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Lanthanide Doping Effect Of The Thermoelectric Transport Properties Of Ca1-xRxMnO3

Posted on:2017-03-25Degree:MasterType:Thesis
Country:ChinaCandidate:X H ZhangFull Text:PDF
GTID:2271330488952631Subject:Condensed matter physics
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CaMnO3 is a kind of potential high-temperature n-type thermoelectric materials. Because of its high Seebeck coefficient, the ZT value of CaMnO3 is high among n-type oxide materials. At present, a series of experiments are available to improve the thermoelectric transport properties of CaMnO3, such as Ca site doping, Mn site doping, as well as double doping. Now the physical mechanism of the doping effects remains to be further investigated. Based on the density functional theory and the semi-classical boltzmann transport theory, the thesis systematically studied the effects of Ca site doping and oxygen vacancies on the structure, phase transfer and thermoelectric propretties of CaMnO3. The conclusions of the research are as follows:1. Ca site doping with 25% of Lanthanide elements affects the properties of CaMnO3.● The G-type anti-ferromagnetic phase is the most stable one among five intrinsic anti-ferromagnetic phases. However, the C-type phase is the most stable phase after Lanthanides 25% doping in the A-site of CaMnO3.● The conductive mechanism changes from semiconductor-type (intrinsic) to metal-type (doped). Electrons transfer from Lanthanides to Mn d orbital in 25% Lanthanides doped materials, which results that part of conductive bands are occupied by Mn d electrons. This is the physical mechanism for the change of conductive mechanism.● The electrical conductivity is enhanced by Lanthanides doping, about 2 orders of magnitudes higher than that of intrinsic CaMnO3. 2. The doping concentration induces the Jahn-Teller distortion of the Mn-O octahedra and affects the thermoelectric properties of Ca1-xLaxMnO3.● The doping concentration of La element induces Ca1-xLaxMnO3 to change from orthogonal G-type anti-ferromagnetic phase (G-AFM) to monoclinal C-type anti-ferromagnetic phase (C-AFM). The critical value of the doping concentration is 12.5%. When the doping concentration is between the range 0<x<12.5%, the space group of crystal structure is Pnma with orthogonal G-type anti-ferromagnetic. When the doping concentration x is above 12.5%, the space group of crystal structure is P21/m with monoclinic C-type anti-ferromagnetic phase. When the doping concentration x is equal to 12.5%, G-AFM and C-AFM coexist.● Lanthanide doping leads electrons to transfer from doping elements to Mn element. Mn atom receive an electron, which leads to the valence of Mn change from+4 to +3~+4. The splitting of eg and t2g orbit is caused by electron transfer and part of the splitting orbits are occupied by electron, which leads to the Jahn-Teller distortion of Mn-O octahedra is the physical mechanism for the phase change.● Electrons transfer from Lanthanides to Mn d orbital in Lanthanides doped materials, which results that part of conductive bands are occupied by Mn d electrons. Compared with intrinsic CaMnO3, the conductive mechanism of Ca1-xLaxMnO3 transfer from semiconductor to metal. Finally the thermoelectric properties of Ca1-xLaxMnO3 with the critical value of phase transformation x= 12.5% could achieved the best value is predicted in theory. 3. The influences of crystal structure, band structure and thermoelectricproperties for single doping and dual doping in Ca site have been studied.● The volume of Ca0.875R0.125MnO3 and Ca0.875La0.0625Dy0.0625MnO3 reduces with the decreasing of lanthanide radius, and the volume of single doping and dual doping is larger than that of intrinsic CaMnO3.● The Fermi level of doped systems moves upward and across the bottom of the conduction band, which results that part of conductive bands are occupied by Mn d electrons. The conductive mechanism of Ca0.875R0.125MnO3 and Ca0.875La0.0625Dy0.0625Mn03 is metal-type. Because of the smooth trend of conduction band of dual doping system, the electrical conductivity of dual doping system is loewr than single doping system.● Both single doping and dual doping can improve the thermoelectric performance of CaMnO3. However, the thermoelectric properties of dual doping system are better than those of single doping system. And the best thermoelectric properties are obtained in the CaMnO3 co-doped with Yb, Dy at the ratio 1:1.4. The presence of the oxygen vacancy can reduce the resistivity of the CaMnO3 material, and the appearance of oxygen vacancy affects the crystal structure, band structure and the thermoelectric performance of CaMnO3.● By testing the position of oxygen vacancies in the super cell, we find that oxygen vacancies arrange in the straight line in Ca16Mn16O48 super cell is the most stable structure.● The binding energy increases with the increasing of oxygen vacancy. When the content of oxygen vacancy between 0.125-0.1875, the formation energy change from negative to positive. It means that the critical value of the content of oxygen vacancy in CaMnO3 is in the range of 0.125-0.1875.● From the band structure, we can conclude that the conductive mechanism of CaMnO3-δ5 (δ≤0.1875) and CaMnO3 is the same, and the band gap of CaMnO3-δ (δ≤0.1875) decreases with thenumber of oxygen vacancy increasing. The electrical conductivity is enhanced by oxygen vacancy, about 1 orders of magnitudes higher than that of intrinsic CaMnO3. The reason for the electrical conductivity increasing is that the appearance of hybridization band near by Fermi level leads to the decrease of band gap. Finally we forecast that the little exist of oxygen vacancies in the CaMnO3 could improve the thermoelectric properties in theory.
Keywords/Search Tags:Thermoelectric materials, CaMno3, thermoelectric property, First principles calculation, Lanthanide doping
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