Progress and Opportunities in the Application of Cold Sintering to Solid-State, Electrochemical, and Ceramic Devices

Abstract

The high temperature conventional sintering process has been practiced by humans for thousands of years, but only in recent decades has there been significant progress in modifying the sintering process to achieve lower temperatures or shorter dwell times.In this talk, we focus on the newest addition to the family of alternative sintering techniques; cold sintering. Cold sintering is characterized by the addition of a small amount of a transient sintering aid, such as an alkali hydroxide, to the parent ceramic powder. The mixture is then subjected to low temperatures (Tsinter < 400°C) and moderate uniaxial pressure (100 - 300 MPa) which results in a remarkable degree of densification for a wide range of ceramics. Broadly speaking, cold sintering is able to densify even the most refractory technical ceramics at around 30% of the melting temperature, whereas conventional wisdom asserts that the solid-state sintering process requires temperatures near 70% of the melting temperature.To illustrate the benefits of this dramatic reduction in sintering temperature, we will present recent examples of cold sintering applied to conductive ceramics for high temperature electrochemical devices. These examples include sodium ion electrolytes for solid-state batteries (e.g. Na3Zr2Si2PO12, β’’-Al2O3) and oxygen-ion electrolytes for solid oxide fuel cells (e.g. CeO2). For these electrolytes, we illustrate how the density and conductivity of the cold sintered electrolytes is comparable to the conventionally fired ceramics. In doing so, we also discuss the opportunities and open questions surrounding these ceramic materials which have been densified at low temperatures.These examples show how the elimination of elemental volatilization and grain size control can be easily achieved with lower temperature processing. We will conclude with a forward-looking assessment of some of the opportunities that low temperature sintering may enable, such as co-firing device layers and the inclusion of thermally fragile additives into the sintered ceramics. The low temperatures afforded by cold sintering presents new opportunities for the processing of many ceramic-dominated solid-state electrochemical devices.Figure Caption: The relative density is plotted as a function of sintering temperature for the prototypical refractory solid electrolyte, sodium-ion conducting β’’-Al2O3. Conventional sintering methods include hot pressing, spark plasma sintering, liquid phase sintering, and solid-state sintering. Cold sintering achieves high densities at exceedingly low temperatures for this refractory material. Figure 1