Thermal management is becoming a very important issue in the development of electrified components because heat will limit their effectiveness and durability. A case in point is battery electric vehicles where reducing the operating temperature will extend operating range and improve efficiency.

The customary approach for cooling a stationary or moving object is to use air conditioning, but this may not reduce the temperature in the environments surrounding that object. Xiulin Ruan, professor of mechanical engineering at Purdue University in West Lafayette, Ind., says, “While effective in cooling a specific area, air conditioning emits heat into the outside air, which stays on the earth and unfortunately increases the temperature in the environment surrounding the object.”

Providing additional ways to cool electrical devices can potentially lead to better thermal management. The challenge is how to deflect the intense amount of heat generated by the sun. Ruan says, “The sun sends heat at a temperature of 5,800 K that represents an energy flux of more than 1,000 watts per square meter. This has a significant impact on the surface of the earth, which has an average temperature of 300 K.”

An alternative strategy for heat dissipation is to utilize a passive cooling technology known as radiative cooling. Ruan says, “Radiative cooling involves how a surface both responds to the solar radiation and emits infrared heat. A radiative cooling surface is very effective at reflecting solar irradiation into deep space, which effectively minimizes solar heating. Meanwhile, a radiative cooling surface emits heat to an atmospheric transparent window between 8-13 microns, which is in the infrared range. If the surface is highly reflective, then the surface loses more heat than is absorbed, which can lead to a net cooling of the surface. The surface can, in turn, cool down the surrounding environment and the earth.”

Finding a suitable white paint that can cover the exterior surface of an object offers a significant opportunity to dissipate heat and assist with thermal management. Commercially available white paint primarily uses titanium dioxide as the pigment. Ruan says, “Titanium dioxide reflects
80%-90% of the sunlight, but that still means the remainder (10%-20%) is absorbed into an object as heat. This pigment also absorbs ultraviolet light, which does not help to reduce the temperature of an object.”

Ruan and his colleagues developed a paint based on barium sulfate that reflects up to 98.1% sunlight. He says, “Barium sulfate has a good bandgap and does not absorb ultraviolet light. The problem is that thickness of the barium sulfate-based paint must be 400 microns after drying. This is more than double the thickness of a commercial paint based on titanium dioxide. Such a paint will be effective to reduce the heat absorbed in stationary objects (such as buildings) and will be more durable, but the added weight to a moving object makes it nonideal.”

A new approach has now been taken to devise a lighter weight paint that is suitable for use on moving objects.

Hexagonal boron nitride
In an effort to produce a paint that can be used in transportation applications, the researchers turned to using hexagonal boron nitride as a pigment. Ruan says, “We found a number of attractive characteristics with boron nitride. This material has a white color with good pigment morphology, is lightweight and does not absorb ultraviolet light. The refractive index is nearly as ‘‘high as titanium dioxide and better than barium sulfate.”

When hexagonal boron nitride is introduced into a paint formulation that displayed a thickness of 150 microns, a solar reflectance of 97.9% was obtained with a sky window emissivity of 0.83.

Ruan says, “Hexagonal boron nitride is a layered material where non-spherical platelets slide by each other. The asymmetric characteristics of these particles leads to more effective solar reflection as compared to the spherical particles present in barium sulfate and titanium dioxide-based paints.”

Figure 2 shows a schematic comparing how conventional paints reflect sunlight on the left versus the hexagonal boron nitride-based paint on the right.


Figure 2. The difference in the ability of commercial paints versus the lightweight paint in reflecting solar radiation is shown. Lightweight paint emits more infrared heat than it absorbs reducing the surface below ambient temperature making it attractive for assisting with the thermal management of mobile devices such as electric vehicles. Figure courtesy of Purdue University.

The researchers prepared paint with a 60:40 volume ratio of hexagonal boron nitride to the acrylic binder. Ruan says, “This ratio represents the highest treat rate for hexagonal boron nitride that can be prepared without sacrificing the paint’s mechanical properties. If the ratio is increased, then the paint would become too brittle and readily crack.”

An added benefit with the hexagonal boron nitride-based paint is the presence of pores. Ruan says, “Once the solvent used, dimethylformamide, evaporated, we noticed that pores formed within the paint. The pores assist with the paint’s ability to reflect sunlight and their presence reduces weight.”

Outdoor testing of the paint was performed for three days at temperatures ranging between 20-30 C, an average relative humidity of 73% and an average dew point of 19 C. The temperature of the aluminum substrate averaged 6-8 C lower than ambient over the three-day period and 5-6 C lower during daylight hours. This temperature difference may make a significant difference in assisting with the thermal management of mobile devices such as battery electric vehicles.

The light weight of the hexagonal boron nitride-based paint makes it very attractive for transportation applications. Ruan is working to make the paint even more sustainable. He says, “Our next objective is to eliminate the volatile organic compound (VOC) solvent and develop a water-based paint. We also intend to improve the ability of the paint to be self-cleaning.”

Additional information can be found in a recent article¹ or by contacting Ruan at ruan@purdue.edu.

REFERENCE
1. Felicelli, A., Katsamba, I., Barrios, F., Peoples J., Chu, G. and Ruan, X. (2022), “Thin layer lightweight and ultrawhite hexagonal boron nitride nanoporous paints for daytime radiative cooling,” Cell Reports Physical Science, 3 (10), 101058.