1.1. Introduction
A ceramic is an inorganic, nonmetallic solid prepared by the action of
heat and subsequent cooling. Ceramic materials may have a crystalline or
partially crystalline structure, or may be amorphous (e.g.: glass). Because,
most common ceramics are crystalline, the definition of ceramic is often
restricted to inorganic crystalline materials, as opposed to the non-crystalline
glasses.
Electro ceramics form an important class of material of great technical
value used in numerous device applications, for instance, high dielectric
constant capacitors, piezoelectric sonar and ultrasonic transducers, power
engineering, radio and communication, medicine and health-care and
pyroelectric security surveillance devices [1, 2]. The thin film technology has
further broadened the area of application into random access memory
(DRAM), ferroelectric random access memory (FRAM), pyroelectric image
arrays and other devices [3-5]. There is growing interest and miniaturization
integrating electro-ceramic functions on to conventional semiconductor chips
and evolution of multi function components and systems. Further, nano size
effects and nano structuring of electro-ceramics hold much promise studies on
these aspects are currently pursued worldwide. The current world wide
explosion
in
the
development
of
microwave
based
communication
technologies; the production of dielectric resonators has emerged as one of
the most rapid growth areas in electronic ceramics manufacturing [6]. Wireless
communication systems utilize microwave dielectrics for coupling, selecting
and filtering microwaves. Over the past several years there has been an
1
increased demand for smaller, lighter and temperature stable devices. Due to
the incessantly increasing demands of miniaturizing the important microwave
components such as dielectric resonators and frequency filters, intensive
researches have focused on development of highly dielectric ceramics [7].
Properties of ceramics
The physical properties of perovskite such as barium titanate, strontium
titanate etc. are discussed in this chapter. Most of the useful ferroelectrics,
such as barium titanate (BT), strontium titanate (ST) and barium strontium
titanate (BST) have the perovskite structure. The structure BT, SZ and BST
are very tolerant to cation substitution at both A and B sites of lattice, and
hence can form more complex compounds. Also ferroelectricity is mostly
observed in certain temperature regions delineated by transition (or Curie
temperature, Tc) points above which the crystals are no longer ferroelectrics.
Properties
Dielectric
Applications
Multi-Layer Ceramic Capacitor (MLCC), Dielectric Resonators
(DR), thin film resistors
Optical
Electro optic modulators, laser host switch
Ferroelectric/
Piezoelectric transducers, PTC thermistor, electrostritive
Piezoelectric
actuators
Magnetic
Magnetic memory, ferromagnets
Ionic/protonic
Solid electrolyte, SOFC electrolyte, hydrogen sensor
2
Types of ceramics
On the basis of its properties ceramics are broadly classified in to the
following types
Dielectric ceramics
Dielectric ceramics are insulating material or a very poor conductor of
electric current. Dielectrics have no loosely bound electronics, and so no
current flows through them. When they are placed in an electric field, the
positive and negative charges within the dielectric are displayed minutely in
opposite directions, which reduce the electric field within the dielectric.
Examples of dielectric ceramics are BaTiO3, SrTiO3 etc.
Microwave ceramics
Microwave ceramics are the type of ceramic material used in
microwave device application. The important characteristics of microwave
ceramics are as follows.
(1) A high dielectric constant
(2) High Q value
(3) A low temperature coefficient
Optical ceramics
Optical materials derive their utility from their response to infrared,
optical and ultraviolet light. The most obvious optical materials are glasses,
which are described in the article industrial glass, but the ceramics have been
developed for a number of optical applications. In their pure state, most
3
ceramics are wide-band gap insulators. This means that there is a large gap
of forbidden states between the energy gap of highest filled electron levels
and the energy of the next highest unoccupied level. If this ban gap is larger
than optical light energies, these ceramics will be optically transparent
(although powders and porous compacts of such ceramics will be white and
opaque due to light scattering). Two applications of optically transparent
ceramics are windows for bar-code readers at supermarkets and laser
windows.
Electroceramics
Electroceramics is a class of ceramic materials used primarily for their
electrical properties. Electroceramics that have low dielectric constants (i.e.,
low electric resistivity) are made into substrates for integrated circuits, while
electroceramics with high dielectric constants are used in capacitors. Other
electroceramics materials exhibit piezoelectricity (the development of strain
under an applied field, or vice versa) and are employed in transducers for
microphones and other products, while some posses good magnetic
properties and are suitable for transformer cores or permanent magnets.
Some electroceramics exhibit optical phenomenon, such as luminescence
( useful in fluorescent lighting) and lasing (exploited in lasers), and still others
exhibit changes in optical properties with the application of electric fields and
are therefore used extensively as modulators, demodulators and switches in
optical communications.
4
Magneto ceramics
Magnetic ceramics are oxide materials that exhibit a certain type of
permanent magnetization called ferrimagnetism. Commercially prepared
magnetic ceramics are used in variety of permanent magnet, transformer,
telecommunications and information recording applications. This article
describes the composition and properties of the principal magnetic ceramic
materials and surveys their main commercial applications.
Bio-ceramics
Bio-ceramics are ceramic products or components employed in
medical and dental applications, mainly as implants and replacements. This
article briefly describes the principal ceramic materials and surveys the uses
to which they are put in medical and dental applications.
A major category of medical ceramics is those which repair or replace
musculoskeletal hard connective tissues. For load-bearing hip prostheses, the
principal ceramic is high-density, high purity, fine grinded polycrystalline
alumina (aluminum oxide (Al2O3)). Alumina has excellent corrosion resistance,
good biocompatibility, high wear resistance and high strength. Other clinical
applications include knee prostheses, bone screws, segmental bone
replacements and components for maxillofacial reconstruction.
In contrast to dense alumina, which is nearly inert in the human body,
other Bio-ceramics can serve as porous media to support in growth of new
bone tissue, as materials that bioreact with bone, or as “scaffolds” that are
5
completely restored after establishing a template for tissue growth. When
pores exceed 100 µm in size and re interconnected, bone will grow within
pore channels and maintain vascularity. One of the best examples for bio
ceramics is hydroxyl apatite (HA).
ABO3 Perovskite
Oxide ceramics, ABO3 perovskite type in a particular, are the most
extensively used electroceramics having a variety of functionalities. No other
class of materials exhibits such a wide range of properties (dielectric,
ferroelectricity, piezoelectricity, semi-and superconductivity, ferro and antiferromagnetism, colossal magneto resistance (CMR) etc.) which can be
exploited for electronic applications. ABO3-type oxides are known to stabilize
with a wide range of A (Ca, Ba, Sr and Mg) and B (Ti, Mn, Zr and Sn) ions
with ‘A’ having larger ionic radius then that of ‘B’. These oxides exist in
structurally different forms, which are broadly classified into perovskite and
ilmenite on the basis of tolerance factor ‘τ’ by Goldschmidt [8]
τ=
r +r
2 r +r
1.1
Where rA, rB are the radii of cations ‘A’ and ‘B’’
r0 = radius of the anion O2In general, when τ = 1 ideal perovskite structure (e.g.: SrTiO3) results.
