Synthesis and Self-Assembly of Barium Titanate Nanoclusters

Marc Y. H. Landeweer

Augustana College

Advising Professors: Ron Andres
and Elliott Slamovich

Introduction

Barium titanate is a commonly used material for capacitors and transistors because of its high dielectric constant. Current commercial methods of synthesizing barium titanate are either by high temperature (~1000oC) solid diffusion processes or by hydrothermal processing. Hydrothermal processing is a lower temperature (~ 80-500oC) method of fabricating ceramic powders in an aqueous solution. Its lower temperature processing is one of the distinct advantages over conventional methods. It also can provide very fine submicron powders of high purity. The size of the powders is dependent on the processing method employed for fabrication. By varying the starting reagents, relative concentrations thereof, reaction time and temperature, the size of the particle can be greatly varied. Barium titanate exhibits an unusual behavior of nanoparticles clustering of 10-50nm crystals to form larger agglomerated crystals ranging from 100-500nm. One of the approaches in this project was to adjust the size and separation of the titanium dioxide precursor in an attempt to eliminate interactions of these nanocrystals during formation. If the titania molecules are separated (using a surfactant) by a distance such that intermolecular forces become negligible, then the particle interactions of reacted barium titanate should not occur. As following the present trend of miniaturization in microelectronics, barium titanate based capacitors continue to reduce in size. The hope is that in the near future, we will be able to develop a hydrothermal method of producing nanoparticles of 20nm or less for use in these increasingly smaller circuits.

Objectives

  • To investigate the reaction mechanism of hydrothermal barium titanate synthesis
  • To synthesize barium titanate nanoparticles using a hydrothermal method or a variation using titania (TiO2) nanoparticles

Experimental Approach

The experimental approach to this project was by reacting barium chloride with either stock titania, or titania created in the Distributed Arc Cluster Source (DACS). (The DACS uses elemental titanium, heated it to a plasma arc then reacting it with oxygen in the air to form TiO2.) These reaction were processed under alkaline (basic) conditions for at least 40 hours at temperatures between 80-90oC. Most of the reactions using the stock titania provided ample product to characterize using X-ray Diffraction (XRD). The three primary characterization tools were X-ray diffraction, laser diffraction for volume calculations, and electron microscopy. Experimental results were analyzed by several methods depending upon the size and quantity of the product. The most important tool for identification of these sub-micron powders was the Transmission Electron Microscope (TEM). The TEM provided visual data, as well as an electron diffraction pattern to further identify the sample. X-ray diffraction can be used to identify crystal structures of a sample and a rough estimation of sample purity. The final tool used was a particle size analyzer that uses the diffraction of a laser to determine the size distribution of a sample of powder suspended in water. The last two forms of characterization require at least 0.5g of sample and could not be used for any of the powders reacted using the DACS titania.

Research Findings

After inspecting most of the powders under XRD, particles size distribution and TEM the results were not the anticipated nanoparticles. The standard hydrothermal powders synthesized using BaCl2, were not as well defined as desired. Although, they lacked a definitive particle size, they did, however demonstrate a high level of particle dispersion. The samples synthesized using the DACS powders did reveal the presence of barium titanate, however, again there was a lack of any definite pattern to the particle size distribution. These powders created were not as small as desired, closer 50-100nm rather than the desired 20nm range.

Although barium titanate was formed in nearly all of the samples, the morphology and size distribution of all of the powders was unpredictable, and would require further processing techniques to refine these powders.


Fig. 1. Hydrothermally processed barium titanate. Note the rounded , "berry" morphology due to the agglomeration of smaller crystals. Barium titanate has cubic crystal structure.


Fig 2. Hydrothermally reacted barium titanate using DACS titania. The large agglomeration of smaller particles suggests initial formation of smaller dispersed crystals.

Final Research Presentation