In many ways, Moore’s Law – the famous prediction by Gordon Moore, co-founder of chip manufacturer Intel, that microprocessor complexity will grow exponentially without an increase in price – has held for four decades. But that complexity has come with a hidden “cost”: heat.
Packing more and more components and circuits onto a chip requires more electrical power to run it. And most of that power turns into heat, so that the latest chips can quickly exceed 100 degrees Centigrade, if not properly cooled.
The problem is getting so serious that last year Intel canceled a high-speed-CPU project, in part because it found no practical way to cool down the energy-consuming chips (see accompanying Notebook). Overheated chips don’t work reliably, possibly leading to computer crashes, mangled files, graphical glitches, and even permanent damage.
“There is a great demand for compact, cost-effective cooling solutions,” says Suresh V. Garimella, director of the Cooling Technologies Research Center at Purdue University in Indiana. According to him, the fans traditionally used to cool personal computers are “reaching their limits.”
One potential solution to this growing problem is more commonly associated with nuclear reactors: liquid metal cooling. Spearheaded by NanoCoolers, a startup in Austin, TX, the technology takes advantage of a unusual compound of metals that remains liquid at room temperature. Currently, their mixture of gallium and indium (and a pinch of tin) flows freely at temperatures above 7 degrees C. And a new formula could go as low as minus 10 degrees C, according to product manager Mick Wilcox.
“This technology is one among a number of new and promising cooling solutions that are being proposed and pursued lately,” says Garimella.
The liquid metal flows in a loop around a PC or graphics card. First, it picks up heat from the top of the heated chip. Then the liquid gets pumped through pipes to a radiator (usually with a fan blowing on it), where the heat is released into the air. Finally, the cooled fluid circulates back to the chip.
The pump that moves the liquid metal around in the system is one of the invention’s main advantages over a rival technology, water cooling. Taking advantage of the metallic nature of the coolant, the pump pushes the liquid around electromagnetically. Unlike water cooling, the process requires no moving parts, consumes little power, and is silent. In a patent application, NanoCoolers even suggests powering the pump solely from the waste heat produced by the computer. While the liquid metal compound is non-toxic, according to Wilcox, it is corrosive to some metals, notably aluminum.
The attraction of liquid metal itself is its excellent conduction of heat. According to Sapphire Technology, which has adopted the NanoCoolers invention for a PC graphics card, it is 65 times more thermally conductive than water – and 1,600 times better than air cooling.
In short, liquid metal is able to absorb heat more rapidly, and thus cool down chips faster. This property has led to its use as an “ultimate” coolant in some nuclear reactors, which are cooled with liquid sodium or potassium, as well as in the manufacture of high-quality machine components, such as gas turbine blades, where the components are rapidly “cooled” to 660 C with molten aluminum to prevent the formation of defects.
“[Liquid metals] could certainly provide higher cooling capacities,” says Garimella. Concerning their practical applications, however, he is more cautious: “[It’s] very much a function of how they are implemented, and the controlling thermal resistance in the package.”