【作者】 彭珍珍；

【导师】 郭瑞松；

【作者基本信息】 天津大学 ， 材料学， 2007， 博士

【摘要】 锆基钙钛矿型高温质子导体具有良好的化学稳定性和较高的晶粒电导率,是固体氧化物燃料电池用电解质隔膜的重要候选材料。然而,烧结温度高、总电导率偏低是其实用化面临的最大障碍。本论文在详细研究和对比钇掺杂锆酸锶和锆酸钡烧结性能和电化学性能的基础上,采用ZnO为烧结助剂,大幅度降低了这类质子导体的烧结温度,缩短了保温时间,进一步以无机盐为第二相添加剂,设计制备了具有新型微观结构的锆酸钡/无机盐复相质子导体,详细研究了它们的烧结性能和电化学性能。采用高温固相法制备了SrZr_xY_1-xO_3-δ（SZY）和BaZr_xY_1-xO_3-δ（BZY）（x = 0.05, 0.10, 0.15）系列单相粉体,在烧结温度为1600°C,保温时间大于6 h的条件下,得到密度为95 %左右的质子导体。电性能测试表明,在固溶度范围内,随着Y₂O₃掺杂量的增加,SZY和BZY的质子导电性能提高。Y₂O₃掺杂量为10 %的SZY₁₀在800°C的直流电导率为1.2×10-3 S/cm,相对来说,同样掺杂量的BZY材料的直流电导率较低,仅为5.32×10-4 S/cm。活化能分析表明, SZY比BZY具有更高的直流电导活化能,这主要与SZY晶格结构的正交畸变有关。ZnO是SZY₁₀(SrZr_0.9Y_0.1 O_2.95)和BZY₁₀(BaZr_0.9Y_0.1 O_2.95)的有效烧结助剂。研究表明,适量的ZnO（< 5 mol%）可以大幅度降低两者的烧结温度达250°C以上,保温时间可以缩短到4 h。ZnO主要以固溶取代的形式存在于SZY₁₀和BZY₁₀晶格中,其在晶格中固溶使晶格缺陷增加是主要的促烧机理。在电性能方面,ZnO对SZY₁₀和BZY₁₀导电性能的影响不同。在BZY₁₀中,材料的直流电导率随着ZnO添加量的增加单调下降;在SZY₁₀中,材料的直流电导率先随ZnO的添加量增加而增加,当添加量超过5 mol%以后,电导率出现大幅下降。电动势测试结果表明,添加5 mol%以下ZnO不会显著改变SZY₁₀和BZY₁₀的质子导电特性,离子传导系数在0.85以上。但是,当ZnO在SZY₁₀中的添加量达到6 mol%时,材料的离子传导系数急剧下降。重点从缺陷化学角度对ZnO在SZY₁₀和BZY₁₀中的不同影响进行了讨论。在1320¹350℃/4 h的烧结条件下,制备了符合预期设想结构的系列BaZr_0.9Y_0.1O_2.95/无机盐（硫酸盐、碳酸盐和氢氧化钠）复相质子导体,重点研究了BaZr_0.9Y_0.1O_2.95/Na₂SO₄复相质子导体的烧结性能、微观结构和电性能等。研究结果表明,BaZr_0.9Y_0.1O_2.95/Na₂SO₄复相材料以BaZrO₃的等轴晶粒为主晶相,Na₂SO₄主要分布于晶界处。对比单相材料BaZr_0.9Y_0.1O_2.95,复相材料的直流电导率提高了一个数量级以上,接近10^-2 S/cm,离子传导系数在0.9以上,是纯质子导体。BaZr_0.9Y_0.1O_2.95/Na2SO4复相材料具有不同于单相材料的导电机理,这是硫酸钠在晶界处的分布造成的。烧结温度、保温时间、ZnO含量以及第二相无机盐类型是影响BaZr_0.9Y_0.1O_2.95/无机盐复相质子导体电性能的关键因素。选择较低的烧结温度和较短的保温时间,可以避免无机盐在体系中的挥发,得到具有较高电导率的复相质子导体。在满足烧结性能要求的基础上,应该尽量减小ZnO的添加量,以避免其对体系导电性能的不良影响。实验所涉及的几种无机盐与氢氧化钠都对主晶相的直流电导率产生影响,按照对直流电导率提高幅度从大到小的顺序为:硫酸盐>碳酸盐>氢氧化钠。更多还原

