A team of researchers from the University of Colorado Boulder has discovered a new method to increase the efficiency of thermoelectric materials used in solar panels and air conditioning machines.

Thermoelectric materials are interesting components that can be used to generate electricity or generate differences in temperature by taking advantage of a phenomenon known as the thermoelectric effect. Scientists discovered this phenomenon in the 1800s. When a thermoelectric material is exposed to a temperature difference with one side of the material exposed to heat and the opposite side exposed to cold, an electric current is produced. When the process is reversed and an electric current is applied to a thermoelectric material, one side cools down while the other side heats up.

Thermoelectric materials have a wide variety of applications. They are used to generate electricity from solar energy or even other heat sources. They are also used in air conditioning devices. However, the current crop of thermoelectric materials comes with certain limitations. The biggest problem is that these materials conduct both heat and electricity. Since a temperature difference is important to the production of electricity, the fact that the material can conduct heat gradually interferes with the temperature difference thereby reducing the efficiency of the system. Scientists have been looking for ways to get around this problem. 

"Until 20 years ago, people were looking at the chemistry of the materials," said Mahmoud Hussein, an assistant professor of aerospace engineering sciences from the University of Colorado Boulder. "And then nanotechnology came into the picture and allowed researchers to engineer the materials for the properties they wanted." 

Scientists have already been using nanomaterials to decrease a thermoelectric material's capacity to conduct heat while maintaining its capacity to conduct electricity. However, this is easier said than done and previous experiments have led to both reduced heat conductivity and electrical conductivity. 

The researchers from the University of Colorado Boulder were able to come up with a new method of using nanotechnology to overcome the limits of the current generation of thermoelectric materials. To achieve this, the scientists used an array of nanoscale pillars built on top of the surface of a thermoelectric material. This resulted in a new type of material that the researchers have called a "nanophonic metamaterial."

The nanoscale pillars are able to reduce the material's heat conductivity by vibrating at certain frequencies. Since heat is transmitted though vibrations also referred to as phonons, the vibrations of the tiny pillars can interact with the phonons effectively slowing their rate of propagation. While the nanoscale pillars interfere with phonon propagation, scientists believe that the nanostructures will not have any effect on the flow of an electric current. 

"If we can improve thermoelectric energy conversion significantly, there will be all kinds of important practical applications," said Hussein. 

The researchers published their findings in the online journal Physical Review Letters.