INTRODUCTION: In 1880 Jacques and Pierre Curie discovered an unusual characteristic of certain crystalline materials .When subjected to mechanical stress the crystals became electrically polarized .This phenomenon was termed Piezoelectricity .The name has its origin from Greek word Piezein which means to press or to squeeze. Quartz, Rochelle salt, tourmaline are some crystals which exhibit piezoelectricity. Piezoelectricity finds applications in a number of fields like in actuators, sensors, transformers, ultrasonic transducers etc.

Piezoelectricity and its inverse Piezoelectricity is the appearance of electric field or potential across certain type of crystals when subjected to tensile or compressive stresses. The piezoelectric effect is reversible .If a crystal is polarized by electric field then a strain is produced that is its dimensions change. This is termed as inverse piezoelectricity .The deformation of about 0.1% of original dimension in piezocrystals finds useful applications such as in production and detection of sound, electronic frequency generation, microbalance.1 cm3 of quartz with 2kN of applied force on it can produce a voltage of 2500 volts.Piezoelectricity is direction sensitive that is tensile and compressive forces produce voltages of opposite polarity.

Piezoelectric crystals

Of the thirty two crystals classes twenty one are non centrosymetric and of these twenty exhibit piezoelectricity. Piezo electric crystals classes: 1, 2, m, 222, mm2, 4,-4, 422,4mm.Piezo crystals can be broadly classified into two types: a) Natural b) Synthetic.

Natural crystals:-Quartz, Rochelle salt, Ceramics A&BThe ceramic materials are poly crystalline in nature. They are made up of Sodium Titanate.Synthetic group:-Ammonium dihydrogen phosphate,lithium sulphate,dipotassium tartarate,potassium dihydrogen phosphate,lithium niobate.

Single crustals of natural or manmade materials exhibit desirable piezoelectric properties.Materials used to fabricate single-crystal piezoelectric elements include lithium niobate(LiNbO3), Lithium Tetraborate(Li2B4O7) and quartz single crystal PMN-PT and PZM-PT elements exhibit ten times the strain of comparable poly crystalline Lead-Zirconate-Titanate elements.Relaxor materials are also being used for piezoelectric materials. Lead magnesium niobate,lead nickel niobate are currently being studied among the most studied relaxor materials.

How are piezoelectric ceramics made?

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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(fig1.1) .Under conditions that confer tetragonal or rhombohedral symmetry on the crystals, each crystal has a dipole moment (1.1b).

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

Above a critical temperature, the Curie point, each perovskite crystal in the fired ceramic element exhibits a simple cubic symmetry with no dipole moment (Figure 1.1a). At temperatures below the Curie point, however, each crystal has tetragonal or rhombohedral symmetry and a dipole moment (Figure 1.1b). Adjoining dipoles form regions of local alignment called domains. The alignment gives a net dipole moment to the domain, and thus a net polarization. The direction of polarization among neighboring domains is random, however, so the ceramic element has no overall polarization (Figure 1.2a).

The domains in a ceramic element are aligned by exposing the element to a strong, direct current electric field, usually at a temperature slightly below the Curie point (Figure 1.2b). Through this polarizing (poling) treatment, domains most nearly aligned with the electric field expand at the expense of domains that are not aligned with the field, and the element lengthens in the

direction of the field. When the electric field is removed most of the dipoles are locked into a configuration of near alignment (Figure 1.2c). The element now has a permanent polarization, the remanent polarization, and is permanently elongated.

Analogous to corresponding characteristics of ferromagnetic materials, a poled ferroelectric material exhibits hysteresis. Figure 1.3 shows a typical hysteresis curve created by applying an electric field to a piezoelectric

Ceramic element until maximum polarization, Ps, is attained, reducing the field to zero to determine the remanent polarization, Pr, reversing the field to attain a negative maximum polarization and negative remanent polarization, and re-reversing the field to restore the positive remanent polarization. The tracing below the hysteresis curve plots the relative change in the dimension of the ceramic element along the direction of polarization, corresponding to the change in the electric field. The relative increase / decrease in the dimension parallel to the direction of the electric field is accompanied by a corresponding, but approximately 50% smaller, relative decrease / increase in the dimension perpendicular to the electric field.

