Study On Electrical And Thermal Transport Performance Of 1-2-2 Type Zintl Phase Thermoelectric Materials

Thermoelectric materials,which can realize the direct conversion between heat and electricity,play an important role in energy conversion.An ideal thermoelectric material should have a low thermal conductivity and high electrical properties.The1-2-2 type Zintl phase studied in this paper is a kind of such materials.The 1-2-2type Zintl phase materials belong to the hexagonal CaAl₂Si₂ type crystal structure.Such a complex crystal structure leads to a very low intrinsic lattice thermal conductivity,while the covalent network in the crystal provides a good channel for electrical transport.However,due to the existence of the crystal field effect,the valence band degeneracy of 1-2-2 type Zintl phase materials is only 1 or 2.This severely reduces the thermoelectric properties of p-type materials.In addition,due to the existence of intrinsic cation vacancies,there are only a few reports on n-type Zintl phase compounds.In this paper,carrier concentration optimization,band engineering,and phonon engineering are used to optimize the thermoelectric properties of 1-2-2 type Zintl phase alloy.The main results are as follows:The effects of crystal field splitting energy（35）E and carrier concentration on the density-of-states effective mass are systematically studied.The carrier scattering effect introduced in the process of regulating（35）E is clarified,and the important role of point defects in phonon scattering is revealed.The density-of-states effective mass of CaCd₂Sb₂ is～0.60m₀.With the valence band convergence by introducing Mg at the Cd site,the density-of-states effective mass of CaCd_1.5Mg_0.5Sb₂ increases to～1.00m₀.Similarly,when the carrier concentration is increased to～5×10¹⁹ cm^-3by doping Ag at the Cd site,the density-of-states effective mass of CaCd_1.99Ag_0.01Sb₂ increases to 0.90m₀.The incorporation of Mg not only optimizes the band structure but also causes alloy scattering,which makes the improvement of electrical properties limited.However,the Mg/Cd substitution point defects can effectively suppress phonon transmission,thereby reducing the lattice thermal conductivity.Finally,the z T value of CaCd_1.496Mg_0.5Ag_0.004Sb₂ reaches～1.3 at 700K.The ΔE and lattice thermal conductivity is adjusted by alloying YbZn₂Sb₂ with Mg Zn₂Sb₂ and Yb Mg₂Sb₂.With the increase of x,the（35）E of(Yb_0.9Mg_0.1)Zn_2-xMg_xSb₂ gradually tends to 0.The peak density-of-states effective mass is 0.85m₀ at x=0.8,which is 30%higher than that at x=0.The power factor of(Yb_0.9Mg_0.1)Zn_1.198Ag_0.002Mg_0.8Sb₂ reaches 13.6 W cm^-1 K^-2 at 773 K with the optimized hole concentration by Ag doping.In addition to reducing the mobility,the incorporation of Mg/Yb and Mg/Zn substitution point defects also caused more serious phonon scattering.Finally,the lattice thermal conductivity of the material is0.52 W m^-1 K^-1,and the z T value is 1.5 at 773 K.With the study on the substitution point defects of Mg/Cd,Mg/Yb,and Zn/Cd in Yb Cd₂Sb₂,it is found that in addition to the mass fluctuation and strain field fluctuation,the point defects also cause lattice distortion.The scattering effect of lattice distortion on phonons is revealed in this work.In addition,the degenerate band structure and high mobility obtained due to the matching electronegativity effectively improve the average z T.Debye-Callaway model is usually used to analyze the lattice thermal conductivity of materials.This model only considers the effect of mass and strain fluctuations caused by point defects,and overestimates the lattice thermal conductivity of(Yb_0.9Mg_0.1)Cd_1.2Mg_0.4Zn_0.4Sb₂ by 20%.A distortion of the distance a’about-11.7%～16.0%and a distortion of c about-5.0%～4.6%are observed in(Yb_0.9Mg_0.1)Cd_1.2Mg_0.4Zn_0.4Sb₂.It is close to the lattice distortion in high entropy alloys.With the contribution of mass fluctuation,strain field fluctuation and lattice distortion,the lattice thermal conductivity of(Yb_0.9Mg_0.1)Cd_1.2Mg_0.4Zn_0.4Sb₂ is only～0.45 W m^-1 K^-1 at 700 K,which is lower than that of all reported 1-2-2 Zintl phase materials.Finally,the z T value of(Yb_0.9Mg_0.1)Cd_1.2Mg_0.4Zn_0.4Sb₂ reaches～1.4 at 700 K,and an average z T is as high as～0.9 in the range between 300 to 700 K.An n-type SmMg₂Sb₂ is designed and developed in this work.It is found that the n-type transformation of SmMg₂Sb₂ occurred near 450 K with the incorporation of excess Mg.When Te is doped at the Sb site,the materials is transformed into n-type over the entire temperature range and obtains high electron concentration.The electron concentration of SmMg_2.3Sb_1.97Te_0.03 reaches～7×10¹⁹ cm^-3,and the z T value reaches～0.5 at 773 K.The thermal properties of SmMg_2.3Sb_1.97Te_0.03 were further optimized by the incorporation of Bi at the Sb site.With the incorporation of Bi/Sb substitutional point defects,the lattice thermal conductivity of SmMg_2.3Sb_1.72Bi_0.25Te_0.03 decreases to～0.82 W m^-1 K^-1 at 873 K,and the z T value reaches～1.0.The SmMg₂Sb₂ reported in this work is the second n-type Zintl phase with z T above unity.Our results are important for the future discovery of other potential n-type 1-2-2 Zintl phase materials.