UNIVERSITI PUTRA MALAYSIA

STRUCTURAL ANALYSIS AND ELECTRICAL CHARACTERISATIONS OF Sr(1-x)CaxTiO3 AND Sr(1-x)BaxTiO3 CERAMICS

AZIZAH BINTI ISHAK

FS 2008 20

STRUCTURAL ANALYSIS AND ELECTRICAL CHARACTERISATIONS

OF Sr(1-x)CaxTiO3 AND Sr(1-x)BaxTiO3 CERAMICS

By

AZIZAH BINTI ISHAK

Thesis submitted to the School of Graduate Studies, Universiti Putra Malaysia, in Fulfilment of Requirement for the Degree of Master of Science

February 2008

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DEDICATION

This dedication especially goes to my husband Samsudin Bin Mohd Said, my beloved parents Hj Ishak Shamsuddin and Hjh Yusti Isrin; my loving

children Hazim, Haziq, Nabilah and Azfar; my brother Aziz, sisters Rozita, Rozana, Hanita and also to my mother-in-law

Hjh Indon Shaari and all my dear friends.

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Abstract of thesis presented to the Senate of Universiti Putra Malaysia in fulfilment of the requirements for the degree of Master of Science

STRUCTURAL ANALYSIS AND ELECTRICAL CHARACTERISATIONS OF Sr(1-x)CaxTiO3 AND Sr(1-x)BaxTiO3 CERAMICS

By

AZIZAH BINTI ISHAK

February 2008

Chairman : Associate Professor W. Mohamad Daud W. Yusof, PhD.

Faculty : Science

Ceramic samples of a structural formula Sr1-xCaxTiO3 (SCT) and Sr1-xBaxTiO3 (SBT)

were prepared from the starting materials CaCO3, BaCO3, SrCO3 and TiO2 through

the conventional solid state reaction technique. The XRD measurements reveals that

the lattice parameter of SrTiO3 changes after the substitution of Ba2+ and Ca2+ while

the specimens, Sr1-xBaxTiO3 and Sr1-xBaxTiO3 showed the typical XRD patterns of

perovskite polycrystalline structure without a preferred orientation; no second phase

was observed.

The crystalline structure and dielectric properties of SCT and SBT ceramics with

various compositions of x were investigated. The a- and c-axis lattice constants of

SCT and SBT were calculated. It is found that the crystal structures of SBT are

cubic phase when x ≤ 0.6 and tetragonal phase when x ≥ 0.7 at room temperature.

However, the crystal structures of SCT is orthorhombic when x ≥ 0.1 at room

temperature.

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The dielectric properties such as real permittivity or dielectric constant (ε′),

imaginary permittivity or loss factor (ε″) and dielectric loss (tan δ) have been

measured on the sintered disks with respect to frequency in the range 0.01 Hz to 1

GHz at room temperature and varying temperatures from 40oC to 200oC. Sample

SBT-5 shows the largest value of the real permittivity(ε′ = 4800), low dielectric loss

tangent (tan δ = 2.77) and very low ac conductivity (σ = 7.39×10-9 S/cm) at 0.01 Hz.

Ceramic sample SCT shows very low dielectric conductivity,σ. The conductivity

increases with frequency over the entire temperature range reflecting higher

dielectric loss at higher temperature. In the impedance and modulus relaxation

spectroscopy, the magnitude of imaginary impedance, Z″ maxima decrease with

temperature indicating increasing loss in the resistive property of the sample. This

behaviour of impedance pattern arises probably due to the presence of space charge

in the material. The activation energy decreases from 0.301 eV to 0.221 eV when the

composition of Ca2+ is increased from x = 0.3 to x = 0.9. The electric behavior of

sample SBT showed that the activation energy has the value of 0.337 eV and 0.097

eV.

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Abstrak tesis yang dikemukakan kepada Senat Univesiti Putra Malaysia sebagai memenuhi keperluan untuk ijazah Master Sains

PENGANALISAAN STRUKTUR DAN PENCIRIAN ELEKTRIK BAGI SERAMIK Sr(1-x)CaxTiO3 DAN Sr(1-x)BaxTiO3

Oleh

AZIZAH BINTI ISHAK

Februari 2008

Pengerusi : Professor Madya W. Mohamad Daud W. Yusof, PhD.

