Superhard and superconductive polycrystalline boron doped diamond synthesized with different concentration of B4C

Abstract

Boron doped diamond are potential superhard materials with good electric conductivity. In this work, polycrystalline boron doped diamond (p-BDD) with different boron concentration were synthesized from the mixture of graphite and boron carbide under ultra-high pressure (15 GPa) and temperature (2500–2700 K). The results indicate that no more boron atoms can be inserted into diamond when the boron concentration is above 4 at%. The EELS spectra indicated that the boron atoms have been doped into the diamond grains, and surplus boron carbide locate at the grain boundary. The hardness measurements reveal that the p-BDD doped with 1 at% and 23 at% boron atoms exhibit high hardness of 78.5 GPa and 45 GPa, respectively, which is equivalent to the hardness of single crystal diamond and cubic boron nitride. The p-BDD is a semiconductor and transforms into superconductive state below 2.5 K. According to microstructure measurements, highly dense p-BDD and B4C bonded diamond exhibit superhard behavior because of the strong bonding of diamond grains by B4C additives. The oxidation temperature of these p-BDD specimens is around 1538 K in the air, which is far higher than diamond. B4C is demonstrated as a superior additive to sinter superhard and electric conductive p-BDD, which are extremely useful in areas where high hardness and electric conductive are highly desired.