Figure 1.3. Effects of Electric Field (E) on Polarization (P) and Corresponding Elongation / Contraction of a Ceramic Element

What can piezoelectric ceramics do?

Mechanical compression or tension on a poled piezoelectric ceramic element changes the dipole moment, creating a voltage. Compression along the direction of polarization, or tension perpendicular to the direction of polarization, generates voltage of the same polarity as the poling voltage (Figure 1.4b). Tension along the direction of polarization, or compression perpendicular to the direction of polarization, generates a voltage with polarity opposite that of the poling voltage (Figure 1.4c). These actions are generator actions -- the ceramic element converts the mechanical energy of compression or tension into electrical energy. This behavior is used in fuel-igniting devices, solid state batteries, force-sensing devices, and other products. Values for compressive stress and the voltage (or field strength) generated by applying stress to a piezoelectric ceramic element are linearly proportional up to a material-specific stress. The same is true for applied voltage and generated strain.

If a voltage of the same polarity as the poling voltage is applied to a ceramic element, in the direction of the poling voltage, the element will lengthen and its diameter will become smaller (Figure 1.4d). If a voltage of polarity opposite that of the poling voltage is applied, the element will become shorter and broader (Figure 1.4e). If an alternating voltage is applied, the element will lengthen and shorten cyclically, at the frequency of the applied voltage. This is motor action -- electrical energy is converted into mechanical energy. The principle is adapted to piezoelectric motors, sound or ultrasound generating devices, and many other products.

Generator action is used in fuel-igniting devices, solid state batteries, and other products; motor action is adapted to piezoelectric motors, sound or ultrasonic generating devices.

Note: In a piezoelectric crystal the positive and negative charges are separated but symmetrically distributed. So crystal is electrically neutral. When a mechanical stress is applied symmetry is disturbed and the charge asymmetry generates a voltage across the material.

Properties of piezoelectric crystals:

1. Charge Sensitivity (d):

It is defined as the ratio of charge induced to force applied. It is expressed in coulombs per Newton.

d=Q/F

Eg:-The charge sensitivity of quartz is 2 pC/N

2. Voltage sensitivity (q):

It is defined as the ratio of electric field applied to the stress produced. It is expressed as Volt-meter per Newton.

g=E/P

Eg: - The voltage sensitivity of quartz is 50*10 -3 Vm/N

3. 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.

4. 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.

5. 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.

For a particular ceramic material, the pyroelectric charge constant - the change in polarity for a given change in temperature - and the pyroelectric field strength constant - the change in electric field for a given change in temperature - are indicators of the vulnerability of the material to pyroelectric effects. A high piezoelectric charge constant: pyroelectric charge constant ratio or piezoelectric voltage constant: pyroelectric field strength constant ratio indicates good resistance to pyroelectric effects.

Note : Quartz is the most stable piezoelectric material .However its output is quite small .On the other hand Rochelle salt provides the highest output but it can be worked over a limited

humidity range and has to be protected against moisture .The highest temperature is limited to 450 C.

Applications of piezoelectricity

Piezoelectric crystals because of their small size, low power consumption ,absence of electromagnetic waves find applications in wide range of fields.

One of the first 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 coworkers developed an ultrasonic submarine detector. The detector consisted of a 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 chirp 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, and the success of that project, created intense development interest in piezoelectric devices. Over the next few decades, new piezoelectric materials and new applications for those materials were explored and developed.Some of these applications are

1)Piezoelectric Motors

2)Piezo Generators

3)Actuators

4)Ultrasonic Transducers

5)Piezo Transformers

6)Other Applications

PIEZOMOTORS

A piezoelectric motor or piezo motor is a type of motor based upon the change in shape of a piezoelectric material when an electric field is applied..Piezomotor is commonly known under the names of Piezowalk motors or Inchworth motors.

