Last Updated on May 16, 2023 by You Ling
1、What is high-temperature piezoelectric ceramics?
High-temperature piezoelectric ceramics are piezoelectric ceramic materials with high Curie temperature, high piezoelectric coefficient, high resistivity, low dielectric loss, and the ability to operate stably at higher temperatures. As the core sensitive element of high-temperature piezoelectric vibration sensors, high-temperature piezoelectric ceramic materials are widely used in fields such as aviation, aerospace, nuclear energy, metallurgy, petrochemicals, and geological exploration.
2、What are the challenges of developing high-temperature piezoelectric ceramics?
PZT-based piezoelectric ceramics have a low Curie temperature (300-400℃) but a large piezoelectric coefficient (300-700 pC/N), mainly used for sensor series below 200℃; bismuth-layered structure piezoelectric ceramic materials have a high Curie temperature (500-940℃) but a lower piezoelectric coefficient (about 20 pC/N) and can be used for sensor series above 400℃. For higher temperature environments, piezoelectric crystals, which do not undergo high-temperature phase transitions, exhibit high resistivity and a wide temperature range of use, and are commonly used in ultra-high-temperature piezoelectric vibration sensors at or above 650℃, but their production and processing costs are relatively high, and their piezoelectric coefficients are also low (<10 pC/N). Overall, bismuth-layered structure piezoelectric ceramic materials are the preferred material for the core sensitive elements of high-temperature piezoelectric vibration sensors and the universal material for 482℃ high-temperature piezoelectric vibration sensors and a candidate material for 650℃ ultra-high-temperature piezoelectric vibration sensors, with great potential for research and application. Bi4Ti3O12, CaBi4Ti4O15, Bi3TiNbO9, CaBi2Nb2O9, SrBi4Ti4O15, SrBi2Nb2O9, etc. are all bismuth-layered structure piezoelectric materials that can be used for sensors. However, the development and production of bismuth-layered structure piezoelectric ceramic materials also face some technical challenges:
a、The coercive field strength is too high, which is not conducive to polarization, and batch polarization technology is difficult.
b、The piezoelectric activity is low, and the piezoelectric coefficient is small, resulting in a narrow frequency response range of the sensor.
c、The temperature stability of the piezoelectric response is poor, resulting in a significant decrease in the long-term maximum operating temperature of the sensor. The modification and improvement of bismuth-layered structure piezoelectric ceramic materials are the main direction of current research on piezoelectric ceramic materials.
3、Types of High-Temperature Piezoelectric Ceramics
High-temperature piezoelectric ceramics can be divided into four types based on their crystal structures: perovskite, layer-structured bismuth, tungsten bronze, and alkali metal niobate.
a、Perovskite-type High-Temperature Piezoelectric Ceramics
Perovskite structure materials have become a hot topic in the field of material science, and they belong to a type of composite metal oxide with a cubic crystal structure. As early as 1942, the special perovskite structure crystal, barium titanate (BaTiO3), was discovered by countries such as the United States and Japan. It is known as the pillar of the electronic ceramics industry and is widely used in fields such as multilayer ceramic capacitors, thermistors, and electro-optical devices. In addition, barium titanate ceramics have a low Curie temperature of only 120℃, but their sintering temperature can reach 1300℃.
Lead titanate (PT) is a ferroelectric material with a high Curie temperature and perovskite structure. During the cooling process of pure lead titanate grains, the crystal structure changes from cubic to tetragonal. However, the internal stress generated during the transformation process can easily cause ceramic fracture, making it difficult to polarize. Therefore, lead titanate is difficult to sinter in preparation. To solve this problem, a suitable amount of modified additives must be added to obtain high-performance piezoelectric ceramic materials.
Currently, the most widely studied perovskite-type high-temperature piezoelectric ceramics, in addition to modified PbTiO3 ceramics, is lead zirconate titanate (PZT), which is a functional material with great development prospects. It is usually used to make various electronic components, and has the advantages of simple preparation process, easy availability of raw materials, and low price. However, its Curie temperature is only 386℃, which limits its application in high-temperature fields. Therefore, a new type of BSPT ceramics has been developed.