Fakulti : Sains

Sampel seramik yang mempunyai formula struktur Sr1-xCaxTiO3 (SCT) and Sr1-

xBaxTiO3 (SBT) telah disediakan daripada bahan-bahan berikut iaitu CaCO3, BaCO3,

SrCO3 and TiO2 menggunakan teknik konvensional tindakbalas keadaan pepejal.

Pengukuran XRD menunjukkan parameter kekisi bagi SrTiO3 berubah setelah

didopkan dengan Ba2+ dan Ca2+, manakala sampel Sr1-xBaxTiO3 dan Sr1-xBaxTiO3

menunjukkan corak spektrum yang biasa bagi struktur polikristilin perovskit di mana

tiada perubahan orientasi; tiada fasa kedua.

Struktur kristal dan ciri-ciri dielektrik bagi sample seramik SCT dan SBT dengan

komposisi x telah dikaji. Daripada pengiraan pemalar kekisi paksi-a dan paksi-c

bagi sample SCT dan SBT, didapati struktur kristal bagi SBT adalah dalam fasa kiub

apabila x ≤ 0.6 dan fasa tetragonal apabila x ≥ 0.7 pada suhu bilik. Manakala

struktur kristal bagi SCT adalah ortorombik apabila x ≥ 0.1 pada suhu bilik.

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Sifat dielektrik seperti permittiviti nyata atau pemalar dielektrik (ε′), permittiviti

maya atau faktor kehilangan (ε″) dan kehilangan dielektrik (tan δ) telah diukur ke

atas disk yang telah disinter, terhadap frequensi di dalam renj 0.01 Hz ke 1 GHz pada

suhu bilik dan juga perubahan suhu dari 40oC ke 200oC. Sampel SBT-5 mempunyai

nilai nyata pemalar dielektrik yang maksimum (ε′ = 4800), kehilangan tangent

dielektrik yang rendah (tan δ = 2.77) dan nilai minima bagi kekonduksian (σ =

7.39×10-9 S/cm) pada 0.01 Hz.

Kekonduksian dielektri, σ, bagi sample seramik SCT menunjukkan nilai

kekonduksian yang sangat rendah. Kekonduksian juga bertambah dengan frekuensi

disepanjang renj suhu dan ini menunjukkan kehilangan dielektrik yang tinggi pada

suhu yang tinggi. Dalam bahagian spektroskopi relaksasi bagi impedans dan

modulus, magnitud bagi impedans khayalan Z″ maksima didapati berkurangan

dengan suhu menunjukkan peningkatan dalam kehilangan sifat resistif pada sample

tersebut. Perubahan pada corak impedans seperti ini adalah disebabkan oleh

kewujudan cas ruang di dalam bahan. Didapati tenaga pengaktifan berkurangan dari

0.301 eV ke 0.221 eV apabila komposisi Ca2+ bertambah dari x = 0.3 ke x = 0.9.

Ciri dielektrik bagi sampel SBT-1 menunjukkan tenaga pengaktifan mempunyai nilai

0.337 eV dan 0.097 eV.

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ACKNOWLEDGEMENTS

Firstly, I am very grateful to ALLAH s.w.t for giving me the strength and patience to

complete this research. It is my pleasure to gratefully acknowledge the enthusiastic

supervision of my research advisor, Associate Professor Dr. W Mohamad Daud W

Yusof for reviewing my thesis and giving insightful comments. His knowledge and

depth of commitment to this area of research impressed me greatly and were critical

to the completion of this research work.

I am also deeply grateful to my co-supervisors, Professor Dr. Abdul Halim Shaari

and Professor Dr. W Mahmood for their valuable support and helpful comments

throughout my research. I owe special thanks to our XRD lab officer Miss Yusnita

Osman, lab assistants, Mr. Razak Harun and Mr. Suhaimi Ibrahim, the technical staff

of Electron Microscopy Unit for their kind assistance. I am also indebted to all the

lecturers and staff in the Physics Department especially Dr. Halimah Mohamed

Kamari whose friendship and knowledge I admire.

