Energy Conversion via Ferroic Materials: Materials, Mechanisms, and Applications

Abstract

Energy conversion is a process converting one form of energy into another. Significant research effort has been dedicated to energy conversion mechanisms for portable energy conversion. Specifically, mechanisms based on ferroic materials have been widely explored for this goal.

Ferroic materials include ferromagnetic, ferroelectric and ferroelastic materials. This thesis is focused on two ferroic materials: ferromagnetic TbxDy1-xFe2 (x ~ 0.3, Terfenol-D), and ferroelectric barium strontium titanate (BST) including its paraelectric phase, for their energy conversion mechanisms. We grew and characterized these materials, followed by device fabrication to study potential energy conversion mechanisms in resulting devices.

With Terfenol-D, we demonstrated a wireless energy-conversion process via the Villari effect, i.e. magnetic flux change induced by mechanical input. A new technique of transfer-printing a Terfenol-D film onto a flexible substrate was developed to study this mechanism. The transferred Terfenol-D showed a high saturation magnetization (~ 1.3 T) and flexibility (strain ~ 1.9 %). Subsequently, the Villari effect was successfully utilized to convert mechanical energy, from a mechanical source and a simulated biomechanical source, into electricity.

For next projects, another ferroic material, a high-permittivity (dielectric constant ~ 200) BST was sputtered on Pt/SiO2/Si or stainless steels to form a metal-insulator(BST)-metal heterostructure. The BST was found to be paraelectric when grown upon Pt/SiO2/Si, whereas it was ferroelectric when grown on the stainless steel. Two different mechanisms were therefore studied on these two modifications.

In the paraelectric BST we found a new thermal-electric response via a flexoelectricity-mediated mechanism, which was enabled by a large strain gradient (> 104/m) produced by lattice mismatch. With the enhanced flexoelectricity from the large strain gradient, electrical output was generated under thermal cycling, showing a new approach for thermal-electrical conversion. A theoretical model was also developed to understand this mechanism.

We also observed a photovoltaic effect on the ferroelectric BST grown on stainless steel. The high-permittivity and the crystalline structure were preserved, with additional flexibility obtained (strain ~ 0.25%). The photovoltaic effect was verified and characterized with different illumination sources. A flexible imager was then demonstrated via the photovoltaic effect.

Finally, the outlook for further studies of these mechanisms is discussed.