Electron Density Optimization and the Anisotropic Thermoelectric Properties of Ti Self-Intercalated Ti_1+xS₂ Compounds

Abstract

Polycrystalline TiS (0.111 ≤ x ≤ 0.161) with high density and controllable composition were successfully prepared using solid-state reaction combined with plasma-activated sintering. TiS showed strong (00 l) preferred orientation with Lotgering factor of 0.32-0.60 perpendicular to the pressing direction (⊥), whereas the preferred orientation was not obvious along the pressing direction (∥). This structural anisotropy resulted in distinct anisotropic thermoelectric transport properties in TiS. At 300 K, while the Seebeck coefficient was weak anisotropic, the power factor and lattice thermal conductivity of TiS was much larger in the perpendicular direction as compared to that of the parallel direction, with an anisotropic ratio of 1.8-2.7 and 1.3-1.7, respectively. Theoretical calculations of formation energy of defects suggested that the excess Ti was most probably intercalated into the van der Waals gaps in metal-rich TiS, consistent with X-ray diffraction, high-resolution transmission electron microscopy characterization and transport measurements. With increasing x, the carrier concentration and power factor of TiS dramatically increased because of the donor behavior of Ti interstitials, which was accompanied by a significant decrease in the lattice thermal conductivity owing to the strengthened phonon scattering from structural disorder. Because of its strongest (00 l) preferred orientation and largest carrier mobility among all samples, TiS had the highest power factor of 22 μW cm K at 350 K perpendicular to the pressing direction, close to the value (37.1 μW cm K) achieved in single-crystal TiS. We found out that the maximum power factor and dimensionless figure of merit ZT could be achieved at an optimum carrier concentration of about 5.0 × 10 cm. Finally, TiS acquired the highest ZT value of 0.40 at 725 K perpendicular to the pressing direction because of the beneficial preferred orientation, improved power factor, and reduced lattice thermal conductivity.

Strong Anisotropy and Bipolar Conduction-Dominated Thermoelectric Transport Properties in the Polycrystalline Topological Phase of ZrTe5

“…On the contrary, we did not observe any preferred orientations along the //P direction. The calculation of the orientation factor LF indicated that 38 the LF factor of the sintered bulk ZrTe 5 reaches a significant value of 0.13 along the (041) plane. The scanning electron microscopy (SEM) fractured surface morphologies verified the microscopic crystalline orientation, as depicted in Figure 2d−f.…”

Realization of an Ultrahigh Power Factor and Enhanced Thermoelectric Performance in TiS2 via Microstructural Texture Engineering

“…For example, Aoki et al 15 applied a centrifugal heating method for sintering (SnS) 1.2 (TiS 2 ) 2 , which only promoted the LF value from 0.085 to 0.13 with the centrifugal acceleration increased from 1000 to 8000 g. Sever et al 14 obtained TiS 2 bulk samples with LF ∼ 0.64 by pulsed electric current sintering (PECS) under a high pressure of 200 MPa; however, the high sintering temperature of 800 °C could have led to an obvious loss of sulfur. Besides, Tang et al 13 prepared Ti-selfintercalated Ti 1+x S 2 bulks with LF ∼ 0.6 sintered using a plasma-activated process, but it seemed to be readily affected by the intercalation amount, which is difficult to control precisely. By contrast, as can be seen from the similar high LF values for our pulverization samples, the powder pretreatment of shear pulverization processing we demonstrate here exhibits itself as a very reliable and simple technique for achieving high texturing for TiS 2 bulks.…”

Engineering covalently bonded 2D layered materials by self-intercalation

“…Moreover, an intercalation phase diagram that correlates the density and spatial distribution of intercalation atoms with mesoscopic properties of the intercalated compound is currently lacking. Compared with foreign atom intercalation, intercalation of a TMD by its native metal atoms has so far received scant attention 11,29,30 . Such self-intercalated TMD compounds may exist as local energy minima in the part of the phase diagram where a metal-rich stoichiometry is promoted by growth conditions using high chemical potential of metal atoms.…”