Study of nanocrystalline gamma-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, tra nslucent, and dense samples. An improvement of these properties was observe d for samples compacted at higher temperatures up to 565 degrees C. The nan ocrystalline structure of gamma-Al2O3 is retained, and the samples became t ransparent, 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 (FT IR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) was cond ucted 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 s amples were submitted to thermal treatments up to 450 degrees C. For sample s compacted at RT, a reduction in the content of adsorbed water was observe d, compared to the original powder. Also, the surface hydroxyl groups becam e 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 sample s compacted simultaneously with temperature, the FTIR and TGA results did n ot show water release up to 500 degrees C. The compaction at temperatures h igher than 565 degrees C yielded the formation of an aluminum hydroxide (di aspore) and the phase transformation from gamma- to alpha-Al2O3. All these results support strongly the idea that the compaction at I-IF has caused th e formation of a strong structure, with closed pores containing trapped wat er and hydroxylated internal surfaces, which confirms a proposed model for "cold-sintering". At temperatures higher than 565 degrees C, this kind of s tructure is responsible for the formation of diaspore plus alpha-Al2O3.