The battle against heat in electronics is not new. IBM began using water-cooling technologies in its systems as early as the 1970s. But as semiconductor manufacturers continue to produce increasingly smaller microprocessors with greater density, both the heat and the battle against it have become more intense, demanding innovative cooling solutions. Two new technologies that are available today seek to alleviate these problems.

Thin-film thermoelectrics from nanoCoolers, Austin, Texas, are a low-cost, power-dense alternative to traditional thermoelectric cooling methods. TEs, or Peltier devices, remove heat by exploiting the Peltier effect, named for the French watchmaker who discovered it in 1834. Peltier devices contain two dissimilar conductive materials — one that is heavily doped with an excess of electrons and another that is electron-deficient. .

At room temperature, most TEs are based on n-doped and p-doped bismuth-telluride semiconductor materials. When DC current runs from the n-doped material to the p-doped material, heat is absorbed. When current runs from the p-doped material to the n-doped material, heat is dissipated.
The construction of the device provides for the flow of current from the n-doped material to the p-doped material on one side, while the current runs inversely on the other side, allowing one side of the device to get hot, while the other becomes cold. When the direction of the current is reversed, the heat and electron polarities of the sides are also reversed..

Conventional TEs are built by assembling discrete blocks of n-doped and p-doped TE elements. The TEs are usually placed on ceramic plates using pick and place assembly equipment, and the plates have metal traces that route the electrical current. The metal traces are soldered to the plates, and the plates are stacked, with one plate on top and the other on the bottom.

Miniature, power-dense cooler

Tec2 refrigerators on chip.

The elements used in these TEs are 1 mm to 2 mm high, making the entire module approximately 4 mm high and 4 cm long by 4 cm wide. Using thin-film materials in a typical semiconductor construction, nanoCoolers has significantly increased the power density of conventional TEs as well as shrunk the height of the elements to 300 microns to 500 microns high, providing for an actual cooler size of approximately 1.5 mm by 1.5 mm and increased power density..

“Our cooling densities are significantly higher than conventional thermoelectrics,” says Mick Wilcox, product line manager for nanoCoolers. “If you can get 10 W per cm2, that’s doing very well with conventional thermoelectrics. With our thermoelectrics we can scale into the hundreds or even thousands of W per cm2.”.

To increase power, modules can be added in side-by-side and pyramid arrangements as well as by building upward. .

Because the circuit interconnections within the TE are based on standard CMOS metal layer processes, the solder is not necessary. By eliminating the solder requirement, nanoCoolers has eliminated a significant source of mechanical failure — the solders tend to break apart at temperatures of 85 DegC and higher in conventional TEs. And because the manufacturing technology has been on the market for some time, the coolers can be produced at a lower cost than many of its market counterparts..

NanoCooler’s thin-film cooling technology is suitable for a number of applications, including lasers, electronics, optoelectronics, sensors and potentially small areas on processors..

“The optoelectronics and sensor cooling industries already use thermoelectrics — they just use conventional thermoelectrics. So there is no education, and there is no redesign, other than to perhaps shrink the package of the product for the incorporation of our device,” Wilcox says.

Foam solutions

Fig. 1. SEM image of graphite foam derived from Koppers Mesophase Pitch.

Graphite foams are another alternative to traditional electronic cooling technologies. .

A piece of foam is soldered to a microprocessor to better dissipate heat. .

In a cooling experiment, discussed in the paper, “The Role of Structure on the Thermal Properties of Graphitic Foams,” researchers at Oak Ridge National Laboratory, Oak Ridge, Tenn., used pure carbon foams in planes of hexagons, and stacked the planes on top of one another. .

The planes form a graphite sheet and align along the cell walls. The graphite sheet had very good bonding properties and high thermal conductivity. This results in the foams exhibiting an overall bulk thermal conductivity as high as aluminum, but at 1/5 the weight..

