温差核电池（Radioisotope thermoelectric generator, RTG）具有长寿命、小体积和高能量密度的特点，在深空探测任务中展现出巨大的应用潜力。然而，其较低的输出功率、能量转换效率以及有限的服役稳定性，限制了RTG在深空探测中的广泛应用。随着探测任务对电源需求的多样化以及制备技术的不断进步，促使研究人员从高性能热电材料研发、小型化、几何结构优化、稳定性提升等多方面改进RTG的设计，以提升电学输出并延长服役寿命。本文针对使用高温热源或服役于具有较大温度梯度环境中的双级RTG，对其电学输出和机械稳定性进行了综合优化，并深入分析了其在冷/热冲击条件下电学输出、机械稳定性和热力学性能的瞬态变化特性；提出了两种具有特殊几何形状的热电腿来改善RTG的电学输出性能，并采用直写式3D打印技术实现了制备。主要研究内容及成果如下：

（1）使用非支配排序遗传算法-II（Non-dominated Sorting Genetic Algorithm-II, NSGA-II）对双级热电器件的电学输出和机械稳定性进行了综合优化。将能量转换效率（η_two）设定为电学性能的优化目标，将冷级与热级热电器件中热电腿内的最大von Mises应力（Von_BT, Von_SKD）设定为机械稳定性的优化目标，对这三个目标赋予权重并组成权重因子，采用逼近理想解排序法（Technique for Order Preference by Similarity to Ideal Solution, TOPSIS）从最优解集中筛选出合适的最优解。优化结果表明，三个目标之间具有显著的相互影响，三者的重要性排序为Von_SKD>η_two>Von_BT，且热级热电器件具有比冷级热电器件更大的重要性与影响力。根据不同权重因子选择得到的热电器件具有不同的结构参数，从而表现出不同的电学输出性能和机械稳定性，以及不同的电学输出特性。

（2）分析了双级热电器件在冷/热冲击条件下电学输出与机械稳定性的瞬态变化特性。设置了正弦波和方波两种冲击模型，每种模型中包含五种冲击能量相同、而冲击时间和功率不同的冲击模式。电学输出性能在正弦波模型的冷/热冲击的波峰时刻达到最大值和最小值，而在方波模型的冷/热冲击的结束时刻达到最大值和最小值。机械稳定性在正弦波和方波模型中的差异较小，Von_SKD和Von_BT在两种模型中达到的最大值相差小于2%。在相同冲击能量下，热冲击对电学输出和机械稳定性的影响明显高于冷冲击。此外，电学输出和机械稳定性的变化幅值随冲击时间的延长而增加。由于碲化铋级热电器件的阻挡作用，方钴矿级热电器件的电学输出与机械稳定性的变化幅值低于碲化铋级热电器件。

（3）分析了双级热电器件在冷/热冲击条件下热力学性能的瞬态变化及内部Cu电极的疲劳行为。在热冲击作用时，双级热电器件的温度升高，导致进入器件的有效能减少，而消耗在Cu电极和陶瓷板等非能量转换部件中的有效能比例有所增加；在冷冲击作用时，则呈现相反的趋势。这种变化规律与电学输出的变化相互印证。双级热电器件的熵增速率和不可逆性在热冲击过程中降低，而在散热和冷冲击过程中升高，在整个冲击过程中呈上升趋势。靠近冷端的Cu电极因承受较大的温度变化，导致其循环次数较低；而靠近热端的Cu电极则因承受较高的温度，存在Cu原子扩散的风险。因此，需要针对不同问题采取相应措施，以增强RTG的输出性能并延长服役寿命。

