Chapter 1 Introduction - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/27695/9/09... · 2018. 7. 9. · Chapter 1Chapter 1 2 1.2. Microwaves Microwaves are electromagnetic

Chapter 1Chapter 1Chapter 1Chapter 1

IntroductionIntroductionIntroductionIntroduction


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CHAPTER 1

INTRODUCTION

1.1. Dielectric Materials

A material is something with properties that give it the potential for a particular

application, either structural, as with a building material; functional, as with materials

used to make devices (electronic, optical, or magnetic); or biological, with biomedical

applications. Every class of material has distinctive properties that reflect the

differences in the nature of bonding and hence conductivity. Based on bonding and

conductivity, materials can be classified as metals (conductors), semiconductors and

insulators (dielectrics). The term “dielectric” was coined by William Whewell (from

“dia-electric”) in response to a request from Michael Faraday.

Dielectric materials include covalent and ionic bonded materials, such as polymers,

glasses and ceramics. Due to strong bonds between valence electrons and atoms,

dielectrics are good electric insulators. Hence dielectrics and insulators can be defined

as materials with high electrical resistivity. When a dielectric is placed in an electric

field, electric charges do not flow through the material, as in a conductor, but only

slightly shift from their average equilibrium positions causing dielectric polarization.

Thus a dielectric is an electrical insulator that may be polarized by an applied electric

field. A good dielectric is, of course, necessarily a good insulator, but the converse is by

no means true.1

Although the term “insulator” refers to a low degree of conduction, the term

“dielectric” is typically used to describe materials with a high polarizability. The latter

is expressed by a number called the dielectric constant (ε). The dielectric constant (ε)

of a material indicates the degree to which the medium can resist the flow of electric

charge and is defined as ε = D/E where D is the electric displacement and E is the

electric field strength. Dielectric constant (ε) is an essential piece of information for

designing Capacitors, Radio Frequency(RF) Lines, Optical fibres (e.g. waveguide to

form filters), Printed Circuit Boards(PCBs), and Microwave Communication Systems.


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1.2. Microwaves

Microwaves are electromagnetic radiations. The electromagnetic spectrum ranges

from 1 MHz to 106 GHz and can be broken down into ranges as shown in figure 1.1

The radio frequency is in the range of 300 KHz to 300 MHz. Radio waves are used to

carry information in amplitude modulation (AM) and frequency modulation (FM)

broadcast radio, short wave radio, and very high frequency (VHF) television channels.

These signals can be carried using traditional transistors, tubes and circuits.

Microwaves lie in the region from 300 MHz to 300 GHz. The microwave region can

be broken down further into the ultra high frequency (UHF) region from 300 MHz to

3 GHz, the super high frequency (SHF) region from 3 GHz to 30 GHz, and the

extremely high frequency (EHF) region from 30 GHz to 300 GHz.

Fig. 1.1: Electromagnetic region in that microwaves lie in the region of 10 -1 to 10 -3 m.

MSR CMR ATCT GPS ST MH MR SCD PR SCU MS

0.3GHz 1GHZ 2 3 4 6 8 10GHz 20 30 80 100GHz 200 300

UHF SHF EHF

FREQUENCY

MSR= Military Search Radar CMR = Cellular Mobile Radio ATCT= Air Traffic Control Transponder

GPS = Global Positioning System ST= Space Telemetry MH= Microwave Heating

MR= Microwave Relay SCD= Satellite Communication Downlink

SCU= Satellite Communication Uplink PR = Police Radar MS = Missile Seeker

Fig. 1.2. The microwave spectrum and some of the applications utilizing microwaves..2


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Microwave energy has emerged as the most versatile form of energy applicable in

numerous diverse fields. Since, its first use in radar in World War II, it is now being

applied in communication, chemistry, rubber vulcanization, drying, food processing,

medical treatment and diagnosis and variety of materials processing fields. The most

recent development of microwave research is the use of separation of E and H fields

at 2.45 GHz in materials processing that has produced dramatic enhancements in

reaction kinetics and phase transformations.3 Recently, wireless communication

systems using microwaves to carry information have become increasingly popular.

Microwaves are able to carry more information than radio waves because of their

higher frequency. The higher frequency gives a wider bandwidth capacity, enabling it

to carry more information. Modern communication systems which include cellular

phones and satellite communications have moved to the microwave (MW) frequency

region, where advanced dielectric ceramics are frequently used in resonators and

filters. New communication systems have been investigated extensively for the

practical uses, such as wideband code division multiple access (W-CDMA),

multimedia mobile access communication (MMAC), intelligent transport system

(ITS), home radio frequency (RF), and high-speed wireless i.e. Local Area Network

(LAN). Because of current increase in demand for these systems, investigation of

microwave dielectrics used in RF devices has become more important.4 The demand

for microwave dielectric resonators increase rapidly due to the development of

microwave technique in recent years.5

1.3. Dielectric Resonator

A dielectric resonator is an electronic component that exhibits resonance for a narrow

range of frequencies in the microwave band. The electronic component (ceramic

element) acts as a resonator due to multiple total internal reflections at the high

dielectric constant material and air boundary. In 1939, Richtmyer6 showed that

unmetallized dielectric objects could function similarly to metallic cavity resonators;

he called them dielectric resonators. The dielectric resonator generally consists of a

‘Puck’ of ceramic which is able to resonate at the frequency of the carrier signal to

allow that signal to be efficiently separated from other signals in the microwave band;


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this frequency is called the resonant frequency (fo). The resonant frequency depends

on the dielectric material and the size of the resonator.

Ceramic pucks suitable for single mode resonator applications are cylindrical and

based on the original concepts of Richtmyer. The puck is designed to sustain a

standing wave within its body of a specific resonant frequency and may therefore act

as either a filter or a transmitting resonator.6 Ceramic resonators of differing

geometries are shown in Fig. 1.3, in which single mode pucks are indicated.

Multimode resonators are also manufactured, often with unusual geometries to induce

several resonant MW modes within their body at any one time. These materials find

extensive applications after the growth of the mobile phone market in the 1990s.

Mobile communications have been extended rapidly all over the world. Mobile phone

networks allow communication from cell to cell via antennas located on masts and

associated base stations. Within a cellular network, the average base station coverage

is a diameter of 35 and 18 km at 900 and 1800 MHz, respectively. Each base station

houses microwave (MW) resonators that are used to carry signals of a specific

frequency and remove (filter) spurious signals and sidebands, that interfere with the

quality of the transmitted/received frequency band.7

Fig. 1.3: Various geometries of ceramic puck used as single and multimode resonators and filters.7

In service, the ceramic conventionally rests within a silver-coated square cavity, as

illustrated in Fig. 1.4. Typically, many hundreds of these cavities will reside in the

base stations of a cellular network.7


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Fig. 1.4: Silver-coated cavity with pucks used in resonator technology.7

Fig. 1.5 (below) shows the most commonly utilized three dominant modes8 for dielectric

resonators. The TEM mode dielectric resonator is characterized by a guided mode field

distribution of a TEM mode with standing wave of a quarter wavelengths. This mode

dielectric resonator causes significant size-reduction of the component. The TM010 mode

dielectric resonator is characterized by a TM mode field distribution. This mode resonator

has the middle levels of unloaded Q and size reduction effect between the TE01δ and TEM

mode resonators. The TE01δ mode dielectric resonator is characterized by a dominant TE

mode field distribution, the field of which leaks in the direction of wave propagation. A

high unloaded quality factor can be achieved using this mode.

Fig. 1.5. Three dominant modes for dielectric resonators.8


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The quality of the resonator depends mainly upon the dielectric property of the

material.9 There are three main, very important properties of a dielectric resonator

material: relatively high dielectric constant, low dielectric loss, and temperature

stability. Thus for a ceramic to be usable as a dielectric resonator/filter, the three key

parameters need to be optimized10 as such:

i. Relative permittivity (εr): 20 < εr < 50

ii. Temperature coefficient of resonant frequency: τf ≈ + 0 ppm/0C and

iii. Low dielectric loss i.e. high quality factor which means Q > 30,000 at 1 GHz

Temperature-stable, medium-permittivity dielectric ceramics have been used as

resonators in filters for microwave (MW) communications for several decades and

perovskites have been extensively investigated11for use as MW dielectric materials

because of the outstanding and tuneable dielectric properties present in a number of

compositions. Complex perovskite ceramics with a general formula as A(B’1/3B”2/3)O3

have been reported to be excellent for MW dielectric resonators with sufficient high

dielectric constant, very low dielectric loss, and small temperature coefficient of resonant

frequency.12, 13, 14

1.4. Microwave Dielectric Properties

The various microwave dielectric properties are described as below.

1.4.1. Dielectric Constant or Relative Permittivity (εr)

Basically, dielectric constant is used to express polarizability of materials and it is

independent of frequency.8 In the modern usage dielectric constant refers to relative

permittivity (εr) and it may be either the static or the frequency-dependent relative

permittivity depending on context.

The relative static permittivity, εr, can be measured for static electric fields as follows:

first the capacitance of a test capacitor, C0, is measured with vacuum between its

plates. Then, using the same capacitor and distance between its plates the capacitance

Cx with a dielectric between the plates is measured. The relative dielectric constant

can be then calculated as

εr = Cx / C0 … (1)


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Thus permittivity is ratio of the amount of stored electrical energy in capacitor at an

applied voltage relative to that in vacuum.

This quantity becomes frequency dependent for time-variant electromagnetic fields

and in general it is regarded as relative permittivity. Thus the relative permittivity is

related to the resonant frequency, f0, by the following equations;

… (2)

where c is the speed of light in a vacuum and λd is the wavelength of the standing

wave along the diameter (D) of a resonator. Consequently, if the permittivity is

increased, the size of the resonator may be decreased while still maintaining a specific

resonant frequency, i.e. larger permittivities enable miniaturization.7 The resonant

frequency can be changed by manipulation of the volume of the resonating body and

the dielectric constant. Frequency selection can be achieved by altering the sample

volume and dielectric constant. Higher dielectric constant materials are preferred due

to the ability to make smaller volume components, which is an important

consideration when expensive materials are (e.g. Ta2O5) are required. However, the

relative permittivity (εr) is expected to be small for higher frequency millimetre wave

region, because of reducing the delay time of electronic signal transmission and

improvement of accuracy for production.

Dielectric materials are purposely doped with impurities so as to control the precise

value of εr within the cross-section. This controls the modes of transmission. It is

technically the relative permittivity that matters, as they are not operated in the

electrostatic limit.

1.4.2. Quality Factor (Q)

Quality factor (Q) is a measure of the selectivity of a resonator to a given frequency

and hence can be considered as the definition of how good our component is. Two

definitions are employed, depending on the frequency range of the measurement. At

sub-microwave frequencies the quality factor is a measure of the efficiency or power

loss. At microwave frequencies a peak occurs in the transmitted signal amplitude at


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the resonant frequency and has a finite width in a microwave dielectric system. A

high Q means a narrow peak, giving high selectivity to a given frequency. Higher

selectivity enables an increased density of channels in a given frequency band. Peak

resonant frequency (fo) divided by the peak width (∆f) is equal to Q or in sub-

microwave frequency measurements is equal to 1/tan δ.

