A Novel Cold Sintering Method for Ceramics

Engineered high-performance ceramics and hobbyists' pottery alike are usable only after they are fired for hours at high temperatures, usually above 1,000 °C. The sintering process that takes place causes the individual particles to "bake" together, making the material more compact and giving it the required properties, such as mechanical strength.

Pennsylvania State University researchers have demonstrated that a range of inorganic materials and composites can also be sintered between room temperature and 200 °C. This cold sintering process is based on the addition of small amounts of water to aid the transport processes that densify the material.

In conventional high-temperature sintering processes, individual ceramic powder particles densify into a hard, compact object. The driving force for this process is the reduction of the high surface free energy of the powder by material diffusion—a process that occurs only at high temperatures.

"In contrast, cold sintering relies on interfacial solution effects in water for the densification of the material," says Clive Randall, professor of materials science and engineering, "a process that occurs at low temperatures and over much shorter time frames—minutes instead of hours—when pressure is applied."

(Click to enlarge.) Cold sintering relies on interfacial solution effects in water for the densification of the material. Image credit: Penn State University.

According to Randall, the small amounts of interfacial water serve as a transient liquid phase. The details vary for different systems. But for a number of ceramic materials, the first step is dissolution of the sharp edges of the boundary surfaces between the particles, reducing the surface free energy of the powder. Then, under appropriate pressure and temperature combinations, the dissolved material diffuses through the liquid and preferentially precipitates away from the contact areas between the particles. This process closes the pores and makes the material more compact.

"Cold sintering functions for a broad palette of inorganic compounds, including metal oxides, carbonates, halides, phosphates and multimaterial systems (composites)," Randall says.

The researchers examined the process in detail using sodium chloride, alkali molybdates and vanadium oxide, among other materials. The properties of the cold-sintered samples were equivalent to those sintered by conventional methods.

According to Randall, composites of ceramics with metals, polymers or other ceramics are in high demand, but because of their different thermal stabilities, shrinkage and possible chemical incompatibilities, systems made from different materials are not easy to sinter at high temperatures.

"These problems are minimized in cold sintering and, most importantly, this process opens exciting sustainable and low-cost manufacturing possibilities for ceramics and their composites," he says.