| Using energy efficiently and maintaining a clean environment have been two important concerns worldwide during the past few decades. They stimulated vigorous research activity directed towards identifying novel and more efficient energy sources and the development of clean energy technologies. As an effective means, thermoelectric(TE) technology has become attractive for the great significance to complete heat-electricity mutual conversion without any noise and costly maintenance. Mg2Si1-xSnx based solid solutions, possessing great merits of abundant raw materials, nontoxicity and so on, show great potential for thermoelectric generation in the temperature range of 500-800 K. So far, n-type Mg2Si1-xSnx materials have achieved high ZT value of 1.2-1.3. However, this material system is still confronted with some challenges: the high saturated vapor pressure and reaction activity of Mg causing the great difficulty in the material synthesis process(especially for mass production); the intrinsic non-continuous solid solution region in Mg2Si1-xSnx leading to the inhomogeneous multiphase structure which has the detrimental effect in mobility, power factor and figure of merit; low figure of merit ZT of p-type Mg2Si1-xSnx materials impeding the thermoelectric couple matching for the power generation application. Concerning the problems above, some investigations have been conducted and the results are presented below:Mg2Si1-xSnx(x = 0, 0.5 and 0.7) solid solutions were prepared by combustion synthesis(self-propagating high-temperature synthesis and thermal explostion) combined with plasma activated sintering(PAS). The mechanisms of phase transition in self-propagating high-temperature synthesis(SHS) for Mg2Si1-xSnx(x = 0, 0.5 and 0.7) have been investigated through DSC tests and rapid quenching experiments. Raw material power Mg and Si can directly react into Mg2 Si compound at around 800-850 K(depending on the heating rate). Raw material powder Mg and Sn, among which Sn will melt at 500 K(the melting point of Sn) accompanied by slight formation of Mg2 Sn, will react to form Mg2 Sn in large scale when temperature reaches at 700 K. The early stage of SHS process for Mg2Si1-xSnx(x = 0.5 and 0.7) is the same with the Mg2 Sn compound's, and then with the further increase of temperature, Mg2 Si appears and dissolves in Mg2 Sn forming Mg2Si1-xSnx solid solution.A series of Sb-doped Mg2 Si compounds were prepared by self-propagating high-temperature synthesis(SHS) combined with plasma activated sintering(PAS) method, which totally takes less than 20 min. EPMA and HRTEM show that there are different phases with varied Sb content. Nanoprecipitates were observed for the samples with Sb doping. With the increase of Sb content, electrical conductivity ? rises markedly while Seebeck coefficient ? varies with the opposite trend to electrical conductivity ?, attributed to the increase of the carrier concentration. Carrier mobility ?H declines slightly with nH increasing. The thermal conductivity ? still decreases with increasing x despite the enhanced carrier conductivity, because of the significantly dropped lattice thermal conductivity ?L on account of the strengthened point defect scattering induced by the doping of Sb and possible interface scattering caused by the nanoprecipitates. As a result, the sample with x = 0.02 achieves the thermoelectric figure of merit ZT ~ 0.65 at 873 K, one of the highest in Sb doped binary Mg2 Si compounds investigated so far. Subsequent annealing treatment on the sample x = 0.02 at 773 K for 7 d does not exert noticeble influence over the thermoelectric transport properties, indicating the excellent thermal stability of the sample prepared by SHS method. Thereafter, Mg2(Si0.3Sn0.7)1-ySby(0 ? y ? 0.025) solid solutions are also prepared by SHS combined with PAS. XRD patterns indicate that all samples are single-phase. BEI and element distribution WDS map of polished surface exhibit many different phases with varied Si/Sn ratio. The results of thermoelectric properties measurement show that Sb can adjust carrier concentration and optimize figure of merit ZT. The highest ZT ~ 1.15 is achieved for sample y = 0.025 at 800 K, with a range of 10% lower than that of sample with same composition synthesized by SSR.Sb doped Mg2(Si1-xSnx)1-ySby(x = 0.5 and 0.7, 0 ? y ? 0.025) are prepared by thermal explosion(TE) combined by PAS. XRD patterns show the singe phases with FCC structure for all the samples. However, according to the BEI and composition element distribution map, all samples are multiphase, especially for samples x = 0.5 presenting extremely obvious contrast difference caused by the Si-rich and Sn-rich regions. The doping of Sb can optimize the carrier concentration and power factor, and meanwhile, suppress the intrinsic excitation effectively at high temperature, together leading to enhanced ZT of 1.1 and 1.0 at 800 K, respectively for x = 0.7 with y = 0.02 and x = 0.5 with y = 0.025.Compared with the samples prepared by SSR, the samples fabricated by ultra-fast method(SHS and TE) exhibit multiphase structure due to the inferor solid solution, leading to the reduced mobility and increased lattice thermal conductivity. As a result, the ZT value of sample prepared by ultra-fast method is lower than that of sample prepared by SSR. Even so, the ultra-fast method is still of great significance in the future mass production due to the exciting merits of low energy consumption, time saving and low requirement on equipment.All above, homogeneous microstructure of Mg2Si1-xSnx material is favorable for thermoelectric performance. And thus, Sb doped Mg2Si0.3Sn0.7 solid solution is prepared by melt spinning(MS) combined with PAS. A nonequilibrium melt spinning technique, in contrast to the SSR process, greatly improves the structural homogeneity by rapidly quenching the homogeneous melt. As a consequence, the carrier mobility and electrical conductivity is much enhanced leading to a record-high power factor PF of 5.18×10-3 Wm-1K-2 at around 600 K, a 15% improvement over the SSR-PAS sample. The dimensionless figure of merit ZT reaches a value of 1.30 at 750 K. Hence, improving the homogeneity of material system can serve as an effective approach to increase carrier mobility??, PF and ZT value.To optimize thermoelectric performance of p-type Mg2Si0.3Sn0.7 solid solution, we use Li doping on Mg sites in an attempt to enhance and control the concentration of hole carriers. We show that Li as well as Ga is a far more effective p-type dopant in comparison to Na or K. With the increasing content of Li, the electrical conductivity rises rapidly on account of a significantly enhanced density of holes. While the Seebeck coefficient decreases concomitantly, the power factor retains robust values supported by a rather high mobility of holes. Theoretical calculations indicate that Mg2Si0.3Sn0.7 intrinsically possesses the almost convergent double valence band structure(the light and heavy band), and Li doping retains a low density of states(DOS) on the top of the valence band, contrary to the Ga doping at the sites of Si/Sn. Low temperature specific heat capacity studies attest to a low DOS effective mass in Li-doped samples and consequently their larger hole mobility. The overall effect is a large power factor of Li-doped solid solutions. Although the thermal conductivity increases as more Li is incorporated in the structure, the enhanced carrier density effectively shifts the onset of intrinsic excitations(bipolar effect) to higher temperatures, and the beneficial role of phonon Umklapp processes as the primary limiting factor to the lattice thermal conductivity is thus extended. The final outcome is the figure of merit ZT ~ 0.5 at 750 K for x = 0.07. This represents a 30% improvement in the figure of merit of p-type Mg2Si1-xSnx solid solutions over the literature values. Hence, designing low DOS near Fermi level EF for given carrier pockets can serve as an effective approach to optimize PF and thus ZT value. |
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