Electrical and structural properties of compositions in the systems Y₁₋ₓCaₓMnO₃ and La₁₋ₓCaₓMnO₃

Author

Jeffry W. Stevenson

Abstract

"Electrical conductivity, Seebeck coefficient, and thermogravimetric behavior of compositions in the system Y_1-xCa_xMnO₃ were studied as functions of temperature and ambient oxygen activity in order to determine the mechanism of electrical transport and defect structure. Thermal expansion was also measured. Electrical conduction appeared to occur via a small polaron hopping mechanism for compositions with 0.30 ≤ x ≤ 0.80. Seebeck coefficient data did not support the traditional assumption that Mn is present in Mn perovskites only in the +3 and +4 valence states. A new defect model was developed which included the thermally excited disproportionation of Mn³⁺ into Mn²⁺ and Mn⁴⁺ pairs. The Seebeck data could be explained with this model by assuming that divalent Mn cations were site blockers in the conduction process. Since compositions in Y_1-xCa_xMnO₃ exhibited very slight oxygen activity-dependent behavior as the ambient oxygen activity was reduced, several compositions in the system La_1-xCa_xMnO₃ were studied in order that the defect model could be applied to the oxygen activity-dependent region in manganites. The disproportionation model was successful in explaining the experimentally observed dependence of oxygen stoichiometry, electrical conductivity, and Seebeck coefficient upon ambient oxygen activity"--Abstract, page iii.

Advisor(s)

Anderson, H. U. (Harlan U.)

Committee Member(s)

Sparlin, Don M.
Nasrallah, Magdi M.
Parris, Paul Ernest, 1954-
Rahaman, M. N., 1950-
Pringle, Oran Allan

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Ceramic Engineering

Publisher

University of Missouri--Rolla

Publication Date

Fall 1991

Pagination

xi, 119 pages

Note about bibliography

Includes bibliographical references (pages 112-118).

Rights

© 1991 Jeffry Wayne Stevenson, All rights reserved.

Document Type

Dissertation - Restricted Access

File Type

text

Language

English

Thesis Number

T 6329

Print OCLC #

26980625

Recommended Citation

Stevenson, Jeffry W., "Electrical and structural properties of compositions in the systems Y₁₋ₓCaₓMnO₃ and La₁₋ₓCaₓMnO₃" (1991). Doctoral Dissertations. 484.
https://scholarsmine.mst.edu/doctoral_dissertations/484