## 3 Results and Discussion  
MS process is widely used in nanostructuring and defect-engineering technology due to its high cooling rate (\(\sim\)10\({}^{6}\) K s\({}^{-1}\)) [14], and it can generate unexpected (or metastable) structures and phases in a solidified state. Generally, nanogenized structure can be obtained in conventional TE materials such as Bi-Te- and skutterudite-based alloys [15,16]. Figure 1a,b shows SEM images for the melt spun ribbons(\(\sim\)10 mm long, \(\sim\)1 mm wide, and \(\sim\)10 \(\upmu\)m thick) of Hf\({}_{0.35}\)Zr\({}_{0.35}\)7 Ti\({}_{0.3}\)NiSn\({}_{0.99}\)Sb\({}_{0.01}\). At the contact surface (where the ribbons makes a direct contact to the Cu wheel), nano-scale (\(\sim\)100 nm) grains were observed, while relatively larger grains (\(\sim\)200 nm) were formed at the free surface (opposite surface to the contact surface) since the solidification was initiated at the contact surface and propagated to the free surface (compared with the solidification at the free surface, more rapid solidification occurred at contact surface due to the higher cooling rate). To reduce the size of grains in melt spun ribbons, we also fabricated the ribbons with a higher Cu wheel rotating speed (60 m s\({}^{-1}\)). However, the size of grains at the contact surface was limited to about 100 nm despite of the more rapid solidification. Limited grain size reduction in level about 100 nm has been reported in filled skutterudites [15], whereas grains smaller than 5 nm can be obtained in melt spun ribbons of Bi-Te-based alloys [16]. Amorphous phase was also observed at contact surface. These results suggest that the limit of grain size reduction by MS process is considered to be related with the intrinsic crystallization behavior of different alloy as well as the rate of solidification. On the other hand, well dispersed white nanodots were clearly observed both in contact and free surfaces of melt spun ribbons (Figure 1a,b). In a previous report [13], we experimentally demonstrated that the formation of a full Heusler phase ((Hf,Zr,Ti)Ni\({}_{2}\)Sn) can be effectively suppressed by using TRMS technique since the full Heusler phase is produced at a high-temperature of \(\sim\)2000 K. We also confirmed that compositions of small (\(<\)10 nm) white dots observed in melt spun ribbons were Ti\({}_{6}\)Sn\({}_{5}\) and Sn by SEM-EDS (energy-dispersive X-ray spectroscopy) analysis. By using the TRMS process, sub-micron grain (200-400 nm) structure without abnormal grain growth was formed after SPS, as shown in Figure 1c (the SEM image for the fractured surface of SPSed Hf\({}_{0.35}\)Zr\({}_{0.35}\)7 Ti\({}_{0.3}\)NiSn\({}_{0.99}\)Sb\({}_{0.01}\)), suggesting a much higher grain boundary density compared to that observed in micrograined bulks. It was noted that no inclusions and impurity phases were observed in melt spun ribbons, suggesting that the complete single phase of the half-Heusler compound was fabricated during the SPS process. Formation of single-phase in SPSed bulks was also confirmed by XRD analysis. Figure 2 shows the XRD patterns of the SPSed bulks for Hf\({}_{0.35}\)Zr\({}_{0.35}\)Ti\({}_{0.3}\)NiSn\({}_{1-x}\)Sb\({}_{x}\) (\(x\) = 0, 0.01, 0.02, 0.03). XRD pattern of ZrNiSn (F\(\overline{4}\)3m, ICSD ID 80486) is also shown for comparison. All samples showed a pattern of a single half-Heusler phase without any secondary impurities such as those of a full-Heusler phase, _M_-Sn binary alloys, and Sn. Lattice constants of the samples were almost the same due to the small doping amount of Sb. A complete single-phase can be prepared by combined techniques of TRMS and SPS without a long annealing process. This is considered to be related to both the controlled phase evolution (mainly suppressed formation of full Heusler phase) and inhibited grain growth in melt spun ribbons prepared by using TRMS process [13; 15]. Reduced diffusion distance in the presence of nano-scale grains and good dispersion of secondary phases (Figure 1a,b) triggered the formation of a single half-Heusler phase.