Study of Nanocrystalline γ-Al 2 O 3 Produced by High-Pressure Compaction Tania M. H. Costa, ²,‡ Ma ´ rcia R. Gallas,* ,² Edilson V. Benvenutti, ‡ and Joa ˜ o A. H. da Jornada ² Instituto de Fı ´sica, UFRGS, Caixa Postal 15051, 91501-970 Porto Alegre (RS), Brazil, and Instituto de Quı ´mica, UFRGS, Caixa Postal 15003, 91501-970 Porto Alegre (RS), Brazil ReceiVed: September 21, 1998; In Final Form: February 10, 1999 Using a high-pressure (HP) technique, samples of γ-Al 2 O 3 were obtained by compaction at 4.5 GPa, 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 °C. The nanocrystalline structure of γ-Al 2 O 3 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 γ-Al 2 O 3 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 °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 °C. For samples compacted simultaneously with temperature, the FTIR and TGA results did not show water release up to 500 °C. The compaction at temperatures higher than 565 °C yielded the formation of an aluminum hydroxide (diaspore) and the phase transformation from γ- to R-Al 2 O 3 . All these results support strongly the idea that the compaction at HP 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 °C, this kind of structure is responsible for the formation of diaspore plus R-Al 2 O 3 . Introduction The γ-Al 2 O 3 is one of the transition aluminas widely used in technology, in the form of powder or thin film, as adsorbents, catalysts or catalysts carriers, coatings, and soft abrasives because of their fine particle size, high surface area, and catalytic activity. The possibility of obtaining γ-Al 2 O 3 nanocrystalline bulk samples from nanosize particles has been investigated over the last few years 1-3 in order to produce a new material with interesting specific properties. It was not possible to prepare this material in bulk some years ago due to phase transformation sequences that occur during the conventional sintering process at ambient pressure: γ(750 °C) f δ(900 °C) f θ(>1000 °C) f R. The use of the HP technique provides the means to overcome this difficulty. Gallas et al. 3 reported the fabrication of transparent and hard γ-Al 2 O 3 samples (about 0.2 mm of diameter) using a diamond anvil HP cell (DAC) to compact a nanosize powder at pressures up to 3 GPa, followed by pressureless heat treatment at 800 °C. Recently, using another HP technique with a nearly hydrostatic pressure transmitting medium at pressures up to 5.6 GPa and RT, it was possible to produce larger (3.0 mm of diameter) γ-Al 2 O 3 samples, trans- lucent and with high density, indicating a process of “cold- sintering”. In this paper, the results of density measurements vs applied pressure fit a model based mostly on a mechanism of plastic deformation. 4 Costa et al. 5 studied this process for SiO 2 gel powders treated at HP and RT. They proposed a model for the “cold-sintering” process based on condensation reactions of silanol groups at the surface of the nanoparticles to form siloxane bonds between the particles and water, resulting in a stiff body with closed nanopores contained trapped water. It was clear from this model that the adsorbed water plays an important role in the mechanism of consolidation of this kind of powder, which is covered by OH groups. A small number of works involving compaction studies of γ-Al 2 O 3 at HP and high temperature (HT) are found in the literature. Mishra et al. 1 studied the HP sintering of γ-Al 2 O 3 samples over the temperature range 650-1050 °C and pressures of 1 GPa; however, they did not specify the physical appearance of the compacts and did not study the microscopic mechanism involved in the consolidation process. Several techniques based on FTIR spectroscopy have been widely used to characterize γ-Al 2 O 3 films and powders made by different methods 8,9 and to obtain a microscope understanding of the processes involved in these systems. 8-12 The FTIR spectrum of γ-Al 2 O 3 is well-known, and its principal feature is a broad band between 950 and 500 cm -1 , ascribed to Al-O stretching. The surface of γ-Al 2 O 3 is covered by OH groups that cause water adsorption, and the band due to these species is between 3800 and 3000 cm -1 , centered at 3460 cm -1 . This band is ascribed to OH stretching of the adsorbed water, to the bridged hydroxyl group with molecular water or with other OH groups, and to isolated OH groups. 10 The study of all these bands * Corresponding author. Phone: +55-51 316 6542. Fax: +55-51 319 1762. E-mail: marcia@if.ufrgs.br. ² Instituto de Fı ´sica. ‡ Instituto de Quı ´mı `ca. 4278 J. Phys. Chem. B 1999, 103, 4278-4284 10.1021/jp983791o CCC: $18.00 © 1999 American Chemical Society Published on Web 05/06/1999