【Abstract】 Protonic conductor of Y-doped zirconates exhibits good chemical stability and high bulk conductivity. However, its low grain boundary conductivity and high refractory nature, hence a low total conductivity and high sintering temperature, remain the main obstacles in its high-drain applications as electrolytes of solid state fuel cells （SOFC）. In this study, we investigated the sintering behaviors and DC electrical conductivities of Y₂O₃ doped SrZrO₃ and BaZrO₃. The influences of sintering additive ZnO on the properties of Y-doped zirconates were thoroughly studied. Furthermore, heterogeneous composites were designed and fabricated with Y-doped BaZrO₃ as matrix and some inorganic salts as dispersants by a conventional powder processing. The electrical conductivities of the composites were investigated, and the enhancements were interpreted from the point of view of microstructure.SrZrxY1-x O₃-δ（SZY） and BaZrxY1-x O₃-δ（BZY） （x = 0.05, 0.10, 0.15） powders were synthesized by conventional solid-state reaction. Densified protonic conductors （95 % theoretical density） were achieved after sintering at 1600°C for more than 6 h. DC electrical measurements indicated that the protonic conductivities of SZY and BZY increased with increasing the Y₂O₃ content. The DC conductivity of SZY₁₀ （with 10 % Y₂O₃） is 1.2×10-3 S/cm at 800°C in wet hydrogen, while BZY₁₀ only achieved 5.32×10-4 S/cm under the same conditions. SZY has higher DC electrical activation energy than BZY, which closely relates to its deviation of orthorhombic lattice from cubic.Transition metal oxide ZnO is an effective sintering additive for zirconates. Addition of more than 2 mol% ZnO in SZY and BZY lowered their sintering temperatures by 250°C. XRD analysis confirmed the formation of solid-solution upon ZnO addition into SZY and BZY. The increased lattice defects by Zn substitution on Zr-site may account for the enhanced sintering ability. As for electrical properties, ZnO impart different impacts on SZY₁₀ and BZY₁₀. In BZY₁₀, addition of ZnO leads to a monotonically decreased DC electrical conductivity. In SZY₁₀, addition of 2⁵ mol% ZnO had benecial effects on its protonic conduction. Excess ZnO content （>5 mol%） in SZY₁₀ led to a decreased protonic conduction. Ionic transport number more than 0.85 at 800°C by electromotive force （EMF） measurements under fuel cell conditions revealed that SZY₁₀ and BZY₁₀ with low ZnO contents （less than 5 mol%） are nearly pure protonic conductors. The different influences of ZnO addition on the properties of SZY₁₀ and BZY₁₀ are discussed according to defect chemistry.With BZY₁₀ as the main phase, sulphates, carbonates and sodium oxyhydrate as the second phase respectively, several kinds of barium zirconate/salt composites were designed and fabricated at sintering temperature of 1320¹350 ?C and keeping time of 4 h, aiming to improve the total electrical conductivity of single phase barium zirconate. The sintering behavior, microstructure and electrical conductivity of BaZr_0.9Y_0.1O_2.95/Na2SO4 composites are thoroughly studied and compared with those of the single phase BZY₁₀. The BaZr_0.9Y_0.1O_2.95/Na₂SO₄ composites exhibit multi-phase microstructures, with BZY₁₀ grains as the main phase and small amounts of sulfates distributed at the grain boundaries, which is consistent with the expected microstructure. The electrical conductivity of BZY₁₀ increased by more than one order of magnitude, close to 10-2 S/cm, upon introducing Na2SO4 at the grain boundary. The ionic transport number of more than 0.9 indicates a pure protonic conductor at temperature range of interest. The proton transport mechanism is discussed based on the different mechanisms of zirconate and sulphates.Key factors of influencing the electrical conduction of the barium zirconate/salt composites involve sintering temperature, keeping time, ZnO content and species of salts. Low sintering temperature and short sintering time would help to avoid the evaporation of salts from the composite, and result in high conductivity. ZnO content should be minimized because of its detrimental influence on the proton conductivity of the composites. Of the salts in this study, we observed an improvement of electrical conductivity of barium zirconate in the sequence of sulphates > carbonates > sodium hydroxide.更多还原