（4）提出了螺旋构型和轮辐构型两种具有特殊几何形状的热电腿，通过增加侧面积、强化侧面散热来提升电学输出性能。螺旋构型热电腿在热电腿的朝向和热流方向之间产生夹角来增加侧面积；轮辐构型热电腿在截面上设置轮辐结构来增加截面周长，从而增加侧面积。分析了两种特殊构型热电腿在恒定功率和恒定温度两种热源条件下的电学输出参数和内部最大von Mises应力随几何参数的变化规律。结果表明，两种特殊构型热电腿的电学输出性能随几何参数的变化而变化，存在最优几何参数使其电学输出性能达到最佳。然而，特殊构型设计使热电腿的侧面出现尖锐的几何形状，从而尖锐部位处产生更大的von Mises应力，在一定程度上降低了机械稳定性。与传统圆柱、方形构型相比，特殊构型热电腿在单腿和集成在热电器件内两种情况下均表现出更高的电学输出性能。

（5）采用直写式3D打印技术实现了上述特殊构型热电腿的成功制备。采用有机溶剂和粘结剂合成一种粘性溶液，并以碲化铋合金粉末作为热电填料、Cu粉作为导电助剂，成功制备出一种适用于直写式3D打印的热电浆料。热电浆料和烧结后的热电材料的测试结果表明，该热电浆料具有适合于3D打印的剪切稀化和粘弹逆变的流变特性；Cu在热电材料中均匀分布，没有与碲化铋反应生成新物质，在为载流子提供导电通路的同时又能促进碲化铋在烧结过程中的结晶行为。P型热电材料的ZT_max为0.91；N型为0.24。采用直写式3D打印技术制备了不同构型的热电腿，并将其拼接成热电模块进行电学性能测试。测试结果与模拟结果的变化趋势一致，表明该热电浆料具有优异的均匀性和稳定性，能够用于特殊形状热电腿的批量制备。

本论文的研究工作涉及温差核电池的输出性能优化、几何结构设计和3D打印制备技术，能够为深空探测任务中的温差核电池的设计、制备和性能优化提供新的研究思路和发展方向。

Radioisotope thermoelectric generator (RTG) have the characteristics of long life, small size and high energy density, showing great application potential in deep space exploration missions. However, the low output power, energy conversion efficiency and the limited service stability restrict the widespread application of RTG in deep space exploration. With the diversification of power requirements in exploration missions and the continuous advancement of fabrication technology, researchers have improved the design of RTG in many aspects, such as the development of high-performance thermoelectric materials, miniaturization, optimization of geometrical shapes, and improvement of stability, all of which are aimed at improving electrical output performance and extending service life. This paper comprehensively optimizes the electrical output and mechanical stability of the two-stage RTG which equipped with high-temperature heat source or serving in the environments with large temperature differences, and analyzes in detail the transient changes in its electrical output, mechanical stability and thermodynamics performance under cold and hot shock conditions; two kinds of thermoelectric legs with special geometrical shapes are proposed to improve the output performance of the thermoelectric generators, and the thermoelectric legs with special geometrical shapes are successfully fabricated with direct writing 3D printing technology. The main research contents and results are as follows:

(1) Electrical output and mechanical stability of the two-stage thermoelectric generator are comprehensively optimized using non-dominated sorting genetic algorithm-II (NSGA-II). The energy conversion efficiency (η_two) is set as the optimization objective about electrical performance, and the maximum von Mises stress of the thermoelectric legs in the cold and hot stage thermoelectric generator (Von_BT, Von_SKD) are set as the optimization objectives about mechanical stability. Weights are assigned to the three objectives to form weight factor, and the appropriate optimal solution are selected from the optimal solution set with technique for order preference by similarity to ideal solution (TOPSIS). Optimization results show that there are significant mutual influences among the three objectives, and the importance is ranked as: Von_SKD>η_two>Von_BT. Hot-stage thermoelectric generator has greater importance and influence than the cold-stage thermoelectric generator. The thermoelectric generators selected according to different weight factors have different structural parameters, thereby showing different electrical output and mechanical stability, as well as different electrical output characteristics.

