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.

4.017X = 1 -

0.108

a

Composition Chart

Powder Composition vs. Solution Composition

0.0000

0.1000

0.2000

0.3000

0.4000

0.5000

0.6000

0.7000

0.8000

0.9000

1.0000

0.70 0.75 0.80 0.85 0.90 0.95

mole fraction of BaCl2 in sln.

mo

le f

rac

tio

n o

f B

aT

iO3

in

po

wd

er

Experimental Data

1:1 Function

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.

D = cosFWHM

Particle Size Data

Particle Size vs. Composition

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20

Mole fraction Ba

Par

ticl

e si

ze i

n n

m

SEM Pictures – SrTiO3

SEM Pictures – Ba0.53Sr0.47TiO3

SEM Pictures – BaTiO3

Particle Size Data

SrTiO3 Particle Size vs. NaOH Concentration

30.00

35.00

40.00

45.00

50.00

55.00

60.00

65.00

70.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00

[NaOH]

Pa

rtic

le s

ize

in

nm

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

Questions