MIT Engineers Uncover Material With Reversible Thermal Conductivity

Materials possess inherent thermal conductivities; plastic typically conducts heat poorly, while substances like marble exhibit efficient heat transfer. Generally, a material's thermal conductivity is a fixed property, requiring re-manufacturing for alteration. However, MIT engineers have identified a common material capable of reversibly switching its thermal conductivity.

This innovative material, an olefin block copolymer (OBC), significantly increases its heat conductance when stretched. It transitions from a baseline similar to plastic to a capacity closer to marble, then reverts to its lower conductivity when it returns to an unstretched state.

Rapid Thermal Switching Observed

This thermal switching occurs remarkably fast, with the material's heat conductance more than doubling within 0.22 seconds.

This is reported as the fastest thermal switching observed in any material to date.

Unlocking Real-Time Applications

The discovery suggests diverse applications in systems that need to adapt to temperature fluctuations in real time. Examples include switchable fibers for apparel that could dissipate body heat when stretched, or crucial components for laptops and infrastructure to prevent overheating.

Researchers are continuing to optimize the polymer and develop new materials with similar properties. Svetlana Boriskina, a principal research scientist at MIT's Department of Mechanical Engineering, highlighted the pressing need for "inexpensive, abundant materials that can quickly adapt to environmental temperature changes," noting this finding directs new avenues for adaptive material development.

The study results were published in the journal Advanced Materials.

The Microscopic Mechanism

The material's ability to switch thermal conductivity is attributed to changes in its microscopic structure. When stretched, these structures align, allowing heat to pass through more easily. In its unstretched form, the microstructures are tangled, which impedes heat flow.

The research initially focused on finding sustainable alternatives to spandex, exploring materials like polyethylene. While polyethylene's carbon atoms form chains that are good heat conductors, they are typically arranged in a disordered amorphous phase, leading to low overall thermal conductivity. Previous work by MIT Professor Gang Chen and collaborators achieved permanent increases in polyethylene's thermal conductivity by inducing a shift to a more aligned, crystalline phase.

However, the MIT team's experiments with OBC revealed a different and highly reversible mechanism. Duo Xu, a co-author and MIT graduate student, observed that OBC's thermal conductivity was consistently high when stretched and low when relaxed, over thousands of cycles. This reversibility was notable because the material remained largely amorphous.

Further analysis using X-ray and Raman spectroscopy showed that when OBC is stretched, its few ordered crystalline domains align, and its amorphous tangles straighten out. Crucially, these straightened tangles remain in an amorphous state rather than fully crystallizing, allowing them to reversibly switch between straightened and bunched configurations as the material is stretched and relaxed.