Engineers manufacture continuously adjustable thermal regulators for batteries and electronic devices
(Nanowerk News) Battery safety and performance in electronic devices and systems such as battery thermal management, space conditioning, vehicle thermal comfort, and thermal energy storage can be improved thanks to continuously adjustable thermal regulators built at Purdue University School of Engineering.
Xiulin Ruan and Amy Marconnet have invented a patent-pending, solid-state, continuously tunable thermal device based on a compressible graphene foam composite. The device can dissipate heat, protect against cold and function over a wide range of temperatures. Ruan is a professor at the School of Mechanical Engineering. Marconnet is an associate professor in the School of Mechanical Engineering and a Perry Academic Excellence Scholar; he also has an honorary appointment at the School of Materials Engineering.
“As batteries and electronic devices become more powerful, managing heat becomes an even more crucial issue,” said Ruan. “We all know that humans have a narrow temperature range in which to live comfortably, and that is why we wear shirts in summer to stay cool and coats in winter to stay warm. Likewise, batteries and electronic devices have a narrow temperature range to function properly, and are even more ‘picky’ than humans.”
Marconnet said, “Batteries perform poorly when they overheat. As it heats up, a chemical reaction occurs causing it to heat up faster. The development of this unstable reaction is called ‘thermal escape’ and can even cause fires and explosions. On the other hand, if the temperature is too low, the battery will suffer internal damage. This leads to poor performance such as a shorter driving range for electric vehicles and less time on the phone.”
A conventional thermal switch, analogous to an electrical switch that moderates current flow, adjusts the path of battery heat dissipation simply by changing the conduction between the on and off states. Ruan said the thermal regulator Purdue invented improves this technology by changing the thickness of the material inside the regulator, which helps the battery constantly adapt to different climates and seasons.
“Unlike people who can wear a coat when it’s cold or a swimsuit when it’s warm, batteries wear the same ‘clothes’ everywhere with conventional thermal management technologies,” says Marconnet. “By using a thermal regulator to adjust the path between the battery and the environment, we can protect the battery in cold conditions and make it very easy to dissipate heat in warm conditions.”
The commercially available compressible graphene foam that Ruan and Marconnet used is constructed from nanoscopic carbon particles deposited in a specific pattern with tiny air voids in between. When not compressed, the foam acts as an insulator; air pockets keep heat in place. When compressed, air escapes and heat is completely transferred. The amount of heat transfer can be adjusted precisely depending on how much the foam is compressed.
Marconnet and Ruan measured the thermal conductance of foam at Purdue’s Birck Center for Nanotechnology. They sandwiched a 1.2-millimeter-thick sample of graphene foam between the heater and the heat sink, and placed the system under an infrared microscope to measure temperature and heat flow. When they actually compressed the foam to a thickness of 0.2 millimeters, the thermal conductance went up by a factor of 8. They also conducted an experiment in a room at Purdue’s Flex Lab that could create certain environmental conditions and achieve similar results with an ambient temperature of zero degrees Celsius. (32 degrees Fahrenheit) to 30 degrees Celsius (86 degrees Fahrenheit).
Ruan said the patent-pending regulator was originally made for electric vehicle batteries, but there are other applications.
“The same approach can be applied to sensors and detectors for scientific or industrial applications that need to be maintained at the right temperature, as well as electronic devices in various applications,” said Ruan. “It can also help maintain suitable temperatures for space vehicles, which face harsh environments with extremes of heat and cold.”
Ruan and Marconnet’s research is published in two peer-reviewed journals. Battery app research published in Nature Communications (“wide continuously adjustable range and fast thermal switching based on compressible graphene composite foam”); fundamental research published in ACS Applied Materials & Interfaces (“Unconventional and Dynamic Anisotropic Thermal Conductivity in Compressed Flexible Graphene Foam”).