If the tolerance τ is in between 0.75 and 1.0 distortions in perovskite structure
such as tetragonal, orthorhombic, rhombohedral, monoclinic and triclinic type
get included. Such an asymmetry in the structure is necessary pre-condition
6
for properties such piezoelectricity and ferroelectricity if the tolerance factor
τ < 0.75 ilmenite-type structures (e.g.: FeTiO3, MgTiO3, CdTiO3) results. For
τ > 1 hexagonal structures (e.g.: caTiO3) results.
Crystal structure of perovskites
Perovskite in general refer to a family of compounds whose structures
are related to that of the material perovskite (CaTiO3). The ideal perovskite
structure has a cubic unit cell of side about 3.9 Ao, space group Pm3m and
contains one formula unit. The ‘B’ ions have octahedral oxygen coordination
and the ‘A’ ions have 12 fold coordination. The oxygen ions are linked to six
cations, 4A and 2B [9]. The structure of a perovskite can be visualized in
several ways. In one such representation, ‘B’ is located at the center of the
cube with oxygen at the face centers and ‘A’ at the corners. This is referred to
as B-type unit cell and is shown in figure 1.1(a). In another depiction, known
as A-type unit cell as shown in figure 1.1 (b), ‘A’ is located at the center of the
cube, oxygen at the middle of cube edges and ‘B’ at the corners. The most
useful approach is to consider it to be a framework of corner-shared (BO6)8octahedral forming dodecahedral void in which ‘A’ cations are situated (figure
1.1(c)). The dimensions of the frame work are determined by the BO distance
which in turn controls the permissible size of the‘A’. if the relative sizes of A, B
and O are appropriate, an ideal perovskite, for example SrTiO3 results;
otherwise distortion occurs leading to the formation of such related structures
as tetragonal, rhombohedral, orthorhombic, monoclinic or triclinic.
7
Figure 1.1 The Perovskite structure, ABO3
(a) B-type unit cell,, (b) A-type
A
unit cell and (c) [BO6]8- octahedral frame
work
1.2. Barium titanate
1.2.1. Introduction
Barium titanate (BaTiO3) has been of practical interest for more than 60
years because of its attractive properties. Firstly, because it is chemically and
mechanically very stable,
stable secondly because it exhibits ferroelectric properties
propertie
at and above room temperature and finally because it can be easily prepared
8
and used in the form of ceramic polycrystalline samples [10]. Due to its high
dielectric constant and low loss characteristics, barium titanate has been used
in applications, such as capacitors and multilayer capacitors (MLCs). Doped
barium titanate has found wide application in semiconductors, PTC
thermistors and piezoelectric devices, and has become one of the most
important ferroelectric ceramics. The properties of BaTiO3 have been reported
in a number of papers. Barium titanate is a member of large group of
compounds which is called the perovskite family. Ceramic materials with a
perovskite structure are very significant electronic materials. In this review
paper, a study on the barium titanate structure and the most often used
preparation methods is presented.
1.2.2. Barium Titanate Structure
The perovskite structure is adopted by many oxides that have the
chemical formula ABO3. Barium titanate is a member of this perovskite family.
This structure takes its name from the mineral perovskite, CaTiO3. The
general crystal structure is a primitive cube, with the A-larger cation in the
corner, the B-smaller cation in the middle of the cube, and the anion,
commonly oxygen, in the centre of the face edges, where A is a monovalent,
divalent or trivalent metal and B a pentavalent, tetravalent or trivalent element,
respectively. Geometrical packing of ions in the lattice is a very important
factor determining the structure type. The perovskite structure can be
considered as a three-dimensional frame work of BO6 octahedra (figure 1.2
a), but it can also be regarded as a cubic close packed arrangement of A and
O ions, with the B ions filling the interstitial positions (figure 1.2 b) [10, 11].
9
The unit cell of the cubic perovskite type lattice is shown on figure 1.2 [12]. It
can be detected that the coordination number of cation A is 12 and for cation
B is 6.
(a)
(b)
Figure 1.2 Cubic perovskite-type structures ABO3
1.2.3. Structural phase transitions in barium titanate
Barium titanate is the first discovered ferroelectric perovskite. Its
ferroelectric properties are connected with a series of three structural phase
transitions. The Curie point TC, of barium titanate is 120 oC. Above 120 oC the
original cubic cell is stable up to 1460 oC. Above this temperature a hexagonal
structure is stable [13]. When the temperature is below the Curie point,
crystallographic changes in BaTiO3 occur, first at about 120 oC a ferroelectric
transition between the cubic, paraelectric and ferroelectric phase of tetragonal
structure takes place. At 5 oC, the transition to a phase of the orthorhombic
structure goes on and at -90 oC to the low temperature phase having a trigonal
10
structure [10, 14]. Figure 1.3 illustrates crystallographic changes of BaTiO3
[10]. The dotted lines in (b), (c) and (d) delineate the original cubic cell [10]
Figure 1.3 Unit cells of the four phases of BaTiO3
(a) Cubic, stable above 120 oC,
(b) Tetragonal, stable between 120 oC and 5 oC
(c) Orthorhombic, stable between 5 oC and -90 oC
(d) Rhombohedral, stable below -90 oC
At the Curie point Ti-ions are all in equilibrium positions in the center of
their octahedra, but with a decrease of the temperature, Ti-ions jumps
between energetically favorable positions out the of octahedron center, as
shown in figure 1.4 [12].
11
Figure 1.4 Ion positions in tetragonal BaTiO3
These changes can be related to structural distorsion, lengthening of
the bonds or their shortening, so crystallographic dimensions of the barium
titanate lattice change with temperature [14]. A lot of papers indicate that the
mechanisms responsible for these phase transitions in BaTiO3 are complex.
The temperature dependence of the lattice constants of BaTiO3 in the
four phases is depicted in figure 1.5 [10].
Figure 1.5 Lattice constants of BaTiO3 as function of temperature
12
BaTiO3 was the first material used for manufacturing dielectric
ceramics capacitors, multilayer capacitors etc. It is used for this application
due to its high dielectric constant and low dielectric loss. The values of the
dielectric constant depend on the synthesis route, which means purity,
density, grain size etc [15]. The dielectric constant is also dependent on
temperature, frequency and dopants. Figure 1.6 shows the temperature
dependence of the dielectric constant measured with a small field along the
pseudo-cubic edge [16]. In this figure, only the values of the dielectric
constant in the tetragonal phase have a clear meaning, as they were
measured on carefully selected single-domain crystals with the proper
orientation.
Figure 1.6 Dielectric constants of BaTiO3 as a function of temperature
13
1.3. Strontium titanate
SrTiO3 is high temperature ceramic material and used in microwave
devices; such as tunable microwave phase shifters, filters and resonators [1719]. Dielectric permittivity makes strontium titanate (ST) an attractive material
for applications in tunable electronic components, particularly in several
microwave devices including filters and phase shifting elements in phased
array antennas. Due to the high dielectric permittivity, these devices may be
miniaturized and cost-effective. However, ST has limited application in the
microwave electronic industry, since adequate tunability (change in the
permittivity induced by a DC field) is achieved only below 80 K. The
temperature range of high tenability can be shifted towards high temperature;
such ceramics have a number of advantages over single crystals. Apart from
the ease and speed with which they can be made, requiring less expensive
and time – consuming melting and pulling processes, it is possible to
manufacture solid solutions over a very wide range, modifying electrical and
mechanical properties. Ferroelectric and non–ferroelectric ceramics based on
the perovskite structure (ABO3) but using different combinations of A and B
ions and exploiting solid solutions have become used for many applications,
including electoptic and pyroelectric.
