Reversed piezoelectric effect

The left-hand side is the forward piezoelectric coefficient, in units of coulombs/newton. The right-hand side is the reverse piezoelectric coefficient, in units of meters/volt. They are equal. The coexistence of forward and reverse piezoelectric effects provides a simple method to test the piezodrive used in STM, which is discussed in Section 9.6. 

The simplest SAW sensor is a two-terminal transmission (delay) line in which the acoustic (mechanical) wave is piezoelectrically launched in one oscillator, called the transmitter. It travels along the surface of the substrate and is then transformed back into an electrical signal by the reverse piezoelectric effect at the receiving oscillator (Fig. 4.18). 

If a quartz plate is subjected to an alternating electric field, the reverse piezoelectric effect causes it to expand and contract at the field frequency. If this field frequency is made to coincide with the natural elastic frequency of the crystal, the plate resonates the direct piezoelectric effect then augments the applied electric field. This is the basis of the crystal oscillator and the quartz clock. See also CRYSTAL MICROPHONE CRYSTAL PICK-UP. 

Fig. 5.6 Piezoelectric effects in a dielectric material. Direct piezoelectric effect due to strain there is an outward shift either of cations or anions on opposite surfaces, which generates surface charge and potential difference between the surfaces (a). Reverse piezoelectric effect surface charge caused by application of an external EM field attracts ions with an opposite charge in the material expansion of the material Is the result (b)...

Fig. 7.4 An intelligent active system to monitor and adjust combustion in engines. A sensor (electrochemical cell) monitors the partial pressure of oxygen, its output voltage depends on the ratio of partial pressure of oxygen in air (po2 (ref)) to partial pressure of oxygen in the combustion gas (po2(exhaust An increase of po2(exhaust at incomplete combustion is indicated by the sensor with increased voltage V. In response, the control component sends a voltage signal to the actuator. Thanks to the reverse piezoelectric effect (see Pig. 5.6b) the actuator expands and sets ajar the valves supplying fuel to the engine...

The piezoelectric effect is the generation of an electrical charge in a material as the result of a mechanical force exerted on it. Many materials exhibit the piezoelectric effect, for example, quartz, synthetic ceramics such as lead titanate, poly(vinyli-dene fluoride) and sucrose the opposite is called the reverse piezoelectric effect. If an alternating voltage of the required frequency is applied to a piezoelectric material, then a mechanical resonance occurs that emits an electrical signal at a very precise frequency. Such crystal oscillators, nowadays nearly all based on... 

The reverse piezoelectric effect can be similarly explained. On bringing a quartz plate into an electric field, it will be deformed due to displacement of the charges the sign on deformation changes to the opposite when the direction of the field is changed. 

Direct and reverse piezoelectric effects have found very wide application in practice for measuring pressure in rapidly proceeding processes, for transformation of electric vibrations in mechanical and in acoustoelectronics, etc. 

From the very beginning of their experiments, the Curie brothers realized that the linear piezoelectric effect only exists in anisotropic crystals, such as quartz, tourmaline, and Rochelle salt (Curie and Curie, 1880). In fact, as seen from Fig. 9.3, if the +z and -z directions of the plate are equivalent, by reversing the direction of the electric field E-s, the strain should be the same. In such cases, the strain should be proportional to Es 1 instead of E3. In other words, there should be no linear piezoelectric effect. 

Ferroelectrics. Among the 32 crystal classes, 11 possess a centre of symmetry and are centrosymmetric and therefore do not possess polar properties. Of the 21 noncentrosymmetric classes, 20 of them exhibit electric polarity when subjected to a stress and are called piezoelectric one of the noncentrosymmetric classes (cubic 432) has other symmetry elements which combine to exclude piezoelectric character. Piezoelectric crystals obey a linear relationship P,- = gijFj between polarization P and force F, where is the piezoelectric coefficient. An inverse piezoelectric effect leads to mechanical deformation or strain under the influence of an electric field. Ten of the 20 piezoelectric classes possess a unique polar axis. In nonconducting crystals, a change in polarization can be observed by a change in temperature, and they are referred to as pyroelectric crystals. If the polarity of a pyroelectric crystal can be reversed by the application on an electric field, we call such a crystal a ferroelectric. A knowledge of the crystal class is therefore sufficient to establish the piezoelectric or the pyroelectric nature of a solid, but reversible polarization is a necessary condition for ferroelectricity. While all ferroelectric materials are also piezoelectric, the converse is not true for example, quartz is piezoelectric, but not ferroelectric. 

