Phase transitions to 120 GPa for shock-compressed pyrolytic and hot-pressed boron nitride

Abstract

Shock-compression characteristics of two types of hexagonal graphitelike boron nitride have been investigated. Highly oriented very pure pyrolytic boron nitride exhibits shock-velocity versus particle-velocity discontinuities that appear to be manifestations of the initiation of a sluggish phase transition. This transition begins at 20 GPa and is driven to completion (melting) at 75 GPa. Discontinuities in the plot for impure hot-pressed boron nitride indicate initiation at 10 GPa and completion at 20 GPa. The ($U_s,U_p$) plots follow essentially the same paths for $4.0<\mathrm{}{U}_p<5.2\mathrm{}$ km/sec. No evidence for a transition to a metalliclike state was seen. Temperature calculations indicate that the material is liquid above ∼80 GPa.