Biju Patnaik Central Library

Thesis (Ph.D)- National Institute of Technology, Rourkela

High permittivity barium titanate zirconate (BTZ) i
s often used for dielectrics in
commercial multilayer ceramic capacitors, actuators
applications, and is a highly
promising material for dynamic random access memory
(DRAM) and
microelectromechanical system (MEMS) applications d
ue to its very stable, high insulating
characteristic against voltage. Especially, the mat
erial is promising for environmental
friendly application in compared to Lead (Pb) based
compositions.
Different solid solutions in the system have been
synthesized via solid state
reaction route. Detailed phase formation behaviors
of the solid solutions were investigated
through; (i) phase identification, (ii) phase quant
ity determination, (iii) change in lattice
parameters and crystallite sises, (iv) evolution of
phase formation kinetics and reaction
mechanism, and (v) structure and microstructure evo
lution by Rietveld refinement method.
The study on reaction mechanism concludes that; tit
anates form more easily in the system
than zirconates. Then titanate inter-diffuses into
zirconate, where latter phase acts as
diminishing core
Coming to the Rietveld structural refinement on the
four-component system, the
solid solution system Ba
1-x
Sr
x
(Ti
0.5
Zr
0.5
)O
3
remains cubic up to x<0.6 and becomes
tetragonal in the range x >0.6 to x=1.0. Compositio
n with x=0.6 contains both the cubic
and tetragonal phases. The solid solution system (B
a
0.5
Sr
0.5
)(Ti
1-x
Zr
x
)O
3
remains cubic up
to x≤0.6, and the solid solution breaks around x=0.
8 with the formation of second phase of
tetragonal type. Compositions with x=0.8 contain bo
th the cubic and tetragonal phases
and suggest to have a super-lattice structure due t
o the presence of two dissimilar
structures. The Ba
0.5
Sr
0.5
ZrO
3
has orthorhombic structure.
Solubility of Ca and Mg in BaTi
0.6
Zr
0.4
O
3
was also studied by Rietveld method. The
Study concludes that Mg forms single phase solid so
lution with BaTi
0.6
Zr
0.4
O
3
in the
studied composition range and the structure remains
cubic at room temperature. But
solubility of Ca with BaTi
0.6
Zr
0.4
O
3
breaks at around 20 atom % of Ca. The composition
Ba
0.9
C
0.1
Ti
0.6
Zr
0.4
O
3
remains cubic. With increase in Ca content, a diff
erent phase,
orthorhombic CaTiO
3
, grows along with tetragonal Ba
0.8
C
0.2
Ti
0.6
Zr
0.4
O
3
.
Detailed composition dependency microstructure and
dielectric behavior of the
different solid solution in the system has been stu
died. Temperature dependency
dielectric behaviors of some of the selected compos
itions have been studied.
iv
The substitution of Sr in Ba (Ti
0.5
Zr
0.5
) O
3
and Zr in (Ba
0.5
Sr
0.5
) Ti O
3
decreases the
grain size, dielectric constant and dielectric loss
due to the decrease in charge defects by
the substitution. The effect of Ca and Mg on the co
mposition BaTi
0.6
Zr
0.4
O
3
has been
studied in details. Similar dielectric behavior and
microstructure are observed with the
addition of Ca and Mg at room temperature.
The temperature dependency dielectric study on the
composition BaTi
0.6
Zr
0.4
O
3
,
Ba
0.9
Ca
0.1
Ti
0.6
Zr
0.4
O
3
, Ba
0.93
Mg
0.07
Ti
0.6
Zr
0.4
O
3
and Ba
0.86
Mg
0.14
Ti
0.6
Zr
0.4
O
3
was carried out in
the temperature range 130 K to 333 K. The compositi
ons show a diffuse phase transition
having its Curie range of temperature much below th
e room temperature. The Ca-
containing composition shows that there is a shift
of transition temperature towards room
temperature but the Mg containing compositions show
s shift of transition temperature
further below. A clear deviation from Curie-Weiss l
aw is observed for all the compositions,
and degrees of deviations were also calculated. To
study the diffuseness, the data were
fitted with a modified Curie-Weiss law, and it was
found that the degree diffuseness
decreases with Ca substitution, but increases with
Mg substitution. In order to analyze the
relaxation feature, the experimental curves were fi
tted with Vogel-Fulcher formula and the
experimental data were found to be in good agreemen
t with the theoretical fitting.
To get into the realm of physics, relaxor ferroelec
tric is treated as a close analogy
with Ising model of Spin glass system. Further exte
nsion of Ising model to spherical
random bond–random field model is discussed. Both t
he models are extensively reviewed
and presented towards end of the thesis. Finally, f
uture aspects of theoretical models on
present experimental result are proposed. To get mo
re information on the studied
materials for practical application, further extens
ion of experimental work are also
proposed.