smart functions, such as ferroelectric, piezoelectric, pyroelectric, ferromagnetic, electromechanical, etc. One book cannot cover all of them, but those belong to ferroic materials and devices especially in the form of thin films will be extensively introduced in this book. Before going into details, some application examples of ferroic materials in smart systems are given in this chapter.
Figure 1.5 Diagram showing coupling between different moduli and the clarification of smart materials.
1.2 Device Application of Ferroelectric Materials
When people talk about applications of ferroelectric materials, the first thing jumps out is most possibly the PZT (lead–zirconium–titanate with chemical formula Pb(Zrx Ti1−x)O3), which is known as an excellent piezoelectric material. As the most popular ferroelectric, PZT is also the most important piezoelectric material in commercial applications. Piezoelectric materials have very broad applications in many fields, from medical ultrasound imaging to ultrasonic wire bonding machine in semiconductor industry, from pressure sensors to accelerometer, etc. The market size of piezoelectric materials is more than US $1 billion now and is expected to be US $1.68 billion by 2025 (GRAND VIEW RESEARCH).
Another field of application of ferroelectric materials is the infrared sensors based on their pyroelectric property, which is also one of the most important properties of a ferroelectric material. Beyond these well‐known applications, another important application based on the switching of ferroelectric polarization is the non‐volatile memory device such as FeRAM. Examples are given in the following and details will be introduced in the following chapters.
1.2.1 Piezoelectric Device Applications
An example of smart system using piezoelectric material is the distance radar system in a car or a sonar system in submarines as shown in Figure 1.6, where the key sensing element is based on piezoelectric material to realize the conversion between electrical energy and acoustic energy for sending and receiving sound waves. Other application examples of piezoelectric devices include active damping system, micro‐scanning system in scanning probe imaging instrument (such as AFM), force sensor, accelerometer, energy harvesting, etc.
Figure 1.6 Piezoelectric materials‐based sonar system for car (a) and submarine (b).
Medical ultrasound imaging system with piezoelectric material as the transducer to convert electrical and acoustic energies is another very good example of device application where the piezoelectric material plays the roles of sensing and actuating functions. Figure 1.7 shows photos of ultrasound transducers developed in our group. Knowledge in ultrasound transducer fabrication, characterization, and applications will be intensively introduced in Chapter 6.
Figure 1.7 (a) Transducers and (b) B‐mode image of a wire phantom acquired with PolyU‐made array ultrasound transducer.
A very new application example is piezoelectric‐based fingerprint ID system in mobile phone. The currently used finger identification system is based on capacitance measurement to obtain two‐dimensional (2D) information of fingerprint, but it faces the problem of difficulty to identify the fingerprint when the finger is dirty or wet. Ultrasound fingerprint identification system based on piezoelectric ultrasonic transducer and imaging system can obtain a three‐dimensional image of fingerprint with a certain depth. This can overcome the problems of the current fingerprint identification system in most mobile phones. InvenSense, Inc. is one of the main suppliers of this solution, and Figure 1.8 is an illustration of the ultrasonic fingerprint system.
Figure 1.8 Illustration of concept of a ultrasonic transducer‐based fingerprint ID system based on complementary metal‐oxide‐semiconductor micro‐electro‐mechanical systems (CMOS‐MEMS) technology.
1.2.2 Infrared Sensor
An infrared sensor is usually made of a ferroelectric material, which is also pyroelectric that generates surface electric charges when exposed to heat in the form of infrared radiation. A pyroelectric‐based infrared sensor can detect the temperature change but produce no response for a steady temperature since the pyroelectric sensing element can only produce polarization change‐induced electric charge when the sensor is subject to temperature change. Figure 1.9a shows a photo of a real infrared detector with its internal device structure illustrated in Figure 1.9b, where the active element is made of pyroelectric materials such as LiTaO3. Those pyroelectric materials with their polarization able to be switched are ferroelectrics. Therefore, pyroelectric sensors that are widely used as infrared detectors are important device applications for ferroic materials in a smart system.
Figure 1.9 A photo of an infrared detector (a) and illustration of its internal structure (b).
1.2.3 Ferroelectric RAM (FeRAM)
Ferroelectric RAM (FeRAM, F‐RAM, or FRAM) is a random‐access memory that is similar to Dynamic Random Access Memory (DRAM) in structure but uses a ferroelectric layer instead of a dielectric layer to achieve non‐volatility. FeRAM is one of a growing member of alternative non‐volatile random‐access memory technologies that offers the same functionality as flash memory.
Advantages of FeRAM over flash memory include lower power usage, faster write performance, and much greater maximum read/write endurance (about 1010–1014 cycles). FeRAMs have data retention of more than 10 years at +85 °C (up to many decades at lower temperatures). Market disadvantages of FeRAM are much lower storage densities than flash devices and higher cost.
A ferroelectric material has a nonlinear relationship between the applied electric field and the stored charge. Specifically, the ferroelectric characteristic has the form of a hysteresis loop, which is very similar in shape to the hysteresis loop of ferromagnetic materials. The dielectric constant of a ferroelectric is typically much higher than that of a linear dielectric because of the effects of electric dipoles formed in the crystal structure of the ferroelectric material. When an external electric field is applied across a dielectric, the dipoles tend to align themselves with the field direction. This alignment process is produced by small shifts in the positions of ions and shifts in the distributions of electric charges in the crystal structure. After the charges are removed, the dipoles retain their polarization state. Binary “0”s and “1”s are stored as one of the two possible electric polarizations in each data storage cell. For example, in Figure 1.10,
