Researchers at the University of Virginia School of Engineering and Applied Sciences have found a way to make a universal heat conductor that promises more energy-efficient electronic devices, green buildings and space exploration.
They showed that a certain material used in electronic equipment can now be used as a thermostat when in very pure form. This new class of materials gives engineers the ability to increase or decrease thermal conductivity on demand by changing the thermal insulator into a conductor and vice versa.
The team published its findings earlier this spring in Natural communications: Observation of bidirectional switching of thermal conductivity in solid state in antiferroelectric lead zirconate.
Bidirectional control or “tuning” of thermally conductive materials will be especially useful in electronics and devices that need to operate at extreme temperatures or withstand extreme temperature fluctuations. One of the scenarios where devices have to work in such harsh conditions is space.
“Temperature fluctuations in space can be quite intense,” said Kiumars Ariana, who won her doctorate. in mechanical and aerospace engineering at UVA this spring and is the first author of Nature Communications. “This type of thermal transport technology can be a huge advantage as we build vehicles and space exploration devices.
“The rover is a great example,” Ariana said. The ground temperatures at the rover’s landing sites can reach 70 degrees Fahrenheit during the day and minus 146 degrees at night. To keep electronic devices operating at these wide temperature fluctuations, the rover relies on an insulating box and heaters to protect components from freezing and radiators to prevent them from burning out.
“This new way of heat management is significantly less complicated and means that heat regulation is easier to manage – and faster. When the radiator or insulation takes a long time to start heating or cooling, the solid state mechanism would be almost instantaneous. the ability to cope with rapid temperature changes also makes things safer. “Because heating and cooling can be maintained, the chances of heat or cold causing malfunctions – or worse – are reduced,” Ariana said.
Meanwhile, here on Earth, promising applications include large-scale heating and cooling control, such as buildings, and small-scale, such as electronics boards. Less energy equals greener technology and lower costs.
This progress continues the long collaboration between John Ilefeld, Associate Professor of Materials Science and Engineering at UVA Engineering and Electrical and Computer Engineering, and Patrick E. Hopkins, Professor of Whitney Stone Engineering and Professor of Mechanical and Aerospace Engineering and Adviser to Ariana.
The Ihlefeld-Hopkins team has been a pioneer in adjustable thermal conductivity in crystalline materials for a decade, first at Sandia National Laboratories and now at UVA.
The tuning capability is unique to a class of functional materials called ferroelectrics, a specialty of Ihlefeld’s multifunctional thin film research group.
“Ferroelectric material is like a magnet, except that instead of the north and south poles, you have a positive and a negative charge,” Ilefeld said. An electric field or voltage, when applied to a ferroelectric material, “converts” the polarity of the surface of the material to its opposite state, where it remains until the opposite voltage is applied.
“Thermal conductivity is generally considered a static property of a material,” Hopkins said. “If you want to change a heat conductor into an insulator, you have to constantly change its structure or integrate it with a new material.
Previous research by Ilefeld and Hopkins has shown how to reduce thermal conductivity with an electric field and how to integrate the material into the device to increase thermal conductivity, but they cannot make the same material do both.
For this project, the team uses an antiferroelectric material in which heat and voltage come into play.
“What makes this interesting material, in addition to being a high-quality crystal that has thermal conductivity trends like amorphous glass, in addition to being solid, is that it gives us two unique buttons to change thermal conductivity,” Hopkins said. “We can quickly heat the crystal with a laser or apply voltage to actively adjust the thermal conductivity and heat transfer.”
“We tried to use a commercial sample of lead zirconate to test bidirectional thermal conductivity, but it didn’t work,” Ariana said. Lane Martin, the Chancellor’s professor of materials science and engineering and chair of the University of California, Berkeley, provided an extremely clean sample of lead zirconate. “Using the Lane test, we achieved a 38% two-way change in thermal conductivity in one explosion, which is a huge leap,” Ariana said.
The structures of antiferroelectric materials are bidirectional in nature. In the smallest repeating unit of the crystal lattice, one half has a polarity directed upwards and the other half downwards, so that the positive and negative charges compensate each other. When heated, the crystal structure changes and the antiferroelectricity disappears, increasing the thermal conductivity. The application of an electric field does the opposite – it causes the material to be transformed into a ferroelectric and the thermal conductivity decreases. The net polarity returns to zero when the voltage is removed.
Reversing the polarity and arrangement of the atoms in the crystal that support the antifreeroelectric structure leads to observable and measurable heat dissipation events – something like heat signature – which means that energy diffuses through the material in ways that can be predicted and controlled.
Members of Hopkins’ experiments and simulations in the thermal engineering research group have made great strides in laser measurement of materials. Nature Communications presents an innovation in optical thermometry-based experiments in which students use a third laser to induce rapid heating to modulate the antiferroelectric film through the transition from an antifreeroelectric to a paraelectric structure, giving it the ability to polarize under an applied electric field.
To have an impact on technology, engineers will need a larger on / off switch to move quickly or store a much higher percentage of heat. The next steps for the research team include working to better define material constraints so that they can design new material with higher switching ratios, accelerating the use of actively adjustable thermally conductive materials.