(2) Transient change of the electrical output and mechanical stability of the two-stage thermoelectric generator under cold/hot shock conditions are analyzed. Two shock models: sinusoidal wave and square wave are set up, and each model contains five shock modes with the same shock energy but different shock time and power. Electrical output performance reaches the maximum and minimum values at the peak of the cold/hot shock in the sinusoidal wave model, but reaches the maximum and minimum values at the end of the cold/hot shock time in the square wave model. The difference in mechanical stability between the sinusoidal wave and square wave models is small, and the maximum values reached by Von_SKD and Von_BT in the two models differed by less than 2%. Under the same shock energy, the effect of thermal shock on electrical output and mechanical stability is significantly greater than that of cold shock. In addition, the change amplitude of electrical output and mechanical stability increases with the extension of shock time. Due to the block of bismuth telluride-stage thermoelectric generator, the change amplitude of the electrical output and mechanical stability of the skutterudite-stage thermoelectric generator is lower than that of the bismuth telluride-stage thermoelectric generator.

(3) Transient changes of the thermodynamic performance of the two-stage thermoelectric generator under cold/hot shock conditions as well as the fatigue behavior of the Cu electrode are analyzed. During the thermal shock, the temperature of the two-stage thermoelectric generator increases, resulting in a decrease of the exergy entering the thermoelectric generator, while the proportion of exergy consumed in non-energy conversion components such as Cu electrode and ceramic plate increases. During the cold shock, the opposite trend is presented. This change pattern is mutually confirmed by the change in the electrical output. The entropy generation rate and irreversibility of the two-stage thermoelectric generator decrease during the thermal shock, but increase during the heat dissipation and cold shock, showing an upward trend during the entire cold/hot shock process. Cu electrodes near the cold side are subjected to greater temperature changes, resulting in the lower number of cycles; while the Cu electrodes near the hot sides are subjected to higher temperatures, which poses a risk of Cu atom diffusion. Therefore, corresponding measures need to be taken for different problems to enhance the output performance and extend the service life of RTG.

(4) Two kinds of thermoelectric legs with special geometrical shapes, named helix-shaped and spoke-shaped, are proposed to improve the electrical output performance by increasing the side area and the heat dissipation occurred on the side. The helix-shaped thermoelectric leg increases the side area by creating an angle between the orientation of the thermoelectric leg and the direction of heat flow; and the spoke-shaped thermoelectric leg increases the side area by increasing the circumference of cross-section by setting spokes in cross section. Variation trends of the electrical output parameters and the internal maximum von Mises stress with geometrical parameters of the two thermoelectric legs with special geometrical shapes under two heat source conditions of constant power and constant temperature are analyzed. Results show that the electrical output parameters vary with the geometrical parameters, and there are optimal geometrical parameters to achieve the best electrical output performance. However, the special geometrical design makes the sides of the thermoelectric legs appear sharp, which generates greater von Mises stress at these sharp and reduces the mechanical stability to a certain extent. Compared with traditional cylindrical and square shapes, the thermoelectric legs with special geometrical shapes show higher electrical output performance both in single-leg cases and integrated into thermoelectric generators.

(5) The above thermoelectric legs with special geometrical shapes are successfully fabricated using direct writing 3D printing technology. A thermoelectric slurry suitable for direct writing 3D printing is successfully prepared, in which an organic solvent and binder are used to synthesize a viscous solution, bismuth telluride alloy powder is used as the thermoelectric filler, and Cu powder is used as the conductive additive. Test results of the thermoelectric slurry and sintered thermoelectric material show that the thermoelectric slurry has the rheological properties of shear thinning and viscoelastic transformation, which is suitable for 3D printing; Cu is evenly distributed in the thermoelectric materials and does not react with bismuth telluride to generate new substances; Cu can provide conductive paths for carriers and promote the crystallization of bismuth telluride during the sintering process. ZT_max of P-type thermoelectric material is 0.91; and that of N-type is 0.24. Thermoelectric legs with different geometrical shapes are fabricated using direct writing 3D printing technology and assembled into thermoelectric modules for testing electrical output characteristics. Variation trends in the test results are consistent with those in the simulation results, which indicates that the thermoelectric slurry has excellent uniformity and stability and can be used to fabricate thermoelectric legs with special shapes in batches.

The research work in this paper involves the output performance optimization, geometrical shape design and 3D printing fabrication technology of RTG, which can provide new research ideas and development directions about the design, fabrication and performance optimization of RTG used in deep space exploration missions.

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