Piezoelectric materials are materials that exhibit a linear relationship between electric and mechanical variables. The direct piezoelectric effect can be described as the ability of materials to convert mechanical stress into an electric field and the reverse, to convert an electric field into a mechanical stress. The use of the piezoelectric effect in sensors is based upon the latter property. 

Piezoelectric materials are materials that exhibit a linear relationship between electric and mechanical variables. Electric polarization is proportional to mechanical stress. The direct piezoelectric effect can be described as the ability of materials to convert mechanical stress into an electric field, and the reverse, to convert an electric field into a mechanical stress. The use of the piezoelectric effect in sensors is based on the latter property. For materials to exhibit the piezoelectric effect, the materials must be anisotropic and electrically poled ie, there must be a spontaneous electric field maintained in a particular direction throughout the material. A key feature of a piezoelectric material involves this spontaneous electric field and its disappearance above the Curie point. Only solids without a center of symmetry show this piezoelectric effect, a third-rank tensor property (14,15). 

The piezoelectric effect is an electromechanical effect in which mechanical evoke and reverse an electric reaction in a ferroelectric material and vice versa. The word piezo has been derived from the Greek piezein which means press . Compounds are composed of positive and negative ions and are electrically neutral as a whole. The fact that electrically charged particles are still present in the crystal can for example be demonstrated by means of the electric... 

If a piezoelectric plate (Fig. 6.1), polarized in the direction indicated by P, carries electrodes over its two flat faces, then a compressive stress causes a transient current to flow in the external circuit a tensile stress produces current in the opposite sense (Fig. 6.1(a)). Conversely, the application of an electric field produces strain in the crystal, say a negative strain reversal of the field causes a positive strain (Fig. 6.1(b)). The changes in polarization which accompany the direct piezoelectric effect manifest themselves in the appearance of charges on the crystal surface (see Eq. (2.71)) and, in the case of a closed circuit, in a current. 

In Fig. 9-9,b is shown the corresponding situation in the piezoelectric effect, which we shall return to. It will be seen that because the atoms are pulled into position by their neighboring atoms rather than pushed against them, the charge transfers are reversed and the effective charge is reduced this can be seen in the figure. 

In a normal dielectric, the observed polarisation of the material is zero in the absence of an electric field, and this does not change if the material is heated or subjected to mechanical deformation. In a piezoelectric solid a surface electric charge develops when the solid is subjected to a mechanical stress such as pressure, even in the absence of an external electric field. This is called the direct piezoelectric effect. The effect is reversible and the inverse (or converse) piezoelectric effect, in which a voltage applied to a crystal causes a change in shape, also occurs in piezoelectric crystals. The piezoelectric effect generally varies from one direction to another in a crystal, and in some directions a crystal may show no piezoelectric effect at aU whereas in other directions it is pronounced. 

Piezoelectric effect was discovered in 1880 and is a phenomenon that produce electrical charges across material boundaries with application of a mechanical stress. This effect is obtained from specific materials, named piezoelectric materials and the relationship between induced electrical field and the mechanical stress is linear and reversible (Ramadan... 

Electric energy is converted into mechanical vibrations in modem machinery using the piezoelectric effect. It concerivs the reversible property of special ceramic materials to deliver an electric voltage when affected by external forces. This characteristic is used for the generation of ultrasonic vibrations in such a way that the applied voltage is converted into mechanical vibrations. Modern sonic converters usually contain several piezoceramic disks of lead zirconate titanate restricted by two final masses that are mechanically prestressed by a centric screwing. 

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