Finally, I must acknowledge my parents Haji Ishak and Hajjah Yusti for their

constant love and support throughout my life. My special thanks to my children

M.Hazim, M.Haziq, Aniqah Nabilah and M.Azfar for their love, understanding and

time they missed their mother. Also to my mother-in-law Hajjah Indon Shaari for her

love and support. During my graduate studies, my beloved husband, Samsudin Bin

Mohd Said, has been the source of endless encouragement. Without his support, love

and presence, this study could not have been completed.

May Allah Bless You All!

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I certify that an Examination Committee has met on 28 February 2008 to conduct the final examination of Azizah Binti Ishak on her Master of Science thesis entitled “Structural Analysis And Electrical Characterisations of Sr1-xCaxTiO3 and Sr1-

xBaxTiO3 Ceramics” in accordance with Universiti Pertanian Malaysia (Higher Degree) Act 1980 and Universiti Pertanian Malaysia (Higher Degree) Regulations 1981. The Committee recommends that the candidate be awarded the degree of Master of Science. Members of the Examination Committee are as follows: Zainal Abidin Bin Sulaiman, PhD Associate Professor Faculty of Science Universiti Putra Malaysia (Chairman) Sidek Bin Hj Abdul Aziz, PhD Associate Professor Faculty of Science Universiti Putra Malaysia (Internal Examiner) Halimah Binti Mohamed Kamari, PhD Lecturer Faculty of Science Universiti Putra Malaysia (Internal Examiner) Sinin Hamdan, PhD Associate Professor Faculty of Science Universiti Malaysia Sarawak (External Examiner)

________________________________ HASANAH MOHD GHAZALI, PhD Professor and Deputy Dean School of Graduate Studies Universiti Putra Malaysia Date : 22 July 2008

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This thesis was submitted to the Senate of Universiti Putra Malaysia and has been accepted as fulfillment of the requirement for the degree of Master of Science. The members of the Supervisory Committee were as follows: W. M. Daud W Yusof. PhD Associate Professor Faculty of Science Universiti Putra Malaysia (Chairman) Abdul Halim Bin Shaari, PhD Professor Faculty of Science Universiti Putra Malaysia (Member) W. Mahmood Mat Yunus, PhD Professor Faculty of Science Universiti Putra Malaysia (Member) _______________________________ AINI IDERIS, PhD Professor and Dean School of Graduate Studies Universiti Putra Malaysia Date: 14 August 2008

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DECLARATION

I hereby declare that the thesis is based on my original work except for quotations and citations which have been duly acknowledged. I also declare that it has not been previously or concurrently submitted for any other degree at UPM or other institutions. __________________________ AZIZAH BINTI ISHAK Date : 20 August 2008

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TABLE OF CONTENTS

Page DEDICATION ii ABSTRACT iii ABSTRAK v ACKNOWLEDGEMENTS vii APPROVAL SHEETS viii DECLATION FORM x LIST OF TABLES xiii LIST OF FIGURES xiv LIST OF ABBREVIATIONS xvii CHAPTER I INTRODUCTION 1 1.1 Electronic Ceramics 3 1.2 Scope of Study 6 1.3 Objectives of This Research 8 II LITERATURE REVIEW 2.1 Introduction 9 2.2 Previous Works on Ferroelectric Ceramics 10 2.3 Ferroelectric Materials 14 2.4 Strontium Titanate 15 2.5 Strontium Barium Titanate System 17 2.6 Strontium Calcium Titanate System 22 III THEORY 3.1 Introduction 25

3.2 Synthesis and Processing 25 3.3 Perovskite Structure 26 3.4 Principles of Dielectrics 31 3.5 Dielectric Behavior 34 3.6 Impedance Spectroscopy 39 3.7 AC Conductivity 44 3.8 Principal types of dielectric functions 45 3.9 X-ray Powder Diffraction (XRD) 48

IV MEHODOLOGY 4.1 Introduction 52 4.2 Materials 52 4.3 Sample Preparation 53 4.4 Structural and Morphological Analysis 58 4.5 Dielectric Spectroscopy 61

