The piezoelectric effect is a physical phenomenon that some crystals present due to which a difference in electrical potential (voltage) appears between certain faces of the crystal when it is subjected to mechanical deformation and is called direct piezoelectric effect (if the opposite occurs, it is the inverse piezoelectric effect).[5]​.

When a mechanical stress is applied to the crystal, the ionized (charged) atoms present in the structure of each crystal-forming cell move, causing charges to appear on the surfaces of the material. Due to the regularity of the crystalline structure, and as the effects of cell deformation occur in all cells of the crystal body, these charges add up and an accumulation of electrical charge occurs, producing a difference in electrical potential between certain faces of the crystal that can be many volts.

The effect occurs only in crystals that do not have a center of symmetry in the unit cell (symmetric center), that is, the center of negative charges cannot coincide with the center of positive charges at the unit cell level. This asymmetry in charges is what gives rise to the formation of dipoles and this arrangement is called the perovskite structure.

The fact that there are dipoles in the crystal makes it possible that when an electric field is applied to the material, the crystalline structure is deformed, expanding or contracting depending on the direction of the current and that when a force is applied to the crystal, subjecting it to mechanical deformation, an electrical displacement of the dipoles is generated, creating a potential difference between their ends. This potential difference between the extremes is called piezoelectricity and the materials that have this property are piezoelectric materials.

Within this type of materials, we can distinguish the group of those that have the piezoelectric character naturally (such as quartz, tourmaline or Rochelle salts), which usually have a weaker character and the group that presents piezoelectric properties after being subjected to a polarization process, the piezoelectric ceramics (lithium tantalum, lithium nitrate, bernylite in the form of monocrystalline materials and ceramics or polar polymers in the form of micro crystals. oriented), called ferroelectrics.[6]​.

Piezoelectric ceramics are solid bodies similar to those used in electrical insulators; they are made up of countless microscopic ferroelectric crystals, becoming known as polycrystalline. Particularly in ceramics of the Piezoelectric Ceramic type, these small crystals have perovskite-type crystalline structures, and can present simple tetragonal, rhombohedral or cubic symmetry, taking into account the temperature at which the material is found.

Being below a critical temperature, known as the Curie Temperature, the perovskite structure will present tetragonal symmetry where the center of symmetry of the positive electric charges does not coincide with the center of symmetry of the negative charges, giving rise to an electric dipole. The existence of this dipole causes the crystalline structure to deform in the presence of an electric field and generate an electrical displacement when subjected to mechanical deformation, characterizing the inverse and direct piezoelectric effect respectively.[7]​.

The mechanical deformation or the variation of the electrical dipole of the crystalline structure of the ceramic does not necessarily imply macroscopic effects, since the dipoles are organized in domains, which in turn are randomly distributed in the polycrystalline material. For macroscopic manifestations to occur, a preferential orientation of these domains is required, known as polarization. Even this polarization fades with time and use, rendering the material unusable for the transformation of electrical energy into mechanical energy.[8]​.

In piezoelectric materials, dipoles with parallel orientation are grouped in the so-called Weiss domains. In piezoelectric ceramics these Weiss domains are randomly oriented. The responses of the dipoles to an electric field applied from the outside will tend to cancel each other, producing no significant changes in the dimensions of the ceramic and no macroscopic piezoelectric behavior being observed.

To obtain a piezoelectric response at a macroscopic level, the dipoles must be permanently aligned with each other, for which the polarization process is used. A characteristic of piezoelectric materials is that they have a Curie temperature, above which the dipoles of the crystals can change their orientation within the solid phase of the material. In the polarization process, the material is heated above this Curie temperature and a powerful electric field is applied in whose direction (polarization direction) the dipoles will align.

By keeping the electric field constant, the material cools below its Curie temperature, causing the dipoles to remain permanently aligned, having already suppressed the electric field, making it possible to say that the material is polarized. The dipoles will not be completely aligned or parallel to each other, but this alignment will be enough for them to exhibit piezoelectric properties.

With the piezoelectric material polarized, the Weiss domains increase their alignment proportionally to the voltage applied to it, with the result of a change in the dimensions of the material.

