Three-dimensional Piezoceramics Fabrication for Energy Transducers


Student thesis: Doctoral Thesis

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Awarding Institution
  • Ji-jung KAI (Supervisor)
  • Zhengbao YANG (External person) (External Co-Supervisor)
  • C W LIM (Co-supervisor)
Award date18 Oct 2023


Bulk piezoceramics are important functional materials and widely used in electronics and energy devices, thanks to their high electromechanical coupling effect, stable mechanical properties, good thermal stability, and low cost. Normally, these piezoceramics with advanced properties are applied onto supporting bases to construct execute functions like sensing, actuation and energy conversion. Piezoceramic components are usually formed to planar shapes such as plates and discs due to their poor processibility that originates from the strong iono-covalent bonds between constitutive atoms. However, such monotonous planar shapes hinder the application and development of piezoceramic-based electronics as most real objects possess irregular non-plannar geometries. Despite huge enhancements in improving the intrinsic properties of piezoceramics, techniques towards shaping piezoceramics still moves strugglingly. Conventional high-precision machining is one promising method to process ceramics to complex shapes, but it requires costly machinery and generates a number of residual materials. As potential alternatives, msot existing ceramic shaping techniques, 3D printing, come at the cost of physical properties such as piezoelectricity and thermal stability. A general and economic friendly method that can facilely fabricate geometrically complex piezoceramics while maintaining their intrinsic material properties is challenging but highly desired.

To solve the aforementioned problems of fabricating high-performance piezoceramics in complex shapes, in this thesis, I prepare various complex shape piezoceramics via exploiting the thermal creep phenomenon of ceramic powder compacts during heating and sintering. The shape forming principle is studied and the merits of the ceramic shaping techniques proposed in this thesis are evaluated via systematic material property characterizations and comparisons with other processing methods. To demonstrate the advantages of the 3D shape piezoceramics, these as-fabricated piezoceramic are applied on irregular real objects to construct high-performance 3D electronics. This thesis is divided into four chapters on the purpose of helping readers clearly understand the proposed novel ceramic shaping strategy.

First, in the beginning of this thesis, the current piezoceramic shaping techniques, flexible piezoceramic-based structures (piezoceramic composites and array mainly), complex shape piezoceramics and their applications are introduced. Compared with other mentioned techniques (machining and 3D printing), the ceramic shaping strategy proposed in this thesis is highlighted as a scalable, low-cost and stable process that facilely fabricates geometrically complex piezoceramics possessing compact sintered bodies and holding intrinsic piezoelectric properties.

Second, a gravity-driven sintering (GDS) process is first proposed for shaping piezoceramics during sintering. Normally, compact and geometrically complex piezoceramics are required by a variety of electromechanical devices owing to their outstanding piezoelectricity, mechanical stability and extended application scenarios. The GDS process directly fabricates curved and compact piezoceramics by exploiting gravitational force and high-temperature viscous behavior of sintering ceramic specimens. As a demonstration of the GDS process, lead zirconate titanate (PZT) ceramics were used to study the feasibility and capacity of the proposed method. The sintered lead zirconate titanate (PZT) ceramics possess curve geometries that can be facilely tuned via the initial mechanical boundary design, and exhibit high piezoelectric properties comparable to those of conventional-sintered compact PZT (d33=595 pC/N). In contrast to 3D printing technology, the GDS process is suitable for scale-up production and low-cost production of piezoceramics with diverse curved surfaces. The proposed GDS strategy is an universal and facile route to fabricate curved piezoceramics and other functional ceramics with no compromise of their functionalities.

Third, although the performance of fabricating curved piezoceramics of the GDS process has been verified, it is remaining hard for those bulk piezoceramics to be shaped to adapt to complex 3D surfaces. In other words, only simple arch shape piezoceramic sheets cannot meet the demands of constructing advanced piezoceramic-based 3D electronics. Therefore, to fabricate 3D piezoceramic sheets, I develop a mold-assisted sintering (MAS) method that can replicate 3D surfaces with nonzero Gaussian curvatures while maintaining the intrinsic properties. Assisted with supporting molds, the MAS exploits elastic-to-viscous transition of piezoceramic powder compacts during sintering, bypassing the boundaries of deformation and surface development in an elastic continuum. I successfully replicate shapes to sintered lead zirconate titanate (PZT) ceramics from designed molds with saddle, sine and spherical surfaces, respectively. The PZT ceramic replicas show high piezoelectricity and excellent conformability towards nonplanar surfaces (maximum deviation 90 μm). I further demonstrate the advantage of the conformal curved piezoceramics by using them for wirelessly transferring power over a pipeline. The MAS method is low-cost, simple, scalable and applicable to various 3D electronics.

Finally, I present a general route that exploits the thermomechanical field engineering to endow piezoceramics with versatile three-dimensional (3D) shapes and surface architectures. Specifically, deformable carbon felts are used as Joule heating element and mechanical carrier that allows for directly tailoring and regulating the thermomechanical field. By fine-tuning the deformation rates and processing temperature, in minutes, I deform the ceramic powder compacts to 3D shapes and then sinter them to dense ceramics. As a proof of concept, I fabricate Barium titanate (BT) piezoceramics with twisted geometry and stamped patterns (100- to 600-um resolution). The twisted BT ceramics hold well dense bodies (relative density of around 96%) as well as high piezoelectric constants (d33 of 380 pC/N).

In summary, this research introduces the concept of forming 3D compact piezoceramics by thermomechanical field engineering. Following this concept, I start producing simply curved piezoceramics by GDS process; and then the upgraded the process (MAS method) for fabricating conformal piezoceramics to conform with various irregular bases has been proposed; finally, a more powerful and efficient ceramic shaping method (3D forming) has been developed that allows users to design and fabricate complex shape piezoceramics with a high level of freedom. I envision that the new design principle proposed in this research extends piezoceramic-based devices to a wide range of geometries and functions. And these low-cost, simple, and scalable strategies will attract the interest from academia and industry.