**4. Applications of low-temperature-processed PZT films**

Thanks to multifunctional properties, PZT films can be utilized for various applications in sensor, actuator, nonvolatile memory, and energy harvesting as well. Once PZT films can be deposited at a low temperature, which is compatible with Si technology, direct integration of PZT films to other active control element, i.e., Si-CMOS, becomes possible. Further lowering of PZT process temperature to be compatible with flexible substrates enables flexible PZTbased electronics. This part briefly summarizes emerging applications of low-temperature PZT films.

## **4.1. Piezoelectric actuator**

a template with an antistick layer. The template with PZT was then placed onto the substrate and applied a pressure. After that, the substrate was separated from template directly. The transferred sample was then crystallized by thermal annealing in air at 650°C for 15 min. The patterned lines appeared to be reasonably straight and well ordered with few defects. The height of the gratings was in the range between 125 and 250 nm depending on the template mold.

**Figure 15.** PZT films nanoembossing process and results of embossed profiles. (a) A schematic diagram illustrating a one-step embossing process to form a stagger shape in a PZT film with two different thicknesses and (b) AFM images of

embossed stagger like profiles of PZT film [67].

104 Ferroelectrics and Their Applications

Shen et al applied a nanoembossing technique to form a stagger structure in PZT film [67]. **Figure 15(a)** illustrates the nanoembossing process of PZT film. **Figure 15(b)** displays the embossed PZT film profiles measured by AFM; showing an embossed depth of about 160 on a 450-nm thick PZT film with well-quadrate-patterned profiles. After crystallization, the PZT films exhibited a tetragonal structure with (111) preferred orientation. In addition, the morphology of the embossed region remained stable. Chen group demonstrated a low-press, low-temperature direct NIL method for patterning PZT films [68, 69]. In general, conventional direct NIL utilizes ultrahigh pressure or temperature to form patterns on the film. The A diaphragm-type piezo actuator was demonstrated using a low-temperature (450°C) solutionprocessed PZT film [15]. Cross-sectional structure and SEM image of the fabricated actuator is given in **Figure 16(a)** and **(b)**. The observed experimental data agreed well with simulation result in which the actuator displacement linearly increased as the increase of applied voltage (**Figure 16(c)**). The maximum displacement was approximately 130 nm at 10 V. These results verify that the low-temperature-processed PZT film can be applied for actuator applications.

**Figure 16.** (a) Cross-sectional structure and (b) SEM image of the fabricated PZT actuator. (c) Comparison of the displacement between experiment and simulation results [15].

### **4.2. Piezoelectric energy harvester**

Energy harvesting, which collects useful energy from wasted energy sources, is the most promising technology to provide solutions for the shortage of a fossil fuel, various environmental problems, and the improvement of energy efficiency in smart grids. Piezoelectric energy harvesters have been actively studied due to their easy integration with MEMS and integrated circuit technologies [70–74]. The cantilever type is the most typical structure of a piezoelectric energy harvester, in which a piezoelectric material is deposited onto a rigid Si cantilever, and a proof mass is located at the free end of the cantilever. The electricity can be generated by a conversion of kinetic energy from the mechanically stimulated proof mass via the piezoelectric material. However, the typical rigid-body-based cantilever type energy harvester has a large resonance frequency due to its high spring constant. Thus, it is not for harvesting energy from human activity or low-frequency ambient vibration. In this regard, flexible PZT appears as a promising candidate for flexible energy harvesting applications.

device was able to generate output voltage and power of 1.2 V and 117 nW, respectively. Through the introduction of indium-tin-oxide (ITO) and polyethylene terephthalate (PET) substrates to laser lift-off PZT-based energy harvester, a transparent flexible device (TFD) was implemented [71]. The TFDs based on PZT films generated an AC-type output signal and

Recently, single-cell hybrid nanogenerators (NGs) that can simultaneously harvest mechanical and thermal energies have received great attention [72–74]. Jung group reported the development of a hybrid piezoelectric-pyroelectric NG using PZT material to simultaneously harvest mechanical and thermal energies from extreme resources [74]. By the combination of

stably generate electric power in harsh environment, and at elevated temperatures (**Figure 17**). The success of flexible piezoelectric energy harvesters lies in its packaging density, output voltage and power, resonance bandwidth, lifetime, and cost. Among these, the two biggest challenges are wider bandwidth and higher power density. Advances in novel piezoelectric materials such as giant coefficient and lead-free piezoelectric as well as harvester structural

