Electrically heated silicate glass seems to defy Joule's first law

The discovery that, under certain conditions, the electrically heated silicate glass defies a long-recognized law of physics and known as Joule's first law should interest a wide range of scientists, engineers and even the big world. public, according to Himanshu Jain, Diamond's distinguished president. of the Department of Materials Science and Engineering at Lehigh University.

The English physicist and mathematician James Prescott Joule laid the foundation for electric heating in 1840. Joule demonstrated that heat is generated when the electric current passes into a resistance. His conclusion, known as Joule's first law, simply states that heat is produced in proportion to the square of an electric current flowing through a material.

"This has been verified time and time again on homogeneous metals and semiconductors that heat uniformly, as does an incandescent light bulb," says Jain.

He and his colleagues, including Nicholas J. Smith and Craig Kopatz, both of Corning Incorporated, and Charles T. McLaren, a former Ph.D. Jain student, now a researcher at Corning – have written an article published today in Scientific reports this details their discovery that the homogeneous and electrically heated homogeneous silicate glasses seem to defy Joule's first law.

In the document entitled "Development of a very inhomogeneous temperature profile in electrically heated alkali silicate glasses", the authors write: "In contrast to electrically conductive metals and semiconductors, ion-conduction glass heating becomes extremely inhomogeneous with the formation of an alkali at the nanoscale The depletion zone, such as the glass melts near the anode, evaporates even while remaining solid elsewhere. "

"In our experiments, the glass got warmer by more than 1,000 degrees Celsius near the positive side than in the rest of the glass, which was very surprising considering the fact that the glass was totally homogeneous to begin with," says Jain. . "The cause of this result is the modification of the structure and chemistry of glass on the nanoscale by the electric field itself, which then heats this nano region much more strongly."

Jain says that Joule's classic physics law enforcement needs to be carefully re-examined and adapted to reflect these findings.

These observations reveal the origin of glass softening by induction induced by an electric field. In a previous article, Jain and his colleagues reported the phenomenon of electric field induced softening. They demonstrated that the softening temperature of heated glass in an oven could be reduced by a few hundred degrees Celsius simply by applying a 100 volt sample to an inch of thickness.

"The calculations did not work out to explain what we thought was a simple standard Joule heater," says Jain. "Even under very moderate conditions, we have observed glass fumes requiring a temperature several thousand degrees higher than that provided by Joule's law!"

The team then undertook a systematic study to monitor the temperature of the glass. They used high resolution infrared pyrometers to map the temperature profile of the entire sample. The new data as well as their previous observations have shown that the electric field dramatically changed the glass and that it was necessary to modify the application of Joule's law.

The researchers believe that these works show that it is possible to produce heat in a glass on a much finer scale than by the methods used up to now, or even at the nanoscale. This would then create more precisely than previously new structures and optical and other complex devices on the glass surface.

"In addition to demonstrating the need to qualify Joule's law, the results are critical to the development of new technology for the manufacture and manufacture of glass and ceramic materials," Jain said.

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More information:
Charles T. McLaren et al, Development of a very inhomogeneous temperature profile in electrically heated alkali silicate glasses, Scientific reports (2019). DOI: 10.1038 / s41598-019-39431-8