The High Temperature Thermoelectric Performance Of N-type ZrPtSn And P-type Yb₁₄MnSb₁₁

Thermoelectric(TE)materials can realize the conversion between heat and electricity directly,and provide novel solution for waste heat recovery.Both the thermoelectric performance(the dimensionless figure of merit,z T)of TE materials and the conversion efficiency of TE devices are closely associated with temperature,and developing high-temperature TE materials with excellent performance are in urgent need.In this work,n-type half-Heusler(HH)and p-type Yb₁₄Mn Sb₁₁ compounds are studied.Although ZrNiSn,as a well-studied n-type HH,shows excellent z T at mediate temperatures,its TE performance at high temperatures is great deteriorated by bipolar diffusion due to its small band gap.In this work,an alternative HH compound ZrPtSn,which has an intrinsically wider band gap as compared with ZrNiSn,is systematically studied,focusing on the optimization of carrier concentration for improving high temperature TE properties.In the other hand,Yb₁₄Mn Sb₁₁ Zintl phase is well studied as a p-type TE material for the high temperature applications.However,it is hard to obtain single phase Yb₁₄Mn Sb₁₁ and the element sublimation at high temperature is also an obstacle for practice applications.In this thesis,efforts have been also made on both the optimization of synthesis process and improvement of high temperature stability of Yb₁₄Mn Sb₁₁ Zintl phase.The obtained main results are summarized as follows:1.Arc melting and Levitation melting are used to synthesize ZrPtSn half-Heusler.The XRD and SEM analysis indicated that the Levitation melted samples are single phase ZrPtSn.The optical band gap of ZrPtSn is measured?0.25 e V by infrared absorption spectroscopy,which is larger than that of ZrNiSn(?0.14 e V).The carrier concentration is significantly increased from 10¹⁹ cm^-3 to 10²⁰ cm^-3 by doping Sn,leading to an enhancement of room-temperature electrical conductivity by two orders of magnitude.For the composition of ZrPtSn_0.92Sb_0.08,the power factor reaches 22.8?Wcm^-1K^-2,and the z T increases monotonously with temperature and reaches 0.47 at1000 K,which is improved by 300%as compared with the pristine ZrPtSn.2.The effects of Ni alloying on the lattice thermal conductivity of ZrPtSn is investigated.The XRD and SEM results show that the maximum solubility of Ni in ZrPtSn is about 10%.Due to the enhanced fluctuation of mass and stress fields introduced by Ni alloying,the lattice thermal conductivity?_L at medium and low temperature are greatly reduced.For example,the room-temperature?_L is decreased by40%.However,the decreased mobility is also observed because of the enhanced carrier scattering,and the resulted zT of ZrPt_0.9Ni_0.1Sn_1-xSb_x is comparable with that of ZrPtSn0.92Sb0.08.3.A series of Zintl phase Yb₁₄Mn_xSb₁₁ samples with different Mn content are synthesized by ball-milling and annealing process.The XRD results show that the Zintl phase can be obtained as the stoichiometry of Mn is above 1.The z T value of Yb₁₄Mn_1.1Sb₁₁is enhanced by 10%at 1000 K as compared with Yb₁₄Mn Sb₁₁.A series of Yb₁₄Mn_1.5Sb₁₁/W composites are further synthesized,and experimental results indicated that the high temperature oxidation resistance is improved by introducing W particles forming composite.