A Study of electromechanical behavior of Piezo ceramic Smart materials and

application of Piezo ceramic (PZT) for vibration alerts in mobile phones.

Dheepan T, Arun Prasad S

VI semester B. E. Manufacturing Engineering, College of Engineering, Anna University, Guindy,

Chennai-25.

Abstract:

Advances in material science have opened up exciting possibilities for creating novel devices

and structures which respond in safe, effective and speedy manners to changing environmental

and operational conditions. Smart materials and structures is a new field that is being cited as one

of the key technologies for the 21st Century. Typically these SMART materials exploit the

coupling of elasto mechanics and fluid mechanics with electro-physical influences such as piezo-

electric effects, magnetostrictive effects. While millimeter and larger scale devices based on such

principles are beginning to find their way into consumer products, prospects for miniaturization

and micro-miniaturization are emerging that further widen the area of applicability of these smart

materials. As these materials become increasingly smaller for the next generation of smart

materials systems, the need to understand and predict material response becomes critical. This

paper mainly focuses on the study of electro mechanical behavior of piezo ceramic crystals and

designing a basic circuit for incorporating Piezo ceramic (PZT) in vibration alerts in mobile

phones. ’Low battery’ is common term among mobile users ,by applying this smart material

based systems(SMBS) to mobile phones substantial amount of battery will be saved.

Piezo ceramic PZT is a universally acclaimed smart material - which produces motion by

receiving electric potential across their polarized surfaces. Strain output is directly proportional

to the input potential and optimal vibration sense is possible with minimal power consumption.

A basic circuit comprising of receiver, comparator, PZT ceramic, amplifiers is designed to

incorporate the PZT in mobile phone vibration system. The resonant frequency of the ceramic is

too high, so a metal plate (substrate) must be placed along with the ceramic that vibrates with the

contraction and expansion of the piezo ceramic. Metal substrate aids the desired strain transfer

process. This application using smart material can be effectively used in a mobile phone

replacing classic motor operated vibration and since it’s a miniature device it can also be

extended to wrist watch (with some changes in circuit) as a low power consumption vibration

alerts.

Key words: Smart material, Piezo ceramic PZT, Resonant frequency.

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INTRODUCTION:

The field of smart materials is catching up slowly. Though it’s not as famous as nanotechnology

these materials have their own venture in multifaceted sectors. Potential applications are

aerospace structure monitoring systems, automotive monitoring and control devices, fluid control

devices, biomedical equipment, manufacturing process monitoring devices. Today’s people

admire automations and miniaturizations. Smart material based systems which combines sensor

and actuators is the key technology to achieve this at optimal cost, through system integration

and compact design, systems with less complexity, lower cost, and higher reliability .In this

paper we have studied the electro mechanical behavior of the piezo ceramic smart material and

we designed a basic circuit for employing this smart material in vibration alerts in mobile

phones.

SMART MATERIALS -DEFINITIONS:

Materials that respond with a significant change in a property upon application of

external driving forces.

Smart materials respond to differences in light or temperature. They sense conditions in

their environment and respond to them.

Smart materials are materials which can respond and change their properties depending

upon sensing external factors such as Temperature, Current flow and Light levels.

TYPES OF SMART MATERIALS:

Property Actuators Driving energy

Shape memory Nitinol T , S

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Piezoelectric PZT,PVDF E,S

Magnetostriction Tetrofenol –D H

T - Thermal energy, E- electrical field S- mechanical strain H – magnetic field;

PIEZOELECTRICITY:

Piezoelectricity, discovered in on Rochelle salt 1880 by the brothers Jacques and Pierre

Curie is defined as a change in electric polarization. Historically, Rochelle salt and quartz are the

most frequently used piezoelectric materials.

The most popular material systems being used for sensors and actuators are piezoelectric

materials, magnetostrictive materials, shape memory alloys, electrorheological fluids and optical

fibers. Magnetostrictive materials, shape memory alloys and electrorheological fluids are used as

actuator materials. Among all these active materials, piezoelectric materials are most widely used

because of their fast electromechanical response, low power requirements and relatively high

generative forces.

PIEZOELECTRIC BEHAVIOR CAN BE MANIFESTED IN TWO DIFFERENT WAYS:

Direct piezoelectric effect: Occurs when piezoelectric crystal is electrically charged when

subjected to mechanical stress. These devices can be used to detect the strain, movement, force,

pressure, vibration by developing appropriate electrical responses, as in case in the case of

ultrasonic and acoustic sensors.