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V RESULTS AND DISCUSSION 5.1 Introduction 63 5.2 Structural Analysis 64 5.3 Electrical Properties of SCT and SBT at Room Temperature 80 5.4 Sr1-xBaxTiO3 (SBT) System 86 5.5 Sr1-xCaxTiO3 (SCT) System 99 5.6 Dielectric losses ( tan δ ) 106 5.7 AC Conductivity 110 5.8 Impedance and Modulus Relaxation Spectroscopy 114 VI CONCLUSION 6.1 Structural Analysis 133 6.2 Dielectrical Analysis 134 6.3 Suggestions for Future Research 136 REFERENCES 139 BIODATA OF STUDENT 147

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LIST OF TABLES

Table Page 1.1 Electrical Property constants of different ceramic materials. 6 3.1 Relation between the impedance and other dielectric functions 41 3.2 Capacitance values and their possible interpretation 43 3.3 Interplanar spacings dhkl for different crystal systems and their 50 dependency on Miller indices hkl (Connoly, 2007). 4.1 Molecular weight for the starting materials. 51 4.2 Compounds and weight of substitution (i) Sr1-xCaxTiO3 and 56 (ii) Sr1-xBaxTiO3. 5.1 Properties of Sr1-xBaxTiO3 system sintered at 1200oC 67 5.2 Properties of Sr1-xCaxTiO3 system sintered at 1350oC 69 5.3 Energy dispersive x-ray parameters for the phase of Sr0.7Ca0.3TiO3 73 5.4 Average Grain Size of sample Sr1-xCaxTiO3 79 5.5 Dielectric properties values for sample (a) Sr1-xBaxTiO3 85 (b) Sr1-xCaxTiO3 at room temperature 5.6 Real permittivity values for samples SBT-1 at temperatures (40-200oC) 88 5.7 Values of tan δ for sample SBT-1 at different temperatures 93 and frequencies 5.8 Values of conductivity for sample SBT-1 at different temperatures 95 and frequencies 5.9 Activation energy for various composition in the sample Sr1-xCaxTiO3 132

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LIST OF FIGURES

Figure Page 2.1 Classification of crystals according to the symmetry and polarization 15 (Haertling, 1999) 2.2 Variation of dielectric constant with temperature for bulk and thin 19 film barium strontium titanate (Shaw et al., 1999). 3.1 (a) A cubic ABO3 (BaTiO3) perovskite-type unit cell and (b) three 27 dimensional network of corner sharing octahedral of O2- ion (Xu, 1991). 3.2 The crystal structure of perovskite barium titanate, (a) above the Curie 29 temperature the unit cell is cubic; (b) below the Curie temperature the unit cell structure is tetragonal with Ba2+ and Ti4+ ions displaced to the O2- ions (Safari et al., 1996). 3.3 The variation of the relative permittivity with temperature for BaTiO3 30 Ceramics with (a) 1 mm grain size and (b) 50 mm grain size (Safari et al., 1996) 3.4 Equivalent circuit of a ferroelectric element (non-resonance operation) 33 (Kingery et al., 2000) 3.5 Schematic representation of different mechanisms of polarization. 38 (Brown et al., 2006) 3.6 Contributions to the frequency-dependent dielectric constant from the 38 Different charge configuration (Brown et al., 2006) 3.7 The real and imaginary components of the complex dielectric 46 susceptibility for the Debye process (Jonscher, 1999). 3.8 A near-Debye response showing the characteristic deviation from the 46 Debye shape of χ′(ω) (Jonscher, 1999). 3.9 Bragg’s Law reflection. 49 4.1 Flow chart of the synthesis of SBT and SCY by conventional 54 solid-state route. 5.1 XRD diffractogram with indexing of samples Sr1-xBaxTiO3 sintered 66

at 1200oC. 5.2 Dependence of the lattice parameter and c/a ratio of Sr1-xBaxTiO3 67 sintered at 1200 oC. 5.3 XRD diffractogram of Sr1-xBaxTiO3 system for x = 0.7 sintered at 68

