Sponsors: NSF POETS Center, ARPA-E
Collaborators: Google, UIUC, SRC 3D ASCENT Collaborators, U Arkansas
Students: Tanya Liu, Chi Zhang, Qianying Wu

Some of the best thermal management technologies use capillary forces to bring the liquid phase to hotspots for evaporative cooling, for example heat pipes and vapor chambers. The increasing complexity and power density of electronic systems demand the scaling of capillary cooling technologies to dimensions where dryout and boiling crisis strongly reduce the peak heat removal capability. We are addressing this problem using micro/nanofabrication to dramatically improve the wick structures and thereby improve control and routing of the liquid and vapor phases.

A particularly challenging goal is high performance, deeply-scaled vapor chambers with thicknesses approaching 0.1mm, which would represent a major advancement for portables and 3D logic. We are using inverse opal structures (common as photonic meta materials), which yield precision and consistency of pore size for the metallic wick that far exceeds those of conventionally-sintered wicks. Template sintering enlarges the “necks”, or hydraulic vias, that bridge adjacent IO pores. The enhanced neck size increases the hydraulic permeability for vapor extraction by an order of magnitude, and subsequently the CHF can be lifted by an order of magnitude. We are modeling the impact of wick structure on boiling crisis and permeability and demonstrating the potential of this material for ultrathin and low superheat solutions for high-power-density electronic devices.

Another application is passive thermal regulators, which are promising as heat routing components that can mitigate large temperature spikes (common in vehicles and power devices) by transitioning between high and low resistance states without external actuation. Existing regulators, however, are often either limited to fixed temperature regulation ranges or are difficult to package in a compact form factor. We are developing compact and tunable thermal regulators that employ the dynamics of vapor transport through a noncondensable gas cavity. These devices have already achieved a switching resistance ratio of 4 in response to variations in the input power, and we are using our copper inverse opal technology to target an order of magnitude level of control. These devices can set the temperature difference across the hot and cold sides to a fixed, “clamped” value that is reasonably independent of heat flow.

The impact of this work on improved capillary control is very strong in both 3D integrated logic circuits, which demand very compact heat spreaders, as well as in power devices where heat routing can be a major benefit.

Copper inverse opal (CIO) structures with pore size 5-10 microns are having a big impact on the precision of wick construction as well as the peak heat transfer rates through evaporation at hotspots.
Modification of the CIO through sintering can fine tune the balance between fluid permeability (and the viscous limits to heat removal) viscous losses against the thermal conductivity of the capillary structure.