Substitution of silicon within the rhombohedral boron carbide (B4C) crystal lattice through high-energy ball-milling

Boron carbide (B4C) is a ceramic with a structure composed of B-12 or B11C icosahedra bonded to each other and to three (C and/or B)-atom chains. Despite its excellent hardness, B4C fails catastrophically under shock loading, but substituting other elements into lattice sites may change and possibly improve its mechanical properties. Density functional theory calculations of elemental inclusions in the most abundant polytypes of boron carbide, B-12-CCC, B-12-CBC, and B11Cp-CBC, predict that the preferential substitution site for metallic elements (Be, Mg and Al) is the chain center atom and that for non-metallic elements (N, P and S) it is generally the chain end atom of the three-atom chain in B4C's rhombohedral crystal lattice. However, Si, a semi-metal, seems to prefer the chain center in B-12-CCC and icosahedral polar sites in both B-12-CBC and B11Cp-CBC. As a first step to testing the feasibility of elemental substitutions experimentally, Si atoms were incorporated into B4C at low temperatures (similar to 200-400 degrees C) by high-energy ball-milling. High-resolution transmission electron microscopy showed that the Si atoms were uniformly dispersed in the product, and the magnitude of the lattice expansion and Rietveld analysis of the X-ray diffraction data were analyzed to determine the likely sites of Si substitution in B4C. Further corroborative evidence was obtained from electron spin resonance spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy characterizations of the samples. Thus, a simple, top-down approach to manipulating the chemistry of B4C is presented with potential for generating materials with tailored properties for a broad range of applications.