High-pressure Synthesis And Properties Of Mg₂Si Based Thermoelectric Materials

Thermoelectric materials,which can convert thermal energy into electrical energy and vice versa,have an appealing application potential in power generation and refrigeration.Mg₂Si-based thermoelectric materials have the advantages of eco-friendliness,low cost and abundant earth reserves.Moreover,these materials have high melting point,low density,and small thermal expansion coefficient,making them an important class of thermoelectric materials for mid-temperature applications.The traditional syntheses at ambient pressure usually suffer from the volatilization and oxidation of Mg,leading to difficulties in composition control and thermoelectric properties optimization.High-pressure synthesis?HPS?can regulate the formation energy of the synthesis reaction,accelerate the reaction rate,inhibit the volatilization and oxidation of Mg,thus provide an effective approach to improving the thermoelectric properties.In this thesis,the effects of high pressure and doping on the thermoelectric properties of Mg₂Si-based thermoelectric materials are investigated.Mg_2+ySi?y=0,0.05,0.1?samples were prepared with the high pressure synthesis method.The structure and thermoelectric properties of the samples were studied.Either the anti-fluorite cubic phase or the hexagonal phase of Mg₂Si can be formed depending on the applied high pressure.Single-phase anit-fluorite structured cubic Mg₂Si samples were produced under 2 GPa.High pressure effectively eliminated the oxidation or volatilization of Mg,and promoted the formation of interstitial Mg,leading to larger lattice constant with increasing Mg content.The thermoelectric properties measurements show that,with increasing Mg content,the electrical resistivity and lattice thermal conductivity of Mg_2+ySi samples decreases.ZT of y=0.1 sample reaches 0.3 at 723 K,which is 50%higher than that of y=0 sample.Mg_2.05Si_1-xSb_x samples with different Sb doping levels?x=0,0.005,0.01,0.015? were prepared with high pressure synthesis method followed by spark plasma sintering?SPS?.The phase composition,microstructure and thermoelectric properties were investigated.Single phase anti-fluorite structured Mg₂Si_1-xSb_x samples were successfully produced.The lattice constant increases with elevating Sb doping level.The doped Sb atoms and the interstitial Mg atoms introduce extra electrons to the system and greatly increase the carrier concentration.Compared with those samples synthesized under ambient pressure,HPS samples show much improved thermoelectric properties.The optimal Mg₂Si_0.985Sb_0.015 sample shows a significantly enhanced power factor(3300?Wm^-1K^-2 at 773K)and greatly suppressed lattice thermal conductivity(ca.1.0 Wm^-1K^-1at high temperature),leading to a maximum ZT of 0.94 at 873 K.HPS samples also show a nice thermal stability.Mg₂Si_1-xBi_x?x=0,0.01,0.015,0.02?compounds were prepared with high-pressure synthesis followed by SPS.After Bi doping,the carrier concentration increases,contributing to decreasing electrical resistivity and increasing power factor.The power factor of the Mg₂Si_0.985Bi_0.015 sample is greater than 2600?Wm^-1K^-22 at temperatures higher than 600 K,and reaches a peak value of 3050?Wm^-1K^-2 at 843 K.The lattice thermal conductivity is great suppressed due to Bi doping as well as the interstitial Mg.ZT of the optimal Mg₂Si_0.985Bi_0.015 sample reached 0.98 at 883 K,which is the highest value of single-element doped Mg₂Si.The doping and alloying effects were investigated for Sb-doped Mg₂Si_0.4Sn_0.6samples prepared from high pressure synthesis followed by SPS.Due to the mass and atomic radius fluctuations of Si and Sn atoms,the thermal conductivity is greatly suppressed for Mg₂Si_0.4Sn_0.6 alloy.After Sb doping,with increasing carrier concentration,the electrical resistivity conductivity decreases,and the power factor increases.The power factor of the Mg₂(Si_0.4Sn_0.6)_0.985Sb_0.015 sample was greater than 4000?Wm^-1K^-2 at high temperature and reached 4300?Wm^-1K^-2 at 673 K.The lattice thermal conductivity further decreased with higher Sb doping level.As a result,ZT of Mg₂(Si_0.4Sn_0.6)_0.985Sb_0.015 sample reached 1.48 at 823 K.HPS samples show a nice thermal stability.