Non-stoichiometric ordered niobate and tungstate perovskites for
microwave applications
Hui Wu,
University of Pennsylvania
Ordered perovskite oxides such as
Ba(Zn1/3Ta2/3)O3 (BZT)
are widely utilized as frequency filters in commercial microwave based
wireless communications systems. However, the processing of these
materials to yield an optimum performance is notoriously difficult and
large changes in the dielectric loss properties result from very small
changes in bulk stoichiometry, cation ordering of Zn and Ta, and the
volatilization of Zn at high temperature. Although the bulk crystal
chemistry of BZT and other closely related phases have been widely
studied, many inconsistencies remain as to how the Zn loss affects the
crystal chemistry and properties. In this talk, I will present a
systematic study of the effect of small degrees of non-stoichiometry
on the structure, stability and dielectric response of
Ba(Zn1/3Nb2/3)O3 (BZN)
and related phases. This work began with the discovery and structure
refinement of the first family of non-stoichiometric 1:2 ordered
perovskites in the
(1-x)A2+(Li1/4Nb3/4)O3(x)A2+(Li2/5W3/5)O3,
A2+=
Ba, Sr, and Ca, systems. In contrast to all known
A(BI1/3BII2/3)O3
perovskites, these 1:2 ordered solid solutions do not include any
composition with a 1:2 cation distribution and the structure exhibits
extensive non-stoichiometry. Tungstate-based substitutions were found
to be very effective in stabilizing the order and improving the
dielectric loss properties of
Ba3ZnNb2O9 (BZN).
For example in the
(1-x)Ba3ZnNb2O9(x)Ba3W2O9
system the incorporation of very small concentrations of vacancies
on the B-site, compensated by the partial replacement of Nb by W,
increases the ordering temperature of BZN and dramatically enhances
the kinetics of the B-site ordering reaction. The ceramic samples
of these solid solutions exhibit some of the lowest dielectric
losses recorded for niobate perovskite dielectrics with
Q.f's > 110,000
at 8 GHz. Finally, the effect of cation non-stoichiometry on
phase stability, microstructure, and dielectric properties of pure
BZN will be discussed. Non-stoichiometry was induced by forming
solid solutions of BZN and related phases in
BaO-Nb2O5-ZnO ternary; it was found that BZN can exhibit a limited
range of non-stoichiometry and exceptionally small changes in bulk
chemistry produce order of magnitude alterations in the dielectric
loss properties. The role of cation vacancies on the stabilization
of the cation ordering, microstructure and sintering will be
discussed together with the correlations between the phase stability
and microwave dielectric properties.
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