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CHAPTER# 2

LITERATURE REVIEW

2.1 Introduction

Piezoelectric ceramics materials play an important role in the field of smart

structures. The degradation in their internal characteristics affects their efficiency and

performance. This chapter reviews a brief historical background. Piezoelectric crystal

classes, their characteristics. Fabrication and processing techniques have also been

described. A comprehensive review on the degradation behavior of piezoelectric ceramic

subjected to thermal cycling and shocking condition is presented.

2.2 Piezoelectricity

The phenomenon of piezoelectricity was discovered in the late nineteenth century. It was

observed that certain materials generate an electric charge or voltage when they are under

mechanical stress. Alternately, these materials produce a mechanical stress when they are

subjected to an applied voltage [2].

In 1880, Pierre and Jacques Curie experimentally discovered the direct piezoelectric

effect in various naturally occurring substances. In 1881, Hermann Hankel suggested

using the term piezoelectricity, which is derived from the Greek piezen meaning to

press. In 1893, Willam Thomson published seminal papers on the theory of

piezoelectricity. It was mathematically hypothesized and then experimentally proven that

a material exhibiting both, the generation and actuation effect.

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Piezoelectricity is a property of certain classes of crystalline materials including natural

crystals and manufactured ceramics such as barium titanate and lead zirconate titanate

(PZT). The piezoelectricity phenomenon was developed and applied in sonar and quartz

oscillation crystals. In 1921, Walter Cady invented the quartz crystal-controlled oscillator

and the narrow band quartz crystal filter used in communication systems. Two important

artificial piezoelectric crystals, barium titanate, and lead zirconate titanate were invented

in the early 1950s [3]. The surface charge leads to mechanical strain either compressive

or tensile depends upon the direction of applied polarity of the applied voltage. The

phenomena occur only in those crystals having no centre of symmetry. As piezoelectric

materials have excellent capability to convert an electrical signal to mechanical and

mechanical to electrical and therefore have high electromechanical coupling factor. A

especially cut electroded piezo crystal detect the longitudinal transverse vibration in

solid. These mechanical vibrations converted to electrical signal and can be displayed on

oscilloscope. One of the most important applications of the piezoelectric material is in the

frequency control of oscillator and filters whenever a mechanical force is setup in these

materials they vibrate at certain frequency. The frequency of these mechanical vibrations

has certain wavelength. These mechanical vibrations have very small losses and therefore

have a high quality factor Q. With the excitation of PZT materials, Impedance at

maximum and minimum frequency is the measure of coupling factor. Higher is the

difference between these two referenced frequencies, higher is the coupling factor

between fm and fn the response of the transducer is controlled by the mass of the material

[4]. Piezoelectric materials are being used in MEMS sensors and actuators. Think-film

piezoelectric materials have been explored for use as on-chip acoustic transducers,

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pumps, accelerometers, and microphones and mainly as actuators and sensors in the

aerospace and marine industries [2]. The effect also useful application for the production

and detection of sound, generation of high voltages, electronic frequency generation,

microbalance, and ultra fine focusing of optical assemblies [5].

2.3 Polarization

Many important properties of piezoelectric materials stem from their crystalline

structures. Piezoelectric crystals can be considered to be a mass of minute crystallites

(domains). The macroscopic behavior of the crystal differs from that of individual

crystallites, due to the orientation of such crystallites. The direction of polarization

between neighboring crystal domains can differ by 900

or 1800. Owing to the random

distribution of domains throughout the material, no overall polarization or piezoelectric

effect is exhibited. A crystal can be made piezoelectric in any chosen direction by poling,

which involves exposing it to a strong electric field at an elevated temperature. Under the

action of this field, domains most nearly aligned with the field will grow at the expense

of others. The material will also lengthen in the direction of the field, when the field is

removed, the dipoles remain locked in an approximate alignment, and crystal becomes

polarized.

The poling treatment is usually the final step of crystal manufacturing. Care must be

taken in all subsequent handling and use to ensure that the crystal is not depolarized,

since this will result in a partial or even total loss of its piezoelectric effect.

For static fields, the threshold is typically between 200-500 V/mm.

Mechanical depolarization occurs when mechanical stress on a piezoelectric element

becomes high enough to disturb the orientation of the domains and hence destroy the

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alignment of the dipoles. If a piezoelectric element is heated to a certain threshold

temperature, the crystal vibration may be so strong that domains become disordered and

the element becomes completely depolarized. This critical temperature is called the Curie

point or the Curie temperature. A safe operating temperature would normally be halfway

between 00C and the Curie point. The properties of piezoelectric elements are time

dependent and the stability of a piezoelectric as a function of time is of particular interest

[2]. Piezoelectric materials are crystals. The microscopic origin of piezoelectricity is the

displacement of ionic charges with a crystal, leading to the polarization and electric field.

