What is Piezoelectric Ceramic?

Piezoelectric ceramic (piezoelectric ceramic chip) is a polycrystalline material with piezoelectric effect, named after its similar production process to ceramics. It is a general term for ferroelectric ceramics with piezoelectric effects, which are made by mixing oxides (zirconia, lead oxide, titanium oxide, etc.), high-temperature sintering, and solid-state reaction to produce polycrystalline polarization treatment. Piezoelectric ceramics have excellent mechanical properties and stable piezoelectric properties. As an important functional material for force, heat, electricity, and light sensing, they have been widely used in electronic components such as amplitude rods, ultrasonic transducers, and micro displacement devices.

 

The Development History of Piezoelectric Ceramics:

In 1880, the Curie brothers first discovered the piezoelectric effect of tourmaline, marking the beginning of the history of piezoelectricity.

In 1881, the Curie brothers experimentally verified the inverse piezoelectric effect and gave the same positive and negative piezoelectric constants as quartz.

From 1842 to 1949, high dielectric constant, ferroelectricity, and piezoelectricity were discovered on BaTiO3 piezoelectric ceramics. The polarization issue was subsequently resolved.

In the 1950s, the United States and Japan conducted research on the use of BaTiO3 piezoelectric ceramics to produce ultrasonic transducers, high-frequency transducers, pressure sensors, filters, and other applications.

In 1954, the United States B Jaffe et al. discovered that lead zirconate titanate (PZT) has very strong and stable piezoelectricity, which greatly advances the application research of piezoelectric devices.

Later, in order to protect the Earth and human living space, prevent environmental pollution, non lead piezoelectric ceramics became the direction of future research and application.

So far, the application of piezoelectric ceramics has been extremely extensive, from space development to household life.

Basic concepts of piezoelectric ceramics:

Spontaneous polarization
Below 120 ℃, the BaTiO3 crystal structure is slightly distorted and exhibits a tetragonal structure. Ba2+Ti4+undergoes a displacement relative to O2-, resulting in the misalignment of positive and negative charge centers and polarization (spontaneous polarization). This transition temperature is commonly referred to as the Curie temperature or Curie point (Tc).

Artificial polarization
Artificial polarization is the process of applying a sufficiently high direct current electric field to a piezoelectric ceramic and maintaining it at a certain temperature and time, forcing its electric domains to turn or, in other words, forcing its spontaneous polarization to make directional arrangements. The following diagram illustrates the changes in electric domains in ceramics before polarization treatment.

Ferroelectric ceramics
Some materials exhibit spontaneous polarization within a certain temperature range. Moreover, its spontaneous polarization can be reversed by the action of an external electric field, and this property of the material is called ferroelectricity. Ceramic materials with this characteristic are called ferroelectric ceramics.

The principle of piezoelectric ceramics: piezoelectric effect

The piezoelectric effect refers to the deformation of certain media under the action of force, causing the surface of the media to become charged, which is the positive piezoelectric effect. On the contrary, when an excitation electric field is applied, the medium will undergo mechanical deformation, known as the inverse piezoelectric effect. The essence of the positive piezoelectric effect is the polarization of the medium caused by mechanical action; The essence of the inverse piezoelectric effect is the polarization of the medium caused by the action of an electric field.

Manufacturing of piezoelectric ceramics
The main steps in the production process of piezoelectric ceramics are: batching pretreatment pre firing granulation forming firing machining electrode polarization aging testing.

 

raw material
Raw materials are the foundation for preparing piezoelectric ceramics. For PZT, its main raw materials are Pb3O4, ZrO2, and TiO2. When selecting raw materials, attention should generally be paid to their chemical composition and physical state. The purity requirements for raw materials should be moderate. High purity raw materials are expensive, and the sintering temperature is high with a narrow temperature range. Impurities in raw materials with slightly lower purity can act as mineralizers and melting aids, but instead lower the sintering temperature and widen the temperature range. However, raw materials with excessively low purity contain more impurities and are not suitable for use.

 

Impurity

Impurities are divided into harmful impurities and beneficial impurities. PZT doping modification can be divided into equivalent substitution and heterovalent substitution; Heterovalent substitution can be divided into soft substitution modification, hard substitution modification, and other substitution modifications.

Equivalent substitution
Equivalent substitution refers to the substitution of Pb2+ions with divalent ions such as Ca2+, Sr2+, Mg2+, etc., which have smaller radii than Pb2+ions. As a result, the dielectric constant ε of PZT ceramics increases ↑, the electromechanical coupling coefficient KP increases ↑, and the piezoelectric constant d increases, thereby improving the piezoelectric performance of PZT ceramics.

 

Heterovalent substitution
Soft substitution modification in heterovalent substitution refers to the addition of some additives to the raw material that can reduce the coercive field strength Ec by ↓, making polarization easier. Therefore, under the action of electric field or stress, the material properties become "soft". (The porcelain body after firing turns yellow)

The hard substitution in heterovalent substitution refers to the addition of some additives that can increase the coercive field strength Ec and make polarization more difficult, resulting in the material properties becoming "hard" under the action of electric field or stress. After firing, the porcelain body appears black

 

The market of piezoelectric ceramics

Piezoelectric ceramics, as important functional materials, occupy a considerable proportion in the field of electronics. In 2000, the global sales of piezoelectric ceramics reached approximately 3 billion US dollars. In recent years, the annual sales of piezoelectric ceramics worldwide have grown at a rate of 15%. In order to protect the Earth from environmental pollution, the European Parliament passed a law in 2001 on the restriction of harmful substances in electrical and electronic equipment, which includes piezoelectric devices containing lead in the restricted substances. At present, products in the domestic piezoelectric ceramic market still operate using traditional technology and production methods, and lead-free products have not yet formed an industrial advantage. Lead free is the main direction of efforts in the industry.

 

Application of piezoelectric ceramics

The application of piezoelectric ceramics is very extensive. For example, piezoelectric detonators, ultrasonic detectors, piezoelectric drivers, piezoelectric igniters, piezoelectric converters, piezoelectric ceramic fans, piezoelectric touch screens, ultrasonic reversing radars, piezoelectric ceramic relays, and so on all use piezoelectric ceramics. Our company generally uses PZT-4 and PZT-8 piezoelectric ceramics because these two types have high electromechanical conversion efficiency and are suitable for high-power ultrasonic generator ultrasonic machining and ultrasonic cleaning of transducer components.