What is Lead-free piezoelectric ceramics ?

1、What is lead-free piezoelectric ceramics?

Traditional piezoelectric ceramics have many outstanding performance advantages, but the main component of lead oxide PbO (with a content of more than 60-70%) is an easily volatile toxic substance. The sintering temperature is generally above 1200℃, which often causes the chemical stoichiometry of lead zirconate titanate PZT ceramics to deviate, making it difficult to control their properties. This is easy to cause pollution to the environment and can seriously harm the health of humans and other organisms.

Therefore, researchers have developed lead-free piezoelectric ceramics, which mainly include barium titanate-based ceramics, bismuth sodium titanate-based ceramics, and potassium sodium niobate-based ceramics.

However, no material has been found so far that can match the properties of lead-based PZT ceramics (such as high piezoelectric coefficient under low driving electric field, excellent temperature stability and fatigue resistance, and low hysteresis).

2、The necessity of developing lead-free piezoelectric ceramics

Since the early 1960s, the most important material for ultrasonic transducers has been piezoelectric ceramics based on the PZT system. However, due to the well-known toxicity of lead, legislative efforts have been made to reduce the use of lead in electronic products, at least since the first European “RoHS” directive came into effect in 2002. Several countries outside of Europe have also implemented similar legislation, including China, Japan, and South Korea. The RoHS exemption for PZT in Europe will be revised in 2022.

Currently, the deadline seems to be extended for another 5 years, but it may take a long time for lead-free devices to be launched, as current lead-free alternatives have different properties from PZT. This means that the next 5 years should be used to test and evaluate lead-free materials available in practical equipment and applications.

Traditional piezoelectric ceramics have many outstanding performance advantages, but the main component of lead oxide PbO (with a content of more than 60-70%) is an easily volatile toxic substance. The sintering temperature is generally above 1200℃, which often causes the chemical stoichiometry of lead zirconate titanate PZT ceramics to deviate, making it difficult to control their properties. This is easy to cause pollution to the environment and can seriously harm the health of humans and other organisms.

The purpose of developing lead-free ceramics is to obtain piezoelectric ceramics that are both satisfactory in performance and environmentally friendly. The material system itself should not contain substances that may cause damage to the ecological environment, and should not produce substances that may be harmful to the environment during preparation, use, and disposal.

3、Main Systems of Lead-Free Piezoelectric Ceramics:

The lead-free piezoelectric ceramic system mainly includes three types of materials: tungsten bronze structure, bismuth layer structure, and perovskite structure.

Currently, the perovskite structure is the most widely studied in lead-free piezoelectric ceramics. This type of lead-free piezoelectric ceramics includes barium titanate-based ceramics, bismuth sodium titanate-based ceramics, and potassium sodium niobate ceramics. Let’s take a closer look at them:

?

a、Barium titanate-based lead-free piezoelectric ceramics

BaTiO3 (referred to as BTO) is the earliest discovered piezoelectric ceramic and is a well-studied lead-free piezoelectric ceramic material. Barium titanate ceramics have four crystal phases, which are cubic phase, tetragonal phase, orthorhombic phase, and rhombohedral phase from high temperature to low temperature, respectively. The corresponding phase transition temperatures are 120℃, 5℃, and -80℃. Although barium titanate ceramics were widely used in transducers, audio transducers, pressure sensors, filters, resonators, and other piezoelectric devices, its disadvantages are also obvious: low piezoelectric performance (d33=190pC/N), low Curie temperature (120℃), and poor temperature stability near room temperature due to phase transitions. Therefore, after PZT ceramics with better performance emerged, BaTiO3 ceramics are generally only used as dielectric materials. Of course, in recent years, some scholars have used BaTiO3 micropowder synthesized by hydrothermal method as raw material and prepared barium titanate ceramics with piezoelectric constants of 360, 460, and 788pC/N by microwave sintering, two-step sintering, and TGG technology.

?

b、Bismuth sodium titanate-based lead-free piezoelectric ceramics

(Na,Bi)TiO3 (referred to as NBT) ceramics have high Curie temperature (Tc=320℃), large residual polarization (Pr38μC/cm2), small dielectric constant, high frequency constant, and large thickness electromechanical coupling coefficient. It is considered one of the most potential lead-free piezoelectric material systems. However, the coercive field at room temperature is large (Ec73kV/cm), and the bismuth element is easy to volatilize, which causes the density and resistivity of ceramics to decrease. It makes it difficult to polarize bismuth sodium titanate ceramics and shows low piezoelectric performance (d33＜100pC/N), making it difficult to truly achieve practical applications.

