon tetrachloride (CCl4) is one of a number of volatile organic chemicals (VOCs) of interest as pollutants which are regulated at very low levels (ppm to ppb) by the EPA. A need exists to measure these chemicals at these regulatory levels rapidly and, in many cases, in situ. Raman spectroscopy is one method which has the potential to be a useful tool for remote monitoring and has already found a number of applications for process and field monitoring of chemicals. While carbon tetrachloride has been detected with these remote sampling systems, it is doubtful that the requisite sensitivity could be obtained with visible or near-IR lasers because of the relatively small cross section for the Raman effect. Large increases in the Raman cross section have been observed when the excitation frequency is at or near an electronic absorption of the molecule. Resonance enhancement of the Raman cross sections can be many orders of magnitude (103 to 106). Hence, large gains in sensitivity might be expected. Such measurements are complicated by background florescence as well as the need to use lasers in the UV region of the spectrum to access the electronic transitions of simple VOCs.

Ceramic Coatings for PBR Components

Particle bed reactors (PBRs) have the potential for providing compact power sources for both space power and propulsion applications. The reactors operate in the temperature range of 2000 to 3000 K and, for propulsion systems, utilize hydrogen as a coolant. Given these overall operating conditions, the need exists to develop and verify the performance of various reactor components, particularly those which must operate at the more extreme conditions of temperature and hydrogen flow rate. These components must undergo multiple thermal cycles between cryogenic and exit temperatures. In addition, some of the components are subject to a thermal gradient of thousands of degrees Kelvin over a few centimeters. A number of approaches directed towards meeting these needs have been developed. These include both monolithic ceramics (e.g. boron nitride) as well as coated composites consisting of refractory carbides (ZrC, NbC and TaC) on carbon-based substrates. Full and partial scale components have been fabricated and tested under prototypic conditions. The results of this developmental effort will be described.