UC Engineer Advances Electronic Cooling

By: Liz Daubenmire

University of Cincinnati graduate, Mohammed T. Ababneh, PhD, and Senior Associate Dean for the College of Engineering and Applied Science, Frank M. Gerner, PhD worked together to reduce the size of standard heat pipes in order to implement ultra-thin, sheet like systems for cooling electronics.

Professor Frank Gerner and Mohammed T. Ababneh, PhD

Professor Frank Gerner and Mohammed T. Ababneh, PhD

The world of technology is rapidly advancing with hand-held devices getting thinner and faster.  In order to maintain optimal processing speed, it is important to keep electronics at an appropriate temperature to keep them from overheating and losing function.  The issue then becomes finding a cooling system small enough to keep the overall size of electronics as compact as possible.

Mohammed Ababneh, PhD, who is currently working as a Research and Development (R&D) Engineer, Defense/Aerospace Division, at Advanced Cooling Technologies, Inc. in Lancaster, PA, graduated from the University of Cincinnati College of Engineering and Applied Science with his PhD in 2012.  During his time at UC, Ababneh collaborated with Professor Frank M. Gerner, PhD, to find an effective solution to the standard bulky heat pipes which are traditionally used to cool microelectronics or computer processors. 

Heat pipes, despite their contradictory name, are not actually used to heat devices.  Instead they are used as a means to transfer heat away from electronic and mechanical devices.  The heat is captured within the pipes, cooling the operating system and allowing it to function efficiently, hence the name “heat pipe.”

Imagine a thin, flat copper pipe or vapor chamber upon which computer processors or other heat generating electronics are mounted.  The inside of this thin heat pipe contains a wick; a porous lining on the inside of the pipe, which is usually made of sintered (sponge like) copper.  The key component to a heat pipe is the liquid inside the wick, which is often water for the normal operating temperatures of microelectronics.

The liquid turns into vapor from the waste heat generated by an electronic device.  That vapor then travels through the pipe due to the pressure change- cooling the electronics and condensing into a liquid form on its path. 

Once the condensation has reached the “cool side” of the pipe, it passively flows back down the wick to the “hot side” on the pipe. This process is repeated as a means of transferring heat from the electronic chips to the larger surface area outside of the heat pipe thermal spreader where more conventional heat transfer technologies may be utilized.

The biggest challenge in reducing the size of a heat pipe is finding materials that are both lightweight and that can handle the stress of heat transfer.

After vigorous prototype testing Ababneh and Professor Gerner and their partners at GE Global Research and Wright Patterson AFB were able to match ideal materials and structure to create a heat pipe system that is less than 1 mm thick.

This new technology with a total thickness of less than 1 mm can operate in any gravitational orientation, and in fact have been tested in adverse gravity conditions exceeding 13g.  A “g” is a measurement of gravitational force.  To put 13 g’s into perspective; a manned aircraft is only designed to pull up to 9 g’s, the force you feel on a roller-coaster is usually somewhere between 3 and 4 g’s. The pair published their findings in a paper which was recently picked up by Thermal News.  This trade publication is mostly used for engineers in the field who are looking to share and receive knowledge on best practices and field advancements.  Professor Gerner explains, “Getting published in this kind of trade publication means our work will have a real-world impact on the industry.” 

The remarkably small heat pipe prototypes developed by Ababneh and Gerner have a wide variety of potential applications.  The heat transfer system, which is only 1 mm thick and can operate in any gravitational orientation, therefore, can be used in anything from phones and tablets, to aircrafts, avionics and UAV’s.

Figure 1 is a photograph for a thin flat thermal ground plane (TGP) heat pipe with a surface pattern etched circuit directly onto the surface. The heat pipe TGP has been gold plated and pads of Gold-Tin solder have been deposited for direct attachment of vertical cavity surface emitting laser (VCSEL) chips.

Figure 1: Heat pipe TGP with etched electrical circuitry, gold plated with gold-tin solder pads ready for direct attach of 1 cm² vertical cavity surface emitting laser (VCSEL) chips. A representative sample of the converging wick structure is shown in the lower right photograph (Courtesy of Advanced Cooling Technologies, Inc.).

Figure 1: Heat pipe TGP with etched electrical circuitry, gold plated with gold-tin solder pads ready for direct attach of 1 cm² vertical cavity surface emitting laser (VCSEL) chips. A representative sample of the converging wick structure is shown in the lower right photograph (Courtesy of Advanced Cooling Technologies, Inc.).

Ababneh, PhD, under the supervision of Professor Frank M. Gerner developed numerical models that predict the heat pipe TGP’s performance accurately. The models were used to predict temperature distribution, effective thermal conductivity, and heat loads before fabrication and testing.

This tool helps designers to determine the effects of vapor space, wick thickness, condenser temperature, heat pipe external geometry, etc. Figure 2 shows the steady state temperature distribution for a 9 cm long TGP when 30 Watts were applied to the evaporator. This result confirms that the TGP has an extremely high axial thermal conductivity, in the range of 5,000 W/mK - 10,000 W/mK, or approximately 12.5 - 25 times higher than bulk copper (kcopper=400 W/mK) and 5 - 10 times higher than diamond (kdiamond=1000 W/mK).

Figure 2: Steady state temperature distribution for 9 cm length TGP when apply 30 Watt at the evaporator section.

Figure 2: Steady state temperature distribution for 9 cm length TGP when apply 30 Watt at the evaporator section.

Ababneh explains, “Two permanent trends in the mobile electronics industry are increased processing speed and compactness… The trends are most evident in the speedily growing cell phones and tablets markets.” 

With the recent release of the iPhone 6, it is increasingly evident that consumers want their devices as thin and as fast as possible.  Professor Gerner and Ababneh are doing their part to ensure our devices run with efficiency, keeping our technologically advanced society in order.

Ababneh sites prweb.com and explains, “The global tablet PC market is expected to reach $77.5 billion by 2016 with a compound annual growth rate (CAGR) of around 35%.  While Tablets and Cell Phones represent an important market, they are not the only technologies that could benefit from increased processing speed and compactness.”

Potential applications extend beyond the devices we carry in our pockets.  “The military also represents a significant market. For example, electronic components that are used in many military applications are required to operate in harsh environments, and as a result, increased heat dissipation is very difficult to manage,” says Ababneh.

Ababneh explains, “High-power commercial-off-the-shelf (COTS) components need innovative cooling solutions in order to keep the lower operating temperatures necessary for optimal performance like the present ultra-thin heat pipe TGPs”.   

Thanks to Frank Gerner, PhD, and Mohammed T. Ababneh, PhD, for keeping things cool, the world of technology can continue its climb towards the thinnest, fastest devices ever created.