14
1.4 Literature review
S. Balakumar et al [20] studied the single crystals of BaSrTiO3 and the
crystals are grown by flux technique using KF as the flux. Morphology
variation with Sr content and their growth conditions have also been
investigated. Growth mechanism of the grown crystals was discussed with the
help of micromorphologies observed on the habit faces. J.W. Liou et al [21]
reported on Ba1-xSrxTiO3 (x = 0 to 1) ferroelectric ceramics doped with 1.0
mol.% MgO and 0.05 mol.% MnO2 with a rate-controlling of sintering profile.
The decrement in lattice constant was observed as the strontium molar
fraction increases in this system. This is due to the smaller strontium ionic
radius than that of barium ion. They measured the temperature dependence
of the dielectric constant at 10 KHz and observed a linear relation of the Curie
temperature of the BST system to the Sr fraction for x = 0.75 and the dipole
relaxation of the composition with x = 0.25 at frequencies above 1 MHz.
L. Zhang et al [22] discussed the preparation of BaxSr1-xTiO3 ceramics
by solgel method with different grain size. The dielectric relaxation of the
samples was observed at room temperature and the experimental results
showed that the relaxation frequency increases with decreasing grain size. An
explanation of this phenomenon was given by an electric potential model that
qualitatively agrees with the experimental results. The structures and
dielectric properties of Ba1-xSrxTiO3 ceramic solid solutions in the whole x
range and their dependence on the material processing are investigated by
Liqin Zhou et al [23]. They noted that, a short time pre-calcination milling–
15
mixing of the precursor powders causes compositional inhomogeneity and
thus a diffuse phase transition.
M. Viviani et al [24] prepared the barium titanate powders by wet
chemical route method. The surface composition of ceramic powders
prepared via the low temperature Aqueous Synthesis, with formula
Ba(1-x)SrxTiO3 (x=0-1), have been investigated. They measured the total
relative amount of carbonate by X-ray diffraction and compared to that
present on surface. The influence of Sr on the crystal structure,
microstructure, and thermal expansion of Ba0.98Sr0.02Ti1-xMnxO3 ceramics
(0≤x≤0.02) was investigated by H.T Langhammer et al [25]. Compared to Sr“free” samples the transition region between tetragonal and hexagonal phase
shifting was observed for higher concentrations of Mn. They used the
Goldschmidt tolerance factor to discuss the stabilization of the tetragonal
phase by the Sr impurity.
DC electric field effects on the real part of the relative dielectric
constant and the loss factor of barium strontium titanate for application in
phased array antennas have been studied by Yih-Chien Chen et al [26]. The
real part of the relative dielectric constant of the specimens decrement was
observed with increasing applied DC field and the loss factor under bias DC
electric field is slighter than the real part of the relative dielectric constant
does. They got the tunability is about 24% for barium strontium titanate doped
with 1 wt.% Al2O3 content.
16
S. Suasmoro et al [27] synthesized the mixed titanates (Ba1-xSrxTiO3)
via calcination of oxalate co-precipitated precursors. On heating there were
three thermal events observed: T<250 °C corresponds to the evaporation of
trapped water and dehydration, T = 250 °C–450 °C corresponds to the
decomposition of oxalate and the formation of an intermediate phase where
the composition is close to [Ba1-xSrx]2Ti2O5.CO3 and T=600 °C–700 °C
corresponds to the carbonate decomposition and the formation of
Ba1-xSrxTiO3 phase. From the XRD data they observed a single phase of
perovskite structure in which their tetragonality decrement with increasing
concentration of Sr2+ incorporated in Ba2+ site. The role of hydrogen in
strontium titanate and barium strontium titanate is analysed J.F. Scott et al
[28], for both ceramics and single crystals. From infrared and secondharmonic generation experiments established the relationship between
hydrogen and oxygen vacancies. The infrared data exhibit strong absorption
in the 3000–6000 cm-1 region due to polarons.
O.P Thakur et al [29] reported the detailed dielectric studies carried out
on a Barium strontium titanate (BST) (95:5) composition. The material was
synthesized by conventional ceramic method and microwave processing. In
the microwave processing technique the resulting material have high density
and improved microstructure and dielectric properties. They studied the
dielectric properties as a function of frequency and temperature and welldefined ferroelectric behavior. They attained slightly higher Curie temperature
for microwave sintered material.
17
In
barium
strontium
titanate
(BaxSr1-xTiO3)
ceramics,
obvious
tunabilities of both dielectric permittivity and loss tangent under external DC
bias field were observed by Xiaoyong Wei et al [30]. They studied the
temperature and frequency dependences of nonlinear dielectric properties
and the nonlinear variation of dielectric properties under external DC bias
field. The nonlinear variation of dielectric properties under external DC bias
field was tentatively explained. Xiaofeng Liang et al [31] reported the effect of
doping Al2O3 on the Ba0.6Sr0.4TiO3 ceramics. A strong correlation was
observed between the average grain size and Al2O3 content. His results
shows that Al3+ behaves as a grain-growth helper below a certain doping level
and at the same time lowers the dielectric constant. He modified both the
dielectric loss and tunable properties by the addition of Al2O3. At Al2O3 doping
level up to 0.8 wt.% the minimum of the dissipation factor was achieved by
them.
Sol–gel synthesis of Ba1-xSrxTiO3 (x=0.1, 0.2) ceramic fibers with a
diameter of 6–10 µm using catechol-complexed titanium isopropoxide, barium
acetate hydrate and strontium acetate hydrate as precursors has been
investigated by Qifang Lu et al [32]. They studied the Microstructural
development of barium strontium titanate (Ba1-xSrxTiO3) ceramic fibers as a
function of strontium concentrations. From X-ray diffraction a well-developed
cubic phase of (Ba, Sr)TiO3 was noted at 1100 °C. C.B Samantaray et al [33]
studied the Eu3+-doped BST (65:35), and BST (80:20) at different
compositional ratios of BaTiO3, SrTiO3 and Eu2O3. Both structural and
microsurface analyses were done using X-ray diffraction and scanning
18
electron microscopy, respectively. At room temperature they investigated the
photoluminescence properties of doped samples.
H.V Alexandru et al [34] reported the barium–strontium titanate (BST),
ternary ceramic compounds with the molar formula (Ba1-xSrx)TiO3 with
x= x=0.25, 0.50 and 0.75. The samples were prepared by solid-state reaction
method from raw materials. They manufactured and analyzed the samples
with two type of thermal treatment at 1230°C and 1260 °C. When the
strontium ion (Sr2+) enters substitutionally on the Ba2+ site, a small decrease
in the unit cell volume with the Sr content was observed. They performed the
dielectric measurements at low and high frequencies and the increasing
concentration of Sr ion leads to a shift of the Curie point below room
temperature. S. Tusseau-Nenez et al [35] investigated the effect of grain size
on the dielectric properties of Ba0.6Sr0.4TiO3 ceramics. Attrition milling was
chosen to obtain nanometre particle size from micrometre particle size
powders. Fine grained ceramics are obtained by hot uniaxial pressing. The
properties of these samples are evaluated in the range of MHz.