An important detail in piezoelectric materials is that if, once polarized, the Curie temperature is exceeded, their piezoelectric properties will be lost or reduced.[9]​.

Materials that have piezoelectric properties are divided into two groups: those that have these properties naturally and those that need to be polarized (which are piezoelectric ceramics).

The former have a very small piezoelectric effect, which is why the latter were developed, with a much greater effect, of which the most used in industry are barium titanate (BaOTiO2), and a combination of lead zirconate (PbZrO3) and lead titanate (PbTiO3). This combination is called piezoelectric ceramics (lead titanate zirconate) in the industry and they are manufactured by compression of powder at high temperature, molded and fired in an oven.

To get an idea of ​​the difference in the piezoelectric effect of one and the other, we can observe their piezoelectric constants. Barium titaniate was discovered in the 1940s and its use was mainly focused on the manufacture of ultrasonic sensors, usually for sonars. Currently, although it is still used in industry as a main component of memory devices, it has been widely replaced by the Piezoelectric ceramic (and its derivatives), which have a much higher critical (Curie) temperature.

Contenido

La fabricación de cerámicas piezoeléctricas es muy similar sea cual sea la cerámica que se quiere obtener. El proceso consta de los siguientes pasos:

1. Se selecciona el material y se pesa acorde a las proporciones para ser manufacturado.

2. Se muele en un molino de bolas para conseguir un grano muy fino

3. La mezcla se calienta hasta el 75 % de la temperatura de síntesis para acelerar la reacción de los componentes.

4. El polvo calcinado es molido de nuevo para incrementar su reactividad.

5. Se prensa y se elimina el sobrante.

6. Se calienta hasta la síntesis entre 1250 °C y 1350 °C.

7. Se corta, se pule y se le da la forma final.

8. Se somete al proceso de polarización.

Piezoelectric ceramics

Piezoelectric ceramics were developed in 1952 at the Tokyo Institute of Technology. It is a synthesized solid solution of lead titaniate with lead zinconate and is the most widely used piezoelectric ceramics due to its critical temperature, its piezoelectric coefficient and its relatively low operating temperature (200 °C).

It has the advantage over other ceramics that it can be manufactured at a very low price, it is physically strong and chemically inert, and it has also been shown to have more piezoelectric sensitivity than other ceramics, which is verified by observing its piezoelectric coefficient.[9]​.

Navy Type I (“Hard”) Recommended for medium and high power applications under conditions of continuous and repetitive use. This is capable of generating high amplitudes of vibrations while keeping mechanical and dielectric losses low. Featured properties: d, dielectric dissipation and Q. Main applications: Ultrasonic and sonar cleaning systems. Commercially known as Piezoelectric Ceramic-4.

Navy Type II (“Soft”) High sensitivity, ideal for the transmission and reception of low-power devices. It presents dielectric and mechanical losses that prevent continuous excitation with high intensity. Featured properties: d, g, N and TC. Main applications: Devices for non-destructive testing, hydrophones and accelerometers. Commercially known as Piezoelectric Ceramic-5A.

Navy Type III (“Hard”) Similar, but less sensitive than Navy Type I; It is capable of converting twice the power while keeping mechanical and dielectric losses low. Recommended for applications that require high power. Featured properties: Dielectric dissipation, Q and maximum power conversion. Main applications: ultrasonic welding systems and materials processing. Commercially known as Piezoelectric Ceramic-8.

Navy Type IV (“Soft”) Suitable for medium power applications. It became obsolete with the arrival of Piezoelectric Ceramics, being replaced mainly by Navy Type I (according to note 5,[5]​ the Navy Type IV subgroup is made up of BT and not Piezoelectric Ceramics). It has low TC. Main applications: maintenance of old equipment. Commercially known as barium titanate.

Navy Type V (“Soft”) Suitable for applications requiring high energy and potential difference. Featured properties: d, K and g. Main applications: impact detonators. Commercially known as Piezoelectric Ceramic-5J.

Navy Type VI (“Soft”) Suitable for applications that require large mechanical deformations. Featured properties: d and K. Main applications: actuators and positioners. Commercially known as Piezoelectric Ceramic-5H.

The main properties of piezoelectric materials from the application point of view are [3,4,6]:.

