摘要
微型直接甲醇燃料电池(Micro Direct Methanol Fuel Cell,μDMFC)具有高比能、高效率、使用方便和环境友好等优点,具有广阔的应用前景。可靠的封装对于燃料电池有着举足轻重的作用。本文设计、制作了环氧树脂整体封装型μDMFC,并结合力学实验和数值模拟方法,对热-机械载荷作用下环氧封装结构的蠕变特性展开研究,初步阐明了μDMFC膜电极接触状态随封装结构蠕变过程的对应变化规律。
首先,设计并制作出环氧树脂整体封装型被动式μDMFC。通过灌封工艺制作的环氧树脂封装结构“包裹”由膜电极和两片集流板组成的电池内部结构,构成整体μDMFC被动式发电单元。采用课题组搭建的DMFC测试系统测试了电池性能,结果显示,在室温、常压、甲醇溶液浓度为4mol/L的条件下,μDMFC最大功率密度为5.256mWcm-2,验证了环氧树脂封装方法的可行性。
第二,介绍了高聚物的蠕变特性,给出了求解蠕变问题的基本理论和影响蠕变特性的主要因素。采用时间硬化理论,研究了环氧树脂封装结构的蠕变过程,基于力学实验方法,测定了μDMFC封装用环氧树脂的弹性模量、热膨胀系数和蠕变曲线,求解出环氧树脂时间硬化蠕变方程的基本参数。
第三,在ANSYS软件平台上建立了μDMFC封装结构蠕变特性分析的有限元模型,求解了热-机械载荷作用下μDMFC膜电极法向应力的变化过程。计算结果显示,由于蠕变特性的存在,μDMFC膜电极工作面法向应力随着热-机械载荷循环次数的增加而增大。电池状态稳定后,膜电极工作面的平均法向应力增大0.08Mpa左右。
最后,采用正交试验法,系统考察了集流板通孔直径、环氧封装结构外圆直径和封装厚度对膜电极接触状态的影响。分别以膜电极工作面法向应力均值变化和分布均匀性作为设计输出指标,计算了25组环氧封装μDMFC结构有限元模型。计算结果表明,集流板通孔直径对膜电极接触状态的影响最大,封装环氧厚度次之,封装环氧外圆直径影响最小。通过正交分析实验,确定了较合理的电池结构参数。
Micro direct methanol fuel cell (μDMFC) has broad application prospects because of its high efficiency and cleanness. A reliable package is essential to DMFC. AμDMFC with an entire epoxy resin package structure was designed and fabricated. For the reason that epoxy resin has creep behavior under thermal-mechanical load, this paper studied the change process of the working plane in membrane electrode assembly (MEA) with the creep behavior of epoxy resin package structure, in the method of mechanics experiments and numerical simulation.
Firstly, a passiveμDMFC with an entire epoxy resin package structure was designed and fabricated. The fuel cell structure was divided into two parts:internal structure and package structure. The internal structure, which was composed of MEA and two current collectors, was clamped by epoxy resin package structure that was made by potting. The test system for DMFCs established by the research group was used to test the performance of theμDMFC, and experimental results showed that the passiveμDMFC with 4mol/L methanol had a maximum power density of 5.256mWcm-2 at room temperature, which indicated that epoxy resin was feasible forμDMFC packaging.
Secondly, the paper introduced the creep of polymer. Some basic theories to solve creep problem and some primary factors influencing creep behavior were presented. The time hardening theory was selected to research the creep process of epoxy resin package structure. In addition, the elastic tensile tests and tensile creep tests for package epoxy resin were accomplished. Besides that, the test for determining the coefficient of thermal expansion was also conducted. The paper processed the experimental data and got the relevant parameters for the simulation based on time hardening theory.
Thirdly, this paper built theμDMFC finite element model for analyzing creep behavior of package structure, and adopted ANSYS software to simulate the change process of the MEA contact condition under thermal-mechanical load. Simulation results showed that because of the effect of creep, the overall normal stresses of the working plane in MEA increased with the increase of thermal-mechanical load cycles. After the cell became stable, the average normal stresses increased by about 0.08Mpa.
Finally, using orthogonal test, the paper analyzed the effects of structure parameters on the contact state of the working plane in MEA, which contained the hole diameter of the current collector, the outer diameter and the thickness of the epoxy package structure. By evaluating the mean change and the uniformity of the normal stresses of the working plane in MEA, the paper calculated the 25 sets of finite element models ofμDMFC packaged by epoxy. The results showed that the most important parameter was the hole diameter of the current collector, the second important parameter was the thickness of the epoxy package structure and the least important parameter was the outer diameter of the epoxy package structure. Through orthogonal test, the author gained the optimized parameters of cell structure.
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