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The Influence of La Doping and Heterogeneity on the Thermoelectric Properties of Sr 3 Ti 2 O 7 Ceramics
Author(s) -
Lu Zhilun,
Sinclair Derek C.,
Reaney Ian M.
Publication year - 2016
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/jace.13998
Subject(s) - materials science , doping , perovskite (structure) , sintering , ceramic , thermoelectric effect , seebeck coefficient , analytical chemistry (journal) , oxygen , mineralogy , electrical resistivity and conductivity , conductivity , chemical engineering , metallurgy , composite material , chemistry , thermal conductivity , thermodynamics , physics , electrical engineering , optoelectronics , organic chemistry , chromatography , engineering
La‐doping mechanisms and thermoelectric properties of Sr 3 Ti 2 O 7 Ruddlesden–Popper ( RP ) ceramics sintered under air and flowing 5% H 2 at 1773 K for 6 h have been investigated. Changes in lattice parameters and conductivity revealed a limited interstitial anion mechanism (~1 at.%) based on La 3+ + ½O 2− →Sr 2+ , which resulted in insulating samples when processed in air. In contrast, electronic donor‐doping (La 3+ + e − → Sr 2+ ) and oxygen loss [O 2− → ½ O 2 (g) + 2 e − ] are the dominant mechanism(s) in 5% H 2 ‐sintered ceramics with a solution limit of ~5 at.%. The increased solubility limit is attributed to the formation of Ti 3+ during reduction, which compensates for the extra positive charge associated with La on the A‐site and also to the occurrence of oxygen loss due to the reducing conditions. For 5% H 2 ‐sintered samples, an insulating surface layer was formed associated with SrO volatilization and oxygen uptake (during cooling) from the sintering. Unless removed, the insulating layer masked the conductive nature of the ceramics. In the bulk, significantly higher power factors were obtained for ceramics that were phase mixtures containing highly conductive perovskite‐based (Sr,La)TiO 3−δ ( ST ). This highlights the superior power factor properties of reduced perovskite‐type ST phases compared to reduced RP ‐type Sr 3 Ti 2 O 7 and serves as a precaution for the need to identify low levels of highly conducting perovskite phases when exploring rare‐earth doping mechanisms in RP ‐type phases.