B 3 C

1. Intrinsic Properties of Boron Carbide PowdersWilliam G. Fahrenholtz, Missouri University of Science and Technology, DMR 0906584 * (003) B4C Boron carbide (typically represented as B4C) has a crystal structure that consists of B12 or B11C icosahedra joined by 3 atom chains of C and/or B. This structure is stable over a range of boron (B) to carbon (C) ratios from about B4.3C to B12C. Commercial boron carbide is produced by carbothermal reduction, resulting in “B4C” that consists of C-saturated B4.3C and excess carbon. Commercial powders appear black due to excess C along with N and O impurities on the chain sites. In this study, high purity powders were synthesized across the stability range, which allowed the intrinsic properties of the compound to be studied. B-rich compositions were gray and had lower band gaps (~2.2 eV) while C-rich compositions were lighter in color and had higher band gaps (~3.7 eV). Above: Deconvolution of x-ray diffraction peaks for B6.5C showing extra peaks that arise is site occupancy due to the B-to-C ratio. Lower Left: Optical images of boron carbide powders with different B-to-C ratios. C-rich compositions have lighter colors. Below: Tabulated properties of boron carbide powders with varying B-to-C ratios. (012) B4C (101) B4C * = extra peaksfrom systematicdifferences inlattice siteoccupancy * * B3C B4.3C B5C B6.5C B10C B12C

2. Ultra-High Purity Diboride CeramicsWilliam G. Fahrenholtz, Missouri University of Science and Technology, DMR 0906584 The thermal conductivity of ZrB2 ceramics depends on compositional and microstructural factors. Due to the high temperatures (i.e., on the order of 2000°C or higher) used to synthesize and densify ZrB2, impurities such as carbon and oxygen are present in most diboride powders. The current research is aimed at determining the intrinsic properties of diborides by independently evaluating the effects of porosity, compositional variations, and grain size. One of the main technical challenges is producing high purity, dense ceramics. If the inherent properties can be determined from high purity materials, then existing models can be applied to understand the effects of factors such as impurities, porosity, and grain size on the thermal properties. Thermal conductivity decreases as the amount of porosity in a material increases. In addition, impurities/additives, such as carbon, and microstructural factors, such as grain size, also affect thermal conductivity. Without knowing the inherent properties of ZrB2, models applied to the system can give seemingly inconsistent results (e.g., thermal conductivity of fully dense ZrB2 differs for different models, which are shown by lines in the plot above).

3. Broader Impacts William G. Fahrenholtz, Missouri University of Science and Technology, DMR 0906584 Improved materials are needed for use in extreme environments such as those associated with hypersonic flight (temperature >2000°C, reactive gases) and armor (high-velocity impact). While diboride ceramics such as ZrB2 are candidates for these types of applications, the typical synthesis and densification methods produce materials with impurities such as carbon and oxygen that affect the behavior of the ceramics. NSF DMR 0906584 focuses on minimizing impurity content so that the intrinsic thermomechanical properties can be studied. Success in this project could enable the production of improved armor ceramics or shape-stable leading edges for highly maneuverable hypersonic aerospace vehicles. Camdenton, MO high school student Kevin Bird worked with undergraduate Andrea Els and graduate student Greg Harrington on a project for the Junior Science and Humanities Symposium (JSHS). Kevin’s project on laser-induced synthesis of magnesium won a prize at the Missouri regional competition. He advanced to the national competition, which was held April 27-May 1, 2011 in San Diego, CA. Kevin drew on the experience that the Missouri research group has with reactive synthesis of ceramics to make mixtures of carbon and magnesium oxide for self-propagating, high temperature synthesis of magnesium, which was ignited with a laser pulse. High school student Kevin Bird working in the laboratory at Missouri S&T Image from NASA Image Exchange web archive Artist rendering of the hypersonic concept vehicle X-43A. To date, the length of hypersonic flights has been limited to just a few seconds due to factors such as lack of materials for robust leading edges.