A stress (tensile or compressive) applied to a piezoelectric crystal will alter the spacing

between centers of positive and negative charge sites in each domain cell; this leads to a

net polarization manifested as open circuit voltages measurable at the crystal surface.

Compressive and tensile stresses will generate electric fields which will exert a force

between the centers of positive and negative charges, leading to an elastic strain and

changes of dimensions depending on the field polarity. The direction of the induced

polarization depend on the direction of applied stress generally the applied stress in one

direction can give rise to induced polarization in other direction and reversing of stress

reverse the polarization direction [4]

2.4 Mathematical Description of Piezoelectric Effect

In a piezoelectric crystal, the constitutive equation that relates electrical polarization (D)

and applied mechanical stress (T) is

D= dT + E (1.1)

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Where d is the piezoelectric coefficient matrix, the electric permittivity matrix, and E

the electrical field. The electrical polarization is contributed by two parts-one stemming

from electrical biasing and one from mechanical loading.

If no electric field is present (i.e.., E=0), then the second term on the right-hand side of

Equation (1.1) can be eliminated.

The General Constitutive Equation can be written in the full matrix form:

T1

D1 d11 d12 d13 d14 d15 d16 T2

11

12

13 E1D2 = d21 d22 d23 d24 d25 d26 T3 + 21 22 23 E2D3 d31 d32 d33 d34 d35 d36 T4 31 32 33 E3

T5T6

The terms T1 through T3 are normal stress along axes 1, 2, and 3, whereas T4 through T6

are shear stresses. The units of electrical displacement (Di) stress (Tj), permittivity (i),

and electrical field (Ej) are C/m

2, N/m

2, F/m, and V/m, respectively. The unit of the

piezoelectric constant dij is the unit of electric displacement divided by the unit of the

stress namely.

[ ] [ ][ ]

[ ] [ ][ ] N

Columb

mN

mm

T

E

VF

T

Ddij ====

2

(1.3)

(1.2)

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Equation 1.4 can be expanded to a full matrix from:

(1.5)

(1.6)

The inverse effect of piezoelectricity can be similarly described by a matrix-form

constitutive equation. In this case, the total strain is related to both the applied electric

field and any mechanical stress, according to

S = ST + dE, (1.4)

Wheres is the strain vector and Sis the compliance matrix.

If there is no mechanical stress present (Ti,i=1,6=0), the strain is

related to the electric field by

Note that, for any given piezoelectric material, the dijcomponents connecting the strain

and the applied field in the inverse effect are identical to the dij connecting the

polarization and the stress in the direct effect [2]. The unit of dij can be confirmed from

Equation (1.6) as well. It is (m/m)/(V/m = m/V = C/N.

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The electromechanical coupling coefficient k is a measure of how much energy is

transferred from electrical to mechanical energy, or vice versa, during the actuation

process and is calculated as defined [6].

Energy

Converted

Input

EnergyK =

2(1.7)

This relation holds true for both mechanical-to-electrical and electrical-to-mechanical

energy conversion. The magnitude ofkis a function of not only the materials, but also the

geometries of the sample and its oscillation mode [2]

2.5 Historical Background

The piezoelectric effect dates back to thousands of years was first noticed in rocks

which would repel other rocks when they were heated. These rocks, which were actually

Tourmaline crystals, eventually found their way into Europe. Once the crystals arrived in

Europe, they were scrutinized by the scientists. In the mid 1700s, this effect was given

the name of pyroelectricity, which means electricity by heat. Pyroelectricity is the ability

of certain mineral crystals to generate electrical charge when heated, was known as early

as the 19th century, and was named by David Brewster in 1824. In 1880, the brothers

Pierre Curie and Jacques Curie predicted and demonstrated piezoelectricity using tinfoil,

glue, wire, magnets, and a jeweler's saw. They showed that crystals of tourmaline, quartz,

topaz, cane sugar, and Rochelle salt generate electrical polarization from mechanical

stress. Quartz and Rochelle salt exhibited the most piezoelectricity effects. The second

practical application for piezoelectric devices was sonar, first developed during World

War I. In France in 1917, Paul Langevin (whose development now bears his name) and

his fellows developed an ultrasonic submarine detector. The detector consisted of a

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transducer, made of thin quartz crystals carefully glued between two steel plates, and a

hydrophone to detect the returned echo. By emitting a high-frequency chip from the

transducer, and measuring the amount of time it takes to hear an echo from the sound

waves bouncing off an object, one can calculate the distance to that object. The use of

piezoelectricity in sonar, create an interest in piezoelectric devices. Over the next few

decades, new piezoelectric materials and new applications for those materials were

explored and developed. Development of piezoelectric devices and materials in the

United States was kept within the companies involved in the development, mostly due to

the wartime beginnings of the field, and in the interests of securing profitable patents.

