February 27, 2018
February 27, 2018
Speaker: Atta Ullah Khan, Ph.D.
Rutgers University-New Brunswick
Title: Borides Based Ceramic Materials for Armor
Location: McLaren Center for Ceramic Research CCR-201
Abstract: Boron rich compounds are known for their low density and high hardness, making them the materials of interest for armor. Among those, B4C and B6O are particularly interesting due to extremely high hardness. However, the well-documented formation of amorphous bands in boron carbide (B4C) under contact loading has been identified in the literature as one of the possible mechanisms for its catastrophic failure under high velocity impacts. To mitigate amorphization, Si-doping is suggested by a theoretical study and substantiated through an experimental study. However, there have been discrepancies between theoretical and experimental studies, about Si replacing one or more atoms in B12 icosahedra or the C-B-C chain. Dense phase pure Si-doped boron carbide is produced through a conventional scalable route. A powder mixture of SiB6, B4C, and amorphous boron is reactively sintered, yielding a dense Si-doped carbide material. A combined analysis of Rietveld refinement on XRD pattern coupled with electron density difference Fourier maps and theoretical DFT simulations was performed in order to investigate the location of Si atoms in B4C lattice. Si atoms are found to reside around the chain and result in a kinked chain. These Si atoms lie close to the boron atoms of neighboring icosahedra. The bonding distance between the two, suggests weak bonding and Si is anticipated to stabilize the icosahedra through electron donation and is expected to result in mitigated amorphization. Possible supercell structures are suggested along with the most plausible structure for Si-doped boron carbide composed of (B11Cp)CBC, (B12)CBC, (B12)CSiC and (B12)BVB with 1:3:1:1 ratio.
Sintering of B6O has been challenging over the years. We successfully synthesized boron suboxide by mixing amorphous boron and boric acid powders, and heating at 1300 °C for 2 hours, followed by heating at 1400 °C for 1 hour. This synthesized powder was then sintered with different sintering aids, in spark plasma sintering (SPS) at 1850 °C under 50 MPa pressure, in a graphite die, with BN sprayed on all surfaces in contact with the sample. More than 99% of the theoretical density was achieved. Elastic constants were measured by employing ultrasound waves. Average Vickers hardness was found to be ~30 GPa under 1 Kg load. Sintered pellets were characterized by X-ray diffraction, scanning electron microscope, and Raman spectroscopy.
Bio: Atta Ullah Khan is a Postdoctoral Researcher in the Department of Materials Science & Engineering at Rutgers University. Atta completed his undergraduate degree in Chemistry from the University of the Punjab, Pakistan in 2005. Atta then went on to pursue his Ph.D. in Chemistry at the University of Vienna under the supervision of Professor Peter Rogl, graduating in 2011. At the University of Vienna, Atta contributed to the study of crystallography and phase equilibria of various intermetallic systems and reporting many novel crystal structures. After graduating Atta was appointed as the ThermoMag FP-7 Postdoctoral Fellow at the University of Cyprus. There, he investigated the thermoelectric properties of Mg2Si based system and held a world record of the highest ZT for this class of compounds. Later on, he joined National Institute for Materials Science (NIMS) and studied skutterudites for thermoelectric applications and holds a world record of highest ZT for rare-earth free skutterudites. Since 2016, he is a part of the armor ceramic research in the group of Prof. Richard A. Haber at Rutgers University. His research interests broadly lie at the intersection of solid-state synthesis, crystallography, phase equilibria, thermoelectrics and ceramics for energy conversion and armor applications.
For more information, please contact Sheela Sekhar at firstname.lastname@example.org or 848.445.2159.