High pressure compaction of nanosize ceramic powders

Abstract

High-density ceramic materials from nanosize ceramic powders were produced by high pressure under nearly hydrostatic environment up to 5.6 GPa, on a special configuration in a toroidal-type apparatus, at room temperature. Attempts to use a common solid pressure transmitting medium, as NaCl, resulted in cracked samples. Lead and indium, which have an extremely low shear strength, proved to be the suitable choices as a pressure-transmitting medium to compact these ceramic materials, in order to obtain high-density samples. Transparent amorphous SiO2-gel and translucent γ−Al2O3 samples, in bulk, with volumes about 40 mm3, hard and crack-free were obtained. Densities over 90% of full density for the γ−Al2O3 samples and over 80% for the compacted SiO2-gel samples were obtained. In addition, from the density-pressure curve, the yield strength (σ) for γ−Al2O3 was estimated, for the first time, as 2.6 GPa. Vickers microhardness values were in the range of 5.7 GPa for the γ−Al2O3 samples, and 4.0 GPa for the SiO2-gel samples, under loads of 50 g. An important and practical application of these results is the possibility of producing bulk γ−Al2O3, a new alumina material, which was not possible to prepare before due to the conversion to a phase during the normal sintering process. Additionally, specially for SiO2-gel, a very important application of this study is the possibility of incorporation of organic substances in an inorganic matrix, using high pressure at room temperature.

The production of advanced fine-grained alumina by carbon nanotube addition

“…The final grain size in heat-treated composite was almost the same as the starting powders. This method uses less energy as a result of reduced pressure and processing times as compared to previously researched methods [46][47][48][49]. Characterization of other grain-size dependant properties and utilization of direct electrical heating to oxidise CNTs in composites will be the subjects of future research.…”

“…Such results were previously only possible when very high pressures (1)(2)(3)(4)(5)(6)(7)(8) were used during the sintering of alumina [46][47][48][49]. For example, Liao et al [49] applied 8 GPa pressure during conventional lengthy sintering process to produce a microstructure having a mean grain size of 50 nm using 18 nm starting powder alumina.…”

Synthesis of ORMOSIL silica/rhodamine 6G: Powders and compacts

“…The compaction was accomplished in a toroidal-type high-pressure chamber at 7.7 GPa and room temperature, during 10 min to promote the cold sintering process [16,17]. The pressure calibration was performed by the 'fixed points' technique, using Bi and Yb, which allowed calibrating the pressure in the following three fixed points: Bi with phase transitions at 2.5 GPa and 7.7 GPa, and Yb with a phase transition at 4.0 GPa.…”

“…Fluorescent compacts were also obtained, through the high-pressure processing of these powders, at 7.7 GPa, at room temperature. This kind of compaction promotes a cold sintering process and improves the samples physical properties [15][16][17]. The microstructure, chemical composition, thermal stability and fluorescence emission of these powders and compacts were comparatively studied, using N 2 adsorption/ desorption isotherms, FTIR spectroscopy, thermogravimetric analyses (TGA), CHN elemental analyses, and fluorescence spectroscopy.…”

Fluorescent compacts prepared by the entrapment of benzoxazole type dyes into a silica matrix at high pressure

“…At this point the resulting monoliths were comminuted in an agate mortar to obtain a silica-doped powder. For the high pressure compaction, the silica-gel doped powders were firstly precompacted in a piston-cylinder type apparatus to approximately 0.1 GPa, and then placed inside a lead container with internal diameter of 8 mm, which acts as a quasi-hydrostatic pressure transmitting medium [17,18]. The compaction was then accomplished in a toroidal-type high pressure chamber at 4.5 GPa during ten minutes to promote the cold sintering process [19,20].…”