When it comes to getting a lot of heat fast, it looks as though a thin layer of graphite may be better than ordinary heaters made with nichrome wire. This realization may soon transform the construction of heating equipment in areas from fast food to plastic molding, and make practical new capabilities in airplanes and ground vehicles.
The material that is stimulating such speculation is flexible expanded graphite foil, also called vermiform graphite. It is generally laminated to an outer heat-conducting layer on one side and to an insulating layer on the other. The handy thing about the graphite is that it works like an electrical conductor but without the need to include wire elements, metals, or heat-conducting fibers in the graphite. A heater is made by simply putting a thin layer of laminated graphite onto a substrate and attaching electrodes.
The resistivity of the graphite is about 1 to 8 × 10-4 Ω-in. This is roughly eight times higher than that of metallic heater materials such as nickel-chromium-plated (nichrome) steel wire. Flexible graphite also has a thermal conductivity approaching that of brass. This allows graphite heating elements to use far less energy than any previous type of thermoelectric heater. These qualities, plus a low thermal mass because it is only a few mils thick, let graphite excel at heating up large surface areas rapidly.
The first real application for graphite heaters has been on the wings of small airplanes to prevent ice build-up. Here, Q*Foil film heaters from EGC Enterprises Inc., Chardon, Ohio, have performed better than similar devices made with serpentine-etched metal heaters. Low thermal mass lets the graphite heat wing surfaces rapidly enough to prevent moisture run-off from refreezing on the wing surface. (See www.airplanedeice.com and Scanning the field for ideas, MACHINE DESIGN, 2/21/02.)
Other applications for the material are emerging where a rapid rise to working temperature can provide advantages. For example, Q*Foil heaters are now one-for-one replacements for nichrome in restaurant heating equipment. It once took about a half-hour for such devices to get hot enough to cook pizza, tortillas, and similar dishes. Moreover, they had to stay on continuously to be ready for use throughout restaurant hours. And their heating was uneven, warmest near where the heating wire resided but cooler elsewhere.
In contrast, devices equipped with heating platens incorporating Q*Foil can stay off most of the time. They hit operating temperature so quickly that they can flash-heat food. Heating equipment makers say the energy savings over devices with wire-heated platens are significant. And the low thermal mass of graphite means no dead spots near the platen edges.
It is interesting to compare a graphite heater to a typical metallic resistor element. Metallic resistor elements are designed using a formula that calculates resistance in terms of circular mils/feet. For example, copper has a resistance of 10.37 Ω/circular mil/ft. This is a low number, considering 1 circular mil equates to 7.853 × 10-7 in.2 and typical heater wire has a diameter of 1,020 circular mils or less.
Heater materials range from a resistance as high as 1k Ω/circular mil/ft to as low as 9.755 Ω/circular mil/ft (silver). Low resistivity forces metallic heating elements to be fairly long, because dissipated power is proportional to resistance.
In contrast, flexible graphite has a relatively high resistivity, 3.1 × 10-4 Ω-in. or 3,800 Ω/circular mil/ft, about 380 times that of copper. This high resistivity, plus a thermal conductivity approaching that of brass, allows graphite heating elements to use far less energy than other types of thermoelectric heaters.
In addition, ordinary heater wire has a positive temperature coefficient that depends on the wire material. This may need to be factored into heater-sizing calculations for operation at extended temperatures. Graphite, on the other hand, has a small negative temperature coefficient that can often be ignored in sizing calculations.
An example shows some of the important differences in designs employing heating wire and those using graphite. Say the goal was a 1k-W heater operating from 110-Vac lines. Ohm's law shows that the heating element, regardless of the technology, must be 12.1 Ω. In the case for a nichrome-wire heater, 24-awg wire (0.0201-in. diameter) has a resistance of 1.609 Ω/ft as read from standard tables. Solving for the length that yields 12.1 Ω gives 7.52 ft of wire. The application determines the exact geometry of the wire layout.
The calculations for Q*Foil are not as straightforward but offer several options in the length, width, and thickness of the graphite element. The resistance of Q*Foil comes from
where R = resistance, Ω; L = graphite element length, in.; W = element width, in.; T = element thickness, in.; and C = a resistivity
constant, Ω-in. Its value typically is 3.1 × 10-4.
