Cold sintering of the Mg–C–O–H system

Abstract

The cold sintering process reproduces the formation of sedimentary rocks in the Earth's crust by molding raw powder and a small amount of solvent at temperatures of about 300 °C or less under uniaxial pressure of several hundred megapascals. Generally, carbonates and hydroxides cannot be hardened by conventional sintering process due to thermal decomposition. In contrast, when this cold sintering process is selected, since the densification of the starting powder can be achieved by a dissolution-precipitation reaction with water as a solvent, carbonates and hydroxides can be hardened at temperatures below their decomposition temperatures. In the Mg–C–O–H system, magnesium hydroxide and a basic magnesium carbonate were selected as starting compounds, and the mechanism of their densification by a cold sintering process was investigated. The average compressive strength of the obtained magnesium hydroxide solidified products after cold sintering at 250 °C and 270 MPa for 60 min, and basic magnesium carbonate solidified products after cold sintering at 150 °C and 270 MPa for 60 min, were 121 MPa (84% relative density) and 275 MPa (88% relative density), respectively. The added water was found to play an important role in promoting a solution-precipitation process inside the magnesium hydroxide or basic magnesium carbonate powder compact, resulting in low-temperature sintering to form hardened bodies in the Mg–C–O–H system.