James W. Klett

High-thermal-conductivity, mesophase-pitch-derived carbon foams: effect of precursor on structure and properties

Abstract Pitch-based carbon foams are not new, but the development of high thermal conductivity foams for thermal management applications has yet to be explored. The research reported here focused on a novel foaming technique and the evaluation of the foaming characteristics of two mesophase pitches (Mitsubishi ARA24 and Conoco Dry Mesophase). After graphitization to 2800°C, densities of the graphite foams ranged from 0.2 to 0.6 g/cm3, with average pore diameters ranging from 275 to 350 μm for the ARA24-derived foams, and from 60 to 90 μm for the Conoco-derived foams. Scanning electron microscopy and polarized light optical microscopy were performed to characterize the cell walls, revealing highly aligned graphitic-like structures along the axis of the ligaments. Analysis of X-ray diffraction results determined that the foams exhibited average interlayer (d002) spacings as low as 0.3355 nm, stack heights (Lc) up to 80 nm and crystallite sizes (La) up to 20 nm. Finally, thermal diffusivity measurements were performed revealing that the bulk thermal conductivity varied with density from 40 to 150 W/m K. The specific thermal conductivities of the graphitized foams were more than six times greater than solid copper.

Carbon foams for thermal management

A unique process for the fabrication of high-thermal-conductivity carbon foam was developed at Oak Ridge National Laboratory (ORNL). This process does not require the traditional blowing and stabilization steps and therefore is less costly. The resulting foam can have density values of between 0.2 and 0.6 g/cc and can develop a bulk thermal conductivity of between 40 and 180 W/m K. Because of its low density, its high thermal conductivity, its relatively high surface area, and its open-celled structure, the ORNL carbon foam is an ideal material for thermal management applications. Initial studies have shown the overall heat transfer coefficients of carbon foam-based heat sinks to be up to two orders of magnitude greater than those of conventional heat sinks.

Carbon/Carbon Composite Bipolar Plate for Proton Exchange Membrane Fuel Cells

Carbon/carbon‐composite bipolar plates for proton exchange membrane fuel cells (PEMFC) have been fabricated by slurry molding a chopped‐fiber preform followed by sealing with chemically vapor‐infiltrated carbon. The resulting component is hermetic with respect to through‐thickness leakage and has a high electronic conductivity (200–300 S/cm) as a result of the deposited graphitic carbon. The material has a low density due to retained porosity resulting in a low‐weight component. Biaxial flexure strength was measured to be 175 ± 26 MPa. Cell testing of a active area, single‐sided plate indicated very low cell resistance and high efficiency, but with a somewhat steep drop‐off in voltage with current at high values. Corrosion testing indicated minimal corrosion in fuel cell environments.

Finite-element modeling of heat transfer in carbon/carbon composites

A finite-element model has been developed to predict the thermal conductivities, parallel and transverse to the fiber axis, of unidirectional carbon/carbon composites. This versatile model incorporates fiber morphology, matrix morphology, fiber/matrix bonding, and random distribution of fibers, porosity, and cracks. The model first examines the effects of the preceding variables on the thermal conductivity at the microscopic level and then utilizes those results to determine the overall thermal conductivity. The model was able accurately to predict the average thermal conductivity of standard pitch-based carbon/carbon composites. The model was also used to study the effect of different composite architectures on the bulk thermal conductivity. The effects of fiber morphology, fiber/matrix interface, and the ratio of transverse fiber conductivity to matrix conductivity on the overall composite conductivity was examined.

Parametric investigation of a graphite foam evaporator in a thermosyphon with fluorinert and a silicon CMOS chip

High thermal conductivity graphitic foam was utilized as the evaporator in a modified thermosyphon. The foam was soldered directly to the back of a silicon CMOS die and mounted in a standard PGA. Fluorinert FC-87 and FC-72 were evaluated as the working fluids of choice and a variety of variables on the foams were explored. It was found that the density of the foam evaporators affected the thermal performance of the system. However, the fluid level and fluid type had very little effect on the overall performance in the system, making fabrication of a commercial device less challenging. The most significant effect on performance was the modifications to the foam structure. Slotted patterns were found to enhance the rate of return of fluid to the foam closest to the die, thus improving performance. With a slotted foam evaporator, a heat flux of 150W/cm/sup 2/ resulted in wall superheats of only 11/spl deg/C. The experimental setup used in this research gives accurate measurements of the actual active layer in the chip and temperatures less than 71/spl deg/C have been achieved at heat fluxes of 150 W/cm/sup 2/. This performance is significantly better than any prior literature data. In fact, the graphite foam thermosyphons were shown to outperform spray cooling. In addition, it was found that critical heat flux was not reached in these experiments with graphite foam evaporators at heat fluxes as high as 150 W/cm/sup 2/.

