Alloying And Compositing Of Mg₂(Si, Sn) Based Thermoelectric Materials

Thermoelectric(TE) materials are semiconducting functional materials,which can convert heat energy directly to electricity or reversely.TE power generators and cooling devices have been extensively applied in waste heat harvesting,space flight,microelectronic cooling with the merit of pollution-free,noiseless,easy-maintenance and reliability.Mg₂Si based compounds have high effective mass,carrier mobility and low lattice thermal conductivity,which are beneficial for power generation in 200℃～500℃temperature range. Because of their advantages of abundant raw materials,low cost and non-toxicity,they are considered to be promising as one of environmentally friendly next-generation TE materials.In the present work,extensive theoretical and experimental work has been conducted on Mg₂Si based compounds.Many methods have been tried,such as forming solid solutions, doping and compositing to tune the electron and phonon transport and thereby improve the ZT values.First principles calculations based on density functional methods were used to analyze the formation energy,the electrical structures and the TE properties.The main results are summarized as follows:New type of isoelectronically substituted solid solutions Mg_2-xCa_xSi and Mg_2-xYb_xSi_0.6Sn_0.4 were designed and prepared in this work.It was found that the solid solubility of Ca and Yb in Mg₂Si and Mg₂Si_0.6Sn_0.4 are 10%and 2.5%,respectively.The phonon thermal conductivity(K_ph) can be lowered by enhanced point defect scattering due to Ca subsititution.The electrical conductivity can be increased by enhanced carrier concentration.Theσ/k values are then increased obviously.The conduction mechanism is changed by the solubility of Yb.The highest ZT value exists in the solid solution Mg₂Si_0.6-xGe_xSn_0.4 with x=0.05,which is 87%higher than that of Mg₂Si_0.6Sn_0.4.Rare earth(RE) elements were chosen as the donor dopants in Mg₂Si based TE materials. The doping effects of light rare earth elements(La,Ce,Pr,Nd,Sm) and heavy rare earth elements(Gd,Dy,Er,Yb) were systematically studied.It was shown that all the nine RE elements could increase the electrical conductivity,and resulted in higher ZT values.It was found that the effect of doping is connected with the atomic number.From La to Yb,the atomic number increases,while the atomic radius decreases because of the lanthanide contraction,and weakens the ability of loss of electrons and the effect of donor doping.On the other hand,the ionic radius of RE³⁺ lowers with the increasing atomic number,which reduces their radius difference from Mg²⁺,and hence decreases the scattering of phonons.The light RE elements are better than the heavy ones in both electrical conductivity and thermal conductivity.A highest ZT value～0.75 at 750 K for La doped Mg_1.995La_0.005Si_0.6Sn_0.4 was obtained.Utilizing the peritectic reaction featured in the pseudo-binary phase diagram of Mg₂Si and Mg₂Sn,we fabricated self-assembled Mg_2-xLa_xSi_0.58Sn_0.42 TE composites in which the micrometer Mg₂Si-rich bulk grains were in situ coated by hundreds of nanometer Mg₂Sn-rich thin layers.The Mg₂Sn-rich phase was selectively doped with La.It was found that both phases obtained in this way are semiconductors with good TE properties estimated by the factorβfor excellent TE materials.Hashin-Shtrikman model was used to discuss the TE properties of the composites.As a result,a dimensionless figure of merit ZT～0.81 has been obtained at 810 K for the Mg_1.995La_0.005 Si_0.58Sn_0.42 in situ composite.The possibility of phase separation in Mg₂Si-Mg₂Sn system is predicted by First principles calculations.Combination with the TE materials evaluation factorβ,Sn-rich series materials Mg₂Si_0.4-xSn_0.6Sb_x(0≤x≤0.015) were designed.Then Sb doped Mg₂(Si,Sn) bulk in situ nanocomposites were prepared by nanoscale phase separation.Microstructure study shows that a fairly large amount of in situ formed nanostructures exist in these samples,such as the nanoscale compositional modulation,scale-like nanoscale structural modulation with the feature size of over ten nanometers and the compositional fluctuation zone with the characteristic length scale of several nanometers,which can provide scattering mechanism for the mid-to-long-wavelength phonons and contribute to the marked reduction of K_ph.The experimental minimum K_ph is 1.22 Wm^-1K^-1 at 780 K,which is still 60%higher than the theoretical minimum value estimated by Debye model.It was found that the Mg₂Si_0.395Sn_0.6Sb_0.005 and Mg₂Si_0.3925Sn_0.6Sb_0.0075 in situ nanocomposites have highest ZT values higher than 1.0.