Synthesis and Colloidal Properties of Anisotropic Hydrothermal Barium Titanate
Open Access
- Author:
- Yosenick, Timothy James
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 21, 2005
- Committee Members:
- James Hansell Adair, Committee Chair/Co-Chair
Clive A Randall, Committee Chair/Co-Chair
Susan E Trolier Mckinstry, Committee Member
Thomas R Shrout, Committee Member
Darrell Velegol, Committee Member - Keywords:
- Barium Titanate
hydrothermal synthesis
surface chemistry
colloidal properties - Abstract:
- Nanoparticles of high dielectric constant materials, especially BaTiO3, are required to achieve decreased layer thickness in multilayer ceramic capacitors (MLCCs). Tabular metal nanoparticles can produce thin metal layers with low surface roughness via electrophoretic deposition (EPD). To achieve similar results with dielectric layers requires the synthesis and dispersion of tabular BaTiO3 nanoparticles. The goal of this study was to investigate the deposition of thin BaTiO3 layers using a colloidal process. The synthesis, interfacial chemistry and colloidal properties of hydrothermal BaTiO3, a model particle system, was investigated. After characterization of the material system particulates were deposited to form thin layers using EPD. In the current study, the synthesis of BaTiO3 has been investigated using a hydrothermal route. TEM and AFM analyses show that the synthesized particles are single crystal with a majority of the particle having a <111> zone axis and {111} large face. The particles have a median thickness of 5.8  3.1 nm and face diameter of 27.1  12.3 nm. Particle growth was likely controlled by the formation of {111} twins and the synthesis pH which stabilizes the {111} face during growth. With limited growth in the <111> direction, the particles developed a plate-like morphology. Physical property characterization shows the powder was suitable for further processing with high purity, low hydrothermal defect concentration, and controlled stoichiometry. TEM observations of thermally treated powders indicate that the particles begin to loose the plate-like morphology by 900 °C. The aqueous surface chemistry of BaTiO3 is complex and difficult to model using current models due to the pH dependent dissolution/readsorption of Ba2+ at the surface. In addition the precipitation of BaCO3 at high pH influences the surface chemistry. In the current study a model was developed to account for the effect of dissolved Ba2+ as a function of pH. Three distinct regions in the surface chemistry are observed as a function of pH. At low pH, the dissolution of Ba2+ results in a TiO2 surface which can be described using the MUSIC model. As pH increases the affect of dissolved Ba2+ becomes more prominent. The adsorption of Ba2+ onto the TiO2 is observed and can be modeled using a modified Stern isotherm. In basic environments (>pH 9.5) the precipitation of BaCO3 on the surface of the BaTiO3 particles requires the use of a Nernst-Gouy-Stern charging model to described the surface. The aqueous passivation, dispersion, and doping of nanoscale BaTiO3 powders was investigated. Passivation BaTiO3 was achieved through the addition of oxalic acid. The oxalic acid selectively adsorbs onto the particle surface and forms a chemically stable 2-3 nm layer of barium oxalate. The negative surface charge of the oxalate effectively passivated the BaTiO3 providing a surface suitable for the use of a cationic dispersant, polyethylenimine (PEI). Rheological properties indicate the presence of an oxalate-PEI interaction which can be detrimental to dispersion. With a better understanding of the aqueous surface chemistry of BaTiO3 the surface chemistry was manipulated to control the adsorption of aqueous soluble complexes of Co, Nb, and Bi, three common dopants in the processing of BaTiO3. Surface charge, TEM, and EDS analysis showed that while in suspension the dopants selectively absorbed onto the particle surface forming an engineered coating. The electrophoretic deposition of two different BaTiO3 nanoparticle suspensions was investigated. The effect of solution chemistry on dispersion, deposition kinetics, and film microstructure is addressed. The conditions necessary for optimum dispersion results in low deposition rates and poor film adhesion. High dispersant concentration leads to electrochemical inhibition at the electrode and reduced field drop in the bulk of solution. Low effective fields in the bulk of the suspension results in low electrophoretic velocities and reduced deposition kinetics. Strong repulsive interactions between the particles and electrode lead to poor adhesion for the particles that do deposit. The addition of an indifferent electrolyte reduces the repulsion and improves adhesion. However, the indifferent electrolyte reduces the zeta potential of the particles in suspension, leading to aggregation prior to deposition. Deposited films comprised of aggregates exhibit inhomogeneous microstructures.