Piezoelectric charge constant d.

Piezoelectric voltage constant g.

Coupling coefficient.

Mechanical quality factor Q.

Dielectric dissipation factor Tan δ.

Curie Temperature TC.

Frequency constants N.

Acoustic impedance Z.

The first serious applications of piezoelectrics (natural, of course, piezoelectric ceramics had not yet been developed) took place during World War I, where they were incorporated into submarines to serve as ultrasonic detectors (sonars). However, at that time, large companies did not give any importance to this success, although it is true that this first use that was given to these materials is still present today.

A few years later, materials testing methods based on the propagation of ultrasonic waves and new methods of transient pressure measurements were developed, useful for studying, for example, the internal combustion of an engine. In addition, the first microphones, record players, accelerometers, etc. began to be manufactured.

Already during the Second World War, the first piezoelectric ceramic material, barium titaniate and later the lead zirconate titaniate family (Piezoelectric Ceramics), was manufactured. Thanks to the development of these ceramics, sonar could be improved, they were also used to simplify electrical circuits, microphones were improved and the first ceramic tone transducer was created.

A very important application of piezoelectrics, more specifically quartz, is in precision electronic oscillators, devices that use the natural resonant frequency of the crystal excited by a voltage to make clocks, radios or internal computer clocks.

Their use as memory devices (called FRAM) is currently being investigated, whose advantages over conventional RAM and ROM memories are that they are non-volatile, low-consumption memories that do not require refresh processes and are easy to read/write. However, integration and speed problems currently do not allow this technology to compete with other types of memories (semiconductor technology).

Its use is extended to many more areas. Thanks to these ceramics there are smoke detectors, television controls, motion sensors, the classic “click” lighters, which generate a spark when pressed, and even today dozens of products based on piezoelectrics continue to be patented every year.[9]​.

The main applications of piezoelectric ceramics are:

The most common shapes and dimensions of commercial piezoelectric ceramics are [1]:.

Piezoelectric ceramics are also popularly known as piezoelectric crystals, piezo crystals, piezoelectric crystal and piezo crystal.

The piezoelectric effect is a physical phenomenon that some crystals present due to which a difference in electrical potential (voltage) appears between certain faces of the crystal when it is subjected to mechanical deformation and is called direct piezoelectric effect (if the opposite occurs, it is the inverse piezoelectric effect).[5]​.

When a mechanical stress is applied to the crystal, the ionized (charged) atoms present in the structure of each crystal-forming cell move, causing charges to appear on the surfaces of the material. Due to the regularity of the crystalline structure, and as the effects of cell deformation occur in all cells of the crystal body, these charges add up and an accumulation of electrical charge occurs, producing a difference in electrical potential between certain faces of the crystal that can be many volts.

The effect occurs only in crystals that do not have a center of symmetry in the unit cell (symmetric center), that is, the center of negative charges cannot coincide with the center of positive charges at the unit cell level. This asymmetry in charges is what gives rise to the formation of dipoles and this arrangement is called the perovskite structure.

The fact that there are dipoles in the crystal makes it possible that when an electric field is applied to the material, the crystalline structure is deformed, expanding or contracting depending on the direction of the current and that when a force is applied to the crystal, subjecting it to mechanical deformation, an electrical displacement of the dipoles is generated, creating a potential difference between their ends. This potential difference between the extremes is called piezoelectricity and the materials that have this property are piezoelectric materials.

Within this type of materials, we can distinguish the group of those that have the piezoelectric character naturally (such as quartz, tourmaline or Rochelle salts), which usually have a weaker character and the group that presents piezoelectric properties after being subjected to a polarization process, the piezoelectric ceramics (lithium tantalum, lithium nitrate, bernylite in the form of monocrystalline materials and ceramics or polar polymers in the form of micro crystals. oriented), called ferroelectrics.[6]​.

Piezoelectric ceramics are solid bodies similar to those used in electrical insulators; they are made up of countless microscopic ferroelectric crystals, becoming known as polycrystalline. Particularly in ceramics of the Piezoelectric Ceramic type, these small crystals have perovskite-type crystalline structures, and can present simple tetragonal, rhombohedral or cubic symmetry, taking into account the temperature at which the material is found.