L Szymczak et al [36] studied the sintering effect on transition
parameters and dielectric characteristics of Ba0.8Sr0.2TiO3 ceramics were
prepared by the conventional mixed-oxide processing technique. By
measuring the dielectric constant, the loss factor and the remanent
polarization as a function of temperature, the phase transitions and dielectric
properties were examined. Ultrafine Ba0.5Sr0.5TiO3 powders were prepared by
Cai Shen et al [37] using barium nitrate, strontium nitrate, tetrabutyl titanate,
19
and ammonia via citrate–nitrate combustion process at low temperature (500
°C). Spark plasma sintering was carried out to obtain the ultrafine crystalline
BST and to improve the dielectric property. It was found that the sintered
barium strontium titanate showed ultrafine crystalline microstructure. At 25 °C,
the dielectric constant and dissipation factor of the sintered sample were 1533
and 0.0063 at 10 kHz.
Jae-Ho Jeon [38] investigated the effect of SrTiO3 concentration and
sintering temperature on the microstructure and dielectric constant of
Ba1-xSrxTiO3 materials at Curie temperature. The maximum dielectric constant
values are obtained at the Curie temperature, which was sintered 1400°C
temperature for the composition x=0.4. They discussed the dielectric
properties of Ba1-xSrxTiO3 materials in terms of SrTiO3 concentration and
microstructure. P. Bomlai et al [39] studied the antimony and manganesedoped barium strontium titanate ceramics, containing silica and titania
sintering additives, were prepared using various heating rates between 1–20
°C/min. They found the crystalline secondary phases of Ba6Ti17O40 and
Ba2TiSi2O8, due to the sintering additives in all samples. The amount of these
secondary phases depended on the heating rate. There was a progressive
decrease in grain size from a maximum of 19.7 to 8.3 µm and also an
apparently more uniform grain size with increasing heating rate.
Jian Quan Qi et al [40] reported the wet chemical synthesis technique
for large-scale fabrication of perovskite barium strontium titanate nanoparticles near room temperature and under ambient pressure. Particle size
20
and crystallinity of the particles were controllable by changing the processing
parameters. These particles are well-crystallized, chemically stoichiometric
and ~50 nm in diameter. Ceramic composites of barium strontium titanate
(Ba0.3Sr0.7TiO3) mixed with magnesium titanate (MgTiO3, in a range of 5 to 40
mol%) were prepared and studied by T.N. Lin et al [41]. When sintered at
1250 °C, the dielectric constant and microwave property Qxf values of
composites decrement was observed with increasing MgTiO3 content due to
the dilution effect of MgTiO3 phase. When sintered at 1350 °C, microwave
property enhancement was noted due to the single phase of Ba0.3Sr0.7TiO3
and large grained structures.
The crystalline structure and dielectric properties of BaxSr1-xTiO3
ceramics with x = 0.45, 0.5, 0.6, 0.65, 0.8, 0.9 were investigated by Chunlin
Fu et al [42]. The a- and c-axis lattice constants of BaxSr1-xTiO3 ceramics were
calculated and it is found that the crystal structures are tetragonal phase when
x = 0.65 at room temperature. The values of coercive electric field and
remanent polarization (Pr) increase as Ba/Sr ratio of BaxSr1-xTiO3 ceramics
increment was found except for x = 0.5, which are due to the decrease in
grain size and the difference between the radius of Ba2+ and that of Sr2+. H.
Frayssignes et al [43] investigated the variation of internal friction and shear
modulus versus temperature in a low frequency range for BaxSr1-xTiO3
systems (x = 0.4, 0.55, 0.7 and 0.8). An experimental phase diagram has
been established for BaxSr1-xTiO3 systems.
21
Effects of MgO doping have been investigated for the composition x =
0.8. Helmi Abdelkefi et al [44] reported the detailed dielectric studies carried
out on barium strontium titanate BaxSr1-xTiO3 (BST) (80:20) and (60:40)
compositions. He also presented the data on dielectric permittivity
measurements made in the temperature range from 80 to 600 K with
frequencies from 0.1 to 200 KHz, respectively. It follows the Curie–Weiss law
above the transition temperature in the paraelectric region. Rui-Hong Liang et
al [45] studied the acceptor (Mn-, Co-) doped and donor Y-doped
Ba0.6Sr0.4TiO3. The samples were prepared by traditional ceramic processing
and their structural, surface morphological, dielectric and tunable properties
were investigated. The results show all dopants to have a strong effect on the
average grain size. The Curie temperature of all doped specimens decreases
and their temperature spectrum broadens. Loss tangent at 15 °C and low
frequency (10 KHz) increases for all doped specimens whereas the loss
tangent at high frequency (100 MHz) is not affected.
X.G. Tang et al [46] examined the ferroelectric-relaxor behavior of
(Ba0.65Sr0.35)(Zr0.35Ti0.65)O3 (BSZT) ceramics in the temperature range from 80
to 380 K. A broad dielectric maximum, which shifts to higher temperature with
increasing frequency, signifies the relaxor-type behavior of these ceramics. At
300 K and 10 kHz, the dielectric constant and loss tanδ are 1100 and 0.0015,
respectively. Huyong Tian
et al [47] performed a study on the dielectric
properties of BaxSr1-xTiO3–Mg0.9Zn0.1O (BST–MZO) composite ceramics
derived from core–shell structured nanopowders with the shell of zinc doped
MgO and core of BaxSr1-xTiO3. It was found that the ceramics exhibit a
22
significant improvement in dielectric response under a DC electric field and
the Curie temperature decrement more significantly in the BST–MZO
composite ceramics compared to that of pure BST ceramics.
Yih-Chien Chen et al [48] studied the effect of DC biasing field on the
real part of the relative dielectric constant of (Ba0.6Sr0.4)TiO3 and ZrO2-doped
(Ba0.6Sr0.4)TiO3 for application in phased array antennas. The maximum grain
size was obtained for (Ba0.6Sr0.4)TiO3 doped with 0.5 wt.% ZrO2 content. The
real part of the relative dielectric constant of the specimens’ decrement was
observed with increasing applied DC biasing field. The measured tunability of
the real part of the relative dielectric constant for (Ba0.6Sr0.4)TiO3 doped with
0.5 wt.% ZrO2 content is about 45.74% and 45.88% at 20 and 40 MHz,
respectively. S. Kongtaweelert et al [49] studied the effects of calcining and
sintering conditions on the relative permittivity and sintering behaviors of
BSTs which were prepared by the solid solution method. BST (BaxSr1-xTiO3, x
= 0.6, 0.7 and 0.8) was synthesized using an established solid-state reaction
method. Crystallinity and lattice parameter of the calcined powders was
improved by increasing the calcining temperature, as indicated by the
increase in intensity of the X-ray diffraction peak. The particle size distribution
increasing at higher calcining temperature was also observed.
Influence of La2O3 additions on the microwave dielectric properties of
Ba0.6Sr0.4TiO3
(BST)
mixed
with
magnesium
oxide
composites
was
investigated by Xiaohong Wang et al [50]. With increasing quantities of
lanthanum oxide x (wt%), the lattice constant of BST–MgO material
23
decrement was observed. The lattice constant is minimum at x = 0.4 and then
increment was found up to a maximum at x = 1.0. The solubility of MgO in
BST is analyzed from X-ray diffraction patterns with increasing amounts of
La2O3. The grain sizes of the BST decrement was found with increasing
amounts of La2O3. The dielectric properties of BST–MgO–La2O3 indicate that
La2O3 additives shift the Curie point towards lower temperatures. When the
doping amount of La2O3 is 0.2 wt%, BST–MgO composite has the following
properties: dielectric constant = 94.05, tanδ = 0.012 (at 2.853 GHz) and the
dielectric constant tunability = 16.26% (under electric field 3.57 kV/mm), which
is suitable for ferroelectric phase shifter.