… (3)

Evidently, the quality factor (Q) is approximately the inverse of the loss tangent

(tanδ). Thus, high Q implies low loss and vice versa. Higher Q values reduce the risk

of cross-talk within a given frequency range.

Q decreases with increasing frequency; Fig. 1.6 shows some of the results. It should be

stressed that ceramics of Ba(Mg1/3Ta2/3)O3 (BMT) and Ba(Zn1/3Ta2/3)O3 (BZT) still

keep a high value of Q more than 6000 even at 20 GHz.

Fig. 1.6: Q vs. frequency 9

The theoretical relationship between the two is such that Q x f0 should be constant for

any given material, and, often, Q x f0 values are quoted when comparing ceramics.10


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The radio frequency devices require narrow frequency selectivity and low power

consumption which are determined by the Q-factor of the materials. It is, therefore,

significant to develop and study dielectrics with the high Q-factor in the microwave

regime.4 The Q value of ceramics is structure-sensitive and strongly depending upon

the sintering condition.9 The quality factor (Q) is affected by intrinsic factors such as

crystal structure and by extrinsic factors such as grain growth, impurity, and so on.

Higher Q values can be obtained in materials by ordering through prolonged sintering

at high temperature.13 The various factors responsible for high Q values, for e.g., in

Ba(Zn1/3Ta2/3)O3 are:

i. Inducing 1:2 cation

ii. Volatilization of ZnO at extended high temperature

The possible mechanisms for ZnO loss as proposed by Desu et al.: 14

Ba3(ZnTa2)O9 Ba3Ta2O8 + ZnO ↑ ... (i)

Ba3(ZnTa2)O9 (1-x)Ba3(ZnTa2)O9 + xBa3Ta2O8 + xZnO ↑ ... (ii)

Ba3(ZnTa2)O9 (1-x)Ba3(Zn1-xBaxTa2)O9 + 4/3xZnO ↑ + x/3Ta2O5 ... (iii)

iii. Stabilization of the domain boundary and lowering of the free energy of the

antiphase boundary15

iv. Increase in the grain size and relative density16

Degradation of Q arises due to the creation of oxygen vacancies (VO) and possibly the

onset of electronic conduction. The formation of vacancies is a plausible explanation

from the point of view of phonon–photon interactions and it is intuitively easy to imagine

a distribution of VO increasing the anharmonicity of vibrations and dampening of phonon

modes, both of which are classic explanations for extrinsic dielectric loss. A direct link

between electronic conductivity and less Q is clear from a mechanistic perspective.7

1:2 Ordering in Perovskites

The perovskite related oxides are disordered (cubic) at low temperatures but gets

ordered (-B’-B”-B”-B’-) to a hexagonal structure at high temperatures.17,18 Ordering

behaviour in A(B’1/3B”2/3)O3-type complex perovskite ceramics has been extensively


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studied and it has been reported that the microwave dielectric properties of the

ceramics are strongly dependent on the cationic ordering in the perovskite structure.19

The ability of A(B’ B”)O3 perovskites to order depends mainly on the differences in

size and valency of the two B-site ions: large differences in these favour order.20 For

A(B’1/3B”2/3)O3 in general, when charge differences ≥4 the cations order but when the

charges differ by less than 4 disordered structures are found unless there is a large

difference in radii.21

A study of Ba(B’0.5Nb0.5)O3-type compounds demonstrated that size and charge

difference of the B position ions had an important, effect on their ordering.22, 23 In an

investigation of A(B’0.33Ta0.67)O3-type compounds, where A is a barium or strontium

ion and B’ is a smaller divalent ion, the results not only were substantiated but it also

was found that the degree of long-range ordering in the B position ions decreased as

the difference in the size of these ions became smaller.24

There are two kinds of ordering of the B-site cations.25 One is a complete ordering

between B’II

and B”v as can be found in Ba(Zn1/3Ta2/3)O3 which is often called 1:2

ordering and the space group is P-3m1 (D33d). The other which is often called 1:1

ordering, is ordering between B’II

and B”v

as can be found26 in Pb(Mg1/3Nb2/3)O3

and27 Pb(In1/2Nb1/2)O3; the space group is Fm3m (O5

h). The A(B’1/3B”2/3)O3 -type

complex perovskite may also have a disordered structure. In the case of complete

disordering 25 between B’

II and B”

v, the space group is Pm3m (O

1h).

B-site cation ordering of A(B’1/3B”2/3)O3 systems has been investigated for its effect

on the microwave Q-factor (Table1.1). In general, it is found that higher ordering

correlates with a higher Q-factor. 4 Measurements of microwave dielectric properties

showed that permittivity (εr) and temperature coefficient of resonant frequency(τf)

decreased with ordering, and quality factor (Q) increased with ordering.28 However,

Seung-Hyun Ra and Pradeep P. Phule,29 reported that the B-site cation ordering is not

a primary factor that influences the observed microwave loss in BMT ceramics.


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Table 1.1: Structure and Microwave dielectric properties of Ba(B’1/3B”2/3)O3 ceramics.4

Materials εεεεr Qf (GHz) τf (ppm/oC) Structure

Ba(Zn1/3Ta2/3)O3 30 168000 0 ORD

Ba(Mg1/3Ta2/3)O3 25 360000 4 ORD

Ba(Zn1/3Nb2/3)O3 41 86900 31 DIS/ORD

Ba(Mg1/3Nb2/3)O3 32 55400 33 ORD

Ba(Co1/3Nb2/3)O3 31 60000 -6 DIS/ORD

Ba(Co1/3Ta2/3)O3 25 46200 -16 ORD

Ba(Mn1/3Nb2/3)O3 39 900 27 DIS

ORD= Ordered, DIS= Disordered

Most of the studies on order–disorder transitions in perovskite structure have been

focussed on the transitions due to compositional variations. But, of the late, in some

A(B’1/3B”2/3) O3-type ceramics it has been observed that their ordering structure may

be changed locally by sintering temperatures.19

The ordering in the A(B’1/3B”2/3)O3 –type complex perovskite compounds can be

investigated from superlattice reflections either by an X-ray diffraction (XRD)

technique30 or by an electron diffraction technique. The electron diffraction technique

provides us evidence for local ordering which is more microscopic than XRD.31 The

other feasible tools25for the probing of the ordering in A(B’1/3B”2/3)O3 –type complex

perovskite compounds might be those based on vibrational spectroscopy, which is

highly sensitive to the short-range ordering. The 1:2 ordering might also be detected

by using Raman spectroscopy.

1.4.3. Dielectric Loss (or Dissipation Factor)

The dielectrics are associated with loss.32 It is the ratio of the energy dissipated to the

energy stored in the material. Dissipated energy is typically turned into heat from

conduction of electrons flowing through the material or through the anharmonicity of

the lattice vibrations.33 It is also known as the loss tangent or dielectric loss. A lower

dissipation factor is optimal in order to retain maximum signal and to prevent

excessive heat generation in devices.


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Mathematically, it can be expressed as the ratio of the imaginary portion of

permittivity (ε′′) and the real portion of permittivity (ε′).

tan δ = ε″/ε′ … (4)

The dielectric loss tangent increases proportionately to frequency8 in the frequency

range from 109 to 1011 Hz. Since more customers can be accommodated at a given

frequency range at low loss, the dielectric loss (tanδ or Q-1 [Refer Eqn (3)] needs to be

minimized to retain maximal frequency resolution.34

It is observed that the loss increases with increasing permittivity.35 Since the Q-factor

generally varies inversely with frequency (f) in the microwave region, the product Q x f

rather than Q alone is used to evaluate dielectric loss.

For a filter, the effect of losses (1/Qd) in the dielectric material used for the resonator,

radiative losses (1/Qr), conductive losses in the cavity walls (1/Qc), losses occurring in

the support used to mount the dielectric resonator (1/Qsup), and losses encountered in

coupling and tuning (1/Qk,t) must be considered. [The subscript to Q refers to the

source, e.g., d in Qd refers to the dielectric.] These Q factors are related by the

following equation (5):36, 37

… (5)

As the dielectric characteristics at microwave frequencies mainly depend upon an

ionic polarization brought about by lattice vibration, information concerning lattice

vibration is indispensable for understanding dielectric loss in a crystal.38 The

mesoscopic scale of disorder of the charge distributions plays a major role in

dielectric loss properties of complex perovskite oxides at microwave frequencies.39

Loss has also been attributed to defects and grain boundaries in polycrystalline

materials.9 Loss increases with porosity (P), which is defined as P = 1 – D where D is

the fraction of the material’s theoretical density. Losses and dielectric dispersion are

divided into two categories, intrinsic and extrinsic. Intrinsic losses are related to the

crystal structure (e.g. atomic masses, atomic charges and bond strengths) and the

interaction of the phonon with the AC electric field. Extrinsic losses are caused by


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imperfections in the crystal including impurities, grain boundaries, porosity, oxygen

vacancies, microstructural defects and random crystallite orientation.40

Dielectric loss in sintered pellets is dominated by extrinsic factors and thus is highly

dependent on sintering conditions. Hence the extrinsic contribution can be minimized

by using proper processing conditions. Ordering of the octahedral cations in

perovskites leads to weak coupling and a low energy leakage between the modes,

which can result in lowered intrinsic losses and a large increase in Q.33

The studies on dielectric properties of the materials are sometimes controversial41 which

is primarily due to the interference of extrinsic dielectric loss on the measured dielectric

characteristics. The most promising approach to understand the intrinsic microwave

dielectric properties of the materials is to study their higher frequency response,

including the submillimeter and infrared regime, since the intrinsic losses are

overwhelmingly stronger than the extrinsic ones in the far-infrared regime. 42-45

1.4.4. Temperature Coefficient (τf)

Temperature coefficient of the resonant frequency (τf), the permittivity (τε), or the

capacitance (τc) can be used to indicate the thermal stability of dielectric properties of

a material. The temperature coefficient of resonant frequency (τf ) determines how

well a resonator will function when there are fluctuations in temperature. Thus τf is a

measure of the ‘‘drift’’ with respect to the temperature of the resonant frequency. It is

self-evident that a material with a significantly non-zero τf is useless in an MW

circuit. Thus the temperature coefficient of resonant frequency (τf) is expected to be

near zero ppm/oC for receiving the RF signals in all places in the world.

