New Technology to Improve Heat Flow

Purdue University, West Lafayette, IN

Mechanical engineers at Purdue University in West Lafayette IN, have proposed a new technology that, they say, could control the flow of heat the way electronic devices control electrical current, an advance that could have applications in a diverse range of fields from electronics to textiles.

Fig. 1 – Triangular graphene nanoribbons (a) are proposed as a new thermal rectifier, in which the heat flow in one direction is larger than that in the opposite direction. Thermal rectification (b) can be seen in other materials including thin films, pyramidal quantum dots, nanocones, and triangles. (Credit: Purdue University)

Their research uses tiny triangular structures to control “phonons,” quantum-mechanical phenomena that describe how vibrations travel through a material’s crystal structure. Findings in research using advanced simulations show the tiny triangular or T-shaped structures are capable of permitting a greater flow of heat in one direction than in the opposite direction, called “thermal rectification.” (See Figure 1) However, their findings also indicate that thermal rectification is not limited to graphene alone, but could be seen in other materials in structures such as pyramidal, trapezoidal, or T-shaped designs.

Rectification is used in transistors, diodes, and memory circuits central to the semiconductor industry. The new devices are thermal rectifiers that could perform the same function but use phonons instead of electrical current.

Using an advanced simulation method called molecular dynamics, the team of researchers demonstrated thermal rectification in asymmetric graphene nanoribbons. Molecular dynamics simulations can be used to simulate the vibrations of atoms and predict the heat flow in a material.

While the researchers had already published a paper four years ago proposing asymmetric graphene nanoribbons as a thermal rectifier in research using the molecular dynamics simulations, they did not know the mechanism behind thermal rectification at that time. Their new findings show that this mechanism works by restricting vibrations as they travel through the small lateral direction of an asymmetrical structure. They demonstrated that other asymmetric materials, such as asymmetric nanowires, thin films, and quantum dots of a single material can also be high-performance thermal rectifiers, as long as you have lateral confinement.

Thermal rectification is not seen in larger triangular-shape structures because they lack lateral confinement. In order for lateral confinement to be produced, the cross section of the structure must be much smaller than the “mean free path” of a phonon, or only a few to hundreds of nanometers depending on the material, they said.