A. Ioachim et al [51] studied the Ba1-xSrxTiO3 solid solutions by solidstate reaction from raw materials. Four compositions with x = 0.25, 0.50, 0.75
and 0.90 have been investigated. The morphology, grain size distribution,
porous structure and elemental composition of the sintered ceramics were
analyzed by using scanning electron microscopy (SEM) and energydispersive X-ray (EDX) microanalysis. The temperature dependence of
permittivity and of dielectric loss tangent at low frequency (1 kHz) showed that
the decrease of Curie temperature with increase of Sr content. The dielectric
and
ferroelectric
properties
of
Ba1-xSrxTiO3
(BST)
ceramics
and
Ba1-2xSrxCaxTiO3 (BSCT) ceramics have been investigated by Sining Yun et al
[52]. At low temperature phase transitions of BST ceramics disappearance
was noted after Ca2+ substitution while the high temperature transition is
diffused and relaxed, which becomes more obvious with increasing.
24
Zhao Chen et al [53] reported the effects of rare earth oxide (Y2O3 and
Dy2O3) on the microstructure and dielectric properties of Ba0.7Sr0.3 TiO3 (BST).
The strong correlation was observed between the dopant concentration and
materials properties. It was also found out that different rare earth oxides and
amount had different influences on BST. Tao Hu et al [54] studied composite
made of ferroelectric ceramic (barium strontium titanate, BST) and
thermoplastic polymer (polyphenylene sulfide, PPS) with various BST
loadings up to 70 wt.%. The microstructures exhibited uniform distribution of
ceramic particles in the polymer matrix. RF measurements showed that the
relative permittivity and loss tangent of the composites gradually increase with
increasing BST loading. At 1 GHz a composite with 70 wt.% BST loading had
relative permittivity and a loss tangent of 13.5 and 0.0025, respectively.
Shaojun Liu et al [55] reported the results of an investigation of the
structure–property relationship of vanadium (donor) and scandium (acceptor)
doped Ba0.7Sr0.3TiO3. The Curie temperature (Tc) of Ba0.7Sr0.3TiO3 decrement
was observed with increasing dopant concentration from a Tc of 40 °C for
undoped material to 18 °C for 4 mol.% V and 22 °C for 4 mol.% Sc (i.e.,
Ba0.7Sr0.3Ti0.96V0.04O3, Ba0.7Sr0.3Ti0.96Sc0.04O3). Adelina Ianculescu et al [56]
synthesized the Ba1-xSrxTiO3 (x = 0, 0.20, 0.25, 0.30 and 0.35) nanopowders
by Pechini method. Ceramic pellets with relative density of 85–93% were
obtained after sintering at 1350 °C for 3 h. High values of the dielectric
constants (of 1500–12,000), low losses at the room temperature and a shift of
the ferro-para phase transition temperature in the range of 7–127 °C with x
decreasing were found.
25
Zheng Wang et al
[57]
synthesized
Ba1-xSrxTiO3 (x =
0.3)
nanopowders, by citric acid gel method. Infrared (IR) spectroscopy, differential
scanning calorimetry (DSC), X-ray diffraction (XRD) Thermo gravimetric
analysis (TGA) and scanning electron microscopy (SEM) were used to
characterize the thermal decomposition behavior, the crystallization process
and the particle size and morphology of the calcined powders. D.R. Patil et al
[58] studied the barium strontium titanate (BST) samples with the molar
formula Ba1-xSrxTiO3 in which x varies as 0.1, 0.2 and 0.3 mol%. The samples
were prepared by standard double sintering ceramic method. They confirmed
the tetragonal perovskite phase formation by XRD. The lattice parameters a
and c were calculated from the XRD data. The DC resistivity was measured
as a function of temperature. The dielectric constant and loss tangent were
studied as a function of frequency and temperature. To understand the
conduction mechanism in the samples AC conductivity was measured from
dielectric data at room temperature in the frequency range 100 Hz to 1 MHz.
Jong-Yoon Ha et al [59] studied the (Ba0.6Sr0.4)(Ti1-xZrx)O3 0.05 ≤ x =
≤ 0.3 ferroelectric materials. Curie point shifting was observed to a negative
value as increasing Zr content in (Ba0.6Sr0.4)(Ti1-xZrx)O3 system. When Zr
substituted 0.1 mol, the dielectric constant, dielectric loss, tunability and Curie
point were 4500, 0.0005, 63%, -1.6 °C and 1260, respectively. This
composition shows excellent microwave dielectric properties than those of
(Ba0.6Sr0.4)TiO3 ferroelectrics, which are limelight materials for tunable devices
such as varactors, phase shifters and frequency agile filters, etc. Chen Zhang
26
et al [60] studied the Sb2O3-doped (Ba0.992-xSrxY0.008)TiO3.004 dielectric
ceramics . The samples were prepared by conventional solid state ceramic
route. The structure was identified by X-ray diffraction method and SEM was
also employed to observe the surface morphologies. The dielectric properties
were investigated with variation of Sb2O3 doping content and Ba/Sr ratio. The
studies indicate that the relative dielectric constant as well as dielectric loss
initially increases with increasing Sb2O3 content and then decreases.
Chaoliang Mao et al [61] studied the pure perovskite phase
Ba0.70Sr0.30TiO3 (BST) powders. The samples were successfully synthesized
by molten-salt method in NaCl–KCl flux at a low temperature of 850 °C for 2
h, which is 300 °C lower than that of the conventional solid-state reaction.
This simple process involved mixing of the raw materials and salts in a certain
proportion. Subsequent calcining of the mixtures led to BST powders at 800–
900 °C. XRD and SEM techniques are used to characterize the phase and
morphology of the fabricated BST powders, respectively. R. Pazik et al [62]
synthesised the Ba1-xSrxTiO3 (BST) nanopowders with composition x=0.1–0.4.
The samples were prepared by using microwave driven hydrothermal
synthesis (MDHS). MDHS method allows obtaining the nanocrystalline
powder samples during relatively short period of time (10 min) and therefore
MDHS was optimized. In case of the phase evolution studies the XRD
measurements were performed. The average size of crystallites was
estimated using Scherrer equation. TEM and SEM were taken for the detailed
analysis of the grain size, surface and morphology.
27
Sining Yun et al [63] studied the barium strontium titanate (BST) with
the molar formula (Ba0.8Sr0.2TiO3). The samples were prepared by two
different processing methods: mixed-oxide (BST-MO) and reaction-sintering
(BST-RS). X-ray powder diffraction was performed to show the differences in
grain size and crystal symmetry for both these ceramics. Shanming Ke et al
[64] reported the Ba0.6Sr0.4TiO3-MgTiO3 (BST-MT) composite ceramics. The
samples were prepared by reaction of BST powders with molten MgCl2. The
sintered composites exhibit a five orders of magnitude decrease in the DC
resistivity as the relative humidity (RH) was increased from 5 to 92%.
