When Band Convergence is Not Beneficial for Thermoelectrics

Band convergence is considered a clear benefit to thermoelectric performance because it increases the charge carrier concentration for a given Fermi level, which typically enhances charge conductivity while preserving the Seebeck coefficient. However, this advantage hinges on the assumption that interband scattering of carriers is weak or insignificant. With first-principles treatment of electron-phonon scattering in CaMg$_{2}$Sb$_{2}$-CaZn$_{2}$Sb$_{2}$ Zintl system and full Heusler Sr$_{2}$SbAu, we demonstrate that the benefit of band convergence can be intrinsically negated by interband scattering depending on the manner in which bands converge. In the Zintl alloy, band convergence does not improve weighted mobility or the density-of-states effective mass. We trace the underlying reason to the fact that the bands converge at one k-point, which induces strong interband scattering of both the deformation-potential and the polar-optical kinds. The case contrasts with band convergence at distant k-points (as in the full Heusler), which better preserves the single-band scattering behavior thereby successfully leading to improved performance. Therefore, we suggest that band convergence as thermoelectric design principle is best suited to cases in which it occurs at distant k-points

Structure and Failure Mechanism of the Thermoelectric CoSb_3/TiCoSb Interface

The brittle behavior and low strength of CoSb_3/TiCoSb interface are serious issues concerning the engineering applications of CoSb_3 based or CoSb_3/TiCoSb segmented thermoelectric devices. To illustrate the failure mechanism of the CoSb_3/TiCoSb interface, we apply density functional theory to investigate the interfacial behavior and examine the response during tensile deformations. We find that both CoSb_3(100)/TiCoSb(111) and CoSb_3(100)/TiCoSb(110) are energetically favorable interfacial structures. Failure of the CoSb_3/TiCoSb interface occurs in CoSb_3 since the structural stiffness of CoSb_3 is much weaker than that of TiCoSb. This failure within CoSb_3 can be explained through the softening of the Sb–Sb bond along with the cleavage of the Co–Sb bond in the interface. The failure mechanism the CoSb_3/TiCoSb interface is similar to that of bulk CoSb_3, but the ideal tensile strength and failure strain of the CoSb_3/TiCoSb interface are much lower than those of bulk CoSb_3. This can be attributed to the weakened stiffness of the Co–Sb framework due to structural rearrangement near the interfacial region

Observation of valence band crossing: the thermoelectric properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution

CaAl_2Si_2 type Zintl phases have long been known to be promising thermoelectric materials. Here we report for the first time on the thermoelectric properties of CaMg_2Sb_2 along with the transport properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution. The charge carrier tuning in this system was carried out by substituting divalent Ca^(2+) with monovalent Na^+. To check a possible band convergence, we applied an effective mass analysis to our samples and found an abrupt doubling of the samples' effective masses as the composition switches from Zn-rich to Mg-rich. We further analyzed the effect that alloy scattering plays in the lattice thermal conductivity of our samples with a Modified Klemens model. We showed that the reduction seen in the lattice thermal conductivity of the alloyed samples can be well explained based on the mass difference of Mg and Zn in the poly-anionic metal site. Our best p-doped sample with a composition of Ca_(.99)Na_(.01)MgZnSb_2 displays a relatively high peak zT of 0.87 at 850 K

Observation of valence band crossing: the thermoelectric properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution

CaAl_2Si_2 type Zintl phases have long been known to be promising thermoelectric materials. Here we report for the first time on the thermoelectric properties of CaMg_2Sb_2 along with the transport properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution. The charge carrier tuning in this system was carried out by substituting divalent Ca^(2+) with monovalent Na^+. To check a possible band convergence, we applied an effective mass analysis to our samples and found an abrupt doubling of the samples' effective masses as the composition switches from Zn-rich to Mg-rich. We further analyzed the effect that alloy scattering plays in the lattice thermal conductivity of our samples with a Modified Klemens model. We showed that the reduction seen in the lattice thermal conductivity of the alloyed samples can be well explained based on the mass difference of Mg and Zn in the poly-anionic metal site. Our best p-doped sample with a composition of Ca_(.99)Na_(.01)MgZnSb_2 displays a relatively high peak zT of 0.87 at 850 K

Superstrengthening Bi_2Te_3 through Nanotwinning

Bismuth telluride (Bi_2Te_3) based thermoelectric (TE) materials have been commercialized successfully as solid-state power generators, but their low mechanical strength suggests that these materials may not be reliable for long-term use in TE devices. Here we use density functional theory to show that the ideal shear strength of Bi_2Te_3 can be significantly enhanced up to 215% by imposing nanoscale twins. We reveal that the origin of the low strength in single crystalline Bi_2Te_3 is the weak van der Waals interaction between the Te1 coupling two Te1─Bi─Te2─Bi─Te1 five-layer quint substructures. However, we demonstrate here a surprising result that forming twin boundaries between the Te1 atoms of adjacent quints greatly strengthens the interaction between them, leading to a tripling of the ideal shear strength in nanotwinned Bi_2Te_3 (0.6 GPa) compared to that in the single crystalline material (0.19 GPa). This grain boundary engineering strategy opens a new pathway for designing robust Bi_2Te_3 TE semiconductors for high-performance TE devices

Metal phosphides as potential thermoelectric materials

There still exists a crucial need for new thermoelectric materials to efficiently recover waste heat as electrical energy. Although metal phosphides are stable and can exhibit excellent electronic properties, they have traditionally been overlooked as thermoelectrics due to expectations of displaying high thermal conductivity. Based on high-throughput computational screening of the electronic properties of over 48 000 inorganic compounds, we find that several metal phosphides offer considerable promise as thermoelectric materials, with excellent potential electronic properties (e.g. due to multiple valley degeneracy). In addition to the electronic band structure, the phonon dispersion curves of various metal phosphides were computed indicating low-frequency acoustic modes that could lead to low thermal conductivity. Several metal phosphides exhibit promising thermoelectric properties. The computed electronic and thermal properties were compared to experiments to test the reliability of the calculations indicating that the predicted thermoelectric properties are semi-quantitative. As a complete experimental study of the thermoelectric properties in MPs, cubic-NiP2 was synthesized and the low predicted lattice thermal conductivity (∼1.2 W m^(−1) K^(−1) at 700 K) was confirmed. The computed Seebeck coefficient is in agreement with experiments over a range of temperatures and the phononic dispersion curve of c-NiP_2 is consistent with the experimental heat capacity. The predicted high thermoelectric performance in several metal phosphides and the low thermal conductivity measured in NiP_2 encourage further investigations of thermoelectric properties of metal phosphides