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John O | March 2019

Scientists demonstrate that electrically-heated silicate glass defies long accepted law in physics


By Josh Perry, Editor
jperry@coolingzone.com

 

Researchers at Lehigh University (Bethlehem, Pa.) demonstrated that applying a direct current field across common silicate glass reduces its melting temperature and enables the glass to be shaped with greater precision than heat alone, according to a report from the university.

 


Charles T. McLaren (left), with Himanshu Jain, says applying a direct current field across glass also reduces its melting temperature and makes it possible to shape glass with greater precision than can be done using heat alone. (Lehigh University)

 

This research showed that under certain circumstances silicate glass defies Joule’s first law of heating, which states that the heat is produced in proportion to the square of an electrical current passing through the material. Silicate glass is used in a number of applications from display screens to medical treatments and drug-delivery methods to the fibers that power the internet.

 

Adding heat to homogenous silicate glass created nanoscale regions where the glass melted near the anode to the point of evaporation and remained solid in other sections.

 

Infrared imaging showed that seconds of the glass were 1,000 degrees warmer than the rest of the sample indicating that the structure of the glass had been changed at the nanoscale and those regions were heated to a higher level than the rest of the glass.

 

“The team then undertook a systematic study to monitor the temperature of glass,” the article explained. “They used high-resolution infrared pyrometers to map out the temperature profile of the whole sample. New data together with their previous observations showed that electric field modified the glass dramatically and that they had to modify how Joule's law can be applied.”

 

The research was recently published in Scientific Reports. The abstract stated:

 

“According to Joule’s well-known first law, application of electric field across a homogeneous solid should produce heat uniformly in proportion to the square of electrical current.

 

“Here we report strong departure from this expectation for common, homogeneous ionic solids such as alkali silicate glasses when subjected even to moderate fields (~100 V/cm). Unlike electronically conducting metals and semiconductors, with time the heating of ionically conducting glass becomes extremely inhomogeneous with the formation of a nanoscale alkali-depletion region, such that the glass melts near the anode, even evaporates, while remaining solid elsewhere. 

 

In situ infrared imaging shows and finite element analysis confirms localized temperatures more than thousand degrees above the remaining sample depending on whether the field is DC or AC. These observations unravel the origin of recently discovered electric field induced softening of glass.

 

“The observed highly inhomogeneous temperature profile point to the challenges for the application of Joule’s law to the electrical performance of glassy thin films, nanoscale devices, and similarly-scaled phenomena.”

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