Piezoelectricity

Nanotechnology

UJI David Mateu Lluesma

Andrés Orenga Montoliu

Index

1. Piezoelectricity 2. Piezoelectric effect 3. What causes piezoelectricity 4. History of the piezoelectric effect 5. Piezoelectric materials 6. Piezoelectric applications

1.Piezoelectricity

Piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure. It is derived from the Greek piezo, which means to squeeze or press, and electric, which means amber, an ancient source of electric charge.

2.Piezoelectric effect

Piezoelectric effect or piezolectricity has some characteristics like this effect is reversible, meaning that materials exhibiting the direct piezolectric effect, also exhibit the converse piezolectric effect. It can generate stress when an electric field is applied.

In practice, the crystal becomes a kind of tiny battery with a positive charge on one face and a negative charge on the opposite face; current flows if we connect the two faces together to make a circuit. In the reverse piezoelectric effect, a crystal becomes mechanically stressed (deformed in shape) when a voltage is applied across its opposite faces.

3. What causes piezoelectricity?

In most crystals, the unit cell is symmetrical; in piezoelectric crystals, it isn't. Normally, piezoelectric crystals are electrically neutral: the atoms inside them may not be symmetrically arranged, but their electrical charges are perfectly balanced: a positive charge in one place cancels out a negative charge nearby. However, if you squeeze or stretch a piezoelectric crystal, you deform the structure, pushing some of the atoms closer together or further apart, upsetting the balance of positive and negative, and causing net electrical charges to appear. This effect carries through the whole structure so net positive and negative charges appear on opposite, outer faces of the crystal. The reverse-piezoelectric effect occurs in the opposite way. Put a voltage across a piezoelectric crystal and you're subjecting the atoms inside it to "electrical pressure." They have to move to rebalance themselves—and that's what causes piezoelectric crystals to deform (slightly change shape) when you put a voltage across them. 4. The history of the piezoelectric effect

The direct piezoelectric effect was first seen in 1880, and was initiated by the brothers Pierre and Jacques Curie. By combining their knowledge of pyroelectricity with their understanding of crystal structures and behavior, the Curie brothers demonstrated the first piezoelectric effect by using crystals of tourmaline, quartz, topaz, cane sugar, and Rochelle salt. Their initial demonstration showed that quartz and Rochelle salt exhibited the most piezoelectricity ability at the time.

Over the next few decades, piezoelectricity remained in the laboratory, something to be experimented on as more work was undertaken to explore the great potential of the piezoelectric effect. Initial use of piezoelectricity in sonar created intense international developmental interest in piezoelectric devices. Over the next few decades, new piezoelectric materials and new applications for those materials were explored and developed.

During World War II, research groups in the US, Russia and Japan discovered a new class of man-made materials, called ferroelectrics, which exhibited piezoelectric constants many times higher than natural piezoelectric materials. Although quartz crystals were the first commercially exploited piezoelectric material and still used in sonar detection applications, scientists kept searching for higher performance materials. This intense research resulted in the development of barium titanate and lead zirconate titanate, two materials that had very specific properties suitable for particular applications.

5. Piezoelectric materials There are many materials, both natural and man-made, that exhibit a range of piezoelectric effects. Some naturally piezoelectric occurring materials include Berlinite (structurally identical to quartz), cane sugar, quartz, Rochelle salt, topaz, tourmaline, and bone (dry bone exhibits some piezoelectric properties due to the apatite crystals, and the piezoelectric effect is generally thought to act as a biological force sensor). An example of man-made piezoelectric materials includes barium titanate and lead zirconate titanate.

In recent years, due to the growing environmental concern regarding toxicity in lead-containing devices and the RoHS directive followed within the European Union, there has been a push to develop lead free piezoelectric materials. To date, this initiative to develop new lead-free piezoelectric materials has resulted in a variety of new piezoelectric materials which are more environmentally safe.

Examples: Quartz (SiO2), Berlinite (AlPO4), Gallium orthophosphate (GaPO4), Tourmaline,Barium titanate (BaTiO3)Lead zirconate titanate (PZT)

Quartz SiO2

Quartz shows a strong piezoelectric effect perpendicularly to the prism axis. Applying pressure on a quartz crystal generates an electrical polarization along the pressure direction. Alternatively, applying an electrical tension leads to a mechanical deformation of the crystal.