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(A) 1250oC (B) 1350oC. 5.4 XRD diffractogram with indexing of samples Sr1-xCaxTiO3 sintered 71

at 1350oC. 5.5 Energy dispersive x-ray images of Sr0.7Ca0.3TiO3 73 5.6 Scaaning electron microscope (SEM) images of Sr1-xCaxTiO3 76 as prepared powder at different compositions: (a) x= 0.2 (b) x= 0.3 (c) x= 0.4 (d) x= 0.5 (e) x= 0.6 (f) x= 0.8 (g) x= 0.9 (h) x= 1.0 5.7 Permittivity versus frequency for Sr0.5Ca0.5TiO3 and Sr0.5Ba0.5TiO3 81 at temperature 400C (a) ε′ vs frequency (b) ε″ vs frequency 5.8 Variation of tan δ for Sr0.5Ca0.5TiO3 and Sr0.5Ba0.5TiO3 samples at 82 room temperature as a function of frequency. 5.9 Variation of ac conductivity for Sr0.5Ca0.5TiO3 and Sr0.5Ba0.5TiO3 83 Samples at room temperature as a function of frequency. 5.10 Frequency response of permittivity (a) log ε′ (b) log ε″ for sample 87 Sr0.9Ba0.1TiO3 at different temperature. 5.11 Frequency response of ε′, ε″ for SBT-1 sample. 90 5.12 Frequency response of tan δ for sample Sr0.9Ba0.1TiO3. 94 5.13 Frequency response of conductivity for sample Sr0.9Ba0.1TiO3. 94 5.14 Modulus M″ relaxation spectroscopy at varying temperatures for sample 98 Sr0.9Ba0.1TiO3 (SBT-1) 5.15 Plot of log τ versus 1000/T for sample Sr0.9Ba0.1TiO3 (SBT-1) 98 5.16 Frequency response of permittivity (a) ε′(b) ε″ for sample SCT-3 at 101 different temperatures. 5.17 Frequency response of permittivity (a) ε′(b) ε″ for sample SCT-4 at 102 different temperatures. 5.18 Frequency response of permittivity (a) ε′(b) ε″ for sample SCT-6 at 103 different temperatures. 5.19 Frequency response of permittivity (a) ε′(b) ε″ for sample SCT-9 at 104 different temperatures. 5.20 Frequency dependence of the dielectric loss tangent as a function of 108 temperature for (a) SCT-3 (b) SCT-4.

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5.20 Frequency dependence of the dielectric loss tangent as a function of 109 temperature for (c) SCT6 (d) SCT-9. 5.21 Variation of ac conductivity with frequency at different temperatures 112 for (a) SCT-3 (b) SCT-4. 5.21 Variation of ac conductivity with frequency at different temperatures 113 for (c) SCT-6 (d) SCT-9. 5.22 Real part of impedance as a function of frequency at different 116 temperatures for (a) SCT-3 (b) SCT-4.

5.22 Real part of impedance as a function of frequency at different 117 temperatures for (c) SCT-6 (d) SCT-9. 5.23 Imaginary part of impedance as a function of frequency at different 118 temperatures for (a) SCT-3 (b) SCT-4. 5.23 Imaginary part of impedance as a function of frequency at different 119 temperatures for (c) SCT-6 (d) SCT-9. 5.24 Variation of Z″ and M″ as a function of frequency measured at varying 121 temperatures from 40oC to 200oC for sample SCT-9 5.25 Modulus M″ relaxation spectroscopy at different temperatures for 129 (a) SCT-3 (b) SCT-4.

5.25 Impedance M″ relaxtion spectroscopy at different temperatures for 130 (c) SCT-6 (d) SCT-9. 5.26 Plots of log τ vs 1000/T for sample Sr1-xCaxTiO3 at different 131

Compositions.

5.27 The relationship between activation energy, E against composition, x for 131 sample Sr1-xCaxTiO3.

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LIST OF ABBREVIATIONS

ε Dielectric permittivity

ε′ Real part of permittivity

ε″ Imaginary part of permittivity

εr Relative dielectric permittivity

ε(ω) Dielectric permittivity as a function of angular frequency

σ Conductivity

σ (ω) Conductivity as a function of angular frequency

τ Relaxation time (sec)

