At the National Institute of Standards and Technology (NIST), a group of researchers headed by Kris Bertness and her colleagues developed a new fabrication technique for converting heat into electricity.
The First of Its Kind in Energy Conversion
The technique involves coating the top of a silicon wafer with hundreds of thousands of microscopic columns made from gallium nitride. Then the silicon layers are removed from the lower part of the wafer until it only reveals a thin sheet of the material.
Allowing the pillars and the silicon sheet to interact will slow heat transfer in the silicon material. Thus, more heat is converted into electric current. The study is inspired by a phenomenon discovered by German physicist Thomas Seebeck in the early 1820s. This phenomenon later called the Seebeck effect, shows how the difference in the temperature of two different electrical conductors or semiconductors can produce a voltage between the two substances.
Ideally, the Seebeck effect can be used to recycle heat lost during the process. However, the material must be a poor conductor of heat to keep the difference in temperature and, at the same time, a good conductor of electricity to carry out the conversion process effectively. The problem with this idea is that most substances' heat and electrical conductivity are directly proportional to each other.
To address this problem, theorist Mahmoud Hussein from the University of Colorado tried to separate heat and electrical conductivity by covering a thin membrane with nanopillars.
Hussein collaborated with Bertness, and their colleagues successfully uncoupled the two properties in the silicon sheet. This is a milestone in physics as the heat conductivity of a material was separated from its electrical conductivity for the first time. The team also reports that they could lower the silicon sheet's heat conductivity without contradicting the Seebeck effect.
How the Novel Device Works
In most solids, such as silicon, chemical bonds limit the motion of atoms, so they are not allowed to transmit heat freely. As a result, heat transfer is carried out by phonons or the vibrational motion of the atoms. In the device developed by Bertness, phonons are carried by gallium nitride nanopillars and silicon sheets. Interacting with the nanopillars' vibrations slows down phonons' travel in the silicon sheet. As a result, it becomes hard for heat to pass through the material, and thermal conductivity is reduced due to the increased temperature difference between both ends. Most importantly, this phonon interaction process happens while the silicon sheet's electrical conductivity remains the same.
Upon perfecting the fabrication method, it will be possible to wrap the silicon sheets around heat-generating materials such as steam or exhaust pipes. The heat emitted by these materials can be converted to electricity and used to power nearby devices or delivered to a power grid instead. The researchers hope their discovery can be applied on a larger scale to benefit various industries.
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