Quartz crystals were the first commercially exploited piezoelectric material, but scientists

searched for higher-performance materials. Piezoelectric devices found homes in many

fields. Ceramic phonograph cartridges simplified player design, were cheap and accurate,

and made record players cheaper to maintain and easier to build. Ceramic electric

microphones could be made small and sensitive. The development of the ultrasonic

transducer allowed for easy measurement of viscosity and elasticity in fluids and solids,

resulting in huge advances in materials research. Ultrasonic time-domain reflecto-meters

(which send an ultrasonic pulse through a material and measure reflections from

discontinuities) could find flaws inside cast metal and stone objects, improving structural

safety. However, despite the advances in materials and the maturation of manufacturing

processes, the United States market had not grown as quickly. Without many new

applications, the growth of the United States' piezoelectric industry suffered. In contrast,

Japanese manufacturers shared their information, quickly overcoming technical and

manufacturing challenges and creating new markets. Japanese efforts in materials

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research created piezoceramic materials competitive to the U.S. materials, but free of

expensive patent restrictions. Major Japanese piezoelectric developments include new

designs of piezoceramic filters, used in radios and televisions, piezo-buzzers and audio

transducers that could be connected directly into electronic circuits, and the piezoelectric

igniter which generates sparks for small engine ignition systems (and gas-grill lighters)

by compressing a ceramic disc. Ultrasonic transducers that could transmit sound waves

through air had existed for quite some time, but first saw major commercial use in early

television remote controls. These transducers now are mounted on several car models as

an echo location device, helping the driver determine the distance from the rear of the car

to any objects that may be in its path. Historically, well known applications of

piezoelectric sensors have included phonograph pickups, microphones, acoustic modems,

and acoustic imaging for underwater, underground objects and medical instrumentation

[7]. The first ceramic to be developed commercially was BaTiO3. By the 1950s the solid

solution system Pb (Ti, Zr)O3 (PZT), which also the perovskite structure, was found to be

ferroelectric and PZT compositions are now the most widely exploited of all piezoelectric

ceramics [8].

2.6 General Characteristics, Fabrication and Processing of PZT

The Pb (Zr1-x Tix)O3 phase diagram is shown in figure 2.1. The morphotropic

phase boundary (MPB) defines the composition at which there is an abrupt structural

change, the composition being almost independent of temperature. That is the phase

boundary between the high temperature rhombohedral and tetragonal forms is, practically

speaking is a vertical line. As in figure2.1, the piezoelectric activity peaks in the region of

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the MPB composition and considerable effort has been directed to elucidating the reasons

for this technically very important phenomena.

Figure 2.1: Phase Stability in the System Pb(Ti1-x Zrx)O3 [5]

The current understanding is that the MPB is not a sharp boundary but rather a

temperature dependant compositional range over which there is a mixture of tetragonal

and monoclinic phases. At room temperature (300K) the two phases coexist over the

range 0.455 x 0.48. The enhanced piezoelectric activity of the commercial

compositions (x=0.48) can be rationalized in terms of the relatively large ionic

displacements associated with stress (electrical or mechanical) induced rotation of the

monoclinic polar axis.

Coupling coefficient and permittivity values across the PZT has been shown in Fig.2.2

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Figure 2.2: Coupling Coefficient kp and Permittivity r Values Across the PZT

Compositional Range

Depoling can be achieved by applying a field in the opposite direction to that used

for poling or in some cases by applying a high ac field and gradually reducing it to zero,

but there is a danger of overheating because of high dielectric loss at high fields. Some

compositions can be poled by applying a compressive stress (10-100 MPa). Complete

depoling is achieved by raising the temperature to well above the Curie point and cooling

without a field.

