Clive A. Randall, Sucking the Heat Out of Ceramic Processing

Clive A. Randall

The Pennsylvania State University, PA, USA

For over 30,000 years, the general practice of sintering ceramics has involved a high temperature thermal treatment to drive the transport processes to densify the particles and minimize the surface energy of the material. Typical sintering temperatures consider 0.6 to 0.8 of the melting temperature (Tm) for many oxides; this means we sinter around 800 ºC to 1200 ºC for 2 to 10 hours. Here we introduce a broad body of systems that utilize a transient aqueous based liquid phase (1 to 10 wt%) that sinters under a uniaxial pressure, while being heated from room temperature to 250 ºC, over a time period of 10 to 60 minutes. During this process, there are all the aspects of liquid phase sintering, namely particle rearrangement, dissolution precipitation, and grain growth. We believe transport processes are enhanced through mechanisms such as diffusiophoresis, which in turn is driven by concentration gradients. This phenomenon has very fast transport velocities, and therefore does not require high temperatures ~ 10 micrometers/sec. The driving for precipitation is also enabled by the transient evaporation of the water and a sustained supersaturation under local hydrothermal conditions. These mechanisms work together to create a pathway to sinter ceramics under extremely low temperatures and fast times; many of the systems that we will show are sintered at 120 ºC, 15 minutes, under 300 MPa uniaxial pressure. We have termed this fabrication method the Cold Sintering Process (CSP). We fully realize that there are many subtle differences in the CSP of each system, but as this is the start of a new approach, we will share our qualitative understanding, as determined from microstructural observations. We have also benchmarked and compared properties where possible, and it will be seen that the properties are in comparison to conventionally processed materials; we will in particular contrast conduction and dielectric properties.
Given the massive drop in sintering temperature of the ceramic, this offers many new opportunities in material design, especially in composites. We will show three different types of polymer ceramic composites with high percentages of ceramic, 100% to 60%, with the thermoplastic polymers for dielectric applications, ionic electrolytes, and semiconducting composites. We will also show preliminary data with CSP with multilayer ceramics and printable electronics.

Plenary lectures - YUCOMAT 2017

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