Graphite foams can be prepared from coal, petroleum pitch and synthetic pitch. The foams made of coal and petroleum have similar thermal conductivity, but the synthetic materials have higher thermal properties, due to their purity.

Fig. 2. SEM image of graphite foam derived from AR Mesophase Pitch.

“We aligned the graphite sheets so that they laid parallel to the surface walls, or ligaments of the foam. This gives very high bulk thermal conductivity,” says James Klett, senior research staff member at Oak Ridge. “The result is we have a material that is extremely conductive along those walls. You have a bulk material that is very conductive, but it has graphite properties — it can take very high temperatures, and it is very lightweight.”.

The bulk properties of the graphite foam in this case are nearly equivalent to aluminum, but the foams are about 1/5 the weight of aluminum, 1/16 the weight of copper, with about 1/3 the conductivity of copper..

The technology is being used by Lockheed Martin as an electronic cooling device for satellite equipment, and is licensed to foam manufactures Poco Graphite, Decatur, Texas, and Koppers, Inc., Pittsburgh.

Improving heat transfer

During their experimentation with graphite foams, discussed in the paper, “Parametric Investigation of a Graphite Foam Evaporator in a Thermosyphon with Fluorinert and a Silicon CMOS Chip,” researchers at Oak Ridge National Laboratory, Oak Ridge, Tenn. determined that by increasing the surface area of graphite foams they were able to gain high heat fluxes at minimal temperatures..

In this project, the graphite foam technology developed at Oak Ridge was used as an evaporator in a thermosyphon, a heat spreading device that internally uses gravity and evaporative cooling to rapidly spread heat from one surface to another, thus maximizing heat fluxes. .

The liquid at the bottom of the system boils, vaporizes, rises to the top of the system and then condenses. The heat moves from the boiling part of the system to the condensing point, and gravity feeds it back down. The thermosyphon transfers the heat from a small surface to a larger surface and dissipates it. .

Unlike a refrigeration system, a thermosyphon does not utilize a pump, instead it has a vacuum. Oak Ridge used 3M’s Fluorinert fluid as the coolant for their system. The system was designed to cool computer chips, but can easily be modified for just about any other type of cooling..

The graphite foams were bonded to a silicon chip during the experiment. The graphite foam is wet with Fluorinert and it boils, causing the cooling effect on the chip. Just like sweat evaporation, the boiling effect rapidly moves the heat from the chip to the condenser, thus behaving like a very efficient heat sink..

By soldering the foam to the surface that is to be cooled, the researchers were able to reduce thermal resistance, by eliminating contact resistance, and increase heat transfer from the silicon chip to the graphite foam.

Plot of the thermal conductivity versus density of graphite foams derived from various precursors using the ORNL process.

Foams with higher densities have better heat transfer properties, but with smaller pores, their boiling performance is decreased. In the event the pores are too small, the system is unable to produce enough boiling, resulting in a loss of heat transfer capability..

Modifications in fluid level play a minor role in comparison. By increasing the fluid level, the researchers were able to increase the rate at which the foam condenses from the pores, but the effect was virtually negligible..

With the introduction of machined fins, the surface area of the foam was increased. In turn, the liquid’s access to the internal porosity of the foam was increased. Consequently, by enhancing the surface area, the researchers were able to provide for more boiling and better heat transfer..

The graphite foams perform better than water-cooling technologies, allowing heat transfer at much lower temperatures. As the researchers were able to decrease the thermal resistance, they were able to increase the performance in their comparisons..

Klett indicated that when used in the consumer market, the graphite foam technologies developed by Oak Ridge, should be individually tailored to the application.
“Appliance manufacturers need to take a step back from their applications, examine where they have a need to improve heat transfer, reduce weight, or improve their thermal uniformity,” he says. “They can attack the problem using the unique properties of the foam, along with the various technologies we have developed with the foams, and possibly come up with a novel solution. They need to not be afraid of radical changes. In many cases, it is these radical changes that have proven very successful.”