Synthesis And Thermoelectric Properties Of Defective Heusler-based Compounds

Heusler compounds are a large family of face centered cubic intermetallics.Traditionally,they can be roughly divided into two categories,termed L2₁ full-Heusler(FH)compounds XY₂Z with a 1:2:1 stoichiometry and C1_b half-Heusler(HH)compounds XYZ with a 1:1:1 stoichiometry.The X and Y atoms stand for transition metals,while the Z atom is a p-electron element.There exist half-metallic ferromagnets among FH compounds,that are of great interest in spintronics field.On the other hand,HH compounds typically show semiconducting properties with potential application in thermoelectrics and solar cells.The research on fundamental physical properties of these Heusler-based compounds has promoted the development of spintronics,optoelectronics,and thermoelectrics.Previous studies have mainly focused on the stoichiometric Heusler-based compounds mentioned above.Recently,several non-stoichiometric Heusler compounds have been discovered,in which the intrinsic lattice defects owing to the off-stoichiometric induce interesting structure character.The investigation on both the structure and transport behaviors of these defective Heusler compounds have greatly promoted the research in the fields of thermoelectric and other functional materials.In this dissertation,the phase composition,crystal structures and transport properties of defective phases in M-Co-Sn(M=Ti,Zr,Hf)and Er-Ni-Sb systems were investigated.By virtue of DFT calculations,the influences of defects on transport properties were clarified.Finally,their thermoelectric performance was optimized through element doping or alloying.The main achievements are listed as follows:1.MCo_1.5Sn(M=Ti,Zr,Hf)compounds were found to crystalize in the Heusler-like cubic structure.Both the high-angle annular dark field images and X-ray refinement confirmed that their crystal structures can be viewed as the ordered half-Heusler sublattice with the four empty tetrahedral holes being disorderedly occupied by two extra Co atoms.Electronic structure calculations and experimental results revealed that these compounds are metallic ferromagnets.The magnetic ordering below the Curie temperature greatly affects the electrical transports due to the scattering by spin fluctuations.MCo_1.5Sn compounds have ultralow lattice thermal conductivities compared with other typical Heusler-based compounds due to the partial occupation of Co at the 4d sites.2.The magnetic Co atoms were substituted with Cu in ZrCo_1.5Sn samples for the purpose of clarifying how the magnetism influences the electrical transport properties.Significantly lowered ferromagnetic transition temperature and decreased saturated magnetic moment were observed in Cu-doped ZrCo_1.5Sn due to the diluted magnetic Co atoms.Likewise,in the ferromagnetic phase,the dominant carrier scattering mechanism changed from spin fluctuation scattering to magnetic impurity scattering.The electrical transport behavior in the paramagnetic phase gradually turned from metallic character to semiconducting character with a negative temperature dependence of resistivity,which is consistent with the appearance of a narrow band gap in the first-principles DFT calculations.The maximum Seebeck coefficient of ZrCo_0.85Cu_0.65Sn reached 95?V/K.Sb-doped ZrCo_0.85Cu_0.65Sn samples were prepared to improve the thermoelectric performance.The doping limit reached 20%according to the XRD analysis.The power factor of ZrCo_0.85Cu_0.65Sn_0.8Sb_0.2 was doubled comparing with that of the pristine sample due to the enhanced Seebeck coefficient.The highest z T value of0.35 was achieved in ZrCo_0.85Cu_0.65Sn_0.8Sb_0.2 at 800 K.3.About 8%Ni vacancies were observed in 18-electron ErNiSb samples.The maximum value of Ni deficiency reached 18%by adjusting the composition intentionally.The selected area electron diffraction patterns indicate that the Ni sites are randomly occupied by Ni and vacancies.The formation energy calculations show that the formation of Ni vacancies in ErNiSb is energetically favorable.The increased Ni vacancies not only dampen lattice thermal conductivity,but also induce the formation of non-bonding states for Er near the valance band maximum which decreases the band gap.As the results,the overall high temperature thermoelectric performance was deteriorated.In order to optimize carrier density,Sn-doped Er Ni_1-xSb samples were synthesized.The optimal carrier density was reached according to the single parabolic model.High carrier density suppressed the bipolar diffusion effect.The highest z T value of 0.58 was achieved in ErNiSb_0.97Sn_0.03 at 950 K,improved by 150%as compared with ErNiSb.4.A series of Er_1-xY_xNi Sb and Er_1-xSm_xNi Sb samples were synthesized.With the same alloy concentration,the influence of Y to the band gap is less significant than Sm.The lattice thermal conductivity of Y-alloyed samples is lower than that of Sm-alloyed samples.The power factor of Er_0.8Y_0.2Ni Sb was further improved by 80%through Sn doping.The lattice thermal conductivity of Er_0.8Y_0.2Ni Sb_1-xSn_x is lower than the Ni-deficient Er Ni_0.9Sb_1-xSn_x,and the temperature corresponding to the occurrence of intrinsic excitation is higher.The highest z T value of 0.5 was achieved in Er_0.8Y_0.2Ni Sb_0.97Sn_0.03 at 750 K.