Piezoelectric materialsIntroduction: the piezoelectric effectThe piezoelectric effect describes the relation between a mechanical stress and an electrical voltage in solids.It is reversbile: an applied mechanical stress will generate a voltage and an applied voltage will change the shape of the solid by a small amount (up to a 4% change in volume).In physics, the piezoelectric effect can be described as the the link between electrostatics and mechanics.

HistoryThe piezoelectric effect was discovered in 1880 by the Jacques and Pierre Curie brothers. They found out that when a mechanical stress was applied on crystals such as tourmaline, tourmaline, topaz, quartz, Rochelle salt and cane sugar, electrical charges appeared, and this voltage was proportional to the stress.First applications were piezoelectric ultrasonic transducers and soon swinging quartz for standards of frequency (quartz clocks).An everyday life application example is your car's airbag sensor. The material detects the intensity of the shock and sends an electricla signal which triggers the airbag.Piezoelectric materialsThe piezoelectric effect occurs only in non conductive materials. Piezoelectric materials can be divided in 2 main groups: crystals and cermaics. The most well-known piezoelectric material is quartz (SiO2).Quartz SiO2IntroductionThe trigonal crystallized silica of SiO2 is known as Quartz and is one of the most common crystals on earth's surface. It has an hexagonal structure made of ,PropertiesQuartz is a very pure crystal and contains only traces of other elements. Natural quartz contain: Al3+: 13 to 15,000 ppm, usually only 100 ppm Fe3+: Ti4+: P5+: H+: Li+: Na+: 9 to 1400 ppm K+: 3 to 300 ppmDensity: 2.65 g/cm3Mineral class: Oxides/hydroxidesPiezoelectricityQuartz 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.Aluminum orthophosphate - Berlinite - AlPO4IntroductionAluminum orthophosphate, AlPO4, is a rhombic crystal made of sodium phosphate and aluminum salt. In this chemical configuration, aluminum is present as three-way positively charged cation (Al3+) reciprocal electrostaticaly to the phosphate anion (PO43).

PropertiesDensity: 2560 kg/mMol mass: 121,95 g/molGallium orthophosphate GaPO4IntroductionGallium 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.TourmalineIntroductionTourmaline has trigonal crystal properties with the following complex chemical composition (one of the most complicated groups of silicate minerals): Na(Al,Fe,Li,Mg,Mn)M3Al(Si6O18)(BO3)3(OH,F)4 XY3Z6 [(BO3) 3Si6O18 (OH, F) 4] X= (approx., well, K, Mn) Y= (mg, left, aluminium, Mn, Fe II) Z= (aluminium, mg, Ti, CR, V, Fe III)Chemical properties: Alumina borosilicate with fluorine.Color: commonly black but can range from violet to green and pink.Crystals show piezoelectric, pyroelectric effect and strong pleochroismus.Lead zirconate titanate PZTLead zirconate titanate (PZT) is a ceramic material made of lead (Pb), oxygen (O) and titanium (Ti) or zirconium (Zr).

Chemical formula: Pb [ZrxTi1-x] O3 ; x = 0,52The atoms are arranged in a cubical structure.At temperatures below the Curie temperature (depending on the material between 150C and 200C) the titanium atom moves from its central position and the electrically neutral lattice becomes a dipole.This dipole lattice presents now piezoelectric characteristics and is considered as one of the most economical piezoelectric material.By doping the PZT material, its piezoelectric characteristics can be modified: especially the hardness or softness of the material.Barium titanate BaTiO3Barium titanate belongs to the group of electrical ceramics. It is an oxide of barium and titanium.As a piezoelectric material, it is usually replaced by lead zirconate titanate (PZT).It is used as a piezoelectric material for microphones and transducers. Crystal Structure: tetragonal Molar mass 233.192 g/molApplicationsPiezoelectric SensorsPiezoelectric 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.Principle of operations3 modes of operation exist depending on how a piezoelectric material is cut: transverse longitudinal shearUltrasonic transducersUltrasonic transducers (10kHz to 70kHz) are used in imaging and high power applications. The transducer can act as a sensor and an actuator in imaging applciations In high power applications such as sonochemistry or medical treatment, the transducer sends ultrasound waves in the body and detect the returning echo.Piezoelectric motorsA 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 ActionWith 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.Other Piezoelectric applications High-voltage sources In lighters or portable sparkers with a piezo fuze a sudden and strong pressure is used in order to produce a short high voltage. The spark then ignites the gas. Frequency standard Reduction of vibrations Piezoelements are used in music for acoustic instruments. They are inserted in stringed instruments such as guitar, violin or Mandoline. The dynamic deformation of the instrument (vibration of the cords) is converted into a small alternating voltage.