Effect of the sintering conditions on the morphology of La9.33Si2Ge4O26 oxyapatite for SOFCs electrolytes
Effect of the sintering conditions on the morphology of La9.33Si2Ge4O26 oxyapatite for SOFCs electrolytes
复制标题
烧结条件对SOFCs电解质La9.33Si2Ge4O26氧磷灰石形貌的影响
DOI：
10.1017/s1431927613001207
复制
发表时间：
2013
影响因子：
2.8
通讯作者：
B. Trindade
中科院分区：
文献类型：
--
作者：
M. Macatrão;M. Santos;C. Alves;F. Oliveira;T. Marcelo;J. Mascarenhas;B. Trindade
固体氧化物燃料电池（SOFC）是允许通过电化学反应以比传统工艺更清洁和更有效的方式将化学能直接转化为电能的装置（例如，燃料电池）。燃气轮机）。它们的特征在于使用固体氧化物材料作为电解质。氧化钇稳定的氧化锆（YSZ）传统上用于温度在850-1000 °C范围内的SOFC电解质中。最近的研究重点是开发在中温（500-800ºC）下具有更高离子传导率的新材料，作为YSZ的替代材料。具有磷灰石型结构的稀土硅酸盐，例如通式La 10（MO 4）6 O2的掺杂氧化镧，其中M = Ge、Co、Si、Al或P，是这些材料中的一种。与制造这些材料相关的主要限制是它们的烧结性差，这需要高烧结温度（1600ºC）。本工作涉及由La 2 O3（99.9%）、SiO2（99.4%）和GeO 2（99.9%）粉末通过机械合金化（MA）随后在1350ºC下进行常规和微波混合烧结来生产La9.33Si2Ge4O26致密材料。在保护气氛（氩气，200 kPa）中通过使用350 rpm的转速进行干法机械研磨15小时。在合成La9.33Si2Ge4O26混合物之前，在氩气中在50 kPa下以350 rpm单独研磨起始材料，以获得具有更大烧结能力的低粒度分布。MA混合物通过在390 MPa下单轴压制，随后在1350 °C下烧结1小时来压实。通过沸腾试验法测定La 9.33Si 2Ge 4 O 26颗粒的密度。采用SEM/EDS、XRD和原子力显微镜（AFM）对样品进行了表征。粒料的堆积密度和开口孔隙率（分别用于常规和微波混合烧结的EF和MW）示于表1中。在这两种情况下都获得了致密的颗粒。然而，样品MW显示出比样品EF更高的密度，并且因此显示出更低的孔隙率。两种机械合金化样品均由磷灰石相形成。该相在烧结过程中保持稳定。然而，烧结后检测到微量的第二相（La 4GeO 8）。这一阶段已经在以前的工作中提到过（Serra等人，2012年）。图1显示了通过SEM和AFM获得的两种样品的形态以及相应的粒度分布。将烧结的样品抛光并在空气中在1300 ℃下热蚀刻10分钟，进行详细的显微结构观察。低放大倍数下的SEM图像证实样品EF比样品MW多孔（分别见图1（a）和（B））。尽管在两种烧结工艺中使用相同的烧结温度，但样品MW具有较粗的晶粒尺寸（图1（c）至（f））。该特征已经在先前的工作中报道（Fang等人，1994年）。测定样品EF和MW的平均值分别为0.9和1.9 m。因此，微波复合烧结是一种新的适用于固体氧化物燃料电池电解质用La9.33Si2Ge4O26粉末的致密化工艺。根据PTDC/EME-PME/102837/2008号合同，本研究部分由FEDER基金通过竞争力操作因素计划赞助，部分由国家基金通过FCT赞助。还感谢向Cátia Alves、Márcio桑托斯和Mafalda Macatrão提供研究金。
Solid oxide fuel cells (SOFCs) are devices that allow direct conversion of chemical to electrical energy through an electrochemical reaction in a cleaner and more efficient way than conventional processes (eg. gas turbines). They are characterized by the use of a solid oxide material as the electrolyte. Yttria-stabilised zirconia (YSZ) has traditionally been used in SOFCs electrolytes at temperatures in the range of 850-1000 °C. Recent research is being focused on the development of new materials with increased ionic conductivity at intermediate temperatures (500-800ºC) as alternative materials to YSZ. Rare earth silicates with an apatite-type structure, such as doped lanthanum oxides of general formula La10(MO4)6O2, where M = Ge, Co, Si, Al, or P, are among these materials. The major limitation associated with the manufacture of these materials is their poor sinterability, which requires high sintering temperatures (1600ºC). The present work concerns the production of La9.33Si2Ge4O26 dense materials from La2O3 (99.9%), SiO2 (99.4%) and GeO2 (99.9%) powders by mechanical alloying (MA) followed by conventional and microwave hybrid sintering at 1350ºC. Dry mechanical milling was carried out in protective atmosphere (argon at 200 kPa) by using a rotating speed of 350 rpm for 15h. Prior to the synthesis of the La9.33Si2Ge4O26 mixture, the starting materials were milled separately at 350 rpm in argon at 50 kPa in order to achieve low particle size distributions with a greater ability for sintering. The MA mixture was compacted by uniaxial pressing at 390 MPa followed by sintering for 1 h at 1350 °C. Density of the La9.33Si2Ge4O26 pellets was determined by the boiling test method. SEM/EDS, XRD and atomic force microscopy (AFM) were used for samples characterization. The bulk densities and the open porosities of the pellets (EF and MW for conventional and microwave hybrid sintering, respectively) are presented in Table 1. Dense pellets were obtained in both cases. However, sample MW showed higher density and, consequently, lower porosity than sample EF. Both mechanically alloyed samples are formed by an apatite phase. This phase remains stable during sintering. However, traces of a second phase (La4GeO8) were detected after sintering. This phase was already referred in previous work (Serra et al., 2012). Figure 1 shows the morphology of the two samples obtained by SEM and AFM as well as the corresponding particles size distributions. The sintered samples were as-polished and thermal etched in air at 1300 ºC, for 10 min., for detailed microstructural observation. SEM images at low amplification confirm that sample EF is more porous than sample MW (Figure 1 (a) and (b), respectively). Although the same sintering temperature was used in both sintering processes, sample MW has a coarser grain size (Figure 1 (c) to (f)). This feature was already been reported in previous work (Fang et al., 1994). Mean values of 0.9 and 1.9 m were determined for samples EF and MW, respectively. As conclusion, one may say that microwave hybrid sintering is a novel suitable process for the densification of La9.33Si2Ge4O26 powders for application in SOFCs electrolytes. This research is partially sponsored by FEDER funds through the program COMPETE – Programa Operacional Factores de Competitividade – and by national funds through FCT – Fundação para a Ciência e Tecnologia – under the contract PTDC/EME-PME/102837/2008. The research fellowships granted to Cátia Alves, Márcio Santos and Mafalda Macatrão are also gratefully acknowledged.