This review on low-temperature processing of solution-derived PZT films summarized major approaches to decrease the crystallization temperature below 450°C. The success would mitigate the integration of PZT films in electronics devices and diminish a possible loss of stoichiometry and consequent worsened functional properties due to either evaporation of volatile species (lead-, alkali-oxides) or possible interface reactions. The approaches described here include chemical and physical treatments for the precursor solutions, as-deposited, and as-pyrolyzed films by solvothermal synthesis, UV-light treatment, laser, microwave, and flash-lamp-assisted annealing. It reveals that design of functional precursor solutions, which are photo-sensitive and possibly decomposable at a low temperature, is critical to the produc-

Combination of low-temperature solution-processed PZT films with facile micro-/nanopatterning techniques would open new opportunities for low-cost, large-area transparent, flexible ferroelectric/piezoelectric devices such as nonvolatile memory, piezoelectric sensor/

The authors would like to thank JST, ERATO Shimoda Nano-Liquid Process project and JST-CREST project for financial support. Members of the Center for Single Nanoscale Innovative Devices, Japan Advanced Institute of Science and Technology are acknowledged for their

design are expected to bring us closer to battery-free autonomous systems.

, at periodically bending and releasing motion.

Lead Zirconium Titanate Films and Devices Made by a Low-Temperature Solution-Based Process

and Ni-Cr metal foil as a bottom electrode and a flexible substrate, they

(28 μC/cm2

), high piezoelectric constant

K). The PZT-based NG was proven to

http://dx.doi.org/10.5772/intechopen.79378

107

output power of 8.4 nW/cm2

have successfully grown a PZT film with a large *Pr*

(140 pC/N), and high pyroelectric coefficient (50 nC/cm<sup>2</sup>

perovskite LaNiO<sup>3</sup>

**5. Conclusion**

tion of high-quality PZT films.

**Acknowledgements**

technical assistance.

actuator/transducer, and energy harvesters.

Cho et al. presented a microfabricated flexible and curled PZT cantilever using d33 piezoelectric mode for vibration-based energy harvesting applications [70]. The proposed energy harvester consists of PI layer, PZT thin film, and interdigitated IrO<sup>x</sup> electrodes. The PZT thin film on PI layer showed 2*Pr* and 2*Ec* of approximately 47.9 μC/cm2 and 78.8 kV/cm, respectively. At optimal conditions of resistive load (6.6 MΩ) and resonant frequency (97.8 Hz), the fabricated

**Figure 17.** Illustration of flexible PZT film-based NG [74].

device was able to generate output voltage and power of 1.2 V and 117 nW, respectively. Through the introduction of indium-tin-oxide (ITO) and polyethylene terephthalate (PET) substrates to laser lift-off PZT-based energy harvester, a transparent flexible device (TFD) was implemented [71]. The TFDs based on PZT films generated an AC-type output signal and output power of 8.4 nW/cm2 , at periodically bending and releasing motion.

Recently, single-cell hybrid nanogenerators (NGs) that can simultaneously harvest mechanical and thermal energies have received great attention [72–74]. Jung group reported the development of a hybrid piezoelectric-pyroelectric NG using PZT material to simultaneously harvest mechanical and thermal energies from extreme resources [74]. By the combination of perovskite LaNiO<sup>3</sup> and Ni-Cr metal foil as a bottom electrode and a flexible substrate, they have successfully grown a PZT film with a large *Pr* (28 μC/cm2 ), high piezoelectric constant (140 pC/N), and high pyroelectric coefficient (50 nC/cm<sup>2</sup> K). The PZT-based NG was proven to stably generate electric power in harsh environment, and at elevated temperatures (**Figure 17**).

The success of flexible piezoelectric energy harvesters lies in its packaging density, output voltage and power, resonance bandwidth, lifetime, and cost. Among these, the two biggest challenges are wider bandwidth and higher power density. Advances in novel piezoelectric materials such as giant coefficient and lead-free piezoelectric as well as harvester structural design are expected to bring us closer to battery-free autonomous systems.
