Location of Al and Si atoms in substituted boron carbide

Abstract

Boron carbide is a material of interest for personal body armor, but its low fracture toughness and amorphization limits its widespread use. Al and Si atoms in doped boron carbide reduce this problem. Passage of the substitution reaction in boron carbide powders with Al and Si vapors in vacuum was found. Certification methods: chemical analysis, full-profile XPA (Powder Cell for Windows. Version 2.4 FREE, W. Kraus & G. Nolze) and modeling in format of the 15-atomic unit cell B12(C-C-C) of trigonal syngony, spatial group R3 ̅m, Z = 3. A mixture of powders of boron carbide, aluminum or silicon is heat treated in vacuum at conventional evaporation temperatures of Al (1520 K) or Si (1640 K) for 1–5 h. The samples were purified with alkali and analyzed by arbitration chemical analysis for boron, carbon, aluminum and silicon. The formula composition of the input powders of boron carbide was determined as B12[(C-В-C)x(C-C-C)1-x], where x = 0.4–0.6. The aluminum substitution reaction takes place in both types of boron carbide chains and corresponds to the formula B12(C-Al-C) or AlB12C2. In the presence of silicon, the reaction took place exclusively at the positions of the tri-carbon chains. The composition of the obtained solid solution corresponds to - B12[(C-B-C)0.4(C-Si-C)0.6], starting powder B12[(C-B-C)0.4(C-C-C)0.6]. The absence of boron phases of silicide, such as SiB3 (SiB2.89), SiB6, SiBn (n ≈ 23) indicated greater resistance of C–B–C chains to interaction with vaporous Si. The content of Al and Si in the substituted phases is equal to 13.3 and 4.0 (% at.). Equivalent molar amounts of Al8B4C7 and SiC of gas-nano-phase origin were measured in the reaction products with vapor-like Al and Si. The area of tolerance chains of the boron carbide structure in the format of the average specific electronegativity (χN-Sh/rai) was found. It is in the range of values: 2.79 ≥ ССС ≥ СВС ≥ CSiC ≥ ВВС ≥ 2.18.