Converse piezoelectric effect: Occurs when piezoelectric crystal is strained when placed in an

electric field .These property can be used to generate strain, force, and vibration by application.

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PZT – THE SUITABLE PIEZOELECTRIC CERAMIC FOR A MOBILE PHONE

VIBRATION ALERT

Let us study the basic properties of PZT

Lead zirconate titanate,PZT is a ceramic material used in a variety of memory applications,

microphones, transducers, actuators. They are also employed in MEMS devices.

PZT is Lead-Zirconate-Titanate or Pbx(Ti, Zr)1-xO3. A typical commercial composition has

~47 Mole-% PbTiO3 and ~53 Mole-% PbZrO3

PZT has a perovskite structure with an ABO3– structure type.

PZT exhibits good ferroelectric properties and excellent piezoelectric properties.

PEROVSKITE STRUCTURE

Virtually all piezoelectric materials crystallize in the perovskite structure .A representation of

eight perovskite unit cells on which most commercially available piezoceramics are based on.

The generic formula is ABO3– Oxygen sits in the octahedral sites (red small dots), an A++

material (e.g. Pb) in the cube corners (green big dots) and a small B++++ cation (e.g. Zr, Ti) in the

center (small black dots). The unit cell is electrically neutral.

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EFFECT OF CURIE TEMPERATURE ON PIEZO ELECTRIC CERAMIC CRYSTAL

STRUCTURE :

A traditional piezoelectric ceramic is a mass of perovskite crystals, each

consisting of a small, tetravalent metal ion usually titanium or zirconium, in a lattice of larger,

divalent metal ions, usually lead or barium, and O2- ions.

a) Temperature above Curie point :

Cubic lattice, symmetrical arrangement of positive and negative charges.

b) Temperature below Curie point:

Tetragonal (orthorhombic) lattice, crystal has dipole moment. A piezo electric crystal

must always be operated below curie temperature ,since it loses its piezo electric capabilities

totally beyond this temperature.

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O 2+ oxygen

Ti, Zr, other smaller, tetra valent metal ion

Pb , Ba, other larger divalent metal ion

WHY PARTICULARLY PZT IS SELECTED FOR OUR CIRCUIT? :

Among all piezoelectric materials, lead zirconate titanate (PZT) has been most extensively used

in transducers. This family of ceramics has high values for the piezoelectric charge coefficient

(d33), electromechanical coupling coefficient (kt) and dielectric constant (K). They also have low

electrical losses. Piezopolymers like Polyvinylidene Fluoride copolymers (PVDF) are an

alternative to piezoceramics. Their limitations for use as transducers include a low dielectric

constant (K) and coupling coefficient (kt), high electrical losses.

PRODUCTION OF PZT

To prepare a piezoelectric ceramic, fine powders of the component metal oxides are mixed in

specific proportions, and then heated to form a uniform powder. The powder is mixed with an

organic binder and is formed into structural elements having the desired shape (discs, rods,

plates, etc.). The elements are fired according to a specific time and temperature program, during

which the powder particles sinter and the material attains a dense crystalline structure. The

elements are cooled, then shaped or trimmed to specifications, and electrodes are applied to the

appropriate surfaces.

WORKING OF PZT

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Small tetra valent ion is shifted from centre ,so that crystal loses its symmetry.

The displacement of electron densities occurs when the electrical voltage potential is

applied across the opposite crystal faces that could be used as an "electrical current" The reason

this "electrical current" is possible rests in the net movement of negative anion charges (-) in one

direction within the crystal, which is reinforced by the net movement of positive cation charges

(+) in the opposite direction. So, with the application of a "voltage potential", the electric field

compresses the crystal and a "electric potential" is created in one direction. If you now reverse

this voltage potential by releasing the compression, the ions or atoms will move in the opposite

direction which in turn causes the crystal to expand back to its natural form (Ellis et al).

ALTERNATIVE PIEZO ELECTRIC MATERIAL: PVDF, or PolyVinylidine DiFluoride,

is a highly non-reactive and pure thermoplastic fluoropolymer. It is also known as KYNAR.

PVDF is very expensive; its use generally reserved for applications requiring the highest purity,

strength, and resistance to solvents, acids, bases and heat. When poled, PVDF is a ferroelectric

polymer, exhibiting piezoelectric and pyroelectric properties. These characteristics make it useful

in sensor and battery applications. To give the material its piezoelectric properties, it is

mechanically stretched to orient the molecular chains and then poled under tension.

