High‐Pressure Synthesis and Thermal Conductivity of Semimetallic θ‐Tantalum Nitride

Abstract

The lattice thermal conductivity (κ ph ) of metals and semimetals is limited by phonon-phonon scattering at high temperatures and by electron-phonon scattering at low temperatures or in some systems with weak phonon-phonon scattering. Following the demonstration of a phonon band engineering approach to achieve an unusually high κ ph in semiconducting cubic-boron arsenide (c-BAs), recent theories have predicted ultrahigh κ ph of the semimetal tantalum nitride in the θ-phase (θ-TaN) with hexagonal tungsten carbide (WC) structure due to the combination of a small electron density of states near the Fermi level and a large phonon band gap, which suppress electron-phonon and three-phonon scattering, respectively. Here, measurements on the thermal and electrical transport properties of polycrystalline θ-TaN converted from the ε phase via high-pressure synthesis are reported. The measured thermal conductivity of the θ-TaN samples shows weak temperature dependence above 200 K and reaches up to 90 Wm −1 K −1 , one order of magnitude higher than values reported for polycrystalline ε-TaN and δ-TaN thin films. These results agree with theoretical calculations that account for phonon scattering by 100 nm-level grains and suggest κ ph increase above the 249 Wm −1 K −1 value predicted for single-crystal WC when the grain size of θ-TaN is increased above 400 nm.

Electron-Induced Nonmonotonic Pressure Dependence of the Lattice Thermal Conductivity of θ -TaN

“…Figure 1 shows the calculated pressure dependence of the total thermal conductivity (κ) of TaN compared to diamond [40], BAs [14,16], c-BN [14], BP [15], and Al [41], which have been reported in the literature. At ambient pressure, the room-temperature total κ of TaN is 1037 and 824 W m −1 K −1 along the a and c axes, respectively, consistent with the values reported earlier [17,19]. The electronic contributions to the thermal conductivity (κ el ) at ambient pressure are 27 and 15 W m −1 K −1 , respectively.…”

“…The κ of TaN can surpass that of BAs in the range ∼ 20−70 GPa. Finally, it is worth noting that TaN has been synthesized and remains stable at high pressures [19,52,53]. Our reported anomalous pressure dependence of κ occurs at around 60 GPa, and thus within the experimentally accessible range.…”

“…Our research also revealed that the mechanism responsible for TaN's ultrahigh κ is similar to that of BAs [18], with an additional factor of weak phonon-electron scattering. Recent experiments have indicated that this predicted high ambient-pressure value of κ for TaN can be achieved by increasing the grain size of samples and decreasing their defect concentrations [19].…”

Thermal decomposition of oxygen‐containing Ta3N5${\rm {Ta}}_3{\rm {N}}_5$

“…The Raman spectra of $$\utheta$$‐TaN and $${\rm {Ta}}_2{\rm {O}}_5$$ phases, as shown in Figure 4B, corroborate the findings from the X‐ray diffraction analysis. Relatively broad peaks at around 219 and 594 $${\rm {cm}}^{-1}$$ and a sharp peak at around 437 $${\rm {cm}}^{-1}$$, owing to the vibrations of Ta and N atoms, are the characteristic Raman modes of $$\utheta$$‐TaN 35 . The broad peaks of the $$\utheta$$‐TaN phase can be associated with dissolved oxygen 33 (please refer to Section 3.3.2).…”

“…Relatively broad peaks at around 219 and 594 cm −1 and a sharp peak at around 437 cm −1 , owing to the vibrations of Ta and N atoms, are the characteristic Raman modes of θ-TaN. 35 The broad peaks of the θ-TaN phase can be associated with dissolved oxygen 33 (please refer to Section 3.3.2). The low energy peak at 106 cm −1 can be attributed to the external modes due to the interaction between Ta polyhedra and TaO 5−2𝑛 𝑛 -Ta 6 O 6+ 12 clusters.…”