Aging effects are known to be significantly changed when the concentration of

vacant oxygen sites is increased either by doping or by heating in mildly reducing

atmospheres. The dipoles then provide an internal field stabilizing the domain

configuration thereby reducing ageing rate. The material features extremely large

dielectric constants. These properties make PZT-based compounds one of the most

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prominent and useful electroceramics. Commercially, it is usually not used in its pure

form, rather it is doped with either acceptor dopants, which create oxygen (anion)

vacancies, or donor dopants, which create metal (cation) vacancies and facilitate domain

wall motion in the material. In general, acceptor doping creates hard PZT while donor

doping creates soft PZT. In general, soft PZT has a higher piezoelectric constant, but

larger loss in the material due to internal friction. In hard PZT, domain wall motion is

pinned by the impurities thereby lowering the losses in the material, but at the expense of

a reduced piezoelectric constant. It is used to make ultrasound transducers and other

sensors and actuators, as well as high-value ceramic capacitors. PZT is also used in the

manufacture of ceramic resistors for reference timing in electronic circuitry. The

manufacturing process for high-voltage piezoceramic consists of following steps. The

manufacturing process for high-voltage piezoceramic starts with mixing and ball milling

of the raw materials. Next, to accelerate reaction of the components, the mixture is heated

to 75% of the sintering temperature, and then milled again. Granulation with the binder is

next, to improve processing properties. After shaping and pressing, the green ceramic is

heated to about 750 C to burn out the binder. The next phase is sintering, at temperatures

between 1250 C and 1350 C. Then the ceramic block is cut, ground, polished, lapped,

etc., to the desired shape and tolerance. Electrodes are applied by sputtering or screen

printing processes. The last step is the poling process which takes place in a heated oil

bath at electrical fields up to several kV/mm. In this case the ceramic take on

macroscopic piezoelectric properties [8].

Piezoelectric ceramics are fabricated with powder preparation. Powder preparation and

powder calcining and sintering are the more important processes that influence the

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material properties. The powder is then pressed to the required shapes and sizes.

Machining, electroding, poling and application of a DC field to orient the dipoles are the

more steps to induce piezoelectricity.

The most common powder preparation is the mixed oxide route. In this process, powder

is prepared from the appropriate stoichiometric mixture of the constituents oxide route.

In the case of lead zirconate titanate (PZT): lead oxide, titanium oxide, and zirconium

oxide are the main compounds. Depending on application, various dopants are used to

tailor the properties of interest. PZT ceramics are rarely utilized without the addition of

dopants to modify some of their properties. A-site additives tend to lower the dissipation

factor, which affects heat generation, but also lower the piezoelectric coefficients; for this

reason they are mostly used in ultrasonics and other high frequency applications, B-site

dopants increase the piezoelectric coefficients but also increase the dielectric constant

and loss. They are utilized as actuators in vibration and noise control, benders. Mixing of

the powders can be done by dry-ball milling or wet ball milling. Both methods having

advantages and disadvantages: wet ball-milling is faster than dry-milling; however, the

disadvantage is the added step of liquid removal. The most common method for making

PZT ceramics is through wet-ball milling; ethanol and stabilized zirconia media are

added for a wet milling process. A vibratory mill may be used rather than a conventional

ball mill; it was shown by Herner that this process reduces the risk of contamination by

the balls and the jar [9]. Zirconia media are used to further reduce the contamination

risks. The calcinations step is a very crucial step in the processing of PZT ceramics; it is

important that the crystallization be complete and that the perovskite phase forms during

this step. After calcining, a binder is added to the powder, and then the mixture is shaped

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usually by dry-pressing in a die for simple shapes, or extrusion, or casting for more

complicated bodies. Next, the shapes are sintered: placed in an oven for binder burn-out

and densification.

The major problem in the sintering of the PZT ceramic is the volatility of PbO at about

8000C. To minimize this problem, the PZT samples are sintered in the presence of a lead

source, such as PbZrO3, and placed in closed crucibles. The saturation of the sintering

atmosphere with PbO minimizes lead loss from the PZT bodies. Sintering can now be

carried out at temperatures varying between 1200-1300oC. Despite precautions, there is

usually a resulting loss of 2%-3% of the initial lead content.

After cutting and machining into desired shapes, electrodes are applied and a strong DC

field is used to orient the domains in the polycrystalline ceramic. DC poling can be done

at room temperature or at higher temperatures depending on the material and the

composition. The poling process only partially aligns the dipoles in a polycrystalline

ceramic, and the resulting polarization is lower than that for single crystals.

This processing technique presents many uncertainties and the presence of a wide number

of other fabrication techniques is an indication that there is a great need for the

production of reliable PZT ceramics with optimum properties and microstructure. One

problem often encountered is the deviation from stoichiometry. This problem is often due

to impurities present in the raw materials as well as the lead loss during the sintering

processes, which invariably results in substantial alternations of the PZT properties. As a

result, the elastic properties can vary as much as 5%, the piezoelectric properties 10% and

the dielectric properties 20% within the same batch [10]. Also, the piezoelectric and

dielectric properties generally suffer if there is any lack of homogeneity due to poor

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mixing. It is important then that the constituent oxides be intimately mixed. In the method

described above, however, the constituents are solid solutions and it has been shown that

an intimate mixing of solid solutions is difficult if not impossible. More information on

the preparation of piezoelectric ceramics can be found by Moulson [8] and Jaffe [11].

