  Hexagonal boron nitride (h-BN) is brittle, however, its atomic-scale
structural engineering can lead to unprecedented physical properties. Here we
report the bulk synthesis of high-density crystalline h-BN solids by using
high-temperature spark plasma sintering (SPS) of micron size h-BN powders. In
addition to the high mechanical strength and ductile response of such
materials, we have obtained anomalous values of dielectric constant beyond
theoretical limits, high thermal conductivity, and exceptional neutron
radiation shielding capability. Through exhaustive characterizations we reveal
that SPS induces non-basal plane crystallinity, twisting of layers, and
facilitates inter-grain fusion with a high degree of in-plane alignment across
macroscale dimensions, resulting in near-theoretical density and anomalous
properties. Our findings highlight the importance of material design, via new
approaches such as twisting and interconnections between atomically thin
layers, to create novel ceramics with properties that could go beyond their
intrinsic theoretical predictions.