The approach to sizing the graphite is iterative. One assumes two of the dimensions initially and solves for the third, then revisits the assumptions if necessary. Typical figures are 0.375 and 0.003 in. for the width and thickness of the graphite respectively. Plugging these values in the equation above reveals that the graphite heating element with these dimensions would be 43.9 in. long.
In a real application, the designer might at this point do what-if scenarios with different lengths or widths, depending on specifics of the end use. For example, if it were advantageous to use a 100 in. of graphite in this case, putting this value into the above equation reveals the necessary width is 0.854 in.
BAKE THE FLAKES
Flexible graphite is processed from naturally occurring graphite, the crystalline form of carbon. It comes in rolled sheets ranging in thickness from a few mils to tens of mils. The flexible sheets are made from natural graphite flake treated with an oxidizing agent to form a compound that resides both between layers of the graphite structure and within them.
This material then gets rapidly heated to a temperature high enough to cause these compounds in the graphite crystal to form a gas that makes the layers of the graphite separate. Graphite flakes expand or exfoliate in an accordion-like fashion better than 80-fold in the direction perpendicular to the graphite crystalline planes. This expansion (exfoliation) produces wormlike or vermiform structures with highly active, dendritic rough surfaces. This material then gets compressed or calendered into sheets and rolls.
The expansion process removes all unnatural chemicals from the flake. Molding or calendering involves only mechanical interlocking of the expanded flakes. The resulting sheet is essentially pure graphite that is typically well over 90% elemental carbon by weight. It also has a highly aligned structure. Only naturally occurring minerals remain, taking the form of oxides of metals, typically referred to as ash.
To form a resistor, the graphite sheets get cut into the shape desired and then get laminated. Lamination (usually with a phenolic) doesn't affect the composition of the graphite. This means, among other things, that the resistors can withstand relatively high temperatures limited by the upper max of the laminating resin. (Graphite itself is good to about 450°C in air.)
Another use for graphite sheets is as heat spreaders for power electronic components. In this application, the expanded graphite gets impregnated with resin. The composite material consists of natural graphite in a resin matrix that can be pressed into near-net-shape components or machined into the final shape.
The properties of such materials can be changed depending on the amount of resin and graphite in the mix and the molding technique. For example, it is possible to vary the ratio of in-plane to through-thickness thermal conductivity. Materials with such properties can have a different amount of heat flowing in different directions, something not possible with aluminum or copper. Similarly, electrical resistivity also varies with direction through the graphite/resin material. And graphite heat spreaders weigh much less than equivalent copper or aluminum structures, important in applications where weight is at a premium.
Finally, tests have shown that flexible graphite makes a good electromagnetic shielding material. In one case, researchers found that its shielding effectiveness was between about 100 and 130 dB at 2 GHz, higher than that of solid copper. The shielding range depended on the electrical conductivity of the graphite. Higher shielding came with material that was more conductive.
The making of a graphite platen heater
A typical graphite-based heater consists of a thin layer of flexible expanded graphite foil, also known as vermiform graphite, laminated to an outer heat-conducting layer and an insulating layer (usually mica) that seals the interior of the laminate against moisture. The insulating layer may be directly bonded to the laminate or may be a component of the heated surface to which the laminate is applied.
Only a relatively small amount of electrical power produces a rapid rise in temperature, much less than needed to produce the same effect in etched metal or electrical wire heating systems. Tests of Q*Foil heaters, from ECG Enterprises Inc. in Chardon, Ohio, have measured temperature response rates of 100°C/sec. The thin graphite film can be configured to evenly heat a surface area to temperature stability within ±3%. Designers can also create temperature zones on the platen by making minor changes in the thickness and density of the graphite.
The graphite film upon which these heaters are based is made by heating graphite flakes with an oxidizing agent. The resulting reaction expands the graphite into a form characterized by numerous layers. This material gets calendered or compressed into sheets a few mils thick. These sheets are cut to shape for specific applications and then laminated.