Influence of Stoichiometry and Aging at Operating Temperature on Thermoelectric Higher Manganese Silicides
作者:Sylvain Le Tonquesse、Loic Joanny、Quansheng Guo、Erik Elkaim、Valérie Demange、David Berthebaud、Takao Mori、Mathieu Pasturel、Carmelo Prestipino
DOI:10.1021/acs.chemmater.0c03714
日期:2020.12.22
Thermoelectric higher manganese silicides, MnSix, were synthesized by magnesioreduction followed by spark plasma sintering with different nominal compositions (x = 1.65, 1.74, and 1.80) and various postsynthesis annealing durations (0, 48, 96, and 336 h). The composite Nowotny chimney-ladder crystal structures of the resulting samples were investigated by synchrotron X-ray powder diffraction. The modulation vector component γ, generally considered corresponding to the stoichiometry (x) of the material, was accurately determined by Rietveld refinement using a (3 + 1)D superspace approach. Regardless of the initial nominal composition, all the samples have a similar γ ∼ 1.736 after 48 h of annealing at 900 K. This result suggests that MnSix, at a temperature of 900 K, is better described as a defined compound with x close to 1.736, rather than intermediate solid-solution phases with 1.725 < x < 1.75 as predicted by the commonly accepted phase diagram. At the fixed nominal composition MnSi1.74, γ increases significantly from 1.7313(2) to 1.7411(1) after 336 h of annealing, indicating that the thermal history influences the Si stoichiometry. The evolution of γ with time is concomitant with a power factor drop (−19%), attributed to a decrease in charge carrier concentration. The drop of the power factor, partially compensated by a decrease in thermal conductivity, results in a −12% reduction of the maximum figure-of-merit ZT, after prolonged annealing under realistic application conditions.
通过镁还原和火花等离子烧结合成了热电高锰硅化物 MnSix,具有不同的标称成分(x = 1.65、1.74 和 1.80)和不同的合成后退火持续时间(0、48、96 和 336 小时)。同步辐射 X 射线粉末衍射研究了所得样品的复合诺沃特尼烟囱式晶体结构。调制矢量分量 γ 通常被认为与材料的化学计量 (x) 相对应,它是通过使用 (3 + 1)D 超空间方法的里特维尔德精炼精确确定的。这一结果表明,在 900 K 的温度下,MnSix 更适合描述为 x 接近 1.736 的确定化合物,而不是通常公认的相图所预测的 1.725 < x < 1.75 的中间固溶相。在固定的标称成分 MnSi1.74 下,γ 在退火 336 小时后从 1.7313(2) 显著增加到 1.7411(1),这表明热历史会影响硅的化学计量。γ随时间的变化伴随着功率因数的下降(-19%),这归因于电荷载流子浓度的降低。在实际应用条件下,经过长时间退火后,功率因数的下降部分被热导率的下降所补偿,导致最大功率因数 ZT 下降-12%。