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2018 Research Highlight: Locating Si atoms in Si-Doped Boron Carbide

Locating Si atoms in Si-Doped Boron Carbide

 

CMEDE Researchers
Dr. Atta U. Khan 
Rutgers University
Dr. Anthony Etzold 
Rutgers University
Dr. Vlad Domnich 
Rutgers University
Dr. Qi An
University of Nevada, Reno
Dr. Kris Behler
U.S. Army Research Laboratory
Dr. Jerry LaSalvia
U.S. Army Research Laboratory
Professor Richard Haber
Rutgers University

Boron carbide is a well-known compound owing to its high hardness, low density and exceptional resistance to wear, making it a prime candidate for armor materials. However, its low fracture toughness is detrimental for its use as a material for multi-impact armor. At Rutgers, we have enhanced the properties of boron carbide powders through Rapid Carbothermal Reduction processing, giving rise to a carbon particulate free material, which has had an immediate impact on the hardness and strength of armor plates. While these measures solve some macroscopic issues, silicon addition has helped in mitigating the amorphization of boron carbide. As amorphization is a key component in the failure of boron carbide, this reduction signifies advancement in toughening the material for use in armor. However, it is very important to understand the mechanism, by which silicon addition to boron carbide reduces the amorphization under impact. The very first step in solving the mechanism puzzle is the resolution of the crystal structure, including the location of the silicon atoms in the boron carbide lattice.

A suitable sample for Rietveld refinement was synthesized by mixing boron carbide and SiB₆ powders and sintered in SPS for an extended period. Presence of liquid aided in achieving thermodynamic equilibrium. X-ray powder diffraction pattern obtained from this sample confirmed a homogenized sample. Rietveld refinement of this pattern coupled with electron density difference Fourier maps shows the silicon atoms being present in the void between the 3-atomic chain and the icosahedra, resulting in a kinked 3-atomic C-Si-C chain. These silicon atoms lie close to the icosahedra and seem to have bonding with the nearest boron atom of the icosahedra, possibly further stabilizing the icosahedra. This location of Si atoms also bridges the bond between the chain end atom and the icosahedra. As it is reported in the literature that the icosahedra disintegrates first in the event of amorphization, this additional stabilization may have led to the observed reduced amorphization. Moreover, DFT simulations by Dr. Qi An, confirmed the location of Si atoms and these calculations fully support our findings.