Pham Thi Ngoc Mai

Ph.D. thesis

Summary

Ferroelectric materials are important for many industrial applications, ranging from high-dielectric constant capacitors to later developments in piezoelectric transducers, sensors, actuators and memories. To meet stringent requirements for a specific application their physical properties can be tailored, e.g., by varying the synthesis conditions, doping with a foreign element or adding a second phase. In this thesis the preparation and characterisation of dual-phase PZT-Pt composites, in the form of bulk and thin films, have been presented. The basic idea is to enhance the dielectric and ferroelectric properties of PZT by dispersion of Pt in the PZT matrix. The dispersion reduces the effective thickness phase of PZT, hence results in an increase of the effective dielectric constant and a decrease of coercivity. The formation of space charges at PZT/Pt interfaces also contributes to the enhancement of the dielectric constant and the total polarisation of material. The high dielectric constant of such composites is promising for application in super-capacitors or for DRAM devices, while the low coercivity suits low-voltage operation for applications like non-volatile memories.

Dual-phase PZT-Pt composites were prepared successfully by the sol-precipitation route, in which PZT powder is wet mixed with a sol containing Pt nano-particles. The dielectric constant at room temperature of the PZT-Pt composites reaches a 6-times enhancement at 28 vol.% of Pt relative to that observed for bulk ceramic PZT. The composites exhibit higher polarisations than pure PZT, which is attributed to the space charge polarisation formed at PZT/Pt interfaces. Thin films of PZT-Pt were fabricated by Pulsed Laser Deposition. The film microstructure can be controlled through changing the deposition conditions. The highest enhancement factor of the dielectric constant obtained for thin films equals to 8 at 10 vol.% Pt. Films with Pt contents below 10 vol.% exhibit good properties: high dielectric constant, low dielectric loss, high polarisation and low coercive field. For films with Pt contents higher than 10 vol.% both the dielectric and ferroelectric properties degraded. The change from micro to nano-size in domain structure of PZT in the presence of Pt particles is considered to be responsible for the degradation of ferrolectric properties. The dielectric and ferroelectric properties are dependent on the film morphology, such as the spatial distribution of Pt phase and also the film roughness.

Studies on the electronic and ionic conductivities of PZT using complex impedance spectroscopy suggest that lead and oxygen vacancies are the majority lattice defects. The p-type conductivity is found dominant at high temperature above the Curie transition, while below it a significant contribution of ionic conductivity is observed. From bulk to thin film of PZT-Pt the conduction mechanism changes from bulk-controlled to interface-controlled. The I-V characteristics of PZT and PZT-Pt films can be well explained by the thermionic-Schottky emission model. The current at low and intermediate fields is due to the thermionic emission of electrons/holes over the Schottky barrier height from the reverse biased electrode into the film. The increase of leakage current in PZT-Pt films with increasing Pt content is attributed to the earlier onset of the Schottky effect and to the decrease of the barrier height at the film/electrode interface in the presence of Pt phase.