###### Abstract  
The formation of secondary phases of NiTe\({}_{2}\) and NbTe\({}_{2}\) in \(p\)-type Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) thermoelectric alloys was investigated through in situ phase separation by using the melt spinning process. Adding stoichiometric Ni, Nb, and Te in a solid-state synthesis process of Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\), followed by rapid solidification by melt spinning, successfully segregated NiTe\({}_{2}\) and NbTe\({}_{2}\) in the Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) matrix. Since heterointerfaces of Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) with NiTe\({}_{2}\) and NbTe\({}_{2}\) form potential barriers of 0.26 and 0.08 eV, respectively, a low energy carrier filtering effect can be expected; higher Seebeck coefficients and power factors were achieved for Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NiTe\({}_{2}\))\({}_{0.01}\) (250 eV/K and 3.15 mW/mK\({}^{2}\)), compared to those of Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) (240 eV/K and 2.69 mW/mK\({}^{2}\)). However, there was no power factor increase for NbTe\({}_{2}\) segregated samples. The decrease in thermal conductivity was seen due to the possible additional phonon scattering by the phase segregations. Consequently, \(zT\) at room temperature was enhanced to 0.98 and 0.94 for Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NiTe\({}_{2}\))\({}_{0.01}\) and Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NbTe\({}_{2}\))\({}_{0.01}\), respectively, compared to 0.79 for Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\). The carrier filtering effect induced by NiTe\({}_{2}\) segregations with an interface potential barrier of 0.26 eV effectively increased the Seebeck coefficient and power factor, thus improving the \(zT\) of \(p\)-type Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\), while the interface potential barrier of 0.08 eV of NbTe\({}_{2}\) segregation appeared to be too small to induce an effective carrier filtering effect. Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\); heterointerface; phase separation; carrier energy filtering; secondary phase +
Footnote †: journal: Applied Sciences  
energy filtering, and resonance doping have been applied [4; 5; 6]. In addition, a variety of defect structures is introduced to the material to reduce the \(\kappa\) by intensifying phonon scattering [7; 8; 9]. For example, nanograin-composite, dense dislocations formed at the grain boundary, and substitutional doping have been known as effective approaches to reduce \(\kappa\) by intensifying phonon scattering in (Bi\({}_{2}\)Te\({}_{3}\))-based alloys [8; 9; 10; 11; 12]. Among these approaches, introducing nanoparticles in the thermoelectric materials can improve \(zT\) by increasing the electrical transport properties and reduction in \(\kappa\). The reduction in \(\kappa\) can be attained by additional phonon scattering by the embedded nanoparticles [13], and the increase in the electrical transport properties can be attributed to the carrier energy filtering effect caused by the nanoparticles. The carrier filtering effect is a strong energy dependence of the carrier relaxation time caused by band bending at the interface between metallic inclusions and thermoelectric materials [14; 15; 16]. In this study, the formation of secondary phases of NiTe\({}_{2}\) and NbTe\({}_{2}\) in \(p\)-type Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) thermoelectric alloys related with the carrier filtering effect was investigated through in situ phase segregation by using the melt spinning process. Metallic NiTe\({}_{2}\) and NbTe\({}_{2}\) were chosen for the successful segregation in Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) alloys, using the melt spinning process, which was observed in the preliminary experiment. Adding excess stoichiometric composition of NiTe\({}_{2}\) and NbTe\({}_{2}\) in the melt-process of Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) synthesis successfully segregated NiTe\({}_{2}\) and NbTe\({}_{2}\) phases through the rapid solidification process by melt spinning.