2.7 Piezoelectric Crystal Classes

Of the thirty-two crystal classes, twenty-one are non-centrosymmetric (not having

a centre of symmetry), and of these, twenty exhibit direct piezoelectricity. Ten of these

are polar (i.e.. spontaneously polarize), having a dipole in their unit cell, and exhibit

pyroelectricity. If this dipole can be reversed by the application of an electric field, the

material is said to be ferroelectric [5].In a piezoelectric crystal, the positive and negative

electrical charges are separated, but symmetrically distributed, so that the crystal overall

is electrically neutral. The domains are usually randomly oriented, but can be aligned

during poling (not the same as magnetic poling), a process by which a strong electric

field is applied across the material, usually at elevated temperatures. When a mechanical

stress is applied, this symmetry is disturbed, and the charge asymmetry generates a

voltage across the material. Crystal is a solid in which the constituent atoms, molecules,

or ions are packed in a regularly ordered, repeating pattern extending in all three spatial

dimensions. Generally, crystals form when they undergo a process of solidification.

Under ideal conditions, the result may be a single crystal, where all of the atoms in the

solid fit into the same crystal structure. However, generally, many crystals form

simultaneously during solidification, leading to a polycrystalline solid.

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Crystalline structures occur in all classes of materials, with all types of chemical bonds.

Almost all metal exists in a polycrystalline state; amorphous or single-crystal metals must

be produced synthetically, often with great difficulty. Ionically bonded crystals can form

upon solidification of salts, either from a molten fluid or when it condenses from a

solution. Covalently bonded crystals are also very common, notable examples being

diamond, silica, and graphite. Polymer materials generally will form crystalline regions,

but the lengths of the molecules usually prevent complete crystallization. Weak Van der

Waals forces can also play a role in a crystal structure; for example, this type of bonding

loosely holds together the hexagonal-patterned sheets in graphite. Most crystalline

materials have a variety of crystallographic defects. The types and structures of these

defects can have a profound effect on the properties of the materials. Some crystalline

materials may exhibit special electrical properties such as the ferroelectric effect or the

piezoelectric effect [8].

Following are few important piezoelectric types used in various applications

2.7.1 Lead Zirconate Titanate (PZT)

Lead zirconate titanate is a ceramic material that shows a marked piezoelectric

effect compared to other ferroelectric properties. PZT develops a voltage difference

across two of its faces when compressed (This is used for sensor applications), and

physically strained when an external electric field is applied (used for actuators etc). It is

also ferroelectric, in other words, it has a spontaneous polarization which can be reversed

in the presence of an electric field. The lead zirconate titanate is widely used in

polycrystalline (ceramic) from with very high piezoelectric coupling. Depending on the

formula of preparation, PZT materials may have different forms and properties.

Manufacturers of PZT use proprietary formulas for their products [12]. Techniques that

are commonly used for preparing the bulk PZT materials such as (PZT-4, PZT-5) are not

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suited for micro-fabrication. A number of techniques for preparing PZT films have been

demonstrated, including sputtering, laser ablation, jet molding, and electrostatic spray

deposition [13]. Lead zirconate titanate shows a much greater piezoelectricity effect than

quartz. These can readily be fabricated into variety of shapes and sizes and therefore can

be tailored to a particular application [14].

2.7.2 Barium Titanate

Barium titanate is an oxide of barium and titanium with the chemical formula

BaTiO3. It is a ferroelectric ceramic material, with a photorefractive effect and

piezoelectric properties. It has four structures as a solid, starting with the high

temperature to a low temperature structure. These four structures are cubic, tetragonal,

orthorhombic, and rhombohedral crystal structure. All of the structures exhibit the

ferroelectric effect except for the cubic barium titanate structure. Barium titanate can be

manufactured by sintering of barium carbonate and titanium dioxide, optionally with

other materials for doping. Barium titanate is often mixed with strontium titanate. It has

the appearance of a white powder or transparent crystals and is insoluble in water and

soluble in concentrated sulfuric acid. As a piezoelectric material, it was largely replaced

by lead zirconate titanate, also known as PZT. Barium titanate crystals find use in

nonlinear optics. The material has high beam-coupling gain, and can be operated at

visible and near-infrared wavelengths. It has the highest reflectivity of the materials used

for self pumped phase conjugation (SPPC) applications. It can be used for continuous-

wave for wave mixing with milliwatt-range optical power. Barium titanate mostly used

as a dielectric material for ceramic capacitors, and as a piezoelectric material for

microphones and other transducers [15]

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2.7.3 Polyvinylidene Fluoride (PVDF)

Polyvinylidene Fluoride, or PVDF is a highly non-reactive and pure thermoplastic

fluoropolymer. PVDF is very expensive; its use is generally reserved for applications

requiring the highest purity, strength, and resistance to solvents, acids, bases and heat.

