From hybrid and electric vehicles to underground exploration of fossil-fuels, our search for energy solutions has increasingly placed us in situations demanding electricity storage and delivery under extreme conditions. Though batteries are the reigning storage technology, capacitors are an alternative with several advantages: they’re lightweight, they can be charged and discharged relatively quickly, and they don't lose their storage capacity over time.

In order to function properly, capacitors require dielectric materials, which behave as insulators and are essential for charge storage. Polymeric dielectrics have enhanced performance over other materials, and they can operate under more intense electric fields without failing (termed higher breakdown strength) and greater reliability. They also have the added benefit of practicality, being scalable, lightweight, and easily manipulated.

Right now, their major drawback as a material is their inability to work at high temperatures, like those required in many applications. But a composite polymer has finally been developed that seems to break down the traditional limitations of these materials, promising to open up a broader range of uses. Scientists made the new material by crosslinking a traditional polymer embedded with flakes of boron nitride nano sheets.

Dielectrics

An ideal dielectric material would inhibit current leakage and would exhibit a stable dielectric constant, a measure of how much charge it can keep separate. This composite polymer is made of cross-linked divinyltetramethyldisiloxane-bis(benzocyclobutene) surrounding boron nitride nano sheets. Boron nitride is structurally similar to graphene and forms sheets a single atom thick. The resulting material exhibits superior properties compared to anything already out there.

In order to see how good this new dielectric really is, the team compared its properties to the best polymeric dielectrics on the market. They examined the dielectric constant at 104 Hz alternating current, a frequency common in power conditioning, and 300°C. Under these conditions, they saw minor variations (less than 1.7 percent) compared to eight percent for the best existing system. They also looked at how the dielectric constant changed under direct current and found similar results.

As an insulating material, it’s important that dielectrics inhibit loss of energy. The team explored this issue and found that the rate of energy loss (indicated by the dissipation factor) of the composite doesn’t increase drastically when the temperature is raised. At 104 Hz, the rate of energy loss only increases from 0.09 percent to 0.13 percent as the temperatures is raised to 300°C. Though one existing polymer dielectric has similar characteristics, all others lose more energy when the temperature goes up. When the same tests were performed over the frequency range of 102 – 106 Hz, the new composite polymer again offered the most stable characteristic.

The researchers also evaluated two capacitive energy storage properties—the efficiency of the charge-discharge cycle and the discharged energy density—at elevated temperatures. They saw that the composite polymer outperformed the other polymer dielectrics over a large temperature range (150°C to 250°C).

The superior performance of these composite polymer dielectrics can be attributed to their enhanced ability to retain charge in high-fields at elevated temperatures, resulting in reduced leakage current. Any current leaks typically cause dielectric loss, which decreases the discharge energy and charge-discharge efficiency, while often generating heat internally.

Avoiding the heat

Typically, this type of internal heat generation results in failures of the capacitor due to overheating or decomposition. In some cases, elevated temperatures can result in a positive feedback mechanism, causing a further increase in the temperature (thermal runaway). Despite the many advantages of engineered polymer dielectrics already in use, they typically hold onto heat (technically, they have poor thermal conductivities), which significantly raises the risk of thermal runaway.

But the new material has thermal conductivities in the range of 300 to 2000 Wm-1K-1. The large thermal conductivity range indicates that this polymer composite can also be used to dissipate heat as a passive heat exchanger (heat sink), which can be helpful for many electronic devices.

To explore whether heat is generated within these constructs, the scientists used computer modeling to simulate the steady-state internal temperature of the composite polymers and traditional polymeric constructs. They found that the composite polymer dielectrics exhibited much lower internal temperatures than the existing alternatives.

The team also explored other ways of making the material and found that UV-polymerization could also be used to prepare solution-cast films. After further curing, the films exhibited the same dielectric properties as the thermally crosslinked films. This fabrication scheme is very useful for electronic applications because it essentially means that we can form patterned films very easily. By placing a mask above the solution and shining the UV light over the mask, this will allow only some regions to photo-polymerize.

The team also checked how these materials would hold up to the extreme mechanical deformations that are necessary for other flexible electronic devices. Amazingly, they found that the dielectric properties were not affected by bending and twisting, which they tested by characterizing films that had been bent and wound at room temperature and at 250°C.

Taken together, these results show that scientists have fabricated a truly remarkable dielectric material that has the potential to improve electronics that are operated under extreme conditions.

Nature, 2015. DOI: 10.1038/nature14647 (About DOIs).