Synthesis and Stability of Perovskite Oxynitrides
Date
2010-12
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Abstract
In the field of materials, there is continuous search for new compounds with
enhanced performance for a variety of applications. Historically, perovskite oxides have
been of particular interest, due to the ease of compositional-property manipulation and
availability of inexpensive manufacturing processes. A new class of ceramic materials
with a perovskite crystal structure has been synthesized by substituting nitrogen for
oxygen in the anion lattice, e.g. A+2B+5O2N (A=Ca, Sr, Ba and B = Nb, Ta), A+3B+5ON2
(A=Ln and B = Ta, Nb) and A+3B+4O2N (A=Ln and B = Ti, Zr).
Of interest in present work are applications as capacitors and lead-free
piezoelectrics. Kim et al. characterized permittivity, dielectric loss and conductivity of
ATaO2N (A=Ca,Sr,Ba) and BaNbO2N powder compacts (~45% porosity).1 Results
indicated high permittivity for BaTaO2N (κ ~4870) and SrTaO2N (κ ~2870) that was
relatively constant over a range of temperature (273K to 373K) and frequency (0.1 kHz
to 1MHz).
The origin of the high permittivity is not fully understood, since both compounds
possess non-polar crystal symmetries and therefore, cannot exhibit permanent nonzero
dipole moments. Various hypotheses have been proposed. Kim et al. contend that anion
disorder varies the local structure thus inducing Ta+5 displacements and dipoles.
Alternatively, formation of internal barrier layers was considered, but Kim et al. reason
that the impedance results do not support this model. In contrast, Marchand et al. report
that the dielectric response of a BaTaO2N polycrystalline pellet, deduced from infrared
reflection spectra, is not high.2 The contradicting results merit further study and a property-material relationship as undetermined. In addition, dielectric loss was high
(~0.2) and material had insufficient density (~55%) for use in application.
Dense, low defect concentration oxynitride ceramic are essential for electrical
property measurements. However, to control purity, while creating dense ceramic,
requires an understanding of oxynitride synthesis and stability. Therefore, current work
created a fundamental knowledge base for synthesizing oxynitride compounds that
allowed large-batch production of phase-pure oxynitride powders. Unfortunately,
investigations into sintering oxynitride powders identified material instabilities at high
temperatures causing difficulties in forming dense ceramic. This led to development of a
liquid precursor process for fabricating thin-film oxynitrides. Results indicate an
insulating and dense thin-film that allowed a true characterization of electrical properties.
Description
Advisory committee members: Nathan Mellott, Scott Misture, Doreen Edwards, Steven Pilgrim. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Ceramics at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University
Type
Thesis