Study On The Synthesis And Thermoelectric Properties Of Fe_1-xNi_xTiSb

Thermoelectric materials are able to realize the mutual conversion between thermal energy and electrical energy in solid state,possessing prodigious application prospect in many fields such as waste heat recycling,energy supply in some special environment and solid-state refrigeration.The fundamental unit of thermoelectric devices,more often than not,consists of a unicouple of p-type and n-type legs.High-efficient thermoelectric device not only requires a high dimensionless figure of merit for materials,but also the similar thermal expansion coefficient and mechanical strength for the two components.However,for majority of thermoelectric materials,there is huge difference in composition for p/n type components.In some special cases,only one type leg exists.In this way,even if materials are capable to reach a higher in both n-type and p-type,it is high possible that devices would broke down and fail in the process of thermal circling due to the unmatched thermal expansion coefficient and mechanical properties.Half-Heusler compound has become of great research value in the field of middle and high temperature waste heat recovery due to its good thermal stability and high mechanical strength.However,as a thermoelectric material,it suffers from the relatively high lattice thermal conductivity of compared to other classic thermoelectric materials.In fact,there is still a large gap between the theoretical minimum thermal conductivity and the observed experimental value in Half-Heusler compounds.In addition,the Half-Heusler alloys usually contain precious elements,which leads to high production cost.Therefore,this thesis taking the Half-Heusler material Co Ti Sb as the research object,aims to synthesize novel half-Heusler compounds Fe_1-xNi_xTi Sb by the means of are melting and replacing the expensive Co with Fe and Ni.By only altering the ratio of Fe and Ni without foreign element doping,both p and n types of compounds with similar thermal expansion coefficient and mechanical strength are attained.Consequently,with further optimization of electrical performance by adjusting carrier concentration and the reduction of lattice thermal conductivity by high-energy ball milling,the dimensionless figure of merit of both n-type and p-type can be improved simultaneously.The main research results of this study are given as follows:1.The novel Half-Heusler material Fe_0.5Ni_0.5Ti Sb was successfully prepared by arc melting.Through X-ray diffraction and transmission electron microscopy,it was confirmed that Fe_0.5Ni_0.5Ti Sb has the same non-central symmetric cubic structure as Co Ti Sb（space group no.216,F4%3m,C1b）.Moreover,Fe and Ni atoms occupy the sites of Co atoms in the lattice of Co Ti Sb in a random distribution.Through the thermoelectric performance measurement,it was found that the electrical performance of Fe_0.5Ni_0.5Ti Sb was similar to that of Co Ti Sb,while the total thermal conductivity of the former decreased significantly.In addition,adjusting the ratio of Fe and Ni in the chemical formula Fe_1-xNi_xTi Sb can not only optimize the carrier concentration and improve the power factor,but also obtain the n-type and p-type materials at the same time,which is of important research significance and potential application value for the fabrication of thermoelectric devices.2.In order to further optimize the thermoelectric performance of Fe_1-xNi_xTi Sb,the lattice thermal conductivity was reduced by mechanical ball milling,and the influence of ball milling time on the electrical transport of different components Fe_1-xNi_xTi Sb was evaluated by the transport coefficient theory.Finally,the grain size of different components under the optimum ball milling time was checked by SEM images.After determining the optimal ball grinding time for Fe_1-xNi_xTi Sb with different compositions,the electrical and thermal properties of all samples were measured.P-type compound with nominal composition of Fe_0.65Ni_0.35Ti Sb and n-type of Fe_0.3Ni_0.7Ti Sb showed the highest values of 0.48 and 0.40 respectively,at 923 K,which were increased by about 40%and 50%compared to the samples without ball milling.The single parabolic band（SPB）model was used to analyze the electrical transport relationship between carrier concentration and Seebeck coefficient of all samples,and the thermoelectric quality factor B was used to evaluate the thermoelectric potential of Fe_1-xNi_xTi Sb with varied compositions.Finally,it is concluded that compared with Co Ti Sb,the new Half-Heusler compounds Fe_1-xNi_xTi Sb show obvious advantages in thermoelectric performance and environmental friendliness.3.Although the random Fe and Ni substitution for Co in Co Ti Sb lattice can reduce the lattice thermal conductivity effectively,the total thermal conductivity is still high compared with other thermoelectric materials.In order to further improve the thermoelectric performance of these compounds,on the basis of high-energy ball milling,the n-type Fe_0.6Ni_0.4Ti Sb and p-type Fe_0.4Ni_0.6Ti Sb are selected to further alloying with same group element Hf to reduce the lattice thermal conductivity promote the thermoelectric properties.