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类石墨烯2D材料利用量子效应实现超低摩擦

美国T大学工程学院的博士生Peter Serles将一个磁性样本放入原子力显微镜中. 对这种材料的新测量和模拟表明,它的低摩擦特性是由于量子效应(Daria Perevezentsev摄)

Researchers from the University of Toronto's Faculty of Applied Science & 工程学和莱斯大学报告了首次测量一种被称为磁石的材料的超低摩擦行为. 研究结果为设计用于各种领域的低摩擦材料指明了方向, including tiny, implantable devices. 

磁铁矿是一种2D材料,这意味着它由一层原子组成. In this respect, it is like graphene, 这是一种因其特殊性能(包括超低摩擦)而被广泛研究的材料, since it was produced and identified. 

“Most 2D materials are formed as flat sheets,” says Peter Serles, 申请应用科学系机械及工业工程系博士学位 & Engineering, and the lead author of the paper published this month in Science Advances.

“理论是,这些石墨烯薄片的摩擦性能很低,因为它们的结合非常弱,很容易滑动. You can imagine it like fanning out a deck of playing cards. 因为纸牌间的摩擦力很低,所以不需要花太多精力去分散桥牌.” 

研究人员试图通过比较石墨烯和其他二维材料来验证这一理论. 

The team includes: Tobin Filleter, a professor of mechanical and industrial engineering; Chandra Veer Singh材料科学与工程系副教授; Shwetank Yadav, a post-doctoral researcher in materials science and engineering; as well as several current and former students.

While graphene is made of carbon, magnetene is made from magnetite, a form of iron oxide, which normally exists as a 3D lattice. The team’s collaborators at Rice University, in Houston, 使用高频声波小心地分离出仅由几片2D磁铁矿组成的一层. 

然后,德克萨斯大学的工程团队将磁片放入原子力显微镜中. In this device, 一个尖尖的探头被拖过磁感应片的顶部来测量摩擦力. 这个过程就像电唱机的唱针被拖过黑胶唱片的表面一样. 

 “磁铁矿层之间的键比一堆石墨烯薄片之间的键要强得多,” Serles says. “They don’t slide past each other. 让我们惊讶的是探测器尖端和磁石最上层之间的摩擦. It was just as low as it is in graphene.” 

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A schematic showing the lattice structure of magnetene, 暗红色的球体描绘铁,浅红色的球体描绘氧气(图片由Shwetank Yadav拍摄)

Until now, 科学家们将石墨烯和其他二维材料的低摩擦力归因于薄片可以滑动的理论,因为它们只通过被称为范德华力的弱力结合在一起. But the low-friction behaviour of magnetene, which doesn’t exhibit these forces due to its structure, suggests that something else is going on. 

“When you go from a 3D material to a 2D material, 由于量子物理的影响,很多不寻常的事情开始发生,”  Serles says. “根据你切割的角度,它可以很光滑,也可以很粗糙. 原子在第三维度中不再受限制,所以它们可以以不同的方式振动. And the electron structure changes, too. We found that all of these together affect the friction.” 

研究小组通过将他们的实验结果与计算机模拟预测的结果进行比较,证实了这些量子现象的作用. Yadav和Singh建立了基于密度泛函理论的数学模型来模拟探针尖端在二维材料上滑动的行为. 包含量子效应的模型是实验观测的最佳预测器. 

塞尔斯说,该团队的发现的实际结果是,他们为希望有意设计超低摩擦材料的科学家和工程师提供了新的信息. 在各种小型应用中,这些物质可能用作润滑剂, including implantable devices. 

For example, 你可以想象一个微小的泵,将一定量的给定药物输送到身体的特定部位. 其他类型的微型机电系统可以收集跳动的心脏的能量来为传感器供电, 或者为一个微型机器人操纵器提供动力,使其能够在培养皿中对不同类型的细胞进行分类. 

“When you’re dealing with such tiny moving parts, the ratio of surface area to mass is really high,” says Filleter, corresponding author of the new study. “That means things are much more likely to get stuck. 我们在这项工作中所展示的是,正是因为它们的微小尺寸,这些2D材料才有如此低的摩擦力. These quantum effects wouldn’t apply to larger, 3D materials.” 

Serles says that these scale-dependent effects, combined with the fact that iron oxide is non-toxic and inexpensive, 使磁石在可植入的机械装置中非常有吸引力. 但他补充说,在完全理解量子行为之前,还有更多的工作要做. 

“We have tried this with other types of iron-based 2D materials, such as hematene or chromiteen, and we don’t see the same quantum signatures or low friction behaviour,” he says. “So we need to zero in on why these quantum effects are happening, 这可以帮助我们更有意地设计新型低摩擦材料.”

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