Electronic and phonon transport in Sb-doped Ti_0.1Zr_0.9Ni_1+xSn_0.975Sb_0.025 nanocomposites

Abstract

The thermoelectric behavior of n-type Sb-doped half-Heusler (HH)-full-Heusler (FH) nanocomposites with general composition Ti(0.1)Zr(0.9)Ni(1+x)Sn(0.975)Sb(0.025) (x = 0, 0.02, 0.04, 0.1) was investigated in the temperature range from 300 to 775 K. Samples used for structural characterization and transport measurements were obtained through the solid-state reaction of high purity elements at 950 °C and densification of the resulting polycrystalline powders using a uniaxial hot press. X-ray diffraction study of the powder samples suggested the formation of single-phase HH alloys regardless of the Ni concentration (x value). However, high resolution transmission electron microscopy investigation revealed the presence of spherical nanoprecipitates with a broad size distribution coherently embedded in the HH matrix. The size range and dispersion of the precipitates depend on the concentration of Ni in the starting mixture. Well dispersed nanoprecipitates with size ranging from 5 nm to 50 nm are observed in the nanocomposite with x = 0.04, while severe agglomeration of large precipitates (>50 nm) is observed in samples with x = 0.1. Hall effect measurements of various samples indicate that the carrier concentration within the Sb-doped HH matrix remains nearly constant (~7 × 10(20) cm(-3)) for samples with x = 0.02 and x = 0.04, whereas a significant increase of the carrier concentration to ~9 × 10(20) cm(-3) is observed for the sample with x = 0.1. Interestingly, only a marginal change in thermopower value is observed for various samples despite the large difference in the carrier density. In addition, the carrier mobility remains constant up to x = 0.04 suggesting that the small nanoprecipitates in these samples do not disrupt electronic transport within the matrix. Remarkably, a large reduction in the total thermal conductivity is observed for all nanocomposites, indicating the effectiveness of the embedded nanoprecipitates in scattering phonons while enabling efficient electron transfer across the matrix/inclusion interfaces.

Thermoelectric properties of Ge doped n-type TixZr1−xNiSn0.975Ge0.025half-Heusler alloys

“…In our earlier work, [23] [30] we demonstrated that the substitution of a Figure 3E. In our earlier work, [23] [30] we demonstrated that the substitution of a Figure 3E.…”

“…This can be achieved by increasing mass fluctuation or point defects in the sample [23][24][25][26][27] . In our previous research, we focused on coherently embedding full-Heusler (FH) phases as nanostructures in bulk HH alloys [23][24][25][29][30][31] . In our previous research, we focused on coherently embedding full-Heusler (FH) phases as nanostructures in bulk HH alloys [23][24][25][29][30][31] .…”

“…Alternatively, thermal conductivity of HH alloys is reduced using second phases as additional phonon scattering centers in the samples [28][29] . The FH phase, which is structurally related to the HH matrix, was formed by adding excess Ni in the starting mixture, resulting in the formation of a bulk HH/FH composite via solid-state reaction [23,[30][31] . The FH phase, which is structurally related to the HH matrix, was formed by adding excess Ni in the starting mixture, resulting in the formation of a bulk HH/FH composite via solid-state reaction [23,[30][31] .…”

“…The FH phase, which is structurally related to the HH matrix, was formed by adding excess Ni in the starting mixture, resulting in the formation of a bulk HH/FH composite via solid-state reaction [23,[30][31] . Furthermore, we observed that the addition of a small amount of Sb at Sn sites in Ti 0.1 Zr 0.9 Ni 1+x Sn nanocomposites resulted in a constant Seebeck coefficient and electrical conductivity, while the lattice thermal conductivity was effectively reduced to ~ 4.4 WK -1 m -1 at 775 K [30] . For example, the lattice thermal conductivity of Ti 0.1 Zr 0.9 Ni 1+x Sn nanocomposites 4 could be reduced to 4.75 WK -1 m -1 at 775 K [23] .…”

Resolving the true band gap of ZrNiSn half-Heusler thermoelectric materials

“…[7][8][9][10] The XNiSn solid solution is the most extensively studied n-type HH system. 3,4,[10][11][12][13][14][15][16][17] Much of the existing literature focuses on the thermoelectric characterization of n-type XNiSn and p-type XCoSb solid solutions, whereas studies on p-type XNiSn-based HHs are far fewer in number. The most promising p-type HHs are found in the XCoSb system (X ¼ Ti, Zr, Hf).…”