Barczak Sonia A, Kennedy Blair F, da Silva Ivan, Bos Jan-Willem G
Institute of Chemical Sciences and Centre for Advanced Energy Storage and Recovery, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
ISIS Facility, Rutherford Appleton Laboratory, Harwell Oxford, Didcot OX11 0QX, U.K.
Chem Mater. 2023 Apr 26;35(9):3694-3704. doi: 10.1021/acs.chemmater.3c00393. eCollection 2023 May 9.
Half-Heusler alloys are leading contenders for application in thermoelectric generators. However, reproducible synthesis of these materials remains challenging. Here, we have used in situ neutron powder diffraction to monitor the synthesis of TiNiSn from elemental powders, including the impact of intentional excess Ni. This reveals a complex sequence of reactions with an important role for molten phases. The first reaction occurs upon melting of Sn (232 °C), when NiSn, NiSn, and NiSn phases form upon heating. Ti remains inert with formation of TiNi and small amounts of half-Heusler TiNiSn only occurring near 600 °C, followed by the emergence of TiNi and full-Heusler TiNiSn phases. Heusler phase formation is greatly accelerated by a second melting event near 750-800 °C. During annealing at 900 °C, full-Heusler TiNiSn reacts with TiNi and molten TiSn and Sn to form half-Heusler TiNiSn on a timescale of 3-5 h. Increasing the nominal Ni excess leads to increased concentrations of Ni interstitials in the half-Heusler phase and an increased fraction of full-Heusler. The final amount of interstitial Ni is controlled by defect chemistry thermodynamics. In contrast to melt processing, no crystalline Ti-Sn binaries are observed, confirming that the powder route proceeds via a different pathway. This work provides important new fundamental insights in the complex formation mechanism of TiNiSn that can be used for future targeted synthetic design. Analysis of the impact of interstitial Ni on the thermoelectric transport data is also presented.
半赫斯勒合金是热电发电机应用的主要候选材料。然而，这些材料的可重复合成仍然具有挑战性。在这里，我们使用原位中子粉末衍射来监测由元素粉末合成TiNiSn的过程，包括故意过量Ni的影响。这揭示了一个复杂的反应序列，其中熔融相起着重要作用。第一个反应发生在Sn熔化时（232℃），加热时会形成NiSn、NiSn和NiSn相。Ti保持惰性，仅在接近600℃时形成TiNi和少量的半赫斯勒TiNiSn，随后出现TiNi和全赫斯勒TiNiSn相。在750 - 800℃附近的第二次熔化事件极大地加速了赫斯勒相的形成。在900℃退火期间，全赫斯勒TiNiSn与TiNi以及熔融的TiSn和Sn反应，在3 - 5小时内形成半赫斯勒TiNiSn。增加名义上的Ni过量会导致半赫斯勒相中Ni间隙原子浓度增加以及全赫斯勒相的比例增加。间隙Ni的最终含量由缺陷化学热力学控制。与熔体加工不同，未观察到结晶的Ti - Sn二元化合物，这证实了粉末路线通过不同的途径进行。这项工作为TiNiSn复杂的形成机制提供了重要的新基础见解，可用于未来有针对性的合成设计。还介绍了间隙Ni对热电输运数据影响的分析。