Being below a critical temperature, known as the Curie Temperature, the perovskite structure will present tetragonal symmetry where the center of symmetry of the positive electric charges does not coincide with the center of symmetry of the negative charges, giving rise to an electric dipole. The existence of this dipole causes the crystalline structure to deform in the presence of an electric field and generate an electrical displacement when subjected to mechanical deformation, characterizing the inverse and direct piezoelectric effect respectively.[7]​.

The mechanical deformation or the variation of the electrical dipole of the crystalline structure of the ceramic does not necessarily imply macroscopic effects, since the dipoles are organized in domains, which in turn are randomly distributed in the polycrystalline material. For macroscopic manifestations to occur, a preferential orientation of these domains is required, known as polarization. Even this polarization fades with time and use, rendering the material unusable for the transformation of electrical energy into mechanical energy.[8]​.

In piezoelectric materials, dipoles with parallel orientation are grouped in the so-called Weiss domains. In piezoelectric ceramics these Weiss domains are randomly oriented. The responses of the dipoles to an electric field applied from the outside will tend to cancel each other, producing no significant changes in the dimensions of the ceramic and no macroscopic piezoelectric behavior being observed.

To obtain a piezoelectric response at a macroscopic level, the dipoles must be permanently aligned with each other, for which the polarization process is used. A characteristic of piezoelectric materials is that they have a Curie temperature, above which the dipoles of the crystals can change their orientation within the solid phase of the material. In the polarization process, the material is heated above this Curie temperature and a powerful electric field is applied in whose direction (polarization direction) the dipoles will align.

By keeping the electric field constant, the material cools below its Curie temperature, causing the dipoles to remain permanently aligned, having already suppressed the electric field, making it possible to say that the material is polarized. The dipoles will not be completely aligned or parallel to each other, but this alignment will be enough for them to exhibit piezoelectric properties.

With the piezoelectric material polarized, the Weiss domains increase their alignment proportionally to the voltage applied to it, with the result of a change in the dimensions of the material.

An important detail in piezoelectric materials is that if, once polarized, the Curie temperature is exceeded, their piezoelectric properties will be lost or reduced.[9]​.

Materials that have piezoelectric properties are divided into two groups: those that have these properties naturally and those that need to be polarized (which are piezoelectric ceramics).

The former have a very small piezoelectric effect, which is why the latter were developed, with a much greater effect, of which the most used in industry are barium titanate (BaOTiO2), and a combination of lead zirconate (PbZrO3) and lead titanate (PbTiO3). This combination is called piezoelectric ceramics (lead titanate zirconate) in the industry and they are manufactured by compression of powder at high temperature, molded and fired in an oven.

To get an idea of ​​the difference in the piezoelectric effect of one and the other, we can observe their piezoelectric constants. Barium titaniate was discovered in the 1940s and its use was mainly focused on the manufacture of ultrasonic sensors, usually for sonars. Currently, although it is still used in industry as a main component of memory devices, it has been widely replaced by the Piezoelectric ceramic (and its derivatives), which have a much higher critical (Curie) temperature.

Contenido

La fabricación de cerámicas piezoeléctricas es muy similar sea cual sea la cerámica que se quiere obtener. El proceso consta de los siguientes pasos:

1. Se selecciona el material y se pesa acorde a las proporciones para ser manufacturado.

2. Se muele en un molino de bolas para conseguir un grano muy fino

3. La mezcla se calienta hasta el 75 % de la temperatura de síntesis para acelerar la reacción de los componentes.

4. El polvo calcinado es molido de nuevo para incrementar su reactividad.

5. Se prensa y se elimina el sobrante.

6. Se calienta hasta la síntesis entre 1250 °C y 1350 °C.

7. Se corta, se pule y se le da la forma final.

8. Se somete al proceso de polarización.

Piezoelectric ceramics

Piezoelectric ceramics were developed in 1952 at the Tokyo Institute of Technology. It is a synthesized solid solution of lead titaniate with lead zinconate and is the most widely used piezoelectric ceramics due to its critical temperature, its piezoelectric coefficient and its relatively low operating temperature (200 °C).

