A powerful presence -
ceramics in piezo applications




Piezoceramics
are capable of converting mechanical quantities such as pressure and acceleration into electrical quantities or, conversely, of turning electrical signals into mechanical movement or oscillations.

Piezoceramic components are used in a broad spectrum of electromechanical transducers covering a wide frequency range. In sensors, they enable the conversion of forces, pressures and accelerations to electrical signals. In sound generators and ultrasonic transducers, they transform voltages into oscillations or deformations.

In automotive engineering, sensor systems assure the safety of occupants and provide intelligent engine control capabilities.

As gas ignition devices, piezoceramic elements are a widespread volume product found in gas heaters and cigarette lighters.

In ultrasonic applications, piezoceramic components generate high-power ultrasonic waves for ultrasonic cleaning, drilling and welding and for stimulating chemical processes. On the other hand, piezoceramics are found in many signal and information processing solutions in the form of ultrasonic receivers and transmitters. They also play a key role in advanced sonar locating and ranging, non-destructive testing, and medical diagnostic equipment.

The operation of piezoceramic actuators relies on the ability of piezo elements to produce controlled deformation in the micrometer range. This characteristic enables new appli
cations for electromechanical transducers as drivers in hydraulic and pneumatic valves, positioning systems, micromanipulators, and proportioning systems for liquid and gaseous media.

Given this broad diversity of applications, it is recommended to discuss with CeramTec's experts at the development stage of a new product or system. Our experienced staff will gladly assist. In dialogue with your company's experts, CeramTec can help to specify application conditions, requirement criteria and design prerequisites, and to propose costeffective solutions. Advanced manufacturing methods enable CeramTec to produce piezoceramic components of unsurpassed quality and in large volume. CeramTec's companywide quality management system assures that its products and services will be fully in line with customer's expectations.

Active in automotive systems: back-up sensor



Fundamentals of piezoceramics


Direct piezoelectric effect & Inverse piezoelectric effect

Piezoelectrics: performance under strain
Piezoelectricity is based on the ability of certain crystals to emit an electrical charge when mechanically loaded in pressure or tension (direct piezo effect). Conversely, these crystals undergo a controlled deformation when exposed to an electric field - a behavior referred to as the inverse piezo effect.

The polarity of the charge depends on the orientation of the crystal relative to the direction of the pressure.

The perovskite structure
Ceramics exhibiting piezoelectric capabilities belong to the ferroelectric group. Today's systems are based mostly on lead zirconate titanate (PZT), i.e., they consist of mixed crystals of lead zirconate (PbZr03) and lead titanate (PbTi03).

Piezoceramic components have a polycrystalline structure comprising numerous crystallites (domains) each of which consists of a plurality of elementary cells.

The elementary cells of these ferroelectric ceramics exhibit the perovskite crystal structure, which can generally be described by the
structural formula A2+ B4+ O3 2-.

For greater clarity, the following graphics show the alternative view of an elementary cell of this lattice structure. The structure has been rotated out of its previously shown position, so that now the anions lie at the center of the cube faces and the tetravalent cation is body-centered, while the bivalent cations are located at the corners of the cube. At temperatures above the Curie point (Tc), this lattice is of the "body-centered cubic" type (top diagram).

At temperatures below the Curie point, the lattice becomes distorted and undergoes a shift in center of gravity of charge which gives rise to a permanent dipole moment (bottom diagram). This phenomenon is referred to as "spontaneous polarization". In the case of PZT, the lattice distortion may be either tetragonal or rhombohedral, depending on the Zr/Ti molar ratio.

Schematic diagram of an ideal perovskite structure, neglecting distortions due to spontaneous polarization at below - Curie temperatures. The bivalent cation is located at the center of the cube, while the tetravalent cations form the cube corners. The bivalent anions are located at the center of each cube edge in this rendering mode. For the PZT (lead zirconate titanate) mixed crystal, the formula is A: Pbl-, B: Ti4+/Zr°+


Body-centered cubic state (above Tc)


Tetragonally distorted (below Tc)


Fundamentals of piezoceramics


Before polarization

During polarization

After polarisation

Immediately after sintering, the domains of a ceramic body (i.e., the areas consisting of crystallites of uniform dipole direction) will show an arbitrary (statistically distributed) orientation, i.e., the macroscopic body is isotropic and has no piezoelectric properties.

These piezoelectric properties must be implemented by "polarization". In this process, the product is exposed to a strong electric DC field that causes the electric dipoles to become aligned in the field direction. They will maintain this orientation even when the DC field is no longer applied (remanent polarization) - a necessary condition for the piezoelectric behavior of ferroelectric ceramics.