K.A. Razak et al [65] studied the preparation procedure, structural and
dielectric properties of hydrothermally derived BaxSr1-xTiO3 (BST). BST with
initial Ba compositions of 75, 80, 85 and 90 mol.% were prepared by a high
temperature hydrothermal synthesis. The obtained powders were pressed into
pellet, cold isostatically pressed and sintered at 1200 °C for 3 h. The phase
compositions and lattice parameters of the as prepared powders and sintered
samples were analysed using X-ray diffractometry. Dielectric constant and
polarization increment was observed with increasing Ba content but also
affected by the electronic state and grain size of the compositions. Mingli Li et
al [66] discussed the barium strontium titanate (Ba0.6Sr0.4TiO3, BST) nanopowders. The samples were prepared using Ba(NO3)2, Sr(NO3)2, oxalic acid
dehydrate, and tetrabutyl titanate (Ti(OC4H9)4) as precursors by the chemical
co-precipitation method. The product was characterized by thermogravimetrydifferential scanning calorimetry (TG-DSC) thermal analyses, X-ray diffraction
(XRD), and scanning electron microscopy (SEM). The Ba0.6Sr0.4TiO3-MgTiO3
28
(BST-MT) bulk composite ceramics doped by Mn were obtained by the
traditional solid phase method. The XRD patterns demonstrated that Mndoped BST was unable to change the perovskite crystalline structure of BST
materials. Two effects of Mn doping on the dielectric properties of the BST-MT
composite ceramics were observed. At low Mn doping concentrations
(<1.5%), Mn mainly acted as an acceptor dopant to replace Ti at the B site of
ABO3 perovskite structure, leading to a diffused phase transition. It was also
observed that the grain size increased drastically as the Mn content increased
and thus caused the decrease of dielectric loss. At higher Mn doping
concentrations (>1.5%), the grain size decreased and the suppression of
permittivity and the drastic increase of the dielectric losses were observed,
which indicated a “composite” mixing effect.
Manoj Kumar et al [67] reported the structural, dielectric and
ferroelectric properties of zirconium (Zr)-modified barium strontium titanate
(Ba0.9Sr0.1ZrxTi1-xO3) ceramics with varying Zr content as x=0, 0.20, 0.25 and
0.30 synthesized by the sol–gel method. The polarization vs electric field
shows a decrease in remnant polarization with increasing Zr content was
observed. Shanming Ke et al [68] discussed the Ba0.6Sr0.4TiO3–MgTiO3
composites were prepared by reaction of BST powders with molten MgCl2 at
various MgCl2/BST molar ratios at 600 and 800 °C in air. The products were
examined by X-ray diffraction (XRD) and scanning electron microscopy
(SEM). It is shown that the ceramic composites have a relaxor-like
ferroelectric transition as MgTiO3 is above 8 mol%. The addition of MgTiO3
has greatly reduced the dielectric loss of BST ceramics. The BST–MT
29
ceramics could be a promising candidate for tunable microwave device
applications.
Effect of the applied DC control voltage on the capacitance of the
ferroelectric-based ceramic disc capacitor for application in voltage control
oscillator has been studied by Yih-Chien Chen et al [69]. The ceramic disc
capacitor is composed of Al2O3-doped barium strontium titanate. The
capacitance of the ceramic disc capacitor decrement was noticed with
increasing applied control voltage. A simple process has been investigated
by Chaoliang Mao et al [70] to synthesize nanocrystalline Ba0.70Sr0.30TiO3
(BST) powders via a nonhydrolytic sol–gel method by using barium acetate,
strontium
acetate,
titanium
tetrabutoxide,
ethanol,
acetic
acid
and
acetylacetone as the starting materials. Thermogravimetry (TG) and
differential thermal analysis (DTA) were used to examine the decomposition
behaviour of the xerogel. The particle size of BST is close to 30 nm calculated
by X-ray diffraction (XRD) and confirmed by transmission electron microscopy
(TEM). Using these nanocrystalline BST powders, dense BST ceramics with
ultrafine grains were obtained by spark plasma sintering (SPS). The grain size
effect on the dielectric properties was studied. It was shown that as the grain
size decreased, the transition temperature (Tc) and the dielectric constant
decreased and the transition became diffuse.
The effects of cationic substitution of niobium for titanium in
(Ba0.80Sr0.20)TiO3 composition on the structural and dielectric properties as
well as on the electrical conductivity and Seebeck effect were investigated by
30
L. Szymczak et al [71]. The analysis of the results of electric conductivity and
Seebeck coefficient leads to the conclusion that Nb5+ ions, reducing the
conductivity of about two orders, are playing role of donors.
The structural, microstructural and electrical properties of BaCe1-xTixO3
materials were investigated by P. Pasierb Chen et al [72]. The series of
materials with different titanium concentrations x (0–0.3) were prepared by
solid-state reaction method. The structural studies by X-ray diffraction have
shown that undoped material crystallizes in orthorhombic phase, while the
increasing concentration of Ti dopant up to x = 0.2 leads to the ordering of the
structure to phases with higher symmetries (tetragonal and even cubic). The
estimated solubility limit was found to be not higher than 20 at.% of Ti.
Microstructure observations by scanning electron microscopy and linear
contraction determination have shown the strong influence of Ti dopant on
microstructure and an improvement of sinterability. C.R.K. Mohan et al [73]
discussed the BaxSr1-xTiO3 (x=0.6, 0.75, 0.80, 0.85 and 0.9) compositions.
The samples were prepared by solid-state reaction route using controlled
heating and cooling. Density optimization by varying sintering temperature
was achieved. X-ray diffraction (XRD) analysis shows the phase pure
materials. The lattice constant decreases from 3.9868 Å (x=0.90) to 3.9449 Å
(x=0.60) with increasing Sr2+; the tetragonal distortion also decreases. The
peak broadening in Sr2+ rich compositions indicates that diffused transitions
and is attributed to the disorder in the arrangement of cations at A-site.
31
The effect of DC electric field and temperature on dielectric
properties of sol–gel derived (Ba0.65Sr0.35)TiO3 (BST35) ceramics were
investigated by Q.X. Liu et al [74]. It is observed that the phase transition of
BST35 sample is a weak diffusion phase transition. The loss tangent values
were
decreased
significantly
at
temperatures
above
the
transition
temperature. The dielectric constant was greatly suppressed under DC
electric field. At room temperature (293 K) and 20 kVcm-1, the tunability (k)
and figure of merit (FOM) were 60% and 60. Wanhui Zhang et al [75]
discussed the stoichiometric BaxSr1-xTiO3 ceramics (x=0.7, 0.8, and 0.9) have
been prepared by solid-state reaction method. It has been observed in our
experiments that the photoluminescence (PL) spectra in such samples at
room temperature were centered on 800 nm; the results have been found to
differ from observations in current literature. An explanation of this
phenomenon was given, and the origin of such photoluminescence is
ascribed to certain defects existing in these compounds, such as oxygen
vacancies.
