Characterization of hexabarium 17-titanate and its effects on the dielectric properties of barium titanate ceramics

Author

Xilin Xu

Keywords and Phrases

Hexabarium titanate; Barium titanate

Abstract

"The formation of the Ba₆Ti₁₇O₄₀ (B6Tl 7) phase is inevitable in BaTiO3 ceramics with excess TiO₂. And B6Tl 7 phases in a dendritic morphology were frequently reported. In this study, the formation mechanism for B6Tl 7 having a dendritic morphology, and the effect of this B6Tl 7 phase on the dielectric properties of BaTiO₃ ceramics have been investigated.

In non-doped BaTiO₃, the B6Tl 7 phase exhibited a needle-like morphology when sintered above the eutectic temperature. The needle-like B6Tl 7 phase was more likely to occur on the as-sintered surface of the sample, as the eutectic melt was squeezed out during sintering and densification. The topotactic relationship between the BaTiO₃ and B6Tl 7 phases can be used to interpret the formation mechanism for the needle-like B6Tl 7 morphology.

When BaTiO₃ was doped with Nb⁵⁺ or other donor dopants (Ta⁵⁺, La³⁺) and sintered in an oxidizing atmosphere, the B6Tl 7 phase exhibited a dendritic morphology. This dendritic morphology was not observed in Co²⁺ doped samples. The dendritic morphology may be due to the topotactic relationship between the BaTiO₃ and B6Tl 7 phases and the formation of Ti₂O₉ groups for hexagonal stacking.

Based on experimental results, the B6Tl 7 phase in a dendritic morphology in BaTiO₃ ceramics occurs under the following conditions: 1) presence of excess TiO₂; 2) sintering above the eutectic temperature to form a eutectic melt; 3) presence of a suitable donor doping level; 4) sintering in an oxidizing atmosphere; and 5) the presence of a fine grain size BaTiO₃ matrix.

X-ray diffraction analysis was used to characterize the B6Tl 7 phase. The needlelike B6Tl 7 and the dendritic B6Tl 7 have the same preferred (041) reflection showing that they may have the same mechanism for their formation. The content ofB6Tl 7 phase in the BaTiO₃ ceramics was also estimated by theoretical calculation and by X-ray diffraction phase analysis using the Rietveld method.

B6Tl 7 is a linear dielectric material, with a dielectric constant of 59. It has a high breakdown strength of 1387 kV/cm. Its coefficient of thermal expansion is 12.5 ppm/K. Field distribution modeling results showed that for a spherical B6Tl 7 particle in the center of a BaTiO₃ film, the field enhancement factor is 1.51, while for a B6Tl 7 phase in needle-like morphology, a field enhancement factor of 4.20 was determined. Although the field enhancement effect inside the B6Tl 7 phase is not high enough to initiate a failure directly from the B6Tl 7 phase, the field enhancement effect at sites just outside of the B6Tl 7 phase might still lead to a failure within the BaTiO₃ matrix. The modeling results also showed that the field distribution in the B6Tl 7 phase was not uniform. This non uniform distribution is believed to be due to the non-linear dielectric behavior of the BaTiO₃ matrix.

The dielectric properties, especially DC breakdown strength of 1 mol¾ Nb⁵⁺ doped BaTiO₃ ceramics with different quantities of excess TiO₂, have been studied. The breakdown strength was found to decrease with increasing quantity of excess TiO₂ (or increasing quantity of the B6Tl 7 second phase) but could not be simply explained by a relative density or grain size effect. Although the Nb⁵⁺ dopant segregated into the B6Tl 7 second phase, the effect of segregation on conductivity could be neglected. The drop in the breakdown strength from a stoichiometric composition to samples with up to 4 mol¾ of excess TiO₂ is believed to be due to the presence of an increasing quantity of the B6Tl 7 second phase. For BaTiO3 ceramics with 1 mol¾ Nb⁵⁺ doping, no more than 1 mol¾ of excess TiO₂ is favored to provide a dense structure with a high dielectric constant while still maintaining a high breakdown strength compared to other samples near the stoichiometric composition"-- Abstract, pp. iv-v

Advisor(s)

Hilmas, Greg

Committee Member(s)

Fahrenholtz, William
Dogan, Fatih
Schwartz, Robert W.
Stoffer, James O.

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Ceramic Engineering

Publisher

University of Missouri--Rolla

Publication Date

Spring 2004

Journal article titles appearing in thesis/dissertation

• The formation mechanism for dendritic Ba₆Ti₁₇O₄₀ in BaTiO₃ ceramics
• Characterization of Ba₆Ti₁₇O₄₀ in BaTiO₃ ceramics by X-ray diffraction analysis
• The field modeling of dimpled electrode pellets and the Ba₆Ti₁₇O₄₀ phase in BaTiO₃ ceramics
• The effects of Ba₆Ti₁₇O₄₀ on the dielectric properties of BaTiO₃ ceramics
• The rheological behavior of ceramic/polymer mixtures for coextrusion processing

Pagination

xvii, 174 pages

Note about bibliography

Includes bibliographical references (pages 171-173)

Rights

© 2004 Xilin Xu, All rights reserved.

Document Type

Dissertation - Restricted Access

File Type

text

Language

English

Subject Headings

Barium compoundsDielectric measurements

Thesis Number

T 8473

Print OCLC #

56727094

Recommended Citation

Xu, Xilin, "Characterization of hexabarium 17-titanate and its effects on the dielectric properties of barium titanate ceramics" (2004). Doctoral Dissertations. 1526.
https://scholarsmine.mst.edu/doctoral_dissertations/1526