Ceramic-polymer cold fusion
A new cold sintering process enables the creation of previously incompatible ceramic–polymer compound materials at temperatures far lower than traditional sintering techniques. Khai Trung Le reports.
The simple introduction of water has enabled the co-sintering of ceramic and thermoplastic polymer composites in a single step at room temperature, in a process that the Pennsylvania State University, USA, research team speculate has been lost in history.
The cold sintering process (CSP) requires a transient low temperature solvent, such as water, that is used to regulate the dissolution and precipitation of the ceramic, and, as described in the paper, Cold sintering process of composites: bridging the processing temperature gap of ceramic and polymer materials, published in Advanced Functional Materials, ‘effect densification between room temperature and ≈200˚C’, under which conditions thermoplastic polymers and ceramics can be formed into dense composites.
The prevention of thermal decomposition within the polymer stems from the low temperatures involved in the CSP, with Professor Clive Randall, Director of the Penn State Materials Research Institute, stating, ‘It allows you to work with polymers really easily in this whole process. So you can have the polymer within the grain boundaries of a structure, which could be 90% ceramic and just 10% polymer in one block, or have the polymer between layers of ceramics.’
The process shares similarities with liquid phase sintering, which typically requires temperatures above 1,000˚C, although Randall states that the CSP differs through ‘exploiting the transient liquid phase, the system in which water is escaping and applying pressure simultaneously allows you to get densification in incredibly fast times – we’re seeing between 15 to 60 minutes – happening around the boiling point of water.’
Much of the CSP stems from existing understanding of liquid phase sintering, and Randall cites among others work from Japan in the 1980s based on hydrothermal sintering. ‘They were asking all of the right questions, but work in a very closed system, and all of the temperatures in their reported data were very high. Sometimes the densities required additional thermal processes, so they never saw what happens at very low temperatures.’
Although the research team has only been working on the process for 12 months, Randall remarked that it has currently produced 50 different compounds, a result that takes advantage of the ‘types of sciences we’re pulling together, anything from biomineralisation, colloidal science and nanomaterials to solvo-chemistry, equilibrium thermodynamics and geo-chemistry, where people worry about dissolution kinetics and chemical weathering important to this process.’
Despite this, Randall speculates that their discovery of the CSP may not be the first. ‘You almost feel this was to be found serendipitously. You can see evidence of this in filamentous rock, or having a cup of tea on a wet day in England and seeing the sugar sintering and forming clumps on the surface. We were surrounded by so many clues that it’s shocking to me that, given the age of the science of ceramics, other people did not catch this. I joke with some of my students that the method must have been burnt up in the Library of Alexandria.’
With a speciality in electronic ceramics, Randall’s team will explore the compound material’s use in solid-state battery technology, but the team also foresee its application in biomedical and architectural use, with Randall noting, ‘I think we need to understand the surface chemistry and surface exchange better, but some of the devices we’re trying to make will have a big impact.’