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Electric fields create mobile hot spots in heated alkali silicate glasses

By Lauren Borja April 24, 2019
glass softening
Series of infrared images captured from video of an alkali silicate glass sample with 200 V applied for a furnace temperature of 320°C at (a) 41 seconds; (b) 43 seconds, when localized heating during electric field induced softening occurs; (c) 45 seconds, when a flash of light at the anode following intense heating is observed; and (d) 54 seconds, during overall glass softening. The anode is located at the top of the sample and the temperature scale ranges from 250°C to 1300°C. Credit: Nature Publishing Group.

A research collaboration between the materials science group of Himanshu Jain at Lehigh University and Nicholas J. Smith at Corning Incorporated has captured the movement of hot spots formed in glass simultaneously subjected to heating and an electric field. Published recently in Scientific Reports, these results could lead to the development of new materials processing methods.  

Jain’s group investigates electric-field-induced softening (EFIS) in glasses, where the temperature needed to soften glass is drastically reduced by simultaneously applying an electric field during heating. According to Joule’s law, the energy imparted to the glass by applying an electric field should lead to a homogeneous increase in the internal temperature of the sample. By taking advantage of this enhancement in internal temperature, the temperature of the furnace can be reduced.      

The study reveals several anomalies that occur during EFIS. “We observed an order of magnitude higher increase in the sample temperature that could not have been predicted by Joule’s law,” Jain says. Using an infrared thermal imaging setup at Corning, Charles T. McLaren and Craig Kopatz subjected several standard alkali silicate glasses to a DC current while being heated to temperatures between 300°C and 400°C. This allowed the researchers to image changes in temperature over the entirety of the glass sample during the process.   

These infrared images revealed that, instead of increasing homogenously across the sample, extreme temperatures were produced in localized regions in the glass. These hot spots reached temperatures greater than 1000°C when the furnace was maintained at a temperature of only 320°C. The temperature and location of the hottest point in the sample migrated during the experiment.     

The researchers also investigated EFIS using an applied AC current. In these experiments, the temperature of the sample also increased by several hundred degrees Celsius over the temperature of the furnace but without the appearance of a localized hot spot. According to Jain, “The heating is more uniform using AC current,” which could be better for industrial processing where a uniform temperature profile is typically desired.   

To further understand the observed effects, the researchers modeled the results using finite element analysis (FEA). Jain says that these preliminary FEA calculations “added an understanding of the wandering nature of the hot spot.” The modeling revealed that the migration of ions in the glass led to depleted regions whose temperature increased dramatically. These depleted regions are highly resistive, so the current would divert to, and therefore increase in, the surrounding areas. This increase in current would then create another depleted region, which would then increase dramatically in temperature. When AC current was used, however, the alternating polarity of the electric field did not create a single depletion region. As a result, the heating effect for AC-EFIS was more uniform.   

While the effect of the depletion layer had been suggested in previous studies, it had not been clearly observed before the recent work. “This work explained the effect of the depletion layer and the mechanism behind the process,” says Mattia Beisuz of Queen Mary University of London.     

“In the future, we hope to build a complete quantitative description of EFIS in glass,” Jain says. He hopes that his work could also be used to explain similar phenomena observed in other materials processing methods, such as flash sintering of ceramics. Flash sintering is a process where the temperature and time required to form a solid ceramic from a constituent powder is reduced by applying an electric field. “In the two cases, flash sintering and [EFIS in] glasses, we find similar signatures,” says Rishi Raj of the University of Colorado, Boulder. Although he is not connected with the recent publication in Scientific Reports, Raj’s work on flash sintering inspired Jain’s work on EFIS in glass and the two have collaborated closely on it in the past.     

Jain also wants to develop new glass processing methods using EFIS by creating topographies in glass for specific applications. For example, an electric field could be used to pattern the surface of the glass by superimposing electric fields onto a mechanical press. “An electric field is more precise than a mechanical field,” says Jain, “and this could allow the field to form topologies that could not be achieved with the standard processing methods, while saving energy.”   

Read the article in Scientific Reports.