能量存储:基于全新原理的“纳米弹簧”

许多机械装置如钟表、玩具等都采用弹簧来驱动,其能量的存储与释放是通过弹簧内部原子间距的变化来实现的。但是这种原子间距的变化(即弹性变形)所能存储的体能量密度相对很低,如何提高能量的转换效率以及材料存储的能量密度是当前材料科学理论和实验研究共同关注的一个问题。本研究利用金属钨单晶纳米线在加载时独特的孪晶变形行为,提出了一个可以在纳米尺度下高效存储与释放机械能的新原理,并据此设计了相应的纳米装置——纳米弹簧。与块体弹簧不同,本文提出的纳米弹簧通过表面原子的重构来实现能量的存储与释放。进一步的计算还表明,由于金属钨孪晶界面的移动阻力非常小,金属钨纳米弹簧的能量转换效率可以达到98%;同时该纳米弹簧存储的体能量密度可以超过钟表发条的1600倍,并具有30%的应变以及3GPa的驱动应力。

Energy Storage: A Nanowire Spring Surfaces

Machines from wrist watches to toys can be powered by elastic springs,which store energy when bonds between atoms are stretched in response to an external mechanical force.When the spring relaxes,the bond lengths return to normal and energy is released.While they are ubiquitous,elastic springs are also limited in important ways：they can withstand only so much strain,and can store only a relatively small amount of energy in a given volume.Now,we have proposed storing energy in a nanoscale spring,not by changing its atomic bond lengths,but by reconfiguring its surface atoms,resulting in a predicted 16,000 times increase in energy volume density compared to a watch spring.This ＂pseudo-elastic＂ behavior occurs in a thin metallic nanowire.Applying a stretching force causes a twin boundary to develop in the nanowire,and travel down the nanowire length.This in turn causes the atoms at the nanowire surface to assume a higher-energy arrangement,so that the force applied to the nanowire is converted to stored energy.We found that if the nanowire is made of a particular configuration of tungsten metal,the motion of the twin boundary will encounter very low friction,allowing 98% of the stored energy to be recovered when the nanowire relaxes.Further calculations indicate that the nanowire can withstand a strain of 30% and a stress of over 3GPa.