Controllable Synthesis,Transport Properties And Performance Optimization Of Magnesium Based Thermoelectric Materials

Magnesium-based materials have unique advantages serving in thermoelectric(TE)applications,such as low cost,low density and environmentally friendly chemical constituents.Among them,the N type Mg2X?(X?= Si,Ge,Sn)based and Mg3X?2(X?=Sb,Bi)based materials exhibit superior TE performance with zT exceeding unit at around 600?800 K.Further improvement of the zT of n-type Mg2X? materials lies on the effective reduction of its lattice thermal conductivity.Besides,its ideal device assembling from the perspective of materials requires the matchable performance of the p-type Mg2X?,which is still the bottleneck for the researches of the Mg2X? based materials in recent years due to low doping efficiency and low carrier mobility.For the newly developed Mg3Sb2 based materials,the deeper understanding on the TE transport properties is urgently needed,in order to fully exploit their great potentials in TE applications.Based on the above-mentioned viewpoints,the main research works and the major conclusions in this thesis are listed as follow:1)The TE properties of Mg,Sn1-xSbx(x=0.02?0.5)were systematically investigated.It is found that large amount of Sb substitution on Sn site will induce proportional amount of Mg deficiency which is considered as Mg vacancies based on theoretical calculations and the observed reduced electron concentration.Moreover,detailed investigations on the microstructures reveal the existence of complex nano-defects such as interstitial clusters and nano-precipitates.Phonon transport modeling calculations confirm that these multiscale defects bring in additional scattering and account for the low lattice thermal conductivity in this material system.50%enhancement of the zT(0.9 at 750 K)was realized compared with the Sb doped Mg2Sn.2)The low temperature solid state reaction based on MgH2 was used to synthesize high performance n-type Mg2(Si,Sn)solid solutions(Sn rich content).High TE performance with zT=1.1 at 800 K was realized.And this method was then applied in the synthesis of the p-type Mg2(Ge,Sn)solid solutions.The doping effects of NaBH4,LiBH4 and LiH in this material were systematically investigated.It is found that the hole concentration in Mg2(Ge,Sn)is not only related to the dopants but also the Ge contentlower the Ge content is,higher the hole concentration.LiH(inducing Li on Mg site)could act as an ideal p-type dopant which effective increase the hole concentration,while the B element in Borohydrides does no good for the performance improvement.Benefiting from the optimized carrier concentration,maximum zT of 0.6 was realized in the LiH doped Mg2Ge0.4Sn0.6,which is almost one-fold higher than the Ag doped ones.3)Mg3Sb2 and Mg3Bi2 single crystals were successfully synthesized by using Sb/Bi flux method.And their electrical and thermal transport properties were investigated.It is found that the single crystals obtained in the Mg poor chemical environment are both p-type ones.Intuitively weird anisotropic electrical transport property was observed that the out-of-plane electrical conductivity is twice the in-plane electrical conductivity which is consistent with the theoretically calculated valence band structure.The dominant carrier scattering mechanism is only acoustic phonon scattering which gives side proof to the grain boundary scattering in its polycrystalline materials.Low temperature heat capacity measurement along with the analysis based on the Debye-Einstein model confirms the existence of the theoretical predicted low-lying vibration mode.By using the modified Debye-Callaway model,it is found out that the large anharmonicity and the additional low-frequency vibration mode are the main reasons for its low lattice thermal conductivity.