The temperature coefficient of the resonant frequency (τf) is defined as

τf = - (1/2τε + αL ) … (6)

where τε is the temperature coefficient of permittivity and αL is the linear expansion

coefficient. It is related10 to the temperature coefficient of capacitance (τc) by

τf = - ½(τc + αL ) … (7)


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The signs of τf and τε are opposite and the τε controls the τf neglecting the small change

in the linear thermal expansion coefficient. Microwave circuits normally have some

low characteristic τf, so resonator components ideally counteract the inherent drift

with a small τf.33 Obtaining a material with a zero temperature coefficient is one of the

most difficult parts in the development of dielectric ceramic materials. Barium- and

strontium-based complex perovskites generally show positive and negative τf,

respectively, at room temperature.46 Hence Ba and Sr are employed to obtain τf values

zero or near to zero.

From an engineering perspective, the premise used to obtain a temperature-stable

material appears quite simple; a solid solution or composite is created of two materials,

each having opposite signs of τf. Although this approach adequately explains tuning τf

from the perspective of a composite, it does not describe the complexities associated

with solid solutions.7 Within perovskite solid solutions, the true tuning mechanism(s)

is/are a combination of the reduction in the polarizability per unit volume (decrease in

permittivity)47 and inducing a phase transition above room temperature usually

associated with rotations of the O-octahedra.46,48

1.5. Perovskite Structure

A perovskite is any material with the same type of crystal structure as calcium titanium

oxide (CaTiO3) – known as perovskite structure.32 Perovskites take their name from this

compound which was first discovered in the Ural Mountains of Russia by Gustav Rose

in 1839 and named after Russian mineralogist, Lev Aleksevich Von Perovski (1792-

1856). The first synthetic perovskites were produced by Goldschmidt (1926) of the

University of Oslo led to the use of the term perovskite as a description of a class of

compounds sharing the same general stoichiometry and connectivity found in CaTiO3.

Thus the perovskite structure is a term which is used to describe an arrangement of

anions and cations that is isomorphous with CaTiO3 but where the symmetry of each

phase can be quite different.48

It is a category of mixed metal oxide crystal which constitutes a large family of

crystalline ceramics. The ideal perovskite has a primitive cubic structure with the

formula ABO3 where ‘A’ is a (Lanthanide and, or alkaline earth metal) cation of larger


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size than B (a transition metal cation). It is considered a Face Centred Cubic (FCC) -

derivative structure in which the larger A cation and oxygen together form an FCC lattice

while the smaller B cation occupies the octahedral interstitial sites in the FCC array. There

is only the oxygen being B cation’s nearest neighbour. The structure is a network of corner-

linked oxygen octahedra, with the smaller cation filling the octahedral holes and the large

cation filling the dodecahedral holes.49

The symmetry of the ideal perovskite structure is cubic in space group Pm3m. The unit

cell of perovskite cubic structure is shown below in Figure 1.7.

Fig. 1.7(a): The cubic ABO3 Perovskite structure.50

Fig. 1.7(b): The cubic ABO3 perovskite 3D-structure. Grey spheres represent the A-site cations, blue spheres the B-site cations, and smaller red spheres are the anions.51


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Fig. 1.7(c): Perovskite structure52

Fig. 1.8: 1:2 ordering along the [111] in A(B’1/3B”2/3)O3.

In Figure 1.7(b) and 1.7(c), we can see that the coordination number of A is 12, while

the coordination number of B is 6. In most cases, the above figure is somewhat

idealized. In fact, any structure consisting of the corner-linked oxygen octahedra with a

small cation filling the octahedral hole and a large cation (if present) filling the

dodecahedral hole is usually regarded as a perovskite, even if the oxygen octahedra are

slightly distorted.53 Also, it is unnecessary that the anion is oxygen. For example,

fluoride, chloride, carbide, nitride, hydride and sulphide perovskites are also classified


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as the perovskite structures. As a result, we can say that perovskite structure has a wide

range of substitution of cations A and B, as well as the anions, but the substitution must

maintain charge balance and keep sizes within the range for particular coordination

number. Because the variation of ionic size and small displacements of atoms in

perovskite structure lead to the distortion of the structure and the reduction of symmetry

which have profound effects upon the electrical and magnetic properties of the crystals

eventually playing important role in dielectric ceramic.

The perovskite structure accommodates most of the metallic ions in the periodic table and a

significant number of different anions. Perovskites have been reported with all naturally

occurring cations in the periodic table except boron and beryllium. Phosphorus and the

noble gases are also not observed in the perovskite structure. The majority of the perovskite

compounds are oxides or fluorides, but the perovskite structure is also known for the

heavier halides54, 55, sulphides56, hydrides57, cyanides58, 59, oxyfluorides60 and oxynitrides.61

There are varieties of ordered structures known in perovskite oxides, among them the

double-perovskite (A2BB’O6 or AA’BB’O6) and the triple-perovskite (A3BB’2O9)

structures are the most popular. Many of these ordered perovskites contain Ti, Nb, or

Ta as the B-cation, and are interesting due to their optimum dielectric characteristics for

applications in the electronic industry.62 A wide range of multiple ion substitutions

(Fig.1.9) compositions and properties studied by R. Roy.63 provides plausible clues to a

further dielectric materials research.

Fig. 1.9: Chart illustrating types of multiple ion substitution in the perovskite lattice.63


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1.5.1. Distortions in Cubic Symmetry of Perovskites

The parent structure type is referred to as the ‘aristotype’, and distorted perovskites are

designated ‘hettotypes’. Three different types of distortions identified include:

distortions of BO6 octahedral units, B-cation displacements within the octahedra, and

octahedral tilting distortions.64 Distortions of BO6 octahedral units often result from

electronic factors. In octahedral coordination, the 3d transition metal cations Mn3+ and

Cu2+ (high-spin) have electron configurations (t2g)3(eg)

1 and (t2g)6(eg)

3, respectively.

Electronically, this is an unfavourable situation and the Jahn-Teller (J-T) theorem states

the occurrence of a distortion in the geometry. J-T distortions typically occur without a

large deviation of the O-B-O bond angles from ideal values of 90° and 180°.

Interestingly the mineral perovskite, CaTiO3, does not adopt the aristotype cubic

structure. The symmetry of CaTiO3 is lowered from cubic (Pm3m, Z = 1) to

orthorhombic (Pnma, Z = 4) by a cooperative tilting of the titanium centred

octahedral.65 This distortion is driven by the mismatch between the size of the cubo-

octahedral cavity in the corner-sharing octahedral network and the undersized ionic

radius of the Ca2+ ion. The octahedral tilting distortion lowers the coordination number

of Ca2+ from 12 to 8 in order to reduce the tension in the remaining Ca-O bonds66 and

increase the lattice energy. However, there is very little perturbation of the local

octahedral coordination of the Ti4+ ion.

Octahedral tilting is the most common type of distortion the perovskite structure type

undergoes. Displacements of octahedral cations are often observed due to the

combination of structural influences and electronic factors.

In the perovskite structure (ABO3) the A-site cation is located in the cavity formed by

the corner-sharing network of [BO6] octahedra. The fit of the A-site cation is described

by the tolerance factor (Ref: Eqn. 8). If the tolerance factor is greater than unity, often

no octahedral tilting is observed, whereas compositions with a tolerance factor less than

unity typically undergo octahedral tilting distortions.67 Oversized A-cations stretch the

octahedral bonds, resulting in an increase in octahedral volume and reduction of B-O

bonding. Distortion from ideal cubic symmetry of the aristotype cubic perovskite

(Pm3m) occurs by a practically rigid tilting of the octahedral units while maintaining

the corner-sharing connectivity. Distortions in the octahedral angles are minor (typically


19

less than 4° for O-B-O). Octahedral tilting allows greater flexibility in the coordination

of the A-site cation while maintaining a regular coordination environment for the

octahedral cation. Octahedral tilting reduces the symmetry of the A-site cation

coordination environment and leads to a significant change in the A-O bond lengths. B-

site cation ordering causes cell distortion and more stable phase of ordered structure can

enhance cell distortion.4

1.5.2. Tolerance Factor (t)

The tolerance factor or Goldschmidt tolerance factor (Goldschmidt, 1926) is a measure

of the fit of the A-site cation to the cubic corner-sharing octahedral network. Thus it is a

parameter to predict the structural distortion in perovskites. In a cubic perovskite twice

the B-O bond length is the cell edge, and the twice the A-O bond length is equal to the

face diagonal. Thus the tolerance factor is expressed as,

… (8)

where RA, RB, and RO are the radius of the A, B, and O ions respectively7, 64

The tolerance factor value is an approximate guide to the structural stability of the

perovskite phase. In general, when the value of t is close to 1, the perovskite phase will

be formed. If t is very far from 1, then the perovskite phase will not form.48 It has been

found that21 anion-deficient phases have low values of tolerance factor, 0.85< t < 0.99.

An upper limit of the tolerance factor is approximately 1.04, whereas a tolerance factor

less than 0.87 is near the range where the ilmenite (FeTiO3) structure type becomes

more stable compared to the perovskite structure type.7

1.6. Synthesis of Perovskites

Rational synthesis of materials requires knowledge of crystal chemistry besides

thermodynamics, phase equilibria and reaction kinetics. A variety of inorganic solids

have been prepared in the past several years by the traditional ceramic method. A wide

range of conditions, often bordering on the extreme, such as high temperatures and

pressures, very low xygen fugacities and rapid quenching have all been employed in the


20

matrial synthesis. The present day trend is to avoid brute-force methods in order to get

a better control of the structure, stoichiometry and phase purity.

Several methods are known in the literature for the synthesis of perovskites: Solid State

Reaction (mixed oxide) method, Sol-gel method, Molten Salt Synthesis (MSS) or Flux

method, Microwave Processing, Citrate gel method, Ceramic injection moulding

(CIM) method, Spray Pyrolysis Technique, Homogeneous Precipitation Process, Co-

precipitation process, Inverse micro-emulsion process, combustion method,

intercalation, ion-exchange, electrochemical method, and Hydrothermal Synthesis.

Among various methods listed above, the solid state reaction method and molten salt

synthesis method have been used extensively in our studies.

1.6.1. Solid State Reaction (Mixed Oxide) Method

It is one of the simplest and most widely used methods to synthesize perovskites. The

different steps involved in this method are shown in the flow chart (Fig. 1.10).

Fig. 1.10: Flow Chart of the Solid State Reaction Method


21

The most common method of preparing metal oxides and other solid materials is by the

solid state method or ceramic method, which involves grinding powders of oxides,

carbonates, oxalates or other compounds containing the relevant metals manually by

mortar and pestle or mechanically by a grinder. The grinding decreases the particle size

and increases the surface area of the powder, thereby increasing the intimacy between

the reactants, which will facilitate the reaction by diffusion. These milled powders are

then calcined at desired temperatures to bring about effective decomposition

(heterogeneous reaction). The entire reaction has to occur in the solid state, initially by

a phase boundary reaction at the points of contact between the components and later by

the diffusion of the constituents through the product phase. With the progress of the

reaction, diffusion paths become increasingly longer and the reaction rates slower. The

product interface between the reacting particles acts as a barrier. The reaction can be

speeded upto some extent by intermittent grinding between heating cycles.

