1.School of Physics and Electronics, Hunan Key Laboratory for Super-Microstructure and Ultrafast, Central South University, Changsha 410083, China 2.School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
Abstract:Kirigami, the art of cutting paper, recently emerged as a powerful tool to substantially modify, reconfigure and program the properties of material. The development of kirigami technology provides an effective solution for designing the inorganic flexible electronic devices. Pyramid kirigami, as a kind of kirigami structure, shows a large vertical extension characteristic. It has been widely used to demonstrate versatile applications, such as graphene kirigami spiral spring, three-dimensional stretchable supercapacitor, and wearable flexible sensors. In the present work, we construct a polygonal radial symmetric pyramid kirigami by introducing some cuts in the elastic sheet. The mechanical behavior of pyramid kirigami is investigated based on the cantilever formula solved by Galerkin method. In addition, a “beam model” is proposed to explain deformation process of pyramid kirigami, which consists of several “beam elements” containing two cantilever beams. The formula for the relationship between the elastic coefficient K and the structural parameters of the regular N-sided pyramid kirigami of n modules is obtained by combining several cantilever beams. The formula for the linear threshold of deformation DT is obtained based on the comparison between the approximate curve of small deflection and the theoretical curve of a cantilever beam. When the deformation of the structure exceeds the linear threshold, the structure cannot keep the elastic coefficient K value linear any more, and the mechanical behaviors become non-linear. The simple geometric relationship of a single module is used to explain the out-of-sheet distortion of the structure. The proposed theoretical model is confirmed by finite element method simulation and experimental methods, and it is used to analyze the mechanical characteristics of graphene krigami reported. The results indicate that the defined parameters can be adjusted to tailor or manipulate the ductility and mechanical behaviors. This work provides theoretical support for the application of pyramid kirigami in the field of flexible devices. In the macroscopic field, the pyramid kirigami structure is expected to be applied to the field of flexible devices as a flexible structure with controllable elastic coefficient. In the microscopic field, it is expected to use two-dimensional materials to make force measurement devices with a simple visual readout and femtonewton force resolution. Keywords:Kirigami/ radial symmetry/ graphene/ flexible devices
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2.1.金字塔剪纸模型
2015年, Blees等[1]在Nature上发表了利用石墨烯制作的微型剪纸结构的研究, 实验测得金字塔型石墨烯剪纸结构在微小力条件下, 弹性系数K = 2 × 10–6 N·m–1. 其剪纸结构为典型的正四边形金字塔结构, 母图案如图1(a)所示, 主要参数为: 正多边形的边数为N = 4; 模块数为n = 3; 靠近内侧的第一个模块切口长度最短, 定为L; 往外侧的相邻模块的模块切口长度为(b/2 + L + b/2), 即两侧共增加长度为b; 相邻模块的连接长度一般不变, 为x; 梁宽为w; 薄板厚度为t; 材料杨氏模量为E. 固定最外层边缘结构, 在垂直面内施加拉力F后, 在面外的拉伸形变如图1(b)所示. 图示结构(n = 3)在受力形变后, 可以分为三个形变模块, 三个模块的形变长度分别为d1, d2, d3, 总形变为长度D. 图 1 典型金字塔型剪纸结构 (a) 边数N = 4, 模块数n = 3的金字塔结构; (b) 模型在竖直拉力F作用下产生竖直形变 Figure1. Typical pyramid kirigami structure: (a) Pyramid structure with number of edges N = 4 and number of modules n = 3; (b) pyramid model produces vertical deformation under the action of vertical tension F.
为了对金字塔型剪纸结构在竖直方向形变过程进行分析, 建立了如图2的物理模型, 将此结构分为若干个悬臂梁的组合, 采用“梁模型”分析它的受力形变. 根据正四边形剪纸结构某个形变区域, 对应图1(b)拉伸模型中的一个侧面, 建立如图2(a)所示的“梁模型”. 图2(a)中共有3个“梁模块”, 分别为m1, m2, m3. 每个“梁模块”可以看作由4个相同长度为(L – x + 2b)/2的“梁单元”组成. 如图2(b)的“梁单元”所示, 红色虚线区域为组成侧面结构的一个基本的“梁单元”, 近似分割为两个等效的一端受力而另一端固定的悬臂梁Beam A和Beam B. 对于Beam A来说, 可以看作左端固定的悬臂梁, 右端在受到右侧梁Beam B向下拉伸的竖直力fA作用后向下弯曲, 产生的竖直方向挠度为dA, 悬臂梁的长度为lA, 宽度为w, 厚度为t. Beam B可类似定义. 图 2 由“梁单元”构成的“梁模型” (a) 金字塔结构一个形变区域简化成的“梁模型”; (b) 悬臂梁组成的“梁单元” Figure2. “Beam model” consisting of “beam elements”: (a) Simplified “beam model” of a deformed area of the pyramid structure; (b) “beam element” consisting of cantilever beams.
