When a rock sits out in the sun, it slowly warms up, accumulating heat energy until the sun goes down and the temperature drops, causing the rock to slowly release the accumulated energy over time. If heat energy could be similarly collected but retained and released on command, it could prove extremely useful for harnessing energy that is regularly produced and wasted by cars, industrial factories, as well as renewable energy sources such as solar power plants.  

In 2015, researchers at the University of Tokyo described just such heat storage properties in a new ceramic material they synthesized, nanocrystalline trititanium-pentoxide (Ti₃O₅). The material preserved heat energy for any amount of time—even in low temperatures—and released it only when the researchers exposed the material to an external stimulus, such as light or 60 MPa of pressure. This was caused by the triggering of a phase change from λ- to β-phase upon application of pressure. Now, as detailed in a new Scientific Reports study, the researchers report that they have further refined their material to require even lower pressure to release its stored heat energy, bringing it closer to real-world use, especially for transportation.

“There are many voices from the automobile industry calling for some possible heat-storing material they could use,” says Marie Yoshikiyo, a chemist at the University of Tokyo and co-author of the study. “But since cars have many parts, it’s difficult to add a system to apply high pressure.” From speaking with industry representatives, Yoshikiyo and her colleagues knew they needed to design a version of Ti₃O₅ that releases heat at 10 MPa or less.

In 2010, Shin-ichi Ohkoshi, a chemist at the University of Tokyo, created a new form of Ti3O5 by heating TiO₂ nanoparticles under hydrogen flow. Ohkoshi observed the material during phase changes and found that it exhibited a solid-solid phase transition from λ-Ti₃O₅ to β-Ti₃O₅ when he irradiated it with a laser light. Later, he and his colleagues found that pressure also caused the material to undergo this phase change. Thermodynamic studies carried out by the researchers confirmed that an energy barrier existed between the two phases and that it remained in place at all temperatures. When pressure is applied, however, the barrier disappears and the λ-phase transforms into the more stable β-phase. This discovery caused Ohkoshi to realize that λ-Ti₃O₅ could be useful as a heat-storage material for storing and releasing heat energy.

Previously, Ohkoshi and his colleagues had synthesized λ-Ti₃O₅ by starting with rutile TiO₂ nanoparticles, then heating them to 1117°C. This yielded rectangular crystals about 200-nm in length. In their new work, they heated the particles to 1300°C, producing large cubic crystals about 500-nm in length. The new block-type λ-Ti₃O₅ transforms to β-Ti₃O₅ when the researchers apply just 7 MPa of pressure, and it can store up to 237 kJ/L when warmed up to 200°C.

“It seems [that] by increasing the primary crystal size, the necessary pressure to induce a phase transition becomes lower,” Yoshikiyo says. “7 MPa, for example, is lower than the pressure you apply when you write with your pen, so if you were able to write on this material, you could probably change its phase.”   

Others agree that this is a significant step forward. “The key feature is that the energy is not released on cooling, as the system remains in the high temperature λ phase, due to the existence of the energy barrier between two bistable phases at room temperature and ambient pressure,” says Eric Collet, a physicist at the University of Rennes 1 in France, who was not involved in the research. “Interestingly, the authors have observed that this energy can be efficiently released by applying a weak pressure, which drives the exothermic λ to β transition.”

Ohkoshi, Yoshikiyo, and their colleagues plan to continue refining Ti₃O₅ for industrial use, including by lowering the transition temperature at which the material stores heat, set now at 198°C. They also hope to increase the heat energy that the material can store from above its current capacity, 237 kJ per liter. “237 kJ per liter is a large value, but it would, of course, be much better if we could increase it [even] more,” Yoshikiyo says.

Read the article in Scientific Reports.