Study of nanocrystalline γ-Al2O3 produced by high-pressure compaction

Using a high-pressure (HP) technique, samples of gamma-Al2O3 were obtained by compaction at 4.5 GFa, in a toroidal-type apparatus, at room temperature (RT) and at higher temperatures. Compaction at RT produced crack-free, translucent, and dense samples. An improvement of these properties was observed for samples compacted at higher temperatures up to 565 degrees C. The nanocrystalline structure of gamma-Al2O3 is retained, and the samples became transparent, showing high hardness (HV = 17 +/- 1 GPa) and high density (95% of theoretical density). To understand the mechanisms of consolidation, a comparative analytical study by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) was conducted on the compacted gamma-Al2O3 samples and the original powder. An FTIR study was done using the KBr technique and a high-vacuum cell, where the samples were submitted to thermal treatments up to 450 degrees C. For samples compacted at RT, a reduction in the content of adsorbed water was observed, compared to the original powder. Also, the surface hydroxyl groups became bridged, promoting dehydroxylation reactions, which were confirmed by TGA technique. In the dehydroxylation region, a weight loss was observed, and the water was released only at temperatures above 300 degrees C. For samples compacted simultaneously with temperature, the FTIR and TGA results did not show water release up to 500 degrees C. The compaction at temperatures higher than 565 degrees C yielded the formation of an aluminum hydroxide (diaspore) and the phase transformation from gamma- to alpha-Al2O3. All these results support strongly the idea that the compaction at I-IF has caused the formation of a strong structure, with closed pores containing trapped water and hydroxylated internal surfaces, which confirms a proposed model for "cold-sintering". At temperatures higher than 565 degrees C, this kind of structure is responsible for the formation of diaspore plus alpha-Al2O3.