Ph.D Thesis


Ph.D StudentHanady Yassen
SubjectPreparation, Sintering and Characterization of Barium
Titanate Layers by EPD from Aqueous Suspension
DepartmentDepartment of Chemical Engineering
Supervisor Professor Tsur Yoed
Full Thesis textFull thesis text - English Version


Abstract

Thin metal layers with low surface roughness can be achieved via electrophoretic deposition (EPD) using nanoparticles. In order to obtain similar results with dielectric layers, the synthesis and the dispersion of BaTiO3 nanoparticles are required. The goal of this study is to prepare BaTiO3 thin layers by electrophoretic deposition using a colloidal process. Synthesis, interfacial chemistry and colloidal properties of hydrothermal BaTiO3 were performed. After the characterization of the material system, particulates were deposited to form thin layers using EPD.

HRSEM, B.E.T., XRD and particle size distribution measurements show that the nanoparticles were of 24 nm diameter, 62 gr/m2 surface area, and of pseudo cubic crystal structure. High purity, low defect concentration including OH defects, and controlled stoichiometry powder were obtained.

The aqueous passivation, dispersion, and doping of nanoscale BaTiO3 powders were investigated too. Passivation of BaTiO3 was achieved through the addition of oxalic acid. Oxalic acid selectively adsorbs onto the particle surface and forms a chemically stable very thin layer of barium oxalate. The negative surface charge of the oxalate effectively passivated the BaTiO3 providing a surface suitable for the using of a cationic dispersant, polyethylenimine (PEI).

The mechanism and kinetics of electrophoretic deposition of BaTiO3 nanoparticle suspensions were investigated; it is suggested that the deposition was related to the changes in the pH value near the cathode and the decrease in zeta potential and hence the repulsive forces. A linear relation between the deposit and the deposition time was obtained with a slope from which the kinetic constant of the reaction has been calculated. Dense and crack free thin layer of barium titanate with thickness of 150 nm after sintering and with dielectric constant of ~20 was obtained at EPD conditions of, e.g. 5 V, 10 min. deposition time, and 8 cm distance between the two electrodes. No PTCR behavior was noticed for Nb doped barium titanate deposited on Ni using the latter method and conditions.

The value of the dielectric constant of the pure BaTiO3 layer and the resistance-temperature behavior of the doped layer are not fully explored. In particular, the dielectric constant of the layers is surprisingly low. More experimental and theoretical work has to be done in order to investigate and better understand these phenomena.