Flexible towpreg for the fabrication of high thermal conductivity carbon/carbon composites

Abstract A continuous powder coating process was used to produce flexible, preimpregnated towpreg from a heat-treated Mitsubishi AR mesophase pitch powder (AR-120) and three different carbon fibers: T300 PAN-based fiber, P55 pitch-based fiber, and an experimental high thermal conductivity pitch-based ribbon fiber. The towpreg was hot-pressed into unidirectional composites, carbonized at 1100 °C, oxidized and then graphitized at 2400 °C. As expected, the PAN-based fibers developed strong fiber/matrix bonding and the pitch-based fibers developed poor fiber/matrix bonding. This resulted in high flexural strengths (841 MPa) in the graphitized composites reinforced with the T300 fibers and low flexural strengths (196 MPa) in the graphitized composites reinforced with the P55 fibers. In addition, it was found that during consolidation the ribbon fibers oriented normal to the pressing direction. The thermal conductivity (parallel to the fibers) of the graphitized T300/AR-120 and P55/AR-120 composites was 80.5 and 135.5 W/m · K, respectively. These results, along with X-ray analysis, indicated a significant development of preferred crystalline order (parallel to the fibers) upon graphitization at 2400 °C. The composites reinforced with ribbon fibers exhibited three-dimensional anisotropy, with a thermal conductivity (transverse to the fibers) of 213.5 W/m · K, higher than that parallel to the fibers (145 W/m · K). These results indicated that fiber shape can affect matrix properties in carbon/carbon composites. Finally, the towpreg was woven into a two-dimensional fabric, demonstrating that towpreg can be used to produce preimpregnated multidimensional composite preforms. Towpreg may provide a low-cost route for producing carbon/carbon composites.

Heat Exchangers for Heavy Vehicles Utilizing High Thermal Conductivity Graphite Foams

Approximately two thirds of the worlds energy consumption is wasted as heat. In an attempt to reduce heat losses, heat exchangers are utilized to recover some of the energy. A unique graphite foam developed at the Oak Ridge National Laboratory (ORNL) and licensed to Poco Graphite, Inc., promises to allow for novel, more efficient heat exchanger designs. This graphite foam, Figure 1, has a density between 0.2 and 0.6 g/cm 3 and a bulk thermal conductivity between 40 and 187 W/m{center_dot}K. Because the foam has a very accessible surface area (> 4 m 2 /g) and is open celled, the overall heat transfer coefficients of foam-based heat exchangers can be up to two orders of magnitude greater than conventional heat exchangers. As a result, foam-based heat exchangers could be dramatically smaller and lighter.

Estimation of the thermal conductivity of composites

In this article we introduce the concept of homogenization for the approximation of the effective thermal conductivity of composites. A simple algebraic approximation method is proposed and shown to yield an upper bound for the effective conductivity. Numerical results are given for uni-directional carbon-carbon composites which demonstrate the validity of the approach.

Carbon composite for a PEM fuel cell bipolar plate

The current major cost component for proton exchange membrane fuel cells is bipolar plate. An option being explored for replacing the current, nominal machined graphite component is a molded carbon fiber material. One face and the volume of the component will be left porous, while the opposite surface and sides are hermetically sealed via chemical vapor infiltration of carbon. This paper will address initial work on the concept.

Graphite Foams for Lithium-Ion Battery Current Collectors

Graphite open-cell foams, with their very high electronic and thermal conductivities, may serve as high surface area and corrosion resistant current collectors for lithium-ion batteries. As a proof of principle, cathodes were prepared by sintering carbon-coated LiFePO4 particles into the porous graphite foams. Cycling these cathodes in a liquid electrolyte cell showed promising performance even for materials and coatings that have not been optimized. The specific capacity is not limited by the foam structure, but by the cycling performance of the coated LiFePO4 particles. Upon extended cycling for more than 100 deep cycles, no loss of capacity is observed for rates of C/2 or less. The uncoated graphite foams will slowly intercalate lithium reversibly at potentials less than 0.2 volts versus lithium.