Hall Lee J, Coluci Vitor R, Galvão Douglas S, Kozlov Mikhail E, Zhang Mei, Dantas Sócrates O, Baughman Ray H
MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, TX 75083, USA.
Science. 2008 Apr 25;320(5875):504-7. doi: 10.1126/science.1149815.
Most materials shrink laterally like a rubber band when stretched, so their Poisson's ratios are positive. Likewise, most materials contract in all directions when hydrostatically compressed and decrease density when stretched, so they have positive linear compressibilities. We found that the in-plane Poisson's ratio of carbon nanotube sheets (buckypaper) can be tuned from positive to negative by mixing single-walled and multiwalled nanotubes. Density-normalized sheet toughness, strength, and modulus were substantially increased by this mixing. A simple model predicts the sign and magnitude of Poisson's ratio for buckypaper from the relative ease of nanofiber bending and stretch, and explains why the Poisson's ratios of ordinary writing paper are positive and much larger. Theory also explains why the negative in-plane Poisson's ratio is associated with a large positive Poisson's ratio for the sheet thickness, and predicts that hydrostatic compression can produce biaxial sheet expansion. This tunability of Poisson's ratio can be exploited in the design of sheet-derived composites, artificial muscles, gaskets, and chemical and mechanical sensors.
大多数材料在拉伸时会像橡皮筋一样横向收缩，因此它们的泊松比为正。同样，大多数材料在静水压力下压缩时会向各个方向收缩，拉伸时密度会降低，因此它们具有正的线性压缩率。我们发现，通过混合单壁和多壁纳米管，可以将碳纳米管片材（巴基纸）的面内泊松比从正值调整为负值。这种混合显著提高了密度归一化的片材韧性、强度和模量。一个简单的模型根据纳米纤维弯曲和拉伸的相对难易程度预测了巴基纸泊松比的正负和大小，并解释了普通书写纸的泊松比为何为正且大得多。理论还解释了负的面内泊松比为何与片材厚度的大正泊松比相关，并预测静水压力压缩可导致片材双轴膨胀。泊松比的这种可调性可用于设计片状复合材料、人造肌肉、垫圈以及化学和机械传感器。
