经济学人下载:一种受外力时硬度提高的新材料

Science and Technology Materials science Stronger when stricken
科技 材料科学 越敲越结实

A new material that gets stiffer when it is stressed
一种受外力时硬度提高的新材料

ONE of the valuable properties of bone is that when it endures repeated stress it responds by becoming denser and stronger.
骨头有一种很珍贵的属性，那就是当受到反复的击打时，其质地反而会变得更加紧密，坚硬度变得更高。

A living material can do that. A non-living one cannot.
只有活性材料具备这种特征，非活性材料则不具备。

It has no way of adding the extra matter needed to provide the extra density. But it would help engineers a lot if non-living stuff could at least stiffen in response to stress—and that may now be possible.
用添加必要物质来让材料获得更高密度的方法是不可行的，但如果非活性材料起码可以在外力作用下变得坚硬—目前达到这一点是可能的，那么这对工程师们来说有非常大的借鉴意义。

Brent Carey, a graduate student at Rice University in Texas, thinks he has found a way to make it happen.
布伦特凯利是一名德州莱斯大学的研究生，他认为他已找到了实现这一设想的办法。

Mr Carey made his discovery when he was testing the properties of a material made of carbon nanotubes (cylinders of carbon atoms a few billionths of a metre across) and a rubbery polymer called polydimethylsiloxane.
凯利在测试一种材料的属性时有所发现，该材料的成分包含碳纳米管（直径为几纳米的碳原子圆柱体）和一种名为聚二甲基硅氧烷的橡胶聚合物。

He created this composite by growing a forest of nanotubes using hot hydrocarbon gases and an iron catalyst, and then filling the space between the tubes with the polymer.
材料的合成过程是这样：首先，他使高温碳氢混合气体和铁催化剂相互作用产生密集的碳纳米管群，然后再用聚合物充填碳纳米管间的空隙。

The surprise came when he discovered how his new material responded to repeated stress.
当他注意到新材料受到连续作用力而产生反应时，令他惊讶的现象出现了。

He found this did not cause any of the damaging fatigue that would be expected.
据他观察，预期中的材料损毁性疲劳并未产生。

Indeed, his initial inspection suggested the stuff was actually growing stiffer.
实际上，他的初步监测显示材料变得更加坚硬了。

Fascinated by this result, he took his finding to his supervisor, Pulickel Ajayan, and they assembled a team to study the new material.
这个结果让凯利喜出望外，他马上找到了自己的导师普里克尔阿贾扬，随后他们成立了一个小组，专门研究这种新型材料。

They gave the composite a real workout. They compressed it five times a second for a week. That caused its stiffness to increase by 12%.
他们给这种合成材料做了次实验：对该材料施加5.5倍的压力，时间持续一周，结果材料硬度提高了12%，

Moreover, the effect showed no sign of abating,
并且这种状态没有减弱的迹象。

which led them to suspect that if it were exposed to more stress it would grow stiffer still.
研究小组由此设想，如果对其施以更大的作用力，这种材料的硬度是否会变得更高。

Why this happens is still a mystery.
产生这种效果的原因还是个谜。

Mr Carey and his colleagues report in the American Chemical Society's journal Nano that heating the new material did not eliminate the response.
凯利和他的同事们发表在美国化工科学院的期刊杂志《Nano》的文章中说，即使对这种新型材料加热，也不能消除已产生的变化。

This suggests that the self-stiffening is not the result of chemical changes in the polymer, which can usually be undone by heat.
这说明材料强度提高并不是聚合物发生化学反应造成的，一般这种变化在加热状态下是可逆的。

The researchers do have one lead, though. Because of the regular alignment of the nanotubes, they were able to stress the material from various directions.
不过研究人员已有所发现。由于碳纳米管呈规律状排列，他们能够从不同的方向对材料施加作用力。

They found that when the direction of stress was at right-angles to the tubes, it stiffened by 5.9%.
当施力方向与碳纳米管成直角时，材料硬度提高5.9%；

When it was in the direction in which tubes were pointing, the increase was only 4.3%.
当作用力与碳纳米管所指方向一致时，材料硬度仅提高4.3%。

What that means is still unclear, but it may be the key to understanding the phenomenon—and thus being able to replicate it with other ingredients.
现在尚无法对其中的原因作出解释，但或许对于理解材料硬度变化是非常重要的—进而在其他原材料身上复制这类变化也有了可能。