Grain size effect on the phase transformations of higher manganese silicide thermoelectric materials: An in situ energy
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Tiejun Zhu and Xinbing Zhaoa) State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
Eckhard Mueller Institute of Materials Research, German Aerospace Center (DLR), 51147 Cologne, Germany (Received 13 December 2010; accepted 22 February 2011)
Phase structures of microscale and nanoscale higher manganese silicides (HMSs) were investigated using in situ energy dispersive x-ray diffraction at high temperatures or/and high pressure. A few phase transformations accompanied with the presence of MnSi phase were observed in different temperature regions, which were associated with the interevolution of several incommensurate HMS phases. It was found that in nanostructured HMS, the interevolution of HMS was remarkable and accelerated compared to that in the micropowders. Meanwhile, high pressure was able to influence these phase transformations due to giant strain in the materials. The phase transformations were discussed from thermodynamic aspects with respect to the different formation enthalpy of Mn–Si system and the large surface energy and structural instability of the nanopowders. I. INTRODUCTION
Higher manganese silicides (HMSs), chemically represented by MnSix (x 5 1.71–1.75), are one of the most promising semiconductors suitable for applications in thermoelectric energy harvesting systems and optoelectronic devices.1,2 They exist as several incommensurable phases such as Mn4Si7, Mn11Si19, Mn15Si26, and Mn27Si47,3–5 all of which belong to the same tetragonal crystal system deduced from the TiSi2 structure with different numbers of subcells stacking along the c-axis. These incommensurable phases usually coexist with each other. As a promising p-type thermoelectric material, HMSs have been extensively studied due to their advantages such as nontoxicity, low costs of starting materials, high thermal and chemical stability, etc.6 Recently, much more attention has been paid to the synthesis of thermoelectric bulk nanomaterials using methods like powder metallurgy, quenching, and melt spinning. In our previous work on nanostructuring of HMS by ball milling,7 it was found that the high-energy ball milling introduced a mechanochemical decomposition of HMS, which was closely dependant on the grain size of the powders. By controlling a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.80 1900
J. Mater. Res., Vol. 26, No. 15, Aug 14, 2011
http://journals.cambridge.org
Downloaded: 04 Dec 2014
the milling conditions, ultrafine HMS powders within tens of nanometers without obvious impurity phase have been obtained. In the successive investigation, we also found that the single-phase nanopowders (NPs) synthesized by ball milling were unstable at high temperatures. For a better understanding of this phenomenon, we applied energy dispersive x-ray diffraction (EDXRD) for an in situ study of the phase transformations. Possible mechanisms of the transformation behavior were discussed accordingly. II. EXPERIME
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