D.S. Jung et al [76] reported the Nano-sized Ba1-xSrxTiO3 (BST)
powders were prepared by flame spray pyrolysis using “CA-assisted” spray
solution. The effects of the mole ratios of Ba to Sr components on the mean
sizes, morphologies, and crystal structures of the BST powder prepared by
flame spray pyrolysis were investigated. The precursor powders obtained by
flame spray pyrolysis had large size, fractured and hollow structures
irrespective of the mole ratios of Ba to Sr components. The mean sizes of the
milled BaTiO3, Ba0.5Sr0.5TiO3 and SrTiO3 particles were each 110, 32, and 48
32
nm. Phase pure BST powder was obtained at a post-treatment temperature of
1000 °C irrespective of the mole ratios of Ba to Sr components. The BaTiO3
powder had tetragonal crystal structure. On the other hand, the BST except
for the BaTiO3 composition had cubic crystal structures at post-treatment
temperature of 1000 °C.
Huang Jiquan et al [77] discussed the Ba0.55Sr0.45TiO3 (BST)
precursors were synthesized via a polyvinyl alcohol (PVA) modified solprecipitation route. And they obtained precursors were then calcined in air at
temperatures ranging from 400 to 800 °C. The formation mechanism of BST
phase was investigated using X-ray diffraction (XRD), while the effect of PVA
on the particle morphology of the BST phase was studied by using scanning
electron microscopy (SEM). The results show that the introduction of PVA
significantly affects the morphology of the BST powders. High purity
nanocrystalline powders were synthesized by calcination of the BST
precursors encapsulated by PVA gel, with narrow particle distribution (30–80
nm) and being nearly free of agglomeration. While, large particles (40–170
nm) with evident agglomeration were obtained from the solution without PVA.
Hao Xue et al [78] reported the textured barium strontium titanate ceramics
with a high degree of <0 0 1> preferred orientation were prepared by
templated grain growth technique. The structure and dielectric tunable
properties of <0 0 1> textured BST ceramic were investigated. The dielectric
tunability of <0 0 1> textured BST ceramic were significantly increased
compared to random oriented ceramic.
33
ZnO additions to Ba0.3Sr0.7TiO3 ceramics have been studied by Guixia
Dong et al [79] in order to determine the role of this dopant on dielectric
property and energy storage density development The temperature and
frequency dependences of dielectric constant, the breakdown strength, and
the dielectric dissipation were measured. The crystalline structure and
morphology were also investigated by XRD and SEM, respectively. Benoît
Fournaud et al [80] developed a method for preparing nanomaterials shaped
into powder and balls (diameter: 1–2 mm). The synthesis is performed by an
oil-drop process using a sol–gel method. BaTiO3 and Ba0.67Sr0.33TiO3
perovskite-type compounds synthesized by both procedures present the
same physical–chemical properties.
Composite ceramics with a nominal composition of Ba0.6Sr0.4TiO3 + x
wt.% MgO (x = 0–60) were prepared by Qing Xu et al [81] using superfine
Ba0.6Sr0.4TiO3 powder derived from a citrate method and superfine MgO
powder. The sinterability, structure and nonlinear dielectric properties of the
specimens were investigated. Adopting the superfine powders was found to
be effective in promoting the sinterability of the composites. The specimens
sintered at 1230 °C attained relative densities of around 95%. The relatively
low sintering temperature of the composites suppressed the diffusion of Mg2+
into the lattice of the Ba0.6Sr0.4TiO3 phase, which consequently led to a small
shift of the temperature for dielectric constant maximum (Tm) to -10 °C.
Ba0.68Sr0.32TiO3 ceramics of perovskite structure are prepared by solid
state reaction method with addition of x mol% Sm2O3, and their dielectric
34
properties are investigated by Yuanliang Li et al [82]. It is found that,
integrating with the lattice parameters and tolerance factor τ there is an
alternation of substitution preference of Sm3+ for the host cations in perovskite
lattice. Owing to the replacement of Sm3+ ions for Ba2+ ions in the A site, Tc
rises with the increase of Sm2O3 doping when the doping content is below 0.1
mol%; meanwhile, when the content is more than 0.1 mol%, Sm3+ ions tend to
occupy the B-site, causing a drop of Tc. Owing to the modifications of Sm3+
doping, dielectric constant, dissipation factor and temperature stability of
dissipation factor are influenced remarkably, making it a superior candidate
for environment-friendly applications. Moreover, the creation of oxygen
vacancies controls the dielectric constant when the addition is above 0.1
mol%, so the dielectric constant decreases with increasing of samarium.
Siwei Wang et [83] al discussed the composite ceramics based on
Ba0.6Sr0.4TiO3 and BaZn6Ti6O19 are prepared by conventional solid-state
reaction method. The sintering temperatures of the composites can be
decreased by 100–200 °C compared with pure Ba0.6Sr0.4TiO3. The
microstructures and dielectric tunable properties of the composites have been
investigated. Ba0.6Sr0.4TiO3 powder was synthesized by Xiao-Fei Zhang et al
[84] a citrate method. The phase development was examined with respect to
calcining temperature and heating rate during the calcining process. It was
found that keeping relatively low heating rates =0.7 °C/min during the
calcining process after 300 °C was favorable to a sufficient decomposition of
(Ba,Sr)2Ti2O5·CO3 intermediate phase at low temperatures and consequently
led to the formation of a pure perovskite phase at 550 °C. Ba0.6Sr0.4TiO3
35
powder calcined at the temperature under the heating rate of 0.7 °C/min
showed a superfine and uniform particle morphology and high sintering
reactivity.
X.H. Zuo et al [85] studied the Ba0.6Sr0.4TiO3 (BST) nanopowders
preparation by using the modified citrate method with ammonium nitrate as a
combustion promoter, and the formation mechanism, phase evolution, and
particle size have been investigated using TG/DTA, XRD, and SEM. Kensaku
Sonoda et al [86] discussed the ceramic–polymer composites. The samples
were
fabricated
from
barium
strontium
titanate
powder
(BST)
and
polypropylene-graft-poly (styrene-stat-divinylbenzene) (ER) using a twinscrew extruder.
Wei Li et al [87] studied the Ba(1-x)SrxTiO3 (x = 0, 0.1, 0.2, 0.3)
nanopowders were synthesized from alkoxide solution precursor by sol–gel
process. The Ba(1-x)SrxTiO3 powders calcined at 800 °C for 2 h were
maintained in cubic phase and the cell parameters were in the range of
4.0282–3.9786. The effects of Sr doping and sintering temperature on
structure and dielectric characteristics of the Ba(1-x)SrxTiO3 ceramics were
investigated. The increase of Sr content in the Ba(1-x)SrxTiO3 ceramics causes
the inhibition of grain growth and downward shift of Curie temperature (Tc).
R.M. Mahani et al [88] explained the nano-crystalline Ba1-xSr(x = 0.1 and 0.5)TiO3,
(B10ST and B50ST) powders. The samples were prepared by sol–gel
method, using barium acetate (Ba(Ac)2), titanium butoxide (Ti(C4H9O)4), and
strontium bromide as precursors. The dielectric measurements were carried
36
out in the frequency range 42 Hz to 1 MHz, at temperature range between 25
°C and 250 °C. The Curie temperature was detected at 125 °C for BT, while it
is detected at 110 °C and 75 °C for B10ST and B50ST, respectively.
