Cool operators

Developments in cooling technology could be a breath of fresh air
for designers of electronic devices.

From PCs to Laptops to mobile phones, the electronic devices we take for granted are fast approaching meltdown. Our voracious appetite for speedier performance and flash functionality is currently sated by designers cramming more and more chips into products — but only just.

There’s now a high level of concern in the electronics industry about thermal management. New processors are consuming more power, circuit densities are getting higher and there’s growing pressure to reduce the size of devices. Many agree that conventional fan-driven cooling will soon no longer be enough to prevent our gadgets from going up in smoke.

Fortunately, a number of new cooling technologies are under development that, if they fulfil their promise, will enable designers to continue to cater for our expensive tastes.

One of the latest developments has seen aerospace giant Swales join forces with thermal management company Aavid Thermalloy. Under their recently announced agreement the two companies aim to develop and find wider commercial applications for technology originally developed for use on telecommunication satellites.

Swales’ expertise centres on heat pipe and loop heat pipe technology, systems that are used extensively in the aerospace industry, but yet to gain acceptance in commercial terrestrial applications. Essentially this passive technology works by means of capillary forces which remove heat from a source and transport it to a condenser (or radiator) to dissipate the heat.

Swales said: ‘The two companies plan to offer commercial off-the shelf heat removal devices that will further reduce the size, weight and cost of semi-conductor-based products.’

But while heat pipe technology relies on liquid (typically anhydrous ammonia) to transport heat away from its source, many thermal management experts believe that the safest and most efficient hope lies with technology that uses air as a cooling medium.

Enter mechanical engineers from Purdue University, Indiana, who recently announced that they are developing cooling technology based on what they term ‘nano-lightning’ to create tiny wind currents. They claim that the technology could replace the need for a fan and enable each chip to cool itself.

Prof Suresh Garimella, who is heading the project, explained that the technique works by generating ions – or electrically charged atoms – using electrodes placed close to one another on a computer chip. Negatively charged electrodes, or cathodes, are made of carbon nanotubes with tips that are just five nanometers wide – enabling the system to run at low voltages.

Voltage is passed into the electrodes, causing the negatively charged nanotubes to discharge electrons towards the positively charged electrodes. The electrons react with surrounding air, causing the air molecules to be ionised just as electrons in the atmosphere ionise air in clouds. This ionisation of air leads to an imbalance of charges that eventually results in lightning bolts.

Clouds of ions created when electrons react with air can then be attracted by the second region of electrodes and ‘pumped’ forwards by changing the voltages in those electrodes. The voltages are rapidly switched from one electrode to the next in such a way that the clouds of ions move forward and produce a cooling breeze.

More work must be done to measure the cooling performance, perfect the technique and develop a prototype, but the researchers hope to be able to develop cooling devices that are small enough to fit on individual chips, actually making up a layer of the chip. ‘The entire thing would sit on, and be integrated into, a chip that is 10mm by 10mm,’ said Garimella.

He added that the system would have no moving parts, making it quiet and reliable, and would also be easy to manufacture using existing techniques from the semiconductor industry.

Two of the researchers on the project, doctoral students Daniel Schlitz and Vishal Singhal, have formed spin-out company Thorn Micro Technologies to commercialise the technology. ‘There’s definitely a need for more efficient cooling technology. People are acknowledging the need very quickly and there’s plenty of money available for companies that have even semi-plausible solutions,’ said Schlitz.

He said that while the technology is still at a very early stage, companies including Apple, HP and Sony have indicated a high level of interest.

The team is now focusing on making a device for laptop computers. They expect to be able to prove the technology and come up with a prototype before the end of the year, with products to follow about 18 months later.

In a separate development, engineers at Georgia Institute of Technology spin-out company Innovative Fluidics have developed technology that they claim could be used to provide highly efficient air cooling of processors at a fraction of the flow rate of traditional fans. The firm said that its SynJets devices could be used to cool laptops, PDAs and mobile phones.

Based on research carried out by Prof Ari Glezer of the School of Mechanical Engineering at the institute, SynJets work using an air-puffer powered by an inaudible vibrating membrane (150Hz).

Jon Goldman of Innovative Fluidics explained how a vortex is created at every push stroke of the membrane. This vortex, he said, promotes good mixing of air and breaks up the boundary layer that is typically attached to each fin of a heatsink. Goldman added that though the devices operate at a considerably lower flow rate than a fan they nevertheless result in a very high heat transfer coefficient. The SynJet mechanism means that they can be built in a variety of different shapes and, most usefully, designed in a very flat profile. So the devices aren’t limited to the circular shape required by a fan.

Goldman claimed that Synjets have potential where shape and profile militate against the use of fans. ‘In certain applications this technology could have some superior performance to fans particularly from a shape and noise point of view,’ he said.

In other applications, Goldman said that the technology could be used alongside fans. In fact, the company has already investigated this possibility.

‘We had a case where there was a set-top box and it was crunching so many MIPS [millions of instructions per second] that it needed two pretty big case fans. We replaced them with one, directed it with a SynJet and got 24 per cent better performance – and it was quieter,’ he said.

Georgia Tech has now licensed that technology, and productsequipped with it are expected to be available soon. Initial applications are likely to be in laptop batteries and LCD projectors, said Goldman.