Carbon-composite submounts tipped with diamond are being developed as improved means of dissipating heat generated in high-power laser diodes. Copper is the traditional heat-sinking material for many applications other than laser diodes; it is not suitable for heat-sinking submounts for laser diodes because its coefficient of thermal expansion (CTE) is too high to enable an acceptably close match to the CTEs of laser-diode semiconductor materials. Heretofore, heat-sinking submounts for laser diodes have been made from a copper/tungsten alloy, chosen because of its rigidity and its low CTE, which matches the CTEs of the laser-diode semiconductor materials more closely than copper does. Unfortunately, the thermal conductivity of the copper/tungsten alloy is only 45 percent of that of copper. In contrast, the carbon composites of the present development can be made to have both low CTEs and effective thermal conductivities of the order of three times that of copper.
The carbon-composite materials under consideration in the present development effort include, variously, graphitic or vapor-grown carbon fibers in matrices that comprise one or more other forms of carbon and that can include diamond-like carbon. Metals (typically, copper or aluminum) can be used as alternative matrix materials to increase effective thermal conductivities. Like other composite materials, these composites can be formulated to tailor their thermal and mechanical properties within the limits imposed by the intrinsic properties of the constituent materials.
The thermal conductivities of these composites are much higher in the along-fiber directions than in the cross-fiber directions. This anisotropy must be taken into account in designing a heat-sinking submount, as in the example illustrated in the figure. The laser diode is mounted on a wedge made of either chemical-vapor-deposited diamond (which has about twice the thermal conductivity of copper) or single-crystal diamond (which has about five times the thermal conductivity of copper). The diamond wedge conducts heat away from the laser diode. The slanted face of the diamond wedge distributes some of the heat to a mating carbon/carbon composite wedge that contains horizontal fibers and that conducts this portion of the heat into a main carbon/carbon heat-sink body that also contains horizontal fibers. The slanted face of the diamond wedge also distributes some of the heat downward into a larger carbon/carbon composite wedge that contains vertical fibers. These vertical fibers meet the horizontal fibers of the main heat-sink body at mating slanted wedge surfaces. The heat-sink body conducts the heat away horizontally. The far end (the right end in the figure) of the heat-sink body is placed in contact with a heat pipe, radiator panel, or other suitable heat sink.
This work was done by Sang Hyouk Choi and Howard G. Maahs of Langley Research Center.