Compared to other fluoropolymers, it is easier to melt because of its relatively low

melting point.

It is available as piping products, sheet, plate and an insulator for premium wire.

It can be injection molded and welded and is commonly used in the chemical,

semiconductor, medical and defense industries, as well as in lithium ion batteries. When

poled, PVDF is a ferroelectric polymer, exhibiting efficient piezoelectric and pyroelectric

properties. These characteristics make it useful in sensor and battery applications. PVDF

has a glass transition temperature (Tg) of about -350C and is typically 50-60% crystalline.

To give the material its piezoelectric properties, it is mechanically stretched to orient the

molecular chains and then poled under tension. Polyvinylidene Fluoride is a synthetic

floropolymer with monomer chains. It exhibits piezoelectric, pyroelectric and

ferroelectric properties, excellent stability to chemicals, mechanical flexibility and

biocompatibility [16].

2.8 Recent Developments in Piezoelectric Ceramics

There has been a growing interest in recent years in piezoelectric ceramics

materials because of their excellent dielectric, sensing, actuating and efficient process

control applications. Lead zirconate titanate (PZT), barium titanate (BaT1O3), lead

metaniobate (PbNb2O6) and polyvinylidene fluoride (PVDF) polymers are generally

favored as smart sensing materials. These materials are being used in critical engineering

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systems and smart structures. Fatigue failure due to electrical and thermal shocking is a

major issue in degradation of these materials. A lot of work has been done in this area but

still various issues need to investigate. Recent developments and current issues in

piezoelectric materials and deterioration of their properties in different working

conditions have been discussed in a review paper titled Recent Developments in

Piezoelectric Ceramics Materials and Deteriorations of their Properties [Appendix-B].

The new piezoelectric finite element capability available in some commercial packages

like ANSYS makes it convenient to perform static, dynamic, transient and thermal

analysis for the fully coupled piezoelectric and structural response.

In the past two decades many theoretical studies including finite element analysis and

modeling have been conducted to the fracture and damage of piezoelectric ceramics

under electrical, mechanical or combined electromechanical loading modes. The decay of

piezoelectric properties and the degradation mechanisms of piezomaterials due to the

strong coupling effect of the high alternating electric field and mechanical load have been

serious concerns, but have not well characterized. In particular, durability performance of

peizomaterials, in terms of integrity and piezoelectric properties, is always a key issue in

long term for both conventional piezoceramic based actuation system and recently

developed new generation actuation systems. Cyclic domain switching in piezomaterials

caused by the high frequency cyclic electric field and consequently the electric field

induced fatigue crack growth, and the temperature rise due to self heating of the

materials, seriously deteriorate the electromechanical properties of piezomaterials [17].

Ferroelectric ceramics has a broad range of applications due to its enhanced physical

properties such as dielectric coefficients, elastic optical coefficients, piezoelectric

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coefficients, elastic coefficient. All these properties have been investigated [18-20].

There are considerable reports on experimental studies of fatigue induced either by an

electrical or a mechanical load alone [21]. Ferroelectric fatigue was a key problem for the

wide application of ferroelectric materials in non-volatile memories and other

electromechanical devices, such as actuator. Up to now, much work has been carried out

on the research towards understanding ferroelectric fatigue and thus many corresponding

mechanisms have been suggested [22].

In the ferroelectrics literature, the term fatigue generally refers to the gradual degradation

of bulk material properties, such as the saturation remnant polarization, in a cyclically

loaded specimen.

Experiments have shown that cracks grow in ferroelectric ceramics under cyclic electric

fields. The works of Jiang, Cross, and coworkers have shown that high porosity might

cause the severe decrease on polarization under alternating electric loading. Jiang et al.

(1993) have found from their experiments that the smaller the grain size the more

difficult it is to produce and propagate cracks. Jiang have studied the difference on the

electric fatigue behavior caused by conditions of ceramic-electrode interfaces [23, 24].

Hill et al. (1996) have used transmission electron microscopy (TEM) to observe the

fatigue behavior of PZT -8 by measuring acoustic velocity and piezoelectric coefficients

[25]. Tai and Kim have investigated the fatigue of PZT ceramics under cyclic

compressive loading by measuring piezoelectric coefficient and capacitance [26].

