Sol-gel synthesis of highly oriented lead barium titanate and lanthanum nickelate thin films for high strain sensor and actuator applications

Piezoelectric materials, capable of 0.1% strains, have been extensively used in sensor and actuator applications. Ferroelectric materials, a subset of the piezoelectric class, are capable of strains an order of magnitude larger. For a ferroelectric material with tetragonal crystal structure, large strains can be achieved through 90° domain switching between a and c domains. Bulk barium titanate has been shown to produce strains of 0.9% through such domain switching under combined electromechanical loading. Lead titanate has a larger c/a ratio and would be expected to produce 6% strains, though it is prone to brittle fracture. By examining the solid solution lead barium titanate, larger strains can be achieved while maintaining mechanical integrity. The work presented here covers the development of multiple sol-gel processes for producing powder and highly oriented thin film lead barium titanate, and a detailed discussion of their parametric optimization towards low temperature crystallization. Finally, results of early efforts toward integrating these films into useful structures and devices are discussed, including sol-gel synthesis of highly oriented conductive oxide electrodes. Thin film barium lead oxide and lanthanum nickelate electrodes were produced using sol-gel processing. (100)-oriented lanthanum nickelate electrodes were produced on a wide variety of amorphous and crystalline substrates, and subsequently deposited PBT showed excellent (100/001)-orientation regardless of substrate. The ability to produce highly oriented ferroelectric films on oxide electrodes deposited directly on Si promises to improve fatigue characteristics and greatly facilitate efforts to integrate ferroelectric thin films into MEMS process.