Achieving Densification and High Thermoelectric Performance in TiNiSn-Based Alloys with a MoO3 Sintering Aid

Hf-free TiNi0.94Cu0.06Sn-x wt % MoO3 (x = 0, 0.5, 0.75, 1, 1.5) composites were successfully prepared via hot-pressing sintering. Herein, low-melting-point MoO3 was introduced as a multifunctional sintering aid, aiming to increase the density of the alloy while simultaneously providing a scattering medium to suppress thermal transport. Remarkably, even trace additions of MoO3 boosted the density from the original 98.49% to 99.95% and doubled the room-temperature carrier mobility from 11.63 cm2 V-1 s-1 to 24.87 cm2 V-1 s-1. At an optimal composition (x = 0.75), the Seebeck coefficient was significantly improved, which compensated for the electrical conductivity loss induced by reduced carrier concentration, leading to a peak power factor of 45 × 10-4 W m K-2. Concurrently, the total thermal conductivity was reduced to a minimum of 3.81 W m-1 K-1 at x = 1, marking a ∼21% decrease from the pristine alloy. This reduction stemmed from declines in both electronic and lattice contributions, with the latter plummeting from 2.60 W m-1 K-1 to 1.43 W m-1 K-1, primarily due to the multiscale phonon scattering introduced by the addition of MoO3. Consequently, the synergistically optimized power factor and depressed thermal conductivity yielded a maximum figure of merit (zT) of 0.89 at 800 K for the TiNi0.94Cu0.06Sn-MoO3 nanocomposites. This represents a 29% enhancement over the matrix and positions the material competitively among reported Hf-free half-Heusler (HH) alloys. Thus, MoO3 achieves the decoupling of electrical and thermal transport by balancing densification-induced carrier mobility enhancement and second-phase-induced phonon scattering.