Melt-Spun SiGe Nano-Alloys: Microstructural Engineering Towards High Thermoelectric Efficiency AVINASH VISHWAKARMA, 1,2 NAGENDRA S. CHAUHAN , 3 RUCHI BHARDWAJ, 1,2 KISHOR KUMAR JOHARI, 1,2 SANJAY R. DHAKATE, 1,2 BHASKER GAHTORI, 1,2,5 and SIVAIAH BATHULA 1,2,4,6 1.—Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh 201002, India. 2.—Division of Advanced Materials and Devices Metrology, CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India. 3.—Micro and Nanofabrication Department, International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal. 4.—School of Minerals, Metallurgical and Materials Engineering, IIT Bhubaneswar, Bhubaneswar 752050, Odisha, India. 5.—e-mail: bhasker@nplindia.org. 6.—e-mail: sivaiahb@iitbbs.ac.in Silicon-germanium (SiGe) alloys are prominent high-temperature thermo- electric (TE) materials used as a powering source for deep space applications. In this work, we employed rapid cooling rates for solidification by melt-spin- ning and rapid heating rates for bulk consolidation employing spark plasma sintering to synthesize high-performance p-type SiGe nano-alloys. The cur- rent methodology exhibited a TE figure-of-merit (ZT)  0.94 at 1123 K for a higher cooling rate of 3.0 9 10 7 K/s. This corresponds to  88% enhance- ment in ZT when compared with currently used radioisotope thermoelectric generators (RTGs) in space flight missions,  45% higher than pressure-sin- tered p-type alloys, which results in a higher output power density, and TE conversion efficiency (g)  8% of synthesized SiGe nano-alloys estimated using a cumulative temperature dependence (CTD) model. The ZT enhance- ment is driven by selective scattering of phonons rather than of charge car- riers by the high density of grain boundaries with random orientations and induced lattice-scale defects, resulting in a substantial reduction of lattice thermal conductivity and high power factor. The TE characteristics of syn- thesized alloys presented using the constant property model (CPM) and CTD model display their high TE performance in high-temperature regimes along with wide suitability of segmentation with different mid-temperature TE materials. (Received June 11, 2020; accepted October 9, 2020; published online November 2, 2020) Journal of ELECTRONIC MATERIALS, Vol. 50, No. 1, 2021 https://doi.org/10.1007/s11664-020-08560-6 Ó 2020 The Minerals, Metals & Materials Society 364