It has the advantage over other ceramics that it can be manufactured at a very low price, it is physically strong and chemically inert, and it has also been shown to have more piezoelectric sensitivity than other ceramics, which is verified by observing its piezoelectric coefficient.[9]​.

Navy Type I (“Hard”) Recommended for medium and high power applications under conditions of continuous and repetitive use. This is capable of generating high amplitudes of vibrations while keeping mechanical and dielectric losses low. Featured properties: d, dielectric dissipation and Q. Main applications: Ultrasonic and sonar cleaning systems. Commercially known as Piezoelectric Ceramic-4.

Navy Type II (“Soft”) High sensitivity, ideal for the transmission and reception of low-power devices. It presents dielectric and mechanical losses that prevent continuous excitation with high intensity. Featured properties: d, g, N and TC. Main applications: Devices for non-destructive testing, hydrophones and accelerometers. Commercially known as Piezoelectric Ceramic-5A.

Navy Type III (“Hard”) Similar, but less sensitive than Navy Type I; It is capable of converting twice the power while keeping mechanical and dielectric losses low. Recommended for applications that require high power. Featured properties: Dielectric dissipation, Q and maximum power conversion. Main applications: ultrasonic welding systems and materials processing. Commercially known as Piezoelectric Ceramic-8.

Navy Type IV (“Soft”) Suitable for medium power applications. It became obsolete with the arrival of Piezoelectric Ceramics, being replaced mainly by Navy Type I (according to note 5,[5]​ the Navy Type IV subgroup is made up of BT and not Piezoelectric Ceramics). It has low TC. Main applications: maintenance of old equipment. Commercially known as barium titanate.

Navy Type V (“Soft”) Suitable for applications requiring high energy and potential difference. Featured properties: d, K and g. Main applications: impact detonators. Commercially known as Piezoelectric Ceramic-5J.

Navy Type VI (“Soft”) Suitable for applications that require large mechanical deformations. Featured properties: d and K. Main applications: actuators and positioners. Commercially known as Piezoelectric Ceramic-5H.

The main properties of piezoelectric materials from the application point of view are [3,4,6]:.

Piezoelectric charge constant d.

Piezoelectric voltage constant g.

Coupling coefficient.

Mechanical quality factor Q.

Dielectric dissipation factor Tan δ.

Curie Temperature TC.

Frequency constants N.

Acoustic impedance Z.

The first serious applications of piezoelectrics (natural, of course, piezoelectric ceramics had not yet been developed) took place during World War I, where they were incorporated into submarines to serve as ultrasonic detectors (sonars). However, at that time, large companies did not give any importance to this success, although it is true that this first use that was given to these materials is still present today.

A few years later, materials testing methods based on the propagation of ultrasonic waves and new methods of transient pressure measurements were developed, useful for studying, for example, the internal combustion of an engine. In addition, the first microphones, record players, accelerometers, etc. began to be manufactured.

Already during the Second World War, the first piezoelectric ceramic material, barium titaniate and later the lead zirconate titaniate family (Piezoelectric Ceramics), was manufactured. Thanks to the development of these ceramics, sonar could be improved, they were also used to simplify electrical circuits, microphones were improved and the first ceramic tone transducer was created.

A very important application of piezoelectrics, more specifically quartz, is in precision electronic oscillators, devices that use the natural resonant frequency of the crystal excited by a voltage to make clocks, radios or internal computer clocks.

Their use as memory devices (called FRAM) is currently being investigated, whose advantages over conventional RAM and ROM memories are that they are non-volatile, low-consumption memories that do not require refresh processes and are easy to read/write. However, integration and speed problems currently do not allow this technology to compete with other types of memories (semiconductor technology).

Its use is extended to many more areas. Thanks to these ceramics there are smoke detectors, television controls, motion sensors, the classic “click” lighters, which generate a spark when pressed, and even today dozens of products based on piezoelectrics continue to be patented every year.[9]​.

The main applications of piezoelectric ceramics are:

The most common shapes and dimensions of commercial piezoelectric ceramics are [1]:.

Piezoelectric ceramics are also popularly known as piezoelectric crystals, piezo crystals, piezoelectric crystal and piezo crystal.