STUDY OF ELECTROMECHANICAL PROPERTIES

CONSTITUTIVE EQUATIONS

“d” FACTOR

ELECTRO MECHANICAL RELATIONS

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CONSTITUTIVE EQUATIONS:

In order to describe or model piezoelectric materials, one must have knowledge about the

material's mechanical properties (compliance or stiffness), its electrical properties (permittivity),

and its piezoelectric coupling properties. Engineers are already familiar with the most common

mechanical constitutive equation that applies for everyday metals and plastics. This equation is

known as Hook’s Law and is written as:

S = s. T

In words, this equation states: Strain = Compliance × Stress.

Compliance can mean the inverse of stiffness.

Since piezoelectric materials are concerned with electrical properties too, we must also consider

the constitutive equation for common dielectrics:

D= ε.E

where D is volumetric charge density, is permittivity and E is electric field

strength.Permittivity is an intensive physical quantity that describes how an electric field affects

and is affected by a medium.

For piezo electric materials, The above two equations may be combined into so-called coupled

equations, of which the strain-charge form is:

{S}=[SE] {T} + [dt] {E}

{D}=[d]{T}+[ εT ] {E}

Where the superscript E indicates a zero, or constant, electric field; the superscript T indicates a

zero, or constant, stress field; and the subscript t stands for transposition of a matrix.

Also,

T = constant stress = mechanically free

E = constant field = short circuit

D = constant electrical displacement = open circuit

S = constant strain = mechanically clamped

The piezoelectric coupling terms are in the matrix d.

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"d" CONSTANT:

The piezoelectric constants relating the mechanical strain produced by an applied electric field

are termed the strain constants, or the "d" coefficients. The units may then be expressed as

meters per meter, per volts per meter (meters per volt).

d33 - describes the strain parallel to the polarization vector of the ceramics (thickness) and is

used when calculating the displacement of stack actuators; d31 is the strain orthogonal to the

polarization vector (width) and is used for calculating tube and strip actuators .

ELECTRO MECHANICAL RELATIONS:

The relationships between an applied voltage or electric field and the corresponding increase or

decrease in a ceramic element's thickness, length, or width are:

   h = d33V

     l / l = d31E

    w / w =d31E

where l: initial length of ceramic element

       w: initial width of ceramic element

        h: change in height (thickness) of ceramic element

        l: change in length of ceramic element

        w: change in width of ceramic element

         d: piezoelectric charge constant

         E: electric field

d33: piezoelectric charge constant, For a PZT ceramic is d33~ 400*10-12 m/V (Here i=j=3

which simply means that the strain and the voltage gradient are in the same direction.

PROPERTY ENHANCING METHODOLOGIES:

DOPING OF PZT TO FACILITATE DOMAIN WALL MOTION

PIEZO COMPOSITES

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DOPING OF PZT TO FACILITATE DOMAIN WALL MOTION:

The material features an extremely large dielectric constant at the morph tropic phase boundary

(MPB) near x = 0.52. These properties make PZT-based compounds one of the most prominent

and useful electro ceramics. 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.

PIEZO COMPOSITES: Instead of looking for an entirely new class of piezoelectric materials

without the existing limitations, researchers in the last two decades have successfully made

composites of piezoelectric ceramics with inactive polymers. These piezo composites show

excellent electromechanical properties while limiting the various detrimental properties of the

monoliths. The properties of the ceramic/polymer composites can be tailored by changing the

connectivity of the phases, volume fraction of the ceramic in the composite, and the spatial

distribution of the active ceramic phase.

PIEZO STACKS:

Description: A low voltage piezoelectric stack is a monolithic ceramic construction of many

thin piezo ceramic layers which are connected in parallel electrically. The common feature of

stack actuators is that many thin layers of piezoelectric material, typically PZT, are glued or

coifed together with an electrode between each layer. This arrangement allows the mechanical

displacement to sum in series while the electrical properties remain in parallel. This leads to

large displacements, 0.1% strain, for lower voltage levels than would be achievable with a

monolithic element of same length. Stacks can be built with aspect ratios up to 12:1

(length:diameter). This means that the maximum travel range of an actuator with 15 mm piezo

diameter is limited to about 200 µm. Longer travel ranges can be achieved by mechanical

amplification techniques.

