Preparation And Modification Of Oxide Thermoelectric Materials

Oxide thermoelectric materials have attracted people's more attentions in the latest year as noval thermoelectric materias due to their friendly environment?low cost and the simplicity during the preparation process.As one of the oxide thermoelectric materials,CaMnO₃ can be synthesised in the air ambient and used at high temperature,it also owns the strong stability of chemistry and structure.In this thesis,we mainly study the effect of variation doping content of chemical compound and graphene including its derivatives in CaMnO₃ system on its thermoelectric performance.In addition,Ca₂Co₂O₅ is also studied in this thesis,the carbon nanotubes are doped in Ca₂Co₂O₅ system to optimize its thermoelectric performance.The following results are obtained in this thesis:1.The dual doping are implemented at both Ca²⁺ and Mn⁴⁺ sites with Sr²⁺ and Si⁴⁺in CaMnO₃ system respectively.In this experiment,the dual doping of compound is different from most similar work those were using electron doping directly.The thermoelectric performance of the ceramics in this experiment are promoted by the change of carriers concertrate?more electron doping indirectly?occurring via the transformance between Mn⁴⁺ and Mn³⁺,which is induced by radius difference between host ions and donor ions.However,as the doping content is over 3%molar fraction,the thermoelectric performance of modified CaMnO₃ ceramics is reduced to lower level even than that of pure CaMnO₃.The reason may be that more dopant increase the lattice distortion to cause the strong bend of the bond of Mn-O-Mn,finally resulting in the large resistivity and relatively low thermoelectric performance.In consequence,the best doping content of 1%is sought,the power factor value of 225?W/mK2 is achieved at 977 K,about twofold than that of pristine CaMnO₃ and better than those of some similar work.2.The constant graphene oxide dopant with content of 6%weight ratio are doped in CaMnO₃,then the composites are sintered in microwave furnace at different temperatures to seek an optimal sintering temperature.Finally,the test rsults demonstrate that the ceramic sample calcinated at 1173 K behaves the best thermoelectric performance and its microstructure is also the compacted one observing from the images of scanning electron microscope.The experiments of CaMnO₃ doping with graphene and its derivatives will also adopt this sintering temeprature process in the following.3.The graphene oxide are doped into CaMnO₃ system to modify the structure and improve the thermoelectric performance.In this experiment,the dopant with content of 1%weight ratio causes the best performance,the power factor value reaches 324?W/mK2 at 934 K,higher than that of many other work those were doped with relatively expensive element compound.4.The graphene quantum dots are doped into CaMnO₃ system to modify the structure and improve the thermoelectric performance.In this experiment,the dopant with content of 2%weight ratio causes a sharply improving thermoelectric performance,the power factor value reaches 290 ?W/mK2 at 334 K and 455 ?W/mK2 at 654 K,the values are higher remarkably than those of other ceramic samples in this series.5.The carbon nanotubes are doped into Ca₂Co₂O₅ system to modify the structure and improve the thermoelectric performance.In this experiment,the dopant with content of 5%weight ratio makes better improvement in the thermoelectric performance.In this thesis,the thermoelectric properties of modified CaMnO₃ ceramics are studied systematically.The element compound and graphene including its derivatives are doped in CaMnO₃ to promote its thermoelectric performance,which is indicated that CaMnO₃ is a promising thermoelectric material for the application in the future.Additionally,the Ca₂Co₂O₅ as p type oxide thermoelectric material is also studied preliminarily.