Mechanical alloying-spark plasma sintering synthesis and thermoelectric properties of n-type NiSe_2+x semiconductors: analysis of intrinsic defects and phase structures

A series of bulk samples NiSe_2+x (x = 0, 0.01, 0.02, 0.06) were prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The effect of excessive selenium content (x) on the phase structure, microstructure and thermoelectric properties was investigated. The formation energy of intrinsic defects in NiSe₂ was calculated using first principle theory. The added selenium initially entered into the Se vacancy (\(V_{Se}^{ \cdot }\)) which is resulted from the Se volatilization during the MA and/or SPS process and subsequently into the interstitial sites (\(Se_{i}^{\prime }\)). An improved absolute value of Seebeck coefficient (|α|) 9.27 μVK⁻¹ at 323 K for the NiSe_2.06 was reached along with an inversely varied electrical conductivity (σ) as increasing x. A minimum thermal conductivity (κ) value 3.43 Wm⁻¹ K⁻¹ was achieved at 323 K for the NiSe_2.01, along with a high ZT value 0.0049 at 323 K for the NiSe_2.02 among NiSe_2+x system. Our work provides a preliminary approach towards TE performance enhancement in n-type NiSe₂ semiconductors via tailoring Se content in a nonstoichiometry, and will certainly be of value for further exploration of high performance NiSe₂ semiconductors by either doping or microstructure controlling.