Talk about tantalizing tidbits of technology! There is a new article available online that reports on research showing that green-density yttrium-stabilized zirconia can be sintered to full density in only a few seconds at 850°C, when subjected to a dc electrical field above a critical threshold. For a sense of what a radical difference this is, traditional sintering of YSZ would require several hours at 1450°C.
So, could there be another radical development in sintering ahead such as microwave-assisted or spark plasma sintering? It’s much too early to tell, unfortunately, and it is not clear to what extent this phenomenon extends to other materials, but it is interesting to ponder the savings in energy costs and production times that might be possible.
The authors, Marco Cologna, Boriana Rashkova and Rishi Raj, acknowledge they now are testing other materials and hope to report on this soon. But, in the meantime, they say their work is starting to provide insights on how other advanced sintering methods, such as spark plasma and microwave-assisted techniques, are cutting sintering costs and time. Their paper is published as an “Early View” article by the Journal of the American Ceramic Society.
The trio’s technique was fairly straightforward. They made dog-biscuit shaped samples from 3 mol% nanograin YSZ. They then sintered samples in a vertical tubular furnace, applying a constant dc voltage, varying temperature and voltage. (See depiction of the apparatus below.)
In the stages of their tests, they encountered a phenomenon I have written about before: accelerated sintering speeds at lower temperatures, dubbed field-assisted sintering or “FAST.” In fact Raj, Di Yang and Hans Conrad had recently published another paper about how low (20 V/cm) dc electric fields could speed sintering and slow grain growth, noting:
“It has not escaped our attention that these small electrical fields, which consume very little power, can lead to huge savings in energy by lowering the processing temperature of ceramics.”
Then they took the tests a step farther and started to increase the electrical field. As the voltage increase, sintering onset temperatures dropped in a fairly expected manner. However, when they conducted a test at 850°C and upped the voltage to 120 V/cm something very unexpected happened: Nearly instantaneously, the 3YSZ sample was completely sintered.
They described this phenomenon as “flash” sintering and say the rates of sintering they encountered at the point where the electric field triggers this cascade effect is three orders of magnitude faster than FAST sintering.
Initially, they thought that the flash sintering events were the result of even smaller grain-growth rates, but SEM and TEM measurements showed that grains in the flash-sintered samples were approximately the same size as those in FAST-sintered samples. Certainly not enough difference was apparent to explain the speed of flash sintering.
When I spoke with Raj, he told me that they are uncertain about what is occurring. He said they suspect it may have to do with enhanced kinetics and that the threshold voltage triggers a rapid increase of temperatures at the grain boundaries, but said that several other possible explanations are possible.
Raj and the other authors describe the findings as a “huge leap” but many questions remain. “About all we can say at this point for certain that it is a new physics process,” Raj says. “It may not be diffusion process. It may not even be sintering. But I expect some new science may come out of it.”
Raj also tells me that they are only beginning to test to what extent other materials can be flash sintered. “We are trying to find out how general this phenomenon is. We have flash sintered AlO2, and are starting to play with other oxides, doped and undoped zirconia and some spinels. We think that flash sintering of silicon carbide may also be possible.”
If this pans out, Raj believes that flash sintering could greatly simplify manufacturing processes. And, besides reduced energy and capital costs, higher productivity and extended tool life, another advantage of short sintering times is that undesirable reactions between zirconia and other components may be avoided.
Raj warns, however that the flash techniques would pose a new paradigm for ceramics manufacturing. “Quick implementation is most likely if the science of this new sintering process is integrated into the design and tooling of manufacturing processes, from the ground up,” Raj says. “The power surge associated with flash-sintering requires special consideration. For example, how will the current to be supplied to the ceramic and how is the transient heat be dissipated? Manufacturers will also want to know if different tool designs and materials are needed, and if non-contacting electrodes can create, for instance, a plasma to provide the necessary current flow.”
Raj acknowledges that this will take development and investment of time and money, but predicts the payback will be considerable. “The question is how long it will be before these new methods are developed and implemented. If history is a guide it would take ten to twenty years,” he says.
This research is being supported by Department of Energy-Basic Energy Sciences.
Interested in hearing more? These results, along with other reports about the use of electric and magnetic fields, will be presented at upcoming MS&T’10 conference that starts next week in Houston, Texas. In particular, check out the symposium on “New Roles for Electric and Magnetic Fields in Processing, Microstructure Evolution and Performance of Energy and Biosciences” that is part of the Process and Product Manufacturing section of MS&T’10.
The subtopics for this symposium and schedule are as follows (locations are all in the George B. Brown Convention Center, Room 371B):
Tuesday, Oct. 19 10:20 am — Microwave Processing (Materials) I
Tuesday, Oct. 19 2:00 pm — Microwave Processing (Materials) II
Wednesday, Oct. 20 8:00 am — Electrical and Magnetic Phenomena Related to Interfaces
Wednesday, Oct. 20 2:00 pm — Microwave Phenomena and Mechanism
Thursday, Oct. 21 8:00 am — Spark Plasma Sintering