弹性系数K关系式(11)的适用范围受到悬臂梁理论公式的适用范围影响. 悬臂梁理论公式采用了去掉高阶项的小挠度曲线近似, 当结构形变超过保持弹性系数K为线性的最大形变, 即线性阈值DT时, 其力学响应表现为非线性. 图3给出了基于(1)式和(2)式的悬臂梁Beam A理论曲线与小挠度近似曲线对比, 在dA/lA小于0.3的范围内, 两曲线近似相等, 在此范围内梁的受力与形变的关系可以采用“小挠度”近似公式计算. 图 3 基于(1)式和(2)式的悬臂梁理论曲线与小挠度近似曲线对比 Figure3. Theoretical curve of cantilever beam compared with the approximate theoretical curve of small deflection based on Eq. (1) and (2).
金字塔结构剪纸的力学响应可以被切口参数调制, 如(11)式. 为验证各参数的影响, 图4展示了利用FEM软件进行多点验证, 计算正多边形弹性系数受切口参数调制的定量结果. 本文中使用ANSYS Workbench验证, 首先根据物理模型建模, 并在最上层多边形平台中心建立向上凸起表面的圆形平台作为应力施加边界, 在最下层制作和最底层结构大小相同的“回”形平台作为约束边界; 材料属性设置为: 密度300 kg/m3, 杨氏模量1.2 GPa, 泊松比0.3, 材料温度22 ℃; 网格单元大小设置为20 mm. 几何参数设置为: L = 1.26 m, x = 2 cm, 其余各参数设置在对应图中空白处给出了标注. 对模型位移进行求解后, 利用施力F和得到的结构最大形变D可以求出结构弹性系数K. 图4(a)—(c)中的点表示对三个参数验证的FEM仿真值, 弹性系数K分别与w, $ t^3 $及N呈线性变化的关系, 和K值理论公式(11)结果一致. 对于梁宽w来说, 增大w可以使弹性系数增大. 对于厚度t, 弹性系数对厚度t最为敏感, t增大为2倍, K值会增大为8倍, 若需要大幅度增强结构弹性系数, 增加t是最有效的方法. 反之亦然, 对于t为纳米级的二维材料薄膜而言, 利用此类剪纸结构, 可以获得极小的弹性系数K值, 从而测量微小力, 如光压或光镊力[1]. 对于边数N来说, 增大N可以增大K. 但是增大N意味着需要占用更大的面积(如正六边形的金字塔结构面积是正四边形的2.6倍左右), 同时过大的N会限制b的大小, 因此之后研究均采用四边形结构. 图 4 FEM模拟和理论计算验证弹性系数与结构参数关系 (a)?(c) 弹性系数K分别与梁宽w、厚度t的三次方及边数N值呈线性变化关系; (d) 取不同模块切口长度L的增加值b, 验证K值与模块数n的关系, 点为模拟值, 虚线为计算值 Figure4. Verify the relationship between elastic coefficient and structural parameters through FEM simulation and theoretical calculation: (a)?(c) The elastic coefficient K varies linearly with the beam width w, the cube of thickness t, and the number of sides N; (d) take different values b to verify the relationship between the elastic coefficient K and the number of modules n. The points are simulation values, and the dotted lines are calculated values.
为验证弹性系数K计算公式(11)和线性阈值DT计算公式(14), 在弹性纸板上制作了模块数n为3的四边形金字塔结构. 纸板采用A4大小的270 g规格相片纸, 实验结构图形采用CAD (computer aided design)绘制, 用刻刀对打印好图像的纸板进行切割, 完成金字塔结构的制作. 为减小重力影响, 实验中将此结构的底端最外层结构固定在一竖直平面上, 通过水平滑动装置施加变化的水平拉力F, 用数字测力计记录其拉力F与形变D的关系. 图5(a)为实验图, 结构参数为: N = 4, n = 3, L = 7.5 cm, x = 8 mm, b = 1 cm, w = 4.5 mm, t = 280 μm. 图 5 利用实验对K, DT的计算公式(11)和(14)式进行验证 (a) 实验图; (b) 四边形实验数据, 点为测量结果, 虚线红色为线性区域拟合结果, 黑色虚线为计算出的线性阈值; (c) Nature上发表的石墨烯剪纸弹簧在激光驱动下的形变-受力结果[1] Figure5. The K and DT formulas (11) and (14) are verified experimentally: (a) Experimental picture; (b) the experimental data of the quadrangular pyramid structure, the points are the measurement results, the red dotted line is the linear region fitting result, and the black dotted line is the calculated linear threshold; (c) laser-driven deformation of graphene kirigami springs published in Nature[1].