?

C. Potassium sodium niobate-based lead-free piezoelectric ceramics

(K,Na)NbO3 (referred to as KNN) ceramics are a binary solid solution of ferroelectric body KNbO3 and antiferroelectric body NaNbO3. As early as 2004, scholars have used doping modification and template orientation growth method to increase the piezoelectric constant of ceramics in the same system to 426pC/N, which can be comparable to the lead-based ceramic PZT-4 in key indicators such as piezoelectric performance and Curie temperature. Currently, KNN-based lead-free piezoelectric ceramic materials have been applied in ultrasonic transducers, contact sensors, etc. However, KNN-based lead-free piezoelectric ceramics also have disadvantages such as difficult sintering, poor temperature stability, and low piezoelectric activity.

?

4、Preparation of lead-free piezoelectric ceramics

Grain orientation technology

Through process control, the originally randomly oriented ceramic grains are oriented in a specific way, resulting in performance similar to that of single crystals. Grain orientation technology, as a structural modification technique, has the advantage of not altering the Curie temperature of the ceramic, as compared to traditional doping modifications. Currently, the most researched techniques are heat treatment and template grain growth technology.

Spark plasma sintering (SPS) technology

①Introduction to SPS technology

SPS is a new type of material sintering technology. It is a pressure sintering method that utilizes on-off pulsed direct current to directly sinter materials. The main effect of on-off pulsed direct current is to produce discharge plasma, discharge shock pressure, Joule heating, and electric field diffusion.

②Application of SPS technology in the preparation of lead-free piezoelectric ceramics

For NKN series ceramics, conventional sintering methods easily result in losses of Na and K, making it difficult to obtain high-density materials. SPS technology has the advantages of fast heating rate, short sintering time, and lower sintering temperature compared to conventional techniques (920℃), thus it is conducive to controlling the microstructure of the sintered body, and can obtain materials with high density, uniform grain size, and good piezoelectric performance.

Sol-gel technology

The use of sol-gel method to prepare piezoelectric ceramics is becoming increasingly popular. By using this method, various components of the material can be homogeneously mixed at the atomic or molecular level, resulting in highly uniform and dense materials with high piezoelectric properties.

5、Problems faced by lead-free piezoelectric ceramics

In the fields of sensors, actuators, and ultrasonic devices, lead-free piezoelectric ceramics based on lead-free KNN, BNT, and BT have been partially applied due to the rapid improvement in their performance. However, for larger-scale applications, good overall performance and stability must be achieved. However, the development of lead-free piezoelectric materials is faced with a major obstacle, as most lead-free piezoelectric materials involve temperature-related phase boundaries or so-called “same-direction phase boundaries (MPB)”, which lead to inadequate piezoelectric performance. Therefore, to achieve high performance and stability, a true MPB still needs to be obtained; at the very least, the temperature stability of electrical performance should be improved. In terms of temperature stability, BFO and KNN-based ceramics are the most promising lead-free piezoelectric materials for practical applications, but their electrical parameter performance still needs to be improved. For “hard” lead-free piezoelectric materials, it is difficult to achieve high Qm and high d33. The corresponding physical mechanisms need to be elucidated to guide the development of “hard” lead-free piezoelectric materials. For energy storage, increasing density is very important. In addition to electrical parameters, the manufacturing processes of these lead-free piezoelectric materials (such as multi-layer and three-dimensional printing technology) and other characteristics (such as mechanical properties) should also be considered. Domain wall nanoelectronics represents a potential paradigm for the next generation of ferroelectric devices. Stricter environmental, health, and safety regulations aimed at limiting global lead usage are crucial.

6、In summary, lead-free piezoelectric ceramics are a promising alternative to traditional lead-based materials due to their environmental friendliness and potential for high performance. The three main types of lead-free piezoelectric ceramics are tungsten bronze, layered bismuth, and perovskite-based ceramics, with perovskite-based ceramics being the most extensively researched. The preparation of these ceramics involves various techniques such as grain orientation, spark plasma sintering, and sol-gel techniques. However, these materials still face challenges such as the lack of a true morphotropic phase boundary and poor temperature stability. Further research is needed to improve the overall performance and stability of these materials for practical applications in sensors, actuators, and other devices.

Welcome to contact us via [email protected] !