Researchers at the University of Virginia’s School of Engineering and Applied Science have devised a method for creating a flexible thermal conductor that might be used in more energy-efficient electrical equipment, green buildings, and space exploration.
They’ve shown that when a substance used in electrical equipment is in its purest form, it can also be employed as a temperature regulator. Engineers can change a thermal insulator into a conductor and vice versa using this new kind of material, which allows them to control thermal conductivity on demand.
Observation of Solid-state Bidirectional Thermal Conductivity Switching in Antiferroelectric Lead Zirconate was reported in Nature Communications earlier this spring.Electronics and gadgets that must work at severe temperatures or survive significant temperature changes would benefit from bi-directional control or “tuning” of thermal conductive materials. Space is one of the environments in which electronics must work under such extreme circumstances.
“Temperature changes in space may be rather strong,” said Kiumars Aryana, who received his Ph.D. in mechanical and aerospace engineering from the University of Virginia this spring and is the paper’s primary author. “As we create spacecraft and gadgets for space travel, this sort of heat transfer technology might be a big help.”
“A wonderful example is the Mars Rover,” Aryana explained. At the rover landing locations, ground temperatures may exceed 70 degrees Fahrenheit during the day and minus 146 degrees at night. The rover uses an insulating box and heaters to keep electrical components from freezing and radiators to keep them from burning up to keep them operating during these extreme temperature changes.
“This new style of heat management is far less complicated, making heat control both easier and faster to manage.” The solid-state process would be practically immediate when a radiator or insulation takes a long time to heat or cool. It’s also safer to be able to adjust to abrupt temperature fluctuations. As a result of the heating and cooling systems’ ability to maintain a consistent temperature.Because the heating and cooling systems can keep up, the possibilities of breakdowns — or worse — are reduced, according to Aryana.
Meanwhile, on Earth, prospective applications include large-scale heating and cooling, such as buildings, as well as small-scale applications, such as circuit boards for electronics. Greener technologies and reduced prices result from using less energy.Jon Ihlefeld, associate professor of materials science and engineering and electrical and computer engineering at UVA Engineering, and Patrick E. Hopkins, Whitney Stone Professor of Engineering and professor of mechanical and aerospace engineering and Aryana’s advisor, have been working together for a long time.
Over the course of a decade, the Ihlefeld-Hopkins collaboration has pioneered adjustable thermal conductivity in crystalline materials, first at Sandia National Laboratories and now at UVA.Tunability is peculiar to ferroelectrics, a type of functional materials that Ihlefeld’s multifunctional thin-film research group specializes in.
“A ferroelectric substance is similar to a magnet, but it has a positive and negative charge instead of a north and south pole,” Ihlefeld explained. When an electric field, or voltage, is supplied to a ferroelectric material, the polarity of the material’s surface “flips” to the opposite state, where it remains until another voltage is applied.
“Thermal conductivity is usually thought of as a static material attribute,” Hopkins explained. “To turn a thermal conductor into an insulator, you must modify its structure permanently or combine it with a new material.”
Prior research by Ihlefeld and Hopkins showed how to reduce thermal conductivity using an electric field and how to incorporate the material into a device to increase thermal conductivity, but they couldn’t make the same material do both.The researchers employed an antiferroelectric material for their experiment, which involves both heat and voltage.
“What this remarkable material does, in addition to being a high-quality crystal with thermal conductivity trends similar to an amorphous glass and being solid-state, is that it provides us two distinct knobs to adjust thermal conductivity,” Hopkins explained. “We can use a laser to quickly heat the crystal or apply voltage to actively modify thermal conductivity and heat transmission,” says the researcher.”We tried to test bi-directional thermal conductivity using a commercial sample of lead zirconate, but it didn’t work,” Aryana said. A very pure sample of lead zirconate was donated by Lane Martin, Chancellor’s Professor of Materials Science and Engineering and department chair at the University of California Berkeley. “We got a 38 percent bidirectional shift in thermal conductivity in one burst using Lane’s sample, which is a big leap,” Aryana added.
By their very nature, antiferroelectric material structures are bi-directional. 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. The crystal structure changes when heated, and the antiferroelectricity vanishes, enhancing thermal conductivity.
The anti-ferroelectric structure is supported by the polarity flip and the arrangement of atoms in the crystal, which results in observable and quantifiable thermal scattering events — similar to a heat signature — which implies energy diffuses through the material in predictable and regulated ways.
Hopkins’ experiments and simulations in thermal engineering research group has achieved several advancements in laser material measurement. Students used a third laser to cause a quick heating event to modify the antiferroelectric film through the transition from antiferroelectric to paraelectric structure, allowing it to become polarized under an applied electric field, according to the Nature Communications research.Engineers will require a larger “on-off” switch to quickly transport or store a considerably higher percentage of heat to have an influence on technology. The study team’s next steps will be to further identify the material’s limits so that they may create a new material with greater switching ratios, allowing them to employ actively adjustable thermal conductivity materials sooner.