One-step ultra-rapid fabrication and thermoelectric properties of Cu2Se bulk thermoelectric material.

Cu2Se is a promising material for high temperature thermoelectric energy conversion due to its unique combination of excellent electronic properties and low thermal conductivity owing to its ionic liquid characteristics at high temperature. In this paper, fully dense single-phase bulk Cu2Se material was prepared by the combination of self-propagating high-temperature synthesis (SHS) with in situ quick pressing (QP) for the first time. This new approach shortens the duration of the synthesis from days to hours compared to conventional preparation methods. SHS-QP technique is an ultra-fast preparation method, which utilizes the heat released by the SHS reaction and an external applied pressure to achieve the synthesis and densification of materials in one-step. The ultra-fast process of the SHS-QP technique enables the fabrication of single-phase Cu2Se bulk materials with relative density of over 98% and with precise control over the stoichiometry owing to the ability to suppress the Se vapor during the reaction. The SHS-QP prepared Cu2Se samples exhibit excellent thermoelectric figure of merit, ZT ∼ 1.5 at 900 K, which is comparable to those of Cu2Se materials prepared by conventional methods. This study opens a new avenue for the ultra-fast and low-cost fabrication of Cu2Se thermoelectric materials.

Optimizing the average power factor of p-type (Na, Ag) co-doped polycrystalline SnSe.

Despite the achievable high thermoelectric properties in SnSe single crystals, the poor mechanical properties and the relatively high cost of synthesis restrict the large scale commercial application of SnSe. Herein, we reported that co-doping with Na and Ag effectively improves the thermoelectric properties of polycrystalline SnSe. Temperature-dependent carrier mobility indicates that the grain boundary scattering is the dominant scattering mechanism near room temperature, giving rise to low electrical conductivity for the polycrystalline SnSe in comparison with that of the single crystal. Co-doping with Na and Ag improves the electrical conductivity of polycrystalline SnSe with a maximum value of 90.1 S cm-1 at 323 K in Na0.005Ag0.015Sn0.98Se, and the electrical conductivity of the (Na, Ag) co-doped samples is higher than that of the single doped samples over the whole temperature range (300-773 K). Considering the relatively high Seebeck coefficient of 335 µV K-1 at 673 K and the minimum thermal conductivity of 0.48 W m-1 K-1 at 773 K, Na0.005Ag0.015Sn0.98Se is observed to have the highest PF and ZT among the series of samples, with values of 0.50 mW cm-1 K-2 and 0.81 at 773 K, respectively. Its average PF and ZT are 0.43 mW cm-1 K-2 and 0.37, which is 92% and 68% higher than that of Na0.02Sn0.98Se, 40% and 43% higher than that of Ag0.02Sn0.98Se, and 304% and 277% higher than that of the previously reported SnSe, respectively.

Enhanced Thermoelectric Properties of Codoped Cr2Se3: The Distinct Roles of Transition Metals and S.

Pristine Cr2Se3 is a narrow-band gap semiconductor but with an inferior ZT value of 0.22 obtained at 623 K. In this paper, we improve the thermoelectric performance of the Cr2Se3 material by optimizing carrier concentration, suppressing the bipolar thermal conductivity, and reducing the lattice thermal conductivity simultaneously. First, the effect of different dopants (Nb, Ni, and Mn) on the phase composition and thermoelectric transport properties of M2 xCr2-2 xSe3 (M = Nb, Ni, and Mn; x = 0-0.02) compounds are systematically investigated. The roles of those dopants are distinct. Mn-doped samples show superior thermoelectric properties in comparison with those of other-element-doped samples. Doping with Mn significantly increases the carrier concentration, accompanied with a suppression of the intrinsic excitation and a reduction of both the bipolar thermal conductivity and the lattice thermal conductivity of Cr2Se3. To further reduce the thermal conductivity, we have synthesized Mn and S codoped Mn0.04Cr1.96Se3-3 xS3 x ( x = 0-0.1) samples. Alloying with S significantly decreases the lattice thermal conductivity and enlarges the band gap, boosting the Seebeck coefficient. The maximum ZT value of Mn0.04Cr1.96Se2.7S0.3 reaches 0.33 at 823 K. Compared with the pristine Cr2Se3 sample, the maximum ZT value is increased by 50% and the temperature corresponding to the peak value shifts toward higher temperatures by 200 K.

Interpreting the Combustion Process for High-Performance ZrNiSn Thermoelectric Materials.

The ZrNiSn alloy, a member of the half-Heusler family of thermoelectric materials, shows great potential for mid-to-high-temperature power generation applications due to its excellent thermoelectric properties, robust mechanical properties, and good thermal stability. The existing synthesis processes of half-Heusler alloys are, however, rather time and energy intensive. In this study, single-phase ZrNiSn bulk materials were prepared by self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) for the first time. The analysis of thermodynamic and kinetic processes shows that the SHS reaction in the ternary ZrNiSn alloy is different from the more usual binary systems. It consists of a series of SHS reactions and mass transfers triggered by the SHS fusion of the binary Ni-Sn system that eventually culminates in the formation of single-phase ternary ZrNiSn in a very short time, which reduced the synthesis period from few days to less than an hour. Moreover, the nonequilibrium feature induces Ni interstitials in the structure, which simultaneously enhances the electrical conductivity and decreases the thermal conductivity, which is favorable for thermoelectric properties. The maximum thermoelectric figure of merit ZT of the SHS + SPS-processed ZrNiSn1-xSbx alloy reached 0.7 at 870 K. This study opens a new avenue for the fast and low-cost fabrication of half-Heusler thermoelectric materials.