You are browsing the archives of Rishi Raj.

Credit: Marco Cologna & Rishi Raj, Univ. of Colorado.

Words such as “novel” and “revolutionary” get thrown around fairly frequently in science and engineering circles, but the still-young technique of flash sintering, in theory, could deliver a broad revolution in the preparation of ceramic materials, and it appears it will soon be put to a significant manufacturing test by a UK-based company, Ceram.

If you haven’t heard of flash sintering before, you are certainly not alone. Aside from a (growing) body of papers in ACerS’ journals (as well a few other peer-reviewed materials journals), posts in this blog and a feature story in this month’s ACerS’ Bulletin, not a lot has been written as yet in the broad scientific and engineering media about flash sintering. I am not sure why that is. Perhaps it is because sintering and kiln technology have an old fashion ring to them and do not carry the high-tech cachet that accompanies novel electronics, biomedical, energy and defense-oriented glass and ceramic research.

But, I would argue that flash sintering—if it continues to pan out as it is scaled up—could be one of the most profound and disrupting developments in materials in the last 50 years (if not millennia, given how long mankind has been trying to fire ceramic objects or melt glass), at least from an energy-consumption point of view. And, if you have ever seen even modern, relatively energy-efficient ceramic production lines (making, for example, tiles, whiteware, refractory brick, construction brick, etc.), you know the current process still requires long heating (sometime over a day) and the use of energy-intensive tunnel or massive box kilns.

But flash sintering may change nearly all of that.

For the sake of simplicity, you can think of flash sintering as a traditional sintering/kiln process, but with special twist: The heating takes place in the presence of an electrical field. Surprisingly, because of the effects of the electrical field, as the ceramic object reaches a critical (and relatively low) temperature, it suddenly sinters in a few seconds rather than hours and hours. There is a lot of research and speculation about the exact mechanisms that allow flash sintering to occur, but part of the phenomenon likely has to do with the electrical field allowing the ceramic grains to align and slide past each other. A defect avalanche mechanism might also enhance diffusion. Oddly, there is also a photoemission effect that has been detected, which seems to also signify that the formation of electron hole pairs plays of role.

Regardless of the mechanism, the good news is that flash sintering has been tested many times on relatively small scales and appears to work on nearly every type of ceramic material. (It also seems to be an effective way to rapidly melt glass—but that’s a story for another day.)

Despite a lot of initial (and not inappropriate) leeriness and “too good to be true” thinking when reports of flash sintering first appeared, the caution has evolved into general enthusiasm among ceramic scientists and engineers, and some of that enthusiasm has started to spill over into the manufacturing community. That is because the potential energy and CO₂ emissions savings from flash sintering in the ceramics and glass sectors is staggering.

Regarding manufacturing, when I interviewed University of Colorado’s Rishi Raj for my Bulletin story, he mentioned that he is working to scale-up the technology with Ceram, a subsidiary of British Ceramics Research and a well known international company focused on materials testing, analysis and consultancy. Raj predicted that a place to start introducing flash sintering into the business side of things might be to develop a new kiln system for ceramic tile manufacturing, where energy costs are high and business competition is intense. He mentioned that one challenge would be developing a non-contact electrode system to provide the electrical field.

The non-contact electrode challenge must have been solved because the project with Ceram now appears to have started. A few days ago, the Westland (UK) version of TheBusinessDesk.com reported that at Ceram, “Work has started on a kiln that is to be the basis for technology that could cut energy costs for ceramics firms by up to 30 percent.”

The same story says Ceram is calling the effort the Low Energy Firing Project and describes it as an “80-feet-long commercial-scale kiln.” It also reports that construction of the new kiln is supposed to be completed this month, and testing is to begin in May. It goes on to say, “[T]he first commercial-scale results should be available before the end of the year.”

The story intriguingly quotes David Pearmain, the LEFP project manager, who says, “The potential of this work is really exciting. We think we can reduce firing times as well as temperatures, so there could be very, very significant advantages for the sector.”

I am curious how Ceram came up with its projected 30 percent savings figure, and I suspect that is on the conservative side. I have reached out to officials at Ceram for more details on the LEFP and hope to have more details next week.

Share

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 AlO_2, 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

Share

In recent years, the Materials Science & Technology conference has become the broadest materials-oriented meeting in the United States. The 2009 MS&T conference - to be held Oct. 25-29 in Pittsburgh – will feature a novel symposium organized by Rishi Raj, with assistance from Rustum Roy and Dinesh Agrawal. Guest blogger Raj provides the following information about the symposium and encourages potential presenters to submit abstracts as part of MS&T’s call for papers.

“New Roles for Electric and Magnetic Fields in Processing, Microstructure Evolution, and Performance of Material”
Microwave sintering, spark–plasma–sintering, creep, superplasticity, grain growth and  phase–transformation are examples of  the strong influence of electrical and magnetic fields on processing, microstructure evolution and properties of materials. This symposium will provide an impetus to a new era of materials science where mechanical, chemical and electrical driving forces are coupled into a unified framework to explain dramatic new phenomena in the MS&T-world’s most significant materials.

The symposium addresses experimental findings, engineering implications, and fundamental mechanisms of these new transformative discoveries. It includes all materials from polymers, to ceramics, metals,  and semiconductors. The symposium is particularly interested in interfacial reactions in materials systems such as those used in batteries, fuel cells, and hydrogen production from water.   - Rishi Raj

Share