A New Solid-State Material Could be Used in High-Temperature Thermal Diodes

April 13, 2021 by Nicholas St. John

Thermal management is a challenge throughout most, if not all, fields in engineering, on the macro, micro and nanoscopic scales. A research team has found a material that can effectively and efficiently take on this problem.

Approximately 90% of the world’s energy use involves the generation and/or manipulation of heat. This makes the complex issue of thermal management and the transporting of thermal energy extremely important for energy innovation. The most ideal materials in these applications should exhibit very fast transitions between low and high thermal conductance states. According to a news release from the Chinese Academy of Sciences, a joint team led by Professors Tong Peng and Zhang Yongsheng from the Institute of Solid State Physics (ISSP) of the Hefei Institute of Physical Sciences (HFIPS) has found this ideal material.

The new material that was found is a nickel-iron-sulfide (chemical composition Ni1-xFexS) that shows an up to 200% change of thermal conductivity, reaching a maximum of 12 W/m, within a temperature interval of only 40 Kelvin. 

Professor Tong stated that this thermal conductivity change exceeds that of the other current thermal regulation materials, and also, the operating temperature can be adjusted by changing the concentration of iron in the sulfide. The team also observed a large entropy change of 50 J kg-1 K-1 and temperature change of approximately 10 K due to an exerted pressure of 100 MPa. This kind of behavior is known as the Barocaloric (BC) Effect, where compression results in a change in entropy and temperature within a material. This kind of property is illustrated in the picture below.


Plot Showing the Thermal Conductivity Spike in the Sulfide Material based on temperature change. Image Courtesy of the Chinese Academy of Sciences
Plot Showing the Thermal Conductivity Spike in the Sulfide Material based on temperature change. Image Courtesy of the Chinese Academy of Sciences


Furthermore, the theoretical calculations insinuate that the cause for the abrupt change is due to the changing of the electronic structure of the sulfide at the first-order phase transition.

While the thermal properties are fantastic, the material is very brittle, and so the group looked into how to make it more machinable. They found that the addition of silver would improve this, as well as the thermal cycling stability (rapidly changing the temperature of the material from one extreme to the other), and both of these changes make it much better in practical use. On top of all of this, the raw materials are environmentally friendly, and the sulfides have a hexagonal structure that is very easy to synthesize.

This new material can be used to create thermal diodes. Analogous to electrical diodes, they only allow heat to flow in only one direction. One can then also vary the iron concentration to maintain optimal temperature against the environment temperature variation. Furthermore, since this material shows the strongest BC Effect that we know of today, more than doubling that the current popular refrigerant, Vanadium Dioxide (VO2) according to the team’s published abstract, can be an optimal solution as a refrigerant for solid-state cooling applications.

This new Sulfide material can have novel applications throughout thermal management technology. By outperforming all current BC Effect materials, while being environmentally friendly and easy to produce, is quite an improvement.