Rebecca Olsen and Dr. Juliana Boerio-Goates, Chemistry and Biochemistry

Nanomaterials, possessing unique chemical, physical, and mechanical properties, can be used in a wide range of industrial, biomedical, and electronic applications. There are three general categories for producing nanomaterials: mechanical processing (milling), chemical processing (precipitation), or thermal processing (evaporation and condensation). Conventional methods often have drawbacks in controlling size, purity, surface morphologies, production amounts, cost, and time.1,2,3 Additionally, techniques developed for one metal oxide often do not transfer to other nano-oxides. A new method developed here at BYU overcomes many of these difficulties.

Using this BYU method, a 3nm Al2O3 (alumina) sample was synthesized and characterized. Initially, we planned to use low temperature adiabatic calorimetry to analyze the waters of hydration on the particle surfaces. Due to instrumental difficulties, these measurements are still pending. However, upon characterization, it was determined that the particles were agglomerated. Much of the project’s time has been focused on the dispersion of these particles.

Synthesis

The alumina sample was synthesized using the BYU nanosynthesis method. This method consists of mixing metal salts with a solid base. As the solid powders are mixed together a reaction occurs producing a precursor material (containing complex amorphous metal hydroxides and ammonium salt) and gaseous byproducts. The precursor is then heated in an oven at low temperatures for about an hour. The amorphous metal hydroxide reacts to form nanometal oxide particles while the ammonium salt forms mostly nitrous oxide gas and water. All byproducts are gaseous and so the particles are as pure as the starting materials.

Methods of Characterization and Analysis:
The sample was sent to Galbraith Laboratories to test for carbon, hydrogen, nitrogen, chloride, and other trace elements. The sample showed levels of nitrogen ≥2%.

We have performed thermogravimetric analysis on the alumina to determine the temperature at which the bulk of the crystallization occurs. With this data we determined the appropriate calcination temperature to be 300C.

We used X-ray diffraction (XRD) to measure crystallite size using the Scherrer formula from the peak width at half maximum for the principal XRD peak. The alumina was 3nm (±5nm). We also used XRD to determine that the alumina sample was phase pure (gamma phase).

We collected pictures of the alumina particles using transmission electron microscopy (TEM) to verify the size of the crystals and found the results to be consistent with XRD data. We found from the TEM data that the particles were both crystalline and agglomerated.

Thermodynamic Data: Due to necessary repairs on instruments this year, heat capacity measurements have not yet been collected for the sample. We are currently in the process of recalibrating the instruments and will then load the sample for analysis.

We performed several experiments to identify the parameters affecting final particle size. Final size is primarily influenced by completeness of reaction to form the amorphous metal hydroxides in the precursor mixture. If the reaction is incomplete, the unreacted starting materials will decompose during sintering, increasing temperature and leading to larger particles. Other factors which influence particle size are starting materials, calcination temperatures, and milling.

Dispersion

Nanoparticles have high surface energies and clump together (agglomerate) to become more stable. To test dispersion we prepared alumina using our standard method. We found that without treatment, approximately 0.5% of the powder would remain suspended in a solvent (disperse) over a 48 hour time period. To increase dispersion, we tested different methods of breaking up and inhibiting agglomeration.

We have found that ultrasonication increases dispersion to almost 1%.

We have found that grinding powders increases dispersion from 0.5% to over 1%. Preliminary tests in milling with a ball mill result in dispersions over 3%.

We have tested the effects of dispersing in ethanol, water, acetone, hexanes, and ethylene glycol. Dispersing twice in water increases dispersion to over15%. We plan to examine other solvent options, including mixtures of water and ethanol to determine the optimum dispersing solvent.

We tested these methods in both the precursor (powders prior to calcination) and the nanopowders. Though preliminary tests have shown that the preparation of the precursor is crucial in dispersing the resulting particles, due to time restraints, we have focused our efforts on treatment of the powders. Our future research will include changes made during the precursor stage to maximize dispersion.

References

1. C.C. Koch, Nanostruct. Mater. 2(2), 109 (1993).
2. B. Gunther, A. Kumpmann, Nanostruct. Mater. 1(1), 27 (1992).
3. Y. Mizoguchi, M. Kagawa, M. Suzuki, Y. Syono, T. Hirai, Nanostruct. Mater. 4(5), 591 (1994).
4. Woodfield, Brian F.; Liu, Shengfeng; Boerio-Goates, Juliana; Liu, Qingyuan. Preparation of uniform nanoparticles of ultra-high purity metal oxides, mixed metal oxides, metals, and metal alloys. PCT Int. Appl. (2007), 38pp. CODEN: PIXXD2 WO 2007098111 A2 20070830
5. Acknowledgments: BYU Office of Creative Research, Juliana Boerio-Goates and Brian F. Woodfield Lab Group.