ω Angular frequency

ωc Critical angular frequency

ωp Peak angular frequency

ac Alternating current

dc Direct current

C* Complex capacitance

C′ Real part of capacitance

C″ Imaginary part of capacitance

E Activation energy

G Conductance

M Modulus

M* Complex modulus

M′ Real part of modulus

M″ Imaginary part of modulus

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MHz Megahertz

R Resistance

Y* Complex admittance

Y′ Real part of admittance

Y″ Imaginary part of admittance

Z* Complex impedance

Z′ Real part of impedance

Z″ Imaginary part of impedance

XRD X-ray diffraction

SEM Scanning electron microscope

EDX Energy dispersive x-ray

Sr Strontium

Ba Barium

Ca Calcium

ST Strontium titanate

SBT Strontium barium titanate

SCT Strontium calcium titanate

CHAPTER 1

INTRODUCTION

A ceramic has traditionally been defined as "an inorganic, nonmetallic solid that is

prepared from powdered materials, is fabricated into products through the application

of heat, displays such characteristic properties as hardness, strength, low electrical

conductivity and brittleness" (NDT, website). The word ceramic comes from Greek

word "keramikos", which means "pottery." They are typically crystalline in nature

and are compounds formed between metallic and nonmetallic elements.

The atoms in ceramic materials are held together by a chemical bond, The two most

common chemical bonds for ceramic materials are covalent and ionic. Covalent and

ionic bonds are much stronger than in metallic bonds and, generally speaking, this is

why ceramics are brittle and metals are ductile. Many adopt crystalline structures,

but some form glasses. The properties of the materials are a result of the bonding and

structure.

Alkaline earth titanates of general formula ATiO3 ( A = Ca, Sr, Ba and Pb) and

having perovskite structure are useful materials because of their wide applications in

the electroceramics industry. Owing to their ferroelectric and piesoelectic properties,

BaTiO3 and PbTiO3 form the basis of a veriety of electronic devices ( Parkash et. al.,

2003). Besides its relevance for electronics, sensor science, electrochemistry and

photochemistry, SrTiO3 (strontium titanate) is an excellent model material for a

mixed type conductor because of availability of a detailed knowledge ot its defect

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models, chemical and transport parameters. Strontium tianate (SrTiO3) is an incipient

ferroelectric (or quantum paraelectric) in which a ferroelectric phase can be induced

at a low temperature by isotrophic substitution or substitutional impurities as well as

by external stress (Ping et al., 2008).

Perovskite materials with high dielectric permittivity have many applications in

dyamic random access memories and ferroelectric non-volatile memories. High-K

dielectric materials with low dielectric lossess are rewuired today for various

applications in low dimention devices for a large frequency range, including optical

domain (Barberacu et. al., 2008). The permittivity control of theses materials by a

suitable DC bias field ensures a great opportunity of the phase control of the

microwave devices. In this view strontium barium titanate (SBT), bulk ceramics

and/or thin films in paraelectric state, is one of the promising material. Solid solution

oxides of calcium strontium titante (SCT) with high dielectric constant and dielectric

properties of a small temperature coefficient have attracted a great deal of attention

for practical use as capacitor dielectrics (Ping et. al., 2008).

Electroceramics are advanced materials whose properties and applications depend on

the close control of structure, composition, ceramic mixture, dopants and dopant (or

defect) distribution ( Irvine et. al.,1990). In the late 1980s, conventional processing

methods for producing electroceramic materials (powder compacting, tape casting

followed by sintering processes) were increasingly supplemented by thin film

deposition techniques. Their properties can be tailored to operation as insulators,

ferroelectric materials, highly conductive ceramics, electrodes as well as sensors and

actuators.

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1.1 Electronic Ceramics

Advanced materials and particularly advanced ceramics play an important role in the

future of the world economy. The development and use of advanced ceramics are

revolutionizing the field of material science and technology. Advanced ceramics are

inorganic, nonmetallic materials with combinations of fine-scale microstructures,

purity, complex compositions and crystal structures, and accurately controlled

additives. Advanced ceramics can be classified into two groups, structural and

electronic ceramics. The study of electronic ceramics involves many disciplines

including chemistry, physics, metallurgy, mechanical engineering and material

science.

Electronic ceramics can be divided into two main groups :

(i) insulators (dielectrics) and semiconductors, and

(ii) conductors (ionic and mixed).

The use of ceramics in electronics is becoming increasingly popular. Insulating

ceramics are used extensively in consumer and industrial electronic system.