STACK ACTUATORS:

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The principal characteristics of the stack are:

1. high energy conversion efficiency,

2. low voltage operation, large force,

3. Low motion, fast response.

Motion may be increased, at the expense of force, by mechanical amplification. The stack offers

a high energy density in a small package. Due to its superior compressive strength, it provides a

high load bearing capability. However, it is relatively weak in tension. Generally, excitation

should be applied only in the direction of polarization. The common feature of stack actuators is

that many thin layers of piezoelectric material, typically PZT, are glued or coifed together with

an electrode between each layer. This arrangement allows the mechanical displacement to sum in

series while the electrical properties remain in parallel. This leads to large displacements, 0.1%

strain, for lower voltage levels than would be achievable with a monolithic element of same

length.

ACHIEVING MAXIMUM EFFICIENCY OF THE INVERSE PIEZOELECTRIC

EFFECT

RESONANCE FREQUENCY

RESONANCE FREQUENCY:

A piezoelectric ceramic element exposed to an alternating electric field changes

dimensions cyclically, at the frequency of the field. The frequency at which the element vibrates

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most readily in response to the electrical input, and most efficiently converts the electrical energy

input into mechanical energy -- the resonance frequency -- is determined by the composition of

the ceramic material and by the shape and volume of the element.

STUDY OF LIMITATIONS OF PIEZOCERAMICS

STABILITY

ELECTRICAL LIMITATIONS

THERMAL LIMITATIONS

MECHANICAL LIMITATIONS

STABILITY: Most properties of a piezoelectric ceramic element erode gradually, in a

logarithmic relationship with time after polarization. Exact rates of aging depend on the

composition of the ceramic element and the manufacturing process used to prepare it.

Mishandling the element by exceeding its electrical, mechanical, or thermal limitations can

accelerate this inherent process.

ELECTRICAL LIMITATIONS :

  Exposure to a strong electric field, of polarity opposite that of the polarizing field, will

depolarize a piezoelectric material. The degree of depolarization depends on the grade of

material, the exposure time, the temperature, and other factors, but fields of 200-500 V / mm or

greater typically have a significant depolarizing effect. An alternating current will have a

depolarizing effect during each half cycle in which polarity is opposite that of the polarizing

field.

MECHANICAL LIMITATIONS:

Mechanical stress sufficient to disturb the orientation of the domains in a piezoelectric material

can destroy the alignment of the dipoles. Like susceptibility to electrical depolarization, the

ability to withstand mechanical stress differs among the various grades and brands of

piezoelectric materials.The bending forces generated by converse piezoelectricity are extremely

high and usually cannot be constrained. The only reason the force is usually not noticed is

because it causes a displacement of the order of one millionth of an inch (a few nanometers).

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THERMAL LIMITATIONS:

If a piezoelectric ceramic material is heated to its Curie point, the domains will

become disordered and the material will be depolarized. The recommended upper operating

temperature for a ceramic usually is approximately half-way between 0°C and the Curie point.

Within the recommended operating temperature range, temperature-associated changes in the

orientation of the domains are reversible. On the other hand, these changes can create charge

displacements and electric fields. Also, sudden temperature fluctuations can generate relatively

high voltages, capable of depolarizing the ceramic element. A capacitor can be incorporated

into the system to accept the superfluous electrical energy. The system dissipative power is

eventually transformed into internal heat energy in PZT element. An increase in the

temperature of the actuators is thus inevitable. The phenomenon of temperature rise of PZT

actuators has often been observed in experiments. When PZT elements operate at a certain

temperature, piezoelectric properties, such as dielectric constant and the piezoelectric constant,

change because of the strong temperature dependency. In practice, as PZT elements are used

for structural actuation at the system resonance or with a large electrical field, heat generation

throughout PZT actuators may be significant. Therefore, the heat transfer analysis of integrated

PZT elements in large electric field applications are is important.

THE STATE-OF-THE-ART RESEARCH ON PZT:

In the application of PZT materials indicates that numerous issues remain unanswered

about the application of piezoelectric and their limitations. Research on modeling is needed to

predict the behavior. On the materials engineering side investigation for enhancing mechanical

properties of piezoelectric is needed. However, much less research has been performed on

structures with curvatures and further research in this area is needed.