1.6.2. Molten Salt Synthesis (Flux Method)

Molten salt synthesis (MSS) has been reported to be one of the simplest techniques to

prepare pure, stoichiometric ceramic powders of multicomponent oxides since the

diffusivities of the components are much higher by comparison to the solid state

reaction.68 The following are the basic requirement for selecting the salt in the molten

salt synthesis:

i. The melting point of any given salt must be appropriately low compared to the

formation temperature of the main phase.

ii. The solubility of a salt must be sufficient to be eliminated by a simple washing

step.

iii. There must be no undesirable reactions between salt and constituent oxides.

The advantages of this method are significant reduction in the powder formation

temperature and time. Furthermore, control of powder morphology was found to be

much easier using this technique.


22

Factors influencing the final particle

Characteristics at each step

Initial particle size, shape, geometry and morphology

Purity and identity of precursors.

Chemical nature and quantity of salts

Purity of salts

Molar ratio between salt and precursors

Chemical nature and Synthesis temperature/ Reaction time

Heating and cooling rates

Remaining impurities from salt and

precursor components

Presence of agglomerates

Precursor Materials

Synthesis of desired products at melting temperature of salts

Washing of salts with aliquots of deionized water

Drying

Mixing of precursor molecules with salt(s) and/or surfactant

Fig. 1.11: Flow chart illustrating factors influencing the MSS of transition-metal oxide Materials.69

In the synthesis of all oxide materials two steps, calcinations and sintering are referred

to frequently.

Calcination

It is an endothermic decomposition reaction in which an oxy-salt, such as a carbonate or

hydroxide, decomposes, leaving an oxide as a solid product and liberting a gas. The

calcined oxide is finely divided. It is easy to obtain a particle size much finer than 1 µm

and the specific surface are in the range of 100 m2/g. The surface area and the size are

easily modified. Low calcinations temperatures produce very fine, high surface area

powders. Heating at temperatures well above the decomposition temperature reduces the

surface area (and increases the particle size). Calcination is often the final step in the

production of high- purity ceramic powders, for example a homogeneous mixture of

several elements can be made by co-precipitation of solutes using hydroxides, carbonates,

or oxalates, which are later, converted to oxide compounds by calcinations.

Sintering

The pellets are sintered at higher temperatures to achieve a high density. This process

involves the heating of the pellets in a furnace below its melting point until its particles

adhere to each other. During sintering there occurs shrinkage of the calcined powder. It


23

consists of solid particles bonding or neck formation, followed by continuous closing of

pores from a large open porosity to essentially pore-free bodies. Thus it converts the

green microstructure to the microstructure of the dense ceramic component. In this

process the important parameters such as sintering temperature, atmosphere, heating

and cooling rates, impurity concentrations, particle size etc., have to be exactly

controlled to obtain reproducible results. After sintering, the ceramic material is built up

of closely packed grains with random crystallographic orientations. While the sintering

process has an influence on the microstructure of the resulting material, the cooling rate

has a strong influence on the electrical properties of the ceramic material.1

It is the last of ceramic processing step where the ceramist has an influence on micro-

structural development. This influence is limited, however, as the worst in-homogeneities

that pre-exist in the compact are usually exaggerated during sintering; for example, flaws

will persist or even grow, while large particles may induce abnormal grain growth.71

1.7. Characterization Techniques for Perovskites

The Powder X-ray diffraction can be used to characterize the phases formed in the

polycrystalline powder, Newtron diffraction and Transmission Electron Microscope can

also be used to study the structural information.

1 0 2 0 3 0 4 0 5 0 6 0

2 θθθθ

Fig. 1.12: Powdered x-ray pattern showing disordered cubic perovskite A(B’1/3B”2/3)O3.


24

1 0 2 0 3 0 4 0 5 0 6 0

2 θθθθ

Fig. 1.13: The arrows show super structure reflections corresponding to 1:2 ordering in A(B’1/3B”2/3)O3.

1.8. Materials for Dielectric Resonator Applications

Until the late 1960’s the applications of dielectric resonator were limited due to a lack

of suitable materials. Rutile phase (TiO2) was often used but its large resonant

frequency fluctuations with temperature made it impractical for many applications.

Single crystal alumina (sapphire) was also used but found very expensive. By the

1970’s, research and development for temperature stable dielectric resonator materials

had begun worldwide.32

Today a wide variety of dielectric ceramics is available for communications applications.

Among them the complex perovskites with general formula A(B1/3B’2/3)O3, where

A=Ba2+, B=Mg2+, Zn2+, or Ni2+, B’ =Ta5+ or Nb5+ show interesting and commercially

important properties at microwave frequencies. These ceramics typically have high

relative permittivity (or dielectric constant), low dielectric loss (also described in terms of

high dielectric Q value), and small temperature coefficient of resonance frequency

(τf).These materials are commonly used as resonators in microwave devices and systems,

particularly for communications applications. A high dielectric Q-factor combined with

small or preferably zero τf is critical for such applications.


25

Dielectric materials, particularly those used in the microwave frequency applications

are generally grouped into three categories72

1. Containing compounds with highQ, Qxf(GHz)=190,000–350,000 and εr= 25-30

e.g. Ba(M1/3Ta2/3)O3 [where M=Mg, Zn]

2. Containing compounds with intermediate properties Qxf(GHz)=7400–50,000 and

εr ≈ 40, e.g. Ba2Ti9O20 and (Sn, Zr) TiO4.

3. Containing compounds with Q ≤ 7000 and high εr (80-100), e.g. BaO-A2O3-TiO2

[where A=La, Nd etc.]

The lattice constants, theoretical density and sintering temperature of some ceramics are

listed in Table 1.2.

Table 1.2: Lattice constants, theoretical density (ρ) and sintering temperature of perovskite-type ceramics.9

Compounds Structure Lattice constant (Å)

ρ

(g/cm3) Sintering

temperature (oC)

a c

Ba(Mg1/3Nb2/3)O3 (BMN) Hexagonal 5.776 7.089 6.211 1550

Ba(Mg1/3Ta2/3)O3 (BMT) Hexagonal 5.774 7.095 7.637 1550 - 1600

Ba(Zn1/3Nb2/3)O3 (BZN) Cubic 4.093 6.515 1500

Ba(Zn1/3Ta2/3)O3 (BZT) Hexagonal 5.787 7.087 7.944 1550

Ba(Mn1/3Nb2/3)O3 (BMnN) Pseudo-cubic 4.113 6.337 1550

Ba(Mn1/3Ta2/3)O3 (BMnT) Hexagonal 5.814 7.156 7.709 1600

Sr(Mg1/3Nb2/3)O3 (SMN) Hexagonal 5.638 6.920 5.378 1500

Sr(Zn1/3Nb2/3)O3 (SZN) Hexagonal 5.658 6.929 5.698 1500

Rapid progress in electromagnetic simulation methods has enabled a significant

improvement in the performance of state-of- the-art microwave devices. A growing

number of these devices require intricately shaped microwave materials. This has

placed increasing demand on the ceramic research community to develop a low-cost

process that can manufacture high performance microwave materials in complex-

shapes. The use of dielectric ceramics with low loss, high dielectric constants, and near-


26

zero temperature coefficient of resonant frequency is also essential in achieving the

required microwave performance for the devices of interest.73 Although there has been

considerable amount of work in the area of dielectric oxides in the past 50 years,

dielectric materials continue to remain a significant area of scientific research due to

their wide range of applications such as resonators, filters and tuners. The perovskites

found to exhibit the highest Q values are based on the formula Ba(M2+1/3Ta5+

2/3)O3,

where M =Zn, Mg. For the Zn system, εr ≈ 30, and for the Mg system, εr ≈ 25. These

‘Tantalum’ based ceramic systems are: Ba(Zn2+1/3Ta5+

2/3)O3 (BZT) and

Ba(Mg2+1/3Ta5+

2/3)O3 (BMT).

1.8.1. Tantalum (Ta)-based Systems

Ba(Zn2+1/3Ta5+

2/3)O3 (BZT) and Ba(Mg2+1/3Ta5+

2/3)O3 (BMT) are the most common

‘Tantalum’ based ceramic materials with excellent microwave characteristics currently

used for dielectric resonator applications. The BZT has a high dielectric constant (εr

≈30), low temperature coefficient of resonant frequency (τf ≈ 4 ppm/C), and high

quality factor (Q x f ≈80 000–150 000 GHz).13 BMT has permittivity εr ≈ 24, high

quality factor (Q x f) ≈ 250 000 GHz and the low temperature coefficient of the

resonant frequency (τf) ≈ 0 ppm/oC.7 Single phase BMT has, of course, the highest Q

value among microwave materials studied to date,73,74 however, it is difficult to sinter.

Based on the Ginstling–Brounshtein model, the activation energy for the formation

process of BMT was estimated to be 257kJ/mol. Raising sintering temperatures resulted

in an increase in the ordering degree and bulk density of BMT.75 But a decrease in the

1:2 cation ordering and increase of dielectric loss in BMT occurred at sintering

temperatures above 1590 °C.76 Reducing the barium content in BMT substantially

resulted in improved densification and enhanced ordering structure. On the other hand,

an excess barium content in specimens hindered the progress of sintering, and also

induced the disordering structure.75

Both the BZT and BMT belong to a class of compounds often referred to as 1/3:2/3 i.e.

1:2 complex perovskites.77, 78 Since there are two different B-site cations, there are two

different structures that can exist: the disordered (space group Pm3m) and the 1:2

ordered states (i.e. for e.g. –Mg–Ta–Ta–Mg–Ta–Ta–) (space group P¯3m1).79


27

However, the crystal structures reported in the literature indicate significant differences

in the reported O fractional coordinates and cation coordination environments of BZT

and BMT.11 The B-site bond lengths (Å) in BZT21: [Zn-O 1.977(4) x 6, Ta-O 2.023(7) x

3, and 2.141 (10) x 3] whereas that in BMT80: [Mg-O 2.07(2) x 6, Ta-O 2.04(2) x 3 and

2.033(6) x 3.

1.8.1.1. Barium Zinc Tantalate Ba3ZnTa2O9 or Ba(Zn1/3Ta2/3)O3 (BZT)

Neutron diffraction patterns (Fig. 1.14) and the data show the local B cation

environment of hexagonal phase of BZT in detail.

Fig. 1.14: Neutron diffraction profile21 for Ba3Ta2ZnO9.

