Hydrothermal Processing of BST Powders Katherine Frank August 3, 2005 Professor Slamovich
BST Background • BST refers to barium strontium titanate. • BST is a ceramic material, and is produced as a powder or as a thin film. • The properties of BST make it well suited for electronic applications. • High dielectric constant • High capacitance density
Solid Solutions • Barium titanate and strontium titanate make up a solid solution, BST. • A solid solution is formed when molecules of one substance work themselves into the crystal structure of another substance.
Applications • Dynamic random access memory (DRAM) is the focus of Prof. Slamovich’s research. • Information is stored in DRAM cells in capacitors. • Capacitors are layers of conducting material separated by layers of an insulating material (in this case, BST). • The thinner the layers, the more information each capacitor can store.
Research Objective • The size of the BST particles produced hydrothermally varies according to the amount of barium and strontium. • Other factors such as pH and temperature also effect the size of the particles. • The goal of my research is to study the relationships between these variables and the size of the BST particles produced.
Processing • Hydrothermal processing refers to a reaction conducted in an aqueous solution, at an elevated temperature. • The solutions are composed of BaCl2, SrCl2, TiO2, and NaOH powders added to 100 mL of water. • The solutions, once mixed, are left to react in an oven at ~80° Celsius for 48 hours.
Processing • The NaOH is necessary because BaCl2 and SrCl2 are more soluble at higher pH’s. • BaCl2 and SrCl2 dissociate and react with TiO2 to form a solid solution of BaTiO3 and SrTiO3. • Once removed from the oven, the BST powders are washed several times to remove carbonate contamination and left to dry.
Powder Composition • Another empirical function relates the lattice parameter of the material to the composition of the powder.
Peak Position • The lattice parameter of each sample is calculated with Bragg’s Law, using XRD peak positions. • According to this law, peak position varies inversely with the spacing between planes of molecules.
Lattice Parameters • The following equation relates lattice parameter to composition: where a is the lattice parameter and X is the mole fraction of barium.
Particle Size Measurement • For small particles (<100 nm), x-ray diffraction and the Scherrer Equation can be used to determine particle size. where D is the particle diameter, K is a constant evaluated as 0.94, λ is the wavelength of the X-ray, and θ is the angle at which the peak occurs.
Potential Sources of Error • Composition calculation: because the relationship between initial and final composition was determined experimentally, it could be inaccurate. • Experimental error: slight deviations from the initial composition would result in misleading data for the final composition. • XRD sample preparation: with day-to-day differences in preparation, two scans of the same sample had a 13% discrepancy for the particle size measured. • Peak broadening standard: the particle size data has not been calculated with a peak broadening standard determined as part of this experiement.
Conclusions • The correlation between the amount of barium in the powder and the particle size is positive and linear. • At higher pH’s, the particle size increases, but the exact relationship between pH and particle size cannot be determined.
Continuing Research • If more data is collected (i.e., powders of more compositions are created and analyzed), there will be enough information to infer a mathematical relationship with some certainty.
Acknowledgements • Many thanks to: • Professor Slamovich • Hsin-Yu Li • Dave Roberts • Turner Lab denizens • The overwhelmingly awesome MSE faculty and staff