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[Image above] Credit: Andrew Sutherland; Flickr CC BY-SA 2.0

Sintering is literally and figuratively a hot topic.

Sintering—the process of using heat or pressure to compact individual particles together into a densified material, without melting said material—is an integral technique in the world of ceramics and materials science.

Various sintering techniques have been used to produce graphene-based bone replacements, complex spinel components, and all kinds of ceramics.

But conventional sintering is a time- and energy-intensive process. So much recent work has focused on developing and perfecting sintering techniques that can reduce the energy needed to densify ceramic materials. We’ve reported on exciting developments with flash sintering, flash spark plasma sintering, and cold sintering, for instance.

One approach to make the process more efficient is to use smaller starting particles, whose increased surface area promotes sintering. But, those small particles also tend to clump together and prevent homogenous, even densification during sintering—which is undesirable if you want to make a high-quality, uniform material.

Now, researchers at Jožef Stefan Institute (Ljubljana, Slovenia), the National Institute of Chemistry (Ljubljana, Slovenia), and Stockholm University (Stockholm, Sweden) have developed a new method to rapidly and evenly densify nanoceramics, offering incredible potential to save a lot of time and energy in sintering processes.

The team developed a process to sinter 3 mol.% ytttria-stabilized tetragonal zirconia (3YSZ) particles into an evenly dense zirconia nanoceramic in just 2 minutes at 1,300ºC.

The key to their success, where other rapid sintering methods have failed, is that the scientists took a step-wise approach, starting with primary crystallites of 3YSZ and first agglomerating them together into larger secondary grains.

While these grains weren’t quite the final structure that the scientists were going for, secondary grain formation served a different goal: Agglomerating the primary crystallites into secondary larger grains allowed the nanoparticles to pack together more orderly and homogenously, avoiding the clumping problem.

Then, rapidly heating those larger grains rearranged them, reducing pores and forming a uniformly densified material. For this step, the scientists used a spark plasma sintering set-up to deliver electromagnetic radiation to heat the secondary grains, prompting them to agglomerate, coalesce, and slide together.

“In this way nanoceramics exhibiting 91% of theoretical density can be prepared at a heating rate of 250°C/min and with only 2 minutes of dwell time at 1,300°C,” the authors write in the open-access paper, published in Scientific Reports.

Through careful detailed analysis, the team traced the ability of the 3YSZ powder to densify so well during rapid sintering to the presence of an amorphous-like intergranular film, rich with yttrium, that acts like a layer of liquid at the grain boundaries.

The scientists hypothesize that the film enhances sintering by creating hydrostatic pressure at the grain boundaries, pushing those boundaries to slide and rearrange. This rearrangement reduces pores within the material, densifying the resulting nanoceramic.

But is a film between the grains really enough to rearrange their boundaries?

According to the scientists, they calculated that the film can exert pressure exceeding 100 MPa (assuming 100-nm grains), “which can be more than one order of magnitude higher than the typical pressures encountered with micron-sized powders, representing a considerable force—high enough to induce rearrangement to the extent observed in the present study.”

The team’s findings help illuminate the mechanisms of densification and grain-growth that take place during rapid sintering methods, offering important insights that could help reduce energy needs and increase the quality of sintered materials. Ultimately, the strategy could offer a low-cost, pressureless process to rapidly sinter nanoceramics, the authors say.

For more information, see the open-access paper, published in Scientific Reports: “The agglomeration, coalescence and sliding of nanoparticles, leading to the rapid sintering of zirconia nanoceramics” (DOI:10.1038/s41598-017-02760-7).