The Influence Of Thermoelectric Properties On CaMnO₃-based Co-doping Elements With Different Valence System

Thermoelectric material is a kind of functional material which can make electrical power and thermal power convert to each other.And because its converting process cause no mechanical losses and no harm to the environment,it matches the mainstream of the research on some kinds of materials.The past exploration has been finding some thermoelectric alloys with significant properties,and to find materials with higher stability under high temperature condition and lower cost,oxide materials start to be investigated.Since the p-type thermoelectric material which is one of the two kinds of oxide materials has been achieving good properties but the n-type thermoelectric material is still not good enough to match the p-type materials,n-type thermoelectric materials are necessary to be explored.And because that pure CaMnO₃ which is one of the n-type materials has a good Seebeck coefficient,it is claimed as an optimum n-type thermoelectric material,which,however,its resistivity is too high to be apply.According to the previous study,doping method is one of the effective ways to decrease much resistivity of it,which is because doping some trivalent elements in the system introduces electrons and changes the microstructure.While the electrons introduced by doping trivalent elements also make the Seebeck coefficient drop.So,this subject is going to try a new doping method which is co-doping a trivalent element and a univalent element simultaneously to make no difference on carrier concentration and only change the microstructure,thus,the resistivity will drop and the Seebeck coefficient will remain high.In this subject,many doping tries have been done for the CaMnO₃ oxide thermoelectric system and three of them show good data.All of them used solid state reaction to prepare samples.The first try is about Y³⁺/Na⁺ co-doped[Ca_1-x(Na_1/2Y_1/2x]MnO₃ system,the second try is about Dy³⁺/Na⁺ co-doped[Cal-x?Na1/2Dy1/2?x]MnO3 system,and the third try is that on the bases of doping 10%Y³⁺/Na⁺,Sr²⁺ doped[Ca0.9-xSrx?Yi/2Na1/2?0.1]MnO3 system.Although Na was missing in all three systems,the holes introduced by the Na in the system indeed compensate with the electrons introduced by doping trivalent elements,thus,it causes an increase of resistivity and absolute Seebeck coefficient compared to the single trivalent element doped systems.Besides,decreasing temperature of reaction can reduce the volatilization of Na and thus,increase the properties.And doping more kinds of elements make the structure of the system more complicated and raise the lattice heat scattering,thus,the thermal conductivity decreases.And in this subject,the elementary attempt of the re-sintering method has been partially achieved to solve the problem of the samples which are too fragile to be measured.In the Y/Na co-doping and Dy/Na co-doping systems,XRD patterns do not show a pure CaMnO₃ phase,and the second phase belongs to the CaMn2O4,which is because the missing Na makes the number of trivalent elements relatively increase.The geometric distortion happens in all three systems.In the Y/Na/Sr co-doping system,Sr²⁺ has the bigger ionic radius compared to Ca2+,thus,the geometric distortion might decrease and for that.reason,the resistivity increases.In the Y/Na co-doping and Dy/Na co-doping systems,Y³⁺ has a smaller ionic radius than Dy³⁺,thus,the geometric distortion in the Y/Na co-doping system is more severe than the Dy/Na co-doping system and thus,the resistivity is smaller in the Y/Na co-doping system.As results,the highest power factors are 235.9?gW/K²m when x=0.10 at 200?,298.O?W/K²m when x=0.05 at 50? and 12.9?W/K²m when x=0.05 at 250? for[Ca1-x?Nai12Yi/2?x]MnO3,[Cal-x?Dy1/2Na1/2?x]MnO3 and[Ca0.9-xSrx?Y1/2Na1/2?0.1i]MnO3 samples respectively.And because of the limit of time and conditions,the ZT could only be calculated for the Dy³⁺/Na⁺ co-doped CaMnO₃ system and the highest ZT of the system is 0.126 when x=0.15 at 600?.