Xiujian Chou et al [89] reported the Ba0.5Sr0.5TiO3–Zn2TiO4 composite
ceramics with low dielectric constant and high tunability are fabricated at a
relatively low sintering temperature of 1200 °C via the conventional solid-state
reaction route. These composite ceramics are promising candidates for
multilayer low-temperature co-fired ceramics (LTCC) and potential tunable
devices applications. Qing Xu et al [90] studied the Ba0.6Sr0.4TiO3 ceramics
were prepared by a citrate precursor method. The structure and nonlinear
dielectric properties of the resulting ceramics were investigated within the
sintering temperature range 1200–1300 °C. The ceramic specimens sintered
at 1230–1280 °C presented relative densities of around 95%. A significant
influence of sintering temperature on the microstructure and nonlinear
dielectric properties was detected. The discrepancy in nonlinear dielectric
behavior among the specimens sintered at different temperatures was
qualitatively interpreted in terms of the dielectric response of polar microregions under bias electric field.
Jiangying Wang et al [91] et al studied the Structural and dielectric
properties of yMgO–(1-y)Ba0.60Sr0.40TiO3 (BST) (where y = 3, 5, 10, 20, 30
and 40 mol%) ceramics prepared by using sol–gel method. Two phases,
corresponding to BST and MgO phases, are clearly visible when MgO content
up to 20 mol%. The influence of MgO additive on dielectric properties of BST
37
ceramics can be classified into two categories: one is the substitution effect of
Mg2+ ions and the other is the “composite” effect of MgO.
The external stress dependence of dielectric and tunable properties of
BaxSr1-xTiO3 (x = 0.65) ceramics has been investigated by Qiwei Zhang et al
[92]. The results reveal that the Curie peaks of samples are suppressed and
broadened, the Curie temperature (Tc) is slightly shifted to lower temperature
with an increase of external stress. Correspondingly, the tunability decreases
from 66.6% to 58.8% in the vicinity of cubic–tetragonal (C–T) phase transition
at an applied DC electric field of 10 kV/cm. These results could be useful for
the design of devices and practical applications of BaxSr1-xTiO3 ceramics.
Qing Xu et al [93] discussed the composite ceramics of Ba0.6Sr0.4TiO3 + 60
wt.% MgO were prepared from fine constituent powders by sintering at 1200–
1280 °C. The composite specimens sintered at the relatively low temperatures
showed satisfactory densification due to fine morphology of the constituent
powders. The dependence of the dielectric properties on sintering
temperature was explained in relation to the structural evolution. Controlling
the sintering temperature of the composite was found to be important to
achieve the desired nonlinear dielectric properties.
A.E. Souza et al [94] studied the BaxSr1-xTiO3 was prepared by the
microwave-assisted
hydrothermal
characterized
X-ray
by
method.
diffraction,
The
Raman,
ceramic
powder
was
ultraviolet–visible
and
photoluminescence (PL) spectroscopies and scanning electron microscopy
(FE-SEM). The results confirm that the powders consist of perovskite-type
38
cubic and tetragonal crystalline structures. The Raman data show bands
characterizing a multi-phonon process, suggesting the presence of defects,
due to partial Sr/Ba substitution, resulting in different distorted clusters. A
broad-band PL emission (blue light region) confirms the existence of these
defects, which together with the results of ultraviolet–visible spectroscopy
suggests non-uniformity in band structure. The distinct morphologies
observed are associated with different clusters organization during the
nucleation and growth of the BaxSr1-xTiO3 nanoparticles.
Lian-wei SHAN et al [95] reported the barium strontium titanate
(Ba1-xSrxTiO3, BST) and strontium barium niobate (SrxBa1-xNb2O6, SBN).
These are important ferroelectric materials with excellent pyroelectric,
dielectric properties and faster response time of infrared radiation. SBN/BST
composite ceramics with different mole ratios of Nb and Ti were fabricated
using a powder-sol (P-S) method with Nb2O5 fine powders suspended in the
barium strontium titanate (BST in short) sol solution. Juan Li et al [96] studied
the compositionally inhomogeneous Barium Strontium Titanate (BST)
ceramics with greatly improved dielectric temperature stability by the
combination of the solid-state reaction and mixed-phase method with the help
of a sol-assisted sintering technology. It was found that the powders'
calcination temperatures have significant effects on the microstructure and the
dielectric temperature stability of the sintered BST ceramics. Increasing
calcination temperature of either powder slightly degrades the BST ceramics'
sintering properties, but can significantly suppress and broaden their Curie
peaks.
39
Seung Ho Choi et al [97] discussed the nano-sized Ba0.7Sr0.3TiO3
powders are prepared by post-treatment of the precursor powders with hollow
and thin wall structure at temperatures between 900 and 1100 °C. It was
found that Ethylenediamine tetra acetic acid and citric acid improves the
hollowness of the precursor powders prepared by spray pyrolysis. The mean
sizes of the powders post-treated at temperatures of 900, 1000 and 1100 °C
are 42, 51 and 66 nm, respectively. The densities of the Ba0.7Sr0.3TiO3 pellets
obtained from the powders post-treated at 900, 1000 and 1100 °C are each
5.36, 5.55 and 5.38 g/cm3 at a sintering temperature of 1300 °C. Xilin Wang et
al [98] reported the Zr doped Ba0.6Sr0.4TiO3 ceramic was sintered by spark
plasma
sintering
(SPS)
and
thermal
treatment.
Sintering
behavior,
microstructure and dielectric properties of the BST ceramics were investigated
by XRD, SEM and impedance analyzer. High dense Ba0.6Sr0.4(Zr0.2Ti0.8)O3
ceramic with 98.2% of the theoretical density was fabricated at 1270 °C for 5
min. The dielectric constant of this sample reduced to 850 at 100 kHz, the
tunability of the ceramic increased to 57% (1.5 KV/mm, 298 K) and the figure
of merit (FOM) value increased to 475, which is promising material for tunable
devices application.
Qiwei Zhang et al [99] studied the Ba0.4Sr0.6TiO3–Mg2TiO4 composite
ceramics with different grain sizes were prepared by three sintering methods.
The dielectric constant dependences of temperature and frequency showed
an increased degree of diffuseness of the Curie peaks as the grain sizes
decreased. The tunability (T) and quality factor (Q) were found to be strongly
40
dependent on the grain sizes. The tunability significantly decreased with
increasing the grain size. In contrary, the quality factor (Q) at microwave
frequencies increased with increasing grain size.
Yang Yu et al [100] studied the comparative study on the dielectric
properties of Ba1-xSrxTiO3 (x=0.1–0.6) ceramics prepared by microwave
sintering (MS) and conventional sintering (CS). It was found that MS samples
need lower temperature and much shorter time than CS samples to obtain the
same degree of densification. Compared with CS samples, MS samples
possessed smaller grain size, better densification and more uniform grain
growth. The dielectric properties of the samples were measured as a function
of temperature. It was observed that the dielectric constant was higher for MS
samples compared with that of CS samples especially in the ferroelectric
phase. The chemical and structural properties of Ba1-xSrxTiO3 (BST, x=0–1)
nanoparticles synthesised via sol–gel–hydrothermal were analysed by S.
Fuentes et al [101]. Chemical characterisation and oxidation states were
obtained using X-ray photoelectron spectroscopy. Structural information was
acquired by Raman spectroscopy, and calculations to obtain theoretical
Raman spectra associated with the different formed phases of BST were
performed for comparison.
41
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