Recently, Jiang and Sun (1999) have studied the behavior of fatigue crack growth rates

under combined electrical and mechanical loads. They have attempted to use a single

energy parameter to quantify electrical and mechanical loads. They also had extended the

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mechanical fatigue theory and have included an intensity factor of electric displacement

in the Paris law[27].

It is common practice to embed piezoelectric sensors into prototypes because these

sensors can be manufactured with strength and dimensional characteristics that do not

degrade the structural integrity of the materials of the prototype. When thermal effects are

generated through either friction or direct exposure to significant temperature gradients,

the reliability of the electrode layer in these piezoceramics can completely dominate the

performance of the device being investigated [28]. A new 3-D electromechanical-coupled

field finite method has been proposed to accurately predict the resonant frequency and

harmonic response of a system applying the step voltage as input. The simulation of

piezoelectric devices with time domain was modeled by Lerch (1990) [29].

2.9 Thermal Cycling and Shocking in Piezoelectric

Ceramic materials are brittle and susceptible to catastrophic fail under most

conditions of high heat transfer and rapid environmental temperature variations. Thermal

stress resistance of brittle ceramics can be measured by two methods. The first approach

is based on thermo elastic theory [30]. Material properties are selected to avoid the

initiation of fracture by the thermal stresses. In general this requires materials with high

values of tensile strength, thermal conductivity, and thermal diffusivity combined with

low values of thermal expansion coefficient, Youngs modulus of elasticity, Poissons

ratio and emissivity [31-33]. Some workers have reported a reasonable agreement

between calculated and observed thermal stress performance, providing a valid basis for

the thermo-elastic approach to thermal stress fracture [34, 35]. Thermal shock resistance

concerned with the extent of crack propagation and the resulting change in physical

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behavior of the material. Thermal stress resistance may be determined by the relative

change in strength, the loss of weight, or the change in permeability. The change in

elastic behavior or resonant frequency may also be used as a measure of thermal stress

resistance. A new approach to the calculation of the extent of crack propagation in brittle

ceramics as a function of thermal shock treatment has been presented by

D.P.H.Hasselman (1969) [36].

Heat transfer effects in ferro-electric materials, electric impact loading, thermal effects of

piezoelectric sensors and heat generation rate in piezoelectric materials have also been

investigated. Ningning Dul (2006) investigated the energy dissipation mechanisms and

thermal effects in cracked piezoelectric materials [37]. His results showed that the

temperature rise caused by electric saturation or electric impact loading is remarkable and

may play a significant role in fracture of piezoelectric materials especially under

high frequency condition and some electric-waves with higher electric loading rates.

Many piezoelectric structural components are under the influence of transient thermal

loads like in aerospace structures and hence it is necessary to accurately model the

coupled thermal-mechanical-electrical behaviors of piezoelectric ceramics. In thermal

shock conditions during sudden heating or cooling of a solid, development of high

values of stresses are possible. If the thermal transient is severe enough, sudden fracture

may occur. Thermal shocks in a plate of finite thickness have been attempted. A fracture

mechanics analysis having an edge crack in the transient stress analysis, and the degree

of severity of any given thermal shock is characterized in terms of the stress-intensity

factor [38, 39]. The dynamic linear piezothermoelastic theory, the constitutive

formulations, and governing equations for thermal, elastic, and electric fields have been

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discussed [40, 41]. The effective thermal expansion and pyroelectric coefficients of

piezoelectric composites have also been analyzed [42]. The transient thermo-electric-

elastic fields in a hexagonal plate were investigated by Choi et al [43]. Recently,

numbers of coupled thermal-mechanical-electrical finite-element method are being used

to study thermally induced stresses in smart structures. The phenomenon of strength

evaluation of piezoelectric ceramics under transient thermal environment has also been

investigated [44].

Piezoelectric thin films operating in many structural components, like in aerospace

component, are sometime subjected to severe thermal loading which may be produced by

aerodynamic heating, by laser irradiation, or by localized intense heating. The amount of

energy delivered to the thin film surface in short time plays a significant role in

developing thermal stresses. Thermal shock and thermal fatigue of ferroelectric thin film

were investigated by the pulsed laser tests by Zheng et al [45]. Micrographs from

scanning electron microscope show a remarkable difference in microstructure and grain

size after during thermal cycling. They also discussed the possible origins of the thermal

fatigue cracks. Thermal shocks in a plate of finite thickness have been attempted.

