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Hexagonal boron nitride (hBN) ceramics are expected to have wide applications at high temperatures as both a structural and functional material. However, because of its flake structure and general inertness, it is currently impossible to sinter hBN powder to a dense bulk (with a relative density of above 96%) even at 2000 °C. Here, we report dense bulk hBN with 97.6% theoretical density achieved at a lower preparation temperature (1700 °C) via a self-densifying mechanism without sintering additives. During the sintering process, cubic boron nitride particles incorporated into the hBN flake powders transform into BN onions with a significant volume increase, thus filling in voids among the hBN flakes and highly densifying the hBN bulks. The resulting dense hBN ceramics possess 2–3 times the strength of traditional hBN ceramics. This phase-transition-induced volume expansion strategy could lead to dense sintered compacts with high performance in other ceramic systems.

Bistable structures featuring two stable states have been widely applied in designing fast and high-force-output actuators under various types of stimuli, such as mechanical force, swelling, thermal expansion, and so on. In this paper, we designed a magneto-actuated mechanism to realize the reversible shape transition between two curved stable configurations of a buckled beam using magneto actuation. The beam is composed of a silicone elastomer matrix with embedded micro-sized iron particles. The magnetic response of these iron particles endows the composite beam with the ability to snap from one stable shape to the other when the magnitude of the surrounding magnetic field exceeds the threshold value. By separately analyzing the electric-magnetic field and the magnetic-mechanical field, we formulate a simple and efficient computational method to numerically predict the critical current on the onset of snap-through. The computational and experimental critical currents show good agreement for different material and geometrical parameters, including the thickness of the beams, iron particle mixing ratios of the material, and the distances of the beam to the electromagnet. The proof-of-concept design is demonstrated to be efficient in the application of a magneto-responsive soft switch and a catapult for ejecting small objects, providing new insights into designing contactless, low-voltage-actuated bistable structures.

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The coating of BN nanosheets on carbon fibers could enhance the anti-oxidation and microwave absorbing properties of carbon fibers significantly.