Structure and bonding in boron carbide: The invincibility of imperfections

Abstract

Boron carbide, usually described as B 4 C, has the mysterious ability to accommodate a large variation in carbon composition (to as much as B 10 C) without undergoing a basic structural change. We systematically explore how the bonding varies with carbon concentration in this structure and the origin of the fundamental electron deficiency of the phase. As the carbon concentration is reduced, we find that the exo-polyhedral B Eq -C bonds of the icosahedra in the structure become increasingly engaged in multiple bonding, and the repulsive steric interactions between the bulky B 12 units surrounding the carbon atom are reduced. The short bond lengths observed within the three-atom yC-B-Cx chains are then due to substantial p-bonding, while the carbon deficiency weakens its s-framework significantly. We conclude that the idealized framework of boron carbide has to expel some electrons in order to maximize its bonding; disorder in the structure is an inevitable consequence of this partial oxidation. The localization of electronic states arising from the disorder leads to the semiconducting nature of boron carbide throughout its composition range.

Shear-Induced Brittle Failure along Grain Boundaries in Boron Carbide

“…The failure mechanisms of the Fe-doped GB involve the interaction of icosahedra with impurity Fe atoms, leading to deconstruction of the icosahedra within the GB region. Balakrishnarajan et al 49 examined the stability effect of adding or removing an electron from the icosahedron. Our simulations show that the addition of electrons from Fe atoms weakens the icosahedron, which can explain why the existence of impurity (Fe) further weakens the strength of the B 4 C GBs.…”

Electronic structures and mechanical properties of boron and boron-rich crystals (Part I)

“…For B 13 C 2 and less C compounds, Balakrishnarajan et al show that the valence top band has an antibonding character for the bond between the CBC chain and an equatorial B site, and thereby it is a little unstable [110]. This causes releasing of an electron from the chain bond, resulting in replacement of the CBC chain with CBB chain.…”

“…According to recent experiments [103], the band gap has been modified as 2.09 eV. On the other hand, all band calculations so far published show gaps as large as 3 eV [74,[105][106][107][108][109][110]. Let us look in more detail at the calculated energy gap of boron carbide.…”

Melt Infiltration Casting of Alumina Silicon Carbide and Boron Carbide Reinforced Aluminum Matrix Composites

“…It is the hardest third material after diamond and cubic-BN which are very expensive and hard to prepare. At temperatures above 1200°C its hardness value even exceeds that of diamond [1]. Combination of high hardness and low density makes boron carbide top candidate of armor materials.…”