基于CALYPSO方法的新型高能量密度材料设计

The design of new high energy density materials based on CALYPSO method

高能量密度材料的研究由于其重大的研究意义和广泛的实际应用价值受到越来越广泛的关注.在众多含能材料家族中,聚合氮因其生成物为清洁无污染的氮,作为一种环境友好型的清洁能源材料,成为含能材料领域研究的热点.研究者从不同的角度出发,结合CALYPSO结构搜索技术通过第一性原理模拟对纯氮、碱金属叠氮化合物MN3(M=Li,Na,K,Rb,Cs)的高压行为进行了研究,期望得到结构稳定的聚合氮,并降低合成压强.本文在课题组前期研究的基础上,结合其他学者的研究内容,细致总结了以纯氮和碱金属叠氮化合物为前驱物,高压压致聚合氮过程中的相变及伴随相变发生的物理、化学性质的变化.

High energy density materials（HEDM） have attracted more and more attention due to their wide potential applications as propellants and explosives. Among the family of HEDM, polymeric nitrogen has become a hot topic because its emission is nitrogen gas and is considered as a green and environmentally-friendly HEDM. In this field, the key issue is searching for polymeric nitrogen that can stably exist in ambient conditions. CALYPSO is an efficient structure prediction method based on particle swarm optimization algorithm, which only requires chemical composition can get stable or metastable structures at given conditions. Using CALYPSO technology companied by first-principles calculations, the high-pressure behaviors, including phase transition and electronic properties, of pure nitrogen and alkali metal azides MN3（M=Li, Na, K, Rb, Cs） have been studied. Based our previous works and some works done by other researchers, we summarized structural evolution and electronic properties of pure nitrogen and MN3 under high pressure in this review. The CALYPSO combining density functional theory（DFT） has predicted a cagelike diamondoid nitrogen with striking stabilization above 263 GPa, named diamondoid N. The diamondoid N adopts a highly symmetric body-centered cubic structure with 20 nitrogen atoms in a unit cell. Alkali metal azides consist of metal cations M＋ and linear anions N3ˉ. The anions N3ˉ with double bonds N=N is expected to form polymeric structure at lower pressure comparing to N2 gas due to their lower bonding energy relative to N≡N in N2. Under pressure region 0–400 GPa, the N3ˉ have a series of structural transitions from N3ˉ to pseudo-benzene ＂N6＂ ring and then to polymeric nitrogen with different structure depending on the alkali metal cations. The structural transition of N3ˉ is companied by hybridization type of nitrogen atoms from sp, sp2 to partial sp3 induced by pressure. Under high pressure, metal elements in azides play two roles. On the one hand, metal atoms act as additives to nitrogen leading to chemical pre-compression effects, which is expected to decrease synthesis pressure of polymeric nitrogen. On the other hand, they act as electronic donors, which is expected to improve the stability of polymeric nitrogen.