Table 1.3: Interatomic distances and angles in Ba3Ta2ZnO9 from the neutron diffraction data.21

Ba(1)-O(1) 2.830(8)Å O(1)-O(1) 2.890(1)Å

Ba(1)-O(2) 2.895(12) O(1)-O(2) 2.976(4)

Ba(1)-O(2) 2.906(12) O(2)-O(2) 2.851(8)

Ba(2)-O(1) 2.890(1) O(2)-O(2) 2.741(8)

Ba(2)-O(2) 2.956(4) Ta-O(1) 2.023(7)

Zn-O(2) 1.977(4) Ta-O(2) 2.141(10)

O(2)-Zn-O(2) 92.2(5)o O(1)-Ta-O(1) 91.2(4)o

O(2)-Zn-O(2) 87.8(5) O(1)-Ta-O(2) 91.1(6)

O(2)-Ta-O(2) 83.4(8)


28

Fig. 1.15: The B-cation environment in Ba3Ta2ZnO9.21

The long-range cation ordering observed in Ba3Ta2ZnO9 appears to originate because

of the difference in the ionic size of the two cations; Ta5+ =0.64, Zn2+ = 0.74Å

(Shannon & Prewitt, 1969) 21 Zn and Ta compete (Table 1.3) for the same corner-

sharing O atom; the Zn-O distance = 1.98Å (Zn2+ + O2- = 2.09Å) and the O-Ta

distance 2.14Å (Ta5+ + O2- =1.99Å). For this, a conceivable explanation is that the

high electron affinity of Ta5+ (and considerable covalence in the Ta-O bond) modifies

the O ligand field so that some 3d electron donation becomes possible from Zn to O

(in effect into a bond arising from Ta-O overlap). Combined with back donation from

O to the s-p orbitals of Zn this could give rise to an abnormally high Zn-O covalence

and a correspondingly short distance. The effect of delocalizing the d electrons of Zn

would be to transfer electron density to the d-orbital anti-bonding regions of Ta thus

weakening the bonding and increasing the Ta-O distance observed. The reduced

contribution to the lattice energy from an overall increase in the Ta-O distance is

partially compensated in a familiar way by an off-centre Ta displacement which leads

to three short and three long bonds with O.

A cubic perovskite structure belongs to the space group O1h of the cubic system, which

has no Raman-active mode and 3 infrared-active modes.38 The Ba(Zn1/3Ta2/3)O3 crystal

with a hexagonal superstructure, however, has 9 Raman-active modes and 16 infrared-

active modes. Fig. 1.16 and 1.17 show the Raman and infrared spectra of

Ba(Zn1/3Ta2/3)O3 ceramics.


29

Fig. 1.16: Raman spectra38 of ceramic Ba(Zn1/3Ta2/3)O3.

Fig. 1.17: Measured and calculated far-infrared reflectivity38 of ceramic Ba(Zn1/3Ta2/3)O3.


30

The dielectric constant and its temperature coefficient depend mainly on the

composition of the material. But loss quality varies from sample to sample of the same

composition, indicating that the loss quality is sensitive to slight differences in

crystallographic structure and microstructure of the material.13

Since the first report of the outstanding dielectric performance of complex perovskite BZT

many investigations have been conducted on the effect of different processing schemes and

chemical additives on the crystal chemistry, microstructure, and properties of BZT based

ceramics. Both ordered-type with a trigonal space group of P¯3m1 and disordered-type

with a cubic phase space group of Pm3m are formed depending on the synthesis conditions

of Ba(Zn1/3Ta2/3)O3.13 The B-site cations in Ba(Zn1/3Ta2/3)O3 are stoichiometrically ordered

in a hexagonal cell, with one Zn2+ layer and two Ta5+ layers repeat along the <1 1 1>

direction of the parent perovskite cubic cell. This ordering is a matter of considerable

interest because ordering is believed to play an important role in the quality factor (Q) of

BZT.13 The factors influencing Q values of perovskites Ba(M2+ 1/3Ta5+2/3)O3 (M = Mg, Zn)

have been considered to be long-range ordering (LRO) of cations, zinc oxide

volatilization/evaporation, point defects and stabilization of micro-domain boundaries.14,38

Different studies using Ba(Mg1/3Ta2/3)O3 (BMT) with minimized effect of volatilization

of ZnO have shown that the changes in Q originate from cation- ordering13, and the

cation-ordering enhances Q 81 While the other reports describe that the improvements

in Q are not related to cation-ordering, but to volatilization14 of ZnO, stabilization of the

domain boundary and lowering of the free energy of the antiphase boundary. The

deviations from stoichiometry induced by the volatilization of ZnO have been reported

to promote the growth of the ordered domains82 increase the extent of cation ordering83

and the c/a lattice distortion14 and improve the dielectric loss properties.13

Among all these different possibilities, the crystal structural ordering together with the

ceramic microstructure have been found to influence the Q factor of BZT strongly.84 It

has been found that Q improvement corresponds with increased Zn and Ta ordered

structures in the ceramics.13 Usually a significant ceramic microstructure difference

between the ordered and the disordered perovskite has been found.84 Dense ceramics

(density ≈7.76 g/cm3) has been obtained in the ordered perovskite with the relatively

larger grain (1.0–1.5µm). Whereas the disordered perovskite has the lower density


31

(density ≈5.0 g/cm3) with the smaller grain size (0.4–0.6 µm). Thus these two factors

strongly relate each other in the BZT system. Therefore, their influence on the Q factors

can’t be discussed separately.

E. Koga et al., 85 studied large amount of Q factor variation within dense, highly ordered

region of the BZT system by means of crystal structure analysis, micro-structural

analysis and electrical measurements using samples around stoichiometric BZT. The

influences of composition deviation from the stoichiometric BZT on the structural order

and the Q factor were studied and the findings listed as below:

1. The crystal structure phases strongly depend on the slight composition deviation

from the stoichiometric BZT.

2. Single phase ordered perovskite is obtained only in the vicinity of the

stoichiometric BZT. [In this region, an improvement in Q factor (Q×f = 133,000

GHz was found in extended sintering up to 400 h at 1400C.]

3. In the other regions, the single phase of the disordered perovskite or the ordered

perovskite with the secondary phase is formed. [In these regions the Q factor was

found to be low.]

4. Presence of small amount of secondary phase (e.g., less than 1%) affects Q factor

to decrease. This suggests that the structural order and the presence of the

secondary phase play an important role on Q factor in the BZT system. Therefore,

suppressions of small amount of secondary phase (e.g., less than 1%) and increase

of ordering ratio by strict composition control can provide further Q factor

improvements in the BZT system.85

Min-Han Kim et al.,86 however, investigated that the increase in the Q value was not

related to the relative density or the 1:2 ordering, in contrast, the improvement in the Q

value occurred in specimens in which grain growth occurred.

M. Thirumal and P. K. Davies87 have reported the synthesis of hexagonal perovskite

Ba8ZnTa6O24 in single-phase by solid-state technique. High-density ceramics of

Ba8ZnTa6O24 was prepared at temperatures considerably lower (1400oC) than those

used to sinter pure Ba(Zn1/3Ta2/3)O3, and that exhibited very good microwave dielectric

properties with εr =30.5, Q x f = 62300 at 8.9 GHz; however, additives would be needed

to lower the temperature coefficient τf = +36 ppm/oC.87


32

Use of Dopants

The use of additives is seen to improve the microwave dielectric properties of ceramics;

however, their effect on the BZT system is sometimes tricky. For an example, addition

of ZrO2 [i.e. Ba(Zn1/3Ta2/3)O3 + xZrO2 with 0.0 ≤ x ≤ 4.0 mol%] was investigated16 on

crystallographic order, microstructure, and microwave dielectric properties of BZT

ceramics. A small amount of ZrO2 disturbed the 1:2 cation ordering. The average grain

size of the BZT significantly increased with the addition of ZrO2. The relative density

increased with the addition of a small of ZrO2, but it decreased when the ZrO2 content

was increased. Variation of the dielectric constant with ZrO2 addition ranged between

27 and 30, and the temperature coefficient of resonant frequency increased abruptly as

the ZrO2 amount exceeded 2.0 mol%. The Q value of the BZT significantly improved

with the addition of ZrO2, which could be explained by the increased relative density

and grain size. The maximum Q x f value achieved in this investigation was ≈164000

GHz for BZT with 2.0 mol% ZrO2 sintered at 1550oC for 10 hours.16

Nomura and co-workers successfully9 prepared dense ceramics comprised BMT, BZT

with a small amount of Mn addition. The Mn doping not only favourably affected

sinterbility in lowering the required sintering temperature but also advantageously

heightened the unloaded Q value of the resulting sinter in an SHF band. They also

applied this doping method to Ba2Ti9O20 and Zr0.8Sn0.2TiO4 ceramics.9 The values of ε,

Q and τf in 10 GHz band are listed in Table 1.4.

Table 1.4: Values of ε, Q and τf of doped perovskite-type ceramics.9

Compound εεεε Q τf

(ppm/oC)

Remarks

BMN 32 5600 33 Mn 2 mol % , 9.9 GHz

BMT 25 16800 4.4 Mn 1 mol % , 10.5 GHz

BZN 41 9150 31 Anneal in N2, 9.5 GHz

BZT 30 14500 0.6 Mn 1 mol % , 11.4 GHz

BMnN 39 100 27 9.3 GHz

BMnT 22 5100 34 Anneal in N2, 11.4 GHz

SMN 33 2300 -14 Mn 2 mol % , 10.3 GHz

SZN 40 4000 -39 9.2 GHz


33

It is interesting to note that the Ba compound has a positive τf, while the Sr has a

negative one, which corresponds to a negative and a positive temperature coefficient of

the dielectric constant, respectively. The most important result is that the Q values of

BMT and BZT ceramics with 1 mol % Mn exceed 104 in 10 GHz band. Generally, the ε

was not so sensitive to the Mn doping, while the Q showed a strong dependence on it. A

typical example is shown in Fig. 1.18.

Fig. 1.18: Q vs. Mn Concentration.9

The addition of small amount of BaZrO315or ZrO2

16 has been found to decrease the

ordering but increase the Q. Similarly, Ni-doped Ba(Zn1/3Ta2/3)O3, is commonly used

for many microwave materials as it exhibits excellent high frequency properties with a

high dielectric constant (εr ~ 30), a low loss tangent (<2×10−5 at 2 GHz) and has a near-

zero temperature coefficient of resonant frequency (τf).88 Zr is often alloyed with this

compound since it has been found to improve the manufacturability of the material by

allowing shorter annealing times to achieve low loss.89 The doping by BaWO4 has been

reported90to increase the sinterability of BZT ceramics. The highest Qxf values (15,000-

200,000 GHz) were obtained at 0.5-1.5 mol% BaWO4 in samples sintered at 1570-

1580oC for 3h in air. BZT doped with 5 mol% SrGa1/2Ta1/2O3 (SGT)91 has a temperature


34

coefficient of the resonant frequency (τf) tunable through zero, a dielectric constant of

29 and a unloaded quality factor (Q) of 50–80,000 at a resonant frequency (fo) of ≈2

GHz(Q x fo=100–150,000 GHz).