Dielectrics are employed as capacitors to store charge and control the flow of

current; for miniaturization of devices the ceramic layers should be as thin as

possible. Special low loss dielectrics, operating as resonators are the key to the

operation of mobile telephone handsets and base stations. Semiconducting ceramics

(non-linear resistors) are employed as protection devices against power surges in

almost all electronic and electrical system from computers to power distribution

networks. Ceramic superconductors allow the flow of electricity with little or no

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resistance or heat loss. This gives the potential for trains floating on a magnetic

cushion of air running at two to three hundred miles per hour and super computers

the size of desktop calculators performing calculations thousands of times faster than

any computer today.

The applications of ceramics in the electronics industry can be divided into two

groups:

(i) the use of materials for interconnection and packaging of semiconductor

circuits, and

(ii) the use of ceramics in circuit components which perform a function in their

own right, such as capacitors and sensors (Whatmore , 1988).

The former application forms a large market and has been well reviewed elsewhere.

The latter is particularly interesting because the materials which are used for a wide

range of applications are in many cases closely related in crystal structure.

In many circuit functions, the material is required to give an electrical response to

some stimulus. This can be as simple as giving a large change in induced electrical

displacement for a small applied electric field, (i.e. having a large dielectric constant)

if the material is for use as a capacitor dielectric.

Dielectric science has been a long-standing branch of physics, with close links to

chemistry and electrical engineering. Over 130 years ago, Faraday coined the term

“dielectric” to suggest that something analogous to current flow occurs through a

capacitor structure during the charging process when current introduced at one plate

“flows” through the insulator to charge the other plate. It is generally accepted that a

4

dielectric material interacts with an electric field differently than does free space

because the dielectric contain charges that can be displaced. Because all materials

contain charges, essentially every material can be classified as a dielectric .

Dielectric ceramics can be divided into four categories according to their properties:

(i) high permittivity dielectric, ranging between 1000 to 25000, example SrTiO3

and BaTiO3.

(ii) medium permittivity dielectric, ranging between 50 to 200, example CaTiO3.

(iii) low permittivity dielectric, ranging between 4 to 50, example MgTiO3.

(iv) ultra-low permittivity dielectric, below 3, example SiO2, and organic polymer

(i.e. Teflon ® AF, parylene).

Based on the current technology and the increasing use of portable electronics the

need to reduce the capacitance of electronic devices so that higher, less crosstalk and

lower power consumption can be achieved using a very low permittivity insulator

material. Meanwhile, high permittivity dielectrics have important applications in

microelectronics and microwave communication system (Cho et al., 1995).

Good ceramic insulators should have relative dielectric constant or relative

permittivity ε < 30, electrical resistivity ρ > 1012 Ω cm, loss factor, tan δ < 0.001, and

consequently dielectric strength > 5 kV mm-1. The review of dielectric properties of

some ceramics is presented in Table 1.1.

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Table 1.1 : Electrical property constants of different ceramic materials (Herbert, 1985).

Material Dielectric constant at 1 MHz Dielectric strength (kV/cm) Air 1.00059 30

Polystyrene 2.54 - 2.56 240 Glass (Pyrex) 5.6 142

Alumina 4.5 - 8.4 16 - 63 Porcelain 6.0 - 8.0 16 - 157

Titanium dioxide 14 - 110 39 - 83

The trend in research of dielectric materials is to develop more low-dielectric

constant materials for tomorrow integrated circuitary, The benefit to microelectronic

devices are: higher speed, smaller size, higher frequency and lower operational

power requirement.

1.2 Scope of Study

The ceramics form of Sr1-xCaxTiO3 (SCT) and Sr1-xBaxTiO3 (SBT) are of

commercial importance as a ceramic capacitor dielectric and it is of scientific interest

since it provides a straightforward example of the effect of solid solution

composition of ceramics on dielectric properties. In this study, strontium titanate

(ST) powders doped with Ca- and Ba- ions, are conventionally prepared by solid-

state synthesis with different compositions and at very high temperatures, 1200oC to

1350oC.

SCT and SBT powders prepared by this method consist of non-uniform,

submicrometer-sized coarse particle. The control of particle size distribution is not

6