Two months ago, I wrote about how North Carolina State University’s Hans Conrad had apparently discovered that sintered ceramic materials could be deformed and shaped by applying an electric field. According to Conrad, the field interacts with the charges at the grain boundaries and make it easier for the crystals to slide against each other along these boundaries.
Now, Conrad has been again been tinkering with electric fields and ceramics, this time targeting the sintering process itself, and, once more, apparently has come up with some startling conclusions.
In brief, Conrad and his research team introduced an 60 Hz alternating current electric field during sintering of materials made of zirconia. Compared to normally sintered zirconia, the grain size of the ceramics fired under the influence of this electric field was reduced by 63 percent. They were also able to eliminate porosity in the material at 1,250°C rather than the expected 1,500°C.
In their experiments, Conrad’s team created grains with a diameter of 134 nm compared to the 360 nm diameter grains produced using conventional sintering methods.
The team also found that similar but less pronounced effects could be caused by a dc electric field (porosity eliminated at 1,400°C, grain diameter of 217 nm). Both ac and dc fields were 13.9 volts/cm.
“We found that the use of a small electric field – with a current of only six-tenths to eight-tenths of an amp per centimeter squared – can result in improved sintering rates with much finer grain size,” Conrad says. In other words, ceramics manufacturers can make their products more quickly and cheaply by using an inexpensive electric field – and make their product stronger as well.
“You don’t use much energy, and you put it right at the atomic site where it is needed – rather than using more energy to create higher temperatures in a kiln, which is less efficient,” Conrad says. “If you want to make a strong ceramic, you want to eliminate porosity and keep the grain size as small as possible. And you want to do it at the lowest cost – which means using the smallest amount of energy and doing it at the lowest temperature at the fastest rate possible. Using an electric field achieves all of these goals.”
The phenomenon is discussed in “Enhanced sintering rate of zirconia (3Y-TZP) by application of a small AC electric field,” which will be published in a forthcoming issue of letters-oriented journal, Scripta Materialia. The paper’s lead author actually is Di Yang, a senior research associate at NC State who works with Conrad.
For Conrad and Yang, the next steps are to gauge how the electric field’s frequency and strength affect the outcomes, and also to test the electric field on other ceramic materials.