Nanosized phase pure powder of Ba(Zn1/3Ta2/3)O3 (BZT) were obtained by sol–gel

method and subsequent pyrolysis of the dried gel.92 The powders were pyrolyzed at

different elevated temperatures and the particle sizes were found to increase with

temperature. To improve the sintered density, several sintering aids/dopants, in the form

of oxides of metallic elements were added by 1 mol%, to BZT nanopowders and the

powders were subsequently sintered. The effect of different dopants on the microwave

dielectric properties of BZT can be better explained from the following experimentally

observed plots.

Fig. 1.19: Variation of Q x f and τf with ionic radius of the dopants.92

Dopants like Ti, Zr and Sb, showed a reduction of Q x f whereas the dopants like Cr,

Ga, Mn, In, Ce and Bi showed an improvement in Q x f. All dopants were found to

reduce temperature coefficient of resonant frequency (τf) as can be seen in Fig.19.

When the average ionic radius of the dopant was about 0.615Å, τf was found to be zero.

Most of the dopants were found to promote grain growth of sintered microstructure.92

Temperature has been found one of the decisive factors to increase the degree of order

whereby improving/affecting Q. Therefore, knowing the maximum temperature at

which BZT orders prior to disordering is of technological importance. I. M. Reaney et

al., 93 studied and performed a series of quenching experiments to ascertain the


35

approximate temperature of the order–disorder phase transition of BZT. The XRD (Fig.

1.20) revealed in their experiments a sudden decrease (maximum to zero) in the degree

of order in the BZT samples between 1600 and 1625°C. Hence it was assumed that a

reversible order–disorder phase transition in BZT had occurred between 1600 and

1625°C.

Fig. 1.20: XRD patterns from BZT (a) sitered at 1475oC and annealed & quenched from (b)1575oC, (c)1600oC and (d) 1625oC.93, 94

In another example of solid-state reaction, the BZT ceramics being sintered at 1400-

1600oC for 4 hours had low loss in microwave domain.95The increase of the sintering

temperature was found to lead to a normal granular growth, with a polyhedral grains

size development.96 For sintering temperatures higher than 1500°C, the XRD (Fig. 1.21)

patterns revealed the BZT multiphase compositions with the presence of trigonal

supercell peaks and low Zn content secondary phases. The last two disappeared at

temperatures higher than 1600°C. 96

The formation of secondary phases can introduce point defects into the matrix of

sintered body. The probable defects are vacancies at the A and/or the B sites and

interstitials in the perovskite structure.74


36

Fig. 1.21: XRD patterns of Ba(Zn1/3Ta2/3)O3 system vs. sintering temperature.96

The analysis of the order–disorder transition of the BZT material showed a long-range

order with a 2:1 ratio of Ta and Zn cations on the octahedral positions95 of the

perovskite structure with the increase of the sintering temperature. There is a strong

correlation between the cation ordering, domain growth, zinc loss and sintering

parameters. The investigations on dielectric properties revealed 96 that the increase of

the sintering temperature had decreased the dielectric loss especially when an annealing

of 10 hours at 1400°C was performed. The quality factor Q was observed to depend

strongly on the BZT crystalline structure, i.e. on the unit cell distortion and cationic

order. Dielectric loss decreased with the increase of ordering degree in the structure and


37

with the disappearance of secondary phases. Lowest loss was obtained for a Zn and Ta

completely ordered BZT ceramic with a strongly distorted unit cell. Porosity had small

effect on dielectric loss of BZT material.

As a matter of fact the solid state reaction of BaCO3, ZnO, and Ta2O5 is not the most

appropriate method, because a high sintering temperature is required to achieve high-

density Ba(Zn1/3Ta2/3)O3 (BZT) materials (>1500C),which is too high for industrial

applications.13,15 Many efforts have been made to lower the sintering temperature of

BZT ceramics by the addition of sintering agent 97 Wen-Cheng Tzou et al. used BZT

powder, calcined at 1200C for 3 h, as the precursor and 0.1 mol BaTi4O9 (BT4) to

improve the sintering characteristics of BZT ceramics.

Fig. 1.22: (a) The bulk densities and the dielectric constants of BT4-BZT ceramics, as a function of sintering temperature; (b) The quality factor(values) and temperature coefficients of resonant frequency of BT4-BZT ceramics, as a function of sintering temperature.

The quality values (Q × f) of BT4-BZT ceramics were investigated as a function of

sintering temperature [results shown in Fig. 1.22(b)]. As the sintering temperatures were

increased from 1240 to 1320C, the Q × f values also increased critically. The Q × f

values of BT4-BZT ceramics reached a saturation value of 68500 at 1320C sintered

ceramics, then the Q × f values of BT4-BZT ceramics slightly increased with the further

increase of sintering temperature to 1360C. The τf values of BT4- BZT ceramics are also

shown in Fig. 1.22(b). As the sintering temperatures were changed from 1240 to1320C,

the τf values of BT4-BZT ceramics linearly changed from −1.5 ppm/C to 4.1 ppm/C.

(a) (b)


38

Thus the addition of 0.1 mol BaTi4O9 into the Ba(Zn1/3Ta2/3)O3 composition would

improve the sinterability of Ba(Zn1/3Ta2/3)O3 ceramics. The needed sintering temperature

of BT4-BZT ceramics was about 1320C which is much lower than that of

Ba(Zn1/3Ta2/3)O3 ceramics. The BT4-BZT ceramic sintered at 1320C has the microwave

dielectric property of εr = 31.5, Q × f= 68500, and τf = 4.1 ppm/C.98

A technological process has been investigated97 with respect to milling conditions,

calcinations parameters and also lithium salt (Li2CO3, LiF, BaLiF3 and LiNO3)

additions so as to lower the sintering temperature of BZT. First, milling conditions

investigation permits to optimise powder reactivity authorising a sintering temperature

reduction of nearly 100C. Secondly, the lowering of the ceramics sintering temperature

has been explored using the lithium salt additions. The addition of BaLiF3 permitted to

reach a sintering temperature of around 1200C. Permittivities of 29 for the LiF added

sample and 38 for the BaLiF3 added sample were measured with (τε(LiF) =137 ppmC−1

and τε(BaLiF3) =156 ppmC−1, respectively). Thus except for the temperature coefficient

which was increased, the addition of lithium salt did not modify or alter the dielectric

properties (tanδ and ε).97 A drastic decrease of BZT sintering temperature had been

reported99 by lithium salts and glass phase addition. A. Chaouchi et al. studied BZT

perovskite with the objective to decrease their sintering temperature close to 900oC. The

different types of sintering aids (glass phase combined or not with lithium salts) were

employed in order to decrease the BZT sintering temperature below the silver melting

point (961oC). The glass phases based on the SiO2 – ZnO – B2O3 (ZSB) system and LiF

lithium salt were used as the sintering agents which showed a good efficiency on the

sintering temperature of BZT material. The combined addition of 5% wt. of ZSB and

1% wt. of LiF to BZT showed a significant lowering of sintering temperature. The

materials achieved good dielectric properties: εr = 32, τε = -10 ppm/oC, low losses factor

(Tan δ < 10-3), high resistivity (1013Ω.cm). This is partially explained by the fact that

glass added samples had higher density.99

Despite of various efforts, the detailed mechanism of high Q factor of BZT system has

not been fully understood, namely, quite large amount of Q factor variation within

dense, highly ordered single phase region of the BZT system. If we could understand


39

the origin of this large amount of the Q factor variation, then we would suggest methods

for further Q factor improvements of the BZT system. Depending on the conditions of

synthesis, both ordered and disordered-type were formed in case of Ba(Zn1/3Ta2/3)O3

(BZT) and Ba(Mg1/3Ta2/3)O3 (BMT).13 Thus the analysis of the exact relationships

between the composition and the dielectric properties is necessary.

There have been many attempts to explain the material’s excellent microwave dielectric

properties and further to improve its properties. A clear relationship between the

composition and properties will be helpful in investigation of new ceramics with less

costly raw materials. Although tantalate high Q perovskites such as BZT and BMT

have been commercialized, in the last few years, economic factors associated with the

high cost of Ta2O5 have increased the focus on their niobate, a low cost counterpart.

The low cost materials with high performance are in urgent need for development.

1.8.1.2. Barium magnesium tantalate Ba3MgTa2O9 or Ba(Mg1/3Ta2/3)O3 (BMT)

Nomura and co-workers12 reported Ba(Mg1/3Ta2/3)O3 in 1982. Since then there have been

several reports of BMT by solid sate method e.g. H. Matsumoto et al., 100, S.Nomura, 9 M.

Sugiyama et al., 101; M.-H. Liang, et al, 102; wet chemical methods e.g. O. Renoult, et al.;

103, S. Katayama et al., 104 sol-gel powders for microwave dielectric resonators, Ceramics

today- tomorrow ceramics105 P.Vincezini.(Ed.,) Elsevier Publishers B.V. (1991-1997).

There are several reports on the microwave dielectric properties of BMT with ε 24-25,

Q x f upto 430000GHz and τf close to zero ppm/0C.

1.8.2. Niobium (Nb)-based Systems: BZN, BMN, SZN

In A(B’1/3B”2/3)O3, the B-site substitution of the identical ionic radius106 (0.78 Å) Nb5+

for Ta5+ results in crystallization of isostructural 1:2 ordered perovskites; however, the εr

is higher, Q is lower, and τf is more positive. The origins of the dielectric property

differences between the isostructural compounds are not well understood.11

Entanglement of synthetic variables (e.g., chemical composition, reagent purity,

annealing and sintering temperatures and times, initial particle size, processing

conditions, and partial O2 pressure) and experimental observables (e.g., crystal structure,


40

density, cation order inside of a domain, ordered domain size, domain boundaries,

defects, and impurity phases) that influence the dielectric properties (i.e., εr , τf, and Q)

complicate formation of structure-property relationships.

Nb-based perovskites are looked upon as candidates with comparable dielectric

properties to substitute the expensive Ta-based perovskites Ba(Zn1/3Ta2/3)O3 (BZT) and

Ba(Mg1/3Ta2/3)O3 (BMT). The Nb-based ceramics, e.g. Ba(Zn1/3Nb2/3 )O3 (BZN), and

Sr(Zn1/3Nb2/3)O3 (SZN) are cheaper than Ta-based complex perovskite ceramics and

have simple perovskite cubic structures (pm 3– m) with lattice constant a = 4.094 Å at

room temperature.107 The next important Nb-based ceramic is Ba(Mg1/3Nb2/3)O3

(BMN) which has interesting dielectric properties (εr = 32, τf = 33 ppm/C, and Q x f =

56 THz at 10 GHz) but because of its high sintering temperature (Ts) >1500C it is less

used than BZN, which exhibits a lower Ts (1350C) and similar dielectric properties

(εr = 41, τf = 30 ppm/C, Q x f = 54 THz at 10 GHz).108 However, the resonance

frequency temperature coefficient (τf) of the BZN ceramic is relatively high (30

ppm/C), and this limits BZN ceramics also to be used in microwave applications.109

The analysis of the permittivity (εr) and intrinsic loss110 of 1:2-ordered BZN and BMN

shows that the difference in permittivity between BZN and BMN is caused by the

significant influence of the dipole interaction on the nucleus oscillation in BZN.

