Polybutadiene (PB) has a low glass temperature Tg and exhibits rubbery behavior during mechanical perturbation. The corresponding PB-based polyurea (PU) has a higher Tg and fails in a brittle mode for high strain rates. However, unlike in glasses, this brittle failure is accompanied by large energy dissipation. Dielectric relaxation measurements demonstrate that whereas the PB segmental dynamics are faster than the strain rate during impact loading, for PU these motions are on the order of the strain rate, ∼105s−1. Consequently, impact induces a transition to the glassy state, with the accompanying response markedly different from that of a rubber.
Boron carbide (B 4 C) is a ceramic with a structure composed of B 12 or B 11 C icosahedra bonded to each other and to three(C and/or B)-atom chains. Despite its excellent hardness, B 4 C 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 B 11 C p -CBC, predict that the preferential substitution site for metallic elements (Be, Mg and Al) is the center atom and that for non-metallic elements (N, P and S) it is generally the end of the three-atom chain in B 4 C'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 B 11 C p -CBC. As a first step to testing the feasibility of elemental substitutions experimentally, Si atoms were incorporated into B 4 C at low temperatures (~200-400 o 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 B 4 C. 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 B 4 C is presented with potential for generating materials with tailored properties for a broad range of applications.
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