Study On The Thermoelectric Properties Of N-type Mn(Fe)Bi₄S₇ Compounds With Low Lattice Thermal Conductivity

Thermoelectric materials,as a new type of clean energy resources that can directly convert waste heat into electrical energy,are receiving more attention recently due to the serious environmental and energy problems.Thermoelectric materials can not only be used for waste heat recovery,outspace energy supply,but also have broad application prospects in solid-state refrigeration,local cooling for microchips,and power supply for sensors.However,for a long time,thermoelectric materials have low thermoelectric figure of merit,low conversion efficiency of related devices,and most thermoelectric materials with high thermoelectric performance contain toxic or precious elements,which set obstacle for the widespread application of thermoelectric materials.In recent years,sulfur-based compounds have received extensive attentions from thermoelectric community because of their environmental friendliness and earth-abundance.Successive progress has been made in p-type thermoelectric sulfides,such as Sn S,Cu₁₂Sb S₁₃,Cu₂S with z T values over unity.In contrast,meanwhile the thermoelectric performance of n-type sulfur-based compounds is far from satisfactory,which greatly restricts the practical application of sulfur-based thermoelectric materials.Moreover,previous studies mainly focused on thermoelectric sulfides with high crystal symmetry structure and ignored the compounds with low symmetry that account for a large proportion of sulfides.This thesis takes n-type Mn(Fe)Bi₄S₇,which consist of environmentally friendly and earth-abudant elements,as the research object,and explores its thermoelectric transport properties in detail,especially the origin of its low lattice thermal conductivity by linking the unique crystal structure and the heat transport characteristics.The electrical transport properties of these compounds are investigated through first-principles calculations and experimental data.Selenium solid solution at sulfur site is used to improve its electrical properties.The preparation methods of Mn Bi₂Se₄ and Mn Bi₂S₄ are expolored and their thermoelectric transport proerpteis are preliminarily studied.The main contents of this thesis are as follows:(1)A number of Mn_1.03Bi₄S₇,MnBi₄S_6.86 and Mn Bi₄S₇ polycrystalline samples were synthesized by the traditional solid-state synthesis method with fined synthesis procedure.The anisotropic thermoelectric properties of the bulk samples consolidated by spark plasma sintering were studied.It was found that the carrier concentration of prinsitine Mn Bi₄S₇ is relatively low.Through the self-doping strategy by Mn excess or S deficiency,the carrier concentration can be effectively enlarged while S deficiency was found to be efficient than Mn excess.With increased carrier concentration,the power factor of Mn Bi₄S_6.86 at 770 K was boosted from 0.1 to 0.21,and the resultant z T value was increased from 0.14 to 0.27 at 770 K,which was almost doubled.(2)A series of Mn Bi₄S_6.86-7xSe_7x polycrystalline samples were synthesized by the same procedure.It is possible that the Se substitution would lower the formation energy of anion vacancy or the ionization energy,which will further enlarge the carrier concentration.At the same time,the alloying with Se will introduce fluctuations in the mass field and stress field to reduce the lattice thermal conductivity.It was found that the effect of Se alloying is beyond the point defect scattering as indicated by fitting with Debye model and the actual lattice thermal conductivity deviates from the predicted value with increasing Se content,which is ascribed to softening of the phonon mode induced by Se substititon.Due to the dual benefits of Se alloying on the thermoelectric properties of Mn Bi₄S₇,the peak z T value reached 0.31 at 770 K,which is one of the best reported results for the abundant of n-type sulfides.(3)FeBi₄S₇ polycrystalline samples were synthesized in large quantities by the same method as employed for Mn Bi₄S₇.The thermoelectric properties of Fe Bi₄S₇are comparable to that of Mn Bi₄S₇,with similar low lattice thermal conductivity but inferior electrical properties.The results indicate that Fe Bi₄S₇ can be potential n-type thermoelectric materials in the middle temperature region with further optimization in carrier concentration and alloying.(4)Motivated by the successful exploration of thermoelectric properties of Mn(Fe)Bi₄S₇,this thesis also pay attention to Mn-Bi-Y(Y=S,Se)compounds,which also possess a complex crystal structure.After a series of trial,Mn Bi₂Se₄ and Mn Bi₂S₄ were successfully synthesized,and the thermoelectric potential of Mn Bi₂Se₄ was initially evaluated.The thermoelectric performance of such compounds will be further investigated in detail and optimized in the future.