A flexible thermal conductor has been created by the University of Virginia School of Engineering and Applied Science researchers. This discovery has potential for space exploration, more energy-efficient electronic equipment, and green architecture.
They have shown that, in its most pure form, a material that is now utilized in electronic equipment may also be employed as a temperature regulator. Engineers now have the capacity to change a thermal insulator into a conductor or vice versa by using this new class of materials, which allows for variable thermal conductivity.
Observation of Solid-state Bidirectional Thermal Conductivity Switching in Antiferroelectric Lead Zirconate, the team's findings were published in Nature Communications earlier this spring.
Electronics and gadgets that must function in extreme temperatures or withstand significant temperature changes would benefit most from the bi-directional control, or "tuning," of thermal conducting materials. Space is one of the environments where equipment must function under such difficult circumstances.
The first author of the Nature Communications research and UVA graduate in mechanical and aerospace engineering Kiumars Aryana noted, "The temperature variations in space can be fairly intense. As we develop machines and tools for space exploration, this kind of heat transport technology could be a major benefit.
Aryana cited the Mars Rover as an excellent illustration. At the rover landing sites, the ground can get as hot as 70 degrees Fahrenheit during the day and as cold as -146 degrees at night. The rover depends on an insulating box, heaters to keep the electrical components from freezing, and radiators to keep them from burning up to keep them operating through these extreme temperature changes.
"Heat regulation is now much simpler to control and moves more quickly thanks to this new style of heat management. The solid-state process would be nearly instantaneous while a radiator or insulation takes a while to begin heating or cooling. It is also safer if you can adapt to quick temperature changes. The likelihood that faults — or worse — will result from the heat or cold is reduced because the heating and cooling system can keep up "explained Aryana.
On Earth, however, there are also intriguing applications for controlling heating and cooling on both a big scale, such in buildings and a small scale, as in electronic circuit boards. Greener technologies and lower costs are correlated with less energy use.
This development continues a long-standing partnership between Aryana's advisor, Patrick E. Hopkins, Whitney Stone Professor of Engineering and professor of mechanical and aerospace engineering at UVA Engineering, and Jon Ihlefeld, associate professor of materials science and engineering and electrical and computer engineering.
Over the course of a decade, the Ihlefeld-Hopkins collaboration has made significant advancements in adjustable thermal conductivity in crystalline materials, first at Sandia National Laboratories and now at UVA.
Ihlefeld's multifunctional thin-film research group specializes in ferroelectrics, a family of functional materials that are particularly tuned.
Ihlefeld explained that a ferroelectric material is similar to a magnet, but instead of having north and south poles, it has a positive and negative charge. A ferroelectric material's surface "flips" to its opposite state when an electric field or voltage is applied to it. This condition persists until an additional opposite voltage is added.
Hopkins stated that "thermal conductivity is typically thought of as a static material attribute." "You must permanently alter a thermal conductor's structure or incorporate it with a new material if you want to convert it to an insulator."
Prior studies by Ihlefeld and Hopkins showed how to use an electric field to reduce thermal conductivity and how to incorporate the material within a device to increase thermal conductivity, but they were unable to make the same material accomplish both.
The researchers used an antiferroelectric substance for this experiment, which is affected by both heat and voltage.
Hopkins stated, "What this interesting material does is it gives us two distinct knobs to adjust thermal conductivity, in addition to being a high-quality crystal with thermal conductivity trends like an amorphous glass, in addition to being solid-state. We can actively adjust thermal conductivity and heat transmission by applying voltage or quickly heating the crystal with a laser.
We attempted to evaluate the bi-directional thermal conductivity of lead zirconate using a commercial sample, but it was unsuccessful, according to Aryana. A very pure sample of lead zirconate was donated by Lane Martin, the department chair and Chancellor's Professor of Materials Science and Engineering at the University of California, Berkeley. We produced a 38 percent bi-directional change in heat conductivity using Lane's sample, which Aryana described as a significant improvement.
Structures made of antiferroelectric materials are by definition bidirectional. The positive and negative charges cancel each other out in the smallest repetition unit of the crystal lattice because one half has a polarity pointing up and the other half has a polarity pointing down. Thermal conductivity rises with temperature as the crystal structure modifies and antiferroelectricity disappears. The material changes into a ferroelectric and its thermal conductivity drops when an electric field is applied, on the other hand. When the voltage is turned off, the net polarity goes back to zero.
It is possible to observe and measure thermal scattering events, which are akin to heat signatures, as a result of the polarity flip and the arrangement of atoms in the crystal that support the anti-ferroelectric structure. This means that energy diffuses through the material in ways that can be predicted and controlled.
The experiments and simulations in a thermal engineering research group at Hopkins has made significant strides in the field of laser material measurement. The antiferroelectric film was modulated by a third laser to cause a quick heating event that caused it to change from the antiferroelectric to paraelectric structure, giving it the ability to become polarized by an applied electric field, as described in the Nature Communications publication.
Engineers will require a larger "on-off" switch to quickly move or store a considerably larger percentage of heat in order to have an impact on technology. To build a new material with higher switching ratios and accelerate the deployment of actively adjustable thermal conductivity materials, the research team's next steps include better defining the material's limitations.
Materials are provided by the University of Virginia School of Engineering and Applied Science.
Kiumars Aryana, John A. Tomko, Ran Gao, Eric R. Hoglund, Takanori Mimura, Sara Makarem, Alejandro Salanova, Md Shafkat Bin Hoque, Thomas W. Pfeifer, David H. Olson, Jeffrey L. Braun, Joyeeta Nag, John C. Read, James M. Howe, Elizabeth J. Opila, Lane W. Martin, Jon F. Ihlefeld, Patrick E. Hopkins. Observation of solid-state bidirectional thermal conductivity switching in antiferroelectric lead zirconate (PbZrO3). Nature Communications, 2022; 13 (1) DOI: 10.1038/s41467-022-29023-y