【作者】 王东；

【导师】 曹健；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2017， 硕士

【摘要】 固态氧化物燃料电池（SOFC）是一种高效环保的绿色电化学能源转化系统,氧化钇稳定的氧化锆陶瓷（YSZ）是应用最广泛的固体燃料电池电解质材料,铁素体不锈钢（FSS）常被用作连接体材料。根据固态燃料电池的实际工作需求,铁素体不锈钢与YSZ陶瓷之间必须形成紧固密封连接,连接体系必须具备优异的密封性和高温抗氧化性。本课题首先对铁素体不锈钢进行了抗氧化表面改性,然后采用空气活性钎焊方法（RAB）实现了其与YSZ陶瓷的高质量连接,获得了具有较高强度及优异抗氧化性的钎焊接头。首先进行铁素体不锈钢表面抗氧化保护层的制备与性能表征。采用溶胶凝胶法获得亚微米级（Mn,Co）₃O₄尖晶石粉末,在10V/60s的最优电泳沉积工艺下,在铁素体不锈钢表面获得厚度约为20μm的均匀尖晶石沉积层。随后将尖晶石沉积层烧结成尖晶石保护层。采用光学显微镜、扫描电子显微镜/能谱分析（SEM/EDS）、X射线衍射（XRD）、面比电阻测量仪等分析手段对尖晶石保护层的组织形貌、电性能、抗氧化性能进行表征。研究了烧结气氛、烧结温度、保温时间对尖晶石保护层组织形貌的影响。确定最优烧结工艺参数为:氩氢气氛600°C/2h+氩氧气氛1000°C/1h,最优工艺下获得致密保护层的典型界面组成为Fe-Cr/MnCr₂O₄/（Mn,Co）₃O₄。对在最优工艺下获得的被保护不锈钢试样进行高温抗氧化测试及面比电阻测量:经过800°C/200h的高温氧化试验后,尖晶石保护层依然能够对金属实现有效保护,其面比电阻只增加了不到30%;若不对金属采取保护措施,在相同试验条件下,金属基体被完全氧化,面比电阻增加近六倍。最后提出了尖晶石保护层的生长及保护机制:经过还原产生的MnO和Co纳米颗粒的填隙愈合作用及其自身较高的烧结效率共同保证了不锈钢基体在氧化烧结过程中不被氧化腐蚀。采用Ag-CuO体系钎料实现了YSZ陶瓷与表面预制尖晶石保护层的铁素体不锈钢的高质量空气钎焊连接。采用SEM/EDS、XRD、透射电子显微镜（TEM）对接头界面及断口的组织形貌、物相成分进行分析。研究了钎料成分及工艺参数对接头组织形貌、力学性能和线膨胀系数（CTE）的影响。确定最优工艺参数为CuO含量8mol.%,1050°C/30min,在该工艺参数下得到的接头典型界面结构为Fe-Cr/MnCo₂O₄+CuFeMnO₄/CuO/Ag（CuO）/CuO/ZrO₂,CuO与尖晶石材料发生了烧结反应生成Cu_0.5Fe_0.5MnCoO₄的致密尖晶石相,起到了促进连接和保护钎焊接头的作用,此时接头强度达到55MPa。CuO和Ag都能直接与ZrO2实现原子水平的结合,CuO/ZrO₂、Ag/ZrO₂界面的晶格错配度分别只有1.3%和3.4%。测试了YSZ/铁素体不锈钢接头的抗氧化性:经过800°C/200h的高温氧化试验,采用预制保护层的不锈钢与YSZ的接头组织形貌和力学性能几乎不发生变化,接头得到有效保护,断裂形式以塑性断裂为主;未预制保护层的不锈钢与YSZ的钎焊接头经过高温氧化试验后已完全失效,金属基体被严重氧化腐蚀,断裂形式以脆断为主。Ag钎缝强度不足,且CTE值较大,向钎料中添加MgO颗粒制备复合钎料以解决这个问题。采用含12wt.%MgO的复合钎料在1100°C/30min下获得的铁素体不锈钢/YSZ接头的强度达到76MPa,相比添加增强相前提高了38%;钎缝硬度为160HV,提高了90HV;复合钎料CTE值为16.38×10^-6 K-1,降低了11.5%。最后提出了复合钎料空气钎焊连接表面预制尖晶石保护层的铁素体不锈钢与YSZ陶瓷的界面形成机制。更多还原

