Carbon nanotube arrays hold much potential for high-temperature thermal interface applications that require thermal stability and mechanical compliance. A 1D reference bar test rig designed for measurements at and substantially above room temperature is used to perform thermal cycling tests on CNT arrays and to measure their thermal interface resistance at temperatures up to 700˚C. The CNT arrays are synthesized and tested with both thermomechanically matched (Cu-Cu) and mismatched (alumina-Cu) interfaces to examine their mechanical compliance. For both cases, a moderate decrease in thermal interface resistance (TIR) occurs from room temperature to 700˚C. The results reveal good cyclic stability of CNT arrays in both cases, confirming their ability to accommodate large thermomechanical mismatch. Postmortem analysis indicates that CNT free ends conform to the opposing Cu surface. In the Cu-Cu interface, only compressive deformation of the CNT arrays is observed. In the alumina-Cu interface, more complex thermomechanical deformation occurs that is hypothesized to be a combined effect of compressive loading and shear stress.
A scalable roll-to-roll process is employed to produce graphitic petals. A 1-meter sample of graphitic petals on carbon fiber tow is produced with a roll-to-roll radio-frequency plasma chemical vapor deposition method. Microscale characterization reveals increasingly graphitic carbon structures with production time. Correspondingly, macroscale characterization shows enhanced functional performance with production time: decreasing electrical resistance and increasing capacitance. Using optical emission spectroscopy, important species in the plasma are studied and used to determine the plasma gas temperature. The results demonstrate that 1) successful cost-driven production of graphitic petals is possible, 2) a plasma diagnostic technique is capable of real-time process monitoring, and 3) meaningful material characterization methods are amenable to a production setting. Finally, valuable experiential learnings are shared to inform future equipment design and process development.
In order to measure thermal interface resistance (TIR) at temperatures up to 700 °C, a test apparatus based on two copper 1D reference bars has been developed. Design details are presented with an emphasis on how the system minimizes the adverse effects of heat losses by convection and radiation on measurement accuracy. Profilometer measurements of the contacting surface are presented to characterize surface roughness and flatness. A Monte Carlo method is applied to quantify experimental uncertainties, resulting in a standard deviation of thermal resistance as low as 2.5 mm2 K/W at 700 °C. In addition, cyclic measurements of a standard thermal interface material (TIM) sample (graphite foil) are presented up to an interface temperature of 400 °C. The interface resistance results range between approximately 40 and 100 mm2 K/W. Further, a bare Cu–Cu interface is evaluated at several interface temperatures up to 700 °C.
Electroreflectance microscopy is demonstrated as a high-resolution, non-contact method to image dynamic charge distribution in integrated microsupercapacitor structures during fast voltage cycling. Electroreflectance camera images of a gold electrode H3PO4 polymer electrolyte microsupercapacitor reveal time varying charge distribution with submicron spatial resolution, millisecond time resolution, and electroreflectance resolution on the order of 500 nC cm(-2). A model describing changes in the metal electrode's optical constants as a function of free electron concentration shows good agreement with measured electroreflectance. The proposed method can be used for sensitive, non-contact measurements of charge spatial distribution, and defect and performance characterization in electrode-electrolyte microdevices.
This microsupercapacitor ageing study demonstrates the usefulness of the electroreflectance technique by quantifying local charge accumulation. Two separate devices with interdigitated electrodes were evaulated over a period of 4.1 million charge/discharge cycles. The key results are spatial mapping of charge accumulation in the gold electrodes derived from variation in the observed electrode reflectance. The nominal device exhibited little change in spatial distribution throughout the ageing cycle and serves as a comparison for the test device, which exhibited some nonuniform charge accumulation behavior. Further, an accelerated ageing test was completed by applying increasing voltage pulses up to 1.46 V to the device. Visual evidence of electrode ageing emerged in the reflectance distribution. An equivalent circuit model was developed to assess the evolution of individual circuit elements that correlate to the physical causes of ageing.
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