Date of Award

August 2014

Degree Type

Thesis

Degree Name

Master of Science

Department

Engineering

First Advisor

Benjamin Church

Committee Members

Nidal Abu-Zahra, Sudeep Ingole

Keywords

Alumina, Aluminum Borate, Boron, Sintering

Abstract

Alumina has a wide variety of applications, but the processing of alumina based materials can be costly. Mechanically milling alumina has been shown to enhance the sintering properties while decreasing the sintering temperature. Additions of boron have also proven to increase sintering properties of alumina. These two processes, mechanical milling and boron additions, will be combined to test the sintering properties and determine if they are improved upon even further compared to the individual processes.

Multiple samples of pure alumina, 0.2 weight percent boron, and 1.0 weight percent boron are batched and processed in a ball mill for different time intervals. These samples are then characterized to observe the structure and properties of the samples after milling but before sintering. Pellets are dry pressed from the milled powders, sintered at 1200C for one to 10 hours, and characterized to determine the impact of processing.

X-ray diffractometry (XRD) was used on each sample to determine crystallite size and lattice parameters at different stages throughout the experiment. XRD was also used to identify any samples with an aluminum borate phase. Scanning electron microscopy (SEM) was used to observe the powder and pellet morphology and to measure bulk chemical composition. Samples were sputter coated with an Au-Pd coating observed in the SEM to characterize the topography as a function of variables such as milling time, boron composition, and sintering time. Additionally, porosity and change in diameter were measured to track the sintering process.

Milling sample for longer periods of time would be unnecessary due to the crystallite size leveling off between 10 and 12 hours of milling time. Samples of alumina with 0.2 weight percent boron prove to have very little effect on the sintering properties. At 1.0 weight percent boron, there are changes in diffraction patterns and topography after being sintered for one hour. The porosities of all of the sintered samples are larger than anticipated likely due to a poor sinter. Higher boron compositions appear to have the greatest effect on post sintering properties by producing the smallest relative porosities and largest changes in densification.

Reduction of crystallite size during high-energy ball milling is independent of boron composition in the range of compositions that were investigated. The lattice parameters of the main alumina phase as a function of milling time did not follow the same trends shown in literature, indicating any impurities within the literature samples affected the lattice parameter measurements. Increased milling times on the samples that contain 1.0 weight percent boron are observed to have a higher densification during sintering. Higher boron additions increase the densification from sintering while maintaining a relatively lower porosity. Relative to the other samples, the highest sample densification is observed from both 1.0 weight percent boron additions and ball milling for 12 hours and sintering for one and 10 hours.

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