Graphene-Infused Foam Provides Dynamically Tunable Path for Thermal Management

What you’ll learn:

How graphene-infused foam has been evaluated as a tunable thermal resistor for passive adjustable cooling.
The modeling that was done to investigate the concept.
The two test fixtures used to measure and verify performance.

Keeping electronics cool or within a desired temperature band despite internal operating conditions and varying ambient environments continues to be a difficult design challenge. Passive techniques often are preferable but have limited range, while active techniques such as forced air driven by controlled fans adds cost, weight, and complexity.

To address this dilemma, a team based at Purdue University devised a patent-pending alternative approach relying on graphene-infused foam that compresses as needed, thus changing its thermal resistance and associated heat flow. Somewhat counterintuitively, this dynamic cooling “mechanism” is both passive and tunable.

Unlike a thermal on-off switch, the foam provides a continuous thermal-tuning range and therefore adapts smoothly to the thermal situation. (Note that graphene is yet another chemical allotrope of elemental carbon along with graphite, diamond, and others, yet it has unique properties and is enabling many very innovative developments; there’s some parallel to silicon in that respect.)

Their wide-range variable thermal resistor is based on highly compressible, open-pore graphene foam composites that function as a thermal switch or regulator, analogous to a variable electrical resistor (Fig. 1). This graphene foam is a commercially available product, built from nanoscopic particles of carbon deposited in a specific pattern, along with tiny entrapped voids of air.

1. (a) A traditional thermal switch only has two possible thermal conductance states and (b) a traditional thermal regulator has a no-linear thermal performance curve and transitions from low to high thermal conductance passively based on device operating conditions. (c) Schematic of the variable thermal resistor concept. (d) Schematic showing the dual function of the device as both thermal switch and regulator as desired based on compressible graphene foam. The solid lines indicate the thermal performance in the “on” and “off” states (with the slopes being the thermal conductance of each state), and the shaded region indicates the region accessible with the continual tuning of the thermal conductance. As the power level or heat flux varies, the device temperature can be regulated through modulation of the foam thickness (along the dashed green, vertical line). Similarly, if a specific heat flux must be regulated, the thickness of the element can be controlled as the temperature varies (along the dashed purple, horizontal line).

Project leader and mechanical engineering professor Xiulin Ruan noted, “Most devices use passive thermal management such as conduction and convection to move excess heat. But those systems aren’t tunable or adjustable, and don’t help at all in cold conditions.”

When this foam is placed between a heat source and a heatsink, it modulates the flow of heat based on its amount of compression. When the foam is fully compressed, it has maximum thermal conductance (its “on” state). If the foam is fully uncompressed, it conducts heat poorly (“off” state)—with proportional conductance in between. The on/off thermal conductance has a large tuning range of nearly an order of magnitude.

Modeling and Testing

For the first step in the project, the team modeled the thermal performance of the graphene foam to assess its heat-transfer properties under different compression factors (Fig. 2).

2. (a) Schematic of sample stack illustrating the direction of heat flow and compression. (b) 2D temperature maps for the fully uncompressed (bottom) and fully compressed (top) states and the corresponding 1D temperature profiles. The reduced temperature difference across the sample indicates an increase of thermal conductance after compression. As illustrated in the 1D profiles (i.e., the lack of a temperature jump at the interfaces), the interfacial resistance between the sample and the reference layers is negligible compared to the thermal resistance of the sample itself.

However, this sort of theoretical analysis and associated modeling still needs real-world, lab-based testing for verification. As a result, they performed a range of quantitative tests with two different fixtures.

First, the Purdue researchers sandwiched a 1.2-mm-thick sample of graphene foam in-between a heater and heatsink (Fig. 3). They next placed the arrangement under an infrared microscope to measure the temperature and heat flow. When fully compressing the foam to a thickness of 0.2 mm, the thermal conductance went up by a factor of eight.

3. (a) Schematic of the IR Reference Bar Setup. The graphene foam is sandwiched between two layers of material with known thermal conductivity (i.e., the reference layers) and the three-layer structure is placed between two adapter bars that span from a heat source to the heatsink during property testing. Both sides have embedded cartridge heaters to achieve the uniform temperature required for calibration of emissivity. During test, the heaters on the hot side and the chiller on the cold side are used to establish a temperature gradient across the reference-sample-reference stack. The IR microscope measures the 2D temperature map of the top surface of the sample stack. The graphene foam is compressed by moving the caliper. The emissivity is calibrated, and a new temperature map is captured by the IR microscope after the compression to measure properties as a function of compression. (b) Image of the three-layer sandwich structure with the graphene foam in the middle and the acrylic of known thermal conductivity on two sides. The commercially available acrylic is coated by graphite to enhance the emissivity during IR measurements.

For the second set, the team used a chamber at Purdue’s Flex Lab that can create specific environmental conditions, and achieved similar results with ambient temperatures from 0°C (32°F) to 30°C (86°F). They placed the graphene foam on top of a heat source to simulate an electronic device and used thermocouples to measure the temperature (Fig. 4).

4. (a) Schematic and (b) photo of the performance test setup. A film heater is used to mimic a device and placed on a block of hard polymer foam insulation to minimize heat dissipation through the baseplate. An aluminum bar (1 × 1 in. in cross-section and 0.8 in. tall) is placed above the heater with six thermocouples to quantify the heat flow. The thickness of the graphene foam, inserted between the aluminum bar and the heatsink, can be controlled by moving the middle plate shown in panel (b). An indicator measures the movement of the middle plate and is related to the thickness of graphene foam. The temperature of the top thermocouple (T4), which is 0.1 in. from the surface, is taken as the device temperature for the calculation of thermal regulation to remove the effect of the added aluminum bar. Three other thermocouples are distributed within the base of heatsink. Another thermocouple, floating in the air, measures the ambient temperature. The whole system is contained within an environmental chamber where the ambient temperature can be adjusted from 0 to 30°C.

“This is important because the thermal switch adjusts to temperatures both inside and outside the device,” said Ruan. “When the ambient is very hot, our thermal switch can transfer heat out to cool the batteries down, and when the ambient is too cold, our thermal switch can turn off heat transfer to keep the batteries warm.” The team concluded that their variable thermal resistor can stabilize the operating temperature of a heat-generating device while the ambient temperature varies continuously by about 10°C or the heat generation rate varies by a factor of 2.7.

The work is detailed in their readable paper with the lengthy title “Wide range continuously tunable and fast thermal switching based on compressible graphene composite foams” published in Nature Communications, along with Supplementary Information. There are also short videos: “The compression process of graphene-PDMS foam in experiment” and “The compression process of graphene foam in molecular dynamics simulations.” In addition, there’s an untitled .gif clip that’s eerily visually reminiscent of the flesh-eating slime featured in many “sci-fi/horror-thriller genre” movies you’ve probably seen.