PRINCIPLE OF OPERATION

Piezoelectric motors use a piezoelectric ceramic element to produce ultrasonic vibrations of an appropriate type in a stator structure. The elliptical movements of the stator are converted into the movement of a slider pressed into frictional contact with the stator. The consequent movement may either be rotational or linear depending on the design of the structure. Linear piezoelectric motors typically offer one degree of freedom, such as in linear stages. However, these devices can be combined to provide more complex positioning factors. Rotating piezoelectric motors are commonly used in sub-micrometric positioning devices. Large mechanical torque can be achieved by combining several of these rotational units.

OPERATION OF PIEZOMOTOR AS STEPPER MOTOR

Current piezoelectric motors are fundamentally stepping motors, with each step comprising either two or three actions, based on the locking type.

1.Use of three types of crystals

The most common type of piezoelectric motor uses three groups of crystals: two which are Locking and one Motive, permanently connected to either the motor's casing or stator (not both) and sandwiched between the other two, which provides the motion.

2.Locking Mechanism

The non-powered behaviour of a piezoelectric motor is one of two options: Normally Locked or Normally Free. When no power is being applied to a Normally Locked motor, the spindle or carriage (for rotary or linear types, repectively) will not move under external force. For a Normally Free motor, the spindle or carriage will move freely under external force; However, if both locking groups are powered at rest, a Normally Free motor will resist external force without providing any motive force.

A combination of mechanical latches and crystals could be used, but this would restrict the maximum stepping rate of the motor

3.Stepping Action

Stepping Actions

Fig. 1: Stepping stages of Normally Free motor

Regardless of locking type, piezoelectric motors – both linear and rotary – use the same mechanism to provide movement.

First, one group of locking crystals is activated – this gives one locked side and one unlocked side of the 'sandwich'.

Next, the motive crystal group is triggered and held – the expansion of this group moves the unlocked locking group along the motor path. This is the only stage where motor movement takes place.

Then the locking group triggered in stage one is released (in Normally Locking motors, the other is triggered). Then the motive group is released, retracting the 'trailing' locking group. Finally, both locking groups are returned to their default states.

Other Designs

Single Action

Piezo ratchet stepping motor (designed by Cataclysm)

Very simple single-action stepping motors can be made with piezoelectric crystals. For example, with a hard and rigid rotor-spindle coated with a thin layer of a softer material (like a polyurethane rubber), a series of angled piezoelectric transducers can be arranged. When one group of transducers is triggered, the rotor will be pushed around one step. This design is not capable of such small or precise steps as more complex

designs, but can reach higher speeds and are cheaper to manufacture.

SQUIGGLE MOTORS

The SQUIGGLE motor is a revolutionary linear actuator that sets a new benchmark for small size and high performance. The patented ultrasonic motor creates high force and speed with only a few parts. It replaces complex electromagnetic gearhead motors which have hundreds of parts.

PRINCIPLE OF OPERATION

Piezo actuators change shape when electrically excited. A SQUIGGLE motor consists of several piezoelectric ceramic actuators attached to a threaded nut, with a mating threaded screw inside.   Applying power to the actuators creates ultrasonic vibrations, causing the nut to vibrate in an orbit - similar to a person’s hips in a “Hula Hoop.”   The rotating nut turns the threaded screw, creating a smooth in-and-out linear motion. The speed and position of the threaded

screw can be precisely controlled, and the screw holds its position when the power is turned off. This piezoelectric motor has no parasitic drag, no backlash, and very high stiffness.

Applications of Squiggle Motors

1.In MRI Systems

The SQUIGGLE motor generates no magnetic fields, is vacuum compatible, and can be made from non-ferrous metals. This makes it ideal for use in MRI, scanning electron microscopy and focused ion microscopy applications. They can operate in and around MRI systems without affecting image quality or motor performance

2.In Drug Delivery System

They are used in drug delivery system due to high precision.

3.In Mobile Phone Cameras

These motors are small and robust enough to drive autofocus and optical zoom lens in mobile phone cameras.