The remanent polarization produced by this process is illustrated by the graph below. The electric displacement density D is plotted over the applied electric field strength E. When a field is applied, D increases with E along the ^initial curve" (starting from 0) until saturation is achieved. If E is reduced at this point, Dwill decrease insignificantly; at the remanance point it retains a finite value for E=0 which corresponds to the remanent polarization level Pr. In other words, the material exhibits piezoelectric properties. When an opposed field is applied, D will decrease and disappear at a given field strength (the coercive field strength 'Ej. As E becomes more negative, DwiU reach a negative saturation point. If the material goes through several positive and negative field cycles, it becomes apparent that the forward and return graphs do not coincide. The resulting forward/return loop is called the hysteresis curve.



If we plot the strain S of the ceramic body over the field strength E, as in the next illustration, we obtain a "butterfly curve". The strain S initially grows with the "initial curve" (starting from 0) before it reaches a saturation point. At the remanence level, the component will also retain its residual strain, i.e., the polarization process causes a lasting deformation of the ceramic. The strain S decreases with negative E and disappears at -Ec before beginning to rise again. Also, the deformation effect shows a hysteresis. The remanent polarization imparted to the material defines the operating point of the piezoceramic component. In the range of Sr, the piezoceramic body will respond to field strength variations by undergoing a proportional length change that is utilized for application purposes.


A full or partial elimination of the domain alignment achieved by the polarizing process ("depolarization") will degrade the piezoelectric properties of the material.


Butterfly curve

Depolarization may be the result of three factors:

Thermal depolarization due to heat exposure. In application environments, the component temperature should not exceed one-half the Curie temperature stated in the data sheet. Storage temperatures should also not ;exceed' temperature by a significant margin.

Electric depolarization due to electric fields acting against the original polarization direction.

Mechanical depolarization caused by high-pressure loads, especially with short-circuited electrodes. The maximum permissible pressure level varies greatly according to the material used.

Since most applications will involve some form of compound loading (e.g., exposure to an electric field opposed to the polarization direction at elevated operating temperatures) and the depolarization behavior cannot be estimated in this case, the use of life tests under near-field conditions is recommended at the project planning stage.



Piezoceramic applications

Automotive

Knock sensors, back-up sensors, acceleration sensors, gyrometric sensors, sonar transducers for object indentification, sonar transducers for locating/navigation functions.

Knock sensor

Mechanical engineering
Ultrasonic distance sensors, level sensors, flow rate measurement (liquid and gaseous media), ultrasonic cleaning, ultrasonic welding (plastics and metal), ultrasonic machining, non-destructive testing, active vibration control.


Level transducers (reflection type)




Medical techniques
Lithotripters, dental plaque removers, ultrasonic scalpels, ultrasonic nebulizers, ultrasonic diagnostics, therapy.

Actuators
Optical fiber alignment, precision positioning devices, pneumatic/ hydraulic valves.

Monolithic multilayer actuator
Consumer applications
Household gas lighters, piezo cigarette lighters, guitar pickups and burglar alarm sensors.

Terminal connections of piezoceramics
The metallized electrode surfaces of CeramTec piezoceramics are suitable for conductive rubber or spring-type contact systems, as well as for soldered connections. In sensor applications involving small power levels, it is also possible to attach flexible PCB type conductors to the metallized surface with a conductive bonding agent such as specialized epoxies.

Where terminals are to be soldered, a number of special requirements must be observed in order to achieve optimized results. First, a temperature-controlled electronic soldering iron with an output of approximately 30 W should be used. The optimum soldering tip temperature is 320°C. Since the electrode is only several micrometers thick, a solder containing silver must be used on silver electrodes to prevent the silver from diffusing into the solder. CeramTec recommends the use of the following solder:
L-Sn 62 Pb 36 Ag 2 DIN 1707 / F-SW 26, 1.5 DIN 8516


Soldering is generally performed in two steps:
a) tinning the soldering point sensors.
b) soldering the flexible connecting lead into position

The duration of both steps should be minimized and should not exceed 1 second. To achieve this, the use of pre-tinned flexible leads is required. It is often beneficial to preheat the piezoceramic to facilitate and/or accelerate the soldering process. The recommended preheating temperature is 60°C.




Article kindly provided by CeramTec

  
 
 

For details of Sensor Suppliers, click here...

Home - Website - Search - Suppliers - Links - New Products - Catalogues - Magazines
Problem Page - Applications - How they work - Tech Tips - Training - Events - Jobs