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978-3-8439-2385-9, Reihe Materialwissenschaften
Calcium Manganese(IV) Oxides for Thermoelectric Power Conversion
146 Seiten, Dissertation Universität Stuttgart (2015), Softcover, A5
Polycrystalline calcium manganese(IV) oxides (CaMnO3-δ) are promising n-type materials for thermoelectric converters, sustaining temperatures above 1200 K in oxidizing atmosphere.
Highly dense Ca1-xAxMn1-yByO3-δ (0 ≤ x,y ≤ 0.8 A= Dy3+, Yb3+ and B = Nb5+, Ta5+, Mo6+, W6+) n-type thermoelectric materials with low electrical resistivities are prepared from nano-crystalline powders. Their room temperature power factors outperform the best reported results by 30% or more. In combination with the thermal conductivities promising figure-of-merits of ZTM=Ta,x=0.04 = 0.21 and ZTM=W,x=0.04 = 0.20 were achieved at 1160 K. The relative changes and temperature dependencies of the Seebeck coefficient, the electrical resistivity, and the power factor are described with a small-polaron-hopping-based mechanism. At high substitutions the efficiency of the doping decreases presumably due to trapping states caused by the formation of bands from Jahn-Teller lowered eg orbitals of Mn3+. Jahn-Teller distortion of Mn3+ also leaves its footprints in the orthorhombic distortion of the crystal structure along the b-axis.
Redox reactivity and structural phase transitions have a major impact on transport and mechanical properties of Ca1-xAxMn1-yByO3-δ thermoelectrics. The high-temperature oxygen content is widely independent from the substituent. The formation of oxygen vacancies dominates the electron transport at high temperatures. Subsequently, the differences in the Seebeck coefficient vanish above 1200 K.
The orthorhombic distortion of the crustal structure and the upper temperature limit of the stability of the orthorhombic crystal structure show an almost linear dependency. Accordingly, the mechanical stability of all-oxides thermoelectric converters at temperatures exceeding 1000 K will be increased employing materials with high substitution level and substituents inducing a high orthorhombic distortion.
An improved sintering procedure allows fabricating calcium manganese(IV) oxides n-type legs on a large scale (100g). The resulting materials are combined with spark plasma sintered p-type Ca3Co4O9 material to fabricate all-oxide thermoelectric converters. The converters are characterized in a newly constructed testing rig up to 1100 K in air. A modification of the joining procedure from classically brazing to hot-pressing boosts the performance by a factor of more than three to 587 mW/cm2. These modules perform for more than 120 h without any signs of degradation.