EAGER: Stereolithography-based Multi-material Additive Manufacturing of Particle-reinforced Composite Lattices to Achieve Tunable Negative-Thermal-Expansions

固体材料受热时通常膨胀。在广泛的工程环境中，这种特性可能导致严重的热失配问题。制造几乎为零或负热膨胀的材料以减轻热失配是非常理想的。一种很有前途的方法是利用由不同热膨胀系数组成的复合晶格结构中的几何相互作用。然而，要制造具有多个不同成分和高度复杂几何形状的三维复合晶格是很困难的。这项早期概念探索性研究资助（EAGER）奖支持基于立体光刻的多材料增材制造工艺的基础研究，该工艺可以制造具有可调负热膨胀的3D颗粒增强复合材料晶格。研究结果将有利于许多需要仔细管理热应力的应用，包括桥梁接头，微芯片设备，粘合剂填埋物，牙齿填埋物以及经历可变温度的高精度光学或机械设备。在基于立体光刻的多材料增材制造工艺中，逐层制备两种类型的光树脂（带和不带颗粒增强），交替形成两种类型的光树脂梁。当温度升高时，具有不同热膨胀系数的两种梁相互作用，导致复合晶格整体负热膨胀。第一个研究目标是建立颗粒增强光树脂的光固化深度与制造参数（光强度、光曝光时间、颗粒类型和颗粒体积分数）之间的关系。为了实现这一目标，将建立一个正面光聚合模型来阐明增强光树脂在紫外线辐射下从液体到固体的转变。它将用于预测光固化深度作为制造参数的函数。一些模型预测将与不同光强（5-100 W/m^2）、光曝光时间（0.5-300 s）、颗粒类型（铜、二氧化硅、铁和氧化铝）和颗粒体积分数（0- 10%）的光固化实验进行比较。第二个研究目标是了解晶格几何参数（加筋梁长度、加筋梁与未加筋梁之间的夹角）和颗粒体积分数对复合材料晶格负热膨胀的影响。为了实现这一目标，将构建一个分析热弹性模型来描述复合晶格内光树脂梁之间的热诱导几何相互作用。基于该模型，对不同晶格几何参数和颗粒体积分数值的复合材料晶格的负热膨胀进行了分析预测。制备的复合材料晶格的热膨胀实验将在温度控制的热室中进行。加固梁的长度从2到2.8毫米，梁角从60到90度，颗粒体积分数从2%到10%不等。复合晶格的负热膨胀将由数码相机在温度变化过程中拍摄的图像序列来测量。

Solid materials usually expand when heated. This property may induce severe thermal mismatch problems in a wide range of engineering settings. It is highly desirable to manufacture materials with nearly-zero or negative thermal expansions that can mitigate the thermal mismatch. One promising approach is to harness the geometrical interactions within composite lattice structures composed of constituents of distinctive thermal expansion coefficients. However, it is difficult to fabricate 3D composite lattices with multiple distinctive components and highly sophisticated geometries. This EArly-concept Grant for Exploratory Research (EAGER) award supports fundamental research on a stereolithography-based multi-material additive manufacturing process that can make 3D particle-reinforced composite lattices with tunable negative thermal-expansions. Research results will benefit a number of applications where thermal stress should be carefully managed, including bridge joints, microchip devices, adhesive fillers, dental fillings, and high precision optical or mechanical devices that experience variable temperatures.In the stereolithography-based multi-material additive manufacturing process, two types of photoresins (with and without particle reinforcement) are procured layer by layer to form two types of photoresin beams alternately. When experiencing rising temperature, the two types of beams with different thermal expansion coefficients interact with each other to induce an overall negative-thermal-expansion of the composite lattice. The first research objective is to establish the relationships between the photocuring depth of the particle-reinforced photoresins and manufacturing parameters (light intensity, photoexposure time, particle type, and particle volume fraction). To achieve this objective, a frontal photopolymerization model will be developed to elucidate the transition from liquid to solid of reinforced-photoresins under ultraviolet radiation. It will be used to predict photocuring depth as a function of manufacturing parameters. Some model predictions will be compared against with photocuring experiments with varied light intensity (5-100 W/m^2), photoexposure time (0.5-300 s), particle type (copper, silica, iron, and alumina), and particle volume fraction (0-10 percent). The second research objective is to understand the effects of lattice geometric parameters (reinforced beam length, angle between reinforced and unreinforced beams) and particle volume fraction on the negative-thermal-expansion of the composite lattices. To achieve this objective, an analytical thermoelastic model will be constructed to describe the thermal-induced geometrical interactions between photoresin beams within the composite lattices. Based on the model, the negative thermal expansion of the composite lattices will be analytically predicted for different values of lattice geometric parameters and particle volume fraction. Thermal expansion experiments on manufactured composite lattices will be conducted in a thermal chamber with temperature control. Reinforced beam length will be varied from 2 to 2.8 mm, beam angle from 60 to 90 degrees, and particle volume fraction from 2 percent to 10 percent. The negative-thermal-expansion of the composite lattices will be measured from image sequences taken by a digital camera during the temperature variation.