Common high-temperature piezoelectric ceramics such as PZT and BSPT contain lead, which is harmful to humans and the environment. Lead-free piezoelectric ceramic materials generally have lower piezoelectric performance and cannot completely replace PZT ceramics. With the requirement of sustainable development in human society, lead-free piezoelectric ceramics will become the ultimate development direction.
b、Layer-Structured Bismuth Piezoelectric Ceramics
Layer-structured bismuth piezoelectric ceramics are a highly promising lead-free high-temperature piezoelectric ceramic material. In recent years, research on improving the piezoelectric properties of layer-structured bismuth ceramic has become a hot topic. Generally, two methods are used: process modification and doping substitution. Compared with PZT ceramics, layer-structured bismuth piezoelectric ceramics are often made into piezoelectric devices and used in filters, energy conversion, high-temperature and high-frequency fields.
c、Tungsten Bronze-structure Piezoelectric Ceramics
Tungsten bronze-structure piezoelectric ceramics have high Curie temperature, low dielectric constant, and large anisotropy, making them a very promising electro-optical crystal material. Lead magnesium niobate (PbNb2O6) was the earliest discovered tungsten bronze-type ferroelectric material, which has a tetragonal tungsten bronze structure, a high Curie temperature (TC=570℃), low quality factor Qm, and is not easy to depolarize when close to the Curie point. It has a large d33/d31 value, and the longitudinal electromechanical coupling coefficient is much larger than the transverse and planar electromechanical coupling coefficients, making it particularly suitable for preparing high-temperature transducers.
d、 Alkali metal niobate piezoelectric ceramics
In the late 1940s, American scientists first synthesized alkali metal niobate compounds (ANbO3), which have excellent electrical properties and can be used to prepare high-frequency electronic components. They are considered as candidates to replace lead-based materials. Lithium niobate (LiNbO3) is the most representative and widely used among them, with a Curie temperature of about 1210 ℃ and good piezoelectric, ferroelectric, optoelectric, thermoelectric, and nonlinear optical properties. It can be used to make transducers for high-temperature piezoelectric filters, etc. However, the Li element in lithium niobate piezoelectric ceramics is prone to volatilization during sintering, which makes it difficult to prepare and has a narrow sintering temperature range.
Later, lead-free piezoelectric ceramics (K, Na) NbO5 (referred to as KNN) were discovered. This type of piezoelectric ceramics has a lower dielectric constant, higher Curie temperature, and quality factor, but its piezoelectric and electromechanical coupling performance is average. It can be used in optoelectronic materials and high-frequency transducers.
4、Applications of high-temperature piezoelectric ceramics
High-temperature piezoelectric ceramic materials have very important applications in fields such as new energy, nuclear energy, petrochemicals, geological exploration, aerospace, automobile industry, and national defense.
Aerospace: High-temperature piezoelectric sensors are used for real-time monitoring of temperature, vibration acceleration, and blade fatigue of aircraft engines and wings.
Automobile industry: High-temperature piezoelectric valves with high Curie temperature are usually used in internal combustion engines.
Oil exploration: High-temperature piezoelectric sensors are often needed to collect temperature, density, pressure, and chemical composition data.
National defense: High-temperature and high-pressure piezoelectric actuators and sensors are urgently needed for supersonic aircraft.
Currently, Endevco, PCB, Vibro-Meter, and B&K companies in the United States, Switzerland, and Denmark can produce mature high-temperature piezoelectric vibration sensors. Especially, Endevco’s high-temperature piezoelectric vibration sensor for aircraft engines has become the standard for aircraft engine vibration monitoring worldwide.
Most of the ceramic materials used for producing piezoelectric vibration sensors are modified bismuth-layered structure piezoelectric ceramics, which have extremely high research and development potential. Some scientific research units in China have conducted systematic research on high-temperature piezoelectric vibration sensors and achieved small-scale production. However, there is still a certain gap compared with advanced piezoelectric ceramic materials abroad, especially in terms of long-term temperature stability and airborne environmental adaptability. But Chinese company (he-shuai)’s high-temperature piezoelectric ceramics have better cost performance.
5、This article introduces the types and characteristics of high-temperature piezoelectric ceramics. High-temperature piezoelectric ceramics can be classified into perovskite-type, layered bismuth-type, tungsten bronze-type, and alkali metal niobate-type. Perovskite-type ceramics have a wide range of applications and research value, but contain lead elements that are harmful to humans and the environment, thus making lead-free piezoelectric ceramics the development direction. Layered bismuth-type, tungsten bronze-type, and alkali metal niobate-type ceramics are candidate materials for lead-free high-temperature piezoelectric ceramics.