Thermal shock and thermal fatigue of ferroelectric thin film were investigated by the

pulsed laser tests by X.J.Zheng et. al. (2005)

Lead zirconate titanate decreases the dielectric constant and the resonance frequency by

thermal shock. Temperature stability for dielectric constants and resonance frequencies is

an important phenomenon. Tolerance to thermal shocks is strictly required in piezoelectric

resonators and filters. Resonance frequency of vibration mode has also been investigated

earlier [46].

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Fatigue studies shows that material degradation is strongly influenced by temperature and

by the electromechanical fatigue. Temperature plays an important role in dictating the

electromechanical response of piezoelectric materials. In typical actuators, the operating

temperature is less than 100C0

and work show that the relative permittivity varies linearly

with temperature up to 120C0

and it shows that linear approximation of relative

permittivity with temperature is valid. Behavior of piezoelectric ceramics used in actuator

application was discussed by Donny Wang and his fellows [47]

The extent of aging has been expressed as total normalized frequency change over a

specific time period. Aging mechanism and high frequency modes of piezoelectric

resonator was earlier analyzed [48].

Thermal shock resistance of the materials was evaluated by water quenching and a

subsequent three point bending test to determine flexure strength degradation. In the

investigation it was analyzed that fracture toughness can be improved. By considering

specific heat treatment the ceramics materials can be shock resistance [49].Transient

thermal analysis of thin strips used in various applications had been investigated since

long. Thin strip with or without crack was determined for its behavior in transient thermal

environment [50].

The ageing process in any ceramic can be accelerated by exposing the ceramic to high

mechanical stress, strong electric depoling field and high temperature approaching the

Curie point. Most of the properties of piezoelectric ceramics changes gradually with time.

The changes tend to be logarithmic with time after poling. The ageing rate of various

properties depends on the ceramic composition and on the way the ceramic is processed

during manufacture. Because of ageing, exact values of various properties such as

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dielectric constant, coupling, and piezoelectric constants may only be specified for a

standard time after poling. The longer the time period after poling, the more stable the

material becomes.

2.10 Effect of Water and Moistures in Piezoelectric Ceramics

Transient thermal analysis of thin strips has been studied [50]. Piezoelectric thin

films operating in many structural components such as in aerospace applications can

experience severe thermal loading which may be produced by aerodynamic heating, laser

irradiation, or incidental heating from other electrical components. The amount of energy

delivered to the thin film surface in a short time plays a significant role in developing

thermal stresses. Recently Jiang et. al (2006) studied the effect of water induced

degradation on soft PZT piezoelectric ceramics using electromechanical charging in a

NaOH solution. They observed the effect of electrolysis of water on property changes of

the PZT [51]. Other researchers have studied the affect of applying a 50Hz AC voltage on

the degradation in properties of a PZT ceramic ring in NaOH solution. The rings treated

with AC voltage were found to degrade in material properties [52].

As the performance of PZT materials used in various applications may be affected by

changes in temperature and water condition, therefore, in this study the effect on

performance of a PZT material have been analyzedat different frequencies in different

water conditions.

Ceramic materials are brittle and susceptible to catastrophic failure under conditions of

rapid environmental temperature variations [30]. Relative change in strength, the loss of

weight, or the change in permeability, the change in elastic behavior or resonant

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frequency is the measure of thermal stress resistance [36]. Thermal shocks in a plate of

finite thickness have been attempted. Fatigue studies show that material degradation of

PZT ceramics are strongly influenced by temperature and by the electromechanical

fatigue. Lead zirconate titanate ceramics shows a decrease in dielectric constant and the

resonance frequency when subjected to thermal shock. Thermal shock resistance of the

materials was evaluated by water quenching and a subsequent three point bending test to

determine flexure strength degradation. Degradation of various properties of the piezo

devices in the presence of water & AC voltage was investigated and concluded that water

is an important cause for the degradation of PZT piezoelectric ceramics [52].

Dielectric constant is an important parameter, especially in the piezoelectric device such

as resonators and filters used in the electronic circuits. Impedance is also dependent on the

dielectric constant of the piezoelectric. Currently there is limited data available for the

thermal shocking and quenching effect of a thin PZT disc. Therefore there is a scope to

investigate various parameters which are still unattended. In this research work, the focus

is to investigate the degradation of thin PZT disc due to thermal shocking and its

quenching effect. A noticeable change in capacitance and dielectric constant has been

observed which is further changing other piezoelectric properties.

By considering all of the above discussions, it is concluded that fatigue behavior of

piezoelectric ceramics materials either by electrical, mechanical, electromechanical has

been investigated extensively, whereas there is a scope of work during thermal

cycling/shocking of piezoelectric material in variable conditions. In chapter 4

experimentations performed, analyzed and comprehensive discussions have been

presented.