Nacre-inspired boron nitride/sodium alginate composite with enhanced mechanical properties by prestress

Prestressing strategies are widely adopted in the formation of biological structures to modify their mechanical property. Inspired from the prestress generation during the formation of biominerals, a nacre-inspired boron nitride (BN) nanosheets strengthened by prestressed sodium alginate (SA) matrix is fabricated. The BN/SA (BS) films are formed by freeze-casting and pressing, then immerse in ethanol (BS-E) or CaCl2 solution (BS-Ca). Several megapascals contractile stress is generated in film that is attributed to the contraction and crosslink of SA chains. Subsequently, artificial bulk composites are fabricated by stacking of these prestressed films via hot -pressing. Benefit from the contractile stress, the orientations of BN nanosheets in BS-E and BS-Ca are improved, and the (002) interplanar spacing of BN nanosheets in BS-Ca bulk is slight expansion compared with BS. Finally, the flexural strength of BS-E bulk is enhanced from 145 MPa (BS) to 174 MPa when the loading direction is parallel to stacking.