Furthermore, such influence is attributed to the covalent interaction among Zn, O, and

Nb which is correlated with the Zn 3d orbital. The investigation of intrinsic loss shows

that the measured loss is close to the intrinsic loss in both BZN and BMN, and that the

difference in the measured loss is mainly caused by the difference in the intrinsic loss.

Since the intrinsic loss increases rapidly with the permittivity, it seems that the

difference in permittivity between BZN and BMN is the origin of the difference in the

measured loss.110

Research works are in progress to obtain the near-zero temperature coefficient of resonant

frequency (τf) and also improve other dielectric properties of BZN. A good combination

of microwave dielectric properties: εr=36, Q x f =16,170 GHz, and τf = −12 ppm/°C has

been reported111 in the composition (Ca1−xBax)(Zn1 / 3Nb2 / 3)O3 when x =0.1.


41

Fig. 1.23: τf and εr values variation as a function of x for (Ca1−xBax)(Zn1 / 3Nb2 / 3)O3 ceramics.111

The τf value was found to increase sharply corresponding to the increased εr at x= 0.1,

where the primitive cubic structure of Ba(Zn1/3Nb2/3)O3 distorted to hexagonal structure.

For x=0.3–0.9, the τf value decreased when the Ca-rich secondary phase appeared. The

secondary phase with orthorhombic structure as Ca (Zn1/3Nb2/3)O3 (τf = −35 ppm/°C)

had a negative effect on the τf value. The rattling of Ca2+ and the cell volume variation

led to the anomalous variation of εr and τf in the present system for the Ba-rich

compositions where the decreased Q x f value was obtained.111

Various methods have been applied to improve the properties of BZN ceramics112 such as

doping with different elements like Ti 113or making composite of BZN ceramics with other

Ba(B’1/3B”2/3)O3 microwave dielectric ceramics (B’ = Mg, Zn, Ni; B” =Ta, Nb) like

Ba(Zn1/3Nb2/3)O3 – Ba(Ni1/3Nb2/3)O3.114 0.35[Ba(Ni1/3Nb2/3)O3] – 0.65[Ba(Zn1/3Nb2/3)O3]

composite microwave dielectric material which was sintered at 1450oC for 4 hours and

annealed for at 1300oC for 72h exhibited good dielectric properties: τf = +0.6ppmoC-1, εr =

35 and quality factor (Q) in excess of 25,000GHz.114 Various amounts of different dopants

such as oxides of monovalent, divalent, trivalent, tetravalent, pentavalent and hexavalent

elements were employed in the synthesis of BZN. Effect of these dopants on microwave

dielectric properties of BZN was investigated.115 Some of the dopants were found to

increase quality factor Q×f and slightly alter the temperature coefficient of resonant


42

frequency (τf). The quality factor was found to depend on the dopant ionic radii and its

concentration. The quality factor was observed to increase when the ionic radius of the

dopant was close to the ionic radii of the B site ions Zn and Nb.115 In a synthesis of Bi, Ce

and In doped Ba(Zn1/3Nb2/3)O3(BZN), by conventional mixed oxide technique, the effect

of each dopant on the sintering temperature and dielectric properties was investigated.116

‘In’ doping between 0.2 and 4.0 mol% increased the density of BZN at 1300oC, but Ce

doping caused a decrease in density at 1250oC. Levels of Bi2O3 up to 1.0 mol% had

negative effect on densification, while high level doping significantly improved the

densification of the specimens. It was found that In, Ce and Bi doping resulted in single

phase formation at all concentrations, except 0.5 mol% Bi. In and Ce doping increased

the dielectric constant from 41 to around 66 at 1MHz, however, Bi doping decreased

the dielectric constant to about 37 at 0.2 mol%, and then higher doping116 led to

increase to about 63.

Calcinations conditions also play an important role in the formation of an ordered phase

and on the quality factor (Q) of BZN. Table 1.5 summarizes the microwave dielectric

properties of BZN ceramics under various calcination and sintering conditions. The

relative permittivity varies little around 35–41, but the Qxf values were significantly

affected by the calcinations and sintering conditions.

Table 1.5: Microwave Dielectric Properties of BZN Ceramics under different Calcination & Sintering Conditions 117

Calcination condition

Sintering condition

Relative density

(%)

Second phase εr Q x f (GHz)

1000oC/6h

1250oC/4 h 97.2 None 39.4 5749 1300oC/4 h 97.6 None 40.9 7094 1350oC/4 h 97.8 None 40.2 8915 1350oC/48 h 96.2 Ba5Nb4O15 39.2 11560 1350oC/4 h† 97.5 None 40.1 3720

1250oC/12h 1350oC/4 h 85.5 Ba5Nb4O15 35.5 9027 1450oC/4 h 96.5 Ba5Nb4O15 39.9 66278 1550oC/4 h 93.8 Ba5Nb4O15 38.2 41639

1000oC/6h and

1250oC/12h

1300oC/4 h 96.7 None 40.6 47101

1350oC/4 h 96.4 None 39.4 112286

Double calcinations

1400oC/4 h 95.4 None 41.1 69675 1350oC/48 h 95.0 Ba5Nb4O15 39.7 236 753

1350oC/48 h‡ 93.8 Ba5Nb4O15 37.7 67 714

†Annealing at 1250oC/12 h after sintering of 1350oC/4 h. ‡Without muffling powder during sintering. BZN, barium zinc niobate (Ba(Zn1/3Nb2/3)O3).


43

For BZN specimens single calcined at 1000oC/6 h and sintered for 4 h, there was no

second phase formation, and all had a high relative density (>97%) but relatively low

Q x f values. The Q x f value did increase at higher sintering temperature (1250o–

1350oC) but not by much. Prolonged soaking time (1350oC/ 48 h) also increased the Q

x f value slightly. For sintered BZN specimens after 1250oC/12 h of single calcination,

the relative density decreased due to second phase formation, but the Q x f value

increased markedly. For the specimens double calcined at 1000oC/6 h and 1250oC/12 h,

the Q x f values became much higher than those with single calcination. The highest Q x

f of 236,753 GHz was observed for the specimen sintered at 1350oC/48 h. Raising the

sintering temperature (1300o–1350oC) or prolonging the soaking time could increase the

Q x f value, but not when sintered at 1400oC or without powder muffling during

sintering to prevent ZnO loss.

Fig. 1.24: Dark-field transmission electron microscopy (TEM) images of the barium zinc niobate (BZN) specimen after double calcinations and sintering at (a)1350oC/4h and (b)1350oC/48h.117

Figure 1.24 shows a dark-field TEM micrograph for the sintered specimen of double-

calcined BZN powder (at 1000oC/6 h and 1250oC/12 h). The dark regions did not

correspond to the disordered regions but rather to the ordered domains whose super

lattice reflections were not enclosed by the objective aperture of TEM. Figure 1.24(a)

shows that ordered domains, typically 50–100 nm in size, were present in the specimen

of a shorter soaking time (4 h). The ordered domains grew by an order of magnitude


44

when the soaking time was increased from 4 to 48 h at 1350oC, as shown in Fig. 1.24(b)

of lower magnification. Apparently, the synthesis condition had a pronounced effect on

the growth of ordered domain in the sintered BZN specimen. Thus by examining the

effects of calcination condition on the ordered structure and the quality factor for BZN

microwave dielectric ceramics, the above study verified that the calcination condition

had played an important role in the formation of an ordered phase and the growth of an

ordered domain. The quality factor was found closely related to the size of ordered

domain, the degree of 1:2 ordering, and relative density. A BZN specimen sintered at

1350oC/48 h from the double-calcined powder possessed an εr of 39.7, a τf of

15 ppm/oC, and a Q x f value of 236753 GHz117

As a part of investigation of BZN-based ceramics, cooling rate after sintering has also

been found a factor to affect dielectric properties significantly. R. Freer et al., examined

the effect of the cooling rate (2°C–240°C/h) after sintering on the microwave dielectric

properties of the ceramics 0.35Ba(Ni1/3Nb2/3)O3–0.65Ba(Zn1/3Nb2/3)O3 being prepared

by mixed oxide route. The perovskite ceramics Ba(Zn1/3Nb2/3)O3 (BZN) and

Ba(Ni1/3Nb2/3)O3 (BNN) exhibited similar relative permittivities (≈40 and 29,

respectively), but τf values of opposite polarity. When BZN and BNN were combined

in 0.65:0.35 ratio (i.e., 0.35Ba(Ni1/3Nb2/3)O3 0.65Ba(Zn1/3Nb2/3)O3, a temperature-stable

product was achieved, with τf ≈ 0 ppm/ °C, although the dielectric losses varied

significantly with processing conditions.114

The variation in the microwave dielectric properties of 0.35BNN-0.65BZN ceramics

was examined as a function of cooling rate after sintering. While the extrinsic factors

(porosity, Inhomogeneity) were found to dominate the dielectric properties in the low-

frequency regime, they had very limited effect in the terahertz frequency regime.

A slower cooling rate resulted in enhanced Q x f values for the materials, but had

comparatively little effect on the relative permittivity. This correlates very well with the

increase in B-site occupancy, while the cell volume changed little as the cooling rate is

decreased. These phenomena are consistent with the facts that, as the sample cools

more slowly after sintering, the Raman shifts are changed insignificantly.118


45

In an another example, solid solutions of the (x)Ba(Zn1/3Nb2/3)O3 – (1-

x)Ba(Co1/3Nb2/3)O3 (BZCN) system were produced by the mixed oxide route.119

Compositions near to 0.6[Ba(Zn1/3Nb2/3)O3]– 0.4[Ba(Co1/3Nb2/3)O3] exhibited zero

temperature dependence of resonant frequency (τf ≈ 0).Sintering temperatures of 1300–

1500oC with cooling rates of 3–180oC/h were used. Sintered densities of ≤95% were

achieved. A second phase deficient in Co and Zn was detected. Slow cooling after

sintering, down to 900oC, induced B-site ordering which increased the dielectric Q

values. The material would offer a new design option for filter engineers in the

development of 3G and other similar telecommunication systems. BZCN ceramics

would offer a high Q solution for base station filtering at a greatly reduced cost

compared to BZT ceramics.119

Small deviations in the stoichiometry of Ba(Zn1/3Nb2/3)O3 (BZN) and other related

microwave perovskites have a profound effect on their ordering, sintering, and

dielectric properties. The cubic perovskite structure is well known to accommodate

large degrees of non-stoichiometry and vacancy formation on the A-site and anion

positions. While the B sites readily accept multiple cations and the degree of vacancy

formation on this position is limited to concentrations less than 2%. (The exact relations

between the B-site ions and the ceramics dielectric properties are still unclear.120 The

bulk crystal chemistry of BZN has been extensively investigated, and it undergoes a

transition from a 1:2 ordered to disordered B-site arrangement at 1375oC.25,76,121-123

Much higher Q factor has been observed and reported in the disordered phase sintered

below 1350C than the ordered-phase sintered above1350C in case of Ba(Zn1/3Nb2/3)O3

(BZN), unlike BZT.124 The influence of the crystal structural ordering, microstructure,

secondary phase and lattice defect has been investigated on the high Q factor of the

BZN, using series of the samples with their compositions near the stoichiometric

BZN.125 The observations can be listed as:

1. The structural order and the crystal phases depended on the slight composition

deviation from the stoichiometric BZN and on the heat treatment.

