Processing Aluminum Oxide for the Control of Microstructural Texture and Optical Properties
thesisposted on 01.05.2020, 20:12 by Andrew P Schlup
Transparent polycrystalline aluminum oxide is a promising optical material, particularly in applications that require ballistic protection. However, the rhombohedral crystal structure of alumina limits its transparency due to birefringent scattering. One method of reducing birefringent scattering is to align the particles along the same crystallographic direction, minimizing the refractive index mismatch. This dissertation explores the use of high aspect-ratio platelet-morphology alumina powder in order to process a crystallographically aligned polycrystalline alumina part, with improved optical properties. The optimal hot-pressing parameters of non-pre-aligned platelet alumina were explored, showing that a low pre-load pressure (0MPa), a high maximum temperature (1800°C), a low maximum pressure (10MPa), and a long isothermal hold time (>5hrs) yields dense, transparent parts. These parameters resulted in samples with a high in-line transmission (>65%) despite a large grain size (>60μm). This is due to a high degree of crystallographic orientation, which minimizes the refractive index mismatch between grains, reducing birefringent scattering. Pre-alignment resulted in a further increase in crystallographic orientation, indicating that the pre-alignment procedure effectively aligns the platelets along the same crystallographic orientation. However, pre-alignment resulted in a minimal improvement in optical properties due to the pre-aligned platelets decreasing the densification. Mechanical properties were characterized, resulting in a flexure stress and Vickers hardness of approximately 175MPa and 17GPa, respectively. These low mechanical properties are due to the large grain size. The Vickers hardness was also characterized along different alignment/hot-pressing directions, showing that the hardness matches that of sapphire along corresponding crystallographic directions. Modifications to the Rayleigh-Gans-Debye model were made, accounting for crystallographic orientation. The modified model more closely matches the experimental optical data, illustrating the importance of accounting for crystallographic alignment. This dissertation emphasizes the importance of characterizing optical losses in transparent ceramics and how they relate to the microstructure, as well as the significance of crystallographic alignment in a birefringent transparent ceramic like alumina.