【Abstract】 Solid Oxide Fuel cell（SOFC）is a highly efficient and environmentally friendly green electrochemical energy conversion system.Yttria-stabilized zirconia（YSZ）is the most widely used solid fuel cell electrolyte material.Ferritic stainless steel（FSS）is often used as connecting material.According to the actual working demand for solid fuel cell,a tight connection of ferritic stainless steel and YSZ ceramic must be formed,the connection system must own excellent airtightness and high temperature oxidation resistance.In this project,the ferritic stainless steel was pre-modified to obtain anti-oxidative capacity,and then it was brazed to YSZ ceramic by reactive air brazing（RAB）,and the brazing joint with high strength and excellent oxidation resistance was obtained.Firstly,the preparation and characterization of the anti-oxidation protective layer on the surface of ferritic stainless steel were carried out.（Mn,Co）₃O₄ spinel powder with the particle size in submicron range was obtained by sol-gel method.Under the optimal electrophoretic deposition process of 10 V / 60 s,a homogeneous spinel deposit with thickness of around 20μm was obtained on the surface of ferritic stainless steel.The spinel deposit is then sintered into a compact spinel layer.The microstructure,electrical properties and oxidation resistance of the spinel layer were then studied by means of optical microscopy,scanning electron microscopy / energy dispersive spectroscopy（SEM/EDS）,X-ray diffraction（XRD）and area specific resistance（ASR）measuring instrument.The effects of sintering atmosphere,sintering temperature and holding time on the microstructure of spinel layer were studied.The optimum sintering process parameters were as follows: 600°C / 2 h at argon-hydrogen atmosphere + 1000°C / 1 h at argon-oxygen atmosphere.The phase structure at the interface between the spinel layer and the base metal was Fe-Cr/MnCr₂O₄/（Mn,Co）₃O₄.High temperature oxidation test and ASR measurement of the protected stainless steel samples obtained under the optimum process were carried out: after high temperature oxidation test at 800°C / 200 h,the spinel layer could still protect the metal matrix effectively and the increase of ASR was no less than 30%;if the metal was not protected,under the same test conditions,the metal matrix is completely oxidized,the ASR increased six-fold.Finally,the growth and protection mechanism of the spinel protective layer is put forward: the pore-healing effect of the MnO and Co nanoparticles produced by the reduction process and their high sintering efficiency both ensure that the stainless steel matrix will not be oxidized during the oxidative sintering process.The Ag-CuO brazing filler metal was used to realize the high quality RAB joint of YSZ ceramic and ferritic stainless steel with prefabricated spinel protective layer.The microstructure and phase composition of the joint interface and fracture surface were analyzed by SEM / EDS,XRD and TEM.The effects of brazing filler metal composition and process parameters on the microstructure,mechanical properties and linear expansion coefficient（CTE）of the joint were studied.Both CuO and Ag could directly bind to ZrO₂ at the atomic level.The lattice mismatches at the interfaces of CuO/ZrO₂ and Ag/ZrO₂ interfaces were only 1.3% and 3.4%,respectively.The oxidation resistance of YSZ / ferritic stainless steel joint was tested.After 800°C / 200 h high temperature oxidation test,the microstructure and mechanical properties of the joint were not changed and the joint was effectively protected.The fracture form is mainly plastic fracture.The brazing joint of YSZ and stainless steel without prefabricated protective layer completely cracked after high temperature oxidation test.The metal matrix is seriously oxidized and corroded,and the fracture form is mainly brittle fracture.The strength of Ag-based joint is insufficient,and its CTE value is too large in comparison to them of the substrates on both sides,adding tiny MgO particles to brazing filler metal is promising to solve this problem.The strength of the joint of ferritic stainless steel / YSZ obtained at 1100°C / 30 min with composite brazing filler metal containing 12 wt.% MgO is 76 MPa,which is 38% higher than that before adding enhanced particles.The hardness of brazing joint is 160 HV,which is 90 HV higher.The CTE value of the composite filler metal is 16.38×10^-6 K^-1,which is 11.5% lower.Finally,the interface forming mechanism of the RAB joint between ferritic stainless steel with prefabricated spinel layer and YSZ ceramic using composite brazing materials was proposed.更多还原