2. For the stoichiometric BZN, the order-disorder phase transition occurred between

1300oC and 1400C (as evident from the plots in the Fig. 1.25).


46

3. Regardless the crystal structural ordering the stoichiometric BZN with high

density and large grain size exhibited the significant high Q factors (Q x f) ≈ 9-10

× 104 GHz). This result suggests that the pore and grain boundary around the

crystal grain should affect the variation of the Q factor than the crystal-structural

ordering.

4. In the other regions, by the slight composition deviation from stoichiometry, the

ordered BZN with the secondary phase or the non-stoichiometric disordered BZN

with low density were formed.

Fig. 1.25: Q x f, density and average grain size dependence of the stoichiometric BZN as functions of various heat treatments. Ord:Ordered perovskite(trigonal), Dis:Disordered perovskite (Cubic). 125


47

Since the Q factors were found to be low, the presence of the secondary phases, the

lattice defect and low density were considered to be causes of the decrease of the Q

factors in this system. From these results for BZN system, the highly dense ceramics

with large grain size in the microstructure and the suppressions of the secondary phase

and of the lattice defect by strict composition control would provide the further Q factor

improvements in BZN system.125 Thus the influence of the crystal-structural ordering

and the microstructure on the high Q factor seems far from fully understanding.

The dielectric loss properties of perovskite microwave materials are notoriously

sensitive to a number of different processing and structural variables. The losses can

vary from sample to sample even when they have the same nominal composition due to

small differences in the intrinsic crystal structure, microstructure, density, and impurity

concentration. In an investigation126 2 wt. % of ZnO–SiO2–B2O3 glass phase and

1 wt.% of LiF-added BZN sample sintered at 900oC exhibited a relative density higher

than 95% and attractive dielectric properties: a dielectric constant (εr) of 39, low

dielectrics losses (tan(δ) < 10-3) and a temperature coefficient of permittivity (τε) of 45

ppm/oC. The 2 wt.% ZnO–SiO2–B2O3 glass phase and 1 wt.% of B2O3-added BZN

sintered at 930oC showed also attractive dielectric properties (εr=38, tan(δ) < 10-3) and

it was more interesting in terms of temperature coefficient of the permittivity (τε = -5

ppm/oC).126 The lowest microwave losses have been reported in alkaline-earth based

niobate and tantalate members of the’’1:2’’ A2+ (B1/32+B2/3

5+)O3 family of mixed-metal

perovskites (A2+ = Ca, Sr, Ba; B2+ = Mg, Ca, Sr, Mn, Fe, Co, Ni, Cu, Zn; and B5+ = Nb,

Ta).89 The ionic radii of Sr2+ is 0.116 nm and that of Ba2+ is 0.136 nm127, the difference

between them are less than 15%, moreover these two ceramics have opposite signal of

the temperature coefficient of the capacitance (τc); negative and positive respectively.

Therefore, a temperature stable solid solution can be fabricated by the substitution of Sr2+

to Ba2+. A study 107 of the microwave dielectric properties of (Ba1-xSrx)(Zn1/3Nb2/3)O3

(BSZN) solid solutions revealed that the samples with (Ba0.35Sr0.65)(Zn1/3Nb2/3)O3 and

(Ba0.30Sr0.70)(Zn1/3Nb2/3)O3 had near zero τc values (-6.9 x 10-6 and -2.1 x 10-6ppm/oC

respectively), suitable dielectric constants (εr ≈ 42.4 and 39.4 respectively) and

relatively high values of Q (3000 and 3365, respectively at about 3.85 GHz), indicating

the suitability of these ceramics for the applications as microwave components.


48

Another family of perovskites128 with the general stoichiometry (A2+2/3La1/3)

(B1+1/3B

5+2/3)O3

at 1 MHz (Sr2/3La1/3) (Li1/3Nb2/3)O3 showed εr =30, τc=146 ppm/°C;

(Sr2/3La1/3) (Li1/3Ta2/3)O3 εr =25 and τc=156 ppm/°C. Whereas at microwave

frequencies (Sr2/3La1/3) (Li1/3Nb2/3)O3 showed εr =29, and τf = -75.5 ppm/°C and quality

factor (Q x f ) = 6300 at 8.9 GHz; (Sr2/3La1/3) (Li1/3Ta2/3)O3 showed εr =25, and τf = -

25.1 ppm/°C and Q x f = 25 200 at 10.2 GHz. Measurements on (Ca2/3La1/3)

(Li1/3Nb2/3)O3 at microwave frequencies yielded Q x f =26 500 at 8.7 GHz, εr =30 and τf

= -25.7 ppm/°C. While the quality factors of these dielectrics are not as high as those of

the known 1:2 ordered high Q perovskite microwave dielectrics, they are quite

respectable and their negative values of τf coupled with their lower sintering

temperatures may make them attractive candidates for tuning the properties of the BZN

(or BZT)-type systems.128

Ba(Mg1/3Nb2/3)O3 (BMN) is known as a good dielectric material for a microwave

resonator9 since has εr of 32, Q value of 5600 at 10.5 GHz, and τf of 33 ppmoC-1.

Although this resonator has a high dielectric constant, its Q value is relatively lower

than those of other complex perovskites. In order to enhance the Q value, additives were

often used.129 However, the additives form a second phase, which is sometimes

detrimental to microwave dielectric properties. One way of improving the Q value is to

enhance the sinterability of the specimen through the change of the materials

stoichiometry.130 Paik et al. investigated130 the sinterability and microwave dielectric

properties of Mg-deficient BMN specimens. Mg deficiency has a significant effect on

the microwave dielectric properties. An improvement in both density and the dielectric

Q value when x = 0.02 in Ba(Mg1/3−xNb2/3)O3 has been reported. A 1:2 ordering of B-

site ions in a complex perovskite compound is formed due to the differences in size and

charge valencies of the two B-site ions.24 Furthermore, a 1:2 ordering produces the

deviation of c/a ratio from the ideal value for hexagonal structure (√3/√2 = 1.2247),

which introduces the lattice distortion in the hexagonal unit cell. The lattice distortion

can cause the splitting in (422) and (226) reflections.130 It has been found that 1:2

ordering and lattice distortion are sensitive to the Mg deficiency.

SEM results (Fig. 1.26) indicate that grain growth is not completed in the specimens

with x>0.02. Therefore, it is suggested that a small number of Mg vacancies enhances


49

the sinterability, which leads to the increase of relative density of the specimen.

However, a large number of Mg vacancies inhibits the sintering and thus decreases the

relative density of the specimen.

Fig. 1.26: SEM micrographs130 of Ba(Mg1/3-xNb2/3)O3 at: (a) x = 0, (b) x = 0.01, (c) x=0.02, (d) x= 0.03, (e) x= 0.04.

The variation of dielectric constant with Mg deficiency is not significant. For all the

specimens, it is about 32. On the other hand, the temperature coefficient of resonant

frequency decreases from 32 to 22 as x varies from 0 to 0.03. However, for the

specimen with x =0.04, it increases to 45. The variation of τf with x is not clear. The Q

values are normalized values to a frequency of 8 GHz. The variation of Q value with


50

Mg deficiency is very similar to that of relative density and lattice distortion. Therefore,

the improvement of Q value with Mg deficiency could be related to the increase of both

relative density and 1:2 ordering. The highest Q value achieved in this investigation was

about 12 000 at 8 GHz for Ba(Mg1/3-xNb2/3)O3.

Young-Woong Kim et al.,19 investigated the effect of heat treatment on the cationic

ordering behaviour in Ba(Mg1/3Nb2/3)O3 (BMN) complex perovskite microwave

ceramics. The effect of sintering temperature and time on the cationic ordering in the

BMN ceramics was studied by using transmission electron microscopy (TEM) and

energy-dispersive spectroscopy (EDS). The relationship between the local structure and

chemical composition, and the effect of the structural variation on microwave dielectric

properties of the ceramics were studied.

Table 1.6. Microwave dielectric properties of BMN ceramics sintered with various sintering conditions.19

Sintering condition

Dielectric constant

Quality factor (Q x f: GHz)

τf

(ppm/oC) Sintered density

1350oC - 4h 31.2 46,000 +18 96.9

1500oC - 4h 31.5 15,300 +20 96.0

1350oC - 40h 31.0 28,600 +18 96.0

The BMN ceramics show very diverse local cationic ordering behaviour according to

the sintering conditions, such as the development of domain twinning, “core-shell”

structured grains and local disordered structure, though having 1:2 cationic ordering

structure basically. When the BMN ceramics were sintered at 1350oC for 40 h and

1500oC for 4 h, a local order-disorder transition was observed in Mg-excess regions,

especially where the ratio of Nb/Mg is less than 1.8. It is postulated that the formation

of BaNb2O6-like second phase at grain boundary triple points resulted in local

compositional change in the 1:2 ordered matrix grain and thus caused the local

disordering. The microwave dielectric quality factor of the ceramics decreases greatly

with the increase of the structural and chemical inhomogeneity.


51

Aim of the Thesis

The objective of the thesis is to understand how the dielectric properties in the

microwave dielectrics are affected by the processing conditions, and by synthesizing

methods and to synthesize and characterize the phase that forms during the syntheisis

and sintering of cubic perovskites.

Chapter 2

This chapter deals exclusively on the various methods of initial calcinations condition

in the synthesis of Ba(Zn1/3Ta2/3)O3 and we had compared between various methods.

• Comparison between Agate grinding and ball milling

• Comparison between various solvents in ball milling

• Comparison between various calcinations time in Agate grinding

• Comparison between various calcinations time in Ball-milling

Chapter 3

Flux method is one of the well known method in the literature, we explored this method

for synthesizing Ba(Zn1/3Nb2/3)O3 and Ba(Mg1/3Nb2/3) and studied the effect of

temperature and soaking time on the 1:2 ordering of these systems.

Chapter 4

In this chapter we studied the solid solution and composites of hexagonal perovskites,

which forms during the sintering of cubic perovskites, the various system studied are:

• (1-x) (Ba8ZnTa6O24) – x (Sr8ZnTa6O24)

• (1-x) [Sr(Zn1/3Ta2/3)O3] – x[Sr(Zn1/8Ta3/4)O3]

• (1-x) [Ba(Co1/3Ta2/3)O3] – x[Ba(Co1/8Ta3/4)O3]


52

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