In a quartz clock or watch, the reverse-piezoelectric effect is used to keep time very precisely. Electrical energy from a battery is fed into a crystal to make it oscillate thousands of times a second. The watch then uses an electronic circuit to turn that into slower, once-per-second beats that a tiny motor and some precision gears use to drive the second, minute, and hour hands around the clock-face.

Gallium orthophosphate GaPO4

Gallium orthophosphate (GaPO4) is a colorless crystal crystallizing in a trigonal crystal system. Similar to quartz due to the fact that the silicon atoms are replaced alternating by gallium and phosphor. Therefore this crystal has almost the same characteristics, however it has twice its piezoelectricity, which is an interesting advantage especially in mechanical applications.

Contrary to quartz GaPO4 is not found in nature, but has to be synthetised.

6. Piezoelectric applications

Piezoelectric Sensors Piezoelectric sensor are devices using the piezoelectric effect to measure acceleration, pressure, strain or force and converting them to an electrical signal. Piezoelements are suitable for the detection of dynamic processes. In static applications the piezoelectric charges are too small, in order to be detected. An amplifier is used to convert the piezoelectric charges into a measurable electrical tension.

Piezoelectric motors

A piezo motor is based on the change in mechanical shape of a piezoelectric material when an tension is applied. The material produces ultrasonic or acoustic vibrations and produces a linear or rotary motion. A few design exists currently: Locking mechanisms Stepping Actions Single Action With a fast response of the material (up to 5Mhz), it is possible to obtain a linear speed of 800mm/second. According to manufacturers, precision can be of a few nanometers.

Ultrasonic Technology Piezoceramics can be used to generate ultrasonic waves in the frequency range of power ultrasound (20 to 800 kHz). They can be used in different diagnostic and therapeutic applications, for example in tartar removal or lithotripsy, but also in ultrasonic technology. Industrial Ultrasonic Cleaning, Sonar Technology and Hydroacoustics, Ultrasonic surgery

Scientific Instrumentation Piezo components have become firmly established in modern science as drives and ultrasonic transducers. They work reliably even under extreme conditions such as magnetic fields, cryogenic temperatures or ultrahigh vacuum, which they have proven worldwide in many applications in space, in laboratories and in large-scale research installations such as synchrotrons. High-precision measurement and testing systems in industry also rely on piezo components as drives. Examples: Scanning Probe Microscopy, Laser tuning

Pumping and Dosing

Piezo elements pump and meter small liquid or gas volumes reliably and precisely in the range of a few hundred milliliters to a few nanoliters. Different types of pumps, such as membrane or peristaltic hose pumps, are actuated by different drive principles. The piezo elements can be adapted perfectly to each specific application environment, for example miniaturized lab-on-a-chip solutions for mobile analytical instruments. Examples: Printing technology, Piezo Micropumps, Aerosol Production

Medical Technology Medical technology and related life-science disciplines require drive components that have to be fast, reliable and energy-saving. In these fields as well, progress goes hand in hand with increasing miniaturization. Piezoceramic drives combine exactly these characteristics. The piezo components and piezo actuators used are as different as their applications. Ultrasonic applications that use simple disks are in use, for example, in cosmetics, but also in medical tooth cleaning and for metering tasks. Examples: Implantable Medical Devices, Ultrasonic surgery, Medical diagnostics: Imaging

Energy Harvesting

The term "energy harvesting" refers to the generation of energy from sources such as ambient temperature, vibration or air flow. Converting the available energy from the environment allows a self-sufficient energy supply for small electric loads such as sensors or radio transmitters. Piezo igniters

Metrology

Ultrasonic sensors emit high-frequency sound pulses beyond the human hearing threshold and receive signals reflected from objects. The time the echo signals take to arrive is processed electronically and can be used for a wide range of applications in metrology. Examples: Level Measurement, Non-contact Measurement with Air Ultrasound, Ultrasonic Proximity Sensors as Park Distance Control