PIEZO CERAMICS IN MEMS

Current electronic devices which are based on piezoelectric properties, according to Ballato

(1996), have expanded in their application beyond that of crystal oscillators. Such piezoelectric

devices are being used as transducers in telephone speakers and sonar arrays as well as

mechanical actuation and sensing microstructures on electronic micro-chips. These MEMS

devices can provide signal sensing, processing, and output functions that were unattainable by

electronic or photonic methods alone.

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Another compelling aspect of piezoelectric application, suggests, is that of micro- and nano-

electronics which are constructed on the behavior of charged species when they are subjected to

an electric field. The extreme miniaturization of these structures is based on their ability to take

on a "capacitor-like" form, thereby removing the need for bulky inductors and replacing them

with "thin, planar electrodes" which now introduce electric fields for circuit operations. These

"introduced electric fields" can also provide forces needed to drive mechanical motions in a

piezoelectric device by making use of the voltages resident on the micro-chips by way of the

piezo effect. The phenomenon of piezoelectricity is also evident in ultra sensitive micro-

positioning.

CLASSIC VIBRATION ALERTS IN MOBILE PHONES:

A Micro motor is used in the mobile phones for vibration alerts. It has an eccentric weight

mounted on the drive shaft which attains resonance when rotated at some frequency thus

produces vibrations. Its common that mobile phone in vibration mode will run out of battery

soon. But the strain produced in the piezo ceramics is very large with very minimal voltage

consumption .They also have low electrical losses because of its material properties. So this

application will give effective reduction in battery power consumption.

BASIC CIRCUIT DESIGN FOR APPLICATION OF USING P Z T IN MOBILE PHONE

VIBRATION:

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THE FUNCTIONS OF THE CIRCUIT:

1. Signals are received by the antenna.

2. Band pass filter and RF amplifier will filter out the desired band width from signals.

3. Mixer combines the sine wave from local oscillator to the filtered signals thus

producing an output having frequency as difference between the signal frequency and

oscillator frequency.(i.e. intermediate frequency)

4. I F amplifier provide most of the receiver’s gain and selectivity.

(Selectivity is ability to separate signals from interference and noise)

5. Modes of switch operation :

No call or message: S1 and S2 open.

Call alert : S1 close and S2 open

Call attended/rejected:S1 open

Call attended: S2 close.

Call rejected: S1 open.

6. When switch S1 is closed, electric field is provided to PZT in polarization direction

and it vibrates.

7. When messages are received only preset number of pulses is sent to PZT so it

vibrates a few number times and automatically stops on the end of pulses.

PZT

Antennae

RF amp

Band pass filter

Mixer

IF amp

Detector

Local oscillator

Audio amp

Metal substrate

Band pass filter

S 1

S 2

LS

Mob

il

e

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8. Since PZT has high resonance frequency the vibration are passed through metal

substrate to produce good sense of vibration.

ADVANTAGES:

Classic micro motor is replaced by PZT ceramic, hence less moving parts.

very reliable

current consumption is nearly 8 to 10 mA (in motor its 120-160mA)

low power requirements (if used ideally)

fast-acting and highly controllable

EPILOGUE:

Significant progress has been made in recent years in the field of smart materials and

structures. However, further research is needed before these materials can be utilized as viable

options for engineering applications. The most promising areas in the field of smart material and

structures include integration and miniaturization, active control and self-adaptation, and

diagnostic and self-repairing. Our design is still in its nascent stage lot of enhancements are

feasible .To conclude, smart materials will lead a major role in building up intelligent home and

unmanned factory, which are the vision of today’s engineering ad technology.

Acknowledgements:

We sincerely thank the following dignified staffs of our college for their kind help and guidance

to finish this paper successfully.

[1].Th.G.Sakthinathan, Lecturer, Dept. of Manufacturing Engg ,CEG, Anna Varsity.

[2]. Dr. P. Vanaja Rajan ,Assistant Professor, Dept. of EEE, CEG, Anna Varsity.

[3]. Th.Pughazalendi Sukumaran , Lecturer, Dept. of EEE, CEG, Anna varsity.

References:

[1]. Smart talk given by Dr. Siva Kumar, Dept of Physics, Anna varsity

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[2]. Technical papers from 1996 ASME International Mechanical Engineering Congress and

Exhibition in Atlanta.

[3].www.efunda.com ,www.intellimat.com

[5] .Web site of APC international limited.

[6].Piezoelectric effect and its applications, by Sherri Garcia Edward (1998)

[7].Crystalline Dielectrics, volume 2, By I.S, Zheludev, Institute of crystallography, Moscow.

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