Unlike naturally occurring dense ceramics, man-made ceramics often require sintering temperature in excess of 1400 °C for densification to occur. Various biomineralization-inspired processes have been trialled to bring the temperature down, but many have been limited to thin film and particle fabrications. Now, a couple of researchers from ETH Zürich, Switzerland have created a geologically-inspired densification approach that operates at room temperature, by reducing the size of the particles to nanoscale dimensions and utilising higherpressures throughout the compaction process in the presence ofwater.
The ability to perform densification methods at ambient temperatures has been a long process. The ability to simultaneously process inorganic and organic components into bulk ceramic materials at room temperature has been the goal of many a researcher. There have been many advances in recent years in producing methods that can mimic the biomineralization process of naturally occurring ceramics, with a small degree of success. However, many of these have needed temperatures greater than 1400 °C, and have only produced thin film and particle structures.
An alternative approach to biomineralization processes is that of geologically-inspired methods. A natural geological process to produce ceramics involves the formation of strong and dense carbonaceous rocks from inorganic particle sediments. These processes operate at mild temperatures and have inspired a way to synthetically produce dense ceramics.
Geological-based processes provide an attractive method for ceramic production as the processes involved are much simpler than those in biomineralization processes. The driving parameters in geological-based processes revolve around composition, pressure and temperature, all of which are tuneable processes, scalable processes and are compatible with state-of-the-art machinery. The main thing with these methods that requires addressment, is the reduction of the geological timescales to fit a synthetic method timeframe.
There have been some recent developments to bring down the process temperatures of inorganic oxides to between 120-180 °C by cold sintering methods; and sintering methods using sodium chloride. Whilst this is a significant improvement, and a step in the right direction, they didn’t yield room temperature results than many aspire to obtain.
Obtaining Room-Temperature Ceramics
The team from Zürich produced a method of accelerated sintering in water at 25 °C, using an inorganic compound (nanovaterite) that is only moderately soluble in aqueous solvents. The limited solubility lowered the sintering temperatures and eliminated the need for hydrothermal conditions, which allowed the sintering of the inorganic compounds into the water.
The production of nanovaterite agglomerates was achieved by wet chemical methods using water, calcium chloride and sodium carbonate. The process involved many dissolutions, mixing and agitation methods and centrifugal steps. The structures were characterised by Broad Ion Beam milling (IM4000, Hitachi), electron microscopy (LEO1530, Zeiss) and transmission electron microscopy (FEI Talos F200X). Creep tests were performed on a Instron 8562 universal testing machine and mechanical testing was performed on a P/O/Weber uniaxial press.
By using water, the researchers managed to create a material that exhibits a relative density up to 90% instead of 68% and 64% when using air and paraffin oil, respectively. This can also be achieved within 30 minutes. The researchers also used up to 800 MPa of pressure to achieve such results, although the optimal pressure was found to be 500 MPa. The material itself exhibits an elastic modulus of 30 GPa, a flexural strength of 50 MPa and a compressive strength of 225 MPa.
The cold sintering of the nanopowder at high pressures allowed the formation of strong and dense structural materials to be produced at room temperature. The process is also scalable and allows for the processing of inorganic materials under inexpensive conditions and can produce a material that mimics the design principles of functional biological materials. The timescales are also comparable to those currently utilised in industrial manufacturing processes, affording the potential for a commercially viable process.
Bouville F., Studart A., Geologically-inspired strong bulk ceramics made with water at room temperature, Nature Communications, 2017, 8, 14655