The growing demand for flexible electronics and wearable devices in today’s market booms an extensive study on flexible piezoelectric materials, especially piezoelectric ceramic thin films. Many studies show that piezoelectric ceramic thin films epitaxially grown on single crystalline rigid substrates, such as silicon, magnesium oxide, and sapphire, have a controllable crystal structure and good piezoelectric properties. However, piezoelectric materials grown on rigid and hard substrates are difficult to take advantage of the unique flexibility of the thinned ceramics, hinders the application in energy harvesting and sensing on flexible devices. Notably that with the sophistication of large apparatus and microdevices, the conventional experience of preparing thin films on small and flat wafers cannot fully fit the requirements of practical thin film materials fabrication on complex object surfaces. Furthermore, in the process of transferring the material from the growth substrate for obtaining freestanding thin film, the strong acid and alkali solution used in chemical etching can easily destroy the ceramic structure, while the low efficiency of physical etching significantly increases the processing cost, making it difficult to mass-produce the piezoceramic thin-film devices.

To address the aforementioned challenges, in this thesis, I prepare piezoceramic thin films via a modified sol-gel technology on flexible and flat substrates, 3D freeform substrates, and furthermore develop a simple method to obtain the freestanding piezoceramic films without substrates. The materials properties and piezoelectric performances of thin films derived via different routes are studied through systematic experiments. The derived piezoceramic thin films with different substrate conditions can be used in diverse applications, such as human motion monitoring, energy harvesting, and structural health monitoring. Based on the research contents, this thesis is divided into the following five chapters.

First, this thesis reviews the state of the art of flexible piezoelectric materials, including the properties and applications of 1D nanowires, 2D structural thin films, and 3D composites. It mainly introduces the fabrication process of thin-film materials, the selection of substrate materials, and the current status of the transfer technology for piezoceramic films.

Second, to address the challenge that traditional rigid substrate cannot be directly used in flexible applications, the translucent PZT thin films are successfully prepared on planar mica substrates via a modified sol-gel process. The traditional solution-based method usually requires at least 20 coating cycles to fabricate 2 μm thick PZT thin films. To save cost and improve fabrication efficiency, a simplified thin film fabrication method assisted by PZT powder is developed. The new method can fabricate 2 μm thick PZT films in a single step, i.e., one spin coating and annealing. The measured piezoelectric modulus d₃₃ of solution-based and powder-based PZT thin film is 130 pm/V and 47 pm/V, respectively. Furthermore, I fabricate lead-free piezoelectric material, Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT), that shows high biocompatibility and competitive piezoelectric performances. The d₃₃ of the BCZT thin films grown on mica evaluated from our established system reaches 150 pC/N, which is comparable to films grown on single-crystal substrates. The BCZT-based PENG can charge capacitors under mechanical excitation, indicating broad application prospects in the field of flexible self-powered electronics.

Third, a fast, energy-efficient, and cost-effective approach, named flame treated spray (FTS) coating, is developed to solve the issue of preparing piezoceramic films on 3D freeform surfaces. The flame treatment significantly enhances the hydrophilicity of a substrate, assisting in forming a uniform and continuous thin film. The followed spray coating deposits hundreds of nanometers to several micrometers -thick films on 3D freeform surfaces. Given the size controllability and arbitrary surface compatibility of the FTS method, we assembled a highly conformal piezoelectric tactile sensor array ("4×4") on a spherical surface for mimicking robot fingers and an on-site thin-film sensor on the wing of an aircraft model to monitor the vibration in real-time during flight. The FTS film deposition offers a highly promising methodology for the application of functional thin films from micro- to macro-scale devices, regardless of conformal problems.

Finally, freestanding piezoceramic thin films are prepared in a simple and environmentally friendly route facilitated by water capillary forces. Piezoceramic thin films that offer unique features on energy conversion are usually not considered alternatives to piezoelectric polymers in soft applications due to their inherently brittle nature. The emerging transfer process has brought the dawn to applying ultra-thin ceramic films in the flexible electronics field. To date, however, no matter for physical laser lift-off or chemical etching, removing the film from the growth substrate is always accompanied by enormous process difficulty and high cost. Therefore, there is an urgent need for a universal method to transfer ceramic thin films easily and non-destructively. Here, we introduce a simple, green, and inexpensive method of peeling off large-area Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT) thin films with a giant piezoelectric modulus d₃₃=209±10 pm/V" from mica substrates by water, facilitated by the surface tension-driven capillary force. The freestanding ceramic thin film remains highly integrity, serving as a sticker that can be conformally transferred to any freeform substrates. The BCZT film show outstanding biocompatibility that is comparable to commonly in-vivo used polydimethylsiloxane (PDMS) membranes. The film sticker can then be used to fabricate self-powered flexible devices.

In summary, I introduce modified sol-gel processes to overcome the limitation of substrates and prepare high-quality piezoelectric ceramic films on arbitrary materials or without substrates for meeting different application demands. Among them, films on flexible, transparent, and planar mica substrates are developed as energy harvesters. Films on 3D topology substrates show their potential in the field of structural health monitoring and tactile sensing for robotics. The biocompatible freestanding thin films serving like stickers allow implantable devices. I believe that this study will promote the development of piezoelectric films in flexible electronics and medical devices.